A METHOD FOR PROGRAMMING A SINGLE EPROM OR FLASH MEMORY CELL TO STORE MULTIPLE LEVELS OF DATA THAT UTILIZES A FORWARD-BIASED SOURCE-TO-SUBSTRATE JUNCTION

Abstract: Multiple logic levels can be programmed into a single EPROM or FLASH memory cell by applying one of a corresponding number of programming voltages to the control gate of a memory cell that has a forward-biased source-to-substrate junction and a reverse-biased drain-to-substrate junction. During programming, the bias conditions form substrate hot electrons which, in addition to the channel hot electrons, accumulate on the floating gate. By utilizing the substrate hot electrons, a much lower control gate voltage can be utilized during programming. More importantly, however, once the channel hot electrons cease to exist, the substrate hot electrons and holes converge to a stable charge that is related to the control gate voltage used during programming and the programmed threshold voltage of the cell.

Description: A METHOD FOR PROGRAMMING A SINGLE EPROM OR FLASH MEMORY CELL TO STORE MULTIPLE LEVELS OF DATA THAT UTILIZES A FORWARD-BIASED SOURCE-TO-SUBSTRATE JUNCTION RELATED APPLICATION The present application is related to commonly-assigned U.S. Patent Application Serial Number 08/341, 171. filed by Albert Bergemont et al. of even date herewith, for A METHOD FOR PROGRAMMING A SINGLE EPROM OR FLASH MEMORY CELL TO STORE MULTIPLE LEVELS OF DATA THAT UTILIZES A FLOATING SUBSTRATE. BACKGROUND OF THE INVENTION 1. Field of the Invention. The present invention relates to floating gate memory cells and, more particularly, to a method for programming a single EPROM or FLASH memory cell to store multiple levels of data, i.e., more than a logic "1" and a logic "0". 2. Description of the Related Art. FIG. 1 shows a conventional n-channel, floating gate memory cell 10. As shown in FIG. 1, memory cell 10 includes an n-type source region 12 and an n-type drain region 14 which are formed a distance apart in a p-type substrate 16. The surface area of substrate 16 between the source and drain regions 12 and 14 defines a channel region 18 which is typically doped with additional p-type dopants (typically boron atoms) to adjust the threshold voltage of the cell. Formed over the channel region 18 is a stacked gate structure which includes a gate oxide layer 20, a polysilicon floating gate 22 formed over gate oxide layer 20, an oxide-nitride-oxide (ONO) layer 24 formed over floating gate 22, and a polysilicon control gate 26 formed over ONO layer 24. With 0.8 micron technology, where the channel length is approximately 0.8 microns, memory cell 10 is conventionally programmed to store one of two logic levels, i.e., a logic "1" or a logic "0", by grounding the source region 12 and substrate 16, applying a bias voltage VD in the range of 6-8 volts to the drain region 14, and applying a programming voltage VG of approximately 12 volts to control gate 26. FIG. 2 shows the result of these bias conditions on memory cell 10. As shown in FIG. 2, when the programming voltage is applied to control gate 26, a positive potential is induced on floating gate 22. The positive potential on floating gate 22, in turn, attracts electrons from the doped p-type atoms in the channel region 18 to the surface of substrate 16 to form a channel 30, and also repels holes from the doped p-type impurity atoms to form a depletion region 32. When the bias voltage VD is applied to the drain region 14, an electric field is established between the source and drain regions 12 and 14 in channel region 30 and depletion region 32. The electric field accelerates the electrons in channel 30 which, in turn, have ionizing collisions that form "channel hot electrons". The positive potential of floating gate 22 attracts these channel hot electrons which penetrate gate oxide layer 20 and begin accumulating on floating gate 22. The negative charge on floating gate 22 that results from the accumulated electrons, in turn, directly corresponds to the threshold voltage required to induce a defined current to flow through memory cell 10. Thus, when a large negative charge has accumulated on floating gate 22, the threshold voltage of the cell is large because a larger positive voltage must be applied to control gate 26 to compensate for the negative charge on floating gate 22. Similarly, if the cell has not been programmed, the threshold voltage is small because a smaller voltage will induce current to flow through the cell. The above-described process is self-limiting because, as the number of electrons on floating gate 22 increases, the potential of floating gate 22 decreases until the potential on floating gate 22 is insufficient to create channel 30. When memory cell 10 is read, a reference threshold voltage is applied to control gate 26 to again induce a potential on floating gate 22. If memory cell 10 has not been programmed, the positive potential on floating gate 22 will cause channel 30 to again be formed. As a result, current flows from the drain region 14 to the source region 12 through channel 30. If memory cell 10 has been programmed, the potential on floating gate 22 is reduced by the accumulated electrons so that a much smaller current flows through channel 30. By then comparing