Abstract:
A die has a positional location in a wafer defined by first and second coordinates, the first and second coordinates identifying a respective horizontal and vertical location where the die was formed. An index formed on the die has a first comb structure of a first contiguous arrangement of first dots, and a second comb structure of a second contiguous arrangement of second dots. A first marker at a selected one of the first dots indicates a first digit of the first coordinate, and a first additional marker at a selected one of the first dots indicates a second digit of the first coordinate. A second marker at a selected one of the second dots indicates a first digit of the second coordinate, and a second additional marker at a selected one of the second dots indicates a second digit of the second coordinate.

Description:
RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 12/900,994, filed on Oct. 8, 2010, entitled “Indexing of Electronic Devices with Multiple Weight Markers,” which claims the priority benefit of Italian patent application number MI2009A001728, filed on Oct. 9, 2009, entitled “Indexing of Electronic Devices with Multiple Weight Markers,” both of which are hereby incorporated by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The solution according to one or more embodiments of the present invention generally relates to the electronic field. More specifically, this solution relates to the indexing of electronic devices. 
         [0003]    Electronic devices are generally integrated in dice, which are formed in a large number of portions of a wafer. Particularly, in a production process of the stepper shot type, each stage of the production process is not performed concurrently on the whole wafer, but step by step repeating the same operations on different shot areas thereof (by moving a smaller photolithographic mask accordingly). 
         [0004]    In this context, it is of the utmost importance to be able to determine the original position of the dice of the electronic devices in the wafer (before their separation). For example, this information is very useful for a quality management of the production process. Indeed, several characteristics of the electronic devices (for example, their functional parameters, performance and reliability) depend significantly on the position of the corresponding dice in the wafer (for example, because of changes in the crystallographic structure of the wafer through its extent). Therefore, when some electronic devices are subject to failures during their operation and are then returned to a corresponding manufacturer, the knowledge of their position in the wafer facilitates the analysis of the failures and the development of corresponding improvements in their production process. The same information may also be useful during a test of the electronic devices at the wafer level, known as Electrical Wafer Sorting (EWS). Indeed, in this way it is possible to store the position of any defective electronic devices that did not pass the test, so that they may be identified and discarded after the corresponding dice have been cut (without the need of marking the dice of the defective electronic devices during the test to discriminate them—for example, with ink dots). 
         [0005]    For this purpose, it is known in the art to form an index on each die (when it is still included in the wafer), which index indicates the position of the die in the wafer. Particularly, in the case of the production process of the stepper shot type, the index has a composite structure with a shot index (indicating the position of the corresponding shot area in the wafer) and a die index (indicating the position of the die in the corresponding shot area). Typically, each (shot and die) index is defined by a row index and a column index, which define a row coordinate and a column coordinate, respectively, in a corresponding matrix. Particularly, a generic index may be implemented with a ruler (for example, being formed in a surface metallic layer of the die), which ruler defines an ordered alignment of locations (referred to as dots) each one associated with a corresponding number; a marker selects a specific dot (for example, by means of the erasure of a corresponding portion of the metallic layer), and then the corresponding number. In this case, the die indexes and the rulers of the shot indexes of all the dice may be formed during a selected stage of the production process by means of a corresponding mask (which replicates the same structures in the different shot areas); the markers of the shot indexes are instead formed by exploiting an additional mask (designed to form them at the same position in all the corresponding dice), which additional mask is however slightly displaced at every shot so as to form these markers at different positions in every shot area. An example of the above-mentioned indexing technique is described in United States Patent Publication No. 2008/0153250, which is hereby incorporated by reference. 
         [0006]    A drawback of the indexing techniques known in the art is their limitation in the number of dice that can be indexed in the same wafer. Indeed, the dots of the rulers cannot be smaller than a minimum size (for example, 1 μm×1 μm), in order to allow their correct inspection; in addition, the area of the dice that can be used to form each ruler is constrained (for example, with a length of 15 μm). As a result, the maximum value of each row and column index is relatively low (about 15 μm/1 μm=15), with a corresponding limitation in the range of each shot and die index (15×15=225 in the example at issue). This drawback is particularly acute in the die indexes, since the modern production processes easily exceed the above-mentioned number of dice in each shot area; in this case, it is not possible to implement any indexing of the dice (with a detrimental effect on the quality of the corresponding production process). 
