IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 41, NO. 3, MARCH 1994 607

Selective Emission of Electrons from Patterned Negative Electron Affinity Cathodes

Edval J. P. Santos, Member, IEEE, and Noel c. MacDonald, Senior Member, IEEE

Abstract- We have observed electron emission into vacuum from the exposed areas of a patternedp++-GaAs substrate which was coated with cesium and oxygen. The emission barrier is a double layer of Titanium-TungstedSilicon Nitride. The exposed areas of the cathode were activated to the negative electron affinity (NEA) condition. It has been an open question whether it would be possible to activate the exposed areas of a patterned GaAs cathode. This result opens the possibility of utilizing NEA cathode technology for projection electron beam lithography tools, NEA-based vacuum microelectronics devices, combination of bulk devices with NEA emitters. A picture of an emission pattern projected onto a phosphor screen is presented. Auger depth profile was used to determine the stability of the TiW/GaAs interface through the activation procedure. Short and long term current stability were measured. A technique for cathode recov- ery and reactivation has been developed.

I. INTRODUCTION

LECTRON sources are a critical component of electron E beam devices and instruments, including electron beam based displays and electron beam lithography tools. Because of its serial nature, traditional electron beam based lithography tools have low through-put and therefore are not used for production lithography. To increase the through-put of electron beam lithography tools, the variable shape electron beam architecture was developed [ 11. This architecture allows for exposure of a large number of pixels in parallel. A pattemed cathode, such as the cathode described in this paper, can be thought of as the integration of an electron gun with the variably shaped aperture as found in the variable shape electron beam machine.

The negative electron affinity (NEA) effect is routinely observed on clean (nonpattemed) semiconductor surfaces, after deposition of a thin layer of an alkaline material (or its oxide). NEA emitters have several desirable properties, including: room temperature operation, small energy spread (-200 meV) [2], [3], small angular spread (half angle -5 ' ) [4], [5], and shot noise limited emission [6]. NEA cathodes differ from typical photocathodes because of the higher photo yield. Brightness

Manuscript received May 12,1993; revised September 16,1993. The review of this paper was arranged by Associate Editor I. Brodie. This work was supported in part by the Semiconductor Research Corporation (S.R.C.) via the Comell Program for Microscience and Technology. The chips were processed at the National Nanofabrication Facility (N.N.F.), which is supported in part by the National Science Foundation, Come11 University, and industrial affiliates.

E. J. P. Santos is with the Universidade Federal de Pernambuco, Departa- mento De Fisica, 50000-Recife, PE-Brazil.

N. C. MacDonald is with the School of Electrical Engineering and the National Nanofabrication Facility, Comell University, Ithaca, NY 14853.

IEEE Log Number 92 15 146.

greater than lo7 A/cm2sr has been achieved from membrane type NEA cathodes in pulsed operation [6].

It has been an open question whether it would be possible to achieve NEA-activation on pattemed GaAs surfaces. Here we show that it is possible to pattem a GaAs substrate with standard microelectronic processing techniques and to activate the exposed GaAs pattem to the negative electron affinity condition. This paper describes a process to achieve the negative electron affinity effect only in selected areas of the cathode. The patterned NEA cathode technology offers the possibility of new projection electron beam lithography architectures for high through-put electron beam lithography, NEA-based vacuum microelectronics, and a combination of bulk devices with NEA emitters.

11. THE NEGATIVE ELECTRON AFFINITY EFFECT It is observed experimentally that if an alkaline material

is deposited onto a metallic surface the work function is reduced [7]. To explain this phenomena, Taylor and Langmuir [SI suggested that the cesium atom loses one electron to the surface, forming a dipole layer. The amount of work function reduction within this model is proportional to the dipole concentration, Ndipole,

A d z -

For Ndipole - 6.25 X AX N 4 A, E G ~ A ~ = 13.186, [9] yields A$ N -3.43 V. The linear decrease of the work function with the dipole concentration, as expressed by ( l ) , is only valid at low concentrations. As the dipole concentration increases, the work function stops decreasing, this effect is explained as a depolarization caused by the interaction among the dipoles. This ionic view of the work function reduction effect was first disputed by Gumey [lo]. Despite this early criticism, the ionic model became widely accepted. It was only recently that the ionic model was finally discarded [ 1 11-[ 131. The experimental evidence and theoretical calculations indicate that almost no charge transfer occurs.

