Patent Publication Number: US-11031331-B2

Title: Phase-change material (PCM) radio frequency (RF) switches with trench metal plugs for RF terminals

Description:
CLAIMS OF PRIORITY 
     This is a divisional of application Ser. No. 16/231,121 filed on Dec. 21, 2018. Application Ser. No. 16/231,121 filed on Dec. 21, 2018 (“the parent application”) is a continuation-in-part of and claims the benefit of and priority to application Ser. No. 16/103,490 filed on Aug. 14, 2018, titled “Manufacturing RF Switch Based on Phase-Change Material.”. The parent application is also a continuation-in-part of and claims the benefit of and priority to application Ser. No. 16/103,587 filed on Aug. 14, 2018, titled “Design for High Reliability RE Switch Based on Phase-Change Material,”. The parent application is also a continuation-in-part of and claims the benefit of and priority to application Ser. No. 16/103,646 filed on Aug. 14, 2018, titled “PCM RF Switch Fabrication with Subtractively Formed Heater,”. The parent application is further a continuation-in-part of and claims the benefit of and priority to application Ser. No. 16/114,106 filed on Aug. 27, 2018, titled “Fabrication of Contacts in an RF Switch Having a Phase-Change Material (PCM) and a Heating Element,”. The disclosures and contents of all of the above-identified applications are hereby incorporated fully by reference into the parent application and the present divisional application. 
    
    
     BACKGROUND 
     Phase-change materials (PCM) are capable of transforming from a crystalline phase to an amorphous phase. These two solid phases exhibit differences in electrical properties, and semiconductor devices can advantageously exploit these differences. Given the ever-increasing reliance on radio frequency (RF) communication, there is particular need for RF switching devices to exploit phase-change materials. However, the capability of phase-change materials for phase transformation depends heavily on how they are exposed to thermal energy and how they are allowed to release thermal energy. For example, in order to transform into an amorphous state, phase-change materials may need to achieve temperatures of approximately seven hundred degrees Celsius (700° C.) or more, and may need to cool down within hundreds of nanoseconds. 
     It is sometimes desirable to avoid fabricating ohmic contacts for connecting to RF terminals of an RF switch. In those instances, a robust capacitive (and non-ohmic) contact can be a good choice. However, capacitance fabrication techniques applicable to conventional semiconductor devices may not be optimum for, or easily compatible with, PCM RF switches, and may not properly utilize or take advantage of the unique structure, layout, and geometry of PCM RF switches. As such, fabricating capacitors in PCM RF switches can present significant manufacturing challenges. 
     Thus, there is a need in the art for capacitive (and non-ohmic) contacts to connect with RF terminals of PCM RF switches while preserving or improving RF performance. 
     SUMMARY 
     The present disclosure is directed to phase-change material (PCM) radio frequency (RF) switches with capacitively coupled RF terminals, substantially as shown in and/or described in connection with at least one of the figures, and as set forth in the claims. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  illustrates a cross-sectional view of a portion of a radio frequency (RF) switch according to one implementation of the present application. 
         FIG. 2  illustrates a top view of a portion of an RF switch according to one implementation of the present application. 
         FIG. 3  illustrates a cross-sectional view of a portion of an RF switch according to one implementation of the present application. 
         FIG. 4  illustrates a cross-sectional view of a portion of an RF switch according to one implementation of the present application. 
         FIG. 5A  illustrates a cross-sectional view of a portion of a metal-oxide-metal (MOM) capacitor structure for use in one implementation of the present application. 
         FIG. 5B  illustrates a top view of a portion of a MOM capacitor structure corresponding to the MOM capacitor structure of  FIG. 5A . 
         FIG. 6  illustrates a cross-sectional view of a portion of an RF switch according to one implementation of the present application. 
         FIG. 7  illustrates a cross-sectional view of a portion of a metal-insulator-metal (MIM) capacitor structure for use in one implementation of the present application. 
         FIG. 8  illustrates a cross-sectional view of a portion of a stacked MIM capacitor structure for use in one implementation of the present application. 
         FIG. 9  illustrates a cross-sectional view of a portion of an RF switch according to one implementation of the present application. 
     
    
    
     DETAILED DESCRIPTION 
     The following description contains specific information pertaining to implementations in the present disclosure. The drawings in the present application and their accompanying detailed description are directed to merely exemplary implementations. Unless noted otherwise, like or corresponding elements among the figures may be indicated by like or corresponding reference numerals. Moreover, the drawings and illustrations in the present application are generally not to scale, and are not intended to correspond to actual relative dimensions. 
       FIG. 1  illustrates a cross-sectional view of a portion of a phase-change material (PCM) radio frequency (RF) switch according to one implementation of the present application. RF switch  100  shown in  FIG. 1  includes substrate  102 , lower dielectric  104 , heating element  106 , thermally conductive and electrically insulating layer  108 , PCM  110  having active segment  112  and passive segments  114 , RF terminal dielectric segment  116 , RF terminals  118  having lower metal portions  120  and upper metal portions  122 , upper dielectric  124 , and capacitors  130 . 
     Substrate  102  is situated under lower dielectric  104 . In one implementation, substrate  102  is an insulator, such as silicon oxide (SiO 2 ). In various implementations, substrate  102  is a silicon (Si), silicon-on-insulator (SOI), sapphire, complementary metal-oxide-semiconductor (CMOS), bipolar CMOS (BiCMOS), or group III-V substrate. In various implementations, a heat spreader is integrated with substrate  102 , or substrate  102  itself performs as a heat spreader. Substrate  102  can have additional layers (not shown in  FIG. 1 ). In one implementation, substrate  102  can comprise a plurality of interconnect metal levels and interlayer dielectric layers. Substrate  102  can also comprise a plurality of devices, such as integrated passive elements (not shown in  FIG. 1 ). 
     Lower dielectric  104  is situated on top of substrate  102 , and is adjacent to the sides of heating element  106 . In the present implementation, lower dielectric  104  extends along the width of RF switch  100 , and is also coplanar with heating element  106 . In various implementations, lower dielectric  104  can have a relative width and/or a relative thickness greater or less than shown in  FIG. 1 . Lower dielectric  104  may comprise a material with thermal conductivity lower than that of thermally conductive and electrically insulating layer  108 . In various implementations, lower dielectric  104  can comprise silicon oxide (SiO 2 ), silicon nitride (Si X N Y ), or another dielectric. 
     Heating element  106  is situated in lower dielectric  104 . Heating element  106  also underlies active segment  112  of PCM  110 . Heating element  106  generates a crystallizing heat pulse or an amorphizing heat pulse for transforming active segment  112  of PCM  110 . Heating element  106  can comprise any material capable of Joule heating. Preferably, heating element  106  comprises a material that exhibits minimal or substantially no electromigration, thermal stress migration, and/or agglomeration. In various implementations, heating element  106  can comprise tungsten (W), molybdenum (Mo), titanium (Ti), titanium nitride (TiN), titanium tungsten (TiW), tantalum (Ta), nickel chromium (NiCr), or nickel chromium silicon (NiCrSi). For example, in one implementation, heating element  106  comprises tungsten lined with titanium and titanium nitride. Heating element  106  may be formed by a damascene process, a subtractive etch process, or any other suitable process. Heating element  106  can be connected to electrodes of a pulse generator (not shown in  FIG. 1 ) that generates a crystallizing current pulse or an amorphizing voltage or current pulses. 
     Thermally conductive and electrically insulating layer  108  is situated on top of heating element  106  and lower dielectric  104 , and under PCM  110  and, in particular, under active segment  112  of PCM  110 . Thermally conductive and electrically insulating layer  108  ensures efficient heat transfer from heating element  106  toward active segment  112  of PCM  110 , while electrically insulating heating element  106  from RF terminals  118 , PCM  110 , and other neighboring structures. Thermally conductive and electrically insulating layer  108  can comprise any material with high thermal conductivity and high electrical resistivity. In various implementations, thermally conductive and electrically insulating layer  108  can comprise aluminum nitride (AlN), aluminum oxide (Al X O Y ), beryllium oxide (Be X O Y ), silicon carbide (SiC), diamond, or diamond-like carbon. 
     PCM  110  is situated on top of thermally conductive and electrically insulating layer  108 . PCM  110  also overlies heating element  106 . PCM  110  includes active segment  112  and passive segments  114 . Active segment  112  of PCM  110  approximately overlies heating element  106  and is approximately defined by heating element  106 . Passive segments  114  of PCM  110  extend outward and are transverse to heating element  106 , and are situated approximately under RF terminals  118 . As used herein, “active segment” refers to a segment of PCM that transforms between crystalline and amorphous states, for example, in response to a crystallizing or an amorphizing heat pulse generated by heating element  106 , whereas “passive segment” refers to a segment of PCM that does not make such transformation and maintains a crystalline state (i.e., maintains a conductive state). With proper heat pulses and heat dissipation, active segment  112  of PCM  110  can transform between crystalline and amorphous states, allowing RF switch  100  to switch between ON and OFF states respectively. 
     PCM  110  can be germanium telluride (Ge X Te Y ), germanium antimony telluride (Ge X Sb Y Te Z ), germanium selenide (Ge X Se Y ), or any other chalcogenide. In various implementations, PCM  110  can be germanium telluride having from forty percent to sixty percent germanium by composition (i.e., Ge X Te Y , where 0.4≤X≤0.6 and Y=1−X). The material for PCM  110  can be chosen based upon ON state resistivity, OFF state electric field breakdown threshold, crystallization temperature, melting temperature, or other considerations. PCM  110  can be provided, for example, by physical vapor deposition (PVD), sputtering, chemical vapor deposition (CVD), evaporation, or atomic layer deposition (ALD). It is noted that in  FIG. 1 , current flowing in heating element  106  flows substantially under active segment  112  of PCM  110 . 
     RF terminal dielectric segment  116  is situated over PCM  110  and over thermally conductive and electrically insulating layer  108 . In various implementations, RF terminal dielectric segment  116  is SiO 2 , boron-doped SiO 2 , phosphorous-doped SiO 2 , Si X N Y , or another dielectric. In various implementations, RF terminal dielectric segment  116  is a low-k dielectric, such as fluorinated silicon dioxide, carbon-doped silicon oxide, or spin-on organic polymer. RF terminal dielectric segment  116  can be provided, for example, by plasma enhanced CVD (PECVD), high density plasma CVD (HDP-CVD), or spin-on processes. 
     Lower metal portions  120  of RF terminals  118  extend partially through RF terminal dielectric segment  116 . Notably, lower metal portions  120  do not connect to passive segments  114  of PCM  110 . That is, lower metal portions  120  are ohmically separated from passive segments  114  of PCM  110 . In one implementation, a metal layer is deposited in and over RF terminal dielectric segment  116 , and then planarized with RF terminal dielectric segment  116 , for example, using chemical machine polishing (CMP), thereby forming lower metal portions  120 . In an alternative implementation, a damascene process is used to form lower metal portions  120 . In various implementations, lower metal portions  120  can comprise tungsten (W), aluminum (Al), or copper (Cu). Lower metal portions  120  are part of RF terminals  118  that provide RF signals to and from PCM  110 . As described below, lower metal portions  120  are capacitively coupled to passive segments  114  of PCM  110 . In an alternative implementation, only one lower metal portion  120  is capacitively coupled to its respective passive segment  114  of PCM  110 , while the other lower metal portion  120  is directly and ohmically connected to its respective passive segment  114 . As such, only one RF terminal is ohmically separated from and capacitively coupled to a respective passive segment of the PCM, while the other RF terminal is ohmically connected to its respective passive segment of the PCM. 
