Abstract:
According to an aspect of the invention an integrated circuit is conceived which comprises a physical unclonable function which is at least partially implemented in a passivation layer of said integrated circuit. According to a further aspect of the invention, a corresponding method for manufacturing an integrated circuit is conceived. According to a further aspect of the invention, an electronic device is conceived which comprises an integrated circuit of the kind set forth.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims the priority under 35 U.S.C. §119 of European patent application no. 12168100.1, filed on May 15, 2012, the contents of which are incorporated by reference herein. 
     FIELD OF THE INVENTION 
     The invention relates to an integrated circuit. The invention also relates to a method for manufacturing an integrated circuit and to an electronic device comprising an integrated circuit. 
     BACKGROUND OF THE INVENTION 
     Many integrated circuits contain important data which have to be kept secret, for examples codes which are used for encryption, identification and/or authentication purposes. These integrated circuits (chips) are used, for example, in banking applications, ticketing applications or pay TV applications. 
     Chips which are used in these fields have to be security-certified. Basically, a security certification of this kind proves that the integrated circuit meets a certain security standard. In order to meet a security standard it is usually required that the integrated circuit is sufficiently resistant against tampering. In other words, the integrated circuit must be resistant against all kinds of attacks which aim at retrieval of codes or uncontrolled change of functionality of said integrated circuit. 
     One important form of attack consists of reverse engineering of the integrated circuit. Another possible attack is to modify the integrated circuit in such a way that certain security measures, for example electrical shields, are circumvented. This kind of attacks typically requires physical or chemical modification of the chip, for example the removal of passivation layers. Unfortunately, such attacks typically go unnoticed because conventional integrated circuits are not capable of detecting removal of a passivation layer autonomously. 
     SUMMARY OF THE INVENTION 
     It is an object of the invention to improve the tamper resistance of an integrated circuit by enabling said integrated circuit to detect attacks autonomously using relatively simple on-chip electrical measurements. This object is achieved by an integrated circuit as defined in claim  1  and a corresponding method for manufacturing an integrated circuit as defined in claim  7 . 
     According to an aspect of the invention an integrated circuit is conceived which comprises a physical unclonable function which is at least partially implemented in a passivation layer of said integrated circuit. 
     According to an exemplary embodiment of the integrated circuit, the physical unclonable function is implemented by randomly distributed conducting particles in said passivation layer. 
     According to a further exemplary embodiment of the integrated circuit, the randomly distributed conducting particles comprise aluminum. 
     According to a further exemplary embodiment of the integrated circuit, the integrated circuit further comprises a detector which is arranged to evaluate the physical unclonable function by distinguishing between shorted and open connections between said randomly distributed conducting particles and to produce a corresponding evaluation result. 
     According to a further exemplary embodiment of the integrated circuit, the integrated circuit is arranged to compare said evaluation result with a reference value. 
     According to a further exemplary embodiment of the integrated circuit, the reference value is stored in a memory of said integrated circuit. 
     According to a further aspect of the invention, a method for manufacturing an integrated circuit is conceived which comprises implementing a physical unclonable function at least partially in a passivation layer of said integrated circuit. 
     According to a further exemplary embodiment of the method, the physical unclonable function is implemented by randomly distributing conducting particles in said passivation layer. 
     According to a further exemplary embodiment of the method, the randomly distributed conducting particles comprise aluminum. 
     According to a further exemplary embodiment of the method, said randomly distributed conducting particles are deposited substantially above a detector for evaluating the physical unclonable function. 
     According to a further aspect of the invention, an electronic device is conceived which comprises an integrated circuit of the kind set forth. 
     According to a further exemplary embodiment of the method, the electronic device is one of an electronic device for banking applications, an electronic device for ticketing applications and an electronic device for pay television applications. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The invention will be described in more detail with reference to the appended drawings, in which: 
         FIG. 1  shows a portion of an integrated circuit comprising a physical unclonable function in accordance with an exemplary embodiment of the invention; 
         FIG. 2  shows a method of producing a portion of an integrated circuit comprising a physical unclonable function in accordance with an exemplary embodiment of the invention. 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  shows a portion of an integrated circuit  100  comprising a physical unclonable function in accordance with an exemplary embodiment of the invention. 
