Patent Publication Number: US-2015060527-A1

Title: Non-uniform heater for reduced temperature gradient during thermal compression bonding

Description:
TECHNICAL FIELD 
     Embodiments described herein generally relate to thermal compression bonding. Some embodiments relate to a non-uniform heater used during thermal compression bonding of integrated circuit dies. 
     BACKGROUND 
     Integrated circuit dies may be attached to substrates or circuit boards using a process commonly referred to in the art as thermal compression bonding. Solder balls may be attached to various points of the die that are desired to be anchored to the substrate. The die may then be heated to melt the solder balls and the die and substrate are compressed such that, when the solder balls cool, the die may be attached to the substrate. 
     A heater may be used during a fabrication process to heat the die, substrate, and solder balls in order to perform the bonding. One problem that may occur with present heaters is that the edges and/or corners of the die/substrate combination are more exposed to ambient air temperatures than the remainder of the die/substrate combination creating a relatively large temperature gradient across the die/substrate combination. Thus, some areas of the die/substrate combination may be cooler than other areas. The cooler areas may not be hot enough to melt the solder balls. 
     In order to compensate for this large temperature gradient across the die/substrate combination, the overall temperature of the heater may be increased such that the edges and/or corners of the die/substrate combination are at a temperature that is adequate for properly melting the solder balls. However, this also increases the temperature of the inner portions of the die/substrate combination such that the inner portion is now hotter than is typically used to accomplish the task of melting the solder balls. This may result in yield loss from solder bridging on inner portions. 
     Since the solder balls attached to the inner portion of the die may be heated to a much greater temperature than its melting temperature, they may have a longer cool down time as well. The relatively large temperature gradient across the die/substrate combination may thus lead to longer fabrication times as well as negatively impact the solder ball joint quality. 
     There are general needs for reducing the temperature gradient across a die/substrate combination during a thermal compression bonding process. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a diagram of an embodiment of a non-uniform heater concept. 
         FIG. 2  illustrates a diagram of an embodiment of a non-uniform heater with heating elements. 
         FIGS. 3A-3C  illustrate cross-sectional views of an embodiment of a thermal compression bonding process in accordance with the embodiments of  FIGS. 1 and 2 . 
     
    
    
     DETAILED DESCRIPTION 
     The following description and the drawings sufficiently illustrate specific embodiments to enable those skilled in the art to practice them. Other embodiments may incorporate structural, logical, electrical, process, and other changes. Portions and features of some embodiments may be included in, or substituted for, those of other embodiments. Embodiments set forth in the claims encompass all available equivalents of those claims. 
     A relatively large temperature gradient across an integrated circuit die and substrate during a thermal compression bonding process may result in various problems. For example, a higher temperature in certain areas of the die/substrate combination may result in reduced solder joint quality as well as a slower bonding process since the hotter solder balls take longer to cool and solidify. 
     A heater that generates a non-uniform temperature pattern may be used to reduce the relatively large temperature gradient across the die/substrate combination. Since edges and/or corners of the die/substrate combination may be cooler due to cooling by ambient air temperatures, a heater with a non-uniform heat pattern may heat these areas of the die/substrate combination differently such that an inner portion of the die/substrate combination is heated by a lower temperature than an outer portion of the die/substrate combination. This may be accomplished by different densities of heating element segments on the heater or different resistances for the heating element segments. 
       FIG. 1  illustrates a diagram of an embodiment of a non-uniform heater concept. This diagram illustrates how a heater may be configured to generate a non-uniform temperature output in order to reduce a relatively large temperature gradient resulting from the heating of a die/substrate combination during a thermal compression bonding process. The diagram of  FIG. 1  is for purposes of illustration only as different thermal compression bonding process embodiments may use different configurations for a non-uniform heater. 
     The heater  100  is shown with an outline of a typical die  101  located over the heater  100 . As described previously, the edges and corners of the die  101  and/or substrate (not shown) may be cooler during the thermal compression bonding process due to the proximity of these areas to the cooler ambient air. Thus, certain areas  110 - 115  of the heater  100  associated with the edges and corners of the die  101  may be configured to generate a higher temperature than the remainder  120  of the heater  100 . 
     The non-uniform heater concept is shown in  FIG. 1  having multiple areas  110 - 115 ,  120  with different temperatures. For example, the corners of a die  101  may experience the most ambient cooling due to both sides of the die  101  being exposed to the cooler ambient air. Thus, the areas  110 - 113  of the heater  100  associated with the corners of the die  101  may generate the highest temperatures of the heater  100 . 
     Each edge of the die  101  may experience less cooling than the corners but may still experience greater cooling than the remainder of the die. Thus, certain areas  114 ,  115  of the heater  100  associated with the edges of the die  101  may generate temperatures that are less than the corner areas  110 - 113  but still higher than the remainder  120  of the heater  100 . 
       FIG. 1  shows only the areas  114 ,  115  of the heater  100  associated with the vertical edges of the die as generating a relatively higher temperature. Another embodiment may also generate relatively higher temperatures along areas of the heater  100  associated with the horizontal edges of the die along with the vertical edges. Another embodiment may generate relatively higher temperatures only along areas of the heater associated with the horizontal edges of the die. 
     As shown in  FIG. 1 , the size of the heater  100  may also be relatively larger than the size of the die  101 . This may extend the heating of the die  101  beyond the edges of the die  101  and, thus, reduce the cooling caused by interaction of the corners and edges of the die  101  with the ambient air. 
       FIG. 2  illustrates a diagram of an embodiment of a non-uniform heater  200  with a heating element  201 . The diagram of  FIG. 2  illustrates different methods for generating a non-uniform temperature pattern across the surface of the thermal compression bonding heater. 
     One illustrated method for generating a non-uniform temperature pattern may use different distances between multiple segments of the heating element  201 . For example, the segments of the heating element  201  in end portions  204 ,  205  near the vertical edges of the heater  200  are closer together than the heating element  201  segments in the central portion  206  of the heater  200 . Placing the heating element  201  segments closer together may generate a higher temperature in those portions  204 ,  205  of the heater  200  than the temperature generated in the central portion  206  where the segments are further apart. 
     Another illustrated method for generating a non-uniform temperature pattern may use different cross sectional areas of the heating element  201 . As illustrated in  FIG. 2 , the heating element  201  segments in the end portion  205  near the right vertical edge of the heater have a smaller cross sectional area and/or depth than heating element  201  segments in inner portions  204 ,  206  of the heater  200 . The smaller cross sectional area may increase the resistance of the segments in that particular portion  205 . An increased resistance may result in an increased heat generated and, thus, an increased temperature for those particular traces. 
     Referring to  FIG. 2 , the central portion  206  and left end portion  204  of the heater  200  of  FIG. 2  may generate less heat than the right end portion  205  of the heater  200 . Since the right end portion  205  comprises heating element  201  segments having both a smaller cross sectional area  211  as well as being placed closer together, the right end portion  205  may generate the highest temperature of the heater  200 . The left end portion  204  may generate the next highest temperature as a result of the heating element  201  segments being placed closer together than the central portion  206  but having the same cross sectional area  210  as the segments of the central portion. The central portion  206  may generate the lowest temperature of the heater  200  as a result of the segments having a larger cross sectional area and also being placed further apart than the other portions  204 ,  205  of the heater  200  (i.e., having a larger element pitch). Pitch may be defined as a distance between the various segments of the heating element  201 . 
     Another method for generating a non-uniform temperature pattern may use different chemical compositions for different segments of the heating element  201 . Different chemical compositions for different segments of the heating element  201  may change the resistance of the heating element  201  in those areas of the heater  200 . A predetermined temperature, in a particular portion  204 - 206  of the heater  200 , may thus be generated by a predetermined resistance in that particular portion  204 - 206 . 
     For example, a heating element  201  may typically include a pure metal, a metal alloy, or a paste-like material. For example, the heating element  201  may include gold (Au), copper (Cu), or tungsten/alumina (W/ALN). Introducing different compositions into different segments of the heating element may change the resistance in those segments. 
     Thus, referring to  FIG. 2 , if it is desired to increase the temperature generated in the left end portion  204  of the heater  200 , metal alloy of higher resistivity may be introduced into the segments of that portion  204  of the heater  200  while the remaining heating element  201  segments may consist of metal alloy of lower resistivity. 
     Additional examples for increasing the temperature of the heater  200  in a non-uniform manner may include a constant pitch and constant heating element  201  cross-sectional area in portions  204 - 206  having different chemical compositions; a constant chemical composition, varied pitch, and constant heating element  201  cross sectional area; and a constant chemical composition, constant pitch, and varied heating element  201  cross sectional area. 
     The embodiments of  FIG. 2  for increasing the temperature of the heater  200  in a non-uniform manner are for purposes of illustration only. Other embodiments may use various combinations of these methods. Still other embodiments may use different methods for generating a non-uniform temperature pattern. 
     For example, the illustrated embodiment shows only a single, continuous heating element  201  that is formed into multiple segments to generate non-uniform temperatures on the heater. In such an embodiment, power is applied to only the single, continuous heating element  201 . 
     Another embodiment may have multiple, separate heating elements where each heating element is a separate segment to generate the non-uniform temperatures on the heater. In such an embodiment, each separate segment may be powered separately such that increasing the power to one segment to increase its temperatures would not affect the power applied to any of the other segments nor the temperatures generated by those segments. 
     Examples to illustrate the non-uniform heating of the different portions  204 - 206  may include the left end portion  204  having an average output power of 1×10 6  W/m 2 , the center portion  206  having an average output power of 7×10 5  W/m 2 , and the right portion  205  having an average output power of 3×10 6  W/m 2 . These values for average output power are for purposes of illustration only as other embodiments may have different average output powers. 
     As another example of the non-uniform heating of the different portions  204 - 206  may be illustrated by comparisons of each portion to another portion. For example, the left portion  204  may have a first coil density, the right portion  205  may have a second coil density, and the third portion  206  may have a third coil density. It can be seen that the first coil density is less dense than the second coil density such that the two portions  204 ,  205  taken together have a non-uniform coil density. It can also be seen the second coil density is more dense than either of the first or the third coil densities. Thus the two portions  204 ,  206  taken together have a non-uniform coil density. 
       FIGS. 3A-3C  illustrate cross-sectional views of an embodiment of a thermal compression bonding process in accordance with the embodiments of  FIGS. 1 and 2 .  FIG. 3A  illustrates a die  301  to be attached to a substrate  302  by a thermal compression bonding process using a non-uniform heater  300  as discussed previously. The die  301  may have the solder balls  310 - 312  attached and positioned over the substrate  302  using a vacuum force through one or more vacuum ports (not shown) in the heater to temporarily hold the die  301  to the heater  300 . 
     The die  301  may be attached to the substrate  302  by a plurality of solder balls  310 - 312 . The non-uniform heater  300  may be used to generate a non-uniform temperature pattern such that the solder balls  310 - 312  melt and cool at a substantially uniform rate. 
       FIG. 3B  illustrates a mid-point at which the heater  300  has heated the solder balls  310 - 312  to their melting point and a compression force has begun to push the die  301  and the substrate  302  together. 
       FIG. 3C  illustrates the final step during which the solder balls  310 - 312  may now be fully compressed and cooling at a substantially uniform rate. The die  301  may now be attached to the substrate  302 . 
     In the interest of simplicity, the solder used in the present embodiments may be referred to as solder balls. However, the solder is not limited to a spherical shape. The solder may have any of one or more different shapes including spherical. 
     While the above disclosure refers to die-to-substrate bonding, the disclosed heater is not limited to such an embodiment. The non-uniform heater may operate to bond any apparatus to any other apparatus as well as embodiments using a non-uniform temperature pattern to heat an apparatus without bonding. 
