Patent Publication Number: US-2023152501-A1

Title: Bragg reflector for photonic chip security structure

Description:
BACKGROUND 
     The present disclosure relates to semiconductor structures and, more particularly, to reflecting material which surrounds a photonics component and methods of manufacture. 
     Photonic semiconductors have many applications in modern consumer electronics. For example, photonic semiconductors include optical modulators, quantum well (QW) lasers, photodiodes, and waveguide structures, etc. Silicon waveguides are of special interest as they have unique guiding properties. For example, due to their unique guiding properties, silicon waveguides can be used for communications, interconnects, and biosensors. 
     Silicon photonic devices can be made using existing semiconductor fabrication techniques, and because silicon is already used as the substrate for most integrated circuits, it is possible to create hybrid devices in which the optical and electronic components are integrated onto a single microchip. However, unlike electronic devices which have known protection techniques, the silicon photonic devices remain vulnerable to both physical and non-invasive attacks aimed at obtaining cryptographic encryption keys, certificates, intellectual property and other critical or sensitive data. 
     SUMMARY 
     In an aspect of the disclosure, a structure includes an optical component over a substrate material, and at least one vertical wall including a reflecting material within a dielectric stack of material and surrounding the optical component. 
     In an aspect of the disclosure, a structure includes a silicon-on-insulator (SOI) substrate which includes a semiconductor layer on an insulator layer on a handle substrate, an optical component over the semiconductor on the insulator substrate, a dielectric stack of material covering the optical component, and a vertical wall within the dielectric stack of material and surrounding the optical component. 
     In an aspect of the disclosure, a method includes detecting a resistance of a Bragg reflector of a photonic chip security structure; comparing the detected resistance of the Bragg reflector with a predetermined acceptable limit; and generating a tamper signal in response to the detected resistance of the Bragg reflector being outside the predetermined acceptable limit. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present disclosure is described in the detailed description which follows, in reference to the noted plurality of drawings by way of non-limiting examples of exemplary embodiments of the present disclosure. 
         FIG.  1    shows an optical component, amongst other features, and respective fabrication processes in accordance with aspects of the present disclosure. 
         FIG.  2    shows trenches formed through a dielectric stack and surrounding the optical component, amongst other features, and respective fabrication processes in accordance with aspects of the present disclosure. 
         FIG.  3    shows the trenches filled with reflecting material, amongst other features, and respective fabrication processes in accordance with aspects of the present disclosure. 
         FIGS.  4 - 17    show various alternate embodiments security structures including reflecting material, amongst other features, and respective fabrication processes in accordance with additional aspects of the present disclosure. 
         FIG.  18    shows a flowchart depicting a method of detecting a security breach using the photonic chip security structure in accordance with aspects of the present disclosure. 
     
    
    
     DETAILED DESCRIPTION 
     The present disclosure relates to semiconductor structures and, more particularly, to a Bragg reflector for photonic chip security and methods of manufacture. In embodiments, the photonic chip security structure includes a semiconductor material composed of reflecting material which surrounds a photonics component. In embodiments, the reflecting material may include a Bragg reflector. The reflecting material may include one or more vertical walls in combination with one or more lateral walls to provide an enclosure surrounding the photonics component. In particular, the term surrounding may be interpreted as walls on sides of the photonics component, on sides and over the photonics component or a complete enclosure about the photonics component. As to the latter example, the enclosure may include sidewalls, a top wall, and a bottom wall. Advantageously, the reflecting material may reflect optical hacking signals (e.g., incoming radiation) away from the photonics component to provide security for sensitive optical parts without impacting optical performance and functionality of the photonics component. 
     More specifically, the semiconductor reflecting material may form a Bragg reflector including vertical walls and lateral walls that surround an optical component, e.g., optical photonic waveguides or other critical optical components. The semiconductor reflecting material (e.g., vertical Bragg reflector) may surround the optical component on both sides and above the optical component in order to reflect or block incoming radiation (e.g., to absorb or reflect/deflect any optical hacking signal). In embodiments, the semiconductor reflecting material may be poly silicon material and oxide material, either of which is capable of reflecting incoming radiation. The security structure can also include a top layer of reflecting material and, in embodiments, a bottom layer of reflecting material, to form a security box around the optical component. The optical component may be front end of the line (FEOL) silicon components or a back end of the line (BEOL) silicon nitride components, amongst other optical components. 
