Patent Publication Number: US-2009236664-A1

Title: Integration scheme for constrained seg growth on poly during raised s/d processing

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     The present application is a divisional of U.S. patent application Ser. No. 11/150,923, filed on Jun. 13, 2005, entitled “INTEGRATION SCHEME FOR CONSTRAINED SEG GROWTH ON POLY DURING RAISED S/D PROCESSING,” (Attorney Docket No. 1458-H1956) which is incorporated by reference in its entirety herein. 
    
    
     FIELD OF THE INVENTION 
     The present disclosure relates generally to a semiconductor device and manufacturing process, and more particularly to epitaxial silicon layers formed during a semiconductor manufacturing process. 
     BACKGROUND 
     The selective epitaxial growth (SEG) of thin silicon or silicon-germanium films can be used to achieve a raised source/drain region in complementary metal oxide semiconductor (CMOS) and bipolar CMOS (BiCMOS) devices utilizing silicon-on-insulator (SOI) technology. 
     However, epitaxial growth on the gate stack polysilicon results in a “mushroom-shaped” cap, due to unconstrained lateral growth during the epitaxial silicon growth process. When the lateral growth of this cap is excessive, a potential path for increased leakage current between the polysilicon of the gate structure and contacts landed on the source/drain area is created. In the worst case, such cap growth provides a potential for shorts between the polysilicon of the gate structure and the landed source/drain contacts, due to their close proximity. 
     Therefore, a method which overcomes the problems of excessive cap growth which is easily integrated into existing production facilities would be useful. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure may be better understood by referencing the accompanying drawings. It will be appreciated that for simplicity and clarity of illustration, elements illustrated in the Figures have not necessarily been drawn to scale. For example, the dimensions of some of the elements are exaggerated relative to other elements for clarity. 
         FIGS. 1 through 11  illustrate, in cross section, semiconductor device manufacturing process steps according to at least one embodiment of the present disclosure; and 
         FIG. 12  is a flow diagram illustrating a method for determining a desired thickness of an epitaxial layer according to at least one embodiment of the present invention. 
     
    
    
     The use of the same reference symbols in different drawings indicates similar or identical items. 
     DESCRIPTION OF THE PREFERRED EMBODIMENT(S) 
     The present disclosure provides methods for constraining epitaxial growth on polysilicon gates to the vertical direction during an epitaxial silicon growth process. The methods are readily integrated into existing manufacturing process flows in production facilities. Further, the methods avoid problems created by epitaxial overgrowth, e.g., the “mushroom cap,” resulting from unconstrained epitaxial lateral growth on gate structures. 
       FIGS. 1 through 6  illustrate, in cross-section, a location  200  of a semiconductor device during a manufacturing process according to an embodiment of the present disclosure. At the manufacturing stage presented in  FIG. 1 , the following blanket layers have been formed overlying a substrate  10 : gate dielectric layer  12 ; a conductive gate layer  14 , and a sacrificial layer  16 . 
     Semiconductor substrate  10  can be a mono-crystalline substrate or a gallium arsenide substrate. Alternately, substrate  10  can be semiconductor-on-insulator (SOI), silicon on sapphire, or the like. Gate dielectric layer  12  can be formed by an oxide or other insulative material. Conductive gate layer  14  typically comprises doped poly silicon, but may comprise other conductive materials. Sacrificial layer  16  is used, at least in part, as a sacrificial layer to define the height of a subsequently formed sidewall spacer. The thickness of sacrificial layer  16  can be selected to depend upon the desired thickness of the silicon epitaxial growth layer that is grown in later processing steps. The sacrificial layer  16  can comprise a hard mask, such as an oxide comprising tetra ethyl orthosilicate (TEOS), amorphous carbon or other insulative material. The term “sacrificial layer” is used because, relative to the final gate structure, the layer is used for sacrificial purposes. However, the sacrificial layer may have other functions, such as an anti-reflective or etch stop layer for other processes not illustrated. 
     In  FIG. 2 , photolithography and etch processes have been conducted on portion  200  to form gate structure  18 . Gate structure  18  comprises a gate stack  20  and a sacrificial gate layer  26 . Gate stack  20  includes gate dielectric  22  and conductive gate layer  24  formed from layers  12  and  14 , respectively. 
     A conformal spacer layer  30  has been formed overlying the gate structure  18 . Spacer layer  30  can be formed by various deposition techniques such as PECVD (Plasma Etch Chemical Vapor Deposition), CVD (Chemical Vapor Deposition), LPCVD (Low Power Chemical Vapor Deposition), and the like. The spacer layer  30  is typically formed using a material that allows the use of an etch process selective to the material of the spacer layer  30 . In an embodiment, an immediately adjacent top portion of the gate stack  20  is formed of a material that is selective to the sacrificial layer  26 . In embodiments where the sacrificial layer  26  is an oxide, spacer layer  30  may be a nitride such as silicon nitride. In another embodiment, spacer layer  30  may be an oxide and sacrificial layer  26  may be a nitride. The spacer layer  30  will subsequently be etched to form spacers, as seen in  FIG. 3 . 
