Abstract:
To prevent formation of entrapped underfill material between solder balls and bonding bumps, fast temperature ramping is employed during a chi assembly after application of an underfill material prior to bonding. Voids formed within the underfill material are subsequently removed by curing the underfill material in a pressurized environment. Temperature cycling on the underfill material is limited during the bonding process in order to maintain viscosity of the underfill material prior to the cure process in the pressurized environment. Further, the underfill material is subjected to the pressurized environment until the cure process is complete to prevent re-formation of voids. The cure process can be a constant temperature or a multi-temperature process including temperature ramping. Further, the cure process can be a constant pressure process or a pressure cycling process that accelerates removal of the voids.

Description:
BACKGROUND 
     The present disclosure relates to methods of curing underfill material, and particularly to methods of curing underfill material between a semiconductor chip and a substrate under a pressurized condition. 
     A semiconductor chip can be bonded to another semiconductor chip or a packaging substrate by employing Controlled Collapse Chip Connection (C4) process. Each C4 ball contacts a C4 pad on the semiconductor chip and another C4 pad on the other semiconductor chip or the packaging substrate. Each C4 pad is a contiguous metal pad typically formed out of the last metal layer of a metal interconnect structure during a semiconductor manufacturing sequence. Each C4 pad is large enough to accommodate the bottom portion of a C4 ball. Typically, an array of C4 balls can be employed to provide input/output (I/O) connections between the semiconductor chip and another semiconductor chip or a packaging substrate. 
     The C4 connections are susceptible to mechanical stress created by a mismatch in the coefficients of thermal expansion (CTE&#39;s) between the semiconductor chip and the other semiconductor chip or the packaging substrate. Such mechanical stress may cause cracks in the C4 balls or the semiconductor chip, causing the semiconductor chip to fail during usage. An underfill material, which easily deforms under stress, i.e., has a low value of Young&#39;s modulus, is employed to fill the space around the array of the C4 balls between the semiconductor chip and the other semiconductor chip or the packaging substrate. The underfill material absorbs the stress during the thermal expansion or contraction of the semiconductor chip, the array of C4 balls, and the other semiconductor chip or the packaging substrate, thereby preventing cracks in the bonded structure. In order to distribute the stress uniformly, however, it is necessary to have a uniform distribution of the underfill material without voids therein. 
     Recently, processes that apply the underfill material on a semiconductor chip or on a packaging substrate have been proposed. These processes enable filling spaces between an array of C4 balls with fewer and/or smaller cavities by applying the underfill material either on the substrate to which C4 balls are attached or to the other substrate to which C4 balls are not attached at the beginning of the bonding process. See, for example, U.S. Pat. No. 6,746,896 to Shi et al. and U.S. Pat. No. 6,902,954 to Shi. 
     One method is commonly referred to as the No Flow Underfill (NFU) method, in which the underfill material is pre-applied to a substrate. The other method is commonly referred to as the Wafer Level Underfill (WLU) method, in which the underfill material is pre-applied to a wafer, i.e., at least one semiconductor chip on a wafer. According to both methods, the pre-applied underfill material is cured in order to minimize the stress on low-k material layers in the semiconductor chip(s) during the flip chip assembly. 
     The pre-applied underfill material contains a fluxing component or flux capability in itself, and is subjected to the solder temperature, which is above the melting temperature of the solder material. For example, the melting temperature of lead-free solder is about 217° C. The relatively high soldering temperature causes large amounts of voids to form within the underfill material. The voids remain entrapped between the chip bumps on the semiconductor chip and the substrate bumps (or pads) on the substrate. The voids entrapped inside the underfill material are considered as a failure factor for the flip chip process because the areas including the voids do not absorb the mechanical stress. Further, some underfill material can be entrapped between a solder ball and a chip bump or a substrate bump, causing an electrical open. 
     On one hand, a low rate of temperature ramping can trigger an early cure of the underfill material, which minimizes formation of voids at the expense of maximizing entrapped underfill material between solder balls and the chip bumps or the substrate bumps during a flip chip bonding process. On the other hand, a high rate of temperature ramping can trigger a delayed cure of the underfill material, which minimizes formation of entrapped underfill materials between solder balls and the chip bumps or the substrate bumps voids at the expense of maximizing formation of voids due to aggressive reaction of underfill material during a flip chip bonding process. To date, a solution that reduces formation of entrapped underfill material on chip bumps or substrate bumps while reducing formation of voids in the underfill material has not been provided in the art. 
     BRIEF SUMMARY 
     To prevent formation of entrapped underfill material between solder balls and bonding bumps, fast temperature ramping is employed during a chip assembly after application of an underfill material prior to bonding. Voids formed within the underfill material are subsequently removed by curing the underfill material in a pressurized environment. Temperature cycling on the underfill material is limited during the bonding process in order to maintain viscosity of the underfill material prior to the cure process in the pressurized environment. Further, the underfill material is subjected to the pressurized environment until the cure process is complete to prevent re-formation of voids. The cure process can be a constant temperature or a multi-temperature process including temperature ramping. Further, the cure process can be a constant pressure process or a pressure cycling process that accelerates removal of the voids. 
     According to an aspect of the present disclosure, a method of bonding substrates is provided, which includes: applying an underfill material onto a surface of a first substrate; placing a second substrate on the underfill material, wherein an array of C4 balls is present between the first substrate and the second substrate; bonding the first substrate to the second substrate by heating the array of C4 balls to a reflow temperature of the array of C4 balls; and curing the underfill material at a cure temperature greater than room temperature and at a cure pressure greater than 1.0 atm after the bonding. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         FIGS. 1A and 1B  compare C-mode scanning acoustic microscopy (CSAM) pictures of an underfill material in a bonded chip assembly.  FIG. 1A  is a CSAM picture after bonding and cool down to a room temperature, and  FIG. 1B  is a CSAM picture after a 2 hour long exposure to a 2 atm pressurized environment at room temperature. 
