Abstract:
A method and apparatus is used to determine the thickness of a layer deposited on a specimen. For example, the thickness of a layer of polycrystalline may be measured as it is deposited over silicon oxide on a silicon wafer. The intensity of radiation emission at the top of the silicon wafer is detected. The temperature of the silicon wafer is measured and the variation in the intensity of radiation emission due to variation of the temperature is subtracted from the intensity of radiation emission detected at the top of the silicon wafer. The resultant signal is used to calculate the thickness of the polycrystalline silicon layer.

Description:
This is a continuation of copending application Ser. No. 07/564,995 filed on Aug. 9, 1990, now abandoned. 
    
    
     BACKGROUND 
     The present invention relates to the in situ monitoring and control of the thickness of a thin film during deposition on a wafer. 
     In conventional processes, polycrystalline silicon is deposited on oxidized silicon by chemical vapor deposition. It is important to accurately monitor and control the thickness of the layer of polycrystalline silicon as it is deposited. Typically this is done by complicated laser measurement system. 
     At least one attempt has been made to monitor the thickness of polycrystalline silicon deposition during its deposition using an infrared detector. See &#34;In-Process Thickness Monitor for Polycrystalline Silicon Deposition&#34;, T. I. Kamins and C. J. Dell&#39;Oca, Journal Electrochemical Society, January 1972, p. 112. This article describes the use of an infrared detector to observe the radiation emitted from an oxide-covered silicon wafer during the deposition of a polycrystalline silicon film on the oxide. However, the process did not provide a means to compensate for variations in radiation emissions which are due to variations in temperature or gas flow during processing. 
     SUMMARY OF THE INVENTION 
     In accordance with the preferred embodiment of the present invention, a method and apparatus are presented for determining the thickness of a layer deposited on a specimen. For example, the thickness of a layer of polycrystalline silicon may be measured as it is deposited over silicon oxide on a silicon wafer. 
     The intensity of radiation emission at the top of the silicon wafer is detected. This is done, for example, with one or more optical pyrometers. The emissivity varies with the thickness of the layer of polycrystalline silicon and with the temperature of the silicon wafer. In the present invention, the temperature of the silicon wafer is measured and the variation in the intensity of radiation emission due to variation of the temperature is subtracted from the intensity of radiation emission detected at the top of the wafer. The measurement of the temperature may be done, for example, by one or more optical pyrometers at the back of the wafer, or it may be done by a thermocouple. Once the emissivity variation due to variation of the temperature is subtracted, the resultant signal is used to calculate the thickness of the polycrystalline silicon layer. 
     The present invention allows for the monitoring of the thickness of a layer of polycrystalline silicon at the time of deposition. This monitoring may be done despite variations in temperature during processing time. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a simplified cross-sectional view of a thermal reactor used for chemical vapor deposition. 
     FIG. 2 shows a top view of part of the infrared heating system of the thermal reactor shown in FIG. 1. 
     FIG. 3 shows a cross section of a silicon wafer upon which has been deposited a layer of polycrystalline silicon on top of an silicon oxide layer in accordance with the preferred embodiment of the present invention. 
     FIG. 4 shows a plurality of optical pyrometers arranged to monitor thickness of a polycrystalline silicon layer deposited in the thermal reactor shown in FIG. 1 in accordance with the preferred embodiment of the present invention. 
     FIG. 5 shows an alternate embodiment of the present invention in which some of the optical pyrometers shown in FIG. 4 are replaced with a thermocouple in accordance with an alternate embodiment of the present invention. 
     FIGS. 6, 7 and 8 shows signals plotted on graphs illustrating operation of the preferred embodiment in the present invention. 
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENT 
     In FIG. 1 is shown a cross-sectional view of a thermal reactor 11 used in chemical vapor deposition process. In the process, a layer of polycrystalline silicon is deposited upon a wafer 14. A wafer support structure 15 is used to support wafer 14. Wafer support structure 15 extends through a bottom aperture 16 of thermal reactor 11. This allows a drive assembly (not shown) to rotate wafer support structure 15 and thus wafer 14 during processing. This is done to enhance process uniformity. 
     During processing, gasses enter thermal reactor 11 through an entry port 12 and are removed through an exit port 13. Also during processing, heat is provided by infrared radiation bulbs, for example, an infrared radiation bulb 17, an infrared radiation bulb 18, an infrared radiation bulb 19 and an infrared radiation bulb 20. The infrared radiation bulbs are mounted on a support system 24 connected to housing 10. The walls of thermal reactor 11 are transparent allowing infrared radiation from the infrared radiation bulbs to freely enter thermal reactor 11 and heat wafer 14. 
     FIG. 2 shows a top cutaway view of infrared radiation bulbs including infrared radiation bulb 17 and infrared radiation bulb 18. For a more complete description of thermal reactors used in semiconductor processing, see U.S. patent application Ser. No. 07/491,416 entitled Double-Dome Reactor for Semiconductor Processing. 
     As illustrated by FIG. 3, during processing a layer 61 of silicon oxide and a layer 62 of polycrystalline silicon may be deposited on silicon wafer 14. A typical thickness of layer 61 is 1000 Angstroms. A typical thickness of layer 62 is 3000 Angstroms, although layer 62 may typically range from 50 to 10,000 Angstroms. 
