Abstract:
A method for removing oxides from the bottom surface of a contact hole is provided. The method provides efficient cleaning of the bottom surface without distortion of the contact hole upper and sidewall surfaces.

Description:
BACKGROUND OF THE INVENTION 
       [0001]    1. Field of the Invention 
         [0002]    Embodiments of the present invention generally relate to methods for forming electronic devices. More particularly, embodiments of the present invention generally relate to methods for cleaning contact hole bottom surfaces in the formation of electronic devices. 
         [0003]    2. Description of the Related Art 
         [0004]    In the fabrication of an active electronic device, such as a Metal Oxide Silicon Field Effect Transistor (“MOSFET”), the electrodes and interconnecting pathways include silicide layers formed by depositing a refractory metal on bare silicon and annealing the layer to produce the metal silicide layer. A dielectric layer is then deposited over the metal silicide, and a contact hole is formed through the dielectric layer to the surface of the metal silicide. The contact hole is then filled with a bulk metal to complete the contact. 
         [0005]    In a typical fabrication process, the metal silicide may be formed on a substrate in one vacuum environment and, after contact hole formation, transferred to another vacuum environment for further processing. As a result, a native oxide may develop on the contact hole bottom surface. The cleanliness of the contact hole bottom surface is critical for reducing contact resistance and ensuring optimal device performance. Therefore, the contact hole bottom surface must be cleaned and the native oxide removed prior to further processing. 
         [0006]    Sputter etch processes have been used in an attempt to clean contact hole bottom surfaces; however, such techniques may damage the underlying surface. Sputter etch techniques may also alter the contact hole geometry due to the physical bombardment of ions on the surface surrounding the contact hole. For example, the contact opening may become widened or tapered, often referred to as “faceting.” 
         [0007]    A more recent approach to remove native oxide films involves forming a fluorine/silicon-containing salt on the substrate surface that is subsequently removed by thermal anneal. According to this approach, a thin layer of the salt is formed by reacting a fluorine-containing gas with the oxide surface. The salt is then heated to an elevated temperature sufficient to dissociate the salt into volatile by-products, which are then removed from the processing chamber. 
         [0008]    However, during this process, the upper and sidewall surfaces of the contact opening are exposed to the oxide cleaning chemicals for a longer period of time than the bottom contact oxide surface. Prior art recipes etch the upper and sidewall surfaces more than the bottom contact surface during this time period. This results in significantly lower oxide removal at the contact hole bottom surface than at the upper and sidewall surfaces; hence, the contact hole bottom surface cleaning efficiency is very low. Accordingly, the contact resistance may be elevated, resulting in inefficiency of the device, and/or the geometry of the contact opening may be compromised, resulting in current leakage. 
         [0009]    Therefore, a need exists for a dry clean process that removes oxides from a bottom surface of a contact hole with increased efficiency without over etching the upper and sidewall surfaces of the contact hole. 
       SUMMARY OF THE INVENTION 
       [0010]    In one embodiment of the present invention, a method of cleaning a contact surface of an electronic device comprises positioning a substrate having an at least partially oxidized contact surface disposed thereon in a processing chamber, generating a reactive species from a gas mixture comprising nitrogen trifluoride and ammonia within the chamber, wherein the molar ratio of nitrogen trifluoride to ammonia is greater than about 2:1, directing the reactive species to the at least partially oxidized contact surface to react with the oxide thereon and form a film on the at least partially oxidized contact surface, and heating the substrate within the chamber to dissociate and remove the film. 
         [0011]    In another embodiment, a method of cleaning a bottom surface of a contact hole comprises positioning a substrate having a contact hole with an oxidized bottom surface in a processing chamber, generating a reactive species from a gas mixture comprising nitrogen trifluoride, ammonia, and a carrier gas, wherein the molar ratio of nitrogen trifluoride to ammonia is greater than about 2:1, directing the reactive species to the bottom surface to react with the oxide thereon and form a film on the oxidized bottom surface, and heating the substrate within the chamber to dissociate and remove the film. 
