FILLING AND CUTTING METHOD
FIELD
The present disclosure relates to filling micro-sized holes in a substrate with liquid metal, and to cutting the liquid metal from a pool of liquid metal once the micro-sized holes have been filled. In particular, the present disclosure relates to filling via holes during through-silicon via (TSV) fabrication.
BACKGROUND
A TSV is an electrical connection passing through a substrate for connecting electrical components on either side of the substrate. TSV techniques can be used to create 3-dimensional circuits on a silicon substrate.
A known method of TSV fabrication is shown in Figure 1. In a first step A, a number of vias 102 are etched into a silicon wafer 100, and at step B an isolation layer 104 is deposited on the sidewalls of each of the vias 102. The isolation layer 104 electrically isolates the conducting material that will fill the via 102 from the substrate.
Secondly, a copper plating method is performed. In the copper plating method, firstly at step C a barrier layer 106 is deposited over the isolation layer 104 on the sidewalls of each of the vias 102. The barrier layer 106 prevents atoms of the conducting material that will fill the via 102 from migrating into the substrate. Then, at step D a seed layer 108 is deposited over the barrier layer 106. Alternatively, a seed layer wafer may be attached to an opening of the via 102. Finally, at step E a copper electroplating 110 is grown from the seed layer 108 on the sidewall of the via or from the seed layer wafer, until the copper electroplating fully fills the via 102.
Thirdly, at step F a chemical mechanical polishing (CMP) process is performed on the silicon wafer 100. A redistribution process is then performed. Finally, the silicon wafer 100 after TSV fabrication is sliced into silicon interposers.
The copper plating method used in TSV fabrication described above has a number of disadvantages. Firstly, the copper electroplating 110 takes a relatively long time (several hours) to grow from the seed layer 108 and fully fill the via 102. Secondly, because growing the copper electroplating 110 is a slow process, it is impractical to use the above copper plating method to fabricate relatively large TSVs -the process either takes too long or results in very low yields. Thirdly, the copper plating process is complex, involving many parameters that need to be careflully chosen for a particular via size, and is therefore inflexible.
SUMMARY
An invention is set out in the claims.
A method of filling a via hole in a substrate is provided. A filling hole in a filling component is aligned with the via hole in the substrate, and a surface ofa pool of liquid is in contact with a second face of the filling component. A pressure condition ofPl > P2 and P1 > P3 is achieved such that liquid is drawn through the filling hole and into the via hole, wherein P1 is a pressure at the second face of the filling component, P2 is a pressure between a first face of the filling component and a second face of the substrate, and P3 is a pressure at a first face of the substrate.
A method of providing a break in a liquid filling a via hole in a substrate and a filling hole in a filling component is provided. A dimension of the filling hole is less than a dimension of the via hole. A pressure condition of P1 > P3 and P2> P3 is achieved such that the liquid breaks in the filling hole, wherein P1 is a pressure at a second face of the filling component, P2 is a pressure between a first face of the filling component and a second face of the substrate, and P3 is a pressure at a first face of the substrate.
An apparatus for filling a via hole in a substrate is provided. The apparatus comprises a filling component, the filling component comprising a filling hole; a first pressure region for maintaining a pressure at a second face of the filling component; a second pressure region for maintaining a pressure between a first face of the filling component and a second face of the substrate; and a third pressure region for maintaining a pressure at a first face of the substrate.
An apparatus for filling a via hole in a substrate is provided. The apparatus comprises a filling component having a tilling hole; and a substrate having a via hole. A dimension of the filling hole is less than a dimension of the via hole.
