Method for measuring lifetime of semiconductor material and apparatus therefor

A method and apparatus measure the lifetime of a semiconductor material by directing microwave energy into the semiconductor material and by producing carriers within the semiconductor material by impinging light thereon. A non-metal material is interposed between the semiconductor material and a metallic surface, such that a portion of the microwave energy travels through the semiconductor material and the non-metal material and reflects off of the metallic surface and back through the non-metal material and the semiconductor material. Additionally, a heating member is provided for heating the semiconductor material, whereby the lifetime of the semiconductor material is determined according to characteristics of the reflected microwave energy and the temperature of the semiconductor material.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
The present invention relates to a method for measuring the lifetime of a 
semiconductor material and an apparatus therefor, and in particular, to a 
method and a device for determining the lifetime of the semiconductor 
material in which pulse energy is injected into the semiconductor material 
in order to generate a particular attenuation of a carrier, and by 
measuring the attenuation characteristics of the carrier, highly reliable 
and effective detection of a defective semiconductor material crystal and 
a very small heavy metal taint is enabled. 
2. Description of the Prior Art 
Semiconductor materials, such as Si, Ga and As, are processed according to 
several hundreds of processing steps, including raw material preparation 
steps through to semiconductor device manufacturing steps. These raw 
material preparation and the semiconductor device manufacturing steps 
include material washing, impurity diffusion, thermal treatment, 
patterning and etching, and each disadvantageously introduces the 
possibility of the generation of defective crystals and heavy metal taints 
in the semiconductor material. Furthermore, since the number of such 
troublesome processing steps is very high, the control or management of 
these steps, which necessitate many laborious and precise operation, is 
very difficult. 
Nevertheless, it is inevitable that some semiconductor materials or chips 
having defective crystals and/or heavy metal taints or pollutants will be 
produced at a certain rate. As a result, a number of devices for 
identifying defective crystals and fine metal taints in semiconductor 
materials or chips have been widely marketed. A general and conventional 
process for measuring and analyzing defective crystals in the 
semiconductor materials or chips has been used for determining the 
respective lifetimes of the chips. 
FIG. 1 is a block diagram of the structure of a prior art apparatus for 
measuring the lifetime of a semiconductor material. 
As shown in FIG. 1, microwave energy generated by a microwave oscillator 1 
is guided by a magic tee 4 to a wave guide 8 via an impedance matching 
device 2 and an E-H tuner 3, and is irradiated onto a semiconductor 
material 10 which is the object of the measurement. The microwave energy a 
irradiated onto the semiconductor material 10 is reflected from the near 
surface of the material 10, from within the material 10, and from the 
reverse surface of the material 10, respectively, and returns to the magic 
tee as microwave energy b which is guided by the magic tee 4 to the E-H 
tuner 6 and detected by a detector 7. 
The principle of the lifetime measurement will now be explained referring 
to FIG. 2, where the reference a denotes microwave energy which is 
constantly applied to the semiconductor material 10. Carriers are 
operatively produced when external energy is fed to the semiconductor 
material 10 in the form of pulses from a laser diode 9 at a time of the 
measurement. Since this area producing the carriers is equivalent to the 
semiconductor turning into a conductor, and since the microwave energy is 
reflected from the produced carriers (microwave energy is reflected 100% 
on a metal), the reflection of the microwave energy is detected as a 
temporary increase in the region. The chronological change of the 
increased microwave energy coincides with the chronological attenuation 
waveform of the produced carriers. Therefore, the crystal in the 
semiconductor material 10 can be evaluated by measuring the attenuation 
waveform (lifetime) of the produced carriers. 
