Patent Publication Number: US-2005140030-A1

Title: Scribe street width reduction by deep trench and shallow saw cut

Description:
FIELD OF THE INVENTION  
      The present invention is related in general to the field of semiconductor devices and more specifically to a method of dicing semiconductor wafers.  
     DESCRIPTION OF THE RELATED ART  
      With most semiconductor products, for example integrated circuits, transistors and diodes, a large number of elements are manufactured simultaneously on a large semiconductor wafer of silicon, gallium arsenide, gallium phosphide etc. The semiconductor industry employs the terms “singulation”, “dicing technologies” or “scribing technologies” to refer to those techniques for obtaining a large number of functional chips from each semiconductor wafer. Two dicing methods are particularly well known in the art: the grinding-cutting method, using a blade or wire saw, and the scribing method, using a diamond point. Modern silicon technology prefers the cutting method using high-speed rotating blades. For reasons of mechanical stability at high rotating speeds, the blades have to possess a particular thickness, which cannot safely be reduced. When laying out the pattern of circuit chips on the surface of the semiconductor wafer, manufacturing efficiency requires that the distance between adjacent circuit chips be small so that the number of obtainable chips can be increased.  
      The technology of dicing has been developed to a high standard. Still, three restrictions exist with respect to the distance permissible between adjacent chips. The first restriction is the actual dicing width (for instance, thickness of the rotating blade), the second restriction is the degree of precision to which the cutting machine can be adjusted, and the third restriction is the cracks and chip-outs extending laterally from the dicing line into the semiconductor and insulating materials. In particular the third of these restrictions, namely the generation of cracks, creates the most significant limitation with respect to decreasing the distance between adjacent circuit chips. In addition, those cracks represent significant reliability risks, since they tend to grow and widen under thermal and mechanical stress and thus eventually imperil the functionality of the integrated circuit.  
      In typical processes, the scribe street for wafer sawing represents a space of about 50 to 70 μm between individual ships on a wafer. For a 200 mm wafer which is used for 1 mm 2  chips, the scribe streets will represent a total of around 12% of the wafer area. For logic chips, which can be as small as 200×600 μm 2 , the scribe streets represent around 33% of the wafer.  
      A need has therefore arisen for an efficient, low cost and high yield method to drastically reduce the loss of semiconductor area lost to the scribe streets, and to eliminate the reliability hazards caused by the semiconductor chip-outs, particles and micro-cracks. The innovative method should use the installed equipment base so that no investment in new manufacturing machines is needed. The method should be flexible enough to be applied for different semiconductor materials and products, and should achieve improvements towards the goal of process reliability and handling simplification.  
     SUMMARY OF THE INVENTION  
      One embodiment of the invention is a method to singulate a semiconductor wafer into chips; the wafer has a first, active surface and an opposite second surface. Trench streets of predetermined depth are formed across the first wafer surface to define the outline of the chips. Thereafter, the fabrication of the active first wafer surface is completed and protected. Then, the wafer is flipped to expose the second wafer surface, and the wafer is subjected to a cutting saw. The saw is aligned with the trenches in the first surface so that the saw is cutting the second surface along streets which extend the trenches. The saw is stopped cutting when the saw streets just coalesce with the trench streets, respectively, whereby the chips have been completely singulated.  
      In another embodiment of the invention, a method is disclosed to singulate a semiconductor wafer with a first and a second surface into chips. In the first surface, the active semiconductor device is fabricated and a photomask is applied which permits consecutive etch steps for opening the bond pad windows into the protective overcoat and forming trench streets of predetermined depth in the semiconductor material. The active wafer surface is then protected, the wafer is flipped to expose the second surface, and subjected to a cutting saw. The saw is aligned with the trenches in the first surface so that the saw is cutting the second surface along streets which extend the trenches. The saw is stopped cutting when the saw streets just coalesce with the trench streets, respectively, whereby the chips have been completely singulated.  
      Embodiments of the invention are related to integrated circuit chips and to discrete device chips. The technical advantage of the invention to save valuable semiconductor real estate comes to bear progressively more, the smaller the chip area is. In addition, the reliability of the singulated chips is enhanced by the fact that the singulation of the active zone of the chip is achieved by etching (employing chemical or plasma techniques) and not by mechanical means such as sawing or scribing. Particles, chip-outs, and micro-cracks as deleterious side-effects of the singulation techniques are thus eliminated from the active zones of the chip.  
      It is a technical advantage of one or more embodiments of the invention that the embodiments can reach the goals of the invention with a low-cost manufacturing method without the cost of equipment changes and new capital investment, by using the installed fabrication equipment base, specifically the established wafer-fab etching techniques and automated sawing machines. Further, one or more embodiments of the invention can reach the goal of the invention without specific effort on aligning the etched trenches with the sawed streets, making the implementation of the invention in semiconductor manufacturing easy.  
