An Image Intensifier for the Electron Microscope

An Image Intensifier for The Electron Microscope M. E. HAINE, A. E. ENNOS, AND P. A. EINSTEIN

Research Laboratory, Siemens, Edison Swan Ltd.. Harlow, Essex, England

INTRODUCTIOS

An important requirement for the successful operation of the electron microscope is the easy observation of the final image. There are three main requirements which depend in the first instance on visual observation: these are searching of the specimen to find areas of particular interest, focusing of the image, and the application of tests for the correction of astigmatism and other defects. At high magnifications, the low screen intensity makes viewing difficult and it is important to make the best use of the image by any means which can facilitate visual observation. The intensity of the image is limited by the maximum current density which can be applied at the object. This is only sometinies limited by object heating but is always limited by the limitations in the electron gun. The beam intensity a t the fluorescent screen is, of course, reduced to 1/M2 of that a t the object where M is the instrument magnification. Thus to maintain a high fluorescent screen image brightness the magnification should be kept to a minimum consistent with the resolved object detail being enlarged to a size at least as great as the resolution of the fluorescent screen (or photographic plate as the case may be). Thus if d is the instrumental resolving power (in object space) and 6 the screen resolution,

M 2 6/d. The maxiinuni value of current density at the final image ( p , ) is prescribed by the limitations in the electron gun. The maximum current density per unit solid angle (8) which can be produced in a gun operating a t an accelerating potential v o is given by'

B

=

p,vo/nX'T,

where pc is the cathode emission density. T its temperature e17/"K). k Boltzman's constant ( = 8.6 x Thus the current density in the object plane is pe = p,~o.'ra,2/.'rkT. 31i

(OK),

and

318

M. E. RAINE, A. E . ENNOS, AND P. A . EINSTEIN

where a, is the illuminating beam semi-angle (a,<
Ps = P o / M -

and putting M

=

~

M2kT '

S/d,

I n normal operation the illuminating beam aperture angle is made smaller than the full objective aperture (2a,) to introduce some degree of coherence into the illumination. Let a, = Ka, (where K is fractional), then putting d = O.6lh/a0 (the well-known expression for microscope resolving power), A (the electron wavelength) = 1/(15O/cp,) x cm., and k = 8 . 6 ~ 1 0 - eV/"K, ~ equation (1) becomes ps = 6.4 x lo-'' pcK2/S2T.

Now po cannot exceed about 1 A/cm2, otherwise the cathode life is severely limited by evaporation of the tungsten2 ( -30 hours a t 1 A/cm2). The corresponding cathode temperature is about 2700°K. and for normal operation K must be not greater than about 0.3 giving pn = 2

x 10-16/62.

(2)

Typical values of screen resolution have been given by von Borries3 and Hinderer4 as 70 microns at 50 keV and 120 microns a t 100 keV. The maximum value of ps is thus about 2xlO-l' A/cm2 a t 75 keV corresponding to a power at the screen of 1.5 x watts/cm2. A good phosphor will convert electron energy with an efficiency of about 100 lumens/watt6 (25%) and thus a screen brightness of 1.5 x lamberts can be expected, if the electron gun is operating with theoretical efficiency. Figure 1 shows the relation between brightness and acuity of vision.6 For the above brightness of lamberts for an image contrast of 20% the resolution of the eye is 700 microns. In consequence, it is an advantage to use a x 10 telescope to view the image on the fluorescent screen so that loop detail on the screen may be resolved by the eye. Since the light produced a t the screen is diffuse, the magnification does not produce a decrease in brightness provided the aperture of the telescope is large enough to fill the eye pupil with light. Practical experience has shown that the lowest limit of brightness for effective viewing of electron microscope images lies around 5 x 10-4 lamberts. Even with dark adaption, lo-4lamberts is too low an intensity

ELECTRON MICROSCOPE I M A G E INTENSIFIER

319

for reasonably efficient or comfortable viewing of the electron microscope image and it has long been apparent that some form of image intensification would be advantageous. Apart from increasing the

