Abstract:
A semiconductor fabricating method including: placing the semiconductor wafer having a film thereon inside of a chamber; generating plasma; detecting a quantity of interference lights for each of at least two wavelengths obtained from a surface of the wafer for a predetermined time period during the etching of the wafer; detecting a first time point at which the detected quantity of interference lights for one of the two wavelengths becomes a maximum and a second time point at which the detected quantity of interference lights for the other wavelength becomes a minimum; determining a state of etching based on a result of comparing a predetermined value with an interval between the first and second time points, wherein both time points are detected by using outputs of a detector for detecting a quantity of the interference lights; and controlling etching in accordance with the determining.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation application of U.S. Ser. No. 11/289,394, filed Nov. 30, 2005, now U.S. Pat. No. 7,259,866 which is a continuation application of U.S. Ser. No. 10/377,823, filed Mar. 4, 2003 (now U.S. Pat. No. 6,972,848). 
     The present invention is related to U.S. patent application Ser. No. 09/946,504 filed on Sep. 6, 2001 and U.S. patent application Ser. No. 10/230,309 filed on Aug. 29, 2002. 
    
    
     BACKGROUND OF THE INVENTION 
     The present invention relates to an apparatus that fabricates semiconductor devices through etching processing and more particularly to a semiconductor fabricating apparatus that has a function of determining the etching processing state such as an etched depth. 
     Dry-etching has been widely used in the semiconductor device formation process to remove layers of various materials, such as dielectric materials and insulating materials, formed on the surface of a semiconductor wafer or to form patterns on those layers. In the dry-etching process, it is important to adjust etching during the processing of the layers so that an etched depth desired for a layer may be obtained or so that a thin film desired for a layer may be obtained. Therefore, it is required to accurately detect the end points of etching process and the film thickness. 
     When dry-etching a semiconductor wafer using plasma, it is known that the light emission intensity of a specific-wavelength light included in a plasma light changes with the progress of the etching process of a specific layer. One of known technologies for checking the etching states, such as the end point of etching process and the film thickness on a semiconductor wafer, takes advantage of this characteristics to detect a change in the light emission intensity of a specific-wavelength light included in the plasma light during the dry-etching process and, based on this checking result, detects the end point of etching process on a specific layer and the film thickness of the layer. To increase detection precision, a misdetection caused by a fluctuation in the detected waveform generated by noises should be reduced. 
     Recently, as the wiring pitch of a semiconductor becomes finer and its density becomes higher, the open area ratio (non-etched area on a semiconductor wafer) becomes lower. This decreases the light emission intensity of a specific-wavelength light sent from the light sensor to the light detector. As a result, the level of the sampling signal from the light detector becomes lower, making it difficult for the end point determination unit to correctly detect the end point of etching process based on the sampling signal from the light detector. 
     When stopping processing upon detection of the end point of etching process, it is important that the thickness of the remaining dielectric layer should be equal to a predetermined value. In the conventional process, the time thickness control method is used to monitor the whole process on the assumption that the etching speed of each layer is constant. An etching speed is obtained, for example, by processing a sample wafer in advance. In this method, the etching process stops at the same time the elapsed time measured by the time monitor method becomes equal to the time corresponding to a predetermined film thickness (remaining film thickness in etching process). 
     However, it is known that the thickness of an actual film, for example, an SiO 2  layer formed by the LPCVD (Low Pressure Chemical Vapor Deposition) method, varies from time to time. An allowable thickness error caused by a process fluctuation during LPCVD corresponds to about 10% of the initial thickness of the SiO 2  layer. This means that the actual final thickness of the remaining SiO 2  layer on the silicon substrate cannot be measured accurately by the time monitor method. The actual thickness of the remaining layer is measured finally by the standard emission spectroscopy. If excess etching is found, the wafer is rejected and discarded. 
     A technology for detecting the end point of etching process on a semiconductor wafer by measuring the surface of a wafer with the use of an interferometer is known. This technology is disclosed, for example, in JP-A-5-179467 (document 1), U.S. Pat. No. 5,658,418 (document 2), JP-A-2000-97648 (document 3), and JP-A-2000-106356 (document 4). 
     JP-A-5-179467 (document 1) discloses a method for detecting the end point of etching process by detecting an interference light (plasma light) using three color filters (red, green, blue). U.S. Pat. No. 5,658,418 (document 2) discloses a method for counting the extreme values (maximum and minimum of waveform: zero-pass point of differential waveform) of an interference waveform using a change in the interference waveform of two wavelengths with respect to time and its differential waveform. The etching speed is calculated by measuring the time required until the count reaches a predetermined value, the remaining etching time required until a predetermined film thickness is attained is calculated based on the calculated etching speed, and, based on the calculated time, the etching process is stopped. 
     JP-A-2000-97648 (document 3) discloses a method that obtains a difference waveform (that uses a wavelength as a parameter) between a light intensity pattern (that uses a wavelength as a parameter) of an interference light before processing and a light intensity pattern of the interference light after or during processing and compares the obtained waveform with the difference waveform read from the database for measuring a difference in level (film thickness). JP-A-2000-106356 (document 4), which relates to a rotary coating apparatus, discloses a method for finding the film thickness by measuring a change in the interference light of multiple wavelengths with respect to time. 
     When stopping processing upon detection of the end point of etching process, it is important that the thickness of the remaining film layer is close to a predetermined value as much as possible. The conventional technology monitors the film thickness by adjusting the time on the assumption that the etching speed of each layer is constant. The reference etching speed value is obtained, for example, by processing a sample wafer in advance. According to this technology, the etching process stops when the time corresponding to the predetermined film thickness elapses. 
     SUMMARY OF THE INVENTION 
     However, when semiconductor wafers with the film structure of several different types are processed in small amounts at a time for fabricating semiconductors, a database for a multiple-wavelength interference pattern of differential coefficients must be created, one for the wafer to be processed and to be fabricating into products. Therefore, when etching processing is performed on a trial basis using a wafer with the same film structure as that of an actual wafer, the test cost becomes too high for small-amount fabrication because the wafer is expensive and as many extra wafers as the number of tests are required. This results in an increased device fabrication cost. 
     In the prior art described above, a sample wafer is required also when the film thickness of a wafer to be used is detected for use in setting up the operating conditions of a semiconductor fabricating apparatus when the thickness detection result is used to process a wafer for fabricating a product therefrom. For example, a wafer is selected from one lot for measuring. This requires a measuring time and a wafer, decreasing the throughput of semiconductor fabrication. 
     It is an object of the present invention to provide a semiconductor fabricating apparatus that solves the problems with the prior art described above. 
     It is another object of the present invention to provide a semiconductor fabricating apparatus that increases processing throughput. 
     There is described a semiconductor fabricating apparatus etching a semiconductor wafer, placed in a chamber and having films thereon, using plasma generated in the chamber. The semiconductor fabricating apparatus comprises a detector that detects a change in an amount of light with at least two wavelengths obtained from a surface of the wafer for a predetermined time during the processing; and a determination function that compares an interval between a time at which an amount of light with one of the two wavelengths is maximized and a time at which an amount of light with the other wavelength is minimized with a predetermined value to determine a state of the etching. 
     There is described a semiconductor fabricating apparatus comprising a detector that, when a semiconductor wafer placed in a chamber is etched using plasma generated in the chamber, detects a light interference from a surface of the wafer for a predetermined time during the etching processing; and a control unit that compares an interval between a time at which an amount of light with one of at least two wavelengths output from the detector is maximized and a time at which an amount of light with the other wavelength is minimized with a predetermined value to control the etching processing. 
     In addition, there is described that a thickness of the film being etched is determined if the interval is determined to be smaller than the predetermined value. 
     In addition, there is described that the etching processing is stopped if the interval is determined to be smaller than the predetermined value. 
     There is described a semiconductor fabricating apparatus etching a semiconductor wafer, placed in a chamber and having a multiple-layer film composed of a first film formed a surface thereof and a second film formed on the first film, using plasma generated in the chamber. The semiconductor fabricating apparatus comprises a light detector that detects a change in an amount of light with a plurality of wavelengths obtained from a surface of the wafer for a predetermined time during which the second film is etched; and a detection function that detects a thickness of the first film based on a specific waveform obtained from an output of the detector. 
     There is described a semiconductor fabricating apparatus etching a semiconductor wafer, placed in a chamber and having a multiple-layer film composed of an oxide film formed a surface thereof and a film formed on the oxide film, using plasma generated in the chamber. The semiconductor fabricating apparatus comprises a light detector that detects an amount of light with a plurality of wavelengths obtained from a surface of the wafer for a predetermined time during which the film formed on the oxide film is etched; and a detection function that detects a thickness of the oxide film based on a specific waveform obtained from an output of the detector. 
     There is described that, when a change in an amount of interference light from the surface of the wafer with respect to time is detected for the plurality of wavelengths, a characteristic change in an output of the light detector is detected. 
     To detect the maximum value and the minimum value from a light from a wafer, a theoretical interference analysis is made in advance using the optical physical characteristic values of a material to be etched to obtain a change in the interference waveform for each wavelength. Then, for a predetermined film thickness, the two different wavelengths whose interference waveforms have the peak-trough correlation are selected. In addition, the interference waveforms of the two wavelengths are measured and, through differentiation, the zero-cross times of the differential values are found. If the zero-cross times are within a predetermined value, it may be determined that a desired film thickness is attained. 
     A differential interference pattern may be created for an interference light with multiple wavelengths that are obtained during the etching processing to attain a desired film thickness. With this multiple-wavelength differential interference pattern stored each time a wafer is etched, the data values of the interference patterns may be averaged to give more reliable multi-wavelength differential interference data. By using this data to select two interference waveforms with different wavelengths, which satisfy the peak-trough correlation for the predetermined film thickness described above, the etching condition such as the film thickness may be detected and determined more accurately. 
     Other objects, features and advantages of the invention will become apparent from the following description of the embodiments of the invention taken in conjunction with the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a diagram showing the general configuration of a semiconductor wafer etching apparatus with a film thickness measuring apparatus in one embodiment of the present invention. 
         FIG. 2  is a diagram showing an example of the vertical section of a member to be processed  40  for which gate etching is performed. 
         FIG. 3  is a diagram showing an example of a change in an interference light with respect to time for multiple wavelengths during the etching processing is  FIG. 2 . 
         FIG. 4  is a flowchart showing a procedure for finding the film thickness of a member to be processed when etching processing is performed by the film thickness measuring apparatus in  FIG. 1 . 
         FIGS. 5A-5D  are diagrams showing the under-coating oxide film dependence of a poly-silicon film thickness and the undercoating oxide film dependence of a wavelength when etching processing is performed by the film thickness measuring apparatus in  FIG. 1 . 
         FIG. 6  is a diagram showing a multiple-wavelength differential interference pattern and the wavelength dependence of a differential value in a poly-silicon film thickness in another embodiment of the present invention. 
         FIGS. 7A and 7B  are diagrams showing a multiple-wavelength differential interference pattern and the wavelength dependence of a differential value in a poly-silicon film thickness in the other embodiment of the present invention. 
         FIG. 8  is a flowchart showing the operation of the other embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE EMBODIMENTS 
     Some embodiments of the present invention will be described with reference to the attached drawings. 
     In the description of the embodiments below, the same reference numerals as those in a first embodiment are used to denote the same components with the similar functionality, and their detailed descriptions are omitted. In the embodiments below, there is described a semiconductor fabricating apparatus according to the present invention and a method for measuring etching conditions such as the etching amount (etched depth or etched film thickness) during etching processing of a wafer that is a member to be processed. In the description below, “film thickness” refers to a remaining film thickness in the etching processing. 
     A first embodiment of the present invention will be described with reference to  FIGS. 1-3 .  FIG. 1  is a diagram showing, with the use of its vertical section and the blocks, the general configuration of the first embodiment of a semiconductor fabricating apparatus of the present invention.  FIG. 2  is a diagram schematically showing the configuration of a wafer to be processed and the overview of light interference in the first embodiment.  FIG. 3  is a graph showing an example of data obtained through light interference in the first embodiment. 
     First, the general configuration of a semiconductor wafer etching apparatus with a film thickness measuring apparatus according to the present invention will be described. 
     An etching apparatus  1  has a vacuum chamber  2  containing etching gas that is decomposed by microwave power into plasma  3 . This plasma  3  is used for etching a member to be processed  4  such as a semiconductor wafer on a wafer table  5 . A multiple-wavelength light, emitted from a measuring light source (for example, a halogen light source) of a film thickness measuring apparatus  10 , is guided into the vacuum chamber  2  through an optical fiber  8  and is focused onto the member to be processed  4  at a right incident angle. The member to be processed  4  has a poly-silicon layer. The emitted light is formed into an interference light by combining the light refracted on the surface of the poly-silicon layer with the light refracted on the boundary between the poly-silicon layer and the undercoating layer. The interference light is guided through the optical fiber  8  into a spectroscope  11  of the film thickness measuring apparatus  10  for use in measuring the film thickness or in determining the end point based on the state. 
