Patent Publication Number: US-2023135765-A1

Title: Rf mos varactor

Description:
BACKGROUND 
     A metal-oxide-semiconductor (MOS) varactor, also known as a variable capacitor, can be fabricated using the structure analogous to that of a MOS field effect transistor (FET), wherein what would otherwise be source and drain regions operate as bias terminals to bias a well region with respect to a gate electrode. When the voltage applied between the terminals and the gate is varied, the charge layer beneath the gate oxide will vary either by charge carrier depletion or accumulation. This creates a voltage-dependent capacitance between the heavily doped polysilicon gate electrode and the bias terminals that can be tuned by varying the DC voltage between the gate electrode and the bias terminals. 
     SUMMARY 
     Disclosed examples provide a method of fabricating a MOS varactor in which both sides of a polysilicon gate structure terminate over shallow trench isolation (STI) structures, providing physical separation between a channel region and contacts to doped regions that provide a conductive connection to the channel region. A buried layer may be provided below the well structure to change the resistance between the channel region and the doped regions. The disclosed method may use up to three mask layers not previously utilized in MOS varactor fabrication. One implementation will be discussed using a BiCMOS technology, which generally uses these mask layers in other regions of the chip, so these “new” mask layers may be incorporated into many new designs without additional costs. The three masks include a composite mask, a well mask, and a buried layer (BL) mask. The composite mask is used to add the STI structures underneath the ends of gate structures. The well mask and the BL mask may block a respective implant over portions of a planned channel region and can be used to adjust the dopant levels in the channel region, e.g., to make depletion in the channel region more three dimensional and to optimize performance. The regions blocked respectively by the well mask and the BL mask can be adjusted in size for different performance optimization, depending on the fabrication technology and the particular application, and provides designers with new options. 
     In one aspect, an example of an integrated circuit is disclosed. The integrated circuit includes a substrate having a first conductivity type, the substrate having an upper surface; a well formed at the upper surface, the well having a second conductivity type that is opposite the first conductivity type, the well having a first dopant concentration; a first shallow trench isolation (STI) structure formed at the upper surface and within the well; a second STI structure formed at the upper surface and within the well; a first polysilicon gate structure formed over the upper surface between the first STI structure and the second STI structure, a first portion of the first polysilicon gate structure extending over a first side of the first STI structure and a second portion of the first polysilicon gate structure extending over a first side of the second STI structure; a first doped region formed within the well at the upper surface and on a second side of the first STI structure; and a second doped region formed within the well at the upper surface and on a second side of the second STI structure, the first and second doped regions each having the second conductivity type and a second dopant concentration that is greater than the first dopant concentration. 
     In another aspect, an example of a method of fabricating an integrated circuit is disclosed. The method includes forming STI structures at a first surface of a substrate, the substrate having a first conductivity type, the STI structures including a first STI structure and a second STI structure; forming a well at the first surface of the substrate, the well having a well surface and surrounding the STI structures, the well having a second, opposite conductivity type and a first dopant concentration; forming a first polysilicon gate structure over the well surface, a first portion of the first polysilicon gate structure extending over a first side of the first STI structure and a second portion of the polysilicon gate structure extending over a first side of the second STI structure; and forming bias regions having the second conductivity type located at the first surface and within the well, the bias regions having a second dopant concentration that is greater than the first dopant concentration, the bias regions including a first bias region and a second bias region, the first bias region on a second, opposite side of the first STI structure and the second bias region on a second, opposite side of the second STI structure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Examples of the present disclosure are illustrated by way of example, and not by way of limitation, in the figures of the accompanying drawings in which like references indicate similar elements. It should be noted that different references to “an” or “one” example in this disclosure are not necessarily to the same example, and such references may mean at least one. Further, when a particular feature, structure, or characteristic is described in connection with an example, it is submitted that it is within the knowledge of one skilled in the art to effect such feature, structure, or characteristic in connection with other examples whether or not explicitly described. As used herein, the term “couple” or “couples” is intended to mean either an indirect or direct electrical connection unless qualified as in “communicably coupled” which may include wireless connections. Thus, if a first device couples to a second device, that connection may be through a direct electrical connection, or through an indirect electrical connection via other devices and connections. 
       The accompanying drawings are incorporated into and form a part of the specification to illustrate one or more example of the present disclosure. Various advantages and features of the disclosure will be understood from the following Detailed Description taken in connection with the appended claims and with reference to the attached drawing figures in which: 
         FIG.  1    depicts a number of masks that may be used for the disclosed MOS varactor according to an implementation of the disclosure; 
         FIGS.  1 A- 1 K  depict cross-sections of various stages in the fabrication of a varactor using a BiCMOS process flow according to an implementation of the disclosure; 
         FIG.  2    depicts a cross-section of a varactor fabricated using a CMOS process flow according to an implementation of the disclosure; 
         FIG.  2 A  depicts an overhead view of a MOS varactor according to an implementation of the disclosure; 
         FIG.  3 A  shows a graph of a TCAD simulation of the Q versus the gate voltage on the MOS varactor of  FIG.  1 I  for gates of various lengths according to an implementation of the disclosure; 
         FIG.  3 B  depicts a graph of a TCAD simulation of the tuning ratio versus the gate length for a MOS varactor that contains a blanket NBL and an NWell that was implanted using a well mask; 
         FIG.  3 C  shows the dopant concentration below the polysilicon gate structure of a MOS varactor that contains a blanket NBL and an NWell that was implanted using a well mask according to another implementation of the disclosure; 
         FIG.  3 D  shows the dopant concentration below the polysilicon gate structure of a MOS varactor containing both an NBL implanted using a BL mask and an NWell implanted using a well mask according to another implementation of the disclosure; 
         FIG.  4    depicts a method of fabricating a MOS varactor according to an implementation of the disclosure; 
         FIG.  5 A  depicts a perspective view of a baseline MOS varactor; 
         FIG.  5 B  depicts a cross-sectional view of a baseline MOS varactor containing multiple gate fingers; 
         FIG.  5 C  depicts an overhead view of a baseline MOS varactor; and 
         FIG.  6    depicts a circuit modelling the parasitic elements affecting a MOS varactor. 
