Patent Publication Number: US-2023148420-A1

Title: Die attach systems, and methods for integrated accuracy verification and calibration using such systems

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a divisional application of U.S. patent application Ser. No. 16/561,224, filed Sep. 5, 2019, which claims the benefit of U.S. Provisional Application No. 62/727,447, filed Sep. 5, 2018, the content of both of which is incorporated herein by reference. 
    
    
     FIELD 
     The invention relates to systems and methods for attaching a die to substrate, and more particularly, to improved systems and methods for accuracy verification and calibration for a die attach system. 
     BACKGROUND 
     In connection with the placement of a die on a substrate (e.g., the placement of a semiconductor die on a substrate), many conventional applications utilize a “pick and place” operation. In such operations, a die is “picked” from a semiconductor wafer or other die supply source, and then the die is moved to (and “placed” on) a target substrate. Such operations may also utilize one or more transfers between a “pick” tool and a “place” tool. 
     Certain die attach applications do not utilize a pick and place operation. For example, a die supply source (e.g., a wafer including a plurality of die) may be positioned between a bond tool and a substrate. Die included in the die supply source may be attached to a film or the like. After alignment between the bond tool, the die to be attached, and a placement location of the substrate—the bond tool presses the die against the placement location of the substrate. 
     Accuracy of a die attach operation tends to be performed after the operation is complete, using equipment separate from the die attach system. Such accuracy determinations tend to be time consuming and costly. Thus, it would be desirable to provide improved systems and methods for verifying the accuracy of die attach operations, and similar processes. 
     SUMMARY 
     According to an exemplary embodiment of the invention, a die attach system is provided. The die attach system includes a verification substrate configured to receive a plurality of die, the verification substrate including a plurality of substrate reference markers. The die attach system also includes an imaging system for determining an alignment of the plurality of die with the verification substrate by imaging each of the plurality of die with respective ones of the plurality of substrate reference markers. 
     According to another exemplary embodiment of the invention, a method of operating a die attach system is provided. The method includes the steps of: providing a verification substrate configured to receive a plurality of die, the verification substrate including a plurality of substrate reference markers; and imaging each of the plurality of die with respective ones of the plurality of substrate reference markers using an imaging system of the die attach system for determining an alignment of the plurality of die with the verification substrate. 
     According to yet another exemplary embodiment of the invention, another die attach system is provided. The die attach system includes: a die supply source holding a die supply form including a first plurality of reference markers; a first motion system for moving the die supply source; a bond head including a bond tool and an imaging system; a second motion system for moving the bond head; a substrate including a second plurality of reference markers; and wherein the imaging system is configured for imaging ones of the first plurality of reference markers and ones of the second plurality of reference markers in a single field of view. 
     According to yet another exemplary embodiment of the invention, a method of calibrating a die attach machine is provided. The method includes the steps of: providing a calibration die supply form including a calibration die supply, the calibration die supply including a first plurality of reference markers; providing a bond head including a bond tool and an imaging system; providing a substrate including a second plurality of reference markers; and imaging ones of the first plurality of reference markers and ones of the second plurality of reference markers in a single field of view with an imaging system of a die attach machine. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawings. It is emphasized that, according to common practice, the various features of the drawings are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. 
         FIG.  1 A  is a block diagram top view of elements of a die attach system in accordance with an exemplary embodiment of the present invention; 
         FIG.  1 B  is a side view of elements of the die attach system of  FIG.  1 A ; 
         FIG.  2    is a top view of a verification substrate in accordance with an exemplary embodiment of the present invention; 
         FIG.  3    is a top view of a portion of the substrate of  FIG.  2   , indicating an ideal die attach location, in accordance with an exemplary embodiment of the present invention; 
         FIGS.  4 - 6    are top view illustrations of a verification substrate used in connection with a die placement accuracy process in accordance with an exemplary embodiment of the present invention; 
         FIGS.  7 A- 7 B  are top view illustrations of a calibration operation in accordance with an exemplary embodiment of the present invention; 
         FIGS.  8 A- 8 B  are top view diagrams illustrating an pre-bond inspection operation in accordance with an exemplary embodiment of the present invention; and 
         FIGS.  9 A- 9 B  are top view diagrams illustrating a calibration operation in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION 
     As used herein, the term “die” is intended to refer to any structure including (or configured to include at a later step) a semiconductor chip or die. Exemplary “die” elements include a bare semiconductor die (such as a bare LED semiconductor die), a semiconductor die on a substrate (e.g., a leadframe, a PCB, a carrier, a semiconductor chip, a semiconductor wafer, a BGA substrate, a semiconductor element, etc.), a packaged semiconductor device, a flip chip semiconductor device, a die embedded in a substrate, amongst others. 
