Abstract:
A method is provided for bonding a semiconductor chip to a packaging substrate while minimizing the variation in the solder ball heights and controlling the stress in the solder balls and the stress in the packaging substrate. During the solder reflow, the warp of the packaging substrate, including the absolute warp, thermal warp, and substrate to substrate variations of the warp, is constrained at a minimal level by providing a clamping constraint to the packaging substrate. During cool down of the solder balls, the stresses and strains of the solder joints are maintained at levels that do not cause tear of the solder joints or breakage of the packaging substrate by removing the clamping constraint. Thus, the bonding process provides both uniform solder height with minimized solder non-wets and stress minimization of the solder balls and the packaging substrate.

Description:
BACKGROUND 
     The present disclosure relates to a method for bonding substrates, and particularly, to a method of bonding substrates while minimizing stress induced by a mismatch in the coefficients of thermal expansion (CTE&#39;s) between the substrates during a cool down step, and an apparatus for effecting the same. 
     Substrate warp poses a challenge for the attachment of a semiconductor chip to a packaging substrate using flip-chip technology. A large warp in magnitude at reflow is a cause for unreliable C4 bonding, leading to potential non-wets or unequal C4 solder heights. In addition, a large sample to sample variation (sigma) of warp is a problem during bond and assembly even for substrates having a nominal mean warp. The thermal warp, i.e., the change in warp with temperature, is another undesirable problem for bond and assembly. A large thermal warp means that the packaging substrate changes shape during the critical cool down period after the reflow of the solder balls. Such change in the shape of the substrate could lead to defects in the solder ball joints such as hot tears. 
     One solution to the warp problem is the use of a mechanical constraint in the form of a temporary glass carrier (TGC) throughout the bonding process, including the entire duration of the cool down period. In this example, the packaging substrate is attached, with an adhesive, to a thick flat glass plate under pressure such that the inherent absolute warp of the substrate is flattened out. Because of the mechanical constraint of the glass carrier, the absolute and thermal warps are reduced close to zero, and so is the sample to sample variation (sigma) of this warp. However, modeling studies have shown that the stresses and shear strains in the C4 balls and back-end-of-line (BEOL) metallurgy are increased by constraining the free movement in the Z-direction of the packaging substrate during the cool down process. The difference in the coefficients of thermal expansion (CTE&#39;s) between the packaging substrate and the semiconductor chip causes a differential contraction between the two during cooling. This results in the build-up of shear stresses in the C4 balls and the semiconductor chip. 
     The shear stresses in the C4 balls can be naturally minimized by the bending of the bonded assembly including the semiconductor chip and the packaging substrate if the bonded assembly is free to bend. If the free bending of the bonded assembly including the semiconductor chip and the packaging substrate is restricted by use of the TGC, the reduction of stresses and strains cannot be achieved. Limiting the free bending of the bonded assembly can thus lead to an effective increase in failures in the C4 balls and BEOL metallurgy. 
     A factor that can mitigate the increase in stresses and strains, while using a TGC, is the potential to constrain the X-Y expansion of the substrate and thereby reduce the effective CTE difference between the die and substrate. Modeling studies have shown that the CTE of the constrained packaging substrate has to be reduced by nearly 40% for the stresses to be lowered to the stresses for a packaging substrate that is free to bend. Measurements on actual TGC mounted substrates have shown that such a large reduction in CTE is very difficult to achieve, and is highly dependent on the adhesive being used. Soft adhesives allow slippage between the substrate and the glass, and do not achieve a sufficient reduction in CTE. Harder adhesives can reduce the CTE more, but result in either permanently deforming the substrate or breaking the glass. 
     The TGC also has the drawback of requiring an adhesive to attach the packaging substrate to the glass. The adhesive contributes to contamination of a ball grid array (BGA) or land grid array (LGA) side of the substrate. Due to this phenomenon, the TGC technique cannot be used with LGA substrates where contamination is not tolerable. The removal of the packaging substrate from the TGC is also an issue. The technique of shearing a bonded package off the TGC is stressful to the packaging substrate. Through substrate vias in the packaging substrate are particularly vulnerable to damage during this process. 
     BRIEF SUMMARY 
     A method is provided for bonding a semiconductor chip to a packaging substrate while minimizing the variation in the solder ball heights and controlling the stress in the solder balls and the stress in the packaging substrate. During the solder reflow, the warp of the packaging substrate, including the absolute warp, thermal warp, and substrate to substrate variations of the warp, is constrained at a minimal level by providing a clamping constraint to the packaging substrate. During cool down of the solder balls, the stresses and strains of the solder joints are maintained at levels that do not cause tear of the solder joints or breakage of the packaging substrate by removing the clamping constraint. Thus, the bonding process provides both uniform solder height with minimized solder non-wets and stress minimization of the solder balls and the packaging substrate. 
     According to an aspect of the present disclosure, a method of bonding substrates is provided, which includes: clamping a back side surface of a first substrate to a planar surface, wherein the back side surface remains coplanar with the planar surface while the first substrate is clamped; bonding the first substrate to a second substrate through an array of solder balls that contact by reflowing the array of solder balls at a reflow temperature, wherein the first substrate, the second substrate, and the array of solder balls form a bonded assembly; cooling the bonded assembly below the reflow temperature to a release temperature that is greater than room temperature while the back side surface of the first substrate remains clamped; and releasing of the clamping of the back side of the first substrate at the release temperature. 
