Ohmic Contacts for High Power and High Temperature Microelectronics

Ni 2 Si Ohmic contacts were fabricated via pulsed laser deposition on n-SiC. The contacts electrical, structural, compositional, and surface morphological properties were investigated as a function of annealing temperatures ranging from 700 to 950 °C. The as-deposited and 700 °C annealed contacts were non-Ohmic. Annealing at 950 °C yielded excellent Ohmic behavior, an abrupt void free interface, and a smooth surface morphology. No residual carbon was present within the contact metallization or at the contact-SiC interface and the contact showed no appreciable thickness increase as a result of the annealing process. Our results demonstrate that aside from maintaining the desirable electrical integrity associated with Ni and Ni/Si Ohmic contacts, the Ni2Si Ohmic contacts possessed improved interfacial, compositional, microstructural, and surface properties which are required for reliable high temperature and high power device operation.

Wide band gap semiconductors have become very attractive for power electronics because of their excellent properties at high power and high junction temperatures above 300°C. However, the maximum operation temperatures of conventional packaging materials, like tin-based solders or even tin-lead solders, are limited to around 220°C. Thus, a new packaging material with a higher melting temperature must be developed. Sintered silver paste die attachment has been regarded as one of the most promising interconnection technologies for high temperature and high power applications because of its excellent electrical connection and thermal dissipation abilities and the low processing temperature and high operating temperatures up to 700°C. Nevertheless, some critical problems still exist in this technology, for example high porosity, poor wettability at interfaces and severe oxidation problem when aging at temperatures above 200°C in air. These serious issues cannot be neglected before this promising technology steps to practical application. Our research aims at solving these problems of sintered nano-silver die attachment by introducing indium into the sintered nano-silver joint. In our study, we put a slice of indium foil on nano-silver paste, which had already been stencil-printed with a certain thickness on substrate. Indium foil would begin to melt at the beginning of the bonding process and then permeated and filled into the pores between silver nanoparticles. Indium reacted quickly with silver and transformed the whole joint into a silver-indium intermetallic, which has a high melting temperature above 600°C. Then, a much lower porosity joint was produced and the wettability between sintered joint and substrates also became better, which showed a better connection at interface. To compare the high temperature reliability between pure sintered nano-silver joint and silver-indium joint, we conducted high temperature storage test at 300°C in air. The results showed that after thermal aging at 300°C in air for 100h, the shear strength of pure silver joints dramatically reduced because of the oxidation of copper substrates. In contrast, no copper oxide could be observed inside the silver-indium joints, and even had twice higher shear strengths than the as-bonded ones. Therefore, we suggest that by introducing indium the high temperature reliability of the sintered silver joint can be significantly improved. Furthermore, the silver-indium joint also passed high temperature storage test at 300°C in air for 1000h, and was proven to have anti-tarnishing property by other researchers. To sum up, we provide a simple method and an ideal packaging material that can successfully overcome some serious problems of the sintered silver paste die attachment technology and bring this technology much closer to practical use for high temperature and high power applications.

Low resistivity ohmic contacts on 4H-silicon carbide for high power and high temperature device applications