BRIEF DESCRIPTION OF THE DRAWINGS
Specific cmbodimcnts and examples arc shown in the accompanying drawillgs, in which: Figure 1 illustrates a known method of TSV fabrication and a liquid metal via filling and cutting method according to thc present disclosurc; Figure 2A is a transparent perspective view of an apparatus for performing the via filling and cuffing method; Figure 2B is a solid perspective view of the apparatus for performing the via filling and cutting method; Figure 3A is a cross-sectional view of the apparatus for performing the via filling and cutting method; Figurc 3B is a cross-scctional view of the apparatus for performing thc via filling and cutting method after the filling method has been performed; Figure 3C is a cross-sectional view of the apparatus for performing the via filling and cuffing method during the cutting process; Figure 3D is a cross-sectional yiew of the apparatus for performing the via filling and cutting method after the cutting method has been performed; Figure 3E is a cross-sectional view of a substrate wafer after the via filling and cutting method has been performed; Figure 4A shows a simulation of liquid metal in a filled via hole and nozzle hole at approximately a first pressure condition; Figure 4B shows a simulation of liquid metal in a filled via hole and nozzle hole at approximately a second pressure condition; Figure 5 is a cross-sectional view of a first embodiment of an apparatus for performing the via filling and cutting method; Figure 6 is a cross-sectional view of a second embodiment of an apparatus for performing the via filling and cutting method; Figure 7 is a cross-sectional view of a third embodiment of an apparatus for performing the via filling and cuffing method; Figures SA to SG show various different profiles in vertical section for nozzle holes; Figures 9A to 9D show various configurations of metal rings around a via hole; Figures 9E to 9D show various configurations of metal rings around via hole having metal-coated sidewalls; Figure IOA shows a filled via hole having metal-coated sidewalls; Figure lOB shows a filled via hole having metal-coated sidewalls and a metal ring around one end of the via hole; and Figure 11 shows an apparatus for performing the via filling and cutting method in which a nozzle wafer is bending under a first pressure condition.
OVERVIEW
The method described herein uses a filling component, such as a nozzle wafer, which allows micro-sized holes (having diameters in the range of tens to hundreds of microns) in a substrate to be filled with a liquid metal, which is then cut and solidified. The substrate may be a silicon/glass wafer. The method can be used for via filling in through-silicon via (TSV) fabrication. The nozzle wafer has nozzle holes having a hole dimension of less than that of the holes in the substrate wafer, which allows thc liquid mctal to be cut reliably in the nozzle wafer by a pinch-off effect. The hole dimension may be the cross-sectional width or diameter of a hole, taken in a direction perpendicular or transverse to the direction of flow of the liquid metal as the liquid metal fills the hole.
DESCRIPTION
Figures 2A and 2B show respectively a transparent and solid perspective view of a substrate wafer 100. The substrate wafer 100 is provided with an array of via holes 102 to be filled that extend through the substrate wafer 100 from a first face of the substrate wafer 100 to a second face of the substrate wafer 100. The via holes 102 can be cylindrical or tapered; in the example shown in Figures 2A and 2B the via holes 102 are circular in cross-scction but the via holes 102 can be any shape.
A first face of a nozzle wafer 200 is provided adjacent to the second face of the substrate wafer 100. The nozzle wafer 200 is provided with an array of nozzle holes 202 that extend through the nozzle wafer 200 from its first face to a second face of the nozzle wafer 200. The nozzle holes 202 can also be cylindrical or tapered; in the example shown in Figures 2A and 2B the nozzle holes 202 are circular in cross-section but the nozzle holes 202 can be any shape. The cross-seclional shape of the nozzle holes 202 may be different to that of the via holes 102. The via holes 102 and the nozzle holes 202 each have a diameter measured in a direction perpendicular to the direction of flow of the liquid metal 300 as the liquid metal 300 fills the via holes 102 and the nozzle holes 202. The diameter of the nozzle holes 202 at the first face of the nozzle wafer 200 is smaller than the diameter of the via holes 102 at the second face of the substrate wafer 100. It is sufficient that the diameter of each of the nozzle holes 202 is smaller than the diameter of the via holes 102 at at least one point within the nozzle hole 202.
The array of nozzle holes 202 is aligned with the array of via holes 102, such that each of the nozzle holes 202 is aligned with a via hole 102, as shown in Figure 2A.