         FIGS. 2A-2H  are sequential schematic vertical cross-sectional views of an exemplary structure illustrating a first method of bonding and curing substrates according to an embodiment of the present disclosure. 
         FIGS. 3A-3D  are sequential schematic vertical cross-sectional views of another exemplary structure illustrating a second method of bonding and curing substrates according to another embodiment of the present disclosure. 
         FIG. 4A  is a set of graphs illustrating conditions for a first cure process according to a first embodiment of the present disclosure. 
         FIG. 4B  is a set of graphs illustrating conditions for a variation of the first cure process according to the first embodiment of the present disclosure. 
         FIGS. 4C and 4D  compare C-mode scanning acoustic microscopy (CSAM) pictures of an underfill material in a first bonded chip assembly.  FIG. 4C  is a CSAM picture after bonding and cool down to a room temperature, and  FIG. 4D  is a CSAM picture after a 2 hour long exposure to a 2 atm pressurized environment at 150° C. 
         FIG. 5A  is a set of graphs illustrating conditions for a second cure process according to a second embodiment of the present disclosure. 
         FIG. 5B  is a set of graphs illustrating conditions for a variation of the second cure process according to the second embodiment of the present disclosure. 
         FIGS. 5C and 5D  compare C-mode scanning acoustic microscopy (CSAM) pictures of an underfill material in a second bonded chip assembly.  FIG. 5C  is a CSAM picture after bonding and cool down to a room temperature, and  FIG. 5D  is a CSAM picture after a combination of a first 1 hour long exposure to a 2 atm pressurized environment at 100° C. and a second 1 hour long exposure to a 2 atm pressurized environment at 150° C. 
         FIGS. 5E and 5F  compare C-mode scanning acoustic microscopy (CSAM) pictures of an underfill material in a third bonded chip assembly.  FIG. 5E  is a CSAM picture after bonding and cool down to a room temperature, and  FIG. 5F  is a CSAM picture after a combination of a first 1 hour long exposure to a 2 atm pressurized environment at 125° C. and a second 1 hour long exposure to a 2 atm pressurized environment at 150° C. 
         FIG. 6A  is a set of graphs illustrating conditions for a third cure process according to a third embodiment of the present disclosure. 
         FIG. 6B  is a set of graphs illustrating conditions for a variation of the third cure process according to the third embodiment of the present disclosure. 
         FIG. 7A  is a set of graphs illustrating conditions for a fourth cure process according to a fourth embodiment of the present disclosure. 
         FIG. 7B  is a set of graphs illustrating conditions for a variation of the fourth cure process according to the fourth embodiment of the present disclosure. 
         FIG. 8A  is a set of graphs illustrating conditions for a fifth cure process according to a fifth embodiment of the present disclosure. 
         FIG. 8B  is a set of graphs illustrating conditions for a variation of the fifth cure process according to the fifth embodiment of the present disclosure. 
         FIG. 9A  is a set of graphs illustrating conditions for a sixth cure process according to a sixth embodiment of the present disclosure. 
         FIG. 9B  is a set of graphs illustrating conditions for a variation of the sixth cure process according to the sixth embodiment of the present disclosure. 
         FIG. 10A  is a set of graphs illustrating conditions for a seventh cure process according to a seventh embodiment of the present disclosure. 
         FIG. 10B  is a set of graphs illustrating conditions for a variation of the seventh cure process according to the seventh embodiment of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     As stated above, the present disclosure relates to methods of curing underfill material between a semiconductor chip and a substrate under a pressurized condition, 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. 
     Referring to  FIGS. 1A and 1B , C-mode scanning acoustic microscopy (CSAM) pictures are shown for an underfill material in a bonded chip assembly before and after a 2 hour long exposure to a 2 atm (atmospheric pressure) pressurized environment at room temperature. As used herein, “room temperature” refers to a continuous temperature range from 20° C. to 25° C. and including 20° C. and 25° C.  FIG. 1A  is the CSAM picture after bonding and cool down to room temperature and before the pressurized treatment at room temperature. The dark area represents regions in which a fill material is present without voids, and the white area represents regions in which voids are present.  FIG. 1B  is the CSAM picture after the pressurized treatment at room temperature.  FIGS. 1A and 1B  demonstrate that subjecting an underfill material including voids therein to a pressurized environment up to 2 atm at room temperature does not significantly reduce the voids, and is thus ineffective in removing the voids within the underfill material. 
     Referring to  FIGS. 2A-2H , a first method of bonding substrates according to an embodiment of the present disclosure is sequentially illustrated. A first substrate  100  includes a first substrate layer  110  and a first surface dielectric layer  120 . An array of first C4 pads  130  is formed on the first surface dielectric layer  120 . An array of C4 balls  300  is attached to the array of first C4 pads  130  employing methods known in the art. Subsequently, a fill material is applied over the array of C4 balls  300  on a front surface, which is a bottom surface (as seen with the first substrate  100  upside down in  FIG. 2A ) of the first substrate  100 , i.e., the exposed surface of the first surface dielectric layer  120 . For example, the fill material can be over bump applied resin (OBAR) as known in the art, and the thickness of the fill material can be greater than the height of the array of C4 balls by an additional thickness from 5 microns to 50 microns. The first substrate  100  is then held upside down over a second substrate  200  by a bonder head  60 . 
     The second substrate  200  includes a second substrate layer  210  and a second surface dielectric layer  220 . An array of second C4 pads  230  are provided on the second surface dielectric layer  220 . A solder material portion  240 , which can have the same composition as the array of C4 balls  300 , can be applied to each second C4 pad  230 . 
