Abstract:
A laser marking apparatus and method for marking the surface of a semiconductor chip are described herein. A laser beam is directed to a location on the surface of the chip where a laser-reactive marking material, such as a pigment containing epoxy, is present. The heat associated with the laser beam causes the laser-reactive marking material to fuse to the surface of the chip, creating a visibly distinct mark in contrast to the rest of the surface of the chip. Only reactive material contacted by the laser fuses to the chip surface and the remaining residue on the non-irradiated portion can be readily removed.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
   This application is a continuation of application Ser. No. 09/928,314, filed Aug. 13, 2001, pending, which is a continuation of application Ser. No. 09/825,262, filed Apr. 3, 2001, now U.S. Pat. No. 6,461,690, issued Oct. 8, 2002, which is a continuation of application Ser. No. 09/625,938, filed Jul. 26, 2000, now U.S. Pat. No. 6,217,949, issued Apr. 17, 2001, which is a continuation of application Ser. No. 09/358,178, filed Jul. 20, 1999, now U.S. Pat. No. 6,113,992, issued Sep. 5, 2000, which is a continuation of application Ser. No. 08/944,684, filed Sep. 30, 1997, now U.S. Pat. No. 5,985,377, issued Nov. 16, 1999, which is a continuation of application Ser. No. 08/584,246, filed Jan. 11, 1996, abandoned. 

   BACKGROUND OF THE INVENTION 
   1. Field of the Invention 
   This invention relates generally to laser marking techniques and, more specifically, to an apparatus and method for marking the surface of a bare or packaged semiconductor device, comprising one or more dice, using a laser and a laser-reactive marking material. 
   2. State of the Art 
   Since the first semiconductor devices became commercially available, manufacturers have found it necessary to mark each chip or assembly of chips (bare die or package) with the company name, a part or serial number, or other information such as lot number or die location. Conventional marking methods utilize a mechanical device to transfer ink contained in an ink pad to the surface of a stamp. An individual chip is then stamped, and the automated process is repeated for subsequent chips. 
   Because of its mechanical nature and the drying time associated with ink, an ink stamping process is relatively slow. Moreover, if the mark is accidentally touched prior to complete drying, the mark will smudge. In chip manufacturing processes using such an ink stamping method, the ink marking operation may have to be included at a relatively early stage of production (if the die itself is to be marked) or just after post-encapsulation processing (if the package is to be marked) to allow for drying time without affecting the production rate. Such early marking may result, however, in marking defective chips that never make it completely through the manufacturing process. 
   Another problem associated with ink stamping methods is that the quality of ink stamped marks may substantially vary over time. This variation may be dependent upon the quantity of ink applied, ambient temperature and humidity, and/or the condition of the surface of the stamp. In any event, the consistency of a stamped mark may vary widely from chip to chip. 
   As a result of the deficiencies associated with ink stamping, it has become increasingly popular to use a laser beam to mark the surface of a chip. Unlike ink stamping, laser marking is very fast, requires no curing time, has a consistently high quality, and can take place at the end of the manufacturing process so that only good chips are marked. 
   Various machines and methods have been developed for marking a chip with a laser. As illustrated in U.S. Pat. No. 5,357,077 to Tsuruta, U.S. Pat. No. 5,329,090 to Woelki et al., U.S. Pat. No. 4,945,204 to Nakamura et al., U.S. Pat. No. 4,638,144 to Latta, Jr., U.S. Pat. No. 4,585,931 to Duncan et al., U.S. Pat. No. 4,375,025 to Carlson, a semiconductor device is placed in a position where a laser beam, usually produced by a carbon dioxide, Nd:YAG, or Nd:YLF laser, inscribes various characters or other information on a surface of the semiconductor device. Basically, the laser beam bums the surface of the chip such that a different reflectivity from the rest of the chip surface is formed. By holding the chip at a proper angle to a light source, the information inscribed on the chip by the laser can be read. 
