KR980012540A - Low Volatile Solvent Substrate Method for Forming Thin Film Nanoporous Aerogel on Li-Semiconductor Substrate - Google Patents

Abstract

The present invention enables a novel simple thin film nanoporous dielectric manufacturing method. In general, the present invention uses glycerol or other low volatility compounds as solvents. The novel process of the present invention allows for the preparation of thin film aerogels / low density xenogels without critical drying, freeze drying or surface modification steps prior to drying. Prior art aerogels required at least one of the above steps to prevent the voids from substantially collapse during drying. Therefore, the present invention allows for the fabrication of nanoporous dielectrics at room temperature and atmospheric pressure without separate surface modification steps. In general, the novel method of the present invention is compatible with most prior art airgel techniques. The novel process of the present invention allows the preparation of aerogels without substantial collapse of voids during drying, but there may be some permanent shrinkage during aging and / or drying. The present invention allows for porosity control of thin film aerogels deposited, gelled, aged and dried without atmospheric control. In another aspect, the present invention allows the control of porosity of deposited, gelled, rapid aging at elevated temperatures and dried thin film nanoporous aerogels by only passive atmospheric control, such as limitation of the aging chamber volume.

Description

Low Volatile Solvent Substrate Method for Forming Thin Film Nanoporous Aerogel on Li-Semiconductor Substrate

1 is a graph showing variation in evaporation rate with respect to saturation ratio and solvent form.

2 is a graph showing evaporation rates for glycerol as a function of temperature and atmospheric saturation ratio

3 is a graph showing the theoretical relationship between porosity, refractive index and dielectric constant for nanoporous silica dielectric.

4 is a graph showing the change in gelation time (without solvent evaporation) for a bulk ethylene glycol based gel as a function of the substrate catalyst.

5 is a graph showing modulus variation with respect to density in non-glycol based gels and ethylene glycol based gels.

6 is a graph showing the distribution of pore sizes of the bulk glycerol based nanoporous dielectrics according to the present invention.

7 is a graph showing evaporation rates for ethylene glycol as a function of temperature and atmospheric saturation ratio

8 is a graph showing the change in vapor pressure with temperature.

9 is a graph showing thin film shrinkage when dried in a 5 mm thick container.

10 is a graph showing shrinkage of a thin film when dried in a 1 mm thick container.

11A and 11B are graphs showing viscosity variation as a function of alcohol volume fraction for some ethylene glycol / alcohol and glycerol / alcohol mixtures.

12A and 12B are cross-sectional views of a semiconductor substrate at some point during the deposition of a thin film in accordance with the present invention.

Figure 13 is a flow chart showing the teaming process for the nanoporous dielectric according to the present invention.

14 is a graph showing the theoretical molar ratio of glycerol molecules to metal atoms versus the porosity of nanoporous dielectrics according to the present invention.

15 is a graph showing relative film thickness and relative film porosity as a function of time for one embodiment of the present invention.

16A and 16B are sectional views and partial reduction views of the sol-gel thin film manufacturing apparatus according to the present invention, respectively.

16C is a cross sectional view of the device in contact with a substrate.

17A and 17B are cross-sectional views of a device incorporating a substrate and a cross section of a dyne device according to the present invention without a substrate, respectively.

Figure 18a 18b shows a cross-sectional view of another device according to the invention, each without an engine, and a sectional view of the device with a substrate.

19a, 19b and 19c illustrate cross-sectional views of additional device arrangements illustrating other aspects of the present invention.

* Explanation of symbols for main parts of the drawings

10 semiconductor substrate 12 conductor

13: gap 14: sol thin film

18: membrane 20: body

22 plate 24: condensation unit

26 substrate 28 substrate surface

30: chamber surface 32: chamber

34, 38: temperature control means 36: holder

40: first solvent supply pipe 42: first solvent layer

44: atmospheric control means 46: port

[Purpose of invention]

[Technical Field to which the Invention belongs and Prior Art in the Field]

The present invention generally relates to precursors and deposition methods for forming thin film nanoporous airgels on semiconductor substrates, including deposition methods suitable for the production of airgel thin films of nanoporous dielectrics.

Aerogels are porous silica materials that can be used for a variety of purposes, including as membranes (eg, electrical insulators or optical coatings on semiconductor devices) or in bulk (eg, thermal insulator) forms. For ease of discussion, examples herein will primarily be for use as electrical insulators on semiconductor devices.

Semiconductors are widely used in integrated circuits for electronic devices such as computers and televisions. In addition to semiconductors and electronics, end users require integrated circuits that can be achieved in smaller packages in less time with less power. However, many of these requirements are contrary to each other. For example, a simple shrinkage of the minimum wiring width from 0.5 microns to 0.25 microns on a given circuit can increase energy usage and heat generation rate by 30%. Miniaturization also generally increases electrostatic coupling or crosstalk between conductors carrying signals through the chip. Both of these effects limit the speed attainable and reduce the noise margin used to ensure proper device operation. One way to reduce energy usage / heat generation and crosstalk effects is to reduce the dielectric constant of the insulator, or the dielectric separating the conductor. U.S. Patent 5,470,802, issued to Gnade et al., Provides a background for some of these schemes.

A group of materials, nanoporous dielectrics, includes some of the most promising new materials for semiconductor manufacturing. Such dielectric materials include solid structures, such as silica, which typically penetrates a network of pores, typically several nanometers in diameter. Such materials can be formed with extremely high porosities, which typically have a dielectric constant of less than half the dielectric constant of dense silica. However, despite their high porosity, it has been found that nanoporous dielectrics having high strength and excellent compatibility can be produced by most existing semiconductor manufacturing processes. Therefore, nanoporous dielectrics provide a viable low dielectric constant in place of conventional semiconductor dielectrics such as dense silica.

A preferred method for forming nano multiphoto dielectrics is to use sol-gel techniques. Sol-gel refers to a reaction mechanism, not a product, where a sol is a colloidal suspension of solid particles in a liquid which is converted into a gel due to the growth and interconnection of the solid particles. The reaction allows one or more molecules of the sol to reach the macroscopic level, forming a solid network that extends substantially throughout the sol. The substance at this time (called the gel point) is called the gel. By this definition, a gel is a substance containing a continuous solid skeleton that surrounds a continuous liquid phase. As the backbone is porous, the term "gel" as used herein means a pore open-pored solid structure surrounding the pore fluid.

One method of forming the sol is through hydrolysis and condensation reactions, which can cause polyfunctional monomers in solution to polymerize into relatively large, branched particles. Many monomers suitable for this polymerization are metal alkoxides. For example, tetraethoxysilane (TEOS) monomers can be partially hydrolyzed in water by the following Scheme 1

Scheme 1

The reaction conditions can be adjusted so that, on average, each monomer performs the desired number of hydrolysis reactions to partially or completely hydrolyze the monomers. Fully hydrolyzed TEOS becomes Si (OH) ₄ . Once the molecules have been at least partially hydrolyzed, two molecules can be joined together in a condensation reaction to form oligomers and release water or ethanol molecules, as in Schemes 2 or 3 below.

Scheme 2

Scheme 3

The Si-0-Si arrangement of the oligomers formed by this scheme has three sites available at each end for further hydrolysis and condensation reactions. Therefore, additional monomers or oligomers can be added to these molecules in a slightly random manner to actually produce polymer molecules with very high side chains from thousands of monomers. Oligomeric metal alkoxides as defined herein include molecules formed from at least two alkoxide monomers, but do not include gels.

Sol-gel reactions form the basis of xerogel and aerogel film deposition. In a typical thin film xenogel manufacturing method, an ungelled precursor sol is coated onto a substrate (e.g. spray coating, dip coating or spin coating) to form a thin film of several microns or less, gelled and dried. To form a dense film. The precursor sol often includes a raw material solution and a solvent, and it is also possible to include a gelling catalyst that modifies the pH of the precursor sol for rapid gelation. During and after coating, the volatile components in the sol thin film typically evaporate rapidly. Therefore, the deposition, gelling, and drying steps can occur simultaneously (at least to some extent) as the film rapidly disintegrates into a dense film. In contrast, the airgel preparation method differs considerably from the xenogel preparation method by avoiding the collapse of voids during drying of the wet gel. Some methods to avoid pore collapse include treating the wet gel with condensation inhibiting modifiers (described in Gnade's' 802 patent document) and supercritical pore fluid extraction.

For aerogels and xenogels, aerogels are preferred as gel materials dried twice for semiconductor thin film nanoporous dielectric applications. Typical thin film xenogel methods form films with limited porosity (large pore sizes of at least 60%, but substantially less than 50% of pore sizes useful for submicron semiconductor fabrication). Some prior art xenogels have porosities greater than 50%, while these prior art xenogels have substantially large pore sizes (typically over 100 nm). Such large pore size gels have significantly less mechanical strength. In addition, these large pore sizes make them unsuitable for filling small (typically less than 1 mm, and potentially less than 100 nm) patterned gaps on microcircuits, limiting their optical film applications to only long wavelengths. do. On the other hand, the nanoporous airgel thin film can be combined with a very fine pore size to form almost any desired porosity. In general, as used herein, nanoporous materials have an average pore size of less than about 25 nm in diameter, but are preferably less than 20 nm (more preferably less than 10 nm, particularly more preferably less than 5 nm). In many manufacturing methods using this method, typical nanoporous materials for semiconductor applications are often at least 3 nm, although the average pore size can be at least 1 nm in diameter. Nanoporous inorganic dielectrics include nanoporous metal oxites, in particular nanoporous silica.

In many nanoporous thin film applications such as aerogels and xenogels used as optical membranes or in microelectronics, it is desirable to precisely control film thickness and aerogel density. Some important properties of the membrane are related to the airgel density, including mechanical strength, pore size and dielectric constant. We have found that both aerogel density and film thickness are related to the viscosity of the sol when applied to the substrate. This presents a problem that has not been identified so far. The problem is that it is extremely difficult to control the airgel density and film thickness independently and accurately compared to conventional precursor sol and deposition methods.

Nanoporous dielectric thin films can often be deposited on patterned wafers beyond the level of patterned conductors. The inventors have found that sol deposition should be terminated before the initiation of the gelling reaction so that the gap between the conductors is adequately filled and the gel surface remains substantially flat. For this purpose, it is desirable that no significant evaporation of the pore fluid occurs after the gelling reaction, such as during aging. Unfortunately, it is necessary to reach the gel point as soon as possible after deposition for simple processing, and one method for high speed gelling of thin films is to allow evaporation. It has been found that precursor sol suitable for airgel deposition must allow control of film thickness, airgel density, gap filling and top view, must be relatively stable before deposition, and also gelate relatively immediately after deposition and age without substantial evaporation.

We have found a method that allows for controlled deposition of aerogel thin films from a multisolvent precursor sol. In this method, the sol viscosity and the film thickness can be controlled relatively independently. This method allows the aerogel density to be changed quickly by changing the film thickness from a known first value to a known second value which can be set by solvent ratio and spin conditions. Very independently maintains film thickness and allows for rapid gelling. At the same time, however, the solid: liquid ratio (and airgel density) present in the film upon drying can be accurately measured in the precursor sol before deposition, independent of spin conditions and film thickness.

Although the deposition problem is newly separated into the lower problem of viscosity control and density control, the inventors have found that thin film sol-gel techniques for xenogel and aerogel formation are generally limited to evaporation prior to drying, such as during air conditioning, after gelling and during aging. I know that it requires some method such as In general, such evaporation rate control can be achieved by adjusting the solvent evaporation concentration beyond the wafer. However, we know that solvent evaporation rate is very sensitive to small changes in vapor concentration and temperature. In an effort to better understand this method, we modeled isothermal evaporation of some solvents from the wafer as a function of percent saturation. The ambient temperature evaporation rate for some of these solvents is shown in FIG. 1 For evaporation without processing problems, the product of the evaporation rate and processing time (preferably in the order of minutes) should be significantly less than the membrane thickness. This suggests that for solvents such as ethanol, an atmosphere larger than the wafer should be maintained at a saturation rate of at least 99%. However, there may be a problem associated with the atmosphere reaching saturation or supersaturation. Some of these problems relate to the condensation of atmospheric components on thin films. In gelled or non-gelled thin films

It has been found that condensation on either one causes defects in insufficiently aged membranes. Therefore, it is generally desirable to adjust the atmosphere to such an extent that the components are not saturated.