     As provided above, certain die attach applications do not utilize a pick and place operation. For example, a die supply source (e.g., a wafer including a plurality of die, such as an LED wafer of other source of LED die) may be positioned between a bond tool and a substrate. The die supply source may include a plurality of die (e.g., an LED die) attached to a film or the like. Transfer of the die from the die supply source to the substrate may be accomplished using a number of processes. Two exemplary processes are described below. 
     According to a first exemplary process, after alignment of the bond tool, the die to be attached, and a placement location of the substrate—the bond tool presses the die against the placement location of the substrate. Adhesive on a lower surface of the die (and/or on the placement location of the substrate) is provided such that the die is now secured to the substrate. Such bond tools may include a needle, a plurality of pins (e.g., separably actuatable pins), etc. for contacting the die in connection with a transfer from the die supply source to the substrate. 
     According to a second exemplary process, after alignment of the bond tool, the die to be attached, and a placement location of the substrate—a laser or other light source (e.g., where the laser may be carried by the bond head) is used to transfer the die from the die supply source to the substrate. Adhesive on a lower surface of the die (and/or on the placement location of the substrate) is provided such that the die is now secured to the substrate. 
     While two exemplary processes are described above, it will be appreciated that other transfer processes are contemplated. Regardless of the transfer process, aspects of the invention may be utilized to improve the relevant die attach system and related processes. 
     According to certain exemplary embodiments of the present invention, accuracy verification may be integrated into a die attach system, for example, using a camera of the die attach system. Conventional substrates or dedicated substrates may be used for die attachment. Dedicated substrates have high relative accuracy of local references versus global substrate alignment references. Exemplary dedicated substrates include state-of-the-art glass substrates with lithographically applied reference markers, or metal substrates with laser engraved references. The camera (or other imaging system) of a die attach system may be used to register the x, y, and theta deviations of attached dies versus their respective substrate references. By imaging both the substrate reference and the die in the same camera image field-of-view, measurement errors are minimized and typically only depend on the quality of the camera and the relative accuracy of the substrate reference markers. 
     Thus, aspects of the invention may relate to obtaining x, y, and theta deviation data. Such deviation data may be used, for example: (i) to assemble an accuracy report on the die attach system; (ii) to determine systematic deviations and feed back into the system to improve the die attach accuracy; and (iii) to derive diagnostic information to invest root cause for accuracy related non-conformities. 
     This is different from conventional systems, for example, in that die attach accuracy related verification and calibration is integrated with the system that is used to attach the dies. 
     This is a significant improvement as compared to conventional systems and methods because, for example, additional measurement equipment (and related management) is not required to obtain the accuracy data. Shorter calibration/verification loops may be provided because of the integration with the die attach system itself. 
     Referring now to the drawings,  FIG.  1 A  illustrates a die attach system  100 . Die attach system  100  includes a support structure  110  for supporting a substrate  112 , a die supply source  108  including a plurality of die  108   a  (where the plurality of die  108   a  are provided on a film/foil  108   b  included as part of die supply source  108 ) configured to be attached to substrate  112 , and a bond head  104  including a bond tool  104   b  for contacting die  108   a  during a transfer of die  108   a  from die supply source  108  to substrate  112 . Die attach system  100  also include a bond head support  102  and a supply support  106 . Bond head support  102  and supply support  106  are each mounted on machine structure  150  such that bond head support  102  and supply support  106  are independently moveable relative to machine structure  150 . Bond head support  102  supports moveable bond head  104 . Bond head support  102  includes a motion system (e.g., a robot) for moving bond head  104  along the x, y directions. Bond head  104  includes a camera  104   a  (and other visions system components) for use in connection with alignment and/or inspection operations. Die supply source  108  is moveably mounted on supply support  106 . Supply support  106  includes a motion system (e.g., a robot) for moving die supply source  108  along the x, y directions. In the exemplary embodiment of the invention shown in  FIG.  1 A  (and in  FIG.  1 B ), during a die attach operation, die supply source  108  is positioned between bond tool  104   b  and substrate  112  supported by support structure  110 . 