     
    
    
     DETAILED DESCRIPTION 
     Specific examples will now be described in detail with reference to the accompanying figures. In the following detailed description of examples, numerous specific details are set forth in order to provide a more thorough understanding of the disclosure. However, it will be apparent to one of ordinary skill in the art that the disclosure may be practiced without these specific details. In other instances, well-known features have not been described in detail to avoid unnecessarily complicating the description. 
       FIGS.  5 A and  5 B  depict views of two prior art N-type MOS (NMOS) varactors.  FIG.  5 A  depicts a perspective view of a baseline MOS varactor  500 A that is formed similar to the design of an NMOS transistor. In this implementation, the baseline MOS varactor  500 A includes a P-type substrate  502 , an N-type well  504  (also referred to as an NWell), and two N-type bias terminals  506  in the N-type well  504 . The bias terminals  506  are formed analogous to forming source/drain regions of a MOS transistor, e.g. by heavily doped NSD implants. In the case of the varactor, these regions act as terminals to bias the N-type well  504  since they are the same conductivity type. A gate oxide  508  is formed over the channel region  512  and a polysilicon gate structure  510  is formed over the gate oxide  508 . Each of the bias terminals  506  and the polysilicon gate structure  510  form elongated “fingers” that extend in parallel and can be replicated multiple times as part of the baseline MOS varactor  500 A. Although not explicitly shown, the bias terminals are tied together, so that when a DC voltage is applied between the common bias terminal and the gate terminal, the charge in the channel region  512  varies and changes a capacitance between the gate electrode and the N-type well  504  that can be tuned by varying the DC voltage. 
       FIG.  5 B  depicts a cross-sectional view of a prior art MOS varactor  500 B in which multiple fingers are implemented and extend in parallel when embodied in silicon. The prior art MOS varactor  500 B also includes P-type substrate  522 , an N-type well  524 , and five N-type bias terminals  526  in the N-type well  504 . Four polysilicon gate structures  530  are each formed over a respective gate oxide  528  and include sidewall spacers  531 . Two deep trenches  532 , each topped by an STI structure  534 , isolate the N-type well  524  from other regions on the chip and reduce the parasitic capacitance with adjacent regions. A substrate contact  536  can be seen outside of the deep trench  532 . The use of the fingers to form the prior art MOS varactor  500 B is especially important when designing a varactor for use in radio frequency (RF) to increase the efficiency. If the plates become too large, resistive elements can particularly affect high frequency applications such as RF. 
       FIG.  5 C  is an overhead view of a prior art MOS varactor  500 C such as the prior art MOS varactor  500 B and is provided to illustrate where the contacts are typically placed when the device in formed in silicon. An explanation for the placement is discussed with reference to  FIG.  6   . A semiconductor substrate  542  includes an N-type well  544  and bias terminals  546 . A portion of the N-type well  544  forms the channel regions  548 , which are outlined by dotted lines and lie under respective polysilicon gates  550  and adjacent the respective bias terminals  546 . A series of gate contacts  554  can be placed across the length of the polysilicon gate  550 , e.g., at each end, while the bias terminal contacts  556  are placed along the width of the bias terminals  546 . The gate contacts  554  are generally not placed over the channel region  548  to avoid causing gate oxide reliability problems; the gate contacts  554  may be placed over STI lying just outside of the channel regions  548 . 
       FIG.  6    graphically depicts the capacitances and resistances in a varactor lumped-element model of a varactor, such as the prior art MOS varactor  500 A ( FIG.  5 A ) or the prior art MOS varactor  500 B ( FIG.  5 B ), as a circuit having an RF signal input to an upper plate  601  of a capacitor  600  and an RF signal output from a lower plate  603  of the capacitor  600 . The upper plate  601  may be formed by the gate, while the lower plate  603  may be formed by the NWell. Each of the other symbols shown represents a parasitic resistance or parasitic capacitance; collectively these parasitic elements prevent the varactor from acting as an ideal capacitor. Resistance in the polysilicon gate is represented by resistor R POLY    602  and resistance in the NWell and bias terminals is represented by resistor R NB+NW    604 . The gate, in addition to the bias terminals and the N-type well that form the lower plate, experience both parasitic capacitance with the substrate and the contact to the substrate, represented by capacitors C DT , and resistance, represented by resistors R SUB . 