         [0012]    In yet another embodiment, a method of forming a metal contact comprises depositing a metal on a substrate in a first vacuum environment, annealing the substrate at conditions sufficient to provide a metal silicide layer, depositing an insulating cover layer on the metal silicide layer, forming a contact hole in the insulating cover layer exposing a portion of the metal silicide layer, transferring the substrate from the first vacuum environment to a second vacuum environment, wherein the exposed portion of the metal silicide layer at least partially oxidizes during the transfer, and exposing the exposed portion of the at least partially oxidized silicide layer to a reactive species in the second vacuum environment to remove the at least partially oxidized silicide layer while substantially maintaining the shape of the contact hole. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0013]    So that the manner in which the above recited features of the present invention can be understood in detail, a more particular description of the invention, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. 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. 
           [0014]      FIG. 1  is a cross section view of an illustrative clean chamber  100  for removing native oxides from a contact surface according to the present invention. 
           [0015]      FIG. 2  is a schematic top-view diagram of an illustrative multi-chamber processing system  200 . 
           [0016]      FIGS. 3A-3N  are sectional schematic views of an illustrative fabrication sequence for forming an illustrative active electronic device, such as a MOSFET structure  300 . 
       
    
    
     DETAILED DESCRIPTION 
       [0017]    As will be explained in greater detail below, a substrate having a contact surface at least partially disposed thereon is treated to remove metal oxides or other contaminants prior to contact level metallization. The term “contact surface” as used herein refers to a layer of material that includes a metal silicide that can form part of a gate electrode. In one or more embodiments, the metal silicide can be nickel silicide, cobalt silicide, titanium silicide or any combinations thereof. The metal silicide can also include tungsten, Ti/Co alloy silicide, Ti/Ni alloy silicide, Co/Ni alloy silicide and Ni/Pt silicide. 
         [0018]    The term “substrate” as used herein refers to a layer of material that serves as a basis for subsequent processing operations and includes a “contact surface.” For example, the substrate can include one or more conductive metals, such as aluminum, copper, tungsten, or combinations thereof. The substrate can also include one or more nonconductive materials, such as silicon, silicon oxide, doped silicon, germanium, gallium arsenide, glass, and sapphire. The substrate can also include dielectric materials such as silicon dioxide, organosilicates, and carbon doped silicon oxides. Further, the substrate can include any other materials such as metal nitrides and metal alloys, depending on the application. In one or more embodiments, the substrate can form part of an interconnect feature such as a plug, via, contact, line, and wire. 
         [0019]    Moreover, the substrate is not limited to any particular size or shape. The substrate can be a round wafer. The substrate can also be any polygonal, square, rectangular, curved or otherwise non-circular workpiece, such as a glass substrate used in the fabrication of flat panel displays. 
         [0020]      FIG. 1  is a cross sectional view of an illustrative clean chamber  100  for conducting bottom contact cleaning. The chamber  100  may be particularly useful for performing a plasma assisted dry etch process according to the present invention. The chamber  100  provides both heating and cooling of a substrate surface without breaking vacuum. In one embodiment, the chamber  100  includes a chamber body  112 , a lid assembly  140 , and a support assembly  180 . The lid assembly  140  is disposed at an upper end of the chamber body  112 , and the support assembly  180  is at least partially disposed within the chamber body  112 . 
         [0021]    The chamber body  112  includes a channel  115  formed therein for flowing a heat transfer fluid therethrough. The heat transfer fluid may be a heating fluid or a coolant and is used to control the temperature of the chamber body  112  during processing and substrate transfer. Exemplary heat transfer fluids include water, ethylene glycol, or a mixture thereof. An exemplary heat transfer fluid may also include nitrogen gas. 
         [0022]    The chamber body  112  may further include a liner  120  that surrounds the support assembly  180 . The liner  120  may be removable for servicing and cleaning. The liner  120  may be comprised of a metal such as aluminum, a ceramic material, or any other process compatible material. The liner  120  may be bead blasted to increase surface roughness and/or surface area to increase the adhesion of any material deposited thereon in order to prevent flaking and contamination of chamber  100 . In one embodiment, the liner  120  includes one or more apertures  125  and a pumping channel  129  formed therein that is in fluid communication with a vacuum system. The apertures  125  provide a flow path for gases into the pumping channel  129 , which provides an egress for the gases within the chamber  100 . 
