Patent Publication Number: US-2005136580-A1

Title: Hydrogen free formation of gate electrodes

Description:
RELATED APPLICATIONS  
      This application is related to U.S. patent application Ser. No. ______, (TI Attorney Docket No. TI-35222) filed on Dec. 22, 2003, entitled HYDROGEN FREE INTEGRATION OF HIGH-K GATE DIELECTRICS, wherein the entirety of this application is hereby incorporated by reference as if fully set forth herein. 
    
    
     FIELD OF INVENTION  
      The present invention relates generally to semiconductor processing, and more particularly to fabricating semiconductor devices employing high-k dielectric materials.  
     BACKGROUND OF THE INVENTION  
      Field effect transistors (FETs) are widely used in the electronics industry for switching, amplification, filtering, and/or other tasks related to both analog and digital electrical signals. Most common among these are MOSFETs, wherein a metal or (doped) polysilicon gate contact or electrode is energized to create an electric field in an underlying channel region of a semiconductor body, by which current is allowed to conduct between a source region and a drain region of the semiconductor body.  
      The source and drain regions are typically formed by adding dopants to targeted regions on either side of the channel region in a semiconductor substrate. A gate dielectric or gate oxide, such as silicon dioxide (SiO 2 ), is formed over the channel region to physically separate the gate electrode from the substrate, and more particularly the channel region. A patterned gate electrode and gate dielectric is commonly referred to as a gate structure or stack.  
      The gate dielectric has electrically insulative properties and, as such, serves to retard the flow of large electrical currents between the gate electrode and the source/drain regions or channel of the substrate when a voltage is applied to the gate contact. The gate dielectric also serves to allow the applied gate voltage to set up an electric field in the channel region in a controllable manner.  
      A continuing trend in the manufacture of semiconductor products is toward a steady reduction in the size of electrical devices (known as scaling), together with improvements in device performance in terms of device switching speed, power consumption, reliability, etc. New materials and processes have been developed and employed in silicon processing technology to accommodate these requirements, including the ability to pattern and etch smaller device features. Recently, however, electrical and physical limitations have been reached in the thickness of gate dielectrics, particularly those formed of silicon dioxide.  
      By way of example,  FIG. 1  illustrates a conventional complementary MOS (CMOS) device  2  with PMOS and NMOS type transistor devices  4  and  6 , respectively, formed in or on a silicon substrate  8 . Isolation structures  10 , such as shallow trench (oxide) isolation structures (STI), are formed within the substrate  8  to electrically isolate the devices from one another as well as from other surrounding devices. For example, one or both of the transistors may be included as part of an integrated circuit or used in any other appropriate manner.  
      The substrate  8  in the above example is lightly doped p-type silicon with an n-well  12  formed therein under the PMOS transistor  4 . The PMOS device  4  includes two laterally spaced p-doped source/drain regions  14   a  and  14   b  with a channel region  16  located therebetween in the n-well  12 . A gate is formed over the channel region  16  comprising an SiO 2  gate dielectric  20  overlying the channel  16  and a conductive polysilicon gate contact structure  22  formed over the gate dielectric  20 .  
      The NMOS device  6  includes two laterally spaced n-doped source/drain regions  24   a  and  24   b  outlying a channel region  26  in the substrate  8  (or alternatively a p-well region (not shown)) with a gate formed over the channel region  26  comprising an SiO 2  gate dielectric layer  30  and a polysilicon gate contact  32 , where the gate dielectrics  20  and  30  may be patterned from the same oxide layer. Both the PMOS device  4  and the NMOS device  6  include sidewall spacers  18  that aid in doping the respective source/drain regions  14   a ,  14   b  and  24   a ,  24   b.    
      Referring to the NMOS device  6 , for example, the resistivity of the channel  26  may be controlled by the voltage applied to the gate contact  32 , where changing the gate voltage changes the amount of current through channel  26 . The gate contact  32  and the channel  26  are separated by the SiO 2  gate dielectric  30 , which is an insulator. The gate dielectric, thus, allows little or no current to flow between the gate contact  32  and the channel  26 . The gate dielectric  30  allows the gate voltage at the contact  32  to induce an electric field in the channel  26 , by which the channel resistance can be controlled by the applied gate voltage.  
