Nondestructive Depth Determination of Subsurface Microdefects in Silicon Wafers

Abstract

Nondestructive short wavelength laser scattering tomography ͑LST͒ is proposed for determining the depths and dimensions of microdefects to a depth of several micrometers in the subsurface region of silicon wafers. Measurements were made at two different temperatures, i.e., 23 and 83°C, utilizing the temperature dependence of the absorption coefficient of silicon at the laser wavelength ͑680 nm͒. From a comparison between the depths of the defects determined by this new LST method and those by transmission electron microscopy, it is verified that the depths of microdefects less than 5 m deep are determined well by our method.The quality of the subsurface region ͑to a depth of a few micrometers͒ in silicon wafers is one of the dominant factors for successful ULSI fabrication. The dimensions and densities of microdefects in this region are very important to the quality. Nondestructive methods have, therefore, been required for detecting subsurface microdefects. Short wavelength laser scattering tomography ͑S-LST͒ was developed to detect these defects, but it is incapable of measuring the depths and dimensions of individual defects. 1 An optical shallow defect analyzer is very useful for detecting microdefects in the subsurface region to a depth of 0.5 m, but the measurable region is too shallow. 2 In this paper, we propose an original method using nondestructive laser scattering tomography for measuring the depths and dimensions of microdefects in the region to a depth of several micrometers, utilizing the temperature dependence of the absorption coefficient of silicon at the laser wavelength. We demonstrate the depth determination of subsurface defects.When a laser beam with intensity, I 0 , and energy larger than the silicon bandgap ͑1.12 eV at 300 K͒ 3 is incident on the surface of a silicon wafer close to the Brewster angle ͑75.3°͒, 1,4,5 the intensity, I, of the scattered light due to a microdefect at depth d beneath the surface is given bywhere is the differential scattering cross section, t i is the energy transmittance of the incident laser through the air/silicon interface close to the Brewster incidence, t s is the energy transmittance of the scattered light through the silicon/air interface at normal incidence, h is the instrument function, and ␣(T) is the absorption coefficient ͑at the laser wavelength͒ as a function of the sample temperature, T. Note that the scattered light is observed just above the sample wafer.Although the optical path of the incident laser from the incident surface to the defect is approximately 4% longer than the depth, d, of the defect due to the incidence close to the Brewster angle, we neglect the difference in deriving Eq. 4, because the difference is small. If the scattering intensity, I, is measured at two different temperatures, T 1 and T 2 , the measured intensities, I 1 and I 2 , are expessed as I 1 ϭ ͑ I 0 t i t s h ͒ exp͑Ϫ2␣͑T 1 ͒d ͒ ͓2͔ I 2 ϭ ͑ I 0 t i t s h ͒ exp͑Ϫ2␣͑T 2 ͒d ͒ ͓3͔The d and the quantity A (ϭI 0 t i t s h) are derived as follows d ϭ ln͑I 1 /I 2 ͒/2͓␣͑ T ...

Topic Review: Application of Raman Spectroscopy Characterization in Micro/Nano-Machining

“…A new laser scattering method was also used to detect subsurface defects. Goto et al [ 13 , 14 ] measured the depth and dimension of subsurface microdefects in Czochralski-grown silicon wafers and epitaxial silicon wafers using the laser scattering method. They found out that this method could determine the depth of subsurface defects which are less than 5 μm deep, and the boundaries between the epitaxial layers and substrates were detected successfully.…”

Nondestructive Observation of Depths and Dimensions of Subsurface Microdefects in Czochralski-Grown and Epitaxial Silicon Wafers

“…3 The detected intensity I(d) of the light scattered by a defect at a depth of d decreases exponentially with d as follows…”

“…The measurement procedures of the two-temperature S-LST method were described in the previous paper. 3 The measurement area (10 ϫ 5 mm) was approximately at the center of the wafer. Figure 2a shows the correlation ͑d-log A plot͒ between d and log A of microdefects detected in the subsurface region of the CZ wafer, and depth d. In Fig.…”

Non-destructive evaluation methods for subsurface damage in silicon wafers: a literature review

“…The laser scattering method has been used by Goto et al (2001) to measure subsurface microdefects in Czochralski-grown (CZ) silicon wafers and epitaxial silicon wafers. Utilising the temperature dependence of the absorption coefficient of silicon at the laser wavelength, they could measure the depths and dimensions of microdefects in the region to a depth of 5 µm.…”