GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> Fin-FETs. Other reported vertical Schottky diodes are not feasible for lateral integration. <s> This paper characterizes of spiral inductor on silicon wafer using post-IC process. There are two critical factors to affect Quality factor of on-chip spiral inductor; one is a resistance of inductor, the other is substrate loss induced by eddy current. This paper demonstrated 10 mum thick Cu film of inductor structure, to reduce the inductor resistance, and 10 mum thick BCB dielectric material, which is a low-k material, separates inductor structure and silicon wafer to reduce the substrate loss. The Quality factor is over 45 at 2.4 GHz with inductance of 0.5 nH. In application, this technology can provide fully CMOS compatible and low temperature process. <s> BIB001 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> Fin-FETs. Other reported vertical Schottky diodes are not feasible for lateral integration. <s> This paper proposes a 3D stacked buck converter with a 15μm thick spiral inductor on a silicon Interposer, which is suitable for fine-grain power-supply voltage control in SiP's. Our newly developed silicon interposer technology realizes a fine-pitch design rule (Line/Space=20μm/20μm, via hole diameter =30um) at the metal thickness of 15μm as well as conventional interposers with 5μm metal thickness. The measurement result shows that the 15μm thick inductor improves the power efficiency of the buck converter by 12% at the output current of 100mA compared with that by the conventional 5-μm thick inductor. <s> BIB002 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> Fin-FETs. Other reported vertical Schottky diodes are not feasible for lateral integration. <s> In this letter, a novel post-CMOS silicon-embedded coreless power inductor is proposed and demonstrated. The inductor is fabricated in the thick bottom layer of a silicon substrate and connected to the front side through vias opened in the thin top layer where control circuits can be fabricated for chip area saving. A 0.8- coreless inductor fabricated using this monolithic inductor technology shows a low dc resistance of 87 and an inductance of 13.1 nH with a quality factor of 3.9 at 100 MHz. A high inductor efficiency of 93% was estimated for 2.4-1.5-V 0.6-A power conversion at 100 MHz. This technology is very suitable for power-supply-on-chip applications. <s> BIB003 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> Fin-FETs. Other reported vertical Schottky diodes are not feasible for lateral integration. <s> In this letter, we report a concept of integrating a monolithic buck converter power IC with in-package bondwire inductors. The power IC containing all switching devices, driver circuitry, and control logic was designed and fabricated with a standard 0.5-μm CMOS process. Mutliturn bondwires with and without ferrite epoxy glob cores are used as the filter inductor in the buck converter. A prototype system-in-package converter with an output voltage and current of 2.5 V and 120 mA was built to operate at frequencies up to 5 MHz. The power level of the prototype buck converter is scalable by increasing the size of the active power switches. <s> BIB004 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> Fin-FETs. Other reported vertical Schottky diodes are not feasible for lateral integration. <s> On a GaN smart power IC platform, a robust GaN high electron mobility transistor (HEMT) with integrated gate protection is demonstrated by embedding a depletion-mode HEMT (D-HEMT) into the gate electrode of an enhancement-mode HEMT (E-HEMT). This protection scheme allows large input bias (e.g. 20 V) without gate-overdrive-induced reliability issues, and yields no penalties on the ON-state current and OFF-state breakdown voltage. Mixed-signal functional blocks, such as a 2-level quantizer circuit and a set/reset flip-flop, are demonstrated with operation up to 250°C to expand the design library for implementation of GaN smart power ICs. <s> BIB005 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> Fin-FETs. Other reported vertical Schottky diodes are not feasible for lateral integration. <s> We have developed an AlGaN/GaN-on-Si metal–oxide–semiconductor heterojunction field-effect transistor (MOSHFET) integrated with a clamp circuit to enable normally-off operation. A clamp circuit consisting of a multilayer MIM capacitor and a Schottky barrier diode was monolithically integrated with a normally ON AlGaN/GaN MOSHFET. The integrated clamp circuit shifted the input driving signal from (0, 20 V) to (−19.2, 0.8 V), allowing the normally ON AlGaN/GaN MOSHFET with a pinchoff voltage of −14 V to be operated as a normally-off device. The multichannel device with a 5-mm channel width exhibited a drain current density of $\sim 470$ mA/mm and a breakdown voltage of >900 V. In comparison with other conventional normally-off GaN-based FETs, a higher threshold voltage with high current density (i.e., low ON-resistance) can be achieved with easy device processing, no need for gate recess or complicated epitaxial growth. <s> BIB006 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> Fin-FETs. Other reported vertical Schottky diodes are not feasible for lateral integration. <s> In this paper, we present a comprehensive reviewand discussion of the state-of-the-art device technology and application development of GaN-on-Si power electronics. Several device technologies for realizing normally off operation that is highly desirable for power switching applications are presented. In addition, the examples of circuit applications that can greatly benefit from the superior performance of GaN power devices are demonstrated. Comparisonwith other competingpower device technology, such as Si superjunction-MOSFET and SiC MOSFET, is also presented and analyzed. Critical issues for commercialization of GaN-on-Si power devices are discussed with