Reaction Mechanism

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 below). 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 cross-linking so that molecules with higher molecular masses are obtained. The PMMA is turned into a negative resist. This effect is depicted in the diagram on the right 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聚合物在電子束照射下主鏈斷裂,分子量從原來950,000 g/mol(950k)降為5,000 – 10,000 g/mol. 這種主鏈斷健主要來自於根基反應. (如左下圖). 在適合的曝光劑量下,自由基重組形成分子量約5,000g/mol的聚合物. 然而,大量提高曝光劑量,產生的大量自由基會形成架橋反應,分子量提高,反而變成負型阻劑. 右下圖顯示在標準製程(AR-P 671.05, 490nm film thickness, 30kV, developer AR 600-56), 高曝光劑量使阻劑轉為負型.

The sensitivity of a PMMA resist (AR-P 671.05) strongly depends on the acceleration voltage. At 100 kV a major part of the energy passes the resist without any interaction and the resist is consequently less sensitive. At 5 kV however, all electrons are absorbed.

PMMA阻劑,的敏感度與加速電壓有直接關係,下圖以AR 671.05為例, 在100kV曝光參數下,能量直接穿透阻劑而無反應,敏感明顯下降. 在5kV下,所有電子都可被吸收.