Patent Publication Number: US-9847314-B2

Title: Bond heads for thermocompression bonders, thermocompression bonders, and methods of operating the same

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     This application is a continuation of U.S. patent application Ser. No. 14/746,065, filed Jun. 22, 2015, which is a continuation of U.S. patent application Ser. No. 14/314,149, filed Jun. 25, 2014, which claims the benefit of U.S. Provisional Application No. 61/842,081, filed Jul. 2, 2013, the contents of which are incorporated herein by reference. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to semiconductor bonding machines, and more particularly, to improved bond head assemblies for bonding machines for forming electrical interconnections. 
     BACKGROUND OF THE INVENTION 
     The present invention relates to bonding machines and has particular applicability to thermocompression bonding machines and bonding therewith. 
     Thermocompression bonding machines are be used in bonding a plurality of conductive regions on one substrate to a plurality of conductive regions on another substrate. For example, such bonding machines may be used to bond a semiconductor die (e.g., including conductive regions such as bumps or pillars formed on the die) to another substrate (e.g., where the another substrate may be another die, a wafer, a leadframe, or any other substrate used in packaging). In certain exemplary thermocompression bonding machines, a placer tool (also referred to as a place tool, a placing tool, a bonding tool, or simply a tool to hold or bond a workpiece) is used to bond the one substrate (e.g., a die, also referred to as a workpiece) to the another substrate. In connection with the bonding of the die/workpiece, it may be desirable to heat the die/workpiece, for example, to heat the conductive regions on the die/workpiece. 
     According to the present invention, novel structures and methods are provided which permit heating of a bonding tool (carrying a workpiece such as a die) to allow proper bonding of the workpiece to an underlying substrate, and then a controlled and rapid cool down of the heater (and thus the bonding tool) before transfer of another workpiece onto the bonding tool to again repeat the cycle. 
     SUMMARY OF THE INVENTION 
     According to an exemplary embodiment of the present invention, a bond head for a thermocompression bonder comprises a tool configured to hold a workpiece to be bonded, a heater configured to heat the workpiece to be bonded, and a chamber proximate the heater, the chamber configured to receive a cooling fluid for cooling the heater. 
     According to another exemplary embodiment of the present invention, a thermocompression bonder comprises a workpiece supply station including a plurality of workpieces, a bonding station, a placer tool for receiving a workpiece from the workpiece supply station and for bonding the workpiece to a substrate at the bonding station, and a cooling station for cooling the placer tool after bonding the workpiece to the substrate. 
     According to another exemplary embodiment of the present invention, a thermocompression bonder comprises a bond head including (a) a tool configured to hold a workpiece to be bonded, (b) a heater configured to heat the workpiece to be bonded, and (c) a chamber proximate the heater, the chamber configured to receive a cooling fluid for cooling the heater. 
     According to another exemplary embodiment of the present invention, a thermocompression bonder comprises a bond head including (a) a tool configured to hold a workpiece to be bonded, (b) a heater configured to heat the workpiece to be bonded, and (c) a chamber proximate the heater, the chamber configured to receive a cooling fluid for cooling the heater, the chamber adapted to move between a first position in contact with the heater and a second position out of contact with the heater. 
     According to another exemplary embodiment of the present invention, a thermocompression bonder comprises a bond head including (a) a tool configured to hold a workpiece to be bonded, (b) a heater configured to heat the workpiece to be bonded, and (c) a chamber proximate the heater, the chamber configured to receive a cooling fluid during a first operational phase, and a second fluid during a second operational phase. 
     According to another exemplary embodiment of the present invention, a thermocompression bonder comprises a bond head including (a) a tool configured to hold a workpiece to be bonded, (b) a heater configured to heat the workpiece to be bonded, (c) a chamber proximate the heater, the chamber configured to receive a cooling fluid for cooling the heater, (d) a support structure above the heater, and (e) at least two flexures disposed between the support structure and the heater. 
     According to another exemplary embodiment of the present invention, a method of thermocompressively bonding a workpiece to a substrate comprises the steps of (1) bonding a workpiece to a substrate using a bond head of a thermocompression bonder, the bond head including a heater and (2) providing a cooling fluid into a chamber of the bond head proximate the heater to reduce a temperature of the heater after step (1). 
     Additional exemplary methods are disclosed herein, such as methods of operating any of the bond heads or bonding machines disclosed or claimed herein. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily deformed or reduced for clarity. Included in the drawing are the following figures: 
         FIG. 1  is a block diagram side view of elements of a conventional thermocompression bonder; 
         FIGS. 2A-2D  are block diagram views of bonding structures on a lower substrate, bonding structures on an upper substrate, and a conventional method of bonding the upper substrate to the lower substrate; 
         FIG. 3A  is a block diagram side view of a bond head assembly in accordance with an exemplary embodiment of the present invention; 
         FIG. 3B  is a detailed view of a first example lower bond head for the bond head assembly of  FIG. 3A  in accordance with an exemplary embodiment of the present invention; 
         FIG. 3C  is a detailed view of a second example lower bond head for the bond head assembly of  FIG. 3A  in accordance with another exemplary embodiment of the present invention; 
         FIGS. 3D-3E  are block diagrams illustrating a chamber of a bond head assembly having a fluid therein, in accordance with an exemplary embodiment of the present invention; 
         FIG. 4  is block diagram illustrating recirculation of a cooling liquid fluid through a chamber of a bond head assembly in accordance with an exemplary embodiment of the present invention; 
         FIGS. 5A-5B  illustrate a chamber being moved in and out of contact with a heater of a bond head assembly in accordance with an exemplary embodiment of the present invention; 
         FIGS. 6A-6C  illustrate bond head assemblies including flexures in accordance with various exemplary embodiments of the present invention; and 
         FIG. 7  is a block diagram illustrating operations of elements of a bonding machine in accordance with an exemplary embodiment of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     According to certain exemplary embodiments of the present invention, a bond head for a thermocompression bonder (e.g., a die attach bonder) is provided which performs, for example, a local reflow solder attach process. A bonding tool of the bond head places and bonds a first workpiece (e.g., a die, an interposer, etc.) to a second workpiece (e.g., a substrate, a chip, a wafer, etc.) by melting and re-solidifying of solder bumps on the workpiece (e.g., die) being placed. Typically, the bonding tool is in contact with the workpiece being placed during the entire heating cycle (and possibly the cooling cycle), making the temperature ramp up and ramp down using a serial process affecting machine throughput (e.g., units per hour, UPH). Therefore, it is typically desirable to achieve the temperature ramp up and ramp down in the shortest possible time. A challenge is to quickly switch from a heating phase (where a minimal heat loss out of the bonding tool is typically desired) to a cooling phase (where a maximum heat loss out of the bonding tool is typically desired). In accordance with the present invention, bond head structures/assemblies (e.g., for solder die attach) for rapidly switching from a relatively low cooling rate to a much higher cooling rate are provided. For example, a cooling heat sink (e.g., a cooling chamber) is mechanically separated from the heated tool holder during heating, and then brought into contact during cooling. This combines a very low heat transfer into the heat sink during heating, with a very high heat loss into the heat sink during cooling, effectively increasing the speed at which the bonding process can be performed. Since the bonding times are typically several seconds, any time savings is a direct benefit to total machine UPH. 
     In other exemplary embodiments of the present invention, a cooling heat sink (e.g., a cooling chamber which may be a low thermal mass, high thermal conductive structure) is attached directly to the non-process side of the heater (a side away from the tool which holds the workpiece to be placed). This structure may be filled with different fluids depending upon the present portion of the process. For example, when cooling is desired a cooling fluid will be circulated though the cooling heat sink to remove heat from the heater. During the temperature ramp up segment of the process the cooling heat sink may be purged (e.g., using air) to minimize the effective thermal mass of the heat sink which must be heated during the temperature ramp up segment of the process. Such an exemplary configuration combines a very low heat transfer out of the heat sink during heating with a very high heat loss from the heat sink during cooling, effectively increasing the speed at which the bonding process can be performed. 
