電漿蝕刻磁性薄膜的影響

Abstract

現代電子元件與資訊儲存技術的進步，人類社會對高密度、高穩定性功能性材料需求日益提升。其中，磁性薄膜材料的磁學特性與多功能可調控性，在高密度磁性儲存、自旋電子學（Spintronics）、磁阻式隨機存取記憶體（MRAM）。磁性薄膜的性能除取決於其內部層狀結構與元素組成外，與基板表面結構特性、粗糙度以及成膜過程中的介面品質有密切關聯。基板表面形貌與微觀結構對磁性薄膜之晶粒成長機制、磁疇結構形成以及磁各向異性等特性均產生深遠的影響。本實驗以射頻氬電漿（Argon Radio Frequency Plasma）乾式刻蝕技術為核心，針對磁性薄膜沉積常用之兩種絕緣基板材料:藍寶石（Al₂O₃）與二氧化矽（SiO₂）進行表面結構調控，系統性研究不同基板經氬電漿蝕刻後的表面形貌演變行為。實驗研究經氬電漿蝕刻鈷鈀多層膜Pd(3nm)/[Co(0.6nm)/Pd(0.8nm)]₅/Pd(4nm)/Al₂O₃磁性薄膜後的磁性特性所造成的影響。不同於傳統單一層或無蝕刻參數優化之研究，實驗研究使用原子力顯微鏡（Atomic Force Microscopy, AFM）與磁光柯爾效應（Magneto-Optical Kerr Effect, MOKE）兩種實驗量測方法，搭配不同區域的磁性掃描分析，分析氬電漿蝕刻對磁性薄膜微觀與宏觀磁學行為的深層影響。實驗過程中，採用RF射頻電漿刻蝕系統，用以交流射頻功率與氬電漿為蝕刻方式，分別對藍寶石與二氧化矽基板進行10分鐘與30分鐘的乾式氬電漿蝕刻處理。藍寶石作為高硬度單晶基板，具六方緊密堆積（Hexagonal Close-Packed，HCP）晶格結構，具有高莫氏硬度（Mohs 9）、高熱穩定性與良好光學性能，在高品質磁性薄膜生長中廣泛應用；二氧化矽具有化學穩定性高與介面光滑特性，常用於磁性多層結構與自旋元件製程中的隔離層或緩衝層。透過氬電漿物理性濺射作用，氬電漿實驗產生基板表面粗糙度、蝕刻深度與微觀表面形貌變化，藉以探討不同基板特性對電漿蝕刻行為之反應性差異。蝕刻後樣品經AFM進行奈米尺度三維表面形貌量測，並利用Ra（算術平均粗糙度）、Rq（均方根粗糙度）作為表面粗糙度之定量參數，分析不同蝕刻時間與基板材料對表面粗糙度與形貌特徵之影響。AFM量測結果顯示，隨蝕刻時間增加，兩種基板表面粗糙度均有明顯提升，但其表面反應機制存在顯著差異：藍寶石基板因其高結晶性與硬度，受氬離子蝕刻後表面僅形成輕微凹洞狀起伏，Ra與Rq值增加幅度較緩；二氧化矽基板由於密度與莫氏硬度比藍寶石基板較小，二氧化矽在氬電漿蝕刻後表面更容易蝕刻成微奈米凹洞，表面粗糙度提升幅度更為明顯。藍寶石基板上沉積鈷鈀多層膜結構（Pd(3nm)/[Co(0.6nm)/Pd(0.8nm)]₅/Pd(4nm)/Al₂O₃），經氬電漿功率100W，氬電漿蝕刻5分鐘後，實驗採用MOKE磁光柯爾顯微鏡進行不同區域磁滯曲線量測，探討氬電漿蝕刻後蝕刻去除鈷鈀多層膜對磁性特性之影響，特別關注矯頑力（Hc）、剩磁（Mr）、磁滯曲線特徵。實驗結果顯示，經氬電漿蝕刻（中央區域，100 W持續5分鐘以上）之基板表面，沉積後之磁性薄膜幾乎完全喪失鐵磁特徵，MOKE量測之磁滯曲線呈現線性趨勢，無剩磁且矯頑力趨近於零，表現出典型順磁行為。分析推測，可能因為氬離子的高能量物理撞擊作用已去除鈷鈀磁性層結構。觀察邊緣輕蝕刻區（蝕刻深度較淺），磁性薄膜保持明顯垂直磁各向異性，呈現方形磁滯曲線，剩磁接近未蝕刻區域水平，矯頑力雖略有下降，但仍維持可觀磁性翻轉特徵。特別值得注意的是，過渡區域之磁滯曲線則呈現典型混合特性，整體曲線於高磁場區域展現斜率變化，而於低磁場區域維持一段平台狀特徵，反映此區域內同時存在部分未被完全破壞之磁性層與蝕刻損耗之非磁性層，導致磁性特徵逐步過渡與轉變。深層蝕刻區表面具明顯蝕刻坑洞。實驗研究結果顯示，藉由氬電漿蝕刻處理，即可於單一樣品內部實現磁性梯度結構，具備從鐵磁性、高矯頑力區，到低矯頑力磁性區，甚至完全順磁性區域之漸變分佈，無需複雜多道製程或額外遮罩步驟，即可快速製備多功能磁性元件所需之微區磁性結構。綜合分析，實驗研究提供氬電漿蝕刻對磁性薄膜的磁學特性調變效果，藉由調控氬電漿、氬電漿蝕刻時間與基板選擇，可有效實現磁性薄膜從順磁到鐵磁，以及矯頑力與剩磁之連續可調特性，為未來發展高密度磁性儲存、自旋電子學與可重構磁性功能元件提供幫助。

