Abstract:
This application describes, among others, wafer designs, testing systems and techniques for wafer-level optical and optoelectronic testing by coupling probe light to/from the top of the wafer. In the described exemplary implementations, wafers are designed with one or more optical alignment features or structures. The alignment structures are alongside the devices to be tested, but are easier to find or locate by optical means, than the devices to be tested. An optical diffraction grating structure such as a Littrow grating may be used as reflective alignment structures. (FIG.  6 B and paragraph 56.) A Littrow grating as an alignment structure produces a retro-reflection. A Littrow grating is a one-port optical device, with input and output beams going along the same path. Various exemplary implementations are shown in FIGS.  3 C,  5, 6 A,  6 B,  7 A and  7 B and described in paragraphs 18–21, 39–40 and 53–64.

Description:
This application is a continuation of U.S. Non-Provisional Utility patent application Ser. No. 10/820,631 pending, filed Apr. 7, 2004, which claims priority from U.S. Provisional application No. 60/461,041 filed Apr. 7, 2003. 

   BACKGROUND 
   This application relates to testing of integrated optical devices, integrated optoelectronic circuits and systems. 
   Compact optical components, devices, and systems may be monolithically fabricated on various substrates to form integrated optical components, devices, and systems. Such fabrication may be achieved using various fabrication processes to process wafers, including micro fabrication processes used for fabricating integrated circuits (ICs), micro-electro-mechanical systems (MEMS), and other microstructures. The integrated optical components, devices, and systems may be monolithically integrated with ICs, MEMS, and other microstructures on the same chip to provide various useful functions. Fabrication of such integrated devices with optical components or devices generally includes wafer-level fabrication processes and packaging after dice on a single wafer are separated into chips by, e.g., dicing or sawing. The individually packaged chips are then tested. 
   In fabrication of integrated electronic circuits, a portion of the IC testing can be performed at the wafer level prior to separation of the dice. This wafer level testing measures the performance or identifies defective components and dice. Hence based on such testing at the wafer level, defective dice may be removed from subsequent processes including the packaging and testing of individual chips. Labor and costs associated with subsequent processes for the defective dice, therefore, can be significantly reduced. Various wafer-level testing systems have been developed for testing ICs at the wafer level. 
   SUMMARY 
   This application describes, among others, exemplary designs of wafer layouts with dice for chips having at least one integrated optical component, and associated testing systems and techniques for testing such chips at the wafer level. In the described exemplary implementations, wafers are designed with one or more optical alignment features or structures alongside the components and devices of the chips that are easier to find or locate by optical means than an optical component or device to be tested. Each of the optical component or device, and the optical alignment structure or structures may be optically accessed from a position above the wafer so that optical alignment and testing can be performed at the wafer level prior to separating the wafer into individual chips. In addition, as part of the wafer design, each optical alignment feature or structure has a known, predetermined positional relationship with one or more optical components or devices to be tested. Therefore, the optical component or device to be tested can be found by first optically locating a nearby optical alignment mark and then using the known positional relationship between the optical alignment mark and the optical component or device to accurately locate the optical component or device. Next, optical testing of the optical component or device can be performed at the wafer level. 
   The wafer level testing of optical components and devices can identify defective optical components and associated host dice at the wafer level. Therefore, defective dice may be removed from subsequent fabricating processes including the packaging of separated dice into chips and testing of individual chips. In this regard, the wafer level optical testing can spare the labor and costs associated with subsequent processes for the defective dice. In addition, certain optical testing procedures, which are conducted during testing individual chips in other chip fabrication and testing methods, may now be performed at the wafer level prior to separating the wafer into individual dice. The wafer level optical testing may be automated and systematically performed at a high speed. Therefore, the wafer level optical testing may be used to reduce the testing at the chip level and hence reduce the testing time, labor, and associated cost. Furthermore, the optical alignment and testing techniques described here may be used for attaching fibers to optical ports on wafers and chips after the proper alignment is achieved between an optical port on the chip and a fiber to be bonded to the optical port. 
   In one implementation, a probe beam from an optical probe located above the surface of a wafer is directed to the surface of the wafer. An optical alignment structure on the wafer is used to direct a portion of the probe beam along a direction above the wafer. The portion of the probe beam from the optical alignment structure is used as a guide to adjust the position of the optical probe relative to the optical alignment structure. 
   Based on the above implementation, a known spatial relationship between the positions of each component on the wafer relative to the optical alignment structure may be used to align the optical probe with at least one optical component on the wafer. After this alignment, the probe beam from the optical probe is used to optically test the optical component. 
   In another implementation, a device is described to include a wafer having a wafer surface patterned to comprise an integrated component and an optical coupler operable to couple light incident from a device above the wafer to the integrated component. The device also includes at least one optical alignment structure on the wafer surface operable to reflect incident light along a predetermined direction above the wafer surface. 
   System implementations for the wafer level optical testing are also described. For example, one described system includes a wafer holder, a wafer positioner, an optical probe, an optical probe positioner, and a system control. The wafer holder is used to hold a wafer comprising dice pattered to include optical devices and optical alignment marks that reflect incident light. The wafer positioner is engaged to the wafer holder to control positions and orientations of the wafer. The optical probe is used to deliver a probe beam to the wafer at an incident direction above the wafer and to receive optical reflection of the probe beam from the wafer. The optical probe positioner is engaged to the optical probe to control the position of the optical probe. The control system is used to control the wafer positioner and the optical probe in aligning the optical probe to a selected position on the wafer according to reflection of the probe beam from the optical alignment marks. 
   In another implementation, an optical probe head is provided to include an array of optical waveguides and to be positioned above a wafer. At least one optical waveguide in the array is used to direct a probe beam to the wafer which directs at least a portion of the probe beam back to the array. One of the optical waveguides in the array is used to receive the portion of the probe beam. The received light in the one of the optical waveguides is used as at least one of (1) a guide to adjust a position of the optical probe head relative to a position on the wafer to align different waveguides to different predetermined positions on the wafer and (2) a monitor signal to test an optical component on the wafer. 
   These and other implementations, their variations, applications, and associated advantages and benefits are described in greater detail in the attached drawings, the detailed description, and the claims. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
       FIG. 1  illustrates a wafer patterned with microstructures in dice. 
       FIG. 2  shows one die in  FIG. 1  that has optical components or devices. 
       FIGS. 3A and 3B  show operation of optical couplers in the terminal ports of optical components or devices in  FIG. 2 . 
       FIG. 3C  illustrates light exiting a fiber probe used in  FIGS. 3A and 3B  in interaction with a flat surface on a wafer where the angled facet and the tilting of the fiber probe prevents the reflection and scattering of the light from entering the fiber probe. 
       FIG. 4  shows an example of a wafer-level testing system. 
       FIG. 5  shows examples of optical alignment structures in the form of dot and line optical alignment marks in a wafer. 
       FIG. 6A  illustrates optical operation of an optical alignment structure in  FIG. 5  according to one implementation. 
       FIG. 6B  shows a Littrow grating as one exemplary implementation of an alignment structure. 
       FIGS. 7A ,  7 B,  8 A,  8 B,  8 C,  8 D, and  9  show examples of optical alignment structures and their operations. 
       FIGS. 10 ,  11 A,  11 B, and  11 C show an optical probe with an array of fibers and optical alignment of the array of fibers using various optical alignment structures. 
