Home
Search
Collections
Journals
About
Contact us
My IOPscience
Phototitus: image intensifier and converter for an electron microscope
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1977 J. Phys. E: Sci. Instrum. 10 520 (http://iopscience.iop.org/0022-3735/10/5/029) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 139.184.14.159 This content was downloaded on 02/10/2015 at 04:36
Please note that terms and conditions apply.
P G Cracen and D J Fegan References Adam G R 1971 Publs R. Obs. Edinb. 8 43 Beaver E A, McIlwain C E, Choisser J P and Wysoczanski N 1972 Proc. 5th Symp. on PhotoeIectronic Image DeGices, London, 1971 (London: Academic) Fegan D J and Craven P G 1977 J. Phys. E: Sei. Instrum. 10 510-15 Jelley J V 1973 Obsewatory 93 9 Kobler H 1975 J. Phys. E: Sci. Instrum. 8 426 Whitford A E and Kron G E 1937 Rev. Sci. Instrum. 8 78
Phototitus: image intensifier and converter for an electron microscope
F Dumontt, D Rossierf, P Bonhommet, A Beorchiat and B Meuniert t Laboratoires d’tilectronique et de Physique Appliquee, 94450 Limeil-Brevannes, France $ Faculte des Sciences de Reims, France Receiced I I October 1976, in finalform 20 December 1976 Abstract We describe an image converter, Phototitus, consisting of a photoconductor deposited on an electrooptical dielectric crystal. The electron microscope beam incident on the photoconductor creates electron-hole pairs which modulate the transparency of the electro-optical dielectric crystal by means of the Pockels effect. The converter can be read out with a fairly high-intensity light source, making Phototitus a solid state image intensifier. The read-out light can be coherent, allowing direct processing of the image, e.g. a real-time Fourier transform of the image which permits fast, accurate focusing of the electron microscope. Furthermore, Phototitus can be used as an ‘analogue optical computer’ making possible applications such as sharpening of high-resolution images.
Journal of Physics E : Scientific Instruments 1977 Volume 10 Printed in Great Britain 0 1977 520
1 Introduction The observation of delicate objects with an electron microscope requires exposures low enough to avoid the destruction of thin object structures by radiation damage. Furthermore, focusing and stigmating the objective lenses is often a laborious, timeconsuming task during which the object receives a high electron dose. These considerations apply to the conventional transmission electron microscope as well as to the scanning transmission electron microscope. Various methods have been proposed in order to overcome the limitations just outlined: channel plate electron multipliers have been used as image intensifiers (Goetze and Laponsky 1971), and several focusing and stigmating methods have been proposed including the observation of the Fourier transform (Thon 1965) or of the autocorrelation function (Frank 1975) of the image of a ‘white noise’ test pattern. However, it is generally difficult to use these methods in real time. For instance, analogue devices performing a Fourier transform by optical diffraction require the use of an optical transparency and thus are very time-consuming. On the other hand, it is in principle possible to digitize the image and to process it by a computer which displays on a cathode ray tube either the Fourier transform or the autocorrelation function of the image. Both functions permit one to estimate the focusing and stigmating of the microscope, but an image digitizer and a computer are
Electron microscope image intensifier and cont‘erter needed; moreover, the computation time may be fairly long depending on the size of the computer and the precision of the computation (Frank 1975). The previous discussion shows that an analogue device capable of performing the Fourier transform in real time would be very useful and would avoid the hardware required by the digital methods. Such a device has recently been realized using the technologies developed in the field of optical light valves. In this paper we describe the application of Phototitus - a light valve which has been used for image processing (Dumont et a1 1974, Marie et a1 1974) - in the field of electron microscopy. Phototitus makes it possible to increase the brightness of the image, thus allowing a reduction in exposure of the object to radiation damage. Furthermore, the real-time imageprocessing capability of Phototitus makes it easy to display the two-dimensional Fourier transform of an image. The Fouriermode display presents to the operator a new field of investigation of the electron images and yields new, powerful methods for making such adjustments as sharp focusing of highresolution images, corrections of astigmatism and spatial frequency filtering in the Fourier plane. Another important application involves the use of Phototitus in the field of image improvement by holographic methods. These methods have been widely used to de-blur and sharpen the images given by high-resolution microscopes of both the scanning and the conventional type (Hanszen 1973). For example, very thin slices of material (such as unstained specimens of biological matter) examined with a high-resolution electron microscope behave as phase objects and the image has no contrast when the microscope is focused. Thus the microscope is often defocused so as to give a contrasted image (‘defocusing phase contrast’). The image thus obtained is always blurred and can be de-blurred by means of holographic filtering methods (Stroke and Halioua 1973). In this context, Phototitus is useful because it can be read out by coherent light so that, when associated with a coherent light source and a holographic filter, it behaves like a fast ‘analogue optical computer’, making real-time image restoration possible (Bonhomme et a1 1976a, Bonhomme et a1 1976b). 2 Writing-in with an electron beam Phototitus is a light valve which uses the Pockels effect in a DKDP (deuterated potassium diphosphate) crystal addressed by photoconduction through an amorphous selenium layer (Dumont et a1 1974). The electron-hole pairs required to produce the photoconduction can be created by a beam of fast electrons such as that of an electron microscope. As shown in
figure 1, Phototitus is then placed in the microscope with its photoconducting member exposed to the electron beam, at the place normally occupied by the phosphor. The operation is as follows. In the first step the voltage Vis applied to the sandwich (photoconductor and DKDP)while the electron beam is absorbed by the photoconductor after passing through a very thin electrode. The electron-hole pairs created are separated by the applied field and the carriers of one type (electrons or holes according to the polarity of the applied voltage) drift in the photoconductor, to be trapped in deep centres near the interface between the photoconductor and the dielectric mirror. This produces an image-like pattern of trapped charges which modulates the voltage applied to the DKDP.
In the second step, the electron beam is switched off or deflected and the electrodes are short-circuited so that the voltage applied to the DKDP is produced only by the trapped charges. The DKDP,having its optical axis in the direction of the incident light, is illuminated by a beam of polarized light through a polarizing beam splitter and a projection lens. On passing twice through the DKDP,the light beam becomes elliptically polarized so that the polarizing beam splitter directs towards the screen the light having its polarization perpendicular to the polarization of the incident light. The intensity of the light falling on the screen is
I = 10sin2 (Vk/Vh!Z)
(1)
where IO is the intensity of the incident light, V k the voltage applied to the DKDP by the pattern of trapped charges ( V k is spatially variable according to the intensity of the primary electron beam), and V h p is the half-wave voltage of the DKDP. In order to reduce V h / 2 the DKDP is cooled by Peltier elements to a temperature slightly above its Curie temperature (- 503C), making it passible to operate Phototitus with only 150-200 V (instead of about 2000 V at room temperature). In these conditions the modulation efficiency by the Pockels effect in DKDP can be as high as 75 %. 3 Characteristicsof the converter This new mode of conversion of an electron microscope image provides several advantages as compared to the normal case where the image is made visible by use of a phosphor screen.
3.1 Gain A fast electron creates a large number of electron-hole pairs when it loses its energy in the photoconductor. Owing to this multiplication process a high conversion efficiency is achieved.
Mu1 t I 1 aye r dielectric mirror
i
Transparent electrodes
micposcope
column
Figure 1 Set-up of Phototitus used with an electron microscope
Depth below surface (pm)
Figure 2 Calculated energy dissipation of fast electrons in amorphous selenium. About 25 % of the primary beam energy is backscattered
521
F Dumont, D Rossier, P Bonhomme, A Beorchia and B Meunier From Spear (1956) we know that an energy of 18-25 eV is required to create one electron-hole pair in selenium, so we can estimate the number of secondary electron-hole pairs created by one primary electron and the exposure necessary to write an image in Phototitus. However, data concerning the penetration of fast electrons must also be taken into account. The plots in figure 2 show the energy dissipation of a fast electron in amorphous selenium, calculated according to the formula given by Klein (1966). These plots show that electrons having an energy less than 100 keV lose most of their energy (except for the backscattered energy) in the thickness of the selenium layer currently used in Phototitus (about 15 pm). However, the more energetic electrons lose only a fraction of their energy in the selenium layer, resulting in a lower gain. From these data we can estimate the sensitivity of Phototitus to a beam of fast electrons. The results are shown in table 1.
The calculations have been made assuming that the DKDP is 0 5 ° C above its Curie temperature, so that the dielectric constant and half-wave voltage are ~=600
v= 300 Vh12.
(2) (3)
Measurements of the sensitivity gave results in fair agreement with these values. This sensitivity is less than that of the devices using Vidicon or silicon Vidicon tubes which can be operated down to 10-9 or 10-10 A m-2. However, the image is free of line structure since there is no scanning beam (except in the case of the scanning electron microscope) and Phototitus allows one to attain a much higher resolution. The sensitivity to high-energy electrons could be increased in principle by having a thicker selenium layer on the DKDP crystal, but this would impair the resolution. Thelimitingresolution of Phototitus addressed with visible light is 70 lines/mm (Donjon et a1 1975, Roach 1974). When Phototitus is addressed with an electron beam, a resolution decrease results from the spreading of the electron trajectories in selenium. Figure 3 shows the image of a metallic grid made of threads with a diameter of 18 pm and spaced by 45 pm. The threads are clearly visible, so the limiting resolution has probably not been reached. The ultimate resolution is discussed further in the next section. 522
Figure 3 Shadow images of electron microscope metallic grids. The grids were placed near the photoconductor to obtain unit magnification on the Phototitus target plane. (a) The grid is made of threads of 22 pm diameter, 90 pm apart. (b) The threads are 18 pm in diameter and 45 pm apart. The magnification in the plane of the photograph is x 50 for both (a) and (b)
3.2 Dimensions and brightness ofthe image The image written in Phototitus by the electron beam is magnified and projected on to the screen S by the projection lens L (figure 1). The image displayed on S can have large dimensions and a high brightness since this brightness depends only on the luminous power emitted by the light source. The sensitive area of Phototitus, 27 mm x 38 mm, is projected on to S with a magnification of 2-7 (practically limited only by the luminous power of the read-out light source). The image displayed by Phototitus is much brighter than if it were directly viewed by means of a phosphor screen. For instance, for an electron beam of 1O-6 A m-2 (energy 75 keV)
Electron microscope image intensifjerand converter
falling on a phosphor whose energy conversion efficiency is 5-15%, the density of radiant flux emitted is 4-10 mW m-2. If Phototitus is used instead, with a read-out light flux density of 1 W m-2 on the DKDP and a 50% DKDP modulation efficiency in order to preserve the grey-scale capability, the radiant flux density on the observation screen is 250 mW m-2 for unity magnification and 10 mW m-2 for x 5 magnification, so that either a gain of brightness or a magnification of the image or both can be obtained.
Figure 4 Image of lattice plane spacings of catalase crystals (both 8.75 nm and 6.85 nm pitches). The magnifications in the plane of the photoconductor and of the observation screen are respectively x 9700 and x 155 OOO
Thus the use of Phototitus makes the observations more convenient; furthermore, owing to the memory, the image can be observed or processed for several minutes and photographs can be taken without exposing the object to the electron beam. Figure 4 shows the image of lattice plane spacings of catalase crystals (pitch 8.75 nm), which were not visible on the fluorescent screen of the electron microscope (using a binocular eyepiece) with a magnification of x 6700, but were clearly visible on the projection screen of Phototitus owing to the increased brightness and dimensions of the image. The periodicity in the other direction (6.75 nm) is also clearly discernible. Therefore, the resolution is at least 16 lines/" for a thin, low-contrast preparation. For a more contrasted object it is higher than 25 lines/" (as shown by the metallic grid of figure 3), but the limit resolution has not yet been determined. Figure 5 shows a Fresnel fringe around a hole in an amorphous carbon film.
3.3 Memory An electron image can be stored by Phototitus as long as the charge carriers remain trapped near the photoconductordielectric-mirror interface (several minutes), allowing examination of the image with the electron beam switched off to avoid radiation damage of the sample. As the dielectric mirror is not completely opaque, 2-3% of the read-out light reaches the photoconductor, generating electron-hole pairs which recombine with the trapped charges, gradually erasing the stored image. The memory time is limited by the luminous power of the read-out light source and the wavelength of the read-out light since the reflection of the dielectric mirror and the photosensitivity of the photoconductor are strongly wavelengthdependent (amorphous selenium is nearly insensitive to yellow
Figure 5 Fresnel fringe around a hole in an amorphous carbon film. (a) Image displayed on the phosphor screen of the electron microscope; (b) the same image displayed by Phototitus. The contrast enhancement has been obtained by a small uniform background subtraction
and red light making this part of the visible spectrum convenient for reading out Phototitus). In practice, if the projected image is 90 mm x 60 mm, the memory time is between 5 and 10 min assuming a luminous intensity high enough for visual examination in dark-room conditions (with a yellow or red read-out light). 3.4 Possibility of direct image processing The optical image-processing ability of Phototitus has been described by Marie et al( 1974). Most of the conclusions remain valid as far as the electron images are concerned. It should be emphasized that the advantage of Phototitus consists of performing whole image processing in real time without the need of a point-by-point image analysis, in contrast with photographic or electronic techniques. Addition or subtraction of images can be performed by the addition or subtraction of the pattern of trapped charges: addition of two images is obtained by successive projections on
523
F Dumont, D Rossier, P Bonhomme, A Beorchia and B Meunier lifetimes were unchanged, and only a small decrease in the photogeneration efficiencyfor the visible light (30%after 100 h and 50% after 200 h) was observed. Therefore after being exposed for 200 h or more to an electron beam in normal conditions, the resolution, contrast and memory of Phototitus should be unchanged (since these properties do not depend on the photogeneration efficiency). The sensitivity will be slightly decreased, but this could be compensated by increasing the exposure time. In fact, the device was operated in an electron microscope for several months, and no continuous degradation was observed, indicating the reliability of the electron-beamaddressed Phototitus. Conclusion Phototitus exhibits several features which make it useful in the field of electron microscopy. The sensitivity is good (of the same order as that of usual microscope plates) owing to the gain of the photoconductor. High-quality images can be displayed. The contrast ratio is high: more than lO3:l with a highly collimated read-out beam. The resolution is better than 25 lines/" for high-contrast objects and the grey-scale capability is good. The memory makes it possible to observe and image for several minutes while not producing radiation damage to the observed sample. However the most interesting property of Phototitus is its coherent read-out capability, permitting this device to act as a fast analogue optical computer, allowing real-time image improvement by holographic methods. Such experiments are now in progress and the results will be reported in another paper. 4
Figure 6 Zinc oxide crystals (magnification in the plane of Phototitus: x 10 000). The image has been differentiated to show the contours
the photoconductor with the same applied voltage; and subtraction of two images is obtained by successive projection with reversal of the polarity of the applied voltage V (figure 1) between the exposures. The image subtraction capability allows spatial differentiation and display of equiluminance contours (by subtracting a uniform background), resulting in contrast or in small detail enhancement (figures 5, 6). An important feature is the image-processing capability of Phototitus in coherent light as described in the introduction. If a coherent light source is used, the two-dimensional Fourier transform of the stored image is displayed in the focal plane of the projection lens L (figure 1). This feature can be used, for instance, to focus the electron microscope and to correct the astigmatism, following the method discussed by Thon (1965). This method consists of displaying the Fourier transform of an object, such as a carbon film, which contains all the spatial frequencies up to a certain limit (a 'white noise' object). The Fourier transform of the well focused image of this object has the shape of a disc the radius of which is proportional to the highest spatial frequency present in the object. Conversely if the microscope is not correctly focused, the Fourier transform is fringed by dark and bright concentric circles, and if there is some astigmatism the Fourier transform is no longer circular but distorted. Displaying the Fourier transform of an image is just one simple example of the possibilities given by the use of coherent light. Other interesting applications consist in image improvement by holographic filtering methods (Hanszen 1973). In this field of applications, Phototitus has the advantage of making real-time operations possible. 3.5 Reliability ~ ~ e n i u m c ~ s t a ~ ~ i z e s v however, e ~ e a s i thecrystallization ~~; rate is drastically reduced by working at a IOW temperature (- 50°C in our case). Nevertheless care must be taken to avoid contamination by oil from the vacuum pump; for instance, it is advisable to trap the oil vapour by a chilled metal plate situated near the selenium. There is also the possibility of radiation damage. We performed preliminary experiments during which amorphous selenium samples were exposed to an electron beam (75 kV, 10-6 A m-2). These samples showed only very little changes in their electrical properties: the electron and hole mobilities and 524
Acknowledgments The authors wish to thank Dr J P Hazan for many stimulating suggestions concerning the feasibility of Phototitus directly addressed by an electron beam, and Dr S Boronkay who performed the measurements of the photogeneration and transport properties of the irradiated samples. References Bonhomme P, Beorchia A and Meunier B 1976a C.R. Acad. Sci., Paris B 286 63 Bonhomme p, Beorchia A, ~~~~i~~ B, Dumont F and ~~~~i~~ D 1976b optik 4 2 Donjon J, Decaesteker M, Monod B and Petit R 1975 Acta Electron. 18 Dumont F, I-Iazan J p and Rossier D 1974 Phil@ Tech- Reu. 34 247 Frank J 1975 J. Phys. E: Sci. Instrum. 8 582 Goetze G w and Laponsky A B 1971 Photoe[ectronic zmaging Deuices vo1 2, eds L Biberman and s Nudelman (New York: Plenum) p 217 Hanszen K J 1973 Image Processing and Computer Aided Design in Electron Optics, ed P W Hawkes (London: Academic) p 16 Klein C A 1966 Appl. Opt. 5 1922 Marie G, Donjon J and Hazan J P 1974 Aduances in Image pick-up and Disp[ay ~ 0 1, 1 ed B Karin (New York: Academic) p 225 Roach w R 1974 IEEE Trans. E~ectronDeu. ED-21453 Spear W E 1956 Proc. Phys. Soc. B 69 139 Stroke G W and Halioua M 1973 Optik 37 192 non F 1965 Z. Natur. A 20 154 Journal of Physics E: Scientific Instruments 1977 Volume 10 Printed in Great Britain 0 1977
Search
Collections
Journals
About
Contact us
My IOPscience
Phototitus: image intensifier and converter for an electron microscope
This content has been downloaded from IOPscience. Please scroll down to see the full text. 1977 J. Phys. E: Sci. Instrum. 10 520 (http://iopscience.iop.org/0022-3735/10/5/029) View the table of contents for this issue, or go to the journal homepage for more
Download details: IP Address: 139.184.14.159 This content was downloaded on 02/10/2015 at 04:36
Please note that terms and conditions apply.
P G Cracen and D J Fegan References Adam G R 1971 Publs R. Obs. Edinb. 8 43 Beaver E A, McIlwain C E, Choisser J P and Wysoczanski N 1972 Proc. 5th Symp. on PhotoeIectronic Image DeGices, London, 1971 (London: Academic) Fegan D J and Craven P G 1977 J. Phys. E: Sei. Instrum. 10 510-15 Jelley J V 1973 Obsewatory 93 9 Kobler H 1975 J. Phys. E: Sci. Instrum. 8 426 Whitford A E and Kron G E 1937 Rev. Sci. Instrum. 8 78
Phototitus: image intensifier and converter for an electron microscope
F Dumontt, D Rossierf, P Bonhommet, A Beorchiat and B Meuniert t Laboratoires d’tilectronique et de Physique Appliquee, 94450 Limeil-Brevannes, France $ Faculte des Sciences de Reims, France Receiced I I October 1976, in finalform 20 December 1976 Abstract We describe an image converter, Phototitus, consisting of a photoconductor deposited on an electrooptical dielectric crystal. The electron microscope beam incident on the photoconductor creates electron-hole pairs which modulate the transparency of the electro-optical dielectric crystal by means of the Pockels effect. The converter can be read out with a fairly high-intensity light source, making Phototitus a solid state image intensifier. The read-out light can be coherent, allowing direct processing of the image, e.g. a real-time Fourier transform of the image which permits fast, accurate focusing of the electron microscope. Furthermore, Phototitus can be used as an ‘analogue optical computer’ making possible applications such as sharpening of high-resolution images.
Journal of Physics E : Scientific Instruments 1977 Volume 10 Printed in Great Britain 0 1977 520
1 Introduction The observation of delicate objects with an electron microscope requires exposures low enough to avoid the destruction of thin object structures by radiation damage. Furthermore, focusing and stigmating the objective lenses is often a laborious, timeconsuming task during which the object receives a high electron dose. These considerations apply to the conventional transmission electron microscope as well as to the scanning transmission electron microscope. Various methods have been proposed in order to overcome the limitations just outlined: channel plate electron multipliers have been used as image intensifiers (Goetze and Laponsky 1971), and several focusing and stigmating methods have been proposed including the observation of the Fourier transform (Thon 1965) or of the autocorrelation function (Frank 1975) of the image of a ‘white noise’ test pattern. However, it is generally difficult to use these methods in real time. For instance, analogue devices performing a Fourier transform by optical diffraction require the use of an optical transparency and thus are very time-consuming. On the other hand, it is in principle possible to digitize the image and to process it by a computer which displays on a cathode ray tube either the Fourier transform or the autocorrelation function of the image. Both functions permit one to estimate the focusing and stigmating of the microscope, but an image digitizer and a computer are
Electron microscope image intensifier and cont‘erter needed; moreover, the computation time may be fairly long depending on the size of the computer and the precision of the computation (Frank 1975). The previous discussion shows that an analogue device capable of performing the Fourier transform in real time would be very useful and would avoid the hardware required by the digital methods. Such a device has recently been realized using the technologies developed in the field of optical light valves. In this paper we describe the application of Phototitus - a light valve which has been used for image processing (Dumont et a1 1974, Marie et a1 1974) - in the field of electron microscopy. Phototitus makes it possible to increase the brightness of the image, thus allowing a reduction in exposure of the object to radiation damage. Furthermore, the real-time imageprocessing capability of Phototitus makes it easy to display the two-dimensional Fourier transform of an image. The Fouriermode display presents to the operator a new field of investigation of the electron images and yields new, powerful methods for making such adjustments as sharp focusing of highresolution images, corrections of astigmatism and spatial frequency filtering in the Fourier plane. Another important application involves the use of Phototitus in the field of image improvement by holographic methods. These methods have been widely used to de-blur and sharpen the images given by high-resolution microscopes of both the scanning and the conventional type (Hanszen 1973). For example, very thin slices of material (such as unstained specimens of biological matter) examined with a high-resolution electron microscope behave as phase objects and the image has no contrast when the microscope is focused. Thus the microscope is often defocused so as to give a contrasted image (‘defocusing phase contrast’). The image thus obtained is always blurred and can be de-blurred by means of holographic filtering methods (Stroke and Halioua 1973). In this context, Phototitus is useful because it can be read out by coherent light so that, when associated with a coherent light source and a holographic filter, it behaves like a fast ‘analogue optical computer’, making real-time image restoration possible (Bonhomme et a1 1976a, Bonhomme et a1 1976b). 2 Writing-in with an electron beam Phototitus is a light valve which uses the Pockels effect in a DKDP (deuterated potassium diphosphate) crystal addressed by photoconduction through an amorphous selenium layer (Dumont et a1 1974). The electron-hole pairs required to produce the photoconduction can be created by a beam of fast electrons such as that of an electron microscope. As shown in
figure 1, Phototitus is then placed in the microscope with its photoconducting member exposed to the electron beam, at the place normally occupied by the phosphor. The operation is as follows. In the first step the voltage Vis applied to the sandwich (photoconductor and DKDP)while the electron beam is absorbed by the photoconductor after passing through a very thin electrode. The electron-hole pairs created are separated by the applied field and the carriers of one type (electrons or holes according to the polarity of the applied voltage) drift in the photoconductor, to be trapped in deep centres near the interface between the photoconductor and the dielectric mirror. This produces an image-like pattern of trapped charges which modulates the voltage applied to the DKDP.
In the second step, the electron beam is switched off or deflected and the electrodes are short-circuited so that the voltage applied to the DKDP is produced only by the trapped charges. The DKDP,having its optical axis in the direction of the incident light, is illuminated by a beam of polarized light through a polarizing beam splitter and a projection lens. On passing twice through the DKDP,the light beam becomes elliptically polarized so that the polarizing beam splitter directs towards the screen the light having its polarization perpendicular to the polarization of the incident light. The intensity of the light falling on the screen is
I = 10sin2 (Vk/Vh!Z)
(1)
where IO is the intensity of the incident light, V k the voltage applied to the DKDP by the pattern of trapped charges ( V k is spatially variable according to the intensity of the primary electron beam), and V h p is the half-wave voltage of the DKDP. In order to reduce V h / 2 the DKDP is cooled by Peltier elements to a temperature slightly above its Curie temperature (- 503C), making it passible to operate Phototitus with only 150-200 V (instead of about 2000 V at room temperature). In these conditions the modulation efficiency by the Pockels effect in DKDP can be as high as 75 %. 3 Characteristicsof the converter This new mode of conversion of an electron microscope image provides several advantages as compared to the normal case where the image is made visible by use of a phosphor screen.
3.1 Gain A fast electron creates a large number of electron-hole pairs when it loses its energy in the photoconductor. Owing to this multiplication process a high conversion efficiency is achieved.
Mu1 t I 1 aye r dielectric mirror
i
Transparent electrodes
micposcope
column
Figure 1 Set-up of Phototitus used with an electron microscope
Depth below surface (pm)
Figure 2 Calculated energy dissipation of fast electrons in amorphous selenium. About 25 % of the primary beam energy is backscattered
521
F Dumont, D Rossier, P Bonhomme, A Beorchia and B Meunier From Spear (1956) we know that an energy of 18-25 eV is required to create one electron-hole pair in selenium, so we can estimate the number of secondary electron-hole pairs created by one primary electron and the exposure necessary to write an image in Phototitus. However, data concerning the penetration of fast electrons must also be taken into account. The plots in figure 2 show the energy dissipation of a fast electron in amorphous selenium, calculated according to the formula given by Klein (1966). These plots show that electrons having an energy less than 100 keV lose most of their energy (except for the backscattered energy) in the thickness of the selenium layer currently used in Phototitus (about 15 pm). However, the more energetic electrons lose only a fraction of their energy in the selenium layer, resulting in a lower gain. From these data we can estimate the sensitivity of Phototitus to a beam of fast electrons. The results are shown in table 1.
The calculations have been made assuming that the DKDP is 0 5 ° C above its Curie temperature, so that the dielectric constant and half-wave voltage are ~=600
v= 300 Vh12.
(2) (3)
Measurements of the sensitivity gave results in fair agreement with these values. This sensitivity is less than that of the devices using Vidicon or silicon Vidicon tubes which can be operated down to 10-9 or 10-10 A m-2. However, the image is free of line structure since there is no scanning beam (except in the case of the scanning electron microscope) and Phototitus allows one to attain a much higher resolution. The sensitivity to high-energy electrons could be increased in principle by having a thicker selenium layer on the DKDP crystal, but this would impair the resolution. Thelimitingresolution of Phototitus addressed with visible light is 70 lines/mm (Donjon et a1 1975, Roach 1974). When Phototitus is addressed with an electron beam, a resolution decrease results from the spreading of the electron trajectories in selenium. Figure 3 shows the image of a metallic grid made of threads with a diameter of 18 pm and spaced by 45 pm. The threads are clearly visible, so the limiting resolution has probably not been reached. The ultimate resolution is discussed further in the next section. 522
Figure 3 Shadow images of electron microscope metallic grids. The grids were placed near the photoconductor to obtain unit magnification on the Phototitus target plane. (a) The grid is made of threads of 22 pm diameter, 90 pm apart. (b) The threads are 18 pm in diameter and 45 pm apart. The magnification in the plane of the photograph is x 50 for both (a) and (b)
3.2 Dimensions and brightness ofthe image The image written in Phototitus by the electron beam is magnified and projected on to the screen S by the projection lens L (figure 1). The image displayed on S can have large dimensions and a high brightness since this brightness depends only on the luminous power emitted by the light source. The sensitive area of Phototitus, 27 mm x 38 mm, is projected on to S with a magnification of 2-7 (practically limited only by the luminous power of the read-out light source). The image displayed by Phototitus is much brighter than if it were directly viewed by means of a phosphor screen. For instance, for an electron beam of 1O-6 A m-2 (energy 75 keV)
Electron microscope image intensifjerand converter
falling on a phosphor whose energy conversion efficiency is 5-15%, the density of radiant flux emitted is 4-10 mW m-2. If Phototitus is used instead, with a read-out light flux density of 1 W m-2 on the DKDP and a 50% DKDP modulation efficiency in order to preserve the grey-scale capability, the radiant flux density on the observation screen is 250 mW m-2 for unity magnification and 10 mW m-2 for x 5 magnification, so that either a gain of brightness or a magnification of the image or both can be obtained.
Figure 4 Image of lattice plane spacings of catalase crystals (both 8.75 nm and 6.85 nm pitches). The magnifications in the plane of the photoconductor and of the observation screen are respectively x 9700 and x 155 OOO
Thus the use of Phototitus makes the observations more convenient; furthermore, owing to the memory, the image can be observed or processed for several minutes and photographs can be taken without exposing the object to the electron beam. Figure 4 shows the image of lattice plane spacings of catalase crystals (pitch 8.75 nm), which were not visible on the fluorescent screen of the electron microscope (using a binocular eyepiece) with a magnification of x 6700, but were clearly visible on the projection screen of Phototitus owing to the increased brightness and dimensions of the image. The periodicity in the other direction (6.75 nm) is also clearly discernible. Therefore, the resolution is at least 16 lines/" for a thin, low-contrast preparation. For a more contrasted object it is higher than 25 lines/" (as shown by the metallic grid of figure 3), but the limit resolution has not yet been determined. Figure 5 shows a Fresnel fringe around a hole in an amorphous carbon film.
3.3 Memory An electron image can be stored by Phototitus as long as the charge carriers remain trapped near the photoconductordielectric-mirror interface (several minutes), allowing examination of the image with the electron beam switched off to avoid radiation damage of the sample. As the dielectric mirror is not completely opaque, 2-3% of the read-out light reaches the photoconductor, generating electron-hole pairs which recombine with the trapped charges, gradually erasing the stored image. The memory time is limited by the luminous power of the read-out light source and the wavelength of the read-out light since the reflection of the dielectric mirror and the photosensitivity of the photoconductor are strongly wavelengthdependent (amorphous selenium is nearly insensitive to yellow
Figure 5 Fresnel fringe around a hole in an amorphous carbon film. (a) Image displayed on the phosphor screen of the electron microscope; (b) the same image displayed by Phototitus. The contrast enhancement has been obtained by a small uniform background subtraction
and red light making this part of the visible spectrum convenient for reading out Phototitus). In practice, if the projected image is 90 mm x 60 mm, the memory time is between 5 and 10 min assuming a luminous intensity high enough for visual examination in dark-room conditions (with a yellow or red read-out light). 3.4 Possibility of direct image processing The optical image-processing ability of Phototitus has been described by Marie et al( 1974). Most of the conclusions remain valid as far as the electron images are concerned. It should be emphasized that the advantage of Phototitus consists of performing whole image processing in real time without the need of a point-by-point image analysis, in contrast with photographic or electronic techniques. Addition or subtraction of images can be performed by the addition or subtraction of the pattern of trapped charges: addition of two images is obtained by successive projections on
523
F Dumont, D Rossier, P Bonhomme, A Beorchia and B Meunier lifetimes were unchanged, and only a small decrease in the photogeneration efficiencyfor the visible light (30%after 100 h and 50% after 200 h) was observed. Therefore after being exposed for 200 h or more to an electron beam in normal conditions, the resolution, contrast and memory of Phototitus should be unchanged (since these properties do not depend on the photogeneration efficiency). The sensitivity will be slightly decreased, but this could be compensated by increasing the exposure time. In fact, the device was operated in an electron microscope for several months, and no continuous degradation was observed, indicating the reliability of the electron-beamaddressed Phototitus. Conclusion Phototitus exhibits several features which make it useful in the field of electron microscopy. The sensitivity is good (of the same order as that of usual microscope plates) owing to the gain of the photoconductor. High-quality images can be displayed. The contrast ratio is high: more than lO3:l with a highly collimated read-out beam. The resolution is better than 25 lines/" for high-contrast objects and the grey-scale capability is good. The memory makes it possible to observe and image for several minutes while not producing radiation damage to the observed sample. However the most interesting property of Phototitus is its coherent read-out capability, permitting this device to act as a fast analogue optical computer, allowing real-time image improvement by holographic methods. Such experiments are now in progress and the results will be reported in another paper. 4
Figure 6 Zinc oxide crystals (magnification in the plane of Phototitus: x 10 000). The image has been differentiated to show the contours
the photoconductor with the same applied voltage; and subtraction of two images is obtained by successive projection with reversal of the polarity of the applied voltage V (figure 1) between the exposures. The image subtraction capability allows spatial differentiation and display of equiluminance contours (by subtracting a uniform background), resulting in contrast or in small detail enhancement (figures 5, 6). An important feature is the image-processing capability of Phototitus in coherent light as described in the introduction. If a coherent light source is used, the two-dimensional Fourier transform of the stored image is displayed in the focal plane of the projection lens L (figure 1). This feature can be used, for instance, to focus the electron microscope and to correct the astigmatism, following the method discussed by Thon (1965). This method consists of displaying the Fourier transform of an object, such as a carbon film, which contains all the spatial frequencies up to a certain limit (a 'white noise' object). The Fourier transform of the well focused image of this object has the shape of a disc the radius of which is proportional to the highest spatial frequency present in the object. Conversely if the microscope is not correctly focused, the Fourier transform is fringed by dark and bright concentric circles, and if there is some astigmatism the Fourier transform is no longer circular but distorted. Displaying the Fourier transform of an image is just one simple example of the possibilities given by the use of coherent light. Other interesting applications consist in image improvement by holographic filtering methods (Hanszen 1973). In this field of applications, Phototitus has the advantage of making real-time operations possible. 3.5 Reliability ~ ~ e n i u m c ~ s t a ~ ~ i z e s v however, e ~ e a s i thecrystallization ~~; rate is drastically reduced by working at a IOW temperature (- 50°C in our case). Nevertheless care must be taken to avoid contamination by oil from the vacuum pump; for instance, it is advisable to trap the oil vapour by a chilled metal plate situated near the selenium. There is also the possibility of radiation damage. We performed preliminary experiments during which amorphous selenium samples were exposed to an electron beam (75 kV, 10-6 A m-2). These samples showed only very little changes in their electrical properties: the electron and hole mobilities and 524
Acknowledgments The authors wish to thank Dr J P Hazan for many stimulating suggestions concerning the feasibility of Phototitus directly addressed by an electron beam, and Dr S Boronkay who performed the measurements of the photogeneration and transport properties of the irradiated samples. References Bonhomme P, Beorchia A and Meunier B 1976a C.R. Acad. Sci., Paris B 286 63 Bonhomme p, Beorchia A, ~~~~i~~ B, Dumont F and ~~~~i~~ D 1976b optik 4 2 Donjon J, Decaesteker M, Monod B and Petit R 1975 Acta Electron. 18 Dumont F, I-Iazan J p and Rossier D 1974 Phil@ Tech- Reu. 34 247 Frank J 1975 J. Phys. E: Sci. Instrum. 8 582 Goetze G w and Laponsky A B 1971 Photoe[ectronic zmaging Deuices vo1 2, eds L Biberman and s Nudelman (New York: Plenum) p 217 Hanszen K J 1973 Image Processing and Computer Aided Design in Electron Optics, ed P W Hawkes (London: Academic) p 16 Klein C A 1966 Appl. Opt. 5 1922 Marie G, Donjon J and Hazan J P 1974 Aduances in Image pick-up and Disp[ay ~ 0 1, 1 ed B Karin (New York: Academic) p 225 Roach w R 1974 IEEE Trans. E~ectronDeu. ED-21453 Spear W E 1956 Proc. Phys. Soc. B 69 139 Stroke G W and Halioua M 1973 Optik 37 192 non F 1965 Z. Natur. A 20 154 Journal of Physics E: Scientific Instruments 1977 Volume 10 Printed in Great Britain 0 1977
