JPH11294948A - Manufacture of minute device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To effectively prevent the fracture of a minute structure body caused by volume change on freezing, by using water and liquid that is soluble in water and at the same time has a negative volume change rate on freezing when performing freezing/drying. SOLUTION: A precursor for forming the self-supporting structure of a minute device is dipped into etching liquid 110 consisting of fluoric acid, an oxide film that is a sacrifice layer is eliminated, and a minute structure body 111 is formed, where the minute structure body is opposite to the main surface of a silicon substrate 100 that is a support substrate and is of free standing type while being separated by a minute gap 112. Then, the minute structure body 111 is sufficiently washed in pure water without drying, is rinsed in mixed liquid where water and tertiary butyl alcohol are mixed by a capacity ratio of 20% where volume change becomes smaller on freezing, and is frozen while a liquid film remains on the surface, thus effectively preventing fracture caused by the volume change on freezing.

Description

DETAILED DESCRIPTION OF THE INVENTION

[0001]

The present invention relates to a method for forming a mechanical microstructure on a substrate material such as a semiconductor substrate, and more particularly to a freeze-drying method after a sacrificial etching step.

[0002]

2. Description of the Related Art The history of freeze-drying in the semiconductor industry is relatively short, and applications have been filed as a technique for preventing the occurrence of watermarks during wafer drying (see, for example, Japanese Patent Application Laid-Open Nos. Showa 60-165727
JP-A-63-222433, JP-A-1-96624
Issue).

[0003] On the other hand, in 1982, Petersen
con as a Mechanical Material ”(Proceeding of The
IEEE, vo1.70, No.5, pp.420-457, May1982), demonstrating the potential of silicon as a mechanical material. Since then, a new trend has been created in the semiconductor industry to form micromachines using semiconductor processes. Although a technique called sacrifice etching is used to form the micromachine, there occurs a problem that the microstructure adheres to the surface of the support substrate in the drying process. Guckel proposes to apply a freeze-drying method as a method for preventing such sticking (Japanese Patent Laid-Open No. 3-50 / 1990).
No. 2268, USP-5013693). Then Fu
jita converts water to tert-B having a freezing point near room temperature.
a method of replacing with uOH (Tranducers 1991, p.63),
Or a method to replace water with paradichlorobenzene, a sublimable substance that is solid at room temperature (MEMS 1992, p.214)
Has been announced.

[0004] In addition, a method of replacing water with an organic solvent such as cyclohexane or a sublimable substance such as naphthalene has been reported to other researchers. However, instead of removing the liquid by evaporation, the liquid is frozen. In terms of sublimation and removal of solids, both can be said to be methods of the same concept.

When the liquid such as pure water used for cleaning is evaporated and dried after the sacrificial etching without performing the freeze-drying as described above, the distance between the microstructure and the supporting substrate is extremely small (about several μm). Therefore, there is a problem that the microstructure adheres to the surface of the support substrate and a self-standing structure cannot be formed stably. This is due to the fact that wet fallen leaves stick firmly when dried, that when wet books and bills are dried while overlapping, they stick together. To stick to and shrink as a whole,
It is a phenomenon similar to the phenomenon observed on a daily basis.

[0006] In the conventional freeze-drying method for solving the above-mentioned problems, the method of freezing water has a problem that the microstructure is damaged at the time of freezing and the yield is reduced. This phenomenon is similar to the phenomenon often experienced on a cold day, such as the water inside the water pipe freezes and the water pipe ruptures.
This is because the structure expands by almost 0% and gives a large stress to the structure. The tendency of the microstructure to break largely depends on the structure of the microstructure or the manner of freezing during freeze-drying. The more open the gap, the less likely it is to break,
The closer the gap is, the more likely it is to break. Also, when the frozen liquid is frozen in such a manner that the inside of the gap is frozen after the opening of the gap is closed, the liquid is easily damaged. Further, a structure that is weaker to the application of stress is more likely to be damaged.

Further, when tert-BuOH or paradichlorobenzene is used instead of water,
Cracks are generated in the frozen liquid, and the microstructure is damaged, resulting in a problem of lowering the yield. This is because the volume shrinks by 10 to 20% during freezing, and a stress such as being pulled or torn is applied to the microstructure. The tendency of the frozen liquid to crack and the tendency to break the microstructure depend on the structure of the microstructure, the thickness of the liquid film to be frozen, and the like. Cracking is less likely to occur as the number of anchor structures for preventing shearing of the frozen liquid film at the time of volume shrinkage on the surface of the support substrate, and tends to occur more easily as the surface becomes flatter or the liquid film becomes thicker. Further, a structure that is weaker to the application of stress is more likely to be damaged.

