FR2517470A1 - LASER STIMULATED HIGH-DENSITY CURRENT ELECTRON GENERATOR AND METHOD OF MANUFACTURING SAME - Google Patents

Abstract

ELECTRON BEAM GENERATOR DESIGNED PARTICULARLY FOR LITHOGRAPHY APPLICATIONS ON SEMICONDUCTOR IN DIRECT WRITING INCLUDING A PHOTOEMISSIVE CATHODE, A MODULAR LASER TO LIGHT THE CATHODE AND AN OPTICAL TO CREATE AN OPTICAL DRAWING ON THE CATHOD. CATHODE 16 IS COMPOSED OF A TRANSPARENT SUBSTRATE 40 ON WHICH AN ELECTRICALLY CONDUCTIVE SEMI-TRANSPARENT FILM 46 IS DEPOSITED. THIS FILM IS COATED WITH A THIN LAYER OF A PHOTO-EMISSIVE SUBSTANCE 48 SUCH AS CESIUM ANTIMONIDE, SO THAT THE CATHODE EMITS A SENSITIVELY MONOCHROMATIC BEAM OF ELECTRONS WHEN ITS LASER ILLUMINATION. THE ELECTRON BEAM IS SHAPED ACCORDING TO THE OPTICAL DRAWING CREATED ON THE CATHODE AND, THROUGH SUCCESSIVE ELECTRONIC OPTICAL DEVICES, IS AGAIN CONFORM AND DIMENSIONED FOR USE FOR EXAMPLE IN LITHOGRAPHY MANUFACTURING OF SEMI-CONDUCTIONS OR VLSI CIRCUITS.

Description

The invention relates to beam generators for electrons to

  photoemission that emit electron beams of high current density when illuminated by a laser d E

  appropriate wavelength and energy, and which are particularly suitable

  especially as sources of electrons for lithography by

  electron beam on semiconductor.

  As more and more elements are placed on the semiconductor wafers, it is necessary to develop the lithographic systems with an ever greater resolution in order to generate a greater number of elements on the

  pellets Optical lithographic systems operating at long

  visible wavelengths have resolution limits

  1.25 micron Viron Electron beams have been proposed and successfully used to reduce the dimensions of elements below this limit. Such systems may have resolutions much less than 1 micron because of longer wavelengths. short

  associated with high energy electrons.

  Because modern lithographic systems must have fast write times (high output) in addition to their high resolution, their electron beams must also

  have a high brightness which, in the case of a beam of

  This property is particularly important for direct-write applications in which the electron beam is directed and modt quickly to effect complex circuit diagram exposure directly onto a semiconductor chip. directly opposed conventional projection techniques in which a mask is used

  to define the scheme as a whole for a similar exhibition

  multanée of all the elements on the pellet.

  Light sources of electrons that can be used in lithography are known for example thermionic tungsten cathodes and La B 6, compensated cathodes at the bars.

  and W / O / Zr emitters by heated electric field were used.

  Such emitters by electric field have reached a nominal brightness of 5 × 10 7 A / cm 2 / sr (amperes / square centimeters, steradian). Each of these electron sources, however, has some unfavorable characteristics. The tungsten filament

  suffers from a high evaporation rate at the operating temperature

  La 6 is easily contaminated by impurities

  surrounding areas, remains difficult to link stably to the temperatures

  The compensation cathodes tend to evaporate at operating temperatures and are also easily contaminated. In addition, the support systems of heated cathodes are subject to distortion at high temperatures. Such distortions may cause configuration changes in electron beams. Finally, electric field transmitters are also easily contaminated, may suffer from

  fuzziness or occasional migration, require frequent reprocessing

  not foreseeable and, if heated, may also

  introduce beam errors due to geometric distortions

  The hot transmitters are further limited by the time required to heat them, thus punctuating rapid modulation of such sources.

  of electrons For lithographic systems with

  heaters the beam modulation at the target plane is produced

  electrostatically and requires the additional complexity of

  removal trodes arranged in the lithographic column.

  Cold electron emitters are known, such as cesium and palladium iodide environment stable cathodes. These photocathodes have been irradiated with a light.

  ultraviolet to obtain the electrons for lithographic columns

  graphs running in low vacuum conditions the

  range from 104 to 10 5 torr but the low luminosity (approx.

