SUN Xinli, GUO Hui, ZHANG Yuming, GUO Bingjian, LI Xingpeng, CAO Zhen

( 1.State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology, School of Microelectronics, Xidian University, Xi’an 710071, China; 2. Xi’an Zhongjing Semiconductor Materials Co., Ltd, Xi’an 710071, China)

Abstract: The effects of initial oxygen concentration on the reverse leakage current of PIN rectifier diodes were studied. We fabricated the PIN rectifier diodes with different initial oxygen concentrations, and analyzed the electrical properties, anisotropic preferred etching by means of optical microscopy, Fourier transform infrared spectroscopy and transmission electron microscopy. It is pointed out that the reverse leakage current increases exponentially with the increasing initial oxygen concentration. Furtherly, we researched and analyzed the mechanism of the effects of initial oxygen concentration on the reverse leakage current of PIN rectifier diode. It is shown that the oxygen precipitations present in an "S" curve with increasing initial oxygen concentration after high temperature diffusion. The main reason is that the nucleation and growth of oxygen precipitation at high temperature induce bulk oxidation-induced defects (B-OSF), which are mainly dislocations,and a small amount of rod stacking faults. The density of B-OSF increases with the increasing initial oxygen concentration. The existence of B-OSF has great effects on the reverse leakage current of PIN rectifier diode.

Key words: oxygen concentration; PIN rectifier diode; induced defect; reverse leakage current

1 Introduction

Integrated circuits and semiconductor power devices are two widely used electronic components.Semiconductor power devices are the core components of power electronic systems. Power diodes are important semiconductor power devices. Silicon-based PIN power diodes are widely used[1,2]in household appliances, industrial electronic systems, automobiles,power locomotive electronic systems, smart grids,ships, and airplanes due to their high voltage resistance,high current, and low cost.

Impurity oxygen is an important impurity in monocrystalline silicon[3,4]. In an integrated circuit,impurity oxygen can improve the mechanical strength and inhibit the high-temperature deformation of the silicon wafer[5-7]. Oxygen precipitations can induce secondary defects[8], attract metal impurities, and generate high-quality surface denude zones (DZ) in silicon wafers[9]. The internal structure and processing route of PIN power diodes are completely different from those of integrated circuits. Integrated circuits are generally fabricated on epitaxial layers or silicon wafer surface layers. A PIN power diode is an axial structure,which works based onp-type andn-type diffusion on both sides of a silicon wafer. Monocrystalline silicon has a great impact on the electrical performance of PIN power diodes. With the development of different integration and miniaturization techniques for power conversion systems, the size of PIN power diodes is becoming smaller; thus, the effects of impurities and defects on the electrical properties of PIN power diodes have become prominent. Therefore, it is necessary to study the effects of impurity oxygen on the electrical performance of PIN power diodes, especially on the reverse leakage current. The present paper focused on the effects of different initial oxygen concentrations on the reverse leakage current of a PIN rectifier diode. We here studied the effects of initial oxygen concentration on the intrinsic-region-induced defects and the patterns of oxygen impurity in detail. We here expound upon the mechanism by which initial oxygen concentration affects reverse leakage current of the PIN rectifier diode with a focus on the micro-mechanism, which is of great significance to the research into PIN rectifier diodes.

2 Experimental

2.1 Samples

An N-type phosphorus-doped ＜111＞ monocrystalline silicon ingot of 100-mm diameter was cut into silicon wafers with different oxygen concentrations,which were grown by Czochralski method under different conditions of oxygen control. In order to minimize the effects of impurity carbon, impurity metal, and as-grown swirl defects on the experiment,the monocrystalline silicon ingot with lower carbon content and heavy metal concentration and without as-grown swirl defects was selected to prepare test samples (Table 1).

Table 1 Wafer properties

Oxygen and carbon concentrations in the samples were measured according to the ASTM F 1188:1993 standard, which was based on the test principle that the interstitial oxygen concentration of silicon was calculated by using the absorption coefficient of oxygen in silicon absorption band at 1 107 cm-1of Fourier infrared light. The samples were cut into 340 μm thick slices and 2 mm thick plates. The 2 mm thick plates were polished with a mixed solution of HNO3and HF.The 340 μm thick slices were chamfered and ground to 290 ± 5 μm thick surfaces after being heat-treated at 650 ℃.

