Semiconductor memory device capable of storing plural-bit data in a single memory cell

Abstract: Each of memory cells has one MOS transistor including a drain region, a source region, a channel region and a gate electrode. An impurity-introducing area of the channel region is varied in the width direction of the channel region, thereby storing data of plural bits in the memory cell.

Description: The present invention relates to a method for programming data of plural bits into a semiconductor memory device. In a conventional read-only memory (ROM), one memory cell is constituted by one transistor. There are two methods of storing data in each memory cell. In a first method, data is stored in the memory cell by selecting one of high-level and low-level threshold voltages (Vth) of the transistor as the memory cell. In a second method, data is stored in the memory cell by whether the drain of the transistor as the memory cell is connected to a column line or not. When data was stored by the first method, the data can be read by applying a predetermined potential to a row line connected to the gate of the associated memory cell transistor. In this case, the transistor with the high-level threshold voltage is turned off and the transistor with the low-level threshold voltage is turned on. Thus, data "1" or data "0" is read from the memory cell. When data was stored by the second method, the data can be similarly read by applying a predetermined potential to the row line. When a predetermined potential is applied to the row line, the transistor is rendered conductive. If the drain of the transistor is connected to the column line, the column line is discharged through this memory cell transistor. If the drain is not connected to the column line, the column line is not discharged even if the memory cell transistor is rendered conductive. Thus, the column line has two potential levels, i.e. a discharged level and a charged level, depending on whether the drain of the memory cell transistor is connected to the column line. Thereby, data "1" or data "0" is read out. In either method, however, data of only one bit can be stored in one memory cell. Thus, in this prior art, in order to increase the memory capacity, the chip size increases inevitably. In order to reduce the chip size, a memory cell in which data of two bits is stored has been proposed. FIG. 16 shows an example of this type of conventional semiconductor memory device. The data of two bits is stored in a single memory cell. In FIG. 16, reference numeral 1 denotes a column decoder for decoding column address signals a 0 , /a 0 , a 1 , /a 1 ,...; numeral 2 denotes insulated gate type field effect transistors (hereinafter referred to as "MOS transistors") for column selection, which are controlled by decode outputs from the column decoder 1; numeral 3 denotes column lines; numeral 4 denotes a row decoder for decoding row address signals A 1 , /A 1 , A 2 , /A 2 ,..., excluding lowest-bit signals A 0 and /A 0 ; numeral 5 denotes row lines; numeral 6 denotes MOS transistors constituting memory cells driven selectively by the row lines 5; numeral 7 denotes a load MOS transistor for charging the column lines 3; and symbol P denotes a common node of the MOS transistors 2. The MOS transistors 2 and 6 are N-channel enhancement type transistors, and the transistor 7 is an N-channel depletion type or a P-channel enhancement type transistor. As is shown in FIG. 17, for example, the threshold voltage Vth of each memory cell transistor 6 is preset at one of four threshold voltages Vth1 to Vth4 (Vth4 < Vth3 < Vth2 < Vth 1) in accordance with data units D0 and D1 of two bits to be stored. FIG. 18 shows the structure of a circuit for outputting data stored in each memory cell by detecting the potential of each column line 3. In FIG. 18, a terminal 11 is connected to the common node P of the column selection transistors 2. A potential generating circuit 12 comprises an enhancement type MOS transistor 15, an enhancement type MOS transistor 16 and a depletion type MOS transistor 17. The threshold voltage of the transistor 15 is set at one of the aforementioned four thresholds, i.e. Vth4. A power supply voltage VC is constantly applied to the gate of the transistor 15, and the transistor 15 is equivalent to the selected memory cell transistor 6 having the threshold voltage Vth4. The enhancement type MOS transistor 16 has the same dimensions as the column selection MOS transistor 2, and the transistor 16 is normally set in the on-state with voltage VC applied to the gate thereof. The depletion type MOS transistor 17 has the same dimensions as the load MOS transistor 7. The potential generating circuit 12 generates a potential V 1 equal to the potential of the node P at the time the column line 3 has been discharged through the memory cell transistor 6 having the threshold voltage Vth4. Like the potential generating circuit 12, potential generating circuits 13 and 14 generate potentials V 2 and V 3 , respectively. The potentials V 2 and V 3 are equal to the potential of the column line 3 at the time the column line 3 has been discharged through the memory cell