Improvement of Light Collection Efficiency ofLens-Coupled YAG Screen TV System for aHigh-Voltage Electron MicroscopeKAZUO YAMAMOTO,1* TAKAYOSHI TANJI,2 MICHIO HIBINO,2 PETER SCHAUER,3 AND RUDOLF AUTRATA3

1Department of Electronics, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan2Center for Integrated Research in Science and Engineering, Nagoya University, Furo-cho, Chikusa-ku, Nagoya 464-8603, Japan3Institute of Scientific Instruments, Academy of Science of Czech Republic, 612 64 Brno, Czech Republic

KEY WORDS YAG 1 glass hemisphere 1 lens coupling method; high-voltage electron micro-scope TV system; light collection

ABSTRACT A new lens coupling television (TV) system using a YAG (Yttrium AluminumGarnet: Y3Al5O12 : Ce31) single crystal screen has been developed for a high-voltage electronmicroscope (HVEM), and its performance is examined. The system, using a combination of YAG andlenses, is less damaged by high-energy electron irradiation and reduces the influence of X-rays onthe image. YAG screens have not been used for lens-coupling systems, because the high refractiveindex (n 5 1.84) of YAG results in a low light collection efficiency for emitted light. Thisdisadvantage is overcome by combining a thin YAG disk screen (thickness; 100 mm) with a glasshemisphere whose refractive index is 1.81. We found that the light intensity is almost the same asthat obtained with a conventional P22 powder screen and lenses system. The resolution is about 55mm on the YAG screen, and this value is 1.3 times higher than that measured by the conventionalsystem. Shading and distortion do not affect TV observation. Detection quantum efficiency,obtained after correction of the channel mixing effect, is about 0.1. Microsc. Res. Tech. 49:596–604,2000. © 2000 Wiley-Liss, Inc.

INTRODUCTIONAs recording devices for a transmission electron mi-

croscope (TEM), photographic film, imaging plates(Ayato et al., 1990; Mori et al., 1990), TV cameras, andslow-scan CCD (charge coupled device) cameras(Barbe, 1980; Krivanek and Mooney, 1993) have gen-erally been used in many studies. The TV camera is anespecially influential device because it is useful fordynamic observation to clarify physical phenomena.Moreover, it is useful in focusing images, correctingastigmatism, and so forth. The TV system consists oftwo processes: converting electrons to photons in ascintillation screen, and transferring photons to TVtubes or CCDs. So far, two types of coupling methodshave generally been used for the electron microscopeTV system. One is a lens coupling (LC) method using afluorescent powder screen and a transfer lens system(Fan and Ellisman, 1993; Mooney et al., 1994), and theother is an optical fiber coupling (OFC) method using aYAG single crystal screen and an optical fiber plate(Daberkow et al., 1991; Krivanek and Mooney, 1993;Weickenmeier et al., 1995). However, there are a num-ber of problems in utilizing these TV systems for high-voltage electron microscopes (HVEMs). The powderscreen of the LC method and the optical fiber of theOFC method are damaged by high-energy electron ir-radiation. Moreover, the influence of X-rays (emitted inthe screen) on the image is serious in the OFC methodbecause the distance between the screen and the TVtube is very short compared to the LC method.

We developed a new LC method using the YAG diskscreen. The YAG disk screen has not been used with

the LC method because its luminous efficiency is lowerthan that of the powder screen and its emitted lightdiverges at the exit surface of the disk because of YAG’shigh refractive index (n 5 1.84) (Autrata et al., 1983),resulting in reduced light collection efficiency. The newsystem, consisting of a thin YAG disk and a glasshemisphere, has been developed to avoid the effect ofthe high refractive index of YAG.

In this paper, the design and the construction of thisnew system will be introduced and its performance willbe evaluated for the following aspects: (1) Resolution,(2) Light intensity, (3) Shading, (4) Distortion, and (5)Detection quantum efficiency (DQE). The results willalso be compared with those of a P22 fluorescent pow-der screen.

