MICROPROCESSOR CONTROLLED REMOLDING OF FIELD EMITTERS

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MICROPROCESSOR CONTROLLED REMOLDING OF FIELD EMITTERS

F. Hasselbach, M. Nicklaus, K. Zeuner

To cite this version:

F. Hasselbach, M. Nicklaus, K. Zeuner. MICROPROCESSOR CONTROLLED REMOLD- ING OF FIELD EMITTERS. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-453-C7-458.

�10.1051/jphyscol:1986776�. �jpa-00225971�

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MICROPROCESSOR CONTROLLED REMOLDING OF FIELD EMITTERS

F. HASSELBACH, M. NICKLAUS and K. ZEUNER

Institut fur angewandte Physik der Universitat Tubingen, _Aufder Morgenstelle 1 2 , 0-7400 Tubingen, F.R.G.

Abstract

A fully automatic, microprocessor controlled remolding system is presented. I t has been tested with <loo>-oriented tip-shaped tungsten field electron emitters. Many of its features a n d capabilities wouldibe equally relevant for other types of field electron emitters or in t h e context of field ion emission. I t can produce with high reliability a single spot emission p a t t e r n , which is most frequently desired because i t normally represents highest brightness. Field emission tips t h a t were in such bad conditions t h a t they would normally have t o be replaced could b e restored t o good emission by t h e system. Some results on t h e remolding process itself which were obtained through t h e use of t h e remolding system are presented.

1. I n t r o d u c t i o n

Field emission electron guns with pointed emitters fabricated from single crystal tungsten wires electrolytically etched t o a very fine tip can reach t h e highest brightness values (> 1 0 ~ ~ / c r n ~ s r ) known. However, a brightness a s high a s this is not normally achieved with a newly etched tip.

Moreover, t h e brightness often deteriorates after a certain operation time. In these cases, t h e t i p can be restored t o good emission conditions by a so-called remolding process, achieved by t h e simultaneous application of a high electric field a n d high t e m p e r a t u r e t o t h e tip / I / .

However, u p t o now remolding facilities are not used in commercial instruments d u e t o t h e rather cumbersome and unreliable application of t h e process when performed manually. These diffi- culties have been described in detail in a previ- ous paper / 2 / . In t h e s a m e paper we presented a microprocessor aided remolding system. T h i s system has now been developed into a fully auto- matic microprocessor controlled remolding sys- tem.

It h a s been tested with <loo>-oriented tungsten t i p emitters. However, many of its features and capabilities would b e equally relevant for other types of field electron emitters or in t h e context of field ion emission.

2. T h e m i c r o p r o c e s s o r r e m o l d i n g s y s t e m ( h a r d w a r e )

A block diagram of t h e system is shown in Fig. 1.

All components are controlled by t h e microprocessor, which is a t present a 16 bit microprocessor Intel 8088. Any o t h e r suitable processor could b e used a s well. Two externally pro- grammable high tension cascades provide t h e remolding and field emission voltages resp.. Both cascades a r e controlled by t h e microprocessor via 12 bit D/A-converters. T h e actual switching of t h e high voltages t o t h e field emitter is performed by two high voltage Reed-relays. T h e t o t a l field emission current is measured by a sensing resistor in t h e extraction voltage lead.

T h e resulting voltage d r o p is amplified a n d then

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986776

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C7-454 JOURNAL DE PHYSIQUE

DEFLECTION

FAWAY

FLUORESCENT SCREEN

Figure 1: Block diagram of the remolding system.

transmitted via a voltage-to-frequency converter, a high voltage opto-coupler a n d a frequency- to-voltage converter t o a n 8 bit AID-converter a t t h e input port of t h e microprocessor.

T h e emission p a t t e r n c a n b e observed o n a fluorescent screen. A Faraday cage is situated be- hind a probe hole in its center. T h e emission p a t t e r n is scanned across t h e Faraday cage by two deflection coil pairs (for x a n d y deflection).

These a r e driven by t h e microprocessor via two 8 bit D/A-converters a n d two current amplifiers.

T h e current in t h e Faraday cage is amplified and read into t h e RAM of t h e microprocessor after digitalization by a n 8 bit AID-converter.

T h e high t e m p e r a t u r e of t h e tip required for t h e remolding process is produced by resistive heating of t h e hairpin filament carrying t h e tip. T h e heating current is generated by a current source a t t h e same potential a s t h e emitter, fed by a 12 V accumulator. It is controlled by t h e microprocessor via a high voltage opto-coupler.

