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Fast electron resist contrast definition by "fitting before measurement" approach
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Fast electron resist contrast definition by "fitting before measurement" approach

Knyazev M.A., Dubonos S.V., Svintsov A.A., Zaitsev S.I.

Institute of Microelectronics Technology, Russian Academy of Sciences,
142432, Chernogolovka, Moscow district, Russia

A new method for fast definition of an electron resist contrast is presented. The development velocity of an electron resist can be written [1]:

where is the development rate of the electron resist exposed to dose , is the development rate of the electron resist exposed to dose (resist sensitivity), and is the resist contrast. Using (1), a test structure (dose wedge) for contrast definition can be designed. The dose wedge is a set of small rectangles closely following each other (Fig.1). During exposure, the dose increase for each sequential rectangle. The dose for the first (left edge) rectangle is zero, reaching the maximum for the last (right edge) one, . If the number of rectangles is N, then the exposure dose for the n-th rectangle is:

Three different profiles can, generally, be revealed after the development of the exposed dose wedge (Fig. 1). As shown in Fig. 2, when is higher than the real resist contrast exp the profile is concave, when < exp the profile is convex, and when and exp are equal the profile is plane. Therefore, when = exp the change in the residual resist thickness is linear. The real test structures consist of several dose wedges. The exposure dose for each wedge was calculated for different (from 1.5 to 4.5) using (2). Fig 3 shows the result of test structure development in a MMA copolymer resist exposed by a JEOL 840A under NanoMaker hardware/software system [2] control at 25 KV and developed in IPA-H2O 8:1 solution. The exposure data were prepared using the software part of the NanoMaker according to (2). Fig 3. is an optical microscope photo obtained using a red light filter. Generally, the dose wedge looks as shown in Fig. 3 in an optical microscope with any light filter. When the residual resist thickness grows linearly, the distance between neighboring intensity minima or maxima is constant. In the case > exp, the distance decreases with a thickness increment and, vice versa, if < exp the distance between minima or maxima increases with a thickness increment. For the test structure in Fig. 3, the value of the resist contrast is between 3 and 3.5. The considered methods is similar to approach developed for measurements of proximity function parameters [3].
In comparison to other methods the considered one is extremely fast. In particular steps (design, exposure, development and observation) take typically 30 minutes.

The partial support of the EC-funded project NaPa (Contract no. NMP4-CT-2003-500120) is gratefully acknowledged. The content of this work is the sole responsibility of the authors.

[1] A.R. Neureuther, D. F. Kyser and C. H. Ting. IEEE Trans. Electron Dev. ED-26, 686, 1979.
[2] www.nmaker.ru (this site)
[3] L.I. Aparshina, S.V. Dubonos, S.V. Maksimov, A.A. Svintsov, and S.I. Zaitsev, J. Vac. Sci. Technol. B, Vol. 15, No. 6, Nov/Dec 1997 (pp. 2298-2302)

Fig. 1 Schematic image of the dose wedge

Fig. 2 Dose wedge profile form at different correlations between the
resist contrast and parameter

Fig. 3 Test structure for resist contrast definition exposed
in the MMA copolymer resist (dose wedges)
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