     Upper metal portions  122  are situated over RF terminal dielectric segment  116  and over lower metal portions  120 . Notably, in the present implementation, upper metal portions  122  are ohmically connected to lower metal portions  120 . Together, lower metal portions  120  and upper metal portions  122  make up RF terminals  118  that provide RF signals to and from PCM  110 . Upper metal portions  122  are made from an interconnect metal of generally any multi-layer stack of interconnect metals and interlayer dielectrics suitable for semiconductor devices. Upper metal portions  122  facilitate external connections for RF switch  100  and also improve signal handling. In the present implementation, upper metal portions  122  are a first interconnect metal (i.e., M 1 ). In other implementations, upper metal portions  122  may be any other interconnect metal of a multi-level metallization. In one implementation, a first interconnect metal is deposited over RF terminal dielectric segment  116  and over lower metal portions  120 , and then a middle segment thereof overlying active segment  112  is patterned, thereby forming upper metal portions  122 . In an alternative implementation, a damascene process is used to form upper metal portions  122 . In various implementations, upper metal portions  122  can comprise W, Al, or Cu. In one implementation, lower metal portions  120  can comprise W, and upper metal portions  122  can comprise Al or Cu. Although lower metal portions  120  and upper metal portions  122  are aligned in  FIG. 1 , in various implementations, upper metal portions  122  can have an offset towards active segment  112  of PCM  110  or can have an offset away from active segment  112  of PCM  110 . 
     Upper dielectric  124  is situated on top of upper metal portions  122  and RF terminal dielectric segment  116 . Upper dielectric  124  is an interlayer dielectric of generally any multi-layer stack of interconnect metals and interlayer dielectrics suitable for semiconductor devices. Upper dielectric  124  provides insulation for upper metal portions  122 . In the present implementation, upper dielectric  124  is the top interlayer dielectric. In various implementations, RF switch  100  can include more interconnect metal levels and/or more interlayer dielectrics than shown in  FIG. 1 . In various implementations, upper dielectric  124  can comprise SiO 2 , silicon nitride, or another dielectric. 
     Because passive segments  114  of PCM  110  maintain a conductive crystalline state, capacitors  130  are formed by passive segments  114  of PCM  110 , RF terminal dielectric segment  116 , and lower metal portions  120  of RF terminals  118 . Capacitors  130  capacitively couple lower metal portions  120  to passive segments  114 , creating part of an RF signal path of RF switch  100 , despite that lower metal portions  120  and passive segments  114  are ohmically separated from each other. 
       FIG. 2  illustrates a top view of a portion of an RF switch according to one implementation of the present application.  FIG. 1  illustrates a cross-sectional view along line “ 2 - 2 ” in  FIG. 2 . RF switch  200  includes heating element  206  having heater line  226  and heater contacts  228 , PCM  210 , and lower metal portions  220  of RF terminals. For purposes of illustration, the top view in  FIG. 2  shows selected structures. RF switch  200  may include other structures not shown in  FIG. 2 . 
     Heating element  206  extends along RF switch  200  transverse to PCM  210 , and includes heater line  226  and heater contacts  228 . Heater line  226  is approximately centered along heating element  206 . Heater line  226  underlies PCM  210 , and is seen through PCM  210 . Heater contacts  228  are situated at the two ends of heating element  206 . In the present implementation, heater contacts  228  occupy a relatively a large area. In other implementations, heater contacts  228  may have any other size or shape. Heater contacts  228  provide for connection of, for example, a voltage or current pulse generator (not shown in  FIG. 2 ) to heater line  226 . Heater line  226  provides Joule heating for converting current pulses into heat pulses. In  FIG. 2 , heating element  206  generates a crystallizing or an amorphizing heat pulse for transforming an active segment of PCM  210 , as described above. Heating element  206  in  FIG. 2  generally corresponds to heating element  106  in  FIG. 1  and may have any implementations and advantages described above. 
     PCM  210  in  FIG. 2  generally corresponds to PCM  110  in  FIG. 1  and may have any implementations and advantages described above. In response to a crystallizing or an amorphizing heat pulse generated by heating element  206 , an active segment of PCM  210  can transform from a crystalline phase that easily conducts electrical current to an amorphous phase that does not easily conduct electrical current and, thus, can transform the state of RF switch  200  to an ON state or an OFF state, as described above. 
     Lower metal portions  220  of RF terminals are coupled to passive segments of PCM  210 . According to the implementation of  FIG. 1 , lower metal portions  220  of RF terminals would be capacitively coupled to passive segments of PCM  210 . In other implementations, lower metal portions  220  of RF terminals can be ohmically connected to passive segments of PCM  210 , as described below. Lower metal portions  220  of RF terminals provide RF signals to and from PCM  210 . In various implementations, lower metal portions  220  of RF terminals can comprise W, Al, or Cu. 
       FIG. 3  illustrates a cross-sectional view of a portion of an RF switch according to one implementation of the present application.  FIG. 3  can represent a cross-sectional view along line “ 2 - 2 ” in  FIG. 2 . RF switch  300  includes substrate  302 , lower dielectric  304 , heating element  306 , thermally conductive and electrically insulating layer  308 , PCM  310  having active segment  312  and passive segments  314 , optional contact uniformity support layer  332 , RF terminal dielectric segment  316 , RF terminals  318  having lower metal portions  320  and upper metal portions  322 , pre-metal dielectric  334 , upper dielectric  324 , and metal-oxide-metal (MOM) capacitors  330 . Substrate  302 , lower dielectric  304 , heating element  306 , thermally conductive and electrically insulating layer  308 , PCM  310  having active segment  312  and passive segments  314 , RF terminal dielectric segment  316 , and upper dielectric  324  in RF switch  300  in  FIG. 3  are similar to corresponding structures in RF switch  100  in  FIG. 1 , and may have any implementations and advantages described above. 