3. Operation of the system (software) T h e microprocessor controls t h e remolding process by varying t h e remolding voltage and t h e t e m p e r a t u r e course of t h e tip. As t h e process is more reproducible when t h e tip is heated by a

sequence of s h o r t current pulses / 3 / , t h e micro- processor usually generates a sequence of pulses of variable duration and interval, which drive t h e heating current source. T h e microprocessor controls t h e field emission by varying t h e extraction voltage.

A feedback mechanism is achieved by feeding t h e values of t h e t o t a l field emission current a n d t h e current in t h e Faraday cage t o t h e microprocessor, thus providing it with a knowledge of t h e current s t a t e of t h e process. In order t o perform a n evaluation of t h e emission p a t t e r n , t h e microprocessor scans t h e pattern across t h e Faraday cage. This is done stepwise by discretely varying t h e current in t h e two deflection coil pairs, controlled by t h e microprocessor via two 8 bit D/A-converters which drive two current amplifiers. T h i s scanning procedure yields - in t h e present s t a t e of o u r system - a digital representation of t h e field emission p a t t e r n with 32x32 picture elements ("pixelsn), with a quantization of 8 bit each. This is read into t h e memory of t h e microprocessor and can then be evaluated with p a t t e r n recognition 'algorithms or in some other way.

A local a r e a network connection t o other com- puters (e.g. a P D P 11/40) is provided in o u r sys- t e m . T h i s enables one t o further process (plot

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SWITCHING OF THE PEED RELAYS, FIELD EMISSION VOLTACE OFF AEIIOLDING VOLTAGE ON

RE3OLDING (25 HEATING PULSES)

< 3 K V ?

Figure 4: Flowchart of a remolding program.

etc.) or t o s t o r e t h e d a t a , if this is desired. I t must b e noted, however, t h a t t h e remolding system is a n independent system t h a t does not re- quire this feature for correct functioning.

T h e fact t h a t all components of t h e system a r e controlled by t h e microprocessor renders possi- ble remolding procedures which a r e by f a r t o o complex or wearisome, o r need t o o fast reactions t o b e performed manually. Furthermore, t h e use of a microprocessor a s t h e main control element provides a virtually unlimited flexibility of t h e system by simply writing o r changing programs.

T h e programs we have been mainly working with a n d which we want t o present here all search for a single s p o t emission p a t t e r n , which normally represents highest brightness. T h e y perform a

cyclic remolding process t h a t is repeated until this p a t t e r n is reached. T h e y typically proceed in t h e foliowing way: Before t h e first remolding cycle, t h e t i p is rounded by heating it without any voltage applied. T h i s produces well-defined initial conditions of t h e t i p which have proven t o b e crucial for t h e further course of the remolding process. T h e n a remolding cycle is started.

O n e cycle h a s t h e following typical course: T h e tip is heated by a sequence of typically 25 pulses of 1 4 ms duration a n d 100 ms interval with ap- prox. 6 A , t o a t e m p e r a t u r e of a b o u t 2500 K. A high positive voltage is simultaneously applied t o t h e tip, which produces forces acting on t h e surface a t o m s of t h e e m i t t e r in t h e direction of t h e t i p apex. T h e remolding voltage applied in t h e first remolding cycle is usually t h e highest one of all cycles (see below). After comple-

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C7-456 JOURNAL DE PHYSIQUE

tion of this remolding period t h e t i p is switched t o field emission. T h e negative extraction voltage is increased slowly by t h e microprocessor, until a predefined t o t a l field emission current (usually 3 PA) is reached. T h e n t h e microprocessor s t a r t s t h e scanning of t h e emission pattern across t h e Faraday cage and reads in t h e current values of t h e pixels successively. T h e

Figure 2: Field emission pattern (top) and its digital representation on an oscilloscope (bottom). The slight difference is due to the variation of the field emission during the time delay between the registra- tion of the two pictures.

digital representation gained in this way (see Fig. 2) is now examined for the numbers of emission s p o t s , which is essentially done by counting the number of intensity increases in t h e whole p a t t e r n in various directions. If only one single emission spot is detected, t h e microprocessor stops t h e process a n d gives, for example, a mes- sage t o t h e operator. A timing diagram of t h e remolding cycle is shown in Fig. 3.

Usually, however, t h e first cycle does n o t yield a single s p o t emission p a t t e r n . In this case, t h e microprocessor repeats t h e whole remolding cycle with subsequent evaluation of t h e field emission p a t t e r n under different remolding voltages until t h e desired p a t t e r n is reached.

OFF HEATING CURRENT /A

t

VOLTAGE /KV FIELDEMISSION

RENILDING VOLTAGE /KV

7.0 6.9 - I

Figure 3: Timing diagram of a remolding cycle.

