Patent Publication Number: US-2022223714-A1

Title: Rf switch device with a sidewall spacer having a low dielectric constant

Description:
REFERENCE TO RELATED APPLICATIONS 
     This Application is a Continuation of U.S. application Ser. No. 16/815,131, filed on Mar. 11, 2020, which claims the benefit of U.S. Provisional Application No. 62/892,115, filed on Aug. 27, 2019. The contents of the above-referenced Patent Applications are hereby incorporated by reference in their entirety. 
    
    
     BACKGROUND 
     Modern day integrated circuits (ICs) comprise millions or billions of semiconductor devices formed on a semiconductor substrate (e.g., silicon). ICs may use many different types of transistor devices, depending on application. In recent years, the increasing market for cellular and radio frequency (RF) devices has resulted in a significant increase in the demand for RF switch devices. A smartphone may, for example, incorporate 10 or more RF switch devices in its receive chain to switch a received signal to appropriate bands. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Aspects of the present disclosure are best understood from the following detailed description when read with the accompanying figures. It is noted that, in accordance with the standard practice in the industry, various features are not drawn to scale. In fact, the dimensions of the various features may be arbitrarily increased or reduced for clarity of discussion. 
         FIG. 1  illustrates a cross-sectional view of some embodiments of a radio frequency (RF) switch device with a sidewall spacer having a low dielectric constant. 
         FIG. 2  illustrates a cross-sectional view of some embodiments of an integrated circuit (IC) in which the RF switch device of  FIG. 1  is on a full depletion (FD) semiconductor-on-insulator (SOI) substrate for RF. 
         FIGS. 3A-3E  illustrate cross-sectional views of some alternative embodiments of the IC of  FIG. 2 . 
         FIG. 4  illustrates a cross-sectional view of some embodiments of an interconnect structure overlying an RF switch device with a sidewall spacer having a lower dielectric constant. 
         FIGS. 5-12  illustrate a series of cross-sectional views of some embodiments of a method for forming an RF switch device with a sidewall spacer having a low dielectric constant. 
         FIG. 13  illustrates a flow diagram of some embodiments corresponding to the method of  FIGS. 5-12   
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure provides many different embodiments, or examples, for implementing different features of this disclosure. Specific examples of components and arrangements are described below to simplify the present disclosure. These are, of course, merely examples and are not intended to be limiting. For example, the formation of a first feature over or on a second feature in the description that follows may include embodiments in which the first and second features are formed in direct contact, and may also include embodiments in which additional features may be formed between the first and second features, such that the first and second features may not be in direct contact. In addition, the present disclosure may repeat reference numerals and/or letters in the various examples. This repetition is for the purpose of simplicity and clarity and does not in itself dictate a relationship between the various embodiments and/or configurations discussed. 
     Further, spatially relative terms, such as “beneath,” “below,” “lower,” “above,” “upper” and the like, may 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. 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. The apparatus may be otherwise oriented (rotated 90 degrees or at other orientations) and the spatially relative descriptors used herein may likewise be interpreted accordingly. 
     In some embodiments, a radio frequency (RF) switch device comprises a drain region, a source region, a gate electrode laterally between the drain and source regions, and a sidewall spacer comprising, for example, silicon nitride on sidewalls of the gate electrode. However, silicon nitride has a high dielectric constant, such as, for example, a dielectric constant between about 7-11. The high dielectric constant results in a large parasitic capacitance from the gate electrode to the drain region. The large parasitic capacitance provides a leakage path from the gate electrode to the drain region and also leads to insertion losses. Hence, the large parasitic capacitance degrades overall switch performance and degrades electrical isolation. 
     In some embodiments, to form the RF switch device, a gate electrode is formed on a substrate. A spacer layer comprising, for example, silicon nitride is deposited over the substrate and the gate electrode, and an etch back is performed into the spacer layer, thereby resulting in the sidewall spacer. Further, in some embodiments, the etch back is performed by a solution comprising 1% hydrogen fluoride by weight. However, such a solution has a high etch rate for the silicon nitride of the spacer layer, such that it is difficult to control the etch back. This, in turn, may result in over etching and/or damage to a substrate upon which the RF switch device is on. Damage to the substrate may, in turn, lead to leakage, thereby further degrading the overall switch performance of the RF switch device. 
     Various embodiments of the present application are directed towards an RF switch device comprising a sidewall spacer with a low dielectric constant relative to silicon nitride, as well as a method for forming the RF switch device. It has been appreciated by employing a low dielectric constant for the sidewall spacer, parasitic capacitance from the gate electrode to the drain region is low. As a result, leakage is low at this parasitic capacitance and isolation is high, whereby performance of the RF switch device is high. In some embodiments, the sidewall spacer is or comprises a dielectric material such as silicon oxycarbonitride (SiOCN), silicon oxycarbide (SiOC), or some other suitable material. It has been further appreciated that a solution comprising 1% hydrogen fluoride by weight has a low etch rate for the dielectric material (e.g., SiOCN) relative to silicon nitride. As such, the etch back to form the RF switch device may be better controlled. This, in turn, may reduce over etching and/or damage to a substrate upon which the RF switch device is on and may hence reduce leakage, thereby further increasing the overall switch performance of the RF switch device. 
       FIG. 1  illustrates a cross-sectional view  100  of some embodiments of an RF switch device  102  that comprises a sidewall spacer  104  with a low dielectric constant. 