0

I

I

0.01

I

0.1

I

1.0

Brighfness (mi Ili -Lamberts)

I

10

FIG.1 . Plot of acuity of vision versus brightness for different contrasts. from Agar". from Lukiesh and MossG;

-

- -- - -

effectiveness and comfort of viewing, an intensifier of the type described offers the following possible advantages: (1) Facility for cinematography from an auxiliary display screen. (2) Reduction of current density a t the object. (3) Possibility of electronic contrast expansion. (4) Easy and rapid recording of pictures from an auxiliary wreen as well as the simultaneous display of the image on a number of tubes. Method of Intens$cation There are a variety of possibilities for intensifying the electron microscope image. The image may be reproduced by a static system, or alternatively it may be scanned and displayed in the manner of television. The pick-up device may be a completely self-contained unit viewing the electron microscope screen via an optical system, or it,may form an integral part of the instrument, such as would be the case where the electron microscope phosphor is deposited directly on a thin end window of an image tube.

320

M. E . HAINE, A. E . ENNOS, AND P. A. EINSTEIN

A disadvantage in the former case would be the large loss of available light which results from the use of auxiliary optical systems, as well as the limitation of the final definition to that of the microscope fluorescent soreen, which lies in the region of loop. Alternative suggestions include the application of a light amplifier such as was described by C ~ s a n o , ~ using a suitable photoconductor, or again a photoconductive-electroluminescent sandwich combination such as described by Kazan and NicolP may ultimately provide sufficient light amplification without loss of image detail. The attraction of the latter two devices, if they could be made t o work, would lie in their inherent simplicity and compactness. An alternative scheme, which is the subject of this paper, uses a scanned image tube with the sensitive element directly responsive to the electron beam. The principle of the tube is shown in Fig. 2. A 50 kV imaging beam

Thin plastic support membrane

I

\ \

LToandamplifier display

Selenium Low voltage scanning beam

FIQ.2. Schematic diagram of image intensifier.

thin film of selenium, which is normally of very high resistivity but which becomes relatively conducting on bombardment with high energy electrons (electron bombardment induced conductivity-“E.B.C.”), is mounted on a thin conducting signal-plate transparent to the primary electrons and supported on a thin plastic membrane, t also transparent to the primary electrons. The free (lower) surface of the film is scanned by a television raster with a low velocity electron beam. The function of this beam is to stabilize the surface potential. Thus if the surface starts off a t a potential (w.r.t. cathode) lower than the first secondary emission crossover potential (secondary emission ratio

t

Melinex, I.C.T. Ltd.

ELECTRON MICROSCOPE IMAGE INTENSIFIER

32 1

6 < 1) the beam will reduce the surface to cathode potential, after which the scanning beam is repelled. If the surface starts off a t a potential greater than the first secondary emission cross-over potential (6 > l ) , the beam will raise the surface potential to that of the anode of the scanning system or to the second crossover potential, whichever is greater. If during the interval between one scan and the next (frame period T ) charge leaks across the layer, the surface potential will rise by an amount given by

dV=

C-,

PT

(3)

where p is the leakage current density and C the capacity of one square centimetre of the layer. When the scanning beam passes over the discharged area it teturns t,he surface to cathode potential by depositing electrons. The arriving electrons induce a displacement current to the signal plate which passes through the load resistance to give a signal voltage proportional to the current density. The sensitivity of amorphous selenium to light has been well known for a long time and requires no special comment. The change of conductivity produced by high energy electrons was first reported by Pensake and was later investigated in more detail by Spear.’” The following briefly summarize the results of various investigations : (1) Amorphous selenium films are highly insulating in the dark but

become photoconductive when irradiated by high velocity electrons. Electrons with 50-100 keV energy are each capable of producing several thousand charge carriers. (2) The conduction current is carried mainly by “holes” which have an abnormally long lifetime and high mobility, and are capable of migrating several tens of microns from their place of excitation. (3) Selenium films become conducting only in one direction, i.e. with positive potential applied to the bombarded side, unless the bombarding electrons penetrate completely, in which case, the selenium conducts under either polarity.