     In further embodiments, the security structure may be a Bragg reflector including vertical walls of alternative material with different refractive indices (i.e., polysilicon and oxide). Also, the width of each of the vertical walls can be chosen for a predetermined light wavelength. Further, a height of the Bragg reflector may be greater than a height of the waveguide. For example, the height of the Bragg reflector consisting of the vertical walls can be approximately 4.5 to 5 times taller than the height of the waveguide. 
     The photonic chip security structure can be manufactured in a number of ways using a number of different tools. In general, though, the methodologies and tools are used to form structures with dimensions in the micrometer and nanometer scale. The methodologies, i.e., technologies, employed to manufacture the photonic chip security structure of the present disclosure have been adopted from integrated circuit (IC) technology. For example, the structures are built on wafers and are realized in films of material patterned by photolithographic processes on the top of a wafer. In particular, the fabrication of the photonic chip security structure uses three basic building blocks: (i) deposition of thin films of material on a substrate, (ii) applying a patterned mask on top of the films by photolithographic imaging, and (iii) etching the films selectively to the mask. 
       FIG.  1    shows an optical component, amongst other features, and respective fabrication processes in accordance with aspects of the present disclosure. More specifically, the structure  10  includes a substrate  12  and dielectric stack of materials  14  with one or more electronic components  16  and an optical component  18 . In embodiments, the electronic components  16  may be any passive or active device including, e.g., transistors with contacts and metal wiring layers, etc. The optical component  18  may be any optical component such as, e.g., a waveguide or other photonic devices amongst many different examples. 
     The substrate  12  is preferably a semiconductor-on-insulator (SOI) substrate. For example, the substrate  12  includes a semiconductor handle substrate  12   a , an insulator layer  12   b  and a semiconductor layer  12   c . In embodiments, the semiconductor handle substrate  12   a  and semiconductor layer  12   c  may be composed of any suitable material including, but not limited to, Si, SiGe, SiGeC, SiC, GaAs, InAs, InP, and other III/V or II/VI compound semiconductors. The semiconductor layer  12   c  may also include any suitable crystallographic orientation (e.g., a (100), (110), (111), or (001) crystallographic orientation). The insulator layer  12   b  may include a dielectric material such as silicon dioxide, silicon nitride, silicon oxynitride, boron nitride or a combination thereof and, preferably, a buried oxide layer (BOX) supported on the semiconductor handle substrate  12   a.    
     Still referring to  FIG.  1   , the dielectric stack of materials  14  may include alternating layers of dielectric material  14   a ,  14   b . For example, the dielectric stack of materials  14  may include alternating layers of oxide material  14   a  and nitride material  14   b . In more specific embodiments, the oxide material  14   a  may include SiO 2  and the nitride material  14   b  may include SiN. The alternating layers of dielectric material  14   a ,  14   b  may be deposited by conventional deposition methods such as, e.g., chemical vapor deposition (CVD). 
     In  FIG.  2   , trenches  20  are formed through the dielectric stack of materials  14 . In more specific embodiments, the trenches  20  are formed on sides of the optical component  18 . In this layout scheme, the optical component  18  may be a front end of the line Si optical component; although, it is also contemplated that the optical component  18  may be a back end of the line SiN optical component within the dielectric stack of materials  14 . 
     The trenches  20  may be formed by conventional lithography and etching methods known to those of skill in the art. For example, a resist formed over the dielectric stack of materials  14  is exposed to energy (light) to form a pattern (opening). An etching process with a selective chemistry, e.g., reactive ion etching (RIE), will be used to transfer the pattern from the resist layer to the dielectric stack of materials  14  to form one or more trenches  20  in the dielectric stack of materials  14 . In embodiments, the trenches  20  will extend to the buried insulator layer  12   b ; however, it is also contemplated that the trenches  20  can extend into the insulator layer  12   b . Although  FIG.  2    shows four trenches  20  being formed, any number of trenches can be formed for a Bragg reflector. 
