THE COMPUTER-CONTROLLED FIELD ION MICROSCOPE WITH ATOM-PROBE AT THE HAHN-MEITNER-INSTITUTE

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1 THE COMPUTER-CONTROLLED FIELD ION MICROSCOPE WITH ATOM-PROBE AT THE HAHN-MEITNER-INSTITUTE P. Mertens, U. Vidic, Hanna Becker To cite this version: P. Mertens, U. Vidic, Hanna Becker. THE COMPUTER-CONTROLLED FIELD ION MICRO- SCOPE WITH ATOM-PROBE AT THE HAHN-MEITNER-INSTITUTE. Journal de Physique Colloques, 1984, 45 (C9), pp.c9-309-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 C9-309 THE COMPUTER-CONTROLLED FIELD ION MICROSCOPE WITH ATOM-PROBE AT THE HAHN-MEITNER-INSTITUTE P. Mertens, U. Vidic and H. Becker Bahn-Meitner-Institut fiir Kemforschung Berlin, Glienieker Str. 100, D-1000 Berlin 39, F.R.G. Résumé - Un microscope ionique de champ fonctionnant sous un contrôle informatisé à grande échelle est décrit. Les considérations de base relatives à la conception et à la structure du hardware et aux programmes de mesure sont soulignées. Abstract - A field ion microscope operated under large scale computer control is described. Basic design consideration and the structure of electronic hardware and of measuring programs are outlined. I - INTRODUCTION The field ion microscope with atom-probe briefly sketched here was developed for the routine analysis of alloys. In field ion microscopy an experiment with an alloy is usually stopped by the rupture of the tip specimen at high voltages. Therefore a relatively large number of samples is involved in an analysis. Thus, an effective tip exchange facility is required which does not cause long-term interruptions. Similarly, a high availability of the instrument has to be provided. An additional aspect governing the design of this instrument was, that it is to be operated by personnel not necessarily experienced in field ion microscopy. Right from the beginning it was planned to be used as a tool to do research with, and not to do research at. The basic considerations for this instrument's design are compiled in the following table. Basic Considerations for the instrument's design Requirements Easy Maintenance Provisions No liquid coolant for tip Reduction of LN2-use High stability and short repair periods Temperature control by closed cycle refrigeration Turbomolecular pumps instead of diffusion pumps Use of commercial components wherever possible, instrument and support built of two easily detachable parts Article published online by EDP Sciences and available at

3 JOURNAL DE PHYSIQUE Table cont. Easy Operati on Frequent and rapid tip exchange for short-1 ived a1 loy specimens FIM usable by unexperienced personnel Daylight operation Simp1 e a1 ignment and focussing of the ion beam Atom-probe operation independent of pulser type or timedependent performance Straightforward operation Easy data access and evaluation Five-tip-storage at 10-7 Torr and electrical transport into tip holder Computer control for crucial functions (voltages etc.) Video display of FIM image Electrostatic deflection plates, reasonably we1 1 focussing einzellens, focus control via ratemeters and additional video camera On-1 ine pul se-height analysis of HV-pulses Large-scale computer control of the experiment, individual programs for atomprobe, desorption microscope, and TOFcalibration Instrument coupled to a POP 11/44 via UNIBUS, files stored on disks, on-line and off-1 ine display of mass spectra and depth distributions Reliability and easy maintenance of the instrument are a result of the mechanical hardware, while versatility and effectivity of the operating are most of all due to the electronic hardware and the programs available. Here only electronic hardware and the programs will be outlined. A comprehensive description of the instrument including mechanical hardware is given in ref PROCESS INSTRUMENTATION FOR MEASUREMENT AND CONTROL The field ion microscope is controlled by a process computer PDP 11/44 as manufactured by Digital Equipment Corporation, Maynard, Mass. The computer in coupled to the experiment vis a CA 11-F-interface, which is directly attached to the UNIBUS with a length of 75 ft. At the same time, this interface acts as a crate controller for the CAMAC sytem. CAMAC (Computer Application for Measurement And - Control) is an internationally standardtzed modutar instrumentatton system f<r electronic measurements. As a great varity of modules is available, CAMAC is the most versatile system by far to interface an experiment's electronic peripherals to the computer. A block diagram of the hardware components is given in fig. 1. All voltages applied to the instrument can be adjusted manually as well as under computer control. As this option is set by a hardware switch, an operator can change from manual to automatic control during a measurement, if it is necessary. Before a new HV-pulse is triggered, the actual values for tip voltage and pulser charging voltage are read automatically, so that this manual intervention does not hamper the measurement. Each experiment is started with the manual adjustment of the voltages for tip, pulser and lens. If automatic control is selected, these values are read by the computer, and appropriate control voltages are set. After the change to automatic control, the computer initiates all further adjustments. The higher the degree of automatization in a system is, the more important facilities become by which an operator can influence the course of an experiment. Here, the operator's intervention and the experiment's status control signals are treated in a common priority interrupt register (Jorway Model 63), which is