The properties of AlGaN/GaN heterostructures have been a subject of great activity because of their application in high frequency, high power, and high temperature devices. Magnetotransport measurements give the possibility to study the properties of a two-dimensional electron gas. Indeed, Shubnikov-de Haas oscillations of a two-dimensional electron gas can be observed at high magnetic fields. Moreover, magnetoresistance measurements close to zero magnetic field give the possibility to investigate the weak localization and weak antilocalization arising from AlGaN/GaN heterostructures. The latter is related to the spin-orbit interaction on the spin of the carriers present in these heterostructures and is a key-feature of the spin-transistor proposed by Datta and Das [1]. As a matter of fact, the spin orientation between the electrodes of this novel device should be manipulated by the controllable strength of the spin-orbit interaction in the two-dimensional electron gas. In order to obtain information on spin-orbit effects in AlGaN/GaN heterostructures we therefore analyzed the weak antilocalization observed in the magnetoresistance.Polarization-doped AlGaN/GaN heterostructures were grown by metalorganic vapor phase epitaxy (MOVPE) on the (0001) surface of sapphire substrates. First a 3 mm-thick GaN buffer layer was grown, followed by a Al0.3Ga0.7N layer with a thickness of 20 nm. Magnetotransport measurements were performed over a magnetic field range from –50 mT to +50 mT at various temperatures between 0.1 and 18 K. The Hall-bars were prepared by optical lithography and Ar+-ion-beam-etching technique. The metals used for the ohmic contacts were Ti/Al/Ni/Au. The mobility and carrier concentration in the single occupied subband were obtained from the Shubnikov-de Haas oscillations with the respective values of 9100 cm2/Vs and 6.2×1012 cm-2.In our case, the Shubnikov-de Haas oscillations of the two-dimensional electron gas reveal the occupation of a single subband in the nearly triangular quantum well. At low magnetic field, weak localization as well as weak antilocalization were observed, showing that strong spin-orbit interaction is present in our structures. A previous report [2] explained the weak-antilocalization as being related to the intersubband scattering due to the occupation of a second subband in a modulation-doped quantum well. We show that weak antilocalization is also present in a polarization-doped quantum well with a single subband being occupied. In this perspective, temperature-dependent weak antilocalization measurements will be presented and analyzed using adequate theoretical models. Finally, the relevant scattering times like elastic scattering time, dephasing time as well as spin-orbit scattering time have been extracted. [1] S. Datta and B. Das, Appl. Phys. Lett. Vol. 56, pp. 665-667, 1990. [2] J. Lu et al., Appl. Phys. Lett., Vol. 85, pp. 3125-3127, 2004

Barrier height requirements for leakage suppression in diamond power Schottky diodes

Power electronic systems have become a key component in the efficient generation, distribution, storage, and use of energy. This increasingly places power electronic systems in harsh environments under elevated stress conditions where, in many cases, their reliable operation is now mission critical. The semiconductor material conventionally used, silicon, possesses limited capability for high temperature applications due to its small bandgap. Power electronic components based on wide bandgap (WBG) semiconductors, such as SiC and GaN, can be operated reliably at higher temperatures. Furthermore, WBG devices have higher breakdown voltages and switching frequencies, rendering them more ideal switches and rectifiers for highefficiency as well as low-volume and low-weight systems. Yet, new packaging concepts are required to make use of the high temperature capability of power electronic systems based on WBG semiconductor devices. In this paper, a summary of the progress in the development and commercialization of wide bandgap power components and power electronic systems is given. The properties of SiC and GaN components are compared to those based on silicon. The shortcomings of conventional power electronic packaging technologies are analyzed, and alternatives introduced and compared. An overview of research is given on Transient Liquid Phase Sintering (TLPS), a promising interconnect technology for the manufacturing of WBG and high temperature power electronic systems. TLPS is a liquid-assisted sintering process during which a low melting temperature constituent melts, surrounds, and diffuses in a high melting temperature constituent. Intermetallic compounds with high melting temperatures are formed by liquid-solid diffusion. Interconnect systems can be processed at low temperatures but are capable of operating at the high melting temperatures of the intermetallic compound. Different process approaches are contrasted and material systems compared. Latest results of research on paste-based low temperature TLPS are presented. The pastes assessed are based on the Cu-Sn and Ni-Sn material systems. It is shown that joints formed by TLPS remain strong up to 600°C and exhibit softening temperatures far higher than conventional interconnect technologies. They show minimum creep and possess very limited voiding. The process does not require an inert or reducing atmosphere or vacuum and is compatible with standard reflow soldering equipment. It shows good wettability to common metallizations such as Cu, Ni, Ag, and ENIG. This attach technology shows the potential to enable reliable operation of wide bandgap power electronic systems under high stress conditions.