     According to another aspect of the present disclosure, an apparatus for bonding substrates is provided. The apparatus includes: a heating means for heating an assembly of a first substrate, an array of solder balls, and a second substrate; a supporting means for supporting the first substrate, the supporting means including a planar surface; a clamping means for clamping a back side surface of the first substrate to the planar surface and configured to hold the back side surface coplanar with the planar surface while the first substrate is clamped; and a control means for releasing of the clamping of the back side of the first substrate at a release temperature that is greater than room temperature. 
     According to yet another aspect of the present disclosure, an apparatus for bonding substrates is provided. The apparatus includes: a heating means for heating an assembly of a first substrate, an array of solder balls, and a second substrate; a supporting means for supporting the first substrate, the supporting means including a planar surface; a release means for releasing a back side surface of the first substrate from the planar surface from a clamped state in which the back side surface is held to coincide with the planar surface while the first substrate is clamped; and a control means for activating the release means at a release temperature that is greater than room temperature. 
     According to still another aspect of the present disclosure, a substrate carrier is provided. The substrate carrier is configured to form an encapsulated vacuum enclosure therein by combining with at least one substrate in a geometry wherein either the back side or the front side surface of the at least one substrate is combined with surfaces of a cavity within the substrate carrier to form and seal the encapsulated vacuum enclosure. The substrate carrier can further include a vacuum release mechanism configured to break the sealed encapsulated vacuum enclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIG. 1  is a schematic vertical cross-sectional view of a first exemplary apparatus that can be employed for bonding a first substrate to a second substrate according to a first embodiment of the present disclosure. 
         FIG. 2  is a schematic vertical cross-sectional view of a second exemplary apparatus that can be employed for bonding a first substrate to a second substrate according to a second embodiment of the present disclosure. 
         FIG. 3  is a graph illustrating the mean warp of a packaging substrate under various bonding methods. 
         FIG. 4  is a graph illustrating the stress on solder balls under various bonding methods. 
         FIG. 5  is a bird&#39;s eye view of an exemplary substrate carrier configured to hold substrates for bonding according to an embodiment of the present disclosure. 
         FIG. 6A  is a vertical cross-sectional views of a first exemplary assembly of a bonded substrate mounted to a substrate carrier according to an embodiment of the present disclosure. 
         FIG. 6B  is a vertical cross-sectional views of a second exemplary assembly of a bonded substrate mounted to a substrate carrier according to an embodiment of the present disclosure. 
         FIG. 6C  is a vertical cross-sectional views of a third exemplary assembly of a bonded substrate mounted to a substrate carrier according to an embodiment of the present disclosure. 
         FIG. 7A  is a vertical cross-sectional view of an assembly of a bonded substrate, a vacuum chuck, and a pair of mechanical clamps while a packaging substrate is clamped according to an embodiment of the present disclosure. 
         FIG. 7B  is a vertical cross-sectional view of an assembly of a bonded substrate, a vacuum chuck, and a pair of mechanical clamps while a packaging substrate is unclamped according to an embodiment of the present disclosure. 
         FIG. 8  is a vertical cross-sectional view of an assembly of a bonded substrate, an adhesive layer, a transparent chuck, and an irradiation means configured to remove or deactivate the adhesive layer according to an embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As stated above, the present disclosure relates to a method of bonding substrates while minimizing stress induced by a mismatch in the coefficients of thermal expansion (CTE&#39;s) between the substrates during a cool down step, and an apparatus for effecting the same, which are now described in detail with accompanying figures. It is noted that like and corresponding elements are referred to by like reference numerals. The drawings are not in scale. 
     In an embodiment of the present disclosure, the warp of a packaging substrate is controlled during reflow of solder balls, but is not constrained during a latter period of a cool down process. The combination of the two warp control methods allows planar alignment of solder balls at the time of bonding to minimize non-bonding of solder balls, and allows stresses in various portions of a bonded assembly including the packaging substrate, an array of solder balls, and a semiconductor substrate to be partially released by free bending of the substrate package during the latter period of the cool down process. 
     Referring to  FIG. 1 , a first exemplary apparatus according to a first embodiment of the present disclosure is illustrated, which can be employed for bonding a first substrate  100  to a second substrate  200 . 
     The first substrate  100  can include a first substrate layer  110 , a first surface dielectric layer  120 , and an array of first bonding pads  130  embedded within the first surface dielectric layer  120 . The first substrate  100  can be a packaging substrate or a semiconductor chip or an interposer. If the first substrate  100  is a packaging substrate, the first substrate  100  can include layers of wiring structures (metal interconnect structures), and can be a ceramic substrate, an organic laminated substrate, a silicone substrate, a metal substrate, or a flexible film substrate. If the first substrate  100  is a semiconductor chip, the first substrate  100  includes semiconductor devices therein. 
     The second substrate  200  includes a second substrate layer  210 , a second surface dielectric layer  220 , and an array of second bonding pads  230  embedded within the second surface dielectric layer  220 . Each bonding pad within the array of second bonding pads  230  can be electrically connected to the layers of wiring within the second substrate  200 . The second substrate  200  can be any of a semiconductor chip, an interposer, or a packaging substrate. Each bonding pad within the array of second bonding pads  230  can be electrically connected to at least one semiconductor device within the second substrate layer  210  through metal interconnect structures (not shown) embedded in the second substrate layer  210 . The second substrate layer  210  includes semiconductor devices that can be, for example, field effect transistors, bipolar transistors, diodes, and/or optical semiconductor devices. 
