Xiaoyan Liang a b c, Liang Yin a, Jijun Zhang a b c, Wenxuan Yang a, Chen Xie a, Chao Yu a, Linjun Wang a b c, Jiahua Min a
a School of Materials Science and Engineering, Shanghai University, Shanghai, 200444, China
b Zhejiang Institute of Advanced Materials, SHU, Jiashan, 314113, China
c Shanghai Collaborative Innovation Center of Intelligent Sensing Chip Technology, Shanghai, China
## Abstract
Cadmium Zinc Telluride (CZT) crystals are widely regarded as the optimal semiconductor materials for producing room temperature high-energy radiation detectors. The goal of polishing is to enhance the quality of the crystal surface. This paper adopts a three-step polishing process, where the first two steps involve mechanical polishing with Al2O3 polishing slurries of different particle sizes. The third step employs the chemical mechanical polishing (CMP) of CZT crystals with a polishing slurry of silica sol and sodium hypochlorite. The results indicate that the introduction of 300 nm Al2O3 alkaline transition polishing eliminates the deep scratches, which lay a foundation for the next CMP process. The experimental results of orthogonal CMP indicate that when the NaClO content is 5 %, the crystal roughness reaches its optimum over a large range of 800 × 700 μm2, with an average roughness of 1.61 nm and minimal leakage current. Compared to the first step using 3 μm Al2O3 mechanical polishing, the energy resolution is significantly improved from 24.3 % to 10.5 %. Based on the analysis of CMP mechanisms, during the CMP process, NaClO forms an oxide layer through chemical reaction, softening the surface of CZT, which is easily removed by the physical action of silicon sol. When the NaClO content is 5 %, a balance between mechanical and chemical actions is achieved, leading to the optimal quality of the crystal surface. Therefore, it provides an effective surface treatment technique for preparing high-performance X-ray and γ-ray detectors by this three-step polishing method.
## Introduction
Cadmium Zinc Telluride (CZT) crystals, due to their high electron mobility, large atomic number, and high charge collection efficiency, exhibit excellent photoelectric responsiveness and stability at room temperature [[1], [2], [3]]. Thus, they are widely used in the manufacture of X-ray and γ-ray detectors, playing an important role in various fields such as medical imaging, nuclear radiation detection, aerospace, and military weapon [4,5]. In order to improve the performance of CZT detectors, surface polishing technology is widely used [[6], [7], [8]], with the goal of reducing or eliminating defects [1,4,9] on the surface of materials to obtain better energy resolution, sensitivity and spatial resolution [10,11].
Due to the brittleness and vulnerability of CZT crystals [12], with a hardness of 1.21 GPa and a fracture toughness of 0.158 MPa m [13], traditional chemical etching and mechanical lapping processes can easily cause surface damages like etch pits, embedded hard abrasive particles, deep scratches [12,14,15]. Chemical mechanical polishing (CMP) technology [16], which achieves surface planarization through the dual action of chemical etching and mechanical wear, has been widely applied in semiconductor wafer processing in recent year [[16], [17], [18]].
Although existing studies on CZT crystal CMP have achieved lower surface roughness, these results are mostly obtained on smaller test surface areas. Yan Li et al. [19] using silica sol as the abrasive and comparing nitric acid, bromine crystal, and hydrogen peroxide as oxidants, showed that nitric acid as the oxidant obtained the best polishing surface, with a surface roughness of 0.67 nm within a 70.6 × 63.0 μm2 test range, but nitric acid is not stable with strong oxidation and easy to pollute the environment. Lezhen Zhang et al. [20] discarding traditional strong acid, strong alkali, and bromo methanol polishing slurries, used silica sol and weak oxidant H2O2, tannic acid in a certain proportion to prepare a new type of environmentally friendly green polishing slurry, which is harmless to the environment and operators. After CMP processing of CZT crystals, a surface with Ra of 0.29 nm within a 200 × 200 μm2 test range was achieved. Menghan Ao et al. [21] obtained a surface with a PV value of 2.49 μm and roughness of 1.42 nm using a silica sol polishing slurry with 6 % sodium hypochlorite at a pH of 8–9, but did not specify the test range. Compared to acidic polishing slurries, alkaline polishing slurries have lower corrosiveness to CZT, helping to protect the material surface and having less environmental impact, thus easier to manage and recycle [21].
This paper introduces a three-step polishing process, the polishing process flow chart is shown in Scheme 1. The first two steps are pretreatment processes, which involve mechanical polishing; the third step involves CMP. The study investigates the impact of oxidant concentration in the alkaline polishing liquid on the surface roughness, electrical properties, and energy spectrum of CZT crystals during the CMP process. Furthermore, XPS is utilized to conduct a more in-depth analysis of the CMP process mechanism.
CdZnTe Association (CdZnTe.com)
https://www.cdznte.com/thesis/effect-of-abrasive-particle-size-and-oxidant-concentration-on-cdznte-crystal-properties-during-cmp-proces.html