N O T I C E 

THIS DOCUMENT HAS BEEN REPRODUCED FROM 

MICROFICHE. ALTHOUGH IT IS RECOGNIZED THAT 

CERTAIN PORTIONS ARE ILLEGIBLE, IT IS BEING RELEASED 

IN THE INTEREST OF MAKING AVAILABLE AS MUCH 

INFORMATION AS POSSIBLE 
CR 1747

DEVELOPMENT PROGRAM

ON A COLD CATHODE

ELECTRON GUN

Annual Report

January 1985

By: C.A. Spindt, Senior Research Engineer

C.E. Holland, Research Physicist

Physical Electronics Laboratory

Prepared for:

National Aeronautics and Space Administration

Lewis Research Center

21000 Brookpark Road

Cleveland, Ohio 44135

Contract NAS 3-23776 f ,^ ^ 1 t • t

W. ,

t^. D460 ^°

8S ^^^

^,

 ^y

F

6%,

SRI Project 5723

(NASA-Cb-174792) LEVELCPMENT kLiOGRAM ON A NEij-14459

CGLU CA1ti(,LG LLE%.:7FCN GUN ALLUdl Report

J58I IntErn3tiocal C01F.) 7C F

HC A04/MF AJ1 CSCL 09A Unclds

G3/3J 16292

SRI International

333 Ravenswood Avenue

Menlo Park, California 94025-3493

(415) 326-6200 SRI

i,

Telex: 334486
CR 1747

DEVELOPMENT PROGRAM

ON A COLD CATHODE

ELECTRON GUN

Annual Report

January 1985

By: C.A. Spindt, Senior Research Engineer

C.E. Holland, Research Physicist

Physical Electronics Laboratory

Prepared for:

National Aeronautics and Space Administration

Lewis Research Center

21000 Brookpark Road

Cleveland, Ohio 44135

Contract NAS 3-23776 f ,^ ^ 1 t • t

W. ,

t^. D460 ^°

8S ^^^

^,

 ^y

F

6%,

SRI Project 5723

(NASA-Cb-174792) LEVELCPMENT kLiOGRAM ON A NEij-14459

CGLU CA1ti(,LG LLE%.:7FCN GUN ALLUdl Report

J58I IntErn3tiocal C01F.) 7C F

HC A04/MF AJ1 CSCL 09A Unclds

G3/3J 16292

SRI International

333 Ravenswood Avenue

Menlo Park, California 94025-3493

(415) 326-6200 SRI

i,

Telex: 334486
0

0

00

J

0

FcDEVELOPMENT PRr AM

ON A COLD CATHOk.

ELECTRON GUN

Annual Report

January 1985

By: C.A. Spindt, Senior Research Engineer

C.E. Holland, Research Physicist

Physical Electronics Laboratory

Prepared for:

National Aeronautics and Space Administration

Lewis Research Center

21000 Brookpark Road

Cleveland, Ohio 44135

Contract NAS 3-23776

SRI Project 5723

111

f

i i I

SRI

dernabonal

Approv9d:

Ivor Brodie, Director

Physical Electronics Laboratory

W.F. Greenman, Vice President 

Advanced Technology Division

Y
ME

1. Report No. 2. Government Accession No. 3. Recipient's Catalog No. 

CR174792

d. Title end Subtitle S. Report Data

January 1985

Development Program on a Cold Cathode Electron Gun 6. Performing Organisation Code

506-58-22

•. Performing Organisation Report No.

C.A .

7.

r e Spindt and C. Holland

SRI Project 5723 

9. Performing Organisation Name and Address

_

10. Work Unit No.

SRI International

333 Ravenswood Avenue 11. Contract at Grant No.

Menlo Park, California 94025 Contract NAS 3-23776

13. Typo of Rupert and Period Covered

12. Sponsoring Agency Name end Address

National Aeronautics and Space Administration Annual

Lewis Research Center

14. sponsoring Agency Code Cleveland, Ohio 44135

15. Supplementary Notes 

Project Manager - E.G. Wintucky

16. Abstract 

During this phase of the cathode development program, SRI improved the 

multiple electron beam exposure system used to print hole patterns for the cathode

arrays, studied anisotropic etch processes, conducted cathode investigations

using an emission microscope, reviewed possible alternate materials for cathode

fabrication, studied cathode storage techniques, conducted high power operation

experiments, and demonstrated high-current-density operation with small arrays of

tips.

17. Key Words 10. Distribution Statement

For General Release

19. Security Clossif. (of this report) 20. Security Clessif. (of this pogo) 21• No. of Pages 22. Price

U U 63

'For sale by the National Technical Information Service, Springlield, Virginia 22161 

__

as ,'s 
CONTENTS

S(JMMARY ......................................................... ix

LISTOF ILLUSTRATIONS ............ .........................0000.. v

LISTOF TABLES .................................................. vii

I INTRODUCTION .............................................. 1

II FABRICATION TECHNOLOGY .................................... 5

A. State-of-the-Art Cathodes ............................ 5

B. Advanced Fabrication Processes........ 0 .............. 7

I. Active Area Patterning .0 ...... 0 ................. 7

2. Anisotropic Etching....... 0 ..................... 10

3. Materials ....... . ....................0........a. 13

IIICATHODE TESTING ....................................... ... 15

A. Initial Emission Tests ..............................0 15

B. Cathode Storage and Retesting .............0......000. 17

C. The Emission Microscope * ...... o ....... 0.............. 21

D. High-Power Tests ..................................... 24

E. High-Current-Density Tests .......... 0.0 ..... 0 ....... 0 35

IVSUMMARY AND CONCLUSIONS ................................... 39

REFERENCES ...................................................... 41

APPENDIX ........................................................ 43

iii
ILLUSTRATIONS

1 1 Conventional Field Emitter and Anode ...................... 2

'. 2 Thin-Film Field Emission Cathode ..0 ...... 6....6......0.6.0 4

3 Cathode Identification Code .......6.... ...... 6.......6.... 6

4 Multiple Electron Beam Exposure System .................... 8

S Hole Patterns in Molybdenum Films ......................... 9

6 RIE-Etched S102 Layer, Molybdenum

Top Layer Etched With Wet Chemistry ....... ..0.6....0. .... 0 12

7 Standard Setup for Emission Test .......................... 16

8 Emission Microscope Setup.. .......0.0.. .... .0. ....... 0.... 21

9 Schematic of Collector System to Reduce Power

Handling Problems at the Collector... o .... o ... o.6o.00 .... o 24

10 Combination Half-Wave, Full-Wave, and CW Supply........... 27

11 Full Wave 60 Hz Emission. .... 0. ...... 6.. ........ 0......... 28

12 Full Wave 60 Hz Emisison--Average Current

Equal to 20 mA CW.....0.. .... 0.0.0..0 ..................... 30

13 20C-171-6C Operating at 10 mA CW

With Increasing Collector Voltage. .... . .... 0.0 ...... 60.... 32

14 Emission Level of Cathode 20C-171-6F Operating

at 10 mA CW with +2000 Volts on the Anode ................. 33

15 Emission Level as a Function of Time

for Cathode 20C-181-6F .......... 0...0.0.000 ... . ...... 0.... 34

F 16 Cathode 29C-177-3T Operating in the 60-Hz

Pulse Mode at 10.8 mA Peak Emission Current ............... 37

s 17 Fowler/Nordheim Plot of the Emisison Data

for Cathode 29C-177-3T Operating

up to 10-mA Peak Emission ..............0........0......... 38

,;',`CLDING PAGE BLANK NOT MM6'

v

!.
E

.0 1 _t 1.

-

TABLES

1 Results of Retesting Cathodes After Long Periods

of Storage in Air ......... . .............0.000............. 18

2 Results of Retesting Cathodes After Storing

in Dessicator with Rough Vacuum for Several Months........ 19

3 Repeat Tests, Stored in Situ............ 0 ................. 20

4 Relative Brightness of the Cathode Tips

During Testing in the Emission Microscope. ** ... ****.* ..... 23

5 Emission Tests with Small Area Emitter Arrays

Using 60 Hz Pulse ......................................... 36

?RECEDING PAGE BLANK NOT FILMED

vii
SUMMARY

•1

During this phase of the cathode development program, SRI improved

the multiple electron beam exposure system used to print hole patterns

for the cathode arrays, studied anisotropic etch processes, conducted

cathode investigations using an emission microscope, reviewed possible

alternate materials for cathode fabrication, studied cathode storage

techniques, conducted high power operation experiments, and demonstrated

high-current-density operation with small arrays of tips.

• A new multiple-electron beam exposure system has produced packing densities of 107 holes/ cm2 , with hole size uniformities

within about + 1.5 percent.

3 • Anisotropic etching using plasma-based dry etching processes has

produced good hole geometries, but more work is needed to reduce

process-related contamination problems.

• Experiments on the effects of storage on cathode performance

suggest that the cathodes should be stored in a good vacuum when

not in use.

• Only preliminary tests were made with the emission microscope

because of changing mission priorities. Only one cathode was

tested; 75 percent of the tips were observed to be contributing

to the total emission when the cathode was operating at the

14 A/cm2 level.

• High-power tests were designed with a goal of 20 mA CW into a

3 kV anode. We achieved 10 mA CW and demonstrated 42 mA peak

with a full wave rectified 60-Hz pulsed driving voltage. The

average power at this level is equivalent to 20 mA CW.