The inventors have found that using a low volatility solvent with less atmospheric control than using a high volatility solvent and precisely controlling the solvent atmosphere is a better solution. By studying this premise, we found that glycerol is an excellent solvent.

Glycerol is used to loosen the desired atmospheric control (relative to prior art solvents) during deposition, gelling and / or aging. This is because even when it is desirable to still avoid saturation, the atmospheric solvent concentration can be lowered without excessive evaporation. 2 shows how the evaporation rate of glycerol varies with temperature and atmospheric solvent concentration. We have found that glycerol can be used to form acceptable gels by depositing, gelling and aging in an uncontrolled or substantially uncontrolled atmosphere.

In the preparation of nanoporous dielectrics, it is desirable to perform a process known as aging on wet gel thin films. The hydrolysis and condensation reactions do not stop at the gelation point and continue to restructure or age the gel until the reaction is stopped intentionally. During aging, it is believed that some of the solid structures provide advantageous results, including high strength, high uniformity of pore size, and high resistance to pore collapse during aging, preferably by dissociation and redeposition. Unfortunately, the present inventors have found that the conventional aging techniques used for bulk gels are inadequate for aging thin films in semiconductor processing, partly because they require immersion of the substrate, and in part may terminate for several days or It takes a few weeks. One aspect of the present invention involves vapor phase aging, which avoids premature drying of the immersion or wet gel thin films, and can surprisingly age the thin films in moisture.

In other words, aerogels are nanoporous materials that can be used as membranes or for a variety of purposes, including in bulk. However, it should be noted that the problems that arise in film fabrication processing are different from the bulk processing problem, and for practical purposes film processing is not similar to bulk fabrication processing.

In general, the inventors have found that aging in saturated atmosphere avoids the problems associated with immersion aging. In addition, this aspect of the invention provides some approaches to aging the wet gel at elevated temperatures. This method can be used even when the wet gel contains essentially low boiling pore fluid. However, they work better with low volatility solvents. Finally, this aspect of the invention provides a method of adding any vapor phase aging catalyst to the aging atmosphere for rapid aging.

Aging of the wet gel in the form of a thin film is difficult because very small amounts of pore fluid must be held fairly constant for a predetermined time in order to generate aging. If the void fluid evaporates from the membrane before aging strengthens the network, the membrane will be densified in xenogel form. On the other hand, if excess void fluid is condensed from the atmosphere into the thin film before the network is strengthened, this can locally disrupt the aging process and cause membrane defects.

Accordingly, the inventors have found that some methods of controlling the pore fluid evaporation rate during aging are advantageous for the production of aerogel thin films. In general, evaporation rate control during aging can be achieved by actively controlling the pore fluid vapor concentration beyond the wafer. However, for example, 70% porosity of 1 mm thick deposited on a 150 mm wafer

The total amount of void fluid contained in the wet gel with is only about 0.012 ml, which amount will easily fit into a single fluid liquid of 3 mm diameter. Typical thin films used as nanoporous dielectrics on semiconductor wafers are about 1000 times thinner. Therefore, allow void fluid vapor concentrations (eg by 1% or less) (by adding or removing solvent to the atmosphere).

To be actively regulated and void fluid evaporation during aging provides a difficult proposal, the surface area of the thin film is high and the allowable resistance to void fluid fluctuations is extremely small. In particular, the control of evaporation and condensation reactions is particularly important for rapid aging at elevated temperatures, where the process of film formation has certainly not been practically possible until now.

The inventors have overcome the problem of controlling the evaporation rate by making no attempt to actively adjust the pore fluid vapor concentration beyond the wafer. Instead, the wafer is processed in an extremely small volume chamber so that through natural evaporation of the relatively small amount of pore fluid contained in the wet gel membrane, the processing atmosphere is substantially not saturated with the pore fluid. If the wafer is not cooled at any point in the substantially saturated processing atmosphere, this method also naturally avoids the problems associated with condensation, especially during high temperature processing.

[Technical problem to be achieved]

Disclosed herein is a method for forming a thin film nanoporous dielectric on a semiconductor substrate. The method includes providing a semiconductor substrate and depositing a nanoporous airgel precursor sol on the substrate. The airgel precursor sol comprises a metal based airgel precursor reactant and a first solvent comprising glycerol, wherein the molar ratio of glycerol molecules to metal atoms in the reactants is at least 1:16. The method also includes forming a dry nanoporous dielectric by allowing the creation of a gel consisting of a porous solid and a pore fluid from the deposited sol and removing the pore fluid in a dry atmosphere without substantially disrupting the porous solid. In this way, the pressure of the dry atmosphere during the forming step is below the critical pressure of the void fluid, preferably near atmospheric pressure.

Preferably, the aerogel precursor reactant may be selected from the group consisting of metal alkoxides, at least partially hydrolyzed metal alkoxides, particulate metal oxides, and mixtures thereof. Preferably, the aerogel precursor reactant comprises silicon. In some embodiments, the aerogel precursor reactant is TEOS. Typically, the molar ratio of glycerol molecules to metal atoms in the reactants is less than 12: 1, and preferably the molar ratio of paracerol molecules to metal atoms in the reactants is from 1: 2 to 12: 1. In some embodiments, the molar ratio of glycerol molecules to metal atoms in the reactants is 2.5: 1 to 12: 1. In this method, the porosity of the nanoporous dielectric is preferably greater than 60% and the average pore diameter is less than 25 nm. In some embodiments, the airgel precursor also includes a second solvent. Preferably, the boiling point of the second solvent is lower than that of glycerol. In some embodiments, the second solvent can be ethanol. In some embodiments, the first solvent consists of glycol and is preferably selected from the group consisting of ethylene glycol, 1,4-butylene glycol, 1,5-pentanediol and mixtures thereof. In some embodiments, the aging fluid is replaced with a drying fluid after aging but before drying. This allows, for example, drying quickly, low temperatures (eg room temperature) so that the fluid evaporates faster and the surface tension is suitably small. Examples of dry fluids are heptane, ethanol, acetone, 2-ethyl butyl alcohol and some alcohol-water mixtures.

Therefore, the present invention allows for control of the porosity of deposited, gelled, aged and dried thin film nanoporous aerogels without atmospheric control. In another aspect, the present invention allows for the control of porosity of deposited, gelled, rapid aging at elevated temperature and dried thin film nanoporous aerogels by passive atmospheric control such as volume limitation of the aging chamber.

[Configuration and Function of Invention]

Typical sol-gel thin film manufacturing processes provide gels that disintegrate and densify upon drying to form xenogels with only a few percent porosity. Under uncontrolled drying conditions of xenogel film formation, it was not important or possible to completely separate the deposition, flocculation, gelling and drying steps during thin film formation because the whole process could be terminated in minutes. However, we find that this method is generally unsuitable for depositing high porosity thin films with controllable low densities, where in the drying step in the form of aerogels the membrane remains substantially undensified after drying, and its final density is alternately It was found that this is because it is determined by the solid: liquid ratio in the membrane at the gel time. The inventors have found that the following criteria are particularly desirable for aerogel thin film deposition and / or gap filling of patterned wafers where thin film planarization is desired.

1) initial viscosity suitable for spin-on application,

2) stable viscosity during deposition,

3) stable film thickness at gelation time,

4) a predetermined solid: liquid ratio at the gelling time, and

5) Gelation immediately after deposition.

Prior art precursor sols and methods have not been found to meet the above conditions. However, it has been found that the above conditions can be met using a sol prepared in a specific proportion of at least two solvents.

Methods of depositing and gelling the precursor sol described above will be well understood with reference to FIG. 15.

As shown in FIG. 15, at time t = 0, the multisolvent precursor sol can be spun onto the wafer at the initial film thickness Do and the initial viscosity ho, which is preferably low, which greatly inhibits evaporation of the low volatility solvent from the wafer. It is carried out in a controlled atmosphere with a partial pressure of volatile solvents. Therefore, after spin-on application, it is desirable to remove the high volatility solvent from the wafer during the evaporation time period T _1, while maintaining the low volatility solvent to reduce the film thickness to D ₁ . The viscosity also changes during this time, preferably mainly due to the removal of the solvent, to h _1. At the end of T ₁ , substantially all of the high volatile solvent has to be evaporated and at this time the film thickness shrinks at a stable or even reduced rate. To provide the desired liquid: solid ratio and thickness for the thin film at gelation time.

The time period T ₂ is primarily intended to provide a separation between the end point of the evaporation time period T ₁ and the gelling junction occurring during the gelation time period T ₃ . Preferably, time period T ₂ is greater than zero. However, the precursors with some precursors, especially solvents such as glycerol to accelerate the gelation reaction, will be gels at the end of cycle T ₁ . In addition, a vapor phase catalyst such as ammonia can be introduced into the established atmosphere during the time period T ₁ or T ₂ . This catalyst can diffuse into the thin film to make the sol more active and to promote a rapid crosslinking reaction. Although it is desirable that little or no evaporation occurs during T ₂ , the crosslinking reaction must continue to increase substantially since the crosslinking reaction continues to bond the polymerizable clusters.

Evaporation after the gel point can exhibit poor gap filling and planarity for the patterned wafer. As a result, after the gelling time T ₃ , the film thickness is preferably kept substantially constant until it passes through the gel point by limiting evaporation. Occasionally, during the time period T ₃ , a surprising change in viscosity occurs as the sol approaches the gel point, where the macropolymerizable clusters finally combine to create spanning clusters that cross the thin film continuously.

Some advantages of this novel approach are apparent from FIG. 15. Both the sol viscosity and the film thickness allow for a rapid change, but generally not at the same time. In addition, the film thickness can be quickly changed from a known first value to a known second value, which can be set independently by solvent ratio and spin conditions. This method can be used to coat low viscosity films, quickly reduce to a desired thickness and gel quickly to the desired density.

The above paragraphs teach how to change the precursor sol viscosity independently of the dried gel density. However, the problem of which solvent is most suitable remains. We have found that solvent evaporation rates for typical airgel solvents are very sensitive to small changes in vapor concentration and temperature. In an effort to better understand this method, we modeled isothermal evaporation of some solvents from the wafer as a function of saturation percentage. This modeling is based on mass transfer theory. RB Bird, WE Stewart and EN Lightfoot, Trasport Phenomena , in particular Chapters 16 and 17, are good references to mass transfer theory. Calculations were performed here for a range of solvents. The ambient temperature evaporation rate for some of these solvents is shown in FIG. 1. For evaporation without processing problems, the product of evaporation rate and processing time (preferably in the order of minutes) must be significantly less than the film thickness. This suggests that for solvents such as ethanol, an atmosphere larger than the wafer should be maintained at a saturation rate of about 99% or more. However, there may be a problem associated with the atmosphere reaching saturation or supersaturation. Some of these problems relate to the condensation of atmospheric components on thin films. It has been found that condensation on either gelled or non-gelled thin films causes defects in poorly aged films. Therefore, it is generally desirable to adjust the atmosphere to such an extent that the components are not saturated.

The inventors have found that using a high volatility solvent and using a low volatility solvent with less atmospheric control than accurately controlling the solvent atmosphere is a better solution. By studying this premise we found that glycerol is an excellent solvent.