     The film thickness measuring apparatus  10  comprises a spectroscope  11 , a first digital filter circuit  12 , a differentiator  13 , a second digital filter circuit  14 , a differential waveform database (differential zero-cross time accumulator)  15 , a differential waveform comparator (differential zero-cross time comparator)  16 , a processing state determination unit  26 , and a result display  17 .  FIG. 1  shows the functional configuration of the film thickness measuring apparatus  10 . Actually, the film thickness measuring apparatus  10  may comprise a CPU; storage units such as a ROM containing a film thickness measuring processing program and various types of data such as an interference light differential waveform pattern database, a RAM containing measurement data, and an external storage device; data input/output devices; and a communication controller. 
     When a member to be processed such as a semiconductor wafer is plasma-etched, the semiconductor fabricating apparatus according to the present invention calculates in advance the light interference waveform using the optical physical property values of the member to be processed and selects in advance the wavelength groups whose differential value of the interference light for a predetermined film thickness crosses the zero-value (or takes an extreme value). Data on the positive-to-negative zero-cross (or maximum value) wavelength group λ 1  and the negative-to-positive zero-cross (or minimum value) wavelength group λ 2  is stored or recorded in a storage unit or a recording unit provided in, or configured for communication with, the semiconductor fabricating apparatus main body. When the member to be processed  4  is actually processed, the intensity of the interference light of waveform groups λ 1  and λ 2  is measured to detect the time at which the differential value of each wavelength group for the measured interference light intensity crosses the zero-value (takes an extreme value). Then, the zero-cross time is compared with a predetermined value to find the film thickness of the member to be processed. 
     The light emission intensity of an interference light with a wavelength included in the wavelength group λ 1 , obtained by the spectroscope  11 , becomes a current detection signal corresponding to the light emission intensity. This signal is then converted to a voltage signal. A plurality of specific-wavelength signals output by the spectroscope  11  as sampling signals are stored in a storage unit, such as a RAM, as time series data yij. This time series data yij is then smoothed by the first digital filter circuit  12  and is stored in a storage unit, such as a RAM, as smoothed time series data Yij. Based on this smoothed time series data Yij, the differentiator  13  calculates time series data dij of a differential coefficient value (first differential value or second differential value), which is then stored in a storage unit such as a RAM. The time series data dij of the differential coefficient value is smoothed by the second digital filter circuit  14  and is stored in a storage unit, such as a RAM, as smoothed differential coefficient time series data Dij. A real pattern of the differential values of each wavelength of the interference light intensity is obtained from this smoothed differential coefficient time series data Dij. 
     On the other hand, the light emission intensity of an interference light with a wavelength included in the wavelength group λ 2 , obtained by the spectroscope  11 , becomes a current detection signal corresponding to the light emission intensity. This signal is then converted to a voltage signal. A plurality of specific-wavelength signals output by the spectroscope  11  as the sampling signals are stored in a storage unit, such as a RAM, as time series data y′ij. This time series data y′ij is then smoothed by a first digital filter circuit  22  and is stored in a storage unit, such as a RAM, as smoothed time series data Y′ij. Based on this smoothed time series data Y′ij, a differentiator  23  calculates time series data d′ij of a differential coefficient value (first differential value or second differential value), which is then stored in a storage unit such as a RAM. The time series data d′ij of the differential coefficient value is smoothed by a second digital filter circuit  24  and is stored in a storage unit, such as a RAM, as smoothed differential coefficient time series data D′ij. A real pattern of the differential values of each wavelength of the interference light intensity is obtained from this smoothed differential coefficient time series data D′ij. 
     Differential data storage units  15  and  25  store the interference light intensity data on the wavelengths, which vary with the film thickness, or the values of waveforms of the differential data and their times. In particular, the maximum values and minimum values, corresponding to the peaks and troughs of differential data, and their times Tm and T′n are stored. The length of the time between Tm and T′n is compared with a predetermined value pre-set or stored/recorded in the comparison operation unit  16 . If the length of the time between Tm and T′n is smaller than the predetermined value, it is determined that the remaining film thickness has reached or almost reached a predetermined size and the remaining film thickness of the member to be processed is determined. The result is displayed on the result display  17 . 
     The inventor and the colleagues have found the following. When a film formed on a semiconductor wafer is etched, a group of a plurality of wavelengths, where one of differential waveform data is the maximum value and the other is the minimum value, may be selected in the neighborhood of the time at which a specific film thickness is attained. The length of the time between the time when a waveform with a wavelength belonging to the groups becomes the maximum or the minimum gets shorter as the film thickness becomes smaller. This means that, for a specific film thickness, the interval between the time when one of the waveforms becomes the maximum and the time when the other becomes the minimum before or after the maximum time is fixed. With this interval pre-set as the base, whether or not the specific film thickness is attained may be determined. 
     The differential data storage unit  15  stores the change in the differential waveform data of the interference light from the wafer  4  that is the member to be processed as well as the times at which the value of differential waveform data becomes the extreme value (maximum, minimum). That is, data on the times of the peak and the trough of the interference light intensity that varies with the film thickness is stored. In addition, for a specific range of wavelengths at etching processing time, the differential data value of interference light data at a predetermined time during processing is sent from this differential data storage unit  15  to the display  17  and is displayed on the display  17 . The data on the times at which the differential value of the interference light with multiple wavelengths becomes the maximum and the minimum may also be displayed on the display  17 . 
     The differential waveform comparator  16  compares the time Tm and the time T′n to obtain the film thickness of the member to be processed. The result is displayed by the result display  17 . 
     Although only one spectroscope  11  is shown in this embodiment, a plurality of spectroscopes  11  may be used to measure and control a wide range of the area of a member to be processed. 
       FIG. 2  shows an example of the vertical section of the member to be processed  4 , such as a semiconductor wafer, to be used in gate etching processing. Referring to  FIG. 2 , a material to be processed (poly-silicon)  40 , which is the film to be processed, is formed on an oxide film  42  that is the undercoating material provided on the wafer (substrate)  4 . In addition, a masking material  41  is laminated on the material to be processed  40 . For example, when etching a gate film, the undercoating material of the material to be processed  40  is an SiO 2  insulation film, and a poly-silicon gate layer is formed on a poly-crystal undercoating material corresponding to the source and the drain. To ensure the independent operation of a gate electrode section  48  of each device, a device separation trench  49  is formed by an oxide film. In this embodiment, the gate electrode section  48  is provided in the side in which a Shallow Trench Isolation (STI)  42  is provided below a mask  71 . 
     A multiple-wavelength light emitted from the spectroscope  11  or the plasma  3  is focused, at an almost right incident angle, on the member to be processed  4  including the laminated structure of the material to be processed  40  and the oxide film  42  that is the undercoating material. The emitted light guided into the gate electrode section  48  where the undercoating material  42  is thin is formed into an interference light  95 A composed of the light reflected on the surface of the material to be etched  40  and the light reflected on the boundary between the material to be processed  40  and the undercoating material  42 . Similarly, the emitted light guided into the device separation trench  49  where the undercoating material  42  is thick is formed into an interference light  95 B composed of the light reflected on the surface of the material to be processed  40  and the light reflected on the boundary between the material to be processed  40  and the undercoating material  42 . The interference intensity of these interference lights is reduced as the undercoating oxide film becomes thinner. Therefore, the interference lights  95 A and  95 B are in the relation  95 B&gt; 95 A. 
     The reflected light is guided into the spectroscope  11  to generate a signal whose intensity varies according to the thickness of the layer of the material to be etched  40  that is being etched. Among the interference lights detected by the spectroscope  11 , the interference light  95 B from the thicker processing film part is dominant over the interference light  95 A. In this embodiment, the etching state, such as the film thickness and the etching trench depth detected by the interference light, may be obtained more accurately from the interference light reflected on the material above the device separation trench  42  (for example, film thickness  46 ). 
     The display  17  is a liquid crystal display or a CRT display, a notification unit that notifies that a predetermined film thickness or an end point is reached with the use of light or sound, or a combination of them. In this embodiment, the display  17  having a display capable of displaying the measurement data as a graph and a unit notifying the state with the use of light and sound is provided. 
     In addition, the apparatus in this embodiment has a function that, with the use of measurement data displayed on the display  17 , displays specific information desired by the user who views the displayed data or allows the user to specify information necessary to detect specific information or to perform calculation. Included in this function are the specification function, such as a pointer allowing the user to specify a specific or any point on the time-wavelength coordinate displayed on the display  17  or the data at that point; the function used to detect data values at a specified point or to calculate specific amounts, such as the time between specific times or a wavelength and amounts indicating the etching state such as an etching speed and a film thickness; and the function to display those amounts in an easy-to-identify position. 
     The unit calculating the amounts described above may be a calculator in the apparatus or a separate calculator located remotely from the apparatus and capable of sending and receiving measured or detected data via the communication unit. 
       FIG. 1  shows the functional configuration of the apparatus for measuring an etching amount. Actually, the measuring apparatus  10 , except the display  17  and the spectroscope  11 , may comprise a CPU; storage units such as a ROM containing an etching depth and film thickness measuring processing program and various types of data such as an interference light differential waveform pattern database, a RAM containing measurement data, and an external storage device; data input/output devices; and a communication controller. This applies also to the other embodiment described below. 
       FIG. 3  is a graph showing a change in multiple wavelengths with respect to time for data generated by differentiating interference light data detected by the semiconductor fabricating apparatus in this embodiment. As shown in this figure, from several waveforms each with a wavelength, a pair of waveforms may be selected wherein, in the neighborhood of the time at which one of the waveform reaches the extreme value (maximum), the other reaches the minimum. The inventor and the colleagues have found that the difference between the time at which a waveform with one wavelength reaches the maximum and the time at which the other waveform reaches the minimum is reduced as the processing progresses (as the remaining film thickness is reduced), that there is a correlation between the time difference and the film thickness, and that the remaining film thickness (etching (trench) depth) and the end point may be determined by checking this time difference. The present invention is based on this fact. 
       FIG. 4  shows the operation flow of the semiconductor fabricating apparatus shown in  FIG. 1 . In particular, the figure shows the flow of operation in which the etching state of a material to be processed is detected for use in adjusting etching processing. 
     In this embodiment, the semiconductor fabricating apparatus receives in step  800  the etching processing condition for the poly-silicon film that is the material to be processed  40 . In this step, the semiconductor fabricating apparatus may receive data from the processing condition database stored or recorded in a storage unit or a recording unit in advance or may receive data entered by the user via an input device such as a keyboard or a mouse provided on the display  17 . The semiconductor fabricating apparatus may also read film configuration data from the cassette of the semiconductor wafer  4  or from the wafer  4  itself and detect the data with an operation unit not shown. 
     Next, in step  801 , the wavelength groups λ 1  and λ 2  for determining the etching state are detected by using data recorded on the differential data storage unit or by comparing the wavelengths with the differential data on intensities stored/recorded on a separate storage/recording unit. In addition, the base time difference ΔT is set for the time difference between the times corresponding to the opposed extreme values described above. 
     In steps  802 ,  803 , and  804 , the waveform data on the interference light obtained by processing the actual wafer  4  is detected, the interference waveforms of determination wavelength groups λ 1  and λ 2  that are set in step  801  are differentiated, and the times T 1  and T 2  at which the differential data becomes the extreme value in each wavelength group are calculated. 
     In step  818 , the comparator  16  is used as described above to compare the time difference between T 1  and T 2  calculated in step  804  (T 1 −T 2  or T 2 −T 1 ) with the base time difference ΔT that is set in step  801 . If T 1 −ΔT□T 2 □T 1 +ΔT is not satisfied, that is, if it is found the time difference ΔT is smaller than the time difference between T 1  and T 2 , the apparatus determines that the desired film thickness is not reached and passes control back to step  803  to continue processing on the material to be processed  40 . On the other hand, if T 1 −ΔT□T 2 □T 1 +ΔT is satisfied, that is, if it is found that ΔT is equal to or larger than the time difference between T 1  and T 2 , the apparatus determines that the desired film thickness has been reached or exceeded and passes control to step  814  to end etching and sampling. 
     In this embodiment, etching stops at this point. The sampling of interference lights belonging to the wavelength groups  1  and  2 , which is performed via the spectroscope  11 , also stops. 
     The inventor and the colleagues have conducted a theoretical interference analysis of the material to be processed (poly-silicon film)  40  considering the effect of the undercoating material (oxide film)  42  to find that the interference waveform appearing when the film thickness changes as a result of the etching of the material to be processed  40  depends on the thickness of the undercoating film (oxide film)  42 . 
       FIGS. 5A-5D  are diagrams showing the undercoating oxide film dependence of the poly-silicon film thickness, where the differential value of the measurement wavelength 400 nm and the measurement wavelength 380 nm zero-crosses from negative to positive when etching processing is executed by the film thickness measuring apparatus shown in  FIG. 1  and, with that film thickness, the undercoating oxide film dependence of the wavelength where the differential value zero-crosses from positive to negative. 
     For a poly-silicon film about 60 nm thick,  FIG. 5A  shows the poly-silicon film thickness with respect to the undercoating oxide film thickness for a light with the wavelength of 400 nm that zero-crosses from negative to positive (minimum value is reached). As shown in the figure, the poly-silicon film thickness changes cyclically at about 130 nm increments in the undercoating oxide film thickness. This is because the undercoating oxide film interference occurs immediately after the poly-silicon film interference. 
     That is, the poly-silicon film thickness changes cyclically every sin(4πnd/λ), where n is the refraction index of the undercoating oxide film, d is the thickness of the undercoating oxide film, and λ is the wavelength.  FIG. 5B  shows the wavelength group of waveforms that zero-cross from positive to negative (maximum value is reached) when the light with the wavelength of 400 nm zero-crosses from negative to positive for the poly-silicon film thickness. As shown in the figure, the wavelength group ranges from about 430 to 500 nm. Therefore, the approximate thickness of the undercoating oxide film may be obtained by measuring the light with the wavelength of 400 nm and the wavelength group of about 430 nm to 500 nm. 
     For example, when the extreme value of the wavelength of 400 nm matches the extreme value of the wavelength of 440 nm, the thickness of the undercoating oxide film is several nm to 130 nm, about 170 nm to 260 nm, or about 330 nm to 380 nm. 
     When the zero-cross of the wavelength of 400 nm, the wavelength of 440 nm, and 480 nm match, the undercoating oxide film thickness is found to be about 140 nm to 170 nm and about 300 nm to 330 nm. With the wafer product specifications taken into consideration, the undercoating oxide film thickness is further limited. When the measurement wavelength of 380 nm shown in  FIG. 5C  zero-crosses from negative to positive using the undercoating oxide film thickness (about 300 nm to 320 nm) obtained as described above, the wavelength group λ 2  that zero-crosses from positive to negative is narrowed to about 410 nm to 420 nm (about 410 nm to 450 nm) (see  FIG. 5D ). At the same time, the accuracy of the poly-silicon film thickness is increased from about 48 nm-56 nm to about 52 nm-55 nm. 
     A plurality of spectroscopes may be used to measure and control a wide range of the area of a member to be processed. 
     Without using a light source that supplies a light to the vacuum chamber  2  as in the above embodiment, an interference light may also be measured by a measuring instrument via the spectroscope  11  using a light from the plasma  3  generated in the vacuum chamber  2 . In this case, the plasma light reflected on the surface of the wafer  4  is supplied to the spectroscope  11 . In addition, to measure a change in the plasma light, a measurement port and an optical transmission unit are provided on the side wall of the vacuum chamber  2  such that the light inside the chamber may be received. The signal detected by them is used as a reference light. This reference light must be a light that does not pass through the light path into which the light from the wafer surface directly enters and that can detect a change in the plasma light. In this embodiment, the plasma light is received by the light receiver provided on the side wall. 
     Next, a second embodiment of the present invention will be described with reference to  FIGS. 6 ,  7 A,  7 B, and  8 . 
       FIGS. 6 ,  7 A, and  7 B are diagrams showing interference waveforms detected by a semiconductor fabricating apparatus in the second embodiment of the present invention. 
     The left-hand side graph in  FIG. 6  shows a change in the interference light intensity with the wavelength on the vertical axis and the film thickness (processing time) on the horizontal axis. The right-hand side graph shows a change in the intensity in the wavelength area ranging from 300 nm or shorter to 700 nm or longer at a specific time indicated by the dotted line in the left-hand side graph. 
     This figure shows that the interference light greatly changes around a specific wavelength and that the differential data on the intensity fluctuates widely. As with  FIG. 6 ,  FIGS. 7A and 7B  show a change in the interference light intensity with the wavelength on the vertical axis and the film thickness (processing time) on the horizontal axis.  FIG. 7A  shows the change when the thickness of an oxide film  42  formed below the poly-silicon film that is a material to be processed  40  is 290 nm, while  FIG. 7B  shows the change when the thickness of the undercoating oxide film is 330 nm. The inventor and the colleagues have found that, as shown in the figures, the wavelength value at which the intensity of the interference light differential data fluctuates widely depends on the thickness of the undercoating oxide film  42 , that is, there is a correlation between the wavelength and the thickness of the undercoating oxide film  42  and, therefore, the thickness of the undercoating oxide film may be detected using interference light data obtained when the film, which is above the oxide film and which is to be processed, is etched. This embodiment of the present invention is based on this fact. 
