Abstract:
A tamper-resistant microchip package contains fluid- or nanofluid-filled capsules, channels, or reservoirs, wherein the fluids, either alone or in combination, can destroy circuitry by etching, sintering, or thermally destructing when reverse engineering of the device is attempted. The fluids are released when the fluid-filled cavities are cut away for detailed inspection of the microchip. Nanofluids may be used for the sintering process, and also to increase the thermal conductivity of the fluid for die thermal management. Through-vias and micro vias may be incorporated into the design to increase circuitry destruction efficacy by improving fluid/chip contact. Thermal interface materials may also be utilized to facilitate chip cooling.

Description:
FIELD OF THE INVENTION 
       [0001]    The present invention relates to tamper-resistant microchip technology and, more particularly, to microchip packages comprising fluid- or nanofluid-filled capsules, channels, or reservoirs in which fluid released during a tampering event will destroy dies or circuitry via etching, sintering, or thermal destruction. 
       BACKGROUND OF THE INVENTION 
       [0002]    The development of microelectronic circuitry is presently one of the most active areas of technological advancement, and therefore demands substantial economic and infrastructure resources. To recoup their investment and maintain a competitive position in the marketplace, microelectronic device manufacturers strive to protect their cutting-edge technologies from unauthorized disclosure. Once the development of microelectronic components reaches the manufacturing stage, however, and the devices are distributed to customers or otherwise fall into the hands of third parties, manufacturers lose a great deal of control over their respective technologies. At that point, undesired reverse engineering by third parties becomes a significant concern. 
         [0003]    It is the microprocessor element of the chip package that generally contains the sensitive, cutting-edge technology that is sought to be protected from disclosure. With prior art microchip devices, third parties may isolate the microprocessor for detailed evaluation by removing it from the chip package by, for example, grinding or separating (cutting) the substrate or lid away to expose the microprocessor chip. The exposed microprocessor chip may then be analyzed with a suitable device, such as a microscope, thereby enabling the unauthorized third party to gain access to valuable trade secret information. Lockable cases or the like are insufficient because these may be cut away and removed to expose the chip package, which may then be reverse engineered as described above. 
         [0004]    Presently, no known effective method prevents reverse engineering; third parties, after coming into physical possession of the sensitive technological devices, can take them back to a laboratory and manipulate them at will. There is a need, therefore, to provide a chip level package that will frustrate or defeat third party attempts to reverse engineer sensitive technologies, even when the third parties have uninhibited access to the microprocessor chip package. 
         [0005]    The most effective method of protection would be a quality inherent in the chip package itself, which would serve to destroy microprocessor components if attempts at reverse engineering are made. In particular, this could be accomplished by a chip package that would result in destruction of the microprocessor upon attempts to separate the chip from the chip package components, such as the substrate or lid, before attempts could be made to analyze it under a microscope. Destruction of the microprocessor, as described, would further prevent the extraction of sensitive software or memory stored in the chip. 
         [0006]    Integrated circuit boards may be constructed by depositing a conductive or semi-conductive layer onto a non-conductive board or substrate. A chip trace is then laid out on the board with a protective, chemically-inert coating. An etchant is applied to the surface of the board, which dissolves the portion of the conductive or semi-conductive layer not protected by the coating. The coating is then removed, leaving the bare chip trace behind. 
         [0007]    Certain chemical processes typically used in the microprocessor manufacturing industry may be incorporated into tamper-resistant microprocessor chip packages. One of these chemical processes involves the etching of the chip circuitry. 