   Various materials are known in the art that are laser reactive (e.g., capable of changing color when contacted by a laser beam). As described in U.S. Pat. No. 4,861,620 to Azuma et al., U.S. Pat. No. 4,753,863 to Spanjer, and U.S. Pat. No. 4,707,722 to Folk et al., the part or component may be partially comprised of the laser markable material or have a coating of the material on the surface of the part or component to be marked. 
   Using a laser to mark a chip is a fast and economical means of marking. There are, however, certain disadvantages associated with state-of-the-art laser marking techniques that merely burn the surface to achieve the desired mark in comparison to ink stamping. For example, ink stamping provides a clearly visible image on the surface of a chip at nearly every angle of incidence to a light source. A mark burned in a surface by a laser, on the other hand, may only be visible at select angles of incidence to a light source. Further, oils or other contaminants deposited on the chip surface subsequent to marking may blur or even obscure the mark. Additionally, because the laser actually burns the surface of the work piece, for bare die marking, the associated burning may damage the internal circuitry of the chip directly or by increasing internal die temperature beyond acceptable limits. Moreover, where the manufactured part is not produced of a laser-reactive marking material, laser-reactive coatings applied to the surface of a component may take hours to cure. 
   Thus, it would be advantageous to provide a marking technique that combines the speed and precision of laser marking with the contrast and distinctiveness of ink stamping, without any substantial curing or drying time. Moreover, it would be advantageous to develop a method and apparatus for marking the surface of a semiconductor chip that does not harm the circuitry enclosed therein. 
   BRIEF SUMMARY OF THE INVENTION 
   According to the present invention, a laser marking apparatus and method are disclosed wherein an object is subjected to a laser beam or other suitable energy source for marking purposes. While the laser beam is actively marking, a substance is introduced into the marking work area that interacts with the laser beam. The substance reacts with the localized heat created by the laser and forms a new compound on the surface of the package or surface of the chip. This new compound is selected to contrast highly with the color and/or surface texture of the surface that has been marked. 
   In another particular aspect of the invention, the surface of a chip is at least partially covered with a laser-reactive substance prior to being contacted by a laser beam. The substance may be in either liquid or powder form and may be rolled on, sprayed on, or otherwise applied by means known in the art. When subjected to the localized heat created by the laser, a semi-permanent, solvent-removable mark is formed and bonded to the surface of the chip. The excess material on the non-irradiated portion, that is, the portion of the surface not contacted by the laser beam, is readily removed by an exhaust or residue removal system and may be recycled for future marking. 
   In another, more particular aspect of the invention, an ink bearing material, or other pigmented or laser-reactive substance-bearing material, is disposed adjacent to an exposed surface of a chip. The laser beam transfers ink contained in the ink bearing material to the exposed surface of the chip. For example, the ink bearing material may comprise a ribbon contained in a ribbon dispenser. During the marking process, as the laser beam transfers ink from one point on the ribbon to the chip, another segment of the ribbon may be exposed to the laser beam for subsequent markings. Such an ink bearing material may also help to reduce heat produced by the laser beam from substantially penetrating the surface of the marked chip. 
   In a more particular aspect of the invention, a stream of atomized particles of B-stage epoxy with an added pigment of a desired color (white for example) is directed at the surface where the laser is actively marking the specimen. The epoxy reacts to the heat of the laser and cures to a visible white image coincident with the path of the laser. The excess particles, those which have not been directly irradiated by the laser beam, may be removed along with other debris from the work area by a debris removal system. 
   In another, more particular aspect of the invention, much of the epoxy is destroyed by the laser. A thermal gradient, however, along the trailing edge of the laser path causes the epoxy to cure normally into a final and permanent state, thus producing the desired mark. 
   In another particular aspect of the invention, the laser-reactive marking material absorbs most of the heat produced by the laser. As a result, the delicate internal circuitry of the chip is not exposed to this potentially damaging heat. 
   In another aspect of the invention, subsequent to, or while being marked, the chip is subjected to a jet of coolant to rapidly cool the markings and prevent or reduce the potential for heat damage to the chip. The coolant may be in a liquid, gas, or solid state. In this manner, any residual heat contained in the marking material or present in the surface of the chip may be rapidly dissipated. The markings are thus completely cured and/or cooled before exiting the marking apparatus. 
   In another, more particular aspect of the invention, the laser marking apparatus is computer controlled. In addition to controlling the laser beam, chip location, and other process parameters, the central processing unit (CPU) may control the quality of markings. If so, the marked chips may be subjected to a camera which feeds an image of each chip to the CPU. The CPU compares the pixels of the captured image to a given resolution standard. If the marking is of a sufficiently high quality, the chips are automatically accepted. If not, the chips are automatically rejected for rework and remarking. 

   
     BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       FIG. 1  is a schematic side view of a laser marking apparatus in accordance with the present invention; 
       FIG. 2  is a perspective view of a chip contained in a first embodiment of a chip carrier in accordance with the invention shown in  FIG. 1 ; 
       FIG. 3  is a close-up perspective view of a magazine and chips contained therein in accordance with the invention shown in  FIG. 1 ; 
       FIG. 4  is a perspective view of a second embodiment of a chip carrier in accordance with the present invention; 
       FIG. 5  is a perspective view of a portion of track in accordance with the chip carrier shown in  FIG. 4 ; 
       FIG. 6  is a close-up schematic side view of a first embodiment of a laser marking apparatus in accordance with the present invention; 
       FIG. 7  is a close-up schematic side view of a second embodiment of a laser marking apparatus in accordance with the present invention; 
       FIG. 8  is a close-up schematic side view of an alternate embodiment of a roller-type applicator in accordance with the present invention; and 
       FIG. 9  is a perspective view of a packaged semiconductor device positioned on a track in accordance with the present invention. 
   

   DETAILED DESCRIPTION OF THE INVENTION 
   Referring to  FIG. 1 , a laser marking apparatus  10  in accordance with the present invention is illustrated. Generally, the chips (the term “chips” as used herein refers to both bare and packaged dice, as the invention has equal utility in the marking of both)  12  are automatically fed through the laser marking apparatus  10  for marking purposes. The chips  12  may be fed by a belt, chain, or pneumatic conveyor system as known in the art, gravity fed as shown in  FIG. 1 , or delivered by other means known in the art. The chips  12  are first stacked in a feed magazine  16  ( FIG. 3 ). When released from feed magazine  16  by a mechanical release mechanism as known in the art, the chips  12  exit through an opening  18  located proximate the bottom  20  of the feed magazine  16  onto the low-friction track  14 . 
   As shown in  FIG. 2 , the chips  12  are secured in carriers  11 , preferably made of a statically dissipative material, such as certain plastics and other materials known in the art. The chip carriers  11  may be used to handle the chips  12  during many phases of the manufacturing process, up to and including shipment. The chips  12  are placed on a base  17  and held in place by projections  19 ,  21 ,  23 , and  27 . Legs  33 ,  35 ,  37  and  39  extend downwardly from the bottom  41  of the base  17 . The legs  33  and  35 , as well as legs  37  and  39 , are separated by a distance D 1  sufficient to allow passage of the track  14 . Moreover, legs  35  and  39 , as well as legs  33  and  37 , are separated by a distance D 2  to allow projections  23  and  19  to fit respectively therein whenever the chips  12  are stacked in their respective carriers  11 . 
   As seen in  FIG. 3 , the chips  12  are stacked in the feed magazine  16 . The chips  12 , suspended above the track  14  by the feed magazine  16 , are individually released onto the track  14  and allowed to slide by the force of gravity down the track  14 . The feed magazine  16  automatically releases the chips  12  at constant or selectively variable intervals dictated by process requirements. The feed magazine  16  may vary in size to accommodate large or small numbers of chips  12  and each carrier  11  may vary in size to accommodate one or more dice. 
