US 20020031909 A1
A silicide processing method for a thin film SOI device including depositing a metal or an alloy on a gate and a source/drain structure formed in a silicon-on-insulator film, reacting the metal or alloy at a first temperature with the silicon-on-insulator film to form a first alloy, etching the unreacted layer of the metal (or alloy) selectively, depositing a Si film on the first alloy, reacting the Si film at a second temperature to form a second alloy, and etching the unreacted layer of the Si film selectively.
1. A method for fabricating a silicide for a semiconductor device, said method comprising:
depositing a buried oxide layer on a substrate;
applying a silicon layer to said buried oxide layer;
forming a source and drain in said silicon layer;
forming a gate on said layer of silicon; and
depositing a metal or an alloy on said gate and said source and drain, to form said silicide for said semiconductor device.
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reacting said metal or said alloy with said silicon to form a first alloy at said gate and said source/drain structure.
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etching said unreacted layer of said metal or said alloy selectively;
depositing a Si film on said first alloy; and
reacting said Si film to form a second alloy.
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etching said unreacted layer of said Si film selectively.
16. A silicide processing method for a thin film silicon-on-insulator (SOI) device, said method comprising:
depositing a metal or an alloy on a gate and a source and drain formed in a silicon-on-insulator (SOI) film;
reacting said metal or said alloy at a first temperature with said SOI film to form a first alloy;
selectively etching said unreacted layer of said metal or said alloy;
depositing a Si film on said first alloy; and
reacting said Si film at a second temperature to form a second alloy.
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 The subject matter of the present Application was at least partially funded under the Grant No. N66001-97-1-8908 from the U.S. Defense Advanced Research Projects Agency (DARPA).
 1. Field of the Invention
 The present invention generally relates to silicon-on-insulator (SOI) MOSFETs and specifically, a self-aligned silicide (salicide) process for thin film SOI MOSFETs having low resistivity contacts.
 2. Description of the Related Art
 Conventionally, a reduction of a short channel effect in a silicon-on-insulator (SOI) MOSFET has been addressed by using ultra-thin silicon films (e.g., having a thickness substantially within a range of about 50 nm to about 3 nm). However, using an ultra-thin SOI film can result in high source/drain series resistance. A portion of the high source/drain series resistance can be reduced by using a self-aligned silicide (salicide) contact (e.g., for a discussion of salicides, see Lisa T. Su et al., “Optimization of the series resistance in sub—0.2 μm SOI MOSFET's”, Electron Device Letters, 15(9), p. 363, September 1994).
 The conventional salicide process has been limited to bulk or thick SOI films (e.g., for purposes of the invention, a “bulk” or “thick” SOI film is thicker than 100 nm). Reduction of a SOI film thickness to an estimated 10 nm precludes the use of conventional salicide. That is, if the amount of silicon consumed by the formation of the silicide alloy becomes a large portion of the initial SOI film thickness, then the contact area will decrease, leading to an increase in the contact resistance. Further, even if a conventional salicide was used with thin films, there is no guarantee of low parasitic resistance because an ultra-thin silicon film may be completely consumed during the silicide formation. Further, the conventional salicide process can form a metal-rich silicide which is characterized by higher resistance.
 In the case of a thin SOI film, the percentage of the SOI consumed by the silicide considerably affects the series resistance. It has been demonstrated that when 80% or more of the SOI layer is consumed, the series resistance begins to increase as a result of a reduction in the contact area (e.g., see Su et al., supra).
 Alternatively, if the silicide layer is made extremely thin (e.g., less than 30 nm) to avoid consuming the thin SOI film, then the silicide layer loses its efficiency in reducing the series resistance. For example, if the silicide is in the thick regime, then a reduction of the silicide thickness would roughly yield a proportional increase in the series resistance. This linear behavior would hold down to about 20 nm (depending on the silicide metal). A thinner silicide film may exhibit nucleation problems and some of the phases may not form. All of this would lead to a very steep increase in the contact resistance.
 The series parasitic resistance must be minimized in order to facilitate the fabrication of high performance thin film SOI MOSFETs. The conventional salicide process is not applicable to the production of ultra-thin SOI MOSFETs, and therefore a new salicide process is required to overcome the problems of the conventional method.
 Further, the conventional method and structures are deficient in their silicide/SOI interface roughness.