[0008]

As described above, in the method of evaporating water and drying without using the freeze-drying method after the sacrificial etching, there is a problem that the microstructure adheres to the surface of the supporting substrate. In the freeze-drying method that solves this problem, microstructures are formed by volume expansion during freezing when water is used as the liquid to be freeze-dried, or by volume shrinkage during freezing when an organic solvent is used. May be damaged.

The present invention has been made in order to solve the problems of the prior art as described above, and the microstructure does not adhere to the surface of the support substrate, and the damage due to the volume change during freezing is effectively prevented. It is an object of the present invention to provide a method for manufacturing a micro device which can be prevented from occurring.

[0010]

Means for Solving the Problems In order to achieve the above-mentioned object, the present invention has a structure as described in the claims. That is, in the invention described in claim 1, water and a liquid that dissolves in water and has a negative rate of volume change during freezing (volume during freezing) are used as the liquid that is frozen and removed by sublimation during lyophilization. Liquid that shrinks)
Is used. By using a mixture of water that expands volume during freezing and liquid that contracts volume as described above, the volume change of the liquid during freezing becomes very small, preventing the destruction of the microstructure due to the volume change. You can do it.

Further, in the invention according to claim 2,
The liquid to be removed by freezing and sublimation is mainly water, and the ratio of the liquid having a negative volume change rate during freezing is 20 to 40% by volume. As will be described in detail in an embodiment described below, the volume change rate at the time of freezing in a liquid obtained by mixing water and a liquid having a negative volume change rate at the time of freezing is as follows. It cannot be simply calculated from the volume change rate when frozen alone. That is, since the rate of change in deposition when the mixed solution is frozen does not have a linear correlation with the volume ratio, it is not easy to calculate the volume ratio at which the volume change rate decreases. As a result of consideration and experiments by the present inventor, a composition in which the volume change rate when a mixture of water and a liquid having a negative volume change rate freezes is extremely small is a type of liquid having a negative volume change rate. Regardless, the ratio was found to be in the range of 20 to 40% by volume. By using this ratio, an effective mixing ratio can be easily obtained.

The invention according to claim 3 uses a so-called compatible organic solvent which dissolves in water and dissolves in water at an arbitrary volume% as a liquid having a negative volume change rate during freezing. ing. Further, the invention according to claims 4 to 6 is:
Examples of liquids that dissolve in water and have a negative rate of change in volume during freezing are given, and these are also compatible organic solvents according to the third aspect.

[0013]

According to the present invention, since the freeze-drying method is used, the microstructure does not adhere to the surface of the supporting substrate, and the rate of volume change when the liquid freezes during freeze-drying is suppressed. Therefore, the microstructure can be prevented from being damaged, the yield can be maintained at a high level, and the final cost can be reduced.

[0014]

1 and 2 are a sectional view and a plan view, respectively, showing a manufacturing process of an embodiment according to the present invention. The process shown in FIG. 2 is followed by the process shown in FIG. 1, (A) and (C) show cross-sectional views, (B) and (D) show plan views, and in FIG. 2, (E) to (G) show cross-sectional views, and (H) shows a plan view. Show. Hereinafter, each step will be described.

First, as shown in FIGS. 1A and 1B, an oxide film 101 serving as a sacrificial layer is formed on a main surface of a silicon substrate 100 by a plasma CVD method, and photolithography and dry etching are performed. By the method, the oxide film 101
Is patterned.

Next, as shown in (C) and (D),
A polycrystalline silicon film 102 is formed on the main surface of the above structure by LP-C.
The polycrystalline silicon film 102 is formed by a VD technique, and is patterned by photo and dry etching techniques.

Through the above steps, a precursor for forming a self-supporting structure of a micro device, comprising a silicon substrate 100 as a supporting substrate, an oxide film 101 as a sacrificial layer, and a polycrystalline silicon film 102 for forming a micro structure, is formed. Is done. In the present invention, the above steps are the same as those in the conventional example, and do not require a change in design such as a pattern rule.

