KR100772125B1 - Mitigation of radiation induced surface contamination - Google Patents

Abstract

The present invention relates to a process for reducing or eliminating contamination and / or degradation of surfaces having conventional and any atmospheric contaminants that are absorbed thereon and exposed to radiation. Gas or gas mixtures are injected into the environment of the surface to be protected. The type of gas to be injected (typically hydrocarbon gas, water vapor, oxygen or mixtures thereof) is selected depending on the contaminants as well as the activity of the type of gas bound to the surface to be protected. When surfaces and bond constrained species are exposed to radiation, reactive species form to form surface contaminants, such as carbon or oxide films, to form volatile products (eg, CO, CO ₂ ) that are absorbed and removed from the surface. React with

Emissions, Surface Contamination, Oxidation, Hydrocarbons, Water Vapor, Ethanol

Description

Mitigation of radiation induced surface contamination             

The present invention relates to a process for reducing degradation and contamination of surfaces caused by destruction of molecules by radiation injection and subsequent reaction and / or deposition on the surface. More specifically, the process disclosed herein relates to protecting the surface of a part of a lithographic stepper, such as a mask and imaging light, from molecular destruction by radiation injection.

Photolithography is a well known technique for applying a pattern to the surface of a workpiece, for example a circuit pattern on a semiconductor chip or wafer. This technology has the advantage of being able to faithfully provide small and sophisticated patterns. Conventional exposure techniques have openings formed therein such that light or radiation passing through the openings is applied to a predetermined area of the workpiece surface coated with a radiation-sensitive material, for example a photoresist. Applying electromagnetic radiation to a mask (transparent mask). This mask pattern is reproduced on the surface of the workpiece by removing exposed or unexposed photosensitizers. However, the performance of conventional exposure techniques is severely limited by the increase in circuit density and high resolution characteristics. As more integration is required, shorter emission wavelengths are required to reproduce the desired pattern. As the radiation wavelength applied becomes shorter, the radiation energy becomes larger, which can damage, degrade or contaminate a surface by the decomposition of molecules absorbed on or near the surface. It also provides reactive species.

Short-wavelength radiation can dissociate molecules directly, and the secondary electrons produced by interactions with their surface are the major factors that dissociate molecules. Low energy (5-10 eV) secondary electrons are known to be very promising in breaking chemical bonds by direct ionization of absorbed electrons or by electron attachment, where secondary electrons are dissociated by-products. It binds to molecules that provide reactive anions that de-excite. Any kind of radiation (photons, electrons, ions, and particles) with enough energy to escape electrons can produce secondary electrons; Typically an energy of about 4-5 eV is required. As a result, the radiation-injected contamination, that is, the contamination of the surface according to the reaction model provided by the secondary electrons due to the radiation interaction, is determined by the ultra-violet exposure (photon energy of about 100 eV) and the projection electron exposure (about 50 to 100 keV). Electron energy), ion beam exposure (ion energy> 10 keV), 193 nm exposure (photon energy of about 6.4 eV), and 157 nm exposure (photon energy of about 7.9 eV). Will occur without Thus, the potential for contamination of critical exposure components, such as masks and light surfaces, and deterioration of their operating performance are seen in all improved exposure processes.

A mechanism for surface contamination with gaseous species absorbed on top and exposed to the incident flux of radiation is schematically illustrated in FIG. 1. Here, surface 110 has both hydrocarbon and water molecules absorbed thereon. The term "hydrocarbon" may include any kind containing carbon. Exposure to the radiant flux releases secondary electrons from the surface 110, which releases the absorbed hydrocarbon molecules to form reactive carbon particles that can form a graphite layer on the surface. . For example, graphite having a thickness of about 230 GPa when exposed to a Si-terminal Mo / Si multilayer mirror at a flux density of about 330 mA / mm 2 of 13.4 nm radiation under a background pressure of 1 × 10 ⁻⁷ Torr for about 45 hours. The carbon layer grows. The graphite carbon film provided by secondary electron injected dissociation of hydrocarbon molecules absorbed on the surface from this environment reduces the mirror reflectance from 66% to 12%, which decreases the reflectance stepper of the multilayer mirror. You can disable it from within.

