Abstract:
A method for forming an interferometer is disclosed. The method involves forming a ring interferometer and a fiber optic gyroscope on a single semiconductor substrate.

Description:
GOVERNMENT INTEREST STATEMENT 
       [0001]    The U.S. Government may have certain rights in the present invention under contract no. N00030-05-C-0007 (Prime) and DL-H-551019 (Subcontract) awarded by the United States Navy. 
     
     BACKGROUND 
       [0002]    A fiber optic gyroscope (FOG) contains a coil (for example, up to 2 miles) of wound optical fiber. FOGs measure angular rotation by determining the phase difference in light waves that propagates in opposite directions through the coil of optical fiber. Light waves that propagate through the coil in the opposite direction of the rotation take a shorter time than light waves that propagate in the direction of rotation. 
         [0003]    Typically, an optical system with a beam splitter directs two light beams on a photodetector. With a zero attitude rate, the phase shift between the two beams is 180°; the two beams cancel each other and output photocurrent is minimized. FOGs provide extremely precise rotational rate information, due in part to a lack of cross-axis sensitivity to vibration, acceleration, and shock. FOGs will typically show higher resolution than a traditional ring laser gyroscope, and are commonly used to measure rotation in navigation applications such as aircraft, missiles, satellites, and other vehicles. 
         [0004]    With the attitude rate oriented along the fiber (that is, around the coil&#39;s axis) the original phase shift changes. This phase shift change occurs because of an increase in the light path for one beam and a decrease in the light path for another beam. As a result, the photodetector&#39;s current responds to the increased illumination and becomes larger. This is typically known as the Sagnac effect. 
         [0005]    The Sagnac effect is illustrated in what is commonly referred to as ring interferometry. Similar to the FOG, ring interferometry involves a beam of light (for example, a laser) split into two beams. The two beams are made to follow a trajectory in opposite directions. To act as a ring, the trajectory must enclose an area. In normal laser operation, light inside the laser cavity is several frequencies at first, and one frequency quickly outperforms other frequencies (after that the light is monochromatic). The frequency that outperforms the others fits well in the laser cavity; a multiple of its wavelength is the length of the cavity. When a ring laser is rotating, the effective path lengths of the two counter-propagating beams of laser light are different. At this point, the laser process generates two frequencies of laser light. The two resulting frequencies are such that at all times, the same number of cycles exist in both directions of propagation (that is, a standing wave). This standing wave is stationary with respect to the local inertial frame of reference when the ring laser is not rotating. The standing wave is also stationary with respect to the local inertial frame of reference when the ring laser interferometer is rotating. This property makes the ring interferometer the electronic counterpart of a mechanical gyroscope. 
         [0006]    Combining a ring interferometer with a FOG currently involves integrating a separate laser source/detector and the coil of optical fiber with a wave guide for the two light beams. Such a combination requires numerous mechanical connections. The additional interactions involved lead to decreased reliability and is prone to numerous usability and capability issues (for example, more frequent calibrations). Also, the end product is typically bulky with demanding operating requirements (for example, energy consumption). These characteristics are not conducive to many present and future applications. 
       SUMMARY 
       [0007]    The following specification addresses recording orientation with an electronic device. Particularly, in one embodiment, a method for forming an interferometer is provided. The method involves forming a ring interferometer and a fiber optic gyroscope on a single semiconductor substrate. 
     
     
       DRAWINGS 
         [0008]    These and other features, aspects, and advantages will become better understood with regard to the following description, appended claims, and accompanying drawings where: 
           [0009]      FIG. 1  is a block diagram of an embodiment of an electronic package assembly incorporating an interferometer fiber optic gyroscope; 
           [0010]      FIG. 2  is a block diagram of an embodiment of an interferometer fiber optic gyroscope; 
           [0011]      FIG. 3  is a cross-sectional view of an embodiment of a wave guide for an interferometer fiber optic gyroscope shown in a partially-formed state with a substrate structure comprising at least one semiconductor substrate layer; 
           [0012]      FIG. 4  is a cross-sectional view of the substrate structure of  FIG. 3  comprising an enclosed wave guide formed on the at least one semiconductor substrate layer; 
           [0013]      FIG. 5  is a cross-sectional view of an embodiment of a light source for an interferometer fiber optic gyroscope shown in a partially-formed state with a substrate structure comprising a compound junction and at least one semiconductor substrate layer; 
           [0014]      FIG. 6  is a cross-sectional view of the substrate structure of  FIG. 5  with at least one additional masking layer formed on the at least one semiconductor substrate layer; 
           [0015]      FIG. 7  is a cross-sectional view of the substrate structure of  FIG. 6  with at least one additional doping layer formed on the at least one semiconductor substrate layer; 
           [0016]      FIG. 8  is a cross-sectional view of the substrate structure of  FIG. 7  with an additional metallization layer formed on the at least one semiconductor substrate layer; 
           [0017]      FIG. 9  is a cross-sectional view of an embodiment of a light detector for an interferometer fiber optic gyroscope shown in a partially-formed state with a substrate structure comprising a compound junction and at least one semiconductor substrate layer; 
           [0018]      FIG. 10  is a cross-sectional view of the substrate structure of  FIG. 9  with at least one additional masking layer formed on the at least one semiconductor substrate layer; 
           [0019]      FIG. 11  is a cross-sectional view of the substrate structure of  FIG. 10  with at least one additional doping layer formed on the at least one semiconductor substrate layer; 
           [0020]      FIG. 12  is a cross-sectional view of the substrate structure of  FIG. 1  with an additional metallization layer formed on the at least one semiconductor substrate layer; and 
           [0021]      FIG. 13  is a cross-sectional view of an embodiment of the substrate structure of  FIG. 2 ; and 
           [0022]      FIG. 14  is a block diagram of an embodiment of a system for recording orientation with an electronic device. 
       
    
    
