Etching Characteristics of Top Metal Electrode and IGZO channel layer in Inductively Coupled Plasma System

The oxide thin film transistors (TFTs) are interested by many researchers and industries due to its higher performance and superior oxide material properties compared to those of Si-based TFTs [1]. However, most of researchers are using the chemical wet etch [2] and lift- off processes for the patterning of the oxide channel and electrode layer. In order to scale down the oxide TFTs and make better control of patterning and processing, the dry etch process is preferred. In this work, the dry etching characteristics using inductively coupled plasma (ICP) was studied for pattering of oxide and metal electrode layer in the application to oxide TFTs fabrication. The In-Ga-ZnO (IGZO) thin film was deposited on Si substrate using rf magnetron sputter at room temperature. The IGZO target was composed with an atomic ratio of In:Ga:Zn=1:1:1. Next, metal layer, such as TiN or Al, was depostied on IGZO using rf-magnetron sputter as one of the top electrode stacks. Etching experiments were performed using a planar ICP system [3]. The chemistries used for etching were CF4 and Ar mixed gases. To study the etching characteristics of both IGZO and metal layers, the gas mixing ratios, RF power, DC voltage and process pressure were varied. The etch rate was measured by a depth profiler. For the surface morphology and vertical profile were measured using AFM and SEM, respectively. The surface bonding structure was analyzed by using XPS because the surface chamemical bonding affect the stability of oxide TFTs [4]. Figure 1 show the etch rate of IGZO and TiN as well as the selectivity of IGZO respect to TiN as a function of CF4/Ar gas mixing ratio. As the CF4 gas content increased the etch rate of IGZO decreased, but that of TiN remained at the similar values and increased sharply when only Ar gas was used. This results show that TiN can be used as good etch stopper when top metal electrode is etched on oxide channel layer. The other process conditions were fixed at RF power of 700 W, bias voltage of -150 V, process pressure of 2 Pa, substrate temperature of 40°C, and etch time of 60 sec. Figure 2 shows the etch rate of IGZO and TiN as a function of RF power, and Fig.3 shows the etch rate of IGZO and TiN as a function of DC voltage. The etch rate of IGZO changed less with varing RF power or DC voltage compared to those of TiN. Therefore, it can be possible to achieve the better selectivity during the etching of top electrode material of TiN. In addition to the etching characteristics of both IGZO and TiN, the surface chemical characteristics of IGZO channel layer was studied by using XPS because it can reveal the oxygen bonding structure which may affect the stability of oxide TFTs.