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.

Claims

1. 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 set of semiconductor devices; 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 second set of semiconductor devices; forming at least one interconnection layer on the second device region for electrically interconnecting the semiconductor devices of the second device region; and forming at least one via extending through the wafer stack from the at least one 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 2, 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.

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 material layer, or a stack including a group III-V material layer formed on a Ge-layer.

6. The process according to claim 2, wherein a third device region including a third set of semiconductor devices is formed on the front side of the semiconductor layer.

7. The process according to claim 6, wherein at least one interconnection layer is formed on the third device region for electrically interconnecting the semiconductor devices of the third device region.

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 material layer, or a stack including a group III-V material 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, wherein at least one interconnection layer is formed on the first device region for electrically interconnecting the semiconductor devices of the first device region.

14. The process according to claim 1, wherein the second wafer includes a device region formed on the front side of the second wafer and including a third set of semiconductor devices.

15. The process according to claim 14, wherein at least one interconnection layer is formed on the device region of the second wafer for electrically interconnecting the semiconductor devices of the device region of the second wafer.

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 set of semiconductor devices; 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 second set of semiconductor devices; forming at least one interconnection layer on the second device region for electrically interconnecting the semiconductor devices of the second device region; forming at least one via extending through the wafer stack from the at least one interconnection layer and through the first wafer; forming a third device region including a third set of semiconductor devices on a front side of a semiconductor layer; and bonding the 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.

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.

Description

BRIEF DESCRIPTION OF THE FIGURES

[0111] The above, as well as additional, features will be better understood through the following illustrative and non-limiting detailed description of example embodiments, with reference to the appended drawings. In the drawings like reference numerals will be used for like elements unless stated otherwise.

[0112] FIG. 1 is a schematic illustration of an existing 3D integration process.

[0113] FIG. 2 is a schematic illustration of an existing 3D integration process.

[0114] FIGS. 3a, 3b, and 3c, are schematic illustrations of a sequential integration process, according to an example embodiment.

[0115] FIGS. 4a, 4b, 4c, and 4d are schematic illustrations of a sequential integration process, according to an example embodiment.

[0116] FIGS. 5a, 5b, 5c, and 5d are schematic illustrations of a sequential integration process, according to an example embodiment.

DETAILED DESCRIPTION

[0117] Example embodiments will now be described more fully hereinafter with reference to the accompanying drawings. That which is encompassed by the claims may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided by way of example. Furthermore, like numbers refer to the same or similar elements or components throughout.

[0118] A number of example sequential integration processes will now be described with reference to FIGS. 3a-c, 4a-d, and 5a-d. In the figures, various wafer stacks are schematically shown in a cross-section taken vertically through the stacks. Only portions of the layers forming part of the stacks are shown and the layers may extend laterally/horizontally beyond the illustrated portions. It should also be noted that for the purpose of clarity the various layers and other features of the stacks are not drawn to scale and their relative dimensions, in particular their thickness, may differ from a physical stack.

[0119] FIG. 3a through 3c illustrate an example sequential integration process which is particularly suitable for a homo-integration approach.

[0120] FIG. 3a shows a first wafer 110. The first wafer 110 may be of a semiconductor material such as Si, Ge, SiGe, SiC, GaAs or InP. The first wafer 110 may also be a SOI wafer, a GeOI wafer, SiGeOI wafer or a III-V-OI wafer. A first device region 112 is formed on a front side 110a of the first wafer 110. At least one interconnection layer 114 is formed on the first device region 112. A bonding layer 116 is formed on the interconnection layer 114.

[0121] The first device region 112 includes a set of semiconductor devices and defines a first FEOL layer of the first wafer 110. The set of semiconductor devices may include planar or non-planar devices. By way of example, the set of semiconductor devices may include metal-oxide-semiconductor field-effect transistors (MOSFETs), metal-insulator-semiconductor field-effect transistors (MISFETs), bipolar transistors (BJTs), heterojunction BJTs (HBTs), fin field-effect transistors (FinFETs), nanowire FETs, high electron mobility transistors (HEMT), junction less transistor (JLT), and/or tunnel Field effect transistor (TFET). The semiconductor devices may be formed using conventional device fabrication/FEOL processing in accordance with the particular type of devices.

[0122] Although in the drawings the device regions are generally illustrated as layers, it is noted that a device region typically may present a limited horizontal extension on a wafer side. Semiconductor devices need hence not be present throughout the layers indicated in the drawings. However, the designation device/FEOL layer may be understood to refer to a thickness portion of a wafer/substrate in or on which a device region (for instance the first device region 112) is formed.

