Abstract:
A tamper sensing system mounted with respect to a protected structure so as to have corresponding stress changes occur therein in response to selected kinds of tamperings with said protected structure comprising a first pair of stress affected magnetoresistive memory devices each capable of having a magnetic material layer therein established in a selected one of a pair of alternative magnetization states if in a first kind of stress condition and of being established in a single magnetization state if in an alternative second kind of stress condition, and the magnetic material layer in each having a magnetization in a first direction in one of the pair of alternative magnetization states and in a second direction in that remaining one of the pair of magnetization states. A first magnetizing electrical conductor extends adjacent to each of the first pair of stress affected magnetoresistive memory devices to establish said magnetic material layer in that one of said pair of alternative magnetization states thereof so as to have its said corresponding magnetization be oppositely directed with respect to said magnetization of that other. The first pair of stress affected magnetoresistive memory devices can each be provided by a spin dependent tunneling device having differing numbers of magnetization states available thereto depending on whether being in differing ones of alternative stress conditions

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application is a division of U.S. application Ser. No. 12/075,148 filed Mar. 10, 2008 for STRESSED MAGNETORESISTIVE TAMPER DETECTION DEVICES by J. Deak, which in turn claims the benefit of Provisional Patent Application No. 60/906,152 filed Mar. 9, 2007 for STRESSED MAGNETORESISTIVE TAMPER DETECTION DEVICES by J. Deak. U.S. application Ser. No. 12/075,148 is hereby incorporated by reference in its entirety. 
     
    
     BACKGROUND 
       [0002]    The present invention is directed towards electrical power source free tamper detection sensors and, more particularly, sensors that detect and record sufficient mechanical change in the packaging of, or other protective enclosure of, items such as integrated circuits or electronic systems to indicate tampering therewith. 
         [0003]    The prevention of unauthorized handling, or the removal from rightful custody, of personal, corporate, or military items, components, systems or other entities, or the illicit obtaining of information therefrom or thereabout, particularly expensive electronic goods or electronic devices that contain sensitive data, has become increasingly important. This growing importance of such physical security for those entities against such tampering comes about as the numbers of such entities has grown, and as their economic value and the competitive value of sensitive information often contained therein has also increased. The ongoing trend of storing vast amounts of personal and business data and software in a computer hard disc drive, a portable memory device, or other electronic systems makes theft and tamper prevention all the more important. Computers, personal digital assistants (PDAs), cell phones, portable data storage devices, and smart cards, are increasingly available in the home and in the business place. Military systems increasingly contain sensitive hardware and algorithms that cannot be permitted to fall into the hands of a foreign government, or agents thereof, that might attempt to reverse engineer a critical military system for its own use. 
         [0004]    Even larger systems, such as desk-top computers or hardware systems that cannot easily be removed from a home or facility, are also susceptible to theft or compromise: there is a likelihood that valuable information or sensitive information bearing internal components such as non-volatile memory chips, computer hard drives, programmable gate arrays, or application specific integrated circuits may be removed after the exterior cover thereof has been removed. Most such thefts are never solved, and the property is rarely recovered. Therefore there is a need to protect certain entities, such as a computer or other electronic systems, from being opened to remove sensitive components or to recover valuable data including but not limited to security encryption keys and computer algorithms. 
         [0005]    Tamper protection for systems falls into at least three categories; tamper-evident packaging; packaging that is difficult to remove without destroying the protected device; and sensors to detect intrusion to allow a system to erase sensitive data or to destroy critical hardware in the event of tampering. Tamper-evident packaging schemes are limited to low cost devices or low importance data, and do not provide any protection in the situation in which an entire system can be stolen. Anti-tamper coatings or housings that are difficult to remove without destroying the components contained within the housing also do not protect data stored within the enclosure as the data can be recovered from damaged components. Sensors that allow a system to destroy critical data or disable critical components often need electrical power such as from a battery in order to operate. Different types of sensors, including switches to mechanically detect the removal of a lid, optical sensors to detect light, and fabrics containing wires that must be cut to access a component, always require electronics hardware that must be electrically powered to record the tamper event. There is thus a security risk in that a clever tamperer, such as a person skilled in “reverse engineering” or “hacking”, may disable the sensor before the system records the tamper event and responds. There is thus a need for a tampering sensor that can record a tamper event in the absence of applied electrical power. Such a sensor is often made even more useful if it can destroy critical data, such as, but not limited to, an encryption key, in the absence of applied power. 
       SUMMARY 
       [0006]    The present invention provides a tamper sensing system mounted with respect to a protected structure so as to have corresponding stress changes occur therein in response to selected kinds of tamperings with said protected structure comprising a first pair of stress affected magnetoresistive memory devices each capable of having a magnetic material layer therein established in a selected one of a pair of alternative magnetization states if in a first kind of stress condition and of being established in a remaining magnetization state if in an alternative second kind of stress condition, and the magnetic material layer in each having a magnetization in a first direction in one of the pair of alternative magnetization states and in a second direction in that remaining one of the pair of magnetization states, the first pair of stress affected magnetoresistive memory devices being mounted together such that at least one of the first and second directions thereof at least in part parallels an aligning direction that parallels at least in part at least one of said first and second directions of that other one remaining. A first magnetizing electrical conductor extends adjacent to each of the first pair of stress affected magnetoresistive memory devices such that providing an electrical current therethrough to generate a corresponding magnetic field thereabout establishes said magnetic material layer thereof in that one of its pair of alternative magnetization states so as to have its corresponding magnetization thereof be oppositely directed with respect to said magnetization of that other magnetic material layer insofar as components thereof substantially parallel to said aligning direction when each of the first pair of stress affected magnetoresistive memory devices are provided with a first kind of stress condition. 
         [0007]    The first pair of stress affected magnetoresistive memory devices can each be provided by a spin dependent tunneling device having differing numbers of magnetization states available thereto depending on whether being in differing ones of alternative stress conditions. Such a spin dependent tunneling device has an electrically insulative material intermediate layer with two major surfaces on opposite sides thereof, and both a magnetization reference layer on one of said intermediate layer major surfaces a relatively fixed direction of magnetization and a memory film of an a magnetostrictive, anisotropic ferromagnetic material on that remaining one of the intermediate layer major surfaces having a magnetization directed at an angle with respect to said relatively fixed direction due to an effective magnetic bias field being present to result in said device having an electrical resistance versus applied external magnetic field characteristic in a first kind of stress conditions with unequal coercivities for external magnetic fields applied in opposite directions. The magnetostrictive, anisotropic ferromagnetic material is such that the device in one kind of stress conditions has a coercivity with a magnitude exceeding that of the effective magnetic bias field and in another kind of stress conditions has no coercivity with a magnitude exceeding that of the effective magnetic bias field. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0008]      FIGS. 1A and 1B  show graphs with plots of magnetoresistance versus magnetic field for ferromagnetic films, 
           [0009]      FIG. 2  shows schematic representations of a spin dependent tunneling device and a representation of a magnetoresistance versus externally applied magnetic fields characteristic therefor, 
           [0010]      FIG. 3  shows schematic representations of a spin dependent tunneling device having a fixed magnetization layer, 
           [0011]      FIG. 4  shows a graph with plots of magnetoresistance versus magnetic field for a stressed spin dependent tunneling device, 
           [0012]      FIGS. 5A ,  5 B and  5 C show schematic representations of spin dependent tunneling device interlayer magnetic coupling and auxiliary magnetic couplings, 
           [0013]      FIGS. 6A ,  6 B and  6 C show graphs with plots indicating the effects of stress on magnetization states of a patterned ferromagnetic film, 
           [0014]      FIG. 7  shows a schematic representation of a two LATSS element die and the arrangement of the programming conductor used to initialize the device into selected magnetization states, 
           [0015]      FIG. 8  shows cross-section views schematically representing the layers in LATSS elements in a selected interconnection arrangement, 
           [0016]      FIG. 9  shows a schematic diagram of an interconnection arrangement used for a two LATSS element device, 
           [0017]      FIG. 10  shows a mechanical housing arrangement for use with LATSS elements, 
           [0018]      FIG. 11  shows a cutaway view schematically representing a mechanical housing arrangement for LATSS elements in a first condition, 
           [0019]      FIG. 12  shows a cutaway view schematically representing the mechanical housing arrangement of  FIG. 11  in a second condition, 
           [0020]      FIG. 13  shows a schematic diagram bridge circuit arrangement portion used with an analog circuit operated LATSS device, 
           [0021]      FIG. 14  shows a schematic diagram bridge circuit arrangement portion used with a digital circuit operated LATSS device, 
           [0022]      FIGS. 15A and 15B  show a circuit schematic diagram and a corresponding die layout used with analog circuit operated LATSS device arrangement comprising series connected LATSS elements, 
           [0023]      FIG. 16  shows a schematic representation of LATSS memory device arrangement to protect an electronic circuits system, 
           [0024]      FIG. 17  shows a schematic representation of interconnected devices in a system arrangement embodying the present invention, and 
           [0025]      FIG. 18  shows a schematic representation of an LATSS memory device arrangement to protect an electronic circuits system. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    A latching anti-tampering stress based sensor (LATSS) apparatus, and methods for employing same, is useable to provide tamper protection for a protected arrangement through detecting and recording the occurrence of a physical intrusion into that arrangement such as a housing for that device or an encapsulated electronic assembly or the like. LATSS devices are stress sensors operated without supplying electrical power thereto during the time they are used to monitor whether an unacceptable degree of stress has been applied to the protective barrier enclosing the items that the LATSS devices are intended to protect with such monitoring, whether through attempted or actual movement thereof or otherwise, such monitoring beginning after those devices have been electronically set to a monitoring state, i.e., initialized. Once initialized, a LATSS will detect and record whether a significant change in some mechanical aspect of the protective barrier has ever occurred since initialization, and it will then be capable of providing a digital signal that will permanently latch the device into a tamper detected state. The mechanical change will be detected by the sensor as a change in stress occurring in its substrate, which for a properly positioned sensor will result from the removal (and possibly replacement) of a cover that protects an critical electronics system, the removal (and possibly replacement) of a PC board from a rack, or removal of a potting compound, or the like. Once such a mechanical change has been detected, a LATSS device cannot be reset. 
         [0027]    The sensor elements of the LATSS devices herein are based on spin-dependent tunneling (SDT) junctions utilizing magnetostrictive ferromagnetic free layers. SDT junctions are advantageous in that they are compatible with standard semiconductor processing, thereby allowing low cost device fabrication of them. In addition, they are intrinsically radiation hard, permitting the development of radiation hard antitamper (AT) sensors. Magnetostrictive SDT junctions provide an effective combination of capabilities for an AT sensor. The hysteretic magnetic behavior of the SDT electrodes provides the capability for the magnetization of a LATSS device to irreversibly latch into a state indicating the protective barrier has been breached, even in the absence of applied power. The combination of magnetoresistance and magnetostriction together provide for a small, highly sensitive, stress-to-resistance transducer. Here, the magnetoresistance of the SDT junctions permits resistive readout of the orientation of the magnetizations of the ferromagnetic sensor layers, and the magnetostrictive effect couples the magnetization orientation to the stress in the sensor substrate. Magnetic hysteresis allows the devices to irreversibly change the electrical resistance therethrough when the stress on the sensor substrate passes a selected threshold. 
         [0028]    Magnetostriction can be described as a change in dimensions, or strain, exhibited by ferromagnetic materials when they are subjected to a magnetic field. Magnetostrictive ferromagnetic materials also exhibit an inverse effect, known as a magnetomechanical effect, that is, a change in their magnetic behavior as a function of applied stress. The present invention is directed towards passive tamper-detection sensors that couple this stress sensitivity of magnetostrictive materials with the magnetoresistive effect in order to provide a latching stress sensor that will latch into one of two alternative magnetic states to allow a digital readout. For the purpose of the LATSS device disclosed herein, a mathematical description of the latching response of a magnetostrictive sensor element to an applied stress is provided below. This involves showing the dependence of the switching field (H c ) of the sensor on applied stress (σ) in addition to numerical simulation of the magnetic state of the sensor as a function of increasing σ. 
         [0029]    The stress-strain relationship for a thin-film (f) plated onto a thick cantilever beam substrate (s), that is, a thin-film (f) on a thick substrate (s), is described by the following expression known as Stoney&#39;s formula, 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       σ 
                       f 
                     
                     = 
                     
                       
                         1 
                         
                           6 
                            
                           C 
                         
                       
                       · 
                       
                         
                           
                             E 
                             s 
                           
                            
                           
                             d 
                             s 
                             2 
                           
                         
                         
                           
                             ( 
                             
                               1 
                               - 
                               
                                 v 
                                 s 
                               
                             
                             ) 
                           
                            
                           
                             d 
                             f 
                           
                         
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   1 
                   ) 
                 
               
             
           
         
       
     
         [0000]    where E s  is the Young&#39;s modulus of the substrate, d s  the substrate thickness, C the bending radius of the substrate, v s  the Poisson ratio of the substrate, σ f  the film stress, and d f  the film thickness. Stoney&#39;s formula provides a convenient way to measure the stress of a thin-film on a thick substrate with just measuring the bending radius, which is easier to measure than is stress directly. 
         [0030]    The magnetomechanical effect can be described in terms of a magnetic anisotropy field H s  induced by stress or strain that may be expressed as 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       H 
                       s 
                     
                     = 
                     
                       
                         3 
                          
                         
                           σ 
                           f 
                         
                          
                         λ 
                       
                       
                         M 
                         s 
                       
                     
                   
                   , 
                 
               
               
                 
                   ( 
                   2 
                   ) 
                 
               
             
           
         
       
     
         [0000]    or as an anisotropy constant as 
         [0000]    
       
         
           
             
               
                 
                   
