Fabrication of monolithic diffractive optical elements by the use of e-beam direct write on an analog resist and a single chemically assisted ion-beam-etching step

Abstract

We present a method to fabricate high-quality and environmentally rugged monolithic diffractive optical elements (DOE's). Analog direct-write e-beam lithography was used to produce analog resist profiles that were transferred into their substrates by the use of chemically assisted ion-beam etching (CAIBE) in one single etching step. An iterative method was used to compensate for the proximity effect caused by electron scattering in the resist and from the substrate during the e-beam exposure. Slope-dependent differential etch rates that occur during the transfer process were characterized and compensated for. Finally, the DOE was divided into regions with different period ranges, and the exposure dosages were set to achieve even and accurate etch depths in the final element. The presented fabrication method will increase manufacturability and reduce processing time, which will result in a general cost reduction per element.

<title>Transfer of micro-optical structures into GaAs and diamond</title>

“…The main factor responsible for the distortion is the dependence of the etch rate with surface relief angle [15,16]. We do not know the exact dependence, but by trying various likely dependencies we were able to find a good agreement between etched results and simulated etched profiles.…”

Multi-step electron beam technology for the fabrication of high performance diffractive optical elements

“…the mask consisting of clear and opaque areas only, is used to copy the pattern on the substrate followed by wet or dry etching. (2) Analog methods [7][8][9][10][11][12][13], where variable-dose writing is used to achieve a multi-level or even semi-continuous relief in the resist layer, then transferred into the substrate by proportional dry etching. In order to form a structure with n phase levels, n different doses have to be used.…”

<title>Precise proximity correction for fabricating chirped diffraction gratings with direct-writing electron-beam lithography</title>

“…To predict the electron dosage suitable for a desired profile, it is necessary to consider an electron scatter in a resist layer and resist characteristics in development process. There are some dose-correction methods taking account of the electron scatter effect and nonlinear feature of the resist development [5], [6], [7].…”

“…A typical proximity correction method called the contrast curve method, which is based on a double-Gaussian distribution of electron scatter and a remaining resist thickness curve [5]. The absorbed energy density in the resist layer is determined by convolution of the double-Gaussian electron scatter profile and the electron exposure dose profile.…”