ANALYSIS OF HV-PULSES FROM KRYTRON AND RELAY PULSERS

Transcription

1 ANALYSIS OF HV-PULSES FROM KRYTRON AND RELAY PULSERS P. Mertens, B. Kell, H. Krüger-Elencwajg To cite this version: P. Mertens, B. Kell, H. Krüger-Elencwajg. ANALYSIS OF HV-PULSES FROM KRYTRON AND RELAY PULSERS. Journal de Physique Colloques, 1984, 45 (C9), pp.c9-323-c < /jphyscol: >. <jpa > HAL Id: jpa Submitted on 1 Jan 1984 HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.

2 JOURNAL DE PHYSIQUE Colloque C9, supplément au n 12, Tome 45, décembre 198* page C9-323 ANALYSIS OF HV-PULSES FROM KRYTRON AND RELAY PULSERS P. Mertens, B. Kell and H. Kriiger-Elencwajg Hdhn-Meitner-Institut fiir Kernforsohicng Berlin, Glienioker Str. 100, D-1000 Berlin 39, F.R.G. Résumé - L'analyse directe de hauteur d'impulsion est appliquée à l'étude de la distribution en amplitude et en hauteur d'impulsion des signaux H.T. produits par les pulseurs à relais et à krytron communément utilisés sur les microscopes ioniques de champs équipés d'une sonde à atomes. Les fonctions V, ^,,-,. (V,,,, ) sont dérivées dans hauteur d impulsion v de charge de pulseur' les deux types de pulseurs. Les fluctuations de hauteur typiques de ces pulseurs sont décrites. Abstract - Direct pulse-height analysis is applied to investigate the amplitude and pulse-height distribution of HV-pulses produced by krytron and relay pulsers as commonly used in field ion microscopes equipped with an atom-probe. The functions V pu i se height ( v pulser charging voltage) are derived for both the pulser types. Pulse-hevght fluctuations typical for these pulsers are described. 1 - INTRODUCTION The majority of the field ion microscopes equipped with an atom-probe employs a high voltage (HV)-pulser for desorbing ions from the tip specimen. The mass resolution achieved in the time-of-flight analysis of desorbed species is mainly determined by the shape of the HV-pulse. It has been sucessfully demonstrated by Panitz /l/ that even an instrument using a small flight path can exhibit an excellent mass resolution, if the HV-pulse is produced and guided carefully to be of almost rectangular shape at the tip. To fully make profit of an atom-probe's inherent mass resolution additionally the precise height of the probe has to be known. This pulse-height, however, commonly is accessed by a rather indirect method, registering the HV-DC-input instead of the pulse actually produced. Indication that this way how the pulse-height is derived from the DC-voltage can be unprecise, may be found in the sometimes directly observable fluctuations of the pulses produced. In order to achieve a more quantitative understanding of the HV-pulse production, direct pulse-height analysis (PHA) has been applied here for HV-pulsers typically used for an atom-probe. 2 - EXPERIMENTAL SETUP FOR THE HV-PULSE ANALYSIS The process of the HV-pulse production is illustrated in the upper part of fig. 1. Via a resistor, a 50 n cable is charged to a DC-voltage up to 5 kv. By means of an externally triggerable HV-switch, this cable is discharged into a 50 n load, part of which is the FIM tip assembly. The pulse risetime depends on the type of the cables and the HV-switch used, whereas the pulse duration is given by the length of the charging cable. Provided the HV-switch is ideal, the HV-pulse-height amounts to half of the cable's charging voltage. Deviations from this pulse-height and from a rectangular pulse shape are produced by the interruption of the 50 fi impedance inside the field ion microscope. Such an Article published online by EDP Sciences and available at

3 C9-324 JOURNAL DE PHYSIQUE [HV-PULSE ANALYSIS 1 Idetectoble by eye for Fig. 1 - HV-pulse production and analysis TOF-c~l,br~t#on no? dependent of pulrer-type ond performonre no decreore 8n morr resslutlon due t~ HV-pa11e va~catlons interruption usually results in a reduction of the pulse amplitude and in the jittering of the pulse shape due to reflections. The request for the HV-switch to be "ideal" implies an instantaneous transition from the open to the closed state without any intrinsic voltage loss. Generally desirable but indispensable only in the case of the pulse applied to the imaging plates in desorption mode is a short and constant response time to the triggering signal. Two types of HVswitches are widely used: the krytron tube and the mercury wetted relay. The krytron swith offers a fast response to the triggering pulse, while HV-switching by the relay incorporates an undefined delay period between triggering and operation that is due to the inclusion of a mechanical contact. But, on the other hand, the shape of the relay-pulse is closer to the desired rectangular form. A further advantage of the relay pulser will become obvious by means of the HV-pulse analysis. Due to the problems inherent in the direct determination of the amplitudes of the HV-pulses, the pulse-height often is derived by assuming it to be a constant fraction of the charging voltage. This assumption is justified, if there is no voltage loss in the HV-switch. In the following, the HV-pul se-height analysis wi 11 be applied primarily to examine the val idity of this assumption for a krytron pulser and a relay pulser. In order to reduce the pulse voltage from some kilovolts to some volts for the analysis, commercial attenuators were employed here (see fig. 1). They are normally used to diminish the amplitude of high-frequency signals, and therefore they are constructed to resist high electrical power better than high voltages. If the attenuators appropriate for 100 watts or higher power dissipation are selected, the high voltage pulses, too, are reliably attenuated. In this instrument, the three consecutive attenuators add up to a total voltage reduction of ( ) db = 70 db. The resulting pulse height of less than 1 volt is well suited for processing in nuclear electronics. Yet, for pulse analogue-todigital converters, the risetime of these signals is too small, so that a pulse stretcher has to be applied. Eventually, these final pulses have risetimes of about 100 ns. It must be emphasized that this process of pulse stretching, which results in shaped signals, is a sort of manipulation that only keeps the highest

