Abstract:
An apparatus and process for depositing a barrier film on a substrate is disclosed. In particular, deposition of the barrier film is carried out on the substrate having an applied pressure. This applied pressure flexes the substrate to reduce in-plane stresses, wherein removal of the applied pressure after deposition of the barrier film modifies the in-film stress for the thin-film. With the above-described arrangement, it is possible to minimize the deterioration of electric characteristics of a semiconductor device and the occurrence of defects, such as film delamination, substrate cracks, and the like.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to methods of controlling in-film stress in thin films of the type used in semiconductor fabrication, and, more particularly, to a method of controlling the state and amount of in-film stress of a barrier material provided on a semiconductor substrate.  
         BACKGROUND OF THE INVENTION  
         [0002]    In many areas of semiconductor processing, it is often necessary to provide consecutive layers of materials that are not stable in contact with each other. For example, aluminum (Al) reacts with silicon at a few hundred ° C. to form “spikes” of an eutectic alloy which can penetrate into the silicon through the source or drain layer causing shorts to the body if a direct Al—Si contact is made. Additionally, silicon (Si) must also be protected during tungsten deposition, as the copious amounts of fluorine present will combine with hydrogen to form hydrofluoric acid (HF), which can attack silicon or silicon dioxide to form “wormholes” under the tungsten layer. Furthermore, copper (Cu) used in IC metallization must not encounter silicon dioxide passivants, as Cu+ ions will diffuse readily through the oxide and contaminate the underlying silicon.  
           [0003]    In all the above cases and more, the situation is rescued by employing barrier materials, which are typically metals or nitrides of such metals in most applications that conduct electricity but do not permit interdiffusion and reactions of neighboring materials. However, certain barrier materials exhibit tensile or compressive stress when deposited as a thin film. In some cases, stress will build up because of the processing conditions, thermal expansion, or the mismatch of various characteristics of neighboring materials. As an example, low-Cl and low resistivity TiN films (TiCl4-based) exhibit very high in-film tensile stress when deposited on a silicon substrate.  
           [0004]    The conventional method of depositing such thin films includes continuous deposition of a barrier material onto a semiconductor substrate until the desired thickness has been attained. The prior art continuous deposition method results in a structure as seen in prior art FIG. 1: a thin film of the barrier material  10 , deposited onto a semiconductor substrate  12 . When the thin barrier film made using the prior art process consists of TiN, for example, the thin barrier film  10  after heat treatment exerts a tensile stress  14  on the underlying substrate  12 . The force exerted in compressive stress  16  by the substrate  12  is equal and opposite in directionality to the tensile stress  14  of the thin barrier film  10 .  
           [0005]    If the magnitude of the in-film tensile stress is sufficient, the thin barrier film  10  may crack, buckle, delaminate or pull away from the surface of the substrate  12 , or even cause stress-related breakage of metal interconnects limiting applications of the used barrier material. This in-film stress also limits the thickness of such thin barrier films in applications because thicker films have more potential energy to crack and peel. Additionally, high stress levels in such thin barrier films can affect many material properties such as dielectric constant and crystallographic orientation. These damaging effects may occur during the course of the integrated circuit manufacturing process, or at any time throughout the useful lifetime of the integrated circuit device, resulting in yield loss and seriously affecting the reliability of the product seriously.  
           [0006]    It would be, therefore, desirable to provide a method of depositing thin barrier films on semiconductor substrates in a manner that addresses in-film stress such that the thin barrier films exhibit reduced tensile or compressive stress following deposition.  
         SUMMARY OF THE INVENTION  
         [0007]    The present invention addresses the above need by providing a method of depositing a thin film of a barrier material on a substrate so that the thin barrier film has a reduced amount of in-film stress. By reducing the amount of stress in the thin barrier film, cracking and delamination of barrier film from the semiconductor device may be addressed.  
           [0008]    In one embodiment, provided is a method of modifying in-film stress of a thin barrier film comprising preloading a substrate with a preloaded stress, depositing a barrier material as a thin film on the substrate, and unloading the preloaded stress applied to the substrate.  
           [0009]    In another embodiment, a method for fabricating a thin-film structure body is provided. The method comprises flexing a semiconductor substrate, depositing a thin film of a barrier material on the flexed semiconductor substrate, and unflexing the substrate.  
           [0010]    In still another embodiment, a method for fabricating a thin-film structure body is provided. The method comprises providing a substrate to a sample holder, flexing the substrate to preload the substrate with tensile stress, depositing a barrier material on the flexed substrate, and unflexing the substrate.  
           [0011]    In still another embodiment, a method for fabricating a thin-film structure body is provided. The method comprises providing a substrate to a sample holder, flexing the substrate to preload the substrate with compressive stress, depositing a barrier material on the flexed substrate, and unflexing the substrate.  
           [0012]    In yet another embodiment, a method of modifying in-film stress of a thin barrier film is provided. The method comprises providing a substrate to a sample holder, raising a pin to flex the substrate to preload the substrate with a predetermined tensile stress, depositing a barrier material as a thin film on the substrate, and lowering the pin to unload the tensile stress applied to the substrate.  
           [0013]    In still yet another embodiment, a method of modifying in-film stress of a thin barrier film is provided. The method comprises providing a substrate to a sample holder, raising a pin to flex the substrate to preload the substrate with a predetermined compressive stress, depositing a barrier material as a thin film on the substrate, and lowering the pin to unload the compressive stress applied to the substrate.  
           [0014]    In another embodiment, a method for fabricating a thin-film structure body is provided. The comprises mounting a substrate by clips to a sample holder, situating the sample holder in a deposition chamber, pumping the deposition chamber to a base pressure, gradually heating the substrate to a desired temperature, and raising a pin to flex the substrate to preload the substrate with tensile stress. The method further includes depositing a barrier material on the flexed substrate, and lowering the pin to unflex the substrate.  
           [0015]    In another embodiment, a method for fabricating a thin-film structure body is provided. The comprises mounting a substrate by clips to a sample holder, situating the sample holder in a deposition chamber, pumping the deposition chamber to a base pressure, gradually heating the substrate to a desired temperature, and applying a vacuum to flex the substrate to preload the substrate with compressive stress. The method further includes depositing a barrier material on the flexed substrate, and removing the vacuum to unflex the substrate.  
           [0016]    In still another embodiment, an apparatus for manufacturing a semiconductor device and for carrying out a process of depositing barrier materials so as to form deposited thin barrier films having a modified in-film stress is provided. The apparatus comprises a chamber in which a semiconductor substrate can be contained, a sample holder adapted to support the semiconductor substrate within the chamber, and a tool for flexing the semiconductor substrate when supported by the sample holder.  
           [0017]    In another embodiment provided is a method of forming a DRAM cell. The method comprises providing a substrate having CMOS structures to a sample holder, situating said sample holder in a deposition chamber, flexing the substrate, depositing a thin film of a barrier material on the flexed substrate, and unflexing the substrate.  
           [0018]    In another embodiment, provided is a memory device structure comprising a semiconductor substrate having a lightly doped P-type crystal silicon substrate, and having field oxide areas and drain regions and source regions. The memory device structure further includes transistor gate members, including a wordline bounded by insulative material, formed on the surface of the semiconductor substrate, and a barrier film which was disposed over the semiconductor substrate when preloaded with a tensile stress.  
           [0019]    In another embodiment, provided is a memory device structure comprising a semiconductor substrate having a lightly doped P-type crystal silicon substrate, and having field oxide areas and drain regions and source regions. The memory device structure further includes transistor gate members, including a wordline bounded by insulative material, formed on the surface of the semiconductor substrate, and a barrier film which was disposed over the semiconductor substrate when preloaded with a compressive stress.  
