- 學位論文
軟磁性鈷鐵硼異質結構之高頻特性研究
High Frequency Characteristics in CoFeB Based Soft Magnetic Heterostructures
摘要
With the development of magnetic information storage technology, especially when data rates approach 1 GHz and above, new insight into the magnetization dynamics in ferromagnetic materials becomes a more pressing need. In this thesis, our recent studies of the static/ dynamic magnetic properties in high-resistive CoFeB based heterostructures and their effects on high frequency characteristics of coplanar microstrip inductors are presented. First of all, the Co40Fe40B20/ MgO multilayer thin films (MLs) annealed at 225~275oC show high resistivity (ρ = 524.0 μΩ-cm), high saturation magnetization (4πMS = 16 kG), high real-part permeability (μ' = 450), and a rather low damping parameter (α = 0.0054 ± 0.0001). The extrinsic contributions of dynamic damping, determined by using the 3-GHz permeameter with the presence of a biasing magnetic field are reduced when multilayer thin films are annealed at temperatures below 300oC, leading to a decreased α. The CoFeB/ MgO multilayer thin films, annealed at the temperatures equal to or higher than 300oC show the enhanced saturation magnetization as well as a significant increase in damping parameter α due to the occurrence of bcc (200) texture in CoFeB induced by (200) textured MgO layers. The correlation between Q-factor (μ'/ μ", inverse of magnetic loss tangent) and the damping parameter α is experimentally demonstrated and verified that α is one of key parameters to determine Q-factors of highly-resistive soft magnetic heterostructures used for the GHz range. The knowledge of chemical and magnetic conditions at the Co40Fe40B20/ MgO interface is important to interpret the annealing temperature dependence of static/ dynamic magnetic properties of MgO/ CoFeB/ MgO multilayer thin films, which are improved with annealing temperature ranging from 175oC to 275oC, and are degraded after annealing above 300oC. In the second topic, we present results from an x-ray photoemission spectroscopy study of MgO/ CoFeB/ MgO multilayer thin films. The interfacial Fe2O3 at the top CoFeB/ MgO interface due to oxidation of CoFeB during MgO deposition is found in the as-grown samples, and it is partially reduced after annealing at 275oC. The reduction of interfacial oxides improves static/ dynamic magnetic properties. However, the static/ dynamic soft magnetic properties of the CoFeB/ MgO multilayer thin films are significantly deteriorated for higher annealing temperature, which cannot be attributed to Fe2O3 reduction but the crystallization of CoFeB. Moreover, the significant amounts of diffused B as BOX are observed at the interface in the as deposited samples, and annealing further incorporates B in to the MgO forming a composite MgBXOY. Inserting a thin Mg layer between CoFeB and MgO introduces an oxygen sink, providing increased control of interface quality in the MgO/ CoFeB/ MgO multilayer thin films. We finally describe the class of coplanar microtrip inductors with magnetic cores consisting of [Co40Fe40B (5 nm)/ MgO (5 nm)]40 multilayer thin films. The highly-resistive CoFeB-based soft magnetic heterostructures is chosen for its good combination of static and dynamic magnetic properties that minimizes the hysteresis losses, eddy-current losses, and dynamic relaxation losses. Therefore, the increase in inductance L of 60% is obtained, and further enhancement can be achieved by increasing the stacking number and/ or reducing the thickness of the MgO layer. The quality factor Q (ωL/ R) of the inductors can be manipulated by controlling the damping parameter α of the CoFeB/ MgO MLs, and reaches up to 7.5-7.7 at frequencies of about 500 MHz. The magnetic field tunable microstrip inductors with a large tunable Q (ΔQ/ Q0) up to 70-90% are also obtained, by annealing the devices at the temperatures from 225oC to 275oC. Such tuning ability of Q is due to the significant modification of dynamic relaxation losses (damping parameter α and ferromagnetic resonance frequency fres), since the effects of applied DC magnetic field on hysteresis losses and eddy-current losses are not pronounced at the same frequency (500 MHz). The concept of magnetic-field tunable inductors using highly-resistive and low-α materials as the magnetic cores may lead to a brand-new vision of compact thin film inductors with better performance and minimum power consumption.
並列摘要
近年來由於高速無線數據產業與多重無線傳輸技術(多模組)之蓬勃發展，整合新穎高頻鐵磁薄膜與深次微米CMOS工藝技術發展高品質、小體積的平面薄膜電感器，將有助於放寬相關射頻電路設計的要求，因此可採用更複雜且完整的系統技術，減少耗電量、生產成本、電路版面積與測試成本，同時提供整體性能和製造良率，甚至是兼顧綠色環保概念的提倡。 本論文之目的主要為開發應用於微波頻段通訊技術之鐵磁薄膜。第一部分，利用超高真空直流/ 射頻濺鍍系統製備新型鈷鐵硼/ 氧化鎂異質結構多層膜系統，不僅具備相當高的電阻率，更表現出符合高頻應用之絕佳軟磁特性。其中我們特別強調與動態磁化損耗相關之磁性阻尼係數的探討，發現在施予適當磁場退火處理之後(225o至275oC)，磁性阻尼係數產生大幅下降的趨勢，進而提升新型鈷鐵硼/ 氧化鎂異質結構多層膜系統之高頻磁性品質。此研究顛覆了長久以來視磁性阻尼係數為現象學參數的傳統概念，確定其為可藉由實驗設計而調整的重要技術因子。 於本論文的第二部分，藉由X光光電子能譜以及高解析穿透式電子顯微技術之輔助，我們完整地釐清於新型鈷鐵硼/ 氧化鎂異質結構多層膜系統中，磁性阻尼係數隨磁場退火處理變化的原因，其中包含著界面三氧化二鐵的還原(低於275oC)以及鈷鐵硼成核結晶/ 晶粒成長(高於300oC)等相關機制。因此，我們進一步設計於鈷鐵硼/ 氧化鎂界面置入金屬鎂(硼亦可)做為氧離子吸收層以抑制界面三氧化二鐵生成之系列實驗，更證實上述所提機制之可靠性，將可做為未來於相關高頻鐵磁異質結構材料開發的指導準則。 整合新型鈷鐵硼/ 氧化鎂異質結構多層膜系統之共平面微帶型電感器於本研究論文之最終階段被開發，一方面確定鐵磁薄膜對電感器感值提升之效果，另一方面成功地驗證材料端鐵磁薄膜之磁性阻尼係數與元件端電感器之品質因子的相關性。同時，我們亦針對整合新型鈷鐵硼/ 氧化鎂異質結構多層膜系統之共平面微帶型電感器的可調性進行探討，觀察到磁性電感器的品質因子實可藉由外在磁場的施加產生相對提升，而其改善比例似與鐵磁薄膜之磁性阻尼係數的數值大小有關。此研究論文之成果預期將提供相關技術團隊一嶄新的視野，刺激搭配鐵磁薄膜之射頻電路以及系統基頻處理元件集成乃至於未來產品的實現。