Abstract:
An automatically programmable tilting mechanism for holding a scribe tool at varying angles using an aerostatic bearing. The air bearing secures the scribe tool about its longitudinal shank and generally allows for free axial movement while the tool holding structure prevents rotation of the scribe tool, thus providing extremely fine compliance and force application of the scribe tool point against the substrate. During a scribing process, the tilting mechanism regularly adjusts the angle of the scribe tool relative to the substrate so that a fresh cutting edge is always being employed in the scribing process.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   Not applicable. The present application is an original regular national patent application. 
   STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
   Not applicable. 
   REFERENCE TO A MICROFICHE APPENDIX 
   Not applicable. 
   TECHNICAL FIELD 
   The present invention also relates to methods and apparatus for semiconductor wafer processing and more generally to an apparatus and method for scribing and breaking frangible materials such as semiconductor wafers. More particularly the invention relates to application of a scribe tool to the task of making a scribe line in frangible material to give a controlled break along that scribe line. The apparatus for applying the scribe tool is an automatically programmable tilting mechanism which holds the tool with an aerostatic bearing while tilting it regularly during a scribing operation. The aerostatic air bearing affords extraordinary compliance and force application of the scribe tool point against the surface being scribed. The frictionless compliance, precise force application and continuous angle adjustment combine to give improved scribing and much greater working life to the scribe tools points. 
   BACKGROUND INFORMATION AND DISCUSSION OF RELATED ART 
   Semiconductors are manufactured in an integral fashion on a wafer of semiconductor material. Such wafers are commonly, but not exclusively single crystals of silicon, gallium arsenide, indium phosphide, gallium nitride, germanium etc. The economy of manufacture is created by producing hundreds or thousands of the same semiconductor device or circuit, in mass, on a single wafer at one time. The devices are organized on the wafer in rows and columns. 
   After the semiconductors are manufactured on the wafer, the devices must be separated from each other (singulated) so they can be used individually. This processing is called wafer dicing. Wafer dicing is performed by cutting or scribing and breaking along the separation areas (streets) between the rows and columns. Some of the apparatus for cutting are rotary saws and laser burning. Some of the apparatus for scribing and breaking are sharp tool and laser scribing. 
   Sharp tool scribing is the oldest technique and been practiced since the Semiconductor Industry began in the 1960s before that when glass was invented. A scribing method is described in U.S. Pat. No. 4,095,344, dated Jun. 29, 1978, and entitled “Scribe Tool and Mount Therefore”, to James W. Loomis, one of the present inventors. An improved method of dicing scribed wafers was shown in U.S. Pat. No. 5,458,269, to James W. Loomis, dated Oct. 17, 1995. Each of the forgoing patents is incorporated in its entirety herein by reference. 
   There have been many promising innovations in semiconductor separation methods since the Loomis &#39;344 patent was issued, and even since the &#39;269 patent was issued, particularly in the area of laser cutting technology. However, the scribing and breaking method of wafer singulation continues to have several advantages over the sawing and cutting methods. In particular, the scribing and breaking of wafers does not create appreciable particle and dust contamination. Thin semiconductor wafers are exquisitely sensitive to contamination by small charged particles, and both abrasive sawing and laser cutting techniques generate a considerable volume of particles and dust that tend to redeposit on the wafer surface. Cleaning of such particles is challenging because the particles adhere to the wafer surface with remarkable tenacity through the van der Waals force, electrostatic forces, and capillary action. The mechanical forces required to overcome the attaching forces and to remove the contaminant particles are often more than sufficient to damage the devices by compromising wire bonds or generating short circuits. 
   Accordingly, methods were devised to protect the wafer from dust and particle contamination. One method employs a thin protective layer of photo resist, which is peeled from the wafer after singulation through an etching process. Another employs rinsing the wafer as it is sawn in a wet sawing process. Yet another entails covering the wafer with a thin sheet of DI water during sawing. All are expensive and time consuming and the latter two produce a slurry which itself may contaminate the wafer, thus producing a poor product yield. 
   Sharp point scribing and breaking of thin semiconductor wafers does not generate appreciable contaminant dust and small particles. It is relatively fast and inexpensive, and it reduces the method steps employed in the fabrication processes. As an older and well-established method, it has also reached a stage of considerable refinement. Thus, the method is still preferred by many manufacturers. 