       FIGS. 12A ,  12 B,  13 , and  14  illustrate examples of alignment structures having spatially-varying optical properties and their operations. 
       FIGS. 15A ,  15 B, and  15 C show an exemplary workflow for aligning a wafer for optical testing. 
       FIG. 15D  illustrates operation of an exemplary search based on moving the optical probe in a circular motion. 
       FIGS. 16 ,  17 , and  18  illustrate examples for optical alignment devices that facilitate alignment of certain optical devices such as wavelength-sensitive devices and optical detectors. 
       FIG. 19  shows one example of an optical probe suitable for the system in  FIG. 4  having a capacitance sensor. 
       FIG. 20  shows an exemplary workflow for adjusting the relative vertical spacing between the optical probe and the wafer in a lateral adjustment during optical testing. 
   

   DETAILED DESCRIPTION 
   The wafers for implementations of testing techniques and systems described in this application may be made of various solid-state materials suitable for micro fabrication processes. Examples of substrate materials include semiconductor materials (e.g., silicon or GaAs), silicon-on-insulator materials, glass materials, and others.  FIG. 1  shows an example of a wafer  100  that is patterned to include dice  110 ,  120 , etc. Each die may include one or more micro structures. Each micro structure may be designed to perform one or more specific functions or operations. Examples of a microstructure include, but are not limited to, an optical component or device, an electronic component or circuit, a magnetic component or device, a mechanical component or device, an opto-electronic component or device, and a MEMS component or device. At least one die on the wafer  100  includes an optical component that is to be tested based on the wafer level optical testing described in this application. All dice on the wafer may be identical in some implementations. In other implementations, dice on the same wafer may be different. For example, although at least one die has at least one optical component integrated in the die, one or more dice on the same wafer  100  may not include an optical component, or alternatively, include one or more different optical components. 
     FIG. 2  shows one example of the die  110  in  FIG. 1  that has both optical and electronic components integrated within the die area  201 . Alternatively, a die may be all optical. In this particular example, integrated circuits  210  are fabricated along with the optical components. One optical component is a 2×2 optical switch  220  having optically coupled waveguides. A control mechanism, e.g., one of the circuits  210 , may be implemented to control the switching operation. The units  212  may be two optical transmitters to generate optical signals to be switched. Alternatively, the units  212  may be two detectors to receive optical signals. 
   The die  110  in  FIG. 2  further includes a second optical component  230  illustrated as an optical Mach-Zehnder modulator formed with optical waveguides. The unit  232  on one terminal of the modulator  230  may be an optical detector to receive output of the modulator  230 , or alternatively an optical transmitter to send an optical signal to be modulated by the modulator  230 . The other terminal of the modulator  230  is an optical port  240  which includes an optical coupler as a vertical optical interface. 
   An optical component or device on the die  110  may be optically passive or active, and may be implemented in a variety of configurations. Examples include but are not limited to an optical switch, an optical modulator, an optical detector, an optical splitter, an optical wavelength-division multiplexer or demultiplexer, an arrayed waveguide grating, an optical amplifier, an optical transmitter, an optical attenuator, an optical filter, an optical resonator, an optical interferometer, etc. 
   Notably, the die  110  includes optical ports  240  that are located at predetermined positions and operate to input or output optical signals. In some implementations, an optical port may be located at or near an edge of a die on the wafer. In other implementations, an optical port may be located away from an edge of a die. For example,  FIG. 2  shows that all of the optical ports  240  are located on or near one or more edges of a die. In certain applications, some optical ports may be located at or near an edge of a die and at least one of the optical ports  240  may be located away from an edge within the die. Each optical port  240  is located at a terminal end of a waveguide or an optical device on the die  110  and includes an optical coupler for coupling light into the waveguide or device from the top of the die  110  or coupling light out of the waveguide or device and directing the light to the top of the dice  110 . This optical coupler may be designed to couple light incident to the coupling port from a light source, a waveguide or fiber positioned above the die  110 . The incident light may be in a direction with a predetermined fixed incident angle with respect to the normal direction of the wafer surface of the die  110 . In one implementation, for example, the incident light may be at a direction with a fixed acute angle with respect to the normal direction of the wafer surface, such as an angle of several degrees. In another implementation, the incident light may be at a direction substantially perpendicular to the wafer surface. 
   Conversely, each optical coupler within a port  240  may also be operated to couple output light from the waveguide or device on the wafer as an output light beam towards the top of the wafer surface at a predetermined output angle with respect to the normal direction of the wafer surface. For example, the output angle may be at an acute angle with respect to the normal direction of the wafer surface or may be substantially perpendicular to the wafer surface. 
   Such optical couplers allow for an external optical unit located above the die  110  or the wafer  100  to optically interface with optical waveguides, optical components or optical devices on the die  110  or the wafer  100 . The external optical unit located above the die  110  or wafer  100  may be, for example, an optical waveguide or an optical fiber having an end facet that receives light from or directs light to an optical coupler on the die  100 . This external optical unit may include multiple fibers or waveguides for sending light to and receiving light from the die or wafer. The multiple fibers or waveguides may form a 1- or 2-dimension array. In these and other implementations of optical coupling through the top of the die  110  or wafer  100 , the individual die  110  need not be separated from the wafer  100  in order to perform an optical test on an optical component or device on the die  110 . The die  110  and other dice on the same wafer  100  may be tested for their optical operations at the wafer level. The wafer level optical testing may be used to achieve a number of advantages, including early identification of defects in optical components and devices and thus defective dice, automated testing at the wafer level, reduced testing costs and time, and savings in labor and time associated with packaging and testing at the chip level. 
     FIGS. 3A and 3B  illustrate two modes of operation of the optical coupler within an optical port  240  described above. In  FIG. 3A , the coupler  240  operates as an input coupler to a waveguide or an optical device on one of the dice on the wafer  100 . A beam incident from the top of the wafer surface is directed into the coupler  240 . The coupler  240  receives the incident beam and redirects the incident beam in a direction substantially parallel to the wafer surface into an associated waveguide or device on the wafer  100 . The coupler  240  may be implemented in various forms. For example, a grating coupler may be used as the coupler  240  to redirect the beam by optical diffraction.  FIG. 3B  illustrates the operation of the coupler  240  as an output coupler where an output beam from the waveguide or device generally parallel to the wafer surface is redirected by the optical coupler  240  to the top of the wafer surface in a predetermined direction. 
   In both  FIGS. 3A and 3B , a single fiber is illustrated as a fiber probe and represents one specific example for the external optical unit above the wafer surface that optically communicates with an optical component or device on the wafer  100 . Such a fiber may be a single-mode fiber, a polarization maintaining fiber, or a multi-mode fiber. With a properly designed grating coupler as the coupler  240 , the fiber probe may be used to guide and direct a probe beam from the top of the wafer  100  into the coupler  240  and to receive output light from the coupler  240  through a fiber end facet of the fiber. The fiber may have an angled end facet that is positioned above the wafer surface. The fiber probe may direct light to the wafer at a small angle with respect to the normal direction of the wafer surface, or may receive light output from the wafer at the small angle, or may both direct light to the wafer and receive light from the wafer along the direction with this small angle. Hence, a probe beam from a light source can be delivered through this fiber to the die  110  and an output from the same optical coupler  240  may be directed back to the fiber for optical detection or diagnosis. The light used with the optical probe may be at a suitable spectral range for the intended applications, such as the near infrared range including but not limited to wavelengths at or near 850 nm, 980 nm, 1150 nm, 1310 nm, 1480 nm, and 1550 nm. 
