Patent Publication Number: US-2005116261-A1

Title: In-pixel memory for display devices

Description:
The present invention relates to in-pixel memories and in-pixel memory circuits, particularly for display devices. The present invention also relates to methods of forming such in-pixel memories and in-pixel memory circuits. The present invention is particularly suited to, but not limited to, providing in-pixel memory circuits in active matrix liquid crystal display devices.  
      Known display devices include liquid crystal, plasma, polymer light emitting diode, organic light emitting diode, and field emission. Such devices comprise an array of pixels, usually in rows and columns. In active matrix display devices, each pixel is typically associated with one or more respective switching devices, such as thin film transistors, to provide an array of pixels and switching devices. In operation, the pixels are addressed according to an addressing scheme in which each pixel is regularly refreshed for each frame to be displayed with display data (e.g. video) specifying the intensity level the pixel is to display. Usually the addressing scheme selects the pixels on a row-by-row basis and provides individual intensity levels on a column-by-column basis.  
      One development in the field of display devices is to provide in-pixel memories, whereby a respective memory device is provided for each pixel, the memory devices being arranged in an array corresponding to the pixel array. Static images may then be displayed without a need to refresh, thereby saving power. This is potentially particularly attractive for display devices for portable devices such as mobile telephones, cordless telephones, personal digital assistants, and so on.  
      It is known to use static random access memory (SRAM) and dynamic random access memory (DRAM) circuits for such in-pixel memory. Conventionally only one memory device (formed by a circuit) is provided for each pixel. A separate array of SRAM or DRAM circuits is provided in addition to the pixel and switching device array. This involves either a further entire manufacturing process in addition to that used for the pixel and switching device array, or the need for a large number of additional masking stages.  
      Quite separate from display device technology, one type of memory device is magnetoresistive random access memory (MRAM), in which a tunnel current depends on a magnetisation direction of two so-called magnetic electrodes. MRAM provides non-volatile memory. Use of such a memory (in applications unrelated to displays) is described for example in “Magnetoelectronic memories last and last . . . ”, Mark Johnson, IEEE Spectrum, February 2000, pages 33-40.  
      One problem with the use of MRAM is that in operation MRAM provides, as its output, different resistance states (as opposed to e.g. a voltage change). Furthermore, the difference between the resistance states is low, usually less than 35%.  
      The present invention uses MRAM technology to provide in-pixel memory for display devices, in ways that alleviate the problems described above.  
      In a first aspect, the present invention provides a memory circuit comprising one or more MRAMs coupled to a read-out circuit. The read-out circuit is preferably a flip-flop circuit. Preferably the memory circuit comprises two MRAMs, the flip-flop circuit comprises two inputs, and each of the two MRAMs is coupled to a respective one of the flip-flop circuit inputs.  
      In a further aspect the present invention provides a display device comprising a plurality of pixels and a plurality of memory circuits according to the first aspect, each pixel associated with or comprising a respective one of the memory circuits.  
      In a further aspect the present invention provides a drive line arrangement for an in-pixel memory in which a drive line, for example a bit line, is arranged to pass over and contact a first MRAM in a first direction and a second MRAM in a second direction, the first and second directions being in the plane of drive line and substantially opposite to each other. This provides opposite resistance states in the two MRAMs. Preferably the bit line is laid out such that it passes over the first MRAM then turns or meanders back on itself before passing over the second MRAM.  
      In a further aspect the present invention provides a drive line arrangement for an in-pixel memory in which a bit line is arranged such that it avoids passing over a gate line, thereby avoiding or reducing gate overlap capacitance losses.  
      In a further aspect the present invention provides an in-pixel memory structure for an active matrix display device and a method of forming thereof in which a word line for the in-pixel memory is formed during a same masking stage as a display driving line, for example a gate line.  
      In a further aspect the present invention provides an in-pixel memory structure for an active matrix display device and a method of forming thereof in which a bit line for the in-pixel memory is formed during a same masking stage as a display drive line, for example a column address line.  
      In further aspects the present invention provides memory circuits or structures including one or more MRAMs and a flip-flop circuit for use in applications other than display applications, for example as sensors, preferably medical sensors.  
      Further aspects are as claimed in the appended claims. 
    
    
      Embodiments of the present invention will now be described, by way of example, with reference to the accompanying drawings, in which:  
       FIG. 1  is a schematic illustration (not to scale) of a liquid crystal display device;  
       FIG. 2  is a schematic illustration of a sample 2×2 portion of an array of pixels;  
       FIG. 3  is a schematic illustration of a simple MRAM stack;  
       FIG. 4  is a circuit diagram of an in-pixel memory circuit;  
       FIG. 5  shows further details of the overall pixel circuitry for a pixel;  
       FIG. 6  shows a schematic diagram, not to scale, of a constructional layout employed for a pixel;  
       FIG. 7  is a flowchart showing certain process steps used to form an in-pixel memory structure;  
       FIG. 8  shows a cross-section between points X-X indicated in  FIG. 6 ;  
       FIG. 9  shows a preferred MRAM stack in cross-section (not to scale); and  
       FIGS. 10 and 11  show the results of simulations performed for the in-pixel memory circuit described with reference to  FIG. 4 . 
    
    
       FIG. 1  is a schematic illustration (not to scale) of a liquid crystal display device  1 , comprising two opposed glass plates  2 ,  4  (or any other suitable transparent plates). The glass plate  2  has an active matrix layer  6 , which will be described in more detail below, on its inner surface, and a liquid crystal orientation layer  8  deposited over the active matrix layer  6 . The opposing glass plate  4  has a common electrode  10  on its inner surface, and a liquid crystal orientation layer  12  deposited over the common electrode  10 . A liquid crystal layer  14  is disposed between the orientation layers  8 ,  12  of the two glass plates. Except for any active matrix details described below, in particular in relation to in-pixel memory, the structure and operation of the liquid crystal display device  1  is the same as the liquid crystal display device disclosed in U.S. Pat. No. 5,130,829, the contents of which are contained herein by reference.  
      Certain details of the active matrix layer  6 , relevant to understanding this embodiment, are illustrated schematically in  FIG. 2  (not to scale). The active matrix layer  6  comprises an array of pixels. Usually such an array will contain many thousands of pixels, but for simplicity this embodiment will be described in terms of a sample 2×2 portion of the array of pixels  20 - 23  as shown in  FIG. 2 .  
      In the field of display devices, there is often some variation in what is intended to be covered by the term “pixel”. For convenience, in this example each pixel  20 - 23  is to be considered as comprising those elements of the active matrix layer  6  relating to that pixel in particular. The pixel  20  includes, inter-alia, a thin-film-transistor (TFT)  24 , an in-pixel memory circuit  25 , a drive circuit  26  and a pixel electrode  27 . The TFT  24  and pixel electrode  27  are conventional, and may for example be as described in the earlier mentioned U.S. Pat. No. 5,130,829. The in-pixel memory circuit  25  and drive circuit  26  are not found in conventional liquid crystal devices, and will be described in more detail below.  
      The other pixels  21 - 23  comprise respective TFTs  28 ,  32 ,  36 , in-pixel memory circuits  29 ,  33 ,  37 , drive circuits  30 ,  34 ,  38  and pixel electrodes  31 ,  35 ,  39 .  
      Also provided as part of the active matrix layer  6  is a plurality of addressing lines, as follows. Pixels  20  and  21  form a first row of the array of pixels, and pixels  22  and  23  form a second row of the array. The first row is provided with a polarity line  40 , a refresh line  41 , a read line  42 , a word line  43  and a gate line  44  extending across the whole row. Also, a bit line  45  is provided for pixel  20 , and a bit line  46  is provided for pixel  21 . Likewise, the second row is provided with a polarity line  47 , a refresh line  48 , a read line  49 , a word line  50  and a gate line  51  extending across the whole row, a bit line  52  for pixel  22 , and a bit line  53  for pixel  23 .  
      Pixels  20  and  22  form a first column of the array of pixels, and pixels  21  and  23  form a second column. The first column is provided with a column line  54 . Likewise, the second column is provided with a column line  55 .  
      By way of example, further details of the connections of the various pixel components and addressing lines, and operation of the pixels, will now be described for the case of pixel  20 , but the following description applies in corresponding fashion to the other pixels  21 - 23 .  
