Integrated semiconductor-magnetic random access memory system

[0002] This is a divisional application of U.S. application Ser. No. 09/260,920, filed on Mar. 2, 1999 now U.S. Pat. No. 6,219,273.

[0003] This application claims benefit of the priority of U.S. Provisional Application Ser. No. 60/076,524, filed Mar. 2, 1998 and entitled “JPL's Magnetic Random Access Memory: MagRAM.”

[0004] The invention described herein was made in performance of work under a NASA contract, and is subject to the provisions of Public Law 96-517 (35 U.S.C. 202) in which the Contractor has elected to retain title.

[0005] The present specification generally relates to memory devices. More particularly, the present specification describes an integrated semiconductor-magnetic random access memory.

[0006] A demand for increase in data processing rate has led to search for faster and denser random access memory (RAM). Semiconductor memories such as dynamic RAM and static RAM have very fast access times but are also volatile. Electrically erasable read only memories (EPROM) are non-volatile but have very long write times and offer a conflict between refresh needs and radiation tolerance.

[0007] The concept of using magnetic material for a non-volatile RAM has been implemented before, e.g., in core memory and in magnetic RAM. A non-volatile magnetic random access memory is described in U.S. Pat. No. 5,289,410, the disclosure of which is herein incorporated by reference to the extent necessary for understanding.

[0008] The present disclosure describes a non-volatile magnetic random access memory (RAM) system having a semiconductor control circuit and a magnetic array element. The integrated magnetic RAM system uses a CMOS control circuit to read and write data magnetoresistively. The system provides a fast access, non-volatile, radiation hard, high density RAM for high speed computing.

[0009] According to the present disclosure, magnetic storage array cells have certain hysteresis characteristic that allows data to be written to or read from the cell accurately without interference from surrounding cells. Semiconductor circuits operate to read data from and write data to the magnetic storage array cells by generating currents to apply electromagnetic fields to the cells. A preferred embodiment of the magnetic array cells uses magnetoresistive material.

[0010] A method for writing data to the magnetic memory array cells includes selecting cell address and applying appropriate currents to address lines.

[0011] The details of one or more embodiments are set forth in the accompanying drawings and the description below. Other embodiments and advantages will become apparent from the following description and drawings, and from the claims.

[0012] These and other aspects will be described in reference to the accompanying drawings wherein:

[0013] FIG. 1 shows a block diagram of an integrated semiconductor-magnetic random access memory (RAM) system;

[0014] FIG. 2 shows a more detailed block diagram of the integrated system showing an array of magnetoresistive elements;

[0015] FIG. 3 shows a schematic diagram of the integrated semiconductor-magnetic RAM system containing a 4×4 magnetic memory array;

[0016] FIG. 4A shows a layout of the magnetic element array;

[0017] FIG. 4B shows a resistance hysteresis curve of a magnetic bit operating to write data;

[0018] FIG. 5 shows a plot of memory state switching threshold versus sense current;

[0019] FIG. 6A shows distribution of switching thresholds in a memory array with no overlap of thresholds;

[0020] FIG. 6B shows distribution of switching thresholds in a memory array with overlap of thresholds;

[0021] FIG. 7 shows a schematic diagram of a voltage sense amplifier, sample and hold, a comparator and a data latch operating to read data from a memory array;

[0022] FIG. 8 shows a timing diagram of a read process;

[0023] FIG. 9A shows a more detailed timing diagram of a read cycle; and

[0024] FIG. 9B shows a detailed timing diagram of a write cycle.

[0025] Like reference numbers and designations in the various drawings indicate like elements.

[0026] The advantageous features of a magnetic RAM include fast access, non-volatility, radiation hardness, and high density. The inventors found that by using the integrated semiconductor-magnetic elements, all the advantageous features of the magnetic RAM can be achieved at lower cost and having a faster access time.

[0027] A block diagram of a preferred embodiment of the integrated semiconductor-magnetic RAM system, showing data inputs and outputs for semiconductor electronics and magnetic elements, is shown in FIG. 1. The magnetic elements are used for storing and sensing data. The preferred magnetic array element used for data storage is described in U.S. Pat. No. 5,659,499.

[0028] The integrated system 100 writes and reads data to and from the magnetic elements 104 magnetoresistively using hysteresis characteristics. The semiconductor electronics 102 perform magnetic element array addressing, voltage sense amplifying, and data latching functions. Currents in word lines generate magnetic fields to write data. The data signals on sense lines are amplified and converted to digital form.

[0029] FIG. 2 shows a more detailed block diagram of the integrated system showing an array of magnetoresistive elements 200. The word lines 202 and sense lines 204 form row and column addresses respectively. The word lines 202 and sense lines 204 address and access the array of magnetoresistive elements 200. The word line 202 applies approximately 15 to 25 mA of current to read the data and approximately 40 to 50 mA to write the data. The sense line 204 applies approximately 2 mA of current. The areal density of the integrated semiconductor-magnetic RAM chip is mainly determined by the semiconductor electronics 102 rather than the magnetic elements 104 because the word-line and the sense-line sizes are determined by the dimensions of the current drivers 206 and sense amplifiers 208.