 In view of the foregoing and other problems of the conventional methods and structures, an object of the present invention is to provide a new salicide process applicable to the production of ultra-thin SOI MOSFETs (e.g., having a thickness substantially within a range of about 3 nm to about 100 nm).
 It is a further object to provide a new salicide process in which less of the thin SOI film is consumed, produces a thicker SOI film in a source/drain region, and is a self-aligned process.
 Additionally, a further object of the invention is to stay within the thermal budget allowed for the production of conventional MOSFETs using the conventional salicide process. The thermal budget consists of both the temperature and the time length at which the wafer was held at a given temperature. Typically, to minimize the thermal budget the wafer is annealed by rapid thermal annealing (RTA) to form the silicide alloy. For example, to form the CoSi2 phase from the CoSi phase the wafer is annealed at about 750° C. for 60 seconds.
 In a first aspect of the invention, a method for fabricating a semiconductor device, includes depositing a buried oxide layer on a substrate, applying a silicon layer to the buried oxide layer, forming a source and drain in the silicon layer, forming a gate on the layer of silicon, and depositing a metal on the gate and the source and drain, to form the silicide for the semiconductor device.
 In a second aspect of the present invention, a silicide processing method for a thin film silicon-on-insulator (SOI) device, includes depositing a metal on a gate and a source and drain formed in a silicon-on-insulator (SOI) film, reacting the metal at a first temperature with the SOI film to form a first alloy, selectively etching the unreacted layer of the metal, depositing a Si film on the first alloy, reacting the Si film at a second temperature to form a second alloy, and selectively etching the unreacted layer of the Si film.
 In the method of the present invention, preferably a thin-film of cobalt (Co) is deposited on a substrate and is reacted with silicon (Si) at a low temperature to form an alloy of Co2Si (e.g., having a metal-rich phase). The Co which is not reacted is removed by selective etching.
 This step is similar to the etching step in the conventional salicide processing, but in the conventional process, a higher temperature anneal is used so the etching is usually performed at the CoSi formation stage. After the etching step, a non-crystalline film of Si or poly-Si is deposited and subsequently annealed to form the alloy (CoSi2) followed by selective etching of the un-reacted silicon.
 In this manner, a reaction of Co to initially form Co2Si, minimizes the silicon consumption of the thin SOI film. The consumption of the thin SOI film is additionally reduced by the deposition of a silicon or poly-silicon film on the Co2Si.
 The present invention extends the use of a salicide-like process to thin SOI films, which are expected to be used in future SOI MOSFETs. Such thin-film SOI films will be advantageous in making the devices smaller, reducing the source/drain to substrate overlap capacitance, and eliminating the floating body voltage.
 Further, the invention provides a superior solution to the alternative method(s) which include a raised source/drain by epitaxy.
 The foregoing and other objects, aspects and advantages will be better understood from the following detailed description of a preferred embodiment of the invention with reference to the drawings, in which:
 FIGS. 1-6 illustrate a self-aligned method for forming low resistivity contacts to thin film SOI MOSFETs, and more specifically:
FIG. 1 shows a conventional MOSFET device to be silicided;
FIG. 2 shows a thin film of metal (e.g., Co) deposited on the device of FIG. 1;
FIG. 3 shows the formation of an alloy including the cobalt after exposure to a low-temperature processing;
FIG. 4 shows an amorphous Si film deposition on the alloy;
FIG. 5 shows the device after an annealing step at high temperature; and
FIG. 6 shows the device after un-reacted silicon has been removed by selective etching.
 Referring now to the drawings, and more particularly to FIGS. 1-6, there is shown a preferred embodiment of the method of making of a self-aligned silicide which is applicable to the standard MOSFET structure, and also to non-conventional MOSFETs and structures according to the present invention.
 Referring now to FIG. 1, a conventional MOSFET structure 100 is shown having a substrate 1 formed of silicon, a buried oxide layer 2 (e.g., silicon oxide layer), an SOI layer 3 which thickness noted by tsi, a gate dielectric 6A (e.g., SiO2), sidewall spacers 6B formed of nitride or oxide, a gate 7 (e.g. doped poly-Si, or metal), and a source 4 and a drain 5 maid into the SOI film 3, typically by an implant.