The production process of the PIN rectifier diode is illustrated in Fig.1. Each group of silicon wafers was ground on both sides, and four pieces from each group were put on the PIN rectifier diode. The size of the PIN rectifier diode on the silicon wafer was 1.143 mm × 1.143 mm. In order to analyze the effects of impurity oxygen on the high-temperature performance of the diode, the diffusion process of the 2-mm samples during phosphorus pre-sintering (600 ℃, 2 h) and boron and phosphorus diffusion (1 260 ℃, 23 h) were studied.

Fig.1 Flowchart of the PIN rectifier diode production process

2.2 Test equipments

The thickness and resistivity of the resultant silicon wafers were measured by a JXNRT-1 noncontact thickness tester and a SDY-4 four-probe tester, respectively. The initial oxygen and carbon contents in monocrystalline silicon were measured by a Perkin Elmer PE-C99158 Fourier-transform infrared(FTIR) spectrometer. The minority carrier lifetime of monocrystalline silicon was measured by a LT-100C digital minority carrier lifetime tester. A silicon PT-301 point tester and a SD-DS-600S tester were used to evaluate the electrical properties of the wafers (VB[0-1800 V],VF[I = 1 A], and reverse leakage current [VB=-1 100 V, 0-5 μA]). An Agilent B1505A power device analyzer/curve tracker was used to detect the IV curve of the packaged die, and a JEM-2100F transmission electron microscope (TEM) was employed to observe oxygen precipitations and induced defects. All the above tests were executed at 23 ℃.

3 Results and discussion

3.1 Effects of oxygen concentrations on the reverse leakage current of the PIN rectifier diode

It is noticeable in Fig.2 that the reverse leakage current of the diode changed in two stages with increasing initial oxygen concentration. In the first stage, when the initial oxygen concentration was lower than 8.1 × 1017at/cm3, the reverse leakage current was smaller (0.08-0.1 μA) and did not change greatly with the increase of the initial oxygen concentration.When the initial oxygen concentration was higher than 8.1 × 1017at/cm3, the reverse leakage current grew exponentially between 0.1-0.6 μA. In addition,the forward voltage drop (VF) varied from 1.17 V to 1.21 V with increasing initial oxygen concentration.Furthermore, the reverse breakdown characteristics of the diode were compared under different reverse leakage currents. It is evident from Fig.3 that the increase of the reverse leakage current at the breakdown voltage made the characteristics of switch cut-off worse and led to soft breakdown characteristics.

Fig.2 Relationship between reverse leakage current and oxygen concentration at the backward voltage of -1 100 V

Fig.3 IV curves normalization of PIN rectifier diodes with different reverse leakage current at the backward voltage of-1 100 V

3.2 Analysis of induced defects in the intrinsic region of the PIN rectifier diode

In order to further analyze the effects of initial oxygen concentration on the leakage current of the PIN rectifier diode, glass passivation was carried out to remove the grid of the intrinsic region of the PIN rectifier diode, and subsequently, the induced defects after anisotropic preferential etching were observed. It is noticeable from Fig.4 that the density of dislocation etch pits increased rapidly with increasing initial oxygen concentration. When the oxygen concentration was high, uniformly-distributed dislocation etch pits were observed, and when the initial oxygen concentration was low, almost no dislocation etch pits were detected. These defect pits were mainly dislocations, and a small amount of stacking faults appeared when the initial oxygen concentration was higher than 1.1 × 1018at/cm3(Fig.5). As the samples were strictly selected to control the impurity carbon concentration, heavy metal impurities, and as-grown defects during crystal growth, dislocations and stacking faults were generated after the diffusion process.The “impurity piercing pipe” effect by these induced defects increased the reverse leakage current[10]. The increase of the initial oxygen concentration increased the defect density and the reverse leakage current and led to macro-electrical characteristics, such as soft breakdown.