       FIG. 3  illustrates location  200  following etch of spacer layer  30  to form D-shaped spacers  31  adjacent to gate structure  18 . The spacers  31  are formed to have an uppermost portion that is above the interfacial region between layers  24  and  26 . In  FIG. 3 , the uppermost portion of spacer  31  is shown to be substantially the same height as the uppermost surface of sacrificial layer  26 . Generally, location  200  of  FIG. 2  is subjected to an anisotropic etch process to etch spacer layer  30  to form spacers  31 . Although spacers  31  are shown as being D-shaped spacers, in another embodiment, L-shaped spacers can be formed, such as are illustrated in  FIG. 11 . 
     In embodiments where spacer layer  30  is an oxide, the etch processes employed to form spacers  31  are preferably selective to the underlying sacrificial layer  26 , i.e., nitride. For example, when the sacrificial layer  26  is a nitride and the spacer layer  30  is an oxide, an etch chemistry utilizing CF4/CHF3 to etch the oxide may be used. Alternatively, in embodiments where spacer layer  30  is a nitride, CF4/HBr may be used in the etch process, in order to remove the spacer layer  30  and leave the sacrificial layer  26 . In specific embodiments, spacer etch processes can be monitored to detect an etch endpoint. 
       FIG. 4  illustrates location  200  after removal of the sacrificial layer  26  to leave gate stack  20 . Following the spacer  31  formation, the sacrificial layer  26  is removed. Typically the sacrificial layer  26  is removed with an etch selective to the sidewalls  31 . For example, in embodiments where spacers  31  are a nitride, the selective etch to remove sacrificial layer  26  is typically an etch process that has a high oxide-to-nitride selectivity, thereby leaving the sidewall substantially unetched. In embodiments where the protective layer  26  is amorphous carbon, the layer can be removed by being selectively ashed away in a plasma stripping process. 
     As a result of the removal of sacrificial layer  26 , an uppermost portion of the sidewalls  31  extends above the top surface of the gate conductive layer  24 , and will cause subsequent silicon epitaxial growth on the conductive gate portion  24  to proceed in a vertical direction, with no lateral growth, as seen in  FIG. 6 . Once the selective removal of the protective layer is complete, portion  200  is ready for further processing, for example, a dopant implantation process to create doped regions in gate conductive layer  24  and in active regions adjacent to the gate structure  20 , as seen in  FIG. 5 . 
       FIG. 5  illustrates location  200  during a dopant implantation process  35 . The doping process creates doped active regions  32 , which may be deep source/drain (S/D) regions. Note that extension regions, not shown in  FIG. 5 , may be previously formed. In addition, a doped portion  34  is formed within the conductive gate layer  24 . The dotted line within conductive gate layer  24  is intended to indicate one possible outcome for the extent of the doped region  34 . In other embodiments, the dopant implantation process may occur following the formation of an epitaxial growth layer, or the dopant implantation may be through the entire thickness of the conductive gate structure. Note that the conductive gate structure  24  may be doped or undoped silicon prior to the implantation illustrated in  FIG. 5 . 
     When undoped prior to  FIG. 5 , the conductive gate layer  24  would be in a semiconductive state, and the doping process  35  would be used to facilitate making the conductive layer  24  conductive. As illustrated in  FIG. 5 , a region  33  is shown underlying the gate structure  20  in the substrate  10 . Typically, this region will include extension implants (not shown). In accordance with one embodiment, the substrate  10  is a SOI substrate, with layer  11  being a semiconductive layer, and layer  13  being an insulative layer. 
       FIG. 6  illustrates, in cross-section, the location  200  of  FIG. 5  following a silicon epitaxial growth process used to create layers  38  and  36  overlying silicon-comprising portions  32  and  34  respectively. Layer  38  is a raised source/drain region. Because of the selective removal of the sacrificial layer  26 , the sidewalls  31 , which extend above the gate structure  24 , constrain epitaxial layer  36  to having a vertical component, and no lateral component relative to the footprint of the conductive gate  24 . Therefore, as shown in  FIG. 5 , the epitaxial layer  36  overlying the gate conductive layer  34 , where the sacrificial layer  26  was formed, is constrained to the footprint of the top of the gate conductive layer  34 . Hence, no “mushroom cap” is present when utilizing the teachings of the present disclosure. 
     The epitaxial layers  38  and  36  may be doped, or undoped, depending upon device requirements. For example, the epitaxial layers  38  and  36  in  FIG. 6  can remain undoped, when the underlying S/D regions  32  were doped prior to growth of the silicon epitaxial layers  138  and  136 . In other embodiments, the epitaxial layers  138  and  136  may be doped. 