In Fig. 3 some of Spear’s results are reproduced showing the induced current multiplication ratio plotted against bombarding electron energy for several values of applied field. The curves all show a saturation of induced current when the primary voltage corresponds to a depth of penetration equal to that of the selenium film thickness. The primary current density had a fixed value of approximately A/cm2. A curve is also shown for reverse applied field, demonstrating the difference in mobility between holes and electrons. Thus conduction Y

322

M. E . HAINE, A. E . ENNOS, AND P. A. EINSTEIN

only occurs when the primary electrons begin to penetrate the layer and holes are generated near the positive electrode. The current amplification factors obtained by Spear have been confirmed, and it also appears that a somewhat increased gain is obA/cm2. tained for lower current densities in the region of

i

j keld acros!

Bombarded electrode positive

I

layer J

7000 c

P

l5 1500

s

c

9

1000

=x

500

0

*-Penetrhan cmp/ete = 5.7p

FIG.3. Plots of amplification factor against electron energy (kV)for different fields across the layer (abstracted from Spearlo).

Basic Xensitivity There are three main factors which may be expected to limit the sensitivity of this device. (1) The change in potential of the surface due to charge leakage between scans must be large enough t o allow a sufficient number of electrons to be accepted during the subsequent scan to substantially discharge the surface. (2) The signal across the load resistance must substantially exceed the amplifier input noise. (3) The quantum noise in the imaging beam must be negligible. At this stage in the development of the device these factors will not be analysed fully, but it can be said that with a current multiplication of 1000 and a frame repetition frequency of 25 per second, the discharge voltage is several volts a t 10-11 A/cm2 of primary intensity with the 15 micron thick films now used, and it is hoped to increase this surface

ELECTRON MICROSCOPE IMAGE INTENSIFIER

323

potential by an order of magnitude by the use of thicker and more sensitive films. The signal voltage appearing across the amplifier input resistance is dependent on the area of the scanning spot. I n the present case, with a spot 100 microns diameter and a current density of 10-l1 A/cm2, it is already in the region of 5 mV, giving a signal-to-noise ratio of 50. It is believed that appreciable improvement is still possible. The ultimate limitation by the quantum noise in the electron beam is of fundamental importance. It is well known that any series of measurements of a number n of particles arriving a t random time intervals will yield a statistical error of 100/z/nyo. Each image point (diameter 6) receives a number of electrons each frame period (1/25 second) given by n = -P P 25e '

where e is the electronic charge = 1 . 6 ~ 1 0 - 'coulombs. ~ There will thus be a fluctuating noise contrast on each image point given by

To make this quantum noise negligible, C,yo should be less than say

5% or

Ps

'1.6 x

10-7

62

,

( 8 in microns);

e.g. for a 100 micron spot, ps > 1.6 x 10-11 A/cm2, and for a 20 micron spot, ps > 4 x 10-lo A/cm2. It is seen therefore, that quantum noise already provides a limitation to the image on the fluorescent screen and does not allow visual operation, even with an intensifier, much below 10-l' A/cm2 a t 25 frames per second. It is important here to remember that quantum noise is lessened if exposure time is increased. For visual operation the effective exposure is determined by the relaxation time of the eye (1/25+1/10 sec.); this could be artificially increased by reducing the frame repetition frequency, but an easier way might be to use a long afterglow phosphor on the display tube. There would result the disadvantage that all operations such as focusing would need to be slowed down by a corresponding amount. Apparatus and Results I n the experimental apparatus the selenium layer with its associated electrodes is mounted on an aluminium annulus of 3-in. diameter; the

324

M. A. HAINE, A. E. ENNOY, AND P. A. EINSTEIN

selenium film is supported on the thin Melinex membrane, both sides of which are coated with thin evaporated metal deposits acting as conducting electrodes. This forms a fairly rugged construction and can be inserted readily below the retractable fluorescent screen in an electron microscope, the free selenium surface facing downwards as in Fig. 2. The scanning arrangement consists of a tungsten hair-pin filament cathode gun, magnetic focusing lens and electrostatic deflector plates positioned below the layer. The signal current is fed into a preamplifier in close proximity to the signal-plate, and passed thence to a main video amplifier and a commercial 405 line television display tube scanned in synchronism with the image tube.

