Patent Publication Number: US-11043411-B2

Title: Integration of air spacer with self-aligned contact in transistor

Description:
DOMESTIC PRIORITY 
     This application is a continuation of U.S. application Ser. No. 16/128,674 filed Sep. 12, 2018, the disclosure of which is incorporated herein by reference in its entirety. 
    
    
     BACKGROUND 
     The present invention relates to a semiconductor device, and more specifically, to integration of an air spacer with a self-aligned contact in a transistor. 
     In the continuing effort to decrease the size of transistors, gate pitch is decreasing along with gate size. That is, the distance between the gate and a contact above a source or drain region is smaller. This has led to the formation of a self-aligned contact (SAC). By recessing the gate metal and forming an etch stop or SAC cap above the gate, an etch of the trench that defines the contact can be allowed to self-align without a danger that the contact will connect to the gate metal. In certain devices, not every source and drain region requires a contact. For example, in a not-and (NAND) circuit, n-type field effect transistors (NFETs) are electrically connected in series by shared source and drain regions. In this type of device, forming a contact on internally connected sources and drains is not necessary. The same is true for a not-or (NOR) circuit. 
     SUMMARY 
     Embodiments of the present invention are directed to a method of fabricating a semiconductor device includes forming a series of two or more gates, and forming a gate spacer on each side of each gate of the series of two or more gates. The method also includes forming a source region on a side of each of the two or more gates and forming a drain region on an opposite side of each of the two or more gates. The source region or the drain region between two adjacent ones of the two or more gates is shared by the two adjacent ones of the two or more gates and only the source region or the drain region on one side of a first gate in the series of two or more gates and the source region or the drain region on one side of a last gate in the series of two or more gates are unshared source or drain regions. An interlayer dielectric (ILD) layer is deposited above each of the source regions and each of the drain regions, and the ILD layer is removed above the unshared source or drain regions. The method also includes removing the gate spacer on the one side of the first gate in the series of two or more gates and removing the gate spacer on the one side of the last gate in the series of two or more gates, and forming a self-aligned contact (SAC) on the unshared source or drain regions while retaining the ILD layer above all other ones of the source regions and the drain regions. An air spacer is formed between the SAC on the one side of the first gate and the first gate and between the SAC on the one side of the last gate and the last gate. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The examples described throughout the present document will be better understood with reference to the following drawings and description. The components in the figures are not necessarily to scale. Moreover, in the figures, like-referenced numerals designate corresponding parts throughout the different views. 
         FIGS. 1-3  show devices that are fabricated with integration of an air spacer with a self-aligned contact according to one or more embodiments of the invention, in which: 
         FIG. 1  is a circuit diagram of a two-input not-and (NAND) device; 
         FIG. 2  is a circuit diagram of a two-input not-or (NOR) device; and 
         FIG. 3  is a circuit diagram of a three-input NAND device; 
         FIGS. 4-12  show aspects of a process flow of fabricating a semiconductor device that include integration of an air spacer with self-aligned contacts (SACs) according to one or more embodiments of the invention, in which: 
         FIG. 4  is a cross-sectional view of a structure that results after formation of a SAC cap over a recessed gate  445  in three nanosheet field effect transistors (FETs); 
         FIG. 5  shows the structure that results from formation of trenches above unshared source or drain regions; 
         FIG. 6  shows a structure that results from removal of the low-k spacers exposed in the structure shown in  FIG. 5 ; 
         FIG. 7  shows the structure that results from the deposition of liners; 
         FIG. 8  shows the structure that results from removal of the sacrificial spacer material on horizontal surfaces of the structure shown in  FIG. 7 ; 
         FIG. 9  shows the result of forming SACs; 
         FIG. 10  shows the structure that results from recessing the SACs; 
         FIG. 11  shows the structure that results from removing the sacrificial spacer material; and 
         FIG. 12  shows the air spacers between each SAC and adjacent gate; 
         FIG. 13  shows a cross-sectional view detailing the air spacer between an SAC and gate according to one or more embodiments of the invention; 
         FIG. 14  is an overhead view of aspects of a semiconductor device with air spacers according to one or more embodiments of the invention; 
         FIG. 15  is a cross-sectional view through vias of the device shown in  FIG. 14 ; and 
         FIG. 16  is a cross-sectional view through a gate contact of the device shown in  FIG. 14 . 