The respective views FIGS. 3A to 3C show examples of microwave irradiation 
set ups of a conventional apparatus for measuring the semiconductor 
lifetime. In particular, FIG. 3A depicts a microwave irradiation set up 
provided with a measurement table 21 made of non-metal material and a 
waveguide 8 for emitting and receiving the microwave energy, and a 
semiconductor material 10 to be measured being placed on the measurement 
table 21. FIG. 3B shows another microwave irradiation set up provided with 
the waveguide 8 described above and a measurement holder 22 made of 
non-metal material on which a semiconductor material 10 to be measured is 
held, the measurement holder 22 being used in place of the measurement 
table 21 shown in FIG. 3A. FIG. 3C shows still another example of a 
microwave irradiation set up having a metal plate 23 from which the 
microwave energy reflects, in addition to the measurement table 21 having 
the semiconductor material 10 placed thereon and the waveguide 8. 
A contactless inspection method, such as a DLTS method (Deep Level 
Transient Spectroscopy), has been widely employed to detect fine or very 
small metal taints in a semiconductor material. According to the DLTS 
method, a diode is formed on a substrate and a voltage is impressed on the 
diode, generating a response to the impression after the voltage is cut 
off. The response as shown in FIG. 13D is measured or determined in the 
form of a changed amount I in the electric signal between the instants 
t.sub.1 and t.sub.2. The relation between the values of the changed amount 
I and temperature are plotted to determine the degree of metal taints 
using particular thermal peak values of the changed amount I based on 
various metals. 
Although the object materials of the lifetime measurement are usually 
semiconductor materials, such as a Si-wafer, the resistivity of the 
materials ranges fairly extensively depending on the usage of devices. 
The waveguide 8 which is used in the prior art lifetime measuring apparatus 
can be equivalently replaced with a distribution circuit as shown in FIG. 
4A. If the distribution circuit is terminated with a terminal resistance 
Z.sub.1, a reflected signal corresponding to the terminal resistance 
Z.sub.1 is produced as shown in FIG. 4B. Accordingly, in the case of the 
apparatus shown in FIG. 1 for example, the horizontal axis Z in FIG. 4B 
may be replaced with the resistivity .rho..sub.s since the terminal 
resistance Z.sub.1 is equivalent to the resistivity of the Si-wafer. The 
signal of the reflected microwave energy measured in the regions a, c and 
b corresponding to the resistivities .rho..sub.a, .rho..sub.o1 and 
.rho..sub.b shown in FIG. 4B becomes as shown in FIGS. 5A, 5B and 5C, 
respectively. The region c is where the measurement is impossible. In 
other regions, such as d, e and f, the reflected microwave signals to be 
measured are greatly influenced by the non-linear characteristics of the 
above mentioned waveguide to thereby have significantly changed in signal 
intensity and deteriorated in data reproducibility. The prior art method 
is problematic since it cannot measure some of the object materials with a 
high reliability. 
From another standpoint the prior art is problematic in that according to 
the conventional process for analyzing a character of the semiconductor 
material or for locating any defective crystals as shown in FIG. 3A, the 
effective amount of the reflected microwave signal is small due to the 
effects of the microwave energy a' passing disadvantageously through the 
semiconductor material 10 and the microwave energy b' reflecting from the 
measurement table 21. According to the conventional device as shown in 
FIG. 3B, the microwave energy b has a small S/N ratio value because the 
microwave energy passes through the semiconductor material 10 and is 
reflected on the measurement holder 22. In order to improve the S/N ratio 
of the signal by making the microwave a' passing through the semiconductor 
material 10 reflected on the metal plate 23 so as to increase the volume 
of the microwave e,crc/b/ reflected from the semiconductor material 10, 
the device shown in FIG. 3C is utilized. This device essentially has 
disadvantages in that the signal output is unstable in its amplitude and 
period since the two microwave signals overlap in their phases. 
Especially, with regard to the device shown in FIG. 3C, the waveshape of 
the reflected microwave signal changes as shown in FIGS. 5A to 5C 
according to respective positions d.sub.1, d.sub.2 and d.sub.3 of the 
metal plate 23 as shown in FIG. 6, thus disadvantageously generating 
non-effective signals. In brief, the reflected microwave signal is apt to 
change according to the particular thickness of the semiconductor material 
10 to be measured, the position of the metal plate 23, the thickness of 
the measurement table 21, the positional relationship between the 
measurement table 21 and the waveguide 8, and the like, thus generating 
data of little reliability. 
