Stacked-substrate FPGA semiconductor devices

A stacked-substrate FPGA device is described in which a second substrate is stacked over a first substrate. Logic transistors (e.g., semiconductor devices and at least some conductive interconnections between them) are generally fabricated on (or over) a first substrate and memory transistors (e.g., SRAM cells and SRAM arrays) are generally fabricated on a second substrate over the first substrate. This has the effect of physically disposing elements of a CLB and a programmable switch on two different substrates. That is a first portion of a CLB and a programmable switch corresponding to logic transistors are on a first substrate and a second portion of these components of an FPGA corresponding to SRAM transistors is on a second substrate.

BACKGROUND

Field programmable gate array (FPGA) devices are integrated circuit devices that have become increasingly common and useful in many applications. FPGA devices include various elements that can be programmed (and in some cases re-programmed) by a user. These various elements include configurable logic blocks (CLBs), memory cells, and reconfigurable interconnects (also referred to as programmable switches) that can change the connectivity between CLBs themselves as well as between CLBs and memory cells. Some FPGAs are fabricated as multiple die (or “chips”) on a common electronic package substrate (whether indirectly on a silicon interposer or directly on a laminate board), where for example one chip is primarily associated with memory cells and another chip is associated with the CLB (i.e., logic transistors). More recently, some FPGAs are fabricated as a “system on a chip,” in which logic transistors (associated with reconfigurable CLBs), memory cells or arrays, and reconfigurable interconnections are all disposed on a common semi conductor substrate.

The figures depict various embodiments of the present disclosure for purposes of illustration only. Numerous variations, configurations, and other embodiments will be apparent from the following detailed discussion. Furthermore, as will be appreciated, the figures are not necessarily drawn to scale or intended to limit the described embodiments to the specific configurations shown. For instance, while some figures generally indicate straight lines, right angles, and smooth surfaces, an actual implementation of the disclosed techniques may have less than perfect straight lines and right angles, and some features may have surface topography or otherwise be non-smooth, given real-world limitations of fabrication processes. In short, the figures are provided merely to show example structures.

It is noted that designations such “above” or “below” or “top” or “bottom” or “top side” or “bottom side” are not intended to necessarily implicate a limitation as to orientation of the embodiments described herein. Rather, such terminology is simply used in a relative sense to consistently describe a structure as it exists in any one particular orientation and as illustrated herein. In addition, designations such as “over” are not intended to necessarily implicate a limitation as to one structure being distinct from another underlying structure. For example, if an integrated circuit integrated circuit is over a first semiconductor substrate and below a second semiconductor substrate, the integrated circuit may be formed on a topside of the first substrate by depositing additional materials onto the topside of the first substrate, or by shaping (by way of lithography and etching) a portion of the first substrate into one or more parts of the integrated circuit, or by a combination such depositing and shaping. In still other embodiments, the integrated circuit may be formed on a backside of the second substrate by depositing additional materials onto the backside of the second substrate, or by shaping (by way of lithography and etching) a portion of the second substrate into one or more parts of the integrated circuit, or by a combination such depositing and shaping.

DETAILED DESCRIPTION

Techniques are disclosed for fabricating a stacked-substrate FPGA integrated circuit device in which logic devices and memory cells (e.g., SRAM cells) associated with configurable logic blocks (CLBs) and programmable switches are disposed on different substrates. In one example such embodiment, logic devices are over a first substrate and SRAM cells are over a second substrate that itself is over the first substrate, thus forming an FPGA integrated circuit device having a stacked-substrate assembly. The second substrate has a thickness of less than 200 nm in some example cases, or less than 100 nm in still other example cases. Vias are fabricated to pass through the second substrate and into a dielectric layer between the first substrate and the second substrate. The vias connect the components of the CLBs together, connect the components of the programmable switches together despite the disposition of these components on two different, stacked substrates. In some examples, I/O blocks are disposed on yet another substrate and connected to CLBs and programmable switches by through-substrate vias. This stacked-substrate configuration, as variously described herein, can increase a density of transistors (whether associated with logic functions or memory functions) relative to FPGA devices in which CLBs, programmable switches, I/O blocks and memory arrays (and their individual components) are all on a same substrate. Furthermore, by having a second substrate that is less than 100 nm or less than 200 nm thick, the vias passing through the substrate can have a depth that is less than 200 nm (e.g., less than 150 nm). Vias having such a relatively short length can result in favorable electrical characteristics (e.g., a low resistance of between 1 Ohm and 40 Ohms).