           [0020]    In still another embodiment, provided is a DRAM cell comprising a semiconductor substrate having a lightly doped P-type crystal silicon substrate, and having field oxide areas and drain regions and source regions. The DRAM cell further includes transistor gate members, including a wordline bounded by insulative material, formed on the surface of the semiconductor substrate, and a barrier film which was disposed over the semiconductor substrate, the thick field oxide areas, and the transistor gate members when preloaded with a tensile stress. The barrier film has bitline contacts contacting the source regions for electrical communication with a bitline, and, further, has capacitor contacts contacting the drain regions for electrical communication with capacitors.  
           [0021]    In still another embodiment, provided is a DRAM cell comprising a semiconductor substrate having a lightly doped P-type crystal silicon substrate, and having field oxide areas and drain regions and source regions. The DRAM cell further includes transistor gate members, including a wordline bounded by insulative material, formed on the surface of the semiconductor substrate, and a barrier film which was disposed over the semiconductor substrate, the thick field oxide areas, and the transistor gate members when preloaded with a compressive stress. The barrier film has bitline contacts contacting the source regions for electrical communication with a bitline, and, further, has capacitor contacts contacting the drain regions for electrical communication with capacitors.  
           [0022]    These and other features and objects of the present invention will be apparent in light of the description of the invention embodied herein. 
       
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0023]    [0023]FIG. 1 is an enlarged schematic cross-sectional view of a portion of a prior art semiconductive structure;  
         [0024]    [0024]FIGS. 2-5 illustrate process steps according an embodiment of the present invention, and depict cross-sectional views of a portion of a semiconductor substrate having a thin barrier film deposited thereon;  
         [0025]    [0025]FIGS. 6-7 illustrate process steps according another embodiment of the present invention, and depict cross-sectional views of a portion of a semiconductor substrate having a thin barrier film deposited thereon; and  
         [0026]    [0026]FIG. 8 is a schematic cross-sectional side view of a memory array of a DRAM chip formed by a method to reduce in-film stress according to an embodiment of the present invention. 
     
    
       [0027]    In the drawings, the thicknesses of the various layers of material have been modified for clarity of illustration and are not necessarily true to scale.  
       DETAILED DESCRIPTION OF THE INVENTION  
       [0028]    For purposes of the invention, the term “thick barrier film” will be understood to mean a layer of barrier material, which has a thickness greater than or equal to 5000 Å.  
         [0029]    For purposes of the invention, the term “thin barrier film” will be understood to mean a layer of barrier material, which has a thickness less than 5000 Å.  
         [0030]    For purposes of the invention, suitable materials for the barrier film are any materials which conduct electricity but do not permit inter-diffusion and reactions of neighboring films, and which possess properties of a low electrical resistivity, high melting point, thermal stability, and good adhesion properties. Such barrier films include, for example, Ti, TiW, TiN, TaN, Ta-based materials, WN, MoN, AlN, CrN, ScN, and any other barrier metal and metal alloy films suitable for the intended application.  
         [0031]    For the purposes of this invention, a semiconductor substrate may comprise a silicon wafer, optionally with various components formed therein, including active devices, dielectric layers, barrier layers, underlying metal lines, oxide-filled barrier trenches, and the like.  
         [0032]    Referring to FIGS. 2-5, one embodiment of the invention provides a method of modifying in-film (intrinsic) stress for thin barrier films deposited on a surface  20  of a substrate  22 , such that thicker barrier films than that found in the prior art may be employed without having the negative effect of increased in-film stress.  
         [0033]    As illustrated in FIG. 2, substrate  22  is mounted by clips  24  to a sample holder  26 , which is then situated in a deposition chamber  28 . It is assumed for the purposes of this discussion that substrate  22  has no intrinsic stress from prior processes, and is generally flat. The deposition chamber  28  is then pumped to a base pressure prior to deposition, such as for example, between about 1 to about 10 Torr. Prior to deposition, substrate  22  is gradually heated to a desired temperature, such as for example, between about 250° C. to about 700° C., and may be cleaned if desired by conventional cleaning processes, such as for example, sputtering.  
