GB2157884A - Photoemissive cathode for high current density photoelectron generators - Google Patents

Abstract

A photoemissive cathode (16) for an electron beam generator suitable for direct-write semiconductor lithography applications comprises a light transmissive substrate (40), e.g. of quartz, glass, or sapphire, onto which is deposited an optically semitransparent, electrically conductive film (46), e.g. of chromium. This film (46) in turn is coated with a thin layer (48) of a photoemissive substance comprising cesium antimonide, sodium potassium antimonide, cesiated gallium phosphide, or cesiated gallium arsenide phosphide, and this photoemissive layer (48) will emit an intense and substantially monochromatic beam of electrons upon illumination with laser light of appropriate wavelength and energy. As shown, a thick film 42 of chromium is included. Steps for making the cathode are described. <IMAGE>

Description

1 GB 2 157 884A 1

SPECIFICATION

Photoemissive cathode for high current density photoelectron generators This invention relates to a photoemissive cath ode for an electron beam generator which is capable of producing a high current density beam of electrons and is suitable for use in electron beam semiconductor lithography and also relates to a method of making the cath ode.

As more and more elements are placed on semiconducting chips, lithography systems for making the chips must be developed having ever greater resolution in order to generate the increased number of elements on the chips. Optical lithographic systems operating at visible wavelengths have resolution limits approximating 1.25 micrometers, but electron beams have been proposed and successfully used to reduce feature size below this limit.

Such systems can have resolutions well below 1 micrometer because of the shorter wave lengths associated with high energy electrons.

Since modern semiconductor lithographic systems must achieve fast writing times (high throughput rates) in addition to high resolu tions, their electron energy beams must also have a high brightness, which means a high current density. This property is particularly important for so called direct-write systems wherein the electron beam is rapidly steered and modulated so as to effect an exposure of the highly complex circuit pattern directly 100 onto a semiconducting chip. Direct-write methods contrast with conventional projection lithographic techniques in which a mask is used to define the entire pattern for simulta neous exposure onto the chip.

Bright electron sources for use in semicon ductor lithography are known. For example, tungsten and LaB, thermionic cathodes, bar ium dispenser cathodes, and heated W/0/Zr field emitters have been used. Such field emitters have attained a nominal brightness value of 5 X 101 A /CM2 /sr (amperes/square centimeter/steradian).

However, each of these electron sources has some drawback. The tungsten filament suffers from a high evaporation rate at its operating temperature. LaB6 is easily poisoned by environmental impurities, has difficulty in remaining stably bonded at operating temper atures and forms undesirable current intensity lobes. Dispenser cathodes tend to evaporate at operating temperatures and are, moreover, easily poisoned. Furthermore, the support sys tems of heated cathodes are subject to high temperature distortions, which are likely to cause configurational changes in the electron beams. Finally, field emitters are also easily poisoned, may suffer from spot migration or flicker, require frequent unpredictable repro- cessing and, if heated, may similarly introduce 130 beam errors through geometrical distortions caused by the hot support system. Hot emitters are further limited by the finite time required to heat them, thereby precluding rapid intensity modulation of such electron sources. For semiconductor lithographic systems with heated emitters, beam modulation at the target plane is effected electrostatically and requires the additional complexity of blanking electrodes located in the lithographic column.

Cold electron emitters, such as environmentally stable cesium iodide and palladium photocathodes, are also known. Such photoca- thodes have been irradiated by ultraviolet light to provide electrons for lithography columns operating at a vacuum in the range of 10-4 to 15 - 5 torr, but the low brightness (approximately 10-50 A/CM2/sr) of electron beams from these cathodes has restricted their use to projection lithography.

Another criterion for high resolution lithography is that the electron source exhibits uniform and substantially monochromatic (low spread in electron energy) emission. A low spread in electron energy is necessary to provide high resolution imaging by allowing the electron beam to be focussed to a minimum sized spot.

In our Application No. 8231900 (Serial No. 2 111 299), from which the present application is divided, we describe and claim an electron beam generator for an electron beam semiconductor lithography system, the generator comprising a continuous wave laser, a modulator for varying the intensity of, or deflecting the optical output beam of the laser, a photoemissive cathode which has a semitransparent film of a photoemissive sub- stance comprising any one or more of cesium antimonide, sodium potassium antimonide, cesiated gallium phosphide, and cesiated gallium arsenide phosphide, and which is positioned for illumination by the output beam of the laser so that the film of the photoemissive substance is caused to emit electrons, and a light optical train positioned between the laser and the cathode for producing with the output beam of the laser a pattern of laser light on the photoemissive cathode such that the electrons emitted by the film form an electron image which is determined by the pattern and which is suitable for semiconductor lithography.

Such a generator is capable of producing a high current density electron beam from a photoemissive source (i.e. the cathode) operated at low temperatures, the electrons in the generated electron beam being substantially monochromatic (monoenergetic), thus permitting high resolution imaging by allowing the electron beam to be focussed to a minimum sized spot. Furthermore the intensity of the electron beam may be modulated by modulating the activating laser beam, thereby avoid- ing the need for electron beam blanking and reducing proximity effects. Also, the emitted electron beam is spatially uniform and is shapable by shaping the optical illuminating beam.

In a preferred embodiment the laser is an argon ion continuous wave laser operable at a discrete wavelength between 454.5 and 514.5 nanometers, and the photoemissive cathode for generating the electron beam upon illumination by the laser light comprises a substrate which is optically transmissive at the lasing wavelength and which has a back face facing towards the optical output beam of the laser and a front face facing away from the optical output beam of the laser, and an electrically conductive, optically semitransparent film deposited on the front face of the substrate and the semitransparent film of pho- toemissive substance is deposited on the electrically conductive film. The photoemissive cathode is operated within a high vacuum environment and is preferably oriented such that the photoemissive surface will emit elec- trons upon back-illumination by the laser light. Although the back- illuminated cathode is preferred for lithographic applications, a front- illuminated cathode formed by deposition of the photoemissive surface film onto an opaque, electrically conductive substrate is an alternative configuration.

The construction of the cathode in fact forms the subject of the present invention, according to which a photoemissive cathode for generating, in response to suitable laser light illumination, a substantially monochromatic high intensity electron beam which can be modulated and shaped, comprises an optically transmissive substrate, an electrically conductive, optically semitransparent film deposited upon the substrate, and a semitransparent film of a photoemissive substance deposited on the electrically conductive film, the photoemissive substance comprising any one or more of cesium antimonide, sodium potassium antimonide, cesiated gallium phosphide, and cesiated gallium arsenide phosphide, and being operable to emit electrons upon illumination by suitable laser light.

The substrate of the photoemissive cathode may be made of quartz, glass, or sapphire, and the semitransparent electrically conductive film may be of an electrically conductive metallic material, such as chromium. The pre- ferred photoemissive surface film is cesium antimonide (Cs,sb) formed by consecutive depositions of antimony and cesium. Other suitable photoernissive surface films for the source may be formed of sodium potassium antimonide (NaKSb), or of single crystal com- 125 pounds composed of gallium, phosphorus, and arsenic coated with cesium or cesium and oxygen.

A preferred method of making a Cs,Sb photoemissive cathode in accordance with the 130 GB 2 157 884A 2 invention includes the formation of electrical connections to the cathode by the deposition onto a transparent substrate of a thick electrically conductive metallic coating of, for example, chromium. The metallic coating covers the substrate surface except for a portion thereof, for example a small central region, which is masked prior to deposition of the coating. This region will subsequently contain the photoemissive surface. The mask is removed after deposition of the thick metallic layer, and a thinner coating of chromium or other electrically conductive material, sernitransparent to the illuminating laser wave- length, is deposited over at least the unmasked region and preferably over the entire substrate surface. A thin layer of antimony is then deposited onto the thin electrically conductive film, and cesium is vapour-deposited onto the antimony to complete the manufacture of the Cs,Sb photoemissive cathode. A Na,KSb photoemissive cathode may be made in the same way except that sodium and potassium are vapour deposited onto the anti- mony layer in a predetermined ratio instead of cesium.

Photoemissive cathodes in accordance with the invention have a spectral response which is compatible with existing optically mono- chromatic visible light continuous wave lasers. Also, they possess a photoemissive surface which is easy both to prepare and to restore.

An example of a photoemissive cathode in accordance with the invention and of a method of manufacturing the cathode will now be described with reference to the accompanying drawings in which:- Figure 1 is a schematic representation of the essential components of an electron beam lithography system employing an electron beam generator h ving a photoemissive cathode in accordance?with the invention; Figure 2 is a graph illustrating the spectral response of various photoemissive materials, sensitive to visible and.n - ear infrared radiation, as a function of the irradiating wavelength, with appropriate stimulating laser wavelengths indicated; Figure 3 is a cross-sectional view of a pohtoemissive cathode in accordance with the invention and suitable for use in the system shown in Fig. 1; and, Figure 4 illustrates a method of fabricating the photoemissive cathode of Fig. 3.

The electron beam lithography system represented in Fig. 1 includes a laser 10, such as an argon ion laser which is operable to generate a beam of coherent light at one of several radiating frequencies, namely 454.5, 457.9, 465.8, 472.7, 476.5, 488.0, 496.5, 501.7, and 514.5 nanometers. The strongest radiating frequencies are 488.0 and 514.5 nanometers, and a suitable laser is a series 550 argon ion laser available from Control Laser Corporation of Orlando, Florida.

3 GB 2 157 884A 3 Positioned in the lasing cavity of the laser 10 are at another location near the laser is a beam modulator 11. The modulator 11 may be any optical, electro-optical, or acousto- optical device suitable for regulating the inten- 70 sity of the beam or deflecting it.

The light beam radiating from the laser 10 is guided by a light optical train 12 including a plate 13 having an aperture 14 of specified geometry, for example, a square. A lens 15 focuses the laser light as an image of the aperture 14 onto a photoemissive cathode 16 which will be described in detail hereinafter. The photoemissive cathode 16 and electron optical components for processing the electrons emitted by the cathode 16 are housed within a vacuum chamber schematically illustrated by the dotted enclosure 18. A high vacuum such as a pressure of 10-9 torr or less is maintained in the vacuum chamber 18. 85 On the opposite side of the photoemissive cathode 16 from the laser 10 is an anode 20 which operates to accelerate electrons emitted by the cathode 16. An additional negatively charged Wehnelt electrode (not shown) may be positioned between the photoemissive cathode 16 and the anode 20. From the anode 20 the electron beam next passes through various known electron optical components, which shape and position the elec- tron beam as it is directed towards a target 21. After being accelerated by the anode 20, the electron beam passes through an electron lens 22 and then through an electrostatic beam shaping deflector 26 and a beam-shap- 100 ing aperture 28. The beam shaping deflector 26 operates to alter the position of the elec tron image of the photoemissive electron source on the beam shaping aperture 28 to create a variably shaped and sized electron beam. The beam next passes through a demagnification lens 29, and subsequently through a beam limiting aperture 30. Immersed in a final projection lens 32 are dy- namic focussing coils 34 which focus the beam onto the target 21, dynamic stigmators 36 which provide astigmatic correction to the beam, and a deflection yoke 38 which scans the beam over the target.

Because the photoemissive cathode 16 responds instantaneously to illumination by the laser 10, the electron beam intensity can be modulated by modulating the laser beam intensity. Modulation of this optical beam is facilitated by location of the beam modulator 11 outside of the high vacuum chamber 18. In prior art lithographic electron beam devices, beam modulation is accomplished by special blanking electrodes which must be located between an electron source and a target within a vacuum enclosure. The lithography system incorporating an electron beam generator of the present invention takes advantage of the general proposition that replacing any component located in the vacuum enclosure by a similar functional element positioned outside the vacuum enclosure simplifies the overall lithographic column fabrication and operation.