                     K 
                     s 
                   
                   = 
                   
                     
                       
                         3 
                          
                         
                           σ 
                           f 
                         
                          
                         λ 
                       
                       2 
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   3 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Here, M s  is the saturation magnetization and λ=ΔL/L is the magnetostriction constant, which refers to the change in length ΔL of a ferromagnetic object relative to the object&#39;s length L at a zero applied magnetic field as the result of a magnetic field large enough to saturate the ferromagnetic object being applied. 
         [0031]    The stress-induced anisotropy, K s , changes the coercivity of the ferromagnetic object. To predict the change in the coercivity, H c , of a micron-sized patterned magnetostrictive ferromagnetic film as a function of σ, the stress anisotropy relation in equation 3 may be added to the Stoner-Wohlfarth description of the coercivity of a ferromagnetic object to yield 
         [0000]    
       
         
           
             
               
                 
                   
                     E 
                      
                     
                       ( 
                       θ 
                       ) 
                     
                   
                   = 
                   
                     
                       
                         - 
                         
                           H 
                           x 
                         
                       
                        
                       
                         M 
                         s 
                       
                        
                       
                         Cos 
                          
                         
                           ( 
                           θ 
                           ) 
                         
                       
                     
                     - 
                     
                       
                         H 
                         y 
                       
                        
                       
                         M 
                         s 
                       
                        
                       
                         Sin 
                          
                         
                           ( 
                           θ 
                           ) 
                         
                       
                     
                     + 
                     
                       
                         K 
                         A 
                       
                        
                       
                         
                           sin 
                           2 
                         
                          
                         
                           ( 
                           
                             θ 
                             - 
                             
                               θ 
                               A 
                             
                           
                           ) 
                         
                       
                     
                     + 
                     
                       
                         
                           
                             N 
                             x 
                           
                            
                           
                             M 
                             s 
                             2 
                           
                         
                         2 
                       
                        
                       
                         
                           Cos 
                           2 
                         
                          
                         
                           ( 
                           θ 
                           ) 
                         
                       
                     
                     + 
                     
                       
                         
                           
                             N 
                             y 
                           
                            
                           
                             M 
                             s 
                             2 
                           
                         
                         2 
                       
                        
                       
                         
                           Sin 
                           2 
                         
                          
                         
                           ( 
                           θ 
                           ) 
                         
                       
                     
                     + 
                     
                       
                         K 
                         S 
                       
                        
                       
                         
                           
                             Sin 
                             2 
                           
                            
                           
                             ( 
                             
                               θ 
                               - 
                               
                                 θ 
                                 S 
                               
                             
                             ) 
                           
                         
                         . 
                       
                     
                   
                 
               
               
                 
                   ( 
                   4 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Here, E is the magnetostatic energy of the patterned magnetic film, N x  and N y  are the demagnetizing factors in the x and y directions in the plane of the film, H x  and H y  the fields applied along the x and y directions, and K A  is the material&#39;s intrinsic anisotropy constant. Note also that θ is the angle of M s  with respect to the x-axis, θ A  is the angle of the ferromagnetic material&#39;s intrinsic easy-axis with respect to the x-axis, and θ S  is the axis along which stress σ is applied. 
         [0032]    Assuming stress σ and the magnetic field are applied along the easy axis α-axis), the field H c  where the magnetization as a result irreversibly flips into the opposite direction in these circumstances can be found by taking the first and second derivative of equation 4 with respect to θ, and solving the resulting system of equations, dE/dθ=0 and d 2 E/dθ 2 =0, for H x =H c . This procedure yields the following dependence of H c  on σ 
         [0000]    
       
         
           
             
               
                 
                   
                     
                       H 
                       c 
                     
                      
                     
                       ( 
                       
                         σ 
                         c 
                       
                       ) 
                     
                   
                   = 
                   
                     
                       
                         2 
                          
                         
                           { 
                           
                             
                               K 
                               A 
                             
                             + 
                             
                               
                                 3 
                                  
                                 
                                   σ 
                                   f 
                                 
                                  
                                 λ 
                               
                               2 
                             
                             + 
                             
                               
                                 μ 
                                 0 
                               
                                
                               
                                 
                                   M 
                                   S 
                                   2 
                                 
                                  
                                 
                                   ( 
                                   
                                     
                                       N 
                                       y 
                                     
                                     - 
                                     
                                       N 
                                       x 
                                     
                                   
                                   ) 
                                 
                               
                             
                           
                           } 
                         
                       
                       
                         M 
                         s 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   5 
                   ) 
                 
               
             
           
         
       
     
         [0000]    This equation is inverted to find the critical stress, σ c , that is required to reverse the magnetization direction in the film. If an external field H x &lt;H c  is applied, the difference between H x  and H c  represents the anisotropy field that must be overcome by the applied stress. Thus, substituting H c -H x  for H c  in this equation and inverting for σ f =σ c , provides a definition for the critical stress for reversal 
         [0000]    
       
         
           
             
               
                 
                   
                     σ 
                     c 
                   
                   = 
                   
                     
                       
                         
                           
                             M 
                             s 
                           
                            
                           
                             ( 
                             
                               
                                 H 
                                 c 
                               
                               - 
                               
                                 H 
                                 x 
                               
                             
                             ) 
                           
                         
                         - 
                         
                           2 
                            
                           
                             { 
                             
                               
                                 K 
                                 A 
                               
                               + 
                               
                                 
                                   μ 
                                   0 
                                 
                                  
                                 
                                   
                                     M 
                                     S 
                                     2 
                                   
                                    
                                   
                                     ( 
                                     
                                       
                                         N 
                                         y 
                                       
                                       - 
                                       
                                         N 
                                         x 
                                       
                                     
                                     ) 
                                   
                                 
                               
                             
                             } 
                           
                         
                       
                       
                         3 
                          
                         λ 
                       
                     
                     . 
                   
                 
               
               
                 
                   ( 
                   6 
                   ) 
                 
               
             
           
         
       
     
         [0033]    Clearly, σ c  can be increased or decreased by the application of a magnetic field. The magnetostriction coefficient, λ, may be positive or negative depending on the kind of ferromagnetic material used in the film, which allows much freedom in configuring the stress response of a LATSS device. For positive λ, compressive (−) stress along the easy-axis reduces H c  while tensile stress increases H c . For negative λ, compressive (−) stress applied along the easy-axis increases H c  while tensile stress reduces H c . The effect of arbitrary orientation of σ and applied fields, rather than just along the easy axis as in the example above, may be simulated numerically by minimization of equation 4. 
         [0034]    The basic dependence for λ&lt;0 and λ&gt;0 with σ applied along the easy-axis of a patterned ferromagnetic film is shown in  FIGS. 1A and 1B . Shown there are the results of a simulation of the effects of no stress,  200 , tensile stress,  220 , and compressive stress,  210 , on 3 μm by 1.5 μm by 5 nm thick elliptical patterned ferromagnetic films. The result for a negative magnetostriction coefficient of λ=−10 −5  is shown in  FIG. 1A  and for a positive magnetostriction coefficient of λ=10 −5  in  FIG. 1B . The magnetic field is applied along the easy-axis and parallel to the stress. Note that the combination of stress and the polarity of the magnetostriction coefficient may be used to increase or reduce the coercive force H. 
         [0035]    Exemplary magnetostrictive materials include alloys of Ni, Fe, Co with the possible addition of other elements including but not limited to Tb, Dy, Ga, Al, B. Hf, and Pd. The magnetostriction of various ferromagnetic materials is given in Table 1. The values given in Table 1 are only approximate. The actual values may differ dependent on material microstructure, film thickness, etc. 
         [0000]    
       