     As compared to  FIG.  1 A , the side (partial cross sectional) view of  FIG.  1 B  illustrates bond head  104  (including camera  104   a  and bond tool  104   b ) having been moved to a position over die  108   a  which, in turn, is positioned over substrate  112 . Two “bonded” die  108   a ′ have been attached to substrate  112  at respective bonding locations and bond tool  104   b  is shown engaging another die  108   a  above a third respective bonding location on substrate  112 . Bond tool  104   b  (e.g., including a needle, a plurality of pins, etc.) presses die  108   a  against the third bonding location on substrate  112  to complete another transfer. 
     While  FIGS.  1 A- 1 B  utilize a bond tool  104   b  for completing transfer of ones of a plurality of die  108   a  from a film  108   b  to substrate  112 , other types of transfer are contemplated (e.g., the aforementioned laser transfer). 
       FIGS.  1 A- 1 B  illustrate substrate  112 , which is a substrate for use in connection with typical die attach processes. Aspects of the invention utilize verification substrates useful in connection with operations such as (i) die attach accuracy verification, (ii) pre bond alignment, (iii) calibration operations, among others. Exemplary verification substrates are labelled herein with reference number “112a”. In exemplary embodiments of the invention, verification substrate  112   a  is utilized in connection with a die attach machine (e.g., die attach machine  100  shown in  FIGS.  1 A- 1 B ). In connection with certain exemplary methods described herein, verification substrate  112   a  will be located on support structure  110  of die attach system  100  (in place of substrate  112  from  FIGS.  1 A- 1 B ). 
       FIG.  2    illustrates verification substrate  112   a . Verification substrate  112   a  includes a plurality of local substrate reference markers  112   a   1 , and a plurality of global alignment reference markers  112   a   2 . Verification substrate  112   a  may be a glass substrate, a metal substrate, etc. Reference markers  112   a   1 ,  112   a   2  may be lithographically applied reference markers, laser engraved reference markers, among other types of reference markers. Accuracy performance (e.g., in the x, y, and theta dimensions) of a die attach system (such as die attach system  100 ) may be provided relative to verification substrate  112   a . Relative measurements taken by a camera of a die attach system (e.g., camera  104   a  of die attach system  100 ) may be converted using verification substrate  112   a . Such measurements may also involve reference coordinate system  200  also shown in  FIG.  2   , where reference coordinate system is an xy coordinate system of a die attach system (e.g., die attach system  100 ). 
       FIG.  3    illustrates an exemplary die  108   a  attached to a die attach location on verification substrate  112   a . More specifically, die  108   a  is attached between four (4) local substrate reference markers  112   a   1  on verification substrate  112   a . In  FIG.  3   , die  108   a  is shown at an ideal location.  FIG.  3    shows a die center  108   a   1  centered within a theoretical ideal location  108   a   2  for die center  108   a   1  (where ideal location  108   a   2  is, in the example shown in  FIG.  3   , at a center of substrate reference markers  112   a   1 ). 
       FIGS.  4 - 6    illustrate steps of a method of determining (and/or verifying) the accuracy of a die attach operation.  FIG.  4    illustrates a plurality of die  108   a  attached to bonding locations (target locations) of verification substrate  112   a . For example, the plurality of die  108   a  may be attached to verification substrate  112   a  using die attach machine  100  shown in  FIGS.  1 A- 1 B  (e.g., using bond tool  104   b ). The die attach process may be accomplished by imaging one or more plurality of global alignment reference markers  112   a   2  before bonding. The relative location of the local substrate reference markers  112   a   1  to the global alignment reference markers  112   a   2  is accurately known (e.g., either by prior measurement or precise manufacturing). 
