Abstract:
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 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.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims benefit of the priority of U.S. Provisional Application Serial No. 60/076,524, filed Mar. 2,1998 and entitled “JPL&#39;s Magnetic Random Access Memory: MagRAM.” 
    
    
     ORIGIN OF INVENTION 
     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 (35U.S.C. 202) in which the Contractor has elected to retain title. 
    
    
     BACKGROUND 
     The present specification generally relates to memory devices. More particularly, the present specification describes an integrated semiconductor-magnetic random access memory. 
     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. 
     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. 
     SUMMARY 
     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. 
     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. 
     A method for writing data to the magnetic memory array cells includes selecting cell address and applying appropriate currents to address lines. 
     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. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     These and other aspects will be described in reference to the accompanying drawings wherein: 
     FIG. 1 shows a block diagram of an integrated semiconductormagnetic random access memory (RAM) system; 
     FIG. 2 shows a more detailed block diagram of the integrated system showing an array of magnetoresistive elements; 
     FIG. 3 shows a schematic diagram of the integrated semiconductor-magnetic RAM system containing a 4×4 magnetic memory array; 
     FIG. 4A shows a layout of the magnetic element array; 
     FIG. 4B shows a resistance hysteresis curve of a magnetic bit operating to write data; 
     FIG. 5 shows a plot of memory state switching threshold versus sense current; 
     FIG. 6A shows distribution of switching thresholds in a memory array with no overlap of thresholds; 
     FIG. 6B shows distribution of switching thresholds in a memory array with overlap of thresholds; 
     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; 
     FIG. 8 shows a timing diagram of a read process; 
     FIG. 9A shows a more detailed timing diagram of a read cycle; and 
     FIG. 9B shows a detailed timing diagram of a write cycle. 
    
    
     Like reference numbers and designations in the various drawings indicate like elements. 
     DETAILED DESCRIPTION 
     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. 
     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. 
     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. 
     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 . 
     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. 
     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 WORD    404  and I S    402  on word line and sense line respectively are used for addressing during read and write cycles. 
     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 W ,  412 . A “1” bit is written when the word current  404  applied is −I W    414 . 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. 
     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. 
     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 . 
     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. 
     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. 
     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 . 
     A nondestructive read consists of a three-phase cycle. During the first phase, the voltage developed across the column of cells passing I S    720 , 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 n    708 , functions as a reference level for the comparator  702 . During the second phase, the word current is switched to +I R  and 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.  4 B. The change in resistance is converted to a change in voltage by sense current I S    720 , and amplified  700  to generate the read signal, V p    706 . The read signal, V p    706 , is compared to the reference level, V n    708 , by comparator  702  generating a digital signal  712 . A positive change in voltage V p    706  results in an output voltage  712  corresponding to a digital “ 1 ” and a negative change in voltage V p  results 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. 
     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 n  has stabilized. Next, the word current transitions from −I R  to +I R  at  800  generating read signal V p . The comparator  712  generates a digital output  712  based on the values of V p  and V p . Finally, the digital value is latched to complete the read cycle. 
     FIGS. 9A and 9B show timing diagrams of the read and the write processes respectively. The 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. 
     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: