Determining carbon density in silicon single crystal by FT-IR

Abstract: A method of determining the substitutional carbon density in a silicon single crystal by the Fourier transform infrared spectroscopy (FT-IR), comprising the steps of determining by the FT-IR the infrared absorbance spectrum of a sample of a silicon single crystal, determining by the FT-IR the infrared absorbance spectrum of a reference of a substantially carbon-free silicon single crystal having substantially the same degree of free carrier absorption and produced by the same process as the sample, computing a difference coefficient from the infrared absorbance spectra of the sample and the reference, computing a difference absorbance spectrum between the sample and the reference using the computed difference coefficient, and determining the substitutional carbon density in the sample from a distance of the absorption peak of the difference absorbance spectrum of the localized vibration of a substitutional carbon in the sample from a base line of the difference absorbance spectrum. An FT-IR carbon density determination apparatus embodying the method is also disclosed.

Description: The present invention relates to determining the carbon density in a silicon single crystal by the Fourier transform infrared spectroscopy (FT-IR). It relates more particularly to a method and an apparatus of determining the substitutional carbon density in a silicon single crystal by the FT-IR using a reference of a silicon single crystal. Carbon and oxygen in a silicon single crystal wafer are important factors determining the quality of the wafer. The FT-IR is widely employed for determining the carbon density in the wafer, - see for instance document EP-A-0250707. FIG.9 illustrates an FT-IR optical system. The system comprises a source 1 of infrared light, an aperture device 2, a collimator 3 in the form of concave mirror, a Michelson interferometer 4, a concave or converging mirror 8, an aperture device 10 and a sensor 11. The aperture device 2 allows a divergent light from the source 1 of infrared light to pass. The collimator 3 receives the divergent light passing through the aperture device 2 and reflects a beam of collimated infrared light to the Mickelson interferometer 4. The Michelson interferometer 4 comprises a beam splitter 5, a fixed mirror 6 and a movable mirror 7. The beam splitter 5 receives the collimated beam from the collimator 3 and splits it into a beam of reflected infrared light and a beam of transmitted infrared light. The fixed mirror 6 reflects the beam of reflected infrared light from the beam splitter 5 back to the beam splitter 5. The movable mirror 7 reflects the beam of transmitted infrared light from the beam splitter 5 back to the beam splitter 5. The beam of reflected infrared light from the fixed mirror 6 and the beam of reflected infrared light from the movable mirror 7 meet at the beam splitter 5 and interfere with each other. The converging mirror 8 receives the two beam interference from the Michelson interferometer 4 and transmits it to a sample 9 to be determined. The sensor 11 receives a light transmitting through the sample 9 and the aperture device 10. An analog/digital convertor (not shown) converts an interferogram output by the sensor 11 into a digital form which is then Fourier-transformed. Thus, an FT-IR density determination apparatus detects the infrared absorbance spectrum of the sample 9 and also the infrared absorbance spectrum of a reference in the same manner. FIG.10 shows the infrared absorbance spectrum of a sample (p-type, 10 Ωcm) of a silicon single crystal produced by the Czochralski method (referred to as CZ-method produced silicon single crystal hereinafter). FIG.11 shows the infrared absorbance spectrum of a reference (p-type, 2000 Ωcm) of a silicon single crystal produced by the floating zone method (referred to as FZ-method produced silicon single crystal hereinafter). Usually, the FT-IR density determination apparatus previously detects the infrared absorbance spectrum of the reference and stores data thereof. After producing the data of the infrared absorbance spectra of both the sample and the reference regarding the substitutional carbon Cs in a silicon single crystal wafer, the FT-IR density determination apparatus computes a difference coefficient f for computing a difference absorbance spectrum between the infrared absorbance spectra of the sample and the reference. Subtracting a product of the difference coefficient and the infrared absorbance spectrum of the reference from the infrared absorbance spectrum of the sample provides the difference absorbance spectrum between the infrared absorbance spectra of the sample and the reference. FIG.12 shows the difference absorbance spectrum between the infrared absorbance spectra of the sample and the reference of FIGS.10 and 11. Then, the FT-IR density determination apparatus determines the substitutional carbon density in the silicon single crystal wafer from the distance (i.e. the height) of the absorption peak of the localized vibration of the substitutional carbon appearing at 605 cm -1 from a base