Abstract:
An integrated circuit including an intrusion attack detection device. The device includes a single-piece formed of a conductive material and surrounded with an insulating material and includes at least one stretched or compressed elongated conductive track, connected to a mobile element, at least one conductive portion distant from said piece and a circuit for detecting an electric connection between the piece and the conductive portion. A variation in the length of said track in an attack by removal of the insulating material, causes a displacement of the mobile element until it contacts the conductive portion.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims the priority benefit of French patent application  number 08/55552, filed on Aug. 13, 2008, entitled DEVICE FOR DETECTING AN ATTACK AN INTEGRATED CIRCUIT,” which is hereby incorporated by reference to the maximum extent allowable by law. 
       BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    The present disclosure relates to a device for detecting attacks by contact on an integrated circuit. This type of attacks is generally called an “intrusion attack” and comprises applying conductive probes directly on areas of the integrated circuit to sample signals therefrom. In an integrated circuit, the active areas contain circuits for processing data which may be confidential, such as for example in bank card chips or access-control chips in toll television applications. 
         [0004]    2. Discussion of the Related Art 
         [0005]    To protect the circuits located in the active area from any fraudulent intrusion, it is known to use a shield covering the entire circuit surface or part of it. This shield is generally formed of one or of several conductive paths formed in one or several metallization levels above the active area to be protected. 
         [0006]    The aim of such conductive paths is to detect a discontinuity or any electric modification of their properties, for example, their resistance or capacitance. The conductive paths run along the entire surface or only an area of the circuit to be protected, in an irregular and random manner. If a “hacker” attempts to cross the metallization level containing the path, by introduction of one or several probes, a detection circuit is supposed to detect a rupture in the conductive path. 
         [0007]    A disadvantage of such a solution is that it does not enable to detect the removal of the insulating layers covering the conductive path. Once the conductive path has been exposed, a “hacker” might double this conductive path with an external section to simulate an electric continuity. The conductive path could then be interrupted without this being detected. 
       SUMMARY OF THE INVENTION 
       [0008]    At least one embodiment of the present invention aims at improving the detection of attacks by intrusion on an integrated circuit by enabling to detect the removal of insulating layers covering the integrated circuit. 
         [0009]    Thus, an embodiment of the present invention provides an integrated circuit comprising an intrusion attack detection device. The device comprises a single-piece formed of a conductive material and surrounded with an insulating material and comprising at least one stretched or compressed elongated conductive track, connected to a mobile element, at least one conductive portion distant from said piece and a circuit for detecting an electric connection between the piece and the conductive portion. This results in a variation in the length of said track in an attack by removal of the insulating material, causing a displacement of the mobile element until it contacts the conductive portion. 
         [0010]    According to an embodiment of the present invention, the single-piece comprises a first bar extending along a first direction, a second bar extending along a second direction inclined with respect to the first direction and connected to a first lateral surface of the first bar at the level of a first junction area, and a third bar extending along a third direction inclined with respect to the first direction and connected to a second lateral surface of the first bar, opposite to the first lateral surface, at the level of a second junction area, the first and second junction areas being shifted along the first direction. The at least one conductive portion is distant from the first, second, and third bars and arranged opposite to the first or second lateral surface. This results in a variation in the length of the second and third bars in the attack by removal of the insulating material, causing a pivoting of the first bar until it contacts the conductive portion. 
         [0011]    According to an embodiment of the present invention, the first bar comprises first and second ends. The conductive portion is arranged opposite to the first lateral surface, at the level of the first end, the circuit comprising an additional conductive portion arranged opposite to the second lateral surface, distantly from the first, second, and third bars, at the level of the second end. This results in a variation of the length of the second and third bars in the attack by removal of the insulating material, causing a pivoting of the first bar so that the first end comes into contact with the conductive portion and that the second end comes into contact with the additional conductive portion. 
         [0012]    According to an embodiment of the present invention, the second bar continues in a first track of the conductive material of larger cross-section and the third bar continues in a second track of the conductive material of larger cross-section. 
         [0013]    According to an embodiment of the present invention, the second and third bars belong to a conductive path. 
         [0014]    According to an embodiment of the present invention, the cross-section of the second bar decreases at the level of the first junction area and the cross-section of the third bar decreases at the level of the second junction area. 
         [0015]    According to an embodiment of the present invention, the first bar comprises a tapered surface for bearing against the conductive portion during the pivoting of the first bar. 
         [0016]    According to an embodiment of the present invention, the second and third directions are parallel to each other and perpendicular to the first direction. 
         [0017]    An embodiment of the present invention also provides a method for manufacturing an integrated circuit comprising a device for detecting an attack by contact, comprising the steps of forming a single-piece formed of a conductive material and surrounded with an insulating material and comprising at least one elongated conductive track connected to a mobile element, at least one conductive portion distant from said piece and a circuit for detecting the creation of an electric connection between the piece and the conductive portion and of annealing. This results in the creation of tensile or compressive stress in said conductive track and in a variation of the length of said track in an attack by removal of the insulating material, causing a displacement of the mobile element until it contacts the conductive portion. 
         [0018]    The foregoing objects, features, and advantages of the present invention will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is intended to show the state of the art; 
           [0020]      FIG. 2  is a simplified top view of a device for detecting attacks by contact with an integrated circuit according to an embodiment of the present invention; 
           [0021]      FIG. 3  is a simplified cross-section view of the detection device of  FIG. 2 ; 
           [0022]      FIG. 4  is a cross-section view similar to  FIG. 3  illustrating the principle of an attack by contact; 
           [0023]      FIG. 5  is a view similar to  FIG. 2  illustrating the operation of the detection device according to the present embodiment of the invention in an attack by contact; 
           [0024]      FIG. 6  shows the variation of an operating parameter of the detection device of  FIG. 2  according to a characteristic dimension of the detection device; and 
           [0025]      FIGS. 7 and 8  are top views of details of detection devices according to other embodiments of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0026]    For clarity, the same elements have been designated with the same reference characters in the different drawings and, further, as usual in the representation of integrated circuits, the various drawings are not to scale. In the following description, an integrated circuit comprising a substrate (for example, a solid semiconductor substrate, a silicon-on-insulator substrate (SOI), etc.) covered with a stack of layers of an insulating material at the level of which metal tracks of different metallization levels are provided is considered. The metal tracks of a given metallization level may be arranged on the surface of an insulating layer or formed in an insulating layer, leveling the surface thereof. The metal tracks most remote from the substrate are called metal tracks of the last metallization level, or upper metallization level. The metal tracks of the last metallization level are covered with an insulating layer, generally called a passivation layer. 
         [0027]    According to an embodiment of the present invention, a device for detecting the removal of an insulating layer covering metal tracks of a given metallization level is formed, at least partly, by metal tracks of the given metallization level. Advantageously, according to an embodiment, the metal tracks forming the detection device may also belong to a conductive path of a protection shield. An embodiment of the present invention is based on the fact that a conventional method for manufacturing an integrated circuit causes the appearance of tensile stress in the metal tracks of the different metallization levels. This is due to the fact that an anneal is generally performed once the metal tracks and the insulating layers have been formed. The anneal causes a growth of the grains of the metal tracks and a decrease in the grain boundary density. This creates tensile stress in the metal tracks, given that the insulating material which surrounds them forces them to keep their shape. The removal of the insulating material surrounding the metal tracks relieves the stress in these tracks. An embodiment of the present invention provides a device for detecting the removal of the insulating material in an intrusion attack. The device comprises a mechanical switch comprising a mobile element which is displaced on relief of the stress to establish an electric contact. The creation of the electric contact is detected and is representative of the removal of the insulating material. 
         [0028]      FIG. 1  is a perspective view of an integrated circuit conventionally provided with a protection shield. The circuit comprises a substrate S covered with a stack of insulating layers to which are associated different metallization levels M 1  . . . Ms. A conductive path L delimited by terminals V and W is formed in one of these levels, for example, upper metallization level Ms. Conductive path L is connected at its ends to a circuit, not shown, capable of detecting an interruption of conductive path L. 
         [0029]      FIG. 2  shows a detection device  5  according to an embodiment of the present invention. Device  5  comprises a mechanical switch  10  connected to a detection circuit C. More specifically,  FIG. 2  is a partial simplified top view of metal tracks of a same metallization level of an integrated circuit forming switch  10  according to an embodiment of the present invention, detection circuit C being schematically represented by a block. Switch  10  may be duplicated at several locations at the level of a same metallization level. It may further be duplicated in several metallization levels of the same integrated circuit. 
         [0030]    Switch  10  is formed at the level of a conductive path L comprising first and second metal tracks  12 ,  14 . It comprises an arm  16  corresponding to a metal track of length L 1 , of width l 1 , and extending along a rectilinear central axis Δ 1 . Arm  16  continues track  12  and has a cross-section smaller than the cross-section of track  12 . Switch  10  further comprises an arm  18  corresponding to a metal track of length L 2 , of width l 2 , and extending along a rectilinear central axis Δ 2 . Arm  18  continues track  14  and has a cross-section smaller than the cross-section of track  14 . Axes Δ 1  and Δ 2  are, for example, parallel. In the present embodiment, arms  16 ,  18  substantially have a constant cross-section. 
