ultramicroscopy ELSEVIER Ultramicroscopy 54 (1994) 261-267

Development of real-time defocus-modulation-type active image processing (DMAIP) for spherical-aberration-free

TEM observation

Toshiyuki Ando ‘, Yoshifumi Taniguchi ‘,I, Yoshizo Takai a, Yoshihide Kimura a, Ryuichi Shimizu ‘, Takashi Ikuta b

a Department of Applied Physics, Faculty of Engineering, Osaka UniL,ersity, Yamada-oka 2-1, Suifa, Osaka 565, Japan h Depurtment of Applied Electronics, Osaka Electra-Communication Uniwrsity, 18-8 Hatsu-machi, Neyaguwa, Osaka 572, Japan

Received 2 December 1993; accepted 10 January 1994

Abstract

A new technique for spherical-aberration-free observations under Transmission Electron Microscope (TEM) by real-time defocus-modulation-type active image processing (DMAIP) has been developed. This technique is based on a sophisticated accelerating-voltage modulation which enables both rapid through-focusing and functionized irradiation-time control to be performed. The functionized irradiation-time control which we have newly developed for the present DMAIP, is as follows: The through-focusing, Af(t>, is controlled so that the irradiation time of the primary beam per unit defocus value, dt/dAf, is proportional to the weighting function, W(Af), at each defocus value, Af. The simple accumulation of the image signal per unit time, i(Af(t)), expressed by the integral, li(Af(t)) dt, is proved to lead to the real-time DMAIP. The driving signal for the accelerating-voltage modulation was simply supplied to the feedback circuit of the high-voltage stabilizing unit of the TEM, and the processed images were successively displayed on a cathode ray tube (CRT) in real time at the video-frame rate (4 s). The experimental confirmation has been made through the Thon-diagramming technique with a commercial type TEM, the JEM-200CX. The Thon diagram constructed from these real-time processed images have clearly revealed that the spherical aberration of the objective lens has been successfully corrected in the range of intermediate spatial frequency ( < 3.0 nm- ‘1.

1. Introduction

In a previous paper we reported the elimina- tion of spherical aberration by defocus-modula- tion-type image processing (DMIP) [l]. This DMIP is based on an integration of the through-

’ Present address: Instrument Division, Hitachi Ltd., 882 Ichige, Katsuta, lbaraki 312, Japan.

focused images with a bipolar (i.e. having both positive and negative values) weighting function, enabling the amplitude and phase components of the sample to be obtained separately. This ap- preach has been applied for high-resolution transmission electron microscopic observations with considerable success [l]. In practice, the through-focusing used for the DMIP has been done by changing the lens current for the objec- tive lens as an extension of the conventional

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262 7: Ando et ul. / Ul~ramicrosc.opy 54 (1994) 261-267

through-focusing method, which does not require any modification in basic performance of the commercial type ‘EM.

As reported in : previous paper [2], however, this through-focusing method has basic problems for DMIP as follows. Misalignment between the current center and the optical axis causes the image to shift during through-focusing, which cannot be distinguished from drift of the speci-

men. Another problem is that the hysteresis of the magnetic lens does not allow a rapid through-focusing that is essential to realize real- time processing. DMIP is, in principle, most suit- able for the real-time processing, because this processing is achieved only by the simple integra- tion of through-focus images with a weighting function, /r(AfW(Af> dAf.

It is well known that charge-storage-type im- age detection elements mounted in a TV camera integrate the image signals over the video-frame period (f s). Hence the image integration of each through-focused image can be realized over the video-frame period. This leads to the possibil- ity of realizing real-time processing by developing accelerating-voltage modulation [3] called defo- cus-modulation-type active image processing (DMAIP), which enables us to perform both rapid through-focusing and functionized irradiation-

time control. DMAIP is, therefore, a unified technique of

image detection and processing, which should be essentially distinguished from conventional image processing in which image detection and process- ing are performed separately [41.

The present paper reports the development of the first real-time DMAIP for spherical-aberra- tion-free observation using a commercial TEM.

