鐵鉑薄膜經快速熱退火處理後之結構、相變、優選方位與內應力之研究

Study on structure, phase transition, preferred orientation, and internal stress of FePt thin films treated by rapid thermal annealing

梅瑞國(Jui-Kuo Mei)

指導教授 ： 林鴻明

大同大學/工程學院/材料工程學系所/博士(2012年)

摘要

本研究係在室溫下，非晶基板上鍍製不同厚度的鐵鉑薄膜，經快速熱退火處理，以了解其序化條件及(001) 優選方位形成的機制，研究結果發現：在升溫速率80˚C /秒、持溫120秒、溫度700、800、900˚C的條件下，鐵鉑膜厚10至100 nm試片完全序化，膜厚30 nm以下，有強烈優選方位，升溫速率40˚C /秒，溫度900˚C，在膜厚20 nm以下，出現(001)優選方位。具最佳(001)垂直優選方位(LOF=0.92)發生在溫度800˚C、升溫速率80˚C /秒、持溫120秒，膜厚30 nm 的樣品。
對於序化度大於0.54的試片，水平方向的矯頑磁力都介於0.50 至1.38T之間；然而低序化度的試片，水平方向的矯頑磁力介於0.8至2.0 kOe之間。 (001)優選方位對水平方向跟垂直方向矯頑磁力的影響非常明顯，當LOF大於0.78，垂直方向矯頑磁力大於水平值的 50%，其中升溫速率80˚C /秒，800˚C退火，厚度30 nm的試片，LOF高達0.92，其垂直方向矯頑磁力8.1 kOe，水平方向矯頑磁力1.9 kOe，飽和磁化量526emu/cm3，殘餘磁化量447emu/cm3。
微結構的觀察發現退火溫度高（900˚C）、升溫速率快(80˚C /秒)，試片會形成不連續的島狀或網狀結構，試片表面的平均粗糙度增加數十奈米，相對的在退火溫度低（≦600˚C）的條件下，試片表面形貌呈連續狀，平均粗糙度約十幾奈米，即使在膜厚較低的試片中亦然。磁區的分佈形態為單晶粒構成單磁區或多晶粒構成單磁區，磁性對比強，只存在高膜厚( 100 nm)及具有(001)優選方位的試片中。
具(001)優選方位試片的微結構特徵是平均晶粒尺寸較大，磁區分佈形態傾向單晶粒構成單磁區，且磁性對比很強。基於上述所發現優選方位跟微結構之間的特定關聯性去做應力分析，發現在初始應力屬拉伸應力狀態，應力值接近1GPa，跟膜厚沒有明顯的關係，在那些具有(001)優選方位的試片中，我們發現拉伸應力大幅的提升，範圍從1.6GPa到8.9GPa，直接證明內應力對(001)優選方位形成的關鍵影響力。
在熱處理溫度900oC，升溫速率80°C/秒，持溫時間縮短成60秒時，在膜厚40~100 nm發現了一種序化介穩結構，此介穩態為具有fcc序化相的軟磁結構，我們發現適當的退火條件才能形成此序化介穩結構。
藉由調控不同的沉積參數，例如背景工作壓力、偏壓，濺鍍功率等，我們發現可以控制室溫下沉積FePt薄膜的初始應力態，從1.01Gpa壓縮到0.18 GPa拉伸，FePt薄膜不同的初始應力態，亦被證實會產生明顯的優選方位，初始壓縮應力態，會導致晶粒的等向性成長，而拉伸應力卻能促進(001)優選方位的形成。