     During processing, a certain amount of infrared radiation is absorbed by layer 61 and layer 62 and a certain amount is emitted. On the top of wafer 14, the amount of infrared radiation varies with the thickness of layer 62 and with the temperature of wafer 14. On the bottom of wafer 14, the amount of infrared radiation varies with the temperature of wafer 14. In the preferred embodiment of the present invention the radiation level at the top of wafer 14 and the radiation level at the bottom wafer 14 are measured. Using the measured level of infrared radiation it is possible to subtract out the change in infrared radiation from the top of wafer 14 resulting from variations in temperature. Once the subtraction has been performed, it is possible to determine the thickness of layer 62 based on the infrared radiation at the top of wafer 14. 
     FIG. 4 shows an optical pyrometer 31 placed to measure emission of infrared radiation at a point 41 on top of wafer 14. An optical pyrometer 32 is placed to measure emission of infrared radiation at a point 42 on top of wafer 14. An optical pyrometer 33 is placed to measure emission of infrared radiation at a point 43 on top of wafer 14. While a single optical pyrometer may be used to measure emission of infrared radiation at the top of wafer 14, a plurality of optical pyrometers may be used when it is desirable to monitor deposition uniformity of layer 62 of polycrystalline silicon. 
     Similarly, an optical pyrometer 34 is placed to measure emission of infrared radiation at a point 44 on the bottom of wafer 14. An optical pyrometer 35 is placed to measure emission of infrared radiation at a point 45 on the bottom of wafer 14. While a single optical pyrometer may be used to measure emission of infrared radiation from the bottom of wafer 14, a plurality of optical pyrometers may be used when it is desirable to monitor temperature uniformity across wafer 14. 
     Optical pyrometers 31-35 are, for example, able to measure temperature in the range of 500 degrees centigrade to 1250 degrees centigrade and detect a wavelength between two microns and four microns, for example 3.3 microns. Such an optical pyrometer is available from Ircon, Inc., having a business address of 7300 North Natchez Ave. Niles, Ill. 60648, or from Linear Labs, having a business address of 1290 Hammerwood Avenue, Sunnyvale, Calif. 94089. 
     Output of optical pyrometers 31-35 is received and evaluated by a computer 51. Computer 51 generates a current output which is converted to a voltage between zero and ten volts that is proportional to the temperature detected by optical pyrometers 31-35. 
     For example, FIG. 6 shows a measured signal 64 plotted on a graph 60. Graph 60 has a horizontal axis 61 which represents the passage of time, and a vertical axis 62, which represents the temperature optical pyrometer 31, optical pyrometer 32 and/or optical pyrometer 33 measure at the top of wafer 14. A reference line 63 indicates a measured temperature of 700 degrees centigrade. 
     FIG. 7 shows a measured signal 74 plotted on a graph 70. Graph 70 has a horizontal axis 71 which corresponds to the passage of time as represented by horizontal axis 61 of graph 60. Graph 70 also has a vertical axis 72, which represents the temperature optical pyrometer 34 and/or optical pyrometer 35 measure at the bottom of wafer 14. A reference line 73 indicates a measured temperature of 700 degrees centigrade. 
     In order to generate an output which indicates changes in radiation emissions detected by optical pyrometer 31, pyrometer 32 and/or optical pyrometer 33 which are not due to changes in the temperature, computer 51 subtracts the amplitude of measured signal 74 from voltage amplitude of measured signal 64 to produce a calculated temperature differential signal 84 on a graph 80 as shown in FIG. 8. Graph 80 has a horizontal axis 81 which corresponds to the passage of time as represented by horizontal axis 61 of graph 60 and horizontal axis 71 of graph 70. A vertical axis 82 represents the measured radiation emitted from the top of wafer 14 which is not caused by changes in temperature. A reference line 83 is also shown. 
     Calculated temperature differential signal 84 may be used to monitor the thickness of layer 62 at processing time. For example, calculated temperature differential signal 84 may be generated for a specific process. Parameters of the process may include, for example, type of oxide film deposited, pattern in which a deposit is made, the deposition rate, temperature of deposit and the pressure within thermal reactor 11 or other deposition chamber during deposition. 
     Once the parameters have been set, one or more calibration test runs are performed and temperature differential signal 84 is calculated for each test run. The process parameters may be varied for each of the test runs. The thickness of deposited layer 62 is also measured for each test run. 
     Data from the calibrated test runs is used to determine at what time, relative to temperature differential signal 84, a desired thickness of the deposited layer 62 is reached for a given set of process parameters. The slope of the change in relative differential signal 84 may be calculated for the location where an optimal thickness of deposited layer 62 is reached. In subsequent production runs, for the specified parameters, the waveform and slope of relative differential signal 84 may be used to monitor and control the thickness of the deposited layer. 
     For example, for a selected set of parameters, the measured thickness of layer 62 was measured during a calibration run to be 600 Angstroms at a point 85 on calculated temperature differential signal 84, 3000  Angstroms at a point 86 and 6000 Angstroms at a point 87. In future production runs using the same selected set of parameters, the calculated temperature differential signal 84 can be used to monitor and control the deposition process, for example, allowing the process to be stopped when the desired thickness of deposited layer 62 is reached. 
     Also, once calculated temperature differential signal 84 is stored in computer 51 for a specific set of parameters, on subsequent production runs using the same specific set of parameters, the calculated temperature differential signal 84 for the production run may be compared to the calculated temperature differential signal 84 for the test run. The parameters of future production runs may then be adjusted to compensate for any deviations in the signals. 
     In FIG. 5 optical pyrometers 34 and 35 are shown replaced by a thermocouple 37 in accordance with an alternate embodiment of the present invention.