     An array of solder balls  300  is bonded to one of the first substrate  100  and the second substrate  200 . Specifically, the array of solder balls  300  is bonded either to the array of first bonding pads  130  or the array of second bonding pads  230 . The other of the first substrate  100  and the second substrate  200  is aligned to the array of solder balls  300 . Specifically, if the array of solder balls  300  is bonded to the array of first bonding pads  130 , the second substrate  200  is aligned to the array of solder balls  300  such that the array of second bonding pads  230  is aligned to the array of solder balls  300 . If the array of solder balls  300  is bonded to the array of second bonding pads  230 , the first substrate  100  is aligned to the array of solder balls  300  such that the array of first bonding pads  130  is aligned to the array of solder balls  300 . The first substrate  100 , the array of solder balls  300 , and the second substrate  200  form an aligned pre-bonding assembly that include two physically detachable structures, one of which includes a structure of the array of solder balls  300  and one of the first and second substrates ( 100 ,  200 ). The array of solder balls  300  can be an array of C4 balls. 
     The aligned pre-bonding assembly is placed in an enclosure  500 , which can be a furnace including heating means (not shown explicitly). The enclosure  500  includes enclosure walls  510  and a door  520 , which can be configured to form a sealed volume when the door  520  is shut. Alternatively, the enclosure  500  can have be an open-ended system that is configured to continuously process multiple aligned pre-bonding assemblies through a reflow process and a subsequent cool down process. 
     The heating means can be any type of heater element known in the art. The heating means can be embedded in the enclosure walls  510  and/or the door  520 , located within the cavity of the enclosure  500 , or located outside the enclosure walls  510  and the door  520 . The heating means is configured to heat structures loaded within the enclosure  500 . Thus, the heating means is configured to heat the assembly of the first substrate  100 , the array of solder balls  300 , and the second substrate  200 . The enclosure  500  is configured to confine heat at least during a reflow process. 
     A vacuum chuck  530  can hold the first substrate  100  tightly in place during the heat-up cycle, during a solder reflow process, and during an initial period of the cool down cycle. The vacuum chuck  530  embeds a portion of a vacuum manifold  620 , and has a top planar surface that supports the first substrate  100 . Thus, the vacuum chuck  530  functions as a supporting means for supporting the first substrate  100  with a planar surface. Optionally, a pedestal  540  can be provided to mechanically support the vacuum chuck  530  depending on the configuration of the vacuum chuck  530 . A vacuum pump  610  is connected to the vacuum manifold  620 , which extends from the vacuum pump  610  to the top surface of the vacuum chuck  530 . 
     The combination of the vacuum pump  610 , the vacuum manifold  620 , and the vacuum chuck  530  collectively functions as a clamping means for clamping the back side surface of the first substrate  100  to the planar top surface of the vacuum chuck  530 . The vacuum pump  610  and the vacuum manifold  620  are configured to hold the back side surface of the first substrate  100  coplanar with the planar top surface of the vacuum chuck  530  while the first substrate  100  is clamped. 
     The aligned pre-bonding assembly is heated up above the melting temperature of the material of the array of solder balls  300  while the back side surface of the first substrate  100  is held flat against the planar top surface of the vacuum chuck  530 . The clamping is provided by a vacuum suction provided to the back side surface of the first substrate  100 . The vacuum suction is provided by the vacuum manifold  620  that extends from the vacuum pump  610  to the back side surface of the first substrate  100 . Thus, the back side surface of the first substrate  100  is clamped to a planar surface, i.e., the planar top surface of the vacuum chuck. The back side surface of the first substrate  100  remains coplanar with the planar top surface of the vacuum chuck  530  while the first substrate  100  is clamped. 
     The array of solder balls  300  reflows above the melting temperature of the material of the array of solder balls  300 , thereby wetting and bonding to both the first and second substrates ( 100 ,  200 ). The first substrate  100  is bonded to the second substrate  200  through the array of solder balls  300  by reflowing the array of solder balls  300  at the reflow temperature. The array of solder balls  300  contacts the front (top) surface of the first substrate  100 . Upon cooling below the melting temperature of the material of the array of solder balls  300 , a bonded assembly is formed, which includes the first substrate  100 , the second substrate  200 , and the array of solder balls  300  bonded to the first and second substrates ( 100 ,  200 ). 
     The bonded assembly ( 100 ,  200 ,  300 ) is cooled below the reflow temperature to a release temperature while the back side surface of the first substrate  100  remains clamped. The release temperature is greater than room temperature. As used herein, room temperature refers to a temperature range from 10 degrees Celsius and 35 degrees Celsius. The clamping of the back side of the first substrate  100  is released at the release temperature. The supporting means and the clamping means are configured to allow free bending of the assembly upon release of the clamping. 
     The vacuum chuck  530  is configured to release the first substrate  100  at an appropriate time during cool down. For example, the clamping of the back side of the first substrate  100  is released by venting the vacuum enclosure at the release temperature. Upon releasing of the clamping, the bonded assembly ( 100 ,  200 ,  300 ) is allowed to bend freely thereafter. 
     The timing of the release of clamping can be controlled by a control means  80 , which can be, for example, a microprocessor, a microcontroller, or a computer. The control means  80  is configured to release the clamping of the back side of the first substrate  100  at the release temperature that is greater than room temperature. The control means  80  can be pre-programmed to release the clamping after a predefined time period following the reflow process. The predefined time period can be determined based on measured cooling characteristics of the enclosure  500 . Alternately, the control means  80  can be configured to monitor the temperature of the enclosure  500  through sensors (not shown) and to control the operation of the vacuum pump  610  so that the vacuum pump  610  is turned off or the vacuum manifold  620  is vented at the release temperature. 
