Method and apparatus for semiconductor profiling using an optical probe

A method and apparatus of determining defects in semiconductors, by scann with small spot of light. As the semiconductor is scanned, a voltage is generated which may be used to indicate gross defects as well as the reduction in carrier lifetime due to the defects.

BACKGROUND OF THE INVENTION 
The current trend in microelectronic technology of decreasing device size 
and increasing integrated circuit complexity is placing ever increasing 
demands on the level of spatial uniformity required of the semiconductor 
starting material. Inhomogeneities in the semiconductor, present either 
after slice preparation or introduced during circuit manufacture can lead 
to unacceptable variations in device characteristics which may manifest 
themselves as either an initially unacceptable IC chip or perhaps even 
worse, the subsequent failure of an initially acceptable circuit. A need 
then exists for some means of semiconductor profiling to enable regions of 
suspect material to be identified early in the production cycle. Also the 
technique itself should introduce a minimum of damage into the sample 
under scrutiny. 
SUMMARY OF THE INVENTION 
The present invention provides for a method and apparatus for a scan 
measurement of the photo voltage generated in the space charge region at 
the surface of a semiconductor. Apparatus is provided for optically 
scanning with a small spot of light the sample under consideration. 
Accordingly, it is an object of the present invention to provide a method 
and apparatus for the detection of semiconductor inhomogeneities by a 
scanned measurement of the surface photo voltage generated in the surface 
space charge region of the semiconductor of interest. 
Other objects and many of the attendant advantages of this invention will 
be readily appreciated as the same becomes better understood by reference 
to the following detailed description when considered in connection with 
the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Referring now to FIG. 1 wherein there is shown a schematic illustration of 
the optical probe scanner, a variable wavelength monochromatic beam is 
generated from a broadband Quartz Hallogen source 10 by passing the 
radiation through a grating monochromator 12, low-pass filter 14 (to 
remove unwanted defraction orders) and collimator lens 16. Following 
modulation by a mechanical chopper 18 a portion of the emergent beam is 
diverted by a small mirror 19 to a thermocouple detector 32 to provide a 
reference signal. A demagnifying microscope (lenses 21, 23) column is 
placed in this path to enchance the intensity of the signal from the 
thermocouple 32 and to balance the attenuation of the glass optics in the 
main beam. The main beam passes through an intermediate demagnifying 
microscope object lens 22 onto a scanning mirror 24 which deflects it 
through approximately 90.degree. into a demagnifying microscope column. 
The beam spot size at the focus of the microscope objective lens 28 may be 
varied by changing eithr or both the intermediate lens 22 and objective 
lens 28. Typical objective lenses used provided a diffraction limited spot 
size of approximately 5.mu.m for 1.mu.m illumination. This large 
diffraction limit results from using long working distant objective 
lenses. Spot size may be measured by scanning the spot over a knife edge 
onto a large area photo detector. The intensity of the spot may be 
adjusted by varying the voltage to the source 10. Typical maximum 
intensities into a 100.mu.m spot where 1.mu.m watt at 1.mu.m wavelength. 
The spot for exciting the sample (lenses 26, 28) is scanned in a raster 
pattern by tilting the single scanning mirror 24 about two orthogonal 
axis. A transducer 30 drive by sawtooth voltages provided the signals for 
tilting the scanning mirror 24. The maximum frequency of scan is 
determined by the resonant frequency of the galvonometers 30. In practice 
typical maximum Y rates were found to be 10Hz while the X deflection 
movement could be driven to approximately 300Hz. The semiconductor sample 
27 is positioned on an X-Y-X mechanical stage 34 for ease of either being 
scanned in a raster format or being held stationary while the intensity 
and wavelength are varied. 
Referring now to FIG. 2, the reference signal from thermocouple 32 and the 
sample signal from sample 27 are fed to phase locked amplifiers 31 and 33, 
respectively. The reference signal from chopper 18 is also fed to lock in 
amplifiers 31 and 33. 
Lock in amplifier 31, referenced to the mechanical chopper 18 is used to 
measure the signal from the reference thermocouple 32 and lock in 
amplifier 33 is used to measure the signal from the sample 27 under study. 
As shown in FIG. 2, the apparatus may be operated in two distinct modes. 
In the first mode the beam is held stationary and data taken at various 
wavelengths at one point on the sample 27. As shown in FIG. 1 sample 27 is 
mounted on an X-Y-Z micro positioning stage 34 and by observation and 
visible light through the trinocular head of the microscope it is possible 
to position the light beam on a particular point on the sample while 
adjusting the position of the sample for best beam focus. In the second 
mode, the beam is scanned over the sample at a fixed wavelength. In the 
second mode the data is displayed on a CRT 41, the beam of which is driven 
in synchronism with the scanning light beam by means of reference signals 
from the generators driving the galvonometer movements of scanning mirror 
24. 
