The effect of quantity reduction of photosensitive polyimide on the uniformity and film quality of the spin-coated passivation layer in integrated circuits
|關鍵字:||感光性聚亞醯胺;旋轉塗佈;鈍化層;均勻性;光阻減量;photosensitive polyimide;spin coating;passivation layer;uniformity;photoresist reduction|
The expense of photoresist materials is estimated to occupy more than 20% of the total cost of chemicals used for IC fabrication. In order to reduce the fabrication cost and environmental pollution, the reduction of the photoresist quantity has recently become a popular research topic. This thesis reports the influence of optimization of the spin-coating process of the polyimide (PI) on 8-inch wafers, including spin-up speed, deceleration, spin-off speed and the control of the dispensed PI weight, on the film uniformity, and verifies the feasibility of the optimized PI reduction process for the on-line mass production. The study shows that the PI reduction without any process optimization may degrade the uniformity of the PI film. However, the decrease in the spin-up speed combined with an increase in the spin-off speed can mitigate the non-uniformity problem. We can produce a stable PI passivation film, which passes the pin-hole test and the cross-sectional examination, by reducing the used PI weight from 3.5 g to 2.7g per wafer, saving the material cost by 22.8%. The dependence of the PI uniformity on the optimized spin-coating parameters is summarized as follows. The spin-up speed greatly affects the film uniformity although a direct relation between the speed and the film thickness is not obvious. It is important to set the spin-up speed within an appropriate range for a better film uniformity. The increase in the deceleration speed reduces the film thickness, and the deceleration speed is found to be related to the film thickness by a power of -0.07. However, the film uniformity becomes worsen after the speed is increased to a critical value; this is due to the occurrence of PI backflow at the high speed, deteriorating the film uniformity. The increase in the spin-off speed reduces the film thickness, and the speed is related to the thickness by a power of -1.01. The film uniformity, however, degrades with increasing spin-off speed; this may result from an unbalance between the center and the edge centrifugal forces. The increase in the PI amount used for the spin coating has little effect on the film thickness and uniformity. This is because the excessive PI solution is spun out of the wafer at the beginning of the spin process and, therefore, the PI amount has a small influence on the film quality of the PI layer.