     A good varactor is expected to have a low on-resistance, which comes both from the polysilicon gate, which forms the upper plate, and from the lower plate. The polysilicon gate structure typically has a layer of silicide, giving the overall gate structure a sheet resistance of about 8 ohms per square, while NWell sheet resistance is typically much greater, e.g., 500-1000 ohms per square. To compensate for this disparity in resistance, a typical varactor layout contacts the top plate from the shorter end of the polysilicon gate structure, while the contacts for the NWell are placed along the longer structure of the bias terminals  546 . A good varactor will also have a high tuning ratio and a high Q factor, both of which are affected by the parasitic capacitances that exist, e.g., between the polysilicon gate and the heavily doped N+ contacts for the bias terminals that typically extend under the gate. 
     In the prior art, all of the resistances and capacitances shown in  FIG.  6    occur near the surface of the varactor. Although there are parameters for the arrangement and the doping of the varactor that can be adjusted to achieve a desired tuning ratio and/or Q factor, there is a large parasitic fringing capacitance near the edge of each gate that is not very susceptible to adjustment because of the heavily doped bias terminal that is partially under the gate. This parasitic capacitance limits the ability to get a favorable capacitance on/off (C on /C off ) tuning ratio for the varactor. If the length of the gate structure is too short, the parasitic capacitance limits the ability to tune the device; if the length of the gate structure is too large, the desired frequency performance is limited, especially in the radio frequency range. 
     Some such deficiencies of the prior art varactor are addressed by examples described hereinbelow. The parasitic capacitance between the gate structure and the N+ bias terminals is addressed in the present disclosure by placing the edges of the gate structure over respective dielectric isolation structures. The dielectric structures may be STI structures, although a local oxidation of silicon (LOCOS) structure can also be used. Not only does the addition of STI structures under the edges of the gate structure greatly reduce the parasitic capacitance between the gate and the substrate, but the reduced parasitic capacitance also increases the ability to implement other changes that may improve the tuning ratio and/or the Q of the varactor. 
     The resistance of the lower plate of the equivalent capacitor is also addressed in the present disclosure in optional examples, initially by implanting a buried layer beneath the channel region that is more heavily doped than the channel region. An NBL typically has a resistance of about 20-30 ohms per square, which is much closer to the resistance in the upper plate. Also disclosed are masks that can be used to block portions of the well implant and/or the buried layer implant; these masks may be used to provide options in shaping the doping of the channel region to make the capacitance less of a solely surface effect and more of a three-dimensional effect. 
     A result of these changes is that the polysilicon gate structure may now be made wider while still achieving an improved resistance in the lower plate under the polysilicon gate structure, because instead of 600 ohms per square in the lower plate, the resistance may be 25 ohms per square. The disclosed modifications may also provide the ability to change the ratio of the perimeter to area of the channel region to make the perimeter impact less strong. Capacitance from the perimeter is reduced by placing the STI structures between Well contacts (the bias terminals) and the gate, but because there is a buried layer underneath, the resistance is not significantly increased. Reduction of this capacitance may improve the ratio of C on /C off . 
       FIG.  1    depicts three masks that may be added to the fabrication process for a MOS varactor  100  according to one implementation. In many applications, these three masks are not added to the overall fabrication process, although the use of one or more of the masks in the region of the MOS varactor  100  to provide adjustable parameters is a new feature. The dimensions of each of the respective regions masked by these additional masks may be varied as desired in order to provide electrical parameters—e.g., resistance and/or capacitance—that are best suited for a specific application. 
     The MOS varactor  100  may be formed on a substrate  102 , which may include a conventional single crystal substrate or a silicon-on-insulator (SOI) substrate. In one implementation, the substrate  102  includes P-type silicon and the MOS varactor  100  includes N-type doping, although it is within the scope of the disclosure for the substrate to include an N-type substrate and the MOS varactor  100  to include P-type doping. A gate polysilicon mask  104 , which is used to form a polysilicon gate structure  106 , may be part of a baseline process flow, but is shown herein for reference. Within substrate  102 , a deep trench structure  108  surrounds the MOS varactor  100  to provide isolation from the surrounding regions of the substrate  102 . Although seen as two separate structures, the deep trench structure may extend on all sides of the MOS varactor  100 . An optional N-type buried layer (NBL)  110  lies under an N-type well  112 , while N+ sinker regions  114  act as bias terminals to couple the buried layer  110  to an upper surface  116  of the substrate. In implementations in which a buried layer  110  and/or sinker regions  114  are not used, the sinker regions  114  may also be referred to interchangeably as contact regions  114  and doped accordingly. 
     Three STI structures  118 A- 118 C are shown and may be referred to collectively as the STI structures  118 . While the STI structure  118 A that lies over the deep trench structure  108  may be conventional or formed by a future-developed process, the STI structure  118 B and the STI structure  118 C, which are provided under lateral edges of the polysilicon gate structure  106 , have been added to the MOS varactor  100  to improve the parasitic capacitance between the lateral edges of the polysilicon gate structure  106  and the contact regions  114 . Although only the polysilicon gate structure  106  and the two STI structures  118 B and  118 C under the lateral edges of the polysilicon gate structure  106  are shown, there may be tens or hundreds of the polysilicon gate structures  106  in a finger-type configuration with corresponding STI structures  118  under each lateral edge. 
