GB2413645A

GB2413645A - Vacuum treatment for lithography wafer

Info

Publication number: GB2413645A
Application number: GB0409587A
Authority: GB
Inventors: Robert Bruce Grant
Original assignee: BOC Group Ltd
Current assignee: BOC Group Ltd
Priority date: 2004-04-29
Filing date: 2004-04-29
Publication date: 2005-11-02
Also published as: GB0409587D0

Abstract

Extreme ultraviolet (EUV) lithography apparatus comprises a lithography tool (10) housed in a vacuum chamber (12) for exposing a wafer to EUV radiation to transfer a pattern to a photoresist layer applied to the layer. Before the wafer is inserted into the lithography tool, it is subjected to a vacuum treatment to promote outgassing of a solvent from the photoresist, thereby reducing the level of solvent contamination within the lithography tool.

Description

24 1 3645

LITHOGRAPHY APPARATUS

This invention relates to lithography apparatus.

s Photolithography is an important process step in semiconductor device fabrication.

In overview, in photolithography a circuit design is transferred to a wafer through a pattern imaged on to a photoresist layer deposited on the wafer surface. The wafer then undergoes various etch and deposition processes before a new design is transferred to the wafer surface. This cyclical process continues, building up the to multiple layers of the semiconductor device.

In lithographic processes used in the manufacture of semiconductor devices, it is advantageous to use radiation of very short wavelength, in order to improve optical resolution, so that very small features in the device may be accurately reproduced.

In the prior art, monochromatic visible light of various wavelengths have been used, and more recently radiation in the deep ultra violet (DUV) range has been used, including radiation at 248 nm, 193 nm and 157 nm. In order to further improve optical resolution, it has also been proposed to use radiation in the extreme ultra violet (EUV) range, including radiation at 13.5 nm.

The use of EUV radiation for lithography creates many new difficulties, both for the optics in the lithography tool, and also in the EUV radiation source.

One problem is that EUV radiation has poor transmissibility through most gases at atmospheric pressures, and therefore much of the mechanical, electrical and optical equipment involved in the lithography process must be operated in a high- purity vacuum environment. In many cases, gas purge flows are used to prevent contaminating materials (such as photoresist and photoresist by- products) reaching the optical components, and to provide cooling and to prevent migration so of particles. Gases may also be used in hydrostatic or hydrodynamic bearings in order to allow mechanical motion of the wafer or the mask.

2 BVlv- A further problem is that lens materials used for projection and focussing of radiation in DUV lithography, such as calcium fluoride, are not suitable for transmission of EUV radiation, and it is usually necessary to use reflective optical devices (mirrors) in place of transmissive optical devices (lenses). These mirrors s generally have multilayer molybdenum-silicon surfaces, which are extremely sensitive to contamination. In the presence of EUV radiation, secondary electrons are released from the multi-layer mirror (MLM) surface, which interact with contaminants on the surface, reducing their reflectivity. Adsorbed water vapour on the mirror surface causes oxidation of the uppermost silicon layer. Adsorbed lo hydrocarbon or other carbonaceous deposits can be cracked to form graphitic carbon layers adhering to the surface. The resulting loss of reflectivity leads to reduced illumination and consequent loss of tool productivity. Due to the high cost of these optical components, it is always undesirable to replace them, and in many cases it is completely impractical.

A significant improvement in the oxidation resistance of MLMs has been achieved by "capping" the MLM with ruthenium but this does not mitigate the cracking of residual organic material.

to The deposition and in-situ removal of carbonaceous layers on MLMs has been studied by many groups. For example, in H Meiling et al, Proceedings of SPIE Vol 4506 (2001) pages 93-104, the deposition rates of carbonaceous films under simulated EUV lithography conditions were measured. It was demonstrated that high molecular weight organic contamination presented the greatest problem to mirror cleanliness, and that the addition of oxygen at levels of approximately 1 x1 o-6 mbar could reduce or even reverse the rate of carbonaceous film growth.

However, in this case surface oxidation of the mirrors becomes a problem. In K Sugisaki et al, Proceedings of the 2nd EUVL Symposium 2003, it was demonstrated that a pinhole will become completely blocked by the build up of so carbonaceous deposits on exposure to EUV radiation in the presence of residual organic material. However, the addition of oxygen to the system increases the risk of oxidation of MLM surfaces which do not have carbonaceous deposits 3 - AA^An.4 An,. __ formed thereon, thereby increasing the risk of losing reflectivity and the resultant loss of performance.