  10-50 A / cm / sec) of these cathodes limited their use to the

projection lithography.

  Another criterion for high resolution lithography

  is that the electron source has a uniform emission

  monochromatic (low dispersion in the energy of electri-

  trons) A low dispersion in the electron energy is needed to provide a high resolution image by allowing the electron beam to be focused on a dimension point

minimal.

  One of the objects of the present invention, therefore, is to provide an electron beam generator that produces a high current density electron beam from a

  photoemissive source used at low temperature.

  Another object of the invention is to provide such

  source of electrons in which the electrons of the electron beam

  tronic product are substantially monochromatic (monoenergetic)

  ticks), thus enabling such a high resolution image by allowing the electron beam to be focused on a point of

minimum dimension.

  Another object of the invention is to provide a photoemissive cathode whose spectral response is compatible with lasers (CW) with continuous emission of optically visible light

monochromatic.

  Another object of the invention is to provide a surface

  photoemissive that is easy to both prepare and repair.

  Another object of the invention is to provide a device suitable for use as a lithographic electron source which forms an electron beam whose density can be modulated by modulating the activation laser beam, thus providing a

  beam suppression and reducing proximity effects.

  Another object of the invention is to provide a source of electrons whose emitted beam is spatially uniform and can

  be shaped by shaping the illumination optical beam.

  The present invention provides a light emitting cathode capable of emitting an electron beam of high current density when illuminated by a suitable laser energy, and a

  electron beam generator which includes the photodecode

  The electron beam generator comprises, in addition to the light emitting cathode, a continuous emitting laser, a modulator for deflecting and modifying the intensity of the laser output beam and a set of emissive electrodes which can be used for a semiconductor lithography system. optics for light to create a drawing with the laser beam on the cathode so the cathode

  emits an electron beam that can be used for lithography.

  In a preferred embodiment, the photo cathode

  emissive to generate an electron beam during its illumination.

  by a laser light comprises an optically

  transparent to the wavelength of the laser, an optically semi-

  electrically conductive transparent film deposited on the substrate and a photoemissive semi-transparent surface film deposited on the electrically conductive film The photoemissive cathode is implemented in a high vacuum environment and is oriented so that the surface emits electrons during its illumination -4 - from the rear by a laser light Although the cathode may be illuminated by the rear is preferably used for

  lithographic applications, a cathode likely to be illuminated

  formed by deposition of a photo-surface film

  emissive on an opaque, electrically conductive substrate, can

constitute a variant.

  According to a preferred embodiment of the invention, the laser is a continuous emission argon ion laser capable of operating at discrete wavelengths between 454.5 and 514.5 nanometers. A substrate suitable for the photoemissive cathode is constituted quartz, glass or sapphire The transparent electrically conductive layer is formed by depositing a film of a

  electrically conductive material such as chromium on the

  The preferred photoemissive surface film is cesium antimonide (Cs 3 Sb) formed by successive depositions of antimony and cesium. Other photoemissive surface films suitable for the cathode may be formed from sodium antimonide and potassium (Na 2 K Sb), or monocrystalline compounds composed of gallium,

  phosphorus and arsenic coated with cesium or cesium and oxygen.

  A preferred method for producing a photoemissive cathode

  Cs 3 Sb according to the present invention comprises the formation of

  electrical connections to the cathode by deposition on the transparent substrate of a thick electrically conductive coating, for example of chromium The chromium coating covers the surface of the susbstrate except for a part thereof such as a small

  central area that is masked before filing to exclude chromium.

  This area therefore contains the photoemissive surface The mask is removed after deposition of the thick chromium layer and a thinner semitransparent chromium layer at the wavelength

  illumination laser is deposited on the entire surface of the substrate.

  A thin layer of antimony is then vapor deposited on the

  chromium coating and the cesium is vapor deposited on the anti-

  monk to complete the manufacture of the light-emitting cathode

Cs 35 b.