     The three added, adjustable masks include a composite mask  120 , a buried layer (BL) mask  122 , and a well mask  124 . The composite mask is used during etching of the upper surface  116  of the substrate to form STI structures  118 . As noted previously, the STI structures  118  may be conventionally formed, but the added STI structures  118 B/ 118 C are hitherto unknown. By suitable modification of the composite mask  120 , the width of the STI structures  118 B and  118 C can be varied, as can the overlap of the polysilicon gate structure  106  with the STI structures  118 B and  118 C to reduce the parasitic capacitance between the polysilicon gate structure  106  and the contact regions  114 . 
     In addition to the novel use of dielectric isolation regions under the gate edges, a buried layer is not known to have been previously used in a MOS varactor. The thickness and doping of the N-type buried layer  110  can now be used to help determine the electrical characteristics of the MOS varactor. The BL mask  122  can also be used to create a “hole” within the buried layer  110  to provide further flexibility in the electrical characteristics of the MOS varactor  100 . Although some lateral diffusion is expected of the NBL dopant into the region from which the NBL implant is excluded, a lower doping density remains, creating a “hole” in the NBL that ensures that when the channel region depletes the depletion extends deeper in the channel region wherever the hole has been formed. 
     Finally, in the baseline process, the N-type well  112  would extend across the entire MOS varactor  100  within the region enclosed by the deep trench structure  108 . However, the use of the well mask  124  provides the ability to block the NWell implant from regions in which a lower doping profile or a shaped profile are desired, creating a hole in the implanted NWell that can be designed, in common with the design of the NBL, to form a depletion region that best meets the needs of the application. 
       FIGS.  1 A- 1 H  depict different stages in the fabrication of a MOS varactor  100  that is fabricated in BiCMOS technology. The fabrication stages are presented in one non-limiting example using the dimensions and the masks shown in  FIG.  1   . Specifically, the gate polysilicon mask  104  and the polysilicon gate structure  106 , which is etched using the gate polysilicon mask  104 , each have a gate length of 0.94 μm. The composite mask  120  includes both a first composite mask portion  120 A, which is 0.50 μm wide, and second composite mask portions  120 B that are 0.42 μm wide; each of the STI structures  118  are separated from adjacent STI structures  118  by either a distance of 0.42 μm or a distance of 0.50 μm. Given these dimensions, the polysilicon gate structure  106  will overlap the adjacent STI structures by 0.22 μm. The well mask  124  blocks a width of 0.46 μm, so that in the region underlying the polysilicon gate structure  106 , the dopant for the well will only be implanted into the 0.2 μm nearest each of the STI structures  118 . The BL mask  122  extends 0.1 μm beyond the point at which the polysilicon gate structure  106  overlaps the STI structures  118 , and in this implementation blocks 0.52 μm of the region under a channel region  134  ( FIG.  1 A ) from receiving the buried layer implant. 
       FIG.  1 A  depicts a first stage  100 A of the MOS varactor  100  having the substrate  102  over which a pad oxide  126  has been formed. In one implementation, the substrate  102  may be a bulk silicon substrate having P-type conductivity on which N-type doping is used to form the MOS varactor  100 , although in one implementation, the substrate  102  can have N-type conductivity on which P-type doping is used to form the MOS varactor  100 . A silicon-on-insulator (SOI) may also be used for the substrate. 
     The fabrication process continues with forming the buried layer  110  ( FIG.  1   ), which in one implementation is part of a conventional process for fabricating an NPN bipolar transistor, and may be performed simultaneously with forming other devices over the substrate  102 . The BL mask  122  is implemented as a photoresist layer  128 , which has been formed and patterned over the substrate  102 . A BL implant  130  provides dopants to the substrate  102  to form a doped region  132  having a conductivity type that is opposite to the substrate  102 . In the implementation shown as the first stage  100 A of the MOS varactor  100 , the photoresist layer  128  was patterned to block dopants in a planned channel region  134  near the center of the illustrated cross-section, so that the doped region  132  is formed as two separate doped regions. In one implementation, the BL implant  130  uses arsenic, which is implanted at about 5.0×10 15  cm 2 . 
       FIG.  1 B  depicts a second stage  100 B of the MOS varactor  100  after an epitaxial layer  136  has been grown over the doped region  132  and the substrate  102 . In most BiCMOS processes, the thickness and doping of the epitaxial layer  136  are generally optimized for the performance of bipolar junction transistors fabricated on the chip, although in one implementation, the thickness and doping of the epitaxial layer  136  can be optimized for the performance of the MOS varactor. In one implementation, the epitaxial layer  136  has a thickness in the range of about 8300 Å to about 9300 Å. 
     The epitaxial layer  136  has the same conductivity type as the substrate  102 , e.g., a P-type epitaxial layer  136  for a P-type substrate  102 . The doped region  132  diffuses in both vertical and lateral directions, during the epitaxial growth process and/or by a separate diffusion step, to form the buried layer  110  that has an opposite conductivity type from the substrate. It can be seen from the second stage  100 B of the MOS varactor  100  that the two doped regions  132  have diffused until they overlap each other and form a continuous buried layer, although the buried layer  110  does not have the same concentration of dopants across the width of the MOS varactor  100 . In one implementation, the BL mask  122  ( FIG.  1 A ) is not used in the MOS varactor and the buried layer  110  includes a similar doping profile across the width of the buried layer  110 . 
