Abstract:
A method for filling vias, and in particular initially blind vias, in a wafer, and various apparatus for performing the method, comprising evacuating air from the vias; trapping at least a portion of the wafer and a paste for filling the vias between two surfaces; and pressurizing the paste to fill the vias.

Description:
FIELD OF THE INVENTION 
   This invention is directed to a process and apparatus, and in particular tooling, for enabling the creation of filled, and preferably conductive, vias and through-vias in a semiconductor substrate. More particularly, it is directed to a process and apparatus for enabling the filling of such vias. 
   BACKGROUND OF THE INVENTION 
   There are many advantages to using silicon as a substrate for electronic packaging, rather than traditional ceramic and organic laminate packaging. Some of the key features of the silicon carrier include: the ability to create high performance wiring and joining at much finer pitch than typical packaging, the ability to join heterogeneous technologies or different generation technologies for high speed applications, the ability to integrate passives, MEMS or optical fibers, the ability to add silicon functionality to the carrier package in addition to wiring, the ability to dramatically increase the I/O density, and for many applications, the ability to reduce overall system cost when compared to other system on package (SOP) approaches which do not use Si as the carrier. 
   Elements and structures of semiconductor packages have been described in U.S. Pat. No. 5,998,292 to Black et al. and U.S. Pat. No. 6,593,644 to Chiu et al. In order to attain the advantages outlined above several key steps are necessary, as shown in  FIGS. 1A to 1F . As illustrated in  FIG. 1A , first, deep blind vias  10  (several hundred microns in depth) are etched into a silicon wafer  12 , and sidewall insulation  14  is deposited. As shown in  FIG. 1B , vias  10  must be completely filled with a conductor  16 . Once the vias are filled, as shown in  FIG. 1C , standard BEOL wiring levels  18  can be built on top of the silicon wafer  12 , and the whole wafer can be thinned by backside grinding to expose the via conductors on the backside, as shown in  FIG. 1D . As shown in  FIG. 1E , solder connections, such as C 4  solder balls  20  may then be built on the carrier back, and chips  22  may be joined to the front, by any one of a number of conventional techniques such as flip chip bonding as illustrated in  FIG. 1F , completing the high performance silicon carrier package  24 . 
   At this point there are several options, one of which is illustrated in  FIG. 2 , where the high performance silicon carrier package  24  is joined to a ceramic module  26  by means of solder balls  20 , and then to a PC board  28  by means of, for example, additional C 4  solder balls  30 . 
   Of all the key technology elements described above, that which is most problematic is the filling of high aspect ratio blind vias with conductor. Filling with common metals by PVD or CVD methods is impractical, while plating becomes extremely difficult due to the tendency for the plated side walls to “breadloaf” at the top, cutting off the via from further filling, and trapping plating solution in a central void. Even if these or other methods of solid metal filling, such as filling with molten metal, could be made to work, typical metals have a large coefficient of thermal expansion (CTE) mismatch with silicon. There are three potential problems associated with large CTE mismatches between the vias and the silicon substrate: delamination at the via side walls; cracking of the silicon substrate between vias; and piston-like rupture of any overlying or underlying structures or thin films in contact with the top/bottom surfaces of the vias. Accordingly it is advantageous to use a material which is simultaneously conductive with a good CTE match to silicon. 
   One such material which has been used by International Business Machines Corporation in the production of glass ceramic multi-chip modules (MCM) is a paste containing a mixture of copper and glass particles suspended in a mixture of organic solvents and binders. Such pastes are typically applied to a patterned ceramic greensheet by a screen printing method, after which the sheets are stacked and sintered at high temperature, during which the organic components are burned off, and the glass and Cu components coalesce to form conductive lines and vias. 
