Thin semiconductor device having embedded die support and methods of making the same
View Patent ↗Ultra-thin semiconductor devices, including piezo-resistive sensing elements can be formed a wafer stack that facilitates handling many thin device dice at a wafer level. Three embodiments are provided to form the thin dice in a wafer stack using three different fabrication techniques that include anodic bonding, adhesive bonding and fusion bonding. A trench is etched around each thin die to separate the thin die from others in the wafer stack. A tether layer, also known as a tether, is used to hold thin dice or dice in a wafer stack. Such as wafer stack holds many thin dice together at a wafer level for handling and enables easier die picking in packaging processes.
1. A wafer stack comprising:
a silicon device wafer and a silicon substrate wafer;
a silicon die having a circuit formed thereon the device wafer, the die extending over a cavity in the device wafer and having an outer perimeter defined by a trench formed in the device wafer, the trench separating the die from the device wafer; and
a single tether attached to the die, extending across the trench and ending on the device wafer.
2. A wafer stack comprising:
a silicon device wafer and a silicon substrate wafer;
a silicon die having a circuit formed thereon the device wafer, the die extending over a cavity in the device wafer and having an outer perimeter defined by a trench formed in the device wafer, the trench separating the die from the device wafer; and a die tether layer attached to the die, extending across the trench and ending on the device wafer.
3. The wafer stack of claim 1 or 2 wherein the die tether layer is a substantially planar layer formed on the bottom of the silicon die facing to the cavity, and wherein the die tether layer is comprised of at least one of: silicon oxide, silicon nitride and poly silicon.
4. The wafer stack of claim 1 or 2 , wherein the die tether layer is perforated.
5. The wafer stack of claim 1 or 2 , wherein the die tether layer has a thickness less than about two microns.
6. The wafer stack of claim 1 or 2 , wherein the die tether layer is a layer formed using at least one of:
thermal oxidation;
low pressure chemical vapor deposition (LPCVD); and
plasma enhanced chemical vapor deposition (PECVD).
7. The wafer stack of claim 1 or 2 , wherein the trench has a width less than about ten microns.
8. The wafer stack of claim 1 or 2 , wherein the die has a nominal thickness and wherein the open trench has a depth substantially equal to the die nominal thickness.
9. The wafer stack of claim 1 or 2 , wherein the trench has a bottom and wherein the trench bottom is comprised of material of the die tether layer.
10. The wafer stack of claim 1 or 2 , wherein the die tether layer is an etch stop layer for an etching process used to form the trench.
11. The wafer stack of claim 1 or 2 , wherein the trench is formed by etching.
12. The wafer stack of claim 1 or 2 wherein the circuit on the die forms part of pressure sensor.
13. The wafer stack of claim 1 or 2 , wherein the wafer stack is made of two silicon wafers attached to each other by a fusion bond, the trench separating the silicon die from the device wafer, and a single tether or a die tether layer attaching the silicon die to the device wafer.
14. The wafer stack of claim 1 or 2 wherein the trench is etched onto the top surface of the device wafer.
15. The wafer stack of claim 1 or 2 wherein the trench formed in the device wafer is around the circuit.
16. A wafer stack comprising:
a thin silicon device layer having top and bottom surfaces and a cap wafer;
at least one die formed on a portion of the thin silicon device layer, and having a circuit on the die;
a tether covering at least a portion of the top surface of the die and extending over the die;
an anodic bonding between the cap wafer and top surface of the thin silicon layer;
wherein the cap wafer is over the die and forms a sealed gap between the top surface of the die and the cap wafer;
wherein the die has an outer perimeter defined by a trench formed from a bottom surface of the thin silicon device layer and surrounding the die, the die being attached to the wafer stack by the tether extending over the trench, said tether extending across the trench.
17. The wafer stack of claim 16 , wherein the tether is a substantially planar layer formed on the topside of the die and at least portion of the top surface of the thin silicon layer.
18. The wafer stack of claim 17 , wherein the tether is an etch stop layer for an etching process used to form the trench.
19. A wafer stack comprising:
a thin silicon device layer having top and bottom surfaces and a cap wafer;
a die formed on a portion of the thin silicon device layer and having a circuit on the top surface of the die;
a tether covering at least a portion of the top surface of the die and extending over the die;
wherein the cap wafer is suspended over the die and forms a sealed gap, and wherein the die has an outer perimeter defined by a trench formed from the bottom surface of the thin silicon device layer and surrounding the die, the die being attached to the wafer stack by the passivation layer extending over the trench;
an anodic bonding between the cap wafer and top surface of the thin silicon layer;
wherein said tether extends across the trench attaching the die to the wafer stack.
20. The wafer stack of claim 19 , wherein the cap wafer is comprised of a borosilicate glass.
21. The wafer stack of claim 19 , wherein the cap wafer is comprised of single crystalline silicon.
22. The wafer stack of claim 19 , 20 or 21 , further comprising an adhesive bond in an adhesive bonding zone between the cap wafer and the top surface of the thin silicon layer.
23. The wafer stack of claim 22 , wherein an inner perimeter of the adhesive bond zone defines an outer perimeter of a gap between the cap wafer and the top surface of the die, the thickness of the adhesive layer defining a height of the gap.
24. The wafer stack of claim 19 , wherein the trench is configured to be formed after fabricating the circuit and tether.
25. The wafer stack of claim 19 , wherein the cap wafer encloses at least one die having a circuit on the top surface of the die within the outer perimeter of the cap wafer.
26. The wafer stack of claim 19 , wherein the tether includes at least one layer comprised of at least one of:
silicon oxide;
silicon nitride;
polyimide;
polysilicon;
a film of ductile metal.
27. The wafer stack of claim 19 , wherein the tether has a thickness less than about two microns.
28. The wafer stack of claim 19 , wherein the tether is formed using at least one of:
thermal oxidation;
low pressure chemical vapor deposition (LPCVD);
plasma enhanced chemical vapor deposition (PECVD);
sputtering of ductile metals;
evaporation of ductile metals;
coating with polyimide.
29. The wafer stack of claim 19 wherein the trench has a width less than about ten microns.
30. The wafer stack of claim 19 wherein the die has a nominal thickness and wherein the trench has a depth substantially equal to the die nominal thickness.
31. The wafer stack of claim 19 , wherein the trench has a bottom and wherein the trench bottom is comprised of material of the tether layer.
32. The wafer stack of claim 19 , wherein the tether layer is an etch stop layer for a an etching process used to form the trench.
33. The wafer stack of claim 19 , wherein the trench is formed by silicon etching.
34. The wafer stack of claim 19 , wherein the tether layer is perforated.
35. The wafer stack of claim 19 , wherein the circuit forms part of a pressure sensor.