Abstract:
A dry etching method and apparatus are described. A workpiece supports silicon nitride and silicon dioxide. The workpiece is exposed to a plasma containing at least one of sulfur hexafluoride and nitrogen trifluoride and ammonia to selectively remove the silicon nitride in relation to the silicon dioxide. In one feature, the plasma contains sulfur hexafluoride and ammonia. In another feature, the plasma contains nitrogen trifluoride and ammonia.

Description:
BACKGROUND 
       [0001]    The present invention is related generally to the field of selective etching using a plasma and, more particularly, to selectively etching silicon nitride in the presence of silicon dioxide and an associated apparatus. 
         [0002]    The formation, for example, of modern integrated circuits can require many process steps. In the manufacture of some state-of-the-art integrated circuits, there is a need to selectively remove silicon nitride in the presence of silicon dioxide. In some cases, a layer of silicon dioxide may support an overlying layer of silicon nitride where it is desired to remove the silicon nitride in selected regions, whereby to expose the underlying silicon dioxide without causing significant damage to the silicon dioxide. One example of a situation in which this need arises resides in silicon nitride gate spacer etching where, at one point in the process, a silicon nitride layer surrounds a gate electrode that is itself supported on a gate silicon dioxide layer. The objective is to remove the silicon nitride from the gate silicon dioxide layer which surrounds the gate electrode, without significantly damaging the gate silicon dioxide layer. 
         [0003]    Another example of this situation is seen in the formation of a floating gate electrode in an ONO (Oxide Nitride Oxide) film stack used in flash memory. Typically, an EEPROM device includes a floating-gate electrode upon which electrical charge is stored. In a flash EEPROM device, electrons are transferred to a floating-gate electrode through a dielectric layer overlying the channel region of the transistor. The ONO structure is in wide use in state-of-the-art non-volatile memory devices. At one point during formation of the floating gate structure, a substrate supports a silicon dioxide, silicon nitride, silicon dioxide (i.e., ONO) layer structure. A gate electrode is supported on this ONO layer structure. In particular, the gate electrode is located directly on an outer layer of silicon dioxide. Initially, the outer layer of silicon dioxide, surrounding the gate electrode, is removed. This exposes the inner, silicon nitride layer which is itself supported on a bottom layer of silicon dioxide that is supported directly on the substrate. At this point, the silicon nitride layer, surrounding the gate electrode, must be removed to expose the underlying, bottom layer of silicon dioxide, but without adversely affecting the bottom layer of silicon dioxide. 
         [0004]    Having set forth several examples of processing scenarios in which it is necessary to selectively remove silicon nitride in the presence of silicon dioxide, the state-of-the-art will now be considered, as it addresses this need. Turning to  FIG. 1 , one recent approach that has been used for the purpose of selectively removing silicon nitride, relative to silicon dioxide uses a plasma that is formed from sulfur hexafluoride (SF 6 ) and Hydrogen (H 2 ). This prior art process is illustrated by way of a plot  1  of silicon nitride to silicon dioxide selectivity versus hydrogen gas flow. Process conditions include a pressure of 20 millitorr, 1000 watts of RF power applied to the plasma source, no power applied to the wafer pedestal, a 30 sccm flow of SF 6 , a 170 sccm flow of Argon, a process temperature of 25 degrees Centigrade and a process time of 30 seconds. While the combination of SF 6  and H 2  gas has demonstrated acceptable selectivity, as can be seen from the plot of  FIG. 1 , the use of hydrogen gas can be a significant concern at least with respect to its flammability. 
         [0005]    The foregoing examples of the related art and limitations related therewith are intended to be illustrative and not exclusive. Other limitations of the related art will become apparent to those of skill in the art upon a reading of the specification and a study of the drawings. 
       SUMMARY 
       [0006]    The following embodiments and aspects thereof are described and illustrated in conjunction with systems, tools and methods which are meant to be exemplary and illustrative, not limiting in scope. In various embodiments, one or more of the above-described limitations have been reduced or eliminated, while other embodiments are directed to other improvements. 
         [0007]    A dry etching method and associated apparatus are described. In one aspect of the present disclosure, a workpiece supports silicon nitride and silicon dioxide. The workpiece is exposed to a plasma containing (i) at least a selected one of sulfur hexafluoride and nitrogen trifluoride and (ii) ammonia to selectively remove the silicon nitride in relation to the silicon dioxide. In one feature, the plasma contains sulfur hexafluoride and ammonia. In another feature, the plasma contains nitrogen trifluoride and ammonia. 