      The input to TFT  24  is connected to the column line  54 , and the gate of the TFT is connected to the gate line  44 , as in a conventional active matrix liquid crystal device. The output of the TFT  24  is connected to the bit line, which is connected to both the in-pixel memory circuit  25  and the pixel electrode  27 . The word line  43  is connected to the in-pixel memory circuit  25 . The read line  42  is connected to the in-pixel memory circuit. The polarity line  40  and the refresh line  41  are each connected to the drive circuit  26 . The in-pixel memory circuit has two separate connections to the drive circuit  26 . The drive circuit  26  is connected to the pixel electrode.  
      In operation, as with conventional active matrix display devices, row selection is performed via the gate line  44  and intensity level data is provided via the column line  54 . The output of the TFT  24 , i.e. in effect the intensity level data, is delivered to the pixel electrode via the bit line  45 . This in itself corresponds to conventional operation of an active matrix display device. However, here, additionally the output from the TFT  24  is also delivered by the bit line  45  to the in-pixel memory circuit, and driving of the pixel electrode  27  by the drive circuit  26  is controlled by the resulting memory setting of the in-pixel memory circuit  25 , as will be described in more detail below. The drive circuit  26  and the in-pixel memory circuit  25  are further controlled by inputs provided via the polarity line  40 , the refresh line  41  and the read line  42 , as will also be described in more detail below.  
      Before describing the above mentioned features in further detail, it will be helpful to provide an outline summary of the operation of a MRAM structure.  FIG. 3  shows a schematic illustration of a simple MRAM stack. The MRAM stack comprises two ferromagnetic layers, namely a free layer  102  and a pinned layer  106 , each made for example of Ni 81 Fe 19  and having a thickness of several nanometres, separated by an insulation layer  104 , being for example 1 to 2 nm thick and made for example from A 1   2 O 3 . The free layer  102  and the pinned layer  106  are each often referred to as magnetic electrodes. The insulation layer  104  serves as a tunnel barrier layer. An electrical contact is made with the free layer  102  and with the pinned layer  106 . In this example, these are the bit line  45  and a contact  108  (in the pixel array embodiment shown in  FIG. 2 , such a contact of each MRAM is connected to the flip-flop circuit  64  via a respective flip-flop connection as will be described in more detail below). A further electrical supply line is provided below the MRAM stack but insulated therefrom. This further electrical supply line runs orthogonal to the bit line  45 , i.e. in and out of the page in  FIG. 3 . In this example, this further electrical supply line is the word line  43 .  
      The MRAM stack operates as follows. The pinned layer  106  has a fixed magnetisation orientation shown by arrow  110 . The free layer is capable of being switched between two magnetic orientations, as indicated by double-headed arrow  112 . A write current  114 ,  116  is applied to the bit line  45  and the word line  43  to control or set the magnetic orientation  112  of the free layer. This may be set either parallel to or anti-parallel to the magnetic orientation  110  of the pinned layer  106 . These two possibilities are each stable when set if no further write current  114 ,  116  is applied.  
      These two states are distinguishable, i.e. capable of being read-out, as follows. A read-out current  118 ,  120 ,  122  may be passed through the MRAM stack from the bit line  45  to the contact  108  due to tunnelling of electrons through the tunnel barrier layer  104 . The resistance encountered by this current depends upon the tunnelling resistance of the tunnel barrier layer  104 , which itself directly depends upon whether the magnetic orientation  112  of the free layer  102  is parallel to or anti-parallel to the magnetic orientation  110  of the pinned layer  106 . The maximum resistance variation of present MRAM stacks is however typically only about 35%.  
      Further details of the MRAM stacks employed in the present embodiment will be described later below, but these outline details should assist in understanding details of the pixel array being described, in particular the function of the word line  43  which passes under the MRAM stacks but does not directly connect to them, and the bit line  45  and contact  108  (connected in this embodiment to the flip-flop circuit  64 ) which are in direct contact with respective ends of the MRAM stack.  