General Overview

For all types of integrated circuit devices, increasing a density of circuits per unit area of substrate is often accomplished by shrinking the dimensions of transistors and their associated interconnections. Shrinking these dimensions often requires changes to one or more of circuit design ground rules, semiconductor device configuration, and processing techniques. This method of increasing density can be challenging when applied to field programmable gate array (FPGA) integrated circuit devices. This is because FPGA devices are composed of different types of integrated circuit groups that are configured for coordinated operation. A first type of integrated circuit group is a configurable logic block (CLB) that includes a variety of random logic transistors that include, but are not limited to transistor pairs, NAND gates, OR gates, look up tables (LUTs), multiplexers, among others. CLBs can also include some memory elements, whether dynamic access memory (DRAM) or static random access memory (SRAM). LUTs within the CLBs are often instantiated using static SRAM cells or SRAM arrays (i.e., arrangements of multiple SRAM cells), whether 4 transistor (4T), 6 transistor (6T), or some other type of SRAM configuration. A second type of integrated circuit group is a programmable switch that includes SRAM cells, multiplexers, and conductive interconnections. These programmable switches are associated with conductive interconnections that connect the programmable switches to the CLBs, and therefore (indirectly) connect the CLBs to one another. Input/output (I/O) circuits and structures (e.g., I/O contact pads) are generally connected to programmable switches and enable signals to be transmitted between the FPGA device and computing resources external to the FPGA device. Given the diversity of these groups of integrated circuits and the diversity of connectivity between the various arrays of CLBs, programmable switches, and I/O circuits generally employed in FPGA devices, increasing transistor density in FPGA devices has been challenging.

Thus, techniques are described herein for fabricating a stacked-substrate FPGA device in which a second substrate is stacked over a first substrate. Logic transistors (e.g., semiconductor devices and at least some conductive interconnections between them) are generally fabricated on (or over) a first substrate and memory transistors (e.g., SRAM cells and SRAM arrays, among optionally other types of memory cells in some embodiments) are generally fabricated on a second substrate over the first substrate. This has the effect of physically disposing elements of a CLB on two different substrates, and similarly physically disposing elements of a programmable switch on two different substrates. That is a first portion of a CLB corresponding to logic transistors is on a first substrate and a second portion of the CLB corresponding to SRAM transistors is on a second substrate. The same arrangement applies to a programmable switch, thus disposing most (if not all) SRAMs in an FPGA on a same substrate, regardless of whether the SRAMs are associated with a CLB or a programmable switch (or an I/O circuit). Vias passing through the second substrate and through an interlayer dielectric between the first substrate and the second substrate, connect the logic integrated circuits of CLBs and programmable switches on the first substrate to their corresponding memory components on the second substrate. In this way, a density of transistors per unit area of substrate is increased. Furthermore, because some examples of the second substrate are less than 200 nm or less than 100 nm in thickness, the vias passing through the second substrate have a length that can also be less than 200 nm, thus enabling a low enough electrical resistance (e.g., from 1 Ohm to 40 Ohms) to support high rates of data transmission (e.g., in the gigahertz range).

Example Device

FIGS.1A,1B, and1Cillustrate schematic plan and cross-sectional views of an example stacked-substrate FPGA device100of the present disclosure in which SRAM integrated circuits associated with CLBs and programmable switches are disposed on a second substrate, between a first substrate and a third substrate. It will be appreciated that the examples described herein refer to SRAM integrated circuits for convenience of explanation and that different types of memory cells/arrays can be included as a replacement for and/or a complement to SRAM memory cells/arrays.

Turning first toFIG.1A, a plan view is shown in which a second (or middle) substrate is exposed. It will be appreciated that the example device100includes three stacked substrates, and the presentation of the exposed second substrate is to facilitate explanation.

The view inFIG.1Aincludes a first substrate102, a second substrate103, a configurable logic block (CLB)104, and a programmable switch108. The first substrate102is disposed below the second substrate103and thus is shown using a broken (“dashed”) line to indicate a perimeter. Similarly, the semiconductor structures associated with the first substrate102and disposed between the first substrate102and the second substrate103are also indicated using a broken line. These structures include (but are not limited to) logic transistors106,110associated with the CLBs104and programmable switches108, respectively.

SRAMs114associated with the CLBs104and SRAMs120associated with the programmable switches108are also disposed on the second substrate103. The SRAMs114,120are connected to their corresponding logic transistors106,110by vias116,124, respectively. The vias116,124can be configured to pass through the second substrate103and be in contact with either the logic transistors106,110directly or through an intervening conductive interconnection.

This configuration thus not only illustrates a stacked-substrate FPGA device, but also the stacked-substrate CLBs and stacked-substrate programmable switches that collectively constitute the stacked-substrate FPGA device100.

FIG.1Billustrates a third substrate126over the second substrate103(thus placing the second substrate103between the first substrate102and the third substrate126) on which are disposed various input/output (I/O) blocks that facilitate communication between the FPGA device and external computing resources (e.g., power supply, processors, clock modules).

FIG.1Cillustrates a cross-sectional view of the device inFIG.1B. The stacked-substrate configuration of CLB104and programmable switch108can be seen in this figure. As shown, integrated circuits106,110of CLB104and switch108, respectively, are disposed on the first substrate103while associated SRAMs114,120are disposed on the second substrate103. I/O blocks128are disposed on the third substrate126, placing the second substrate103over the first substrate102and between the first substrate102and the third substrate126. Vias116and124pass through the second substrate103so as to place the SRAMs114,120into contact with the corresponding logic transistors106,110(via conductive interconnects132,134). Via136passes through the third substrate126to place I/O block128into contact with, in this example, CLB104.

