Patent Publication Number: US-2015077143-A1

Title: Apparatus for testing resistivity of semiconductor and method for testing resistivity of semiconductor

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is based upon and claims the benefit of priority from Japanese Patent Application No. 2013-194311, filed on Sep. 19, 2013; the entire contents of which are incorporated herein by reference. 
     FIELD 
     Embodiments described herein relate generally to a apparatus for testing resistivity of semiconductor and a method for testing resistivity of semiconductor. 
     BACKGROUND 
     Methods for testing the resistivity of a semiconductor include electrical test methods such as the four-point probe method and C-V measurement, for example. However, the electrical test methods require a relatively long measurement time. In addition, they need destructive measurement in which an object to be measured is processed to prepare a measurement sample, like electrode formation or the like, for example. 
     Other test methods include optical test methods such as Fourier transform infrared spectroscopy (FT-IR) and ellipsometry, for example. Such optical test methods use infrared light emitted from a lamp light source. However, in the optical test methods using a lamp, although non-destructive measurement is possible, measurement accuracy is low as compared to the measurement accuracy of the electrical test methods. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a block diagram showing an apparatus for testing a resistivity of a semiconductor according to an embodiment of the invention; 
         FIG. 2  is a graph illustrating an example of a relationship between an wavelength and a reflectance of reflected light reflected at SiC (silicon carbide); 
         FIG. 3  is a block diagram showing another apparatus for testing a resistivity of a semiconductor according to an embodiment of the invention; 
         FIG. 4A  and  FIG. 4B  are schematic diagrams showing signals outputted by a detector and a first lock-in amplifier; 
         FIG. 5  is a block diagram showing still another apparatus for testing a resistivity of a semiconductor according to an embodiment of the invention; 
         FIG. 6  is a flow chart showing a method for testing a resistivity of a semiconductor according to an embodiment of the invention; and 
         FIG. 7  is a flow chart showing another method for testing a resistivity of a semiconductor according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     In general, according to one embodiment, an apparatus for testing the resistivity of a semiconductor configured to test a resistivity of a semiconductor formed on a matrix. The apparatus includes: a first chopper configured to chop infrared light with a first wavelength emitted from a first light source at a first chopping frequency, the first wavelength being a wavelength between a transverse optical phonon frequency and a longitudinal optical phonon frequency; a second chopper configured to chop infrared light with a second wavelength emitted from a second light source at a second chopping frequency, the second wavelength being a wavelength different from the first wavelength; an irradiation unit configured to cause synthetic light produced by synthesizing the infrared light with the first wavelength and the infrared light with the second wavelength together to be applied to the semiconductor; a detector configured to detect an intensity of reflected light of the synthetic light reflected at the semiconductor; a first lock-in amplifier configured to detect a signal with the first chopping frequency out of a signal transmitted from the detector; a second lock-in amplifier configured to detect a signal with the second chopping frequency out of a signal transmitted from the detector; and a computer configured to estimate a ratio between a reflectance of the infrared light with the first wavelength out of the reflected light and a reflectance of the infrared light with the second wavelength out of the reflected light on the basis of signals transmitted from the first lock-in amplifier and the second lock-in amplifier. 
     Hereinbelow, embodiments of the invention are described with reference to the drawings. In the drawings, like components are marked with the same reference numerals, and a detailed description is omitted as appropriate. 
       FIG. 1  is a block diagram showing an apparatus for testing the resistivity of a semiconductor according to an embodiment of the invention. 
     The apparatus for testing the resistivity of a semiconductor according to the embodiment tests the resistivity of a semiconductor formed on a matrix (for example, a semiconductor wafer). An apparatus for testing the resistivity of a semiconductor  100  shown in  FIG. 1  includes a first light source  111 , a second light source  112 , a first chopper  131 , a second chopper  132 , an irradiation unit  120 , a detector  151 , a first lock-in amplifier  161 , a second lock-in amplifier  162 , a signal processing device  165 , a computer  170 , and a stage  181 . 
     The irradiation unit  120  of the apparatus for testing the resistivity of a semiconductor  100  shown in  FIG. 1  includes a first lens  121 , a second lens  122 , a first dichroic mirror  141 , a second dichroic mirror  142 , a third lens  125 , and a third dichroic mirror  145 . The apparatus for testing the resistivity of a semiconductor  100  shown in  FIG. 1  includes a fourth lens  126 , a fifth lens  127 , and a fourth dichroic mirror  146  as appropriate. The installation configuration of the lenses and the dichroic mirrors is not limited to the installation configuration shown in  FIG. 1  but may be altered as appropriate. The second dichroic mirror  142  is not limited to a dichroic mirror, and any mirror capable of reflecting infrared light L2 may be used, for example. The third dichroic mirror  145  is not limited to a dichroic mirror, and any mirror capable of reflecting infrared light L3 in which infrared light L1 and infrared light L2 are synthesized with each other may be used, for example. The fourth dichroic mirror  146  is not limited to a dichroic mirror, and any mirror capable of reflecting reflected light L4 reflected at a semiconductor of a sample  200  may be used, for example. 
     The apparatus for testing the resistivity of a semiconductor  100  according to the embodiment may not necessarily include the first light source  111  and the second light source  112 . 
     The first light source  111  is a semiconductor laser such as a quantum cascade laser, for example, and emits infrared light L1 of a single wavelength λ1. The second light source  112  is a semiconductor laser such as a quantum cascade laser, for example, and emits infrared light L2 of a single wavelength λ2. 
     The wavelength λ1 of the infrared light L1 emitted by the first light source  111  is different from the wavelength λ2 of the infrared light L2 emitted by the second light source  112 . The wavelength of each of the infrared light L1 and the infrared light L2 is approximately 8 micrometers (μm), approximately 10 μm, approximately 12 μm, or approximately 14 μm, for example. 
     The first chopper  131  opens and closes the infrared light L1 emitted from the first light source  111  at a prescribed frequency (chopping frequency). That is, the first chopper  131  produces a light quantity change in accordance with the chopping frequency by periodically chopping the infrared light L1 emitted from the first light source  111 . 
     The second chopper  132  opens and closes the infrared light L2 emitted from the second light source  112  at a prescribed frequency (chopping frequency). That is, the second chopper  132  produces a light quantity change in accordance with the chopping frequency by periodically chopping the infrared light L2 emitted from the second light source  112 . 
     The chopping frequency f1 of the first chopper  131  is different from the chopping frequency f2 of the second chopper  132 . One of the chopping frequency f1 and the chopping frequency f2 is not an integral multiple of the other of the chopping frequency f1 and the chopping frequency f2. The chopping frequency f1 of the first chopper  131  is approximately 30 kilohertz (kHz), for example. The chopping frequency f2 of the second chopper  132  is approximately 50 kHz, for example. However, the chopping frequency f1 of the first chopper  131  and the chopping frequency f2 of the second chopper  132  are not limited thereto. 
     In the apparatus for testing the resistivity of a semiconductor  100  shown in  FIG. 1 , the irradiation unit  120  synthesizes the infrared light L1 emitted from the first light source  111  and the infrared light L2 emitted from the second light source  112  together, and applies the synthesized infrared light (synthetic light) to a prescribed range of a sample  200 . Specifically, the first lens  121 , the first dichroic mirror  141 , the third lens  125 , and the third dichroic mirror  145  cause the infrared light L1 emitted from the first light source  111  to be applied to the prescribed range of the sample  200 . The second lens  122 , the second dichroic mirror  142 , the first dichroic mirror  141 , the third lens  125 , and the third dichroic mirror  145  cause the infrared light L2 emitted from the second light source  112  to be applied to the prescribed range of the sample  200 . 
     At this time, the infrared light L1 that has passed through the first dichroic mirror  141  and the infrared light L2 reflected at the first dichroic mirror  141  are synthesized with each other. The infrared light L3 (synthetic light) in which the infrared light L1 and the infrared light L2 are synthesized with each other is applied to the prescribed range of the sample  200  coaxially. That is, the infrared light L3 having a plurality of wavelengths is applied to the prescribed range of the sample  200  coaxially. 
     The sample  200  includes a semiconductor to be tested and a matrix. The semiconductor to be tested is formed on the surface of the matrix. The semiconductor to be tested is formed on the surface of the matrix by epitaxial growth, for example. The sample  200  is mounted on the stage  181 . The stage  181  can move in the horizontal direction on the basis of a signal transmitted from the computer  170 . 
     The reflected light L4 reflected at the semiconductor of the sample  200  is reflected at the fourth dichroic mirror  146 , and passes through the fourth lens  126  and the fifth lens  127  to be guided to the detector  151 . The detector  151  detects the reflected light L4 reflected at the semiconductor of the sample  200 . The detector  151  outputs a voltage signal corresponding to the intensity of the reflected light L4 (detected signal) to the first lock-in amplifier  161  and the second lock-in amplifier  162 , for example. 