   However mature it may be in relation to other singulation methods, sharp point scribing has not transcended the need for improvement. One feature of the scribe and break method that limits its efficiency is that the cutting edge of diamond tipped scribes quickly dull through use. After even a single pass over the surface of a wafer, the cutting edge begins to dull and degrade and its ability to scribe the surface sufficiently for damage-free breaking diminishes. In the case of diamond tipped scribe tools, it is a common practice to routinely change the angle of the tool manually after a predetermined number of passes depending on the nature of the substrate, the depth of the scribing, and the quality of the cutting edge. The durability under any set of circumstances can now be fairly accurately predicted from numerous prior microscopic observations of scribe points in use. 
   Because diamond tips have multiple scribe edges formed in the lapping and polishing process, changing the angle very slightly can bring a new portion of a cutting edge or an altogether new edge into engagement with the wafer, thereby ensuring optimum cutting efficiency. Thus, there has arisen a need to automatically move and control cutting edge engagement with wafer surface during the singulation process. 
   Scribing and breaking is a phenomenon not well understood. A proper scribe line is a ductile deformation created in the scribed surface. A ductile formed scribe will break without creating dust and cracking. Brittle materials will behave in a ductile fashion when scribed with a microscopically sharp point. The ductile deformation freezes immediately as the deforming point passes a spot x/t. Upon freezing very high stress is created lateral to the scribe line. If the point is sharp and is replicated in the frozen deformation a vertical crack will form under the scribe line. This crack, under the frozen deformation, is a controlled fracture. Applying tensile strain to the crack causes the crack to grow through the wafer. 
   Creating this deformation and the resulting scribe line causes very high wear on the sharp point. Because of this high wear, the material of choice for sharp points is diamond. Diamond is the hardest material in nature, has a low coefficient of friction and has a thermal conductivity greater than copper. If the scribe point is formed in the proper crystalline structure of the diamond, the point will have optimum wear. Loomis Industries has developed manufacturing techniques that provide scribe tools that are durable and consistent. This consistency in manufacture gives consistency in scribing and consistency in wear life. Consistency is the essence of all manufacturing; it is extremely important for scribe dicing. Scribing must be 100% consistent if break yield is to be high. Knowing how long a point will last is critical so the point can be removed/changed before end of life. If the longevity of a point is ninety meters, the point must be replaced before that. When should a point be replaced? Usually replacement is at the completion of a wafer. A wafer that is 150 mm diameter with dice that are 1×1 mm requires 34 meters of scribing. If point replacement is effected prior to 90 meters and the point is changed after wafer completion, then the point must be replaced at 68 meters (only having produced two wafers). However, if the scribe point lasts 350 meters, then ten wafers can be scribed. 
   Increased point life creates the following advantages: (1) reduced tool cost (perhaps as little as one fifth the costs for conventional scribing methods; (2) greater than 99 percent yield; (3) greater machine productivity; and (4) improved product quality. 
   BRIEF SUMMARY OF THE INVENTION 
   The present invention solves the foregoing problem by providing a new and improved scribe tool holder having an automatic, motorized tool tilting apparatus, and also having an air bearing supporting the scribe tool stylus (shank) and controlling the pressure placed on the wafer surface by the scribe tool tip. Additionally, the present invention provides an improved diamond scribe tool stylus especially adapted for use with the motorized tilting apparatus. 
   It is therefore an object of the present invention to provide a new and improved method and apparatus for scribing semiconductor wafers. 
   It is another object of the present invention to provide a new and improved method and apparatus for automatically controlling the angle of a scribe tool stylus. 
   A further object or feature of the present invention is a new and improved method and apparatus for automatically and, selectively, continually changing angle of the scribe tool tip so that undulled cutting edges can come into use for scribing a semiconductor wafer during fabrication. 
   An even further object of the present invention is to provide a novel air bearing system for supporting a scribe tool stylus such that the stylus can respond with generally axial movement as the stylus tip passes over minute irregularities on the wafer surface. 
   A still further object is to provide an improved scribe tool having a diamond tip with a truncated tip and a cutting edge configuration that allows for heel scribing. 
