Abstract:
Methods arc provided for forming a contact in an integrated circuit by chemical vapor deposition (CVD). The methods include forming titanium silicide in the contact. One method includes forming titanium silicide by combining a titanium precursor in the presence of hydrogen, H 2 . Another method includes forming titanium silicide by combining titanium tetrachloride, TiCl 4 , in the presence of hydrogen. A further method includes forming titanium silicide by combining tetradimethyl amino titanium, Ti(N(CH 3 ) 2 ) 4 , in the presence of hydrogen. The methods may further include forming titanium in the contact.

Description:
This application is a Divisional of U.S. application Ser. No. 08/915,986, filed Aug. 21, 1997. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to a method for manufacturing semiconductor devices, and in particular, to a method for forming titanium silicide within electrical contacts and apparatuses including such electrical contacts. 
     BACKGROUND OF THE INVENTION 
     Device density in integrated circuits (ICs) is constantly being increased. To enable the increase in density, device dimensions are being reduced. As the dimensions of device contacts get smaller, device contact resistance increases, and device performance is adversely affected. Methods for decreasing device contact resistance in ICs are needed to obtain enhanced device and IC performance. 
     Device contacts with reduced resistance may be created by forming certain metals on a silicon semiconductor base layer. These metals react with the underlying silicon, for example, to form silicides. Silicide device contacts are desirable because they reduce the native oxide on silicon. The native oxide is undesirable because it increases the contact resistance. 
     Titanium is preferably used to form silicide device contacts for two reasons. First, titanium silicide has superior gettering qualities. Also, titanium silicide forms low resistance contacts on both polysilicon and single-crystal silicon. 
     Device contacts are normally formed with the following process. First, a thin layer of titanium is formed on top of the silicon base layer, such as a substrate. The titanium adjoins active regions exposed by contact holes in an isolating layer, such as an oxide, above the silicon base layer. Then, the silicon base layer is annealed. As a result, the titanium reacts with the active regions of silicon to form titanium silicide. 
     Ultimately, an electrically conductive plug material, such as tungsten, fills the contact hole to facilitate external electrical connection to the contact. However, plug materials, such as tungsten, adhere poorly to titanium silicide. Additionally, to ensure low contact resistivity, aluminum or polysilicon plug materials should not be intermixed with the titanium silicide and underlying silicon base layer. Accordingly, a barrier layer is formed over the titanium silicide to prevent diffusion of the titanium silicide and silicon base layer into the plug material. The barrier layer also causes the plug material to adhere to the IC. 
     Titanium nitride is a desirable barrier layer because it is an impenneable barrier for silicon, and because the activation energy required for the diffusion of other impurities is very high. Titanium nitride is also chemically and thermodynamically stable, and has a relatively low resistivity. Titanium nitride can be formed on the substrate by (1) evaporating titanium in a nitrogen ambient, (2) reactively sputtering titanium in an argon and nitrogen mixture, (3) sputtering from a titanium nitride target in an inert argon ambient, (4) sputter depositing titanium in an argon ambient, and converting the titanium to titanium nitride subsequently by plasma nitridation, or (5) low pressure chemical vapor deposition (CVD). 
     The resistance of device contacts can also be adversely increased by the formation of titanium silicide having small step coverage in the contact hole. As device dimensions shrink, the contact holes become relatively deeper and narrower. Also, the walls of the contact holes become steeper, and closer to vertical. As a result, most metal deposition techniques form conductive layers having relatively small step coverage. As a result, a void, or keyhole, forms in the plug material. The void increases contact resistivity and diminishes contact reliability. Hence, IC performance is degraded. Thus, there is a need for forming contacts with reduced resistivity. Specifically, there is a need for a method of forming contacts without voids. 
     SUMMARY OF THE INVENTION 
     The present invention solves the above-mentioned problems in the art and other problems which will be understood by those skilled in the art upon reading and understanding the present specification. The present invention includes a method for forming titanium silicide and/or titanium by chemical vapor deposition (CVD), and apparatus formed by such a method. The method comprises cleaning a contact hole. A titanium precursor and a silicon precursor are combined in the presence of hydrogen (H 2 ). Titanium silicide is formed by CVD. 
     In another embodiment, the method includes combining a titanium precursor in the presence of hydrogen (H 2 ). Then, titanium silicide is formed by CVD on an exposed silicon surface of a contact hole. 
     In another embodiment, the method includes forming titanium by CVD on a conductor according to the following chemical process: 
     
       
         TiCl 4 +H 2 →Ti+HCl. 
       