Glycerol is used to loosen the desired atmospheric control (relative to conventional solvents) during deposition, gelling and / or aging. This is because even when it is desirable to still avoid saturation, the atmospheric solvent concentration can be lowered without excessive evaporation. 2 shows how the evaporation rate of glycerol varies with temperature and atmospheric solvent concentration. We have found that glycerol can be used to form acceptable gels by depositing, gelling and aging in an uncontrolled or substantially uncontrolled atmosphere. In this most preferred approach, controlling the atmosphere (substantially uncontrolled atmosphere) is limited to controlling the clean room temperature and humidity during deposition and gelling reactions, although the temperature of the wafer and / or precursor sol can be controlled independently.

One attractive feature when using glycerol as a solvent is that the evaporation rate is sufficiently small at ambient temperature so that at ambient conditions several hours will not provide detrimental shrinkage for the thin film. We have found that glycerol can be used to form acceptable gels by depositing, gelling and aging in an uncontrolled or substantially uncontrolled atmosphere. With glycerol, the ambient temperature evaporation rate is sufficiently small that several hours at ambient temperature will not provide harmful shrinkage to the thin film. In addition, the inventors have found that ethylene glycol can be used to form acceptable gels by depositing and gelling in an uncontrolled or substantially uncontrolled atmosphere. With ethylene glycol, the ambient temperature evaporation rate is greater than glycerol, but still small enough so that moisture at ambient temperature will not provide harmful shrinkage to the thin film. However, ethylene: glycol based sols have a significantly lower viscosity compared to glycerol based sols, so that the deposition reaction is simple. In addition, the pore fluid in the glycerol based sol has a significantly greater surface tension compared to the ethylene glycol based sol and therefore is more difficult to dry in low shrinkage.

In addition to serving as a low vapor pressure and water miscible solvent, ethylene: glycol and .glycerol may also be involved in the sol-gel reaction. The exact scheme of this method has not been fully studied, but one can expect one. If tetraethoxysilane (TEOS) is used as the precursor, ethylene glycerol can be exchanged with ethoxy groups as in Scheme 4 below.

Scheme 4

Similarly, if tetraethoxysilane (TEOS) is used as a precursor with a glycerol solvent, glycerol can be exchanged with ethoxy groups as in Scheme 5 below.

Scheme 5

In general, the presence and concentration of such chemical groups change precursor reactivity (ie, gelation time), denature gel microstructures (surface area, pore size distribution, etc.), change aging characteristics, or Other characteristics can be changed.

The use of novel solvent systems can change a wide range of processing parameters including gelation time, viscosity, aging conditions and dry shrinkage. Many of these properties, such as gelation times, are difficult to measure in thin films. Bulk film and thin film properties may be different, but this is because performing a series of experiments on bulk samples (eg, approximately 5 mm in diameter and 30 mm in length) is often a matter of changing the solvent system in nanoporous silica processes. It is often useful to provide a better understanding of the impact.

Glycerol can be reacted with TEOS to produce dry cells having properties that are surprisingly different from those of ethanol / TEOS gels. Unexpected property changes in gels based on glycerol / TE0S include the following (at least in most formulations):

Lowering of the viscosity can be obtained without supercritical drying or surface modification before drying.

Greatly simplified aging.

Shorter gelling time even when no catalyst is used.

The strength of the bulk sample (at a certain density) that is approximately over the size of a conventional TEOS gel.

Very large surface area (˜1,000 m 2 / g).

High optical clarity of bulk samples (due to narrow pore size distribution).

<Low density>

With the present invention, it is possible to form dry gels at very low densities without surface thickening or supercritical drying before drying. Such low densities can generally be lowered to about 0.3 to 0.2 g / cm 3 (nonporous SiO 2 has a density of 2.2 g / cm 3) or, if noted, to less than 0.1 g / cm 3. In terms of porosity (porosity is a percentage of hollow structure), it represents about 86 to 91% porosity (about 95% porosity at density 0.1 g / cm 3). As shown in FIG. 3, the light intensity corresponds to a dielectric constant of about 1.4 for 86% porosity and 1.2 for 91% porosity. The actual mechanisms that enable such high porosity are not fully known. However, it may be because the gel has high mechanical strength and does not have OH (hydroxyl) groups, combinations thereof, or some other factors at many surfaces. This method also appears to yield superior uniformity throughout the wafer.

If desired, this method can be adjusted (by varying the TEOS / solvent ratio) to provide porosity from about 90% to about 50%. Typical prior art dry gels with small pore sizes required a supercritical drying or surface modification step prior to drying to achieve this low density. Some prior art dry gels have more than 50% porosity; These prior art dry gels have a fairly large pore size (typically at least 100 nm). Such large pore size gels have significantly lower mechanical strength. In addition, this large size makes the gel unsuitable for filling small (typically less than 1 μm) patterned gaps on microcircuits. If necessary, this method can be adjusted to provide a porosity of less than 50% (by varying the TEOS / solvent ratio). Less than 20% porosity is possible with care to prevent immature gelling.

Thus, the present invention enables a novel simple nanoporous low density dielectric manufacturing method. This novel glycerol based method allows the preparation of both bulk membrane and thin film aerogels without supercritical drying or surface modification steps prior to drying. Prior art aerogels require at least one of the above steps to prevent substantial void collapse during drying.

<Density prediction>

By changing the ratio of glycerol to silicon (or other metals), the density after drying can be accurately predicted. This accuracy is believed to be due to evaporation as well suppressed as possible by low volatility glycerol solvents. Since the method shows superior suppression of shrinkage during aging and drying, this allows an accurate prediction of the density of the dry gel (and thus porosity). Density prediction was generally not considered a major problem with bulk gels, but it was typically difficult to predict the final porosity of a thin film gel. Even for low porosity dry gels, this accurate density prediction is why this new method is preferred over the existing dry gelation methods for forming low porosity gels.

<Simplified Aging>

The inventors have found that it is desirable to carry out a method known as aging on wet gel thin films in the manufacture of nanoporous dielectrics. The hydrolysis and condensation reactions do not stop at the gel point, and the gel is reconstituted or aging continues until the reaction ultimately ends. It is believed that preferential dissolution and redeposition of a portion of the solid structure during aging produces favorable results. Such advantageous results include high strength, large uniformity of pore size, and greater ability to prevent void collapse during drying. However, it is difficult to age the wet gel in a thin film form because very few voids are contained in the membrane, which must be kept almost constant for the time required to cause aging. If the void fluid evaporates from the membrane before aging strengthens the network, the membrane tends to dense in the form of a dry gel. On the other hand, if excess void fluid is condensed from the atmosphere onto the thin film before the network is strengthened, this can locally interfere with the aging process and cause membrane defects.

The novel glycerol based method of the present invention greatly simplifies aging of thin film nanoporous dielectrics. Other thin film nanoporous dielectric aging processes require significant evaporation, fluid condensation, or require a controlled aging atmosphere. During deposition and gelation these glycerol based methods behave similarly to the ethylene glycol based methods described below to some extent. However, ethylene glycol based gels typically require atmospheric control to prevent significant evaporation during aging even at room temperature. In contrast, glycerol based gels dramatically reduce evaporation and shrinkage during aging. This allows to loosen or eliminate atmospheric regulation during aging. The present invention allows the production of high quality thin film glycerol based nanoporous dielectrics using only passive air conditioning during room temperature or high temperature aging.

<Shortened gelation time>

The use of glycerol substantially shortens the gelling time. Many typical ethanol based precursors have a gelation time of at least 400 seconds when catalyzed (much longer than W / O catalyzed). However, the inventors have found that some glycerol based precursors can be gelled during wafer spin on without catalysis. Such rapid gelation is not only much faster than ethanol based gels, but also surprisingly much faster than ethylene glycol based gels. 4 shows the gelation time for two different ethylene glycol based compositions as a function of the amount of ammonia catalyst used. This gelling time is for bulk gels without evaporation of ethanol and / or water, which may be possible for thin films. Evaporation increases the silica content, thus reducing the gelation time. The gelation time as shown in FIG. 4 is approximately smaller in size than conventional ethanol based precursors. Gelation time generally indicates a primary dependence on the concentration of the ammonia catalyst. This means that the gelation time can be easily controlled.

In the case of thin films of such novel glycerol based gels, it is common to obtain gelation without using a gelling catalyst in a few seconds. Applicants have identified a number of mechanisms that can be used to initiate gelation in thin films without the addition of a catalyst. . One method is concentration of the precursor sol by evaporating the volatile solvent. Another method is to increase the pH by evaporating the acid in the precursor sol. This evaporative basicization method is due to an increase in precursor sol pH to aid in initial gelation. However, this basicization method typically does not require a pH change from less than 7 to more than 7. This evaporative basicization works similarly to typical base catalysis methods and greatly promotes gelation. Some acids, such as nitric acid, at room temperature and pressure have an evaporation rate comparable to that of Ethanol. Changing the high volatile solvent type and / or concentration and / or stabilizing acid provides a simple, flexible method for controlling gelation time.

〈High Strength〉

The properties of glycerol based samples appear significantly different from conventional gels, as evidenced by both differences in the quantitative treatment of wet and dry gels and their low shrinkage dry shrinkage. Thus, upon physical irradiation, glycerol based dry gels appear to have improved mechanical properties compared to both conventional and ethylene glycol based dry gels. FIG. 5 shows the bulk elasticity measured during Isostatash fill measurements of samples made using dry gels based on ethylene glycol and conventional ethanol based dry bulk gels, both having the same initial density. After the initial change, which contributes to the deformation of the structure, both samples show the law of elasticity with respect to density. This power law dependency is typically observed in dry gels. However, what is surprising is the strength of ethylene glycol based dry gels. At certain densities (and hence dielectric constants), the elasticity of such ethylene glycol based dry gel samples is on the order of magnitude greater than conventional dry gels. Preliminary evaluations indicate that glycerol based gels are much stronger than ethylene glycol based gels. These evaluations include the quantitative treatment of data and tests based on shrinkage during drying. The reason for such strength increases is not entirely clear. However, the results of preliminary experiments teach that the rapid gelation time and / or narrow pore size distribution may be responsible for high strength.

〈High surface area〉

We measured the surface area of some dried bulk gels. Their surface area was on the order of 1,000 m 2 / g compared to typical ethanol based gels with a surface area in the range of 600 to 800 m 2 / g. Their high surface area can impart smaller pore sizes and improved mechanical properties. The reason why their high surface area is obtained using glycerol based dry gels is not clear to date.

〈Pore size distribution〉

The optical clarity of this dry bulk gel was greater than any ethanol based dry gel at this density already prepared. Such superior optical clarity may be possible due to the very narrow pore size distribution. However, it is not clear why glycerol has this effect. Preliminary experiments indicate that one possible explanation is that rapid gelling times can be linked to narrow pore size distributions. 6 shows the pore size distribution (as measured by BJH nitrogen desorption measurement) of a bulk gel sample having a density of about 0.57 g / cm 3. The average pore diameter (desorption method) of this sample was 3.76 nm. As typical pores are not completely cylindrical, the diameter used herein means substantially the same cylindrical diameter with the same surface area to volume ratio as the total gelling surface area to volume ratio.

As noted above, the same properties of gels based on glycerol apply to both bulk gels and thin films. However, some advantages are most apparent when applied to thin films such as nanoporous dielectric films on semiconductor wafers. One important advantage is that this novel method allows high quality nanoporous membranes to be processed without atmospheric control during deposition or gelling.

It is important to be able to deposit and gel nanoporous thin films without atmospheric control, but it is also necessary to age the nanoporous thin films without atmospheric control. It has been found that this can be a major challenge over deposition. The main reason is that deposition and room temperature gelation can occur in minutes or in seconds; Room temperature aging typically requires several hours. Thus, evaporation rates that provide acceptable shrinkage during the shortened process can lead to unacceptable shrinkage if the process time is significantly extended.