2. Theoretical

As described in a previous paper [5], DMIP is achieved by performing the integration

j cc I(Af)WAf) dAf> -cc (1)

where Z(Af> is the through-focused image and W(Af) is the weighting function representing the imaging characteristics of an electron-optical sys-

tern. Using image signals per unit time, i(Af>, and the frame time, T, for obtaining each through-focus image, pi, represents I(Af>, and Eq. (1) is rewritten as

j = i(Af )bWAf >I dAf. -r (2)

Next, consider that one controls the irradiation time of the primary beam, dt, per unit defocus- value, dA f, at each defocus value, A f. Then, we obtain image signals

ji(Af(l)) dt= j= i(Af)(dt,‘dAf) dAf, pm

(3a)

for a monotonic increasing function, t(Af >. For a monotonic decreasing function, t(Af >, we also obtain

ji(Af(t)) dt = j-=i(Af)(dr/dAf) dhf a

X = j

i(Af)( -dt/dAf) dAf. pz

(3b)

Eqs. (3a) and (3b) are unified by

ji(Af(r)) dl=jm i(Af)Idt/dAf ldhf, (4) --XI

for the monotonic function, t<Af ). Comparing Eq. (2) with Eq. (4) the functionized irradiation- time control satisfies the following equation:

(dt/dAfl =M(Af). (5)

Hence, the integration of the image signals per unit time, i( A f >, is observed under the condition that Eq. (5) is exactly equivalent to DMIP.

The solutions of Eq. (5) are

r(Af) =~j%‘(Af’) dAf’, --r.

for a monotonic increasing function,

(6a)

t(Af) =~j~-=,w(Af’) dAf’,

for a monotonic decreasing function.

(6b)

T Ando et al. / Ultramicroscopy 54 (I 994) 261-267 263

Fig. 1. Accelerating-voltage modulation for DMAIP. (a) Ef-

fective transfer function (ETF) for the present DMAIP exper-

iment. (b) Weighting function for the phase component of the

sample, B’,(Af). (c) Cumulative function of the weighting

function rthf) = jt+‘(Af) dhf for functionized irradiation-

time control.

t(Af> is depicted in Fig. lc for the weighting function plotted in Fig. lb. According to Eq. (5) the cumulative function of the negative part in Fig. lc is calculated by changing the negative sign to the positive sign for convenience of practical processing.

This t versus Af curve in Fig. lc enables us to realize the defocusing as a function of irradiation time, i.e. Af(t>. The functionized irradiation-time

Electron Microscope

control is the system, supplying such accelerating- voltage modulation, Av(t), that provides the de- focusing, Af(t), according to the t versus Af curve shown in Fig. lc.

3. Experiment

Fig. 2 shows a block diagram of the system developed in the present work. The TEM used in this experiment was a JEM-200CX operated at an accelerating voltage of 160 kV. The third-order spherical aberration coefficient of the objective lens was 1.5 mm measured by the method pro- posed by Krivanek [6]. A Gatan model-622 TV camera with an image intensifier was used to take high-resolution micrographs at the video rate & s. Since the charge-storage-type image detection elements mounted in the camera integrate image signals over the period of a video frame, image integration with a unipolar defocus weight has been realized over the video-frame period ($ s). In order to integrate the through-focus image, /i(Af(t)) dt, w e h ave devised a functionized irra- diation-time control as follows.

Fig. lc shows the intraframe time of the weighting implementations, ?(A f 1, for image in- tegration with a bipolar weighting function; dif- ferent video frames are used to perform respec-

erating Voltage Modulation Electron Gun or Rapid Defocus Modulation)

Condenser tens IxlKl

Modulation

Controller I /lJ CRT

Sample Stage l-u A

Objective Lens II8 Ix]<

Continuous DefocusControl

Projection Lens NY

for constructing

I ( Personal

Thon diagram Computer

Fluorescent - A Screen

TV-Camera V

Video Realtime

Video Sig. ' Amplifier - Image Subtractor

Fig. 2. Block diagram of the present real-time DMAIP system.

264 T Ando et al. / Ultrurnicrosco~~y 54 (1994) 261-267

tively the integrations corresponding to the posi- tive and negative parts of the weighting function. In the present work, an image intensifier (Gatan model-622) was used, in which an image is read by scanning over the complete array of pixels in & s. For every pixel to integrate over one complete

cycle of the through-focus, A f, it becomes neces- sary to actually perform two identical defocusing cycles in succession, synchronized with transfer of the pixel array. The image read during the second cycle corresponds to integration over a single defocus cycle. Finally, the interframe subtraction was done between those two kinds of integrated images so that image integration with bipolar weighting function was consequently completed

with four video frames (f s>.

subtracter, and displayed on a cathode ray tube (CRT). A personal computer, NEC PC-BOlDA, was used to control the whole system.

Fig. 1 shows (a) the effective transfer function (ETF), tbl the weighting function for the phase component of the sample and (c> the functionized irradiation-time control for weighting implemen- tation. The ETF decides the range of the spatial frequency to be corrected by the DMAIP. In the present experiment the maximum spatial fre- quency of the ETF was set to 3.0 nrn-’ for the following reason: The present approach for accel- erating-voltage modulation did not allow us to generate the modulation of rapid response due to the limited temporal response of the high-voltage stabilizer.

The modulation drive for the accelerating-volt- Fig. 3a shows the waveform of the functionized

age modulation was simply supplied to the feed- weighting implementation used in this experi-

back circuit of the high-voltage stabilizing unit of ment. In order to avoid very large jumps in volt-

a TEM. The TV camera enables the two kinds of age between the two kinds of weighting imple-

image integrations for the positive weight and the mentations, the positive part of the weighting

negative weight, to be alternately performed with implementation can be flipped over to give the

an interval of $ s. The subtraction of these two broken curve shown in Fig. 3a. Fig. 3b shows the

kinds of integrated images was carried out at an drive signal for this functionized accelerating

interval of 6 s by utilizing an interframe image modulation used in this experiment, which is an

Positive Part Negative Part A

F >I 1 Video Frame (1/30[secl)

time

4 1 Video Frame 11/30[secl) time

Fig. 3. Scheme of the digitized waveform of the modulation drive for functionized irradiation-time control

T. Ando et al. / Ultramicroscopy 54 (1994) 261-267 265

Fig. 4. Original images of an amorphous carbon film observed under different defocus values; (a) - 65 nm, (b) 0 nm and (c) 65 nm (Scherzer focus) (positive values represent underfocus).

approximation to the weighting implementation shown in Fig. 3a.

4. Results

Fig. 4 shows the original images of an amor- phous carbon of about 15 nm thick and the processed phase images obtained by the present DMAIP is shown in Fig. 5. The interval for the observation was $ s. The defocus values are (a> - 65 nm, (b) 0 nm and (c) 65 nm (Scherzer focus), respectively (the positive values represent under- focus). All these images were taken at a direct magnification of 400000 X .

Experimental confirmation has been made through Thon diagramming [7], leading to the conclusion that the correction for the spherical aberration of the objective lens has been success- fully done. In conventional Thon diagramming each through-focused image is Fourier trans- formed to derive the power spectrum as a func- tion of spatial frequency, and the power spectrum is depicted for a series of defocus values. In order to assess the present DMAIP we proceed with a similar procedure using the processed images ob- tained by DMAIP at each defocus value. In the present assessment the through-focused images were taken with a defocus step of 4.27 nm by changing the objective lens current. Thus, Thon

Fig. 5. Real-time processed images obtained by the present DMAIP for different defocus values: (a) - 6.5 nm, (b) 0 nm and (c) 65 nm (Scherzer focus) (positive values represent underfocus),

266 T. Ando et ul. / Ultramicroscopy 54 (1994) 261-267

I I I I I I I I I I

-200 -100 0 100 200 -200 -100 0 100 200

Defocus-value Af [nm] Fig. 6. Thon diagrams constructed from (a) the original through-focus images and (b) the through-focus processed images obtained

by real-time DMAIP.

diagrams were constructed for the original images and the processed images under the same experi- mental condition by recording each image on a video-tape recorder.

Fig. 6a shows the Thon diagram constructed from the original images. This diagram demon- strates that the dependence of the image contrast is oscillating against the defocus values. A central black curve indicates the condition where the wave aberration function, y(g, Af), becomes zero. This diagram clearly indicates the influence

of the spherical aberration. Fig. 6b shows the Thon diagram made from

the processed through-focus images obtained by DMAIP for the phase component. The diagram is symmetric about the line Af = 0, confirming that the spherical aberration is completely cor- rected. A straight bright line at the center con- firms that the phase component is observed un- der just-focus condition by eliminating the spheri- cal aberration.

5. Conclusion

A real-time DMAIP has been developed. This technique enables spherical-aberration-free ob-

servation of the phase component of the sample to be realized with TEM. Since the present sys- tem has been based on the use of a commercial type TEM without any modification of its basic performance, the application has been restricted to spatial frequencies in the intermediate region (< 3.0 nm-‘>.

It is noted, however, that this approach can easily be extended to higher spatial frequency so as to achieve high-resolution observations free of spherical aberration. For instance, a functionized irradiation-time controller can be attached through optical fibers to the high-voltage power supply of a TEM. Such an arrangement is now under construction.

Acknowledgements

The authors are very grateful to Professor H. Hashimoto, Okayama University of Science, for his continuous stimulation of work on high-reso- lution TEM observation. Stimulating discussions with Dr. T. Tanji, Electron Wavefront Project, JRDC, are also gratefully acknowledged.

T. Ado et al. / Ultramicroscopy 54 (1994) 261-267 267

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