HIGH-SPEED, NON-VOLATILE ELECTRICALLY PROGRAMMABLE AND ERASABLE CELL AND METHOD

Abstract: A non-volatile programmable circuit having programming bitlines (PBL; 36) and read bitlines (RBL; 37), a non-volatile memory cell (30), and a read select transistor (34), and a method for its operation. The non-volatile memory cell is programmable through the progamming bitline. The read select transistor is connected between the non-volatile memory cell and the read bitline. During read operation, the programming bitline is grounded and programmed information is readable onto the read bitline. During programming operation, the read bitline is grounded, and programmed information is programmable into the non-volatile memory cell for storage and retention.

Description: Description High-Speed, Non-Volatile Electrically Programmable and Erasable Cell and Method Technical Field The field of the invention is that of high-speed, non-volatile, electrically programmable and erasable cells and methods for their operation. Background Art Electrically programmable and erasable cells of many kinds are well known in the semiconductor arts. Such cells find widespread application as key elements in programmable logic devices, field programmable gate arrays, flash memories and electrically erasable program¬ mable read only memories. Figure 1 illustrates a typical non-volatile cell 10 of the prior art for use with electrically erasable programmable read only memories and flash memories. The non-volatile cell 10 shown includes a select transistor 12 and a floating gate transistor 14, which are connected in series between a bitline 16 and a ground connection. Floating gate transistor 14 of Figure 1 is an n-channel depletion-type device. Select transis¬ tor 12 and floating gate transistor 14 are typically nmos field effect transistors. Each has a control gate effec¬ tive for controlling the passage of electric current depending upon the voltage applied to the control gate. The control gate of select transistor 12 is driven by a wordline 20, and the control gate of floating gate transistor 14 is connected to a reference line 22 supplying a reference voltage effective for selective activation of floating gate transistor 14. Select transistor 12 shown in Figure 1 permits selection and accessing of selected non-volatile cell 10 and is connected to bitline 16 through which the logical state of non-volatile cell 10 can be detected or sensed. Select transistor 12 is preferably an n-channel device, and its drain is connected to bitline 16. The source of select transistor 12 is further connected to floating gate transistor 14 which holds the signal information subject to detection or sensing. A select signal on wordline 20 is effective for causing select transistor 12 to conduct current, permitting access to floating gate transistor 14. This permits the logical state of float- ing gate transistor 14 to be detected or sensed through bitline 16. Floating gate transistor 14 is fabricated with tunnel oxide and n+ doping on its drain connection. The source connection of floating gate transistor 14 is grounded in the typical case. Certain disadvantages characterize the struc¬ ture of the typical non-volatile cell 10 of the prior art as shown in Figure 1. These disadvantages apply particu¬ larly with respect to programmable logic device applica¬ tions, in which speed of operation is particularly essen- tial. Speed of operation of select transistor 12 is critical, especially with regard to the reading and the programming of non-volatile cell 10. In order for select transistor 12 to perform key functions in the reading and programming of non-volatile cell 10, it is fabricated with a thick oxide layer and a long channel length between source and drain. Select transistor 12 is not optimized for speed of operation, because the long channel and the thick oxide layer tend to slow operation. Select transistor 12 is further hampered by its low current drive capabilities. This makes it particularly slow at three (3) volts, which is the new power supply standard for many semiconductor applications. The non-volatile cell 10 shown in Figure 1 further suffers in the construction of floating gate transistor 14. Floating gate transistor 14 relies upon tunnel oxide and n+ diffusion in its drain region. This establishes a high level of capacitive loading on bitline 16 with attendant disadvantages during high speed operation. This is particularly troublesome, because in the case of programmable logic devices, half of the wordlines are "on," at any given time. Thus, a selected non-volatile cell 10 has to charge or discharge the n+ and tunnel oxide capacitance of the connected bitlines in order effectively to operate. However, the difficulty in effectively discharging against the tunnel oxide capacitance considerably slows operation. It is accordingly an object of the invention to develop a fast and effective cell structure and organiza¬ tion for high speed operation in the fields of program¬ mable logic devices, field programmable gate arrays, flash memories, and electrically erasable programmable read only memories. It is further an object of the invention to avoid the effects of tunnel oxide capacitance and n+ con¬ centrations in the transistors of typical cell arrange¬ ments of the prior art. Summary of the Invention The above objects have been achieved in an arrangement and architecture of a non-volatile elec¬ trically programmable and erasable cell for high speed devices, which provides separate programming and read paths to different bitlines. In particular, according to the arrangement of the invention, in order to increase operating speed, separate programming and read bitlines are employed to program the non-volatile cell and read its memory contents. Additionally, read operation is conducted through the drain of the cell arrangement rather than through the source of the cell, the drain being defined as the side of the cell arrangement connected to the read bitline. In order to implement the invention herein, an additional read select transistor is connected to the non-volatile cell in series therewith and leading to a read bitline. The cell may include a programming select and a floating gate transistor. Accordingly, the cell and the additional read select transistor are connected respectively to the programming and read bitlines, effective to accomplish both programming and read func¬ tions for the cell along different and separate paths, one path leading to the programming bitline and the other leading to the read bitline. Thus, cell programming may be conducted through the programming select transistor in the cell or directly with the cell's floating gate transistor, depending on whether the floating gate transistor is an n-channel depletion device or a p-channel enhancement device and whether the cell includes a programming select transistor. On the other hand, read operation is conducted through the read select transistor which is connected to the read bitline. This two-bitline approach considerably enhances the opera¬ tional speed of the non-volatile cell and reduces the power consumed by the cell during operation. Brief Description of the Drawings Fig. 1 shows a traditional non-volatile cell of the prior art which can be employed with an electrically erasable programmable read only memory or with flash memories, for example. Fig. 2 shows a non-volatile electrically erasable programmable read only memory cell, for high speed applications in accordance with the invention. Fig. 3 shows another non-volatile electrically erasable programmable read only memory cell according to the invention, which is suited for high speed operation. Fig. 4 shows the topography of the non-volatile electrically erasable read only memory cell of Fig. 2. Best Mode for Carrying Out the Invention Figure 2 illustrates the architecture of an electrically erasable programmable read only memory arrangement 29, including a non-volatile cell 30, according to the invention herein. Non-volatile cell 30 may include a programming select transistor 32 and a floating gate transistor 33. Programming select transis¬ tor 32 preferably has an oxide layer about 300 to 400 Angstroms thick. The oxide layer for programming select transistor 32 may however be from about 250 to 450 Angstroms thick. The programming select transistor 32 is also characterized by a channel which is at least 1.8 μm wide. Memory arrangement 29 further includes a read select transistor 34, connected in series with the non-volatile cell 30 between a programming bitline 36 and a read bitline 37. Read select transistor 34 preferably has an oxide layer about 150 Angstroms thick, much thinner than the oxide layer of the programming select transistor 32. The preferred range of thickness of the read select transistor 34 according to the invention is between approximately 100 to 200 Angstrom. The read select transistor 34 has a narrow channel, typically between 0.6 and 0.8 μm wide. These characteristics ensure fast read operation. Transistors 32, 33, and 34, according to a preferred version of the invention, are field effect transistors, and each of them has a control gate. The control gate of select transistor 32 is connected to a programming wordline 40. Further, the control gate of the floating gate transistor 33 is connected to a reference line 42 which is selectively applied to activate floating gate transistor 33. Finally, the control gate of read select transistor 34 is connected to a read wordline 43. Programming select transistor 32 is connected to programming bitline 36. Programming select transistor 32 is preferably an n-channel device. The drain of programming select transistor 32 is connected to programming bitline 36. The source of programming select transistor 32 is connected to floating gate transistor 33. A signal on programming wordline 40 is effective for permitting programming select transistor 32 to activate, permitting the logical state of floating gate