     In some embodiments, the RF switch device  102  includes a gate electrode  106 , a gate dielectric layer  108 , the sidewall spacer  104 , and a pair of source/drain regions  112 . The gate dielectric layer  108  underlies the gate electrode  106  and spaces the gate electrode  106  from a semiconductor layer  110  upon which the RF switch device  102  is arranged. The semiconductor layer  110  may, for example, be or comprise silicon or some other suitable semiconductor material. The sidewall spacer  104  is disposed along sidewalls of the gate electrode  106  and sidewalls of the gate dielectric layer  108 . The sidewall spacer  104  further overlies a pair of source/drain extensions  112   e . The source/drain extensions  112   e  are part of the pair of source/drain regions  112  respectively on opposite sides of the gate electrode  106 . The source/drain regions  112 , and hence the source/drain extensions  112   e , are doped regions of the semiconductor layer  110  having an opposite doping type as an adjoining portion of the semiconductor layer  110 . In some embodiments, the source/drain extensions  112   e  have a lower doping concentration than a remainder of the source/drain regions  112 . 
     In some embodiments, the sidewall spacer  104  comprises a sidewall spacer dielectric material with a low dielectric constant relative to silicon nitride. In further embodiments, the sidewall spacer dielectric material may, for example, be or comprise silicon oxycarbonitride (SiOCN), silicon oxycarbide (SiOC), or some other suitable material. In yet further embodiments, the low dielectric constant is between about 4.2-5.5, about 4.2-4.5, about 4.5-5.0, about 5.0-5.5, or some other suitable value. In some embodiments, the low dielectric constant of the sidewall spacer  104  is less than about 6, less than about 7, or some other suitable value. In some embodiments, the sidewall spacer  104  comprises or consists essentially of silicon, carbon, nitrogen, and oxygen, where silicon is about 30-35 atomic percent of the sidewall spacer  104 , carbon is about 3-10 atomic percent of the sidewall spacer  104 , nitrogen is about 16-20 atomic percent of the sidewall spacer  104 , and oxygen is about 40-45 atomic percent of the sidewall spacer  104 . Other atomic percentages and/or other materials are, however, amenable. In some embodiments, the gate dielectric layer  108  has a higher dielectric constant than the sidewall spacer  104 . 
     Further, a sidewall liner  114  lines the sidewall spacer  104  and separates the sidewall spacer  104  from the gate electrode  106 , the gate dielectric layer  108 , and the source/drain extensions  112   e . The sidewall liner  114  may, for example, be or comprise silicon dioxide or some other suitable material and/or may, for example, have a lower dielectric constant than the sidewall spacer  104  and/or the gate dielectric layer  108 . 
     By employing a relatively low dielectric constant (e.g., the dielectric constant is less than or equal to 5.5) for the sidewall spacer  104 , parasitic capacitance from the gate electrode  106  to a drain region of the pair of source/drain regions  112  is low. This is because as the dielectric constant decreases, the capacitance between the gate electrode  106  and the drain region of the pair of source/drain regions  112  decreases. As a result, leakage is low at this parasitic capacitance and isolation is high, whereby performance of the RF switch device  102  is high. In addition, reducing the parasitic capacitance from the gate electrode  106  to the drain region of the pair of source/drain regions  112  also reduces insertion loss of the RF switch device  102 . In further embodiments, if, for example, the dielectric constant of the sidewall spacer  104  is less than about 4.2, then an etching rate of the sidewall spacer  104  may be increased (e.g., by a wet etch process). This may result in over etching the sidewall spacer  104  during fabrication, thereby decreasing isolation between the RF switch device  102  and other semiconductor devices (not shown) disposed within/over the semiconductor layer  110 . 
     In some embodiments, if, for example, the atomic percentage of carbon in the sidewall spacer  104  is less than about 3, then the dielectric constant of the sidewall spacer  104  is increased. This may increase a parasitic capacitance between the drain region of the pair of source/drain regions  112  and the gate electrode  106  of the RF switch device  102 . In further embodiments, if, for example, the atomic percentage of nitrogen in the sidewall spacer  104  is greater than about 20, then the dielectric constant of the sidewall spacer  104  may be increased. This may further increase a parasitic capacitance between the drain region of the pair of source/drain regions  112  and the gate electrode  106  of the RF switch device  102 . In yet further embodiments, if, for example, the atomic percentage of oxygen in the sidewall spacer  104  is less than about 40, then the dielectric constant of the sidewall spacer  104  is increased. This may even further increase a parasitic capacitance between the drain region of the pair of source/drain regions  112  and the gate electrode  106  of the RF switch device  102 . In alternative embodiments, if, for example, the atomic percentage of oxygen in the sidewall spacer  104  is greater than about 45, then a rate in which the sidewall spacer  104  (e.g., the sidewall spacer layer  904  of  FIG. 9 ) is etched (e.g., by a wet etch) by a solution comprising about 1% hydrogen fluoride by weight is increased. This may result in over etching the sidewall spacer  104  (e.g., the sidewall spacer layer  904  of  FIG. 9 ) during the etching process, thereby decreasing isolation between the RF switch device  102  and other semiconductor devices (not shown) disposed within/over the semiconductor layer  110 . 
