Abstract:
A method and an apparatus to determine characteristics of a film on a substrate in a processing chamber. An example of a method in accordance with one embodiment of the present invention includes impinging optical radiation upon the film, sensing optical radiation reflected from the film to form spectral signals containing information concerning interference fringes, and obtaining thickness information of the film as a function of a periodicity of the interference fringes. The apparatus includes a detector in optical communication with the processing chamber to sense optical radiation generated by the plasma, and a spectrum analyzer in electrical communication with the optical detector. The spectrum analyzer resolves the spectral bands and produces information corresponding thereto. A processor is in electrical communication with the spectrum analyzer, and a memory is in electrical communication with the processor. The memory includes a computer-readable medium having a computer-readable program embodied therein that controls the system to carry out the method.

Description:
BACKGROUND OF THE INVENTION  
         [0001]    The present invention relates to monitoring of semiconductor processes. More particularly, the present invention relates to a method and apparatus to measure characteristics of a film during semiconductor processing.  
           [0002]    The semiconductor processing industry continues to strive for larger production yields while increasing the uniformity of layers deposited on substrates having increasing larger surface areas. These same factors in combination with new materials also provide higher integration of circuits per unit area of the substrate. As circuit integration increases, the need for greater uniformity and process control regarding layer thickness rises. As a result, process diagnostics and control are important to determine the characteristics of films during processing. This has led to the development of many process control and diagnostic techniques to facilitate determination of film characteristics.  
           [0003]    One prior art technique is optical endpoint detection technique. Optical endpoint detection ascertains a process endpoint by monitoring one or more narrow bands of optical emission from process plasmas. A drawback of this technique concerns the limited information regarding the characteristics of the processed films, such as only being able to determine the characteristics of the last film deposited.  
           [0004]    The test wafer measurement is another prior art process control and diagnostic technique. Test wafer measure involves direct measurement of a film on a substrate undergoing processing. As a result, the test wafer measurement technique evaluates the last process step performed by examination of one or more test wafers that are processed within a group of production wafers. A drawback of this technique is that it does not provide means to measure film characteristics in situ and in real-time. This may result in the loss of a great number of processed wafers. Another drawback with this technique is that the test wafer measurement technique is, in some cases, destructive in nature, substantially reducing the operational life of the test wafer.  
           [0005]    What is needed, therefore, is an improved technique to measure film characteristics during semiconductor processing.  
         SUMMARY OF THE INVENTION  
         [0006]    An exemplary embodiment of the present invention is directed to a method to determine characteristics of a film on a substrate in a processing chamber by impinging optical radiation upon the film, sensing optical radiation reflected from the film to form spectral signals containing information concerning interference fringes, and obtaining thickness information of the film as a function of a periodicity of the interference fringes. The apparatus includes a detector in optical communication with the processing chamber to sense optical radiation generated by the plasma, and a spectrum analyzer in electrical communication with the optical detector. The spectrum analyzer resolves the spectral bands and produces information corresponding thereto. A processor is in electrical communication with the spectrum analyzer, and a memory is in electrical communication with the processor. The memory includes a computer-readable medium having a computer-readable program embodied therein that controls the system to carry-out the method. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0007]    [0007]FIG. 1 is a simplified plan view of a plasma-based semiconductor processing system in accordance with the present invention;  
         [0008]    [0008]FIG. 2 is a detailed cross-sectional view of a substrate shown above in FIG. 1;  
         [0009]    [0009]FIG. 3 is a graphical representation of optical radiation levels reflected from a substrate and sensed by a detector using the processing system shown above in FIG. 1, in accordance with the present invention;  
         [0010]    [0010]FIG. 4 is a graphical representation of a reciprocal pattern of the optical radiation levels shown above in FIG. 3, in accordance with the present invention;  
         [0011]    [0011]FIG. 5 is a detailed cross-sectional view of the substrate shown above in FIG. 3, including a layer of photo-resist thereon;  
         [0012]    [0012]FIG. 6 is a graphical representation reciprocal pattern of the optical radiation levels measured from the substrate shown above in FIG. 5, in accordance with the present invention;  
         [0013]    [0013]FIG. 7 is a frequency domain representation of the reciprocal pattern shown above in FIG. 6, in accordance with the present invention;  
         [0014]    [0014]FIG. 8 is a flow diagram showing a method for measuring the characteristics of a film in a semiconductor process;  
         [0015]    [0015]FIG. 9 is a simplified plan view of a semiconductor processing system in accordance with an alternate embodiment of the present invention;  
         [0016]    [0016]FIG. 10 is a detailed view of the semiconductor processing system, shown above in FIG. 1; and  
         [0017]    [0017]FIG. 11 is a perspective view of a processing environment in which the processing chambers, shown above in FIGS.  1 - 3 , may be employed. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0018]    Referring to FIG. 1, a plasma-based semiconductor processing system  12  includes a housing  14  that defines a processing chamber  16 . A lens assembly  18  is provided that is in optical communication with processing chamber  16  via a window  20  disposed in the housing  14 . A spectrum analyzer  22  is in optical communication with the lens assembly  18  via a fiber-optic cable  24 . A processor  25  is in data communication with the spectrum analyzer. The spectrum analyzer may include any known detector in the art, such as a charged-coupled-device (CCD), photo-multiplier tube and the like, and typically has a dispersive grating disposed between the detector and the window  20 . Were a CCD detector employed, the dispersive grating would correspond each of the pixels associated with the CCD device to a set of wavelengths that differs from the set of wavelengths with which the remaining pixels of the CCD device correspond. The system  12  may be any plasma-based system known in the semiconductor art, e.g., plasma enhanced chemical vapor deposition system, sputter deposition system, etch system and the like. For purposes of the present discussion, the system  12  will be described as a plasma source chamber to, inter alia, implement etch processes.  
         [0019]    Referring to FIGS. 1 and 2, substrate  34  will typically include one or more films, shown as a film  66 , disposed on a wafer  68 . The wafer  68  may be formed from any material suitable for semiconductor processing. In this example, wafer  68  is formed from silicon. Similarly, film  66  may be formed from any material suitable for semiconductor processing. In the present example, film  66  is formed from silicon dioxide, SiO 2 . Characteristics of film  66  are measured as a function of spectral emission of optical radiation reflected therefrom. In this example, the aforementioned optical radiation is produced by plasma  70 , or external light source, discussed more fully below.  
         [0020]    Specifically, optical radiation, shown by arrows  72 , impinges upon substrate  34 . A portion of the optical radiation, shown as rays  74 , reflects from a first interface  76  defined by a film surface  66   a  and ambient  78 . Another portion of the optical radiation, shown as rays  80 , reflects from a second interface  82 , defined by the interface of film  66  and wafer  68 . The difference in the length of an optical path length, Λ, that is traveled by rays  74  and  80  is given by:  
         Λ=2 n   f   t cos θ  (1)  
         [0021]    where n f  is the refractive index of the film, t is thickness of film  66  in nm, and θ is the beam angle with the wafer surface. To ensure that θ is a small angle, lens assembly  18  is a collimating lens that is positioned to be disposed opposite of substrate  34  to sense cylindrical radiation reflecting from a subportion of substrate  34 . The area of subportion is dependent upon several factors, such as length and numerical aperture of fiber  24 . In one example, the area of subportion was 1 cm in diameter. In this manner, cylindrical light is collected by lens assembly  18  and collimated light is sensed by the detector in spectrum analyzer  22 , ensuring that θ is very small. Assuming very small or 0° angle θ, equation (1) may be expressed in simplified form as follows:  
         Λ=2 n   f   t   (2)  
         [0022]    A relative phase shift, δ, between rays  74  and  80  may be defined as follows:  
             δ   =         k   0        Λ     =         4      π                   n   f        t     λ     ±   π               (   3   )                               
 
         [0023]    where k 0 =2π/Λ and λ is the wavelength of radiation produced by plasma  70 . The interference of rays  74  with rays  80  forms an interference pattern, referred to as reflectance fringes, which are sensed by the detector in spectrum analyzer  22 . The reflectance fringes, shown as  84  in FIG. 3, are obtained from emission spectra over a range of wavelengths.  
         [0024]    Reflectance fringes  84  are characterized by a periodicity, defined by the distance between minima or maxima of reflectance fringes  84 , discussed more fully below. For a fixed index of refraction and thickness of film  66 , in this example 1000 Å, the periodicity was found to vary as a function of wavelength. One manner in which to determine the periodicity of reflectance fringes  84  is to identify minima  86   a - e  or maxima  88   a - e  among the reflectance fringes  84 . For the case where δ has a value that is even multiples of π the thickness “t mx ”, in nm, of film  66  may be related to the position of maxima of fringes  84  as follows:  
                 t   mx     =       (       2      m     +   1     )            λ   f     4         ,           (   4   )                               
 
         [0025]    where m is an integer number associated with one of the fringes  84  of interest and Λ f  is the wavelength, in nm, of radiation in the film  66 , i.e., Λ f =λ/n f  wherein Λ is the wavelength of radiation produced by plasma  70  and n f  is the index of refraction of film  66 . For the case where δ has a value that is odd multiples of π the thickness “t mn ”, in nm, of film  66  may be related to the position of minima of fringes  84  as follows:  
               t   mn     =     m            λ   f     2     .               (   5   )                               
 
         [0026]    For a given thickness, t, the maxima and minima will occur at all wavelengths satisfying equations 4 and 5, respectively, when reflectance fringes are plotted as a function of λ. The width of the fringes in λ domain is proportional to λ. Thus, for shorter wavelengths the fringes are narrower and vice versa. For a fixed index of refraction and thickness of film  66 , in this example 1000 Å, the distance between adjacent minima  86   a - e  or adjacent maxima  88   a - e  was found to vary as a function of wavelength. This is shown comparing distances d 1  and d 2 . Distance d 1  is the distance between maxima  88   c  and  88   d  that correspond to the intensity measured at λ=490 nm and λ=580 nm, respectively. Distance d 2  is the distance between maxima  88   d  and  88   e  that correspond to the intensity measured at λ=580 nm and λ=725 nm, respectively. Comparing d 1  and d 2  it is seen that the distance between adjacent maxima varies as a function of wavelength. The same conclusion holds true concerning the distance between adjacent minima.  
         [0027]    The distance between adjacent minima, or adjacent maxima, also varies as a function of thickness of film  66 , as shown by reflectance fringes  184  in FIG. 3. Reflectance fringes  184  show intensity in arbitrary units for optical radiation reflected from film  66  having a thickness of approximately 500 Å. The distance between adjacent minima  186   a - b  and adjacent maxima  188   a - c  varies as a function of wavelength, as discussed above with respect to reflectance fringes  84 . This is shown comparing distances d 3  and d 4 . Distance d 3  is the distance between maxima  188   a  and  188   b , which correspond to the intensity measured at λ=350 nm and λ=490 nm, respectively. Distance d 4  is the distance between maxima  188   b  and  188   c , which correspond to the intensity measured at λ=490 nm and λ=725 nm, respectively. Comparing d 3  and d 4  it is seen that the distance between adjacent maxima depends on wavelength. In addition, however, it is also seen that comparing the combined distances d 1  and d 2  with the combined distances d 3  and d 4 , we see that the distance between maxima also depends on the thickness of film  66 .  
         [0028]    Referring to FIGS. 2, 3, and  4 , to determine the thickness of the film  66  as a function of the periodicity of the reflectance fringes it is desirable to transform the data to a domain in which the distance between adjacent minima or adjacent maxima of reflectance fringes is independent of wavelength. It was found that this may be achieved by producing a reciprocal pattern  90  and  190  of the reflectance fringes  84  and  184  that is defined as 1/Λ. To that end, the data contained in reflectance patterns  84  and  184  is replotted to form reciprocal patterns  90  and  190 , respectively. Specifically, the intensity values are replotted as a function of 1/Λ, instead of λ. Reciprocal pattern  90  corresponds to intensity measured from radiation reflecting off of film  66  having a thickness of approximately 1000 Å, and reciprocal pattern  190  corresponds to intensity measured from radiation reflecting off of film  66  having a thickness of approximately 500 Å. Assume that the distance, d mxt  between adjacent maxima of periodic fringes may be defined as follows:  
               d   mxt     =         (       2      m     +   1     )          λ   f       4             (   6   )                               
 
         [0029]    where m is an integer, n f  is the index of refraction of film  66 , and giving a periodicity of 2dn f  in the 1Λ domain.  
         [0030]    As one could readily appreciate, the distance between adjacent pairs of minima  92   a - e  or adjacent pairs of maxima  94   a - e  of reciprocal pattern  90  is constant. This is shown by comparing distances d 5  and d 6 , where d 5  is the distance between maxima  94   a  and  94   b  and distance d 6  is the distance between maxima  94   b  and  94   c . Distances d 5  and d 6  are substantially equal. Similarly, the distance between adjacent minima or adjacent maxima in reciprocal pattern  190  are substantially constant. This is shown by comparing distances d 7  and d 8 , where d 7  is the distance between maxima  194   a  and  194   b  and distance d 8  is the distance between maxima  194   b  and  194   c . The difference in the distance between adjacent minima or adjacent maxima varies only as a function of film thickness, which can be shown by comparing d 7  or d 8  with d 5  or d 6 . As shown, the thinner film  66  becomes, the greater the distance between adjacent minima or adjacent maxima. Thus, assuming a substantially constant index of refraction for film  66 , characteristics of the film, such as thickness, may be measured as a function of the distance between adjacent minima or adjacent maxima of interference fringes produced by optical radiation reflecting from substrate  34  employing the reciprocal patterns  90  and  190 . It should be noted that identifying maxima or minima and determining the distance between adjacent minima or adjacent maxima may be done using any mathematical technique known in the art. The thickness may then be given as the distance between adjacent minima or adjacent maxima, of interference fringes multiplied by two times the refractive index of film  66 . However in the present example, the reciprocal pattern  190  is mapped into the frequency domain employing a Fast Fourier Transform (FFT), discussed more fully below.  
         [0031]    Referring to FIG. 5, difficulty arises when determining the thickness of a layer among a plurality of layers on a substrate  134 . As shown, substrate  134  includes two layers. Layer  166  is a layer of SiO 2 , and layer  167  is photo-resist. As discussed above, optical radiation reflects from various interfaces. The presence of layer  167  presents an additional interface from which optical radiation is reflected. For example, rays  174  represent optical radiation reflected from a first interface  176  defined between film  166  and photo-resist  167 . Rays  180  represent optical radiation reflected from a second interface  182 , defined by the interface of film  166  and wafer  168 . A third interface is defined by the interface of photo-resist  167  with the ambient  178 . Rays  183  are reflected from this interface. The interference of rays  174 ,  180  and  183  form an interference pattern from which a reciprocal pattern is formed, shown in FIG. 6 as  290 . Interface pattern  290  includes curves  284  and  288 , each of which contains characteristic information concerning either film  166  or photo-resist  167 . Determining the characteristic information contained by one of the curves  284  and  288  may be computationally intensive. To that end, the reciprocal pattern  290  is transformed to a frequency domain. This may be done employing fourier analysis. In this example the reciprocal pattern is transformed into the frequency domain employing a Fast Fourier Transform (FFT).  
         [0032]    Referring to FIGS. 5, 6, and  7 , the FFT of reciprocal pattern  290  includes a series of peaks shown as  96  and  98  having differing amplitudes and ranges of frequencies associated therewith. As shown the amplitude of peak  96  is much less than the amplitude of peak  98 . With a priori knowledge it may be determined which peak corresponds to which film, as well as certain characteristics of the film. In the present example it is known that photo-resist  167  has a greater area exposed to plasma  70 , compared to film  166 . It becomes evident that the peak with the greater amplitude, in this example peak  98 , contains information concerning photo-resist  167 . The remaining peak, peak  96  contains information concerning film  166 . In addition, knowing the indices of refractions of film  166  and photo-resist  167 , the thickness of the same may be derived knowing the center frequency of peaks  98  and  96 , respectively. Thickness information may also be derived empirically.  
