Research On High - Resistivity Epitaxial Ultra - Low Capacitor TVS With Double - Deep Groove

Transient Voltage Suppressor (TVS) is a semiconductor device used to protect circuits from electrostatic discharge, electrical fast transient and surge.TVS diodes has many advantages, such as low clamp voltage coefficient,small size, quick responce,low leakage current,high reliability, etc. It is widely used in surge protection. But for the surge protection of high frequency circuit, low capacitance or ultra low capacitance TVS (LCTVS) is needed to preserve signal integrity.LCTVS is a combination of low breakdown voltage (VBR) avalanche diode and low capacitance steering diodes.The steering diodes are PIN diodes to get low capacitance. The low VBR avalanche diode needs heavily doped Si,while the I layer of PIN diodes needs very low doping concentration, then it is necessary to deposit N type EPI with high resistivity on heavily doped P type substrate, and let N type dopants diffuse through N type EPI into P type substrate to form an avalanche diode, which needs very high thermal budget.And the diodes are isolated by PN junctions,which occupy large area of the chip. The N type EPI resistance of traditional LCTVS is not high enough, so gold diffusion is needed to increase the effective EPI resistance.The purpose of this thesis is to achieve high density ultra low capacitance TVS by dual deep trenches and high resistance EPI processes, so as to improve the performance of traditional LCTVS.The studies of this thesis included:(1)Auto doping suppression by back sealing and N type buffer EPI layer to get high enough resistivity N type EPI layer on heavily doped P type substrate, so that the gold diffusion is not needed. (2)Diodes isolation the by deep trench instead of PN junctions. (3)Avalanche diode formed by deep trench on P type substrate. (4) Improvement of the capacitance under bias by electron irradiation.The conclusions of this thesis are:(1)It is an effective way to suppress the auto doping of P type substrate by back sealing and N type buffer EPI layer to achieve high resistivity N type EPI layer on heavily doped P type substrate.(2)The stress between the in-situ doped polycrystalline silicon is one of the main causes of avalanche diode’s leakage current,and high temperature anneal can reduce the leakage current dramatically.And the optimized denuding process can improve the leakage current,too.(3)The capacitance-voltage characteristic of LCTVS mainly depends on the leakage current of the avalanche diode and minority carrier lifetime in the EPI layer. Electron irradiation is an effective way to reduce minority carrier lifetime, so as to improve the capacitance-voltage characteristic of LCTVS dramatically.High density ultra low capacitance TVS is achieved by dual deep trenches and high resistance EPI processes through the study of this thesis. Compared with traditional LCTVS, the high density ultra low capacitance TVS by this process has significant advantage,the chip size can be shrunk by 50%. It will inevitably find wide applications in the field of high frequency circuit protection.