It will be appreciated the example device100is presented for convenience of illustration and explanation and that other configurations of substrates (e.g., four or more substrates, two substrates) are within the scope of the present disclosure. Furthermore, not every element inFIGS.1A,1B,1Cis labeled for clarity of depiction.

Architecture and Methodology

FIG.2illustrates an example method200by which embodiments of the present disclosure can be fabricated.FIGS.3A-3Gillustrate cross-sectional views of various stages of fabrication of the example method depicted inFIG.2. Concurrent reference toFIG.2andFIGS.3A-3Gwill facilitate explanation.

The method200includes providing204a first substrate302(as shown inFIG.3Awith various logic circuits, described in more detail below) and providing205a second substrate304(as shown inFIG.3B). As will be described below in more detail, in some embodiments, logic integrated circuits associated with CLBs can be fabricated on the first substrate302. These logic circuits can be used to perform various random logic functions associated with FPGAs and their associated CLBs, including communicating with associated SRAMs. In some embodiments, the SRAM cells (and more generally SRAM arrays formed by groups of individual SRAM cells) are fabricated on the second substrate304.

For both the first substrate302and the second substrate304, any number of suitable substrate types and materials can be used. The substrate may be, for example, a bulk semiconductor wafer (e.g., bulk silicon, germanium, gallium arsenide or other III-V materials, etc.) or an on-insulator configuration (e.g., silicon on-insulator, germanium on-insulator, silicon germanium on-insulator, indium phosphide on-insulator, etc.). In other examples, the substrate (whether one or both of the first substrate302and the second substrate304) may be: gallium nitride (GaN); c-axis aligned indium gallium zinc oxide (c-IGZO); transition metal dichalcogenides including but not limited to MoS2, MoSe2, CuS2, CuSe2, WS2, WSe2, among others; black phosphorus; or InGaAs. The substrate may be p-type, n-type, neutral-type, high or low resistivity, off-cut or not off-cut, etc. The substrate may have a vicinal surface that is prepared by off-cutting the substrate from an ingot, wherein substrate is off-cut at an angle between, for instance, 2° and 8° (e.g., 4° off-cut silicon). Note, however, the substrate need not have any such specific features, and that embodiments of the present disclosure can be implemented using any one of more of numerous different substrates. The thickness of the first substrate302can vary and in some embodiments, for example, is in the range of 100 nm to thousands of nanometers. In some cases, the substrate may be subsequently thinned or removed (e.g., by way of backside polish or other suitable thinning/removal process), after formation of the interconnect structure and application of protective layer such as etch stop, passivation layer, inter-layer dielectric (ILD), capping layer, etc. The thickness of the second substrate304as provided205can be of similar dimension to that of the first substrate302. Techniques for thinning the second substrate304so as to maintain a height of a via through the second substrate to less than 100 nm or less than 200 nm are described below.

Once the first substrate302has been provided204, semiconductor devices306A,306B (collectively306, corresponding to CLB and programmable switch logic circuits) can be formed208on the first substrate302. Examples of logic integrated circuits that the semiconductor devices306A,306B can be configured to form include, but are not limited to a variety of logic gates (e.g., AND gates, OR gates, NOR gates), multiplexers, power select circuitry, among others. Conductive vias and conductive lines314(described below) can connect various semiconductor devices306together, thus connecting various semiconductor devices together to form larger scale integrated circuits.

In some example embodiments, the semiconductor devices306A,306B can be formed208using standard metal oxide semiconductor (MOS) processing, although any desired process technology can be used. In some examples, such as the one shown inFIG.3A, the semiconductor devices306can be formed208using fin-based semiconductor devices, including field effect transistors (also known as “FinFETs”). A FinFET is a MOSFET transistor built around a thin strip of semiconductor material (generally referred to as a fin) that extends from and above an underlying substrate (in this case, the substrate302). The conductive channel of the FinFET device resides on the outer portions of the fin adjacent to the gate dielectric. Specifically, current runs along/within both sidewalls of the fin (sides perpendicular to the substrate surface) as well as along the top of the fin (side parallel to the substrate surface). Because the conductive channel of such configurations essentially resides along the three different outer, planar regions of the fin, such a FinFET design is sometimes referred to as a tri-gate transistor. Other types of FinFET configurations are also available, such as so-called double-gate FinFETs, in which the conductive channel principally resides only along the two sidewalls of the fin (and not along the top of the fin).

