# Patent

## Title
Device for detecting a laser attack in an integrated circuit chip

## Abstract

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, including a detector of the variations of the current flowing between the base of said NPN bipolar transistor and the substrate.

## Background
CROSS REFERENCE TO RELATED APPLICATIONS
This application claims the priority benefit of French patent application number 12/51151, filed on Feb. 8, 2012, which is hereby incorporated by reference to the maximum extent allowable by law.
BACKGROUND
1. Technical Field
The present disclosure relates to the protection of an integrated circuit chip against laser attacks.
2. Discussion of the Related Art
In certain secure devices such as payment cards, integrated circuits chips are likely to process and/or store critical data, for example, encryption keys. Such chips may be fraudulently manipulated 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 frauds, 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 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 its resetting, 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 for example require creating new structures in the chips 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 twice smaller than that of the P-channel MOS transistor.
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.



BRIEF DESCRIPTION OF THE DRAWINGS
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.



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.
DETAILED DESCRIPTION
FIG. 1 is a cross-section view schematically showing a portion of an integrated circuit chip comprising an NPN bipolar transistor having an N-type buried layer. 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 Vdd 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 Vdd. This turns on PNP transistor 21 and a current IPNP then flows between emitter contact 11 and collector contact 15 of PNP transistor 21. The voltage at node 22 switches from Vdd to Vdd−RIPNP, 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 IPNP 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 twice 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 μm2, for example being on the order of 4 μm2.
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 mm2. 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.

## Claims

1. A device for detecting a laser attack in an integrated circuit chip formed in an upper P-type portion of a semiconductor substrate incorporating an NPN bipolar transistor having an N-type buried collector layer comprising a detector of variations of current flowing between a base of said NPN bipolar transistor and said semiconductor substrate.

2. The device of claim 1, wherein the substrate comprises a substrate contact provided to be grounded, and wherein 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.

3. The device of claim 1, wherein a collector and the base of the NPN bipolar transistor are interconnected.

4. The device of claim 2, wherein the resistor is embodied by a P-channel MOS transistor.

5. The device of claim 2, wherein the comparator comprises an inverter.

6. The device of claim 5, wherein 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 twice smaller than that of the P-channel MOS transistor.