Abstract:
A system to imprint patterns on impressionable materials by generating a pressure differential within an imprinting chamber by creating a substantial vacuum in an imprinting area is provided. This system can be used to create conductive traces in a substrate onto which integrated circuit chips and dies can be mounted to create semiconductor packages. A low pressure line evacuates air from a material receiving area of a vessel creating a pressure differential across pistons in the vessel thereby causing the pistons to press microtools into impressionable material layers. The low pressure line helps the microtools conform to any thickness variations in the imprinted material and prevents air pockets from developing between the microtool and the imprinted material.

Description:
TECHNICAL FIELD 
   This disclosure relates to substrates on which semiconductor chips and dies can be mounted and, more specifically, to a system and method for creating connection patterns in such substrates using vacuum generated imprinting. 
   BACKGROUND 
   Semiconductor integrated circuit chips and dies are relatively fragile. To protect chips and dies from damage, they are typically confined in a semiconductor package. For example, Ball Grid Array (BGA) packages typically include at least one integrated circuit chip or die mounted and electrically connected to a substrate with conductive trace lines in the substrate connecting the chip or die to electrical contacts on the bottom surface of the substrate. The chip and the substrate are then encapsulated with resin to protect the chip while leaving the electrical contacts on the bottom surface of the substrate exposed. 
   Substrates used in semiconductor packages can be made from various materials, typically insulators, including ceramic, plastic, and organic, for instance. Electrical trace lines are integrated into or onto the substrate to provide proper power and signals paths for the chip. One way to form electrical traces is illustrated in  FIG. 1 . 
   In  FIG. 1 , an imprinting system  10  is illustrated. The system  10  includes upper and lower platens  12  and  14  that provide pressure for upper and lower rigid microtools  22  and  24 , respectively. The microtools  22  and  24  each include several embossings  25  that project from a base of the corresponding microtool. A substrate  30  is covered on two surfaces by impressionable material layers  32  and  34 , which can be made of uncured thermal-setting epoxy. As illustrated in  FIG. 2 , in operation, the microtools  22  and  24  are each pressed by the platens  12  and  14  into the impressionable material layers  32  and  34 . Embossings  25  on the microtools  22  and  24  leave impressions in the material layers  32  and  34 . The material layers  32  and  34  are then cured. After the microtools  22  and  24  are removed, the impressions remain in the material layers  32  and  34 . 
   The impressions are later filled with an electrically conductive material, such as copper or gold metal, and machined or otherwise processed to provide the electrical traces in the substrate. 
   After a first set of material layers  32  and  34  is imprinted and the electrical traces created in the layer, another set of material layers of impressionable material (not shown) can be deposited over the first and the cycle can be repeated, resulting in a multi-layer substrate. 
   Although the platens  12  and  14  are typically compliant or otherwise float to match imperfections of the substrate  30  and the impressionable material layers  32  and  34 , the microtools  22  and  24  shown in  FIGS. 1 and 2  are relatively rigid. With reference to  FIG. 2 , when the microtools  22  and  24  are pressed into the impressionable material layers  32  and  34 , the embossings  25  do not always imprint to a consistent depth. Because of the imperfections in the substrate  30  and impressionable material layers  32  and  34 , known as Total Thickness Variation (TTV), some of the impressions left in the material layers  32  and  34  after the microtools  22  and  24  are removed are relatively shallow or non-existent, while other impressions are relatively deep. This variation in impression depth can cause problems when the impressions are later filled with the conductive material machined to make the electrical traces in the substrate. 
   During processing of the impressions, the conductive material filling some of the more shallow impressions can be completely or mostly removed, either of which will cause inferior or inoperative electrical connections with the chip or die to be mounted on the substrate. 
   Another problem with the imprinting process as described is that air or other gasses can be trapped in the material layers  32  and  34 , depending on the sequence of events, due to out-gassing of the imprint materials and air pockets formed between the microtools  22 ,  24  and the material layers  32  and  34 . 
   A soft tooling pressing system is illustrated in  FIGS. 3-5 . In  FIG. 3 , a system  40  includes an upper heater  42 , lower heater  52 , an upper microtool  44  and a lower microtool  54 . Differently from the above embodiment, the microtools  44  and  54  are soft tools and have a degree of flexibility and conformity. Soft tool microtools have the ability to conform to variations in the thickness of the impressionable layers. Soft tool microtools can be made from nickel which is relatively flexible. The views of the flexing of the soft microtools  44  and  54  in  FIGS. 3-5  are exaggerated for illustration purposes. The substrate  60  can be covered by impressionable materials on both sides. 
   In a first operation, illustrated in  FIG. 4 , the microtools  44  and  54  are held in place by a vacuum generated to pull the microtools toward the heaters  42  and  52 . As illustrated in  FIG. 3 , this vacuum action causes a deformation in the soft microtools  44  and  54 . Such vacuum action also creates air pockets  70  (shown in  FIG. 5 ) between the microtools  44  and  54  and the substrate  60 . 
   In a next step, imprint pressure is applied to the backsides of the microtools  44  and  54 , as illustrated in  FIG. 5 . As the imprint pressure is applied, the air pockets  70  are partially dissipated, but some of the air from the pockets  70  is forced into the impressionable material on the substrate  60 . Other portions are not forced into the material but instead create fluid back pressure that presses on the front sides of the microtools  44  and  54 , which can prevent the imprint regions of the microtools from fully pressing into the impressionable material. As described above, this causes connection problems when the impressions are filled and processed into electrical connection lines. 
   Additionally, because edges of the microtools are clamped in position during the impression period, boundary conditions exist around the microtools  44  and  54  that prevent the tools from ever being able to possibly be perfectly flat and that may adversely affect the impression depth and uniformity. 
   Embodiments of the invention address these and other disadvantages in the prior art. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The description may be best understood by reading the disclosure with reference to the accompanying drawings. 
       FIGS. 1 and 2  are cross-sectional views of a substrate impression system. 
       FIGS. 3-5  are cross-sectional views of another substrate impression system. 
       FIG. 6  is a cross-sectional diagram of an inventive material imprinting system according to an embodiment. 
       FIG. 7  is a cross-sectional diagram of an inventive material imprinting system according to another embodiment. 
       FIGS. 8-10  are cross-sectional diagrams illustrating an imprinting process performed by the imprinting system of  FIG. 6 . 
       FIG. 11  is a detailed cross-sectional view of the material imprinting system of  FIG. 6  showing soft microtools. 
       FIG. 12  is a detailed cross-sectional view of the material imprinting system of  FIG. 6  showing dished microtools. 
   

   DETAILED DESCRIPTION 
     FIG. 6  is a cross-sectional diagram illustrating an inventive material imprinting system  100  according to an embodiment of the invention. The system  100  includes a chamber vessel  110  formed of a material such as steel that is strong enough to support various pressures and vacuums that will be generated within the vessel. 
   Two or more vent/pressure lines  112  extend through walls of the vessel  110  and at least one low pressure (vacuum) line  116  also extends through the walls of the vessel  110 . 
   In the illustrated embodiment, the vent/pressure lines  112  are located near the ends of the vessel  110 , while the vacuum line  116  is located near the center of the vessel adjacent to the material receiving area  117 . 
   An upper piston  122  and a lower piston  124  travel within the vessel based on a pressure differential in the vessel, as described below. Adjacent to the pistons  122  and  124  are microtools  132  and  134 , which can be similar to those described above, each having embossings in imprint regions (not illustrated in  FIG. 6 ) on a face opposite the pistons  122  and  124 . 
   A substrate  140  is positioned in the material receiving area  117 . The substrate  140  includes impressionable material layers  142  and  144  on opposing sides, but not all embodiments require two-sided substrates. Embodiments of the invention are equally applicable to substrates  140  having a single impressionable material layer as shown in  FIG. 7 . 
   As described below, the microtools  132  and  134  may be made of soft tooling that can conform around TTV variations in the material layers  142  and  144 , or the microtools may be made out of relatively stiff materials. 
   The pistons  122  and  124  may be made of traditional rigid materials, or may be relatively soft. In some embodiments the pistons  122  and  124  may be formed of a bladder or highly elastic fixed membrane such as rubber or some other flexible polymer that can deform and apply pressure to the microtools  132  and  134 . The pistons  122  and  124 , or seals around the pistons (not shown) may be sealed air tight to the inside surface of the vessel  110 , to ensure adequate vessel pressure. 
   The vessel  110  may also include a sealed door (not shown) to access the material receiving area  117  for inserting and removing the substrate  140 . 
     FIG. 7  is a cross-sectional diagram illustrating a material imprinting system  200 . The system  200  includes a chamber vessel  210  formed of material strong enough to support various pressures and vacuums that will be generated within the vessel. At least one vent/pressure line  212  extends through the walls of the vessel  210  and at least one low pressure (vacuum) line  216  extends through the wall of the vessel  210 . 
   In  FIG. 7 , a substrate  240  is positioned in the material receiving area  217 . The substrate  240  has an impressionable material layer  242  on one side facing the upper piston  222 . The upper piston  222  travels vertically responsive to a pressure differential within the vessel  210 . 
   Adjacent to the piston  222  is microtool  232 , which can be similar to those described in  FIG. 6 . The microtool  232  has an imprint region (not illustrated in  FIG. 7 ) on a face opposite the piston  222 . 