         [0008]    U.S. Pat. No. 7,494,608 issued to Li et al. for STABILIZED SILVER NANOPARTICLE COMPOSITION discloses an example of a fluid that may be suitable for use with the present invention. Therein is described a composition comprising a liquid and a plurality of silver-containing nanoparticles with a stabilizer, wherein the silver-containing nanoparticles are a product of a reaction of a silver compound with a reducing agent comprising a hydrazine compound in the presence of a thermally removable stabilizer in a reaction mixture comprising the silver compound, the reducing agent, the stabilizer, and an organic solvent wherein the hydrazine compound is a hydrocarbyl hydrazine, a hydrocarbyl hydrazine salt, a hydrazide, a carbazate, and sulfonohydrazide, or a mixture thereof, and wherein the stabilizer includes an organoamine. Electronic circuit elements may be fabricated by depositing a composition comprising a liquid and a plurality of silver-containing nanoparticles with a stabilizer on a substrate by a liquid deposition technique to form a deposited composition. 
         [0009]    It is believed that the present invention contains a high degree of novelty in the field of microelectronic devices. No known prior art is closely related to the technology disclosed herein. A brief discussion of prior art references is presented below, however, as a point of comparison. 
         [0010]    In U.S. Pat. No. 7,685,438 issued to Knudsen for TAMPER-RESISTANT PACKAGING AND APPROACH USING MAGNETICALLY-SET DATA there is described a tamper-resistant packaging approach that protects an integrated circuit from undesirable access. According to an example embodiment, data is encrypted as a function of the state of a plurality of magnetically-responsive circuit element and then decrypted as a function of the state. A package is arranged to prevent access to the integrated circuit and having magnetic particles therein. The magnetic particles are arranged to cause the magnetically-responsive circuit elements to take on a state that is used to encrypt the data. The state of these elements is again accordingly used to decrypt the data (e.g., as a key). When the magnetic particles are altered, for example by removing a portion of the package, the state of one or more of the magnetically-responsive circuit elements is changed, thus rendering the state incapable of being used for decrypting the data. 
         [0011]    Under certain circumstances, encryption of integrated circuit data and subsequent decryption by use of a magnetic key could offer sufficient protection of sensitive data electronically stored on a chip. In other circumstances, however, a more robust form of protection may be necessary. Magnetic encryption/decryption technology would not, for instance, protect the chip architecture from reverse engineering by third parties. Moreover, damage or destruction to the chip components, as described hereinbelow, will permanently render the information contained thereon unreadable. 
         [0012]    U.S. Pat. No. 4,236,463 issued to Westcott for TAMPER PROOF CASE FOR THE PROTECTION OF SENSITIVE PAPERS describes a technology analogous to the present invention for protecting sensitive papers. A carrying case having a hinged lid, with handle and locks, contains internal components that will be destroyed during a tampering event. Within the interior of the case and hingedly connected to the case is a liner, within which are stored sensitive papers. A thermite charge is disposed within a removable boat in the liner and has igniters electrically connected, through a selectively positionable key operated switch, to a battery and through various switch members so that the igniters are triggered upon the occurrence of one of various tampering events. Upon the making of an electrical circuit, the igniters ignite the thermite charge to burn or char the papers, the gases escaping through an opening in the lid of the case. The similarity between this and the present invention is the permanent destruction of sensitive, internal components upon tampering. While conceptually similar, the present invention obviously operates on a much smaller scale and achieves its intended function by very different means. 
         [0013]    The inventors are not aware of any technologies that provide tamper-resistance for a microprocessor package, in which the anti-tampering mechanism is integrated into the chip package and capable of destroying the microprocessor. In particular, there are no known technologies that make use of fluids and/or nanofluids to etch, sinter, or thermally destroy microprocessor chip components during a tampering event. 
       SUMMARY OF THE INVENTION 
       [0014]    It is a primary object of the present invention to enhance the art of tamper-resistant electronic devices, which will be destroyed during a tampering event. A variety of fluids and nanofluids may be incorporated into the chip level package, such fluids being contained in capsules, channels, or reservoirs (collectively, “cavities”) that will be compromised when disassembly of the chip package is attempted, the released fluid then performing a function that causes destruction of a microprocessor chip. 
