Abstract:
A device for adjusting an integrated circuit before encapsulation includes a first MOS transistor having a gate and a source connected together, and a body connected to a voltage reference. A first resistor is connected in parallel with the first MOS transistor. A second MOS transistor is connected in series with the first MOS transistor. The second MOS transistor has a gate and a source connected together, and a body connected to the voltage reference. A second resistor is connected in parallel with the second MOS transistor. A first terminal is connected to the source of the first MOS transistor, and a second terminal is connected to the source of the second MOS transistor. The first terminal is accessible externally after the integrated circuit has been encapsulated.

Description:
This application is a national phase of PCT International Application No. PCT/FR02/00503 filed on Feb. 11, 2002 under 35 U.S.C. § 371. 

   FIELD OF THE INVENTION 
   The present invention relates to analog and digital integrated circuits. These circuits desirably use the smallest possible silicon area to reduce costs while still maintaining high precision. 
   BACKGROUND OF THE INVENTION 
   A silicon wafer that has undergone various steps of etching and/or deposition of conductive, semiconductor or insulation layers is put through a sorting step intended to remove defective circuits. The sorting step is followed by a packaging or encapsulation step. 
   During the sorting step, each circuit on a wafer is tested to check its conformance,to specifications. A circuit can be considered satisfactory, rejected, or alternatively, a candidate for adjustment. Adjustment is performed by imposing given electrical voltages and/or currents on terminals of the integrated circuit. Some of these terminals may no longer be accessible once the circuit is encapsulated. 
   SUMMARY OF THE INVENTION 
   In view of the foregoing background, an object of the present invention is to provide a device for adjusting an integrated circuit during the sorting step, prior to its encapsulation. 
   This and other objects, advantages and features in accordance with the present invention are provided by a device forming part of an integrated circuit, and is disposed between an external contact terminal accessible even after encapsulation and the remainder of the circuit that is intended to perform a specific function. 
   The integrated electronic circuit may comprise a plurality of MOS transistors. The circuit comprises at least a first and a second MOS transistor arranged in series. Each transistor comprises a gate and a source short-circuited together, and a base connected to ground of the integrated circuit. The circuit may advantageously comprise a first resistance connected in parallel with the first transistor, and a second resistance connected in parallel with the second transistor. 
   The circuit may further comprise a third transistor connected in series with the first and second transistors. This transistor comprises a gate and a source short-circuited together, and a body connected to ground of the integrated circuit. The circuit may also further comprise a third resistance connected in parallel with the third transistor. This is the global ground of the circuit, which is necessary for its satisfactory operation. 
   In one embodiment of the invention, the circuit may comprise a connection terminal that is connected to the source of the first transistor and is accessible after the circuit is encapsulated. Alternatively, a resistance can be connected between the terminal and the source of the first transistor. 
   In another embodiment of the invention, the circuit may comprise a connection terminal that is connected to the source of the second transistor and is not accessible after the circuit is encapsulated, and a connection terminal that is connected to the drain of the second transistor and is not accessible after the circuit is encapsulated. 
   In yet another embodiment of the invention, the circuit may further comprise a connection terminal that is connected to the drain of the third transistor and is not accessible after the circuit is encapsulated. More generally, the connection terminal connected to the drain of the nth transistor can be connected to the rest of the circuit. The term “series-arranged MOS transistor” is understood to mean transistors in which the source of the n+1th transistor is connected to the drain of the nth transistor. 
   The MOS transistors may be isolated or non-isolated transistors. The body connector is preferably adjacent the drain. 
   Another aspect of the present invention is directed to a method of adjusting an electrical resistance in an integrated electronic circuit comprising a plurality of series-connected MOS transistors, each provided with a parallel-connected resistance. The bodies of the MOS transistors may be connected to one another. A first voltage is applied to a MOS transistor at its body, its gate and its source and a second voltage is applied to its drain in order to break down the MOS transistor. 
   The bodies of the MOS transistors are preferably connected to a global ground of the circuit, and the bodies of the MOS transistors are short-circuited to the gate and to the source of the MOS transistor that is to be broken down. 
   The first voltage is preferably constant and the second voltage is a monotonic ramp. The first voltage can be zero and the second voltage can be increasing. The breakdown of the MOS transistor can be effected by avalanche of the drain/substrate junction, irreversible breakdown of the drain/substrate junction and a short-circuit between the drain and the source. The difference between the first and second voltages is about 16 V. The breakdown current can be less than 100 mA. 
   The invention applies to both n-MOS and p-MOS transistors. 
   The use of “snapback” MOS transistors makes it possible to achieve a short-circuit and thus a resistance inside an integrated circuit by acting on the pins or terminals of the integrated circuit that can be accessed prior to encapsulation. A component formed in this manner takes up little space on a silicon wafer and is therefore inexpensive. The fact that the gate and the source of the MOS transistor are short-circuited ensures permanent blocking of the MOS transistor and keeps it from affecting operation of the rest of the electronic circuit. After breakdown, the MOS transistor can be considered the equivalent of an open circuit. 
