Read only memory

Abstract: A read only memory is provided in which the memory cells (24) are single metal gate or silicon gate field effect transistors (26). Each FET (26) has one of several different thresholds or states. The size or area of the FET gates (64) at the surface of the semiconductor chip are substantially the same regardless of the cells threshold or state. The input to the gates is a ramp, and the cells are rendered conducting by the amplitude of the ramp at a given instance. The output of a cell is fed to several flip-flops (42,44,46), which are synchronized with the input ramp, thereby setting the flip-flops in accordance with the threshold or state of the cell. An encoder (48) converts the output from the flip-flops to a bit binary signal. This permits a very high density ROM, e.g. 128k on a single chip.

Description: The invention relates to semiconductor integrated circuits and particularly to read only memories. The read only memory (ROM) is a well-known circuit used in great quantities in computers, calculators and in almost all digital systems. Conventional integrated circuit ROMs include on a single chip not only the ROM array, which is made of ROM or memory cells interconnected in a matrix, but also attendant circuits, such as input and output ports, address register and decoder, control, multiplexer, buffers, timing circuits and the interconnections. The size of the ROM, related to the number of cells in the array, is constantly growing and 32,000 bit ROMS are presently commercially available. Current activity is to design still larger ones, 64k, and it is desirable to have still larger ones, all on a single chip. A typical ROM cell has a single field effect transistor (FET) with a source-drain connection that is either complete or open, e.g. inoperable. When the ROM cell is interrogated, and if it has an open circuit, then it gives an output signal of one logic level, e.g. a signal representing "1"; and if it has a completed circuit, it gives the other logic level, e.g. a signal representing "0". In one type of ROM circuit, the FETs that make up the ROM array are fixed during fabrication. During one masking step, selected ROM cells which are to give the logic "1" have their source drain circuit rendered effectively inoperable. Thus, each ROM cell has a permanently stored logic 1 or logic 0, and gives the logic 1 or logic 0 response to an input signal. An introductory explanation of ROMs, the mask programmable ROM cell, the complete ROM array, the ROM chip, may be found in Brice Ward, MICRO PROCESSOR/MICRO PROGRAMMING HANDBOOK, 1975, Blue Ridge Summit, Pennsylvania. A classic and historic goal in the design and manufacture of ROMs is to have more data on a single chip. Single chip ROMs in the past decade have increased from 256 bits to approximately 1,000 to 2,000, 4,000, 8,000, 16K, 32K, and talk is currently of 64K and larger. In US-A-4,202,044, WO-A-80/01119 and an article in "Electronique Industrielle", No: 7, 1st December 1980, page 22, Paris, FR, it has been proposed, in order to increase the capacity of a ROM chip without increasing the number of FETs making up the ROM array, to use a FET in each cell which has one of more than two different thresholds, e.g. four different thresholds. Thus, four states are available for a given cell in the array, and these four states can be compared with the cell first described which had only two states, and, in the first described ROM, to get four states two cells are required. The invention is directed to a read only memory of the general type referred to in the pre-characterising part of the appended independent claims and disclosed in US-A-4,202,044 which comprises a plurality of field effect transistors (FETs) interconnected in a matrix, each of said matrix FETs having one of N conditions where N is greater than 2, and input means for applying to a selected FET, an input signal for stimulating an output signal from said FET in accordance with the state of said FET, the FET output signal, in operation of the memory, being applied to output means for producing a binary output in dependence upon the FET output signal. In operation of the ROM disclosed in US-A-4,202,044, the input means applies a fixed amplitude pulse to a selected matrix FET, the pulse amplitude being large enough to trigger the FET irrespective of its threshold level. The triggered FET produces an output pulse, the voltage amplitude of which assumes one of four values corresponding to that one of the four possible threshold values possessed by the selected FET. The output pulse is detected by the output means, compared with reference voltage levels, and the result of the comparison is then used to derive the binary information corresponding to the threshold or state of the selected FET. In the ROM known from WO-A-80/01119, sense amplifiers adapted to compare a read signal representing the conductance value of a selected FET with different reference levels are provided, which sense amplifiers output respective comparison signals