Patent Publication Number: US-11640972-B2

Title: Process for fabricating a high-voltage capacitive element, and corresponding integrated circuit

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 16/802,871 filed Feb. 27, 2020, which claims the priority benefit of French Application for Patent No. 1902277, filed on Mar. 6, 2019, the contents of which are hereby incorporated by reference in their entireties to the maximum extent allowable by law. 
    
    
     TECHNICAL FIELD 
     Implementations and embodiments relate to integrated circuits and, in particular, to the fabrication of integrated circuits including high-voltage capacitive elements. 
     BACKGROUND 
     The term “high voltage” is understood to mean, for example, a voltage of the order of 3.5 V to 12 V. 
     There exist conventional capacitive elements formed on one face of a well formed in a semiconductor substrate, typically made of silicon. These conventional capacitive elements include a conductive layer, typically made of polycrystalline silicon, which is insulated from the substrate by a dielectric layer that is thick enough to allow operation at high voltages. 
     One electrode of the capacitive element is formed by the well and the other electrode is formed by the conductive layer. The term MOS (metal-oxide-silicon) capacitive element is then used. 
     The methods for forming such an electrode including a layer of a conductive material on one face of the well typically implement a step of levelling the conductive material, for example using a chemical-mechanical polishing (CMP) process or using a controlled-etch process such as a polysilicon-etchback (poly-etchback, PEB) process. 
     One improvement to such a capacitive element comprises an operation of forming an additional conductive layer, on top of the initial conductive layer. The conductive layers are insulated from one another by another dielectric layer. 
     The additional conductive layer is coupled to the substrate and belongs to the first electrode of the capacitive element, in what is referred to as a “sandwich” structure. 
     This has been made possible thanks to the existence of fabrication processes making provision for a second operation of forming a conductive layer on top of a first, and a required second levelling step. For example, the technologies for fabricating floating-gate transistors typically make provision for these steps. 
     That being said, it is difficult, in the processes for fabricating integrated circuits, to reiterate operations of forming conductive layers and the corresponding levelling steps, which are generally carried out over the entire surface of the substrate (or wafer). 
     Specifically, in the processes for fabricating integrated circuits, it is desirable to combine the fabrication steps for various elements in various parts of the integrated circuit, both for cost reasons and for reasons of architectural compatibility. 
     Meanwhile, it is desirable to improve the density of the capacitive elements, such as to decrease the footprint of the embodiments of capacitive elements. 
     It is also desirable to improve the performance of the capacitive elements, in particular to limit the parasitic effects introduced by the capacitive elements into neighboring elements, and to improve the linearity of the capacitive values of the capacitive elements. 
     SUMMARY 
     According to one aspect, an integrated circuit includes a semiconductor substrate having a front face, a first dielectric region extending into the substrate from the front face, and at least one capacitive element. 
     The capacitive element according to this aspect includes, on a surface of the dielectric region at the front face, a stack of a first conductive region, of a second conductive region and of a third conductive region, the second conductive region being electrically insulated from the first conductive region by a second dielectric region and electrically insulated from the third conductive region by a third dielectric region. 
     In other words, the capacitive element includes a stack of three conductive layers and is produced on one surface of a dielectric volume, and does not include any electrical connection with the semiconductor substrate. 
     Consequently, since it is isolated from the substrate, the capacitive element does not introduce any parasitic effect transmitted to neighboring components via the substrate and, since it does not use the semiconductor substrate electrically, the linearity and the density of the capacitive element are improved. 
     According to one embodiment, the second dielectric region and the third dielectric region are configured to withstand voltages of higher than 3.5 volts. 
     Advantageously, the second dielectric region and the third dielectric region are configured to withstand voltages of higher than 10 volts. 
     The second dielectric region may include a high-voltage oxide having a thickness of between 10 nm and 20 nm, for example 15 nm to within 10%, or includes a tunnel oxide having a thickness of between 5 nm and 15 nm, for example 10 nm to within 10%. 
     The third dielectric region may include a stack of a silicon oxide layer, of a silicon nitride layer and of a silicon oxide layer, the stack having a thickness of between 10 nm and 17 nm, for example 13 nm to within 10% or 15 nm to within 10%. 
     According to one embodiment, the capacitive element comprises a first electrode comprising the first conductive region and the third conductive region, and a second electrode comprising the second conductive region. 
     According to one embodiment, the first conductive region, the second conductive region and the third conductive region each comprise polycrystalline silicon. 
     The integrated circuit may further include an analog or radiofrequency-receiver device incorporating said at least one capacitive element, and a nonvolatile memory device incorporating at least one high-voltage transistor and/or at least one memory cell comprising a floating-gate transistor and/or a buried vertical-gate transistor. 
     Advantageously, and according to the respective “and/or” alternatives of this exemplary integrated circuit, said high-voltage transistor comprises a gate formed in the material of the second conductive region, said buried vertical-gate transistor comprises a vertical gate formed in the material of the first conductive region, said floating-gate transistor comprises a floating gate formed in the material of the second conductive region and/or comprises a control gate formed in the material of the third conductive region. 
     Advantageously, and likewise according to the respective “and/or” alternatives of this exemplary integrated circuit, said high-voltage transistor comprises a high-voltage gate oxide formed in the material of the second dielectric region or else said floating-gate transistor comprises a tunnel oxide formed in the material of the second dielectric region, and/or said floating-gate transistor comprises a control-gate dielectric region formed in the material of the third dielectric region. 
     According to another aspect, a process for fabricating an integrated circuit on a semiconductor substrate having a front face, comprising an operation of fabricating at least one capacitive element comprises: forming a first dielectric region comprising an operation of etching a trench in the substrate from the front face and an operation of filling the trench with a dielectric material; forming a first conductive region on a surface of the dielectric region at the front face; forming a second dielectric region on the first conductive region; forming a second conductive region on the first dielectric region; forming a third dielectric region on the first conductive region; and forming a third conductive region on the second dielectric region. 
     According to one implementation: forming the first conductive region takes place together with an operation of forming a conductive gate region of a buried vertical-gate transistor; forming the second dielectric region takes place together with an operation of forming a high-voltage gate oxide of a high-voltage transistor or with an operation of forming a tunnel oxide of a floating-gate transistor; forming the second conductive region takes place together with an operation of forming a conductive floating-gate region of the floating-gate transistor; forming the third dielectric region takes place together with an operation of forming a control-gate dielectric region of the floating-gate transistor; and forming the third conductive region takes place together with an operation of forming a control-gate conductive region of the floating-gate transistor. 
     This implementation has, in particular, the advantage of using fabrication steps that are already envisaged for fabricating a buried vertical-gate transistor and a floating-gate transistor, for example for producing a memory cell of a nonvolatile memory. Thus, costs specific to producing the capacitive element are minimal. 
     According to one implementation, forming the second dielectric region includes forming a high-voltage oxide having a thickness of between 10 nm and 20 nm, for example 15 nm to within 10%, or includes forming a tunnel oxide having a thickness of between 5 nm and 15 nm, for example 10 nm to within 10%. 
     According to one implementation, forming the third dielectric region includes an operation of forming a stack of a silicon oxide layer, of a silicon nitride layer and of a silicon oxide layer, the stack having a thickness of between 10 nm and 17 nm, for example 13 nm to within 10% or 15 nm to within 10%. 
     According to one implementation, the process further comprises forming a first electrode of a capacitive element comprising a coupling with the first conductive region and the third conductive region, and forming a second electrode of the capacitive element comprising a coupling with the second conductive region. 
     According to one implementation, forming the first conductive region, forming the second conductive region and forming the third conductive region each comprise forming polycrystalline silicon. 
     According to one implementation, fabricating said at least one capacitive element is incorporated within an operation of fabricating an analog or radiofrequency-receiver device of the integrated circuit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Other advantages and features of the invention will become apparent on examining the detailed description of completely non-limiting implementations and embodiments with reference to the appended figures, in which: 
         FIGS.  1 - 9    illustrates steps of a process for one exemplary implementation of producing an integrated circuit capacitive element. 
     
    
    
     DETAILED DESCRIPTION 
       FIGS.  1 - 9    show the results of steps of one exemplary process for fabricating a capacitive element, implemented successively in the order in which the figures are numbered. The figures illustrate a section through a semiconductor substrate SUB, from the side of a front face FA of the substrate SUB. The front face FA is the face located at the FEOL (front end of line) of the integrated circuit during fabrication. 
     Two parts of the integrated circuit are schematically shown: one part PCHV corresponds to a part in which an advantageous capacitive element CHV is produced ( FIG.  9   ). Another part PMEM corresponds to a part that is ultimately intended to include another device of the integrated circuit. 
     In the exemplary process described with reference to  FIGS.  1  to  9   , the fabrication steps that are intended to form the device in the part PMEM are advantageously used in parallel to fabricate the capacitive element CHV. To this end, the device of the integrated circuit in the part PMEM may advantageously include buried vertical-gate transistors TA and floating-gate transistors FGT, as for example in certain known embodiments of memory cells of a nonvolatile memory. 
       FIG.  1    shows the result of a step ST 01  comprising an operation of forming a dielectric region STI. 
     The dielectric region STI is advantageously a shallow trench isolation, and extends into the substrate SUB from the front face FA. Unlike a dielectric region having a layered structure, the dielectric region STI occupies a volume in the substrate. 
     The operation of forming the dielectric region STI comprises an operation of etching a trench in the substrate SUB from the front face FA and an operation of filling the trench with a dielectric material. 
     Other dielectric regions STI (not shown) are formed elsewhere in the integrated circuit, typically for the purpose of lateral electrical insulation between neighboring components. 
     The step ST 01  further comprises an operation of forming a hardmask HM layer, for example of silicon nitride, on the front face FA of the substrate SUB. 
       FIG.  2    shows the result of a step ST 2  comprising an operation of etching part of the hardmask HM located above the dielectric layer STI, at the site at which the capacitive element CHV will be formed. The etching operation comprises an operation of forming a photolithography mask MSK 2  including an aperture OUV defining the zone in which the hardmask HM layer is etched. Once etched, the hardmask HM includes this same aperture OUV thus transferred, which exposes the corresponding part of the dielectric region STI, at the front face FA. 
       FIG.  3    shows the result of a step ST 3  comprising an operation of etching a plurality of, at least one, trench(es) TRTA in the part PMEM of the substrate SUB. 
     The trenches TRTA extend into the substrate perpendicularly to the front face FA. The position of the trenches TRTA may be defined by a photolithography step including a mask (not shown). 
     Step ST 3  also includes an operation of vertically anisotropically implanting a counter-implant layer CTI, the conductivity type of which (for example, n-type conductivity) is opposite that of the substrate SUB (for example, p-type conductivity) in the semiconductor parts that are not covered by the hardmask HM (such as, in this example, into the bottom of the trenches TRTA). 
     The trenches TRTA are, for example, intended to accommodate buried vertical-gate transistors, and the implantation of the counter-implant layer CTI is intended to form the source regions of said buried vertical-gate transistors. 
       FIG.  4    shows a result of a step ST 4  comprising an operation of forming a first conductive region P 0  on the dielectric region STI. 
     The operation of forming the first conductive region P 0  on the dielectric region STI comprises an operation of filling the aperture OUV ( FIG.  2   ) with a conductive material P 0 , which extends out of the aperture over the hardmask HM. 
     This overrun may be obtained by overfilling with the conductive material P 0 . 
     For example, the conductive material P 0  may include polycrystalline silicon. 
     Advantageously, the operation of filling the aperture OUV is carried out together with an operation of filling the trenches TRTA envisaged in a step of forming the vertical gates of the buried transistors in the part PMEM of the integrated circuit. An insulating gate-dielectric envelope, not explicitly illustrated, has been formed beforehand on the flanks and at the bottom of said trenches TRTA. 
       FIG.  5    shows the result of a step ST 5  including an operation of levelling the excess conductive layer P 0  deposited in step ST 4 . 
     The levelling operation comprises an operation of removing the excess deposit of conductive material P 0  until the surface of the hardmask HM layer, which may act as a stop layer here, is reached. 
     The levelling operation may be implemented by means of a chemical-mechanical polishing (CMP) technique, or possibly by means of an etching technique such as a polysilicon-etchback (poly-etchback, PEB) technique parametrized over time according to the thickness of the excess. 