line, for example, between 595 cm -1 and 615 cm -1 . A determination of the substitutional carbon density provides 0.5 ppma from difference absorbance spectra of FIG.12. The lower detective limit of the substitutional carbon density is on the order of 0.05 ppma according to the ASTM designation: F123-81. The process for producing a silicon single crystal substrate for semiconductor devices is categorized into the Czochralski method and the floating zone method. The Czochralski method comprises the steps of placing a raw polysilicon in a quartz crucible, melting the raw polysilicon by a carbon heater, immersing a seed crystal of silicon single crystal in the surface of the melt, and lifting the seed crystal while rotating it to grow a silicon single crystal. The floating zone method comprises the steps of melting part of a raw polysilicon rod by a melting coil to produce a melting zone and moving the melting zone to grow a silicon single crystal. In comparison, the CZ-method produced silicon single crystal contains more amounts of carbon and oxygen than the FZ-method produced silicon single crystal. It is assumed that carbon invades the CZ-method produced silicon single crystal from the carbon heater, etc. and oxygen invades the CZ-method produced silicon single crystal from the crucible. On the other hand, the FZ-method produced silicon single crystal is substantially carbon-free and oxygen-free. Thus, it has been considered that the FZ-method produced silicon single crystal is more appropriate to the reference for determining the carbon density and the oxygen density in the CZ-method produced silicon single crystal, - see for instance document EP-A-0473130. However, the present inventors researched and discovered that an employment of the FZ-method produced silicon single crystal as the reference had problems described below. As shown in FIG.13, the absorption peaks of the localized vibration of the substitutional carbon in silicon single crystals appear at 605 cm -1 and overlap the intense absorption peaks of the silicon phonon. In addition, the forms of the absorption peaks of silicon phonon depend on degrees of free carrier absorption of a dopant and are different when silicon single crystal wafers have resistivities of 3 Ωcm and 20 Ωcm. Thus, the other effects on differences absorbance between the sample and the reference except the localized vibration peaks of the substitutional carbon and, particularly, the effect of the absorption by the intense silicon phonon and free carriers must be as reduced as possible so that the FT-IR density determination apparatus can exactly extract the absorption peak of the localized vibration of the substitutional carbon as the difference absorbance spectrum between the sample and the reference. Therefore, a reference having essentially the same degree of absorption by the silicon phonon and essentially the same degree of free carrier absorption of a dopant which overlap the absorption peak of the localized vibration of the substitutional carbon as a sample must be employed. However, since the resistivity of the sample of the CZ-method produced silicon single crystal is 20 or less Ωcm in almost all cases and the resistivity of the reference of the FZ-method produced silicon single crystal, which is normally produced without a dopant, is 1000 or more Ωcm, degrees of free carrier absorption of the dopant in the sample and the reference have a large difference. In addition, the dopant density and the oxygen density in the FZ-methodical silicon single crystal are much smaller than those in the CZ-method produced silicon single crystal. Thus, degrees of free carrier absorption of the dopant and the forms of the absorption peak curves by the silicon phonon of the sample and the reference largely differ from each other. Thus, the absorption peak of the localized vibration of the substitutional carbon is deformed and it is difficult to precisely determine the substitutional carbon density of 0.1 ppma or less. In addition, conventional methods of determining the difference coefficient have a problem in improving the precision. That is, a conventional FT-IR carbon density determination apparatus computes the difference coefficient f as a simple ratio of the infrared absorbance (As(κ)) of a sample to the infrared absorbance (Ar(κ)) of a reference at a specified wave number κ by the following equation (1): As (κ) - f × Ar (κ) = 0 or as a ratio of an integral of the infrared absorbance (As(κ)) of a sample to an integral of the infrared absorbance (Ar(κ)) of a reference between the limits κ=κl and κ=κh in terms of wave number by the following equation (2): κ=κ l κ=κ h As (κ) d κ - f × κ=κ l κ=κ h Ar (κ) d κ = 0 Since wave number κ is actually determined by a certain resolution, wave numbers are not analog values but discrete values κn (n=1, 2, 3 ---). Thus, the equation (2) is actually transformed into the equation (3): κ= l κ= h As (κ) - f × κ= l κ= h Ar (κ) = 0 When computing the difference coefficient by the equation (1), (2) or (3) and the difference absorbance spectrum using the difference coefficient, the conventional FT-IR carbon density