It is worldwide accepted that a real breakthrough in the Power Electronics field mainly comes from the development and use of Wide Band Gap (WBG) semiconductor devices. WBG semiconductors such as SiC, GaN, and diamond show superior material properties, which allow operation at high switching speed, high voltage and high temperature. These unique performances provide a qualitative change in their applications for energy processing. From energy generation to the end-user, the electric energy undergoes a number of conversions, which are currently highly inefficient to the point that it is estimated that only 20% of the whole energy involved in energy generation reaches the end-user. WGB semiconductors increase the conversion efficiency thanks to their outstanding material properties. The recent progress in the development of high voltage WBG power semiconductor devices, especially SiC and GaN, is reviewed. Future trends in device development and industrialization are also addressed.

SiC power semiconductors can safely operate at a junction temperature of 500degC. Such a high operating temperature range can substantially relax or completely eliminate the need for bulky and costly cooling components commonly used in silicon-based power electronic systems. However, a major limitation to fully realizing the potential of SiC and other wide band-gap semiconductor materials is the lack of qualified high-temperature packaging systems, particularly those with high-current and high-voltage capabilities required for power conversion applications. This paper proposes a new hybrid power module architecture that allows wide bandgap semiconductor power devices to operate at a junction temperature of 300degC. The concept is based on the use of double metal or DCB leadframes, direct leadframe-to-chip bonding, and high temperature encapsulation materials. The leadframes, serving as both the external leads and the internal interconnect to the semiconductor chips, need to provide excellent high temperature stability, adequate electrical and thermal conductivity, and a coefficient of thermal expansion (CTE) closely matching that of SiC. The SiC chips are sandwiched between and bonded to the top and bottom leadframes using a brazing or adhesion process. Extensive electrical, thermal, and mechanical modeling has been performed on this new concept. Several prototypes are fabricated, and a finite element model is evaluated. Packaging architecture and materials considerations are discussed.

GaAs PHEMT are used abroad in high temperature and high power RF/Microwave field; it is the most promising microwave devices. In long-term use, the PHEMT device may cause parameter drift and output power decrease. In this paper, based on the analysis of device structure and degradation mechanism, we design structure doing high-temperature accelerated lift testing, and analyze using SIMS, FIB, EM and other equipment, finally evaluating Ohmic contact of GaAs devices.

Thermal stability study of n-type and p-type ohmic contacts simultaneously formed on 4H-SiC

Due to its large bandgap (~4.6-4.8 eV), wide range of n-type doping, and single-crystal wafer availability, β-Ga2O3 holds promise for high-efficiency power devices. R&D of β-Ga2O3 has been conducted in Japan for more than a decade; however, funded research programs on this semiconductor material began in the U.S. only five years ago, highlighting its early stage of development. Device development of a semiconductor technology requires the ability to produce suitable ohmic and rectifying (Schottky) contacts. Whereas ohmic and Schottky contacts have been demonstrated on n-type β-Ga2O3, certain characteristics of contacts to β-Ga2O3 differ from contacts to other wide bandgap semiconductors. For example, we found that morphology was problem with many ohmic contact metals, and it was concluded that metal work function is not a dominant predictive factor for forming an ohmic contact to β-Ga2O3 [1]. Furthermore, thermal instability of the standard ohmic contact metallization (Ti/Au) is a concern for operation of devices at elevated temperatures. Schottky contacts to β-Ga2O3 have shown dependence on the orientation, growth method, and choice of metal. For example, on (-201) β-Ga2O3 Schottky barrier heights calculated from I-V measurements were typically between 0.9 and 1.3 eV and displayed little dependence on the metal work function [2]. Our preliminary measurements of selected metals on (100) β-Ga2O3 substrates yielded lower ideality factors and a correlation between barrier height and metal workfunction. The results for Schottky contacts on the (100) surface show some similarities to those previously reported on the (010) surface. In this presentation specific examples of ohmic and Schottky contact metallizations to β-Ga2O3 will provide a platform for more detailed discussion. [1] Y. Yao, R.F. Davis, and L.M. Porter, J. Electron. Mater. 46(4), 2053-2060 (2016). [2] Y. Yao, R. Gangireddy, J. Kim, K. Das, R.F. Davis, and L.M. Porter, J. Vac. Sci. Technol. B 35(3), 03D113.1-7 (2017).