     In other exemplary embodiments of the present invention, a bond head structure is provided that allows for the bonding tool to rapidly heat and cool (where the temperature shift may be hundreds of degrees Celsius) while maintaining a desirable degree of precision positioning of the workpiece to be placed (e.g., die placement to single digit micron or smaller levels). For example, a heater, tool (that holds the workpiece), and the workpiece itself (e.g., a die) are supported on a flexure system whose primary compliance is designed to be radially symmetric from the center of the workpiece. For example, floating screws may pierce portions of the heater and facilitate movement of the heater and flexures during heating, and cooling. Such a configuration may be used to constrain motion of the heater, tool and workpiece to be radially symmetric about the center of the workpiece. This provides compliance to allow for differential expansion to occur in a desirable and predictable fashion. 
     In thermocompression bonding, it is often desirable to heat the tool of the placer while holding the workpiece to be bonded (e.g., during the bonding process—which may be considered a non-cooling phase)—but it is also desirable to have the tool of the placer at a lower temperature during other operational phases (e.g., a cooling phase) of the process such as during (1) the picking (or transfer) of another workpiece to the tool from a workpiece supply (e.g., a wafer), and/or (2) during initial placing of the workpiece before the high temperature bonding. In accordance with various exemplary embodiments of the present invention, chambers are provided as part of the bond head (also referred to as a bond head assembly), where a cooling fluid may be provided in the chamber. In certain embodiments, two different fluids may be used in such a chamber (e.g., a liquid cooling fluid during the cooling phase, and a fluid such as air during a non-cooling phase). In other embodiments, the chamber (which may use one or more fluids, as desired) may be moved in and out of contact with the heater, or a force between the chamber and heater may be varied. 
       FIG. 1  is a block diagram side view of elements of thermocompression bonder  100 . Die  102  is to be bonded to substrate  104  using placer  106  (including a bond tool) as illustrated in  FIG. 1 . Placer  106  holds die  102 , and substrate  104  is supported by bond stage  110 . Camera  114  is shown interposed between placer  106  and bond stage  110  (but could be in other locations), and has split field vision, that is it images both upwardly, towards die  102 , and downwardly, towards substrate  104 . Of course, other positions and configurations of a camera may be used. Using appropriate mechanisms and visual information provided by camera  114 , placer  106  (and/or bond stage  110 ) is repositioned as necessary to align bonding structures on die  102  with corresponding bonding structures on substrate  104  (see below). Camera  114  is moved so that placer  106  with aligned die  102  may be lowered over substrate  104 , and die  102  is bonded to substrate  104 . The bonded die  102 /substrate  104  structure is indexed, or moved, to output indexer  112  with intermediate elements, not shown but represented by double headed arrow  118 . Then, another substrate  104  is taken from input indexer  108 , with intermediate elements, not shown but represented by double headed arrow  116 , and placed on bond stage  110 , and placer  106  takes another die  102  from, for example, a semiconductor wafer or another structure having indexed die  102 . Another die  102  is aligned with another substrate  104 , using information from repositioned and interposed camera  114 , and the process repeats. Of course, many different or additional elements may be included in thermocompression bonder  100 , and as such it is understood that the present invention is not limited to integration with the illustrated exemplary configuration of  FIG. 1 . 
     The present invention relates to bonding of a first workpiece to another workpiece. The term “substrate” is interchangeably used with the term “workpiece”. The workpieces/substrates described herein may be, for example, semiconductor dice, wafers, leadframe devices, interposers (e.g., silicon, glass, etc.), amongst others. In one specific example illustrated herein, the workpiece being bonded by the bond head is a semiconductor die and the substrate to which the semiconductor die is being bonded is a substrate. The conductive regions (also referred to as bonding structures) on each workpiece may be, for example, conductive pillars (e.g., Cu pillars), metalized pads, amongst others. 
       FIGS. 2A-2D  illustrate details of bonding structures on lower substrate  204 , bonding structures on upper substrate  202  (e.g., a die  202 ), and the method of bonding die  202  to substrate  204 . Specifically, as illustrated in  FIG. 2A  substrate  204  has bonding structures  228  that may be, for example, copper (Cu) structures  228 , such as copper pillars or metallized pads, grown on substrate  204  that collectively form a target conductive region. As shown, optional plating layer  220  may cover Cu structures  228  to facilitate bonding. Plating layer  220  may comprise, for example, solder (e.g., tin (Sn)-based solder) or other materials to facilitate bonding.  FIG. 2B  illustrates Cu structures  228  and substrate  204  covered with a non-conductive paste (NCP) layer  222 . For example, NCP layer  222  may be an encapsulant material marketed by Henkel AG &amp; Co. A non-conductive film (NCF) (not shown) may be provided over upper substrate  202 , that is, over pillars  224  and overlying layer  226 . As is known to those skilled in the art, pillars  224  tend to push through such an NCF (not shown) during bonding of pillars  224  to corresponding structures  228 . 