       FIG. 4  is a circuit diagram of the in-pixel memory circuit  25 . The in-pixel memory circuit  25  comprises two MRAMs  60 ,  62  and a flip-flop circuit  64 . The flip-flop circuit comprises two p-type transistors, implemented as TFTs and hereinafter referred to as a first p-type TFT  66  and a second p-type TFT  67 ; and two n-type transistors, implemented as TFTs and hereinafter referred to as a first n-type TFT  68  and a second n-type TFT  69 . The TFTs are arranged to provide in effect two input chains, a first input chain, in this example comprising the first p-type TFT  66  and first n-type TFT  68 , connected to the first MRAM  60 , and a second input chain, in this example comprising the second p-type TFT  67  and the second n-type TFT  69 , connected to the second MRAM  62 . The remaining end of each of the input chains of the flip-flop circuit  64  is connected to the read line  42 . The respective other ends of the first MRAM  60  and the second MRAM  62  are connected to the bit line  45 . (Operation of the MRAMs also involves the word line  43 , as will be described later below, but for clarity this is not shown in  FIG. 4 .) The flip-flop circuit comprises two output connections, hereinafter referred to as a first output connection  70  and a second output connection  71 , which provide the two (complementary) flip-flop circuit outputs, represented, as is conventional, as D and {overscore (D)} in  FIG. 4 .  
      In this example the detailed connections of the flip-flop circuit  64  components are as follows. Each TFT  66 - 69  comprises, in conventional fashion, one gate and two source/drain terminals (hereinafter referred to as a first and a second terminal). In operation, one of the source/drain terminals functions as the source of the TFT and the other of the source/drain terminals functions as the drain of the TFT. The question of which source/drain terminals serves as the source and which serves as the drain at any particular moment is determined by the polarity of the applied voltage at that moment.  
      The first terminal of the p-type TFT  66  and the first terminal of the second p-type TFT  67  are connected to each other and to the read line  42 . The gate of the first p-type TFT  66 , the gate of the first n-type TFT  68 , the second terminal of the first p-type TFT and the first terminal of the second n-type TFT  69  are connected to each other and to the first output connection  70 . The second terminal of the first p-type TFT  66 , the first terminal of the first n-type TFT  68 , the gate of the second p-type TFT  67  and the gate of the second n-type TFT  69  are connected to each other and to the second output connection  71 . The second terminal of the first n-type TFT  68  is connected to the first MRAM  60 . The second terminal of the second n-type TFT  69  is connected to the second MRAM  62 .  
      In operation, the MRAMs are set at particular resistance states using the bit line  45  and word line  43 , and these states are read-out by the flip-flop circuit  64  operating as follows. Initially the bit line  45  and the read line  42  are at the same potential, for example 0V. The voltages on the two nodes of the flip flop,  70  and  71 , will be substantially the same. In order to read the state of the MRAMs the read line is made positive with respect to the bit line, for example by switching it from 0V to 3V, thus applying a power supply voltage to the flip flop circuit. The voltages on both nodes of the flip flop circuit will initially start to charge towards the mean value of the voltages on the bit and read lines, 1.5V. The rate of change of the voltages on the nodes will depend on the resistance of the MRAM elements, the resistance of the TFTs and the capacitance of the nodes of the circuit. One of the MRAM elements will have a lower resistance than the second. For example the resistance of MRAM element  60  may be lower than MRAM element  62 . In this case the voltage on the flip flop node  70  will become more positive than that on node  71 . This voltage difference is then amplified by the positive feedback within the flip flop circuit so that node  70  settles at the potential on the read line, 3V, and node  71  settles at the voltage on the bit line, 0V.  
       FIG. 5  shows further details of the overall pixel circuitry for the pixel  20 . In addition to those items already described above (and indicated by the same reference numerals as used above),  FIG. 5  shows further details of the drive circuit  26 , and its connection, along with that of the bit line  45 , to the pixel electrode  27 . This connection to the pixel electrode  27  is shown in circuit terms, as is conventional, as connection to a storage capacitor  80  of capacitance C s  and a capacitance C LC  of the liquid crystal cell formed by the liquid crystal layer  14  between the pixel electrode  27  and the opposing common electrode  10 .  
      The drive circuit  26  comprises, in this example, four transistors, implemented as TFTs and hereinafter referred to as a first drive circuit TFT  75 , a second drive circuit TFT  76 , a third drive circuit TFT  77  and a fourth drive circuit TFT  78 . The second drive circuit TFT  76  is a p-type TFT; the other three drive circuit TFTs  75 ,  77 ,  78  are n-type TFTs. The drive circuit TFTs  75 - 78  are arranged to provide a single drive input to the pixel electrode  27  based on the two outputs D and {overscore (D)} from the flip-flop circuit  64 .  