       FIG.  5    illustrates a field of view  500  (e.g., taken with a camera of a die attach system, such as die attach system  100 ), including measurement axes  500   a . After the plurality of die  108   a  are attached to verification substrate  112   a  (as in  FIG.  4   ), images are taken of each bonded die  108   a  and its corresponding local substrate reference markers  112   a   1  in a single field of view, as shown in  FIG.  5   . This imaging (using field of view  500 ) results in initial measurement results obtained in the camera measurement coordinate system. While  FIG.  5    shows a single field of view measurement of a single die  108   a  bonded to a portion of verification substrate  112   a , it is understood that multiple images may be taken of multiple bonded die  108   a.    
     After the imaging of  FIG.  5   , the location of die  108   a  relative to the local substrate reference system (including local substrate reference markers  112   a   1 , center point  112   a   1   a ) is determined, as shown in  FIG.  6   . As shown in  FIG.  6   , the measurement results are illustrated after conversion to the substrate reference system, which typically involves de-rotation of the camera angle relative to the substrate. This makes measurement results invariant to incidental camera orientation and position. As shown in  FIG.  6   , in addition to an angular theta deviation (about the theta axis shown in  FIG.  6   ), an x-offset and a y-offset are determined. 
     While  FIGS.  4 - 6    illustrate a method of determining the accuracy of die placement on a substrate (which may be used as feedback for corrections in future die placement operations), other improvements may be provided in accordance with the inventive systems and methods described herein.  FIGS.  7 A- 7 B  illustrate a method of determining (and correcting) systematic deviations in a die attach operation. Such a systematic error is typically obtained as the mean or median value of the measured die-offsets.  FIG.  7 A  illustrates a plurality of die  108   a  bonded to verification substrate  112   a  (where the die are bonded on a die attach machine, such as die attach machine  100  including bond tool  104   b ). As can be seen in  FIG.  7 A , each of the plurality of die  108   a  are offset from desired bonding location between adjacent local substrate reference markers  112   a   1  (the systematic error is illustrated in  FIG.  7 A  as an offset that is off center from the cross at the bottom of  FIG.  7 A ). 
     After determining (and/or applying) the systematic error (for example, by correcting for a mean or median die offset), another bonding operation may be completed.  FIG.  7 B  illustrates the plurality of die  108   a  bonded to a substrate  112  (e.g., see substrate  112  shown in  FIG.  1 A ) after feedback from the systematic error determination. In  FIG.  7 B , the local substrate reference markers  112   a   1 ′ are shown in “phantom” format because they are likely not on substrate  112  (as they would be on verification substrate  112   a ). Nonetheless, the feedback results in improved bonding as shown in  FIG.  7 B , with the bonded die  108   a  more accurately placed between adjacent local substrate reference markers  112   a   1 ′ (the systematic error is illustrated in  FIG.  7 B  as being corrected, with a “dot” centered on the cross at the bottom of  FIG.  7 B ). Of course, it is understood that the feedback may be accomplished by bonding the die  108   a  to another verification substrate  112   a  to confirm the improved accuracy (as opposed to the substrate  112  shown in  FIG.  7 B ). 
     In addition to the die attach accuracy verification of  FIGS.  4 - 6   , and the systematic error determination and correction shown in  FIGS.  7 A- 7 B , additional embodiments of the invention are contemplated. For example, certain accuracy related diagnostics may be performed within the scope of the invention.  FIGS.  8 A- 8 B  illustrate an example of such a diagnostic technique in connection with a pre-bond accuracy inspection.  FIG.  8 A  is a block diagram view of die attach system  100  where camera  104   a  is imaging a die  108   a  while still attached to film  108   b . That is, the imaging is performed before a die attach operation is completed. The plurality of die  108   a  are still on the film  108   b  of die supply source  108 . The film is typically transparent (or semi transparent, or translucent), such that the markers of a substrate below the film are well visible and can be measured accurately. In  FIGS.  8 A- 8 B , verification substrate  112   a  is positioned below die supply source  108  (where the plurality of die  108   a  are still on film  108   b ). More specifically, die supply source  108  (and/or verification substrate  112   a ) is moved such that at least one of the plurality of die is positioned above a corresponding bonding location of verification substrate  112   a  (i.e., between respective local substrate reference markers  112   a   1 ). In this orientation, a field of view of the camera is imaged to show the relative position of the die  108   a  to the respective local substrate reference markers  112   a   1 .  FIG.  8 B  illustrates a single die  108   a  in such a field of view. Using the image taken from this field of view, accuracy data may be obtained before the die attach process occurs. This image data may be used as feedback to correct for any accuracy issue. 