         [0023]    The vacuum system may include a vacuum pump  130  and a throttle valve  132  to regulate flow of gases through the chamber  100 . The vacuum pump  130  is coupled to a vacuum port  131  disposed on the chamber body  112  and in fluid communication with the pumping channel  129  formed within the liner  120 . The terms “gas” and gases” are used interchangeably, unless otherwise noted and refer to one or more precursors, reactants, catalysts, carrier, purge, cleaning, and combinations thereof, as well as any other fluid introduced into the chamber body  112 . 
         [0024]    The lid assembly  140  includes at least two stacked components configured to form a plasma volume or cavity therebetween. In one embodiment, the lid assembly  120  includes a first electrode  143  disposed vertically above a second electrode  145  confining a plasma volume or cavity  150  therebetween. The first electrode  143  is connected to a power source  152 , such as a radio frequency (RF) power supply, and the second electrode  145  is connected to ground, forming a capacitance between the two electrodes  143 ,  145 . 
         [0025]    In one embodiment, the lid assembly  140  includes one or more gas inlets  154  that are at least partially formed within an upper section  156  of the first electrode  143 . The one or more process gases enter the lid assembly  140  via the one or more gas inlets  154 . The one or more gas inlets  154  are in fluid communication with the plasma cavity  150  at a first end thereof and coupled to one or more upstream gas sources and/or other gas delivery components, such as gas mixers, at a second end thereof. 
         [0026]    In one embodiment, the first electrode  143  has an expanding section  155  that houses the plasma cavity  150 . In such embodiment, the expanding section  155  is an annular member with an inner surface  157  that gradually increases from an upper portion  155 A to a lower portion  155 B. As such, the distance between the first electrode  143  and the second electrode  145  is variable. 
         [0027]    In one embodiment, the expanding section  155  resembles a cone or “funnel.” In another embodiment, the inner surface  157  of the expanding section  155  gradually slopes from the upper portion  155 A to the lower portion  155 B of the expanding section  155 . The slope or angle of the inner surface  157  may vary depending on process requirements. The length or height of the expanding section  155  may also vary depending on specific process requirements. 
         [0028]    The expanding section  155  is in fluid communication with the gas inlet  154 . The first end of the one or more gas inlets  154  may open into the plasma cavity  150  at the upper most point of the inner diameter of the expanding section  155 . Similarly, the first end of the one or more gas inlets  154  may open into the plasma cavity  150  at any height interval along the inner surface  157  of the expanding section  155 . Although not shown, two gas inlets  154  may be disposed at opposite sides of the expanding section  155  to create a swirling flow pattern or “vortex” flow into the expanding section  155  which helps mix the gases within the plasma cavity  150 . 
         [0029]    The lid assembly  140  may include an isolator ring  160  to electrically isolate the first electrode  143  from the second electrode  145 . The isolator ring  160  may be made from aluminum oxide or any other insulative, process compatible material. Preferably, the isolator ring  160  substantially surrounds at least the expanding section  155 . 
         [0030]    The lid assembly  140  may further include a distribution plate  170  and blocker plate  175  adjacent the second electrode  145 . The second electrode  145 , distribution plate  170  and blocker plate  175  may be stacked and disposed on a lid rim  178  which is connected to the chamber body  112 . A hinge assembly (not shown) may be used to couple the lid rim  178  to the chamber body  112 . The lid rim  178  may include an embedded channel or passage  179  for housing a heat transfer medium. The heat transfer medium may be used for heating, cooling, or both, depending on the process requirements. 
         [0031]    In one embodiment, the second electrode  145  may include a plurality of gas passages or apertures  165  formed beneath the plasma cavity  150  to allow gas from the plasma cavity  150  to flow therethrough. The distribution plate  170  is substantially disc-shaped and also includes a plurality of apertures  172  or passageways to distribute the flow of gases therethrough. The apertures  172  may be sized and positioned about the distribution plate  170  to provide a controlled and even flow distribution to the chamber body  112 , where the substrate to be processed is located. Furthermore, the apertures  172  prevent the gases from impinging directly on the substrate surface by slowing and re-directing the velocity profile of the flowing gases, as well as evenly distributing the flow of gases to provide an even distribution of gases across the surface of the substrate. 
         [0032]    In one embodiment, the distribution plate  170  includes one or more embedded channels or passages  174  for housing a heater or heating fluid to provide temperature control of the lid assembly  140 . A resistive heating element (not shown) may be inserted within the passage  174  to heat the distribution plate  170 . A thermocouple may be connected to the distribution plate  170  to regulate the temperature thereof. The thermocouple may be used in a feedback loop to control electric current applied to the heating element. 
