Abstract:
An apparatus for depositing films on a substrate, the apparatus including: a plasma deposition chamber having a first electrode and a second electrode that defines a target plane in which the substrate is held during deposition, and wherein during deposition a plasma is sustained between the first and second electrodes, said deposition chamber also including an input window and an output window; and a monitoring system which includes a light source and an optical detector, both located outside of the deposition chamber, said optical monitoring system also including an optical system that directs a beam from the light source through the input window and into the deposition chamber as a measurement beam, wherein the measurement beam arrives at the target plane along a path that is approximately normal to the target plane and wherein during operation said measurement beam interacts with the substrate to generate a return measurement beam that passes from the substrate out of the chamber through the output window, wherein said optical system directs the measurement return beam onto the optical detector.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]    This application claims benefit of U.S. Provisional Application Ser. No. 60/470,608, filed on May 15, 2003, entitled “PECVD In-Situ Monitoring System,” which is incorporated herein by reference. 
     
    
     
       TECHNICAL FIELD  
         [0002]    This invention relates to in-situ monitoring of thin film growth.  
         BACKGROUND  
         [0003]    A new category of optical filter has been designed. This new optical filter is thermo-optically tunable and constructed of multiple thin films to produce one or more Fabry-Perot cavities, which function as an optical interference filter. These filters have been fabricated in a plasma enhanced chemical vapor deposition (“PECVD”) reactor, by changing the gas compositions that enter the reactor at precise times in order to form the thin-film layers with precisely controlled thicknesses. They are described in U.S. Ser. No. 10/174,503, filed Jun. 17, 2002, and U.S. Ser. No. 10/211,970, filed Aug. 2, 2002, both of which are incorporated herein by reference.  
         SUMMARY  
         [0004]    In general, in one aspect, the invention features an apparatus for depositing films on a substrate. The apparatus includes: a plasma deposition chamber having an input window and an output window and also having a first electrode and a second electrode that defines a target plane in which the substrate is held during deposition, and wherein during deposition a plasma is sustained between the first and second electrodes; and a monitoring system which includes a light source and an optical detector, both located outside of the deposition chamber. The optical monitoring system further includes an optical system that directs a beam from the light source through the input window and into the deposition chamber as a measurement beam, wherein the measurement beam arrives at the target plane along a path that is approximately normal to the target plane and wherein during operation the measurement beam interacts with the substrate to generate a return measurement beam that passes from the substrate out of the chamber through the output window, wherein the optical system directs the measurement return beam onto the optical detector.  
           [0005]    Other embodiments include one or more of the following features. The light source is a laser, e.g., a tunable laser. The plasma deposition chamber is a PECVD chamber, wherein the first electrode is a showerhead through which process gases are introduced into the chamber during operation to form a film on a surface of the substrate. The second electrode has a hole extending therethrough, wherein the hole is located in the second electrode behind a position in which the substrate is held during operation. The input window is located behind the second electrode so that the measurement beam passing through the input window passes through the hole and impinges on the backside of the substrate. In some embodiments, the input window and the output window are located on the same side of the second electrode and the measurement return beam is a reflected beam produced by the measurement beam interacting with the substrate. In some of those embodiments, the input window is the output window. In other embodiments, the output and input windows are located on opposite sides of the second electrode and the measurement return beam is a transmitted beam produced by the measurement beam interacting with the substrate.  
           [0006]    In still other embodiments, the optical monitoring system further includes a coupler that splits a beam from the light source into first and second beams that are spaced apart, wherein the optical system directs the first beam into the deposition chamber as the measurement beam and the second beam into the chamber as a reference beam. The optical monitoring system also includes a second detector located outside of the deposition chamber and wherein during operation the reference beam produces within the chamber a reference return beam that passes out of the output window, and the optical system directs the reference return beam onto the second detector. In some of the embodiments, the optical system includes a second detector and an optical splitter that splits the beam from the light source into first and second beams, wherein the optical system directs the first beam into the deposition chamber as the measurement beam and the second beam onto the second detector. The monitoring system further includes a second light source and the optical system directs light from the second light source through the input window into the deposition chamber as a reference beam that is spaced apart from the measurement beam. The monitoring system also includes a second detector located outside of the deposition chamber and wherein during operation the reference beam produces within the chamber a reference return beam that passes out of the output window, and wherein said optical system directs the reference return beam onto the second detector.  