  An embodiment of the present invention will now be described.

  invention with reference to the accompanying drawings in which

  Figure 1 is a schematic representation of

  essential components of a lithographic system with an electron beam

  trons using the photoemissive cathode according to the invention,

D 2517470

  Figure 2 is a graph showing the answer

  spectrum of various radiation-sensitive photoemissive materials

  visible and near infra-red depending on the incident wavelength with the indication of the appropriate stimulation laser wavelengths; FIG. 3 is a sectional side view of the light emitting cathode of the invention, and Figure 4 shows a method of manufacturing the

  Photoemissive cathode of Figure 3 for lithogra applications.

cal.

  FIG. 1 is a schematic representation of an electron beam lithographic system using a laser-stimulated back-illuminated electron-emitting light source according to the invention. This system comprises a laser 10 such an argon ion laser which can be used to generate a coherent light beam at one of the various 454.5 radiation frequencies; 457.9; 465.8; 472.7; 476.5488.0; 496.5; 501.7; and 514.5 nanometers The highest radiation frequencies are 488.0 and 514.5 nanometers A suitable lase is a 550 series Argon ion laser of the Company.

  Control Laser Corporation of Orlando, Florida.

  A beam modulator is arranged in the cavity of

  laser 10 or at another location near the laser.

  11 may be any optical device, electro-optical or acoustooptic suitable for regulating the intensity of the beam or

deflect it.

  The beam of light radiating from the laser 10 is guided by an optical light assembly 12 comprising a plate 13 having an aperture 14 of predetermined geometry, for example square. A lens 15 focuses the laser light in the form of an image of the aperture 14. on a light-emitting cathode 16 which will be described in detail below The light-emitting cathode 16 and electronic optical components for treating the electrons emitted by the

  16 are housed in a vacuum chamber illustrated schematically

  The high vacuum, such as a pressure of 109 torr or less, is maintained in the chamber.

empty 18.

  On the opposite side with respect to the light emitting cathode 16 of the laser 10 is an anode 20 whose function is to accelerate the electrons emitted by the cathode 16. An additional Wehnelt electrode (nor-represented) negatively charged can be arranged between the photoemissive cathode 16 and the anode 20 from the anode 20, the electron beam then passes through the various

  known components of electronic optics that conform and posi-

  the electron beam as it moves towards a target 21 After being accelerated by the anode 20, the beam

  of electrons passes through an electronic lens 22 and then a deflec-

  electrostatic beam generator 26 and an opening

  The deflector 26 of conformation 28

  Beam effect has the effect of changing the position of the image

  electron source of the light emitting electron source on the opening

  beam conformation 28 to create a beam of elec-

  The beam then passes through a raster lens 29 and then a beam-limiting aperture 30 disposed in a final projection lens 32 are dynamic focusing coils 34

  which focus the beam on the target 21, dynamic devices

  36 which provide an astigmatic correction to the beam and a deflection collar 38 which causes the target to be scanned.

by the beam.

  Since the photoemissive cathode 16 responds instantaneously

  At the time of illumination by the laser 10, the density of the electron beam can be modulated by modulating the intensity of the laser beam. The modulation of this optical beam is made possible by the placement of the beam modulator 11 outside the beam. vacuum chamber 18 In known electron beam lithographic devices, the beam modulation is performed by special suppression electrodes which must be arranged between an electron source and a target in a vacuum chamber. The lithographic system according to the present invention takes advantage of the general idea that replacing any component

  located in the vacuum chamber by a similar functional element

  arranged outside the vacuum chamber simplifies the

  seems from the manufacture and implementation of the column

lithographic.

  As will be described in more detail below, the light emitting cathode 16 comprises a photoemissive surface formed, for example, of cesium antimonide Cs 3 Sb, which emits electrons.

  when illuminated by the light of an argon ion laser.

  FIG. 2 is a graph of the spectral response (milliamps of electronic current per watt of illuminating radiation) of various photoemissive materials as a function of the wavelength of illumination. It will be noted that at the highest wavelengths of illuminated by a 488.0 and 514.5 nm argon ion laser, cesium antimonide has high sensitivity with quantum efficiencies of 6% or higher. The combination of high monochromatic optical emission from ionized argon and a good adaptation of argon laser wavelengths to the spectral response of cesium antimonide results in high current density emission from the photocathode Other lasers operating at lengths waves below about 520 nanometers would also be suitable for

this application.