         [0033]    Additionally, observing variations in the peaks over time also facilitates process control of semiconductor processes. For example, during an etch process the center frequency of peak  98  was found to change over time at a greater rate than the change in the center frequency of peak  96 . An example of this is shown in FIG. 7, where peak  98 ′ represents the thickness measurement of photo-resist  167  after being exposed to plasma  70  forty seconds after the thickness measurement represented by peak  98  occurred. The shift to the lower frequency represents a thinning of photo-resist  167 . Peak  96 ′, however, superimposes peak  96 , which indicates very little, if no change, in the thickness of film  166 . From this information, information concerning plasma may be obtained and the characteristics of the same adjusted to selectively vary the etch rate of film  166  and photo-resist  167 . For example, the characteristics of plasma  70  may be adjusted so that film  166  is etched at a faster rate than photo-resist  167 . In addition, the etch rate exhibited by film  166  and photo-resist  167  may provide information from which diagnostic data concerning the processing system may be derived.  
         [0034]    Referring to FIG. 5, as mentioned above, the exact thickness represented by the differing frequencies in the frequency domain may be determined empirically. In this manner, the thickness of either film  166  or photo-resist  167  may be determined as a function of frequency. Thickness measurements may be obtained for substrates having other layers of films thereon, in addition to layer  166  and photo-resist  167 . As a result, an exemplary embodiment of the present invention includes a method for measuring characteristics of films on a substrate during a semiconductor process, such as etching.  
         [0035]    Referring now to FIG. 8, the method includes impinging optical radiation upon the film at step  200 . At step  202 , the spectrum analyzer senses optical radiation reflecting from the film to form signals that contain information concerning interference fringes. At step  204 , the processor received the signals and derives an inverse transform of the interference fringe information, forming transformed fringes. At step  206 , the processor  25  obtains thickness information of the film as a function of a periodicity of the transformed fringes.  
         [0036]    The thickness information may be used advantageously in a feedback loop to control process conditions during processing. For example, in a deposition process the thickness information over time could be measured over time to determine the change in film thickness per unit time. This would facilitate control of the deposition rate in a deposition process or etch rate in an etch process.  
         [0037]    Although the foregoing invention has been discussed with respect to sensing optical radiation produced by plasma  70 , it should be understood that a light source may be employed. To that end, a lamp,  170  may be placed in optical communication with processing chamber  16 , via an optical fiber  124 , as shown in FIG. 9.  
         [0038]    Referring to FIG. 10 in an exemplary semiconductor process in which the present invention may be employed etches wafer  34  in order to form, inter alia, trenches thereon. To that end, processing chamber  16  has a grounded, conductive, cylindrical sidewall  28  and a shaped dielectric ceiling  30 , e.g., dome-like. Disposed within processing chamber  16  is a wafer pedestal  32  to support semiconductor wafer  34 . A cylindrical inductor coil  36  surrounds dielectric ceiling  30  and, therefore, an upper portion of processing chamber  16 . A grounded body  38  shields inductor coil  36 . An RF generator  40  is in electrical communication with inductor coil  36  through a conventional active RF match network  42 . A winding of coil inductor  36  furthest away from pedestal  32  is connected to the “hot” lead of RF generator  40 , and the winding closest to pedestal  32  is grounded. An additional RF power supply or generator  46  is in electrical communication with an interior conductive portion  48  of pedestal  32 . An exterior portion  50  of pedestal  32  is dielectric material.  
         [0039]    One or more gas sources, shown as  52 , are placed in fluid communication with processing chamber  16  through a feed line  54 . A pumping system  56  controls the chamber pressure. To that end, sidewall  28  includes an exhaust port  58  that places pumping system  56  in fluid communication with processing chamber  16  via an exhaust conduit  60 .  
         [0040]    Etchant gas, such as NF 3 , SF 6 , SiF 4 , Si 2 F 6 , and the like can be employed, either alone, or in combination with, a non-fluorine containing gas such as HBr, oxygen or both. The etchant gas exits gas source  52 , traverses feed line  54  and enters processing chamber  16 . The RF generators are activated to create a high-density plasma. To that end, in one example, RF generator  40  may provide up to about 3000 watts at 12.56 MHz. The RF generator  46  may supply up to 1000 watts at a frequency in the range of 400 kHz to 13.56 MHz to the interior conductor  48 . The chamber pressure is typically in the range of 1 to 100 millitorr.  
         [0041]    A processor  25 , in data communication with a memory  64 , controls the operation of the system  12 . To that end, processor  25  is in data communication with the various components of the system, such as signal generators  40  and  46 , RF match network  42 , gas source  52 , pump system  56 , and spectrum analyzer  22 . This is achieved by having the processor  25  operate on system control software that is stored in a memory  64 . The computer program includes sets of instructions that dictate the timing, mixture of fluids, chamber pressure, chamber temperature, RF power levels, and other parameters of a particular process, discussed more fully below. The memory  64  may be any kind of memory, such as a hard disk drive, floppy disk drive, random access memory, read-only-memory, card rack or any combination thereof. The processor  25  may contain a single-board computer (SBC), analog and digital input/output boards, interface boards and stepper motor controller boards that may conform to the Versa Modular European (VME) standard that defines board, card cage, and connector dimensions and types. The VME standard also defines the bus structure as having a 16-bit data bus and a 24-bit address bus.  
         [0042]    Referring to both FIGS. 10 and 11, the interface between a user and the processor  25  may be via a visual display. To that end, two monitors  239   a  and  239   b  may be employed. One monitor  239   a  may be mounted in a clean room wall  240  having one or more semiconductor processing systems  12   a  and  12   b . The remaining monitor  239   b  may be mounted behind the wall  240  for service personnel. The monitors  239   a  and  239   b  may simultaneously display the same information. Communication with the processor  25  may be achieved with a light pen associated with each of the monitors  239   a  and  239   b . For example, light pen  241   a  facilitates communication with the processor  25  through monitor  239   a , and light pen  241   b  facilitates communication with the processor  25  through monitor  239   b . A light sensor in the tip of the light pens  241   a  and  241   b  detects light emitted by CRT display in response to a user pointing the same to an area of the display screen. The touched area changes color, or a new menu or screen is displayed, confirming communication between the light pen and the display screen. Other devices, such as a keyboard, mouse, or other pointing or communication device may be used instead of or in addition to the light pens  241   a  and  241   b  to allow the user to communicate with the processor  25 .  
         [0043]    As discussed above, the computer program includes sets of instructions that dictate the timing, mixture of fluids, chamber pressure, chamber temperature, RF power levels, and other parameters of a particular process, as well as analyzing the information obtained by the spectrum analyzer  22 , discussed more fully below. The computer program code may be written in any conventional computer readable programming language: for example, 68000 assembly language, C, C++, Pascal, Fortran, and the like. Suitable program code is entered into a single file, or multiple files, using a conventional text editor and stored or embodied in a computer-readable medium, such as a memory system of the computer. If the entered code text is in a high level language, the code is compiled, and the resultant compiler code is then linked with an object code of precompiled Windows® library routines. To execute the linked and compiled object code the system user invokes the object code, causing the computer system to load the code in memory. The processor  25  then reads and executes the code to perform the tasks identified in the program.  
         [0044]    Although the invention has been described in terms of specific embodiments, one skilled in the art will recognize that various modification and improvements may be made. For example, the present invention may be employed to dynamically control process conditions in response to the spectra sensed by the spectra analyzer via feedback control. Therefore, the scope of the invention should not be based upon the foregoing description. Rather, the scope of the invention should be determined based upon the claims recited herein, including the full scope of equivalents thereof.