Mems-Enabled Retarding Potential Analyzers for Hypersonic in-Flight Plasma Diagnostics

Abstract

We report the design, fabrication, and preliminary characterization of a microfabricated retarding potential analyzer (RPA) that ensures unprecedented grid alignment accuracy. Through refined manufacturing methods, improved tolerances upon assembly serve to increase the signal to noise ratio (SNR). Furthermore, microfabrication permits smaller features overcoming sensor limitations previously barring RPAs from use in very dense plasmas such as those during reentry. Preliminary results show more than a twofold increase in signal strength compared to conventional RPAs. Finally, a new batch-fabricated RPA design that uses MEMS springs for grid assembly is demonstrated to help drive down cost while improving device reliability.

Batch-fabricated MEMS retarding potential analyzer for high-accuracy ion energy measurements

“…Improving upon in-plane assembly methods reported by Gassend et al [11], our MEMS RPA extends our previous work [10] by replacing the alumina guide rails and a stainless steel housing with nitride-coated silicon springs. The physical dimensions of our RPA are thus reduced while further refining the sensor's assembly precision by approximately an order of magnitude (conventional machining tolerances are on the order of tens of microns, while Gassend et al demonstrated accuracies of a few microns [11]).…”

“…The process flow to fabricate the electrodes uses contact lithography, RIE, and DRIE as main processing technologies. The process flow was reported in [10]; however, the electrodes of our fully microfabricated sensors are coated with sputtered gold instead of sputtered tungsten. Integrating the grids and collector to the housing using the deflection springs completes the fabrication of the RPAs.…”

“…In these experiments, we tested a conventional RPA, a hybrid RPA, i.e., an RPA with microfabricated grids and housing machined using standard techniques [10], and our MEMS RPA. The results of these experiments are shown in Figure 5.…”

“…By utilizing microelectromechanical systems (MEMS) fabrication methods, we were able to implement RPAs with the required micron scale features for small-Debye length plasmas without affecting the output signal strength of the sensor. This work builds on previous results presented at Hilton Head 2012 [10]. Figure 2 shows a schematic of our MEMS RPA, where curved silicon springs are utilized to fasten and reference a set of grids to achieve proper stack alignment; the gap between successive electrodes is simply defined by the difference in thickness between the grid and each housing layer.…”

“…Ion energy distribution of an ion beam using our MEMS RPA, our previously reported hybrid RPA[10] and a conventional RPA. A tenfold increase in signal strength is apparent when compared against the conventional RPA.…”