Abstract:
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.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a division of U.S. patent application Ser. No. 12/643,537, filed Dec. 21, 2009, which claims the priority benefit of French patent application No. 08/59055, filed on Dec. 24, 2008, which applications are 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 thereto-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. 
     The foregoing objects, features, and advantages of the present invention 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 block diagram of a device according to an embodiment of the present invention; 
         FIG. 2  is an electric diagram of a conventional memory cell constitutive of EEPROMs; 
         FIG. 3  is a cross-section view of the cell of  FIG. 2 ; 
         FIG. 4  shows a charge storage element that an be used in a circuit according to an embodiment of the present invention; 
         FIG. 5  shows a thermo-mechanical switch and a charge evacuation element that can be used in a circuit according to an embodiment of the present invention; 
         FIG. 6  shows two curves of the elongation of metals according to temperature; and 
         FIG. 7  is an electric diagram of a circuit according to an embodiment of the present invention comprising a reading device. 
     
    
    
     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. 1  is a block diagram of an embodiment of a device for monitoring the temperature of a product. 
     The device comprises an electric charge storage element  10  and an element for evacuating charges  12 . Storage element  10  and evacuation element  12  are connected via a thermomechanical switch  14  which is provided to close when the temperature exceeds a selected temperature threshold. Storage element  10  is associated with a reading device  16  enabling to determine the amount of charges present in charge storage element  10 . 
     Initially, thermomechanical switch  14  is off The device is powered to store charges in charge storage element  10 . The power is then cut off. If the temperature exceeds the threshold of thermomechanical switch  14 , said switch turns on and the charges stored in element  10  evacuate through element  12 . As long as the temperature remains below the threshold, thermomechanical switch  14  remains off and the charges remain stored in element  10 . 
     The amount of charges present in element  10  is 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 device  16  is powered when this amount of charges is desired to be determined after the monitoring phase. 
       FIG. 2  is an electric diagram of a conventional memory cell  20  constitutive of EEPROMs. 
     Memory cell  20  comprises a selection transistor T 1  or a storage transistor or memory point T 2 . Transistor T 1  is a MOS transistor comprising a drain D 1 , a source S 1 , and an insulated gate G 1 . Memory point T 2  is of dual-gate type. It comprises a drain D 2 , a source S 2 , and two insulated gates, that is, a floating gate F 2  and a control gate G 2 . A memory point T 2 , having its floating gate insulator F 2  comprising 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 S 1  of transistor T 1  is connected to drain D 2  of memory point T 2 . 
       FIG. 3  is a cross-section view of memory cell  20  of  FIG. 2 . Memory cell  20  is formed in an active region of a semiconductor substrate  22 , typically made of silicon, and laterally delimited by field insulation areas (not shown). Above semiconductor substrate  22  are formed the gate structures of transistor T 1  and of memory point T 2 . The gate of transistor T 1  is formed of a stacking of a first insulating portion  24 , of a first conductive portion  26 , of a second insulating portion  28 , and of a second conductive portion  30 . It may be desirable for the operation of selection transistor T 1  to be similar to that of a single-gate conventional MOS transistor. For this purpose, conductive portions  26  and  30  are short-circuited by metal tracks and vias, not shown. 
     The gate of memory point T 2  is formed of a stack of a first insulating portion  32 , of a first conductive portion  34 , of a second insulating portion  36 , and of a second conductive portion  38 . Conductive portion  34  forms floating gate F 2  of memory point T 2 . Insulating portion  32  comprises a relatively thick portion forming the non-tunnel portion of the insulator of floating gate F 2  and a relatively thin portion  40  forming the tunnel oxide area. Spacers  42  are formed on either side of selection transistor T 1  and of memory point T 2 . 
     On either side of selection transistor T 1  and of memory point T 2 , in substrate  22 , are formed drain and source implantation areas  44  of these elements. Two other implantation areas  46  are formed at the surface of substrate  22  and partly extend under insulating portion  32  of the memory point. 
