Abstract:
Methods for monitoring and detecting optical emissions while performing photoresist stripping and removal of residues from a substrate or a film stack on a substrate are provided herein. In one embodiment, a method is provided that includes positioning a substrate comprising a photoresist layer into a processing chamber; processing the photoresist layer using a multiple step plasma process; and monitoring the plasma for a hydrogen optical emission during the multiple step plasma process; wherein the multiple step plasma process includes removing a bulk of the photoresist layer using a bulk removal step; and switching to an overetch step in response to the monitored hydrogen optical emission.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a continuation of co-pending U.S. patent application Ser. No. 10/776,672, filed Feb. 11, 2004, which claims benefit of U.S. provisional patent application Ser. No. 60/447,625, filed Feb. 15, 2003. Each of the aforementioned related patent applications is herein incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates generally to a method for semiconductor substrate processing. More specifically, the invention relates to a method for monitoring and detecting optical emission endpoint(s), for photoresist stripping and removal of residues from a substrate or a film stack on a substrate.  
         [0004]     2. Description of the Related Art  
         [0005]     As a part of semiconductor manufacturing, various layers of dielectric, semiconducting, and conducting films, such as silicon dioxide, polysilicon, and metal compounds and alloys, are deposited on a silicon substrate. Features are defined in these layers by a process including lithography and etching. Such a process comprises coating a substrate with photoresist, patterning the photoresist, and then transferring this pattern to the underlying layers during etching by using the patterned photoresist as an etch mask. Many of these etch processes leave photoresist and post-etch residues on the substrate and must be removed before performing the next process step.  
         [0006]     Patterned photoresist also serves as an ion implant mask for preferentially doping semiconductor substrates in selected areas. The doping or implantation process includes exposing the substrate to ions or an electronic beam of implant species, for example, arsenic (As), boron (B, BF 2 , BF 4 ), phosphorous (P), indium (In), antimony (Sb) and hydrogen (H). The ion implantation process dehydrogenates the photoresist material, resulting in a hydrogen deficient, carbonized crust layer that is typically one to several thousand angstroms thick on top of the bulk photoresist. This makes the characteristics of the photoresist material vertically non-uniform such that uniform removal (stripping) of the photoresist can be difficult. As such, the photoresist removal process may result in non-uniform removal and substantial post-implant residue remaining on the substrate. Consequently, a technique for monitoring removal of the photoresist is necessary such that the photoresist removal process can be controlled as the characteristics of the material change.  
         [0007]     Optical emission spectroscopy is commonly used to detect the endpoint of plasma etch processes. Plasma transitions of reactant or by-product species emit photons which can be detected in the ultraviolet, visible and near-infrared ranges. Thus, the endpoint is usually based on increasing signal for reactants or decreasing signal for by-products. The endpoint is identified when either the reactants or by-products attain a specific concentration (i.e., the respective signals cross a threshold level). However, such an endpoint detection technique does not account for the variations in the characteristics of a photoresist layer that has been exposed to an ion beam.  
         [0008]     Therefore, there is a need in the art for a method and apparatus for performing optical emission endpoint detection for photoresist strip and residue removal especially when using a chamber having a remote plasma source.  
       SUMMARY OF THE INVENTION  
       [0009]     The invention relates to a method for monitoring and detecting optical emission endpoint(s), more particularly hydrogen emissions within a plasma, for photoresist stripping and removal of residues from a substrate or a film stack on a substrate. In one embodiment, a method is provided that includes positioning a substrate comprising a photoresist layer into a processing chamber; processing the photoresist layer using a multiple step plasma process; and monitoring the plasma for a hydrogen optical emission during the multiple step plasma process; wherein the multiple step plasma process includes removing a bulk of the photoresist layer using a bulk removal step; and switching to an overetch step in response to the monitored hydrogen optical emission.  
         [0010]     In another embodiment, a method is provided that includes positioning a substrate comprising a photoresist layer into a processing chamber; processing the photoresist layer using a multiple step plasma process; and monitoring the plasma for both by-product optical emission and a reactant optical emission during the multiple step plasma process; wherein the multiple step plasma process includes removing a bulk of the photoresist layer using a bulk removal step; and switching to an overetch step in response to the monitored hydrogen optical emission. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0011]     So that the manner in which the above recited features of the present invention are attained and can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to the embodiments thereof which are illustrated in the appended drawings.  