transistor 33 to be programmed through programming bitline 36. Floating gate transistor 33 is fabricated with tunnel oxide in its drain region. During read operation, the programming wordline 40 is set to VCC; the programming bitline 36 is grounded; and reading is accomplished via the read bitline 37. Since the read select transistor 34 never sees high volt¬ age, its input wordline, i.e., read wordline 43, can be optimized for speed. Further, since read wordline 43 is not subject to high voltages, e.g., up to 20 volts, required for programming, any input circuitry connected to read wordline 43 can be speed optimized by having shortened channel widths and thinner oxide layers. In particular, the tunnel oxide and n+ diffusion are on the source side during read operation. Accordingly, there is no need for charge-up prior to read operation, considera¬ bly speeding read operations. Likewise, the read bit line 37 never sees high voltage, so both it and the sense amplifier circuitry connected to it can be optimized for speed. Fig. 3 shows an alternate embodiment of the electrically erasable programmable read only memory arrangement 29 according to the invention, in which the select transistor 32 of Figure 2 is eliminated as unnec- essary, and floating-gate transistor 33 is an n-channel enhancement transistor which is gate driven directly by programming wordline 40. Further, floating-gate transis¬ tor 33 is connected directly to programming bitline 36. Otherwise, the advantages and features of Figure 3 are similar to those of Figure 2. Fig. 4 shows the silicon topography for imple¬ menting a version of the invention as set forth in Fig. 2. The topography shown is directed toward a single cell including floating-gate transistor 33, programming select transistor 32 and read select transistor 34. These three devices are shown the topography of Figure 4. Transistor 32 has source and drain regions, respectively, 32' and 32". Similarly, transistor 33 has drain and source regions, respectively, 33' and 33". Finally, transistor 34 has source and drain regions, respectively, 34' and 34". The arrangement includes a diffusion layer 50 deposited on a substrate 51. A tunnel oxide patch 52 is deposited over the diffusion layer 50 according to a defined, preferably rectangular pattern to comprise part of floating-gate transistor 33. Three vertical strips, respectively 54, 56, and 58 are the remnants of poly 2 and poly 3 semiconductor layers (i.e., polysilicon layers), each of which is deposited according to well-known processes in the semiconductor arts. A central vertical strip 53 under vertical strip 54 and comprised of poly 1 layer within the bounds of diffusion layer 50 is deposited directly over diffusion layer 50. The poly 1 layer of central vertical strip 53 is deposited over tunnel oxide patch 52 of the floating gate transistor. Each poly layer is preceded during fabrication by predeposit of an oxide layer thereunder. As vertical strips 53 and 54 comprise, respectively, two poly layers, poly 1 and poly 2, there will, in addition to tunnel oxide patch 52, be applied two other oxide layers under corresponding poly layers 1 and 2. The oxide thicknesses applicable are preferably about 85 Angstroms thick for the tunnel oxide patch 52, about 200-300 Angstroms for the oxide under the poly 1 layer, and about 300-400 for the oxide under the poly 2 layer. The topography of Figure 4 additionally includes first and second contacts, respectively 61 and 62, which are fabricated on diffusion layer 50. First contact 61 is connected to the programming bit line and second contact 62 is connected to the read bitline. Third vertical strip 58 is made from the poly 3 layer and is relatively thin in terms of its underlying oxide layer, which is preferably about 150 Angstroms thick and may according to the invention be between about 100 Angstroms to about 200 Angstroms thick. Vertical strip 58 serves as the gate of the read select transistor, substantially increasing its speed as a result of the thinner oxide layer under the poly 3 layer. The gate of the read select transistor is activated by voltages on the order of 5 volts during read select operation. Vertical strip 54 is the reference or gate input to the floating gate transistor. Vertical strip 56 is the programming select input for the program select transistor. Region 55 of diffusion layer 50 is the connection between program select transistor and the floating gate transistor. Region 57 of diffusion layer 50 is the connection between the floating gate transistor and the read select transistor. In summary, according to the multibitline arrangement of the invention, high speed operations are enabled by providing separate paths for information flow to and from non-volatile cell 30 during reading and pro¬ gramming phases of operation. This flexibility of opera¬ tion, provided by two-bitline structural enhancements results in effective operation of non-volatile cell 30 at higher speeds than possible in the prior art.