With the advancement of modern electronic devices and information storage technologies, the demand for functional materials exhibiting high density and excellent stability has grown significantly. Magnetic thin-film materials, known for their distinctive magnetic properties and versatile adjustability, play crucial roles in high-density magnetic storage, spintronics, and magnetoresistive random-access memory (MRAM). The performance of magnetic thin films not only depends on their intrinsic layered structure and elemental composition but is also strongly influenced by the substrate's surface structure, roughness, and interface quality during deposition. Substrate surface morphology and microstructure profoundly impact grain growth mechanisms, magnetic domain structure formation, and magnetic anisotropy. This research employs argon radio frequency (RF) plasma dry etching to systematically modulate the surface structure of two insulating substrates commonly utilized in magnetic thin-film deposition—sapphire (Al₂O₃) and silica (SiO₂). We investigated the morphological evolution of these substrates after argon plasma etching and its subsequent impact on the magnetic properties of cobalt-palladium multilayers, specifically Pd(3 nm)/[Co(0.6 nm)/Pd(0.8 nm)]₅/Pd(4 nm)/Al₂O₃. Unlike traditional studies involving single-layer structures or those lacking optimized etching parameters, this work integrates atomic force microscopy (AFM) and magneto-optical Kerr effect (MOKE) measurements to perform comprehensive microscopic and macroscopic analyses of magnetic behavior.The RF plasma etching system was utilized with argon plasma at RF power to treat sapphire and silica substrates for durations of 10 and 30 minutes. Sapphire, characterized by its hexagonal close-packed (HCP) lattice, high Mohs hardness (9), excellent thermal stability, and optical properties, is widely applied in growing high-quality magnetic films. In contrast, silica substrates, due to their chemical stability and smooth interfaces, serve commonly as buffer layers or isolation layers in multilayer magnetic structures and spintronic devices.Through physical sputtering induced by argon plasma, notable differences in surface roughness, etching depth, and morphology were observed between substrates. AFM three-dimensional nanoscale measurements quantified surface roughness changes using arithmetic mean roughness (Ra) and root-mean-square roughness (Rq). Results indicated significant increases in roughness with extended etching durations. Sapphire substrates showed moderate increases in Ra and Rq values, exhibiting shallow pit-like topographical features due to their high crystallinity and hardness. Conversely, silica substrates, characterized by an amorphous structure, displayed more substantial roughness increases with deeper, variable micro-nanometric pits.The cobalt-palladium multilayer deposited on sapphire substrates was etched using argon plasma at 100 W for 5 minutes. MOKE microscopy analyzed magnetic hysteresis curves across different regions, assessing coercivity (Hc), remanence (Mr), and general hysteresis behavior. Results indicated that heavily etched central regions experienced near-complete loss of ferromagnetic properties, presenting linear hysteresis curves with negligible coercivity and remanence, indicative of paramagnetic behavior. This behavior is attributed to the removal of magnetic Co-Pd layers due to high-energy ion bombardment.In mildly etched edge regions, distinct perpendicular magnetic anisotropy persisted, evidenced by square hysteresis loops and remanence similar to unetched regions, albeit with slightly reduced coercivity. Transitional regions exhibited mixed magnetic characteristics, with hysteresis curves displaying a platform-like feature at low magnetic fields and sloped variations at higher fields, reflecting partial erosion and coexistence of ferromagnetic and non-magnetic layers.This research demonstrates the ability to create internal magnetic gradient structures within a single sample using argon plasma etching, ranging continuously from ferromagnetic, high-coercivity zones to fully paramagnetic regions without the need for complex processing steps or additional masking. The findings highlight argon plasma etching as an effective method for tuning magnetic properties from paramagnetic to ferromagnetic states and continuously adjusting coercivity and remanence, offering promising prospects for advanced magnetic storage, spintronics, and reconfigurable magnetic functional devices.