Scanning Infrared Polariscope Research Articles

10 cm Diameter Mono Cast Si Growth and its Characterization

To get the optimized condition and ideal furnace structure, we have performed seed cast growth of mono-crystalline Si by using unidirectional solidification furnace. More than 20 ingots of 10 cm diameter and 10 cm height were grown under different growth conditions. The quality of ingots was characterized by using Fourier transform infrared spectroscopy (FTIR), infrared microscopy, scanning infrared polariscope (SIRP), X-ray topography, etc. We have realized reduction of carbon, residual strain and extended defects, which may contribute the increase of solar cell efficiency.

Residual strain distribution in casting ingot of multi-crystalline silicon

Abstract By using a scanning infrared polariscope (SIRP), the residual strain distribution has been characterized in specially-prepared vertical slices of rectangular blocks cut from two conventional casting ingots in production scale. All the blocks clearly revealed local strain increase in the shapes of straight-pillar and long-pyramid but its distribution was significantly different from block to block. The averaged vertical profiles exhibited common W-shape in the center blocks, while it was more distorted in the peripheral blocks, which may reflect casting process condition such as thermal stress applied to the whole ingot.

Grain Boundaries in Multicrystalline Si

We report the electrical, structural and mechanical properties of grain boundaries (GBs) in multicrystalline Si (mc-Si) based on electron-beam-induced current (EBIC), transmission electron microscope (TEM), and scanning infrared polariscope (SIRP) characterizations. The recombination activities of GBs are clearly classified with respect to GB character and Fe contamination level. The decoration of Fe impurity at boundary has been approved by annular dark field (ADF) imaging in TEM. Finally, the distribution of residual strain around GBs, and the correlations between strain and electrical properties are discussed.

Quantitative imaging of residual strain profile in large diameter GaAs substrates

AbstractA near‐infrared imaging polariscope (NIRIP) has been developed quantitatively to measure residual strain profiles in large‐diameter GaAs substrates, in which a near‐infrared camera connected to an image‐processing system is used as a 2D multi‐channel sensor and also a collimated light beam is used to illuminate the whole area of substrate, in addition to a pair of computer‐controlled rotating polarizer and analyser. The measurement time is less than 5 minutes for the GaAs substrate with the diameter of 6‐inch, which is drastically reduced in comparison to that of the scanning infrared polariscope (SIRP) using a single channel sensor and a focused light beam. By comparing the residual strain profiles measured in the same substrates with NIRIP and SIRP, it is confirmed that the NIRIP developed here has the sensitivity equivalent to the SIRP. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Photoelastic strain measurement in GaP (100) wafers under external stresses

The sign of the absolute value of residual strain measured in LEC-grown GaP wafers with scanning infrared polariscope (SIRP) has been determined by measuring the residual strain in the round-shape wafer with and without compressive load to the diametric edges. Since the external stress induced by the compressive load is additive to the residual strain in wafer, the residual strain measured in the wafer with the compressive load is increased if the sign is negative while it is decreased if the sign is positive. The measurement of residual strain was successively performed in various LEC-grown GaP (100) wafers clamped their diametric edges in a specially-made vise, in which a movable jaw is pulled by a coil spring to slide parallel toward a fixed jaw with a compressive load. It is found from their results that the residual strain component of (S r − S t), where S r is the radial strain component and S t is the tangential strain component defined in the cylindrical coordinate system matching to the round-shape wafer, is negative; that is, radially compressive in most wafers.

Photoelastic characterization on multicrystalline silicon substrates for solar cell

By using a scanning infrared polariscope (SIRP), the absolute difference of refractive index |Δ n| and the principal direction ψ of strain-induced birefringence have been characterized in a multicrystalline silicon substrate sliced from an ingot grown by conventional casting technique. The |Δ n| is concentrated with gradual variation of ψ in the shapes of distorted square-pillar and straight needle along and around a part of the grain boundaries. The averaged value of |Δ n| is 2.9×10 −5 over whole the substrate and the maximum value of |Δ n| attains 2×10 −4. The |Δ n| is also decreased with drastic change of ψ in the shapes of straight line and cell-like structure inside the grain, which corresponds plastic deformations caused by crystal defects such as dislocation and/or subgrain. Since the concentration of strain-induced birefringence as well as its local relief by plastic deformations observed here is considerably associated with the mechanical strength of the substrate, we can conclude that the SIRP measurement is useful for practical diagnoses of commercial substrates of multicrystalline silicon for solar cell.

Comparative study on residual strain and thermal stress in 4-in GaAs crystals grown by LEC and VB techniques

Residual strains of 4-in diameter GaAs crystals grown by liquid encapsulated Czochralski (LEC) and vertical Bridgman (VB) techniques have been measured by scanning infrared polariscope (SIRP). It was found that the residual strain of the substrate cut at the shoulder portion of a LEC-grown crystal was remarkably larger than that cut at the middle portion of the crystal. It was also found that the residual strains of the substrates cut at the shoulder and middle portions of a VB-grown crystal were both smaller than those of the LEC-grown crystal, and showed the similar values and profiles with each other. Von Mises stress and excess shear stress according to the Jordan's criterion were calculated to understand these experimental results. It was found that large thermal stress was caused during the LEC growth especially in the early stage, while thermal stresses caused during the VB growth were considerably smaller than those caused during the LEC growth from the shoulder to the middle. These experimental and analytical results suggest that the difference in the residual strain along the growth axis between LEC and VB crystals is due to the difference of the thermal stress caused during the crystal growth between LEC and VB techniques.

Optical anisotropy in dislocation-free silicon single crystals

The optical anisotropy in silicon single crystal was evaluated by measuring the birefringence accurately. In measurement, the use of dislocation-free silicon crystals effectively avoided the influence due to dislocation-induced birefringence. The extreme small birefringence, which was supposed to be due to the point defects, was also measured and taken into consideration, by using scanning infrared polariscope (SIRP) with high sensitivity. The random error, which may occur in 1-point measurement, was eliminated by carrying scanning measurement over the checking plane of the cuboid sample. As the result, the birefringence due to the optical anisotropy in dislocation-free silicon crystals was measured to be (3.34±0.09)×10 −6, when probing light ( λ=1.52 μm) was propagating along the [ 1 10] direction (at the [ 1 10] direction). The birefringences due to the optical anisotropy were also measured at other crystallographic directions with a cylindrical silicon sample.

Optical anisotropy and strain-induced birefringence in dislocation-free silicon single crystals

Birefringence measurements, using a scanning infrared polariscope (SIRP) with high sensitivity, have been carried out in as-grown dislocation-free silicon single crystal ingots. A small amount of birefringence due to 〈110〉 optical anisotropy, which is predicted not by the classical optics theory but by the Lorentz's spatial dispersion theory, is observed in the θ– Z optical configuration, for which a probing polarized light is introduced from the as-grown cylindrical ingot surface and then the polarization of the light transmitted through the opposite surface is analyzed. On the other hand, in the R– θ optical configuration, for which the probing polarized light is introduced along the 〈001〉 crystal growth direction from the (001) sliced and polished surface and then the polarization of the light transmitted through the opposite (001) surface is analyzed, the birefringence is still observed although it is extremely small. It is found that this 〈001〉 optical anisotropy is not constant over the (001) cross-section of the ingot but consists of two different components, i.e. a symmetric component distributed over the whole sample, probably caused by point defects, and an asymmetric component distributed randomly near the peripheral region, related to dents and chips on the as-grown cylindrical surface.

Photoelastic characterization of Si wafers by scanning infrared polariscope

A small amount of birefringence caused by the photoelastic effect from residual strains, crystal-defects-induced strains, and process-induced strains in Si wafers has been measured by using an improved version of a scanning infrared polariscope (SIRP). The SIRP presented here has high sensitivity sufficient to detect the small amount of strain induced near the wafer-supporting finger by the wafer weight itself. It is found that an anomalous amount of strain is induced by slip-line generation during the thermal process and also that a concentric ring pattern of strain is induced by OSF rings. From these results, it is suggested that SIRP is very useful for Si wafer inspection and Si process evaluation in various phases.

Residual strain in annealed GaAs single-crystal wafers as determined by scanning infrared polariscopy, X-ray diffraction and topography

We have compared the strain data in GaAs wafers, as-grown as well as annealed, determined by means of the scanning infrared polariscope (SIRP) with data of high-resolution X-ray diffraction (HRXD) and qualitative results of a synchrotron based, single-crystal X-ray transmission topography (SXRTT) study. The in-plane strain component | ε r− ε t| measured by SIRP throughout the wafer thickness was about 10 −5, while it derived from the single components ε xx, ε yy , and ε xy determined by HRXD at a penetrated layer close to the surface was above 10 −4. Consequently, we assume that a strong strain gradient exists between the surface and the bulk.

Photoelastic characterization of residual strain in GaAs wafers annealed in holders of different geometry

The spatial distribution of residual strain in undoped 2 inch GaAs wafers multi-step annealed in holders of different geometry was characterized by the scanning infrared polariscope (SIRP) method. The SIRP maps reveal that the distribution of strain is significantly influenced by the symmetry of annealing, in particular by the points of contact between wafer and holder. In contrast to the as-grown state, the annealed wafers show fine patterns of slip lines. The lowest level and the most homogeneous distribution of residual strain were achieved by annealing in a vertically positioned holder of graphite rings. The radial temperature differences in the wafers caused by heating and cooling were checked by means of thermocouples on dummies of graphite. Temperature gradients up to 30 K cm −1 were measured depending upon the rates of cooling and heating.