Next, as shown in FIG. 2E, the precursor formed in FIG. 1 is replaced with an etching solution 11 made of hydrofluoric acid.
0, and the oxide film 101 serving as the sacrificial layer is removed.
A self-supporting microstructure 111 is formed opposite to the main surface of the silicon substrate 100 serving as a support substrate, and separated by a minute gap 112 (for example, about 10 μm to about 10 μm), which is the same as the thickness of the oxide film 101. . The gap 112 is filled with an etching solution 110 (dotted portion) made of hydrofluoric acid. The interval between the gaps 112 is, for example, about 1 μm, the thickness of the microstructure 111 is about several to 10 μm, and the length is about 100 to 400 μm.

Next, as shown in FIG.
Continue to wash thoroughly in pure water without drying,
Subsequently, water: tert-BuOH = 4: 1 (volume ratio), that is, tert-BuOH was added to water at a volume ratio of 20%.
Rinse with the mixed liquid mixture, and freeze with the liquid film remaining on the surface. 113 (dotted portion) is a frozen liquid mixture. The complete freezing temperature at which the liquid film completely freezes is 10 ° C. below freezing.

Next, as shown in (G) and (H), the above structure is held in an environment having a low vapor partial pressure of water such as vacuum or dry air, and the frozen liquid 113 is removed by sublimation. As a result, a free-standing microstructure 111 is obtained on the surface of the silicon substrate 100 with a minute gap therebetween. In this example, the microstructure 111 has a doubly supported structure. Further, the inside of the gap 112 is filled with air or gas or kept in a vacuum depending on the surrounding atmosphere where the microstructure is installed and the state of the package.

In the present embodiment, the above step (F)
When the mixed solution is frozen in the freezing step, the volume of the mixed solution hardly expands or contracts, the volume is maintained before and after freezing, and no crack occurs in the frozen liquid film. Therefore, the microstructure is not subjected to stress such as being pushed, pulled or torn. Therefore, even in the case of a microstructure having a structure that is weak against stress or a structure in which a gap is closed, the microstructure can be prevented from being damaged, and the yield can be maintained at a high level. Becomes

Next, the mixed solution used in the above-described manufacturing process will be described. Water that is the first liquid, and the volume change rate when the liquid mixture of the second liquid is frozen, the water that is the first liquid, and when each of the second liquid is frozen alone It cannot be simply calculated from the volume change rate. That is, since the rate of change in deposition when the mixture is frozen has no linear correlation with the volume ratio, it is not easy to calculate the volume ratio at which the volume change does not expand or contract. As a result of various considerations and experiments, the inventor has found that the volume change rate when the mixture of the first liquid water and the second liquid is frozen, the composition that does not expand or contract, It has been found that the amount is 20 to 40% by volume regardless of the type of liquid (organic solvent).

Hereinafter, the experiments performed will be described. First, water was selected as the liquid A whose volume expands during freezing. Next, MeOH (methanol), IP as a liquid whose volume shrinks during freezing and which dissolves in the liquid A
A (isopropyl alcohol), tert-BuOH
(Dartal butyl alcohol) and TEGB (Triethylene glycol monobutyl ether).
The above-mentioned MeOH is representative of an organic solvent whose molecular structure is closest to water, TEGB is a semiconductor having a large molecular weight and only one hydroxyl group, and IPA is a representative of an organic solvent whose molecular structure is far apart from water. As a representative of organic solvents used in large quantities in the industry and having high purity and low cost, t
ert-BuOH was selected as a representative of the organic solvents in which the complete freezing point of the mixed solution is not expected to be much lower than the freezing point.

FIG. 3 is a characteristic diagram showing the results of the above experiment, in which the horizontal axis represents the volume percentage occupied by the organic solvent, and the vertical axis represents the volume change rate (%) during freezing. As can be seen from FIG. 3, in any of the mixed liquids with the liquid B (organic solvent), the optimum composition that reduces the volume change during freezing is in the range of 20 to 40% by volume. Thus, it can be said that the organic solvent used almost completely falls within this range, from those having a molecular structure very close to that of water to those very far apart. Representative MeOH having a molecular structure closest to water has a high first-order correlation. On the other hand, other organic solvents have a secondary or tertiary correlation, and tend to be flat or rather convex downward on the high concentration side. Therefore, it can be understood from the volume change rates of the liquid A (water) and the liquid B (organic solvent) that the optimum composition is on the lower concentration side than the composition ratio simply estimated from the first order.