[0123] By way of example, FEOL processing for forming a set of planar FETs may include a number of different processing stages or module including for instance forming of device isolation using the shallow trench isolation (STI) technique on the front side of the first wafer 110. Forming of STI regions may be followed by formation of the gate, which could be formed by either a gate first or a gate last process (also known as replacement metal gate process). In a gate first process, the gate dielectric and the gate conductor are formed prior to forming of source and drain regions. In a gate last process, the final gate dielectric and gate conductor are formed after forming of source and drain regions. In an example gate last process, a dummy gate oxide and a dummy gate (e.g. of amorphous/poly Si) may be deposited and patterned. Source and drain region may then be formed by implantation and subsequent annealing or by in-situ epitaxial growth of a doped semiconductor layer. An interlayer dielectric (ILD) deposition may thereafter be performed. Following CMP of the ILD, the dummy gate and dummy gate oxide may be removed to create a trench in the ILD. A gate dielectric and a gate conductor may be deposited to fill the trench. Portions of the gate conductor deposited outside the trench may be removed by CMP. Contact windows may be opened above the source and drain regions and filled with metal. In the gate last process, spacers may be formed on the dummy gate or the final gate by spacer material deposition.

[0124] The above discussion concerning the first device region 112 and the FEOL processing is applicable also to the further device regions discussed below. The discussion will therefore not be repeated in connection with the further device regions.

[0125] The at least one interconnection layer 114 is formed on the first device region 112 for interconnecting the semiconductor devices of the first device region 112. The at least one interconnection layer 114 forms a BEOL layer with respect to the first device region 112. The at least one interconnection layer 114 may include a plurality of dielectric layers including conductive paths. The conductive paths may be formed by a conductive material, for instance metallic conductors such as Cu, W, Au, Ag, Al, Co or Ru. The dielectric layers may be formed for instance by silicon oxide, silicon oxide nitride, silicon carbide nitride or silicon nitride.

[0126] Also shown in FIG. 3a is a second wafer 120. Similar to the first wafer 110, a device region 122, an interconnection layer 124, and a bonding layer 126 is formed on a front side 120a of the second wafer 120. The first wafer 120 and the second wafer 120 may be formed of a material of a same or a different type. The device region 122 and the interconnection layer 124 may be formed to implement desired type(s) of devices and circuitry.

[0127] At the right-hand side of FIG. 3a is shown a wafer stack formed by bonding the first wafer 110 to the second wafer 120 with the front side 110a of the first wafer 110 facing the front side 120a of the second wafer 120. As seen in FIG. 3a, a back side 110b of the first wafer 110 thereby forms a front side of the wafer stack.

[0128] The bonding is achieved by the bonding layers 116, 126. The bonding layers 116, 126 may be oxide or nitride layers, wherein the wafers 110, 120 may be bonded by oxide-oxide or nitride-nitride bonding. More generally, the bonding may be achieved by molecular bonding. The bonding layers may also be adhesive layers, wherein the wafers 110, 120 may be glued together (adhesive bonding). The bonding layers 116, 126 may be metallic bonding layers (such as Au or Ag), wherein the wafers 110, 120 may be bonded by thermocompression.

[0129] In FIG. 3b, a second device region 132 is formed on the back side 110b of the first wafer 110. The second device region 132 includes a set of semiconductor devices and defines a second FEOL layer of the first wafer 110. At least one interconnection layer 134 is formed on the second device region 132. The at least one interconnection layer 134 forms a BEOL layer with respect to the second device region 132.

[0130] The second device region 132 and the at least one interconnection layer 134 may be implemented in a similar manner as outlined in connection with the first device region 112 and the at least one interconnection layer 114, respectively. However it should be noted that, in an example embodiment, all FEOL processing steps to form the second device region 132, as well as BEOL processing steps to form the at least one interconnection layer 134, are performed at temperatures below 600 C. or in an example embodiment below 500 C. to avoid affecting BEOL layers and other FEOL layers already present in the wafer stack. More generally, the limit on temperature could vary depending upon metal and type of inter metal dielectric used in the BEOL layer and time for which the wafer stack is subjected to these temperature.

[0131] As shown in FIG. 3b, prior to forming the second device region 132, an optional thinning of the first wafer 110 may be performed. The thinning may include grinding and/or (selective) etch back from the back side 110b of the first wafer 110, optionally followed by CMP. The second device region 132 may thereafter be formed on the thinned wafer 110.