As an example, we have found that when using some glycerol based gels, a satisfactory aging time at room temperature is on the order of one unit. However, Table 1 shows that the higher temperature can be used to age the thin film in a few minutes. Such aging times are comparable to the preferred aging times of many typical ethanol based and ethylene glycol based gels. Thus, when such times and temperatures are combined with the evaporation rates of FIGS. 1, 7 and 2, they represent approximate thickness loss during aging as shown in Table 2. This estimated thickness loss needs to be compared with the allowable thickness loss, especially for thin film applications. While there are no clear guidelines for acceptable thickness loss, for some microcircuit applications, such as nanoporous dielectrics, one possible guideline is that the thickness loss should be less than 2% of the film thickness. If the small film thickness is assumed to be 1 μm (the actual film thickness can typically vary from 0.5 μm and considerably smaller to several μm), this represents an acceptable thickness reduction of 20 nm. As shown in Table 2, glycerol based gels can achieve this preliminary purpose without atmospheric control at room temperature. Thus, the present invention allows deposition, gelation, aging and drying of the porous nanoporous airgel with reduced porosity without atmospheric control. And drying with only passive atmospheric control, such as limiting the volume of the aging chamber.

TABLE 1

Approximate aging time as a function of temperature for some thin film glycerol based gels

TABLE 2

Thickness Loss During Aging to Saturation Ratio

Measures of improved yield and reliability may require thickness losses of less than 2%, such as less than 0.5% or 0.1%. By using passive air conditioning, the present invention can provide them and even reduce evaporation losses. Such inert control involves placing the gel in a relatively small closed container at least during aging. In one aspect of the invention, evaporation from the wafer serves to raise the saturation ratio of the atmosphere in a closed vessel. At any given temperature, this evaporation continues until the partial pressure of steam increases enough to be equal to the vapor pressure of the liquid. Thus, a low vapor pressure solvent / temperature combination will not evaporate the liquid solvent as vapor pressure allows. 8 shows how solvent pressure changes with temperature for many solvents. If the vessel size is known, the amount of evaporation can be calculated. FIG. 9 shows an approximation of the thickness of a solvent layer that can potentially evaporate when a 70% porous gel is placed in a 5 mm play cylindrical container of the same diameter as the wafer. 10 shows a similar approximation for a container with 1 mm air space above the wafer. The figures show that using a 5 mm space, the glycerol-based gel having a preliminary purpose of 20 nm is 150 ° C. or less, and ethylene glycol based gel is only 80 ° C. or less. Using a 1 mm air space, glycerol based gels with a preliminary purpose of 20 nm can be ignored up to 120 ° C. or less, but only 50 ° C. or less for ethylene glycol based gels. Can be reduced. Inert evaporation suppression using a 1 mm container results in a thickness loss of less than 1 nm (0.1% of 1 μm membrane) even at 100 ° C. for glycerol based gels.

There are many variations of this inactivity control. One variation is to increase the container size. The thickness loss will increase in proportion to the vessel volume. However, a 1000 cm 3 container typically only allows evaporation of 5 nm at 80 ° C. Another variant is gel porosity. Low porosity gels generally exhibit slightly smaller thickness losses, while high porosity gels have large thickness losses.

One disadvantage of glycerol is its relatively high viscosity, which can cause problems with gap filling and / or planarization. As described above, a low viscosity, high volatile solvent can be used to lower the viscosity. 11A shows the calculated viscosity at room temperature of some ethanol / glycerol and methanol / glycerol mixtures. As shown in this figure, alcohols can significantly reduce the viscosity of these mixtures. 11B shows the calculated viscosity at room temperature of some ethanol / ethylene glycol and methanol / ethylene glycol mixtures. As shown in this figure, ethylene glycol is much less viscous than glycol, and small amounts of alcohol significantly reduce the viscosity of these mixtures. In addition, when the viscosity of using ethanol in the raw material solution is higher than necessary, further improvement can be made by using methanol in the precursor solution. The viscosity as disclosed in FIGS. Lla-11b relates to pure fluid mixtures. In fact, depending on the membrane precursor solution, the precursor solution may contain glycerol, alcohols, water, acids and partially reacted metal alkoxides. Of course, since the viscosity can be increased prior to deposition by catalyzing the condensation reaction, the values reported in FIGS. 11A-11B represent a lower limit.

The multisolvent method can be combined or replaced with alternative methods. This replacement reduces the sol viscosity in use at elevated temperatures. The viscosity of the precursor sol can be significantly lowered by heating and / or diluting the precursor during deposition (eg, by heating the transfer line and deposition nozzle in the wafer spin state). This preheating not only lowers the sol viscosity, but also speeds up the gelation time and accelerates the evaporation of any high volatile solvent. It may be desirable to preheat the wafer. Such wafer preheating can improve process control, especially for more viscous precursors, which can improve gap filling. However, for many uses. No wafer preheating is required, which simplifies the process flow. If spin-on coating is used without such a wafer preheating method, the spin state will not require a temperature controlled spinner.

Dry gels prepared by such a simple thin film aerogel manufacturing method can be used in many applications. Some of these uses may not be cost effective using the methods of the prior art. Such applications include low dielectric constant thin films (particularly on semiconductor substrates), modeled chemical sensors, thermal isolation structures, and thermal isolation layers (including thermal isolation structures in infrared detectors). As a general rule, many low dielectric constant thin films preferably have a porosity of greater than 60%, and for critical applications preferably a porosity of greater than 80 or 90%, thus exhibiting a substantial reduction in dielectric constant. However, structural strength and condition measurements can limit the actual porosity to 90% or less. Some uses, including thermally isolated structures and thermally isolated layers, may require the loss of some porosity for high strength and stiffness. These higher stiffness requirements may require low dielectric constants with a porosity of 30 or 45%. In other high strength / toughness applications, particularly applications such as sensors, it may be desirable to use low porosity gels with a porosity of 20-40% for applications where surface area may be more important than density.

The thin films discussed above were focused on thin film aerogels for microelectronic circuits. Aerogels, however, are useful for other applications, such as thin films on passivation substrates. These new high strength, easy-to-manufacture gels currently serve many of these applications. For this purpose, the passivating substrate is defined as a substrate with or without a microelectronic circuit, or at least no interaction between the aerogel and the electronic component. Many of these uses are disclosed in Sol-Gel Science, C. J. Brinker and G. W. Scherer, Chapter 14. Such passivating applications may partially include some types of optical coatings, some types of protective coatings, and some types of porous coatings.

Anti-reflective (AR) coatings may require a wide range of porosity. These may typically range from 20% porosity to 70% porosity, but higher porosity (90% or more) may be useful if suitable surface protection is present, and is more preferred for high performance coatings or coatings on substrates with high refractive indexes. Low porosity (less than 10%, or less) may be useful. For some single layer AR coatings, it may be desirable to use gels having a porosity of 30% to 55%. For some monolayer AR coatings, it may be desirable to use gels having a porosity of 30% to 55%. High performance, multilayer AR coatings have a higher density of layers adjacent to the substrate (eg, multiplicity of 10% to 30%), less dense layers adjacent to the air interface (eg, 45% to 90% porosity). Is preferred. For high strength / toughness applications, it may be desirable to use low porosity gels with a porosity of 20% to 40%, especially where high strength and surface area are the main objectives. Other thin film coatings may require the lowest density and therefore require porosity of greater than 85%, 90% or even 95%.

There are also a number of bulk gel applications where such novel high strength, easy to prepare airgels may be useful. These bulk gel applications include, but are not limited to, nanoporous (eg, molecular) bodies, heat sinks, catalyst supports, adsorbents, sound insulation materials, and optical separation membranes. In general, many bulk applications preferably have a porosity greater than 60% and critical applications preferably have a porosity greater than 80% or 90%. However, structural strength and condition measurements can be limited to less than 95% of actual porosity. Some applications involving sieves may require some loss of porosity for high strength and stiffness. These high stiffness requirements may require dielectrics with porosities as low as 30 or 45%. For high strength / rigid applications including catalyst supports and sensors, the surface area may be important beyond density, and it may be desirable to use low porosity gels with a porosity of 20% to 40%.

Typical sol-gel thin film processes collapse and densify on drying to form dry gels with limited porosity (up to 60% for large pore sizes, generally 50% for pore sizes of interest). Under uncontrolled drying conditions of dry gel film formation, many internal voids are permanently collapsed. However, for thin film aerogel formation, there may be a small amount of shrinkage during aging and / or drying affecting the final density, but the voids remain substantially unbroken.

12A shows a semiconductor substrate 10 (typically in the form of a wafer). Typical substrates include silicon, germanium, and gallium arsenide, and the substrate can include effective devices, low level wiring and insulation layers, and many other conventional structures not shown but known in the art. A number of patterned conductors 12 (eg, Al-0.5% Cu compositions) are shown on the substrate 10. The conductor 12 typically travels at least partially parallel to its longitudinal direction, separated by a gap 13 of a predetermined width (typically a micron fraction). Both conductors and gaps may have a height-to-width ratio above that shown, and typically larger ratios are commonly found in devices with smaller characteristic sizes.

According to the first embodiment of the present invention, 61.0 ml tetraethoxysilane (TEOS), 61.0 ml glycerol, 4.87 ml water and 0.2 ml 1M HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours to form a raw material solution. Let's do it. Similarly, 0.27 mol TEOS, 0.84 mol glycerol, 0.27 mol water and 2.04E-4 mol HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours. After cooling the raw material solution, the solution may be diluted with ethanol to reduce the viscosity. One suitable raw material solution: solvent ratio is 1: 8. However, this ratio depends on the desired film thickness, spin rate and substrate. It is mixed vigorously, stored in a refrigeration apparatus at ˜7 ° C. to maintain stability until use. The solution is typically warmed to room temperature before film deposition. 3 to 5 ml of this precursor sol can be dispensed to the substrate 10 at room temperature and then spun at l500 to 5000 rpm (depending on the target film thickness) for about 5 to 10 seconds to form the sol thin film 14. Deposition can be carried out in the atmosphere without particular control of solvent saturation (eg in a clean room using non-pyrogenic humidity control). During and after such deposition and spinning, ethanol, water and nitric acid are evaporated from the membrane 14, but due to the low volatility of glycerol, no substantial evaporation of glycerol occurs. This evaporation temporarily cools the thin film, but the film temperature rises within seconds after the evaporation rate is lowered. The cooling delays but does not prevent gelation. This evaporation shrinks the thin film 14 and concentrates the silica content of the sol to form a film 18 having a reduced thickness. 12B shows the reduced thickness sol film 18 obtained after substantially all of the ethanol has been removed (at least about 95%). Concentration, evaporative basicization, and / or reheating of such membranes typically results in gelation in seconds.

Membrane 18 has an approximately known ratio of silicon to pore fluid at the gel point. This ratio is about the same as the ratio of TEOS to glycerol in the arsenic deposited sol (accompanied by slight changes due to residual moisture, continued reaction and incidental evaporation). Since this method prevents the gel from permanent collapse, this ratio determines the density of the aerogel membrane that can be prepared from the sol thin film.

After gelation, the thin film wet gel 18 comprises a porous solid and a pore fluid, and preferably time for aging at one or more controlled temperatures, such as, for example, about one day at room temperature, can be tolerated. Note how much void fluid changes during processing. The changes may be due to continued reactions, evaporation / condensation, or chemical additives to the thin film. Aging may preferably be obtained by allowing the substrate and gel to stand at about 25 ° C. for about 24 hours or by heating to 130-150 ° C. for about 1 minute in a closed container.

