Manufacturing process for high capacity capacitor

Abstract: The invention relates to a high-capacitance capacitor (1) which is monolithically integratable on a semiconductor substrate (3) doped with a first type (P) of dopant and accomodating a diffusion well (4) which is doped with a second type (N) of dopant and has a first active region (5) formed therein. A layer (6) of gate oxide is deposited over the diffusion well (4) which is covered with a first layer (POLY1) of polycrystalline silicon and separated from a second layer (POLY2) of polycrystalline silicon by an interpoly dielectric layer (7). Advantageously, the high-capacitance capacitor (1) of the invention comprises a first capacitor element (C1) having the first (POLY1) and second (POLY2) layers of polycrystalline silicon as its conductive plates, and the interpoly dielectric layer (7) as the isolation dielectric, and a second capacitor element (C2) having the first layer (POLY1) of polycrystalline silicon and the diffusion well (4) as its conductive plates and the gate oxide layer (6) as the isolation dielectric.

Description: Field of the Invention This invention relates to a process to integrate a high-capacitance capacitor. In particular, the invention relates to a semiconductor MOS type process comprising the step of integrating monolithically a high-capacitance capacitor on a semiconductor substrate doped with a first type of dopant, the process further comprising the steps of: forming a diffusion well doped with a second type of dopant in the semiconductor substrate, forming an active region in said diffusion well, depositing a gate oxide layer onto said diffusion well which is covered with a first layer of polycrystalline silicon and separated from a second layer of polycrystalline silicon by a dielectric layer. The invention relates, specifically but not exclusively, to a high-capacitance capacitor method adapted to be integrated to a current step-up circuit for non-volatile semiconductor memories, and the description which follows will make specific reference to this field of application for convenience of illustration. Background 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 have been employed for the purpose which can provide such higher voltages, even internally of the integrated circuit. In particular, electrically programmable non-volatile memories, e.g. of the Flash EEPROM type, require a write voltage far above the conventional 5-volt supply. In programming integrated read-only memories of this kind, in fact, 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, and the 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 is to increase the area for it on the circuit, and hence 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 could be used, connected in series or parallel configurations to provide an equivalent capacitor of greater overall capacitance. 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. In Japanese Patent Abstract No. 58-23470 is shown a semiconductor device comprising a first capacitor having junction capacitance shaped by a P type diffusion layer and a deeper N type epitaxial layer formed in a semiconductor substrate, a second capacitor, which uses a first layer metallic film and the P type diffusion layer as both electrodes and the first insulating layers as insulating layer, and a third capacitor which employs a second layer metallic film and the first layer metallic film as both electrodes and the second insulating layer as insulating layers. In the U.S. Patent No. 4,914,546 it is disclosed a stacked structure comprising a various polisilicon layers and diffused region utilised in the formation of a serial capacitance. The underlying technical problem of this invention is to provide a high-capacitance capacitor method 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 The solution idea on which this invention stands is to provide a high-capacitance capacitor process by utilizing the interpoly oxide in a structure integrated on a semiconductor with a process of the MOS type. Based on that solution idea, the technical problem is solved by a MOS process to integrate a capacitor as previously indicated and as defined in Claim 1 foll.. The problem is further solved by process as defined in Claims 4 foll including a voltage step-up circuit of the charge pump type which uses such high-capacitance capacitors. The features and advantages of a capacitor according to the invention will be apparent from the following detailed description of an embodiment thereof, given by way of non-limitative example with reference to the accompanying drawings. Brief Description of the Drawings In the drawings: Figure 1 shows an example of a high-capacitance capacitor; Figure 1A is a vertical cross-section view showing schematically an embodiment of the high-capacitance capacitor process of this invention; Figure 1B is an equivalent electrical diagram of the high-capacitance capacitor of Figure 1A, in a series layout; Figure 1C is an equivalent electrical diagram of the high-capacitance capacitor of Figure 1A, in a parallel layout; and Figure 2 shows a voltage step-up circuit of the charge pump type using high-capacitance capacitors formed according to the invention. Detailed Description Referring to the drawing figures, generally shown at 1 is a high-capacitance capacitor, as formed on an integrated structure 2 basically of a sandwich type. The structure 2 comprises 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 higher concentration of the second dopant type N+. However, a diffused region 5 doped P+ could be provided instead. Thus, within a possibly P-doped diffusion well 4, a diffused region 5 of either the N+ or P+ type may be formed. An oxide layer 6, known as the gate oxide, is deposited over the diffusion well 4, and is covered with a first layer of polycrystalline silicon, POLY1. A dielectric layer 7, known as the interpoly oxide, is deposited onto the first layer POLY1, and is covered with a second layer of polycrystalline silicon, POLY2. Shown in Figure 1A is an embodiment of the integrated structure 2 on which the high-capacitance capacitor 1 of this invention is formed. The second layer POLY2 consists of first POLY2A and second POLY2B layer portions which are structurally independent and isolated from each other. The first portion POLY2A has a first contact terminal TC1. Advantageously, the second portion POLY2B is in direct contact with the first layer POLY1 of polycrystalline silicon and includes a second contact terminal TC2. 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 TC2 is, therefore, separated from the first contact terminal TC1. By contrast, in the example of Figure 1, the first layer POLY1 of polycrystalline silicon does not contact the second layer POLY2, and has the first contact terminal TC1 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 the skilled ones 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 TC3. 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 comprising a first C1 and a second C2 elementary capacitor. In fact, a first elementary capacitor C1 formed from the layers POLY1 and POLY2A which provide the conductive plates, and from the layer 7 of interpoly oxide providing the dielectric between the plates, can be readily discerned. Also discernible is a second elementary capacitor C2 formed from the first layer POLY1 and the diffusion well 4, which provide its plates, with a dielectric layer being provided by the gate oxide layer 6. The overall capacitance of the resultant capacitor is given by, C = C 1 + C 2 = ε 1 A 1 t 1 + ε 2 A 2 t 2 where: ε 1 is the value of the dielectric constant of the interpoly oxide layer 7 which forms the isolation dielectric of the first elementary capacitor C1; t 1 is the thickness of the oxide 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 C1; 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 C2; 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 diffusione well 4, which form the conductive plates of the second elementary capacitor C2. For a simple approach, 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 relation (1) becomes: And assuming, for simplicity, that t 2 =kt 1 , relation (2) reduces to: It can be seen at once from relation (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 anyhow, 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 relation (3), that the capacitance of the inventive capacitor increases as k becomes smaller. Shown in Figure 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 C1 and C2. In fact, to obtain the parallel layout of Figure 1C, it will be sufficient that the contact terminals TC1 and TC3 be shorted by placing the first portion POLY2A 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 Figure 1C. 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 process 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 Figure 2. It comprises an oscillator OSC; 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 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 D2 having its anode connected to the supply voltage reference VS. The cathode of the diode D2 is also connected to the anode of another diode D1 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 ccapacitor 1 is utilized, as shown in Figure 1C, in the voltage step-up circuit of Figure 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 process 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 embodiment described, the metallization step for making the contact terminals of the high-capacitance capacitor according to this invention only affects the second layer POLY2 of polycrystalline silicon.

