Process for the manufacture of a Zener Diode for flash-EEPROM devices

Abstract: A process for the manufacture of a zener-diode as an integrated voltage limiter and stabilizer component in a flash EEPROM memory device comprises a step of formation of an N type lightly doped well (2) on a single-crystal silicon substrate (1); a step of formation of an active area (4) on the surface of said N type well (2); a step of growth of a thin gate oxide layer (5) over said active area (4); a step of implantation of a first heavy dose of N type dopant into said N type well (2) to obtain an N type region (6; 18; 19); a step of implantation of a second heavy dose, higher than said first heavy dose, of N type dopant into said N type region (6; 18) to obtain an N+ contact region (7; 20) to both the N type well (2) and said N type region (6; 18; 19); a step of implantation of a third heavy dose, higher than said first heavy dose, of P type dopant into said N type region (6; 19) to form a P+ region (8; 21). The N+ region (7; 20) is of annular shape. The zener-diode can also be gated. All manufacturing steps for the zener-diode correspond to manufacturing steps of the flash EEPROM memory device.

Description: The present invention relates to a process for the manufacture of an integrated voltage limiter and stabilizer in flash EEPROM memory devices. Electrically erasable and programmable memory devices, such as EEPROM and flash EEPROM, must handle and control relatively high internal voltages, assuring at the same time a high degree of stability: in order to accomplish a programming or an erasing operation, a voltage of 12V or more must in fact be supplied to the memory cells. It is therefore advisable to provide such devices with integrated components suitable to both limit any possible overvoltage, which could damage the memory cells or the circuitry connected to them, and stabilize internal voltages against temperature variations and statistical distribution of the process parameters. These components become essential for memory devices belonging to particular logic families, such as 5V-only devices, wherein the voltage necessary for programming and erasing the memory cells is generated internally. Integrated voltage limiters are commonly obtained in MOS devices by means of chains of reverse-biased planar junction diodes or diode-connected MOS transistors. Both these solutions have drawbacks: in the case of a chain of reverse-biased junction diodes the overall clamping voltage, even if it shows a small dependence on the temperature and on the process parameters, is affected by a decrease with time due to the injection of charge in the oxide layer covering the junction surface (walk-out phenomenon); in the case of a chain of diode-connected MOS transistors, instead, the overall clamping voltage depends on both the temperature and the variations of the process parameters due to their statistical distribution. In the European Patent Application No. 0426241 there is described a process for the manufacture of a zener diode to limit and stabilize the programming voltage supplied to EEPROM memory cells, suitable to be integrated with the EEPROM memory cells in a single chip. Such process does not involve additional steps with respect to those usually necessary for the accomplishment of EEPROM memory cells: the junction subjected to break down is constituted by a P+ region with dopant concentration of 10 20 atoms/cm 3 obtained in a process step corresponding to the implantation of source and drain areas of the P-channel transistors, and by an N- region with dopant concentration of 10 19 atoms/cm 3 obtained in a process step corresponding to the implantation of condensers of the EEPROM cells; this last step requires a litographic mask, and is performed before the gate oxide layer is formed. Since however the manufacture of EEPROM and flash EEPROM involves different fabrication processes, the integration of the zener diode described in the cited Patent Application in a flash EEPROM memory device would require additional process steps with respect to those usually necessary for the accomplishment of a flash EEPROM, namely the step of implantation of condensers. In view of the state of art described, the object of the present invention is to accomplish a process for the manufacture of an integrated component suitable to limit the overvoltages and to stabilize the internal voltages in flash EEPROM memory devices, but also employable in EEPROM, which assures a clamping voltage stable over time and independent as far as possible from temperature and process parameters variations, said process requiring no additional steps with respect to the usual process utilized for the accomplishment of flash EEPROM memory devices. According to the present invention, such object is attained by means of a process for the manufacture of an integrated voltage limiter and stabilizer component, as set forth in claim 1. Thanks to the present invention, it is possible to exploit some steps of the process usually employed to manufacture flash EEPROM memory devices in order to obtain a zener diode suitable to stabilize the programming/erasing voltage applied to the memory cells against temperature and process parameters variations, and to limit any possible overvoltage. More particularly, step a) is normally necessary to obtain P-channel MOS transistors, step b) is carried out to define the areas wherein MOS transistors and memory cells are to be obtained, and step c) is clearly inherent to the fabrication of the flash EEPROM memory cells. Step d) is coincident with the step of implantation of N type dopants to form the source region of the flash EEPROM cells. Step e) corresponds to the step of implantation of N type dopants to form the source and drain regions of N-channel transistors. Step f) corresponds to the step of implantation of P type dopants to form the source and