O u r investigations showed t h a t t h e single s p o t p a t t e r n is obtained with t h e highest probabil- ity when t h e remolding voltage is very high in t h e beginning and then lowered successively af- t e r each remolding period. In this way one produces an "overremolding" first, which is worked down t o the single spot p a t t e r n in t h e following remolding cycles. T h e remolding voltage used in t h e first cycle is typically 7 kV and is t h e n lowered in steps of a b o u t 50 V . O n e entire cycle lasts for a b o u t 17 sec in the normally applied programs. T h i s time is for t h e most p a r t due t o t h e duration of t h e physical processes involved (cooling down intervals etc. 121). A flowchart of a typical program is shown in Fig. 4.

4. Results

W i t h t h e programs described, a single spot emission p a t t e r n was usually reached with < l o o > - oriented tungsten t i p s a t a final remolding voltage of 4-5 kV in 15-20 minutes. This t i m e is determined by t h e duration of a single cycle a n d t h e value of t h e remolding voltage steps (from which follows t h e number of t h e cycles performed). A sequence of emission p a t t e r n s

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spot p a t t e r n is reached within one program run in most of t h e cases, if one s t a r t s from initial tip conditions t h a t fall i n t o t h e usual, though wide, range of t i p conditions (e.g. deteriorated emission after long operation). However, if the whole program was applied several times (which can, of course, again b e programmed), even very poorly etched or badly damaged tips could b e restored t o good emission conditions. T h i s procedure, which may take u p t o several hours, is very unlikely t o b e performed manually

-

t h a t is, t h a t tip would normally have t o b e replaced.

MAX. CURRENT DENSITY (ARBITRARY UNITS)

Figure 5: Sequence of emission patterns a t decreas- ing remolding voltage recorded during one program

0

. .

7.0 6.0 5.0 REMOILDING

VOLTAGE / KV

Figure 6: Current density vs remolding voltage.

FIEU) EMISSION VOLTAGE / KV

0.0

7.0 6.0 5.0 RENlDlNG

VOLTAGE /KV

Figure 7: Extraction voltage for a field emission current of 3 PA, recorded during one program run.

T h e increase in current density in t h e bright- est s p o t of each subsequent emission p a t t e r n vs remolding voltage, a s recorded during one pro-

run. gram run, is plotted in Fig. 6. T h e last - a n d

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C7-458 JOURNAL DE PHYSIQUE

highest - value corresponds t o t h e single s p o t emission p a t t e r n , a t which t h e microprocessor has stopped t h e process.

T h e a u t o m a t i c course of t h e process m a d e it pos- sible t o obtain a great a m o u n t of d a t a a b o u t t h e remolding process. O n e interesting result t h a t was gained from these d a t a is illustrated in Fig. 7: When t h e remolding voltage is lowered successively, t h e extraction voltage for a given field emission current decreases continously for most of t h e time, b u t always shows a slight increase a s h o r t t i m e before t h e single s p o t pattern is reached. T h i s could eventually b e used as a n (additional) criterion for gauging success- ful completion of t h e remolding process, e.g. if t h e emission p a t t e r n itself cannot b e registered for technical reasons.

T h e system is made most flexible through t h e use of a microprocessor a s t h e control element for all p a r t s of t h e system. T h e evaluation of t h e field emission is, a t present, performed in o u r system by a simple form of image processing applied t o t h e emission p a t t e r n . However, t h e microprocessor can, of course, b e programmed t o t a k e other parameters (e.g. t h e emission current or t h e course of t h e extraction voltage) a s a measure for t h e quality of t h e field emission achieved, in cases where t h e emission p a t t e r n is not conveniently accessible, as, for example, in an electron microsope. Methods of solving t h e problems caused by t h e high acceleration poten- t i a l when creating a n electronic environment for a field emission gun in a n electron microscope a r e well known 141.

5. Conclusion

A fully automatic, microprocessor controlled re-

/I/

I . L. Sokolovskaia, J. Tech. Phys. (USSR), molding system has been described. I t h a s been 26(1956), 1177

tested with <loo>-oriented tip-shaped tungsten

field electron emitters. I t produces t h e single / 2 / F. Hasselbach and M. Nicklaus, J. ~ h y s . E:

s p o t emission p a t t e r n , which is most frequently Sci. Instrum., 17(1984), 782

desired because it normally represents highest / 3 / R. Speidel a n d F. Vorster, Optik 42(1974), brightness, with high reliability. Even field emis-

sion t i p s in such b a d conditions t h a t they would 383

normally have t o b e replaced can b e restored t o / 4 / H. P i n n a , K. Liang, M. Denisart and B.

good field emission. Jouffrey, Revue Phys. Appl. 18(1983), 659