     The integrated circuit  100  (also referred to as “chip”) comprises a pad  106  for external connections such as supply voltage connections and input/output signal connections. Furthermore, the integrated circuit  100  comprises internal connections  108  for carrying active signals within the integrated circuit  100 . In addition, the integrated circuit  100  comprises a physical unclonable function (PUF) detector  110  which contains connections for making contact with randomly distributed conducting particles  104  in the passivation layer  102  of the integrated circuit  100 . According to this exemplary embodiment, the randomly distributed conducting particles  104  implement the PUF. The PUF detector  110  is arranged to distinguish between shorted and open connections between said randomly distributed conducting particles  104 . In this way the PUF detector  110  evaluates the PUF. All elements except the pad  106  for the external connections are covered by the passivation layer  102 . 
     In general, undesirable modifications of an integrated circuit  100  may be detected by, for example, measuring the resistance or capacity of parts of the circuit  100 . Some modifications might be reversible: if, for instance, the removal of the passivation layer  102  of a chip  100  cannot be detected by the chip  100 , a possible attack would be to remove the passivation layer  102 , modify the circuit  100  and add a new passivation layer  102 . According to an aspect of the invention a PUF is implemented in the passivation layer  102  of the chip. The PUF introduces a factor which is random from chip to chip, but as stable as possible for a single chip. 
     According to an exemplary embodiment of the invention, the passivation layer  102  contains an inhomogeneous characteristic, i.e. a plurality of randomly distributed conducting particles  104 , which can be evaluated by the chip  100  autonomously, i.e. by a PUF detector  110  which also forms part of the chip  100 . During production of the chip  100 , a reference value representing the expected evaluation result of the PUF may be stored in the chip&#39;s internal non-volatile memory (not shown). An attempt to remove the passivation layer  102 , for example in case of reverse engineering of the chip  100 , will irreversibly damage the PUF (i.e. the random structure of conducting particles  104 ) and the evaluation result of the PUF will permanently deviate from the expected result. Thus, the chip  100  may evaluate the PUF by means of the PUF detector  110  and compare the evaluation result with the reference value in order to determine whether or not the passivation layer  102  has been removed. 
     If the passivation layer  102  is removed and/or replaced, the change can be detected by the chip  100  autonomously in a relatively easy way. PUFs implemented by poorly matching devices like small diodes or transistors have the disadvantage, that they usually do not notice a modification of the top or bottom surface of the chip, while a protection of these parts against unnoticed modification would raise the security hardness of the chip. PUFs using the properties of the surface, such as optical or electrical properties, are difficult to implement. A PUF implemented by randomly distributed conducting particles  104  in the passivation layer  102  of the chip  100  protects the surface of the chip  100  in combination with a relatively easy detection scheme. 
       FIG. 2  shows a method of producing a portion of an integrated circuit comprising a physical unclonable function in accordance with an exemplary embodiment of the invention. 
     In step (a) a basic structure is provided which comprises a pad  106  for external connections, internal connections  108  and PUF detector connections  110 . For example, these elements may form a regular aluminum pattern. In step (b) this basic structure is covered with a photoresist layer  200 , which is part of a normal photolithographic process. However, the PUF detector connections  110  are not covered by the photoresist layer  200 . Subsequently, in step (c) conducting particles are randomly distributed over the entire surface of the intermediate product. For example, these particles form a random aluminum pattern  202  on the entire surface. The random aluminum pattern  202  may for example be created by stopping the deposition of aluminum at an arbitrary moment in time. In step (d) the photoresist layer  200  is removed such that only some randomly distributed conducting particles  104  remain on the surface of the basic structure. In particular, only those particles which are substantially above the PUF detector connections  110  remain on the surface. Finally, in step (e) the passivation layer  102  is deposited on all elements except the pad  106  for the external connections. 
     The above-mentioned embodiments illustrate rather than limit the invention, and the skilled person will be able to design many alternative embodiments without departing from the scope of the appended claims. In the claims, any reference sign placed between parentheses shall not be construed as limiting the claim. The word “comprise(s)” or “comprising” does not exclude the presence of elements or steps other than those listed in a claim. The word “a” or “an” preceding an element does not exclude the presence of a plurality of such elements. The mere fact that certain measures are recited in mutually different dependent claims does not indicate that a combination of these measures cannot be used to advantage. 
     LIST OF REFERENCE SIGNS 
     
         
           100  integrated circuit 
           102  passivation layer 
           104  random aluminum pattern 
           106  pad for external connections 
           108  internal connections 
           110  PUF detector 
           200  photoresist layer 
           202  random aluminum pattern