   The fiber shown in  FIGS. 3A and 3B  is one example of the external optical unit above the wafer surface that optically communicates with an optical component or device on the wafer  100 . This external optical unit above the wafer surface may be implemented in various ways. As described in later sections of this application, multiple fibers such as one or more arrays of fibers or a bundle of fibers may be used in some implementations to simultaneously couple multiple probe beams into a die and to receive multiple output beams from the die. A light source or an array of light sources such as diode lasers (e.g., VCSELs) and LEDs may be directly placed above the wafer surface to direct probe light to the wafer surface. A single or an array of optical detectors may be placed above the wafer surface to receive light coupled out of the wafer surface for testing. Certainly, a combination of two or more above and other optical components may be used as this external optical unit above the wafer surface for the wafer level testing. 
   In the example illustrated in  FIGS. 3A and 3B , the fiber probe may be oriented to be slightly off the normal direction of the wafer surface so that the output light from the fiber probe exiting at the angled end facet is incident to the wafer surface at an angle to prevent specular reflection of the incident light from the wafer surface from hitting the angled end facet and entering the fiber probe.  FIG. 3C  illustrates a situation where the incident probe beam from the fiber probe hits a flat surface or a location other than an optical port  240  having a grating coupler. In absence of the diffraction by the grating coupler, the incident probe beam is reflected into a main reflection based on the specular reflection and possibly weak secondary reflections. Tilting the fiber probe allows the reflection to be directed away from the end facet of the fiber probe. In one implementation, for example, the fiber probe with an angled end facet may be set at about 8 degrees from the normal direction. 
   This mechanism for preventing reflection of incident probe light from returning to the fiber probe in part facilitates the use of the fiber probe to receive output light from an optical component or device on the wafer via a grating coupler  240  and the optical alignment of the optical probe using optical alignment marks, features, or structures described in this patent application. For example, in some implementations, the grating coupler in the optical port  240  shown in  FIGS. 3A and 3B  may be designed to direct output light to the fiber probe or an optical alignment mark on the wafer may be designed to direct input light back to the angled fiber probe along the original input path by using a diffraction grating such as a Littrow grating. Therefore, unless the fiber probe is positioned above the wafer  100  at or near an optical port  240  or an optical alignment mark, the fiber probe ideally does not receive strong light from reflection from the wafer surface. This application provides some examples of using received light from an optical port  240  for optical detection or alignment of the fiber probe and examples of using received reflection from an optical alignment mark for alignment of the fiber probe. 
   A die on the wafer  100  having an electronic component may have input and output electrical contact pads located at, e.g., near edges of the die. Testing of such electronic components may also be included in the wafer level testing described here and may be performed either simultaneously or at a different time with the optical wafer level testing. The locations of the contact pads and optical couplers  240  may be designed to accommodate simultaneous optical and electronic testing in some chips so that the optical probe and electronic probe(s) do not interfere with each other. For example, an optical detector connected to an I/O port  240  with a grating coupler is tested with an optical input and an electrical output. As another example, an electrically controlled optical modulator has at least one electrical input, one optical input and an optical output. Such a modulator is tested with both electrical and optical probes. 
   In the examples shown in  FIGS. 3A ,  3 B, and  3 C, a fiber collimator lens may be included in the fiber facet. In an implementation, where one fiber is used to deliver a probe beam to the wafer and a separate fiber is used to receive the reflected or output beam from wafer, such a fiber collimator lens may be used to improve the collection efficiency of the fiber for receiving light. 
     FIG. 4  illustrates one exemplary implementation of a wafer-level testing system  400  for testing wafers like the wafer  100  in  FIG. 1  prior to separation of the dice. This system  400  may also be used to test diced wafers. The testing system  400  includes a support base that holds a wafer positioner, an electronic probe positioner, and an optical probe positioner. The wafer positioner supports a wafer holder or chuck configured to hold a wafer under test and has positioning mechanisms to adjust and control the lateral and vertical positions, and orientations of the wafer under test. The wafer positioner operates to move the wafer to position a selected die under test at a predetermined fixed position at which electronic and optical tests are conducted. The wafer positioner further operates to position components within the selected die under test at desired positions relative to the optical or electronic probe for testing. 
   The electronic probe positioner is used to hold and position an electronic probe above the wafer for testing electronic, optoelectronic, or electro-optical components in a die on the wafer. The electronic probe may be an RF or DC probe with one or more electrically conductive tips or needles as the contact leads. The electronic probe may include one or more electronic contact leads for supplying one or more testing electronic signals to the die and receiving one or more output electronic signals from the die. An electronic signal output from the die may be generated in response to light, e.g., an optical detector on the die. An electronic probe card with multiple contact leads, e.g., may be used for testing the electronic components. The electronic probe positioner may be controlled manually by an operator or automatically by a motorized control mechanism. The electronic probe positioner may be used to provide fine position adjustments to the electronic probe within a die area to ensure a proper alignment between the testing contact leads and the contact pads on the die for testing. 
   The optical probe positioner holds an external optical unit such as an optical probe at an approximate position above the wafer or a diced die for performing optical testing. As an example, the optical probe may include one or more optical probe heads that deliver one or more probe beams from the top of the wafer to the wafer at a predetermined height above the wafer surface. The optical probe may also include one or more optical probe receivers to receive output light beams coming out of the wafer surface. An optical waveguide may be used as the optical probe receiver, the optical probe head, or both, e.g., the optical fiber probe illustrated in  FIGS. 3A and 3B  which may be held in a V-groove in a substrate or a waveguide fabricated in a substrate. 
   For optical testing, the wafer positioner is used to adjust the lateral and vertical positions of the wafer to an initial position under the optical probe. The wafer positioner may include a translational positioner or a combination of translational positioners to control the lateral positions and the vertical position of the wafer held on a chuck. For the lateral control, for example, two one-dimensional positioners may be combined to provide adjustments in two orthogonal lateral directions X and Y. A third one-dimensional positioner may be used to control the vertical position Z of the wafer. The wafer positioner may further include a yaw adjustment to adjust the orientation of the wafer around the vertical axis Z. The wafer positioner may be designed to have two predetermined fixed vertical positions, a “testing position” and a lower “separate position” for controlling the vertical positions of a wafer. At the lower separate position, the wafer positioner separates the wafer from the electronic probe and the optical probe. At the testing position, the wafer positioner raises the wafer to be in contact with the electronic probe and close to the optical probe with a desired spacing in the vertical direction so that either or both of electronic testing and optical testing may be conducted at this position. After the wafer positioner sets the wafer at the testing position, the electronic probe positioner can be used to adjust the vertical position of the electronic probe to contact the wafer for testing. The optical probe positioner may be adjusted to finely tune the lateral positions (X and Y) of the optical probe and the relative vertical spacing between the optical probe and the wafer at a predetermined distance for the optical testing. The optical probe positioner may also be used to finely tune the yaw angle of the optical probe, for example when the optical probe is a linear array of fibers. 
   The accuracy of many commercial wafer positioners, however, may not be sufficient for properly aligning an I/O optical coupler  240  to the optical probe for optical testing in part because optical alignment in optical testing demands higher alignment accuracy. For example, an optical fiber end facet in the optical probe may need to be aligned to an input or output optical coupler  240  with a lateral tolerance of about one micron or less in various applications. This accuracy is difficult to achieve in many commercial wafer positioners in wafer testing systems designed for testing electronic circuits only. Hence, the system  400  implements the optical probe positioner to finely adjust the position and orientation of the optical probe relative to the wafer under test to optimize the optical alignment. As an example, a PZT-driven precision piezo positioner with a displacement range of about 100 microns may be implemented as part of the optical probe positioner for fine adjustment of position, and a combination of a rotation stage for yaw orientation and a manual pitch/roll stage may be used for fine adjustment of orientation. Such an optical positioner provides up to six degrees of freedom in adjusting the position and orientation of the optical probe relative to the wafer. 
   The system in  FIG. 4  may include two separate optical probes held by two different optical probe positioners. As an example, one optical probe may include at least one fiber to send probe light to the wafer and the other optical probe may include a fiber to receive reflected probe light or light output from an optical I/O port  240  on the wafer. Additional optical probes may also be added when needed. Two or more electronic probes may also be included in the system in  FIG. 4 . 
   In  FIG. 4 , a tester and control system is shown in the system  400  to control the system operations and tests. This system may include one or more micro processors or computers and other electronics for carrying out various processing and control operations. For a given design of die patterns on a wafer, a position map may be generated from the given design in which each component, optical, electronic, or otherwise, is registered and represented by position coordinates. The data for this position map may be made available to the tester and control system by, e.g., being stored in an internal memory or disk of the system or a portable memory media or device. The wafer positioner may be controlled according to this map to move any die and any component within the die to the fixed position under the optical probe for testing. The electronic probe positioner may also be controlled based on this map to align the electronic contacts between the electronic probe and the die on the wafer. 
   The tester and control system in  FIG. 4  may include testing functions for testing the electronic and optical components. This part of the system may include one or more electronic signal sources for generating desired electronic probing signals, electronic interfaces for outputting and receiving electronic signals, and testing analyzers for performing the testing. For optical testing, this part of the system may include one or more light sources for producing probe light to the wafer, one or more optical receivers for receiving optical signals from the wafer and converting into electronic signals, and optical testing analyzers for performing optical testing. With at least one electronic probe and one optical probe, the system in  FIG. 4  may perform optical testing, electronic testing, and optoelectronic testing on a chip or a wafer mounted on the wafer positioner. The optical testing, electronic testing, and optoelectronic testing may be conducted at the wafer level prior to dicing the wafer into chips. 
   Optical alignments for optical testing in the system in  FIG. 4  may need an alignment accuracy higher than that for aligning electronic pads for electronic tests. The tester and control system in  FIG. 4 , thus, may be designed to include fine alignment control mechanisms to achieve such accuracy not readily available from many wafer testing systems designed for testing electronic IC chips or electronic devices, components, and circuits. The entire optical alignment control mechanism of the system in  FIG. 4  is implemented in different parts of the system and these different parts operate in combination with one another. The following describes exemplary implementations of the optical alignment mechanisms in the system  400  in connection with optical alignment structures such as alignment marks or optical devices that are designed and fabricated on wafers. Various alignment features and mechanisms in the system  400 , including coarse optical alignment control for the wafer positioner, and fine optical alignment control for the optical probe positioner, may be used to fully explore the benefits of such optical alignment structures on the wafers as described in greater detail below. It is understood that various alignment features and mechanisms in the system  400  may also be used for optical alignments for optical testing, including wafer-level optical testing, based on other optical alignment techniques without such optical alignment structures on the wafers. 
   In the examples described below, an optical alignment structure on a wafer may be an optically reflective or diffractive structure that directs the majority or all of light energy in an optical probe beam incident from above the wafer to a predetermined output direction above the wafer. This optical output from the optical alignment structure may be collected at a position above the wafer by an appropriately designed optical probe positioned above the wafer. The received light from the optical alignment structure may be used as a guide or indicator to align the position of the optical probe with reference to the optical alignment structure. The use of a single optical alignment structure on the wafer may be sufficient to align the optical probe. In some implementations, more than one alignment structures on the wafer may be used in combination to achieve the desired optical alignment and to improve reliability and accuracy of the optical alignment. In some implementations, after the optical probe is aligned with reference to at least one optical alignment structure, the optical probe may be moved relative to the wafer to align with a target optical device such as an optical coupler in an optical I/O port  240  described above. This alignment step may be achieved according to a known relative positioning relationship between the optical alignment structure and the target optical device. 
     FIG. 5  shows one example of a wafer  500  that is fabricated with optical alignment marks as the optical alignment structures used in optical testing. A portion of the wafer  500  is illustrated to include the exemplary die  110  shown in  FIG. 2 , and optical alignment marks  510 ,  520 ,  530 ,  540 ,  550 , and  560  surrounding the four sides of the die  110 . Each alignment mark is designed to be optically reflective to reflect an incident optical beam from the top of the wafer back to the top of the wafer either in its original optical path or in a different but predetermined direction so that the reflected light can be collected by the optical probe positioned above the wafer  500  and analyzed for optical alignment. 
   Each optical alignment mark may be designed to reflect incident light back in a retro reflection mode or to reflect light to a predetermined known direction. In the latter non-retro-reflection design, a separate optical receiver positioned above the wafer  500  may be used to receive and collect the reflected probe beam for optical alignment. When each optical alignment mark is retro reflective, an optical probe can use the same optical waveguide such as a fiber that delivers the probe beam to the wafer  500  to collect the reflected probe beam to an optical detector coupled to the optical waveguide. The optical reflection of an alignment mark is designed to be much stronger than optical reflection or scattering at other parts of the wafer  500 . Under this design, the reflection from an alignment mark indicates whether the probe beam hits an alignment mark and the strength of reflection indicates whether the optical probe is properly aligned with reference to the optical alignment mark. 
     FIG. 6A  illustrates an alignment in connection with the wafer  500  in  FIG. 5  where a single fiber probe  610  positioned above the wafer  500  is used to both direct a probe beam  611  and to receive a reflected probe beam  612  from the wafer  500 . In this specific illustration, the fiber probe  610  is under alignment with the optical alignment mark  510 . The optical alignment mark  500  receives the probe beam  611  from the fiber probe  610  and reflects the probe beam back to the fiber probe  610  as the reflected probe beam  612 . The fiber probe  610  collects the reflected probe beam  612  and delivers it to an optical receiver used in the optical alignment control. At a given height, the relative lateral position between the mark  510  and the fiber probe  610  is adjusted to maximize the received optical power of the reflected probe beam  612 . 
     FIG. 6B  shows one exemplary implementation of the optical alignment marks used in  FIG. 5  where an optical diffraction grating structure such as a Littrow grating may be used as the reflective alignment marks. The Littrow grating as the alignment structure operates to produce a retro reflection under the Littrow condition of d=mO/(2n g sinT) where m is an integer for the diffraction order, n g  is the effective index in the fiber probe, and T is the angle of the fiber probe with respect to the normal direction of the wafer surface. A Littrow grating may be directly fabricated on the wafer  500 . 
   In designing the wafer  500  with alignment marks, the positions of the alignment marks are predetermined and are known. In addition, the relative positions of each component within each die with respect to the optical alignment marks are known. These positional relationships are included in the position map. Hence, the alignment system may use the known location of at least one alignment mark to align the optical probe to that alignment mark and then use the alignment mark as the reference to move the wafer to place a selected I/O optical port  240  for an optical component under the optical probe. 
   The shape and location of an optical alignment structure for a die may be designed to allow for unambiguous identification of a location on the die. A single optical alignment structure may be sufficient to define a reference location on the die. An optically reflective dot at a known reference location on the wafer, for example, may be used as an optical alignment mark. 
   The example shown in  FIG. 5  uses a set of 2 alignment marks to achieve this unambiguous identification of a reference location on the die. The marks  510  and  520  form one set. The mark  510  is an elongated line mark that is spaced from die  110  and is parallel to one side of the die  110 . The mark  520  is a dot mark located at the corner formed by the two adjacent sides of the die  110 . The dot mark  520  is separated from but is aligned with the line mark  510 . In this set, the dot mark  520  is the “defining” mark whose location is to be found by using the “referencing” line mark  510  as a guide leading to the dot mark  520  for convenience and ease of alignment because a line does not uniquely define a location but a dot does and a line is easier to find than a dot. 
   In operation, the wafer positioner is controlled to move the wafer to the fixed position so that a selected die is visually under the optical probe. With this initial position as a starting point, the wafer is moved along one axis to search for a strong optical reflection from an alignment mark until a reflection is received by the optical probe and is detected. As described previously with an example shown in  FIGS. 3A ,  3 B, and  3 C, the optical probe is designed to receive only retro-reflected light from an alignment mark or light from an optical port  240 . A fiber probe with an angled end facet in a tilted position relative to the normal direction of the wafer surface may be used as illustrated in  FIGS. 3A through 3C  and  FIG. 6A . Next, the wafer is moved around to determine whether the mark is a dot mark or a line mark. If the reflection is from a dot mark, the alignment with respect to the dot mark is finely adjusted and optimized by maximizing the reflection. If the reflection is determined to be from a line mark, the wafer is moved along the found reflection line until the reflection line ends and the strong reflection signal drops to the value close to zero. The design with dot  520  and line  510  represents the scheme in which the line mark  510  is used as a “runway” leading the system to the dot  520 . Once the dot  520  is found, the system can use the position map to move wafer from the known location of the dot  520  to place the selected optical device or component under the optical probe. 
   There are additional three sets of alignment marks surrounding the remaining sides of the die  110  as shown in  FIG. 5 . The second set includes the dot  520  and the line mark  530 , the third set includes dot  550  and the line  540 , and the fourth set includes dot  550  and the line  560 . These redundant sets of alignment marks are optional and may be used to further facilitate the optical alignment, especially when the wafer positioner has some inherent alignment inaccuracy in its movement. For example, three alignment marks may be used when the x, y, and z translational stages in the wafer positioner are not orthogonal to one another. 
     FIGS. 7A and 7B  show two different examples of optical alignment marks for implementing automated optical alignment. In  FIG. 7A , three reflective optical alignment dot marks  720  are formed on the wafer for aligning a fiber probe  701  to a grating coupler in an optical I/O port  240  connected to a waveguide  710 . Depending on the specific design of each alignment mark, the light from the fiber probe  701  may be reflected back to the same fiber or to a different fiber by an alignment mark. The alignment marks or structures may be lithographically defined structures like reflective gratings, structures defined by material depositions such as a Bragg reflector made of various dielectric layers, or structures defined both lithography and by material deposition such as a metal layer patterned into small reflective structures and a set of metal layers patterned to form an effective angle surface. Spatial separation may be obtained by, e.g., designing a reflective structure that sends light into another fiber or a predetermined, known direction to be collected and measured. If an alignment mark or structure is retro reflective to send light back to the fiber probe  701 , optical devices such as optical circulators, beam splitters, and directional couplers may be coupled to the other end of the fiber probe to properly route the outgoing and incoming optical signals. Interferometric techniques may also be used to improve the contrast of the optical measurements of light received from the wafer. 
   The use of optical gratings as the reflective alignment marks or structures may be used to make the alignment system sensitive to optical polarization. Hence, when a polarization maintaining (PM) fiber is used for delivering a probe beam to the wafer or receiving light from the wafer, the optical grating reflective alignment marks may be designed to allow for optimizing the orientation of the PM fiber based on the sensitivity to the optical polarization and to maximize the optical coupling. 
     FIG. 7B  show another example where reflective optical alignment lines and dots in an arrangement different from the example shown in  FIG. 5  based on a combination of line and dot alignment marks. The reflective alignment dots and lines are retro reflective and hence a single fiber probe is used to deliver the probe light and to collect reflected light. A light source and an optical detector are optically coupled to the other end of the fiber probe to respectively produce the probe light and to measure the received reflected probe light. An optical circulator is coupled between the other end of the fiber probe and the light source and the detector for routing the light from the light source into the fiber and received reflected probe light form the fiber to the detector. The output of the optical detector may be used to form a feedback control to the positioning mechanism so that high reflected power is maintained as the fiber is controlled to move on top of an alignment line mark.  FIG. 7B  further shows an optical component in the die that is connected through waveguides to two optical I/O ports  240  with grating couplers. After the fiber probe is aligned with respect to an alignment mark, it can be moved to align with one of the I/O ports  240  for optical testing. 
     FIGS. 8A ,  8 B,  8 C, and  8 D show an implementation where an alignment algorithm may be used to automate the alignment process based on the alignment marks with minimal user intervention. The alignment marks have lines and dots on the four sides of a wafer as illustrated ( FIG. 8A ). The x and y axes of the wafer positioner are aligned with the x and y axes of the wafer. After the automated alignment process is initialized, the wafer positioner is adjusted to move the fiber probe relative to the wafer to search along one axis (e.g., the x axis) until a strong reflection is detected.  FIG. 8B  shows the power measurement during this step. Next, the fiber is brought back to a position where the reflected power was at or near the maximal power. Subsequently, the fiber position is moved along the y direction where the reflected power remains at the maximal value until the power drops.  FIG. 8C  illustrates this situation. Due to the arrangement of the dot and line marks in this example, this position indicates that the fiber in between an end of a line and a dot. Under this condition, the fiber is continuously moved along this direction until the reflected power is back again. At this point, the fiber is approximately above a dot mark. Next, a local search is performed to adjust the relative position between the fiber and the dot mark to find a position where the reflected power is maximized.  FIG. 8D  illustrates a circular search path around the dot mark for maximizing the reflected power. After the position for the maximum reflected power is found, the alignment with respect to this dot mark is completed. The above process can be repeated to align the fiber to other dot marks on the wafer. 
     FIG. 9  shows another example for the alignment marks on a wafer where an alignment mark is purposely located near an I/O optical port  240  on the wafer with known positional offsets. Such an alignment mark is “local” to the corresponding optical port  240  and may be used in combination with the “global” alignment marks on the wafer, such as those shown in  FIG. 8A . Such a “local” alignment mark or structure is useful when a particular nearby I/O optical port  240  may not be optically responsive due to some reason. For example, the optical device or the waveguide connected to the optical port  240  experiences large optical losses; the waveguide connected to that optical port  240  is damaged; that optical port is for receiving an optical input only; or the optical device connected to that port  240  has a response sensitive to the light wavelength and happens to fail to produce a detectable response at the probe wavelength. In such circumstances, a local realignment may be performed by using the one or more alignment marks near that optical port  240  that does not produce an optical output in response to the optical probe beam to ensure that the fiber probe is positioned at the right place, i.e., aligned with that optical port  240 . 
   If the optical port  240  that does not itself produce an optical response to a received probe beam is connected to an optical detector or a device that produces an electronic signal in response to light received from that optical port  240 , the electronic probe may be used to receive that electronic signal from the wafer as an output to assist the optical alignment between the optical probe and the optical port such as fine alignment in addition to the use of one or more “local” alignment structure near the optical port  240 . The electronic output may also be used during the testing of the optical port  240  and the device that receives light from the optical port  240 . 
   This use of both the electronic probe and the optical probe in alignment and testing may be applied to alignment and testing of various optoelectronic devices and components on the wafer. As an example, an optoelectronic component such as a LED or a laser diode may generate light in response to an applied electronic signal. The generated light may be directed to an optical port  240  which directs the light as an output above the wafer through the corresponding optical coupler therein. In this situation, the electronic probe may be used to supply the electronic signal and the optical probe may be used to receive the output light from the optical port  240  during the testing. In another example, an optical port  240  may be connected to an optical switch through a waveguide and the optical switch operates in response to an electronic control signal. Hence, both the electronic probe and the optical probe may be used in aligning and testing of the optical switch. 
   An optical probe for optical testing may include multiple fibers or an array of fibers that are designed to be respectively aligned with different optical components on a wafer for optical testing. Optical waveguides formed in a substrate may be used as an alternate to the multiple fibers or the fiber array. This use of multiple fibers/waveguides or a fiber/waveguide array in the optical probe allows for simultaneous testing of multiple ports and multiple optical elements on a die or a wafer. At least one of the fibers in the optical fiber probe may be dedicated for optical alignment with respect to an optical alignment structure which may be an optical alignment mark or an optical device for either alignment or some optical function on the wafer. Once the dedicated fiber for alignment is aligned with the optical alignment structure, all other fibers in the optical probe are also respectively aligned with their corresponding optical components on the wafer. A fiber array for an optical probe may a one-dimensional array or a two-dimensional array. A fiber in the fiber array may be a single-mode fiber, a polarization maintaining fiber, or a multi-mode fiber. The array may include one or more such fibers. As an example, a single fiber array may include a combination of the two or all three types of fibers. 
     FIG. 10  shows one example of using a fiber array with two or more fibers for alignment and optical testing. In this example, three fibers  1010 ,  1020 , and  1030  are shown to form a linear fiber array corresponding to a dot-shaped alignment reflector  1001  for optical alignment, a first optical I/O port  240 A for receiving light and a second optical I/O port  240 B for output light, respectively. The fiber  1010  is dedicated for alignment with the alignment dot  1001  and the fibers  1020  and  1030  are dedicated for optical testing of the ports  240 A and  240 B. The fiber array is adjusted relative to the wafer to maximize the reflected power from the alignment mark  1001  that is collected by the fiber  1010  to achieve the optical alignment of the fibers  1020  and  1030  for optical testing. As illustrated, the fibers in the array are engaged to a fiber array mount. A substrate with parallel V grooves with proper spacings may be used to hold and fix the fibers in position to form the fiber array. 
   The optical alignment of a fiber array with respect to the corresponding optical I/O ports, optical components, or alignment structures on the die or wafer, however, presents a challenge in aligning all fibers. One technical issue is the yaw alignment of the fiber array by adjusting the rotational position of the fiber array around an axis that is perpendicular to the wafer surface.  FIG. 11A  shows a situation where one fiber in the fiber array  1120  is aligned with a corresponding optical port or an alignment structure in an array  1110  of ports or alignment structures on the die  1100  while the other fibers are misaligned with their corresponding ports due to a yaw of the fiber array  1120  relative to the array  1110  of the ports on the die  1100 . 
   The wafer positioner may have a rough yaw adjustment to change the angle of the wafer around the vertical Z axis. This rough yaw adjustment may be used in the initial alignment of the wafer after the wafer is placed on the wafer positioner. However, this rough yaw alignment in the wafer positioner may not be sufficient to address the above alignment issue. In recognition of this, a fine yaw optimization mechanism based on a yaw adjustment in the optical probe positioner and an alignment loop on the wafer may be implemented and operated in combination to address this alignment issue. 
   Referring back to  FIG. 4 , the optical probe positioner may have its own translational and orientation adjustments that are independent of the adjustments in the wafer positioner to provide fine adjustments to the relative position and rotation between the optical probe and the wafer. The fine yaw adjustment in the optical probe positioner is used here to achieve the fine yaw alignment of the fiber array in the optical probe. 
     FIG. 11B  shows the die  1100  on a wafer that includes an optical alignment loop  1130  having two optical I/O couplers  1131  and  1132  separated from each other to define a line parallel to the array  1110  of optical ports or optical components on the die  1100  to be aligned with the corresponding fiber array  1120  of an optical probe. The two couplers  1131  and  1132  are spaced to correspond to two fibers in the fiber array  1120 . The optical alignment loop  1130  includes an optical waveguide  1133  connecting the two optical I/O couplers  1131  and  1132 . The relative position between the array  1110  of optical ports or components and the alignment loop  1130  is known and the wafer positioner may be used to move the fiber array  1120  from its aligned position with the alignment loop  1130  to align with the array  1110  of optical ports or components according to this known relative position without additional optical alignment. 
   In operation, the fiber array  1120  is first aligned with the alignment loop  1130  by aligning two couplers  1131  and  1132  to two corresponding fibers in the fiber array  1120 . One of the two fibers is used to send a probe beam to the alignment loop  1130  and the other fiber is used to receive light output from the alignment loop  1130 . When the fiber array  1120  is perfectly aligned with the alignment loop  1130 , the optical power coupled from the fiber array  1120  into one side of the alignment loop  1130  is received from the other side of the alignment loop  1130  and is maximized. Any yawing of the fiber array  1120  that deviates the fiber array  1120  from this alignment condition can cause the output power from the alignment loop  1130  to decrease or to disappear. Hence, the fine yaw adjustment in the optical probe positioner is used to vary the yawing of the fiber array relative to the fixed wafer to monitor the optical power received in the receiving fiber in the fiber array. This yawing is performed until the optical power in the receiving fiber is optimized. After this yaw optimization, the fiber array may be moved to align with the array of optical ports to perform optical tests. 
   Alternatively, the optical alignment loop  1130  may be formed in two of the optical ports within the array  1110  of ports on the die  1100 .  FIG. 11C  illustrates one example of this design. In this case, after the yaw alignment, the fiber array  1120  is the aligned position and is ready for optical testing of other ports without any further adjustment of the fiber array  1120 . 
   The fine yaw optimization for a fiber array may also be implemented with two or more reflective or diffractive alignment structures that each receive light from a fiber in the fiber array and direct received light back to the fiber. For example, the wafer may be fabricated with a line of two or more optical alignment marks such as dot marks that spatially correspond to two or more fibers in the fiber array, respectively. Referring to  FIG. 11B , the two optical ports  1131  and  1132  for the alignment loop may be replaced by two reflective optical alignment dot marks in implementing this technique. More than two dot marks may be used. After an initial alignment to align the fibers with the alignment marks to receive returned light in the fibers, the optical probe positioner may be adjusted to finely vary the yaw angle of the fiber array relative to the wafer. The total optical power received by all receiving fibers in the fiber array may be maximized to optimize the yawing. Next, the wafer positioner is used to change the lateral position of the wafer to place the fiber array above the array of optical ports to perform optical tests. 
   The above line and dot alignment marks and the alignment loops exemplify the use of one or more optical reflective or diffractive alignment structures with a distinctive spatial pattern to define a reference position. When two or more alignment structures are used in combination for alignment, one or more the alignment structures in the combination may be used as referencing structures (e.g., one or more line marks) for optically guiding the optical probe to an optical alignment structure that defines that reference position (e.g., a dot alignment mark) in the combination. Any combination that produces a distinctive pattern may be used. As another example, two orthogonal reflective alignment line marks that intercept at a crossing point may be used as a combination where the line marks guide the optical probe and the cross point defines the reference position. 
   As further examples, a single alignment structure may also be used for aligning the optical probe a defined reference position.  FIGS. 12A ,  13 , and  14  show three different examples where an alignment structure is formed close to a target I/O port  240  on the wafer with a spatially varying reflectivity to indicate the location of the port  240 . Such a spatially-varying alignment structure may surround the port  240 . Once the reflective alignment structure is located, a local, more precise spatial scan of the fiber position over the found alignment structure may be used to precisely position the fiber to the port  240  under the guidance of the varying reflectivity from the alignment structure. For example, a reflectivity gradient may be built in to the alignment structure so that a positioning control feedback mechanism may be used to automatically follow the gradient to the desired final position of the fiber relative to the port  240 . 
     FIG. 12A  shows an alignment structure surrounding a port  240  with a gradually-increasing reflectivity profile as the distance to the port  240  decreases.  FIG. 12B  further shows the reflected power from this alignment structure as the fiber position is scanned along x and y directions through the port  240 . A dip in the reflected power indicated the position of the final fiber position if the port  240  does not send out light during the alignment. 
     FIG. 13  shows another alignment structure surrounding a port  240  with a step-wise changing reflectivity profile as the distance to the port  240  changes. Concentric reflective circles  1320  with varying optical reflectivities may be used as such an alignment structure on a wafer. The optical reflectivities of the different circles may increase or decrease with the radius from the center of the circles so that a change in the reflected power with position may be used to indicate position of the fiber over the circles. An optical coupler  240  is placed at the center of the concentric reflective circles and is optically connected to a waveguide  1310  or an optical device. In operation, the wafer positioner is controlled to perform a circular or spiral scan to determine the position of the fiber relative to the concentric reflective circles  1320 . The varying reflected optical power from the circles may be used to determine the center of the circles. Next, the wafer is moved to place the fiber above the center of the circles to align with the coupler  240 . 
     FIG. 14  shows yet another example of an alignment structure surrounding an I/O port  240  on the wafer. In this example, the alignment structure uses multiple layers to achieve a spatially varying reflectivity profile that can guide the fiber to the surrounded port  240 . As the distance to the port  240  changes, the layer structure of the alignment structure and thus the resultant reflectivity change accordingly. 
   The use of the above and other alignment structures is in part based on the recognition that a wafer positioner may have insufficient accuracy for optical alignment. The wafer positioner is initially used to move from one die to another on the wafer. One or more optical alignment structures allow for an accurate identification of a “local” reference for a die, i.e., the dot mark  520  in the above example, and uses the local reference to make further small movements of the wafer within the scale of a die to accurately locate the final position of a target location in that die. At this location, the optical probe and the selected optical coupler  240  within the die are approximately aligned and hence an optical or electronic response caused by the optical probe can be generated. 
   Referring back to the system  400  in  FIG. 4 , the fine alignment for the optical probe may be achieved by using the optical probe positioner. In operation, the wafer positioner sets the initial relative position between the wafer and the optical probe and maintains the wafer at this position. The optical probe positioner, which was set at a fixed position in prior alignment process, is controlled to move the optical probe relative to a corresponding optical I/O coupler to find an optimized location. The movement of the optical probe is controlled by the optical probe positioner according to a search pattern, such as a spiral or circle pattern. The optical probe positioner may include high resolution mechanical stages such as stepper motors, or piezo stages. Upon completion of this search, the optical probe is fixed at the optimized location, such as one that yields maximum received optical or electrical signal. At this point, the optical alignment is completed and the optical test can be carried out at this location. After completion of the test, the system then moves the wafer to place the probe at another location for optical tests. 
     FIGS. 15A ,  15 B, and  15 C show an exemplary workflow of this alignment process. As discussed above, a single alignment structure on the wafer may suffice to establish an initial x, y, and z position when the wafer positioner has positioning stages orthogonal to each other. More than one alignment structures may also be used in this alignment process. 
   In finely adjusting the optical probe in a selected search pattern by using the optical probe positioner, different search mechanisms may be used. In one implementation of a “hill climbing” algorithm, for example, the optical probe positioner may be moved through a sequence of discrete positions and measurements of the received responses are taken at each of the discrete positions. The measurements are used to estimate the gradient of the received responses as a function of the discrete positions and to predict the next position to move the optical probe to increase the received response. 
   Alternatively, the fiber probe may be driven by the optical probe positioner to spatially move in a circular pattern around a center at a low frequency sufficient for the optical detector in the optical probe to respond the change of optical power in the received light. When an optical alignment dot mark or an optical I/O port  240  is located at the center of the circle, the received optical power or a corresponding electronic response from the dot mark or the port  240  remains substantially at a constant when the optical probe at different positions of the circle. Under this condition, the search is completed and the optical probe positioner is controlled to place the optical probe to the center of the circle. If, however, the received optical power or the generated electronic response varies in amplitude with the position of the optical probe in the circle, the dot mark or the port  240  is not located at the center of the circle. At this off-the-center position, the output response is a sinusoidal function of time and the X or Y position is also a sinusoidal function of time. 
     FIG. 15D  illustrates an example of the measured Y position and the signal power as a function of time and their relative phase. The relative phase between the position and the power indicates the direction along which the center of the circle deviates from the dot mark or the port  240 . Based on this relative phase, the optical probe positioner may be controlled to adjust the optical probe to move the center of the circle. At the new position, the measurements are taken and additional adjustments may be made if needed. This process continues until the received power is a constant and the center of the circular motion coincides with the dot mark or the port  240 . This algorithm may operate at a higher speed than the hill climbing method. One implementation of this algorithm a system under a trade name of “NanoTrak” and marketed by Melles Griot, Ltd., of London. See also U.S. Pat. No. 6,555,983 assigned to Melles Griot. 
   Certain optical components or devices in a die may not produce a sufficiently measurable response to the optical probe even though the optical alignment may be perfect. For such an optical component, it is difficult to determine whether the optical probe is satisfactorily aligned to the corresponding optical coupler  240  connected to the optical component. Examples of such optical devices include certain wavelength-sensitive optical devices such as optical interferometers, resonators, optical wavelength filters when the wavelength of the probe beam happens to be between two spectral peaks in the spectral response. An Arrayed Waveguide Grating (AWG) is also such an optical device. In addition, optical detectors such as photodetectors and devices that produce a non-optical signal in response to received light. 
   To address this technical issue in optical alignment, certain optical devices that are insensitive to wavelength or exhibit some dependence on wavelength but still can produce a measurable response may be fabricated on the die or wafer as an optical alignment device for fine alignment. In some implementations, such an optical alignment device may be located close to the target optical device as an optical alignment reference. Hence, if the coupler for the alignment device is aligned with the optical probe, the coupler for the target device near the alignment device can also be aligned with the optical probe after a known, pre-determined, short move that can be sufficiently accurate. Since the move is very short due to the closeness between the alignment device and the target device by the design, an error in alignment caused by the move may be negligible. 
   In one implementation, the following process may be used to achieve the alignment based on the above alignment device. First, a coarse alignment using the wafer positioner is performed to align the optical probe to the optical alignment device adjacent to the target optical device. This alignment step may use one or more optical alignment structures on the wafer. Second, the optical probe is finely adjusted by controlling the optical probe positioner to optimize the optical alignment between the optical probe and the optical alignment device. Next, a short, accurate move to the target device is performed and the target device is presumed to be aligned with the optical probe. Optical testing on the target device can then be performed. 
     FIG. 16  shows one example of an optical alignment device  1600  adjacent to a wavelength-sensitive optical element  220  such as a Mach-Zehnder interferometer device. The device  1600  includes an optical splitter or tap  1620  to split a fraction of a beam coupled into the device  220  from the coupler  1610  into a waveguide  1630 . The waveguide  1630  terminates at the same edge of the die  110  where the coupler  1610  is located. An optical coupler  1640  is implemented at the terminal port of the waveguide  1630 . Hence, as long as there is an optical input to the port  1610 , the port  1640  will produce an optical output, or vice versa. Therefore, the optical output at the coupler  1640  may be used to align the optical probe at the coupler  1610 . The alignment for the coupler  1611  at another port of the device  220  may be achieved by moving the optical probe aligned with the coupler  1610  to the coupler  1611  in a short and known position offset. 
     FIG. 17  shows another example of an optical alignment device  1700  located close a target device  230  with an I/O optical coupler  240  in die  110 . The alignment device  1700  a waveguide loop having a waveguide  1710  connected between two terminal ports having two I/O couplers  1711  and  1712  on the same edge of the die  110  and close to the coupler  240 . The optical probe may be designed to have a linear array of three fibers spatially corresponding to the ports  240 ,  1711 , and  1712 . The fiber probe is aligned by sending a probe beam into the waveguide loop  1700  and optimizing the output to achieve the alignment with the port  240  of the adjacent device  230 . In this design, the alignment with the alignment device  1700  and the alignment with the target device  230  are completed at the same time. 
   Another application of the optical alignment loop  1700  shown in  FIG. 17  is to assist alignment of the optical input to an optical detector  1820  that receives light from an I/O coupler  240  through a waveguide  1810  as shown in  FIG. 18 . The optical detector  1820  produces an electrical output in response to an optical input. In absence of an electronic probe to measure the detector output, it can be difficult to align the optical probe to the port  240  for the optical detector  1820 . The alignment loop  1700  may be placed adjacent to the port  240  for aligning the optical probe to the port  240 . The optical probe may be designed to have a linear array of three fibers spatially corresponding to the ports  240 ,  1711 , and  1712 . The fiber probe is aligned by sending a probe beam into the waveguide loop  1700  and optimizing the output to automatically align the third fiber with the port  240  for the optical detector  1820 . 
   In the above optical alignment with the optical probe on top of the wafer, the height of the end facet of the optical probe and local wafer surface may be set at a close distance, e.g., from about 5 microns to about 25 microns, to ensure efficient coupling of the probe light into the optical device under test. This close spacing between the probe and the wafer might cause a collision of the probe to the wafer during a lateral movement between the optical probe and wafer because the wafer held down on a vacuum chuck is not perfectly flat. The typical unflatness of the wafer may be, for example, about 20 to 30 microns or more in some commercial probe stations. This collision may damage the optical probe and the devices on the wafer. 
   This collision may be avoided by controlling the vertical position of the optical probe relative to the wafer at a safely large distance to separate the optical probe from the wafer when laterally moving the probe from one location on the wafer to another. After the optical probe is placed at a desired lateral position relative to the wafer, the distance between the optical probe and the wafer may be adjusted to the desired small spacing for optical testing. In this regard, the two vertical positions in the wafer positioner for the “testing position” and the “separate position” for setting the vertical positions of a wafer may be used. During the optical testing, the wafer positioner is set to the “testing position” to be close to the optical probe. Before a lateral adjustment of the relative position between the optical probe and the wafer, the wafer positioner is set to the lower separate position to separate the wafer from the optical probe. At this lowered separate position, the lateral adjustment can be safely made without colliding the optical probe into the wafer. 
   During the optical testing, the spacing between the optical probe and the wafer is set to a close distance, e.g., from about 5 microns to about 25 microns to achieve efficient optical coupling. It is beneficial to keep this close distance during optical testing as a constant in testing different optical components at different locations on the wafer. The optical probe positioner may be used to finely adjust and maintain this close distance between the optical probe and the wafer for the optical testing. 
   In this regard, a spacing monitor and an active feedback control may be used in the optical probe positioner to monitor the distance between the optical probe and the wafer when aligning the optical probe and to control the distance at the predetermined constant distance based on the monitored spacing. In one implementation, a spacing sensor may be used to measure the height of the optical probe from the wafer surface. This sensor may be a capacitance spacing sensor that is mounted at the optical probe. 
     FIG. 19  shows an example of an optical probe  1900  suitable for implementing the above capacitance sensor using the wafer-level testing system in  FIG. 4 . This particular probe  1900  is built on a fiber array holder  1910 . A fiber array  1920  with multiple fibers with predetermined spacings is fixed to the holder  1910 . The fiber array  1920  is designed to correspond to the optical couplers for one or more optical components or devices in certain dice on a wafer design. As an example, some of the fibers in the array  1920  may correspond to the couplers  1611 ,  1610  and  1640  on the die in  FIG. 16  or the couplers  240 ,  1711 , and  1712  on the die in  FIG. 17 . As illustrated, a capacitance sensor  1930  may be engaged to the fiber holder  1910  near the fiber probe facets to provide a measurement for the height above the wafer surface. Based on the measurement from the sensor  1930 , the optical probe positioner controls its vertical adjustment to set the spacing between the optical probe and the wafer surface. 
     FIG. 20  shows an exemplary workflow for adjusting the vertical spacing between the optical probe and the wafer based on the separate vertical controls in the wafer positioner and the optical probe positioner. 
   The above optical alignment and optical testing techniques may also be used for attaching one or more fibers to optical ports on surface a wafer or a chip. The fibers may be polarization maintaining fibers, single-mode fibers, multi-mode fibers, or a combination of these and other fibers. For example, the system  400  in  FIG. 4  may be used to first align a single fiber to an optical port on the fiber or align a fiber array to an array of optical ports on the wafer. Next, an adhesive is applied to an optical port. Subsequently, a corresponding aligned fiber is lowered in its position to contact the optical port and the adhesive thereon. This contact is maintained until the adhesive is properly cured so that the fiber and the optical port are bonded together. The adhesive may be a UV curable adhesive so that UV light may be used to illuminate the contact interface between the optical port and the fiber to cure the adhesive. As another example, the adhesive may be a thermal curable adhesive and heat may locally applied to the interface to cure the adhesive. Alternatively, the adhesive may be applied to the contact interface between the optical port and the fiber after the fiber is brought to contact with the optical port. Other applications of the above optical alignment and testing techniques are also possible. 
   Although the present invention has been described in detail, it should be understood that various changes, substitutions and alterations can be made without departing from the spirit and scope of the invention.