       FIG. 2  illustrates a cross-sectional view  200  of some embodiments of an integrated circuit (IC) in which the RF switch device  102  of  FIG. 1  is on a semiconductor-on-insulator (SOI) substrate  202 . 
     The SOI substrate  202  comprises a high resistivity layer  204 , a trap-rich layer  206  overlying the high resistivity layer  204 , an insulator layer  208  overlying the trap-rich layer  206 , and a device layer  210  overlying the insulator layer  208 . The high resistivity layer  204  has a high resistance to reduce RF losses from the RF switch device  102 , thereby increasing a performance of the RF switch device  102 . The high resistance may, for example, be between about 1,000-10,000 ohms, about 1,000-5,000 ohms, about 5,000-10,000 ohms, or some other suitable value. The high resistivity layer  204  may, for example, be or comprise silicon or some other suitable semiconductor material. 
     The trap-rich layer  206  comprises a high concentration of electron traps to reduce the parasitic surface conduction (PSC) effect along a top surface of the high resistivity layer  204 . The PSC effect may, for example, be caused by fixed charge in the insulator layer  208 . The trap-rich layer  206  may, for example, be or comprise polysilicon or some other suitable material. Further, the trap-rich layer  206  may, for example, have a thickness of about 1.0-2.0 micrometers, about 1.0-1.5 micrometers, about 1.5-2.0 micrometers, or some other suitable value. 
     The insulator layer  208  provides electrical isolation between the trap-rich layer  206  and the device layer  210 . In some embodiments, the insulator layer  208  has a lower dielectric constant than the sidewall spacer  104 . In other embodiments, the insulator layer  208  has a higher dielectric constant than the sidewall spacer  104 . The insulator layer  208  may, for example, be or comprise silicon dioxide and/or some other suitable dielectric material. The insulator layer  208  may, for example, have a thickness of about 1,000-4,000 angstroms, about 1,000-2,500 angstroms, about 2,500-4,000 angstroms, or some other suitable value. 
     The device layer  210  is a semiconductor layer within which the RF switch device  102  is formed. The device layer  210  corresponds to the semiconductor layer  110  of  FIG. 1 . Further, the device layer  210  is sufficiently thin to promote full depletion of the RF switch device  102 . That is to say, a depletion region of the RF switch device  102  extends through a full thickness of the device layer  210 . The depletion region may, for example, form in the device layer  210  due to the gate electrode  106  being polysilicon having an opposite doping type as a portion of the device layer  210  directly under the gate electrode  106 . The device layer  210  may, for example, have a thickness of about 50-100 nanometers, about 50-75 nanometers, about 75-100 nanometers, or some other suitable value. The device layer  210  may, for example, be or comprise silicon or some other suitable semiconductor material. In some embodiments, the device layer  210  and the high resistivity layer  204  are or comprise a same semiconductor material. 
     An isolation structure  212  extends into the device layer  210  to the insulator layer  208 . Further, the isolation structure surrounds the RF switch device  102 . The isolation structure  212  provides electrical isolation between the RF switch device  102  and other devices (not shown) on the SOI substrate  202 . The isolation structure  212  is or comprises silicon dioxide and/or some other suitable dielectric material. The isolation structure  212  may, for example, be a shallow trench isolation (STI) structure, a deep trench isolation (DTI) structure, and/or some other suitable isolation structure. 
       FIGS. 3A-3E  illustrate cross-sectional views  300   a - 300   e  of some alternative embodiments of the IC of  FIG. 2 . As illustrated by the cross-sectional view  300   a  of  FIG. 3A , the source/drain extensions  112   e  are omitted. As illustrated by the cross-sectional view  300   b  of  FIG. 3B , the sidewall liner  114  is omitted. As illustrated by the cross-sectional view  300   c  of  FIG. 3C , the device layer  210  is thicker so the depletion region of the RF switch device  102  extends partially, not fully, through a thickness of the device layer  210 . As illustrated by the cross-sectional view  300   d  of  FIG. 3D , the trap-rich layer  206  is omitted. As illustrated by the cross-sectional view  300   e  of  FIG. 3E , a bulk semiconductor substrate  302  is used in place of the SOI substrate  202  and corresponds to the semiconductor layer  110  of  FIG. 1 . 
     While  FIGS. 3A-3E  each illustrate a different variation to the IC of  FIG. 2 ,  FIG. 2  may be modified with any combination of the variations in  FIGS. 3A-3E . For example, the source/drain extensions  112   e  may be omitted (as in  FIG. 3A ) and the trap-rich layer  206  may be omitted (as in  FIG. 3D ). 
       FIG. 4  illustrates a cross-sectional view  400  of some embodiments of an IC having an interconnect structure  401  overlying an RF switch device  102  with a sidewall spacer  104  having a low dielectric constant. 
     In some embodiments, the IC includes the RF switch device  102  disposed within/over the SOI substrate  202 . In further embodiments, the SOI substrate  202  comprises the high resistivity layer  204 , the trap-rich layer  206  overlying the high resistivity layer  204 , the insulator layer  208  overlying the trap-rich layer  206 , and the device layer  210  overlying the insulator layer  208 . The SOI substrate  202  of  FIG. 4  may, for example, be illustrated and/or described as the SOI substrate  202  of  FIG. 2 . Further, the RF switch device  102  is disposed over and/or on the device layer  210  of the SOI substrate  202  and/or may be configured as the RF switch device  102  of  FIGS. 1 through 3E . 
