| Summary: | 博士 === 國立中山大學 === 材料與光電科學學系研究所 === 107 === In recent years, flat panel display industry has developed rapidly and is widely used in electronic products (e.g. television, notebook computers, digital cameras and intelligent mobile devices) and car display. In these applications, thin film transistors (TFTs) play important role in pixel switch and current drive. Besides, As the resolution increases with the development of displays, the requirements for thin film transistors are also increasing. Amorphous metal oxide semiconductors have many advantages, such as high carrier mobility, high uniformity, and low process temperature, these advantages making them become the focus of attention in the corporations. Among the many kinds of metal oxide semiconductors, amorphous zinc oxide doped with indium and gallium (a-InGaZnO) in this study is the most widely studied. However, due to the instability which induced by environment, temperature and bias, degradation behavior can be easily found in metal oxide TFTs under displays pixel circuit operating. Therefore, it is crucial to clarify the physical mechanism of devices degradation and instability. In this thesis, the electrical degradation behavior caused by electrical instability, self-heating effect and environment of amorphous InGaZnO TFTs with different structures and different passivation layers are discussed in order.
In the first part of this thesis, negative bias instability of copper electrode a-InGaZnO TFTs has been investigated. Because of the copper ions diffusion in copper electrode back channel etching (BCE) devices process, negative bias attracts copper ions in active layer and let these ions drift to the interface between active layer and gate insulator. This phenomenon causes the on current and sub threshold swing degradation of devices. To further confirm this degradation, some experiments are carried out in this thesis. First, verifying the internal copper ions drift away from the interface phenomenon by the recover behavior of on current and S.S. induced by low temperature heating or positive bias applying in degraded devices. Second, verifying the internal copper ions drifting phenomenon by applying different frequencies bias. Because copper ions drifting phenomenon takes time, changing the frequencies is helpful for observing the phenomenon that the drift of internal copper ions affects the characteristic of devices. Finally, to further confirm that there are indeed copper ions in active layer, the devices crossection sample made by Focus Ion Beam (FIB) instrument has been produced for Transmission Electron Microscope elements proportion analysis. There are some copper element signals in TEM element proportion analysis results which also directly indicating the reason of devices degradation.
In second part of this thesis, the S.S. degradation and the increase of gate side leakage phenomenon in large width copper electrode BCE devices which applied with self-heating stress has been investigated. By the current crowding phenomenon in devices output properties, we assume that self-heating stress causes the copper ions in drain electrode diffusing into the interface and leading the bad electrode contact. Besides, the forward and revers I-V curves indicate that the gate leakage is induced by the diffusion of copper ions from gate electrode to gate insulator. Finally, different frequencies self-heating stress can also confirm this phenomenon.
In third part of this thesis, the device degradation with positive and negative bias operation under moisture environment has been investigated. A Drain Induced Barrier Lowering(DIBL) like phenomenon was observed in devices I-V curves under moisture environment. It is assumed that the H2O molecular in atmospheric environment would diffuse into the passivation layer due to the loose passivation structure. The drain bias arranges these polar H2O molecular in passivation directionally and also decreases the source barrier. Therefore, the lowered barrier leads the DIBL phenomenon. To further clarify this phenomenon, positive and negative bias stress under moisture environment are utilized. The experiment result shows that the negative threshold voltage shift was observed both in positive and negative bias operation. In addition, the parasitic transistor phenomenon was also observed in negative bias operation. By applying different frequencies bias, we can realize that gate positive bias attracts H2O molecular drift to the interface between passivation and active layer. For InGaZnO, these H2O molecular acts like electron dopant and also leads the threshold voltage shift phenomenon. In the other hand, under negative operation, H2O molecular would generate the hydrogen dangling bond and then rebind with InGaZnO. These hydrogen bonds supply extra electrons to the back channel and also cause the parasitic transistor phenomenon. Finally, this mechanism can be explained by the devices recover phenomenon under different bias operation.
In final part of this thesis, the organic passivated devices degradation under high temperature negative bias operation has been investigated. After the high temperature negative bias operation, parasitic transistor phenomenon was observed. Due to the donor like hydrogen ions which attracted by bias drift to the back channel and bind with active layer, the back channel electron concentration is higher than front channel. Thus, the parasitic transistor phenomenon was observed. The hydrogen ions diffusion behavior is similar to the Negative Bias Temperature Instability(NBTI) degradation which can be observed in traditional metal oxide semiconductor field effect transistors. Therefore, the Power’s Laws which used to explain NBTI model is cited to further confirm this degradation.
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