         [0015]    It is another object of the invention to provide a chip package with cavities containing a single fluid or nanofluid that is capable of etching circuitry. This fluid may attack and destroy components of the chip, such as silicon, copper, or gold components, when released due to a tampering event. 
         [0016]    It is another object of the invention to provide a chip package with cavities containing multiple fluids or nanofluids where individual fluids are stable and inactive, but mixing the fluids together results in a chemistry that is capable of etching circuitry. The individual fluids may be contained in neighboring cavities for substantially simultaneous release upon tampering. 
         [0017]    It is another object of the invention to provide a chip package with cavities containing multiple fluids or nanofluids where individual fluids are stable and inactive, but mixing the fluids together can cause chemical reactions sufficient for thermally damaging die structures. 
         [0018]    It is another object of the invention to provide a chip package with an aqueous or organically based nanofluid in which, upon release, the aqueous or organic base fluid will evaporate even at room temperature, leaving the nanoparticles behind to sinter and short circuit the chip traces. 
         [0019]    It is another object of the invention to provide a chip package whereby nanoparticles are added to the base fluid to increase the overall thermal conductivity of the fluid for die thermal management. These nanoparticles may include conductive or ceramic materials. High thermal conductivity nanofluids may be used in combination with the aforementioned etching, heat releasing, or sintering devices to provide die thermal management benefits during normal operation of the electronic components. 
         [0020]    It is yet another object of the invention to provide a chip package with through-vias and/or micro vias to facilitate fluid or nanofluid access to circuitry components, thereby enhancing the chip destruction efficacy during a tampering event. 
         [0021]    It is still another object of the invention to provide a chip package comprising thermal interface materials to enhance die thermal management. These thermal interface materials may be in thermal communication with the chip, the fluid or nanofluid, and a heat sink (radiator) component. 
         [0022]    It is possible to incorporate one or more fluid-holding devices into the chip package design to provide the anti-tampering functionality described herein: capsules, channels, or reservoirs. The term “cavities” as used herein includes all three types of fluid-filled cavities. The cavity bodies comprise inert materials that will not react with the contained fluids. Upon tampering, the cavity walls will be compromised, and fluid will be released. When the fluid(s) contact the microprocessor, the etching, sintering, or thermal destruction properties of the fluids will be activated, thus destroying the circuitry and rendering difficult or futile any attempts at reverse engineering or data extraction. Through-vias and micro vias may also be utilized to facilitate fluid contact with microprocessor elements requiring destruction upon release of the fluid(s) from respective cavities. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0023]    These and other features and advantages of this invention will be more readily understood from the following detailed description of the various aspects of the invention taken in conjunction with the accompanying drawings in which: 
           [0024]      FIG. 1  is cross sectional view of a prior art microchip device offering no protection against tampering events; 
           [0025]      FIG. 2  is a cross sectional view of a chip package mounted on a substrate and comprising a heat sink and fluid bag; 
           [0026]      FIG. 3  is a cross sectional view of a chip package mounted on a substrate and comprising a heat sink, fluid bag, and thermal interface materials to enhance die thermal management. 
           [0027]      FIG. 4  is a cross sectional view of a basic chip package with through vias and micro vias included; 
           [0028]      FIG. 5  is a cross sectional view of a chip package mounted on a substrate and comprising a heat sink, fluid bag, through vias, and micro vias; 
           [0029]      FIG. 6  is a cross sectional view of a chip package mounted on a substrate and comprising a heat sink, fluid bag, through vias, micro vias, and fluid capsules; and 
           [0030]      FIG. 7  is a cross sectional view of a chip package mounted on a substrate and comprising a heat sink, fluid bag, through vias, micro vias, fluid capsules, and thermal interface materials. 
       
