Abstract:
A system, including: a first current mirror having a first current, formed of multiple devices disposed on a substrate, where, when a stress is present, a behavior of a device of the multiple devices forming the first current mirror depends on a direction in which the device of the multiple devices forming the first current mirror is disposed on the substrate; a second current mirror having a second current, formed of multiple devices disposed on the substrate, where, when the stress is present, a behavior of a device of the multiple devices forming the second current mirror depends on a direction in which the device of the multiple devices forming the second current mirror is disposed on the substrate; and a device for measuring a ratio of a difference between the first current and the second current to a sum of the first current and the second current.

Description:
STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
       [0001]    The present invention was made with the support of the government of the United States under contract NBCH020055 awarded by the Defense Advanced Research Projects Administration. The United States government may have certain rights in the present invention. 
     
    
     BACKGROUND 
       [0002]    A typical computer system includes at least a microprocessor and some form of memory. The microprocessor has, among other components, arithmetic, logic, and control circuitry that interpret and execute instructions necessary for the operation and use of the computer system. In addition to the microprocessor and memory, a computer system has integrated circuits (ICs) that have various functionalities and communication paths, i.e., buses and wires that are necessary for the transfer of data among the aforementioned components of the computer system. 
         [0003]    An integrated circuit (IC) may have hundreds of thousands up to millions of individual devices (e.g. transistors) deposited on a single semiconductor substrate. Such an IC is commonly referred to as a “Very Large Scale Integration” (VLSI) IC or “chip.” 
         [0004]    A common building block of an IC, such as a VLSI chip, is a device known as a “transistor.” A transistor is a semiconductor device often partially made from silicon that regulates current or voltage and acts as a switch or gate for electronic signals. One type of transistor has three terminals that are electrically attachable to other electrical devices, such as, for example, resistors and power supplies, and even to other transistors in order to carry out an electrical circuit&#39;s function. A transistor&#39;s attributes, for example, length (L), width (W), and carrier mobility (μ) (a ratio of a carrier&#39;s velocity to an applied external voltage) dictate how well the transistor conducts electrical current and under what circumstances particular behavior occurs. 
         [0005]    One factor that may affect the operation of a transistor is the stress on the transistor. Stress is a measure of the force producing or tending to produce deformation in a body. Components of stress may be measured in units of pressure. The metric unit for pressure is the Pascal (Pa). Stress on a silicon-based transistor affects the transistor&#39;s carrier mobility (μ). Stresses as large as 10 8  Pa may produce mobility changes (Δμ/μ) on the order of 10%. The change in carrier mobility (Δμ/μ) has a component dependent on the direction in which the transistor is positioned (δ) (also referred to as an “direction-dependent mobility change”) and a component that is independent of the direction in which the transistor is positioned (α) (also referred to as an “direction-independent mobility change”), i.e., Δμ/μ is equal to (α+δ): 
         [0000]    
       
         
           
             
               
                 Δ 
                  
                 
                     
                 
                  
                 μ 
               
               μ 
             
             = 
             
               α 
               + 
               
                 δ 
                 . 
               
             
           
         
       
     
         [0000]    The above expression may often be approximated for computational simplicity as: 
         [0000]    
       
         
           
             
               
                 Δ 
                  
                 
                     
                 
                  
                 μ 
               
               μ 
             
             = 
             
               
                 α 
                 + 
                 δ 
               
               ≈ 
               
                 δ 
                 . 
               
             
           
         
       
     
         [0006]    A transistor may be designed such that the orientation of its parts cancels the direction-dependent mobility change (δ=0) effect of a stress. In such cases, only the direction-independent mobility change (α) is present: 
         [0000]    
       
         
           
             
               
                 Δ 
                  
                 
                     
                 
                  
                 μ 
               
               μ 
             
             = 
             
               α 
               . 
             
           
         
       
     
         [0007]    When in use, VLSI chips may be subject to a variety of stresses. Typical stresses include mechanical loading due to the packaging of these chips, thermal loading due to heat generated by circuits through power dissipation, and stresses applied to materials in close proximity to the VLSI chip. These stresses may lead to mechanical failure of the chip and/or mechanical failure of the material in close proximity to the chip. The larger the stress, the higher the chance for mechanical and/or circuit failure. 
       SUMMARY 
       [0008]    According to one aspect of one or more embodiments of the present invention, a computer system comprises: a substrate; a first current mirror formed of at least two devices disposed on the substrate, wherein, when a stress is present, a behavior of at least one of the at least two devices forming the first current mirror is dependent on a direction in which the at least one of the at least two devices forming the first current mirror is disposed on the substrate; and a second current mirror formed of at least two devices disposed on the substrate, wherein, when the stress is present, a behavior of at least one of the at least two devices forming the second current mirror is dependent on a direction in which the at least one of the at least two devices forming the second current mirror is disposed on the substrate, wherein the at least one of the at least two devices forming the first current mirror is disposed in a non-parallel orientation to the at least one of the at least two devices forming the second current mirror. 
         [0009]    According to another aspect of one or more embodiments of the present invention, a method of manufacturing comprises: disposing a first device on a substrate, the first device forming part of a first current mirror, wherein, when a stress is present, a behavior of the first device is dependent on a direction in which the first device is disposed on the substrate; and disposing a second device on the substrate in a non-parallel orientation to the first device, the second device forming part of a second current mirror, wherein, when the stress is applied, a behavior of the second device is dependent on a direction in which the second device is disposed on the substrate. 
         [0010]    According to another aspect of one or more embodiments of the present invention, a method of operation comprises: propagating a first current through a first current mirror formed of at least two devices disposed on a substrate, wherein, when a stress is present, a behavior of at least one of the at least two devices is dependent on a direction in which the at least one of the at least two devices forming the first current mirror is disposed on the substrate; and propagating a second current through a second current mirror formed of at least two devices disposed on a substrate, wherein, when the stress is applied, a behavior of at least one of the at least two devices forming the second current mirror is dependent on a direction in which the at least one of the at least two devices forming the second current mirror is disposed on the substrate, wherein the at least one of the at least two devices forming the first current mirror and the at least one of the at least two devices forming the second current mirror are disposed in a non-parallel orientation with respect to each other. 
         [0011]    Other aspects of the present invention will be apparent from the following description and the appended claims. 
     
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
         [0012]      FIG. 1  shows a stress measuring circuit in accordance with an embodiment of the present invention. 
           [0013]      FIG. 2  shows a stress measuring circuit in accordance with an embodiment of the present invention. 
           [0014]      FIG. 3  shows a stress measuring circuit in accordance with an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0015]    Exemplary embodiments of the invention will be described with reference to the accompanying drawings. Like items in the drawings are shown with the same reference numbers. 
         [0016]    In one or more embodiments of the present invention, numerous specific details are set forth in order to provide a more thorough understanding of the present invention. In other instances, well-known features have not been described in detail to avoid obscuring the invention. 
         [0017]    Embodiments of the present invention relate to a circuit that may be used to produce a signal that is strongly dependent on a stress applied to the circuit and/or on a stress applied to a material in close proximity to the circuit. In order words, a circuit in accordance with one or more embodiments of the present invention amplifies a signal that is dependent on a stress applied to the circuit and/or to a material in close proximity to the circuit. 
         [0018]      FIG. 1  shows an exemplary circuit  100  in accordance with an embodiment of the present invention. The circuit  100  includes eight transistors T 1 -T 8  and two resistors R 1  and R 2 . In the exemplary embodiment of  FIG. 1 , resistors R 1  and R 2  are of equal value (R 1 =R 2 ), transistors T 1  and T 4  are of equal width (W T1 =W T4 ), and transistors T 2  and T 3  are of equal width (W T2 =W T3 ). The two sets of transistors T 1 -T 2  and T 3 -T 4  have widths related by a factor of ω 2  (i.e., ω 2 W T1 =ω 2 W T4 =W T2 =W T3 , where ω≧1). Selection of ω is a tradeoff between sensitivity and the maximum value of stress (δ MAX ) desired to be measurable. In  FIG. 1 , transistors T 1 -T 8  are of equal length (L). 
         [0019]    Transistors T 1 , T 2 , T 5  and T 6  (referred to as forming the “left side” of the circuit  100 ) are interconnected such that the electrical current flowing through transistor T 1  is approximately equal to the electrical current flowing through transistor T 2 . This electrical current is denoted I L . In one or more embodiments of the present invention, the value of ω may also be selected to keep I L &gt;0 over a desired range of measurable stresses. 
         [0020]    Transistors T 3 , T 4 , T 7  and T 8  (referred to as forming the “right side” of the circuit  100 ) are configured such that the electrical current flowing through transistor T 3  is approximately equal to the current flowing through transistor T 4 . 
         [0021]    This electrical current is denoted I R . In one or more embodiments of the present invention, the value of ω may also be selected to keep I R &gt;0 over a desired range of measurable stresses. 
         [0022]    Resistors R 1  and R 2  provide a resistive path through which electrical currents I L  and I R , respectively, flow to ground. The values of resistors R 1  and R 2  are determined by desired values for I L  and I R , respectively, when the circuit is not under stress. δ MAX  can also be understood as the direction-dependent mobility change which reduces I L  and/or I R  to a negligible value. 
         [0023]    Those skilled in the art will recognize the configurations of transistors T 1 , T 2 , T 5 , and T 6 , and T 3 , T 4 , T 7  and T 8  are current mirrors. Further, those skilled in the art will recognize that transistors T 1 -T 4  are negative-channel metal-oxide semiconductor (NMOS) transistors and transistors T 5 -T 8  are positive-channel metal-oxide semiconductor (PMOS) transistors. 
         [0024]    Stress has the effect of increasing or decreasing the differences in the strength of transistors. Mathematically, stress is equivalent to some of the transistors growing longer and others being reduced in length. A small increase in the carrier mobility ratio of transistors in the left side of circuit  100  (μ T2 /μ T1 ) may yield a large increase in the electrical current I L . The opposite effect may occur in the right side of circuit  100 —namely, a large decrease in the electrical current I R  resulting from of a small change in μ T3 /μ T4 . Simultaneous measurements of I L  and I R  may be used to infer the magnitude of the stress applied to the circuit. In particular, a direction-dependent mobility change (δ) may be inferred from a measurement of the relative difference in the current through the two sides of the circuit (Φ): 
         [0000]    
       
         
           
             Φ 
             = 
             
               
                 
                   
                     I 
                     L 
                   
                   - 
                   
                     I 
                     R 
                   
                 
                 
                   
                     I 
                     L 
                   
                   + 
                   
                     I 
                     R 
                   
                 
               
               . 
             
           
         
       
     
         [0025]    In one or more embodiments of the present invention, T 1  and T 4  may each be designed such that when a stress is present, the change in carrier mobility of both transistors is not affected by the direction in which the transistors are positioned (δ=0). Transistors T 2  and T 3  are oriented perpendicularly so that the carrier mobility of both transistors is affected by the direction in which the transistors are positioned, but with an opposite polarity, i.e., 
         [0000]    
       
         
           
             
               
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                     3 
                   
                 
               
             
             ≈ 
             
               - 
               
                 δ 
                 . 
               
             
           
         
       
     
         [0000]    In such an embodiment, the relative difference in the current through the two sides of the circuit  100  (Φ) reduces to an expression in terms of ω and δ. This allows a precise determination of δ for a measured value of Φ. 
         [0026]    Although transistors T 2  and T 3  are described as being oriented perpendicularly in  FIG. 1 , in one or more other embodiments of the present invention, these transistors may be oriented in any non-parallel orientation. For example, in one embodiment of the present invention, transistor T 2  may be oriented with a 75 degrees offset with respect to transistor T 3 . 
         [0027]    In one or more embodiments of the present invention, transistors T 2  and T 3  may each be designed such that when a stress is present, the carrier mobility of both transistors is not affected by the direction in which the transistors are positioned (δ=0). Transistors T 1  and T 4  are oriented perpendicularly so that the carrier mobility of both transistors is affected by the direction in which the transistors are positioned, but with an opposite polarity, i.e., 
         [0000]    
       