The aged membrane 18 can be dried without substantial densification by one of a number of methods including supercritical fluid extraction. However, when using such a novel glycerol based gel, one alternative is to replace the aging fluid with a drying fluid using a solvent exchange method and then air dry the membrane 18 from this drying fluid. This drying method uses a solvent exchange method to replace the aging fluid with a different fluid. Whether this fluid is the same as or different from the aging liquid, the void fluid present during drying is often referred to as a "drying fluid". In use, the solvent exchange method replaces aging fluids with predominantly high surface tension with glycerol and dry fluids with low surface tension. This solvent exchange can be carried out as a one or two step process. In a two step process, the first step aliquots approximately 3 to 8 ml of ethanol into the aged thin film 18 at room temperature (or higher), and then spins the wafer for about 5 to 10 seconds at approximately 50 to 500 rpm. Thereby replacing the aging fluid with an intermediate. Sometimes 3 to 6 spin on sequences are required to replace most aging fluids. The second step preferably replaces the intermediate fluid with a dry fluid such as heptane. This step preferably comprises aliquoting approximately 3-8 ml of heptane into the aging thin film 18 at room temperature (or higher), followed by spinning the wafer at approximately 50-500 rpm for about 5-10 seconds. . Sometimes 3-6 spin on sequences are required to replace most intermediate fluids. This solvent exchange method allows the removal of almost all glycerol containing fluids prior to drying. The dry fluid (in this case heptane) is finally evaporated from the wet gel 18 to form a dry nanoporous dielectric (dry gel). If the membrane can be satisfactorily dried from a liquid soluble in the aging fluid, no intermediate will be needed. In many cases, the wet gel can be dried directly from ethanol or from another suitable solvent.

This evaporation can be accomplished by exposing the wafer surface to an atmosphere that is nearly saturated with a drying fluid. For example, the wafer can be present in a substantially uncontrolled atmosphere or a dry gas can be introduced into the atmosphere. To prevent boiling, drying should preferably begin at a temperature slightly below the boiling point of the drying fluid, such as room temperature. If a high boiling point drying fluid such as glycerol is used (eg drying without solvent replacement), the drying start temperature can be raised to or near the aging temperature. Since the thin film is significantly dry (typically in seconds), the temperature must then be raised above the boiling point of both the aging fluid and the drying fluid. This method prevents destructive boiling and is safe until all fluid is removed. Glycerol and other fluids are decomposed at or near isothermal with or instead of boiling. With these fluids, care must be taken not to overheat evaporated fluids or undried wafers, especially glycerol-like fluids that can decompose into toxic substances. After drying, it is desirable to bake the nanoporous dielectric for a short time (eg, 300 ° C. for l5 to 60 minutes) to remove any residual material such as organics present in or on the dielectric. The theoretical dielectric constant (before surface modification) of this embodiment is 1.3.

In order to reduce the dielectric constant, it is desirable to hydroxyl remove (anneal) the dry gel. This can be done by placing the wafer with a formulation such as hexamethyldisilazane (HMDS) or hexaphenyldisilazane vapor in a dry atmosphere. HMDS can associate most water and / or heatoxyls bound to the surface of gel pores dried with methyl groups. This replacement can be carried out at or above room temperature. This replacement can remove water and / or hydroxyl as well as make the dry gel hydrophobic (water repellent). Hexaphenyldisilazane can also remove water and / or heat hydroxyls, making the dry gel hydrophobic. However, the phenyl group has high temperature stability over the methyl group due to the slightly nominal dielectric constant.

FIG. 13 includes a flow chart of a general method for obtaining an aerogel thin film from a precursor sol according to one embodiment of the present invention. Table 3 is a brief summary of some of the materials used in this method.

TABLE 3

Substance summary

According to a second, high density embodiment of the invention, 150.0 ml TEOS, 61.0 ml glycerol, 150.0 ml ethanol, 12.1 ml water and 0.48 ml 1M HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours to prepare the raw material solution. To form. Similarly, 0.67 mol TEOS, 0.84 mol glycerol, 2.57 mol ethanol, 0.67 mol water and 4.90E-4 mol HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours. After cooling the raw material solution, the solution may be diluted with ethanol to reduce the viscosity. One suitable raw material solution: solvent ratio is 1: 8. It is mixed vigorously, typically stored at refrigeration units at -7 ° C to maintain stability until use. The solution is allowed to warm to room temperature before film deposition. 3-5 ml of this precursor sol can be aliquoted to the substrate 10 at room temperature, followed by spin at l500-5000 rpm (depending on the target film thickness) for about 5-10 seconds to form the sol thin film 14. Deposition can be performed in the atmosphere without solvent control (eg, standard exhaust in a clean room using non-pyrogenic humidity control). During and after such deposition and spinning, ethanol (viscosity reducing additive and reaction product from TEOS and water) and water are evaporated from the membrane 14, but due to the low volatility of glycerol no substantial evaporation of glycerol occurs. This evaporation shrinks the thin film 14 and concentrates the silica content of the sol to form a film 18 of reduced thickness. 12B shows the reduced thickness sol film 18 obtained after substantially all (at least about 95%) of water has been removed. Such concentration typically causes gelation in minutes.

Subsequent processes generally follow the method described in the first embodiment. After gelling, the thin film wet gel 18 comprises a porous solid and a pore fluid, and may preferably allow time for aging at one or more controlled temperatures. The aged membrane 18 can be dried without substantial densification using one of a number of methods including supercritical fluid extraction. However, using low density formulations of these novel glycerol based gels, it is possible to perform non-critical drying, such as solvent exchange followed by air drying of the membrane 18 from the drying fluid as described in the first embodiment described above. It is preferable because it can. The nanoporous dielectric may then be post-dry baked and / or surface modified as described in the first embodiment. The theoretical dielectric constant (before surface modification) of this embodiment is 1.6.

According to a third, high density embodiment of the invention, 208.0 ml TEOS, 61.0 ml glycerol, 208.0 ml ethanol, 16.8 ml water and 0.67 ml 1M HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours to form a stock solution Let's do it. Similarly, 0.93 mol TEOS, 0.84 mol glycerol, 3.56 mol ethanol, 0.93 mol water and 6.80E-4 mol HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours. After cooling the raw material solution, the solution may be diluted with ethanol to reduce the viscosity.

One suitable raw material solution: solvent ratio is 1: 8. It is mixed vigorously, typically stored at refrigeration units at -7 ° C to maintain stability until use. The solution is allowed to warm to room temperature before film deposition. 3-5 ml of this precursor sol can be aliquoted to the substrate 10 at room temperature and then spun at l500-5000 rpm (depending on the target film thickness) for about 5-10 seconds to form the sol thin film 14. Deposition can be performed in the atmosphere without solvent control (eg, standard exhaust in a clean room using non-pyrogenic humidity control). During and after such deposition and spinning, ethanol and water are evaporated from the membrane 14, but due to the low volatility of glycerol no substantial evaporation of glycerol occurs. This evaporation shrinks the thin film 14 and concentrates the silica content of the sol to form a film 18 of reduced thickness. 12B shows the reduced thickness sol film 18 obtained after substantially all (at least about 95%) of water has been removed. Such concentration typically causes gelation in minutes.

Subsequent processes generally follow the method described in the first embodiment. After gelation, the thin film wet gel 18 comprises a porous solid and a pore fluid, and preferably there may be an aging period at one or more controlled temperatures. Aging can be performed by allowing the device to stand at 25 ° C. for approximately 24 hours. Aged membrane 18 can be dried without substantial densification using one of a number of methods, solvent exchange involving supercritical fluid extraction or air drying. However, especially for high density formulations of these novel glycerol based gels, it is desirable to air dry the membrane 18 from the aging fluid. In this direct drying method, the wafer surface is exposed to an atmosphere that is almost saturated with a drying fluid. A simple method is to remove the cover from the low volume aging chamber, exposing the gel surface to a substantially uncontrolled atmosphere. In other methods, the drying gas is introduced into the aging chamber or the atmosphere. Using this direct drying method, it may be desirable to raise the drying start temperature to or near the aging temperature. This high temperature drying reduces surface tension and accompanying shrinkage, speeds up drying and simplifies the process. Since the thin film dries significantly (typically in seconds for high temperature drying), the temperature must then be raised above the boiling point of both the aging fluid and the drying fluid (these are often the same fluid). This method prevents destructive boiling and is safe until all fluid is removed. The drying fluid of this method contains glycerol, which can be broken down into toxic substances, so care must be taken not to overheat the evaporated fluid or the undried wafer. Nanoporous dielectrics may be post-dry baked and / or surface modified as described in the first embodiment. The theoretical dielectric constant (before surface modification) of this embodiment is 1.76.

According to the fourth embodiment of the present invention, 278.0 ml TEOS, 61.0 ml glycerol, 278.0 ml ethanol, 22.5 ml water and 0.90 ml 1M HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours to form a stock solution. Similarly, 1.23 mol TEOS, 0.84 mol glycerol, 4.76 mol ethanol, 1.25 mol water and 9.1E-4 mol HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours. After cooling the raw material solution, the solution may be diluted with ethanol to reduce the viscosity. One suitable raw material solution: solvent ratio is 1: 8. It is mixed vigorously, typically stored at refrigeration units at -7 ° C to maintain stability until use. The solution is allowed to warm to room temperature before film deposition. 3 to 5 ml of this precursor sol can be aliquoted to the substrate 10 at room temperature and then spun at l500 to 5000 rpm (depending on the target film thickness) for about 5 to 10 seconds to form the sol thin film 14. Desorption can be carried out in the atmosphere without solvent control (eg standard exhaust in a clean room using non-pyrogenic humidity control). Ethanol and water are evaporated from the membrane 14 during and after such deposition and spinning, but no substantial evaporation of glycerol occurs due to the low volatility of glycerol. This evaporation shrinks the thin film 14 and concentrates the silica content of the sol to form a reduced thick film 18. 12B shows the reduced thickness sol film 18 obtained after substantially all (at least about 95%) of water has been removed. Such concentration typically causes gelation in minutes.

Subsequent processes generally follow the method described in the third embodiment. After gelation, the thin film wet gel 18 comprises a porous solid and a pore fluid, and preferably there may be an aging period at one or more controlled temperatures. The aged membrane 18 can be dried without substantial densification using one of a number of methods including supercritical fluid extraction. However, it may be desirable to air dry the membrane 18 from the aging fluid as described in the third embodiment. The nanoporous dielectric may then be post-dry baked and / or surface modified as described in the first embodiment. The theoretical dielectric constant (prior to surface modification) of this embodiment is 1.96.

According to a fifth embodiment of the present invention, 609.0 ml TEOS, 6100 ml glycerol, 609.0 ml ethanol, 49.2 ml water and 1.97 ml 1M HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours to form a stock solution. Similarly, 2.73 mol TEOS, 0.84 mol glycerol, 10.4 mol ethanol, 2.73 mol water feather 2.70E-3 mot HNO1 are mixed and refluxed at ˜60 ° C. for 1,5 hours. After cooling the raw material solution, the solution may be diluted with ethanol to reduce the viscosity. One suitable raw material solution: solvent ratio is 1: 8. It is mixed vigorously, typically stored at refrigeration apparatus at ˜7 ° C. and maintained stable until use. The solution is allowed to warm to room temperature before film deposition. 3-5 ml of this precursor sol can be aliquoted into the engine 10 at room temperature, followed by spin at l500-5000 rpm (depending on the target film thickness) for about 5-10 seconds to form the sol thin film 14. Deposition can be performed in the atmosphere without solvent control (eg, standard exhaust in a clean room using non-pyrogenic humidity control). Ethanol and water are evaporated from the membrane 14 during and after such deposition and spinning, but no substantial evaporation of glycerol occurs due to the low volatility of glycerol. This evaporation shrinks the thin film 14 and concentrates the silica content of the sol to form a film 18 of reduced thickness. 12B shows the reduced thickness sol film 18 obtained after substantially all (at least about 95%) of water has been removed. Such concentration typically causes gelation in minutes.