 The inventive method is directed to making a self-aligned silicide which is applicable to the standard MOSFET structure, and also to non-conventional MOSFETs and structures. For ease of discussion, the present invention will be applied to the conventional MOSFET of FIG. 1.
 However, although the process flow is demonstrated using a conventional MOSFET structure, it is applicable to a wide variety of structures. Metals, other than Co, that are used for silicides (e.g., Ti, Ni, Pd, Pt and alloys thereof) can be used with the present invention.
 Referring to FIG. 2, a metal 20 (e.g., Co, Ni, Ti, Pd, Pt or alloys thereof) is deposited in a thickness within a range of about 7 nm, and is reacted with silicon in the source 4, drain 5, and gate 7 regions at a low temperature T1. It is noted that if the temperature is too low, no reaction will take place. On the other hand, if the temperature is too high, then the silicide phase of CoSi will be formed. Since the temperature window over which the metal-rich phase Co2Si is formed is narrow, it is difficult to achieve only this phase during the first anneal. To extend the temperature window, a mixture of 80% Co and 20% Si may be deposited (e.g., by co-sputtering or evaporation from a Si0.2Co0.8 target). The temperature window for the formation of the Co2Si out of the Si0.2Co0.8 mixture is about 337° C. to about 487° C. The use of a 80% Si and 20% Co to extend the temperature window is described in U.S. patent application Ser. No. ______, Cyril Carbal et. al, “Method for self-aligned formation of silicide contacts using metal silicon alloys for limited silicon consumption and for reduction of bridging”, filed on ______, having IBM Docket No. YOR900-0044, incorporated herein by reference.
 As shown in FIG. 3, the alloy Co2Si 30 is formed, as a result of applying the low temperature (e.g., T1) in the anneal process to the structure of FIG. 2. (The thickness depends on the initial Co film thickness. One angstrom of Co yields 1.47 angstroms of Co2Si. The typical Co film thickness is about 7 nm. Using the conversion ratio stated above, a 10.3 nm thick film of Co2Si will be obtained after the first anneal.) An upper layer of the cobalt 20 (e.g., over the alloy Co2Si 30) is unreacted Co 20.
 That is, in the standard process, all of the Co which is deposited over a Si surface will react with the silicon surface and will form a silicide. On the other hand, the Co that was deposited over dielectric surfaces such as the oxide or nitride sidewalls cannot react with the Si surface and will remain as unreacted Co. FIG. 3 demonstrates a case in which some Co is left unreacted on top of the silicide. As described above, this is not desirable in a manufacturing process. Yet, even if this does happen, the overall process of the invention will not be affected except that the silicide film will be thinner than targeted. As such, the robustness of the process is clearly demonstrated. It is noted that in most cases, there will be no unreacted Co.
 For example, the unreacted cobalt 20 has a thickness dependent on the anneal time/temperature, and is removed through selective etching. That is, a too short anneal may leave some of the Co unreacted. If the temperature is too low, then the Co will not react with the SOI film. This step is similar to the etching step in the conventional salicide process with the exception that in the conventional salicide process the un-reacted Co is etched at the stage where CoSi is formed. It is noted that the deposition of the a-Si may be carried out over the CoSi phase. However, by forming the CoSi phase, much more of the Si in the SOI layer will be consumed. An example of a selective etching solution is 10:1 H2O2:H2SO4 at 65° C.
 Next, referring to FIG. 4, an amorphous Si (a-Si) or a poly-Si film 40 is deposited over the alloy Co2Si 30. The a-Si film thickness depend on the initial Co film thickness. One unit of Co would require 0.91 units of Si to form Co2Si, 1.82 units of Si to form CoSi and 3.64 units of Si to form CoSi2. For example, suppose the process starts with a 7 nm Co film which are then reacted with the SOI film to form 10.3 nm film of Co2Si. Assuming that all the Co will diffuse into the top deposited a-Si, then it requires the a-Si film to be about 19 nm thick. A more realistic assumption is that more than half but not all the Co will diffuse into the top film, so that a thinner film is actually needed. The amorphous silicon or polysilicon film 40 is annealed at a high temperature T2, (e.g., T2>T1). The temperature window for the formation of CoSi is about 481° C. to about 625° C. (at about 625° C., CoSi2 will start to form). Typical annealing temperature (T2) for the second anneal is about 750° C. These temperatures may vary slightly depending on the doping species and concentration that were implanted into the SOI film.
