420732 Investigation of Film Porosity and Hydrogen Content Dependence on Deposition Conditions in a PECVD Process Using a Multiscale Model

Wednesday, November 11, 2015: 1:10 PM
Salon G (Salt Lake Marriott Downtown at City Creek)
Marquis Crose, UCLA, Los Angeles, CA, Joseph Sangil Kwon, Chemical Engineering, UCLA, Los Angeles, CA and Panagiotis D. Christofides, Department of Chemical and Biomolecular Engineering and Department of Electrical Engineering, University of California, Los Angeles, Los Angeles, CA

Plasma enhanced chemical vapor deposition (PECVD) is widely used in the microelectronics and solar cell industries to deposit thin films from a mixture of gas-phase species onto a solid substrate [1]. Specifically, in the solar cell industry, PECVD is broadly used in the production of thin film silicon solar cells to deposit amorphous silicon semiconductor layers due to reduced manufacturing costs via silane recycling and the possibility for extremely low operating temperatures (<250  ͦ C) while achieving optimal dielectric properties of amorphous silicon thin films.  The material of interest here, hydrogenated amorphous silicon (a-Si:H), differs from crystalline silicon by the lack of long-range order and the high content of bonded hydrogen (typically around 10% in device-quality a-Si:H). Although useful for opto-electronic devices, this material suffers from degradation via the well characterized Staebler-Wronski effect; whereby, in the first six months of operation the defect density increases under intense light, accompanied by a 10-15% drop in efficiency [2,3].

Motivated by the above considerations, a multiscale modeling and operation framework is developed to explore the relationship between standard operating conditions (e.g., 150 < T < 250   ͦ C, 1 < P < 5 bar) and the quality of deposited films, in an effort to develop deposition techniques that are capable of producing high-quality materials at reasonable growth rates. In the present work, quality is a function of two conditions: the hydrogen content which affects the stability and optical band-gap, and the void fraction which plays a role in the electronic and structural properties of the film [4]. Specifically, the continuous gas phase is modeled via a system of one dimensional reaction and transport equations at discrete locations across the wafer surface. The resulting concentration of silane and hydrogen radicals is then applied to a hybrid kinetic Monte Carlo (kMC) model which captures the complex surface chemistry involved in the growth of a-Si:H films [5]. Unlike traditional solid-on-solid schemes, the triangular, two-dimensional lattice employed in this study creates the possibility for overhangs and voids to develop within the growing film. Additionally, both silicon and hydrogen radicals interact on the surface and within the lattice bulk, yielding a more accurate growth mechanism than in previous studies. After completion of the deposition process, post batch measurements allow for the hydrogen content and void fraction to be characterized. Simulations across a wide range of deposition conditions reveal that the reactor temperature and the ratio of SiH4 to H2in the feed are critical parameters which play a key role in determining the quality of the film microstructure and the concentration of trapped hydrogen species.

[1] Kern W. Thin Film Processes II. Academic Press. 1991

[2] Rech B, Wagner H. Potential of amorphous silicon for solar cells. Appl. Phys. A. 1999;69:155-167.

[3] Nelson J. The Physics of Solar Cells. Imperial College Press. 2003

[4] Smets AHM, Kessels WMM, Van de Sanden MCM. Vacancies and voids in hydrogenated amorphous silicon. Appl. Phys. Let. 2003;82:1547-1549.

[5] Crose M, Kwon JS, Nayhouse M, Ni D, Christofides PD. Multiscale modeling and operation of PECVD of thin film solar cells. Chem. Eng. Sci. 2015, in press.

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