   The invention makes use of a natural characteristic of MOS transistors, that of having parasitic components, particularly a bipolar transistor. In some configurations, such parasitic components are harmful. During electrostatic discharge, circuits can be seriously damaged by turn-on of the parasitic transistor. 
   Conversely, the invention utilizes the parasitic bipolar transistor of the MOS transistor to make it a short circuit and obtain a resistance having a predetermined value between the drain and the source of the MOS transistor, i.e., between the collector and the emitter of the parasitic bipolar transistor. This component can be considered an antifuse. A fuse is a closed circuit in the normal state and an open circuit after breakdown. Here, the MOS transistor is an open circuit before breakdown and a closed circuit after breakdown, with a low residual resistance value. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The present invention will be better understood from a study of the detailed description of a few embodiments, taken strictly as non-limiting examples and illustrated by the appended drawings, wherein: 
       FIG. 1  is a characteristic operating curve of a MOS transistor according to the prior art; 
       FIG. 2  is a cross-sectional view of a MOS transistor according to the present invention; 
       FIG. 3  is a diagram of the device according to the present invention; and 
       FIG. 4  is a cross-sectional view of a variation of a MOS transistor according to the present invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   As can be seen in  FIG. 1 , where the drain voltage is plotted on the abscissa (horizontal axis) and the drain current is plotted on the ordinate (vertical axis), an n-MOS transistor has four operating regions. Region  1  is the conventional linear operation of a MOS transistor. Region  2  is a saturation-mode operation, in which current varies only very slightly with voltage. Region  3  is known as the avalanche region, with a weakening of the drain/substrate junction caused by avalanche of the junction. Finally, Region  4  is the turn-on of the parasitic bipolar transistor, with the curve showing a first breakdown, referenced  5 , which is reversible, and a second breakdown, referenced  6 , which is destructive and therefore irreversible. 
   Beyond the second breakdown  6 , the current varies extremely rapidly with voltage, with the slope of the curve being almost vertical. Since the breakdown process, also known as a second breakdown is irreversible, it is possible to move along the curve beginning at the second breakdown!  6  by moving up, which translates into a decrease in the resistance offered by the MOS transistor broken down in this way, insofar as the current can be seen to increase against a substantially constant drain voltage. 
     FIG. 2  shows the structure of the various components of the MOS transistors. The MOS transistor comprises a drain  8 , a source  9  and a gate  10  formed on a body  11 , also known as the bulk. A parasitic bipolar transistor  12  forms in the body  11 . Its collector is formed by the drain  8  and its emitter by the source  9 , and its base can be modeled as connected to ground by a substrate resistance  13  and by a current source  14  connected to the drain  8 . 
   In the arrangement according to the invention, the drain  8  is connected to a first supply voltage, while the source  9 , gate  10  and body  11  are short-circuited and are connected to a second supply voltage. When the MOS transistor reaches saturation, a high voltage on the drain triggers avalanche of the drain/body junction by generating electron-hole pairs, thus creating a body current. The voltage at the terminals of the body resistance increases, thereby biasing the source/body junction. The parasitic bipolar transistor thus undergoes flashover and the phenomenon of breakdown occurs. 
   At high currents the component goes into the irreversible second breakdown state, represented by destruction of the polysilicon crystal lattice of the channel formed between the drain and the source. After avalanche of the collector/body junction of the parasitic bipolar transistor, the emitter connected to ground serves to forward-bias the body/emitter junction, which causes the snapback effect. To trigger the avalanche phenomenon, a sufficient voltage must be imposed on the drain to reverse-bias the drain/body junction. This voltage depends on the doping characteristics and is proportional to the square of the electrical field. 
   The current generator  14  shown in  FIG. 2  between the collector and the body of the parasitic bipolar transistor simulates the leakage currents of the drain/body junction in an initial phase. Thereafter, it serves to simulate avalanche of the junction and biasing of the parasitic bipolar npn-type transistor. 
   By way of example, tests were performed using HF 4  CMOS technology with an n-MOS transistor having the following channel dimensions: width (W)=1 μm and length (L)=0.7 μm. The source was grounded, and a voltage ramp ranging from 8 to 18 V with current limitation was applied to the drain. With a current of 2 mA, a post-breakdown resistance of 300 ohms was created. With a current of 10 mA, a post-breakdown resistance of 60 ohms was obtained, and with a current of 100 mA, a post-breakdown resistance of 11 ohms was obtained. It will be noted that with a drain voltage of less than 11 V, the drain/base junction is not in avalanche, and therefore no current passes through the drain/source channel. Beyond this voltage the phenomenon sets in, with the creation of a conductive path allowing the passage of current. Once the breakdown voltage is reached, all the available current flows into the channel and a resistance is created. 
