Redundant address memory and test method therefor

Abstract: A redundancy implementation circuit has a set of memory cells each storing an address bit of an address identifying a redundant memory location and a set of comparator circuits each connected to compare the address bit stored in a memory cell with an incoming address bit. A switch selectively connects the output of the memory cell to a redundant address line supplying the incoming address bit during a test mode. A redundant address line driver is activated for supplying an incoming address bit onto the redundant address line in a normal mode, and a test line output driver is connected to the redundant address line in a test mode for utilising the redundant address line to supply test signals onto a test path. This arrangement reduces the number of wires which are requires on a memory chip.

Description: This invention relates to a memory and a method of testing a memory. The invention is particularly but not exclusively concerned with flash EPROMs (electrically programmable read only memories) which comprise a plurality of memory cells. These memory cells include programmable and erasable floating gate single transistor memory cells and other cells which can be programmed only once, referred to herein as UPROM (unerasable programmable read only memory) cells. For implementing redundancy on a flash EPROM, these UPROM cells contain respective bits identifying a redundant address. As is well known in the art, the bit in each UPROM cell is compared with a respective address bit to detect if an incoming address matches the address to be repaired stored in the UPROM cells. Figure 1 illustrates a redundancy implementation circuit according to the prior art. Figure 1 illustrates a plurality of address pads A0,A1,A2,A3 for receiving address bits. The address pads A0...A3 are each connected to a line driver 4 0 ...4 3 for driving the address bits onto redundant address lines via an address bus 31. Two groups of redundant address lines are shown in Figure 1, GROUP0,GROUP1. They are denoted RAL 00 ,RAL 01 etc. in GROUP0 and RAL 10 ,RAL 11 etc. in GROUP1. It can be seen that redundant address lines RAL 00 ,RAL 10 share a common address line AL0 connected to pad A0 and that this also occurs for other pairs of redundant address lines in the groups. The redundant address lines are connected to respective comparators, 8a,8b,8c,8d, four of which are shown in Figure 1 in each group. It will be appreciated that there may be any different number of comparators depending on the number of bits in the address. The comparators 8a to 8d compare the address bits on redundant address lines RAL with the outputs from a corresponding plurality of memory cells 10a,10b,10c,10d. In a flash memory, these memory cells will normally be UPROM cells. Each UPROM cell 10a to 10d has an output line 12a to 12d respectively on which the bit stored in that memory cell is output for comparison at the comparator 8a to 8d. The outputs of the comparators 8a to 8d in each group are fed to respective compare circuits 14 0 ,14 1 which generate a hit signal HIT0,HIT1 when all of the bits stored in the memory elements 10a to 10d match the incoming bits of the redundant address. The hit signals are supplied on line 16 0 ,16 1 to decoding logic to access a spare element in a memory array rather than a defective element when the incoming address is the address of a defective element. It is clearly desirable to test the data stored in the memory cells 10a to 10d after they have been programmed and before the chip is used. With the prior art circuit of Figure 1 this is done by examining the hit signals HIT0,HIT1 for a plurality of incoming addresses. However, to test the chip properly, it can be necessary to cycle through all possible addresses if the expected result is not attained because it is difficult to know which of the particular memory cells 10a to 10d is at fault without trying all possible addresses. An alternative would be to supply the outputs on line 12a to 12d individually to a test bus for the chip. This would enable the output of each memory cell 10a to 10d to be individually examined during a test, but it proliferates the number of wires which are required on the chip. The present invention seeks to provide a redundancy implementation circuit which can readily be tested but which minimises the number of connections which need to be routed in layout, in order to minimise the chip area. According to the present invention there is provided a redundancy implementation circuit comprising: a memory cell storing an address bit of an address identifying a redundant memory location; a comparator circuit connected to compare the address bit stored in the memory cell with an incoming address bit supplied on a redundant address line; a switch for selectively connecting the output of the memory cell to said redundant address line during a test mode; a redundant address line driver activated for supplying an incoming address bit onto said redundant address line in a normal mode; and a test line output driver connectable to said redundant address line in a test mode for driving signals on said redundant address line onto a test path when said redundant address line drivers are not activated. The test path may be a test bus or any other suitable test path. The provision of the switch allows the redundant address line to perform a dual function of supplying address bits or outputting stored bits for test, so that the chip area consumed is reduced, while nevertheless allowing the output of each memory cell to be tested individually. The concept of the invention can also be extended to include memory cells which do not store redundant address bits but which nevertheless store bits which are required to be tested. The output of these memory cells can be connected through appropriate switches to the redundant address lines in the same manner as the outputs of memory cells containing redundant address bits. For a better understanding of the present invention and to show how the same may be carried into effect reference will now be made by way of example to the accompanying drawings in which:- Figure 1 is a circuit diagram of a known redundancy implementation circuit; Figure 2 is a diagram of a redundancy implementation circuit according to the present invention; Figure 3 is a transistor level