Corresponding author: Kirill D. Shcherbachev ( chterb@gmail.com ) © 2019 Vladimir T. Bublik, Marina I. Voronova, Kirill D. Shcherbachev, Mikhail V. Mezhennyi, Vladimir Ya. Reznik.
This is an open access article distributed under the terms of the Creative Commons Attribution License (CC BY 4.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Citation:
Bublik VT, Voronova MI, Shcherbachev KD, Mezhennyi MV, Reznik VYa (2019) Regularities of microdefect formation in silicon during heat treatment for internal getter synthesis. Modern Electronic Materials 5(3): 133-139. https://doi.org/10.3897/j.moem.5.52812
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Gettering is defined as a process by which metal impurities in the device region are reduced by localizing them in predetermined, passive regions of the silicon wafer. Internal or intrinsic gettering is an effective way to reduce the contamination in active regions. The generation of internal getters is based on the decomposition of the supersaturated oxygen solid solution in silicon, which favours the formation of a complex defect system in silicon that consists of various precipitate/dislocation agglomerates. Regularities of microdefect formation during oxygen solid solution decomposition in silicon have been studied. We show that actual solid solution supersaturation, temperature and heat treatment duration determine the structure of the solid solution. Combining these factors, including heat treatment parameters, one can control solid solution decomposition rate and SiOx precipitate sizes and quantity. The methods of X-ray diffuse scattering and transmission electron microscopy have shown high efficiency for studying the effect of heat treatment in crystals. For annealing at 450 °C, solid solution decomposition occurs at high supersaturation degrees, and concentration inhomogeneity regions may form at an early decomposition stage over the actual annealing time (up to 40 h). With an increase in the temperature of subsequent annealing to 650 °C, local regions with above-average oxygen supersaturation degrees increase the efficiency of oxygen solid solution decomposition. Further, an increase in annealing temperature to T > 1000 °С results in a more intense generation of the largest precipitates at the expense of the smaller ones. Once the precipitate sizes become sufficiently large, the elastic stresses start to relax, leading to partial incoherence and the generation of dislocations around the particles. This type of defect structure seems to be the most efficient getter.
X-ray diffuse scattering, silicon, heat treatment, getter
Heat treatments for internal getter synthesis through the decomposition of a supersaturated oxygen solid solution in silicon matrix require combining temperature and duration so as to produce an efficient gettering environment (sizes and quantity of microdefects) providing for reliable trapping of detrimental impurities during further treatments [
A theoretical analysis of the thermodynamic and kinetic conditions controlling the formation of microdefects, i.e., oxygen-containing precipitates in silicon, was reported earlier [
Analysis [
Thus, selection of the optimum internal getter synthesis mode should take into account the abovementioned factors which may control the oxygen solid solution decomposition rate and the size and quantity of precipitates which determine the gettering properties of the structure. Unfortunately the solubility limit of oxygen in silicon, its dependence on the formation of complexes and the energy parameters of the abovementioned factors have not yet been studied well. It is therefore impossible to simulate the resultant structure after gettering heat treatments. Most getter formation works are experimental ones. Therefore objective and information valuable structural control methods are required.
Thus, given similar initial supersaturation degree and high-temperature heat treatment duration the decomposition rate of oxygen solid solution in silicon and the resultant crystal structure will mostly depend on the processes occurring during low-temperature heat treatments. Obviously, as the molecule attachment energy at 1000 °C is not the determinant factor, diffusion processes will dominate. For example, the formation of high local supersaturation regions reduces the probability of the formation and growth of silicon oxide precipitates through diffusion.
The structure of crystals after the complete heat treatment cycle for internal getter synthesis is of great interest since the decomposition of an oxygen solid solution in silicon leads to the formation of various microdefect complexes: SiOx precipitates, precipitate/dislocation associations and ~2–3 µm stacking faults. Further heat treatments do not change the quantity of microdefects and the type of oxygen-containing precipitates but dramatically increase the quantity of small microdefects that form during the growth of the oxide particles and originate from intrinsic point defects [
Below we present experimental data for a series of silicon crystals. When choosing the experimental heat treatment modes we took into account earlier results [
Our experimental data differ from earlier ones [
The test materials were double-side chemically polished fragments of 150 mm dislocation-free Cz-Si(100) wafers grown in “vacancy” mode. The oxygen concentration in the crystals was approx. (7–9) · 1017 cm-3 and the electrical resistivity was 1–5 Ohm·cm. The specimens were heat treated in various modes as summarized below.
Specimen # Heat treatment mode
1 650 °C (40 h)
2 650 °C (2 h) + 1000 °C (16 h)
3 650 °C (16 h) + 1000 °C (0.5 h)
4 650 °C (16 h) + 1000 °C (16 h)
The structure was studied using X-ray diffuse scattering on a D8 Discover multifunctional X-ray diffractometer (Bruker-AXS, Germany) in triple-crystal setup. The X-ray source was a 1.6 kW copper tube. A parallel beam was formed by a Goebel mirror. The measurements were carried out in a high-resolution setup. A (n; +n) monochromator with two Ge(220) crystals (double-reflection channel-cut monochromators) cut off the CuKα1 radiation component. The instrumental function width of the device is ~12 arc sec in this setup. A Ge(220) channel-cut triple analyzer crystal was installed before the detector.
