Patent Publication Number: US-6909830-B2

Title: Method and apparatus for on-wafer testing of an individual optical chip

Description:
CROSS REFERENCE TO RELATED APPLICATIONS 
   This application claims priority from Provisional patent Application Ser. No. 60/396,509 filed on Jul. 16, 2002. 

   FIELD OF THE INVENTION 
   The invention is directed to optics and in particular to the capability to utilize optics to test optical chips while they are still part of a wafer. 
   BACKGROUND OF THE INVENTION 
   Planar optical devices are optical devices based on waveguide technology, which are fabricated on planar substrates of various types. An optical chip based on this technology could contain only simple waveguides, single devices like couplers, or complex planar lightwave circuits consisting of a multitude of devices and functions. These optical chips are fabricated on planar substrates. Typically individual chips are fabricated in bulk on semiconductor wafers made from, for example, Si or InP. These wafers are subjected to various fabrication steps, like deposition of materials and etching, typically involving microlithography, to form the discrete optical devices in the substrate material or on top of the substrate. 
   Optical waveguides serve as connections between the planar optical devices as well as input and output terminals. After or during the fabrication it is desirable to test the functionality of the optical chips to prevent shipment, sale or use of non working chips. Some chips may include optical sources like lasers and detectors on the chip itself, which allows for standard electronic on-wafer testing utilizing electrical connections to the wafer. However, other chips may require the tester to couple light into the chips or out of the chips in order to perform the tests. In this case, the chip usually must be separated from the wafer to allow access to the chip to couple light from optical fibers or other chips into the waveguides at the facets of the chip. In many cases, the facets of the chip to be tested have also to be polished. A typical example is the preparation of laser bars from wafers which allows testing of edge-emitting lasers. The wafer is separated in bars to keep the number of pieces to be handled low. Nevertheless, it would be advantageous to test such lasers en masse on complete wafers because the handling of bars or chips after separation is tedious and time-consuming. Accordingly, there is a need to test the optical chips prior to separation from the wafer. 
   It is known in the art to test optical devices that emit or receive light vertically (perpendicular to the wafer plane). An example of a vertically accessed optical device is a VCSEL (Vertical Cavity Surface Emitting Laser). A vertical testing method of a VCSEL is disclosed in US Patent Publication 2001/0021287 A1. However, this process does not allow for vertical testing of optical devices whose input and output ports are in the plane of the wafer, with waveguides in the same plane. 
   Some have tried to allow for optical access from the surface of the wafer by including devices on the wafer, which redirect the light from a planar direction (parallel to the surface) to a direction perpendicular to the surface. To access the light vertically from the planar optical devices with waveguide in the wafer plane, Kuwana et al. in Japanese Journal of Applied Physics, Vol. 38 (1999) has fabricated a triangular shaped Al mirror underneath the waveguide to deflect the propagation direction of the light in the waveguide. The light is deflected by the mirror, travels upward. A similar process occurs in the reverse direction. Thus vertical access to the planar waveguide is achieved in this prior art. Another example is U.S. Pat. No. 6,337,871 B1 for a multiple edge-emitting laser components located on a single wafer and the on-wafer testing of the same, where the inventors include special devices on the wafer to redirect the light of laser perpendicular to the surface. However, these constructions require additional structure to be integrated with the chips as part of the wafer adding to the complexity and time required to manufacture such wafers. 
   Accordingly, a method and apparatus for testing chips on the wafer, which overcomes the shortcomings of the prior art is desired. 
   SUMMARY OF THE INVENTION 
   A method for testing an optical chip formed on an uncut wafer, the wafer including at least a substrate and said optical chip including at least a core disposed above such substrate and a cladding layer disposed on said core, the optical chip being a planar optical waveguide comprising the steps of removing some portion of the waveguide to form an access point; and inserting an optical probe adjacent to the optical chip at said access point. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     For a fuller understanding of the invention, reference is had to the following description taken in connection with the accompanying drawings in which 
       FIG. 1  is a sectional view of a waveguide formed on an uncut wafer in accordance with the prior art; 
       FIG. 2  is a sectional view of an optical chip formed on an uncut wafer in accordance with the present invention; 
       FIG. 3  is a sectional view of an optical probe constructed in accordance with the invention; 
       FIG. 4  is a sectional view of an optical probe constructed in accordance with a second embodiment of the invention; 
       FIG. 5  is a sectional schematic view of an optical probe and optical chip used in accordance with the testing method of the invention; 
       FIG. 6  is a sectional view of an optical chip formed on an uncut wafer in accordance with a second embodiment of the invention; 
       FIG. 7  is a sectional view of an optical probe constructed in accordance with another embodiment of the invention; 
       FIG. 8  is a sectional view of an optical probe constructed in accordance with yet another embodiment of the invention; 
       FIG. 9  is a sectional view of an optical probe constructed in accordance with yet another embodiment of the invention; 
       FIG. 10  is a sectional view of the use of a probe for testing an optical chip on a wafer in accordance with another embodiment of the invention; 
       FIG. 11  is a sectional view of a probe optically coupled to an optical fiber in accordance with the invention; and 
       FIG. 12  is a sectional view of an optical chip separated from the wafer after having been tested in accordance with the invention. 