As shown inFIG.3A, the fins318(318A,318B in the example shown) of the semiconductor devices306include a salient feature composed of a semiconductor material and configured to extend from and above a substrate surface. In some examples, a fin318of semiconductor material can be patterned (e.g., lithographically masked and then etched) from the first substrate302itself or from a layer of material formed on the substrate302that is compositionally different from the first substrate302. Materials that are “compositionally different” or “compositionally distinct” as used herein refers to two materials that have different chemical compositions. This compositional difference may be, for instance, by virtue of an element that is in one material but not the other (e.g., SiGe is compositionally different than silicon), or by way of one material having all the same elements as a second material but at least one of those elements is intentionally provided at a different concentration in one material relative to the other material (e.g., SiGe having 70 atomic percent germanium is compositionally different than from SiGe having 25 atomic percent germanium). In addition to such chemical composition diversity, the materials may also have distinct dopants (e.g., gallium and magnesium) or the same dopants but at differing concentrations. In still other embodiments, compositionally distinct materials may further refer to two materials that have different crystallographic orientations. For instance, (110) silicon is compositionally distinct or different from (100) silicon. Creating a stack of different orientations could be accomplished, for instance, with blanket wafer layer transfer.

Regardless of the composition, a gate structure322(322A,322B on corresponding fins318A,318B) comprising at least one layer of gate dielectric326(326A,326B on corresponding fins318A,318B) and at least one layer of gate electrode330(330A,330B on corresponding fins318A,318B) can be fabricated on the one or more the fins318.

Examples of materials that can be used to form the gate dielectric layer326include, but are not limited to, dielectric materials used for interlayer dielectric (ILD) described below, as well as “high-k” materials, or both. High-k dielectric materials are those generally considered to have a dielectric constant greater than that of silicon dioxide and include, but are not limited to hafnium oxide, hafnium silicon oxide, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide, tantalum oxide, titanium oxide, barium strontium titanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate. In some embodiments, additional processing may be performed on the high-k dielectric layer, such as an annealing process, to improve the quality of high-k material.

The portions of the fin318not covered by the gate structure322can be replaced with semiconductor material and/or doped to function as a source region334(334A,334B on corresponding fins318A,318B) and a drain region338(338A,338B on corresponding fins318A,318B). The portion of the fin318between the source region334and the drain region338and between the substrate302and the gate structure322is sometimes referred to as a semiconductor body and can be characterized as including a channel region342(342A,342B on corresponding fins318A,318B) through which charge carriers can flow. A voltage can be applied to the gate structure to control the flow of charge carriers from the source region334to the drain region338.

A layer of dielectric material346(sometimes referred to as interlayer dielectric) can be formed212on the semiconductor devices306. The dielectric layer (ILD) may include any number of conventional dielectric materials commonly used in integrated circuit applications, such as oxides (e.g., silicon dioxide, carbon doped oxide), silicon nitride, or organic polymers (e.g., perfluorocyclobutane or polytetrafluoroethylene), fluorosilicate glass, and organosilicates (e.g., silsesquioxane, siloxane, or organosilicate glass). The dielectric material may be low-k or high-k depending on the desired isolation, and may include pores or other voids to further reduce its dielectric constant. Examples of high-k materials have been described above. The dielectric layer thickness can vary and in some example embodiments is in the range of 50 nm to 5000 nm. In some embodiments, the dielectric layer may actually include multiple layers having the same or differing thicknesses. Likewise, in some embodiments, each ILD layer is implemented with the same dielectric material, but in other embodiments, at least some of the ILD layers are implemented with differing dielectric materials.

Techniques for forming212the layer of dielectric material346can be any of a wide range of suitable deposition techniques, including but not necessarily limited to: physical vapor deposition (PVD); chemical vapor deposition (CVD); spin coating/spin-on deposition (SOD); and/or a combination of any of the aforementioned. Other suitable configurations, materials, deposition techniques, and/or thicknesses for base ILD layer346will depend on a given application and will be apparent in light of this disclosure. Numerous ILD configurations and dimensions will be apparent in light of this disclosure and the claimed invention is not intended to be limited to any particular configurations and dimensions. In some embodiments after forming, the ILD can be planarized and/or polished using any suitable technique including chemical-mechanical planarization/polishing (CMP) processes, for example.

As also shown inFIG.3A, conductive interconnect structures (e.g., contact structures or “vias”310A,310B,315A,315B) and conductive lines (314A,314B,317) can then be formed216within the dielectric layer346. The electrical connection established by the conductive interconnect structures310,315,314,317between semiconductor devices306can be used to place semiconductor devices in contact with one another to form larger scale integrations of semiconductor devices. The interconnect structures310,315,314,317can also be used to place semiconductor devices in contact with SRAMs and I/O blocks using vias disposed through at least a second substrate and/or a third substrate, as described below in more detail. In some examples, the conductive interconnections can be formed of the same materials, and in other cases, may include compositionally different materials, whether different from layer to layer and/or within the same layer. For example, one or more of the vias310,315and the conductive lines314,317can include one or more layers of a conductive liner (e.g., silicon nitride, graphene, tantalum nitride) and one more layers of a conductive metal (e.g., copper, aluminum).