         [0008]    In another aspect of the present disclosure, a dry etching system is configured for selective etching of silicon nitride in the presence of silicon dioxide. The system includes a chamber defining a chamber interior. A workpiece support arrangement supports a workpiece in the chamber interior. The workpiece supports silicon nitride and silicon dioxide. A plasma generator is configured for producing a plasma containing (i) at least a selected one of sulfur hexafluoride and nitrogen trifluoride and (ii) ammonia and for exposing the workpiece to the plasma to selectively remove the silicon nitride in relation to the silicon dioxide. In one feature, the plasma generator is configured to produce the plasma containing sulfur hexafluoride and ammonia. In another feature, the plasma generator is configured to produce the plasma containing nitrogen trifluoride and ammonia. 
         [0009]    In addition to the exemplary aspects and embodiments described above, further aspects and embodiments will become apparent by reference to the drawings and by study of the following descriptions. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0010]    Exemplary embodiments are illustrated in referenced figures of the drawings. It is intended that the embodiments and figures disclosed herein are to be illustrative rather than limiting. 
           [0011]      FIG. 1  is a plot of process results for a prior art process for use in selective removal of silicon nitride with respect to silicon dioxide. 
           [0012]      FIG. 2  is a diagrammatic view, in elevation, of a system that is configured for selective removal of silicon nitride in the presence of silicon dioxide. 
           [0013]      FIG. 3  illustrates silicon nitride to silicon dioxide selectivity versus flow of ammonia (NH 3 ) gas and includes a plot of the selectivity that is obtained with the use of the combination of sulfur hexafluoride and ammonia as well as a plot of the selectivity that is obtained with the use of the combination of nitrogen trifluoride and ammonia. 
           [0014]      FIG. 4  illustrates silicon nitride to silicon dioxide selectivity versus process pressure gas and includes two plots of the selectivity that is obtained with the use of the combination of sulfur hexafluoride, ammonia and argon as well as a plot of the selectivity that is obtained with the use of the combination of sulfur hexafluoride and argon. 
           [0015]      FIG. 5  illustrates silicon nitride to silicon dioxide selectivity versus process pressure gas and includes one plot of the selectivity that is obtained with the use of the combination of nitrogen trifluoride, ammonia and argon as well as another plot of the selectivity that is obtained with the use of the combination of nitrogen trifluoride and argon. 
       
    
    
     DETAILED DESCRIPTION 
       [0016]    The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the described embodiments will be readily apparent to those skilled in the art and the generic principles taught herein may be applied to other embodiments. Thus, the present invention is not intended to be limited to the embodiment shown but is to be accorded the widest scope consistent with the principles and features described herein including alternatives, modifications and equivalents, as defined within the scope of the appended claims. It is noted that the drawings are not to scale and are diagrammatic in nature in a way that is thought to best illustrate features of interest. Further, like reference numbers are applied to like components, whenever practical, throughout the present disclosure. Descriptive terminology such as, for example, upper/lower, right/left, front/rear and the like may be adopted for purposes of enhancing the reader&#39;s understanding, with respect to the various views provided in the figures, and is in no way intended as being limiting. 
         [0017]    Turning again to the figures, wherein like components may be designated with like reference numbers throughout the various figures, attention is immediately directed to  FIG. 2  is a diagrammatic view, in elevation, of a system that is configured according to the present disclosure, generally indicated by the reference number  10 , for selectively removing silicon nitride in the presence of silicon dioxide. The system includes a plasma source  12 , that is diagrammatically illustrated, for generating a plasma  14  (diagrammatically shown) that is suitable for use in an etching process. By way of example, the plasma source may use an inductively coupled configuration. One such suitable plasma source is described in U.S. Pat. No. 6,143,129 which is incorporated herein by reference. Accordingly, an induction coil  16  couples RF energy into the source vessel from a first RF power supply  18  through a matching network which is not shown. A gas inlet  20  is configured for introducing a combination of a fluorine containing gas  22 , as will be further described, and ammonia (NH 3 ) gas  24  into the plasma source. A processing chamber  26  is located below plasma source  12  and includes a pedestal  30  that supports a workpiece  32  such as, for example, a semiconductor wafer. The workpiece supports a silicon nitride region  34  which overlies a silicon dioxide region  36 , the dimensions of which are greatly exaggerated for illustrative purposes. Gate arrangements  38  each include a gate electrode  40  with an underlying layer of silicon dioxide  42 , that is supported on silicon nitride region  34 . By way of example, it is desired to remove silicon nitride region  34  using plasma  14 , except for those portions which are directly below gate arrangements  38 . A second RF power source  50  can provide RF power to pedestal  30 , generally at one of the ISM (Industry, Scientific, Medical) standard frequencies (i.e., 13.56 MHz, 27.12 MHz or 40.68 MHz. Power source  50  biases the pedestal appropriately, for example, to enhance anisotropic etching. An exhaust port  60  is provided for pumping purposes in maintaining process pressure and removal of process by-products. 