     In RF switch  300 , optional contact uniformity support layer  332  is situated over PCM  310 . In one implementation, optional contact uniformity support layer  332  comprises Si X N Y . In another implementation optional contact uniformity support layer  332  is a bi-layer that comprises oxide and nitride, such as SiO 2  under Si X N Y . Optional contact uniformity support layer  332  can be provided, for example, by PECVD or HDP-CVD. RF terminal dielectric segment  316  is situated over optional contact uniformity support layer  332 . Otherwise, RF terminal dielectric segment  316  in  FIG. 3  generally corresponds to RF terminal dielectric segment  116  in  FIG. 1 . 
     Lower metal portions  320  of RF terminals  318  extend through RF terminal dielectric segment  316  and through optional contact uniformity support layer  332  (in case optional contact uniformity support layer  332  is used). In contrast to lower metal portions  120  in  FIG. 1 , lower metal portions  320  in  FIG. 3  are ohmically connected to passive segments  314  of PCM  310 . In the present implementation, forming lower metal portions  320  of RF terminals  318  may comprise two different etching actions. In the first etching action, RF terminal dielectric segment  316  can be aggressively etched without having to accurately time the etching action. This etching action can use a selective etch, for example, a fluorine-based plasma dry etch, and optional contact uniformity support layer  332  can perform as an etch stop while RF terminal dielectric segment  316  is selectively etched. 
     In the second etching action, optional contact uniformity support layer  332  is punch-through etched. As used herein, “punch-through” refers to a short etching action that can be accurately timed to stop at the top surface of PCM  310 . In RF switch  300 , lower metal portions  320  are narrow and optional contact uniformity support layer  332  is thin. Thus, only a small volume of optional contact uniformity support layer  332  is etched, and the punch-through etching action is short and can be accurately timed. In one implementation, a chlorine-based plasma dry etch is used for this etching action. Optional contact uniformity support layer  332  is optional in that the inventive concepts of the present application may be implemented without optional contact uniformity support layer  332 , and lower metal portions  320  can extend through RF terminal dielectric segment  316  into PCM  310 . Because the ON state resistance (R ON ) of RF switch  300  depends heavily on the uniformity of contact made between lower metal portions  320  and PCM  310 , the R ON  will be significantly lower when optional contact uniformity support layer  332  is used. 
     Pre-metal dielectric  334  is situated over RF terminal dielectric segment  316  and over lower metal portions  320 . Pre-metal dielectric  334  aids formation of upper metal portions  322  of RF terminals  318  and processing of interconnect metals in a multi-level metallization. In various implementations, pre-metal dielectric  334  can comprise borophosphosilicate glass (BPSG), tetra-ethyl ortho-silicate (TEOS), silicon onynitride (SiO X N Y ), SiO 2 , Si X N Y , or another dielectric. 
     Upper metal portions  322  of RF terminals  318  are situated over pre-metal dielectric  334  and in upper dielectric  324 . In the present implementation, upper metal portions  322  are a first interconnect metal (i.e., M 1 ). In other implementations, upper metal portions  322  may be any other interconnect metal of a multi-level metallization. Notably, upper metal portions  322  do not physically connect to lower metal portions  320 . That is, upper metal portions  322  are ohmically separated from lower metal portions  320 . Otherwise, upper metal portions  322  in  FIG. 3  generally correspond to upper metal portions  122  in  FIG. 1 . 
     MOM capacitors  330  are formed by lower metal portions  320  of RF terminals  318 , pre-metal dielectric  334 , and upper metal portions  322  of RF terminals  318 . MOM capacitors  330  capacitively couple upper metal portions  322  to lower metal portions  320 , creating part of an RF signal path of RF switch  300 , despite the fact that upper metal portions  322  and lower metal portions  320  are ohmically separated from each other. MOM capacitors  330  in  FIG. 3  are formed farther away from PCM  310  along the RF signal path, relative to capacitors  130  in  FIG. 1 . In an alternative implementation, only one MOM capacitor  330  is formed by a respective lower metal portion  320  of a respective RF terminal  318 , pre-metal dielectric  334 , and upper metal portion  322 . In this alternative implementation, the other lower metal portion  320  is directly and ohmically connected to its respective upper metal portion  322 . The direct connection can be formed using standard CMOS via damascene processes. As such, only one RF terminal  318  is capacitively coupled to a respective passive segment  314  of PCM  310 , while the other RF terminal  318  is ohmically connected to its respective passive segment  314  of PCM  310 . 
       FIG. 4  illustrates a cross-sectional view of a portion of an RF switch according to one implementation of the present application.  FIG. 4  can represent a cross-sectional view along line “ 2 - 2 ” in  FIG. 2 . RF switch  400  includes substrate  402 , lower dielectric  404 , heating element  406 , thermally conductive and electrically insulating layer  408 , PCM  410  having active segment  412  and passive segments  414 , optional contact uniformity support layer  432 , RF terminal dielectric segment  416 , RF terminals  418  having lower metal portions  420  and upper metal portions  422 , interlayer metal levels  436  and  440 , interlayer dielectrics  438  and  444 , interconnect metals  442 , and MOM capacitors  430 . Substrate  402 , lower dielectric  404 , heating element  406 , thermally conductive and electrically insulating layer  408 , PCM  410  having active segment  412  and passive segments  414 , optional contact uniformity support layer  432 , and RF terminal dielectric segment  416  in RF switch  400  in  FIG. 4  are similar to corresponding structures in RF switch  300  in  FIG. 3 , and may have any implementations and advantages described above. 