     The release temperature is selected to be a temperature between the melting temperature of the array of solder balls  300  and the upper limit of room temperature, i.e., 35 degrees Celsius. In one embodiment, the ratio of the difference between the release temperature and 25 degrees Celsius to the difference between a melting temperature of the array of solder balls and 25 degrees Celsius is from 0.50 to 0.90. In another embodiment, this ratio can be from 0.60 to 0.80. For example, if the melting temperature of the array of solder balls  300  is 225 degrees Celsius, the release temperature can be in a range from 125 degrees Celsius and 205 degrees Celsius, and optionally can be in a range from 145 degrees Celsius and 185 degrees Celsius. 
     Since the first substrate  100  is held by vacuum without employing any adhesive, contamination of the bonded assembly ( 100 ,  200 ,  300 ) by adhesive materials is prevented. Thus, both ball grid array (BGA) substrates and land grid array (LGA) substrates can be employed for the first substrate  100 . Further, because the bonded assembly ( 100 ,  200 ,  300 ) remains in place only by gravity and friction between the bottom surface of the first substrate  100  and the top surface of the vacuum chuck  530 , the bonded assembly ( 100 ,  200 ,  300 ) can be picked up after cool down. Thus, no external force for freeing the bonded assembly ( 100 ,  200 ,  300 ) is needed in contrast to other methods employing adhesives to fix the first substrate  100  in place during a bonding process. 
     Referring to  FIG. 2 , a second exemplary apparatus according to a second embodiment of the present disclosure includes a chip bonder configured to hold, and bond, a first substrate  100  and a second substrate  200 . The second exemplary structure includes a bonder head  60 , the first substrate  100 , an array of solder material portions  300 , the second substrate  200 , and a base plate  460  that functions as a platform to support the first substrate  100 . A back side of the second substrate  200  can be attached to the bottom side of the bonder head  60  by vacuum suction. A bonder head movement actuator  10  configured to provide vertical movement of the bonder head  60  relative to the base plate  460  can be located above the bonder head  60 . The first substrate  100  can be any of a semiconductor chip, an interposer, or a packaging substrate. The second substrate  200  can be any of a semiconductor chip, an interposer, or a packaging substrate. 
     The bonder head movement actuator  10  can operate to enable the vertical movement of the bonder head  60  and the second substrate  200  relative to the base plate  460  and the first substrate  100 . The array of solder material portions  300  is bonded to one of the first substrate  100  or the second substrate  200  prior to the bonding step. Optionally, the bonder head movement actuator  10  may be configured to enable a lateral movement of the bond head  60  and the second substrate  200 . The bonder head movement actuator  10  can be configured to provide upward and downward vertical movement of the bonder head  60  relative to the base plate  460 . 
     The bonder head  60  can include a bonder head thermally conductive plate  40 , a bonder head heater assembly  30  that includes at least one bonder head heating coil  33 , and a chip support mechanism  20 . The bonder head heater assembly  30  can be configured to generate and transmit heat to the bonder head conductive plate  40  by conduction and/or radiation and/or convection. 
     The chip support mechanism  20  holds the second substrate  200  upside down. The chip support mechanism  20  can include a vacuum manifold  22  that extends through the bonder head conductive plate  40  and to at least one hole located on the bottom surface of the bonder head conductive plate  40 . The opposite end of the vacuum manifold  22  includes at least one vacuum connection port that is configured to be connected to a vacuum pump (not shown). The vacuum suction provided to the back side of the second substrate  200  can mechanically support the second substrate  200 . Alternately, any other chip support mechanism  20  that is capable of mechanically supporting the second substrate  200  can be employed instead of the substrate support mechanism employing vacuum pumping. 
     The bonder plate  460  is configured to hold the first substrate  100  upright under the horizontal plane of the bottommost surface of the second substrate  200 . The first substrate  100  is held upright so that an exposed surface of the first surface dielectric layer  120  is located at the topmost portion of the first substrate  100  and faces the bottommost surface of the second substrate  200 . 
     The base plate  460  includes a base thermally conductive plate  440 , a base plate heater assembly  430  that includes at least one base plate heating coil  433 , and a substrate support mechanism  420 . The first substrate  100  can be held at a fixed location by the base plate  460 . A base plate movement actuator  410  may be located beneath the base plate  460 , and may be configured to move the base plate  460  laterally and/or vertically. 
     The base plate heater assembly  430  is configured to generate and transmit heat to the base conductive plate  440  by conduction and/or radiation and/or convection. Typically, the base plate heater assembly  430  is configured to transmit heat to the base conductive plate  440  by conduction. The at least one base plate heating coil  433  is electrically connected to an electrical power source (not shown) that provides electrical power in the form of direct current (DC) or alternate current (AC). 
     The substrate support mechanism  420  holds the first substrate  100  in the upright position. The substrate support mechanism  420  can include a vacuum manifold  620  that extends through the base conductive plate  440  and to at least one hole located on the bottom surface of the base conductive plate  440 . The opposite end of the vacuum manifold  620  includes at least one vacuum connection port that is configured to be connected to a vacuum pump  610 . The vacuum suction provided to the back side of the first substrate  100  can mechanically support the first substrate  100 . Alternately, any other substrate support mechanism  420  that is capable of mechanically supporting the first substrate  100  can be employed instead of the substrate support mechanism employing vacuum pumping. 
