Line-of-Sight Sealed Silicon Carbide Diaphragms for Harsh Environment Sensors

Silicon carbide (SiC) is currently being developed as a platform material for micro scale devices that operate in harsh environments. One vision of the future is an all-SiC sensor chip composed of crystalline SiC substrate, doped polycrystalline SiC structures, and sputtered amorphous SiC sealing layers. Such a chip would have minimal internal stresses over a wide thermal range. The chemical inertness, as well as the electrical and mechanical properties of SiC in harsh environments overcomes the limitations of traditional silicon-based platforms. The development of robust SiC devices requires fabrication techniques and a materials system that are compatible with the thermal budget and operational environment.
This thesis describes a process for depositing wafer-level, SiC sealing layers for harsh environment sensors. An emerging method of diaphragm fabrication is a sealing technique that utilizes deposition of two thin films: a porous scaffold layer and a second sealing layer. This approach offers many advantages in comparison to traditional bulk etch methods such as improved wafer-level uniformity and reduction in die size. This work describes a novel approach for the fabrication of diaphragms by the utilization of an ion beam sputter deposition of amorphous SiC for line-of-sight sealing. The approach was developed to be compatible with harsh environment specifications and can be applied to applications such as vacuum encapsulation and pressure sensing.
This thesis also presents the etching of SiC films and substrates, which is important for devicefeature definition. A high density plasma etch system was used to further explore the etching of SiC with SiO2 etch masks in HBr/Cl2 etch chemistries. An etch recipe was developed for the fabrication of a thin film SiC resonant strain gauge. In addition, the use of aluminum nitride (AlN) as an etch mask with SF6/O2 was investigated for the first time and demonstrated improved etch rates, selectivities and sidewall slopes.
The characteristics of ion beam, sputter deposited SiC were investigated for the development of a low temperature, directional deposition technique. Amorphous SiC thin films were sputter deposited at low temperature (below430◦C), high vacuum (order of10^-6 Torr) conditions. Thematerial properties such as uniformity, composition, residual stress, strain gradient, and stoichiometry were investigated. The results demonstrated that low temperature and ion-bombarded filmsresulted in high compressive stresses and strain gradients. Therefore, annealing was used to re-duce the film stresses and gradients. The deposition process also demonstrated that controllable SiC stoichiometries could be obtained with this process as well as low temperature pinhole free films. In addition to the film properties, the film deposition topography was observed with substrate tilting. The results confirmed line-of-sight deposition topology which can be utilized as vacuum sealing films. The integrity of the sealing was tested with a high temperature chemical etch. In addition, a fabrication process to obtain line-of-sight sealed SiC diaphragms to be used for vacuum encapsulation and pressure sensing applications was demonstrated. The techniques developed in this work add to the library of SiC processes for harsh environment applications.
Debbie Gahaton Jones
Alice Agogino
Oscar Dubon
Publication date: 
December 31, 2007
Publication type: 
Ph.D. Dissertation
Jones, D. G. (2007). Line-of-sight Sealed Silicon Carbide Diaphragms for Harsh Environment Sensors. (n.p.): University of California, Berkeley.

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