Abstract:
A solution to failure mechanisms caused by mechanical sawing of a mechanical semiconductor workpiece entails use of a laser beam to cut and remove the electrically conductive and low-k dielectric material layers from a dicing street before saw dicing to separate semiconductor devices. A laser beam forms a laser scribe region such as a channel in the electrically conductive and low-k dielectric material layers, the bottom of the channel ending on a laser energy transparent stop layer of silicon oxide lying below all of the electrically conductive and low-k dielectric material layers. The disclosed process entails selection of laser parameters such as wavelength, pulse width, and fluence that cooperate to leave the silicon oxide layer stop layer completely or nearly undamaged. A mechanical saw cuts the silicon oxide layer and all other material layers below it, as well as the substrate, to separate the semiconductor devices.

Description:
COPYRIGHT NOTICE 
     © 2010 Electro Scientific Industries, Inc. A portion of the disclosure of this patent document contains material that is subject to copyright protection. The copyright owner has no objection to the facsimile reproduction by anyone of the patent document or the patent disclosure, as it appears in the Patent and Trademark Office patent file or records, but otherwise reserves all copyright rights whatsoever. 37 CFR §1.71(d). 
     TECHNICAL FIELD 
     This disclosure relates to laser scribing of patterned semiconductor workpieces and, in particular, to use of a laser energy transparent stop layer to effect, with minimal laser scribing debris generation, scribing of a channel in a multilayer patterned workpiece. 
     BACKGROUND INFORMATION 
     Semiconductor devices are multilayer structures that are produced on a substrate, such as a silicon wafer, and then diced into individual chips by a mechanical saw or laser beam before packaging. The trend in semiconductor devices is to replace silicon dioxide dielectric layers with low-k dielectric material layers. Low-k dielectric materials are not mechanically strong; therefore, mechanical sawing of low-k dielectric material layers can cause a unique set of device failure mechanisms. A problem with laser scribing semiconductor devices is that the laser beam interacting with the multilayer structures generates a large amount of debris that must be removed or managed. Laser-generated debris is hot and contains molten material. When it lands on the wafer surface, the molten material or slag becomes fused onto the surface. Laser generated debris resulting from scribing is managed by either cleaning the wafer after scribing, or applying a water-based coating to the wafer surface before scribing, to prevent the hot slag from sticking to the wafer surface, and then cleaning the coating along with the resulting debris after scribing. Coating and cleaning add cost and complexity to the scribing process. What is needed is a method of scribing semiconductor devices quickly and completely, with a minimum of debris generation. 
     SUMMARY OF THE DISCLOSURE 
     One solution to failure mechanisms caused by mechanical sawing entails use of a laser beam to cut and remove the electrically conductive and low-k dielectric material layers from a dicing street before saw dicing. The disclosed process uses a laser beam to form a laser scribe region such as a channel or groove (hereafter “channel”) in a semiconductor workpiece that includes electrically conductive (e.g., copper) and low-k dielectric material layers, the bottom of the channel ending on a laser energy transparent stop layer of silicon oxide (SiO x ), preferably silicon dioxide, lying below all of the electrically conductive and low-k dielectric material layers. The silicon oxide layer can be prepared by a number of common processes, such as physical and chemical deposition, spin on glass (e.g., tetraethyl orthosilicate (TEOS)), or oxidation of silicon (such as thermal oxide). The semiconductor workpiece can also contain electrical circuitry. The disclosed process entails selection of laser parameters such as wavelength, pulse width, and fluence that cooperate to leave the silicon oxide layer stop layer completely or nearly undamaged. The result is a channel floor that conforms to the silicon oxide stop layer. 
     There are two preferred embodiments of scribing a semiconductor workpiece and thereafter dicing it to separate the semiconductor devices. One embodiment entails laser cutting a channel with side boundaries that define a channel width that is wider than the saw blade width to remove the device layers down and to the channel floor and thereafter using a mechanical saw to dice the semiconductor devices in the resulting channel. Another embodiment entails cutting on both side margins of the dicing street two scribe lines depthwise to the silicon oxide stop layer and then dicing the semiconductor devices with a mechanical saw between the scribe lines. 
     Additional aspects and advantages will be apparent from the following detailed description of preferred embodiments, which proceeds with reference to the accompanying drawings. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  is a fragmentary plan view of a patterned semiconductor workpiece that includes multiple, mutually spaced apart semiconductor devices that are separated by dicing streets. 
         FIG. 2  is a magnified image showing a cross-sectional view of a semiconductor device of  FIG. 1  that includes a multilayer structure composed of a silicon dioxide lower layer covered by layers of electrically conductive and low-k dielectric materials. 
         FIGS. 3A and 3B  are modified replicas of  FIG. 1  that show two preferred laser scribing regions configured to separate the semiconductor devices of  FIG. 1 . 
         FIG. 4A  is an electron micrograph tilted image of a debris field after performing laser scribing of a silicon wafer, and  FIG. 4B  is a simplified block diagram showing generally the multilayer structure of the silicon wafer scribed to produce the debris field shown in  FIG. 4A . 
         FIG. 5  is an electron micrograph of a debris field created by laser scribing a scribe line in a silicon substrate in accordance with a prior art technique. 
     