  Another suitable photoemissive surface for cathode 16 is sodium and potassium bialkalin antimonide (Na 2 K Sb). Although this surface is more difficult to achieve than Cs-Sb because a well defined ratio of It is necessary to use sodium-potassium, this cathode does not use highly volatile cesium and is perfectly stable. A preferred method for producing Na K Sb is substantially similar to that which will be described hereinafter for Cs 3 Sb, and the spectral response. of these two surfaces is similar as shown in Figure 2 Therefore, Na 2 K Sb is also sensitive to

  Argon ion laser radiation.

  Other suitable photoemissive surfaces can be made from monocrystals composed of elements of groups III and V of the Periodic Table such as gallium, phosphorus and arsenic, coated with either cesium or with cesium and 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 of energy which is not particularly dispersed. Those of these compounds which

  are the easiest to manufacture as photoemissive surfaces

  sive for lithographic applications are gallium phosphide (Ga P), or gallium phosphide arsenide (Ga (Asx Pl x))

  which requires only cesium (instead of cesium and oxy-

  gene) for activation In a mode of manufacture of these surfaces, a transparent layer of Ga P is first deposited on an optically transparent substrate, the photoemissive surface being deposited on this layer Either an argon ion laser, or a suitable semiconductor injection laser, can be used to stimulate electron emission The argon ion laser emits energy at a radiation wavelength close to the optimum quantum yield for photoemissive phosphide phosphide Gallium phosphide arsenide, which maximizes electron emission An injection laser emits energy at much lower power levels but can be built to operate near the long-term threshold

  of these photoemissive materials-which minimizes the

  energetic persion of emitted electrons.

  cathodes with photoemissive surfaces composed of

  Group III and V compounds are, however, offset by increased difficulties in the manufacture of

  such surfaces for their use in transmission mode.

  The rear-illuminable light-emitting cathode described herein and a preferred method of manufacturing the cathode are now described with reference to Figs. 3 and 4. Referring first to Fig. 3, the cathode Photoemissive 16 comprises a substrate transparent to light 40

  which is preferably quartz or sapphire but which can also

  As will be described more fully with reference to FIG. 4, a thick metal coating 42 is deposited on one side of the substrate 40. Suitable materials are, for example, chromium, tungsten and aluminum. see in these figures, the layer 42 does not extend in a central zone 44 which is obtained by keeping the zone 44

  masked during the deposition of the coating 42 A thin layer semitrans-

  parent and electrically conductive 46 for example chromium is

  then deposited on the top of the layer 42 and on the zone 44.

  (This electrically conductive layer may not be necessary

  for cathodes that use gallium phosphide or

  gallium phosphide arsenide as photoemissive surfaces).

  Finally, a layer 48 of photoemissive material, such as

  Cesium antimonide is produced in Zone 44.

  We will now describe the manufacture of the cathode 16 in

  Figure 4 First of all, a transparent substrate con-

  Viable 40 such as quartz, sapphire or glass is chosen.

  On a selected side of the substrate 40, a coating is deposited under vacuum.

  For example chromium, sufficiently thick to allow the attachment of external electrical conductors and to serve as a low resistance electrical path to the central zone 44 This deposit can be achieved by evaporation of chromium from a wire. The central zone 44, which may have an area of about 0.02 square millimeters or more, is masked to prevent the thick chromium layer from being deposited in the zone 44. The mask is then removed. and an

  thin layer 46 electrically conductive and optically semitransparent

  The parent chromium is deposited under vacuum over the entire selected side of the substrate including the previously masked central zone 44. This chromium can also be provided by the chrome nickel wire 52.

  heated by resistance to evaporate chromium on the substrate 40.

