Abstract:
The present invention relates to a method of etching tungsten or tungsten nitride in semiconductor structures, and particularly to the etching of gate electrodes which require precise control over the etching process. We have discovered a method of etching tungsten or tungsten nitride which permits precise etch profile control while providing excellent selectivity, of at least 175:1, for example, in favor of etching tungsten or tungsten nitride rather than an adjacent oxide layer. Typically the oxide is selected from silicon oxide, silicon oxynitride, tantalum oxide, zirconium oxide, and combinations thereof. The method appears to be applicable to tungsten or tungsten nitride, whether deposited by physical vapor deposition (PVD) of chemical vapor deposition (CVD). In particular, an initial etch chemistry, used during the majority of the tungsten or tungsten nitride etching process (the main etch), employs the use of a plasma source gas where the chemically functional etchant species are generated from a combination of sulfur hexafluoride (SF 6 ) and nitrogen (N 2 ), or in the alternative, from a combination of nitrogen trifluoride (NF 3 ), chlorine (Cl 2 ) and carbon tetrafluoride (CF 4 ). Toward the end of the main etching process, a second chemistry is used in which the chemically functional etchant species are generated from Cl 2  and O 2 . This final portion of the etch process may be referred to as an “overetch” process, since etching is carried out to at least the surface underlying the tungsten or tungsten nitride. However, this second etch chemistry may optionally be divided into two steps, where the plasma source gas oxygen content and plasma source power are increased in the second step.