dependent on the relative magnitude of said read signal and said reference levels. It is an object of the invention to provide an improved ROM. One aspect of the invention is defined in appended claim 1. Another aspect of the invention is defined in appended claim 3. A first plurality of the matrix FETs may be enhancement mode devices, a second and different plurality of said matrix FETs may be depletion mode devices, a third and different plurality of said matrix FETs may have a degree of gate region doping intermediate to that of the enhancement and depletion mode devices, and a plurality of said matrix FETs may be inoperable as a FET. If, for example, three thresholds are available (-2.0 volts, +0.1 volts, and +0.7 volts) then four conditions are now available for a given matrix FET (the fourth condition being an inoperable, or non-existent FET). The surface area of the gate region of each matrix FET is the same as that of each other matrix FET, and the threshold level of each FET is not affected by this common size. Thus the memory size of the ROM array can be doubled without increasing its physical size. Put it another way, the silicon area is halved. Further, this invention makes possible ROMs of 128K bits by using existing metal gate processing technique. It had been believed by some that to manufacture ROMs of 128K new process techniques would have to be perfected. In the invention, the reduction in area is achieved by circuit innovation. The layout rules have not been compromised. Thus as new geometric shrink advances become available, the ROMs using the invention can take advantage of them. In the present array, capacitance within the array is reduced; it is approximately halved. This has an effect on speed. Moreover, less power is required thus avoiding attendant problems of heating, and particularly heat dissipation. This is a classic goal in any memory array. The manufacture of ROMs embodying the invention is straight-forward. It does not require any radically different or new fabrication techniques from those currently in use, and which have been tried and tested. Further, as all the gate regions are of the same size, masking is straight-forward and thus easy to manufacture. In order that the invention may be more readily understood, reference will now be made to the accompanying drawings, in which:- Fig. 1 is a schematic drawing of a prior art ROM cell; Fig. 2 is a schematic drawing of a portion of a ROM embodying the invention; Figs. 3A and 3B are plan and cross-sectional views, respectively, not to scale, of a metal gate FET of a ROM cell embodying the invention; and Fig. 4 is a schematic drawing of a portion of a ROM according to an alternative embodiment of the invention. Fig. 1 shows a ROM cell 10 with a row select conductor 12 and a bit line conductor 14 passing therethrough. As is conventional, but not shown in Fig. 1, there are many cells adjacent one another arranged in an array, with row select and bit line conductors passing through each row and column of the array. In each bit line there is a column select gate 16, which is connected to a voltage V at a terminal 18. A source of the gate 16 is connected to an output lead 20, and on which is provided the output from each of the cells in that column. In ROM cell 10, a metal gate FET 22 has its source connected to the bit line 14, its drain connected to ground, and its gate is connected to row select 12. The transistor 22 is fabricated to be either normally operable, or not operating, e.g. during fabrication it has been altered in the source drain connection such that it always presents an open circuit to bit line 14. When column select gate 16 is turned on, and the row select is on, there is provided at output 20, one of two voltages depending on whether FET 22 is an operable transistor or an effectively inoperable one. If it is effectively inoperable, an output voltage at 20 is, for example, a logic 1 if we are using positive logic and it would approach the voltage V. If transistor 22 is an operable one, then it conducts and there is a conducting path between its source and drain, and the potential on output 20 approaches ground which represents a logic 0. As noted above, there are many ROM cells in the ROM array, and they are interconnected in a matrix with column select and bit line conductors. There is also circuitry on the ROM chip for ushering around the bits to their proper positions. FIG. 2 is a schematic diagram showing part of an embodiment of the ROM of the invention. It includes a plurality of ROM cells, one of which 24 is shown. In the cell is a FET 26, which has one of three possible different thresholds at which it will turn on (for example -2.0 volts, +0.1 volts, or +0.7 volts), or is always inoperable, e.g. open circuit (for example, it has a cut in its source drain circuit). Thus FET 26 may present four different states. A ramp generator 28 provides a ramp signal on a row select line 30 to cell 24. The ramp, for example, has a five volt rise,-2.5 to +2.5 volts in one micro