以Ar2500+-O2+共濺射增強軟材料之二次分子離子質譜

Abstract

氣體簇離子團 (gas cluster ions)是由許多原子共同組成的離子團，做為濺射離子源時，由於其在相同電壓下，濺射能量會分散於各粒子且集中於分析物表面，可使表面飛濺出的二次離子產率增加，亦不易形成小片段分子，因此近年來引起廣大的研究，已被證明是分析生物樣本及軟性材料頗具發展潛力的一項技術。但一般而言，分子量大的訊號通常較弱，將不利於分析，因此，若可以增強這些分子離子訊號，則可使SIMS的應用更為廣泛。 本研究以聚對苯二甲酸乙二酯 (polyethylene terephthalate, PET)為基板，使用時間飛行二次離子質譜 (ToF-SIMS)偵測表面二次離子，比較Ar2500簇離子源與使用低能量O2離子源共同濺射PET基板時，觀察其特徵破片的訊號強度變化，主要目的為在作濺射的同時，不僅可以保存其化學結構不受破壞，也能增強其飛濺出的二次正離子產率，在最後，也分別使用表面輪廓儀與原子力顯微鏡，量取濺射速率及濺射後的表面形貌。 研究過程中觀察到，單獨以Ar2500+或O2+濺射時，分子離子訊號強度均隨濺射時間有降低的現象，但當利用共濺射技術，由於濺射速率的提升，以及表面受氧化所形成的OH可促進質子之交換，因此整體的訊號強度達到穩定，實驗也顯示與低電壓的O2+做共濺射時，隨著O2+電流密度的增加，氧增益的效應逐漸顯現出來，相對的於高電壓下，越高電流密度的O2+做共濺射時，反而因有額外的表面損傷未能完全移除，訊號增益的強度因此受壓抑。 綜合以上討論，本實驗發現使用10 kV 2×10-6 A/cm2 Ar2500+與低電壓、高電流密度 (200V 32×10-5 A/cm2) 或高電壓、低電流密度 (500V 2×10-5 A/cm2) 的O2+進行共濺射時，可得到理想的縱深分布分析。

Over the last decade, cluster ion beams displays its capability for depth profiling organic materials and biological specimens. Compared with monatomic ion beams, cluster ion beams possess non-linear enhancement of sputter yield, minimum damage accumulation and generate high mass fragments during sputtering. These properties allow the successful secondary ion mass spectrometry (SIMS) analysis of soft materials beyond the static limit. While the intensity of molecular ions of high mass is still low, enhancing the intensity of these secondary ions while preserve samples in their original state is the key to high sensitivity molecular depth profiles. In this work, bulk poly(ethylene terephthalate) (PET) was used as modeling materials and was analyzed using a time-of-flight (ToF) SIMS with a pulsed Bi32+ primary ion. 10 kV Ar2500+ gas cluster ion beam (GCIB) and low kinetic energy (100-500 V) oxygen ion beam (O2+) were employed to co-sputtering for depth profile and examining the effect of beam parameters to the yield of positive secondary ions. The result of depth profile showed that the characteristic signal of PET declines steadily during GCIB sputtering. In other words, damage accumulation was observed. On the other hand, the secondary ion intensity can reach a steady state and was enhanced during GCIB-O2+ cosputtering. Since the secondary ions were generated by a fixed Bi32+ bombardment and was independent to the sputtering beam parameter, this enhancement of intensity is attributed to the enhanced ionization yield. This enhanced ionization yield is attributed to the oxidation of molecules and the formation of hydroxyl group that serve as a proton donator to particles emitted from the surface. The enhanced ion intensity also masked the damage to the chemical structure hence steady signal intensity is obtained. However, as O2+ is known to alter chemical structure and damage accumulation is apparent beyond the static limit, high kinetic voltage and fluence leads to the suppression of molecular ion intensity. Nevertheless, with optimized Ar2500+ and O2+ ratio that leads to enhanced ionization yield while the damage is masked, improved depth profile is obtained. The results indicated that optimized GCIB-O2+ cosputtering could provide improved depth profiles from soft materials in terms of sensitivity and preserve the chemical structural in the remaining surface.