the current to a reference current, the magnitude of the current can be inteφreted to be either a logic "1" or a logic "0". In other words, if the threshold voltage of the cell is larger than the reference threshold voltage, then one logic level is present, whereas if the threshold voltage of the ceil is smaller than the reference threshold voltage, then the other logic level is present. Historically, the density of memory arrays has been increased by reducing the feature sizes of the cells. As the feature sizes become ever smaller, however, this approach becomes more costly and more difficult to implement. Another approach to increasing the density of a memory array is to program each cell to store more than two logic levels. As stated above, memory cells are conventionally programmed to store one of two logic levels, i.e., a logic " 1 " or a logic "0". However, if each memory cell could be programmed to store a logic "00", "01", "10", or "11", then the density of an array could be doubled without changing the physical size of the array. In theory, multi-level programming could be accomplished by varying the length of time that the programming voltage is applied to the control gate. Thus, for example, if the programming voltage was applied for a first time period, the floating gate would reach a corresponding first negative charge level. Similarly, if the programming voltage was applied for either a second, third, or fourth time period, the floating gate would reach either a corresponding second, third, or fourth negative charge level. The problem with this type of multi-level programming, however, is that it is difficult to precisely control the number of electrons that accumulate on the floating gate because the reduced floating gate potential that results from the electrons accumulating on the floating gate causes fewer electrons to be attracted to the floating gate, thereby causing the number of electrons accumulated on the floating gate to vary over time. The greater the variation, the more difficult it is to compare current levels during a read operation and determine which logic level is present. As a result, there is a need for a method to accurately program a cell to store multiple logic levels. SUMMARY OF THE INVENTION The present invention provides a method for accurately programming a single memory cell to store one of three or more logic levels. As a result, rather than storing either a logic "1" or a logic "0", a single memory cell can store, for example, either a logic "0-0", a "0-1", a "1-0", or a "1-1". In the present invention, the memory cell includes a source formed in a substrate, a drain formed in the substrate a distance apart from the source, a floating gate formed over the substrate, and a control gate formed over the floating gate. A method for programming the memory cell to store one of three or more logic levels as one of three or more threshold voltages includes the step of selecting one of three or more programming voltages where the three or more programming voltages correspond to the three or more threshold voltages. After one of the programming voltages is selected, a first voltage is applied to the substrate, and a second voltage is applied to the source so that the source-to-substrate junction is forward-biased. In addition, a third voltage is applied to the drain so that the drain-to-substrate junction is reverse-biased. After this, the selected programming voltage is applied to the control gate of the memory cell for a predetermined time. A better understanding of the features and advantages of the present invention will be obtained by reference to the following detailed description and accompanying drawings which set forth an illustrative embodiment in which the principals of the invention are utilized. BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a cross-sectional drawing illustrating a conventional one-micron, n-channel, floating gate memory cell 10. FIG. 2 is a cross-sectional drawing illustrating the bias conditions that result when memory cell 10 is programmed. FIG. 3 is a cross-sectional drawing illustrating an n-channel, floating-gate memory cell 100 in accordance with the present invention. FIG. 4 is a graphical representation illustrating a cell programming characterization curve. FIG. 5 is a graphical representation illustrating the current and the amount of negative charge injected onto the floating gate for initial and final floating-gate voltages V j and V2- FIG. 6 is a graphical representation illustrating the current of negative charge injected onto the floating gate for initial floating-gate voltages V -VJ J and final voltage V4. FIG. 7 is a graphical representation illustrating a series of experimental results. DETAILED DESCRIPTION FIG. 3 shows an n-channel, floating-gate memory cell 100 in accordance with the present invention. As described in greater detail below, one of three or more logic levels can be programmed into memory cell 100 by applying one of a corresponding three or more programming voltages to the control gate of memory cell 100 when the cell is biased to enhance the formation of substrate hot electrons. As shown in FIG. 3, when formed in a p-type substrate 110, memory cell 100 includes an n-type source region 1 12, an n-type drain region 114, and a channel region 116 formed between the source and drain regions 112 and 1 14. Channel region 116, in turn, is doped with p-type impurity atoms to adjust the threshold voltage of the cell. Memory cell 100 also includes a first insulation layer 120 formed over channel region 116, a floating gate 122 formed over insulation layer 120, a second insulation layer 124 formed over floating gate 122, and a control gate 126 formed over insulation layer 124. In accordance with the present invention, memory cell 100 is programmed to store one of three or more logic levels by forward-biasing the source-to-substrate junction, reverse-biasing the drain-to- substrate junction, and applying one of a corresponding three or more programming voltages to the control gate during programming. With 0.6 micron technology, memory cell 100 preferably utilizes a source voltage which is 0.3 to 0.8 volts less than the substrate voltage, and a drain voltage which is 4 to 6 volts greater than the substrate voltage. Thus, for example, a source voltage of -0.7 volts, a drain voltage of 5 volts, and a substrate voltage of 0 volts satisfy these conditions. When more advanced technologies with smaller feature sizes are utilized, the range of preferable bias voltages can be reduced accordingly. Thus, for example, with 0.3 micron technology, memory cell 100 preferably utilizes a source voltage which is 0.2 to 0.6 volts less than the substrate voltage, and a drain voltage which is 2 to 4 volts greater than the substrate voltage. As a result of these bias conditions, one advantage of the present invention is that substantially lower voltages can be utilized to program cell 100 than the 12 volts that are conventionally used to program a cell. Thus, the method of the present invention is particularly applicable to low-power applications. In operation, when one of the programming voltages is applied to control gate 126, a positive potential is induced on floating gate 122 which, in turn, attracts electrons from the doped p-type atoms in channel region 116 to the surface of substrate 1 10 to form a channel 130. This potential repels holes from the doped impurity atoms and forms a depletion region 132. When the source and drain voltages are applied, an electric field is established between the source and drain regions 112 and 114. The electric field, as with conventional programming, accelerates the electrons in channel 130 which, in turn, have ionizing collisions that form channel hot electrons. The positive potential on floating gate 122 attracts these channel hot electrons which penetrate insulation layer 120 and begin accumulating on floating gate 122. In addition to the formation of channel hot electrons, the present invention utilizes the bias conditions to enhance the formation of substrate hot electrons which also collect on floating gate 122. Due to the reverse-bias across the drain-to-substrate junction, a small, essentially constant leakage current flows from drain 114 to substrate 112. The leakage current results from thermally-generated minority carriers in the depletion region of the drain-to-substrate junction where electrons are swept to drain 114 and holes are swept to substrate 110 under the influence of the electric field. The magnitude of the leakage current is limited by the number of available minority carriers that are present in substrate 110. The number of minority carrier electrons which are present in substrate 1 10, in turn, can be greatly increased by forward-biasing the source-to-substrate junction. With forward-biasing, the potential energy barrier across the source-to-substrate junction is lowered. This, in turn, allows a larger number of electrons to diffuse or be injected from source 112 to substrate 110 than is possible under equilibrium conditions. Until saturation is reached, the larger the forward-bias, the larger the number of electrons diffusing from source 112 to substrate 110. Thus, by increasing the number of minority carrier electrons in substrate 1 10 as a result of the forward-biased source-to-substrate junction, the number of minority carrier electrons which are accelerated towards drain 1 14 under the influence of the electric field is also increased. Some of the minority electron carriers in substrate 1 10 that are accelerated by the electric field, in turn, have ionizing collisions that form "substrate hot electrons". The positive potential on floating gate 122 also attracts these substrate hot electrons which penetrate insulation layer 120 and begin accumulating on floating gate 122. Thus, as a result of this "bi-polar transistor effect", the number of minority carrier electrons which are accelerated towards drain 1 14 under the influence of the electric field, and thereby the number of substrate hot electrons that are formed, are controlled by the magnitude of the forward-bias across the source-to-substrate junction and the magnitude of the electric field. For example, with a constant electric field, a 0.3 volt forward-bias across the source-to-substrate junction will cause a relatively low number of substrate hot electrons to be formed, whereas a 0.8 volt forward-bias will cause a substantially larger number of