• We routinely operated small area cathodes at over 50 A/cm 2 CW

and 100 A/ cm2 pulsed. A peak emission of 10 mA (60-Hz pulse

drive) was achieved with a 29-tip cathode. This is over

400 A/cm2.

tt.. DING PAGE BLANK NOT g^NiEII

ix
I INTRODUCTION

This report summarizes the past year's work in an ongoing program

at SRI International (SRI) to develop a thin-film field-emission cathode

(TFFEC) array and a cold-cathode electron gun based on the emitter

array. The objective is to produce a microwave-tube gun that uses the

thin-film field-emission cathode as an electron source.

During this report period, the project effort has been directed to

fabricating state-of-the-art cathodes for evaluation, studying and

improving the fabrication technology with a view toward increasing reliability and performance, and demonstrating high emission-current

capability.

The TFFEC, which is based on the well-known field-emission effect,

was conceived to exploit the advantages of that phenomenon while minimizing the difficulties associated with conventional field-emission

structures, i.e. limited life and high voltage requirements. Field

emission has been shown to follow the Fowler-Nordheim equation (Fowler

and Nordheim, 1928):

AF Bv(y)o3/2

J eXp - I

t2 (Y)^ F

where J is the emission-current density in A/cm2 , A and B are constants,

F i g the field at the tip, 0 is the work function in eV, and v(y) and

t(y) are slowly varying functions of y, where

3.79 x 10

-4 

F 

1/2

Y = 0— (2)

Both v( y) and t ( y) are tabulated in the literature ( Burgess et al.,

1953). The field at the tip is

F - (3V V/cm (3)

where V is the voltage applied to the diode structure, and

R = f(r,R,9)cm-1

(4)

1

op

A,,

-- — - 

► ..r f

(1)
R^1mm

NOT TO SCALE

ANODE

IED WIRE

TER TIP

HAIRPIN FILAMENT — 0.003 inch diem.

The relationship between a and the tip radius ( r), the anode-to-tip

spacing ( R), and emitter cone half angle ( 0) is complex ( Dyke and Dolan,

1956) and difficult to determine accurately. For the purpose of our

work it is sufficient to note that p increases as r, R, and 0 become

smaller. Thus, smaller cathode/anode structure reduces the voltage

required for a given emission current.

The conventional field emitter, shown in Figure 1, consists of a

short segment of fine wire (usually tungsten) etched electrolytically to

form a sharp point ( i.e. small r). The segment is mounted on a hairpin

filament for support and in then cleaned and annealed in vacuum by passing current through the hairpin filament and heating the tip to incandescence. After cooling, the point produces cold electron emission when

a positive voltage is applied to a ring or aperture anode spaced at a

macroscopic distance ( R) (approximately 1.0 mm) from the emitter tip.

FIGURE 1 CONVENTIONAL FIELD EMITTER AND ANODE

Electric fields of the order of 107 V/cm are required for field

emission, and with this geometry anode potentials of the order of kilovolts are usually needed to produce the required field. A common difficulty in operating the conventional field emitter cathode arises from

the effects of positive ions formed in the interelectrode region being

directed toward the emitter tip by the curved field lines that terminate

at the tip. Some of these ions have energies in the keV range and can

therefore roughen or sharpen the tip by sputtering. [The sputtering

rate is determined by the rate of ion formation (which is directly

related to the local vacuum pressure and emission current drawn) and by

the ion energies (which depend on the applied voltages).] Tip sharpening by ion sputtering during operation increases the local field (for

the same applied voltages) and thus progressively increases the emission

current and the sputtering rate until an arc or resistive heating of the

tip leads to its destruction. Although this effect limits useful

2

lY

+i A
lifetimes for conventional emitters, Brodik (1975) has shown that the

sputtering effect can be greatly reduced if the emission voltage is

below 150 V, and reducing the applied voltage to 50 V might effectively

eliminate the sputtering.

Another difficulty with the etched-wire emitter is that fabricating

large arrays of tips for high-current applications would be very diffii cult, and in densely packed arrays neighboring tips shield each other

electrostatically. Thus, large emitter arrays that produce high current

densities have not yet been achieved by these means.

In an effort to overcome these difficulties, SRI developed the

field emission cathode shown in Figure 2, which consists of a

conductor/insulator/conductor sandwich with dielectric thickness of

approximately 1.5 μm, holes approximately 1.5 μm in diameter in the top

conductor (metal gate film), undercut cavities in the dielectric layer,

and metal cones within the cavities. Field emission from the tips of

the cones is obtained when the tips are driven to a negative voltage

with respect to the gate film. Because of the field enhancement of the

tip (small r) and the close spacing between the rim of the hole in the

gate film and the tip (small R), potentials as low as 100 V to 200 V

across the sandwich can produce large field-emission currents. In addition, dense arrays of the tips can be operated without the tips

Influencing one another, because the gate film that surrounds each cone

prevents reduction of the electric field that would otherwise result

from mutual shielding between the tips.

This configuration also reduces the problem of ion bombardment from

other high-voltage electrodes, because the tips are well shielded electrostatically by the gate film; that is, the equipotentials contoured

about the tip are essentially confined within the cavity, and the potentials between the gate film and other external acceleration electrodes

are essentially uniformly spaced and plane parallel. Thus, unlike the

single etched-wire configuration, most ions formed between the gate film

and an external acceleration electrode are directed toward the gate

film, rather than the tip. The ionization volume between the gate film

and the tip is very small; any ions formed within this region will have

low energies (100 to 200 eV) and will be unlikely to cause significant

sputtering damage by striking the tip. This assumption appears to be

well-justified: Experimental results show currents averaging over 50 μA

peak/tip for more than 65,000 hours from a 100-tip array driven with a

60-Hz pulse. The tips were operating at room temperature and unknown

pressure, but probably in the 10-9 torr range.
T

I y, N m ME TAL GATE

MFTAI TIPS I F I I M SiO_ DIELECTRIC

la) SCHEMATIC OF A THIN FILM FIFLD EMISSION CATHODE (TFFEC) ARRAY

^ w

w ^ ^ w

w

w w

w

w ^ w ^ w ^ 

R ^

'

n

^

^ w w

^ w w

w ^

w

w ^

w

^

^ ^

w ^ ^ w ^ w

w ^

w ^

w ^ ^ w

^ ^ ^

w

w ^ ^ w 

411% ^

w ^ ^ 

w ^ ^

w ^

w w ^

w

Ibi SEM MICROGRAPH OF TFFEC ARRAY

r

I

r

f

r

(c) SEM MICROGRAPH OF TFFEC CATHODE

FIGURE 2 THIN-FILM FIELD-EMISSION CATHODE (TFFEC)

4

O
II FABRICATION TECHNOLOGY

A. State-of-the-Art Cathodes

The state-of-the-art cathode for this phase of the program Lovers

an area 1 mm in diameter on an n-type, 0.01 Q • cm, (111) orientation

silicon chip. The silicon chip is nominally 2.5-mm square, and the 1-mm

diameter active area consists of 5,000 emitter tips spaced on 12.7-μm

centers. This cathode geometry is designated "type 20" in the cathode

series (see Figure 3).

The basic cathode-fabrication process has been modified several

times during the program. Developments continue as the research

progresses and new technologies become available. The basic state-ofthe-art cathode-fabrication process at this time is as follows:

(1) Two-inch-diameter silicon wafers are oxidized to adegth of

about 1.5 μ m in a wet oxygen atmosphere at about 1000 C.

(2) Molybdenum is deposited over the oxide to a thickness of

approximately 4000 A using an electron-bombardment-heated

evaporator.

(3) The 2-inch (5-cm) wafer is cut into 1/2-inch (1.27-cm) square

substates with a dicing saw.

(4) An electron-sensitive resist, poly(methyl-methacrylate) or

PMMA, is spun onto the substrates, and the desired pattern of

holes is exposed in the PMMA using an SRI-developed multiple

electron beam exposure system. Typically, 25 patterns on

0.25-cm centers are formed on the 1.25-cm square substrate.

(5) The PMMA is developed. The molybdenum film is then etched

away where it is exposed by the developed holes in the PMMA

layer.

(6) The silicon dioxide layer is etched down to the silicon substrate through the etched holes in the molybdenum film; the

PMMA is then removed.

(7) Cones are formed in the holes using a dual deposition technique developed at SRI for this purpose (Spindt et al., 1976).

(8) The gate film pattern is etched using photolithography. Typically, this pattern consists of 25 1.78-mm square pads, with a

cathode array centered in each pad.

5 V

i

M,

i

sF 
WAFER NUMBER

2 6 B 13 1 B

ARRAY TYPES ARRAY MATERIAL AND TECHNOLOGY

CODE TIPS CENTERS

Ipm1

20 5AOC 12.7

21 1• -

22 20AW 5,35

23 100 12.7

24 400 6.35

26 16 12'

26 40 6.35

27 40,000 4.5

28 10,000 9

29 32 9

30 5,000 t 12.7

31 16 t 12.7

32 64 t 12.7

33 100 t 12.7

34 100 9

36 ' 3,500 4.2

36 3,500 r 12.7

37 7=: 9

38 400 t 12.7

39 1 -

40 65,000 2.5

41 200 I 9

E r

ARRAY

CODE

DESCRIPTION

No Letter P-type (100), vet chemetlry

A P-type (111), vet chemntry

/ P-type (111), R I DE etch

C n-type (111), wet cMmtttry

D n-type 011) RIBE etch

SUBSTRATE

12.7 x 12.7 mm SUBSTRATE AND CHIP

SILICON

ACTIVE

r;l

1.

AREA F70

MOLYBDENUM

GATE FILM PAD

/ ; i • ; L^ S• V•

CATHODE

CHIP

N• M•

7

0

E.

• W.daa, mm lone

D• rel ro 

t Wtadea, 10t urn long r x. 1.76 mm i 0.1 mm X a mm army

2.5 mm E• F• O• F• Y•

—^ 2.S mm

FIGURE 3 CATHODE IDENTIFICATION CODE

Illustrated code indicates 16-tip array on 12.7-,um centers,

fabricated on p-type (111) silicon using reactive ion beam

etching I RIBEI. Wafer is number 136, substrate number

is 4, chip is in "L"

6
(9) The 1.27-cm square substrate is cut into the desired number of

cathode chips to correspond with the array pattern. The

state-of-the-art cathode is cut into 25 cathode chips, each

about 0.25-cm square with the 1.78-mm square molybdenum pad

and cathode array centered in it.

(10) The chips are cleaned and mounted for testing.

These processes have been described in detail in previous reports on

this development program (NASA CR-134888, CR-159866, and CR-165401).

B. Advanced Fabrication Processes

During this phase of the pLugram work was done on the multiple

electron beam exposure system used for patterning the active area,

anisotropic etching techniques, and materials studies for cone

fabrication.

1. Active Area Patterning

During the previous report period, SRI modified the multiple electron beam exposure system (MEBES) used for printing arrays of spots in

an electron sensitive polymer [poly(methyl-methacrylate) or PMMA] by

adding a motor driven stage that could be used to precisely position the

beams on the substrate. This system was used to print hole arrays having high packing densities by making a series of step-and-repeat exposures in the PMMA.

During this report period an advanced

al. 1981) became available and was adapted

process. Figure 4 is a diagram of the sys

tungsten hairpin filament, an einzel lens,

aperture, first and second deflectors, the

stage.

form of the MEBES (Brodie et

to the cathode fabrication

tem, which consists of a

a beam blanker, an object

screen lens, and the target

The advantages of this system over the original MEBES are many. It

has a cryopumped vacuum chamber, which is much cleaner than the oilpumped system used with the original MEBES. This gives an improved

result and reduces downtime for cleaning.

The system also has a well-defined object aperture that is

illuminated with a well-defined beam formed by the einzel lens system.

The beam blanker can be used to make rapid exposures of short duration

and rapid cycle times. The final major improvement is the dualdeflection system that can be used to very accurately position stepand-repeat exposure patterns, or to write patterns such as lines rather

-L__ --1t8.

7
CATHODE

GRID

ANODE C== IC=== ^/ LENS I

BLANKER

I mo— OBJECT APERTURE

1 kV

/ ^ \ FIRST DEFLECTOR

V2 VI Zo-1m // i \

\\ SECOND DEFLECTOR

\\ BEAM-LIMITING APERTURE,

\/ RADIUS ra

1 kV ( ^` SCREEN LENS

LENS APERTURE, RADIUS R

Z,-2 cm IMAGE PLANE 9 kV

WAFER

LASER-CONTROLLED STAGE

MOTIONS (X, Y, 8)

FIGURE 4 MULTIPLE ELECTRON BEAM EXPOSURE SYSTEM

(MEBES)

Using deflectors to position the spot on the target rather than

mechanically moving the target and screen has the advantage of keeping

the target in the same portion of the flood beam during all of the exposures that are made in a step-and-repeat multiple exposure sequence.

This improves uniformity because each exposure is more likely to produce

the same electron dosage to the polymer film if the flood beam is not

moved across the screen.

An illustration of the improved uniformity achieved with the

deflector system can be seen in Figure 5. Figure 5(a) is an SEM of a

r pattern of holes formed with the original screen lens and a step-andrepeat procedure done by mechanically repositioning the screen and target in the flood beam between each exposure. The alignment of the holes

is acceptable, but the uniformity of the hole sizes is very poor. Mechanical alignment is not too difficult because the system demagnification

factor is about 735 to 1. Thus moving the screen and target 7351.m

moves the image only 1 μm, and very precise positioning is possible.

8
c

E

r

i

OF POOR QUALITY

(a) GENERATED WITH ORIGINAL SCREEN LENS

11 • • • • • • • • 4b • • • • p a • • • *so • • • • • • • • • • •

a • • • • • • • • • • • 000 • • •

• • • 000 • 000 O • • • • • • • • • • • • • • • • • • • • • • •

u

• • • • • • • • • • • • • • • 04 ^• • • • • • • • 000 • 000 • • 4

• • 0000 •••• 0000, ••4

• •

M-Z..iw

• • • • • • • • •• •

• • • • • • • • • •• • • •

• • • • • ••• t • • • •

• •••• • ••• i ••r Ili'

- 

rte ^-_ . .

W GENERATEDWiTH UPGRADEDSYSTFr%l

FIGURE 5 HOLE PATTERN.; IN MOLYBDENUM FILMS

9

1

6
j

But moving the screen and target large distances in the flood beam can

lead to nonuniform exposures if the flood beam is not very homogeneous

and stable over long periods of time. The result, shown in Figure 5(a),

indicates that the beam in the original system was indeed not very uniform or stable; as a result, there is a large variation in the size of

the holes that printed ( w + 25 percent), and some did not print at all.

Figure 5(b) shows much-improved hole size uniformity, with the hole

size variation in the + 1.5 percent range. With the new MEBES and its

dual deflector system, the exposures can be done much more rapidly, and

the same portion of the flood beam is used for each exposure. Figure

5(b) is an SEM of a sample fabricated with 16 exposures, each of about

1-1/4 second duration, and a small deflection of the beam between exposures. The spacing between holes is about 2.5 μm between centers, and

the groups of 16 holes are spaced on 12.7-Mm centers. The packing density is 107 holes/cm2 . By filling in the open space between the groupps

of holes, the packing density can be increased to 1.55 x 10 7 holes/cY

Emitter arrays with these packing densities would require an average

current of only about 6.5 μA/tip to produce a current density of

100 A/cm2.

2. Anisotropic Etching

Referring to Figure 2(c), it is clear that the isotropic wet etch

used to form cavities in the silicor. dioxide layer produces an undercut

of the molybdenum gate film that is much too large for hole spacings of

2.5 μm. Thus, in order to make use of the increased packing density

available with the improved MEBES, it will be necessary to develop an

anisotropic etch for the silicon dioxide layer. It may be advantageous

to have a reduced undercut from a structural point of view as well: The

large area of molybdenum overhanging the hole in the silicon dioxide

forms a trap for contaminants and is probably more vulnerable to damage

in the event of a discharge because it is not supported.

Recently there has been a great deal of effort to develop dry etch- 

ing systems based on plasma etching. These etching processes can be

tailored to produce anisotropic etching in most materials. In an effort

to see how this technique could be applied to the cathode structures,

SRI established a cooperative effort with the Varian Central Research

Labs, so that their sophisticated reactive ion beam etch (RIBE) system

could be used to develop an anisotropic etch process for the cathode

structure. By the beginning of this report period, we had obtained a

good result using CC14 in the RIBE system with a chromium resist mask on

the sample. This result was described in the preceding report (NASA

CR-168212).

During this report period, we made several attempts to repeat this

result in the Varian RIBE system. None were successful; and problems

with the vacuum system then forced a shutdown of the RIBE system.

10
Samples were then sent to Plasma Therm s applications laboratory to

see how their reactive ion etch (RIE) system would do with the cathode

structures. The results were excellent with regard to selectivity of

the silicon dioxide etch with respect to molybdenum, and the anisotropy

was also excellent. Figure 6 shows SEHs of these results. In Figure

6(a) the substrate was deliberately fractured to provide a view of a

hole in cross section. Figure 6(b) shows another cavity viewed at 200

from normal to the surface. This cavity geometry is exactly what we had

hoped for. The hole in the molybdenum is nicely tapered; the side wall

of the hole through the silicon dioxide is very nearly vertical and does

not undercut the molybdenum at all; the etch was stopped right at the

silicon/silicon dioxide interface; and the molybdenum has been essentially untouched by the process.

The differences between these samples (with the Plasma Therm RIE)

and the samples etched successfully in the Varian RIBE system are that

the molybdenum was etched almost all the way through in the Varian RIBE

system, and the edges of the holes etched in the RIBE system were not as

smooth as those etched with the RIE system.

The Varian RIBE used CC1 4 as the reacting gas and the RIE uses CHF3

+ 02 in the ratio 10:1. The beam energy in the RIBE etch was 500 V; the

bias on the target during the RIE etch at Plasma Therm was not reported.

Other variables, such as system pressure, are inherent in the differences between the two processes and cannot be meaningfully compared.

Cones have been successfully formed in samples etched in both systems, and emission tests have been made. The results of the emission

tests have been very poor with samples from both systems, in that there

is always excessive electrical breakdown between the two electrodes

(base and gate), frequently even before significant emission can be

detected.