DESIGN AND CONSTRUCTIONOne of the most important points for the YAG and

LC TV system is its light collection efficiency. As shownin Figure 1a, the emitted light in the YAG disk isrefracted at the exit surface of the disk, so that themaximum angle (u1) collecting the light of the lenssystem reduces to u1/1.84. If the YAG disk is combinedwith a glass hemisphere whose refractive index is thesame as the YAG, as shown in Figure 1b, the refractionof the light is avoided. Consequently, the image inten-sity obtained with the YAG disk and glass hemisphere

*Correspondence to: Kazuo Yamamoto, Department of Electronics, NagoyaUniversity, Furo-cho, Chikusa-ku, Nagoya 464 8603 Japan.E-mail: h973203d@mbox.media.nagoya-u.ac.jp

Received: 1 September 1999; accepted in revised form 23 February 2000

MICROSCOPY RESEARCH AND TECHNIQUE 49:596–604 (2000)

© 2000 WILEY-LISS, INC.

is 3.4 (61.842) times higher than that obtained withthe YAG disk alone.

The new TV system constructed for HVEM is shownin Figure 2. A YAG disk with a thickness of 100 mm wasplaced on a glass hemisphere with a radius of curva-ture of 53 mm. Aluminum was evaporated to a thick-ness of 100 nm on the top surface of the YAG disk. Thespecial glass hemisphere was made by combining acylinder glass with a flat-convex lens. The refractiveindex of the glass hemisphere was n 5 1.81 (SFL6). Alight image converted from a 1,000-keV electron imagein the YAG disk was transferred to a photocathode of aTV camera through two optical lenses (F 5 1.2, f 5 50mm and F 5 1.0, f 5 50 mm). The pair of lenses wereused to control the magnification of the light image onthe photocathode. The TV camera was a HamamatsuPhotonics C-1000 type 12 Silicon intensifier tube (SIT);the resolution, determined by the width of scanninglines, was 40 mm on the photocathode. In this lenssystem, the glass hemisphere makes a virtual image ofthe light on the YAG disk plane at a magnification of1.8. The pair of lenses reduces the image to proper size,and forms the image magnified by a factor of 1.35 onthe photocathode. The light collection angle of the sys-tem is about 10°. The size of the TV frame that we couldobserve corresponds to an area 5.6 3 4.2 mm on theYAG disk plane. Since the light emits from almost thecenter of the flat surface of the glass hemisphere, wecould ignore spherical aberration, astigmatism, andcoma of the glass hemisphere. Figure 3 shows an illus-tration of the lens system. The distance between theYAG disk and the photocathode is very long, about 460mm, compared to the YAG and OFC method (in whichit is less than 10 mm), so that the influence of X-rays

reduces to less than 1/2,100 (61/462) that of the YAGand OFC method. An outer view of the new TV systemis shown in Figure 4.

EXPERIMENT AND DISCUSSIONResolution

The deterioration of the resolution in the new TVsystem was mainly caused by (1) the electron beamspread in the YAG disk screen, (2) the curvature ofimage field due to the glass hemisphere, and (3) themisalignment of the lens system. The influence of (2)and (3) was analyzed light-optically without the YAGdisk. Figure 5a shows the setup of the light-opticalexperiment. In order to reproduce the same situationas that of the YAG screen, in which the light emitsradiantly, we set a ground glass plate over the newsystem and introduced a light emitted in a fluorescentscreen mounted on a 200-kV TEM (Hitachi H-8000; thewavelength of the light was almost the same as that ofthe light emitted in YAG). A small knife was put on theglass hemisphere. The resolution was measured as the20–80% width of the output profile of a small knifeedge image. Figure 6 shows the resolution as a functionof distance from the center of the flat surface of theglass hemisphere. The curving solid line indicates the

Fig. 1. Light collecting angle of (a) YAG disk 1 LC system and(b) YAG disk 1 glass hemisphere 1 LC system. The angle u1 is themaximum at which a lens system can collect emitted light. The lightcollection angle of the YAG disk 1 LC system is u1/1.84, because of therefraction at the exit surface of the disk. The hemisphere of thehigh-refraction glass increases the light collection angle.

Fig. 2. New LC TV system using the YAG disk and the glasshemisphere. Combining a cylinder glass with a flat-convex lens makesthe glass hemisphere.