         [0034]    In FIG. 3, substrate  22  is then warped or flexed into a convex configuration, such as by a retractable pin  30  engaging the undersurface  23  of substrate  22  and being raised in the direction indicated by symbol a. Substrate  22  may be warped or flexed by a number of alternative methods such saddling substrate  22  over pin  30  and lowering clips  24  in the direction indicated by symbol b, by squeezing clips  24  together in the direction indicated by symbol c, or any other means which applies a pressure to deform substrate  22  into a convex configuration to preload tensile stress. The flexing of substrate  22  induces tensile stress, thereby reducing the resulting intrinsic compressive stress realized after depositing a thin barrier film. The range of the preloaded tensile stress is from about 10% to about 200%, and the actual amount of the preloaded tensile stress depends on the intended barrier layer material and desired thickness of the film.  
         [0035]    With substrate  22  flexed into a convex configuration illustrated in FIG. 4, a thin barrier film  32  is then provided over surface  20  of substrate  22 . The thin barrier film  32  may be provided using any conventional deposition method, such as by reactive ion sputtering, electron beam evaporation, physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer chemical vapor deposition (AL-CVD), and ion-beam assisted deposition (IAD) techniques. The method may also be used to grow a monolayer of the thin barrier film  32  on surface  20 . Regardless of the technique used for depositing the thin barrier film  32 , it is important the deposited layer(s) adhere securely to one another and/or to substrate  22 , as the case may be.  
         [0036]    As illustrated by FIG. 5, because of the preloading of tensile stress within substrate  22  prior to deposition and crystallization of the thin barrier film  32 , the level of stress in the thin barrier film  32  is reduced, or, in some cases, the state of the stress in the layer is changed. The state and level of stress in the thin barrier film  32  after deposition varies primarily as a function of preloading of tensile stress on substrate  22  and the composition and thickness of the thin barrier film  32 . Thus, by appropriate selection of the amount of preloaded tensile stress for substrate  22  and materials for the thin barrier film  32 , and by depositing the barrier film to an appropriate thickness, the state and level of stress in the film may be controlled.  
         [0037]    In another embodiment, the substrate  22  may be warped or flexed into a concave configuration, such as by raising clips  24  in the direction indicated by symbol d, applying a vacuum to engage the undersurface  23  of substrate  22 , such as through retractable pin  30 , and lowering pin  30  in the direction indicated by symbol e. Substrate  22  may be warped or flexed by any other means which applies a pressure to deform substrate  22  into a concave configuration to preload a compressive stress. The range of the preloaded compressive stress is from about 10% to about 200%, and the actual amount of the preloaded compressive stress depends on the intended barrier layer material and desired thickness of the film.  
         [0038]    Such flexing of substrate  22  induces compressive stress, thereby reducing the resulting intrinsic tensile stress realized after depositing the thin barrier film  32 , as illustrated by FIG. 7. Preloading the substrate before deposition of the thin barrier film helps to reduce the amount of in-film stress of barrier material which exhibits very-high in-film compressive stress when deposited on a substrate without a preloaded stress. As before, the thin barrier film  32  may be provided using any conventional deposition method, such as by reactive ion sputtering, electron beam evaporation, physical vapor deposition (PVD), chemical vapor deposition (CVD), atomic layer chemical vapor deposition (AL-CVD), and ion-beam assisted deposition (IAD) techniques. The method may also be used to grow a monolayer of the thin barrier film  32  on surface  20 . Regardless of the technique used for depositing the thin barrier film  32 , it is important the deposited layer(s) adhere securely to one another and/or to substrate  22 , as the case may be.  