       FIG. 8  is a flowchart showing the flow of the operation of a semiconductor fabricating apparatus in the embodiment based on the interference waveforms shown in  FIGS. 6 ,  7 A, and  7 B. In this embodiment, the intensity of an interference light with the wavelength ranging from about 300 nm to 700 nm is obtained during poly-silicon gate etching and its differential data is calculated. Instead of a zero-crossing (extreme value is reached) wavelength is shifted sequentially to the short wave side as the film thickness of the poly-silicon film to be processed  40  is decreased, the wavelength in the short wave side is reversed before the wavelength in the long wave side because of the undercoating oxide film thickness. That is,  FIGS. 6 ,  7 A and  7 B&#39;s differential data exhibits vacillation (i.e., vacillates). This characteristic is used to calculate the undercoating oxide film thickness from the reversed wavelength. 
     This enables the thickness of the undercoating film (undercoating oxide film), which has been determined by measuring a sample wafer, to be detected using the interference light data obtained when the film above the undercoating film is processed. This minimizes a throughput decrease or a fabrication cost increase that has been caused by the measurement of a sample. 
     Using the undercoating oxide film thickness calculated as described above can also increase the accuracy in the thickness measurement of a poly-silicon film that is the material to be processed. 
     In  FIG. 8 , the etching condition (etching discharge condition, etching remaining film thickness condition) for the material to be processed (poly-silicon film) is entered (step  900 ). Based on the etching remaining film thickness condition, the two determination wavelength groups λ 1  and λ 2 , their zero-cross directions, and the determination time width ΔT are set from data stored in the data storage unit (step  901 ). 
     Next, the etching and sampling of the wafer  4  is started (step  902 ), the differential coefficient time series data Di,j is calculated from the multiple-wavelength output signal yi,j received from the spectroscope (step  903 ), and the wavelength λi at which the zero-cross times Ti and Tm of the differential coefficient time series data Di,j and Dm,j are reversed is calculated ( 904 ), where i is the measured wavelength in the long wavelength side and m is the measured wavelength in the short wavelength side. 
     The times Ti and Tm satisfy the relation Ti&lt;Tm in the wavelength area where there is no effect of the undercoating oxide film, while the times Ti and Tm satisfy the relation Ti&gt;Tm in the wavelength area where there is the effect of the undercoating oxide film. From the undercoating oxide film thickness and the distortion wavelength value which have this wave-length λi in the database in advance, the undercoating oxide film thickness is calculated (step  905 ). Then, using the calculated undercoating oxide film thickness, the two determination wavelength groups λ 1  and λ 2  are reset from the differential zero-cross table (step  906 ). 
     The interference waveform of the two determination wavelength groups λ 1  and λ 2  that have been reset is differentiated (step  907 ). Next, the time difference ΔT that is the base value set in step  901  is compared with the time difference between the zero-cross times T 1  and T 2 , that is, a check is made if the relation T 1 −ΔT□T 2 □T 1 +ΔT is satisfied (step  908 ) to determine if etching and sampling should be ended (step  909 ). If the comparison between the time difference ΔT and the time difference between the zero-cross times T 1  and T 2  (check if the relation T 1 −ΔT□T 2 □T 1 +ΔT is satisfied: step  908 ) results in “NO”, the operation is repeated beginning in step  903 . If the undercoating oxide film thickness is calculated, steps  903  to  905  need not necessarily be executed. On the other hand, if the comparison results in “YES” in step  908 , control is passed to step  909  to end etching and sampling. 
     In contrast to the prior art in which the processing throughput and the cost are high because a sample wafer must be measured, the apparatus according to the present invention allows data, obtained during the processing of an upper-layer film, to be used to detect the film thickness of a lower-layer film, thus increasing the processing throughput of the whole apparatus and reducing the cost. 
     It should be further understood by those skilled in the art that although the foregoing description has been made on embodiments of the invention, the invention is not limited thereto and various changes and modifications may be made without departing from the spirit of the invention and the scope of the appended claims.