利用射頻反應磁控濺鍍方式來研究氧化鋅薄膜之材料特性與電漿特性之關係

Abstract

本研究主要利用射頻反應磁控濺鍍系統來沈積氧化鋅(ZnO)薄膜在玻璃基板上。我們分別利用蘭牟爾探針(Langmuir probe)來量測電漿特性、X光繞射儀 (XRD) 量測材料結構特性、X光光電子能譜儀(XPS)量測材料成份及鍵結特性，可見光-紫外光光譜儀(UV-VIS spectrophotometer)量測光學特性以及接觸角量測儀量測表面親疏水特性。結果顯示，電漿密度、電子溫度、沈積速率以及所計算出之離子轟擊能量均隨著射頻功率(RF power)的增加而遞增。而在薄膜成長的過程中，沈積速率與離子轟擊能量的大小同時會影響氧化鋅薄膜之結構特性。由實驗的結果，我們觀察到三個不同的射頻功率範圍與電漿、材料結構特性有強烈的影響。在低功率的情況下(50-100瓦)，可以觀察到最大的晶粒(grain size)，這主要是因為沈積速率較慢而產生的結果；在功率次之的情況下(100-200瓦)，由於沈積速率與離子轟擊能量的效應相互抵銷下，導致最小的晶粒產生；而在高功率的情況下(200-400瓦)，因為離子轟擊能量增加，間接使基板溫度升高，進而提供能量使表面不穩定之原子重新排列，所以又可觀察到晶粒尺寸逐漸增大。所以當射頻功率在400瓦時，可以得到材料特性最佳的氧化鋅薄膜。此外，在光學特性部分，不管是氧化鋅薄膜或是氧化鋅薄膜沈積在玻璃上，它們在可見光範圍(400-700奈米)的平均穿透率均可超過85 ％，而且在紫外光範圍(280-400奈米)也得到較佳的抗紫外光之特性。薄膜厚度的效應在光學特性的影響上比起射頻電源的效應來的更重要。最後，當氧化鋅薄膜沈積在玻璃基板上，它同時也顯示出疏水性特徵。 對於不同氣體比例的影響，沈積速率均隨著氧氣含量的增加而減少，主是係因為氧氣屬於負電漿，進而導致電漿密度降低所致。當氧氣與氬氣之氣體比例為0.3時，XRD的結果指出它屬於多晶系的氧化鋅薄膜，且同時具有較小的半高寬(FWHM)以及最大的晶粒尺寸。另一方面，藉由X光光電子能譜儀得知，在此條件下亦同時擁有較多的O-Zn鍵結以及較強的鍵結強度。 最後，經由退火處理後，可以同時得到較小的半高寬、較大的晶粒尺寸以及具有較少殘餘應力薄膜。然而，適當的退火溫度是一個相當重要的因素對於有效改善材料的特性上，像是薄膜品質、應力、晶粒大小、穿透率、抗紫外線能力等等。其中，當退火溫度在300度以下時則會有較明顯的改善趨勢。反之當溫度超過400度時，則對材料的改善效果就趨於和緩。因此，我們推測在玻璃上較佳的退火溫度大約是在400度。 綜合目前的結果，我們可以同時得到高穿透率、較佳的抗紫外光能力以及疏水性特徵於氧化鋅薄膜沈積在玻璃基板上，這些特性相當適合用在建築玻璃、車用玻璃等相關產業上。未來，亦可針對不同的沈積參數做進一步的研究及討論，包括不同的靶材與基板間之距離、操作壓力等參數。

Zinc oxide (ZnO) thin film was deposited on glass substrate by RF reactive magnetron sputtering. The plasma parameters, structural, chemical, optical and hydrophilic/hydrophobic properties of the film were measured using a Langmuir probe, x-ray diffractometry (XRD), X-ray Photoelectron Spectroscopy (XPS), a UV-VIS spectrophotometer, and contact angle measurement, respectively. Results show that plasma density, electron temperature, deposition rate, and estimated ion bombardment energy all increased with increasing applied RF power. The deposition rate and ion bombardment energy were both influenced during ZnO thin film growth. Based on the measurements above, there are three distinct power regimes, which are strongly correlated with plasma properties. In the low power regime (50-100 W), a slow deposition rate produced the largest grain size. The smallest grain size appeared in the medium power regime (100-200 W). This is attributed to insufficient time for the adatoms to migrate on the substrate surface. In the high power regime (200-400 W), a relatively larger grain size appeared due to very large ion bombardment energy, which heated up the substrate and enhanced the thermal migration of adatoms. The optimal level of RF power for obtaining the optimum material properties of ZnO thin film is RF 400 W. However, the average transmittance is over 85% in the visible region of 400-700 nm for both pure ZnO thin film and ZnO on glass. This thesis also shows that ZnO thin film possessed better UV-shielding characteristics. For all of these optical properties, the thickness effect was more important than the power effect. The ZnO thin film exhibited hydrophobic characteristic when coated on glass substrate. For the O2/(Ar+O2) ratio effect, the results of this study show that the deposition rate decreased with an increasing O2/(Ar+O2) ratio. This was caused by a reduction in plasma density due to the addition of electronegative oxygen. At an O2/(Ar+O2) ratio of ~0.3, XRD results show that ZnO thin film forms a polycrystalline structure with the lowest FWHM (largest grain size). At the same time, the content fraction of the OⅠ peak (O-Zn bond) and the corresponding binding energy reached their highest levels at this O2/(Ar+O2) ratio, according to XPS analysis. The smallest FWHM, largest grain size, and less relaxation stress were simultaneously created by annealing treatment. The moderation annealing temperature is an important factor for improving material properties such as film quality, stress, grain size, transmittance, and UV-shielding characteristics, etc. An annealing treatment below 300℃ offers significant improvements, whereas it has no obvious variance when the annealing temperature exceeds 400℃. Finally, we deduced that the best annealing temperature is at 400℃ for glass substrate. In summary, we have obtained the excellent properties of ZnO thin film on glass with the high transparency, good UV-shielding, and hydrophobic characteristics for glass industry applications. The final section of this thesis provides directions for future research on different deposition parameters (distance between target and substrate, working pressure effect, etc.).