For semiconductors, the electron affinity is defined as the difference in energy between the vacuum level and the con- duction band minimum,

E A = E,, - Ec (2)

When a p++ GaAs substrate is coated with cesium and oxygen, the vacuum level will, effectively, lie below the conduction

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608 IEEE TRANSACTIONS ON ELECTRON DEVICES, VOL. 41, NO. 3, MARCH 1994

Conduction band

Bandgap

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A

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Vacuum level

band level. When this happens the electron affinity becomes negative [ 141. A graphical representation of the adsorption process on a semiconductor surface and subsequent reduction of the work function is shown in Fig. 1. The effect was first observed on a GaAs surface coated with cesium [15].

It has been demonstrated that NEA emitters have several advantages: room temperature operation, small energy spread (- 200 meV) [2], [3], small angular spread (half angle N 5") [4], [ 5 ] , and shot noise limited emission [6]. These properties make the NEA cathode a low noise, high brightness source. Since the emission occurs at a flat surface (and not a tip), one can easily fabricate extended (parallel) emission sources. The negative electron affinity effect is routinely observed on clean (nonpattemed) semiconductor surfaces, after deposition of a thin layer of an alkaline material (or its oxide). To activate a GaAs surface to the negative electron affinity condition, one must satisfy two critical requirements: ultrahigh vacuum (UHV; lop8 - torr), and use a high quality, smooth, crystal surface. A UHV environment is also required to achieve stable, low noise emission from field emitter cathodes; UHV reduces the possible sputter erosion of the tip by ions accelerated by the strong electric field used to achieve field emission. Fabrication of high quality GaAs crystal surfaces has been made possible with the development on epitaxial growth (MOCVD, MBE) in the last two decades.

111. PATTERNED NEA CATHODES FABRICATION AND OPERATION

A. Patterned Cathode Fabrication Steps

The process is designed to protect the emission areas from processing damage and is accomplished by using a lift-off fabrication procedure. The substrate is a highly doped p-type (Zn doped) GaAs (100) wafer (Sumitomo, N , = 2.8 - 3.2 x lo1' ~ m - ~ , E.P.D. < 2000 cm-2). The first step is to spin-on

t _ -

Fig. 2. obtained with an ammonia based image reversal process.

Patterned cathode after TiW sputtering step. The lift-off profile is

a layer of photoresist. The lift-off profile is obtained by an image reversal process, which uses ammonia. With the lift- off profile defined, a 1000-8, layer of titanium-tungsten (2.0 kW, 10 mtorr, 40 sccm of Ar) and a 300-A layer of silicon nitride (1.1 kW, 2 mtorr, 24 sccm of Ad16 sccm of N2) are sputter-deposited on the p++ GaAs substrate. Refer to Fig. 2. The resist is removed and the device is now ready to undergo surface cleaning and be loaded in the UHV chamber for activation. The cleaning step consists of etching the open areas in a solution of NH*OH:H20:H202 (4: 100: 1). This etch step removes some of the surface layers, which may be damaged during processing, and leaves a thin oxide layer, which acts as a passivation layer (the surface is further oxidized when the cathode comes into contact with air). Immediately after the cleaning/oxidization etch-step, the patterned chip is dried with nitrogen and soldered with indium onto the molybdenum cup, as shown in Fig. 3. The molybdenum cup is mounted back in the UHV compatible probe station [16] and the chamber is pumped down to UHV (< 1 x 10-l' torr). After careful out-gassing of the cesium dispenser and the molybdenum cup, the oxide covering the emission areas is desorbed by heating the surface to about 590°C. After a cooling down period the chip is activated by exposing it to cesium and oxygen flows, alternatively.

B. Pattem Projection

The experimental setup to project the cathode pattern, uses a simple parallel plate arrangement. The patterned cathode faces a phosphor screen mounted on the anode side. After the sample is activated, a potential difference of 3 kV is applied between the sample holder and the screen. Illumination of the cathode is achieved with a laser. For these experiments, a low power (4 mW) CW HeNe (1.98 eV) laser and a (pyridine) dye laser (tunable range: 1.58 eV - 1.82 eV) pumped by a Nd:YAG laser (82 MHz) were used. To illuminate the whole area of the cathode it is necessary to produce a divergent laser beam. A divergent beam illuminates the cathode nonuniformly, because of the gaussian shape of the TEMoo mode. The

SANTOS AND MACDONALD: SELECTIVE EMISSION OF ELECTRONS FROM AFFINITY CATHODES 609

Molvbdenum cup

0 SOW IWOO Tim. (seconds)

Phosphor screen

Fig. 3 . cup.

