Abstract:
An EPROM structure for a nonvolatile semiconductor memory includes a plurality of memory cells that each include a floating gate transistor ( 6 ) that can be programmed by hot electrons and erased by UV light. An additional, common gate capacitance ( 7 ) is associated with each memory cell to raise the potential at the floating gate transistor ( 6 ) to the level required for writing by applying to the gate capacitances a predetermined voltage, common to all the memory cells.

Description:
BACKGROUND OF THE INVENTION 
     The invention relates to an electronic memory device, and in particular an EPROM (erasable and programmable read-only memory) structure for a nonvolatile semiconductor memory with a plurality of memory cells, each of which has a floating gate transistor, which can be programmed by hot electrons and can be erased by UV light. 
     EPROM structures are used to build non-voltile semiconductor memories, especially for integrated circuits (embedded EPROM) and generally in computers or in microprocessor-controlled devices for storing programs and/or data that must be retained without a supply voltage. 
     Each memory cell of an EPROM structure generally has two transistors, namely a selection or access transistor that selects the memory cell, and a floating gate transistor, whose floating gate represents an erased or programmed state according to its positive or negative charge. 
     To program the memory cells, voltages of at least eight (8) volts are generally needed at the cell level and consequently voltages of about ten (10) volts for the pass gates. In the case of known EPROM structures, these voltages must be applied selectively to each memory cell that is to be programmed or erased. This implies a necessity for transistors that can switch these voltages. Since each memory cell must be selected, these transistors furthermore must be sufficiently small so that the entire structure and thus the memory will not become disproportionately large. 
     One problem is that conventionally produced integrated circuits as well as their production methods are designed for five (5) volts or less. For a conventional production process for integrated circuits to handle the high voltages necessary to embed EPROM structures, numerous additional process steps (generally about five to eight masking steps) are generally necessary. This makes the entire process and thus the integrated circuit substantially more expensive. 
     U.S. Pat. No. 5,212,541 discloses an EPROM structure, each of whose memory cells includes a floating gate transistor, which can be written (programmed) by hot electrons and erased by UV light. These memory cells can be rather easily manufactured with a known CMOS production processes. However, their disadvantage is that a voltage of thirteen (13) volts must be applied to a memory cell so as to program it selectively. 
     Therefore, there is a need for an EPROM structure that operates utilizing a voltage not much greater than 5 volts applied selectively to the individual memory cells. 
     SUMMARY OF THE INVENTION 
     Briefly, according to an aspect of the present invention, an EPROM structure includes a common gate capacitance disposed at each memory cell to raise the potential at the floating gate transistor to the level required for writing by applying to the gate capacitances a voltage, common to all the memory cells. 
     An advantage of this solution is that the common voltage does not need to be decoded. For this reason, high voltage transistors with other gate oxide thicknesses and diffusions are not necessary. This saves numerous mask and process steps in production, so that the electrical parameters of the standard transistors are not changed by additional process steps. On the other hand, voltages in excess of 20 volts can be present at the additional gate capacitance, since these need not be switched. 
     An economic advantage is that the production steps for the EPROM structure can be inserted without complication and without great expense into a conventional CMOS production process for integrated circuits. 
     The additional gate capacitance is preferably disposed above the floating gate of each memory cell. Furthermore, the floating gate transistor in particular is a depletion n-channel transistor, whose gate is charged negatively by hot electrons, so the floating gate transistor assumes an “off” state. 
     These and other objects, features and advantages of the present invention will become apparent in light of the following detailed description of preferred embodiments thereof, as illustrated in the accompanying drawings. 
    
    
     BRIEF DESCRIPTION OF THE DRAWING 
     FIG. 1 illustrates a schematic top view of a memory cell according to the present invention; 
     FIG. 2 illustrates a cross sectional view of the memory cell taken along the line A—A′ in FIG. 1; and 
     FIG. 3 illustrates an EPROM structure with a 6×4 bit memory cell arrangement. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     As part of a preferred EPROM structure, FIG. 1 illustrates a top view of a memory cell with a floating gate transistor and an access transistor. This structure includes, in a substrate, an n-doped diffusion layer  1 , which is contacted through a first metal contact  4 . Next to this is a first poly-layer  2   a , forming the access transistor  5 . This is followed by a second poly-layer  2   b , which creates the floating gate transistor  6 . On this is situated a capacitance layer  3 , forming the entire gate capacitance  7 . 