   Carriers  11  may also be in elongated form to accommodate a plurality of chips  12  to be marked. As depicted in  FIG. 4 , an empty chip carrier  82  is capable of holding at least four (4) chips  12 . The chip carrier  82  may also be modified to hold several dice that have not been cut apart (if increased in size) or an entire wafer (if modified to hold round rather than rectangular objects). Chips  12  are held in the carrier  82  by elements  84  which provide an interference or resiliently-biased fit, as desired, between the carrier  82  and a chip  12 . Moreover, the chips  12  rest upon the lip  86  so that each chip  12  held by the carrier  82  extends equally above the top surface  88  of the carrier  82 . 
   The carrier  82  is adapted to slide along a track positioned in several different orientations, such as a track  90  shown in  FIG. 5 . The carrier  82  has legs  92  and  94  depending from and separated by cross-members  96 ,  98 ,  100 ,  102  and  104  extending the length of the carrier  82 . The legs  92  and  94  are parallel to each other, have lateral extensions  106  and  108 , respectively, spaced from the cross-members  96 ,  98 ,  100 , and  102  running the length of the legs  92  and  94  and projecting inwardly for grasping the elongate rails  110  and  112  of the track  90 . 
   The rails  110  and  112  of the track  90  are shown oriented back-to-back and having a “C” shaped cross-section and are spaced apart by members  107 . When the carrier  82  is riding on the top of the track  90 , the lateral extensions  106  and  108  grasp the top portions  114  and  116  of the rails  110  and  112 , respectively. If the carrier  82  is suspended from the bottom of the track  90  (in an inverted orientation), the lateral extensions  106  and  108  grasp the bottom portions  118  and  120 , respectively. Moreover, because the carrier  82  is designed to actually grasp the track  90  rather than merely ride on it, the track may be placed in any orientation. 
   When the chips  12  are placed in the carrier  82  and the carrier  82  is positioned on the track  90 , the marking operation may occur on either side. That is, because both sides of the chip  12  are exposed, neither the top nor the bottom of the chip  12  has any substantial portion covered by the carrier  82 . If the chips  12  in the carrier  82  are automatically inspected, defective chips  12  may be automatically popped out of the carrier  82 . A solvent or other substance, or even a de-marking laser, may be used to remove the defective mark and the chip  12  may then be reloaded into a carrier  82  and remarked. Thus, the requirements of the process and of the marking and inspection apparatus can dictate the orientation of the track  90 , the carriers  82  thereon, and the chips  12  in the carriers  82 . 
   The carrier  82  is also suited for stacking with other similar carriers. Extending longitudinally along the length of the top surface  88  of the outside edges  103  and  105  of the carrier  82  are channels  95  and  97  sized and shaped to receive extensions  99  and  101  extending downwardly from legs  92  and  94 , respectively. The extensions  99  and  101  also extend longitudinally the length of the carrier  82  along the bottom  93  of the carrier  82 . The extensions  99  and  101  extend downwardly from the lateral extensions  106  and  108 , respectively, a sufficient distance so that when stacked, the lateral extensions  106  and  108  are spaced above the chips  12  contained in the carrier  82 . 
   For typical packaged dice (chips)  122 , such as that shown in  FIG. 6 , the chip  122  can ride directly on the track  14  without being placed in a carrier. The connecting tabs  124  located on the sides  126  and  128  of the chip  122  keep the chip  122  properly aligned on the track  14 . Moreover, the track  14  is of a width W so that the chips  122  stay in longitudinal and latitudinal alignment with the track  14 . The chips  122  can also be loaded onto the track  14  by a feed magazine of a modified version of feed magazine  16  and loaded into a shipping magazine such as tubular shipping magazine  50  ( FIG. 1 ). 