   

   DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
   The current invention allows testing of planar optical waveguide chips, including edge-emitting lasers, from the surface of the wafer and without separating the wafer into individual chips. The invention does not require the inclusion of special devices on the wafer which redirect the light processing perpendicular to the surface. Instead, the redirection of the light is achieved in optical probes described here. The wafer under test has to be prepared only with very simple means readily available to any fabricator of such devices. 
   A first embodiment of the invention is a method and device for testing optical chips on the wafer level without separating the chips from the wafer. One optical terminal of the chip is shown in FIG.  1 . As is known in the art, a wafer consists of the substrate  1000  with a plurality of the waveguides  1010  formed thereon. The waveguide  1010  includes a lower cladding layer  1020 , a core  1040 , and an upper cladding layer  1030 . 
   As shown in  FIG. 2 , a trench is etched in accordance with the invention at the end of the waveguide where optical access is needed. Trench  1150  provides sidewalls, one of which forms facet  1160  on waveguide  1110  through which the light can be coupled into the waveguide. The depth of this trench is at least below the core  1140  of waveguide  1110 , but could extend deeper into substrate  1000 . The width of trench  1150  need only be sufficient to allow access of the test device as described below. 
   The angle of facet  1160  should be close to 90° relative to the waveguide (core), but even with angles deviating from 90° the results can be sufficient for the purposes of wafer-level testing. For example, one may even want to have facet  1160  to be at 8° from verticality to reduce back reflection. The roughness of the sidewalls should be as low as possible but some roughness can be tolerated. The method by which this trench  1150  can be formed includes, but is not limited to, etching or physical removal, e.g., a shallow dice-saw cut or a scribe line that does not go through the entire substrate. Milling, such as, ion milling and Focused Ion Beam (FIB), or laser ablation can also be used for creating this trench. Making a trench on-wafer in order to open up an access point to the horizontal waveguide that is otherwise closed, is an important part of this invention. 
   To couple light into and out of the now exposed waveguides an optical probe is used. This optical probe is brought in close proximity to the ends of the waveguides at facet  1160 . The light is coupled into an input end of the optical probe by means of optical fibers, waveguides, or by directly coupling at the input end, a light source. The output or exit end of the optical probe, which is disposed in trench  1150 , is designed to change the direction of the light into the wafer plane, thereby allowing coupling into the end of waveguide  1110  on the chip. The change of direction can be accomplished by either a narrow bend or by a mirror. 
   One specific embodiment of the current invention is shown in FIG.  3 . The optical probe  2000  consists of a substrate  2010  with a waveguide  2060  disposed thereon. Waveguide  2060  may be a dielectric waveguide. The waveguide  2060  consists of a lower cladding  2020 , a core layer  2030 , and an upper cladding layer  2040 . The end of the optical probe has been polished to have substantially a 45-degree angle relative to the core, providing facet  2050 . In one non-limiting example, facet  2050  is covered with metal layer  2070 , thereby providing a mirror. The facet  2050  can also be covered with a dielectric mirror stack that provides reflection at the wavelength that is being used. However, it should be noted that facet  2050  can also be used without any coating if the reflectivity at the facet is sufficient for deflecting enough light into the wanted direction. For example, if the facet  2050  is polished to have a smooth surface and has an interface with sufficient index difference between the waveguide  2060  and air, it will act as a mirror and provide a good reflectivity. The reflected light traveling through upper cladding  2040  is unguided in cladding  2040  in this embodiment. Probe  2000 , similar to what will be discussed below, can also be made so that there is a waveguide that guides the reflected light substantially perpendicular to core layer  2030  within upper cladding layer  2040 . 
