Abstract:
Disclosed is a method of fabricating a semiconductor device, comprising: (a) providing a bare semiconductor substrate, the substrate having a frontside and a backside; (b) forming one or more protective films on the backside of the substrate; and (c) performing one or more wafer fabrication steps. Some or all the protective films may be removed and the method repeated multiple times during fabrication of the semiconductor device.

Description:
FIELD OF THE INVENTION  
         [0001]    The present invention relates to the field of semiconductor manufacturing; more specifically, it relates to a method of protecting the backside surface of semiconductor substrates.  
         BACKGROUND OF THE INVENTION  
         [0002]    In the manufacture of semiconductor devices, semiconductor substrates, most notably, silicon wafers, are subjected a wide variety of process steps during formation of devices on the frontside of the substrate. These process steps include forming process films, etching process films, etching the substrate itself and placing dopants into the substrate to name a few. During these processes, it is important to protect and/or seal the backside of the substrate from damage or contamination. For example, mechanical damage to the backside or formation of rails on the backside during frontside only film formation can affect photolithographic processes, the backside of the substrate can become contaminated and then contaminate the frontside of the adjacent substrate or backside charging can occur, leading to defective or inoperable devices.  
           [0003]    With many traditional film formation tools, process films are formed on the backside of the substrate as a by-product to frontside film formation. These incidentally formed backside films are used to protect or seal the backside of the substrate.  
           [0004]    With the advent of frontside only deposition tools such as those used in high-density plasma (HDP) and rapid thermal chemical vapor deposition (RTCVD) tools, incidental backside films are no longer available to protect and/or seal the backside of the substrate. The lack of incidental protective films will create a series of undesirable effects including: inability to remove some process films formed on the backside, inability to prevent or remove backside defects, inability to minimize charging events at subsequent processing steps, attack of the backside of the substrate or backside process films during backside rail removal, and contamination of the backside of the substrate.  
           [0005]    Therefore, there is a need to provide purposefully formed backside films at strategic steps in the fabrication processes in order to avoid the undesirable effects cause by the lack of incidental backside films.  
         SUMMARY OF THE INVENTION  
         [0006]    A first aspect of the present invention is a method of fabricating a semiconductor device, comprising: (a) providing a bare semiconductor substrate, the substrate having a frontside and a backside; (b) forming one or more protective films on the backside of the substrate; and (c) performing one or more wafer fabrication steps.  
           [0007]    A second aspect of the present invention is method of fabricating a semiconductor device, comprising: (a) providing a bare semiconductor substrate, the substrate having a frontside and a backside; (b) forming a protective film on the frontside and the backside of the substrate; (c) removing the protective film from the frontside of the substrate; and (d) performing one or more wafer fabrication steps.  
           [0008]    A third aspect of the present invention is method of fabricating a semiconductor device, comprising: (a) providing a bare semiconductor substrate, the substrate having a frontside and a backside; (b) forming a first protective film on the frontside and the backside of the substrate; (c) forming a second protective film over the first protective film; (d) removing the first and second protective films from the frontside of the substrate; and (e) performing one or more wafer fabrication steps. 
       
    
    
     BRIEF DESCRIPTION OF DRAWINGS  
       [0009]    The features of the invention are set forth in the appended claims. The invention itself, however, will be best understood by reference to the following detailed description of an illustrative embodiment when read in conjunction with the accompanying drawings, wherein:  
         [0010]    [0010]FIG. 1 is a flowchart of the method of providing backside protective films according to the present invention;  
         [0011]    [0011]FIGS. 2A through 2D are cross-sectional views of a first embodiment of the present invention;  
         [0012]    [0012]FIGS. 3A through 3E are cross-sectional views of a second embodiment of the present invention;  
         [0013]    [0013]FIGS. 4A through 4D are cross-sectional views of a third embodiment of the present invention;  
         [0014]    [0014]FIGS. 5A through 5E are cross-sectional views of a fourth embodiment of the present invention; and  
         [0015]    [0015]FIGS. 6A through 6D are cross-sectional views of a fifth embodiment of the present invention. 
     
    
     DETAILED DESCRIPTION OF THE INVENTION  
       [0016]    [0016]FIG. 1 is a flowchart of the method of providing backside protective films according to the present invention. In step  100 , a bare semiconductor substrate is provided. In one example, the semiconductor substrate is a silicon wafer. In step  105 , one or more process films are formed on the backside and frontside of the wafer. The frontside of the wafer is defined as the side of the wafer on which semiconductor devices and integrated circuits are fabricated. Process films are defined as films formed on the wafer during fabrication of semiconductor devices or integrated circuits as a necessary part of or a by-product of the fabrication process. Step  105  may include surface clean processes. Examples of surface clean processes, well known in the art, include: acid treatments, solvent treatments, chemical oxidation/reduction treatments and etch treatments. Step  105  is an optional step.  
         [0017]    In step  110 , a first protective film is formed on the frontside and backside of the wafer. Protective films are defined as strategic films that are formed on the wafer solely to protect and/or seal wafer surfaces and/or process films formed on wafer surfaces, from damage, contamination or unwanted removal during the fabrication process. If optional process films are formed on the wafer in step  105 , then the first protective film is formed on top of the process films, otherwise the first protective film is formed directly on the frontside and backside surfaces of the wafer. The first protective film may serve one of two purposes depending upon whether the first protective film is used alone or with a second protective film formed on top of the first protective film. If the first protective film is the only protective film then the first protective film is used for protection of the backside of the wafer (or process films formed on the backside of the wafer), against mechanical damage, as a contamination barrier, especially for metallic contamination from wafer chucks, or as a contamination getter, for protection of the backside from frontside etchants, as a diffusion barrier, especially from frontside out-diffusion, or to reduce charging. If the first protective film is used with a second protective film, then the first protective film is used to protect the frontside of the wafer (or process films formed on the frontside of the wafer), from the second protective film, as an adhesion promoter for the second protective film or to allow for removal of the second protective film from the frontside of the wafer. Step  110  may include surface clean processes.  
         [0018]    In step  115 , an optional second protective film is formed on the frontside and backside of the wafer. The second protective film is used for protection of the backside of the wafer (or process films formed on the backside of the wafer), against mechanical damage, as a contamination barrier, especially for metallic contamination from wafer chucks, or as a contamination getter, for protection of the backside from frontside etchants, as a diffusion barrier, especially from frontside out-diffusion, or to reduce charging. Step  115  may include surface clean processes.  
         [0019]    In step  120 , the first protective film (and optional second protective film, if one was formed) is removed from the frontside of the wafer. Any process films formed in step  105  are not removed.  
         [0020]    In step  125 , one or more wafer process steps necessary for fabrication of semiconductor devices is performed. Wafer processing steps include: process film depositions and evaporations, thermal oxidations (forms a process film), ion implants, diffusions, wet and plasma etching, photoresist apply, develop, and strip, photolithography, cleans and anneals. The exact processes performed depend on the design of the semiconductor devices and integrated circuits being fabricated and where in the fabrication sequence the wafer is.  
         [0021]    In step  130 , the second protective film is optionally removed or both the first and second protective films are optionally removed. In step  135 , it is determined if further wafer processing is required. If in step  135  further wafer processing is required it is determined if additional protective films are to be formed on the frontside and backside of the wafer. If in step  140 , additional protective films are to be formed, the method loops to step  105 , otherwise the method loops to step  125 . If in step  135 , it is determined no further wafer processing is required then in step  145 , it is determined if one or more backside films are to be removed from the backside of the wafer. Backside films are defined as protective films and process films. If in step  145 , it is determined that backside films are to be removed then, in step  150  one or more protective films and/or one or more process films are removed. Examples of removal processes include wet and dry etching, chemical-mechanical-polishing (CMP) and grinding. Then, in step  155 , the wafer is optionally tested and/or diced into integrated circuit chips or semiconductor devices. If in step  145 , it is determined that no backside films are to be removed then the method proceeds to step  155  and the method terminates.  
         [0022]    [0022]FIGS. 2A through 2D are cross-sectional views of a first embodiment of the present invention. In FIG. 2A, a bare semiconductor substrate wafer  160  has a frontside  165 , a backside  170  and an edge  175 .  
         [0023]    In FIG. 2B, a first protective film  180  is formed on frontside  165 , backside  170  and edge  175  of wafer  160 . First protective film  180  has a backside outer surface  185  where the first protective film is formed on backside  170  of wafer  160 .  
         [0024]    In FIG. 2C, a second protective film  190  is formed on backside outer surface  185  of first protective film  180 . In a first example, first protective film  180  is TEOS (tetraethoxysilane) oxide about 50 Å to 7000 Å thick formed using any number of well known chemical vapor deposition (CVD) processes and second protective film  190  is silicon carbide about 350 Å to 5000 Å microns thick formed only on backside outer surface  185  of first protective film  180  using a tool/process such as AMAT Producer for BLOK by AMAT, Santa Clara, Calif. In a second example, first protective film  180  is TEOS oxide about 350 Å to 5000 Å microns thick formed by any well known CVD process and second protective film  190  is dual layer of about 350 Å to 5000 Å of diamond over about 350 Å to 5000 Å of silicon nitride, silicon carbide or silicon formed only on backside outer surface  185  of first protective film  180 . At the present state of the technology, diamond is best formed over silicon, silicon nitride or silicon carbide. Diamond may be formed by a plasma enhanced chemical vapor deposition (PECVD) process under low pressure using H 2  and CH 4  gases using a process offered by P1 Diamond Inc, Santa Clara, Calif. In either example, a thermal oxide may be substituted for TEOS.  
         [0025]    In FIG. 2D, first protective film  180  is removed from frontside  165  and edge  175  of wafer  160  by etching in dilute HF. At this point, one or more wafer processing steps may be performed as illustrated in FIG. 1 and described in step  125  above.  
         [0026]    First protective film  180  protects frontside  165  of wafer  160  from mechanical damage during formation of second protective film  190 . Second protective film protects backside  170  of wafer  160  from mechanical damage and acts as a diffusion barrier. Since silicon carbide and diamond are impervious to nearly all standard semiconductor wet etches, first protective film  180  sandwiched between second protective film  190  and backside  170  of wafer  160  is maintained during wafer process steps such as HF etches. The presence of first protective film  180  sandwiched between second protective film  190  and backside  170  of wafer  160  minimizes wafer charging. Second protective film  190  may removed by mechanical means such as CMP or grinding.  
         [0027]    [0027]FIGS. 3A through 3E are cross-sectional views of a second embodiment of the present invention. In FIG. 3A, a bare semiconductor substrate wafer  260  has a frontside  265 , a backside  270  and an edge  275 .  
         [0028]    In FIG. 3B, one or more process films are formed on wafer  260 . In one example, the process films comprise a frontside pad oxide film  295 A formed on frontside  265  and a backside pad oxide film  295 B formed on backside  270  of wafer  260  and a frontside pad silicon nitride film  300 A formed on frontside pad oxide film  295 A and a backside pad silicon nitride layer  300 B formed on backside pad oxide film  295 B. Frontside and backside pad oxide films  295 A and  295 B are about 50 Å to 150 Å thick and may be formed by any number of well-known CVD processes. Frontside and backside pad silicon nitride films  300 A and  300 B are about 700 Å to 1800 Å thick and may be formed by any number of well-known CVD processes.  
         [0029]    In FIG. 3C, first protective film  280  is formed on frontside pad silicon nitride film  300 A, backside pad silicon nitride film  300 B and edge  275  of wafer  260 . A first protective film  280  has a backside outer surface  285  where the first protective film is formed on backside pad silicon nitride film  300 B.  
         [0030]    In FIG. 3D, a second protective film  290  is formed on backside outer surface  285  of first protective film  280 . In one example, first protective film  280  is TEOS oxide about 50 Å to 7000 Å thick formed by any number of well known CVD processes and second protective film  290  is silicon carbide about 350 Å to 5000 Å thick formed only on backside outer surface  185  of first protective film  280  using a tool/process such as AMAT Producer for BLOK by AMAT, Santa Clara, Calif. A thermal oxide may be substituted for TEOS.  
         [0031]    In FIG. 3E, first protective film  280  is removed from frontside pad silicon nitride film  300 A and edge  275  of wafer  260  by etching in dilute HF. At this point, one or more wafer processing steps may be performed as illustrated in FIG. 1 and described in step  125  above.  
         [0032]    First protective film  280  protects frontside  265  of wafer  260  from mechanical damage during formation of second protective film  290 . Second protective film protects backside  270  of wafer  260  from mechanical damage and acts as a diffusion barrier. Since silicon carbide and diamond are impervious to nearly all standard semiconductor wet etches, first protective film  280  sandwiched between second protective film  290  and backside  270  of wafer  260  is maintained during wafer process steps such as HF etches. The presence of first protective film  280  sandwiched between second protective film  290  and backside  270  of wafer  260  minimizes wafer charging. Second protective film  290  may removed by mechanical means such as CMP or grinding.  
         [0033]    [0033]FIGS. 4A through 4D are cross-sectional views of a third embodiment of the present invention. In FIG. 4A, a bare semiconductor substrate wafer  360  has a frontside  365 , a backside  370  and an edge  375 .  
         [0034]    In FIG. 4B, a protective film  380  is formed on frontside  365 , backside  370  and edge  375  of wafer  360 . In one example, protective film  380  is a thick silicon oxide formed by any number of well known oxidation or deposition methods. Protective film  380  has a thickness “T1.” The thickness of protective film  380  depends upon the number and length of oxide etching process steps to which wafer  360  will be subjected. The thickness of protective film  380  is chosen such that about 1000 Å to 5000 Å of silicon oxide remains after wafer processing. Depending upon the processes from which backside  370  of wafer  360  is to be protected, protective film  380  may also be formed from silicon nitride, TEOS oxide, thermal oxide or combinations thereof.  
         [0035]    In FIG. 4C, protective film  380  is removed from frontside  365  and edge  375  of wafer  360  by wet etching or frontside CMP. At this point, one or more wafer processing steps may be performed as illustrated in FIG. 1 and described in step  125  above.  
