Interface and microstructural characterization of phosphorus doped silicon on silicon substrate for contact plug

Abstract

Semiconductor devices have been developing with reduced design rules, increased density, and improved performance. Device size is at the limit of lithography techniques-20 nm technology. In such nanosized devices, the interfaces and crystalline defects are strongly in need of control. The manufacturing process for fabricating phosphorus (P) doped polysilicon (poly Si) and silicon (Si) selective epitaxial growth (SEG) plugs continues to receive more attention as a key factor for high-performance semiconductor devices. Native oxide removal prior to Si contact and crystal defect curing of defective Si contact is the most critical technology to ensure process and device performances for dynamic random access memory (DRAM), flash memory, and logic device. In this research, three big categories of analysis were organized to provide high quality P-doped Si thin film for reliable poly Si plug module in memory and logic device. The first is the investigation of the interface characterization in Si thin film/Si substrate by taking account of surface chemical state after dry cleaning process, including the subsequent Si growth behavior. The second is the experimental results and the proposal of mechanism about the planar defect annihilation of P-doped Si thin film through post heat treatment, which is deposited on damaged Si surface. Finally, the third is dynamical study about the crystallization behavior and planar defect generation/annihilation behavior of P-doped Si by real time in-situ TEM (Transmission electron microscopy). The detailed accomplishments are listed as follows. First, dry cleaning process for interfacial oxide removal has attracted a world-wide attention due to its superior passivation properties to conventional wet cleaning processes. Thus, the surface states of Si substrate after dry cleaning process and the role of atomic elements including fluorine and hydrogen on the properties of subsequent deposited Si layer were investigated using SIMS (Secondary-ion mass spectroscopy), XPS (X-ray induced photoelectron spectroscopy), HR-TEM (High resolution transmission electron microscopy), and FT-IR (Fourier transform infrared spectroscopy) analysis. The amounts of residual fluorine on the Si surface after dry cleaning are a key factor for maintaining clean interface. The mechanism of native oxide re-growth caused by residual fluorine after dry cleaning is proposed based on analytical results. Second, the planar defect behavior and P diffusion of P-doped Si thin film on mono-crystalline Si with annealing was investigated. It is observed that as-deposited P-doped Si thin film on Si substrate crystallizes with many planar defects, such as stacking faults and twin boundaries, due to interface defects. From SIMS analysis of post-annealed samples, it is estimated that P atoms are segregated to the planar defects at as-deposition and diffuse out the Si substrate at $600^circ C$ . The solubility of P atoms has an influence on the rearrangement of Si atoms, which leads to the annihilation of the defects in the deposited Si thin film. The concentration of P atoms in Si thin film increased proportionally with temperature and vacancy generation in Si thin films was promoted by a P solubility increase, and the planar defects were eliminated by Si self-diffusion through the vacancies. Third, the crystallization and planar defect generation/annihilation behavior of P-doped Si were investigated by real time in-situ TEM. Nucleation is taking place primarily at the a-$Si/SiO_2$ interface by interface stress at low temperature, while nucleation on the top surface generates at high temperature due to high concentration of P on top surface. In addition, by studying for defect annihilation behavior of P-doped a-Si on Si substrate with imperfect surface state, the planar defects started to vanish at $750^circ C$ . In comparison to the case of undoped poly whose planar defects start to remove at above approximately $1000^circ C$ , the defect annihilation of P-doped Si at low temperature can be explained to be associated with P segregation at the top surface by P diffusion during heat treatment. Therefore, we suggest that the high concentration of P at top surface enhances the Si self diffusion at that area, which lead to activate Si atom movement and finally, annihilate planar defects.