Patent Publication Number: US-10770407-B2

Title: IC structure with interdigitated conductive elements between metal guard structures

Description:
BACKGROUND 
     The present disclosure relates to circuit structures, and more specifically, to an integrated circuit (IC) structure for moisture and chip integrity monitoring having interdigitated conductive elements. 
     During IC packaging and package qualification, monitoring of chip package integrity is an important part of the process. Similar monitoring of crack propagation or moisture ingress may be valuable after a device is manufactured and deployed. IC chips are formed on a semiconductor wafer and separated into individual chips by cutting that can damage the chips. Any moisture or cracks/defects in an IC structure can lead to chip failures and/or performance degradation. Sensing and monitoring moisture and cracks/defects will prevent chip failures from these issues. Various IC chips include one or more metal fills positioned near the perimeter edge for stemming crack propagation or moisture ingress form the edge of the chip. As IC devices continue to shrink and densify, space on an IC chip has become more valuable. It has become more difficult to accommodate each of the various structures for monitoring and preserving the structural integrity of a circuit. In particular, the physical space for accommodating metal fills to obstruct cracks and moisture must compete with the metal wires and circuitry for monitoring the progress of cracks and/or moisture into the structure of an IC chip. 
     SUMMARY 
     A first aspect of the disclosure is directed to an integrated circuit (IC) structure, including: a back end of line (BEOL) stack on a substrate, the BEOL stack having a plurality of metal layers therein and a plurality of inter-level dielectric (ILD) layers therein, wherein the plurality of metal layers includes a lowermost metal layer and an uppermost metal layer; active circuitry coupled to at least one of the plurality of metal layers of the BEOL stack; a pair of metal guard structures positioned between the active circuitry and an outer perimeter of the BEOL stack, the pair of metal guard structures concentrically surrounding the active circuitry, wherein each metal guard structure includes a continuous metal fill between the lowermost metal layer and the uppermost metal layer of the plurality of metal layers; and a set of interdigitating conductive elements within one of the plurality of metal layers, the set of interdigitating conductive elements including a first plurality of conductive elements electrically coupled to one of the pair of metal guard structures interdigitating with a second plurality of conductive elements electrically coupled to the other of the pair of metal guard structures. 
     A second aspect of the disclosure includes an integrated circuit (IC) structure, including: a back end of line (BEOL) stack on a substrate, the BEOL stack having a plurality of metal layers therein and a plurality of inter-level dielectric (ILD) layers therein, wherein the plurality of metal layers includes a lowermost metal layer an uppermost metal layer, and at least one sensor layer between the lowermost metal layer and the uppermost metal layer; active circuitry coupled to at least one of the plurality of metal layers of the BEOL stack; a first metal guard structure positioned between the active circuitry and an outer perimeter of the BEOL stack, wherein the first metal guard structure includes a first continuous metal fill between the lowermost metal layer and the uppermost metal layer of the plurality of metal layers; a second metal guard structure between the active circuitry and the first metal guard structure, and separated from the first metal guard structure by a separation distance, the second metal guard structure concentrically surrounding the active circuitry, wherein the second metal guard structure includes a second continuous metal fill between the lowermost metal layer and the uppermost metal layer of the plurality of metal layers; a first plurality of conductive teeth within the at least one sensor layer and extending horizontally from the first metal guard structure toward the second metal guard structure without contacting the second metal guard structure; and a second plurality of conductive teeth within the at least one sensor layer and extending horizontally from the second metal guard structure toward the first metal guard structure without contacting the first metal guard structure, the second plurality of conductive teeth interdigitating with the first plurality of conductive teeth. 
     A third aspect of the disclosure relates to a method for monitoring an integrated circuit (IC) structure, the method including: applying a voltage to a pair of metal guard structures within a back end of line (BEOL) stack having a plurality of metal layers, wherein each metal guard structure includes a continuous metal fill between a lowermost metal layer and a uppermost metal layer of the plurality of metal layers, and wherein the pair of metal guard structures includes a set of interdigitating conductive elements within one of the plurality of metal layers, the set of interdigitating conductive elements including a first plurality of conductive elements electrically coupled to one of the pair of metal guard structures interdigitating with a second plurality of conductive elements electrically coupled to the other of the pair of metal guard structures; measuring a test current or capacitance through the pair of metal guard structures based on the applied voltage; calculating a differential current or capacitance between the test current or capacitance and a reference value for the pair of metal guard structures; in response to the differential current or capacitance not exceeding a threshold, repeating the transmitting and calculating; and in response to the differential current or capacitance exceeding the threshold, transmitting a signal to active circuitry of the IC structure to indicate current leakage through the pair of metal guard structures. 
     The foregoing and other features of the disclosure will be apparent from the following more particular description of embodiments of the disclosure. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The embodiments of this disclosure will be described in detail, with reference to the following figures, wherein like designations denote like elements, and wherein: 
         FIG. 1  shows a top view of an IC structure according to embodiments of the disclosure. 
         FIG. 2  shows an enlarged top view of a set of interdigitated conductive elements in an IC structure according to embodiments of the disclosure. 
         FIG. 3  shows a cross-sectional view of the IC structure according to embodiments of the disclosure. 
         FIG. 4  shows an enlarged top view of the IC structure with a sensing wire and vias according to embodiments of the disclosure. 
         FIG. 5  shows an enlarged top view of the IC structure with a sensing wire discontinuous from the plurality of conductive elements according to embodiments of the disclosure. 
         FIG. 6  shows a top view of three metal guard structures and two sets of interdigitated conductive elements according to embodiments of the disclosure. 
         FIG. 7  shows a top view of three metal guard structures and one set of interdigitated conductive elements according to embodiments of the disclosure. 
         FIG. 8  provides an illustrative flow diagram for implementing methods according to embodiments of the disclosure. 
         FIG. 9  provides an illustrative flow diagram for implementing methods according to further embodiments of the disclosure. 
     
    
    
     It is noted that the drawings of the disclosure are not to scale. The drawings are intended to depict only typical aspects of the disclosure, and therefore should not be considered as limiting the scope of the disclosure. In the drawings, like numbering represents like elements between the drawings. 
     DETAILED DESCRIPTION 
     An integrated circuit (IC) structure with interdigitating conductive elements, and related methods of use, are disclosed. Embodiments of the IC structure use structural and functional elements of a continuous metal fill to prevent the ingress of cracks or moisture, and monitor chip integrity with the same structure. “IC structure” as used herein may include circuitry in or on a semiconductor substrate and provided as a complete IC, IC chip or a partial (IC) such as a test site. The IC structure may include a back end of line (BEOL) stack on a substrate. The BEOL stack may include several metal layers, each alternating with inter-level dielectric (ILD) layers. One or more of the metal layers may be designated a sensor layer for monitoring crack propagation and/or moisture ingress through the IC structure. A pair of metal guard structures may concentrically surround active circuitry of the IC structure. Each metal guard structure may provide a continuous metal fill between a lowermost and uppermost metal layer of the BEOL stack. A set of interdigitating conductive elements may include a first plurality of conductive elements electrically coupled to one metal guard structure interdigitating with a second plurality of conductive elements electrically coupled to the other metal guard structure. In some cases, each conductive structure may take the form of a conductive spine, which along with respective plurality of conductive elements electrically coupled thereto, forms a conductive comb structure. 
     Referring to  FIG. 1 , a top view of an IC structure  100  is illustrated. IC structure  100  includes a back end of line (BEOL) stack  102 , which may extend vertically into the page of  FIG. 1 , and may be positioned over various devices, e.g., transistors, resistors, capacitors, etc. BEOL stack  102  as noted herein may include several metal wires within each layer, in addition to several insulating layers. The various layers of BEOL stack  102  may include active circuitry  104  including, conductive interconnects, metal wires, vias, etc., extending horizontally or vertically therethrough.  FIG. 1  shows a top view of IC structure  100 , e.g., along an uppermost metal layer MN of BEOL stack  102 . Uppermost metal layer MN of BEOL stack  102  may function as a sensor layer, and thus may include conductive features, described herein, near a perimeter  108  of IC structure  100  in BEOL stack  102 . Uppermost metal layer MN may be one of several layers of BEOL stack  102 , and multiple sensor layers may be vertically interconnected as will be described herein using metal guard structures, vias, and/or other vertically extending conductive materials. 
     Referring to  FIG. 2 , which shows an enlarged view of a section of IC structure  100  from  FIG. 1 , uppermost metal layer MN may include a first metal guard structure  110  located near perimeter  108  of IC structure  100 , and a second metal guard structure  112  disposed parallel to first metal guard structure  110  and perimeter  108  of IC structure  100  ( FIG. 1 ). Each metal guard structure  110 ,  112  may be a continuous metal fill extending between a lowermost metal level M 1  and an uppermost metal level MN of BEOL stack  102 , as shown in  FIG. 3  and discussed elsewhere herein. Metal guard structures  110 ,  112  may be provided in the form of one or more metal crack stops (e.g., inner and first metal crack stops, respectively), and/or may include other protective metallic elements such as a moisture oxidation barrier (MOB), guard ring, etc. In any case, IC structure  100  includes a set of interdigitating conductive elements  120  (shown in phantom) electrically coupled to metal guard structures  110 ,  112 . Set of interdigitating conductive elements  120  may include a first plurality of conductive elements  122  electrically coupled to first metal guard structure  110  interdigitating with a second plurality of conductive elements  124  electrically coupled to second metal guard structure  112 . As illustrated, first metal guard structure  110  and first plurality of conductive elements  122  are configured in a comb arrangement, and second metal guard structure  112  and second plurality of conductive elements  124  are configured in a comb arrangement. Other arrangements, shapes, sizes, etc., are possible as noted below. First plurality of conductive elements  122  interdigitate with second plurality of conductive elements  124 , i.e., they may mesh like the fingers of two clasped hands without) eking. 