     The first substrate  100  can be a semiconductor chip and the second substrate  200  can be a packaging substrate. Alternately, the first substrate  100  can be a packaging substrate and the second substrate  200  can be a semiconductor chip. Yet alternately, the first substrate  100  can be a first semiconductor chip and the second substrate  200  can be a second semiconductor chip. The relative sizes of the first substrate  100  and the second substrate  200  can vary, although the second substrate  200  is depicted to be larger in size than the first substrate  100  in  FIGS. 2A-2H . The array of C4 balls  300  is bonded to the array of the first C4 pads  130 , which are bond pads, located on the first substrate  100  prior to applying the underfill material  310  to the first substrate  100 . 
     The second substrate  200  is placed under the first substrate  100  either by moving the first substrate  100  or by moving the second substrate  200 . In one embodiment, the second substrate  200  can be directly mounted to a fixture, and the first substrate  100  may be mounted to the bonder head  60 , which can move vertically and/or laterally by a movement actuator (not shown). Alternately or in conjunction, the second substrate  200  can be mounted in a way that enables lateral movement of the second substrate  200 . The first substrate  100  and/or the second substrate  200  are moved relative to each other so that a C4 ball  300  directly overlies a corresponding solder material portion  240  and a corresponding second C4 pad  230 . Any alignment scheme known in the art can be employed so that components in each pair of a C4 ball  300  and a corresponding second C4 pad overlie or underlie each other. A finite vertical spacing is maintained between the bottom surface of the underfill material  310  and the topmost surfaces of the solder material portions  240  until the alignment of the first and second substrates ( 100 ,  200 ) relative to each other is complete. 
     Referring to  FIG. 2B , the array of C4 balls  300  is raised to an elevated temperature that does not exceed the melting temperature of the material of the C4 balls  300 . The temperature of the array of C4 balls  300  can be raised by applying heat through the bonder head  60  to the first substrate  100 , which transmits the heat to the array of C4 balls  300  and the underfill material  310 . Alternately, the temperature of the array of C4 balls can be raised by any other means known in the art. As the temperature of the underfill material  310  increases, the outer surfaces of the underfill material may develop a convex shape due to a decrease in viscosity and the effect of surface tension. 
     Referring to  FIG. 2C , the first and second substrates ( 100 ,  200 ) are moved closer to each other until a surface of the underfill material  310  contacts the topmost surface of the second substrate  200 . The relative movement of the first and second substrates ( 100 ,  200 ) can be effected by a vertical downward movement of the first substrate  100 , a vertical upward movement of the second substrate  200 , or a combination thereof. Initially, the array of C4 balls  300  does not make a contact with the solder material portions  240  located on the second substrate  200 . 
     As the temperature of the underfill material  310  approaches the melting point of the solder material of the array of C4 balls  300 , voids  317  from within the underfill material  310  because the fluxing component within the underfill material  310  decomposes and generates gas. The composition of fluxing components is described, for example, in U.S. Pat. No. 7,009,009 to Crane et al. By-products such as oxygen or water may be additionally generated through a reaction of reducing an oxide layer of the metal bumps. See, for example, U.S. Pat. No. 7,759,162 to Karashima et al. In addition, volatile gases from substrate may contribute to formation of voids  317  within the underfill material  310 . For example, moisture absorption of semiconductor device affects the void generation. See U.S. Pat. No. 7,687,319 to Nishimura et al. 
     Referring to  FIG. 2D , the first and second substrates ( 100 ,  200 ) are moved even closer to each other until the array of C4 balls  300  contact the second substrate  200 , i.e., the solder material portions  240  of the second substrate  200 . 
     Referring to  FIG. 2E , after the array of C4 balls  300  contacts the second substrate  200 , the array of C4 balls  300  is further heated to a reflow temperature of the solder material of the C4 balls  300 . Upon heating of the array of C4 balls  300  to the reflow temperature of the solder material therein, the solder material of the C4 balls  300  and the solder material of the solder material portions  240  on the second substrate  200  begin to reflow, and the material of each solder material portion  240  is incorporated into a C4 ball  300 . Upon reflow of the array of C4 balls  300 , the first substrate  100  can be moved closer to the second substrate  200  to more effectively incorporate the material of the solder material portions  240  and to increase the solder bonding areas. The voids  317  may grow in size as the temperature of the underfill material  310  increases. 
     The processing steps of  FIGS. 2B-2E  can be performed in a continuous sequence in which the temperature of the underfill material  310  ramps up continuously. For example, the temperature of the array of C4 balls  300  can be ramped up at a rate between 10° C./sec and 100° C./sec from the melting temperature of the solder material of the array of C4 balls  300  to the reflow temperature during the bonding step of  FIG. 2E . Further, the temperature of the array of C4 balls  300  can be ramped up at a rate between 10° C./sec and 100° C./sec from a temperature lower than the melting temperature of the solder material of the array of C4 balls  300 , e.g., from room temperature or any temperature between room temperature and the melting temperature, to the reflow temperature during the bonding steps of  FIGS. 2B-2E . 
     Referring to  FIG. 2F , the first substrate  100  can be moved away from the second substrate  200  after the moving of the first substrate  100  closer to the second substrate  200  to provide an optimal and uniform shape for a fillet of the underfill material  310 , i.e., for the portion of the underfill material  310  that laterally extrudes away from the sidewalls of the first substrate  100 . Optionally, the first substrate  100  can be moved away from the second substrate  200  after the moving of the first substrate  100  closer to the second substrate  200  to provide an optimal and uniform shape for the solder joint  300 . Thus, the first and second substrates ( 100 ,  200 ) are bonded through the array of C4 balls  310  and the underfill material  310 . 
     The temperature of the underfill material  310  can be maintained at the reflow temperature for a time period during the processing steps of  FIGS. 2G and 2F . The time period for maintaining the reflow temperature can be, for example, between 1 second and 3 seconds. The reflow temperature for a lead-free solder material can be about 250° C. Subsequently, the temperature of the underfill material  310  is ramped down at a relatively high temperature ramp rate, which can be between 10° C./sec and 30° C./sec from the reflow temperature to the melting temperature during the bonding. Further, the temperature of the underfill material  310  can be further ramped down to room temperature or a cure temperature, which is higher than room temperature and lower than the melting temperature of the solder material of the array of C4 balls  300 . The combination of the relatively high temperature ramp up rate, the relatively short time period at the reflow temperature, and the relatively high temperature ramp down rate minimizes the exposure of the underfill material  310  to thermal treatment, and thereby minimizes curing of the underfill material due to the heat during the bonding steps of  FIGS. 2B-2F . 
     At the end of the step of  FIG. 2F , at least 15% of the underfill material  310  remains uncured. The percentage of the remaining uncured fraction of the underfill material  310  can be from 15% to 99%, and typically from 20% to 80%. The curing process changes the crosslink density and molecular weight distribution of the underfill material. Thus, the uncured fraction of the underfill material  310  can be analytically measured by determining the residual exotherm of the underfill material  310  at any stage of a curing process. 
     Referring to  FIG. 2G , the bonded assembly of the first substrate  100  and the second substrate  200  is placed in an enclosure  360 , which is provided with heating instrumentation  380 . The enclosure  380  is a pressure chamber configured to enable pressurization therein with a range including at least 1 atm and 2 atm. The enclosure  380  can be configured to provide pressures greater than 2 atm. For example, the enclosure  380  can be configured to enable pressurization in a continuous pressure range including 1 atm and 25 atm, although pressurization at a greater pressure can be employed. The heating instrumentation  380  is an apparatus configured to provide heat within the enclosure  360 , for example, by convection, conduction, radiation, or a combination thereof. For example, the assembly of the enclosure  360  and the heating instrumentation  380  can be a pressurized oven with electrical heaters, a thermostat mechanism, and a processor for temperature control. 
     Upon placement within the enclosure  360 , the underfill material  310  is cured at a cure temperature greater than room temperature and at a cure pressure greater than 1.0 atm. A plurality of cure temperatures and/or a plurality of cure pressures can be employed. In general, the cure temperature(s) is/are less than the melting temperature of the solder material of the array of C4 balls  300 . The cure pressure(s) is/are can be between 1.0 atm and 25 atm, although pressures greater 25 atm can be employed if the enclosure  380  supports operation at such pressures. The total volume of voids  317  in the underfill material  10  decreases during the curing of the underfill material  310  at the cure temperature(s) and the cure pressure(s). 
     Referring to  FIG. 2H , the enclosure  360  is de-pressurized and the bonded assembly of the first substrate  100  and the second substrate  200  can be removed from inside the enclosure  360 . The pressurized cure process removes the voids in the underfill material  310 . At the end of the cure process, at least 90% of the underfill material  310  is cured before pressure applied to the underfill material is reduced from the cure pressure to 1.0 atm. Further, at least 99% of the underfill material  310  can be cured before pressure applied to the underfill material is reduced from the cure pressure to 1.0 atm. In practice, more than 99% of the underfill material  310  can be cured by the pressurized cure process if the temperature and the pressure are selected optimally. 
     Referring to  FIGS. 3A-3D , a second method of bonding substrates according to another embodiment of the present disclosure is sequentially illustrated. Referring to  FIG. 3A , a first substrate  100  includes a first substrate layer  110  and a first surface dielectric layer  120 . An array of first C4 pads  130  is formed on the first surface dielectric layer  120 . A solder material portion  140 , which can have the same composition as an array of C4 balls  300  to be formed on a second substrate  200 , can be applied to each first C4 pad  130 . 
     The second substrate  200  includes a second substrate layer  210  and a second surface dielectric layer  220 . An array of second C4 pads  230  are provided on the second surface dielectric layer  220 . The array of C4 balls  300  is attached to the array of second C4 pads  230  employing methods known in the art. The second substrate  100  is then held upside down over the first substrate  100  by a bonder head  60 . 
     A fill material  310  is applied over the first surface dielectric layer  120  and the solder material portions  140  located on the top surface of the first substrate  100 . For example, the fill material  310  can be over bump applied resin (OBAR) as known in the art, and the thickness of the fill material  310  can be greater than the height of the array of C4 balls by an additional thickness from 5 microns to 50 microns. 
     The first substrate  100  can be a semiconductor chip and the second substrate  200  can be a packaging substrate. Alternately, the first substrate  100  can be a packaging substrate and the second substrate  200  can be a semiconductor chip. Yet alternately, the first substrate  100  can be a first semiconductor chip and the second substrate  200  can be a second semiconductor chip. The relative sizes of the first substrate  100  and the second substrate  200  can vary, although the first substrate  100  is depicted to be larger in size than the second substrate  200  in  FIGS. 3A-3D . The array of C4 balls  300  is bonded to the array of the second C4 pads  230 , which are bond pads, located on the second substrate  200  prior to applying the underfill material  310  to the first substrate  100 . 
     The first substrate  100  is placed under the second substrate  200  either by moving the second substrate  200  or by moving the first substrate  100 . In one embodiment, the first substrate  100  can be directly mounted to a fixture, and the second substrate  200  may be mounted to the bonder head  60 , which can move vertically and/or laterally by a movement actuator (not shown). Alternately or in conjunction, the first substrate  100  can be mounted in a way that enables lateral movement of the first substrate  100 . The second substrate  200  and/or the first substrate  100  are moved relative to each other so that a C4 ball  300  directly overlies a corresponding solder material portion  140  and a corresponding first C4 pad  130 . Any alignment scheme known in the art can be employed so that components in each pair of a C4 ball  300  and a corresponding first C4 pad  140  overlie or underlie each other. A finite vertical spacing is maintained between the bottom surface of the underfill material  310  and the topmost surfaces of the solder material portions  240  until the alignment of the second and first substrates ( 200 ,  100 ) relative to each other is complete. 
