Abstract:
A method for testing an integrated circuit includes, in a burn-in test mode, two steps during which gate oxides of conductive high voltage MOS transistors of the integrated circuit are subjected to a first test voltage, and blocked high voltage MOS transistors of the integrated circuit are subjected to a second test voltage. The first test voltage is set to a value higher than a high supply voltage supplied to the high voltage MOS transistors in a normal operating mode, to make the gate oxides of transistors considered as insufficiently robust break down. The second test voltage is set to a value lower than the first test voltage and which can be supported by the blocked transistors, the states of the transistors being changed between the two steps.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to burn-in tests of integrated circuits like EEPROM memories, at the end of manufacture. These tests include in particular subjecting, during some time, an integrated circuit to unfavorable operating constraints, to cause a failure of the circuit if it has some manufacture defects. That way, the integrated circuits passing such tests have a failure rate which may reach low values. 
     2. Description of the Related Art 
     One of the main causes of failure of an EEPROM memory is gate oxide breakdown of transistors which may be subjected to a relatively high voltage. The rate of failure tends to increase with memory miniaturization. In an EEPROM memory, such high voltage transistors are present in bit line latches, gate control latches and word line selection latches, as well as in each memory cell and in gate control switches of a group of memory cells.  FIG. 1  schematically shows a conventional EEPROM memory. In  FIG. 1 , the memory MEM 1  comprises a memory array CELM comprising memory cells distributed into lines and columns transverse to lines. Line and column decoders YDC 1 , XDC 1  allow one or more words comprising several memory cells belonging to a same line to be selected. Decoders XDC 1 , YDC 1  are controlled by an address register AREG allowing a word at the intersection of a line and a group of columns to be selected. The memory array CELM comprises a bit line BL for each column of memory cells, a word line WL for each line of memory cells and a gate control line CGL for each word column, each gathering several bit lines. Decoders XDC 1 , YDC 1  are powered by a voltage VPP, VPE, VPW by a high voltage generation circuit HVG 1 , and control the memory cells for reading, programming and erasing by supplying voltages adapted to the bit lines BL, the gate control lines CGL for decoder XDC 1  and the word lines WL for decoder YDC 1 . For reading, the voltage VPP is equal to the supply voltage of the memory, and for programming and erasing, the voltage Vpp is equal to a high voltage, around 15 V. 
     Some memories have a burn-in test mode. To that end, they comprise a mode control circuit TMC 1  to place the memory either in burn-in test mode, or a normal operating mode as a function of a test signal TST supplied to the memory. Circuit TMC 1  controls circuit HVG 1  and decoder YDC 1 , in particular to adjust the value of high voltage VPP as a function of the operating mode. 
       FIG. 2  shows a memory cell MC of the memory array CELM. In  FIG. 2 , each cell MC comprises an access transistor AT and a floating gate transistor FGT connected in series, the drain of transistor AT being connected to a bit line BL, and the source of transistor FGT being connected to the ground. The gate of transistor AT is connected to a word line WL and the gate of a gate control transistor CGT whose drain is connected to a gate control line CGL, and whose source is connected to the gate of transistor FGT. To program and erase the cell MC, transistors AT and CGT must be able to support the high voltage generated by circuit HVG 1 . 
       FIGS. 3A ,  3 B show a bit line latch of decoder XDC 1 . The bit line latch comprises two inverters mounted back-to-back, each formed by a high voltage P-channel MOS transistor, referred to as P 1 , P 2 , and a high voltage N-channel MOS transistor, referred to as N 1 , N 2 . Transistors P 1 , P 2  receive voltage VPP on their source and their well bias terminal. The sources of transistors N 1 , N 2  are connected to a low voltage line SWG. The drains of transistors P 1 , N 1  and the gates of transistors P 2 , N 2  are connected to the low voltage line SWG through an N-channel MOS transistor referred to as N 3 , whose gate is controlled by a reset signal RST. The drains of transistors P 1 , N 1  and the gates of transistors P 2 , N 2  are also connected to the gate of an N-channel MOS transistor referred to as N 6 , receiving on its drain a write command voltage VPW and whose source is connected to a bit line BL. The gates of transistors P 1 , N 1  and the drains of transistors P 2 , N 2  are connected to the low voltage line SWG through two N-channel MOS transistors referred to as N 4 , N 5  connected in series. The gate of transistor N 4  is controlled by a word column selection signal COL and the gate of transistor N 5  is controlled by a data signal DT supplying the value of a bit to be written in a selected memory cell MC. Therefore, a bit line BL can be subjected to the high voltage only if the corresponding bit line latch is in the active state, if the data to be programmed DT is at 1, and if the corresponding column is selected (signal COL at 1). In some memories, the sources of transistors N 3  and N 5  may be connected to the ground GND. 