A pool of liquid metal 300 comprising the filling material, for example molten solder or molten silver, is provided directly underneath the second face of the nozzle wafer 200, such that the surface of the pool of liquid metal 300 contacts the second face of the nozzle wafer 200. The substrate wafer 100 and the nozzle wafer 200 are unwettable to the liquid metal 300. The manner in which filling is achieved will now be described.
Turning to Figure 3A, three chambers are formed in which different pressures can be applied to the apparatus such that there are regions of pressure differential. The structure of the chambers will be described in more detail below with reference to Figures 5 to 7. A first chamber encloses the second face of the nozzle wafer 200 and contains the pool of liquid metal 300, and has a first pressure P1. A second chamber encloses the first face of the nozzle wafer 200 and the second face of the substrate wafer 100 (and therefore the space between the nozzle wafer 200 and the substrate wafer 100), and has a second pressure P2. A third chamber encloses the first face of the substrate wafer 100, and has a third pressure P3.
Initially, the second and third chambers are connected to one another, whereas the first and second chambers are isolated from one another by the liquid metal 300, and the first P1, second P2 and third P3 pressures are substantially equal.
A via filling process will now be described. A first pressure condition ofPl > P2 = P3 is created. Such a condition maybe created, for example, by reducing the second P2 and third P3 pressures using a vacuum pump, whilst maintaining the first pressure P1 at a constant value. As a result of the first pressure condition, the liquid metal 300 is drawn from the pool, through the nozzle holes 202 and into the via holes 102, as shown in Figure 3B.
For the liquid metal 300 to be drawn through the nozzle holes 202, the difference between the first pressure P1 and the second pressure P2 must be greater than 2y/r, where y is the surface tension of the liquid metal 300 and r is the radius of the nozzle holes 202.
A mechanism may be provided to prevent liquid metal 300 spilling out of the via holes 102 at the first face of the substrate wafer 100. For example, a stop layer can be provided on the first face of the substrate wafer 100. The stop layer may comprise a wafer having an array of stop holes, each of the stop holes having a diameter smaller than the diameter of the nozzle holes 102 at the first face of the substrate wafer 100, for example approximately 10 microns.
The second face of the substrate wafer 100 and the first face of the nozzle wafer 200 arc provided sufficiently close to one another to prevent liquid metal from filling the gap between the substrate wafer 100 and the nozzle wafer 200.
After the filling process has been completed, the liquid metal 300 in the via holes 102 isolates the third chamber from the second chamber.
As shown in Figure 3B, the liquid metal 300 in the via hole 102 is still connected to the poll of liquid metal 300. In order to achieve a substrate wafer 100 with isolated filled via holes 102, the liquid metal 300 in the via holes 102 must be cut from the liquid metal 300 in the pool. A cutting process will now be described.
The cutting process uses a pinch-off effect, in which a cylinder of liquid metal 300 filling a via hole 102 and continuously therewith a respective nozzle hole 202 separates or breaks at its narrowest point. Because the diameter of the nozzle holes 202 is smaller than the diameter of the via holes 102 at the interface between the nozzle wafer 200 and the substrate wafer 100, the breakage or separation will occur in the nozzle holes 202.
To break the liquid metal 300 in the nozzle holes 202, the first P1, second P2 and third P3 pressures must be adjusted from the first pressure condition to a second pressure condition, in which the first P1 and second P2 pressures are both greater than the third pressure P3 (P1, P2 > P3), and in which the difference between the first P1 and the second P2 pressures is less than yfr ((P1 -P2) <y/r, (P1-P2,) can he negative). The second pressure condition can be reached, for example, by gradually increasing the second pressure P2 whilst maintaining the first P1 and third P3 pressures at a constant value, as shown in Figure 3C.
Figure 4A shows a simulation of the liquid metal 300 in a filled via hole 102 and nozzle hole 202 at approximately the first pressure condition. Figure 4B shows a simulation of the liquid metal 300 in a filled via hole 102 and nozzle hole 202 at approximately the second pressure condition. As shown in Figure 4B, as the second pressure condition is approached, the liquid metal 300 is pinchcd in the nozzle hole 202. The stop layer prevents the liquid metal from being pushed out of the substrate.