         
               
               
               
             
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                   
                 Material 
                 Saturation Magnetostriction (ppm) 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Fe 
                 7.5 
               
               
                   
                 Ni 
                 −35 
               
               
                   
                 Alfenol 13 
                 40 
               
               
                   
                 Galfenol 
                 400 
               
               
                   
                 Terfenol-D 
                 4000 
               
               
                   
                 Co 70 Fe 30   
                 130 
               
               
                   
                 Ni 45 Fe 55   
                 30 
               
               
                   
                 Ni 80 Fe 20   
                 2 
               
               
                   
                 Ni 90 Fe 10   
                 −12 
               
               
                   
               
             
          
         
       
     
         [0036]    Spin dependent tunneling (SDT) devices provide a simple means for determining the magnetic state of a magnetostrictive ferromagnetic thin-film used as a free layer therein. A SDT device minimally comprises two planar ferromagnetic thin-films, a free layer and a pinned magnetization direction layer, that are separated by a thin insulating tunneling barrier, as represented schematically in  FIGS. 2 and 3 , and with the magnetoresistance versus externally applied magnetic field data from such a SDT device also being shown in  FIG. 2 . The applied field is provided parallel to the pinned direction shown in  FIGS. 2 and 3 . Tunneling magnetoresistance (TMR) is defined as the maximum change in device electrical resistance divided by the device minimum resistance. The minimum resistance occurs when the magnetizations of both magnetic layers are parallel, and the maximum resistance when the magnetizations of the two layers are oriented anti-parallel to one another. The offset,  300 , in the loops plot is due to Nèel coupling between the pinned and free layers. A typical layered structure of a SDT device with a top-pinned ferromagnetic film is schematically represented in  FIG. 3 . The free layer  330  and pinned layer ferromagnet  310  are typically composed of CoFeB or NiFeCo. The tunnel barrier  310  is usually either Al 2 O 3  or MgO. The antiferromagnet  305 , used to set the orientation of the pinned layer ferromagnet, is typically of IrMn or CrPtMn. 
         [0037]    The insulating barrier, although commonly composed of Al 2 O 3  or MgO, the fabrication thereof need not be limited to these insulating materials. The ferromagnetic layers are usually alloys of Ni, Fe, and Co and are often alloyed also with non-ferromagnetic materials like B, Cr, Zr, Al, Mo, and Ta, but need not be restricted to these materials. One ferromagnetic layer, as the free layer, has a magnetization that is free to rotate in response to an applied magnetic field. The other ferromagnetic layer, as the pinned layer, has a magnetization that is not free to respond to magnetic fields of magnitudes at which the free layer begins to respond. The pinning is accomplished either with a single antiferromagnetic pinning layer, such as FeMn or IrMn (see  FIG. 3 ) exchange coupled to a ferromagnetic layer, or through the use of a high coercivity ferromagnetic layer. The orientation of the free layer&#39;s magnetization relative to the pinned layer&#39;s magnetization determines the electrical resistance through the SDT device, and serves as a probe of the stress in the free layer if the free layer is composed of a magnetostrictive material. 
         [0038]    Electrical conduction through a SDT device is of a different nature from that of an anisotropic magnetoresistance (AMR) formed of a single ferromagnetic layer or common giant magnetoresistive (GMR) effect of a ferromagnet—normal metal—ferromagnet structure, in which the electrical current flows parallel to the plane of the substrate. Electrical conduction through a SDT device is similar to that of a vertical giant magnetoresistance (VGMR) device insofar as the current flows instead perpendicular to the plane of the substrate. In both the VGMR and SDT arrangements, the resistance of the devices is proportional to the cosine of the angle between the magnetizations of the two ferromagnetic films. The magnetoresistance is a minimum when the magnetizations of the two magnetic layers are parallel and greatest when they are anti-parallel. In the VGMR effect, this field-varying portion of the resistance typically represents an increase from the nominal (magnetizations parallel) resistance on the order of 1%, whereas in SDT junctions it represents 20-70% for Al 2 O 3  and can approach 400% for SDT devices utilizing an MgO barrier. 
         [0039]    Due to the perpendicular to the substrate direction of the electrical conduction therein, the resistance of VGMR and SDT devices decreases with increasing planar areas of the devices. In contrast to VGMR elements, however, the SDT junction “resistance-area product” (RA) can be adjusted over many orders of magnitude by controlling the tunneling barrier thickness. SDT materials thus provide great flexibility in that they can be deposited on silicon wafers with RA ranging from 0.05 MΩ-μm 2  to 10,000 MΩ-μm 2 . The improved control over cell resistance and improved magnetoresistance of SDT devices allow for a large cell signal and optimal matching with peripheral sensing circuitry. 
         [0040]    The results of measuring of the effect of stress on the magnetoresistive hysteresis loop [R(H)] of a SDT junction using a CoFeB as free layer is shown in  FIG. 4 . Voltage values representing resistance values in three TMR traces for the SDT conducting a constant measurement basis electrical current therethrough are each provided for the SDT being under one of three corresponding stress conditions. The SDT sample is formed as a Si(100)-Si 3 N 4 —Ru—CoFeB—Ta—CoFeB—Al 2 O 3 —CoFeB—Ru—FeCo—CrMnPt SDT junction pair after annealing at 250° C. for one hour. 
         [0041]    CoFeB has a positive λ=30 ppm; thus, when a large tensile stress is applied, CoFeB develops an easy axis in the direction parallel to a tensile stress. In the situation shown in  FIG. 4 , when tensile stress is applied perpendicular to the applied field direction of the zero-stress easy-axis R(H) loop, it transforms into a hard axis R(H) loop, that is it becomes linear and closed. This means that the intrinsic magnetic anisotropy of the free magnetic layer with no strain becomes small compared to the stress-induced anisotropy. When a tensile stress is applied parallel to the applied field direction of the zero-stress easy-axis R(H) loop, the coercivity increases and the R(H) loops become more square. The easy-axis coercivity is thus enhanced because the stress-induced anisotropy then adds to the material anisotropy. This experiment compares well with the simulated results for positive magnetostriction in  FIG. 1B . 
         [0042]    The magnetic behavior of the free layer in a SDT junction is altered by the presence of the pinned layer. In a typical SDT device material layers stack shown in  FIG. 3 , free ferromagnetic layer  330  tends to show an offset due to magnetic coupling thereof to pinned layer  310 . This magnetic coupling is a result of the stray magnetic field emanating from the edges of the pinned layer and also due to an exchange coupling through the non-magnetic layer separating the free and pinned layers. These coupling fields are illustrated in a corresponding layered magnetic structure in  FIG. 5A  showing the coupling fields  400  and  410  between pinned layer  310  and free ferromagnetic layer  330 , and they are a result of free magnetic charges  405  at the pinned-layer  310 /tunnel barrier  320  interface, H coup    400 , and those that exist at the edges of the pinned layer, H stray    410 . Free magnetic charges  405  form due to the roughness of the tunnel barrier/pinned layer interface and the edges of the pinned layer. The field  400  due to the interface roughness is called Nèel coupling, and it tends to favor parallel alignment of the free and pinned layer magnetizations. The stray field coupling  410  tends to favor anti-parallel alignment of the free and pinned layer magnetizations. 
         [0043]    The stray field coupling, H stray    410 , decreases with increasing bit size, decreasing pinned layer thickness, and decreasing pinned layer saturation magnetization. The magnetic exchange coupling (Nèel coupling, H coup    400 ) between the two magnetic electrodes in these SDT devices can be controlled through the roughness of the seed layer, the thickness of the barrier, and the kinds of ferromagnetic materials used. The dependence of H coup    400  on the magnetic and physical properties of the SDT junction is described by the well-known equation for Nèel coupling, 
         [0000]    
       
