APPENDIX I

KLHCTROHICS FOR MAGIHG ATOM PROBE

1.1 ZHTRODDCTIOH

The major limitation of PIM lies in its inability to chemically

identify the imaging species. The atom probe PIM^ can give us the information about the chemical nature of a single atom (or a group of atoms) on the specimen tip surface. Panitz '-^ introduced a new technique known as Imaging

Atom Probe (lAP), which gives the spatial distribution of the surface species.

It mass analyzes the field evaporated species from the specimen surface on the basis of time of flight mass spectroscopy. The lAP, which is essentially a field desorption microscope with time gated microchannel plate (MCP) detector, is a powerful tool for studying a number of metallurgical problems.

The electronic circuitry built in this laboratory for the lAP (figure

1.1) consisted of (i) two high voltage nanosecond pulsers, one for field evaporation from the tip and another for MCP activation, (ii) a variable delay network, and (iii) a master pulser to synchronize the whole operation. The two high voltage pulsers were identical, except the output pulse amplitude. Each of them comprised of the Krytron pulser and the avalanche pulser used for grid triggering the Krytron. The combination of the master pulser and the variable delay network was used to get the two pulses having an adjustable time interval between them. These pulses were used to tri^er the avalanche transistor pulsers. Thus, the high voltage pulse for field evaporation and the

MCP activation pulse after a presettable time delay "t" were obtained. The time gated images of the desired species are obtainable by deciding the delay time from following equation

73 CRYOSTAT ChteVRON MICROCHANNEL ^ PLATE - SCREEAi

'4

;5

^ lOM —WWW -• +5 kV ^Vdc -vv5/J^ -• + 1-3kV 50M -VWWw -• + 0-3 kV 500 pP 5wi i:_--500p F

X

KRYTRON KRYTRON PULSER PULSER

AVALANCHE VARIABLE AVAL/INCHE TR-ANSISTOR TRANSfS TOR PELAY PULSER PULSER NETWORK

M/^STER

PULSER.

FIGURE 1.1 The schematic of Imaging Atom Probe (lAP) FIM. d^ / ^ t = M 2eV

where t = time delay (sec),

d = tip to screen distance (m) ,

m/n = mass to charge ratio of the desired species (amu per unit charge), e = electronic charge (coulomb)? V = accelerating voltage (volts), dc + pulse. The detailed working of the circuitry is explained below.

1.2 DESCRIPTIOir OP ELBCTROHIC CIRCDITRY

1.2.1 MASTER PUI^ER AND VARIABLE DELAY RETWORK

The master pulser (figure 1.2) produces the astable multivibrator pulses of 5V amplitude and 50 (or 100) Hz frequency with pulse rise time better than 70 nS. The manual override provides the facility of "single pulse" operation. The output pulses of the master pulser drive the variable delay network (figure 1.3). It is based on the operation of the edge triggered monostable multivibrators. The emitter follower stages are provided at the outputs. This circuit finally produces two output pulses of 12V amplitude. The time interval between these two output pulses is decided by the setting of the 10 turn helical (10 kji.) and the timing (10 pP to 5 nP), and can be adjusted in a range 50 nS to lOjuS. The timing are polycarbonate type which have very low temperature drift and small aging effect.

The output pulses, from the combination of the master pulser and the variable delay network, trigger the two very similar high voltage pulsers.

1.2.2 AVALANCHE TRANSISTOR PULSER

The first stage of the high voltage pulser is the avalanche transistor pulser, which is basically a "marx bank". The principle of marx bank is to

74 *|_ MANUAL l' OVERRIDE

DEBOUNCE CIRCUIT

ASTA&LE X I^ANUAL MONOSTA&LE MULTI - • ^/n fyiULTI - " /' - VIBRATOR -\/IBRATOR AUTO

FIGURE I. 2 : The Master Pulser.

+ Ve EDGE niFFBREN- TRI&GEReD AMPUFIEf^ s • "/ /p • -riATOR. > • /p

• 1 r

-Ve ELGE -•-Ve EDGE TfilG&ER£l> TRIGCERED APnPLlFIER X ^ ^ '^ M MV

CT

FIGl JRE 1.3 : The v;ariabl e Delay Network charge the capacitors in parallel, and then by means of the spark gaps connect them in series to get the voltage multiplication. Only one is required to be triggered, remaining spark gaps breakdovm due to excessive over voltage. The marx bank using spark gaps, however, does not provide a reliable operation below 1 kV. We have, therefore, used the avalanche in the place of spark gaps. Figure 1.4 shows the the circuit of the avalanche s transitor pvilser which is a transistorized version of the marx bank. This type of configuration allowed the dc isolation of the transistors'^ and eliminated the danger of mass failure of transistors as in the case of series string arrangement-^. This circuit avoids the use of pulse , thus minimizing the danger of spontaneous pulsing. The transistors (2N 5019 or 2N 3020) used in this circuit were carefiilly chosen. The circuit shown in figure

1.5 was used for finding out the breakdown voltage BVQJJ^ for the transistors.

Those transistors having BVQJJ^ in a narrow range 160 + 10 V were selected. These transistors were subsequently subjected to the "burn in" test^, using the circuit shown in figure 1.6. After operating the transitors in avalanche mode in this circuit for about 40 hours, the BVQ-gpj of the transistors were again found out. Only those transistors which did not show any significant change in BVQ-gp were finally chosen. The (PCB) for the avalanche transistor pulser was carefully designed following the standard norms for designing high frequency and high voltage circuits. The PCB design was found to have strong influence on the quality of the output pulse. The supply voltage of the avalanche transistor pulser was adjijsted to a value just below the voltage at which the entire chain starts self oscillating. Figure 1.7 shows the output waveform of the avalanche transistor

75 1^

u o a. O -v/ww- O- h. 7V -I ^h ^^ O -WWVv- -vV»A\VV^ r"

CD

a. O o -P o CO 1 •H IP 0) > in o -wwwv- O EH in yi O 0 lo s:o -AWW- ^ o 2 o

01 -WVAAAr- x; 0 EH ^ 5

-vWVW- ou .00- -in 0 -VA/WV- o •Vs/WW ZA O eg C5 ^ O I—I o •O [in 10 •10

a, O o O GJ -\AAM/V- /-

u. •^WWVAr-

.1 V>

H • V ( VARI/^BLE) OTO 300V

IM

_, TO OSCILLOSCOPE + IIV I 1 _, SOOpF 2N3020 ''-^:Hy SOO pF T • .. BYIZ7 N. i^s ^SO SOHj.

/////>

FIGURE 1.5 : The test circuit used for measuring BVQJJ^ of the trprislstors.

f '^*2.-A kV

10M 500,

77777} 8-a k

•> TO OSCILLOSCOPE

Ql2 47 a

Q II 47

Q1 Q12 -^ 2M5020

+ 4S Hh nli IOOHJ >• so (0

77777 FIGURE 1.6 : The circuit u£5ed to carry out the "burn-in" test. J FIGURE 1.7 : Output waveform of the avalanche transistor pulser piilser recorded using the storage oscilloscope . The output pulses were found to have (i) capacitor discharge type pulse shape, (ii) amplitude of about 800

V, and (iii) rise time less than 5 nS. The time delay between the input trigger pulse and the output pulse of the avalanche transistor pulser was found to be approximately 10 nS with negligible jitter.

1.2.3 KRYTRON PUIfiER

The second stage of the high voltage pulser is a Krytron pulser.

Krytron used as a high voltage has a distinct advantage of achieving well synchronized operation over other types of high voltage namely, mercury wetted ' and spark gap°. Krytron KN-22 " (E.G. & G., USA) is a

^s filled tube with specially designed . The circuit diagram of the

Krytron pulser has been given in figure 1.8. A transmission line was charged to a voltage twice the desired amplitude of the output pulse, in the circuit of the tube. The "keep alive" current of about 200 xiA was allowed to flow in the circuit. This keep alive current keeps the environment inside the tube "ready" for breakdown. The tube also contains a small amount of radioactive material which assists the plasma build up during "turn on".