         [0033]    Alternatively, a heat transfer medium may be passed through the passage  174 . The one or more passages  174  may contain a cooling medium, if needed, to better control temperature of the distribution plate  170 , depending on the process requirements within the chamber body  112 . Any heat transfer medium may be used, such as nitrogen, water, ethylene glycol, or mixtures thereof, for example. 
         [0034]    In one embodiment, the lid assembly  140  may be heated using one or more heat lamps (not shown). Typically, the heat lamps are arranged about an upper surface of the distribution plate  170  to heat the components of the lid assembly  140  including the distribution plate  170  by radiation. 
         [0035]    An optional blocker plate  175  may be disposed between the second electrode  145  and the distribution plate  170 . The blocker plate  175  may be removably mounted to a lower surface of the second electrode  145 . The blocker plate  175  may be in thermal and electrical contact with the second electrode  145 . In one embodiment, the blocker plate  175  may be coupled to the second electrode  145  using a bolt or similar fastener. The blocker plate  175  may also be threaded or screwed onto an outer diameter of the second electrode  145 . 
         [0036]    The blocker plate  175  includes a plurality of apertures  176  to provide a plurality of gas passages from the second electrode  145  to the distribution plate  170 . The apertures  176  may be sized and positioned about the blocker plate  175  to provide a controlled and even flow distribution the distribution plate  170 . 
         [0037]    The support assembly  180  may include a support member  185  to support a substrate (not shown) for processing within the chamber body  112 . The support member  185  may be coupled to a lift mechanism  186  through a shaft  187  which extends through a centrally-located opening  114  formed in a bottom surface of the chamber body  112 . The lift mechanism  186  may be flexibly sealed to the chamber body  112  by a bellows  188  that prevents vacuum leakage from around the shaft  187 . The lift mechanism  186  allows the support member  185  to be moved vertically within the chamber body  112  between a process position and a lower, transfer position. The transfer position is slightly below the opening  114  of the slit valve formed in a sidewall of the chamber body  112 . 
         [0038]    In one embodiment, the support member  185  has a flat, circular surface or a substantially flat, circular surface for supporting a substrate to be processed thereon. The support member  185  may be constructed of aluminum. The support member  185  may include a removable top plate  190  constructed of some other material, such as a ceramic material, for example, to reduce backside contamination of the substrate. 
         [0039]    In one embodiment, the substrate (not shown) may be secured to the support member  185  using a vacuum chuck. In another embodiment, the substrate (not shown) may be secured to the support member  185  using an electrostatic chuck. 
         [0040]    The support member  185  may include one or more bores  192  formed therethrough to accommodate a lift pin  193 . Each lift pin  193  is typically constructed of ceramic or ceramic-containing materials and is used for substrate-handling and transport. The lift pin  193  is moveable within its respective bore  192  by engaging an annular lift ring  195  disposed within the chamber body  112 . The lift ring  195  is movable such that the upper surface of the lift-pin  193  may be located above the substrate support surface of the support member  185  when the lift ring  195  is in an upper position. Conversely, the upper surface of the lift-pin  193  is located below the substrate support surface of the support member  185  when the lift ring  195  is in a lower position. Thus, part of each lift-pin  193  passes through its respective bore  192  in the support member  185  when the lift ring  195  moves from either the lower position to the upper position. 
         [0041]    The support assembly  180  may further include an edge ring  196  disposed about the support member  185 . In one embodiment, the edge ring  196  is an annular member that is adapted to cover an outer perimeter of the support member  185  and protect the support member  185  from deposition. The edge ring  196  may be positioned on, or adjacent to, the support member  185  to form an annular purge gas channel between the outer diameter of support member  185  and the inner diameter of the edge ring  196 . The annular purge gas channel may be in fluid communication with a purge gas conduit  197  formed through the support member  185  and the shaft  187 . The purge gas conduit  197  may be in fluid communication with a purge gas supply (not shown) to provide a purge gas to the purge gas channel. Any suitable purge gas such as nitrogen, argon, or helium, may be used alone or in combination. In operation, the purge gas flows through the conduit  197 , into the purge gas channel, and about an edge of the substrate disposed on the support member  185 . Accordingly, the purge gas working in cooperation with the edge ring  196  prevents deposition at the edge and/or backside of the substrate. 
