Abstract:
A method for testing a strip of MEMS devices, the MEMS devices including at least a respective die of semiconductor material coupled to an internal surface of a common substrate and covered by a protection material; the method envisages: detecting electrical values generated by the MEMS devices in response to at least a testing stimulus; and, before the step of detecting, at least partially separating contiguous MEMS devices in the strip. The step of separating includes defining a separation trench between the contiguous MEMS devices, the separation trench extending through the whole thickness of the protection material and through a surface portion of the substrate, starting from the internal surface of the substrate.

Description:
BACKGROUND 
     1. Technical Field 
     The present disclosure relates to a method for strip testing of MEMS (Micro Electro Mechanical System) devices, to a testing strip of MEMS devices, and to a MEMS device configured to be used with the method. 
     2. Description of the Related Art 
     MEMS devices are playing an increasingly important role in the electronic industry, especially in the consumer electronic field of portable electronics, thanks to the reduced size and power consumption. 
     As is known, a MEMS device includes one or more dice of semiconductor material (e.g., in the case of a MEMS sensor device, a first die integrating a mechanical sensing structure and a second die integrating a related electronic interface made as an ASIC—Application Specific Integrated Circuit), encapsulated in a package, which protects and covers the dice and provides suitable electrical connections to the outside, e.g., for soldering to an external printed circuit board. 
     Common packages are the so called BGA (Ball Grid Arrays) or LGA (Land Grid Array) packages, which offer reduced area occupation and high density of the electrical connections. 
       FIG. 1  schematically shows a MEMS device, denoted as a whole with reference  1 , provided with a BGA or LGA package  2 . Package  2  includes a substrate  3 , having an internal surface  3   a  to which the dice of the MEMS device are attached, and an external surface  3   b , which carries suitable electrical connection elements  4  to the outside of the package  2 , in the form of an array of “balls” or “bumps” (in the case of BGA packages) or “lands” (in the case of LGA packages, as is the case shown in  FIG. 1 ). The substrate  3  is usually made of a multi-layer structure, composed of several layers of conductive material (generally metal) separated via dielectric layers; electrical traces are provided through the substrate  3  to connect the dice to the external electrical connection elements  4 . A covering and protection material, generally a mold compound  5 , is provided on the substrate  3  and covers the dice, protecting them from the external environment. 
     In particular, in the example shown in  FIG. 1 , MEMS device  1  comprises a sensor die  6 , including a micromechanical detection structure, and an ASIC die  7 , including a related interface electronics. Dice  6  and  7  are stacked, with suitable electrical connections in the form of wires (using the so called “wire bonding” technique) designed to electrically connect sensor die  6  to ASIC die  7 , and ASIC die  7  to the substrate  3 ; moreover, vias  8  and suitable traces are provided through the various layers of the substrate  3  to route the signals between the ASIC die  7  and the electrical connection elements  4  (these being either detection signals or power supply signals, or any other kind of signals exchanged between the MEMS device  1  and external devices). Clearly, other arrangements are possible for dice  6  and  7 , which may be placed side-by-side on the substrate  3 ; or sensor die  6  may be attached to the ASIC die  7  with the flip-chip technique, with direct electrical connections being provided between the two dice. 
     In the semiconductor industry, testing of MEMS devices, in order to assess the electrical and mechanical performances of the finished products, accounts for an important part of the manufacturing costs, especially due to the amount of time and the expensive systems and apparatuses, which have to be provided for performing the testing operations. Testing generally envisages providing a stimulus (e.g., in the form of a physical stress) to a MEMS device and detecting an output electrical signal generated by the MEMS device in response thereto. 
     In order to reduce testing costs and increase the overall efficiency, so called “strip testing” procedures have been proposed, envisaging simultaneous parallel testing of a number of MEMS devices arranged in strips, according to matrix layouts, instead of separately testing single MEMS devices. These testing procedures allow to achieve a great productivity improvement and a reduction of the time used for testing, and thus a reduction of the final manufacturing costs. 
     In this connection,  FIGS. 2   a  and  2   b  schematically show a strip  10  of MEMS devices, again denoted with reference  1  (each one being for example a sensor device as shown in  FIG. 1 , or any other kind of known MEMS device). MEMS devices  1  are in a matrix arrangement, aligned along a first and second directions x, y of a plane xy: strip  10  in the example has a main extension along the first direction x. Testing systems may be designed for parallel testing of several devices, for example of a group thereof, as shown enclosed by the dashed box in  FIG. 2   a.    
     As depicted in  FIG. 2   b , the various MEMS devices  1  are enclosed in the same mold compound  5  and attached to the same substrate  3 , after manufacturing (but before the final singulation step); therefore, it is considered that the substrate  3  includes a plurality of portions, each corresponding to a single MEMS device, each portion being separated by the others by a boundary region where the final cut during singulation will be performed. 
     However, especially in the case of MEMS sensors, parallel testing of devices arranged in strips implies some difficulties, due to the need to carry out suitable physical stimulation of the various sensors during testing (e.g., providing a test acceleration for acceleration sensors, or a test pressure for pressure sensors), and particularly due to the fact that the stresses acting on the devices in strip form are different from the stresses acting on the single devices, separated from the others. Moreover, the various devices in the strip are to be electrically insulated, in order to perform electrical tests on the individual devices. 
     Physical stimulation of the various devices during testing is achieved through the use of suitable testing equipments, configured to exert specific stresses on the devices, for example envisaging the use of support tables providing accelerations along a plurality of axes. 
     Various solutions have already been proposed in order to solve the problem related to the stresses acting on the devices while in strip form, and their electrical insulation. 
     In particular, a proposed solution envisages first the singulation of the various MEMS devices  1 , and then their placing in a suitable carrier structure (or tray), provided with a plurality of housings, each adapted to house a respective singulated device. The housings in the carrier structure are arranged so as to define a strip of MEMS devices  1 , which may undergo a parallel testing procedure. 
     This method is advantageous since no undesired stresses act on the singulated MEMS devices  1  during parallel testing operations. However, huge investments for preparing the support structures are needed to house the various MEMS devices  1 , which have to be designed and manufactured for each possible package size; also, continuous maintenance of the carrier structures is conducted to assure correct alignment of the MEMS devices  1  during testing. 
     A further testing procedure has been already employed by the present Applicant, the so called “pre-cut” method, envisaging, as shown schematically in  FIG. 3 , sawing of the common substrate  3  from the external surface  3   b  and of part of the overlying mold compound  5 , so as to create trenches (or openings, or cut-out portions)  11  extending through the whole substrate  3  and through part of the mold compound  5 . In a way that is not shown in  FIG. 3 , these trenches  11  extend in the strip  10  both along the first and the second directions x and y, so as to define a continuous hollow portion, separating and surrounding MEMS devices  1 . 
     This cutting process also electrically insulates the various MEMS devices  1  from each other and allows testing in strip-form of the same devices, which are still hold together via the residual portions of the mold compound  5 ; in particular, thickness of this residual portion (starting from the surface thereof not originally contacting the internal surface  3   a  of the substrate  3 ) is configured to have sufficient rigidity to achieve the result of holding together the MEMS devices  1  during handling and testing operations. Moreover, the resulting physical separation achieved between MEMS devices  1  in the strip  10  limits the amount of reciprocal stresses during testing. 
     The Applicant has realized that, although advantageous, this testing procedure suffers from some drawbacks. 
     In particular, in the “pre-cut” process the final full separation of the various MEMS devices  1  (the so called “singulation” operation) is carried out after their testing in strip form. The singulation process releases the stresses exerted by the residual portion of the mold compound  5  on the devices, thus offsetting the devices from the previously calibrated values; indeed, the pre-cut process leaves a certain amount of residual stress acting on the devices during the calibration step, and after singulation the offset distribution is widely spread, possibly driving a part of the population out of a specification. 
     This offset thus implies the need of providing a second testing procedure, after the singulation operation, designed to guarantee that all finished MEMS devices  1  are within the specified tolerance values. 
     The Applicant has realized that this second testing step represents a non-value adding process, since it is performed only because of the remaining stresses acting on the MEMS devices  1  during the strip testing/calibration procedures. 
     Especially when MEMS production volume is increased, the investments in strip testing equipments and second test equipments have also to increase correspondingly and may come to represent an important part of the overall manufacturing costs. Therefore, the need is clearly felt for a testing procedure that would allow testing of MEMS devices in strip form, while reducing or possibly avoiding the need of performing a further testing step after singulation, thus drastically improving the overall testing costs and times. 
     BRIEF SUMMARY 
     One embodiment of the present disclosure is a strip testing method for MEMS devices, having improved overall performances and costs, allowing to solve, at least in part, the problems discussed above in connection with known testing methods. 
     One embodiment of the present disclosure is a method that includes testing a strip of MEMS devices, each MEMS device including a common substrate having an internal surface and an external surface, a die of semiconductor material coupled to the internal surface of the common substrate, and a protection material configured to cover the die. The method also includes detecting output values generated by each of the MEMS devices in response to a testing stimulus and before the detecting, partially separating contiguous MEMS devices in the strip. The separating includes forming a separation trench between the contiguous MEMS devices, the separation trench extending through an entire thickness of the protection material and through a surface portion of the substrate, starting from the internal surface of the substrate. 
    
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
       For a better understanding of the present disclosure, preferred embodiments thereof are now described, purely by way of non-limiting example and with reference to the attached drawings, wherein: 
         FIG. 1  shows a schematic cross section of a known MEMS device; 
         FIG. 2   a  shows schematically a top view of a strip of MEMS devices according to the prior art; 
         FIG. 2   b  shows a schematic cross section through the strip of  FIG. 2   a;    
         FIG. 3  shows a schematic cross section through a strip of MEMS devices, designed for a known “pre-cut” testing method; 
         FIG. 4  shows a schematic cross section through a strip of MEMS devices, according to an embodiment of the present disclosure; 
         FIG. 5  shows a more detailed cross section through a portion of a substrate of the strip of  FIG. 4 ; 
         FIG. 6   a  shows a schematic top view of the substrate portions of two contiguous MEMS devices, according to an embodiment of the present disclosure; 
         FIGS. 6   b  and  6   c  show cross sections of the substrate portions of  FIG. 6   a , before and, respectively, after a sawing process; and 
         FIG. 7  shows plots related to testing performances of the MEMS devices of the strip. 
     
    
    
     DETAILED DESCRIPTION 
     The present Applicant has realized that the common mold compound surrounding the various MEMS devices in the testing strip is a cause of the residual stresses acting on the same devices and offsetting the testing procedures. 
     Accordingly, as shown in  FIG. 4  (where same numerals denote same elements as disclosed above, which are not described again hereinafter), a first aspect of the present disclosure envisages, before carrying out of the testing operations, removing of the whole thickness of the mold compound  5  surrounding the MEMS devices  1  of the strip  10 , e.g., via a cutting or sawing operation. The cut performed also extends through an internal surface portion (starting from the internal surface  3   a  on which the dice of the MEMS devices  1  are arranged) of the substrate  3 , which is removed, leaving only residual substrate portions, denoted with  12 , between contiguous MEMS devices  1 . These residual substrate portions  12  (constituted by external surface portions of the substrate  3 , starting from the external surface  3   b , opposite to the internal surface  3   a ) hold together the various MEMS devices  1  during testing in strip form. In particular, the removal operation thus defines separation trenches, here denoted with  14 , separating the various MEMS devices  1  (and arranged between contiguous MEMS devices  1 , and surrounding each one of them), extending through the whole thickness of the mold compound  5  and through the internal surface portion of the substrate  3 . In a manner that is not shown, these separation trenches  14  extend in the strip  10  both along the first and the second directions x and y, so as to define a continuous hollow (or cut-out) portion. 
     According to an embodiment, the separation trenches  14  may extend through the substrate  3  from the internal surface  3   a  up to half the thickness of the same substrate  3 . 
     The presence of separation trenches  14  not only allows to reduce the stresses acting on the various MEMS devices  1  (the Applicant has indeed realized that the residual substrate portions  12  originate a much lesser mechanical stress on the MEMS devices  1 , and less stress is thus released during subsequent full singulation of the devices), but also allow to electrically insulate the various MEMS devices  1  in the strip  10 , by removing any possible electrical connections between the devices in the substrate. Indeed, these electrical connections may be conveniently arranged in the internal surface portion of the substrate  3 , at the boundary of the various MEMS devices, which is removed in the material removal process defining the separation trenches  14 . The residual portions  12  of the substrate  3  instead do not carry any electrical connection between contiguous MEMS devices  1 , so that the same MEMS devices  1  are electrically insulated and ready to be tested. 
     Given that generally the substrate  3  is made of a plurality of layers (conductive and dielectric stacked layers), the discussed removal operation is designed to affect only some of the layers, in particular the first layers, i.e., the layers more proximate to the internal surface  3   a , to which the dice of the MEMS devices  1  are attached. 
     In particular, it is known that the multilayer structure of a substrate includes a central layer, and symmetrical stacked conductive and dielectric layers overlying and underlying the central layer; each conductive layer (made of a metal material, e.g., copper) is defined, e.g., via an etching process, to create the pattern of electrical connections through the substrate, and is separated from the other conductive layers by means of contiguous dielectric layers (e.g., made of BT or FR-4). 
     An exemplary multilayer structure for a substrate, again denoted with  3 , is shown in  FIG. 5 , where: the central layer, made of a dielectric material (e.g., BT or FR-4) is denoted with  20 ; the suitably defined conductive layers are denoted with  21 ; and the other dielectric layers are denoted with  22  (as it is shown, each dielectric layer  22  is arranged between two conductive layers  21 , so as to separate them). A first conductive layer  21  is considered to be arranged at the internal surface  3   a  of the substrate  3 , while a last conductive layer  21  is considered to be arranged at the external surface  3   b  of the same substrate  3 . 
     Conductive through vias, here denoted with  24 , are conveniently provided through the substrate  3 , so as to define electrical connections between internal conductive pads  25  on the internal surface  3   a  and external connection elements  4  on the external surface  3   b . Conductive traces  26  are conveniently provided in the various conductive layers  21  to route the signals through the substrate  3 ; likewise, internal vias  27  are provided to connect the various conductive layers  21 , within the substrate  3 . 
     Moreover, a first protective layer  28   a  (so called “solder mask”) is provided on the first conductive layer  21 , above the internal surface  3   a  of the substrate  3 , to cover and protect the traces and pads provided therein, except at access openings to the same first conductive layer  21 , e.g., at bonding pads  30  (so called bonding fingers, arranged at the internal surface  3   a  for bonding to the ASIC dice  7  of the MEMS devices  1 , here not shown, via the wire boding technique). Analogously, a second solder mask layer  28   b  is provided on the last conductive layer  21 , above the external surface  3   b  of the substrate  3 , except at access openings to the same last conductive layer  21 , e.g., at the external electrical connections  4 . 
     As previously discussed, separation trenches  14  made at the periphery of each MEMS device  1  (i.e., in the boundary portions of the substrate  3  between contiguous MEMS devices) may have a height h, in the direction transverse to the internal surface  3   a  of the substrate  3 , such as to extend at least through the whole first conductive layer  21 , and up to reach the central layer  20  (extending through part of the same central layer  20 ); for reasons of structural stability, separation trenches  14  are not designed to extend below the central layer  20 . 
     Various design possibilities may thus be envisaged, according to which separation trenches  14  may have different extensions through the substrate  3 , so that a different number of substrate layers are removed or preserved by the cutting operation (the double-arrow in  FIG. 5  denotes an exemplary range of variability for the height h of the separation trenches  14  within the substrate  3 ). 
       FIGS. 6   a - 6   c  show, by way of example, a portion of the substrate  3  regarding two contiguous MEMS devices  1 , according to an embodiment of the present disclosure (it is clear, however, that a greater number of devices are provided in the same substrate  3  and belong to the testing strip  10 ). 
     Substrate  3  here includes: the central layer  20 ; a first conductive layer, here denoted with M 1  (as it is usual in the field), contiguous to the internal surface  3   a  of the substrate  3 ; and a second conductive layer, here denoted with M 2 , contiguous to the external surface  3   b  of the same substrate  3 . Conductive traces  26  are provided in the first conductive layer M 1 , e.g., to define electrical connections between bonding pads  30 , and internal conductive pads  25  and conductive through vias  24 . Conductive traces  26  are also provided in the second conductive layer M 2 , to define electrical connections between the through vias  24  and the external electrical connections at the external surface  3   b  of the substrate  3 , here in the form of lands and again denoted with  4 . 
     Moreover, in a known manner, plating traces  32  are defined in the first conductive layer M 1 , to provide electrical connections between the first conductive traces  26  (and/or the bonding pads  30  and/or the internal conductive pads  25  and/or the conductive through vias  24 ) and a plating bar  34 . 
     Plating bar  34  is arranged at the periphery of each MEMS device  1  and defines a boundary between contiguous devices (in the embodiment of  FIG. 6   a , plating bar  34  defines a square ring around each MEMS device  1 ). In a known manner, plating bar  34  is defined in the first conductive layer M 1  and may be defined also in the second conductive layer M 2 , and it is biased with a suitable electrical voltage during an electroplating process, so as to electrically plate the conductive traces  26 , bonding pads  30 , internal conductive pads  25  and/or conductive through vias  24  with a thin layer of protective material via a chemical ionization process. 
     According to a particular aspect of the present embodiment, all plating traces  32  are formed in the first conductive layer M 1 , and no plating traces are formed in the second conductive layer M 2 , so that no electrical connection to the plating bar  34  is provided in the same second conductive layer M 2 . 
     Since plating bar  34  is common to all MEMS devices  1 , its presence electrically short circuits traces and pads of different devices. However, the removal process defining the separation trenches  14  is designed to remove the first conductive layer M 1  in the boundary regions between the various MEMS devices  1  (in correspondence to the plating bar  34 ), thus removing the same plating bar  34  and at least part of the plating traces  32  connected to the plating bar  34 ; in this manner, electrical insulation is provided between the MEMS devices  1  before testing in strip form. 
     The result of the selective removal of the portions of the first conductive layer M 1  is schematically shown in  FIG. 6   c , where separation trenches  14  and residual portions  12  of the substrate  3  are shown between contiguous MEMS devices  1 . 
     It is clear that similar considerations apply when the substrate  3  includes a higher number of layers, in which case plating traces  32  connecting to the plating bar  34  are to be designed only in the conductive layers  21  that will be removed in the removal operation before testing of the MEMS devices  1  in strip form. Therefore, if peripheral portions of first and second conductive layers  21  are removed in the cutting operation (e.g., in the case of a substrate having five layers), then all plating traces  32  will be routed in the same first and second conductive layers  21 , so as to achieve electrical insulation between the various MEMS devices  1  before testing is carried out. 
     The present Applicant has verified, through tests and simulations, that the present solution allows to achieve improved performance during strip testing (with a reduced number of failures and of devices falling out of desired specifications); in particular, it is possible to avoid second testing procedures after singulation of the MEMS devices  1 . 
       FIG. 7  shows plots relating to the testing performance in terms of offset from desired specification values; the distribution of the MEMS devices  1  according to the present solution (shown in continuous line) is compared to a distribution of devices tested using the “pre-cut method” (shown in dashed lines) and to a distribution of singulated devices after a second test procedure (shown in dotted line).  FIG. 7  also shows, as a box, the desired specification outer limits, so that it is evident that the present solution allows to satisfy the specification, even when a second testing procedure is not carried out, as opposed to previously known methods. 
     From what has been described and illustrated, the advantages that the present solution allows to achieve are evident. 
     In particular, it is again underlined that, during strip testing, the residual portions  12  of the substrate  3  exert less stress on the MEMS devices  1 , thus providing a minimal offset between calibration and final performances of the devices; second testing procedures may even be avoided, drastically improving the costs for testing of the MEMS devices. 
     No carrier is used during testing, and the placement tolerances of the various devices in the strip  10  are related only to the substrate manufacturing tolerances (which are usually very low). 
     There may be modification of the usual substrate design, in terms of placement of the plating traces  32 , which have to be concentrated in the layer(s) of the substrate  3  that are partially removed before testing, this does not entail an increase of the design and manufacturing complexity. 
     Finally, it is clear that modifications and variations may be made to what has been described and illustrated herein, without thereby departing from the scope of the present disclosure. 
     In particular, the structure of the substrate  3  may differ from the specific embodiments previously shown and discussed, e.g., in terms of the number of layers and constituent materials; also the depth and extension of the separation trenches  14  in the substrate (and the extent of removal of the surface portion of the same substrate) may change accordingly. 
     Moreover, the discussed testing solution may be applied when testing any kind of MEMS devices in strip form, even if it is particularly advantageous in case of MEMS sensors, which may be greatly affected by residual stresses during testing operations. 
     The various embodiments described above can be combined to provide further embodiments. These and other changes can be made to the embodiments in light of the above-detailed description. In general, in the following claims, the terms used should not be construed to limit the claims to the specific embodiments disclosed in the specification and the claims, but should be construed to include all possible embodiments along with the full scope of equivalents to which such claims are entitled. Accordingly, the claims are not limited by the disclosure.