       FIG. 3A  shows the latch in normal operation, in the reset state during a programming or write cycle. This state is previously reached by the temporary switching of transistor N 3  to the conductive state, after a pulse of signal RST, maintaining transistors N 4  and N 5  in the blocked state (signals COL and DT at 0). During the pulse of signal RST, the drains of transistors P 1 , N 1  and the gates of transistors P 2 , N 2  receive the voltage SWG. Transistors P 1  and N 2  therefore switch to the blocked state, while transistors P 2 , N 1  switch to the conductive state. The voltage SWG may be chosen different from zero, for example equal to 3 V, so as to limit the drain-well and drain-source voltages of transistors P 1 , P 2  and drain-source voltages of transistors N 1 , N 2 . In practice, voltage SWG may not be higher to avoid programming non-addressed memory cells. When voltage VPP is set at the programming value (for example 15 V), transistors P 2  and N 1  are subjected to a maximum gate oxide stress voltage of 15−3=12 V. 
       FIG. 3B  shows the latch in normal operation, in the active state during a programming or write cycle. This state is previously reached by temporary and simultaneously switching transistors N 4  and N 5  to the conductive state, after a pulse of signals COL and DT, while maintaining transistor N 3  in the blocked state. During the pulse of signals COL and DT, the gates of transistors P 1  and N 1  and the drains of transistors P 2 , N 2  receive the voltage SWG. Transistors P 1  and N 2  switch to the conductive state, while transistors P 2 , N 1  switch to the blocked state. When voltage VPP is set to the programming value, transistors P 1  and N 2  are subjected to a maximum gate oxide stress voltage of 15−3=12 V. 
     The gate control line latches CGL of decoder XDC 1  and the word line latches WL of decoder YDC 1  have architectures similar to that of the bit line latch previously described. 
     To reduce circuit failure rate due to gate oxide breakdown of high voltage transistors, it is known to subject these circuits, at the end of manufacture, to burn-in tests that subject the gate oxides of high voltage transistors to sufficiently high test voltages below the intrinsic breakdown voltage of these gate oxides. The aim of these tests is to prematurely make weak microelectronic structures break down, so as to discard faulty or insufficiently robust circuits, and thus avoid them from being prematurely faulty during their use. A burn-in test is all the more efficient to discard the circuits having manufacture defects as test voltages are much higher than the voltages applied in normal operation, even near the intrinsic breakdown voltage of the gate oxides of high voltage transistors. However, the test voltages should not be chosen too close to this intrinsic breakdown voltage to avoid discarding circuits having acceptable defects. 
       FIGS. 4A ,  4 B show the bit line latch previously described, respectively in the reset state and the active state, burn-in test voltages being indicated. In burn-in test, voltages VPP and SWG are set to values allowing transistors P 1 , P 2 , N 1 , N 2  to be subjected to a maximum gate oxide stress voltage without exceeding a breakdown threshold voltage of transistors P 1 , P 2 . In the example of  FIGS. 4A ,  4 B, voltage VPP is set to 14 V and voltage SWG to 0 V. The weakness of P-channel MOS transistors formed in a well, is their drain-well junction which cannot admit more than 14 V in the blocked state. At voltages superior or equal to this value, leaks may also appear between the drain and source of these transistors. It is therefore not possible to subject these transistors to voltages significantly higher than normal operating voltages (VPP−SWG=12 V). However, the weakness of N-channel MOS transistors is between the drain and source, whose voltage cannot exceed 17 V in the blocked state. 
     When the latch is in the reset state shown by  FIG. 4A , transistors P 1  and N 2  are blocked, while transistors P 2  and N 1  are conductive. The transistors whose gate oxides are subjected to a voltage stress of 14 V are therefore the conductive transistors P 2 , N 1  (surrounded by a dotted line). 
     When the latch is in the active state shown by  FIG. 4B , transistors P 1  and N 2  are conductive, while transistors P 2 , N 1  are blocked. The transistors whose gate oxides are subjected to a voltage stress of 14 V are therefore the conductive transistors P 1 , N 2  (surrounded by a dotted line). Transistor N 6  which is subjected to voltage VPP, is also conductive. If the voltage VPW is equal to 0, this transistor is also subjected to a gate oxide stress voltage at 14 V. 
     Although the gate oxides of transistors considered to be of sufficient quality may support 17 V, it is not possible to go beyond 14 V due to the presence of P-channel transistors in the blocked state whatever the state of the circuit. 
     To compensate for the small difference between test voltages and normal operation voltages, it is known to increase the time during which each transistor to be tested is subjected to a test voltage. This solution also has a limit since it cannot be considered to penalize production rates by increasing burn-in test durations beyond an acceptable limit. 
     BRIEF SUMMARY 
     One embodiment of the present disclosure reduces the premature failure rate of integrated circuits comprising MOS transistors subjected to high voltages, without increasing burn-in test durations. One embodiment of the present disclosure increase the burn-in test voltage for all the high voltage transistors of such integrated circuits, while maintaining this voltage below the breakdown voltage of the gate oxides of these transistors. 
     Embodiments relate to a method for testing an integrated circuit, the method comprising in a burn-in test mode, two steps during which gate oxides of high voltage MOS transistors in the conductive state of the integrated circuit are subjected to a first test voltage, and high voltage MOS transistors in the blocked state of the integrated circuit are subjected to a second test voltage, the first test voltage being set to a value higher than a high supply voltage supplied to the high voltage MOS transistors of the integrated circuit in a normal operating mode, to make the gate oxides of high voltage transistors considered as insufficiently robust break down, the second test voltage being set to a value lower than the first test voltage, which can be supported by the high voltage transistors in the blocked state, and sufficient to ensure a normal operation of the circuit, the states of the high voltage transistors being changed between the two steps. 
     According to an embodiment, the first test voltage is subjected by supplying a first voltage to source terminals of high voltage P-channel MOS transistors in the conductive state, and the second test voltage is subjected by supplying a second voltage to source terminals of high voltage P-channel MOS transistors in the blocked state, a same high voltage being supplied in normal operation to the source terminals of the high voltage P-channel MOS transistors in the conductive or blocked state. 
     According to an embodiment, the first test voltage is subjected by supplying a third voltage to source terminals of high voltage N-channel MOS transistors in the conductive state, and the second test voltage is subjected by supplying a fourth voltage higher than the third test voltage, to source terminals of high voltage N-channel MOS transistors in the blocked state, a same high voltage being supplied in normal operation to the source terminals of the high voltage N-channel MOS transistors in the conductive or blocked state. 
     According to an embodiment, the integrated circuit is a non-volatile memory comprising bit line latches, gate control latches, word line latches, and a memory array comprising memory cells, each of the latches and the memory array comprising high voltage transistors which are subjected to the first and second test voltages in burn-in test mode, all the latches being in a reset state during the first step of the burn-in test, and in an active state during the second step of the burn-in test, the high voltage transistors of the memory array being in the conductive state during the second step of the burn-in test and subjected to the first test voltage. 
     According to an embodiment, each of the first and second steps of the burn-in test mode has a duration corresponding to several standard erase and write cycles of the memory in normal operation. 
     According to an embodiment, write and erase control signals are forced to zero during the second step of burn-in test, these signals being at the high voltage respectively during write and erase operations of the memory in normal operation. 
     Embodiments also relate to an integrated circuit comprising MOS transistors configured to support a high voltage higher than a supply voltage of the integrated circuit, the integrated circuit being configured to operated either in a test mode, or in a normal mode, configured to implement the method as above-defined. 
     According to an embodiment, the integrated circuit comprises a generation circuit for generating a high voltage from a supply voltage of the integrated circuit, a control circuit for controlling the generation circuit so that it generates in the burn-in test mode a first high voltage higher than a high voltage generated in the normal operating mode, and a switching circuit controlled by the control circuit to generate from the high voltage supplied by the generation circuit, a second high voltage lower than the first high voltage, and transmit them to the source terminals of high voltage P-channel MOS transistors as a function of the conductive or blocked state of the P-channel MOS transistors. 
     According to an embodiment, the integrated circuit comprises two high voltage supply lines, each connected by source terminals to the high voltage P-channel MOS transistors in a same conductive or blocked state at a given time. 
     According to an embodiment, the integrated circuit comprises two low voltage supply lines, connected by source terminals to the high voltage N-channel MOS transistors in a same conductive or blocked state at a given time. 
     According to an embodiment, the integrated circuit comprises a memory comprising bit line latches, gate control latches and word line latches and a memory array, the latches and the memory array comprising high voltage MOS transistors to be tested during the burn-in test. 
     According to an embodiment, each latch comprises two high voltage supply lines, each connected by source terminals to the high voltage P-channel MOS transistors in a same conductive or blocked state at a given time. 
     According to an embodiment, each latch comprises two low voltage supply lines connected by source terminals to the high voltage N-channel MOS transistors in a same conductive or blocked state at a given time. 
     According to an embodiment, selection circuits for selecting memory cells are configured to reset all the memory cell selection latches during the first burn-in test step, and to select all the cells of the memory during the second burn-in test step. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       Embodiments of the disclosure will be described hereinafter, in relation with, but not limited to the appended figures wherein: 
         FIG. 1  previously described schematically shows a conventional EEPROM memory; 
         FIG. 2  previously described shows a memory cell of the memory of  FIG. 1 ; 
         FIGS. 3A ,  3 B previously described, show a bit line latch of the memory of  FIG. 1 , in normal operation; 
         FIGS. 4A ,  4 B previously described, show the bit line latch, in burn-in test; 
         FIG. 5  schematically shows an EEPROM memory, according to one embodiment; 
         FIGS. 6A ,  6 B show a bit line latch, according to one embodiment; 
         FIGS. 7A ,  7 B show a gate control line latch, according to one embodiment; 
         FIGS. 8A ,  8 B show a word line latch, according to one embodiment; 
         FIG. 9  schematically shows a circuit of the memory of  FIG. 8 ; 
         FIGS. 10A ,  10 B schematically show a bit line latch, according to another embodiment. 
     
    
    
     DETAILED DESCRIPTION 
       FIG. 5  shows an EEPROM memory referred to as MEM, according to one embodiment. Memory MEM comprises a memory array CELM comprising memory cells distributed into lines and columns transverse to lines. Line YDEC and column XDEC decoders allow one or more words comprising several memory cells of a same line to be selected. Decoders XDEC, YDEC are controlled by an address register AREG allowing a word at the intersection of a line and a group of columns to be selected. A high voltage generation circuit VPGN supplies a high voltage VPP and voltages VPE, VPW to decoder XDEC. Decoders XDEC, YDEC control memory cells for reading, programming or erasing by providing adapted voltages, to bit lines BL, and gate control lines CGL for decoder XDEC, and word lines WL for decoder YDEC. A mode control circuit TMCL transmits a control signal CMD to the circuit HVGN to activate a burn-in test mode or a normal operating mode of the memory. 
     According to one embodiment, the memory MEM is modified with respect to that shown in  FIG. 1 , to generate different high voltages VP 1  and VP 2 . To that end, a switching circuit VCOM supplies, to decoders XDEC, YDEC, high voltages VP 1 , VP 2  which may differ from voltage VPP supplied by generator HVGN. The circuit TMCL transmits control signals IN 1 , IN 2  to circuit VCOM allowing it to determine the values of voltages VP 1  and VP 2  to be supplied to decoders XDEC, YDEC. In a burn-in test mode, the high voltage P-channel transistors of the circuits of memory MEM receive one or the other of high voltages VP 1 , VP 2  whether they are conductive or blocked. 
       FIGS. 6A ,  6 B show a bit line latch BLT of decoder XDEC, according to one embodiment. The bit line latch comprises two inverters mounted back-to-back, each formed by a high voltage P-channel MOS transistor, referred to as P 1 , P 2 , and a high voltage N-channel MOS transistor, referred to as N 1 , N 2 . Transistor P 1  comprises a source and a well bias terminal receiving voltage VP  1 . Transistor P 2  comprises a source and a well bias terminal receiving voltage VP 2 . The sources of transistors N 1 , N 2  are connected to a low voltage line SWG. The drains of transistors P 1 , N 1  and the gates of transistors P 2 , N 2  are connected to the low voltage line SWG or the ground GND through an N-channel MOS transistor referred to as N 3 , whose gate is controlled by a reset signal RST. The drains of transistors P 1 , N 1  and the gates of transistors P 2 , N 2  are also connected to the gate of a high voltage N-channel MOS transistor referred to as N 6 . Transistor N 6  comprises a drain receiving a write command voltage VPW and a source connected to a bit line BL of the memory array. The gates of transistors P 1 , N 1  and the drains of transistors P 2 , N 2  are connected to the low voltage line SWG or the ground GND through two N-channel MOS transistors referred to as N 4 , N 5 , connected in series. The gate of transistor N 4  is controlled by a word column selection signal COL and the gate of transistor N 5  is controlled by a data signal DT supplying the value of a bit to be written in a selected memory cell. 
       FIGS. 6A ,  6 B show two steps of the burn-in test of the bit line latch. In burn-in test, each voltage VP 1  and VP 2  is set to a maximum value that the blocked transistors receiving the voltage can support. In  FIG. 6A , the latch is in the reset state. This state is previously reached by the temporary switching of transistor N 3  to the conductive state, after a pulse of signal RST, maintaining transistors N 4  and N 5  in the blocked state (signals COL and DT at 0). Transistors P 2  and N 1  are in the conductive state, while transistors P 1 , N 2  and N 6  are in the blocked state. Voltage VP 1  is set to a value VTL which can be supported by transistors P 1 , P 2  in the blocked state, and voltage VP 2  is set to a maximum value VTH which can be supported by transistors N 1 , N 2 , N 3  in the blocked state. Voltage VTH may also correspond to a value lower than the average gate oxide breakdown voltage of the transistors in the conductive state of memory MEM. Voltage VTH may also correspond to a voltage that the gate oxides of the high voltage transistors in the conductive state must support to be considered as acceptable. Voltage SWG is set to a voltage VG corresponding to the circuit ground or a voltage near the ground. Tested transistors N 1 , P 2  (surrounded by a dotted line) are subjected to a gate oxide stress voltage equal to VP 2 −SWG=VTH−VG. 