Fig.4 Etch pit photos of oxygen precipitation-induced defects in the intrinsic region at the initial oxygen concentration of (a) 1.29 × 1018 at/cm3, (b) 1.1 × 1018 at/cm3, (c) 8.1 ×1017 at/cm3, and (d) 6.6 × 1017 at/cm3 under 100× optical microscopic resolution, respectively

Fig.5 Dislocation etch pits of B-OSF (a) and stacking fault corrosion pits of B-OSF (b) under 400× optical microscopic resolution

3.3 Microscopic mechanism analysis of the effects of oxygen concentrations on the reverse leakage current of the PIN rectifier diode

Impurity oxygen atoms existed in the form of interstitial states in as-grown monocrystalline silicon.The Fourier-transform infrared absorption peaks at 1 107 cm-1, 1 203 cm-1, and 515 cm-1appeared from the normal vibration of chemical bonds between interstitial oxygen atoms and two adjacent silicon atoms[11-13],and the absorption peak at 1 107 cm-1was used as the measurement peak[14]. After different thermal treatment stages, the concentration of interstitial oxygen atoms in monocrystalline silicon changed and a part of these atoms was converted into oxygen precipitations;thus, the absorption peak of interstitial oxygen atoms decreased. The difference in the absorption peaks of interstitial oxygen atoms at 1 107 cm-1was compared before and after heat-treatment diffusion (600 ℃ for 2 h + 1 260 ℃ for 23 h). It is clear from Figure 6 that the absorption peaks at 1 107 cm-1and 515 cm-1decreased greatly after high-temperature diffusion(1 260 ℃ for 23 h), and their decrease rates were similar. However, the absorption peak at 1 225 cm-1corresponding to amorphous oxygen precipitation(SiOx;x＜ 2) did not change significantly. Therefore,the reduction of interstitial oxygen atoms did not form amorphous oxygen precipitation; however, they existed in the form of stable oxygen precipitation. The relationship between initial oxygen concentration and oxygen precipitation is presented in Fig.7. When the initial oxygen concentration was lower than 8.1 × 1017at/cm3, little or no oxygen precipitation was detected.When the initial oxygen concentration was between 8.1 × 1017at/cm3and 1.1 × 1018at/cm3, the amount of oxygen precipitation increased rapidly with increasing initial oxygen concentration. When the initial oxygen concentration was higher than 1.1 × 1018at/cm3, the amount of oxygen precipitation decreased slowly with increasing initial oxygen concentration. Therefore, the amount of oxygen precipitation in the PIN rectifier diode increased gradually with increasing initial oxygen concentration after high-temperature diffusion(1 260 ℃ for 23 h), and this changing trend followed a“s” curve. This is consistent with Chiou’s findings on the relationship between initial oxygen concentration and oxygen precipitation[15].

Fig.6 Comparison of FTIR spectra before and after diffusion

Fig.7 Relationship between oxygen precipitation and initial oxygen concentration after diffusion

In order to analyze the relationship between induced defects and oxygen precipitation, transmission electron microscopy (TEM) was carried out at the initial oxygen concentration of 1.29 × 1018at/cm3.Polyhedral oxygen precipitation with a particle size of about 40 nm was detected (Fig.8), which was similar to the polyhedral oxygen precipitation observed by Xu J[16]. Clear electron diffraction patterns were observed in the oxygen precipitation matrix in the [111] direction(Fig.9), indicating the existence of fewer stacking fault defects.

Fig.8 TEM image of polyhedral oxygen precipitation with a particle size of about 40 nm in the [111] direction

Fig.9 Electron diffraction patterns of the oxygen precipitated matrix in the [111] direction

Due to the absence of heterogeneous nucleation,interstitial oxygen atoms precipitated in a supersaturated form during the high-temperature diffusion process.According to the classical homogeneous nucleation rate formula proposed by Becker-Boering-Zedovich[8,17],

where,αcis a coefficient (less than 1),rcis the critical radius, 2D/α2is the frequency factor of the oxygen diffusion coefficient D,noiis the oxygen concentration,andF*is the activation energy for critical nucleation.According to Eq.(1), the nucleation rate increases monotonically with the initial oxygen concentration.Therefore, in the sample with a high initial oxygen concentration, the nucleation rate of oxygen precipitation was higher and the amount of interstitial oxygen atoms converted into oxygen precipitation was larger.

