Pattern displacement measurements for Si stencil reticles

Abstract

We have fabricated Si stencil reticles that are employed by a new type of e-beam projection lithography system ͑EB stepper͒. We applied a stress reduction technique to the Si membrane to improve the pattern placement accuracy. The residual stress of Si membranes which were fabricated by anisotropic etching of B-doped Si wafers in KOH aqueous solution was reduced by annealing at 1150°C. We carried out pattern-displacement measurements for a Si stencil reticle made of a Si membrane where the residual stress was reduced to 10 MPa, and we observed that the pattern displacement error was reduced to less than 20 nm. Furthermore, the pattern displacement in the stencil reticle had a high correlation with the displacement determined from a simulation based on a finite element model. However in the same reticle, we discovered additional, comparatively small displacements in random directions, which was not expected in a membrane that had a homogeneous tensile stress. As a cause of the pattern displacement in random directions, we identified a pattern-width broadening in the dry etching process.

Image Placement Distortions in EPL Masks

“…Any type of compressive film is undesirable. The optimal film stress for EPL is less than 10 MPa, as others have similarly identified [2,8,9]. Boron is typically either diffused or included in a silicon epitaxial layer at a concentration of about 1x10 20 atoms/cm 3 to act as an etch stop to form the membrane in typical wet anisotropic etchants, such as potassium hydroxide.…”

“…Annealing of the boron doped membranes has been shown to reduce the stress to lower levels [9]. The inclusion of germanium with the boron during the diffusion or epitaxial growth process can likewise reduce the final stress of the film without sacrificing its etch stop capability.…”

Analysis of critical parameters for EPL mask fabrication

“…This has been achieved by several groups through doping the silicon layers. [16,17] 4.2 Mask chucking EPL masks are produced on wafers which are less rigid than thick quartz substrates. This dictates that the chucking of EPL masks be well defined during the ebeam writing, image placement metrology, and wafer exposure processes.…”

EPL reticle technology

“…Because SOl layer on SOl wafer has compressive stress, we needed to control membrane stress. 3 We doped an impurity like Boron and Phosphorus to SOl layer in order to control the membrane stress. The relations between internal membrane stress and impurity concentration due to the thermal diffusion and the ion implantation methods are shown in FIG.8.…”