Integrated Amorphous and Polycrystalline Silicon TFTs with a Single Silicon Layer
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ABSTRACT Selective exposure of an a-Si:H film to a room temperature hydrogen plasma using a patterned SiN,, capping layer and a subsequent anneal at 600"C, resulted in polycrystalline and amorphous silicon regions in a single silicon layer on the same glass substrate. Top-gate nonself-aligned TFTs were fabricated in both the amorphous and polycrystalline regions with all shared processing steps and no laser processing using a re-hydrogenation step. The TFTs had good characteristics, with field-effect mobilities upto 1.2 cm 2/Vs and 15 cm2/Vs for the a-Si:H and the poly-Si TFTs, respectively, and ON/OFF ratios >105 in either case.
INTRODUCTION For large-area electronics, there has been considerable interest to integrate both polycrystalline and a-Si:H TFTs. This can be done by selective laser crystallization of a-Si:H [1], or using two layers of silicon, one polycrystalline and second amorphous [2], to integrate the aSi:H and the poly-Si TFTs on the same substrate. In active-matrix flat panel displays, a-Si:H TFTs provide low leakage in the OFF state and poly-Si TFTs high drive currents. Integration of a-Si:H and poly-Si TFTs is traditionally difficult for three reasons. First, the conventional a-Si:H TFT fabrication process is a low temperature process (0.5 pA/tm, with ON/OFF ratios of both types of devices >105 (Fig. 8(b)). This is the first demonstration of the integration of a-Si:H and poly-Si TFTs on the
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same substrate in a single layer without laser processing. These results compare favorably with other work on integrated a-Si and poly-Si TFTs using laser processing which resulted in a-Si TFTs with field-effect mobility of -0.9 cm 2/Vs and poly-Si TFTs with field-effect mobility of -20 cmZ Vs [1]. These are also the best results for a-Si:H top gate TFTs after a 600 "C anneal step. Integrated a-Si and poly-Si TFTs on
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Figure 8. TFT" characteristics of (a) the best a-Si:H top-gate TFT fabricated after 600 'C anneal with linear field2 effect mobility of ~1.2cm /Vs, and (b) optimized integrated a-Si:H and poly-Si TFTs on the same glass substrate.
CONCLUSION We have successfully integrated high performance a-Si:H and poly-Si TFTs in a single Si layer on the same glass substrate, using a hydrogen-plasma selective crystallization technique. The TFTs share all processing steps and no laser processing is involved. Re-hydrogenation is critical after the crystallization anneal at 600 TC to obtain a device quality a-Si:H film. ACKNOWLEDGEMENTS This work was supported by DARPA and the Princeton Program in Plasma Science and Technology (DOE contract no. DE-AC02-76-CHO-3073). REFERENCES [1] P. Mei, J.B. Boyce, D.K. Fork, G. Andersen, J. Ho, J. Lu, M. Hack and R. Lujan, Mat. Res. Soc. Symp. Proc. 507, 3 (1998). [2] T. Aoyama, K. Oga
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