Hele Väinölä

Experimental and theoretical study of heterogeneous iron precipitation in silicon

Heterogeneous iron precipitation in silicon was studied experimentally by measuring the gettering efficiency of oxide precipitate density of 1×1010cm−3. The wafers were contaminated with varying iron concentrations, and the gettering efficiency was studied using isothermal annealing in the temperature range from 300to780°C. It was found that iron precipitation obeys the so-called s-curve behavior: if iron precipitation occurs, nearly all iron is gettered. For example, after 30min annealing at 700°C, the highest initial iron concentration of 8×1013cm−3 drops to 3×1012cm−3, where as two lower initial iron concentrations of 5×1012 and 2×1013cm−3 remain nearly constant. This means that the level of supersaturation plays a significant role in the final gettering efficiency, and a rather high level of supersaturation is required before iron precipitation occurs at all. In addition, a model is presented for the growth and dissolution of iron precipitates at oxygen-related defects in silicon during thermal proces...

Sensitive Copper Detection in P-type CZ Silicon using μPCD

12 cm 23 can be detected by this method. It is demonstrated that positive corona charge can be used to prevent out-diffusion of interstitial copper, while negative charge enables copper to freely diffuse to the wafer surfaces. It was observed that the precipitation rate of copper increased significantly when the bias-light intensity is raised above a certain critical level. In addition, the copper precipitation rate was discovered to be much higher in samples which have internal gettering sites. These findings suggest that (i) high intensity light reduces the electrostatic repulsion between positively charged interstitial copper ions and copper precipitates enabling copper to precipitate in the wafer bulk even at a low concentration level, and ( ii) during high intensity illumination, oxygen precipitates provide effective heterogeneous nucleation sites for copper.

Quantitative copper measurement in oxidized p-type silicon wafers using microwave photoconductivity decay

We propose a method to measure trace copper contamination in p-type silicon using the microwave photoconductivity decay (μ-PCD) technique. The method is based on the precipitation of interstitial copper, activated by high-intensity light, which results in enhanced minority carrier recombination activity. We show that there is a quantitative correlation between the enhanced recombination rate and the Cu concentration by comparing μ-PCD measurements with transient ion drift and total reflection x-ray fluorescence measurements. The results indicate that the method is capable of measuring Cu concentrations down to 1010cm−3. There are no limitations to wafer storage time if corona charge is used on the oxidized wafer surfaces as the charge prevents copper outdiffusion. We briefly discuss the role of oxide precipitates both in the copper precipitation and in the charge carrier recombination processes.

Modeling of heterogeneous precipitation of iron in silicon

A model is presented for the growth and dissolution of iron precipitates at oxygen-related defects in silicon during thermal processing. The heterogeneous nucleation of iron is taken into account by special growth and dissolution rates, which are inserted into a set of modified chemical rate equations. This approach allows us to calculate the size distribution of iron precipitates and the residual iron concentration. By comparing the simulated results with experimental ones, it is proven that this model can be used to estimate the internal gettering efficiency of iron under a variety of processing conditions.

Gettering in silicon-on-insulator wafers: experimental studies and modelling

Buried oxide, which separates the device area from the substrate in silicon-on-insulator (SOI) wafers, forms a diffusion barrier for transition metals in silicon. The impact of this barrier on the efficiency of traditional gettering techniques for iron and copper is evaluated using computer modelling. Several parameters essential for the modelling, such as the diffusivity of iron in SiO2 and the segregation coefficient of iron and copper in SiO2, are verified experimentally. It is found that all available data for the diffusivity of iron in SiO2 (including data points from the literature and our own value of 1.4 × 10−13 cm s−2 at 1100 °C) could be fitted by the equation D(Fe in SiO2) = 2.2 × 10−2 × exp(−3.05 eV/kBT)(cm2 s−1). The solubility of iron in silicon dioxide was found to be 4.5 times to 5.5 times less than that in silicon at temperatures from 1020 °C to 1100 °C, which indicates that iron does not segregate in SiO2. The solubility of Cu in silicon dioxide was determined to be half of that in silicon at 1150 °C and 3.3 times higher than in silicon at 690 °C. Modelling of gettering using these parameters revealed that buried oxide prevents iron from diffusing to gettering sites in the substrate at typical processing temperatures, thus rendering the substrate gettering techniques inefficient. On the other hand, the diffusion barrier protects the device area from contamination from the backside of the wafer. Copper has sufficiently high diffusivity in SiO2 to diffuse through the buried oxide within a short time at 1000 °C; however, it may be difficult to remove copper from the device area because heavily doped areas of the devices could provide competitive gettering sites for copper. Possible gettering strategies for the SOI wafers are discussed.