       FIG.  1 C  depicts a third stage  100 C of the in-process MOS varactor  100  in which deep trenches are being formed, which may also be part of a baseline process for fabricating an NPN bipolar transistor. A pad oxide  138  has been formed over the substrate  102  and epitaxial layer  136 , then a hardmask layer  140  is deposited over the pad oxide  138  and a photoresist layer  142  is deposited over the hardmask layer  140 . In one implementation, the hardmask layer  140  includes a silicon nitride layer and a silicon oxide layer (not shown separately). The thickness of the hardmask layer  140  and the photoresist layer  142  depends on the depth to which the trenches  146  are being etched. The photoresist layer  142  is patterned and the pattern is reproduced in the hardmask layer  140 , followed by etching the substrate  102  to an endpoint below the buried layer  110 . In the third stage  100 C of the MOS varactor  100 , etch process  144  is shown near the end of the process of forming trenches  146 . In one implementation, the trenches  146  are etched to a depth that is in the range of about 5 μm to about 9 μm. In one implementation in which the substrate is an SOI substrate, the trenches  146  can stop on the SOI for a fully isolated varactor. In one implementation, a channel stop implant (not explicitly shown) is performed to implant a dopant into the bottom of the trenches  146  to form a heavily doped region (not explicitly shown) at the tip of the trenches  146 . In one implementation, the channel stop implant includes boron at about 3×10 13  cm 2 . When used, the heavily doped region has the same conductivity type as the substrate. 
       FIG.  1 D  depicts a fourth stage  100 D of the in-process MOS varactor  100  once the etch process  144  and the implant process (if used) are complete, the photoresist layer  142  and the hardmask layer  140  are removed, and the trenches  146  are filled to form the deep trench structure  108 . In one implementation, filling the trenches  146  includes growing a thin layer of silicon oxide (not explicitly shown), followed by depositing a polysilicon layer  148  to overfill the trenches  146 . In one implementation, the trenches  146  may instead be filled by growing a thin oxide liner and depositing a layer of an oxide, e.g., tetraethyl orthosilicate (TEOS) to overfill the trenches  146 . A planar surface is then formed using chemical-mechanical polishing (CMP) to remove the excess filling. The deep trench structure  108  can be used to reduce the parasitic capacitance at the perimeter of the MOS varactor  100 . 
       FIG.  1 E  depicts a fifth stage  100 E of the MOS varactor  100  in which shallow trenches  150  are being formed. In the implementation shown, the shallow trench  150 A is formed at the outer perimeter of the MOS varactor  100 , over the deep trench structure  108 ; additional shallow trenches  150  are formed on either side of the planned channel region  134 . A new pad oxide layer  152  is formed over the epitaxial layer  136  and a photoresist layer  154  is formed over the pad oxide layer  152 . A hardmask layer (not explicitly shown) may also be formed between the pad oxide layer  152  and the photoresist layer  154 , as was shown in  FIG.  1 C . The photoresist layer  154  and the hardmask layer, if present, are patterned. An etch process  156  removes portions of epitaxial layer  136  to form the shallow trenches  150 . 
       FIG.  1 F  depicts a sixth stage  100 F of the MOS varactor  100  after the photoresist layer  154  and a hardmask layer, if present, are removed. The shallow trenches  150  have been filled and the surface has been planarized using CMP, completing formation of the STI structures  118 . In one implementation, a deposited silicon dioxide is used to fill shallow trenches  150 . In one implementation, a furnace field oxide is used to fill the shallow trenches  150 . Both implementations use CMP to provide a planarized surface to complete the formation of the STI structures  118 . Although the process is disclosed in terms of STI structures, it is expected that the process would also work with local oxidation of silicon (LOCOS) structures under the lateral edges of the gates if desired. 
     As also shown in the sixth stage  100 F, sinker regions  114  that may serve as the terminals of the varactor  100  are being formed. A pad oxide  158  is again grown on the upper surface  116  of the substrate  102  and a photoresist  160  is deposited and patterned to expose portions of the substrate  102  between pairs of the STI structures  118  that will not have the channel region  134  between the pairs. In the sixth stage  100 F as shown, exposed portions include the region between a first occurrence of the STI structure  118 A and STI structure  118 B and also between the STI structure  118 C and a second occurrence of the STI structure  118 A. A sinker implant process  162  implants a dopant having the second conductivity type into the exposed portions of the substrate  102  to form the sinker regions  114 . In one implementation, the dopant is phosphorus, which is deposited at a dosage of about 3.5×10 13  cm 2  and energy of about 110 keV. 