transistor 6 having the threshold voltage Vth3 and Vth2, respectively. In the potential generating circuit 13, the MOS transistor 15 is replaced with an enhancement type MOS transistor 18 set at the threshold voltage Vth3. In the potential generating circuit 14, an enhancement type MOS transistor 19 set at the threshold voltage Vth2 is substituted. The generated voltages V 1 to V 3 have the relationship, V 1 < V 2 < V 3 . Each of voltage comparing circuits 20, 21 and 22 comprises two enhancement type MOS transistors 23 and 24 and depletion type MOS transistors 25 and 26. The voltage comparing circuit 20 compares potential Vp at the node P applied to the terminal 11 with the output potential V 1 of the potential generating circuit 12. When the potential Vp is equal to or lower than V 1 , the potential comparing circuit 20 outputs a "1"-level signal a, and when the potential Vp is higher than V 1 , it outputs a "0"-level signal a. The voltage comparing circuit 21 compares the potential Vp at node P with the output potential V 2 of the potential generating circuit 13. When the potential Vp is equal to or lower than V 2 , the potential comparing circuit 21 outputs a "1"-level signal b , and when the potential Vp is higher than V 2 , it outputs a "0"-level signal b . The voltage comparing circuit 22 compares the potential Vp at node P with the output potential V 3 of the potential generating circuit 14. When the potential Vp is equal to or lower than V 3 , the potential comparing circuit 22 outputs a "1"-level signal c , and when the potential Vp is higher than V 3 , it outputs a "0"-level signal c. Reference numerals 27, 28 and 29 denote NOR logic circuits, and numeral 30 denotes an inverter circuit 30. The output signal c of the voltage comparing circuit 22 and the lowest-bit row address signal A 0 are supplied to the NOR logic circuit 27. The output signal b of the voltage comparing circuit 21 is supplied to the NOR logic circuit 28 via the inverter circuit 30, and the row address signal /A 0 is also supplied to the NOR logic circuit 28. Output signals from the NOR logic circuits 27 and 28 as well as the output signal a of the voltage comparing circuit 20 are supplied to the NOR logic circuit 29. An output buffer circuit (BC) 31 detects the output signal from the NOR logic circuit 29 and outputs "1"-level data or "0"-level data. A chip selection signal CS controls output of data from the output buffer circuit 31. All the transistors are N-channel transistors, like those shown in FIG. 16. The operation of the circuit with the above structure will now be described. When row address signals are input to the row decoder 4, the row decoder 4 selects one of the row lines 5 and sets it at "1" level. When column address signals are input to the column decoder 1, the column decoder 1 selects and activates one of the column selection transistors 2. The memory cell transistor 6 located at the intersection of the selected column line 3 and row line 5 is driven, and this column line 3 is charged or discharged through the memory cell transistor 6. If the threshold voltage of the transistor 6 is Vth4, the potential of the column line 3 is V 1 at the time the column line 3 has been discharged. The potential V 1 of the column line 3 is compared with voltages V 1 , V 2 and V 3 in the voltage comparing circuits 20, 21 and 22. As a result, all signals a , b and c are set at "1" level. At this time, since the "1" level signal a is input to the NOR logic circuit 29, the output of the NOR logic circuit 29 is "0" level, irrespective of the output signals of the NOR logic circuits 27 and 28. If the output buffer circuit 31 is activated by the chip selection signal CS, the "0" level signal is output as data stored in the selected memory cell transistor 6 from the output buffer circuit 31. In other words, whether the address signal A 0 is at "0" level or "1" level, the output signal of the NOR logic circuit 29 is at "0" level and a "0" level signal is output from the output buffer circuit 31. Thus, data D0 and D1 (D0 = D1 = "0") of two bits, as shown in FIG. 17, are output from one memory cell. If the threshold voltage of the memory cell transistor 6 located at the intersection of the selected column line 3 and row line 5 is Vth3, the potential of the column line 3 is V 2 at the time the column line 3 has been discharged by the memory cell transistor 6. In this case, only the output signal a of the voltage comparing circuit 20 is at "0" level, and the output signals b and c of the other two voltage comparing circuits 21 and 22 are at "1" level. If the row address signal A 0 = "1" and the row address signal /A 0 = "0", the output signal of the NOR logic circuit 28 is "1" level since the output signal of the inverter circuit 30 is "0" level. The output signal of the NOR logic circuit 29 is "0" level. Accordingly, the "0" level signal is output from the output buffer circuit 31. On the other hand, if the row address signal A 0 = "0" and the row address signal /A 0 = "1", the output signals of both NOR logic circuits 27 and 28 are "0" level. Since the signal a is also "0" level, the output signal of the NOR logic circuit 