determination apparatus cannot suppress aging and the effects of state differences between the sample and the reference (e.g. differences in the thickness and the resistivity) to be least enough so as to be free from the effects of the state differences. Thus, the repeatability of the determination of the same apparatus is poor and errors in the determination between any two conventional FT-IR carbon density determination apparatuses of different types are intolerable or intolerably high. In wafers of silicon single crystal of today, a required level of the carbon density has come down to under 0.05 ppma and the resolution power of the FT-IR carbon density determination apparatus has been impressive because of a high quality substrate in use for highly integrated devices and a high-purity device manufacturing process. A first aspect of the present invention in a method of determining a substitutional carbon density in a silicon single crystal by the FT-IR comprises the steps of determining by the FT-IR the infrared absorbance spectrum of a sample of a silicon single crystal the substitutional carbon density of which is to be determined, determining by the FT-IR the infrared absorbance spectrum of a reference of a substantially carbon-free silicon single crystal of substantially the same degree of free carrier absorption as the sample, the reference being produced by the same process as the sample, computing a difference coefficient from the infrared absorbance spectra of the sample and the reference by the equation (1), (2) or (3): As (κ) - f × Ar (κ) = 0 κ=κ l κ=κ h As (κ) d κ - f × κ=κ l κ=κ h Ar (κ) d κ = 0 or κ= l κ= h As (κ) - f × κ= l κ= h Ar (κ) = 0 computing a difference absorbance spectrum between the infrared absorbance spectra of the sample and the reference using the computed difference coefficient, and determining the substitutional carbon density in the sample from a distance of the absorption peak of the difference absorbance spectrum of the localized vibration of a substitutional carbon in the sample from a base line of the difference absorbance spectrum. A second aspect of the present invention in an apparatus of determining a substitutional carbon density in a silicon single crystal by the FT-IR comprises means for determining by the FT-IR the infrared absorbance spectrum of a sample of a silicon single crystal the substitutional carbon density of which is to be determined, means for storing data of the infrared absorbance spectra determined by the FT-IR of multiple references of substantially carbon-free silicon single crystals of different degrees of free carrier absorption, means for selecting from the data of the infrared absorbance spectra of the references in said storage means data of the infrared absorbance spectrum of a reference of substantially the same degree of free carrier absorption as the sample, means for computing from the infrared absorbance spectra of the reference and the sample a difference coefficient by the equation (1), (2) or (3), or by the least square approximation so that within lower and higher ranges of wave number than the absorption peak of the difference absorbance spectrum of the localized vibration of the substitutional carbon a relational expression between wave number and infrared absorbance is approximate to an equation, means for computing a difference absorbance spectrum between the infrared absorbance spectra of the sample and the reference using the computed difference coefficient, and determining the substitutional carbon density in the sample from the computed difference absorbance spectrum. The apparatus of the second aspect of the present invention may further comprise means for determining by the FT-IR the infrared absorbance spectra of the references. The apparatus of the second aspect of the present invention may be automated. Another object of the present invention is to provide a method and an apparatus of precisely determining a substitutional carbon density in a CZ-method produced silicon single crystal. In order to achieve this object, a third aspect of the present invention in a method of determining a substitutional carbon density in a CZ-method produced silicon single crystal by the FT-IR comprises the steps of determining by the FT-IR the infrared absorbance spectrum of a sample of the CZ-method produced silicon single crystal the substitutional carbon density of which is to be determined, determining by the FT-IR the infrared absorbance spectrum of a reference of a substantially carbon-free CZ-method produced silicon single crystal of substantially the same degree of free carrier absorption as the sample, computing a difference coefficient from the infrared absorbance spectra of the sample and the reference by the equation (1), (2) or (3), computing a difference absorbance spectrum between the infrared absorbance spectra of the sample and the reference using the computed difference coefficient, and determining the substitutional carbon density in the sample from a distance of the absorption peak of the difference absorbance spectrum of the localized vibration of a substitutional carbon in the sample from