     In some embodiments, the device layer  210  may comprise a first doping type (e.g., p-type) with a first doping concentration and the pair of source/drain regions  112  of the RF switch device  102  may comprise a second doping type (e.g., n-type) opposite the first doping type with a second doping concentration. In some embodiments, the first doping type may be p-type and the second doping type may be n-type, or vice versa. In further embodiments, the first doping concentration of the device layer  210  is less than the second doping concentration of the pair of source/drain regions  112 . In some embodiments, the gate electrode  106  may, for example, be or comprise polysilicon, doped polysilicon, or a metal, such as tungsten, titanium nitride, tantalum nitride, titanium, tantalum, a silicide, another conductive material, or any combination of the foregoing. In some embodiments, the gate dielectric layer  108  may, for example, be or comprise an oxide, such as silicon dioxide, a high-k dielectric material, such as silicon oxynitride, hafnium oxide, hafnium oxynitride, hafnium aluminum oxide, zirconium oxide, another suitable dielectric material, or any combination of the foregoing. As used herein, a high-k dielectric material is a dielectric material with a dielectric constant greater than 3.9. 
     The interconnect structure  401  overlies the SOI substrate  202 . The interconnect structure  401  comprises a plurality of conductive vias  402 , a plurality of conductive wires  404 , and an interconnect dielectric structure. The plurality of conductive vias and wires  402 ,  404  are disposed within the interconnect dielectric structure and are configured to electrically couple devices disposed within and/or over the SOI substrate  202  to one another and/or to another IC (not shown). The interconnect dielectric structure comprises a plurality of inter-level dielectric (ILD) layers  406  and a plurality of dielectric protection layers  408 . A bottommost layer of conductive vias  402  extend from a bottommost layer of the conductive wires  404  to the pair of source/drain regions  112  and the gate electrode  106  of the RF switch device  102 . The conductive vias  402  extend through the dielectric protection layers  408  (which can be made of dielectric material and/or act as etch stop layers during manufacturing of the IC). In some embodiments, the conductive vias and/or wires  402 ,  404  may, for example, respectively be or comprise aluminum, copper, tungsten, titanium nitride, tantalum nitride, another suitable conductive material, or any combination of the foregoing. In some embodiments, the ILD layers  406  may, for example, be or comprise an oxide, such as silicon dioxide, a low-k dielectric material, an extreme low-k dielectric material, another dielectric material, or any combination of the foregoing. As used herein, a low-k dielectric material is a dielectric material with a dielectric constant less than 3.9. Thus, in some embodiments, the sidewall spacer  104  has a dielectric constant greater than the ILD layers  406 . In further embodiments, the dielectric protection layers  408  may, for example, be or comprise silicon nitride, silicon carbide, silicon oxynitride, another suitable dielectric material, or the like. Therefore, in some embodiments, the sidewall spacer  104  has a dielectric constant less than the dielectric protection layers  408 . 
       FIGS. 5-12  illustrate cross-sectional views  500 - 1200  of some embodiments of a method for forming an RF switch device with a sidewall spacer having a low dielectric constant according to the present disclosure. Although the cross-sectional views  500 - 1200  shown in  FIGS. 5-12  are described with reference to a method, it will be appreciated that the structures shown in  FIGS. 5-12  are not limited to the method but rather may stand alone separate of the method. Further, although  FIGS. 5-12  are described as a series of acts, it will be appreciated that these acts are not limiting in that the order of the acts can be altered in other embodiments, and the methods disclosed are also applicable to other structures. In other embodiments, some acts that are illustrated and/or described may be omitted in whole or in part. 
     As illustrated by the cross-sectional view  500  of  FIG. 5 , an SOI substrate  202  is provided. In some embodiments, the SOI substrate  202  comprises a high resistivity layer  204 , a trap-rich layer  206  overlying the high resistivity layer  204 , an insulator layer  208  overlying the trap-rich layer  206 , and a device layer  210  overlying the insulator layer  208 . In such embodiments, the SOI substrate  202  may be illustrated and/or described as the SOI substrate  202  of  FIG. 2 . In alternative embodiments, the SOI substrate  202  is replaced with a bulk semiconductor substrate, an example of which is in  FIG. 3E . In alternative embodiments, the trap-rich layer  206  is omitted, an example of which is in  FIG. 3D . 
     As illustrated by the cross-sectional view  600  of  FIG. 6 , an isolation structure  212  is formed extending into a top surface of the device layer  210  to the insulator layer  208 . The isolation structure  212  surrounds and demarcates a device region. For example, the isolation structure  212  may extend laterally in a closed path along a border of the device region when viewed top down. In some embodiments, a process for forming the isolation structure  212  includes: depositing (e.g., by a chemical vapor deposition (CVD) process, a physical vapor deposition (PVD) process, an atomic layer deposition (ALD) process, etc.) a pad oxide layer covering the SOI substrate  202 ; depositing (e.g., by a CVD process, a PVD process, an ALD process, etc.) a pad nitride layer covering the pad oxide layer; patterning the pad oxide and nitride layers to form a trench with a layout of the isolation structure  212 ; performing an etch (e.g., a wet etch and/or a dry etch) into the SOI substrate  202  with the pad oxide and nitride layers in place to extend the trench into the SOI substrate  202 ; depositing (e.g., by a CVD process, a PVD process, an ALD process, thermal oxidation, etc.) a dielectric layer filling the trench and covering the device layer  210 ; performing a planarization process (e.g., a chemical mechanical planarization (CMP) process) into a top of the dielectric layer; and removing the pad oxide and/or nitride layers. Other processes are, however, amenable for forming the isolation structure  212 . 