29 is "1" level. Accordingly, the "1" level signal is output from the output buffer circuit 31. In this case, data D0 and D1 (D0 = "0", D1= "1") of two bits, as shown in FIG. 17, are output from one memory cell in accordance with the "1" level and "0" level of the address signal A 0 . If the threshold voltages of the memory cell transistors are Vth2 and Vth1, the potentials of the column line 3 are V 3 and V 4 , respectively, at the time the charge or discharge of the column line 3 has been completed. When the potential of the column line reaches V 3 , the output signals a and b of both voltage comparing circuits 20 and 21 are "0" level and the output signal c of the voltage comparing circuit 22 is "1" level. In this case, the output signal of the NOR logic circuit 29 is "1" level, whether the row address signal A 0 = "1" and the row address signal /A 0 = "0", or the row address signal A 0 = "0" and the row address signal /A 0 = "1", because all input signals of this NOR logic circuit 29 are "0" level. Thus, data D0 and D1 (D0 = D1 = "1") of two bits, as shown in FIG. 17, are output from one memory cell. On the other hand, when the potential of column line 3 reaches V 4 , data DO and D1 (D0 = "1", D1 = "0") of two bits, as shown in FIG. 17, are output from one memory cell. Specifically, when all the signals a , b and c are "0" level and when the address signal A 0 = "1" and the address signal /A 0 = "0", the output of the NOR logic circuit 27 is "0" level and the output of the inverter circuit 30 is "1" level. Accordingly, the output of the NOR logic circuit 28 is "0" level. Thus, all input signals to the NOR logic circuit 29 are "0" level, and the output of the NOR logic circuit 29 is "1" level. Furthermore, if the address signal A 0 = "0" and the address signal /A 0 = "1", all inputs to the NOR logic circuit 27 are "0" level. Thus, the output of the NOR logic circuit 27 is "1" level. Since one of the inputs to the NOR logic circuit 29 is "1" level, the output from the NOR logic circuit 29 is "0" level. Accordingly, when the address signal A 0 = "1", the "1" level signal is output as the stored data of the selected memory cell from the output buffer circuit 31. When the address signal A 0 = "0", the "0" level signal is output as the data from the output buffer circuit 31. In other words, if the column line potential is V 4 , data D0 and D1 (D0 = "1", D1 = "0") of two bits, as shown in FIG. 17, are output from one memory cell. With the above circuit, two-bit data of two addresses is stored in one memory cell. Thus, double data can be stored in the memory cell without increasing the chip size. In other words, the chip size can be remarkably reduced, with the same memory capacity. In the above circuit, the potential of the column line 3 at the time of completion of charge or discharge is determined by presetting the threshold voltage of the memory cell transistor 6 at one of four threshold voltages. The potential of the column line 3 at the time of completion of charge or discharge can be determined by providing four kinds of channel widths W1, W2, W3, W4 of the transistors 6, as shown in FIG. 19, or by providing four kinds of channel lengths L1, L2, L3, L4, as shown in FIG. 20, instead of the four kinds of threshold voltages Vthl, Vth2, vth3 and Vth4. When the potentials of the column lines 3 are determined by the channel widths of the transistors 6, if the channel widths have the relationship of W4 < W3 < W2 < W1, the potentials of the column lines 3 at the time of completion of charge or discharge, i.e. the potentials Vp at the node P, have the relationship of Vp1 < Vp2 < Vp3 < Vp4. The potentials Vp1 to Vp4 are equal to the potentials of the column lines charged or discharged by the transistors of channel widths W1 to W4, respectively. When the potentials of the column lines 3 are determined by the channel lengths of the transistors 6, if the channel lengths have the relationship of L1 < L2 < L3 < L4, the potentials Vp at the time of completion of charge or discharge have the relationship of Vp1 < Vp2 < Vp3 < Vp4. The potentials Vpl to Vp4 are equal to the potentials of the column lines charged or discharged by the transistors of channel lengths L1 to L4, respectively. When the potentials of the column lines 3 are to be set by the channel widths or channel lengths of the transistors 6, the channel widths of the transistors 15, 18 and 19 of the potential generating circuits 12, 13 and 14 need to be set at W1, W2 and W3, respectively, or the channel lengths thereof need to be set at L1, L2 and L3, respectively. In each case, the memory size can be reduced. In the case of setting the potentials of the column lines 3 at the time of completion of charge or discharge at four levels by the threshold voltages of the transistors 6, the memory cell size can be reduced to a minimum. On the other hand, at least three manufacturing steps need to be additionally provided at the time of setting four threshold voltages, i.e. at the time of writing data. However, if the data of the memory cell is determined by providing four channel widths or four channel lengths of the transistors 6, the increase in number of manufacturing steps can be prevented. In the case of storing data by varying the channel width of the transistor, the number of manufacturing steps does not increase. However, since the step of setting the channel width is performed at the early stage of manufacture, a long period of time of manufacture is required from the reception of a client's order to the delivery of a finished product. In the case of varying the channel length, data is stored at the time of forming gate electrodes. Thus, the time of manufacture is less than in the case of varying the channel width, but is greater than in the case of storing data by varying the threshold voltage. Specifically, in the case of storing data by varying the threshold voltage, impurities for determining the threshold voltage of the memory cell are introduced by ion implantation in the channel region through the gate electrode after the gate electrode of the memory cell is formed. Accordingly, the time of manufacture is made less than in the case of varying the channel width or channel length. However, since data is stored by varying the threshold voltage, a plurality of masks need to be used. In addition, ion implantation with different dosages must be performed with different masks according to data to be stored, and thus the number of manufacturing steps increases. A method for programming data of plural bits into a memory cell by ion implantation is known from EP-A-0 448 141. The object of the present invention is to provide a method for programming data of plural bits into a semiconductor memory device wherein an increase in manufacturing steps can be prevented, a time period for delivery can be decreased, data of plural bits can be stored in one memory cell, and stored data of plural bits can be read out exactly. This object is achieved by the present invention as defined in any one of the appended claims 1 to 3. More specifically, in the present invention, data of plural bits is stored in one memory cell by varying an impurity-introducing area of a channel region, in which impurities are introduced through a gate electrode, in accordance with data to be stored. The impurity-introducing region is, for example, a region extending from a drain region to a source region with a predetermined width in the channel width direction, or a region extending over the entire channel width and having a predetermined length in the channel length direction. The impurity-introducing area is set in one of four states, thereby storing two-bit binary data. This invention can be more fully understood from the following detailed description when taken in conjunction with the accompanying drawings, in which: FIGS. 1A and 1B show a memory cell for explaining a first embodiment of the present invention, in which FIG. 1A is a plan view and FIG. 1B is a cross-sectional view taken along line 1B-1B in FIG. 1A; FIGS. 2A and 2B illustrate a step of writing data in the memory cell shown in FIGS. 1A and 1B, in which FIG. 2A is a plan view and FIG. 2B is a cross-sectional view taken along line 2B-2B in FIG. 2A; FIGS. 3A and 3B illustrate a step of writing data in the memory cell shown in FIGS. 1A and 1B, in which FIG. 3A is a plan view and FIG. 3B is a cross-sectional view taken along line 3B-3B in FIG. 3A; FIGS. 4A and 4B illustrate a step of writing data in the memory cell shown in FIGS. 1A and 1B, in which FIG. 4A is a plan view and FIG. 4B is a cross-sectional view taken along line 4B-4B in FIG. 4A; FIGS. 5A and 5B illustrate a step of writing data in the memory cell shown in FIGS. 1A and 1B, in which FIG. 5A is a plan view and FIG. 5B is a cross-sectional view taken along line 5B-5B in FIG. 5A; FIGS. 6A and 6B show a memory cell for explaining a second embodiment of the present invention, in which FIG. 6A is a plan view and FIG. 6B is a cross-sectional view taken along line 6B-6B in FIG. 6A; FIGS. 7A and 7B illustrate a step of writing data in the memory cell shown in FIGS. 6A and 6B, in which FIG. 7A is a plan view and FIG. 7B is a cross-sectional view taken along line 7B-7B in FIG. 7A; FIGS. 8A and 8B illustrate a step of writing data in the memory cell shown in FIGS. 6A and 6B, in which FIG. 8A is a plan view and FIG. 8B is a cross-sectional view taken along line 8B-8B in FIG. 8A; FIGS. 9A and 9B illustrate a step of writing data in the memory cell shown in FIGS. 6A and 6B, in which FIG. 9A is a plan view and FIG. 9B is a cross-sectional view taken along line 9B-9B in FIG. 9A; FIGS. 10A and 10B illustrate a step of writing data in the memory cell shown in FIGS. 6A and 6B, in which FIG. 10A is a plan view and FIG. 10B is a cross-sectional view taken along line 10B-10B in FIG. 10A; FIGS. 11A and 11B show a memory cell for explaining a third embodiment of the present invention, in which FIG. 11A is a plan view and FIG. 11B is a cross-sectional view taken along line 11B-11B in FIG. 11A; FIGS. 12A and 12B illustrate a step of writing data in the memory cell shown in FIGS. 11A and 11B, in which FIG. 12A is a plan view and FIG. 12B is a cross-sectional view taken along line 12B-12B in FIG. 12A; FIGS. 13A and 13B illustrate a step of writing data in the memory cell