METHOD AND APPARATUS FOR MEASURING BY INFRARED ABSORPTION THE CONCENTRATION OF MICROCRYSTAL DEFECTS IN A SILICON WAFER USED IN THE MANUFACTURE OF A SEMICONDUCTOR ELEMENT

Abstract: In a method for predicting a density of micro crystal de­ fects to be generated in a semiconductor element, an infrared absorption spectrum associated with a silicon wafer (30), which is used for the manufacture of the semiconductor el­ ement, is formed by a spectrum forming system (10, 18, 22, 34). The spectrum has a first oxygen absorption peak at the wavenumber range of 1150 to 1050 cm⁻¹ and a second oxygen absorption peak at 530 to 500 cm⁻¹. First and second coefficients indicating oxygen concentrations at the first and second peaks are read by a reading unit (12). The density of the micro crystal defects are predicted by using a ratio of the first and second coefficients as a monitor.

Description: The present invention relates to a method for predicting the yield of semiconductor elements by determining the density of micro crystal defects which may be produced when a silicon semiconductor element is formed from the silicon wafer. A substrate used in the manufacture of a silicon semiconductor element, i.e., a wafer, is normally prepared by the Czochralsky (CZ) method. A silicon wafer prepared by the CZ method contains about 10¹⁸ atoms/cm³ of oxygen, and almost all oxygen atoms (about 95%) are located between lattices. These interstitial oxygen atoms are infrared active, and absorb incident infrared rays. An infrared absorption spectrum obtained by irradiating infrared rays on the wafer has a peak absorption coefficient of 30 cm⁻¹ and a half-value width of 36 cm⁻¹ at a frequency corresponding to wavenumbers of 1106 cm⁻¹ at room temperature. An absorption coefficient α 1106 at this peak is proportional to the oxygen concentration of the wafer over a wide range. Oxygen atoms contained in the silicon wafer in an over-saturation state are precipitated when the wafer is subjected to a thermal treatment during the manufacture of the semiconductor element, thus causing micro crystal defects. When these defects appear on an element-forming region of the wafer, the yield of the properly manufactured semiconductor elements is considerably reduced. For this reason, conventionally, the oxygen concentration of wafers is specified in accordance with the manufacturing processes of semiconductor elements, and wafers having an oxygen concentration within a specified range are selected for the manufacture of semiconductor elements. Oxygen concentration [Oi] is calculated from the following equation and absorption coefficient α1106 measured as a monitor of an oxygen concentration by an infrared absorption method: [Oi] = 3.01 x 10¹⁷ x α1106 atoms/cm³ However, when a plurality of wafers having the same oxygen concentration [Oi] are subjected to identical processes, the resulting micro crystal defect compositions cannot always be the same. For example, if a plurality of wafers, whose oxygen concentration is selected to fall within the range of 7 to 10 x 10¹⁷ cm⁻³, are subjected to an identical thermal treatment, a micro crystal defect density after the thermal treatment varies over the range of 10⁷ to 10¹⁰ cm⁻³ for each wafer. Micro crystal defects are greatly influenced by residual carbon in the wafer. Normal wafers prepared by the CZ method contain an amount (< 2 x 10¹⁶ cm⁻³) of carbon which cannot be detected by the infrared absorption method. Nevertheless, the micro crystal defects cannot yet be controlled. For this reason, condition management during crystal growth and during manufacture, e.g., management of stability of a solid/liquid interface and thermal hysteresis, must be strictly performed. Therefore, the manufacture of semiconductor elements requires time and cost. Prior art document IBM-TDB, vol. 23, no. 4, September 1980, pages 1389, 1390, G. K. AGOPIAN et al.: "Determination of interstitial oxygen in silicon using internal calibration with two phonon peaks", discloses a method of determination of interstitial oxygen in silicon using internal calibration with two phonon peaks. In this method in a first step an IR spectrum of an oxygen-free, float zoned wafer is obtained at a required resolution. Then, a two-phonon peak around 613 cm⁻¹ is removed and is replaced by a straight line between 641 and 577 wave numbers. An IR spectrum of the sample wafer is obtained at the same resolution. The spectrum of the oxygen-free wafer without the 613 cm⁻¹ peak is subtracted from the spectrum of the sample wafer with appropriate scaling. Phonon peaks between 1046 and 700 wave numbers are useful in obtaining perfect null. Peaks at 1107 cm⁻¹ and 613 cm⁻¹ are substituted with straight lines, one between 1145 and 1060 cm⁻¹ and other between 641 and 577 cm⁻¹, respectively. This base line spectrum is substracted from the spectrum resulting from the foregoing step to obtain oxygen and silicon peaks from the sample wafer. Thus, the ratio of the peak heights or that of the peak areas is obtained. This is a measure of interstitial oxygen. The peak-area ratio is more accurate than the peak-height ratio because the two-phonon