Formation216of these conductive interconnect structures can be performed by standard processes of photolithographic patterning and etching (e.g., removal of portions of the dielectric layer346), followed by deposition of one or more layers in the patterned and etched regions of the dielectric layer346. Example deposition techniques applicable to conductive material include but are not limited to sputtering, physical vapor deposition (PVD), chemical vapor deposition (CVD), metallo-organic CVD (MOCVD), among others. In some examples, metal lines and vias can be implemented as a dual damascene structure fabricated in the corresponding dielectric layer (ILD). Other embodiments may employ other interconnect structures (e.g., single damascene or other suitable metal interconnect pathway where interconnect metal can be provisioned). The dual damascene trench can be formed in the dielectric layer, for example, using standard lithography including via and trench patterning and subsequent etch processes followed by polishing, cleans, etc., as typically done. As with the preceding examples, the patterning and etch processes can be carried out, for instance, using wet and/or dry etch techniques.

The trenches in the ILD in which vias and conductive lines are formed can have various dimensions depending on the application. In one example case, an upper trench opening (i.e., corresponding to a conductive line portion of a dual damascene via/conductive line structure) is about 10 nm to 100 nm (e.g., 20 to 50 nm) and a lower via opening is about 5 nm to 50 nm (e.g., 10 to 25 nm), and the entire structure has an aspect ratio in the range of about 10:1 to 1.25:1 (e.g., 5:1). As will be appreciated, however, the dimensions and aspect ratio of the damascene trench will vary from one embodiment to the next, and the present disclosure is not intended to be limited to any particular range of dimensions, or any particular trench configuration.

In a more general sense, conductive interconnect cross-sections can include a profile having a first lateral width and a second lateral width that is smaller than the first lateral width, while in another example case the metal interconnect pathway cross-section demonstrates a profile having a substantially uniform lateral width from top to bottom.

In other examples, rather than forming trenches in ILD, the conductive interconnections can be formed by first forming a blanket layer of conductive material followed by patterning and etching of the blanket layer into the conductive interconnect structures illustrated inFIG.3A. The conductive interconnect structures formed in this way can be electrically isolated from one another by subsequent deposition of dielectric material, as described above.

Note that the layout shown inFIG.3A(and subsequent figures) is not intended to implicate any particular feature spacing or density. Rather, this layout is simply an arbitrary example, and any number of other layout designs can benefit from an embodiment of the present invention. In some examples, the layers of interconnection (e.g., vias and conductive lines) are demarcated from neighboring layers by an etch stop layer (e.g., a nitride layer) and may also include a barrier layer to prevent electromigration of material between adjacent conductive interconnect structures (e.g., nitride layers, among others).

As mentioned above, the method200also includes providing205a second semiconductor substrate304, which in various embodiments can be any one of the substrate compositions or types previously described in the context of the first substrate302. As indicated above, the final thickness of the second substrate as integrated into a stacked-substrate FPGA device is less than 200 nm or less than 100 nm so as to facilitate a similar depth (e.g., less than 200 nm or less than 100 nm) of a via connecting the SRAM memory cells to the logic devices on the first substrate.

To accomplish this thinning, the second substrate304can be first implanted209with, for example, accelerated hydrogen ions through one of the exposed surfaces. In some examples, as shown inFIG.3C, the accelerated hydrogen ions cause the formation of a sub-surface layer350with crystallographic defects that can be between from 5 nm to 500 nm below the surface through which the ions penetrate. More generally, the accelerated ions interact with atoms within the lattice of the second substrate304so as to displace some of the atoms from crystal lattice sites, thus forming vacancies within the crystalline lattice as one type of crystallographic defect. In some examples, dislocations and dislocation loops can form as another type of crystallographic defect. Regardless of the type or types of crystallographic defects that are formed by ion implantation, the defective layer350formed is a location at which the substrate304will cleave upon heat treatment.

In some examples, implantation can be accomplished by accelerating hydrogen ions (formed by exposure of hydrogen gas to an electrically biased filament so as to form a hydrogen plasma) at voltages of anywhere between 20 keV to 100 keV. Implantation densities of on the order of 1015atoms/cm3to 1022atoms/cm3can be used to form the defective layer350. In some examples, the implantation current is from 5 mA/cm2of substrate to 10 mA/cm2. In some examples, accelerated hydrogen ions within the previously indicated acceleration voltages and implantation densities are capable of producing on the order of from 1022vacancies/cm3to 1023vacancies/cm3. In a specific example, an accelerating voltage of 40 keV can produce an approximately normal distribution of vacancies from 1 nm to 700 nm below the exposed surface through which the hydrogen ions are implanted, with a median of the distribution at about 500 nm below the surface (+/−50 nm).

In some examples, the defective layer350can have a thickness (measured perpendicular to the surface of the substrate304through which the accelerated ions pass) within any of the following ranges: from 5 nm to 50 nm; from 5 nm to 25 nm; from 5 nm to 10 nm; from 10 nm to 50 nm; from 25 nm to 50 nm. It will be appreciated that the accelerating voltage of the parting (hydrogen) ions can be selected so that a depth below the surface of the defective layer350is within a desired range according to standard techniques. For example, a greater accelerating voltage of the hydrogen ions will cause the hydrogen ions to impinge to a distance greater from the exposed surface than a lower accelerating voltage. In some embodiments, the accelerating voltage is selected to cause the defective layer350to be within the range below the surface indicated above (i.e., from 5 nm to 500 nm below the exposed surface).

