WO1999048129A1

WO1999048129A1 - Tandem optical scanner/stepper and photoemission converter for electron beam lithography

Info

Publication number: WO1999048129A1
Application number: PCT/US1999/005767
Authority: WO
Inventors: Marian Mankos; Tai-Hon Philip Chang; Lee H. Veneklasen
Original assignee: Etec Systems, Inc.
Priority date: 1998-03-20
Filing date: 1999-03-16
Publication date: 1999-09-23

Abstract

A combined photo and electron beam lithography tool takes advantage of the high throughput of optical lithography and the high spatial resolution of electron lithography. The tool includes two subsystems, the first of which is a conventional photolithography tool, for instance a stepper or scanner, and the second of which is a demagnifying electron beam column. These two subsystems are coupled by a photoemission cathode. This allows device fabrication entailing pattern delineation of minimum feature size significantly smaller than 0.5 νm. The total demagnification of this system is achieved in two steps where first a photo image of a mask is generated by the conventional photolithography tool, with unity magnification or demagnification, and the photon image is converted into an electron beam image by the photoemission cathode, and subsequently this image is demagnified by an electron beam column.

Description

TANDEM OPTICAL SCANNER/STEPPER AND PHOTOEMISSION CONVERTER FOR ELECTRON BEAM LITHOGRAPHY

BACKGROUND OF THE INVENTION Field of the Invention

This invention relates to lithography and more specifically to electron beam lithography for semiconductor device fabrication.

DESCRIPTION OF PRIOR ART

There are two general categories in the field of lithography. The first is photolithography (light lithography) which images patterns on a substrate, typically a semiconductor wafer, using a mask which is a pattern through which a beam of light is passed and imaged onto the surface of the substrate . The surface of the substrate carries a layer of photosensitive resist which is thereby exposed by the mask pattern. Later steps of developing the photoresist and etching the substrate are performed to form a pattern replicating the image of the mask on the wafer.

A second category of lithography is electron beam (or charged particle beam) lithography in which a beam of e.g. electrons from an electron source is directed onto a substrate. The electrons expose a resist layer (in this case an electron sensitive resist) on the substrate surface. Electron beam lithography uses what are called "electron lenses" to focus the electron beam. These are not optical (light) lenses but are either electro-static or magnetic. Typically electron beam lithography is used for making masks; however it can also be used for direct exposure of semiconductor wafers .

Typically electron beam lithography does not use a pattern (mask) but instead is "direct write" in which the beam is scanned and turned on and off (blanked) to determine the patterns imaged on the substrate. It is also known to use electron beams in conjunction with masks. The chief disadvantage of electron beam direct write lithography is its relatively slow exposure rate, making it generally uneconomic for semiconductor wafer fabrication. The systems used in photolithography or electron beam lithography are well known and include a source of light or electrons, optical or electron beam lenses, and stages for supporting the substrate and the mask (reticle) .

As is well known, the primary goal in lithography in the semiconductor field is to define smaller feature sizes, where feature size is usually the minimum width of a portion of a transistor or interconnection. Generally photolithography and electron beam lithography have followed different evolutionary steps. Photolithography has achieved its present dominant position in semiconductor device fabrication by concentrating on mask techniques using a mask (reticle) which defines the actual image. These techniques utilize a highly efficient parallel projection scheme whereby a single reticle is used repeatedly to project the identical image onto different portions of the semiconductor wafer. In contrast, typical applications of high resolution electron beam lithography are limited to mask-making and to limited manufacturing of specialized (low production) integrated circuits due to the inherent low throughput in direct write lithography and high equipment cost. However, since the general trend in semiconductor fabrication is to reduce minimum feature size progressively, it is expected that a typical minimum feature size will be less than 100 nanometers (nm) in about ten years and at that time optical lithography may become too expensive and not offer sufficient resolving power due to the relatively large wavelength of light .

At the same time, current electron beam technology is not regarded as economic even in the long term for mass production of semiconductor devices .

SUMMARY

In accordance with this invention, a combined light and electron lithography process and apparatus take advantage of the high throughput of photolithography and the high spatial resolution of electron beam lithography. A system for carrying out the combined method combines two subsystems, the first of which is a conventional photolithography tool, for instance a stepper or scanner, and the second of which is a demagnifying electron beam column. These two subsystems are coupled by a photoemission cathode.

The photon and electron beam subsystems are arranged serially. The photolithography subsystem transfers one to one or a demagnified image

^■3- (demagnified for instance four to five times) of the conventional mask (reticle) onto the photoemission cathode, which couples the photon subsystem to the electron beam subsystem. The photoemission cathode converts the incident light (photons) into an electron beam emission pattern and the electron optics project a demagnified electron image of the mask onto the wafer surface .

