Device for detecting the thinning down of the substrate of an integrated circuit chip

A device for detecting the thinning down of the substrate of an integrated circuit chip, including, in the active area of the substrate, bar-shaped diffused resistors connected as a Wheatstone bridge, wherein: first opposite resistors of the bridge are oriented along a first direction; the second opposite resistors of the bridge are oriented along a second direction; and the first and second directions are such that a thinning down of the substrate causes a variation of the imbalance value of the bridge.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of French patent application number 09/53968, filed on Jun. 15, 2009, entitled “DEVICE FOR DETECTING THE THINNING DOWN OF THE SUBSTRATE OF AN INTEGRATED CIRCUIT CHIP,” which is hereby incorporated by reference to the maximum extent allowable by law.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to the protection of an integrated circuit chip against laser attacks. It more specifically aims at the detection of the thinning down of the chip substrate which is carried out before performing a laser attack.

2. Discussion of the Related Art

FIG. 1is a simplified cross-section view of an integrated circuit chip1comprising a semiconductor support substrate3, comprising in its upper portion an active layer5in which are formed electronic components, not shown. Substrate3is currently covered with a stack of insulating layers7. Conductive interconnection tracks9are formed between insulating layers. There generally exist several successive interconnection levels, for example, three, M1 to M3, in the shown example. Conductive vias, not shown, cross the insulating layers to connect the conductive tracks to one another, to input-output terminals11of the chip, and to components of active area5, thus forming the circuit interconnects.

In secure devices, for example, payment cards, some regions of active area5are capable of processing and/or storing critical data, for example, ciphering keys. Such devices may be subject to fraudulent manipulations aiming at obtaining protected confidential data.

Among known attacks, so-called “fault attacks” comprise deliberately disturbing the operation of a chip, and analyzing the effect of the disturbances on its behavior. The attacker is especially interested in the influence of disturbances on data such as output signals, power consumption, or response times. He is likely to deduce therefrom, by statistical studies or others, critical data such as the algorithms used and possibly the ciphering keys. To deliberately cause faults in the circuits of a chip, an attack mode comprises bombarding areas of the chip with a laser beam. It is thus possible to inject faults into certain memory cells and/or to alter the operation of certain components. It should be noted that in a laser attack, the chip should be powered.

FIG. 2is a simplified cross-section view of chip1ofFIG. 1illustrating a preliminary step where substrate3is thinned down, which is necessary to carry out a laser attack. To make the components of active region5accessible to the laser beam, the attacker needs to remove part of the thickness of support substrate3. As an example, a chip formed from a substrate having a 180-μm thickness will have its thickness decreased by on the order of 150 μm before a laser attack.

To ensure protection against tampering, an attack detection device coupled to a protection circuit is generally provided in secure chips. When an attack is detected, the protection circuit implements measures of protection, alienation, 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 reset it, in order to reduce the time for which the attacker can study the chip response to a disturbance.

Attack detection solutions may be logical. They, for example, comprise regularly introducing into the calculations integrity tests enabling to make sure that the data have not been modified from the outside. Such solutions have the disadvantage of introducing additional calculation steps, thus increasing the chip response times. Further, integrity tests cannot detect all the disturbances caused by an attacker. The latter thus has a leeway likely to enable him to acquire critical data.

Other so-called physical attack detection solutions especially comprise sensors sensitive to temperature variations, to ultraviolet rays, or to X rays, enabling to detect suspicious activities. Like logic solutions, such solutions are not perfectly reliable. Indeed, the attacker has a leeway before the attack has been detected, during which he may be able to obtain critical data. Further, such solutions are expensive and difficult to implement.

SUMMARY OF THE INVENTION

Thus, an object of an embodiment of the present invention is to provide a device for detecting a laser attack, which overcomes at least some of the disadvantages of prior art solutions.

An object of an embodiment of the present invention is to provide a device enabling to detect the attack before the attacker is able to analyze the chip response to laser disturbances.

An object of an embodiment of the present invention is to detect the thinning down of the chip support substrate, which is carried out before performing a laser attack.

An object of an embodiment of the present invention is to provide a low-cost solution, easy to implement with usual manufacturing methods.

Thus, an embodiment of the present invention provides a device for detecting the thinning down of the substrate of an integrated circuit chip, comprising, in the active area of the substrate, bar-shaped diffused resistors connected as a Wheatstone bridge, wherein: first opposite resistors of the bridge are oriented along a first direction; the second opposite resistors of the bridge are oriented along a second direction; and the first and second directions are such that a thinning down of the substrate causes a variation of the imbalance value of the bridge.

According to an embodiment of the present invention, the main surfaces of the substrate are in plane [001] of the crystal structure of the substrate.

According to an embodiment of the present invention: the diffused resistors are formed in a substrate region of a first conductivity type; and the first and second directions respectively correspond to directions (100) and (110) of the crystal structure of the substrate.

According to an embodiment of the present invention: the first opposite resistors are formed in a substrate region of a first conductivity type; said second opposite resistors are formed in a substrate region of the second conductivity type; and the first and second directions respectively correspond to directions (100) and (010) of the crystal structure of the substrate.

