| AVS 55th International Symposium & Exhibition | |
| Plasma Science and Technology | Friday Sessions |
| Session PS1-FrM |
| Session: | Plasma-Surface Interactions in Materials Processing II |
| Presenter: | R. Ding, University of Wisconsin-Madison |
| Authors: | R. Ding, University of Wisconsin-Madison M.G. Friesen, University of Wisconsin-Madison L.I. Klein, University of Wisconsin-Madison M.A. Eriksson, University of Wisconsin-Madison A.E. Wendt, University of Wisconsin-Madison |
| Correspondent: | Click to Email |
Plasma etching is a critical tool in the fabrication of Si/SiGe heterostructure quantum devices, but it also presents challenges, including 1) control of etch profiles and 2) damage to etched feature sidewalls that affects device performance. 1) Fluorine-based plasma etching often results in device profiles with undercuts due to preferential etching of SiGe over silicon. A C4F8/N2/Ar etch plasma gas mixture introduced here has been successfully used to achieve straight sidewalls through heterostructure layers by formation of a fluorocarbon inhibitor film on feature sidewalls to prevent undercutting. 2) Chemical and structural changes in the semiconductor at feature sidewalls associated with plasma-surface interactions are considered damage, as they affect band structure and electrical conduction in the active region of the device. Here we report the results of experiments designed to better understand the mechanisms of plasma-induced sidewall damage in modulation-doped Si/SiGe heterostructures containing a two-dimensional electron gas (2DEG). Damage to straight wires was characterized both by the width of the non-conductive “sidewall depletion” region at the device sidewall and by the noise level factor, γH/N, determined from spectra of the low frequency noise. Observed increases in sidewall depletion width with increasing etch depth are tentatively attributed to the increase in total number of defects with increased plasma exposure time. Excess negative charge incorporated into the fluorocarbon inhibitor film could be another contributing factor. Other factors considered, including defects at the bottom of etched features as well as leakage current bypassing the wire, appear to contribute minimally. The noise level shows a minimum at an ion bombardment energy of ~100 eV, while the sidewall depletion width is independent of bias voltage, within experimental uncertainty. A proposed explanation of the noise trend involves two competing effects as ion energy increases: the increase in damage caused by each bombarding ion and the reduction in total ion dose due to shorter etch times and reduced ion flux to the sidewalls.