Patent Publication Number: US-2006000641-A1

Title: Laser metallization for ceramic device

Description:
FIELD OF THE INVENTION  
      The present invention is related to formation of structures associated with semiconductor devices. More specifically, the present invention relates to methods and apparatus for forming a ceramic device.  
     BACKGROUND OF THE INVENTION  
      Integrated circuits have been manufactured for many years. Manufacturing integrated circuits involves integrating various active and passive circuit elements into a piece of semiconductor material, referred to as a die. The die is encapsulated into a ceramic or plastic package. In some applications, these packages are directly attached to a printed circuit board by connecting pins, which are arranged along the periphery of the package. An electronic system can be formed by connecting various integrated circuit packages to a printed circuit board.  
      As advances in semiconductor manufacturing technology led to substantially increased numbers of transistors on each integrated circuit, it became possible to correspondingly increase the functionality of each integrated circuit. In turn, increased functionality resulted in the need to increase the number of input/output (I/O) connections between the integrated circuit and the rest of the electronic system of which the integrated circuit was a part. One adaptation designed to address the increased need for I/O connections was to simply add additional pins to the package. Unfortunately, adding pins to the package increased the area consumed by the package.  
      A further adaptation designed to address the increased need for I/O connections without consuming an unacceptably large amount of area was the development of ball grid array (BGA) packages. BGA packages include a plurality of solder bumps formed by a process commonly referred to as controlled collapsed chip connection (C 4 ). In such a package, a large number of I/O connection terminals are disposed in a two dimensional array over a substantial portion of a major surface of the package. In some instances, BGA packages are directly attached to a supporting substrate such as a printed circuit board. In other instances, an interposer is directly attached to the printed circuit board, and the BGA package is attached to the interposer. The interposer includes routing traces and vias that connect the solder bumps of the BGA to contacts that are attached to the printed circuit board. The interposer “fans-out” the relatively small die pad pitch of the integrated circuit to the larger contact pad pitch of the printed circuit board. In many applications, the interposer material has a coefficient of thermal expansion value between the value of the coefficient of thermal expansion of the printed circuit board and the value of the coefficient of thermal expansion of the BGA package. The interposer, therefore, reduces mechanical stress induced by different coefficients in thermal expansion between the package and the printed circuit board. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The invention is pointed out with particularity in the appended claims. However, a more complete understanding of the present invention may be derived by referring to the detailed description of some of the embodiments when considered in connection with the figures, wherein like reference numbers refer to similar items throughout the figures, and:  
       FIG. 1  is a top view of a printed circuit board having a package with an interposer attached to the printed circuit board, according to an embodiment of the invention.  
       FIG. 2  illustrates a schematic cross-sectional view of a package having a die, an interposer and a substrate, according to an embodiment of this invention.  
       FIG. 3  illustrates a schematic cross-sectional view of an interposer or electrical device, according to an embodiment of this invention.  
       FIG. 4  illustrates a schematic exploded side view of an interposer or electrical device having a plurality of layers, according to an embodiment of this invention.  
       FIG. 5  illustrates a schematic side view of an interposer or electrical device with multiple laminated layers, according to an embodiment of this invention.  
       FIG. 6  illustrates a schematic side view of an interposer or electrical device with multiple laminated layers having an opening formed in the interposer, according to an embodiment of this invention.  
       FIG. 7  illustrates a schematic side view of an interposer or electrical device with multiple laminated layers having an opening therein, according to an embodiment of this invention.  
       FIG. 8  illustrates a schematic side view of an opening in the interposer or electrical device undergoing laser treatment, according to an embodiment of this invention.  
       FIG. 9  illustrates a schematic cross-sectional view of an opening in the interposer or electrical device after laser treatment, according to an embodiment of this invention.  
       FIG. 10  illustrates a schematic cross-sectional view of an opening in the interposer or electrical device substantially filled with high conductivity paste, according to an embodiment of this invention.  
       FIG. 11  illustrates a flow diagram of the fabrication of the interposer, according to an embodiment of this invention.  
       FIG. 12  illustrates a cross-sectional view of a laminated and sintered ceramic interposer or electrical device, undergoing laser ablation according to an embodiment of this invention.  
       FIG. 13  illustrates a schematic side view of an opening being formed in the interposer or electrical device by laser ablation, according to an embodiment of this invention.  
       FIG. 14  illustrates a flow diagram of the fabrication of an interposer or electrical device, according to an embodiment of this invention.  
       FIG. 15  is a cross-sectional schematic diagram of laser forming an electrical trace on the surface of a ceramic device, according to another embodiment of this invention. 
    
    
      The description set out herein illustrates some embodiments of the invention, and such description is not intended to be construed as limiting in any manner.  
     DETAILED DESCRIPTION  
      In the following detailed description of the preferred embodiments, reference is made to the accompanying drawings that form a part hereof, and in which are shown by way of illustration specific embodiments in which the invention can be practiced. The embodiments illustrated are described in sufficient detail to enable those skilled in the art to practice the teachings disclosed herein. Other embodiments can be utilized and derived therefrom, such that structural and logical substitutions and changes can be made without departing from the scope of present inventions. The following detailed description, therefore, is not to be taken in a limiting sense, and the scope of various embodiments of the invention is defined only by the appended claims, along with the full range of equivalents to which such claims are entitled.  
       FIG. 1  is a top view of a printed circuit board  100  having a component with an interposer, according to an embodiment of the invention. The printed circuit board (“PCB”)  100  is a multi-layer plastic board that includes patterns of printed circuits on one or more layers of insulated material. The patterns of conductors correspond to wiring of an electronic circuit formed on one or more of the layers of the printed circuit board  100 . The printed circuit board  100  also includes electrical traces  110 . The electrical traces  110  can be found on an exterior surface  120  of the printed circuit board  100 , and also can be found on the various layers within the printed circuit board  100 . Printed circuit boards also include through holes (not shown in  FIG. 1 ), which are used to interconnect traces on various layers of the printed circuit board  100 . The printed circuit board  100  can also include planes of metallized materials such as ground planes, power planes, or voltage reference planes (not shown in  FIG. 1 ).  
      The printed circuit board  100  is also populated with various components  130 ,  132 ,  134 ,  136 ,  138 . The components  130 ,  132 ,  134 ,  136 ,  138  can either be discrete components or semiconductor chips which include thousands of transistors. The components  130 ,  132 ,  134 ,  136 ,  138  can use any number of technologies to connect to the exterior surface  120  of the printed circuit board  100 . For example, pins may be inserted into plated through holes, or pins may be extended through the printed circuit board  100 . An alternative technology is surface mount technology, where an electrical component, such as component  136 , mounts to an array of pads on the exterior surface  120  of the printed circuit board  100 . For example, component  136  could be a ball grid array package or device, that has an array of balls or bumps that interact or are connected to a corresponding array of pads on the exterior surface  120  of the printed circuit board  100 . The printed circuit board  100  can also include connectors for making external connections to other electrical or electronic devices. The component  136  could be a processing chip or microprocessor.  
      As shown in  FIG. 1 , the printed circuit board  100  includes a first edge connector  140  and a second edge connector  142 . As shown in  FIG. 1  there are external traces, such as electrical trace  110 , on the external surface  120  of the printed circuit board  100 , that connect to certain of the outputs associated with the first edge connector  140 . Other traces that connect with the edge connectors  140 ,  142  will have traces internal to the printed circuit board  100 .  
       FIG. 2  illustrates a schematic cross-sectional view of a package  200  having a die  210 , an interposer  300  and a substrate  220 , according to an embodiment of this invention. The die  210  includes electrical circuitry of various types. One common use for a die  210  is for the circuitry of a microprocessor. The die  210  actually includes layers upon layers of electrical devices, such as transistors and other logic devices. The die  210  includes a number of electrical outputs, such as output  212  and electrical inputs, such as electrical input  214 . As shown in  FIG. 2 , the electrical outputs and the electrical inputs, such as  212 ,  214 , are in a ball grid array. A ball grid array includes an array of solder balls formed on one of the major surfaces of the die  210 . The ball grid array, which includes an output  212  and an input  214 , is also known as a flip chip that includes a number of bumps for the various inputs and outputs of the circuitry within the die  210 . The bumps, such as output  212  and input  214 , provide for a much more dense packing of inputs and outputs for the die  210 .  
      The interposer  300  is connected between the die  210 , and more specifically the inputs and outputs of the die  210 , and the substrate  220 . As shown in  FIG. 3 , the interposer  300  includes a plurality of through holes  310 ,  312 ,  314 ,  316  and  318 . The plurality of through holes  310 ,  312 ,  314 ,  316 ,  318  are filled with high conductivity metallic paste, as depicted by reference numbers  320 ,  322 ,  324 ,  326 ,  328 . The paste  320 ,  322 ,  324 ,  326 ,  328  has substantially no adhesion-promoting fillers, since the through openings  310 ,  312 ,  314 ,  316 ,  318  include a metalized inner surface. The metalized inner surface of the through holes or vias  310 ,  312 ,  314 ,  316 ,  318  lessens or eliminates the need for an adhesion-promoting filler within the metallic paste in the vias or through openings. The interposer  300  also includes a number of sheets or layers  340 ,  342 ,  346 . Although  FIG. 3 , shows five layers or sheets, only three of the layers carry reference numerals. Any number of layers can be used to form the interposer  300 . The interposer  300  is typically formed of a material that has a coefficient of thermal expansion with a value that is between the value of the coefficient of thermal expansion of the die  210  and the value of the coefficient of thermal expansion of the substrate  220  (see  FIG. 2 ). In some embodiments of the interposer  300 , the individual layers may include electrical or conductive traces or pathways. In addition, the exterior major surfaces  350  and  360  of the interposer  300  may include electrical traces which fan out, further separate, or lessen the density of the bumps of the ball grid array associated with the die  210 .  
      Now returning to  FIG. 2 , the substrate  220  also includes solder balls or contact points for solder balls on a surface for connecting the interposer  300  to a corresponding pad  223  on an opposite major surface. An electrical trace  222  connects the contact point  221  and the pad  223  of the substrate  220 . A pin or other output or input device  224  may be in electrical communication with the pad  223 . A solder ball, such as solder ball  321  (see  FIG. 3 ), electrically connects the via  318  and the paste within the via  328  of the interposer  300  with the substrate  220  at an attachment point  221 . As shown in  FIG. 2 , electrical traces, such as electrical trace  222  within the substrate  220 , fan out so that each pin, such as pin  224 , can be further spaced from an adjacent pin. The larger spacing between the pins, such as pin  224 , also provides for a more substantial input or output pin which can be plugged into a connector associated with the printed circuit board  100 . The output of the substrate  220  need not be a pin  224 , as shown in  FIG. 2 . The output of the substrate  220  can be any other number of commonly used electrical outputs, such as solder bumps or the like.  
      Now turning to  FIGS. 4-10 , the process for fabricating an electrical device such as the ceramic interposer  300  will now be discussed.  FIG. 4  illustrates a schematic exploded view of a plurality of substantially ceramic layers that form an interposer  500 , according to an embodiment of this invention. It should be noted that any number of layers can be used to form the electrical device, such as the interposer  500 . As shown in  FIG. 4 , the electrical device includes a first layer  410 , a second layer  412 , a third layer  414 , and a fourth layer  416 . It should be noted that the electrical device, such as the interposer  500 , can be varied in thickness by using either more or fewer layers. Each of the substantially ceramic layers is formed of or includes aluminum nitride. An amount of aluminum nitride (AlN) within the ceramic allows for laser direct metallization of the ceramic. In other words, by directing a laser at the surface or at the ceramic that includes aluminum nitride in an appropriate concentration intensity, a metal surface is formed. The layers  410 ,  412 ,  414 ,  416  can be formed from green, unfired sheets of ceramic, hereinafter referred to as sheets. The sheets  410 ,  412 ,  414 ,  416  come off of rolls of ceramic material, such as those used in a tape-casting process. In other words, the sheets  410 ,  412 ,  414 ,  416  are made of a material which, in the presence of a laser, converts the originally insulative ceramic (such as aluminum nitride AlN) into a conductor, such as a aluminum, by selective photo ablation. A material that converts from an originally insulative state into a conductive state in the presence of a laser of appropriate type, power and wavelength is referred to as a laser active conversion material. Other materials can be included in an appropriate concentration intensity so that in the presence of a laser, a metal surface is formed. Other such materials include silicon carbide (SiC) and silicon nitride (Si 3 N 4 ) and similar materials placed within a ceramic. The green, unfired sheets  410 ,  412 ,  414 ,  416  are malleable.  
       FIG. 5  illustrates a schematic side view of an electrical device or interposer  500  in which the various layers  410 ,  412 ,  414 ,  416  have been laminated. The number of green, unfired sheets used  410 ,  412 ,  414 ,  416  produces a device or interposer  500  of a required or selected thickness. The number of sheets used can be varied to vary the thickness of the electrical device or interposer.  
       FIG. 6  illustrates a schematic side view of an interposer or electrical device with multiple laminated layers, as an opening is being formed in the device or interposer  500 , according to an embodiment of this invention. As shown in  FIG. 6 , the electrical device having multiple layers  410 ,  412 ,  414 ,  416  is in the process of mechanical punching of the green, unfired sheets to form a through hole or via within the electrical device  500 . A punch  600 , as shown in  FIG. 6 , travels in a direction  610  relative to the electrical device or interposer  500 . The punch  600  has progressed through layer  410  and slightly into layer  412  of the interposer  500 .  
       FIG. 7  illustrates a schematic side view of an interposer or electrical device  500  having a through hole or via  720  formed therein. The through hole or via  720  extends through each of the layers  410 ,  412 ,  414 ,  416 . After the through hole or via  720  is punched or formed by the mechanical punch  600  (see  FIG. 6 ), the electrical device or interposer  500  with the through hole  720  is sintered. The sintering process heats the ceramic, laminated sheets  410 ,  412 ,  414 ,  416  to form a hard, ceramic device  500  having a through hole  720 . Sintering of the punched ceramic device  500  essentially fires the ceramic. Forming the through hole or via  720  using the punch  600  on green sheets of ceramic  410 ,  412 ,  414 ,  416  (see  FIG. 6 ) permits the vias or through holes  720  to be formed accurately and easily when compared to forming through holes in a ceramic device that has been fired.  
       FIG. 8  illustrates a schematic view of an opening or via  720  within the electronic device or ceramic interposer  500  undergoing laser treatment, according to an embodiment of this invention. As shown in  FIG. 8 , a laser beam  800  having a diameter, D, and a focal length, FL, is applied to the via or opening  720  in the electrical device or interposer. The laser has a depth of focus, DF. The focal plane  810  occurs approximately midway along the length of the via or through opening and can be adjusted along any specific location according to the specific design of a given device  720 . Laser interaction with the ceramic interposer or ceramic device  500  forms a laser direct metalized phase along the length of the through hole or via  720 . In other words, a direct-metalized phase is formed on the walls  820  of the through hole or via  720 . The laser is applied to the walls  820  of the through opening or via  720  with a sufficient intensity to form the metallic conductive phase along the through hole wall. The laser induced metallic phase is embedded at the surface of the laser-metalized ceramic and acts as an inner connect line or conductive trace along the walls or in the wall  820  of the via or through hole  720 .  
       FIG. 9  illustrates a schematic cross sectional view of an opening  720  after the laser treatment or selective laser irradiation of the opening  720 , according to an embodiment of this invention. As shown in  FIG. 9 , the walls  820  near the opening or through hole or via  720  are rich with aluminum. The aluminum-rich walls act as a conductor embedded within the walls  820  of the through opening or via  720 .  
       FIG. 10  is a schematic cross sectional view of an opening  720  substantially filled with a high conductivity paste  1000 . The high conductivity paste  1000  is devoid or substantially devoid of adhesion-promoting filler. When adhesion-promoting fillers are used, the electrical properties of the paste plug  1000  are less desirable than the high-conductivity paste  1000  which is shown in  FIG. 10 . The adhesion properties between the conductive filler paste  1000  and the ceramic interposer or device  500  are enhanced by creating the metallic conductive phase along the wall  820  of the through hole or via  720 . This either allows for elimination of an adhesive promoting agent or substantial reduction in an adhesive promoting agent in the conductive paste. As a result, the high conductivity paste has enhanced electrical conductivity properties, when compared to filler pastes that have adhesion-promoting fillers. The improved adhesion between the high conductivity filler paste, such as silver-based paste, and the laser-metalized wall  820  of the through opening  720 , eliminates or reduces the need for filler particles in the providing conductive paste.  