4 computer FIELD ION MICROSCOPE PDP 1114L Interface C A 11 F C C t I t 'I ttme-ot-t~lght measurement TDC I L > r.,,, channel 0-7 nv.dlu.lment DAC cd-.tatus.iper,nynt 'lal". pulse and count~n. Interrupt d- \ A - I,,em lo" maso experiment I and I cl~sorptlon mkrossoo.. detector I turn.-or- tllght path I Ilp lo be an.1yz.d lnuoh~ plat.. I L -_ ' + Fig. 1: Block diagram of the microscope with electronic supplies and controls. read before a new HV-pulse is triggered. This means that the status control is accomplished within the measuring program and not by a separate task, which eventually could not react to status changes instantaneously. For the FIM experiment, the status control register is merely needed to transfer scatus information to the measuring program. Direct hardware protection of, for example, the channel plates in the case of a leakage in the vacuum system is provided by relays, which are set by the pressure monitoring system. The CAMAC modules used for the time-of-flight measurement are a Le Croy 2228 A clock with ranges of 2 ps, 5 ~s and 10 ps, an output register for the triggering of the pulse, and a parllel output register for the setting of a digital delay. A HV-pulse is triggered, if a bit is set in the output register. A synchronous pulse is derived from the HV-pulse, which then represents the actual start signal for the timing circuitry. This signal triggers the digital delay, the output signal of which then initiates the timing cycle in the 200 MHz clock and resets a shift register. The pulses to stop the timing cycle are derived from the detector signals of the impinging particles. In order to measure up to eight particles desorbed by one HV-pulse, the shift register is employed. Thi-s register directs the signal of the first particle detected after the reset pulse into.the first channel of the clock, the second signal into the second channel and so on. When after the readout the i-th channel of the clock is found to have reached its maximum content (11 bit), it can be concluded that i-1 particles have been detected within the range of 10 ys. If even the contents of the 8th channel is less than 211-1, more than eight particles might have been desorbed. Such an event cannot be included in the construction of a mass spectrum. For a normal event with 1 ( i 6 8, the masses of the particles are calculated and accumulated into a mass histogram. Subsequently, another HV-pulse is triggered.

5 JOURNAL DE PHYSIQUE 3 - THE SOFTWARE SYSTEM The program system comprises three tasks, running under the control of the operating system RSX-11M version 4.0: '- MUMFIM = task for control and measuring operations - FIMSTO = task for data storage - FIMGRA = task for the graphic input and output on the display. Actual experiment control is performed by MUMFIM in a dialogue with the operator who can choose between two different modes: - atom probe mode - desorption mode (with integrated atom probe mode). In addition to these two modes, there is a third program - calibration which can be used to adjust the parameters for atom probe mode. FIMSTO and FIMGRA are loaded only if required by the measuring programs. The function of FIMSTO is to store mass spectra and depth profiles on the disk in the format which is generally used for spectra in the HMI (CESSNA format as defined in the internal report by W.L. Bauerfeld, DM 52 - July 1977). FIMSTO is also employed to read mass spectra from the disk and to make them available for calibration purposes. The function of FIMGRA is to plot mass spectra and concentration depth profiles on the graphic display. MUMFIM is programmed in MUMTI, which is a HMI-written interpretative language, whereas FIMSTO and FIMGRA are FORTRAN programs. The program for atom probe mode is designed to analyze the elemental composition of the tip under investigation. Fig. 2 presents a rough diagram of this mode. INSTRUMENT FIELD ION MICROSCOPE IFIMI AND ATOMPROBELAPI J FIM AND AP OPERATION AN0 CONTROL POP :-I HVadpsldon-lin ornlrr~rd mmbwsndw. thod*o(-fimsw mompob. mod. Fig. 2 - Software components for atom probe mode

6 When the measuring program is started via the terminal, it sets the experimental parameters and controls the hardware components applied. Then the program triggers the tip pulse at a pre-selected frequency. Before a new pulse is triggered, the status of th'e experiment is checked and the actual high voltages for the tip and for the pulser are measured. From the time-of-flight data of the ions registered at the ion detector, the respective mass-to-voltage ratios m/n are calculated, accumulated to a spectrum and listed on the graphic display (atom probe listing). By means of manual intervention by the operator ('show spectrum'), the spectrum is plotted onto the graphic display during the measurement. The m/n limits for the graphic display are normally chosen by the program in a way that cuts off the parts of the spectrum before the first and after the last peak. But other sections can be selected alternatively by the operator ('enter mass display limits'). A depth profile of the tip can be compiled by registering the number of ions per atom layer for.specific elements. Up to 8 elements with up to 3 charge states each can be included. In the presence of imaging gas, the signal 'layer desorbed' can be given manually or automatically (see 3.3). For operations without imaging gas, an atom layer is arbitrarily defined by a fixed number of atoms per layer. Every atom layer is listed on the terminal. A graphic display in the form of concentration profiles can be requested manually ('show depth profile'). Spectra and depth profiles (in the form of individual spectra for selected specimens) can be stored on the disk. The close connection of the field ion microscope with a computer might be regarded as a restriction on performing tests and electronic adjustments at the instrument. But a multitude of functions can, in fact, be operated manually, if desired. All voltages can be applied in this way, and all pulses can be triggered by an off-1 ine pulser with preselectable pulse numbers and repetition rates. Only the TOF spectrum cannot be obtained like that. But the number of particles arriving at the detector within the TOF range of 10 ps can be registered instead. For this purpose, the gate of the NIM counter for the detector pulses is opened for 10 ps, simultaneously with the pulse starting the clock. Thus, virtually all functions of the instrument can be operated off-line. Computer programs, on the other hand, can be debugged without the experiment attached. For the voltages to be read, default values are programmed. The TOF events can be simulated by a pulse generator, which is triggered by the CAMAC output register like the HVpulser. Employed in burst mode, this generator (HP 8015 A) produces a preselectable number of output pulses simulating the desorbed particles with adjustable delay times between them. By this off-1 ine operation and TOF simulation, instrument and program can be tested independently in an effective way. Having operated this field ion microscope with atom-probe for two years, it can be stated now that the initial goal of building a reliable instrument which is easy to service and to operate has been fully reached. Examples for the performance in imaging and atom probing are given in ref. /I/. REFERENCES /I/ P. Mertens, U. Vidic and H. Becker, A Computer-Controlled Field Ion Microscope with Atom-Probe, Report of the Hahn-Meitner-Institut Fur Kernforschung Berlin CmbH