       FIG. 7  illustrates a cross sectional view of a location  300  of a semiconductor device after silicon epitaxial growth of layers  138  and  136 , but before any doping has occurred. In  FIG. 7 , the regions  132  in substrate portion  111 , and the conductive gate layer  134  have not undergone dopant implantation. Various options exist for dopant implantation, depending upon device requirements. As previously mentioned, it will be appreciated that there may be one or more doping steps prior to forming epitaxial layers followed by subsequent doping of epitaxial layers. Alternatively, doping semiconductor layer  111  can occur prior to epitaxial  138 ,  136  layer growth, with no doping of the epitaxial layer  138 ,  136 . Another alternative is to not dope prior to epitaxial layer  138 ,  136  growth. 
     As illustrated in the embodiment of  FIG. 8 , the epitaxial layer  236  has been formed to have a thickness that is less than the thickness of the sacrificial layer previously formed and removed overlying the conductive gate layer  134 , thereby resulting in the top of the epitaxial layer  236  being below the topmost portion of the sidewall spacers  131 . In another embodiment, e.g.,  FIG. 7 , the thickness of the epitaxial layer is substantially the same as the thickness of the sacrificial layer  26 . 
       FIG. 9  illustrates an embodiment where the thickness of a top portion of the epitaxial layer  336  is greater than the sacrificial layer originally formed while the bottomed portion is laterally constrained. This may result in a slight lateral spread during the epitaxial process, as seen in mushrooming of the epitaxial layer  336  over the tops of sidewalls  131 . However, the amount of lateral spread is reduced over that previously obtainable. The amount by which the epitaxial layer thickness is greater than the thickness of the sacrificial layer formed will vary based upon the device technology, e.g., the proximity of contacts, but will typically be less than 50%. 
       FIG. 10  illustrates a cross-sectional view of a location  400  of a semiconductor device manufactured according to an embodiment of the present disclosure.  FIG. 10  is a simplified illustration which does not show all of the features of the device in order to keep the illustration from being cluttered. In  FIG. 10 , the regions  438  and the gate region  436 , created using the epitaxial process as disclosed herein, are shown integrated into the transistor at location  400 . Location  400  is further illustrated to have interconnects  441  and  442 , which are connected to vias/contacts  443  and  444  within an interconnect dielectric region  440 . A passivation layer  450  has been formed overlying portion  400 . 
     In portion  400 , a gate stack overlies a silicon layer  411  of a semiconductor substrate  410 . The stack includes a gate dielectric  422 , conductive gate portion  434 , and an epitaxially-grown layer  436  having an interface surface immediately adjacent to the upper surface of the gate portion  434 . Substrate  410  includes a semiconductor layer  411  that comprises channel region  433 , and source/drain regions  432 . When substrate  410  is an SOI wafer, the layer  413  represents a buried oxide (BOX) layer. D-shaped spacers  431  immediately adjacent to the conductive gate portion  434  have been formed according to the teachings herein. 
     The present disclosure provides for many variations in the finished device. For example, in an alternate embodiment illustrated by the location  500  of  FIG. 11 , L-shaped spacers  537  may be utilized instead of the standard D-shaped spacers  531  shown in  FIGS. 3-10 . The principle difference between  FIG. 10  and  FIG. 11  is the choice of shape of the spacer. 
       FIG. 12  is a flow diagram illustrating a methodology to determine a desired thickness of an epitaxial layer to be formed over an SOI substrate. At step  601 , a determination is made as to the desired thickness of the raised source/drain (S/D) layer, that is, the thickness of the epitaxial layer which will form the raised S/D. At step  602 , a thickness of a sacrificial layer to be formed overlying a polysilicon gate portion is determined based upon the desired height of the epitaxial layer. These desired height and thickness values would then be provided to a semiconductor device fabrication facility at step  603 . The manufacturer would, in turn, fabricate a device based upon the desired height and thickness values. 
     The method and apparatus herein provides for a flexible implementation. Although described using certain specific examples, it will be apparent to those skilled in the art that the disclosure is not limited to these few examples. For example, the disclosure is discussed herein primarily with regard to nitride spacer formation and use of a sacrificial layer to serve as a hard mask for subsequent epitaxial silicon growth for a CMOS device, however, the disclosure can be employed with other device technologies to create spacers and avoid exceeding lateral boundaries created by SEG processes during device manufacture. Additionally, various types of deposition and etch devices are currently available which could be suitable for use in employing the method as taught herein. In addition, although this disclosure has been discussed primarily with regard to the use of an oxide sacrificial and nitride spacers, this does not preclude the use of a nitride sacrificial layer and oxide spacers, or the use of an organic anti-reflective coating (ARC) layer as the sacrificial layer or hard mask over the gate structure. Other combinations of materials and suitable etch chemistries suitable to practice the teachings herein may be utilized. Note also, that although an embodiment of the present disclosure has been shown and described in detail herein, along with certain variants thereof, many other varied embodiments that incorporate the teachings of the invention may be easily constructed by those skilled in the art. Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. Accordingly, the present invention is not intended to be limited to the specific form set forth herein, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents, as can be reasonably included within the spirit and scope of the invention. 