 Hereinbelow, the amount of required Si in angstroms per angstroms of metal is described. Forming 1 angstrom of Co2Si will take 0.91 angstrom of Si, CoSi will take 1.82 angstroms of Si, and CoSi2 will take 3.64 angstrom of Si. If the a-Si layer is deposited on top of the Co2Si film, then the Si consumption may be reduced by at least a half, since the Co2Si film would be reacting on both top and bottom interfaces.
 It is important to clean the top surface of the Co2Si and remove any native oxide before the a-Si film deposition. The existence of such an oxide at the interface may prevent the Co2Si to react with the deposited a-Si layer. The cleaning of the surface and the stripping of a native oxide may be achieved by Ar (argon) sputtering in the a-Si deposition chamber or by a short dip in a diluted HF acid.
 As shown in FIG. 5, as a result of the annealing operation at a high temperature T2, a layer of CoSi2 50 is formed under an unreacted layer/portion 40A of the amorphous silicon or polysilicon film 40. The thickness of the layer of un-reacted amorphous silicon/polysilicon layer 40A depends on the initial thickness of the top a-Si layer 40. It is desirable that all of the Co2Si is transformed into CoSi2, without consuming the entire a-Si layer. The unreacted layer/portion 40A results from the supply of Si from the a-Si exceeding that which is needed to form CoSi2. In other words, the a-Si layer was too thick.
FIG. 6 illustrates the selective etching of the layer of un-reacted a-Si or poly-Si film 40 in a last phase.
 Thus, with the invention, the reacting of the metal (e.g., cobalt) in an annealing operation to initially form the alloy Co2Si 30 minimizes the silicon consumption of the SOI film 3.
 Additionally, the deposit of the amorphous silicon or the polysilicon film 40 on top of the alloy Co2Si 30 further reduces consumption of the SOI film 3 by a factor of two since at least half of the Co contained in the Co2Si 30 will diffuse into the top amorphous silicon/polysilicon film layer 40 at the high temperature anneal which forms the CoSi2.
 The diffusivity of cobalt in polysilicon may be larger than in single crystal (mono-crystal) silicon. Due to this diffusivity difference between polysilicon and single crystal silicon, the high temperature anneal will consume more of the top polysilicon layer than that of the bottom single crystal SOI film.
 In an alternative embodiment, the invention can be modified so that the first anneal is at an intermediate temperature, T3 (e.g., T3 is about 550° C.), where CoSi is formed (T1<T3<T2). The anneal process at this temperature will consume more of the SOI film than a Co2Si formation. However, it may provide a larger temperature window for the anneal.
 The larger the temperature window, the easier it is to form a given silicide phase without the risk of obtaining a mix phase. The temperature window for Co2Si is about 20° C. wide if pure Co is used. It may be widened to about 100° C. by using Co0.8Si0.2. The window may shift and vary depending on the SOI doping. This makes it difficult to obtain the Co2Si phase if the window is narrow. If pure Co rather than Co0.8Si0.2 is used, then it is easier to form CoSi due to its large temperature window of about 150° C.
 Also, the etch selectivity of CoSi, with respect to Co, is higher than that of Co2Si with respect to Co. The advantages of this selectivity include better reliability and precision of the resulting product. After the Co is reacted to form CoSi, the unreacted Co must be etched away. Otherwise, the source/drain regions will be shortened to the gate. The etchant should be selective to CoSi. That is, it should only remove the Co and leave the CoSi alloy intact. The etching selectivity is typically higher if the alloy contains less Co and more Si. Thus, CoSi is expected to be more resistant to the etch of Co than Co2Si. The remainder of the steps in the process are the same.
 Thus, the present invention is optimized over the conventional techniques. That is, alternative conventional methodologies (e.g., which are less desirable when contrasted to the embodiments of the invention described above), include thickening a SOI layer (by at least the amount that will be consumed by the silicide in source and drain regions by using selective epitaxial growth of silicon on these regions, fabricating different silicide thicknesses over gate, source and drain regions by laser melting, and the deposit of a silicon alloy (Co1-xSix, where x<0.2) to limit the amount of silicon consumed at source, drain, and gates during silicide formation.
 As mentioned above, the epitaxial growth of Si is performed by thickening a SOI layer in the source/drain regions through selective epitaxial growth of Si in these regions. This alternative has several disadvantages when compared to the process of the present invention.
 First, the epitaxial growth has to be selective, otherwise Si growth will take place on the sidewalls of the device. This condition can lead to shorting the gate to the source and the drain. To avoid this problem, the choice of the sidewall material to use is narrowed because only growth-resistant materials can be selected.
 Further, the growth temperature is an important parameter in determining the selectivity of the growth. The Si epitaxial growth loses selectivity at low growth temperatures. “Low growth temperatures” depend on the growth technique, and the sillicon source. The most selective source is SiCl4, but it also requires the highest deposition temperature (about 900° C. to 1200° C.). Silane (SiH4) can be used for low temperature deposition (as low as about 650° C.), but it exhibits very little selectivity, if any. Therefore, a sufficiently high growth temperature (e.g., about 900° C.) is required to guarantee selectivity. The required high growth temperature may be in excess of the thermal budget incurred in the conventional salicide process.
 A further problem with producing a raised source/drain by epitaxial growth of silicon is the process robustness. Silicon epitaxy is very sensitive to surface preparation and cleaning. Different surface treatments can lead to different defects in the film. Oxide residuals (e.g., even an atomic monolayer) can prevent epitaxial growth.
 Another problem with the epitaxial growth approach, known as growth rate dependency on feature size, can occur. In a chemical vapor deposition (CVD)-type epitaxy, the growth rate may be dependent on the topography, the dimensions of the growth area, and the ratio between the growth to non-growth areas. This may lead to a growth of different film thicknesses in devices that are embedded in different circuit layouts. This condition is an additional dimension that must be included in a manufacturing process, and hence require additional costs. The present invention does not require epitaxy, and is therefore not limited by the difficulties imposed by epitaxy.
 A second approach is siliciding by laser melting. This is a relatively new technique that allows the fabrication of different silicide thicknesses over the gate and over the source/drain region. This technique has not been applied in a manufacturing context and therefore its usage in practice is unknown. The throughput of the technique may be lower than that achieved with the other techniques such as raised source/drain, deposition of CoSi alloy, etc. Laser annealing is carried out per wafer, that is the wafers are processed sequentially, one at a time. Raised source/drain epitaxy and the present invention are parallel techniques in the sense that the entire wafer lot may be processed together (e.g., a single deposition is carried out on all wafers).
 The invention uses conventional fabrication techniques, and does not have a throughput problem. The thermal budget required by the invention is the same as in a conventional salicide process.
 A third approach employs deposition of a Co1-xSix alloy to limit the source and drain silicon consumed during silicide formation. This approach is limited to a composition that has a sufficiently small concentration of Si (x<0.2) permitting the alloy deposited on the oxide sidewalls to be removed by selective etching.
 It is noted that the invention can utilize this technique to further decrease the Si consumption by depositing Co1-xSix instead of pure Co as mentioned above. The second advantage of Co1-xSix is the larger temperature window which is available for the formation of the metal-rich phase.
 Thus, the present invention overcomes the above-mentioned and other problems of the conventional techniques and allows forming a silicon-on-insulator (SOI) MOSFET having ultra-thin silicon films and while preventing (or at least minimizing) the high source/drain series resistance and maintaining its efficiency. Thus, bulk or thick SOI films are unnecessary with the inventive method. Further, series parasitic resistance is minimized in order to facilitate the fabrication of high performance thin film SOI MOSFETs with the inventive method.
 While the invention has been described in terms of preferred embodiments, those skilled in the art will recognize that the invention can be practiced with modification within the spirit and scope of the appended claims.
 The method and structure of the present invention are not limited to a specific silicide-forming metal. Further, the invention, is not limited to one particular device as described above, but can also be used in devices with a non-planar source/drain region, such as a polysilicon sidewall source drain (e.g., see P. M. Solomon, H. -S. P. Wong, “Method for Making Single and Double Gate Field Effect Transistors with Sidewall Source Drain Contacts”, U.S. Pat. No. 5,773,331, Jun. 30, 1998, incorporated herein by reference; and T. Yoshimoto et al., “Silicided Silicon-Sidewall Source and Drain Structure for High Performance 75-nm gate length pMOSFETs,” 1995 Symposium on VLSI Technology, digest p.11).