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Non-Destructive Testing of Silicon Wafers by Monitoring Changes in Electron-Hole Plasma Reflectivity Caused by Wafer Defects

IP.com Disclosure Number: IPCOM000038691D
Original Publication Date: 1987-Feb-01
Included in the Prior Art Database: 2005-Jan-31
Document File: 4 page(s) / 41K

Publishing Venue

IBM

Related People

DiStefano, TH: AUTHOR [+3]

Abstract

When a plasma of density N is created (as with a laser pump beam) in a well-defined spatial region of a Si wafer, Drude contributions modify the crystal's optical dielectric susceptibility e(l) in that region. Changes in e(l) or N are then monitored by measuring changes in the reflectivity (wR) and state of polarization of a probe beam incident on the excitation volume (V). Lattice defects which influence N are viewed as a change in Drude (plasma) contribution to (wR,(wP) in V. Therefore, by monitoring (wR,wP) everywhere on a wafer surface, one obtains a map of those wafer defects which influence carrier mobility or lifetime.

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Non-Destructive Testing of Silicon Wafers by Monitoring Changes in Electron-Hole Plasma Reflectivity Caused by Wafer Defects

When a plasma of density N is created (as with a laser pump beam) in a well-defined spatial region of a Si wafer, Drude contributions modify the crystal's optical dielectric susceptibility e(l) in that region. Changes in e(l) or N are then monitored by measuring changes in the reflectivity (wR) and state of polarization of a probe beam incident on the excitation volume (V). Lattice defects which influence N are viewed as a change in Drude (plasma) contribution to (wR,(wP) in V. Therefore, by monitoring (wR,wP) everywhere on a wafer surface, one obtains a map of those wafer defects which influence carrier mobility or lifetime. A laser pump beam of intensity Ip, wavelength gp and beam cross- section Ap (at wafer surface) generates an electron-hole (e-h) plasma density N which, in a perfect crystal, depends principally on the following parameters: 1. The absorption depth (L) which depends on gp through the absorption coefficient a (gp). We

take L between 10 and 50 and 1 and 10 mm. 2. The ambipolar diffusion time out of L, t=L2/D, where D is the ambipolar diffusion

coefficient. 3. Ip and Ap . If in the excitation volume (VNApL) there exist lattice defects (with associated strain

fields) which cause efficient non-radiative e-h

recombination, and drain carriers over a

significant fraction of V, then the effective

plasma density and its effect on e(l) are

proportionately reduced. Consequently, plasma

contributions to probe beam reflectivity and state

of polarization are reduced by local crystal

defects. Spatially superimposed pump and probe beams can be simultaneously scanned over a wafer surface while the probe beam parameters (wR,wP) are continuously monitored. A perfect wafer surface will show no spatial variation of Drude reflectivity, while certain types of wafer defects that may be contained in V will greatly alter Drude contributions. A map of lattice defects can therefore be obtained by monitoring spatial variations in the plasma contribution to probe beam parameters (wR,wP) in a pump and probe measurement. The probe depth is determined by gr/n, where gr is the wavelength of the probe beam and n is the real part of the index of refraction of Si at gr . Estimate of wR: For simplicity, it is assumed that the probe beam is at near normal incidence and that the imaginary component of e(l) is negligible at gr . Then, where e(l) is the optical dielectric constant in the absence of the e-h plasma is the free carrier plasma frequency which the e-h concentration is
N. es is the static dielectric constant of Si, and m*e, m*h are electron and hole effective masses. If the Drude contribution is small compared to eo(l), the relative reflectivity change associated with an e-h plasma density N is given approximately by wR N we N l _P 2

1

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R e 16l If N = 5 x 1017cm-3 (a plasma density easily obtainable with mode...