         [0042]    The temperature of the support assembly  180  may be controlled by a fluid circulated through a fluid channel  198  embedded in the body of the support member  185 . In one embodiment, the fluid channel  198  is in fluid communication with a heat transfer conduit  199  disposed through the shaft  187  of the support assembly  180 . The fluid channel  198  may be positioned about the support member  185  to provide a uniform heat transfer to the substrate receiving surface of the support member  185 . The fluid channel  198  and heat transfer conduit  199  may flow heat transfer fluids to either heat or cool the support member  185 . Any suitable heat transfer fluid may be used, such as water, nitrogen, ethylene glycol, or mixtures thereof. The support assembly  185  may further include an embedded thermocouple (not shown) for monitoring the temperature of the support surface of the support member  185 . For example, a signal from the thermocouple may be used in a feedback loop to control the temperature or flowrate of the fluid circulated through the fluid channel  198 . 
         [0043]    The support member  185  may be moved vertically within the chamber body  112  so that a distance between support member  185  and the lid assembly  140  may be controlled. A sensor (not shown) may provide information concerning the position of support member  185  within chamber  100 . 
         [0044]    In operation, the support member  185  may be elevated to a position in close proximity of the lid assembly  140  to control the temperature of the substrate being processed. As such, the substrate may be heated via radiation emitted from the distribution plate  170 . Alternatively, the substrate may be lifted from the support member  185  to a position in close proximity to the heated lid assembly  140  using the lift pins  193  activated by the lift ring  195 . 
         [0045]      FIG. 2  is a schematic top-view diagram of an illustrative multi-chamber processing system  200  that may be adapted to perform processes as disclosed herein. The system  200  may include one or more load lock chambers  202 ,  204  for transferring substrates into and out of the system  200 . A first robot  210  may transfer the substrates between the load lock chambers  202 ,  204 , and a first set of substrate processing chambers  212 ,  214 ,  216 ,  218 . Each processing chamber  212 ,  214 ,  216 ,  218 , may be outfitted to perform a number of substrate processing operations including the dry etch processes described herein in addition to atomic layer deposition (ALD), chemical vapor deposition (CVD), physical vapor deposition (PVD), etch, pre-clean, and other substrate processes. 
         [0046]    The first robot  210  may also transfer substrates to and from transfer chambers  222 ,  224 . The transfer chambers  222 ,  224  may be used to maintain vacuum conditions while allowing substrates to be transferred within the system  200 . A second robot  230  may transfer the substrates between the transfer chambers  222 ,  224  and a second set of processing chambers  232 ,  234 ,  236 ,  238 . Similar to processing chambers  212 ,  214 ,  216 ,  218 , the processing chambers  232 ,  234 ,  236 ,  238  may be outfitted to perform a variety of substrate processing operations. Any of the substrate processing chambers  212 ,  214 ,  216 ,  218 ,  232 ,  234 ,  236 ,  238  may be removed from the system  200  if not necessary for a particular process. 
         [0047]      FIGS. 3A-3N  are sectional schematic views of an illustrative fabrication sequence for forming an active electronic device, such as a MOSFET structure  300  using the bottom contact clean process described. The MOSFET structure  300  may include a combination of (i) dielectric layers, such as silicon dioxide, organosilicate, carbon doped silicon oxide, phosphosilicate glass (PSG), borophosphosilicate glass (BPSG), silicon nitride, or combinations thereof; (ii) semiconducting layers such as doped polysilicon, and n-type or p-type doped monocrystalline silicon; and (iii) electrical contacts and interconnect lines formed from layers of metal or metal silicide, such as tungsten, tungsten silicide, titanium, titanium silicide, cobalt silicide, nickel silicide, or combinations thereof. Each layer may be formed using any one or more depositions techniques, such as atomic layer deposition (ALD), physical vapor deposition (PVD), chemical vapor deposition (CVD), or plasma enhanced chemical vapor deposition (PECVD), for example. 