It is noted that the amount of the reflected microwave signal is very small 
when a short lifetime of the semiconductor chip is measured, so that the 
reliability of data deteriorates because that the microwave signal is 
amplified through an amplifier and the lifetime data is electrically 
delayed. 
From still another standpoint the prior art is problematic, in that 
according to the conventional inspection method a contact breakage 
inspection must be carried out for the semiconductor chips or material. 
While the conventional process for measuring the lifetime of the 
semiconductor material has considerable positive achievements concerning 
the measurements of defective crystal lattices, metal taint O.sub.2 swirls 
and the like, it has problems in the inspection of very small metal taints 
and surface and bulk lifetimes. Accordingly, the DLTS method of a breakage 
type has been used unavoidably to detect very small metal taints. 
It is apparent from above that it has not been possible to detect very 
small metal pollutants in semiconductor chips using a non-contact method. 
SUMMARY OF THE INVENTION 
The present invention is provided considering the above situation. It is 
accordingly a primary object of the present invention to provide a method 
for measuring the lifetime of a semiconductor material which can detect 
crystals existing in the material with a high reliability. 
It is another object of the present invention to provide a method and an 
apparatus for the lifetime measurement of the semiconductor material which 
is improved in measurement reliability through improved data 
reproducibility and adaptable to the semiconductor material to be 
measured. 
It is still another object of the present invention to provide a method and 
an apparatus for measuring the lifetime of the semiconductor material by 
detecting the very small metal taints in the semiconductor material or 
chips. 
According to one aspect of the present invention, for achieving the objects 
described above, there is provided a method for measuring the lifetime of 
a semiconductor material which includes positioning a metal face on which 
microwave energy passing through the semiconductor material reflects so as 
to be opposed to the semiconductor material, determining a particular 
distance between the semiconductor material and the metal face, such a 
distance making the effect of a microwave energy portion reflected on the 
metal face to another portion of the microwave energy smallest, 
positioning a non-metal material or member having a thickness identical to 
the distance in a space between the semiconductor material and the metal 
face, and finely adjusting the distance between the wave detector or 
waveguide irradiating the microwave energy and the semiconductor material. 
According to another aspect of the present invention, there is provided a 
method for measuring the lifetime of a semiconductor material using 
equivalent distribution circuit characteristics of a waveguide which is 
used for irradiating microwave onto the semiconductor material variable, 
shifting regions where the measurement is impossible due to 
characteristics outside of a measurable area on the semiconductor 
material, and conducting the measurement within the region of the 
equivalent distribution circuit characteristics which are linearized. 
There is provided an apparatus for measuring the lifetime of the 
semiconductor material including an adjusting means on the waveguide for 
irradiating the microwave onto the semiconductor materials for making the 
equivalent distribution circuit characteristics thereof variable. 
According to still another aspect of the present invention, there is 
provided a method for measuring the lifetime of a semiconductor material 
which includes changing the temperature of the semiconductor material when 
a lifetime of the semiconductor material is measured. 
The nature, principle and utility of the present invention will become more 
apparent from the following detailed description when read in conjunction 
with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENT 
The preferred embodiment of the method for measuring the lifetime of a 
semiconductor material and the apparatus therefor according to the present 
invention will be explained with reference to the drawings. 
According to the apparatus of the present invention, as shown in FIG. 7A, a 
quartz glass plate 24 is placed on a metal table 25, and a semiconductor 
material 10 to be measured is placed on the quartz glass plate 24. A more 
detailed view is shown in FIG. 7B. 
