Electricity: measuring and testing – Fault detecting in electric circuits and of electric components – Of individual circuit component or element
Reexamination Certificate
2000-09-28
2003-01-28
Sherry, Michael (Department: 2829)
Electricity: measuring and testing
Fault detecting in electric circuits and of electric components
Of individual circuit component or element
C324S765010, C324S762010
Reexamination Certificate
active
06512384
ABSTRACT:
FIELD OF INVENTION
This invention relates to determining minority carrier diffusion lengths, and particularly to fast and accurate determination of minority carrier diffusion lengths.
BACKGROUND OF INVENTION
The performance and reliability of semiconductor electronic and optoelectronic devices, and the integrated circuits into which they are incorporated, depends upon the purity of the semiconductor from which the devices are made. In particular, the level of heavy metal contaminants (e.g., Fe, Cr, and other metals) which may be introduced into the semiconductor during manufacturing and processing degrades performance and reduces the manufacturing yield.
One measure of semiconductor contamination is the minority carrier diffusion length, L. This parameter is the effective distance that excess minority carriers diffuse into a semiconductor during their lifetime. The value of minority carrier diffusion length is used as an indicator of the purity of semiconductor materials. L gives a measure of the contaminant concentration in the semiconductor because heavy metals function as recombination centers which reduce the minority carrier lifetime. As a result, higher concentrations of contaminants decrease the minority carrier diffusion length. Typically, the diffusion length in silicon wafers is measured at various stages of fabrication of microelectronic chips to measure the concentration of potentially harmful impurities inadvertently introduced into the wafer. Frequent monitoring of the minority carrier diffusion length helps to identify when a given process or a given tool starts to contaminate wafers above a permissible level. Preventive maintenance of processing equipment or replacement of chemicals done at this stage helps to avoid large manufacturing loses.
A general technique for measuring diffusion length includes directing a light signal onto a semiconductor to create a surface photovoltage. A surface photovoltage results when the energy of the incident photons is above the semiconductor band gap such that it produces excess carriers (holes and electrons). As a result of photogeneration, recombination, and diffusion, a concentration profile of the excess carriers is established beneath the surface of the semiconductor wafer. Larger excess carrier concentrations near the surface of the semiconductor reduce the electric field of the surface-space charge region and thereby produce larger surface photovoltage signals.
In certain diffusion length measurements, intensity modulation of the light signal produces an ac-surface photovoltage which, in turn, produces an ac-electrical signal in a capacitor formed by the semiconductor wafer and an electrode placed near the semiconductor's surface. The ac-electrical signal is subsequently measured using a lock-in amplifier tuned to the light modulation frequency to determine the surface photovoltage. Devices for determining diffusion length via surface photovoltage measurements are described in U.S. patent application Ser. Nos. 5,025,145 and 5,663,657, the contents of which are herein incorporated in their entirety.
Typically, the diffusion length measuring techniques implemented in commercial instruments use a sequence of successive illuminations of the semiconductor with monochromic beams each being intensity modulated at the same frequency but having different wavelengths. Different wavelengths of light generate minority carriers in regions extending to different depths beneath the wafer surface. The corresponding ac-surface photovoltages produced by the different wavelengths are measured sequentially, i.e., one after another, and the resulting data of the surface photovoltage, V, vs. the excitation depth, z (i.e. the light penetration depth), are then used to calculate the minority carrier diffusion length.
The American Society for Testing and Materials (ASTM) recommends two methods to determine diffusion length by employing successively measured surface photovoltage signals. See, for example, ASTM F391-96. In both of these methods, the diffusion length calculation is based on the steady-state equation for excess minority carrier concentration at the surface and is valid for low light modulation frequencies and for minority carrier diffusion lengths that are short in comparison to the semiconductor wafer thickness.
This steady-state equation is given by the expression:
Δ
n
=
Φ
1
-
R
(
D
/
L
-
S
)
1
(
1
+
zL
-
1
)
(
1
)
where &Dgr;n is the excess minority carrier concentration; L is the diffusion length; z is the penetration depth; &PHgr; is the incident photon flux; R is the reflectivity of the semiconductor; D is the minority carrier diffusion constant, D=kT/q&mgr;, where k is Boltzman's constant, T is the temperature, q is the elemental charge, and &mgr; is the minority carrier mobility; and S is the surface recombination velocity on the front surface of the semiconductor. This expression is derived in Moss, J. Electronics and Control, 1, 126, (1955).
In the constant magnitude method, one of the ASTM-recommended methods, the photon flux, &PHgr;, is measured and it is adjusted to obtain the same surface photovoltage value for each wavelength. The method assumes that the excess carrier concentration, &Dgr;n is constant because the photovoltage is constant. The diffusion length is then obtained, using Equation (1), for &Dgr;n=const. From a plot of the photon flux, &PHgr;, as a function of the light penetration depth, z, the diffusion length is determined as the intercept value, L=−z
int
at &PHgr;=0. The constant magnitude method is further discussed, for example, in Goodman, J. Appl. Phys. Vol. 32, p. 2550, 1961, and U.S. Pat. No. 4,333,051.
The second ASTM-recommended method, the linear constant photon flux, relies on measurements of the surface photovoltage only. These measurements are performed with low light intensity so that resulting surface photovoltage is a linear function of the photon flux. Under these conditions, the surface photovoltage is directly proportional to the minority carrier concentration, or V=const•&Dgr;n, where the constant depends on the semiconductor doping and the surface charge, but does not depend on the photon flux. In this method the measuring apparatus is built in such a way that the effective photon flux entering a semiconductor, &PHgr;
eff
=&PHgr;(1−R), is constant for all wavelengths and, thus, for all penetration depths, z. For short L values, the diffusion length is obtained by plotting the inverse of the photovoltage signal, &PHgr;
eff
/V, as a function of penetration depth, z. An intercept value, at &PHgr;
eff
/V=0 gives L=z
int
. This method is further discussed in U.S. Pat. Nos. 5,025,145 and 5,177,351, and in Solid State Technol. 35, 27 (1992) and in Semicond. Sci. Technol. 7, A185 (1992). An improved version of this method suitable for long diffusion length, i.e., when the minority carrier diffusion length exceeds the wafer thickness, is also discussed in U.S. Pat. No. 5,663,657 and ASTM stock # STP1340 p. 125 (1998), by Lagowski et. al.
SUMMARY OF THE INVENTION
The present invention provides a surface photovoltage (SPV) apparatus and method for measuring the minority carrier diffusion length, L, which is faster, e.g., at least twice as fast, than existing methods. The enhancement in speed is achieved by replacing a plurality of successive surface photovoltage measurements with a single simultaneous measurement of all surface photovoltage signals (for all penetration depths) at the same moment of time.
The apparatus of the invention uses a linear constant photon flux method described in previous section to determine the minority carrier diffusion length. The apparatus illuminates simultaneously a semiconductor wafer with a beam containing an entire set of wavelengths rather than with consecutive beams of different wavelengths. Each component of the beam, i.e., each monochromatic wavelength in the set, is modulated with a different frequency, (i.e. &lgr;
1
with the f
Aleinikov Andrei
Faifer Vladimir
Lagowski Jacek
Fish & Richardson P.C.
Patel Paresh
Semiconductor Diagnostics, Inc.
Sherry Michael
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