     As shown in  FIG. 1 , IC structure  100  may also include a terminal  126  coupled to various portions of active circuitry  104 , in addition to first and second metal guard structures  110 ,  112 . Each metal guard structure  110 ,  112  may include a single terminal such that a resistance or capacitance between metal guard structures  110 ,  112  is electrically measurable as discussed herein. To wire second metal guard structure  112  to terminal  126 , a gap (shown in phantom) may be formed in one layer of first metal guard structure  110 . The gap within first metal guard structure  110  may be any chosen layer of BEOL stack  102 . Such gaps may not appear in first metal guard structure  110  at other layers of IC structure  100 . Metal guard structures  110 ,  112 , and set of interdigitating conductive elements  120  may be formed in BEOL stack  102  of IC structure  100  using any now known or later developed semiconductor fabrication process appropriate for the particular dimensions of a particular layer, e.g., electroplating; or dielectric layer photolithographic patterning, etching, conductor deposition and planarization, etc. While terms such as “elements” and “structures” have been used herein to differentiate features of IC structure  100 , the features will typically include the same conductive material, e.g., copper, aluminum, etc., and depending on the deployment of IC structure  100  in which the features are used, they may be positioned within any appropriate dielectric material of IC structure  100 , e.g., silicon oxide, low-k dielectric, etc. 
     In the embodiments shown, first plurality of conductive elements  122  extend horizontally perpendicularly from first metal guard structure  110  and second plurality of conductive elements  124  extend perpendicularly from second metal guard structure  112 . Although not shown, it will be appreciated that conductive elements  122 ,  124  may extend in parallel fashion at an angle other than perpendicular from respective metal guard structures  110 ,  112  while interdigitating with each other. In any event, first metal guard structure  110  and second metal guard structure  112  may be each individually wider than any conductive element  122 ,  124  in set of interdigitated conductive elements  120 . In further embodiments, interdigitating conductive elements  122 ,  124  may have at least one of: different sized conductive elements, wholly or partially linear or curvilinear conductive elements, different spacing between conductive elements and different density of conductive elements. Through selection of different sizes, spaces, density, etc., sets of interdigitating conductive elements  122 ,  124  within a particular IC structure, the sensor can be highly customized for a particular application. 
     Turning to  FIG. 3 , a cross-sectional view of IC structure  100  in plane X-Z is shown to further illustrate embodiments of the disclosure. IC structure  100  may include a substrate  130 , serving as a base material to form one or more devices and a foundation for subsequently-formed metal wiring levels of a structure. Substrate  130  may include, e.g., one or more currently-known or later developed semiconductive substances generally used in semiconductor manufacturing, including without limitation: silicon (e.g., crystal silicon), germanium, silicon germanium, silicon carbide, and those consisting essentially of one or more III-V compound semiconductors having a composition defined by the formula Al X1 Ga X2 In X3 As Y1 P Y2 N Y3 Sb Y4 , where X1, X2, X3, Y1, Y2, Y3, and Y4 represent relative proportions, each greater than or equal to zero and X1+X2+X3+Y1+Y2+Y3+Y4=1 (1 being the total relative mole quantity). Other suitable substrates include II-VI compound semiconductors having a composition Zn A1 Cd A2 Se B1 Te B2 , where A1, A2, B1, and B2 are relative proportions each greater than or equal to zero and A1+A2+B1+B2=1 (1 being a total mole quantity). Substrate  130  may be situated in or beneath a first level M 1  of BEOL stack  102 . One or more active components  132  (including, e.g., doped semiconductor material) may define a device layer D, also known as an active region, of IC structure  100 . Device layer D may include various active components  132  (e.g., transistors, capacitors, resistors, etc.) defining a portion of, or otherwise connected to, active circuitry  104  of IC structure  100 . Device layer D may also include one or more memory elements  133  for storing and/or retrieving various forms of data. In the example of  FIG. 3 , memory element  133  is shown as an electrically programmable fuse, or “e-fuse.” An e-fuse refers to a thin portion of wiring materials configured to break down when a circuit passes therethrough. The e-fuse being programmed or non-programmed may denote a one or a zero in a data catalogue. Memory element  133  may additionally or alternatively include, e.g., one or more memory cells including without limitation static random access memory (SRAM), dynamic random access memory (DRAM), transistor-based memory arrays, and/or other currently known or later developed memory storage or retrieval devices. 
     First metal layer M 1  may include an inter-level dielectric (ILD)  134  formed on substrate  130  and active components  132  to physically and electrically separate device layer D from overlying wiring layers, e.g., lowermost metal layer M 1  and uppermost metal layer MN. ILD  134  may be formed of any currently-known or later developed substance for providing electrical insulation, and as examples may include: silicon nitride (Si 3 N 4 ), silicon oxide (SiO 2 ), fluorinated SiO 2  (FSG), hydrogenated silicon oxycarbide (SiCOH), porous SiCOH, boro-phospho-silicate glass (BPSG), silsesquioxanes, carbon (C) doped oxides (i.e., organosilicates) that include atoms of silicon (Si), carbon (C), oxygen (O), and/or hydrogen (H), thermosetting polyarylene ethers, a spin-on silicon-carbon containing polymer material, near frictionless carbon (NFC), or layers thereof. 