In addition, according to the critical radius formula[8,18]of classical nucleation theory, oxygen precipitations are formed at a critical radius. When the radius is larger than the critical radius, the resultant aggregates grew into oxygen precipitation, and when it is smaller than the critical radius, aggregates dissolve.

where,xis the particle volume of each oxygen atom, γ is the interfacial energy per unit area,Coxis the actual concentration of interstitial oxygen atoms,is the equilibrium concentration of interstitial oxygen atoms,δ= 0.3 represents the linear mismatch between Si and SiO2, ξ is the strain of precipitation compression,Vpis the volume of silicon atoms in the precipitation,nvis the number of absorbed vacancy atoms,nIis the number of released interstitial atoms,Cvis vacancy concentration,is vacancy equilibrium,CIis the concentration of self-interstitial atoms, andis the equilibrium concentration of self-interstitial atoms.

It is clear from Eq.(2) that the equilibrium concentration of interstitial oxygen atoms was consistent at the same temperature. The higher the initial oxygen concentration, the smaller the critical radius of oxygen precipitation and the easier the formation of oxygen precipitation. The growth process of oxygen precipitation worked under a stress. The supersaturation of interstitial oxygen (initial oxygen concentration) was the driving force for the continuous growth of oxygen precipitation. The higher the initial oxygen concentration, the greater the driving force for oxygen precipitation and the larger the oxygen precipitation. In the process of oxygen precipitation nucleation and growth, self-interstitial atoms or dislocations were released continuously to reduce the stress for oxygen precipitation, forming oxidationinduced defects, such as dislocations, stacking faults,and dislocation loops. The defect type, the defect density, the defect size, the initial oxygen concentration,the impurity concentration, and surface conditions were related to the thermal process history[19,20]. Generally,two types of oxidation-induced stacking faults (OSF)existed in the diode: surface OSF and internal OSF.Surface OSF nucleated at the exposed positions of mechanical damage, metal contamination, and microdefects (oxygen precipitation), and bulk oxidationinduced stacking faults (B-OSF) nucleated from oxygen precipitation[21].

In the PIN rectifier diode, the intrinsic region of monocrystalline silicon was located in the middle of boron and phosphorus diffusion; therefore, the induced defects in the intrinsic region were the nucleation of B-OSF in oxygen precipitation[22]. According to Eqs.(1) and (2), with the increase of the initial oxygen concentration, the critical radius of oxygen precipitation becomes smaller and the homogeneous nucleation rate increases, facilitating the formation and growth of oxygen precipitation; thus, the oxygen precipitation density increases. The higher the initial oxygen concentration, the greater the amount of oxygen precipitation, the greater the density, and the larger the volume of oxygen precipitation after diffusion. As the volume of oxygen precipitation was 2.25 times that of silicon substrate[23], the volume was gradually expanded and the volume strain energy also increased with the growth of oxygen precipitation, resulting in dislocations and other internal oxidation-induced defects (B-OSF). B-OSF could form defect energy levels and micro region impurity enrichment, which enhanced micro region conduction ability and formed an “impurity piercing pipe”[10]. Impurity piercing pipe was formed by the enrichment of impurities along the defect diffusion at high temperature, which was easy to conduct. Therefore, the impurity concentration near B-OSF was higher, the leakage current increased,the reverse cut-off characteristic became worse,and the power consumption increased, showed a soft breakdown characteristic, and the electrical performance of the device became worse.

4 Conclusions

In conclusion, the influence of different initial oxygen concentrations on the reverse leakage current of pin rectifier diodes was studied in macro and micro,and the related mechanism was analyzed. It was pointed out that the initial oxygen concentrations were between 8.1 × 1017at/cm3and 1.29 × 1019at/cm3, the reverse leakage current of PIN rectifier diodes increased exponentially in the range of 0.1 - 0.6 μA , the critical initial oxygen concentration was 8.1 × 1017at/cm3.Combined with the electrical performance testing,anisotropic preferential etching, Fourier transform infrared spectroscopy testing, TEM testing and classical nucleation theory analysis, it was pointed out that the effects mechanism of initial oxygen concentration on the reverse leakage current of PIN rectifier diode was that the nucleation radius of oxygen precipitation decreased，the growth driving force increased, which was easier to nucleate and grow, and the oxygen precipitation presented an “S” curve with increasing initial oxygen concentration after high temperature diffusion. Due to the “impurity piercing pipe” effect of oxygen precipitation-induced B-OSF in monocrystalline silicon, the reverse leakage current of the PIN rectifier diode increased, the power consumption increased and the reverse cut-off characteristics and electrical performance of the device get worse.