In the operation of the measurement apparatus shown in FIGS. 7A and 7B, a 
portion of the microwave energy irradiated from a waveguide 8 (for 
outputting and receiving microwave energy) reflects on the semiconductor 
material 10 and another portion of the microwave energy passes through the 
semiconductor material 10. A portion of the microwave energy which has 
passed through the semiconductor material 10 reflects on the quartz glass 
plate 24 and another portion of this microwave energy passes through the 
quartz glass plate 24 and reaches the surface of the metal table 25. It is 
understood that the reflected microwave energy received by the waveguide 8 
is mainly that which has reflected from the semiconductor material 10 and 
the metal table 25. The phasic relation between these two reflected 
microwave energy is determined by the distance between the semiconductor 
material 10 and the metal table 25. Accordingly, it is possible to 
determine this distance using the effect of the portion of the microwave 
energy reflected from the metal table 25 to the portion of the microwave 
energy reflected from the semiconductor material 10 by continuously 
checking the phasic relation. When the thickness of the semiconductor 
material 10 to be measured is previously determined, the distance between 
the semiconductor material 10 and the metal table 25 is controlled by 
adjusting the thickness (for example, 2 to 3 mm) of the quartz glass plate 
24. Accordingly, it is possible to maximize the intensity of the effective 
reflective microwave energy. Further, when the thickness of the 
semiconductor material 10 varies a little (for example, it is shown by P 
or Q), the position of the waveguide 8 for outputting and receiving the 
microwave is finely adjusted to instantly obtain the maximum reflective 
microwave energy as shown in FIG. 8. 
According to the preferred embodiment of the measurement method of the 
present invention, the metal plate 23 shown in FIG. 3C is used and a 
quartz glass plate having a fixed-thickness is introduced. The quartz 
glass plate is employed in the preferred embodiment as anon-metal material 
of the measurement table 21 in FIG. 3C. However, it is apparent that any 
non-metal material other than quartz glass may be used in the method of 
the present invention. 
It is noted that the method for measuring the lifetime of the semiconductor 
material according to the present invention enables an increased output 
precision by more than several fold in comparison to the conventional 
measurement method and a constant generation of highly reliable lifetime 
signals having no-strain through the reflective microwave energy. When the 
semiconductor material to be measured by the present invention is the 
widely employed CZ-silicon, the signals containing the lifetime 
information can be treated without applying any amplifying steps, can 
enjoy a wide range of measurable proportional resistivities and can result 
in a significantly improved S/N ratio. In addition, notwithstanding the 
relatively easy and simple treatment of the output signals, it is possible 
to obtain an improved reliability, economy and maintenance. 
In FIG. 9, the microwave energy oscillated by a microwave oscillator 1 is 
directed to a waveguide 8 via a magic tee 4 and irradiated onto a 
semiconductor material (not shown) which is an object of the measurement. 
The microwave energy is reflected by the semiconductor material to return 
to the waveguide 8, passed through the magic tee 4 and detected by a 
detector 7. The waveguide 8 is provided with a stub tuner 12. 
The stub tuner 12 has a structure which is shown in the enlarged view of 
FIG. 10 wherein the distance D between three screws 13.sub.1, 13.sub.2 and 
13.sub.3 is determined by the frequency of the microwave energy to be 
used. The distribution circuit of the waveguide 8 may be made variable by 
providing the stub tuner 12 on the waveguide 8 and by adjusting the 
lengths l.sub.1, l.sub.2 and l.sub.3 inserted within the waveguide 8. The 
above arrangement can also transform the characteristic curve of the 
reflected microwave signals from the curve a denoting the arrangement 
without the stub tuner (to position where measurement is impossible being 
at Z.sub.01) to the curve b as shown in FIG. 11. The point where a 
measurement is impossible may be avoided for almost all materials by 
setting the resistivity, for example, at 100 .OMEGA.m in the case of a 
Si-wafer. Accordingly, the reflected microwave signals are outputted as an 
ideal waveform as shown in FIG. 5A. The curve b in FIG. 11 is improved to 
assume a relatively linear form relative to the non-linear characteristics 
in the region extending toward a point Z.sub.02, and the amplitude 
variation or distortion of the reflected microwave can also be restricted. 