         
           
             
               
                 
                   
                     H 
                     coup 
                   
                   = 
                   
                     
                       
                         
                           π 
                           2 
                         
                          
                         
                           h 
                           2 
                         
                          
                         
                           M 
                           P 
                         
                       
                       
                         
                           2 
                         
                          
                         Γ 
                          
                         
                             
                         
                          
                         
                           d 
                           f 
                         
                       
                     
                      
                     
                       exp 
                       ( 
                       
                         
                           
                             - 
                             2 
                           
                            
                           π 
                            
                           
                             2 
                           
                            
                           
                             t 
                             b 
                           
                         
                         Γ 
                       
                       ) 
                     
                   
                 
               
               
                 
                   ( 
                   7 
                   ) 
                 
               
             
           
         
       
     
         [0000]    Here, M P  is the pinned layer magnetization, d f  is the free layer thickness, t b  the tunnel barrier of thickness, Γ the roughness wavelength, and h the roughness amplitude. The magnitude of the coupling can easily be varied from 1 to 20 Oe. The competition between H coup    400  and H stray    410  can be used to control the offset of the freelayer hysteresis loop. Generally, SDT devices, having free layer  330  and pinned layers  310  composed of alloys of Ni, Fe, and Co with a thickness on the order of 1 to 20 nm and patterned with dimension much less than 1 μm, will be dominated by H stray    410 , while those larger than 1 μm will be dominated by H coup    400 . 
         [0044]    The bias or offset field may also be produced by the addition of other magnetic components layers due to either being provided with the semiconductor die or external thereto. One possibility is shown in  FIG. 5B . Here, a spacer layer (S)  350  and bias layer (BL)  340  are added to the stack. BL  340  is to have its magnetic moment pinned. This can be accomplished by adding an additional antiferromagnet adjacent to BL  340 . The BL  340  can offset the hysteresis loop of the freelayer (FL)  330  by two possible means. One method would be to use the stray field  380  emanating from the edges of the BL. In this case, the S layer  350  can be a thick metal layer or an additional tunnel junction. Alternatively, the S layer could be composed of a thin metal such as Cu. In this case, RKKY exchange coupling could be used to produce a bias field  390  parallel to the moment of BL  340 . 
         [0045]    Yet another technique for producing the bias or offset field would be to add bias structures (BS)  370  adjacent to the LATSS element. This is illustrated in  FIG. 5C . In this case, the BS devices  370  could either be composed of a high coercivity ferromagnet or they could be a ferromagnet that is pinned by an antiferromagnet. These bias structures would be used to produce a stray field  380  that offsets the hysteresis of the LATSS element&#39;s FL  330 . 
         [0046]    The stress dependent switching behavior of a magnetostrictive SDT element in the presence of a small bias field  600 , as might be produced by one of the device arrangements shown in  FIG. 5 , is the basis on which the LATSS devices will operate. The graphs of  FIG. 6A  show the magnetization response to an applied magnetic field at 0 (inner loop) and 100 MPa (outer loop) of applied tensile stress. Note the M(H) loops are offset from zero field, typically through one of various methods for providing bias field  600  described in, but not limited to those in,  FIG. 5 . That is, the bias field is built into the device using one of these various techniques, including roughness dependent Nèel coupling between the pinned and free layer or the addition of bias structures. The bias field  600  must have sufficient magnitude such that the stressed state has two stable remanent values  601  and  602 , and the unstressed state only has one possible remanent value  603 . Alternatively, the bias field  600  must be less then the coercivity of the LATSS element under one stress condition  604  and greater than the coercivity in the other stress condition  605 .  FIG. 6B  shows a simulation of the effect of stress on the magnetic behavior of the LATSS element. The sequence of events is as follows:
   1) The film is stressed at 60 MPa and a magnetic field is applied to set the magnetization into the positive direction. A magnetic field in excess of 350 Oe is applied and then removed to set the freelayer of the LATSS element in a preferred magnetic state. This is the “SET” condition.   2) If stress is removed from the substrate, the LATSS free layer coercivity will decrease from 350 Oe to 50 Oe. At some critical value of the applied stress, the coercivity falls below the bias field&#39;s magnitude  600 , and the magnetization irreversibly changes direction. This is defined as the “TAMPERED” state.   3) At zero applied stress, the LATSS is in the reversed magnetization TAMPERED state.   4) If the stress is reapplied, the magnetization stays in the low state indicating tampering has occurred. This is the “TAMPERED” state.
 
The output of the device is indicated by the resistance measured across the tunnel barrier. Resistance data measured as a function of stress on a LATSS element prototype is shown in  FIG. 6C  The configuration for which  FIG. 6C  is provided indicates a low resistance when tampering occurs.
   