The patterned cathode is goldered with indium onto the molybdenum

0.5 a o iX1o4 2x104 3x10~ 4x10~

Time (seconds) (b)

Fig. 5. Photoemission current as a function of time. (a) Photocurrent at activation and about three hours after. (b) IO-hours measurement for a cathode that was kept emitting for -290 hours. In both cases the Cs How was kept on all the time.

both cases data was collected automatically, every ]Os, and stored in a computer. These measurements show that NEA cathodes can have stable emission for long periods of time. Previous work on nonpattemed cathodes has observed that a suitably prepared NEA cathode is shot noise limited 16).

Fig. 4. Example of a projected pattern. Distance from the cathode to the phosphorous screen is about 4 cm. The screen size is I in x 1 in . On the top right, the original pattern IS shown.

nonuniform illumination causes the emission to be stronger at the center of the cathode than at the borders. Electrons are emitted only from the activated GaAs patterns. An example of a projected pattern is shown in Fig. 4. The distance between the cathode and the phosphorous screen is about 4 cm. It should be emphasized that there are no electron lenses, only the electrostatic voltage applied between the cathode and the screen. This is a demonstration of the collimation of the emitted NEA electron beam.

C. Photorrizission Stabiliv

Emission stability is most critical for practical applications. In electron beam lithography, the amount of charge per unit time per area (dose) is used to determine exposure levels. Emission 5tability translates into reproducible exposure times. A 3-hour emission current measurement, including cathode activation, is shown in Fig. 5(a). A 10-hour measurement done over a period of 10 hours for a cathode that was kept activated for -290 hours is shown in Fig. 5(b). For these measurements the cesium flow was kept on at all times. In

D. TiW/GaAs Interface Stability

TiW is a refractory alloy traditionally used as diffusion barrier for metalization layers and plugs in integrated circuit fabrication processes. This alloy was chosen for the "emission barrier" not only because of its refractory properties, but also because Ti is quite reactive with GaAs. The reaction, GaAs + 2 Ti 4 TiAs + TiGa, starts at temperatures as low as 400" C [ 17, 18, 191. The reactivity of titanium may help the adherence of the TiW layer onto the GaAs surface.

The sputtering target is 10 wt% Ti and 90 wt% W. The sta- bility of the TiW/GaAs layer was studied with auger electron spectroscopy (AES). An AES depth profile of the TiW-GaAs structure is shown in Fig. 6. This AES depth profile was performed on a cathode after the oxide desorption step (in which the cathode is heated to 590" C). It shows that the interface is stable.

E. Surface Roughness

The collimation of the emitted beam is affected by the roughness of the emission surface. A rough surface adds to the angular spread of the emitted beam [4], [5]. Refer to Fig. 7. Therefore, to achieve the level of collimation predicted by the momentum conservation [20], the fabrication process must yield smooth surfaces.

The atomic force microscope (AFM) is high resolution surface profiler, capable of resolving surface features smaller than 1 nm. A surface scan measurement performed with an

IEEE TRANSACTIONS Oh' ELIXTKON I)IJVIC'l:S. V O L 41. KO. 3. MARCfl I993

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Fig. 6. mcasurement through the SiNITiWiGaAs interfaces.

( a ) AES surface wrvcy on the SiN layer. ( b ) AES depth profile

Fig. 7. Emission from a rough surface. Electron are emitted within a cone around the local surface nornial. Therefore. a rough surface will have a larger aneular snread than a Dei-fcctlv f a [ wrface.

Fig. 8. Atomic forcc microscopy of the emitting surface near the TiW layer.

surface), it is not possible to activate the cathode to the negative electron affinity condition. This indicates that the topmost arsenic layer plays an important role in achieving the NEA condition, and suggests a method to recover the cathode. Therefore, to increase the cathode lifetime we studied the possibility of refreshing the topmost arsenic layer.

Based on the observations described in the previous para- graph, a thin arsenic layer was deposited onto the cathode. The cathode was then taken to the UHV chamber and the excess arsenic was removed by heating the substrate to about 300" C. This recovery method has been proven successful. The arsenic layer not only replenishes the GaAs surface with arsenic atoms. but it also acts as a passivation layer. This passivation layer is necessary to transport the cathode from the MBE chamber to the test chamber. In addition, the use of an arsenic passivation layer (instead of the most common oxide layer) has the added advantage of requiring a lower temperature for desorption.