In other examples, other accelerated particles, including but not limited to helium ions, can be used to create the defective layer350.

As shown inFIG.3C, for convenience of explanation, the defective layer350can be considered to divide the second substrate304into a first portion354and a second portion358on opposing sides of the defective layer350. As can be appreciated in light of the present disclosure, a depth of the defective layer350essentially defines a thickness of the second portion358.

Surfaces of the first substrate302(e.g., the dielectric layer346) and the second substrate304(in particular, the exposed surface of the second portion358of the second substrate304) ultimately to be joined together are then exposed222to a plasma. In one example, exposure222is to an oxygen plasma. In other examples, exposure222can be to a UV enhanced ozone (O3) plasma, an aqueous (H2O) ozone solution plasma, or any other plasma chemistry capable of terminating the exposed surfaces in hydroxyl (OH) groups. As schematically shown inFIG.3D, exposure222to the oxygen plasma creates layers362A,362B on the treated surfaces that include a plurality of hydroxyl groups.

The method200continues by placing226the plasma treated surface of the second portion358of the second substrate304in contact with the plasma treated surface of the first substrate302, thus bringing the layers362A,362B into contact with one another. This initial contact between the plasma treated surfaces can be performed at ambient temperature (e.g., between 20° C. and 25° C.) and ambient atmospheric pressure (e.g., 1 atmosphere +/−5%).

Placing226the layers362A,362B into contact with one another and then heating the two substrates302,304causes a condensation reaction to occur between hydrogen and hydroxyl groups that terminate the confronting surfaces. In some examples, the two substrates302,304are heated226at temperatures anywhere between 300° C. and 400° C. (within normal equipment variation and measurement tolerances of approximately +/−2° C.) for between 10 minutes and 60 minutes in an inert atmosphere (e.g., N2, Ar). As water is produced by the reaction and removed from the interface between the layers362A,362B by vaporization, a covalent bond is formed between the second portion358of the second substrate304and the ILD346of the first substrate302, thus joining the two substrates together. In the case of a second substrate304fabricated from silicon, and a dielectric layer346fabricated from silicon dioxide, the covalent bonds formed are silicon-oxygen-silicon bonds. The silicon-oxygen-silicon bonds that bridge the interface are strong enough so that no additional adhesive or connection is needed to bond the first substrate302to the second substrate304.

The heat applied to the attached first substrate302and second substrate304also causes the separation230of the first portion354of the second substrate304from the layer with crystallographic defects350. In some cases, this can be described as “exfoliation” of first portion354from the second substrate304. As shown inFIG.3E, in some cases some or all of the layer with crystallographic defects350remains on the second portion358after the heat induced separation230of the first portion354. In some embodiments, any remaining layer350with crystallographic defects can be removed by standard polishing and planarization techniques, including but not limited to chemical mechanical polishing (CMP). In some examples, some of the material comprising the second portion358can be removed through continued chemical mechanical polishing so that a thickness (indicated inFIG.3Fas dimension a) can be within any of the following ranges: from 5 nm (nm) to 200 nm; from 5 nm to 100 nm; from 10 nm to 100 nm; from 10 nm to 50 nm; from 15 nm to 30 nm. The resulting structure, referred to herein as a substrate assembly366, is shown inFIG.3F.

In another embodiment, the fabrication of the second portion358can include the use of a silicon germanium (SiGe) layer. For example, a SiGe layer (of from 10 nm thick to 30 nm thick) on the second substrate can be composed and formed to have a coherent and epitaxial interface (i.e., a lattice parameter mismatch of less than 2%) with the underlying substrate (e.g., a silicon substrate). A silicon layer of from 10 nm to 20 nm thick corresponding to the second portion358can then be formed (epitaxially and coherently) on the SiGe layer. The defective layer350can then be formed via ion implantation (as described above) on a side of the SiGe layer opposite that of the silicon layer corresponding to the second portion358. After separation of the second portion358from the substrate, the crystallographically defective layer can be removed via CMP until the SiGe layer is exposed. Silicon can be selectively removed from SiGe using CMP processes that include ammonium hydroxide (NH4OH) or tetramethyl ammonium hydroxide (N(CH3)4OH). In this way, the SiGe acts an “etch stop.” In other words, the processes used to remove the silicon layer are composed so as to not remove the SiGe layer or remove it at a rate that is at least 5 times slower than silicon is removed. Once the SiGe layer is exposed and subsequently detected, the etch composition can be changed so that the SiGe layer can be removed using a selective etch that removes SiGe but does not remove Si m (e.g., KOH). Using a SiGe layer as an etch stop as described above can, in some examples, enhance control of a thickness α of the second portion358. It will be appreciated that the second portion358essentially becomes a semiconductor substrate on which semiconductor devices can be fabricated, as is explained below

Having thus prepared the substrate assembly366, SRAM cell components and memory arrays can be formed234on the exposed surface of the second portion358on a side opposite that of the first substrate302. Vias through the second portion358can connect the SRAM cells with corresponding logic semiconductor devices on the underlying first substrate302to form CLBs and programmable switches, as illustrated inFIGS.1A-1C. This forms a stacked-substrate FPGA device372, an example of which is shown inFIG.3G. Similar to the preceding cross-sections, the cross-section ofFIG.3Gis taken perpendicular to the gates of some of the semiconductor devices on the first substrate302.

