Liang-Yu Chen, OAI/NASA GRC, Liangyu.Chen@grc.nasa.gov

Patrick McCluskey, Department of Mechanical Engineering, University of Maryland, College Park

Abstract

Microsystems composed of microelectronics and micro-electro-mechanical systems (MEMS) are being considered increasingly for high temperature applications, such as aerospace engine monitoring and space exploration. While electronic and MEMS devices exist to perform the needed functions, little attention has been paid to the packaging technology for high temperature environment operation. This paper discusses the issues involved in high temperature packaging of microsystems, and describes a ceramic chip level package developed and evaluated by NASA Glenn Research Center.

Introduction

Cost, size, weight, and performance advantages have driven the incorporation of distributed control systems in aircraft and automobiles in the past few years. Such systems require microelectronics that can operate at temperatures well above the traditional maximum allowable operating temperatures of 70 °C for commercial electronics and 125 °C for military electronics [1]. Interest is now growing in utilizing microsystems composed of electronics and MEMS sensors/actuators in these high temperature applications, where these microsystems may, in fact, see temperatures significantly higher than the upper limits for conventional electronics.

NASA is especially interested in using high temperature microsystems. Here the desire is to perform in situ characterization of the combustion processes of aerospace engines and of the atmosphere of inner solar planets such as Venus. In aeronautic engine applications, the microsystems must operate at temperatures up to 500 °C and under pressures up to 20 MPa in a gas ambient composed of chemically reactive species such as oxygen (in air) and hydrocarbon/hydrogen (in fuel), along with catalytically poisoning species such as NOx and SOx from combustion products. The microsystems needed to characterize the atmosphere of Venus must operate in acid gas at 500 °C under 9 MPa. For a landing space probe, the microsystems must also withstand high (de)acceleration.

These are indeed high temperature environments compared to the standard operating conditions for most commercial sensors/electronics. In addition to the basic electronics for signal conditioning and intelligent control, these applications require pressure sensors, gas flow sensors, gas chemical sensors, and accelerometers. In recent years, silicon carbide (SiC) high temperature electronic and MEMS devices that can perform these functions have been demonstrated; however, most of these devices were demonstrated in laboratory conditions because packaging technology was not available. High temperature packaging technology is thus a key element of in situ testing and commercialization of high temperature operable microsystems.

When addressing packaging for high temperature environments, the design elements and materials used must be selected for their ability to perform their intended function over time in the high temperature environment. For example, in NASA’s high temperature applications, the substrate, metallization material(s), electrical interconnections, and die-attach must in each case be able to withstand 500 °C temperature, chemically corrosive gases, high dynamic pressure, and high acceleration. Much of the work that has gone into developing packaging for high temperature electronic control systems can be utilized to develop microsystem packages. Each of these elements is described in further detail below.

Die Attach

Microsystems are, at least partially, mechanical devices, and as such are sensitive to external mechanical forces. A major external force that can act on a microsystem is the thermo-mechanical stress arising from thermal expansion mismatches between the die, die attaching (bonding) layer, and the substrate. This thermally induced stress can generate unwanted device response to temperature changes, and, in the extreme case, permanent mechanical damage to the die attach. This stress must be minimized, through careful selection of the die attach, to achieve precise and reliable device operation.

Commercial plastic encapsulated microelectronics use silver-filled epoxies to attach the die to the substrate. Epoxies minimize the stress on the die, but degrade at temperatures before reaching 200 °C, thereby eliminating them as candidates for use in high temperature microsystems. The alternatives are silver-filled glasses and solders. Silver-filled glasses are quite stiff, often placing considerable thermomechanical stress on the chip. Therefore, for 200 °C applications, the best alternatives appear to be solders. There are several high temperature solders that have been used successfully as die attaches, including Pb95Sn05, Pb90Sn10, 95Sn5Sb, and Sn65Ag25Sb10. Gold-based eutectic alloys are another option for high temperature die attach. These alloys have excellent creep and corrosion resistance together with high strength; however, their stiffness causes them to transmit most of the thermo-mechanical stress to the die, as with silver-filled glasses [2], and they are soft at temperatures above 400 °C. Gold (Au) thick-film material has been tested as die attaching material for 500 °C applications.

Substrates and Metallization

Ceramic substrates and precious metal thick-film metallization have been proposed for chip level packaging of high temperature and other harsh environment devices based on their excellent stabilities at high temperatures and in chemically reactive environments [3, 4]. Three ceramic materials are typically used as substrates for high temperature microsystems: aluminum oxide, beryllium oxide, and aluminum nitride. Aluminum oxide (Al2O3) is the cheapest and has a reasonable thermal expansion match to SiC and silicon (Si), but it has poor thermal conductivity. Beryllium oxide (BeO) has a significantly higher thermal conductivity, but its thermal coefficient of thermal expansion (7.2) provides a poorer match to silicon, increasing the thermomechanical stresses induced during power or thermal cycling. Other drawbacks include its high cost and potential toxicity. The third alternative, alumimum nitride (AlN), combines the best expansion match to SiC and Si with the thermal conductivity of BeO [5]. In recent years, its cost has declined and issues related to its metallization have been solved, thus rendering it the substrate of choice for high temperature microsystems [4].

Wirebonds and Flip Chip Attachment

High temperature applications can weaken both the wire and the wirebond by annealing the wire and causing intermetallic reactions at the bond site, respectively. Annealing increases the grain size in the wire, reducing the wire’s strength and fatigue resistance. Intermetallic reaction can lead to the formation of voids and brittle compounds that cause fatigue fracture at the bond interface. Intermetallic formation is most often seen at temperatures exceeding 125 °C in Au wire/aluminum (Al) bond pad, interconnects [2]. Other wirebond systems, such as Al wire/nickel (Ni) bond pad, have a slower rate of intermetallic formation, and thus a higher allowable use temperature [6]. Temperatures in excess of 300 °C require monometallic wirebond systems such as Al wire/Al bond pad or Au wire/Au bond pad.

In order to achieve higher packaging density, in many cases, microsystem electronics are directly joined to the substrate by flip chip solder joints instead of wirebonds. The high lead solders used in flip chip joints will not degrade at temperatures up to 250 °C. Nevertheless, the CTE mismatch between the microsystem (chip), die-attaching material, and the substrate must be kept low, as the underfills typically used to lower thermomechanical stress in commercial ICs are not stable above 150 °C [7].

Issues Unique to High Temperature Microsystems

There are also a host of packaging issues that are unique to microsystems. Thermal expansion can narrow or widen tolerance gaps, leading to interferences or loose joints, respectively. For example, thermal expansion can cause jamming of the mechanical feed-through for an actuator, or conversely cause an opening in the gasket of the feed-through so that it no longer provides a seal to protect the electronics inside the package. Another possible effect of temperature is to change the internal diameter of microfluidic tubes thereby affecting the flow rate and the measurement of fluid flow. Such effects would need to be considered in design and calibration.

Elevated temperatures also accelerate chemical reactions. This can affect sensor chips not only by increasing the rate of environmentally-induced sensor degradation, but also by increasing the sensing reaction rate in a manner similar to having a higher level of the sensed chemical, thereby requiring re-calibration. In the limit, it is possible that elevated temperatures might change the nature of the chemical reaction that is used by the sensor, rendering it inoperable.

NASA Ceramic Packages

Researchers at NASA Glenn Research Center have developed chip level prototype electronic packages for high temperature and other harsh environment microsystems using AlN and Al2O3 ceramic substrates and gold (Au) thick-film metallization (see Figure 1). The electrical interconnection system of this advanced packaging system, including the thick-film metallization and wirebonds, has been successfully tested in a 500 °C oxidizing environment for over 5,000 hours. A compatible low resistance die-attach scheme using Au thick-film material as a conductive bonding material was also developed that is compatible with SiC devices [8]. This complete electrical interconnection system was tested with an in-house-fabricated SiC Schottky diode test chip in an oxidizing environment in a temperature range from room temperature to 500 °C for more than 1,000 hrs. These test results set lifetime records for both high temperature electronic packaging and high temperature electronic device testing. As required, the thick-film-based interconnection system demonstrated electrical resistance characteristics that were both low (2.5 times the 25 °C resistance of the Au conductor) and stable (decreased 3% in the first 1,500 hrs. of continuous testing) at 500 °C in an oxidizing environment. Also as required, the electrical isolation impedance between printed conductors/wires of the prototype packages (shown in Figure 1) remained high (>0.4 GW) at 500 °C in air. The attached SiC diode demonstrated low (< 3.8 W- mm2) and relatively consistent forward dynamic resistance from room temperature to 500 °C as indicated by Figure 2. These results indicate that this prototype package meets the design requirements for low power, long term operation in high temperature, chemically corrosive environments. This technology will be further developed and evaluated for packaging various SiC high temperature microsystems.

Figure 1. Prototype high temperature SiC microsystem electronic package.
Conclusion In conclusion, packaging of microsystems for temperatures as high as 500 °C has been demonstrated. However, it requires very careful attention to material compatibility issues and design tolerances. Furthermore, package design can vary from the simple to the complex based on the amount of interaction between the microsystem and the environment. As a result, further research is needed to lower the cost, improve the reliability, and increase the flexibility of microsystem packaging for high temperature applications.

Acknowledgments

The authors would like to acknowledge the more than 50 members of the CALCE Electronic Products and Systems Center for their support of this research. They would also like to thank the NASA Electronic Parts and Packaging (NEPP) Program, the Glennan Microsystems Initiative (GMI), the DARPA High Temperature Distributed Control Systems TRP, and the ONR PEBB Program for their support. For more information on high temperature MEMS research at NASA Glenn, please contact Dr. Lawrence G. Matus at NASA Glenn Research Center, Cleveland, OH 44135. For more information on high temperature microsystems research at CALCE, please contact Dr. Patrick McCluskey.

References

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