     
    
    
     DETAILED DESCRIPTION 
     It is understood in advance that although this invention includes a detailed description of exemplary gate-all-around (GAA) nanosheet FET architectures having silicon (Si) channel nanosheets and SiGe sacrificial nanosheets, embodiments of the invention are not limited to the particular FET architectures or materials described in this specification. Rather, embodiments of the present invention are capable of being implemented in conjunction with any other type of nanosheet/nanowire FET architecture or materials now known or later developed. In this detailed description and in the claims, the terms nanosheet and nanowire are treated as being synonymous. 
     For the sake of brevity, conventional techniques related to semiconductor device and integrated circuit (IC) fabrication may or may not be described in detail herein. Moreover, the various tasks and process steps described herein can be incorporated into a more comprehensive procedure or process having additional steps or functionality not described in detail herein. In particular, various steps in the manufacture of semiconductor devices and semiconductor-based ICs are well known and so, in the interest of brevity, many conventional steps will only be mentioned briefly herein or will be omitted entirely without providing the well-known process details. 
     Turning now to a description of technologies that are more specifically relevant to the present invention, transistors are semiconductor devices commonly found in a wide variety of ICs. A transistor is essentially a switch. When a voltage is applied to a gate of the transistor that is greater than a threshold voltage, the switch is turned on, and current flows through the transistor. When the voltage at the gate is less than the threshold voltage, the switch is off, and current does not flow through the transistor. 
     Typical semiconductor devices are formed using active regions of a wafer. The active regions are defined by isolation regions used to separate and electrically isolate adjacent semiconductor devices. For example, in an IC having a plurality of metal oxide semiconductor field effect transistors (MOSFETs), each MOSFET has a source and a drain that are formed in an active region of a semiconductor layer by incorporating n-type or p-type impurities in the layer of semiconductor material. Disposed between the source and the drain is a channel (or body) region. Disposed above the body region is a gate electrode. The gate electrode and the body are spaced apart by a gate dielectric layer. 
     MOSFET-based ICs are fabricated using so-called complementary metal oxide semiconductor (CMOS) fabrication technologies. In general, CMOS is a technology that uses complementary and symmetrical pairs of p-type and n-type MOSFETs to implement logic functions. The channel region connects the source and the drain, and electrical current flows through the channel region from the source to the drain. The electrical current flow is induced in the channel region by a voltage applied at the gate electrode. 
     The wafer footprint of an FET is related to the electrical conductivity of the channel material. If the channel material has a relatively high conductivity, the FET can be made with a correspondingly smaller wafer footprint. A known method of increasing channel conductivity and decreasing FET size is to form the channel as a nanostructure. For example, a so-called gate-all-around (GAA) nanosheet FET is a known architecture for providing a relatively small FET footprint by forming the channel region as a series of nanosheets. In a known GAA configuration, a nanosheet-based FET includes a source region, a drain region and stacked nanosheet channels between the source and drain regions. A gate surrounds the stacked nanosheet channels and regulates electron flow through the nanosheet channels between the source and drain regions. GAA nanosheet FETs are fabricated by forming alternating layers of channel nanosheets and sacrificial nanosheets. The sacrificial nanosheets are released from the channel nanosheets before the FET device is finalized. For n-type FETs, the channel nanosheets are typically silicon (Si) and the sacrificial nanosheets are typically silicon germanium (SiGe). For p-type FETs, the channel nanosheets can be SiGe and the sacrificial nanosheets can be Si. In some implementations, the channel nanosheet of a p-type FET can be SiGe or Si, and the sacrificial nanosheets can be Si or SiGe. Forming the GAA nanosheets from alternating layers of channel nanosheets formed from a first type of semiconductor material (e.g., Si for n-type FETs, and SiGe for p-type FETs) and sacrificial nanosheets formed from a second type of semiconductor material (e.g., SiGe for n-type FETs, and Si for p-type FETs) provides superior channel electrostatics control, which is necessary for continuously scaling CMOS technology down to seven (7) nanometer node and below. The use of multiple layered SiGe/Si sacrificial/channel nanosheets (or Si/SiGe sacrificial/channel nanosheets) to form the channel regions in GAA FET semiconductor devices provides desirable device characteristics, including the introduction of strain at the interface between SiGe and Si. 
