BACKGROUND 
       [0001]    The present invention generally relates to integrated circuits (IC), and more particularly to fabricating IC chips having a programmable shelf life. 
         [0002]    New IC technologies may include individual IC chips (i.e., “dies”) arranged into a three dimensional integrated circuit, also known as a three dimensional semiconductor package (3D package). One type of 3D package may include two or more layers of active electronic components stacked vertically and electrically joined with some combination of through substrate vias and solder bumps. Current IC chips may never expire, they may exhibit a long lifespan and may even last forever if not powered up. 
         [0003]    To continue the miniaturization trend in current IC technology, copper (Cu) metallization may be extensively used due to its low resistivity and high migration resistance. Owing to the rapid diffusion of copper into silicon (Si) and silicon dioxide (SiO 2 ), copper structures may be covered with barrier metals and barrier insulators to prevent degradation of the IC. In the presence of oxygen and at relatively low temperatures, copper may act as a catalyst during the oxidation of silicon to form silicon dioxide. 
       SUMMARY 
       [0004]    The ability to manufacture integrated circuit (IC) chips having a programmable shelf life that may allow the IC chips to regain operability after expiring may prevent, among other things, misuse of sensitive data stored in the IC chips and/or stop unauthorized use of the IC chips after a certain period of time. 
         [0005]    According to an embodiment of the present disclosure, a structure may include a first interconnect structure and a second interconnect structure each located within an interlevel dielectric (ILD), a first top metal layer and a second top metal layer disposed on and in direct electrical connection with the first interconnect, a third top metal layer and a fourth top metal layer disposed on and in direct electrical connection with the second interconnect, a silicon dioxide layer above the first, second, third and fourth top metal layers, the silicon layer is in direct contact with the first and fourth top metal layers, and a barrier layer separating the silicon dioxide layer from each of the second and third top metal layers, a high resistance connection exist between the third top metal layer and the fourth top metal layer due to the presence of the silicon dioxide layer. 
         [0006]    According to another embodiment of the present disclosure, a structure may include a plurality of top metal layers in an interlevel dielectric (ILD), the plurality of top metal layers are electrically connected to one or more interconnect structures of an IC chip, a barrier layer directly above two adjacent top metal layers, the two adjacent top metal layers being located between two outer top metal layers, and an oxidized silicon layer above the two adjacent top metal layers and the two outer top metal layers, the oxidized silicon layer being directly on top of the outer top metal layers, the oxidized silicon layer is separated from the two adjacent top metal layers by the barrier layer, the oxidized silicon layer, the two adjacent top metal layers and the barrier layer includes a sensing circuit, the oxidized silicon layer damages the sensing circuit and makes the IC chip inoperable. 
     
    
     
       BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS 
         [0007]    The following detailed description, given by way of example and not intended to limit the invention solely thereto, will best be appreciated in conjunction with the accompanying drawings, in which: 
           [0008]      FIG. 1  is a cross-sectional view of an integrated circuit (IC) chip, according to an embodiment of the present disclosure; 
           [0009]      FIG. 2  is a cross-sectional view of the IC chip depicting forming a barrier layer and a silicon layer, according to an embodiment of the present disclosure; 
           [0010]      FIG. 3  is a cross-sectional view of the IC chip depicting forming a protective layer, according to an embodiment of the present disclosure; 
           [0011]      FIG. 4  is a cross-sectional view of the IC chip depicting forming a solder structure, according to an embodiment of the present disclosure; and 
           [0012]      FIG. 5  is a cross-sectional view of the IC chip depicting oxidizing the silicon layer, according to an embodiment of the present disclosure. 
       
    
    
       [0013]    The drawings are not necessarily to scale. The drawings are merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention. In the drawings, like numbering represents like elements. 
       DETAILED DESCRIPTION 
       [0014]    Detailed embodiments of the claimed structures and methods are disclosed herein; however, it may be understood that the disclosed embodiments are merely illustrative of the claimed structures and methods that may be embodied in various forms. This invention may, however, be embodied in many different forms and should not be construed as limited to the exemplary embodiments set forth herein. Rather, these exemplary embodiments are provided so that this disclosure will be thorough and complete and will fully convey the scope of this invention to those skilled in the art. 