     Referring to  FIG. 3B , the array of C4 balls  300  is raised to an elevated temperature that does not exceed the melting temperature of the material of the C4 balls  300 . The temperature of the array of C4 balls  300  can be raised by applying heat through the bonder head  60  to the first substrate  100 , which transmits the heat to the array of C4 balls  300  and the underfill material  310 . Alternately, the temperature of the array of C4 balls can be raised by any other means known in the art. At this step, the array of C4 balls  300  does not make a contact with the solder material portions  140  located on the first substrate  100 . 
     Referring to  FIG. 3C , the first and second substrates ( 100 ,  200 ) are moved closer to each other until a surface of the underfill material  300  contacts the bottommost surface of the first substrate  100 . The relative movement of the first and second substrates ( 100 ,  200 ) can be effected by a vertical downward movement of the first substrate  100 , a vertical upward movement of the second substrate  200 , or a combination thereof. As the temperature of the underfill material  310  approaches the melting point of the solder material of the array of C4 balls  300 , voids  317  form within the underfill material  310  for the reasons discussed above in the first method of bonding substrates. The first and second substrates ( 100 ,  200 ) are moved even closer to each other until the array of C4 balls  300  contact the first substrate  100 , i.e., the solder material portions  140  of the first substrate  100 . After the array of C4 balls  300  contact the first substrate  100 , the array of C4 balls  300  is further heated to a reflow temperature of the solder material of the C4 balls  300 . Upon heating of the array of C4 balls  300  to the reflow temperature of the solder material therein, the solder material of the C4 balls  300  and the solder material of the solder material portions  140  on the first substrate  100  begin to reflow, and the material of each solder material portion  140  is incorporated into a C4 ball  300 . Upon reflow of the array of C4 balls  300 , the second substrate  200  can be moved closer to the first substrate  100  to more effectively incorporate the material of the solder material portions  140  and to increase the solder bonding areas. The voids  317  may grow in size as the temperature of the underfill material  310  increases. 
     The processing steps corresponding to  FIG. 3C  can be performed in a continuous sequence in which the temperature of the underfill material  310  ramps up continuously. For example, the temperature of the array of C4 balls  300  can be ramped up at a rate between 10° C./sec and 100° C./sec from the melting temperature of the solder material of the array of C4 balls  300  to the reflow temperature. Further, the temperature of the array of C4 balls  300  can be ramped up at a rate between 10° C./sec and 100° C./sec from a temperature lower than the melting temperature of the solder material of the array of C4 balls  300 , e.g., from room temperature or any temperature between room temperature and the melting temperature, to the reflow temperature during the bonding steps of  FIG. 3B-3C . 
     Referring to  FIG. 3D , the second substrate  200  can be moved away from the first substrate  100  after the moving of the second substrate  200  closer to the first substrate  100  to provide an optimal and uniform shape for a fillet of the underfill material  310 , i.e., for the portion of the underfill material  310  that laterally extrudes away from the sidewalls of the first substrate  100 . Optionally, the first substrate  100  can be moved away from the second substrate  200  after the moving of the first substrate  100  closer to the second substrate  200  to provide an optimal and uniform shape for the solder joint  300 . Thus, the first and second substrates ( 100 ,  200 ) are bonded through the array of C4 balls  310  and the underfill material  310 . 
     The temperature of the underfill material  310  can be maintained at the reflow temperature for a time period during the processing steps of  FIGS. 3C and 3D  immediately after the temperature ramp up. The time period for maintaining the reflow temperature can be, for example, between 1 second and 3 seconds. Subsequently, the temperature of the underfill material  310  is ramped down at a relatively high temperature ramp rate, which can be between 10° C./sec and 30° C./sec from the reflow temperature to the melting temperature during the bonding. Further, the temperature of the underfill material  310  can be further ramped down to room temperature or a cure temperature, which is higher than room temperature and lower than the melting temperature of the solder material of the array of C4 balls  300 . The combination of the relatively high temperature ramp up rate, the relatively short time period at the reflow temperature, and the relatively high temperature ramp down rate minimizes the exposure of the underfill material  310  to thermal treatment, and thereby minimizes curing of the underfill material due to the heat during the bonding steps of  FIGS. 3B-3D . 
     At the end of the step of  FIG. 3D , at least 15% of the underfill material  310  remains uncured. The percentage of the remaining uncured fraction of the underfill material  310  can be from 15% to 99%, and typically from 20% to 80%. The curing process changes the crosslink density and molecular weight distribution of the underfill material. Thus, the uncured fraction of the underfill material  310  can be analytically measured by determining the residual exotherm of the underfill material  310  at any stage of a curing process. 
     The processing steps of  FIGS. 2G and 2H  are subsequently employed to cure the assembly of the first substrate  100  and the second substrate  200  as in the first method of bonding substrates. 
     Referring to  FIG. 4A , a set of graphs illustrate conditions for a first cure process according to a first embodiment of the present disclosure. At time t 0 , which is herein referred to as a process starting time, the temperature of an assembly of a first substrate  100  and a second substrate  200 , as provided in  FIG. 2F  or in  FIG. 3D , is placed in a pressurization chamber, which can be the enclosure  360  illustrated in  FIG. 2G . Once the pressurization chamber is sealed, the assembly of the first substrate  100  and the second substrate  200  with an underfill material  310  and voids  317  is subjected processing conditions. 
     The pressure of the processing conditions is shown in a pressure curve  400 , and the temperature of the processing conditions is shown in a temperature curve  410 . The pressure of the processing conditions can be modified as shown in an alternate pressure curve  402 , or any curve between the pressure curve  400  and the alternate pressure curve  402 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  411  or any curve between the temperature curve  410  and the alternate initial temperature curve  411 . In other words, the assembly of the first substrate  100  and the second substrate  200  can be placed within the pressurization chamber immediately after bonding or at any time after bonding before the assembly cools down to room temperature. Alternately, the bonding process may be performed within the pressurization chamber to obviate the need to transport the assembly ( 100 ,  200 ) to initiate the cure process. Further, the temperature of the processing conditions can also be modified as shown in an alternate terminal temperature curve  412  or any curve between the temperature curve  410  and the alternate terminal temperature curve  412 . 