     It will be appreciated that the above disclosure can be implemented using a variety of techniques. For example, it will be appreciated that any number of substrate preclean steps can occur before the formation of any epitaxial layer. For example, U.S. patent application having Ser. No. 10/791,346, which is hereby incorporated in its entirety by reference, discloses several substrate preclean techniques appropriate for cleaning a substrate prior to forming an epitaxial layer. 
     In one example, contaminates on the surface of a substrate are subjected to a cleaning process comprising applying a plasma to a surface of the active regions produce a reduction reaction with the contaminates in an upper portion of the surface of the active regions. In an embodiment, the plasma comprises H 2 . While the plasma is being applied to the upper portion of the exposed active regions, the resultant products or vapor byproducts of the reduction reaction are removed by the normal vacuum process within the chamber. Therefore, contaminates contained in the vapor byproducts and are vented away, leaving the upper portion of the surface of the active regions suitably clean for the ensuing epitaxial process. In one embodiment, the plasma process parameters comprise a gas flow of 450 sccm H2 and 300 sccm argon, at a chamber temperature of 400 degrees Celsius, with an high frequency (HF) power setting of 700 W, and a low frequency (LF) power setting of between approximately 50 to 100 W. Chamber pressure is 1 Torr, and the spacing between the surface of the active region and the faceplate of the tool (not shown) should be 300 mils. In other embodiments, plasma process parameters comprise a gas flow ranging from between 100-800 sccm H 2  and from between 100 and 600 sccm argon. Chamber temperatures can range between 300 to 450 degrees Celsius, and HF power settings from between 400-900 W, with LF power settings varying from between 0-150 W. Chamber pressures can range from between 1 mT-5 Torr, with spacing between the surface of the active region and the faceplate of the tool varying from between 200 to 400 mils. Exposure times for the various embodiments utilizing plasma range from between approximately 10 seconds up to approximately 120 seconds. 
     Various tool types are suitable for this cleaning, for example, CVD (Chemical Vapor Deposition) equipment, HDP (High Density Plasma) tools, etch chambers, or the like. Differences in chamber design, power settings, and species, e.g., H 2  with or H 2  without helium or nitrogen, will result in different thickness of the layer after anneal. Typically the layer after anneal will be between 20 and 50 Angstroms thick. This plasma cleaning process also results in passivation of Si—H bonds in the layer after anneal. No wet cleaning dip with hydrofluoric (HF) acid prior to SEG is necessary. 
     In addition to no longer requiring an HF dip prior to SEG, the reduced temperature of this H 2  plasma cleaning treatment results in a reduction of the SEG process thermal budget of more than 100 degrees Celsius. Typically pre-SEG cleaning processes are conducted at approximately 900 degrees Celsius or greater. In an embodiment of the present disclosure, the cleaning process occurs at less than approximately 800 degrees Celsius. In another embodiment, the cleaning process occurs at less than approximately 500 degrees Celsius or less. In addition, the cleaning processes of the present disclosure could be conducted at less than approximately 700 degrees Celsius or less, or even at less than approximately 600 degrees Celsius or less. 
     In another embodiment, location including includes a gate structure and active regions is subjected to a cleaning process utilizing a low-power dry etch to selectively remove an upper atomic layer of material from the active regions. The thickness of the upper atomic layer of material to be removed ranges from between 20 to about 50 Angstroms. In one embodiment, the dry etch process is an anisotropic dry etch utilizing a carbon-free gas as an etchant gas. In another embodiment, the anisotropic dry etch utilizes an oxygen- and carbon-free gas as an etchant gas. The etchant gas can comprise HBr, NF 3 , SF 6 , gaseous fluorine-interhalogenics such as ClF 3 , or any gas containing fluorine, suitable to disassociate F-radicals, which does not contain oxygen and carbon. Prior to undergoing the anisotropic dry etch process, location  200  is subjected to a standard wet etch chemistry process utilizing a dilute HF solution (100:1) at room temperature, e.g., 20 to 26 degrees Celsius, for a time period ranging from 50 to 200 seconds. Following the HF clean, a low-power dry etch utilizing a temperature of approximately 400 degrees Celsius, RF power of approximately 375 W, pressure of approximately 150 mTorr, and a gas flow rate ranging from 50 to 100 sccm, is conducted. In other embodiments, the low-power dry etch utilizes a temperature ranging from between 300-500 degrees Celsius, with RF power ranging from between 200-700 W, a pressure ranging between 0-1 Torr, and a gas flow rate ranging from between 10-300 sccm, for a time ranging between 10 to 60 seconds. 
     This low-power dry etch removes carbon and oxygen contamination, and provides a very clean surface for SEG. The low temperature HF clean followed by the low-power dry etch does not require a high temperature bake. This results in a reduction of thermal budget for SEG of more than 100 degrees Celsius. 
