Abstract:
Embodiments of the invention relate generally to semiconductor wafer technology and, more particularly, to the use of conformal grounding for active charge screening on wafers during wafer processing and metrology. A first aspect of the invention provides a method of reducing an accumulated surface charge on a semiconductor wafer, the method comprising: grounding a layer of conductive material adjacent a substrate of the wafer; and allowing a mirrored charge substantially equal in magnitude and opposite in sign to the accumulated surface charge to be induced along the conductive material.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a divisional of currently pending U.S. patent application Ser. No. 13/368,630 filed on 8 Feb. 2012. The application identified above is incorporated herein by reference in its entirety for all that it contains in order to provide continuity of disclosure. 
     
    
     BACKGROUND 
       [0002]    Embodiments of the invention relate generally to semiconductor wafer technology and, more particularly, to the use of conformal grounding for active charge screening on wafers during wafer processing and metrology. 
         [0003]    Wafer charging during manufacture and measurement is a major challenge that can have dramatic impacts on manufacturing yields. Charge build-up on and in wafers during manufacture and measurement is attributable to their non-conductive materials, including silicon, common resist materials, dielectric materials, and low-k materials. Surface charge and bulk charge can produce electrical potentials peaking at several hundred volts, especially around sharp and high aspect ratio features. Silicon-on-insulator (SOI) wafers tend to suffer from greater charging than bulk silicon wafers due to the presence of oxide insulation. 
         [0004]    Wafers can accumulate and retain charge for a variety of reasons, some of which are unavoidable in wafer processing and metrology. Simple handling of wafers, including their loading onto and unloading from various machines can result in charge accumulation. Other processing and metrology techniques necessarily employ electrical current, which can result in wafer charging. For example, high-current ion implanters deliver around 25 mA, e-beam lithography tools deliver about 10 μA, and critical dimension scanning electron microscopes (CDSEMs) deliver around 10 pA to a wafer. Other charging sources include ultra-violet (UV) and X-ray irradiation. 
         [0005]    The effects of wafer charging are varied and significant. For example, e-beam-based metrology techniques can cause registration, alignment, and automation failure, as well as distortion in e-beam-based image formation. Plasma processing technologies and ultra-low energy implanters can cause process excursion. Poor device performance can result from electrical discharge or permanent trapping around a device or memory area. Imaging using low landing-energy e-beams is more susceptible to wafer surface charging, resulting in the need to reduce the landing voltage of CDSEM beams to limit resist shrinkage. Photomasks and atomic force microscopy (AFM) also result in substrate and wafer charging. 
         [0006]    Known attempts to address wafer charging suffer from at least two drawbacks. First, they are typically voltage-based, rather than charge-based. That is, known attempts apply a voltage to the wafer, which is premised on two assumptions, neither of which is generally true. The first assumption is that the wafer capacitance is constant. The second assumption is that the trapped charge is static. 
         [0007]    A second drawback of known attempts to address wafer charging is that they are tool-based. This necessarily adds to the cost of manufacture and metrology and shifts the solution from the manufacturing facility to the tool vendor side of the business. 
         [0008]    For example, surface-charge potential measurement (SPM) uses a set of electrostatic probes in the wafer transfer path to map an electric potential attributable to trapped charges. A corrective voltage corresponding to the mapped potential is superimposed on the wafer locally. This technique necessarily only corrects for charges accumulated before wafer loading and does not correct or otherwise address wafer charge accumulation during processing or loading, and does not account for charge diffusion in the wafer bulk after loading into the tool chamber. 
         [0009]    Other attempts to correct wafer charging have used electron showers or floods following some implant operations. However, this only neutralizes positive charges and runs the very real risk of negatively over charging the wafer. 
         [0010]    Still other attempts involve washing wafers with deionized water or carbon dioxide. Aside from the high cost, such techniques necessarily only neutralize surface charges and have no effect on charges trapped in the wafer bulk. 
       SUMMARY 
       [0011]    The invention provides a method of reducing an accumulated charge in a semiconductor wafer and a wafer structure therefor. 
         [0012]    A first aspect of the invention provides a method of reducing an accumulated surface charge on a semiconductor wafer, the method comprising: grounding a layer of conductive material adjacent a substrate of the wafer; and allowing a mirrored charge substantially equal in magnitude and opposite in sign to the accumulated surface charge to be induced along the conductive material. 
         [0013]    A second aspect of the invention provides a method of reducing an accumulated bulk charge in a semiconductor wafer, the method comprising: grounding a layer of conductive material adjacent a substrate of the wafer; and allowing a mirrored charge substantially equal in magnitude and opposite in sign to the accumulated bulk charge to be induced along the conductive material. 
         [0014]    A third aspect of the invention provides a semiconductor wafer comprising: a substrate including a semiconductor material; and a layer of conductive material, wherein the layer of conductive material, when connected to a ground, is capable of developing an induced charge opposite in sign to an accumulated charge along a surface of the substrate or within the substrate. 
         [0015]    The illustrative aspects of the present invention are designed to solve the problems herein described and other problems not discussed which are discoverable by a skilled artisan. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0016]    These and other features of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings that depict various embodiments of the invention, in which: 
           [0017]      FIGS. 1 and 2  show, respectively, a cross-sectional side view and a top view of an accumulated charge on a semiconductor wafer. 
           [0018]      FIGS. 3A and 3B  show, respectively, critical dimension scanning electron microscope (CDSEM) images of a charged and an uncharged semiconductor wafer. 
           [0019]      FIG. 4  shows a cross-sectional side view of a semiconductor wafer according to an embodiment of the invention. 
           [0020]      FIGS. 5 and 6  show top views of, respectively, an accumulated charge and an induced charge on the semiconductor wafer of  FIG. 4 . 
           [0021]      FIG. 7  shows a cross-sectional side view of a semiconductor wafer according to another embodiment of the invention. 
           [0022]      FIG. 8  shows a cross-sectional side view of a semiconductor wafer according to another embodiment of the invention. 
           [0023]      FIG. 9  shows a cross-sectional side view of a semiconductor wafer according to another embodiment of the invention. 
           [0024]      FIG. 10  shows a cross-sectional side view of a semiconductor wafer according to another embodiment of the invention. 
           [0025]      FIGS. 11A-G  show graphical representations of electron beam ray traces at various degrees of wafer charging. 
           [0026]      FIGS. 12A-G  show graphical representations of electron beam resolutions at various degrees of wafer charging. 
           [0027]      FIGS. 13A-E  show CDSEM images at various degrees of wafer charging. 
           [0028]      FIG. 14  shows a graph of electron beam radius as a function of wafer charging in semiconductor wafers with and without a grounded back coat. 
       
