Reliability Without Hermeticity: Further Development in Commercial Vapor Deposited Coatings for High-Frequency RF Microelectronics
(Room 204)
17 Oct 18
1:45 PM
-
2:15 PM
It is challenging for today’s advanced RF systems to meet size, weight, power and cost requirements set by consumer market or defense agency demand, what the military calls “SWaP-C”, due to the need for reliable packaging of the RF components and Monolithic Microwave Integrated Circuits (MMICs). Reliability concerns traditionally necessitate the environmental protection of these high-frequency RF devices at either the component or the assembly level. Protection is achieved by hermetically sealing the device in a bulky, heavy, and expensive metal enclosure to prevent degradation caused by hazards in operating environments. Standard conformal coatings are unusable due to the significant signal degradation which occurs if applied directly over an RF circuit. There is a critical need for coating alternatives which are compatible with high-frequency RF devices to provide a lighter and smaller environmental protection scheme with high reliability and negligible performance impact for RF devices. This approach would allow RF device integration on SWaP-C effective PCBs and eliminate the hermetically packaged chip-and-wire assemblies currently in use. Coating properties (dielectric constant and thickness) become ever more important as the operating frequency of the device being coated increases, since the immediate dielectric environment surrounding the circuit significantly impacts device performance. A performance impact is typically seen with conventional conformal coatings at X-band frequencies (8-12 GHz) and increases over higher RF frequencies, at which point hermetic packaging is seen as the only high-reliability option for environmental protection. Another important factor to consider is the coatings’ thermal stability, in order for these technologies to be compatible with hot-running semiconductor technologies such as gallium nitride (GaN) components. As opposed to standard coating application methods such as spraying or brush-on, the chemical vapor deposition (CVD) technique helps minimize RF performance impact by keeping the coating as thin as possible. This technique has been used for industry-standard coatings such as parylene. However, in addition to thickness the surface adhesion of the coating is also important. An ideal coating will have a conformal, molecular-level coverage of the surface which ensures excellent adhesion to the device. This paper describes several different types of CVD processes suitable for conformal coating. Additionally we describe the requirements for the coating’s chemical structure and how molecular-level adhesion to the device surface provides environmental protection, even from very thin (<1 micron) coatings. The testing of these coatings is also described, including measurements of RF signal integrity when applied directly to the surface of Monolithic Microwave Integrated Circuits (MMICs) operating up to 100 GHz. The results of other environmental testing at high heat and humidity is also presented.