Abstract:
An SRAM cell is formed of FDSOI-type NMOS and PMOS transistors. A doped well extends under the NMOS and PMOS transistors and is separated therefrom by an insulating layer. A bias voltage is applied to the doped well. The applied bias voltage is adjusted according to a state of the memory cell. For example, a temperature of the memory cell is sensed and the bias voltage adjusted as a function of the sensed temperature. The adjustment in the bias voltage is configured so that threshold voltages of the NMOS and PMOS transistors are substantially equal to n and p target threshold voltages, respectively.

Description:
PRIORITY CLAIM 
     This application claims the priority benefit of French Patent Application No. 1457800, filed on Aug. 13, 2014, the contents of which is hereby incorporated by reference in its entirety to the maximum extent allowable by law. 
     TECHNICAL FIELD 
     The present disclosure relates to a method of minimizing the operating voltage of an SRAM (“Static Random Access Memory”) cell formed on FDSOI (“Fully Depleted Silicon On Insulator”). 
     BACKGROUND 
       FIG. 1  schematically shows a circuit of an SRAM cell. The memory cell comprises two cross-coupled inverters Inv 0  and Inv 1  in antiparallel, output terminal Out 0  of inverter Inv 0  being connected to input terminal In 1  of inverter Inv 1 , and output terminal Out 1  of inverter In 1  being connected to input terminal In 0  of inverter Inv 0 . Inverter Inv 0  comprises a P-channel MOS transistor (PMOS)  1  and an N-channel MOS transistor (NMOS)  3  series-connected between a power supply potential Vdd connected to the source of PMOS transistor  1 , and a ground potential Gnd connected to the source of NMOS transistor  3 . The drains of transistors  1  and  3  are coupled together to output terminal Out 0  of inverter Inv 0 , the gates of transistors  1  and  3  being coupled together to input terminal In 0  of inverter Inv 0 . Inverter Inv 1  comprises a PMOS transistor  5  and an NMOS transistor  7  series-connected between potential Vdd connected to the source of PMOS transistor  5 , and ground potential Gnd connected to the source of NMOS transistor  7 . The drains of transistors  5  and  7  are coupled together to output terminal Out 1  of inverter Inv 1 , the gates of transistors  5  and  7  being coupled together to input terminal In 1  of inverter Inv 1 . 
     Terminals In 0  and Out 1  are connected to a bit line  9  via an NMOS transfer transistor  11  and terminals Out 0  and In 1  are connected to a bit line  13  via an NMOS transfer transistor  15 . Thus, a value stored by the memory cell may be written into or read from via bit lines  9  and  13  by applying a control potential Ctrl to the gates of transfer transistors  11  and  15 . 
     In this memory cell, the PMOS transistors of the inverters inv 1  and inv 0  are all identical and have a same threshold voltage VtP, and the NMOS transistors of the inverters inv 1  and inv 0  are all identical and have a same threshold voltage VtN. 
     The electric dynamic power consumption of a circuit comprising an SRAM memory whose memory cells are of the type described in relation with  FIG. 1  particularly depends on the square of the power supply voltage of this circuit which is generally limited by the power supply or operating voltage Vdd of its SRAM memory. It would be desirable to provide an SRAM cell having an operating voltage Vdd as low as possible to decrease the power consumption of a circuit comprising an SRAM memory. 
     SUMMARY 
     Thus, an embodiment provides a method of minimizing the operation voltage of an SRAM cell formed of FDSOI-type NMOS and PMOS transistors, a doped well extending under an insulating layer of the FDSOI structure, in front of said transistors, a bias voltage being applied to the well, the method comprising adjusting the bias voltage according to the state of the memory cell. 
     According to an embodiment, the memory cell is an element of an array of identical memory cells, the doped well being shared in common to all the memory cells in the array. 
     According to an embodiment, the method comprises the successive steps of: carrying out measurements representative of the threshold voltage of the PMOS transistors and of the threshold voltage of the PMOS transistors; and adjusting the bias voltage of the well so that the threshold voltage of the NMOS transistors and of the PMOS transistors are substantially equal (i.e., to within a margin of 10%) to target threshold voltages of the NMOS and PMOS transistors respectively. 
     According to an embodiment, the target threshold voltages of NMOS and PMOS transistors are equal. 
     According to an embodiment, the method further comprises the successive steps of: measuring the operating temperature of the memory cell; and controlling the bias voltage with difference between the operating temperature and a reference temperature to correct the increase of the operating voltage resulting from a variation of said difference. 
     According to an embodiment, the well is P-type doped and the bias voltage is increased when the operating temperature decreases, and the bias voltage is decreased when the operating temperature increases. 
     According to an embodiment, the measurement representative of the threshold voltages is a measurement of a frequency of an oscillator formed of a chain of inverters. 