         [0031]    Switch  10  further comprises a bar  20  corresponding to a metal track having a length L 3 , a width l 3 , and extending along a rectilinear central axis Δ 3  perpendicular to axes Δ 1  and Δ 2 . Bar  20  comprises a central portion  22  and two free ends  23 ,  24 . Central portion  22  comprises two opposite lateral surfaces  25 ,  26 . Switch  10  is symmetrical with respect to a plane of symmetry P which corresponds to the plane perpendicular to axis Δ 3  equidistant from ends  23  and  24 . Call O the intersection point between plane P and axis Δ 3 . Call P′ the plane perpendicular to axes Δ 1  and Δ 2  and containing axis Δ 3 . At the end opposite to track  12 , arm  16  is connected to surface  25  of central portion  22  at the level of a junction area  27  and, at the end opposite to conductive track  14 , arm  18  is connected to surface  26  of central portion  22  at the level of a junction area  28 . Arms  16 ,  18  are thus arranged on either side of bar  20 . Further, arms  16 ,  18  are located on either side of plane P. Call spacing ec the distance, measured along direction Δ 3 , between the middle of junction area  27  and the middle of junction area  28 , that is, in the present embodiment, between axes Δ 1  and Δ 2 . 
         [0032]    Switch  10  further comprises metal tracks  29 ,  30 . Track  29  is arranged on the same side of bar  20  as arm  16  and extends opposite to a portion of surface  25  of central portion  22  close to end  23 . Track  30  is arranged on the same side of bar  20  as arm  18  and extends in front of a portion of surface  26  of central portion  22  on the side of end  24 . Tracks  29  and  30  are connected to circuit C, which is capable of detecting whether tracks  29 ,  30  are electrically connected to each other. 
         [0033]    In the embodiment shown in  FIG. 2 , arms  16 ,  18 , tracks  12 ,  14 , and bar  20  are made of a single piece. 
         [0034]      FIG. 3  shows a partial simplified cross-section view of switch  10  along line A-A. Switch  10  is formed at the level of an integrated circuit  31  comprising a semiconductor substrate  32 , for example, made of silicon, covered with a stack of insulating layers at the level of which are arranged metal tracks of different metallization levels. As an example, metal tracks  34 ,  36  of metallization level Ms−1 leveling the surface of an insulating layer  38  have been shown. An insulating layer  40  covering insulating layer  38  and metal tracks  36  and  34  has further been shown. The metal tracks of metallization level Ms  20 ,  29  extend on insulating layer  40 . As an example, metal tracks  34 ,  36  are made of copper and metal tracks  20 ,  29  are made of aluminum. An insulating layer  46 , called passivation layer, covers metal tracks  20 ,  29  and insulating layer  40 . Layers  40 ,  46  are made of a same insulating material, for example, silicon oxide. 
         [0035]    The method for manufacturing integrated circuit  31  comprises an anneal step which comprises the step of, after the forming of insulating layer  46  covering metal tracks  20 ,  29 , heating integrated circuit  31  up to a temperature, for example, on the order of a few hundreds of degrees, for example, 400° C., for several tens of minutes, for example, 50 minutes. The anneal step causes an increase in the size of the metal grains of the metal tracks, in particular the metal grains in arms  16 ,  18  by decrease of the density of the grain boundaries. This increase in the grain size creates tensile stress in the metal grains of arms  16 ,  18 . This tensile stress cannot be relieved because of the presence of insulating layers  40 ,  46  which surround arms  16 ,  18  and force them to keep their initial shapes. 
         [0036]      FIG. 4  is a view similar to  FIG. 3  and illustrates the principle of an attack by contact. Such an attack comprises an initial step of etching an opening  50  at the surface of integrated circuit  31  where the contacts are desired to be taken. Opening  50  is formed by etching of insulating layer  46 . Since insulating layers  40  and  46  are formed of the same insulating material, it is not possible to accurately control the depth of opening  50 . If the attack is performed in the region of circuit  31  containing switch  10 , for example, to duplicate conductive path L, opening  50  will at least partially penetrate into insulating layer  40 . After the forming of opening  50 , bar  20  is completely disengaged and arms  16  and  18  are at least partially disengaged. 
         [0037]    The removal of the insulating material surrounding arms  16 ,  18  results in a relieving of the tensile stress in arms  16 ,  18 , that is, a decrease of lengths L 1  and L 2 . Since arms  16 ,  18  remain anchored at one end to tracks  12 ,  14 , their shortening causes a pivoting of bar  20  around center O, with axis Δ 3  substantially remaining in a plane perpendicular to plane P. Bar  20  pivots enough to come into contact with metal tracks  29 ,  30 . 
         [0038]      FIG. 5  is a detail view of  FIG. 2  which shows the state of switch  10  after the etching of opening  50 . Since arms  16  and  18  are not in prolongation of each other, their shortening has caused a pivoting of bar  20 . A slight deformation of arms  16 ,  18  by buckling can be observed. Call clearance D of bar  20  the distance between the edge of end  24  closest to plane P′ and plane P′ after the stress has been relieved. 