It is known that difficult-to-remove polymers can be formed during

reactive ion processes (Flamm, 1981), and it seems very likely that the

electrical breakdown is due to contamination of the samples by polymers

formed during the etch process. This notion is supported by the good

result obtained with the one sample that was coated with chromium during

the etch process. The chromium layer was removed in subsequent processing, which would also remove any polymer layers that might have been

formed on the chromium during the etch process.

In summary, dry etching processes have been shown to produce ideal

structures for the cathode application, but more work needs to be done

to eliminate contamination buildup during the etch, protect the surface

from polymerized layers with a coating (i.e. chromium), or remove contamination from the surface by subsequent processing that is nondamaging

to the cathode structure.

1.`

11
6

n 1

1-1,

12

r

ORfCi^:AL PACZ

OF POOR QUALITY

lum

(a) CROSS SECTION OF HOLE ETCHED

WITH RIE IN SiO2 125,000 XI

L

1 VIII

(b) HOLE VIEWED AT 20 0 , 20,000 X

FIGURE 6 RIE-ETCHED S O2 LAYER, MOLYBDENUM

TOP LAYER ETCHED WITH WET CHEMISTRY

o
3. Materials

Field emission is very sensitive to emitter surface conditions

because of the strong effect the surface work function and microtopography have on the emission current (see Fowler/Nordheim equation in Section I).

Molybdenum is a convenient material for field emission cathode

fabrication because it is refactory; easily evaporated; adheres well to

the silicon and silicon dioxide; is selectively etchable with respect to

silicon dioxide, silicon, and aluminum oxide; and can be easily etched

in a polish-etch mode. Its use is not without drawbacks, however. The

most important negative characteristic of molybdenum for this application is that it forms oxides that are unstable and are volatile at relatively low temperatures (8000C), and that oxide formation begins at very

low temperatures (2000C). This may be a source of electrical breakdown

in some of our tests because of dissociation, migration, and evaporation

during operation of the cathode. In addition, the sticking coefficient

for gases on metals such as tungsten and molybdenum is much higher than

it is for some other materials, such as the carbides (Eisinger, 1958).

Thus it would seem that passivating the molybdenum surface with, for

example, the transition metals (Davydova, 1979) or coating with stable

materials such a titanium carbide which has been reported to yield

stable emission (Adachi, 1983), could reduce the incidence of breakdown.

A second approach would be to form the entire cone from a more stable

material like a carbide. This may be possible because the composition

of many carbides is not stoichiometric, and the ncnstoichiometric carbides are also very stable (Strome, 1967). It is reported that titanium

carbide can be expressed as TiC x where x can vary from 0.95 to 0.5

without appreciably changing the stability of the material. This being

the case, it should be possible to evaporate TiCO.95 and obtain a suitably stable TiCx . }

2

ri Other carbides are also interesting in that they are very refac-

.^ tory, chemically stable, have a low capacity for gas adsorption, and

have relatively low reported work functions. For example, RFC has a

' melting point of 38900C and a reported work function of 2.04 eV ( Samsonov and Vinitskii, 1980). This compares extremely well to molybdenum,

which has a melting point of 2600 0 C and a reported work function of at

best 4.1 eV ( Kohl, 1967). NbC, ZrC, and TaC are also very interesting.

Some other materials are interesting but difficult to use for this

". application because of problems with evaporation or chemical properties

(e.g. LaB6 and TaN).

Experimental investigations of some of these materials will be considered in the next report period.

t

f+

13

§#3

F

.a
^1.

III CATHODE TESTING

A

SRI performed a variety of cathode tests during the report period.

Routine quality control tests were performed on all cathodes. In addition, SRI tested for the effects of storage environment on the cathode's

performance. A brief test was made in an emission microscope to investigate emission uniformity. Tests were also made on high total current

operation with large arrays, and on low total current but high-currentdensity operation using small arrays.

A. Initial Emission Tests

All cathodes that complete the fabrication process successfully are

tested in SRI's standard test setup before being subjected to other

tests or being shipped to users. This is done to provide a standard

level of operation by which the cathodes can be judged and to have a

basis by which trends resulting from process changes can be evaluated.

The Appendix lists all cathodes that were tested (including those

on other programs, to give as large a data base as possible) by test

group. The listing includes bakeout temperature, cathode mount, gate

current, applied voltage, maximum emission, number of the tips blown

during the test, number of shorts between the base and gate during the

test, and final condition of the cathode, [i.e. OK or NG (no good)].

Figure 7 shows the circuit used for the initial tests. It consists

of a simple power supply that delivers a 60-Hz half-wave rectified output that is continuously variable between 0 and -500 V peak; a dutycycle-control gating circuit that is used to operate the cathode at

reduced duty cycle (to prevent overheating the collector when operating

at emission currents above about 10 mA); and a variable series padding

resistor (to prevent excessive emission bursts when the cathode is initially turned on).

The cathode tips are driven to a negative voltage by the power supply, and the gate film is grounded. A collector is biased to about

+1200 V with respect to ground to help in overcoming space-charge

effects in the emitted beam. The collector is a 316 stainless-steel

tube, 3/16 inch in diameter and about 1-1/2 inches long. The tube is

bent in a gentle curve, as shown in Figure 7, and the emitted electron

beam is directed into one open end of the tube, so as to spread out the

landing impact area to reduce the power density at the collector. The

tube also acts as a Faraday cage to minimize errors resulting from

-' secondary electron emission. It is open at both ends to pump out any

desorbed gases.

15

PRECEDING PAGE BLANK NOT FILMED
R 

CHANNEL B (I4

100K

HORIZONTAL

DRIVE (V)

51K

/ 1

1

1

1

1

1

1

1

5.1K

av—

ov—u

0

FIGURE 7 STANDARD SETUP FOR EMISSIC

DUAL TRACE COLLECTOR

OSCILLOSCOPE

COLLECTOR

i CHANNEL A (1) BIAS – 1 kV

V ---- 1--------- -^

16
The emission process is monitored with a dual-trace oscilloscope,

as shown in Figure 7. The voltage applied to the emitter tips is used

to drive the horizontal sweep of the oscilloscope through a 100:1 voltage divider; the gate current is monitored by Channel B of the oscilloscope, while the emission current is measured with Channel A. Thus, the

applied voltage, the emission current, and the gate current are all monitored simultaneously and can be observed at a glance.

SRI's three test sites, each having space for six cathodes, can all

be operated simultaneously; thus, as many as 18 cathodes can be tested

at a time. (Usually only 12 are tested at a time because of the characteristics of the vacuum systems and the number of power supplies

available.)

Under normal conditions, the cathodes are taken up to 20-mA peak

emission with the duty cycle reduced to one pulse in ten through the

duty cycle control. Operating at levels much higher than 10 mA with the

full 60-Hz driving signal and the usual +1200 V on the collector

overheats the collector (and subsequently the cathode, by radiation from

the hot collector). Collector temperatures to 900 0C have been observed,

caused by bombardment from the cathode at peak emission levels of about

20 mA and a 60-Hz driving voltage. These extreme temperatures can be

damaging and should be avoided.

B. Cathode Storage and Retesting

Historically the cathodes have been stored in ordinary aluminum

trays kept in plastic parts boxes, or wrapped in aluminum foil and

placed in glass bottles for shipping. An early test to determine the

suitability of these measures by retesting three cathodes that had been

stored in the plastic boxes for several months showed no significant

change in emission characteristics as compared to the original cathode

tests.

Difficulties experienced at NASA with cathodes that had been

shipped to the Lewis Research Center led to additional tests of cathodes

that had been stored in air. Table 1 shows the results of three successive tests with six cathodes. The cathodes were stored in aluminum

trays inside plastic parts boxes between tests. It is remarkable that

three of the cathodes showed no change in the second test--the same

result as our first three tests, many months before. However, the other

three showed significant change, and all six showed significant change

after the third test. Clearly a better storage system is needed.

A second test was done with cathodes that had been stored a short

time in air, but most of the time in a standard dessicator pumped out

with an oiless mechanical pump to - SO millitorr. There was no intentional difference in the way the cathodes were handled, stored, or

tested. The time between tests was much longer (6 months) than the time

between the tests on cathodes stored in air. The results were again

mixed, as shown in Table 2, with four out of nine cathodes that were

17
Table 1

RESULTS OF RETESTING CATHODES

AFTER LONG PERIODS OF STORAGE IN AIR

(Cathodes were hydrogen fired

at 6000C 1 /2 hour before each test)

Cathode

Test

Number

Voltage

( V)

Emission

(mA)

Time Between

Tests

(mo)

Total Tips

Blown

28A-140-2K 1 152 20 5

2 156 20 1 5

3 174 20 2 52

28A-140-2M 1 158 20 52

2 160 20 1 79

3 166 20 2 >200

28A-140-2N 1 156 20 22

2 157 20 1 22

3 160 20 2 70

28A-140-21 1 153 20 10

2 163 20 1 50

3 205 20 2 -300

28A-140-1J 1 151 20 9

2 150 20 1 9

3 98 0.001 2 10 failed

28A-140-1P 1 160 20 13

2 164 20 1 35

3 176 20 2 52

tested giving better performance in the second test and suffering an

insignificant loss of tips ( three or less). However, the remainder

showed either a large increase in the voltage required to reach 20-mA

emission or a large loss of tips, or both. In every case except one

(where a contact failed that had nothing to do with the cathode), the

cathodes reached the 20-mA goal, and this is encouraging. In fact, the

same can be said for the group that was stored in air, but the storage

times were much shorter, and the one failure was an odd short at very

low voltage and with very minimal damage. This is a very rare failure

mode.