      In this example the detailed connections of the drive circuit TFTs  75 - 78  are as follows. The gates of the first drive circuit TFT  75  and the third drive circuit TFT  77  are connected to each other and to the refresh line  41 . The gates of the second drive circuit TFT  76  and the fourth drive circuit TFT  78  are connected to each other and to the polarity line  40 . The first terminal of the first drive circuit TFT  75  is connected to the first flip-flop output connection  70 . The first terminal of the third drive circuit TFT  77  is connected to the second flip-flop output connection  71 . The second terminal  75  of the first drive circuit TFT  75  is connected to the first terminal of the second drive circuit TFT  76 . The second terminal of the third drive circuit TFT  77  is connected to the first terminal of the fourth drive circuit TFT  78 . The second terminal of the second drive circuit TFT  76  and the second terminal of the fourth drive circuit TFT  78  are connected to each other and to the pixel electrode  27 , i.e. to the storage capacitor  80  and the liquid crystal capacitance  82 .  
      In operation, signals are applied to the polarity line  40 , the refresh line  41 , the read line  42 , the word line  43 , the gate line  44  and the column line  54  as follows, and consequently the drive circuit operates as follows to provide the required input to the pixel electrode  27 , i.e. to the storage capacitor  80  and the liquid crystal capacitance  82 . One way in which the circuits of  FIG. 5  may be operated in order to provide appropriate drive signals for the liquid crystal capacitance is as follows. The liquid crystal normally requires a drive voltage waveform which alternates in polarity with respect to the common electrode of the display. This is achieved by driving the pixel with positive and negative drive signals in successive pixel refresh periods. In order to refresh the pixel electrode with a positive drive signal the data must first be read from the MRAMs. Initially the word line and the read line are at the same potential, for example 0V. The read line is then switched to a positive voltage level, for example 3V, and the flip flop circuit  64  takes on a state determined by the state of the MRAMs. If MRAM  60  has a higher resistance than MRAM  62  then node  70  will settle at a voltage level of 0V and node  71  will settle at a voltage of 3V. The pixel is refreshed by taking the signal on the refresh line from a low voltage level to a high voltage level. This turns on the two transistors  75  and  77  allowing the data voltages generated by the flip flop circuit to be passed to the liquid crystal capacitance. During the positive refresh period the polarity line is held at a high voltage level. This turns on transistor  78  so that the liquid crystal capacitance becomes charged to the voltage present on node  71  which in this example is 3V. After the liquid crystal capacitance has been charged the refresh line is returned to a low voltage level, turning off transistors  75  and  77  and the voltage on the read line is returned to 0V.  
      In order to refresh the pixel electrode with a negative drive signal the data must again be read from the MRAMs but in this case this is achieved by taking the word line to a negative voltage level, for example −3V. If MRAM  60  has a higher resistance than MRAM  62  then node  70  will settle at a voltage level of −3V and node  71  will settle at a voltage of 0V. The pixel is refreshed by once again taking the signal on the refresh line from a low voltage level to a high voltage level. During the negative refresh period the polarity line is held at a low voltage level. This turns on transistor  76  so that the liquid crystal capacitance becomes charged to the voltage present on node  70  which in this example is −3V. After the liquid crystal capacitance has been charged the refresh line is returned to a low voltage level, turning off transistors  75  and  77  and the voltage on the read line is again returned to 0V.  
      In the case where the resistance of MRAM  60  is higher than that of MRAM  62  the liquid crystal capacitance is driven with a voltage waveform having an amplitude of 6V. In the case where a normally white transmissive TN LC effect is being employed this would cause the pixel to be dark. If the relative resistance of the MRAMs is reversed so that MRAM  60  has a lower resistance than MRAM  62  then the voltages generated on the two nodes of the flip flop,  70  and  71 , would also be reversed. As a result a voltage of 0V would be applied to the liquid crystal capacitance in both the positive and negative, refresh periods. This would cause the liquid crystal pixel to appear light.  
      While the pixel is being operated using data from the MRAM rather than data supplied via the column line the gate line is held at a low voltage in order to keep transistor  24  in a non-conducting state.  
      In the above described version of drive circuit  26 , in some circumstances the status of the flip-flop may not be completely determined initially, or it may not be completely discharged between frames. This may leave remaining charge which may skew a read-out from the MRAMs. This is avoided or alleviated in another possible version of the drive circuit  26 , in which the p-type TFT  76  and the n-type TFT  77  are omitted, i.e. the drive circuit instead comprises just the n-type TFT  75  and the n-type TFT  78 . Then, although these TFTs  75 ,  78  may normally be alternated to change the polarity on the liquid crystal, they may instead both be switched on so as to reset the flip-flop circuit  64 .  
       FIG. 6  shows a schematic diagram, not to scale, of the constructional layout employed for the pixel  20  in this embodiment. For clarity, the drive circuit  26 , the polarity line  40 , the refresh line  41  and the read line  42  are not shown. Indeed, the benefits of the constructional layout to be described below are achieved independently of these items that are not shown. Those items already mentioned above which are shown in  FIG. 6  are the word line  43 , the gate line  44 , the TFT  24 , the column line  54 , the bit line  45 , the pixel electrode  27  and the flip-flop circuit  64 .  
      The various components and lines are each formed using conventional thin film deposition, masking and etching processes, as for conventional active matrix display devices.  FIG. 7  is a flowchart showing certain process steps used to form the in-pixel memory structure shown  FIG. 6 .  
      At step s 2 , the word line  43  and the gate line  44  are formed in the same masking stage. Thus, advantageously, the word line  43 , which is used in relation to operation of the in-pixel memory and would not be present in a conventional active matrix display device without in-pixel memory, is provided during a masking stage that is anyway needed for the conventional device (to provide the gate line  44 ), i.e. without the need for an additional masking stage. Also, the gate dielectric may be used to form a dielectric layer between the MRAM and the word line  43 .  
      At step s 4 , the first MRAM  60  and the second MRAM  62  are formed as respective MRAM stacks above the word line  43 , using a half tone mask. This represents one of only two additional mask steps (compared to a conventional active matrix display device) required in this embodiment to add the additional features shown in  FIG. 6 . The positions of the MRAM stacks of the first MRAM  60  and the second MRAM  62 , as viewed from above, are indicated by items  84  and  85  respectively.  
      At step s 6  the bit line  45  and the column line  54  are formed in the same masking stage as each other. Thus, advantageously, the bit line  45 , which is used in relation to operation of the in-pixel memory and would not be present in a conventional active matrix display device without in-pixel memory, is provided during a masking stage that is anyway needed for the conventional device (to provide the column line  54 ), i.e. without the need for an additional masking stage.  
      Also formed at step s 6 , i.e. this masking stage, are two connections hereinafter referred to as a first flip-flop connection  86  and a second flip-flop connection  87 . The first flip-flop connection  86  connects the flip-flop circuit  64  to a first contact-via connected to the bottom of the first MRAM  60 , i.e. effectively connects the first n-type TFT  68  of the flip-flop circuit  64  to the first MRAM  60 . The position of the first contact-via as viewed from above is shown by item  88  in  FIG. 6 . Likewise, the second flip-flop connection  87  connects the flip-flop circuit  64  to a second contact-via connected to the bottom of the second MRAM  62 , i.e. effectively connecting the second n-type TFT  69  of the flip-flop circuit  64  to the second MRAM  62 . The position of the second contact-via as viewed from above is shown by item  89  in  FIG. 6 . (Forming the contact-vias represents the second of the two additional mask steps, compared to a conventional active matrix display device, required in this embodiment to add the additional features shown in  FIG. 6 .)  
      Returning to considering the bit line  45 , another optional advantageous feature is included in this embodiment, as follows. The bit line  45  is arranged so that a current flowing along it passes or crosses over the first MRAM  60  in a first direction (in terms of  FIG. 6 , in the direction up the page as indicated by arrow  90 ) and passes or crosses over the second MRAM  62  in a second direction (in terms of  FIG. 6 , in the direction down the page as indicated by arrow  91 ), the first and second directions being substantially opposing directions (in the plane of the bit line). This has the effect of producing different, i.e. opposite resistance states between the first MRAM  60  and the second MRAM  62 , since in one MRAM stack the current will produce a magnetic field into the page (i.e. down the respective MRAM stack) and in the other MRAM stack the current will produce a magnetic field out of the page (i.e. up the other MRAM stack). This arrangement of the bit lane advantageously increases the distinction achieved in the overall resistance states of the pair of MRAMs.  
