Development of a PECVD Line Source for the Roll-to-Roll Deposition of Thin Film Silicon Solar Cells

Roll-to-Roll (R2R) production of thin film silicon based solar cells & transistors (TFT’s) combine the advantages of the use of inexpensive, lightweight & flexible substrates with high throughput production. Significant cost reduction opportunities can also be found in terms of utilized substrate area and process gas flow when compared with batch processing systems. The thin amorphous and microcrystalline silicon thin films used in these devices are conventionally deposited using Plasma Enhanced Chemical Vapor Deposition (PECVD) as a consequence of the deposited material’s low electronic defect density and the ability to tune the deposited layer’s optical and electronic properties via variation of the process gas chemistry. Applied Materials has developed a linear PECVD source technology for use in the R2R manufacture of thin film silicon based electronic devices providing high through-roll process stability, low particulate contamination and reduced clean process induced downtime. The work presented in this paper describes the development of a 40.68 MHz PECVD line source for the high rate deposition of thin film silicon films for use in flexible solar cells. The plasma source design was optimized to produce the necessary plasma density, electron temperature and ion energy for the deposition of device quality material. This optimization was driven by the use of simple particle and energy balance modeling, computational fluid dynamics and chemical kinetic modeling of the deposition process. Experimental plasma characterization (retarding field energy analysis, plasma impedance monitoring) was subsequently used to confirm the source performance. Highly uniform layers were deposited at high dynamic deposition rates using this source technology. These layers were subsequently integrated within PIN and NIP device structures on both rigid and flexible substrates resulting in single junction solar cells with equivalent performance to those deposited within traditional cluster tools. The results therefore indicate the suitability of this linear source technology in the high throughput R2R manufacture of thin film based solar cells and other silicon based flexible electronic devices.