Thermal stability of internal gettering of iron in silicon and its impact on optimization of gettering

The redissolution behavior of gettered iron was studied in p-type Czochralski-grown silicon with a doping level of 2.5×1014 cm−3 and an oxide precipitate density of 5×109 cm−3. The concentrations of interstitial iron and iron–boron pairs were measured by deep level transient spectroscopy. It was found that the dependence of redissolved iron concentration on annealing time can be fitted by the function C(t)=C0[1−exp(−t/τdiss)], and the dissolution rate τdiss−1 has an Arrhenius-type temperature dependence of τdiss−1=4.01×104×exp[−(1.47±0.10) eV/kBT] s−1. Based on this empirical equation, we predict how stable the gettered iron is during different annealing sequences and discuss implications for optimization of internal gettering.

Detection of low-level copper contamination in p-type silicon by means of microwave photoconductive decay measurements

In order to achieve a better understanding of the behaviour of copper in p-type silicon, studies of the recombination of copper were carried out by the microwave photoconductive decay measurement method (μ PCD) using high-intensity bias light. It was observed that in the presence of small oxygen precipitates, high-intensity light could be used to activate precipitation of interstitial copper. It is suggested that high-intensity light changes the charge state of interstitial copper from positive to neutral, which enhances the precipitation. The precipitation follows Hams kinetics and results in an increase in the recombination rate, which is detectable even with very low copper concentrations. This phenomenon can be used to detect low levels of copper contamination by the μ PCD method. In addition, it was observed that out-diffusion as well as in-diffusion of interstitial copper could be affected by an external corona charge. Thus, it is suggested that copper atoms do not form stable bonds at the Si–SiO2 interface after out-diffusion from bulk silicon.

Modeling and Optimization of Internal Gettering of Iron in Silicon

We present a model for heterogeneous precipitation of iron in silicon. In the model Fokker-Planck Equation is used to simulate the evolution of size distribution of iron precipitates. From the simulation results we may conclude, that in case of low levels of initial iron concentration (<1×1012 cm-3), internal gettering is difficult to achieve just by cooling. The low level of initial iron concentration can be gettered by using an additional nucleation step, which can be just a fast ramp to room temperature, before isothermal gettering anneal. We also analyze the effect of competitive gettering on the final iron concentration and the iron precipitate density profile. We found that internal gettering can reduce iron concentration and the particular advantage is the reduction of the iron precipitate density in the device layer. The iron precipitation in the device layer can also be reduced by increasing the doping concentration as the segregation increases.

Experimental Study of Internal Gettering Efficiency of Iron in Silicon

We have studied internal gettering efficiency of iron in silicon by Deep Level Transient Spectroscopy (DLTS) and standard lifetime – methods (SPV, PCD). Conventional high–low–high anneals were performed to produce a series of wafers with varying denuded zone (DZ) width and oxygen precipitation density. The wafers were intentionally iron contaminated to a level of about 3–5 ∗ 1013 cm−3. After contamination the wafers were annealed at 900 ◦ C and then slowly cooled to 850, 800, 750, 700 or 600 ◦ C. After cooling the remaining interstitial iron concentration was measured by SPV, -PCD and DLTS. The experimental results are compared with simulations. Our results indicate that with this contamination level, the gettering is effective only at temperatures below 750 ◦ C when iron is supersaturated over a factor of twenty. For temperatures above 750 ◦ C the gettering is limited by iron precipitation in the bulk.

Light Activated Copper Defects in P-Type Silicon Studied by PCD

We have studied copper defects in p-type silicon by measuring its precipitation kinetics by means of the microwave photoconductive decay (µPCD) technique. Copper atoms precipitated during high intensity light treatment at room temperature. We used the total reflection X-ray fluorescence (TXRF) and the transient ion drift (TID) techniques to determine the bulk concentration of copper. We estimated the density and the radius of the copper precipitates as well as the average capture cross-section for precipitated copper atoms from the measured copper precipitation time constant, bulk concentration of copper, and the change in the recombination rate. We also studied how the density of oxygen defect affects the copper precipitation. Our results show that copper precipitates at two different kinds of defects.