      Additionally, the formation of the metalized phase in the wall  820  of the through opening  720  decouples the curing of the conductive paste step from the sintering step. It should be noted that in another embodiment of this invention, rather than using aluminum nitride within the ceramic, other ceramics can also be used. For example, silicon carbide (SiC) can also be used. The selective laser irradiation of the opening results in a metal or semi metal species of silicon in the walls  820  near the opening  720  of the ceramic electrical device  500 . In still further embodiments, another material such as silicon nitride_(Si 3 N 4 ) can also be used to produce a metalized or semi metalized species of material in the walls  820  near the opening or through hole or via  720  in the electrical device or interposer  500 .  
       FIG. 11  illustrates a flow diagram of a method  1100  for fabrication of an electrical device, such as an interposer, according to an embodiment of this invention. The method  1100  for forming an electrical device includes obtaining layers of green, unfired ceramic  1110 , laminating the layers  1112 , punching a through hole or via in the laminated layers  1114 , sintering or firing the laminated layers  1116 , and applying a laser to the laminated ceramic material to form a conductive material  1118 . In one embodiment, applying a laser to the ceramic material  1118  to form conductive material includes directing a laser toward a surface of the ceramic material. In another embodiment, applying a laser to the ceramic material  1118  to form conductive material includes directing a laser toward a via within the ceramic material. In yet another embodiment of the method  1100 , applying a laser to the ceramic material  1118  to form conductive material includes directing a laser toward a surface of the ceramic material, and moving one of the laser and the ceramic material to form a conductive path on the surface of the ceramic material. Applying the laser  1118  also includes directing a laser toward a surface of the ceramic material, and directing a laser toward a via within the ceramic material. In some embodiments, the method  1110  further includes moving one of the laser and the ceramic material to form a conductive path on the surface of the ceramic material. The shape and the dimensions of the conductive phases formed in each of those different embodiments are well controlled and can be defined  
       FIG. 12  illustrates a cross sectional view of a laminated and sintered ceramic electrical device or interposer  1200  undergoing laser ablation by a laser  1220 . The laser  1220  is used to produce an opening, or laser ablate material of the ceramic interposer or electrical device. As the opening is formed, the laser focal point is moved further down into the layers of the ceramic electrical device or interposer  1200 . As shown in  FIG. 12 , the laser  1220  is being directed into a layer  1210  of the ceramic interposer electrical device  1200 . The ceramic interposer or device also includes a layer  1212 , a layer  1214 , and a layer  1216 . As laser ablation occurs, an opening is formed.  
       FIG. 13  illustrates a schematic side view of an opening  1320  formed by laser ablation, according to an embodiment of this invention. The opening  1320  extends through layers  1212 ,  1214 ,  1216 ,  1218  of the ceramic electrical device or interposer  1200 . A metal metallic or (conductive) phase  1322  is formed in the walls near the opening  1320 . Forming an opening by laser ablation also forms an opening having a metal (conductive) phase or semi-metallic phase in the walls near the opening, as depicted by reference numeral  1322 . It should be noted that the layers  1212 ,  1214 ,  1216 ,  1218  are all formed of laser convertible material, such as AlN, SiC, or Si 3 N 4 . The opening  1320  can then be filled with a high conductivity metallic paste such as paste  1000 , shown in  FIG. 10 .  
