Device for measuring semiconductor characteristics

A device for measuring semiconductor characteristics, wherein electrodes are installed maintaining a gap on the front and back sides of a semiconductor specimen of which the characteristics are to be measured, at least one of the electrodes being transparent, the surface of the semiconductor specimen is scanned with a pulsed narrow photon beam via the transparent electrode, and a photovoltage generated between the front and back surfaces of the semiconductor specimen is taken out from the two electrodes via the capacitive coupling, in order to observe the distribution of characteristics in the surface of the semiconductor specimen.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a device for measuring semiconductor 
characteristics relying upon a photovoltaic method. 
2. Description of the Prior Art 
The photovoltaic method has long been employed in the field of 
semiconductor measurements, owing to its advantage as a non-contact 
measuring method over, for example, the four-point-probe method for 
measuring resistivity. FIG. 1 is a diagram for illustrating the 
fundamental principle of a conventional method of measuring resistivity 
distribution of semiconductor specimens utilizing a photon beam. 
When a surface 2' of a semiconductor specimen 2, which spreads in a 
two-dimensional manner, is irradiated with a photon beam 1, electron-hole 
pairs consisting of holes 3 and electrons 4 usually develop on the surface 
2' of the specimen 2, and diffuse toward the back surface 2" of the 
specimen 2 as indicated by arrows 3', 4'. In the case of silicon, as is 
well known, however, the electrons 4 have greater mobility than that of 
the holes 3. In other words, the electrons 4 move in larger number than 
the holes 3 toward the back surface 2". Therefore, the holes 3, having 
positive charge, are left in large amounts on the surface 2' of the 
semiconductor specimen 2 and, consequently, the surface 2' of the specimen 
2 is positively charged. This phenomenon was reported in 1931 by H. Dember 
of Germany, and has, since then, been known as the Dember effect. The 
voltage produced by the Dember effect, i.e., the Dember voltage, however, 
is much smaller than the voltage that develops when the p-n junction is 
irradiated with light, and has not heretofore been utilized for any 
specific purposes. 
The inventors of the present invention have found that the following result 
is obtained from the n-type wafers, such as those formed of silicon, 
##EQU1## 
where .DELTA.V.sub.D denotes a Dember voltage, and each of the symbols has 
the following meaning: 
b: mobility of electrons/mobility of holes, 
S: area of wafer, 
.rho.(0): resistivity of the wafer surface, 
e: electric charge of the electron, 
I: intensity of the photon beam (photon flux/sec), 
.alpha.: photon beam absorption coefficient, 
L.sub.p : diffusion length for minority carriers, 
V.sub.p : diffusion velocity for minority carriers 
S.sub.f : recombination velocity of carriers on the wafer surface. 
As is obvious from the equation (1) above, the Dember voltage is dependent 
upon many factors. If all of the factors except for the resistivity 
.rho.(0) are regarded as being constant, the above equation can be written 
as, 
EQU .DELTA.V.sub.D =K.multidot..rho.(0) (2) 
where K is a constant. 
Namely, if the semiconductor specimen (wafer) 2 without the junction is 
scanned by converging the photon beam 1 and if the distribution of 
photovoltage at that time is measured, the measured result is a Dember 
voltage, which, finally, is equal to the measurement of resistivity 
distribution on the surface of the specimen 2. 
A Schottky junction has heretofore been used to detect the distribution of 
resistivity. FIG. 2 illustrates a fundamental principle thereof. An ohmic 
electrode 6 is attached to the back surface 2" of the semiconductor 
specimen 2, a metal probe 5 is erected on the surface of the specimen 2, 
and the vicinity of the probe 5 is irradiated with the photon beam 1. As 
is well known, a photovoltage develops in the Schottky junction 5' and is 
measured by a voltmeter 7. Usually, the intensity of the photovoltage 
depends upon the resistivity of the portion of the specimen 2 to which the 
metal probe 5 is opposed. Therefore, the indication of the voltmeter 7 
varies in proportion to the resistivity. As for the surface 2 of a wide 
wafer, the metal probe 5 needs to be simply moved. In practice, however, 
this operation is not practical. As shown in FIG. 3, therefore, mesh 
electrodes 8 are pressed with pressure onto the specimen 2 to form a 
Schottky junction 8" on the whole surface. By scanning the specimen 2 with 
the photon beam 1, it is possible to detect the distribution of 
resistivity on the surface 2'. 