         [0048]    Fabrication of the active electronic device begins by forming electrical isolation structures that electrically isolate the active electronic device from other devices. Several types of electrical isolation structures exist as generally described in VLSI Technology, Second Edition, Chapter 11, by S. M. Sze, McGraw-Hill Publishing Company (1988), which is incorporated herein by reference. Referring to  FIGS. 3A-3N , the illustrative MOSFET structure  300  may be formed on a semiconductor material, for example a silicon or gallium arsenide substrate  325 . A field oxide layer (not shown) having a thickness of about 2,000 Å is first grown over the entire substrate  325  and portions of the oxide layer are removed to form the field oxide barriers  345 A, B which surround exposed regions in which the electrically active elements of the device are formed. The exposed regions are thermally oxidized to form a thin gate oxide layer  350  having a thickness of from about  50  to 300 Å. A polysilicon layer is then deposited, patterned, and etched to create a gate electrode  355 . The surface of the polysilicon gate electrode  355  may be reoxidized to form an insulating dielectric layer  360 . 
         [0049]    Referring to  FIG. 3B , the source and drain  370 A, B may next be formed by doping the appropriate regions with one or more suitable dopant atoms. For example, on p-type substrates, an n-type dopant species comprising arsenic or phosphorous may be used. The doping may be performed by an ion implanter and may include, for example, phosphorous ( 31 P) at a concentration of about 10 13  atoms/cm 2  at an energy level of from about 30 to 80 Kev, or Arsenic ( 75 As) at a dose of from about 10 15  to 10 17  atoms/cm 2  and an energy of from 10 to 100 Kev. After the implantation process, the dopant may be driven into the substrate  325  by heating the substrate, for example, in a rapid thermal processing (RTP) apparatus. Thereafter, the oxide layer  350  (shown in  FIG. 3A ) covering the source and drain regions  370 A, B may be stripped in a conventional stripping process to remove any impurities caused by the implantation process, which are trapped in the oxide layer. 
         [0050]    Referring to  FIGS. 3C and 3D , a silicon nitride layer  375  may be deposited on the gate electrode  355  and the surfaces on the substrate  325  by low-pressure chemical vapor deposition (LPCVD) using a gas mixture of SiH 2 , Cl 2 , and NH 3 . The silicon nitride layer  375  may then be etched using reactive ion etching (RIE) techniques to form sidewall spacers  380  on the sidewall of the gate electrode  355 , as shown in  FIG. 3D . The electrical isolation sidewall spacers  380  and overlayers may be fabricated from other materials, such as silicon oxide. The silicon oxide layers used to form sidewall spacers  380  may be deposited by CVD or PECVD from a feed gas of tetraethoxysilane (TEOS) at a temperature in the range of from about 600° C. to about 1,000° C. 
         [0051]    Referring to  FIG. 3E , a native silicon oxide layer  385  may be formed on exposed silicon surfaces by exposure to atmosphere during transfer of the substrate  325  between processing chambers and/or processing systems. The native silicon oxide layer  385  may increase the electrical resistance of the semiconducting material and adversely affect the silicidation reaction of the silicon and metal layers that are subsequently deposited. Therefore, it is necessary to remove this native silicon oxide layer  385  prior to forming metal silicide contacts or conductors for interconnecting active electronic devices. A clean process, such as an NH 3 /NF 3  clean process, as described in U.S. patent application Ser. No. 11/063,645 filed on February  22 ,  2005 , which is herein incorporated by reference, may be used to remove the native silicon oxide layers  385  to expose the source  370 A, drain  370 B, and the top surface of the gate electrode  355  as shown in  FIG. 3F . 
         [0052]    Referring to  FIG. 3G , a PVD sputtering process may be used to deposit a layer of metal  390 . Suitable conductive metals include cobalt, titanium, nickel, tungsten, platinum, and any other metal that has a low contact resistance and that can form a reliable metal silicide contact on both polysilicon and monocrystalline silicon. Alloys or a combination of two or more metals may also be used. 
         [0053]    Conventional furnace annealing may then be used to anneal the metal and silicon layers to form metal silicide in regions in which the metal layer  390  is in contact with silicon. The anneal is typically performed in a separate processing system. Accordingly, a protective cap layer (not shown) may be deposited over the metal  390  prior to the anneal step. The cap layer is typically of a nitride-containing material and can include titanium nitride, tungsten nitride, tantalum nitride, hafnium nitride, and silicon nitride. The cap layer may be deposited by any deposition process, such as by PVD. Annealing typically involves heating the substrate  325  to a temperature of between 500° C. and 800° C. in an atmosphere of nitrogen for about  30  minutes. Alternatively, a rapid thermal annealing process can be used in which the substrate  325  is rapidly heated to about 1,000° C. for about 30 seconds. 