     In RF switch  400 , interlayer metal level  436 , interlayer dielectric  438 , interlayer metal level  440 , and interlayer dielectric  444  are sequentially situated over RF terminal dielectric segment  416 . Interlayer metal levels  436  and  440  provide layers in which interconnect metals can be built. Interlayer dielectrics  438  and  444  provide insulation between interlayer metal levels  436  and  440 . In the present implementation, interlayer metal level  440  is the top interlayer metal level. In various implementations, RF switch  400  can include more interlayer metal levels and/or more interlayer dielectrics than shown in  FIG. 4 . 
     Upper metal portions  422  are situated in interlayer metal level  436  over RF terminal dielectric segment  416  and over lower metal portions  420 . In the present implementation, upper metal portions  422  are a first interconnect metal (i.e., M 1 ). In other implementations, upper metal portions  422  may be any other interconnect metal of a multi-level metallization. Notably, upper metal portions  422  are ohmically connected to lower metal portions  420 . 
     Interconnect metals  442  are situated in interlayer metal level  440  over interlayer dielectric  438 . In the present implementation, interconnect metals  442  are a second interconnect metal (i.e., M 2 ). In other implementations, interconnect metals  442  may be any other interconnect metal of a multi-level metallization. Notably, interconnect metals  442  are ohmically separated from upper metal portions  422 . 
     MOM capacitors  430  are formed by upper metal portions  422  of RF terminals  418 , interlayer dielectric  438 , and interconnect metals  442 . MOM capacitors  430  capacitively couple interconnect metals  442  to upper metal portions  422 , creating part of an RF signal path of RF switch  400 , despite the fact that interconnect metals  442  and upper metal portions  422  are ohmically separated from each other. As described above, lower metal portions  420  are narrow in order to punch-through etch optional contact uniformity layer  432 . Narrower lower metal portions  420  also decrease parasitic capacitive coupling to heating element  406 . In  FIG. 3 , because lower metal portions  320  are narrow, small misalignment of lower metal portions  320  or upper metal portions  322  can impact the value of MOM capacitors  330 . In  FIG. 4 , MOM capacitors  430  are formed farther away from heating element  406 , decreasing parasitic capacitive coupling to heating element  406 , and allowing for wider upper metal portions  422  and wider interconnect metals  442 . When upper metal portions  422  and interconnect metals  442  are wider, misalignment of upper metal portions  422  and interconnect metals  442  has less impact on the value of MOM capacitors  430 . Thus, RF switch  400  affords some tolerance for misalignment. Additionally, because MOM capacitors  430  are formed by interconnect metals  422  and  442  and interlayer dielectric  438 , MOM capacitors  430  are easily integrated in multi-level metallization. In an alternative implementation, only one MOM capacitor  430  is formed by a respective interconnect metal  422  of a respective RF terminal  418 , interlayer dielectric  438 , and interconnect metal  442 . In this alternative implementation, the other interconnect metal  422  is directly and ohmically connected to its respective interconnect metal  442 . The direct connection can be formed using standard CMOS via damascene processes. As such, only one RF terminal  418  is capacitively coupled to a respective passive segment  414  of PCM  410 , while the other RF terminal  418  is ohmically connected to its respective passive segment  414  of PCM  410 . 
       FIG. 5A  illustrates a cross-sectional view of a portion of a MOM capacitor structure according to one implementation of the present application.  FIG. 5A  represents a MOM capacitor structure that can be used instead of the structure shown in outline  446  in  FIG. 4 . MOM capacitor structure  500 A includes interlayer metal levels  536 ,  540 , and  548 , interlayer dielectrics  538  and  544 , interconnect metals  522 ,  542 , and  550 , fingers  552   a ,  552   b ,  552   c , and  552   d , vias  556  and  558 , and capacitors  560 ,  562   a ,  562   b ,  562   c , and  564 . 
     In  FIG. 5A , interconnect metal  522  is a first interconnect metal situated in interlayer metal level  536 . Interlayer dielectric  538  is situated above interconnect metal  522  and interlayer metal level  536 . Interconnect metal  542  is a second interconnect metal situated in interlayer metal level  540  above interlayer dielectric  538 . Interconnect metal  542  has fingers  552   a ,  552   b ,  552   c , and  552   d . In other implementations, interconnect metal  542  can have more or fewer fingers than shown in  FIG. 5A . In the cross sectional view of  FIG. 5A , fingers  552   a ,  552   b ,  552   c , and  552   d  are substantially rectangular and approximately equally spaced from one another in interlayer metal level  540 . In various implementations, fingers  552   a ,  552   b ,  552   c , and  552   d  can have different shapes and/or spacing than shown in  FIG. 5A . Interlayer dielectric  544  is situated above interconnect metal  542  and interlayer metal level  540 . Interconnect metal  550  is a third interconnect metal situated in interlayer metal level  548  above interlayer dielectric  544 . 
     Via  556  extends through interlayer dielectric  538 , connecting interconnect metal  522  to interconnect metal  542  (and more specifically to finger  552   d ). As described below, fingers  552   b  and  552   d  are connected to each other. Thus, interconnect metal  522 , via  556 , and fingers  552   b  and  552   d  together form one node. Likewise, via  558  extends through interlayer dielectric  544 , connecting interconnect metal  542  (and more specifically finger  552   a ) to interconnect metal  550 . As described below, fingers  552   a  and  552   c  are connected to each other. Thus, interconnect metal  550 , via  558 , and fingers  552   a  and  552   c  together form another node. 
     Capacitor  560  is formed by interconnect metal  522 , interlayer dielectric  538 , and interconnect metal  542 . Capacitor  560  represents a combination of a capacitors formed between interconnect metal  522  and finger  552   a , and between interconnect metal  522  and finger  552   c . Capacitors  562   a .  562   b , and  562   c  are formed between adjacent fingers of interconnect metal  542 . Specifically, capacitor  562   a  is formed between fingers  552   a  and  552   b , capacitor  562   b  is formed between fingers  552   b  and  552   c , and capacitor  562   c  is formed between fingers  552   c  and  552   d . Capacitor  564  is formed by interconnect metal  542 , interlayer dielectric  544 , and interconnect metal  550 . Capacitor  564  represents a combination of a capacitors formed between interconnect metal  550  and finger  552   b , and between interconnect metal  550  and finger  552   d . MOM capacitor structure  500 A utilizes areas below, between, and above interconnect metal  542  to form capacitors  560 ,  562   a ,  562   b ,  562   c , and  564  between two nodes. When these nodes are used in the RF signal path for an RF switch, such as in outline  446  in  FIG. 4 , MOM capacitor structure  500 A increases the capacitance value of capacitors integrated in the RF terminals of the RF switch. 
       FIG. 5B  illustrates a top view of a portion of a MOM capacitor structure corresponding to the MOM capacitor structure of  FIG. 5A  according to one implementation of the present application.  FIG. 5A  illustrates a cross-sectional view along line “ 5 A- 5 A” in  FIG. 5B . MOM capacitor structure  500 B includes interlayer metal level  540 , interconnect metal  542  having fingers  552   a ,  552   b ,  552   c , and  552   d  and runners  554   a  and  554   b , and capacitors  562   a ,  562   b , and  562   c . Interlayer metal level  540 , interconnect metal  542  having fingers  552   a ,  552   b ,  552   c , and  552   d , and capacitors  562   a ,  562   b , and  562   c  in  FIG. 5B  correspond to similarly labeled structures in  FIG. 5A . 
     In MOM capacitor structure  500 B, fingers  552   b  and  552   d  are connected to each other by runner  554   b . Thus, runner  554   b  and fingers  552   b  and  552   d  are parts of the same node. Likewise, fingers  552   a  and  552   c  are connected to each other by runner  554   a . Thus, runner  554   a  and fingers  552   a  and  552   c  are parts of another node. In various implementations runners  554   a  and  554   b  may connect additional fingers. Finger  552   b , finger  552   d , and/or runner  554   b  may connect to a lower interconnect metal through a via, such as via  556  in  FIG. 5A . Likewise, finger  552   a , finger  552   c , and/or runner  554   a  may connect to a higher interconnect metal through a via, such as via  558  in  FIG. 5A . 
     As shown in  FIG. 5B , the fingers of interconnect metal  542  are interdigitated. Capacitors  562   a ,  562   b , and  562   c  are formed between adjacent fingers of interconnect metal  542 . More specifically, capacitor  562   a  is formed between fingers  552   a  and  552   b , capacitor  562   b  is formed between fingers  552   b  and  552   c , and capacitor  562   c  is formed between fingers  552   c  and  552   d . Although not explicitly labeled in  FIG. 5B , it is noted that capacitors are also formed between runner  554   a  and finger  552   b , between runner  554   a  and finger  552   d , between runner  554   b  and finger  552   a , and between runner  554   b  and finger  552   c . When the nodes of interconnect metal  542  are used in the RF signal path for an RF switch, such as in outline  446  in  FIG. 4 , MOM capacitor structure  500 B increases the capacitance value of capacitors integrated in the RF terminals of the RF switch. 
       FIG. 6  illustrates a cross-sectional view of a portion of an RF switch according to one implementation of the present application.  FIG. 6  can represent a cross-sectional view along line “ 2 - 2 ” in  FIG. 2 . RF switch  600  includes substrate  602 , lower dielectric  604 , heating element  606 , thermally conductive and electrically insulating layer  608 , PCM  610  having active segment  612  and passive segments  614 , optional contact uniformity support layer  632 , RF terminal dielectric segment  616 , RF terminals  618  having lower metal portions  620  and upper metal portions  622 , interlayer metal levels  636  and  640 , interlayer dielectrics  638  and  644 , interconnect metals  642 , metal-insulator-metal (MIM) capacitors  630  having insulating films  666  and top MIM metals  668 , spacers  670 , and vias  672 . Substrate  602 , lower dielectric  604 , heating element  606 , thermally conductive and electrically insulating layer  608 , PCM  610  having active segment  612  and passive segments  614 , optional contact uniformity support layer  632 , RF terminal dielectric segment  616 . RF terminals  618  having lower metal portions  620  and upper metal portions  622 , interlayer metal levels  636  and  640 , interlayer dielectrics  638  and  644 , and interconnect metals  642  in RF switch  600  in  FIG. 6  are similar to corresponding structures in RF switch  400  in  FIG. 4 , and may have any implementations and advantages described above. 
     In RF switch  600 , insulating films  666  are situated on upper metal portions  622 . In various implementations, insulating films  666  are high-k dielectrics, such as tantalum pentoxide, aluminum oxide, hafnium oxide, zirconium oxide, zirconium aluminum silicate, hafnium silicate, hafnium aluminum silicate or other dielectrics with a relatively high dielectric constant. In one implementation, the thickness of insulating films  666  can range from approximately two hundred angstroms to approximately six hundred angstroms (200 Å-600 Å). 
     Top MIM metals  668  are situated over insulating films  666 . Top MIM metals  668  can comprise, for example, titanium nitride, tantalum nitride, or a stack comprising aluminum and titanium nitride or tantalum nitride. In one implementation, the thickness of top MIM metals  668  can range from approximately one thousand angstroms to approximately three thousand angstroms (1,000 Å-3,000 Å). Top MIM metals  668  can be provided, for example, by PVD or CVD techniques. It is noted that, unlike interconnect metals  642 , top MIM metals  668  are not formed in interlayer metal levels, such as interlayer metal levels  636  and  640 . As used herein, “MIM capacitor” refers to a capacitor having a top metal formed within an interlayer dielectric where conventionally no metal exists, such as within interlayer dielectric  638 . 