The conventional method shown in FIG. 3, however, has defects. First, 
characteristics of the Schottky junction 8" depend on the mechanical 
pressure of the metal, surface conditions of the metal (roughness, oxide 
layer, etc.), and surface conditions of the semiconductor (oxide layer, 
humidity, dust, etc.), which make it difficult to form a uniform junction 
over wide areas. Second, portions of the surface are covered with mesh 
electrodes 8, so the whole surface of the specimen 2 is not irradiated 
with the photon beam 1. Third, attachment of the ohmic electrode 6 damages 
the specimen 2, and makes it difficult to carry out a perfect 
non-destructive insepection. 
Measuring the characteristics of the specimen 2 by forming a Schottky 
junction using an electrolyte 13 such as Na.sub.2 SO.sub.4 as one 
electrode as shown in FIG. 4, based upon the same principle as the method 
of FIG. 3 has also been reported. The electrolyte 13, however, involves 
clumsy operation if it is attempted to use it as a transparent electrode. 
Further, the ohmic electrode 6 must be attached onto the back surface, as 
in the above-mentioned prior art. In FIG. 4, reference numeral 12 denotes 
an electrode, and 14 denotes a side wall of a vessel for storing the 
electrolyte. 
As mentioned above, there has not heretofore been known any method of 
photovotaically measuring the resistivity distribution of the surface of 
the silicon wafer without damaging the specimen being measured. 
SUMMARY OF THE INVENTION 
The object of the present invention is to provide a device for 
photovotaically measuring the distribution of characteristics in the 
surface of the semiconductor specimen without damaging the semiconductor 
specimen. 
In order to accomplish this objective, the device according to the present 
invention is characterized in that electrodes are attached to both 
surfaces of a semiconductor specimen of which the characteristics are to 
be measured maintaining a gap, at least one of the electrodes is 
transparent, the surface of the semiconductor specimen is scanned by a 
narrow pulsing photon beam through said transparent electrode, and the 
photovoltage generated between the two electrodes is measured through the 
capacitive coupling to determine the distribution of characteristics of 
the surface of the semiconductor specimen. 
In a more advantageous setup of the present invention, transparent 
electrodes are attached to both sides of the specimen, and the light which 
passes through the semiconductor specimen is also detected to measure the 
absorbency as well as the photovoltage.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
FIG. 5 shows the principle of a measuring method according to the present 
invention. Namely, according to the present invention, a Dember voltage 
can be correctly measured when a given portion of the specimen 2 is 
irradiated with photon beam 1. As mentioned already, the Dember voltage 
generates between the front surface 2' and the back surface 2" of the 
specimen 2. Therefore, electrodes 8, 9 are attached to such surfaces 
maintaining a clearance. If the photon beam 1 is converted into pulses, 
the Dember voltage is also generated in the form of pulses. Therefore, 
even if the electrodes 8, 9 are separated from the specimen 2, it is 
possible to detect the Dember voltage owing to the capacitive coupling 
that results from air gap capacitances 10, 11. 
FIG. 6 illustrates an equivalent circuit of FIG. 5. Capacitances 10 and 11 
are present above and below the specimen 2, generating voltage upon 
irradiation with the photon beam 1, and a voltmeter 7 is connected to 
generating specimen 2 via capacitances 10, 11. 
Referring to FIG. 5, a transparent electrode 8 is formed by, for example, 
coating the surface of glass with indium oxide. Owing to the transparent 
electrode 8, the photon beam 1 is permitted to reach the specimen 2 
without being greatly absorbed. The electrode 9 may be either a 
transparent one like electrode 8 or an opaque one. 
In principle, therefore, the semiconductor specimen 2 is placed between 
electrode 8 and electrode 9 without being in direct contact with them. 