     As illustrated by the cross-sectional view  700  of  FIG. 7 , a gate electrode  106  and a gate dielectric layer  108  are formed stacked on the device region. As noted above, the device region is a region of the device layer  210  surrounded and demarcated by the isolation structure  212 . In some embodiments, a process for forming the gate electrode  106  and the gate dielectric layer  108  includes: depositing (e.g., by a CVD process, a PVD process, an ALD process, thermal oxidation, etc.) a dielectric layer covering the device layer  210 ; depositing (e.g., by a CVD process, a PVD process, an ALD process, sputtering, electroless plating, electroplating, etc.) a conductive layer covering the dielectric layer; and pattering the dielectric layer and the conductive layer respectively, thereby defining the gate dielectric layer  108  and the gate electrode  106 . Other processes are, however, amenable. For example, the gate electrode  106  and/or the gate dielectric layer  108  may be formed by a high-k metal gate (HKMG) replacement process. In such embodiments, the sidewall spacer  104  of  FIG. 10  may be formed before depositing the gate electrode  106 . 
     As illustrated by the cross-sectional view  800  of  FIG. 8 , a top portion of the device layer  210  is lightly doped to sides of the gate electrode  106 . The doping forms a pair of source/drain extensions  112   e  having an opposite doping type as adjoining portions of the device layer  210 . In alternative embodiments, the light doping is omitted and the source/drain extensions  112   e  are not formed. In further embodiments, the device layer  210  may comprise a first doping type (e.g., p-type), and the pair of source/drain extensions  112   e  may comprise a second doping type (e.g., n-type) opposite the first doping type. In alternative embodiments, the light doping is omitted and the source/drain extensions  112   e  are not formed. The doping may, for example, employ the gate electrode  106  (or a hard mask (not shown) on the gate electrode  106 ) as a mask. The doping may, for example, be performed by ion implantation and/or some other suitable doping process. 
     As illustrated by the cross-sectional view  900  of  FIG. 9 , a sidewall liner layer  902  is deposited (e.g., by a CVD process, a PVD process, an ALD process, thermal oxidation, etc.) over the device layer  210  and the gate electrode  106 , and further lining sidewalls of the gate electrode  106 . In alternative embodiments, the depositing of the sidewall liner layer  902  is omitted (see, for example,  FIGS. 3B, 3C and 3E  where the sidewall liner  114  is omitted). In some embodiments, the sidewall liner layer  902  may, for example, be or comprise silicon dioxide, another suitable dielectric material, or a combination of the foregoing. 
     Also illustrated by the cross-sectional view  900  of  FIG. 9 , a sidewall spacer layer  904  is deposited over the sidewall liner layer  902 . In embodiments in which the sidewall liner layer  902  is omitted, the sidewall spacer layer  904  further occupies the space presently occupied by the sidewall liner layer  902 . In some embodiments, the sidewall spacer layer  904  comprises a sidewall spacer dielectric material with a low dielectric constant relative to silicon nitride. In further embodiments, the sidewall spacer dielectric material may, for example, be or comprise silicon oxycarbonitride (SiOCN), silicon oxycarbide (SiOC), or some other suitable material. In yet further embodiments, the low dielectric constant is between about 4.2-5.5, about 4.2-4.5, about 4.5-5.0, about 5.0-5.5, or some other suitable value. In some embodiments, the low dielectric constant of the sidewall spacer layer  904  is less than about 6, less than about 7, or some other suitable value. In some embodiments, the sidewall spacer layer  904  comprises or consists essentially of silicon, carbon, nitrogen, and oxygen, where silicon is about 30-35 atomic percent of the sidewall spacer layer  904 , carbon is about 3-10 atomic percent of the sidewall spacer layer  904 , nitrogen is about 16-20 atomic percent of the sidewall spacer layer  904 , and oxygen is about 40-45 atomic percent of the sidewall spacer layer  904 . Other atomic percentages and/or other materials are, however, amenable. In some embodiments, the gate dielectric layer  108  has a higher dielectric constant than the sidewall spacer layer  904  and/or the sidewall liner layer  902  has a lower dielectric constant than the sidewall spacer layer  904 . Further, the sidewall spacer layer  904  may, for example, have a dielectric constant less than that of the pad nitride layer described with regard to  FIG. 6  and/or greater than that of the pad oxide layer described with regard to  FIG. 6 . 
