Patent Publication Number: US-7589015-B2

Title: Fabrication of semiconductor devices using anti-reflective coatings

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   The present application is a divisional application of U.S. patent application Ser. No. 11/414,348, filed May 1, 2006, now U.S. Pat. No. 7,390,738 which is a continuation of U.S. patent application Ser. No. 10/684,431, now U.S. Pat. No. 7,067,894, filed Oct. 15, 2003, which is a divisional application of U.S. patent application Ser. No. 09/252,448, now U.S. Pat. No. 6,713,234, filed Feb. 18, 1999, the disclosures of which are each herewith incorporated by reference in their entirety. 

   BACKGROUND 
   The present invention relates generally to the manufacture of semiconductor devices and, in particular, to methods for fabricating such devices using anti-reflective layers, as well as devices including anti-reflective coatings. 
   The fabrication of integrated circuits requires the precise positioning of a number of regions in a semi-conductor wafer, followed by one or more interconnection patterns. The regions include a variety of implants and diffusions, cuts for gates and metallizations, and windows in protective cover layers through which connections can be made to bonding pads. A sequence of steps is required for each such region. 
   Photolithographic techniques, for example, can be used in the performance of some or all of the foregoing operations. Typically, for example, the surface of a wafer to be processed is pre-coated with a photoresist. The photoresist then is exposed to a light source with a suitably patterned mask positioned over the wafer. The exposed resist pattern is used, for example, to open windows in a protective underlying layer to define semiconductor regions or to delineate an interconnection pattern. 
   To improve the degree of integration and to obtain high density devices, performing photolithographic operations at shorter wavelengths is desirable. Currently, i-line techniques with a wavelength of about 365 nanometers (nm), KrF excimer laser techniques with a wavelength of about 248 nm, and KnF excimer laser techniques with a wavelength of about 193 nm are used. However, at those wavelengths, optical reflections at the interfaces of previously-formed layers on the semiconductor wafer can cause notching of the photoresist. 
     FIG. 1  illustrates the general nature of the problem. A semiconductor wafer  10  includes one or more previously-formed layers  12  covered by a thick layer of boro-phospho-silicate glass (BPSG)  14 . The BPSG layer  14  serves as a protective layer for the underlying layers  12  and also provides a more planar surface. A photoresist film  16  is coated over the BPSG layer  14 , and a mask  18  is positioned over the photoresist prior to exposure of the photoresist to an appropriate source of radiation  20 . The mask  18  can be used to define, for example, contact holes for one of the previously-formed layers  12 . 
   Ideally, when the photoresist film  16  is exposed to the radiation  20 , the mask  18  precisely defines the dimensions of the exposed regions of the photoresist film. However, the BPSG layer  14  is transparent to the wavelengths typically used in photolithography, including 248 nm and 365 nm. Thus, a significant amount of the radiation  20  that passes through the mask  18  travels through the BPSG layer  14  and is reflected at the interface between the BPSG layer and one or more of the previously-formed underlying layers  12 . Some of the reflected radiation (indicated by arrow  22 ) contributes to exposure of the photoresist film  16 . 
   In some situations, a dielectric anti-reflective coating is provided above the BPSG layer to reduce reflections from the underlying layers. However, if the structures in the previously-formed underlying layers  12  have varying dimensions or varying shapes and the level of reflected light is relatively high, the reflected light  22  will expose the photoresist film  16  non-uniformly leading to the formation of notching. 
   SUMMARY 
   In general, techniques are disclosed for fabricating a device using a photolithographic process. The techniques are particularly advantageous for transferring an optical pattern by photolithography to one or more layers which are transparent to the wavelength(s) at which the photo-lithography is performed. 
   According to one aspect, a method includes providing a first anti-reflective coating over a surface of a substrate. As used herein, the term “substrate” refers to one or more semiconductor layers or structures which may include active or operable portions of semiconductor devices. Various films or other materials may be present on the semiconductor layers or structures. A layer which is transparent to a wavelength of light used during the photolithographic process is provided over the first anti-reflective coating, and a photosensitive material is provided above the transparent layer. The photosensitive material is exposed to a source of radiation including the wavelength of light. Preferably, the first anti-reflective coating extends beneath substantially the entire transparent layer. 
   According to another aspect, a semiconductor device includes a layer that is transparent to light having a wavelength, for example, of approximately 193 nm, 248 nm or 365 nm. A first anti-reflective coating extends substantially entirely beneath the transparent layer. 
   One advantage of providing an anti-reflective coating beneath the transparent layer is that the anti-reflective coating can help reduce notching of the photosensitive material that may occur during the photolithographic process. 
   In general, the complex refractive index of the first anti-reflective coating can be selected to maximize (or increase) the absorption at the first anti-reflective coating to minimize (or reduce) the amount of light transmitted through the first anti-reflective coating and reflected back from the underlying structures. Therefore, the effects of the non-uniform structures in the layers below the first anti-reflective coating can be reduced or eliminated. That, in turn reduces the amount of light that is reflected back toward the photosensitive material and, therefore, further reduces notching. 
   Various implementations include one or more of the following features. The transparent layer can include a material such as BPSG, PSG and TEOS. Other materials, including various oxides and nitrides, also can be used as the transparent layer. Depending on the properties of the photosensitive material, it can be exposed to radiation at various wavelengths including approximately 193 nm, 248 nm or 365 nm. Portions of the photosensitive material selectively can be exposed to the radiation. 
   The first anti-reflective coating can include various materials, including a material comprising silicon and nitrogen; silicon and oxygen; or silicon, oxygen and nitrogen. Other materials, such as organic polymers, also can be used as the first anti-reflective coating. 
   According to another aspect, in addition to the first anti-reflective coating, a second anti-reflective coating can be provided above the transparent layer. The photosensitive material then can be provided over the second anti-reflective coating. Providing the second anti-reflective coating between the photosensitive material and the transparent layer can further help reduce the effects of light that is reflected from the interface of the first anti-reflective coating and the transparent layer. 
   Other features and advantages will be readily apparent from the following description, the accompanying drawings, and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  is a partial cross-section of a semiconductor wafer during performance of a photolithographic process. 
       FIG. 2  illustrates a cross-section of an exemplary, partially-fabricated semiconductor device. 
       FIG. 3  illustrates a cross-section of the device of  FIG. 2  during performance of a photolithographic process according to the invention. 
       FIG. 4  illustrates a cross-section of the device of  FIG. 3  following formation of a contact hole. 
       FIG. 5  illustrates a cross-section of a device having multiple layers to which a photolithographic pattern is to be transferred and which are transparent to the light used during the photolithographic process. 
       FIG. 6  illustrates a cross-section of a device during performance of a photolithographic process according to another embodiment of the invention. 
   

