Modeling and analysis of extrusion-spin coating : an efficient and deterministic photoresist coating method in microlithography

Thesis (Ph. D.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2001.Includes bibliographical references (p. 173-178).In the fabrication of microelectronic chips, microlithography is used to transfer a pattern of circuit geometry from mask to semiconductor wafer. An important step in this process is the deposition of a thin and uniform layer of photoresist (often called resist) on which the lithographic image is exposed. Typical photoresist layers are less than 1 pum thick with a variation of 5 [angstroms] for advanced chips. Spin coating is the prevalent coating method to produce the required thickness and uniformity, but it typically wastes over 90% of the photoresist applied. A more efficient method needs to be developed for two reasons. The first is that 80% of the photoresist is an environmentally hazardous solvent. The second is the cost increase of photoresist. As the target of semiconductor industry moves toward the fabrication of smaller devices with larger capacity, the trend in photoresist shifts from i-line to deep UV resists, which allow for narrower linewidths on a chip. The price of this new resist is four to ten times higher than that of i-line resists. Reducing photoresist waste is desirable for both environmental and economical reasons. The current spin coating method has another problem in addition to low coating efficiency. Results from spin coating are unpredictable. The relationships between the inputs (process variables) and outputs (coating thickness and uniformity) can only be obtained by trial and error. Thus, a number of experiments have to be conducted to attain a certain coating thickness and uniformity. A more effective method would yield the predictable coating thicknesses and uniformities for given inputs.(cont.) Both the cost and time required for process development can be reduced this way. Extrusion-spin coating achieves high coating efficiency with predictable coating results. This new method uses an efficient extrusion coating technique to apply a thin film of resist to a wafer before spinning. spinning. This initial layer of photoresist eliminates the spreading phase, the most inefficient step of spin coating. The initial layer also provides the existing spin coating models with determined initial conditions and thereby renders its results predictable. A prototype extrusion-spin coater has been designed and fabricated. Initial experiments have been conducted to determine, test and optimize process variables. One variable, the solvent concentration degree in the environment, is most critical. As the initial coating layer deposited by extrusion coating is only 20-40 [mu]m, solvent contained in the photoresist evaporates rapidly at the absence of a solvent concentration in the environment. Evaporation causes the viscosity of photoresist to be nonuniform over the wafer. The outcome of the spin coating process becomes less uniform. Experimental results are compared with Emslie et al.'s predictive models of spin coating. A solvent concentration of 80% or higher in the environment was found to be necessary to attain a predictable coating thickness with 5 [angstrom] uniformity. With optimized process variables, mean coating thickness matches theoretical predictions with a variation of 0.01 [mu]m. Defect-free coating results with coating efficiencies as high as 40% were achieved.by Sangjun Han.Ph.D

Optimization of process variables in extrusion-spin coating

Thesis (M.S.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 1997.Includes bibliographical references (p. 83-84).by Sangjun Han.M.S