     In some embodiments, the sidewall spacer layer  904  is deposited by ALD, low pressure chemical vapor deposition (LPCVD), or some other suitable deposition. In further embodiments, the sidewall spacer layer  904  may be formed solely by an ALD process or solely by a LPCVD process. In some embodiments in which the sidewall spacer layer  904  is deposited by a first ALD process, deposition of the sidewall spacer layer  904  comprises sequentially flowing the following gases over the device layer  210  in the following order: 1) hexachlorodisilane (e.g., Si 2 Cl 6 ); 2) propene (e.g., C 3 H 6 ); 3) diatomic oxygen (e.g., O 2 ); and 4) ammonia (e.g., NH 3 ), thereby forming the sidewall spacer layer  904  such that the sidewall spacer layer  904  comprises, for example, silicon oxycarbonitride (SiOCN). In some embodiments, the foregoing sequence is performed at an ambient temperature of about 580-650 degrees Celsius (° C.), about 580-615° C., about 615-650° C., or some other suitable temperature. In further embodiments, the hexachlorodisilane of step  1 ) above is a high-purity hexachlorodisilane (HCD). By performing the first ALD process at the ambient temperature, the sidewall spacer layer  904  comprises or consists essentially of silicon, carbon, nitrogen, and oxygen (e.g., silicon oxycarbonitride (SiOCN)), where silicon is about 30-35 atomic percent of the sidewall spacer layer  904 , carbon is about 3-10 atomic percent of the sidewall spacer layer  904 , nitrogen is about 16-20 atomic percent of the sidewall spacer layer  904 , and oxygen is about 40-45 atomic percent of the sidewall spacer  104 . This, in some embodiments, ensures that the sidewall spacer layer  904  comprises silicon oxycarbonitride (SiOCN) and/or has the low dielectric constant that is between about 4.5-5.5. 
     In some embodiments, if, for example, the atomic percentage of carbon in the sidewall spacer layer  904  is less than about 3, then the dielectric constant of the sidewall spacer layer  904  is increased. This may increase a parasitic capacitance between a drain region and the gate electrode  106  of the RF switch device ( 102  of  FIG. 11 ). In some embodiments, if, for example, the atomic percentage of carbon in the sidewall spacer layer  904  is greater than about 10, then a rate in which the sidewall spacer layer  904  is etched (e.g., by a wet etch) during subsequent processing steps may be increased. This may, for example, result in over etching the sidewall spacer layer  904  during the subsequent processing steps, thereby decreasing isolation between the RF switch device ( 102  of  FIG. 11 ) and other semiconductor devices (not shown) disposed within/over the SOI substrate  202 . In some embodiments, if, for example, the atomic percentage of nitrogen in the sidewall spacer layer  904  is greater than about 20, then the dielectric constant of the sidewall spacer layer  904  may be increased. This may further increase a parasitic capacitance between a drain region and the gate electrode  106  of the RF switch device ( 102  of  FIG. 11 ). In further embodiments, if, for example, the atomic percentage of oxygen in the sidewall spacer layer  904  is less than about 40, then the dielectric constant of the sidewall spacer layer  904  is increased. This may further increase a parasitic capacitance between a drain region and the gate electrode  106  of the RF switch device ( 102  of  FIG. 11 ). In alternative embodiments, if, for example, the atomic percentage of oxygen in the sidewall spacer layer  904  is greater than about 45, then a rate in which the sidewall spacer layer  904  is etched (e.g., by a wet etch) during subsequent processing steps is increased. This may result in over etching the sidewall spacer layer  904  during the subsequent processing steps, thereby decreasing isolation between the RF switch device ( 102  of  FIG. 11 ) and other semiconductor devices (not shown) disposed within/over the SOI substrate  202 . 
     In alternative embodiments, the sidewall spacer layer  904  is deposited by a second ALD process that comprises sequentially flowing the following gases over the device layer  210  in the following order: 1) hexachlorodisilane (e.g., Si 2 Cl 6 ); 2) propene (e.g., C 3 H 6 ); and 3) diatomic oxygen (e.g., O 2 ), thereby forming the sidewall spacer layer  904  such that the sidewall spacer layer  904  comprises, for example, silicon oxycarbide (SiOC). In some embodiments, the second ALD process sequence is performed at an ambient temperature of about 580-650 degrees Celsius (° C.), about 580-615° C., about 615-650° C., or some other suitable temperature. This, in some embodiments, ensures that the sidewall spacer layer  904  comprises silicon oxycarbide (SiOC) and/or has the low dielectric constant between about 4.2-4.5. This may decrease a parasitic capacitance between a drain region and the gate electrode  106  of the RF switch device ( 102  of  FIG. 11 ), thereby increasing the overall switch performance of the RF switch device ( 102  of  FIG. 11 ). In yet another alternative embodiments, the sidewall spacer layer  904  may, for example, be or comprise silicon carbide (SiC) and/or another suitable dielectric material. 
     In further embodiments, the second ALD process may omit the propene flow at step number two and instead may include performing a polishing process to remove impurities after performing the hexachlorodisilane flow. Thus, in such embodiments, the second ALD process may include: 1) flowing hexachlorodisilane (e.g., Si 2 Cl 6 ) over the device layer  210 ; 2) performing the polishing process; and 3) flowing diatomic oxygen (e.g., O 2 ) over the device layer  210 , thereby forming the sidewall spacer layer  904 . In some embodiments, the polishing process may include exposing the structure of  FIG. 9  to one or more etchants, such as hydrofluoric acid, nitric acid, glacial acetic acid, another suitable etchant, or any combination of the foregoing. 