Subsequent processes generally follow the method described in the third embodiment. After gelation, the thin film wet gel 18 comprises a porous solid and a pore fluid, and preferably there may be an aging period at one or more controlled temperatures. The aged membrane 18 can be dried without substantial densification using one of a number of methods including supercritical fluid extraction. However, it may be desirable to air dry the membrane 18 from the aging fluid as described in the third embodiment. The nanoporous dielectric may then be post-dry baked and / or surface modified as described in the first embodiment. The theoretical dielectric constant (before surface modification) of this embodiment is 2.5.

The ratio of solvent to other reactant ratios can be used to provide different porosity / dielectric constants. FIG. 14 shows the theoretical relationship between the molar ratio of glycerol molecules to metal atoms and the porosity of the nanoporous dielectric when all ethanol evaporates from the deposited sol. Typically, high porosity glycerol based gels (generally less than about 0.51 g / dl) are preferred by solvent exchange or other methods of reducing shrinkage during drying. On the other hand, low porosity gels require attention to prevent premature gelation. This may include pH control, temperature control or other methods known in the art. In some applications, evaporation of the high volatility solvent may also be acceptable.

As noted above, gels based on high density glycerol (generally greater than about 0.64 g / dl) can be aged and dried almost without shrinkage without the use of solvent exchange. Unaged wafers can be placed in small volume furnaces, or in small containers that can be run on hot plates. After selective draining, the container is sealed at room temperature. When a temperature gradient is achieved with the vessel sealed, the membrane quickly ages and the aging / drying fluid viscosity is lowered. After sufficient aging (during the temperature gradient), the gel is easy to dry. At temperatures near the boiling point of glycerol, the glycerol viscosity should be low enough to remove glycerol in the furnace atmosphere and allow the membrane to dry directly (relative to the porosity of the aging membrane of pre-measured porosity). A slightly lower surface tension can be obtained by raising the drying temperature above the boiling point of glycerol in the most desired low density applications. In this case, the furnace needs to maintain pressure (most subcritical dry conditions can be treated with a pressure of 1 to 3 MPa). In addition, it is necessary to pay attention to the removal of glycerol in the furnace atmosphere, especially at first. Glycerol in the furnace atmosphere can be removed, for example, by evacuating the pressure, by a vacuum pump, or by evacuating the glycerol using a gas. The furnace temperature can be kept constant, or the temperature can be kept elevated while the glycerol is removed (the furnace can be ramped below the baking temperature while glycerol is released as a gas). Some glycerol can be introduced during heating to minimize evaporation from the membrane, but the preferred is that the furnace volume is low enough so that evaporation does not significantly reduce the film thickness without the introduction of glycerol during heating. If the membrane requires supercritical drying to eliminate temporary shrinkage, preference is given to using CO ₂ solvent exchange known in the art.

The same stock solution can be used for bulk aerogels as thin film aerogels, but the process is substantially different. When using different raw material solution mixtures, the following examples can be adapted to provide bulk gels with different porosities. According to the bulk aerogel of the present invention, 278.0 ml TEOS, 61.0 ml glycerol, 208.0 ml ethanol, 16.8 ml water and 0.67 ml 1M HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours to form a stock solution. Similarly, 0.93 mol TEOS, 0.84 mol glycerol, 3.56 mol ethanol, 0.93 mol water and 6.80E-4 mol HNO ₃ are mixed and refluxed at ˜60 ° C. for 1.5 hours. It is typically stored in refrigeration units at -7 ° C to maintain stability until use. Before pouring the raw material solution into the mold, it is preferably warmed to room temperature. After pouring into the template, ethanol, water and acid are evaporated, but due to the low volatility of glycerol no substantial evaporation of glycerol occurs. This evaporation reduces the volume of the raw material solution precursor sol and concentrates the silica content of the sol. At least some evaporation may occur prior to filling the mold. This pre-evaporation is particularly useful when the form of the mold cannot be substantially evaporated on its own after filling, such as a mold form that cannot shrink or a mold with less surface area. This evaporation is not necessary, but has many advantages, including rapid gelation without the use of a catalyst and a reduction in shrinkage after gelation.

After this evaporation, the sol has a nearly known silicone ratio to the pore fluid at the gel point. This ratio is almost identical to the ratio of TEOS to glycerol in the precursor mix (small changes due to residual water, continued reactions and incidental evaporation). This method prevents the gel from permanently decaying, so this ratio determines the density of the aerogel to be produced. If the sol does not gel during evaporation, the sol gels immediately after substantially all water, ethanol and acid have evaporated.

Alternatively, the precursor can be catalyzed with 5 M ammonium nitrate before filling the template. In these mixtures, typically the sol gels in minutes. The wet gel is removed from the mold and ethanol and water are evaporated. Typically, the gel shrinks while evaporating. However, when the evaporation is substantially terminated as other elements complete, the sol has a nearly known silicon ratio for the pore fluid at the gel point. This ratio is almost identical to the ratio of TEOS to glycerol in the precursor mix (residual water, small changes due to subsequent reaction tail incidental evaporation). Since this method prevents the gel from permanently decaying, these ratios determine the density of the aerogel to be produced.

After gelation, the wet gel may comprise porous solids and pore fluids and preferably allow time to age at one or more controlled temperatures. It is preferred to age the substrate and gel by standing at about 25 ° C. for about 24 hours or by heating to 130-150 ° C. for 5 minutes in a closed container. Such high temperature aging parameters are valid for bulk aerogels of 5 mm diameter. However, due to the low thermal conductivity of the wet gel, high temperatures accelerate the aging time and the temperature combination depends largely on the shape of the bulk gel.

After initial aging, the gel is removed from the mold and dried directly from the mother liquor (ie, the void fluid remains at the end of aging with no solvent change upon aging or drying). Warm up slowly to dry the gel at about 500 ° C.

Instead of drying directly from the mother liquor, preference is given to solvent exchange, in particular using more porous gels.

Such solvent exchange can be done in a one or two step process. In the first step, the aging liquid is exchanged for an intermediate fluid, and in the second step, the intermediate fluid is exchanged for a dry fluid having a low surface tension such as heptane. In this way, the gel is removed from the mold and placed in a sealed tube containing ethanol and the pore fluid exchanged at 50 ° C. for 8 hours. At the end of the 8 hour interval, the gel is rinsed with ethanol and then stored in pure ethanol in a 50 ° C. oven. After 3 to 6 times at the same interval as above, ethanol is exchanged for hexane in a similar manner. Almost all glycerol-containing fluids can be removed by this solvent exchange method before drying. Finally, the dry fluid (in this case heptane) is evaporated from the wet gel to form a dry airgel. If the membrane can be satisfactorily dissolved from a liquid that can be dissolved with the aging fluid, no intermediate fluid is required. In many cases, the wet gel can be dried directly from ethanol, or other suitable solvent.

It is often desirable to bake the airgel for a short time (eg, 15-60 minutes at 300 ° C.) to facilitate the removal of any residual material such as organics in or on the airgel after drying. In some cases, it is desirable to hydroxyl remove (anneal) the dried gel. This is done by placing a dry airgel in a dry atmosphere consisting of surface modifiers such as trimethylchlorosilane (TMCS), hexamethyldisilazane (HMDS), or hexaphenyldisilazane vapor. HMDS exchanges large amounts of water and / or hydroxyl bound to the pore surface of the dried gel with methyl groups. This exchange can be done at room temperature or higher. This exchange not only removes water and / or hydroxyl groups, but also makes the dried gel hydrophobic (water repellent). Hexaphenyldisilazane also removes water and / or hydroxyl and makes the dried gel hydrophobic. However, phenyl groups have higher temperature stability than methyl groups.

In accordance with an ethylene glycol based embodiment of the present invention, chetraethoxysilane (TEOS), ethylene glycol, ethanol, water and acid (1M HNO ₃ ) are mixed in a molar ratio of 1: 2.4: 1.5: 1: 0.042, Reflux at 1.5 ° C. for 1.5 hours. After cooling the mixture, the solution is diluted with ethanol to a composition of 70% by volume raw material solution and 30% by volume ethanol. The composition is mixed vigorously and typically stored in a refrigeration apparatus at about 70 ° C. to maintain stability until use. The solution is allowed to warm to room temperature before film deposition. The mixture of stock solution and 0.25M NH ₄ 0H catalyst (volume ratio 10: 1) are combined together and mixed. 3-5 ml of such precursor sol is dispensed onto the substrate 10 at room temperature and then spun at l500-5000 rpm (depending on the target film thickness) for about 5-10 seconds to produce the sol thin film 14. Can be deposited in an uncontrolled atmosphere. However, it is desirable to deposit and gel the sol in a clean room under standard humidity control. During and after such deposition and spinning, the ethanol / water mixture is evaporated from the membrane 14, but due to the low volatility of ethylene glycol the ethylene glycol is not substantially evaporated. This evaporation concentrates the silica of the sol, which shrinks the thin film 14 and forms a reduced thickness film 18. 12B shows a reduced thickness sol film 18 obtained after substantially all of the ethanol (at least about 95%) has been removed. This concentration combined with the catalyst typically gels within minutes or seconds.

Membrane 18 has a nearly known ratio of silicon to void fluid at the gel point. This ratio is almost identical to the ratio of TEOS to ethylene glycol in the deposited sols (there are minor changes due to residual water, continued reactions and incidental evaporation). To the extent that the gel is prevented from collapsing, these ratios will determine the density of the aerogel film to be prepared from the sol thin film.

After gelling, the thin film wet gel 18 comprises a porous solid and a void fluid, and preferably a time of aging for about one day at one or more controlled temperatures, such as room temperature, may be tolerated. It should be noted that the void fluid changes somewhat during processing. This change is due to continued reaction and / or evaporation / concentration. Preferably, the device is allowed to age by standing in a low volume aging chamber at about 100 ° C. for 5 minutes.

The aging film 18 may be dried without substantially densification by one of a number of methods including supercritical fluid extraction or solvent exchange after air drying. However, it is preferable to air dry the membrane 18 from the aging fluid, preferably as described in the third glycerol embodiment. The nanoporous dielectric may then be subjected to post-dry baking and / or surface modification as described in the first glycerol embodiment.

According to another ethylene glycol based embodiment of the invention, tetraethoxysilane (TEOS), ethylene glycol, water and acid (1M HNO ₃ ) are mixed in a molar ratio of 1: 4: 1: 0.042 and at about 60 ° C. Reflux for 1.5 hours. Typically stored in a refrigeration apparatus at about 7 ° C. to maintain stability until use. Preferably, the solution is allowed to warm to room temperature before film deposition. 3-5 mL of the precursor sol was dispensed on the substrate 10 (without catalyst) at room temperature and then rotated at 1500-5000 rpm (depending on the target film thickness) for about 5-10 seconds to sol thin film 14 Manufacture. Can be deposited in an uncontrolled atmosphere. However, it is desirable to deposit and gel the sol in a clean room under standard humidity control. During and after such deposition and spinning, ethanol and water are evaporated from the membrane 14, but due to the low volatility of ethylene glycol the ethylene glycol is not substantially evaporated. This evaporation concentrates the silica of the sol, which shrinks the thin film 14 and forms a reduced thickness film 18. 12B is obtained and gelled after substantially all water (at least about 95%) is removed.

After gelling, the thin film wet gel 18 comprises a porous solid and a void fluid, and preferably a time of aging for about one day at one or more controlled temperatures, such as room temperature, may be tolerated. It should be noted that the void fluid changes somewhat during processing. Preferably, the device is allowed to age by standing in a low volume aging chamber at about 100 ° C. for 5 minutes.

The aging film 18 may be dried without substantially densification by one of a number of methods including supercritical fluid extraction or solvent exchange after air drying. However, it is preferable to air dry the membrane 18 from the aging fluid, as described in the third glycerol embodiment. The nanoporous dielectric may then be subjected to post-dry baking and / or surface modification as described in the first glycerol embodiment.