    
    
       [0031]    The drawings of the invention are not to scale, but merely schematic representations, not intended to portray specific parameters of the invention. The drawings are intended to depict only typical embodiments of the invention, and therefore should not be considered as limiting the scope of the invention. In the drawings, like numbering represents like elements among the drawings. In other words, for the sake of clarity and brevity, like elements and components of each embodiment bear the same designations throughout the description. 
       DETAILED DESCRIPTION OF THE INVENTION 
       [0032]    The present invention is a microchip package including fluid- or nanofluid-filled capsule, channel, or reservoir. When the package is compromised, the fluid destroys dies or circuitry on the chip. 
         [0033]    An anti-tampering approach described herein uses the same chemical processes to remove the chip trace that is left over from the manufacturing process. Because the protective coating is no longer present on the chip, the bare chip trace is susceptible to etching in the same respect as the original conductive or semi-conductive deposition layer during the manufacturing process. Release of etchants onto the bare chip surface, therefore, will compromise the chip architecture as well as the memory and software stored on the chip. 
         [0034]    Chip traces may also be damaged if an excessive amount of heat is applied to the chip. Heat may be generated by mixing a variety of two or more chemicals that will react together to create an exotherm. As can be appreciated by one skilled in the art, a multitude of known exothermic chemical reaction mechanisms may be utilized for this purpose. 
         [0035]    Yet another means to compromise microchip integrity is through the novel use of nanofluids containing electrically conductive nanoparticles. Upon release of the nanofluid, the base fluid will evaporate and leave the conductive nanoparticles behind. These nanoparticles will sinter onto the chip traces causing short circuits. Certain other advantages can be realized from the use of nanofluids, particularly the ability of nanoparticles to increase fluid thermal conductivity. 
         [0036]    The chip package is capable of self-destruction of the chip upon undesired attempts of reverse engineering by third parties. Fluids capable of attacking chip circuitry could be contained within cavities in the chip package. Removal of the lid or substrate for chip isolation would compromise the cavity walls and release the fluid(s). The chip would then be damaged by etching, sintering, or heat release. Various chemicals that may be used in conjunction with the present invention are presented below. These are presented by way of example only. It can be appreciated by one skilled in the art that numerous similar mechanisms may be employed without departing from the spirit of the present invention. 
       I. Single Fluids Capable of Etching Circuitry 
       [0037]    In one embodiment of the present invention, a single fluid is contained within a cavity or multiple cavities incorporated into the chip package. Etchants known within the art could be used for etching at least silicon and copper, as described below. 
         [0038]    a. Silicon Etching 
         [0039]    As is known in the art, certain strong acids or bases of pH&gt;12 may be used to etch silicon. For example, potassium hydroxide will react with silicon. The hydroxyl group will combine with silicon according to the following equation. 
         [0000]      Si+4OH − →Si(OH) 4 +4 e   − 
 
         [0040]    Another strong base that may be used as a single fluid to etch silicon is tetramethylammonium hydroxide (TMAH), (CH 3 ) 4 NOH. 
         [0041]    Also commonly known in the art is silicon etching with hydrofluoric acid (HF). 
         [0000]      Si+6HF→H 2 SiF 6 +2H 2  
 
         [0042]    The most suitable chemical selection may be dependent upon the intended application of the chip sought to be protected. For instance, HF would likely be unsuitable for use in a consumer-oriented device due to health and safety concerns and associated product liability. It may, however, be a suitable fluid for use in devices where secrecy is of paramount importance and users can be adequately trained to mitigate the potential health and safety concerns. 
         [0043]    b. Copper Etching 
         [0044]    Ferric chloride may be used to etch copper according to the following reaction. 
         [0000]      Fe 3+ Cl − +Cu→Fe 2+ +Cu (1+ or 2+) +Cl − 
 
         [0045]    The ferric ion reacts with the metallic copper on the circuit board in a redox reaction, producing a ferrous ion (Fe 2+ ) and cuprous or cupric (Cu 1+  or Cu 2+ ) copper. The copper ion, unlike metallic copper, is soluble so it leaves the circuit board and goes into solution. 
       II. Multiple Fluids, Each Stable and Inactive but Fluid Mixture Will Etch Circuitry 
       [0046]    a. Silicon Etching 
         [0047]    In addition to the single chemical components above, the following chemical mixtures are capable of etching silicon. Mixing of the chemicals can occur when both are contained within separate cavities and are released due to a tampering event.
       Mixture of nitric acid [HNO 3 ], ammonium fluoride [NH 4 F], and H 2 O   Mixture of ethylenediamine [C 2 H 4 (NH 2 ) 2 ], hydroquinone [C 6 H 4 (OH) 2 ], and H 2 O   Mixture of HF, NH 4 F, and H 2 O   Mixture of HF and HNO 3          
 
         [0052]    b. Gold Etching 
         [0053]    Aqua regia or chrome regia can be use to etch gold components. Aqua regia is a mixture of hydrochloric acid (HCl) and nitric acid (HNO 3 ). Both of these chemicals are individually stable and non-reactive with gold, but when mixed together (generally at a ratio of 3:1 HCl:HNO 3 ) will be effective for gold etching. Similarly, chrome regia is a mixture of HCl and chromium trioxide (CrO 3 ), both components individually stable and non-reactive with gold but the mixture effective for gold etching. 
       III. Multiple Fluids, Each Stable and Inactive but Mixing Fluids Will Cause Chemical Reaction Sufficient for Thermally Damaging Die Structures 
       [0054]    Another method to achieve the desired destruction of circuitry is heat release resulting from a chemical reaction when multiple fluids are mixed together. For example, an acid-base reaction (e.g., reacting hydrochloric acid with ammonium hydroxide) can release a relatively large amount of heat. It is also possible to use acid base reaction to reach the temperature favorable for etching. 
         [0000]      HCl+NaOH→NaCl+H 2 O+heat
 
         [0055]    Multiple destruction pathways may also be utilized simultaneously. For instance, some individual acids and bases are effective for etching circuitry and will also generate heat when mixed together. Multiple pathways may increase the rate of the etching reaction(s) or enhance the overall efficacy of chip destruction. The following chemical reactions can be used for this purpose. 
         [0000]      HNO 3 +triethanolamine→amine chloride+heat
 
         [0000]      HNO 3 +HCl+triethanolamine→amine chloride+amine nitrate+heat
 
         [0000]      HNO 3 +HCl+tetramethylammonium hydroxide(TMAH)→tetramethylammonium chloride+tetramethylammonium nitrate+heat
 
         [0000]      HNO 3 +HCl+KOH→KCl+KNO 3 +heat
 
         [0000]      Perchloric acid(HClO 3 )+KOH→potassium perchlorate (KClO 3 )+H 2 O+heat
 
         [0056]    It is also possible that heat generated during initial chemical reaction can further exothermically decompose product and generated even higher amount of energy. The decomposition of perchlorate is highly exothermic. Various other chemicals may also be selected for generating the requisite exotherm. The amount of heat released for chip thermal destruction will be determined by chip size and thickness. This, in turn, will play a role in selecting the appropriate chemistry for a given application. 
         [0000]    IV. Nanofluids that can Act as Thermal Fluids and Sinter Upon Release, Causing Short Circuits 
         [0057]    In addition to the aforementioned chip destruction capabilities, nanofluids may be incorporated into the present invention. Nanofluids include novel electronic, magnetic, optical, thermal, and biological properties. They are formed when nanoparticles are dispersed in a base fluid medium. A nanofluid is best achieved with different nanoparticles having low viscosity, in the range of 10-50 cP. Nanofluids generally contain from 1-20% nanoparticles, but it is possible to use a higher nanoparticle loading. The key to optimal nanofluid selection is to achieve stability and homogeneity of the nanoparticles in solution. Depending on the size of particles and the dispersing medium, a range of function fluids can be generated. For example, certain conductive or ceramic nanoparticles in aqueous ethylene glycols are proposed as the next generation heat transfer fluids because of their very high thermal transport capacities. This property can also be used in a microprocessor chip package for die thermal management. 
         [0058]    Moreover, nanofluids may contain electrically conductive nanoparticles within an aqueous or organic base, wherein upon release of the nanofluid, the aqueous or organic base evaporates (even at room temperature), concentrating the nanoparticles into solution. When substantially all of the base fluid has evaporated, the nanoparticles sinter onto the surface of the circuit board causing short circuits with concomitant malfunctions of the circuitry. Metal nanoparticles, such as gold or silver, would make effective sintering agents. 
         [0059]    In addition to the sintering function, nanoparticles may also be introduced into the fluids to increase thermal conductivity, thereby facilitating die thermal management. Several metal or ceramic nanoparticles, including gold, silver, platinum, ZnO, silicon carbide, carbon, nanotubes, and graphite are effective for this purpose. Similarly, a fluid with aqueous base may contain polyethylene glycol (PEG) for enhanced thermal conductivity. 
         [0060]    In certain cases, even nanofluids with enhanced thermal conductivity may insulate the chip circuitry more than desired. In that case, thermal interface materials may be incorporated into the chip package design. These could be “pillars” of high thermal conductivity materials positioned to be in thermal communication with the chip and a heat sink (radiator) component of the chip package. Filled Silicone based materials are commonly used as thermal interface material. It can be thermally conducting silicone based paste, grease, sheet or sponge. It is also possible to use highly filled thermally conducting adhesives as thermal interface materials. In one example, it will be 90 wt % silver filled epoxy adhesives where silver can be nano particle, micro particle, or mixture of nano and micro particles. High thermal conductivity nanofluids may also be used in conjunction with thermal interface materials, if desired. 
         [0061]    A variation of the present invention is a chip package having a passivation layer on the underside of the microprocessor chip, and through-vias in the microprocessor chip and/or micro vias in the passivation layer. The vias act as fluid pathways to facilitate contact of the circuit board with the fluids when they are released due to tampering. As such, they may improve the efficacy of, or reduce the time required for, chip destruction. Through-vias and micro vias may be incorporated into the design for any of the chip destruction pathways described above. 
         [0062]    In  FIG. 1  there is schematically illustrated a prior art internal chip package  10  comprising chip  12  and metal pads or metal limiting pads  16  electrically adhered to selected portions of chip  12  through recesses in passivation layer  14 . Solder bumps  18  are electrically adhered to metal pads or metal limiting pads  16  for electrical connections to external components. 
         [0063]      FIG. 2  schematically illustrates a tamper-resistant chip package additionally comprising top surface metal (TSM)  22 , such as copper pads, electrically adhered to solder bumps  18 . Also included is underfill  20  as is commonly used in the art to protect the integrity of solder joints, with all of the components mounted on substrate  24 . Heat sink  28  is mounted on top of fluid bag or reservoir  26 . A single fluid bag  26  in this embodiment could contain a single fluid or nanofluid capable of etching or sintering circuitry upon release. The fluid would be released upon cutting or prying away of the lid (here, heat sink  28 ). Moreover, a nanofluid of enhanced thermal conductivity could be contained within fluid bag  26  for die thermal management. Additionally, fluid bag  26  could comprise a plurality of fluid bags to take advantage of a fluid mixture destruction technology as described herein. 
         [0064]      FIG. 3  schematically illustrates a chip package further comprising a plurality of thermal interface materials  30  in thermal communication with chip  12  and heat sink  28  for die thermal management. Fluid bag  26  may contain various circuitry-destroying fluids, as described herein, and may also contain nanofluids of enhanced thermal conductivity for die thermal management. 
         [0065]      FIG. 4  schematically illustrates an internal chip package comprising through-vias  33  (tunneling through chip  12 ) and micro vias  34  (tunneling through passivation layer  14 ). Passivation layer  14  can be a filled polymer, and the filler can be etched to access the circuitry. Through-vias  33  and micro vias  34  can enhance the efficacy of chip destruction by providing fluid pathways, whereby released fluids can travel through the vias to achieve better contact with the microprocessor chip. This enhances the etching, sintering, or thermal destruction functions of the present invention. 
         [0066]      FIG. 5  shows the device of  FIG. 4  incorporated into a chip package design. Chip destruction capabilities are included with the addition of fluid bag  26 , through-vias  33  and micro vias  34 . 
         [0067]      FIG. 6  schematically illustrates a chip package further comprising a plurality of fluid capsules  36  contained within under fill  30 . Inclusion of fluids on the underside of the chip package will facilitate chip destruction in the event that access through the bottom (or substrate) side of the chip package is attempted. Fluids that are released from capsules  36  travel into micro vias  34  and/or through vias  33  to contact chip  12 . In addition, multiple capsules can contain multiple fluids for die destruction initiated by the fluid mixing process. Capsules  36  may also take the form of channels or the like. As can be appreciated by one skilled in the art, a variety of cavity shapes and sizes may be successfully incorporated into the present invention. 
         [0068]      FIG. 7  shows the chip package of  FIG. 6  with the inclusion of thermal interface materials to facilitate die thermal management. 
         [0069]    Since other combinations, modifications and changes varied to fit particular operating requirements and environments will be apparent to those skilled in the art, the invention is not considered limited to the chosen preferred embodiments for purposes of this disclosure, but covers all changes and modifications which do not constitute departures from the true spirit and scope of this invention. 
         [0070]    Having thus described the invention, what is desired to be protected by Letters Patent is presented in the subsequently appended claims.