Appendix I Klhctrohics for Magihg Atom Probe 1.1
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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 transistor 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 potentiometer (10 kji.) and the timing capacitor (10 pP to 5 nP), and can be adjusted in a range 50 nS to lOjuS. The timing capacitors 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 spark gap 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 transistors 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 transformer, 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 printed circuit board (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 + <zv 500 pF J -TL ^/i Qi 1>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 switch has a distinct advantage of achieving well synchronized operation over other types of high voltage switches namely, mercury wetted relay' and spark gap°. Krytron KN-22 " (E.G. & G., USA) is a ^s filled tube with specially designed electrodes. 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 anode circuit of the tube. The "keep alive" current of about 200 xiA was allowed to flow in the cathode 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.