peak at 613 cm⁻¹ is a resultant peak with many component gaussian peaks. Finally, a calibration is established between the oxygen present and the peak-height ratio or the peak-area ratio and is used in the calculation of the oxygen in the sample wafer. Further, the book by D.A. SKOOG "Principles of instrumental analysis", 3rd edition, 1984, pages 124-128, 332-336, CBS College Publishing, describes several infrared Fourier transform spectrometers which use laser and white ligth interferometers for analyzing samples such as germanium or silicon deposited on cesium iodide or bromide. Finally, prior art document SOLID STATE TECHNOLOGY, vol. 28, no. 4, April 1985, pages 309-313, Port Washington, New York, US; H.J. HUMECKI "Infrared spectroscopy and its application to caontamination anaysis", describes a similar Fourier transform infrared spectrometer which consists of two mirrors (one fixed and one reciprocating), a beamsplitter, a light source, and a detector. In operation, radiation from the light source is split such that one-half is reflected into the fixed mirror and one-half transmitted to the moving mirror. If the mirrors are spaced equidistant from the beam-splitter, their beams reinforce one another (in phase). When they are out of phase they interfere destructively. For a continuous light source, interference and reinforcement can be represented by equations involving the wavelength of the radiation. It is an object of the present invention to provide a method which can predict the yield of semiconductor element by determining the density of micro crystal defects to be produced when a semiconductor element is formed from the silicon wafer, before the manufacture of the semiconductor element. To solve this object the present invention provides a method as defined in claim 1. The present inventors found that an infrared absorption spectrum obtained by irradiating a silicon wafer with infrared rays had first and second oxygen absorption peaks and a ratio of oxygen concentrations obtained from these absorption peaks was closely related to the density of micro crystal defects producing during the manufacturing process of semiconductor elements. When the above ratio is used as a monitor, the density of micro crystal defects to be generated in a semiconductor element after the manufacturing process can be easily predicted by use of a wafer before the process. An infrared absorption measurement apparatus comprises spectrum forming means for forming an infrared absorption spectrum of a silicon wafer used in the manufacture of a semiconductor element, means for reading first and second coefficients indicating oxygen concentrations from first and second oxygen absorption peaks appearing in the spectrum, means for outputting a ratio of the first and second coefficients, and means for displaying the ratio. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: Fig. 1 is a schematic view showing a first measurement apparatus; Fig. 2 is a view showing an infrared absorption spectrum obtained by the apparatus; and Fig. 3 is a schematic view showing a second measurement apparatus. An embodiment of the present invention will now be described in detail with reference to the accompanying drawings. Fig. 1 shows a Fourier transform type infrared absorption measurement apparatus. This apparatus comprises optical system 10 for obtaining an interferogram, CPU 12 having an operational function, e.g., Fourier transform and a memory function for storing data, CRT 14 for displaying data, and X-Y plotter 16. Optical system 10 has light source 18 for emitting infrared rays. Infrared rays emitted from light source 18 are reflected by reflection mirror 20 and converted to parallel beams, and are then incident on Michelson interferometer 22. Interferometer 22 has beam splitter 24, stationary mirror 26a, and movable mirror 26b. interferometer 22 splits an incident light beam into two beams and, thereafter, allows them to interfere with each other. Interfered light beams (interferogram) from interferometer 22 are converged on test sample 30, i.e., a silicon wafer, by reflection mirror 28, and are partially absorbed by the sample. After passing through sample 30, the light beams are reflected by reflection mirror 32 to become incident on detector 34. The beam incident on detector 34 is photoelectrically converted thereby and is detected as an electrical signal. Detector 34 has cryostat 36 and photoconductive cell 38 which is housed in cryostat 36 together with liquid nitrogen. Cryostat 36 has window 36a for transmitting the interfered light beam from reflection mirror 32. Beam splitter 24 of interferometer 22 and window 36a of detector 34 are preferably formed of CsI (cesium iodide) which has a high transmittance for infrared rays and low noise within the wavenumber range of 530 to 500 cm⁻¹. Hg x Cd l-x Te, having a high resolution within an avaiable wavenumber range, is preferably used for photoconductive cell 38 of detector 34. An interferogram detected by detector 34 is supplied to CPU 12 through D/A converter 40, and is stored therein. The interferograms are accumulated and averaged, and