Characteristic study of multi-layer using Hybrid method for digital X-ray detector

Recently digital X-ray detector, often called Active-Matrix Flat Panel Detector (AMFPD), have been researched by using either the direct or the indirect conversion method in order to form the digital image. In this paper, we introduced the concept of Hybrid method using phosphor layer to complement a weak point of direct conversion method having low conversion efficiency about X-ray. The Hybrid receptor scheme consists of a photoconductor layer, phosphor layer and top/bottom electrodes to collect charges produced in the photoconductor layer. A light reflective layer was deposited under the phosphor as lower layer. Mercury Iodide (HgI₂) is used as photoconductor layer to detect X-ray and light from the phosphor, and Cesium Iodide (CsI) is used as phosphor layer to convert X-ray to light. We fabricated a photoconductor (HgI₂ 200um) and a phosphor (CsI 50um) using PIB(Particle In Binder) method. As top and bottom electrodes, ITO (Indium Tin Oxide) layers were deposited to photoconductor layer and light reflective layer(Al 15 to 25um) in order to improve photon conversion efficiency. As results, higher X-ray sensitivity of Hybrid method exhibited comparing with that of direct conventional digital X-ray detector. The Hybrid method shows about 1.5 times higher sensitivity than the direct conversion method. This effect is caused by simultaneous detection of electric signal induced by the direct X-ray absorption in photoconductor layer and by the light absorption produced in phosphor layer. The Hybrid method can solve problems such as lower conversion efficiency of photoconductor, breakdown because of high voltage application in the direct conversion method as well as low fill factor of TFT, low electric image signals and low SNR in indirect conversion method.