     Spacers  670  are situated adjacent to the sidewalls of insulating films  666  and top MIM metals  668  of MIM capacitors  630 . In various implementations, spacers  670  can comprise Si X N Y , or another dielectric. Spacers  670  protect the sidewalls of insulating films  666  from aggressive etch chemistry used to etch metals during fabrication of RF switch  600 . Thus, there is no increase in leakage current of MIM capacitors  630  or decrease in the breakdown voltage of MIM capacitors  630  due to an increase in defects or voids within insulating films  666 . As such the capacitance value of MIM capacitors  630  is more precisely controlled, and RF switch  600  is more reliable. 
     Vias  672  connect top MIM metals  668  of MIM capacitors  630  to interconnect metals  642 . Thus, for each of MIM capacitors  630 , top MIM metal  668 , vias  672 , and interconnect metals  642  together form one node. In various implementations, each of MIM capacitors  630  can be connected by more or fewer vias than shown in  FIG. 6 . Notably, because top MIM metals  668  are within interlayer dielectric  638 , vias  672  only extend partially through interlayer dielectric  638 . 
     MIM capacitors  630  formed by upper metal portions  622 , insulating films  666 , and top MIM metals  668  have a capacitance with significantly improved density. MIM capacitors  630  do not require addition of interlayer metal levels, such as interlayer metal level  548  in  FIG. 5A , and also do not use up lateral die space. MIM capacitors  630  advantageously increase routing capability because MIM capacitors  630  utilize the space amply available between interlayer metal levels  636  and  640 . 
     In an alternative implementation, only one MIM capacitor  630  is formed by a respective upper metal portion  622  of a respective RF terminal  618 , insulating film  666 , and top MIM metal  668  connected to a respective interconnect metal  642 . In this alternative implementation, the other interconnect metal  642  is directly and ohmically connected to its respective upper metal portion  622  without using a MIM capacitor. The direct connection can be formed using standard CMOS via damascene processes. As such, only one RF terminal  618  is capacitively coupled to a respective passive segment  614  of PCM  610 , while the other RF terminal  618  is ohmically connected to its respective passive segment  614  of PCM  610 . 
       FIG. 7  illustrates a cross-sectional view of a portion of a metal-insulator-metal (MIM) capacitor structure according to one implementation of the present application.  FIG. 7  represents a closer view of the structure shown in outline  674  in  FIG. 6 . MIM capacitor structure  700  includes upper metal portion  722 , interlayer metal levels  736  and  740 , interlayer dielectric  738 , interconnect metal  742 , MIM capacitor  730  having insulating film  766  and top MIM metal  768 , spacers  770 , and vias  772 . Upper metal portions  722 , interlayer metal levels  736  and  740 , interlayer dielectric  738 , interconnect metal  742 , MIM capacitor  730  having insulating film  766  and top MIM metal  768 , spacers  770 , and vias  772  in MIM capacitor structure  700  in  FIG. 7  are similar to corresponding structures in RF switch  600  in  FIG. 6 , and may have any implementations and advantages described above. 
       FIG. 8  illustrates a cross-sectional view of a portion of a stacked MIM capacitor structure according to one implementation of the present application.  FIG. 8  represents a stacked MIM capacitor structure than can be used instead of the structure shown in outline  674  in  FIG. 6 . Stacked MIM capacitor structure  800  is formed by stacking two distinct MIM capacitors, i.e. MIM capacitor  830  and MIM capacitor  831 . Stacked MIM capacitor structure  800  includes upper metal portion  822 , interlayer metal levels  836 ,  840 , and  848 , interlayer dielectrics  838  and  844 , interconnect metals  842   a ,  842   b , and  850 . MIM capacitor  830  includes insulating film  866  and top MIM metal  868 , spacers  870  and vias  872  and  856 . MIM capacitor  831  includes insulating film  867  and top MIM metal  869 , spacers  871 , and vias  873  and  858 . Upper metal portion  822 , interlayer metal levels  836 ,  840 , and  848 , interlayer dielectrics  838  and  844 , interconnect metal  850 , and vias  856  and  858  in MIM capacitor structure  800  in  FIG. 8  are similar to corresponding structures in MOM capacitor structure  500 A in  FIG. 5A , and may have any implementations and advantages described above. MIM capacitor  830  having insulating film  866  and top MIM metal  868 , spacers  870 , vias  872 , and interconnect metal  842   a  in stacked MIM capacitor structure  800  in  FIG. 8  are similar to corresponding structures in MIM capacitor structure  700  in  FIG. 7 , and may have any implementations and advantages described above. 
     As shown in stacked MIM capacitor structure  800  in  FIG. 8 , MIM capacitor  831  is stacked over MIM capacitor  830 . Insulating film  867  of MIM capacitor  831  is situated on interconnect metal  842   a . Top MIM metal  869  of MIM capacitor  831  is situated over insulating film  867  and is in interlayer dielectric  844 . Spacers  871  are situated adjacent to the sidewalls of insulating film  867  and top MIM metal  869  of MIM capacitor  831 . 
