Device for monitoring the temperature of an element

A device for monitoring the temperature surrounding a circuit, including: a charge storage element; a charge evacuation device; and a thermo-mechanical switch connecting the storage element to the evacuation element, the switch being capable of closing without the circuit being electrically powered, when the temperature exceeds a threshold.

CROSS REFERENCE TO RELATED APPLICATIONS

This application claims the priority benefit of French patent application Ser. No. 08/59055, filed on Dec. 24, 2008, entitled “DEVICE FOR MONITORING THE TEMPERATURE OF AN ELEMENT,” 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 a device and a method for monitoring the temperature of an element. More specifically, the present invention relates to a device and a method enabling to check that the temperature of the element has not exceeded a threshold.

2. Discussion of the Related Art

Various techniques for monitoring the temperature of a product are known. A first technique comprises placing, on a product, an electronic circuit which periodically stores the product temperature in a memory. An appropriate reading device then enables extraction of the stored values. It can thus be checked that the product has not been overheated. Such a technique requires that the circuit is powered.

A second technique comprises placing, on a product that should remain chilled, a color chip which irreversibly changes color when the product temperature has exceeded a threshold. Such chip are generally formed of temperature-sensitive microorganisms. Thus, consumers are informed of the freshness of the product. This solution is not reversible and only works for temperatures below 10° C.

It would be desirable to have a reusable device and a method for monitoring the temperature of an element, with a design adaptable according to the temperature threshold which is desired to be detected and requiring no permanent powering.

SUMMARY OF THE INVENTION

An object of an embodiment of the present invention is to provide a device and a method for monitoring the temperature of an element, which does not need continuous powering.

Another object of an embodiment of the present invention is to provide a reusable device.

Another object of an embodiment of the present invention is to provide a device which is manufactured with conventional electronic component manufacturing steps.

Thus, an embodiment of the present invention provides a device for monitoring the temperature surrounding the circuit, comprising a charge storage element, a charge evacuation device, and a thermo-mechanical switch connecting the storage element to the evacuation element, the switch being capable of turning on without the circuit being electrically powered, when the temperature exceeds a threshold.

According to an embodiment of the present invention, the circuit further comprises a device for reading the amount of charge in the storage element.

According to an embodiment of the present invention, the thermo-mechanical switch is formed of two metal strips having opposite ends separated by a distance adapted to decrease as the temperature increases and to come down to zero when the temperature exceeds the threshold.

According to an embodiment of the present invention, the circuit comprises an interconnect stack in which the metal strips are formed, the ends of the strips being separated by a region filled with air formed in the interconnect stack.

According to an embodiment of the present invention, the metal strips are made of copper or aluminum.

According to an embodiment of the present invention, the storage element is formed of a dual-gate MOS transistor comprising a control gate and a floating gate, the floating gate being connected to the thermo-mechanical switch.

According to an embodiment of the present invention, the charge evacuation element is formed by a contact between the thermo-mechanical switch and the substrate in and on which the circuit is formed.

Another embodiment of the present invention provides a method for monitoring the temperature of an element, comprising the steps of: powering a circuit attached to the element to inject charges into a charge storage element of this circuit; removing the power; causing an evacuation of the charges if the temperature exceeds a temperature threshold; powering the circuit; and reading the amount of charges present in the storage element.

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 drawings are not to scale.

The inventors provide a circuit and a method for monitoring the temperature surrounding the circuit, requiring no continuous powering during the monitoring. Such a circuit may, for example, be used to make sure that the temperature of a product, in a transportation phase, does not exceed a threshold above which the product is altered. It may also be used to detect reverse engineering operations on a product. Indeed, such operations generally provide the steps of heating at high temperatures, which can thus be detected. It may also be desired to know the history of the temperature of a product in case a product is returned by a customer to verify that the product has been used by the customer in normal temperature conditions.

FIG. 1is a block diagram of an embodiment of a device for monitoring the temperature of a product.

The device comprises an electric charge storage element10and an element for evacuating charges12. Storage element10and evacuation element12are connected via a thermomechanical switch14which is provided to close when the temperature exceeds a selected temperature threshold. Storage element10is associated with a reading device16enabling to determine the amount of charges present in charge storage element10.

Initially, thermomechanical switch14is off. The device is powered to store charges in charge storage element10. The power is then cut off. If the temperature exceeds the threshold of thermomechanical switch14, said switch turns on and the charges stored in element10evacuate through element12. As long as the temperature remains below the threshold, thermomechanical switch14remains off and the charges remain stored in element10.