         [0036]    [0036]FIG. 4D illustrates the thinning of protective layer  380  during wafer processing. Thinning occurs as protective film  380  is etched by process chemicals during fabrication. In FIG. 4D, protective layer is reduced a thickness of “T2”, where “T1&gt;T2.” 
         [0037]    Protective film  380  protects backside  370  of wafer  360  from mechanical damage and acts as a diffusion barrier.  
         [0038]    [0038]FIGS. 5A through 5E are cross-sectional views of a fourth embodiment of the present invention. In FIG. 5A, a bare semiconductor substrate wafer  460  has a frontside  465 , a backside  470  and an edge  475 .  
         [0039]    In FIG. 5B, one or more process films are formed on wafer  460 . In one example, the process films comprise frontside pad oxide film  495 A formed on frontside  465  and backside pad oxide film  495 B formed on backside  470  of wafer  460  and frontside pad silicon nitride film  500 A formed on frontside pad oxide film  495 A and backside pad silicon nitride layer  500 B formed on backside pad oxide film  495 B. Frontside and backside pad oxide films  495 A and  495 B are about 50 Å to 150 Å thick and may be formed by any number of well-known CVD processes. Frontside and backside pad silicon nitride films  500 A and  500 B are about 700 Å to 1800 Å thick and may be formed by any number of well-known CVD processes.  
         [0040]    In FIG. 5C, a protective film  480  is formed on frontside pad silicon nitride film  500 A, backside pad silicon nitride film  500 B and edge  475  of wafer  460 . In one example, protective film  480  is a TEOS oxide formed by any number of well known CVD processes. Protective film  480  has a thickness “T3.” The thickness of protective film  480  depends upon the number and length of etching process steps to which wafer  460  will be subjected. The thickness of protective film  480  is chosen such that about 1000 Å to 5000 Å of TEOS oxide remains after wafer processing. Depending upon the processes from which backside  470  of wafer  460  is to be protected, protective film  480  may also be formed from polysilicon.  
         [0041]    In FIG. 5D, protective film  480  is removed from frontside  465  and edge  475  of wafer  460  by wet etching or frontside CMP. At this point, one or more wafer processing steps may be performed as illustrated in FIG. 1 and described in step  125  above.  
         [0042]    [0042]FIG. 5E illustrates the thinning of protective layer  480  during wafer processing. Thinning occurs as protective film  480  is etched by process chemicals during fabrication. In FIG. 5E, protective layer is reduced to a thickness of “T4”, where “T3&gt;T4.” 
         [0043]    Protective film  480  protects backside  470  of wafer  460  from mechanical damage and acts as a diffusion barrier.  
         [0044]    [0044]FIGS. 6A through 6D are cross-sectional views of a fifth embodiment of the present invention. In FIG. 6A, a bare semiconductor substrate wafer  560  has a frontside  565 , a backside  570  and an edge  575 .  
         [0045]    In FIG. 6B, one or more process films are formed on wafer  560 . In one example, the process films comprise a frontside pad oxide film  595 A formed on frontside  565  and a backside pad oxide film  595 B formed on backside  570  of wafer  560  and a frontside pad silicon nitride film  600 A formed on frontside pad oxide film  595 A and a backside pad silicon nitride layer  600 B formed on backside pad oxide film  595 B. Frontside and backside pad oxide films  595 A and  595 B are about 50 Å to 150 Å thick and may be formed by any number of well-known CVD processes. Frontside and backside pad silicon nitride films  600 A and  600 B are about 700 Å to 1800 Å thick and may be formed by any number of well-known CVD processes.  
         [0046]    In FIG. 6C, a protective film  580  is formed on frontside pad silicon nitride film  600 A, backside pad silicon nitride film  600 B and edge  575  of wafer  560 . In one example protective film  580  is polysilicon about 350 Å to 7000 Å formed by any number of well known CVD processes such as a 620° C. deposition in SVG (Orange, Calif.)VTR600 or VTR700 at 150 millitor and a silane flowrate of about 3 to 500 ccm.  
         [0047]    In FIG. 6D, protective film  580  is removed from frontside pad silicon nitride film  600 A and edge  575  of wafer  560  by etching in dilute HF/HNO 3 . At this point, one or more wafer processing steps may be performed as illustrated in FIG. 1 and described in step  125  above.  
         [0048]    Protective film  580  protects backside  570  of wafer  560  from mechanical damage.  
         [0049]    The description of the embodiments of the present invention is given above for the understanding of the present invention. It will be understood that the invention is not limited to the particular embodiments described herein, but is capable of various modifications, rearrangements and substitutions as will now become apparent to those skilled in the art without departing from the scope of the invention. For example, instead of one or two protective films, three or more may be employed. Therefore it is intended that the following claims cover all such modifications and changes as fall within the true spirit and scope of the invention.