a base line of the difference absorbance spectrum. Specifically, FIG.2 shows an example of the difference absorbance spectrum computed by the method of the third aspect of the present invention. The method of the third aspect of the present invention employs a sample of a p-type CZ-method produced silicon single crystal of a resistivity of 10 Ωcm and a carbon density of about 0.05 ppma as the sample of FIG.1 and a reference of a substantially carbon-free p-type CZ-method produced silicon single crystal of a resistivity of 20 Ωcm and computes the difference absorbance spectrum by the equation (1), (2) or (3). FIG.2 destinctively shows the absorption peak of the localized vibration of the substitutional carbon. On the other hand, FIG.1 shows an example of the difference absorbance spectrum determined by a prior-art method of determining the carbon density. This method employs the same sample of the p-type CZ-method produced silicon single crystal of the resistivity of 10 Ωcm and the carbon density of about 0.05 ppma and a reference of a p-type FZ-method produced silicon single crystal of the resistivity of 2000 Ωcm and computes a difference absorbance spectrum by the equation (1), (2) or (3). In the fourth aspect of the present invention, the difference coefficient computing step of the first and third aspects of the present invention comprises the step of computing the difference coefficient by the least square approximation so that within lower and higher ranges of wave number than the absorption peak of the difference absorbance spectrum of the localized vibration of the substitutional carbon a relational expression between wave number and infrared absorbance is approximate to a linear or quadratic equation. As described above, subtracting a product of the difference coefficient f and the infrared absorbance spectrum of the reference from the infrared absorbance spectrum of the sample provides the difference absorbance spectrum. Thus, the value of the difference coefficient directly changes the form of the difference absorbance spectrum. FIG.3 shows different forms of difference absorbance spectra when the difference coefficient has different values. In a case of FIG.3, the p-type CZ-method produced silicon single crystal of the resistivity of 10 Ωcm and a substitutional carbon density of about 0.05 ppma was employed as the sample and a substantially carbon-free p-type CZ-method produced silicon single crystal of the resistivity of 20 Ωcm was employed as the reference. The difference coefficients f were 0.975, 0.985, 0.995, 1.005, 1.015 and 1.025. In particular, a difference coefficient f appropriate to the determination of the carbon density was 0.995 at which a part of a curve having a wave number higher than the absorption peak of the localized vibration of the substitutional carbon and a part of the curve having a wave number lower than the absorption peak of the localized vibration of the substitutional carbon are flattest. However, since a commercially sold in-line FT-IR carbon density determination apparatus for determining the carbon densities of a large number of samples computes the difference coefficient by the equation (1), (2) or (3), it cannot provides the optimum difference coefficient (i.e. f=0.995). The determined value is relatively stable because the absorption peak of the localized vibration of the substitutional carbon is high, when the substitutional carbon density exceeds 0.1 ppma. On the other hand, when the substitutional carbon density in the sample is 0.1 ppma or lower, the in-line FT-IR carbon density determination apparatus has a large error in the determination of the carbon density. Therefore, the third aspect of the present invention provides the method of computing the optimum difference coefficient (i.e. f=0.995) by the equation (4): S = κ=κ l1 κ=κ h1 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 d κ + κ=κ l2 κ=κ h2 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 d κ Thus, both the part of the curve of the difference absorbance spectrum having a wave number more than the absorption peak of the localized vibration of the substitutional carbon and the part of the curve of the difference absorbance spectrum having a wave number less than the absorption peak of the localized vibration of the substitutional carbon are substantially flat. Since wave number κ is actually determined by a certain resolution, wave numbers are not analog values but discrete values κn (n=1, 2, 3 ---). Thus, the equation (4) is actually transformed into the equation (5): S = κ=κ l1 κ=κ h1 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 + κ=κ l2 κ=κ h2 κ=κ h2 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 Specifically, a range of wave number of κl1 to κh1 is selected as a range of larger wave numbers than the absorption peak of the localized vibration of the substitutional carbon and a range of wave number of κl2 to κh2 is selected as a range of less wave numbers than the absorption peak of the localized vibration of the substitutional carbon. The method of the fourth aspect of the present invention computes the difference coefficient by the