Reel-to-Reel Cassette Cluster Tool System for Thin Film Transistor and Four Terminal Solar Cell Fabrication
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Reel-to- Reel Cassette Cluster Tool System for Thin Film Transistor and Four Terminal Solar Cell Fabrication Arun Madan, MVSystems, Inc., 17301 W. Colfax Ave., Suite 305, Golden, CO 80401, USA. Email: [email protected] www.mvsystems.info ABSTRACT We present a new type of system architecture to fabricate thin film silicon devices on flexible substrates, which uses the inherent advantages of a cluster tool normally used in the production of amorphous silicon thin film transistors, solar cells, etc. In this, the flexible substrate material is contained within a cassette which includes a reel to reel operation. As in the current cluster tools used for planar substrates, the cassette is transported to a process chamber using a robotic arm. When the entire roll in the cassette has been processed, it is transported into other chambers for further processing. We also show that a four terminal device structure (e.g. amorphous Si and stable low band gap nano-crystalline Si cells) potentially can lead to high efficiency (>15%), stable, low cost solar cells on a plastic substrate. Lastly we discuss the use of a pulsed PECVD deposition technique, which allows that the structure of the nano-crystalline Si films can be altered from 111 to 220 in a controllable way at a low temperature of 15%),stable, low cost devices could be manufactured on plastic substrates. REEL TO REEL CLUSTER TOOL FOR TFT FABRICATION Thin film Silicon TFT’s essentially consist of gate metal (Cr)-SiNx- intrinsic – n typemetal (source and drain contacts -Al) while a solar cell is constructed in the following configuration: SnO2 (or ZnO)-p type- intrinsic – n type- Al contact. a-Si:H films are normally deposited, at the rate of 1-3 Å/sec using the PECVD technique (e.g. using SiH4 and H2 gasses) capacitively coupled, operating at a fixed continuous frequency of 13.56 MHz. Doping is achieved by the adding diborane and methane for the p- type layer and phosphine for the n-type layer. SiNx is normally deposited at T>300C using the PECVD technique using gas mixtures of SiH4 and NH3 (and /or N2) [4]. Other materials, necessary to complete these device structures require metallization and transparent conducting oxides (e.g. ZnO, ITO etc.) which are deposited using the sputter deposition technique. As cross contamination of even 1 ppm of B or P can have a profound deleterious effect [5] on the performance of a device, hence it necessitates the use of a cluster tool as shown in Fig. 1; in this, multiple process chambers (Modular Process Zones-MPZ's) are stationed around a central circular evacuated isolation and transfer zone (ITZ). The ITZ houses an accurate and precise robotic arm which works on a “pick and place" principle and serves to insert, extract and transfer substrates from one MPZ to another in any sequence. As each MPZ has a gate valve present between it and the ITZ, then any cross contamination is thus prevented. For a-Si:H TFT’s on rigid plates, we have provided systems to customers which have yielded field effect mobility in excess of 0.7 c
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