TFTs are employed as switching and/or driving devices in many electronic circuits. As an example, TFTs are used as control devices for pixels in display applications such as flat panel displays (FPD), whether based on active-matrix-liquid-crystal displays (AMLCD), or active-matrix-organic-light-emitting-displays (AMOLED). These FPD are used in televisions, computer monitors, smart phones, tablets, etc. Traditionally, TFTs based on amorphous silicon technology (a-Si) have been used due to the low cost and ease of manufacture. However, a-Si-based TFTs have a number of issues such as low mobility, low ON/OFF current ratios (e.g. higher power), and limited durability. Additionally, TFTs based on a-Si are not transparent, thereby limiting the size of the TFT within the pixel so that the display characteristics are not compromised. With the market moving to higher resolution, higher refresh rate, lower power consumption, lower cost, larger displays, smaller bezel size, higher touch quality, higher image quality, and new applications (e.g. flexible displays) there is a need to replace a-Si.
Metal-based semiconductor materials (e.g. metal oxides, metal oxy-nitrides, metal oxy-chalcogenides, metal chalcogenides) are candidates for replacing a-Si in display applications. The metal-based semiconductor materials may be amorphous, crystalline, or polycrystalline. Some examples of metal-based semiconductor materials include those based on In—Ga—Zn—O (IGZO) and related materials, like In—Zn—O (IZO), In—Ga—O (IGO), Zn—Sn—O (ZTO), In—Sn—Zn—O (ITZO), Hf—In—Zn—O (HIZO), and Al—Zn—Sn—O (AZTO). Some examples of metal oxy-nitrides include Zn—O—N (ZnON), In—O—N (InON), Sn—O—N (SnON). Examples of crystalline metal-based semiconductor materials include c-axis aligned crystalline (CAAC) materials like CAAC-IGZO, or polycrystalline materials like ZnO and In—Ga—O (IGO). In addition to the application of these materials into TFTs, these materials are also being considered for memory (e.g. non-volatile random access memory (RAM)), sensor applications (e.g. image sensors), and central processing units (CPU). Some of these materials exhibit stable amorphous phases, high mobility (e.g. >5 cm2/Vs), low threshold voltage (close to zero, e.g. in a range of −1.0V to +2.0V), low carrier concentrations (e.g. 1016-1017 cm−3), high ON/OFF current ratios (e.g. >106), and high durability (e.g. negative bias temperature illumination stress NBTIS with threshold voltage shift in a range of −1.5V to +0.5V). However, since these materials are multinary compounds (e.g. three or more elements), their performance and properties are sensitive to factors such as composition, concentration gradients, deposition parameters, post-deposition treatments, interactions with adjacent materials, and the like. Further, since the electrical, physical, and chemical behavior of these materials is difficult or impossible to model, much of the development and optimization must be accomplished empirically. Comprehensive evaluation of the entire composition range and deposition parameter space for the formation of a TFT device utilizing these materials requires thousands or millions of experiments.
The traditional film stack for a metal-based semiconductor TFT requires an etch stop layer to protect the metal-based semiconductor material during the patterning of the source and drain electrodes. If an etch process can be developed that does not degrade the metal-based semiconductor material performance, the number of processing steps and the cost of manufacture of the TFT device can be reduced because the etch stop layer deposition and patterning can be avoided. Furthermore, back-channel-etch (BCE) devices (no etch stopper) can be made smaller than etch-stopper-based devices, and parasitic capacitance can be reduced, both improving display performance.