   Other novel features which are characteristic of the invention, as to organization and method of operation, together with further objects and advantages thereof will be better understood from the following description considered in connection with the accompanying drawings, in which preferred embodiments of the invention are illustrated by way of example. It is to be expressly understood, however, that the drawings are for illustration and description only and are not intended as a definition of the limits of the invention. The various features of novelty that characterize the invention are pointed out with particularity in the claims annexed to and forming part of this disclosure. The invention does not reside in any one of these features taken alone, but rather in the particular combination of all of its structures for the functions specified. 
   There has thus been broadly outlined the more important features of the invention in order that the detailed description thereof that follows may be better understood, and in order that the present contribution to the art may be better appreciated. There are, of course, additional features of the invention that will be described hereinafter and which will form additional subject matter of the claims appended hereto. Those skilled in the art will appreciate that the conception upon which this disclosure is based readily may be utilized as a basis for the designing of other structures, methods and systems for carrying out the several purposes of the present invention. It is important, therefore, that the claims be regarded as including such equivalent constructions insofar as they do not depart from the spirit and scope of the present invention. 
   Further, the purpose of the Abstract is to enable the U.S. Patent and Trademark Office and the public generally, and especially the scientists, engineers and practitioners in the art who are not familiar with patent or legal terms or phraseology, to determine quickly from a cursory inspection the nature and essence of the technical disclosure of the application. The Abstract is neither intended to define the invention of this application, which is measured by the claims, nor is it intended to be limiting as to the scope of the invention in any way. 
   Certain terminology and derivations thereof may be used in the following description for convenience in reference only, and will not be limiting. For example, words such as “upward,” “downward,” “left,” and “right” would refer to directions in the drawings to which reference is made unless otherwise stated. Similarly, words such as “inward” and “outward” would refer to directions toward and away from, respectively, the geometric center of a device or area and designated parts thereof. References in the singular tense include the plural, and vice versa, unless otherwise noted. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
     The invention will be better understood and objects other than those set forth above will become apparent when consideration is given to the following detailed description thereof. Such description makes reference to the annexed drawings wherein: 
       FIG. 1  is an upper left front perspective view of the automatic tool tilting protractor for a scribe tool of the present invention; 
       FIG. 2  is an upper right front perspective view thereof, while  FIG. 2A  is an exploded view thereof; 
       FIG. 3  is a cross-sectional top plan view thereof; 
       FIG. 4A  is a cross-sectional right side view in elevation, showing the tool holder mount tilted at a first angle; 
       FIG. 4B  is a cross-sectional side view in elevation showing the tool holder mount tilted at a second angle; 
       FIG. 5  is a cross-sectional front view in elevation of the inventive apparatus; 
       FIG. 6A  is a schematic partial top plan view showing details of the air supply for the air bearing system of the present invention; 
       FIG. 6B  is a schematic partial cross-sectional side view in elevation taken along line  6 B- 6 B of  FIG. 6A ; 
       FIG. 7A  is a schematic top plan view showing further details of the air supply for the inventive air bearing system; 
       FIG. 7B  is a detailed schematic cross-sectional front view of the air bearing system for supporting the scribe tool stylus; 
       FIG. 8A  is a perspective view showing the improved scribe tool stylus tip of the present invention; 
       FIG. 8B  is a perspective view showing a conventional scribe tool stylus tip; and 
       FIG. 9  shows the heel scribing made possible by the inventive tip and the tool tilting apparatus of the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIGS. 1 through 9 , wherein like reference numerals refer to like components in the various views, there is illustrated therein a new and improved automatic tool tilting protractor for a scribe tool, generally denominated  100  herein. 
   The figures collectively illustrate a first preferred embodiment of the inventive apparatus and show that it comprises a protractor  110  describing an interior arc and having an exterior side  120  to which a mounting bracket  130  is integrally affixed. The mounting bracket is for secure and rigid attachment to a linear translation system for semiconductor wafer scribing, as is shown in U.S. Pat. No. 5,458,269, incorporated in its entirety herein by reference. The protractor includes an elongate arcuate hole  140 . 