         [0015]    In the following description, numerous specific details are set forth, such as particular structures, components, materials, dimensions, processing steps, and techniques, in order to provide a thorough understanding of the present invention. However, it will be appreciated by one of ordinary skill of the art that the invention may be practiced without these specific details. In other instances, well-known structures or processing steps have not been described in detail in order to avoid obscuring the invention. 
         [0016]    It will be understood that when an element as a layer, region, or substrate is referred to as being “on” or “over” another element, it may be directly on the other element or intervening elements may also be present. In contrast, when an element is referred to as being “directly on” or “directly over” another element, there are no intervening elements present. It will also be understood that when an element is referred to as being “beneath,” “below,” or “under” another element, it may be directly beneath or under the other element, or intervening elements may be present. In contrast, when an element is referred to as being “directly beneath” or “directly under” another element, there are no intervening elements present. 
         [0017]    In the interest of not obscuring the presentation of embodiments of the present invention, in the following detailed description, some processing steps or operations that are known in the art may have been combined together for presentation and for illustration purposes and in some instances may have not been described in detail. In other instances, some processing steps or operations that are known in the art may not be described at all. It should be understood that the following description is rather focused on the distinctive features or elements of various embodiments of the present invention. 
         [0018]    An integrated circuit (IC) chip may include a plurality of interconnected electronic circuits formed on a semiconductor substrate. IC chips may have a long lifespan, in some cases they may never expire. The extended life of current electronic devices, including IC chips, may put at risk sensitive data stored in such devices if unauthorized access occurs. Limiting the life of an IC chip may find applications in areas such as weapon systems, chip security, and/or cyber security where sensitive information may need to be destroyed after a certain period of time in order to avoid security threats that may arise from inappropriate use of the stored information. Accordingly, fabricating IC chips with a programmable shelf life that may allow the IC chip to stop working after a determined period of time but may be reprogrammed to regain operability may, among other potential benefits, enhance data security in many industry and government sectors. 
         [0019]    Referring now to  FIG. 1 , a cross-sectional view of an integrated circuit (IC) chip  100  after completion of a semiconductor metallization step is shown, according to an embodiment of the present disclosure. At this stage of the fabrication process, the IC chip  100  may include numerous electronic devices  140  formed on a substrate  160 . In an exemplary embodiment, the electronic devices  140  may include field effect transistor (FET) devices such as transistors, capacitors, and the like. The electronic devices  140  may include gate structures  14 , source-drain regions  16  and a plurality of metal contacts  18  (hereinafter “contacts”). Typically, the gate structures  14  may be energized to create an electric field in an underlying channel region of the substrate  160 , by which charge carriers may be allowed to travel through the channel region between the source-drain regions  16  of the substrate  160 . The contacts  18  may be subsequently formed to electrically connect the electronic devices  140  to subsequently formed metallization layers. The contacts  18  may typically include tungsten (W). 
         [0020]    The substrate  160  may be, for example, a semiconductor-on-insulator (SOI) substrate, where a buried insulator layer (not shown) separates a base substrate (not shown) from a top semiconductor layer (not shown). The components of the IC chip  100 , including the electronic devices  140 , may then be formed in or adjacent to the top semiconductor layer. In other embodiments, the substrate  160  may be a bulk substrate which may be made from any of several known semiconductor materials such as, for example, silicon, germanium, silicon-germanium alloy, carbon-doped silicon, carbon-doped silicon-germanium alloy, and compound (e.g. III-V and II-VI) semiconductor materials. Non-limiting examples of compound semiconductor materials include gallium arsenide, indium arsenide, and indium phosphide. 