           [0007]    In general, in another aspect, the invention features a method of monitoring film thickness on a substrate during deposition. The method includes: in a plasma deposition chamber, plasma depositing a film onto a surface of the substrate; while the film is being deposited, introducing a measurement beam into the deposition chamber from outside of the deposition chamber; delivering the measurement beam to the substrate from a direction that is approximately normal to said surface of the substrate; interacting the measurement beam with the substrate to generate a measurement return beam; delivering the measurement return beam to outside of the deposition chamber; and detecting the measurement return beam that has exited from the deposition chamber.  
           [0008]    Other embodiments include one or more of the following features. Plasma depositing also involves introducing a process gas into the plasma and forming the film from the process gas. Interacting the measurement beam with the substrate involves reflecting the measurement beam off of the substrate. Introducing the measurement beam into the deposition chamber involves passing the measurement beam through an input window in the plasma deposition chamber. Delivering the measurement return beam to outside of the plasma deposition chamber involves passing the measurement return beam through an output window in the plasma deposition chamber. The output window is the input window. The method also includes: while the film is being deposited, introducing a reference beam into the deposition chamber from outside of the chamber, the reference beam being spaced apart from the measurement beam inside of the chamber; providing a reference inside of the chamber; interacting the reference beam with the reference to generate a reference return beam; delivering the reference return beam to outside of the deposition chamber; and detecting the reference return beam that has exited from the deposition chamber. The reference is a mirror and wherein interacting the reference beam with the reference involves reflecting the reference beam off of the mirror to generate the reference return beam. The method also involves using the detected reference return beam to compensate for fluctuations in the detected measurement beam due to fluctuations in the measurement beam. The method also involves using both the detected measurement return beam and the detected reference return beam to determine thickness of the film as it is being deposited, wherein the detected reference return beam is used to compensate for changes in detected measurement beam that are due to changes in the measurement beam.  
           [0009]    Other embodiments may also include one or more of the following features. The reference is a thermally tunable optical filter, and the method further involves using the detected reference return beam to compensate for changes in the detected measurement beam due to changes in temperature of the substrate. Interacting the reference beam with the reference involves reflecting the reference beam off of the thermally tunable optical filter to generate the reference return beam. The reference is a thermally tunable optical filter and the method further involves using both the detected measurement return beam and the detected reference return beam to determine thickness of the film as it is being deposited, wherein the detected reference return beam is used to compensate for changes in the detected measurement beam that are due to changes in temperature of the substrate. The method further includes putting the reference into thermal contact with the substrate. Putting the reference into thermal contact with the substrate involves resting the reference against the backside of the substrate.  
           [0010]    The details of one or more embodiments of the invention are set forth in the accompanying drawings and the description below. Other features, objects, and advantages of the invention will be apparent from the description and drawings, and from the claims. 
       
    
    
     DESCRIPTION OF DRAWINGS  
       [0011]    [0011]FIG. 1 illustrates a PECVD reactor with an in-situ monitoring system.  
         [0012]    [0012]FIG. 2 illustrates an embodiment of a monitoring system for monitoring film growth on a wafer.  
         [0013]    [0013]FIG. 3 illustrates a second embodiment of a monitoring system for monitoring film growth on a wafer.  
         [0014]    [0014]FIG. 4 illustrates how interference fringes shift with temperature changes.  
         [0015]    [0015]FIG. 5 illustrates one way that data are generated from the signals coming from the in-situ monitoring system.  
         [0016]    [0016]FIG. 6 illustrates how the monitored reference signal changes as a function of temperature.  
         [0017]    [0017]FIG. 7 illustrates how thermal noise is removed from the monitored signal.  
     
    
     DETAILED DESCRIPTION  
       [0018]    The family of in-situ monitoring systems described below enables precise measurements of thin film growth, especially of optical thicknesses of deposited films, and aids in the deposition of multi-layered optical structures, including multicavity Fabry-Perot optical filters. Deployable in Plasma Enhanced Chemical Vapor Deposition (PECVD) reactors, the in-situ monitoring systems leave the geometry of the growth space, including the plasma gap spacing, unchanged. The optical signals for monitoring the thin-film growth are sourced and collected outside of the chamber, and both transmission and reflection monitoring configurations are possible. Systems and methods using remote measurements for compensating for a variety of noise sources, including mechanical jitter, power drift in the system lasers, and temperature drift in the chamber, are presented. Finally, various approaches to signal processing are described.  