The data clearly show that, although there are no overwhelming

problems, there is a trend toward increased failures with stor age time

and perhaps also with the number of tests or total operating time.

18

F
RESULTS OF RETESTING CATHODES AFTER STORING

IN DESSICATOR WITH ROUGH VACUUM FOR SEVERAL MONTHS

(Cathodes were hydrogen fired

at 6000C 112 hour before testing)

Cathode

Test

Number

Voltage

(V)

Emission

(mA)

Time Between

Tests

(mo)

Total Tips

Blown

20A-129-3B 1 118 20 0

2 215 20 10 7

20C-150-1G 1 No contact 0

2 205 20 6 80

20C-150-1B 1 162 20 0

2 140 20 6 2

20C-150-1C 1 164 20 1

2 155 20 6 2

20C-150-1D 1 164 20 4

2 155 20 6 7

20C-150-1F 1 164 20 28

2 162 20 6 30

2BA-140-20 1 121 20 9

2 158 20 6 >100

28A-140-2R 1 110 20 65

2 139 20 6 71

28A-140-2Q 1 110 20 67

2 No Contact 6

To help sort some of these variables, a third test was set up in an

appendage to our standard test chamber with a bakeable valve on it, so

the appendage could be sealed off with the cathodes and kept under

vacuum between tests. Of course, it is not possible to examine the

cathodes between tests when storing them in this way, but it is possible

to do several repeat tests to see if a large number of failures occur.

Table 3 shows the results of these tests. There is a distinct

reduction in the total number of tips damaged and improvement in the

consistency of performance from one test to the next with regard to the

voltage required to achieve a given current. The cathodes were in the

chamber for a total of almost 10 months and were taken up to 20 mA three

19

.+ -
Table 3

REPEAT TESTS, STORED IN SITU

Cathode

Test

Number

Voltage

(V)

Emission

(M)

Time Between

Tests

(mo)

Total Tips

Blown

20C-157-3B 1 100 20

2 95 20 1

3 92 20 1 1/2

4 60 0.01 3 1/2

5 60 0.01 3 0

20C-157-3C 1 116 20

2 109 20 1

3 103 20 1 1/2

4 60 0.01 3 1/2

5 60 0.01 3 5

20C-157-3D 1 90 20

2 90 20 1

3 91 29 1 1/2

4 60 0.01 3 1/2

5 62 0.01 3 4

20C-157-3F 1 95 20

2 118 20 1

3 139 20 1/2

4 58 0.01 3 1/2

5 60 0.01 3 8

20C-157-3H 1 100 20

2 90 20 1

3 89 20 1 1/2

4 Lost contact

5 3 (contact

repaired)

20C-157-3H 1 109 20

2 98 20

3 100 20

4 68 0.01

5 65 0.01 3

times during that period. These cathodes were taken out of the chamber,

Inspected, and immediately placed in a 1/2-inch-diameter copper pinchoff tube, which was pumped down on a vac-ion system and pinched off for

,j long-term storage in a sealed-off, evacuated container. The tube will

=-f be opened and the cathodes will be remounted in the test chamber at a

later date to test the effectiveness of this storage technique.

20

R <^ 'PlOW4110pbr ,s r'
CATHI

ACC

FLA

HIGH

VOLTAGE

SOW

C. The Emission Microscope

An emission microscope has been assembled and tested with good

results, following some early difficulties with high-voltage breakdown.

The design is shown in Figure 8. The original idea was to mount the

entire structure on one flange, including electrical contacts and

feedthroughs. in this way, the entire column could be worked on easily

by simply removing one flange from the vacuum system. However, this

design was troubled with electrical breakdown between the high-voltage

flight tube and the leads to the cathode and first lens. The electrical

feedthroughs for the first lens and cathode were then moved to a header

near the cathode end of the microscope, as shown in Figure 8. This made

mounting of the cathode a little more cumbersome; but, more important,

it eliminated the voltage breakdown problem.

FLIGHT TUBE PHOSPHOR 

FIGURE 8 EMISSION MICROSCOPE SETUP

Typical values for first trial: Gate: at ground,

Base: at -155 V, Emission: 200 pA total,

Lens 1: -175 V, Lens 2: + 5 kV, Tube: + 5 kV.

Cathode 25A-156- 5T was tested in the emission microscope. This

cathode had a 3 x 3 array of tips on 12.7-r:m centers. The entire system

was baked at 6000C for cleaning prior to turning the cathode on. The

cathode was first operated with our standard 60-Hz half-wave rectified

^. power supply and +1200 V on the flight tube and phosphor. The voltage

on the flight tube, second lens, and phosphor was then increased to

21
+5 kV and the cathode emission brought up to 250 μA with the 60-Hz

half-wave driving voltage. The cathode was then changed to a CW driving

voltage and brought up to emission levels in the 20 NA range. The gate

was at ground potential, and the tips were driven negative with the do

power supply.

A do potential of -130 V applied to the tips produced 20 μA of

emission. With 4.5 kV on the phosphor and second lens, -120 V was

required on the first lens to focus an image of the cathode on the phosphor. This is undesirable, because a negative voltage on the first lens

can create a problem with electrons returning to the positive gate film

under the influence of space charge effects when the cathode is operated

at high current-density levels. Bombardment of the gate in this way can

cause heating, desorption, and electrical breakdown and should be

avoided. However, an image was obtained, and a decision was made to

carry on as far as possible with this configuration and modify the system later, to see if operation with a positive potential on the first

electrode could be achieved.

The first image of the cathode showed three spots, one of which was

very bright compared to the other two. There was some radial flaring or

"beaconing" from the bright spot, but this effect was not as prominent

as has been observed in the NRL emission microscope.

As the driving voltage was raised to increase the total emission

level, the original three tips became brighter and others began to

appear. The emission was slowly increased to a total of 200 μA, where

the gate current reached an excessive level because of the negative

potential on the first electrode. At this point, spots could be seen on

the phosphor corresponding to seven of the nine tips. The original

three remained the most prominent throughout the eight days during which

the test was run. The three prominent tips took turns as the brightest

_ emitter of the group. Each tip went through a cycle of low initial

brightness, slowly increasing to maximum brightness, followed by a slow

decline to an intermediate brightness that seemed to be very stable.

Table 4 lists the total current from the cathode at various voltages and

the relative intensities of the images of the tips on the phosphor. The

intensity ranges from 1 to 10, with 10 being the brightest. The central

tip (number 5) was the first dominant tip and was always quite visible.

Tips 4 and 8 were also always visible, but not strong at the beginning.

All of the others, except 1 and 6, were visible at various times but

were not continuously visible. Number 9 was continuously visible after

it first appeared, but did not appear until the total current was up to

100 μA. It also continuously improved after first appearing and, much

like 4, 5, and 8, gave the impression that it would become dominant if

given the time. In other words, it would have eventually scored 10 if

the experiment had not been terminated.

As mentioned earlier, the first electrode required a negative voltage to achieve focus. This was unfortunate, because gate current due

to space charge effects rose to dangerous levels when the lens was

focused, and eventually a breakdown in the gun terminated the

22

3
Table 4

RELATIVE BRIGHTNESS OF THE CATHODE TIPS

DURING TESTING IN THE EMISSION MICROSCOPE

Date Voltage

Total

Current

Tip Number and

And Relative Intensity*t

(1983) (Volts) (uA) 1 2 3 4 5 6 7 8 9

6/10 130 20 0 0 0 1 6 0 0 1 0

6/13 140 30 0 1 1 2 6 0 0 2 0

6/14 155 90 0 1 1 6 10 0 0 4 0

6/14 150 100 0 1 1 6 10 0 0 4 1

6/15 150 70 0 1 1 5 8 0 0 4 1

6/15 160 130 0 3 3 8 10 0 1 8 3

6/15 158 160 0 0 3 6 5 0 0 10 3

6/17 158 230 0 0 0 10 5 0 0 5 4

6/18 155 200 0 4 4 10 5 0 0 5 4

*10 = very bright, 5 moderately bright, 1 = very dim.

trip positions: O O ^`

O O

O O O

experiment. The lens will be modified to reduce this effect, and additional tests will be made to see if more tips can be brought to maturity

on a similar cathode. The impression gained from this first test is

that the tips go through a turn-on sequence that goes from very dim

first emission to very bright to moderate. If the tip survives the very

bright phase, then it will be very stable and long-lived in the moderate

stage.

More tests will be done to determine the effects of various treatments on uniformity, stability, and other phenomena such as the "beaconing" or flashing effect seen here and to a larger extent at NRL. Huwever, the time at which this will be conducted is uncertain because of a

shift in project emphasis to demonstrating high emission current levels

in the CW mode.

23

^r
Nine tips emitting 200 μA CW is an average of about 20 MA/tip CW.

This is 5 times greater than the CW emission that would be required to

produce 20 mA CW in a 5000-tip array. At this level of average emission

(20 AA/tip), about 75 percent of the tips were contributing. More tests

should be made to see if this is typical, and to study techniques for

improving uniformity if necessary.

