Sequential Integration Process

Abstract

A sequential integration process is described. An example process involves forming a wafer stack by bonding a first wafer to a second wafer with a front side of the first wafer facing a front side of the second wafer, the first wafer including a first device region formed on the front side of the first wafer and including a set of semiconductor devices. The example process involves, subsequent to forming the wafer stack, forming a second device region on a back side of the first wafer, the second device region including a set of semiconductor devices. The example process involves forming at least one interconnection layer on the second device region for electrically interconnecting the semiconductor devices of the second device region. The example process also involves forming at least one via extending through the wafer stack from the at least one interconnection layer and through the first wafer.

What is claimed is: 
     
         1 . A process comprising:
 forming a wafer stack by bonding a first wafer to a second wafer with a front side of the first wafer facing a front side of the second wafer, the first wafer including a first device region formed on the front side of the first wafer and including a first set of transistors;   subsequent to bonding the first wafer to the second wafer, forming a second device region on a back side of the first wafer, the second device region including a second set of transistors that are distinct from the first set of transistors;   forming an interconnection layer on the second device region for electrically interconnecting the second set of transistors; and   forming a via extending through the wafer stack from the interconnection layer and through the first wafer.   
     
     
         2 . The process according to  claim 1 , further comprising bonding a semiconductor layer to the back side of the first wafer with a front side of the semiconductor layer facing the back side of the first wafer, wherein the forming of the second device region includes forming the second device region on a back side of the semiconductor layer. 
     
     
         3 . The process according to  claim 1 , wherein a semiconductor material forming the second set of transistors is different from a semiconductor material forming the first set of transistors. 
     
     
         4 . The process according to  claim 2 , wherein the semiconductor layer is formed on a bottom substrate, and wherein the process further comprises:
 subsequent to the bonding of the semiconductor layer to the back side of the first wafer, removing the bottom substrate from the semiconductor layer to expose the back side of the semiconductor layer and thereafter forming the second device region on the back side of the semiconductor layer.   
     
     
         5 . The process according to  claim 4 , wherein the bottom substrate includes a Si-layer and the semiconductor layer is formed by a group IV-layer, a group III-V layer, or a stack including a group III-V layer formed on a Ge-layer. 
     
     
         6 . The process according to  claim 2 , further comprising forming a third device region including a third set of transistors on the front side of the semiconductor layer. 
     
     
         7 . The process according to  claim 6 , further comprising forming an additional interconnection layer on the third device region for electrically interconnecting the third set of transistors. 
     
     
         8 . The process according to  claim 1 , wherein the first wafer includes a semiconductor layer, the first device region being formed on a front side of the semiconductor layer, and wherein the process further comprises:
 subsequent to the bonding to the second wafer, forming the second device region on a back side of the semiconductor layer.   
     
     
         9 . The process according to  claim 8 , wherein the first wafer includes a bottom substrate, the back side of the semiconductor layer facing the bottom substrate, and wherein the process further comprises:
 subsequent to the bonding to the second wafer, removing the bottom substrate from the semiconductor layer to expose the back side of the semiconductor layer and thereafter forming the second device region on the back side of the semiconductor layer.   
     
     
         10 . The process according to  claim 9 , wherein the bottom substrate includes a Si-layer and the semiconductor layer is formed by a Ge-layer, a group III-V layer, or a stack including a group III-V layer formed on a Ge-layer. 
     
     
         11 . The process according to  claim 1 , wherein the forming of the second device region comprises forming the second device region directly on the back side of the first wafer. 
     
     
         12 . The process according to  claim 11 , further comprising thinning the first wafer subsequent to the bonding of the first wafer to the second wafer, wherein the second device region is formed on the back side of the thinned first wafer. 
     
     
         13 . The process according to  claim 1 , further comprising forming an additional interconnection layer on the first device region for electrically interconnecting the first set of transistors. 
     
     
         14 . The process according to  claim 1 , further comprising forming a third device region on the front side of the second wafer that includes a third set of transistors. 
     
     
         15 . The process according to  claim 14 , further comprising forming an additional interconnection layer on the third device region for electrically interconnecting the third set of transistors. 
     
     
         16 . A sequential integration process comprising:
 forming a wafer stack by bonding a first wafer to a second wafer with a front side of the first wafer facing a front side of the second wafer, the first wafer including a first device region formed on the front side of the first wafer and including a first set of transistors;   subsequent to bonding the first wafer to the second wafer, forming a second device region on a back side of the first wafer, the second device region including a second set of transistors that are distinct from the first set of transistors;   forming an interconnection layer on the second device region for electrically interconnecting the second set of transistors;   forming a via extending through the wafer stack from the interconnection layer and through the first wafer;   forming a third device region including a third set of transistors on a front side of a semiconductor layer; and   bonding the semiconductor layer to the back side of the first wafer with the front side of the semiconductor layer facing the back side of the first wafer, wherein the forming of the second device region includes forming the second device region on a back side of the semiconductor layer.   
     
     
         17 . A wafer stack comprising:
 a first wafer and a second wafer, the first wafer being bonded to the second wafer with a front side of the first wafer facing a front side of the second wafer, wherein the first wafer includes:
 a first device region formed on the front side of the first wafer and including a set of semiconductor devices; 
 a second device region formed on a back side of the first wafer and including a second set of semiconductor devices; and 
 at least one interconnection layer arranged on the second device region configured to electrically interconnect the semiconductor devices of the second device region; 
   wherein the wafer stack further includes at least one via extending through the wafer stack from the at least one interconnection layer and through the first wafer.   
     
     
         18 . The wafer stack of  claim 17 , wherein a semiconductor material forming the semiconductor devices of the second device region is different from a semiconductor material forming the semiconductor devices of the first device region. 
     
     
         19 . The wafer stack of  claim 17 , further comprising a semiconductor layer bonded to the back side of the first wafer with a front side of said semiconductor layer facing the back side of the first wafer. 
     
     
         20 . The wafer stack of  claim 17 , wherein the semiconductor layer is formed on a bottom substrate.