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention pertains to a method of etching tungsten or tungsten nitride electrode gates in semiconductor structures. In particular, one etch chemistry is used during the majority of the etching process and a second etch chemistry is used toward the end of the etching process. 
     2. Brief Description of the Background Art 
     Semiconductor devices as a whole typically include self-aligned contact structures and gate electrodes which are fabricated from multiple film layers of differing compositions. Tungsten nitride films have previously been used as barrier layers, and tungsten has been used as a conductor in various semiconductor device structures. Recently, both tungsten and tungsten nitride have been developing as gate materials, as a result of smaller device geometries. 
     In many instances, the tungsten or tungsten nitride film (layer) is deposited over a thin (less than about 50 Å thick) silicon oxide inorganic dielectric layer. During patterned etching of the multiple film layer structure, it is desired to plasma dry etch through the tungsten or tungsten nitride layer and to stop etching at the surface of the silicon oxide layer. This makes it important that the etch selectivity for etching of tungsten or tungsten nitride (in preference over silicon oxide) be high. (The term “selectivity” is typically used to refer to a ratio of etch rates of two materials.) Further, as the device geometries become smaller, etching of layers of material must be more precise, providing a profile which permits placement of more devices over a given surface area. In the case of a tungsten gate, for example, the gate may be in the form of a thin line or pad and the cross-sectional profile of the etched gate feature is preferably one where the sidewalls of the etched feature are essentially perpendicular to an underlying silicon oxide substrate layer, for example. This means the tungsten must be completely etched to the surface of the silicon oxide substrate layer (no residual “feet” at the bottom of the etched tungsten sidewall). Control of the etch process is critical in providing proper etched tungsten feature profile while avoiding etching away critical thickness of the underlying silicon oxide film substrate. 
     U.S. Pat. No. 5,295,923 to Hori et al., issued Nov. 9, 1993, describes a dry etching method, wherein a multilayer film including one selected from tungsten, molybdenum, and a silicide thereof, is etched as the first layer. Underlying the “first layer” is a second layer of polycrystal silicon, which overlies a silicon oxide insulation film. The etching step for the first layer uses a plasma etchant source gas made up of a first gas selected from fluorine, sulfur hexafluoride, and nitrogen trifluoride, or a mixture gas containing the first gas and a second gas selected from hydrogen chloride, hydrogen bromide, chlorine, bromine, and carbon tetrachloride. Etching of the second layer of polycrystalline silicon is carried out using a plasma etchant source gas made up of the second gas and a third gas selected from an inert gas, nitrogen gas, oxygen gas, silicon tetrachloride gas and carbon monoxide gas. In the second etch step, the amount of the third gas added to the second gas should preferably be in the range between 0 and 10 volume % of the total etching gas mixture. 
     U.S. Pat. No. 5,599,725 to Dorleans et al., issued Feb. 4, 1997, discloses a method for fabricating a silicon based MOS transistor having an inverse-T refractory metal gate structure. The gate fabricated according to the invention comprises a main CVD tungsten portion and a lower sputtered tungsten portion outwardly extending from the bottom of the CVD portion. A Cl 2 /O 2  plasma etch is used to etch the CVD tungsten layer and a chemical etch (KH 2 PO 4 /KOH/K 3 Fe(CN) 6 ) is used to etch the sputtered tungsten portion. The sputtered tungsten layer is said to act as a shield to protect the underlying gate oxide layer from ion damage throughout the fabrication process. In particular, the sputtered tungsten is said to be more resistant to Cl 2 /O 2  reactive ion etch than is CVD tungsten. 
     U.S. Pat. No. 6,033,962 to Jeng et al. describes a method of fabricating sidewall spacers for a self-aligned contact hole. Although the principal subject matter relates to the formation of silicon nitride spacers on the sides of gate structures, there is also the description of the deposition of a metal such as tungsten which is RIE etched using a conventional etchback procedure, without the use of a photoresist masking, using a Cl 2 —SF 6 —BCl 3 —Ar etchant gas mixture for plasma generation. 
     SUMMARY OF THE INVENTION 
     The present invention relates to a method of etching tungsten or tungsten nitride in semiconductor structures, and particularly to the etching of gate electrodes which require precise control over the etching process. We have discovered a method of etching tungsten or tungsten nitride which permits precise etch profile control while providing excellent selectivity in favor of etching tungsten (or tungsten nitride) rather than a thin underlying oxide layer. Typically the oxide is selected from silicon oxide, silicon oxynitride, tantalum oxide, zirconium oxide, and combinations thereof. The method appears to be applicable to tungsten or tungsten nitride, whether deposited by physical vapor deposition (PVD) or chemical vapor deposition (CVD). 