The portion of the embodiment of the stacked-substrate FPGA device372shown inFIG.3Gincludes the substrate assembly366, described above, as well as SRAM cells375, interconnect380, and inter-substrate via391. SRAM cells375can be formed using standard techniques, such as those described above, for the formation of the transistors and interconnections that constitute the SRAM cells (e.g., lithography, etching, implantation, planarization, single and dual damascene interconnection fabrication processes).

The SRAM cells/arrays375and semiconductor devices306A,306B (and/or corresponding circuits) can be placed into contact (e.g., electrical communication) with one another by forming238a via391that extends from the conductive interconnection380, through the dielectric layer390, the second portion358of the second substrate, and the dielectric layer346. In this example, the via391is connected to the conductive interconnection317. It will be appreciated that in other examples, a via391can extend to different levels of conductive interconnect or even to a contact on a drain region of a semiconductor device (e.g., contact structure310B on drain region338B). The via391can be formed238using standard techniques, such as lithographic patterning and etching. These have been described above in the context of interconnections310,314,315,317, among others, and need no further explanation.

The placement of the via391can be selected using standard optical alignment techniques used to align various levels of interconnections. In some examples in which the second portion358of the second substrate304is optically transparent (due to its thickness α of less than 200 nm, less than 100 nm, or in some cases even less than 50 nm), standard alignment techniques can be used to align the via391with corresponding structure (whether a conductive interconnection or a semiconductor device) on the first substrate302. In some examples, these standard alignment techniques involve the use of alignment structures and/or patterns that can be used as reference points by which lithographic masks and other process tools can be aligned. In other examples, some of the second portion358of the second substrate304can be removed (e.g., by lithographic patterning and etching) so that these alignment structures on the first substrate302(sometimes colloquially referred to as “fiducial structure” or “alignment marks”) can be detected.

As described above, because the thickness α of the second portion is less than 200 nm (and in some cases less than 100 nm), the formation of the via391can use techniques similar or the same as those used to form interconnections315A,315B,310A,310B. That is, standard single damascene or dual damascene techniques for the fabrication of vias between metal levels in a “back end of line” (BEOL) of an integrated circuit can be adapted to the via391by selecting processes that can pattern dielectric material390, second substrate second portion358(whether silicon or some other composition), and then dielectric material346. As also indicated above, a depth of the via391can correspond to the thickness of the second portion358plus 5-10 nm above the surface (to contact to the conductive interconnection380) and plus 5-10 nm below the second portion358(to an interconnections). In embodiments, a depth of the via391is less than 200 nm, less than 100 nm, less than 50 nm, or less than 20 nm. In embodiments, the depth of via391corresponding to (and in some cases 5 nm-20 nm greater than) the thickness α of the second portion358leads to a via391electrical resistance of from 1 Ohm to 40 Ohms or within any of the following sub-ranges: 1 Ohm to 5 Ohms; 1 Ohm to 10 Ohms; 10 Ohms to 30 Ohms; 15 Ohms to 35 Ohms.

As indicated in the figure, this arrangement can form a stacked-substrate CLB382, as described above. It will be appreciated that a similar arrangement can be used to form a stacked-substrate programmable switch also on the substrates302and358, as shown inFIG.1C.

Optionally, another dielectric layer can be formed242on the SRAM memory arrays and a third substrate placed246on the dielectric layer. The third substrate can be processed according to the elements described above in the method200so as to produce a thickness that is less than 200 nm, less than 100 nm, or even less than 50 nm thick (e.g., elements209,222,226,230). I/O blocks can be formed250on the third substrate and placed into contact with SRAMs375and/or the logic circuits (e.g., semiconductor devices306A,306B).

Example System

FIG.4is an example computing system implemented with one or more of the integrated circuit structures as disclosed herein, in accordance with some embodiments of the present disclosure. As can be seen, the computing system400houses a motherboard402. The motherboard402may include a number of components, including, but not limited to, a processor404and at least one communication chip406, each of which can be physically and electrically coupled to the motherboard402, or otherwise integrated therein. As will be appreciated, the motherboard402may be, for example, any printed circuit board, whether a main board, a daughterboard mounted on a main board, or the only board of system400, etc.

Depending on its applications, computing system400may include one or more other components that may or may not be physically and electrically coupled to the motherboard402. These other components may include, but are not limited to, volatile memory (e.g., DRAM), non-volatile memory (e.g., ROM), a graphics processor, a digital signal processor, a crypto processor, a chipset, an antenna, a display, a touchscreen display, a touchscreen controller, a battery, an audio codec, a video codec, a power amplifier, a global positioning system (GPS) device, a compass, an accelerometer, a gyroscope, a speaker, a camera, and a mass storage device (such as hard disk drive, compact disk (CD), digital versatile disk (DVD), and so forth). Any of the components included in computing system400may include one or more integrated circuit structures or devices configured in accordance with an example embodiment (e.g., to include one or more stacked-substrate FPGA devices, as variously provided herein). In some embodiments, multiple functions can be integrated into one or more chips (e.g., for instance, note that the communication chip406can be part of or otherwise integrated into the processor404).

The communication chip406also may include an integrated circuit die packaged within the communication chip406. In accordance with some such example embodiments, the integrated circuit die of the communication chip includes one or more integrated circuit structures or devices as variously described herein. As will be appreciated in light of this disclosure, note that multi-standard wireless capability may be integrated directly into the processor404(e.g., where functionality of any chips406is integrated into processor404, rather than having separate communication chips). Further note that processor404may be a chip set having such wireless capability. In short, any number of processor404and/or communication chips406can be used. Likewise, any one chip or chip set can have multiple functions integrated therein.

Further Example Embodiments

Example 1 is an integrated circuit including a field programmable gate array (FPGA) device, the integrated circuit comprising: a first semiconductor substrate; a second semiconductor substrate that is less than 200 nm thick over the first semiconductor substrate; a dielectric layer between the first semiconductor substrate and the second semiconductor substrate; a configurable logic block (CLB) including a logic transistor between the first semiconductor substrate and the second semiconductor substrate, and a memory cell over the second semiconductor substrate; and a via connecting the logic transistor and the memory cell, the via at least partially disposed through the second semiconductor substrate.

Example 2 includes the subject matter of Example 1, wherein the second semiconductor substrate is from 15 nm to 50 nm thick.

Example 3 includes the subject matter of Example 1 or 2, wherein the via has a depth of from 15 nm to 50 nm.

Example 4 includes the subject matter of any of the preceding Examples, wherein the via has an electrical resistance of from 1 Ohm to 40 Ohms.

Example 5 includes the subject matter of Examples 1 and 4, wherein the second semiconductor substrate is from 10 nm to 20 nm thick.

Example 6 includes the subject matter of Examples 1, 4, and 5, wherein the via has a depth of from 10 nm to 20 nm.

Example 7 includes the subject matter of any of the preceding Examples, wherein the second semiconductor substrate is optically transparent.

Example 8 includes the subject matter of any of the preceding Examples, further comprising: a third semiconductor substrate that is less than 200 nm thick over the second semiconductor substrate; one or more of an additional logic transistor or a memory array over the third semiconductor substrate; wherein the via further comprises a first via connecting the logic transistor and the memory cell and a second via connecting the one or more of the additional logic transistor or the memory array to the CLB or a programmable switch; and wherein the dielectric layer comprises a first dielectric layer between the first semiconductor substrate and the second semiconductor substrate and a second dielectric layer between the second semiconductor substrate and the third semiconductor substrate.

Example 9 includes the subject matter of Example 8, wherein the first via and the second via comprise an electrical resistance of from 1 Ohm to 40 Ohms.

Example 10 includes the subject matter of any of the preceding Examples, wherein one or more of the logic transistor and the memory cell comprise a source region, a drain region, and a semiconductor body between the source region and the drain region.

Example 11 includes the subject matter of any of the preceding Examples, wherein one or more of the logic transistor and the memory cell comprise a fin of a semiconductor material.

Example 12 includes the subject matter of any of the preceding Examples, further comprising: an additional semiconductor substrate that is less than 200 nm thick over the second semiconductor substrate; at least one additional dielectric layer between the second semiconductor substrate and the additional semiconductor substrate; and a plurality of input/output blocks on the additional semi conductor substrate.

Example 13 includes the subject matter of Example 12, further comprising an additional via through the additional semiconductor substrate connecting the input/output blocks to at least the CLB.

Example 14 includes the subject matter of any of the preceding Examples, wherein the via comprises a first via and a second via disposed through the second semiconductor substrate and the dielectric layer between the first semiconductor substrate and the second semiconductor substrate.

Example 15 includes the subject matter of any of the preceding Examples, wherein the memory cell is a first memory cell, the integrated circuit further comprising a programmable switch, the programmable switch comprising: a switch transistor between the first semiconductor substrate and the second semiconductor substrate; and a second memory cell over the second semiconductor substrate.

Example 16 includes the subject matter of Example 15, further comprising a third via connecting the switch and the second memory cell.

Example 17 includes the subject matter of Example 16, the first via comprises a plurality of first vias disposed in a first pattern having a first periodicity; the first via comprises a plurality of first vias disposed in a first pattern having a first periodicity; the second via comprises a plurality of second vias disposed in a second pattern different from the first pattern and having a second periodicity different from the first periodicity; and the third via comprises a plurality of third vias disposed in a third pattern different from the first pattern and the second pattern and having a third periodicity different from the first periodicity and the second periodicity.

Example 18 includes the subject matter of any of the preceding Examples, wherein the memory cell is an SRAM cell.