     The temperature of the assembly can be ramped from a starting temperature, which is the temperature at t 0 , to a cure temperature at a rate between 1° C./min and 50° C./min. The cure temperature depends on the composition of the underfill material, and is generally between 70° C. and 180° C., and typically between 90° C. and 160° C., although lower and higher temperatures can also be employed. The ambient gas in the pressurization chamber can be dried air, nitrogen, argon, or any other inert gas. 
     Once the temperature of the assembly reaches the cure temperature, pressurization of the pressurization chamber is initiated at time tp, which is herein referred to as a pressurization initiation time. While the temperature is maintained at the cure temperature, the pressure inside the pressurization chamber is ramped to a cure pressure, which is between 1.0 atm and 25 atm, although pressures exceeding 25 atm can also be employed. Typically, the cure pressure is between 1.0 atm and 10 atm. The ramp up rate during pressurization can be from 0.1 atm/min to 3 atm/min, although lesser and greater pressure ramp rates can also be employed. 
     At time ti, which is herein referred to as an initiation time, the pressure reaches the cure pressure. The processing conditions can be held steady until time tf, which is herein referred to as a finish time. The difference between the finish time and the initiation time is herein referred to as a steady state cure time period, which can be from 30 minutes to 24 hours, and typically from 1 hour to 3 hours, although lesser and greater time periods can also be employed. 
     At finish time tf, at least 15% of the underfill material  310  remains uncured. The percentage of the remaining uncured fraction of the underfill material  310  can be from 15% to 99%, and typically from 20% to 80%. The curing process changes the crosslink density and molecular weight distribution of the underfill material. 
     The pressurization chamber is depressurized after the finish time tf by ramping down the pressure to room temperature. The ramp down rate during pressurization can be from 0.1 atm/min to 3 atm/min, although lesser and greater pressure ramp rates can also be employed. The depressurization is completed at time td, which is herein referred to as a depressurization time. 
     The temperature can be ramped down after the depressurization time td or during any time period between the finish time tf and a time tc, which is herein referred to as a completion time. The processing conditions at the completion time tc is room temperature and 1 atm. The assembly of the first substrate  100  and the second substrate  200  can be removed from the pressurization chamber. 
     Referring to  FIG. 4B , a set of graphs illustrate conditions for a variation of the first cure process according to the first embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  420 , and the temperature of the processing conditions is shown in a temperature curve  430 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  431  or any curve between the temperature curve  430  and the alternate initial temperature curve  431 . In other words, the assembly of the first substrate  100  and the second substrate  200  can be placed within the pressurization chamber immediately after bonding or at any time after bonding before the assembly cools down to room temperature. Alternately, the bonding process may be performed within the pressurization chamber to obviate the need to transport the assembly to initiate the cure process. Further, the temperature of the processing conditions can also be modified as shown in a first alternate terminal temperature curve  432 , in a second alternate terminal temperature curve  434 , or any curve between the temperature curve  430  and the second alternate terminal temperature curve  434 . In this variation, the beginning of the cool down of the temperature precedes the beginning of the depressurization. The temperature of the assembly is cooled down to room temperature at time tcd, which is herein referred to as a end-of-cool-down time, which precedes, or is simultaneous with, the end of depressurization that occurs at the completion time tc. 
     Referring to  FIGS. 4C and 4D , C-mode scanning acoustic microscopy (CSAM) pictures are shown for an underfill material in a bonded chip assembly before and after a 2 hour long exposure to a 2 atm pressurized environment at 150° C.  FIG. 4C  is the CSAM picture after bonding and cool down to a room temperature and before the cure process employing pressurization at 150° C.  FIG. 4D  is the CSAM picture after the cure process employing pressurization.  FIGS. 4C and 4D  demonstrate that subjecting an underfill material including voids therein to a pressurized environment up to 2 atm at 150° C. significantly reduces the volume of the voids in the underfill material, and is thus effective in removing the voids within the underfill material. 
     Referring to  FIG. 5A , a set of graphs illustrate conditions for a second cure process according to a second embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  500 , and the temperature of the processing conditions is shown in a temperature curve  510 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  511  or any curve between the temperature curve  510  and the alternate initial temperature curve  511 . Further, the temperature of the processing conditions can also be modified as shown in an alternate terminal temperature curve  512  or any curve between the temperature curve  510  and the alternate terminal temperature curve  512 . The pressure of the processing conditions can be modified as shown in an alternate pressure curve  502  or any curve between the pressure curve  500  and the alternate pressure curve  502 . 
     In the second cure process, the single cure temperature employed in the first cure process is modified to include a plurality of cure temperatures with temperature ramps therebetween during pressurization. Thus, the underfill material is cured at a plurality of cure temperatures greater than room temperature that can be different from one another. For example, if the plurality of cure temperatures includes two cure temperatures, the two cure temperatures can be a first cure temperature and a second cure temperature. To cause more flow of the underfill material while the viscosity of the underfill material is higher, i.e., while a greater fraction of the underfill material remains uncured, the sequence of the plurality of cure temperatures can be selected so that curing steps at lower temperatures precede curing steps at higher temperatures. 
     Each of the plurality of cure temperatures depends on the composition of the underfill material, and can be generally between 70° C. and 180° C., and typically between 90° C. and 160° C., although lower and higher temperatures can also be employed. The ambient gas in the pressurization chamber can be dried air, nitrogen, argon, or any other inert gas. At finish time tf, at least 15% of the underfill material  310  remains uncured. The percentage of the remaining uncured fraction of the underfill material  310  can be from 15% to 99%, and typically from 20% to 80%. Other processing parameters can be substantially the same as in the first cure process. 
     Referring to  FIG. 5B , a set of graphs illustrate conditions for a variation of the second cure process according to the second embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  520 , and the temperature of the processing conditions is shown in a temperature curve  530 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  531  or any curve between the temperature curve  530  and the alternate initial temperature curve  531 . In other words, the assembly of the first substrate  100  and the second substrate  200  can be placed within the pressurization chamber immediately after bonding or at any time after bonding before the assembly cools down to room temperature. Alternately, the bonding process may be performed within the pressurization chamber to obviate the need to transport the assembly to initiate the cure process. Further, the temperature of the processing conditions can also be modified as shown in a first alternate terminal temperature curve  532 , in a second alternate terminal temperature curve  534 , or any curve between the temperature curve  530  and the second alternate terminal temperature curve  534 . 
     Referring to  FIGS. 5C and 5D , CSAM pictures are shown for an underfill material in a bonded chip assembly before and after a multi-temperature cure process employing a 1 hour long exposure to a 2 atm pressurized environment at 105° C. followed by a 1 hour long exposure to a 2 atm pressurized environment at 150° C.  FIG. 5C  is the CSAM picture after bonding and cool down to a room temperature and before the multi-temperature cure process.  FIG. 5D  is the CSAM picture after the multi-temperature cure process.  FIGS. 5C and 5D  demonstrate that subjecting an underfill material including voids therein to this multi-temperature cure process significantly reduces the volume of the voids in the underfill material, and is thus effective in removing the voids within the underfill material. 
     Referring to  FIGS. 5E and 5F , CSAM pictures are shown for an underfill material in a bonded chip assembly before and after a multi-temperature cure process employing a 1 hour long exposure to a 2 atm pressurized environment at 125° C. followed by a 1 hour long exposure to a 2 atm pressurized environment at 150° C.  FIG. 5E  is the CSAM picture after bonding and cool down to a room temperature and before the multi-temperature cure process.  FIG. 5F  is the CSAM picture after the multi-temperature cure process.  FIGS. 5E and 5F  demonstrate that subjecting an underfill material including voids therein to this multi-temperature cure process almost eliminates the voids in the underfill material, and is thus effective in removing the voids within the underfill material. 
     Referring to  FIG. 6A , a set of graphs illustrate conditions for a third cure process according to a third embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  600 , and the temperature of the processing conditions is shown in a temperature curve  610 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  611  or any curve between the temperature curve  610  and the alternate initial temperature curve  611 . Further, the temperature of the processing conditions can also be modified as shown in an alternate terminal temperature curve  612  or any curve between the temperature curve  610  and the alternate terminal temperature curve  612 . The pressure of the processing conditions can be modified as shown in an alternate pressure curve  602  or any curve between the pressure curve  600  and the alternate pressure curve  602 . 
     In the third cure process, the single cure pressure employed in the first cure process is modified to include a plurality of cure pressures with pressure ramps therebetween during pressurization. Thus, the underfill material is cured at a plurality of cure pressures greater than 1 atm. The plurality of cure pressures can be different from one another. For example, if the plurality of cure pressures includes two cure pressures, the two cure pressures can be a first cure pressure and a second cure pressure. To cause more flow of the underfill material while the viscosity of the underfill material is higher, the sequence of the plurality of cure pressures can be selected so that curing steps at lower pressures precede curing steps at higher pressures. 
     Each of the plurality of cure pressures depends on the composition of the underfill material, and can be generally between 1 atm and 25 atm, and typically between 1.5 atm and 10 atm, although higher pressures can also be employed. At finish time tf, at least 15% of the underfill material  310  remains uncured. The percentage of the remaining uncured fraction of the underfill material  310  can be from 15% to 99%, and typically from 20% to 80%. Other processing parameters can be substantially the same as in the first or second cure process. 
     Referring to  FIG. 6B , a set of graphs illustrate conditions for a variation of the third cure process according to the third embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  620 , and the temperature of the processing conditions is shown in a temperature curve  630 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  631  or any curve between the temperature curve  630  and the alternate initial temperature curve  631 . Further, the temperature of the processing conditions can also be modified as shown in a first alternate terminal temperature curve  632 , in a second alternate terminal temperature curve  634 , or any curve between the temperature curve  630  and the second alternate terminal temperature curve  634 . 
     Referring to  FIG. 7A , a set of graphs illustrate conditions for a fourth cure process according to a fourth embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  700 , and the temperature of the processing conditions is shown in a temperature curve  710 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  711  or any curve between the temperature curve  710  and the alternate initial temperature curve  711 . Further, the temperature of the processing conditions can also be modified as shown in an alternate terminal temperature curve  712  or any curve between the temperature curve  710  and the alternate terminal temperature curve  712 . The pressure of the processing conditions can be modified as shown in an alternate pressure curve  702  or any curve between the pressure curve  700  and the alternate pressure curve  702 . 
     In the fourth cure process, the single cure temperature employed in the first cure process is modified to include a plurality of cure temperatures with temperature ramps therebetween during pressurization as in the second cure process described above. Thus, the underfill material is cured at a plurality of cure temperatures greater than room temperature that can be different from one another. Further, the single cure pressure employed in the first cure process is modified to include a plurality of cure pressures with pressure ramps therebetween during pressurization as in the third cure process described above. Thus, the underfill material is cured at a plurality of cure pressures greater than 1 atm. The plurality of cure pressures can be different from one another. The transition time between different cure temperatures and the transition time between different cure pressures can be coincident or different. 