     In particular, an initial etch chemistry, used during the majority of the tungsten or tungsten nitride etching process (the main etch), employs the use of a plasma source gas where the chemically functional etchant species are generated from a combination of sulfur hexafluoride (SF 6 ) and nitrogen (N 2 ), or in the alternative, from a combination of nitrogen trifluoride (NF 3 ), chlorine (Cl 2 ) and carbon tetrafluoride (CF 4 ). Toward the end of the main etching process, a second chemistry is used in which the chemically functional etchant species are generated from Cl 2  and O 2 . This final portion of the etch process may be referred to as an “overetch” process, since etching is carried out to at least the surface underlying the tungsten or tungsten nitride. However, this second etch chemistry may optionally be divided into two steps, where the plasma source gas oxygen content and plasma source power are increased in the second step. 
     We have discovered that an unexpectedly high etch selectivity for tungsten in preference over silicon oxide (in the range of 175:1, for example) may be obtained when a sufficiently high concentration of O 2  is used in combination with a sufficiently high plasma density. In particular, when the O 2  concentration is greater than about 20% by volume in the plasma source gas, further increases in O 2  content have a limited effect at plasma densities below about 8×10 10  e − /cm 3 , because there is insufficient power input to energize the active oxygen species. To obtain selectivity in favor of etching tungsten or tungsten nitride relative to an underlying oxide, it is necessary to increase both the oxygen content of the plasma source gas and the source power applied to create and maintain the plasma. For example, at a plasma density of about 1.6×10 10  e − /cm 3 , an increase in plasma source gas oxygen content from about 30 volume percent to about 40 volume percent produces an increase in selectivity from about 40:1 to about 75:1. However, at the 40 volume percent O 2 , if the plasma density is increased to about 1.8×10 10  e − /cm 3 , the selectivity increases from about 75:1 to about 160:1. 
     Although carrying out the “overetch” step at the conditions which produces the highest selectivity protects the underlying layer of oxide, the etched tungsten or tungsten nitride feature profile may be affected by the rapid tungsten etch rates obtained.(about 1,800 Å/min. at the conditions which produce 175:1 selectivity). To enable maintenance of feature profile while removing residual tungsten “feet” at the base of a feature, it may be advantageous to etch under conditions which provide a lower selectivity, of about 30:1, for example, and a tungsten etch rate of about 1,000 Å/min and then change process conditions to those which provide a selectivity of 175:1 for a limited time at the end of the etch, to clean residue off the oxide flat surface surrounding the etched feature in general. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 shows a schematic of the plasma processing apparatus which was used to carry out the etching processes described herein. 
     FIGS. 2 A and  2 B show schematics of photomicrographs of etched tungsten when no overetch step is used, i.e. there is no change in the etch chemistry toward the end of the tungsten etching, and the etch is stopped in sufficient time to avoid etching an underlying silicon oxide substrate. 
     FIG. 3 shows a schematic of a photomicrograph of etched tungsten where there is a change in the etch chemistry toward the end of the tungsten etching, and etching is permitted to continue to the surface of the silicon oxide substrate. 
     FIG. 4 is a graph showing that an increase in oxygen content of a plasma source gas has a diminishing effect on the etch rate of a silicon oxide substrate, when all other process variables are held constant. 
     FIG. 5A is a graph showing the effect of increasing the plasma source power on the etch rate of tungsten; the etch rate of silicon oxide; and on the selectivity (in terms of an increase in etch rate of tungsten relative to the etch rate of silicon oxide) during an overetch step, when the oxygen concentration is about 20% by volume. 
     FIG. 5B is a three dimensional graph showing tungsten etch rate in an overetch step, as a function of plasma source power and oxygen flow rate, all other variables held constant. 
     FIG. 5C is a three dimensional graph showing silicon oxide etch rate in an overetch step, as a function of plasma source power and oxygen flow rate, all other variables held constant. 
     FIG. 5D is a three dimensional graph showing selectivity (etch rate ratio of W: SiO x ) as a function of plasma source power and oxygen flow rate, all other variables held constant. 
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     The present disclosure relates to a method of etching tungsten or tungsten nitride in semiconductor structures. In particular, the method pertains to the etching of feature sizes of less than 0.5 μm, where control of etch selectivity of tungsten or tungsten nitride relative to a thin layer (typically less than about 50 Å) of an underlying oxide is of primary importance. The term “feature” typically refers to metal lines, trenches and openings in a dielectric layer, as well as other structures which make up the topography of the substrate surface. 