The X-ray diffuse scattering distribution was studied in the vicinity of the Si(004) reciprocal lattice site. The reciprocal space maps were built along with their sections perpendicular to the diffraction vector (qx sections) and at a small angle to the diffraction vector (qz sections). The symmetrical and asymmetrical X-ray diffuse scattering components were separated in the reciprocal space maps. The measurement method was described in detail earlier [
Structural defects forming due to the decomposition of the supersaturated oxygen solid solution in silicon during heat treatments were additionally studied using TEM. Silicon specimens were chemically polished to thin foils for TEM and examined under a JEM 200-CX electron microscope at a 200 kV acceleration voltage. The origin of lattice defects was studied by analyzing the electron microscopy images for different diffraction contrast conditions occurring at variation of the diffraction vector g and the Bragg angle deviation θ.
Figure
X-ray diffuse scattering intensity distribution for Si Specimen 1 as-annealed at 650 °C (40 h): (a) reciprocal space map (diffuse scattering isointensity contours and pseudopeak); (b) and (c) asymmetrical and symmetrical X-ray diffuse scattering components, respectively.
(a) qx and (b) qz reciprocal space map sections for Si specimens after different heat treatments: (1) 650 °C (40 h); (2) 650 °C (16 h) + 1000 °C (0.5 h); (3) 650 °C (2 h) + 1000 °C (16 h); (4) 650 °C (16 h) + 1000 °C (16 h); (5) instrumental function.
TEM studies of Specimen 2 heat treated at 650 °C (2 h) + 1000 °C (16 h) showed that the crystals contained 130–150 nm plate-like precipitates and emerging dislocations, which formed complex-shaped linear/globular pile-ups (Fig.
Diffraction contrast TEM image of precipitates in Si specimens: (a) dislocation loops diverging from a precipitate in Specimen 3; (b) dislocation loop around a particle in Specimen 2; (c) coherent particle in Specimen 4.
Now we analyze X-ray diffuse scattering intensity distribution for the (400) reciprocal lattice site of Specimen 2. The position of the diffuse scattering isointensity contours relative to the reciprocal lattice site suggests that the microdefects had a positive power (Fig.
X-ray diffuse scattering intensity distribution for Si Specimen 2 as-annealed at 650 °C (2 h) + 1000 °C (16 h): (a) reciprocal space map; (b) asymmetrical X-ray diffuse scattering component.
Obviously all the microdefects forming at 650 °C can act as precipitation centers that attract SiOx molecules. They largely control the oxygen solid solution decomposition rate at 1000 °C. This assumption is confirmed by the data on Specimen 3 for which the duration of the low-temperature anneal was increased to 16 h. TEM showed that this crystal contained ~200 nm plate-like precipitates (Fig.
X-ray diffuse scattering intensity distribution for Si Specimen 3 as-annealed at 650 °C (16 h) + 1000 °C (16 h): (a) reciprocal space map; (b) asymmetrical X-ray diffuse scattering component.
Thus the main result of an increase in the duration of low-temperature 650 °C anneal from 2 h to 16 h was the formation of more decomposition center precipitates. The data for Specimen 1 suggest that the microdefects remained small and coherent with the matrix.
These results suggest that, most likely, an increase in annealing time for a specific supersaturation degree favors homogeneous precipitation of coherent particles whose quantity grows but size growth is limited due to the low diffusion mobility of oxygen (in the form of single atoms or complexes with interstitial silicon). The precipitates really grow at 1000 °C for which the diffusion rate is higher. Compare the X-ray diffuse scattering data for Specimens 2 and 4. The X-ray diffuse scattering intensities of Specimens 2 and 4 are almost similar. Thus an increase in the quantity of the precipitates due to an increase in annealing time from 2 h to 16 h is compensated by the fact that the microdefect volumes are comparable (Figs
X-ray diffuse scattering intensity distribution for Si Specimen 4 as-annealed at 650 °C (16 h) + 1000 °C (0.5 h): (a) reciprocal space map; (b) asymmetrical X-ray diffuse scattering component.
Indeed, TEM studies suggest that the precipitate sizes in Specimen 4 reached 25 nm (Fig.
On the other hand, comparison of X-ray diffuse scattering data showed that the total volume of the largest microdefects producing asymptotic scattering (for qx ~ 1 µm-1 the lg I = f (lg qx) curve slope is ~3, Fig.
Combined TEM, metallographic and X-ray diffuse scattering studies of the structures produced by the decomposition of oxygen solid solution in silicon revealed regularities of structural transformations occurring during the decomposition of oxygen solid solution in silicon at different temperatures. These regularities originate from the differences in the energy parameters of different process stages.
For annealing at 450 °C, solid solution decomposition occurs at high supersaturation degrees, and concentration inhomogeneity regions may form at an early decomposition stage over the actual annealing time (up to 40 h).
With an increase in the temperature of subsequent annealing to 650 °C, local regions with above-average oxygen supersaturation degrees noticeably increase the efficiency of oxygen solid solution decomposition in comparison with that for annealing with the same duration and at the same temperature but without a preliminary 450 °C anneal (X-ray diffuse scattering resolved large microdefects). TEM-invisible coherent oxide precipitates form after annealing for 40 h and their quantity increases with annealing duration.
Probably, in accordance with the phase diagram, at a higher oxygen concentration and a somewhat higher annealing temperature the solid solution supersaturation may remain high and coherent precipitation may intensify due to higher diffusion rates. Further increase in annealing temperature to T > 1000 °С reduces the supersaturation but increases the diffusion rate. This results in a more intense formation of the largest precipitates at the expense of the smaller ones. Once the precipitate sizes become sufficiently large the elastic stresses start to relax leading to partial incoherence and the generation of dislocations around the particles. It is this type of defect structure that seems to be the most efficient getter.