Abstract:
A device for detecting a laser attack made on an integrated circuit chip comprises a bipolar transistor of a first type formed in a semiconductor substrate, that bipolar transistor comprising a parasitic bipolar transistor of a second type. A buried region, forming the base of the parasitic bipolar transistor, operates as a detector of the variations in current flowing caused by impingement of laser light on the substrate.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a divisional of U.S. patent application Ser. No. 14/531,025 filed Nov. 3, 2014 (now U.S. Pat. No. 9,070,697), which is a continuation of U.S. patent application Ser. No. 13/751,549 filed Jan. 28, 2013 (now U.S. Pat. No. 8,907,452) which claims priority from French Application for Patent No. 1251151 filed Feb. 8, 2012, which are hereby incorporated by reference to the maximum extent allowable by law. 
    
    
     TECHNICAL FIELD 
     The present disclosure relates to the protection of an integrated circuit chip against laser attacks. 
     BACKGROUND 
     In certain secure devices such as payment cards, integrated circuit chips are likely to process and/or store critical data, for example, encryption keys. Such chips may be fraudulently manipulated in order to obtain protected confidential data. 
     To intentionally cause disturbances in the circuits of a chip, an attack mode comprises bombarding chip areas with a laser beam while the chip is operating. Due to the presence of interconnection metal tracks on the front surface side of the chip, laser attacks are often carried out on the back side. 
     To avoid fraud, chips comprising attack detection devices have been provided. The attack detection device is coupled to a chip protection circuit. When an attack is detected, the protection circuit implements certain measures of protection, modification, or destruction of the critical data. For example, it may be provided, when an attack is detected, to interrupt the power supply of the chip or to cause it to reset, in order to reduce the time during which the attacker can study the chip response to a disturbance. 
     Existing detection devices have various disadvantages. They require, for example, creating new structures on chip to enable the detection of a laser attack. Further, they may increase the bulk and/or the complexity of secure devices. 
     SUMMARY 
     An embodiment provides a device for detecting a laser attack in an integrated circuit chip, which overcomes at least some of the disadvantages of the above-described devices. 
     Thus, an embodiment provides a device for detecting a laser attack in an integrated circuit chip, formed in the upper P-type portion of a semiconductor substrate incorporating an NPN bipolar transistor having an N-type buried layer, comprising a detector of the variations of the current flowing between the base of said NPN bipolar transistor and the substrate. 
     According to an embodiment, the substrate comprises a substrate contact provided to be grounded, and the base contact of the NPN bipolar transistor is connected to a comparator and to a terminal of application of a bias voltage by a resistor. 
     According to an embodiment, the collector and the base of the NPN bipolar transistor are interconnected. 
     According to an embodiment, the resistor is embodied by a P-channel MOS transistor. 
     According to an embodiment, the comparator comprises an inverter. 
     According to an embodiment, the inverter comprises an N-channel MOS transistor and a P-channel MOS transistor, the gate width of the N-channel MOS transistor being at least two times smaller than that of the P-channel MOS transistor. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings, wherein: 
         FIG. 1  is a cross-section view schematically showing a portion of an integrated circuit chip; 
         FIG. 2A  is an electric diagram illustrating a device for detecting a laser attack; and 
         FIG. 2B  illustrates a variation of  FIG. 2A . 
     
    
    
     DETAILED DESCRIPTION 
     For clarity, the same elements have been designated with the same reference numerals in the different drawings and, further, as usual in the representation of integrated circuits,  FIG. 1  is not drawn to scale. 
       FIG. 1  is a cross-sectional view schematically showing a portion of an integrated circuit chip comprising an NPN bipolar transistor having an N-type buried layer  3 . Such a transistor is a component commonly provided in existing integrated circuit structures. 
     The NPN transistor is formed in the upper P-type doped portion of a semiconductor substrate  1 . An N-type doped buried layer  3  and a ring-shaped wall  5 , also N-type doped, which extends from the upper surface of the substrate to buried layer  3 , delimit a P-type doped well  7 . 
     A heavily-doped N-type region  9  extends at the surface and at the center of P-type well  7 . Regions  11 , more heavily P-type doped than well  7 , extend at the surface of well  7  and surround region  9 . Contact regions  13 , more heavily N-type doped than regions  5 , extend at the surface of regions  5 . 
     A vertical NPN transistor with an N-type buried layer, having its emitter formed of region  9 , its base formed of well  7  embedded with base contact region  11 , and its collector formed of buried layer  3  connected by wall  5  to collector contact region  13 , is thus obtained. 
     Substrate contact regions  15 , more heavily P-type doped than substrate  1 , extend at the surface of substrate  1  and surround regions  5 . Substrate contact regions  15  are, for example, intended to be grounded. 
     To detect a laser attack in an integrated circuit chip of the type illustrated in  FIG. 1 , incorporating an NPN bipolar transistor having an N-type buried layer, the present inventors provide a detection device capable of detecting variations of the current flowing between the base of the NPN bipolar transistor and the substrate. 
       FIG. 2A  is an electric diagram illustrating an example of a device for detecting a laser attack. 
     The detection device is based on the use of a parasitic PNP bipolar transistor  21  present in an integrated circuit chip incorporating an NPN bipolar transistor having an N-buried type layer, of the type illustrated in  FIG. 1 . The emitter of PNP transistor  21  corresponds to P-type well  7 , that is, to the base of the NPN transistor. The base of PNP transistor  21  corresponds to N-type buried layer  3 , that is, to the collector of the NPN transistor. The collector of PNP transistor  21  corresponds to substrate  1 . Thus, the emitter, base, and collector contacts of PNP transistor  21  approximately correspond to regions  11 ,  13 , and  15 . 
     Collector contact  15  of PNP transistor  21  is grounded. Base  13  of PNP transistor  21  is floating. A node  22  corresponding to emitter contact  11  of PNP transistor  21  is connected to a bias voltage V dd  by a resistor  23 . Resistor  23  may be embodied, as shown, by a transistor assembled as a resistor, for example, having a P channel, or by a current source. Node  22  is also connected to an input of a comparator  24 . In the shown example, comparator  24  is formed of a simple inverter  27 . 
     When a laser beam reaches the rear surface of the chip, electron/hole pairs are photogenerated in substrate  1 . The electrons cross N-type buried layer  3  and are attracted by regions  11  connected, via resistor  23 , to positive voltage V dd . This turns on PNP transistor  21  and a current I PNP  then flows between emitter contact  11  and collector contact  15  of PNP transistor  21 . The voltage at node  22  switches from V dd  to V dd −R*I PNP , R being the value of resistance  23 . The output of comparator  24  then switches from a low level to a high level, which corresponds to a laser attack detection signal. Various measures of protection, modification, or destruction of the confidential data of the chip may then be implemented. 
       FIG. 2B  illustrates a variation of the device for detecting a laser attack illustrated in  FIG. 2A . Base  13  of PNP transistor  21  is connected to emitter  11  of PNP transistor  21 . This enables to decrease the sensitivity of the detection device with respect to that of the detection device illustrated in  FIG. 2A . 
     As an example of order of magnitude, in the case where resistor  23  is embodied by a P-channel MOS transistor, the gate length and width of the MOS transistor, for example, respectively range between 3 and 5 μm and between 2 and 4 μm, for example, respectively, being on the order of 4 μm and 3 μm. This corresponds to a current I PNP  of approximately 10 μA. 
     In the case where comparator  24  is embodied by a simple inverter  27 , the gate width of the N-channel MOS transistor of inverter  27  is selected to be small as compared with the gate width of the P-channel MOS transistor of this inverter, to avoid that a laser attack directly affects the transistors. For example, the gate width of the N-channel MOS transistor of the inverter is at least two times smaller than the gate width of the P-channel MOS transistor. 
     The surface of an NPN transistor of the type illustrated in  FIG. 1 , for example, ranges between 2 and 25 μm 2 , for example, being on the order of 4 μm 2 . 
     Tests have shown that, in the case where a laser attack is performed with a beam having a diameter of approximately 5 μm, a detection device using such NPN transistors having an N-type buried layer enables to detect this attack over a radius for example ranging between 300 and 500 μm around the impact point of the beam on the rear surface of the chip, for example, over a radius on the order of 400 μm around the impact point of the beam. 
     An integrated circuit chip used for the processing or the storage of critical data for example has a surface area ranging between 2 and 3 mm 2 . To be able to detect a laser attack whatever its impact point on the chip, the present inventors provide integrating several NPN transistors having an N-type buried layer of the above-described type in the chip. An array of 20 NPN transistors having N-type buried layers, distributed in rows and in columns and spaced apart by a distance between 150 and 250 μm, for example, on the order of 200 μm, is, for example, formed. 
     An advantage of a laser attack detection device of the type described in relation with  FIGS. 1, 2A, and 2B  is that it can be formed by only using components commonly provided in integrated circuit chips used for the processing or the storage of critical data. 
     Another advantage of such a detection device is that it enables local detection of a laser attack. 
     Another advantage of such a detection device is that the surface area of the integrated circuit chip is almost unchanged with respect to that of a similar integrated circuit chip incorporating no laser attack detection device. 
     Another advantage is that the static power consumption of such a laser attack detection device is almost non-existent. 
     Specific embodiments have been described. Various alterations, modifications, and improvements will readily occur to those skilled in the art. In particular, instead of associating a comparator with each N-type buried layer NPN transistor, a single comparator may be used for a set of NPN transistors, if the laser attack is not desired to be accurately located. 
     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.