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
       FIG. 1  is a fragmentary plan view of a patterned semiconductor workpiece  10  embodied as a silicon wafer that includes multiple, mutually spaced apart semiconductor devices  12  (portions of four devices  12  shown) that include multilayer structures  14  ( FIG. 2 ) fabricated on a silicon substrate  16  ( FIG. 2 ) and that are separated by dicing streets  18 . Alternative substrates  16  include glass, strained silicon, silicon on insulator, germanium, gallium arsenide, and indium phosphide.  FIG. 1  also shows a variety of alignment targets  20  and other sacrificial test structures  22  occupying the areas within dicing streets  18 . 
       FIG. 2  is a cross-sectional view of a semiconductor device  12  that includes multilayer structure or stack  14  composed of a silicon dioxide lower layer  30  covered by layers of copper wires  32  surrounded by layers  34  of low-k dielectric material. Silicon dioxide layer  30  surrounds tungsten interconnect wires  36 . Copper wires  32  and tungsten interconnect wires  36  extend into dicing street  18 . Layers of copper wires  32  and layers  34  of low-k dielectric material are characterized by weak thermo-mechanical strength properties and, therefore, constitute mechanically weak layers  38  of multilayer stack  14  as compared to silicon dioxide layer  30 , which is mechanically strong in that it is about ten times better than low-k material layers in almost every category of thermo-mechanical properties. These thermo-mechanical properties include interlayer adhesion; adhesion to copper; thermal stability; tensile strength; modulus, hardness; cohesive strength; and etch selectivity. Multilayer stack  14  is fabricated such that mechanically weak layers  38  are located depthwise farther from silicon substrate  16  and thermo-mechanically strong layers  40 , including silicon dioxide layer  30  and any layer formed below it, are located depthwise closer to silicon substrate  16 . 
     Laser scribing of semiconductor workpiece  10  with minimal debris generation entails emitting a pulsed laser beam of temporally spaced apart laser pulses and aligning them with one of dicing streets  18  for incidence on mechanically weak upper layers  38  of semiconductor workpiece  10 . The laser pulses are characterized by a wavelength, pulse width, and fluence such that mechanically weak upper layers  38  of multilayer stack  14  absorb and the mechanically strong lower layers  40  transmit the energy of the laser beam propagating through semiconductor workpiece  10 . Silicon dioxide layer  30  functions as a layer energy transparent stop layer for mechanically weak upper layers  38 . 
     The reason why silicon dioxide layer  30  functions as a laser energy stop layer is that it is in thermal contact with silicon substrate  16 . Silicon substrate  16  acts as a heat sink for silicon dioxide stop layer  30 , which consequently remains intact during laser scribing. In contrast, one or more silicon dioxide passivation layers included in the stack of weak upper layers  38  are different from silicon dioxide layer  30  because the former silicon dioxide layers are surrounded by other dielectric materials that are poor heat conductors. This allows heat buildup in silicon dioxide layers forming portions of weak upper layers  38  so that they can be removed by laser energy. 
     A laser beam positioning system (not shown) imparting relative motion between semiconductor workpiece  10  and the pulsed laser beam aligned with dicing street  18  effects depthwise removal of mechanically weak upper layers  38  with minimal debris generation and thereby forms a laser scribe region with side boundaries extending lengthwise along dicing street  18 . The side boundaries of the laser scribe region formed are defined by exposed portions of laser energy transparent stop layer  30  in accordance with either one of two preferred embodiments, which are described with reference to  FIGS. 3A and 3B  showing replicas of  FIG. 1  with the alignment targets and sacrificial test structures removed for clarity. 
       FIG. 3A  shows a laser scribe region in the form of a channel  44  cut by one or more passes of a pulsed laser beam directed along dicing street  18  in accordance with a first preferred embodiment. Channel  44  has side boundaries  46  separated by a distance  48  that defines a channel width. The laser beam removes between side boundaries  46  mechanically weak upper layers  38  of material to form channel  44 , with silicon dioxide layer  30  as a floor  50  that remains substantially undamaged by the laser beam. Separation of semiconductor devices  12  is performed by using a positioning stage or other device to impart relative motion between a mechanical saw and semiconductor workpiece  10  lengthwise along dicing street  18 . The mechanical saw has a saw blade of a thickness that is less than the channel width so that the mechanical saw cuts through no mechanically weak material of upper layers  30  to separate semiconductor devices  12  located on either side of channel  44 . 
       FIG. 3B  shows a laser scribe region  52  in which each of its side boundaries is formed by a scribe line  54  that is cut by one or more passes of a pulsed laser beam directed along a side margin  56  of dicing street  18 , in accordance with a second preferred embodiment. Scribe lines  54  establish a distance  58  that defines a laser scribe region cutting width. The laser beam removes mechanically weak upper layers  38  of material to form each scribe line  54 , with silicon dioxide layer  30  as a floor  60  that remains substantially undamaged by the laser beam. Mechanically weak upper layers  38  of material are present in the space between scribe lines  54 . 
     Separation of semiconductor devices is performed by using a positioning stage or other device to impart relative motion between a mechanical saw and semiconductor workpiece  10  lengthwise along dicing street  18 . The mechanical saw has a saw blade of a thickness that is within the laser scribe region cutting width  58  but does not extend beyond either side margin  56  of dicing street  18  so that the mechanical saw cuts through no mechanically weak material of upper layers  38  of semiconductor devices  12  as they are being separated. The mechanical saw cutting region exceeds the saw blade thickness to allow for x-y saw blade position variation and blade deflection. The saw blade can cut into scribe lines  54  but not cut outside of side margins  56 . There is no physical attachment between weak upper layers  38  of material on either side of scribe lines  54 , so the mechanical saw can cut anywhere inside laser scribe region  58  without causing damage to weak upper layers  38  outside of side margins  56 . 
     Skilled persons will appreciate that cutting with a mechanical saw generates substantial amount of debris in the form of cold particles that do not fuse to the wafer surface. Such debris can readily be washed away during a post-sawing cleanup operation. Moreover, mechanical sawing is performed under a stream of water (sometimes mixed with a liquid lubricant) that prevents the possibility of hot slag formation. 
       FIG. 4A  is an electron micrograph tilted image of a debris field  70  after performing laser scribing of a silicon wafer  72  ( FIG. 4B ) in accordance with the first preferred embodiment relating to  FIG. 3A .  FIG. 4B  is a simplified block diagram showing generally the multilayer structure of silicon wafer  72 . Silicon wafer  72  includes a 0.5 μm-thick layer of copper wire  32  surrounded by Black Diamond 1 low-k dielectric layer  34 . Copper wire  32  and dielectric layer  34  are formed on a 50 nm-thick silicon carbide layer  76 , which is formed on a 0.5 μm-thick TEOS silicon dioxide layer  30 . Two passes of a 30 μm spot diameter, round-top-hat-shaped 355 μm laser beam at 20 KHz repetition rate, 200 mW power, about 20 ns pulse width, and 20 mm/sec scan velocity removed only mechanically weak layers  38  of copper wires  32  and low-k dielectric layers  34  and  76 . Mechanically strong lower layer  40  including silicon dioxide layer  30  was left uncut for a mechanical saw to complete a singulation process.  FIG. 4A  shows that the above-described process significantly eliminates debris and slag generated by the laser scribing process.  FIG. 4A  also shows that floor  50  of channel  44  is very flat. The flatness of channel floor  50  is related to the flatness of silicon dioxide layer  30  as it was deposited during fabrication of semiconductor device  12 . Because the disclosed process removes weak upper layers  38  of material down to bottom silicon dioxide layer  30 , channel floor  50  is as flat as silicon dioxide layer  30  was at the start of the laser scribing process. 
       FIG. 5  is an electron micrograph of a debris field  80  created by laser scribing a scribe line  82  in a silicon substrate  84  in accordance with the prior art technique. The debris generated by laser scribing substrate  84  extends several hundred microns on either side of scribe line  82 . 
     In general, the laser wavelength, pulse width, and fluence can be selected so that, after laser scribing in accordance with the disclosed technique, the silicon oxide stop layer is completely or nearly undamaged. The result is a channel floor that conforms to the silicon oxide stop layer. This is possible because, by proper selection of laser parameters, the silicon oxide material is transparent to the laser, but the metal and low-k structures are not. 
     The exact laser parameters required to scribe to the silicon oxide stop layer are device dependent. The laser parameters required depend on the composition, orientation, and thickness of the different layers of the multilayer structure of the semiconductor devices. For any given semiconductor workpiece  10 , there is a range of laser parameters that define a process window for scribing down to the silicon oxide stop layer. Typically, the parameters of wavelength, spot size, spot shape, and repetition rate are fixed, while laser power, scribe velocity, and number of scribe passes are varied until the scribe-to-oxide process window is revealed. If the resulting process window is too small, the spot size, spot shape, repetition rate, or laser wavelength can be adjusted as necessary to improve the process. Table 1 below summarizes the parameter ranges. 
     