         [0012]     It is to be noted, however, that the appended drawings illustrate only typical embodiments of this invention and are therefore not to be considered limiting of its scope, for the invention may admit to other equally effective embodiments.  
         [0013]      FIGS. 1A and 1B  is an illustrative graph of a hydrogen emission peak for a blanket photoresist and an arsenic implanted photoresist;  
         [0014]      FIG. 2  is an illustrative graph of a hydrogen emission peak for three arsenic implanted substrates during a substrate test showing repeatability of an hydrogen emission peak;  
         [0015]      FIG. 3  is a flow diagram of one embodiment of a method of the present invention;  
         [0016]     FIGS.  4 A-B are illustrative graphs of hydrogen and oxygen emission traces for stripping of unimplanted photoresist ( FIG. 4A ), arsenic implanted photoresist ( FIG. 4B ), phosphorous implanted photoresist ( FIG. 4C ) and boron implanted photoresist ( FIG. 4D ); and  
         [0017]      FIG. 5  is a schematic diagram of one embodiment of an illustrative chamber used to perform the method of the present invention. 
     
    
     DETAILED DESCRIPTION  
       [0018]     The invention relates to a method for monitoring and detecting optical emission endpoint(s), more particularly hydrogen emissions, for photoresist stripping and removal of residues from a substrate or a film stack on a substrate. In one embodiment of the invention, a method determines and uses a hydrogen optical emission peak for identifying an endpoint of a photoresist stripping process, including blanket and patterned photoresist, post-implant photoresist, and post-plasma etch photoresist. In addition, the invention comprises a method to use optical emission endpoint in general, and hydrogen peak specifically, to monitor the transition from crust removal to bulk photoresist removal for post-implant stripping. By this method, the hydrogen endpoint trace is a more direct measure of stripping for patterned implant substrates (compared to other peaks such as oxygen).  
         [0019]     The present invention uses, in one embodiment, the hydrogen optical emission peak at 656 nm to monitor endpoint for ion implant strip, and can be applied to other reducing chemistry based stripping processes for strip and residue removal after the etching of low dielectric constant films (low k films), and for other applications.  
         [0020]     For the crust removal process in post-implant strip, the hydrogen signal can be especially useful because the crust layer is hydrogen-depleted relative to the bulk photoresist. Thus, in accord with an embodiment of the present invention, monitoring for the rise and leveling-off of the hydrogen peak (656 nm) indicates that the hydrogen-depleted crust layer is removed and that the hydrogen-rich bulk photoresist has been reached. The ability to accurately identify the crust removal clearing time is of use for identifying changes in substrate conditions or in situations where a multi-step stripping recipe is beneficial.  
         [0021]     FIGS.  1 A-B depict the hydrogen emission trace that occurs during removal of an unimplanted photoresist layer ( FIG. 1A ) and arsenic implanted photoresist layer ( FIG. 1B ). The graphs  100  and  102  depict emission intensity (axis  104 ) versus time (axis  106 ). During the stripping of photoresist from the unimplanted substrate, the hydrogen emission trace  108  increases (portion  110 ) then levels off (portion  112 ), and then decreases (portion  114 ), allowing endpoint detection as the photoresist clears. For the implanted substrate, the clearing of the crust layer can be easily identified in trace  116 . The crust layer is hydrogen deficient as described above, such that the hydrogen emission is low at the beginning of the stripping process (portion  118 ). As the crust is removed, the hydrogen emission increases (portion  120 ) until a plateau is reached ( 122 ). Finally, the bulk photoresist is removed and the hydrogen emission decreased (portion  124 ). The repeatability of the hydrogen emission peak during a  100  substrate run with implanted blanket photoresist monitor substrates is evident in the emission graphs for substrates  2 ,  49 , and  99  shown in  FIG. 2 .  