 
         [0051]    The LATSS element can be designed to signal a tamper event during operation in response to either tensile or compressive stress by varying the geometry of the LATSS elements and/or orientation of the applied stress. There are three important axes in a LATSS sensor that may be oriented to achieve a desired sensor response. These are the intrinsic (zero applied stress) easy-axis of the free layer, the direction of the bias magnetic field, and the direction of an applied stress. 
         [0052]    The intrinsic easy-axis is due to the magnetic anisotropy of the SDT junction free layer in zero applied stress. This anisotropy is due in part to the anisotropy of the ferromagnetic material from which the free layer is composed, and in part due to the shape of the free layer. Generally the easy-axis would be set parallel to bias and reset fields, but there may be some advantage in terms of the micromagnetic state that results in the LATSS freelayer in rotating it away from the direction of the set or bias fields. Generally, the switching response can be shown to be more reproducible and the switching field can be reduced if the field is not perfectly aligned with the intrinsic easy-axis of the freelayer. Also, the device could rely solely on the stress-induced anisotropy to hold the initially predetermined SET state stable. In this case, the orientation of the intrinsic easy-axis may be changed in order to find an optimal orientation for robust device operation. In any case, an easy axis may be induced in the material by deposition, including sputtering, evaporation, and electroplating, in an applied magnetic field, or annealing the ferromagnetic materials in the presence of an applied magnetic field may induce it. Alternative techniques, including but not limited to irradiation, annealing, or deposition in the presence of applied stress may also be used. The intrinsic easy-axis is also controlled by the shape into which the free layer is patterned. If the shape is patterned so that one axis is longer than the other, then the magnetization will tend to align along the long axis. 
         [0053]    The magnetic bias field axis can be set during device manufacture using on-chip magnets or hard magnetic layers or through the use of an electromagnet or permanent magnet that might be placed off-chip but in the vicinity of the LATSS device during use. A very simple means for providing a biasing field that is set during manufacture is through the use of controlled roughness of the pinned layer resulting in Nèel coupling as described by equation 7. Additionally, bias layers as illustrated in  FIG. 8  might be applied on top of, beneath, or adjacent to the LATSS elements an on the die during manufacture. 
         [0054]    Stress on the LATSS die can be created by several means, including but not limited to curvature of the LATSS substrate, from strain due to mismatch between a seed layer and the materials in the SDT junction stack, through the use of a stressing layer above or below the stack. The stress of the LATSS device may be changed by changing the deformation of the LATSS substrate as shown in  FIGS. 7 ,  10 ,  11 , and  12 . There are many mechanical means to induce curvature in the substrate that are obvious to those skilled in the art. In addition to mechanical means, other methods might include temperature induced strain through a large mismatch in thermal coefficient of expansion of the various materials in the device. 
         [0055]    Table 2 below shows the flexibility in LATSS configurations that can be obtained by varying material properties, and orientation, of the various axes. The “λ” heading indicates the sign and magnitude of the magnetostriction constant of the LATSS element freelayer; the “init σ orient” heading describes the orientation of the axis along which stress is applied relative to the magnetic easy-axis (EA) of the LATSS freelayer; the “Hcoup” heading refers to direction of the bias field with respect to the magnetic moment of the LATSS element, where ∥ means parallel to the remanent magnetic moment and AP means anti-parallel to the remanent magnetic moment; and the “stable σ” heading refers to the stress condition that produces a stable predetermined SET state. The parameter “σ c ” is the stress threshold for switching triggering the LATSS into the TAMPERED state as defined in equation 6. Violation of the “stable σ” condition irreversibly triggers the TAMPERED state. 
         [0000]    
       
         
               
               
               
               
               
             
               
               
               
               
               
               
             
           
               
                 TABLE 2 
               
               
                   
               
               
                   
                 λ 
                 init σ orient 
                 Hcoup 
                 stable σ 
               
               
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 &gt;0 
                 ∥ EA 
                 ∥  
                 Mrem 
                 all 
               
               
                   
                 &gt;0 
                 ∥ EA 
                 AP  
                 Mrem 
                 σ &lt; σ c     
               
               
                   
                 &gt;0 
                 ⊥ EA 
                 ∥  
                 Mrem 
                 all 
               
               
                   
                 &gt;0 
                 ⊥ EA 
                 AP  
                 Mrem 
                 σ &gt; −σ c   
               
               
                   
                 &gt;0 
                 none 
                 ∥  
                 Mrem 
                 all 
               
               
                   
                 &gt;0 
                 none 
                 AP  
                 Mrem 
                 σ x  &lt; σ c,  σ y  &gt; −σ c   
               
               
                   
                 &lt;0 
                 ∥ EA 
                 ∥  
                 Mrem 
                 all 
               
               
                   
                 &lt;0 
                 ∥ EA 
                 AP  
                 Mrem 
                 σ &gt; −σ c   
               
               
                   
                 &lt;0 
                 ⊥ EA 
                 ∥  
                 Mrem 
                 all 
               
               
                   
                 &lt;0 
                 ⊥ EA 
                 AP  
                 Mrem 
                 σ &lt; σ c     
               
               
                   
                 &lt;0 
                 none 
                 ∥  
                 Mrem 
                 all 
               
               
                   
                 &lt;0 
                 none 
                 AP  
                 Mrem 
                 σ x  &gt; −σ c,  σ y  &lt; σ c   
               
               
                   
               
             
          
         
       
     
