Reconfigurable Multi-level Sensing Scheme for Semiconductor Memories

• US 20120063195A1
• Filed: 09/12/2011
• Published: 03/15/2012
• Est. Priority Date: 09/13/2010
• Status: Active Grant

• Current Assignee: Globalfoundries US Incorporated (GlobalFoundries, Inc.)
• Original Assignee: International Business Machines Corporation
• Inventors: Lam, Chung H., Lewis, Scott C., Li, Jing
• Granted Patent: US 8,717,802 B2
• Time in Patent Office: Days
• US Class Current: 365/148
• CPC Class Codes: G11C 11/5642 Sensing or reading circuits; Data output circuits
• CPC Class Codes: G11C 11/5678 using amorphous/crystalline phase transition storage elements
• CPC Class Codes: G11C 13/0004 comprising amorphous/crystalline phase transition cells
• CPC Class Codes: G11C 13/003 Cell access
• CPC Class Codes: G11C 13/004 Reading or sensing circuits or methods
• CPC Class Codes: G11C 13/0061 Timing circuits or methods
• CPC Class Codes: G11C 2211/5634 Reference cells
• CPC Class Codes: G11C 2211/5644 Multilevel memory comprising counting devices
• CPC Class Codes: G11C 23/00 Digital stores characterised by movement of mechanical parts to effect storage, e.g. using balls; Storage elements therefor

Abstract

A method for sensing at least one parameter indicative of a logical state of a multi-level memory cell includes the steps of: measuring the parameter of the multi-level memory cell; comparing the measured parameter of the multi-level memory cell with a prescribed reference signal, the reference signal having a value which varies as a function of time; and storing a time value corresponding to a point in time at which the reference signal is substantially equal to the measured parameter of the multi-level memory cell, the stored time value being indicative of a sensed logical state of the multi-level memory cell.

Claims

1. A method for sensing at least one parameter indicative of a logical state of a multi-level memory cell, the method comprising the steps of:

measuring the at least one parameter of the multi-level memory cell;

comparing the measured at least one parameter of the multi-level memory cell with a prescribed reference signal, the reference signal having a value which varies as a function of time; and

storing a time value corresponding to a point in time at which the reference signal is substantially equal to the measured at least one parameter of the multi-level memory cell, the stored time value being indicative of a sensed logical state of the multi-level memory cell.

2. The method of claim 1, further comprising the steps of:

storing a start time indicative of a start of a sensing operation of the multi-level memory cell; and

storing a stop time indicative of the reference signal being substantially equal to the measured at least one parameter of the multi-level memory cell, wherein the stored time value corresponding to the sensing operation is computed as a function of a difference between the start time and the stop time.

3. The method of claim 2, wherein the step of storing the start time is performed by recording a first value of a clock cycle counter at the start of the sensing operation and the step of storing the stop time is performed by recording a second value of the counter when the reference signal is substantially equal to the measured at least one parameter of the multi-level memory cell.

4. The method of claim 3, further comprising modifying a resolution of the sensing operation by varying a number of clock cycles supplied to the clock cycle counter during the sensing operation.

5. The method of claim 2, wherein the time value comprises an elapsed time measured from a start of the sensing operation to when the reference signal is substantially equal to the measured at least one parameter of the multi-level memory cell.

6. The method of claim 1, further comprising modifying a resolution of the sensed at least one parameter by varying a rate of change of the reference signal relative to time.

7. The method of claim 6, wherein the reference signal is generated as a monotonically increasing ramp function.

8. The method of claim 6, wherein the reference signal is generated as a monotonically increasing step function.

9. A sense circuit for sensing at least one parameter indicative of a state of a multi-level memory cell, the sense circuit comprising:

a reference generator operative to generate a reference signal having a value which dynamically varies as a function of time;

a monitor circuit operative to measure the at least one parameter of the multi-level memory cell and to generate a sensed parameter signal indicative of a state of the multi-level memory cell;

a comparator operative to receive the reference signal and the sensed parameter signal and to generate an output signal as a function of a difference between the reference signal and the sensed parameter signal; and

memory operative as a function of the output signal generated by the comparator to store a time value corresponding to a point in time at which the reference signal is substantially equal to the sensed parameter signal, the stored time value being indicative of a sensed logical state of the multi-level memory cell.