         [0007]    Different indexing techniques are also known in the art. For example, in Japanese Application 10012527, hereby incorporated by reference, each index is represented with a binary code; the bits of the index are defined in corresponding locations (arranged along a straight line) to the value 1 in presence of a predefined slit or to the value 0 in its absence. The same document also describes other embodiments wherein the index is represented with a number in a base higher than 2; particularly, the digits of the index are represented by the length multiple of a predefined value of corresponding teeth (in this case, with the index that has a variable length), or by the depth multiple of a predefined value of slits at the corresponding locations, with the value 0 that is represented by the absence of any slit (in this case, with the slits that extend transversally to the arrangement of the locations). 
         [0008]    Alternatively, in Japanese Application 61142734, incorporated by reference, each bit of the index is represented by the length of a corresponding bar (a long bar for the value 0 and a short bar for the value 1). 
         [0009]    Moreover, in United Stated Patent Application No. 2003/127718, incorporated by reference, each bit of the index is represented at a corresponding location by the presence or the absence of a recess. 
       SUMMARY OF THE INVENTION 
       [0010]    In its general terms, the solution according to one or more embodiments of is based on the idea of using more markers with different weights (for the definition of a corresponding index). 
         [0011]    Particularly, one or more aspects of a solution according to specific embodiments are set out in the independent claims. Advantageous features of the same solution are set out in the dependent claims. 
         [0012]    More specifically, an aspect of a solution according to an embodiment comprises an electronic device, which includes a die integrating an electronic circuit. The die has at least one index; the index includes a reference defining an ordered alignment of a plurality of locations on the die (for example, a ruler with a comb-like structure), and marker means for defining a value of the index according to an arrangement of the marker means with respect to the reference (in other words, the locations are placed in a straight linear arrangement, wherein a line best fitting them is straight). In the solution according to an embodiment, the marker means includes a plurality of markers each one arranged at a selected one of the locations; the selected location of the marker defines a value of a digit associated with a corresponding power of a base (higher than 2) within a number in a positional notation in said base, which number represents the value of the index (for example, in a decimal notation with a marker for the units digit and a marker for the tens digits, each one associated with a corresponding subset of adjacent locations along the ruler). 
         [0013]    A different aspect of a solution according to an embodiment comprises a corresponding indexing method (with the same advantageous features recited in the dependent claims for the electronic device that apply mutatis mutandis to the method). 
         [0014]    A further aspect of a solution according to an embodiment comprises a software program including code means for causing a processing system (for example, a stepper) to perform the steps of this method when the software program is executed on the processing system; a still further aspect of the solution according to an embodiment proposes a software program product including a non-transitory computer readable medium embodying a software program, the software program including code means directly loadable into a working memory of a processing system thereby configuring the processing system to perform the same method. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    A solution according to one or more embodiments, as well as further features and the advantages thereof, will be best understood with reference to the following detailed description, given purely by way of a non-restrictive indication, to be read in conjunction with the accompanying drawings (wherein corresponding elements are denoted with equal or similar references and their explanation is not repeated for the sake of brevity, and the name of each entity is generally used to denote both its type and its attributes—such as its value, content and representation—for the sake of simplicity). Particularly: 
           [0016]      FIG. 1  schematically shows a wafer at an intermediate stage of a production process of electronic devices to which the solution according to an embodiment may be applied; 
           [0017]      FIG. 2  shows an enlarged portion of an electronic device implementing an indexing scheme known in the art, and 
           [0018]      FIGS. 3A-3E  show an enlarged portion of different examples of an electronic device implementing an indexing scheme according to an embodiment. 