It has been found that by far the greatest source of hydrocarbon material in an s EUV lithography system is the wafer itself. Prior to entry into the photolithography process tool, the wafer undergoes a number of prelithography processing steps on what is commonly referred to as a "track tool", which is separate from the photolithography process tool. On the track tool, the wafer is coated with a photoresist, which in turn contains organic solvents like toluene, xylene, anisole to and NMP etc. These solvents are used to dissolve the photoresist and deposit a photoresist solution onto the wafer in a spin coating process. The wafer then undergoes a 'pre-exposure bake' to remove the bulk of the remaining solvent, and is stored prior to insertion into the lithography tool. However, significant quantities (up to 20%) of solvent remain trapped within the photoresist layer and are released when the wafer enters the EUV lithography tool vacuum environment, contributing significantly to the levels of hydrocarbon contamination in the vacuum system. Typically, around 80 to 120 wafers will pass through the lithography tool per hour. The relatively high levels of oxygen required to control the resulting carbonaceous film growth when the optics are illuminated by EUV radiation will lead to MLM oxidation and a reduced lifetime.

In at least its preferred embodiment, the present invention aims to significantly reduce the level of solvent outgassing from wafers within a lithography tool, and thereby reduce the level of hydrocarbon contamination within the lithography tool, in order to lower the oxygen partial pressure required to control the build up of carbonaceous films on the MLM elements and reduce the risk of mirror oxidation.

In accordance with one aspect of the present invention, there is provided lithography apparatus comprising a lithography tool for exposing a wafer to so electromagnetic radiation, and means for vacuum treating the wafer prior to its insertion into the lithography tool.

4 a A The rate q(t) of outgassing from a surface under vacuum obeys an exponential decay law in the form: quit) = const x to s where a is typically between 1 and 1.3 for desorption-limited outgassing from, for example, a metal surface, and is less than 1 for diffusion-limited outgassing which is to be expected from solvent outgassing from a photoresist-covered wafer.

lo By vacuum treating the wafer prior to its insertion into the lithography tool, the amount of time for which the wafer is exposed to a vacuum prior to being inserted into the lithography tool, typically also under vacuum, is dramatically increased.

As a result, by the time the wafer is inserted into the lithography tool, the rate of solvent outgassing from the photoresist layer will have greatly decreased. Is

The vacuum treatment means preferably comprises a vacuum chamber for receiving the wafer, and pump means for evacuating the vacuum chamber. The rate of solvent outgassing from the photoresist whilst the wafer is located within the chamber can be controlled by controlling the solvent partial pressure above the so wafer, and the speed of evacuation of the chamber.

The vacuum chamber may conveniently house at least one of a processing tool for performing a pre-exposure process to the wafer, and wafer storage means for storing the processed wafer prior to its insertion into the lithography tool. This us processing tool may comprise a pre-exposure heating tool for baking the wafer to remove the bulk of the solvent from the photoresist applied to the wafer.

Performing this baking under vacuum can increase the outgassing rate of the solvent from the photoresist.

so The lithography tool may be housed within a second vacuum chamber, with the apparatus comprising means for evacuating the second vacuum chamber, or may - 5 housed in the same vacuum chamber as the processing tool and/or wafer storage means.

In the preferred embodiments, the radiation is extreme ultraviolet radiation. Thus, in a second aspect the present invention provides extreme ultraviolet (EUV) lithography apparatus comprising means for applying to the surface of a wafer a layer of photoresist, means for vacuum treating the wafer to promote outgassing of a solvent from the photoresist, and a lithography tool housed in a vacuum chamber for exposing the wafer to EUV radiation to transfer a pattern to the lo photoresist layer.

The present invention also provides a method of processing a wafer, the method comprising the steps of applying a photoresist layer to the wafer, performing a vacuum treatment of the wafer to promote outgassing of a solvent from the photoresist, and subsequently exposing the wafer to electromagnetic radiation in a vacuum environment to transfer a pattern to the photoresist layer.

Features described above in relation to apparatus aspects of the invention are equally applicable to method aspects, and vice versa.

By way of example, an embodiment of the invention will now be further described with reference to the following Figures in which: Figure 1 illustrates schematically an example of a conventional lithography apparatus; Figure 2 illustrates schematically a first embodiment of a lithography apparatus according to the invention; and so Figure 3 illustrates schematically a second embodiment of a lithography apparatus according to the invention. - 6

With reference first to Figure 1, a conventional lithography apparatus comprises a lithography tool 10 housed with a chamber 12. The lithography tool comprises an optical system of multi-layer mirrors (MLMs) which generate from incident EUV radiation a EUV radiation beam for projection on to a mask or reticle for the selective illumination of a photoresist on the surface of a semiconductor wafer.

The MLMs comprise a plurality of layers, each layer comprising, from the bottom a first layer of molybdenum and a second layer of silicon. A metallic layer, typically formed from ruthenium, is formed on the upper surface of each MLM to improve the oxidation resistance of the MLMs whilst transmitting substantially all of the lo EUV radiation incident thereon. The source of EUV radiation may be a discharge plasma source or a laser-produced plasma source located in a chamber adjoining the chamber 12. In a discharge plasma source, a discharge is created in a medium between two electrodes, and a plasma created from the discharge emits EUV radiation. In a laser-produced plasma source, a target is converted to a Is plasma by an intense laser beam focused on the target. A suitable medium for a discharge plasma source and for a target for a laserproduced plasma source is xenon, as xenon plasma radiates EUV radiation at a wavelength of 13.5 nm.

Due to the poor transmissibility of EUV radiation through most gases, a vacuum go pumping system 14 is provided for generating a vacuum within chamber 12. In view of the complex variety of gases and contaminants, such as water vapour and hydrocarbons, which may be present in chamber 12, the pumping system may include both a cryogenic vacuum pump and a transfer pump, such as a turbomolecular pump, backed by a roughing pump. Such a combination of pumps us can enable a high vacuum to be created in the chamber 12.

The lithography tool 10 receives wafers from track tool 16. The track tool 16 comprises a number of processing tools for performing pre-lithography and post- lithography processing of the wafers. In the example shown in Figure 1 a first so pre-lithography processing tool 18 performs a surface priming or preparation treatment of a wafer. In a second pre-lithography processing tool 20, the wafer is coated with a photoresist, which contains an organic solvent, such as toluene, xylene, anisole and/or NMP, to dissolve the photoresist and deposit a photoresist solution on to the wafer in a spin coating process. A third pre-lithography processing tool 22 performs a pre-exposure bake of the wafer to remove the bulk of the solvent remaining in the photoresist, and stores the processed wafer prior to its insertion into the lithography tool 10. Following the exposure of the wafer to EUV radiation within the lithography tool 10, the wafer is removed from the lithography tool 10 and is subject to a post-exposure baking in the first post- lithography processing tool 24, development in the second post- lithography processing tool 26 and a final, hard baking in the third post- lithography processing lo tool 28.

In this conventional lithography apparatus, significant quantities of hydrocarbon solvent can remain trapped within the photoresist following the pre-exposure bake of the wafer. When the wafer is inserted into the evacuated lithography tool 10, the trapped hydrocarbons are released and become adsorbed on the surfaces of the MLMs within the lithography tool 10. In the presence of EUV radiation within the lithography tool 10, secondary electrons are released from within the surfaces of the MLMs, which electrons interact with the hydrocarbon contaminants on the surfaces, reducing their reflectivity. Cracking of adsorbed hydrocarbon to contaminants can form graphitic type carbon layers adhering to the MLMs, with the resulting loss of reflectivity leading to reduced illumination and consequent loss of tool productivity. In order to control the level of carbonaceous film growth on the MLM surfaces, oxygen is typically introduced into the chamber 12. However, the increased oxygen partial pressure within the chamber 12 results in MLM oxidation, reducing tool lifetime.

In order to reduce the oxygen partial pressure within the chamber 12, a first embodiment of a lithography apparatus according to the invention, as shown in Figure 2, houses at least part of the track tool 16 in a vacuum chamber 30. In the so example shown in Figure 2, the third prelithography processing tool 22 is housed within vacuum chamber 30 to enable one, or both, of the pre-exposure bake of the wafer and the subsequent storage of the wafer to be performed in a vacuum O _ Am A A

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environment. A suitable pumping system 32 is provided for evacuating the vacuum chamber 30.

By vacuum treating the wafer prior to its introduction into the lithography tool 10, s the outgassing of solvent from the photoresist applied in the second pre- lithography processing tool 20 can be promoted before the wafer is inserted into the evacuated lithography tool 10. For example, if the wafer is subjected to a pre- lithography vacuum treatment for 600 seconds or more before it is inserted into the lithography tool 10 for lithographic processing for a duration of, typically, 60 lo seconds or less, the rate of solvent outgassing from the wafer whilst it is located within the lithography tool 10 can be reduced by as much as 85% compared to a situation where the wafer was not subjected to any pre-lithography vacuum treatment. This can greatly reduce the oxygen partial pressure required to control carbonaceous film growth on the MLM surfaces, thereby reducing the risk of mirror oxidation.

The rate of solvent outgassing from the photoresist whilst the wafer is located within the evacuated chamber 30 can be controlled by controlling the solvent partial pressure above the wafer, and the speed of evacuation of the chamber 30.

so These can be controlled by purging the vacuum chamber, as indicated at 34 in Figure 2, with a purge gas, for example, an ultra-high purity gas, whilst simultaneously evacuating the chamber 30, and/or by throttling the vacuum pump 32 during chamber evacuation.

In an alternative embodiment shown in Figure 3, rather than providing a separate vacuum chamber 30 for that part of the track tool 16 in which solvent outgassing is to be performed, the vacuum chamber 12 of the lithography tool 10 can be extended into the track tool so that prelithography solvent outgassing can be performed in the same vacuum environment as the lithography tool 10. In order so to prevent outgassed solvent from adsorbing on to the MLMs surfaces, slit valves or the like may be provided for isolating the vacuum environment of the lithography tool 10 from the vacuum environment of the track tool 10, with processed wafers - 9 - being transferred between the vacuum environments as required. - 10

Claims

1. Lithography apparatus comprising a lithography tool for exposing a wafer to electromagnetic radiation, and means for vacuum treating the wafer prior to its insertion into the lithography tool.

2. Apparatus according to Claim 1, wherein the vacuum treatment means is arranged to promote outgassing of a hydrocarbon or other lo solvent from a photoresist layer applied to the wafer prior to its insertion into the lithography tool.

3. Apparatus according to Claim 2, wherein the vacuum treatment means comprises a vacuum chamber for receiving the wafer, and is pump means for evacuating the vacuum chamber.

4. Apparatus according to Claim 3, comprising means for controlling the rate of evacuation of the vacuum chamber to control the rate of solvent outgassing.

5. Apparatus according to Claim 3 or Claim 4, comprising means for supplying a purge gas to the vacuum chamber to control the rate of solvent outgassing.

6. Apparatus according to any of Claims 3 to 5, wherein the vacuum chamber houses at least one of a processing tool for performing a pre- exposure process to the wafer, and wafer storage means for storing the processed wafer prior to its insertion into the lithography tool.

7. Apparatus according to Claim 6, wherein the processing tool comprises a pre-exposure heating tool.

- 11 - ''I'd' '^'-^

8. Apparatus according to any of Claims 3 to 7, wherein the lithography tool is housed within a second vacuum chamber, the apparatus comprising means for evacuating the second vacuum chamber. s

9. Apparatus according to any of Claims 2 to 8, comprising means for applying to the wafer the photoresist layer.

10. Apparatus according to any preceding claim, wherein the radiation is to extreme ultraviolet radiation.

11. Extreme ultraviolet (EUV) lithography apparatus comprising means for applying to the surface of a wafer a layer of photoresist, means for vacuum treating the wafer to promote outgassing of a solvent from the photoresist, and a lithography tool housed in a vacuum chamber for exposing the wafer to EUV radiation to transfer a pattern to the photoresist layer.

12. A method of processing a wafer, the method comprising the steps of applying a photoresist layer to the wafer, performing a vacuum treatment of the wafer to promote outgassing of a solvent from the photoresist, and subsequently exposing the wafer to electromagnetic radiation in a vacuum environment to transfer a pattern to the photoresist layer.

13. A method according to Claim 12, wherein the vacuum treatment is performed within a chamber housing at least one of a processing tool for performing a pre-exposure process to the wafer, and wafer storage means for storing the processed wafer.

14. A method according to Claim 13, wherein the wafer is subject to a pre- exposure bake within the chamber. - 12

15. A method according to Claim 13 or Claim 14, wherein the rate of evacuation of the chamber is controlled so as to control the rate of solvent outgassing from the layer.

16. A method according to any of Claims 13 to 15, wherein a purge gas is supplied to the chamber to control the rate of solvent outgassing from the layer.

lo

17. A method according to any of Claims 12 to 16, wherein the radiation is extreme ultraviolet radiation.