       [0023]    Like reference numbers and designations in the various drawings indicate like elements. 
       DETAILED DESCRIPTION 
       [0024]    The following detailed description discusses at least one embodiment for combining a ring interferometer with a fiber optic gyroscope on a semiconductor substrate. This combination is referred to as an interferometer fiber optic gyroscope (IFOG). Advantageously, the IFOG serves as a building block in conjunction with peripheral electronics on the same semiconductor substrate. The final result is a miniaturized gyroscope for a plurality of applications that require navigation-related data from a measurement device. Application possibilities range from unaccompanied navigation drones and ballistic trajectory measurement sensors to physiological data recorders for anatomy studies. 
         [0025]      FIG. 1  is a block diagram of an embodiment of an electronic package assembly  100  incorporating an IFOG. Electronic package assembly  100  comprises device substrate  102 , IFOG substrate  104 , and substrate logic components  106 . Examples of electronic package assembly  100  include any logic device such as an application-specific integrated circuit (ASIC) and the like. Device substrate  102  is composed of electrically conductive material known to one of skill in the art of conventional semiconductor wafer fabrication. IFOG substrate  104  resides within device substrate  102  and is in communication with substrate logic components  106  along communication interface  108 . Substrate logic components  106  include, but are not limited to, peripheral electronic components known to one of skill in the art. For example, substrate logic components  106  include resistors, transistors, capacitors, inductors, etc. formed on one or more semiconductor chip die substrates using conventional semiconductor wafer fabrication methods. It is noted that for simplicity in description, a single IFOG substrate  104  is shown in  FIG. 1 . However, it is understood that device substrate  102  is capable of accommodating any appropriate number of IFOG substrates  104  (for example, at least one IFOG substrate in a single device substrate  102 ). IFOG substrate  104  is described in further detail below with respect to  FIG. 2 . 
         [0026]    In operation, electronic package assembly  100  is incorporated into one or more electronic devices. The one or more electronic devices (one example, electronic device  1404 , is provided below with respect to  FIG. 14 ) will measure location and orientation with the capabilities provided by IFOG substrate  104 . In one implementation, electronic device  1404  retains a form factor suitable for use in applications requiring miniaturized navigational aids and operates with a minimal amount of power. Electronic package assembly  100  is suitable for packaging in a variety of electronic devices, such as low power navigation drones, ballistic tracing equipment, tracking aids for internal medicine, and the like. Incorporating the necessary components for an interferometer fiber optic gyroscope in IFOG substrate  104  eliminates integrating separate components (for example, a separate light source and detector with a separate length of optical fiber) into a hybrid system or device. 
         [0027]      FIG. 2  is a cross-sectional view  200  of an embodiment of IFOG substrate  104 . IFOG substrate  104  comprises light source  202 , light detector  204 , wave guide coupler  206 , and ring interferometer wave guide  208 . In the example embodiment of  FIG. 2 , wave guide coupler  206  further includes integrated optical circuits (IOCs)  210   1  and  210   2 . In one implementation, each of IOCs  210   1  and  210   2  serve as beam splitters to direct a single light beam in at least two separate and opposite directions. Light source wave guide  212  couples light source  202  to IOC  210   1 . Light detector wave guide  214  couples IOC  210   1  to light detector  204 . IOC  210   2  is coupled to each end of ring interferometer wave guide  208 . In one implementation, IOC  210   2  is a mirror image of IOC  210   1  (as illustrated in  FIG. 2 ). Alternate implementations for wave guide coupler  206  are possible. Light source  202  (light detector  204 ) is fabricated within IFOG substrate  104  using standard semiconductor wafer fabrication processes, as further described below with respect to  FIGS. 5  ( 9 ),  6  ( 10 ),  7  ( 11 ), and  8  ( 12 ). In the example embodiment of  FIG. 2 , wave guide coupler  206 , ring interferometer wave guide  208 , light source wave guide  212 , and light detector wave guide  214  form at least one continuous fiber-equivalent optical wave guide. In one implementation, the at least one continuous fiber-equivalent optical wave guide does not consist of optical fiber. 
         [0028]    In the example embodiment of  FIG. 2 , ring interferometer wave guide  208  consists of a length of concentric coils whose shape is fitted to represent a ring interferometer. The length of concentric coils of ring interferometer wave guide  208  is at least a portion of the at least one continuous fiber-equivalent optical wave guide discussed above. In one implementation, the length of concentric coils of ring interferometer wave guide  208  reside in one or more semiconductor substrate layers for coupling with wave guide coupler  206  (as illustrated in  FIG. 2 ). In alternate implementations, the length of concentric coils of ring interferometer wave guide  208  (along with light source  202 , wave guide coupler  206 , light source wave guide  212  and light detector wave guide  214 ) reside in a particular plane (for example, the x-y plane) and light detector  204  resides in a separate plane (for example, the z-plane) of a single semiconductor substrate layer. 
         [0029]    Wave guide coupler  206 , ring interferometer wave guide  208 , light source wave guide  212 , and light detector wave guide  214  are formed by one or more electron-beam etching processes. At least one electron-beam etching process is described in further detail below with respect to  FIGS. 3 and 4 . A cross-sectional view of IFOG substrate  104  along line AA is illustrated in further detail below with respect to  FIG. 13 . 
         [0030]    In operation, IFOG substrate  104  receives electrical power to activate light source  202 . Light source  202  emits a light beam along light source wave guide  212  and into wave guide coupler  206 . IOC  210   2  splits the emitted light beam into two beams traveling in a clockwise (CW) and counter-clockwise (CCW) direction (as illustrated) through ring interferometer wave guide  208 . Ring interferometer wave guide  208  encompasses an optical path represented by an area vector {right arrow over (A)}. Wave guide coupler  206  separates the previously-emitted light beam in IOC  210   1  from at least one returning light beam of ring interferometer wave guide  208 . Once light detector  204  detects the at least one returning light beam on light detector wave guide  214 , IFOG substrate  104  establishes a rotational rate vector {right arrow over (r)}. After traveling through ring interferometer wave guide  208 , the at least one returning light beam experiences a phase shift (phase differential) illustrated by Equation 1 below: 
         [0000]    
       
         
           
             
               
                 
                   
                     ΔΦ 
                      
                     
                         
                     
                      
                     R 
                   
                   = 
                   
                     
                       
                         4 
                          
                         ω 
                       
                       
                         c 
                         2 
                       
                     
                      
                     
                       
                         A 
                         → 
                       
                       · 
                       
                         r 
                         → 
                       
                     
                   
                 
               
               
                 
                   ( 
                   
                     Equation 
                      
                     
                         
                     
                      
                     1 
                   
                   ) 
                 
               
             
           
         
       
     
         [0031]    With respect to Equation  1  above, ΔΦR represents a phase differential between the emitted light beam of light source  202  and the at least one returning light beam, ω represents an angular frequency of the emitted light beam of light source  202 , and c represents velocity of light in a vacuum. ΔΦR is proportional to the rotational rate vector {right arrow over (r)} combined vectorially with area vector {right arrow over (A)}. In one implementation, the phase differential is used to calculate the orientation of electronic device  1404  of  FIG. 14 . The length of the concentric coils of ring interferometer wave guide  208  controls sensitivity (that is, measurement resolution) of IFOG substrate  104  based on a magnitude of area vector {right arrow over (A)}. IFOG substrate  104  is suitable for large-scale semiconductor wafer integration. The construction of each major component of IFOG substrate  104  is described in further detail below with respect to  FIGS. 3 to 12 . 
         [0032]      FIG. 3  is a cross-sectional view of an embodiment of wave guide  206  for IFOG substrate  104  shown in a partially-formed state with a substrate structure  300  comprising at least one semiconductor substrate layer. Substrate structure  300  comprises IFOG substrate  104 , wave guide coupler  206  (ring interferometer wave guide  208 ), and trough opening  302 . In the example embodiment of  FIG. 3 , a vapor deposition glass oxide creates wave guide coupler  206  (ring interferometer wave guide  208 ) and trough opening  302  using electron-beam etching. In the vapor deposition glass oxide process, substrate structure  300  is exposed to one or more volatile agents. The one or more volatile agents react to (for example, decompose) substrate structure  300  enough to produce wave guide coupler  206  (ring interferometer wave guide  208 ) and trough opening  302 . Wave guide coupler  206  (ring interferometer wave guide  208 ) is formed as a rounded glass trough of a number of concentric coils created by the electron-beam etching process. The electron-beam etching process allows a direct image of wave guide coupler  206  (ring interferometer wave guide  208 ) to be formed without a mask as substrate structure  300  is modified by the etching process. 
         [0033]      FIG. 4  is a cross-sectional view of a substrate structure  400  comprising an enclosed wave guide  206  formed on at least one semiconductor substrate layer. Substrate structure  400  further comprises capping material  402 . As described above with respect to  FIG. 3 , wave guide coupler  206  (ring interferometer wave guide  208 ) is perfectly rounded on IFOG substrate  104  by the vapor deposition glass oxide and electron-beam etching processes. Wave guide coupler  206  (ring interferometer wave guide  208 ) is sealed with capping material  402 . In the example embodiment of  FIG. 4 , capping material  402  is constructed of an N-P doping material, a metal layer, or the like. 
         [0034]      FIG. 5  is a cross-sectional view of an embodiment of light source  202  for IFOG substrate  104  shown in a partially-formed state with a substrate structure  500  comprising a compound junction and at least one semiconductor substrate layer. Substrate structure  500  further comprises at least one field oxide layer  502  and at least one semiconductor substrate layer  504 . The at least one field oxide layer  502  serves as at least one doping layer when light source  202  is fabricated. 
         [0035]      FIG. 6  is a cross-sectional view of a substrate structure  600  with an additional masking layer formed on the at least one semiconductor substrate layer  504  of  FIG. 5 . Substrate structure  600  further comprises at least one semiconductor dioxide layer  602  and at least one masking layer  604 . The at least one semiconductor dioxide layer  602  and the at least one masking layer  604  are deposited during a series of patterning and layering operations that define a location of light source  202  on IFOG substrate  104 . 
         [0036]      FIG. 7  is a cross-sectional view of a substrate structure  700  with at least one additional doping layer formed on the at least one semiconductor substrate layer  504  of  FIG. 5 . Substrate structure  700  further comprises source P-N junction  702 . Source P-N junction  702  is formed during at least one doping operation performed on substrate layer  600 . The at least one doping operation creates a plurality of pockets in substrate structure  700  that are either rich in electrons (N-type) or rich in electron holes (P-type). The plurality of pockets forms an electrically-active region. 
         [0037]      FIG. 8  is a cross-sectional view of a substrate structure  800  with an additional metallization layer formed on the at least one semiconductor substrate layer  504  of  FIG. 5 . Substrate structure  800  further comprises metallization layer  802 , shown as  802   1  and  802   2 . In the example embodiment of  FIG. 8 , substrate structure  800  represents light source  202  and light source wave guide  212 , with light source wave guide  212  enclosed by capping material  402 . Metallization layer  802  forms during an additional layering operation that provides an electrical connection for light source  202  in at least one design pattern of IFOG substrate  104 . In the example embodiment of  FIG. 8 , light source  202  comprises a laser diode on IFOG substrate  104 . 
         [0038]    In operation, electrical current passes through substrate structure  800  from metallization layer  802   1  to metallization layer  802   2 . The electrical current flows through source P-N junction  702  from P-layer to N-layer, releasing electrical energy that creates a plurality of photons. The plurality of photons emit laser light into light source wave guide  212  (as illustrated). The emitted laser light from light source  202  is transferred to light detector  204  as described above with respect to  FIG. 2 . 
         [0039]      FIG. 9  is a cross-sectional view of an embodiment of light detector  204  for IFOG substrate  104  shown in a partially-formed state with a substrate structure  900  comprising a compound junction and at least one semiconductor substrate layer. Substrate structure  900  further comprises at least one field oxide layer  902  and at least one semiconductor layer  904 . The at least one field oxide layer  902  serves as at least one doping layer when light detector  204  is fabricated. 
         [0040]      FIG. 10  is a cross-sectional view of substrate structure  1000  with at least one additional masking layer formed on the at least one semiconductor substrate layer  904  of  FIG. 9 . Substrate structure  1000  further comprises at least one semiconductor dioxide layer  1002  and at least one masking layer  1004 . The at least one semiconductor dioxide layer  1002  and the at least one masking layer  1004  are deposited during a series of patterning and layering operations that define a location of light detector  1002  on IFOG substrate  104 . 
         [0041]      FIG. 11  is a cross-sectional view of substrate structure  1100  with at least one additional doping layer formed on the at least one semiconductor substrate layer  904  of  FIG. 9 . Substrate structure  1100  further comprises source P-N junction  1102 . Source P-N junction  1102  is formed during at least one doping operation performed on substrate layer  1000 . The at least one doping operation creates a plurality of pockets in substrate structure  1100  that are either rich in electrons (N-type) or rich in electron holes (P-type). The plurality of pockets forms an electrically-active region. 
         [0042]      FIG. 12  is a cross-sectional view of substrate structure  1200  with an additional metallization layer formed on the at least one semiconductor substrate layer  904  of  FIG. 9 . Substrate structure  1200  further comprises metallization layer  1202 , shown as  1202   1  and  1202   2 . In this example embodiment, substrate structure  1200  represents light detector  204  and light detector wave guide  214 , with light detector wave guide  214  enclosed by capping material  402 . Metallization layer  1202  forms during an additional layering operation that provides an electrical connection for light detector  204  in at least one design pattern of IFOG substrate  104 . In this example embodiment, light detector  204  comprises a photodiode on IFOG substrate  104 . 
         [0043]    In operation, light detector  204  detects returning laser light from light detector wave guide  214  (as illustrated). The returning laser light from light detector wave guide  214  flows into detector P-N junction  1102 , absorbing the plurality of photons described earlier with respect to  FIG. 8 . Light detector  204  produces a photocurrent (that is, an electrical current) passing through metallization layer  1202   1  to metallization layer  1202   2 . In this example embodiment, assembly  100  processes the photocurrent for at least one IFOG measurement. 
         [0044]      FIG. 13  is a cross-sectional view of an embodiment of the substrate structure  1300  along line AA of  FIG. 2  (IFOG substrate  104 ). Substrate structure  1300  comprises light source (source diode)  202 , light detector (detector diode)  204  and ring interferometer wave guide  208 . As described above with respect to  FIGS. 3 to 12 , substrate structure  1300  is fabricated with one or more standard semiconductor wafer processes (for example, one or more silicon wafer fabrication methods). Alternate methods of fabricating substrate structure  1300  comprise a heterojunction composed of one or more layers of one or more dissimilar semiconductor material. The one or more dissimilar semiconductor materials have non-equal bandgaps (that is, energy differences between junctions). In one implementation, a sequence of aluminum gallium arsenide-gallium arsenide-aluminum gallium arsenide (AlGaAs—GaAs—AlGaAs) form a double heterojunction. With a heterojunction, characteristics of modern laser diodes (for example, source diode  600  and detector diode  1200 ) closely approach those of an idealized diode. Furthermore, diode model parameters for source diode  600  and detector diode  1200  that define the diode current vs. voltage response are tunable by adjusting the thicknesses and bandgaps of the one or more layers of dissimilar semiconductor material. 
         [0045]      FIG. 14  is a block diagram of an embodiment of a system  1400  for recording data with an electronic device. System  1400  comprises area  1402 , electronic device  1404 , base station  1408 , and user  1412 . In this example embodiment, base station  1408  further comprises display  1410 . Electronic device  1404  comprises electronic package assembly  100 , including at least one IFOG substrate  104 , as described earlier with respect to  FIG. 1 . Examples of electronic device  1404  include, without limitation, a miniature camera, a miniature navigation drone, and a minute remote sensor suitable for providing base station  1408  with navigation-related data surrounding or within area  1402 . In one implementation, electronic device  1404  is appropriately sized (for example, the size of a common housefly or a bumblebee) for unrestricted travel within area  1402 . Electronic device  1404  is self-powered and travels throughout area  1402 , continuously recording navigation-related data. One example of navigation-related data is a position estimate to be determined by electronic device  1404 . Another example of navigation-related data is an estimate of an attribute related to motion within area  1402  (for example, the distance traveled, velocity, acceleration, location, etc.) 
         [0046]    Electronic device  1404  transmits the navigation-related data along wireless transmission link  1406 . Wireless transmission link  1406  comprises secure wireless communication transmissions between electronic device  1404  and base station  1408 . Communication between electronic device  1404  and base station  1408  over wireless transmission link  1406  occurs when electronic device  1404  is sufficiently close to base station  1408 . In one implementation, display  1410  displays the navigation-related data in real time to user  1412 . In other implementations, alternate methods for conveying the navigation-related data include a database, a network server, and the like. 
         [0047]    The methods and techniques described here may be implemented in digital electronic circuitry, or with a programmable processor (for example, a special-purpose processor or a general-purpose processor such as a computer) firmware, software, or in combinations of them. An apparatus embodying these techniques may include appropriate input and output devices, a programmable processor, and a storage medium tangibly embodying program instructions for execution by the programmable processor. A process embodying these techniques may be performed by a programmable processor executing a program of instructions to perform desired functions by operating on input data and generating appropriate output. The techniques may advantageously be implemented in one or more programs that are executable on a programmable system including at least one programmable processor coupled to receive data and instructions from, and to transmit data and instructions to, a data storage system, at least one input device, and at least one output device. Generally, a processor will receive instructions and data from a read-only memory and/or a random access memory. Storage devices suitable for tangibly embodying computer program instructions and data include all forms of non-volatile memory, including by way of example semiconductor memory devices, such as EPROM, EEPROM, and flash memory devices; magnetic disks such as internal hard disks and removable disks; magneto-optical disks; and DVDs. Any of the foregoing may be supplemented by, or incorporated in, electronic package assembly  100  of  FIG. 1 .