       
    
    
     DETAILED DESCRIPTION 
       [0019]    With reference in particular to  FIG. 1 , there is schematically shown a wafer  105  at an intermediate stage of a production process of electronic devices, to which a solution according to an embodiment may be applied. The wafer  105  comprises of a circular slice (for example, with a radius of 8 inches), which is mainly made of semiconductor material; an identical electronic circuit  110  is integrated in a large number of portions of the wafer  105  (for example, from some hundreds to some thousands). The production process of these integrated circuits  110  is executed in a sequence of stages, each one involving the patterning of one or more exposed layers of the wafer  105  (for example, made of semiconductor, insulating, and/or conductive material) by means of a corresponding photolithographic mask. Particularly, in a production process of the stepper shot type, at each stage of the production process a mask being smaller than the wafer  105  is used to pattern a corresponding shot area  115  thereof, which only includes a subset of the integrated circuits  110  that are patterned concurrently (with a single shot) according to this mask. The wafer  105  is then moved so as to position another shot area  115  thereof under the mask, and the same operations are repeated for the corresponding integrated circuits  110 . The process is reiterated until the whole wafer  105  has been patterned. At the end of the production process, the portions of the wafer  105  (wherein the desired integrated circuits  110  are formed) are cut by means of a sawing operation so as to obtain corresponding dice  120 . 
         [0020]    Each die  120  also includes (in addition to the corresponding integrated circuit  110 ) an index  125 , which is used to indicate a position of the die  120  in the wafer  105  uniquely. Typically, the index  125  is formed in one of the last layers of the wafer  105 , so as to be visible (directly or indirectly) with a non-invasive inspection (for example, optically by means of a low power microscope). Particularly, when the above-described production process of the stepper shot type is used, this (global) index  125  has a composite structure with a shot index  125   s  (which identifies the position of the corresponding shot area  115  in the wafer  105 ) and a die index  125   d  (which identifies the position of the die  110  in the corresponding shot area  115 ). The die index  125   d  (being equal for all the dice  110  in the same position within the different shot areas  115 ) may be formed during a selected stage of the production process by means of a corresponding mask (which replicates the same die indexes  125   d  in the different shot areas  115 ). Conversely, the shot index  120   s  (changing for the different shot areas  115 ) is formed by exploiting an additional (service) mask, which creates one or more markers at the same position in all the corresponding dice  120 . At every shot, the wafer  105  is slightly displaced with respect to the service mask, so that the markers move accordingly in the dice  120  of the corresponding shot area  115 ; in this way, the position of the markers in each die  120  (with respect to a predefined reference being formed thereon) allows distinguishing the different shot areas  115 . These operations are typically controlled by a software program that manages operation of a stepper being used to create the electronic circuits  110  in the wafer  105  (for example, being installed on a non-volatile memory of a corresponding control unit, for example, from a removable storage device, and loaded at least in part into its working memory when this control program is running). 
         [0021]    Typically, the shot areas  115  are arranged in a matrix with Rs rows and Cs columns (for example, Rs=Cs=6-12); therefore, each shot area  115  may be identified in the corresponding shot index  125   s  by a pair of row and column coordinates. Likewise, the dice  120  are arranged in a matrix with Rd rows and Cd columns (for example, Rd=Cd=10-100); therefore, each die  120  may be identified in the corresponding die index  125   d  by another pair of row and column coordinates. For example, the specific die  120  being enlarged in the figure is identified by the shot index  125   s =(2,3)—to indicate the shot area  115  in the 2nd row and the 3rd column of the wafer  105 —and by the die index  125   d =(4,5)—to indicate the die  120  in the 4th row and the 5th column of this shot area  115 . 
         [0022]    An enlarged portion of an electronic device  200  implementing an indexing scheme is shown in  FIG. 2 . The electronic device  200  includes an integrated circuit  210  that is formed in a die  220 ; the position of the die  220  in the corresponding wafer (not shown in the figure) is indicated by a (global) index, which includes a (row shot) index  225 Rs and a (column shot) index  225 Cs—indicating the position of the corresponding shot area in the wafer by means of its row and column, respectively—and a (row die) index  225 Rd and a (column die) index  225 Cd—indicating the position of the die  210  in the corresponding shot area by means of its row and column, respectively. 
         [0023]    More in detail, each index  225 Rs, 225 Cs, 225 Rd, 225 Cd includes a reference (for example, made of a reflective material such as metal); this reference is formed by a ruler  230 R with a comb-like structure for the (row) indexes  225 Rs and  225 Rd, and by another ruler  230 C for the (column) indexes  225 Cs and  225 Cd—for example, being obtained by patterning a metal layer of the die  220  with the mask being used at the corresponding stage of its production process. The rulers  230 R and  230 C are arranged at a corner of the die  220 ; particularly, the rulers  230 R and  230 C extend perpendicularly to each other (in parallel with corresponding borders of the die  220 ) from an origin pad  235 . 
