VHF On-chip Coupled Inductors Based On Nanostructured Magnetic Films

Due to the slowdown of the Dennard’s scaling law,power density now becomes a key factor that limits computing capability.In accompany with the development of 3D integration and multi-core architecture,power supply systems tend to be integrated on chip to get high voltage precision,fast transient response and multi-voltage domain in different cores.On-chip coupled inductor is a key component of integrated voltage regulators（IVRs）.The performance of the inducotor has critical effect on the power efficiency and power density of IVRs.In this dessertation,very high frequency（VHF）coupled inductors have been studied,including the preparation of magnetic core films,the fabrication proceses of devices,design of 2D magnetic closure loop,test methods and analysis of high frequency loss of inductors.First,FeCoTiO nanogranular magnetic films have been deposited by magnetron sputtering.An in-plane uniaxially anisotropy field was induced by either applying in-situ biasing magnetic field during growth or tilting the substrate to a certain angle.The effects of different deposition methods on the anisotropic field,the effective damping,high frequency magnetic loss were compared.A high saturation magnetization of 15 k Gs has been achieved with a relatively permeability of 200～300 and a high resistivity of 800μΩ·cm.The coercive field is as low as 1.7 Oe,and the effective damping factor is about0.016.It is pointed out that the deposition method should be selected according to the targeting application frequency.The loss factor of magnetic film deposited with in-stu magnetic biasing field is lower than that deposited by oblique sputtering within 200 MHz.Secondly,an“above IC”integration solution was selected.Open-loop decoupled and reactangular close-loop coupled inductors have been designed and fabricated on silicon wafers,based on FeCoTiO magnetic core films and half-embedded fabrication processes.An inductance of 2×7.4 n H,a DC resistance of 0.27Wand a peak ac qulity factor of17@300 MHz were obtained for the closed-loop coupled inductor.However,the couping factor is less than 0.1.For the opened-loop core decoupled inductor,the inductance is 7.2n H,the DC resistance is 0.65W,and the peak AC qulity factor is 7.5@130 MHz.Thirdly,in order to futher reduce the ohmic loss and improve the saturation current of the inductor,a fabrication process based on thick SU8 photoresist was developed.The thickness of the winding coil was increased from 8mm to 25mm,which significantly reduced the DC resistance.The minimum measured DC resisrtance was only 18 mW（13）In addition,a novel coupled inductor with spliced anisotropic and isotropic magnetic cores has been proposed in order to increase the coupling factor of hard-axis excited coupled inductors with a uniaxial anisotropy.Based on the classic reluctance theory,a 2D closed-loop magnetic circuit model was established.Finally,the design,simulation and fabrication of multiple batches on-chip coupled inductors are completed.The spliced-core coupled inductor based on Ni₈₁Fe₁₉/SiO₂ magnetic multilayers exhibits a 51x inductance gain compared to the air core inductor with the same dimension.The L/R_DC reaches 323 nH/W,the coupling coefficient is 0.5,and the inductance density reaches 127 n H/mm².The spliced-core coupled inductor based on Ni₄₅Fe₅₅/SiO₂ magnetic multilayers shows a high L/R_DC of 295 n H/Wand a high peak ac quality factor of 23.5.The performance of the coupled inductor has been calculated for a standard 1.8 to 0.9V interleaved converter.The power efficiency of the coupled inductor is more than 96% across a wide range of phase current between 0.27 A（light load） and 1.6 A（heavy load）,and the peak power efficiency reaches 97.7%.Our experiment results above not only verify the accuracy of the 2D closed-loop magnetic circuit model,but also lay a solid foundation for the inductive on-chip power supply systems.