Abstract:
Disclosed herein are methods for forming photolithography alignment markers on the back side of a substrate, such as a crystalline silicon substrate used in the manufacture of semiconductor integrated circuits. According to the disclosed techniques, laser radiation is used to remove the material (e.g., silicon) from the back side of a substrate to form the back side alignment markers at specified areas. Such removal can comprise the use of laser ablation or laser-assisted etching. The substrate is placed on a motor-controlled substrate holding mechanism in a laser removal chamber, and the areas are automatically moved underneath the laser radiation to removal the material. The substrate holding mechanism can comprise a standard chuck (in which case use of a protective layer on the front side of the substrate is preferred), or a substrate clamping assembly which suspends the substrate at its edges (in which case the protective layer is not necessary). Alternatively, a stencil having holes corresponding to the shape of the back side alignment markers can be placed over the back side of the substrate to mitigate the need to move the substrate to the areas with precision.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS  
       [0001]     This application is a divisional of U.S. patent application Ser. No. 10/840,733, filed May 6, 2004, which is incorporated herein by reference in its entirety and to which priority is claimed.  
         [0002]     This application is related to U.S. patent application Ser. No. 10/840,324, filed May 6, 2004, and which is incorporated by reference in its entirety. 
     
    
     FIELD OF THE INVENTION  
       [0003]     Embodiments of this invention relate to improved methods for forming back side alignment markers useable in semiconductor photolithography.  
       BACKGROUND  
       [0004]     When fabricating an integrated circuit, and as is well known, a series of layers are deposited on a substrate (usually a crystalline silicon substrate) and are patterned and etched to form a circuit. For the circuit to work properly, it is important that each subsequent layer be aligned with the previously formed layer or layers, at least within some permissible tolerance.  
         [0005]     To align the various layers, and referring to  FIG. 1A , a substrate  12  having a photoresist applied thereon (not shown) is placed in a photolithographic chamber  10 , sometimes referred to as a “stepper” or “scanner.” In the stepper  10 , a mask or reticle  27  is used to pattern the photoresist. As the patterned photoresist ultimately dictates the positioning of the underlying circuit layer to be etched, its alignment is critical.  
         [0006]     To bring the substrate  12  into alignment with the mask  27 , an image of some structure on the mask and some structure  24  on the wafer are compared using well-known optical analysis equipment  14 , with such images being received by optical sensors  20 . If alignment is needed, the optical analysis equipment  14  can control the positioning of a chuck  16  on which the substrate  12  sits via motor stages  18 , which, for example, can move the chuck  16  along the X-axis, Y-axis, or rotational θ-axis as appropriate. Such alignment is usually assessed at numerous locations around the substrate  12 &#39;s perimeter, which accordingly requires reference to a plurality of alignment structures  24  on the substrate  12 , as shown in  FIG. 1B . However, reference to a single alignment marker could also be used.  
         [0007]     Although alignment structures  24  can constitute an actual active portion of the circuit being fabricated, a dedicated inactive structure is usually formed for this purpose—what is referred to as an alignment marker. Referring to  FIG. 1B , such alignment markers  24  are typically formed outside of the active integrated circuit area  22  on the wafer, i.e., in the area in which the substrate will be scribed or “diced” for later insertion into packages. A simple “cross” pattern is illustrated for the alignment marker  24 , but as one skilled in the art will understand, such markers can come in a variety of different shapes and sizes (e.g., chevrons, gratings, squares, etc.), depending on the alignment task be performed. Typically, more than one alignment marker  24  is fabricated on the substrate  12  as shown, which may range from approximately 20 to 500 microns in size.  
         [0008]     However, alignment markers appearing on the front side of the substrate suffer from the problem that such markers may eventually become covered with opaque materials during later processing steps, and hence may become difficult for the optical sensors  20  to “see,” as discussed at length in above-incorporated U.S. patent application Ser. No. 10/840,324.  