         [0024]    Each ruler  230 R and  230 C is used to measure a linear distance from the origin pad  235  (along a straight line extending in parallel with the corresponding border of the die  220 ). For this purpose, each ruler  230 R, 230 C is formed by an elongated spine  240 R, 240 C and a plurality of teeth  245 R, 245 C, which project transversally from the spine  240 R, 240 C (outwards); a portion of the spine  240 R, 240 C between each pair of adjacent teeth  245 R, 245 C (or after a last tooth  245 R, 245 C being distal from the reference pad  235 ) defines a corresponding inter-tooth  250 R, 250 C. A separation pad  255 R, 255 C splits the (global) ruler  230 R, 230 C into a shot ruler  230 Rs, 230 Cs for the (shot) index  225 Rs, 225 Cs and a die ruler  230 Rd, 230 Cd for the (die) index  225 Rd, 225 Cd; particularly, the ruler  230 Rs, 230 Cs extends between the origin pad  235  and the separation pad  255 R, 255 C, while the ruler  230 Rd, 230 Cd extends from the separation pad  255 R, 255 C away from the origin pad  235 . The teeth  245 R, 245 C and the inter-teeth  250 R, 250 C of each ruler  230 Rs,  230 Rd,  230 Cs, and  230 Cd define an ordered alignment of locations thereof (referred to as dots), each one associated with a corresponding number; in the example shown in the figure, each ruler  230 Rs, 230 Rd, 230 Cs, 230 Cd includes 15 dots (numbered from 1 to 15)—moving upwards from the origin pad  235  for the ruler  230 Rs, upwards from the separation pad  255 R for the ruler  230 Rd, leftwards from the origin pad  235  for the ruler  230 Cs, and leftwards from the separation pad  255 C for the ruler  230 Cd. 
         [0025]    A marker  260 Rs,  260 Rd,  260 Cs, and  260 Cd is used to select a corresponding dot (and then its number) in the ruler  230 Rs,  230 Rd,  230 Cs, and  230 Cd, respectively. Each marker  260 Rs, 260 Rd, 260 Cs, 260 Cd is defined by the erasure of the corresponding dot—i.e., the missing of the corresponding tooth  245 R, 245 C or inter-tooth  250 R, 250 C that exposes an opaque material (below the reflective material of the ruler  230 Rs, 230 Rd, 230 Cs, 230 Cd). The markers  260 Rd and  260 Cd for the (die) indexes  225 Rd and  225 Cd, respectively, may be formed by selectively removing the metal layer of the corresponding rulers  230 R and  230 C with an additional mask (producing different markers  260 Rd, 260 Cd in each die  220  of the shot area, with the same markers  260 Rd, 260 Cd that are repeated in the same positions of the different shot areas); the markers  260 Rs and  260 Cs for the (shot) indexes  225 Rs and  225 Cs, respectively, may likewise be formed by selectively removing the metal layer of the corresponding rulers  230 R and  230 C with a suitable service mask that is slightly displaced at every shot (producing the same markers  260 Rs, 260 Cs in all the dice  220  of the shot area, with the markers  260 Rs, 260 Cs that change in the different shot areas). For example, in the figure the marker  260 Rs selects the dot 11 (for the index  225 Rs) and the marker  260 Cs selects the dot 10 (for the index  225 Cs)—so as to define the shot index (11,10); moreover, the marker  260 Rd selects the dot 2 (for the index  225 Rd) and the marker  260 Cd selects the dot 4 (for the index  225 Cd)—so as to define the die index (2,4). 
         [0026]    An enlarged portion of different examples of an electronic device  300  implementing an indexing scheme according to an embodiment is shown in  FIG. 3A-FIG .  3 E. With reference in particular to  FIG. 3A , the electronic device  300  includes an integrated circuit  310  that is formed in a die  320 ; the position of the die  320  in the corresponding wafer (not shown in the figure) is indicated by a (global) index, which includes the same reference being formed by the rulers  230 R and  230 C. As above, the global index includes the (row shot) index  225 Rs and the (column shot) index  225 Cs. However, the global index now includes a (row die) index  325 Rd and a (column die) index  325 Cd each one defined by multiple markers, for corresponding digits that define a positional notation in a predefined base higher that 2. 