D. High-Power Tests

During this report period, there has been an emphasis on demonstrating an emission level of 20 mA in the CW mode with 3 kV on the

anode. The standard test system (Figure 7) cannot support power levels

of this kind so a new collector/anode structure has been built with

water-cooled electrodes to prevent overheating. Figure 9 shows the

WATER LINES

+1000

(COLLECTOR)

COPPER TRAJECTORIES OF ELECTRODES F-

PRIMARY AND

SECONDARY ELECTRONS

+3 kV

(ANODE)

CATHODE

GROUND (GATE)

200 V (BASE)

FIGURE 9 SCHEMATIC OF COLLECTOR SYSTEM TO REDUCE POWER

HANDLING PROBLEMS AT THE COLLECTOR

The system does not have rotational symmetry. The opening in

the anode is rectangular in cross section (a slot ), and the "VEE"

groves in the collector are long, straight grooves parallel to the

slot in the anode.

structure that was built for this purpose. It consists of two electrodes electrically insulated from each other and separately water —

cooled. The idea is to accelerate electrons emitted from a cathode to

!t

24
3000 V through an opening in the first electrode (anode) and collect

them on the second electrode (collector), which is at a reduced potential (- 1000 V). Secondary electrons produced at the collector will

return to the back side (collector side) of the anode and be collected

there. The electrons impacting the collector, where reflected primaries

can return to the cathode, are at 1000 eV of energy, which is no more

than is normally seen in the standard test setup with stainless steel

collectors. The higher-energy impact of secondaries on the back side of

the anode should not cause any difficulty because the impact area is

between the two water-cooled electrodes; any ions formed should merely

proceed to the other water-cooled electrode and not involve the cathode

in any way.

The total power that must be handled is 60 W, which is well within

the capabilities of water-cooled copper electrodes of this size. For

example, water-cooled electron-beam-bombarded evaporators used in the

laboratory routinely handle 1000 W and only rarely suffer a voltage

breakdown.

The first high-power tests were with two tubes having these watercooled electrodes, with the cathodes placed 5/8 inch from the anode.

For the initial cathode start-up, 1200 V was applied to both the collector and anode. The plan was to increase the voltage slowly to +3 kV

after 20 mA was reached. One cathode (20A-120-3D) had been tested in

our usual test setup with stainless steel collector tubes and low-dutycycle pulsing to 20-mA peak emission. The other (20B-137-2G) was one of

the new RIBE etched group and was not pretested. (At this time, none of

the RIBE group had been tested, so we were not yet aware of the poor

performance of this group caused by contamination problems.)

Cathode 20A-129-3D had performed very well in its initial test and

produced 20-mA peak current with 119 V applied between the base and

gate. There were no tip failures, and the gate interception was less

than 1 μA with 1200 V on the collector. However, the cathode had been

stored in a plastic parts box under laboratory air for over a year since

its initial test, and, as reported earlier, we have since found that

many cathodes do not do well after storage under these conditions.

Thus, it is not too surprising that 20A-129-3D shorted at 0.1-mA emission and, although we were able to clear it and achieve 10-mA peak eventually, it never behaved well.

Cathode 20B-137-2G also reached 10 mA, which was probably better

than we could reasonably expect in the light of the generally poor performance of that particular group of RIBE cathodes.

For the next trial, two new untested cathodes from a known good

group were mounted in the tubes (20C-157-3L and 20C-157-3M). The

cathodes were again 5/8 of an inch from the anode, and, for the initial

startup, the anode and collector were biased at +1200 V. Both tubes

were on the same vacuum system, with valves that could be closed to

maintain vacuum in the tubes if the system had to be opened for any

reason. 

25

f 

t

— a
Cathode 20C-157-3L was taken to 5 mA with our usual 60-Hz half-wave

rectified driving voltage. The emission was very stable during this

time, with no visible fluctuations on the monitoring oscilloscope. The

gate current was also very stable and was -7 μA-peak at 10-mA peak emission current. [minus gate current indicates electron flow from the gate

to the anode.) The emission was then shut down, and the anode was

increased to 3 kV while the collector was left at 1200 V). The emission

was then slowly increased.

At 10 mA there was, without warning, a very sharp, loud, arc in the

power supply, and the cathode shorted irreparably. Subsequent examination of the cathode showed a large area of the gate film had been melted

and blown away, including a large area outside of the active area.

There were also discharge marks on the gate hold-down disc. An

interesting feature in the damaged area was that many of the tips were

r undamaged, even though the gate film was melted away. It seemed as

though the arc was between the anode and gate, rather than tips and

gate.

A special power supply was assembled for slowly increasing the duty

- cycle at any given peak emission level. Figure 10 is a schematic of the

circuit. With the circuit shown, the emission can be brought to the

desired level with the usual 60-Hz half-wave rectified driving voltage.

Then the second half-wave of a full-wave rectified driving voltage can

be slowly brought up to the level of the first half-wave, while the

first half-wave is held at the desired value. Finally, a CW voltage can

be applied to slowly fill in the valleys between the full-wave rectified

driving pulses, and full CW operation can be approached while continually operating at the peak level with the 120-Hz pulse (full-wave rectified) driving voltage.

In the first test with this circuit, cathode 20C-157-3M was taken

to 20-mA peak, 60-Hz half-wave emission with 1200 V on both the anode

and collector. The voltage on the collector was then increased slowly,

while the emission was maintained at 20-mA peak, 60-Hz half -wave and the

anode was kept at 1200 V. During this time, the emission was very

stable and quiet, but the gate current was higher than we would like

(-30 μA). The emission was left at 20-mA peak, 60-Hz half-wave over the

weekend for 63 hours, with 3 kV on the collector and 1200 V on the

anode, during which time the emission was uninterrupted.

The second half-wave of the 60-Hz full wave supply was then slowly

brought up to 20-mA peak. Again, the emission current was very stable,

but the gate current was slightly erratic. The full-wave emission is

shown on the oscillograph in Figure 11. The emission current was

divided between the anode and collector, with 35 percent going to the

anode when the collector was at 3000 V and the anode was 1200 V. After

two hours of stable emission at 20-mA peak, 60-Hz full-wave emission, CW

voltage was applied to the cathode while the full wave emission was held

F at 20-mA peak.
DC SUPPLY

AC

SUPPLY

CHANNEL A DRIVING

(EMISSION) VOLTAGE

MONITOR

TIPS

GATE

ANODE

CHANNELS ANODE

(GATE) SUPPLY

HALF WAVE: ^^-

FULL WAVE: ` J

FULL WAVE ^Mf

PLUS DC:

FIGURE 10 COMBINATION HALF-WAVE, FULL-WAVE,

AND CW SUPPLY

The amplitude of each half-wave and the CW

can be varied independently.

The CW emission was brought up to 1 mA with the full —wave left at

20 mA. For 2-1/2 hours the cathode was stable under these conditions,

and then, without warning, it shorted. Examination showed that a devastating arc had left a large crater in the cathode, well into the silicon. The damage was quite different from that seen on 20C -157-3L, which

had no damage to the silicon.

Ideally all of the emission should go to the collector, rather than

have the large interception by the anode that we measured with these two

tubes. Therefore, for the next trial we moved the cathode very close to

27
0—

20 rnA —

GniLiA1 ,i1 `r;. " i^,'

Of POOR QUALITY

FIGURE 11 FULL-WAVE 60-Hz EMISSION

Tot) trace (emission). 5 mA cm Vertical, 5 ms cm horizontal

Bottom trace (gate current): 20 NA cm vertical, 5 ins cm

horizontal; Cathode 20C-157-3 M, Peat Voltage, 164 V,

Anode, 3 kV: Collector 1.2 kV.

the anode (- 1-1/2-mm spacing) in one tube, and in the second tube, we

added lenses to try to shape the beam so that it could miss the anode.

For this trial, rectangular-shaped emitter arrays were used, rather than

the usual circular array. The arrays are 100-μm wide and 6-mm long and

have about 3700 tips. These were used because in earlier experiments

this type of cathode had produced 50-mA peak emission with the 60-kiz

half-wave driving voltage rather routinely, and the ribbon-like geometry

is better suited to the rectangular geometry of the collector system.

Cathode 36C-149-5F was mounted in the tube with a lens meant to

keep the beam from hitting the anode. The lens system was not entirely

satisfactory, because when sufficient voltage was applied to the focus

electrode to steer the beam through the anode aperture, current to the

extractor electrode (closest to the cathode) and gate increased sharply.

Thus we were again forced to accept what turned out to be about 35 percent interception by the anode. Nevertheless, the cathode was well

behaved and gave very stable emission up to 20-mA peak, with the halfwave 60-11z driving voltage. There was one short at 12-au% peak, which

came without warning and was cleared easily. After clearing, the

cathode returned to the emission current voltage characteristic that it

had exhibited before the short. The cathode was operated for 5 days at

20-mA peak emission with the half-wave 60-Hz driving voltage. The emission was very quiet and stable, but the gate current varied from -14 μA

28

O
to -20 μA and had periods of noisy behavior. The second half-wave was

then phased in while the first was held at 20-mA peak. With both half

cycles at 20-mA peak, the emission was still very quiet and stable, but

the gate current was disturbingly erratic: The peak value varied from

-18 μA to -26 μA. After eight hours, the emission had not changed and

was still very stable. The cathode was left at this level overnight and

was found to be shorted in the morning. Attempts to clear the short 

were unsuccessful.

