Patent Document

TECHNICAL FIELD OF THE INVENTION 
     One or more embodiments of the present invention pertain to method and apparatus for process endpoint detection in processing chambers including, without limitation, semiconductor processing chambers. In particular, one or more embodiments of the present invention pertain to method and apparatus for process endpoint detection of cleaning processes in semiconductor processing chambers. 
     BACKGROUND OF THE INVENTION 
     As is well known, processing chambers (for example, and without limitation, processing chambers used to deposit semiconductor films and processing chambers used to etch semiconductor films) need to be cleaned periodically to remove residue formed whenever wafers or substrates are processed therein (for example, such processing chambers may be cleaned after one or more wafers are processed). To clean the processing chambers, a cleaning process is run for a period of time (“clean time”) that is dictated typically by a requirement that substantially all residue built up in the processing chambers be removed. Such cleaning processes typically include plasma processes. 
     Detecting an endpoint for a plasma cleaning process may be performed by monitoring radiation output from a plasma formed within the processing chamber. The endpoint is identified by detecting the presence, or absence, of particular chemical compositions within the processing chamber, as evidenced by an analysis of the monitored radiation. However, such plasma cleaning processes have been found to be disadvantageous in certain environments due to physical bombardment of interior components of the processing chamber by constituents of the plasma, such physical bombardment causing deterioration of these interior components. 
     A high density plasma, chemical vapor deposition (“HDP CVD”) processing chamber (such as one manufactured by Applied Materials, Inc. of Santa Clara, Calif., “Applied”) can be used in a wide range of applications, for example, and without limitation, to deposit a fluorine-doped silicon glass (“FSG”) film, to deposit an undoped silicon glass (“USG”) film, to deposit a phosphorus-doped silicon glass (“PSG”) film, to deposit a film used for shallow trench isolation (“STI”), and so forth. A periodic cleaning process is carried out after one or more deposition processes used in these applications. To avoid the above-described physical bombardment of interior components of the processing chamber, a typical cleaning process for the Applied HDP CVD processing chamber is a “dark” cleaning process, i.e., a chemical process wherein a plasma is formed remotely, i.e., external to the processing chamber, and wherein the remotely generated plasma is admitted to the processing chamber to perform the cleaning process. 
       FIG. 1  shows a pictorial representation of an Applied HDP CVD chamber. As shown in  FIG. 1 , HDP CVD chamber  100  includes heating-cooling plate  110 , coil assembly  120 , interior chamber walls  130 , wafer support  140 , throttle &amp; gate valve assembly  150 , turbo pump  160 , foreline  165 , roughing valves  170  and  180 , turbo valve  190 , remote plasma applicator  200 , remote plasma injection tube  210 , and remote plasma injection manifold  220 . During a typical deposition process, deposition precursor gasses enter chamber  100  through nozzles pictorially shown as  230 ,  240 , and  250 , and gaseous deposition residues are exhausted from chamber  100  through throttle &amp; gate valve assembly  150  and turbo pump  160 . During such a typical deposition process, roughing valve  170  and turbo valve  190  are closed, and roughing valve  180  is open under the control of a controller (not shown). Further, during the deposition process, residues are formed on interior chamber walls  130 . During a typical cleaning process, a plasma is generated in remote plasma applicator  200 , the plasma flows through remote plasma injection tube  210 , and through remote plasma injection manifold  220  into chamber  100 . During such a typical cleaning process, roughing valve  170  and turbo valve  190  are open, and roughing valve  180  is closed under the control of the controller. The constituents of the remotely generated plasma interact with the residues to produce gaseous byproducts that are exhausted from chamber  100  through foreline  165  by a roughing pump (not shown). 
     As is well known, an optimum clean time for each application is a complex function of a number of variables including, without limitation: thickness of residue on interior surfaces of the processing chamber; temperature of interior components of the processing chamber at the inception of, and during, the cleaning process; deposition/sputter ratios used during a deposition process; and chemical composition of the residue. In accordance with prior art techniques, the above-described dark cleaning process is terminated (clean endpoint) after a predetermined time, i.e., the clean endpoint is determined in accordance with a “by-time” algorithm. However, such by-time algorithms are unreliable because, among other reasons, chamber cool down causes temperature variation that produces deposition process variation. Some prior art solutions for determining a clean endpoint for a dark cleaning process entail utilizing “burn boxes” to strike a plasma in the gaseous deposition byproducts. However, such solutions are problematic because they typically require the use of high voltages, are unreliable, and produce electrical noise problems. 
     In light of the above, there is a need in the art for method and apparatus for determining a clean endpoint for a dark cleaning process. 
     SUMMARY OF THE INVENTION 
     One or more embodiments of the present invention advantageously satisfy the above-identified need in the art, and provide a method and apparatus for determining an endpoint of a cleaning process running in a chamber. In particular, one embodiment of the present invention is a method that comprises steps of: (a) directing radiation absorbed by a byproduct of the cleaning process into an exhaust line of the chamber; (b) detecting a measure of absorbance of the radiation by the byproduct; and (c) determining the endpoint when the measure of absorbance falls within a predetermined window. 
    
    
     
       BRIEF DESCRIPTION OF THE FIGURE 
         FIG. 1  shows a pictorial representation of an Applied Materials high density plasma, chemical vapor deposition (“HDP CVD”) process chamber; 
         FIG. 2  shows a pictorial representation of the HDP CVD process chamber shown in  FIG. 1  which further comprises an endpoint detector that is fabricated in accordance with one embodiment of the present invention; 
         FIG. 3  shows a block diagram of EDP  300  that is fabricated in accordance with one embodiment of the present invention; 
         FIG. 4  shows a pictorial representation of the HDP CVD process chamber shown in  FIG. 2  wherein an interior surface of the chamber and an interior surface of a cleaning process exhaust port are indicated; 
         FIG. 5  shows a graphical representation of how a clean endpoint is determined in accordance with one embodiment of the present invention; and 
         FIG. 6  is a graph that shows particle adders as a function of undercleaning or overcleaning the HDP CVD chamber shown in FIG.  1 . 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 6  is a graph that shows particle adders as a function of undercleaning or overcleaning high density plasma, chemical vapor deposition (“HDP CVD”) processing chamber  100  shown in  FIG. 1  (The term particle adders as used herein refers to particles that are added to a wafer as a result of performing a processing step on the wafer. For example, one might measure the number of particles on the wafer prior to undergoing the processing step, and measure the number of particles on the wafer after undergoing the processing step. The difference in the two numbers is a number of particle adders.). As was described in the “Background of the Invention,” a typical cleaning process for an HDP CVD processing chamber manufactured by Applied Materials, Inc. of Santa Clara, Calif. (“Applied”) is a “dark” cleaning process, i.e., a chemical process wherein a plasma is formed remotely, i.e., external to chamber  100 , and wherein the remotely generated plasma is admitted to chamber  100  to perform the cleaning process. As shown in  FIG. 6 , the inventors have discovered that poor “particle” performance, i.e., a large number of particle adders, occurs if the cleaning process (i.e., a process to remove residue deposited on interior chamber walls  130  of chamber  100  during a deposition process) ends too soon (an “underclean”), i.e., before 100% of the residue has been removed. The inventors believe that a source or cause of the poor particle performance for an underclean following a deposition process that deposits a silicon glass film to be SiO 2  particles. The inventors have also discovered that poor particle performance occurs if the cleaning process ends too late (an “overclean”), i.e., some time after 100% of the residue has been removed. The inventors believe a source or cause of the poor particle performance for an overclean to be aluminum fluoride particles produced by interaction of the cleaning plasma with the surface of interior chamber walls  130 . As a result, and as can be seen in  FIG. 6 , for a particular cleaning process, there is an optimum cleaning window, window  510 , during which particles generated as a result of an underclean or an overclean are avoided. 
       FIG. 2  shows a pictorial representation of HDP CVD process chamber  100  shown in  FIG. 1  which further comprises endpoint detector  300  that is fabricated in accordance with one embodiment of the present invention. One such embodiment is directed to detect an endpoint for a processing chamber cleaning process wherein SiF 4  gas is exhausted from chamber  100  as a byproduct of the cleaning process. Such an embodiment is useful, for example, with a wide range of applications such as, for example, and without limitation, to deposit a fluorine-doped silicon glass (“FSG”) film, to deposit a undoped silicon glass (“USG”) film, to deposit a phosphorus-doped silicon glass (“PSG”) film, to deposit a film used for shallow trench isolation (“STI”), to deposit a silicon nitride (“SiN”) film, and so forth. 
     In accordance with this embodiment, infrared radiation (“IR”) spectroscopy is used to track, and to detect a clean endpoint at a predetermined level of the SiF 4  gas. As such, this embodiment may be used to determine a clean endpoint for a cleaning process wherein substantially no light is generated for use in a standard optical spectroscopy endpoint technique. For example, this embodiment may be used to determine a clean endpoint for a cleaning process wherein a remote plasma generator generates a plasma that is injected into chamber  100 . It is well known to those of ordinary skill in the art how to fabricate such a remote plasma generator. For example, and without limitation, a remote plasma generator may comprise a microwave generator that emits microwaves into a cavity through which a gas passes. 
     As shown in  FIG. 2 , endpoint detector  300  (“EDP  300 ”) is disposed about foreline  165  of chamber  100 .  FIG. 3  shows a block diagram of EDP  300  that is fabricated in accordance with one embodiment of the present invention. As shown in  FIG. 3 , IR source  400  outputs infrared radiation having wavelengths substantially overlapping at least a portion of an absorption band of SiF 4  gas (for example, an absorption band of vibrational modes of the SiF 4  gas). In accordance with one embodiment, the wavelengths are substantially equal to about 1020 nm. The IR radiation output from IR source  400  is injected through a window (not shown) into a predetermined volume in foreline  165  of a chamber exhaust system of chamber  100  shown in FIG.  2 . As further shown in  FIG. 3 , EDP  300  comprises filters  420  and  425  disposed on chopper wheel  410 . Chopper wheel  410  is disposed on an opposite side of foreline  165  from IR source  400  to receive radiation transmitted through another window (not shown) in foreline  165 . Filter  420  is constructed to transmit radiation in a band of wavelengths substantially at wavelengths output by SiF 4  molecules that have absorbed the infrared radiation output from IR source  400  (for example, filter  420  may transmit radiation in a band of wavelengths centered at about 972 nm), and filter  425  is a neutral density filter that transmits radiation in a band of wavelengths close to the band of wavelength transmitted by filter  420  (for example, filter  425  may transmit radiation in a band of wavelengths centered at about 909 nm). As still further shown in  FIG. 3 , chopper wheel  410  (and hence filters  420  and  425 ) is rotated by motor  430  in response to signals from controller  440 . 
     As further shown in  FIG. 3 , radiation transmitted by filters  420  and  425  is processed by Fourier Transform Raman (“FTR”) spectrometer  460  to provide output signals that are applied as input to controller  440  for analysis. For example, an output signal received by controller  440  corresponding to radiation passing through filter  425  is used to determine a background that is subtracted from an output signal received by controller  440  corresponding to radiation passing through filter  420 . In accordance with one embodiment of the present invention, a signal output from FTR spectrometer  460  (a “1×signal”) is multiplied (for example, by amplifying the 1× signal in accordance with any one of a number of methods that are well known to those of ordinary skill in the art) by a factor of 20 to provide a second signal (a “20× signal”), and the 20× signal is applied as input to controller  440  for analysis. Then, the background-corrected 20× signal is monitored, and an inventive algorithm is used to determine a clean endpoint. 
     As will be described in detail below, the inventive algorithm is based on removal of SiF 4  from chamber  100 , and a correlation of the background-corrected 20× signal with particle performance. IR source  400 , chopper wheel  410 , filter  420 , filter  425 , FTR spectrometer  460  are fabricated in accordance with any one of a number of methods that are well known to those of ordinary skill in the art, and may be obtained commercially, for example, from MKS Instruments, Inc. of East Hartford, Conn. Controller  440  may be a personal computer, or it may be a controller computer that runs chamber  100 . Those of ordinary skill in the art should understand that a 20× signal is used to provide a convenient signal level (for example, a 20× signal provides an appropriate voltage resolution), however, embodiments of the present invention are not limited to use of a 20× signal. In fact, other embodiments exist where a 1× signal is monitored, or any other signal level is monitored. In addition, although an embodiment has been described wherein a chopper wheel was utilized to perform background correction, further embodiments of the present invention are not limited thereby. In fact other embodiments exist wherein background correction may be performed using beamsplitters in a manner which is well known to those of ordinary skill in the art. 
     As those of ordinary skill in the art will readily appreciate, the 1× signal, and hence the 20× signal, provides a measure of SiF 4  gas IR absorbance in foreline  165  during the cleaning process. In accordance with one embodiment of the present invention, an inventive algorithm detects a clean endpoint whenever the 20× output signal reaches a predetermined voltage level. In accordance with this embodiment, the predetermined voltage level corresponds to a predetermined percentage of SiF 4  removal from chamber  100  and an exhaust port of foreline  165 .  FIG. 4  shows a pictorial representation of HDP CVD process chamber  100  wherein an interior surface of chamber  100  (Chamber Surface Area “B”) and an interior surface of the exhaust port (Exhaust Port Surface Area “A”) are indicated. In accordance with the inventive algorithm, the percentage SiF 4  removal is determined by controller  440  as follows: 
           (     Chamber   ⁢           ⁢   Surface   ⁢           ⁢   Area   ⁢           ⁢     “   B   ”       )     *   100               (     Chamber   ⁢           ⁢   Surface   ⁢           ⁢   Area   ⁢           ⁢     “   B   ”       )     +               (     Exhaust   ⁢           ⁢   Port   ⁢           ⁢   Surface   ⁢           ⁢   Area   ⁢           ⁢     “   A   ”       )               
 
     In accordance with one embodiment of the present invention, chamber  100  is deemed to be 100% clean, and a clean endpoint is found, whenever 97.5% removal of SiF 4  that is generated by the cleaning process from the interior surface of chamber  100  and the interior surface of exhaust port is achieved. Advantageously, in accordance with this embodiment, EPD  300  adjusts the clean time dynamically to provide a consistent level of chamber clean by tracking SiF 4  removal during the cleaning process. Thus, if Exhaust Port Surface Area A changes, the determination of the percentage SiF 4  removed to achieve a 100% chamber clean will also change. 