As will be described in greater detail below, the photoemissive cathode 16 includes a photoemissive surface formed, for example, of cesium antimonide, CS3S1J, which emits electrons when illuminated by the argon ion laser light. Fig. 2 is a graph of the spectral response (milliamperes of electron current per watt of illuminating radiation) of various photoemissive materials as a function of the wavelength of illumination. Note that at the strongest argon ion laser illuminating wavelengths of 488.0 and 514.5 nanometers, cesium antimonide has high sensitivites with quantum efficiencies of 6 percent or more. The combination of strong monochromatic optical emission from the argon ion laser and the good match of the argon ion laser wavelengths to the spectral response of cesium antimonide result in the high current density emission from this photocathode. Other lasers operating at wavelengths below approximately 520 nanometers would also be suited for this application.

Another suitable photoemissive surface for the cathode 16 is the bialkali- sodium po- tassium antimonide (Na,KSb). Although this surface is more difficult to fabricate than a CS3Sb surface because a well-defined ratio of sodium to potassium is required, this cathode does not need any highly volatile cesium, and thus is more stable. The preferred method of making the Nafflb cathode is basically similar to that to be described hereinafter for CS3S1J, and the spectral response of these two surfaces is similar as is indicated in Fig. 2.

Consequently, the Na,KSb cathode is also sensitive to argon ion laser irradiation.

Other suitable photoemissive surfaces may be formed from single crystals composed of elements from groups Ill and V of the Periodic Table such as gallium, phosophorus, and arsenic coated with either cesium or cesium and oxygen. Such surfaces can be made to have a negative electron affinity and therefore substantially increased electron escape depths.

This characteristic results in an emission of electrons with especially low energy spread. The easiest of these compounds to fabricate into photoemissive surfaces for lithographic applications are gallium phosphide (GaP), or gallium arsenide phosphide (Ga(As,Pl-j), which require only cesium (instead of cesium and oxygen) for activation. In one manner of fabricating these surfaces, a transmissive layer of GaP is first grown on an optically transpar- ent substrate, with the photoemissive surface grown on this layer. Either an argon ion laser or an appropriate semiconductor injection laser can be used to stimulate electron emission. The argon ion laser emits energy at a radiative wavelength near the optimum quan- 4 GB 2 157 884A 4 turn efficiency for photoemissive surfaces of gallium phosphide and gallium arsenide phos phide thereby maximizing the emission of electrons. An injection laser will emit energy at much lower power levels but can be con- 70 structed to operate near the long wavelength threshold of these photoemissive materials, thereby minimizing the emitted electron en ergy spread. The superior characteristics of cathodes having photoemissive surfaces com- 75 posed of compounds from elements of groups 111 and V are, however, balanced by the increased difficulty in fabricating such sur faces for use in the transmissive mode.

The back-illuminated photoemissive cathode 80 disclosed herein and a preferred method of fabricating the cathode will now be described with reference to Figs. 3 and 4. With refer ence first to Fig. 3, the photoemissive cathode 16 includes a light transmissive substrate 40 85 which is preferably quartz or sapphire, but which may instead be glass. As will be dis cussed more completely with reference to Fig.

4, a thick metallic coating 42 is deposited onto one face of the substrate 40. Suitable materials are, for example, chromium, tung sten, and aluminum. As can be seen in these figures, the layer 42 does not extend into a central region 44, a structure achieved by keeping the region 44 masked during deposi- 95 tion of the coating 42. A thin semitransparent electrically conducting layer 46 of, for example, chromium is next deposited on top of the layer 42 and the region 44. (This electrically conducting layer may not be 100 necessary for cathodes which utilize gallium phosphide or gallium arsenic phosphide as photoemissive surfaces). Finally, a layer 48 of the photoemissive material, such as cesium antimonide, is produced within the region 44.