     Although nanosheet channel FET architectures provide increased device density over planar FET architectures, there are still challenges when attempting to fabricate nanosheet channel FETs that provide the performance characteristics required for a particular application. Some of these challenges apply, as well, to other types of FETs (e.g., fin FETs, nanowire FETs). For example, as previously noted, some devices have transistors that are electrically connected in series by shared sources and drains. Placing contacts on the internally connected sources and drains increases parasitic capacitance between the gate and contact without any benefit. This is because self-aligned contacts address the reduction in gate pitch but increase parasitic capacitance between the source or drain contact and the metal gate. While the parasitic capacitance issue can be addressed by replacing the typical low-k dielectric spacer between the contact and the gate with an air spacer, removing the low-k spacer selective to surrounding materials such as the SAC cap is challenging. 
     Turning now to an overview of aspects of the invention, embodiments of the invention address the above-described shortcomings of the prior art by integrating of an air spacer with a self-aligned contact. Air spacers are formed only in the region where contacts are present. As detailed, low-k spacers damaged during an etch of the contacts are replaced with a sacrificial material (e.g., amorphous SiGe or Ge) that can easily be removed selective to contact metal and dielectric. In regions where no contact is needed, the low-k spacers remain to improve mechanical stability of the structure. While nanosheet FETs, specifically, three nanosheet transistors in a NAND circuit configuration, are used to illustrate exemplary embodiments of the invention, the processes detailed are equally applicable to other types of transistors (e.g., finFETs, nanowire FETs) and devices. 
       FIGS. 1-3  show devices that are fabricated with integration of an air spacer with a self-aligned contact according to one or more embodiments of the invention.  FIG. 1  is a circuit diagram of a two-input NAND device  100  with two p-type FETs (pFETs), pFET 1  and pFET 2 , and two nFETs, nFET 1  and nFET 2 . The nFETs, nFET 1  and nFET 2 , share a source or drain region, as indicated. This region does not require a contact. Thus, the formation of a contact and the air spacer integrated with each contact can be limited to only one side of each of the nFETs.  FIG. 2  is a circuit diagram of a two-input NOR device  200  with two nFETs, nFET 1  and nFET 2 , and two pFETs, pFET 1  and pFET 2 . The pFETs, pFET 1  and pFET 2 . share a source or drain region, as indicated. Thus, the formation of a contact and the air spacer integrated with each contact can be limited to only one side of each of the pFETs. 
       FIG. 3  is a circuit diagram of a three-input NAND circuit  300  with three p-type FETs (pFETs), pFET 1 , pFET 2 , and pFET 3 , and three nFETs, nFET 1 , nFET 2 , and nFET 3 . The nFETs, nFET 1 , nFET 2 , and nFET 3 , share two source and drain regions, as indicated. That is, both the source and drain regions of nFET 2  are shared. These shared regions do not require a contact. Thus, the formation of a contact and the air spacer integrated with each contact can be limited to only one side of each of the nFETs, nFET 1  and nFET 3 . The formation of source and drain contacts with integrated air spacers, according to exemplary one or more embodiments, is detailed for the three-input NAND circuit  300  with reference to  FIGS. 4-12 . While aspects of the fabrication of the nFETs of the three-input NAND circuit  300  are specifically detailed for explanatory purposes, the processes are not limited to any particular device or type of device. In addition, while nanosheet FETs are shown for explanatory purposes, other types of transistors can benefit from integration of an air spacer with a self-aligned contact according to one or more embodiments of the invention. 
       FIGS. 4-12  show aspects of a process flow of fabricating nanosheet FETs that include integration of an air spacer  1210  ( FIG. 12 ) with self-aligned contacts  910  ( FIG. 9 ) according to one or more embodiments of the invention.  FIG. 4  is a cross-sectional view of a structure  400  that results after formation of a SAC cap  450  over a recessed metal gate  445  in three nanosheet FETs. A dielectric layer  415  is formed on a substrate  410 . The dielectric layer  415  is a spacer material that isolates the source and drain regions  420  from the substrate  410 . The substrate  410  can include a bulk semiconductor, such as silicon, germanium, silicon germanium, silicon carbide, and those consisting essentially of III-V compound semiconductors having a composition defined by the formula Al X1 Ga X2 In X3 As Y1 P Y2 N Y3 Sb Y4 , where X1, X2, X3, Y1, Y2, Y3, and Y4 represent relative proportions, each greater than or equal to zero and X1+X2+X3+Y1+Y2+Y3+Y4=1 (1 being the total relative mole quantity). Other suitable substrates  410  include II-VI compound semiconductors having a composition Zn A1 Cd A2 Se B1 Te B2 , where A1, A2, B1, and B2 are relative proportions each greater than or equal to zero and A1+A2+B1+B2=1 (1 being a total mole quantity). The semiconductor substrate  410  can also comprise an organic semiconductor or a layered semiconductor such as, for example, Si/SiGe, a silicon-on-insulator or a SiGe-on-insulator. A portion or entire semiconductor substrate  410  can be amorphous, polycrystalline, or monocrystalline. In addition to the aforementioned types of semiconductor substrates  410 , the substrate  410  can also comprise a hybrid oriented (HOT) semiconductor substrate in which the HOT substrate has surface regions of different crystallographic orientation. The substrate  410  can be doped, undoped, or contain doped regions and undoped regions therein. The substrate  410  can contain regions with strain and regions without strain therein, or contain regions of tensile strain and compressive strain. In one or more embodiments, the substrate  410  can be a semiconductor-on-insulator (SOI) substrate. The substrate  410  can further include other structures (not shown) such as shallow trench isolation (STI), fins, nanowires, nanosheets, resistors, capacitors, etc. 
     As shown, three sets of nanosheets  425  are formed above the dielectric layer  415 . Adjacent nanosheets  425  of each set are separated by inner spacers  430 . A gate  445  is shown in the gate-all-around configuration in which the gate  445  surrounds each nanosheet  425 . Each gate  445  can include a gate dielectric and a gate conductor. The gate dielectric can include any suitable dielectric material such as, for example, silicon oxide, silicon nitride, silicon oxynitride, high-k materials, or any combination of these materials. Exemplary high-k materials include metal oxides such as hafnium oxide, hafnium silicon oxide, hafnium silicon oxynitride, lanthanum oxide, lanthanum aluminum oxide, zirconium oxide, zirconium silicon oxide, zirconium silicon oxynitride, tantalum oxide, titanium oxide, barium strontium titanium oxide, barium titanium oxide, strontium titanium oxide, yttrium oxide, aluminum oxide, lead scandium tantalum oxide, and lead zinc niobate. The high-k material can also include dopants such as lanthanum, aluminum, magnesium. The gate dielectric material can be formed by any suitable process or any suitable combination of multiple processes like thermal oxidation, chemical oxidation, thermal nitridation, plasma oxidation, plasma nitridation, atomic layer deposition (ALD), and chemical vapor deposition (CVD. The gate dielectric can have a thickness ranging from 1 nanometer (nm) to 5 nm, although less thickness and greater thickness are also conceived. The gate conductor can include any suitable conducting material like doped polycrystalline or amorphous silicon, germanium, silicon germanium, a metal (e.g., tungsten (W), titanium (Ti), tantalum (Ta), ruthenium (Ru), hafnium (Hf), zirconium (Zr), cobalt (Co), nickel (Ni), copper (Cu), aluminum (Al), platinum (Pt), tin (Sn), silver (Ag), gold (Au), a conducting metallic compound material (e.g., tantalum nitride (TaN), titanium nitride (TiN), tantalum carbide (TaC), titanium carbide (TiC), titanium aluminum carbide (TiAlC), tungsten silicide (WSi), tungsten nitride (WN), ruthenium oxide (RuO2), cobalt silicide (CoSi), nickel silicide (NiSi)), transition metal aluminides (e.g. Ti3Al, ZrAl), TaC, TaMgC, carbon nanotube, conductive carbon, graphene, or any suitable combination of these materials. The conductive material can also include dopants that are incorporated during or after deposition. The gate can also include a workfunction setting layer between the gate dielectric and gate conductor. The workfunction setting layer can be a workfunction metal (WFM). The WFM can be any suitable material like a nitride (e.g., titanium nitride (TiN), titanium aluminum nitride (TiAlN), hafnium nitride (HfN), hafnium silicon nitride (HfSiN), tantalum nitride (TaN), tantalum silicon nitride (TaSiN), tungsten nitride (WN), molybdenum nitride (MoN), niobium nitride (NbN); a carbide, including but not limited to titanium carbide (TiC) titanium aluminum carbide (TiAlC), tantalum carbide (TaC), hafnium carbide (HfC), and combinations thereof). A conductive material or a combination of multiple conductive materials can serve as both the gate conductor and WFM. The gate conductor and WFM can be formed by any suitable process or any suitable combination of multiple processes such as ALD, CVD, physical vapor deposition (PVD), sputtering, plating, evaporation, ion beam deposition, electron beam deposition, laser assisted deposition, and chemical solution deposition. 