substrate hot electrons to be formed. FIG. 4 shows a graphical representation that illustrates a cell programming characterization curve. As shown by lines L j and I_2 in FIG. 4, the voltage (V) on the floating gate influences the number of hot electrons (I) that are injected onto the floating gate. Conventionally, the primary consideration in programming memory cells is the time required to place a defined amount of negative charge on the floating gate of the cell. As a result, the typical memory cell is designed to utilize an initial floating gate voltage V j and a final floating gate voltage V 2 that are positioned on opposite sides of the peak of the curve shown in FIG. 4, thereby taking advantage of the maximum injection of hot electrons onto the floating gate. As described, the initial floating gate voltage V j represents the voltage capacitively coupled to the floating gate from the control gate, while the final floating gate voltage V 2 represents the initial voltage V j reduced by the accumulated negative charge. FIG. 5 illustrates the amount of negative charge injected onto the floating gate for initial and final voltages V, and V 2 - As shown in FIG. 5, the amount of charge injected on the floating gate can be determined by integrating under the curve from the initial voltage V, at time _Q to the final voltage V 2 at time t | . More importantly, however, FIG. 5 illustrates that any variation in the timing will cause a greater or lesser amount of negative charge to be injected onto the floating gate. Thus, for example, if the programming is terminated at time - rather than time t j , a greater amount of charge will be injected. With conventional programming, this additional (or lesser) amount of negative charge does not present any problems because the cell is only being programmed to one of two logic levels. Thus, as long as the cell is programmed to have a minimum amount of charge, any additional charge is acceptable. However, with multi-level programming, the accumulation of additional negative charge makes it difficult to determine which logic level is represented by the charge. Thus, to insure that the charge is within a defined range, the timing must be precisely controlled. As stated above, this timing is very difficult to control. The present invention achieves multiple levels of injected charge by utilizing one of a plurality of initial voltages. Since the initial voltages are defined by the voltage capacitively coupled to the floating gate from the control gate, the initial voltages are selected by selecting one of a plurality of control voltages. For example, referring again to FIG. 4, voltage VQQ, VQJ, V|Q, or V j • may be selected as the initial voltage by selecting a corresponding control gate voltage. As further shown in FIG. 4, the present invention utilizes voltage V 4 as the final voltage. The significance of utilizing voltage V 4 as the final voltage can be seen in FIG. 6. FIG. 6 shows a graphical representation that illustrates the amount of negative charge injected onto the floating gate for initial voltages VQQ-V j J and final voltage V 4 . As shown in FIG. 6, the amount of negative charge injected on the floating gate can be determined by integrating under the curve from each of the initial voltages VQQ-V- - at time tø to the final voltage V 4 at time t 4 . FIG. 6, however, also illustrates that because the injected charge converges towards zero for each initial voltage Q-V , , , the injected charge is largely insensitive to timing variations. Thus, the floating gate can still have one of a plurality of discrete levels of injected charge if programming is terminated anytime between time t 4 and t«, because the amount of additional charge during this time is so small. One drawback of the approach, as described, is that the magnitude of the injected current drops as the voltage on the floating gate approaches the final voltage V 4 (see FIG. 4). As a result, it takes a greater amount of time to program the cell. In accordance with the present invention, however, line LI of FIG.4 can be altered, as shown by line L2, by increasing the formation of substrate hot electrons as described above. Thus, although the time required to program a memory cell in accordance with the present invention remains longer than conventional programming, the formation of substrate hot electrons substantially narrows the time difference. FIG. 7 shows a graphical representation that illustrates a series of experimental results. As shown in FIG. 7, when zero volts were applied to the control gate, the charge on the floating gate converged to a threshold voltage (VTP) of approximately 2.5 volts from an initial threshold voltage of 1.5 volts within approximately 2 seconds. Similarly, when one, two, and three volts were applied to the control gate, the charge on the floating gate converged to threshold voltages of approximately 3.5, 4.5, and 5.5 volts, respectively, within approximately 2 seconds. Although an initial threshold voltage of 1.5 volts was utilized in the above experiment, any initial threshold voltage after erase may be utilized. Since the charge on the floating gate converges to a stable value which corresponds to a defined threshold voltage within approximately 2 