shown in FIGS. 11A and 11B, in which FIG. 13A is a plan view and FIG. 13B is a cross-sectional view taken along line 13B-13B in FIG. 13A; FIGS. 14A and 14B illustrate a step of writing data in the memory cell shown in FIGS. 11A and 11B, in which FIG. 14A is a plan view and FIG. 14B is a cross-sectional view taken along line 14B-14B in FIG. 14A; FIGS. 15A and 15B illustrate a step of writing data in the memory cell shown in FIGS. 11A and 11B, in which FIG. 15A is a plan view and FIG. 15B is a cross-sectional view taken along line 15B-15B in FIG. 15A; FIG. 16 shows a part of a data read-out circuit applied to a conventional memory cell; FIG. 17 shows a relationship between data stored in the conventional memory cell and threshold voltages; FIG. 18 shows another part of the data read-out circuit as shown in FIG. 16; FIG. 19 is a plan view showing an example of the conventional memory cell; and FIG. 20 is a plan view showing another example of the conventional memory cell. Preferred embodiments of the present invention will now be described with reference to the accompanying drawings. FIGS. 1A and 1B show a memory cell for explaining a first embodiment of the present invention and illustrate a method of writing data in the memory cell. In FIG. 1A, symbol G denotes a gate, S a source, and D a drain. FIG. 1B shows the state of the memory cell prior to data write. Field insulating films 52 and 53 are formed in a surface region of a substrate 51. A gate insulating film 54 is formed between the field insulating films 52 and 53. The gate G is formed on the field insulating films 52 and 53 and gate insulating film 54. Impurities 55 are introduced into the substrate 51 below the field insulating films 52 and 53. Impurities 56 are introduced into a channel region CH within the substrate 51 to set a predetermined threshold voltage. For example, when the memory cell is constituted by an N-channel transistor, the threshold voltage of the memory cell may be determined by making use of ion implantation which is performed in a channel region of a peripheral N-channel enhancement transistor to set the threshold voltage of this peripheral transistor. In the present invention, after the gate electrode of the memory cell is formed, impurities are introduced by the ion implantation into the channel region of the memory cell through the gate electrode thereof. The substantial channel width is varied by the introduction of the impurities. Thus, only a manufacturing step of introducing impurities is added, and the delivery date can be made earlier. In the present invention, data of two bits are stored by four states shown in FIGS. 2A to 5B. Specifically, glass masks with different opening areas are used to control the location and amount of impurities to be introduced into the channel region of the memory cell. In FIG. 2A, numeral 61 denotes a mask. In the mask 61, a resist 62 is coated on a chip. An opening 63 is an area where the resist is exposed through a glass mask (not shown) and removed. The opening 63 has an area corresponding to the entire channel region CH. Using the mask 61, impurities are introduced into the channel region CH. FIG. 2B is a cross-sectional view taken along line 2B-2B in FIG. 2A, and shows the state in which impurities 64 are introduced into the entire channel region CH. The amount of introduced impurities is suitably controlled to set the threshold voltage of the memory cell so that the memory cell may not be turned on when it is selected. It may be considered that this memory cell has a channel width of zero, i.e. this memory cell has no channel. In FIG. 3A, a part of the channel region CH is covered with the resist 62 so that the impurities may not be introduced into the part of the channel region CH. In FIG. 4A, too, a part of the channel region CH is covered with the resist 62 so that the impurities may. not be introduced into the part of the channel region CH. In the structure of FIG. 4A, however, the width of the channel region covered with the resist 62 is greater than in the structure shown in FIG. 3A. Thus, the substantial channel width in FIG. 4A is greater than that in FIG. 3A. A larger current flows through the memory cell shown in FIGS. 4A and 4B, when selected, than through the memory cell shown in FIGS. 3A and 3B. In FIG. 5A, the entire surface of the memory cell is covered with the resist 62 so that the impurities 64 may not be introduced into the channel region CH of the memory cell. Thus, the threshold voltage of the memory cell of FIGS. 5A and 5B remains at an initially set value and therefore the largest current can flow through the memory cell when it is selected. In this embodiment, the substantial channel widths have the relationship of FIG. 2A < FIG. 3A < FIG. 4A < FIG. 5A. As has been described above in connection with the prior art, it should suffice if electric currents of four different levels flow through the selected memory cells of this type. Accordingly, the memory cell with the entire channel region doped with the impurities 64, as shown in FIG. 2A, may remain turned on, when it is selected. However, if this memory cell is so designed as to be turned off