Use of vacuum chucks to hold a wafer or wafer sub-stack

1. A method of forming a wafer stack, the method comprising:

using a vacuum chuck to hold a wafer sub-stack that comprises a plurality of wafers stacked on one another, the vacuum chuck including a first surface with a vacuum groove, wherein the first surface of the vacuum chuck includes a central region that is encircled by the vacuum groove and an outer region that encircles the vacuum groove, and wherein the central region and the outer region of the first surface of the vacuum chuck are at substantially the same height, wherein a first surface of the wafer sub-stack that faces the vacuum chuck includes a plurality of features thereon that contact the central region of the vacuum chuck's first surface, the first surface of the wafer sub-stack further including a sealing ring that contacts the outer region of the vacuum chuck's first surface; and

bringing a second wafer into contact with a second surface of the wafer sub-stack, wherein the second surface of the wafer sub-stack is on a side of the wafer sub-stack opposite its first surface.

2. The method of claim 1 wherein the features on the first surface of the wafer sub-stack comprise bonding pads.

3. The method of claim 2 wherein each of the bonding pads and the sealing ring comprises an electrical contact and solder paste.

4. The method of claim 1 wherein each of the features and the sealing ring is composed of the same materials and extends substantially the same distance beyond the first surface of the wafer sub-stack.

5. The method of claim 1 including generating a vacuum to hold the wafer sub-stack to the vacuum chuck.

6. A method of forming a wafer stack, the method comprising:

holding, by a first vacuum chuck, a wafer sub-stack that comprises a plurality of wafers stacked on one another, the first vacuum chuck including a first surface with a vacuum groove, wherein the first surface of the first vacuum chuck includes a central region that is encircled by the vacuum groove and an outer region that encircles the vacuum groove, and wherein the central region and the outer region of the first surface of the first vacuum chuck are at substantially the same height, wherein a first surface of the wafer sub-stack that faces the first vacuum chuck includes a plurality of features thereon that contact the central region of the vacuum chuck's first surface, the first surface of the wafer sub-stack further including a sealing ring that contacts the outer region of the first vacuum chuck's first surface;

holding, by a second vacuum chuck, a second wafer, wherein a first surface of the second wafer that faces the second vacuum chuck includes features that project toward the second vacuum chuck, and wherein the second vacuum chuck includes a central recessed surface on which a non-sticky, soft, elastic and non-abrasive material is disposed, and wherein the features of the second wafer on its first surface are in contact with the non-sticky, soft, elastic and non-abrasive material, and wherein part of the first surface of the second wafer near its periphery is in contact with an elevated section of the second vacuum chuck that includes a vacuum channel; and

bringing the second wafer into contact with a second surface of the wafer sub-stack, wherein the second surface of the wafer sub-stack is on a side of the wafer sub-stack opposite its first surface.

7. The method of claim 6 wherein the non-sticky, soft, elastic and non-abrasive material is a silicone mat.

8. The method of claim 6 wherein the features on the wafer sub-stack comprise bonding pads composed of the same materials as the sealing ring.

9. The method of claim 8 wherein the features on the second wafer comprise lens elements.

10. A method of forming a wafer stack, the method comprising:

mounting a first wafer on dicing tape, wherein a first surface of the first wafer includes a plurality of features thereon and a sealing ring encircling the plurality of features, wherein the plurality of features and the sealing ring extend substantially the same distance beyond the first surface of the first wafer; and

using a vacuum chuck to hold the first wafer, wherein the dicing tape is disposed between the vacuum chuck and the first wafer, wherein the vacuum chuck includes a first surface with a vacuum groove, wherein the first surface of the vacuum chuck includes a central region that is encircled by the vacuum groove and an outer region that encircles the vacuum groove, and wherein the central region and the outer region of the first surface of the vacuum chuck are at substantially the same height, wherein the plurality of features on the first surface of the first wafer contact the central region of the vacuum chuck's first surface, and the sealing ring contacts the outer region of the vacuum chuck's first surface.

11. The method of claim 10 including bringing a second wafer into contact with a second surface of the first wafer, wherein the second surface of the first wafer is on a side of the first wafer opposite its first surface.

12. The method of claim 11 wherein the first wafer includes one or more through-holes, and the second wafer includes UV-curable adhesive material that enters the one or more through-holes when the first and second wafers are brought into contact with one another.

13. The method of claim 10 wherein the plurality of features on the first surface of the first wafer comprise bonding pads, and wherein each of the bonding pads and the sealing ring comprises an electrical contact and solder paste.