This discussion shows some of the benefits of aging in closed containers. Since there is no suitable aging chamber, we will describe the chamber invented for carrying out this method. One embodiment of an aging vessel is described in FIGS. 16A, 16B and 16C. In these embodiments, the processing apparatus includes a body 20, a substantially flat plate to which the resilient seal 24 is bonded. The plate 22 needs to be flat to the extent necessary to provide a thin film in the gap during operation, and a material with high thermal conductivity such as stainless steel, glass or aluminum is preferred but an underlying process (eg semiconductor manufacturing) It can be made of any material that is compatible with it. The resilient seal 24 is preferably designed to withstand wet gel processing temperatures and void fluids; Many suitable materials are known to those skilled in the art, including materials based on TEFLON and neoprene. Depending on the nature of the temperature control used in the device, the seal 24 is preferably substantially thermally insulating or thermally conductive.

In operation, the body 20 may simply be disposed on the substrate 26 as shown in FIG. 16C. These substrates may be optical substrates such as glass or plastic, or semiconductor substrates such as Si wafers. In this embodiment, the seal 24 acts as an atmosphere seal and as a spacer that sets the volume of the chamber 32 formed by the substrate surface 28, the chamber surface 30 and the seal 24. . For example, the seal 24 can be designed to be pressed to a thickness of about 1 mm under the weight of the plate 22, so that when the body 20 is placed on the substrate 26, a chamber 32 of 1 mm thick can be obtained. Can be. In many thin film applications, a small amount of water vapor leakage during processing of the substrate 26 does not significantly affect the final properties of the film, so only the chamber 32 needs to be substantially sealed.

The body 20 is applied in many parts of the airgel thin film manufacturing process. Before the sol membrane is gelled, it can be used to limit evaporation as an aging chamber for the wet gel thin film, as a storage or transport chamber for these membranes, or as a drying chamber. In all such applications, it can be seen that both the sol and gel thin films contain very small amounts of liquid to the extent that a limited volume of chamber is necessary to prevent substantial evaporation from the membrane.

In other embodiments, the body 20 may further include other components as shown in FIGS. 17A and 17B. In this embodiment, the body 20 further comprises a substrate holder 36 and a substrate temperature adjusting means 34. This embodiment shows an additional aspect of the seal 24 placed outside of the substrate (and in some cases, the seal 24 may be omitted), which forms a thin film over the entire substrate surface 28. When the chamber 32 is closed, the temperature regulating means 34 is used to simultaneously adjust the temperature of the body 20, the substrate 26 and the chamber 32, and the flat plate 22 and the wafer holder 36. This can be thermally combined.

In another embodiment of FIGS. 18A and 18B, suture 24 provides some degree of thermal insulation between flat plate 22 and wafer holder 36. This allows the temperature control means 34 to adjust the substrate temperature, while the separate temperature control means 38 is used to control the temperature of the flat plate. Since the temperature of the planar plate 77 can be arbitrarily lowered to promote condensation on the chamber surface 30, this embodiment has the advantage of drying the wet gel membrane.

19A, 19B and 19C show additional aspects of these aging chambers. For example, in FIG. 19A, the substrate 26 is processed in reverse position. In this embodiment, the condensate can be collected without dropping accidental or intended condensate on the chamber surface 30 over the substrate surface 26. In FIG. 19B, the substrate 26 is not reverse processed as well as the first solvent layer 42 (preferably a layer of the same composition as the one or more pore fluids) is not before the chamber is closed, for example a first solvent. It is distributed from the feed pipe 40 to the tambour surface 30. In this embodiment, layer 42 can be used to help saturate the processing atmosphere and reduce evaporation of the pore fluid from substrate 26.

In FIG. 19C, some atmospheric conditioning means 44 is connected to the chamber 32 via one or more ports (possibly sealed) 46. Atmosphere conditioning means 44 may be used to create a vacuum, or to adequately overpressure chamber 32, to exchange atmosphere in chamber 32, or to supply void fluid vapor to chamber 32. . This embodiment can be used to age the thin film above the boiling point of the pore fluid, for example by operating the chamber 32 above atmospheric pressure. In addition, the present embodiment can be used to dry the thin film by removing at least a portion of the void fluid vapor from the chamber 32 after aging.

Although the present invention has been described in some embodiments, many of these steps may be modified within the scope of the present invention and other steps may be included to improve the overall process. For example, the initial thin film can be deposited by other common methods, such as dip coating, flow coating, or spray coating, instead of spin coating. Similarly, solvent exchange may use dip coating, spray coating, or dip in a liquid or vapor phase solvent instead of spin coating. When using vapor phase solvents, the wafer can be cooled to a lower temperature than the atmosphere, thus facilitating condensation on the wafer. While water is considered another solvent in the process, for the purposes of the present application water is not considered a solvent.

Glycerol and ethylene glycol each have unique advantages, but there are other low volatility solvents that may be useful for making nanoporous dielectrics. It is preferred to analyze the solvent to determine the expected evaporation rate, but preference is given to selecting a low volatility solvent. Almost all solvents with small evaporation rates at room temperature will have boiling points above 140 ° C. Some solvents having a boiling point below 140 ° C. are useful, but typically more preferred evaporation rates are found in solvents having boiling points above 160 ° C., more preferably above 190 ° C. Solvents with boiling points above 230 ° C. may have evaporation rates small enough to be suitable for deposition and / or aging in a short period of time at 40-80 ° C. with little or no atmospheric control. For processing at 100-150 ° C., preference is given to using solvents having boiling points above 270 ° C. For this reason a lower limit of the desired boiling point is slightly preferred. In addition, higher limits of preferred boiling points are slightly preferred. Most solvents with boiling points above 500 ° C. are viscous and require more attention during processing. Typically, more useful solvents have boiling points below 350 ° C., preferably below 300 ° C. If it is not easy to dilute or heat the sol during deposition, it is preferred to use a low volatility solvent below the boiling point of 250 ° C. If one solvent does not provide all the desired properties, two or more solvents may be mixed to improve performance. Thus, an initial preliminary preference for the selection of low volatility solvents is that they can be mixed with both water and ethanol (for TEOS based gels) and a boiling point range of 175-270 ° C. Based on these preliminary preferences, some suitable low volatility solvents other than glycerol and ethylene glycol are 1,4-butylene glycol and 1,5-pentanediol.

If deposition and aging are easy above room temperature, this opens up additional possibilities. One variation is to use a solvent that is a solid, not liquid, at room temperature. This can potentially use more material. Many of these high melting point materials have lower volatility than the volatility of low volatility “room temperature liquid solvents” (liquid solvents) have at high deposition and aging temperatures. Although no higher melting point is required, the simplicity of processing indicates that the “room temperature solid solvent” (solid solvent) should have a melting point of less than 60 ° C., preferably less than 40 ° C. A further desirable property for potential solid solvents is that they easily solidify into the amorphous phase. Such amorphous solidification reduces gel damage upon sudden cooling. In addition, this allows the solvent to be removed by freeze drying. An alternative to maintaining the precursor temperature above the melting point of the solid solvent is to dissolve the solid solvent in the carrier liquid. These carrier liquids may be water, alcohols, or other liquids typically used in thin film aerogel / xenogel processes. The carrier liquid may be a suitable liquid introduced only as a carrier.

The very good action of glycerol and ethylene glycerol provides some clues for the different solvents that are different. Some solvents have been identified that provide somewhat different properties from ethylene glycol or glycerol, but still have many disadvantages. The most promising additional solvents are 1,2,4-butanetriol; 1,2,3-butanetriol; 2-methyl-propanetriol; And 2- (hydroxylmethyl) -1,3-propanediol; 1-4, 1-4, butanediol, and 2-methyl-1,3-propanediol. Other potential solvents include polyols alone or polyols combined with ethylene glycol, glycerol, or other solvents.

The use of such low volatility solvents alleviates the control of the atmosphere required during deposition, gelling and / or aging. This is preferably because saturation should be avoided, but the atmospheric solvent concentration can be reduced without excess evaporation. Such wider concentration ranges can be used to allow wider temperature variations in the deposition chamber (especially near the wafer and near the evaporative cooling effect). The initial goal is to allow a temperature change of at least 1 ° C. That is, the vapor concentration of the low volatility solvent in the atmosphere should be such that the condensation temperature (similar to dew point) of the solvent vapor is at least 1 ° C. below the substrate temperature.

Indeed, the decisive factor is the surface of the deposited sol and / or gelled sol. However, the thin film properties of the sol keep the temperature difference between the sol and the substrate small. Since it is easier to measure the substrate temperature, these two temperatures can be used interchangeably in the present patent. Although 1 ° C. temperature uniformity can be obtained under some conditions, volume production requires at least 3 ° C., preferably 10 ° C. tolerance. However, the ultimate goal is to deposit, gel and age in an unregulated or nearly unregulated atmosphere. In this most preferred approach (substantially uncontrolled atmosphere), atmospheric control during deposition, gelling and aging limits standard clean room temperature control and humidity control, even though the wafer and / or precursor sol is independently temperature controlled. If substantially uncontrolled atmospheres allow excessive evaporation, passive or less preferably aggressive air conditioning is required. For this application, manual adjustment limits the position of the wafer in a relatively small container. These containers may be partially or fully sealed and may or may not include a liquid reservoir of solvent. However, these containers do not contain new environmental controllers for wafers, container atmospheres and / or storage rooms.

Another variation on the basic method is that the thin film wet gel 18 before drying (and, but not necessarily, but generally after aging) has a void surface modified with a surface modifier. This surface modification step replaces a significant amount of molecules on the pore walls with molecules of another species. If a surface modifier is used, it is generally desirable to remove water from the wet gel (l8) before the surface modifier is added. The wafer may be washed in pure ethanol to remove water, preferably by slow rotational rotation as described in the solvent exchange of the first embodiment. Since water reacts with many surface modifiers such as HMUS, such water removal is advantageous but not essential. In the novel glycerol-based method, the surface does not need to be modified to prevent void breakage, but can be used to impart desirable properties to the dried gel. Some examples of potentially desirable properties are hydrophobicity, reduced dielectric constant, increased resistance to certain chemicals, and feather improved temperature stability. Some potential surface modifiers that can impart desirable properties are hexamethyldisilazane (HMDS). Alkyl chlorosilanes (trimethylchlorosilane (TMCS), dimethyldichlorosilane, etc.), alkylalkoxysilanes (trimethylmethoxysilane, dimethyldimethoxysilane, etc.), phenyl compounds and fluorocarbon compounds. One useful phenyl compound is hexa phenyldisilazane. Other useful phenyl compounds typically follow the basic formula PhxAySiB _(4-xy) (Ph is a phenolic group, A is a reactor such as Cl or OCH ₃ and B is a residual ligand and two may be the same or different groups). . Examples of these phenyl surface modifiers include compounds having one phenolic group, such as phenyltrichlorosilane, phenyltrifluorosilane, phenyltrimethoxysilane, phenyltriethoxysilane, phenylmethylchlorosilane, phenylethyldichlorosilane, phenyl Dimethylethoxysilane, phenyldimethylchlorosilane, phenyldichlorosilane, phenyl (3-chloropropyl) dichlorosilane, phenylmethylvinylchlorosilane, phenethyldimethylchlorosilane, phenyltrichlorosilane, phenyltrimethoxysilane, phenyltris ( Trimethylsiloxy) silane, and phenylallyldichlorosilane. Other examples of these phenyl surface modifiers include compounds having two phenolic groups such as diphenyldichlorosilane, diphenylchlorosilane, diphenylfluorosilane, diphenylmethylchlorosilane, diphenylethylchlorosilane; Diphenyldimethoxysilane, diphenylmethoxysilane, diphenylethoxysilane, diphenylmethylmethoxysilane, diphenylmethylethoxysilane and diphenyldiethoxysilane. These phenyl surface modifiers also include compounds having three phenyl groups, such as triphenylchlorosilane, triphenylfluorosilane, and triphenylethoxysilane. Another important phenyl compound, 1,3-diphenyltetramethyldisilazane, is an exception to this basic formula. These lists are not exhaustive, but represent the basic characteristics of the group. Useful fluorocarbon based surface modifiers are (3,3,3-trifluoro-propyl) trimethoxysilane), (tridecafluoro-1,1,2,2-detrahydrooctyl) -1- Dimethylchlorosilane and other fluorocarbon groups having a reactor such as Cl or 0CH3grill to form covalent bonds with hydroxyl groups.