Subsequent examination showed that a group of about 150 tips had

blown, with large areas of gate film blown away, including a section out

of the active area. There was little damage to the silicon base or silicon dioxide.

The second tube had cathode 36C-149-5E mounted 1/16 inch from the

anode. This cathode was started with +1200 V on both the anode and collector. (Only one 3-kV supply was available, and it was being used on

the first tube at this point.) The cathode was brought up to 10-mA peak

with the half-wave supply. The anode was then increased to +3 kV, and

the second half-wave was phased in, while holding the first at 10-mA

peak. After the second half-wave reached 10-mA peak, the voltage was

increased on both half-waves together to increase the peak full-wave

emission to 20 mA over a period of two days. The emission was very

quiet and stable during this time, and the gate current was also stable

at -9 μA when the peak emission was 20 mA.

At this time the decision was made to attempt to achieve a power

` y level equivalent to 3 kV and 20 mA CW in the full-wave mode. The feeling was that this would be a good way to condition the tube at the

desired power level without having to introduce untried power su2plies.

The area under the emission trace on the oscilloscope (Figure 11) was

used to determine a pulsed equivalent of 20 mA CW power current level.

This turned out to be 42-mA peak full wave. The emission from cathode

_ 36C-149-5E was then increased from 20-mA peak full wave to 42-mA peak

Full wave over a period of four days. Again, the emission and gate

current were very quiet and stable. When the emission was at 42-mA

peak, the gate current was -20 μA as shown in Figure 12. The anode was

+3 kV and the collector was at +1200 V. The equivalent CW emission was

calculated to be 47 percent of the peak or about 20 mA.

While operating at 42-mA peak full-wave, a partial power supply

failure caused the anode voltage to drop to 1200 V. This, in turn,

caused the gate current to go from -20 μA to -50 μA because of the

effects of space charge at the cathode with the reduced accelerating

field. Fortunately there was no evidence of damage, and the emission

remained stable at 42-mA peak with 196 V applied to the cathode when the

+3 kV anode supply was restored. The anode supply failed again on the

next day, and shortly thereafter the cathode shorted. The emission was

stable and quiet up to the time of the sudden failure. The gate current

was also stable except during the anode supply failures.

29
0

40 mA

FIGURE 12 FULL-WAVE 60-Hz EMISSION, AVERAGE CURRENT

EQUAL TO 20 mA CW

Top trace (ernission). 10 mA/cm vertical, 5 ms!cm horizontal;

Bottom trace (gate current): 20 pA/cm vertical, 5 ms'cm

horizontal; Cathode, 36C-149-5E; Peak voltage, 196 V;

Anode, 3 kV; Collector 1.2 kV.

This result is very encouraging: The cathode produced the average

power required with peak current and power levels well in excess of the

CW goals. However, the fact that there was a sudden discharge after

several days of operation is disappointing, and at this time we still do

not know if we are dealing with a discharge that is initiates at the

cathode or at the anode.

We conducted additional trials with the usual 5000-tip arrays in

different configurations in the tubes with no different results. The

highest level of CW emission that has been achieved was with two

cathodes placed about 1/4 inch from the anode. The cathodes were 20C171-6C and 20C-171-6F. Both were baked at 310 0 0 for 36 hours and then

turned on after cooling to room temperature. 20C-171-6C was brought up

to 10-mA peak 60-Hz pulse with +1200 V on the anode and collector. At

that point, the gate current was observed to be -38 μ A, which was judged

to be higher than we would like.

The only way to reduce the gate current was to increase the collector voltage. We also want to operate in the CW mode, so the decision

was made to increase collector voltage to +2000 V and reduce the emission current to 1 mA CW. This is approximately the same power level

that we had with the 10-mA peak 60-Hz pulse, and 1200 V on the

30

0 
fI

collector. The water was also turned on at this time. The cathode was

slowly brought to 10 mA CW with +1200 V on the collectors and found to

be very stable. However, the gate current was again high, -27 μA.

The decision was made to slowly increase the voltage on the collector in order to reduce gate current while maintaining 10 mA CW. This

was done by increasing the collector voltage in 200-V increments and

 leaving it at each level for a few hours to see if any changes occurred.

The gate current diminished with each increase in collector voltage, and

at +2800 V on the collector the emission was unchanged at 10 mA CW and

gate current was down to -11 μA. The emission was left at 10 mA CW with

+2800 V on the collectors overnight, with a chart recorder monitoring

the emission. At about 22:00 hours, the emission suddenly ceased and

the cathode was found to be shorted in the morning. Figure 13 shows the

chart recording of the emission while the cathode was operating at 10 mA

CW and the voltage on the collector was being increased. The emission

was very stable and no sign of trouble could be detected from the emission current or the gate current. The emission had been at 10 mA CW for

about 30 hours when the short occurred, and the collector voltage had

_ been at +2800 V for about 10 hours.

The cathode (20C-111-6F) in the other water-cooled system was also

taken to 10 mA CW with +2000 V on the collector and left at that level

for 63 hours over the weekend. It had low gate current (- -3 μA at 10

mA CW with +2000 V on the collector), and the plan was to increase the

emission to 20 mA CW, or until the gate current increased to an uncomfortable level, before increasing the collector voltage. Unfortunately,

this cathode also shorted without apparent warning after about 40 hours

of emission at 10 mA CW. Figure 14 shows the chart recording of cathode

20C-171-6F operating at 10 mA CW prior to shorting. It was even more

stable than 20C-171-6C and gave no sign of difficulty before failure.

There were some problems with the power supply and one of the amplifiers

used for the chart recorder, but the cathode was very well behaved. The

oscillographs in Figure 15 show the stability of the emission from the

cathode under these conditions.

Both cathodes were cleared of shorts by the usual technique of

discharging a 1 μf capacitor charged to 20 V through the short. They

were operated only briefly after clearing because it seemed important to

see what had happened, and additional operation could result in more

damage that might obliterate important evidence.

Examination of the cathodes showed that the damage was relatively

limited compared to most of our prior failures at high power levels.

Cathode 20C-171-6C had four failure sites, involving a total of 51 tips.

Thirty-eight of the failures were within a single failure site, and it

was probably this area that resulted in the short. Cathode 20C-171-6F

had a total of seven failure sites--the largest of which involved about

16 tips (- 50 μm square).

31

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ORIGINAL PAGE IS

OF POOR QUALITY

.0 - - 'o ago,
In) HORIZONTAL 10 s/cm

0

10 mA

^^> mmmm

mommomm

mmmmmmm

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FIGURE 15 FMISSION I EVE L ASA FUNCTION OF TIME

FOR CATHODE 20C 171 6F

No1r very low low Iteyuent y rime anti esse.n1wily

no high-frequen(, y node

I i4

O
In both cathodes, roughly half of the tips that were damaged blew

out with a fairly energetic discharge that damaged both the gate and the

silicon substrate. The others seemed to be rather mild events and were

probably secondary failures resulting from the more violent events that

damaged their neighbors. The thing that is puzzling is that both

cathodes were very stable for waeks and at 10 mA Cwt for many hours and

then suffered damage without warning.

Three points are encouraging In reviewing these results:

• Both cathodes operated at 10 mA CW for many hours.

• The failure damage was less severe then with any previous highpower test--even though this is the highest CW level we used.

• If each of the damaged areas was the result of a single tip

failure triggering an event, about 99.9 percent of the tips

operated without failure.

E. H10 _';irrent-Densit Tests

Small-area cathodes have been fabricated and tested for use in the

emission microscope and to test high cathode loading without having to

deal with high power to the anode and collector system. Two geometries

have been used: an array of approximately 16 tips spaced on 12.7a

centers (6.4 tips

12.7-Ma

2 ) and an array of approximately 32 tips spaced on

9-;m centers (1.2 x 10 6 tips/cm2 ). (The number of tips on an individual

cathode often varies from 16 or 32 due to small differences in the diameter of masks used to define the active areas.)

Both geometries have consistently produced much higher cathode

loading than we have ever achieved with the more common 5000-tip arrays.

Table 5 shows the results of tests with a recent group of cathodes

fabricated for high-cathode-loading tests on another program. Eighteen

cathodes were tested in three groups: One group of six had a goal of 1

mA total emission; the other two groups had a target of 2.7 mA, which is

a current density of 100 A/c22 for a 32 tip array with 9-Mm spacing

between tips.

Ironically, the group that was meant to go to only 1 mA had twice

the failure rate of the two groups that were taken to 2.7 mA. All of

the cathodes are from the same substrate and were processed together. A

significant difference, however, was that the two groups that were taken

to 2.7 mA were placed in the test chamber the day after processing was

completed, while the group that only went to 1 mA was stored for 18 days

in an aluminum tray enclosed in a glass Petri dish (before being placed

in the test chamber). The petri dish was kept in a desiccator that was

pumped out with an oiless mechanical pump, but was opened several times

during the storage time. This is additional evidence that the cathodes

-` should always be stored in vacuum, such as in the pinch-off tubes

- described in Section III-B.