       FIG. 14  illustrates a flow diagram of the fabrication of an electrical device such as an interposer, according to an embodiment of this invention. A method  1400  for forming an electrical device includes providing a ceramic material  1410 , and applying a laser to the ceramic material to form a conductive material  1412 . The ceramic material is a laser-conversion active ceramic material. In one embodiment, applying a laser to the ceramic material to form conductive material  1412  includes directing a laser toward a surface of the ceramic material. In another embodiment, applying a laser to the ceramic material to form conductive material  1412  includes directing a laser toward a via within the ceramic material. A conductive paste can be added to the via  1414 . Adding a conductive paste to the via  1414  includes lowering the concentration of adhesion-promoting materials when compared to a conductive paste formulated to adhere to a nonmetallic surface on a via. In some embodiments, applying a laser to the ceramic material to form conductive material  1412  includes directing a laser toward a surface of the ceramic material, and moving one of the laser and the ceramic material to form a conductive path on the surface of the ceramic material. In some embodiments, providing the ceramic material further includes punching out a via from the ceramic material, and firing the ceramic material.  
       FIG. 15  is a cross-sectional schematic diagram of laser forming an electrical a conductive trace trace on the surface of a ceramic device, according to another embodiment of this invention. An apparatus  1500  includes a substrate  1510  of ceramic material, and a conductive path  1520  associated with the substrate  1510  formed by a laser  1530  directed at the ceramic material. The conductive path  1520  is on a surface  1540  of the substrate. The conductive path can also be associated with a via  1550  in the substrate. The substrate  1510  includes a first major surface  1541 , a second major surface  1542 , and a via  1550  positioned between the first major surface  1541  and the second major surface  1542 . The substrate  1510  is formed from a laser-conversion active ceramic material. At least one of the first major surface  1541  and the second major surface  1542  includes a conductive path  1520  formed by a laser. The via  1550  includes a conductive portion formed by a laser. In some embodiments, at least one of the first major surface  1541  and the second major surface  1542  includes a conductive path  1520 , and the via  1550  also includes a conductive portion. Both the conductive path  1520  and the conductive portion of the via  1550  are formed by the laser irradiation  1530 . The laser irradiation  1530  is directed at the at least one of the first major surface  1541  and the second major surface  1542 , and at a surface of the via  1550 . The substrate  1510 , in some embodiments, also includes an electrical device  1560  electrically attached to at least one of the conductive portion of the via  1550  or the conductive path  1520  of the substrate  1510 . It should be noted that although the laser  1530  and the other electrical device  1560  are shown together, most generally the laser  1530  would be used to form traces before the electrical device  1560  is attached to the substrate. It is also worth to mention that due to the non contact nature of the laser metallization process, laser irradiation can be used after device attachement to the substrate for some additional purposes, such as repair or trimming of a conductive trace or similar operations.  
      The apparatus  1500  includes a substrate  1510  including a laser-conversion active ceramic material, and a device for directing a laser  1530  toward the substrate  1510  to form a conductive material associated with the substrate  1510 . The device for directing a laser toward the substrate  1510 , in some embodiments, further includes a device for moving one of a laser or the substrate l  150  to produce relative motion between the laser and the substrate. In other embodiments, the device for directing a laser toward the substrate further includes a device for directing a laser at the substrate to form a via. Different type of lasers can be used in embodiments of the invention depending upon: the type of material, the level of conductivity needed and the dimensions of the traces required. In an embodiment, ultraviolet lasers (UV-lasers) with short wavelength (&lt;350 nm) and short pulse width (100-20 nano second) can be used. In this range of wavelength the laser type can either be solid state laser (such as third harmonic or forth harmonic Nd:YAG laser) or it can be gas laser such as the Excimer laser with different wavelength and lasing mediums. With respect to the thickness of materials of the via diameter there is a limit to the size when the substrate is formed and fired and the laser is used to form a via in the fired ceramic substrate. The limit of the diameter of the via will be in the range of 20 microns to 50 microns. The limit is dependent on the wavelength of the laser used as well as the power needed to produce a via in a desired amount of time.  
      In some embodiments, a laser is directed at the ceramic material to form a via. In other embodiments, an opening is punched in the ceramic substrate, and the ceramic substrate is fired.  
      The foregoing description of the specific embodiments reveals the general nature of the invention sufficiently that others can, by applying current knowledge, readily modify and/or adapt it for various applications without departing from the generic concept, and therefore such adaptations and modifications are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments.  
      It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Accordingly, the invention is intended to embrace all such alternatives, modifications, equivalents and variations as fall within the spirit and broad scope of the appended claims.