氬簇離子團(Arn+)能量密度與O2+共濺射對軟材料二次離子質譜縱深分析之影響

Abstract

近年研究顯示，以氣體簇離子團作為濺射離子源去分析生醫材料及軟性材料是一項具發展性的二次離子質譜分析技術，由於簇離子團轟擊表面後大部分的能量集中在材料表面，因此表面靈敏度高。相較於以碳六十和單原子離子作為入射離子源，氣體簇離子團的濺射率高，二次離子強度 (Secondary Ion Intensity) 高，損傷累積小而高分子量資訊易被保留下來，因此在許多高分子軟材料及生物材料的分析上具有優異的解析能力。簇離子源雖然相較於其它離子源有優異的表現，但以不同的能量密度 (E/n) 去濺射分析物樣品時，不但對表面的損傷累積及破壞程度不同，得到的縱深結果亦有所不同。因此，本實驗目的在了解氣體簇離子之能量密度配合低能量之O2+共濺射對表面性質的影響及縱深分析的變化，不僅可了解氣體簇離子團與樣品之間的交互作用，亦可使此分析技術更實際地應用在分析各類樣品上。 本研究使用不同能量密度 (E/n = 2.5 - 20) 之Arn+簇離子團及低能量 (200 V, 500 V) 之O2+在對苯二甲酸乙二酯 (Polyethylene Terephthalate, PET) 基板上以Bi32+為一次離子源進行質譜縱深分析，並觀察其特徵破片之相對強度變化。接著利用原子力顯微鏡 (Atomic Force Microscope) 進行彈性模數之測量，同時得知表面形貌後，再利用探針式表面輪廓儀 (Alpha Step) 進行濺射深度之量測。 研究結果顯示，單獨以Arn+簇離子團或與低電壓低電流密度 (200 V, 5A/cm2)之O2+共濺射，進行縱深分析時，由Bi32+所得到的二次離子相對強度隨著能量密度增加而提升，而E/n = 20時能量過高，使樣品達到膠凝點 (Gel Point)，二次離子相對強度則急劇下降。加入低電壓高電流密度 (200 V, 80A/cm2) 或高電壓低電流密度 (500 V, 5A/cm2) 之O2+後，在適中的能量密度 (E/n = 3.75, 5 - 10) 下能有較佳的二次離子訊號增益效應，其餘條件則引入了更多的損傷累積或增益效應不顯著。而在彈性模數部分，相較於原始表面，除了達到膠凝點時之彈性模數急劇上升之外，其餘表面進行濺射後的彈性模數均下降，此一結果可能來自於解聚作用 (Depolymerization) 的發生。特別的是使用200 V, 80A/cm2之O2+輔助濺射時彈性係數整體上升，而使用500 V, 5A/cm2之O2+輔助濺射時彈性係數整體下降，暗指高能量O2+可能造成更嚴重的解聚作用。不僅如此，使用O2+進行輔助濺射時，對表面粗糙度亦有抑制的功效，不僅抹平了Arn+簇離子團所帶來的表面形貌，還能提高縱深解析度，有利未來高解析縱深分析之應用。 在本實驗研究中亦發現，Arn+簇離子團所造成的破壞及濺射率增益為一非線性結果，因此在最佳化分析時，在適中的能量密度下選用高加速電壓及大簇離子尺寸能有較佳的分析結果。

Over the past few years, gas cluster ion beams (GCIB) has shown great capability of dealing with bio-materials and soft materials owing to its high sputter yield and low damage accumulation that preserved the molecular structures during depth profiling, therefore molecular ion of high mass can be obtained in subsequent analysis. However, although GCIB has lower damage accumulation comparing with C60+ and monoatomic ions, the inevitable alteration in chemical structure can still change the property of remaining surface and gradually affect the accuracy of depth profile. As a result, artifacts can still be observed in the resulting depth profiles. In order to further improve the depth profile of soft materials, low energy O2+ can be used to cosputter the surface to enhance the ionization yield and mask the damage accumulation. While the energy per atom (E/n) is known to be another important factor to the sputter process and previous works concluded higher E/n is beneficial, how the change in E/n affects GCIB-O2+ cosputter depth profile is not clear yet. In this study, bulk (Polyethylene Terephthalate, PET) was chosen as the modeling material and 10 - 20 kV Ar1000-4000 (E/n = 2.5 - 20) with or without O2+ beam was used to cosputter the surface. Spectra at different depths were obtained by a time-of-flight secondary ion mass spectrometer (ToF-SIMS) with pulsed Bi32+ as primary ion to construct the depth profile. After sputtering, the craters were measured by alpha step and atomic force microscope (AFM) with quantitative imaging mode. The result shows that with higher E/n, the resulting surface is more rough and the surface Young’s modulus became smaller compared with the pristine surface. This result suggested that depolymerization took place. Also, relative intensity (I) was comparatively weak in the depth profile, which indicated more damage accumulated. Furthermore, at E/n = 20, the sputter rate and secondary ion intensity decreased rapidly. Because the Young’s modulus increased significantly with low roughness, the result suggested that the system reached gel point rapidly. In other words, radical induced cross-link dominated the damage process. With the auxiliary O2+ as cosputter ion, it helps to break the ion-beam induced morphology and enhance the ionization yield that masked damage. As a result, steady-state can be obtained and depth profiles with less artifact were observed.