The output pulse from the avalanche transistor pulser was applied to the grid to trigger the Krytron into conduction,causing transmission line to discharge in the matched load (Zj^ = ZQ, the characteristic impedence of the transmission line = 50 Ji-). The load resistance R-^ was chosen 39-n-, because the Krytron offers impedence of about 12-fL in the conduction state. Figure 1.9 shows the

The storage oscilloscope facility [100 MHz, Tektronics, U.S.A., Model No.

7633 and 100 X high frequency compensated attenuator probe, Tektronics,

U.S.A., Model No. P6009] was kindly made available by the Head, Plasma Physics

Division, Bhabha Atomic Research Center, Bombay.

76 + HV

50 M

€ 3 R^8U rill/ r KNa2 ^/p + 200V ® TH^^H(F s-'ik: ^^KA 82.k C 3x BY ia7

IniT O/p

31

FIGURE 1.8 : The Krytron Pulser. FIGURE 1.3 : Output waveform of the Krytron pulser. output waveform of the Krytron pulser. It is reported that , if the Krytron is grid triggered by a pulse having rise time less than 10 nS, the output pulse given "by the Krytron has rise time of less than 1 nS -'. In our case, the grid trigger pulse had rise time less than 5 nS, the Krytron pulser output piLLses, therefore, were thought to have rise time less than 1 nS. Due to the limited band width of the oscilloscope and the attenuator probe, however, it was not possible to record properly the rising and falling portions of the Krytron output pvilse. These pulses were found to be free from overshoot, undershoot, and ringing.

The output pulse width of the high voltage pulser was solely determined by the length of the co-axial cable used as the transmission line.

About 1 meter cable (RG 8/U) length was found to give pulse width of about

10 nS. By varying the charging voltage of the cable, the output pulse amplitude coixld be varied without affecting the pulse shape.

The amplitude of the output pulse of the high voltage pulser which was given to the tip for field evaporation was kept variable from 500 V to 2.5 kV.

The output pulse height of another high voltage pulser was kept fixed at 800 V and was given to the MCP. The terminating or load of both high voltage pulsers were located close to the lAP chamber, in order to avoid reflections and interferences.

Figure 1.10 shows the photograph of the entire electronic circuitry, named as "BHASKAR".

1-5 PEIELIMIKART OEERATIOH OF lAP

The system that was used for testing the lAP has been discussed in section 5.1 • The pilot experiments on lAP were carried out on the carbon contaminated molybdenum specimen tip. After evacuation to the ultimate pressure of ~ 2x10-^ mbar, the specimen tip was cooled to liquid nitrogen

77 FI&UEE 1.10 : Photograph of the mit "BHASKAR". • temperature. The field ion images were observed using helium as the imaging gas. Substantial do field evaporation was done to get a good "end form". The chamber was reevacuated, and specimen was manipulated to face the chevron MCP image intensifier. In order to observe the field evaporated ion image, a large gain of about 10° is necessary, which is provided by the chevron MCP.

The electrical connections were done as shown in the schematic of lAP (figure 1.1). The distance between the tip and the chevron MCP was about 130 mm. Initially, the dc field desorption image was observed (various potentials used for this purpose have been listed in Table 1.1). This was found to be highly beneficial in orienting the tip in a proper direction and in determination of the approximate amplitude of pulse required for desorption. Subsequently, the time gated field desertion images were observed by varying the delay time and were recorded at two different values of the delays. Except at these two settings of delay, no image could be obtained. These images (figure 1.11 (a) and (b)) were found to be of very low intensity and were not steady. The time jitter of about 100 nS was found to be present in the delay network. This time jitter was thought to be responsible for the observed flickering images.

The variable delay network used in the present case was based on TTL (transistor transitor logic) monostable multivibrators. The inherent limitation of TTL logic family, namely rise and fall times of the pul^s of the order of 100 nS may cause the observed time jitter.

This problem of time jitter can be avoided by using the scheme shown in figure 1.12. It is based on the BCL (emitter coupled logic) family ICs. The delay generated by such a digital circuit will have the time resolution solely governed by the width of the basic clock, and it will be 10 nS in the proposed scheme. This circuit is under construction.