drain regions of P-channel transistors. It is worth noting that in the zener diode obtained by means of the process according to the invention, the junction made to break down is represented by a P+ region having a dopant concentration identical to that of the source and drain regions of P-channel transistors, and by an N region having a dopant concentration lower than that of the source and drain regions of N-channel transistors; this allows to obtain zener diodes with higher breakdown voltage, typically of 5V, whereas conventional zener diodes, whose cathode region is obtained exploiting the implantation step that also forms the source and drain regions of N-channel transistors, have breakdown voltages of about 2.5.V. The features of the present invention shall be made more evident by the following detailed description of two embodiments, illustrated as non-limiting examples in the annexed drawings, wherein: Figure 1 is a top plan view of a zener diode according to one embodiment of the present invention, at the level of the silicon surface; Figure 2 is a top plan view of a zener diode according to another embodiment of the present invention; Figures 3 to 8 show a succession of process steps for the accomplishment of the zener diode of Figure 1; Figures 9 to 16 show a succession of process steps for the accomplishment of the zener diode of Figure 2. With reference to Figures 3 to 8, into a semiconductor substrate 1, for example of the P conductivity type, there is created, by means of implantation of phosphor ions in a dose of about 7*10 12 atoms/cm 2 , a lightly doped N type well 2 (Fig. 3). A successive step provides for the selective creation of a thick field oxide layer 3 over the surface of the semiconductor substrate 1; the portion of the substrate surface not covered by the field oxide layer 3 defines an active area region 4. A thin gate oxide layer 5 is then made to grow over the substrate surface (Fig. 4). After the etching of the thin gate oxide layer 5, N type dopant ions are implanted into the N type well 2 to form an N type region 6 (Fig. 5); this step corresponds to the step of implantation of ions such as phosphorous to form the source regions of flash EEPROM memory cells (not shown), the dose of implanted ions being typically 10 15 atoms/cm 2 . The N type region 6, much more heavily doped than the N type well 2, will constitute a cathode region for the zener diode to be fabricated. In a successive step, after the creation by means of oxide deposition and etching, or alternatively by means of reoxidation, of an oxide layer 14, a heavy dose of N type dopant ions is implanted into the N type region 6 to form an N+ region 7 (Fig. 6). As shown in Figure 1, the N+ region 7 has an annular shape, and is suitable to provide contact areas to the N type region 6 and to the N type well 2. This step corresponds to the step of implantation of ions such as arsenic to form the source and drain regions of N-channel transistors (not shown), the dose of implanted ions being typically 5*10 15 atoms/cm 2 . A heavy dose of P type dopant ions is then implanted into the N type region 6 to form a P+ region 8 which will constitute an anode region of the zener diode (Fig. 7). This step corresponds to the step of implantation of ions such as boron to form the source and drain regions of P-channel transistors (not shown), the implanted dose being typically 2.5*10 15 atoms/cm 2 , and can alternatively be performed before the above described step of Fig. 6. The final steps of the process provide, in a conventional way, for the deposition of a dielectric layer 13, for the creation of electrical contacts 9 and 10 to the N+ annular region 7 and to the P+ region 8 respectively, for the deposition of electrical interconnections lines 11 and 12, and for the passivation of the entire surface of the semiconductor substrate (Fig. 8). The N+ annular region 7 will be connected, by means of the interconnection line 11, to a high voltage power supply line HV (either supplied externally to the flash EEPROM memory device or internally generated by a voltage booster in the case of a 5V-only device), while the P+ region 8 will be connected, by means of the interconnection line 12, to a ground reference voltage. In Figures 2 and 9 to 16 there is shown a second embodiment of the present invention, suitable to obtain a so-called "gated zener diode". The fabrication process shown in Figures 9 to 16 comprises all the steps described with reference to the first embodiment of the invention, plus one intermediate step which is however typical of the manufacturing process of flash EEPROM memory cells, and consequently does not imply additional process steps. Figures 9 and 10 respectively show the creation of the N type well 2 into the P type semiconductor substrate 1, and the formation of the active area region 4 defined by a portion of the substrate surface not covered by the thick field oxide layer 3. A thin gate oxide layer 5 is grown over the semiconductor surface, and a polysilicon layer is deposited over the gate oxide layer 5. The polysilicon layer is then doped to increase its conductivity, and, together with the underlying gate oxide layer 5, is selectively etched to form a polysilicon strip 51 having a ring portion 16 inside the active area region 4, with a lateral extension 50 (Fig.s 2 and 11). N type dopant ions are then implanted into the N type well 2, the polysilicon strip 51 acting as a mask for the implanted ions. This step corresponds to the formation of the N type region 6 in the previous embodiment of the invention, and gives rise to an N type region made up of a substantially ring-shaped region 18 external to the ring portion 16 of the polysilicon strip 51, and of a circular region 19 internal to said ring portion 16 (Fig. 12). An oxide deposition and etching step, or alternatively a reoxidation step, allows the formation of spacers 17 at