         [0009]     Accordingly, the prior art has experimented with the use of back side alignment markers. As their name suggests, back side alignment markers are located on the opposite side of the substrate from the front side where the active circuitry is formed. The processing steps used to form the active circuitry on the front side generally do not appreciably affect the back side; for example, materials deposited on the front side of the substrate will generally not find their way to the back side, except in trace amounts. Accordingly, back side alignment markers generally remain unaffected during processing of the substrate, and therefore remain visible to the optical sensors  20  for alignment purposes.  
         [0010]     An exemplary stepper chamber  30  relying on the use of back side alignment markers  27  is shown in  FIG. 2 . Such a chamber is generally similar to the chamber  10  of  FIG. 1 , but includes holes  17  in the bottom of the chuck  16  aligned with the backside alignment markers  27 . Mirrors  15  direct light between the back side alignment markers  27  and the optical sensors  20  to allow the alignment markers  27  to be “seen” for alignment purposes. Alternatively, channels through the chuck and parallel to the substrate  12 &#39;s surface can carry the optical path between the back side alignment markers  27  and the optical sensors  20 , such as is disclosed in http://www.minanet.com/documents/ASML.pdf (Sep. 25, 2003), which is submitted herewith and which is incorporated by reference in its entirety.  
         [0011]     However, back side alignment markers still suffer from processing difficulties, as illustrated by the process of  FIG. 3 , which shows how such back side alignment markers are traditionally formed.  FIG. 3A  shows in cross section a blank or stating substrate  12 , which again is usually a silicon crystalline substrate. The substrate  12  has a front side  12   a  and a bottom side  12   b . Prior to fabrication of the integrated circuit on the front side  12   a , the front side is highly polished, rendering the front side  12   a  to near perfect smoothness at the atomic level that is appropriate for the formation of transistors and the like. The back side  12   b  is generally also smooth, but usually not as smooth as the front side  12   a.    
         [0012]     Traditionally, the back side alignment markers  27  are formed using traditional photolithography techniques. However, care must be taken to protect the near-perfectly smooth front side  12   a , as this surface is easily scratched. If scratched, the electrical structures (such as transistors) eventually formed at the front side  12   a  will “leak” current and otherwise may perform poorly from an electrical standpoint. Accordingly, before formation of the back side alignment markers  27 , a protective layer  40  is formed on the front side  12   a , as shown in  FIG. 3B . Typically, this protective layer  40  constitutes a silicon dioxide or silicon nitride layer.  
         [0013]     With the front side  12   a  protected, photolithography processing on the back side  12   b  can now begin. Accordingly, and referring to  FIG. 3C , the substrate  12  is inverted and placed front side  12   a  down onto a work surface  42 , which varies throughout the back side alignment marker formation process but which initially would comprise a photoresist spinning apparatus. A photoresist  41  is deposited (spun) on the back side  12   b , and is moved to a stepper apparatus, where it is exposed using a mask having the desired back side alignment marker  27  pattern (not shown) and developed to expose the back side  12   b  through the photoresist ( FIG. 3D ). The substrate  12  is then moved to an etching chamber (not shown) where the alignment marker is etched into the back side  12   b  using the remaining photoresist as a masking layer ( FIG. 3E ). Thereafter, the remaining photoresist  41  is stripped (not shown), and the protective layer  40  is etched away (not shown), thus leaving the back side alignment marker  27  on the back side  12   b  of the substrate  12  ( FIGS. 3F and 3G ). Thereafter, the substrate  12  can be processed to form an integrated circuit, using the back side alignment marker  27  (or markers) to align the substrate  12  with each mask during sensitive photolithography steps in chamber  30  ( FIG. 2 ).  
         [0014]     However, it should be appreciated from the foregoing that formation of the back side alignment markers  27  involves a lot of preparation—protective layer formation, photoresist deposition, patterning and removal, etching, removal of these layers, etc.—before processing of the substrate  12  can begin in earnest to form active useful structures on the front side  12   a . Accordingly, the art would be benefited by improved methods for forming back side alignment markers, and in particular methods that forego these additional steps. This disclosure provides solutions.  