     According to another aspect, an embodiment provides an integrated circuit chip comprising: an SRAM cell formed of FDSOI-type NMOS and PMOS transistors; a doped well extending under an insulating layer of the FDSOI structure, in front of said transistors; a device for measuring the operating temperature of the memory cell; and a device for controlling the bias voltage of the well according to the operating temperature. 
     According to an embodiment, the memory cell is an element of an array of identical memory cells, the doped well being shared in common to all the memory cells in the array. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The foregoing and other features and advantages will be discussed in detail in the following non-limiting description of specific embodiments in connection with the accompanying drawings. 
         FIG. 1 , previously described, shows a simplified circuit of an SRAM cell; 
         FIG. 2  is a simplified cross-section view showing an embodiment on FDSOI of elements of the memory cell of  FIG. 1 ; as usual in the representation of semiconductor components, this drawing is not to scale; 
         FIG. 3  is a diagram indicating the critical operating voltage of an SRAM cell according to the threshold voltages of the transistors forming this memory cell; and 
         FIG. 4  is a simplified representation of an embodiment of an integrated circuit chip comprising an array of SRAM cells having transistors of the type in  FIG. 2 . 
     
    
    
     DETAILED DESCRIPTION 
     In a memory cell of the type described in relation with  FIG. 1 , there exists a limiting value Vdd R  of operating voltage Vdd below which errors may occur during read operations. Similarly, there exists a limiting value Vdd W  of operating voltage Vdd below which errors may occur during write operations. For the memory cell to operate without errors, operating voltage Vdd should be selected to be greater than or equal to a critical operating voltage Vdd CR  which should be greater than limiting values Vdd R  and Vdd W . To decrease the power consumption of a memory cell, operating voltage Vdd is selected to be equal to or substantially greater than the critical operating voltage Vdd CR . 
     As described in relation with  FIG. 11  of article “Process Architecture for Spatial and Temporal Variability Improvement of SRAM Circuits at the 45 nm node” of N. Planes et al., disclosed in 2008 at the SSDM Conference (incorporated by reference), the fact of bringing as close as possible to each other the absolute values of threshold voltages VtP of the PMOS transistors and VtN of the NMOS transistors results in low values of critical operating voltage Vdd CR . 
       FIG. 2  partially and schematically shows an embodiment on FDSOI (fully-depleted, silicon on insulator) of transistors of a memory cell of the type in  FIG. 1 . Only NMOS transistor  1  and PMOS transistor  3  of the memory cell of  FIG. 1  are shown in  FIG. 2 . 
     PMOS transistor  1  comprises, in a thin silicon layer  21 , P-type doped source and drain regions S 1  and D 1  separated from each other by a channel-forming region C 1 . An insulated gate stack G 1  is formed above channel-forming region C 1 . NMOS transistor  3  comprises, in thin silicon layer  21 , N-type doped source and drain regions S 3  and D 3  separated from each other by a channel-forming region C 3 . An insulated gate stack G 3  is formed above channel-forming region C 3 . 
     Silicon layer  21  is separated from a silicon support  25  by an insulating layer  27 . The memory cell transistors are insulated from one another by insulating trenches  29  crossing silicon layer  21  all the way to insulating layer  27 . Under the memory cell transistors, the support comprises a P-type doped well  31 , insulating layer  27  separating well  31  from silicon layer  21 . 
     Heavily-doped P-type silicon regions  33  (P + ) contact well  31 . Regions  33  are insulated from thin layer  21  by insulating trenches  29 . Thus, well  31  may be biased to a bias potential Vpol via connections connected to regions  33 . 
     All the memory cell transistors are formed above the same well  31 . In an SRAM comprising an array of memory cells, well  31  is shared in common to all memory cells. This type of memory cell and this type of memory will be called hereafter: “single-well memory cell” and “single-well memory”. 
     Beyond the surface occupied by the memory transistors, other transistors, for example, logic circuit transistors, may be formed inside and on top of thin layer  21 , and these transistors can then be arranged above wells different from the well common to the SRAM transistors. 
       FIG. 3  is a diagram indicating, for a single-well memory cell, the value of critical operating voltage Vdd CR  according to threshold voltages VtP and VtN of the transistors forming this memory cell. The absolute values of voltage VtP (in abscissas) and of voltage VtN (in ordinates) are indicated along an arbitrary linear scale. 
     On design of a memory cell, to obtain a critical operating voltage Vdd CR , and thus a power consumption, as low as possible, it is aimed at obtaining equal threshold voltages VtP and VtN for the P-channel and N-channel transistors. In other words, it is aimed at obtaining an operating point on a straight line  35  of  FIG. 3  corresponding to the case where the absolute values of threshold voltages VtP and VtN are equal. For example, in the context of a given technology, it is desired to obtain target values VtP A  and VtN A  of these threshold voltages, corresponding to a point A of the diagram of  FIG. 3  for which the critical operating voltage is equal to Vdd A . The diagram of  FIG. 3  can thus be divided into four quadrants defined by horizontal and vertical lines running through point A. 
     In a quadrant I, also named quadrant FF (“Fast-Fast”) in the art, the values of VtP and VtN are smaller than VtP A  and VtN A . If values VtP and VtN become too low, the transistors will significantly leak. 
     In a quadrant II, also named quadrant SS (“Slow-Slow”) in the art, the values of VtP and VtN are greater than values VtP A  and VtN A . As a result, the operating voltages of the memory cells of quadrant II should be greater than Vdd A . 
     In a quadrant SF, the values of VtP are smaller than VtP A  and the values of VtN are greater than VtN A . As a result, the memory cells risk exhibiting write errors if their operating voltages are not greater than Vdd A . 
     In a quadrant FS, the values of VtP are greater than VtP A  and the values of VtN are smaller than VtN A . As a result, the memory cells risk exhibiting read errors if their operating voltages are not greater than Vdd A . 
     Of course, in the practical forming of an SRAM containing elementary transistors, due to manufacturing dispersions, it is impossible to exactly be in the conditions corresponding to point A (VtP A , VtN A , Vdd A ). A tolerance area  37  is defined within a closed curve  39  where the corresponding memory points can still operate at a voltage substantially equal to Vdd A . In a direction orthogonal to that of straight line  35 , this tolerance area substantially corresponds to memory cells for which the threshold voltage VtP is equal to VtP A  to within 10%, and the threshold voltage VtN is equal to VtN A  to within 10%. 
     If the manufactured circuit is such that its operating point is located outside of tolerance area  37  in a direction orthogonal to that of straight line  35 , it is here provided, in the case where the SRAM is a single-well memory, to modify bias voltage Vpol of the well to take the operating point back inside of tolerance area  37 . More specifically, to take an operating point B located outside of tolerance area  37  on the side of quadrant SF back towards a point B′ located inside of this area, bias voltage Vpol of the well is increased, and to take a point C located outside of tolerance area  37  on the side of quadrant FS back towards a point C′ located inside of this area, bias voltage Vpol of the well is decreased. 
     The fact for an operating point to be outside of tolerance area  37  may result from many reasons. 
     A first reason is that, as previously indicated, there inevitably are manufacturing dispersions. In this case, the bias voltage correction is determined after initial tests during which values VtP and VtN are directly or indirectly measured to determine in which quadrant, more particularly quadrant SF or quadrant FS, the operating point is located. After these initial tests, bias voltage Vpol is modified as indicated previously. The values of threshold voltages VtP and VtN are for example deduced from the measurement of the frequency of an oscillator formed of a chain of inverters having transistors identical to those of the memory cells, and formed above the same well  31  common to the transistors of these memory cells. 
     A second reason is that there are parameter variations during the operation of a memory cell, for example, temperature variations inevitably appear. In this last case, to perform the corresponding correction, it is provided to insert into the integrated circuit chip containing the SRAM cell a temperature sensor and bias voltage Vpol will be temperature-controlled. More specifically, for an operating point C′ situated in the tolerance area  37  and in the quadrant FS, a temperature increase results in a displacement of the operating point towards the point C into the quadrant FS. Bias voltage Vpol is then decreased to take the operating point back into tolerance area  37 . Conversely, for an operating point B′ situated in the tolerance area  37  and in the quadrant SF, a temperature decrease causes the operating point moves toward the operating point B into the quadrant SF. Bias voltage Vpol is then increased to take the operating point back into tolerance area  37 . 
       FIG. 4  shows a simplified example of an integrated circuit chip  50  comprising a single-well SRAM-type memory  52 . According to an embodiment, chip  50  further comprises a temperature sensor  54  and a device  56  for controlling bias voltage Vpol of the single well. Control device  56  is powered with a voltage Vcc and delivers bias voltage Vpol to well  31  of SRAM  52 . Bias voltage Vpol is determined by control device  56 , for example, based on value T of the operating temperature of the single-well memory, T being delivered by temperature sensor  54 . 
     Specific embodiments have been described. Various alterations and modifications will occur to those skilled in the art. In particular, although a single-well SRAM cell where transfer transistors  11  and  15  are N-channel MOS transistors has been described, the present description also applies to the case where these transistors are P-channel MOS transistors. 
     Although an embodiment of a method of minimizing the operating voltage of an SRAM memory formed above a P-type doped well  31  has been described, what has been described above applies to an N-type doped well  31 . 
     Furthermore, the method of minimizing the consumption of an SRAM memory cell has been described above for equal target threshold voltages VtP A  and VtN A . This method also applies to target voltages VtP A  and VtN A  thresholds not being equal. 
     The method of minimizing the consumption of an SRAM memory cell has been described for a six transistors SRAM memory cell. This method can also be applied to SRAM memory cells having a different number of transistors.