Similarly, secondary electrons emitted from the surface 110 in response to the radiation flux can dissociate the absorbed water molecules to form reactive oxygen species, which oxidize the surface to degrade the reflectivity of the mirror by absorption of radiation. The oxide film can be formed. Thus, oxidation resulting from dissociation of the radiation injected water molecules can cause very serious damage to the light surface. For example, a SiO ₂ layer grows when exposed to a Si-terminal Mo / Si multilayer mirror at a flux density of about 330 mA / mm2 of 13.4 nm radiation under a water vapor pressure of 1 x 10 ^-7 Torr for about 24 hours. . This SiO ₂ layer, which cannot be removed without damaging the Mo / Si multilayer structure, reduces the reflectance from 66% to 59%, resulting in unacceptable operating conditions.

Therefore, what is needed is a process that eliminates or significantly reduces the contamination and / or degradation of the surface, particularly the surface of critical exposure components exposed to radiation in the presence of conventional contaminants such as hydrocarbons and water vapor. This is especially true for modern exposure processes using shorter wavelengths and thus higher energy radiation due to the serious potential for deterioration of critical components and their high sensitivity.

It is an object of the present invention to solve the above-mentioned problems, to provide a process for reducing or eliminating surface contamination with molecular compounds of conventional and any atmosphere that are dissociated by exposure to radiation flux.

Another object of the present invention is to provide a process that operates efficiently in an atmosphere where partial pressure is applied.

In the process of the invention disclosed herein the gas or gas mixture is injected into the environment of the surface (surfaces) to be protected. The type of gas to be injected is selected according to the contamination as well as the activity of the type of gas bound to the surface to be protected. The criterion for the activity of the gas species depends on whether the secondary electrons emitted from the surface in response to incident radiation can dissociate the absorbed model. When the surface and the bound bond groups are exposed to radiation, reactive species are formed to react with surface contaminants such as carbon or oxide films to form volatile products (e.g. CO, CO ₂ ) that are absorbed and removed from the surface. do.

1 is a diagram schematically illustrating a process in which surface contamination occurs.

2 is a diagram schematically illustrating an embodiment of the present invention.

3A is a diagram illustrating electron beam irradiation and oxidation of a silicon (Si) surface exposed to water vapor.

Figure 3b is a view showing that the reduction of oxidation occurs by the addition of ethanol (ethanol).

FIG. 4 shows the effects of water vapor and ethanol on silicon surfaces exposed to ultra-ultraviolet (EUV) radiation.

5 shows the effect of oxygen molecules on carbon deposits.

 The present invention relates to a process for reducing or eliminating contamination and / or deterioration of surfaces with contaminants of conventional and any atmosphere that are absorbed thereon and exposed to energetic radiation. These contaminants may be inherently present in the atmosphere of any system and may include such types as hydrocarbons and water vapor which may often arise from material sources of structures, vacuum pump oils and greases. Radiation with energy can be photons, electrons, or ions. The present invention is particularly useful for exposure processes that employ radiation with high energy, such as ultra-ultraviolet radiation, and show significant potential for contamination and consequently deterioration of important part performance.

The process of the present invention is illustrated as being applied to ultra-ultraviolet (EUV) exposure, in particular the reduction of carbon deposition and oxidation, two conventional contaminant forms that are potent therein, but its use is intended for exposure operations or removal of carbon deposition or oxidation. It should be viewed as not applicable and generally applicable.

The process of the present invention is illustrated and described with reference to FIG. 2, which shows a Si surface 210 of a Mo / Si multilayer mirror. Frequently, water vapor can be present in any system, in particular in a non-baking vacuum system. Thus, any vapor is generated from the degassing process of the various system components and these vapor molecules can be absorbed on any surface, as illustrated in FIG. 2. Splitting the surface 210 on high energy radiation, such as by EUV radiation, may cause secondary electrons to be emitted from the surface 210. As described above, these electrons can react with the absorbed water molecules to dissociate into reactive oxygen groups, which in turn can react with the Si surface. Prior to exposing the surface 210 to incident radiation, a small amount of hydrocarbon gas, which may also be bound to the surface 210, is received in the system. In the example illustrated in FIG. 2, the hydrocarbon gas is ethanol (C ₂ H ₅ OH). As in the case of bonding with water vapor, C ₂ H ₅ OH molecules bound to the surface 210 can also be dissociated by secondary electrons emitted from the surface. From the dissociated C ₂ H ₅ OH, reactive carbon particles may react with reactive oxygen species on the surface 210 to form volatile products such as CO and CO ₂ . In this way, the Si surface can be oxidized, and reactive oxygen species that degrade its reflectivity can be removed.