         [0021]    The IC chip  100  may also include various metal interconnect structures  120  (hereinafter “interconnect structures”) that may be formed in an interlevel dielectric (ILD)  180 . The interconnect structures  120  may typically be formed by depositing a dielectric layer (such as the ILD  180 ) above the electronic devices  140 , etching a recess (not shown) in the dielectric layer and filling the recess with a metal. The electronic devices  140  may be coupled to the interconnect structures  120  through the contacts  18  to conduct current though the different circuit layers of the IC chip  100 . In some embodiments, multiple layers of the ILD  180  may be formed above the electronic devices  140 . The interconnect structures  120  may include, for example, wires, trenches, or vias. In the depicted embodiment, the interconnect structures  120  may include a copper-rich material. It should be noted that the process of forming the electronic devices  140  and the interconnect structures  120  is typical and well known to those skilled in the art. 
         [0022]    The IC chip  100  may further include top metal layers  110   a - 110   e  formed in the ILD  180 . The top metal layers  110   a - 110   e  may be disposed on and electrically connected to the interconnect structures  120  providing a pad to be used during a subsequent chip bonding process such as, for example, a Controlled Collapse Chip Connection (C4) process which may be conducted to couple or join the IC chip  100  to a circuit board (not shown). The top metal layers  110   a - 110   e  may include a copper-rich material that may be deposited by any suitable deposition technique, such as, for example, by chemical vapor deposition (CVD). The top metal layers  110   a - 110   e  may have a thickness ranging from approximately 0.5 μm to approximately 2 μm. 
         [0023]    The top metal layers  110   b - 110   e  may alternatively be referred to as “terminals”. More specifically, the top metal layers  110   b,    110   e  may be referred to as “oxidizing terminals”, and the top metal layers  110   c,    110   d  may be referred to as “sensing terminals”. 
         [0024]    Referring now to  FIG. 2 , a barrier layer  150  may be formed above the sensing terminals  110   c,    110   d.  The barrier layer  150  may prevent the copper of the sensing terminals  110   c,    110   d  from coming into direct contact with a subsequently formed silicon layer. By doing so, electrical contact may be maintained between the sensing terminals  110   c,    110   d  and the silicon layer  220  ( FIG. 3 ) forming a sensing circuit in the IC chip  100 . This sensing circuit may behave similarly to a fuse, as will be described in detail below. 
         [0025]    In one embodiment, the barrier layer  150  may include titanium nitride (TiN). In another embodiment, the barrier layer  150  may include tantalum nitride (TaN). The barrier layer  150  may be formed by any suitable deposition technique known in the art, such as, for example, CVD. The barrier layer  150  may have a thickness ranging from approximately 10 nm to approximately 100 nm. 
         [0026]    With continued reference to  FIG. 2 , a silicon layer  220  may be formed and patterned above the ILD  180 . More specifically, the silicon layer  220  may be in direct contact with the oxidizing terminals  110   b,    110   e  and above the sensing terminals  110   c,    110   d  such that the oxidizing terminals  110   b,    110   e  may cause the silicon layer  220  to oxidize. In one exemplary embodiment, the silicon layer  220  may be formed by CVD of an amorphous silicon (a-Si) material. The silicon layer  220  may have a thickness ranging from approximately 100 nm to approximately 1 μm and a length ranging from approximately 100 μm to approximately 10,000 μm. 
         [0027]    By intentionally forming the silicon layer  220  in direct contact with the oxidizing terminals  110   b,    110   e,  properties of the chemical reaction between copper and silicon in the presence of oxygen may be used to limit the life of the IC chip  100 . Under ambient (room) conditions of pressure and temperature, copper atoms from the oxidizing terminals  110   b,   110   e  may diffuse to the silicon layer  220  having a catalytic effect on the oxidation of the silicon layer  220  which may lead to the formation of a silicon dioxide compound (not shown). Owing to these properties, as the IC chip is exposed to an oxygen-containing environment and time progresses, the chemical reaction between silicon, oxygen, and copper may take place until the silicon layer  220  may be consumed and replaced by a silicon dioxide layer. This may result in a substantially high electrical resistance connection between the sensing terminals  110   c,    110   d,  since the silicon dioxide layer formed as a result of the oxidation of the silicon layer  220  may act as an insulator between the sensing terminals  110   c,    110   d.  The high electrical resistance between the sensing terminals  110   c,    110   d  may limit current flow between these terminals of the sensing circuit creating an electrical open or open circuit within the IC chip  100  which may cause the IC chip  100  to be inoperable. The electrical configuration of the IC chip  100 , the location of the silicon layer  220  and the properties of the oxidation reaction between silicon and copper in the presence of oxygen may help imposing a shelf life to the IC chip  100  as will be described in detail below. 
