Computer Image and Spectrum Acquisition on an HB501A STEM

Abstract

A DEC VAXstation 3200 color graphics computer workstation has been interfaced to a Vacuum Generators HB501A Scanning Transmission Electron Microscope (STEM) for on-line image and spectrum acquisition. Electron Energy Loss Spectra (EELS) or Auger spectra may be recorded (serially) using pulse or analog modes (using the voltage to frequency converters supplied by VG). High resolution bright field (BF) / annular dark field (ADF) image pairs may be recorded simultaneously as well as simultaneous pairs of Auger / secondary images. A resolution of 2Å (or better) has been achieved with an experimental pole piece (Cs = 1.2mm. and 0.7mm.). The image quality obtained with this system is greatly improved over the Polaroid system supplied with the microscope.A schematic of the overall system is shown in FIG 1. To avoid possible ground loop noise the entire computer system has been optically isolated from the STEM. There are four 16 bit digital to analog converters (DAC) for controlling the x,y position, the EELS energy and the Auger energy. The output of four detectors is connected back to the computer for pulse counting using a Qbus board manufactured by Codar Technologies. This board has three AMD 9513 counter IC's (with five 16 bit counters each) to count the electron or VFC pulses. The EELS spectra are acquired using two 32 bit counters (16 bit counters concatenated) and may function in a constant dwell time or constant count (constant signal to noise ratio) mode. The Auger and BF/ADF image counters work in a constant dwell time 16 bit mode. The average CPU overhead has been measured at 50 microsec per point in polled mode (images) and 500 microsec in interrupt driven mode (spectra).