[0030] FIG. 3 is a schematic diagram of one embodiment of the integrated semiconductor-magnetic RAM system having a 4×4 magnetic memory array 300, a CMOS active memory array chip 302, analog amplification electronics 304, and digital control electronics 306. The CMOS array 302 controls row addresses and supplies word current. An analog multiplexer 308 controls column addresses.

[0031] FIG. 4A shows a layout of the magnetic element array 400. The array 400 is arranged in a 4×4 configuration allowing 16 bits to be addressed. The currents I_WORD404 and I_S402 on word line and sense line respectively are used for addressing during read and write cycles.

[0032] FIG. 4B shows a resistance hysteresis curve of a magnetic bit operating to write or read data. Writing occurs when a sense current 402, I_S, is applied to the sense line and a word current 404, I_WORD, is applied to the word line. A “0” bit is written to a magnetoresistive memory cell when the word current 404 applied is +I_W412. A “1” bit is written when the word current 404 applied is −I_W414. Nondestructive reading occurs when a sense current 402, I_S, is applied to the sense line and a negative read current followed by a positive read current is applied to the word line.

[0033] To write to an addressed bit corresponding to a specific memory cell, the magnitude of the word current 404 must be greater than the magnitude of a switching threshold with non-zero sense current 416 of the cell on an active sense line, I_ts. However, the magnitude of the word current 404 on inactive sense lines must be less than a “zero sense current” switching threshold 418, I_t0, to avoid an erroneous write. Therefore, the word current 404 has a write margin which is half of the magnitude of the difference between the zero sense current threshold 418 and the non-zero sense current threshold 416. The hysteresis characteristics allow the widening of the write margin by decreasing the non-zero sense current threshold 416 as sense current 402 increases.

[0034] FIG. 5 shows a plot showing how the switching threshold decreases as the sense current 402 increases. The decrease in the switching threshold of the cell crossed by the active sense line 402 allows data to be written while the other cells crossed by the active word line 404 will not be written but will retain their memory. Correct write/read functionality of all cells in the memory array requires that the word current 404 magnitude be larger than the memory cell with the largest non-zero sense current threshold 416, but smaller than the memory cell with the smallest zero sense current threshold 418.

[0035] FIGS. 6A and 6B show the distribution of the switching thresholds of the memory cells in a memory array. FIG. 6A shows no overlap of the distribution of the zero sense current 418 and the non-zero sense current thresholds 416. The memory array of FIG. 6A is fully functional because it has write margin for the worst-case pair of memory cells.

[0036] FIG. 6B, on the other hand, has no write margin for the cells corresponding to the overlapping region of the switching threshold distributions. The cells with the magnitude of the zero sense current threshold 418 less than the magnitude of the word current 404 could change their state erroneously 604. Cells with the magnitude of the non-zero sense current greater than the magnitude of the word current cannot change their state 602. Therefore, the distribution of the memory cells with the switching thresholds like FIG. 6A is desired.

[0037] FIG. 7 shows a schematic diagram of the memory read electronics which consists of a voltage sense amplifier 700, a sample and hold employing capacitor 714 and switch controlled by Hold signal 710, a comparator 702 and a data latch 704.

[0038] A nondestructive read consists of a three-phase cycle. During the first phase, the voltage developed across the column of cells passing I_s720, while the word current is at −I_R, is amplified 700 and held on capacitor 714 by opening switch controlled by Hold signal 710. The voltage held, V_n708, functions as a reference level for the comparator 702. During the second phase, the word current is switched to +I_Rand causes the resistance of the selected cell 724 to decrease if it contains a “0” or to increase if it contains a “1”. This change in resistance is seen in the hysteresis curves shown in FIG. 4B. The change in resistance is converted to a change in voltage by sense current I_s720, and amplified 700 to generate the read signal, V_p706. The read signal, V_p706, is compared to the reference level, V_n708, by comparator 702 generating a digital signal 712. A positive change in voltage V_p706 results in an output voltage 712 corresponding to a digital “1” and a negative change in voltage V_presults in a digital “0”. The final phase consists of latching the digital value 712 to the data out pin 718 when a Latch Enable signal 716 is asserted.

[0039] FIG. 8 shows the timing diagram of the read process. While the word current is −I_R, the Hold signal 710 is activated after signal V_nhas stabilized. Next, the word current transitions from −I_Rto +I_Rat 800 generating read signal V_p. The comparator 712 generates a digital output 712 based on the values of V_nand V_p. Finally, the digital value is latched to complete the read cycle.

[0040] FIGS. 9A and 9B show timing diagrams of the read and the write processes respectively. advantageous features of the hybrid semiconductor-magnetic RAM system include high density, low power and fast read-write operations. Read and write cycle times of 50 and 20 nanoseconds, respectively, are achievable. Also, virtually no power is consumed during a standby mode while the active power consumption is held to below 100 mW.

[0041] Although only a few embodiments have been described in detail above, those of ordinary skill in the art certainly understand that modifications are possible. All such modifications are intended to be encompassed within the following claims, in which: