Abstract:
An electrical check executed on wafer tests for the correct positioning or alignment of the probes of a probe card on the pads or bumps of the electronic devices integrated on the wafer. A signal is applied to cause a current to circulate in at least part of a seal ring of at least one of the electronic devices. In a case where the current flows between and through multiple electronic devices, the seal rings of those electronic devices are suitably interconnected to each other by electronic structures that extend through the scribe line between electronic devices.

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
     This application is a continuation of U.S. application for patent Ser. No. 12/974,957 filed Dec. 21, 2010, now U.S. Pat. No. 9,146,273, which claims priority to Italian Patent Application No. MI2009A002332, filed Dec. 30, 2009, the foregoing applications incorporated herein by reference in their entireties. 
    
    
     TECHNICAL FIELD 
     An embodiment relates to a method for checking the correct positioning of test probes on pads of electronic devices integrated on a semiconductor. 
     An embodiment also relates to an electronic device integrated on a semiconductor wafer and equipped with means for the actuation of the above method. 
     An embodiment may be applied, in particular, to the process of electric selection of the wafers or testing EWS (Electrical Wafer Sort or more in general Wafer Sort) or for the WLBI (Wafer Level Burn-In) i.e. electric testing that executes the test (also in temperature) with stress on the devices subjected to the test. 
     BACKGROUND 
     As it is known in this specific technical field, for the final quality check, a process of electrical sorting of the semiconductor electronic devices is carried out by executing a testing EWS directly on wafer. To this aim it is often necessary to electrically connect a testing apparatus, called tester ATE, that executes the measures on the devices of the wafer. 
     This situation is schematically shown in  FIG. 1 . 
     Between the apparatus ATE and the wafer  1  there is an interface comprising a probe card  2  having an end  3  (probe head) with which also several hundreds (or thousands) of probes  4  are associated that electrically connect the ATE to the almost totality of the pads of the devices to be tested. 
     In general the wafer is positioned on a support  5  called chuck belonging to an apparatus called prober. 
     In consequence, the probes  4  of the ATE should be temporarily connected to all the pads  6  of the electronic device to be tested, as shown in  FIG. 2 , or of the devices since the testing in parallel of several electronic devices simultaneously is more and more frequent. 
     The number of probes  4  of the resources necessary for the testing may also be smaller or identical to the number of pads  6  present on the device. 
     In a similar way checks are carried out also on integrated circuits for which it is necessary to contact the respective bumps. 
     Within this domain it may be very important that the generic probe  4  is well centred on the corresponding pad  6 . 
     Last generation electronic devices have a very high number of pads to be contacted which have a more and more reduced area and are often very close to each other. 
     In particular the more a pad has a small size, the more probable the breakage of the passivation surrounding the pad is further to an incorrect contact by the relative probe. 
     It may be very important that all the probes are planar with each other on the vertical axis Z and have a high alignment with respect to the horizontal plane XY. This allows to reduce the damaging problems of the pad and breakages of the passivation. 
     At present the testing in production of the correct alignment between the generic probe and the pad may be made by observing the signs that the probe leaves on the pad after having contacted it, but this check occurs after having executed the testing step and is not preventive. 
       FIGS. 3B to 3D  show examples of signs left on a pad by the incorrect positioning or alignment of a test probe. 
     The testing operation may, however, be executed on the prober in a manual way or in an automatic way. 
     Apparatuses also exist that are different from the prober that test the alignment and the planarity of the probes of the probe card. 
     However, recently, for meeting the growing requests of electronic applications able to sustain higher and higher temperatures, new materials have been introduced for realizing the end pads of the electronic circuits and for the connections between these pads and the substrate of the package, which is the container of the circuit, so as to ensure a good electric connection. 
     Some of these materials for the pads are also used to strengthen the pad itself and in consequence these materials have a greater hardness than the materials traditionally used like the aluminum. 
     This implies that the signs left by the probe may not always be seen on these particular hard materials, even after different mechanical actions of contacting of the probes on the pads. 
     In consequence, a check in production of the correct centering of the probes on the pads may become difficult, requiring long times and with consequent breakage problems of the passivation that implies the production performance loss with a consequent economic damage. 
     The adoption of pads realized with hard metals thus may make it difficult to execute a process of reliable probing and in consequence it is possible to lose production electric performance with a consequent increase of the costs. 
     Production problems are added to all this, these problems further burdening the costs in case, for example, testing problems of a non-immediate solution arise, such as for example a bad electric contact between the probe and the pad. 
     SUMMARY 
     An embodiment is a process for checking the correct positioning of test probes on pads of electronic devices integrated on semiconductor having such functional characteristics as to allow to execute this check in a completely automatic way and by means of an electric testing, devices provided with pads realized with hard metals being also present. 
     An embodiment is based on the assumption that it is not possible to visually value the correct alignment between a generic probe and its corresponding pad; therefore, it has been thought to execute a check of the probing process counting on a methodology of the electric type. 
     An embodiment for checking the correct positioning of test probes on pads of electronic devices integrated on semiconductor wafer, in which a plurality of said probes associated with an interface of a testing apparatus are guided mutually approaching and departing from said pads of the electronic device/s for executing quality checks based on electric tests, includes at least one control probe chosen in said plurality of probes being put into contact with a corresponding additional control pad of the device subjected to the testing for executing a preliminary electric test and deriving an estimate of alignment or correct positioning of the other probes with the corresponding pads. 
     The preliminary electric test and contact step may be executed by means of a pair of control probes on a corresponding pair of additional control pads. 
     The control probes may be put into contact with the additional control pads prior to other probes and simultaneously with them. 
     The probes of said pair of control probes may be put across said plurality of probes or in positions being considered as critical in relation to the specific application so as to better detect a possible drift of the correct alignment of positioning. 
     Moreover, the electric test may provide the application of a current flow that crosses said pair of control probes; this electric test may be followed by means of a test circuit housed in a seal ring region of the electronic device. 
     An embodiment also relates to a semiconductor electronic device integrated with other devices on a substrate of a semiconductor wafer and comprising electric connection pads also intended for being contacts by a plurality of probes associated with an interface of a testing apparatus for quality checks based on electric tests, comprising at least one additional control pad specifically provided for being contacted by a control probe chosen among probes for executing a preventive electric test and deriving an estimate of alignment or correct positioning of the other probes. 
     The electronic device may comprise at least one pair of additional control pads arranged on opposite parts of said device. 
     More in particular, said additional control pads may be arranged on opposite angular regions of the electronic device. 
     Moreover, said additional control pads may be placed outside a seal ring delimiting, with respect to one another, said devices on said wafer. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       Characteristics and advantages of a process and of a device according to one or more embodiments will be apparent from the following description given by way of indicative and non-limiting example with reference to the figures of the annexed drawings, wherein: 
         FIG. 1  shows a schematic view of a wafer whereon semiconductor electronic devices are realized subjected to final quality checks by means of an interface of a testing apparatus ATE; 
         FIG. 2  shows a schematic view of an electronic device contacted by a plurality of probes of the interface of  FIG. 1 ; 
         FIG. 3A  is a schematic, enlarged view form above of tracks left by a correct contact of test probes on pads of an electronic device to be tested; 
         FIGS. 3B to 3D  are respective schematic, enlarged views from above of tracks left by a wrong contact of test probes on pads of an electronic device to be tested; 
         FIG. 4  shows a schematic view from above of a semiconductor wafer and of an interface of a testing apparatus ATE as modified according to an embodiment; 
         FIG. 5  shows a perspective, schematic view of an electronic device integrated on semiconductor and incorporating additional control pads according to an embodiment; 
         FIG. 6  shows a schematic, enlarged view from above of a group of electronic devices adjacent to each other on a semiconductor wafer according to an embodiment; 
         FIG. 7  shows a schematic, enlarged view from above of an electronic device and of the near structures according to an embodiment; 
         FIGS. 8 and 9  are further schematic, enlarged views from above of the group of devices of  FIG. 6  according to an embodiment; 
         FIG. 10  shows a perspective, schematic view of the group of electronic devices of  FIG. 9  incorporating two additional control pads according to an embodiment; 
         FIGS. 11A, 11B and 12  show respective schematic views of enlarged details of some electronic devices integrated on semiconductor wafer and structured according to an embodiment; 
         FIG. 13  shows a schematic view from above of a semiconductor wafer and of an interface of a testing apparatus ATE modified according to a further embodiment; 
         FIGS. 14 to 21  are respective schematic, enlarged views of angular regions of near electronic devices on the wafer of  FIG. 6  and relative to different embodiments; 
         FIGS. 22 to 25  are respective schematic, vertical section views of details of an electronic device according to an embodiment; and 
         FIGS. 26 and 27  are respective schematic views of alternative embodiments of a device. 
     
    
    
     DETAILED DESCRIPTION 
     With reference to the figures, and in particular to the examples of  FIG. 4  and following ones,  10  globally and schematically indicates a semiconductor wafer whereon several electronic devices  11  are integrated according to an embodiment. These devices  11  are arranged in an orderly way on the wafer and delimited with respect to one another by demarcation lines  12  so called scribe lines. 
     Each electronic device  11  may be electrically contacted by means of a plurality of peripheral pads  13 , in contact to which probes  4  of an interface  2  or probe card are guided mutually approaching and departing, the probe card being associated with a testing apparatus ATE, this latter being not shown since conventional. 
     As explained above, the presence of very hard pads  13  makes it difficult to identify a sign due to the contact of a probe  4  and, in consequence, it is difficult to value visually and in an easy way if the probing and testing process is occurring correctly. 
     In these circumstances it may be probable that breakages occur in the passivation protective layer that covers the electronic devices  11  to be tested; and devices with the broken passivation layer may have to be discarded. 
     According to an embodiment it has been provided that at least one control probe  24  chosen in the group or array  22  of probes  4  of the interface  15  is put into contact with an additional control pad  16  connected to a test circuit  17  associated with or internal to the corresponding device  11  through which a test procedure will test the correct contacting of the probe  24  on the pad  16 . On the basis of the result of this check, information will be derived about the fact that the contacting or probing process does not have serious drifts, i.e. that the process is under control. 
     Considering a generic array  22  of probes  4  of a probe card  2  that are intended for contacting at least one device  11  of the wafer  10 , it has been thought to use, for example, at least two control probes  24 ,  25  that may be placed at the ends of the array  22  of the probes  4  so as to be more sensitive and thus better detect a drift of the probing process. 
     In fact if the probing process has a drift, in general it will be more marked at the ends of the array  22  with respect to the other regions of the array  22  of probes  4 . 
     The two control probes  24 ,  25  are suitably connected to two respective control pads  16 ,  18  belonging to at least one same device  11  on the wafer  10 . 
     Moreover, for housing the test circuit  17  of the probing process, a portion of the wafer called a seal ring  20  ( FIG. 6 ) may be, for example, used to which said two control pads  16 ,  18  will be connected. Through the seal ring  20  a current will be made flow during the test step. The flow of this current will supply the information that the probing process does not have a drift to be considered as dangerous. 
     For completeness of the description and for better understanding all the aspects of an embodiment, it is now suitable to illustrate more in detail the edge structure of the electronic devices  11  integrated on the wafer  10 ; this will be made with reference to  FIGS. 6 and 7 . 
     After an electric testing step of the various devices  11  present on the wafer  10 , they are separated from each other through a conventional cutting operation of the wafer itself by means of the blades of a saw or a laser ray. 
     The electronic devices  11  being realized on the wafer  10  are normally already delimited with each other by a so called scribe line  12  that represents also the interstice whereon the cutting means that physically separate the devices during the cutting operation act.  FIG. 7  shows an enlarged view of this scribe line  12 . 
     The scribe lines  12  are often exploited also for introducing elemental structures  21  or TEG (Test Element Group) intended for the testing of some process parameters, that are measured in general prior to the electric test on wafer, as shown in  FIG. 6 . 
     To protect the electronic devices  11  from the mechanical actions exercised by the blade during the cutting, each device  11  is peripherally surrounded by a so called seal ring  20  that has the aim of sealing the device  11  itself and strengthening it mechanically for ensuring the reliability also after the cutting. 
     The seal ring  20  is put in general between the area where the pads  13  are placed, or pad ring of the device  11 , and the scribe line  12  that surrounds the device  11  itself. 
     The seal ring  20  is formed with more metallic layers and vias and also serves for blocking ions and contaminative substances too that can jeopardize the good operation of the device  11 . 
     Several implementations of seal ring are however known, where for example dielectric layers and metallic layers are alternated. 
     Sometime for avoiding problems of interferences in radiofrequency, open loop seal rings are realized instead of closed loop ones so as to avoid that the seal ring behaves like a coil. 
     The angles of the seal ring  20  may be the most critical regions and they are usually reinforced, for example, by using metals of greater size and possibly rounded off/smoothed. 
     Situations are also known in which at the angles of the seal ring  20  there are suitable patterns of structures that avoid the damaging of the angles of the seal ring itself. 
     An embodiment also provides that the additional pads  16 ,  18  may be suitably positioned outside the seal ring  20 . 
     More in particular, these additional pads  16 ,  18  may be realized in correspondence with opposite angular regions of the electronic device  11  outside the seal ring  20 , which in correspondence with these opposite angular regions is deviated to form substantially a part of a polygonal path. 
     With this structure it may be possible to execute a preventive test to check the correct alignment of the probes before starting with the electric test for checking the correct operation and the quality on a given electronic device contacted by the array of probes  4 . 
     For example, by first exploiting the additional pads  16 ,  18  with the corresponding control probes  24 ,  25  it may be possible to make a current circulate through these two control probes  24 ,  25 . If this current does not circulate correctly, this means that there is a drift of the probing process due to for example to probe-pad alignment problems, or that there are problems of electric continuity or other. 
     To make this initial check more sensitive, the above two control probes  24 ,  25  may be subjected to a stress being identical or greater than the stress to which the generic probe  4 , which is considered the more critical than the array  22  of probes, is subjected. 
     For example if there are probes  4  crossed by high currents during the test of at least one device, a current will be made to circulate being identical or greater in the control probes  24 ,  25  of the probing process with respect to the most critical probe  4  during the test of that given device; in this way the drift of the probing process may be detected in advance. 
     The two additional pads  16 ,  18  intended for the check of the alignment may also have smaller size with respect to the other pads  13  of the device, so as to make the check even more sensitive. 
     Moreover, if these two additional pads  16 ,  18  are placed outside the seal ring  20  there may also be the advantage of not increasing the area of the device. 
     It is to be noted that the presence of the additional pads  16 ,  18  on opposite angles of the device  11  may confers a greater strength of the seal ring  20  that passes nearby, indirectly protecting this angle of the device during the cutting of the wafer. 
     All these considerations may be naturally extended to the case of simultaneous testing of more devices in parallel. 
     To facilitate the testing in parallel it has been thought to add suitable interconnection structures  23  ( FIG. 9 ) that connect to each other at least two additional pads  16 ,  18  placed in two bordering angular regions near two distinct seal rings  20 . 
     As shown in  FIGS. 8 and 9  these interconnection structures  23  may be extended only to the angular regions  15  of adjacent devices and, being near the seal ring  20 , they do not interfere with the TEG  21  placed in the scribe lines  12  of the wafer  10 . 
     With the above described measure, a test current may be made to circulate between at least two devices  11  placed on the wafer  10  although applying the sole two probes  24 ,  25 , as shown in  FIG. 10 . 
     The interconnection structure  23  may be realized by means of at least one metallization track that will pass through the scribe line  12  or above the same. 
     The use of at least two metallization levels connected by at least one vias, for realizing the interconnection  23 , may allow reducing the stress that the seal ring  20  will sustain when this interconnection  23  is cut by cutting means that physically separate the devices  11  during the cutting operation. 
     Suitably, a control pad  27  may be provided on the interconnection structure  23  being substantially positioned between the bordering angular regions of two devices  11 , as shown in  FIG. 11A . 
     In general these structures may be positioned also along the scribe line  12  as shown in  FIG. 11B  according to the application. 
     As shown in  FIG. 12 , also plural control pads may be provided, for example a pair of pads  28 ,  29 , arranged on the interconnection structure  23  between two bordering angular regions, so as to make the electric check of the probing process more sensitive. 
     To facilitate the testing operations provided by the process of an embodiment it may be possible to equip the probe card  2  connected to the testing apparatus ATE with control probes placed in any angle of the array of probes  4 ; for example,  FIG. 13  shows a rectangular array  22  of probes with four control probes  24 ,  25 ,  26  and  31  arranged in the corners. 
     It may also be possible to have a control pad  16 ,  18 ,  28 ,  29  for each corner of the device, and each corner may be electrically connected to the corners close to it, as shown in  FIG. 14 . 
     A plurality of control pads may be present and suitably positioned so as to have control probes not only in the angles/corners of the array  22  but for example along its perimeter so as, for example, to have more types of alarm. 
     Possibly, the control probes may also be far from the array of probes  22  due to constructive problems of the probe card  2 , or for increasing the check sensitivity. 
     In case test circuits  17  devoid of closed coils are to be realized, for example for avoiding radiofrequency problems, the structure may be complicated a little by integrating suitable circuits. For example, as shown in  FIG. 15 , it is possible to integrate at least one diode D on the interconnection structure  23  between control pads  16 ,  18  or  28 ,  29 . This allows making at least one path through which the current flows unidirectional, as shown in  FIG. 15 . 
     For example as shown in  FIG. 15 or 16  by connecting the four angular regions of four adjacent devices two diodes D will be positioned, and by rigidly repeating this structure, also according to the design of the seal ring that is an open loop one, the creation of coils will be avoided, however allowing that the current can have at least one possible path between the at least two control probes  24 ,  25  or  26 ,  31 . 
     The control pads may have a circular form so that the check does not depend on the different orientation of the control probes, as shown in  FIG. 17 . 
     Now, with reference to  FIGS. 18 to 21  embodiments are shown in which the control pads  35  are realized inside the single electronic device  11 , they may thus increase the area of the device itself with a consequent slight increase of the costs. 
     In this case it may be suitable to connect to each other at least two adjacent seal rings  20  for allowing the testing in parallel of at least two devices  11 , as shown in  FIG. 18  with the interconnection lines  33 . 
     If this connection occurs between two angular/corner regions of the device, the generic angle  34  of the seal ring  20  may be advantageously strengthened so that it is less likely to be damaged by the cutting of the wafer, as shown in  FIG. 19 . 
     It may thus be advantageous to reduce, as much as possible, the sizes of the connection metals so as to facilitate the cutting process of the wafer by using, for example, at least two metal levels and at least one via as discussed for the connections  23  of  FIGS. 9  and  10 , but however this has a general validity considering the connections between the at least two seal rings  20  of at least two different devices  11 . 
     However, several forms of connections are possible between at least two adjacent devices  11  incorporating in their interior the control pads  35 . 
     Nothing forbids providing several control pads  35  for each device  11  and not necessarily in the angles of the seal ring  20 , according to the application, as shown in  FIG. 21 . 
     By increasing the number of control pads  35  the arising of false alarms may be reduced, and also different levels of severity of the alarms may be attained. 
     The various seal rings may be connected to each other in various ways, for example as shown in  FIG. 20  and not necessarily in the angles of the same seal ring, as shown in  FIG. 21 . 
     In the example of  FIG. 20  if one of the connections between two seal rings  20  is faulty, for example it has a non desired interruption, the other connections between the other seal rings  20  will however allow the electric check of the position of the probes. 
     By incorporating these control pads directly in the seal ring  20  it may be avoided that they may be internal to the device and it is avoided that they may hinder the circuits  21  of the TEG, if these control pads are housed for example inside the scribe line  12 , as shown in  FIG. 21 . 
     This depends on the circuits  21  of the TEG and on which probes of the probe card they tend to drift earlier during the probing and testing process. 
     The generic control pad may be thus housed or even be part of the seal ring  20 . Also the mirrored structure or suitable modifications thereof, for example further to rotations may be valid. 
     A diode may be realized, for example, through a junction PN  37  in the substrate  36  of the wafer  10  between the seal ring  20  and the metal path of the interconnection structure  23 , as shown in  FIG. 22 . 
     In  FIG. 22  and in the following ones, the place of the P doped material and of the N doped material may be exchanged, according to the circuit that is to be implemented, according to the dopant of the substrate  36  and according to specific design needs. With respect to the scribe line  12 , also the placement of the seal ring  20  with the metal path may be changed, as shown in  FIG. 23 . 
     Also the control pad outside the seal ring  20  may be added, as schematically shown in  FIG. 24 . 
     The control pad  35  may be formed on the same seal ring  20 , possibly locally increasing its size, as shown in  FIG. 25 . 
     Moreover, where the seal ring  20  is interrupted for avoiding the creation of coils, reinforcement mechanical structures may be placed, as shown in  FIG. 26 . 
     For example interdigited structures may be realized, that may be used also for their capacitive behavior, and thus for the check of the probing process a radiofrequency signal may be made to circulate in the seal ring  20 , this may be advantageous if for example the most critical probe of the array  22  is a probe for radiofrequency signals. 
     Alternatively, in the seal ring  20  suitable structures may be formed for signals RF. 
     Thanks to the at least two control pads  16 ,  18  connected to the seal ring  20 , the seal ring may be possibly divided into more parts for transporting signals and supplying to circuits connected to the seal ring itself. 
     In consequence, in general circuits may be connected to the seal ring, for example those shown with  40  and  41  in  FIG. 27 . The circuit  40  may be, for example, formed by at least one diode that may be used for the probing process check and through which the current necessary for the needed tests flows. 
     The presence of the diode may allow for identification of the problems of contact resistance between the probe and the pad thanks to a suitable test that measures some characteristics of the diode from which information on the contact resistance may be derived. 
     The circuit  41  may be used for different aims and supplied through a current that comes from the device  11  or through the control pads  16 ,  18  making a current circulate being opposite to the direction of the diode of the circuit  40  that will thus be akin to an open circuit. 
     This circuit  41  may be advantageously used, for example, for containing in a memory useful production data such as the number of lot, the number of the wafer and the coordinates XY of the device on the wafer. 
     In consequence these control pads  16 ,  18  may remain on the device  11  and being not removed by the cutting of the wafer and being used again afterwards, for example during the analysis of the failure of the device indicated by a client. 
     The circuit  41  may also be an interface, for example serial, towards the BIST circuits of the device, and thus these control pads  16 ,  18  may be used also for the test of the device itself. 
     Moreover, the elemental structures, the circuits and the methodology described may be possibly and advantageously adapted also for incorporating some structures of the TEGs for parametric measures of the technological process. 
     For example, the system in one of its implementations may have a long metallic path between two control pads being enough far from each other, and this may for example allow to obtain a measure of the resistance of this metallic path. 
     It is possible to provide also an index of contact problems between the probe and the pad. 
     An embodiment makes it possible an electric check of the probing process of the wafer. 
     Moreover an embodiment of a structure allows for strengthening the device during the cutting step of the wafer  10 . 
     Moreover the seal ring  20  may be used also as a circuit element of the device itself. 
     Moreover it may be possible that the current for the check of the position of the probes instead of flowing between two pads  16  and  18 , flows for example between the substrate  36  and the pad  16 , and this current will be injected, for example, through the chuck  5  of the prober in electric contact with the substrate  36 . Also a vice versa case may be valid. 
     The seal ring  20  or at least part of the same may have the possibility to be electrically insulated or not by the substrate  36  of the wafer  10  according to the specific circuit that is to be realized, according to the various design ties of the specific device  11  considered. 
     Naturally for meeting incidental and specific needs, a technician of the field will have the possibility to apply several modifications to the previously described embodiments. It is clear that several omissions, substitutions, and modifications in the form and in the details, as other embodiments are possible; it is also expressly intended that specific elements and/or process steps described in relation to any embodiment of the invention described may be incorporated in any other embodiment as general aspect of design choices. 
     An embodiment of an integrated circuit described above may be a processor, and may be coupled to another integrate circuit (e.g., a processor) to form a system. 
     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.