| AVS 54th International Symposium | |
| Plasma Science and Technology | Wednesday Sessions |
| Session PS2-WeM |
| Session: | Plasma-Surface Interactions I |
| Presenter: | J.L. Shohet, University of Wisconsin-Madison |
| Authors: | G.S. Upadhyaya, University of Wisconsin-Madison J.L. Shohet, University of Wisconsin-Madison J.B. Kruger, Stanford University |
| Correspondent: | Click to Email |
Electron Shading, or topography-dependent charging, is believed to occur during plasma exposure when the depth-to-diameter ratio (aspect ratio) of pattern features is greater than 0.5. We present preliminary direct experimental evidence of the existence of electron shading. In addition, we present evidence of removal of the electron shading by exposure of the charged structure to UV radiation produced by a Hg-Ar lamp A patterned test structure was exposed to a d.c. nitrogen plasma operating at a pressure of 120 mTorr. The pattern was composed of a layer of thermally grown oxide with circular pits of 800 nm diameter and 1 micron depth. The pit pitch was 1.6 microns. The structure was placed on the cathode of the discharge to which -500 V was applied to break down the gas and to bombard the surface with ions. The structure was exposed to the plasma for a total of 10 seconds. A Digital Instruments multimode Atomic Force Microscope was modified to operate as a Kelvin probe in order to measure the surface potentials with sufficient spatial resolution to determine the potential inside the pit regions. To enhance the resolution beyond the standard AFM tip dimensions a carbon nanotube was attached to the apex of the AFM tip. Three two dimensional surface potential scans were made. First, a scan of the unexposed test structure showed that the surface potential was less than 10 mV over the surface of the unexposed structure. After plasma exposure, a second scan showed d.c. surface potential of the order of 6 volts over the entire structure. In addition, higher potentials (of the order of 200 mV) were observed directly over the pit regions, thus showing that electron shading appears to be present. The third scan (after UV exposure) shows removal of all surface potentials inside and out of the pit region and returns the structure to the uncharged state before plasma exposure.
1 Work Supported by NSF under grant DMR-036582 and under grant ECS-9731293.