         [0019]    I. Beam Architecture  
         [0020]    Referring to FIG. 1, one embodiment of a PECVD reactor with an in-situ monitoring system is depicted generally at  1000 . Many of the optical components of monitoring system  1000  are external to reactor chamber  1040 . An optical signal is introduced into chamber  1040  through port window  1050 . The signal then passes through a via hole on backplate  1060  of substrate  1070 , interacts with substrate  1070 , and is detected. By monitoring this optical signal, information concerning wafer and thin-film properties, including film thickness, is obtained.  
         [0021]    In this embodiment, part of the optical signal is reflected off of substrate  1070 , and part is transmitted through it, so detectors can be placed to monitor either the transmitted or reflected component. In the reflection-monitoring configuration, the optical signal, originating with laser  1010 , passes through fiber optic  1015  and collimator optic  1030 , enters chamber  1040  through window  1050 , interacts with substrate wafer  1070 , and then exits back through window  1050 . Upon exiting, the signal passes through collimator optic  1035 , passes through fiber optic  1025 , and impinges upon detector  1020 . In the transmission-monitoring configuration, the optical signal enters chamber  1040 , passes through the via in backplate  1060 , interacts with substrate wafer  1070 , and afterwards continues through a hole in upper showerhead electrode  1080 , through window  1090  in bottom showerhead plate  1100 , passes through a via in plenum  1110 , and exits through port window  1120  to be detected by detector  1130 .  
         [0022]    Other elements of the PECVD reactor are standard and known to those skilled in the art. Halogen lamps  1140  heat the substrate, which is held by the substrate holder  1150 . Holder  1150 , which is made of titanium to have a thermal coefficient of expansion similar to that of silicon wafers, has a lip upon which substrate wafer  1070  sits. Clips (not shown) serve to fix wafer  1070  and backplate  1060  in place. Holder  1150 , which also serves as one of the plasma electrodes, is grounded. The other electrode, spaced about one inch away in the illustrated embodiment, is upper showerhead electrode  1080 , which is electrified by an electrical connection passing through electrical port  1180 . Plasma confinement wall  1160 , which confines the plasma to between the two electrodes, is adjustable to adjust the rate of gas flow out of the space between the two electrodes. This adjustability is achieved by threading the inside of plasma wall  1160  and the exterior of plenum  1110 , and by screwing or unscrewing wall  1160  onto plenum  1110  as needed. The gas enters the space between the two electrodes through gas inlet  1170  and then passes through the holes in bottom showerhead plate  1100  and upper showerhead electrode  1080 . Chamber  1040  is evacuated by a vacuum pump connected at vacuum port  1190 .  
         [0023]    II. Compensating for Drift in the Laser Signal  
         [0024]    Referring to FIG. 2, one embodiment of the in-situ monitoring system, generally indicated at  5 , uses a second beam as a reference to improve the measurement precision of the beam that interacts with the substrate. As seen in FIG. 2, PECVD reactor chamber  10  contains a target wafer  20 , held in a substrate holder (not shown) that serves as one of the plasma-forming electrodes, and a showerhead structure  30  that serves as the other plasma-forming electrode. Primary beam  130  interacts with wafer  20  itself. The second, reference beam  135  is reflected off mirror  40 , which is resting on the backside of substrate wafer  20 . Beam  135  enters and exits chamber  10 , and is detected in a similar fashion, as beam  130 . Using a dual beam architecture and beam splitters, system  5  can compensate for power drift in the laser or for changes in the intensity of the laser beam due to mechanical vibration.  
         [0025]    In this embodiment, no backing plates are used with the silicon wafer, as silicon has a high enough thermal conductivity to not make these necessary to achieve the required uniformity of temperature over the silicon substrate. If the target wafer is not sufficiently thermally conductive, as with glass wafers, for example, a silicon wafer is used as a backplate to achieve the required uniformity. Such silicon wafer backplates are coated with an anti-reflection coating, such as silicon nitride ¼ of a wavelength thick, to decrease any interference fringe effect.  