     As shown in  FIG.  3   , reflecting material  24  (i.e., a Bragg reflector) may be formed within the trenches  20  to form the photonic chip security structure. In this way, the photonic chip security structure includes a vertical wall of the reflecting material  24  which surrounds the optical component  18 , e.g., photonic waveguides or functional optical devices. Further, the vertical wall of the reflecting material  24  may extend into the insulator layer  12   b . In embodiments, the vertical wall of the reflecting material  24  may be polysilicon material and oxide material, as examples. In embodiments, the polysilicon can be doped material, and may be used for shorter wavelengths of incoming radiation. In any scenario, though, the polysilicon material and oxide material provide reflecting properties of the incoming radiation without affecting the optical performance of the optical component  18 . The reflecting material  24  may be deposited using, for example, CVD processes. Any residual material on the surface of the dielectric stack  14  may be removed by conventional chemical mechanical polishing (CMP) processes. 
       FIG.  4    shows an embodiment of a photonic chip security structure  10   a  in which a single vertical wall  24  is provided on each side of the optical component  18 . In this embodiment, the vertical wall of the reflecting material  24  may be polysilicon material. In alternative embodiments, the vertical wall of the reflecting material  24  may be a different material than polysilicon, i.e., oxide material. 
       FIG.  5    shows another embodiment of a photonic chip security structure  10   b  in which three vertical walls  26 ,  24 ,  26   a  are provided on each side of the optical component  18 . In this embodiment, a vertical wall of reflecting material  24  (i.e., a Bragg reflector) may be provided between two vertical walls  26 ,  26   a  including dielectric materials on each side of the optical component  18 . In particular, the vertical walls  26 ,  26   a  including dielectric materials may be oxide; whereas the reflecting material  24  may be polysilicon. Other combinations of reflective material and dielectric material are also contemplated herein. The remaining features are similar to that described in  FIG.  4   . 
       FIG.  6    shows an embodiment of the photonic chip security structure  10   c  in which a lateral wall  28  of reflecting material (i.e., a lateral Bragg reflector) is connected to the vertical walls  24  of the reflecting material (i.e., a vertical Bragg reflector). In this way, an enclosure is formed around (e.g., surrounding) the optical component  18  from sides and a top. In embodiments, the reflecting material for the vertical wall  24  and top wall  28  may be the same material or different material. For example, the reflecting material of the lateral wall  28  may include a lateral stack of polysilicon/oxide. Further, the reflecting material of the lateral wall  28  may be multiple stacks of polysilicon/oxide. The remaining features are similar to that described in  FIG.  4   . 
       FIG.  7    shows another embodiment of the photonic chip security structure  10   d  in which the lateral wall  28  of reflecting material (i.e., a lateral Bragg reflector) is connected to the vertical wall  24  (i.e., a vertical Bragg reflector) within the dielectric stack of materials  14 . In comparison to  FIG.  6   , for example, a height of the vertical wall  24  (i.e., a vertical Bragg reflector) is lower than a top surface of the dielectric stack of materials  14 . The remaining features are similar to that described in  FIG.  6   . 
       FIG.  8    shows an embodiment of the photonic chip security structure  10   e  in which multiple lateral walls  28  of reflecting material (i.e., a lateral Bragg reflector) are connected to multiple vertical walls of the dielectric materials  26  and the vertical wall  24  of the reflecting material (i.e., a vertical Bragg reflector). The photonic chip security structure  10   e  includes multiple walls may be constructed using deep trench etching and deposition processing techniques known to those of skill in the arts. As a result, in the photonic chip security structure  10   e , the multiple walls result in an enhanced Bragg reflection protection effect. The remaining features are similar to that described in  FIG.  4   . 
       FIG.  9    shows another embodiment of the photonic chip security structure  10   f  in which different materials, e.g., dielectric material  39  alternating with dielectric material  30 , are provided for the vertical walls. Further, the lateral reflecting material  28  (i.e., a lateral Bragg reflector) are connected to the vertical walls which include the dielectric material  39  alternating with the dielectric material  30 . In this embodiment, the dielectric materials may be different dielectric materials to enhance a Bragg reflection. The photonic chip security structure  10   f  may be fabricated using a level by level processing integration schemes, e.g., etching and deposition processes, as should be understood by those of skill in the art. The remaining features are similar to that described in  FIG.  8   . 
