Study On Oxide Thin Film Transistors With Back-Channel-Etched Structure

Thin film transistor(TFT) array is the pixel driving component of active matrix liquid crystal display(AMLCD) and active matrix organic light emitting diodes(AMOLEDs), which plays an important role in realizing large-size, high-definition and high frame rate display. Nowadays, the active materials for TFTs include hydrogenated amorphous silicon, low temperature poly-silicon, organic semiconductor and oxide semiconductor, among which the oxide semiconductor exhibits high electron mobility and good uniformity, which makes it suitable for the pixel driving component of AMLCD and AMOLED. To realize high resolution in displays, the size of TFTs should be ―scale down‖, the key to which is to impletment TFT with back-channel-etched structure(BCE-TFT).The fabricating process of BCE-TFTs is simple and low cost. What is more, the define accuracy of the channel size for BCE-TFTs is high, which makes it easier to ―scale down‖. However, in oxide TFTs, the active layer is suspetible to most of commonly used etchants. That is, the etching selectivity between the S/D electrodes and the active channel is very low, which leads to the failure of device fabricating process. Therefore, the key to impletment BCE-TFTs is to increase the etching selectivity between the S/D electrodes and the active layer.This work focused on realizing oxide TFTs with BCE structure. Firstly, we introduced an inert carbon film as an etching barrier to protect the active layer from being damaged during the etching process of S/D electrodes. After then, the etching barrier was removed to realize BCE-like structure. In this approach, the selection of etchant is unrestricted and many materials including Cu can be used as the S/D electrodes. However, the introduction of etching barrier needs an extra film deposition and removal process, thus increases fabricating cost. To avoid introducing etching barrier, we fabricated BCE-TFTs by using hydroperoxide(H2O2) based etchant to pattern Mo S/D electrodes directly on the active layer with an etching selectivity higher than 3000, and successfully fabricated BCE structured a-IZO TFTs. However, as the increase of resolution and panel size of flat panel display, the ―signal delay‖ becomes more serious. Therefore, the use of low resistivity copper(Cu) as the electrodes and wiring material is greatly demanded. We then fabricated BCE-TFTs by using an etchant consisting H2O2 and ammonium salt to etch the Cu S/D electrodes directly on the a-IZO active layer with an etching selectivity of as high as 4800. When using H2O2 based etchant, the fabricating process is simple and the etching selectivity is very high. However, the H2O2 etchant has two disadvantages including short shelf-life and the risk of explosion, which makes it difficult to be used in mass manufacture. Finnally, in order to use conventional etchant without introducing etching barrier, we deposited crystalline Indium-Galium-Oxide(c-In Ga O) film with enhanced corrosion resistance at elevated substrate temperature. Using cIn Ga O as the active channel, BCE-TFTs were fabricated.In BCE-TFTs with etching barrier, the carbon film could effectively protect the active layer from being damaged and could be easily removed by oxygen plasma. The carbon film was ―graphitized‖ after thermal annealing and the resistivity decreased to as low as 0.1 Ωcm, which is benifical for the low contact resistance between the S/D electrodes and the active layer. The contact resistance of Mo/C/a-IZO was only 80 Ωcm. The introducing carbon film had less impact on the device performance. BCE typed a-IZO TFTs with carbon barrier exhibited saturated field effect mobility(μsat) of 14.4 cm2/Vs, a subthreshold swing(SS) of 0.21 V/decade, and a threshold voltage(Vth) of 2.0 V.During the etching process of Mo S/D electrodes by H2O2 etchant, we added KOH to accelerate the etching rate of the Mo electrodes; meanwhile, the ―edge oxidation‖ phenomenon caused by the penerstration of etchant to the interface of photo-resist and Mo film was greatly reduced. A-IZO TFTs fabricated by this method exhibited good performance with a μsat of 11.5 cm2/Vs, a SS of 0.21 V/decade, and a Vth of near 0 V.In BCE-TFTs with Cu S/D electrodes, in order to improve the adhesion of Cu film, we proposed to use a-IZO as the active channel and the adhesive layer of Cu film simultaneously. The adhesive property of Cu film was improved by the a-IZO adhesive layer due to the formation of Cu Ox at the Cu/a-IZO interface. Meanwhile, compared to a-IGZO, the diffusion of Cu in a-IZO film was suppressed due to the higher diffusion barrier for Cu atoms to diffuse across In O2 layer than GZO layer. The contact resistance of Cu/a-IZO was only 33.6 Ωcm due to the matching of band diagrams and the high carrier density of a-IZO. The a-IZO TFTs with Cu S/D electrodes exhibited a μsat of 12.2 cm2/Vs, a SS of 0.22 V/decade and a Vth of-0.4 V.The etching selectivity of Mo S/D electrodes and crystalline In Ga O was higher than 56 by using a conventional H3PO4-based etchant. BCE-TFTs based on crystalline In Ga O active channel exhibited reasonable electrical performance.In addition, we proposed to use Ag/IZO as the reflective anode of top-emitting organic LEDs(TOLEDs). The Ag/IZO composite film had high thermal stability, high storage stability and low surface roughness. What is more, compared to the conventional Ag/ITO reflective anode, the Ag/IZO possessed better patterning property and simpler fabricationg process. Finally, TOLEDs using Ag/IZO anode exhibited good performance.