   Recently, in “Filling the Via Hole of IC by VPES (Vacuum Printing Encapsulation Systems) for stacked chip (3D packaging)”, Atsushi Okuno and Noriko Fujita, 2002 Electronic Components and Technology Conference have described the adaptation of a vacuum printing encapsulation system (VPES) for filling blind vias with conductive paste. The VPES method was originally used to deliver plastic resin in the manufacture of ball grid array (BGA) and CSP packaging, wafer level CSP packaging, transparent resin encapsulating for light emitting diode (LED) displays, flip-chip under-filling, and other processes. For BGA or CSP packaging, following die bonding and wire bonding on a printed circuit board substrate, the printing of liquid resin takes place using a squeegee applied to the substrate under vacuum. The substrate is then cured at a high temperature to solidify the liquid resin. After curing, solder balls for terminals are mounted on the backside of the substrate. Conventional screen printing lacked a process for removing the gas from the resin after the printed after curing, causing cracking or warping during the high temperature process. 
   In the method described by Okuno, a squeegee tool applies conductive paste using a knife edge. In this tool design, a vacuum is pulled inside the enclosure, and paste is delivered, for illustrative example, by a slot in the base of the tool. 
   An example of via filling using such a tool is shown in  FIGS. 3A  through  FIG. 3F . In these figures, a vacuum chamber  34  is evacuated by means of a vacuum pump (not shown) connected to chamber  34  by a vacuum hose  36 . Once sufficient vacuum is created, a squeegee blade  38 , mounted an a moving member  39 , moves across the surface of a via containing wafer  40  from left to right in the figure, held in a wafer holder or base plate  42 , delivering paste  44  at its leading edge. Paste  44  is moved into position by a moving support  46  in a channel or base plate slot  48  to which paste  44  is conducted by a passageway (not shown). As shown in  FIG. 3C , excess paste is deposited over a movable support member  50  in a channel or base plate slot  52 . As shown in  FIG. 3D , support member  52  is moved upwards in channel  52 , while support member  46  is moved downwards in channel  48 . Additional paste is supplied to slot  52  through a second passageway (not shown). As illustrated in  FIG. 3E  and  FIG. 3F , moving member  39  is then moved to cause squeegee blade  38  to again traverse wafer  40 , while moving from right to left in the figure. 
   This method has a number of important shortcomings, the most important of which is that there is not sufficient constraint at the leading edge of the squeegee blade  38  to force the paste  44  to the bottom of a deep blind via in a single, or often, even multiple passes. Whether the paste  44  makes it to the via bottom is dependent on a number of factors including the viscosity of the paste  44 , the down force on the squeegee blade  38 , the quantity of paste  44  built up in front of the squeegee blade  38 , and the blade speed. With respect to the down force, there is no method to fully contain the paste  44  under pressure over a blind via except when the squeegee blade  38  is passing directly overhead, and even then paste  44  is free to smear out both in front of and behind the blade  38 . This makes multiple passes a necessity. For high aspect ratio vias incomplete filling can occur if the vacuum level is not sufficiently low or if the paste  44  is of a very high viscosity. The method is also not well suited to semiconductor processing where substrates are round rather than rectangular. In order to ensure complete coverage of a round substrate, paste  44  must be pushed repeatedly onto and off of the base plate  42  holding the wafer. The linear motion of the squeegee blade  38  then leads to buildup at either end of the tool necessitating some method of regular cleaning, and a great waste of the conductive fill paste. Accordingly there is a need to develop a more efficient method for applying viscous conductive paste to semiconductor wafers containing blind vias. 
   In U.S. Pat. No. 5,244,143 to Ference et al. as well as U.S. Pat. No. 5,775,569 to Berger et al., a tool and method for filling a mold with molten solder are described. Since a mold is obviously a rigid plate containing etched regions of specific shapes, if these shapes take the form of cylinders then the problem is essentially one of filling blind vias. The filling head described in these patents is sealed against the mold surface such that a vacuum can be pulled in a region defined by a O-ring seal underneath the head. Molten solder is then delivered through a central slot in the head such that complete fill of the evacuated solder mold cavities is achieved in a single pass. An important distinguishing feature of this tool and method is that it works well only for very low viscosity materials such as molten solder which have a viscosity on the order of 2 centipoise (for comparison water is by definition 1 centipoise). The conductive pastes used for semiconductor applications by contrast have much higher viscosities ranging from 1,000 centipoise to greater than 50,000 centipoise and thus require much higher internal pressures for them to be effectively delivered to the wafer surface and into the blind vias etched therein. 