       SUMMARY  
       [0015]     Disclosed herein are methods for forming photolithography alignment markers on the back side of a substrate, such as a crystalline silicon substrate used in the manufacture of semiconductor integrated circuits. According to the disclosed techniques, laser radiation is used to remove the material (e.g., silicon) from the back side of a substrate to form the back side alignment markers at specified areas. Such removal can comprise the use of laser ablation or laser-assisted etching. The substrate is placed on a motor-controlled substrate holding mechanism in a laser removal chamber, and the areas are automatically moved underneath the laser radiation to removal the material. The substrate holding mechanism can comprise a standard chuck (in which case use of a protective layer on the front side of the substrate is preferred), or a substrate clamping assembly which suspends the substrate at its edges (in which case the protective layer is not necessary). Alternatively, a stencil having holes corresponding to the shape of the back side alignment markers can be placed over the back side of the substrate to mitigate the need to move the substrate to the areas with precision. Using the disclosed techniques, a separate photolithography step to form the back side alignment markers is not necessary, and additionally the need to use a protective layer on the front side of the substrate is potentially unnecessary, saving time and cost, and reducing potential sources of contamination.  
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0016]     Embodiments of the inventive aspects of this disclosure will be best understood with reference to the following detailed description, when read in conjunction with the accompanying drawings, in which:  
         [0017]      FIG. 1A  illustrates a prior art photolithographic stepper for optically sensing front side alignment markers on a substrate, and  FIG. 1B  illustrates a top-down view of the alignment markers on the substrate.  
         [0018]      FIG. 2  illustrates a prior art photolithographic stepper for optically sensing back side alignment markers on a substrate.  
         [0019]      FIGS. 3A-3G  illustrate a prior art process for forming back side alignment markers using photolithography and a protective layer for the front side of the substrate.  
         [0020]      FIG. 4  illustrates a prior art laser ablation or laser-assisted etch chamber useful in accordance with the disclosed technique for forming back side alignment markers without the need for a photolithography step.  
         [0021]      FIGS. 5A-5D  illustrate using cross-sectional views the disclosed technique for forming back side alignment markers without the need for a photolithography step.  
         [0022]      FIG. 6  illustrates a modified laser ablation or laser-assisted etch chamber having a clamping assembly for suspending the substrate, thus mitigating the need to provide a protective layer on the front side of the substrate.  
         [0023]      FIG. 7  illustrates a top down view of the clamping assembly of  FIG. 6 .  
         [0024]      FIG. 8  illustrates a modification in which a stencil is use to mask the back side of the substrate and useful for forming the back side alignment markers with improved precision.  
         [0025]      FIG. 9  illustrates a back side alignment stepper which has been modified to include lasers for writing the back side alignment markers.  
     
    
     DETAILED DESCRIPTION  
       [0026]     In one embodiment of the disclosed invention, laser-assisted etching or laser ablation is used to form back side alignment markers. The disclosed technique is beneficial over the prior art in that it does not require the use of photolithography to form the back side alignment markers, and additionally in some embodiments does not require the provision of a protective layer on the front side of the substrate. It should be noted that both laser-assisted etching and laser ablation are well-known techniques that have been used to etch materials on integrated circuits. Accordingly, only basic aspects of these techniques are discussed, with the focus of the discussion centering on aspects relevant to the back side alignment marker issues discussed earlier.  
         [0027]     In  FIG. 4 , a laser-assisted etch/laser ablation chamber  50  is shown, and which is used in this embodiment to form the back side alignment markers. The chamber  50  includes an optical sensor  51 , a laser  52 , a lens or lenses  54 , a computer  56 , motor stages  58 , a gas inlet port  60  coupled to an etchant gas source  62  via a valve  64 , and a purge pump  73 . Chambers with these components are well known, and will vary in design depending on whether laser-assisted etching or laser ablation techniques are used. Because a gas inlet port  60 , an etchant gas source  62 , and a purge pump  73  are shown, chamber  50  as illustrated more accurately represents a laser assisted-etching chamber. Were laser ablation to be used for the application in question, such gas- and etching-specific structures may not be necessary. Further details concerning laser-assisted etch/laser ablation chambers  50  are described in above-incorporated U.S. patent application Ser. No. 10/840,324. Also present is the substrate  70  to be processed, which sits upon a substrate holding mechanism  71 , such as a chuck  72  or a clamp assembly  79  to be described in further detail later.  