         [0039]    Resulting stress after the deposition method of the present invention may be computationally determined. As is known, the mechanical stress is a force in the plane of film  32  acting per unit area of the film cross section. The mechanical stress may be compressive or tensile in character. The total mechanical stress s of a thin barrier film  32  consists of two terms:  
           s=s   i   +s   T    (1)  
         [0040]    where s i  is intrinsic stress, which is a fundamental result of the conditions and method of film growth and is to a large degree a reflection of the film structure and the presence of impurities. Intrinsic stress s i  is calculated by the expression  
           s   i   =[E   s /6(1 −n   s )]( d   s   2   /d   f )(1 /R   s −1 /R   f )   (2)  
         [0041]    where E s  and n s  are respectively the Young&#39;s modulus and Poisson ratio of the substrate, d s  and d f  are respectively the substrate and the film thickness, and R s  and R f  are, respectively, the radii of curvature of the substrate without and with the barrier film. The thin barrier film thickness d f  can be measured by profilometric measurements of film steps obtained using appropriate film masks on each sample.  
         [0042]    The thermal stress s T  in the barrier film is calculated by the expression:  
           s   T   =[E   f /(1 −n   f )]( a   f   −a   s )( T   d   −T   M )   (3)  
         [0043]    where E f  and n f  are respectively the Young&#39;s modulus and Poisson ratio for the film, a f  and a s  are average thermal coefficients of the film and the substrate, and T d  and T M  are the film deposition temperature and the temperature during the stress measurements, respectively. Accordingly, the amount of flexing of the substrate before deposition of the barrier material can be derived from the above equations in order to give a desired in-film stress level (tensile or compressive).  
         [0044]    Alternatively, the in-film stress of film barrier film  32  may be determined based on empirical testing. Such testing involves depositing the barrier film  32  at a range of different thicknesses and substrate flexing, and then measuring the in-film stress levels of the barrier film  32  using, for instance, known reflectivity-measuring techniques. By correlating the characteristics of layers  22  and  32  with the type (i.e., convex or concave) and extent of deformation after deposition of the barrier film, the deposition parameters, i.e., barrier film thickness and amount of substrate flexing, required to achieve a thin barrier film  32  that impart the desired in-film stress (tensile or compressive) in substrate  22  may be determined.  
         [0045]    The above-described method of controlling the state and level of in-film stress of thin barrier films deposited on a substrate may be used in the current generation DRAMs. Thus, when used in a DRAM, such as illustrated in FIG. 8, the thin barrier film is deposited in accordance with the deposition techniques discussed above. The stress in such DRAMs may be precisely controlled, as also discussed above. By controlling such in-film stress, the formation of dislocations in the substrate adjacent the barrier layer may be reduced significantly. Because such dislocations apparently provide pathways through which ions may diffuse and charge may leak, by reducing the formation of dislocations, it is believed that the reliability and retention time of the associated DRAM will increase.  
         [0046]    For example, a CMOS structure  200  is illustrated in FIG. 6 as a portion of a memory array in a DRAM chip. The CMOS structure  200  comprises a semiconductor substrate  202 , such as a lightly doped P-type crystal silicon substrate, which has been oxidized to form thick field oxide areas  204  and exposed to implantation processes to form drain regions  206  and source regions  208 . Transistor gate members  212 , including a wordline  214  bounded by insulative material  216 , are formed on the surface of the semiconductor substrate  202 . A barrier film  218  is disposed over the semiconductor substrate  202 , the thick field oxide areas  204 , and the transistor gate members  212  according to the present invention described above. The barrier film  218  has bitline contacts  222  contacting the source regions  208  for electrical communication with a bitline  224 , and, further, has capacitor contacts  226  contacting the drain regions  206  for electrical communication with capacitors  228 .  
         [0047]    Having described the present invention in detail and by reference to the embodiments thereof, it will be apparent that modifications and variations are possible without departing from the scope of the invention in the following claims.