     Vias  873  extend partially through interlayer dielectric  844  and connect top MIM metal  869  of MIM capacitor  831  to interconnect metal  850 . Via  858  extends fully through interlayer dielectric  844  and connects interconnect metal  850  to interconnect metal  842   b . Interconnect metal  842   b  is situated in interlayer metal level  840 , and separated from interconnect metal  842   a . Via  856  extends fully through interlayer dielectric  838  and connects interconnect metal  842   b  to upper metal portion  822 . Thus, top MIM metal  869  of MIM capacitor  831 , vias  873 , interconnect metal  850 , via  858 , interconnect metal  842   b , via  856 , and upper metal portion  822  together form a first node of stacked MIM capacitor structure  800 . Similarly, top MIM metal  868  of MIM capacitor  830 , vias  872 , and interconnect metal  842   a  form a second node of stacked MIM capacitor structure  800 . When used in the RF signal path for an RF switch, such as in outline  674  in  FIG. 6 , MIM capacitor structure  800  further increases the capacitance value of capacitors integrated in the RF terminals of the RF switch. 
       FIG. 9  illustrates a cross-sectional view of a portion of an RF switch according to one implementation of the present application. RF switch  900  includes substrate  902 , lower dielectric  904 , heating element  906 , thermally conductive and electrically insulating layer  908 , PCM  910  having active segment  912  and passive segments  914 , optional contact uniformity support layer  932 , RF terminal dielectric segment  916 , trenches  978 . RF terminals  918  having trench metal liners  920  and trench metal plugs  922 , dielectric liner  934 , upper dielectric  924 , and overhang regions  980  and  982  having trench metal liner extension  920   x , dielectric liner extension  934   x , and trench metal plug extension  922   x . Substrate  902 , lower dielectric  904 , heating element  906 , thermally conductive and electrically insulating layer  908 , PCM  910  having active segment  912  and passive segments  914 , optional contact uniformity support layer  932 , RF terminal dielectric segment  916 , and upper dielectric  924  in RF switch  900  in  FIG. 9  are similar to corresponding structures in RF switch  300  in  FIG. 3 , and may have any implementations and advantages described above. 
     In RF switch  900 , trenches  978  extend through RF terminal dielectric segment  916  and through optional contact uniformity support layer  932  (in case optional contact uniformity support layer  932  is used). Trench metal liners  920  of RF terminals  918  line trenches  978 , and are ohmically connected to passive segments  914  of PCM  910 . In various implementations, trench metal liners  920  can comprise W, Al, or Cu. 
     Dielectric liner  934  is situated in trenches  978 , over trench metal liners  920 . Dielectric liner  934  lines trench metal liners  920 . In various implementations, dielectric liner  934  is a high-k dielectric, such as tantalum pentoxide, aluminum oxide, hafnium oxide, zirconium oxide, zirconium aluminum silicate, hafnium silicate, hafnium aluminum silicate or another dielectric with a relatively high dielectric constant. In one implementation, the thickness of dielectric liner  934  can range from approximately two hundred angstroms to approximately six hundred angstroms (200 Å-600 Å). 
     Trench metal plugs  922  are situated in trenches  978  over dielectric liner  934 . In various implementations, trench metal plugs  922  can comprise W, Al, or Cu. As described below, trench metal plugs  922  are ohmically separated from, but capacitively coupled to, trench metal liners  920 . 
     Outside trenches  978 , trench metal liner extension  920   x , dielectric liner extension  934   x , and trench metal plug extension  922   x  are sequentially stacked over RF terminal dielectric segment  916  in overhang regions  980  and  982 . Overhang regions  980  overhang away from active segment  912  of PCM  910 , while overhang regions  982  overhang towards active segment  912  of PCM  910 . Overhang regions  980  and  982  are optional in that the inventive concepts of the present application may be implemented without either or both, and trench metal plugs  922  would still capacitively couple to trench metal liners  920 . However, capacitive coupling between trench metal plugs  922  and trench metal liners  920  is strengthened when overhang regions  980  and  982  are used. 
     Although trench metal plugs  922  are illustrated as integrally formed with trench metal plug extensions  922   x , in one implementation they may be different formations. For example, trench metal plugs  922  may be situated in trenches  978  and a first interconnect metal (i.e., M 1 ) may be subsequently formed over trench metal plugs  922  to form metal plug extensions  922   x . In this example, trench metal plugs  922  can comprise W, and trench metal plug extensions  922   x  can comprise Al or Cu. 
     Trench metal plugs  922  are ohmically separated from, but capacitively coupled to, trench metal liners  920 . Because RF switch  900  utilizes trench metal liners  920  and trench metal plugs  922 , more interface area is available to capacitively couple, and capacitance values are increased compared to RF switch  300  in  FIG. 3 . Additionally, because RF switch  900  utilizes a thin high-k dielectric liner  934 , the capacitive coupling between trench metal liner  920  and trench metal plug  922  is significantly increased. Moreover, because RF switch  900  utilizes overhang regions, capacitive coupling between trench metal liner  920  and trench metal plug  922  is further increased. 
     In an alternative implementation, only one trench metal plug  922  is capacitively coupled to a respective trench metal liner  920 . In this alternative implementation, the other trench metal plug  922  is directly and ohmically connected to its respective trench metal liner  920  without using an intervening dielectric liner  934 . As such, only one RF terminal  918  is capacitively coupled to a respective passive segment  914  of PCM  910 , while the other RF terminal  918  is ohmically connected to its respective passive segment  914  of PCM  910 . 
     Thus, various implementations of the present application achieve reliable RF switches that overcome the deficiencies in the art by providing capacitive contacts to connect with RF terminals of PCM RF switches while preserving or improving RF performance. 
     From the above description it is manifest that various techniques can be used for implementing the concepts described in the present application without departing from the scope of those concepts. For example, a single capacitor can be formed in the RF path near one RF terminal, while the other RF terminal only employs ohmic connections. Moreover, while the concepts have been described with specific reference to certain implementations, a person of ordinary skill in the art would recognize that changes can be made in form and detail without departing from the scope of those concepts. As such, the described implementations are to be considered in all respects as illustrative and not restrictive. It should also be understood that the present application is not limited to the particular implementations described above, but many rearrangements, modifications, and substitutions are possible without departing from the scope of the present disclosure.