     In particular, an initial etch chemistry, used during the majority of the tungsten or tungsten nitride etching process, is preferably one employing a plasma source gas where the chemically functional etchant species are generated from sulfur hexafluoride (SF 6 ) and nitrogen (N 2 ) or from a combination of NF 3 , Cl 2 , and CF 4 . This etch chemistry provides a rapid tungsten etch rate with excellent etch profile characteristics. 
     For example, in one embodiment, the main etch is carried out using a source gas consisting of SF 6  and N 2  The volumetric flow rates for SF 6  typically range from about 30 sccm to about 100 sccm in a CENTURA® DPS™ processing chamber. The volumetric flow rates for N 2  typically range from about 30 sccm to about 100 sccm as well. The preferred volumetric ratio of SF 6 :N 2  generally ranges from about 20:50 to about 60:10. The process chamber pressure ranges from about 2 mTorr to about 20 mTorr, and is preferably maintained at a pressure within a range of about 2 mTorr to about 10 mTorr. The substrate temperature ranges from about 20° C. to about 100° C., with lower temperatures being preferred, since apparatus costs are lower and the possibility of damage to a gate oxide is reduced.. The applied plasma source power ranges from about 200 W to about 2,000 W, and the applied substrate biasing power ranges from about 40 W to about 200 W. (The term “plasma source power” typically refers to the power that is responsible for sustaining the plasma by providing a major portion of the energy to ionize the neutral species in the chamber; while the term “substrate bias power” typically refers to the power applied to the substrate to attract high energy plasma species toward the substrate.) Using conditions within the ranges just described, adjusted for apparatus variables, a tungsten or tungsten nitride etch rate ranging from about 1,500 Å/min. to about 4,000 Å/min. is obtained. During the etching of fine (about 0.15 μm in width) lines, the profile of the line sidewall relative to the underlying substrate surface is vertical at about 88° to 90° (discounting “feet” which may be formed near the base of the sidewall). 
     Toward the end of the etching process, a second chemistry is used in which the chemically functional etchant species are generated from Cl 2  and O 2 . The process during etch of the remaining portion of the tungsten may be referred to as an overetch process, since the etch is carried out to the surface of an underlying film surface and may etch for a limited distance into the underlying film surface. However, this second etch chemistry may optionally be divided into two steps, where the oxygen content and plasma source power are increased during the second step. 
     To obtain a satisfactory etch rate for the tungsten or tungsten nitride and the desired selectivity in favor of etching tungsten relative to an underlying oxide gate layer, the volumetric percentage of the O 2  in a Cl 2 /O 2  source gas mixture ranges from greater than 20% up to about 45%. Preferably, the volumetric percentage of O 2  in a Cl 2 /O 2  source gas mixture ranges from about 35% to about 45%. However, to obtain the benefit of the O 2  content specified above, the plasma density in the etch process chamber must be sufficiently high. We have determined that a plasma density of at least about 8×10 10  e − /cm 3  (a plasma source power of about 800 W in a CENTURA® DPS™ processing chamber available from Applied Materials, Inc. of Santa Clara, Calif.) is required to obtain benefit from increasing the volumetric percentage of oxygen above 20%. (The term “decoupled plasma source” or “DPS” as used herein refers to a plasma etch apparatus with separate controls for the inductively coupled RF source power used to and maintain a plasma and the bias power applied to a semiconductor substrate to direct high energy species toward the substrate.) 
     An alternative to using a single overetch step is to use a two step overetch process. For example, after the main etch step, a first overetch step may be carried out in which the plasma source gas composition is a combination of Cl 2  and O 2 , where the volumetric content of O 2  ranges from greater than 20% to about 35%. The plasma density typically ranges from about 8.0×10 10  e − /cm 3  (800 W applied source power) to about 1.6×10 11  (1,600 W applied source power). The process chamber pressure ranges from about 2 mTorr to about 10 mTorr, and preferably between about 2 mTorr and 6 mTorr. The substrate temperature ranges from about 20° C. to about 100° C. The applied substrate biasing power ranges from about 40 W to about 200 W. Using conditions within these ranges, adjusted for apparatus variables, a “foot” which forms at the bottom of the fine line can be removed, while maintaining the line profile at the vertical 88° to 90° profile. 