regard to cost, reliability, and ease of use. <s> BIB007 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> Fin-FETs. Other reported vertical Schottky diodes are not feasible for lateral integration. <s> In this paper, a new power inductor integration technology using a silicon interposer for DC-DC converter applications is proposed and experimentally demonstrated. Using this technology, coreless spiral inductors can be embedded from the back of a silicon interposer and connected with the front-side metal routing through TSVs (through-silicon vias). Trenches for the inductor windings and TSVs are etched from the back of the silicon interposer and filled with copper by using a two-step electroplating process. In this way, process difficulties and limitations in current TSV based interposer technologies for embedded power inductor applications can be overcome. Using this embedded power inductor integration technology, a 4.2 μΗ power inductor is embedded at the backside of a silicon interposer and integrated with a buck converter IC and LED chips to form a compact LED system. The buck converter in the LED system achieves an overall power efficiency of 69% while the size of the system is reduced by 5 times compared to the discrete counterparts. The silicon interposer embedded power inductor shows a good power efficiency of 89%. <s> BIB008 </s> GaN Power Integration for High Frequency and High Efficiency Power Applications: A Review <s> Fin-FETs. Other reported vertical Schottky diodes are not feasible for lateral integration. <s> On-chip inductive buck converters gain popularity due to their higher efficiency at higher load currents compared to its linear and capacitive counterparts. Through-silicon-via inductors (TSV-Inductors) in 3-D integrated circuit (3-D IC) technology can be used for the buck converter implementation that reduces the metal resource consumption of the inductor. However, in 3-D ICs, the regulated voltage from buck converters might be required for multiple tiers. Simply designing one buck converter per tier is apparently resource consuming. This paper fully utilizes the feature of TSV-Inductor and temporal/spatial sharing techniques to enable single-inductor–multiple-tier regulation for 3-D ICs. Experimental results suggest that under the same design specifications and resource consumption, the TSV-Inductor-based time multiplexing buck converter (TMBC) and the shared inductor buck converter (SIBC) help increase the efficiency by up to 15% and 25%, respectively, compared with the conventional power delivery scheme using one TSV-Inductor-based buck converter per tier. Moreover, the ripples of the TSV-Inductor-based TMBC and SIBC can be reduced by up to $3\times $ and $6\times $ , respectively. To the best of our knowledge, this is the very first work exploring buck converter sharing between multiple tiers in 3-D ICs. <s> BIB009
Passive components can be included in smart power IC platforms through the conventional MMIC IPD process. Thin film resistors include poly silicon, TaN and NiCr deposition. The 2DEG channel with controllable electron density can serve as a resistor but with larger temperature coefficient BIB007 , BIB005 . MIM capacitors use the passivation layer (e.g. SiN x ) or gate dielectric layer (e.g. Al 2 O 3 , HfO 2 ) as insulator. The capacitance density is usually below 1 nF/mm 2 for MIM structure with single or multiple SiN x passivation as insulator BIB006 . Generally the inductor is not preferred in ICs. However in high frequency GaN based power converters, inductance and volume can be reduced when operation frequency is at MHz range. On-chip spiral inductor which is formed by deposition of metal routes can be a practical solution. Fig. 15 illustrates the process flow of integrated passive devices in smart power IC platforms. As the values of inductor required for DC-DC converters are typically in µH scale even for MHz power switching, onchip integration of inductor in converter ICs is challenging. Several techniques have been reported to realize integrated inductor with inductance value from nH to µH range. The conventional planar integrated spiral inductor is shown in Fig. 16(a) . Low resistivity Cu layer with increased thickness is adopted to reduce inductor parasitic resistance. Additional thick isolation layer, such as low κ BCB material, is deposited to separate inductor from substrate to reduce the losses. This method is relatively simple but the realized inductance is low to several nH BIB001 . Fig. 16(b) shows the integrated inductors on Si interposer, which can provide small-size inductors with inductance up to 80 nH. The capacitors can be integrated on Si interposer as well. However, the thickness of inductor metal is limited due to the interposer volume BIB002 . Fig. 16(c) demonstrates the silicon-embedded coreless power inductor. The inductor can be fabricated in the substrate layer and connected to the front side through vias. Alternatively the coreless spiral inductors can be embedded in a silicon interposer and connected with other chips such as GaN HEMTs through the front-side metal routing and vias as show in Fig. 16(d) . The inductance can be up to several µH and the size of the system can be reduced by 5 times compared to the discrete counterparts BIB003 - BIB008 . Fig. 16 (e) and (f) illustrate the reported through-siliconvia (TSV) inductor in a 3D integrated circuit and bondwire inductor, respectively. The fabrication techniques are compatible with GaN HEMT process, however, the realized inductance is still small for DC-DC converters BIB009 , BIB004 . The reported performance of integrated inductors is summarized in Table 2 . Inductance values are still in nH to µH range with Q factor below 50. Especially the embedded coreless power inductor technique can realize inductance in the range of µH, which is a promising solution for integration of large value inductors in power converter ICs.