       
         
               
               
               
             
           
               
                   
                 TABLE 1 
               
               
                   
                   
               
             
             
               
                   
                 Wavelength (±10 nm): 
                 532 nm, 355 nm, and 266 nm 
               
               
                   
                 Spot size ( 1/e 2 ): 
                 3 to 100 μm. 
               
               
                   
                 Spot shapes (spatial): 
                 Gaussian, tophat round, square 
               
               
                   
                 Rep rates: 
                 10 KHz to 2 MHz 
               
               
                   
                 Scribe velocity:  
                 50 to 5000 mm/s 
               
               
                   
                 Number of scribe passes to reach  
                 1 to 25 
               
               
                   
                 lower oxide stop layer: 
                   
               
               
                   
                 Laser power range: 
                 0.1 to 20 W 
               
               
                   
                   
               
             
          
         
       
     
     The thicknesses of the individual semiconductor material layers to be cut, removed, or processed are between 0.5 nm and 10,000 nm. The number of layers is between 1 and 50 (excluding silicon dioxide stop layer  30 ). The semiconductor material layers to be cut, removed, or processed by laser beam contain one or more of the following materials:
         a) Dielectric materials SiO 2  (silicon dioxide), Si 3 N 4  (silicon nitride), silicon carbide, silicon oxynitride, tantalum nitride, or tantalum oxide;   b) Dielectric materials made from any combination of two or more of silicon, oxygen, nitrogen, carbon, hydrogen, and fluorine. These dielectric materials would include low-k materials. Depending on which ones of the elements are selected, the individual chemical bonds of the dielectric material would be C—C, C═C, C C, C—F, C—H, O—H, C—O, C═O, C—N, C═N, C≡N, in addition to “dangling bonds”;   c) Dielectric materials listed above in items a) and b) that have a porous structure so as to lower the dielectric constant of the material (for example, Xerogels or Aerogels);   d) Metal layers of copper (Cu), aluminum (Al), tungsten (W), chromium (Cr), titanium (Ti), nickel (Ni), cobalt (Co), tantalum (Ta), gold (Au), and platinum (Pt) (including the surface oxides on these materials); and   e) Polysilicon and silicon.       

     The following is a list of all low-k materials that would be cut by the laser:
         a) Any of the commercially available low-K dielectric materials made or licensed by the following three companies:
           (i) Applied Materials Black Diamond 1™ Black Diamond 2™, and BLOk™.   (ii) Novellus Coral™   (iii) ASM International Aurora™   
           b) Low-k dielectric materials in the categories of “carbon-doped silicon oxide” or “fluorine-doped silicon oxide.” These materials have chemical stoichiometries comprised of any combination of two or more of the following elements: silicon, oxygen, nitrogen, carbon, hydrogen, and fluorine. The bonding between the elements can be by single, double, or triple bonds depending on the element, i.e., C—C, C═C, C C, C—F, C—H, O—H, C—O, C═O, C—N, C═N, C≡N, in addition to “dangling bonds”;   c) Spin-on polymeric low-k dielectrics, such as SiLK™ from Dow Chemical, polyimides, polynorbornenes, benzocyclobutene, PTFE (Teflon™), and Teflon™-like materials such as PFA, silicone based polymeric dielectric materials, hydrogen silsesquioxane (HSQ), and methylsilsesquioxane (MSQ); and   d) Dielectric materials listed in items above that have a porous structure to lower the materials dielectric constant.       

     It will be obvious to those having skill in the art that many changes may be made to the details of the above-described embodiments without departing from the underlying principles of the invention. The scope of the present invention should, therefore, be determined only by the following claims.