Although a stub tuner is used as the means to make the equivalent 
distribution circuit of the waveguide variable in the above embodiment, 
such means is in no way limited to the above and various modifications are 
possible, without departing from the scope of the appended claims. 
As described in the foregoing, the method and apparatus for measuring the 
lifetime of the semiconductor material according to the present invention 
is highly effective since it can measure all the semiconductor materials 
to obtain accurate reflected microwave signals, it can significantly 
enhance the overall measurement reliability as well as data 
reproducibility and it can realize a flexible measurement arrangement. 
As shown in FIG. 12, a heat-resisting or refractory member 35 is placed on 
an X-Y stage 36 and a heater 37 is buried in the upper portion of the 
refractory member 34. On the refractory member 34, a non-metal refractory 
plate 33 is placed. The semiconductor material 10 to be measured is placed 
on the system consisting of the non-metal refractory plate 33 and the 
refractory member 34 and the X-Y stage 36. In operation, microwave energy 
irradiates through a waveguide 8 (for outputting and receiving the 
microwave energy) which is placed above the semiconductor material 10 and 
excitation rays of wavelengths .lambda..sub.1 and .lambda..sub.2 are 
outputted from the laser diodes 9.sub.1 and 9.sub.2. The semiconductor 
material 10 which is polluted by metal taints is gradually heated by the 
heater 37 embedded in the refractory member 34 then a lifetime of the 
semiconductor 10 is measured using the reflective microwave energy passing 
therethrough. The measurement results as shown in FIG. 13A depict large 
changes in the lifetime of the semiconductor material at a certain 
temperature. This phenomenon is generated because the energy levels of 
very small metal taints contained in the silicon material approach those 
of electrical conductors when the silicon is heated and the excited 
electrons are apt to disappear. FIG. 13B shows an example of the lifetime 
changes of measurement data in which graphed on the abscissas is the 
inverse temperature 1/temperature (1/T) and on the ordinates is lifetime 
.tau.. Measurement data was obtained on a sample a of a semiconductor 
material having metal diffused and a second sample b having no metal. The 
lifetime of the second sample b is lengthened when the temperature of the 
semiconductor material 10 exceeds a certain level. However, the lifetime 
of the first sample a doesn't extend as much, generating a large 
difference between the lifetimes of the two samples. The above result has 
a correlation with the peak in the measurement data obtained the DLTS 
method as shown in FIG. 13C. 
By previously determining on an experimental basis a relation between the 
temperature 1/Ta shown in FIG. 13B and the temperature Tb shown in FIG. 
13C, according to the non-contact and non-destructive method for measuring 
the lifetime of the semiconductor material 10 of the present invention, it 
is possible to judge an existence of very small metal taints and to 
determine the type of such a metal. It is possible to presume that almost 
all of the pollutants in the semiconductor material concentrates in the 
surface of the semiconductor chip, so that heating the semiconductor chip 
and measuring its lifetime results in a lifetime of the much polluted 
surface of the semiconductor material and another lifetime of the little 
polluted interior of the semiconductor chip, and thus a separative 
analysis of the surface and bulk lifetimes is possible at a high S/N 
ratio. 
While the preferred embodiments employs a heater as the heating means as 
herein disclosed, it is to be understood that other forms of heating might 
be adopted. 
By measuring the lifetime of the semiconductor material after it is warmed 
according to the present invention, it is possible to determine the 
existence of fine heavy metal taints which are identified conventionally 
only by destructive type methods, and to separately evaluate the surface 
recombination velocity (surface lifetime), thus obtaining an advantageous 
lifetime measurement system. 
It should be understood that many modifications and adaptations of the 
invention will become apparent to those skilled in the art and that the 
invention is intended to encompass such obvious modifications and changes 
in the scope of the claims appended hereto.