  The nickel-chromium wire 52 is heated until the

  chromium 46 electrically conductive and optically semitransparent

  The thickness of this thin layer of chromium is about 100 Angstroms or less. Such a layer 46 reduces the transmission of visible light through the central zone 44 of, for example, 40 to 50%. The thin layer of chromium 46 serves as an electrical path between the thick annular coating 42 and the central zone 44. Then a drop of antimony 54 melted on a nickel chromium support wire is heated by resistance. under vacuum to evaporate a thin layer of antimony

  on the part of chromium layer 46 in zone 44 The evapo-

  antimony on the portion of the chromium layer 46 outside the central zone 44 does not affect the system's photoemissive behavior The thickness of the antimony film must be such as to reduce the total transmission of visible light in the central zone 44 to, for example, 30 to 40% of the light passing through the transparent substrate 40 Then the substrate 40 with this coating of chromium and antimony stored under vacuum is

  disposed in a high vacuum chamber 50 with a lower pressure

  The substrate 4 Q in the high vacuum chamber 50 is then heated to about 1000 C by means of a heating wire. in nickel

  chromium 58 wrapped around the periphery of the substrate 40 Egale-

  In the vacuum chamber 50 there is disposed a source of cesium 'channel' 60 which contains, for example, a mixture of cesium chromate and a reducing agent such as silicon. Channel 60 is heated by resistance by means of a electrical connection wire 62 for evaporating pure cesium on the antimony film heated in zone 44 The evaporation of cesium on the antimony film and on the chromium layer outside zone 44 does not affect the photoemissive behavior of the system Thus, a thin layer

  or cesium antimonide film 48 is formed in zone 44.

  During the previous cesium evaporation process, the light emitting cathode 16 is illuminated by, for example, the argon ion laser and the photogenerated current is measured in

  collecting the electrons emitted with the nickel chromium wire 52.

  When this current reaches a maximum value, the current in the wire 62 and in the nickel chromium heating wire 58 for heating the substrate are cut off so that the cesium is no longer deposited in the zone 44. the cooling of the substrate, the additional cesium is evaporated on the cold surface 44 If the additional cesium evaporated on the zone 44 does not cause the return of the current to its maximum value can evaporate more antimony on the substrate-and deposit of the cesium additional until the maximum value

be reappeared.

  After a certain period of time both with and without

  emission of its surface, the cathode of Cs 3 Sb can degrade due to the loss of cesium or contamination of antimony by impurities Such degradation can be reversed by a further evaporation of antimony and cesium on the

zone 44 as explained above.

  When the device operates, a laser beam from the laser such as an argon ion laser passes through the transparent substrate 40 and the semitransparent chromium layer 46 and enters the cesium antimonide layer 48 causing the emission of electrons The electron beam thus emitted by the light emitting cathode 16 has a high current density in the range of 1 to 100 milliamps per square centimeter. The current densities in the beam at the plane of the target 28 are - several hundred amperes per square centimeter The values in this range are well suited to direct writing lithographic systems in which the electron beam is directed to form a complex pattern

  semiconductor wafers Such electron beams

  They can also be used to form masks for

  projection lithography and in non-lithographic applications

  graphics such as in electron beam microscopy.

  It is therefore seen that the objects of the present invention are achieved in that the light-emitting cathode 16, when illuminated from the rear through a chromium layer with less than 50 to 60% optical losses, has a performance 3% quantum or more and when deteriorated can be easily repaired in situ by the deposition of cesium or cesium and additional antimony The photoemissive cathode 16 is capable of generating a high current density in the range of 1 to 100 milliamps per square centimeter to provide current densities to the target 21 of several hundred amperes per square centimeter In addition, there is a low energy scattering among the electrons in the range of a few tenths or less of electron -volt This low dispersion is a direct consequence of the low energy of the electrons emitted once they have lost most of their initial energy when they are trans ition from the valence states related to their vacuum level The maximum energy magnitude of the emitted electrons depends on the difference of the laser photon energy and the threshold energy of the electron emission defined by an electron transition between the top of the valence band of the photodmissive material and its vacuum level For the 514.5 nanometer (2.43 electron-volts) argon ion laser lights and a photoemission threshold of the Cs Sb of about 2.0 electronvolts, the maximum emission energy of electrons is 0.43 electronvolt, which is therefore the maximum energy dispersion of electrons The nominal energy dispersion commonly based on the half-width of the distribution curve of a large number of electrons emitted according to their energy is substantially

less than this.