597YAG SCREEN TV SYSTEM FOR A HVEM

resolution calculated from a ray-tracing simulation inwhich the lens system was focused on the flat surface ofthe glass hemisphere. The image blurred at the cornerof the TV monitor view on account of the curvature ofthe image field. However, when proper defocusing wasintroduced, the resolution was almost uniform withinthe area of the TV monitor view and it was 40 to 50 mmon the flat surface of the grass hemisphere. The influ-ence of the misalignment of the lens system was eval-uated from the difference between the calculated andmeasured resolution at the center, and was found to beabout 35 mm. Therefore, the influence of the curvature ofthe image field is estimated to be 30 mm (6=452 2 352).

The overall resolution including the electron beamspread in the YAG disk was measured on an HVEM(Hitachi HU-1000D). The shadow images of a selectedarea aperture in the HVEM were used as the edgeimages, as shown in Figure 5b. The measured resolu-tion was 50–60 mm on the YAG disk plane. We foundfrom the experiments that the influence of each part,(1), (2), and (3), on the resolution was about 30 mm.This value for the influence of the electron beam spreadin the YAG disk (100-mm-thick) conforms to other stud-ies (Kotera and Kamiya, 1994; Nishi et al., 1996a).Table 1 shows a comparison of the resolution betweenthe new TV system and a conventional P22 fluorescentpowder screen and LC TV system. We can observeimages magnified 1.35 times compared to the images ofthe conventional system, because the blurring of thenew system is less. Figure 7 shows an example of a1,000-keV electron microscope image of gold particleson a thin carbon film obtained with the new system.

Light IntensityIn order to verify the improvement of the light col-

lection efficiency with the glass hemisphere, the lightintensity of the new system was compared with that ofthe system from which the glass hemisphere was re-moved, and also with that of the conventional system.The output voltage of the TV camera was measured asthe light intensity, and the comparison was made afteradjusting the magnification of the lens system so thatthe resolution of each system was the same on thephotocathode of the TV camera. Figure 8 shows the

Fig. 3. Illustration of the YAG disk 1 glass hemisphere 1 LCsystem. Since the distance between the YAG disk and the photocath-ode is more than 46 times that of the YAG 1 OFC TV system, theinfluence of X-rays reduces to less than 1/462.

Fig. 4. Outer view of the new TV system.

Fig. 5. Experimental setup for measuring resolution. a: Light-optical experiment. b: 1,000-keV electron beam experiment. The res-olution was measured with the edge image of (a) a small knife edge;(b) a selected area aperture.

598 K. YAMAMOTO ET AL.

light intensity of each system as a function of currentdensity on the specimen plane. We verified that thelight collection efficiency of the new system with theglass hemisphere improved by a factor of 2.9. We alsofound that the light intensity of the new system wasalmost the same as that of the conventional system,although the luminous efficiency of YAG is 1/3 that ofP22. This means that the disadvantage of the LCmethod using the YAG screen has been overcome.

ShadingUniformity of the light intensity was measured by

exposing a uniform electron beam on the YAG disk ofthe new system. Figure 9 shows the light intensitydistribution as a function of the distance from the cen-ter on the disk. Although the light intensity decreaseswith the distance from the center, the shading factorwas below 20% within a circular area having a 3-mmradius at the center, which corresponds to 97% of the

TV monitor view. The shading, which is due to theglass hemisphere, will not affect TV observation.

DistortionGenerally, distortion is evaluated by a distortion fac-

tor

D 5 ~Y 2 Y0!/Y0, (1)

where Y is the distance between the center and adistorted image point and Y0 is the distance betweenthe center and an ideal image point. A mesh sheet(100-mm spacing) was attached to the glass hemisphereinstead of the YAG disk, and the light was illuminatedto the new system. Figure 10 shows the image of themesh sheet. Although a pincushion distortion is ob-served, the distortion factor D is small, within 4% atthe corners of the TV monitor view. The larger theradius of curvature of the glass hemisphere is, thesmaller the distortion. Nevertheless, the measured dis-tortion will not hinder TV observation.