         [0056]    Because the LATSS element freelayers are ferromagnetic, a magnetic field generated externally to the LATSS device could be applied to orient them into a desired state. This method could be used to reset a single LATSS element after it is tripped. In order to make this impossible, an array of LATSS elements is usually built onto the device substrate and configured so that they are written into opposing free layer magnetization direction states when the device is in the untampered or predetermined “SET” condition. Tampering would then be indicated when all elements in the LATSS array are written into the same free layer magnetization direction state from which they cannot all be returned to the initial SET state by an external magnetic field. This can be accomplished using the arrangement shown in  FIG. 7 . Here two LATSS elements  703  and  704  are set into opposing collinear orientations using an on-chip write conductor  700 . The bits need not be perfectly collinear, and misalignment might be preferred for making the device harder to reset with an external field or for improving switching characteristics. Regardless of the deviation from perfect alignment, with respect to the programming conductor, current  705  through the programming conductor  700 , produces opposing magnetic fields  701  at each of the LATSS elements that forces them to have their free layer magnetizations directed into the predetermined opposite directions SET state. Because the LATSS elements are so small and close together, it is impossible to generate an external magnetic field with sufficient gradient to trick the device into returning both to the “SET” state in which the free layer magnetizations of both LATSS elements are oppositely oriented. This can only be accomplished with establishing electrical current through programming conductor  700 . 
         [0057]    There are two additional benefits to using an array of LATSS elements for the device. First, the LATSS elements can each be used as a reference with respect to the other which provides the basis for a straightforward arrangement in providing operational temperature compensation. Second, they permit a tri-state operating mode that has two different acceptable predetermined “SET” conditions. 
         [0058]    An arrangement for determining the magnetization states (or resistance states) of a series connected pair of LATSS elements is illustrated in  FIG. 8 . In  FIG. 8 , “BL” is a thick ferromagnetic bias layer used to offset the hysteresis loop of the “FL” or magnetostrictive free layer. “PL” is the pinned layer. Both the PL and BL magnetizations are fixed in orientation. The “SET  0 ” and “SET  1 ” states are the untampered states that are programmed using a programming conductor  700  arrangement like that shown in  FIG. 7 . In the “TAMPERED” state, the stress on the device causes the coercivity of the magnetostrictive FL layers to be less than the offset field produced by the BL layers. The FL layers are then forced to align parallel to the stray field produced by the BL layers. The tunnel barrier between the FL and PL layers dominates the resistance of the devices which is “R 1 ” for LATSS element  703  and “R 2 ” for LATSS  704 . Thus, R 1 &lt;R 2  is SET 0 , R 1 &gt;R 2  is SET 1 , and R 1 =R 2  indicates the TRIPPED or tampered state. The arrangement shown in  FIG. 8  may be queried by measuring the resistance between connections  800  and  801 . In this configuration, it is impossible to determine the difference between “SET 0  and SET 1 ”, but both the SET states are different from the TRIPPED or tampered state. In both the SET 0  and SET 1  states, the resistance of the two LATSS element arrange would be R 1 +R 2 =High R+Low R. This indicates an untampered condition. When stress is removed from the LATSS substrate as would be expected to occur during tampering, the freelayer of both the  703  and  704  LATSS elements must orient to the left. Due to the stray field form the biasing layers. This produces an output R 1 +R 2 =2 (High R). This is higher than the resistance that occurs when both  703  and  704  are oriented in either the set  1  or set  0  condition, and it thus indicates a tamper event has occurred. 
         [0059]    An alternative digital readout scheme utilizing two LATSS elements  703  and  704  that permits tri-state readout is indicated in  FIG. 9 , and summarized in Table 3. Here, two LATSS elements  703  and  704  are integrated with CMOS circuitry to allow their resistance values to be compared to each other thereby enabling a tri-state digital output. The purpose for doing this would be to make it less likely that a tamperer could restore the device to its original state after tripping the device. Again, programming conductor  700  has a geometry that produces opposing fields at each of the LATSS elements  703  and  704 . This is used create a predetermined SET state with each of the LATSS elements  703  and  704  free layers having their magnetizations forced into opposite directions so that one LATSS element is always in a high resistance state while the other is in a low resistance state. The resistance of each LATSS element is determined by passing a current through the tunnel barrier and measuring the resulting voltage. The Read Bias Circuit  905  is used to source a small measurement basis electrical current though each of the LATSS elements  703  and  704 . The resulting voltage produced at each LATSS elements  703  and  704  are compared in the comparator circuit  906 . 
         [0060]    The output of the device is then determined as indicated in Table 3. 
         [0000]                                          TABLE 3               Voltage measured    Output Value           at LATSS Elements   V out 1 − V out 2   Comment                                V1 &gt; V2   1   Untampered − SET       V1 &lt; V2   −1   Untampered − SET       V1 = V1   0   TAMPERED                    
The values “V1” and “V2” refer to the voltages generated across each LATSS element by the on-chip read bias circuit. The output refers to the output signal from the on-chip comparator.
 
         [0061]    The foregoing LATSS device can be configured for covert deployment by building them into standard DIP or SOIC packages, so they look like conventional circuit elements. Such a housing arrangement can be used to detect the removal of a lid as is illustrated in  FIG. 10 . The LATSS SOIC package is implemented and deployed as follows: (1) A preformed open-cavity package  1000  is made with an additional cavity  1001  under the position where the die  1002  will be attached. (2) The die  1002  is placed into the preformed SOIC package, and it is electrically connected to the lead frame  1003  by one of various techniques including but not limited to wire bonding or flip-chip bonding. (3) The preformed SOIC package is then potted with a plastic or epoxy material  1004 . The potting material is sufficiently plastic that it transfers downward force from the top of the package to the LATSS die  1002 . When pressure is applied to the LATSS package, for example using the enclosure cover  1005 , the encapsulating material forces the LATSS die to bend into the cavity, producing stress on the LATSS elements that is far enough above the critical stress threshold that the LATSS device may be initialized into a predetermined SET state. Removal of the cover  1005  removes stress cause the LATSS die to pull out of the cavity  1001 , relieving stress on the LATSS elements thereby forcing the LATSS device into the TRIPPED or tampered state. 
         [0062]    A covertly deployed LATSS device may be used to protect other devices provided in the vicinity of that LATSS device. A schematic housing illustration for such a covertly deployed LATSS device used to protect other components is shown in  FIGS. 11 and 12  respectively. The protective LATSS devices in a SET state when lid  1005  is in place and trips into a TRIPPED or tampered state when that lid is removed. The LATSS device would be initialized into a SET state as follows:
   1) Set the cover  1005  in place. The cover encloses other components that need to be protected  1130  in addition to the LATSS device and provides the stress on the die  1002  needed to produce the stable initial state in the LATSS.   2) Initialize the magnetization orientation of the sensors using current in the programming conductor  700  to set the individual LATSS elements into opposite states, i.e. one high resistance, the other low resistance. This is the “SET” condition.   3) If needed, burnout a fuse in the LATSS device so that it cannot be reprogrammed through programming conductor  700 . The “fuse” could be implemented in various manners which depend on LATSS implementation. One such arrangement would be to design the write conductor  700  so that a portion of the write conductor  700  vaporizes during the initialization process. Thinning, or “necking down”, a portion of  700  can accomplish this.   
 
         [0066]      FIG. 12  shows the mechanical LATSS devices packaging arrangement of  FIG. 11  with cover removed resulting in the LATSS switching into the TRIPPED state. Removal of the protective lid  1005  relieves mechanical stress on the die  1002 . This can force LATSS element into a low resistance TRIPPED or tampered state through the process described in  FIG. 6B . The LATSS device cannot be reset if a one-time-programmable technology, such as a fuse, is used in the programming circuit. There is probably no need to make a more complicated fusing mechanism for this type of single output LATSS device as it would be a more complicated procedure to defeat the LATSS fuse than finding other methods to invade the other protected components in the absence of a signal coding security arrangement. Thus, for improved security, several LATSS devices could be deployed, and the devices could be programmed into various states “1” or “0”, that is, the collection of LATSS devices would store a code that would be difficult to reproduce. 
         [0067]    For use as an analog tri-state tamper sensor or a more robust digital tamper sensor, an array of 4 or more LATSS elements may be combined into a Wheatstone bridge circuit as shown in  FIGS. 13 through 15 . This arrangement simplifies the electronic system design by increasing the size of the output signal relative to the background signal, and also by better compensating for noise and thermal drift. The output of the bridge circuit is more immune to damage from electrostatic discharge and thus would permit the LATSS output to be directly accessed to thereby eliminate the need for on-chip circuitry. The bridge circuit arrangement  1300  for an analog LATSS bridge is shown in  FIG. 13  and the output thereof, as a function of the resistance values of the different LATSS elements  1310 - 1340 , is given in Table 4. In the SET states, LATSS 1   1310  and LATSS 4   1340  must have the same resistance value, and LATSS 2   1320  and LATSS 3   1330  in those states must also have the same resistance. However, the LATSS pair  1320 / 1330  cannot have the same resistance value as the LATSS pair  1310 / 1340 . The programming field polarity with respect to the programming current required to achieve the SET state is indicated by the “H+” and “H−” labels in  FIGS. 13 and 14 .  FIG. 14  shows a digital LATSS bridge that is formed by adding a comparator  1350  to the output of the bridge circuit. 
         [0068]    Tampering is detected by the bridge circuit arrangement when the stress on the die changes beyond the critical stress σ c , then, all LATTS elements  1310 ,  1320 .  1330 , and  1340  will be forced into the same resistance state producing the TRIPPED or tampered output value. The output state as a function of the LATSS element resistances is summarized in Table 4 for the bridge circuit. The columns in the table are headed as follows: “Set I” is the polarity of the programming or set current; “Tamper” indicates whether or not a change in substrate stress large enough to trip the LATSS elements into a low resistance (L) or high resistance (H) state has occurred; “1” through “4” indicates the resistance of the LATSS elements as labeled in  FIGS. 14 and 14  after programming or after a tamper event; “analog Output” is the value of V1-V2 in  FIG. 13 ; and digital output is the three state output from the comparator  1360  in  FIG. 14 . 
         [0000]    
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
               
