Cu/Cu Barrier Interconnect with Low Resistivity for the Application to the Next-Generation and High-Resolution Display Fabricated Using Microwave-Assisted Sputter
Authors: Jeong Wooseok , Do Jae-chul , Lee Jeong Rak , Park Wan Woo , Shin Jeongchul , Park Sang-Hee Ko 
Published: February 2020

Displays with large size and high resolution are required in many electronic devices such as mobile phones, notebooks, and televisions. In recent times, 8k ultra-high-definition (UHD) display which will be the standards of next-generation display has been developed. While the demand for displays with high-pixel-density is increasing, it is difficult to increase the resolution because of resistor-capacitor (RC) delay issue. The resistance is increased with the display size due to the length of metal lines. When the resolution is increased, the capacitance of metal lines is also increased because of the number of crossings of metal lines. The meaning of RC delay is the signal delay through the circuit wiring as a result of resistance and capacitance effects. This increase induces the severe electrical loads on the gate and data lines. Therefore, controlling these two effects is the major challenge in making high-resolution display.
To minimize the effect of the RC delay, many researchers have been studied the variety of materials which can be used in a metal interconnect lines. There are lots of materials such as Al, Mo, W, Cu, and Ti. Among these materials, copper is the most suitable candidate because it has lower resistivity than aluminum, which has been used as conventional interconnect material. Using a material with low resistivity like copper can reduce the RC delay, in other words, increase the electrical signal speed through the metal lines. Although the copper has a resistivity of 1.7 μΩ∙cm, it shows 2.1 ~ 2.3 μΩ∙cm within the thickness of 1 μm when deposited using DC/RF sputtering method. This is due to the fact that it is difficult to deposit high-quality copper film by using the conventional sputtering method. To overcome these problems, we employed microwave-assisted sputter (MWA sputter), providing high-quality copper films having the resistivity of 1.95 μΩ∙cm within the thickness of 1 μm even after formation of the metal lines. The MWA sputtering is an enhanced sputtering method using electron cyclotron resonance (ECR) phenomenon with the induction of microwave into the DC magnetron sputtering. The MWA sputtering has the advantage of generating highly ionized plasma, having low discharge voltage and low working pressure. Therefore, we can deposit high quality copper film by using MWA sputtering, enabling the reducing RC delay for next-generation display. However, when we deposit copper, the barrier metal is always needed because the copper can easily diffuse into adjacent materials. Unless suitable barrier material is used, it can be cause of degradation of device performance. For these reasons, we fabricated Cu/Cu barrier interconnect as a source/drain electrode and observed various properties including calculation of resistivity.
In this study, we report a study of properties of copper film deposited by DC sputtering and MWA sputtering using scanning electron microscope (SEM), x-ray diffraction spectroscopy (XRD) and atomic force microscope (AFM). The film stress was calculated before and after the formation of the Cu/Cu barrier metal line because film stress is an important parameter when fabricating electronic devices. Mo and Mo-Ti alloy was used as the copper barrier metal and we developed wet etching condition of Cu/Cu barrier interconnect. After the etching process, we measured etching profile using SEM cross-section image and calculate taper angle. Consequently, we obtained resistivity of Cu/Cu barrier interconnect deposited by DC sputtering and MWA sputtering, respectively. It revealed that Cu/Cu barrier interconnect deposited by MWA sputtering shows the lower resistivity than by DC sputtering. From the experimental results, we confirmed that MWA sputtering method makes it possible to deposit high quality metal films with low resistivity, which gives new opportunities for the design and realization of next-generation displays.
Reference
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