Silicon diffusion characteristics of different surface imaging resists

Abstract This paper describes a study of the silylation characteristics of different resists that are suitable forsingle-layer, surface-imaging patterning applications. In particular, the effect of different processparameters on the silicon diffusion in UCB's Plasmask®resist is discussed. The diffusion profileof silicon in the resist is decorated by a staining technique followed by SEM analysis. This allows for two-dimensional resolution of the diffusion profiles and the observation of other process attributes. Links are established among exposure, silylation and etch by observing silylated profiles.It is shown that the silylation profile characteristics are dominated by the resist image created duringexposure. Also, the effects of post-exposure bake and silylating agent temperature are presented.Diffusion profiles for MacDermid's PR1024 are also shown. 1. Introduction Surface imaging lithography relies on the imaging of the top resist layer in a stack of two or threelayers or the top surface of a single layer resist. Another characteristic of surface imaging is the useofplasmas to transfer the imaged layer into the underlying resist film or films. This paper deals withsingle-layer, surface imaging, resists. These resists were described by Taylor and Wolf [1] and oneof best known examples is the Plasmask® resist used in the DESIRE® System [2]. The advantagesand mechanisms ofthis process have been described elsewhere [3-5]. Figure 1 shows a basic schemefor this process, which involves exposing, baking, silylating, and plasma etching the resist. Thispaper deals with the silylation step; that is, the diffusion andreactionofa silylating agent in the resist.In particular, silylation from a gas phase with HMDS is discussed. Liquid silylating agents have beendiscussed [6], while the use of alternate gaseous silylating agents for other single layer processeshas also been described in the literature [7,8].The function of the silylation step is to form 0-Si bonds by displacing hydrogen from the OH pairsin the Novolak resin. In the case of the DESIRE® process, the silylation occurs in the exposed areawhere the photoactive compound has been destroyed and crosslinking cannot occur upon baking ofthe resist. Systems where the exposure induces the crosslinking ofthe resist, directly [8,9] or throughthe use of a crosslinking agent [10,1 1] have also been proposed. The depth, concentration anddiffusion profile of silicon after the silylation play a fundamental role in determining the quality ofthe mask formed during plasma exposure. Furthermore, the two-dimensional characteristics of thediffusion process have a direct impact on the silylation "contrast" of the resist.*