0 Journal of Microscopy, Vol. 103, Pt 1,January 1975,pp. 79-87. R,eceived 7 M a y 1974; revision received 20 August 1974
Photographic aspects of scanning electron microscopy
by D. F. MALIN,Research Department, CZBA-GEZGY ( U K ) Limited, Simonsway, Manchester M22 5LB SUMMARY
Factors governing photographic image quality in the scanning electron microscope are discussed with particular reference to the commonly used EXA camera on the Cambridge Stereoscan IIa. It is shown that the small image on the medium speed film suffers considerable loss of information due to the turbid nature of the photographic emulsion. Inexpensive modifications to the oscilloscope camera supplied with the Stereoscanare described which enable the superior quality of a larger format to be utilized. Appropriate settings of the brightness and contrast controls of the image tube with respect to the photographic system is discussed and the results illustrated in a series of micrographs. INTRODUCTION
I n the extensive literature on the scanning electron microscope (SEM) there are few references to the photographic aspects of image recording. Many papers describe methods of signal processing or instrumental operation to enhance image quality and much attention has been given to improving the electron optics, electron collection and image display systems to give better results. Finally, however, the micrographs must be recorded and it seems logical to expect that the photographic procedures employed would record all of the information generated by the microscope without loss of detail or quality. It is generally assumed that the photography of SEM images is a simple standardized procedure where conditions are empirically adjusted to give a satisfactory picture and that no significant improvement to the usual slightly unsharp photomicrograph is necessary. It will be shown that optimum quality and resolution in the final print requires considerable care in the selection and adjustment of a photographic system. The demands placed on the optics and photographic materials employed are greater than is generally realized, particularly when 35 mm film is used. This paper refers specifically to the Stereoscan Mark IIa, but the principles outlined are applicable in general to the still photography of images on cathode ray tubes. FOCUS I N SEM I M A G E S
There are three distinct stages where the operator of a scanning electron microscope is able to adjust the focus of the image prior to making a photograph. Two of these variables, record tube and camera focus, are normally fixed and all
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D.F. Malin focusing operations are confined to the electron beam incident upon the specimen. This paper is concerned with the image recording stage only and it is assumed that focus at the specimen is optimized. The apparently uncomplicated matter of photographing lines on a cathode ray tube is not as simple as it at first appears but little reference is made to it in the literature on the SEM. However, a lengthy paper by Tyler & Straub (1963), which pre-dates the commercial introduction of the SEM,deals in detail with theoretical and practical aspects of the topic but does not discuss TV type images of any kind. It is not possible to focus the flat-faced record tube of the Stereoscan Mark IIa over the whole of the raster. Careful examination of the screen shows that the smallest possible spot focused at the centre becomes elliptical when moved to the corner of the frame. The long axis of the ellipse always points towards the centre of the screen. This geometric focus error is illustrated in Fig. Id which shows a section of a print of a 500 line raster cut along the diagonal to include a corner of the tube. The centre of the screen is 25 mm off the right hand side of the illustration which has been enlarged by a factor of 2 with respect to the tube size. It can be seen that the tube is incapable of resolving even 500 lines per frame at the corners and about 20% of the image area around the edges of the display is significantly affected. A n y attempt to improve the focus of the spot at the edge of the screen increases its size at the centre with a corresponding fall in overall resolution. The best compromise seems to be to focus the tube with the spot on a diagonal half way between the centre of the screen and one corner. However, the focus achieved using the naked eye is not necessarily the best and a x 10 hand lens reveals that the spot itself consists of a bright central region surrounded by a relatively large halo and several dimmer satellite spots. Using the magnifier and adjusting the tube focus so that the central spot is at its smallest increases the size of the halo. Focusing with the unaided eye tends to reduce the halo but to increase the size of the much brighter central spot. In the work reported here a magniser was used to adjust tube focus so that the bright central spot was at its smallest. The literature supplied with the Stereoscan states that the record tube resolution is approximately 800 lines per frame. With careful adjustment of tube focus as described above, photographs can be made on our microscope which show a resolved line structure at 1500 lines per frame over most of the micrograph. Figure l c represents a small portion of the centre of such a print enlarged x 10 with respect to the tube size. The picture was taken on 120 roll film using the Telford Oscilloscope camera described later. RECORDING REQUIREMENTS
T o ensure that all of the information displayed on the tube face is recorded, it is important that the photographic system employed resolves the finest line structure that this tube can produce and that this detail is produced on the final print (Oatley, Nixon & Pease, 1965). In practice it has been found that the smallest distance the unaided eye can readily detect is 0.2 mm, corresponding to 5 line pairs per mm or 1000 lines on a 200 x 200 mm print. For larger prints which are to be examined closely a finer line structure would be desirable, but for most purposes 1000 lines per frame is adequate, and in the case of the Stereoscan, convenient. Image tube resolution should ideally be the same in both horizontal and vertical directions. T o achieve this the number of picture points along each line must be equal to the total number of lines in a square raster. Ignoring the geometric focus error of the CRT on the Stereoscan, a 1000 line image will therefore contain lo6