     In another embodiment, a cleaning process is used that forms an oxidation layer of between 20 to 50 Angstroms on an upper surface of the active regions using a plasma to produce the oxidation layer on doped active regions. In an embodiment, the plasma is an O 2  plasma. In another embodiment, the plasma is an O 3  plasma. 
     An O 2  plasma production utilizes O 2  gas at a flow rate of 400 sccm, a pressure of 5 Torr, an HF of 300 W, an LF of 100 W, and a temperature of 400 degrees Celsius, with the time ranging from between about 10 to about 120 seconds. The spacing between the surface of the active regions and the faceplate of the vapor deposition apparatus (not shown) should be 400 mils. In other embodiments, the plasma production utilizes O 2  gas at a flow rate of between 100 and 1000 sccm, a pressure ranging from between 2-10 Torr, an HF ranging between 200-500 W, an LF ranging between 50-200 W, a temperature ranging between 300-450 degrees Celsius, for a time ranging from between approximately 10 to approximately 120 seconds. In an embodiment, the spacing between the surface of the active regions and the faceplate of the vapor deposition apparatus ranges from between 200 and 600 mils. The tool type used to generate the plasma could be CVD equipment, HDP tools, or etch chambers. In an embodiment where the plasma is O 3 , plasma production utilizes O 3  gas at a flow rate of 300 sccm, a pressure of 5 Torr, an HF of 300 W, an LF of 100 W, and a temperature of 400 degrees Celsius for a time period ranging from between 10 to 120 seconds. The spacing between the surface of the active regions and the face plate of the vapor deposition apparatus (not shown) should be 400 mils. In other embodiments, plasma production utilizes O 3  gas at a flow rate of between 50 and 600 sccm, a pressure ranging from between 2-10 Torr, an HF ranging between 200-500 W, an LF ranging between 50-200 W, and a temperature ranging from between 300-450 degrees Celsius for a time period ranging from between about 10 to about 120 seconds. In an embodiment, the spacing between the surface of the active regions and the faceplate of the vapor deposition apparatus ranges from between 200 and 600 mils. As was the case with the O 2  plasma, the tool type used to generate the plasma could be HDP tools, CVD equipment, or etch chambers. 
     Forming the oxidation layer facilitates trapping or fixing contamination in the oxide layer overlying the upper layer of the doped active regions for subsequent removal using a wet chemistry process. The wet etch chemistry process utilizes a dilute HF acid solution of 100:1 at room temperature, e.g. 20 to 26 degrees Celsius, for a time ranging from 50 to 200 seconds. Differences in chamber design, power settings and species employed, e.g., O 2  or O 3 , results in differing thickness of the oxidation layer, hence the wide range in times for the HF dip. The use of an O 2  or O 3  plasma to create a contamination-trapping oxidation layer for removal by a room temperature HF dip results in a reduction of the thermal input for location  300 . 
     Another possible pre-clean for use prior to formation of an SEG is disclosed in U.S. patent application having Ser. No. 10/969,769, (Attorney Docket Number 1458-H1962 Concurrently herewith) which is hereby incorporated in its entirety by reference, discloses another substrate preclean technique that facilitates a reduced temperature H 2  bake is performed following formation of any desired spacers, which can comprise one or more nitride or oxide layers and prior to SEG formation. This pre-clean and comprises a first pre-rinse with deionized water, followed by an oxide etch utilizing an aqueous solution of deionized water and hydrofluoric acid (HF or hydrogen fluoride in water) aqueous solution of approximately 30:1 (volumetric ratio) at 21 degrees Celsius, for a time period ranging from between 50-60 seconds. The weight percentage of HF recommended for the HF aqueous solution is 49% in a balance of deionized water (H 2 O). Bulk HF aqueous solution can be purchased from various chemical suppliers in the HF weight percent range of 10% to 49%. In semiconductor fabrication facilities, this aqueous HF aqueous solution is typically diluted in the range 10:1 to 200:1. A 10:1 HF is 1 part aqueous HF (at 49% weight percent) and 10 parts H 2 O. It will be appreciated that the etch rate of the HF aqueous solution is substantially linear with respect to both the concentration of the HF aqueous solution and the etch time. Therefore, various combinations of HF concentrations and etch times can be used to accomplish the oxide etch. Additionally, the temperature may vary. 
     After the HF etch, an overflow rinse utilizing deionized water is performed for a period ranging from approximately 120 to 600 seconds with a typical rinse being about 400 seconds. The cleaning process of portion  100  results in etching away of the surface contamination/debris located on substrate  10  resulting from offset spacer formation and/or dopant implantation. The upper semiconductor surface, i.e. silicon surface, of substrate  10  is also slightly etched, for example, from one to several mono layers of silicon, during the HF etch. 
