Apple patent application US20150035028 "Image sensor with buried light shield and vertical gate" by Xiaofeng Fan, Philip H. Li, Chung Chun Wan, Anup K. Sharma, Xiangli Li proposes a 2-layer stacked global shutter pixel:
The pixel is split into 2 layers: "a buried light shield 710 separates the sensing layer 702 into a first substrate layer 738 and a second substrate layer 740. The sense region 716, the storage region 712, and the pixel transistors can be in the second substrate layer 740 while the photodetector 708 resides in the first substrate layer 738. The buried light shield 710 prevents unwanted charge accumulation in the storage region 712 and in the sense region 716." The vertical transfer gate 714 is made of poly in the trench and has a contact pad 718.