    
    
       [0029]    It is noted that the drawings of the invention are not to scale. The drawings are intended to depict only typical aspects of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements between the drawings. 
       DETAILED DESCRIPTION 
       [0030]    Turning now to the drawings,  FIG. 1  shows a cross-sectional side view of a semiconductor wafer  10 . An accumulated charge  12  is present along a surface of semiconductor wafer  10 , as may be induced, for example, by wafer handling or the use of processing and/or metrology instruments, such as those described above. Although not shown in  FIG. 1 , but will be described in greater detail below, an accumulated charge may be present in the bulk of semiconductor wafer  10  rather than or in addition to along a surface of semiconductor wafer  10 .  FIG. 2  shows a top view of a distribution of accumulated charge  12  along the surface of semiconductor wafer  10 . 
         [0031]    Semiconductor wafer  10  may include any number of semiconducting materials, including, for example, silicon, germanium, silicon germanium, silicon carbide, and those consisting essentially of one or more III-V compound semiconductors having a composition defined by the formula Al X1 Ga X2 In X3 As Y1 P Y2 N Y3 Sb Y4 , where X 1 , X 2 , X 3 , Y 1 , Y 2 , Y 3 , and Y 4  represent relative proportions, each greater than or equal to zero and X 1 +X 2 +X 3 +Y 1 +Y 2 +Y 3 +Y 4 =1 (1 being the total relative mole quantity). Other suitable substrates include II-VI compound semiconductors having a composition Zn A1 Cd A2 Se B1 Te B2 , where A 1 , A 2 , B 1 , and B 2  are relative proportions each greater than or equal to zero and A 1 +A 2 +B 1 +B 2 =1 (1 being a total mole quantity). Furthermore, a portion or entire semiconductor substrate may be strained. 
         [0032]      FIGS. 3A and 3B  show, respectively, critical dimension scanning electron microscope (CDSEM) images of an uncharged semiconductor wafer and a charged semiconductor wafer. As can be seen in  FIG. 3B , wafer charging has resulted in image defocusing, making measurement of wafer features less accurate than is possible with the uncharged wafer in  FIG. 3A . As wafer charging and image defocusing increase, measurement necessarily becomes less accurate and, eventually, impossible. Other metrology problems are associated with wafer charging, including, for example, beam drift and distortion. 
         [0033]      FIG. 4  shows a cross-sectional side view of semiconductor wafer  10  according to one embodiment of the invention. Here, a back coat  20  of conductive material is disposed along a surface of semiconductor wafer  10  opposite accumulated charge  12 . Back coat  20  may include any conductive material or combination of conductive materials, including metals. Suitable conductive materials include, for example, copper, silver, aluminum, gold, tungsten, zinc, nickel, lithium, iron, platinum, tin, lead, titanium, graphite, carbon nanotubes, and carbon nanowires. Similarly, back coat  20  may be applied to semiconductor wafer  10  using any suitable technique, including, for example, chemical vapor deposition (CVD), low-pressure CVD (LPCVD), plasma-enhanced CVD (PECVD), semi-atmosphere CVD (SACVD) and high density plasma CVD (HDPCVD), rapid thermal CVD (RTCVD), ultra-high vacuum CVD (UHVCVD), limited reaction processing CVD (LRPCVD), metalorganic CVD (MOCVD), sputtering deposition, ion beam deposition, electron beam deposition, laser assisted deposition, thermal oxidation, thermal nitridation, spin-on methods, physical vapor deposition (PVD), atomic layer deposition (ALD), chemical oxidation, molecular beam epitaxy (MBE), plating, evaporation. 
         [0034]    Once connected to a ground  30 , back coat  20  develops an induced charge  22  equal in magnitude but opposite in sign to the accumulated charge  12 . Thus, induced charge  22  may be referred to as a mirrored charge, i.e., a charge that is equal to and opposite accumulated charge  12 . This technique, known in electrodynamics as “the method of images,” makes no assumptions as to the charge profile or distribution in accumulated charge  12 . 
         [0035]    Once grounded, the charge is mirrored dynamically in real time. The resulting dipole configuration causes an order of magnitude reduction in the collective field above the wafer and is independent of the original wafer charge profile, polarity or magnitude. 
         [0036]    In essence, semiconductor wafer  10  and grounded back coat  20  forms an effective dipole moment, reducing the interaction between semiconductor wafer  10  and processing plasma or a charged beam being applied to semiconductor wafer. This significantly mitigates distortion in primary electron beam optics and secondary emission beam collection for electron beam imaging and reduces etch and implant irregularities in wafer processing. 
         [0037]    By way of illustration and with the assumption for simplicity that the accumulated charge density on the wafer is constant, then the electric potential caused by induced charge  22  may be calculated according to Equation 1 below, wherein I is the electric potential caused by induced charge  22 , z is a distance from a surface of the semiconductor wafer, d is a thickness of the semiconductor wafer, and D is a diameter of the semiconductor wafer. 
         [0000]    
       
         
           
             
               
                 
                   I 
                   = 
                   
                     - 
                     
                       σ 
                       
                         2 
                          
                         
                           ɛ 
                           ( 
                           
                             
                               
