Abstract:
An embodiment of a method for indexing a plurality of die obtainable from a material wafer comprising a plurality of stacked material layers is disclosed. Each die is obtained in a respective position of the wafer; the plurality of dies is obtained by means of a manufacturing process performed in at least one manufacturing stage using at least one lithographic mask for treating a surface of the material wafer trough an exposition to a proper radiation. Said at least one manufacturing stage comprises at least two steps for treating a respective superficial portion of the material wafer that corresponds to a subset of said plurality of dies using the at least one lithographic mask through the exposition to the proper radiation in temporal succession. The method may include providing a die index on each die which is indicative of the position of the respective die by forming an external index indicative of the position of the superficial portion of the material wafer corresponding to the subset of the plurality of dies including said die. Said forming the external index may comprise forming in a set of material layers of the die a first reference structure adapted to define a mapping of the superficial portions of the wafer; said first reference structure may comprise a plurality of electronic components electrically coupled to each other by means of a respective common control line. The method may further comprise interrupting the control line in a position based on the position of the superficial portion corresponding to the subset of the plurality of dies including the die.

Description:
PRIORITY CLAIM 
       [0001]    The instant application claims priority to Italian Patent Application No. MI2008A002344, filed Dec. 30, 2008, which application is incorporated herein by reference in its entirety. 
       TECHNICAL FIELD 
       [0002]    An embodiment of the present invention generally relates to electronic devices integrated in semiconductor dies, and particularly to the manufacturing thereof. More specifically, an embodiment of the present invention regards a method for indexing dies, which include integrated devices, obtained starting from a semiconductor material wafer. 
       BACKGROUND 
       [0003]    Thanks to the modern integration technologies, it is possible to integrate micro-devices, such as electronic circuits, micro-mechanical structures and/or optical structures, in a die obtained from a semiconductor material (e.g., silicon) wafer. Particularly, the silicon wafer is subjected to a series of selective lithographic processes, at the end of which a plurality of replicas of the desired integrated devices are formed on the semiconductor wafer. At this point, the processed wafer is subjected to dicing operations, in order to obtain the desired dies; at the end of the dicing operations, each resulting die includes a corresponding integrated circuit. Typically, the dies are then encapsulated into respective packages, whose characteristics depend on the use the devices are destined to. 
         [0004]    If the devices integrated in the dies are defective or subjected to faults during the operation, and the dies are returned to the manufacturer in order to carry out a fault analysis, it may be important to be capable of retrieving the original position of the device within the semiconductor wafer (i.e., before the dicing operations for obtaining the dies), since such information may allow one to manage the quality of the manufacturing process in an optimum way. Indeed, the performances, the reliability and different functional parameters (such as electric and/or optic parameters) of the devices integrated in each die may vary with the position of the device itself within the semiconductor wafer, for example because the crystallographic quality of the wafer material may vary with the position within the wafer. As a consequence, by means of analysis of such type, the manufacturer is capable of planning proper strategies for improving the manufacturing quality of the dies and the quality of the circuits integrated therein. 
         [0005]    For this purpose, each die is provided with an indication—in jargon, “die index”—of the zone wherein such die was positioned within the semiconductor wafer before being subjected to the dicing operations. 
         [0006]    At the present time, different indexing methods which exploit “visible” die indexes are known in the art, i.e., visible marks located on each die—for example on a top passivation layer in the stack of material layers of the wafer—during the manufacturing process. In this way, it may be possible to read a die index through a non-invasive inspection even when the electronic device integrated in the die is defective. For example, a die index formed by a number of an ordered sequence of numbers may be assigned to each die obtained from the wafer; such number may be placed in a peripheral portion of the die. 
         [0007]    A process for manufacturing the dies starting from a semiconductor wafer typically comprises a sequence of stages. Photolithographic masks are typically employed during such process stages for defining shapes in the various layers of material forming the wafer. A photolithographic mask—briefly, mask—is a plate including very high definition photographic images (comprising opaque and transparent areas) of shapes and structures useful for implementing the device on the various material layers of the wafer. 
         [0008]    In certain predetermined process stages, the wafer is covered with a photosensitive material layer, in jargon denoted “photosensitive resist” or “photoresist”, and a mask properly aligned with the wafer is enlightened by means an optical system with radiation having wavelengths which are suitable for the exposure of the photoresist. Consequently, the images on the mask are projected and transferred on the photoresist covering the wafer. The exposed photoresist is then developed, generating on the wafer shapes that allow to identify the areas of material to be removed and those to be maintained. 
         [0009]    The manufacturing process previously described may be carried out using a traditional projection system, in which the wafer is positioned and aligned with the required masks for the manufacturing of the devices on the wafer. As long as the size of wafer is relatively small, each mask includes a number of images equal to the number of dies to be obtained from the wafer, and the whole wafer is exposed to the lighting through such mask in a single step. 
         [0010]    By means of such projection system it is possible to index the dies with a visible index, particularly forming the die indexes of all the dies obtainable from the wafer on one among the masks used during the manufacturing process. 
         [0011]    The increasing in size of the semiconductor wafers has resulted in the introduction of step-by-step projection systems (in jargon, “wafer steppers”), by means of which the images of the mask required for the production of the devices are not projected in a single step, but instead are projected on the wafer in different consecutive steps. 
         [0012]    A step-by-step projection system includes the optical system and an alignment system for aligning the optical system, the mask and the wafer in a step-by-step fashion. Particularly, the image projected by the mask typically covers an area that is a portion of the whole area of the wafer (in jargon, such portion is often referred to as “shot”). After each exposition, the wafer positioned under the optical system is shifted by an amount exactly equal to the size of the projection of the mask on the wafer. 
         [0013]    The step-by-step projection systems allow obtaining a higher number of dies, starting from semiconductor wafers larger than the wafers used with the traditional projection systems. Thus, the masks used for the traditional projection systems are not compatible with the size of the semiconductor wafers that can be processed by means of the use of the step-by-step projection systems; consequently, using step-by-step projection systems it may not be possible to project the images of the visible indexes of all the dies of the wafer in at the same time (i.e., in a single step). 