     According to the voltages applied between drain D 2  and source S 2  and on control gate 
     G 2  of the memory point, electrons may be injected into floating gate F 2 / 34  to 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 T 2 . 
     The inventors provide using the charge retention properties of an EEPROM cell, modified to form charge storage element  10  of  FIG. 1 . 
       FIG. 4  shows such a modified cell  10 . Cell  10  comprises a selection transistor T 1  identical to that of  FIG. 3 , formed on a substrate  22  having doped regions ( 44  and  46 ) identical to the regions of  FIG. 3 . Cell  10  also comprises a memory point having its gate formed of a stack of a first conductive layer  50 , of a first conductive layer  52 , of a second insulating layer  54 , and of a second conductive layer  56 . First insulating layer  50  comprises a thinner region to enable the passing of carriers by tunnel effect. 
     First insulating layer  50  and first conductive layer  52  extend, at the surface of substrate  22 , over a greater surface area than second insulating layer  54  and second conductive layer  56 . A contact  58  is taken on top of first conductive layer  52  and is intended to be connected to thermomechanical switch  14 . Thus, by means of selection transistor T 1 , charges may be injected into floating gate  52  of memory point  10  having its gate connected to an initially off thermomechanical switch  14 . 
       FIG. 5  shows an example of a thermomechanical switch that may be used in a device such as that of  FIG. 1 . 
       FIG. 5  shows memory point  10  having its floating gate  52  extending on substrate  22  over a greater distance than control gate  56 . Contact  58  is connected, via several interconnect levels  60  usually comprised in an integrated circuit, to a first metal strip  62  formed in an upper interconnect level. First metal strip  62  forms a first portion of thermomechanical switch  14 . A second metal strip  64 , in the same interconnect level as strip  62 , forms the second portion of switch  14 . As an example, first and second metal strips  62  and  64  may be made of copper or aluminum. First and second metal strips  62  and  64  are separated by a cavity  65  filled with air having its length d selected according to the desired threshold. In a heating, the expansion of the metal forming strips  62  and  64  causes a decrease in distance d. When the heating is sufficient, free ends  621  and  641  of the strips come into contact, which functionally closes the switch. 
     Second metal strip  64  is connected, via metal portions formed in interconnect levels  60 , to substrate  22 . This connection forms charge evacuation element  12  of  FIG. 1 . It should be noted that substrate  22  may be provided in different ways to form a resistor through which the charges stored in floating gate  52  evacuate when thermomechanical switch  14  is 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. 1   0 , ΔT being the temperature difference with respect to a reference temperature,  1   0  being 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 −5  for aluminum and on the order of 1.70·10 −5  for copper. 
     The structure of the switch of  FIG. 5  may be formed by various known techniques. For example, if the metallization level comprising strips  62  and  64  corresponds to the last metallization level of interconnect stacking  60 , metal strips  62  and  64  are 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 cavity  65 . If the metallization level comprising strips  62  and  64  is not the last metallization level, cavity  65  may be formed by carrying out an additional etching of the insulator conventionally formed between metal strips  62  and  64 . It may be provided to form a cavity  65 , around the ends of strips  62  and  64 , larger than that disclosed herein. It may also be provided to only form a single metal strip  62  or  64 , 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. 6  shows two curves  70  and  72  of 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 strip  62  or  64  exhibits 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. 7  is an electric diagram of a circuit according to an embodiment of the present invention and of an example of a reading device  16  associated 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 element  10  (connection to the substrate), symbolized in  FIG. 7  by a capacitor, is connected between floating node F and the ground. A charge evacuation element  12 , symbolized by a resistor, is connected via a thermomechanical switch  14  to the floating node. The second terminal of resistor  12  is grounded. 
     Reading device  16  comprises 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 (switch  14  still off) and at a second state if switch  14  has been closed and the charges of floating node F have been evacuated by resistor  12 . 
     Advantageously, a circuit such as the circuit of  FIG. 7  is 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 device  16  may be different from the reading device shown in  FIG. 7 . It should further be noted that the metal strips forming thermomechanical switch  14  may 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 region  65  between 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. 
     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.