      MOSFET devices produce an output current proportional to the ratio of the width over the length of the channel (W/L), where the channel length is the physical distance between the source/drain regions (e.g., between regions  24   a  and  24   b  in the device  6 ) and the width runs perpendicular to the length (e.g., perpendicular to the page in  FIG. 1A ). Thus, scaling the NMOS device  6  to make the width narrower may reduce the device output current. Previously, this characteristic has been accommodated by decreasing the channel length and decreasing the thickness of gate dielectric  30 , thus bringing the gate contact  32  closer to the channel  26 .  
      Additionally, the thickness and dielectric constant of the gate dielectric layer  30  are typically chosen to create a gate capacitance appropriate for a particular use of the transistor  6 , where the gate capacitance, among other things, controls the formation of the electrical field in channel region  26 . The gate capacitance is directly proportional to the dielectric constant of gate dielectric layer  30  and inversely proportional to the thickness of gate dielectric layer  30 . Therefore, as the other features of transistor  6  are scaled down, the thickness of gate dielectric layer  30  may also be scaled down proportionally to maintain an appropriate gate capacitance (assuming the dielectric constant of the material remains the same).  
      However, making the gate dielectric layer  30  thinner can have undesirable results, particularly where the gate dielectric  30  is SiO 2 . One shortcoming of a thin SiO 2  gate dielectric  30  is increased gate leakage currents due to tunneling through the oxide  30 . Additionally, since the films are literally formed from a few layers of atoms (monolayers), very precise process controls are required to uniformly and repeatably produce the layers. Uniform coverage is important because device parameters may change based upon the presence or absence of even a single monolayer of dielectric material. Also, a thin SiO 2  gate dielectric layer  30  provides a poor diffusion barrier to dopants. In this manner, boron, for example, may be allowed to penetrate into and contaminate the underlying channel region  16  during doping of an overlying poly-silicon gate.  
      Consequently, recent efforts involving MOSFET device scaling have focused on alternative dielectric materials that can be made thicker than scaled silicon dioxide layers and yet still produce the same field effect performance. These materials are often referred to as high-k materials because their dielectric constants are greater than that of SiO 2 , (which is about 3.9). The relative performance of such high-k materials is often expressed as equivalent oxide thickness (EOT), because, while the alternative layer may be thicker, it still provides the equivalent electrical effect of a much thinner layer of SiO 2 .  
      Accordingly, high-k dielectric materials can be utilized to form gate dielectrics, where the high-k materials facilitate a reduction in device dimensions while maintaining a consistency of desired device performance. By way of example, conventional gate dielectrics (e.g., of silicon oxide (SiO 2 )) can have thicknesses of about 1-3 nanometers, whereas high-k gate dielectrics have thicknesses on the order of 2-10 times greater, yet exhibit comparable electrical performance to the thinner SiO 2 . The larger thickness tends to minimize leakage through the gate dielectric, among other things.  
      Referring to  FIG. 2 , one proposed alternative structure is illustrated, in which a high-k gate dielectric material  30 ′ is used to form a gate dielectric layer in an NMOS device  6 ′. A conductive gate electrode structure  32 ′ is formed over the high-k dielectric layer  30 ′. While such a high-k dielectric layer  30 ′ assists in mitigating of some of the issues encountered with device scaling, other issues may persist, however. For example, hydrogen and/or hydrogen containing compounds are commonly utilized in many of the stages of semiconductor fabrication, and hydrogen can react with high-k dielectric materials such as hafnium oxide and adversely affect the construction and/or electrical properties thereof.  
      Hydrogen based precursors, such as SiH 4 , for example, are used extensively in producing epitaxial silicon, polycrystalline silicon and certain dielectrics, such as Si 3 N 4  and SiO 2 . These fabrication processes expose the high-k dielectrics to high concentrations of hydrogen which can etch, embrittle or otherwise react with the high-k dielectric materials to reduce or otherwise adversely affect the high-k materials. Additionally, atomic hydrogen (e.g., H) is often produced in semiconductor fabrication processes as certain (transition) metals utilized in the process are known to “crack” hydrogen gas (H 2 ). Atomic hydrogen is a strong etchant of silicon and silicon based compounds, and thus may undesirably reduce many high-k dielectric materials.  
      Hydrogen can thus reduce the high-k dielectric  30 ′ and can also create point defects  50 ′ therein. Such defects  50 ′ can counteract or negate some of the positive aspects of high-k materials by potentially reducing the electrical thickness of the high-k material  30 ′ and increasing the leakage path through the high-k dielectric at these defects, thus leading to the aforementioned issues at the contaminated locations  50 ′. Such defects can also serve as sinks or reservoirs for dopants and/or other electrically active impurities that can fill in the defects  50 ′ and degrade the electrical properties of the dielectrics, including the reliability thereof.  
      Further, such defects  50 ′ disrupt the uniformity of the high-k dielectric material  30 ′ which can adversely affect the operation of the transistor  6 ′ by, among other things, disrupting electromagnetic fields that are developed between the gate electrode  32 ′ and the source  24   a ′, drain  24   b ′ and/or channel  26 ′ regions of the transistor when a bias voltage is applied to the gate electrode  32 ′. This affects the current flowing through the transistor  6 ′ (e.g., I on -I off ), among other things. It will be appreciated that the defects  50 ′ depicted in  FIG. 2  are merely illustrative and that such defects may have a significantly different physical manifestation in reality.  
     SUMMARY OF THE INVENTION  
      The following presents a simplified summary of the invention in order to provide a basic understanding of some aspects of the invention. This summary is not an extensive overview of the invention. It is intended neither to identify key or critical elements of the invention nor to delineate the scope of the invention. Rather, its primary purpose is merely to present one or more concepts of the invention in a simplified form as a prelude to the more detailed description that is presented later.  
      The present invention pertains to forming a transistor in the absence of hydrogen, or in the presence of a significantly reduced amount of hydrogen. In this manner, a high-k material can be utilized to form a gate dielectric layer in the transistor and facilitate device scaling while mitigating defects that can be introduced into the high-k material by the presence of hydrogen and/or hydrogen containing compounds.  
      To the accomplishment of the foregoing and related ends, the following description and annexed drawings set forth in detail certain illustrative aspects and implementations of the invention. These are indicative of but a few of the various ways in which one or more aspects of the present invention may be employed. Other aspects, advantages and novel features of the invention will become apparent from the following detailed description of the invention when considered in conjunction with the annexed drawings 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a partial side elevation view in section illustrating a conventional semiconductor device with NMOS and PMOS transistors.  
       FIG. 2  is a partial side elevation view in section illustrating point defects in a high-k dielectric in a proposed gate structure of a transistor.  
       FIGS. 3-16  are cross-sectional illustrations of a transistor formed in accordance with one or more aspects of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      One or more aspects of the present invention are described with reference to the drawings, wherein like reference numerals are generally utilized to refer to like elements throughout, and wherein the various structures are not necessarily drawn to scale. In the following description, for purposes of explanation, numerous specific details are set forth in order to provide a thorough understanding of one or more aspects of the present invention. It may be evident, however, that one or more aspects of the present invention may be practiced with a lesser degree of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order to facilitate describing one or more aspects of the present invention.  
      The present invention pertains to forming a transistor in the absence of or in the presence of a significantly reduced amount of hydrogen. In this manner, a high-k material can be utilized to form a gate dielectric layer in the transistor and facilitate device scaling while mitigating defects that can be introduced into the high-k material by the presence of hydrogen and/or hydrogen containing compounds.  
       FIGS. 3-8  are provided below to illustrate various stages of fabrication of a semiconductor device formed in accordance with one or more aspects of the present invention. The device is fabricated in the absence of hydrogen to mitigate defects encouraged thereby. As a result, the device has a greater reliability than conventionally formed devices. The stages are provided to illustrate exemplary structures and fabrication techniques that can be implemented in accordance with one or more aspects of the present invention. It is to be appreciated, however, that suitable variations are contemplated herein and that such variations are deemed to be in accordance with one or more aspects of the present invention  
      Initially, a semiconductor substrate  302  has a layer of high-k gate dielectric material  304  applied  305  there-across in the absence of hydrogen ( FIG. 3 ). It is to be appreciated that the term “semiconductor substrate” as used herein can include a base semiconductor wafer (e.g., silicon, SiGe, or an SOI wafer) and any epitaxial layers or other type semiconductor layers formed thereover or associated therewith. It is to be further appreciated that elements depicted herein are illustrated with particular dimensions relative to one another (e.g., layer to layer dimensions and/or orientations) for purposes of simplicity and ease of understanding, and that actual dimensions of the elements may differ substantially from that illustrated herein.  
