(581b) Mechanical Properties of Polymer Thin Films and Nanostructures

As pattern sizes shrink below 50 nm in semiconductor devices, the resist film thicknesses and resist feature sizes used in the lithography steps for patterning these devices are also rapidly decreasing in size. This reduction in film thickness was originally moivated by limitations in the depth of focus of the optical projection exposue tools used to pattern such resist films. However, it has also become clear that such film thickness reductions are important in mitigating pattern collapse problems that occur due to capillary force induced collapse of such fine polymeric resist nanostructures during wet processing and drying of the resist pattern. This reduction in film thickness is a concern for several reasons such as reduced etch resistance of the thinner resist films, but it is especially important as film thicknesses approach and shrink below 100 nm. The sub-100 nm film thickness regime is where ultra-thin film effects have already been shown to become a major issue due to the confinement of the material at the interfaces and the large surface to volume ratio in such thin films; i.e. the interfacial layer between the substrate and the resist and the interfacial layer at the free surface of the resist film become significant relative to the total film thickness. These ultra-thin film effects have shown to result in dramatic changes in the properties of the materials such as raising or lowering the glass transition temperature, changing the mobility of the material, and orders of magnitude reduction in the diffusivity of small molecules such as acid and water. Among the most important changes for resist and lithography applications may be the potential change in the mechanical properties of these films as the film thickness shrinks and/or the patterned feature size shrinks. This study examines the effect of film thickness and patterned feature size on carefully designed pattern collapse test structures that are used to determine the effective resist polymer modulus and critical stress at the point of pattern collapse for varying film thicknesses, pattern sizes, and types of photoresists. Confinement of the polymer resist in 1-D (thin films), 2-D (line pattern grating structures), and 3-D (resist post structures) were probed using different test structures and experimental methods. For example, film buckling measurements were used to probe the effect of 1-D confinement into thin films on the effective modulus of the thin films. High resolution e-beam lithography was used to generate 2-D line and 3-D post pattern collapse tet structures. Both positive tone (non-crosslinking systems) and negative tone (cross-linking systems) resist systems were studied and compared to determine the influence of cross-linking on such behavior. It will be shown that thin film and nanostructure confinement effects on modulus will be a significant factor affecting the ability to develop future generations of high resolution lithography. A simple mechanical model will be shown that can describe the observed effects and predict the behavior of such systems from knowledge of bulk material properties.