spectrum-Fourier-transformed by CPU 12. A spectrum of an infrared active impurity contained in test sample 30, i.e., an infrared absorption spectrum is obtained by subtracting, from the spectrum of the test sample, an absorption spectrum of a standard sample which is specified in the ASTM, i.e., a pure standard sample which contains only impurities in the amount of a detection limit or lower by the infrared absorption method. The obtained infrared absorption spectrum of test sample 30 is shown in Fig. 2. In this spectrum, peak C appearing at wavenumber 1106 cm⁻¹ is caused by interstitial oxygen atoms in sample 30, and peak N appearing at wavenumber 513 cm⁻¹ is caused by substitutional oxygen atoms. In addition, a peak appearing at wavenumber 607 cm⁻¹ is caused by carbon atoms in sample 30. Absorption coefficient αOi associated with an interstitial oxygen concentration, absorption coefficient αOs associated with a substitutional oxygen concentration, oxygen concentration [Oi] of sample 30, and ratio αOi/αOs of absorption coefficients are obtained by CPU 12 as follows: (1) Average line a is drawn with respect to a spectrum within the wavenumber range of 1250 to 1150 cm⁻¹. (2) Average line b is drawn with respect to a spectrum within the wavenumber range of 1040 to 980 cm⁻¹. (3) Point A corresponding to wavenumber 1200 cm⁻¹ along line a and point B corresponding to wavenumber 1000 cm⁻¹ along line b are connected by a line, and the line is defined as base line c . (4) A vertical height of interstitial oxygen peak point C present at wavenumber 1106 cm⁻¹ from base line c is read, and from the height the absorption coefficient αOi is obtained. (5) Oxygen concentration [Oi] of sample 30 is calculated from the following equation: [Oi] = 3.01 x 10¹⁷ x αOi atoms/cm³ (6) Average line ℓ is drawn with respect to a spectrum within the wavenumber range of 550 to 530 cm⁻¹. (7) Average line m is drawn with respect to a spectrum within the wavenumber range of 500 to 480 cm⁻¹. (8) Point L corresponding to wavenumber 540 cm⁻¹ on line ℓ and point M corresponding to wavenumber 490 cm⁻¹ on line m are connected by a line, and the line is defined as base line n . (9) A vertical height of substitutional oxygen peak point N present at wavenumber 513 cm⁻¹ from base line n is read, and the read height is obtained as absorption coefficient αOs. (10) Ratio αOi/αOs is calculated. Subsequently, in the same manner as in procedures (1) and (4) above, absorption coefficient αOc associated with carbon is obtained from an absorption peak appearing within the wavenumber range of 615 to 600 cm⁻¹. Ratia αOi/αOs obtained as above is displayed on CRT 14 or is printed by a printer (not shown). Data [Oi], αOs, and αOc are also output by the printer, as needed. When oxygen concentration [Oi] and ratio αOi/αOs are measured by the apparatus, a measurement period can be three minutes for each measurement position. The measurement results obtained by the measurement apparatus with the above arrangement are utilized in the manufacture of semiconductor elements as follows. The present inventors measured an infrared absorption spectrum of 256-kb dynamic random-access memories prepared from various silicon wafers at a resolution of 4 cm⁻¹. A lot which was prepared from a silicon wafer with ratio αOi/αOs < 4.5 ([Oi] = 7 to 10 x 10¹⁷ cm⁻³) provided an average product yield of 65%, and a lot prepared from a silicon wafer with ratio αOi/αOs > 4.5 provided an yield of 45%. In intrinsic gettering, in the conventional case of managing only oxygen concentration [Oi], a product yield varied over a range of 38% to 74% due to generation of OSF. However, when management was conducted using ratio αOi/αOs < 4.5 as a monitor in addition to the conventional monitor, the product yield could be improved to 70%. From these findings, it was found that a difference in ratio αOi/αOs appeared as a difference in yield. Therefore, ratio αOi/αOs obtained by the above apparatus is used as a new monitor in addition to oxygen concentration [Oi] of a wafer as the conventional monitor, so that the density of micro crystal defects which may be produced after the manufacture of a semiconductor element can be further accurately predicted. With the above measurement apparatus, a test sample can be easily measured without being cut or broken, and desirable wafers can be easily selected so as to improve a product yield. In the normal CZ method, ratio αOi/αOs of a wafer cut from an ingot cannot be decreased below 4.5. However, when an ingot pulled in a growth furnace by the CZ method is cooled, if a tail portion of the ingot is cooled over four hours at a temperature of 850 to 550°C using a heater provided in the furnace, ratio αOi/αOs of all the wafers cut from this ingot can be decreased below 4.5. As described above, with the measurement apparatus, ratio αOi/αOs of interstitial oxygen absorption coefficient and substitutional oxygen absorption coefficient in a silicon wafer can be easily measured. When the obtained ratio is used as a monitor for managing the manufacture of semiconductor elements together with an oxygen concentration which has been conventionally used, the density of micro crystal defects which may be produced in semiconductor elements manufactured from the silicon wafer can be accurately predicted from the state of wafer before the manufacture. Therefore, manufacturing management can be greatly simplified as compared to conventional one, and a product yield can be significantly improved. In the above embodiment, for example, as a means for splitting infrared rays emitted from a light source, a Michelson interferometer is used. However, the present invention is not limited to this. As shown in Fig. 3, dispersion type beam splitting unit 22 can be used. With this embodiment, infrared rays emitted from light source 18 are converted to parallel beams through collimator lens 42, and then become incident on test sample 30. The parallel beams are partially absorbed by sample 30, and are then reflected by reflection mirrors 44 and 45 to become incident on beam-splitting unit 22. Unit 22 has a pair of reflection mirrors 46 and 47 and diffraction grating 48. In this case, diffraction grating 48 has a blaze wavelength of 15 µm, and a resolution of about 0.5 cm⁻¹. The infrared rays incident on unit 22 are split into spectra by this unit. The beam is reflected by reflection mirror 32 and becomes incident on detector 34. Then, the beam is photoelectrically converted, and is detected as an electrical signal. With the measurement apparatus with the above arrangement, ratio αOi/αOs of absorption coefficients of a test sample can be easily measured in the same manner as in the above embodiment. An absorption coefficient is used as a coefficient indicating an oxygen concentration at an oxygen absorption peak appearing in an infrared absorption spectrum. However, any coefficient may be used as long as it can indicate the oxygen concentration. In the above embodiment, the ratio of an interstitial oxygen concentration and an absorption coefficient is used as a monitor for micro crystal defects. A substitutional oxygen concentration or a carbon concentration can be used as a monitor together with the ratio.

Claims: 1. A method for predicting the yield of semiconductor elements based on the control of semiconductor wafers to be used for the construction of the semiconductor elements, said method being characterized by comprising the step of: measuring an infrared absorption spectrum of the silicon wafer within a wavenumber range of 1500 to 450 cm⁻¹, the spectrum including a first oxygen absorption peak appearing in the wavenumber range of 1150 to 1050 cm⁻¹ and a second oxygen absorption peak appearing within the wavenumber range of 530 to 500 cm⁻¹; reading a first coeffecient indicating an oxygen concentration at the first absorption peak and a second coefficient indicating an oxygen concentration at the second absorption peak; calculating a ratio of the first and second coefficients; displaying the ratio calculated in the calculation step; and determining the density of the micro crystal defects using the display ratio as a monitor, said yield of semiconductor elements being improved when the ratio of the first and second coefficients is below 4.5. 2. A method according to claim 1, characterized in that the spectrum forming step includes the processes of: emitting infrared rays; interfering the infrared rays with each other to form an interferogram; causing the silicon wafer to transmit the interferogram therethrough; detecting the interferogram transmitted through the silicon wafer as an electrical signal; Fourier-transforming the detected electrical signal to form a spectrum; and subtracting, from the spectrum, a spectrum of a standard silicon wafer which does not contain an impurity to obtain an infrared absorption spectrum of the silicon wafer. 3. A method according to claim 1, characterized in that said reading step includes the processes of: reading out a height of the first absorption peak as the first absorption coefficient; and reading out a height of the second absorption peak as the second absorption coefficient. 4. A method according to claim 3, characterized in that the reading step has the process of reading a height of a carbon absorption peak appearing in the wavenumber range of 615 to 600 cm⁻¹ of the infrared absorption spectrum as a carbon absorption coefficient; the calculating step has the process of calculating the oxygen concentration in the silicon wafer from the first absorption coefficient and the process of calculating a carbon concentration in the silicon wafer from the carbon absorption coefficient; the displaying step has the process of displaying at least one of the second absorption coefficient, the oxygen concentration, and the carbon concentration; and the determining step has the process of determining the density of the micro crystal defects using the displayed one of the second absorption coefficient, the oxygen concentration, and the carbon concentration, and the ratio as monitors.

Inventors: KUBOTA, ATSUKO C/O PATENT DIVISION, MATSUHITA, YOSHIAKI C/O PATENT DIVISION, OHWADA, YOSHIAKI C/O PATENT DIVISION
Applicants: KABUSHIKI KAISHA TOSHIBA
Application year: 1987
Main classification: G01N21
IPC codes: G01N21