In-situ exploration of Venus is seriously hampered by its severe environment, which is benign (28oC) at an altitude of 55 km, but rapidly becomes hostile, with increasing temperature and CO2 pressure at lower altitudes, eventually reaching ~465°C and 90 bars at the surface.1 These challenging conditions have limited the previous Venus surface missions, e.g., the Russian Venera series and Vega-2 Landers,2 to barely two hours after deployment with lithium-primary batteries, despite the use of considerable insulation, phase-change materials, and similar heat sinks to isolate batteries and avionics from high surface temperatures. The recent decadal survey, ‘Vision and Voyages for Planetary Science in the Decade as well as the more recent Venus Exploration and Analysis Group (VEXAG) study3 emphasized the need to gather basic information on the crust, mantle, core, atmosphere/exosphere, and bulk composition of Venus, to understand the evolutionary paths of Venus in relation to Earth and recommended long-duration landers and probes for future missions.In order to enable extended surface missions on Venus, e.g., landers, probes and seismometers, NASA has initiated the development of high temperature electronics and power technologies, under its ‘Hot Operating Temperature Technology’ (HOTTech) program. Under this program, we have been developing advanced primary batteries resilient to the hostile conditions on the Venus surface and operational for several days with high specific energy (>100 Wh/kg) and energy density (>150 Wh/l). Here, we will describe the development of high temperature batteries based on lithium alloy (e.g., Li-Al) anodes, molten salt electrolytes containing binary/ternary mixtures of alkali metal halides, cathodes consisting of transition metal sulfides, and designs similar to the aerospace thermal batteries.4 With FeS cathode and appropriate changes in the electrolyte, binder and active material ratios, we have demonstrated the operation of the high temperature battery in prototype cells for 30 days in primary mode, and >150 days in rechargeable mode at 475oC. Further, with suitable thin coatings of inorganic compounds, e.g., Al2O3, AlF3 and AlBO3 on the cathode particles, the utilization of the cathode, and hence the operational life of the cells have been improved by another 50%.AcknowledgementsThe work described here was carried out at the Jet Propulsion Laboratory, California Institute of Technology, under contract with the National Aeronautics and Space Administration (NASA) and supported by the NASA’s HOTTech project. The information in this document is pre-decisional and is provided for planning and discussion only.References Basilevsky, J. W. Head, "The surface of Venus". Rep. Prog. Phys. 66, 1699 (2003).Gilmore, et al., “Venus Surface Composition Constrained by Observation and Experiment”, Space Sci. Rev. 212, 1511–1540 (2017); doi:10.1007/s11214-017-0370-8.A. Bullock, et al., “A Venus Flagship Mission: Report of the Venus Science and Technology Definition Team," 40th Lunar and Planetary Science Conference (Lunar and Planetary Science XL), The Woodlands, TX, March 23-27 (2009).E. Glass, J.P. Jones, A. V. Shevade, D. Bhakta, E. Raub, R. Sim, R. V. Bugga, “High temperature primary battery for Venus surface missions”, J. Power Sources. 449, 227492 (2020). doi:10.1016/j.jpowsour.2019.227492.

There are a variety of RF and microwave electronic devices that can be fabricated from wide bandgap semiconductors such as SiC and GaN. These semiconductors have many properties that make them near ideal for electronic devices intended for high temperature, high frequency, high power, and radiation hard applications. Prototype SiC and GaN-based electronic devices with very good DC and RF performance have been demonstrated and devices such as diodes are commercially available, while RF and high frequency transistors are rapidly approaching the commercialization stage. In this work the performance of microwave power amplifiers fabricated using SiC and GaN-based MESFET's are discussed and investigated. It is demonstrated that these amplifiers can produce RF output power on the order of 4-6 W/mm of gate periphery with near ideal power-added efficiency.

III-V nitrides for electronic and UV applications

Flexible mica films for high-temperature energy storage