Design and fabrication of single grain TFTs and lateral photodiodes for low dose X-ray detection A. Arslan, R. Ishihara, J. Derakhshandeh, C.I.M. Beenakker Delft Institute of Microsystems and Nanoelectronics (DIMES), Delft University of Technology, P.O. Box 5053, 2600 GB Delft, the Netherlands E-mail: a.arslan@tudelft.nl ABSTRACT 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 cm 2 /Vs and 253 cm 2 /Vs, respectively. Keywords: X-ray, thin-film-transistor (TFT), single grain, large area detection, image sensor, μ-Czochralski process 1. INTRODUCTION Patients are exposed to a relatively high amount of X-ray radiation during medical diagnostics and treatment which already carries a risk as a consequence of radiation accumulation in the body. Therefore, to decrease the dose without losing the image quality, X-ray detector performance has to be improved. Digital X-ray radiography detectors are currently using a-Si thin-film-transistors (TFTs) which can be fabricated with lower process temperature compared to IC technology and hence permit use of large area glass substrates. However, due to the amorphous nature, mobility is very low (1 cm 2 / Vs) resulting in an image lag problem. Poly-Si TFTs will improve mobility and thereby enables higher resolution, but due to existence of random grain boundaries [1], sensitivity is low. Motivation of this paper is to develop an indirect X-ray detector [2], which employs a scintillator to convert X-ray to visible light and light sensitive 2D-location controlled silicon islands for photodiode array and readout electronics, as shown in Figure 1. A low-temperature process, based on the μ-Czochralski technique with excimer-laser crystallization of a-Si film, has been developed in TU Delft and used to control the positions of the islands. The proposed X-ray image sensor is a solution to the lag problem and frequency limitation of a-Si TFT via high mobility [3] while achieving a detection area for chest radiography without the tiling process which causes data lost at the connection of the detector arrays. In this paper, first the μ-Czochralski technique is explained and then the SG- TFT and SG-PD designs are presented. SG- PDs were fabricated using 1 μm thick crystallized Si layer to increase the sensitivity of the silicon layer for long wavelengths (>400 nm). For comparison, we fabricated same PD designs using SOI and a-Si deposited wafers, as well. SG-PDs were fabricated in three different sizes to determine the pixel size of the image sensor. Measurement results show that, SG- PD can be substitute of single crystalline PDs with its high sensitivity and low dark current. SG- TFTs were fabricated with 250 nm thick crystallized silicon layer with 1.5 μm gate length. Transfer characteristics of SG- NMOS and SG- PMOS TFTs are demonstrated. Medical Imaging 2011: Physics of Medical Imaging, edited by Norbert J. Pelc, Ehsan Samei, Robert M. Nishikawa, Proc. of SPIE Vol. 7961, 79614N · © 2011 SPIE · CCC code: 1605-7422/11/$18 · doi: 10.1117/12.877959 Proc. of SPIE Vol. 7961 79614N-1 Downloaded from SPIE Digital Library on 20 Oct 2011 to 131.180.130.109. Terms of Use: http://spiedl.org/terms