      Examples of high-k materials that may be used for the gate dielectric layer  304  include, but are not limited to, zirconium silicon oxides, hafnium silicon oxides, aluminum oxide, yttrium oxide, yttrium-silicon-oxides, lanthanum oxide, lanthanum silicon oxides, zirconium aluminate, hafnium aluminate, lanthanum aluminate, aluminum nitride, tantalum oxide, titanium oxide, zirconium oxide, hafnium oxide, zirconium oxynitride, hafnium oxynitride, zirconium silicon oxynitride, and hafnium silicon oxynitride. Any other appropriate high-k dielectric materials may also be used.  
      It will be appreciated that high-k dielectric materials are generally understood to mean materials having a dielectric constant higher than that of silicon dioxide (which is about 3.9). A dielectric material having a k of about 7.8 and a thickness of about 10 nm, for example, is substantially electrically equivalent to an oxide gate dielectric having a k of about 3.9 and a thickness of about 5 nm. It will also be appreciated that the layer of high-k gate dielectric material  304  can be formed  305  across the substrate  302  in any of a number of suitable manners, including, for example, sputtering techniques (e.g., magnetron or ion beam sputtering), chemical vapor deposition (CVD), atomic layer deposition (ALD), etc.  
      A gate electrode layer  306  is then applied  307  over the layer of high-k gate dielectric material  304  ( FIG. 4 ). The gate electrode layer  306  ultimately yields a contact area or surface that provides a means for applying a voltage to the transistor  300  or otherwise biasing the transistor  300 . The gate electrode layer  306  generally includes doped polysilicon, silicon germanium (SiGe) or metal, and can be formed to a thickness of about 200 nanometers or less, for example. A layer of poly-silicon or silicon germanium can be formed via sputtering, chemical vapor deposition (CVD), physical vapor deposition (PVD), molecular beam epitaxy (MBE), atomic layer deposition (ALD) or evaporation, for example, to form the gate electrode layer  306 .  
      In accordance with one or more aspects of the present invention, the gate electrode layer  306  is formed in the absence of hydrogen, or in the presence of a significantly reduced amount of hydrogen (or hydrogen containing compounds). Poly-silicon can, for example, be formed via CVD in the presence of silicon tetrafluoride (SiF 4 ), silicon tetrachloride (SiCl 4 ), silicon tetrabromide (SiBr 4 ) and/or silicon tetra iodide (SiI 4 ). Poly-silicon can similarly be formed in a plasma environment containing argon and/or xenon. A metal gate (e.g., titanium nitride, tantalum silicon nitride, titanium aluminum nitride) can also be formed via CVD using inorganic precursors or using any suitable physical vapor deposition processes (PVD). Since atomic hydrogen can effectively, albeit undesirably, etch high-k materials resulting in point defects, and since some metals can crack molecular hydrogen to form atomic hydrogen, care is taken when metal gates are formed to mitigate the formation of atomic hydrogen.  