      The technical advances represented by certain embodiments of the invention will become apparent from the following description of the preferred embodiments of the invention, when considered in conjunction with the accompanying drawings and the novel features set forth in the appended claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a schematic cross section through a portion of a semiconductor wafer indicating individual chips singulated by a method according to the invention.  
       FIG. 2  is a schematic top view of a semiconductor wafer after chips have been singulated according to an embodiment of the invention.  
       FIG. 3  is a schematic top view of a semiconductor wafer after chips have been singulated according to another embodiment of the invention.  
       FIG. 4  is a schematic top view of a semiconductor wafer after chips have been singulated according to another embodiment of the invention.  
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS  
       FIG. 1  illustrates schematically the cross section of a portion of a semiconductor wafer, generally designated  100 , which has been singulated into a plurality of semiconductor chips  101 . The vertical dimension of the wafer has been expanded in  FIG. 1  for clarity. The semiconductor material of the wafer may be silicon, silicon germanium, germanium, gallium arsenide, aluminum gallium phosphide, indium phosphide, gallium phosphide, or any other semiconductor material used for fabricating semiconductor devices. Each chip  101  is in principle a cuboid, which has a top surface  102 , a bottom surface  103 , and four vertical side surfaces, of which only two surfaces  104  and  105  are indicated in  FIG. 1 . The top surface  102  includes the active electronic device  110 , which may for some wafers include an integrated circuit, for other wafers a discreet device such as a diode, especially a light-emitting diode, or a controlled rectifier, or a power transistor. The bottom surface  103  is the passive surface of the chip.  
      The top surface  102  includes a perimeter  102 a of approximately rectangular cross section, which protrudes beyond the four edge sides  105 . The chip thus exhibits an annulus-shaped protrusion attached to the top portion of the chip. Consequently, the top surface  102  has a larger area than the area of the bottom surface  103 . This increase of the top surface becomes relatively more significant, the smaller the chip area is. The complete enlarged top surface  102  is available to be used for features of the semiconductor device, a significant increase in semiconductor area available for device purposes compared to the area without the annulus-shaped protrusion.  
      The edge side  105  comprises portion  105   a  of the annulus-shaped protrusion, and portion  105   b , which is usually larger than portion  105   a . Portion  105   a  of the edge side  105  is created by etching, preferably by plasma etching, although some embodiments employ chemical etching. This etching process starts at surface  102  and progresses into the semiconductor material to the depth  105   a , creating a trench of width  107 . These etching processes produce no microcracks, which would otherwise stretch from the freshly created surface into the semiconductor material.  
      Depth  105   a  and width  107  of the trench are correlated by the aspect ratio depth-to-width, which is achievable by the selected etching technique. For plasma etching technology, the aspect ratio is preferably  8 : 1  or less (such as 6:1 or 4:1). As an example, a trench depth of 20 μm would require a trench width of approximately 2 to 3μ. For shallower trenches, a trench width of about 1 μm or even 0.5 μm is achievable.  
      Portion  105   b  is created by mechanical sawing, preferably by a rotating blade  120  (a portion of the blade is schematically shown in  FIG. 1  still inserted in one of the freshly cut streets), after the etched trenches have been created. The sawing operation creates a saw “street” of width  106 , determined by the width of the saw blade. In order to cut each saw street, the saw is aligned with the respective trench so that the saw street will be able to coalesce with the respective trench. Where the saw street  106  merges with the etched trench  107 , the saw street forms ridges  106   a.    
      Advanced blades, commercially available for instance from Disco Corporation, Japan, may be as narrow as 25 μm. They create a street of approximately 50 μm width. Somewhat wider saw streets of about 60 μm width and more can be conveniently achieved. At the tip, the saws are typically about rectangular with some rounding; the surface of the blades is covered with diamond grit, especially at the blade tip. Due to the nature of the mechanical sawing operation, the saw street is surrounded by a semiconductor zone afflicted by microcracks. These microcracks originate at the surface, which is freshly created by the sawing operation, and stretch into the semiconductor material. In  FIG. 1 , this microcrack-disturbed zone is designated  108 . Using modern saws, zone  108  is in the range from about 4 to 6 μm, at most 10 to 12 μm. As for the sidewalls of etched trench  107 , they retain a microcrack-free zone  109  from the original trench etching process, but lose a zone of width  108  to the microcrack-affected zone after the sawing operation.  
      The street width can be narrowed, though, by employing “dicing lasers”. Using this technology, a width of about 30 μm is possible. In addition, any microcrack-disturbed zone is narrower.  
      In  FIG. 2 , a semiconductor wafer, generally designated  200 , is schematically illustrated in top view of the first, or active, surface  201 . The line A-A′ in  FIG. 2  is an example, where the cross section of  FIG. 1  may be taken.  