second. It will be understood by those skilled in the art that the voltages given above are by way of example only and that for a typical N-channel MOS array, the ground plane of the array may be at a positive potential in order that all array devices may be cut off. Assuming cell 24 is being sensed by a signal pulse on a column select gate 31, a rising ramp voltage on row select line 30 would trigger and switch transistor 26 when the voltage is either -2.0 volts and 100 nanoseconds (ns) into the signal; or at +0.1 volts and 520 ns into the ramp; or at +0.7 volts and 640 ns into the ramp; or if transistor 26 has an open circuit then it would not switch at all. An output signal from cell 24 appears on output 32, and goes to a sense amplifier 34 which shapes and amplifies the output. The ramp signal is simultaneously applied on the row select to three transistors 36, 38 and 40, which have, respectively, the three different threshold levels which the transistors in the cells can have. In the example, transistor 36 becomes conducting at -2.0 volts; transistor 38 at 0.1 volt and transistor 40 at 0.7 volts; and become conducting approximately 100 ns, 520 ns, and 640 ns after the ramp signal begins. The outputs of the three transistors are applied to three sense amplifiers and then to one input of three flip-flops 42, 44, and 46, respectively. The flip-flops each have a second input connected to receive the output from the cells via 32 and amplifier 34. Coincidence of signals on both inputs of a flip-flop causes it to change state and provide a signal on its output. The states of the three flip-flops thus depend on the state or threshold of transistor 26 in cell 24, and may be summarized in the following chart. State of Transistor 26 F-F 42 Output F-F 44 Output F-F 46 Output -2.0 volt threshold 1 1 1 0.1 volt 0 1 1 0.7 volt 0 0 1 Inoperable 0 0 0 The outputs of the flip-flops 42, 44 and 46 are connected to an encoder 48, which converts them to a binary code signal, which is outputted on a pair of lines 50. A typical encoder provides: ENCODER 48 Input Output 111 11 011 10 001 01 000 00 Thus from a single cell 24, there is provided at lines 50 four states of information. In FIG. 2 it should be understood that the ROM chip contains additional circuitry (not shown), e.g. wave shaping amplifiers, buffers, input and output ports, multiplexing, and sequencing circuits, in addition to the other cells in the array of which there are several, and interconnections. A delay circuit or device 52 may be included between transistor 40 and flip-flop 46. This is because transistor 40 switches very close in time to transistor 38, and it is easier to build a practical circuit if there is a little more delay. In the above example, a delay from device 52 of about 120 ns is desirable, as this spaces the time from the outputs of the three transistors 36, 38 and 40 approximately equally over the total time for reading each cell. Similar delays can be added to transistors 36 and 38. FIG. 3A is a plan view of an integrated circuit chip showing part of two adjacent cells. Each cell has a FET with drain and source diffusions 60 and 62 and a gate region 64 therebetween. FIG 3B is a cross-sectional side view taken along line III-B of FIG. 3A. A thick silicon oxide layer 68 is on top of the silicon surface. A gate metallization 66 corresponds to the row select line 30 of FIG. 2. It is approximately 14 microns (14 x 10 ⁻⁶ meters) wide. An opening in the thick silicon oxide layer 68 defines the gate region 64 and is approximately 6 microns wide, and the distance between the drain and the source regions should be as small as possible, for example, 5.5 microns. The gate surface area (5.5 x 6 microns) for each transistor in each cell is of the same size. For those transistors which are to have the intermediate threshold, in the example 0.1 volt, there is no further impurity in the gate region. For those transistors which are to have the -2.0 volts and +0.7 volts threshold there is an N & P implanted region below the gate for an enhancement and depletion type FET. It should be noted that regardless of what the threshold level is, the size of gate surface area is the same. FIG. 4 shows an alternative embodiment ROM array. It is similar to that of FIG. 2, and like elements bear like legends. A step generator 78 applies a step voltage, e.g. -2.5 to +2.5, with a rapid rise time to one row select 30. This switches all of the operable FETs 26 in the cells 24 on that row select line. Depending upon the threshold of the FETs 26 in the cells 24, or whether they are cut or not, i.e. inoperable, there will be different current flow in the column select line. For example, a depletion FET will draw 300 microamps; an undoped or E zero FET,120 microamps; and an enhancement mode FET 50 microamps. A buffer or current sense amplifier 80 is connected to a column select line, and provides the output from that column to comparison circuits 82, 84, and 86 described below. Included in the ROW select line 30 are the three reference