FIG.4. Photograph of image of grid. Current density 10-11A/cni2,60kT'.

Figure 4 shows a photograph (taken from the display tube) of the electron image of a grid with a current density of 10-l1 A/cm2 (at 60 kV) in the grid squares. The rectangular configurat
ELECTRON MICROSCOPE IMAGE INTENSIFIER

326

marked photoconductive time-lag, so that the image persists after the imaging electron current is switched off. Lags lasting several seconds have been observed, but care in preparing the layers can reduce the lag to less than a sec’ond. Recent work, not yet complete, indicates that this lag can be reduced to less than 0.1 seconds. In addition to this persistence in photlo-conduction, a lag may also result from an inadequate scanning electron current, but in the device so far i t has been possible to supply adequate current to make this effect negligible. Some difficulty may be experienced when attempts, now planned, to improve the resolution to 30 microns, are made. ACKNOWLEDGMENTS The authors wish to acknowledge the invaluable help of several colleagues in the laboratories a t Harlow and a t Aldermaston, where the work was initiated: in particular Messrs. D. H. Cooper, N. A. Alford, R. Thorn and P. Lloyd. REFERENCES 1. Langmuir, D.B., Proc. Irist. Radio Engra 25, 997 (1937). 2. Bloomer, R.N.,Brit.J . uppl. Ph?ys.8, 83 (1957). 3. Borries, €3. v., Optik 3, 321 (1948). 4. Hinderer, K.,2. Phys. 119, 396 (1942). 5. Bril, A,, and Klasens, H. A., PhLiZips Res. Rep. 7, 401 (1952). G. Lukiesh, M.,and Moss, F. K., “The Science of Seeing.” Van Nostrand, New York (1948). 7 . Cusano, D. A.,Phys. Rev. 98, 546 (1955). 8. Kazan, B., and Nicoll, F. H., Proc. Iiist. Rndio Engrs 43, 1888 (1955); Kazan, B., R.C.A. Rev. 19, 19 (1955). 9. Pensak, L.,Phys. Rev. 79, 171 (1950). 10. Spear, W. E., Proc. phys. Soc. B 69, 1139 (1956). 11. Agar, A., Brit. J . nppl. Phys. 8, 410 (1957).

DISCUSSION Have any difficulties been encountered in connection with recrystallization of the amorphous selenium layer? P. A . EINSTEIN: So far we have not noticed any gradual changes in the properties of the selenium over longer periods. Layers which have been stored in air at room temperature for some three months have shown no change in performance, nor does the repeated letting-down to air and re-pumping appear to affect the property of any particular layer. On the other hand, the properties of any individual layer can be affected quite critically if insufficient care is taken during its manufacture. J. A. LODGE: Have you any measurements which indicate that E.B.C. gain using a free surface with a scanning beam is the same as that using a sandwich? P. A . EINSTEIN: We have no direct confirmation at present that the current amplification in a layer is the same as that found in the sandwich experiments. On the 11. BRUINING:

326

M. A. HAINE. A. E. ENNOS, AND P. A. EINSTEIN

ot,her hand, the magnitude of signal obtained from the amplifier for a given imaging beam current density indicates that the expected gain of the order of 1000 is in fact obtained. H. C . LUBYZYNBKI: What was the lag for the short-lag material signal-platss, and which materials gave the shortest lag? P. A. EINSTEIN: The lag was quite short,-certainly less than 0.1 secs. The shortest times seem to occur for a gold or bismuth signal electrode, gold having a slightly shorter time lag.