         [0026]    There are clips (not shown) that hold wafer  20  in the wafer holder. One of these clips also holds monitoring mirror  40  onto wafer  20 . Mirror  40  is reflective at the monitoring wavelength of 1550 nm. In the described embodiment, mirror  40  is ¼ the size of wafer  20 , though it could be larger or smaller than this.  
         [0027]    Exterior to chamber  10  is tunable laser  50 , which has a center wavelength of 1550 nm and a tuning range of ±50 nm. Laser  50  is tuned during deposition when obtaining data regarding film growth. Laser  50  sends a laser beam to coupler  60 , which splits the beam into two beams of equal intensity and couples those beams into corresponding fiber optics  70  and  75 .  
         [0028]    The beam exiting fiber optic  70  passes through collimator lens  80  and into beam splitter  90 . Beam splitter  90  sends half of the beam into collimating optic  100 , which then directs beam  130  into chamber  10 . Beam splitter  90  sends the other half of the beam into optic  110  and photodetector  120 .  
         [0029]    Beam  130  enters chamber  10  through a window (not shown), passes through silicon wafer  20 , and a portion of it is reflected by the thin films that are being deposited on the front of the wafer. The reflected beam passes back through optic  100 , and a portion of it is reflected into optic  140 , which focuses it onto photodetector  150 .  
         [0030]    The beam exiting fiber optic  75  passes through collimator  85  and into beam splitter  90 . Beam splitter  90  sends half of that beam into collimating optic  105 , which then directs beam  135  into chamber  10 . Beam splitter  90  sends the other half of the beam into optic  115  and photodetector  125 .  
         [0031]    Beam  135 , after leaving optic  105 , enters chamber  10  through the same window as beam  130 . Beam  135  strikes mirror  40 , is reflected, passes back through optic  105 , and is partially reflected into optic  145 , which focuses it onto photodetector  155 . The signals acquired by photodetectors  150  and  155  are differenced by differential amplifier  160 , and then fed into data acquisition hardware (“DAQ”)  170  along with signals from compensation photodetectors  120  and  125 . Computer  180  acquires data from DAQ  170 .  
         [0032]    Two sources of beam intensity fluctuations can contaminate the data derived from the signal detected by photodetector  150 , namely, differential mode fluctuations, e.g., changes in coupler  60  that cause it to split the beam unevenly over time, and common mode fluctuations, e.g., power drift in the tunable laser  50 . Data derived from compensation photodetectors  120  and  125  are used to compensate for differential mode fluctuations changes, whereas differencing of the two signals from photodetectors  150  and  155  compensates for the common mode fluctuations.  
         [0033]    III. Compensating for Drift in Temperature  
         [0034]    Referring to FIG. 3, another embodiment of the in-situ monitoring system, generally indicated at  205 , uses a second beam to produce a reference signal to compensate for changes in the measurement signal due to shifts in temperature. In this embodiment, the second beam is generated by a second, separately controllable laser and, instead of using a mirror resting on the backside of the silicon wafer, it uses a thermally tunable thin-film optical filter.  
         [0035]    As seen in FIG. 3, PECVD chamber  210  contains a target wafer  220 . In this embodiment, reference filter  240  rests on the backside of wafer  220 . Filter  240  is a thermo-optically tunable thin-film optical interference filter fabricated as described in detail in U.S. Ser. No. 10/174,503, filed Jun. 17, 2002, and U.S. Ser. No. 10/211,970, filed Aug. 2, 2002, both of which are incorporated herein by reference. The thermo-optically tunable filter  240  has a transmission pass band that is centered at a wavelength that varies as a function of temperature. Thus, the location of the pass band is an accurate indicator of temperature. Filter  240  rests on the backside of the silicon wafer and thus will be at the same temperature as wafer  220 . Filter  240  is ¼ the size of wafer  220 .  