         [0028]    Referring now to  FIG. 3 , a protective layer  310  may be formed above the ILD  180 . The protective layer  310  may function as a passivation or stress buffer layer during a subsequent C4chip packaging process. In an exemplary embodiment, the protective layer  180  may include a polyamide layer. The protective layer  180  may be formed by means of any suitable deposition technique known in the art, such as, for example, CVD. The protective layer  310  may have a thickness ranging from approximately 20 nm to approximately 500 nm. The protective layer  310  may subsequently be patterned to form an opening  320  that may expose the top metal layer  110   a  and uncover the silicon layer  220 . It should be understood that the steps involved in patterning the protective layer  310  are typical and well-known to those skilled in the art. 
         [0029]    Referring now to  FIG. 4 , a solder structure  420  may be formed in the opening  320  ( FIG. 3 ). The solder structure  420  may include a solder bump  46  formed over a solder layer  44 . The solder layer  44  may include TiN, TaN or any other suitable metal. Although the solder layer  44  is shown as a single layer, the solder layer  44  may include one or more layers. A CVD or physical vapor deposition (PVD) process may be used to form the solder layer  44  in the opening  320 . The solder layer  44  may have a thickness ranging from approximately 100 nm to approximately 10,000 nm. 
         [0030]    The solder bump  46  may include, for example, a micro bump, a general bump, a ball grid array (BGA) solder ball, or any other suitable solder structure made of a solder material such as Sn, Ag, Cu, or any combination thereof. Methods of forming the solder bump  46  over the solder layer  44  may include, for example, electroplating, chemical plating, or any other suitable technique. A patterned photoresist layer (not shown) may help to form the solder bump  46  in the opening  320  ( FIG. 3 ). Although one solder bump  46  is shown in  FIG. 4 , it should be understood that there may be multiple solder bumps  46  in the IC chip  100  formed in a similar way. The solder bump  46  may have a substantially spherical shape with a diameter ranging from approximately 60 μm to approximately 150 μm. 
         [0031]    Referring now to  FIG. 5 , the IC chip  100  may be exposed to an oxygen-containing environment  50  so that a silicon dioxide (SiO 2 ) layer  540  may be formed as a result of the catalytic oxidation of the silicon layer  220  ( FIG. 4 ). By exposing the IC chip  100  to the oxygen-containing environment  50 , at ambient conditions of pressure and temperature, oxidation of the silicon layer  220  ( FIG. 4 ) may occur. During the oxidation process copper atoms from the oxidizing terminals (or top metal layers)  110   b,    110   e  may diffuse to the silicon layer  220  and may catalyze the oxidation reaction. The catalytic oxidation of the silicon layer  220  ( FIG. 4 ) may cause spontaneous growth of the silicon dioxide layer  540 . 
         [0032]    The silicon dioxide layer  540  may horizontally grow from an edge region towards a center region of the silicon layer  220  ( FIG. 4 ) until substantially consuming or replacing the silicon layer  220  ( FIG. 4).The  oxidation of the silicon layer  220  ( FIG. 4 ) may result from the segregation of copper atoms from the top metal layers  110   b,    110   e  at an interface between the silicon layer  220  ( FIG. 4 ) and the growing silicon dioxide layer  540  and of oxygen diffusion through the growing silicon dioxide layer  540 . Typically, the rate at which the silicon dioxide layer  540  grows may depend on the amount of copper atoms present at the moving interface between the silicon layer  220  ( FIG. 4 ) and the silicon dioxide layer  540 . 