   DETAILED DESCRIPTION 
   As explained in greater detail below, a technique is described for transferring an optical pattern by photo-lithography to one or more layers which are transparent to the wavelength(s) at which the photolithography is performed. The technique is described with respect to the formation of a contact hole through a BPSG layer. However, the technique is generally applicable to the formation of various features in integrated devices involving the transfer of a pattern by photolithography to other transparent layers as well. Such layers include, for example, phospho silicate glass (PSG), tetra-ethyl-ortho-silicate (TEOS), undoped oxides, among others. 
   As shown in  FIG. 2 , an exemplary partially-fabricated semiconductor device includes an active region  30  formed on a semiconductor wafer  32  and surrounded by an isolation oxide film  34 . A gate electrode  36  is formed in the active region  30  with a gate oxide film  38  disposed between the electrode and the active region. The gate electrode  36  includes a first polycrystalline silicon (poly-Si) film  40  and a first refractory metal silicide film  42 . The gate electrode  36  has its top portion covered with a silicon oxide film  44  and its sidewalls covered with an insulation film  46 . 
   Impurity diffusion layers  48  form source and drain regions at the upper surface of the wafer  32  at either side of the gate electrode to provide a metal-oxide-semiconductor (MOS) field effect transistor structure. A silicon oxide layer  56  is provided over the surface of the substrate. 
   A first interconnection layer  50 , including a second poly-Si layer  52  and a second refractory metal silicide layer  54 , is formed on one of the impurity diffusion layers  48 . As shown in  FIG. 2 , a silicon oxide film  58  is formed over the entire upper surface of the substrate. 
   Techniques for forming the foregoing layers are well-known and, therefore, are not described in further detail. 
   Prior to forming a protective BPSG layer, a first dielectric anti-reflective coating (ARC)  60  is formed over substantially the entire upper surface of the substrate ( FIG. 3 ). In one implementation, the first dielectric ARC  60  has a thickness in the range of approximately 200-500 angstroms (Å) although in general, the thickness will vary depending on the particular application and can be greater than 500 Å or less than 200 Å. 
   Next, a BPSG layer  62  is deposited on the first dielectric ARC  60 , for example, by vapor deposition. The BPSG layer  62  can be heated at a temperature of approximately 850 celsius (° C.) to form an interlayer insulation film having a relatively flat surface. The thickness of the BPSG layer  62  is typically much greater than the thickness of the first dielectric ARC  60  and may be as great as 20,000 Å. Preferably, the first ARC  60  extends substantially beneath the entire BPSG layer  62 . 
   A second dielectric ARC  64  is formed over substantially the entire upper surface of the BPSG layer  62 . The second dielectric ARC  64  also can include, for example, a silicon oxide (Si x O y :H) film, a silicon nitride film (Si x N y :H) or a silicon oxy-nitride (Si x O y N z :H) film. Other materials also can used for the second dielectric ARC  64 . In one implementation, the second dielectric ARC  64  has a thickness in the range of approximately several hundred angstroms (Å), although in general, the thickness will vary depending on the particular application and can be greater or less. 
   A photosensitive film, such as a photoresist  66 , is deposited over the second dielectric ARC  64 , and a mask  68  is positioned over the photoresist prior to exposure of the photoresist to an appropriate source of radiation  70 . Depending on the properties of the photoresist, the radiation which exposes the photoresist can include one or more different wavelengths. Exemplary sources of radiation include those used in i-line techniques with a wavelength of about 365 nanometers (nm), KrF excimer laser techniques with a wavelength of about 248 nm, and KnF excimer laser techniques with a wavelength of about 193 nm. In the illustrated example of  FIG. 3 , the mask  68  is used to define contact holes for one of the previously-formed layers. 
   One advantage of providing the dielectric ARC  60  immediately below the BPSG layer  62  is that the ARC  60  can reduce the amount of notching that may result during the photolithographic process for defining the contact holes. In general, the complex refractive index of the first ARC  60  can be selected to maximize the absorption at the first anti-reflective coating to minimize amount of transmitted light through the first anti-reflective coating back from the underlying structures. Therefore, the effects of the non-uniform structures in the layers below the first ARC can be reduced or eliminated. That, in turn reduces the amount of light that can be reflected back toward the photo-sensitive material and, therefore, further reduces notching. 
   The complex refractive index of a material includes a real part n, known as the refractive index and defining the velocity of light in the material, and an imaginary part k which corresponds to the material&#39;s light absorption coefficient. In the discussion that follows, the real and imaginary parts (n, k) of the complex refractive index for the various layers will be referred to as follows: 
   