         [0018]    Still referring to  FIG. 2 , in one embodiment, fluorine containing gas  22  is sulfur hexafluoride (SF 6 ), along with ammonia (NH 3 )  24  and any suitable additives such as, for example, argon or nitrogen, as will be discussed immediately hereinafter. 
         [0019]    Turning to  FIG. 3 , in conjunction with  FIG. 2 , a vertical axis  70 , in  FIG. 3 , represents the silicon nitride to silicon dioxide selectivity, while a horizontal axis  72  represents the flow of ammonia gas. A plot  76  represents the selectivity that is obtained using 30 sccm of SF 6  for a flow rate of ammonia that ranges from 0-65 sccm. A selected set of supporting process conditions include argon gas at a flow rate of 170 sccm, a pressure of 20 millitorr, RF power applied to induction coil  16  by source  18  at a value of at least approximately 1000 watts, zero power applied to pedestal  30 , a process temperature of 25 degrees centigrade and a process duration of 30 seconds. It should be appreciated that a peak is presented by plot  76  at an ammonia flow rate of approximately 50 sccm. This suggests that an approximately 5 to 3 ratio of flow of NH 3  to SF 6  achieves near optimized process conditions, at least when using the selected set of process conditions described above. In one embodiment, the ratio of ammonia flow to SF 6  flow can be from greater than zero to  4 . That is, acceptable selectivity can be achieved in this range, depending upon other factors that come into play. For example, higher pressure generally enhances selectivity, as is confirmed by the various plots discussed hereinafter. At the same time, however, increasing processing pressure is generally accompanied by a reduction in directionality. That is, the process shifts from some level of anisotropic behavior to being more isotropic (i.e., less directional). 
         [0020]    When plot  76  is compared with plot  1  of  FIG. 1 , it is seen that selectivity is enhanced, over the values that are achieved with the prior art combination of SF 6  and H 2  for values of NH 3  gas flow ranging from greater than zero sccm to just slightly less than 60 sccm. Thus, in one embodiment, the flow of ammonia can be in the range from greater that zero up to approximately 60 sccm or a ratio of ammonia to SF 6  flow from greater than zero up to approximately double the flow of SF 6 . In this regard, it should be appreciated that all other process conditions are unchanged. That is, the same selected set of supporting process conditions was used for purposes of generating plot  1  of  FIG. 1 . 
         [0021]      FIG. 4  includes a vertical axis  80 , which represents the silicon nitride to silicon dioxide selectivity, while a horizontal axis  82  represents process pressure in millitorr. It is noted that, for each plot in  FIG. 4 , the measured selectivity value is given, adjacent to each data point. A plot  90  represents the selectivity that is obtained using process conditions that are identical to those which were used in relation to plot  76  of  FIG. 3 , but with pressure as a variable instead of ammonia flow. For purposes of the present example, 30 sccm of SF 6  and 50 sccm of NH 3  where chosen. The selected set of supporting process conditions again include argon gas at a flow rate of 170 sccm, RF power applied to induction coil  16  by source  18  at a value of at least approximately 1000 watts, zero power applied to pedestal  30 , a process temperature of 25 degrees centigrade and a process duration of 30 seconds. It should be appreciated that the various plots herein may have been generated from different process runs and, therefore, some variation in the results is to be expected from plot to plot. 
         [0022]    Still referring to  FIG. 4 , a process run  100  was performed using SF 6  without NH 3  and with all other process conditions being identical to those which were used in the process run that generated plot  90 . In this case, plot  100  demonstrates a relatively dramatic reduction in selectivity, which establishes that the ammonia, in cooperation with sulfur hexafluoride, is indeed the responsible agent in terms of the enhanced selectivity that is associated with plot  90 . 