         [0014]    European Patent Application EP 1589578, which is incorporated by reference, describes a method for generating visible dies indexes adapted to be employed in the step-by-step projection systems. Briefly, on each die a pair of die indexes is generated, including a first index adapted to identify the position of the die within the wafer, and a second index adapted to identify the position assumed by the mask with respect to the wafer surface during the step in which the wafer portion including the die has been exposed. Such indexes may be implemented using a particular material layer formed on the semiconductor wafer during the preceding phases of the process, for example one among the metallic layers. For example, in one of the embodiments described in the patent application EP 1589578, the first index comprises a reference element and two further index elements whose positions relative to the reference element determine a pair of coordinates which identify the position of the die within the mask. The second index comprises instead a toothed reference structure in which each possible combination of pairs of teeth of the structure is associated with a particular position of the mask in the semiconductor wafer. Selectively etching a pair of teeth of the toothed reference structure, it is thus possible to register the position assumed by the mask with respect to the wafer surface during the step in which the portion of the wafer including the die has been exposed. In this way, by means of such two indexes results, it is possible to univocally retrieve the exact position of the die in the wafer before the dicing operations. 
         [0015]    However, a solution of such type, and generally any solution making use of visible die indexes, may not be advantageous under all applications 
         [0016]    Indeed, since the majority of the dies are sold encapsulated in a respective package, in order to read a visible index it is necessary to open the package, with the risk of further damaging the die. Moreover, once the packaging has been opened, or even using infrared or x-ray sensing systems, distinguishing the structures forming the die indexes from the rest of the components forming the circuit integrated in the die may not be a simple operation, because of the very small size and the very large number of electronic devices which are actually integrated in a single die. A further drawback linked to the use of visible die indexes, which is correlated with the previously cited drawback, regards the costs—in terms of time, too—required for carrying out inspection operations of such type, which may be very high; particularly, a visual inspection of such type, other than requesting sometimes the opening of the packaging, is hardly executable in an automated way, often requiring instead the use of an expert operator, who knows the topology of the integrated device. 
         [0017]    Alternative indexing methods provide instead the integration of die indexes formed by non-volatile memory elements—for example, flash memory cells—in each die, adapted to store the information regarding the coordinates of the die in the semiconductor wafer. For example, during a test phase of the circuit integrated in the die, in the memory elements of the die, indexes of each die are stored as a numeric string corresponding to the coordinates of the die. 
         [0018]    However, even the solutions of such type may increase the costs, and particularly the costs of the manufacturing process of the dies. Indeed, in order to integrate memory cells in the die, it may be necessary to carry out expensive dedicated process steps, and occupy active portions within the die. In addition, a solution of such type may make the die even more to faults; in case the faults are such to completely impair the circuit integrated in the chip, the information stored in the memory elements is lost. 
       SUMMARY 
       [0019]    An embodiment of the present invention overcomes the previously mentioned drawbacks. 
         [0020]    An embodiment of the present invention regards a method for indexing a plurality of dies obtainable from a material wafer comprising a plurality of stacked material layers. Each die is obtained in a respective position of the wafer; the plurality of dies is obtained by means of a manufacturing process performed in at least one manufacturing stage using at least one lithographic mask for treating a surface of the material wafer trough an exposition to a proper radiation. Said at least one manufacturing stage comprises at least two steps for treating a respective superficial portion of the material wafer that corresponds to a subset of said plurality of dies using the at least one lithographic mask through the exposition to the proper radiation in temporal succession. The method includes providing a die index on each die which is indicative of the position of the respective die by forming an external index indicative of the position of the superficial portion of the material wafer corresponding to the subset of the plurality of dies including said die. Said forming the external index comprises forming in a set of material layers of the die a first reference structure adapted to define a mapping of the superficial portions of the wafer; said first reference structure comprises a plurality of electronic components electrically coupled to each other by means of a respective common control line. The method further comprises interrupting the control line in a position based on the position of the superficial portion corresponding to the subset of the plurality of dies including the die. 
         [0021]    A further embodiment of the present invention regards a semiconductor die obtained by a material wafer comprising a plurality of stacked material layers. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0022]    One or more embodiments of the present invention, as well as features and advantages thereof, will be best understood by reference to the following detailed description, given purely by way of a non-restrictive indication, to be read in conjunction with the accompanying drawings. In this respect, it is expressly intended that the figures are not necessary drawn to scale and that, unless otherwise indicated, they are intended to conceptually illustrate the structures and procedures described herein. Particularly: 
           [0023]      FIG. 1A  illustrates a semiconductor wafer aligned with a photolithographic mask during a process for the manufacturing of dies wherein an embodiment of the present invention may be applied; 
           [0024]      FIG. 1B  illustrates the wafer of  FIG. 1A  at the end of the manufacturing process; 
           [0025]      FIGS. 2A-2C  are planar views of external indexes of dies of the wafer of  FIG. 1B  during a stage of the manufacturing process of the dies according to an embodiment of the present invention; 
           [0026]      FIG. 3  illustrates a service lithographic mask usable during the manufacturing process of the dies according to an embodiment of the present invention; 
           [0027]      FIG. 4  is a planar view of an external index of a die during an indexing stage according to an embodiment of the present invention; 
           [0028]      FIG. 5  is a planar view of the external index of  FIG. 4  during a subsequent stage of the manufacturing process; 
           [0029]      FIG. 6  shows an equivalent circuit of the external index according to an embodiment of the present invention; 
           [0030]      FIG. 7  shows an equivalent circuit of the external index according to an alternative embodiment of the present invention; 
           [0031]      FIG. 8  shows an equivalent circuit of the external index according to a further alternative embodiment of the present invention; and 
           [0032]      FIG. 9  shows an example of the final structure of internal indexes of the die obtainable according to an embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION 
       [0033]    Referring in particular to  FIG. 1A , it is schematically illustrated a semiconductor wafer  100  aligned to a photolithographic mask  105  during a process for manufacturing dies from the wafer  100  wherein an embodiment of the present invention may be applied. The dies are represented by areas of the wafer  100  wherein the integrated electronic devices are formed. 
         [0034]    The manufacturing process is carried out in a plurality of stages, during which different masks  105  are employed for selectively defining the material layers stacked on the wafer  100  which form the integrated circuits of the dies. A generic mask  105  includes a set of images  110   a  traced on its surface for the manufacturing of a prescribed number of dies on the wafer  100 . In the considered example, such number is equal to four; particularly, the mask  105  is subdivided in four portions, each one including an image  110   a  dedicated to the manufacturing of a respective die on the wafer  100 . 
         [0035]    Each image  110   a  includes a first portion comprising geometric configurations—formed by opaque and transparent areas—required for defining the integrated circuits of the dies in the material layers of the mask  100  that are processed in the stage making use of the mask  105 . Such first portion of the image  110   a , in the following denoted with the term of “circuit image”, is identified in  FIG. 1A  with the reference  115 . 
         [0036]    During a stage of the manufacturing process in which the mask  105  is employed, the wafer  100  is covered by a photoresist and the mask  105 , aligned to the wafer covered by the photoresist, is enlightened by means of a light source corresponding to a proper wavelength range, for the purpose of exposing the photoresist layer with the image  110   a  projected by the mask  105 . 