10. The sense circuit of claim 9, further comprising at least one storage register operative to store a start time, indicative of a start of a sensing operation of the multi-level memory cell, and a stop time, indicative of the reference signal being substantially equal to the measured at least one parameter of the multi-level memory cell, wherein the stored time value corresponding to the sensing operation is computed as a function of a difference between the start time and the stop time.

11. The sense circuit of claim 10, further comprising at least one counter adapted to receive a clock signal and operative to generate a count value indicative of a number of clock pulses in the clock signal, wherein a first value of the counter indicative of a start time is stored in the at least one register at the start of the sensing operation, and a second value of the counter indicative of a stop time is stored in the at least one register when the reference signal is substantially equal to the measured at least one parameter of the multi-level memory cell.

12. The circuit of claim 11, wherein a resolution of the sense circuit is controlled at least in part by varying a number of clock cycles supplied to the at least one counter during the sensing operation.

13. The sense circuit of claim 9, wherein the reference generator comprises:

a current source operative to generate a bias current; and

a programmable load circuit connected with the current source at a first node, the programmable load circuit being operative to provide an impedance to the current source which varies as a function of at least a first control signal, the reference signal generated by the reference generator having a value which is controllable as a function of the first control signal.

14. The sense circuit of claim 13, wherein the programmable load circuit comprises:

a plurality of switch elements, each switch element being adapted to receive a corresponding control signal supplied thereto; and

a plurality of resistive elements, each resistive element being connected in series with a corresponding one of the switch elements, wherein the impedance of the programmable load circuit is controllable as a function of the respective control signals supplied to the plurality of switch elements to thereby form a resistor selection network.

15. The sense circuit of claim 14, wherein each of the plurality of resistive elements has a resistance associated therewith that is within a prescribed range of resistance of a multi-level memory cell being sensed.

16. The sense circuit of claim 14, wherein each of at least a subset of the plurality of switch elements comprises a metal-insulator-semiconductor field effect transistor.

17. The sense circuit of claim 13, wherein the programmable load circuit comprises:

a counter adapted to receive a clock signal and operative to generate a count value indicative of a duration of a sensing operation of the multi-level memory cell; and

control logic coupled with the counter, the control logic being operative to generate the at least first control signal as a function of the count value.

18. The sense circuit of claim 17, further comprising at least one storage register, wherein a first value of the counter indicative of a start time is stored in the storage register at the start of the sensing operation, and a second value of the counter indicative of a stop time is stored in the storage register when the reference signal is substantially equal to the measured at least one parameter of the multi-level memory cell.

19. The sense circuit of claim 13, wherein the reference generator comprises:

a first storage register operative to receive a data input signal and to generate a first count value;

a counter adapted to receive a clock signal and operative to generate a second count value indicative of a duration of a sensing operation of the multi-level memory cell;

a multiplexer connected with the first storage register and the counter, the multiplexer being operative to receive either the first count value or the second count value and to supply a selected one of the first and second count values to a second register as a function of at least one control signal.

20. A memory circuit, comprising:

a memory array including a plurality of multi-level memory cells and at least one word line and a plurality of bit lines coupled with the memory cells for selectively accessing the cells; and

at least one sense circuit connected with a corresponding one of the bit lines and being operative to sense at least one parameter indicative of a state of a multi-level memory cell, the at least one sense circuit comprising;

a reference signal generator operative to generate a reference signal having a value which dynamically varies as a function of time;

a monitor circuit operative to measure the at least one parameter of the multi-level memory cell and to generate a sensed parameter signal indicative of a state of the multi-level memory cell;

a comparator operative to receive the reference signal and the sensed parameter signal and to generate an output signal as a function of a difference between the reference signal and the sensed parameter signal; and

a storage element operative, as a function of the output signal generated by the comparator, to store a time value corresponding to a point in time at which the reference signal is substantially equal to the sensed parameter signal, the stored time value being indicative of a sensed logical state of the multi-level memory cell.