         [0022]     One advantage of the present invention is that the hydrogen signal is created as a process by-product, rather than a process reactant like oxygen. Thus, the change in optical emission signal is a more direct measure of the photoresist removal process, as opposed to a process reactant which is more of an indirect measure of photoresist removal and may also include additional reactions not related to the photoresist removal process (such as reactions with residues on chamber walls or other locations other than the substrate). A by-product peak should be less sensitive to non-uniformity issues, which, for the bulk strip step of post-implant strip, could lead to overly-short process times. While other by-product signals may also be used to signal the end of the crust removal (e.g., the OH peak at ˜311 nm), the hydrogen signal is significantly stronger in intensity and more well-defined than any of these other peaks and therefore provides a clearer endpoint trace. In addition, when using of the hydrogen peak over the OH peak, it may be advantageous if water vapor is used in the recipe, where the water vapor may mask the OH peak.  
         [0023]     Furthermore, as a process by-product, the hydrogen emission can be monitored near the substrate surface in a remote plasma source reactor as described with respect to  FIG. 5  below.  
         [0024]      FIG. 3  is a flow diagram of a method  300  of the present invention. The method begins at step  302  and proceeds to step  304  where a substrate is positioned in a process chamber capable of performing photoresist stripping. One such chamber is manufactured under the trademark AXIOM™ by Applied Materials, Inc. and described with respect to  FIG. 5  below.  
         [0025]     At step  306 , the method performs a plasma process in the strip chamber. To remove photoresist, an oxygen-based plasma is used. For example, an oxidizing gas such as O 2 , is applied to a remote plasma source at a flow rate of 100 to 10,000 sccm. The oxidizing gas is formed into a plasma when 600 to 6000 watts of RF energy is applied to the source. The gas pressure in the chamber is maintained at 0.3 to 3 Torr. The temperature of the substrate is maintained at 15 to 300 degrees Celsius. In one embodiment of the invention, an RF bias of 100 to 2000 watts is applied to the substrate. Various oxidizing gases can be used including, but not limited to, O 2  O 3 , N 2 O, H 2 O, CO, CO 2 , alcohols, and various combinations of these gases. In other embodiments of the invention, nonoxidizing gases may be used including, but not limited to, N 2 , H 2 O, H 2 , forming gas, NH 3 , CH 4 , C 2 H 6 , various halogenated gases (CF 4 , NF 3 , C 2 F 6 , C 4 F 8 , CH 3 F, CH 2 F 2 , CHF 3 ), combinations of these gases and the like.  
         [0026]     At step  308 , the method  300  monitors the hydrogen emission within the plasma in the chamber. At step  310 , the method responds to the emission magnitude. In one embodiment, the chamber parameters, (e.g., gases, power levels, pressure, temperature and the like) may be altered upon detecting a change in the hydrogen emission. As such, the emission can be used to optimize processing or to cease processing when the photoresist is removed. Alternatively, one chemistry or recipe can be used for photoresist crust removal and a second chemistry or recipe can be used for bulk photoresist removal. Similarly, the bulk photoresist can be removed until another emission change occurs, then a third chemistry or recipe can be used to remove residue that remains from the stripping process. The method  300  ends at step  312 .  
         [0027]     In another embodiment of the present invention, a method uses a combination of a hydrogen optical emission with one (or more) additional emission peak(s) for more robust and/or flexible endpoint control. As such, step  308  can be used to monitor other emissions (shown in phantom).  
         [0028]     The use of the by-product hydrogen signal in combination with other optical emission peaks can provide several advantages. For example, the reactant oxygen signal provides multiple indicators of stripping though transition layers between the crust and bulk photoresist. Also, the method of the present invention permits identification of an early endpoint indicator by monitoring the reactant oxygen peak and a late/final indicator by monitoring the by-product hydrogen peak. FIGS.  4 A-B depicts graphs hydrogen and oxygen optical emission traces during the stripping of blanket unimplanted (graph  400 ), arsenic implanted photoresist (graph  420 ), as well as phosphorous (graph  440 ) and boron (graph  460 ) implanted photoresist. Each graph depicts emission magnitude (axis  404 ) versus time (axis  406 ). In graph  400 , the hydrogen emission is trace  408  and the oxygen emission is trace  410  and, in graph  420 , the hydrogen emission is trace  418  and the oxygen emission is trace  416 . In graph  440 , the hydrogen emission is trace  436  and the oxygen emission is trace  438  and, in graph  460 , the hydrogen emission is trace  456  and the oxygen emission is trace  458 . These data show that the implant species and conditions vary the specific intensity versus time values, but that the general shape of the emissions traces is the same, allowing for use of the method described herein. In this example of an embodiment of the present invention, the hydrogen and oxygen signals mirror each other since the hydrogen is a by-product peak and oxygen is a reactant peak. By measuring and monitoring both wavelengths, the method can incorporate custom endpoint algorithms to minimize risk of mis-processing and maximize throughput by optimizing process duration. In addition, utilization of the present invention can drastically reduce errors by providing a back-up wavelength. In other words, using both signals, simultaneously allows for more robust endpoint capability by providing a backup detection wavelength—if the endpoint is missed at one wavelength, the endpoint can be triggered by the other wavelength. Dual wavelength endpoint triggering occurs when either wavelength meets the endpoint conditions.  
         [0029]     The dual wavelength optical emission can provide advantages for other processes, such as post-silicon etch photoresist strip and residue removal, where the process is switched at step  310  of  FIG. 3  from resist stripping chemistry to residue removal and/or softening chemistry as the photoresist removal is detected. The combination of the reactant oxygen and by-product hydrogen signals is most useful for controlling the plasma-on time for photoresist removal. Because residues are sometimes more difficult to remove when exposed to excessive oxygen radicals, inaccurate endpoint control can result in overly-long plasma-on times to ensure complete photoresist removal, which in turn reduces the efficacy of residue removal post-treatments. Accurate endpoint control limits the oxidizing plasma exposure, thereby increasing the effectiveness of residue-removal post-treatments.  
         [0030]     The present inventive method may be used on a variety of systems as the hardware requirements for the implementation of this invention are not unique.  FIG. 5  depicts a schematic diagram of the AXIOM™ reactor (or chamber)  500  that may be used to practice portions of the method  300 . The AXIOM reactor  500  is described in detail in U.S. patent application Ser. No. 10/264,664, filed Oct. 4, 2002 and incorporated herein by reference. The reactor  500  comprises a process chamber  502 , a remote plasma source  506 , and a controller  508 .  
         [0031]     The process chamber  502  generally is a vacuum vessel, which comprises a first portion  510  and a second portion  512 . In one embodiment, the first portion  510  comprises a substrate pedestal  504 , a sidewall  516  and a vacuum pump  514 . The second portion  512  comprises a lid  518  and a gas distribution plate (showerhead)  520 , which defines a gas mixing volume  522  and a reaction volume  524 . The lid  518  and sidewall  516  are generally formed from a metal (e.g., aluminum (Al), stainless steel, and the like) and electrically coupled to a ground reference  560 . The sidewall comprises a window  594  (quartz) that is used to monitor the optical emissions within the plasma. The window  594  is coupled to a light-collecting device  592  that carries the optical signals to the optical emission spectroscopy (OES) system  590 .  
         [0032]     The substrate pedestal  504  supports a substrate (wafer)  526  within the reaction volume  524 . In one embodiment, the substrate pedestal  504  may comprise a source of radiant heat, such as gas-filled lamps  528 , as well as an embedded resistive heater  530  and a conduit  532 . The conduit  532  provides cooling water from a source  534  to the backside of the substrate pedestal  504 . The substrate sits on the pedestal by gravity or, alternatively, can be mechanically clamped, vacuum clamped, or electrostatically clamped as in an electrostatic chuck. Gas conduction transfers heat from the pedestal  504  to the substrate  526 . The temperature of the substrate  526  may be controlled between about 20 and 400 degrees Celsius.  
         [0033]     The vacuum pump  514  is adapted to an exhaust port  536  formed in the sidewall  516  of the process chamber  502 . The vacuum pump  514  is used to maintain a desired gas pressure in the process chamber  502 , as well as evacuate the post-processing gases and other volatile compounds from the chamber. In one embodiment, the vacuum pump  514  is augmented with a throttle valve  538  to control the gas pressure in the process chamber  502 .  