[0132] In FIG. 3c a number of vias 140 is formed to extend through the wafer stack. Although FIG. 3c shows four vias 140, this represents an example and the number of vias 140 may be implementation specific. By way of example, two vias extend from the at least one interconnection layer 134, through the first wafer 110, the at least one interconnection layer 114, and the bonding layers to the at least one interconnection layer 124. The two vias may accordingly electrically interconnect the at least one interconnection layers 134, 114 and 124 and the device regions 132, 112 and 122. The two other vias extend from the at least one interconnection layer 134, through first wafer 110 to the at least one interconnection layer 114. The other two vias may accordingly electrically interconnect the at least one interconnection layers 134 and 114 and the device regions 132 and 112.

[0133] The vias 140 may be formed by a conductive material, for instance metallic conductors such as Cu, W, Au, Ag, Al, Co, Ni, Ru or by highly doped semiconductor material. The vias 140 may further include a liner material formed by a dielectric material, such as a silicon oxide or silicon nitride. Vias formed by conductive material and dielectric liner may be referred to as TSVs or vias that extend through semiconductor material regions. Vias extending only through dielectric material, such as STI regions, need not be formed with a dielectric liner and may be referred to as inter-tier vias.

[0134] The vias 140 may be formed in a conventional manner, by forming a respective trench through the wafer stack at the intended positions of the vias 140 to be formed. An etch mask (not shown) may be formed above the at least one interconnection layer 134 with openings defining the positions of the trenches. The mask may be patterned using lithography and etching. The trenches may be formed by anisotropic etching through the mask openings. The etching may be continued until trenches with a desired depth have been formed, typically at a depth corresponding to one of the interconnection layers 114 or 124. A via liner material may be deposited in the trenches and subsequently a conductive material may be deposited in the trenches. The liner may function as an insulator to prevent a short between the conductive material and semiconductor layer or it could function as a diffusion barrier along sections not extending through semiconductor material. The mask may be stripped and any material deposited outside the trenches may be removed by CMP. Masking material as well as lithography and etching technique may be selected in accordance with the material systems and device implementation of the first wafer 110. If vias 140 of different materials or dimensions (e.g. height or depth) are to be formed, the forming the vias 140 may be formed by repeating the above process steps a number of times to form each desired via structure.

[0135] Following forming of the structure shown in FIG. 3c, further back end or packaging processing may be performed in a conventional manner, including for instance forming of contact pads above the at least one interconnection layer 134 to provide a contact interface towards the vias 140.

[0136] In an example variation of the process described with reference to FIG. 3a-c, in an example embodiment the at least one interconnection layer 114 may not be present on the first device region 112. Furthermore, in an example embodiment, the device region 122 and/or the at least one interconnection layer 124 may not be present on the second wafer 120. Hence, in a basic variation of the above-described process, device regions may be formed only on the front side 110a and the back side 110b of the first wafer 110/110. In that case, the second wafer 120 may function as a handling wafer for the wafer stack and may accordingly be formed by a non-semiconductor material, for instance a dielectric material such as a glass material. Following formation of the vias 140, the second wafer 120 may be thinned or even removed.

[0137] FIG. 4a through 4d illustrate a further example sequential integration process which is particularly suitable for a hetero-integration approach.

[0138] FIG. 4a shows a wafer stack including a first wafer 210 and a second wafer 220. The layers/wafers 210, 212, 214, 216, 220, 222, 224, 226 forming part of the wafer stack corresponds to the layers/wafers 110, 112, 114, 116, 120, 122, 124, 126 of the wafer stack shown in FIG. 3a. The wafer stack including the first wafer 210 and the second wafer 220 may be formed in the manner described in connection with FIG. 3a.

[0139] Accordingly, a first device region 212 is formed on a front side 210a of the first wafer 210. At least one interconnection layer 114 is formed on the first device region 112. A bonding layer 116 is formed on the interconnection layer 114. Similar to the first wafer 210, a device region 222, an interconnection layer 224, and a bonding layer 226 is formed on the front side 220a of the second wafer 120. The first wafer 210 is bonded to the second wafer 220 with the front side 210a of the first wafer 210 facing a front side 220a of the second wafer 220. A bonding layer 218 similar to bonding layers 216, 226 is formed on top of the wafer stack.

[0140] FIG. 4a also shows a stack of layers including a bottom substrate 250 and a semiconductor layer 252. The semiconductor layer 252 is formed with a back side 252b facing the bottom substrate 250. A bonding layer 256 similar to the bonding layer 218 is formed on the front side 252a of the semiconductor layer 252.