      In one embodiment of the invention, the method to singulate this semiconductor wafer  200  into individual chips  203  comprises the following steps: 
          forming trench streets  202  of predetermined depth across the first wafer surface  201  to define the outline of the chips  203 . In customary fashion, these chips are rectangular, in some instances square. Consequently, the trenches are formed by two pluralities of trenches; within each plurality, the trenches are parallel; relative to each other, the two pluralities are at right angles. Preferably, the trenches are cut by plasma etching or chemical etching. Both techniques allow batch processing;     optionally, filling the trenches with an oxide such as silicon dioxide; a preferred technique is low pressure chemical vapor deposition (in  FIG. 1 , the trench of depth  105   a  would be completely filled with oxide before the fabrication of the electronic device  110  starts);     completing the fabrication of the first wafer surface  201  by building the electronic device. The device may be an integrated circuit or a discrete device. The active surface  201  is protected by a protective overcoat such as silicon nitride or silicon oxynitride (shown in cross section in  FIG. 1 , designated  111 );     removing the oxide from the trenches after the electronic device has been built on active surface  201  (in  FIG. 1 , the trench of depth  105   a  is open again). This step is only necessary for wafers where the trenches have been filled with an oxide;     protecting the whole first wafer surface with a plastic film, which can be easily removed after completion of the sawing operation;     flipping the wafer to expose the second, passive, wafer surface (not shown in  FIG. 2 );     submitting the wafer to a wafer-cutting saw equipment;     aligning the saw consecutively with each trench in the first surface so that the saw cuts the second surface along streets which extend the trenches, respectively; the saw streets  204  are indicated by dashed lines in  FIG. 2 ; and     stopping each saw cutting when the saw street just coalesces with the trench street, respectively, whereby the chips are completely singulated.        

      A number of techniques are available to perform the required alignment of the mechanical saw with the etched trenches before cutting the each individual street. In a preferred approach, the protective film over the first/active wafer surface is transparent in the wavelength range of visible light. In addition, the flexible tape, which supports the wafer during the sawing operation (customarily referred to as the “blue tape”), is transparent in the wavelength range of visible light. After the wafer has been flipped onto the support tape, a camera from the bottom can clearly observe the location of the etched trenches in the first surface. The saw comes in from the top onto the second/passive wafer surface and is computer-controlled by the camera from the bottom. For process control purposes, each completed saw street can be monitored by an operator together with the respective etched trench.  
      Another approach uses infrared alignment equipment, wherein infrared light shines through the semiconductor wafer material to observe the trench locations. The saw is then computer-controlled by the camera operating in the infrared light regime.  
      In another embodiment of the invention, the method to singulate the semiconductor wafer  200  into individual chips  203  comprises the following steps: 
          fabricating electronic devices in the active first wafer surface  201 . The devices may be integrated circuits or discreet devices. The active surface  201  is protected by a protective overcoat such as silicon nitride, silicon oxynitride, silicon carbide, or a combination thereof;     applying a photomask (a photomask portion  130  is schematically shown in the cross section of  FIG. 1 ; the photomask is illustrated in dashed lines, since it is already removed at the process step of chip singulation, which is depicted in  FIG. 1 ). The photomask permits consecutive etch steps, first for opening the bond pad windows into the protective overcoat ( 131  in  FIG. 1 ), and then for forming trench streets  202  of predetermined depth in the semiconductor material (width  107  and depth  105   a  in  FIG. 1 ).     protecting the first/active wafer surface  201  with a plastic film, which can be easily removed after completion of the sawing operation;     flipping the wafer to expose the second wafer surface (not shown in  FIG. 2 );     submitting the wafer to a wafer-cutting saw equipment;     aligning the saw consecutively with each trench in the first surface so that the saw is cutting the second surface along streets which extend the trenches, respectively; the saw streets are indicated by dashed lines in  FIG. 2 ; and     stopping the saw cutting when the saw streets just coalesce with the trench streets, respectively, whereby the chips are completely singulated.        

      In both embodiments described above, the chip singulation on the active, device-bearing surface is accomplished by the narrow etched trench. The electronic device can, therefore, take full advantage of the enlarged area available for the layout of that device, compared with the sacrifice of semiconductor material in connection with mechanical saws. Equally important, any disturbing chipped-out particles or nascent microcracks are kept at safe distance from the electronic device. Particles and microcracks are unavoidable side-effects of mechanical saws. The electronic device can, therefore, take full advantage of the reduced risk of failure mechanisms and thus enhanced reliability expectation.  