It will be appreciated that the process described above may also work in the opposite manner. That is, carbon contamination of the surface may be provided by radiation exposure in an environment rich in carbon containing gas, and the main surface contaminant may be graphite carbon. As mentioned above, the presence of graphite carbon on the mirror surface can greatly shorten its reflectivity to EUV radiation. Thus, in this case, it is advantageous to receive a gas containing oxygen in its environment, such as water vapor or molecular oxygen. By replacing the hydrocarbon gas with water vapor or molecular oxygen, the same process as illustrated in FIG. 2 above can remove carbon contaminants such as carbon oxides (CO and CO ₂ ).

An example of the performance of the novel process described herein for exposing any molecular model to high energy radiation to remove surface contamination is given in the following experimental example.

Experimental Example 1

First, the Si-terminated multilayer mirror was exposed to an electron beam current density of about 5 mA / mm 2 at a beam energy of 2 keV at a water vapor pressure of about 2 × 10 ⁻⁷ Torr. The chemical composition of the mirror surface was monitored as a function of time. Auger peak heights of Si and SiO ₂ (reflecting surface compounds) are shown in FIG. 3A. It can be seen that as the oxidation proceeds with the corresponding deterioration of the reflectance, the surface composition changes rapidly from Si to SiO ₂ .

The experiment was repeated while maintaining the environment except that about 4 × 10 ⁻⁷ Torr of ethanol was added to 2 × 10 ⁻⁷ Torr of water vapor pressure. As before, the chemical composition of the mirror surface was monitored as a function of time and the results are shown in FIG. 3B. It can be seen that the amount of SiO ₂ present on the Si mirror surface is very small compared to the case with only water vapor; The same amount as the original oxide film is found on Si. In addition, post-exposure analysis of the exposed mirror surface showed the presence of a graphite carbon film about 5 mm thick.

The presence of the original oxide film on the Si is what determines the gaseous molecular selection used to prevent surface contamination. One important prerequisite for the selection of gaseous molecules is that they must be combined with the surface to be protected such that dissociation by secondary electron emission occurs, thereby leaving the reactive species on top of the surface itself. Finally, the hydrocarbon molecule is preferably combined with a hydroxylated membrane of the same polarity as can be found on the surface of the original surface component, such as glass, metal, and silicon. Thus, hydrocarbon molecules containing polar functional groups such as ketones, aldehydes, esters, alcohols, amines, thiols, and carboxylic acids that bind well to the hydroxide surface are preferred. Perhaps because these nonpolar molecules cannot be bound to the Si surface, by analogy to any hydroxylated surface, non-hydrogen-bonded hydrogen molecules, such as ethane, can no longer protect any oxidation on the Si surface. In addition, since the binding coefficient of both ethanol and water vapor on the graphite carbon surface is very small, growth of the graphite carbon film was provided when ethanol was used to provide oxidation protection to limit itself to a thickness of about 5 kPa. Thus, the use of ethanol cannot be used to form very thick carbon layers, which is undesirable. The act of limiting itself is a major process for the prevention method.

Experimental Example 2

Si-terminated Mo / Si mirrors were exposed to 13.4 nm EUV radiation at a power level of about 7 mA / mm 2. The mirror was exposed to four different exposure conditions and secondary electron emission is shown as a function of time in FIG. 4. Exposure conditions were as follows: background pressure (about 4 × 10 ^-9 Torr); Ethanol alone at a pressure of 4 × 10 ⁻⁷ Torr; Water vapor alone at 2 × 10 ⁻⁷ Torr pressure; And a mixture of ethanol (4 × 10 ⁻⁷ Torr) and water vapor (2 × 10 ⁻⁷ Torr). Secondary electron emission has been shown to serve as a useful monitor of the surface process, indicating that the onset of oxidation is accompanied by a very rapid increase in secondary electron emission due to EUV absorption by oxygen, with subsequent emission of secondary electrons. Because it is given. Carbon deposition, on the other hand, is given a signal due to the reduction of secondary electron emission due to the poor secondary electron emission characteristics of carbon.

Referring to FIG. 4, it can be seen that the exposure of the Si surface to water vapor in the presence of EUV radiation causes a rapid increase in the signal of the secondary electrons indicating the formation of an oxide film. However, when ethanol accommodates the increase in secondary electron signals due to the surface, it shows that the injected hydrocarbon (ethanol) limits the oxidation of the terminal Si surface and the oxidation is significantly reduced. In addition, adjusting the water / hydrocarbon ratio yields an equilibrium point at which carbon deposition and oxidation are in equilibrium with each other.

As mentioned above, exposure of a carbon containing gas to high energy radiation can form a graphite carbon film on the surface, such as a mirror surface, which may adversely affect the reflectance to EUV radiation. However, the graphite carbon film can be removed by the use of water vapor or oxygen in the disclosed process. This is given by the following experimental example.

Experimental Example 3

Referring now to FIG. 5, it is shown that the electron emission of any hydrocarbon molecules present in the vacuum system can result in carbon deposition. Here, a 2 kV electron beam is incident on the Si-terminal MO / Si multilayer mirror at a current density of about 5 mA / mm 2. The mirror is maintained at a vacuum of 5 × 10 ⁻¹⁰ Torr. As the exposure to the incident electron beam increases, an increase in the atomic weight percent of carbon on the mirror surface occurs. After about 120 minutes of exposure, about 2 × 10 ⁻⁸ Torr of molecular oxygen is received in the vacuum chamber. At that point, the increase in carbon deposition was stopped and the occurrence of additional carbon deposition was not measured.

Those skilled in the art will understand the importance of properly impinging the delivery of radiation to the exposure apparatus, in particular by adding a gas mixture. As exemplified above, the present invention provides an important advantage in this regard because the partial pressure of variously added gas molecule models need to be as high as necessary to affect the surface chemistry. Using gas pressures up to about 1 × 10 ⁻⁵ Torr will have negligible effects on exposure output or uniformity but will often be suitable to provide a protective effect on most contamination problems. For example, with each part at a partial pressure of about 1 × 10 ⁻⁵ Torr and with a 2 meter long water / ethanol gas mixture, EUV radiation transmission at only 13.4 nm was reduced by only about 0.05%.

Consequently, the present invention provides a process for reducing or eliminating the contamination and oxidation of surfaces due to the interaction of high energy radiation on any impurities. In particular, the process disclosed herein can be used to reduce or remove oxide films or carbon films on a surface, such as the surface of a silicon mirror. More specifically, the present invention provides particular advantages in removing or reducing surface contamination in components such as masks and imaging lights used in exposure equipment.

The foregoing methods and experimental examples included herein are merely illustrative of the principles of the present invention, and various other embodiments and modifications may be made by those skilled in the art without departing from the spirit and scope of the invention as defined in the claims. It can be carried out by.

Claims (21)

a) an enclosed space having a surface disposed therein—the atmosphere in the enclosed space comprises water molecules and the surface having water molecules absorbed thereon such that the absorbed water molecules are removed from the surface by ultra-ultraviolet radiation. Providing reactive oxygen species that are dissociated by the generated electrons to form an oxide film on the surface; b) in the atmosphere within the enclosed space by injecting hydrocarbon molecules capable of binding to the surface to form a bond on the surface and dissociating by electrons generated from the surface by ultra-ultraviolet radiation to generate a reactive carbon species. Reacting with water molecules; c) subjecting the contents of the enclosed space to ultra-ultraviolet radiation to form a reactive carbon species by a hydrocarbon molecule which is a surface-bonded bond group, and forming a reactive oxygen species by water molecules by the ultra-violet radiation; And d) combining the formed reactive carbon species and reactive oxygen species to form CO or CO ₂ Removing the contamination of the surface exposed to the radiation comprising a. The method of claim 1, And said enclosed space is at sub-atmospheric pressure. The method of claim 1, The hydrocarbon molecule is at least one functional group selected from the group consisting of ketones, aldehydes, esters, alcohols, amines, thiols, and carboxylic acids. and (functionality) to remove contamination of the surface exposed to radiation. The method of claim 3, And said hydrocarbon molecule is ethanol. delete delete delete a) an enclosed space having a surface disposed therein—the atmosphere in the enclosed space comprises hydrocarbon molecules and the surface having hydrocarbon molecules absorbed thereon such that the absorbed hydrocarbon molecules are removed from the surface by ultra-ultraviolet radiation. Providing reactive carbon species that are dissociated by the generated electrons to produce a graphite film on the surface; b) in the enclosed space by injecting water vapor or oxygen molecules that can bind to the surface to form a bond on the surface and are dissociated by electrons generated from the surface by ultra-ultraviolet radiation to produce a reactive oxygen species. Reacting with the hydrocarbon molecule in the atmosphere; c) subjecting the contents of the enclosed space to ultra-ultraviolet radiation to form a reactive oxygen atom by water vapor or oxygen molecules, which are bonded on the surface, to form a reactive oxygen atom by the ultra-violet radiation; And d) combining the formed reactive carbon species and reactive oxygen species to form CO or CO ₂ Removing the contamination of the surface exposed to the radiation comprising a. The method of claim 8, And said enclosed space is at sub-atmospheric pressure. delete delete delete delete delete delete delete delete delete delete delete delete

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