     Active circuitry  104  of IC structure  100  above first level M 1  may include various wiring materials for providing internal connections within BEOL stack  102 , and/or to form electrical pathways to external components. Active circuitry  104  thus may include metal wires composed of any currently known or later-developed electrically conductive material including, e.g., copper (Cu), aluminum (Al), silver (Ag), gold (Au), combinations thereof, etc. Active circuitry  104  may also include vertically-extending wires, also known as vias, formed of the same or similar conductive materials and positioned between horizontally-extending metal wires within BEOL stack  102 . Each via of active circuitry  104 , in an embodiment, can comprise any standard conductive metal (for example, copper) with a lining material (not shown) thereon, such as tantalum nitride (TaN). Active circuitry  104  may be formed and positioned within various regions of ILD  134  and/or other regions of electrically insulative or semiconductive material (e.g., a region of semiconductor material or an electrically insulating dielectric material), such that active circuitry  104  transmits electricity between other electrically conductive structures in contact therewith. Active circuitry  104  positioned within a lowermost metal level M 1  can extend in a particular direction (e.g., along axis X). Active circuitry  104  positioned within an uppermost metal level MN can similarly extend along axis X in the same direction as active circuitry  104  in lowermost metal level M 1 , or a different direction. Lowermost and uppermost metal levels M 1 , MN are specifically noted in  FIG. 3 , but BEOL stack  102  may include any conceivable number of metal levels. In a partially or fully manufactured product, for example, BEOL stack  102  may include five metal levels, ten metal levels, twenty metal levels, one-hundred or more metal levels, etc. Lowermost metal level M 1  and uppermost metal level MN may be vertically separated from each other (e.g., along axis “Z” shown in  FIG. 3 ), either as directly adjacent metal levels or with intervening metal and insulator levels positioned therebetween. 
     Lowermost and uppermost metal levels M 1 , MN may be separated from one another by one or more intervening metal levels. As suggested by the notations M 1  and MN, the number of metal levels can vary depending on the chosen implementation and any requirements for back end of line (BEOL) processing. BEOL stack  102  may also include additional regions of ILD  134  between each successive metal level e.g., those vertically between or adjacent to M 1 , MN. In one embodiment, one or more vias of active circuitry  104  may extend from one metal level to an adjacent metal level, such that metal wire(s) in lowermost metal level M 1  may be electrically connected to metal wire(s) in uppermost metal level MN of BEOL stack  102 . 
     First and second metal guard structures  110 ,  112  may be located on substrate  130 , concentric about active circuitry  104  near perimeter  108  of BEOL stack  102 . Each metal guard structure  110 ,  112  may provide a continuous metal fill between lowermost metal layer M 1  and uppermost metal layer MN of BEOL stack  102 . As shown, metal guard structures  110 ,  112  may include multiple pillars extending through ILD  134 , and a single wire-like region of metal between ILD regions  134 , though this is not required in all instances. The shape of metal guard structures  110 ,  112  may depend on, e.g., simultaneous fabrication of active circuitry  104  and related processes, e.g., single or dual damascene metal deposition, recessing, etching, patterning, etc. One or more layers of BEOL stack  102  may serve as sensor layers as noted herein, and in the example of  FIG. 3  each layer of BEOL stack  102  provides a sensor layer for monitoring crack and/or moisture propagation from perimeter  108  toward active circuitry  104 . First and second metal guard structures  110 ,  112  include interdigitating conductive elements  122 ,  124  within BEOL Stack  102 . Interdigitating conductive elements  122 ,  124  appear to overlap within the cross-sectional view of  FIG. 3  solely to emphasize that elements  122 ,  124  may project from either metal guard structure  110 ,  112  in plane X-Z. Thus, interdigitating conductive elements  122 ,  124  are depicted with phantom boxes. It is understood that interdigitating conductive elements  122 ,  124  do not interlock as noted elsewhere herein. 
       FIG. 3  shows, as an example, sets of interdigitating conductive elements  122  between metal guard structures  110 ,  112  at each layer including lowermost metal layer M 1  and uppermost metal layer MN. In this case, interdigitating conductive elements  122 ,  124  may be substantially identical to those in other layers, i.e., have the same arrangement of conductive structures and sets of interdigitating conductive elements. In another embodiment however, two or more sensor layers may have interdigitating conductive elements  122 ,  124  with different arrangements. 