         [0054]    The cap layer and unreacted portions of the metal layer  390  may be removed by a wet etch using aqua regia, (HCl and HNO 3 ), which removes the metal without attacking the metal silicide, the sidewall spacer  380 , or the field oxide  345 A, B, thus leaving a self-aligned metal silicide contact  392 A on the source  370 A, a self-aligned metal silicide contact  392 B on the drain  370 B, and a self-aligned metal silicide contact  392 C on the gate  355 , as shown in  FIG. 3H . The sidewall spacers  380  electrically isolate the metal silicide layer  392 C formed on the top surface of the gate  355  from the other metal silicide layers  392 A,  392 B deposited over the source  370 A and drain  370 B. 
         [0055]    Thereafter, an insulating cover layer  393  of, for example, silicon oxide, carbon doped silicon, BPSG, or PSG, may be deposited on the metal silicide  392 A,  392 B,  392 C as shown in  FIG. 3I . The insulating cover layer  393  may be deposited by chemical vapor deposition techniques in a CVD chamber, in which the material condenses from a feed gas at low or atmospheric pressure, as for example, described in commonly assigned U.S. Pat. No. 5,500,249, issued Mar. 19, 1996, which is incorporated herein by reference. Thereafter, the structure  300  is annealed at glass transition temperatures to form a smooth planarized surface. 
         [0056]    The insulating cover layer  393  may then be etched to form contact holes  394 A,  394 B,  394 C as shown in  FIG. 3J . The structure  300  may then be transferred to a wet clean chamber to remove any etch residuals. As a result of this transfer, native oxides  395  may form on the contact surfaces  392 A,  392 B,  392 C, as shown in  FIG. 3K . 
         [0057]    Next, the structure  300  may be subjected to a clean process to remove the native oxides  395  from metal silicide contact surfaces  392 A,  392 B, and  392 C as shown in  FIG. 3L . Preferably, the native oxides  395  are removed using a NF 3 /NH 3  remote plasma process, which saturates at a low etch amount according to the present invention, to prevent over etching of the contact hole upper and sidewall surfaces while fully removing the native oxide from the bottom contact metal silicide surface. This process is described in greater detail as follows. 
         [0058]    In one embodiment of this process, the structure  300  is first cooled below about 65° C., such as between about 15° C. and about 50° C. The structure  300  is preferably maintained below 50° C. In one embodiment, the structure  300  may be maintained at a temperature between about 22° C. (i.e. room temperature) and about 40° C. The nitrogen trifluoride (NF 3 ) and ammonia (NH 3 ) gases are then mixed to form a cleaning gas mixture. The amount of each gas introduced into the chamber  100  is variable and can be adjusted to accommodate, for example, the thickness of the oxide layer to be removed, the geometry of the structure being cleaned, the volume capacity of the plasma, the volume capacity of the chamber body, as well as the capabilities of the vacuum system coupled to the chamber body. 
         [0059]    In one embodiment, the gases are added to provide a gas mixture having a greater concentration of nitrogen trifluoride than ammonia. Preferably, the gases are introduced into the chamber  100  at a molar ratio of from 1.1:1 (nitrogen trifluoride to ammonia) to about 30:1. More preferably, the molar ratio of the gas mixture is of from about 1.5:1 (nitrogen trifluoride to ammonia) to about 5:1. The molar ratio of the gas mixture may fall between 1.1:1 and 1.5:1. The molar ratio of the gas mixture may also fall between about 5:1 (nitrogen trifluoride to ammonia) and about 15:1. 
         [0060]    A purge gas or carrier gas may also be added to the gas mixture. Any suitable purge/carrier gas may be used, such as argon, helium, hydrogen, nitrogen, or mixtures thereof. The overall gas mixture may be from about 0.05% to about 60% by volume of nitrogen trifluoride and ammonia. The remainder of the gas mixture is the purge/carrier gas. In one embodiment, the purge/carrier gas is first introduced into the chamber body  112  before the reactive gases to stabilize the pressure within the chamber body  112 . 