This paragraph contains some of the typical useful properties for many conventional applications. However, potentially different applications apply for nanoporous dielectrics and aerogels with other desirable properties. Examples of some other potentially desirable properties include hydrophilicity, increased electrical conductivity, increased dielectric breakdown voltage, increased or decreased reactivity with certain chemicals, and increased volatility. These lists are not comprehensive. However, many other forms of properties are desirable depending on the application. That is, it is evident that many other materials that form covalent bonds with hydroxyl groups are potential surface modifiers that can impart potentially desirable other properties.

The present invention also includes the use of gelling catalysts such as ammonium hydroxide. The present invention also includes the use of other gelling catalysts in place of ammonium hydroxide and / or the use of gelling catalysts to be added after deposition. Typically, these alternative catalysts change the pH of the sol. Acid catalysts may be used, but preference is given to using catalysts which increase the pH. Typically, acid catalysts give longer processing times and larger dielectric constants than base catalysis processes. Some examples of other preferred gelling catalysts include ammonia, volatile amine species (low molecular weight amines) and volatile fluorine species. When the catalyst is added after deposition, it is preferred to add the catalyst in the vapor, fog or other vapor phase.

The present invention forms nanoporous dielectrics at room temperature and atmospheric pressure without separate surface modification steps. Although no need to prevent substantial densification, this novel method does not exclude the use of supercritical drying or surface modification steps prior to drying. In a manner fast enough to prevent the formation of large cooling (eg 50 nm) crystals, the process can be freeze dried and run. In general, this novel method is compatible with the most advanced aerogel technology.

Other examples of variations include the reaction atmosphere and / or temperature. In addition, coating and gelling need not be performed in the same chamber or in the same atmosphere. For example, the substrate may be warmed to delay gelation or warmed to accelerate surface modification and / or gelation. In addition, the total pressure and / or temperature can be modified to further control the evaporation rate and / or gelation time. The warming process is typically done at 40 ° C; 50 degreeC is preferable and 70 degreeC is more preferable. When done at warming, care must be taken to prevent solvent boiling (eg reaction-partial pressure in the atmosphere must be high enough).

Although TEOS is used as a representative example of the reactants, other metal alkoxides may be used alone or in combination with TEOS or to form silica network with each other. These metal alkoxides are tetramethedoxysilane (TMOS), methyltriethoxysilane (MTEOS), 1,2-bis (trimethoxysilyl) ethane (BTMSE), combinations thereof and silicone-based known in the art. Other metal alkoxides. The sol can also be formed from alkoxides of other metals known in the art, such as aluminum and titanium. Some other precursors known in the art include particulate metal oxides and organic precursors. Two representative particulate metal oxides are pyrogenic (evaporated) silica and colloidal silica. Some representative organic precursors are melamine, phenol furfural, and resorcinol. In addition to the alternative reactants, alternative solvents may be used. Preferred alternatives to ethanol are methanol and other higher alcohols. Other acids may be used as precursor sol stabilizers instead of nitric acid.

Further modifications result in the formation of oligomers of the appropriate size (monomers l5-150 per molecule) in the precursor sol and / or promote formation. Larger oligomers can accelerate the gelation process in the deposited sol. Sols containing large oligomers may have greater viscosity than sols with small oligomers. However, when the viscosity is stable, higher viscosity can be corrected by methods known in the art, such as control of solvent ratio and rotation conditions. To help achieve this desired stable viscosity, oligomerization needs to be slowed down or substantially stopped before deposition. Potential methods for enhancing oligomerization may include heating the precursor sol, evaporating the solvent, or adding small amounts of gelling catalysts such as ammonium hydroxide. Potential methods for delaying oligomerization may include cooling the precursor sol, diluting or condensing the sol with a solvent, and restoring the precursor sol to a pH that minimizes gelation (nitric acid may be used with the ammonium hydroxide above). ).

[Effects of the Invention]

The present invention allows for porosity control of thin film aerogels deposited, gelled, aged and dried without atmospheric control. In another aspect, the present invention allows the control of porosity of deposited, gelled, rapid aging at elevated temperatures and dried thin film nanoporous aerogels by only passive atmospheric control, such as limitation of the aging chamber volume.

While the present invention has been described with some exemplary embodiments, various changes and modifications may be suggested to those skilled in the art. It is intended that the present invention include changes and modifications within the scope of the appended claims.

Claims (47)

Providing an aerogel precursor sol consisting of at least partially hydrolyzed metal alkoxide dispersed in a first solvent and a second solvent, substantially evaporating the second solvent while preventing the first solvent from substantially evaporating Producing a gel of the porous solid and the pore fluid from the sol while continuing to prevent the first solvent from substantially evaporating from the sol until the drying step; And a drying step of removing the pore fluid in a non-supercritical drying atmosphere to form a dry aerogel without substantially disrupting the porous solid, wherein the skeletal density of the dry aerogel is approximately in the first solvent of the aerogel precursor sol. Non-critical method for forming a nanoporous airgel, which is measured by the volume ratio of the metal alkoxide to. The method of claim 1, wherein the gel is produced before the evaporation step is terminated. The method of claim 1, wherein said drying step further comprises solvent exchange. The method of claim 1, further comprising aging the gel before the drying step. Depositing a film of aerogel precursor sol comprising a first solvent and a second solvent on a substrate; Preferably evaporating substantially all of said second solvent from said membrane; And crosslinking the deposited sol to form a wet gel aligned in a pore-opened structure on the substrate, thereby forming an aerogel film on the substrate. The method of claim 5, wherein the second solvent comprises the reaction product of the crosslinking step. The method of claim 5, wherein the vapor pressure of the second solvent is at least two times the vapor pressure of the first solvent. The method of claim 5, wherein said thin film deposition step comprises spin coating said airgel precursor sol over said substrate. 6. The method of claim 5, further comprising drying the wet gel in a non-supercritical atmosphere without substantially densifying the void opening structure. The method of claim 5, wherein the wet gel is formed before the evaporation step ends. 6. The method of claim 5, further comprising aging the wet gel without substantially evaporating the first solvent. a) providing a semiconductor substrate comprising a microelectronic circuit; b) depositing an aerogel precursor sol comprising a metal based aerogel precursor reactant, a first solvent comprising glycerol, and a second solvent, wherein the molar ratio of glycerol molecules to metal atoms in the reactants is at least 1:16; c) producing a gel consisting of a porous solid and a pore fluid from the deposited sol; And d) forming a dry airgel by removing the void fluid in the non-supercritical dry atmosphere. The method of claim 12, wherein the first solvent also comprises a polyol. The method of claim 13, wherein the polyol is glycol. a) milling a semiconductor substrate comprising a microelectronic circuit; b) depositing on the substrate an airgel precursor sol consisting of a metal-based aerogel precursor reactant, a first solvent comprising glycerol, and a second solvent, wherein the molar ratio of glycerol molecules to metal atoms in the reactants is at least 1:16; c) producing a gel of porous solids and pore fluid from the deposited sol, and d) forming a dry nanoporous dielectric by removing the pore fluid in a non-supercritical dry atmosphere. Non-supercritical method for forming a. a) providing a semiconductor substrate comprising a microelectronic circuit; b) an aerogel precursor reactant selected from the group consisting of metal alkoxides, at least partially hydrolyzed metal alkoxides, particulate metal oxides and mixtures thereof and a first solvent comprising glycerol, the metal atoms in the reactants Depositing an aerogel precursor sol on the substrate, wherein the molar ratio of glycerol molecules to is at least 1:16; c) producing a gel consisting of a porous solid and a pore fluid from the deposited sol; And d) forming a dry nanoporous dielectric by removing the pore fluid without substantially disrupting the porous solid, wherein the forming step is performed in a drying atmosphere, wherein the pressure of the drying atmosphere during A non-supercritical method for forming a thin film nanoporous dielectric over a semiconductor substrate, which is below a critical pressure. The method of claim 16, wherein the molar ratio of glycerol molecules to metal atoms in the reactants is 12: 1 or less. The method of claim 16, wherein the molar ratio of glycerol molecules to metal atoms in the reactants is from 1: 2 to 12: 1. The method of claim 16, wherein the ratio of glycerol molecules to metal atoms in the reactants is from 1: 4 to 4: 1. The method of claim 16, wherein the molar ratio of glycerol molecules to slowing atoms in the reactants is 2.5: 1 to 12: 1. The method of claim 16, wherein the porosity of the nanoporous dielectric is greater than 60% and the average pore diameter is less than 20 nm. The method of claim 16, wherein the dielectric constant of the nanoporous dielectric is less than 2.0. The method of claim 16, wherein the dielectric constant of the nanoporous dielectric is less than 1. g. The method of claim 16, wherein the dielectric constant of the nanoporous dielectric is less than 1.4. The method of claim 16, wherein the temperature of the substrate during the forming step is above the freezing point of the pore fluid. The method of claim 16, wherein the temperature of the substrate during the forming step is above the freezing point of the pore fluid and no step of adding a surface modifier prior to the forming step. The method of claim 16, wherein the temperature of the substrate during the formation step is above the freezing point of the pore fluid, the porosity of the nanoporous dielectric is greater than 60%, the average pore diameter is less than 20 nm, and the step of adding a surface modifier before the formation step comprises: How not. The method of claim 16 further comprising the aging step of the gel. The method of claim 28, wherein at least a portion of the aging step is performed in a substantially closed container. 29. The method of claim 28, wherein the temperature of the gel during aging is greater than 30 ° C. 29. The method of claim 28, wherein the temperature of the gel during aging is greater than 80 ° C. The method of claim 28, wherein the temperature of the gel during aging is greater than 130 ° C. 29. The method of claim 16, wherein the rate of permanent volume reduction of the porous solid during removal of the void fluid is less than 2%. The method of claim 16, wherein the porous solid remains substantially uncollapsed after removing the pore fluid. The method of claim 16. The volume reduction rate of the porous solid is less than 5% during removal of the void fluid. The method of claim 16 wherein the rate of volume reduction of the porous solid during removal of the void fluid is less than 1%. The method of claim 16, wherein the gel generation step is performed in a gelling atmosphere, wherein the vapor concentration of the first solvent in the gelling atmosphere is not actively controlled. The method of claim 16, wherein the gel production step is performed in a gelation atmosphere, wherein the vapor concentration of the first solvent in the gelation atmosphere is substantially uncontrolled. The method of claim 16, wherein the reactant is a metal alkoxide selected from the group consisting of tetraethoxysilane, tetramethoxysilane, methyltriethoxysilane, 1,2-bis (trimethoxysilyl) ethane, and mixtures thereof. . The method of claim 16, wherein the reactant is tetraethoxysilane. The method of claim 16 wherein the porosity of the dry porous dielectric is greater than 60%. The method of claim 16, wherein the porosity of the dry porous dielectric is between 60% and 90%. The method of claim 16, wherein the porosity of the dry porous dielectric is greater than 80%. The method of claim 16, further comprising replacing at least a portion of the void fluid with a liquid prior to the void fluid removal step. 45. The method of claim 44, wherein the liquid is hexanol. The method of claim 16, further comprising annealing the dry porous dielectric. The method of claim 16, wherein the pressure of the drying atmosphere during the forming step is less than 3 MPa. ※ Note: The disclosure is based on the initial application.