When the second pressure condition is reached, the liquid metal 300 breaks in the nozzle holes 202 at a break point 204, and the cutting process is complete, as shown in Figure 3D.
After the cutting or breaking process has been completed, the liquid metal 300 in the via holes 102 is cooled and solidifies, as shown in Figure 3E. The nozzle wafer 200 and pool of liquid metal 300 are then removed from the substrate wafer 100.
The substrate wafer 100 with filled via holes 102 can be used in the final stages of fabrication of a silicon interposer described in the background section above. Thus, the filling and cutting methods described herein can be used in the silicon interposer fabrication process as an alternative to the copper plating method described in the background section above, as shown in Figure 1.
A first embodiment 400 of an apparatus for performing the method described above is shown in Figure 5. The substrate wafer 100, nozzle wafer 200 and the pool of liquid metal 300 arc arranged as described above with reference to Figures 2A and 2B.
The first chamber comprises a first housing 402 that extends from the second face of the nozzle wafer 200 and encloses the pool of liquid metal 300. The third chamber comprises a second housing 410 that extends from the first face of the substrate wafer 100.
The second chamber comprises a container 404, a conduit 406 and a seal 408. The seal 408 extends between the first face of the nozzle wafer 200 and the second face of the substrate wafer 100 around the periphery of the substrate 100 and nozzle 200 wafers, enclosing the array of via holes 102 and the array of nozzle holes 202. The container 404 is connected to the space between the array of nozzle holcs 202 on the first face of the nozzle wafcr 200 and the array of via holes 102 on the second face of the substrate wafer 100 via the conduit 406, which extends through the seal 408.
Because the container 404 is separate to the rest of the apparatus in the first embodiment, the second chamber does not need to be removed during the sctup process of inserting thc substrate wafer 100 and arranging the nozzle wafer 200 and the pool of liquid metal 300, providing an easy setup process.
A second embodiment 500 of an apparatus for performing the method described above is shown in Figure 6. The substrate wafer 100, nozzle wafer 200 and the pool of liquid metal 300 are arranged as described above with reference to Figures 2A and 2B.
As in the first embodiment described above, the first chamber comprises a first housing 502 that extends from the second face of the nozzle wafer 200 and encloses the pool of liquid metal 300. Also as in the first embodiment described above, the third chamber comprises a third housing 506 that extends from the first face of the substrate wafer 100. The third housing 506 is contained within the second housing 504.
In the second embodiment, the second chamber comprises a second housing 504 that extends flvm the first face of the nozzle wafer 200 and encloses the substrate wafer 100.
Thus the second embodiment provides a relatively easy setup process and a relatively simple arrangement.
A third embodiment 600 of an apparatus for performing the method described above is shown in Figure 6. The substrate wafer 100, nozzle wafer 200 and the p001 of liquid metal 300 are arranged as described above with reference to Figures 2A and 2B.
As in the first embodiment described above, the first chamber comprises a first housing 602 that extends from the second face of thc nozzle wafer 200 and encloses the pool of liquid metal 300. Also as in the first embodiment described above, the third chamber comprises a second housing 606 that extends flxm the first face of the substrate wafer 100.
In the third embodiment, the second chamber comprises a container 604 that encloses the substrate wafer 100, the nozzle wafer 200, the pooi of liquid metal 300, the first housing 602 and the second housing 606.
Thus the third embodiment provides a simple arrangement.
In each of the first to third embodiments, each chamber comprises a pump or other means for achieving the first and second pressure conditions described above.
Figures 8A to 8G show various different profiles in vertical section for the nozzle holes 202.
Figure SA shows a simple cylinder. This is the easiest nozzle hole profile to fabricate, but it is difficult to predict where in the nozzle hole 202 the liquid metal 300 will break. The profiles shown in Figures SB to SG are more difficult to fabricate, but allow the break point 204 to be specified more precisely within the nozzle holes 202. As discussed above, the cylinder ofliquid metal 300 filling a via hole 102 and a nozzle hole 202 will break at the narrowest point.
In horizontal section, the nozzle profiles shown in Figures 8A to 8G may be circular, or they may be any other shape.
In Figures SB and SE, the nozzle hole profile is tapered from a narrowest point near the top of the nozzle hole 202 (toward the first face of the nozzle wafer 200) to a widest point near the bottom of the nozzle hole 202 (toward the second face of the nozzle wafer 200). Thus, when one of those nozzle hole profiles is used, the cylinder of liquid metal 300 will break in the nozzle hole 202 near the first face of the nozzle wafer 200, thereby minimizing the metal tail protruding from the substrate wafer 100 after cutting and cooling.
In Figures SC and SF, the nozzle hole profile is tapered from a narrowest point near the bottom of the nozzle hole 202 (toward the second face of the nozzle wafer 200) to a widest point near the top of the nozzle hole 202 (toward the first face of the nozzle wafer 200). Thus, when one of those nozzle hole profiles is used, the cylinder of liquid metal 300 will break in the nozzle hole 202 near the second face of the nozzle wafer 200, thereby maximizing the metal tail protruding from the substrate wafer 100 after cutting and cooling.
In Figures SD and SG, the nozzle hole profile is waisted, with the narrowest point of the nozzle hole profile near the middle of the nozzle hole 202 (approximately equidistant from the first and second faces of the nozzle wafer 200). Thus, when one of those nozzle hole profiles is used, the cylinder of liquid metal 300 will break in the nozzle hole 202 near the middle of the nozzle wafer 200, thereby producing a medium-sized metal tail protruding from the substrate wafer 100 after cutting and cooling.
As shown in Figures 9B to 9H, before the filling and cutting method described above is performed, the substrate wafer 100 may be provided with metal rings or pads 112 surrounding one or more of the via holes 102 on the first face of the substrate wafer 100 (as shown in Figure 9B), on the second face of the substrate wafer 100 (as shown in Figure 9C) or on both face of the substrate wafer 100 (as shown in Figure 9D). The sidewalls of the via holes 102 may be provided with a metal coating 114 as shown in Figure 9E. Any combination of the metal coating 114 of the sidewalls and the metal rings or pads 112 may be provided, as shown in Figures 9F to 9H. The metal rings or pads 112 and the metal coating 114 on the sidewalls can provide a better bond to the liquid metal 300.
As described above, the liquid metal 300 is not wettable to the substrate wafer 100. However, where the via holes 102 are provided with metal rings or pads 112 or the sidewalls of the via holes 102 are provided with a metal coating 114, the liquid metal 300 can wet the via hole.
This results in different shapes of the liquid metal 300 after cutting and cooling, as shown in Figures IOA and lOB. For example, in Figure lOB the sidewalls of the via hole 102 are provided with a metal coating 114 and a metal ring or a pad 112 is provided around the via hole 102 on the second face of the substrate wafer 100. As shown, the liquid metal 300 has wetted to the metal ring or pad 112 resulting in a bulb of material 302 at the second face of the substrate wafer 100 after cutting and cooling.
During the filling process described above, as the second P2 and third P3 pressures are decreased (or as the first pressure P1 is increased), the nozzle wafer 200 may bend because of the pressure difference between its first and second faces, and a portion of the first face of the nozzle wafer 200 may touch the second face of the substrate wafer 100 as shown in Figure 11.
The bending of the nozzle wafer 200 serves to eliminate the gap between the first face of the nozzle wafer 200 and the second face of the substrate wafer 100, and thus prevents the liquid metal 300 flowing into the gap. After the filling process has been completed and during the cuffing process described above, as the second pressure P2 is increased, the nozzle wafer 200 may straighten again.
The substrate 100 and nozzle 200 wafers described above can be formed of any suitable material, and can have any suitable shape and thickness. Each of the via holes 102 and the nozzle holes 202 can be any suitable three-dimensional shape.
The first and second pressure conditions can be achieved using any pressure differential mechanism, and may be achieved by increasing and/or decreasing any suitable combination of the pressures P1, P2 and P3.