     In  FIG. 6B , the latch is in the active state. This state is previously reached by the temporary and simultaneous switching of transistors N 4  and N 5  to the conductive state, after a pulse of signals COL and DT, maintaining transistor N 3  in the blocked state (signal RST at 0). Transistors P 1 , N 2  and N 6  are in the conductive state, while transistors P 2 , N 1  are in the blocked state. Voltage VP 1  is set to the voltage VTH, voltage VP 2  is set to the voltage VTL. Tested transistors P 1 , N 2  (surrounded by a dotted line) are subjected to a gate oxide stress voltage equal to VP 1 −SWG=VTH−VG. It is the same for transistor N 6  if signal VPW is set to the voltage VG. 
     In normal operation, voltages VP 1  and VP 2  (VTL and VTH) are for example equal to 15 V and voltage VG is for example set to 3 V during a programming or write cycle. In burn-in test, voltages VTL and VTH may be chosen equal to respectively 14 V and 17 V, and voltage VG may be set to 0, given an average gate oxide breakdown voltage of the high voltage transistors around 20 V. In this example, each high voltage transistor P 1 , P 2 , N 1 , N 2 , N 6  of the circuit of  FIGS. 6A ,  6 B may therefore be tested at a gate oxide stress voltage equal to VTH−VG=17 V, which represents a gain of 3 V with respect to the circuit of  FIGS. 4A ,  4 B. The result is also that the gate oxide of each transistor P 1 , P 2 , N 1 , N 2 , N 6  may be tested at a same value VTH-VG chosen as a function of a desired robustness level, to discard the circuits considered as insufficiently robust. 
     Such a result may also be obtained for gate control latches and word line latches of the memory MEM. Thus,  FIGS. 7A ,  7 B show a gate control line latch CLT of decoder XDEC, according to one embodiment. The gate control line latch comprises two inverters mounted back-to-back, each formed by a high voltage P-channel MOS transistor, referred to as P 7 , P 8 , and a high voltage N-channel MOS transistor, referred to as N 7 , N 8 . Transistor P 7  comprises a source and a well bias terminal receiving voltage VP 1 . Transistor P 8  comprises a source and a well bias terminal receiving the high voltage VP 2 . The sources of transistors N 7 , N 8  are connected to the low voltage line SWG. The drains of transistors P 7 , N 7  and the gates of transistors P 8 , N 8  are connected to the low voltage line SWG or the ground GND through an N-channel MOS transistor referred to as N 9 , whose gate is controlled by a reset signal RST. The drains of transistors P 7 , N 7  and the gates of transistors P 8 , N 8  are also connected to the gate of a high voltage N-channel MOS transistor referred to as N 11 , receiving on its drain an erase command voltage VPE and whose source is connected to a gate control line CGL of the memory array. The gates of transistors P 7 , N 7  and the drains of transistors P 8 , N 8  are connected to the low voltage line SWG or the ground GND through an N-channel MOS transistor referred to as N 10 . The gate of transistor N 10  is controlled by the word column selection signal COL. 
       FIGS. 7A ,  7 B show two steps of the burn-in test of the gate control latch. In burn-in test, each voltage VP 1  and VP 2  is set to a maximum value that the blocked transistors receiving the voltage can support. In  FIG. 7A , the latch is in the reset state. This state is previously reached by the temporary switching of transistor N 9  to the conductive state, after a pulse of signal RST, maintaining transistor N 10  in the blocked state (signal COL at 0). Transistors P 8  and N 7  are in the conductive state, while transistors P 7 , N 8  and N 11  are in the blocked state. Voltage VP 1  is set to the voltage VTL that transistor P 7  in the blocked state can support, voltage VP 2  is set to the voltage VTH, and voltage SWG is set to the voltage VG. Tested transistors N 7 , P 8  (surrounded by a dotted line) are subjected to a gate oxide stress voltage equal to VP 2 −SWG=VTH−VG (i.e., 17 V in the previous example). 
     In  FIG. 7B , the latch is in the active state. This state is previously reached by the temporary switching of transistor N 10  to the conductive state, after a pulse of signal COL, maintaining transistor N 9  in the blocked state (signal RST at 0). Transistors P 7 , N 8  and N 11  are conductive. Voltage VP 1  is set to the voltage VTH, voltage VP 2  is set to the voltage VTL, and voltage SWG is set to the voltage VG. Tested transistors P 7 , N 8  (surrounded by a dotted line) are subjected to a gate oxide stress voltage equal to VP 1 −SWG=VTH−VG (i.e., 17 V in the previous example). It is the same for transistor N 11  if voltage VPE is set to VG. The result is that the gate oxide of each transistor P 7 , P 8 , N 7 , N 8 , N 11  may also be tested at voltage VTH−VG. 
       FIGS. 8A ,  8 B show a word line latch WLT of decoder YDEC, according to one embodiment. The word line latch comprises three inverters, two of which are mounted back-to-back, each inverter being formed by a high voltage P-channel MOS transistor, referred to as P 12 , P 13 , P 14  and a high voltage N-channel MOS transistor, referred to as N 12 , N 13 , N 14 . Transistor P 12  comprises a source and a well bias terminal receiving voltage VP 2 . Each transistor P 13 , P 14  comprises a source and a well bias terminal receiving voltage VP 1 . The sources of transistors N 12 , N 13 , N 14  are connected to the low voltage line SWG. The drains of transistors P 12 , N 12  and the gates of transistors P 13 , N 13 , P 14 , N 14  are connected to the low voltage line SWG or the ground GND through an N-channel MOS transistor referred to as N 15 , in series with several N-channel MOS transistors referred to as N 16  also connected in series. The gate of transistor N 15  is controlled by a decoding signal DEC, and the gates of transistors N 16  receive the bits of an address word ADR. The drains of transistors P 14 , N 14  are connected to a word line WL. The gates of transistors P 12 , N 12  and the drains of transistors P 13 , N 13  are connected to the low voltage line SWG or the ground GND through an N-channel MOS transistor referred to as N 17 . The gate of transistor N 17  is controlled by the latch reset signal RST. 
       FIGS. 8A ,  8 B show two steps of the burn-in test of the word line latch. In burn-in test, each voltage VP 1  and VP 2  is set to a maximum value that the blocked transistors receiving the voltage can support. In  FIG. 8A , the latch is in the reset state. This state is previously reached by the temporary switching of transistor N 17  to the conductive state, after a pulse of signal RST, maintaining at least one of transistors N 15 , N 16  in the blocked state (signals DEC or ADR at 0). Transistors P 12 , N 13  and N 14  are in the conductive state, while transistors N 12 , P 13  and P 14  are in the blocked state. Voltage VP 2  is set to the voltage VTH, that transistor N 12  in the blocked state can support, and voltage VP 1  is set to the voltage VTL, that transistors P 13 , P 14  in the blocked state can support. Voltage SWG is set to the voltage VG. Tested transistors P 12 , N 13 , N 14  (surrounded by a dotted line) are subjected to a gate oxide stress voltage equal to VP 2 −SWG=VTH−VG, i.e., 17 V in the previous example. 
     In  FIG. 8B , the latch is in the active state. This state is previously reached by the temporary and simultaneous switching of all transistors N 15  and N 16  to the conductive state, after a pulse of signals DEC and ADR, maintaining transistor N 17  in the blocked state (signal RST at 0). Transistors N 12 , P 13  and P 14  are in the conductive state, while transistors P 12 , N 13  and N 14  are in the blocked state. Voltage VP 2  is set to the voltage VTL, that transistor P 12  in the blocked state can support, voltage VP 1  is set to the voltage VTH, that transistors N 13 , N 14  and N 17  in the blocked state can support. Tested transistors N 12 , P 13 , P 14  (surrounded by a dotted line) are subjected to a gate oxide stress voltage equal to VP 1 −SWG=VTH−VG, i.e., 17 V in the previous example. The result is that each transistor P 12 -P 14 , N 12 -N 14  may be tested at a same desired value of gate oxide stress voltage. 
     For the case of  FIG. 8B  (when the latch is active), a logic circuit may be provided to force all transistors N 16  to the conductive state in test mode. Alternately, an additional transistor (not shown) may be provided in parallel of transistors N 16 , this transistor being controlled in the conductive state when the latch is active in test mode. 
       FIG. 9  shows an example embodiment of the circuit VCOM. Circuit VCOM comprises two level shifters LS 1 , LS 2  respectively controlled by signals IN 1 , IN 2 , two P-channel MOS transistors referred to as P 21 , P 22  and two N-channel MOS transistors referred to as N 21 , N 22 . Circuits LS 1 , LS 2  are powered between voltages VPP and SWG. The output of circuit LS 1  is connected to the gate of transistor P 21  whose source and well receive voltage VPP. The output of circuit LS 2  is connected to the gate of transistor P 22  whose source and well receive voltage VPP. Transistors N 21  and N 22  are in diode configuration and receive the voltage VPP on their gates and their drains. The drain of transistor P 21  and the source of transistor N 21  supply voltage VP 1 . The drain of transistor P 22  and the source of transistor N 22  supply voltage VP 2 . 
     Various operating modes of circuit VCOM are shown in the following Table 1: 
     
       
         
               
               
               
               
               
               
               
             
           
               
                 TABLE 1 
               
               
                   
               
               
                 Mode 
                 IN1 
                 IN2 
                 O1 
                 O2 
                 VP1 
                 VP2 
               
               
                   
               
             
             
               
                 Normal 
                 0 
                 0 
                 SWG 
                 SWG 
                 VPP 
                 VPP 
               
               
                 Test 1 
                 1 
                 0 
                 VPP 
                 SWG 
                 VPP-VtN 
                 VPP 
               
               
                 Test 2 
                 0 
                 1 
                 SWG 
                 VPP 
                 VPP 
                 VPP-VtN 
               
               
                   
               
             
          
         
       
     
     In Table 1, VtN represents the threshold voltage of transistors N 21 , N 22 , which is for example equal to 3 V. Voltage VPP corresponds to voltage VTH and voltage VPP−VtN corresponds to voltage VTL. 
     In normal operating mode and during the programming or write cycles, voltage VPP supplied by circuit HVGN is for example equal to 15 V and voltage SWG is for example equal to 3 V. Outside the programming and erase cycles, the voltage VPP is set to the supply voltage of the circuit and the voltage SWG is set to 0 V. In burn-in test mode, during tests 1 and 2, voltages VPP and SWG are for example set to 17 V and 0 V. In this example, the values of Table 1 are summed up in the following Table 2: 
     
       
         
               
               
               
               
               
               
               
               
             
               
               
               
               
               
               
               
               
             
           
               
                   
                 TABLE 2 
               
               
                   
                   
               
               
                   
                 Mode 
                 IN1 
                 IN2 
                 O1 
                 O2 
                 VP1 
                 VP2 
               
               
                   
                   
               
             
             
               
                   
               
             
          
           
               
                   
                 Normal 
                 0 
                 0 
                 3 
                 3 
                 15 
                 15 
               
               
                   
                 Test 1 
                 1 
                 0 
                 17 
                 0 
                 14 
                 17 
               
               
                   
                 Test 2 
                 0 
                 1 
                 3 
                 17 
                 17 
                 14 
               
               
                   
                   
               
             
          
         
       
     
     According to one embodiment, all the bit line, gate control and word line latches are in the reset state during Test 1. The result is that transistors N 1 , P 2  ( FIG. 6A ), N 7 , P 8  ( FIG. 7A ), P 12 , N 13  and N 14  ( FIG. 8A ) of the latches of decoders XDEC, YDEC are subjected to a gate oxide stress voltage equal to VP 2 −SWG=VTH−VG. 
     In Test 2, all these latches are in the active state. During this test, the data to be written is set to FF (all the signals DT supplied to the bit line latches are at 1), and all the signals COL supplied to the bit line and gate control latches are at 1. Transistors N 16  of the word line latches are forced in the conductive state (signals AD forced at 1) or the transistor provided in parallel of transistors N 16  of each word line latch is controlled in the conductive state. Voltages VPE and VPW are also set to 0. The result is that transistors P 1 , N 2 , N 6  ( FIG. 6B ), P 7 , N 8 , N 11  ( FIG. 7B ), N 12 , P 13  and P 14  ( FIG. 8B ) are subjected to a gate oxide stress voltage equal to VP 1 -SWG=VTH-VG. It is the same for all the transistors AT and CGT of the memory array CELM which are conductive in these conditions. It is to be noted that contrary to memory cell programming in normal operation, signal VPW is set to 0 in Test 2 to subject transistors AT to the gate oxide stress voltage. The memory cells of the memory array CELM therefore do not switch to the programmed state. 
     All the high voltage transistors of an EEPROM memory may thus be tested at a same desired gate oxide stress voltage value. 
     The duration of tests 1 and 2 may be set to the duration of several standard erase or write cycles (in normal operation), for example to some hundreds of milliseconds. To that end, in addition to the conventional timer of an EEPROM memory, an additional timer controlling the duration of tests 1 and 2 may be provided. 
     In some circuits, N-channel MOS transistors in the blocked state may not support the high voltage VP 1  or VP 2  applied in test mode to the P-channel MOS transistors in the conductive state. In this case, the source and well bias terminal of N-channel MOS transistors intended to support a high voltage may receive a different voltage whether the transistor is conductive or blocked. To that end, the wells in which the N-channel MOS transistors concerned are formed may be insulated from the substrate in which the circuit is formed, for example by the triple well technique.  FIGS. 10A ,  10 B show a bit line latch circuit modified to that end. In  FIGS. 10A ,  10 B, the bit line latch BLT 1  differs from that shown in  FIGS. 6A ,  6 B in that the sources and well bias terminals of transistors N 1 , N 3  receive a voltage SG 1 , and in that the sources and well bias terminals of transistors N 2 , N 4 , N 5  receive a voltage SG 2  which may differ from voltage SG 1 . In normal operation, voltages SG 1  and SG 2  are identical. In test mode, voltages SG 1  and SG 2  are respectively equal to voltages VG 1  and VG 2  during test 1, and to voltages VG 2  and VG 1  during test 2. Transistors P 1 , P 2  are subjected to the same voltages as in  FIGS. 6A ,  6 B. 
     During test 1 ( FIG. 10A ), transistors N 2  and N 4  which are in the blocked state, are subjected to a voltage equal to VP 2 −SG 2 =VTH−VG 2 , while the gate oxide of transistor N 1  in the conductive state, is subjected to a voltage equal to VP 2 −SG 1 =VTH−VG 1 . During test 2 ( FIG. 10B ), transistors N 1  and N 3  which are in the blocked state, are subjected to a voltage equal to VP 1 −SG 1 =VTH−VG 2 , while the gate oxide of transistor N 2  in the conductive state, is subjected to a voltage equal to VP 1 −SG 2 =VTH−VG 1 . 
     In the previous example, voltages VG 1  and VG 2  may be chosen respectively equal to 0 and 3 V. The result is that the N-channel MOS transistors in the blocked state are subjected to a voltage of VTH−VG 2 =14 V, while the gate oxides of the N-channel MOS transistors in the conductive state are subjected to a voltage of VTH−VG 1 =17 V. Although in this example voltage VTL is equal to VTH−VG 2 , this condition is not necessary and depends on the structure of the N- and P-channel MOS transistors to be tested. 
     The gate control latches CLT and the word line latches WLT may be modified similarly to be able to subject the source of the N-channel MOS transistors configured to be subjected to a high voltage in test mode, to voltages VG 1 , VG 2  whether they are conductive or blocked. 
     Admittedly, if in circuit BLT 1 , the P-channel MOS transistors in the blocked state may support the high voltage VTH applied in test mode to the P-channel MOS transistors in the conductive state, it is not necessary to provide two different high voltages VP 1 , VP 2  in burn-in test mode. 
     It will be clear to those skilled in the art that the present disclosure is susceptible of various embodiments and applications. In particular, the disclosure does not only apply to EEPROM memories, but also to Flash memories, and more generally to any other integrated circuit comprising MOS transistors intended to be subjected to voltages higher than the supply voltage supplied to the integrated circuit. Thus, circuits HVGN, TMCL and VCOM may be implemented in any other integrated circuit using a high voltage higher than the supply voltage of the integrated circuit. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.