Design, fabrication and measurement results of single grain (SG) lateral PIN photodiodes and SG thin film transistors
(TFT) are reported in this paper. Devices were developed to be used in indirect X-ray image sensor pixel design. We
have controlled position of 6 μm x 6 μm silicon grains with excimer-laser crystallization of a-Si film. Lateral PIN
photodiode (PD) arrays were designed inside the single grain with 1 μm, 1.5 μm and 2 μm intrinsic region length and 4
μm width. The gate length and the width of the fabricated TFTs are 1.5 μm and 4 μm, respectively. Devices were
fabricated using a-Si, SOI and crystalline silicon layers and electrical measurement results were compared. 100 μm x 100
μm sizes SG-photodiodes have dark and saturation currents on the order of 0.1 nA and 10 nA resulting in a light
sensitivity of 200 with an exposure of white light. Fabricated NMOS and PMOS TFTs inside the grains have field effect
mobility of 526 cm2/Vs and 253 cm2/Vs, respectively.
Existent flat-panel display is mechanically stiff because it requires external connection of IC chips. At its present stage,
displays with a-Si, metal oxide semiconductor or organic TFTs require still external connection of data driver and
controllers, because of their low carrier mobilities. We will review our recent progress on direct formation of high speed Si
circuits fabricated with a plastic compatible temperature. Large Si grains with a diameter of 4 microns were formed on
predetermined positions by a pulsed laser crystallization process with a plastic compatible temperature. High performance
transistors were fabricated inside a single Si grain.
Formation of TFTs inside location-controlled large Si grains with a low temperature process is an attractive approach for realizing system-circuit integration with displays on a large glass substrate. Local structural variations of the substrate using photolithography allows an accurate location-control of the large Si grains in excimer-laser crystallization. Single-crystalline Si (c-Si) TFTs was formed inside a location-controlled large (6 μm) grain by μ-Czochraski process of a-Si film. The c-Si TFTs showed field effect mobility of 450 cm2/Vs on average. Crystallization characteristics, spread of the TFT characteristics and effects of process parameters
will be reviewed and discussed.
This paper reviews advanced excimer-laser crystallization techniques and its application to crystal-Si thin film transistors (TFTs). Combined microstructure and time- resolved optical reflectivity investigations during conventional excimer-laser crystallization showed that explosive crystallization occurs during excimer-laser irradiation. Two methods enabling location-control of large silicon islands will be reviewed. One of the methods uses local thermal relief by modifying locally the heat extraction rate towards the substrate. A small unmolten region remains at the center of high heat extraction part which then acts as a seed for radially grown Si grain with a diameter of 6 micrometers . One of the other methods use geometric selection through a vertical narrow constriction. In this method, upon laser irradiation, a small unmolten Si region remains at the bottom of narrow holes etched in the underlying isolation layer. During vertical regrowth, a single grain is filtered out which subsequently seeds the lateral growth of large grains. We will also discuss the performance of crystal-silicon TFTs that are formed in the location-controlled Si grains. The field-effect mobility for electrons is 450 cm2Vs, which is very close to that of TFTs made with silicon-on-insulator wafers.