                                 
                                   
                                     ( 
                                     
                                       z 
                                       + 
                                       
                                         2 
                                          
                                         d 
                                       
                                     
                                     ) 
                                   
                                   2 
                                 
                                 + 
                                 
                                   D 
                                   2 
                                 
                               
                             
                             - 
                             z 
                             - 
                             
                               2 
                                
                               d 
                             
                           
                           ) 
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   1 
                 
               
             
           
         
       
     
         [0038]    Electric potential caused by accumulated charge  12  may be calculated according to Equation 2 below, wherein A is the electric potential caused by accumulated charge, a is a surface charge density and c is a permittivity constant. 
         [0000]    
       
         
           
             
               
                 
                   A 
                   = 
                   
                     σ 
                     
                       2 
                        
                       
                         ɛ 
                         ( 
                         
                           
                             
                               
                                 z 
                                 2 
                               
                               + 
                               
                                 D 
                                 2 
                               
                             
                           
                           - 
                           z 
                         
                         ) 
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   2 
                 
               
             
           
         
       
     
         [0039]    Thus, induced charge  22  results in a reduction in the total potential above the semiconductor wafer  10  of approximately an order of magnitude. The total potential (V total ) above the semiconductor wafer  10  may be calculated according to Equation 3 below. 
         [0000]    
       
         
           
             
               
                 
                   
                     V 
                     total 
                   
                   = 
                   
                     
                       ( 
                       
                         
                           
                             σ 
                             
                               2 
                                
                               ɛ 
                             
                           
                            
                           
                             
                               
                                 z 
                                 2 
                               
                               + 
                               
                                 D 
                                 2 
                               
                             
                           
                         
                         - 
                         z 
                       
                       ) 
                     
                     - 
                     
                       
                         
                           
                             ( 
                             
                               z 
                               + 
                               
                                 2 
                                  
                                 d 
                               
                             
                             ) 
                           
                           
                             2 
                              
                             
                                 
                             
                           
                         
                         + 
                         
                           D 
                           2 
                         
                       
                     
                     - 
                     z 
                     - 
                     
                       2 
                        
                       d 
                        
                       
                         σ 
                         
                           2 
                            
                           
                             ɛ 
                              
                             
                               [ 
                               
                                 
                                   ( 
                                   
                                     
                                       
                                         
                                           z 
                                           2 
                                         
                                         + 
                                         
                                           D 
                                           2 
                                         
                                       
                                     
                                     - 
                                     z 
                                   
                                   ) 
                                 
                                 - 
                                 
                                   ( 
                                   
                                     
                                       
                                         
                                           
                                             ( 
                                             
                                               z 
                                               + 
                                               
                                                 2 
                                                  
                                                 d 
                                               
                                             
                                             ) 
                                           
                                           2 
                                         
                                         + 
                                         
                                           D 
                                           2 
                                         
                                       
                                     
                                     - 
                                     z 
                                     - 
                                     
                                       2 
                                        
                                       d 
                                     
                                   
                                   ) 
                                 
                               
                               ] 
                             
                           
                         
                       
                     
                   
                 
               
               
                 
                   Equation 
                    
                   
                       
                   
                    
                   3 
                 
               
             
           
         
       
     
