Thermal solutions for discrete and wafer-level RF MEMS switch packages

In discrete radio frequency (RF) microelectromechanical systems (MEMS) packages,, MEMS devices were fabricated on silicon or galium arsenide (GaAs) chips. The chips were then attached to substrates with die attach materials. In wafer-level MEMS,packages, the switches were manufactured directly on substrates. For both types of packages, When the switches close, a contact resistance of approximately 1 Omega exists at the contact area. As a result, during switch operations, a considerable amount of heat is generated in the minuscule contact area. The power density at the contact area could be up to 1000 times higher than that of typical power amplifiers. The high power density may overheat the contact area, therefore affect switch performance and jeopardize long-term switch reliabilities. In this paper, thermal analysis has been performed to study the heat dissipation at the switch contact area. The goal is to control the "hot spots" and lower the maximum junction temperature at the contact area. A variety of chip materials, including Silicon, GaAs have been evaluated for the discrete packages. For each chip material, the effect of die attach materials. has been considered. For the wafer-level packages, various substrate materials, such as ceramic, glass, and low-temperature cofire ceramic (LTCC) have been studied. Thermal experiments have been conducted to measure the temperature at the contact area and its vicinity as a function of dc and RF powers. Several solutions in material selection and package configurations have been explored to enable the use of MEMS with chips or substrates with relatively poor thermal conductivity. For discrete MEMS packages, placing the die inside a copper cavity on the substrate provides significant heat dissipation. For wafer-level packages, thin diamond coatings on the substrate could reduce the hot-spot temperature considerably.