     As illustrated by the cross-sectional view  1000  of  FIG. 10 , an etch back is performed into the sidewall liner layer ( 902  of  FIG. 9 ) and the sidewall spacer layer ( 904  of  FIG. 9 ). The etch back forms the sidewall spacer  104  and the sidewall liner  114  respectively from the sidewall spacer layer ( 904  of  FIG. 9 ) and the sidewall liner layer ( 902  of  FIG. 9 ). In some embodiments, the etch back includes performing a wet etch process with an etchant solution that comprises about 1% hydrogen fluoride by weight to the sidewall liner layer ( 902  of  FIG. 9 ) and the sidewall spacer layer ( 904  of  FIG. 9 ). Other percentages are, however, amenable for the hydrogen fluoride. For example, the etchant solution may be about 0.001-1.0% hydrogen fluoride by weight. In some embodiments, the etchant solution is an aqueous solution in which the solvent is deionized water or some other suitable solvent. In some embodiments, the etch back is performed at an ambient temperature of about 600-650° C., about 630° C., or some other suitable value. 
     In some embodiments, the sidewall spacer layer ( 904  of  FIG. 9 ) is etched at a low rate relative to a high rate in which silicon nitride would be etched during the etch back. In further embodiments, silicon nitride may, for example, be etched at a high rate of about 8 angstroms per minute during the etch back, while the sidewall spacer layer ( 904  of  FIG. 9 ) may, for example, be etched at a low rate of about 5 angstroms per minute during the etch back. Thus, in some embodiments, the etch back etches the sidewall spacer layer ( 904  of  FIG. 9 ) a low rate less than about 8 angstroms per minute or some other suitable rate. For example, in at least embodiments in which the sidewall spacer layer ( 904  of  FIG. 9 ) is or comprises silicon oxycarbonitride and the etch back is performed by an aqueous solution comprising about 1% hydrogen fluoride by weight, the etch back rate may be performed at a rate of about 5 angstroms per minute. By etching back the sidewall spacer layer ( 904  of  FIG. 9 ) at a low rate, the etch back may be better controlled such that a shape and/or width of the sidewall spacer  104  may be controlled. This, in turn, may reduce over etching and/or damage to the device layer  210  and may hence reduce leakage. Further, this may increase isolation between the RF switch device ( 102  of  FIG. 11 ) and other semiconductor devices (not shown) disposed within/over the SOI substrate  202 . In yet further embodiments, the sidewall liner layer ( 902  of  FIG. 9 ) may be etched more quickly than the sidewall spacer layer ( 904  of  FIG. 9 ) during the etch back. In some embodiments, if the etchant solution is, for example, greater than about 1.0% hydrogen fluoride by weight, then the sidewall spacer layer ( 904  of  FIG. 9 ) and/or the device layer  210  may be over etched during the etch back, thereby degrading a performance of the RF switch device ( 102  of  FIG. 11 ). 
     As illustrated by the cross-sectional view  1100  of  FIG. 11 , the device layer  210  is doped to form a pair of source/drain regions  112 , thereby defining the RF switch device  102 . The source/drain regions  112  overlap with the source/drain extensions  112   e  and have a same doping type, albeit a higher doping concentration, than the source/drain extensions  112   e . In alternative embodiments, the doping at  FIG. 11  to form the source/drain regions  112  is performed at  FIG. 8  in place of the light doping to form the source/drain extensions  112   e , such that the source/drain extensions are omitted. The doping may, for example, employ the sidewall spacer  104  and the gate electrode  106  (or a hard mask (not shown) on the gate electrode  106 ) as a mask. The doping may, for example, be performed by ion implantation and/or some other suitable doping process. 
     By employing a low dielectric constant for the sidewall spacer  104 , parasitic capacitance from the gate electrode  106  to a drain region of the pair of source/drain regions  112  is low. As a result, leakage is low at this parasitic capacitance and isolation is high, whereby performance of the RF switch device  102  is high. 
     As illustrated by the cross-sectional view  1200  of  FIG. 12 , an interconnect structure  401  is formed over the SOI substrate  202  and the RF switch device  102 . The interconnect structure  401  comprises a plurality of conductive vias  402 , a plurality of conductive wires  404 , a plurality of inter-level dielectric (ILD) layers  406 , and a plurality of dielectric protection layers  408 . In some embodiments, the conductive vias and wires  402 ,  404  may be formed by a single damascene process and/or a dual damascene process. In further embodiments, the ILD layers  406  and/or the dielectric protection layers  408  may, for example, respectively be deposited by a CVD process, a PVD process, an ALD process, thermal oxidation, or another suitable growth or deposition process. In yet further embodiments, the dielectric protection layers  408  may respectively be configured to function as an etch stop layer during the single and/or dual damascene processes. 
     In some embodiments, the ILD layers  406  may, for example, be or comprise an oxide, such as silicon dioxide, a low-k dielectric material, an extreme low-k dielectric material, another dielectric material, or any combination of the foregoing. Thus, in some embodiments, the sidewall spacer  104  has a dielectric constant greater than the ILD layers  406 . In further embodiments, the dielectric protection layers  408  may, for example, be or comprise silicon nitride, silicon carbide, silicon oxynitride, another suitable dielectric material, or the like. Therefore, in some embodiments, the sidewall spacer  104  has a dielectric constant less than the dielectric protection layers  408 . 