The photon subsystem is based for instance on a conventional stepper or scanner of the type now commercially available, while the electron beam subsystem includes the photoemission cathode, extraction electrode and demagnifying lenses, each of which are essentially conventional. When a scanner is used in the photon subsystem, the wafer is written on the fly, i.e. both the mask (reticle) and wafer move at constant velocities in proportion to total demagnification. In the other case when a stepper is used as the photon subsystem, the wafer is written when both the mask and wafer stop. The exposure begins after the mask and wafer are moved in the appropriate position.

A unique of this composite system is that the optical lenses of the photolithography subsystem can be used to compensate for aberrations in the electron beam lens (or visa versa) . Applications of the system and method in accordance with this invention include electron beam lithography tools for electron beam direct writing of wafers and for mask making. Advantageously a system in accordance with this

-4- invention achieves high throughput by using photolithography and high resolution by using electrons for exposure .

BRIEF DESCRIPTION OF THE DRAWING

Figure 1 shows a schematic side view of a tandem photon and electron beam lithography system in accordance with this invention.

DETAILED DESCRIPTION

Figure 1 shows in a side view a tandem photon and electron beam lithography system ("tool") 10 in accordance with this invention which includes two subsystems, the first of which is a conventional photolithography tool, for instance a stepper or scanner, and the second of which is a demagnifying electron beam column, where the two subsystems are coupled by a photoemission cathode.

A conventional mask 18 (reticle) of the type now used in photolithography is positioned on a conventional stage 24 which may or may not be movable along one or both of the depicted x and y axes, depending on the type of photolithography subsystem. A source of the light is for instance a conventional UV light source or a laser illumination system 14 of the type now used in photolithography which provides a relatively large diameter beam 16 of for instance ultraviolet (UV) light which passes through the transparent portions of the mask 18. It is to be understood that the mask is a substrate transparent to

•5- the incident light 16 on which are located opaque areas. The transparent portions of the substrate define the image which is to be transferred by the mask 18. Typically, one such mask includes the entire pattern of one layer of a single integrated circuit die. The mask is usually, in terms of its X, Y dimensions, some convenient multiple of the size of the actual die being imaged.

A light optical lens system 28 (which is actually a lens system including a large number of individual lens components) focuses the light 26 passed by the mask 18. The light optical lens system 28 is either a 1:1 or demagnifying lens system which demagnifies by e.g. a factor of four or five the image 26 incident thereon to form image 30, which in turn is incident onto the object. A 1:1 ratio is more advantageous when mask size is limited. In this case the object, rather than being a semiconductor substrate, is the photosensitive backside of a photoemission cathode 32. The photoemission cathode 32 defines for instance a minimum feature size of 0.5 micrometers or less, the minimum feature size of course being dependent upon the parameters of the system. The photoemission cathode 32 is for example a thin gold (or other metal) layer deposited on a transparent substrate.

The photoemission cathode 32 (which like the other elements herein is shown in simplified fashion) includes a photoemission cathode layer 34 which absorbs the incident photons 26 and causes electrons present in the photoemission layer 34 to be excited above the

-6- vacuum level. Some portion of the electrons 38 which retain sufficient energy to escape from the photoemission layer 34 are emitted into the vacuum portion 40 of the photoemission cathode downstream from the photoemission layer 34. An electric voltage

(typically several kilovolts to tens of kilovolts) is applied to the extraction electrode 42 associated with the photoemission cathode 32. Extraction electrode 42 extracts the electrons 38 which have escaped from the photoemission layer 34 and accelerates them. Thus the accelerated electrons 46 form a virtual image of the incident photons 30. In effect then the photoemission cathode 32 and extraction electrode 42 form a divergent lens . There may also be, immediately downstream of the extraction electrode 42, a magnetic (or electrostatic) lens (not shown) to reduce aberrations. (A magnetic lens is conventionally a set of coils and magnetic pole pieces, and yokes which focus the electron beam.) Such an electron beam system has been found to offer resolution of below 10 nm. Immediately following (downstream of) this portion of the system is a conventional electron optical lens system 50 consisting of one or more electron lenses and alignment, deflection and blanking systems 52 (shown only schematically in Fig. 1) .

This lens system further demagnifies the virtual image 46 at the writing plane, which is the plane of the principal surface of the wafer 58 (substrate) by a factor determined to achieve the desired minimum

-7- feature size. For instance, if a minimum feature size of 0.5 μm is resolved at the photoemission cathode, an electron beam demagnification factor of five times is needed for a 100 nanometer minimum feature size on the wafer 58. This means that when a total area of approximately 1 mm x 1 mm is exposed on the wafer 58, a total illuminated area of 5 mm x 5 mm is required on the photoemission cathode layer 34. Correspondingly for a 4:1 demagnification ratio an area of 20 mm x 20 mm is illuminated on the mask 18, and a 5 mm x 5 mm area is illuminated for a 1:1 ratio. Of course these are merely illustrative parameters.

The total demagnification factor and exposed wafer area can be varied to achieve the desired minimal feature size and throughput. The wafer 58, including its electron beam resist layer 60, is typically supported on a stage 64 which is movable in the x, y and z axes, as is conventional. Other elements of both the photo and the electron beam subsystems which are well known are not shown, but include positioning measurement systems using for instance laser interferrometry to determine the exact location of the mask on its stage and the wafer on its stage, vacuum systems, air bearing supports for the stages, various vibration absorption and isolation mechanisms to reduce environmental effects, and suitable control systems, all of the type well known in the lithography field.

The deflection system 52 can be used to compensate for positionary errors due to mask/wafer misalignment, vibrations, heating and other effects, and would only use very small deflection amplitudes.

When the photo subsystem is a stepper type system, the required area on the wafer 58 is exposed with both the mask stage 24 and wafer stage 64 in the stationary position. Thus after each single exposure both the mask and wafer stage are moved (stepped) to the next location and stopped before another exposure is started. Each exposure could possibly expose a single die area on the wafer 58, but field size limitations would usually require a die to be composed using several steps. In the case of a scanner, both the mask stage 24 and the wafer stage 64 move at constant velocities and the exposure is continuous, as is well known for lithography scanning. The ratio of the stage velocities is determined conventionally by the total demagnification factor.

This tandem arrangement shown in Figure 1 can be used to optically compensate in light optical lens system 28 for distortions of the electron optical lens system 50. This results in improved resolution and larger exposed areas, which increase throughput of the entire system.

This disclosure is illustrative and not limiting; further modifications will be apparent to those skilled in the art and are intended to fall within the scope of the appended claims.

-9-

Claims

We claim:

1. A method of forming an image on a substrate, comprising: illuminating a mask including opaque and transparent portions with photons to define an image ; transferring the image onto a photocathode using light optics; converting at the photocathode the image into an electron emission pattern; accelerating the electrons of the electron emission pattern to form an electron beam; demagnifying the electron beam; and focusing the demagnified electron beam onto the substrate to expose a resist on a surface of the substrate .

2. The method of Claim 1, wherein the light optics transfers the image of the mask with demagnification.

3. The method of claim 1, wherein the light optics transfers the image of the mask with unity magnification .

4. The method of Claim 1, wherein focusing the demagnified electron beam is performed magnetically or electrostatically.

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5. The method of Claim 1, wherein the illuminating includes generating light from a laser or UV light source.

6. The method of Claim 1, wherein the transferring using the light optics compensates for distortion in the electron beam demagnifying and focusing.

7. The method of Claim 1, wherein a minimum feature size of the image defined by the illumination on the photoemission cathode is less than 0.5 ╬╝m, and a minimum feature size defined by the electron beam on the substrate is less than O.l╬╝m.

8. The method of Claim 1, wherein the substrate and the mask are moved relative to one another during the focusing of the electron beam onto the substrate.

9. The method of Claim 8, wherein the movement is at a constant velocity.

10. The method of Claim 1, wherein the substrate and the mask are stationary relative to one another during the focusing of the election beam onto the substrate .

11. The method of Claim 1, wherein converting the illumination includes focusing the illumination onto a planar or curved photoemission cathode, whereby the

-11- photoemission cathode emits the electron emission pattern.

12. A lithography system comprising: a source of illumination; a support for holding a partially opaque mask to be illuminated by the source of illumination; an optical lens located to focus the illumination that passes through the pattern; a photoconverter device located to receive the focused illumination and convert the illumination into an electron stream; an electron lens located to focus the electron stream into a beam; and a support for a substrate, the support for the substrate being located so that the electron beam is incident on a surface of the substrate.

13. The system of Claim 12, wherein the optical lens is a demagnifying lens system having a plurality of lens components .

14. The system of Claim 12, wherein the optical lens is a unity magnification lens system having a plurality of lens components.

15. The system of Claim 12, wherein the electron lens is a demagnifying lens system.

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16. The system of Claim 12, wherein the source of illumination is a laser or UV light source.

17. The system of Claim 12, wherein the support for the mask and the support for the substrate are movable relative to one another.

18. The system of Claim 17, wherein the movement is at a constant velocity.

19. The system of Claim 12, wherein the support for the mask and the support for the substrate are held stationary to one another while the electron beam is directed onto the substrate.

20. The system of Claim 12, wherein the photo- converter device is a planar or curved photoemission cathode having its photosensitive surface facing the source of the illumination.

21. The system of Claim 12, wherein the electron lens is a magnetic or electrostatic electron lens.

22. The system of Claim 12, wherein the optical lens compensates for an aberration in the electron lens .

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