DQE (Detection Quantum Efficiency)DQE is an effective factor for analyzing characteris-

tics of a detection system (Herrmann and Krahl, 1984).It is the ratio of the square of the SN ratio of the outputsignal to that of the input signal, defined as

DQE 5~S/N!out

2

~S/N!in2 # 1. (2)

The value of DQE is less than 1, because noise exists inevery detection system. An experimental setup formeasuring DQE is shown in Figure 11. Electrons of1,000-keV irradiated uniformly in the new system, andthe output voltage of the TV camera was measured byusing a digital oscilloscope to obtain the (S/N)out. The(S/N)in was calculated from the number of incidentelectrons measured by a Faraday cup. Assuming thatthe number of incident electrons varies according to the

Fig. 6. Resolution depends on the position on the flat surface of theglass hemisphere in the light optical experiment.

TABLE 1. Resolution of the new TV system and theconventional TV system

LC method TV system

Resolution (mm)

On thescreen plane

On thephotocathode plane

YAG 1 glass hemisphere(31.35) 50–60 70–80

P22 (fluorescent powder screen)(31.0) 70 70

Fig. 7. 1,000-kV electron microscope image obtained by the newsystem. (specimen: gold particles on a thin carbon film).

599YAG SCREEN TV SYSTEM FOR A HVEM

probability of a Poisson distribution, we can express(S/N)in as

~S/N!in 5 S n

ÎnD 5 În, (3)

Fig. 8. Comparison of the light intensities of each system. Theglass hemisphere improves light collection efficiency, so the lightintensity of the new system is almost the same as that of the conven-tional system (P22 powder screen 1 LC).

Fig. 9. Uniformity of the light intensity. Light reduction (shadingfactor) is below 20% within a central circular area having a radius of3 mm, which corresponds to 97% of the TV monitor view.

Fig. 11. Experimental setup for DQE measurement. The (S/N)in iscalculated from the number of incident electrons in a pixel. The(S/N)out is measured from the output voltage of the TV camera.

Fig. 10. Image of a mesh sheet (100-mm spacing) put on the flatsurface of the glass hemisphere. The area surrounded by the outerframe shows the TV monitor view. The distortion factor is 4% at thecorners of the TV monitor.

600 K. YAMAMOTO ET AL.

where n is the average number of incident electrons.We have to take conception of pixels to measure the(S/N)in and the (S/N)out of TV systems. Then, we haveto take the channel mixing effect into account to calcu-late DQE. As shown in Figure 12a, the channel mixingeffect is caused by the influence of the emitted light,lens aberration, and so forth, on neighboring pixels andoverestimating the value of DQE because the output ofTV camera is also detected in the neighboring pixels(Ishizuka, 1993). The size of pixels is larger, as shownin Figure 12b, and the measured DQE is closer to theaccurate DQE, but the large pixel size lowers the pre-cision of S/N measurements because the total numberof pixels is less. We defined the pixel size as the sameas the resolution mentioned in Resolution. The pixelsize was defined as 53.0 3 14.5 mm, as shown in Figure11. Though the pixel size of the x-direction can bedefined by the resolution, that of the y-direction isdetermined by the width of scanning lines. Therefore,the channel mixing effect is more serious in the y-direction than in the x-direction. We used the correc-tion method of the channel mixing effect as mentionedin the next paragraph.

First, we have to determine the distribution of theoutput broadening. The distribution was assumed to beexpressed by the Gaussian function. Two-dimensionalGaussian function is

f~x, y! 51

Î2psExp 2

12 Sx2 1 y2

s2 D , (4)

where s is the standard deviation and f( x, y) satisfiesthe equation

Fig. 13. Illustration of the output distribution on the screen whenan electron irradiates the point (0, 0).

Fig. 14. Schematic diagram of the quantum conversion and thelight transferring processes in the new TV system.

Fig. 12. Influence of the channel mixing effect. a: The emittedlight and lens aberration influence the output of the neighboringpixels, leading to the overestimation of DQE. b: A larger pixel sizereduces the effect.