     IC structure  100  may include a seal layer  136  positioned on BEOL stack  102 . Seal layer  136  may include one or more insulators (e.g., nitride and/or oxide insulators) to electrically separate BEOL stack  102  from overlying chip packaging materials and/or components. Terminals  126  for connecting active circuitry  104  and conductive elements  122 ,  124  to various interconnect elements (e.g., conductive solder bumps, not shown) or sensing equipment may initially be formed on BEOL stack  102  before seal layer  136  is deposited thereon. A sensor  150  for applying a voltage to measure current leakage across metal guard structures  110 ,  112  may be coupled to IC structure  100 . Sensor  150  may be embodied, e.g., as any currently known or later developed electronic testing device capable of being manufactured and deployed separately from IC structure  100 . In other embodiments, sensor  150  itself may include an IC structure and/or may be structurally integrated into IC structure  100 . Sensor  150  is depicted for the sake of example as being a device independent from IC structure  100 , though it is understood that sensor  150  may operate substantially identically regardless of the chosen embodiment. Sensor  150  may have two terminals with opposite polarity, which may be coupled to metal guard structures  110 ,  112 , respectively. As discussed elsewhere herein, sensor  150  may be used to monitor current flow through, and/or capacitance across metal guard structures  110 ,  112 . Sensor  150  may be configured to detect moisture ingress and/or crack propagation in methods for monitoring IC structure  100 . Sensor  150  optionally may be electrically coupled to portions of active circuitry  104 , e.g., through one or more wires as shown with phantom lines in  FIG. 3 . In this case, sensor  150  may include a unit gain amplifier and/or similar component for mirroring a voltage of active circuitry  104 , and applying the mirrored voltage to metal guard structures  110 ,  112  through terminals  126 . Sensor  150  may also be configured to store or retrieve data in memory element(s)  133  of active circuitry  104 , e.g., during methods according to the disclosure. 
     In some cases, IC structure  100  optionally may include a pair of electrostatic discharge (ESD) diodes  152  within substrate  130  below BEOL stack  102 . Each ESD diode  152  may be formed by any currently known or later developed process to form diode connections between conductive or semiconductive regions. In many cases, ESD diodes  152  may be formed of oppositely-doped semiconductor regions within substrate  130  to form one or more P-N junctions. In any case, ESD diodes  152  may protect metal guard structures  110 ,  112  from damage during an electrostatic discharge event, e.g., during package assembly and operation. 
     Turning now to  FIG. 4 , an enlarged top view IC structure  100  is shown according to further embodiments of the disclosure. As shown, IC structure  100  may include sensing wires  186  distinct from conductive elements  122 ,  124 . Sensing wires  186  may be configured to electrically connect sensor  150  to selected portions of IC structure  100 . In the example embodiment of  FIG. 4 , IC structure  100  may include several vertically-extending vias  188  positioned on respective conductive elements  122 ,  124 . Vias  188  may electrically connect elements  122 ,  124  to wires, vias, active devices, etc., in other layers of BEOL stack  102  and/or active circuitry  104  for routing test currents through conductive elements  122 ,  124 . In the example shown, conductive elements  122 ,  124  are positioned in a single layer of BEOL  102  and coupled to two sensing wires  186 , in another layer(s) of BEOL stack  102  through vias  188 . Sensing wires  186 , depicted in phantom, may be positioned in any one or more layers different from conductive elements  122 ,  124 . A group of sensing wires  186  additionally may be electrically coupled directly to opposite terminals sensor  150  through vias  188  on conductive elements  122 ,  124 . The embodiment of  FIG. 4  thus may include sensing wires  186  configured to electrically monitor conductive elements  122 ,  124  by measuring resistance at selected locations. In this case, higher-risk portions of IC structure  100  may be monitored for moisture ingress and/or crack propagation without monitoring lower-risk portions of a device. 
     Referring to  FIG. 5 , IC structure  100  is shown with other features for monitoring moisture ingress, crack propagation, etc. In the example of  FIG. 5 , sensing wire  186  may be located within the same layer of BEOL stack  102  as set metal guard structures  110 ,  112  and conductive elements  122 ,  124 . In this case, sensing wire  186  may be discontinuous from conductive elements  122 ,  124  and may traverse a serpentine path between structures  110 ,  112  and elements  122 ,  124  within a single layer. In this manner, sensing wire  186  may be effective to monitor current, capacitance, etc., within BEOL stack  102  between metal guard structures  110 ,  112 . As shown, sensor  150  may be electrically coupled to opposite terminals of sensing wire  186 . Ingress of moisture and/or crack propagation through IC structure  100  may affect the properties of sensing wire  186 , thereby causing an applied voltage to produce varying amounts of current through sensing wire  186 . Similar to the example shown in  FIG. 5 , sensing wire  186  being horizontally between, and discontinuous from, elements  122 ,  124  may allow a quality analyst to monitor moisture ingress and/or crack propagation at selected portions of IC structure  100 . As compared to embodiments that use conductive elements  122 ,  124  as part of a testing pathway, sensing wire  186  may create a closed circuit for measuring resistance instead of capacitance. 