The amount of charges present in element10is thus representative of the fact that the product has been placed in an environment which is overheated with respect to a threshold, since a last initialization step during which charges have been introduced. Reading device16is powered when this amount of charges is desired to be determined after the monitoring phase.

FIG. 2is an electric diagram of a conventional memory cell20constitutive of EEPROMs.

Memory cell20comprises a selection transistor T1and a storage transistor or memory point T2. Transistor T1is a MOS transistor comprising a drain D1, a source S1, and an insulated gate G1. Memory point T2is of dual-gate type. It comprises a drain D2, a source S2, and two insulated gates, that is, a floating gate F2and a control gate G2. A memory point T2, having its floating gate insulator F2comprising at least a portion sufficiently thin to enable a passing by tunnel effect of carriers between the underlying channel and the floating gate is here considered. The floating gate insulator is called “tunnel insulator” or “tunnel oxide”. Source S1of transistor T1is connected to drain D2of memory point T2.

Memory cell20is formed in an active region of a semiconductor substrate22, typically made of silicon, and laterally delimited by field insulation areas (not shown). Above semiconductor substrate22are formed the gate structures of transistor T1and of memory point T2. The gate of transistor T1is formed of a stacking of a first insulating portion24, of a first conductive portion26, of a second insulating portion28, and of a second conductive portion30. It may be desirable for the operation of selection transistor T1to be similar to that of a single-gate conventional MOS transistor. For this purpose, conductive portions26and30are short-circuited by metal tracks and vias, not shown.

The gate of memory point T2is formed of a stack of a first insulating portion32, of a first conductive portion34, of a second insulating portion36, and of a second conductive portion38. Conductive portion34forms floating gate F2of memory point T2. Insulating portion32comprises a relatively thick portion forming the non-tunnel portion of the insulator of floating gate F2and a relatively thin portion40forming the tunnel oxide area. Spacers42are formed on either side of selection transistor T1and of memory point T2.

On either side of selection transistor T1and of memory point T2, in substrate22, are formed drain and source implantation areas44of these elements. Two other implantation areas46are formed at the surface of substrate22and partly extend under insulating portion32of the memory point.

According to the voltages applied between drain D2and source S2and on control gate G2of the memory point, electrons may be injected into floating gate F2/34to set the memory point to a so-called “programmed” state. It is then possible to evacuate the charges stored in the floating gate by applying adapted voltages on the gate, the drain, and the source of memory point T2.

The inventors provide using the charge retention properties of an EEPROM cell, modified to form charge storage element10ofFIG. 1.

FIG. 4shows such a modified cell10. Cell10comprises a selection transistor T1identical to that ofFIG. 3, formed on a substrate22having doped regions (44and46) identical to the regions ofFIG. 3. Cell10also comprises a memory point having its gate formed of a stack of a first insulating layer50, of a first conductive layer52, of a second insulating layer54, and of a second conductive layer56. First insulating layer50comprises a thinner region to enable the passing of carriers by tunnel effect.

First insulating layer50and first conductive layer52extend, at the surface of substrate22, over a greater surface area than second insulating layer54and second conductive layer56. A contact58is taken on top of first conductive layer52and is intended to be connected to thermomechanical switch14. Thus, by means of selection transistor T1, charges may be injected into floating gate52of memory point10having its gate connected to an initially off thermomechanical switch14.

FIG. 5shows an example of a thermomechanical switch that may be used in a device such as that ofFIG. 1.

FIG. 5shows memory point10having its floating gate52extending on substrate22over a greater distance than control gate56. Contact58is connected, via several interconnect levels60usually comprised in an integrated circuit, to a first metal strip62formed in an upper interconnect level. First metal strip62forms a first portion of thermomechanical switch14. A second metal strip64, in the same interconnect level as strip62, forms the second portion of switch14. As an example, first and second metal strips62and64may be made of copper or aluminum. First and second metal strips62and64are separated by a cavity65filled with air having its length d selected according to the desired threshold. In a heating, the expansion of the metal forming strips62and64causes a decrease in distance d. When the heating is sufficient, free ends621and641of the strips come into contact, which functionally closes the switch.