     Low-k spacers  440  are formed on either side of each gate  445  above the set of nano sheets  425 . Some examples of the low-k spacer material include silicon carbide (SiC), silicon oxynitride (SiON), carbon-doped silicon oxide (SiOC), silicon-carbon-nitride (SiCN), boron nitride (BN), silicon boron nitride (SiBN), silicoboron carbonitride (SiBCN), silicon oxycabonitride (SiOCN), silicon oxide, and combinations thereof. The low-k spacer material can have a dielectric constant less than about 7, less than about 5. The low-k spacers  440  can be formed by any suitable techniques such as deposition followed by directional etch. Deposition can include ALD and CVD. Directional etch can include reactive ion etch (RIE). As  FIG. 4  shows, two of the source and drain regions  420  are shared. Specifically, the source or drain region  420  on either side of the center set, among the three sets, of nanosheets  425  are shared. Thus, no contact is needed above these source or drain regions  420 . An interlayer dielectric (ILD) layer  435  is above each source or drain region  420 . As noted, only the ILD layers  435  above the source or drain regions  420  on each end of the structure  400  need to be replaced with SACs  910  ( FIG. 9 ). As such, only the low-k spacers  440  adjacent to these ILD layers  435  on either end need to be replaced with air spacers  1210  ( FIG. 12 ). An SAC cap  450  is formed above each gate  445 . 
       FIG. 5  shows the structure  500  that results from formation of trenches  510  above the unshared source or drain regions  420 . The ILD layers  435  above those source or drain regions  420 , as shown in the structure  400  of  FIG. 4 , are removed to form the trenches  510 . A RIE process is followed by a chemical oxide removal (COR) process to remove any remaining ILD material on the sidewalls of the adjacent low-k spacers  440 . A mask  520  is formed over the SAC caps  450  and low-k spacers  440  to protect those materials during the ME and COR processes. However, because the low-k spacers  440  directly adjacent to the ILD layers  435  that are removed will ultimately be replaced, damage to those low-k spacers  440  does not present issues. Thus, unlike prior process flows, the RIE process need not be carefully tuned to avoid damage to those low-k spacers  440 . 
       FIG. 6  shows a structure  600  that results from removal of the low-k spacers  440  exposed in the structure  500  shown in  FIG. 5 . An isotropic etch is performed to remove the low-k spacers  440  indicated by the dashed lines  610 .  FIG. 7  shows the structure  700  that results from the deposition of liners on the structure  600  shown in  FIG. 6 . The mask  520  is stripped from the structure  600  shown in  FIG. 6 . A thin liner  710  is conformally deposited. This liner  710  can be silicon nitride (SiN), for example, and can have a thickness on the order of 2 nm. A sacrificial spacer material  720  is deposited conformally deposited over the liner  710 . The sacrificial spacer material  720  can be amorphous germanium (aGe), for example, and can have a thickness on the order of 5 nm. 
       FIG. 8  shows the structure  800  that results from removal of the sacrificial spacer material  720  on horizontal surfaces of the structure  700  shown in  FIG. 7 . An ME process is used to remove the exposed sacrificial spacer material  720 .  FIG. 9  shows the structure  900  that results from formation of the SAC  910 , which can also be referred to as a source or drain contact. The liner  710  on the horizontal surfaces is removed and the SACs  910  are formed adjacent to the exposed sacrificial spacer material  720 . The SAC  910  is a metal (e.g., tungsten (W), aluminum (Al), copper (Cu), cobalt (Co), nickel (Ni), titanium (Ti), ruthenium (Ru), or any other suitable conductive material). The SAC  910  can further include a barrier layer. The barrier layer can be titanium nitride (TiN), tantalum nitride (TaN), hafnium nitride (HfN), niobium nitride (NbN), tungsten nitride (WN), tungsten carbon nitride (WCN), or combinations thereof. The barrier layer can prevent diffusion and/or alloying of the metal contact fill material with the top source drain material, and/or anode/cathode material. A chemical mechanical planarization (CMP) process can be performed following deposition of the metal of the SACs  910 . 