     Subsequently, a second overetch or finishing step is carried out, to clean etch residue from the surface of an underlying thin oxide film, without significant etching into the film (less than 10% of the thin oxide film thickness is etched). In the second overetch step, the selectivity in favor of etching tungsten or tungsten nitride relative to an underlying oxide film is critical. It is desired to remove tungsten or tungsten nitride residue from the flat surface of the thin oxide layer surrounding the etched feature without etching through the thin oxide layer. Again, the chemical etchants in the plasma source gas are Cl 2  and O 2 , where the volumetric percentage of O 2  in the mixture typically ranges from about 36% to about 41%. The plasma density typically ranges from about 1.6×10 11  to about 2.0×10 11 , with the other process conditions being the same as those specified above for the first overetch step. A tungsten etch rate under these process conditions is about 1,750 Å/min, while the selectivity (W:SiO x ) is about 175:1. Increased plasma source gas O 2  content and increased plasma densities are expected to be useful as well. The selectivity of 175:1 obtained using the process conditions just described may be compared with the selectivity obtained using other process conditions which, at first glance, do not appear to be significantly different, but which produce surprisingly different results. For example, a change in the O 2  flow rate to provide a volumetric concentration of O 2  of 36%, in combination with a plasma density of 1.6×10 11  e − /cm 3  (1,600W) provides a W:SiO x  selectivity of about 75:1; and a volumetric concentration of O 2  of 20% in combination with a plasma density of 1.5×10 11  e − /cm 3  (1,500W) provides a W:SiO x  selectivity of about 28:1. As this data indicates, there is an unexpected increase in selectivity in favor of etching tungsten or tungsten nitride relative to silicon oxide which occurs as a result of the increasing the oxygen flow rate and the plasma density above particular ranges simultaneously. This is further illustrated in the Examples provided below. 
     As a preface to the detailed description of the Examples, it should be noted that, as used in this specification and the appended claims, the singular forms “a”, “an”, and “the” include plural referents, unless the context clearly dictates otherwise. Thus, for example, the term “a semiconductor” includes a variety of different materials which are known to have the behavioral characteristics of a semiconductor. 
     I. AN APPARATUS FOR PRACTICING THE INVENTION 
     The embodiment etch processes described herein were carried out in a Centura® Integrated Processing System available from Applied Materials, Inc. of Santa Clara, Calif. The system is shown and described in U.S. Pat. No. 5,186,718, the disclosure of which is hereby incorporated by reference. Although the etch process chamber used in the EXAMPLES presented herein is shown in schematic in FIG. 1, any of the etch processors available in the industry should be able to take advantage of the etch chemistry described herein, with some adjustment to other process parameters. The equipment shown in schematic in FIG. 1 includes a Decoupled Plasma Source (DPS) of the kind described by Yan Ye et al. at the Proceedings of the Eleventh International Symposium of Plasma Processing, May 7, 1996 and as published in the Electrochemical Society Proceedings, Volume 96-12, pp. 222-233 (1996). The plasma processing chamber enables the processing of an 8 inch (200 mm) diameter wafer. 
     FIG. 1 shows a schematic of a side view of an individual CENTURA® DPS™ polysilicon etch chamber  100 . The etch chamber  100  consists of an upper chamber  104  having a ceramic dome  106 , and a lower chamber  108 . The lower chamber  108  includes a monopolar electrostatic chuck (ESC) cathode  110 . Gas is introduced into the chamber via gas injection nozzles  114  for uniform gas distribution. Chamber pressure is controlled by a closed-loop pressure control system (not shown) using a throttle valve  118 . During processing, a substrate  120  is introduced into the lower chamber  108  through inlet  122 . The substrate  120  is held in place by means of a static charge generated on the surface of electrostatic chuck (ESC) cathode  110  by applying a DC voltage to a conductive layer (not shown) located under a dielectric film (not shown) on the chuck surface. The cathode  110  and substrate  120  are then raised by means of a wafer lift  124  and sealed against the upper chamber  104  in position for processing. Etch gases are introduced into the upper chamber  104  via gas injection nozzles  114 . The etch chamber  100  uses an inductively coupled plasma source power  126  and matching network  128  operating at 12.56 MHZ for generating and sustaining a high density plasma. The wafer is biased with an RF source  130  and matching network  132  operating at 13.56 MHZ. Plasma source power  126  and substrate biasing means  130  are controlled by separate controllers (not shown). 
     II. EXAMPLES OF EMBODIMENTS OF THE INVENTION 
     FIG. 2A shows a cross-sectional side view schematic of an etched tungsten structure  200 , where the etched pattern is lines  203  and spaces  205 . The line width is approximately 0.165 μm, and the space width is approximately 0.21 μm. The structure includes a thin (≈45 Å) silicon oxide layer  213  on a silicon substrate  202 , overlying silicon oxide layer  213  is a 1,650 Å thick layer of tungsten  204 , a 400 Å thick image focusing antireflective coating layer  206 , and the residue  208  of a photoresist layer which was used to pattern the etched structure  200 . FIG. 2B shows a more three-dimensional view of the same etched structure  200 , showing the surface finish  222  of the etched tungsten in the bottom of a trench (space  205 ), and the smoother upper surface  224  of the overlying photoresist layer residue  208 . Both Figures show a remaining unetched tungsten thickness  214  overlying upper surface  210  of silicon oxide layer  213 . The unetched tungsten thickness  214  is about 0.023 μm. 
