Min Liu

In order to realize high performance and miniaturized integrated voltage regulators (IVRs), the study of ON-chip passive components, especially coupled inductors is necessary. Silicon-embedded coupled three-dimensional solenoidal inductors with 10-turn and 15-turn interleaved windings are designed and batch fabricated in series using micro-electro-mechanical systems (MEMS) and a novel Zn-Al alloy micro-casting technique (MCT) process. The coupled inductors developed in this study incorporating an embedded $200~\mu \text{m}$ -thick ferrite magnetic core, exhibit superior inductance integration densities of 56.7 nH/mm2, and ultrahigh coupling coefficients over 0.96 at lower frequencies of around 10 MHz. Besides, the maximum efficiencies of both 10-turn and 15-turn coupled inductors reach 93.23% and 91%, respectively. These results demonstrate significant performance improvements over the corresponding air-core coupled inductors. In addition, both designs exhibit high current carrying capacity with a measured saturation current over 2.2 A when operating in an inversely coupled mode. The superior performance of proposed coupled inductors is beneficial to the design and application of IVRs.

In response to the emerging demands in the miniaturization and integration of power conversion systems, the stacked-type transformer chips with significantly improved power efficiency for sub-MHz applications are developed in this article. The silicon-embedded high-density multilayer spiral winding structures in the transformer chips are fully metalized using our group developed Zn--Al alloy microelectromechanical system casting technique, which ensures the wafer-level batch fabrication with high consistency. After being assembled with dual E-shape magnetic cores, the fabricated stacked-type transformer chips are measured to demonstrate high inductance integration density of over 1.54 μH/mm ² and superior coupling coefficient of 0.985 at the targeted operating frequency of 100 kHz. Owing to the compact, low-resistance thick-metal structures, Q -factors of 20.5 at 100 kHz and transformer efficiency of above 85% in a wide frequency range are obtained from the two-port small signal measurements. Switching mode fly-back dc–dc converters are constructed to further validate the power conversion capability of the developed device. Benefiting from the implementation of the stacked-type transformers, the isolated dc–dc converters are measured to operate with an overall power efficiency of up to 82.46% at the 7.5 W power output condition, where an ultra-high power density of 17.64 W/cm ³ is also obtained.

This paper proposes a piezoresistive high-temperature absolute pressure sensor based on (100)/(111) hybrid SOI (silicon-on-insulator) silicon wafers, where the active layer is (100) silicon and the handle layer is (111) silicon. The 1.5 MPa ranged sensor chips are designed with the size as tiny as 0.5 × 0.5 mm, and the chips are fabricated only from the front side of the wafer for simple, high-yield and low-cost batch production. Herein, the (100) active layer is specifically used to form high-performance piezoresistors for high-temperature pressure sensing, while the (111) handle layer is used to single-side construct the pressure-sensing diaphragm and the pressure-reference cavity beneath the diaphragm. Benefitting from front-sided shallow dry etching and self-stop lateral wet etching inside the (111)-silicon substrate, the thickness of the pressure-sensing diaphragm is uniform and controllable, and the pressure-reference cavity is embedded into the handle layer of (111) silicon. Without the conventionally used double-sided etching, wafer bonding and cavity-SOI manufacturing, a very small sensor chip size of 0.5 × 0.5 mm is achieved. The measured performance of the 1.5 MPa ranged pressure sensor exhibits a full-scale output of approximately 59.55 mV/1500 kPa/3.3 VDC in room temperature and a high overall accuracy (combined with hysteresis, non-linearity and repeatability) of 0.17%FS within the temperature range of −55 °C to 350 °C. In addition, the thermal hysteresis is also evaluated as approximately 0.15%FS at 350 °C. The tiny-sized high temperature pressure sensors are promising in various industrial automatic control applications and wind tunnel testing systems.

A silicon-chip-based 3D metal solenoidal transformer is proposed and developed to achieve AC-DC conversion for integrated power supply applications. With wafer-level micro electromechanical systems (MEMS) fabrication technique to form the metal casting mold and the following micro-casting technique to rapidly (within 6 min) fill molten ZnAl alloy into the pre-micromachined silicon mold, 45-turns primary solenoid and 7-turns secondary solenoid are fabricated in silicon wafers, where the two intertwining solenoids are located at inner deck and outer deck, respectively. Permalloy soft magnetic core is inserted into a pre-etched channel in the silicon chip, which is surrounded by the solenoids. The size of the chip-style transformer is as small as 8.5 mm × 6.6 mm × 2.5 mm. The internal resistance of the primary solenoid is 1.82 Ω and that of the secondary solenoid is 0.16 Ω. The working frequency of the transformer is 60 kHz. Combined with the testing circuit of the switch mode power supply, the DC voltage of 13.02 V is obtained when the input is 110 V at 50 Hz/60 Hz. Furthermore, the on-chip 3D solenoidal transformer is used for lighting four LEDs, which shows great potential for AC-DC power supply. The wafer-level fabricated chip-style solenoidal AC-DC transformer for integrated power supply is advantageous in uniform fabrication, small size and volume applications.