       FIG. 13  illustrates a flowchart of some embodiments of a method  1300  for forming an RF switch device with a sidewall spacer having a low dielectric constant according to the present disclosure. While the method  1300  of  FIG. 13  is illustrated and described herein as a series of acts or events, it will be appreciated that the illustrated ordering of such acts or events is not to be interpreted in a limiting sense. For example, some acts may occur in different orders and/or concurrently with other acts or events apart from those illustrated and/or described herein. Further, not all illustrated acts may be required to implement one or more aspects or embodiments of the description herein, and one or more of the acts depicted herein may be carried out in or more separate acts and/or phases. 
     At act  1302 , a silicon-on-insulator (SOI) substrate is provided. The SOI substrate comprises a device layer overlying a high resistivity layer.  FIG. 5  illustrates a cross-sectional view  500  of some embodiments corresponding to act  1302 . 
     At act  1304 , an isolation structure is formed within the device layer. The isolation structure surrounds and demarcates a device region in the device layer.  FIG. 6  illustrates a cross-sectional view  600  of some embodiments corresponding to act  1304 . 
     At act  1306 , a gate dielectric layer and a gate electrode are formed over the device layer and laterally within the device region.  FIG. 7  illustrates a cross-sectional view  700  of some embodiments corresponding to act  1306 . 
     At act  1308 , a sidewall liner layer is formed over the gate electrode and the device layer, and a sidewall spacer layer is formed over the sidewall liner layer. The sidewall spacer layer has a low dielectric constant.  FIG. 9  illustrates a cross-sectional view  900  of some embodiments corresponding to act  1308 . 
     At act  1310 , the sidewall liner layer and the sidewall spacer layer are etched, thereby defining a sidewall liner and a sidewall spacer, respectively. In some embodiments, the sidewall liner layer is etched more quickly than the sidewall spacer layer.  FIG. 10  illustrates a cross-sectional view  1000  of some embodiments corresponding to act  1310 . 
     At act  1312 , a pair of source/drain regions are formed within the device layer and on opposing sides of the gate electrode.  FIG. 11  illustrates a cross-sectional view  1100  of some embodiments corresponding to act  1312 . 
     At act  1314 , an interconnect structure is formed over the SOI substrate.  FIG. 12  illustrates a cross-sectional view  1200  of some embodiments corresponding to act  1314 . 
     Accordingly, in some embodiments, the present disclosure provides a radio frequency (RF) switch device disposed on/over a silicon-on-insulator (SOI) substrate, where the RF switch device comprises a gate dielectric layer, a gate electrode, source/drain regions, and a sidewall spacer. The sidewall spacer laterally surrounds sidewalls of the gate electrode and the gate dielectric layer, where the sidewall spacer has a dielectric constant less than that of silicon nitride. 
     In some embodiments, the present application provides a method for forming an integrated circuit (IC), the method including: forming a gate electrode and a gate dielectric stacked over a substrate; depositing a sidewall spacer layer covering the substrate and the gate electrode and further lining sidewalls of the gate electrode; etching back the sidewall spacer layer to form a sidewall spacer on the sidewalls of the gate electrode, wherein etching back is performed at an etch rate less than about 8 angstroms/minute using an etchant comprising hydrogen fluoride; and doping the substrate with the sidewall spacer and the gate electrode in place to form a pair of source/drain regions respectively on opposite sides of the gate electrode. 
     In some embodiments, the present application provides a method for forming an integrated circuit (IC), the method including: depositing a first pad layer covering a substrate; depositing a second pad layer covering the first pad layer; patterning the first and second pad layer with a pattern for an isolation structure; performing an etch into the substrate with the first and second pad layers in place to form a trench that is in the substrate and that has the pattern, wherein the trench surrounds and demarcates a device region; filling the trench with a dielectric material; forming a gate electrode and a gate dielectric layer stacked on the device region; depositing a first dielectric layer covering the substrate and the gate electrode and further lining sidewalls of the gate electrode, wherein the first dielectric layer has a dielectric constant greater than that of the first pad layer and less than that of the second pad layer; etching back the first dielectric layer to form a sidewall spacer on the sidewalls of the gate electrode; and doping the substrate with the sidewall spacer and the gate electrode in place to form a pair of source/drain regions respectively on opposite sides of the gate electrode. 
     In some embodiments, the present application provides an integrated circuit (IC) including: a semiconductor-on-insulator (SOI) substrate including a device layer overlying an insulator layer and a high resistivity layer underlying the insulator layer; a pair of source/drain regions in the device layer; a gate dielectric layer on the device layer and laterally between the source/drain regions; a gate electrode overlying the gate dielectric layer; a sidewall liner disposed along sidewalls of the gate electrode and along an upper surface of the device layer; and a sidewall spacer overlying the device layer on sidewalls and an upper surface of the sidewall liner, wherein the sidewall spacer has a pair of segments respectively on opposite sides of the gate electrode, wherein the sidewall spacer has a dielectric constant less than about 7, and wherein the dielectric constant of the sidewall spacer is greater than that of the sidewall liner and greater than that of the insulator layer. 
     The foregoing outlines features of several embodiments so that those skilled in the art may better understand the aspects of the present disclosure. Those skilled in the art should appreciate that they may readily use the present disclosure as a basis for designing or modifying other processes and structures for carrying out the same purposes and/or achieving the same advantages of the embodiments introduced herein. Those skilled in the art should also realize that such equivalent constructions do not depart from the spirit and scope of the present disclosure, and that they may make various changes, substitutions, and alterations herein without departing from the spirit and scope of the present disclosure.