         [0039]    Detection circuit C connected to tracks  29 ,  30  detects the coming into contact of bar  20  with metal tracks  29 ,  30 . This may be interpreted as corresponding to a contact attack and may cause the stopping of the operation of integrated circuit  31 . 
         [0040]      FIG. 6  shows an example of a curve  51  of the variation, according to spacing ec, of clearance D observed in the absence of tracks  29 ,  30 . Curve  51  is obtained for a switch  10  for which lengths L 1 , L 2 , and L 3  are equal to 100 μm, for which widths l 1 , l 2 , l 3  are equal to 2 μm, and for which the thickness of the metal tracks of metallization level Ms is, for example, on the order of 0.3 μm. It is desirable for clearance D of bar  20  to be as large as possible to ensure for a contact to always be present between bar  20  and metal tracks  29 ,  30  in case of an attack. In the previously-mentioned example, a clearance D on the order of 4.5 μm is obtained for a spacing ec on the order of 4 μm. 
         [0041]    In the present embodiment, switch  10  is formed at the level of a conductive path L belonging to a protection shield. Arms  16 ,  18  and bar  20  electrically connect tracks  12 ,  14  together Tracks  12  and  14  are connected to a circuit capable of detecting an interruption of conductive path L. This provides an additional protection, in addition to the protection provided by switch  10 . According to an alternative embodiment, only metal track  29  is present. In this case, a circuit is capable of detecting whether an electric contact has been created between metal track  29  and one of tracks  12  or  14 . According to another variation, tracks  29  and  30  may correspond to metal pads. This may be advantageous when several switches  10  are arranged adjacent to one another. 
         [0042]      FIG. 7  shows a detail view of a switch  52  according to another embodiment of the present invention. Switch  52  has the same structure as switch  10 , except that arm  16  comprises, at the level of the end connected to bar  20 , a portion  54  with a decreasing cross-section, so that junction area  27  has a reduced cross-section, of width l 4 , with respect to arm  16 . Similarly, arm  18  comprises, at the level of the end connected to bar  20 , a portion  56  with a decreasing cross-section, so that junction area  28  has a reduced cross-section, of width l 5 , with respect to arm  18 . Junction areas  27 ,  28  of switch  52  deform more easily during the pivoting of bar  20  than junction areas  27 ,  28  of switch  10  which have a greater cross-section. This enables, for the same spacing, to obtain a larger clearance D than when arms  16 ,  18  are of constant cross-section. As an example, for a switch  52  for which lengths L 1 , L 2 , and L 3  are equal to 100 μm, for which widths l 1 , l 2 , l 3  are equal to 2 μm, for which the thickness of the metal tracks of metallization level Ms is on the order of 0.3 μm, and for which widths l 4  and l 5  are on the order of 0.2 μm, a clearance D on the order of 10 μm is obtained, in the absence of tracks  29 ,  30 , for a spacing ec on the order of 4 μm. 
         [0043]    Generally, the shape of junction areas  27 ,  28  is determined to enable to obtain the largest possible clearance D of bar  20  in the absence of tracks  29 ,  30  while ensuring a sufficient mechanical resistance of switch  10 . According to an example, each junction area  27 ,  28  may have, in a plane perpendicular to plane P, the shape of a funnel. According to another example, each junction area  27 ,  28  may comprise through openings. 
         [0044]      FIG. 8  shows a switch  60  according to another embodiment of the present invention. Switch  60  comprises, at each end  23  of bar  20 , a tapered surface  62  which is oriented so that, in the pivoting of bar  20 , tapered surface  62  ends up bearing against metal track  29 . This enables to improve the contact between bar  20  and metal track  29 . 
         [0045]    In the previously-described embodiments, switch  10 ,  52 ,  60  is formed by portions of a conductive material in which tensile stress is created during the anneal step of the integrated circuit manufacturing method. As a variation, the switch may be formed of portions of a conductive material in which compressive stress appears in the anneal step. The material is, for example, a semiconductor material such as polysilicon. Thereby, when the stress is relieved, arms  16 ,  18  of the switch tend to lengthen. In this case, as compared with switch  10  shows in  FIG. 2 , arms  16  is arranged to the right of plane P and arm  18  is arranged to the left of plane P so that bar  20  pivots in the right direction when the stress is relieved to ensure the electric connection between metal tracks  29 ,  30 . 
         [0046]    Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, in the previously-described embodiments, switch  10  has a symmetrical shape. However, this is not compulsory. Further, in the previously-described embodiments, arms  16 ,  18  and bar  20  have, before pivoting, rectilinear shapes. However, arms  16 ,  18  and bar  20  may have, before pivoting, curved shapes. 
         [0047]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.