     It should be noted that the amount of material removed during the HF etch is dependent upon the type of material being removed. For example, when native oxide is present, the HF etch will remove approximately 20 to 30 Angstroms of oxide. If a deposited oxide layer is present in addition to a native oxide, an over-etch of approximately 30% is generally desirable. For example, if removal of 100 Angstroms of a chemical vapor deposition (CVD) oxide is desired, the HF etch could be employed to remove approximately 120 to 130 Angstroms oxide removal. This latter example would be applicable in applications where a liner oxide of approximately 100 Angstroms thickness is employed between a conductive gate  25  and a nitride spacer. 
     The next steps in the cleaning process comprise a second pre-rinse with deionized water of approximately 30 seconds duration precedes the performance of a Standard Clean-1 (SC-1), a quick dry rinse (QDR), and a Standard Clean-2 (SC-2). The SC-1 and SC-2 components are followed by a second QDR, and an HF: H 2 O etch, a third rinse, and an isopropyl alcohol (IPA) dry. The amount of material removed by the SC-1 and SC-2 components are implemented such that they etch from approximately one monolayer of silicon to approximately 10 to 100 Angstroms of silicon. 
     In an embodiment, the SC-1 utilizes an aqueous solution of ammonium hydroxide: hydrogen peroxide: deionized water at a ratio of approximately 1:1-4:6-40, at a temperature of approximately 60 degrees Celsius for approximately 72 minutes, to etch approximately 100 Angstroms of silicon. Synonyms for ammonium hydroxide (NH 4 OH) include ammonia solution (typically contains between 12% and 44% ammonia before dilution), dilute ammonia, or concentrated ammonia. A first quick dry rinse is conducted for approximately 3 minutes. In an embodiment, the SC-2 utilizes a solution of hydrochloric acid: hydrogen peroxide: deionized water at an initial ratio of approximately 1:1:50 at a temperature of approximately 60 degrees for about 5 minutes. A second quick dry rinse is then conducted. Synonyms for hydrochloric acid (HCl) are hydrogen chloride, anhydrous hydrogen chloride, aqueous hydrogen chloride, chlorohydric acid, spirit of salts, and muriatic acid. 
     In a particular embodiment, the SC-1 utilizes a solution of ammonium hydroxide: hydrogen peroxide: deionized water at a ratio of approximately 1:4:20 at a temperature ranging of approximately 60 degrees Celsius for approximately 72 minutes. The SC-1 is the step in the clean sequence that etches the silicon. This occurs because the H 2 O 2  (the oxidizer) becomes depleted in the solution with increasing time and increasing temperature. The methods of the present disclosure allow the initial concentration of hydrogen peroxide to be depleted to facilitate etching of the upper-most semiconductor portion. Depletion of the H 2 O 2  is greatly enhanced when the solution temperature rises above 80 degrees Celsius, which can lead to an etch that is difficult to control if not carefully monitored. The temperature range of the SC-1 is expected to be approximately 55 to 85 degrees Celsius, with the etch occurring in a shorter period of time at higher temperatures than at lower temperatures. It is expected that the SC-1 etching will be better controlled at temperatures in the range of 55-80 degrees Celsius and better still at temperatures in the range of 55-75 degrees Celsius. Generally, it is expected that the substrate will be exposed to the SC-1 etch process for longer that 60 minutes. When the oxidizer stops protecting the silicon surface, the ammonium hydroxide (NH 4 OH) starts to etch the silicon. Thus, a small amount of silicon can be etched in a controlled manner. The SC-1 can be performed in a re-usable bath where the solution is re-circulated and heated to maintain the desired temperature. 
     The mechanism of silicon and SiO 2  etching by a NH 4 OH/H 2 O 2  solution occurs when the solution is allowed to be depleted of H 2 O 2 . An alkaline solution, such as NH4OH4 in our example, will attack silicon by water molecules, according to the reaction: 
       Si+2H 2 O+2OH − →Si(OH) 2 (O − ) 2 +2H 2 ↑ 
     A passivation layer formed by the H 2 O 2  prevents this attack by the NH 4 OH. H 2 O 2  decomposes in the course to form O 2  and H 2 O. 
       H 2 O 2 —→H 2 O+½O 2    
     When the concentration of H 2 O 2  is below 3×10 −3 M, then silicon will begin to etch, because of the absence of the inhibition layer. 
     As indicated in the above equations, heat is given off as the H 2 O 2  is depleted. If a bath is used that is not recharged with fresh solution all H 2 O 2  will be depleted, thereby no longer releasing heat. Therefore, the temperature can be monitored on the low end to indicate when the solution should be refreshed, while the temperature on the high end is monitored to prevent unusually rapid decomposition of the H 2 O 2 , which can lead to a process that is difficult to control. 
     The first quick dry rinse is conducted for approximately 3 minutes. The subsequent SC-2 utilizes a solution of hydrochloric acid: hydrogen peroxide: deionized water at a ratio of approximately 1:1:50 at a temperature of approximately 60 degrees for about 5 minutes. A quick dry rinse with deionized water, followed by an IPA dry process, is performed following the SC-2. 
     The IPA dry process uses a heated IPA vapor at approximately 82 degrees Celsius. The IPA vapor is generated in a separate chamber with 100% N 2  bubbled through 100% IPA (heated to 82 degrees Celsius). The IPA condenses on the wafer, and the solution drips off the bottom of the wafer. The IPA vapor concentration is slowly diluted to 100% N 2  before the wafers are removed from the rinsing/drying tank. 
     Subsequent to the SC-1 and SC-2 processes, the substrate will be further recessed (etched) as a result of the cleaning process. Next, an HF: H2O etch can be conducted at an aqueous solution ratio of 200:1 for about 65 seconds, which typically results in approximately 30 Angstroms of oxide removal. The HF: H2O etch 8 is followed by a rinse with deionized water for approximately 10 minutes duration. The deionized water rinse is followed by an IPA dry as described in the preceding paragraph. At this time, the source/drain regions of the substrate are ready for ion implantation or selective epitaxial growth. 
     In a particular embodiment, the SC-1 process comprises a pre-rinse with deionized water of approximately 30 seconds duration. The pre-rinse is followed by a SC-1 solution at a ratio of approximately 1:1-4:6-40, which includes the subranges of 0.25:1:5, 0.5:1:5, 1:1:5, 1:1:6, 1:4:20, and 1:1:40, ammonium hydroxide: hydrogen peroxide: deionized water at a temperature of approximately 60 degrees Celsius for approximately 5 minutes. A quick dry rinse (QDR) is then performed for approximately 3 minutes. 
     Following the SC-1 cleaning process, an SC-2 cleaning process is performed. In an embodiment, the SC-2 cleaning process includes utilizing an aqueous solution of hydrochloric acid: hydrogen peroxide: deionized water at a ratio of approximately 1:1:50 at a temperature of approximately 60 degrees Celsius for approximately 5 minutes. A QDR is then performed, and portion  200  is ready for the third cleaning. The weight percent composition of the hydrochloric acid: hydrogen peroxide: deionized water is 29% (weight percent) hydrochloric acid and 30% (weight percent) hydrogen peroxide in a balance of deionized water. 
     After the SC-1 and SC-2, a third cleaning process comprising an approximate 30 second pre-rinse, an oxide etch, an overflow rinse and an IP dry is performed. The oxide etch is accomplished utilizing a solution of deionized water and hydrofluoric acid at a ratio of approximately 200:1 for a time period ranging from between 450-650 seconds. Following the HF etch, an overflow rinse is performed for approximately 10 minutes. A final isopropyl alcohol (IPA) dry is then performed. Approximately 120-140 Angstroms of the surface of substrate  20  is removed in this process. Portion  200  is ready to undergo selective epitaxial growth. 
     The above-described cleaning process has been found to facilitate formation of an epitaxial layer on a semiconductor surface, specifically silicon. Because various etch processes can etch N- and P-type regions at different rates, it can be useful to amorphize an upper-most surface of the source/drain regions prior to the above-described clean to reduce any preferential etch differences between substrate regions of differing dopant types. 
     For example, the above-described clean process can etch the N-type silicon preferentially, as compared to the P-type silicon, resulting in a quality difference of the SEG between the N and P regions after SEG processing. Etch rate differences between N- and P-type regions can allow for contaminates to remain in the lesser-etched region. For example, an etch process that does not etch P-type regions at the same rate as N-type regions can result in P-regions maintaining embedded carbon that is incorporated from previous process steps. Without appropriate etching of silicon in the P-type regions during the clean, the carbon will remain, and the SEG will grow inconsistently. A high bake temperature of 900° C. can be used to overcome this growth issue on P areas, however, as stated previously, high bake temperatures can be detrimental to the device in that it causes diffusion and deactivation of the dopants. Amorphizing the source/drain regions can reduce etch differences associated with the above-described cleaning process as well as other processes that are used to etch doped substrate regions, thereby improving the quality of both the N and P regions. 
     It has been observed that the selective etching may be P-type over N-type, or N-type over P-type depending on the solution temperature, flow rate of the aqueous ammonia, concentration of the aqueous ammonia, agitation, or illumination of light. By amorphizing the silicon in this manner to a pre-defined depth, it has been observed that unbiased etching to the depth of the amorphized silicon can be achieved. 
     In one embodiment, N- and P-type extensions formed in the source/drain regions are amorphized by being implanted with the Xe, at a dose of 2E14 and energy of 10 keV, to create an amorphous depth of 100A. 