Claims: Claims 1. A non-volatile programmable circuit comprising: (a) a programming bitline for programming a non-volatile memory cell, (b) a read bitline for reading the contents of the non-volatile memory cell, (c) the non-volatile memory cell including a floating gate transistor of the type being a field effect device having one and only one source region and one and only one drain region spaced apart from one another with a thin tunnel oxide patch disposed over said drain region, a first polysilicon layer disposed over an area between said spaced apart source and drain regions and over a portion of said drain region including over said tunnel oxide patch, said first polysilicon layer forming a floating gate for said field effect device with a first oxide layer under said first polysilicon layer exclusive of said tunnel oxide patch, said device also having a second polysilicon layer forming a control gate disposed over said first polysilicon layer with a second oxide layer over said first polysilicon layer and under said second polysilicon layer, the drain region of said floating gate transistor connected to said programming bitline, said non-volatile memory cell being effective for retaining programmed information from said programming bitline, and (d) read select means for selective connection of said non-volatile memory cell to said read bitline, said read select means being connected in series between said source region of said floating gate transistor of said non-volatile memory cell and said read bitline, said read select means having a control gate for controlling, in cooperation with said control gate of said floating gate transistor, the passage of electric current through said non-volatile memory cell and said read select means, and said read select means being effective for enabling reading of said memory cell in response to a read signal on its control gate. 2. A non-volatile programmable circuit according to claim 1, wherein said non-volatile memory cell further includes a programming select transistor. 3. A non-volatile programmable circuit according to claim 2, wherein a source of said programming select transistor is connected to the drain region of said floating-gate transistor. 4. A non-volatile programmable circuit according to claim 1, wherein said read select means includes an n-channel transistor. 5. A non-volatile programmable circuit according to claim 4, wherein the source region of said floating-gate transistor is connected to a drain of said n-channel transistor. 6. A non-volatile programmable circuit according to claim 1, wherein said floating-gate transistor is capable of being programmed from said programming bitline by bringing the control gate of said floating gate transistor and said programming bitline to a high voltage state, while grounding said read bitline. 7. A non-volatile programmable circuit according to claim 1, wherein said floating-gate transistor is capable of being read through the read bitline, while the programming bitline is grounded and a programming wordline connected to said control gate of said floating gate transistor is set to Vcc. 8. A method of operating a non-volatile programmable circuit comprising: (a) connecting a non-volatile memory cell and a read select transistor in series between a programming bitline and a read bitline, said non-volatile memory cell including a floating gate transistor with only one source and only one drain, said read select transistor connected to said source of said floating gate transistor, (b) programming memory state information from said programming bitline onto said floating gate transistor of said non-volatile memory cell through said drain of said floating gate transistor, and (c) reading memory state information from said non-volatile memory cell through said read select transistor for application onto said read bitline. 9. The method of operating a non-volatile programmable circuit according to claim 8, comprising programming said non-volatile memory cell while said read bitline is grounded. 10. The method of operating a non-volatile programmable circuit according to claim 8, comprising reading memory state information while said programming bitline is grounded. 11. A non-volatile programmable circuit cell comprising: (a) a floating gate transistor circuit including a floating gate transistor having only one source and only one drain and a read transistor circuit connected in series between separate programming and read bitlines, said read transistor circuit connected to said source of said floating gate transistor and to said read bitline. 12. The programmable circuit cell according to claim 11, wherein said floating gate transistor circuit consists of only said floating gate transistor, said drain of said floating gate transistor being connected to said programming bitline. 13. The programmable circuit cell according to claim 12, wherein said floating gate transistor circuit comprises a programming gate connection which is drivable by a programming wordline. 14. The programmable circuit cell according to claim 11, wherein said read transistor circuit is a single read transistor. 15. The programmable circuit cell according to claim 14, wherein said read transistor circuit comprises a read gate connection which is drivable by a read wordline. 16. The programmable circuit cell according to claim 11, wherein said floating gate transistor circuit comprises a programming select transistor which is drivable by a programming wordline, said programming select transistor connected to said drain of said floating gate transistor and to said programming bitline. 17. The programmable circuit cell according to claim 11, wherein said floating gate transistor circuit is connected to a reference line. .

Inventors: PATHAK SAROJ, PAYNE JAMES E, PATHAK, SAROJ, PAYNE, JAMES, E.
Applicants: ATMEL CORP, ATMEL CORPORATION
Application year: 1995
Main classification: G11C11
IPC codes: G11C16, H01L21, H01L27, H01L29