The fabrication of the cathode 16 will now be discussed with reference to Fig. 4. First, a suitable transparent substrate 40 such as quartz, sapphire or glass is selected. Onto one selected face of the substrate 40 is deposited in vacuum a coating of, for example, chrom ium, sufficiently thick to allow the attachment of external electrical leads and to act as a low resistance electrical path to the central region 44. This deposition may be performed by evaporating chromium from a resistance heated nichrome wire 52. The central region 44, which may have an area of approximately 0.02 square millimeters or more, is masked to prevent the thick chromium layer from being deposited in the region 44. Thereafter the mask is removed and a thin electrically con ductive, optically semitransparent layer 46 of chromium is deposited in vacuum on the entire selected substrate face, including the previously masked central region 44. This chromium may also be supplied by the ni chrome wire 52, resistance heated to evapo rate chromium onto the substrate 40. The nichrome wire 52 is heated until the electri- cally conductive, optically semitransparent chromium layer 46 is deposited in the region 44. A suitable thickness for this thin layer of chromium is about 100 angstroms or less, and such a layer 46 will reduce the transmission of visible light through the central region 44 to, for example between 40 and 50% of that passing through the transparent substrate 40. The thin chromium layer 46 serves as an electrical path between the thick annular coating 42 and the central region 44. Next, an antimony bead 54 melted onto a nichrome supporting wire 56 is resistance heated in vacuum so as to evaporate a thin layer of antimony onto the portion of the chromium layer 46 in the region 44. Evaporation of antimony onto the part of the chromium layer 46 outside of the central region 44 will not affect the photoemissive behaviour of the system. The thickness of the antimony film should be such as to reduce the overall transmission of visible light in the central region 44 to, for example, about 30 to 40% of that passing through the transparent substrate 40. Next the substrate 40 with the chromium and antimony coatings and kept under vacuum is positioned in a high vacuum chamber 50 pumped to pressures below 2 X 10 - 9 torr which is, or will become, that component of the lithography column containing the photoemissive cathode 16. The substrate 40 in the high vacuum chamber 50 is then heated to approximately 1 00C by means of a nichrome heater wire 58 wrapped around the periphery of the substrate 40. Also disposed within the vacuum chamber 50 is a cesium source -channel- 60 which contains, for example, a mixture of cesium chromate and a reducing agent such as silicon. The channel 60 is resistance-heated by means of an electrical connecting wire 62 to evaporate pure cesium onto the heated antimony film in the region 44. Evaporation of cesium onto the antimony film or chromium layer outside the region 44 will not affect the photoemissive behaviour of the system. In this way a thin layer or film 48 of cesium antimonide is produced in the region 44. During the foregoing cesium evaporation process, the photoemissive cathode 16 is illuminated by, for example, the argon ion laser and the photongenerated current is measured by collecting the emitted electrons with the nichrome wire 52. When the photocurrent reaches a maximum value, the currents through the wire 62 and through the substrateheating nichrome heater wire 58 are stopped so that no more cesium is deposited in the region 44. Should the photocurrent drop during cooling of the substrate, additional cesium is evaporated onto the cold surface 44. If the additional cesium evaporated onto the region 44 does not cause the photocurrent to return to its maximum value, more antimony may be evaporated onto the substrata followed by the GB 2 157 884A 5 deposition of additional cesium until the maximum value is re-attained.

Over a period of time, both with and without photoemission from its surface, the CS3Sb cathode may degrade due to either the loss of cesium or contamination of the antimony by impurities. Such degradation can be reversed by additional evaporation of antimony and cesium onto the region 44 as detailed above.

In operation of the system, a laser beam from the laser 10 passes through the transparent substrate 40 and through the semitransparent chromium layer 46 and penetrates into the cesium antimonide layer 48, causing electrons to be emitted from the cesium antimonide. The electron beam thus emitted from the photoemissive cathode 16 has a high current density in the range of one to one hundred milliamperes per square centimeter.

Current densities in the beam at the plane of the target 21 will be several hundred amperes per square centimeter. Values in this range are well suited for direct-write lithography systems in which the electron beam is steered to generate a complex pattern on a semiconducting chip. Such electron beams may also be used in making masks for projection lithography or in non-lithographic applications such as electron beam microscopy.