       FIG. 10  shows the structure  1000  that results from recessing the SACs  910 .  FIG. 11  shows the structure  1100  that results from removal of the sacrificial spacer material  720 . A wet etch process with a hydrogen peroxide (H 2 O 2 ) solution can be used, for example, to remove an amorphous germanium sacrificial spacer material  720 . This process is highly selective to dielectric and metal. The metal of the gate  445  is protected by the liner  710  that is retained. The prior approach does not involve replacing the low-k spacers  440  with sacrificial spacer material  720  in areas where air spacers  1210  ( FIG. 12 ) are to be formed. As a result, the prior approach requires an aggressive top-down RIE process to remove the low-k spacers  440 , and the ME process risks damage to the metal of the gate  445 . According to one or more embodiments of the invention, SACs  910  are only formed over source and drain regions  420  that are not shared and air spacers  1220  ( FIG. 12 ) are only formed adjacent to the SACs  910 . Thus, the process flow detailed herein, which does not impact the metal of the gate  445 , can be used to form the air spacers  1220 . 
       FIG. 12  shows the structure  1200  that results from forming the air spacers  1210  between each SAC  910  and adjacent gate  445 . Simultaneously, a dielectric cap  1220  is formed on top of SAC  910 . The different materials of the dielectric cap  1220  on top of SAC  910  and SAC cap  450  on top of gate  445  enable the formation of the gate contact  1610  ( FIG. 14 ) and vias  1510  ( FIG. 14 ) to the SACs  910  on the active device region. The non-conformal deposition of the dielectric cap  1220  pinches off an air gap  1225  and form the air spacer  1210 . The dielectric cap  1220  can be silicon oxide nitride (SiON), for example. The size of the air gap  1225  can be controlled by tuning the conformality of the deposition process of the dielectric cap  1220 . A lower conformality results in a larger air gap  1225  than a higher conformality. After removing the sacrificial spacer material  720  a laser anneal can be performed to reduce the contact resistance between the SAC  910  and the source or drain region  420  directly below. 
       FIG. 13  shows a cross-sectional view of aspects of the structure  1200  shown in  FIG. 12 . An overhead cross-sectional view is shown of the right-most portion of the structure  1200 . As indicated, the gate  445  (e.g., a high-k metal) is adjacent to the liner  710  (e.g., SiN). An air spacer  1210  comprised of an air gap  1225  surrounded by material of the dielectric cap  1220  (e.g., SiON) separates the liner  710  from the SAC  910 . The total capacitance (C total ) between the gate  445  and the SAC  910  includes the capacitance of the liner  710  (C liner ), the capacitance of the air gap  1225  (C air ), and the capacitance of the material of the dielectric cap  1220 , which is another liner, (C liner ) in series. Thus: 
                     1     C   total       =       1     C   air       +     2     C   liner                 [     EQ   .           ⁢   1     ]               
As EQ. 1 indicates, the total capacitance is dominated by the capacitance of the air spacer. As such, even a narrow air gap  1225  can result in a significant reduction in parasitic capacitance relative to a spacer without an air gap  1225 . For example, if the total spacer width for the liner  710  and air spacer  1210  is 6 nm, even a 1.5 nm wide air gap  1225  results in a 50 percent reduction in the capacitance between the gate  445  and SAC  910  as compared with not having the air gap  1225 .
 
       FIG. 14  is an overhead view of aspects of a semiconductor device  1400  with air spacers  1220  according to one or more embodiments of the invention. The exemplary semiconductor device  1400  results from additional processing of the structure  1200  shown in  FIG. 12 . One gate  445  is shown with a gate contact  1610 , which is detailed in  FIG. 16 , and two SACs  910  are shown with vias  1510 , which are another set of self-aligned contacts, as shown in  FIG. 15 .  FIGS. 15 and 16  show two different cross-sectional views of the semiconductor device  1400 . 
       FIG. 15  shows a cross-sectional view of the semiconductor device  1400  along A-A, as indicated in  FIG. 14 . As  FIG. 15  indicates, vias  1510  are formed above the SACs  910 . An ILD  1520 , which can be the same material as the ILD layers  435 , is deposited and patterned to form trenches that are then filled with a conductor to form vias  1510 . A CMP process can be performed after filling the trenches with the conductor. As a result of the patterning and CMP process, the ILD  1520  remains only above the SAC caps  450  and ILD layers  435 , as shown. Because the materials of the dielectric cap  1220  on top of SAC  910  and the SAC cap  450  on top of gate  445  are different, vias  1510  can be formed by etching through the ILD  1520  and the dielectric cap  1220 , selective to the SAC cap  450  on top of gate  445 . Self-aligned vias  1510  to the SAC  910  can be formed without electrically shorting the via to the gate  445 , even when the via lands on top of the SAC cap  450 . 
       FIG. 16  shows a cross-sectional view of the semiconductor device  1400  along B-B, as indicated in  FIG. 14 . As  FIG. 16  indicates, gate contacts  1610  are formed above only the gates  445  that are adjacent to SACs  910 . The gate  445  (i.e., the center gate in the exemplary structure shown in  FIG. 16 ) adjacent to shared source and drain regions  420  does not have a gate contact  1610  formed above it. The gate contacts  1610  are formed by depositing the ILD  1520  and patterning the ILD  1520  and SAC caps  450  above the two end gates  445  to form trenches that are filled with a conductor. Because the materials of the dielectric cap  1220  on top of SAC  910  and the SAC cap  450  on top of the gate  445  are different, the gate contact  1610  can be formed by etching through the ILD  1520  and the SAC cap  450 , selective to the dielectric cap  1220  on top of the SAC  910 . The self-aligned gate contact  1610  can be formed on top of the active transistor without electrically shorting the gate contact  1610  to the SAC  910 , even if the gate contact trench lands on top of the dielectric cap  1220 . 
     The methods and resulting structures described herein can be used in the fabrication of IC chips. The resulting IC chips can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case the chip is then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes IC chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     Various embodiments of the present invention are described herein with reference to the related drawings. Alternative embodiments can be devised without departing from the scope of this invention. Although various connections and positional relationships (e.g., over, below, adjacent, etc.) are set forth between elements in the detailed description and in the drawings, persons skilled in the art will recognize that many of the positional relationships described herein are orientation-independent when the described functionality is maintained even though the orientation is changed. These connections and/or positional relationships, unless specified otherwise, can be direct or indirect, and the present invention is not intended to be limiting in this respect. Similarly, the term “coupled” and variations thereof describes having a communications path between two elements and does not imply a direct connection between the elements with no intervening elements/connections between them. All of these variations are considered a part of the specification. Accordingly, a coupling of entities can refer to either a direct or an indirect coupling, and a positional relationship between entities can be a direct or indirect positional relationship. As an example of an indirect positional relationship, references in the present description to forming layer “A” over layer “B” include situations in which one or more intermediate layers (e.g., layer “C”) is between layer “A” and layer “B” as long as the relevant characteristics and functionalities of layer “A” and layer “B” are not substantially changed by the intermediate layer(s). 
     The following definitions and abbreviations are to be used for the interpretation of the claims and the specification. As used herein, the terms “comprises,” “comprising,” “includes,” “including,” “has,” “having,” “contains” or “containing,” or any other variation thereof, are intended to cover a non-exclusive inclusion. For example, a composition, a mixture, process, method, article, or apparatus that comprises a list of elements is not necessarily limited to only those elements but can include other elements not expressly listed or inherent to such composition, mixture, process, method, article, or apparatus. 
     Additionally, the term “exemplary” is used herein to mean “serving as an example, instance or illustration.” Any embodiment or design described herein as “exemplary” is not necessarily to be construed as preferred or advantageous over other embodiments or designs. The terms “at least one” and “one or more” are understood to include any integer number greater than or equal to one, i.e. one, two, three, four, etc. The terms “a plurality” are understood to include any integer number greater than or equal to two, i.e. two, three, four, five, etc. The term “connection” can include an indirect “connection” and a direct “connection.” 
     References in the specification to “one embodiment,” “an embodiment,” “an example embodiment,” etc., indicate that the embodiment described can include a particular feature, structure, or characteristic, but every embodiment may or may not include the particular feature, structure, or characteristic. Moreover, such phrases are not necessarily referring to the same embodiment. Further, when a particular feature, structure, or characteristic is described in connection with an embodiment, it is submitted that it is within the knowledge of one skilled in the art to affect such feature, structure, or characteristic in connection with other embodiments whether or not explicitly described. 
     For purposes of the description hereinafter, the terms “upper,” “lower,” “right,” “left,” “vertical,” “horizontal,” “top,” “bottom,” and derivatives thereof shall relate to the described structures and methods, as oriented in the drawing figures. The terms “overlying,” “atop,” “on top,” “positioned on” or “positioned atop” mean that a first element, such as a first structure, is present on a second element, such as a second structure, wherein intervening elements such as an interface structure can be present between the first element and the second element. The term “direct contact” means that a first element, such as a first structure, and a second element, such as a second structure, are connected without any intermediary conducting, insulating or semiconductor layers at the interface of the two elements. 