         [0036]    Exterior to chamber  210  are two independently tunable lasers  250  and  255 , which each have a center wavelength of 1550 nm and a tuning range of ±50 nm. Lasers  250  and  255  are independently tuned during operation of the monitoring system to obtain data concerning film growth and deposition conditions as deposition occurs. Laser  250  emits a beam, which passes through collimator  280  and enters beam splitter  290  to be split into a first and second part. The first part of the beam passes through optic  310 , which focus it onto photodetector  320 . The second part of the beam passes through collimating optic  300  and through a window into chamber  210  as beam  330 . Laser  255  emits a beam, which passes through collimator  285  and enters beam splitter  290  to be split into a first and second part as well. The first part passes through optic  315 , which focuses it onto photodetector  325 . The second part of the beam passes through collimating optic  305  and through the window into chamber  210  as beam  335 .  
         [0037]    Beam  330 , after entering chamber  210 , passes through silicon wafer  220 , and a portion of it is reflected by the thin films that are being deposited on the front of the wafer. The reflected beam passes back through optic  300 , and a portion of it is reflected into optic  340 , which focuses it into photodetector  350 . Beam  335 , after entering chamber  210 , passes through filter  240 , and a portion of it is reflected by filter  240 . The reflected beam passes back through optic  305 , and a portion of it is reflected into optic  345 , which focuses it onto photodetector  355 . Data from photodetectors  320 ,  325 ,  350 , and  355  pass through DAQ  370  to computer  380  for processing, as discussed below.  
         [0038]    Beam  330  produces a measurement signal that is used to monitor the thickness of the thin film being deposited. Beam  335  produces a reference signal that is used to measure the temperature of the silicon wafer. Since the reflectivity of the stack of thin films that are being deposited will also vary with temperature, knowing the actual temperature fluctuations enables one to compensate for that effect. In other words, knowing the actual temperature fluctuation of the wafer enables one to remove the temperature effect from the measurement signal. Thus, the signal processing software uses the information derived from beam  335  to reduce the impact of variations in temperature on the thin film thickness measurements.  
         [0039]    Data derived from the signals produced by compensation photodetectors  320  and  325  are used to compensate for power drifting in lasers  250  and  255 , as previously described.  
         [0040]    IV. Signal Processing Algorithms  
         [0041]    Beam  130  in FIG. 2 and beam  330  in FIG. 3 are focused on the wafer to be monitored. The data collected by photodetectors  150  in FIGS. 2 and 350 in FIG. 3 are compensated for power drift of the laser. After compensation, these data are used to indicate the thickness that the film growth has achieved on the substrate and underlying thin films.  
         [0042]    In reflection-monitoring configurations a portion of the measurement beam will also reflect off of the backside of the wafer and will interfere with the beam that reflects off of the front side of the wafer when the thin films are being deposited. These two reflected beams will interfere and that interference will change as the temperature of the wafer changes. This is largely because the wafer thickness will expand or contract, thereby changing the size of the reflection cavity produced by the front and backsides of the wafer. This interference has much greater effect in reflection-monitoring configurations than in transmission-monitoring configurations. There are a number of ways to minimize the impact of these changes on the measurements; some of these ways will now be discussed.  
         [0043]    Referring to FIG. 4, interference signal  410  results when the wavelength of the laser is changed. When the temperature changes, interference signal  410  shifts left by an amount  430  to produce interference signal  420 . To eliminate the effect of this fringe and its drift, laser  50  in FIG. 2 and laser  250  in FIG. 3 are swept (i.e., tuned) over a range  440  approximately equally to one or more fringes. The average value of the detected signal is free of the fringe effect.  
         [0044]    Another signal processing approach that helps reduce the temperature effect is illustrated in FIG. 5. As the thin film is being deposited on an existing filter structure the amplitude of the bandpass curve will change. This change is a measure of the thickness of the film being deposited. Unfortunately, changes in temperature will produce shifts in the location of the peak and thus will also cause a change in the measured signal. By tuning the measurement laser over a range that locates the peak of the bandpass curve, the amplitude of the peak can be determined regardless of where it has shifted. Thus, by measuring the amplitude of the peak, the effects attributable to the shifting of the curve are eliminated. The maximum value of the spectrum (i.e., the maximum intensity value) is recorded and plotted on time signal graph  540 . Note that any shift in wavelength of the spectrum does not affect the plot in graph  540 . Graph  540  displays the change in signal intensity over time attributable only to changes in the thickness of the film being deposited on the substrate. Note also that interference fringes arising from the substrate wafer itself will be superimposed on the passband curve. If the substrate wafer is thick, the distance between fringes will be short compared to the size of the passband, and tuning the laser over the passband range will result in high-frequency noise that can be filtered out by using of a low-frequency signal amplifier that does not see the high frequencies caused by the interference signal.  