      The gate electrode layer  306  and the layer of gate dielectric material  302  are then patterned to form a gate structure  308  ( FIG. 5 ). The gate structure  308  thus comprises a gate dielectric  310  and a gate electrode  312 . It will be appreciated that the gate dielectric  210  and gate electrode  312  are patterned in the absence of hydrogen, or in the presence of a significantly reduced amount of hydrogen (or hydrogen containing compounds) to mitigate altering or damaging the high-k material. For example, an etch chemistry of non-hydrogen containing flurocarbons or chlorocarbons plus oxygen can be applied  313  to layers  304 ,  306  to form the gate dielectric  310  and gate electrode  312 . Examples of such etch chemistries include, but are not limited to, CF 2 , CF 4 , C 2 F 6 , C 4 F 6 , C 4 F 8 , CCl 4 , ClF 3 , NF 3  and SF 6 . In addition, CO or CO 2  may be employed in place of, or in addition to, O 2  to tune the selectivity of the etch. Etch chemistries of Si, SiO 2  and Si 3 N 4  can also be utilized to etch the gate dielectric  310  and gate electrode  312 , for example. Such chemistries may not, however, be optimized for selectivity. Additionally, the etching of the high-k gate dielectric  310  is done at a temperature that is elevated relative to that of the gate electrode  312 . For example, the gate electrode layer may be etched at a temperature of less than about 100 C. (e.g., around 50-70 C.), while the high-k dielectric layer may be etched at a temperature range of about 200-400 C. A wet etch may also, however, be utilized to etch the gate dielectric  310 . For example, supercritical flurocarbons and chlorocarbons may be employed in such a wet etch.  
      After the gate structure  308  is formed, a dopant  314  is applied to the substrate  302  to form source and drain extension regions  316 ,  318  therein ( FIG. 6 ). Such extension regions may, for example, be formed according to HDD (highly doped drain) techniques in the absence of hydrogen. The extension regions abut a channel region  320  within the substrate  302  under the gate structure  308  and facilitate absorption of some of the potential associated with the drain. In this manner, some of this potential is directed away from the drain/channel interface, thereby mitigating the occurrence of hot carriers and the adverse affects associated therewith.  
      By way of example, a p-type dopant (e.g., boron) having a concentration of about 1E19 to 5E20 atoms/cm 3  for a PMOS transistor, or an n-type dopant (e.g., phosphorous) having concentration of about 1E19 to 9.5E20 atoms/cm 3  for an NMOS transistor can be implanted to a depth of about 300-350 Angstroms, for example, to establish the extension regions  316 ,  318 . It will be appreciated, however, that other implant concentrations and penetration depths are contemplated by the present invention, as are additional implantation acts (e.g., to form halo regions—not shown).  
      A first oxide layer  328  (e.g., SiO 2 ) is then applied  329  to the gate structure  308  and exposed portions of the substrate  302  ( FIG. 7 ). The first oxide layer  328  can be formed to a thickness of about 10 to 30 Angstroms, for example. The first oxide layer  328  is sometimes referred to a liner oxide and can, for example, be formed utilizing SiF 4 , SiCl 4 , SiBr 4  or SiI 4  plus oxygen. A nitride layer  330  is then applied  331  over the first oxide layer  328  ( FIG. 8 ), and a second SPACER oxide layer  332  is applied  333  over the nitride layer  330  ( FIG. 9 ).  
      The nitride layer  330  can be formed to a thickness of about 50 to 80 Angstroms and the second oxide layer  332  can be formed to about 400 to 800 Angstroms, for example. It will be appreciated that the first oxide layer  328 , the nitride layer  330  and the second oxide layer  332  can be formed in any number of suitable ways in the absence of hydrogen, such as chemical vapor deposition (CVD), for example. The layers may, for example, be formed in an environment of SiF 4 , SiCl 4 , SiBr 4 , SiI 4 , or TEOS plus oxygen or SiF 4 , SiCl 4 , SiBr 4  or SiI 4  plus nitrogen plasma for silicon dioxide and silicon nitride, respectively. SiO 2  and Si 3 N 4  can, for example, also be deposited with non-hydrogen containing precursors, such as oxygen containing compounds and/or deuterated precursors. These processes can be performed at sub-atmospheric pressure (100&#39;s of Torr to mTorr) and at reduced temperatures (&lt;700° C.).  
      The second oxide layer  332  is then processed  335  (e.g., via dry etching or other suitable reduction techniques) in the absence of hydrogen to reveal oxide sidewall spacers  334 ,  336  adjacent to the gate structure  308  ( FIG. 10 ). Such sidewall spacers can have a width of about 300 to 700 Angstroms, for example. It will be appreciated that this processing is substantially selective such that the underlying nitride layer  330  is generally unaffected by the processing.  
      The nitride layer  330  is then processed  337  (e.g., etched) in the absence of hydrogen to remove nitride material not covered/protected by the oxide sidewall spacers  334 ,  336  ( FIG. 11 ). In the example shown the remaining or residual nitride material  330  has an “L” shape. It will be appreciated that this processing is also substantially selective such that the oxide sidewall spacers  334 ,  336  are generally not affected thereby.  