             
           
               
                 TABLE 4 
               
             
             
               
                   
               
               
                   
                   
                   
                   
                   
                   
                   
                 Digital Output 
               
             
          
           
               
                 Set I 
                 Tamper 
                 1 
                 2 
                 3 
                 4 
                 Analog Output 
                 Value 
                 Tamper 
               
               
                   
               
               
                 + 
                 No 
                 R L   
                 R H   
                 R H   
                 R L   
                 
                   
                     
                       
                         
                           
                             
                               R 
                               H 
                               2 
                             
                             - 
                             
                               R 
                               L 
                               2 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   R 
                                   L 
                                 
                                 + 
                                 
                                   R 
                                   H 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                          
                         
                           V 
                           bias 
                         
                       
                     
                   
                 
                 1 
                 0 
               
               
                   
               
               
                 − 
                 No 
                 R H   
                 R L   
                 R L   
                 R H   
                 
                   
                     
                       
                         
                           
                             
                               R 
                               L 
                               2 
                             
                             - 
                             
                               R 
                               H 
                               2 
                             
                           
                           
                             
                               ( 
                               
                                 
                                   R 
                                   L 
                                 
                                 + 
                                 
                                   R 
                                   H 
                                 
                               
                               ) 
                             
                             2 
                           
                         
                          
                         
                           V 
                           bias 
                         
                       
                     
                   
                 
                 0 
                 0 
               
               
                   
               
               
                 Don&#39;t 
                 Yes 
                 R H   
                 R H   
                 R H   
                 R H   
                 ~0 
                 Don&#39;t 
                 1 
               
               
                 Care 
                   
                   
                   
                   
                   
                   
                 Care 
                   
               
               
                 Don&#39;t 
                 Yes 
                 R L   
                 R L   
                 R L   
                 R L   
                 ~0 
                 Don&#39;t 
                 1 
               
               
                 Care 
                   
                   
                   
                   
                   
                   
                 Care 
               
               
                   
               
             
          
         
       
     
         [0069]      FIG. 15A  shows the layout for a LATSS device utilizing a bridge circuit where each leg in the bridge circuit is a series connected string of LATSS elements. The sensor legs  1510 ,  1520 ,  1530 , and  1540  are interconnected as shown in  FIGS. 13 and 14 , and they are each composed of strings of 2 or more individual LATSS elements electrically connected together in series. This is used to decrease the voltage drop across each tunnel junction and, in addition, for use in averaging random defect dependent switching characteristics. A programming conductor  1500  is wrapped into a spiral coil in order to produce the programming field polarities as given in  FIGS. 13 and 14 .  FIG. 15B  shows the layout of a corresponding LATSS die  1560  with an integrated magnetic shield  1570  used to shield the LATSS sensors form ambient magnetic fields. 
         [0070]    One way for protecting a system using a LATSS configured as an anti-tamper sensor is shown in  FIG. 16 . Here a LATSS device  1600  protects the system components located under shield  1005 . A tamper response and LATSS set circuit  1620  is used to initialize the LATSS device through a tamper/set bus  1660  and also to monitor the device status through the tamper/set bus  1660 . The tamper response and LATSS set circuit  1620  are configured to set the LATSS device based on an input line  1650 . Circuit  1620  can also interface with the protected system/components  1630  through a tamper response bus  1670 . The communication between tamper response and LATSS set circuit  1620  and the protected system/components  1630  is to permit the system data to be cleared or to produce some other reset or self-destruct response in order to protect information stored within the protected system/components  1630  block. The protected system/components  1630  is capable of communicating with another system that is external to the protective shield  1005 , through a system I/O bus  1640 . 
         [0071]    LATSS based anti-tamper security may be further improved by configuring an assembly of LATSS elements as a memory device. This provides a memory device that has the capability to respond to a tampering event. The response would be destruction of data stored within the LATSS memory device. One possible means for producing a LATSS memory is shown in  FIG. 17 . Here, a two element LATSS memory element  1700  is composed of three select MOS transistors  1730  and two LATSS elements  1705  that are written into opposing states. This arrangement is often referred to as a 3 transistor, 2 magnetic tunnel junction (3T2MTJ) bit cell. Each 3T2MTJ cell is operated a manner analogous to the operation of the 2-element LATSS described in  FIG. 9 . The select transistors  1730  are used to permit the read and write operation of individual LATSS memory cells within the memory array. These 3T2MTJ LATSS memory cells may be combined with readout circuitry  1710  similar to the comparator shown in  FIG. 9  capable of determining the three state response of the memory element  1700 , and write circuitry  1720  for producing the current used to program the LATSS elements  1705 . Addressing circuitry, not shown in  FIG. 17 , is used to control the state of the select transistors to select a particular 3T2MTJ LATSS element for reading or writing. 
         [0072]    Another way for protecting a system using a LATSS configured as an anti-tamper memory is shown in  FIG. 18 . Here, a LATSS memory device  1805  protects a system or components located under protective cover  1005 . A tamper response and LATSS set circuit  1820  is used to initialize the LATSS memory device  1805  through a read/set bus  1860 . The tamper response and LATSS set circuit  1820  may be configured to program a code into the LATSS device based on an input line  1850 . This code remains intact until tampering that is removal of  1005  occurs. The communication between tamper response and LATSS set circuit  1820  and the protected system/components  1830  is to permit the system data within the protected system block  1830  to be cleared or to produce some other reset or self-destruct response. Alternatively, the protected system block  1830  may interface directly with the LATSS memory device  1805  through a data bus  1815 . In this case, the LATSS memory may be used to hold encryption keys used for encrypting data within the protected system block  1830 , or the LATSS memory may hold sensitive runtime or other data that needs to be destroyed in the event of a tamper. When sensitive data or keys are stored within the LATSS memory  1805 , the tamper event destroys them thereby disabling the system and rendering it difficult to reverse engineer. The protected system/components  1830  may or may not communicate with another system that is external to the protective cover  1005 , through a system I/O bus  1840 . 
         [0073]    Although the present invention has been described with reference to preferred embodiments, workers skilled in the art will recognize that changes may be made in form and detail without departing from the spirit and scope of the invention.