         [0045]    More precise temperature data can be obtained and be used to reduce noise in the thickness data by use of the tuning capability of laser  255 . Referring to FIG. 3, reference laser  255  is swept over the wavelength range of the passband of filter  240  to determine the central wavelength of filter  240 . Referring to FIG. 6, this passband is represented generally in graph  550  as the plot  560 . As the temperature of wafer changes, the passband shifts, for example, to the left as illustrated by plot  580 . This results in a displacement of the central wavelength by an amount  565 . During film growth, the laser frequency sweeping continues and the change in the central wavelength is tracked, measuring the variations in temperature. By relying on the known properties of filter  240  (e.g., the location of the center of the passband as a function of temperature), the precise change in the temperature and the absolute temperature of wafer  220  is known. Referring to FIG. 7, the effects of these variations upon the wafer monitoring beam can then be determined, as shown by plot  630 , and then removed by signal processing  640 , resulting in processed data  650  useful in monitoring the fabrication of thin films. Processing  640  can be subtracting plot  630  from plot  620 , or include more complicated mathematical operations.  
         [0046]    There is another approach to obtaining precise temperature data using the second beam architecture. Referring again to FIG. 3, laser  255  is swept over the passband of filter  240  to determine the central wavelength of filter  240  and the shape of the passband. Referring to FIG. 6, this passband is represented generally in graph  550  as the plot  560 . Then the laser is tuned to and fixed at wavelength  570  at the edge of the passband. As the temperature drifts and the passband shifts left as demonstrated by plot  580 , the signal reflected by filter  240  changes, resulting in a signal difference  585 .  
         [0047]    By monitoring the strength of the signal reflected by filter  240 , the direction and magnitude of the shift in the position of the central wavelength is known, and the effects of the temperature variations of the wafer can then be subtracted out of the signal collected by photodetector  350 . This accurate temperature information is acquired without any physical contact of the wafer  220 .  
         [0048]    In employing the third or fourth monitoring method to collect data containing information regarding temperature, algorithms can be used to further reduce temperature-related noise in the signal detected by photodetector  350  of FIG. 3. Referring to FIG. 7, raw data from photodetector  350  can be represented by plot  620 . Raw data from photodetector  355  are represented by plot  630 . Data  620  have some noise due to the unstable temperature in chamber  210 . Data  630  contain information about the temperature fluctuation, but data  630  also are correlated to the noise in data  620  because filter  240  is thermally sensitive. Exploiting this correlation, the temperature noise in the processed signal  650  is reduced by signal processing  640  to remove these effects, resulting in processed data  650  useful in monitoring the fabrication of thin films. Processing  640  can be subtracting plot  630  from plot  620 , or include more complicated mathematical operations.  
         [0049]    The dual beam architecture and the signal processing techniques can also be used in transmission mode in-situ monitoring systems.  
         [0050]    The equipment and methods described herein are particularly well suited for fabricating the thermo-optically tunable thin film optical filters described in U.S. Ser. No. 10/174,503, filed Jun. 17, 2002, and U.S. Ser. No. 10/211,970, filed Aug. 2, 2002. When used to fabricate those filters, the reference used in the approach of FIG. 3 can be a thermo-optically tunable filter of the very same type that is being fabricated. Thus, its characteristics and response to temperature variations will be very similar to that of the filter being fabricated. In addition, because the filters have transmission/reflection characteristics that very substantially with temperature, the monitoring wavelength should be selected to be located relative to where the passband is located at the higher fabrication temperatures. For example, if substrate is at 200 C, then that means the passband will have shifted by a substantial amount from where it is located at room temperature. Thus, the wavelengths of the measurement beam and the reference beam should be shifted accordingly so that they are located either at the edge of the passband or at its center, whichever is appropriate for the signal processing technique that is being used.  
         [0051]    It should also be understood that the monitoring approaches and various of signal processing approaches can be used in combination within the same system.