      In another embodiment of the invention, the use of a mechanical saw is avoided altogether; instead, the wafer is subjected to a semiconductor material-removing step such as thinning by back-grinding, in order to complete the singulation of the chips from the wafer. The method to singulate the semiconductor wafer into individual chips comprises the following steps: 
          fabricating electronic devices in the first wafer surface. The devices may be integrated circuits or discreet devices. The first surface is protected by a protective overcoat;     applying a photomask which permits consecutive etch steps for opening first the bond pad windows into the protective overcoat, then forming trench streets of a depth equal to the intended thickness of the chips. In this embodiment, the trench streets may have to be etched deeper than in the previously described embodiments, since the trench streets have to penetrate the whole depth of the intended final wafer thickness;     protecting the first wafer surface with a plastic film, which can be easily removed after completion of the singulating process;     flipping the wafer to expose the second wafer surface;     submitting the wafer to a wafer-thinning apparatus. The preferred technique is mechanical back-grinding because of its installed equipment base, high wafer throughput, and low cost. Suitable back-grinding machines are commercially available for example from the companies Disco, TSK, and Okamoto, all of Japan. However, there are several other proven techniques: chemical spin etching; chemical/mechanical wet polishing; and plasma dry etching. From the standpoint of mechanical strength, low stress, minimal mechanical and thermal damage of the singulated chips, chemical etching is the preferred method. From the standpoint of future workability (for instance, extremely thin chips), plasma etching is the preferred method;     removing material from the wafer, starting from the second surface, until the trench streets are just reached; and     stopping the removal process, whereafter the chips are completely singulated.        

      In another embodiment of the invention, which avoids the use of a mechanical saw, a material-removing step such as thinning by back-grinding is employed in order to complete the singulation of the chips from the wafer. The method comprises the following steps: 
          forming trench streets of pre-determined depth across the first wafer surface to define the outline of the chips;     completing the fabrication of the first wafer surface by building the electronic device; the device may be an integrated circuit or a discrete device;     protecting the first wafer surface with a thin film, which can easily be removed after the completion of the singulation process. The wafer is then flipped to expose the second wafer surface; and     removing semiconductor material from the second wafer surface, for example by a grinding or an etching technique. The removing process continues until the trench streets are just reached, at which time the material-removing process is stopped and the chips are completely singulated.        

      When any one of the thinning techniques listed above are employed rather than sawing, the trench streets-to-be-etched can be selected so that the outline of the chips are different from the conventional rectangular or square shape. Examples are hexagonal shape, as illustrated in  FIG. 3 , and circular shape, as illustrated in  FIG. 4 . In the unconventional hexagonal shape, the chip side angles are larger than 90°. In a circular chip, there are no more corners. Chips with these unconventional outlines offer a significant technical advantage, because they avoid the sharp peaks of thermomechanical stress, which appear in electronic device features near the conventional 90° side angles of conventional rectangular chips. The absence of these stresses is a significant advantage for chips of very thin thickness (such as 20 to 50 μm).  
      As an example of these embodiments of the invention, a semiconductor wafer, generally designated  300 , is schematically illustrated in  FIG. 3  in top view of the first, or active surface  301 . Following one of the singulation methods described above in conjunction with  FIG. 2 , trench streets  302  of hexagonal outline and predetermined depth are etched deep into the first wafer surface  301 . The etched streets are at least as deep as the final wafer thickness in order to insure complete chip singulation. As an example, the street depth may be between 20 and 50 μm. As an example, each chip  303  includes an integrated circuit. As another example, each chip  303  is a discrete electronic device such as a light-emitting diode, or a controlled rectifier, or a power transistor.  
      As another example of these embodiments of the invention, a semiconductor wafer, generally designated  400 , is schematically illustrated in  FIG. 4  in top view of the first, or active surface  401 . Applying one of the singulation methods described above in conjunction with  FIG. 2 , trench streets  402  of circular outline and predetermined depth are etched deep into the first wafer surface  401 . The etched streets are at least as deep as the final wafer thickness in order to insure complete chip singulation. For instance, the street depth may be between 20 and 50 μm. Each chip  404  may include, for instance, an integrated circuit. In other wafers, each chip  404  may be a discreet electronic device such as a light-emitting diode, a rectifier, or a power transistor. As  FIG. 4  shows, between the circular-shaped chips  404  remain left-over areas  403 . These areas  403  can be put to good purpose during the device fabrication process, for instance to accommodate test structures, metrology structures, process control monitors and similar functions essential for achieving high fabrication yield. As another example, the small area  404  include electronic devices requiring only little area, such as a sensor.  
      While this invention has been described in reference to illustrative embodiments, this description is not intended to be construed in a limiting sense. Various modifications and combinations of the illustrative embodiments, as well as other embodiments of the invention, will be apparent to persons skilled in the art upon reference to the description. It is therefore intended that the appended claims encompass any such modifications or embodiments.