diagram of one implementation of a UPROM cell; Figure 4 is a schematic diagram of a modified storage block; and Figure 5 is a more detailed schematic of one embodiment of the invention. There follows a description of how the architecture in Figure 1 is modified to implement the present invention in its preferred form. Figure 2 illustrates the case for one address pad and one address line, for example A 0 ,AL 0 in Figure 1. It will readily be understood that the circuitry is repeated for each address pad. The address pad 20 for receiving address bits is connected to an address buffer 22 the output of which is connected to the redundant address line driver 24 (equivalent for example to 4 0 in Figure 1). The redundant address line driver 24 is controlled by complementary READ and READ signals on lines 26 and 28 respectively which are set in normal operation of the chips so that the redundant address line driver 24 is on. The READ and READ signals on lines 26 and 28 are also supplied to an output test line driver 30 but with reverse semantics so that when the redundant address line driver 24 is on, the output test line driver 30 is off and vice versa. It will be appreciated that there is a test line driver associated with each address line driver 4 0 ...4 3 . The output test line driver 30 supplies its signals on line 32 to a test IO bus 34 which is connected to an output pad (not shown) in a test mode for supplying signals off chip. The redundant address line driver 24 drives the incoming address bit via address line 35 and redundant address line 36 to the input of a comparator 38 (for example equivalent to 8a in Figure 1). Address line 35 is equivalent for example to AL 0 in Figure 1. Redundant address line 36 is equivalent for example to RAL 00 or RAL 10 in Figure 1. The comparator 38 receives another input on line 40 from a memory cell 42 (equivalent to memory cell 10a in Figure 1). In a flash memory chip, the memory cell 42 could be a UPROM cell storing a redundant address bit and having the form shown in Figure 3. The UPROM cell shown in Figure 3 comprises first and second floating gate transistors 78,80. The sources of the transistors 78,80 are both connected to ground. The gates of the transistors are connected to a UPROM wordline UWL, which is capable of selectively connecting the gates of the floating gate transistor cells 78,80 to appropriate voltages for programming. The drains of the transistors are connected to bit lines UBL, UBL for the cell. The drains of the flash transistors 78,80 are connected through switch transistors 84,86 to a pair of p-channel cross-coupled transistors 88,90 which cooperate to remove DC current in a known manner when the cell is programmed. These cross-coupled transistors are connected in turn to a power supply voltage Vcc. The output 40 of the UPROM cell is taken between the transistor 90 and a switchable transistor 86 via an inverter 92. The gates of the switch transistors 84,86 receive an RCASC signal for activating the cell. During programming RCASC is low to isolate the p-channel transistors 88,90 and transistor 82 drives the p-channel to a known state in response to a disable signal on line 112. The output of the memory cell 42 on line 40 is also supplied to a switch in the form of a pass gate 44 which is controlled by READ 0 and READ 0 signals on lines 46 and 48 respectively. There is a switch 44 associated with each memory cell in each group. The switches in GROUP0 are all controlled by READ0,READ0 signals. The switches in GROUP1 are all controlled by different read signals READ1, READ1 to ensure that the shared address lines are not connected simultaneously to different redundant address lines (e.g. RAL0 0 ,RAL 10 in the case of AL0). The circuitry including the memory cell 42, pass gate 44 and its control lines 46,48 and comparator 38 is shown inside a dotted line 50. It will readily be understood that the circuitry denoted by the dotted line 50 replaces the UPROM cells 10a...10d and comparators 8a...8d shown in the prior art circuit of Figure 1. Redundant addresses are stored in the memory cells 42 and the outputs of the comparators 38 are fed in a normal mode of operation to compare circuits to generate hit signals, as discussed above with reference to the prior art. No further discussion of the precise manner of redundancy implementation is given herein because it is well known to a person skilled in the art. It will readily be apparent however that the circuit of Figure 2 differs from the circuit of Figure 1 in the provision of a switch 44 associated with each memory cell 42 and in the provision of an output test line driver 30 associated with each address line driver and connected to the shared address line 35. In normal operation of the memory chip, the READ and READ signals on line 26 and 28 are set so that the redundant address line drivers 24 are on. Address bits are supplied via the address pads 20 and address buffers 22 to the comparators 38 and are compared with the outputs of the memory cells 42. When the input addresses match the redundant addresses stored in the memory cells 42, hit signals are generated by the compare circuits. The invention provides for a special test mode. In this test mode, the READ and READ signals change state so that the redundant address line drivers 24 are off and the output test line drivers 30 are on. The output of a selected memory cell 42 can be connected to the redundant address line 36 through the pass gate 44 which is controlled by its control lines 46,48. To select the memory cell 42 shown in Figure 2, the signals READ 0 and READ 0 would be set appropriately. It will readily be appreciated that each switch 44 is controlled by its control lines 46,48 to selectively supply outputs from the memory cells onto the shared address line 35. These signals are then supplied to the test IO bus 34 via the output test line driver 30. This allows the output of each memory cell 42 to be individually tested without the need for extra routing lines on chip. The invention thus allows the redundant address lines to be shared for outputting test data in a test mode onto the test IO bus. The principle of the invention can also be applied where the memory cells 42 are not holding redundancy address information but are holding other bits for controlling operation of the memory device and which are required to be tested. In that case, the output 40 of the memory cell 42 is not supplied to a comparator such as 38 but is supplied directly to another part of the integrated circuit device. Figure 4 shows a modified block 501 where this is the case. Like parts are denoted by like numerals as in Figure 2, but primed. The output 40' is similarly connected to the redundant address line 36 for test purposes through the pass gate 44'. Figure 5 is a fuller schematic of the preferred embodiment of the invention implemented for GROUP0 shown in Figure 1. Like numerals denote like parts as in Figure 2, with suffixes a...d corresponding to those in Figure 1. In addition to blocks 50a...50d for implementing redundancy connected respectively to address lines AL 0 ...AL 3 via redundant address lines RAL 00 ...RAL 03 , two modified blocks 50' are shown in Figure 5 connected to address lines AL 0 and AL 1 respectively. Provided that these address lines AL 0 and AL 1 are not being used in normal operation of the memory or in a special test mode to output test data from the blocks 50a and 50b, then these modified blocks 50' may have their pass gates 44' selected to place the outputs of the memory cells within the modified blocks 50' onto address lines AL 0 and AL 1 for outputting through test output drivers 30a and 30b respectively onto the test bus 34. The address bus 31 and test bus 34 are shown in Figure 5 as 4 bits wide, but it will be clear that the invention can be implemented for any bus width. Each address line driver drives the address line on the address bus and each test output driver drives one test line on the test bus. It will be appreciated that while two groups are shown in Figure 1, the invention can be implemented with any suitable number of groups, provided that no more than one group has access to the common address bus at the same time. This is effected through control of the READ0 and READ0 signals (for Group 0) and equivalent signals for other groups. The present invention can be used not only to test chips before use but also to analyse chip failures of returned chips. It can also be used in later tests to establish what data has been programmed into the UPROM cells.

Claims: A redundancy implementation circuit comprising: a memory cell (42) storing an address bit of an address identifying a redundant memory location; a comparator circuit (38) connected to compare the address bit stored in the memory cell with an incoming address bit supplied on a redundant address line (34); a redundant address line driver (24) activated for supplying an incoming address bit onto said redundant address line in a normal mode; characterized by a switch (44) for selectively connecting the output (40) of the memory cell to said redundant address line during a test mode; and a test line output driver (30) connectable to said redundant address line in a test mode for driving signals on said redundant address line onto a test path when said redundant address line drivers are not activated. A redundancy implementation circuit as claimed in claim 1 comprising a plurality of redundant address lines connected to respective address lines of an address bus, the circuit further comprising a plurality of redundant address line drivers (24a) and test line output drivers (30a) associated respectively with the address lines, the redundant address line drivers being connected for driving incoming address bits onto said redundant address lines. A redundancy implementation circuit as claimed in claim 2 which comprises a plurality of said memory cells, each memory cell having associated therewith said respective comparator (38) and said respective switch (44) wherein said plurality of memory cells and corresponding comparator circuits and switches are arranged in a first group, the outputs of said comparator circuits being supplied to a hit generation circuit which generates a hit signal when the incoming address bits match an address identifying a first redundant memory location. A redundancy implementation circuit according to claim 3 which comprises at least one further plurality of memory cells and corresponding comparator circuits and switches constituting at least one further group, the outputs of the comparator circuits in said at least one further group being supplied to a further hit generation circuit for generating a hit signal when incoming address bits match an address identifying a further redundant memory location. A redundancy implementation circuit according to claim 4 wherein each address line is connected to a redundant address line of each of the first and at least one further group, wherein the redundancy implementation circuit further comprises circuitry for controlling said switches in each group so that the outputs of memory cells in only one of said groups is connected to the address lines at one time. A redundancy implementation circuit according to claim 3 which includes a further plurality of memory cells each storing respective additional bits; a corresponding further plurality of switches for selectively connecting the outputs of said further memory cells to said address lines during said test mode. A redundancy implementation circuit according to any preceding claim implemented on a flash memory chip, wherein said memory cells are unerasable read only memory cells or, short, UPROM cells. A redundancy implementation circuit according to claim 1 wherein the redundant address line driver is connected to receive complementary control signals (READ, READ ) for turning it on and off and wherein said test line output driver is connected to receive said complementary control signals but in a reverse sense so that when said redundant address line driver (24) is on the test line output driver (30) is off, and vice versa. A redundancy implementation circuit according to claim 3 wherein the redundant address line drivers are connected to receive complementary control signals for turning them on and off and wherein said test line output drivers are connected to receive said complementary control signals but in a reverse sense so that when said redundant address line drivers are on, the test line output drivers are off, and vice versa.

Inventors: MCINTYRE HUGH, MCINTYRE, HUGH
Applicants: SGS THOMSON MICROELECTRONICS, STMICROELECTRONICS LIMITED
Application year: 1995
Main classification: G11C29
IPC codes: G11C29, G11C17, G01R31, G11C16