An Additive Soft-Lithographic Submicron-Patterning of PDMS Resists on Electronic Materials

Abstract

We have developed a novel technique for the fabrication of submicron-sized polydimethylsiloxane (PDMS) resist patterns on electronic material substrates using decal transfer lithography (DTL) and reactive ion etching (RIE). The DTL technique is based on the transfer of PDMS decal patterns to a substrate via the engineered adhesion and release properties of a compliant PDMS stamp1. The conventional DTL process is useful for delivering micron-sized PDMS patterns onto targeted substrates. Submicron-sized patterns, however, are more difficult to transfer by CMF and their utility as resists remains limited because of the very thin nature of the PDMS patterns transferred. This limitation is one directly related to the low moduli of the conventional PDMS polymers used in soft lithographic patterning. The present work describes a new advance in one form of DTL patterning that appears to hold exceptional promise as a means for transferring sub-micron feature sizes, ones appropriate for use as resists in electronics fabrication processes, over large substrate areas. This method uses cohesive mechanical failure (CMF) patterning, a class of DTL that exploits the transfer of PDMS patterns to a substrate via a sequence of interfacial adhesion and mechanical decohesion of PDMS from a molded elastomeric patterning tool.2 Figure 1 illustrates procedures for fabricating sub-micron, high-aspect-ratio resist features on a silicon substrate. Briefly, a PDMS prepolymer (Dow Corning, Sylgard 184) is cast onto a master bearing a relief pattern (i.e., patterned photoresist) and cured at 65 °C. The cured PDMS stamp is peeled from the master, the patterned PDMS surface exposed to an unfiltered UV/Ozone (UVO) for 150 sec, and the treated surface is immediately placed in conformal contact with an appropriately modified substrate. Given that CMF-based DTL transfers yield extremely thin PDMS resists when very small features are being patterned (i.e. the decohesive debonding of the PDMS occurs near the interfaces of the bonded decal, Figure 1), we modified the substrate by adding additional layers of thin film materials that would serve to increase the aspect ratio of the resists once developed using an RIE protocol. Towards this end, we used substrates that had first been coated with an organic planarization layer (PL) and subsequently overcoated with a thin-layer of SiO2 deposited using an electron beam protocol. A low pressure mercury lamp (BHK, 173 μW/cm2) was used for UV/Ozone (UVO) treatment of the PDMS and, after curing at 65 °C, the PDMS pattern was transferred by mechanically peeling away the bulk PDMS. In the present study, a 400 nm thick PL layer (Microposit 1805) was spin-cast onto a Si(100) substrate and treated with an oxygen plasma to increase adhesion in conjunction with a 3 nm thick SiO2 film as an adhesion layer and secondary etch stop.