Silicon carbide (SiC) has properties that make it a nearly ideal material for the creation of harsh-environment electronics and microelectromechanical systems (MEMS). The chemical inertness of SiC enhances its robust nature, but it makes it difficult to micromachine. The NASA Glenn Research Center has been investigating problems related to the bulk micromachining of single-crystal SiC by deep reactive ion etching (DRIE), including the formation of high-aspect-ratio structures (ref. 1) and the reduction of imperfections introduced by the DRIE process.
This photograph illustrates typical defects introduced when a circular well is etched into a SiC wafer to produce a pressure-sensing diaphragm. Although this process, which represented the state of the art for SiC DRIE in 2005 (ref. 2), generally provides sufficiently smooth etched surfaces, it introduces other significant nonidealities such as nonvertical sidewalls and microtrenching at the base of the sidewall. Microtrenching is a particularly serious defect for pressure sensors because the trench serves as a stress concentrator that weakens the diaphragm. This research effort developed an improved DRIE process that provides the required characteristics of smooth etched surfaces, vertical sidewalls, and minimal microtrenching, together with a high etch rate for cost-effective manufacturing (ref. 3).
Circular well (1 mm in diameter) etched to a depth of 245 μm using the prior state-of-the-art DRIE process. The structure shows typical defects for this process: that is, microtrenching at the base and the reentrant slope of the sidewalls (diameter of well increases with increasing depth).
A parametric study was used to optimize the DRIE recipe by varying four important parameters: the temperature of the wafer chuck, the pressure within the chamber, and the concentrations of oxygen (O2) and argon (Ar) in a mixture composed of O2, Ar, and sulfur hexafluoride (SF6). Sixteen etches were performed using all combinations of high and low values of each of the four parameters. This experiment was used to determine how microtrench depth, sidewall slope, surface roughness, and etch rate were affected by the key process parameters. Trenches and wells were etched to depths from 100 to 150 μm in single-crystal SiC specimens, which were then cross sectioned and examined using optical microscopy, scanning electron microscopy (SEM), and surface profilometry to characterize the etched structures.
The use of high radiofrequency powers (2500 W applied to the coil and 200 W applied to the platen) resulted in high etch rates (>0.5 μm/min) for all process conditions studied. Oxygen addition was found to be undesirable because it caused increased microtrenching, whereas high temperature was found to be beneficial because it reduced microtrenching. Sidewall slopes became more vertical, as desired, with increasing pressure; however, higher pressure was found to produce increased roughness. This roughness was reduced by using a gas mixture comprising Ar in addition to the principal etchant gas, SF6.
This study enabled us to produce an optimized DRIE process that simultaneously provides minimal microtrenching, vertical sidewalls, and smooth etched surfaces. By using a high fraction of Ar (40 vol%) along with a high temperature (180 °C) and a high pressure (35 mT, or 4.67 Pa), we were able to optimize etch characteristics. The sidewall slope obtained was approximately 90°, with an etch rate of 0.7 μm/min and a surface roughness of 40 Å. The following photograph shows the results achieved using the optimized process. This research is ongoing, with plans for further studies in process repeatability and the characterization of pressure sensors that will be fabricated with this process.
Cross-sectional view of a portion of a 400-μm-wide ring-shaped well that was etched to a depth of 150 μm using the optimized DRIE process. The etched well has minimal microtrenching and has vertical sidewalls and smooth surfaces.
Find out more about silicon carbide electronics research at Glenn: http://www.grc.nasa.gov/WWW/SiC/SiC.html
Laura J. Evans, 216-433-9845, Laura.J.Evans@nasa.gov
Dr. Glenn M. Beheim, 216-433-3847, Glenn.M.Beheim@nasa.gov
Authors: Laura J. Evans and Dr. Glenn M. Beheim
Headquarters program office: Aeronautics Research Mission Directorate
Programs/projects: Subsonics Fixed Wing, Supersonics
Last updated: December 14, 2007
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