     Bonding examples might include die-to-die bonding, die-to-substrate bonding, wafer-to-substrate bonding, substrate-to-substrate bonding, as well as other types of bonding. 
     EXAMPLES 
     The following examples pertain to further embodiments. 
     Example 1 is a method for performing a thermal compression bonding process having a non-uniform temperature pattern. The method comprising positioning a first apparatus, coupled to a plurality of solder balls, over a second apparatus; heating the plurality of solder balls with a heater comprising the non-uniform temperature pattern wherein an outer portion of the first apparatus is heated to a higher temperature than an inner portion of the first apparatus; and compressing the first apparatus towards the second apparatus after the plurality of solder balls have melted. 
     In Example 2, the subject matter of Example 1 can optionally include heating the plurality of solder balls by: heating corners of the first apparatus to a first temperature; heating edges of the first apparatus to a second temperature; and heating a central portion of the first apparatus to a third temperature wherein the first temperature is greater than the second temperature which is greater than the third temperature. 
     In Example 3, the subject matter of Examples 1-2 can optionally include heating the plurality of solder balls with a heater comprising the non-uniform temperature pattern wherein the outer portion of the first apparatus is heated to a higher temperature than the inner portion of the first apparatus comprises heating outside edges of the first apparatus to the higher temperature than the inner portion of the first apparatus. 
     In Example 4, the subject matter of Examples 1-3 can optionally include wherein heating the plurality of solder balls comprises: heating portions of a thermal compression bonding heater associated with corners of the first apparatus to a first temperature; heating portions of the thermal compression bonding heater associated with edges of the first apparatus to a second temperature; and heating portions of the thermal compression bonding heater associated with a central portion of the first apparatus to a third temperature, wherein the first temperature is greater than the second temperature which is greater than the third temperature. 
     In Example 5, the subject matter of Examples 1-4 can optionally include wherein heating the plurality of solder balls comprises generating a plurality of different temperatures across a surface of the thermal compression bonding heater in response to a cross sectional area of heating element segments in each portion of the thermal compression bonding heater. 
     In Example 6, the subject matter of Examples 1-5 can optionally include wherein heating the plurality of solder balls comprises generating a plurality of different temperature across a surface of the thermal compression bonding heater in response to a pitch of heating element segments in each portion of the thermal compression bonding heater. 
     In Example 7, the subject matter of Examples 1-6 can optionally include wherein heating the plurality of solder balls comprises generating a plurality of different temperature across a surface of the thermal compression bonding heater in response to a chemical composition of heating element segments in each portion of the thermal compression bonding heater. 
     In Example 8, the subject matter of Examples 1-7 can optionally include wherein heating the plurality of solder balls comprises generating a plurality of different temperature across a surface of the thermal compression bonding heater in response to one or more of a chemical composition of heating element segments, a pitch of heating element segments, and/or a cross sectional area of heating elements segments in each portion of the thermal compression bonding heater. 
     Example 9 is a heater having a non-uniform temperature pattern. The heater comprising a plurality of heating element segments configured to generate the non-uniform temperature pattern, wherein the configuration comprises one of: a plurality of heating element segment densities or a plurality of heating element segment resistances. 
     In Example 10, the subject matter of Example 9 can optionally include wherein the plurality of heating element segments comprise a single, continuous heating element. 
     In Example 11, the subject matter of Examples 9-10 can optionally include wherein the plurality of heating element segments comprise a plurality of discontinuous heating element segments. 
     In Example 12, the subject matter of Examples 9-11 can optionally include wherein each of the plurality of discontinuous heating element segments is configured to be powered separately from the remaining discontinuous heating element segments. 
     In Example 13, the subject matter of Examples 9-12 can optionally include wherein the heater further comprises a plurality of portions, each of the plurality of portions having a different heating element segment density of the plurality of heating element segment densities in response to a predetermined temperature to be generated by that portion. 