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Photographic aspects of SEM image points. If the tube focus is such that adjacent lines overlap and are not resolved then adjacent image points will also overlap to the detriment of the image. The same concept applies throughout the recording system, including camera lens, film and enlarging optics to the final print, where the scan lines should still be visible. In practice degradation of the image occurs at each stage in the photographic process, so ideally a system should be chosen with a resolution in excess of that normally regarded as adequate for SEM recording. It will be shown later that these requirements are not easily met. Sarson & Stock (1957) have discussed these principles with regard to telerecording on film for subsequent re-broadcast. Choice of line spacing and the ability to resolve the lines can also influence the contrast range available from the record tube as well as ultimate photographic resolution. Figure la-c shows the same area of a silver grid, photographed with 500, 1000 and 1500 lines per frame at an initial instrument magnification of x 5000. The prints on the left of Fig. 1 have been enlarged to x 10,000 (corresponding to a 200 x 200 mm print) whilst those on the right have been further enlarged to ~50,000,to show the line structure. The effect of using only 500 lines is to reduce the contrast range of the print. The x 10 enlargement shows that large strips of the tube phosphor are not illuminated by the electron beam so peak white cannot be reached. However, defocusing either tube or camera will effectively fill the unscanned areas of the picture, and with modification of the enlarger exposure, will restore some of the contrast range, but at the expense of horizontal resolution. The ideal situation is shown in Fig. lc, where at peak white the lines just appear to merge, but everywhere else they are resolved. Under these circumstances maximum resolution and contrast are maintained. Increasing the number of lines to 1500 per frame causes slight blocking of the highlights but extends somewhat the range of mid-tones. With careful control of tube focus as described above, loss of detail in the highlight areas of a 1500 line image is not serious, and excellent 1 m square exhibition prints can be made from 120 size negatives on HP4 film. ADJUSTMENT O F RECORD T U B E LEVELS
The procedure for setting the contrast and brightness levels is relatively simple, but very important if the full photographic potential of the SEM is to be exploited. The following comments apply in particular to the Telford oscilloscope camera, but are valid for any system. With the record CRT focus set as described earlier, the camera is carefully focused at full aperture, using the spot setting on the microscope and preferably a clear glass screen and magnifier to utilize the aerial image. Accurate focus is important since it affects exposure as well as resolution. The camera lens is stopped down to a level where preliminary tests have shown adequate exposure. In the case of the Telford equipment this is f6.3 using HP4 film developed for the normal film speed of 400 ASA. With the record tube contrast control and signal input at zero the tube brightness is increased so that very faint lines can be seen in a darkened room. This is the correct brightness setting for a visual display but optical losses and the film exposure threshold require a higher level than for normal viewing. At this stage the camera shutter is opened and with 1000 lines scanned over (say) 100 sec the brightness is increased in several small but distinct steps. The position on the calibrated brightness control which produces just discernible fogging of the film is the correct one for that particular combination of the line speed, aperture, film and developer. However, the automatic compensation of the record tube brightness for line speeds other than that chosen should be checked but in our case
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Photographic aspects of SEM proved adequate over the range 0.01-1 sec (10-1000 sec frame time at 1000 lines per frame). An appropriate specimen is inserted in the microscope and manipulated to produce a well defined bright vertical edge on the screen. The photomultiplier voltage is adjusted so that the signal from the edge just peaks on the monitoring oscilloscope. Again a running frame is exposed and the contrast control adjusted in steps as before. Separate prints are made from each of the bands of varying contrast large enough for the scan lines to be clearly visible in the mid-tones. The band which shows lines just merging in the area of maximum exposure should indicate the correct contrast setting at optimum image resolution, as in Fig. lb. Ideally, all negatives produced at these settings should produce perfect prints on a given grade of paper without altering the enlarger exposure. In practice some variation at the printing stage is necessary and desirable to accommodate specimens of exceptionally high or low contrast, but both tube settings and film processing conditions must remain constant if optimum quality and consistency are to be maintained. However, if image resolution is degraded at any stage in the photographic process, the lines will merge in the highlights at lower contrast settings and the tonal range of the negative will be restricted. PHOTOGRAPHIC SYSTEM, OPTICS, FORMAT AND F I L M
At first sight, the problems of recording 1000 lines in the 22 mm square format of the EXA Ia do not seem too severe. The line rate is about 50 line pairs per mm in the film plane, well within the capabilities of most modern lenses and medium speed films. Ilford (private communication) quote a high contrast resolution of 150 lines per mm for FP4 film and a recent advertisement describes an objective for a 35 mm single lens reflex camera with a resolution of 100 lines per mm over most of the frame at f 1.8, improving to 150 lines per mm at f4. The Meritar lens fitted to the EXA Ia supplied with our Stereoscan will not resolve 1000 lines per frame at any aperture on a variety of films tried. Indeed the resolution was poorer as the aperture was increased for slower films. Results using FP4 at f8 are shown in Fig. 2a, b and d. Further trials using a 55 mm SMC Takumar lens on a Pentax body showed that some improvement in resolution was obtained on FP4 (125 ASA) but that the lines were still not well separated at an aperture of f8. Slower films such as PanF (50 ASA) and Micro Neg Pan (about 8 ASA) were also used at apertures of about f4-5 and f 2 respectively. Some improvement was noted with PanF and the lines were well enough resolved for the setting up procedure described above to be fully carried out. Micro Neg Pan resolved the lines very well indeed, but its inherently high contrast made printing difficult and the full range of tones could not be accommodated even on very soft paper. This material is a high contrast micro film but for these tests was processed for lower contrast, in I D 11 for 5 min at 20°C. The other films were processed in ID 11 for the recommended times. These simple experiments indicate that adequate line resolution with the 35 mm format is only achieved when a lens of good quality is used in conjunction with a
Fig. 1. SEM micrographs of silver grid at an instrument magnification of ~ 5 0 0 0 .Sections of the 55 x 55mm negatives on 120 roll film enlarged to x 9000 and x 45,000; (a) 500 lines per frame, (b) 1000 lines per frame, (c) 1500 lines per frame, (d) section along the diagonal of a 500 line raster showing the corner of the display. Centre of the tube is 25 mm off the right hand side of the photograph.
