Characterization of anodic bonding

Abstract

Anodic bonding is a common process used in MicroElectroMechanical Systems (MEMS) device fabrication and packaging. Polycrystalline chemical vapor deposited (CVD) silicon carbide (SiC) is emerging as a new MEMS device and packaging material because of its excellent material properties including high strength, hardness, and thermal conductivity. An alternative, low temperature glass to CVD SiC anodic bonding process is required in order to prevent gold tin braze stress relaxation. A novel process recipe, requiring a SiC RMS surface roughness of 45nm, was developed for anodically bonding CVD SiC to bulk and thin-film, lapped Pyrex[TM] and Hoya SD-2[TM] glass substrates. The bond quality, residual curvature, and microstructured interfacial features for CVD SiC anodic bonding were shown to be comparable to single crystal silicon (Si) anodic bonding. The Plaza Test specimen, invented by Plaza et al., was used to assess bulk and thin-film, lapped glass bond quality. A two-part contact/bonding model was used to predict the contact and bonding of the Plaza Test structures. Surface contact was predicted by a parallel plate capacitor pull-in model after the voltage was applied, and linear elastic fracture mechanics (FEA) modeling predicted the toughness or work-of-adhesion of the bonded surfaces after the formation of a permanent silicon dioxide bond. The role of the voltage, structure geometry, work of adhesion, and materials used in the model predicted that the bonding mechanism limited the total number of structures that remained bonded. thin-film, lapped glass bond quality improved when increasing the voltage and time. The calculated, experimental, and modeled thermoelastic curvatures were minimal, indicating low residual stress

(cont.) between the bonded materials. Finally, microscopy and elemental analysis showed distinct differences in elemental depletion band(s) of bulk Pyrex[TM] and Hoya SD-2[TM] glasses bonded to Si, and in interfacial bonding between Pyrex[TM] and CVD SiC compared to Pyrex[TM] and Si. More elements in the glass network are identified as participating in the depletion layer process than identified in previous studies. Overall, the process recipes, modeling, experimental work, and chemical analysis of glass to CVD SiC anodic bonding showed that CVD SiC can be bonded successfully and be a promising packaging material.