   As a result, light entering probe  2000  at an input end  2080  exits probe  2000  at an exit end  2090  in the direction of arrow A. Accordingly, light traveling through probe  2000  in a direction not parallel to a wafer, exits probe  2000  in a direction substantially perpendicular to a facet on wafer chip  1110 . Facet  2050  does not have to be at 45° with respect to the waveguide, and each of its embodiment parts, such as the core, as long as the change in the propagation direction is sufficient for the application. For example, an angle between 35° and 55° may be used if the change in the propagation direction is sufficient for achieving an acceptable coupling loss between the probe and the optical device under test. In fact, an 8-degree angle may be intentionally achieved by having a facet angle of close to 41° or 49°, which is the angle that is used in preparing optical chip facets to reduce back reflection at the interface. Or more generally, the facet  2050  can be curved but smooth to provide a focusing effect. For practical purposes, an angle substantially close to 45° is preferred for facet  2050 . 
   An array of regularly spaced or irregularly spaced waveguides can be included on this probe in order to test multiple waveguide devices at once. One can even have both the input and output waveguides on one probe for sending and receiving the test signals through the same probe. 
   The facet  2050  can also be of a curved shape, rather than having a straight shape. The curve shaped facet may potentially be beneficial since it may provide a focusing or collimating effect. 
   Typical materials that are used in waveguide fabrication can be used in making probe  2000 . The waveguide core can be made of doped silica such as Ge-doped silica, phosphorous doped silica, Silicon Oxynitride, BPSG (Bore Phosphorous Silicate Glass) or undoped silica. The waveguide claddings can be made of undoped silica, doped silica such as BPSG, Flourine doped silica, or Boron doped silica. 
   Reference is now made to  FIG. 4  in which another embodiment of an optical probe generally indicated as  2100  is provided. Probe  2100  includes a cladding block  2130 . Planar waveguide core  2110  extends through block  2130  and includes a waveguide bend  2120  therelong between an input  2150  and an output  2140 . Probe  2100  includes a cladding block  2130  to provide a change in the direction of the optical beam. The cladding block  2130  surrounds the core  211 . 
     FIG. 5  shows the operation of optical probe  2000 . Optical probe  2000  is brought into close proximity to waveguide  1110  and then aligned to allow for coupling of reflected beam  2090  of light into the waveguide. The light traveling through core  2030  of probe  2000  changes propagation direction at the mirror (facet  2050 ) and travels out of core  2030 , through upper cladding  2040 , and exits waveguide  2060  into free space. Then light beam  2090  enters waveguide  1110  that is being tested. Because light beam  2090  that has changed the propagation direction has to exit waveguide  2060 , the facet angle of facet  2050  has to be such that the angle of incidence of the light on the interface of core  2030  and upper cladding  2040  is smaller than the critical angle between core  2030  and upper cladding  2040 , which is determined by the index difference between core  2030  and the upper cladding  2040 . Similarly the angle of incidence of the light on the interface of upper cladding  2040  and region  3060  is smaller than the critical angle between  2040  and  3060 . The index difference between core  2030  and upper cladding  2040  also determines the power loss of light while exiting the waveguide. This is because the power reflectance of the core/cladding interface between  2030  and  2040  is dependent on the index difference. The power reflectance increases as the index difference increases. A low index difference waveguide whose index difference is similar to that of fiber will have a very low power reflectance, resulting in a low power loss when the light exits the waveguide. There will, however, be a reflection between cladding layer  2040  and region  3060  due to index difference. One can fill region  3060  with a material with an index that reduces such reflection. An index matching fluid can be used for this purpose. The index matching fluid can also reduce the scattering losses due to the roughness that may exist on the exit facet of waveguide  1110 . 
   The coupling efficiency of the optical probe can be optionally improved by applying microlens  3090  on the top surface of upper cladding  2040  of probe waveguide  2060 . Microlens  3090  can be placed accurately by coupling light into the probe through the input port and illuminating the optical probe. The top surface will then have a bright spot on which the mircolens can be mounted. This microlens can be made of a polymer such as epoxy. The microlens may have its shape tailored by adjusting the affinity of the monomer to the top surface of the probe before the polymerization. Microlens  3090  can be attached to waveguide  2060  or it can be placed at some distance away from the waveguide facet. 
   In another embodiment, the coupling efficiency of the optical probe can be optionally improved by depositing or forming a Graded Index (GRIN) lens on the top surface of the probe as part of upper cladding  2040 . The GRIN lens can focus the light into waveguide  2060 , when light is traveling from waveguide  1110  to the probe or vice versa. 
   At an output port, the transmission of light in the reverse direction will occur. The light travels out of waveguide  1110 , and then the light can be coupled into optical probe  2000  via the mirror facet via the similar reflective process discussed above. 
   Once the optical probe has been aligned with an optical chip on the wafer, the optical probe can be stepped through all chips on the wafer. Only fine tuning of the optical probe location with regard to the chip being tested is necessary. In this way a large number of chips can be tested efficiently. 