     In Example 14, the subject matter of Examples 9-13 can optionally include wherein the heater further comprises a plurality of portions, each of the plurality of portions comprising a heating element segment having a predetermined heating element segment resistance of the plurality of heating element segment resistances than heating element segments in other portions of the plurality of portions, the predetermined resistance determined in response to a predetermined temperature to be generated by that portion. 
     In Example 15, the subject matter of Examples 9-14 can optionally include wherein the resistance of a heating element segment is determined in response to a composition of the heating element segment. 
     In Example 16, the subject matter of Examples 9-15 can optionally include wherein the resistance of a heating element segment is determined in response to a cross sectional area of the heating element segment. 
     Example 17 is a heater having a non-uniform temperature pattern. The heater comprising a plurality of portions, each portion having a predetermined density of heating element segments wherein each density of heating element segments is determined in response to a temperature to be generated by an associated portion. 
     In Example 18, the subject matter of Example 17 can optionally include wherein the plurality of portions comprise a central portion having a lower density of heating element segments than other portions of the plurality of portions. 
     In Example 19, the subject matter of Examples 17-18 can optionally include wherein the plurality of portions comprise first and second end portions that have a higher density of heating element segments than other portions of the plurality of portions. 
     In Example 20, the subject matter of Examples 17-19 can optionally include wherein the density of heating element segments in each portion is determined in response to a distance of each heating element segment from other heating element segments in each portion. 
     In Example 21, the subject matter of Examples 17-20 can optionally include wherein a higher density of heating element segments within a portion of the plurality of portions is configured to generate a higher temperature by the portion. 
     Example 22 is a heater having a non-uniform temperature pattern. The heater comprising a plurality of portions, each portion having a heating element segment comprising a different predetermined resistance, wherein each predetermined resistance is determined in response to a temperature to be generated by an associated portion. 
     In Example 23, the subject matter of Example 22 can optionally include wherein the predetermined resistance of a first portion of the plurality of portions is determined by a chemical composition of the heating element segment associated with the first portion. 
     In Example 24, the subject matter of Examples 22-23 can optionally include wherein the predetermined resistance of a second portion of the plurality of portions is determined by a cross sectional area of the heating element segment associated with the second portion. 
     In Example 25, the subject matter of Examples 22-24 can optionally include wherein a first portion, having a first heating element segment comprising a higher resistance than a second heating element segment in a second portion, is configured to generate a higher temperature than the second portion. 
     In Example 26, the subject matter of Examples 22-25 can optionally include wherein the heater comprises a size that is larger than a size of an integrated circuit die configured to be heated by the heater. 
     Example 27 is a method for making a thermal compression bonding heater having a non-uniform temperature pattern. The method comprising forming a heating element, comprising a plurality of heating element segments, on a surface of the heater, wherein a temperature of each portion of a plurality of portions of the surface of the heater is set in response to one or more of: a density of heating element segments in an associated portion and/or a resistance of each heating element segment in the associated portion. 
     In Example 28, the subject matter of Example 27 can optionally include changing the resistance of each heating element segment in response to changing a chemical composition of each heating element segment. 
     In Example 29, the subject matter of Examples 27-28 can optionally include changing the resistance of each heating element segment in response to changing a cross sectional area of each heating element segment. 
     Example 30 is a thermal compression bonding heater having a non-uniform temperature pattern, the heater comprising means for positioning a die, coupled to a plurality of solder balls, over a substrate; means for heating the plurality of solder balls with a heater comprising the non-uniform temperature pattern wherein an outer portion of the die is heated to a higher temperature than an inner portion of the die; and means for compressing the die towards the substrate after the plurality of solder balls have melted. 
     In Example 31, the subject matter of Example 30 can optionally include wherein the means for positioning the die comprises a vacuum force. 
     The Abstract is provided to comply with 37 C.F.R. Section 1.72(b) requiring an abstract that will allow the reader to ascertain the nature and gist of the technical disclosure. It is submitted with the understanding that it will not be used to limit or interpret the scope or meaning of the claims. The following claims are hereby incorporated into the detailed description, with each claim standing on its own as a separate embodiment.