[0141] The bottom substrate 250 may include a Si-layer. A semiconductor layer 252 of a Ge-layer, a group III-V material layer may be grown on the Si-layer of the bottom substrate 250 in a conventional manner, with or without an intermediate SRB. The semiconductor layer 252 may also be formed as a layer stack including a Ge-layer epitaxially grown on the Si-layer of the bottom substrate 250, with or without an intermediate SRB, and a group III-V material layer epitaxially grown on the Ge-layer, with or without an intermediate SRB. The bottom substrate 250 may also be formed of a semiconductor material other than Si, for instance Ge, SiGe, SiC, GaAs or InP. The bottom substrate 250 may also be a SOI wafer, a GeOI wafer, SiGeOI wafer or a III-V-OI wafer. The bottom substrate 250 may alternatively function merely as a handling wafer and be formed of a dielectric material, such as a glass material.

[0142] Optionally, a device region (which may be referred to as a third device region) including a set of semiconductor devices may be formed on the front side 252a of the semiconductor layer 252. At least one interconnection layer for electrically interconnecting the semiconductor devices of the device region on the front side 252a may be formed on the device region. The optional presence of a device region and at least one interconnection layer is indicated by the dashed box 254 in FIG. 4a but will be omitted from the further discussion and figures for illustrational clarity.

[0143] At the right-hand side of FIG. 4a is shown a wafer stack formed by bonding the stack including the bottom substrate 250 and the semiconductor layer 252 to a back side 210b of the first wafer 210 with the front side 252a of the semiconductor layer 252 facing the back side 210b of the first wafer 210. The bonding is achieved by the bonding layers 256, 218.

[0144] In FIG. 4b, the bottom substrate 250 has been removed to expose the back side 252b of the semiconductor layer 252. Removing the bottom substrate 250 may include grinding, CMP and/or etch back from the back side bottom substrate 250 until the semiconductor layer 252 is exposed (e.g. the Ge-layer or the SRB, if present).

[0145] As shown in FIG. 4b, an optional thinning of the semiconductor layer 252 may be performed to form a thinned semiconductor layer 252. The thinning may include grinding, CMP and/or etch back from the back side 252b of the semiconductor layer 252.

[0146] In FIG. 4c, a device region 232 (which may be referred to as a second device region of the first wafer 210 since the semiconductor layer 252 is bonded to the first wafer 210) is formed on the back side 252b of the semiconductor layer 252. The device region 232 includes a set of semiconductor devices and defines a second FEOL layer of the first wafer 110. At least one interconnection layer 234 is formed on the device region 232. The at least one interconnection layer 234 forms a BEOL layer with respect to the device region 232.

[0147] The device region 232 and the at least one interconnection layer 234 may be implemented in a similar manner as outlined in connection with the second device region 132 and the at least one interconnection layer 134 in FIG. 3b, respectively. Accordingly, in an example embodiment all FEOL processing steps relating to the device region 232, as well as BEOL processing steps relating to the at least one interconnection layer 234, are performed at temperatures below 600 C. or in an example embodiment below 500 C. to avoid affecting BEOL layers and other FEOL layers already present in the wafer stack.

[0148] In FIG. 4d a number of vias 240 is formed to extend through the wafer stack (by way of example four vias). The vias 240 may be formed with a similar structure and in a similar manner as the above described vias 140.

[0149] Following forming of the structure shown in FIG. 4d, further back end processing may be performed as described in connection with FIG. 3c.

[0150] In an example variation of the process described with reference to FIG. 4a-d, in an example embodiment the at least one interconnection layer 214 may not be present on the first device region 212. Furthermore, in an example embodiment, the device region 222 and/or the at least one interconnection layer 224 may not be present on the second wafer 220. Hence, in a basic variation of the above-described process, device regions may be formed only on the front side 210a of the first wafer 210 and on the back side 252b of the semiconductor layer 252/252 (which is bonded to the back side 210b of the first wafer 210). In that case, the second wafer 220 may function as a handling wafer for the wafer stack and may accordingly be formed by a non-semiconductor material, for instance a dielectric material such as a glass material. Following formation of the vias 240, the second wafer 220 may be thinned or even removed.

[0151] FIG. 5a through 5d illustrate a further example sequential integration process which is particularly suitable for a hetero-integration approach.

[0152] FIG. 5a shows a first wafer including a layer stack comprising a bottom substrate 310 and a semiconductor layer 311. The semiconductor layer 311 is formed with a back side 311b facing the bottom substrate 310. A first device region 312 is formed on a front side 311a of the semiconductor layer 311. The first device region 312 includes a set of semiconductor devices and defines a first FEOL layer of the first wafer. At least one interconnection layer 314 is formed on the first device region 312. A bonding layer 316 similar to any of the above-described bonding layers is formed on the at least one interconnection layer 314.

