A semiconductor device, comprising: a carrier wafer (101, 151, 301, 401); and an integrated circuit, IC (106, 186, 196), formed on a device wafer (103, 153, 303, 403) and bonded to the carrier wafer (101, 151, 301, 401), wherein the IC (106, 186, 196) includes: a front end layer (85, 105, 195, 305, 405) having one or more transistors at a front side of the device wafer (103, 153, 303, 403), and one or more gaps (183, 188) formed by removing components of the one or more transistors; a back end layer (107, 187, 197, 307, 407) having a metal interconnect coupled to the one or more transistors of the front end layer (85, 105, 195, 305, 405); and a capping layer (121, 181, 191, 321, 421) at backside (315, 415) of the device wafer (103, 153, 303, 403) which is opposite the front side of the device wafer (103, 153, 303, 403), filling at least partially the one or more gaps (183, 188) of the front end layer (85, 105, 195, 305, 405), wherein the capping layer (121, 181, 191, 321, 421) is next to the front end layer (85, 105, 195, 305, 405) of the device wafer (103, 153, 303, 403), wherein the capping layer (121, 181, 191, 321, 421) is bonded to the carrier wafer (101, 151, 301, 401); and wherein one or more air gaps (114, 116, 198, 314, 414) are formed within the one or more gaps (183, 188) of the front end layer (85, 105, 195, 305, 405) and between the capping layer (121, 181, 191, 321, 421) and the back end layer (107, 187, 197, 307, 407), wherein the one or more air gaps (114, 116, 198, 314, 414) are to reduce parasitic capacitance of the IC (106, 186, 196) compared to the IC (106, 186, 196) without the one or more air gaps (114, 116, 198, 314, 414). a carrier wafer (101, 151, 301, 401); and an integrated circuit, IC (106, 186, 196), formed on a device wafer (103, 153, 303, 403) and bonded to the carrier wafer (101, 151, 301, 401), wherein the IC (106, 186, 196) includes: a front end layer (85, 105, 195, 305, 405) having one or more transistors at a front side of the device wafer (103, 153, 303, 403), and one or more gaps (183, 188) formed by removing components of the one or more transistors; a back end layer (107, 187, 197, 307, 407) having a metal interconnect coupled to the one or more transistors of the front end layer (85, 105, 195, 305, 405); and a capping layer (121, 181, 191, 321, 421) at backside (315, 415) of the device wafer (103, 153, 303, 403) which is opposite the front side of the device wafer (103, 153, 303, 403), filling at least partially the one or more gaps (183, 188) of the front end layer (85, 105, 195, 305, 405), wherein the capping layer (121, 181, 191, 321, 421) is next to the front end layer (85, 105, 195, 305, 405) of the device wafer (103, 153, 303, 403), wherein the capping layer (121, 181, 191, 321, 421) is bonded to the carrier wafer (101, 151, 301, 401); and a front end layer (85, 105, 195, 305, 405) having one or more transistors at a front side of the device wafer (103, 153, 303, 403), and one or more gaps (183, 188) formed by removing components of the one or more transistors; a back end layer (107, 187, 197, 307, 407) having a metal interconnect coupled to the one or more transistors of the front end layer (85, 105, 195, 305, 405); and a capping layer (121, 181, 191, 321, 421) at backside (315, 415) of the device wafer (103, 153, 303, 403) which is opposite the front side of the device wafer (103, 153, 303, 403), filling at least partially the one or more gaps (183, 188) of the front end layer (85, 105, 195, 305, 405), wherein the capping layer (121, 181, 191, 321, 421) is next to the front end layer (85, 105, 195, 305, 405) of the device wafer (103, 153, 303, 403), wherein the capping layer (121, 181, 191, 321, 421) is bonded to the carrier wafer (101, 151, 301, 401); and wherein one or more air gaps (114, 116, 198, 314, 414) are formed within the one or more gaps (183, 188) of the front end layer (85, 105, 195, 305, 405) and between the capping layer (121, 181, 191, 321, 421) and the back end layer (107, 187, 197, 307, 407), wherein the one or more air gaps (114, 116, 198, 314, 414) are to reduce parasitic capacitance of the IC (106, 186, 196) compared to the IC (106, 186, 196) without the one or more air gaps (114, 116, 198, 314, 414).