the edges of the polysilicon strip 51 (Fig. 13), and a successive implant of N type dopant ions into the N type region 18 allows the formation of a substantially ring-shaped N+ region 20 (Fig. 14). This step is carried out simultaneously to the implantation step into the source and drain regions of the N-channel transistors (not shown), as already described with reference to the first embodiment of the invention. Exploiting the implantation step into the source and drain regions of the P-channel transistors (not shown), a heavy dose of P type dopant ions is then implanted into the N type circular region 19, to form a P+ region 21 which will constitute an anode region of the gated zener diode. The presence of the spacers 17 makes it possible to obtain a P+ region completely surrounded by the N type region 19, so that the junction subjected to breakdown is represented by the P+ region 21 and by the N type region 19; this allows to obtain a zener diode with a stable clamping voltage of about 5V. The final steps are conventional and provide for the deposition of a dielectric layer 22 over the semiconductor surface, for the definition of contact areas 23, 24 and 25 to the ring-shaped N+ region 20, to the P+ region 21 and to the lateral extension 50 of the polysilicon strip respectively, and for the formation of electrical interconnection lines (not shown) (Fig. 16). The N+ region 20 will be connected to the high voltage supply, while an interconnection line will connect the P+ region 21 and the polysilicon strip 51 to the reference ground voltage. The entire semiconductor surface is then passivated. The gated zener diode thus obtained improves the features of the zener diode of the first embodiment of the invention, because the polysilicon strip 51, shortcircuited with the P+ anode region 21, relaxes the superficial electric field and cause the breakdown to occur at a greater depth, thus improving the stability of the clamping voltage value. Even if the process according to the present invention has been described in connection with the manufacturing of flash EEPROM memory devices, it can also be employed to obtain, without additional process steps, zener diodes integrated in some kind of EEPROM memory devices.

Claims: A process for manufacturing of an integrated voltage limiter and stabilizer component in an integrated circuit comprising flash EEPROM memory cells and P channel and N channel MOS transistors, characterized in that it comprises the following steps: a) forming in a single-crystal silicon substrate (1) an N type lightly doped well (2) for the voltage limiter and stabilizer component, and simultaneously forming in said substrate N type lightly doped wells for the P channel MOS transistors; b) forming an active area (4) on the surface of said N type well (2) for the voltage limiter and stabilizer component, and simultaneously forming active areas for the flash EEPROM memory cells and for the P channel and N channel MOS transistors; c) growing a thin gate oxide layer (5) over the surface of said substrate (1) at said active areas to form a gate oxide layer for the flash EEPROM memory cells; d) implanting a first heavy dose of N type dopant into a portion of said N type well (2) for the voltage limiter and stabilizer component to obtain an N type region (6;18,19), and simultaneously implanting the first heavy dose of the N type dopant into the substrate (1) at said active areas for the flash EEPROM memory cells to form source regions of the memory cells; e) implanting a second heavy dose, higher than said first heavy dose, of N type dopant into a first portion of the N type region (6;18,19) to obtain an N+ contact region (7;20) to both the N type well (2) for the voltage limiter and stabilizer component and said N type region (6;18,19), and simultaneously implanting said second heavy dose of the N type dopant into the substrate (1) at said active areas for the N channel MOS transistors for forming source and drain regions of the N channel MOS transistors; f) implanting a third heavy dose, higher than said first heavy dose, of P type dopant into a second portion of said N type region (6;19) said second portion being separate from said first portion, to form a P+ region (8;21), and simultaneously implanting the third dose of the P type dopant into said N type wells for the P channel MOS transistors to form source and drain regions for the P channel MOS transistors. Process according to claim 1, characterized in that the thin gate oxide layer (5) is removed from the substrate surface at said active area (4) for the voltage limiter and stabilizer component before step d) is performed. Process according to claim 1, characterized in that between steps c) and d) there is executed the deposition of a polysilicon layer over the thin gate oxide layer (5), the doping and the etching of the polysilicon layer to form a polysilicon strip (51), insulated from the substrate surface by the thin gate oxide layer (5) and having a portion (16) superimposed over said active area (4) for the voltage limiter and stabilizer component. A process according to claim 3, characterized in that between steps d) and e) spacers (17) are formed at the edges of said polysilicon strip (51). A process according to claim 4, characterized in that said spacers (17) are formed by means of the deposition and etching of an oxide layer. A process according to claim 4, characterized in that said spacers (17) are formed by means of a reoxidation step. A process according to claim 1, characterized in that the step e) is performed before the step f). A process according to claim 1, characterized in that the step e) is performed after the step f).

Inventors: GHEZZI PAOLO, MAURELLI ALFONSO, GHEZZI, PAOLO, MAURELLI, ALFONSO
Applicants: ST MICROELECTRONICS SRL, STMICROELECTRONICS S.R.L.
Application year: 1993
Main classification: H01L21
IPC codes: H01L21, H01L27, H01L29