       FIG.  1 G  depicts a seventh stage  100 G of the MOS varactor  100  during formation of the N-type well  112 , which may be fabricated at the same time as an Well for a PMOS transistor. A new pad oxide  164  has been grown and the well mask  124  ( FIG.  1   ) is implemented as a photoresist  166 , which has been deposited and patterned to block a portion of the channel region  134 , but to expose other portions of the baseline well region; a well implant  168  is in progress. In one implementation, the well implant  168  uses phosphorus, which is implanted at a first dosage of about 1.5×10 13  cm 2  and energy of about 110 keV and at a second dosage of about 3.0×10 12  cm 2  and energy of 110 keV to form a doped region  170 . In the channel region  134 , diffusion of the doped region  170  will form the N-type well  112  adjacent to the surrounding STI structures  118 , while the diffusion in the sinker regions  114  will add to the existing doping level. The portions of the well exposed by the photoresist  166  can be adjusted as desired to further shape doping levels within the N-type well  112 . Masking of portions within the N-type well  112  may be performed either in conjunction with or instead of masking portions of the NBL. 
       FIG.  1 H  depicts an eighth stage  100 H of the MOS varactor  100  after the photoresist  166  has been removed and doped regions  170  have diffused to form the N-type well  112  and to add to the doping in the sinker regions  114 . Additionally, a gate oxide layer  172  has been formed on the upper surface  116  of the substrate and a polysilicon layer  174  has been deposited over the gate oxide layer  172 . The gate polysilicon mask  104  ( FIG.  1   ) has been implemented as a photoresist layer  176 , which has been deposited and patterned to protect the region for the gate, but to expose the remaining regions of the MOS varactor  100 . An etch process  178  is performed to remove the excess portions of the polysilicon layer  174 . 
       FIG.  1 I  depicts a ninth stage  100 I of the MOS varactor  100  after the photoresist  176  and excess portions of the polysilicon layer  174  have been removed, leaving the polysilicon gate structure  106  as shown. The polysilicon gate structure  106  may be fabricated at the same time as the gates for complementary metal-oxide-semiconductor (CMOS) transistors on the wafer. The sidewall spacers that are generally formed on polysilicon gates may be used for the polysilicon gate structure  106 , but are not necessary here because of the overlap of the polysilicon gate structure  106  with the STI structures  118 . It can be noted that many new gate materials and gate dielectrics are being developed. Accordingly, although the present process is disclosed in using a polysilicon gate over a gate oxide layer, other materials, including metal gates—whether existing or future-developed—may be used for the gate and/or the gate dielectric layer while remaining within the disclosure. 
       FIG.  1 J  depicts a tenth stage  100 J of the MOS varactor  100  after a photoresist layer  180  is deposited and patterned and as an N+ source/drain implant  182  is being performed to implant dopants into bias terminals  184 , forming doped regions  186 . In implementations in which no buried layer is used, the N+ source/drain implant  182  may be used to form contacts to the N-type well  112 ; in the present implementation, the N+ source/drain implant will supplement the doping level of the N+ sinker regions  114 . The N+ source/drain implant  182  is part of the doping process for CMOS transistors on the wafer. Although not explicitly shown, a P+ implant that is also part of the doping process for CMOS transistors can be used to form a P+ contact (not explicitly shown) outside of the deep trench structure  108  that enclose the MOS varactor  100  to form a low-resistance ground contact. 
       FIG.  1 K  depicts an eleventh stage  100 K of the MOS varactor  100  after the photoresist  180  has been removed and the doped regions  186  have been diffused to form bias terminal contact regions  188 . An inter-level dielectric  190  has been deposited over the upper surface  116 . Vias  192  have been formed through the inter-level dielectric  190 , with first vias  192 A contacting the sinker regions  114  and second vias  192 B contacting the polysilicon gate structure  106 . Finally, a first metal layer  194  has been deposited and patterned to form first metal layer segments  194 A and second metal layer segments  194 B. The first metal layer segments  194 A are coupled to the sinker regions  114  through the first vias  192 A; likewise the second metal layer segment  194 B is coupled to the polysilicon gate structure  106  through the second vias  192 B. 
     While the above discussion has focused on a specific set of implants that are used in the BiCMOS technology and which may also be used in fabrication of the MOS varactor  100 , other layers that are formed during processing of other devices on the substrate may also be used instead of or in addition to the disclosed layers that form the MOS varactor  100 , a few of which are specifically mentioned here, but which one skilled in the design and fabrication of BiCMOS transistors will understand. In one example, a deep Well process that may be used to fabricate isolated NMOS transistors may also be implemented as part of the fabrication of the disclosed MOS varactor  100 . 
       FIG.  2    depicts a MOS varactor  200  that has been fabricated in a CMOS technology according to an implementation of the disclosure. In one implementation, the MOS varactor  200  is fabricated on a P-type substrate  202  that has an N-type well  204  formed at an upper surface  206  of the substrate. STI structures  208  have been formed across the N-type well  204 , with deep trenches  210  formed at the edge of the N-type well  204 . Three bias terminals  212  have also been formed within the N-type well  204 . Two polysilicon gate structures  216  lie over gate dielectric layer  214  and over portions of N-type well  204 ; the lateral edges of each polysilicon gate structure  216  extend over a respective STI structure  208 . 
