Upon electron irradiation of PMMA resists, the main chain is cleaved and the molecular mass drops from initially 950,000 g/mol (950K) to 5,000 – 10,000 g/mol. This main chain scission is primarily due to radical processes (see figure above). At an optimal dose, radicals recombine and form molecules with a molecular mass of about 5,000 g/ mol. If however the dose is drastically increased, a large number of radicals are produced and undergo crosslinking so that molecules with higher molecular masses are obtained. The PMMA is turned into a negative resist. This effect is depicted in the diagram below which shows the gradation curve of a standard process (AR-P 671.05, 490 nm film thickness, 30 kV, developer AR 600-56). High exposure doses convert the resist into a negative resist.
PMMA阻劑在電子束照射下,聚合物主鏈斷裂,分子量降低,以950k為例,分子量從950,000 g/mol降至5,000-10,000 g/mol. 主鏈斷裂來自於自由基形成過程. 如上圖. 在適當的電子束劑量下, 自由基重新形成分子量約5,000 g/mol的分子.
如果電子束劑量過高,大量自由基架橋使分子量大增, PMMA會變成負型阻劑. 下圖為此變化的曲線圖.
(AR-P 671.05, 膜厚490nm, 30kV, AR 600-56顯影劑)