     
     In yet another embodiment, the method includes forming titanium by CVD on an insulator according to the following chemical process: 
     
       
         TiCl 4 +H 2 →Ti+HCl 
       
     
     In yet another embodiment, the method includes forming titanium silicide according to the following chemical process: 
     
       
         TiCl 4 +Si n H 2n+2 +H 2 →TiSi x +HCl, 
       
     
     wherein TiCl 4  is the titanium precursor, Si n H 2n+2  is the silicon precursor, x is less than or equal to 2, and n is greater than or equal to 1. In yet another embodiment, the titanium silicide is also formed according to the following chemical process: 
     
       
         TiCl 4 +Si+H 2 →TiSi x +HCl, 
       
     
     wherein x is less than or equal to 2. 
     In one embodiment, the apparatus is a memory, comprising a memory array, a control circuit, operatively coupled to the memory array, and address logic, operatively coupled to the memory array and the control logic. The memory array, control circuit and address logic, each including a contact. The contact includes titanium silicide. Titanium nitride is formed on the titanium silicide. A plug material is formed on the titanium nitride. The plug material is substantially solid. In another embodiment, the titanium silicide formed on an exposed silicon base layer, and the exposed silicon base layer is not substantially depleted. 
     In yet another embodiment, the apparatus is a system, comprising a memory, and a processor coupled to the memory. The memory includes a contact. The contact includes titanium silicide. Titanium nitride is formed on the titanium silicide. A plug material is formed on the titanium nitride. The plug material is substantially solid. In yet another embodiment, the titanium silicide formed on an exposed silicon base layer, and the exposed silicon base layer is not substantially depleted. 
     It is an advantage of the present invention that the contacts have reduced resistivity. It is a further benefit that the contacts have increased reliability. 
    
    
     BRIEF DESCRIPTION OF THE FIGURES 
     FIG. 1A is a cross-sectional view of a contact hole that has been etched through an insulator to an underlying semiconductor substrate. 
     FIG. 1B is a cross-sectional view of the contact hole of FIG. 1A, comprising a silicide film formed thereon by one aspect of the present invention. 
     FIG. 1C is a cross-sectional view of the contact hole of FIG. 1B, further comprising a barrier layer and plug material. 
     FIG. 2A is a cross-sectional view of the contact hole of FIG. 1A, comprising a silicide film formed thereon by another aspect of the invention. 
     FIG. 2B is a cross-sectional view of the contact hole of FIG. 2A, further comprising a barrier layer and plug material. 
     FIG. 2C is a cross-sectional view of a contact hole comprising a conductor, a barrier layer, and plug material. 
     FIG. 3A is a cross-sectional view of a contact hole that has been etched into an insulator. 
     FIG. 3B is a cross-sectional view of the contact hole of FIG. 3A, further comprising a silicide film, a barrier layer and plug material formed thereon by another aspect of the invention. 
     FIG. 4 is a block diagram of a memory. 
     FIG. 5 is a block diagram of a memory coupled to an external system. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     In order to manufacture a contact in an integrated circuit (IC)  11 , a contact hole  10 , as illustrated in FIG. 1A, is etched through an insulator  12 , such as borophosphosilicate glass (BPSG) or silicon dioxide, to expose a portion of the underlying silicon base layer  14 , to which electrical contact is to be made. The exposed silicon base layer  14  is generally an active region  15  of a transistor in the IC  11 . An optional in-situ clean of the contact hole  10  may then be performed with a wet chemical clean, or a plasma, such as a high density Ar/NF 3  plasma. 
     Chemical vapor deposition (CVD) is then used to form titanium silicide  16 , such as TiSi x , at the bottom of contact hole  10 , as shown in FIG.  1 B. CVD permits accurately controlled formation of films, including conformal films. 
     CVD techniques are well known by persons skilled in the art, and are described in Panson et al.,  Appl. Phys. Lett ., 53,1756 (1988) and Cowher et al.,  j. Cryst. Growth , 46, 399 (1979), hereby incorporated by reference. Any CVD apparatus design may be used when practicing the present invention including hot wall reactors, cold wall reactors, radiation beam assisted reactors, and plasma-assisted reactors. These CVD apparatuses are disclosed in C. E. Morosanu, “Thin Films by Chemical Vapor Deposition,” Elsevier, N.Y. (1990), pages 42-54; I. P. Herman,  Chemical Reviews , 89, 1323 (1989); U.S. Pat. No. 4,876,112; U.S. Pat. No. 5,005,519; U.S. Pat. No. 4,340,617; U.S. Pat. No. 4,713,258; U.S. Pat. No. 4,721,631; U.S. Pat. No. 4,923,717; U.S. Pat. No. 5,022,905; U.S. Pat. No. 4,868,005; U.S. Pat. No. 5,173,327, and Bachman et al.,  MRS Bull ., 13, 52 (1988), hereby incorporated by reference. 
     For blanket depositions, a cold wall-hot substrate reactor is sometimes preferred, as this design is efficient in regard to precursor consumption. For depositions on selection areas, a radiation beam assisted reactor may be preferred as the radiation beam may be used to selectively deposit metal containing films onto small areas of the substrate. 
     Different embodiments for forming titanium silicide, which may use plasma-assisted CVD (PACVD) and non-plasma CVD, are subsequently described. In a first embodiment, CVD is utilized to deposit a film of titanium silicide  16  on the top and side walls of the insulator  12 , as illustrated in FIG. lB. The titanium silicide  16  is preferably formed as a conformal film, and thus has high step coverage. This embodiment will now be described in further detail. 
     The IC  11  is mounted on a substrate holder in a chamber of the CVD apparatus. A titanium precursor, such as titanium tetrachloride (TiCl 4 ) and a silicon precursor, such as silane (Si n H 2n+2 ), for example either SiH 4  or Si 2 H 6 , are combined with hydrogen (H 2 ) in the CVD apparatus to form the titanium silicide  16  in and around the contact hole  10 , as illustrated in FIG.  1 B. The following general chemical process (I) is used: 
     
       
         TiCl 4 +Si n H 2n+2 +H 2 →TiSi x +HCl,  (I) 
       
     
     wherein, generally, n is greater than or equal to 1, and x is less than or equal to 2. 
     When PACVD is used, the deposition takes place in a water cooled quench chamber having a volume of approximately 6 liters. The power supply for generating a plasma can be any type of energy source, such as radio frequency (RF) or direct current (DC). When a RF power source is used, the radio frequency is approximately 13.6 MHZ. However, the present invention envisions using higher or lower frequencies. The power of the energy source used to create the plasma is between approximately 10 and 1,000 Watts, preferably approximately 600 Watts. The IC  11  is heated to a temperature between approximately 300 and 800 degrees Celsius, preferably approximately 500 degrees Celsius. 
     In the first embodiment, the precursor gases TiCl 4  and Si n H 2n+2  are delivered to a plasma flame. TiCl 4  is introduced at a flow rate of between approximately 1 and 40 sccm, preferably approximately 20 sccm. If SiH 4  is used, the SiH 4  is introduced at a flow rate of between approximately 0.5 and 100 sccm, preferably approximately 50 sccm. Alternatively, Si 2 H 6  may be used, and is introduced at a flow rate of between approximately 0.05 and 50 sccm, preferably approximately 25 sccm. A sheath gas, selected from a group consisting of the noble gases and hydrogen, is employed in order to direct the plasma flame. In this example, hydrogen is introduced at a flow rate between approximately 500 and 5,000 sccm, preferably approximately 3,000 sccm. Optionally, a carrier gas, such as argon, is also introduced at a flow rate between approximately 500 and 5,000 sccm, preferably approximately 2,000 sccm. 
     A precursor compound, including silicon and titanium precursors, becomes a plasma gas. The precursor compound is transported via a reactor tube to the chamber. The precursor plasma, upon coming into contact with the heated IC  11 , pyrolyzes and deposits a film of TiSi x    16  on the exposed surfaces of the insulator  12 . The chamber pressure is between approximately 0.1 and 100 Torr, preferably approximately 5 Torr. The reaction products from the pyrolysis of the precursor compound exit from the chamber via an exhaust manifold. 
     For this embodiment, x is typically 2. However, x may be less than 2 when the silane flow rate is relatively low, and when TiCl 4  precursor flow rate is relatively high. 
     Alternatively, when using a non-plasma CVD method, the process parameters generally remain the same. However, in the absence of the plasma, the process temperature is increased to between approximately 600 and 900 degrees Celsius, preferably approximately 700 degrees Celsius. 
     Titanium silicide is typically formed on the exposed surface of the silicon base layer  14  when the titanium precursor and hydrogen contact the silicon base layer  14 . This reaction is described by the following general chemical process (II): 
     
       
         TiCl 4 +Si+H 2 →TiSi 2 +HCl  (II) 
       
     
     However, process (II) may remove exposed silicon base layer that is the active region  15 . Titanium silicide  16  will then intrude into the active region where the exposed silicon base layer was removed. The active region  15  is highly doped to reduce contact resistance. As a result of the removal, the contact resistance will undesirably increase. Therefore, preferably, sufficient silane is preferably added to the precursor compound by regulating the silane flow rate. As a result, titanium silicide over the exposed silicon base layer is at least partially formed according to process (I). Hence, the exposed silicon will not be substantially depleted. Therefore, the contact resistance will not be detrimentally increased. 
     In a second embodiment, CVD is used to selectively deposit a film of titanium silicide  16  on the exposed silicon base layer according to process (II), described above. If PACVD is used, the IC  11  is heated to between approximately 400 and 800 degrees Celsius. Simultaneously, a substantially thinner layer of titanium  17  is deposited on the sidewalls of the contact hole  10 , as illustrated in FIG.  2 A. The titanium  17  is deposited according to chemical process (III): 
     
       
         TiCl 4 +H 2 →Ti+HCl  (III) 
       
     
     As illustrated in FIG. 2B, the titanium  17  formed on the sidewalls of the insulator  12  is substantially thinner than the titanium silicide  16  formed on the base of the contact hole  10 . Thus, in the event the titanium  17  on the sidewalls is formed with a retrograde, there will be substantially no voids in the subsequently formed plug material  20 , illustrated in FIG.  2 C. Hence, the plug material  20  is substantially solid. The titanium  17  formed on the top of the insulator  12  is substantially thicker than the titanium  17  formed on the sidewalls of the contact hole. 
     The second embodiment may be implemented, for example, at a temperature of approximately 630 degrees Celsius, a pressure of approximately 5 Torr, an H 2  flow rate of approximately 5 slm, and a TiCl 4  flow rate of approximately 40 sccm. A carrier gas, such as argon, having a flow rate of approximately 5 slm, may also be used. As a result, a 500 Angstrom layer of titanium  17  is formed on the exterior surfaces  22  of the insulator  12 . Little or no titanium  17  is formed in the contact hole  10 , such as on the sidewalls  24  and over the active region  15 , as illustrated in FIG.  2 A. Additionally, approximately 2500 Angstroms of titanium silicide  16  is formed over the active region  15 . Alternatively, the second embodiment may be implemented at a temperature of less than about 550 degrees Celsius. As a result, about 1250 Angstroms of titanium  17 , rather than titanium silicide  16 , are formed over the active region  15 . The titanium  17  is subsequently converted to titanium silicide during an anneal. 
     Chemical process (III), described above, may also be used to form a contact to a conductor  21  on an IC  11 , as illustrated in FIG.  2 C. The conductor  21  is typically formed over a second insulator  23  on the IC  11 . The titanium  17  formed on the sidewalls of the contact hole  10  is substantially thinner than the titanium  17  formed on the base of the contact hole  10 . Thus, in the event the titanium  17  on the sidewalls is formed with a retrograde, a there will be substantially no voids in the subsequently formed plug material  20 , illustrated in FIG.  2 C. 
     In a third embodiment, titanium silicide  16  can be formed in a contact hole  10  in insulator  12 , where no silicon base layer  14  is exposed, as illustrated in FIG. 3A. A conformal layer of titanium silicide  16  having high step coverage is formed in the contract hole  10 , according to general chemical process (I) described above. For the first, second and third embodiments, the reaction products of general chemical processes (I) and (II) may also include SiCl 4 . 
     Typically, after the titanium silicide  16  has been formed according to one of the embodiments described above, the barrier layer  18  is then formed in the contact hole, as illustrated in FIGS. 1C,  2 B and  3 B. The IC  11  may be heated before or after barrier layer  18  formation. Heating may be accomplished, for example, in a rapid thermal annealer or a furnace, in a manner known to persons skilled in the art. The heating step can convert titanium  17  proximate to exposed silicon to titanium silicide  16 . Additionally, heating of previously formed titanium silicide  16  is desirable because it reduces native oxides. The plug material  20  is then formed over the barrier layer  18  to complete contact formation. The barrier layer  18  and plug material  20  may each be formed by CVD. 
     The aforementioned processes may be used to form contacts in an integrated circuit  11  that is a memory  400 , such as a dynamic random access memory. The memory  400  may include a memory array  402 , control logic  404 , and address logic  406  coupled in a manner known to one skilled in the art and exemplified in FIG.  4 . Each of the aforementioned elements of the memory  400  includes contacts formed in the manner described above. The memory  400  may be coupled to an external system  524 , such as a processor, as illustrated in FIG.  5 . 
     The present invention provides a method for forming low resistivity, high reliability contacts. It is to be understood that the above description is intended to be illustrative, and not restrictive. Many other embodiments will be apparent to those of skill in the art upon reviewing the above description. For example, other titanium precursors, such as tetradimethyl amino titanium (TDMAT) can be used to form titanium  17  and titanium silicide  16 . Additionally, the present invention may be implemented with any CVD apparatus  29 , including hot wall reactors, cold wall reactors, radiation beam assisted reactors, plasma-assisted reactors, and the like. Hence, the scope of the invention should, therefore, be determined with reference to the appended claims, along with the full scope of equivalents to which such claims are entitled.