     The first lock-in amplifier  161  detects and amplifies a signal with the chopping frequency f1 of the first chopper  131 . In other words, the first lock-in amplifier  161  extracts only a component that changes at the chopping frequency f1 of the first chopper  131 . Thereby, the first lock-in amplifier  161  can perform signal detection with higher accuracy by detecting a small signal in the signal (input signal) transmitted from the detector  151 . Thus, the signal (reference signal) with the frequency transmitted from the first chopper  131  acts on a given signal; thereby, based on the input signal and the reference signal, the first lock-in amplifier  161  detects the amplitude information of the given signal. 
     The second lock-in amplifier  162  detects and amplifies a signal with the chopping frequency f2 of the second chopper  132 . In other words, the second lock-in amplifier  162  extracts only a component that changes at the chopping frequency f2 of the second chopper  132 . The effect of the second lock-in amplifier  162  is similar to the effect of the first lock-in amplifier  161 . 
     The signal processing device  165  includes a digital oscilloscope, an analog I/O device, or the like, for example. The signal processing device  165  receives a signal transmitted from each of the first lock-in amplifier  161  and the second lock-in amplifier  162 , and forms a signal waveform on the basis of the received signal. 
     The computer  170  includes a calculation unit  171  and a memory unit  172 . The calculation unit  171  calculates the ratio between the reflectance R1 of the infrared light L1 out of the reflected light L4 and the reflectance R2 of the infrared light L2 out of the reflected light L4 (reflectance ratio, R1/R2) on the basis of the signals transmitted from the first lock-in amplifier  161  and the second lock-in amplifier  162  via the signal processing device  165 . The memory unit  172  stores the correlation between the reflectance ratio found from a prescribed formula or the like and the resistivity of the semiconductor of the sample  200  (for example, a calibration curve, a correspondence table, etc.) beforehand. The correlation between the reflectance ratio found from a prescribed formula or the like and the resistivity of the semiconductor varies with the semiconductor material. The calculation unit  171  refers to the calculated reflectance ratio (R1/R2) and the correlation stored in the memory unit  172 , and reads the resistivity of the semiconductor corresponding to the calculated reflectance ratio (R1/R2). Details of this are described later. 
     The computer  170  controls the operation of the stage  181 . 
     Next, the operation of the apparatus for testing the resistivity of a semiconductor  100  is further described. 
       FIG. 2  is a graph illustrating an example of the relationship between the wavelength and the reflectance of reflected light reflected at SiC (silicon carbide). 
     The infrared light L1 emitted from the first light source  111  passes through the first lens  121 , is chopped at the chopping frequency f1 by the first chopper  131 , and passes through the first dichroic mirror  141 . 
     The infrared light L2 emitted from the second light source  112  passes through the second lens  122 , is chopped at the chopping frequency f2 by the second chopper  132 , and is reflected at the second dichroic mirror  142  and the first dichroic mirror  141 . 
     The wavelength λ1 of the infrared light L1 emitted by the first light source  111  is different from the wavelength λ2 of the infrared light L2 emitted by the second light source  112 . The chopping frequency f1 of the first chopper  131  is different from the chopping frequency f2 of the second chopper  132 . One of the chopping frequency f1 and the chopping frequency f2 is not an integral multiple of the other of the chopping frequency f1 and the chopping frequency f2. 
     The infrared light L1 that has passed through the first dichroic mirror  141  and the infrared light L2 reflected at the second dichroic mirror  141  are synthesized with each other into infrared light L3, and the infrared light L3 is applied to the prescribed range of the sample  200  coaxially. That is, the infrared light L3 having a plurality of wavelengths is applied to the prescribed range of the sample  200  coaxially. Thereby, the influence of the inclination of the sample  200  and the roughness of the surface of the sample  200  can be suppressed. The infrared light L3 applied to the sample  200  is reflected at the semiconductor of the sample  200 . The reflected light L4 reflected at the semiconductor of the sample  200  is reflected at the fourth dichroic mirror  146 , and passes through the fourth lens  126  and the fifth lens  127  to be incident on the detector  151 . 
     The detector  151  detects the reflected light L4 reflected at the semiconductor of the sample  200 . That is, the reflected light L4 at the semiconductor of the infrared light L3 having a plurality of wavelengths which has been applied to the sample  200  coaxially is detected by one detector (in this example, the detector  151 ). 
     Here, it is known that, depending on the semiconductor material, the permittivity is negative in the band between the TO phonon frequency (transverse optical phonon frequency) and the LO phonon frequency (longitudinal optical phonon frequency) and the reflectance is high in the band as compared to semiconductor materials in which phonon absorption does not occur. Such semiconductor materials include SiC (silicon carbide), GaN (gallium nitride), and the like. 
     The reflectance at the semiconductor of the infrared light L3 depends on the carrier density of the semiconductor in the region irradiated with the infrared light L3. The carrier density of the semiconductor influences the resistivity in the region irradiated with the infrared light L3. 