     The tungsten  204  etch was carried out using only a single etch chemistry, in which the etchant species in the plasma were SF 6  at 30 sccm and N 2  at 50 sccm. (There was no second etch chemistry used to etch the final portion of the tungsten layer  204  to the surface  210  of oxide layer  213 , i.e. there was no overetch carried out.) Other etch process conditions were as follows. The etch process chamber pressure was about 4 mTorr; the substrate temperature was about 50° C.; the plasma source power was about 500 W; the substrate bias power was about 80 W. The etch profile obtained was good, with the sidewall angle of the etched lines  203  from silicon oxide layer  213  surface  210  being about 89°; however, there was some tapering of the profile at the base of the line, since the tungsten was not etched all the way to the surface  210  of silicon oxide layer  203 . The tungsten etch rate was about 1,500 Å/min. The term “tapered” profile, with reference to an etched pattern of lines and spaces, refers to a cross-sectional profile where the width of the line is wider at the base of the line than at the top surface of the line. A “vertical profile” is one where the side walls of the lines are perpendicular to the surface of the silicon oxide substrate. An “undercut” profile is one where the width of the line is more narrow at the base of the line than at the top surface of the line. A vertical profile is typically preferred, because it enables closer placement of device structures on a given surface area. 
     As shown in FIG. 2B, the etched tungsten surface  222  at the bottom of spaces  205  exhibited a rougher finish than the line  203  upper photoresist residue  208  surface  224 . 
     FIG. 3 shows a schematic of a photomicrograph of etched tungsten  300  where there is a change in the etch chemistry toward the end of the tungsten  304  etching, and etching is permitted to continue to the surface  310  of the thin silicon oxide layer  313 . Again, the etched tungsten structure was a pattern of lines  303  and spaces  305 . The line width is approximately 0.120 μm, and the space width is approximately 0.165 μm. The structure included a thin (≈45 Å) silicon oxide layer  313  on a silicon substrate  302 , a 1,650 Å thick overlying layer of tungsten  304 , a 400 Å thick image focusing antireflective coating layer  306 , and a photoresist layer residue  308 . 
     The tungsten  304  etch was carrier out using only a two etch chemistries. During the first portion of the etching, the chemically reactive etchant species in the plasma were SF 6  at 30 sccm and N 2  at 50 sccm. Other etch process conditions were as follows. The etch process chamber pressure was about 4 mTorr; the substrate temperature was about 50° C.; the plasma source power was 500 W; the substrate bias power was about 80 W. The etch profile obtained was good, with the sidewall angle of the etched lines  303  from silicon oxide substrate  302  surface  310  being about 89°. The tungsten etch rate was about 1,650 Å/min. Approximately 1,500 Å of the initial 1,650 Å thickness of the tungsten layer  304  was etched using this first etch chemistry. 
     Etching of the remaining 150 Å of tungsten layer  304  was carried out using a plasma in which the chemically reactive etchant species were produced from a source gas of Cl 2  at a flow rate of about 70 sccm and O 2  at a flow rate of about 40 sccm. Other etch process conditions were as follows. The etch process chamber pressure was about 6 mTorr; the substrate temperature was about 50° C.; the plasma source power was 600 W; the substrate bias power was about 80 W. The etch profile obtained was excellent, with the sidewall angle of the etched lines  303  from silicon oxide substrate  302  surface  310  being about 89° to 90°. The tungsten etch rate was about 1,500 Å/min. The etching was allowed to continue until the upper surface of the silicon oxide substrate  310  was slightly etched. It was determined that the silicon oxide etch rate was about 19 Å/min. The etch rate selectivity of tungsten: silicon oxide was about 79:1. Tungsten residue was cleared off the space  305  open areas, but there was a slight tungsten “foot” (not shown) at the bottom of tungsten lines  303  in the areas where the spacing between lines was less than that shown in FIG. 3 (in the more dense areas). Subsequent experimentation demonstrated that adjustment of the O 2  flow rate to 45 sccm , the plasma source power to 1,800 W, and the process chamber pressure to 3 mTorr results in removal of the “foot”. 
     FIG. 4 is a graph  400  showing the etch rate (shown on axis  404 )of silicon oxide as a function of the plasma source gas oxygen flow rate (shown on axis  402 ). The data shown in graph  400  was obtained by etching a layer of silicon oxide only, which was deposited on a silicon wafer. The total flow of Cl 2  and O 2  used was 110 sccm, and the volume % of oxygen in the plasma source gas may be calculated by dividing the seem shown on axis  402  by the total gas flow 100 sccm and multiplying by 100. The other etch process conditions used during the silicon oxide etching were as follows. The etch process chamber pressure was about 6 mTorr; the substrate temperature was about 50° C.; the plasma source power was about 1,600 W; the substrate bias power was about 80 W. Graph  400  indicates that there is a diminishing effect obtained by increasing the O 2  flow rate, with the etch rate leveling out at about 19 Å/min between about 35 sccm and 40 sccm of O 2 . We later discovered that it was necessary to increase the plasma density (plasma source power) to obtain the full benefit of an increase in the O 2  flow rate over about 35 sccm. 