   It is particularly advantageous to use transistors whose channel is as short as possible, since the shorter the channel, the lower the breakdown voltage, due to the increase in the drain current and the increase in the number of electron-hole pairs generated. The channel width is constant. A decrease in the channel width brings about a decrease in the voltage and the current of the second breakdown  6  illustrated in FIG.  1 . Even if the width of the channel has no effect on the voltage of the first breakdown  5 , a reduced width will increase the heating effect of the second breakdown  6 , since the lines of force will be more unidirectional, implying a decrease in the torque of the second breakdown. It is therefore particularly advantageous to use small-sized MOS transistors. 
   When a MOS transistor is used in the snapback mode, the substrate is connected to the lowest potential of the circuit to reverse-bias all the parasitic diodes existing between drain  8  and source  9 , on the one hand, and body  11  on the other. Source  9  and base  11  are short-circuited. Gate  10  is also short-circuited to source  9  and to the body  11  to deactivate the transistor. 
   Resistances arranged in parallel can be adjusted with this type of snapback MOS transistor. Reference is directed to French Patent No. 2,795,557 for more information. 
     FIG. 3  shows an embodiment of the invention comprising three series-arranged resistances to be adjusted, referenced  15 ,  16  and  17 . Resistance  15  is connected to a ground terminal  18  that will be connected to one of the external pins or terminals of the circuit at the time of encapsulation, and resistance  17  is connected to the rest of the circuit (not shown). The device further comprises three MOS transistors  19 ,  20 ,  21 . Each transistor is provided with a gate, respectively  22 ,  23  and  24 , a drain, respectively  25 ,  26  and  27 , a source, respectively  28 ,  29  and  30 , and a body, respectively  31 ,  32  and  33 . 
   Transistor  19  is connected in parallel with resistance  15 , transistor  20  in parallel with resistance  16  and transistor  21  in parallel with resistance  17 . The gate and the source of each transistor  19 ,  20  and  21  are short-circuited. Bodies  31  to  33  of transistors  19  to  21  are all connected to terminal  18 . Gate  22  and source  28  of transistor  19  are connected to terminal  18 . Drain  25  of transistor  19  and gate  23  and source  29  of transistor  20  are connected to the common point between resistances  15  and  16  and to an adjustment terminal  34  that can be no longer accessible after the circuit is encapsulated. Drain  26  of transistor  20  and gate  24  and source  30  of transistor  21  are connected to the common point between resistances  16  and  17  and to an adjustment terminal  35  that can be no longer accessible after the circuit is encapsulated. Drain  27  of transistor  21  is connected to the other terminal of resistance  17 , to the rest of the circuit (not shown) and to an adjustment terminal  36  that can be no longer accessible after the circuit is encapsulated 
   To break down transistor  21 , terminals  18  and  35  are together connected to ground and a positive voltage ramp is applied to terminal  36 . If terminal  18  were left unconnected, the snapback phenomenon would not occur due to the impossibility of avalanching the drain/body junction and thus of forward-biasing the body/transmitter junction. 
   If transistor  20  is to be broken down, terminals  18  and  34  are connected together and a positive voltage ramp is applied to bump  35 . To break down transistor  19 , a positive voltage is applied to terminal  34  and terminal  18  is connected to ground. 
   A system of low-value series-connected resistances can thus be adjusted by MOS transistors in a reproducible and reliable manner. A non-isolated MOS transistor takes up much less area than an isolated MOS transistor, for example an area of 7 μm out of 14 μm instead of 40 μm out of 40 μm, thus dividing the occupied silicon area by about 16. 
   Breakdown of the isolated snapback MOS transistors is difficult to achieve due to the presence of a second, parasitic bipolar transistor in which the collector is formed by the drain of the MOS transistor, the emitter by the source and the base by the body or bulk. This second, parasitic transistor is capable of stealing most of the current sent by a breakdown terminal to the drain of the MOS transistor. As a result, the post-breakdown resistance can vary between 100 ohms and 1 ohms, compared to the resistance of 10 ohms that is obtained in a reproducible manner when the non-isolated MOS structure is broken down. 
   The term isolated MOS transistor is understood here to mean a MOS transistor whose substrate and body are separated by a dielectric layer. Vertical and especially lateral isolation stresses encourage the use of large silicon areas to increase the dimensions of the body and decrease the gain of the second, parasitic bipolar transistor. 
   Without excluding isolated MOS transistors, it is preferred to use non-isolated MOS transistors that prevent the flow of leakage currents from the body to the substrate. The body and the substrate are at the same potential. Further, it is particularly advantageous to arrange the body connector as close as possible to the drain for reasons of current line distribution during breakdown. The body layer, a p-type layer in an n-MOS transistor, may or may not be ring-shaped. In both cases, the body connector, i.e., the lead-out to the connection levels, is arranged closer to the drain than to the source as illustrated in FIG.  4 . 
   The present invention thus enables series-arranged resistances to be adjusted in a precise and reproducible manner with the aid of economical MOS transistors occupying a reasonable silicon area.