         [0037]    Using a step-by step projection system, the wafer  100  is shifted under the mask  105 , which mask is enlightened more times. In this way the photoresist layer is completely exposed in a plurality of steps (in the considered example, sixteen steps). In greater details, considering the mask  105  of the example of  FIG. 1A  comprising four images  110   a , the process stage exploiting the mask is comprised by sixteen distinct steps, for a total of sixty four images projected on the wafer  100  according to a bi-dimensional arrangement (as shown by the arrows in  FIG. 1A ). The ratio between the area of the images  110   a  of the mask  105  and the area of the images projected on the wafer  100  depends on the optical system of the step-by-step projection system; in the considered example, such ratio is 1:1. 
         [0038]    Each step performed during the considered process stage corresponds to a particular position of the wafer  100  with respect to the mask  105 . The shifting of the wafer  100  may be controlled in a precise way by means of a positioning program loaded in the step-by-step projection system. 
         [0039]    According to the specific geometry of the wafer  100  and of the mask  105 , at a first step (step  1 ) the mask  105  is projected on the wafer  100  in a partial way. At the subsequent step (step  2 ) the wafer  100  is translated along an x axis by a prescribed distance D (equal to the width of the mask  105  that is projected on the surface of the wafer  100 ) for exposing a portion of the wafer  100  which is adjacent to the portion exposed in the previous step. In a similar way, at the two following steps (steps  3 ,  4 ) the wafer  100  is translated along the x axis by the distance D, for exposing the portion of the wafer  100  which is adjacent to the portion exposed in the previous step. According to the considered example, at the step  5  the wafer  100  is translated along an y axis, orthogonal to the x axis, by the distance D; at the steps  6 , 7  and  8  the wafer  100  is shifted again along the x axis, but in the opposite direction with respect to that corresponding to the steps  2 - 4 , and so on. Using as reference system the axis x and y, and associating a pair of coordinates (x,y) with respect to such axis to the position of the mask  105 , the sequence of steps of the generic process stage implies the mask  105  assuming the following (ordered) positions: (0,0)-(1,0)-(2,0)-(3,0)-(3,1)-(2,1)-(1,1)-(0,1)-(0,2)-(1,2)-(2,2)-(3,2) -(3,3)-(2,3)-(1,3)-(0,3). 
         [0040]      FIG. 1B  illustrates the wafer  100  at the end of the manufacturing process. Particularly, the final result of the manufacturing process on the wafer  100  includes a bidimensional arrangement of dies  110   b , obtained by means of the exposition—in the previously described step-by-step modality—of the portions of the wafer  100  area through the plurality of masks used during the various stages of the process. Particularly, each die  110   b  comprises a respective integrated circuit—identified in the figure with the reference  120 —obtained by means of the exposition of the wafer  100  through the circuit images  115  included in the images  110   a  of the masks  105  used during the process stages. 
         [0041]    At each process stage, by exposing the wafer  100  in sixteen steps through a respective mask  105  including four images  110   a , thirty two dies  110   b  are obtained. In  FIG. 1B , each die  110   b  is identified by a respective reference  1 - 32 . 
         [0042]    In order to be capable of recovering the mask position  1 - 16  of the generic die  110   b  within the wafer  100  even after the dicing operations, on each die  110   b  a respective die index is generated, which die index is identified with the reference  130 . 
         [0043]    Each die index  130  is formed in turn by two sub-indexes, and particularly by a first index denoted “external index”—identified in the figure with the reference  140 —and by a second index denoted “internal index”—identified in the figure with the reference  150 . 
         [0044]    The external index  140  of a generic die  110   b  allows identifying the position assumed by the mask  105  with respect to the wafer  100  when such die has been exposed—through the mask—to the enlightening during the manufacturing process. In other words, thanks to the external index  140  it is possible to determine the step of the manufacturing process in which the mask  105  has been exploited for manufacturing the die  110   b  indexed by such external index  140 . In the considered example, the number of such steps is equal to sixteen—indeed, the step-by-step projection system uses a same mask  105  sixteen times during each process stage—and thus the external index  140  may assume sixteen different values. 
         [0045]    The internal index  150  of a generic die  110   b  allows instead to identify the position of the die itself with respect to the mask  105 —i.e., with respect to the image of the mask projected on the wafer. In each step of the process stage that makes use of a mask  105 , such mask  105  is adapted to define a set of adjacent dies  110   b  at the same time—in the considered example, four dies. By means of the internal index  150  located on a generic die  110   b  it is possible to distinguish the position of such die  110   b  among those of the other dies  110   b  belonging to the set defined by the mask  105  in the same step. 
         [0046]    As a consequence, making reference to the considered example, the die  110   b  having the position  1  in the wafer  100  illustrated in  FIG. 1B  has:
       an external index  140  having a value equal to (1,0), corresponding to the step  2  of the manufacturing stage during which the mask  105  is located at the coordinates (1,0); and   an internal index  150  having a value equal to 3, corresponding to the third image  110   a  projected by the mask  105 , i.e. the bottom left image.       
 
         [0049]    Making reference to the considered example, the die  110   b  having the position  4  in the wafer  100  illustrated in  FIG. 1B  has:
       an external index  140  having a value equal to (2,0), corresponding to the step  3  of the manufacturing stage during which the mask  105  is located at the coordinates (2,0); and   an internal index  150  having a value equal to  4 , corresponding to the fourth image  110   a  projected by the mask  105 , i.e. the bottom right image.       
 
         [0052]    As will become more clear in the following description, according to an embodiment of the present invention the external index  140  of each die  110   b  is an integrated electronic circuit—obtained by means of photolithographic manufacturing of the material layers of the wafer  100 —adapted to store the position information in a non-volatile way. 
         [0053]    Making again reference to the  FIG. 1A , the die index  130  is generated by means of the same masks  105  used for generating the integrated circuits  120  during the various process stages. For this purpose, the image  110   a  of each mask  105  includes a second portion comprising geometric configurations adapted to define the die index  130  in the material layers of the wafer  100  that are processed in the process stage that makes use of the mask  105 . Such second portion of the image  110   a , denoted in the following with the term “index image”, is identified in  FIG. 1A  with the reference  160 . Each index image  160  comprises in turn a first reference image  170  for the definition of the external index  140  and a second reference image  180  for the definition of the internal index  150 . 
         [0054]    Although in  FIG. 1A  the sizes of the circuit images  115  are comparable to the sizes of the index images  160 , the ratio between such sizes may be different, for example, where the die index  130  is smaller than the integrated circuit  120 . 
         [0055]    In the following of the present invention will be described in greater detail how the external index  140  is realized according to an embodiment of the present invention. Particularly, making reference to the  FIGS. 2A-2C , the structures of several example external indexes  140  will be illustrated by means of a planar view during a stage of the manufacturing process of the dies  110   b.    
         [0056]    According to an embodiment of the present invention, the external index  140  includes a double array of electronic devices, i.e., a first array—identified in the figures with the reference  205 —adapted to indicate the position of the mask  105  on the x axis, and a second array—identified in the figures with the reference  210 —adapted to indicate the position of the mask  105  on the y axis. 
         [0057]    Particularly, the array  205  comprises a number of electronic components equal to the number of different values the x coordinate of the mask  105  position may assume on the x axis (in the considered example, four, i.e., x=0, 1, 2, 3), while the array  210  comprises a number of electronic components equal to the number of different values the y coordinate of the mask  105  position may assume on the y axis (in the considered example, four, i.e., y=0, 1, 2, 3). 
         [0058]    According to an embodiment of the present invention, the electronic components forming each array  205  and  210  are integrated n-channel MOS transistors. As will be more clear in the following of the present description, the whole structure of the two arrays  205 ,  210  forming the external index  140  may be used for registering a generic position of the mask  105  relative to the semiconductor wafer in the reference system defined by the x and y axes of  FIG. 1A . 
         [0059]      FIG. 2A  illustrates in detail the structure of an external index  140  at the end of one of the stages of the manufacturing process of the dies  110   b  according to an embodiment of the present invention. Particularly,  FIG. 2A  illustrates the structure of such index at the end of a process stage adapted for the generation of the transistors on the semiconductor wafer. 
         [0060]    As may be observed in  FIG. 2A , the array  205  is formed by a plurality of transistors  215 (i) (i=0, 1, n, with n equal to 3 in the considered example). In detail, a first transistor  215 ( 0 ) of the array has a drain region d( 0 ) realized by means of diffusion in the semiconductor wafer, a gate terminal g( 0 ) realized in polysilicon, and a source region s( 0 ) also realized by means of diffusion. The following transistor  215 ( 1 ) in the array has the source region s( 1 ) that coincides with the source region s( 0 ) of the previous transistor  215 ( 0 ), the gate terminal g( 1 ) realized in polysilicon and the drain region d( 1 ) that coincides with the drain region d( 2 ) of the subsequent transistor  215 ( 2 ) in the array. Such transistor  215 ( 2 ) has the gate terminal g( 2 ) realized in polysilicon, and the source region s( 2 ) that coincides with the source region s( 3 ) of the following transistor  215 ( 3 ) in the array. In other words, as may be observed in the equivalent circuit indicated with the reference  220 , the array  205  is formed by an ordered sequence of transistors  215 (i) alternatively having the source and drain regions in common. All the gate terminals of the transistors  215 (i) are connected to a common gate line realized in polysilicon, identified in figure with the reference  230 . 
         [0061]    The array  210  is structured in a way similar to the array  205 . Such array is formed by a plurality of transistors  235 (j) (j=0, 1, m, with m equal to 3 in the considered example) as well. In detail, a first transistor  235 ( 0 ) of the array has a drain region d′( 0 ) realized by means of diffusion in the semiconductor wafer, a gate terminal g′( 0 ) realized in polysilicon, and a source region s′( 0 ) realized by means of diffusion, too. The following transistor  235 ( 1 ) in the array has the source region s′( 1 ) that coincides with the source region s′( 0 ) of the previous transistor  235 ( 0 ), the gate terminal g′( 1 ) realized in polysilicon and the drain region d′( 1 ) that coincides with the drain region d′( 2 ) of the subsequent transistor  235 ( 2 ) in the array. Such transistor  235 ( 2 ) has the gate terminal g′( 2 ) realized in polysilicon, and the source region s′( 2 ) that coincides with the source region s′( 3 ) of the following transistor  235 ( 3 ) in the array. Even in this case, as may be observed in the equivalent circuit indicated with the reference  240 , the array  210  is formed by an ordered sequence of transistors  235 (j) alternatively having the source and drain regions in common. All the gate terminals of the transistors  235 (i) are connected to a common gate line realized in polysilicon, identified in figure with the reference  245 . 
         [0062]    The definition of the gate lines  230 ,  2405 , the source and drain regions d(i), d′(j), s(i), s′(j) of the transistors  215 (i),  235 (j), and the gate regions—non visible in figure—of such transistors have been carried out in the previous stages of the manufacturing process making use of the first reference images  170  located in the various masks  150  used during such process stages. 
         [0063]    By making the array  205  correspond to the x axis, and the array  210  to the y axis, it may be possible to associate with each pair of transistors  215 (i),  235 (j) a corresponding pair of coordinates (x,y) in the reference system determined by such axis, for example according to the relationship (x=i, y=j). 
         [0064]    According to an embodiment of the present invention, in order to register a position of the mask  105  corresponding to the pair of generic coordinates x=i, y=j:
       the gate line  230  is interrupted in a portion thereof included between the gate terminal g(i) of the transistor  215 (i) and the gate terminal g(i- 1 ) of the transistor  215 (i- 1 ); and   the gate line  245  is interrupted in a portion thereof included between the gate terminal g′(j) of the transistor  235 (j) and the gate terminal g′(j- 1 ) of the transistor  235 (j- 1 ).       
 
         [0067]    If the coordinate x=I=0, then the portion of the gate line  230  is interrupted before the gate terminal g( 0 ) of the transistor  215 ( 0 ); if the coordinate y=j=0, then the gate line  245  is interrupted before the gate terminal g′( 0 ) of the transistor  235 ( 0 ). 
         [0068]    The external index  140  illustrated in the  FIG. 2A  corresponds to the pair of coordinates (x=0, y=0). Indeed, the gate line  230  has a first interruption—identified with the reference x 0 —located before the gate terminal g( 0 ) of the transistor  215 ( 0 ), while the gate line  245  has a second interruption—identified with the reference y 0 —located before the gate terminal g′( 0 ) of the transistor  235 ( 0 ). In this situation, the gate terminals of all the transistors  215 (i) are connected to a same portion of the gate line  230 ; similarly, the gate terminals of all the transistors  235 (j) are connected to a same portion of the gate line  245 , too. 
         [0069]      FIG. 2B  illustrates instead the external index  140  corresponding to the pair of coordinates (x=1, y=0). In this case, the gate line  230  has a first interruption—identified with the reference x 1 —between the gate terminal g( 1 ) of the transistor  215 ( 1 ) and the gate terminal g( 0 ) of the transistor  215 ( 0 ), while the gate line  245  has a second interruption y 0  having the same position of the interruption y 0  illustrated in  FIG. 2A . In this situation, the gate terminals of all the transistors  235 (j) are connected to a same portion of the gate line  245 ; furthermore, while the gate terminals of the transistors  215 ( 1 ),  215 ( 2 ) and  215 ( 3 ) are connected to a first portion of the gate line  230 , the gate terminal of the transistor  215 ( 0 ) is connected to a second portion of the gate line  230 , separated from the first portion by means of the interruption x 1 . 
         [0070]    Lastly, the  FIG. 2C  illustrates the external index  140  corresponding to the pair of coordinates (x=2, y=3). In this case, the gate line  230  has a first interruption—identified with the reference x 2 —between the gate terminal g( 2 ) of the transistor  215 ( 2 ) and the gate terminal g( 1 ) of the transistor  215 ( 1 ), while the gate line  245  has a second interruption—identified with the reference y 3 —between the gate terminal g′( 3 ) of the transistor  235 ( 3 ) and the gate terminal g′( 2 ) of the transistor  215 ( 2 ). In this situation, the gate terminals of the transistors  215 ( 0 ) and  215 ( 1 ) are connected to a first portion of the gate line  230 , while the gate terminals of the transistors  215 ( 2 ) and  215 ( 3 ) are connected to a second portion of the gate line  230 , separated from the first portion by means of the interruption x 2 . Moreover, while the gate terminals of the transistors  235 ( 0 ),  235 ( 1 ) and  235 ( 2 ) are connected to a first portion of the gate line  245 , the gate terminal of the transistor  235 ( 3 ) is connected to a second portion of the gate line  245 , separated from the first portion by means of the interruption y 3 . 
         [0071]    According to an embodiment of the present invention, in order to register a position of the mask  105 , i.e., a pair of coordinates (x, y), and obtaining the external index  140  structure illustrated in the previous figures, a service lithographic mask of the type illustrated in  FIG. 3  is employed, which is superimposed on the mask  105  used for generating the gate lines  230  and  245 . 
         [0072]    Each first reference image  170  included in the mask  105  used for generating the gate lines  230  and  245  comprises geometric arrangements adapted to define the gate lines  230 ,  245  in their completeness (i.e., without any interruption). The service mask, identified in figure with the reference  300 , includes a set of identical images  310   a  traced on its surface. The number of the images  310   a  coincides with the number of images  110   a  traced on each one among the masks  105  used during the various stages of the manufacturing process (in the example at issue, such number is equal to four). Each image  310   a  is totally opaque to the light, except for a transparent window having an L shape, identified with the reference  320 . 
         [0073]    During the manufacturing stage of the gate lines  230  and  245 , denoted “indexing stage”, the surface of the wafer  100  is coated by a photoresist layer, and subsequently enlightened in a step-by-step fashion through the service mask  300  superimposed on the mask  105 . 
         [0074]    At the first step, corresponding to the coordinates (x=0, y=0), the service mask  300  is exactly superimposed on the mask  105 , with each one of the images  310   a  that is superimposed on a corresponding die  110   b  of the wafer  100  (with the exclusion of the images corresponding to possible incomplete dies close to the border of the wafer). 
         [0075]    Each window  320  is arranged in such a way that at the first step of the indexing stage such window is superimposed on the external index  140  of a corresponding die  110   b . In detail, at the first step each window  320  is superimposed on the portion of the gate line  230  preceding the gate terminal g( 0 ) of the transistor  215 ( 0 )—corresponding to the coordinate x=0—, and on the portion of the gate line  245  preceding the gate terminal g′( 0 ) of the transistor  235 ( 0 )—corresponding to the coordinate y=0. 
         [0076]    After that the portion of the wafer  100  covered by the service mask  300  superimposed on the mask  105  has been enlightened, the mask  105  is moved along the x axis by the distance D, for reaching the coordinates corresponding to the next step—i.e., the coordinates (1,0); unlike the mask  105 , the service mask  300  is instead moved along the x axis by a distance lower than the distance D, and particularly in such a way that the window is now superimposed on the portion of the gate line  230  comprised between the gate terminal g( 1 ) of the transistor  215 ( 1 )—corresponding to the coordinate x=1—and the gate terminal g( 0 ) of the transistor  215 ( 0 )—corresponding to the coordinate x=0—and on the portion of the gate line  245  preceding the gate terminal g′( 0 ) of the transistor  235 ( 0 )—corresponding to the coordinate y=0. 
         [0077]    According to an embodiment of the present invention, the indexing stage continues in a similar way for the other steps required to completely expose the wafer  100 , with the projection system that at each step shifts the service mask  300  by a distance equal to the prescribed distance D modified by an amount depending on the coordinates (x, y) corresponding to such step. Particularly, at the generic step corresponding to the coordinates x=i and y=j, with i, j different than zero, the service mask  300  is shifted with respect to the position assumed by the mask  105  by an amount along the x and/or y axis such that the window  320  of each image  310   a  is superimposed on the portion of the gate line  230  comprised between the gate terminal g(i) of the transistor  215 (i) and the gate terminal g(i- 1 ) of the transistor  215 (i- 1 ), and on the portion of the gate line  245  comprised between the gate terminal g′(j) of the transistor  235 (j) and the gate terminal g′(j- 1 ) of the transistor  235 (j- 1 ). If the coordinate x=1 is equal to zero, the portion of the gate line  230  which is superimposed by the window  320  is the one preceding the gate terminal g( 0 ) of the transistor  215 ( 0 ); if instead it is the coordinate y=j to be equal to zero, the portion of the gate line  245  which is superimposed by the window  320  is the one preceding the gate terminal g′( 0 ) of the transistor  235 ( 0 ). 
         [0078]    For the sake of clarification,  FIG. 4  illustrates the situation relating the fourteenth step of the indexing stage, corresponding to the coordinates (x=2, y=3). In this case, the window  320  is superimposed on the portion of the gate line  230  comprised between the gate terminal g( 2 ) of the transistor  215 ( 2 ) and the gate terminal g( 1 ) of the transistor  215 ( 1 ), and on the portion of the gate line  245  comprised between the gate terminal g′( 3 ) of the transistor  235 ( 3 ) and the gate terminal g′( 2 ) of the transistor  235 ( 2 ). 
         [0079]    At the end of the last step of the indexing stage, the photoresist layer covering the surface of the wafer  100  is developed, uncovering in each external index  140  of each die  110   b  the contours of the gate lines  230 ,  245  and the two portions thereof corresponding to the pair of coordinates (x,y) of the position assumed by the mask  105  for fabricating such die  110   b . At this point, the layer of material used for the formation of the gate lines  230 ,  245  (in the considered example, a polysilicon layer) is selectively etched, and the portions that are not covered by the photoresist are removed. 
         [0080]    At the end of this operation, in each external index  140  of each die  110   b  it is formed a gate line  230  comprising an interruption xi determined by the x coordinate of the mask&#39;s position, and a gate line  245  comprising an interruption yj determined by the y coordinate of the mask&#39;s position. 
         [0081]    For example, making reference to the case illustrated in  FIG. 4 , the position of the window  320  corresponding to the coordinates (x=2, y=3) allows one to obtain the external index  140  illustrated in  FIG. 2C , wherein the gate line  230  exhibits an interruption x 2  between the gate terminal g( 2 ) of the transistor  215 ( 2 ) and the gate terminal g( 1 ) of the transistor  215 ( 1 ), and the gate line  245  exhibits an interruption y 3  between the gate terminal g′( 3 ) of the transistor  235 ( 3 ) and the gate terminal g′( 2 ) of the transistor  235 ( 2 ). 
         [0082]    After the indexing stage, the external indexes  140  are subjected to further process stages. 
         [0083]    Particularly,  FIG. 5  illustrates in detail the structure of the external index  140  corresponding to the coordinate (x=2, y=3) at the end of a subsequent process stage, and particularly at the end of the process stage adapted to the generation of the metallic connection lines on the semiconductor material wafer. 
         [0084]    At this point of the manufacturing process, the drain regions d( 0 )-d(n) of the transistors  215 (i) forming the array  205  are connected to a common drain region, identified in the figure with the reference  250 . Particularly, the drain line  250  is realized starting from a metallic layer deposed on the semiconductor wafer and defined by means of the first reference images  170  on the various masks  105  used in the previous process stages. The connection between the drain line  250  and the various drain regions d( 0 )-d(n) of the transistors  215 (i) is provided by proper contacts, identified in the figure with the reference  255 . 
         [0085]    In the same way, also the drain regions d′( 0 )-d′(n) of the transistors  235 (j) forming the array  210  are connected to a common drain line, identified in figure with the reference  260 , by means of contacts identified in figure with the reference  265 . 
         [0086]    The source regions s( 0 )-s(n) of the transistors  215 (i) forming the array  205  and the source regions s′( 0 )-s′(m) of the transistors  235 (j) forming the array  210  are connected to a common source line, identified in  FIG. 5  with the reference  270 . The source line  270  is realized starting from a metallic layer deposed on the semiconductor material wafer and defined by means of the first reference images  170  on the various masks  105  used in the previous process stages. The connection between the source line  270 , the various source regions s( 0 )-s(n) of the transistors  215 (i) and the source regions s′( 0 )-s′(m) of the transistors  235 (j) is provided by proper contacts, identified in figure with the reference  275 . 
         [0087]    According to an embodiment of the present invention, a first end of the gate line  230  of the array  205  is connected to the source line  270  by means of contacts identified in figure with the reference  280 , and a second end of the gate line  230  is connected to the drain line  250  by means of further contacts identified in figure with the reference  282 . 
         [0088]    In the same way, a first end of the gate line  245  of the array  210  is connected to the source line  270  by means of contacts identified in figure with the reference  285 , and a second end of the gate line  245  is connected to the drain line  260  by means of further contacts identified in figure with the reference  290 . 
         [0089]    The equivalent circuit of the arrays  205  and  210  is illustrated in  FIG. 5  with the reference  292 . As can be observed by the circuit  292 , at this stage of the manufacturing process each array  205 ,  210  is formed by a set of transistors connected in parallel to each other. 
         [0090]    In detail, the transistors  215 (i) of the array  205  have the drain terminals—identified with the same reference of the corresponding drain regions d( 0 )-d( 3 )—connected to each other, and the source terminals—identified with the same reference of the corresponding source regions s( 0 )-s( 3 )—connected to each other. As previously described, in the external index  140  corresponding to the coordinate (x=2, y=3) the gate terminals g( 0 ), g( 1 ) are connected to each other; in the same way, also the gate terminals g( 2 ) and g( 3 ) are connected to each other. Moreover, the gate terminals g( 2 ) and g( 3 ) are connected to the source terminals s( 0 )-s( 3 ), while the gate terminals g( 0 ) and g( 1 ) are connected to the drain terminals d( 0 )-d( 3 ) of the array  205 . 
         [0091]    In the same way, the transistors  235 (i) of the array  210  have the drain terminals—identified with the same reference of the corresponding drain regions d′( 0 )-d′( 3 )—connected to each other, and the source terminals—identified with the same reference of the corresponding source regions s′( 0 )-s′( 3 )—connected to each other. As previously described, in the external index  140  corresponding to the coordinate (x=2, y=3) the gate terminals g′( 0 )-g′( 2 ) are connected to each other. Moreover, such gate terminals g′( 0 )-g′( 2 ) are connected to the drain terminals d′( 0 )-d′( 3 ), while the gate terminal g′( 3 ) is connected to the source terminals s( 0 )-s( 3 ). 
         [0092]    The source terminals of both the arrays are further connected to each other, in such a way to form a common source line. 
         [0093]    According to an embodiment of the present invention, the drain terminals d(i), d′(j) and the source terminals s(i), s′(j) of the transistors of the arrays  205 ,  210  may be accessed from the outside of the die  110   b  integrating the external index  140  through proper contact pads integrated on the die. A first contact pad  294  is connected to the drain terminals d(i) of the transistors  215 (i) of the first array  205 , and particularly to the drain line  250 ; a second contact pad  296  is connected to the drain terminals d′(j) of the transistors  235 (j) of the second array  210 , particularly to the drain line  260 ; the source line  270 —and, thus, the source terminals s(i), s′(j)—is instead connected to a third contact pad  298 . 
         [0094]    The equivalent circuit of the arrays  205  and  210  corresponding to the considered example is illustrated in figure with the reference  292 . By observing such circuit, it is possible to see that in the array  205  the transistors  215 ( 2 ) and  215 ( 3 ) have the gate terminals g( 2 ), g( 3 ) connected to the source line  270 , and the transistors  215 ( 0 ) and  215 ( 1 ) have the gate terminals g( 0 ) and g( 1 ) connected to the drain line  250 ; in the array  215  the transistor  235 ( 3 ) has the gate terminal g′( 3 ) connected to the source line  270 , and the transistors  235 ( 0 ),  235 ( 1 ) and  235 ( 2 ) have the gate terminals g′( 0 ), g′( 1 ) and g′( 2 ) connected to the drain line  260 . 
         [0095]    Generalizing, according to an embodiment of the present invention, in the external index  140  of a generic die  110   b  corresponding to a pair of coordinates (x=x 0 , y=y 0 ):
       the transistors  215 (x 0 )- 215 (n) of the array  205  have the gate terminals g(x 0 )-g(n) connected to the source line  270 ;   the transistors  235 (y 0 )- 235 (m) of the array  210  have the gate terminals g′(y 0 )-g′(m) connected to the source line  270 ;   the transistors  215 ( 0 )- 215 (x 0 −1) of the array  205  have the gate terminals g( 0 )-g(x 0 −1) connected to the drain line  250 , and   the transistors  235 ( 0 )- 235 (y 0 −1) of the array  210  have the gate terminals g′( 0 )-g′(y 0 −1) connected to the drain line  260 .       
 
         [0100]    According to an embodiment of the present invention, given a generic die  110   b , it is possible to trace back the coordinates (x=x 0 , y=y 0 ) corresponding thereto by means of an electric measurement carried out on the arrays  205 ,  210  of the external index  140  integrated in such die  110   b.    
         [0101]    According to an embodiment, in order to determine the coordinate with respect to the x axis, the source line  270  is biased to a reference voltage—such as the ground voltage—while the drain line  250  is biased to a supply voltage Vdd of the die  110   b.    
         [0102]    In this situation, illustrated in  FIG. 6 , the transistors  215 (x 0 )- 215 (n) of the array  205  are off, since the gate terminals g(x 0 )-g(n) are connected to the source terminals s(x 0 )-s(n), which are biased to the ground voltage. The transistors  215 ( 0 )- 215 (x 0 −1) of the array  205  are instead on, since the gate terminals g( 0 )-g(x 0 −1) are connected to the drain terminals d( 0 )-d(x 0 −1), which are biased to the supply voltage Vdd. Supposing that each transistor  215 (i) of the array  205  is dimensioned in such a way to absorb a current I 0  when the voltage difference between gate g(i) and source s(i) is equal to the supply voltage Vdd, the total current sunk by the array  205 , identified with the reference Ix is substantially proportional to (x 0 )·(I 0 ). 
         [0103]    In a similar way, the transistors  235 (y 0 )- 235 (m) of the array  215  are off, since the gate terminals g′(y 0 )-g′(m) are connected to the source terminals s′(y 0 )-s′(m), which are biased to the ground voltage. The transistors  235 ( 0 )- 235 (y 0 −1) of the array  215  are instead on, since the gate terminals g′( 0 )-g′(y 0 −1) are connected to the drain terminals d′( 0 )-d′(y 0 −1), which are biased to the supply voltage Vdd. Supposing that each transistor  235 (j) of the array  215  is dimensioned in such a way to absorb a current I 0  when the voltage difference between gate g′(j) and source s′(j) is equal to the supply voltage Vdd, the total current sunk by the array  215 , identified with the reference I y  is substantially proportional to (y 0 )·(I 0 ). 
         [0104]    As a consequence, in order to trace back the pair of coordinates (x=x 0 , y=y 0 ) corresponding to a die  110   b  comprising an external index  140  of the type previously described, it is sufficient to properly bias the source line  270  and the drain lines  250 ,  260 , and measure the currents sunk by the two arrays. Particularly, by knowing the value of the current I 0  sunk by each transistor, the coordinate x 0  is retrieved by dividing the measured value of the current I x  by the value of the current I 0 . In the same way, the coordinate y 0  is retrieved by dividing the measured value of the current I y  by the value of the current I 0 . 
         [0105]    Making reference to the example illustrated in  FIG. 5 , the current I x  sunk by the array  205  is given by the current sunk by the transistors  215 ( 0 ) and  215 ( 1 ), and it is thus equal to twice the value of the current I 0 , while the current I y  sunk by the array  210  is given by the current sunk by the transistors  235 ( 0 ),  235 ( 1 ) and  235 ( 2 ), and it is thus equal to three times the value of the current Io. Dividing the two measured values by the value of the current I 0 , it is deduced that in this case x 0  is equal to 2 and y 0  is equal to 3. 
         [0106]    According to an embodiment of the present invention, the biasing voltages may be provided from the outside of the die  110   b  trough proper probes (not illustrated in figure) adapted to cooperate with the contact pads connected to the external index  140 . In this case, a first probe is placed in contact with the contact pad  298  for providing the ground voltage to the source line  270 , a second probe is placed in contact with the contact pad  294  for providing the supply voltage Vdd to the drain line  250  of the array  205  and a third probe is placed in contact with the contact pad  296  for providing the supply voltage Vdd to the drain line  260  of the array  210 . The current I x  sunk by the array  205  is measured by measuring the current flowing through the probe in contact with the contact pad  294 , while the current I y  sunk by the array  210  is measured by measuring the current flowing through the probe in contact with the contact pad  296 . 
         [0107]    An embodiment has several advantages over the use of visible indexes. Indeed, the external index  140  may be read without having to open the package that encapsulate the examined die  110   b , since the reading is performed by measuring a current flowing in a probe that is in electric contact with the contact pads, which contact pads may be accessed from the outside even in presence of packages encapsulating the die. 
         [0108]    Moreover, being that it is not necessary to perform any visual inspection, since current measurements are sufficient, feasible through devices such as amperometers, an embodiment may be implemented in an automatic way. In this way it is possible to drastically reduce the costs—even in terms of employed time—of the dies inspection operations. 
         [0109]    An embodiment may also be advantageous with respect to the known indexing methods which make use of non volatile memory elements adapted to store the information regarding the coordinates. Indeed, an embodiment does not require that memory cells are integrated in the dies, and thus does not require any expensive dedicated process. Moreover, since an embodiment requires only a current measure, feasible through a reading device which is outside the die—for example, an amperometer connected to a probe—the active area occupation of the die may be reduced with respect to the case exploiting memory cells, since in order to read the information included in such memory cells, proper reading devices (to be integrated in the die) are required. 
         [0110]    An additional advantage offered by an embodiment is that the internal index  140  may be considered a visible index, too, since the pair of coordinates (x,y) may by deduced through visible inspection by checking the positions of the interruptions on the gate lines  230 ,  245  of the arrays. In this way it is possible to retrieve the pair of coordinates (x,y) of the die  110   b  even in presence of defects that hinder one from carrying out the previously described current reading. 
         [0111]    In order to further reducing the die&#39;s area occupied by the external index  140 , it may be possible to not provide the use of dedicated contact pads for biasing the drain lines  250 ,  260  and for reading the current—such as the contact pads identified in the figures with the references  296 ,  294 —but instead using contact pads of the integrated circuit  120  by means of proper switching circuits. 
         [0112]    Although in a previously described embodiment the electronic components forming the arrays  205 ,  210  of the external index  140  are n-channel MOS transistors, these electronic components may be of a different type, such as p-channel MOS transistors, npn or pnp bipolar transistors, or resistors. 
         [0113]    For example, according to an alternative embodiment of the present invention, the external index  140  may be formed by arrays of bipolar transistors.  FIG. 7  shows the equivalent circuit  700  of a generic array formed by bipolar transistors, wherein the emitter terminals are connected to each other by means of an emitter line for receiving a ground voltage and the collector terminals are connected to each other by means of a collector line for receiving the supply voltage Vdd. In the same way as for the MOS transistors case, the base terminals of each array are connected to each other by means of a common base line—for example, implemented in a metallic layer—having an end connected to the collector line, and the other end connected to the emitter line; during a particular stage of the manufacturing process, such line is interrupted in a point determined by the coordinates (x,y) of the die. Consequently, in the same way as for the previously described MOS transistors case, the pair of coordinates (x,y) may be read by measuring the total current sunk by each array; such array depends on the number of bipolar transistors of the array having the base terminals connected to the collector line. 
         [0114]    According to a further embodiment of the present invention, the external index  140  is formed by arrays of resistors having the same resistance value, such as it is illustrated in  FIG. 8  with the reference  800 . In this case the resistors of each array have a first terminal connected to a first common line for receiving a reference (e.g.) ground voltage, and a second terminal connected to a second common line—for example formed in a metallic layer—for receiving the supply voltage Vdd; during a particular stage of the manufacturing process, such second line is interrupted in a point determined by the coordinates (x,y) of the die. In this way, the pair of coordinates (x,y) may be read by measuring the current sunk by each array; such current depends on the number of resistors of the array having the second terminal at the supply voltage Vdd. 
         [0115]    According to an embodiment of the present invention, even the internal index  150  of the dies  110   b , i.e., the index that allows to identify the position of the die itself with respect to the mask  105  may be implemented in a way similar to the internal index  140 . Particularly, each internal index  150  may be implemented by means of a double array of electronic devices, such as MOS transistors, associating to each pair of such electronic devices a particular position within the mask  105 . 
         [0116]    Without entering in details, being similar to the case of the external index  140 , a possible implementation of the internal indexes  150  of a die  110   b  is illustrated in  FIG. 9 . Particularly,  FIG. 9  makes reference to the example previously considered, wherein each mask  105  comprises four images  110   a  for the manufacturing of four dies  110   b  on the wafer  100 . Since in this case the images  110   a  of a mask  105  are positioned according to a matrix arrangement, and particularly according to a matrix having two rows and two columns, a possible implementation of the external indexes  150  may provide the use of a double array formed by two MOS transistors. Particularly, with the reference  910  it is indicated a possible implementation of the internal index  150  corresponding to the image  110   a  positioned at the top left in the mask  105 , with the reference  920  the one corresponding to the image positioned at the top right, with the reference  930  the one corresponding to the image positioned at the bottom right, and with the reference  940  the one corresponding to the image at the bottom left. In way similar to that of the external index  140 , the line connecting the gate terminals of the transistors of each array is interrupted in a point determined by the position of the image  110   a  within the mask  105 ; such position may be thus identified by carrying out a measurement of the total current sunk by each array of the internal index  150 . Analogous considerations may be applied in case the transistor&#39;s array of an external index are arranged in another way, for example with the arrays that are parallel to each other. Indeed, in order to form the internal index  150  it is not necessary to use an additional service mask provided with L shaped windows and positioning the two arrays in perpendicular directions, since the internal indexes  150  are replicated in an identical way at each step by only using the second reference images  180  of the mask  105 . 
         [0117]    Naturally, in order to satisfy local and specific requirements, one may apply to the embodiments described above many modifications and alterations. Particularly, although the present invention has been described with a certain degree of particularity with reference to embodiment(s) thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible; moreover, it is expressly intended that specific elements and/or method steps described in connection with any disclosed embodiment of the invention may be incorporated in any other embodiment as a general matter of design choice. 
         [0118]    For example, in order to simplify the operations during the indexing stage, the windows  320  of the service mask may be superimposed on the gates of the transistors of the arrays  205  and  210 . In this way, the interruptions xi, yj on the gate lines  230  and  245  may be formed with an increased ease, by entirely removing the gate terminals of the transistors corresponding to the coordinates to be registered. 
         [0119]    According to the convention employed in the exemplary embodiments described in the present document, the current I x  sunk by the array  205  of an external index  140  corresponding to the coordinate x=x 0  is equal to (x 0 )(I 0 ), while the current I y  sunk by the array  210  of an external index  140  corresponding to the coordinate y=y 0  is equal to (y 0 )(I 0 ). As a consequence, if x 0 =0, the current I x  sunk by the array  205  is null; in the same way, if y 0 =0, the current I y  sunk by the array  210  is null. However, such null values of current may be determined by possible defects in the control and/or biasing circuits of the die. As a consequence, in order not to confuse a current having a null value due to the pair of coordinates (0,0) with a current having a null value due to defects in the control and/or biasing circuits of the die, a possible solution may provide to employ a different convention, for example by associating to the coordinates of values equal to zero current values I x , I y  different than zero, and decreasing such current values as the coordinates values increase (without reaching null current values). In order to implement a convention of such type it may be sufficient to modify the structure of the external indexes  140  previously described, inverting the position of the contacts among the drain/source lines and the gate lines, i.e. contacting the drain line  250  to the gate line  230  at the end adjacent to the transistor associated with the highest coordinate of the array  205  (in the example at issue, the transistor  215 ( 3 )), and contacting the source line  280  to the gate line  230  at the end adjacent to the transistor associated to the lowest coordinate of the array  205  (in the example at issue, the transistor  215 ( 0 )). Similar considerations apply to the array  210 . 
         [0120]    Such a die may be packaged to form an integrated circuit, and may be coupled to another die that is part of the same integrated circuit, or that is part of a different integrated circuit, to form a system. For example, the other die may form at least part of a controller. 
         [0121]    From the foregoing it will be appreciated that, although specific embodiments have been described herein for purposes of illustration, various modifications may be made without deviating from the spirit and scope of the disclosure. Furthermore, where an alternative is disclosed for a particular embodiment, this alternative may also apply to other embodiments even if not specifically stated.