IV. DISCUSSION

For a typical vacuum device, the overall procedure to achieve optimum electron emission can be divided into three major issues: vacuum quality (pumping), desorption (surface contaminants and oxide removal step), and cathode activation. Regarding contaminants. we have shown that the use of arsenic passivation layer provides two beneficial effects: the topmost arsenic layer is renewed, and the desorption temperature for the excess surface arsenic is lower than the temperature needed to desorb surface oxides. I t has been observed that by keeping

AFM is presented in Fig. 8. the average roughness is of the order of 1 0 nm (12 nm rms) over a scan of 12 pm, which seems to be typical of oxide desorbed substrates [21]. The best condition possible would be to have the roughness equivalent to one monolayer step. which is about 0.28 nm. Annealing can be used to smooth the surface 1221. Annealing enhances surface reconstruction.

F. Lijetiine-Cathode R r c o v r i ~

Important issues are the lifetime of the cathode. and cathode reactivation. It is well established [23] that if the GaAs surface temperature is below the congruent temperature, the topmost surface layer is an arsenic layer. In this case, the GaAs surface is said to be arsenic stabilized [24].

It is observed experimentally that if a GaAs substrate is heated above the congruent temperature (gallium stabilized

~~

the cesium flow on after the cathode is activated, long term stable emission is achieved.

The emission area can be defined to submicron tolerances. Patterned cathodes or the ability to achieve selective emission can be used in many electron emission applications. In the case of patterned NEA cathodes. some of these applications are restricted because of the requirement for a UHV environment. For some applications, e.g.. lithography. the gun chamber can be kept at UHV independently of the wafer processing chamber. Thus, electron beam lithography is an example of an area of application for a patterned NEA cathode.

We outlined a process to fabricate patterned NEA cathodes. We have shown that it is possible to activate a pattemed GaAs chip to the negative electron affinity condition. Thus using a patterned NEA cathode it is possible to develop new electron beam devices and systems architectures, including: NEA-based vacuum microelectronics devices, prqjection electron beam lithography tools.

SANTOS AND MACDONALD: SELECTIVE EMISSION OF ELECTRONS FROM AFFINITY CATHODES 61 I

ACKNOWLEDGMENT 1191 M. de Potter, W. De Raedt, M. Van Hove, G. Zou, H. Bender, M. Meuris, .~ and M. Van Rossum, “Characterization of the TiW-GaAs interface after raDid thermal annealing.” J. Aonl. Phvs.. vol. 6. DD. 47754779. 1989. The authors would like to thank Dr. William J. Schaff for . ‘ , ..

depositing the arsenic OverlayerS. The technical assistance of (201 E. 0. Kane, “Implications of crystal momentum conservation in pho- toelectric emission for band-structure measurements,” Phys. Rev. Left., the NNF staff members is acknowledged. vol. 12, PP. 97-98, 1964.

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151 G. Dennis Fisher and U. Ramon Martinelli, “Negative electron affinity

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Edval J. P. Santos (M’87) received the B.S. (cum laude) in electrical engineering from the Universi- dade Federal de Pemambuco, Brazil, in 1984. He received the M.Sc. in physics in 1987, also from the Universidade Federal de Pernambuco, where his thesis focused on pulsed NMR on amorphous materials. In 1988 he received the MSEE from Yale University, and in 1993, he received the Ph.D. from Comell University.

He is now a Visiting Professor at the Universidade Federal de Pernambuco. His present research inter- ests include electron beams and micromechanical devices.

Noel C. MacDonald (S’61-M’67-SMS9 I I received the Ph.D. degree in electrical engineering from the University of California, Berkeley, in 1967.

In 1970, he joined Physical Electronics Ind., Inc., and an entrepreneur. He later served as Division General Manager for the Physical Electronics Divi- sion of Perkin-Elemer Corporation. In 1984, he was appointed Professor of Electrical Engineering, Cor- ne11 University, Ithaca, NY, and Cornell Director of the Semiconductor Research Corporation’h Program on Microscience and Technology. In 198Y, he was

[I71 0. Wada, S. Yanagisawa, and H. Takanashi, “Thermal reaction of Ti evaporated on GaAs,” Appl. Phys. Lett., vol 29, pp. 263-265, 1976.

[I81 K. B. Kim, M. Kniffin, R. Sinclair, and C. R. Helms, ”Interfacial reactions in the Ti/GaAs system,” J. Vac. Sci. Technol. A , vol. 6, pp. 1473- 1477, 1988. “on-a-chip.”

appointed the Director (Chairman of the School of Electrical Engineering, Cornell University. His present research interests include fabrication. prob- ing, and modeling nanometer-scale electromechanical devices and integrated circuits and building electromechanical instruments, sensors, and actuators