84 OPTICS LETTERS / Vol. 15, No. 1 / January 1, 1990

Blazed reflection micro-Fresnel lenses fabricated byelectron-beam writing and dry development

Teruhiro Shiono and Kentaro Setsune

Matsushita Electric Industrial Company, Ltd., Central Research Laboratories, Moriguchi, Osaka 570, Japan

Received June 26, 1989; accepted October 17, 1989

The microfabrication of a blazed reflection micro-Fresnel lens (RMFL) using electron-beam writing and drydevelopment is proposed and demonstrated. The electron-beam-written resist film made of polymethyl isopro-penyl ketone and an aromatic azido compound achieved the blazed structure by 02 plasma development. Thesurface roughness of the RMFL was improved without the bridge and scum. The fabricated RMFL exhibited thediffraction-limited characteristics. A high efficiency of 71% was obtained at a period of 10 Mm. The margin for theelectron-beam writing condition to achieve high efficiency was much broader than in the conventional process.This process would be effective for the fabrication of blazed RMFL's and other grating components.

Blazed micro-Fresnel lenses' are of great interest asthin-film lenses with high focusing efficiency. Wehave studied various types of blazed micro-Fresnellenses with complex specifications, including ellipticallenses2 and rectangular-apertured lenses3 fabricatedby electron-beam (EB) lithography, and have demon-strated that these lenses have diffraction-limited fo-cusing performance with a high efficiency of greaterthan 70%. Theoretical analysis4 indicates that theefficiency of the transmission-type micro-Fresnel lensshould be reduced in the outer portion, where thegroove period is nearly comparable to the wavelength.We recently proposed reflection-type micro-Fresnellenses5 (RMFL's) with better focusing performance.

In EB lithography, especially for the fabrication ofthe blazed structure, the developing method and thechoice of the resist are important. The conventionaldevelopment has been a wet type. When a positiveresist is used, the slope of the grating tends to be roughwhile the depth resolution is better. The negativeresist, chloromethylated polystyrene, that we haveused exhibits high sensitivity and is barely influenced

by the developing conditions (the time and tempera-ture). The grating slope made from the negative re-sist can be smooth, but the blazed pattern is oftenaccompanied by the undesired bridge and scumcaused by the resist swell. The shape of the bridge ispeculiar to the blazed pattern and generally decreasesthe efficiency. If a dry-development process can berealized, it is expected that the reproducibility and theaccuracy of the fabrication can be improved withoutthe bridge and scum. In this Letter we propose anddemonstrate the microfabrication of the blazedRMFL using EB writing and dry development anddiscuss the experimental results.

The structure of our proposed RMFL is shown inFig. 1. This figure shows an elliptical RMFL thatexhibits astigmatic characteristics, where the cross-sectional structure for all types of RMFL's are thesame. The RMFL is composed of chirped gratingscovered with a reflection layer. The optical beam isincident upon the lens from the side opposite the re-flection layer and is focused by a folded optical path.Such a folded optical path reduces the device length.

X ' Y

Y

X

-' S >I

(a) Plane Figure

z z

LR

(b) Cross Section

Fig. 1. Structure of the elliptical RMFL showing (a) the plane figure and (b) the cross section.

0146-9592/90/010084-03$2.00/0 © 1990 Optical Society of America

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January 1, 1990 / Vol. 15, No. 1 / OPTICS LETTERS 85

1.0

coM 0.8

.o 0.6

N 0.4C0:E0.2

10 100Electron Dose [pC/cm 2]

1000

Fig. 2. Resist sensitivity curves obtained by a pattern of 5-gm lines and spaces, where the initial thickness of the resistwas 0.7 gm and the accelerating voltage of the EB was 35 kV.

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:::::.::::. ::::..:.: :.:, - ,, :::*.-.- :: .. ..... . ; B .. . . . .

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be obtained even in the portion where the efficiency ofthe transmission Fresnel lens is reduced, and both aconverging and diverging spherical wave as well as aplane wave can be focused with better performance.Details of the RMFL's are described in our previouspaper. 5

For the dry-development resist,6 we dissolved anaromatic azido compound and polymethyl isopropenylketone in a cyclohexanone solution (in a ratio of ap-proximately 3:10 by weight). From the many kinds ofaromatic azido compounds, we selected 3,3'-diazido-diphenyl sulfone because of its high absorption spec-trum at a short wavelength of 240 nm. This meansthat the resist is transparent and can be used for de-vices in the wide-wavelength region and that the treat-ment of the resist is easy without an exposure underthe visible ray.

Before the fabrication of the RMFL, the resist sensi-tivity was experimentally examined by a process simi-lar to that described below and shown in Fig. 3. Fig-ure 2 shows the resist sensitivity obtained by a patternof 5-grm lines and spaces, where the initial thickness ofthe resist was 0.7 gm and the accelerating voltage ofthe EB was 35 kV. The resist showed negative-typesensitivity depending on the annealing temperatureafter EB writing. The pattern without annealing ex-hibited the best surface smoothness and uniformity,which are important for the fabrication of the RMFL.

The RMFL was fabricated by the process shown inFig. 3. First, the resist was spin-coated on a substratecovered with a transparent conductive layer of indiumtin oxide, which is necessary to avoid an electricalcharge buildup. The EB was circularly scanned at agiven radius for a predetermined time until the speci-fied dose was achieved, and then the radius was in-creased by 0.1-0.2 gm. The electron dose distributionwas achieved by controlling the EB scanning time in

(e) 1 Reflection LayerI Deposition

Fig. 3. Fabrication process of the RMFL from (a) to (e).

The difficulty of using such a structure, caused by thecoincidence of the optical path between the incidentbeam and the focused beam, can be overcome by thecombination5 of the RMFL with a polarizing beamsplitter and a quarter-wave plate. Therefore theRMFL's are expected to have wide application. Thehighest diffraction efficiency is obtained when themaximum film thickness of the RMFL is LR = X/2n,where X is the wavelength and n is the refractive indexof the film. In the case of chloromethylated polysty-rene film, LR = 0.2 ,um for n = 1.6 and X = 0.6328 ,um.It should be noted that the thickness of the RMFL canbe significantly reduced (0.19 times) relative to thethickness of the transmission micro-Fresnel lens.This reduction in thickness results in a higher resolu-tion in the EB lithography and improves the focusingperformance of the lens. According to our theoreticalanalysis of the RMFL, a high diffraction efficiency can

1 OOpm

Fig. 4. Photomicrograph of the fabricated circular RMFL.

5pm

Fig. 5. Cross-sectional scanning-electron-microscope pho-tograph of the RMFL before the deposition of the reflectionlayer.

_ 35kV

i _

F-_

2 _

I I I I I 1111 1 I1

86 OPTICS LETTERS / Vol. 15, No. 1 / January 1, 1990

(a) Light Spot

1.7prm(b) Intensity Profile

Fig. 6. (a) Photograph of the light spot and (b) the intensi-ty profile observed at the focal plane.

consideration of the resist sensitivity curve, which de-pends on the grating period. The dry development ofthe resist under the 02 plasma with 1-Torr pressureachieved the relief structure. Finally, silver was evap-orated onto the blazed film as the reflection layer.Figure 4 shows a photomicrograph of the circularRMFL fabricated on a glass substrate. The lens hasan aperture size of 1 mm and a focal length of 5 mm ata 0.6328-Mm wavelength. The EB writing time andthe development time were approximately 11 and 8min, respectively. A cross-sectional scanning-elec-tron-microscope photograph of the linear RMFL be-fore the deposition of the reflection layer is shown inFig. 5. It is found that the surface roughness wasimproved without the bridge and scum, but the edgewas slightly rounded.

The first-order diffraction efficiency was experi-mentally examined. A TE-polarized expanded He-Ne laser beam was normally incident upon a linearreflection grating with a silver layer that was fabricat-ed for the basic evaluation. The measured efficiencyincluding the metallic reflection loss was as high as71% for the grating with a lO-Am period. Because the

resist has a lower sensitivity contrast than the conven-tional process, as shown in Fig. 2, the margin for theEB writing condition to achieve a high efficiency wasgreatly broadened. This process is confirmed to beeffective for the fabrication of the blazed structure.

The circular Fresnel lens attached to the beam split-ter was illuminated by an expanded He-Ne laser beam(X = 0.6328 Arm), and the focused spot near the side ofthe splitter was projected onto a charge-coupled-de-vice image sensor by a microscope objective lens(10OX). Figure 6 shows the light spot and the intensi-ty profile observed at the focal plane. The FWHM ofthe profile was measured to be 3.3 Am, which agreeswith the diffraction-limited value.

In this Letter we have proposed and demonstratedthe microfabrication of a blazed RMFL using EB writ-ing and dry development. The surface roughness ofthe grating was improved without the bridge and scumobserved when using wet-development techniques.The margin for the EB writing condition to achieve ahigh efficiency was also broadened. The fabricatedRMFL exhibited diffraction-limited characteristics,and a high efficiency of 71% was obtained at a period of10 Mm. This value is close to that (74%) obtained byour conventional method, but further improvementshould be possible by optimization of the resist mate-rials and the processing conditions.

We thank M. Endo of the Semiconductor ResearchCenter of Matsushita Electric for useful discussionsabout dry-development resist.

References

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3. T. Shiono, K. Setsune, 0. Yamazaki, and K. Wasa, Appl.Opt. 26, 587 (1987).

4. E. Enomoto, H. Nishihara, and J. Koyama, Inst. Electron.Inf. Commun. Eng. Tech. Rep. OQE83-89, 15 (1983).

5. T. Shiono, M. Kitagawa, K. Setsune, and T. Mitsuyu,Appl. Opt. 28,3434 (1989).

6. M. Tsuda, S. Oikawa, M. Yabuta, A. Yakota, H. Nakane,K. Yamashita, K. Gamo, and S. Namba, J. Vac. Sci. Tech-nol. B 3, 481 (1985).