       FIGS. 6 and 7  depict further embodiments of IC structure  100 , in which more than two metal guard structures may be used. As illustrated, IC structure  100  may include at least one additional guard structure  190  with an additional set of conductive elements  192  extending perpendicularly from additional guard structure  190 . Additional guard structure  190  may be a distinct structure and/or function from first and second metal guard structures  110 ,  112 . In a first example, additional guard structure  190  may be a moisture oxidation barrier (MOB). Although crack stop and MOB structures each may be formed of metal, MOB structures are specifically designed to prevent moisture ingress from the exterior of a chip from traveling toward active components of a device. An MOB structure thus may have more or fewer vias connecting metal regions in respective metal layers, as compared to the number of via-type connecting metals in a metal crack stop. In still further examples, additional guard structure  190  may include a guard ring specifically designed to protect photonic integrated circuit (PIC) dies. A guard ring may refer to a continuous wall of metal concentrically surrounding the active electrical and optical components on a PIC die. In further examples, additional guard structure  190  may be an additional metal crack stop, either distinct or substantially identical to first and second metal guard structures  110 ,  112 . 
     Regardless of the structure and function of additional metal guard structure  190 , embodiments of the disclosure may include each structure  110 ,  112 ,  190  near perimeter  108  of IC structure  100 . In this arrangement, each structure  110 ,  112 ,  190  may have one or more pluralities of conductive elements  122 ,  124 ,  192  interdigitating with the conductive elements of an adjacent structure. Portions of each guard structure  110 ,  112 ,  190  may extend in parallel with each other to permit interdigitating of conductive elements  122 ,  124 ,  192  as discussed herein. In the example of  FIG. 6 , second metal guard structure  112  has two pluralities of conductive elements  124 A,  124 B extending perpendicularly therefrom. However, it is understood that other metal guard structures  110 ,  190  may additionally or alternatively have multiple pluralities of conductive elements extending therefrom. In the further example of  FIG. 7 , first metal guard structure  110  may lack any conductive elements, while second metal guard structure  112  and additional metal guard structure  190  may feature interdigitating conductive elements  124 ,  192  horizontally inward of first metal guard structure  110 . It is possible for second metal guard structure  112  to lack conductive elements, and/or for multiple additional guard structures  190  to have sets of interdigitating conductive elements  192  therebetween. 
     The resulting IC structure (and/or sensor) described herein can be distributed by the fabricator in raw wafer form (that is, as a single wafer that has multiple unpackaged chips), as a bare die, or in a packaged form. In the latter case, the chip is mounted in a single chip package (such as a plastic carrier, with leads that are affixed to a motherboard or other higher level carrier) or in a multichip package (such as a ceramic carrier that has either or both surface interconnections or buried interconnections). In any case, the chip may be then integrated with other chips, discrete circuit elements, and/or other signal processing devices as part of either (a) an intermediate product, such as a motherboard, or (b) an end product. The end product can be any product that includes integrated circuit chips, ranging from toys and other low-end applications to advanced computer products having a display, a keyboard or other input device, and a central processor. 
     Referring now to  FIGS. 1, 3, and 8  together, embodiments of the disclosure provide methods of monitoring moisture ingress, crack propagation, and/or other characteristics of IC structure  100 . The various processes described herein may be implemented at the time of manufacture, or various processes may be implemented and/or repeated after the time of manufacture to periodically monitor the condition of IC structure  100 . The illustrative flow diagram of  FIG. 8  represents one possible ordering of the various processes discussed herein, and it is understood that some processes may be re-ordered, modified, omitted, etc., without departing from the technical details and features discussed herein. Regardless of whether moisture ingress and/or crack propagation through IC structure  100  is being monitored, the same processes and/or metrics may be used in methods according to the disclosure. Points of reference and/or comparison may be adjusted for closer sensitivity to the source of electrical variation, e.g., moisture or crack propagation. 
     An initial process P 1  (shown in phantom) may include forming an embodiment of IC structure  100 , including BEOL stack  102  with metal guard structures  110 ,  112 . The providing may include forming IC structure  100  with conductive elements  122 ,  124  to monitor a portion of BEOL stack  102  near perimeter  108 . Process P 1  may also include modifying and/or otherwise obtaining IC structure  100  with the various components thereof integrated therein. Process P 1  in some cases may also include coupling metal guard structures  110 ,  112 , or other components of IC structure  100  where applicable, to sensor  150  as discussed herein. Regardless of how IC structure  100  is provided, the following processes discussed herein may be implemented on any embodiment of IC structure  100  discussed herein. 
     Continuing to process P 2 , embodiments of the disclosure allow a user to monitor moisture ingress and/or crack propagation through IC structure  100 . Embodiments of IC structure  100  monitor moisture ingress and/or crack propagation near perimeter  108  of IC structure  100  by testing the flow of electricity through metal guard structures  110 ,  112 . At process P 2 , a user may apply a voltage across metal guard structures  110 ,  112  of IC structure  100 , e.g., using sensor  150  connected to metal guard structures  110 ,  112  at opposite terminals. Sensor  150  may be configured to apply a static voltage, e.g., 50 millivolts (mV) across its terminals, and may be configured to measure an electrical current or capacitance induced across metal guard structures  110 ,  112 . As noted elsewhere herein, the voltage applied in process P 2  may be configured to mirror an operating voltage of active circuitry  104 . In this case, the applying of a voltage in process P 2  may include enabling a unity-gain amplifier of sensor  150  to apply an identical voltage across metal guard structures  110 ,  112 . 
