Resist layer planarizing

Depth of field depends on substrate reflectivity, the degree of partial coherence and the minimum feature size (5). In practice, however, the classical depth of field for the incoherent case (X-f2(N.A.)2) gives a reasonable approximation. Two layer resist processes in which the image is formed in a thin, flat, resist layer on top of a much thicker planarizing layer, alleviate the need for a large depth of field and make it easier to form high resolution, high aspect ratio, resist patterns (6,7). Satisfactory results can be obtained at contrast levels as low as 40%. 

Figure 7. Metal lift-off process using a trilevel-resist scheme, (a and b) The image created in the top-layer resist is transferred via the isolation layer to the bottom planarizing layer by an isotropic etch, (c) The sloped side wall of the planarizing layer has an overhanging transfer layer that breaks up the continuity of the metal film sputter deposited onto the system. (d) Subsequent dissolution of the bottom layer carries off parts of the metal film adhering to the resist layers, and well-defined metal lines are left.

Currently two methods are utilized for transfer of the relief image from the top resist layer to the bottom planarizing layer oxygen reactive ion etching (RIE), and optical flood exposure (1). Oxygen RDE systems have not found widespread acceptance due to the high cost and low throughput of RIE equipment. Optical... 

Figure 9.—Continued. SEM micrographs showing two-layer resist features transferred via O2 RIE into a 1.5 jjn-thick planarizing layer. Graft copoljnner (top resist layer) patterned at 4 J/cm. ...

Trilevel Schemes. Trilevel processing (6, 7) requires planarization of device topography with a thick layer of an organic polymer, such as polyimide or a positive photoresist that has been baked at elevated temperatures ( hard baked ) or otherwise treated to render it insoluble in most organic solvents. An intermediate RIE barrier, such as a silicon dioxide, is deposited, and finally, this structure is coated with the desired resist material. A pattern is delineated in the top resist layer and subsequently transferred to the substrate by dry-etching techniques (Figure 3). 

In order to achieve satisfactory planar surfaces over wide trenches (>10 pm) on a wafer, a double resist coating method was employed. (Refer to Figure 3). The initial resist layer, after application, is exposed with an undersized block-off mask, and, with appropriate processing, will leave the resist only in the depressions of the wide area trenches. The resist thickness of this layer should be about equivalent to, or slightly more than, the depth of the depression - in the present case about 4-5 pm. The resist thickness is also dictated in part by the dimensional difference between the block-off mask size and the trench depression area. 

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