     In another exemplary method of the invention,  FIGS.  9 A- 9 B  illustrates a calibration operation (e.g., a robot stage calibration). The principle of relative marker offset measurement can be applied to robot stage calibration (where a robot stage is a portion of the die attach machine being moved by a motion system such as bond head  104  being moved by a motion system, or die supply source  108  being moved by another motion system). In connection such a robot stage calibration, typical non-linear deviations of robot axes are measured as function of the x,y location in the work-area of the robot stage. For the robot stage having a downwards looking camera (e.g., bond head  104  being moved by a motion system of bond head support  102 ), the substrate marker measurements can be used directly to create a so called error map of the robot stage. For the robot stage without the downwards looking camera (e.g., die supply source  108  being moved by a motion system of supply support  106 ), a similar error map can be obtained using the camera of the other stage (e.g., camera  104   a ) as a measurement device. For this purpose, a calibration die supply form is inserted in place of an actual die supply source, where an exemplary calibration die supply form includes a lithographically manufactured glass plate with markers (a so called “calibration wafer”). The deviations of the calibration die supply form are obtained by measuring the relative offsets of the supply form markers with respect to the substrate reference markers. This may done on various locations covering the work area of the robot stage. Both robot stages can thus be calibrated to the same calibration reference, at the same time. Typically, multiple markers in one field-of-view are used for this purpose (both on substrate and supply form), creating a more accurate and robust measurement capability of positional and angular offsets. 
     Referring to the specific example shown in  FIGS.  9 A- 9 B , a calibration die supply form  108 ′ is provided in place of the die supply source  108  in other drawings provided herein. Calibration die supply form  108 ′ holds calibration die supply  108   c  including supply form markers  108   c   1 . While these supply form markings are square (similar to the shape of die  108   a  from previous drawings), any type of supply form marker shape is contemplated. Substrate  112   a  (including substrate reference markers  112   a   1 ,  112   a   2 ) is positioned below calibration die supply form  108 ′. In this configuration, camera  104   a  may be used to measure the relative offsets of the supply form markers  108   c   1  with respect to the substrate reference markers  112   a   1  and/or  112   a   2 . The relative offsets may be used in connection with the robot stage calibration of each of the robot stages (e.g., bond head  104  being moved by a motion system of bond head support  102 , and die supply source  108  being moved by a motion system of supply support  106 ) as described above. 
     While the invention has been described and illustrated primarily with respect to die attach operations where there is no “pick” operation, it is not limited thereto. The invention has broad applicability in the semiconductor bonding industry including die attach machines (sometimes referred to as die bonders) or other packaging machines (e.g., flip chip machines/operations, advanced packaging operations, etc.). 
     While exemplary embodiments of the invention are illustrated and described with respect to markers having certain shapes (e.g., cross shaped markers, round markers, rectangular markers, etc.), and certain numbers of markers with respect to a single die (e.g., four substrate reference markers  112   a   1  surrounding each die), etc. —these types of details are exemplary in nature, and non-limiting with respect to the scope of the invention. 
     Although the invention has been described and illustrated with respect to the exemplary embodiments thereof, it should be understood by those skilled in the art that the foregoing and various other changes, omissions and additions may be made therein and thereto, without parting from the spirit and scope of the present invention. Rather, various modifications may be made in the details within the scope and range of equivalents of the invention and without departing from the invention.