seconds, a single floating gate memory cell can be programmed to have one of a plurality of threshold voltages by applying the corresponding voltage to the control gate during programming. As a result, a single floating gate memory cell can be utilized to store two or more bits of data. For example, a 0-0 could be represented by a threshold voltage of 2.5 volts, while a 0-1 could be represented by a threshold voltage of 3.5 volts. Similarly, a 1-0 could be represented by a threshold voltage of 4.5 volts, while a 1-1 could be represented by a threshold voltage of 5.5 volts. As further shown in FIG. 7, the experimental results also show that changes in the control gate voltage are linearly related to changes in the threshold voltage, i.e., a one volt increase in the control gate voltage increases the threshold voltage by one volt. As a result, memory cell 100 is not limited to representing two bits, but can represent any number of bits depending on the sensitivity of the current sense detectors utilized to discriminate one threshold voltage from another. Furthermore, even a continuous analog level can be stored in a cell as a threshold voltage. Thus, for example, a 0-0-0 could be represented by a threshold voltage of 2.5 volts, while a 0-0-1 could be represented by a threshold voltage of 3.0 volts. Similarly, a 0-0-0-0 could be represented by a threshold voltage of 2.5 volts, while a 0-0-0-1 could be represented by a threshold voltage of 2.75 volts. As shown in FIG. 7, the threshold voltages converge in about two seconds when -0.7 volts is applied to source region 112 and 5 volts is applied to drain region 1 14. The magnitude of the minority carrier injection, however, can be easily manipulated by varying the source and drain voltage levels. Thus, more electrons can be injected from source region 112 into substrate 110, thereby reducing the convergence time, by reducing the voltage on source region 112, or increasing the voltage on drain region 114. As stated above and as shown in FIG. 7, one advantage of the present invention is that each memory cell can be programmed to store multiple levels by utilizing a programming voltage that is considerably less than the programming voltage typically used, i.e., less than five volts in the present invention compared to the approximately 12 volts that are conventionally used to program a cell. In addition to providing a substantial power savings for low power applications, such as notebook computers, the present invention eliminates the need to form charge pumps on memory chips to produce the programming voltage, i.e., 12 volts. As is well known, charge pumps can consume a significant area, i.e., up to 30% of the total die area of a memory chip. Thus, by eliminating the need for charge pumps, the present invention significantly reduces the area required for a memory cell, and therefore the cost of a memory. The elimination of high programming voltages also leads to an increase in the density of a memory array because less isolation is required between both memory cells and the peripheral circuitry. As a result, the present invention substantially reduces the size of the array and periphery circuits. It should be understood that various alternatives to the embodiment of the invention described herein may be employed in practicing the invention. For example, although the present invention has been described in terms of an n-channel, floating-gate memory cell, the present invention equally applies to a p-channel, floating-gate memory cell. Thus, it is intended that the following claims define the scope of the invention and that methods and structures within the scope of these claims and their equivalents be covered thereby.

Claims: WHAT IS CLAIMED IS: I. A method for programming a single floating-gate memory cell to store one of three or more threshold voltages, the memory cell having a source formed in a substrate, a drain formed in the substrate a distance apart from the source, a floating gate formed over the substrate, and a control gate formed over the floating gate, the method comprising the steps of: selecting one of three or more programming voltages as a selected programming voltage, the three or more programming voltages corresponding to said three or more threshold voltages; applying a first voltage to the substrate; applying a second voltage to the source so that a source-to-substrate junction is forward-biased; applying a third voltage to the drain so that a drain-to-substrate junction is reverse-biased; and applying the selected programming voltage to the control gate of the memory cell for a predetermined time. 2. The method of claim 1 wherein the second voltage is less than the first voltage by 0.3 or more volts. 3. The method of claim 1 wherein a maximum programming voltage is less than or equal to five volts. 4. The method of claim 1 wherein the predetermined time is less than or equal to two seconds. 5. The method of claim 1 wherein the third voltage is greater than the first voltage by two or more volts.

Inventors: BERGEMONT ALBERT, CHI MIN-HWA, BERGEMONT, ALBERT, CHI, MIN-HWA
Applicants: NAT SEMICONDUCTOR CORP, NATIONAL SEMICONDUCTOR CORPORATION
Application year: 1996
Main classification: G11C11
IPC codes: G11C11, G11C16