601YAG SCREEN TV SYSTEM FOR A HVEM

E2`

` E2`

`

f~x, y!dxdy 5 Î2ps. (5)

When an electron irradiates one point ( x 5 y 5 0), theoutput distribution on the YAG screen plane is sche-matically described as shown in Figure 13. The pixelsize is 2a 3 2b, and pj, qj, rj, . . . represent rates of theoutput in the pixel:

Oj52`

`

pj 1 2 Oj52`

`

qj 1 2 Oj52`

`

rj 1 . . . . . . 5 1. (6)

DQE is expressed mathematically by using pj, qj,rj, . . .. Figure 14 shows the conversion processes be-tween the incident electrons and the output signal ofthe TV camera. These processes are as follows: ① theincident electrons irradiate the screen (the number ofelectrons in a pixel: n1), ② the incident electrons con-vert to photons (conversion efficiency: d1), ③ the pho-tons reach the photocathode through the lens system(light transferring efficiency: d2), ④ photons reachingthe photocathode convert to photoelectrons (conversionefficiency: d3), ⑤ the photoelectrons are accelerated byan intensifier and convert to storage electrons in asilicon target of the TV camera (conversion efficiency:d4). Assuming that the number of incident electrons,the quantum conversion, and the light transferring ineach process vary according to the probability of thePoisson distribution, we can express the noise causedby ① in each pixel as

noise in pixel pj: În1 d1d2d3d4 z pj,

noise in pixel qj: În1 d1d2d3d4 z qj,

noise in pixel rj: În1 d1d2d3d4 z rj.

Similarly, the noise caused by ②–⑤ is written by alter-ing =n1 d1d2d3d4 to =n1d1 d2d3d4–=n1d1d2d3d4, re-spectively.

Also, we should consider the noise due to the gainfluctuation and read-out noise of the TV camera. Thegain fluctuation leads to noise that is proportional tothe incident electron density. So, the noise in pixel pj isexpressed as an1d1d2d3d4 z pj, where a is a propor-tional constant. The read-out noise (Nr) in pixel pj isNr z pj.

Since the noises in each pixel are independent ofeach other, the total noise N is expressed by the squareroot of the sum of the squares:

N 5 În1 d1d2d3d4

3 HS Oj52`

`

pj2 1 2 O

j52`

`

qj2 1 2 O

j52`

`

rj2 1 . . .D

z S1 11d1

11

d1d21

1d1d2d3

11

d1d2d3d4

1 a2n1 1Nr2

n1d12d2

2d32d4

2DJ 1/2

. (7)

The value of each parameter (d1–d4, a, Nr) was esti-mated by simple experiments for our TV system andearlier studies (Guldberg and Schroder, 1971; Mi-yashiro and Shirouzu, 1971; Nishi et al., 1996a,b), asshown in Table 2. We found that the noise caused by ①,④, the gain fluctuation, and the read-out have a greaterimpact on the total noise N. Therefore, the total noiseN can be approximated by

N > În1 d1d2d3d4

3 HS Oj52`

`

pj2 1 2 O

j52`

`

qj2 1 2 O

j52`

`

rj2 1 . . .D

z S1 11

d1d2d31 a2n1 1

Nr2

n1d12d2

2d32d4

2DJ 1/2

. (8)

From the output signal (n1d1d2d3d4) and Eq. (3), DQEis expressed as

DQE 5~S/N!out

2

~S/N!in2 5

HS Oj52`

`

pj2 1 2 O

j52`

`

qj2 1 2 O

j52`

`

rj2 1 . . .D

z S1 1 d1d2d3 1 a2n1 1Nr2

n1d12d2

2d32d4

2DJ21

. (9)

If the channel mixing effect does not exist, p0 5 1 andother rates pj, qj, rj, . . . ( pj Þ p0) are equal to 0, thecorrected DQE (DQEcor) is

DQEcor 5~S/N!out

2

~S/N!in2

5 S1 11

d1d2d31 a2n1 1

Nr2

n1d12d2

2d32d4

2D21

. ~10!

In order to obtain the DQEcor, the measured DQE ismultiplied by a correction factor

Cc 5 S Oj52`

`

pj2 1 2 O

j52`

`

qj2 1 2 O

j52`

`

rj2 1 . . .D . (11)

TABLE 2. Parameters of the new TV system

Conversion processConversion

factor Value

1,000 keV electron photon d1 3,600Photon (lens) photon d2 0.015Photon photoelectron d3 0.10Photoelectron storage charge d4 4,000Coefficient of gain fluctuation noise a 0.168Read-out noise (the number of

storage charges) Nr 1.3 3 105

602 K. YAMAMOTO ET AL.

The correction factor Cc is expanded analytically byusing Eqs. (4) and (5) on Figure 13. First term in Eq.(11) is expressed by

Oj52`

`

pj2 5 S 1

Î2psD 4HE2a

a

ExpS212 z

x2

s2DdxJ 2

z Oj52`

` FHE~2j21!b

~2j11!b

ExpS212 z

y2

s2DdyJ 2G .