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Photographic aspects of SEM slow film. The apparent discrepancy between manufacturer’s resolution figures and these results can be explained by considering more carefully the nature of a scanned image consisting of 1000 lines of varying brightness, and width (Tyler & Straub, 1963) on a 105 mm square tube face. It is difficult to define the exact width of a line at any brightness level because of the Gaussian distribution of electrons within the focused spot at the tube face. The effective width must however be less than 100 pm if it is to be seen as a separate line. It can be assumed that 70 pm represents a reasonable estimate of line width in a mid-tone area. A line pair will therefore consist of a bright band 70 pm wide and a dark space 30 pm wide with no clearly defined border between the two, i.e. edge contrast is very low, unlike the test charts used for lens appraisal. If this structure is now reduced to the 22 mm square frame of the miniature camera, the width of the bright line becomes 14.7 pm and that of the dark band only 6.3 pm at the film plane, ignoring optical diffraction and lens aberrations. As the image of the line enters the photographic emulsion it is scattered by the silver halide grains and on development appears wider and more diffuse. Theoretical aspects of line spread and its relation to spatial frequency response have been discussed by Clark-Jones (1958) and practical values for commercially available films have been published by Zweig, Higgins and MacAdam (1958). Ilford (private communication) state that the line spread function of FP4, measured at the normally accepted 10% level, is 9.2 pm. Light scatter within the emulsion will therefore increase the width of the line from a 14.7 pm image to a 23.9 pm developed band causing adjacent lines to overlap. The lines cannot be said to be fully resolved since the dark space between them will have disappeared, but the variation in brightness across the line may still be detected. The quality of the image therefore is restricted more by the nature of the sensitive material itself than any optical difficiencies, except in the case of the EXA Ia, where the three element Meritar lens can be shown to be inadequate. The use of slower films requires that good quality optics are employed with high resolving power at or near full aperture. These are invariably expensive and if suitable lenses in the EXA bayonet fitting are not available, a camera body must be bought as well, with further problems of adapting it to fit the Stereoscan camera support. On the basis of these observations there seems little justification for attempting to compress a high quality image into the restrictive 22 mm square negative of miniature films. USE OF ROLL F I L M FOR IMAGE RECORDING
These problems can be completely avoided if a larger format is used. The 55 mm square negative of 120 or 220 roll film has almost six times the area of the miniature format and since the reduction ratio from the record tube is almost exactly 1:2 compared with 1: 5 for 35 mm film, the demands made on the optical system of both camera and enlarger are less severe. Similarly the line spread effects are less pronounced permitting the use of faster film and smaller camera lens apertures. The improvement of line resolution and picture quality are obvious, even in half-tone reproduction of Fig. la and b are compared with
Fig. 2. SEM micrographs of silver grid at an instrument magnification of x 5000. Sections of the 22 x 22 mm negatives on 35 mm film (EXA Ia) enlarged to x 9OOO and x 45,000; (a) 500 lines per frame, (b) 1000 lines per frame, (c) and (d) same subject at an instrument magnification of x 500 enlarged to x 2250 on (c) 120 roll film, (d) 35 mm film. 85
D.F. Malin Fig. 2a and b. Other workers (Parsons et al., 1973) have experimented with very expensive apparatus to utilize the larger 120 format but their results do not agree with those described here. They report that a high quality optical system (Hasselblad 500C with 120 mm f5.6 Zeiss Planar lens) produces blurred images due to interference from scan lines and that the results from the EXA Ia are preferable. Close examination of their published micrographs can probably explain why. Figure la in their paper purports to show a micrograph taken with the Hasselblad camera at 1000 lines per frame. The line rate, allowing for the x 2 to enlargement quoted, is in fact about 580 lines per frame and a scan fault has produced uneven line spacing, every other line being displaced towards its neighbour. The lines are clearly resolved even on the screened print as published so both record CRT and camera were in reasonable focus. The microscope seems to be poorly focused. I n their illustration lb, using the EXA Ia, the lines are not resolved as individuals, but the scan fault is sufficiently marked for every other line to be counted in the upper part of the photograph. The line rate appears to be the same 580 lines but the EXA Ia has not fully resolved the scan lines. Microscope focus now appears to have been improved resulting in an apparently sharper picture of better contrast. Unfortunately the illustrations are of different areas of the silver grid specimen so direct comparisons are difficult. These effects are best seen if the illustrations are examined at a shallow angle looking along the scan lines. Quite apart from improvement in picture quality the use of the large