      In this embodiment the bit line  45  is arranged to pass over the two MRAMs in substantially opposing directions by laying out the bit line  45  as shown in  FIG. 6 , i.e. if one considers a hypothetical reference line between the positions of the first and second MRAMs, the bit line  45  passes over the first MRAM  60  in a direction substantially perpendicular to the reference line, then turns on itself, and then also passes over the second MRAM  62  in a direction substantially perpendicular to the reference line, but in the opposite sense, i.e. substantially 180° different to the first pass. In other words, the bit line is laid out such that it passes over the first MRAM  60  then turns or meanders back on itself before passing over the second MRAM  62 .  
      Yet another advantageous feature is included in this embodiment, as follows. The word line  43  is positioned between the gate line  44  and the pixel electrode  27 . This means the bit line  45  does not need to pass over the gate line  44 . This reduces the amount of overlap capacitance that would otherwise be caused by the bit line  45  overlapping the gate line  44 .  
      Further details of the construction of the in-pixel memory of this embodiment will now be described with reference to  FIG. 8  which shows a cross-section between the points X-X indicated on  FIG. 6 . The word line  43  runs along the bottom of the section. A dielectric layer  94  is present over the word line  43 , insulating the word line  43  from the MRAM (as mentioned earlier, this dielectric layer  94  may be formed using the gate dielectric layer). A conductor layer, which will serve as a MRAM contact extension  96 , is provided on the dielectric layer  94 . A further dielectric layer  95   a,    95   b,    95   c  is provided over and around the MRAM contact extension  96 . The MRAM stack  97  of the first MRAM  60  is formed at one end of the MRAM contact extension  96 . The bit line  45  is provided over the top of the MRAM stack  97 . A contact-via  98  is provided above the other end of the MRAM contact extension  96 . The first flip-flop connection  86  runs along the further dielectric layer  95   a  to the contact-via  98 . Thus connection is made between the flip-flop circuit  64  and the MRAM stack  97 , via the contact-via  98  and the MRAM contact extension  96 . It will be appreciated that in other embodiments such connection can be made in any other convenient manner.  
      The present invention may be embodied using any appropriate MRAM stacks, for example simple ones as described above with reference to  FIG. 3 . However, in this embodiment a preferred MRAM stack design is employed.  
       FIG. 9  shows this preferred MRAM stack in cross-section (not to scale). The layers will now be described in the order they are deposited during formation of the MRAM stack, this being up the page as shown in  FIG. 9 . The bottom contact is in this embodiment the previously described MRAM contact extension  96 , which extends beyond the edge of the rest of the MRAM stack to allow contact as described earlier. The MRAM contact extension  96  is an approximately 3.5 nm thick Ta layer, and serves also as a buffer layer in terms of the mechanical properties and deposition process for the MRAM stack.  
      The next layer is a (conducting) layer  132  comprising an approximately 2 nm thick layer of Ni 81 Fe 19 . The next layer is an exchange-biasing layer  134  comprising an approximately 20 nm thick layer of Pt 50 Mn 50 .  
      The next layer is a pinned layer  106  (using the same reference numeral as in  FIG. 3 ), i.e. magnetic electrode. This pinned layer  106  is here made up of three layers, i.e. a first Co 90 Fe 10  layer  136  of approximate thickness 3 nm, a Ru layer  138  of approximate thickness 0.8 nm and a second Co 90 Fe 10  layer  140  of approximate thickness 3 nm. The second Co 90 Fe 10  layer  104  has the fixed magnetic orientation  110  described earlier in  FIG. 3 . The first Co 90 Fe 10  layer  136  has a fixed magnetic orientation  141  that is anti-parallel to the fixed magnetic orientation  110  of the second Co 90 Fe 10  layer  104 . The use of two such coupled layers instead of one single ferromagnetic layer is known in the art of ferromagnetism as using an artificial antiferromagnetic layer, also referred to as a synthetic ferrimagnet. Further details of the composition may be found in patent WO99/58994, which is incorporated herein by reference.  
      The next layer is a tunnel barrier layer  104  (using the same reference numeral as in  FIG. 3 ), which here comprises an approximately 0.8 nm thick layer of oxidized Al.  