     
       
         
             
             
             
             
           
             
                 
                 
             
             
                 
               Layer 
               n 
               k 
             
             
                 
                 
             
           
          
             
                 
               Underlayer 
               n 1   
               k 1   
             
             
                 
               First ARC (60) 
               n 2   
               k 2   
             
             
                 
               BPSG (62) 
               n 3   
               k 3   
             
             
                 
               Second ARC (64) 
               n 4   
               k 4   
             
             
                 
                 
             
          
         
       
     
   
   The reflectance and transmittance at the interface of the BPSG layer  62  and the first dielectric ARC  60  are determined by n 2 , k 2 , n 3  and k 3 . To increase the absorption in the first dielectric ARC  60 , k 2  should be relatively high, for example, at least as high as 1.0 and preferably as high as 1.5. 
   The first dielectric ARC  60  can include, for example, a silicon oxide (Si x O y :H) film, a silicon nitride (Si x N z :H) film or a silicon oxy-nitride (Si x O y N z :H) film. Other materials, including titanium nitride and organic compounds such as polymers, also can used for the first dielectric ARC  60 . In one implementation, a Si x O y N z :H layer can be formed as the ARC  60  using a plasma-enhanced vapor deposition technique. A film including a mixture of silicon (Si), oxygen (O), nitrogen (N) and hydrogen (H) atoms can be formed by exciting a plasma in a gas mixture of silane and nitrogen oxide (N 2 O) diluted by helium (He). For example, to obtain a Si x O y N z :H layer with a value of n equal to about 2.13 and a value of k equal to about 1.23 for light having a wavelength of 248 nm, the flow rates of silane, N 2 O and He can be approximately 80 sccm, 80 sccm and 2200 sccm, respectively. The Si x O y N z :H layer is formed at a temperature of about 400° C. and a pressure of about 5.6 Torr. The RF power can be set to approximately 105 watts. 
   If the topography of the underlying structure below the first ARC  60  is not substantially planar, then light reflected from the interface of the first ARC and the BPSG layer  62  will tend to scatter in various directions non-uniformly. Providing the second ARC  64  between the photoresist layer  66  and the BPSG layer  62  can help reduce the effects that any such non-uniform light has on the photoresist. 
   The second ARC  64  also can include, for example, a Si x O y :H film, a Si x N z :H film or a Si x O y N z :H film. Other materials, including titanium nitride and organic compounds such as polymers, also can be used for the second dielectric ARC  64 . In general, the values of n 4  and k 4  for the second ARC  64  should be selected to minimize or reduce the reflectivity at the interface between the photoresist layer  66  and the second ARC  64 . Selecting a suitable material for the second ARC  64  can be determined using known simulated techniques. Generally, however, selection of the material for the second ARC  64  will depend on the BPSG layer  62  as well as on the first ARC  60 . The layers below the first ARC  60  can be ignored due to the high absorption of the first ARC  60 . 
   Once the photolithographic process is performed and the photoresist  66  is developed, the ARC  64 , the BPSG layer  62 , the ARC  60 , the silicon oxide layers  56 ,  58  and the gate oxide film  38  are etched successively to form a contact hole  72  ( FIG. 4 ). The remaining resist  66  then can be removed, and fabrication of the device, including the formation of metallization contacts, can be completed using conventional techniques. Upon completion of the device, the first anti-reflective coating  60  will extend beneath substantially the entire transparent layer  62 . 
   In some cases, the layer which is transparent to the wavelength(s) of light may include multiple vertically stacked layers  62 A,  62 B ( FIG. 5 ) rather than a single layer, where each of the multiple layers  62 A,  62 B is transparent to the wavelength(s) of light used during the photolithographic process. 
   In situations where the topography of the underlying structures below the first anti-reflective coating is substantially planar, the second anti-reflective coating need not be provided. As shown in  FIG. 6 , a semiconductor wafer  80 , which may include one or more previously-formed layers or regions, has a substantially planar topography. An anti-reflective coating  82  is provided over the wafer  80 . The composition of the anti-reflective coating  82  can be similar to those discussed above with respect to the anti-reflective coating  60 . In general, as previously discussed, the complex refractive index of the anti-reflective coating  82  should be selected to maximize the absorption at the anti-reflective coating to minimize the amount of transmitted light through the anti-reflective coating back from the underlying structures formed on the wafer  80 , A BPSG or other protective layer  84  is provided over the anti-reflective coating  82 . A photo-lithographic pattern then is formed by providing a photosensitive film  86 , such as photoresist, over the protective layer  84  and exposing the photoresist to an appropriate source of radiation  90  with a mask  88  in place over the photoresist. Although the protective layer  84  is transparent to the radiation  90 , any light reflected from the interface of the protective layer and the anti-reflective coating  82  will tend to be reflected substantially uniformly. Therefore, notching of the photoresist pattern  86  can be reduced or eliminated. 
   Although the foregoing techniques have been described with respect to a protective layer in a specific semiconductor device, an anti-reflective coating can be provided immediately below any layer to which a photo-lithographic pattern is to be transferred and which is transparent to the wavelength of light used during the photolithographic process. 
   Other implementations are within the scope of the following claims.