   A via filling method using a pressurized paste nozzle is described in U.S. Pat. No. 6,506,332 to J. L. Pedigo and it is clear that while this method has advantages over the squeegee method described by Okuno, it is primarily intended for use in organic printed circuit board (PCB) high-density interconnect (HDI) and sequential build up (SBU) laminate board type applications. The apparatus described makes use of a pressure head in combination comprising an O-ring gasket which is held against the electronic substrate to be filled and moved relative to that substrate such that paste is forced into the via holes as the head passes overhead. The apparatus as described has a number of shortcomings which limit its applicability for use with silicon wafer based packaging. Specifically, the method does not employ vacuum which is a practical necessity for complete filling of small, high aspect ratio blind vias. Instead, the method is described as a means of obtaining “reduced numbers of air pockets formed in the via fill paste while decreasing the amount of processing required per board”. Further, via sizes claimed range from 2 to 25 thousands of an inch (mils) in diameter, a span which covers most important electronic wiring board applications, but which neglects via features smaller than 50 um (2 mils) in diameter which are easily attainable in package substrates made from silicon where blind vias may be on the order of 10 um in diameter with aspect ratios greater than 10:1. Filling such small blind features with viscous paste without the aid of vacuum is highly problematic if not impossible. 
   SUMMARY OF THE INVENTION 
   The present inventors have recognized that there is a need for a method and tooling which employs a combination of pressurized paste delivery in a vacuum environment to enable the complete filling of etched blind features, both lines and vias, in a silicon wafer which may range in size from 10 um (&lt;0.5 mils) to 250 um (10 mils). Furthermore, there is a need for a highly manufacturable process and tooling which is easily adaptable for highly automated batch operation compatible with CMOS back end of the line (BEOL) processing. 
   It is therefore an aspect of the present invention to provide a method for reliably filling vias with a viscous substance. 
   It is another aspect of the present invention to provide apparatus or tooling for reliably filling vias with a viscous substance. 
   In accordance with the invention a method for filling vias, and in particular blind vias, in a wafer, comprises evacuating air from the vias; trapping at least a portion of the wafer and a paste for filling the vias between two surfaces; and pressurizing the paste to fill the via. The method may further comprise forming a seal between the surfaces so as to enclose the portion of the wafer and the paste. The method may further comprises moving the seal over successive portions of the wafer to fill the vias. 
   Further, the method may comprise placing the paste on a planar surface facing the wafer; and moving the planar surface upon which the paste is placed into contact with the wafer. The paste may be injected between one of the surfaces and the wafer. Preferably, the paste is injected between one of the surfaces and the wafer after evacuating the air from the vias. 
   The method may further comprising providing an evacuated space between the surfaces; and forcing the surfaces together to force the paste into the vias. The surfaces can forced together by atmospheric pressure. 
   Preferably, the paste is pressurized to greater than atmospheric pressure, and more specifically to a pressure in the range of 10 to 100 PSI. 
   In accordance with the invention, an apparatus for filing vias in a wafer, comprises a chamber in which to place the wafer, the chamber being capable of being evacuated; a surface upon which to place the wafer; a paste delivery portion for providing a paste to fill the vias; and a paste filling portion for bringing the paste into contact with the vias under pressure so that the paste fills the vias. Preferably, the paste filling portion provides the paste at a pressure with the range of 10 to 100 PSI. 
   The paste delivery portion may comprise a surface onto which the paste is deposited; and a mechanism for applying pressure so that the paste on the surface is forced into contact with the wafer. The paste delivery portion may also comprise a surface onto which the paste is deposited; and a passageway through which the paste is delivered to the surface. The mechanism for applying pressure may comprise a plate which defines the surface; and components for applying a pressure differential to the plate so as to force the plate toward the wafer. A compliant material may be disposed on the surface to which the paste is applied. 
   The paste filling portion may comprise a plate having a portion for receiving the paste; a first seal for sealing the plate to the surface upon which the wafer is placed; a second seal for sealing the paste between the plate and the plate and the wafer; and a mechanism for forcing the plate towards the wafer so that the paste is forced into the vias of the wafer. 
   The mechanism for forcing the plate towards the wafer may comprise a gas removal apparatus for evacuating gas between the plate and the surface upon which the wafer is placed; and gas replacement apparatus for replacing gas evacuated from the chamber. The gas replacement apparatus may comprise an opening through which gas is permitted to enter the chamber. 
   The surface upon which to place the wafer may comprise a base plate having a recess for the wafer or it may be a surface of an electrostatic chuck. 
   The paste delivery portion may comprise a pressurized paste reservoir. 
   Also in accordance with the invention, the paste filling portion may comprise a piston housing having an opening for receiving a piston; a compliant seal for sealing the piston housing to a portion of the wafer so as to confine the paste; a piston disposed in the piston housing; and a piston actuator for forcing the piston toward the wafer; wherein the paste delivery portion provides the paste to the opening. 
   The apparatus in accordance with the invention may further comprise a mechanism for moving the piston housing so that the seal is compressed for filing the vias. The mechanism for moving the piston housing may further move the piston housing to a position wherein the seal is compressed to a lesser degree than when the vias are filled, to thereby allow the piston housing to be moved so that the piston housing is disposed so as to be in a position to fill vias of one or more successive portions of the wafer with paste delivered to the opening. 
   The apparatus may further comprise a mechanism for cleaning the piston of excess paste after vias of a wafer have been filled. 
   The paste filling portion of the apparatus may comprise an elongate member having a surface with a slot through which paste is provided to the wafer; and a compliant seal for sealing the surface to the wafer. 
   In accordance with the invention, the apparatus may further comprise a mechanism for translating the member and the wafer with respect to one another so as to fill vias in successive portions of the wafer and a mechanism for rotating the member and the wafer with respect to one another so as to fill vias in successive portions of the wafer. The mechanism for rotating the member and the wafer with respect to one another may comprise a rotating base having the surface upon which the wafer is placed. 
   The apparatus may be configured to accept a circular wafer, wherein the elongate member is disposed radially with respect to the wafer. Preferably, the elongate member has a length less than that a radius of the wafer, and the further comprises a mechanism for rotating the wafer; and a mechanism for radially translating the member in a radial direction with respect to the wafer. The mechanism for rotating the wafer may include a rotation speed control to control speed of rotation of the wafer. The mechanism for radially translating the member may include a translation speed control to control speed of translation of the member with respect to the wafer. 
   The mechanism for radially translating the member may include a worm gear assembly, and a motor for rotating a drive shaft of the assembly. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects, features, and advantages of the present invention will become apparent upon further consideration of the following detailed description of the invention when read in conjunction with the drawing figures, in which: 
       FIG. 1A  to  FIG. 1F  illustrate, in cross sections, the stages of a prior art process flow for creating a silicon-based chip carrier complete with conductive through vias, topside landing joins or bumps and backside solder connections. 
       FIG. 2  illustrates prior art silicon based carrier populated with chips mounted on a first (ceramic module) and second (PCB) level package. 
       FIG. 3A  illustrates a prior art system using paste delivery through a base plate slot with a vacuum squeegee blade at a home position. 
       FIG. 3B  illustrates the system of  FIG. 3A  with the vacuum squeegee blade at a midway position during a first pass paste filling. 
       FIG. 3C  illustrates the system of  FIG. 3A  with the vacuum squeegee blade at a terminal position after a single pass. 
       FIG. 3D  illustrates the system of  FIG. 3A  with paste delivery through a base plate slot with the vacuum squeegee blade at terminal position. 
       FIG. 3E  illustrates the system of  FIG. 3A  with the vacuum squeegee blade at a midway position during a second (return) pass paste filling. 
       FIG. 3F  illustrates the system of  FIG. 3A  with the vacuum squeegee blade at a home position after one complete cycle including two passes across the surface of a wafer. 
       FIG. 4A  illustrates an apparatus in accordance with the invention with a vacuum piston tool having an upper surface coated with paste, in an initial position. 
       FIG. 4B  illustrates the apparatus of  FIG. 4A  in a configuration wherein an inner vacuum port of the is held open while an outer vacuum port back fills to atmosphere. 
       FIG. 4C  illustrates the apparatus of  FIG. 4A  in a configuration wherein the inner port is set to back fill to atmosphere so that a paste is in the vias of the wafer and an overburden is on the wafer surface. 
       FIG. 5A  is a cross-sectional view of an apparatus in accordance with a second embodiment of the invention wherein a compact piston head is in a starting position on the surface of a wafer. 
       FIG. 5B  is a cross-sectional view of the apparatus of  FIG. 5A  wherein the compact piston head is moved so as to compress a gasket against the wafer. 
       FIG. 5C  is a cross-sectional view of the apparatus of  FIG. 5A  wherein paste is dispensed into an evacuated region between the piston face and the wafer surface. 
       FIG. 5D  is a cross-sectional view of the apparatus of  FIG. 5A  wherein the piston extends downward compressing paste into blind vias of the wafer. 
       FIG. 5E  is a cross-sectional view of the apparatus of  FIG. 5A  wherein the piston is withdrawn and the piston head is in a position resulting in light pressure between the gasket and the surface of the wafer. 
       FIG. 5F  is a cross-sectional view of the apparatus of  FIG. 5A  wherein the piston head is in a second location, while maintaining a light contact force between the gasket and the surface of the wafer. 
       FIG. 5G  is a side elevational view of the piston head apparatus of  FIG. 5A  wherein the piston is at location away from the wafer surface and extends to contact apparatus for removing excess paste. 
       FIG. 6A  is a partial bottom view of an apparatus in accordance with a third embodiment of the invention. 
       FIG. 6B  cross-sectional side elevational view of the apparatus of  FIG. 6A . 
       FIG. 7  is a cross-sectional view a vacuum paste nozzle dispense chamber, utilizing the apparatus of  FIG. 6A  and  FIG. 6B . 
       FIG. 8  is a plan view of a linear nozzle dispense operation inside of a vacuum environment, utilizing the apparatus of  FIG. 6A  and  FIG. 6B . 
       FIG. 9A ,  FIG. 9B  and  FIG. 9C  are plan views of rotary nozzle dispense operations inside of a vacuum environment, utilizing an apparatus in accordance with  FIG. 6A  and  FIG. 6B . 
   

   DESCRIPTION OF THE INVENTION 
   Variations described for the present invention can be realized in any combination desirable for each particular application. Thus particular limitations, and/or embodiment enhancements described herein, which may have particular advantages to the particular application need not be used for all applications. Also, it should be realized that not all limitations need be implemented in methods, systems and/or apparatus including one or more concepts of the present invention. 
   Referring to  FIG. 4A , in a first apparatus and method in accordance with the invention, an outer processing chamber  60  has an outer vacuum port  62  to which a vacuum source (not shown) is connected. Chamber  60  is evacuated, as represented by arrow  65 , through port  62 . Conductive paste  64  is applied to a portion of one face of a top plate or piston  66 , which is coated with Teflon or another nonstick and compliant surface material  68 . 
   As used herein, the term paste refers to any material, and especially to electrically conductive materials, having a viscosity within a broad range, including a range spanning that of traditional pastes, such as highly loaded metal or metal-dielectric filled pastes used in screen printing of printed circuit boards, aqueous suspensions containing fine grains of conducting material, and organo-metallic liquids. 
   An inner O-ring  69  surrounds the surface material and the paste  64 . The wafer  70  being processed is held on the face of a bottom or base plate  72  by means of a countersunk recess or banking pins (not shown). The planar surfaces of piston  66  and base plate  72  are held apart by a by a compliant outer O-ring  74 . Air in the gap separating the paste-coated side from the wafer, is prevented from being trapped in the vias under the paste by achieving a sufficient vacuum in the space  75  between piston  66  and base plate  72 . This is accomplished by evacuating space  75  by means of a passageway  76  in piston  66 , which is connected to a vacuum hose  78 , that is in turn evacuated by a vacuum system (not shown) connected to an inner vacuum port  80 . Thus, in  FIG. 4A , the inner vacuum port  80  and the outer vacuum port  62  are both open so that the space  75  between the paste  64  and the wafer  70  is evacuated without collapse of the plates toward one another. 
   Pressure is then applied to the piston  66 , bringing the paste into contact with the wafer  70 . As shown in  FIG. 4B  this pressure, represented by arrows  82  is easily achieved by maintaining vacuum inside the piston enclosure defining space  75 , while back filling the outer chamber  60  with air, for example, at atmospheric pressure, as represented by arrow  65 A. The inner O-ring  69  forms a baffle enclosure, preventing the paste from escaping at the edge of the wafer  70  and assuring that sufficient pressure is available to force the paste into the evacuated vias of wafer  70 . Once the vacuum is released from the inner piston, by releasing the vacuum at inner vacuum port  80 , and allowing space  75  to revert to, for example, atmospheric pressure, with arrow  80 A representing the flow of air, outer O-ring  74  provides a restoring force which increases the separation between piston  66  and base plate  72 , as shown in  FIG. 4C . 
   Several additional features are available for the apparatus illustrated in  FIG. 4A  to  FIG. 4C . In the case of a lower viscosity liquid paste, as shown in  FIG. 4B , delivery may be achieved via an orifice  84  in the piston  66  and surface material  68  after the vessel is evacuated. In this case a precise amount of paste is delivered, through a preferably flexible paste delivery tube  86  (which may penetrate chamber  60  in an airtight manner) and allowed to flow across the wafer surface and into the evacuated vias before final pressure is applied. In an alternate embodiment, the wafer mounting surface and paste-covered surface are inverted. In this case, the automatic dispensing of the paste takes place through an orifice in the lower plate, and a lower-viscosity paste is allowed to pool for a precise time before the wafer is brought into contact and pressure applied. This may be visualized by inverting  FIG. 4B . In this embodiment, paste overburden must be removed in a subsequent step by any number of methods including but not limited to a squeegee or doctor blade as described above, or a rotary brush cleaning method. 
   Referring to  FIG. 5A  to  FIG. 5F , in the second embodiment of the invention, a compact piston  90  is disposed in a piston housing  91 , of a movable, compact, operating piston head  92 . Piston head  92  is disposed inside a vacuum chamber  94 . Provision is made to move the compact piston head  92  in steps across the surface of a wafer  96  held in, for example, an electrostatic chuck  98 . The filling begins once the chamber  94  is fully evacuated. As illustrated in  FIG. 5B , the piston head  92  is moved to an appropriate starting point ( FIG. 5   a ) and the piston housing  91  is pushed vertically against the wafer surface, by for example, vertical expansion of an actuator  99 , to compress an O-ring gasket  100 . As illustrated in  FIG. 5C , paste, stored in a pressurized paste feed and reservoir  102  is dispensed into the evacuated space  108  underneath the compact piston through a paste feed tube or hose  104  terminating in an opening  106  in the space  108  under piston  90  and above wafer  96 . 
   As illustrated in  FIG. 5D , the piston  90  is then actuated by a piston drive mechanism  110 , which forces piston  90  downward, thus compressing the paste into the vias of wafer  96  below. Piston drive mechanism  110  may be operated in any of several conventional ways, such as by means of an electric motor or a pneumatic or hydraulic drive. Drive mechanism  110  may then be reversed so that piston  90  withdraws. The downward force of the piston housing  91  of head  92  is released by actuator  99 , so that O-ring gasket  100  decompresses but remains lightly in contact with the upper surface of wafer  96 . As illustrated in  FIG. 5F , the entire head  92  is translated across the surface of the wafer  96  to the next delivery location and the process described above is repeated. This may be done at successive locations until vias in the entire wafer accessible by the head are filled. This method is advantageous in that it becomes quite easy to deliver the paste directly to the point of use. Further, as illustrated in  FIG. 5G , it is relatively simple to include a cleaning station, comprising, for example, a rotating cleaning wheel  112 , located away from the wafer chuck  98 , to remove excess paste from the bottom of the compact piston face before subsequent filling. To perform this operation, head  92  is moved to a position removed from chuck  98 , and piston drive mechanism  110  moves piston  90  so that its lower surface extends outside of piston housing  91  and below O-ring gasket  100 . 
   It is noted that while the face of piston  90  may be circular, it is advantageous for it to be a square or rectangular in the case of a x-y translation system. In the case of a rotational system where the head is fixed and the wafer rotates, it is advantageous for the head to assume a shape equal to some reasonable segment of a circle. 
   Referring to  FIG. 6A  (a partial bottom view),  FIG. 6B  (a cross-sectional view) and  FIG. 7 , a cross sectional view), in a third embodiment of the invention, paste is applied using a pressurized nozzle  120 , having an O-ring seal or gasket  121  held firmly in contact with the upper surface of a wafer  122 . Wafer  122  is supported in a countersunk notch or recess  124  of a base plate  126  inside a vacuum environment, such as a vacuum chamber  128 . Conductive paste  129  is applied through a slot  131 . In  FIG. 7 , nozzle  120  is shown moving across wafer  122  in the direction of arrow  130 . As with the first and second embodiment, no filling occurs until the entire chamber  128  has been pumped down to a vacuum level of less than 10 Torr, and preferably closer to 1 Torr. Conductive paste, under pressure, is supplied to nozzle  120  via a delivery tube  132  connected to a paste reservoir  134 , which supplies paste upon movement of a piston assembly  135 . An advantage of this embodiment is that the pressurized paste cartridge supplying the paste to the nozzle via the delivery tube is disposed inside the vacuum chamber and may be electronically or mechanically actuated therein. In this configuration there is no possibility of air seeping into the paste delivery system, and provision is made for preventing air from slowly permeating the paste itself, which is of critical importance for pastes which have been purposefully mixed and dispensed under vacuum specifically for this application. 
   Referring also to the linear scanning operation shown in  FIG. 8 , the nozzle  120  begins at a position to the left of the wafer  122  held in countersunk recess  124  of base plate  126  ( FIG. 7 ), and travels as indicated by arrow  127 . It is preferable that recess  124  either match, or be slightly less deep than, the full thickness of wafer  122  to ensure that the upper surface of wafer  122  is either on grade, or slightly higher (˜1 mil) than, the surface of the base plate over which nozzle  120  moves. This ensures that the compliant nozzle O-ring gasket  121  will remain in compressed contact against the upper surface of wafer  122  throughout the filling operation. As described, for a paste of a given viscosity the controllable filling parameters are vacuum level inside the chamber  128 , pressure applied to the paste inside the nozzle  120 , and scanning speed of nozzle  120  over the surface of wafer  122 . 
   It is noted that with the exception of the rotary embodiments shown in  FIGS. 9A ,  9 B and  9 C, below the wafer fits snugly into a machined, countersunk notch in the tool base plate so that the wafer surface is very nearly planar with respect to the base plate surface. The nozzle moves across the surface filling the evacuated vias in its path and leaving only a very thin overburden on the wafer surface. Alternatively, positioning or banking pins may be used to hold the wafer in place. 
   As shown in  FIGS. 9A and 9B , the pressurized paste nozzle may also be advantageously applied in a rotary configuration wherein the wafer is held on a rotating base plate (not shown in  FIGS. 9A to 9C ) by, for example an electrostatic chuck (also not shown). The wafer  122 A,  122 B,  122 C rotates as represented by arrow  125 . The electrostatic chuck may be of conventional design with respect to the manner in which the wafer is held, but may differ in that provisions are made for applying the voltage used to secure the wafer with electrical connection means that permit rotation of the base plate. 
   A nozzle  120 A is held stationary in a radial direction with respect to a rotating wafer  122 A to apply paste  123 A. This method has the advantage that the nozzle never touches another surface except that of the wafer to be filled. The nozzle may be designed to be less wide than the wafer radius to provide an edge exclusion zone where no paste is applied. Both of these features serve to make this embodiment of the invention particularly compatible with typical CMOS semiconductor processing. 
   The wafer is fixed on a rotating chuck (for example, an electrostatic chuck, as described above) and the paste nozzle is brought into contact with the wafer and moves across the surface filling the evacuated vias in its path and leaving a very thin overburden of the paste  123 A on the surface of wafer  122 A. As shown in  FIG. 9A , a fixed nozzle can have a slot dimension nearly equal to the wafer radius as shown, or the full diameter. 