According to an embodiment of the present invention: the first opposite resistors are formed in a substrate region of a first conductivity type; the second opposite resistors are formed in a substrate region of the second conductivity type; and the first and second directions are parallel to direction (110) of the crystal structure of the substrate.

According to an embodiment of the present invention: the diffused resistors are formed in a P-type doped substrate region; the first and second directions respectively correspond to directions (100) and (110) of the crystal structure of the substrate; and the second opposite resistors are each surrounded with an insulating region covered with a polysilicon layer, the entire resistor being covered with a protection nitride layer.

According to an embodiment of the present invention: the diffused resistors are formed in a N-type doped substrate region; the first and second directions respectively correspond to directions (100) and (110) of the crystal structure of the substrate; and the first opposite resistors are each surrounded with an insulating region covered with a polysilicon layer, the entire resistor being covered with a protection nitride layer.

Another embodiment of the present invention provides a circuit for detecting the thinning down of the substrate of an integrated circuit comprising: at least one device for detecting the thinning down of the substrate according to any of the above-mentioned embodiments; means for measuring the imbalance of this device.

According to an embodiment of the present invention, at least one comparator compares the imbalance value of said at least one device with a threshold, the output state of the detection circuit being based on the output value of this comparator.

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, the various cross-section and top views are not drawn to scale.

FIG. 3shows the electric diagram of a Wheatstone bridge formed of four resistors, for example, of same value R. A first voltage VINis applied to a first diagonal of the bridge, between nodes A and B. An imbalance voltage VOUTmay appear across a second diagonal of the bridge, between nodes C and D.

Normally, the Wheatstone bridge is balanced and output voltage VOUTis independent from the value of VINand from possible temperature variations, for example, close to 0 V.

FIG. 4Ais a top view schematically showing an embodiment of a diffused resistor.FIGS. 4B and 4Care cross-section views ofFIG. 4Aalong axes B-B and C-C. An N-type doped area21is formed in the upper portion of a region23of a lightly-doped P-type semiconductor substrate. In top view, area21has the shape of a rectangular bar. An oxide region25is arranged at the periphery of area21to delimit the resistor. Conductive pads27are arranged in contact with the ends of resistive area21. All the above-mentioned conductivity types may be inverted.

The semiconductor substrate, typically made of silicon, has piezoresistive properties, that is, its conductibility varies according to the mechanical stress to which it is submitted. Now, the substrate thinning preceding a laser attack modifies the stress exerted in the active layer of the chip. Accordingly, the resistance value is likely to vary when the substrate is thinned down. Thus, to detect a thickness variation, it is here provided to detect a resistance variation.

FIG. 5is a simplified top view of an embodiment of a device for detecting the thinning down of a chip substrate. This device, formed in the active chip area, comprises a Wheatstone bridge formed of four resistors31,33,35,37of same value R. Resistors31,33,35,37are diffused resistors, formed in a P-type semiconductor substrate, as described in relation withFIGS. 4A to 4C. A single-crystal silicon substrate having its main surface in a crystal plane [001], that is, a plane orthogonal to crystal direction (001), is here considered. Opposite resistors31and33are parallel to each other and are oriented, in Miller notation, along direction (100). Opposite resistors35and37are parallel to each other and are oriented along direction (110). Resistors31,33,35,37are thus arranged in the same plane [001], directions (100) and (110) forming a 45° angle.

When the substrate is thinned down to be prepared for a laser attack, the stress variations in the active area are likely to affect resistors oriented along different directions differently. Resistors31and33, oriented along axis (100), remain substantially unchanged. However, resistors35and37, oriented along axis (110), undergo a significant variation. Thus, the thinning down of the substrate causes a variation in the imbalance value of the bridge. As an example, output voltage VOUTof the bridge varies by a factor4when 150 μm are removed from a 180-μm substrate (for example, from 20 mV to 80 mV).

If the four resistors were oriented along the same direction, the stress variations in the substrate would affect all resistors substantially in the same way in case of a thinning down of the substrate. The imbalance value of the Wheatstone bridge would thus remain unchanged.

If the resistors were formed in a semiconductor N-type substrate having its main surface also in a plane [001], the resistors oriented along a direction (110) would remain substantially unchanged in case of a thinning down of the substrate. However, the resistors oriented along a direction (100) would undergo a significant variation. Thus, the imbalance value of the bridge would be sensitive to the thinning down of the substrate.

At least one embodiment of the present invention also provides means, not shown, for measuring the imbalance value of the Wheatstone bridge. According to an embodiment, the measurement means comprise comparators comparing output voltage VOUTrespectively with a positive threshold, for example 50 mV, and with a negative threshold, for example, 50 mV. When the bridge imbalance exceeds a threshold, the output of the substrate thinning-down detection circuit changes, thus for example stopping the chip operation. It may, for example, be chosen to perform the imbalance voltage measurements periodically, at each starting of the chip, or continuously, when the chip is powered.

FIG. 6is a simplified top view of an alternative embodiment of a device for detecting the thinning down of a chip substrate. This device, formed in the active chip area, comprises a Wheatstone bridge formed of four resistors41,43,45,47, of same value R. Resistors41,43,45,47, are diffused resistors, formed as described in relation withFIGS. 4A to 4Cin a substrate having its main surface formed in a plane [001]. Opposite resistors41and43are formed in a lightly-doped N-type region of the substrate. Resistors41and43are oriented, in Miller notation, along direction (100). Opposite resistors45and47are formed in a lightly-doped P-type substrate region. Resistors45and47are oriented along direction (010).

When the substrate is thinned down to be prepared for a laser attack, resistors41and43, oriented along direction (100), undergo a significant variation. However, resistors45and47, oriented along direction (010), remain substantially unchanged. Thus, the thinning down of the substrate causes a variation in the bridge imbalance value.

FIG. 7is a simplified top view of an alternative embodiment of a device for detecting the thinning down of a chip substrate. This device, formed in the active chip area, comprises a Wheatstone bridge formed of four resistors51,53,55,57, of same value R. Resistors51,53,55,57are diffused resistors formed as described in relation withFIGS. 4A to 4Cin a substrate having its main surface in a plane [001]. Opposite resistors51and53are formed in a lightly-doped N-type substrate region. Opposite resistors55and57are formed in a lightly-doped P-type substrate region. Resistors51,53,55, and57are oriented along direction (010).

When the substrate is thinned down to be prepared for a laser attack, resistors51and53undergo a significant variation. However, resistors55and57remain substantially unchanged. Thus, the thinning down of the substrate causes a variation of the imbalance value of the bridge.

Of course, the present invention is not limited to the configurations described in relation withFIGS. 5 to 7. It will be within the abilities of those skilled in the art to implement the desired operation by orienting the Wheatstone bridge resistors differently. However, to obtain good detection performances, the bridge should have a good sensitivity to stress variations generated by the substrate thinning-down. Generally, first opposite resistors of the bridge should be particularly sensitive to stress variations caused by the substrate thinning-down. However, the second opposite resistors of the bridge should remain substantially unchanged when the substrate is thinned down. The inventors have determined that the above-described configurations exhibit optimal detection performances for a [001] silicon substrate. Of course, if the main surface of the substrate is not in a plane [001], other orientations providing a maximum variability according to the stress likely to be created by a thinning down will be selected.

FIG. 8Ais a top view schematically representing an embodiment of a diffused resistor, formed in the active layer of a chip.FIGS. 8B and 8Care cross-section views of the resistor ofFIG. 8Aalong axes B-B and C-C. The resistor ofFIGS. 8A to 8Cis similar to the resistor ofFIGS. 4A to 4C. It further comprises a polysilicon layer61on the surface of insulating region25. Layer61corresponds to the maintaining on insulating region25of a portion of an insulated polysilicon layer, forming insulated gates of MOS transistors at other locations of the integrated circuit. Further, a protection nitride layer63may be maintained, to cover the entire resistor except for conductive pads27.

FIG. 9is a simplified top view of a device for detecting the thinning down of a chip substrate. This device comprises a Wheatstone bridge formed of four resistors71,73,75,77of same value R, formed in a P-type semiconductor substrate having its main surface in a plane [001]. Opposite resistors71and73are diffused resistors formed as described in relation withFIGS. 4A to 4C. Opposite resistors75and77are diffused resistors having their insulation region covered with polysilicon, formed as described in relation withFIGS. 8A to 8C. Resistors71and73are oriented along direction (100). Resistors75and77are oriented along direction (110).

Such a device has improved substrate thinning down detection performance with respect to the device described in relation withFIG. 5. Indeed, the polysilicon and nitride layers present in resistors75and77amplify the sensitivity of these resistors to stress variations in the active area.

An advantage of the use of Wheatstone bridges as thinning-down detection elements lies in the fact that imbalance value VOUTis independent from the circuit temperature. Indeed, although the resistance values are likely to vary along with temperature, the undergone drift is the same for all the resistors of a same bridge, at least in the case where all resistors have the same doping type. Thus, the balancing remains unchanged.

According to an advantage of an embodiment of the present invention, the provided solution is compatible with usual manufacturing methods and requires no additional manufacturing steps.

Specific embodiments of the present invention have been described. Various alterations and modifications will occur to those skilled in the art. In particular, the present description mentions a circuit for detecting the thinning down of the substrate of a chip comprising a Wheatstone bridge and means for measuring the imbalance of this bridge. It will also be within the abilities of those skilled in the art to form a detection circuit comprising several Wheatstone bridges. Further, it will be within the abilities of those skilled in the art to implement the desired operation whatever the conductivity type of the semiconductor substrate in which the Wheatstone bridges resistors are formed. Further, the present invention is not restricted to the bridge imbalance detection means discussed in the present description. It will be within the abilities of those skilled in the art to implement the desired operation whatever the Wheatstone bridge imbalance measurement means used. Moreover, the present description mentions Wheatstone bridges formed of four resistors of same value. It will be within the abilities of those skilled in the art to implement the desired operation by using normally balanced Wheatstone bridges in which the resistors do not all have the same value.