     In view of this, the apparatus for testing the resistivity of a semiconductor  100  according to the embodiment makes estimation by separating the reflectance of light with a wavelength that is likely to be influenced by the resistivity and the reflectance of light with a wavelength that is less likely to be influenced by the resistivity. In other words, the apparatus for testing the resistivity of a semiconductor  100  according to the embodiment makes estimation by separating the reflectance of light with a wavelength sensitive to the resistivity (a first wavelength) and the reflectance of light with a wavelength not sensitive to the resistivity (a second wavelength). 
     As shown in  FIG. 2 , in SiC, the band of the first wavelength is approximately not less than 10 μm and not more than 12 μm. In SiC, the band of the second wavelength is a band except approximately not less than 10 μm and not more than 12 μm. That is, in the specification of this application, “the first wavelength” refers to a wavelength between the TO phonon frequency (transverse optical phonon frequency) and the LO phonon frequency (longitudinal optical phonon frequency). In the specification of this application, “the second wavelength” refers to a wavelength except between the TO phonon frequency (transverse optical phonon frequency) and the LO phonon frequency (longitudinal optical phonon frequency), that is, a wavelength different from the first wavelength. The band of the first wavelength and the band of the second wavelength vary with the semiconductor material. 
     When the resistivity of SiC is tested, the wavelength λ1 of the infrared light L1 emitted from the first light source  111  is set to, for example, approximately 12 μm as the first wavelength. When the resistivity of SiC is tested, the wavelength λ2 of the infrared light L2 emitted from the second light source  112  is set to, for example, approximately 14 μm as the second wavelength. 
     The chopping frequency f1 of the first chopper  131  is set to approximately 30 kHz, for example. The chopping frequency f2 of the second chopper  132  is set to approximately 50 kHz, for example. 
     The detector  151  outputs a voltage signal corresponding to the intensity of the reflected light L4 having a plurality of wavelengths to the first lock-in amplifier  161  and the second lock-in amplifier  162 . 
     The first lock-in amplifier  161  detects a signal with the chopping frequency f1 (in this example, approximately 30 kHz) of the first chopper  131  out of the signal outputted by the detector  151 . Here, the detected signal is a signal regarding light with the first wavelength (in this example, approximately 12 μm) (the infrared light L1). The first lock-in amplifier  161  outputs the detected signal to the computer  170  via the signal processing device  165 . 
     The second lock-in amplifier  162  detects a signal with the chopping frequency f2 (in this example, approximately 50 kHz) of the second chopper  132  out of the signal outputted by the detector  151 . Here, the detected signal is a signal regarding light with the second wavelength (in this example, approximately 14 μm) (the infrared light L2). The second lock-in amplifier  162  outputs the detected signal to the computer  170  via the signal processing device  165 . 
     The calculation unit  171  of the computer  170  calculates the ratio between the reflectance R1 of the infrared light L1 out of the reflected light L4 and the reflectance R2 of the infrared light L2 out of the reflected light L4 (reflectance ratio, R1/R2) on the basis of the signals transmitted from the first lock-in amplifier  161  and the second lock-in amplifier  162  via the signal processing device  165 . The calculation unit  171  refers to the calculated reflectance ratio (R1/R2) and the correlation stored in the memory unit  172  (for example, a calibration curve, a correspondence table, etc.), and reads the resistivity of the semiconductor corresponding to the calculated reflectance ratio (R1/R2). 
     By the embodiment, by utilizing the difference between the chopping frequency f1 of the first chopper  131  and the chopping frequency f2 of the second chopper  132 , the reflectances of the reflected light L4 having a plurality of wavelengths are measured simultaneously by one detector (in this example, the detector  151 ). Thereby, the measurement of the reflectances of light having a plurality of wavelengths can be made in a time substantially equal to that of the measurement of the reflectance of light having a single wavelength, and measurement time can be shortened. Furthermore, since the reflected light L4 has a plurality of wavelengths, measurement accuracy can be improved. 
       FIG. 3  is a block diagram showing another apparatus for testing the resistivity of a semiconductor according to the embodiment of the invention. 
       FIG. 4A  and  FIG. 4B  are schematic diagrams showing signals outputted by the detector and the first lock-in amplifier. 
       FIG. 4A  is a schematic diagram showing the signal outputted by the detector.  FIG. 4B  is a schematic diagram showing the signal outputted by the first lock-in amplifier. 