[0153] The bottom substrate 310 may include a Si-layer. A semiconductor layer 311 of a Ge-layer, a group III-V material layer may be grown on the Si-layer of the bottom substrate 310 in a conventional manner, with or without an intermediate SRB. The semiconductor layer 311 may also be formed as a layer stack including a Ge-layer epitaxially grown on the Si-layer of the bottom substrate 310, with or without an intermediate SRB, and a group III-V material layer epitaxially grown on the Ge-layer, with or without an intermediate SRB. The bottom substrate 310 may also be formed of a semiconductor material other than Si, for instance Ge, SiGe, SiC, GaAs or InP. The bottom substrate 310 may also be a SOI wafer, a GeOI wafer, SiGeOI wafer or a III-V-OI wafer. The bottom substrate 310 may alternatively function merely as a handling wafer and be formed of a dielectric material, such as a glass material.

[0154] FIG. 5a also shows a second wafer 320. The wafer/layers 320, 322, 324, 326 corresponds to the wafer/layers 120, 122, 124, 126 of the second wafer 120 shown in FIG. 3a.

[0155] At the right-hand side of FIG. 5a is shown a wafer stack formed by bonding the semiconductor layer 311 of the first wafer to the second wafer 320 with the front side 311a of the semiconductor layer 311 facing a front side 320a of the second wafer 320. The bonding is achieved by the bonding layers 316, 326.

[0156] In FIG. 5b, the bottom substrate 310 of the first wafer has been removed to expose the back side 311b of the semiconductor layer 311. Removing the bottom substrate 350 may include grinding, CMP and/or etch back from the back side bottom substrate 350 until the semiconductor layer 311 is exposed (e.g. the Ge-layer or the SRB, if present).

[0157] As shown in FIG. 5b, an optional thinning of the semiconductor layer 311 may be performed to form a thinned semiconductor layer 311. The thinning may include grinding, CMP and/or etch back from the back side 311b of the semiconductor layer 311.

[0158] In FIG. 5c, a device region 332 (which may be referred to as a second device region of the first wafer) is formed on the back side 311b of the semiconductor layer 311. The device region 332 includes a set of semiconductor devices and defines a second FEOL layer of the first wafer. At least one interconnection layer 334 is formed on the device region 332. The at least one interconnection layer 334 forms a BEOL layer with respect to the device region 332.

[0159] The device region 332 and the at least one interconnection layer 334 may be implemented in a similar manner as outlined in connection with the second device region 132 and the at least one interconnection layer 134 in FIG. 3b, respectively. Accordingly, in an example embodiment, all FEOL processing steps relating to the device region 332, as well as BEOL processing steps relating to the at least one interconnection layer 334, are performed at temperatures below 600 C. or in an example embodiment below 500 C. to avoid affecting BEOL layers and other FEOL layers already present in the wafer stack.

[0160] In FIG. 5d, a number of vias 340 is formed to extend through the wafer stack (by way of example three vias). The vias 340 may be formed with a similar structure and in a similar manner as the above described vias 140.

[0161] Following forming of the structure shown in FIG. 5d, further back end processing may be performed as described in connection with FIG. 3c.

[0162] In an example variation of the process described with reference to FIG. 5a-d, in an example embodiment the at least one interconnection layer 314 may not be present on the first device region 312. Furthermore, in an example embodiment, the device region 322 and/or the at least one interconnection layer 324 may not be present on the second wafer 320. Hence, in a basic variation of the above-described process, device regions may be formed only on the front side 311a of the semiconductor layer 311 of the first wafer and on the back side 311b of the semiconductor layer 311/311. In that case, the second wafer 320 may function as a handling wafer for the wafer stack and may accordingly be formed by a non-semiconductor material, for instance a dielectric material such as a glass material. Following formation of the vias 340, the second wafer 320 may be thinned or even removed.

[0163] While some embodiments have been illustrated and described in detail in the appended drawings and the foregoing description, such illustration and description are to be considered illustrative and not restrictive. Other variations to the disclosed embodiments can be understood and effected in practicing the claims, from a study of the drawings, the disclosure, and the appended claims. The mere fact that certain measures or features are recited in mutually different dependent claims does not indicate that a combination of these measures or features cannot be used. Any reference signs in the claims should not be construed as limiting the scope.