Specific features of the segregation-related redistribution of phosphorus during thermal oxidation of heavily doped silicon layers

A model of the diffusion-segregation redistribution of phosphorus in an SiO₂/Si system during thermal oxidation of highly doped silicon layers is developed taking into account the formation of a peak of surface impurity concentration at the interface. The formation of this surface concentration peak is attributed to a change in the free energy of the impurity atoms near the silicon surface. This process is simulated by a diffusion-segregation equation. It is shown that the developed diffusion-segregation model is quite adequate for describing the phosphorus redistribution occurring during the oxidation of uniformly doped silicon layers. For the oxidation of implanted silicon layers, it was found that the segregation coefficient of the phosphorus at the SiO₂/Si interface is not constant but depends on time in the same way as the efficiency of transient enhanced diffusion in silicon. This phenomenon is explained by the reactivity of the impurity segregation during the thermal oxidation of silicon, when excess point defects in the implanted silicon layer affect both the oxidation process and the capture of impurity atoms by the growing silicon dioxide.     

Reverse current instabilities in amorphous silicon Schottky diodes:modeling and experiments

A new model for high bias transport is reported which describes the time-dependent reverse current variations in amorphous silicon Schottky diodes. This phenomenon is of practical importance in the design and optimization of pixels for large-area optical and X-ray imaging. In the model, the main components of the reverse current, namely thermionic emission and tunneling, are both affected by the electric field at the metal/amorphous silicon interface. Time-dependent variations in this electric field arise due to the release of charges trapped in defect states in the depletion region and to charge trapping at the interface. This effect is analyzed using the approximation that the tunneling component of the current is equivalent to a lowering of the potential barrier at the interface. The calculated time-dependent reverse current is compared with the measured data     

A numerical study of field-aided diffusion

A numerical method for determining the effect of the internal electric field on the diffusion of impurity ions in semiconductors is described. The model of the diffusion process is defined by the flux equations, the continuity equations and Gauss' law. Due to the complexity of this model, there is no known analytical solution. However, the system of nonlinear equations is solved numerically using an iteration technique.Diffusion profiles are presented for boron diffusing in silicon at 1100°C for various values of surface concentration C₀. These profiles are compared to the complementary error function which is the correct solution neglecting the internal electric field. The impurity profiles are also compared to those obtained by solving an approximate diffusion equation derived by using the concept of a concentration dependent “effective” diffusion coefficient.It is shown that the influence of the electric field on the motion of the impurities is a strong function of the surface concentration. For high values of C₀, the impurity profile becomes exponential. The accuracy of the solutions obtained by solving the approximate diffusion equation depends on both the value of C₀ and the diffusion time.     

A model of high-and low-temperature phosphorus diffusion in silicon by a dual pair mechanism

A model of phosphorus diffusion in silicon was developed on the basis of a dual pair mechanism; according to this model, the contribution of the impurity-vacancy (PV) and impurity-self-interstitial (PI) pairs to diffusion is accounted for directly in terms of the phosphorus diffusion coefficient. A violation of thermodynamic equilibrium in relation to native point defects occurs as a result of diffusion of the PI neutral pairs. At the high-temperature diffusion stage, the phosphorus diffusion is described by a single diffusion equation with the diffusion coefficient dependent on both the local and surface phosphorus concentrations; whereas at the next (occurring at lower temperatures) stage, the phosphorus diffusion is described by two diffusion equations for the total concentrations of the components containing phosphorus and self-interstitials. An anomalously high rate of the low-temperature diffusion is ensured by excess self-interstitials accumulated in the doped layer during the preceding high-temperature diffusion. The model makes it possible to quantitatively account for the special features of the phosphorus diffusion in a wide range of the surface concentrations at both the high (900–1100°C) and lower (500–700°C) temperatures.     

界面电荷耐压模型:SOI高压器件纵向耐压新理论

基于求解二维Po isson方程,分析了具有埋氧层界面电荷的SO I结构纵向击穿特性,提出了界面电荷耐压模型。该模型通过埋氧层界面电荷来调制硅层和埋氧层电场,获得极高击穿电压。进一步提出临界界面电荷面密度概念,给出其工程化应用的近似公式。并对文献中的不同结构SO I器件的纵向耐压进行计算。解析结果和试验结果或M ED IC I仿真结果吻合良好。     