         
           
             
               
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         [0000]    In such an embodiment, the relative difference in the current through the two sides of the circuit (Φ) reduces to an expression in terms of ω and δ. This allows a precise determination of δ for a measured value of Φ. 
         [0028]    Although transistors T 1  and T 4  are described as being oriented perpendicularly, in one or more other embodiments of the present invention, these transistors may be oriented in any non-parallel orientation. For example, in one embodiment of the present invention, transistor T 1  may be oriented with an 80 degrees offset with respect to transistor T 4 . 
         [0029]    In one or more embodiments of the present invention, transistor T 2  is oriented perpendicularly to transistor T 3 , transistor T 1  is oriented perpendicularly to transistor T 2  (and thus oriented in parallel or anti-parallel to transistor T 3 ), transistor T 4  is oriented perpendicularly to transistor T 3  (and thus oriented in parallel or anti-parallel to transistor T 2 ), thereby resulting in: 
         [0000]    
       
         
           
             
               
                 Δ 
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         [0000]    Such an embodiment also allows for a determination of δ for a measured value of Φ. Such an embodiment increases the circuit&#39;s sensitivity to stress for any given value of ω. 
         [0030]    In one or more embodiments of the present invention, the circuit may be used as a “trip wire” to provide a binary indication of whether the stress has exceeded a specific threshold. 
         [0031]    In one or more embodiments of the present invention, when the expected range of δ is not symmetric about zero, the width ratios may be chosen to have unequal values, i.e., 
         [0000]    
       
         
           
             
               
                 
                   W 
                   
                     T 
                      
                     
                         
                     
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                     2 
                   
                 
                 
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             , 
           
         
       
     
         [0000]    to produce a symmetric measurement range about some non-zero number. 
         [0032]    In one or more embodiments of the present invention, numerous versions of the circuit shown in  FIG. 1  may be operated in parallel, each operating with a different width ratio and/or a different ω value, and therefore a different δ MAX  value. In such an embodiment, these multiple circuits with differing ranges may be used to achieve good resolution for a desired range of stresses. 
         [0033]    Further, although the devices of  FIG. 1  (and  FIGS. 2 and 3  discussed below) are described as having particular sizes, values, and/or strengths, various differing sizes, values, and/or strengths may be used to effectuate particular behavior. 
         [0034]    In one or more embodiments of the present invention, as shown in  FIG. 2 , an exemplary circuit  200  has eight transistors T 1 -T 8  and three resistors R 3 -R 5 . Transistors T 1 -T 8  are similar in design and operation to those described above with reference to  FIG. 1 . Resistors R 3 -R 5  are laid out in the same orientation so that the stress does not change the ratio of their resistances. With such a configuration, it is still possible to extract δ for a measured value of Φ. 
         [0035]    In one or more embodiments of the present invention, as shown in  FIG. 3 , an exemplary circuit  300  has eight transistors T 1 -T 8  and one resistor R 6 . Transistors T 1 -T 8  are similar in design and operation to those described above with reference to  FIG. 1 . With such a configuration, it is still possible to extract δ for a measured value of Φ. 
         [0036]    In one or more embodiments of the present invention, PMOS transistors instead of NMOS transistors may be used to measure the stress. 
         [0037]    In one or more embodiments of the present invention, a circuit may be formed using an amount of devices that differs from the amounts shown in  FIGS. 1-3 . 
         [0038]    Further, a circuit in accordance with one or more embodiments of the present invention may use devices having characteristics (e.g., W) different than that of those shown in  FIGS. 1-3  to effectuate particular behavior. 
         [0039]    In one or more embodiments of the present invention, a circuit may produce a signal that is an amplified function of a stress on the circuit. Such amplification allows for improved analog and digital measurements. 
         [0040]    While the invention has been described with respect to a limited number of embodiments, those skilled in the art, having benefit of this disclosure, will appreciate that other embodiments can be devised which do not depart from the scope of the invention as disclosed herein. Accordingly, the scope of the invention should be limited only by the attached claims.