AVS 60th International Symposium and Exhibition
    Plasma Science and Technology Tuesday Sessions
       Session PS1-TuM

Paper PS1-TuM3
Investigation of Photo-Assisted Etching in Different Halogen-Containing Plasmas and Synergetic Effects of a Tandem Plasma System

Tuesday, October 29, 2013, 8:40 am, Room 102 B

Session: Plasma Sources
Presenter: W. Zhu, University of Houston
Authors: W. Zhu, University of Houston
L. Liu, University of Houston
S. Sridhar, University of Houston
V.M. Donnelly, University of Houston
D.J. Economou, University of Houston
Correspondent: Click to Email
Photo-assisted etching (PAE) of p-type Si in halogen-containing plasmas was discovered at sub-threshold ion energy in a Faraday-shielded, inductively coupled plasma (ICP). Halogen-containing feed gases (Br2, HBr, Br2/Cl2 and HBr/Cl2) were explored and the strong dependence of PAE on different etchant gases was investigated. Sub-threshold etching rates in 50% halogen /50% Ar cw ICPs were ordered as 25% HBr/ 25% Cl2 > 25% Br2/ 25% Cl2 > 50% Cl2 > 50%HBr > 50% Br2. H-atoms also caused isotropic etching in HBr plasmas; sub-threshold etching of patterned p-Si in other gases was anisotropic. PAE was much less important for Br2, with an etching rate 4 times slower than that for Cl2, under similar conditions. The dependence of PAE on incident photon energy was characterized by filtering incident photons with different wavelength to the sample surface. Vacuum UV was much more important than UV or visible light in inducing PAE. PAE is expected to cause substantial complications for processes that require low ion energies to achieve high selectivity and low damage, such as atomic layer etching. To address this issue, a tandem plasma system was developed to inject one plasma (upper plasma) into another plasma (lower plasma) separated by a grid. A “boundary electrode” (BE) in contact with the upper plasma could be biased to influence the plasma potential. The goal was to manipulate the electron energy distribution functions (EEDF), and possibly enhance ion-assisted etching compared to PAE. Plasma parameters and EEDFs were measured with a Langmuir probe over a wide range of power, pressure and position in cw Ar plasmas. With both plasmas powered (100 W lower plasma/ 500W upper plasma) at 10 mTorr, low energy electrons were depleted in the lower plasma when the BE was grounded. However, with 60 V bias on the BE, low energy electron depletion did not occur, and the high energy tail was enhanced. The reverse behavior was found in the upper plasma under the same conditions. Pulsed lower plasma with cw upper plasma injection was also explored. High plasma density of 2×1011 cm-3 with selectable, nearly-constant Te (e.g. 0.8 eV) was achieved in the afterglow of the lower plasma at 10 mTorr.