   Often an optical chip can have a multitude of input or output waveguides. In this case the optical probe can be designed to have a multitude of input or output channels matched exactly to the chip under test, since the relative positions of the waveguides are exactly known. In this way there is only one alignment necessary to provide access to all channels needed for the test. One can also design the probe and the chip under test such that both input and output can be on one probe, therefore reducing the alignment processing steps. 
   Thus, a large number of chips can be tested on a wafer without separating the chips from the wafer. This can result in increased efficiency in wafer testing. 
   A second embodiment of the invention relates to testing optical chips on the wafer level without separating the chips from the wafer and without etching a trench or otherwise preparing a facet at the end of the waveguide. As shown in  FIG. 6 , according to one aspect of this embodiment, waveguide  4000 , including lower cladding  4020 , core  4030 , and upper cladding  4040 , has been fabricated on top of wafer substrate  4060 . Waveguide  4000  is the optical circuit under test. The waveguide has been extended into an access site  4050 , beyond plane  4010  where the facet for fiber or waveguide coupling will later be prepared for permanent coupling after waveguide  4000  has been separate from the wafer. 
   At access site  4050  upper cladding layer  4010  is partially reduced in thickness, for example, by etching upper cladding layer  4040 . This etching step could be either a dedicated process step, or it could be performed as part of another etching step during the fabrication of the optical chip. The remaining thickness of upper cladding layer  4040  has to be thin enough that there is a sufficient evanescent optical field present outside of the cladding, i.e., to allow evanescent light to reach core  4030  sufficient to test waveguide chip  4000 . The length of the recess has to be long enough to allow optical coupling as described below. However, the length will be mostly determined by practical considerations with regard to the access of an optical probe, as described below. The method by which access  4050  can be formed includes, but is not limited to, etching or physical removal, by way of non-limiting example, e.g., a shallow dice saw cut or scribe line. 
   Optical probe  5000  ( FIG. 7 ) is used to couple light into waveguide  4000  at access site  4050 . Optical probe  5000  features a short piece of waveguide  5010 , which fits into the recess at access site  4050  on the wafer. In a preferred embodiment, the probe is connected to optical fiber  5070 , which can be connected to test equipment.  FIGS. 7 and 8  show two possible embodiments of probes for use in access site  4050 . Optical probe  5000  (FIG.  7 ), by way of example, includes planar optical waveguide chip  5000  with first waveguide core  5020  and second waveguide core  5010  and cladding region  5030 . At region  5040 , chip  5000  has been cut close to core  5010  to allow evanescent coupling. Optical fiber  5070  is optically coupled to chip  5000  to connect to the test equipment. 
   Another embodiment of an optical probe, generally indicated as  5100 , for use with access point  4050  is shown in FIG.  8 . Optical probe  5100  includes an optical chip formed as waveguide  5110 , which is fabricated along the lower edge of optical chip  5100  and second waveguide  5150 . The light in waveguide  5150  is directed into waveguide  5110  through mirror facet  5140 . Waveguide  5150  is optically coupled to optical fiber  5170  so that light from optical fiber  5170  can be directed from waveguide  5110  into core  4030  of a chip to be tested. 
     FIG. 9  shows another embodiment of optical probe, generally indicated as  5200 , for use with access site  4050 . Probe  5200  includes a first waveguide  5250 , a second waveguide  5210  and cladding layer  5230 . Waveguide  5250  is coupled to waveguide  5210  through waveguide bend  5240 . Waveguide  5250  may be connected to a light source such as a laser or a fiber, or a detector for sending or receiving light through waveguide  5250 . Waveguide  5210  that evanescently conducts light sideways in a direction substantially perpendicular to waveguide  5210  enables mode coupling to the optical device under test that will be placed adjacent to  5210 . Waveguide  5210  is close to a chip edge (thinned cladding layer) so that a substantial amount of the evanescent field extends outside the chip for an external device to be placed close to it and achieve evanescent coupling between the two. In this embodiment, waveguides  5250  and  5210  are substantially at 90° relative to each other. For practical purposes, an angle substantially close to 90° is desired, but it is not necessary. For example, one can have an angle between 70° and 110° between waveguides  5250  and  5210  if it yields acceptable performance. 