cells with FETs 36, 38, and 40. The output from these FETs 36, 38,and 40 are fed respectively to three buffers or current amplifiers, and then to one input of the comparison circuits 82, 84, 86, respectively. The second input to the comparison circuits comes from the buffer 80. Coincidence of signals at the comparison circuits triggers the circuit and provides an output signal to the encoder 48. The table of operation is identical to the first one previously set out except that, in this embodiement, the outputs are of the comparison circuits 82,84,86 rather than of the flip-flops 42,44,46 as in the previous embodiement. As with FIG. 2, the support circuitry for the ROM array is not shown, as that is known in the art. Various arrangements may be used for reading out the cells in a given row, e.g. by a tree arrangement, or other control in the bit line or column select. Other techniques are available depending upon the arrangement in which data is to be read out from the ROM. With regard to the reference FETs 36, 38, and 40 in both FIGS. 2 and 4, it has been found desirable, in certain embodiments, to change the gate geometry from that of the geometry of the FETs of the array. In the embodiment of FIG. 2, by changing this geometry, the timing at which the reference FETs trigger or switch can be adjusted; and the triggering of the FETs is more evenly spaced. This would avoid the need for the delay circuit 52. For example, the enhancement FET 40 which triggers at 0.7 volts, has its gate size unchanged. It is the last one, of the reference FETs, to trigger. The reference FET 36 with the undoped gate, might have it considerably larger, for example, 12 x 6 microns (source drain). This would cause it to switch first. A depletion mode FET would not be used at all as a reference, but an enhancement mode FET having a dimension of 10 x 6 microns (source drain) would also be used as the FET 38 which would trigger intermediately. These adjustments of the time of reference FETs switching also accommodate for other delays in the circuit, (e.g. between the outputs from the array FETs reaching the flip flops). It will be appreciated that the above dimensions are for example only, and other modifications may be made in the sizes of the reference FET gates, and using other combinations of doping besides the zero doping, enhancement depletion for the three reference FETs, some of the variables being the degree of doping, enhancement, depletion, or zero, and the size of the gate. Generally it is standard dopants that are used in fabrication and it is desirable to adapt one of those dopants when making the reference FET diffusions. In a modification of the embodiment of FIG. 2, the sequence of signals reaching the flip-flops would be substantially, equally spaced in time, as follows: array depletion mode FETs switching - first reference FET switching; array zero doping FETs switching - second reference FET switching; enhancement mode FETs switching - third reference FET switching; and time for array inoperable FETs, which do not switch. Also in the embodiment of FIG. 4, the reference FETs 36, 38, and 40 need not be of uniform gate dimensions. Also the doping may vary. In the example just given, the reference FETs may be two enhancement mode FETs of different gate dimensions; and one non-doped FET whose gate dimensions are different from those of the array FETs. Other combinations are possible, depending upon the amplitude of the currents to the comparison circuits and the timing of the currents. The turn-on time for all the FETs is relatively quick and at present a conservative cycle would be 300 nanoseconds. It should be noted that faster speeds may be obtained. In the embodiement described three levels in an operating FET have been used. By selected implantations, it is possible to introduce more than three threshold levels. Alternatively, by making different gate oxide thicknesses, different threshold levels may also be achieved; and other techniques are available for different thresholds. The present invention includes multiple threshold levels. Alternatively, to give more precise control a discrete staircase rather than a ramp input could be applied to the gate. Further, any slowly varying with time input signal might be used in FIG. 2 embodiment. "Slowly varying with time" means that the output from the ROM cell is measured or sensed as a function of time. This can be contrasted with the FIG. 4 embodiment in which the output from the ROM cell is measured or sensed as a function of current amplitude from the cell. It will be noted that as techniques are developed for smaller size FETs and interconnections, e.g. as the source and drain lines become closer together, and the metallization and gate regions become smaller there will be an improved speed of operation, more cells per unit area, and larger arrays. The ROM embodying the present invention, which enables the output of each cell to be doubled would also shrink in size with these further developments and take advantage of them.

