Packaging Technologies for High Temperature Electronics and Sensors

This paper reviews ceramic substrates and thick-film metallization based packaging technologies in development for 500°C silicon carbide (SiC) electronics and sensors. Prototype high temperature ceramic chip-level packages and printed circuit boards (PCBs) based on ceramic substrates of aluminum oxide (Al2O3) and aluminum nitride (AlN) have been designed and fabricated. These ceramic substrate-based chip-level packages with gold (Au) thick-film metallization have been electrically characterized at temperatures up to 550°C. A 96% alumina based edge connector for a PCB level subsystem interconnection has also been demonstrated recently. The 96% alumina packaging system composed of chip-level packages and PCBs has been tested with high temperature SiC devices at 500°C for over 10,000 hours. In addition to tests in a laboratory environment, a SiC JFET with a packaging system composed of a 96% alumina chip-level package and an alumina printed circuit board mounted on a data acquisition circuit board was launched as a part of the MISSE-7 suite to the International Space Station via a Shuttle mission. This packaged SiC transistor was successfully tested in orbit for eighteen months. A spark-plug type sensor package designed for high temperature SiC capacitive pressure sensors was developed. This sensor package combines the high temperature interconnection system with a commercial high temperature high pressure stainless steel seal gland (electrical feed-through). Test results of a packaged high temperature capacitive pressure sensor at 500°C are also discussed. In addition to the pressure sensor package, efforts for packaging high temperature SiC diode-based gas chemical sensors are in process. Introduction Various SiC electronics and sensors are currently under development for applications in 500°C high temperature environments such as hot sections of aerospace engines and the surface of Venus. In order to conduct long-term tests and eventually commercialize these SiC devices, compatible packaging technologies for the SiC electronics and sensors are required. This article reviews packaging technologies developed for 500°C SiC electronics and sensors to address both component and subsystem level packaging needs for instrumentation for high temperature environments. The packaging system for high temperature SiC electronics includes ceramic chip-level packages, ceramic PCBs, and edge-connectors. High temperature durable die-attach and precious metal wire-bonding are used in the chip-level packaging process. A high temperature sensor package is specifically designed to address high temperature micro-fabricated capacitive pressure sensors for high pressure environments. This paper reviews development of these electronics and sensor packaging technologies, including some test results of SiC electronics and capacitive pressure sensors using these packaging technologies in laboratory as well as in space and flight environments. Electronics Packaging Compared with other ceramic substrate materials, 96% alumina substrates have better dielectric performance at high temperature [1] while aluminum nitride has a relatively low coefficient of thermal expansion (CTE, 4.5 ppm/K) that is close to that of SiC (4.0 ppm/K). So aluminum nitride substrate based packages provide better thermal-mechanical reliability (compared to alumina with CTE of 7.4 ppm/ K). Selected thickfilm materials designed for alumina and aluminum nitride substrates have been tested as substrate metallization for 500°C packaging applications [2,3]. Prototype high temperature ceramic chiplevel packages based on ceramic substrates of aluminum oxides and aluminum nitride have been developed to address packaging needs for small scale 500°C SiC micro-electronics. These ceramic substrates and Au thick-film metallization based packages have been electrically characterized at temperatures up to 550°C [4]. Figure 1 shows prototype high temperature, low power 8-I/O (Input / Output) chip-level packages based on 96% alumina, 90% alumina, and aluminum nitride. Table 1 shows parasitic capacitance and parallel conductance of two neighboring I/Os (one of two I/Os is the ground with relatively larger metallization area) of high temperature low power 8-I/O surfacemount packages of 96% alumina (shown in Figure 1) in a temperature range from room temperature to 550°C, at 100Hz, 120 Hz, 1 kHz, 10 kHz, 100 kHz, and 1 MHz [4]. The upper numbers in each table entry are capacitances in unit of pF (or as indicated in nF), while the lower numbers are parasitic conductances in unit of μS. The package parasitic parameters increase with temperature because of the changes of the dielectric properties of the substrate material with temperature. The data in the table indicate that parasitic effects between two neighboring I/Os basically increase with temperature, but they are satisfactorily low in the entire temperature and frequency ranges. This 96% alumina prototype package meets the basic requirements for packaging many (currently) envisioned high temperature low power and low frequency SiC electronics. A 96% alumina high temperature circuit board [5] with four SiC devices is shown in Figure 2 [6]. This packaging system has been successfully tested with high temperature SiC JFET (junction gate field-effect transistor) circuits at 500°C for over 10,000 hours. Figure 3 shows I-V curves of a SiC JFET with this packaging system measured at 1, 10, 100, 1,000, and 10,000 hours at 500°C [6]. Without an appropriate packaging, such a long term test of SiC electronics at 500°C would not be possible due to fast degradation of probes. For more detailed analysis of long-term high temperature test results of SiC JFETs see Reference 6. As shown in Figure 2, Au wires were directly and permanently attached to the board for electrical interconnection to the instruments outside of Table 1: Parasitic capacitance (upper numbers, in pF or as indicated in nF) and conductance (lower numbers, in μS) of two neighboring I/Os of 96% alumina packages [4]. Those results not measurable are indicated by -. Figure 1: Prototype high temperature chiplevel packages based on aluminum nitride (left), 96% alumina (center), and 90% alumina (right) [4]. These packages have 8