      A portion of the remaining oxide material  328  is subsequently processed  339  (e.g., etched) in the absence of hydrogen to remove some or all of the exposed portions of the first oxide layer  328  ( FIG. 12 ). Again, this processing is substantially selective such that the remaining (L shaped) nitride material  330  is not affected thereby. In this manner, portions of the first oxide layer  328  underlying the residual nitride material  330  are not affected by the processing. It will be appreciated that variations in the height and/or other dimension(s) of the features  330 ,  334 ,  336  depicted in the Figs. is merely incidental and/or the result of intermediate acts that are generally understood, but that are not shown or described herein.  
      Additional, dopant  340  is then implanted in the absence of hydrogen to form source and drain regions  342 ,  344  adjacent to the channel  320  ( FIG. 13 ). These implants are done at relatively low energies and are substantially blocked by the sidewall spacers  334 ,  336  and the residual nitride material  330 . Accordingly, the sidewall spacers  334 ,  336  and residual nitride material  330  together act as a boundary that guides the dopants  340  in forming the source and drain regions  342 ,  344  in the substrate  302 . By way of example, a dopant of Arsenic or other suitable substance having a concentration of about 5E19 to 5E20 atoms/cm 3  may be implanted at an energy level of about 20 to 50 KeV to provide dopant into silicon to about 300-350 Angstroms to form the source and drain regions  342 ,  344 . It will be appreciated, however, that other implant concentrations, energy levels and/or penetration depths are contemplated as falling within the scope of the present invention.  
      Upper surfaces  350 ,  352 ,  354  of the substrate  302  and the gate electrode  312 , respectively, that are exposed are then salicided ( FIG. 14 ). More particularly, a metal  355  is deposited in a non-hydrogen atmosphere. An annealing process may, for example, also be performed in forming these contacts  350 ,  352 ,  354 , with the un-reacted metal being stripped. These strips are usually performed using wet chemistries that do not contain hydrogen with sufficient activity to affect the high-k dielectric.  
      A layer of nitride material  360  or other pre-metal dielectric (PMD) is then applied  361  over the gate structure  308  and the salicided regions of the substrate  302  in the absence of hydrogen ( FIG. 15 ). This silicon nitride layer can be deposited by PECVD using SiF 4 , SiCl 4 , SiBr 4  or SiI 4  and nitrogen. Vias  362  are then formed within the layer of nitride material  360 , such as by selectively applying one or more non-hydrogen containing etchants  363  such as non-hydrogen containing flurocarbons or chlorocarbons plus oxygen ( FIG. 16 ). The vias can be filled with a conductive material to provide an electrical connection to the gate  308  and the source and drain of the transistor  342 ,  344 .  
      It will be appreciated that the ordering of the stages as set forth herein is not meant to be absolute, and that such ordering can be rearranged, and that any such rearrangement is contemplated as falling within the scope of the present invention. For example, the source  342 , drain  344  and extension regions  316 ,  318  can be formed before or after any of the first oxide  328 , nitride  330  or second oxide  332  layers are formed.  
      Although the invention has been shown and described with respect to one or more implementations, equivalent alterations and modifications will occur to others skilled in the art based upon a reading and understanding of this specification and the annexed drawings. The invention includes all such modifications and alterations and is limited only by the scope of the following claims. In particular regard to the various functions performed by the above described components (assemblies, devices, circuits, etc.), the terms (including a reference to a “means”) used to describe such components are intended to correspond, unless otherwise indicated, to any component which performs the specified function of the described component (i.e., that is functionally equivalent), even though not structurally equivalent to the disclosed structure which performs the function in the herein illustrated exemplary implementations of the invention. In addition, while a particular feature of the invention may have been disclosed with respect to only one of several implementations, such feature may be combined with one or more other features of the other implementations as may be desired and advantageous for any given or particular application. Furthermore, to the extent that the terms “includes”, “having”, “has”, “with”, or variants thereof are used in either the detailed description or the claims, such terms are intended to be inclusive in a manner similar to the term “comprising.”