Low-cost image-capture system for a scanning electron microscope

Authors


Abstract

We describe a PC-based active-capture system for recording digital images from a scanning electron microscope. The system is based on a National Instruments data-acquisition board and a Pentium computer, controlled by software that we have written in Visual Basic.

Introduction

Digital acquisition from a scanning electron microscope (SEM) provides a permanent record of the image without the expense or delay of photographic recording. It also allows convenient retrieval of previous images (via a computer archiving program), rapid transmission of image data between different sites (via a local-area network or the Internet), convenient quantitative analysis of image intensities and various kinds of image processing. As a result, many new SEMs come equipped with digital acquisition, and there exist commercial image-capture systems which can be added to older instruments.

There are two kinds of image-capture system: passive and active. Passive systems record image-intensity data under control of the SEM electronics, the computer acting as a slave to the microscope. To allow the computer to store the signal intensity corresponding to each pixel, attachment points must be found (in the SEM circuitry) for monitoring the line- and frame-scan voltages, as well as the image-intensity signal. The need to continuously monitor the line and frame voltages reduces the maximum acquisition rate, or else requires fast electronics (in addition to a standard data-acquisition board) which increases the cost of the system. On the other hand, it is sometimes possible (by choosing the correct signal-attachment point) to include (within the image) alphanumeric information such as the accelerating-voltage setting or a micrometre-scale bar generated by the microscope electronics. A passive system for acquiring images into a Macintosh computer has been described recently by Postek & Vladar (1997).

Active-capture systems replace the microscope line and frame signals with computer-generated ramp voltages which are applied (through amplifiers in the microscope electronics) to deflect the beam, in place of the SEM-generated raster. Some SEMs have input sockets, provided for X-ray mapping applications, to which the raster signals can be applied. Otherwise, suitable attachment points have to be located in the SEM circuitry and some form of switch installed to implement the change from internal to computer scanning when recording image data. A potential advantage of an active system is that its direct control of the electron beam allows for the possibility of performing electron-beam lithography in the SEM.

This short communication describes an active-capture system based on a standard personal computer (containing a Pentium microprocessor) fitted with a 100-kHz data-acquisition board and programmed in a Windows 3.1 format. One of its advantages is the relatively low hardware cost (less than $US3000, including computer and laser printer), an important consideration in the case of older microscopes. In our case, the system is interfaced to a Philips 505 SEM.

Hardware

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

Figure 1.

. Schematic diagram of the SEM image-capture system. A double-pole mechanical switch applies x- and y-ramp voltages, generated either by the microscope or by the data-acquisition board, to the SEM beam-deflection circuitry.

The data-acquisition board is a National Instruments model AT-MIO-16E-10, which accepts up to 16 analog inputs, multiplexed to allow a total data rate up to 100 kHz. Since we are using only one input, the maximum input rate is 105 pixels per second and a 1000 × 1000-pixel image can be read in 10 s. This is fast enough in most cases; in fact, some images may require longer recording times to achieve satisfactory signal-to-noise ratio. The input is digitized with 12-bit precision (dynamic range 4096:1), allowing the measurement of image intensity (at half-maximum value) to 0.05% accuracy. This is adequate for most purposes, and is more than sufficient for image display since the human eye cannot distinguish more than about 70 grey levels. The AT-MIO-16E-10 board also provides two analog outputs, generated with 12-bit precision, allowing rasters with up to 4096 × 4096 pixels to be generated with appropriate software control (although our implementation allows a maximum 1024 × 1024-pixel image).

Although the AT-MIO-16E-10 board is a standard option for scientific equipment, comparable 100-kHz data-acquisition boards are available from other manufacturers. Higher-speed boards are available but are considerably more expensive and would not have significantly improved the overall data rate of the system, which is also affected by software and computer limitations. Use of a lower-performance board would have reduced the convenience of the system without much reduction in its overall cost. For example, the inexpensive ‘Snappy’ unit described by Postek & Vladar (1997) digitizes the input signal with only 8-bit precision, requiring more accurate setting of the microscope gain and signal-level controls.

Software

The National Instruments board is provided with software which allows its detailed operations to be controlled by the PC. In addition, we have written a master-control program (SMART.EXE) in Visual Basic; a copy of this program is available (without cost) from the authors. The SMART program provides a standard Windows interface which allows the microscope operator to adjust the acquisition parameters; see Fig. 2. Prior to image capture, the data acquisition rate (up to 100 kHz), the horizontal and vertical numbers of pixels, and the input-amplifier gain can, if necessary, be changed from their default values. To enable a micrometre-scale bar to be added to the recorded image, the user also enters the microscope magnification, read from the SEM console. Before image acquisition, the signal gain (contrast) and d.c. level (brightness) controls of the SEM are set to appropriate values, as determined from a light-emitting-diode (LED) display in the case of the Philips 505 SEM.

Figure 2.

. Windows operating panel generated by the SMART.EXE program. The various control boxes are described in the text.

During image capture, intensity data are placed into a computer-memory array. After the raster is completed, these data values are converted to a PCX-format image for display by an image viewer. For the latter, we have used the shareware program L-View, which also provides some image-processing capability (change of brightness, contrast, image orientation, etc.) as well as conversion to a variety of formats (TIFF, GIF, JPEG, etc.) so that more elaborate image analysis and processing can carried out, using a variety of software packages.

While writing data to the PCX file, the SMART.EXE program adds a 1-μm-length scale marker to the image by increasing the image intensity to its maximum value within two lines near the top of the image; see Fig. 3. The length of this marker is calculated from the SEM magnification (entered on the Windows display) and a calibration factor which is specific to the microscope.

Figure 3.

. SEM image of a thin film of gold, captured using the SMART acquisition system and printed using the laser printer. The horizontal white line in the top-left corner is the micrometre bar.

Although we have not yet implemented an electron-beam lithography option, we believe that simple shapes could be generated by defining rectangles on the PCX image (recorded using a low electron dose) using the standard tools provided by the L-View program. The corresponding image coordinates would then be used to define the regions of specimen irradiated by the beam during a subsequent raster scan under high-dose conditions (increased probe current or longer dwell time per pixel). The AT-MIO-16E-10 board provides eight digital outputs, one of which could be used for electron-beam blanking. However, a simpler solution would be to decrease the dwell time per pixel to a low value (corresponding to rapid scanning of the incident beam) while moving from one area of irradiation to another.

Disclaimer

We have no financial interest in the National Instruments Corporation or the Microsoft Corporation (designers of Visual Basic) and are not affiliated with the Independent Jpeg Group (which provided the design basis for L-View).

Ancillary