       FIG.  6    shows the result of a step ST 6  intended to finalize the formation of the vertical gates P 0 GV of the buried transistors in the part PMEM. Step ST 6  comprises an operation of removing, by dry-etching, the part of conductive material P 0  remaining at the front face FA, through the thickness of the hardmask HM. 
     In this step ST 6 , the first conductive region P 0  of the part PCHV is protected by a mask MSK 6  which is impermeable to the dry etch. 
     The dry etch is, for example, typically carried out by means of a controlled-etch process such as a polysilicon-etchback (poly-etchback, PEB) process. The PEB process typically comprises an operation of exposing the integrated circuit, during fabrication, to an ion beam, the energy of which allows a given material (here the conductive material P 0 ) to be removed, for a duration chosen according to the thickness of this material to be etched. 
       FIG.  7    shows the result of a step ST 7  comprising the operation of removing the hardmask HM layer after an operation of removing the mask MSK 6  ( FIG.  6   ). 
     The operation of removing the hardmask HM layer may comprise an operation of selectively wet-etching the material forming the hardmask HM, typically silicon nitride. 
       FIG.  8    illustrates a result of a step ST 8  comprising an operation of forming a first dielectric layer (a second dielectric region) D 1  on the first conductive region P 0  followed by an operation of forming a second conductive region P 1  on the first dielectric layer D 1 . 
     According to one advantageous example, the operation of forming the first dielectric layer D 1  is carried out together with an operation of forming a floating-gate or high-voltage-oxide dielectric layer HV/TN of a floating-gate transistor FGT ( FIG.  9   ) or of a high-voltage transistor, respectively, formed in the part PMEM of the substrate SUB. 
     Thus, according to a first alternative, the first dielectric layer D 1  comprises a high-voltage gate oxide HV of the high-voltage transistor. 
     According to a second alternative, the first dielectric layer D 1  comprises a tunnel oxide TN of the floating-gate transistor FGT. 
     Furthermore, the operation of forming the second conductive region P 1  may advantageously be carried out together with an operation of forming a floating-gate conductive region FG of the floating-gate transistor or a gate region of the high-voltage transistor, respectively. 
     The second conductive region P 1  (and the floating-gate conductive region FG) may comprise polycrystalline silicon and, for example, may be formed by uniformly depositing a layer of polycrystalline silicon, typically by epitaxy or chemical vapor deposition, followed by an etch delimited by a photolithography mask. The etch may use the first dielectric layer D 1  as a stop layer. 
     The floating-gate dielectric layer TN comprises, for example, a silicon oxide resting on the front face FA of the semiconductor substrate SUB. The floating-gate conductive region rests on the floating-gate dielectric layer HV/TN. The floating gate FG allows charge to be stored in a nonvolatile manner in order to store binary data. 
     According to the first alternative mentioned above, the high-voltage oxide HV may be configured to withstand operating voltages of around 11 V so as to be able to route the high voltage to the memory point with a view to injecting positive and negative charges, allowing a binary datum to be written in the floating-gate transistor. 
     The high-voltage oxide may have a thickness of between 10 nm and 20 nm, for example 15 nm to within 10%. 
     According to the second alternative mentioned above, the tunnel oxide TN may be configured to withstand injections or extractions of charge into/from the floating gate by Fowler-Nordheim effect, allowing a binary datum to be written in the floating-gate transistor. It may also be configured to withstand injections of hot carriers, positive and negative charges, allowing a binary datum to be written in the floating-gate transistor. 
     The tunnel oxide may have a thickness of between 5 nm and 15 nm, for example 10 nm to within 10%. 
     In both cases, the first dielectric layer D 1  is configured to withstand voltages of higher than 10 volts, or even 12 volts. 
     Consequently, the first dielectric layer D 1  is capable of withstanding voltages of higher than 3.5 volts, so as to allow the use of the capacitive element CHV ( FIG.  9   ) at high voltages of between 3.5 volts and 10 or even 12 volts. 
     In these advantageous examples, the first dielectric layer D 1  naturally has the same structural characteristics as the high-voltage oxide layer or the floating-gate dielectric layer HV/TN according to the respective chosen alternative. 
     As such, the first dielectric layer D 1  may be produced according to the techniques described above with reference to an operation of fabricating a floating-gate transistor, and have the same structural characteristics, without necessarily being produced together with the floating-gate transistor. 
       FIG.  9    shows one exemplary embodiment of a capacitive element CHV, resulting from a last step ST 9  of the process described above with reference to  FIGS.  1  to  8   . 
     Step ST 9  comprises an operation of forming a second dielectric layer (a third dielectric region) D 2  on the first conductive region P 1  and an operation of forming a third conductive region P 2  on the second dielectric layer D 2 . 
     These formation operations may themselves also be carried out together with operations of forming elements of the floating-gate transistor FGT. 
     Specifically, according to one advantageous example, the operation of forming the second dielectric layer D 2  is carried out together with an operation of forming a control-gate dielectric region ONO of the floating-gate transistor FGT, and the operation of forming the third conductive region P 2  is carried out together with an operation of forming a control-gate conductive region CG of the floating-gate transistor FGT. 
     Thus, the second dielectric layer D 2  may include a stack ONO of a silicon oxide layer, of a silicon nitride layer and of a silicon oxide layer. 
     The stack ONO may have a thickness of between 10 nm and 17 nm, for example 13 nm to within 10% or 15 nm to within 10%. 
     The second dielectric region DI 2  is thus configured to withstand voltages of higher than 10 volts, or even 12 volts. 
     Consequently, the second dielectric layer D 2  is capable of withstanding voltages of higher than 3.5 volts, so as to allow the use of the capacitive element at high voltages of between 3.5 volts and 10 or even 12 volts. 
     The third conductive region P 2  may also include polycrystalline silicon. 
     The third conductive region P 2  may also be formed by a uniform deposition technique followed by an etch delimited by a photolithography mask. 
     Lastly, a first electrode E 1  of the capacitive element CHV is formed and comprises the first conductive region P 0  and the third conductive region P 2 . 
     Meanwhile, the second electrode E 2  of the capacitive element comprises the second electrical region P 1 , which is located between the two conductive regions of the first electrode E 1 , and is electrically insulated therefrom by the first dielectric layer D 1  and by the second dielectric layer D 2 . 
     The operation of forming the second electrode E 2  may simply comprise an operation of forming a film of metal silicide on a portion of the second conductive region P 1 , allowing a contact exhibiting little resistance. 
     The operation of forming the first electrode E 1  may simply comprise an operation of forming a film of metal silicide on portions of the first conductive region P 0  and of the third conductive region P 2 , and an operation of electrically coupling these regions via contacts exhibiting little resistance formed from the films of metal silicide. 
     What has been described is one exemplary fabrication process having the advantage of using fabrication steps that are already envisaged for fabricating a buried vertical-gate transistor TA and a floating-gate transistor FGT for producing a memory cell of a nonvolatile memory PMEM. The costs specific to producing the capacitive element CHV according to this example are minimal. 
     However, the process is not limited to this example but encompasses all variants thereof, for example it is possible to envisage implementing the process described such that it is dedicated to the production of capacitive element, and other materials and other dimensions may be envisaged within the context of the proposed structure. 
     Moreover, since the capacitive element is produced on one surface of the volume of a dielectric region, it is isolated from the substrate and does not introduce any parasitic effect transmitted to neighboring components via the substrate and, since it does not include any semiconductor region in its electrodes, the linearity of the capacitive value and the density of the capacitive element are improved. 
     Consequently, the capacitive element may advantageously incorporate devices in which the capacitive elements perform an essential function and/or are numerous to the point that they have a substantial footprint. 
     Analog parts of the integrated circuit and radiofrequency receive chains typically require linear capacitive values within a wide range of voltages. 
     Thus, the integrated circuit CI, for example fabricated according to the examples described above with reference to  FIGS.  1  to  9   , may advantageously further include an analog or radiofrequency-receiver device in the part PCHV. The analog or radiofrequency-receiver device advantageously incorporates said at least one capacitive element CHV. 
     Of course, the integrated circuit CI may include a nonvolatile memory device PMEM incorporating at least one memory cell comprising a floating-gate transistor FGT and a buried vertical-gate transistor TA. 
     For example, said analog parts of the integrated circuit and radiofrequency receive chains may incorporate capacitive elements according to the invention in a decoupling capacitor, in compensating circuits or else in radiofrequency filter devices.