     The at least one bonder head heating coil  33  and the at least one base plate heating coil  433  collectively constitute a heating means for heating an assembly of the first substrate  100 , the array of solder balls  300 , and the second substrate  200 . The base conductive plate  440  constitutes a supporting means for supporting the first substrate  100 . The base conductive plate  440  includes a planar top surface that maintains flat the back side surface of the first substrate  100  in combination with the vacuum suction provided in the vacuum manifold  620 . The combination of the vacuum pump  610 , the vacuum manifold  620 , and the base conductive plate  440  constitutes a clamping means for clamping the back side surface of the first substrate  100  to the planar top surface of the base conductive plate  440 . The clamping means is configured to hold the back side surface of the first substrate  100  coplanar with the planar top surface of the base conductive plate  440  while the first substrate  100  is clamped. 
     During a heat-up step, a reflow step, and an initial period of a cool down step, the back side surface of the first substrate  100  is clamped to a planar surface, which is the top planar surface of the base conductive plate  440 . The back side surface of the first substrate  100  remains coplanar with the top planar surface of the base conductive plate  440  while the first substrate  100  is clamped. The vacuum suction is provided by the vacuum manifold  620  that extends from the vacuum pump  610  to the back side surface of the first substrate  100 . 
     Subsequently, the first substrate  100  is bonded to the second substrate  200  through the array of solder balls  300  by reflowing the array of solder balls  300  at a reflow temperature, which is a temperature greater than the melting temperature of the array of solder balls  300 . The first substrate  100 , the second substrate  200 , and the array of solder balls  300  form a bonded assembly ( 100 ,  200 ,  300 ) after bonding. 
     The bonded assembly ( 100 ,  200 ,  300 ) is cooled below the reflow temperature to a release temperature that is greater than room temperature while the back side surface of the first substrate  100  remains clamped. The clamping of the back side of the first substrate  100  is released at the release temperature. The supporting means and the clamping means are configured to allow free bending of the assembly upon release of the clamping. The bonded assembly ( 100 ,  200 ,  300 ) is released from bonder head  60  after the reflow step. The bonded assembly ( 100 ,  200 ,  300 ) can be released from the bonder head  60  prior to the release of the clamping on the first substrate  100  from base plate  460 , or simultaneously with the release of the clamping on the first substrate  100  from base plate  460 . 
     The timing of the release of clamping can be controlled by a control means  80 , which can be, for example, a microprocessor, a microcontroller, or a computer. The control means  80  is configured to release the clamping of the back side of the first substrate  100  at the release temperature that is greater than room temperature. The control means  80  can be pre-programmed to release the clamping after a predefined time period following the reflow process. The predefined time period can be determined based on measured cooling characteristics of the chip bonder. Alternately, the control means  80  can be configured to monitor the temperature of the first substrate  100  or the base conductive plate  440  through sensors (not shown) and to control the operation of the vacuum pump  610  so that the vacuum pump  610  is turned off or the vacuum manifold  620  is vented at the release temperature. 
     Referring to  FIG. 3 , the mean warp of a packaging substrate under various bonding methods is schematically illustrated. The mean warp of a packaging substrate is plotted against temperature during a cool down step after a reflow step of a bonding process for various bonding methods that employ different types of constraint on the packaging substrate. For each temperature represented by a linear temperature scale in a bottom horizontal line of the graph, the corresponding time is indicated by a non-linear time scale in a top horizontal line of the graph. 
     A first curve  710  represents the mean warp of a packaging substrate when the packaging substrate is employed as the first substrate  100  and a semiconductor chip is employed as the second substrate  200  in the first and second embodiments described above. A second curve  720  represents the warp of a constrained packaging substrate that is held flat throughout the entire period of the cool down temperature, i.e., until the bonded assembly is cooled to room temperature. The constrained packaging substrate is allowed to laterally slip to prevent breakage of the packaging substrate. A third curve  730  represents the warp of a free packaging substrate that is bonded without flattening the back side surface of the packaging substrate. In other words, the free packaging substrate was not constrained or flattened in any way during bonding. The free packaging substrate suffers from yield loss due to non-uniformity of the height of the array of solder balls between the first substrate and the second substrate. 
     The third curve  730  shows that the free packaging substrate has a mean concave warp of about 12 microns at a reflow temperature of about 225 degrees Celsius, and about 50 microns at 25 degrees Celsius. The thermal warp, i.e., the change in warp, in the temperature range from 225 degrees Celsius and 25 degrees Celsius is about 38 microns. The variations in the absolute warp can be characterized statistically, and 1 sigma in the absolute warp can be on the order of about 25 microns. The third curve  730  shows how the warp behaves in a free-standing unconstrained packaging substrate during a cool down step. 
     The second curve  720  shows the warp behavior for a constrained packaging substrate when held down by a clamping means until the constrained packaging substrate cools down to room temperature. The warp of the packaging substrate, when constrained until the end of the cool down step at room temperature, is nearly perfectly zero throughout the heat-up step, the reflow step, and the cool down step. 
     The first curve  710  shows the warp behavior of a packaging substrate subjected to the processing methods of the first or second embodiments of the present disclosure. The warp of the packaging substrate is nearly perfectly zero throughout the heat-up step, the reflow step, and an initial period of the cool down step until the temperature of the packaging substrate reaches a release temperature, which is 165 degrees Celsius in this example. The clamping is released at the release temperature, and the packaging substrate is allowed to warp during a latter period of the cool down step, i.e., when the temperature of the packaging substrate is below the release temperature. Once the packaging substrate is unclamped, i.e., released from clamping, the warp of the packaging substrate changes toward the warp of a packaging substrate in a free state, i.e., the warp represented by the third curve  730 . However, the warp of the unclamped packaging substrate is less than the warp of a free-standing packaging substrate because some warp is absorbed by the array of solder balls and the second substrate during the initial period of the cool down step. Thus, the difference between the warp of the third curve  730  and the warp of the first curve  710  at 25 degrees Celsius is the result of a forced warp relaxation occurring due to the packaging substrate being forced flat at the reflow temperature. 
     A zero absolute warp at the reflow temperature according to embodiments of the present disclosure means that the sigma of the warp at the reflow temperature is necessarily zero. Hence, use of clamping according to embodiments of the present disclosure provides zero absolute warp, zero thermal warp, and zero sigma warps at reflow. Zero warp at reflow provides a significant benefit for die attachment by removing one of the major influencing factors for C4 non-wetting. Thus, the yield from a bond and assembly (B &amp; A) process can approach 100%. Such a benefit means that tolerance on other processing parameters can be relaxed such as C4 height coplanarity, flip chip attach (FCA) coplanarity, solder volume uniformity, etc. Further, all EG (electrically good) laminates can be used employing the methods of embodiments of the present disclosure irrespective of warp. Thus, substrate suppliers can increase yield, and lower the cost, of laminates employed for the packaging substrates. 
     Referring to  FIG. 4 , the stress on solder balls under various bonding methods is schematically illustrated. A first curve  810  represents the stress on solder balls when the packaging substrate is employed as the first substrate  100  and a semiconductor chip is employed as the second substrate  200  in the first and second embodiments described above. A second curve  820  represents the stress on solder balls when the packaging substrate that is held flat throughout the entire period of the cool down temperature, i.e., until the bonded assembly is cooled to room temperature, while allowing lateral slipping of the packaging substrate to prevent breakage of the packaging substrate. A third curve  830  represents the stress on solder balls when a free packaging substrate is employed, i.e., when the packaging substrate is bonded without flattening the back side surface of the packaging substrate. The free packaging substrate suffers from yield loss due to non-uniformity of the height of the array of solder balls between the first substrate and the second substrate. A fourth curve  840  represents the stress on solder balls when the packaging substrate that is held flat throughout the entire period of the cool down temperature while lateral slipping of the packaging substrate is prevented. By limiting the slippage of the packaging substrate, the thermal shrinkage in the lateral dimension of the packaging substrate is suppressed, and therefore, the thermal contraction of the top surface of the packaging substrate is minimized. Thus, the stress on the solder balls in the fourth curve  840  can be less than the stress on the solder balls in the third curve  830 , which represents a packaging substrate that is allowed to contract during the cool down step. However, the suppression of the thermal contraction in the case of the fourth curve induces substantial increase in the stress within the packaging substrate, which is laterally constrained not to move. Thus, the fourth curve  840  can end when the packaging substrate breaks under severe stress before the bonded assembly cools down to room temperature. The method of the fourth curve  840  typically results in a structural damage to the packaging substrate or the constraint mechanism, which can be, for example, a glass platen or a glass sheet. 
     The first curve  810  represents embodiments of the present disclosure. The packaging substrate is clamped, i.e., constrained, such that the back side surface of the packaging substrate is held flat against a planar surface while being allowed to laterally slip, during a heat-up step, a reflow step, and an initial period of cool down until the release temperature is reached. Thus, the first curve  810  coincides with the second curve  820  until the release temperature is reached. Upon release of the clamping, the packaging substrate becomes unconstrained. Thus, the full effect is manifested of the mismatch of thermal coefficients of expansion between the first substrate (packaging substrate) and the second substrate. The bonded substrate becomes free to bend upon releasing of the clamping at the release temperature, which is reached after the solder balls have solidified sufficiently to be able to maintain substantially the same shapes. Thus, the stress on the solder balls according to the first curve  810  becomes substantially the same as the stress on the solder balls according to the third curve  830  after the release of the clamping, thereby reducing the stress on the solder balls relative to the second curve  820  and preventing cracks on the solder balls or metal interconnect structures attached to the solder balls. 
     Referring to  FIG. 5 , an exemplary substrate carrier according to an embodiment of the present disclosure is schematically illustrated. The exemplary substrate carrier is configured to hold substrates, i.e., a stack of aligned pre-bonding assembly as illustrated in  FIGS. 1 and 2 , for bonding. 
     The exemplary substrate carrier can be employed in a reflow oven. The reflow oven can be operated as a high-throughput linear device such that substrate carriers enter the reflow oven from one end and exits from the other end of the oven. In this configuration, the ability to provide vacuum for a substrate carrier through a fixture, such as a vacuum manifold  622  illustrated in  FIGS. 1 and 2 , may be limited. 
     In this embodiment, vacuum suction to one or more first substrate  100  (See  FIGS. 1 and 2 ) can be provided by the substrate carrier. Specifically, the substrate carrier is designed to include a vacuum enclosure, i.e., a vacuum chamber. The vacuum enclosure is configured to employ the back side or front side surface(s) of one or more first substrates as (a) sealing surface(s). For example, the back side surface of the first substrate  100  as illustrated in  FIGS. 1 and 2  can be employed as a sealing surface. 
     The vacuum enclosure provides a reservoir for the vacuum while the substrate carrier passes through the reflow oven. The size of such a chamber would be chosen such that the vacuum is not depleted (due to leakage) during the time period in which the substrate carrier is in the oven. While the exemplary substrate carrier is illustrated in  FIG. 5  for mounting six packaging substrates, a substrate carrier can have numerous design variations to accommodate various numbers of packaging substrates and/or design variations in the shapes of the packaging substrate(s). The vacuum reservoir is integral with the substrate carrier. 
     In the exemplary substrate carrier illustrated in  FIG. 5 , six packaging substrates (not shown) are held down by vacuum channels that are machined in the frame of the carrier. The frame is shown as ribbed so as to provide the necessary exposure of the bottom of the packaging substrates to the heating medium. Alternately, the frame can be made of solid metal. The vacuum chamber is integral with, and adjacent to, the substrate clamping area. A vacuum port is provided to enable pulling of vacuum in the vacuum enclosure, which is sealed by mounting the six packaging substrates. While the vacuum is pulled, the packaging substrates (corresponding to the first substrate  100  in  FIGS. 1 and 2 ) are positioned over the clamping area, and semiconductor chips are placed on the flip chip area (FCA) of the packaging substrate. At this point the vacuum enclosure is sealed and the mechanism that provides the vacuum is removed, and then the substrate carrier including the six stacks of aligned pre-bonding assembly is sent through the reflow oven. 
     In general, a substrate carrier can be configured to form an encapsulated vacuum enclosure therein by combining with at least one substrate (e.g., packaging substrates). The combination of the substrate carrier and the at least one substrate can be performed in a geometry in which each back side or front side surface of the at least one substrate is combined with surfaces of a cavity within the substrate carrier to form and seal the encapsulated vacuum enclosure. 
     A vacuum release mechanism configured to break the sealed encapsulated vacuum enclosure can also be provided. For example, the vacuum release mechanism can be an orifice in the chamber that is blocked by a needle valve. The needle valve is attached to a vacuum release mechanism. For example, a stationary bar located in the reflow oven can knock this mechanism backwards as the substrate carrier passes beneath it, thereby releasing the vacuum suction. 
     The substrate carrier can be a portable substrate carrier that can hold packaging substrates with a vacuum suction until the packaging substrates cool down to a release temperature or until the substrate carrier passes through a predefined position at which the temperature of the packaging substrates is at a predefined temperature lower than the reflow temperature. Thus, the vacuum suction on the bonded assemblies can be released in a timely manner during the cool down step, i.e., at or around the release temperature. The substrate carrier contains a vacuum reservoir, and is compatible with traditional high through-put linear reflow ovens. 
     Referring to  FIGS. 6A-6C , various means are schematically illustrated for providing a vacuum sealing between a substrate carrier  660  and the first substrate  100  of a bonded substrate. 
       FIG. 6A  illustrates an embodiment in which the vacuum seal is provided between the back side of a first substrate  100  and a planar surface of the substrate carrier  660 . The vacuum seal may be positioned anywhere in the region between the FCA (flip chip area) and the perimeter of the first substrate  100 . The vacuum seal can be implemented employing a vacuum seal element  680 , which can be, for example, an O-ring compatible with vacuum applications. The FCA of the first substrate  100  is clamped coplanar with the planar surface of the substrate carrier  660 . The end of the vacuum manifold  620  that is connected to the vacuum pump  610  can be provided with a self-sealing plug, which can be, for example, of the same type as the self-sealing plug on vehicle tires or on commercial soccer balls or basketballs. 
       FIG. 6B  illustrates another embodiment in which the vacuum seal is provided at the perimeter of the first substrate  100  so that the vacuum sealing occurs at the vertical edge of the first substrate  100 . Here the force that the vacuum seal exerts on the first substrate  100  is oriented along the horizontal plane of the first substrate  100 . 
       FIG. 6C  illustrates yet another embodiment in which the vacuum seal is provided over the first substrate  100  and at the planar top surface of the substrate carrier  660 . The sealing takes place at two places—between the vacuum seal element  680  and the front side of the first substrate  100  and between the vacuum seal element  680  and the top of the planar surface of the substrate carrier  660 . The vacuum seal exerts a downward force, thereby pushing the first substrate  100  against the planar top surface of the substrate carrier  660 . 
     In another embodiment of the present disclosure, mechanical clamping devices can be used to hold a substrate flat against a planar surface in lieu of vacuum suction. One or more mechanical clamping devices, such as one or more clamps, can be used to control the absolute, thermal and sigma of the warp in a manner similar to vacuum clamping. 
     Referring to  FIG. 7 , an assembly is shown of a bonded substrate ( 100 ,  200 ,  300 ), a platen (or a chuck)  770 , and at least one mechanical clamping device  780 . In one embodiment, the at least one mechanical clamping device  780  can be configured to press the back side surface of the first substrate  100  against the planar surface of the platen  770  in one state, and not to press the back side surface of the first substrate  100  against the planar surface of the platen  770  in another state. 
     In one embodiment, the combination of the platen  770  and the at least one mechanical clamping device  780  can replace the vacuum chuck  530  of the first embodiment, or the base conductive plate  440  of the second embodiment. The at least one mechanical clamping device  780  is shown in a clamped position in  FIG. 7A , and is shown in an unclamped (released) position in  FIG. 7B . 
     A control means is provided to enable the release of the at least one mechanical clamping device  780  from the packaging substrate at a release temperature. The timing of the release of the packaging substrate is selected to avoid undue build-up of stress and/or strain in the packaging substrate and the array of solder balls. In one embodiment, the at least one mechanical clamping device  780  can restrict the lateral expansion or contraction of the first substrate  100  relative to the platen  770 . 