when selected, the differences in current value among the other memory cells can be increased. Thus, margins in current value can be advantageously increased at the time of data read-out. In the above embodiment, the threshold voltage of the memory cell is low in the early stage of manufacture. After the gate electrode is formed, the threshold voltage is increased by ion implantation at the time of data write. It is possible, however, to set the threshold voltage of the memory cell at a maximum value in the early stage of manufacture. In this case, the threshold value is decreased by introducing impurities after the gate electrode is formed. In order to increase the threshold voltage of the memory cell by ion implantation, boron, for example, may be introduced as impurities. In order to decrease the threshold value of the memory cell, phosphorus or arsenic, for example, may be introduced. According to the above embodiment, data is written after the gate electrode is formed. Thus, the memory cell may be semifinished up to the stage immediately prior to data write. Therefore, the time period from the reception of a client's order to the delivery of the finished product can be decreased. Furthermore, data is written in the memory cell only by introducing impurities into the channel region with use of a single glass mask having the opening 63 of a different area. Thus, this embodiment is advantageous in that only one manufacturing step is added. FIG. 6A and FIG. 6B show a memory cell for explaining a second embodiment of the invention. The structural elements common to those in the memory cells shown in FIGS. 1A to 5B are denoted by like reference numerals. In the first embodiment, the substantial channel width of the transistor constituting the memory cell is varied. By contrast, in the second embodiment, the threshold voltage is varied to change the substantial channel length. In the second embodiment, data of two bits are stored in four states shown in FIGS. 6A to 9B. In FIG. 6A, the mask 61 is provided with an opening 63 having an area corresponding to the entire channel region CH. Using the mask 61, impurities are introduced into the entire channel region CH through the gate G. FIG. 6B is a cross-sectional view taken along line 6B-6B in FIG. 6A. Impurities 64 are introduced into the entire channel region CH. In the case of this embodiment, the amount of impurities must be determined so as to set a threshold voltage at which the memory cell, when selected, is turned on. In FIG. 7A, a source (S)-side portion of the channel region is covered with the resist 62, thereby preventing impurities from being introduced into the source (S)-side portion of the channel region. If the amount of the impurities 64 is determined so that the memory cell shown in FIG. 6A, when selected, is not turned on, the memory cell is not also turned on in the case shown in FIGS. 7A and 7B. Consequently, data cannot be read out exactly. In FIG. 8A, a drain (D)-side portion of the channel region CH is covered with the resist, thereby preventing impurities from being introduced into the drain-side portion of the channel region. In the case of FIG. 7A, the source-side portion is covered with the resist to introduce impurities into the drain-side portion. Thus, the threshold voltage in the case of FIG. 7A is higher than that in the case of FIG. 8A, owing to a difference in body effect of the MOS transistor. Accordingly, the substantial channel length is greater in FIG. 7A than in FIG. 8A. Therefore, a greater current flows in the memory cell shown in FIG. 8A than in the memory cell shown in FIG. 7A, when these cells are selected. Of course, as in the first embodiment, the region covered with the resist shown in FIG. 7A may be extended toward the drain in FIG. 8A, thereby changing the substantial channel length. In FIG. 9A, the entire surface of the memory cell is covered with the resist 62, thereby preventing impurities 64 from being introduced into the channel region CH of the memory cell MC. Thus, the threshold voltage of the memory cell shown in FIGS. 9A and 9B remains at an initially set value, and the largest current can flow through the memory cell which is selected. Specifically, in this embodiment, the threshold voltages have the relationship, FIG. 6A > FIG. 7A > FIG. 8A > FIG. 9A. Thus, the substantial channel lengths have the relationship, FIG. 6A > FIG. 7A > FIG. 8A > FIG. 9A. It is desirable that the memory cell having the entire channel region CH doped with the impurities 64, as shown in FIG. 6B, is turned off when selected. In this case, the difference in value of a current flowing through the other three kinds of memory cells can be increased, and a data read-out margin can be advantageously increased. Accordingly, only the memory cell having the entire channel region doped with impurities may be designed to be turned off when selected, by preparing and using another glass mask as shown in FIG. 10A and FIG.10B for introducing the impurities. In each case, the threshold voltage of the memory cell is low in the early stage of manufacture. After the gate electrode is formed, the threshold voltage is increased by