     More specifically, a first portion of a first polysilicon gate structure  216 A extends over a first side of a first STI structure  208 A and a second portion of the first polysilicon gate structure  216 A extends over a first side of a second STI structure  208 B. Similarly, a first portion of a second polysilicon gate structure  216 B extends over a first side of a third STI structure  208 C and a second portion of the second polysilicon gate structure  216 B extends over a first side of a fourth STI structure  208 D. A first bias terminal  212 A is formed on a second, opposite side of the first STI structure  208 A, a second bias terminal  212 B is formed between a second, opposite side of the second STI structure  208 B and a second, opposite side of the third STI structure  208 C, and a third bias terminal  212 C is formed on a second, opposite side of the fourth STI structure  208 D. 
     CMOS technology does not generally include a buried layer, although a buried layer may be added as an additional element. Alternatively, as shown in the MOS varactor  200 , an N-type dopant has been implanted below each of the first STI structure  208 A, the second STI structure  208 B, the third STI structure  208 C, and the fourth STI structure  208 D to form respective doped link regions  218 . In one implementation, dopants can be implanted in the doped link regions  218  during the formation of the formation of the STI structure  208 , after the trenches have been etched and prior to deposition of the oxide filler. An inter-level dielectric  220  has been deposited over the upper surface  206  of the substrate  202  and over the polysilicon gate structures  216 . A set of first vias  222 A have been have been formed to contact each of the bias terminals  212  and a set of second vias  222 B have been formed to contact each of the polysilicon gate structures  216 . A first metal layer has been formed and patterned to form first metal segments  224 A and second metal segments  224 B. The first metal segments  224 A are each electrically coupled to a respective bias terminal  212  through respective first vias  222 A and the second metal segments  224 B are each electrically coupled to a respective polysilicon gate structure  216  through respective second vias  222 B. 
       FIG.  2 A  depicts an overhead view of a MOS varactor  200 A according to an implementation of the disclosure. The view is primarily taken at the top of the silicon layer, although the location of the contacts and the gate are shown. A semiconductor substrate  232  includes an N-type well  234  and bias terminals  236 . Channel regions  238  are surrounded on all four sides by STI structures  240 . Differences in the doping within the channel region  238  are not shown in MOS varactor  200 A, but may include the use of a well mask to modify the doping within the channel region  238 . The location of the polysilicon gate structures  242  are shown by dotted lines; the polysilicon gate structures  242  lie over the respective channel regions  238  and extend over portions of the STI structures  240 . The gate contacts  244  are placed within the portions of the polysilicon gate structures  242  that lie over the STI and in this implementation the gate contacts  244  are formed at both of the short sides of the gate (i.e., across the length), but are also formed on one long side of the gate (i.e., along the width). These additional gate contacts  244  may contribute to a reduction of the gate resistance. The bias terminal contacts  246  are placed, as before, along the width of the bias terminals  236 . 
       FIG.  3 A  through  FIG.  3 D  depict graphs and dopant concentration levels for a MOS varactor using the design shown and discussed with regard to  FIG.  1   .  FIG.  3 A  depicts a graph  300 A that plots simulations of the Q of respective MOS varactors against the voltage at the gate (with bias terminals grounded) as the voltage is varied from about −2.5 V to about 2.5 V. Each plot represents a gate having a different length and the series of plots demonstrates how the Q increases as the length of the gate is decreased from 5 μm to 0.5 μm. When the length of the gate is 5 μm, the maximum Q is just over 50; at 4 μm, the maximum Q is about 85; at 3 μm, the maximum Q is about 150; at 2 μm, the maximum Q has increased to about 170; at 1 μm, the maximum Q is about 210, while at a length of 0.5 μm, the Q has reached a maximum of about 270. 
       FIG.  3 B  depicts a graph  300 B that plots a simulation of the tuning ratio of the varactor against the gate length. Curve  305  represents a baseline varactor that does not implement the STI spacers, while curve  307  represents a varactor that uses a blanket NBL and an Well having a hole under the gate region. The implementation using the disclosed features initially has a greater tuning ratio, but reaches a maximum tuning ratio between about 5 μm and about 6 μm and falls. Further refinement of the design changes identified in the present application may be useful in extending the favorable tuning ratio. 
       FIG.  3 C  depicts simulated doping concentrations in a gate region of a varactor  300 C. The varactor  300 C includes a channel region  302  that is separated from the bias terminals  304  by STI structures  306 . Each end of a gate structure  308 , which includes a polysilicon layer with silicide over a thin gate oxide (none of which are separately shown), lies over one of the STI structures  306 . Also shown are the vias  310  to the bias terminals  304  and an inter-level dielectric  312 . The simulation includes a blanket buried layer and a well that was implanted using the well mask  124  ( FIG.  1   ) to reduce the doping level below the gate. The gate region has a reduced doping level, which is most evident near the STI structures  306 , from which the doping concentration quickly drops, but below the channel region, the doping concentrations extend essentially uniformly across the gate region so that depletion will also be relatively even across the channel region  302  of the varactor. 
       FIG.  3 D  depicts simulated doping concentrations in a gate region of a varactor  300 D. The varactor  300 D includes the use of both the well mask  124  ( FIG.  1   ) and the BL mask  122  ( FIG.  1   ), which were used to create a “hole” in the doping concentrations under the gate structure. Instead of having doping concentrations that extend relatively uniformly across the gate region, the channel region  302  of the varactor  300 D has a more complex doping structure that is very lightly doped under the central region of the gate structure  308 , but is somewhat heavier near the STI structures  306 , as well as having lighter doping that extends downward from the channel region  302 . This doping can assist in providing a lower off-state capacitance when there is a large bias present, while providing a low resistance in the pocket. Maintaining a greater doping level near the STI structures  306  may shield any trapped charge present in the STI structures  306  from the device. 