Claims: A semiconductor MOS type process comprising the step of integrating monolithically a high-capacitance capacitor (1) on a semiconductor substrate (3) doped with a first type (P) of dopant, the process further comprising the steps of: forming a diffusion well (4) doped with a second type (N) of dopant in the semiconductor substrate (3), implanting and diffusing an active region (5) in said diffusion well (4), depositing a gate oxide layer (6) onto said diffusion well (4) and covering it with a first layer (POLY1) of polycrystalline silicon separated from a second layer (POLY2) of polycrystalline silicon by a dielectric layer (7), the process being characterized by : forming said dielectric layer (7) as an interpoly dielectric layer used in said MOS process, forming said second layer (POLY2) of polycrystalline silicon consisting of a first (POLY2A) and a second (POLY2B) separated portions, said second separated (POLY2B) portion being formed in direct contact with said first layer (POLY1) and forming a first elementary capacitor (C1) of said high-capacitance capacitor (1) utilizing said first (POLY1) layer and first (POLY2A) separated portion of polycrystalline silicon as its conductive plates and said dielectric layer (7) as its isolation dielectric, and a second elementary capacitor (C2) of said high-capacitance capacitor (1) utilizing said first (POLY1) layer and said diffusion well (4) as its conductive plates and said gate oxide layer (6) as its isolation dielectric, forming a first (TC1) and a second (TC2) contact terminals of the capacitor (1) on said first (POLY2A) and second (POLY2B) separated portions, and a third contact terminal (TC3) of the capacitor (1) on said active region (5) doped with the second dopant type (N). A process according to Claim 1, wherein said first (TC1) and third (TC3) contact terminals of the capacitor are shorted together. A process according to Claim 1, wherein said active region (5) is formed by a diffused region having a concentration of dopant of the second type (N+) higher than the concentration of the diffusion well (4). A process according to any Claims from 1 to 3 further including the formation of a voltage step-up circuit of the charge pump type connected between a first DC supply voltage reference (VS) and a second ground voltage reference (GND) integrated with MOS devices, comprising at least a first charge transfer capacitor (TC) connected between an oscillator (OSC) and an output terminal (OUT) of the voltage step-up circuit, wherein said first charge transfer capacitor (TC) constitutes said high-capacitance capacitor (1). A process according to Claim 4, comprising the formation of a second charge storage capacitor (SC) connected between said ground voltage reference (GND) and the output terminal (OUT) of the voltage step-up circuit, wherein said second charge storage capacitor (SC) constitutes said high - capacitance capacitor (1).

Inventors: BEZ ROBERTO, CAMERLENGHI EMILIO, BEZ, ROBERTO, CAMERLENGHI, EMILIO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1995
Main classification: H01L21
IPC codes: H01L29