     FIG. 2 illustrates a cross sectional view of the memory cell taken along the line A—A′ in FIG.  1 . The n-doped diffusion layers  1  are introduced into the p-substrate  8  by implantation. The first metal contact  4 , a gate  51  of the access transistor  5 . and a floating gate  61  of the floating gate transistor  6  are situated on the p-substrate. On the gate  51  as well as on the floating gate  61  is situated a first dielectric layer  11 , which supports the common gate capacitance  7  in the region of the floating gate  61 , and insulates it from the floating gate. A second dielectric layer  16  is deposited and holes are etched therein for the first metal contact  4  for the n-doped diffusion layer  1 , and for a second metal contact  10  for the common gate capacitance  7  through known photo-lithographic techniques. To produce these contacts, the holes are filled with metal and a first metal layer  14  is applied thereon to make the contact. A third dielectric layer  17  is deposited into which holes are etched, and a second metal layer  15  is added for contacting the first metal layer  14 . In one embodiment, gate capacitance  7  comprises a layer of silicon nitride about 30 nm thick. In another embodiment, gate capacitance  7  comprises a poly-layer about 50 nm thick. In a further embodiment, gate capacitance  7  comprises a titanium/titanium nitride layer about 30 nm thick. 
     FIG. 3 illustrates an electric circuit diagram for a 6×4 bit memory cell arrangement. The access transistors for selecting the individual memory cells are driven through first conductors  20   a - 20   d . The second conductors  18   a - 18   f  represent bit conductors, and are connected to the EPROM memory cells through the access transistors. Two rows of memory cells share a source conductor  19 . A programming conductor  21  is connected to the common gate capacitance  7  for all the memory cells. In this embodiment, the EPROM memory cell is a 2-transistor cell with a floating gate transistor, in the form of a depletion n-channel transistor. The potential rise needed for programming is produced by applying a voltage to the common gate capacitance. Depending on the thickness of the first dielectric layer  11  (FIG. 2) between the common gate capacitance  7  (FIG. 2) and the floating gate  61  (FIG.  2 ), a voltage between about 10 and 30 volts is applied through the programming conductors  21 . The individual memory cells are addressed through one of the access conductors  20   a - 20   d  and through one of the bit conductors  18   a - 18   f . The addressed cell has a drain voltage of Vdd and a source voltage, which is applied through the corresponding source conductor  19 . The high positive potential at the common gate capacitance, which is applied through the programming conductor  21 , pulls the hot electrons created on the drain side to the floating gate  61 . The latter is thereby charged negatively, so that the floating gate transistor goes into the “off” state. 
     To read a memory cell, a voltage Vpp or  0  is applied to the programming conductor  21 . The relevant cell is addressed through one of the bit conductors  18   a - 18   f  and one of the access conductors  20   a - 20   d , and its current flow is detected. If the floating gate  61  is not charged, a current flows (signal “1”); on the other hand, if it is charged negatively, no current flows (signal “0”). 
     To erase the memory cells, they are exposed to UV light, so that the charges on the floating gates are eliminated. Since the floating gate transistors are depletion n-channel transistors, they thereby assume the “one” state, such that a current flows and a signal value “1” is read. 
     An advantage of the invention is that a voltage in excess of 5 volts needs to be applied only to the common gate capacitance. This voltage can be applied in common for all the cells in the memory array, and thus does not need to be decoded. Consequently, no high voltage transistors are needed, for which different gate oxide thicknesses and diffusions would be necessary, so numerous masking and process steps can be saved. Furthermore, the electric parameters of the standard transistors are not changed by additional process steps. The voltages above 5 volts, which are needed for the additional capacitances, can be switched by MOS transistors such as are produced in every CMOS process. These are indeed relatively large, but in each instance only one of them needs to be present. 
     Although the present invention has been shown and described with respect to several preferred embodiments thereof, various changes, omissions and additions to the form and detail thereof, may be made therein, without departing from the spirit and scope of the invention.