Second metal strip64is connected, via metal portions formed in interconnect levels60, to substrate22. This connection forms charge evacuation element12ofFIG. 1. It should be noted that substrate22may be provided in different ways to form a resistor through which the charges stored in floating gate52evacuate when thermomechanical switch14is on. Advantageously, it should be noted that all the steps necessary to the forming of the elements described herein may be carried out by conventional electronic component forming techniques.

The elongation of the copper or aluminum strips according to temperature is equal to α.ΔT.I0, ΔT being the temperature difference with respect to a reference temperature, I0being the size of the metal strip at the reference temperature, and α being the metal elongation coefficient. Coefficient α is on the order of 2.36. 10−5for aluminum and on the order of 1.70. 10−5for copper.

The structure of the switch ofFIG. 5may be formed by various known techniques. For example, if the metallization level comprising strips62and64corresponds to the last metallization level of interconnect stacking60, metal strips62and64are formed on a lower insulating layer. Advantageously, interval d required in most applications is sufficiently low for a passivation layer formed on the metal strips not to penetrate into the confined space between strips, which forms cavity65. If the metallization level comprising strips62and64is not the last metallization level, cavity65may be formed by carrying out an additional etching of the insulator conventionally formed between metal strips62and64. It may be provided to form a cavity65, around the ends of strips62and64, larger than that disclosed herein. It may also be provided to only form a single metal strip62or64, the expansion of this strip enabling a direct contact with metal vias of a lower interconnect level at one end of the strip or in a central portion thereof.

FIG. 6shows two curves70and72of the elongation, respectively, of copper and aluminum strips according to temperature. The reference temperature is arbitrarily set to 25° C. and the strip length, at the reference temperature, is equal to 160 μm for the aluminum strip and to 60 μm for the copper strip.

At 85° C., it can be noted that each strip62or64exhibits an elongation on the order of 230 nm if the strip is made of aluminum and of 60 nm if it is made of copper. Thus, to make sure that a 85° C. temperature is not exceeded, distance d will be provided to be on the order of 460 nm if the strips are made of aluminum and of approximately 120 nm for copper. It should be noted that these values are examples only.

FIG. 7is an electric diagram of a circuit according to an embodiment of the present invention and of an example of a reading device16associated with the circuit.

The circuit comprises a MOS transistor for injecting charges T, having its gate connected to an injection control signal Inj and having its main terminals connected between a positive supply voltage Vdd and a floating node F. A charge storage element10(connection to the substrate), symbolized inFIG. 7by a capacitor, is connected between floating node F and the ground. A charge evacuation element12, symbolized by a resistor, is connected via a thermomechanical switch14to the floating node. The second terminal of resistor12is grounded.

Reading device16comprises a comparator COMP having one of its terminals connected to floating node F and having its other terminal biased to a reference voltage Vref, for example, equal to Vdd/2. Comparator COMP is powered. Output Out of the operational amplifier thus is at a first state if the floating node contains charges (switch14still off) and at a second state if switch14has been closed and the charges of floating node F have been evacuated by resistor12.

Advantageously, a circuit such as the circuit ofFIG. 7is easily reusable. After a reading of the charges stored at the level of the floating node, it is possible to reinject charges into the charge storage element to perform a new temperature monitoring.

Specific embodiments have been described. Various alterations and modifications will occur to those skilled in the art. In particular, it should be noted that reading device16may be different from the reading device shown inFIG. 7. It should further be noted that the metal strips forming thermomechanical switch14may be made of any metal or conductive material expanding as the temperature increases. Further, other storage elements than an EEPROM cell may be provided, provided to respect the described storage and charge evacuation functionalities. Finally, although air is a preferred environment for region65between the strip ends to enable an expansion thereof, other environments (for example, vacuum, a gas or a liquid) may be used, provided to be compatible with the desired deformation of the metal strips.

It should further be noted that the circuit described herein may be integrated in an electronic device implementing other functions than the temperature monitoring, where this electronic device can be powered during the temperature monitoring. It may also be provided to place several monitoring devices such as that disclosed herein on a same product to detect the overheating of this product above different temperatures.

The circuit described herein may also be used to monitor the temperature of an integrated circuit including electronic components. In such a case, the storage and charge evacuation elements could be formed on the same integrated circuit as the one including the electronic components.

Advantageously, the monitoring circuit is formed in CMOS technology. If the monitored circuit is also in CMOS technology, the integration of the monitoring circuit does not involve any specific fabrication step.