Organic sealant materials for quasi-hermetic sealing of MEMS sensor packages

Packaging MEMS chips has always proven to be a major technical challenge as one has to tackle cost reduction pressure, smaller package size requirements, higher package stress and achieving hermeticity at the same time. In particular for hermeticity, many researchers and industry players have tried to lower cost by using organic materials as sealants. Since organic materials cannot hermetically seal the package entirely, these new MEMS packages are termed "near-hermetic" or "quasi-hermetic" packages, while the original definition of hermetic packages describes solid metal-to-metal, metal-to-ceramic or metal-to-glass joints that provide 100% protection of any environmental atmosphere for periods of product life that are far beyond the expectations of durability of consumer and industrial devices. In this paper, a quasi-hermetic transistor outline (commonly known as a TO-can) Thermopile Sensor package is used as the platform for our evaluation. The paper discuss the different material attributes that are critical to the performance of the MEMS device, such as resistance to temperature cycling and humidity. Three different families of chemistries from different suppliers were studied and characterized. The three adhesives chemistries (used for sealing the package) being studied are thermoset epoxy, silicone rubber and olefin epoxy respectively. The influence of the particular chemistry on its performance under various conditions was analyzed. These include thermal analysis using differential scanning calorimetry (DSC), thermodynamic analysis (TMA) and thermogravimetric analysis (TGA). Next, the moisture absorption of the chemistries was studied to evaluate the hydrolytic nature of the specific chemistry. Adhesion push testing was done on metal packages with non-matching CTE enclosure substrates and silicon-based optical filter window materials to characterize the mechanical properties of the chemistries. Any failure modes were then investigated using high-powered micro- scopes, after the adhesion strength test. The three different adhesive chemistries were then built into working thermopile sensor samples. There were verified using a high precision measurement system that can detect a sensitivity deviation of less than 1% during environmental and other stress testing to various nominal reliability standards. The results provide a useful guideline of the type of adhesive chemistries that is most suitable for each type of reliability condition such as temperature cycling and humidity. These results are reported in this paper.