Patent Publication Number: US-6222245-B1

Title: High capacity capacitor and corresponding manufacturing process

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a high-capacitance capacitor. 
     More particularly, the present invention relates to a high-capacitance capacitor which can be monolithically integrated on a semiconductor substrate doped with a first type of dopant, the substrate containing a diffusion well doped with a second type of dopant and formed with a first active region therein, and a gate oxide layer being deposited onto said diffusion well which is covered with a first layer of polycrystalline silicon and separated from a second layer of polycrystalline silicon by an interpoly dielectric layer. 
     The present invention further relates to a process for integrating a high-capacitance capacitor on a semiconductor substrate doped with a first type of dopant, which process includes the steps of defining a well doped with a second type of dopant, implanting and then diffusing a first defined region doped to a higher concentration of dopant of the second type, depositing a gate oxide layer over the diffusion well, covering said oxide layer with a first layer of polycrystalline silicon, and depositing a second dielectric layer referred to as the interpoly dielectric. 
     The present invention further relates, to a voltage step-up circuit for non-volatile semiconductor memories, the voltage step-up circuit including an integrated high-capacitance capacitor. The description which follows will make specific reference to this field of application for convenience of illustration. 
     2. Discussion of the Related Art 
     As is well known, there are many applications, involving electronic circuits integrated on a semiconductor, wherein higher voltages than a supply voltage Vcc need to be generated. Voltage step-up or “booster” circuits provide such higher voltages, even internally of the integrated circuit. 
     In particular, electrically programmable non-volatile memories, e.g., Flash EEPROM type memories, require a write voltage far above the conventional 5-volt supply. In programming integrated read-only memories of this kind, the individual storage cells must be applied a programming voltage Vp of relatively high value, close to about 12 volts. This programming voltage is usually generated inside the integrated circuit by means of a voltage-multiplying circuit or “booster”. 
     A voltage-multiplying circuit is known, for example, from European Patent Application No. EP-A-0 540 948, published on May 12, 1993. The voltage multiplier described in that patent application is of a type known as “charge pump” and requires at least two capacitors. A first capacitor functions to draw and transfer electric charges from the input terminal of the multiplier to the output terminal. A second capacitor functions to store up the charges. A set of switches driven by respective signals that do not overlap in time, allow an output voltage Vout to be generated which is approximately twice as high as the supply voltage. 
     Generating this high programming voltage involves, however, the provision of large-size capacitors, normally outside the integrated circuit. The efficiency of a charge pump circuit is, in fact, proportional to the capacitive value of the capacitors used in the charge pump. 
     But increasing the capacitance of a capacitor requires increasing the area for it on the circuit, and hence increasing the physical size of the memory device which contains it. In addition, enlarging the plate area of a capacitor results in increased risk of breaks within the dielectric and makes its integration more difficult. 
     Alternatively, smaller capacitors connected in series or parallel configurations to provide an equivalent capacitor of greater overall capacitance could be used. However, the last-mentioned approach also implies increased area requirements to accommodate the aggregate capacitors, and hence a device of larger size. 
     In general, for all voltage booster circuits used in apparatus to which only a very low (up to 1 volt) DC supply voltage is available—e.g. many telecommunications line apparatus, portable appliances, etc.—there exists a demand for capacitors of high capacitance and reduced physical size. 
     The underlying technical problem of this invention is to provide a high-capacitance capacitor whereby the ratio of electric charge storage capacity to occupied area can be optimized, overcoming the drawbacks with which prior solutions are beset. 
     SUMMARY OF THE INVENTION 
     According to an embodiment of the invention, a high-capacitance capacitor utilizes the interpoly dielectric in a structure integrated on a semiconductor with a process of the MOS type. 
     One embodiment of the invention directed to a capacitor which includes first and second elementary capacitors formed from two layers of polycrystalline silicon deposited onto an active area of the integrated circuit and separated by an interpoly dielectric layer. 
     Another embodiment of the invention is directed to a fabrication process of the MOS type providing for a double deposition of polycrystalline silicon as previously indicated. 
     Another embodiment of the invention is directed to a voltage step-up circuit of the charge pump type which uses a high-capacitance capacitor. 