It will thus be appreciated that there has been disclosed a photoemissive cathode 16 which, when back-illuminated through a chromium layer with 50 to 60% optical loss, has a quantum efficiency of 3% or more and which upon degradation may be easily restored in situ by the deposition of additional cesium or cesium and antimony. The photoemissive cathode 16 is capable of generating a high current density in the range of one to one hundred milliamperes per square centimeter to provide current densities at a target 21 of hundreds of amperes per square centimeter. In addition, there is a low energy spread among the electrons in the range of a few tenths or less of an electron volt. This small spread is a direct consequence of the low energy of the emitted electrons once they have lost most of their initial energy during their transitions from the bound valence states to the vacuum level. The maximum magnitude of the emitted electrons' energy is dependent on the difference of the laser's photon energy and the electron emission threshold energy, defined by an electronic transition between the top of the photoemissive material's valence band and its vacuum level. For argon ion laser light of 514.5 nanometers (2.43 electron volts) and a threshold of photoemission of Cs,Sb of approximately 2.0 elec tron volts, the maximum emission energy of the electrons is 0.43 electron volts which is, therefore, the maximum energy sp;ead of the electrons. The nominal energy spread, commonly based on the half-width of the distribu- tion curve of numbers of emitted electrons versus their energy, will be substantially lower than this.

The cathode operates at a low temperature such as room temperature so that there are no support problems as would be the case with a heated cathode. Furthermore, no heating up time is required because the electrons are generated instantaneously in response to illumination by the laser light. Modulation of the electron beam can conveniently be accomplished by modulating the laser beam outside the vacuum chamber 18. In addition, beam shaping or patterning into complex shapes is easily accomplished by apertures or masks in the optical train between the laser 10 and the photoemissive cathode 16 outside of the vacuum enclosure 18.

Claims (7)

1. A photoemissive cathode for generat ing, in response to suitable laser light illumi nation, a substantially monochromatic high intensity electron beam which can be modu lated and shaped, the cathode comprising an optically transmissive substrate, an electrically conductive, optically semitransparent, film deposited upon the substrate, and a semitransparent film of a photoemissive substance deposited on the electrically conductive film, the photoemissive substance comprising any one or more of cesium antimonide, sodium potassium antimonide, cesiated gallium phosphide, and cesiated gallium arsenide phosphide, and being operable to emit electrons 100 upon illumination by suitable laser light.

2. A cathode according to Claim 1, in which the substrate is formed of quartz or sapphire.

3. A cathode according to Claim 1 or Claim 2, in which the semitransparent electrically conductive film is of chromium.

4. A method of making a photoemissive cathode according to Claim 1, comprising, in the order given the steps of providing a transparent substrate, masking a selected portion of one face of the substrate, depositing a thick metallic coating upon all of said face of the substrate except the masked portion to create a low-resistance electrical path, unmasking the selected portion of the substrate, depositing a semitransparent, electrically conductive film over at least the selected portion of the substrate, depositing a semitransparent layer of antimony over the electrically conductive film, and evaporating onto the antimony layer either cesium, or sodium and potassium in a predetermined ratio.

5. A method according to Claim 4, in which during the deposition of cesium or sodium and potassium onto the antimony layer, the partial ly-fabricated cathode is illuminated with a laser and the current generated by the partial ly-fabricated cathode is measured, the deposition of cesium or sodium and potassium being terminated when the current 6 3B 2 157 884A 6 reaches a maximum value.

6. A cathode according to Claim 1, substantially as described with reference to Fig. 3 of the accompanying drawings.

7. A method according to Claim 4, substantially as described with reference to Fig. 4 of the accompanying drawings.

Printed in the United Kingdom for Her Majesty's Stationery Office, Dd 8818935, 1985, 4235. Published at The Patent Office, 25 Southampton Buildings, London, WC2A lAY, from which copies may be obtained.