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With the rise and widespread applications of three-dimensional (3D) heterogeneous integration technology, inductive voltage regulators are becoming increasingly important for mobile terminals and high-computing-power devices, while also offering significant development opportunities for high-frequency soft magnetic films. According to the requirements of on-chip power inductors, we first review the advantages and limitations of three types of magnetic core films: permalloy, Co-based amorphous metal films, and FeCo-based nanogranular composite films, with a focus on the technical requirements and challenges of several μm-thick laminated magnetic core films. Secondly, almost all on-chip inductors are hard-axis excited, which means that the magnetic field of inductors should be parallel to the hard axis of the magnetic core. We thus compare the characteristics of two methods of preparing large-area films, i.e. applying an in-situ magnetic field and oblique sputtering, both of which can effectively induce in-plane uniaxially magnetic anisotropy (IPUMA). Their influences on the static and high-frequency soft magnetic properties are also compared. The influences of film patterning on the domain structures and high-frequency magnetic losses of magnetic cores, as well as corresponding countermeasures, are also briefly analyzed. Furthermore, the temperature stability of magnetic permeability and anisotropy of magnetic core films is discussed from the perspectives of process compatibility and long-term reliability. Although the Curie temperatures and crystallization temperatures of the three types of magnetic core films are relatively high, the upper limits of their actual process temperatures are affected by the thermal effects on the alignment of magnetic atomic pairs, microstructural defects, and grain size. Finally, the current bottlenecks in testing high-frequency and large-signal magnetic losses of magnetic core films are discussed, and potential technical approaches to achieving magnetic core films that meet the future demands of on-chip power inductors for higher saturation current and lower magnetic losses are outlined.
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Keywords:
- integrated voltage regulator /
- on-chip inductor /
- soft magnetic film /
- high frequency magnetic loss
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图 2 (a) 软磁材料的发展历程[25], 硅钢片、坡莫合金、软磁铁氧体、Fe(Co)基非晶、纳米晶和复合磁粉芯; (b) 不同软磁材料在1 kHz下的性能对比图[30]; (c) 不同软磁材料在100 kHz功率损耗[30]; (d) 横电东磁DMR 52 W锰锌铁氧体在不同频率下的功率损耗
Figure 2. (a) Development history of soft magnetic materials[25], including silicon steel sheets, Permalloy, soft magnetic ferrites, Fe(Co)-based amorphous, nanocrystalline and composite magnetic powder cores; (b) performance comparison of different soft magnetic materials at 1 kHz[30]; (c) power losses of different soft magnetic materials at 100 kHz[30]; (d) power losses of Heng Dian Group Dmegc Magnetics Co., Ltd. DMR 52 W MnZn ferrite at different frequencies.
图 4 面内单轴各向异性[Ni81Fe19(120 nm)/SiO2(20 nm)]30和的M-H曲线(a)及复数磁导率(b) [16]; 面内单轴各向异性[Ni45Fe55(140 nm)/SiO2(20 nm)]25的M-H曲线(c)及复数磁导率(d) [17]
Figure 4. (a) M-H loop and (b) complex permeability of the in-plane uniaxial anisotropic [Ni81Fe19(120 nm)/SiO2(20 nm)]30 multilayer[16]; M-H loop (c) and complex permeability (d) of the in-plane uniaxial anisotropic [Ni45Fe55(140 nm)/SiO2(20 nm)]25 multilayer [17].
图 6 (a) 薄膜的4πMs以及ρ随x的变化; (b) 薄膜ρ随温度的变化; (c) 实测Fe-Al-O纳米颗粒膜的σ随Fe体积含量ϕ的变化[57]; (d) Fe-Co-Ti-O纳米复合颗粒膜的TEM照片[54]
Figure 6. (a) 4πMs and ρ of the thin film as a function of x; (b) variation of the film ρ with temperature; (c) variation of σ with Fe volume fraction (ϕ) in experimentally measured Fe-Al-O nanogranular films[57]; (d) TEM image of Fe-Co-Ti-O nanocomposite granular films[54].
图 7 不同外加诱导场下沉积的FeCoTiO磁膜的M-H曲线和低场FMR虚部磁导率磁谱 (a), (d) 15 Oe; (b), (e) 70 Oe; (c), (f) 115 Oe
Figure 7. M-H loops and imaginary permeability spectra of FeCoTiO magnetic films deposited under different externally applied bias magnetic fields: (a), (d) 15 Oe; (b), (e) 70 Oe; (c), (f) 115 Oe.