equation (5) so that in the both ranges the difference absorbance spectrum most approximates with respect to wave number to a linear equation. For example, the lower range of wave number covers 550-595 cm -1 (i.e. κl1=550, κh1=595) and the higher range of wave number covers 615-660 cm -1 (κl2=615, κh2=660). In the case of FIG.3, the reference of the same type conductivity and substantially the same resistivity as the sample was employed. When the resistivity of a sample is lower than that of the sample of FIG. 3, a base line of the difference absorbance spectrum is curved. In this case, the difference absorbance spectrum is approximated by the quadratic equation (6) instead of the linear equations (4) and (5) so that the equation (6) provides an optimum difference coefficient: S = κ=κ l1 κ=κ h1 [( As (κ) - f × Ar (κ)) - ( c κ 2 + b κ + a )] 2 d κ + κ-κ l2 κ=κ h2 [( As (κ) - f × Ar( κ)) - ( c κ 2 + b κ + a )] 2 d κ Since wave number κ is actually determined by a certain resolution, wave numbers are not analog values but discrete values κn (n=1, 2, 3 ---). Thus, the equation (6) is actually transformed into the equation (7): FIG.4 shows difference absorbance spectra between the same sample and the same reference computed by the equations (1) and (2) as those of the case of FIG.3. In FIG.4, the curve in S = κ=κ l1 κ=κ h1 [( As (κ) - f × Ar (κ)) - ( c κ 2 + b κ + a )] 2 + κ=κ l2 κ=κ h2 [( As (κ) - f × Ar (κ)) - ( c κ 2 + b κ + a )] 2 dashed line shows the difference absorbance spectrum computed by the equation (1) and the curve in solid line shows the difference absorbance spectrum computed by the equation (2). Neither the curve in dashed line nor the curve in solid line shows the absorption peak of the localized vibration of the substitutional carbon. A fifth aspect of the present invention in an apparatus of determining a substitutional carbon density in a CZ-method produced silicon single crystal by the FT-IR comprises means for determining by the FT-IR the infrared absorbance spectrum of a sample of the CZ-method produced silicon single crystal the substitutional carbon density of which is to be determined, means for storing data of the infrared absorbance spectra determined by the FT-IR of multiple references of the substantially carbon-free CZ-method produced silicon crystals of different degrees of free carrier absorption, means for selecting from the data of the infrared absorbance spectra of the references in said storage means data of the infrared absorbance spectrum of a reference of substantially the same degree of free carrier absorption as the sample, means for computing from the infrared absorbance spectra of the reference and the sample a difference coefficient by the equation (1), (2) or (3), or by the least square approximation so that within lower and higher ranges of wave number than the absorption peak of the difference absorbance spectrum of the localized vibration of the substitutional carbon a relational expression between wave number and infrared absorbance is approximate to an equation, means for computing a difference absorbance spectrum between the infrared absorbance spectra of the sample and the reference using the computed difference coefficient, and determining the substitutional carbon density in the sample from the computed difference absorbance spectrum by a predetermined relation between carbon density and difference absorbance spectrum. The FT-IR carbon density determination apparatus of the fifth aspect of the present invention may be automated. Since the third aspect of the present invention employs the reference of the carbon-free CZ-method produced silicon single crystal of substantially the same degree of free carrier absorption as the sample, the residues of the infrared absorbance spectra of the sample and the reference except the absorption peak of the localized vibration of the substitutional carbon can be substantially the same in extracting the absorption peak of the localized vibration of the substitutional carbon. This will be described with reference to FIGS.1 and 2. As seen in FIG.2, the absorption peak (i.e. the hatched area) of the localized vibration of the substitutional carbon is distinct. On the other hand, the absorption peak of the localized vibration of the substitutional carbon, as seen in FIG.1, is indistinct. In order to achieve the distinct absorption peak of FIG.2, the third aspect of the present invention employs the reference of the same CZ-method produced p-type silicon single crystal as the sample. This reference has substantially the same resistivity as the reference of the FZ-method produced silicon single crystal, so that this reference and the sample have substantially the same degree of free carrier absorption. In addition, since this reference is substantially carbon-free, the absorption peak of the localized vibration of the substitutional carbon in the sample is distinct. Thus, the employment of the reference of the substantially carbon-free CZ-method produced silicon single crystal of substantially the same degree of free carrier absorption as the sample provides the distinct absorption peak of the