negative from 120 roll film offers other important advantages not available with miniature film. In this laboratory it is found convenient to make micrographs in batches of twelve rather than wait for a full 20 or 36 exposure cassette to be used up. 120 films can be developed singly or in groups of two or four to give up to forty-eight negatives for a particularly long investigation. Cutting short lengths of 35 mm film is wasteful and can cause difficulties in filing and storage. The 120 negatives are filed in a commercially available filing system along with a single 203 x 254 mm contact print of all twelve exposures and a data record sheet detailing each shot. The contact prints are large enough for rapid visual assessment of the content and quality of each picture and can be issued as proofs if necessary. Small negatives on 35 mm film demand very high standards of care and cleanliness during processing, handling and storage to avoid ruinous scratches and dust. They are also less suited to the preparation of large display prints. The lower enlargement factors required for roll film allow the use of faster material without an increase in the granularity of the prints which, in turn, means a smaller lens aperture on the camera. In our case 120 HP4 enlarged x 3.7 for a 200 mm square print is less grainy than 35 mm FP4 enlarged x 9 for the same sized print. Both flms were developed in Ilford I D 11 for the recommended times. The use of 120 roll film is undoubtedly more expensive both in terms of film and developer per exposure, but it does have the merit of avoiding the wastage inevitable when making square pictures on 35 mm film in a standard camera. THE T E L P O R D O S C I L L O S C O P E CAMERA
The superiority and convenience of the roll film format can be utilized on the Stereoscan Mark IIa without resorting to very sophisticated and expensive single lens reflex cameras. The Telford oscilloscope camera supplied with the microscope is usually used in conjunction with the Polaroid back. Its lens has been designed to photograph CRTs in the 1:1 x 1:0.5 range and the shutter has the advantage of a ' T' setting. It was modified by replacing the 1:0.7 lens mount supplied by a shorter 1:05 tube and corresponding lens hood to mate with the locking rim
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Photographic aspects of SEM of the camera face plate. A Universal 5 x 4 (127 x 101 mm) camera back in place of the Polaroid fitting and a 12 on 120 roll film holder completed the modification for well under ElOO. All the parts were stock items from Telford Products Limited, Greenford, Middlesex. This arrangement has been in constant use for six years and has proved both convenient and reliable. With the 75 mm f2-8 Dallmeyer lens set at f6-3, 1500 lines per frame can be recorded on HP4 film without loss of detail in the bright parts of the image. The system fully utilizes the excellent image quality available from the Stereoscan and justifies the modest increase in running costs without large capital expenditure. As a final comparison between the two systems, the silver grid used earlier was photographed at an instrument magnification of x 500. Figure 2c taken with the Telford camera on HP4 has been enlarged t O x 2500, equivalent to an 0.5 m square print. Figure 2d illustrates the same area photographed with the EXA Ia on FP4 film. It is clear from these illustrations that the roll film apparatus is capable of recording much finer detail than the EXA Ia and that the difference between the two systems is of practical significance. CONCLUSIONS
T o fully utilize the high quality image of the Stereoscan a photographic system which resolves the scan lines is essential. It has been shown that the small negative on medium speed 35 mm film cannot meet these requirements and that the 35 mm camera supplied with the microscope will not resolve the scan lines under any practical conditions. It is .probable that a system using high quality optics at large aperture with a slow film would be able to record all the information in an SEM image on 35 mm material. However, the difficulties of storing and handling small negatives and the expense of such equipment can be avoided by converting the Telford camera supplied with the microscope to accommodate 120 roll film. The recent introduction of a Polaroid roll film which produces both a print and a negative from one exposure makes this conversion even more attractive, since prints of a useful size and negatives of the whole screen area can be made in one operation. ACKNOWLEDGMENT
The author wishes to thank Mr G. H. Ireland for valuable discussions and helpful comments on the manuscript and CIBA-GEIGY (UK) Limited, for permission to publish this article.
References Clark-Jones, R. (1958) On the point and line spread functions of photographic images.J. opt. SOC.Am. 40,934. Oatley, C.W., Nixon, W.C. & Pease, R.F.W. (1965) Scanning electron microscopy. Adv. In: Electronics Electron Phys. 21, 181. Parsons, Elizabeth, Bole, Barbara, Hall, D.J. & Thomas, W.D.E. (1973) Photographic recording of scanning electron microscope images. Micron, 4, 291. Sarson, A.E. 81 Stock, P.B. (1957) A new approach to telerecording.Brit. Kinematogr. 31,119. Tyler, R.W. & Straub, C. (1963) Photography and photomevy of CRT displays. Photogr. Sci. Engng, 7 , 289. Zweig, H.J., Higgins, C.G. & MacAdam, D.L. (1958). On the information detecting capacity of photographic emulsions. J . Opt. Soc. Am. 48,926.