图 8 不同倾斜角度β溅射沉积的FeCoTiO磁膜的M-H曲线 (a) 10°, (b) 17°, (c) 32°; 不同倾斜角度β溅射沉积的FeCoTiO磁膜低场FMR虚部磁导率测试曲线 (d) 10°, (e) 17°, (f) 32°
Figure 8. M-H loops of FeCoTiO magnetic films sputter-deposited at different oblique angles β: (a) 10°; (b) 17°; (c) 32°; low-field FMR spectra of the imaginary part permeability for FeCoTiO magnetic films sputter-deposited at different tilt angles β: (d) 10°; (e) 17°; (f) 32°.
图 9 根据FeCoTiO磁膜不同诱导磁场(a)与不同倾斜角度(b)下沉积形成的磁膜的$ f_{\text{r}}^{2} $-Hbias曲线; 不同诱导磁场(c)下与不同倾斜角度(d)下沉积的磁膜的Δf-Hbias曲线
Figure 9. Comparison of $ f_{\text{r}}^{2} $-Hbias curves for magnetic films deposited under different inducing magnetic fields (a) and at different oblique angles (b); Δf-Hbias curves for magnetic films deposited under different inducing magnetic fields (c) and at different tilt angles (d).
图 10 (a) 115 Oe磁场诱导与17°倾斜角下沉积的FeCo-Ti-O磁膜的高频磁损耗; (b)不同尺寸的[FeCoSiB(100 nm)/NiFe(5 nm)]3的高频磁损耗[78]
Figure 10. High-frequency magnetic losses of (a) FeCo-Ti-O films deposited under an in-situ 115 Oe field and by oblique sputtering at a tilt angle of 17°; (b) pattered [FeCoSiB(100 nm)/NiFe(5 nm)]3 films with different lateral dimensions[78].
图 12 不同厚度的图形化FeCoSiB薄膜的Moke照片 (a)—(e) 50 nm; (f)—(j) 300 nm. 样品首先沿负磁场方向饱和, 然后降场, 磁矩方向沿箭头方向所示[79]
Figure 12. MOKE images of the full 50 nm layer (a)—(e) and 300 nm layer (f)—(j). The magnetic states are approached from saturation by a magnetic field in negative direction, the magnetization directions are indicated by arrows in the images[79].
图 14 (a) FeCoHfO薄膜的诱导各向异性场和难轴矫顽力随退火温度的变化情况; (b) 不同温度退火后测得的XRD谱, 图中直线对应CoFe(110)峰[84]; (c) 300 K(未退火), (d) 373 K, (e) 473 K, (f) 573 K退火后FeCo(体积分数为30.5%)-SiO2纳米颗粒膜的TEM照片[85]
Figure 14. (a) Annealing temperature dependence of the induced anisotropy field and hard axis coercivity in the FeCoHfO; (b) XRD patterns for different annealing temperatures, the vertical line indicates the position of the CoFe (110) peak[84]; TEM micrographs of FeCo (30.5%)–SiO2 film annealed at (c) 300 K (as-deposited film), (d) 373 K, (e) 473 K and (f) 573 K[85].
表 1 3类磁芯薄膜材料磁参数对比
Table 1. Comparison between magnetic parameters of soft magnetic thin film materials.
坡莫合金
Ni80Fe20
Ni45Fe55
CoNiFe非晶磁膜
CoZr
CoZrTa(B)
CoFeB纳米颗粒膜
FeCo-X-O(N, F)
(X = Si, Al, Hf, Zr, Ti, etc.)4πMs/T 1 ~ 1.5 ~ 2.0 1.5 1.4—2.0 Hk/Oe1) <10 (25) <25 50—100 μr ~1000 (600) 600 100—400 Hch/Oe <1 (2) <0.52) 1—2 ρ/(μΩ·cm) ~20 (45) ~100 500—2000 tthres/nm3) ~100 ~160 ~500 αeff <0.01 <0.01 0.014 注: 1)原位磁场诱导各向异性; 2)取决于靶材质量; 3)出现垂直各向异性的厚度阈值. 
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