Silicon nitride as a mask in phosphorus diffusion

By means of tracer and activation experiments it is shown that during phosphorus diffusion in silicon nitride diffusion profiles are produced, which are very similar to the profiles in silicon dioxide processed in the same experiment. When oxygen was used as part of the carrier gas as is customary in semiconductor technology, a glass was produced in both cases which consisted of SiO₂ and P₂O₅ and whose identity was proved by i.r. absorption and determination of the refractive index. When the medium does not contain oxygen, part of the nitride is transformed to silicon phosphide which is incorporated in the glass or escapes at higher temperatures. When oxygen is present, the silicon phosphide occurs briefly in an intermediate reaction. In this way the normal, slow oxidation of the nitride is accelerated and phosphorus pentoxide acts as a catalyst. The analogous formation of glass from oxide and nitride is the reason why silicon nitride exhibits a similar masking effect as silicon dioxide in phosphorus diffusion, even though it masks better than silicon dioxide against the diffusion of a number of other elements.     

Optical effects during rapid thermal diffusion

The formation of n⁺-p or n⁺-p-p⁺ junctions by rapid thermal diffusion of phosphorus or co-diffusion of phosphorus and aluminum into silicon is opening new possibilities for low-cost and environmentally safe solar cell production. In this work, we analyze the influence of the higher energetic part of the lamp spectrum on phosphorus diffusion, and the impact of evaporated aluminum for back surface field formation during a P-Al co-diffusion step. The diffusion of phosphorus from doped glass films spun onto monocrystalline silicon material in various furnace configurations with front, back, or double sided heating is studied to investigate the influence of the radiation spectra on the dopant profiles. The experiments reveal a relation of the dopant profile to the amount of ultraviolet radiation reaching the surface. Therefore, a modified RTP-System is used for further investigations to demonstrate the influence of the ultraviolet (UV) light on the diffusion profiles. These experiments clearly show that the influence of the UV light is mainly on the densification of the spin-on-glass and not on diffusion kinetics in silicon. Furthermore, the simultaneous formation of a back surface field is of special interest for solar cells. Earlier studies of the simultaneous diffusion of phosphorus and aluminum in order to form a n⁺-p-p⁺ structure show (compared to a single phosphorus diffusion) deeper n⁺ emitters. Using glass densification experiments on such samples, a correlation was found between the decrease in emissivity on the aluminum-coated part of the wafer and the increase in temperature, which seems to be responsible for the deeper profiles.     

埋氧层固定界面电荷对RESURF SOI功率器件击穿特性的影响

通过求解具有界面电荷边界条件的二维泊松方程，建立了埋氧层固定界面电荷Qf对RESURF SOI功率器件二维电场和电势分布影响的解析模型。解析结果与半导体器件模拟器MEDICI数值分析结果相吻合。在此基础上，分别研究了Qf对RESURF SOI功率器件横向和纵向击穿特性的影响规律。在横向，讨论了不同硅膜厚度、氧层厚度和漂移区长度情况下Qf对表面电场分布的影响；在纵向，通过分析硅膜内的场和势的分布，提出了临界埋氧层固定界面电荷密度的概念，这是导致器件发生失效的最低界面电荷密度。     

Generation kinetics of oxide charges and surface states during oxidation of silicon

A simple model to account for the generation kinetics of oxide charges and surface states during oxidation of silicon is presented. In the model, oxide charges and surface states are generated by a reaction between silicon and oxygen within the mono-molecular layer of the Si/SiO₂ interface and are thermally annealed independently of this reaction. A two step oxidation technique was used to investigate the electronic structure of the Si/SiO₂ interface and it was found that centers of fast surface states are located within 1.4–2.0 Å of the interface and that the centroid of oxide charges is located at about 8 Å from the interface. These results agreed with the model.     

The kinetics of open tube phosphorus diffusion in silicon

Using a novel liquid-phase dilution technique, open-tube diffusion of radiophosphorus into silicon was carried out for the first time under intrinsic oxide non-accumulation conditions. The existence of the surface-barrier rate control of the deposition process in the open-tube technique was established from the concentration profiles. An in-depth analysis of the vapor-solid partition coefficient of the phosphorus-silicon system showed the dramatic contribution of oxygen in the incorporation of phosphorus into silicon in the open-tube process. The fitted deposition and redistribution profiles of phosphorus in silicon supported the concept of equivalent diffusion kinetics in both the deposition and redistribution steps. The present study also supported the Kennedy-Murley redistribution kinetics model quantitatively.     