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Photographic aspects of scanning electron microscopy
by D. F. MALIN,Research Department, CZBA-GEZGY ( U K ) Limited, Simonsway, Manchester M22 5LB SUMMARY
Factors governing photographic image quality in the scanning electron microscope are discussed with particular reference to the commonly used EXA camera on the Cambridge Stereoscan IIa. It is shown that the small image on the medium speed film suffers considerable loss of information due to the turbid nature of the photographic emulsion. Inexpensive modifications to the oscilloscope camera supplied with the Stereoscanare described which enable the superior quality of a larger format to be utilized. Appropriate settings of the brightness and contrast controls of the image tube with respect to the photographic system is discussed and the results illustrated in a series of micrographs. INTRODUCTION
I n the extensive literature on the scanning electron microscope (SEM) there are few references to the photographic aspects of image recording. Many papers describe methods of signal processing or instrumental operation to enhance image quality and much attention has been given to improving the electron optics, electron collection and image display systems to give better results. Finally, however, the micrographs must be recorded and it seems logical to expect that the photographic procedures employed would record all of the information generated by the microscope without loss of detail or quality. It is generally assumed that the photography of SEM images is a simple standardized procedure where conditions are empirically adjusted to give a satisfactory picture and that no significant improvement to the usual slightly unsharp photomicrograph is necessary. It will be shown that optimum quality and resolution in the final print requires considerable care in the selection and adjustment of a photographic system. The demands placed on the optics and photographic materials employed are greater than is generally realized, particularly when 35 mm film is used. This paper refers specifically to the Stereoscan Mark IIa, but the principles outlined are applicable in general to the still photography of images on cathode ray tubes. FOCUS I N SEM I M A G E S
There are three distinct stages where the operator of a scanning electron microscope is able to adjust the focus of the image prior to making a photograph. Two of these variables, record tube and camera focus, are normally fixed and all
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D.F. Malin focusing operations are confined to the electron beam incident upon the specimen. This paper is concerned with the image recording stage only and it is assumed that focus at the specimen is optimized. The apparently uncomplicated matter of photographing lines on a cathode ray tube is not as simple as it at first appears but little reference is made to it in the literature on the SEM. However, a lengthy paper by Tyler & Straub (1963), which pre-dates the commercial introduction of the SEM,deals in detail with theoretical and practical aspects of the topic but does not discuss TV type images of any kind. It is not possible to focus the flat-faced record tube of the Stereoscan Mark IIa over the whole of the raster. Careful examination of the screen shows that the smallest possible spot focused at the centre becomes elliptical when moved to the corner of the frame. The long axis of the ellipse always points towards the centre of the screen. This geometric focus error is illustrated in Fig. Id which shows a section of a print of a 500 line raster cut along the diagonal to include a corner of the tube. The centre of the screen is 25 mm off the right hand side of the illustration which has been enlarged by a factor of 2 with respect to the tube size. It can be seen that the tube is incapable of resolving even 500 lines per frame at the corners and about 20% of the image area around the edges of the display is significantly affected. A n y attempt to improve the focus of the spot at the edge of the screen increases its size at the centre with a corresponding fall in overall resolution. The best compromise seems to be to focus the tube with the spot on a diagonal half way between the centre of the screen and one corner. However, the focus achieved using the naked eye is not necessarily the best and a x 10 hand lens reveals that the spot itself consists of a bright central region surrounded by a relatively large halo and several dimmer satellite spots. Using the magnifier and adjusting the tube focus so that the central spot is at its smallest increases the size of the halo. Focusing with the unaided eye tends to reduce the halo but to increase the size of the much brighter central spot. In the work reported here a magniser was used to adjust tube focus so that the bright central spot was at its smallest. The literature supplied with the Stereoscan states that the record tube resolution is approximately 800 lines per frame. With careful adjustment of tube focus as described above, photographs can be made on our microscope which show a resolved line structure at 1500 lines per frame over most of the micrograph. Figure l c represents a small portion of the centre of such a print enlarged x 10 with respect to the tube size. The picture was taken on 120 roll film using the Telford Oscilloscope camera described later. RECORDING REQUIREMENTS
T o ensure that all of the information displayed on the tube face is recorded, it is important that the photographic system employed resolves the finest line structure that this tube can produce and that this detail is produced on the final print (Oatley, Nixon & Pease, 1965). In practice it has been found that the smallest distance the unaided eye can readily detect is 0.2 mm, corresponding to 5 line pairs per mm or 1000 lines on a 200 x 200 mm print. For larger prints which are to be examined closely a finer line structure would be desirable, but for most purposes 1000 lines per frame is adequate, and in the case of the Stereoscan, convenient. Image tube resolution should ideally be the same in both horizontal and vertical directions. T o achieve this the number of picture points along each line must be equal to the total number of lines in a square raster. Ignoring the geometric focus error of the CRT on the Stereoscan, a 1000 line image will therefore contain lo6