關鍵字

鐵鉑薄膜 ； 快速熱退火

並列摘要

Order-disorder transformation and the formation of (001) preferred orientation induced by rapid thermal annealing (RTA) in FePt thin films grown on amorphous glass substrates at room temperature (RT) were systematically investigated. The results showed that the FePt films with thickness (t) in the range of 10 to 100 nm are fully ordered under the post annealing conditions of heating rate (Rheat) of 80˚C/sec, annealing time (τ) of 120 sec, and annealing temperature (Ta) ≥ 700˚C. (001) preferred orientation was obtained in the samples with t ≤ 30 nm. For the samples with Rheat = 40˚C/sec, and Ta = 900˚C, (001)-texture appears at t ≤ 20 nm. Optimized (001)-texture with very high LOF factor of 0.92 was achieved at conditions of Rheat = 80˚C/sec, Ta = 800˚C, τ = 120 sec, and t = 30 nm.
Ordered FePt films with order parameter (Sord) exceeded 0.54 show remarkable hard magnetic properties. The longitudinal coercivity (Hc//) was in the range of 5 kOe to 13.8 kOe. For those samples with Sord < 0.54, the values of Hc are between 0.8 to 2 kOe. Perpendicular coercivity (Hc) depends strongly on LOF. When LOF is higher than 0.78, the value of out-of-plane Hc is greater than the in-plane value by 50%. The optimized sample (LOF = 0.92) shows outstanding perpendicular magnetic properties including large Hc = 8.1 kOe, small Hc// = 1.9 kOe, saturation magnetization of 526 emu/cm3, remanence of 447 emu/cm3.
The results of surface morphology indicate the formation of discontinuous island-like or network structure in the high-Ta annealed (900˚C) films with fast Rheat (80˚C/sec), drastically increases the rms surface roughness (Rrms) to tens of nanometers. On the contrary, low-Ta prepared (≤ 600˚C) samples exhibits continuous layer morphology with relatively small Rrms of about several nm even in the films with reduced t. The magnetic domain structures also show significant differences between the samples. Single domain particles and interaction domains with strong magnetic contrast were found in the films with good (001)-texture and large t (100 nm), respectively. Dependence that the (001)-texture is proportional to the lateral grain size was observed, relating the formation of preferred orientation to the internal stress/strain of the FePt films. The results of the analysis on the residual stress/strain reveal that for the RT-deposited films, the tensile initial stress of about 1 GPa is independent of t; however, for those films with (001)-texture, the tensile residual stress increases drastically from 1.6 to 8.9 GPa, directly evidencing that the internal stress/strain is an important driving force for the development of (001) texture.
In the samples with Rheat = 80˚C/sec, Ta = 900˚C, and τ = 60 sec, and t = 30 nm, a metastable phase of FePt was identified in the thickness range of 40 to 100 nm. The phase is chemically ordered with a face-centered-cubic structure and is magnetically soft. The formation of this metastable structure was found to be very sensitive to annealing condition.
Additionally, we have found that the initial stress/strain of the RT-deposited FePt films can be well controlled with the range of compressive 1.01 GPa to tensile 0.18 GPa by adjusting various deposition parameters such as working pressure, biasing voltage, sputtering powers, etc. Different initial stress/strain state was confirmed to produce distinct preferred orientation of the FePt films. Compressive initial stress/strain results in isotropic grain growth; yet, tensile stress/strain facilitates the development of (001) texture.

並列關鍵字

RTA ； FePt thin films

參考文獻

[64] B. Bian and D. E. Laughlin, J. Appl. Phys. 87, 6962 (2000).

[105] Shouheng Sun, C. B. Murray, Dieter Weller, Liesl Folks, and Andreas Moser, Science 287, 1989 (2000).

[99] D. H. Wei, F. T. Yuan, H. W. Chang, K. L. You, Y. Liou, Y. D. Yao and J. K. Wu, J. Appl. Phys. 103, 07E116 (2008)

[133] S. N. Hsiao, F. T. Yuan, H. W. Chang, H. W. Huang, S. K. Chen, and H. Y. Lee, Appl. Phys. Lett. 94, 232505 (2009).

[199] Yun-Chung Wu, Liang-Wei Wang, and Chih-Huang Lai, Appl. Phys. Lett. 93, 242501 (2008).