Reduction of the parasitic charge generation during silicon nitride deposition in a LOCOS isolation without field implant

MOS integrated circuits use the Local oxidation of silicon to isolate laterally adjacent devices (LOCOS isolation). The insulation structure is typically formed by a semiconductor region doped by ion implantation (field implant) and covered by a thick thermal oxide (field oxide). Other insulators (plasma enhanced chemical vapor deposited (PECVD) silicon oxides and LPCVD silicon nitride) and metal interconnection are subsequently deposited on the field oxide. The ion implant together with the thick insulator ensure a high threshold voltage value of the parasitic MOS transistor formed by source and drain of the adjacent active devices and by the insulator/interconnection gate.However, economical purpose leads to the extension of the application field of lower cost technology, addressing the problem of LOCOS isolation without any field implant. As already shown in a previous work [Fay JL, Beluch J, Allirand L, Brosset D, Despax B, Bafleur M, Sarrabayrose G. Jpn J Appl Phys 38(9A):5012–7] for inter-layer dielectric applications, our PECVD oxides suffer from excessive concentration of fixed positive charges brought about by the silicon nitride deposition, and causing the N-channel field threshold voltage to decrease.Characterization reveals that these charges are generated by diffusion of species coming from the gas phase during the silicon nitride process. These generated charges can be reduced either by increasing the O₂/tetra-ethyl orthosilicate ratio or by doping the oxide with boron and phosphorus. To avoid diffusion and generation of charges, we minimized the thermal budget using a PECVD silicon nitride. With this process, we have achieved a high threshold voltage and an acceptably low leakage current of the NMOS parasitic transistor.     

Phosphorus Removal from Silicon by Vacuum Refining and Directional Solidification

Silicon is widely used as a raw material for production of solar cells. As a major impurity in silicon, phosphorus must be removed to 1 × 10^?5 wt.%. In the present study, based on the distribution of phosphorus in a silicon ingot obtained by vacuum refining and directional solidification, the mechanism for removal of phosphorus from silicon is investigated. The results show that the distribution is controlled not only by segregation at the solid–liquid interface but also by evaporation at the gas–liquid interface, showing some deviation from Scheil’s equation. A modified model which considers both segregation and evaporation is used to simulate the distribution, matching quite well with the experimental results. The temperature and solidification rate are two important parameters that affect the overall mass transfer coefficient and the effective segregation coefficient and thus the distribution of phosphorus. A high removal efficiency and a homogeneous distribution can be obtained by adjusting these two parameters.     

Effect of phosphorus diffusion to the recombination at the Si–SiO2 interface

Measurements of the excess carrier lifetime of diffused and undiffused, thermally oxidized silicon samples are used to show that the presence of a phosphorus diffusion results in a modification of the interface defect properties, resulting in significantly higher surface recombination velocity compared to undiffused samples. In addition, for undiffused samples, positive and negative charges are demonstrated to be equally effective at passivating the silicon surface. Both results hold for (100) and (111) oriented samples, as well as for samples subjected to various post‐oxidation treatments. The results may have practical implications particularly for the design of rear contacted solar cells. Copyright © 2008 John Wiley & Sons, Ltd.     

Tracer investigations on the diffusion of phosphorus from doped oxide sources

Tracer investigations on the diffusion of phosphorus from phosphorus silicate glass (PSG) are described in this paper. Concentration profiles in the silicon and the PSG are given. Investigations were made also on the interaction between PSG and silicon dioxide and between PSG and silicon nitride. The shape of the concentration profiles obtained in the system PSG/silicon dioxide and the loss of phosphorus to the ambient depend on the phosphorus concentration in the PSG. From annealing experiments with PSG/silicon nitride layers it could be derived that phosphorus escapes from the PSG with high phosphorus concentration in the form of the pentoxide.     

Model of doped-oxide-source diffusion in silicon

A new theory of the doped oxide diffusion technique in silicon has been developed. Compared to the existing theories, it uses fore physically realistic boundary conditions. The theory was satisfactorily evaluated with radio phosphorus diffusion into (111) silicon. Between 1100 and 1275°C the transfer of phosphorus into silicon was found to be controlled by a surface barrier process and not by segregation between the oxide and silicon. The kinetics of the interfacial conductance process has been shown to involve chemically transformed elemental dopant and a probable vacancy-related defect. Phosphorus concentration profiles, controlled by its diffusivity in the mixed oxide, were processed to produce surface-induced intrinsic diffusivities. In contrast to the single defect-controlled diffusion processes known in most materials, it is seen that in silicon, under non-oxidizing conditions, the nature of the defect responsible for diffusion of dopants depends on the process employed. Technologically important quantities of surface concentration, junction depth and profile shape by the doped oxide process can bow be predicted accurately for the first time.     