4 pulse voltage. A priori the extent to which this manipulation is still adequate to the ion desorption process cannot be determined. For HV-pulses exhibiting no clear plateau, some peak-averaging procedure might be more realistic. Oscill o- grams of attenuated pulses with and without stretching are plotted in fig. 2 for a krytron and a relay pulser. From the plot it can be concluded, if a pulse can be described real istical ly by just one amplitude. Oscillograms of HV- pulses krytron 2 kv krytron 5 kv Fig. 2 - Attenuated HV-pul ses without and with stretcher krytran 2 kv and pulse stretcher mercury relay 5 kv 3 - MEASUREMENT OF HV-PULSE-HEIGHT DISTRIBUTIONS Pulse-height distributions (not normalized) of the stretched pulses with a pulser charging voltage Vpc of between 1 and 5 kv are displayed in fig. 3a for a krytron pulser and in fig. 3b for a relay pulser. The comparison of the two diagrams shows that a relay pulser delivers a narrower pulse-height distribution at lower voltages, whereas the krytron displays a smaller halfwidth of the gaussian-1 i ke peak at higher voltages. HV- pulse height distribution: krytron - pulser HV-pulse height distr~bution : Hg -relay pulser a b) Fig. 3 - Pulse-height distributions for charging voltages between 1 and 5 kv a: krytron pul ser, b: relay pulser

5 C9-326 JOURNAL DE PHYSIQUE But in either case the gaussian's halfwidth is small enough (< 2%) not to deteriorate the TOF mass resolution significantly. Thus, from this point of view an on-line PHA does not seem to improve mass resolution much. The actual motive for the inclusion of an on-line PHA into this FIM was the observation made on an oscilliscope that some HV-pulses differed from the average by a considerable percentage. This observation is clearly supported by the 5 kv-pulse spectra for both pulsers, since the off-average pulses of the krytron extend to higher voltages, while those of the relay display a tendency towards lower voltages. Both krytron and relay pulsers employed in this comparison were quite new. For pulsers which have been in use for a long time, this effect is much more pronounced at lower charging voltages already. The magnitude of the pulse-height variations then diminishes the attainable mass resolution, so that an on-line PHA could be of considerable help. Besides the pulse-height distributions, in figs. 3a and 3b, the gaussian's peak positions are plotted as a function of the charging voltage. These plots indicate that the linear relation between pulse and voltage is preserved by the stretcher. Moreover, the slope (arbitrary units) is the same for krytron and relay, thus confirming that the pulse manipulation by the stretcher is independent of the individual pulse shapes. Part of the offzero intercept for the pulsers is due to a constant pedestal in the specific stretcher applied and can easily be corrected. For fig. 4 a simple pulse-height analysis employing only attenuator and an oscilloscope was performed for a krytron pulser and a relay pulser. Just like the pul se-height analysis using a nuclear ADC, this simple experimental set-up also delivers a linear function Vpulse (Vpc). While for the relay the commonly used relation Vpulse = a Vpc is obviously a correct assumption, one obtains for the krytron tube Vpulse = a Vpc - Vo, Vo = constant. Here, Vo = 140 volt was found. This value for Vo is just the burning voltage of the plasma in the krytron tube. Analyses relying on Vpulse = a. Vpc in experiments using krytron pulsers therefore sacrifice parts of their mass resolution. HV- pulse height analys~s via oscilloscwe I Krytron - pulser I V, ;( 140 volt pulser chorging voltage Vpc Fig. 4 - Pulse amplitude Vpulse (Vpulser char ing) for krytron and relay puqser as obtained by an osci 11 oscope. Hg - relay - pulser Vpulse ( Vpc 1.2' 4,,/' V, x 0 volt pulser charg~ng vdtage Vpc

6 This simple investigation makes it easy to understand why krytron pulsers are sometimes judged to offer worse mass resolutions than relay pulsers. If in the TOF calibration the correct Vo-value is taken into account, spectra taken with the two pulsers need not necessarily be of different qua1 ity. As Vo only amounts to a low percentage of the total accelerating voltage, the crude way used for its determination is fully sufficient. The TOF calibration now results in a value of a which is indeed valid for all values of Vpc. From the scope of fig. 4, the different behaviour of erratic HV-pulses from the two pulsers can be understood. Since the Hg-relay exhibits no significant voltage drop when closed, an erratic pulse smaller than the average must be attributed to a plasma-like discharge in the enclosed high-pressure hydrogen atmosphere. This discharge is characterized by a not negligible burning voltage. In the krytron tube, an uncontrolled plasma breakthrough results in a reduced voltage drop thereby increasing the pulse voltage. Therefore, for a krytron tube the difference in voltage between erratic and regular pulses is restricted to the height of the burning voltage, whereas for the relay pulser no upper limit for this difference exists. The only way to prevent spectra from being deteriorated by these erratic pulses is connected with the installation of an on-line pulse-height analysis, which directly measures every single pulse in order to calculate a particle's mass. The pul se-height calibration is then reduced to f = constant factor given by pulse attenuation and ADC-conversion; VADC = conversion result by ADC, where f can be calibrated by employing a known mass spectrum, as is common practice. So far, it seems this on-1 ine PHA will not increase the system-inherent resolution of an atom probe, but that it makes the instrument independent of the pulser type and its sometimes random performance. So, by means of on-line PHA the instrument's best operation will become everyday's operation. REFERENCES /I/ J. A. Panitz, Imaging Atom-Probe Mass Spectroscopy, Progress in Surface Science, Vol. 8, 219, Pergamon Press (1978).