         [0040]    Any other similar approach to calculate the total potential above the wafer, analytical or numerical, with generalization of the accumulated charge profile is also implied in this invention without any loss of generality. 
         [0041]    Back coat  20  should preferably be as thin as possible without adversely affecting its ability to develop induced charge  22 . The thickness of back coat  20  will therefore vary, depending on the conductive material(s) included in back coat  20 , as well as the method(s) or technique(s) by which back coat  20  is applied. Typical thicknesses of back coat  20  may range from between about a few (e.g., three) nanometers and about a few (e.g., three) microns. One skilled in the art will recognize, however, that this range is merely illustrative of thicknesses typical of some embodiments of the invention and is not meant to be limiting of the scope of the invention. A back coat  20  of any conductive material of any thickness that is capable of developing induced charge  22  is within the scope of the invention. 
         [0042]      FIGS. 5 and 6  show, respectively, top views of a distribution of accumulated charge  12  on semiconductor wafer  10  (as in  FIG. 2 ) and a distribution of induced charge  22  along back coat  20 . As can be seen in  FIGS. 4-6 , induced charge  22  includes charges opposite in sign to accumulated charge  12 . 
         [0043]    It should be noted that ground  30  may be a non-zero potential. That is, a total charge of semiconductor wafer  10  may be reduced by grounding back coat  20  to a zero potential or a non-zero potential. In some embodiments of the invention, back coat  20  is grounded to a non-zero potential. 
         [0044]      FIG. 7  shows a cross-sectional side view of another embodiment of the invention. Here, a layer of conductive material  21  is disposed beneath an oxide layer  16  within the bulk semiconductor  10 . Functionally, the embodiment in  FIG. 7  is similar to that in  FIG. 4 , with an induced charge  22  opposite in sign to accumulated charge  12  developing upon connecting conductive material  21  to ground  30 . 
         [0045]      FIG. 8  shows a cross-sectional side view of yet another embodiment of the invention. Here, a thin, highly-conductive plate  23  capable of conformal attachment to semiconductor wafer  10  and connection to ground  30  may be alternately attached to and detached from semiconductor wafer  10 . Such an embodiment may be particularly useful, for example, during loading and unloading semiconductor wafer  10  during processing and metrology. As in other embodiments of the invention, upon connecting plate  23  to ground  30 , an induced charge will develop along plate  23  that is opposite in sign to an accumulated charge along or within semiconductor wafer  10 . 
         [0046]      FIG. 9  shows a cross-sectional side view of still another embodiment of the invention. Here, a thin, conductive coating  24  of graphene and/or other conductive material(s) is applied to semiconductor wafer  10  prior to processing and/or metrology. Conductive coating  24  may be stripped from semiconductor wafer  10  after processing and/or metrology. Such an embodiment may be useful, for example, in cases where metal migration into semiconductor wafer  10  is to be avoided. Connecting conductive coating  24  to ground  30  will permit an induced charge to develop along conductive coating  24  that is opposite in sign to an accumulated charge along or within semiconductor wafer  10 . 
         [0047]      FIG. 10  shows a cross-sectional side view of yet another embodiment of the invention. Here, semiconductor wafer  10  includes a highly-doped layer  14  having increased conductivity. When connected to ground  30 , an induced charge  22  develops along the highly-doped layer  14  that is opposite in sign to accumulated charge  12 . 
         [0048]      FIGS. 11A-G  show numerical simulations of an impinging electron beam ray traces at various degrees of wafer charging, both with and without the back coat described above.  FIG. 11A  shows an electron beam ray trace of an uncharged (0 V) wafer.  FIG. 11B  shows an electron beam ray trace of a wafer with 10 V charging. Some distortion in the electron beam ray trace can be observed, as compared to  FIG. 11A .  FIG. 11C  shows an electron beam ray trace of a wafer with 10 V charging and the grounded back coat described above. As can be seen in  FIG. 11C , the electron beam ray trace is more similar to that in  FIG. 11A  than that in  FIG. 11B . 
         [0049]      FIGS. 11D-E  and  11 F-G show electron beam ray traces with greater wafer charging.  FIG. 11D  shows an electron beam ray trace with 50 V wafer charging. Significant distortion, including a rise in the focal point to approximately 200 nm, can be observed, as compared to the uncharged electron beam ray trace of  FIG. 11A .  FIG. 11E  shows the electron beam ray trace with the same 50 V wafer charging, but with the grounded back coat described above. As can be seen, the electron beam ray trace of FIG.  11 E is more similar to that of  FIG. 11A  and, specifically, the focal point is again returned to approximately the working surface. 
         [0050]      FIG. 11F  shows an electron beam ray trace with 300 V wafer charging. Very significant distortion can be seen, including a rise in the focal point to approximately 2000 nm. Distortion to this degree would render the wafer virtually unsuitable for processing or metrology.  FIG. 11G  shows the electron beam ray trace with the same 300 V wafer charging, but with the grounded back coat described above. In  FIG. 11G , distortion is significantly reduced and the focal point returned to approximately the working surface. The difference in appearance of the electron beam ray traces of  FIGS. 11G and 11A  is primarily attributable a difference in the scale of  FIG. 11G , which is made necessary for comparison to the electron beam ray trace of  FIG. 11F . 
         [0051]      FIGS. 12A-G  show the electron beam resolutions at the wafer surface corresponding to each of the electron beam ray traces of  FIGS. 11A-G .  FIG. 12A  shows the electron beam resolution of the uncharged wafer.  FIG. 12B  shows the electron beam resolution of the 10 V wafer charging. A wider, more diffuse electron beam resolution is apparent in  FIG. 12B , as compared to  FIG. 12A .  FIG. 12C  shows the electron beam resolution of the 10 V wafer charging, but with the grounded back coat described above. The electron beam resolution of  FIG. 12C  is narrower and more compact than that of  FIG. 12B  and more closely resembles the electron beam resolution of the uncharged wafer in  FIG. 12A . 
         [0052]      FIGS. 12D and 12E  show uncorrected and corrected electron beam resolutions, respectively, with 50 V wafer charging. Again, the electron beam resolution of  FIG. 12E , where the grounded back coat described above was used, is narrower and more focused. The difference between the electron beam resolutions of  FIGS. 12E and 12A  is primarily attributable to a difference in scale. 
         [0053]    The electron beam resolution of  FIG. 12F , with 300 V wafer charging, is much broader and more diffuse than in  FIG. 12A  (again, taking into account the differences in scale).  FIG. 12G  shows the electron beam resolution with the same 300 V wafer charging, but with the grounded back coat described above. The electron beam resolution of  FIG. 12G  is narrower and more focused. The difference between the electron beam resolutions of  FIGS. 12G and 12A  is primarily attributable to a difference in scale. 
         [0054]      FIGS. 13A-E  show CDSEM images at various wafer chargings.  FIG. 13A  shows a CDSEM image with no wafer charging (0 V).  FIG. 13B  shows the same CDSEM image with 50 V wafer charging. As can be seen in  FIG. 13B , image focus is poorer and wafer features are less clear.  FIG. 13C  shows a CDSEM image with 50 V wafer charging and the grounded back coat described above. In  FIG. 13C , image focus and feature clarity are substantially the same as in  FIG. 13A  and are improved as compared to  FIG. 13B . 
         [0055]      FIG. 13D  shows a numerical simulation of a CDSEM image with 300 V wafer charging. Image focus and wafer features are so poor as to be largely unsuitable for processing or metrology.  FIG. 13E  shows a CDSEM image with the same 300 V wafer charging, but with the grounded back coat described above. As can be seen in  FIG. 13E , image focus and feature clarity are substantially the same as in  FIG. 13A  and are greatly improved as compared to  FIG. 13D . 
         [0056]      FIG. 14  shows a graph of electron beam radius as a function of wafer charging, both with and without a ground back coat as described above. As can be seen in  FIG. 14 , with semiconductor wafers without a grounded back coat, electron beam radius consistently increases with increasing wafer charging. Contrarily, with semiconductor wafers with a grounded back coat, the electron beam radius is substantially constant as wafer charging increases. 
         [0057]    The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. 
         [0058]    The foregoing description of various aspects of the invention has been presented for purposes of illustration and description. It is not intended to be exhaustive or to limit the invention to the precise form disclosed, and obviously, many modifications and variations are possible. Such modifications and variations that may be apparent to a person skilled in the art are intended to be included within the scope of the invention as defined by the accompanying claims.