     Process P 3  of the present disclosure may include measuring a test current or capacitance across metal guard structure  110 ,  112  while applying the test voltage to IC structure  100 . The presence of interdigitating conductive elements  122 ,  124  may create a measurable electrical pathway horizontally across BEOL stack  102  across each metal guard structure  110 ,  112 . In various embodiments, the measured electrical pathway between metal guard structures  110 ,  112  optionally may include and/or rely upon embodiments of sensing wire  186  ( FIGS. 4, 5 ). Sensor  150  thus may include an ammeter, capacitance meter, and/or other component for measuring the electrical current or capacitance induced by a fixed test voltage (e.g., to measure resistance) or an alternating current (AC) voltage in combination with the fixed test voltage (e.g., to measure capacitance). The measured current or capacitance may be identified herein as a test current or capacitance, i.e., the electrical current or capacitance induced by a voltage (e.g., a fixed DC voltage or an applied AC voltage as noted above) during one instance of testing. Although measuring a voltage-induced current may be more responsive for many IC structures  100 . 
     Continuing to process P 4 , embodiments of the disclosure may include analyzing the current or capacitance to determine whether significant moisture ingress and/or crack propagation exists in IC structure  100 . Process P 4  may include calculating a differential current or capacitance between the measured test current or capacitance and one or more predetermined reference values. In various embodiments, a specification for IC structure  100  may include a minimum current or capacitance, and/or range of currents/capacitances indicative of mechanical stability between metal guard structures  110 ,  112 . The specification current or capacitance through metal guard structures  110 ,  112  may be denoted as a reference value, regardless of whether the specification defines a minimum current/capacitances, range of currents/capacitances, etc. The reference value may be obtained via design characteristics of IC structure  100  or similar product units, or may be obtained by performing experiments on IC structure  100  as noted herein relative to further embodiments. In any case, the differential current or capacitance calculated in process P 4  may be a subtractive difference between the test current or capacitance measured in process P 3 , and one or more values representing a reference value for comparison. In the case of a single reference value, the differential current calculated in process P 4  may be a difference in amperes (A) between the measured test current and the predetermined reference value. When conducting a test based on capacitance, a difference in capacitance calculated in process P 4  may be a difference in picofarads (pF) between the measured test capacitance and the predetermined reference value. In the case of a range of reference values, the differential current or capacitance may be a subtractive difference between the measured test current or capacitance and one or more values within the specified reference value range (e.g., maximum value, minimum value, median value, set percentile value, etc.) 
     Embodiments of the disclosure may include process P 5  of determining whether the differential current or capacitance calculated in process P 4  exceeds a threshold for IC structure  100 . As used herein, “threshold” may indicate a minimum safe difference between measured currents or capacitances and reference currents or capacitances. Exceeding the threshold may indicate a significant increase of current or capacitance between metal guard structures  110 ,  112 . The basis for determining whether the calculated differential current or capacitance exceeds, or does not exceed, the threshold may depend at least partially on the calculation(s) implemented in processes P 3 , P 4 . In an example where the differential current or capacitance is calculated as a single value, process P 5  may determine whether the differential current or capacitance indicates that the test current or capacitance exceeds the reference value by a predetermined threshold. The predetermined threshold may represent, e.g., a window of acceptable currents/capacitances indicating that moisture ingress and/or crack propagation is not significant. In another example, the differential current or capacitance may be calculated as a range of values relative to a range of reference values for IC structure  100 . In this case, process P 5  may include determining whether any or all values in the calculated range exceed the corresponding range of reference values by at least a minimum amount. In cases where the calculated differential current or capacitance does not exceed the threshold (i.e., “No” at process P 5 ), the flow may return to process P 2  of applying another voltage to first and second metal guard structures  110 ,  112 , or otherwise may conclude (“Done”). In either case, IC structure  100  may continue to operate without further testing for a predetermined time (e.g., several hours of operating time, several days or hours without regard to operation, etc.) before repeating any or all of processes P 2 , P 3 , P 4 , and/or P 5 . In the case where the differential current or capacitance exceeds the threshold (i.e., “Yes” at process P 5 ), the flow may proceed to additional steps to account for possible moisture ingress and/or crack propagation. 
     To account for possible moisture ingress and/or crack propagation through metal guard structures  110 ,  112  of IC structure  100 , process P 6  may include transmitting a signal to active circuitry  104  to indicate the measured leakage. The transmitting of a signal to indicate leakage may include, e.g., directly transmitting a signal from sensor  150  to active circuitry  104  through corresponding wires, wirelessly signaling active circuitry  104 , etc. Various embodiments of process P 6  may include any other currently known or later developed methods for causing IC structure  100  to notify a user that leakage from moisture ingress and/or crack propagation is detected. The flow may then conclude (“Done”) after signaling the detected leakage to active circuitry  104 . In further embodiments, the method optionally may include process P 7  of ceasing further operation of IC structure  100 . In this case, active circuitry  104  may be configured to terminate further operation upon detecting leakage through metal guard structures  110 ,  112 . According to one example, a power supply of IC structure  100  may be configured to shut down upon receiving one or more signals transmitted from sensor  150 , e.g., in process P 6 . The method may then conclude (“Done”) to allow a user, device, etc., to further examine, repair, and/or otherwise adjust IC structure  100  to account for possible moisture ingress and/or crack propagation. 
     Referring now to  FIGS. 1, 3, and 9  together, further embodiments of the disclosure may optionally include processes P 1 - 1 , P 1 - 2 , P 1 - 3 , P 1 - 4  to prepare IC structure  100  for monitoring of moisture ingress and/or crack propagation. It is understood that processes P 1 - 1 , P 1 - 2 , P 1 - 3 , P 1 - 4  may be implemented only once following process P 1 , and/or periodically during the field lifespan of IC structure  100 . Process P 1 - 1  may include, e.g., applying a calibration voltage to IC structure  100  through sensor  150  immediately after manufacturing of IC structure  100 . The calibration voltage in process P 1 - 1  may be the same or different from the voltage applied in process P 2  and discussed elsewhere herein. In any case, the applied calibration voltage may measure current flow and/or capacitance through metal guard structures  110 ,  112  before dicing, or otherwise before any moisture ingress or crack propagation through IC structure  100  after manufacture. The method may then proceed to optional process P 1 - 2  of measuring one or more the ensuing current flows and/or capacitances through metal guard structures  110 ,  112  as the reference value for IC structure  100 . The flow may then proceed to process P 1 - 3  of recording the reference value in memory element  133  of IC structure  100 . For example, process P 1 - 3  may include programming an e-fuse of memory element  133 , and/or storing the recorded values of current and/or capacitance in memory cells. 
     Further embodiments of the disclosure may include calculating one or more values for the threshold of process P 5 . Calculating the threshold in process P 1 - 4  may include, e.g., analyzing a reliability model for current and/or capacitance for IC structure  100  to predict future performance of a device after dicing or deployment. Based on the reliability model and/or other representation of future device operation, process P 1 - 4  may include calculating a set value or range of acceptable electrical currents and/or capacitances through metal guard structures  110 ,  112  when significant crack propagation and/or moisture ingress does not occur. According to further examples, the calculating of the threshold may be based at least partially on a projected moisture profile of metal guard structures  110 ,  112 . Calculating the threshold may also depend on a projected crack propagation through metal guard structures  110 ,  112 . In either case, the threshold may rely on projected changes to electrical conductance between metal guard structures  110 ,  112  in a model of moisture ingress and/or crack propagation. The calculated threshold may then be stored, e.g., in memory components of active circuitry  104 , elsewhere within IC structure  100 , or may by stored for use in process P 5  by other currently known or later developed techniques for applying data in an analysis. In any case, the flow may proceed to process P 2  after implementing one or more of optional processes P 1 - 1 , P 1 - 2 , P 1 - 3 , P 1 - 4  and/or operating IC structure  100  over a set time span. 
     The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the disclosure. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise. It will be further understood that the terms “comprises” and/or “comprising,” when used in this specification, specify the presence of stated features, integers, steps, operations, elements, and/or components, but do not preclude the presence or addition of one or more other features, integers, steps, operations, elements, components, and/or groups thereof. “Optional” or “optionally” means that the subsequently described event or circumstance may or may not occur, and that the description includes instances where the event occurs and instances where it does not. 
     Approximating language, as used herein throughout the specification and claims, may be applied to modify any quantitative representation that could permissibly vary without resulting in a change in the basic function to which it is related. Accordingly, a value modified by a term or terms, such as “about,” “approximately,” and “substantially,” are not to be limited to the precise value specified. In at least some instances, the approximating language may correspond to the precision of an instrument for measuring the value. Here and throughout the specification and claims, range limitations may be combined and/or interchanged, such ranges are identified and include all the sub-ranges contained therein unless context or language indicates otherwise. “Approximately” as applied to a particular value of a range applies to both values, and unless otherwise dependent on the precision of the instrument measuring the value, may indicate +/−10% of the stated value(s). 
     The corresponding structures, materials, acts, and equivalents of all means or step plus function elements in the claims below are intended to include any structure, material, or act for performing the function in combination with other claimed elements as specifically claimed. The description of the present disclosure has been presented for purposes of illustration and description, but is not intended to be exhaustive or limited to the disclosure in the form disclosed. Many modifications and variations will be apparent to those of ordinary skill in the art without departing from the scope and spirit of the disclosure. The embodiment was chosen and described in order to best explain the principles of the disclosure and the practical application, and to enable others of ordinary skill in the art to understand the disclosure for various embodiments with various modifications as are suited to the particular use contemplated.