Two forms of display are possible as is illustrated in FIG. 2. In the first 
the intensity modulated display, the signal from sample 27 is used to Z 
modulate the CRT beam intensity. In the second form of display a 
pseudo-topographic display is generated by adding the sample signal to the 
reference signal in summing circuit 42 to provide the driving signal for Y 
deflection plates of the CRT 41. For both forms of display retrace 
blanking was accomplished by gating signals available from the deflection 
drivers. 
In order to measure the surface photo voltage generated it is necessary to 
make an electrical contact to both the bulk of the sample and to the 
surface. Ohmic contacts are not necessary. It is only necessary that the 
contact potential follow the potential of the semiconductor. If the light 
beam is modulated then capacitively coupled contacts have been found to be 
satisfactory. Contact to the surface of the sample may be achieved in 
three ways: (1) a metal evaporated onto the surface of the semiconductor 
which will sense .DELTA.C.sub.s directly; (2) an MOS contact may be used 
to sense the signal capacitively; and (3) the photo voltage has been 
detected using a suitable electrolyte. In the latter case no permanent 
contact need be made to the sample being tested. 
In operation and by way of example, all the results presented were obtained 
at ambient room temperature on &lt;100&gt; oriented slices of device grade 
n-type silicon resistivity in the range 1-3.OMEGA. centimeters. The slices 
were 500.mu.m thick with a mirror polish on the front surface and a 
lightly etched saw-cut finish on the back. 
By way of example there is shown in FIG. 3 the photo voltaic response. The 
photo voltaic responsie iterated by a single line scan of a 10.1.mu.m 
diameter light spot across a 400.mu.m diameter semi-transparent Schottky 
barrier. The approximately exponential rise of the signal as the spot 
approaches the electrode as well as the sharp reduction as the spot moves 
on to the electrode are apparent. 
The electrolytic cell of FIG. 4 is used where the photo voltage is being 
detected using an electrolyte. The sample 59 is supported at an edge 
between Delrin (acetal resin) backplate 60 and a phosphor bronze spring 
finger 62, the spring finger serving as the contact to the sample. The 
sample 59 and Delrin support 60 are then immersed in the electrolyte 64 
contained in a glass cell 66. The distance between the sample surface and 
the front window of the cell in actual practice was found to be less than 
2.0Omm. Such a short path into electrolyte is desirable to insure that 
light absorption is minimized. Also it has been found desirable that an 
electrolyte of low absorption over the wavelength range of interest should 
be used. In practicing the present invention the electrolytes used where 
essentially 100% transmitting over the range 0.8 to 1.2.mu.m. A platinum 
reference electrode 68 is attached to the side of the Delrin block 60. The 
photo voltages generated are measured between the phosphor bronze fingers 
62, which should not be immersed in the electrolyte, and the platinum 
electrode 68. The sample signal is then fed to the phase locked amplifier 
33 and the signal processed in the same manner as discussed above. 
In practice it was found that 1% by weight aqueous solution of Na.sub.2 
So.sub.4 electrolyte was found to be satisfactory. 
FIG. 5 shows the scan response at 1.0.mu.m of a piece of silicon immersed 
in the electrolytic cell shown in FIG. 4. FIG. 5a shows the scanned 
response in the intensity mode and FIG. 5b shows the scanned response in 
the X-Y mode. In order to demonstrate the detection of an anomoly in the 
sample FIG. 5 was removed from the electrolyte and a fine scribe line was 
drawn across the back of the sample. The sample was then reimmersed in the 
electrolyte and rescanned as shown in FIG. 5b. It can be seen that the 
scribed line 72 is clearly visible resulting in a reduced response at the 
shorter wavelength, consistent with a reduced value of lifetime, and an 
enhanced response at the longer wavelength. 
As well as providing a qualitative display of these semiconductor 
structures it is also possible to make a quantitative measurement of 
lifetime. FIG. 6 shows the results of a spectral measurement made by 
positioning the light spot (&lt;10.mu.m diameter) between the electrodes of a 
commercially available MOS transistor. As can be seen the measurements of 
the lifeline and the material are linear and by extrapulation with the 
line intersects the abscissa at .alpha..sup.-1 = -32.0.alpha.m. 
Spectral runs were made with the light spot stationary over the scribe line 
and also over the defect region of the sample shown in FIG. 5. The results 
are shown in FIG. 7. Plotted so as to determine diffusion length, the 
results appear as in FIG. 8. It can be seen that the damage introduced by 
scratching the back of the slice has resulted, in the region of the 
scratch, in a decrease in the diffusion length of the sample by a factor 
of approximately 2. 
Obviously many modifications and variations of the present invention are 
possible in the light of the above teachings. It is therefore to be 
understood that within the scope of the appended claims the invention may 
be practiced otherwise than as specifically described.