SHORT COMMUNICATION

Low-cost image-capture system for a scanning electronmicroscope

R. F. EGERTON* & L. WAN†*Physics Department, University of Alberta, Edmonton, Canada T6G 2J1†Public Carrier Networks, Nortel, Ottawa, Canada K1Y 4H7

Key words. Digital image acquisition, scanning electron microscope (SEM).

Summary

We describe a PC-based active-capture system for recordingdigital images from a scanning electron microscope. Thesystem is based on a National Instruments data-acquisitionboard and a Pentium computer, controlled by software thatwe have written in Visual Basic.

Introduction

Digital acquisition from a scanning electron microscope(SEM) provides a permanent record of the image withoutthe expense or delay of photographic recording. It alsoallows convenient retrieval of previous images (via acomputer archiving program), rapid transmission of imagedata between different sites (via a local-area network or theInternet), convenient quantitative analysis of image inten-sities and various kinds of image processing. As a result,many new SEMs come equipped with digital acquisition,and there exist commercial image-capture systems whichcan be added to older instruments.

There are two kinds of image-capture system: passive andactive. Passive systems record image-intensity data undercontrol of the SEM electronics, the computer acting as aslave to the microscope. To allow the computer to store thesignal intensity corresponding to each pixel, attachmentpoints must be found (in the SEM circuitry) for monitoringthe line- and frame-scan voltages, as well as the image-intensity signal. The need to continuously monitor the lineand frame voltages reduces the maximum acquisition rate,or else requires fast electronics (in addition to a standarddata-acquisition board) which increases the cost of thesystem. On the other hand, it is sometimes possible (bychoosing the correct signal-attachment point) to include(within the image) alphanumeric information such as theaccelerating-voltage setting or a micrometre-scale bargenerated by the microscope electronics. A passive system

for acquiring images into a Macintosh computer has beendescribed recently by Postek & Vladar (1997).

Active-capture systems replace the microscope line andframe signals with computer-generated ramp voltageswhich are applied (through amplifiers in the microscopeelectronics) to deflect the beam, in place of the SEM-generated raster. Some SEMs have input sockets, providedfor X-ray mapping applications, to which the raster signalscan be applied. Otherwise, suitable attachment points haveto be located in the SEM circuitry and some form of switchinstalled to implement the change from internal tocomputer scanning when recording image data. A potentialadvantage of an active system is that its direct control of theelectron beam allows for the possibility of performingelectron-beam lithography in the SEM.

This short communication describes an active-capturesystem based on a standard personal computer (containinga Pentium microprocessor) fitted with a 100-kHz data-acquisition board and programmed in a Windows 3.1format. One of its advantages is the relatively low hardwarecost (less than $US3000, including computer and laserprinter), an important consideration in the case of oldermicroscopes. In our case, the system is interfaced to aPhilips 505 SEM.

Hardware

Figure 1 shows a block diagram of the active-capturesystem. A data-acquisition card in the 100-MHz Pentiumcomputer generates a raster scan and records the resultingintensity modulations. The digital image is displayed on thecomputer monitor and can be printed onto plain paperusing an attached laser printer (we used a Brother HL-760DX, capable of 1200 × 600 dpi resolution).

The data-acquisition board is a National Instrumentsmodel AT-MIO-16E-10, which accepts up to 16 analog

Journal of Microscopy, Vol. 191, Pt 2, August 1998, pp. 113–115.Received 4 November 1997; accepted 2 April 1998

113q 1998 The Royal Microscopical Society

inputs, multiplexed to allow a total data rate up to 100 kHz.Since we are using only one input, the maximum input rateis 105 pixels per second and a 1000 × 1000-pixel image canbe read in 10 s. This is fast enough in most cases; in fact,some images may require longer recording times to achievesatisfactory signal-to-noise ratio. The input is digitized with12-bit precision (dynamic range 4096:1), allowing themeasurement of image intensity (at half-maximum value)to 0·05% accuracy. This is adequate for most purposes, andis more than sufficient for image display since the humaneye cannot distinguish more than about 70 grey levels. TheAT-MIO-16E-10 board also provides two analog outputs,generated with 12-bit precision, allowing rasters with up to4096 × 4096 pixels to be generated with appropriatesoftware control (although our implementation allows amaximum 1024 × 1024-pixel image).