It is to be noted that an organic solvent having a too large number of molecules is not suitable for the present invention because it is difficult to dissolve in water. In the present invention, a so-called compatible organic solvent which dissolves in water at an arbitrary volume% is preferable. It is. Each organic solvent shown in FIG. 3 is a corresponding substance. Those not shown in FIG. 3 can use acetone, formaldehyde, and the like. To summarize, alcohols having -OH group such as methanol, ethanol, propanol and butanol,
Carboxylic acids having a -COOH group such as acetone, carbonyl compounds having a -CO-bond or -CHO group such as formaldehyde, ethers having a -O- bond such as diethyl ether, and tert-BuO
H or the like can be used.

Next, the freezing temperature will be described. A substance having a very low freezing temperature complicates the freezing step (F), and thus a substance that can be frozen as easily as possible is desirable. Therefore, the above tertiary organic solvent is expected to have a complete freezing point of the mixed solution that is not expected to be much lower than the freezing point.
The complete freezing temperature was measured using t-BuOH.

FIG. 4 is a characteristic diagram showing the above measurement results, and shows the measurement results of the complete freezing temperature of a mixed solution obtained by mixing tert-BuOH with water. In FIG. 4, the horizontal axis indicates the tert-BuOH mixture ratio (% by volume), and the vertical axis indicates the complete freezing temperature (° C.). As can be seen from FIG. 4, in the range of the mixing ratio (20 to 40% by volume) where the rate of volume change during freezing is small, the complete freezing temperature is about −10 ° C., which is not much lower than the freezing point. It can be seen that the mixture is easy to freeze. Also, the vapor pressure at the freezing point does not decrease so much, and drying by sublimation is easy to perform.

In the above description of the embodiments, specific examples have been described. However, the present invention is not limited to these numerical values, words, or figures, and lyophilization of various micro devices is performed. Applicable to the process.

[Brief description of the drawings]

FIG. 1 is a cross-sectional view and a plan view illustrating a part of a manufacturing process according to an embodiment of the present invention.

FIGS. 2A and 2B are a cross-sectional view and a plan view illustrating another part of the manufacturing process according to one embodiment of the present invention; FIGS.

FIG. 3 is a characteristic diagram showing measurement results of a volume change rate when various mixed solutions are frozen.

FIG. 4 is a characteristic diagram showing measurement results of a complete freezing temperature of a mixed solution (water / tert-BuOH).

[Explanation of symbols]

DESCRIPTION OF SYMBOLS 100 ... Silicon substrate 101 ... Oxide film 102 ... Polycrystalline silicon film 110 ... Etching liquid 111 ... Microstructure 112 ... Micro gap 113 ... Frozen mixed liquid

Claims (6)

[Claims] 1. A multi-layer substrate having at least a support substrate, a sacrifice layer, and a structure film laminated thereon, and after at least a part of the sacrifice layer is removed by etching, a gap between the support substrate and the structure film is formed. A method for manufacturing a microdevice, wherein a substructure is formed by opposing the supporting substrate at a minute interval with the support substrate on the structural film by performing so-called freeze drying by sublimating and removing an existing liquid after the liquid is frozen. The liquid to be removed by freezing and sublimation during the freeze-drying is a liquid obtained by mixing water and a liquid that dissolves in water and has a negative rate of change in volume during freezing. Manufacturing method of micro device. 2. The liquid to be removed by freezing and sublimation is mainly water, and the ratio of the liquid that is dissolved in the water and has a negative volume change rate during freezing is 20 to 40% by volume. The method for manufacturing a microdevice according to claim 1, wherein: 3. The so-called compatible organic solvent, wherein the liquid soluble in water and having a negative rate of change in volume during freezing is dissolved in water at an arbitrary volume%. A method for manufacturing a microdevice according to claim 2. 4. A liquid which is dissolved in water and has a negative rate of change in volume upon freezing is an alcohol having an --OH group, --CO
Carboxylic acids having an OH group, -CO-bond or-
The method for producing a microdevice according to any one of claims 1 to 3, comprising at least one of a carbonyl compound having a CHO group and an ether having an -O- bond. 5. The alcohol having a -OH group is any one of methanol, ethanol, propanol and butanol, the carboxylic acid having a -COOH group is acetone, and a carbonyl having a -CO- bond or a -CHO group. The method according to claim 4, wherein the compound is formaldehyde, and the ether having an -O- bond is diethyl ether. 6. The microdevice according to claim 1, wherein the liquid dissolved in water and having a negative rate of change in volume during freezing is tert-BuOH. Production method.