35 
Table 5

EMISSION TESTS WITH SMALL AREA EMITTER ARRAYS

USING 60 Hz PULSE

Cathode

Number*

Number

of Tips

Peak

Emission

(mA)

Peak

Voltage

(V)

Peak

J

(A/cm2)

Final

Result

29C-177-3A 32 0.45 175 17 Shorted

29C-177-3B 32 2.7 250 i00 OK

29C-177-3E 35 2.7 280 92 OK

29C-177-3F 24 2.7 290 135 OK

29C-177-3F 24 1.7 CW 260 CW 85 CW OK

29C-177-3G 43 2.7 270 75 OK

29C-177-3H 32 2.7 300 100 OK

29C-177-3I 37 2.7 310 87 OK

I

29C-177-3J 20 1.0 185 60 OK

29C-177-3K 31 1.0 192 39 OK

29C-177-3L 32 1.0 210 38 OK

29C-177-3M 32 0.01 140 0.4 Shorted

29C-177-3N 32 1.0 190 38 Shorted

29C-177-30 40 1.0 210 30 Shorted

29C-177-3Q 32 2.7 292 100 OK

29C-177-3R 32 0.5 219 19 Shorted

29C-177-3S 20 0.5 246 30 Shorted

29C-177-3T 29 11.5 368 475 OK

29C-177-3V 32 2.7 294 100 OK

* Cathode 29C-177-3F operated at 85 A/cm2 CW and

29C-177-3T operated at 475 A/cm2 with a pulse drive.

Two of the cathodes in the group that was taken to a minimum of

2.7 mA were subjected to extraordinary tests. One (29C-177-3F) was

tested up to 135 A/cm 2 peak with the usual 60-Hz pulse drive and

then switched to CW and taken to 1.7 mA CW, which is 85 A/cm^ CW.

The cathode had 24 tips, so it was operating at a tip loading of

36
ORIGINAL PACT IS

OF P00? 0'' `-!1Y

70 MA CW/tip. This is almost 20 times higher than the tip loading that

would be required to produce 20 mA CW with a 5000-tip array. These

tests were done in the standard stainless-steel tube collector

apparatus, so it was not possible to check performance at 3-kV

accelerating voltage.

A second cathode (29C-177-3T) was taken to 11.5 mA in the 60-Hz

pulse mode. This is a peak current density of 475 A/cm 2 , and is the

highest loading level measured to date. The cathode has 20 tips, so the

average tip loading was about 575 μA/tip. The 11.5-mA peak 60-Hz pulse

is the maximum level that can be reached with the 3/16-inch diameter

stainless-steel collector system, and the collector was hot enough at

this point to cause a temperature rise measured on the outside wall of

the vacuum vessel from 24 0 C to 30oC.

Figure 16 is an oscillograph of the applied voltage and the emission current as a function of time for the cathode, and Figure 17 shows

the data plotted on a Fowler/Nordheim plot. There are no obvious nonlinearities that can be attributed to the very-high-current-density

operation of the cathode. The voltage on the collector during the test

was +1200 V.

7

FIGURE 16 CATHODE 29C-177-3T OPERATING IN THE 60-Hr PULSE

MODE AT 10 8-mA PEAK EMISSION CURRENT 1450 A crnz)

Collector voltage was + 1200 V. Total number of tips on the

cathode is 29. Top: 2 mA cm, Horizontal 5 ms/cm,

Bottom: 100 Wcm.

37

r

'^
6.6

7.0

/ 416 A/cm2

7.6 •

•

•

8.0

•

s.5

9A

•

9.5

OA

0.51

300 350 400 460 500 550 600

105/V

FIGURE 17 FOWLER/NORDHEIM PLOT OF EMISSION DATA

FOR CATHODE 29C-177-3T OPERATING UP TO

10-mA PEAK EMISSION (415 A/cm2)

It is remarkable that these small-area cathodes can operate at

total emission levels of 10 mA, which is equivalent to that obtained

with the 5000-tip arrays. It is especially interesting when one considers that many 5000-tip arrays have suffered damaging arcs at the 10-ak

level of emission. Again it seems that total power into the collector

is the major limiting factor in the results, rather than a limit on the

cathode loading capabilities.

N

t7

0

J

-1

38
IV SUMMARY AND CONCLUSIONS

i This phase of the cathode development program has been directed

toward increasing cathode array uniformity, improving cathode hole

geometry, conducting preliminary studies of alternate cathode materials,

' performing emission microscope studies, studying storage techniques,

testing high-power CW performance, and conducting high-cathode loading

tests.

An upgraded multiple electron beam exposure system has been adapted

to the cathode fabrication process and has produced arrays of holes with

improved uniformity of size and increased packing density. Arrays having 1.0 x 107 holes / cm2 were demonstrated and hole size variation was

about + 1.5 percent.

- Anisotropic etching techniques based on reactive ion etching have

been shown to produce holes in the silicon dioxide layer with no detect-

_ able undercutting of the molybdenum gate film layer. This produces a

more robust structure because of greatly improved gate film support and

allows us to fabricate higher hole packing densities.

A materials study has shown that several metal carbides are very

attractive cathode materials. Of these TiC, ZrC, NbC, HfC, and TaC are

particularly interesting because they have very high melting points, low

vapor pressure, and low work functions relative to molybdenum. They

also tend to have a low capacity for gas adsorption, which is important

for electrical discharge prevention.

The conditions under which the cathodes are stored have been shown

to have an important effect on their performance. Storage in vacuum,

preferably in sealed-off tubes, has been shown to be the best method for

long-term storage.

An emission microscope has been assembled and tested. The tests

demonstrated that the lens should be modified so that an accelerating

field is formed at the surface of the cathode to prevent excessive gate

current during operation. A nine-tip cathode array was imaged with a

total emission current of 200 μA CW. This is an average tip loading of

about 20 μA/ tip, which is five times the tip loading required to produce

20 mA CW with a 5000-tip array. Seven of the nine tips were seen to

form images on the phosphor: about 75 percent of the tips were contributing to the total emission.

r••'a High-power tests with 5000-tip arrays were made with a goal of 20

^., mA CW into an anode at 3 kV. Water-cooled electrodes were fabricated

for the tests. The 20-mA CW goal has not been achieved, but 42-mA peak,

120-Hz ( full-wave rectified 60-Hz) pulse emission was demonstrated with

39
collectors at 3 kV. The 42-mA peak emission at 120 Hz is equivalent in

power to 20 mA CW. When using pulsed driving voltages, 20-mA peak

{ current is routine, but the highest CW current that has been achieved to

date is 10 mA.

Very high cathode loadings have been demonstrated using small

arrays of tips y p (- 10 to 30 tips). With these small arrays, pulsed

currents averaging 100 MA/tip are common and current densities averaged

over the total area of the array are commonly in excess of 100 A/cm2.

CW emission current densities of 50 A/cm 2 have been achieved many times.

These values are more than an order of magnitude greater than what is

needed to achieve 20 mA with the 5000-tip arrays.

w,

40

^
References

1

Adachi, H., et al. 1983: "Stable Carbide Emitter," Appl. Phys. Lett.,

Vol. 43, No. 7, p. 702.

Brodie, I., 1975: "Bombardment of Field-Emission Cathodes by Positive

Ions Formed in the Interelectrode Region," Intl J. Electron.,

Vol. 38, No. 4, pp. 541-550 (April).

Brodie, I., E.R. Westerberg, D.R. Cone, J.J. Muray, N. Williams, and

L. Gasiorek, 1981: "A Multiple Beam Exposure System for HighThroughput Direct-Write Submicrometer Lithography," IEEE Trans.

Electron. Devices, Vol. ED-28, pp. 1422-1428.

Burgess, R.B., H. Kroemer, and J.M. Houston, 1953: "Corrected Values of

Fowler-Nordheim Field Emission Functions v(y) and t(y)," Phys.

Rev., Vol. 90, No. 4, pp. 173-181 (May).

Darydova, E.I., et al., 1979: Sov. Phys. Tech. Phys., Vol. 49, No. 11,

p. 1307.

Dyke, W.P., and V.W. Dolan, 1956: Advances in Electronics and Electron

Physics, Vol. 8, p. 98 (Academic Press, New York).

Eisinger, J., 1958: J. Chem. Phys., Vol. 20, p. 1154. 

Flamm, D.L., and V.M. Donnelly, 1981: "Design of Plasma Etchants,"

Plasma Chemistry and Plasma Processing, Vol. 1, No. 8, pp. 317-362

(December).

Fowler, R.H., and L.W. Nordheim, 1928: "Electron Emission in Intense 

Electric Fields," Proc. Royal Soc. (London), Vol. A119, pp. 173-181 

(May)

Kohl, W.H., 1967: Handbook of Materials and Techniques for Vacuum Devices, (Reinhold, New York).

Samsonov, G.V., and I.M. Vinitskii, 1980: Handbook of Refractory Compounds (IFI/Plenum, New York).

Spindt, C.A., I. Brodie, L. Huu3hrey, and E.R. Westerberg, 1976: "Physical Properties of Thin-Film Field Emission Cathodes with Molybdenum

Cones," J. Appl. Ph,.., Vol. 47, No. 12, pp. 5248-5263 (December).

Stroms, E.D. 1967: The Refractory Carbides (Academic Press, New York).

41
Appendix

EMITTER CHARACTERISTICS AND TEST DATA

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