   Immediately adjacent protractor  110  is an internal ring gear  150  having gear teeth  160 . The protractor and internal ring gear are coupled with a switch block plate  170  which spans the width of both and is screwed into each with hex socket head screws  180 . A switch block nut plate  190  is demountably and adjustably attached to the switch block plate  170  and a miniature snap action limit switch  200  is then coupled to the switch block nut plate. 
   An angle adjustment sector  210  forms an upper frame member for the apparatus, while a tool holder mount  220  forms a base or lower frame member for the apparatus. The angle adjustment sector  210  has a substantially planar lower surface  210   a  while the tool holder mount has a substantially planar platform  350 , and the two portions are mated at the respective planar surfaces. The upper and lower frame portions are drilled or formed such that when the portions are joined, a cylindrical hole is provided to accommodate a substantially cylindrical electrical stepper motor  230  having a stepper motor shaft  240  which is journalled at its proximal end by a stepper motor shaft bushing  250 . A pinion gear  260  is disposed on the stepper motor shaft and has gear teeth  270  in intermeshing relationship to the gear teeth of internal ring gear  160 . The motor is supplied with power from a power source (not shown) electrically connected to the contacts of the snap action switch. 
   Angle adjustment sector  210  is provided with a cylindrical through hole such that a shaft  290  may be inserted therethrough. The shaft includes a slightly resilient wheel  300  (preferably nylon or rubber) rotatably mounted on its proximal end  310  and a nut and washer  320   a / 320   b  combination at a threaded distal end  330 . The shaft is normal to the protractor  110  and to the elongate arcuate hole  140 , so that the rubber wheel is disposed in the arcuate hole and rolls within it as the tool angle is adjusted. A compression spring  340  is interposed between the nut and the angle adjustment sector  210  and holds the sector flat against the internal ring gear  150 . 
   The tool holder mount  220  includes a substantially planar platform  350  for supporting a tool holder body  370 . The tool holder body includes a substantially planar round interior plate  360  having a top side  380  and a bottom side  390 , and a circumference defined by a crenelated wall  400  having two sets each of opposing crenels  410 / 420 ,  430 / 440 , and merlons  450 / 460 ,  470 / 480 . The interior plate includes a hole  490  which is continuous through an integral cylindrical extension  500  extending downwardly from and at right angles to the plane of the bottom side  390  of plate  370 . The cylindrical extension is inserted through a hole  510  disposed through tool holder mount  220 , and the outside diameter of the cylindrical extension is marginally smaller than the interior diameter of hole  510 , such that the interfacing surfaces form a substantially hermetic seal. Hole  510  preferably includes a chamfered upper edge  510   a  to facilitate the insertion of tightly fitting components during assembly. 
   Means to secure the tool holder in hole  510  are as follows: a taper portion  520  is cut circumferentially around the side of the cylindrical extension; two grub screws  530  are threadably inserted into one or more threaded holes  540  in the side of the tool holder mount  220  and engage the cylindrical extension at the taper portion. 
   The cut away taper portion also forms an annular air space  550 , and because air under pressure will be fed through this space, nitrile O-rings  560 ,  570  are disposed above and below the annular air space to prevent gas escape above and below the rings. A plurality of small diameter air holes  580  are laser drilled into the cylindrical extension, thereby bringing pressurized air into the space immediately interior to the cylindrical extension. The threaded lower end  590  of the cylindrical extension includes a threaded stop washer  600  having a hole to accommodate a scribe tool shank. 
   A cylindrical air bushing  610  having a cylindrical interior wall  610   a , and further having an outside diameter marginally less than the interior diameter of cylindrical extension  500  is removably inserted into the cylindrical extension. The tolerance and fit are determined by the scribe tool behavior and suspension characteristics desired for the scribing system. A channel  620  is cut for placement of a nitrile O-ring to hold the scribe tool and to prevent fluid leak below the O-ring. When fully inserted into the tool holder mount, the upper edge  630  of the scribe tool bushing is slightly recessed from the top side  380  of plate  360 . Further, the air bushing includes a recessed upper end  640 . 
   A top seal ring  650  is bonded around its outside diameter to the top side  380  of plate  360 . The top seal ring includes a center relief hole  660  which brings hole  510  into fluid communication with the outside atmosphere, and it further includes a circumferential recess  670 , which creates an air passage  680  in fluid communication with the space  690  above the upper end  640  of scribe tool bushing  610 . 
   The tool holder body is designed to support a scribe tool  700  having a T-bar  710  at the upper end of a shank  720 , and a sharpened diamond tip  730  at the opposite, lower end. The tip includes a plurality of cutting edges  740 , preferably four, employed to scribe a semiconductor wafer during the scribing process. The scribe tool is removably inserted into the center hole  660  through top seal ring  650  and the hole defined by the cylindrical interior wall  610   a  of the scribe tool bushing. The scribe tool is inserted beyond O-ring  630 , and through threaded stop washer  600 , such that a portion of the shank and the entire tip and cutting edges are exposed a predetermined distance below the stop washer. 
   The foregoing elements of the tool holder body and tool holder mount comprise not only a tool holder apparatus, but the physical and operative elements of an air bearing system. While the tool holder mount  220  is a substantially solid block of material, it is drilled to include a plurality of fluid passageways for the introduction of pressurized air into the air bearing system. A first air circuit includes a first air passageway comprising a first bore  750   a  drilled downwardly from the top of angle adjustment sector  210  to an intersecting bore  750   b  drilled through tool holder mount  220 , and having a first air inlet fitting  760  at its distal end and a dowel pin  770  blocking its proximal end. A first air source (not shown) is connected to air inlet fitting  760 . A third bore  750   c  intersecting second bore  750   b  at a substantially right angle is also plugged at its end with a dowel pin  780 , and brings the first air passageway and the pressurized air source into fluid communication with the air space  550  around cylindrical extension  500 , and thus with scribe tool bushing  610  via holes  580 . This circuit is always on when the machine is in operation, and floats the scribe tool on a film of air to provide relatively frictionless axial movement. 
   A second air circuit comprising a second air passageway  800  comprising a diagonally disposed first bore  800   a  drilled through tool mount holder  220  and intersecting a second bore  800   b  drilled downwardly from the top side  350  of tool mount holder  220 . A second air inlet fitting  810  permits air to be introduced into the second passageway, and an expanded hole  820  in plate  360  brings the passageway into fluid communication with the space  680  underneath top seal ring  650 , and thereafter with the spaces  490  and  690  immediately surrounding the upper portions of the scribe tool shank. A second air source (not shown) is connected to air fitting  810 . 
   Actual movement and downward pressure of the scribe tool is controlled by the second circuit. Air at very low pressure (0-10 psi) is introduced into second air fitting  810  and makes its way to the underside of top seal ring  650  via first and second bores  800   a  and  800   b . After passing through hole  820 , the air enters the air space  680  between the top seal ring  650  and the top side  380  of the tool holder body  370 . The low-pressure air is then forced to flow to the upper portion of the scribe tool shank  700  via an annular groove machined into the bottom of the top seal ring. The scribe tool is exposed to a downward force resulting from the low-pressure air multiplied by the exposed surface area on the top of the scribe tool bushing. A labyrinth seal created around the gap  660  between the top seal ring  650  and the scribe tool  700 , controls the amount of low-pressure air that leaks out. This is the circuit that controls the force on the diamond when scribing wafers. 
   As the assembly is moved across a wafer, a resultant drag force is seen normal to the axis of the scribe tool. This force creates an overturning moment on the floating scribe tool and scribe tool bushing, which attempts to cock the bushing in its bore and jam the assembly. The number of holes in the tool holder body, and their distance apart are designed to prevent any jamming moment during normal operation. Forces as high as 50 grams can be tolerated at the tip of the diamond. 
   Normal scribing is done with the scribe tool  700  tilted back from zero in the 15-30 degree range. The tool holder mount moves slowly and systematically along the internal ring gear when the stepper motor rotates, thus changing the angle of the scribe tool with respect to the wafer plane. It has been found that very fine, yet continuous changes in the angle expose a fresh edge  740  on the diamond tip  720 , allowing for longer life before having to scribe tools. The stepper motor preferably includes reduction gears having a reduction ratio of at least 150:1, and preferably at least 161:1. This ratio may be further increased by the final drive between the gear motor pinion  260  and the internal ring gear  160 . For instance, if there are 110 teeth on the ring gear and 24 teeth on the pinion gear, a final drive ratio of 110/24=4.583:1 is achieved. Multiplied by a stepper motor ratio of 161:1, this provides an overall mechanical reduction of 737.9:1. The small stepper motor is driven electronically to accurately delivers between 10-40 steps per revolution, and preferably 20 steps per revolution. 