Claims: A read only memory comprising a plurality of field effect transistors (FETs) (26) interconnected in rows and columns in a matrix, each of said matrix FETs having one of at least three possible conditions, and input means for applying to a selected FET, an input signal for stimulating an output signal from said FET in accordance with the predetermined condition of said FET, the FET output signal, in operation of the memory, being applied to output means for producing a binary output in dependence upon the FET output signal, the matrix FETs (26) each having a gate region of substantially the same surface area and shape, and at least two of the possible conditions of each FET being determined by different gate threshold levels and a further possible condition being determined by a further different gate threshold level responsive to a further different input signal amplitude, or not being responsive to the input signal at all; characterised in that: reference FETs (36, 38, 40) each having a respective one of the different threshold voltages of the responsive matrix FETs are provided in each row of the matrix; the input means (28) is operable to apply to the matrix FETs of the row comprising the selected FET, during a predetermined read time interval, a ramp signal including the different threshold levels displaced in time from each other and from the beginning of the read time interval; the signal outputted by each reference FET is applied to a first input of a respective flip-flop (42, 44, 46); the signal outputted by the selected matrix FET is applied to a second input of each flip-flop; and the signals outputted by the flip-flops are applied to means (40) which, in dependence upon the flip-flops not changing their state or in dependence upon the time sequence of their changes of state, produces a binary output representative of the condition of the selected FET. A read only memory according to claim 1, wherein the output means further comprising a delay circuit (52) between at least one of said reference FETs (40) and the input to its corresponding flip-flop (46). A read only memory comprising a plurality of field effect transistors (FETs) (26) interconnected in rows and columns in a matrix, each of said matrix FETs having one of at least three possible conditions, and input means for applying to a selected FET, an input signal for stimulating an output signal from said FET in accordance with the predetermined condition of said FET, the FET output signal, in operation of the memory, being applied to output means for producing a binary output in dependence upon the FET output signal, the matrix FETs (26) each having a gate region of substantially the same surface area and shape, and at least two of the possible conditions of each FET being determined by different gate threshold levels and a further possible one of which is determined by a further different gate threshold level responsive to a further different input signal amplitude, or not being responsive to the input signal at all; characterised in that: reference FETs (36, 38, 40) each having a respective one of the different threshold voltages of the responsive matrix FETs are provided in each row of the matrix; the input means (78) is operable to apply a step voltage to the matrix FETs of the row comprising the selected FET; the current flowing through each reference FET is fed to a first input of a respective comparison circuit (82, 84, 86); the current flowing through the selected matrix FET is fed to a second input of each comparison circuit; and the signals outputted by the comparison circuits are applied to means (48) which, in dependence upon no coincidence being detected by said comparison circuits or in dependence upon the detected one, produces a binary output representative of the condition of the selected FET. A read only memory according to claim 1, 2 or 3, wherein each matrix FET has one of at least four conditions including at least three different states which are determined by gate threshold levels responsive to different amplitudes of said input signal, and a further condition which is not operable by said input signal. A read only memory according to claim 4, wherein the matrix FETs have gate regions (64) having different degrees of doping corresponding to said at least three different states. A read only memory according to any of claims 1 to 4, including at least three reference FETs (36, 38, 40) having substantially the same surface dimensions as the matrix FETs. A read only memory according to any of claims 1 to 4 or 6, in combination with claim 5, including at least three reference FETs (36, 38, 40), at least one of which has one of said different degrees of gate region doping of the matrix FETs, and a substantially different gate region surface dimension from that of the corresponding matrix FET. A read only memory according to claim 7, wherein at least two of said at least three reference FETs (36, 38, 40) have gate region surface dimensions different from those of the matrix FETs. A read only memory according to claim 8, wherein one of said reference FETs is an enhancement mode FET, and has the same gate region surface dimension as the matrix FETs, a second of said reference FETs is an enhancement mode FET and has a larger gate region surface dimension, and a third of said reference FETs has an intermediate threshold level and a larger gate region surface dimension.

Inventors: NAIFF KENNETH LESLIE, NAIFF, KENNETH LESLIE
Applicants: GEN INSTRUMENT CORP, GENERAL INSTRUMENT CORPORATION
Application year: 1982
Main classification: G11C17
IPC codes: G11C17, G11C16, G11C11