       FIG.  10    shows an embodiment of the photonic chip security structure  10   g  in which the lateral walls  28  include dielectric material  40  alternating with dielectric material  41 , and vertical walls  24  include dielectric material  42  alternating with dielectric material  43 . Again, the dielectric materials can be different material to enhance the Bragg reflection. The remaining features are similar to that described in  FIG.  8   . 
       FIG.  11    shows an embodiment of the photonic chip security structure  10   h  in which a vertical wall  24  of reflecting material (i.e., the Bragg reflector) is between vertical walls  26 ,  26   a  composed of dielectric material, on each side of the optical component  18 . In addition, a lateral wall  28  including reflecting material (i.e., the lateral Bragg reflector) may be connected to an inner of the vertical walls  26   a . In this way, an enclosure is formed around (e.g., surrounding) the optical component  18  from sides and a top. The dielectric material  26  can be an oxide; whereas the reflecting material  24  may be polysilicon. 
     In embodiments, the lateral wall  28  of reflecting material may be the same material as the dielectric material  26 . In alternative embodiments, the lateral wall  28  of reflecting material may be a different material than the vertical walls  26 ,  26   a . For example, the lateral wall  28  of reflecting material may include a lateral stack of polysilicon/oxide. Further, the lateral wall  28  of reflecting material  28  may be multiple stacks of polysilicon/oxide. The remaining features are similar to that described in  FIG.  4   . 
       FIG.  12    shows an embodiment of the photonic chip security structure  10   i  in which multiple lateral walls  28  of reflecting material are connected to a single, inner vertical wall  26   a  of the reflecting material (i.e., the Bragg reflector). The vertical wall  24  is also between the vertical walls  26 ,  26   a  including dielectric materials on each side of the optical component  18 . As a result, the multiple walls of result in an enhanced Bragg reflection protection effect. The remaining features are similar to that described in  FIG.  11   . 
       FIG.  13    shows an embodiment of the photonic chip security structure  10   j  in which three vertical walls  26 ,  24 ,  26   a  are provided on each side of the optical component  18  at a height below that of the stack of dielectric materials  14 . In this embodiment, the vertical wall of reflecting material  24  (i.e., the Bragg reflector) may be between vertical walls  26 ,  26   a  of dielectric material on each side of the optical component  18 . Also, the lateral wall  28  may be connected to the vertical wall  26   a  of the dielectric material. In comparison to  FIG.  11   , a height of the reflecting material  24  (i.e., the vertical Bragg reflector) may be lower than a top surface of the dielectric stack of materials  14 . The remaining features are similar to that described in  FIG.  11   . 
       FIG.  14    shows an embodiment of the photonic chip security structure  10   k  in which a back end of the line (BEOL) optical component  18  (e.g., silicon nitride waveguide) may be formed within the dielectric stack of materials  14 . In this embodiment, the optical component  18  may be completely surrounded by reflecting material  29  alternating with reflecting material  31  in a lateral orientation, e.g., lateral wall, and dielectric material  44  alternating with dielectric material  30  in a vertical orientation, e.g., vertical wall. As in the previous embodiments, the dielectric materials  26 ,  30  and the reflecting materials  29 ,  31  may be fabricated using conventional lithography, etching and deposition methods as described herein. Further, the reflecting material  29  may be the same material or different material as the reflecting material  31 , and the dielectric material  26  may be the same material or different material as the dielectric material  30 . The remaining features are similar to that described in  FIG.  4   . 
       FIG.  15    shows an embodiment of the photonic chip security structure  10   l  in which the optical component is a back end of the line (BEOL) optical component  18  (e.g., silicon nitride waveguide) within the dielectric stack of materials  14  and which is completely surrounded by vertical and lateral walls including reflecting material  29  alternating with reflecting material  31 . In this embodiment, the vertical wall  24  of the reflecting material (i.e., a Bragg reflector) may be between two vertical walls  26 ,  26   a  of reflecting materials on each side of the optical component  18 . The reflecting material  29 , may be the same material or different material as the reflecting material  31 , and the dielectric material in the vertical walls  26 ,  26   a  may be the same material or different material as the vertical wall of reflecting material  24 , all of which are above the substrate  12 . The remaining features are similar to that described in  FIG.  14   . 
       FIG.  16    shows an embodiment of the photonic chip security structure  10   m  in which the vertical walls  24  may include alternating reflective materials  45 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37 , and  38  of different lengths in the lateral direction. Also, the lateral reflecting material  28  (i.e., a lateral Bragg reflector) is connected to vertical walls. In embodiments, the reflecting material  28  may be the same material as the reflecting materials  45 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37 , and  38 . In alternative embodiments, the lateral reflecting material  28  may be a different material than the reflecting materials  45 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37 , and  38 . For example, the lateral wall  28  of reflecting material may include a lateral stack of polysilicon/oxide. Further, the reflecting material of the lateral wall  28  may be multiple stacks of polysilicon/oxide. Also, the reflecting materials  45 ,  32 ,  33 ,  34 ,  35 ,  36 ,  37 , and  38  may be of a multi-layer stack of alternatively high index and low index films. The remaining features are similar to that described in  FIG.  6   . 
       FIG.  17    shows an embodiment of the photonic chip security structure  10   n  in which lateral wall  28  of reflecting material (i.e., a lateral Bragg reflector) may be connected to the vertical walls  24  including reflecting material (i.e., a vertical Bragg reflector). In comparison to  FIG.  16   , a height of the vertical walls  24  is lower than a top surface of the dielectric stack  14 , with the vertical walls making contact to the substrate  12 . The remaining features are similar to that described in  FIG.  7   . 
       FIG.  18    shows a flowchart depicting the method of detecting a security breach. In particular, polysilicon layers which form a Bragg reflector may be doped and electrically connected to a detector circuit for monitoring a resistance change of the Bragg reflector. For example, any attempt to breach the Bragg reflector can be detected by a change in resistance, with the circuit operation of photonic chip security being terminated upon such detection. More specifically, in step S 200 , a resistance of a Bragg reflector layer can be detected. In step  300 , a detected resistance can be compared with a predetermined acceptable limit. If the detected resistance is within the predetermined acceptable limit (i.e., YES to step  300 ), the method returns to step S 200 . However, if the detected resistance is not within the predetermined acceptable limit (i.e., NO to step  300 ), the method continues to step S 400 . In step S 400 , a tamper signal is generated, and the optical signal can be terminated. 
     In another embodiment of  FIGS.  17  and  18   , a security detect tampering switch may include a capacitance measurement sensor (e.g., a PIN diode or photonic detector) which is positioned adjacent to the reflecting material (e.g., lateral wall  28  and/or vertical wall  24  of  FIG.  17   ). The capacitance measurement sensor may be calibrated to detect reflections off the reflecting material (e.g., lateral wall  28  and/or vertical wall  24  of  FIG.  17   ). For example, if the reflecting material is tampered, altered, or damaged, the reflections off the reflecting material would also be impacted. Thus, the capacitance measurement sensor would sense that a different reflection or magnitude is reflecting off the reflecting material. Further, when the reflecting material has been physically hacked, a kill switch may be triggered to turn off or destroy the circuitry. 
     The photonic chip security structures described herein can be utilized in system on chip (SoC) technology. The SoC is an integrated circuit (also known as a “chip”) that integrates all components of an electronic system on a single chip or substrate. As the components are integrated on a single substrate, SoCs consume much less power and take up much less area than multi-chip designs with equivalent functionality. Because of this, SoCs are becoming the dominant force in the mobile computing (such as in Smartphones) and edge computing markets. SoC is also used in embedded systems and the Internet of Things. 
     The method(s) as described above is used in the fabrication of integrated circuit chips. The resulting integrated circuit chips 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, i.e., surface interconnections and/or buried interconnections). In any case the chip is 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. 
     The descriptions of the various embodiments of the present disclosure 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 embodiments, 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.