     FIG. 5 A is a graph  500  showing the effect of increasing the plasma source power (shown in Watts on axis  502 ), on the etch rate of tungsten (W) (shown by curve  508 ) and on the etch rate of SiO x  (shown by curve  510 ), at a constant O 2  flow rate of about 20 sccm. The etch rate units in each case are shown on axis  504 . Graph  500  also shows the selectivity for W:SiO x  as a function of the plasma source power, illustrated by curve  512  , at the constant O 2  flow rate of 20 sccm, The nominal selectivity is shown on axis  506 . As can be observed from graph  500 , an increase in plasma source power results in an increase in tungsten etch rate, a decrease in SiO x  etch rate, and an increase in selectivity. However, at an O 2  flow rate of 20 sccm, up to a plasma source power of about 1,600 W, the selectivity of W:SiO x  is only about 40:1. The process chamber pressure was about 6 mTorr, and the substrate temperature was about 50° C. 
     FIG. 5B is a three dimensional graph  530  showing tungsten etch rate in an overetch step, as a function of plasma source power and oxygen flow rate, all other variables held constant. In particular, the wafer etched was a sputtered tungsten layer overlying a silicon wafer. The process chamber pressure was 6 mTorr, and the substrate temperature was about 50° C. The tungsten etch rate units (Å/min) are shown on axis  534 , the plasma source power units (W) are shown on axis  532 , and the O 2  flow rate units (sccm) are shown on axis  536 . Curve  538  clearly shows that an increase in O 2  flow rate alone from about 30 sccm to about 40 sccm , with plasma source power held constant at about 1,600 W increased the tungsten etch rate from about 1,170 Å min to about 1,500 Å min. A simultaneous increase in O 2  flow rate from about 30 sccm to about 45 sccm and increase in plasma source power from about 1,600 W to about 1,800 W increased the tungsten etch rate from about 1,170 Å/min. to about 1,750 Å/min. 
     FIG. 5C is a three dimensional graph  540  showing silicon oxide etch rate in an overetch step, as a function of plasma source power and oxygen flow rate, all other variables held constant. In particular, the wafer etched was a silicon substrate having a layer of thermal silicon oxide on its surface. The process chamber pressure was 6 mTorr, and the substrate temperature was about 50° C. The silicon oxide etch rate units (Å/min) are shown on axis  544 , the plasma source power units (W) are shown on axis  542 , and the O 2  flow rate units (sccm) are shown on axis  546 . Curve  538  shows that an increase in O 2  flow rate alone from about 20 sccm to about 40 sccm , with plasma source power held constant at about 1,600 W decreased the silicon oxide etch rate from about 37 Å/min to about 19 Å/min. A simultaneous increase in O 2  flow rate from about 20 sccm to about 45 sccm and increase in plasma source power from about 1,600 W to about 1,800 W decreased the silicon oxide etch rate from about 37 Å/min. to about 10 Å/min. FIG. 5D is a three dimensional graph  550  showing selectivity (etch rate ratio of W:SiO x ) as a function of plasma source power and oxygen flow rate, all other variables held constant. FIG. 5D is derived from FIGS. 5B and 5C, and makes apparent the striking increase in selectivity which can be achieved by increasing both the oxygen flow rate and the plasma source power simultaneously. The selectivity (W:SiO x ) nominal units are shown on axis  554 , the plasma source power units (W) are shown on axis  552 , and the O 2  flow rate units (sccm) are shown on axis  556 . Curve  558  shows that an increase in O 2  flow rate alone from about 30 sccm to about 40 sccm , with plasma source power held constant at about 1,600 W increased the selectivity from about 50:1 to about 80:1. A simultaneous increase in oxygen flow rate from about 30 sccm to about 45 sccm and increase in plasma source power from about 1,600 W to about 1,800 W increased the selectivity from about 50:1 to about 175:1. For example, a selectivity of about 100:1 is obtained when the O 2  content is about 40%, and the plasma source power is about 1,670 W. A selectivity of about 175:1 is obtained when the O 2  content is about 45% and the plasma source power is about 1,800 W. An increase in the selectivity to 175:1 was unexpected in view of the much smaller increase in selectivity obtained when only the oxygen flow rate was increased.. 
     To better appreciate this surprising increase in selectivity, it is helpful to compare FIG. 5D with FIG.  5 A. FIG. 5A showed that when the oxygen flow rate was held constant at about 20 sccm, and the plasma source power was increased from about 800 W up to about 1,600 W, the selectivity of W:SiO x  increased from about 19:1 to about 40:1. Further, as illustrated in FIG. 5D, an increase in oxygen flow rate from about 30 sccm to about 40 sccm with the plasma source power held constant at 1,600 W provided a selectivity increase from about 50:1 to about 80:1. It was only with the synergistic combination of increased oxygen flow rate and increased plasma source power that a selectivity of 175:1 was achieved. 
     Table I, below, provides a summary of various process conditions and the tungsten (or tungsten nitride) etch rate which is expected to be obtained at those process conditions. Table I also shows the selectivity relative to silicon oxide which is expected to be obtained, and the etched sidewall profile angle which is expected to be obtained. 
     
       
         
               
             
               
               
               
             
           
               
                 TABLE 1 
               
             
             
               
                   
               
               
                 Typical Process Conditions During Final Portion Etching (Overetch) 
               
               
                 of Tungsten or Tungsten Nitride 
               
             
          
           
               
                   
                   
                 Process Condition 
               
               
                 Process Condition and 
                 Process Condition Broad 
                 Preferred 
               
               
                 Result 
                 Range and Results 
                 Range and Results 
               
               
                   
               
               
                 Total Gas Flow 
                 50-200 
                  75-125 
               
               
                 (sccm) 
               
               
                 O 2  Flow Rate 
                 &gt;20-60  
                 30-50 
               
               
                 (sccm) 
               
               
                 Vol % O 2  in mixt. of 
                 &gt;20-50  
                 30-40 
               
               
                 O 2  and Cl 2   
               
               
                 Substrate 
                 20-100 
                 40-60 
               
               
                 Temperature (° C.) 
               
               
                 Process Chamber 
                 2-20 
                 3-6 
               
               
                 Pressure (mTorr) 
               
               
                 Source Power (W) 
                  800-3,000 
                 1400-1800 
               
               
                 Bias Power (W) 
                 40-200 
                  80-100 
               
               
                 Etch Rate W or WN 2   
                 1,000-3,000  
                 1,500-2,000 
               
               
                 (Å/min) 
               
               
                 Selectivity 
                 10:1 to 200:1 
                 100:1 to 175:1 
               
               
                 W:SiO x   
               
               
                 Etched Profile, Vertical 
                 88-90  
                 vertical profile 
               
               
                 or Tapered or Undercut 
                   
                 89-90 
               
               
                 (°) 
               
               
                   
               
             
          
         
       
     
     The above described embodiments are not intended to limit the scope of the present invention, as one skilled in the art can, in view of the present disclosure expand such embodiments to correspond with the subject matter of the invention claimed below.