The terms of qj, rj, . . . are written in the similarway. As a result, we obtain

Cc 5 Oi52`

` H 1

Î2ps E~2i21!a

~2i11!a

ExpS212 z

x2

s2DdxJ 2

z Oj52`

` H 1

Î2ps E~2j21!b

~2j11!b

ExpS212 z

y2

s2DdyJ 2

. (12)

The sigma terms correspond to the sums of the contri-butions to each pixel according to one-dimensionalGaussian distributions in the x-direction and the y-direction, respectively.

The output distribution I( x) due to the small edgementioned in Resolution is expressed by using Eq. (4)as follows:

I~x! 5 EX52`

X5x EY52`

Y5`

f~X, Y!dXdY, (13)

The 20–80% width of the profile given by this equationcorresponds to 1.7s. In our experiment, the output

broadening was expressed as a Gaussian distributionof s 5 32 mm (55 mm/1.7). The rates pj, qj, rj, . . . werecalculated as Figure 15a and b and the correction factorCc 5 0.0544. Figure 16 shows the DQEcor as a func-tion of the incident electron density (or the number ofincident electrons) on the YAG disk plane. The fittedcurve is derived by Eq. (10) and the values given inTable 2. The DQEcor decreases as the incident electrondensity increases. This is why the noise, which is pro-portional to the incident electron density, affectsDQEcor. It seems to be the noise caused by the gainfluctuation of the TV camera. In usual TV observations,the incident electron density used for observation isaround 200 pA/cm2, corresponding to DQEcor of about0.1. The DQE of the detector for 100–200 kV TEM isaround 0.7 (Daberkow et al., 1991; Fan and Ellisman,1993; Zuo, 1996). For the HVEM detector, however,DQEs reported so far were about 0.1. For example, theDQE of the photographic film for 1,000 kV is 0.15 to0.25 (Kamiya and Arii, 1990), that of the parallel de-tector of electron energy loss spectrum for 500 kV is0.10 (Yoshida et al., 1991), and that of the imagingplate for 1,250 kV is 0.05 (Taniyama et al., 1997).Therefore, the value of DQEcor of our new TV system isreasonable for HVEM.

CONCLUSIONSA new lens coupling TV system for HVEM using a

YAG disk screen and a glass hemisphere has beendeveloped. The performance of the system is summa-rized as follows.

1. Resolution: The blurring of electron microscope im-ages was caused by the electron beam spread in theYAG disk, the curvature of image field due to theglass hemisphere, and the misalignment of the lenssystem. The resolution was about 55 mm on thescreen plane. The new system has 1.3 times higherresolution than the conventional system.

Fig. 15. Experimental rates of the output in neighboring pixels of (a) x-direction, (b) y-direction.

603YAG SCREEN TV SYSTEM FOR A HVEM

2. Light intensity: Light intensity was improved to thesame value as that of the conventional P22 powderscreen and LC system, although the luminous effi-ciency of YAG is 1⁄3 that of P22.

3. Shading: Uniformity of light intensity was within80% for the TV monitor view.

4. Distortion: The distortion factor was within 4% atthe corners of the TV monitor view.

5. DQE: The detection quantum efficiency (DQE), aftercorrecting for the channel mixing effect, is about 0.1.

The new HVEM TV system, which performs suffi-ciently well for TV observation and reduces the amountof damage caused by high-energy electron irradiationand the influence of X-rays, has been made possible bythe new lens coupling method using a YAG disk andglass hemisphere.

ACKNOWLEDGMENTSWe thank Mr. Chiaki Morita and Mr. Shigeo Arai for

their help with the HU-1000D high voltage electronmicroscope. We are also grateful to Dr. ShigeyasuTanaka, Dr. Takaaki Hanai, and Mr. Kenji Ban fortheir many valuable suggestions regarding the design

and the construction of the new lens system with theglass hemisphere.

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Fig. 16. DQE corrected for the channel mixing effect.

604 K. YAMAMOTO ET AL.