      The next layer is a free layer  102  (using the same reference numeral as in  FIG. 3 ). This free layer  102  is here made up of two layers, i.e. a Co 90 Fe 10  layer of approximate thickness 4 nm and a Ni 80 Fe 20  layer of approximate thickness 10 nm, with two switchable and opposing magnetic orientations shown by double-headed arrow  112  (using the same reference numeral as in  FIG. 3 ).  
      The next layer is a protective (conducting) layer  146  comprising an approximately 10 nm thick Ta layer.  
      The top contact is provided by the bit line  45 , as described earlier above.  
       FIGS. 10 and 11  show the results of simulations performed for the in-pixel memory circuit described with reference to  FIG. 4 .  FIG. 10  shows the results for one of the states of the two MRAMs  60 ,  62 .  FIG. 11  shows the results for the other of the states of the two MRAMs  60 ,  62 . In both  FIGS. 10 and 11 , the x-axis  162  is time in microseconds, the y-axis  160  is voltage in volts, plot  164  shows the first output D of the flip-flop circuit  64 , plot  166  shows the second (complementary) output {overscore (D)} of the flip-flop circuit  64 , plot  168  shows the voltage across the first MRAM  60 , and plot  170  shows the voltage across the second MRAM  62 .The difference in the resistance of the two MRAMs was taken as 24% (i.e. one of the pair has a resistance 12% higher than the average value and the other has a resistance 12% lower than the average) with the average resistance of the two MRAMs being 50 kΩ. The simulation results show that the voltage over the MRAMs is no greater than 0.57V, which is satisfactory since this is below the breakdown voltage level of the tunnel junction which is typically about 1V. The values of the threshold voltages of the TFTs  66 - 69  used in the simulation were about 1V, which represents a low threshold voltage device compared to many used in production. The plots of D ( 164 ) and {overscore (D)} ( 166 ) show successful provision of distinct logical outputs capable of driving the active matrix display device.  
      The embodiment described above comprises a number of advantageous features in combination. However, in other embodiments many of these may be implemented singly or in any combination of two or more, as for example in the following cases.  
      In further embodiments, the circuit arrangements described with reference to  FIG. 2  and/or  FIG. 3  and/or  FIG. 5  are employed, but with any suitable constructional layout and formed by any suitable deposition process being employed rather than those described above. Another possibility is for the. MRAM and flip-flop arrangement to be as described above, but with any suitable drive circuit rather than the drive circuit described above. Similarly, other flip-flop circuit designs, and/or other MRAM stack designs, and/or pixel electrode details, and/or switching component details, and/or drive line details, and so on may be used instead of those described above.  
      In a further embodiment, the use of a flip-flop circuit may be used to fetch out the differing resistance state of a single MRAM serving as in-pixel memory.  
      In further embodiments, more than two MRAMs may be provided for each pixel, and arranged in any suitable manner for providing for example increased read-out capability. For example, if four MRAMs are provided for each pixel, the bit line may be arranged to pass over two of the MRAMs in one direction and over the other two MRAMs in the opposing direction.  
      In further embodiments, two (or more) MRAMs may be provided for a single pixel, to provide increased read-out capability, but using any suitable read-out arrangement rather a flip-flop circuit. In particular, the two (or more) MRAMs may be arranged such that a write current passes over them in opposing directions such that differing resistance states are directly provided.  
      In other embodiments, two (or more) MRAMs may be arranged such that a write current passes over them in opposing directions such that differing resistance states are directly provided, and the arrangement by which the write current passes over in the opposing directions may be implemented in any suitable manner, i.e. not necessarily by means of the bit line pattern or concepts described above.  
      In other embodiments, in the deposition process, the word line is provided at the same stage as the gate line, for any suitable in-memory pixel design.  
      In other embodiments, in the deposition process, the bit line is deposited at the same stage as the column line, for any suitable in-memory pixel design.  
      In other embodiments, the bit line is positioned between the pixel electrode and the gate line, such that the bit line does not pass over the gate line, for any suitable in-memory pixel design.  
      In other embodiments the above possibilities may be applied to other types of active matrix.  
      In other embodiments the above possibilities may be applied to devices using other types of liquid crystal, or indeed any other suitable display device type, including for example plasma, polymer light emitting diode, organic light emitting diode, and field emission display devices.  
      In other embodiments, memory structures or circuits comprising two or more MRAMs and a flip-flop circuit may be employed in applications other than display devices. For example, they may be used for sensors, for example medical sensors.