         [0028]     The laser  52  is used to etch or ablate the bulk substrate  70  material on the backside  70   b  without the need to practice the photolithography steps of the prior art (photoresist deposition, exposure, cleaning and removal of the photoresist, etc.). In this regard, the substrate  70  is initially aligned front side  70   a  down in the chamber  50 . This alignment can be relatively crude (e.g., +/−20 microns), and need not be as sophisticated as the alignment schemes used to align the circuit layers in the device. Thus, initial alignment need only be +/−5 microns for example, and can be performed manually, via operator visual inspection through a microscope, or by automated optical detection schemes, such as automated detection of the edges of the substrate  70  via the use of the optical sensor  51 .  
         [0029]     Once aligned, computer  56  executes a program specific for the substrate  70  in question, and armed with knowledge of the X, Y coordinates of where the back side alignment markers  27  are to be fabricated on the substrate  70 . Accordingly, the computer  56 , moving the substrate holding mechanism  72  via motor stages  58 , brings the desired back side alignment marker areas  75  into alignment ( FIGS. 5A and 5B ) and engages the laser  52  to form laser radiation  53  to etch or ablate the back side  70   b  of the substrate  70 , e.g., of crystalline silicon ( FIGS. 5C and 5D ). Thereafter, the substrate  70  can be cleaned if necessary, and as it would be cleaned in any event prior to further processing of active structures on the front side  70   a  of the substrate  70 . In short, the back side alignment markers  27  are formed without photolithography and all of the steps entailed therein.  
         [0030]     As noted earlier, techniques for using laser-assisted etching and laser ablatement of materials on semiconductor substrates are well known, and hence are not reiterated in much detail herein. Considerations relevant to such selective area processing can be found in Thin Film Processes II, (ed. John L. Vossen &amp; Werner Kern), pp. 621-670, 749-856 (Academic Press 1991), which is submitted herewith and which is incorporated herein by reference.  
         [0031]     Laser ablation is preferably accomplished using an excimer, YAG, or ND-YAG laser which essentially vaporizes the metal layer  30  or other material where it is focused. Suitable ND-YAG lasers have wavelengths of 355 nm, and suitable excimer lasers have wavelengths of 193 nm or 248 nm. Power levels for such lasers are typically in the 1-Watt range. Laser ablation is simpler to implement, and will remove material relatively quickly, but is more difficult to control. Moreover, the vaporized material may need to be cleaned from the substrate  70 &#39;s surface. This being said however, laser ablatement can be a suitable choice for forming back side alignment markers in the substrate  70  in some applications. Exemplary excimer lasers include the PL-1500A Excimer Laser manufactured by Potomac Photonics, Inc., and the xsie200 Excimer Laser manufactured by Xsil Ltd.. An exemplary YAG laser suitable for ablation comprises that LAM 66 manufactured by Heidelberg Instruments Mikrotechnik GmbH. Further details regarding considerations for laser ablation can be found at http://www.me.mtu.edu/˜microweb/chap4/ch4-2.htm, http://www.me.mtu.edu/˜microweb/graph/laser/fluencejpg, and http://www.me.mtu.edu/˜microweb/graph/laser/specmetjpg, which are submitted herewith and which are incorporated by reference in their entireties.  
         [0032]     Laser-assisted etching, by contrast, is slower, but better controlled, and hence is preferred for the application in question. In laser-assisted etching, an etchant gas is introduced into the chamber  50  from an etchant gas source  62  through valve  64  and gas inlet port  60 . The etchant gas is preferably introduced into the chamber  50  as shown proximate to and parallel with the substrate  70 &#39;s surface. Interaction of the laser light and the etchant gas produces a controlled reaction at the surface of the substrate  70  to remove the material in question. Of course, the etchant gas to be used for a particular application, as well as the laser  52  parameters (wavelength; power; spot size) will depend on the composition of the substrate  70 , but again such laser-assisted processes are well known. If silicon or polysilicon is being etched, SF 6  would be a suitable etchant gas and would be used in conjunction with a laser having approximately a 10um wavelength. Other etchant gases and associated laser wavelengths suitable for etching silicon can be found in the above-referenced Thin Film Processes book incorporated above at page 832. An exemplary laser-assist etch chamber  50  can comprise the laser etch and deposition chamber published at http://www.mesofab.com, which is submitted herewith and which is incorporated herein by reference.  
         [0033]     Because the area  75  in which material will be removed will generally be relatively large compared to the spot size of the laser  52 , removal will preferably be accomplished by rastering the area  75  underneath the laser  52 . The laser  52  can either run continuously, or can be turned on and off at each rastered location. Alternatively, if the laser spot size is large enough and comparable with the size of area  75 , rastering may not be necessary.  
         [0034]     As noted earlier, it is important during formation of the back side alignment markers  27  that the front side  70   a  of the substrate  70  not be damaged. In the prior art, protection of the front side  70   a  surface was provided by a protective layer ( 40 ;  FIG. 3B ). Using the laser-assisted etch/laser ablation chamber  50  shown in  FIG. 4 , provision of such a protective layer would be preferred, as the front side  70   a  of the wafer comes into contact with the chuck  72 .  
         [0035]     However, in a preferred embodiment, provision of a front side  70   a  protective surface is rendered unnecessary by making modifications to the laser-assisted etch/laser ablation chamber  50 . Specifically, and as shown in  FIGS. 6 and 7 , the chamber  50   a  has been modified by exchanging a clamp assembly  79  for the chuck  72  of  FIG. 4 . The clamp assembly  79  has the same translational capabilities as the chuck  72 , and again is controlled in its movement by the computer  56  via motor stages  58 . However, through the use of the clamp assembly  79 , the substrate  70  is suspended within the modified chamber  50   a  such that its front side  70   a  does not come into contact with the clamp assembly and does not substantially come into contact with any work surface, except at certain non-critical points  85  along its edge not otherwise suitable for the formation of active circuitry. Accordingly, the back side alignment markers  27  can be fabricated in modified chamber  50   a  without the need to provide a protective layer  40  on the front side  70   a . This save a process step, and allows for processing of active structures on the substrate  70  essentially immediately after the back side alignment markers  27  are formed.  
         [0036]     As best shown in  FIG. 7 , the clamp assembly  79  spans underneath the substrate  70  at cross member  83  and rises along toward the sides of the substrate at risers  82 . Clamp arms  84  are coupled to the risers  82  and contain a bottom arms and top arms, which contact the front side  70   a  and back side  70   b  of the substrate  70  respectively. The clamp arms  84  are coupled to the risers  82  via suitable mechanisms  81 . These mechanisms  81  could comprise the spring mechanism for biasing the top and bottom clamp arms  84  together to pin the substrate  70  therebetween, and additionally can incorporate motors to allow the substrate  70  to be rotated around an axis (φ) joining the two mechanisms  81 . This allows the wafers to be loaded front side  70   a  up, and then rotated to bring the back side  70   b  up, thus providing operational flexibility. Rotational capability also allows for front side laser-based processing as well, e.g., to form alignment markers on the front side  70   a  as well as the back side  70   b . Such rotation and activation of the motors can be accomplished using the computer  56  and the motor stage controls  58 , with wiring to the motors  81  being routed though the body of the cross member  83  and the risers  82 .  
         [0037]     In other embodiments, the substrate  70  need not be flipped as in chamber  50   a , but instead can sit flat on its back side  70   b  when being written to. In such an embodiment, and as shown in  FIG. 9 , a back side alignment chamber  30   a  or stepper such as those discussed above (e.g.,  FIG. 2 ), can be retrofitted with a laser or lasers  52 . The laser  52   a  will allow the back side alignment marker to be written (i.e., by laser ablation or laser-assisted etching) using the same optical path used to “see” the alignment markers once they are formed. The lasers  52  may either appear proximate to the front side  70   a  of the substrate ( 52   a ) or proximate to the back side  70   b  of the substrate  70   b  ( 52   b ). Through this modification, the same tool can be used both for writing the back side alignment markers and as a photolithographic stepper that uses those markers for alignment purposes. Beneficially, through this modification, a protective layer  40  over the front side  70   a  of the substrate need not be used when writing the back side alignment markers with the laser.  