         [0023]    Turning to  FIGS. 2 and 3 , in another embodiment, fluorine containing gas  22  is nitrogen trifluoride (NF 3 ), along with ammonia (NH 3 )  24  and any suitable additives such as, for example, argon or nitrogen, as will be further discussed below. A plot  120  in  FIG. 3  represents the selectivity that is obtained using 30 sccm of NF 3  for a flow rate of ammonia that ranges from 0-80 sccm. Once again, the selected set of supporting process conditions include argon gas at a flow rate of 170 sccm, a pressure of 20 mT, RF power applied to induction coil  16  by source  18  at a value of at least approximately 1000 watts, zero power applied to pedestal  30 , a process temperature of 25 degrees centigrade and a process duration of 30 seconds. It should be appreciated that a peak is presented by plot  120  at an ammonia flow rate of approximately 35 sccm, which is just slightly above the 30 sccm flow rate of the NF 3 . This suggests that near equal flow rates of NF 3  and NH 3  result in near optimized process conditions. This optimization should available, at least within a reasonable approximation, over a relatively wide range of variations in the supporting process conditions. 
         [0024]    When plot  120  is compared with plot  1  of  FIG. 1 , it is seen that selectivity is enhanced over the values that are achieved with the prior art combination of SF 6  and NH 3  for values of H 2  or NH 3  gas flows ranging from approximately 13 sccm to 62 sccm and, certainly, over the range of 20 to 60 sccm. In this regard, it should be appreciated that all other process conditions are unchanged. That is, the same selected set of supporting process conditions was used for purposes of generating plot  1  of  FIG. 1 . Further, for the optimized process, the selectivity is enhanced by approximately 45%, at the same flow of ammonia and with all other conditions being the same. It should also be appreciated that the optimized process conditions for the use of SF 6  and H 2  are obtained at a flow rate of H 2  that is above 60 sccm and results in a selectivity of just slightly over 4.0. When optimized process conditions are compared between NF 3 /NH 3  and SF 6  and H 2 , the former provides approximately a 20% improvement, while avoiding the aforedescribed problems that are associated with the use of hydrogen gas. In one embodiment, the ratio of flow of NF 3  to NH 3  can be in the range from approximately 0.4 to 2.0, which can provide selectivity that is enhanced with respect to the use of an SF 6 /H 2  process. In another embodiment, the ratio of flow of NF 3  to NH 3  can be in the range from approximately 0.4 to 3.5. Again, the selection of particular process conditions such as, for example, pressure can enhance selectivity, however, directionality should also be maintained at a suitable level. 
         [0025]    Referring to  FIG. 5 , a plot  130  represents the selectivity that is obtained using process conditions that are identical to those which were used in relation to plot  120  of  FIG. 3 , but with pressure as a variable instead of ammonia flow. These process conditions include 30 sccm of NF 3  and 50 sccm of NH 3 . The selected set of supporting process conditions again include argon gas at a flow rate of 170 sccm, RF power applied to induction coil  16  by source  18  at a value of at least approximately 1000 watts, zero power applied to pedestal  30 , a process temperature of 25 degrees centigrade and a process duration of 30 seconds. A process run  140  was performed using NF 3  without NH 3  and with all other process conditions being identical to those which were used in the process runs that generated plots  120  ( FIG. 3) and 130 . In this case, plot  140  demonstrates a relatively dramatic reduction in selectivity which establishes that the ammonia is indeed the responsible agent in terms of the enhanced selectivity that is associated with plots  120  and  130 . 
         [0026]    It should be appreciated that the additive gases such as, for example, argon and nitrogen are not introduced for purposes of affecting the etching process itself, but rather for purposes of stabilizing plasma  14 , dependent upon the particular plasma source that is in use. In this regard, it has been empirically demonstrated that reduction in argon flow produces no appreciable difference in selectivity. Further, combinations of sulfur hexafluoride and nitrogen trifluoride, along with ammonia, may be used for purposes of achieving high selectivity. 
         [0027]    While a number of exemplary aspects and embodiments have been discussed above, those of skill in the art will recognize certain modifications, permutations, additions and sub-combinations thereof. For example, it is considered that one of ordinary skill in the art may use sulfur hexafluoride and nitrogen trifluoride together and in combination with ammonia for purposes of achieving high selectivity of silicon nitride relative to silicon dioxide, based on the foregoing teachings. It is therefore intended that the following appended claims and claims hereafter introduced are interpreted to include all such modifications, permutations, additions and sub-combinations as are within their true spirit and scope.