Alternative method of gray-tone lithography with potential for the fabrication of combined continuous 3D surface profiles and subwavelength structures

Abstract

In the last years it has been shown that the gray tone lithography (with HEBS-glass masks) is a suitable technology for fabricating arbitrary continuous surface profiles. However, the resolution of this technology is limited and is in the range of some micrometers dependent on the electron energy used for darkening the HEBS glass mask. To overcome this drawback, we suggest to apply two subsequent lithographic steps on the photoresist layer <i>without</i> a second coating. Non-standard gray tone lithography will be used in a first step for fabricating a pre-form and the second lithographic step which can provide the resolution demanded will correct for the residual deviations. This is possible with an alternative method of gray tone lithography which makes use of the absorption and bleaching of the photoresist. Exposure with a sufficiently short time leaves a bottom part of the resist layer unexposed and develop-ment stops at its face. Thus, the remaining resist profile can be exposed in a second lithographic step without a new re-sist coating. With this method it will be possible to fabricate sub-wavelength structures, e.g. for antireflection, on top of arbitrary continuous surface profiles. Moreover, this method can be used for the fabrication of deep profiles by repeated applica-tion of the first lithographic step. In the paper we will discuss the problems of accuracy, resolution and profile depth of this method and present results of optical elements fabricated by this technology.