  The cathode operates at a low temperature such that the ambient temperature so that there is no problem of support as would be the case with a hot cathode In addition, no heating time is necessary because the electrons are generated instantly in response to illumination by laser light The modulation of the electron beam can

  easily be accomplished by modulating the laser beam externally

  In addition, the conformation of the beam into complex shapes is easily achieved by openings or masks in the optical assembly between the

  laser 10 and the light emitting cathode 16 outside the

to empty 18.

  It will be understood that various variations and modifications of the light emitting cathode in combination with a modulated laser source described above can be carried out without departing from the

of the present invention.

-13-

Claims (13)

  1 Electron beam generator for a system of   electron-beam semiconductor lithography,   by the fact that it comprises a continuous emission laser (CW), a modulator for varying the intensity or deflecting the   optical laser output beam, a light emitting cathode   to be illuminated by the output beam of said laser, said cathode comprising a semitransparent film of a photoemissive substance selected from the group consisting of cesium antimonide, sodium and potassium antimonide, cesium gallium phosphide, cesium gallium arsenide phosphor and mixtures thereof, said film having the effect of emitting electrons when illuminated by said laser, and an optical assembly for the light disposed between said laser and said cathode and having the effect of producing with the output beam of said laser a   laser light pattern on the back of said light-emitting cathode   sive so that the film emits electrons to form an image determined by said drawing and which is suitable for the semiconductor lithography.   2 Electron beam generator according to claim   cation 1, characterized in that the optical assembly for the light comprises at least one optical lens for focusing the output beam of said laser through the rear of said   photoemissive cathode on said film.   3 Electron beam generator according to claim   cation 1, characterized in that the laser is an argon ion laser operating at a wavelength between 454.5 and 514.5 nanometers.   4 Electron beam generator according to claim   cation 1, characterized in that it comprises a vacuum chamber, said light emitting cathode being disposed in said chamber and said laser, said modulator, and said optical assemblies for the light being disposed outside said chamber.   5 Electron beam generator according to claim   cation 1, characterized in that said modulator is a acousto-optical device.   6 Electron beam generator according to claim   cation 1, characterized in that said modulator is a electro-optical device. -14-   7 Electron beam generator according to claim   cation 2, characterized in that said optical lens has the effect of focusing the output beam of said laser on said film so as to substantially form a point electron source. 8 Electron beam generator according to one   any of claims 1 and 4, characterized in that   the light emitting cathode further comprises a substrate optically transparent to said laser light, said substrate having a rear face facing the optical output beam of said laser and a front face facing away from the optical beam of   output of said laser, and an optically semiconducting conductive film   parent deposited on the front face of said substrate, and said semitransparent film of photoemissive substance is disposed on said electrically conductive film.   9 Electron source for generating beams   of high intensity electrons conformable, modular, sensitive   monochromatic when illuminated by a laser light, characterized in that it comprises a substrate   optically transparent to said laser light, an electric film   an optically semitransparent conductor deposited on said substrate and a semitransparent film of a photoemissive substance deposited on said conductive film, said light emitting substance being selected from the group consisting of cesium antimonide, sodium and potassium antimonide, phosphide cesium gallium, the gaseous gallium arsenide phosphide, and mixtures thereof, said photoemissive substance film having the effect of emitting electrons when it is illuminated by a laser light.   Electron source according to claim 9, characterized in that   characterized in that said substrate is formed of a material   selected from the group consisting of quartz and sapphire.   Electron source according to claim 9, characterized in that   achieved by the fact that the film semitransparent electrically driver is chromium.   12 A method for manufacturing a light emitting cathode for a high density electron emission, characterized in that it comprises the successive steps of providing a transparent substrate, masking a portion of a selected side of said substrate, depositing a thin metal coating on all said side of said substrate except for said masked portion to form a low resistance electrical path, unmask said portion of the substrate, depositing a semitransparent electrically conductive film on at least said portion of the substrate, depositing a semitransparent layer of antimony on said electrically conductive film and evaporating on said antimony layer a material selected from the group consisting of cesium, and sodium and   potassium in a specified ratio.   A method of manufacturing a light emitting cathode according to claim 12, characterized in that it further comprises the step of, during said deposition of cesium, or sodium and potassium, to illuminate said partially fabricated cathode with a laser beam, to measure the current generated by the partially manufactured cathode and to terminate said deposit of cesium or sodium and potassium when said current reaches a maximum value.