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Photographic aspects of SEM image points. If the tube focus is such that adjacent lines overlap and are not resolved then adjacent image points will also overlap to the detriment of the image. The same concept applies throughout the recording system, including camera lens, film and enlarging optics to the final print, where the scan lines should still be visible. In practice degradation of the image occurs at each stage in the photographic process, so ideally a system should be chosen with a resolution in excess of that normally regarded as adequate for SEM recording. It will be shown later that these requirements are not easily met. Sarson & Stock (1957) have discussed these principles with regard to telerecording on film for subsequent re-broadcast. Choice of line spacing and the ability to resolve the lines can also influence the contrast range available from the record tube as well as ultimate photographic resolution. Figure la-c shows the same area of a silver grid, photographed with 500, 1000 and 1500 lines per frame at an initial instrument magnification of x 5000. The prints on the left of Fig. 1 have been enlarged to x 10,000 (corresponding to a 200 x 200 mm print) whilst those on the right have been further enlarged to ~50,000,to show the line structure. The effect of using only 500 lines is to reduce the contrast range of the print. The x 10 enlargement shows that large strips of the tube phosphor are not illuminated by the electron beam so peak white cannot be reached. However, defocusing either tube or camera will effectively fill the unscanned areas of the picture, and with modification of the enlarger exposure, will restore some of the contrast range, but at the expense of horizontal resolution. The ideal situation is shown in Fig. lc, where at peak white the lines just appear to merge, but everywhere else they are resolved. Under these circumstances maximum resolution and contrast are maintained. Increasing the number of lines to 1500 per frame causes slight blocking of the highlights but extends somewhat the range of mid-tones. With careful control of tube focus as described above, loss of detail in the highlight areas of a 1500 line image is not serious, and excellent 1 m square exhibition prints can be made from 120 size negatives on HP4 film. ADJUSTMENT O F RECORD T U B E LEVELS
The procedure for setting the contrast and brightness levels is relatively simple, but very important if the full photographic potential of the SEM is to be exploited. The following comments apply in particular to the Telford oscilloscope camera, but are valid for any system. With the record CRT focus set as described earlier, the camera is carefully focused at full aperture, using the spot setting on the microscope and preferably a clear glass screen and magnifier to utilize the aerial image. Accurate focus is important since it affects exposure as well as resolution. The camera lens is stopped down to a level where preliminary tests have shown adequate exposure. In the case of the Telford equipment this is f6.3 using HP4 film developed for the normal film speed of 400 ASA. With the record tube contrast control and signal input at zero the tube brightness is increased so that very faint lines can be seen in a darkened room. This is the correct brightness setting for a visual display but optical losses and the film exposure threshold require a higher level than for normal viewing. At this stage the camera shutter is opened and with 1000 lines scanned over (say) 100 sec the brightness is increased in several small but distinct steps. The position on the calibrated brightness control which produces just discernible fogging of the film is the correct one for that particular combination of the line speed, aperture, film and developer. However, the automatic compensation of the record tube brightness for line speeds other than that chosen should be checked but in our case
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Photographic aspects of SEM proved adequate over the range 0.01-1 sec (10-1000 sec frame time at 1000 lines per frame). An appropriate specimen is inserted in the microscope and manipulated to produce a well defined bright vertical edge on the screen. The photomultiplier voltage is adjusted so that the signal from the edge just peaks on the monitoring oscilloscope. Again a running frame is exposed and the contrast control adjusted in steps as before. Separate prints are made from each of the bands of varying contrast large enough for the scan lines to be clearly visible in the mid-tones. The band which shows lines just merging in the area of maximum exposure should indicate the correct contrast setting at optimum image resolution, as in Fig. lb. Ideally, all negatives produced at these settings should produce perfect prints on a given grade of paper without altering the enlarger exposure. In practice some variation at the printing stage is necessary and desirable to accommodate specimens of exceptionally high or low contrast, but both tube settings and film processing conditions must remain constant if optimum quality and consistency are to be maintained. However, if image resolution is degraded at any stage in the photographic process, the lines will merge in the highlights at lower contrast settings and the tonal range of the negative will be restricted. PHOTOGRAPHIC SYSTEM, OPTICS, FORMAT AND F I L M
At first sight, the problems of recording 1000 lines in the 22 mm square format of the EXA Ia do not seem too severe. The line rate is about 50 line pairs per mm in the film plane, well within the capabilities of most modern lenses and medium speed films. Ilford (private communication) quote a high contrast resolution of 150 lines per mm for FP4 film and a recent advertisement describes an objective for a 35 mm single lens reflex camera with a resolution of 100 lines per mm over most of the frame at f 1.8, improving to 150 lines per mm at f4. The Meritar lens fitted to the EXA Ia supplied with our Stereoscan will not resolve 1000 lines per frame at any aperture on a variety of films tried. Indeed the resolution was poorer as the aperture was increased for slower films. Results using FP4 at f8 are shown in Fig. 2a, b and d. Further trials using a 55 mm SMC Takumar lens on a Pentax body showed that some improvement in resolution was obtained on FP4 (125 ASA) but that the lines were still not well separated at an aperture of f8. Slower films such as PanF (50 ASA) and Micro Neg Pan (about 8 ASA) were also used at apertures of about f4-5 and f 2 respectively. Some improvement was noted with PanF and the lines were well enough resolved for the setting up procedure described above to be fully carried out. Micro Neg Pan resolved the lines very well indeed, but its inherently high contrast made printing difficult and the full range of tones could not be accommodated even on very soft paper. This material is a high contrast micro film but for these tests was processed for lower contrast, in I D 11 for 5 min at 20°C. The other films were processed in ID 11 for the recommended times. These simple experiments indicate that adequate line resolution with the 35 mm format is only achieved when a lens of good quality is used in conjunction with a
Fig. 1. SEM micrographs of silver grid at an instrument magnification of ~ 5 0 0 0 .Sections of the 55 x 55mm negatives on 120 roll film enlarged to x 9000 and x 45,000; (a) 500 lines per frame, (b) 1000 lines per frame, (c) 1500 lines per frame, (d) section along the diagonal of a 500 line raster showing the corner of the display. Centre of the tube is 25 mm off the right hand side of the photograph.
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Photographic aspects of SEM slow film. The apparent discrepancy between manufacturer’s resolution figures and these results can be explained by considering more carefully the nature of a scanned image consisting of 1000 lines of varying brightness, and width (Tyler & Straub, 1963) on a 105 mm square tube face. It is difficult to define the exact width of a line at any brightness level because of the Gaussian distribution of electrons within the focused spot at the tube face. The effective width must however be less than 100 pm if it is to be seen as a separate line. It can be assumed that 70 pm represents a reasonable estimate of line width in a mid-tone area. A line pair will therefore consist of a bright band 70 pm wide and a dark space 30 pm wide with no clearly defined border between the two, i.e. edge contrast is very low, unlike the test charts used for lens appraisal. If this structure is now reduced to the 22 mm square frame of the miniature camera, the width of the bright line becomes 14.7 pm and that of the dark band only 6.3 pm at the film plane, ignoring optical diffraction and lens aberrations. As the image of the line enters the photographic emulsion it is scattered by the silver halide grains and on development appears wider and more diffuse. Theoretical aspects of line spread and its relation to spatial frequency response have been discussed by Clark-Jones (1958) and practical values for commercially available films have been published by Zweig, Higgins and MacAdam (1958). Ilford (private communication) state that the line spread function of FP4, measured at the normally accepted 10% level, is 9.2 pm. Light scatter within the emulsion will therefore increase the width of the line from a 14.7 pm image to a 23.9 pm developed band causing adjacent lines to overlap. The lines cannot be said to be fully resolved since the dark space between them will have disappeared, but the variation in brightness across the line may still be detected. The quality of the image therefore is restricted more by the nature of the sensitive material itself than any optical difficiencies, except in the case of the EXA Ia, where the three element Meritar lens can be shown to be inadequate. The use of slower films requires that good quality optics are employed with high resolving power at or near full aperture. These are invariably expensive and if suitable lenses in the EXA bayonet fitting are not available, a camera body must be bought as well, with further problems of adapting it to fit the Stereoscan camera support. On the basis of these observations there seems little justification for attempting to compress a high quality image into the restrictive 22 mm square negative of miniature films. USE OF ROLL F I L M FOR IMAGE RECORDING
These problems can be completely avoided if a larger format is used. The 55 mm square negative of 120 or 220 roll film has almost six times the area of the miniature format and since the reduction ratio from the record tube is almost exactly 1:2 compared with 1: 5 for 35 mm film, the demands made on the optical system of both camera and enlarger are less severe. Similarly the line spread effects are less pronounced permitting the use of faster film and smaller camera lens apertures. The improvement of line resolution and picture quality are obvious, even in half-tone reproduction of Fig. la and b are compared with
Fig. 2. SEM micrographs of silver grid at an instrument magnification of x 5000. Sections of the 22 x 22 mm negatives on 35 mm film (EXA Ia) enlarged to x 9OOO and x 45,000; (a) 500 lines per frame, (b) 1000 lines per frame, (c) and (d) same subject at an instrument magnification of x 500 enlarged to x 2250 on (c) 120 roll film, (d) 35 mm film. 85
D.F. Malin Fig. 2a and b. Other workers (Parsons et al., 1973) have experimented with very expensive apparatus to utilize the larger 120 format but their results do not agree with those described here. They report that a high quality optical system (Hasselblad 500C with 120 mm f5.6 Zeiss Planar lens) produces blurred images due to interference from scan lines and that the results from the EXA Ia are preferable. Close examination of their published micrographs can probably explain why. Figure la in their paper purports to show a micrograph taken with the Hasselblad camera at 1000 lines per frame. The line rate, allowing for the x 2 to enlargement quoted, is in fact about 580 lines per frame and a scan fault has produced uneven line spacing, every other line being displaced towards its neighbour. The lines are clearly resolved even on the screened print as published so both record CRT and camera were in reasonable focus. The microscope seems to be poorly focused. I n their illustration lb, using the EXA Ia, the lines are not resolved as individuals, but the scan fault is sufficiently marked for every other line to be counted in the upper part of the photograph. The line rate appears to be the same 580 lines but the EXA Ia has not fully resolved the scan lines. Microscope focus now appears to have been improved resulting in an apparently sharper picture of better contrast. Unfortunately the illustrations are of different areas of the silver grid specimen so direct comparisons are difficult. These effects are best seen if the illustrations are examined at a shallow angle looking along the scan lines. Quite apart from improvement in picture quality the use of the large negative from 120 roll film offers other important advantages not available with miniature film. In this laboratory it is found convenient to make micrographs in batches of twelve rather than wait for a full 20 or 36 exposure cassette to be used up. 120 films can be developed singly or in groups of two or four to give up to forty-eight negatives for a particularly long investigation. Cutting short lengths of 35 mm film is wasteful and can cause difficulties in filing and storage. The 120 negatives are filed in a commercially available filing system along with a single 203 x 254 mm contact print of all twelve exposures and a data record sheet detailing each shot. The contact prints are large enough for rapid visual assessment of the content and quality of each picture and can be issued as proofs if necessary. Small negatives on 35 mm film demand very high standards of care and cleanliness during processing, handling and storage to avoid ruinous scratches and dust. They are also less suited to the preparation of large display prints. The lower enlargement factors required for roll film allow the use of faster material without an increase in the granularity of the prints which, in turn, means a smaller lens aperture on the camera. In our case 120 HP4 enlarged x 3.7 for a 200 mm square print is less grainy than 35 mm FP4 enlarged x 9 for the same sized print. Both flms were developed in Ilford I D 11 for the recommended times. The use of 120 roll film is undoubtedly more expensive both in terms of film and developer per exposure, but it does have the merit of avoiding the wastage inevitable when making square pictures on 35 mm film in a standard camera. THE T E L P O R D O S C I L L O S C O P E CAMERA
The superiority and convenience of the roll film format can be utilized on the Stereoscan Mark IIa without resorting to very sophisticated and expensive single lens reflex cameras. The Telford oscilloscope camera supplied with the microscope is usually used in conjunction with the Polaroid back. Its lens has been designed to photograph CRTs in the 1:1 x 1:0.5 range and the shutter has the advantage of a ' T' setting. It was modified by replacing the 1:0.7 lens mount supplied by a shorter 1:05 tube and corresponding lens hood to mate with the locking rim
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Photographic aspects of SEM of the camera face plate. A Universal 5 x 4 (127 x 101 mm) camera back in place of the Polaroid fitting and a 12 on 120 roll film holder completed the modification for well under ElOO. All the parts were stock items from Telford Products Limited, Greenford, Middlesex. This arrangement has been in constant use for six years and has proved both convenient and reliable. With the 75 mm f2-8 Dallmeyer lens set at f6-3, 1500 lines per frame can be recorded on HP4 film without loss of detail in the bright parts of the image. The system fully utilizes the excellent image quality available from the Stereoscan and justifies the modest increase in running costs without large capital expenditure. As a final comparison between the two systems, the silver grid used earlier was photographed at an instrument magnification of x 500. Figure 2c taken with the Telford camera on HP4 has been enlarged t O x 2500, equivalent to an 0.5 m square print. Figure 2d illustrates the same area photographed with the EXA Ia on FP4 film. It is clear from these illustrations that the roll film apparatus is capable of recording much finer detail than the EXA Ia and that the difference between the two systems is of practical significance. CONCLUSIONS
T o fully utilize the high quality image of the Stereoscan a photographic system which resolves the scan lines is essential. It has been shown that the small negative on medium speed 35 mm film cannot meet these requirements and that the 35 mm camera supplied with the microscope will not resolve the scan lines under any practical conditions. It is .probable that a system using high quality optics at large aperture with a slow film would be able to record all the information in an SEM image on 35 mm material. However, the difficulties of storing and handling small negatives and the expense of such equipment can be avoided by converting the Telford camera supplied with the microscope to accommodate 120 roll film. The recent introduction of a Polaroid roll film which produces both a print and a negative from one exposure makes this conversion even more attractive, since prints of a useful size and negatives of the whole screen area can be made in one operation. ACKNOWLEDGMENT
The author wishes to thank Mr G. H. Ireland for valuable discussions and helpful comments on the manuscript and CIBA-GEIGY (UK) Limited, for permission to publish this article.
References Clark-Jones, R. (1958) On the point and line spread functions of photographic images.J. opt. SOC.Am. 40,934. Oatley, C.W., Nixon, W.C. & Pease, R.F.W. (1965) Scanning electron microscopy. Adv. In: Electronics Electron Phys. 21, 181. Parsons, Elizabeth, Bole, Barbara, Hall, D.J. & Thomas, W.D.E. (1973) Photographic recording of scanning electron microscope images. Micron, 4, 291. Sarson, A.E. 81 Stock, P.B. (1957) A new approach to telerecording.Brit. Kinematogr. 31,119. Tyler, R.W. & Straub, C. (1963) Photography and photomevy of CRT displays. Photogr. Sci. Engng, 7 , 289. Zweig, H.J., Higgins, C.G. & MacAdam, D.L. (1958). On the information detecting capacity of photographic emulsions. J . Opt. Soc. Am. 48,926.
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