       FIG.  4    depicts a method  400  of fabricating an integrated circuit that includes a MOS varactor. The method  400  includes forming  405  STI structures at a first surface of a substrate. The substrate has a first conductivity type and the STI structures include a first STI structure and a second STI structure. In the implementation of  FIG.  1   , the first STI structure may be the STI structure  118 B and the second STI structure may be the STI structure  118 C, while in the implementation of  FIG.  2   , the first STI structure may be the STI structure  208 A and the second STI structure may be the STI structure  208 B. In one implementation, the first conductivity type is P-type. The STI structures can also include a third STI structure and a fourth STI structure and may include a large number of STI structures. In the implementation of  FIG.  2   , the third STI structure may be the STI structure  208 C and the fourth STI structure may be the STI structure  208 D. 
     The method  400  also includes forming  410  a well at the first surface of the substrate. The well surrounds the STI structures and has a second, opposite conductivity type and a first dopant concentration. When the first conductivity type is P-type, the second conductivity type is N-type. As has been disclosed, a well mask may be used during fabrication of the well. The well mask may block out a region in planned channel regions so that the dopant concentration is not even across the well. 
     The method  400  forms  415  a first polysilicon gate structure over the first surface of the well. A first portion of the first polysilicon gate structure extends over a first side of the first STI structure and a second portion of the first polysilicon gate structure extends over a first side of the second STI structure, so that first and second portions of the first polysilicon gate structure do not contact the underlying silicon, reducing the parasitic capacitance at the edges of the first polysilicon gate structure. Where additional STI structures have been formed, e.g., a third STI structure and a fourth STI structure, a second polysilicon gate structure is formed over the first surface of the well. A first portion of the second polysilicon gate structure extends over a first side of the third STI structure and a second portion of the second polysilicon gate structure extends over a first side of the fourth STI structure. Although the polysilicon gate structures have typically been doped with the same conductivity type as the well and contact regions, having the two sides of the polysilicon gate structure terminate over the STI structures provides the capability to dope the polysilicon gate structure with either an N+ polysilicon or a P+ polysilicon, regardless of the conductivity type of the rest of the MOS varactor. 
     Another element of the method  400  is forming  420  contact regions at the first surface and within the well. The contact regions have a second dopant concentration that is greater than the first dopant concentration and include a first contact region and a second contact region. The first contact region is on a second, opposite side of the first STI structure and the second contact region on a second, opposite side of the second STI structure. 
     Although not required, the method  400  may form  425  a buried layer having the second conductivity type. The buried layer, if present, is located below the well structure and has a third concentration that is greater than the first concentration. Although mentioned last in the method, the buried layer may be formed prior to the formation of other elements of the integrated circuit. When the buried layer is provided, the contact regions may be formed as sinker regions that extend from the first surface to the buried layer. Fabrication of the buried layer may include using a BL mask, which may block out portions of the buried layer implant over regions of planned channel regions in order to shape the doping levels under these channel regions. 
     Applicants have disclosed an integrated circuit, which includes a MOS varactor, and a method of fabricating an integrated circuit that includes a MOS varactor. Aspects of the disclosed devices and methods include using STI isolation to separate the channel region from the bias terminals and placing the polysilicon gate so that opposite sides of the polysilicon gate terminate over respective STI regions to decrease parasitic capacitance. A buried layer can be added to decrease resistance in the substrate, with sinker regions coupling the buried layer to a first surface of the IC. Both the well region and the buried layer may be fabricated using a well mask and a BL mask respectively to block dopants from regions formed in and under the channel region. The resulting doping of the MOS varactor may reduce resistance of the substrate and/or the size of the bias depletion region, which may be used to adjust the tuning ratio and the Q of the MOS varactor. These changes may provide the ability to shrink the MOS varactor cell size at a fixed capacitance and Q target relative to currently known MOS varactors. These capabilities leverage existing layers in BiCMOS process platforms and can be applied to other technologies, e.g., CMOS processing, with appropriate adjustments. The proposed structures provide device and design engineers tools that may be used to better optimize and/or adjust the MOS varactor. The disclosed layouts may be easily modified to balance tuning of the C on /C off  ratio versus Q enhancement. The disclosed method may also provide better scaling of the width of the elements used for RF simulations as opposed to their implementation in silicon, which may reduce current crowding at high frequencies. 
     Although various examples have been shown and described in detail, the claims are not limited to any particular example or example. None of the above Detailed Description should be read as implying that any particular component, element, step, act, or function is essential such that it must be included in the scope of the claims. Reference to an element in the singular is not intended to mean “one and only one” unless explicitly so stated, but rather “one or more.” All structural and functional equivalents to the elements of the above-described examples that are known to those of ordinary skill in the art are expressly incorporated herein by reference and are intended to be encompassed by the present claims. Accordingly, those skilled in the art will recognize that the exemplary examples described herein can be practiced with various modifications and alterations within the spirit and scope of the claims appended below.