78 TABLE I.l

Varioiis potentials applied during the observation of desorption images

Electrode Potentials for dc Potentials for time gated pulse

desorption image desorption image

Tip Positve high voltage dc +1 to 2.5 kV pulse + BIV

MCP 1 + 1 kV + 300V + 700V pulse

MCP 2 + 2 kV + 1300 V

Screen + 6 kV + 6 kV

i) BIV.... dc voltage required for getting the best helium ion image, ii) All the voltages were applied with respect to ground, iii) Front face of Chevron MCP is at ground potential. FIGURE 1.11 (a),(b) : lAP images of the molyMeniam tip recorded for

two different values of the delay. H CD

0) r-i

•H fn Cti >

-P

•H 1^ 0

0

o s CD o CQ 13 Q) affl o P< 0 x: EH

CM

P4

H iC -J Q O I- in h O REPfflHfCES

1. E.W. Miiller and T.T. Tsong in "Field Ion Microscopy : Principles and Applications", (Am. Elsevier, N.Y., U.S.A., 1969).

2. J.A. Panitz, J. Vac. Sci. & Technol., Vl_. 206 (1974). 3. J.A. Panitz, Prog. Surf. Sci., 8, 219 (1978). 4 E.A. Jang and R.N. Lewis, Nucl, Instrum & Methods, 44, 224 (1966). 5 J. Jethwa, E.E. Marinero, and A. Muller, Rev. Sci. Instrum, 52^, 989 (1981). 6. H.J. Baker, J.J. Bannister, and T.A. King, J. Phys. E: Sci. Instrum., U, 579 (1981).

7. R.R. Alfano and N. Yurlina, Rev. Sci. Instrum, 40, 166 (1969).

8. V.G. Chadbond, J. Phys. E : Sci. Instrum., 1, 1133 (1968). 9. EG & G, U.S.A., Data Sheet K5500B-3.

79 APPHIDII II

II.1 COKPDTER PROGRAM PLOW CHART FOR POP

(START')

READ (i) No. of points, N (ii) Area, AR (iii) Co-ordinates of points, X(I), Y(I) (iv) No. of reference points, M (v) Co-ordinates of reference points, ORGX(J), ORGY(J) (vi) Width of the shell, DR (vii) Minimum and maximum values of R, RMIN, RMAX

'Evaluate planar density PD = N/AR

WRITE PD, N, X, Y, M, ORGX, ORGY, DR, RMIN, RMAX

J=1, M I : I = 1, N

[Evaluate distance DIST(I) = SORT [(Y(l) - ORGY(J)f + (X(I) - OWXiJ)f]

NO YES 1=1+1 K=1

R = RMIN + K*DR

YES NO K=K+11 e

U^ 1=1+1 RANGE (K) = RANGE (K) +1

J=J+1

/ WRITE RANGE /

80 II.2 COHPDTER PEK)GRAM PLO¥ CHABT FOR ADF

(STARTJ

READ (i) No. of points, N (ii) Co-ordinates of points, X(I), Y(I) (iii) No. of reference points, M (iv) Co-ordinates of reference points, ORGX(J), ORGY(J) (v) Nearest neighbour distance, RNN (vi) Value of dG, DTH

RMIN = 0.8*RNN, RMAX = 1.2*RNW, MK = 0, MA = 0, K = 0

J = 1,M

I = 1,N

ih/aluate distance DIST(I) = SQRT [(Y(I) - 0RGY(J))2 + (X(I) - 0RGX(J))2 ]

NO YES I = 1+1 K = K+1 •i OX(K)= X(I) OY(K)= Y(I) I = 1+1

L = 1,K

K1 = L + 1

SL1 = (OY(L) - ORGY(J))/(OX(L) - ORGX(J)) SL2 = (0Y(K1) - 0RGY(J))/(0X(K1) - ORGX(J))

MA = MA + 1 I ANGLE (MA) = ATAN [(SL2 - SL1 )/(1 - SL1*SL2)]

L = L + 1

KR

ai YIS

NO

RANGE (KR) = RANGE (KR) + 1 KR = KR + 1

->-

WRITE ORGX(J), ORGY(J), K

& -<- J = J + 1

WRITE RANGE

f STOPJ

S2.