     NCP layer  222  may be an adhesive type encapsulant material that may be substantially planar as illustrated. As illustrated in  FIG. 2C , die/substrate  202  includes a series of pillars  224  (only one is shown) comprised of, for example, copper (Cu), aluminum (Al), gold (Au), etc. Cu pillars  224  may include an overlying layer  226  of, for example, solder, such as tin-based solder, to facilitate bonding between die Cu pillars  224  with lower substrate Cu structures  228 . While  FIG. 2C  only illustrates one Cu pillar  224  aligned with a corresponding Cu structure  228 , die  202  may comprise many Cu pillars  224  that are each aligned with a lower Cu structure  228 . It is noted that Cu structures  228  on lower substrate  204  may be elongated structures, which is they may extend along a Y-axis that goes in and out of the paper of  FIG. 2C . Regardless, die  202  is lowered such that solder tipped Cu pillar(s)  224  contact, and press into NCP layer  222 . When solder tipped Cu pillar(s)  224  engage corresponding plated Cu structures  228 , pressure and heat is applied such that Cu pillar(s)  224  are bonded to Cu structure(s)  228  as illustrated in  FIG. 2D . In effect Cu pillar solder layer  226  merges/combines with substrate Cu structure plating layer  220  to form interface  229  between Cu pillar  224  and Cu structure  228 . NCP layer  222 , and any NCF film on the upper substrate  202 , may be comprised of a thermal setting material that cures with temperature, so they are kept below a critical temperature, for example below 180° C., as Cu pillars  224  push through NCP layer  222 . NCP and NCF may serve to assist in binding, or fixing, bonded die  202  and substrate  204 . Die  202  with Cu pillars  224  are heated before/during bonding as will be illustrated in the drawings and described below. 
       FIG. 3A  illustrates bond head  306   a  (which is included in a placer system, similar to the placer  106  shown in  FIG. 1 , except that bond head  306   a  includes aspects of the present invention described below). Bond head  306   a  includes Z-motor  332 , theta Z-drive  334 , tilt head control mechanism  336 , and lower bond head  330 . Certain aspects of bond head  306   a  (including Z-motor  332 , theta Z-drive  334 , and tilt head control mechanism  336 ) may be found on the iStack PS ™ die bonder sold by Kulicke &amp; Soffa Pte Ltd. Tilt head control mechanism  336  may permit tilting of lower bond head  330  in either, or both of, X- and Y-axes (see XYZ legend  338  with the Z-axis up/down, the X-axis to the right/left and the Y-axis coming into and out of the paper of  FIG. 3A ). 
       FIG. 3B  illustrates a more detailed view of lower bond head  330  of  FIG. 3A , and includes interface tilt lower bond head  340 , support structure (e.g., distribution and insulation block)  342 , chamber  344 , heater  348 , tool  350  and die  302  held by tool  350 . Interface tilt lower bond head  340  may include a load cell that measures Z-axis force (downward, bonding force), and optionally may provide for direct Z-force measurement and feedback during bonding of the force applied to die  302 . Support structure  342  may represent and include, for example, electrical connections, air connections, water connections, and insulation to isolate lower heater  348 . As will be discussed below in greater detail, heater  348  heats tool  350  which heats die  302  before and/or during bonding (e.g., see  FIG. 2C-2D ). Chamber  344  regulates the temperature, or amount of heat, of heater  348  before, during, and/or after bonding (e.g., assists in ensuring NCP layer  222  shown in  FIGS. 2B-2D  does not cure prematurely from too high a temperature of die  302 /tool  350 ) and serves to cool heater  348  (and thus tool  350  and die  302 ) at predetermined points in the bonding cycle. A cooling fluid is brought through chamber  344  using piping or tubing  346   a ,  346   b , with pipe  346   a  being the inlet pipe and pipe  346   b  the outlet pipe. Cooling fluid may be brought into chamber  344  during a cooling phase (e.g., after bonding of a workpiece) for cooling of heater  348  from one temperature to a lower predetermined temperature (e.g., a predetermined temperature). The cooling fluid absorbs the heat from heater  348 , cooling heater  348  and raising the temperature of the cooling fluid and the higher temperature cooling fluid exits chamber  344  through outlet  346   b .  FIG. 3C  illustrates another embodiment where chamber, heater and tool are integrated into a single structure, chamber/heater/tool structure  352  where the functions are substantially the same. In another example, the heater and tool are integrated into a single structure, with a distinct chamber (where the chamber may take any of the forms disclosed or claimed herein). In yet another example, the chamber and heater are integrated into a single structure, with a distinct tool. 
       FIG. 3D  illustrates chamber  344  filled with a gas, such as air (“fluid  2 ”), and  FIG. 3E  illustrates chamber  344  filled with another fluid (e.g., a liquid cooling fluid) (“fluid  1 ”), having a thermal capacity greater than fluid  2  as fluid  1  is the cooling fluid to cool heater  348 . In either respect, fluid  1  is capable of removing heat from heater  348  by conduction (direct contact between the cooling fluid chamber/chamber  344  and heater  348  (or integrated heater of  FIG. 3C )), convection (through any air or gas between the cooling fluid chamber/chamber  344  and heater  348  without direct contact), or heat transfer mechanisms. Fluid  2 , such as a gas (e.g., air) may have a lower thermal capacity than fluid  1  (such as a liquid having a greater thermal capacity). That is, a cooling liquid fluid would typically require greater energy to raise the temperature of a set volume of cooling liquid fluid than for the equivalent volume of cooling gas fluid, for example. It is contemplated that a fluid  2  gas could be sufficiently cooled, super cooled, etc., to increase or vary its thermal capacity. 
     Examples of fluid  2 , in gas form, include: air filtered to 0.01 μm; air directly from factory supply lines; etc. Examples of fluid  1 , in liquid form, include: water; distilled water; distilled water with a corrosive inhibitor added; ethylene glycol (i.e., automotive antifreeze); non-conductive fluids such fluorinated liquids; etc. Examples of the corrosive inhibitors (that may be added to, e.g., distilled water) are Zalman™ G200 BLUE™ (available from Acoustic PC (www.acoustic.com)); Red Line Water Wetter® available from Redline Synthetic Oil Corporation of Benicia, Calif.; and Valvoline® Zerex® coolant available from Ashland, Inc. of Covington, Ky. Examples of fluorinated liquids are: 3M® Fluorinert® electronic liquids marketed by the 3M Company of St. Paul, Minn.; and Galden® PFPE high performance, inert fluorinated liquids marketed by Solvay Plastics (www.solvayplastics.com). 
       FIG. 4  is a block diagram of recirculation of a cooling liquid fluid (e.g., fluid  1 ) through chamber  444  which may by used in connection with the structure of  FIGS. 3A-3E , where chamber  444  replaces chamber  344 / 352  (or other exemplary embodiments of the present invention illustrated and/or described herein). As illustrated a cooling liquid fluid may be supplied by fluid tank  460  to chamber  444  when valve  474  is open. The cooling liquid enters chamber  444 , travels through chamber  444  (where it may, or may not, cool a heater or the like (not shown)) and exits. The (warmed/heated) cooling liquid passes through check valve  462  (explained in greater detail below) and into cooling liquid reservoir  464 . Fluid pump  466  pumps the fluid  1  through the system and pumps the heated cooling liquid from cooling liquid reservoir  464  to radiator  468 . Radiator  468  permits the heat/excess heat from the heated cooling liquid fluid to be removed by, for example, radiator action, to bring the temperature of the cooling liquid to an acceptable level. The cooled liquid fluid then returns to liquid tank  460  through pipe  472 , as at arrow  478  and may recirculate though the system. Fluid  2  (e.g., air) is provided from tank  470 , and may be used to replace, remove, or blow out, cooling liquid fluid from chamber  444 . During this process, liquid valve  474  is closed and air/gas valve  476  is opened. The air is pumped into chamber  444  and displaces the cooling liquid, with the cooling fluid exiting chamber  444  through check valve  462 . When the air displaces a sufficient amount of the cooling liquid in chamber  444 , the air exits chamber  444  and into check valve  462 . An air exhaust valve (e.g., a bleeder valve) may be provided somewhere in the system (e.g., somewhere between or proximate elements  462 ,  464 ,  466 , and  468 ) to remove, or bleed off, any air left in the system. It is noted that the elements and configuration shown in  FIG. 4  is exemplary in nature, and may be replaced by different or additional elements (e.g., a different cooling structure may be used as opposed to radiator  468 ). 
       FIGS. 5A-5B  illustrate bond head  530  in accordance with an alternate exemplary embodiment of the present invention. Specifically, cavity  580  is defined by support structure  542  and is sized and positioned to accept chamber  544  therein with sufficient room to permit chamber  544  to move upwardly and downwardly within cavity  580 . Z-actuator  582  moves chamber  544  along the Z-axis, so as to bring chamber  544  into, and out of, contact with heater  548  to facilitate cooling of heater  548 . As illustrated, interface tilt lower bond head  540  is over support structure  542 . Support structure  542  includes cavity  580  within which chamber  544  (and inlet and outlet pipes  546   a ,  546   b ) may be moved upwardly, away from underlying heater  548 , and downwardly to contact heater  548 . Inlet pipe  546   a  brings cooling fluid to chamber  544  and outlet pipe  546   b  allows removal of (heated) cooling fluid from chamber  544 . Tool  500  is below heater  548  and holds die  502 . In operation, (e.g., see as  FIG. 5B ) when heater  548  is to be cooled, chamber  544 , with cooling fluid that may be recirculating through chamber  544  (e.g., see  FIG. 4 ), is lowered using Z-actuator  582  to contact heater  548  to absorb heat from heater  548  through, for example, conduction. Cooled recirculating cooling fluid enters chamber  544  through inlet pipe  546   a , absorbs heat from heater  548 , and heated cooling fluid exits chamber  544  through outlet pipe  546   b  to be cooled and recirculated (e.g., see  FIG. 4 ). 
       FIGS. 5A-5B  illustrate chamber  544  being moved in and out of contact with heater  548 . In an alternative embodiment, where the elements (e.g., chamber  544  and heater  548 ) remain in contact, a contact force between chamber  544  and heater  548  may be varied in order to change the heat transfer therebetween. 
       FIGS. 6A-6C  illustrate bond heads  630 ,  630   a ,  630   b  in accordance with exemplary embodiments of the present invention with flexures. In  FIG. 6A , interface structure  640  is over support structure  642 . Linkage  690  and flexures  692   a ,  692   b  are interposed between support structure  642  and heater  648 . Heater  648  retains tool  600  which in turn carries die  602 . During thermocompression bonding, the device holding die  602 , for example, tool  600 , must be rapidly heated from about 130-150° C. to about 250-300° C. in a minimum amount of time to enable acceptable throughput, that is, a rate of bonding a plurality of die  602  to a target conductive region (e.g., see  FIGS. 2A-2D ). During bonding, the bonder is required to hold die  602  in position within, for example, 5 μm in any direction. However, the thermal coefficient of expansion of the material comprising heater  648  should be taken into account. For example, aluminum nitride (with which heater  648  may be made from) has a coefficient of expansion of about 4.3 μm/m. For a typical die size of 10 mm by 10 mm, an aluminum nitride heater may expand upwards of 7 μm for a 170° C. temperature rise (from 130° C. to 300° C.) which may cause the die shift outside of an allowed tolerance band, and produce faulty product. To mitigate this phenomenon, flexures  692   a ,  692   b  are interposed between heater  648  and support structure  642  of the die bonder. While flexures are illustrated as spring members that expand along the z-axis, this is a simplified illustration. 
     Specifically, flexures  692   a ,  692   b  are anisotropically flexible, that is flexures  692   a ,  692   b  are most compliant along a line (and possibly along a vector) that is substantially parallel to the holding surface of the tool, and in a direction from a center of the holding surface of the tool outwards toward the flexures. Stated differently, flexures  692   a ,  692   b  are compliant along the X-axis and Y-axis (e.g., see legend in  FIG. 6A ) in a direction from a center of the holding surface of the tool outwards toward the flexures. 
     Flexures  692   a ,  692   b  may desirably be positioned in a radial pattern between support structure  642  and heater  648 , where any number of flexures may be used (only two flexures are illustrated in  FIGS. 6A-6C  for simplicity). It is desirable that the flexures be configured such that the line/vector of least stiffness points in a direction from the center of the tool to the respective flexure. Each flexure may desirably be designed such that it has the same contribution in resisting the thermal growth of heater  648  as each other flexure. While undergoing thermal growth this will result in a virtual point at the center of the tool to undergo no in-plane motion. 
     In practice, flexures  692   a ,  692   b  may pierce heater  648  using floating screws or the like (not shown). This permits heater  648  to expand laterally (i.e., radially about the virtual center point of the heater), with flexures  692   a ,  692   b  guiding the expansion during a heating cycle while keeping a point in the virtual center of the heater from any X-Y in-plane motion. During a cooling cycle, flexures  692   a ,  692   b  guide, or encourage, heater  648 /tool  600  to return to it/their initial position, centered about, for example, the tool, during the contraction of the heater. This novel construction defines the growth point of the tool, and ensures that the growth point is predictable. 
       FIG. 6A  illustrates an exemplary embodiment that includes linkage  690  also interposed between heater  648  and support structure  642 . Linkage  690  may include, for example, some form of cooling to cool heater  648  as discussed above, and may also include electrical connections, air connections, water connections, cooling fluid connections and insulation to isolate heater  648  (e.g., see support structure  642 ) from other portions of the bond head. Regardless, linkage  690  may serve to cool heater during predetermined portions of a bonding cycle. Flexures  692   a ,  692   b , also interposed between support structure  642  and heater  648  oppose lateral expansion (thermal growth) of heater  648  during rapid heating of heater  648  during bonding, and maintains die  602  within a predetermine tolerance position, and return flexures  692   a ,  692   b  guide die  602  back to its centered position (see above). 
       FIG. 6B  illustrates another exemplary embodiment similar to that of  FIG. 6A  but with external inlet and outlet pipes/lines  646   a ,  646   b  supplying cooled cooling fluid into linkage  690  and removing heated cooling fluid from linkage  690 . Linkage  690  may also include electrical connections, air connections, and water connections. Flexures  692   a ,  692   b  are similar to those illustrated in  FIG. 6A  as to location and function. 
       FIG. 6C  illustrates a further exemplary flexure embodiment similar to that of  FIG. 6B  but with cooling chamber  694  positioned within cavity  680  formed in support structure  642  (e.g., see  FIGS. 5A-5B ). Linkage  690  and flexures  692   a ,  692   b  are interposed between support structure  642  and heater  648 . External inlet and outlet pipes/tubing  646   a ,  646   b  serve to introduce cooled cooling fluid into cooling chamber  694  and remove heated cooling fluid from cooling chamber  694 , respectively. Cooling chamber  694  is configured to move within cavity  680  along at least the Z-axis using Z-actuator  682 . For example, after bonding die  602  to an underlying substrate, cooling chamber may be moved downwardly and brought into direct contact with linkage  690 , the (cooled) cooling fluid removes heat from linkage  690 , to cool, or lower the temperature of, linkage  690 . In turn, heater  648  is cooled by contact with cooling/cooled linkage  690 , and then tool  600  is cooled by contact with cooling/cooled heater  648 . When the cooling process is complete, cooling chamber is moved upwardly using Z-actuator  692  and cooling fluid may still circulate through cooling chamber  694 . 
     For example, in these exemplary flexure embodiments, flexures  692   a ,  692   b  may be placed in an equidistant radial pattern about linkage  690  and between support structure  642  and heater  648 . Such a spacing may ensure that a center point of heater  648  remains fixed relative to linkage  690 , with no in-plane motion so that die  602  has minimal, or no, movement due to the thermal expansion of heater  648  (e.g., see the above discussion for  FIGS. 6A-6C ). While two flexures  692   a ,  692   b  are illustrated in  FIGS. 6A-6C , any number may be used, for example, 3, 4, 5, 6, etc., preferably in a radial pattern. It is also contemplated that the flexures may take the form of a unitary anisotropically flexible annular flexure structure (not shown), or portions of an otherwise anisotropically flexible annular flexure structure. Such an annular flexure structure may also include recirculating cooling fluid to maintain its temperature/reduce its temperature and/or reduce the temperature of heater  648 . Flexures may maintain their positions on the heater by friction. Further, the flexures are anisotropically flexible in that they are flexible substantially along a line/vector to a holding surface of a bonding tool (or perpendicular to a longitudinal axis along the length of the bonding tool) used to bond the die (workpiece) to the substrate. 