   Referring to the operation of the imprinting system  100  of  FIG. 6  as illustrated in  FIG. 8 , the vessel  110  is filled with a gas such as ambient air or an inert gas. Gas pressures within the vessel  110  can be controlled through the vent/pressure lines  112  and the low pressure line  116  to cause pressure differentials on either side of the pistons  122  and  124 . 
     FIG. 9  shows the pressure being reduced near the center of the vessel, i.e., in the material receiving area  117 . The low pressure vacuum line  116  evacuates air and other gasses out of the receiving area  117  reducing the pressure in the receiving area to substantially create a vacuum pressure. The pressure differential forces the pistons  122 ,  124  toward the center of the vessel. 
   In one embodiment, the vent/pressure lines  112  vent the regions  113  to atmospheric pressure and the pressure differential is created by the low pressure vacuum line  116  reducing the pressure in the material receiving area  117 . In another embodiment, the vent/pressure lines  112  supply positive pressure to the regions  113  thereby increasing the pressure differential and thereby increasing the pressure of the microtools  132  and  134  on the impressionable material layers  142  and  144 . 
   As shown in  FIG. 9 , when the pressure differential increases, the pistons  122  and  124  deform and move to press the microtools  132  and  134  into the impressionable material layers  142  and  144 . 
   When pressure is reduced in the material receiving area  117 , gas from this central region is being voided from the vessel  110 . Thus, gas within the vessel  110  is removed in the area where the impression is being made. This gas removal has a dual benefit of preventing air or other gas from being forced into the impressionable material layers  142  and  144 , while also preventing any gas pockets from forming that may prevent the microtools  132  and  134  from fully impressing into the impressionable material layers  142  and  144 . 
   Once the microtools  132  and  134  reach an adequate pressure against the impressionable material layers  142  and  144 , a heater (not shown) heats the impressionable material layers  142  and  144  curing and thereby imprinting patterns of the microtools  132  and  134  in the layers  142  and  144 . 
   A similar process would apply to the system  200  illustrated in  FIG. 7  with the single piston  222  applying pressure to the microtool  232  to imprint the single impressionable layer  242 . 
   As illustrated in  FIG. 11 , the pistons  122  and  124  can be made from a somewhat pliable material such as rubber or some other flexible polymer. Because the microtools  132  and  134  may also be formed from pliable materials, the microtools  132 ,  134  can flex and conform to the surface of the substrate  140  and impressionable material layers  142  and  144 . Thus, by using such a system, uniform impressions across the entire substrate  140  are made by the microtools  132  and  134 . 
   The pistons  122  and  124  may be made from any material that can generate the force within the vessel  110  to press the microtools  132  and  134  into the impressionable material layers  142  and  144 . Preferably, the pistons  122  and  124  are soft enough to conform (or partially conform) to the microtools  132  and  134 , yet hard enough to provide adequate pressure to make a good impression. A hardened rubber or gas-filled bladder can provide pliable conformance and adequate pressure. 
   In a particular embodiment illustrated in  FIG. 12 , the microtools  132  and  134  are somewhat dished such that a central portion of the microtool touches its respective impressionable material layer before the outer regions do. In such an embodiment, gasses that may otherwise be trapped can be extracted by the vacuum line  116 . In other words, as the central portion of the microtool  132  touches the central portion of the impressionable material layer  142 , gasses are being extracted by the vacuum line. By having the outer edges of the microtool  132  touch the material layer  142  last, gasses from inside the vessel  110  would avoid being trapped between the microtool  132  and the material layer  142 .  FIG. 12  shows two microtools  132 ,  134 , however, a single dished microtool could also be used in an embodiment similar to the imprinting system  200  shown in  FIG. 7 . 
   Further, in  FIG. 12  only one vacuum line  116  is illustrated, but many such vacuum lines  116  could be placed around the perimeter of the vessel  110  to extract undesirable gasses during production. An additional benefit to including the vacuum lines  116  near the impressionable material layers  142  and  144  is that any outgassing from the layers can be removed by the vacuum line  116 . 
   Actual pressures to be used within the vessel  110  would depend on the types materials used for the impressionable material layer and for the pistons  122  and  124 . In one embodiment, the imprint pressure of the microtools  132  and  134  on the material layers  142 ,  144  need only be about  29  psi to achieve adequate impressions in the material layers  142 ,  144 . 
   The preceding embodiments are exemplary. Those of skill in the art will recognize that the concepts taught herein can be tailored to a particular application in many other advantageous ways. In particular, those skilled in the art will recognize that the illustrated embodiments are but one of many alternative implementations that will become apparent upon reading this disclosure. 
   Although the specification may refer to “an”, “one”, “another”, or “some” embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment.