溫度與氬簇離子團 (Arn+) 能量密度對有機薄膜二次離子質譜縱深分析之影響

Abstract

由於具備表面及縱深分析的能力，二次離子質譜儀在分析領域已經被使用數十年，然而，單原子離子濺射源容易導致大分子碎裂成小的分子破片以及化學結構上的改變，因此較難建構生物性材料和高分子等軟材料的縱深分析。近年來，氣體團簇離子束 (Gas Cluster Ion Beam, GCIB) 由於其入射至樣品後能量的沉積集中在更表層，在濺射過程中能保持樣品中分子結構的完整性，因此成為分析軟材料的入射離子源之一；此外，GCIB可藉由調整其能量密度 (Energy per atom, E/n) 取得更適合做為濺射離子源的實驗參數。在本實驗中，使用飛行時間二次離子質譜儀 (Time of Flight Secondary Ion Mass Spectroscopy, ToF-SIMS) 作為分析儀器，以脈衝C60+作為分析離子源，並以10-20 keV 的Ar1000-4000+ (E/n = 2.5-20 eV/atom) 作為濺射離子源，以交錯濺射的方式產生縱深分佈，建構矽基材上海藻糖薄膜的縱深分析結果。而實驗結果顯示當入射離子能量密度 (E/n ≥ 10 eV/atom) 越高，入射離子誘發的損傷程度越高，導致樣品內的分子產生化學結構的轉變，所得出的二次離子訊號強度越低；而當入射離子能量密度 (E/n ≤ 4 eV/atom) 越低時，其濺射率越低，因此無法有效的移除入射離子源誘發的損傷，二次離子訊號強度也較低；但在入射離子能量密度 (E/n = 4-10 eV/atom) 適中的情況下，所誘發的損傷和濺射速率較能達到良好的平衡，因此保留了較高的二次離子訊號強度，總結來說，適中的入射離子能量密度可以使得縱深分析結果更加真實。而除了能量密度以外，樣品溫度也是影響縱深分析的因素之一，在較高的溫度下，分子的流動性提高，使得濺射率上升，入射離子誘發的損傷更容易被移除，但同時樣品內的自由基活性提高，使得交聯反應或是分子結構變化更容易發生；相反的，在較低的溫度下，自由基的活性被限制，抑制了化學結構的改變，但同時分子流動性降低使得濺射率降低，入射離子所誘發的損傷較難被移除。而本實驗分別在-90 ℃、25 ℃以及90 ℃下，使用不同能量密度的Arn+對海藻糖薄膜進行縱深分析，結果顯示在高溫時，二次離子的訊號強度因為濺射率的提高而有所提升，但在低溫時，其縱深分析結果並沒有明顯的優於常溫下的結果，顯示在此系統中濺射率提高的影響較損傷降低的影響來的重要。

Secondary Ion Mass Spectroscopy (SIMS), with its ability to analyze chemical compositions at near surface and along the depth, has been used for decades. However, depth profiles of bio-materials and soft materials are more difficult to obtain due to the fragmentation and transformation of molecules induced by atomic ion sputtering gun. Nevertheless, with more surface-localized energy deposition, gas cluster ion beam (GCIB) could preserve molecular structures during sputtering hence is a promising candidate for analyzing soft materials. Furthermore, by adjusting the energy per atom (E/n) of GCIB, experimental parameters can be optimized. In this work, as modeling system, trehalose thin films on silicon were profiled with 10-20 keV Ar1000-4000+ (E/n = 2.5-20 eV/atom). The spectrum during interlaced sputtering was obtained with Time of Flight Secondary Ion Mass Spectroscopy (ToF-SIMS) with pulsed C60+ as primary ion beam to construct the depth profile. It was found that with higher energy density (E/n ≥ 10 eV/atom), lower intensity of molecular ions was obtained due to fragmentation of the molecules and higher degree of damage. Furthermore, with lower energy density (E/n ≤ 4 eV/atom), suppressed intensity is also observed due to the lower sputtering rate that cannot remove damage sufficiently. Nevertheless, with medium energy density (E/n = 4-10 eV/atom), the introduction and removal of structural damage is balanced hence higher molecular ion intensity is retained. As a result, moderate E/n yield more realistic results. In addition to the E/n, temperature is another factor that influences depth profiles. At higher temperature, molecules become more mobile hence higher sputtering rate is expected and could mask the damage. However, radicals are also more mobile and induce more significant cross-linking that suppress the ion intensity. For lower temperature, radicals would be immobilized but the sputtering rate would also decrease hence it is more difficult to remove damage. In this work, depth profiles of trehalose thin films are conducted under -90 ℃, 25 ℃ and 90 ℃ using Arn+ with different energy density as sputter ions. The results show that enhanced intensity is observed at high temperature due to the enhanced sputter yield. However, no obvious differences are found in depth profiles conducted at low temperature as compared with those at room temperature.