Proximity Gettering Design Via Nano-Cavities Induced By Hydrogen-Ion Implantation for Si CMOS Image-Sensor

As the market demand for the application of high resolution of Si CMOS image-sensor (CIS) has been increased rapidly, the pixel size has been scaled down less than 1 μm to achieve high resolution CIS. Reducing the dark current is very important since the sensitivity of photodiode is determined by a dark current. One of the most critical causes for increasing dark current of the CIS cells is a metallic contamination in photodiode region. During the fabrication process of CIS, the photodiode region is easy to be contaminated by metallic impurities such as iron, copper, nickel and cobalt.

In general, p/p⁺⁺ epitaxial wafer has been utilizing for CIS mass production because they can alleviate the harmful effect of the contamination caused by heavy metal ions, resulting from that the heavily doped boron in the p/p⁺⁺ epitaxial wafer enables to capture the metallic ions, such as iron and cobalt. However it is also well known that nickel ions were not effective in segregation gettering. Thus, we came up with Si CIS cells with proximity gettering via forming nano-cavities induced by hydrogen ion-implantation.

We investigated the effect of hydrogen ion-implantation gettering for metallic impurities. Hydrogen ions were implanted at 50-keV and a dose of 1×10¹⁶ atoms/cm² into silicon bulk wafer. To place gettering sites underneath photodiode depletion region, a 3-μm epitaxial silicon layer was grown on the hydrogen-ion implanted wafer. Figure 1 shows the morphology of nano-cavities induced by hydrogen-ion implantation. Figure 1(a) indicated that plate-like shaped cavities were formed at intermediate temperatures, and at high temperature, the cavities took the shape with minimum surface energy, which is similar to morphology of oxygen precipitate. Due to the anisotropy of interface energy, such a preferential shape was the octahedron with eight equivalent (111) faces, as shown in Fig. 1(b). In particular, the boundaries of cavities consist of interstitial silicon atoms and dangling bonds which resulted in the relaxation gettering for metallic ions, for that those were acted as gettering sites.

We also analyzed the profile of impurity concentration in hydrogen-ion implantation wafer using dynamic SIMS, as shown in Fig. 2. It was confirmed that cobalt ions were clearly detected from the surface about 3-μm, which is exact same location we implanted. This is the clear evidence to confirm the gettering ability of hydrogen-ion implanted wafer. Thus, hydrogen-ion implantation gettering is controllable for the depth of gettering site, and hydrogen-ion implanted wafer has great gettering ability for metallic ions.

Based on experiment, we will present how metallic contaminants affect the dark current, photocurrent, sensitivity of photodiode and the sensing margin of a CIS cells. In addition, we will explain the mechanism of hydrogen-ion implantation gettering in detail. Furthermore, we will report the gettering efficiency of nano-cavities induced by hydrogen-ion implantation by fabricating Si CMOS image-sensor.

 * This work was financially supported by the Brain Korea 21 Plus Program in 2015 and SiWEDS (Silicon Wafer Engineering and Defect Science).