     Referring to  FIG. 8A , a set of graphs illustrate conditions for a fifth cure process according to a fifth embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  800 , and the temperature of the processing conditions is shown in a temperature curve  810 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  811  or any curve between the temperature curve  810  and the alternate initial temperature curve  811 . Further, the temperature of the processing conditions can also be modified as shown in an alternate terminal temperature curve  812  or any curve between the temperature curve  810  and the alternate terminal temperature curve  812 . The pressure of the processing conditions can be modified as shown in an alternate pressure curve  802  or any curve between the pressure curve  800  and the alternate pressure curve  802 . 
     In the fifth cure process, the constant cure pressure employed in the first cure process is modified to include at least one pressure cycle in addition to, or in lieu of, a constant pressure step. For example, the pressure in the pressurization chamber during the at least one pressure cycle can be ramped up and down repeated between a first pressure and a second pressure that is greater than the first pressure. In each pressure cycle, the underfill material is subjected to a variable pressure between the first cure pressure and the second cure pressure greater than the first pressure. The first pressure can be 1 atm or can be a pressure greater than 1 atm. The second pressure is greater than 1 atm. The first pressure and the second pressure are selected from a pressure range between 1 atm and 25 atm, and typically between 1 atm and 10 atm, although greater pressures can also be employed. The pressure cycling can be, but does not necessarily need to be, followed a constant pressure step. The pressure at the constant pressure step can be the second pressure, or can be a pressure greater or lesser than the second pressure. The effectiveness of the pressure cycling is derived from the forced movement of the incompletely cured underfill material, which includes at least 15% of uncured fraction remaining prior to the final curing. The repeated increase and decrease of the pressure on the underfill material forces the voids to move out of the underfill material while the underfill material maintains some viscosity, i.e., before the underfill material is completely cured, which occurs toward the end of the cure process. 
     Referring to  FIG. 8B , a set of graphs illustrate conditions for a variation of the fifth cure process according to the fifth embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  820 , and the temperature of the processing conditions is shown in a temperature curve  830 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  831  or any curve between the temperature curve  830  and the alternate initial temperature curve  831 . Further, the temperature of the processing conditions can also be modified as shown in a first alternate terminal temperature curve  832 , in a second alternate terminal temperature curve  834 , or any curve between the temperature curve  830  and the second alternate terminal temperature curve  834 . 
     The fifth cure process can be combined with any of the first through fourth cure processes as needed, i.e., to include features such as multi-cure-temperature feature and/or multi-cure-pressure feature. For example, a sixth cure process can employ the pressure profile of the fifth embodiment and the temperature profile of the second or fourth cure process. 
     Referring to  FIG. 9A , a set of graphs illustrate conditions for the sixth cure process according to a sixth embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  900 , and the temperature of the processing conditions is shown in a temperature curve  910 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  911  or any curve between the temperature curve  910  and the alternate initial temperature curve  911 . Further, the temperature of the processing conditions can also be modified as shown in an alternate terminal temperature curve  912  or any curve between the temperature curve  910  and the alternate terminal temperature curve  912 . The pressure of the processing conditions can be modified as shown in an alternate pressure curve  902  or any curve between the pressure curve  900  and the alternate pressure curve  902 . The at least one pressure cycle can achieve the same effect as in the fifth cure process. The underfill material can be subjected to at least one pressure cycles prior to the curing of the underfill material at a cure temperature, e.g., the second cure temperature. 
     Referring to  FIG. 9B , a set of graphs illustrate conditions for a variation of the sixth cure process according to the sixth embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  920 , and the temperature of the processing conditions is shown in a temperature curve  930 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  931  or any curve between the temperature curve  930  and the alternate initial temperature curve  931 . Further, the temperature of the processing conditions can also be modified as shown in a first alternate terminal temperature curve  932 , in a second alternate terminal temperature curve  934 , or any curve between the temperature curve  930  and the second alternate terminal temperature curve  934 . 
     In general, the temperature can be ramped up or down during a pressurized cure step at a constant pressure greater than 1.0 atm or during at least one pressure cycling. For example, the temperature can be ramped up to a cure temperature during at least one pressure cycling in a seventh cure process. 
     Referring to  FIG. 10A , a set of graphs illustrate conditions for the seventh cure process according to a seventh embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  1000 , and the temperature of the processing conditions is shown in a temperature curve  1010 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  1011  or any curve between the temperature curve  1010  and the alternate initial temperature curve  1011 . Further, the temperature of the processing conditions can also be modified as shown in an alternate terminal temperature curve  1012  or any curve between the temperature curve  1010  and the alternate terminal temperature curve  1012 . The pressure of the processing conditions can be modified as shown in an alternate pressure curve  1002  or any curve between the pressure curve  1000  and the alternate pressure curve  1002 . The at least one pressure cycle can achieve the same effect as in the fifth cure process. 
     The underfill material is subjected to a gradual temperature ramp during at least one pressure cycling. The ramp up rate for the temperature can be from 0.3° C./min to 5° C./min, although lesser and greater ramp up rates can also be employed. The gradual ramping of the temperature during the at least one pressure cycle utilizes the relatively high viscosity of the underfill material that is present while a lesser fraction of the underfill material is cured. Further, the underfill material can be subjected to at least one pressure cycles prior to the curing of the underfill material at the cure temperature. 
     Referring to  FIG. 10B , a set of graphs illustrate conditions for a variation of the seventh cure process according to the seventh embodiment of the present disclosure. The pressure of the processing conditions is shown in a pressure curve  1020 , and the temperature of the processing conditions is shown in a temperature curve  1030 . The temperature of the processing conditions can be modified as shown in an alternate initial temperature curve  1031  or any curve between the temperature curve  1030  and the alternate initial temperature curve  1031 . Further, the temperature of the processing conditions can also be modified as shown in a first alternate terminal temperature curve  1032 , in a second alternate terminal temperature curve  1034 , or any curve between the temperature curve  1030  and the second alternate terminal temperature curve  1034 . 
     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. 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.