     FIG. 9B  illustrates an embodiment that is particularly preferred, where the nozzle  120 B, and thus the slot dimension, is less than the radius of the wafer  122 B. In this embodiment the nozzle  120 B must be moved, for example, in equal steps along the radial direction, such that separate paste delivery tracks  140 A,  140 B,  140 C, etc. are defined. An exemplary mechanism for providing such movement is described below with respect to  FIG. 9C . The combination of vacuum, paste pressure and dwell time of the nozzle over a via or collection of vias are important filling parameters. The embodiment shown in  FIG. 9B  allows wafer rotation speed to be adjusted for each separate paste delivery track to ensure that the average dwell time of the nozzle in any given location is approximately equal across the wafer. Another advantage of the smaller nozzle is that a higher overall paste pressure can be developed for a given amount of nozzle down force. The pressure of the paste multiplied by the area defined by the slot O-ring yields the force with which the nozzle must be held against the wafer surface to avoid any leakage under the O-ring seal. In general, any moving mechanical system such as that shown in  FIG. 9B  will have a maximum structural force at which it can properly operate. If the paste delivery area defined by the nozzle is reduced, the same mechanical down force will allow a higher nozzle pressure to be developed before the paste leakage condition is met. 
   Referring to  FIG. 9C  nozzle  120 C is supported on an arm  142  connected to a block  144  with a threaded hole  145 . A worm gear drive assembly comprises a threaded shaft  146 , supported in fixed bearing blocks  148  and  150 , extends through and engages the threads of hole  145 . Shaft  146  is rotated by a motor  152  controlled by a speed controller  154 . Motion of block  144  resulting from rotation of shaft  146  causes nozzle  120 C to move radially with respect to wafer  122 C. It will be recognized that in addition to depositing separate paste delivery tracks  140 A,  140 B,  140 C, etc., as described above with respect to FIG.  9 B., it is possible, once paste delivery track  140 A has been deposited, to continuously move nozzle  120 C radially outward with respect to wafer  122 C until a desired surface region has been covered with paste. Suitable continuous adjustment in the rotational speed of the wafer is made to assure reasonably uniform paste delivery, as described. 
   It is noted that the worm gear drive mechanism described above with reference to  FIG. 9C  is merely exemplary, and that any other suitable drive mechanism may be used. Further, any such drive mechanisms may be used in any embodiment of the invention described in the various figures, wherein translational motion is required. 
   The three general embodiments outlined describe only the paste application step itself. A production tool based on any of these preferably also comprise the following functions: automated wafer handling from/to a cassette to the paste apply stage (loadlock); provision for cleaning the edge (if necessary) of the wafer (similar to edge bead removal in a resist coater); automated paste pressure control, metering and dispense; some form of automated inspection; and automated loading into a batch vacuum oven for low temperature drying in-situ. 
   The invention described herein has particular application to a semiconductor or glass substrate-based carrier for mounting and packaging multiple integrated circuit chips and/or other devices. The carrier is a freestanding chip or wafer with insulated, conductive through-vias exposed on its top and underside, to connect flip-chip and other device I/O through the carrier to next-level packaging, board, or other flip-chips mounted on the bottom side. However, it may be applied to any situation wherein a via, and in particular a deep via, must be filled with a viscous substance such as a paste. 
   Thus, it is noted that the foregoing has outlined some of the more pertinent objects and embodiments of the present invention. The concepts of this invention may be used for many applications. Thus, although the description is made for particular arrangements and methods, the intent and concept of the invention is suitable and applicable to other arrangements and applications. It will be clear to those skilled in the art that other modifications to the disclosed embodiments can be effected without departing from the spirit and scope of the invention. The described embodiments ought to be construed to be merely illustrative of some of the more prominent features and applications of the invention. Other beneficial results can be realized by applying the disclosed invention in a different manner or modifying the invention in ways known to those familiar with the art. Thus, it should be understood that the embodiments has been provided as an example and not as a limitation. The scope of the invention is defined by the appended claims.