localized vibration of the substitutional carbon even when the sample has a carbon density of 0.1 ppma or lower. The fourth aspect of the present invention can reduce the effects of differences in states (thickness and resistivity) between the sample and the reference even when the sample has the carbon density of 0.1 ppma or lower. The inventive method of computing the difference coefficient by the equation (5) can distinctively extract the absorption peak of the localized vibration of the substitutional carbon when the difference absorbance spectrum is determined using the same sample and reference as those of the case of FIG.4. Thus, the fourth aspect of the present invention can precisely determine the substitutional carbon density in the sample. On the other hand, the both curves, as seen in FIG.4, indicate the effect of the absorption peak by silicon when a conventional computer software for the FT-IR carbon density determination computes the difference absorbance spectrum by the equation (1) or (2), so that this effect makes indistinct the absorption peak of the localized vibration of the substitutional carbon. The second and fifth aspects of the present invention can select the infrared absorbance spectrum of an optimum reference from the infrared absorbance spectra of the references and compute the difference absorbance spectrum, so that even an in-line FT-IR carbon density determination apparatus embodying the fourth aspect of the present invention can precisely determine the carbon density in the CZ-method produced silicon single crystal. The present invention can increase the determination repeatability of the same FT-IR carbon density determination apparatus and reduce a determination error between FT-IR carbon density determination apparatuses. Specifically, the determination repeatability was increased to triple and the determination error was reduced to 1/3 in a test conducted using FT-IR carbon density determination apparatuses of the inventors. Particular embodiments of the present invention will now be described with reference to the accompanying drawings, in which:- FIG.1 is a graph of an infrared difference absorbance, determined by a prior-art method, between a sample of a CZ-method produced silicon single crystal and a reference of a FZ-method produced silicon single crystal; FIG.2 is a graph of an infrared difference absorbance, determined by an inventive method, between a sample of a CZ-method produced silicon single crystal and a reference of a CZ-method produced silicon single crystal; FIG.3 is a graph of difference infrared spectra of different forms with different difference coefficients; FIG.4 is a graph of difference absorbance spectra when a conventional FT-IR carbon density determination computer software determines the carbon density in the CZ-method produced silicon single crystal; FIG.5 is a graph of difference absorbance spectra when the inventive FT-IR carbon density determination method (e.g. a FT-IR carbon density determination computer software) determines the carbon density in the CZ-method produced silicon single crystal; FIG.6 is a graph indicative of the carbon density determination using the difference absorbance spectrum of FIG.5; FIG.7 is a graph of the infrared absorbance spectra of the sample and the reference; FIG.8 is a graph indicative of the computed difference absorbance spectrum and the determination of the carbon density; FIG.9 is a schematic diagram of an FT-IR optical system; FIG.10 is a graph of the infrared absorbance spectrum of the CZ-method produced silicon single crystal; FIG.11 is a graph of the infrared absorbance spectrum of the FZ-method produced silicon single crystal; FIG.12 is a graph of the difference absorbance spectrum between the CZ-method and FZ-method produced silicon single crystals computed by the prior-art method; and FIG.13 is a graph of the infrared absorbance spectra, computed by the prior-art method, of CZ-method produced silicon single crystals of different resistivities. A preferred embodiment of the present invention will be described with reference to the drawings hereinafter. An FT-IR carbon density determination apparatus of the present invention comprises a storage for storing data of the infrared absorbance spectra of different references. The FT-IR carbon density determination apparatus determines the infrared absorbance spectra of samples by the FT-IR, difference absorbance spectra from the infrared absorbance spectra of the determined samples and the stored references, and densities of the substitutional carbon density in the samples from the difference absorbance spectra. When a wafer of a CZ-method ( which is a main method of producing silicon single crystal) produced silicon single crystal is employed as a sample the carbon density of which is to be determined, the inventive FT-IR carbon density determination apparatus and method employ a reference of a CZ-method produced silicon single crystal of a very low carbon density. Since a method of producing a silicon single crystal by the CZ-method comprises the steps of melting a raw polysilicon in a quartz crucible by a carbon heater, immersing a seed crystal in the surface of the melt and gradually lifting the seed crystal to produce a silicon single crystal, the CZ-method produced silicon single crystal takes carbon derived from the raw polysilicon and the carbon heater. An amount of taken-in carbon increases towards the tail of a silicon single crystal rod by the segregation of the carbon. Thus, a silicon single crystal wafer sliced from a head (adjoining the seed crystal) of the silicon single crystal rod which is produced from a raw polysilicon of a minute carbon density provides a substantially carbon-free CZ-method produced reference. The storage of the FT-IR carbon density determination apparatus previously stores data of the infrared absorbance spectra of references of carbon-free CZ-method produced silicon single crystals of different degrees of free carrier absorption which are sliced as described above. Alternatively, the data of the infrared absorbance spectra of the references may be obtained by another method. After the storage of the data, the FT-IR carbon density determination apparatus determines the infrared absorbance spectra of different samples, automatically select data of the infrared absorbance spectrum of a reference of substantially the same degree of free carrier absorption as a sample to be determined from the data of the infrared absorbance spectra of the references in the storage, and computes the difference absorbance spectrum between the infrared absorbance spectra of the sample and the reference. The FT-IR carbon density determination apparatus computes a difference coefficient f by the least square approximation so that within the lower and higher ranges of wave number than the absorption peak of the localized vibration of the substitutional carbon a relational expression of wave number and absorbance of the difference absorbance spectrum is most approximate to a linear or quadratic equation, and the difference absorbance spectrum from the difference coefficient. As see in FIG.6, the absorption peak of the localized vibration of the substitutional carbon appears at 605 cm -1 , so that the lower range of wave number is selected to be 550-595 cm -1 and the higher range of wave number is selected to be 615-660 cm -1 . For example, κl1=550 cm -1 , κl2=595 cm -1 , κh1=615 cm -1 and κh2=660 cm -1 in the equations (4), (5), (6) and (7). The sample was a p-type CZ-method produced silicon single crystal of a resistivity of 10 Ωcm and a carbon density of about 0.05 ppma. The reference was a carbon-free p-type CZ-method produced silicon single crystal of a resistivity of 20 Ωcm. The apparatus determined the infrared absorbance spectra of the sample and the reference. FIG.7 shows the infrared absorbance spectra of the sample and the reference. The apparatus then computed the difference coefficient f by the equation (5) to provide 0.9764 and the difference absorbance spectrum from this value. FIG.8 shows the difference absorbance spectrum provided by this computation. As seen in FIG.8, the apparatus determined the carbon density [Cs] by ASTM designation: F123-81 to be 0.09 ppma using a base line of 595-615 cm -1 . The present invention is not restricted to the determination of the carbon density of the P-type CZ-method produced silicon single crystal but is applicable to the determination of the carbon density in an N-type CZ-method produced silicon single crystal. In the latter case, the apparatus and the method employ a reference of a CZ-method produced silicon single crystal of substantially the same degree of free carrier absorption as a sample. The present invention is also applicable to the determination of the carbon density in a FZ-method produced silicon single crystal. In this case, the apparatus and the method employ a reference of a substantially carbon-free FZ-methodical silicon single crystal.

Claims: A method of determining a substitutional carbon density in a silicon single crystal by the Fourier transform infrared spectroscopy (FT-IR), comprising the steps of: determining by the FT-IR the infrared absorbance spectrum of a sample of a silicon single crystal a carbon density of which is to be determined; determining by the FT-IR the infrared absorbance spectrum of a reference of a substantially carbon-free silicon single crystal of substantially the same degree of free carrier absorption as the sample, the reference being produced by the same process as the sample; computing a difference coefficient from the infrared absorbance spectra of the sample and the reference; computing a difference absorbance spectrum between the infrared absorbance spectra of the sample and the reference using the computed difference coefficient; and determining the substitutional carbon density in the sample from a distance of the absorption peak of the difference absorbance spectrum of the localized vibration of a substitutional carbon in the sample from a base line of the difference absorbance spectrum. A method according to claim 1, in which the sample and the reference are produced by the Czochralski (CZ) method. A method according to claim 1 or 2, wherein the difference co-efficient computing step comprises the step of computing the difference co-efficient by the least square approximation so that within lower and high ranges of wave number than the absorption peak of the difference absorbance spectrum of the localised vibration of the substitutional carbon a relational expression between wave number and infra-red absorbance is approximate to a linear or quadratic equation. An apparatus of determining a substitutional carbon density in a silicon single crystal by the Fourier transform infra-red spectroscopy (FT-IR), comprising: means for determining by the FT-IR the infra-red absorbance spectrum of a sample of a silicon single crystal a substitutional carbon density of which is to be determined; means for storing data of the infra-red absorbance spectra, determined by the FT-IR, of multiple references of substantially carbon-free silicon single crystals of different degress of free carrier absorption; means for selecting from the data of the infra-red absorbance spectra of the references in said storage means data of the infra-red absorbance spectrum of a reference of substantially the same degree of free carrier absorption as the sample; means for computing a difference co-efficient from the infra-red absorbance spectra of the sample and the reference; means for computing a difference absorbance spectrum between the infra-red absorbance spectra of the sample and the reference using the computed difference co-efficient; and, means for determining the substitutional carbon density in the sample from a distance of the absorption peak of the difference absorbance spectrum of the localised vibration of a substitutional carbon in the sample from a base line of the difference absorbance spectrum. An apparatus according to claim 4, for determining a substitutional carbon density in a Czochralski (CZ) method produced silicon single crystal, in which the means for storing data store multiple references of substantially carbon free CZ-method produced silicon single crystals of different degrees of free carrier absorption. An apparatus according to claim 4 or 5, further comprising: means for determining by the FT-IR the infra-red absorbance spectra of the references. An apparatus according to claim 4, 5 or 6, in which the means for computing a difference co-efficient from the infra-red absorbance spectra of the sample and the reference computes this by the least square approximation so that within lower and higher ranges of wave number than the absorption peak of the difference absorbance spectrum of the localised vibration of the substitutional carbon a relational expression between wave number and infra-red absorbance is approximate to a linear or quadratic equation. A method according to claim 3, wherein the linear equation is the following equation (1): As (κ) - f × Ar (κ) = 0 is the following equation (2): κ=κ l κ=κ h As (κ) d κ - f × κ=κ l κ=κ h Ar (κ) d κ = 0 is the following equation (3): κ= l κ= h As (κ) - f × κ= l κ= h Ar (κ) = 0 is the following equation (4): S = κ=κ l 1 κ=κ h 1 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 d κ + κ=κ l 2 κ=κ h 2 [( As (κ) - f × Ar( κ)) - ( b κ + a )] 2 d κ or is the following equation (5): S = κ=κ l 1 κ=κ h 1 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 + κ=κ l2 κ=κ h2 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 A method according to claim 3, in which the quadratic equation is the following equation (6); S = κ=κ l 1 κ=κ h 1 [( As (κ) - f × Ar (κ)) - ( c κ 2 + b κ + a )] 2 d κ + κ=κ l2 κ=κ h2 [( As (κ) - f × Ar (κ)) - ( c κ 2 + b κ + a )] 2 d κ or is the following equation (7): S = κ=κ l 1 κ=κ h 1 [( As (κ) - f × Ar (κ)) - (cκ 2 + b κ + a )] 2 + κ=κ l2 κ=κ h 2 [( As (κ) - f × Ar (κ)) - (cκ 2 + b κ + a )] 2 An apparatus according to claim 7, wherein the linear equation is the following equation (1): As (κ) - f × Ar (κ) = 0 is the following equation (2): κ=κ l κ=κ h As (κ) d κ - f × κ=κ l κ=κ h Ar (κ) d κ = 0 is the following equation (3): κ= l κ= h As (κ) - f × κ= l κ= h Ar (κ) = 0 is the following equation (4): S = κ=κ l 1 κ=κ h 1 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 d κ + κ=κ l2 κ=κ h2 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 d κ or is the following equation (5): S = κ=κ l 1 κ=κ h 1 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 + κ=κ l2 κ=κ h2 [( As (κ) - f × Ar (κ)) - ( b κ + a )] 2 An apparatus according to claim 7, in which the quadratic equation is the following equation (6); S = κ=κ l 1 κ=κ h 1 [( As (κ) - f × Ar (κ)) - ( c κ 2 + b κ + a )] 2 d κ + κ=κ l2 κ=κ h2 [( As (κ) - f × Ar (κ)) - ( c κ 2 + b κ + a )] 2 d κ or is the following equation (7): S = κ=κ l 1 κ=κ h 1 [( As (κ) - f × Ar (κ)) - ( c κ 2 + b κ + a )] 2 + κ=κ l 2 κ=κ h 2 [( As (κ) - f × Ar (κ)) - ( c κ 2 + b κ + a )] 2

Inventors: KITAGAWARA YUTAKA, KUBOTA HIROSHI, TAMATSUKA MASARO, KITAGAWARA, YUTAKA, KUBOTA, HIROSHI, TAMATSUKA, MASARO, Kitagawara, Yutaka, c/o Isobe Factory, Kubota, Hiroshi, c/o Isobe Factory, Tamatsuka, Masaro, c/o Isobe Factory
Applicants: SHINETSU HANDOTAI KK, SHIN-ETSU HANDOTAI COMPANY LIMITED
Application year: 1993
Main classification: G01N21
IPC codes: G01N21