   During testing, optical probe  5000 , by way of example, is aligned to waveguide  4000  at access site  4050 .  FIG. 10  shows how optical probe  5000  is oriented to access site  4050  on the wafer. Waveguide  5010  of optical probe  5000  and waveguide core  4030  of access site  4050  has to be aligned to allow for directional, evanescent coupling. Optical probe  5000  and access site  4050  can be either in direct contact or in close proximity, depending on the design of the respective waveguides and cores. Due to the constraints of the design the coupling may not be optimum, thereby incurring a coupling loss. This loss can be calculated, or it could be measured by means of an on-wafer calibration structure. However, in many cases on-wafer tests are performed to assess wavelength dependent characteristics of the optical chip. In this case the insertion loss is of limited importance. 
   As was discussed above in connection with  FIG. 3 , an index-matching fluid may optionally be used at the access to waveguide  1110  to mitigate the lack of verticality and/or smoothness of the sidewall. The main advantage of index matching fluid is the increased tolerance to errors in positioning and facet preparation allowing users to obtain more consistent coupling efficiency between the probe and the on-chip waveguide. The index matching fluid can be used during testing and be cleaned off before final packaging. 
   The probe can be coated with a soft optically transparent material, which may index match to the input waveguide, to reduce variations in waveguide to waveguide coupling errors. The surface of the probe through which light exits, may be coated with a drop of liquid that can be hardened into a shape of a lens. The lens can then focus or collimate the light coming out into the space  3060 , therefore reducing the coupling loss. 
   The optical probe of the present invention may be used in conjunction with a proximity or force feedback system to prevent the tip of the optical probe from crashing into the chip. It can also be used in conjunction with a mechanically motorized and/or piezo-electrically actuated stage on which the tip is mounted. The relative movement of the tip with respect to the sample that is tested can be automated with software algorithms that control the mechanical motor and/or piezo-electric actuator. 
   The probes illustrated in this embodiment can be manufactured in a relatively large quantity and therefore can be considered as disposable elements (or a consumable). The probes are connected to a fiber or an array of optical fibers at the input (non-inserted) end for connection with the light source and the testing equipment. The whole assembly that includes fibers and the optical probe can be considered as a consumable that can be supplied in large quantities. 
     FIG. 11  shows an exemplary embodiment of such an assembly. The input end of optical probe  7000  is optically connected to optical fiber  7050 . Core  7030  of probe  7000  is aligned to core  7060  of optical fiber  7050  for optical connection with low loss. Typically, an adhesive is used to bond probe  7000  and fiber  7050 , such as UV-curable glue. Multiple fiber (or an array of optical fiber) can be used when the probe contains multiple waveguides. To improve coupling efficiency, the mode size and numerical aperture of the probe waveguide is matched to the mode size and numerical aperture of the optical device connected at the probe input. Similarly, the mode size and numerical aperture of the output of the probe is matched to the mode size and numerical aperture of the optical chip to be tested. 
   When a fiber array is used, they sit on and/or are covered by a silicon or a glass piece for structural support, as commonly used in commercial fiber arrays. Additionally, the optical probe that is supplied may be a consumable item, separate from the fiber array. In this case, a method of easily connecting the optical probe to the fiber array is needed. 
   This on-chip optical probe can also be combined with electrical testing equipment and/or electrical probes to provide high speed electro-optical testing for any chips that include edge emitting or edge receiving devices. This embodiment would be particularly useful for testing high speed electro-optic devices such as edge emitting lasers, waveguide detectors, waveguide modulators and RF photonic components 
   After the on-wafer testing, the optical chips on the wafer are separated from each other by conventional methods, such as sawing or cleaving. At this time the access site can be separated from the actual optical chip, as shown in FIG.  12 . After the separation at facet  8010 , the optical chip can be processed and packaged as any chip that did not comprise the additional access site for on-wafer testing. 
   Thus, a large number of chips can be tested on a wafer without first separating the chips from the wafer. This can result in increased efficiency in wafer testing. 
   In another embodiment, the wafer containing the optical chip may be processed further without sawing or cleaving, after on-wafer testing. This embodiment of the invention allows for the testing of devices on a wafer that is in-process to provide feedback on the process, without having to destroy the wafer. The ability to continue processing a wafer with planar integrated optic waveguides is a big advantage, as it allows trimming and tuning of optical device on the wafer. In addition, it can be used for rapid defect analysis. 
   Thus while there have been shown and described and pointed out novel features of the present invention as applied to preferred embodiments thereof, it will be understood that various omissions and substitutions and changes in the form and details of the disclosed invention may be made by those skilled in the art without departing from the spirit and scope of the invention. It is the intention, therefore, to be limited only as indicated by the scope of the claims appended hereto. 
   It is also to be understood that the following claims are intended to cover all the generic and specific features of the invention herein described and all statements of the scope of the invention, which is a matter of language, might be said to fall therebetween.