     In accordance with certain exemplary embodiments of the present invention described above, a cooling chamber is described for contacting a heater and removing heat therefrom (e.g., see  FIGS. 3A-3E ,  FIGS. 5A-5B , etc.). According to a further exemplary embodiment of the present invention, the tool (e.g., tool  350  of  FIG. 3B ) may be moved to an intermediate cooling stage after bonding a workpiece (e.g., a die) and before picking another such workpiece to be bonded. In such an arrangement (see  FIG. 7 ), the tool may be brought into contact (or at least proximate) a cooling stage to remove heat from the tool. This cooling arrangement may be provided as a replacement for, or in addition to, the various cooling configurations described herein (e.g., including a chamber with a cooling fluid). 
       FIG. 7  illustrates placer  706  (including a tool, not shown) for holding and bonding a workpiece  702  (e.g., die  702 ) to an underlying substrate  704  supported by bond stage  710 . In  FIG. 7 , die  702  has already been bonded to substrate  704 . Placer  706  is then moved from bond stage  710  to cooling stage  754  that is included as part of (or provided proximate to) the thermocompression bonder. Placer  706  is brought into contact with (or at least proximate) cooling stage  754  where heat from placer  706  is absorbed or otherwise removed. For example, a tool (not shown) of placer  706  may be brought into contact with a cooling pad of cooling stage  754 , thereby reducing the temperature of placer  706  (i.e., reducing the temperature of the relevant portion of placer  706  such as the tool and heater, not shown). Cooled placer  706  is then removed from cooling stage  754  and moved to workpiece supply  756  (e.g., a wafer) to obtain another workpiece (e.g., to pick another die from a wafer). 
     As provided above, an intermediate cooling stage (such as stage  754  shown in  FIG. 7 ) may be provided as an alternative to, or in addition to, cooling from a chamber as described in the various embodiments above.  FIG. 7  also illustrates two temperature versus time curves. Curve  1  relates to use of an intermediate cooling stage without an integrated cooling chamber, while curve  2  relates to an arrangement including an integrated chamber cooling (e.g., as in  FIGS. 3A-3E ) plus use of an intermediate cooling stage. Referring to curve  1 , after leaving bond stage  710  the temperature curve is substantially flat (see area A 1 ) as there is no intentional cooling. When placer  706  reaches cooling stage  754  the temperature drops rapidly as shown at area B 1  until the temperature normalizes at area C 1 . Referring to curve  2 , after leaving bond stage  710  the temperature curve is decreasing continuously due to the chamber cooling (see area A 2 ). When placer  706  reaches cooling stage  754  the temperature drops rapidly as shown at area B 2  until the temperature normalizes at area C 2 . The normalized temperature (at areas C 1  and C 2 ) may be below the critical temperature of NCP and/or NCF layers (e.g., see  FIG. 2C ). 
     While certain exemplary devices are illustrated and described herein, it is contemplated that other cooling chambers may have differing structures, means, and methods of cooling. For example, a cooling chamber may be provided sufficiently large to accept at least a heater, with the cooling chamber acting as a deep freezer to rapidly cool down a post-bond heater with or without direct contact. 
     Other alternatives that may be used to vary the heat transfer between the heater and any of the chambers described herein are contemplated. For example, a flow rate of a cooling fluid (e.g., such as the liquid cooling fluids described herein) may be varied depending upon the desired heat transfer and/or depending upon the portion of the bonding process (or other operational phases during non-bonding). 
     Although the invention is illustrated and described herein with reference to specific embodiments, the invention is not intended to be limited to the details shown. Rather, various modifications may be made in the details within the scope and range of equivalents of the claims and without departing from the invention.