     FIG. 1  shows laser marking apparatus  10  of the present invention in a gravity feed arrangement where the track  14  is placed at an angle A relative to the horizon such that the force of static friction between the carriers  11  and the track  14  is less than the force of gravity along the line of the track  14  on the carriers  11 . When the chips  12  are released from the feed magazine  16 , several chips  12  are staged, six (6) in this case, by automated indexing pins  22  and  24  at the initial staging area  13 . Once the chips  12  are staged, indexing pin  24  is retracted to allow the staged chips  12  to slide on the track  14  until stopped by indexing pin  26  at the marking area  25 . The chips  12  are held in place by indexing pin  26  until all of the chips  12  retained by indexing pin  26  are marked by the laser  28 . The laser  28  may be comprised of a carbon dioxide, Nd:YAG, Nd:YLF laser or other suitable lasers or devices, such as an electron beam emitter, known in the art. The laser  28  is longitudinally translatable along the support  30  in at least one direction so that all of the chips  12  retained by indexing pin  26  can be marked by the laser  28  in a single pass. 
   Once the laser  28  marks the chips  12 , indexing pin  26  is retracted and the chips  12  are allowed to slide until retained by indexing pin  32  at the debris removal and inspection area  31 . As the chips  12  pass from indexing pin  26  to indexing pin  32 , they slide under the debris removal system  34 . The debris removal system  34  may employ suction, forced air and/or other methods known in the art to clean the surface  54  of the chip  12  ( FIG. 7 ) without disturbing the markings thereon (not shown). Moreover, any marking material that remains in the recovered residue may be reprocessed for future chip marking. 
   The chip  12  adjacent the indexing pin  32  is then inspected by the camera  36  which may be a CCD camera or other suitable camera known in the art. That is, the camera  36  photographs the image of the surface  54  of the chip  12  and the markings contained thereon and sends this image to a central processing unit, such as CPU  80  in  FIG. 1 . The image received by the CPU  80  is broken down into individual pixels and the pixels are compared to a minimum standard. Once the image is received and compared by the CPU  80 , each chip  12  is released by the indexing pin  32 . The adjacent, upstream chips  12  are maintained in position by the indexing pin  38  until each is released for inspection. If the chip  12  released by the indexing pin  32  is acceptable according to the comparison made by the CPU  80 , then the chip  12  is allowed to slide on the track  14  to the final staging area  40 . If the chip  12  is determined by the CPU  80  to be unacceptable, a trap door  42  is opened and the chip  12  falls into a bin  44  so that the chip  12  may be reworked and remarked. 
   An electronic eye  46  is positioned to identify when a proper number, in this case six (6), of acceptable chips  12  are ready to be packaged. Once the proper number of chips  12  is achieved, the indexing pin  48  is activated until all of the chips  12  held in the final staging area  40  have been loaded into a shipping magazine  50 . 
   The apparatus  10  disclosed herein only requires an operator to load the feed magazine  16  with chips  12  to be marked and to remove and replace the shipping magazine  50  when full. The rest of the marking/inspection operation is completely automated and controlled by the CPU  80 . Moreover, it is possible for the CPU  80  to control multiple track arrangements simultaneously. 
   Referring now to  FIG. 7 , a close-up view of the laser  28  in relation to the chip  12  is shown. The laser  28  projects a movable laser beam  52  onto the surface  54  of the chip  12  to mark the chip  12 . As the laser beam  52  is directed toward the chip surface  54 , a laser-reative marking material  58  is injected through an applicator or nozzle  60  onto the chip surface  54  at the same location  56  that the beam contacts the chip  12 . The heat from the laser beam  52  fuses the laser-reactive marking material  58  onto the chip surface  54 . Laser-reactive marking material  58  present on any non-irradiated portion of the chip  12  that has not been exposed to the laser beam  52  and is, therefore, unreacted, does not bond to the chip surface  54  and is subsequently removed. 
