Conformal, planarizing and bridging AZ5214-E layers deposited by a 'draping'technique on non-planar III-V substrates

• Published in: Journal of micromechanics and microengineering 2006-12, Vol.16 (12), p.2608-2617
• Main Authors: Elias, P, Strichovanec, P, Kostic, I, Novak, J
• Format: Article
• Language: English
• ISSN: 0960-1317; 1361-6439
• DOI: 10.1088/0960-1317/16/12/014

A draping technique was tested for the deposition of positive-tone AZ5214-E photo-resist layers on non-planar (1 0 0)-oriented III-V substrates, which had a variety of three-dimensional (3D) topographies micromachined in them that consisted, e.g., of mesa ridges confined to side facets with variable tilt, inverted pyramidal holes and stubs confined to perpendicular side facets. All objects were sharp-edged. In each draping experiment, an AZ5214-E sheet was (1) formed floating on the water surface, (2) lowered onto a non-planar substrate and (3) draped over it during drying to form either self-sustained, or conformal, or planarizing layers over the non-planar substrates. The draping process is based on the depression of the glass transition temperature Tg of AZ5214-E material induced by penetrant water molecules that interact with AZ5214-E. During the process, the molecules are initially trapped under an AZ5214-E sheet and then transported out through the sheet via permeation. The water-AZ5214-E interaction modifies the stiffness kappa of the sheet. The magnitude of the effect depends on temperature T and on partial water vapour pressure difference p(T, P, kappa): the net effect is that Tg = f(C(T, P), p(T, P, kappa)) is lowered as the concentration C of water increases with T and p, where P is the permeability of the sheet. The interaction depressed the Tg of the sheets as low as or lower than 53 deg C for 6 mum thick sheets. At room temperature T < Tg, the sheet is glassy and too stiff to yield to adhesion and capillary forces. Consequently, it cannot conform to a 3D topography, and it can form a self-sustained, bridging layer over it. By contrast, at T or > Tg, the sheet becomes rubbery and mouldable by adhesion and capillary forces. As a result, it can either contour or planarize the topography depending on its geometry and thickness of the sheet.