Although the AT-MIO-16E-10 board is a standard optionfor scientific equipment, comparable 100-kHz data-acquisi-tion boards are available from other manufacturers. Higher-speed boards are available but are considerably moreexpensive and would not have significantly improved the

overall data rate of the system, which is also affected bysoftware and computer limitations. Use of a lower-perfor-mance board would have reduced the convenience of thesystem without much reduction in its overall cost. Forexample, the inexpensive ‘Snappy’ unit described by Postek& Vladar (1997) digitizes the input signal with only 8-bitprecision, requiring more accurate setting of the microscopegain and signal-level controls.

Software

The National Instruments board is provided with softwarewhich allows its detailed operations to be controlled by thePC. In addition, we have written a master-control program(SMART.EXE) in Visual Basic; a copy of this program isavailable (without cost) from the authors. The SMARTprogram provides a standard Windows interface whichallows the microscope operator to adjust the acquisitionparameters; see Fig. 2. Prior to image capture, the dataacquisition rate (up to 100 kHz), the horizontal and verticalnumbers of pixels, and the input-amplifier gain can, ifnecessary, be changed from their default values. To enable amicrometre-scale bar to be added to the recorded image, theuser also enters the microscope magnification, read fromthe SEM console. Before image acquisition, the signal gain(contrast) and d.c. level (brightness) controls of the SEM areset to appropriate values, as determined from a light-emitting-diode (LED) display in the case of the Philips 505SEM.

During image capture, intensity data are placed into acomputer-memory array. After the raster is completed, thesedata values are converted to a PCX-format image for display

Fig. 1. Schematic diagram of the SEM image-capture system. Adouble-pole mechanical switch applies x- and y-ramp voltages,generated either by the microscope or by the data-acquisitionboard, to the SEM beam-deflection circuitry.

Fig. 2. Windows operating panel generated by the SMART.EXE pro-gram. The various control boxes are described in the text.

Fig. 3. SEM image of a thin film of gold, captured using the SMARTacquisition system and printed using the laser printer. The horizon-tal white line in the top-left corner is the micrometre bar.

114 R. F. EGERTON AND L. WA N

q 1998 The Royal Microscopical Society, Journal of Microscopy, 191, 113–115

by an image viewer. For the latter, we have used theshareware program L-View, which also provides someimage-processing capability (change of brightness, contrast,image orientation, etc.) as well as conversion to a variety offormats (TIFF, GIF, JPEG, etc.) so that more elaborate imageanalysis and processing can carried out, using a variety ofsoftware packages.

While writing data to the PCX file, the SMART.EXEprogram adds a 1-mm-length scale marker to the image byincreasing the image intensity to its maximum value withintwo lines near the top of the image; see Fig. 3. The length ofthis marker is calculated from the SEM magnification(entered on the Windows display) and a calibration factorwhich is specific to the microscope.

Although we have not yet implemented an electron-beamlithography option, we believe that simple shapes could begenerated by defining rectangles on the PCX image(recorded using a low electron dose) using the standardtools provided by the L-View program. The correspondingimage coordinates would then be used to define the regions

of specimen irradiated by the beam during a subsequentraster scan under high-dose conditions (increased probecurrent or longer dwell time per pixel). The AT-MIO-16E-10board provides eight digital outputs, one of which could beused for electron-beam blanking. However, a simplersolution would be to decrease the dwell time per pixel to alow value (corresponding to rapid scanning of the incidentbeam) while moving from one area of irradiation to another.

Disclaimer

We have no financial interest in the National InstrumentsCorporation or the Microsoft Corporation (designers ofVisual Basic) and are not affiliated with the IndependentJpeg Group (which provided the design basis for L-View).

Reference

Postek, M.T. & Vladar, A.E. (1997) Inexpensive digital imaging?Scanning, 19, 197–199.

q 1998 The Royal Microscopical Society, Journal of Microscopy, 191, 113–115

SEM IMAGE-CAPTUR E SYSTEM 115