         [0033]    In one embodiment, the growth rate of the silicon dioxide layer  540 , at ambient conditions and without any additional power supply, may range from approximately 150 nm/month to approximately 1 μm/month. The catalytic effect of interfacial copper atoms from the top metal layers (or oxidizing terminals)  110   b,    110   e  may facilitate the oxidation of the silicon layer  220  ( FIG. 4 ) by changing the atomic bonding arrangement at the interface. It should be noted that a constant supply of oxygen may be required to carry out this reaction at ambient conditions. In one exemplary embodiment, the oxygen-containing fluid  50  surrounding the IC chip  100  may include ambient air. 
         [0034]    As oxidation of the silicon layer  220  ( FIG. 4 ) progresses, the growing silicon dioxide layer  540  may approach the sensing terminals  110   c,    110   d.  Once above the sensing terminals  110   c,    110   d,  the silicon dioxide layer  540  may function as an insulator layer substantially halting current flow through the sensing terminals  110   c,    110   d  thereby creating an electrical open in the IC chip  100 . As mentioned above, the connection between the top metal layers (sensing terminals)  110   c,    110   d  and the silicon layer  220  ( FIG. 4 ) may behave similarly to a fuse. By oxidizing the silicon layer  220  ( FIG. 4 ) and forming the silicon dioxide layer  540 , the fuse may be damaged creating an open circuit that may cause the IC chip  100  to become inoperable. 
         [0035]    However, since only one fuse of the IC chip  100  may be damaged by the formation of the silicon dioxide layer  540 , it may be possible to repair the IC chip  100  to regain operability. For instance, in one embodiment, the IC chip  100  may be repaired by entering a predetermined code that may allow the IC chip  100  to be reprogrammed to function again without the damaged fuse. Stated differently, by reprogramming the IC chip  100 , the damaged region of the circuit (fuse) may be bypassed allowing the IC chip  100  to regain operability. It should be noted that only the designer or manufacturer of the IC chip  100  may have the ability to reprogram the circuit in order to bypass the damaged connection. 
         [0036]    The time required to oxidize the silicon layer  220  ( FIG. 4 ) may be proportional to the length of the silicon layer  220  ( FIG. 4 ). Therefore, the length of the silicon layer  220  ( FIG. 4 ) may define a time for the IC chip  100  to become inoperable and hence the shelf life or expiration date of the IC chip  100 . Accordingly, the larger the length of the silicon layer  220  ( FIG. 4 ) the longer the shelf life of the IC chip  100  may be since oxidation of the silicon layer  220  ( FIG. 4 ), and hence the formation of the silicon dioxide layer  540 , may take longer time to occur. 
         [0037]    Therefore, the catalytic oxidation reaction between copper, oxygen and silicon at ambient conditions of pressure and temperature may be used to fabricate IC chips having a programmable shelf life. The ability to limit the life of IC chips may improve data security in different industry and government sectors. Since the oxidation reaction may take place at ambient conditions additional power supply may not be required which may potentially reduce manufacturing costs. Further, since only a fuse may be damaged by the formation of the silicon dioxide layer, the manufacturer may be able to reprogram the IC chip in order to bypass the damaged connection and reestablish functionality of the IC chip. 
         [0038]    The descriptions of the various embodiments of the present invention have been presented for purposes of illustration, but are not intended to be exhaustive or limited to the embodiments 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 described embodiments. The terminology used herein was chosen to best explain the principles of the embodiment, the practical application or technical improvement over technologies found in the marketplace, or to enable others of ordinary skill in the art to understand the embodiments disclosed herein.
A structure includes a first interconnect structure and a second interconnect structure each located within an interlevel dielectric (ILD), a first top metal layer and a second top metal layer disposed on and in direct electrical connection with the first interconnect, a third top metal layer and a fourth top metal layer disposed on and in direct electrical connection with the second interconnect, a silicon dioxide layer above the first, second, third and fourth top metal layers, the silicon layer is in direct contact with the first and fourth top metal layers, and a barrier layer separating the silicon dioxide layer from each of the second and third top metal layers, a high resistance connection exist between the third top metal layer and the fourth top metal layer due to the presence of the silicon dioxide layer.