Quantitative measurement of resist outgassing during exposure

摘要

Determination of both the identity and quantity of species desorbing from photoresists during exposure at anywavelength – 248nm, 193nm and EUV – has proved to be very challenging, adding considerable uncertainty to theevaluation of risks posed by specific photoresists to exposure tool optics. Measurements using a variety of techniques forgas detection and solid film analysis have been reported but analytical results have not in general been easy to compareor even in apparent agreement, in part due to difficulties in establishing absolute calibrations. In this work we describetwo measurement methods that can be used for any exposure wavelength, and show that they provide self-consistentquantitative outgassing data for 2 all-organic and 2 Si-containing 193 nm resists. The first method, based upon gascollection, uses two primary chromatographic techniques. Organic products containing C, S and Si are determined bycollection of vapors emitted during exposure in a cold trap and analysis by Gas Chromatography-Flame IonizationDetector-Pulsed Flame Photometric Detector-Mass Spectrometry (GC-FID-PFPD-MS). Inorganic products such as SO2are identified by adsorbent bed with analysis by Gas Particle-Ion Chromatography (GP-IC). The calibration procedureused provides reasonable accuracy without exhaustive effort. The second method analyzes the elemental concentrationsin resist films before and after exposure by secondary ion mass spectrometry technique (SIMS), which requires onlyknowledge of the resist compositions to be quantitative. The extent of outgassing of C and S determined by the twomethods is in good agreement for all 4 resists, especially when taking their fundamentally different characters intoaccount. Overall, the gas collection techniques yielded systematically lower outgassing numbers than did SIMS, and theorigins of the spread in values, which likely bracket the true values, as well as detection limits will be discussed. Thedata for Si were found to differ significantly, however, and we show that the discrepancy is due to photo-inducedreactions at the polymer surface with the gas atmosphere present above the resist during exposure. For example,photolytic oxidation of the C-Si bonds in air causes volatile Si-containing products to be formed from an otherwise stablepolymer, showing it is important to take the gas environment during exposure into account when designing resistpolymers for low Si outgassing.