   A coolant  62  may also be injected from a coolant injector or nozzle  64  onto the surface  54  of the chip  12  and onto the laser-reactive marking material  58  present on the chip surface  54 . If a coolant  62  is used, any residual heat contained in the chip  12  or the laser-reative marking material  58  may be quickly dissipated. This may be necessary to help protect the delicate circuitry of a bare die from the heat of the laser beam  52 . The laser  28  is shown without the coolant nozzle  64  in  FIG. 1 . The use of a coolant  62  also prevents or ensures the laser-reactive marking material  58 , which may be an epoxy material that may cure at a relatively low temperature, from curing prematurely, thereby decreasing the need for relatively high curing temperature epoxies to be used in the marking process. 
   As can be seen, both the pigment nozzle  60  and the coolant nozzle  64  are attached to the laser  28  so that any movement of the laser results in movement of the nozzles  60  and  64 . Thus, the laser  28  and the nozzles  60  and  64  translate together, and are thus synchronous, so that a minimum amount of laser-reactive marking material  58  and coolant  62  is required. Moreover, the marking location immediately surrounding the target surface on each chip  12  for laser beam  52  may be placed in a reduced or negative pressure environment with respect to the surrounding work area by means known in the art to reduce overspray that may otherwise settle on the chip  12  or drift onto the track  14  or other parts of the apparatus  10 . 
   In  FIG. 8 , an alternate embodiment is shown having a ribbon dispenser  66  comprised of a feed reel  68  and a take-up reel  70 . The ribbon dispenser  66  dispenses a ribbon or strip of ink bearing ribbon  72  from the feed reel  68  to the take-up reel  70 . The ribbon  72  extends over and is proximate to the surface  54  of the chip  12 . The ribbon  72  may also extend over a number of chips  12  or several ribbon dispensers  66  may be placed side by side so that marking of several chips  12  can occur sequentially or so that multiple colors may be used in the marking process. The chips  12  are allowed to pass under the ribbon  72  as they slide along the track  14 . When the chips have moved to the marking area  25 , the laser  28  projects a laser beam  52  onto the surface of the ribbon  72  and transfers ink from the ribbon  72  onto the surface  54  of the chip  12 . One advantage of the embodiment of  FIG. 8  is the elimination of liquid pigments and coolants, the latter being due to absorbance of the laser energy by the ribbon  72  carrying the marking material. Another advantage is that the marking process using a ribbon  72  is cleaner in that no excess particles of marking material are present in the marking area to contaminate the marking area and chip in undesired areas. 
   Referring to  FIG. 9 , the laser-reactive marking material may be applied by a motorized roller  130  rotatably attached to a roller support  135 . An open-celled sponge or fiber pad  132  is held against the roller  130  by a support member  134 . The support member also supplies the laser-reactive marking material to the pad  132 , the arrangement functioning like a shoe-polish applicator. The roller is held in contact with the top surface  54  of the chips  12  and forces the chips  12  between the roller and the track  14 . Because the pad  132  continually supplies laser-reactive marking material to the roller  130 , each chip  12  receives a consistent layer of material. The chips  12  can then be laser marked. The application of laser-reactive marking material to the roller  130  could also be achieved by spray, drip or other methods known in the art. 
   While the present invention has been described in terms of certain preferred embodiments, it is not so limited, and those of ordinary skill in the art will readily recognize and appreciate that many additions, deletions and modifications to the embodiments described herein may be made without departing from the scope of the invention as hereinafter claimed. As used in the claims, as in the preceding specification, the term “chip” or “chips” is intended to mean and encompass both bare, both the circuit side and/or back (Si) side of the semiconductor dice, and packaged semiconductor dice. 
   Additionally, while the invention has been described in conjunction with the use of a laser as an energy source for the marking of a chip or chips, any suitable energy source may be used in place of the laser energy source, such as a focused ultraviolet light source, electron beam, focused and directed hot air source, etc.