         [0027]    In this way, it is possible to increase the maximum value of each index  325 Rd and  325 Cd, and then the range of the whole index  325 Rd, 325 Cd, for the same size of the rulers  230 Rd and  230 Cd, respectively (or, vice-versa, it is possible to reduce the size of the rulers  230 Rd and  230 Cd for the same maximum value of the indexes  325 Rd and  325 Cd). The proposed solution thus allows indexing the dice  320  even when they are formed in large number in each shot area of the wafer (with a beneficial effect on the quality management of the corresponding production process). 
         [0028]    It is emphasized that this result is achieved (by simply updating the control program of the stepper) without the need of modifying the rulers  230 R and  230 C. Therefore, the proposed solution only requires changing the mask for the new markers (whereas it is possible to continue using the other masks, with a corresponding reduction of the implementation cost). 
         [0029]    Particularly, in an embodiment of the invention each index  325 Rd, 325 Cd is defined by a number in a decimal notation (i.e., with the base is equal to 10). Each number in base 10 is represented with an ordered sequence (from the right to the left) of the digits, which can take any value from 0 to 10−1=9; the value of the number is then defined by the sum of its digit values, each one multiplied by a corresponding power of the base 10 (10.sup.0 for the first digit of the units, 10 1  for the second digit of the tens, 10 2  for the third digit of the hundreds, and so on). In the specific example at issue, the index  325 Rd, 325 Cd includes two digits (one for the units and another one of the tens). 
         [0030]    In order to represent the different digits of the index  325 Rd, 325 Cd, the ruler  230 Rd, 230 Cd is logically partitioned into a component  330 Rdu, 330 Cdu (referred to as units ruler) for the units digits, and a component  330 Rdt, 330 Cdt (referred to as tens ruler) for the tens digits. The ruler  330 Rdu, 330 Cdu includes 9 dots for all the possible non-null value of the units digits from 1 to 9; the ruler  330 Rdt, 330 Cdt includes the remaining 6 dots for the first non-null values of the tens digits from 1 to 6. 
         [0031]    A marker  360 Rdu,  360 Rdt,  360 Cdu, and  360 Cdt is used as above to select a corresponding dot in the ruler  330 Rdu,  330 Rdt,  330 Cdu, and  330 Cdt, respectively; each marker  360 Rdu, 360 Rdt, 360 Cdu, 360 Cdt then selects the corresponding digit value 1-9, while the digit value 0 is represented by the lacking of the marker  360 Rdu, 360 Rdt, 360 Cdu, 360 Cdt in the ruler  330 Rdu, 330 Rdt, 330 Cdu, 330 Cdt. Particularly, the marker  360 Rdu, 360 Cdu selects the units digit of the index  325 Rd, 325 Cd, while the marker  360 Rdt, 360 Cdt selects the tens digit of the index  325 Rd, 325 Cd. Therefore, in the specific example at issue (wherein the tens digit value ranges from 0 to 6), the index  325 Rd, 325 Cd can take any value from 0 to 69. The proposed implementation thus increases the maximum value of the index  325 Rd, 325 Cd (for the same ruler  230 Rd, 230 Cd), with respect to the above-described indexing techniques known in the art being based on a single marker, by (69−15)/16=360%. For example, in the figure the marker  360 Rdu selects the dot of the units digit value 8, while the marker  360 Rdt selects the dot of the tens digit 3—so as to define the index  325 Rd=38; likewise, the marker  360 Cdu selects the dot of the units digit value 3, while the marker  360 Cdt selects the dot of the tens digit value 2—so as to define the index  325 Cd=23. 
         [0032]    A different value of the index  325 Rd, 325 Cd of the same die  300  is illustrated in  FIG. 3B . In this case, the marker for the ruler  330 Rdu is lacking to select the units digit value 0, while the marker  360 Rdt selects the dot of the tens digit value 1—so as to define the index  325 Rd=10; the marker  360 Cdu instead selects the dot of the units digit value 7, while the marker  360 Cdt selects the dot of the tens digit value 5—so as to define the index  325 Cd=57. 
         [0033]    Moving to  FIG. 3C , the marker  360 Rdu selects the dot of the units digit value 9, while the marker for the ruler  330 Rdt is lacking to select the tens digit value 0—so as to define the index  325 Rd=9; the marker  330 Cdu selects the dot of the units digit value 5, while the marker  360 Cdt selects the dot of the tens digit value 3—so as to define the index  325 Cd=35. 
         [0034]    With reference to  FIG. 3D , the marker  360 Rdu selects the dot of the units digit value 7, while the marker for the ruler  330 Rdt is lacking to select the tens digit 0—so as to define the index  325 Rd=7; the marker for the ruler  330 Cdu is lacking to select the units digit value 0, while the marker  360 Cdt selects the dot of the tens digit value 1—so as to define the index  325 Cd=10. In this specific case (i.e., when both the indexes  325 Rd and  325 Cd are lower than or equal to 10) a single marker (i.e., for the units digits for values from 1 to 9, or for the tens digits for the value 10) is present in each ruler  330 Rd, 330 Cd; therefore, the indexing of the die  320  is exactly the same as in the dice known in the art. 
         [0035]    At the end, in  FIG. 3E  the marker for the ruler  330 Rdu is lacking to select the units digit value 0, while the marker  360 Rdt selects the dot of the tens digit value 6—so as to define the index  325 Rd=60; the marker for the ruler  330 Cdu is likewise lacking to select the units digit value 0, while the marker  360 Cdt selects the dot of the tens digit value 3—so as to define the index  325 Cd=30. In this specific case (i.e., when both the indexes  325 Rd and  325 Cd are equal to a power of 10 being 2) a single marker for the tens digits is present in each ruler  330 Rd, 330 Cd; therefore, the index  325 Rd, 325 Cd has again exactly the same structure as in the dice known in the art, but it is now decoded in a different way. 
         [0036]    As a further improvement, it is also possible to encrypt the index with a (secret) encryption key; in this way, the position of the die in the wafer can be recovered only by decrypting the index with the encryption key. This additional feature avoids making the position of the die public, so that this information is available only to authorized persons knowing the encryption key. 
         [0037]    Particularly, in an embodiment the index is encrypted with a simple substitution algorithm, wherein each digit value of the index is replaced with another digit value according to a (secret) substitution alphabet—i.e., with each digit value Di (with i from 0 to the base of the positional notation minus 1) that is replaced with the i th  digit value in the substitution alphabet. For example, the substitution alphabet 5942610387 indicates that the digit values 0, 1, 2, 3, 4, 5, 6, 7, 8 and 9 are replaced by the digit values 5, 9, 2, 4, 6, 1, 0, 3, 8 and 7, respectively:
       0 1 2 3 4 5 6 7 8 9   5 9 4 2 6 1 0 3 8 7       
 
         [0040]    Therefore, as an example, the index value 50 is encrypted to 15. The same operations described above are then repeated in reverse order to recover the actual value of the index from its encrypted version; for example, a row die index equal to 32 and a column die index equal to 07 indicates that the die is at the 73th row and at the 69th column in the corresponding shot area. 
         [0041]    Naturally, in order to satisfy local and specific requirements, a person skilled in the art may apply to the solution described above many logical and/or physical modifications and alterations. More specifically, although this solution has been described with a certain degree of particularity with reference to one or more embodiments thereof, it should be understood that various omissions, substitutions and changes in the form and details as well as other embodiments are possible. Particularly, different embodiments of the invention may even be practiced without the specific details (such as the numerical examples) set forth in the preceding description to provide a more thorough understanding thereof; conversely, well-known features may have been omitted or simplified in order not to obscure the description with unnecessary particulars. Moreover, it is expressly intended that specific elements and/or method steps described in connection with any embodiment of the disclosed solution may be incorporated in any other embodiment as a matter of general design choice. 
         [0042]    For example, similar considerations apply if each electronic device has a different structure or includes equivalent components (either separate to each other or combined together, in whole or in part); particularly, the electronic device may be of the micro-mechanical type, of the opto-electronic type, and the like; the electronic device may also be in the form of a package (including one or more dice), or even in the form of a mere bare die. 
