Optical: systems and elements – Compound lens system – Microscope
Reexamination Certificate
2000-03-09
2002-08-06
Font, Frank G. (Department: 2877)
Optical: systems and elements
Compound lens system
Microscope
C359S368000, C250S458100, C356S326000
Reexamination Certificate
active
06429968
ABSTRACT:
TECHNICAL FIELD
The present invention relates to optical microscopy and, more particularly, to photoluminescence microscopy and spectroscopy used to map material quality in semiconductor wafers and devices.
BACKGROUND OF THE INVENTION
In the information age, communications technology depends on the efficient manufacture of photonic and electronic devices. Optical testing promotes manufacturing efficiency by controlling the quality of incoming materials, providing rapid feedback for process improvements, and analyzing why a product has failed. New, non-destructive optical techniques are being used to measure key properties of semiconductor materials and devices. Optical mapping reveals defective regions in various types of wafers, as well as in optoelectronic devices, such as lasers, modulators and detectors, used in lightwave communications systems.
Spatially resolved photoluminescence is particularly valuable for measuring and characterizing the radiative uniformity of optoelectronic materials, wafers, epitaxial layers and devices. In spatially resolved photoluminescence, the semiconductor wafer or device is irradiated with a beam of highly focused monochromatic light having energy greater than the bandgap energy of the semiconductor. As this so-called “pumping” beam is scanned over an area of the wafer, broadband photoluminescence from the surface is detected, with changes in the intensity of the photoluminescence indicative of defects, such as dislocations. Moreover, displaying the intensity of the photoluminescence on a video monitor creates a spatially resolved photoluminescence image.
In the prior art, confocal laser microscopes have been adapted to detect photoluminescence with high spatial resolution. For example, shown in
FIG. 1
, is a confocal scanning laser microscope
100
which utilizes its confocal properties to separate the photoluminescence originating from different points on the semiconductor. In this latter configuration, light
105
from laser
110
is focused through pinhole
115
by lens
120
, and then into a diffraction-limited spot on the surface of semiconductor
125
by objective lens
130
. Photoluminescence from the semiconductor surface, and within this diffraction limited spot, is directed by dichroic beamsplitter
135
, and focused by objective lens
130
through pinhole
140
onto detector
145
. Dichroic beamsplitter
135
transmits light from laser
110
, but only directs longer wavelength photoluminescence onto detector
145
.
Photoluminescence substantially only originating from focused spot
150
passes through pinhole
140
. Light from any other point on the semiconductor is blocked by the edges of the pinhole inasmuch as focused spot
150
is confocal with pinholes
115
and
140
, giving this configuration the ability to obtain highly spatially resolved photoluminescence images.
Spectrally resolved photoluminescence, on the other hand, is generally used to measure the compositional characteristics of the semiconductor material, such as bandgap energy, donor and acceptor energy levels, phonon energy, and the like. In spectrally resolved photoluminescence, highly monochromatic light is used to excite an area of the semiconductor, with the resulting photoluminescence directed into a monochromator which resolves the spectral components of the photoluminescence. For example, certain photoluminescence systems sold by Waterloo Scientific Inc., focus the photoluminescence onto the entrance slit of a grating monochromator.
Unfortunately, such prior art photoluminescence systems do not readily allow an operator to make both spatially as well as spectrally resolved photoluminescence measurements, particularly with spatial resolutions in the micron (&mgr;m) region. Additionally, such photoluminescence systems only pass a fraction of the collected photoluminescence to the detector. However, a spectrally resolved photoluminescence mapping of semiconductor wafers with good spatial resolution and collection efficiency has been recently disclosed by Dixon et al. in U.S. Pat. No. 5,192,980, which is incorporated herein by reference. Referring to
FIG. 2
, Dixon et al. integrates a monochromator or spectrometer into the detection arm of a confocal microscope adapted to obtain highly spatially resolved photoluminescence spectra of the semiconductor. More specifically, a parallel beam
155
of laser light passes through a beamsplitter
160
to enter objective lens
165
which focuses the beam to a diffraction limited focal spot
170
on the surface of a semiconductor specimen
175
. On-axis photoluminescence
180
from the specimen is collected by objective lens
165
, and is then partially reflected by beamsplitter
160
into a detection arm
185
of the microscope. On-axis photoluminescence
180
strikes diffraction grating
190
which diffracts the light toward lens
195
, placed a focal length, f, in front of a pinhole
200
. That is, pinhole
200
is confocal with focal spot
170
at the focal point of lens
165
.
Inasmuch as diffraction grating
190
separates incoming light
180
into its spectral components along a longitudinal axis, only photoluminescence of a narrow wavelength band passes through pinhole
200
, and reaches detector
205
. Shown in
FIG. 2
are exemplary spectral components
210
(&lgr;
1
+&Dgr;&lgr;),
215
(&lgr;
1
),
220
(&lgr;
1
−&Dgr;&lgr;), with only spectral component
215
(centered at &lgr;
1
) passing through pinhole
200
. Light centered at any other wavelength emitted from focal spot
170
hits the area surrounding pinhole
200
, and is not detected.
Unfortunately, the photoluminescence system of Dixon et al., and similar confocal based photoluminescence systems, are generally ill-suited for semiconductor specimens exhibiting substantial lateral carrier diffusion.
SUMMARY OF THE INVENTION
In accordance with the teachings of the present invention, it has been discovered that when the lateral carrier diffusion area exceeds the diffraction limit spot of the illuminating beam, an optical analysis that includes the extended size of the carrier diffusion area provides a technique for more properly controlling the size of the PL collection region or the axial photoluminescence resolution. On the above basis, the optical system is uniquely characterized in that the optical fiber(s) within the detection arm(s) of the optical system functions as the effective field stop for off-axis photoluminescence. As the field stop, the optical fiber(s) limits the size or field of view of the photoluminescence corresponding to a PL collection region of desired radius. Importantly, it does so, by limiting the cone of light accepted from the off-axis photoluminescence vis-a-via its core size and numerical aperture.
Thus, the underlying rational in controlling the spatial resolution for an extended region of photoluminescence is to judiciously choose the core radius, and the numerical aperture of the optical fiber(s) so as to control the conical bundle of rays from points on the periphery of the desired photoluminescence region so as to reach and enter the optical fiber(s).
In a preferred embodiment, light from a laser is focused into a diffraction limited spot and scanned on the surface of a semiconductor specimen in a predetermined pattern. The photoluminescence from the semiconductor surface is then directed and focused onto the optical fiber, which by judiciously choosing its core diameter and numerical aperture, functions as the field stop to limit the photoluminescence collection region to a desired radius. Photoluminescence spectroscopy and/or microscopy is performed by coupling the collected photoluminescence within the optical fiber into an optical spectrum analyzer and/or photodetector, using optical coupling means or optical switches.
In one embodiment, since collected photoluminescence incident outside the core of the optical fiber is not detected, limiting the PL collection region to a desired radius requires that the photoluminescence from any point outside the desired radius has its conjugate image also outside the
Agere Systems Guardian Corp
Font Frank G.
Law Offices of John. De La Rosa
Punnoose Roy M.
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