     The same methods as the first and second embodiments can be employed to form a bonded assembly of the first substrate  100 , the second substrate  200 , and the array of solder balls  300  by employing mechanical clamping instead of vacuum clamping. Specifically, the same heating means can be employed for heating an assembly of the first substrate  100 , the array of solder balls  300 , and the second substrate as in the first and second embodiments. The platen  770  can be employed as a supporting means for supporting the first substrate  100 . The at least one mechanical clamping device  780  is employed as a clamping means for clamping the back side surface of the first substrate  100  to the top planar surface of the platen  770 , and to hold the back side surface of the first substrate  100  coplanar with the planar top surface of the platen  770  while the first substrate  100  is clamped. Further, the same control means  80  as in the first and second embodiments can be employed for releasing clamping of the back side of the first substrate  100  at a release temperature that is greater than room temperature. The supporting means and the clamping means are configured to allow free bending of the assembly upon release of the clamping. 
     Referring to  FIG. 8 , an assembly of a bonded substrate ( 100 ,  200 ,  300 ), an adhesive layer  880 , a transparent chuck  870 , and an irradiation means  850  configured to remove or deactivate the adhesive layer  880  are shown during a cool down step after a reflow step according to an embodiment of the present disclosure. The combination of the assembly of the bonded substrate ( 100 ,  200 ,  300 ), the adhesive layer  880 , the transparent chuck  870  can be employed in combination with a furnace as in the first embodiment, or in combination with a chip bonder as in the second embodiment. The vacuum clamping of the first and second embodiments or the mechanical clamping of the embodiment of  FIG. 7  can be replaced with clamping of the first substrate  100  with the adhesive layer  880  during a heat-up step, a reflow step, and an initial period of a cool down step until a release temperature is reached. 
     Thus, the clamping is provided by the adhesive layer  880  contacting the back side surface of the first substrate  100  and the planar top surface of the transparent chuck  870 . At the release temperature, adhesive strength of the adhesion layer  880  is reduced at the release temperature by irradiation of electromagnetic radiation  860 , such as a laser beam, emitted by a radiation beam source  850 . The electromagnetic radiation  860  can ablate substantially all or a significant portion of the adhesion layer  880  so that the first substrate  100  is mechanically unconstrained, i.e., unclamped, from the adhesive layer  880 . 
     In one embodiment, the assembly of the bonded substrate ( 100 ,  200 ,  300 ), the adhesive layer  880 , the transparent chuck  870  can be configured to move in a direction relative to the radiation beam source  850 . The movement of the assembly and/or the timing of the emission of the electromagnetic radiation  860  by the electromagnetic radiation source  850  can be controlled by a control means  80 , which can be a computer or a microprocessor configured to monitor the temperature of a furnace or a chip bonder. The radiation beam source  850  can be configured to irradiate the electromagnetic radiation  860  toward the back side surface of the first substrate  100  through the transparent chuck  870 . 
     During the heat-up step, the reflow step, and the initial period of the cool down step above the release temperature, the back side surface of the first substrate  100  is clamped to a planar surface, which is the top surface of the transparent chuck  870 , wherein the back side surface remains coplanar with the planar surface of the transparent chuck  870  while the first substrate  100  is clamped. During the reflow step, the first substrate  100  is bonded to the second substrate  200  through the array of solder balls  300 , and the first substrate  100 , the second substrate  200 , and the array of solder balls  300  form a bonded assembly ( 100 ,  200 ,  300 ). The bonded assembly ( 100 ,  200 ,  300 ) is cooled below the reflow temperature to a release temperature that is greater than room temperature while the back side surface of the first substrate  100  remains clamped to the transparent substrate  870  by the adhesive layer  880 . The clamping of the back side of the first substrate  100  is released at the release temperature by the electromagnetic radiation  860  that passes through the transparent substrate  870  during irradiation of the adhesive layer  880 . 
     The release temperature can be the same in all embodiments. Correspondingly, the ratio of the difference between the release temperature and 25 degrees Celsius to the difference between a melting temperature of the array of solder balls and 25 degrees Celsius can be the same in all embodiments, i.e., can be from 0.50 to 0.90. 
     The adhesive layer  880  and the transparent chuck  870  can be employed as parts of an apparatus for bonding substrates. The apparatus can include a heating means for heating an assembly of the first substrate  100 , the array of solder balls  300 , and the second substrate as in the various embodiment of the present disclosure. The transparent chuck  870  functions as a supporting means for supporting the first substrate  100  such that the back side of the first substrate  100  remains flat when attached to the transparent chuck  870  through the adhesive layer  880 . The electromagnetic radiation source  850  functions as a release means for releasing the back side surface of the first substrate  100  from the planar top surface of the transparent chuck  870  from a clamped state in which the back side surface is held coplanar with the planar top surface while the first substrate  100  is clamped. Further, the apparatus can include a control means  80  for activating the release means at a release temperature that is greater than room temperature. 
     While the disclosure has been described in terms of specific embodiments, it is evident in view of the foregoing description that numerous alternatives, modifications and variations will be apparent to those skilled in the art. Each of the various embodiments of the present disclosure can be implemented alone, or in combination with any other embodiments of the present disclosure unless expressly disclosed otherwise or otherwise impossible as would be known to one of ordinary skill in the art. Accordingly, the disclosure is intended to encompass all such alternatives, modifications and variations which fall within the scope and spirit of the disclosure and the following claims.