     The features and advantages of the invention will be apparent from the following detailed description which is given by way of non-limitative example with reference to the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     In the drawings: 
     FIG. 1 shows a vertical cross-section view of a high-capacitance capacitor according to a first embodiment of the invention; 
     FIG. 1A shows a vertical cross-section view of a high-capacitance capacitor according to a modified embodiment of the invention; 
     FIG. 1B is an equivalent electrical diagram of the high-capacitance capacitor of FIG. 1A, in a series layout; 
     FIG. 1C is an equivalent electrical diagram of the high-capacitance capacitor of FIG. 1A, in a parallel layout; and 
     FIG. 2 shows a voltage step-up circuit of the charge pump type using high-capacitance capacitors according to an embodiment of the invention. 
    
    
     DETAILED DESCRIPTION 
     Referring to the drawing figures, generally shown at  1  is a high-capacitance capacitor according to the invention, as formed on an integrated structure  2  basically of a sandwich type. 
     The structure  2  includes a semiconductor substrate  3  doped with a first dopant, e.g. of the P type, wherein a diffusion well  4  has been formed which is doped with a second dopant of the N type. 
     It would be equally possible to use a semiconductor substrate  3  of the N type formed with a diffusion well  4  of the P type. 
     Provided inside the diffusion well  4 , as by implanting and then diffusing a dopant, is a first diffused region  5  which has a concentration of the second dopant type N+ higher than that of the diffusion well  4 . However, a diffused region  5  doped P+ could be provided instead. 
     Similarly, if the diffusion well  4  is P-doped, a diffused region  5  of either the N+ or P+ type may be formed. 
     An oxide layer  6 , referred to as the gate oxide, is deposited over the diffusion well  4 , and is covered with a first layer of polycrystalline silicon, POLY 1 . 
     A dielectric layer  7 , known as the interpoly dielectric, is deposited onto the first layer POLY 1 , and is covered with a second layer of polycrystalline silicon, POLY 2 . 
     In contrast to a first embodiment shown in FIG. 1, FIG. 1A shows a modified embodiment of the integrated structure  2  on which the high-capacitance capacitor  1  of the invention is formed. In this modification, the second layer POLY 2  has first POLY 2 A and second POLY 2 B layer portions which are structurally independent and isolated from each other. 
     The first portion POLY 2 A has a first contact terminal TC 1 . 
     Advantageously, the second portion POLY 2 B is in direct contact with the first layer POLY 1  of polycrystalline silicon and includes a second contact terminal TC 2 . Thus, only the second polysilicon layer will be subjected to the metallization which leads to the formation of the contact terminals for the capacitor  1 . 
     The second contact terminal TC 2  is, therefore, separated from the first contact terminal TC 1 . 
     By contrast, in the first embodiment shown in FIG. 1, the first layer POLY 1  of polycrystalline silicon does not contact the second layer POLY 2  which has the first contact terminal TC 1  only. In this case, the contact metallization is carried out directly on the first layer of polycrystalline silicon, on either the active or the field oxide. 
     This contact metallization can be carried out by contact making techniques which use metals or alloys of the kind of Al, Al/Si, Al/Si/Cu, or by barrier or plug techniques, well known to those skilled in the art. The use of the DPCC process would be of particular advantage here. 
     Lastly, the diffused region  5  has a third contact terminal TC 3 . This region represents an active area of the semiconductor device to be obtained by an increased concentration of N or P dopant. However, for storage or reversal operated capacitors, N-wells or P-wells would also be workable. 
     Thus, the integrated structure  2  looks like a double capacitor including a first C 1  and a second C 2  elementary capacitor. In particular, as shown in FIG. 1A, a first elementary capacitor C 1  is formed from the layers POLY 1  and POLY 2 A which provide the conductive plates, and from the layer  7  of interpoly dielectric providing the dielectric between the plates. Furthermore, as shown in FIG. 1A, a second elementary capacitor C 2  is formed from the first layer POLY 1  and the diffusion well  4  which provide the conductive plates, and from the gate oxide layer  6  providing the dielectric between the plates. 
     The overall capacitance of the resultant capacitor is given by,              C   =         C   1     +     C   2       =           ɛ   1          A   1         T   1       +         ɛ   2          A   2         t   2                   (   1   )                         
     where: 
     ε 1  is the value of the dielectric constant of the interpoly dielectric layer  7  which forms the isolation dielectric of the first elementary capacitor C 1 ; 
     t 1  is the thickness of the interpoly dielectric layer  7 ; 
     A 1  is the overlap area of the first and second layers of polycrystalline silicon which form the conductive plates of the first elementary capacitor C 1 ; 
     ε 2  is the value of the dielectric constant of the gate oxide layer  6  which forms the isolation dielectric of the second elementary capacitor C 2 ; 
     t 2  is the thickness of the gate oxide layer  6 ; and 
     A 2  is the overlap area of the first polycrystalline silicon layer and the diffusion well  4 , which form the conductive plates of the second elementary capacitor C 2 . 