         [0043]    Moreover, the wafer may be made of another material or it may include a different number of dice. Likewise, the indexes may be formed in another position on the die or with other techniques (for example, with direct writing techniques by means of a laser); the indexes may also be inspected with alternative procedures (even when they are not optically visible)—for example, by means of electromagnetic radiations (such as X-rays, infrared or ultraviolet light), or particle beams (such as electron beams). 
         [0044]    Even though in the preceding description reference has been made to two markers for each ruler, this is not to be interpreted in a limitative manner (with the same concepts that also apply to three or more markers). 
         [0045]    Similar considerations apply if the indexes have a different range (for example, by providing a different number of dots for the tens digits, or even by adding further dots for the hundred digits, and so on). The same technique may also be applied to indexes that are represented with numbers in any other positional notation with any base higher than 2. 
         [0046]    Consequently, the portion of the ruler dedicated to each marker may include any number (&gt;=2) of adjacent locations (down to 2 locations for the digit values 1 and 2 when the base is equal to 3 and the digit value 0 is represented by the lacking of the corresponding marker). An alternative implementation is also feasible wherein the markers have different representations on a common ruler (for example, with the marker for the units digits and the marker for the tens digits that are arranged at opposite sides of the ruler). 
         [0047]    Nothing prevents providing a specific dot for the digit 0 as well. Moreover, in a different implementation of the invention, 10 dots are used for the units digits (from 1 to 10); in this case, the values of the index up to 10 are represented only by the units marker, with the tens marker that is then used for higher values thereof. 
         [0048]    The above-described implementation of the indexes based on the rulers is merely illustrative, and it should not be interpreted in a limitative manner; indeed, similar considerations apply if the indexes are simply defined by the distance of the markers from a predefined reference (which may also comprise of an edge of the die). 
         [0049]    More generally, the ruler may be implemented with any other structure capable of measuring a linear distance from a corresponding origin indicator, which ruler is provided with distance indicators in a straight linear arrangement from the origin indicator, each one defining a corresponding location of the ruler; for example, the ruler may be implemented with the ruler may be implemented with a sequence of small segments each one representing a corresponding dot). Likewise, the markers may be defined in any other way, for example, by deleting all the dots up to the selected one, or by any other sign that is added to the ruler (for example, in the form of a cross). 
         [0050]    In addition or in alternative, the same technique may also be applied to the shot index. In any case, the proposed solution lends itself to be used in standard production processes as well (wherein all the dice are formed concurrently in the whole wafer). 
         [0051]    Similar considerations apply if each index has a different structure (for example, comprising of a single number that directly defines the position of the die in the wafer). 
         [0052]    Moreover, the indexes may be encrypted with any other algorithm (for example, based on a shifted or reversed substitution, of the transposition type, and the like); naturally, this feature is merely optional and in no way limitative. 
         [0053]    It should be readily apparent that the proposed structure might be part of the design of the corresponding integrated circuits. The design may also be created in a programming language; moreover, if the designer does not fabricate dice or masks, the design may be transmitted by physical means to others. 
         [0054]    Moreover, the proposed electronic device may be mounted in intermediate products (such as mother boards), and/or coupled with one or more other electronic devices (such as a processor or a memory). In any case, the electronic device is suitable to be used in complex systems (such as mobile telephones). 
         [0055]    The proposed solution lends itself to be implemented with an equivalent method (by using similar steps, removing some steps being non-essential, or adding further optional steps); moreover, the steps may be performed in a different order, concurrently or in an interleaved way (at least in part). 
         [0056]    The above-described solution may be implemented as a stand-alone module, as a plug-in for the control program of the stepper, or even directly in the control program itself. Moreover, the control program may take any form suitable to be used by the control unit of the stepper (or by any other data processing system) or in connection therewith. In any case, the solution according to an embodiment of the present invention lends itself to be implemented even with a hardware structure (for example, integrated in a chip of semiconductor material), or with a combination of software and hardware. 
         [0057]    Such alterations, modifications, and improvements are intended to be part of this disclosure, and are intended to be within the spirit and the scope of the present invention. Accordingly, the foregoing description is by way of example only and is not intended to be limiting. The present invention is limited only as defined in the following claims and the equivalents thereto.