         [0034]     The process chamber  502  also comprises conventional systems for retaining and releasing the substrate  526 , internal diagnostics, and the like. Such systems are collectively depicted in  FIG. 5  as support systems  540 .  
         [0035]     The remote plasma source  506  comprises a power source  546 , a gas panel  544 , and a remote plasma chamber  542 . In one embodiment, the power source  546  comprises a radio-frequency (RF) generator  548 , a tuning assembly  550 , and an applicator  552 . The RF generator  548  is capable of producing of about 200 to 5000 W at a frequency of about 200 to 600 kHz. The applicator  552  is inductively coupled to the remote plasma chamber  542  and energizes a process gas (or gas mixture)  564  to a plasma  562  in the chamber. In this embodiment, the remote plasma chamber  542  has a toroidal geometry that confines the plasma and facilitates efficient generation of radical species, as well as lowers the electron temperature of the plasma. In other embodiments, the remote plasma source  506  may be a microwave plasma source, however, the stripping rates are generally higher using the inductively coupled plasma.  
         [0036]     The gas panel  544  uses a conduit  566  to deliver the process gas  564  to the remote plasma chamber  542 . The gas panel  544  (or conduit  566 ) comprises means (not shown), such as mass flow controllers and shut-off valves, to control gas pressure and flow rate for each individual gas supplied to the chamber  542 . In the plasma  562 , the process gas  564  is ionized and dissociated to form reactive species.  
         [0037]     The reactive species are directed into the mixing volume  522  through an inlet port  568  in the lid  518 . To minimize charge-up plasma damage to devices on the substrate  526 , the ionic species of the process gas  564  are substantially neutralized within the mixing volume  522  before the gas reaches the reaction volume  524  through a plurality of openings  570  in the showerhead  520 .  
         [0038]     The controller  508  comprises a central processing unit (CPU)  554 , a memory  556 , and a support circuit  558 . The CPU  554  may be any form of a general-purpose computer processor used in an industrial setting. Software routines can be stored in the memory  556 , such as random access memory, read only memory, floppy or hard disk, or other form of digital storage. The support circuit  558  is conventionally coupled to the CPU  554  and may comprise cache, clock circuits, input/output sub-systems, power supplies, and the like.  
         [0039]     The software routines, when executed by the CPU  554 , transform the CPU into a specific purpose computer (controller)  508  that controls the reactor  500  such that the processes (e.g., method  300  of  FIG. 3 ) are performed in accordance with the present invention. The software routines may also be stored and/or executed by a second controller (not shown) that is located remotely from the reactor  500 .  
         [0040]     The AXIOM™ chamber has a window port  594  for attaching a light-collecting device  592  (e.g., a fiber optic probe and cable) to monitor plasma intensity. The window is located slightly above the substrate plane for collecting emission intensity along a line parallel to the substrate. Optical emission spectroscopy hardware  590  based on either a monochromator that can be set to monitor the emission (above the substrate) of a particular wavelength within the entire spectrum or hardware based on bandwidth filter(s), or even a spectrometer, can be used. An exemplary embodiment of the present invention may use a detector unit with two bandpass filters on the chamber. In such an embodiment, one of the filters includes the 656 nm emission, or hydrogen optical emission peak, wavelength.  
         [0041]     In addition to process control and process recipe endpointing, the use of hydrogen, optical emission or hydrogen combined with a second wavelength such as that of oxygen can also be used to monitor chamber health. In such an embodiment of the present invention, a detector unit may be utilized with one or more bandpass filters coupled to the chamber. The oxygen emission peak(s) of 777 nm and/or 845 nm can also be utilized, either singly or jointly in combination with the hydrogen emission peak. The relative intensities of these peaks so measured and monitored could be indicative of the conditions of the plasma sources and chamber surfaces and be used to provide a proper “fingerprint” of a clean or “golden” chamber. The magnitude of the emissions can be used to determine when a cleaning cycle is necessary or whether components within the chamber are degrading, i.e., certain emissions are indicative of chamber health.  
         [0042]     While foregoing is directed to embodiments of the present invention, other and further embodiments of the invention may be devised without departing from the basic scope thereof.