         [0061]    The operating pressure within the chamber  100  may be variable. The pressure may be maintained between about 500 mTorr and about 30 Torr. Preferably, the pressure is maintained between about  1  Torr and about 10 Torr. More preferably, the operating pressure is maintained between about 3 Torr and about 7 Torr. 
         [0062]    An RF power between about 5 and about 600 Watts is preferred to ignite a plasma of the gas mixture. Preferably, the RF power is less than about 100 Watts. More preferably, RF power is between about 50 Watts and about 70 Watts. 
         [0063]    The plasma energy dissociates the nitrogen trifluoride and ammonia gases into reactive species, e.g. fluorine radicals and/or hydrogen radicals, that combine to form a highly reactive ammonia fluoride (NH 4 F) compound and/or ammonium hydrogen fluoride (NH 4 F.HF) in the gas phase. These molecules are then delivered from the remote plasma location to the surface to be cleaned where they combine with the oxide to form a thin, by-product film. The thin, by-product film may be a salt comprising nitrogen and fluorine atoms. In one embodiment, the thin, by-product film may be ammonium hexafluorosilicate (NH 4 ) 2 SiF 6 . A purge/carrier gas may be used to facilitate the delivery of the reactive species to the surface. 
         [0064]    After the thin film is formed on the surface, the surface is annealed to remove the thin film. The anneal temperature should be sufficient to dissociate or sublimate the thin film into volatile ammonia and fluorine-containing products. Typically, a temperature of about 75° C. or more is used to effectively dissociate and remove the thin film from the substrate. Preferably, a temperature of about 100° C. or more is used, such as between about 115° C. and about 200° C. 
         [0065]    This oxide removal process has a very low etch saturation point. That is, the etch reaction is completed at a very low etch amount, such as 20-50 Å. Accordingly, the etch at the upper and sidewall surfaces of the contact hole saturates very quickly, while the bottom contact etch completes the native oxide removal. Therefore, unlike previous recipes, the upper surfaces of the contact opening are minimally affected, while the bottom contact is optimally cleaned. This improved process results in an electronic device with decreased contact resistance and minimal leakage current. 
         [0066]    Thereafter, the cleaned structure  300  may be treated with a silicon-containing compound to recover the metal silicide contact surface  392 A,  392 B,  392 C. 
         [0067]    After the metal silicide contact surface  392 A,  392 B,  392 C has been recovered, one or more liner or barrier layers  396  may be deposited on the substrate, as shown in  FIG. 3M . The barrier layer  396  may contain any one or more refractory metals deposited by any deposition technique capable of providing good step coverage. For example, the barrier layer  396  may include titanium, tantalum, or tungsten deposited by one or more physical vapor deposition techniques. The barrier layer  396  may also include one or more refractory metal nitrides. 
         [0068]    In one embodiment, a first layer  396  (i.e. “liner” layer) containing a refractory metal may be deposited followed by a second layer  397  (i.e. “barrier” layer) containing a refractory metal nitride, as shown in  FIG. 3N . For example, a titanium liner layer may be deposited followed by a titanium nitride layer. In either layer, the refractory metal may be tantalum or tungsten in lieu of or in addition to titanium. 
         [0069]    Thereafter, the contact holes  394 A,  394 B,  394 C are at least partially filled with a bulk metal layer  398 , as shown in  FIG. 3N . Illustrative metals include, but are not limited to, copper, tungsten, titanium, and tantalum. 
         [0070]    Although the process sequence above has been described in relation to the formation of a MOSFET device, the etch process described may also be used to form other semiconductor structures and devices that have other metal silicide layers, for example, silicides of tungsten, tantalum, molybdenum. The cleaning process can also be used prior to the deposition of layers of different metals including, for example, aluminum, copper, cobalt, nickel, silicon, titanium, palladium, hafnium, boron, tungsten, tantalum, or mixtures thereof. Further, the cleaning process can be used to remove oxides formed on a substrate surface in addition to native oxides. For example, oxides may result due to chemical etch processes performed on the substrate, photoresist strip processes performed on the substrate, wet clean processes, and any other oxygen based process. 
         [0071]    While the 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, and the scope thereof is determined by the claims that follow.