Time-dependent MOS breakdown

A general model for time-dependent breakdown in metal-oxide-silicon (MOS) structures is developed and related to experimental measurements on samples deliberately contaminated with Na. A statistical method is used for measuring the breakdown probability as a function of log time and applied field. It is shown that three time regions of breakdown can be explained respectively in terms of silicon surface defects, ion emission from the metal interface, and lateral ion diffusion at the silicon interface.     

Two-dimensional implications of a purely reactive model for plasmaetching

A model for plasma etching of silicon by SF₆ at low ion energy is presented. It is shown how this model, which excludes sputtering effects, allows the simulation of most observed two-dimensional effects (dovetail, field effect, barreling, trench bowing). Ion scattering is ignored and only the equilibrium between adsorption, desorption, and surface diffusion is computed, leading to a differential equation with only three independent parameters. The influence of these parameters on trench shapes is discussed. The role of adspecies diffusion on surfaces is emphasized and the possible extension to materials other than silicon and other other process conditions is considered     

Effect of grain microstructure on P diffusion in polycrystalline-on-single crystal silicon systems

Grain microstructure has a major impact on diffusion of P in polysilicon-on-single crystal silicon systems both within the polysilicon layer and inside the single crystal silicon substrate. During annealing, P diffuses very rapidly along grain boundaries in the polysilicon layer to the interface, where it undergoes very fast diffusion laterally along the polysilicon-single crystal silicon interface, followed subsequently by slow indiffusion into the underlying substrate. However, the extrapolated Secondary Ion Mass Spectrometry profiles for P reveal a discontinuity at the interface, which is caused by anomalous diffusion behavior similar to the well known “kink” effect observed in single crystal silicon during P diffusion. The high diffusivity tail region is also much less pronounced for polysilicon-on-silicon systems compared to single crystal silicon due to a reduction of interstitial supersaturation in the substrate. This reduction is believed to result from the absorption of interstitials by the grain boundaries which act as sinks for the excess interstitials.     

A 3‐in‐1 doping process for interdigitated back contact solar cells exploiting the understanding of co‐diffused dopant profiles by use of PECVD borosilicate glass in a phosphorus diffusion

Boron and phosphorus doping of crystalline silicon using a borosilicate glass (BSG) layer from plasma‐enhanced chemical vapor deposition (PECVD) and phosphorus oxychloride diffusion, respectively, is investigated. More specifically, the simultaneous and interacting diffusion of both elements through the BSG layer into the silicon substrate is characterized in depth. We show that an overlying BSG layer does not prevent the formation of a phosphorus emitter in silicon substrates during phosphorus diffusion. In fact, a BSG layer can even enhance the uptake of phosphorus into a silicon substrate compared with a bare substrate. From the understanding of the joint diffusion of boron and phosphorus through a BSG layer into a silicon substrate, a model is developed to illustrate the correlation of the concentration‐dependent diffusivities and the emerging diffusion profiles of boron and phosphorus. Here, the in‐diffusion of the dopants during diverse doping processes is reproduced by the use of known concentration dependences of the diffusivities in an integrated model. The simulated processes include a BSG drive‐in step in an inert and in a phosphorus‐containing atmosphere. Based on these findings, a PECVD BSG/capping layer structure is developed, which forms three different n⁺⁺−, n⁺− and p⁺−doped regions during one single high temperature process. Such engineered structure can be used to produce back contact solar cells. Copyright © 2016 John Wiley & Sons, Ltd.     

Simulation of near-surface proton-stimulated diffusion of boron in silicon

A quantitative model for near-surface redistribution of doping impurity in silicon in the course of proton-stimulated diffusion is developed for the first time. According to the model, the near-surface peak of the impurity concentration is caused by migration of neutral impurity—self-interstitial pairs to the surface with subsequent decomposition of these pairs and accumulation of the impurity at the silicon surface within a thin layer (referred to as δ-doped layer). The depletion and enhancement regions that are found deeper than the near-surface concentration peak are caused by expulsion of ionized impurity by an electric field from the near-surface region of the field penetration. The field appears due to the charge formed in the natural-oxide film at the silicon surface as a result of irradiation with protons. The diffusion-kinetic equations for the impurity, self-interstitials, vacancies, and impurity—self-interstitial pairs were solved numerically simultaneously with the Poisson equation. It is shown that the results of calculations are in quantitative agreement with experimental data on the proton-stimulated diffusion of boron impurity in the near-surface region of silicon.