     For simplicity, it is reasonable to assume identical overlap areas and dielectric constants, i.e., that A 1 =A 2 =A and ε 1 =ε 2 =ε, so that equation (1) becomes:              C   =       ɛ                   A        [       1     t   1       +     1     t   2         ]         =     ɛ                   A        [         t   1     +     t   2           t   1          t   2         ]                   (   2   )                         
     And assuming, for simplicity, that t 2 =kt 1 , equation (2) reduces to:              C   =           ɛ                 A       t   1            [       1   +   k     k     ]       =         ɛ                 A       t   1            [     1   +     1   k       ]                 (   3   )                         
     It can be seen at once from equation (3) that the overall capacitance of the capacitor  1  grows as a function of the parameter k, compared to a conventional capacitor with the same overlap area A, thickness t 1  of the dielectric, and dielectric constant ε. 
     With k=1, for example, a capacitance is obtained which is twice that of a capacitor having a single dielectric layer, while occupying the same area; thus, the efficiency of the structure is doubled by a high-capacitance capacitor according to the invention. 
     Since k is positive, the high-capacitance capacitor of this invention will always exhibit, for a given occupied area on the integrated circuit, a higher capacitance value than that of conventional capacitors. It is further apparent, from equation (3), that the capacitance of the inventive capacitor increases as k becomes smaller. 
     Shown in FIG. 1B is the circuit equivalent of the resulting high-capacitance capacitor  1 , in a series layout. The presence of three contact terminals also facilitates a parallel connection of the elementary capacitors C 1  and C 2 . In particular, to obtain the parallel layout of FIG. 1C, it will be sufficient that the contact terminals TC 1  and TC 3  be shorted by placing the first portion POLY 2 A of the second layer of polycrystalline silicon in direct contact with the semiconductor substrate  3 . 
     Furthermore, the integrated structure  2  affords considerable savings in space, for a given capacitive value, compared to conventional capacitors formed by utilizing the diffusion well of N+ type alone. 
     Particularly advantageous, in terms of occupied area, is the parallel layout of the high-capacitance capacitor  1  shown in FIG.  1 C. Actual tests carried out by the Applicant have shown that a saving in semiconductor area of about 35% can be obtained with this capacitor over conventional constructions. 
     The high-capacitance capacitor of this invention can be used to produce a voltage step-up circuit of the charge pump type. 
     A well-known construction for such a voltage step-up circuit is shown in FIG.  2 . The step-up circuit includes an oscillator OSC, which is; typically a square-wave oscillator, connected between a first DC supply voltage reference VS and a second voltage reference, specifically a ground voltage reference GND. 
     The oscillator OSC has an output terminal  01  connected to a first terminal of a capacitor TC. In particular, the capacitor TC serves a charge transfer function. 
     A second terminal of the capacitor TC is connected to the cathode of a diode D 2  having its node connected to the supply voltage reference VS. 
     The cathode of the diode D 2  is also connected to the anode of another diode D 1  having its cathode connected to an output OUT of the voltage step-up circuit and to a terminal of a further charge storage capacitor SC, in turn connected to the ground reference GND. 
     In this circuit, the output (no-load) voltage is approximately twice as high as the supply voltage. 
     Advantageously, the charge transfer TC and charge storage SC capacitors are formed from an integrated structure  2  according to the invention. 
     In particular, a parallel type of connection of the high-capacitance capacitor  1  is utilized, as shown in FIG. 1C, in the voltage step-up circuit of FIG.  2 . 
     In this way, the size of this simple circuit, containing only two high-capacitance capacitors, can be greatly reduced. 
     To summarize, the high-capacitance capacitor of this invention has reduced integration area requirements, and accordingly, allows the overall size of devices containing it to be reduced. 
     In addition, by making three contact terminals available, the change from a series layout to a parallel layout to meet contingent requirements has been facilitated. 
     Finally, in the first embodiment described, the metallization step for making the contact terminals of the high-capacitance capacitor according to this invention only affects the second layer POLY 2  of polycrystalline silicon. 
     Having thus described at least one illustrative embodiment of the invention, various alterations, modifications, and improvements will readily occur to those skilled in the art. Such alterations, modifications, and improvements are intended to be within the spirit and scope of the invention. Accordingly, the foregoing description is by way of example only and is not intended as limiting. The invention is limited only as defined in the following claims and the equivalents thereto.