     Spatially relative terms, e.g., “beneath,” “below,” “lower,” “above,” “upper,” and the like, can be used herein for ease of description to describe one element or feature&#39;s relationship to another element(s) or feature(s) as illustrated in the figures. It will be understood that the spatially relative terms are intended to encompass different orientations of the device in use or operation in addition to the orientation depicted in the figures. For example, if the device in the figures is turned over, elements described as “below” or “beneath” other elements or features would then be oriented “above” the other elements or features. Thus, the term “below” can encompass both an orientation of above and below. The device can be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein interpreted accordingly. 
     The terms “about,” “substantially,” “approximately,” and variations thereof, are intended to include the degree of error associated with measurement of the particular quantity based upon the equipment available at the time of filing the application. For example, “about” can include a range of ±8% or 5%, or 2% of a given value. 
     The phrase “selective to,” such as, for example, “a first element selective to a second element,” means that the first element can be etched and the second element can act as an etch stop. 
     The term “conformal” (e.g., a conformal layer) means that the thickness of the layer is substantially the same on all surfaces, or that the thickness variation is less than 15% of the nominal thickness of the layer. 
     As previously noted herein, for the sake of brevity, conventional techniques related to semiconductor device and IC fabrication may or may not be described in detail herein. By way of background, however, a more general description of the semiconductor device fabrication processes that can be utilized in implementing one or more embodiments of the present invention will now be provided. Although specific fabrication operations used in implementing one or more embodiments of the present invention can be individually known, the described combination of operations and/or resulting structures of the present invention are unique. Thus, the unique combination of the operations described in connection with the fabrication of a semiconductor device according to the present invention utilize a variety of individually known physical and chemical processes performed on a semiconductor (e.g., silicon) substrate, some of which are described in the immediately following paragraphs. 
     In general, the various processes used to form a micro-chip that will be packaged into an IC fall into four general categories, namely, film deposition, removal/etching, semiconductor doping and patterning/lithography. Deposition is any process that grows, coats, or otherwise transfers a material onto the wafer. Available technologies include physical vapor deposition (PVD), chemical vapor deposition (CVD), electrochemical deposition (ECD), molecular beam epitaxy (MBE) and more recently, atomic layer deposition (ALD) among others. Removal/etching is any process that removes material from the wafer. Examples include etch processes (either wet or dry), chemical-mechanical planarization (CMP), and the like. Reactive ion etching (RIE), for example, is a type of dry etching that uses chemically reactive plasma to remove a material, such as a masked pattern of semiconductor material, by exposing the material to a bombardment of ions that dislodge portions of the material from the exposed surface. The plasma is typically generated under low pressure (vacuum) by an electromagnetic field. Semiconductor doping is the modification of electrical properties by doping, for example, transistor sources and drains, generally by diffusion and/or by ion implantation. These doping processes are followed by furnace annealing or by rapid thermal annealing (RTA). Annealing serves to activate the implanted dopants. Films of both conductors (e.g., poly-silicon, aluminum, copper, etc.) and insulators (e.g., various forms of silicon dioxide, silicon nitride, etc.) are used to connect and isolate transistors and their components. Selective doping of various regions of the semiconductor substrate allows the conductivity of the substrate to be changed with the application of voltage. By creating structures of these various components, millions of transistors can be built and wired together to form the complex circuitry of a modern microelectronic device. Semiconductor lithography is the formation of three-dimensional relief images or patterns on the semiconductor substrate for subsequent transfer of the pattern to the substrate. In semiconductor lithography, the patterns are formed by a light sensitive polymer called a photo-resist. To build the complex structures that make up a transistor and the many wires that connect the millions of transistors of a circuit, lithography and etch pattern transfer steps are repeated multiple times. Each pattern being printed on the wafer is aligned to the previously formed patterns and slowly the conductors, insulators and selectively doped regions are built up to form the final device. 
     The flowchart and block diagrams in the Figures illustrate possible implementations of fabrication and/or operation methods according to various embodiments of the present invention. Various functions/operations of the method are represented in the flow diagram by blocks. In some alternative implementations, the functions noted in the blocks can occur out of the order noted in the Figures. For example, two blocks shown in succession can, in fact, be executed substantially concurrently, or the blocks can sometimes be executed in the reverse order, depending upon the functionality involved. 
     The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments described. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the described embodiments. The terminology used herein was chosen to best explain the principles of the embodiments, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments described herein.