EEPROM MEMORY CELL WITH A SINGLE POLYSILICON LEVEL AND A TUNNEL OXIDE ZONE

Abstract: The memory cell comprises a selection transistor (51), pickup transistor (52) and a tunnel condenser (53) formed using a single layer of polysilicon (59). The tunnel condenser (53) is formed on an active area (56) distinct and separate from that of the pickup transistor (52).

Description: The present invention relates to an EEPROM memory cell with a single polysilicon level programmable for a Fowler-Nordheim tunnel through a thin oxide zone. In the literature can be found some EEPROM cells with a single polysilicon level which are written and cancelled for Fowler-Nordheim tunnel through a thin oxide or tunnel oxide, utilizing the capacitive couplings between the control gate, floating gate and semiconductor substrate. These cells comprises a selection transistor, a sensing transistor and a tunnel capacitor, defined by a thin oxide zone with implantation of n⁻ phosphorous partially superimposed on the drain diffusion of the sensing transistor. A single layer of polysilicon forms the gate of the selection transistor and, separately, an armature of the tunnel capacitor and the floating gate of the sensing transistor in a single piece with an armature of the coupling capacitor of the control gate formed with an n⁺ diffusion of the substrate. A drawback of these known cells is the fact that the tunnel zone is placed on the same active area (n⁺ drain diffusion) of the sensing transistor. For this reason the thin oxide can be found supporting electrical stresses due to the voltage applied during the reading of the cell and during the writing operations of the other memory cells. EP-A-0 133 667 and EP-A-0 162 737 disclose examples of EEPROM memory cells according to the preamble of claim 1 and EP-A-0 120 303 discloses an EEPROM memory cell having a diffused control gate. The object of the present invention is to achieve an improved structure for an EEPROM memory cell with a single polysilicon level and tunnel oxide zone, which is free of the aforesaid drawback. In accordance with the invention said object is achieved by an EEPROM memory cell as defined in claim 1. In this manner the thin oxide zone is electrically separated from the sensing transistor with resulting reduction of the electrical stresses originating from the reading voltage. In addition, having the sensing transistor serially connected with the n⁺ diffusion of the control gate makes its capacity toward the substrate and toward said diffusion contribute to increasing the effectiveness of the programming voltages applied, going to increase the capacitive couplings which control the voltage drop on the thin oxide. The characteristics of the present invention Will be better understood by observing the annexed drawings wherein: FIG. 1 shows schematically a plan of the conformation of an EEPROM cell with a single polysilicon level in accordance with the known art, FIG. 2 shows a similar plan representation of a cell in accordance with the invention, FIG. 3 shows a cross secton of the cell in accordance with the invention along line III-III of FIG. 2, and FIG. 4 shows a cross section of the cell in accordance with the invention along line IV-IV of FIG. 2. FIG. 1 illustates an EEPROM memory cell in accordance with the known art which comprises a selection transistor 1, a sensing transistor 2 and a tunnel capacitor 3. More precisely, on a semiconductor substrate 4 having active areas 5 and 6 with n⁺ diffusions and including a thin oxide zone 7 on the active area 5 is deposited a single layer of polysilicon 8 formed of a transverse strip 9, which defines the gate of the selection transistor 1 and a U part 10 which has a first longitudinal branch 11 placed on the thin oxide zone 7 and therefore on the active area 5 to form the tunnel capacitor 3, a second longitudinal branch 12 placed similarly on the active area 5 to form the floating gate of the sensing transistor 2 and a space 13 placed on the active area 6 to form a coupling capacitor with the control gate of the transistor 2, defined by the n⁺ diffusion of the active area 6. Two n⁻ diffusions 14 and 15 are made at the thin oxide zone 7 and at the aforesaid control gate to ensure electrical continuity of the cell. Reference number 16 indiates the drain contact and reference number 17 indicates an output contact. In FIGS. 2-4 is illustrated a memory cell in accordance with the invention in which the selection transistor is indicated by reference number 51, the sensing transistor by reference number 52 and the tunnel capacitor by reference number 53. As shown in FIG. 2, on a substrate 54 are provided two active areas 55 and 56 with n⁺ diffusions. On the structure is grown gate oxide 57 (FIGS. 3 and 4) which presents a thin oxide zone 58 at the active area 56 (FIG. 2). A single polysilicon layer 59 further superimposed comprises a transverse strip 60 which defines the gate of the selection transistor 51, and another part 68 formed of a first arm 61 superimposed on the thin oxide zone 58, and then on the active area 56, to form the tunnel capacitor 53, a second arm 62 superimposed on the active area 55 to form the floating gate of the sensing transistor 52 and finally a space 63 further superimposed on the active area 55 to form the coupling capacitor of the control gate, defined by a portion 69 of the n⁺-diffused active area 55. Two n⁻ diffusions 64 and 65 are made in the substrate 54 at the thin oxide zone and the aforesaid control gate to ensure electrical continuity of the cell. Reference number 66 indicates a drain contact and reference number 67 indicates an output contact. Further oxide 70 is superimposed on the polysilicon layer 59 (FIGS. 3 and 4). The manner of operation of the cell in accordance with the invention is substantially identical to that of conventional cells and will therefore not be described in detail. The only exception occurs during the reading phase when the drain contact 67 is brought to high potential, the n⁺ line of the control gate (69) is grounded and the gate 60 of the selection transistor 51 is placed at a potential higher than that of the threshhold.

Claims: EEPROM memory cell formed by a selection transistor (51), a sensing transistor (52) with a floating gate (62) and a control gate (69), and a tunnel capacitor (53) with a thin oxide zone (58), comprising a single layer of polysilicon (59) for the floating gate (62) and the tunnel capacitor (53), a first active area (55) for the control gate (69) and a second active area (56) for said thin oxide zone (58) of the tunnel capacitor (53), characterized in that said first active area (55) has a n⁺ diffusion forming the control gate (69) and said single polysilicon layer comprises a first part (60) defining the gate of the selection transistor (51) and a second part (68) formed of a first arm (61) superimposed on the thin oxide zone (58) on said second active area (56), a second arm (62) superimposed on said first active area (55) to form the floating gate of the sensing transistor (52) and a space (63) further superimposed on said first active area (55) to form a coupling capacitor for the control gate (69).

Inventors: RIVA, CARLO
Applicants: SGS-THOMSON MICROELECTRONICS S.R.L.
Application year: 1987
Main classification: H01L29
IPC codes: H01L21, H01L29