       FIG. 5  shows a graphical representation of how a clean endpoint is determined in accordance with one embodiment of the present invention As shown in  FIG. 5 , an amplitude of the 20× signal is plotted on the ordinate as a function of volts and on the abscissa as a function of time (it will be appreciated that a flat-top of the 20× signal is due to saturation for the voltage scale used). As shown in  FIG. 5 , a voltage amplitude corresponding to 95% SiF 4  removal results in an undercleaned chamber; a voltage amplitude corresponding to 97.5% SiF 4  removal results in a 100% clean chamber; and a voltage amplitude corresponding to 100% SiF 4  removal results in an overcleaned chamber. Thus, as shown in  FIG. 5 , window  500  corresponds to an optimal clean window that minimizes particle adders resulting from an underclean or an overclean. Advantageously, in accordance with this embodiment of the present invention, the optimal clean endpoint occurs before the time for 100% SiF 4  removal from the chamber and the exhaust port. Further, EDP  300  adjusts the clean time dynamically—under varying process and hardware conditions—by measuring IR absorbance of the SiF 4  gas clean byproduct, and by triggering the clean endpoint at the same level each time, independent of chamber conditions, to provide a consistent level of chamber clean. Further, since the optimal clean times are shorter than the times corresponding to 100% SiF 4  removal, use of an embodiment of the present invention may provide reduced cleaning gas usage, and higher throughput due to the shorter cleans. Note that, after using this embodiment of the present invention, although there may be some deposits in foreline  165 , the chamber is 100% clean. Further note that it is not important completely to clean foreline  165  since foreline  165  is not a source of particles in chamber  100 . 
     In further embodiments of the present invention, the inventive algorithm may be extended to provide an overclean as a percentage of the endpoint time. 
     As described above, EPD  300  adjusts clean time dynamically within an optimum clean window that is determined as described below, in accordance with one embodiment, by monitoring SiO 2  and aluminum fluoride particles to provide a consistent, low-particle, high yield chamber clean. The following describes one embodiment of a method to determine voltage levels that correspond to window  500  shown in  FIG. 5  for a predetermined processing step. Step 1: choose a voltage level on a 20× signal to determine an endpoint for a cleaning process (for example, initially, the voltage level should be low enough to correspond to an overclean). Step 2: monitor the cleaning process using EPD  300  and the chosen voltage level to determine the endpoint. Step 3: process a predetermined number of wafers using the predetermined processing step; determine the number of particle adders for each of the wafers in accordance with any one of a number of methods that are well known to those of ordinary skill in the art; and determine an average number of particle adders. Step 4: increase the voltage level used to determine the endpoint for the cleaning process in stages, and repeat step 3 at each stage. The voltage level should be increased eventually to a high enough level to correspond to an underclean. As described above, the average number of particle adders determined at each stage should decrease to a minimum, and then increase as one goes from an overclean condition to an underclean condition. Finally, the voltage levels that correspond to window  500  substantially surround (to within a predetermined amount) the minimum in the average number of particle adders. As those of ordinary skill in the art can readily appreciate, instead of starting at an overclean and moving to an underclean, the above-described process can take place by starting at an underclean and moving to an overclean. 
     Those skilled in the art will recognize that the foregoing description has been presented for the sake of illustration and description only. As such, it is not intended to be exhaustive or to limit the invention to the precise form disclosed. For example, embodiments may be fabricated which utilize any method for determining the absorbance of one or more gaseous byproducts of a cleaning process. In addition, embodiments may be also used with any cleaning process (for example, and without limitation, plasma cleaning processes (dark or not dark), and non-plasma cleaning processes), and with any kind of processing chamber (for example, and without limitation, deposition chambers and etching chambers).

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