Therefore, this method is a completely non-destructive method. 
FIG. 7 illustrates an embodiment of a device using the present invention, 
in which a cathode-ray tube 17 is used as a light source for photon beam 
20. The wavelengths of the photon beam 20 are trimmed to a suitable range 
through an optical filter 18, and it is focused onto the semiconductor 
specimen 2 by an optical lens 19. The photon beam 20 can be scanned by 
scanning the electron beam (not shown) in the cathode-ray tube 17. The 
scanning velocity and scanning area are adjusted by suitably controlling 
the voltage from a scanning voltage source 31 by a controller 32, and 
supplying a current converted from the voltage to a deflection coil 16. 
The same scanning signals are fed to deflection coils 27, 29 of 
cathode-ray tubes 26, 28 for indicating the scanning picture. In the 
cathode-ray tube 28, in particular, the signals from the semiconductor 
specimen 2 are superposed on a deflection current by an adder 30. As is 
well known, the amplitude-modulated scanning picture is obtained if 
signals from the semiconductor specimen 2 are added to the deflection coil 
29. 
FIG. 7 illustrates the specimen 2 sandwiched between the two transparent 
electrodes 8, 8' to detect the intensity and wavelength distribution of 
the photon beam 20' which has passed through the specimen 2. Namely, if 
the transmitted light 20' is detected and analyzed by a detector 21, 
consisting of a photodiode, and if the output is amplified through an 
amplifier 22, it is possible to obtain the impurity concentration by a 
well-known principle. Consequently, which factor among the factors of 
Dember voltage in equation (1) gives the most pronounced effect can be 
more reliably determined, to obtain increase synergistic effects. For 
instance, if the Dember voltage varies greatly while no change develops in 
the intensity of the transmitted light 20', it is proper to consider that 
the surface recombination speed S.sub.f has changed quickly rather than to 
consider that the resistivity .rho.(0) has changed. 
According to the present invention as already mentioned, the Dember voltage 
is measured by capacitive coupling. For this purpose, the photon beam 20 
is converted into pulses. The pulsation is accomplished by pulsing the 
electron beam of the cathode-ray tube 17 by modulating the brightness of 
the cathode-ray tube 17 with a pulse source 15. The pulse voltage is also 
used for phase-sensitive demodulating of the signals. In other words, the 
pulse voltage is used as a reference voltage for a phase-sensitive 
demodulator 25, which markedly improves the signal-to-noise ratio of the 
signals. 
The amplified and phase-sensitive demodulated signals are used for 
modulating the brightness of the cathode-ray tube 26 and also for 
modulating the amplitude of the cathode-ray tube 28. 
Referring again to FIG. 7, spacers 39, 39' are inserted between the 
electrodes 8, 8' and the specimen 2, such that the specimen 2 can be 
brought adjacent to the electrodes 8, 8' without being damaged. The 
spacers 39, 39' will be made of a light-transmitting insulating film such 
as mica, Mylar, polyethylene, or the like, and will have a thickness of 
several tens of microns or smaller. 
An embodiment of the present invention was illustrated in the foregoing 
with reference to FIG. 7. According to the present invention, however, the 
light source is not restricted to cathode-ray tube 17 only, but may be 
another source of light such as a laser. Further, the photon beam 20 can 
be scanned by a moving mirror. 
Further, although the foregoing description has dealt with the case of 
measuring the resistivity distribution only, it is possible to measure any 
of the other characteristics in equation (1). Moreover, the device of the 
present invention can be used to measure the characteristics of wafers 
having a p-n junction formed by ion implantation or having a junction 
consisting of regions of the same type of conductivity but having 
different impurity concentrations, wafers having an oxide film or having 
an oxide film which contain a fixed charge therein, as well as wafers 
having surface (or interface) states. In the case of wafers having a p-n 
junction, for example, the uniformity of junction is displayed on the 
display tube within short periods of time. Therefore, whether the wafer 
can be used for producing solid-state circuit elements or not can be 
readily discriminated, to present great merit from an industrial point of 
view.