ion implantation at the time of data write. It is possible, however, to set the threshold voltage of the memory cell at a maximum value in the early stage of manufacture. In this case, the threshold value is decreased by introducing impurities after the gate electrode is formed. According to the second embodiment, four threshold voltages can be distinguished. Thus, it is possible to detect which one of the four kinds of memory cells is selected by measuring a source voltage of each memory cell having a drain connected to a power supply. Since the threshold voltages of the memory cells have the relationship, FIG. 6A > FIG. 7A > FIG. 8A > FIG. 9A, the source voltages of the memory cells have the relationship, FIG. 6A < FIG. 7A < FIG. 8A < FIG. 9A. FIGS. 11A to 15B show a memory cell for explaining a third embodiment of the invention. The structural elements common to those in the previous memory cells are denoted by like reference numerals. The memory cell employed in the third embodiment comprises two transistors, and the threshold voltages of the two transistors of each memory cell are set according to data. In FIG. 11B, a gate insulating film 54 includes first region 54a and second region 54b in the direction of channel width. The thickness of the first region 54a is less than that of the second region 54b. A gate G is formed on the gate insulating film 54. Accordingly, the memory cell MC includes first transistor T1 and second transistor T2 having gate insulating films of different thicknesses in accordance with the first region 54a and second region 54b, respectively. In FIG. 12A, the mask 61 with an opening 63 having an area corresponding to the entire channel region CH is used to introduce impurities into the entire channel region CH through the gate G. FIG. 12B is a cross-sectional view taken along line 12B-12B in FIG. 12A. Impurities 64 are introduced into the entire channel region CH. The amount of impurities 64 is determined so that both first transistor T1 and the second transistor T2 are turned off when this memory cell is selected. In FIGS. 13A and 13B, the channel region CH of the second transistor T2 is covered with the resist 62, thereby preventing impurities 64 from being introduced into the channel region CH of the second transistor T2. The threshold voltage of the first transistor T1 doped with the impurities 64 is higher than that of the second transistor T2. When the memory cell MC is selected, the first transistor T1 is turned off and the second transistor T2 is turned on. In FIG. 14A, the channel region CH of the first transistor T1 is covered with the resist 62, thereby preventing impurities 64 from being introduced into the channel region of the first transistor T1. Thus, the threshold voltage of the second transistor T2 doped with the impurities 64 is higher than that of the first transistor T1. When the memory cell is selected, the first transistor T1 is turned on and the second transistor T2 is turned off. In the case of the structure shown in FIG. 14B, the substantial channel width is greater than that in the structure shown in FIG. 13B. A greater current flows through the memory cell shown in FIG. 14B than through the memory cell shown in FIG. 13B, when these cells are selected. The reason for this is that the gate insulating film of the transistor T1 is less than that of the transistor T2 and therefore the threshold voltage of the transistor T1 is lower than that of the transistor T2 if the amount of impurities in the channel region CH is the same. Thus, if the transistors T1 and T2 are equal in channel length and channel width, a greater current flows through the memory cell shown in FIG. 14B than through the memory cell shown in FIG. 13B. In FIG. 15A, the entire surface of the memory cell is covered with the resist 62 to prevent impurities 64 from being introduced into the channel regions of the first transistor T1 and the second transistor T2. Since the threshold voltage of the memory cell remains at an initially set value, both the first transistor T1 and the second transistor T2 are turned on when the memory cell is selected and the greatest current can flow through the memory cell. Specifically, in this embodiment, the amounts of currents flowing through the selected memory cells have the relationship, FIG. 12A < FIG. 13A < FIG. 14A < FIG. 15A. As has been described above, in the present invention, the impurity-doped area of the channel region is varied according to data stored in the memory cell. Thereby, the substantial channel width or channel length of the memory cell is varied. In addition, the threshold voltage of the memory cell is varied by changing the impurity-doped area of the channel region. Accordingly, data of plural bits can be stored in one memory cell. In addition, after gate electrodes are formed in a plurality of memory cells, data of plural bits can be written in the memory cells by making use of a single glass mask. Therefore, without substantially increasing the number of manufacturing steps after the formation of the gate electrode, finished products can be delivered to the users in a short time period without an increase in cost.