         [0038]     In an alternative embodiment, the need for the computer  56  to know the precise X, Y coordinates of the areas  75  to be removed on the back side  70   b  is mitigated by the use of a stencil  90 , as shown in  FIG. 8 . According to this alternative, the stencil  90  is aligned with the substrate  70 , and contains holes  92  which correspond to the desired shape of the backside alignment markers  27 . The stencil  90  is raised a distance ‘d’ away from the surface of the substrate  70  by spacers  91 , which distance might range from approximately 2 to 10 microns. Using this approach, the laser  52  can be rastered over the entire surface of the stencil  90 , yet will only have effect to remove the material exposed on the back side  70   b  where it is exposed through the holes  92 . Accordingly, once the stencil  90  is appropriately aligned with the substrate  70 , using any of the techniques mentioned earlier, radiation  53  from the laser  52  can be appropriately directed to form the back side alignment markers  27  into the desired shape.  
         [0039]     Thus, the stencil  90  ensures good alignment of the radiation  53  with the desired area  75 , making laser alignment and spot size considerations less critical. (Indeed, the use of a laser in conjunction with a stencil overlying the wafer has utility to clearing materials over and above clearing the alignment markers, and can be used for patterning active circuits as well). Because the back side alignment markers are relatively large, diffractive effects occurring at the edges of the holes  92  of the stencil  90  should not cause a problem, although optical proximity corrective measures could be incorporated into the stencil  90  if necessary. If used in a laser-assisted etch application, a material should be chosen for the stencil that will not react to the etchant gases in question. For example, for a silicon etchant, quartz (silicon dioxide) would be a good choice for material for the stencil  90 . Likewise, in a laser ablation application, a material should be chosen which will remain impervious to the laser radiation in question.  
         [0040]     Although the stencil  90  is disclosed in the Figures in conjunction with chuck  72 , it should be understood that the stencil  90  can also be used with the clamp assembly  79  disclosed earlier ( FIGS. 6 and 7 ). If so used, the stencil  90  and spacers  91  would also be clamped between the top and bottom clamp arms  84  of the clamp assembly  79 .  
         [0041]     Although the disclosed laser-assisted/laser ablation techniques have been disclosed as useful in the context of forming back side alignment markers, it should be understood that the disclosed techniques can be used to form front side alignment markers, and/or to remove circuit layers from the front side.  
         [0042]     Moreover, while particularly useful to the clearing of materials on semiconductor integrated circuit substrates, the disclosed techniques can have application to other types of substrates and other types of processes.  
         [0043]     While it is preferred to use radiation, and specifically laser radiation, to etch the substrate to form the back side alignment markers, this is not strictly necessary. One skilled in the art will realize that other techniques for selectively removing discrete areas of materials without the use of a photoresist exist in the art, and these could be used as well. For example, an electron or other particle beam (e.g., an ion beam) could be used in much the same way as the disclosed laser radiation  53  is used to remove the substrate material by the use of a rastered beam to directly remove the material without the need for photoresist or photolithography. The use of such alternative beams can also be accompanied by the use of a stencil as disclosed herein. Again, processes for using electron or particle beams to remove materials from semiconductor integrated circuits are well known, and can be found in the Thin Film Processes book incorporated above.  
         [0044]     “Circuit layer” as used herein can comprise any layer used in the formation of integrated circuits on the front side of the substrate, including conductive layers, semiconductive layers, or insulating layers, doped regions of the silicon, etc.  
         [0045]     It should be understood that the inventive concepts disclosed herein are capable of many modifications. To the extent such modifications fall within the scope of the appended claims and their equivalents, they are intended to be covered by this patent.