Corresponding author: Ekaterina A. Gosteva ( gos-3@mail.ru ) © 2019 Vitaliy V. Starkov, Ekaterina A. Gosteva, Dmitry V. Irzhak, Dmitry V. Roshchupkin.
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Citation:
Starkov VV, Gosteva EA, Irzhak DV, Roshchupkin DV (2019) Study of the effect of local photon annealing on stress in silicon wafers. Modern Electronic Materials 5(3): 141-144. https://doi.org/10.3897/j.moem.5.52500
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The effect of photon annealing on deformation in the crystal structure of boron doped Cz-Si wafers has been studied using triple crystal X-ray diffraction. Conventional annealing of the entire surface of double-side polished silicon wafers with halogen lamps (photon annealing mode) and rapid thermal annealing produce compression deformation. Annealing with special phototemplate providing for local annealing of multiple separated wafer areas (local photon annealing mode) at relatively low wafer temperatures (less than 55 °C) produces tensile deformation. This effect however is not observed if the reverse side of the annealed wafer contains a mechanical gettering layer. A mechanism explaining the experimental results has been suggested and can be used for the synthesis of charge pumps in photoelectric converter structures.
photoelectric converters, local photon annealing, charge pumps, triple crystal X-ray diffraction, phototemplate
Silicon wafer treatment in the so-called rapid thermal annealing (RTA) mode is becoming increasingly widely used due to the permanent increase in the size of wafers used for the fabrication of photoelectric converters. The process combines relatively low price, simplicity and high adaptability. It can be successfully used for the synthesis of shallow p-n junctions in photoelectric converter structures [
Photon annealing was implemented on a RTA instrument with halogen lamps. The light power was 45 W/cm2 and the heating rate to 1000 °C was 125 K/s [
We used p conductivity Cz-Si wafers. The boron concentration was approx. 1015 cm-3 (ρv = 8–10 Ohm × cm), the oxygen concentration was (0.8–1.2) × 1018 cm-3 and the surface orientation was (100). The area of each wafer was 2.5 × 2.5 cm2. Specimens 1 and 2 were cut from one wafer. Specimen 3 was cut from another wafer which was one-side polished and had a mechanically ground gettering layer on its reverse side. The parameters of the wafers are summarized in the Table
The type of residual deformation (tension or compression) in the single crystal wafers was determined using triple crystal X-ray diffraction on a Bruker D8 DISCOVER diffractometer. X-ray radiation was monochromated with a Goebel mirror and a four-bounce Ge(400) monochromator. The analyzer was a double-reflecting Ge(400) single crystal. The instrumental error of this optical setup affects the measurement results but slightly. We obtained the required X-ray diffraction intensity maps in the θ–2θ coordinates for the untreated and as-treated silicon wafers. By way of a typical example the Figure
A change in the interplane spacing ∆d/d leads to a change in the position of the X-ray diffraction intensity maximum along the 2θ coordinate on the maps. Wafer bending shows itself in peak broadening along the θ coordinate and a shift of the diffraction intensity maximum along this coordinate (see the Figure
Change of interplane space as a result of photon treatment of silicon wafers
Specimen # | Thickness, µm(specimen surface treatment) | ∆d/d after treatment | |
---|---|---|---|
PA | LPA | ||
1 | 250 (double-side polishing) | – | +2.913 × 10-5 |
2 | 250 (double-side polishing) | –0.887 × 10-5 | – |
3 | 520 (gettering layer on reverse side) | – | –1.267 × 10-5 |
The effect of RTA on the residual stress in the wafers is illustrated in the Table
After PA Specimen 2 had residual stress produced by compression deformation (negative ∆d/d). On the contrary LPA of Specimen 1 (Figure
These experimental results primarily indicate a rearrangement of the defect and impurity structure in the wafers after RTA [
PA increases the wafer temperature to 1000 °C in 8 seconds. Generation and relaxation of defect-impurity complexes occur in the structure of the entire silicon wafer and lead to the formation of residual compression deformation (Specimen 2). The internal stress generated in Specimen 3 by the gettering layer and the rearrangement of the defect and impurity structure during LPA finally cause lattice compression. The residual compression deformation in Specimen 3 is approx. 40% higher than in Specimen 2 which had no gettering layer on the reverse side. However the thickness of Specimen 2 was approx. 2 times smaller than that of Specimen 3.
For LPA of the thinner wafer (Specimen 1) the surface temperature was within 55 °C. One can reasonably assume that the local photon impact generated a cloud-like defect and impurity structure containing discrete defect regions. The most probable mechanism causing residual tensile deformation in the lattice during LPA of Specimen 1 can be below-threshold generation of primary point defects, e.g. interstitial silicon atoms Sii and VSi vacancies as a result of the excitation of the crystal electron system by photogenerated electrons during Auger recombination (2.0–2.5 eV). The level of photon injection during LPA (45 W/cm2) corresponds to an electron concentration of ~1018 cm-3. Further association of point defects as a result of low-temperature migration mechanisms (including hopping migration of Sii+, photostimulated migration of Sii and BI and migration of vacancies and divacancies) leads to the formation of donor or acceptor defect clusters. In oxygen containing Cz-Si interstitial oxygen atoms Oi form high-mobility oxygen dimers O2i which associate into clusters of low-temperature thermal donors TD-1 from (SiOn)+ complexes with n < 10 [
It should be noted that low-energy below-threshold generation of structural defects and complexes in boron doped Cz-Si wafers is a commonly known phenomenon that causes degradation and regeneration of solar cells [
The experimental data showed that conventional heat treatment of the entire surface of double-side polished boron doped Cz-Si wafers in standard RTA mode with halogen lamps generates compression deformation. This was observed in double-side polished specimens and in one-side polished wafers with mechanically ground reverse side. In the latter case the effect was the strongest (typical compression stress was 40% higher than for double-side polished wafers).
LPA in the same RTA mode with phototemplate allowing multiple separated areas of wafer surface to be treated at relatively low wafer temperatures (below 55 °C) generated tensile deformation in double-side polished silicon wafers.
The work was done according to the STATE TASK № 075-00920-20-00.