     An apparatus for testing the resistivity of a semiconductor  100   a  shown in  FIG. 3  includes the first light source  111 , a light source driver  115 , the first chopper  131 , a chopper driver  135 , an irradiation unit  120   a , the detector  151 , the first lock-in amplifier  161 , the second lock-in amplifier  162 , a third lock-in amplifier  163 , the signal processing device  165 , the computer  170 , and the stage  181 . 
     The irradiation unit  120   a  of the apparatus for testing the resistivity of a semiconductor  100   a  shown in  FIG. 3  includes the first lens  121 , the second lens  122 , and the first dichroic mirror  141 . The apparatus for testing the resistivity of a semiconductor  100   a  shown in  FIG. 3  includes the third lens  125 , the fourth lens  126 , and the second dichroic mirror  142  as appropriate. The installation configuration of the lenses and the dichroic mirrors is not limited to the installation configuration shown in  FIG. 3  but may be altered as appropriate. In the apparatus for testing the resistivity of a semiconductor  100   a  shown in  FIG. 3 , the first dichroic mirror  141  is not limited to a dichroic mirror, and any mirror capable of reflecting the infrared light L1 may be used, for example. The second dichroic mirror  142  is not limited to a dichroic mirror, and any mirror capable of reflecting the reflected light L4 reflected at the semiconductor of the sample  200  may be used, for example. The apparatus for testing the resistivity of a semiconductor  100   a  according to the embodiment may not necessarily include the first light source  111  and the second light source  112 . 
     The light source driver  115  modulates the wavelength of the infrared light L1 emitted by the first light source  111 . Specifically, the light source driver  115  can sweep the wavelength of the infrared light L1 emitted by the first light source  111  at a frequency f3 higher than the chopping frequency f1 of the first chopper  131  in a relatively wide range. The light source driver  115  transmits to the first lock-in amplifier  161  a signal with the frequency f3 at which the wavelength of the infrared light L1 is swept. The frequency f3 at which the light source driver  115  sweeps the wavelength of the infrared light L1 is of the order of kilohertz (kHz), for example. 
     The chopper driver  135  controls the chopping frequency f1 of the first chopper  131 , and transmits a signal with the chopping frequency f1 (reference signal) to the second lock-in amplifier  162  and the third lock-in amplifier  163 . The chopping frequency f1 of the first chopper  131  is approximately 100 Hz or less, for example. 
     The irradiation unit  120   a  causes the infrared light L1 emitted from the first light source  111  to be applied to a prescribed range of the sample  200 . 
     Otherwise, the structure is similar to the structure of the apparatus for testing the resistivity of a semiconductor  100  described above in regard to  FIG. 1 . 
     Next, the operation of the apparatus for testing the resistivity of a semiconductor  100   a  is further described. 
     In the method for testing the resistivity of a semiconductor according to the embodiment, the resistivity of a semiconductor formed on a matrix (for example, a semiconductor wafer) is tested. The wavelength of the infrared light L1 emitted from the first light source  111  is swept by the light source driver  115  at the frequency f3 higher than the chopping frequency f1 of the first chopper  131  in a relatively wide range. As described above in regard to  FIG. 2 , it is known that, for example in SiC (silicon carbide), GaN (gallium nitride), and the like, the permittivity is negative in the band between the TO phonon frequency (transverse optical phonon frequency) and the LO phonon frequency (longitudinal optical phonon frequency) and the reflectance is high in the band as compared to semiconductor materials in which phonon absorption does not occur. 
     Thus, the light source driver  115  of the embodiment sweeps the wavelength of the infrared light L1 in a band including a wavelength that is likely to be influenced by the resistivity. In other words, the light source driver  115  of the embodiment sweeps the wavelength of the infrared light L1 in a band including the first wavelength. The band in which the light source driver  115  sweeps the wavelength of the infrared light L1 is approximately not less than 10 μm and not more than 12 μm, for example. The band of the first wavelength is as described above in regard to  FIG. 2 . 
     The infrared light L1 emitted from the first light source  111  passes through the first lens  121 , is chopped at the chopping frequency f1 by the first chopper  131 , and passes through the second lens  122 . The infrared light L1 that has passed through the second lens  122  is reflected at the first dichroic mirror  141 , and is applied to the prescribed range of the sample  200 . 
     Since the wavelength of the infrared light L1 has been swept by the light source driver  115  as described above, the infrared light L1 emitted from the first light source  111  has a plurality of wavelengths. The infrared light L1 having a plurality of wavelengths is applied to the prescribed range of the sample  200  coaxially. Thereby, the influence of the inclination of the sample  200  and the roughness of the surface of the sample  200  can be suppressed. The infrared light L3 applied to the sample  200  is reflected at the semiconductor of the sample  200 . The reflected light L4 reflected at the semiconductor of the sample  200  is reflected at the second dichroic mirror  142 , and passes through the third lens  125  and the fourth lens  126  to be incident on the detector  151 . 
     The detector  151  detects the reflected light L4 reflected at the semiconductor of the sample  200 . That is, the reflected light L4 at the semiconductor of the infrared light L1 having a plurality of wavelengths which has been applied to the sample  200  coaxially is detected by one detector (in this example, the detector  151 ). 
     The signal waveform detected by the detector  151  (the output signal waveform of the detector  151 ) is as shown in  FIG. 4A . The signal waveform shown in  FIG. 4A  has a change in the reflection intensity ΔI resulting from the wavelength sweeping of the light source driver  115  and the average reflection intensity I 0  in the band of the wavelength sweeping of the light source driver  115 . The detector  151  outputs the signal with the waveform shown in  FIG. 4A  to the first lock-in amplifier  161  and the second lock-in amplifier  162 . 
     The first lock-in amplifier  161  detects a signal with the frequency f3 of the wavelength sweeping of the light source driver  115  out of the signal outputted by the detector  151 . In other words, the first lock-in amplifier  161  performs lock-in detection at the frequency f3 of the wavelength sweeping of the light source driver  115 . Then, the first lock-in amplifier  161  outputs a signal with the waveform shown in  FIG. 4B  to the second lock-in amplifier  162 . 
     The second lock-in amplifier  162  detects a signal with the chopping frequency f1 of the first chopper  131  out of the signal outputted by the first lock-in amplifier  161 . In other words, the second lock-in amplifier  162  performs lock-in detection at the chopping frequency f1 of the first chopper  131 . Thereby, the second lock-in amplifier  162  can acquire the change in the reflection intensity ΔI resulting from the wavelength sweeping of the light source driver  115 , and can output the change in the reflection intensity ΔI to the computer  170  via the signal processing device  165 . 
     The third lock-in amplifier  163  detects a signal with the chopping frequency f1 of the first chopper  131  out of the signal outputted by the detector  151 . In other words, the third lock-in amplifier  163  performs lock-in detection at the chopping frequency f1 of the first chopper  131 . Thereby, the third lock-in amplifier  163  can acquire the average reflection intensity I 0  in the band of the wavelength sweeping of the light source driver  115 , and can output the average reflection intensity I 0  to the computer  170  via the signal processing device  165 . 
     The calculation unit  171  of the computer  170  calculates the ratio between the change in the reflection intensity ΔI resulting from the wavelength sweeping of the light source driver  115  and the average reflection intensity I 0  in the band of the wavelength sweeping of the light source driver  115  (intensity ratio; ΔI/I 0 ) on the basis of the signals transmitted from the second lock-in amplifier  162  and the third lock-in amplifier  163  via the signal processing device  165 . The calculation unit  171  refers to the calculated intensity ratio (ΔI/I 0 ) and the correlation stored in the memory unit  172  (for example, a calibration curve, a correspondence table, etc.), and reads the resistivity of the semiconductor corresponding to the calculated intensity ratio (ΔI/I 0 ). 
     In the embodiment, the memory unit  172  stores the correlation between the intensity ratio found from a prescribed formula or the like and the resistivity of the semiconductor of the sample  200  (for example, a calibration curve, a correspondence table, etc.) beforehand. 
     By the embodiment, by utilizing the difference between the frequency f3 of the wavelength sweeping of the light source driver  115  and the chopping frequency f1 of the first chopper  131 , the reflectances of the reflected light L4 having a plurality of wavelengths are measured simultaneously by one detector (in this example, the detector  151 ). Thereby, the measurement of the reflectances of light having a plurality of wavelengths can be made in a time substantially equal to that of the measurement of the reflectance of light having a single wavelength, and measurement time can be shortened. Furthermore, since the reflected light L4 has a plurality of wavelengths, measurement accuracy can be improved. 
     In the embodiment, even if neither the chopper  131  nor the chopper driver  135  is provided, the change in the reflection intensity ΔI resulting from the wavelength sweeping of the light source driver  115  can be found based on the signal outputted by the first lock-in amplifier  161  (the signal with the waveform shown in  FIG. 4B ). On the other hand, the average reflection intensity I 0  in the band of the wavelength sweeping of the light source driver  115  can be found based on the signal outputted by the detector  151  (the signal with the waveform shown in  FIG. 4A ). Thereby, the calculation unit  171  can read the resistivity of the semiconductor by calculating the intensity ratio (ΔI/I 0 ) and referring to the calculated intensity ratio (ΔI/I 0 ) and the correlation stored in the memory unit  172 . Hence, the apparatus for testing the resistivity of a semiconductor  100   a  according to the embodiment may not necessarily include the chopper  131 , the chopper driver  135 , the second lock-in amplifier  162 , and the third lock-in amplifier  163 . 
       FIG. 5  is a block diagram showing still another apparatus for testing the resistivity of a semiconductor according to the embodiment of the invention. 
     An apparatus for testing the resistivity of a semiconductor  100   b  shown in  FIG. 5  includes a diffraction grating  153  and a one-dimensional detector  155  in place of the detector  151 , the first lock-in amplifier  161 , and the second lock-in amplifier  162  in the apparatus for testing the resistivity of a semiconductor  100  shown in  FIG. 1 . The one-dimensional detector  155  is called a line sensor or the like, for example. 
     Otherwise, the structure is similar to the structure of the apparatus for testing the resistivity of a semiconductor  100  described above in regard to  FIG. 1 . 
     The operation from when the infrared light L1 is emitted from the first light source  111  to when it is reflected at the semiconductor of the sample  200  is as described above in regard to  FIG. 1 . The operation from when the infrared light L2 is emitted from the second light source  112  to when it is reflected at the semiconductor of the sample  200  is as described above in regard to  FIG. 1 . 
     Subsequently, the reflected light L4 reflected at the semiconductor of the sample  200  is reflected at the fourth dichroic mirror  146 , and passes through the fourth lens  126  to be incident on the diffraction grating  153 . The diffraction grating  153  can condense the reflected light L4 to certain positions on the one-dimensional detector  155  in accordance with the wavelength by utilizing the diffraction of light. The one-dimensional detector  155  can detect the intensity of the reflected light L4 condensed by the diffraction grating  153 . Here, positions in the linear reflected light L4 and positions in the one-dimensional detector  155  correspond to each other. Therefore, the one-dimensional detector  155  can detect each intensity corresponding to each position of the linear reflected light L4. 
     A first reflected light L41 including the wavelength λ1 out of the reflected light L4 is condensed to a first position A1 in the one-dimensional detector  155 , for example. On the other hand, a second reflected light L42 including the wavelength λ2 out of the reflected light L4 is condensed to a second position A2 in the one-dimensional detector  155 . In other words, the first reflected light L41 including the wavelength λ1 and the second reflected light L42 including the wavelength λ2 are condensed to positions different from each other. 
     The one-dimensional detector  155  detects the intensities of the reflected light L41 condensed at the first position A1 and the reflected light L42 condensed at the second position A2, and outputs the detected signals to the computer  170  via the signal processing device  165 . 
     The calculation that the computer  170  makes is as described above in regard to  FIG. 1 . 
     By the embodiment, by dispersing light using a diffraction grating on the basis of the difference between the wavelength λ1 of the infrared light L1 and the wavelength λ2 of the infrared light L2, the reflectances of the reflected light L4 having a plurality of wavelengths are measured simultaneously by one detector (in this example, the one-dimensional detector  155 ). Thereby, the measurement of the reflectances of light having a plurality of wavelengths can be made in a time substantially equal to that of the measurement of the reflectance of light having a single wavelength, and measurement time can be shortened. Furthermore, since the reflected light L4 has a plurality of wavelengths, measurement accuracy can be improved. 
     Next, methods for testing the resistivity of a semiconductor according to the embodiment of the invention are described with reference to the drawings. 
       FIG. 6  is a flow chart showing a method for testing the resistivity of a semiconductor according to the embodiment of the invention. 
     First, the infrared light L1 with the first wavelength λ1 is emitted from the first light source  111  (step S 101 ). Further, the infrared light L2 with the second wavelength λ2 is emitted from the second light source  112  (step S 101 ). Each of the first wavelength λ1 and the second wavelength λ2 is a single wavelength. The wavelength λ2 is different from the wavelength λ1. 
     Subsequently, the first chopper  131  is used to chop the infrared light L1 emitted by the first light source  111  at the chopping frequency f1 (step S 103 ). Further, the second chopper  132  is used to chop the infrared light L2 emitted by the second light source  112  at the chopping frequency f2 (step S 103 ). 
     Subsequently, the infrared light L1 and the infrared light L2 are synthesized together into the infrared light L3 (synthetic light), and the infrared light L3 is applied to a prescribed range of the sample  200  coaxially (step S 105 ). 
     Subsequently, the reflected light L4 reflected at the semiconductor of the sample  200  is detected by one detector  151  (step S 107 ). 
     Subsequently, the first lock-in amplifier  161  is used to perform lock-in detection in which a signal with the chopping frequency f1 of the first chopper  131  is detected out of the signal outputted by the detector  151  (step S 109 ). Further, the second lock-in amplifier  162  is used to perform lock-in detection in which a signal with the chopping frequency f2 of the second chopper  132  is detected out of the signal outputted by the detector  151  (step S 109 ). 
     Subsequently, the calculation unit  171  of the computer  170  calculates the ratio between the reflectance R1 of the infrared light L1 out of the reflected light L4 and the reflectance R2 of the infrared light L2 out of the reflected light L4 (reflectance ratio, R1/R2) on the basis of the signals transmitted from the first lock-in amplifier  161  and the second lock-in amplifier  162  (step S 111 ). 
     Subsequently, the calculation unit  171  of the computer  170  refers to the calculated reflectance ratio (R1/R2) and the correlation stored in the memory unit  172  (for example, a calibration curve, a correspondence table, etc.), and reads the resistivity of the semiconductor corresponding to the calculated reflectance ratio (R1/R2) (step S 113 ). 
     By the embodiment, by utilizing the difference between the chopping frequency f1 of the first chopper  131  and the chopping frequency f2 of the second chopper  132 , the reflectances of the reflected light L4 having a plurality of wavelengths can be measured simultaneously by one detector. Thereby, the measurement of the reflectances of light having a plurality of wavelengths can be made in a time substantially equal to that of the measurement of the reflectance of light having a single wavelength, and measurement time can be shortened. Furthermore, since the reflected light L4 has a plurality of wavelengths, measurement accuracy can be improved. 
       FIG. 7  is a flow chart showing another method for testing the resistivity of a semiconductor according to the embodiment. 
     First, the infrared light L1 with the wavelength λ1 is emitted from the first light source  111  (step S 201 ). 
     Subsequently, the wavelength of the infrared light L1 is swept at the frequency f3 higher than the chopping frequency f1 in a band including the first wavelength (step S 203 ). 
     Subsequently, the first chopper  131  is used to chop the infrared light L1 emitted by the first light source  111  at the chopping frequency f1 (step S 205 ). 
     Subsequently, the infrared light L1 that has been wavelength-swept and has a plurality of wavelengths is applied to a prescribed range of the sample  200  coaxially (step S 207 ). 
     Subsequently, the reflected light L4 reflected at the semiconductor of the sample  200  is detected by one detector  151  (step S 209 ). 
     Subsequently, the first lock-in amplifier  161  is used to perform lock-in detection in which a signal with the frequency f3 of wavelength sweeping is detected out of the signal outputted by the detector  151  (step S 211 ). Further, the second lock-in amplifier  162  is used to perform lock-in detection in which a signal with the chopping frequency f1 of the first chopper  131  is detected out of the signal outputted by the first lock-in amplifier  161  (step S 211 ). 
     Subsequently, the third lock-in amplifier  163  is used to perform lock-in detection in which a signal with the chopping frequency f1 of the first chopper  131  is detected out of the signal outputted by the detector  151  (step S 213 ). 
     Subsequently, the calculation unit  171  of the computer  170  calculates the ratio between the change in the reflection intensity ΔI resulting from the wavelength sweeping and the average reflection intensity I 0  in the band of the wavelength sweeping (intensity ratio, ΔI/I 0 ) on the basis of the signals transmitted from the second lock-in amplifier  162  and the third lock-in amplifier  163  (step S 215 ). 
     The calculation unit  171  of the computer  170  refers to the calculated intensity ratio (ΔI/I 0 ) and the correlation stored in the memory unit  172  (for example, a calibration curve, a correspondence table, etc.), and reads the resistivity of the semiconductor corresponding to the calculated intensity ratio (ΔI/I 0 ). 
     By the embodiment, by utilizing the difference between the frequency f3 of wavelength sweeping and the chopping frequency f1 of the first chopper  131 , the reflectances of the reflected light L4 having a plurality of wavelengths can be measured simultaneously by one detector. Thereby, the measurement of the reflectances of light having a plurality of wavelengths can be made in a time substantially equal to that of the measurement of the reflectance of light having a single wavelength, and measurement time can be shortened. Furthermore, since the reflected light L4 has a plurality of wavelengths, measurement accuracy can be improved. 
     While certain embodiments have been described, these embodiments have been presented by way of example only, and are not intended to limit the scope of the inventions. Indeed, the novel embodiments described herein may be embodied in a variety of other forms; furthermore, various omissions, substitutions and changes in the form of the embodiments described herein may be made without departing from the spirit of the inventions. The accompanying claims and their equivalents are intended to cover such forms or modifications as would fall within the scope and spirit of the invention.