Claims: A method for programming data of plural bits into a semiconductor memory device having a plurality of memory cells each including a MOS transistor having a drain region (D), a source region (S), a channel region (CH) and a gate electrode (G), by introducing impurities through a specific opening area (63) in a mask (61) into said channel region (CH) by ion implantation, wherein the area is varied according to data to be stored in said memory cell, said area extends from said drain region (D) to said source region (S) and has a width in the width direction of the channel region (CH), and wherein said programming is done by varying a size of said area such that said area assumes one of first to fourth sizes, the first size being such that said area extends over the entire width of the channel region (CH), the second size being such that said area has a first width from one side of the channel region, the third size being such that said impurity-introducing area has a second width from said side of said channel region less than said first width, and the fourth size being such that said impurities are not introduced into said channel region (CH), each of said first to fourth sizes corresponding to two bits of binary data. A method for programming data of plural bits into a semiconductor memory device having a plurality of memory cells each including a MOS transistor having a drain region (D), a source region (S), a channel region (CH) and a gate electrode (G), by introducing impurities through a specific opening area (63) in a mask (61) into said channel region (CH) by ion implantation, wherein the area is varied according to data to be stored in said memory cell, said area has a length in the length direction of the channel region (CH) over the entire width of the channel region (CH), and wherein said programming is done by varying a size of said area such that said area assumes one of first to fourth sizes, the first size being such that said area extends over the entire channel region (CH), the second size being such that said area has such a width as to be in contact with said drain region (D) and out of contact with said source region (S), the third size bein such that said area has such a width as to be in contact with said source region (S) and out of contact with said drain region (D) and the fourth size being such that said impurities are not introduced into said channel region, each of said first to fourth sizes corresponding to two bits of binary data. A method for programming data into a semiconductor memory device having a plurality of memory cells, each including a transistor having a drain region (D), a source region (S), a channel region (CH), a gate electrode (G), and a gate insulating film (54) between said gate electrode (G) and said channel region (CH), said gate insulating film having first and second portions (54a, 54b), said first portion (54a) being formed on a part of said channel region (CH), said first portion having a width between said drain region (D) and said source region (S) and having a first thickness and said second portion (54b) being formed on a part of said channel region (CH), said second portion having a width between said drain region (D) and said source regions and said second portion having a second thickness greater than said first thickness, by introducing impurities through a specific opening area (63) in a mask (61) into said channel region by ion implantation, wherein said programming is done by varying a size of said area such that said area assumes one of first to fourth sizes, the first size being such that said impurities are introduced into said channel region (CH) below said first and second portions (54a, 54b) of said gate insulating film, the second size being such that the impurities are introduced into said channel region (CH) below said first portion (54a) of said gate insulating film, the third size being such that the impurities are introduced into said channel region (CH) below said second portion (54b) of said gate insulating film, and the fourth size being such that the impurities are not introduced into the channel region (CH) below said first and second portions (54a, 54b) of said gate insulating film, each of said first to fourth sizes corresponding to two bits of binary data.

Inventors: IWAHASHI HIROSHI, IWAHASHI, HIROSHI, IWAHASHI, HIROSHI, C/O INT.PROP.DIV. K.K.TOSHIBA
Applicants: TOSHIBA KK, TOSHIBA MICRO ELECTRONICS, KABUSHIKI KAISHA TOSHIBA, TOSHIBA MICRO-ELECTRONICS CORPORATION
Application year: 1995
Main classification: H01L27
IPC codes: G11C16, H01L27, H01L21, G11C11