Hard mask exploration for high aspect ratio deep-reactive ion etching of silicon carbide

The demand for high performance sensors that are small and low power has driven the exponential growth of micro-electromechanical-systems (MEMS) devices. Silicon (Si) is a semiconductor that has led the world as the material of choice for MEMS due to its superior mechanical properties and low cost. However, there are semiconductor materials that can be used for high performance application. In this thesis, bulk monocrystalline silicon carbide (SiC) was used as an alternative for MEMS resonators as it offers advantages to silicon in terms of energy bandgap, environmental robustness, and internal dissipation limits. In particular, the acoustic dissipation limit of high frequency resonant devices for silicon carbide is an order of magnitude lower than that of silicon. It has been demonstrated that ultra-high performing devices can be made from 4H-SiC with room for improvement [5]. This work outlines the challenges that come from creating high performing capacitive devices in 4H-SiC, from the creation of a silicon carbide on insulator platform (SiCOI) to the etching of capacitive gaps with hard mask coverage to the finalized fully released device. Each of these challenges require dedication and focus to successfully circumnavigate, and a lack thereof will lead to subpar device performance or a lack of functionality. One issue with the industry standard electroplated nickel masks as a choice for the deep reactive ion etching of SiC is the surface roughness of the film. Roughness will translate into a phenomenon referred to in this thesis as transverse scalloping. The scalloping can cause issues with electrostatic tuning of devices and could impact the quality factor of a device. To mitigate this issue, electroless plated nickel films were investigated which have a very smooth surface with no noticeable roughness and does not exhibit transverse scalloping. A differing hard mask, aluminum nitride (AlN), is also explored as its selectivity to single crystal silicon ranges up to 5800:1. However, challenges to the patterning of AlN occur as it is difficult to create a highly vertical opening. Any slanted sidewall will translate to the bulk silicon carbide device and current aluminum nitride etching recipes reach tapering angles of 86-87 degrees. Another avenue developed was the creation of high aspect ratio vertical pillars in photoresist reaching as little as 0.8 um width to 9 um in height. For the thick electroplating process, this step is critical, and issues of stability, verticality, and overdevelopment are key factors addressed with techniques employed to improve yield. The creation of a high performing square resonator operating in its Lamé mode is shown with measured quality factors of over one million. The simulated Q value of this device was theorized to be more than an order of magnitude higher, and as such, these devices were used to study the effect process parameters and material properties have on device performance. In summary, this thesis seeks to expand the knowledge of silicon carbide processing challenges and techniques to mitigate and overcome barriers that would inhibit high performing device creation.

Date

2023-08-09

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Text

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