     In accordance with another embodiment, a spacer structure having an undercut can be used to reduce or inhibit facet formation during a selective epitaxial growth process. Such a process can allow for greater lateral uniformity of junction or silicide features during implantation or silicidation processes, and can be accomplished by using a spacer formed with a bi-layer of materials, e.g., a thin liner, such as portion  29  of  FIG. 1 , of one material underlying another layer of material from which the ‘main’ spacer is formed. The thin liner and other material layer are selected such that the two materials are selectively etchable with respect to the other, for example, a thin oxide liner and a nitride layer. By etching the underlying portion of the spacer, an undercut can be formed that reduces facets during epitaxial formation. 
     In another embodiment, a method of germanium-content engineering can be used during a selective epitaxial growth (SEG) process to form raised source drain regions, such that the germanium-content is engineered to facilitate subsequent cobalt-silicidation, or for nickel silicide processes. For example, U.S. patent application having Ser. No. 10/969,774 (Attorney Docket Number 1458-HI955), which is hereby incorporated in its entirety by reference, discloses such a technique. 
     The SEG formation process commences with a germanium content on the order of between approximately 3-5% to ensure good growth conditions at high growth rates for both N- and PMOS. The germanium content is reduced during growth of the upper portion of the SEG layer (raised source/drain region) to provide a good substrate for subsequent cobalt silicidation. Thus the raised source drain region comprises a first portion nearest the semiconductor substrate having a Ge content greater than a second portion of the raised source drain furthest from the substrate. Due to the reduction in germanium during growth of the upper portion of the SEG layer, the germanium-to-silicon ratio in the first portion closest the substrate will be different (typically greater than) from the germanium-to-silicon ratio in the uppermost portion of the SEG layer. 
     This method permits increased throughput for SEG at reduced thermal budget, as well as the ability to continue using cobalt rather than nickel for the silicide layer. Using a graded SEG as described allows for a self-limiting or self-stopping cobalt silicidation process, due to the higher conversion temperatures required to create cobalt-silicide in the presence of germanium. Utilizing the methods during the manufacture of CMOS devices results in reduced junction leakage. A reduction in junction leakage results in improved device performance. 
     In one embodiment, a dopant profile for a Ge-to-Si ratio where a portion of SEG  38  closest to an underlying substrate will typically have a Ge-to-Si ratio in the range from 15% to 35%. While the portion of SEG  38  furthest from the substrate  10  typically has a reduced Ge- to Si ratio in the range from 0% to 2.5%. Note that listed percentages are atomic percentages unless otherwise stated. 
     Following the formation of the Ge-gradient in the source/drain regions, a silicidation process is carried out to form a silicide layer overlying a portion of the raised source/drain region. In an embodiment, the silicide layer comprises cobalt disilicide (CoSi 2 ). In an embodiment, the cobalt disilicide is formed at a temperature ranging from 600 to 800 degrees Celsius. Formation of the cobalt disilicide further comprises depositing a cobalt metal, performing a first anneal at a temperature ranging from 450 to 550 degrees Celsius, performing a wet strip with a sulfuric peroxide mixture followed by a wet strip with an ammonium peroxide mixture, and performing a second anneal at a temperature ranging from 600 to 800 degrees Celsius. 
     In another embodiment, the silicide layer comprises nickel silicide (NiSi). In an embodiment, the nickel silicide is formed at a temperature ranging from 350 to 500 degrees Celsius. The nickel silicide is formed by depositing a nickel metal and performing an anneal at a temperature ranging from 350 to 500 degrees Celsius. Following the anneal, a wet strip with a sulfuric peroxide mixture is performed, followed by a wet strip with an ammonium peroxide mixture. It should be noted that more than one anneal may be utilized to form the nickel silicide, e.g., a two step anneal process such as a first anneal at a temperature ranging from 300 to 400 degrees Celsius following nickel metal deposition, performing the wet strips, then performing a second anneal at a temperature ranging from approximately 400 to 500 degrees Celsius. 
     The method and apparatus herein provides for a flexible implementation. Although described using certain specific examples, it will be apparent to those skilled in the art that the examples are illustrative, and that many variations exist. For example, the disclosure is discussed herein primarily with regard to independent control of the placement of a silicide and the amount of offset of a source/drain region from a gate structure for a CMOS device, however, the invention can be employed with other device technologies to create deep source/drain offsets and determine silicide location during device manufacture. Additionally, various types of deposition and etch devices are currently available which could be suitable for use in employing the method as taught herein. Note also, that although an embodiment of the present invention has been shown and described in detail herein, along with certain variants thereof, many other varied embodiments that incorporate the teachings of the invention may be easily constructed by those skilled in the art. Benefits, other advantages, and solutions to problems have been described above with regard to specific embodiments. However, the benefits, advantages, solutions to problems, and any element(s) that may cause any benefit, advantage, or solution to occur or become more pronounced are not to be construed as a critical, required, or essential feature or element of any or all the claims. Accordingly, the present invention is not intended to be limited to the specific form set forth herein, but on the contrary, it is intended to cover such alternatives, modifications, and equivalents, as can be reasonably included within the spirit and scope of the invention.