   Accordingly, viewed in terms of degrees, the foregoing mechanical elements provide a full output revolution every 737.9 stepper motor revolutions, and divided into 360 degrees per revolution, the angular displacement is 0.4879 degrees per motor revolution. The stepper motor allows for further fine tuning, providing that each full motor revolution can be divided into 1/20, so the final resolution is 0.0244 degrees per step. 
   Typically the system may be programmed so that during scribing the system will tilt back some specified amount (depending on the material being scribed) each time the tool indexes over to scribe a new channel, i.e., after every linear pass is completed and before commencing the next pass. In operation, as the internal ring gear  150  is translated by the pinion gear  260  the arc described by the tilting apparatus locates the center of tool tilting rotation at the scribe tool tip. This can be seen clearly in  FIGS. 4A and 4B . Having the center of rotation located at the cutting tip obviates the need to provide a mechanism to lower the scribe tool tip commensurate with the degree to which it is elevated by a tilting mechanism that has a center of rotation above the cutting tip, as is typical of the tool tilting mechanisms in the prior art. The home position, or most upright angle of the scribe tool range of movement (see  FIG. 4A ), is defined by movement of the angle adjustment sector against the snap-action limit switch  200 . 
   As noted above, the tool tilting apparatus of the present invention provides optimal performance and durability when used in combination with a scribe tool stylus having an improved tip.  FIG. 8A  shows an improved scribe tool stylus tip  900  suitable for use with the inventive tool tilting apparatus and scribe tool. 
     FIG. 8B  is a perspective view showing a conventional scribe tool stylus tip  1000 . This view shows that a conventional diamond tip for a stylus, having four cutting edges  1010 , each extending from a scribe point  1020  to the tip  1030 . As will be readily appreciated, this tip configuration necessitates use in a “toe” scribing system, wherein the scribe tool is essentially dragged across the wafer surface; that is, the scribe tool is tilted in the direction of travel and the scribe point leads the cutting edge in engaging the wafer surface. While it is possible to push the prior art tip rather than drag it across the wafer surface, it generally requires a tilting angle of approximately 45 degrees for effective cutting. This angle is too steep to employ effectively with the air bushing system of the present invention. 
   Accordingly, a specially prepared diamond tip  900  adapted for use specifically with the tool tilting apparatus of the present invention is employed in a “heel” scribing procedure (push scribing as opposed to drag scribing). This is shown schematically shown in  FIG. 9 . In this procedure, the triangular-shaped face  910  of the tip precedes the cutting edge and the scribe tool is tipped away from the direction of travel  920 . That is to say, the scribe tool is effectively pushed along the wafer surface  930 . This is made possible by the structural features of the inventive tip, which include a truncated tip  940 , forming four cutlets (or four small flat facets)  950 , each defining a scribe tip (or scribe point)  960  disposed interiorly relative to the cutting edge  970 . This configuration calls for a heel scribing procedure, as described above, and as shown in  FIG. 9 . It also allows the scribe tool to be tilted upwardly, more toward the vertical than prior art cutting tips. In fact, a suitable starting angle for scribing has been found to be approximately 66 degrees from the horizontal. This relatively upright positioning of the stylus makes it possible to take advantage of the air bushing system of the present invention, wherein pressure on the wafer surface is finely controlled by the pneumatic system and the tool holding apparatus. 
   The above disclosure is sufficient to enable one of ordinary skill in the art to practice the invention, and provides the best mode of practicing the invention presently contemplated by the inventor. While there is provided herein a full and complete disclosure of the preferred embodiments of this invention, it is not desired to limit the invention to the exact construction, dimensional relationships, and operation shown and described. Various modifications, alternative constructions, changes and equivalents will readily occur to those skilled in the art and may be employed, as suitable, without departing from the true spirit and scope of the invention. Such changes might involve alternative materials, components, structural arrangements, sizes, shapes, forms, functions, operational features or the like. 
   Therefore, the above description and illustrations should not be construed as limiting the scope of the invention, which is defined by the appended claims.