METHODS, TEST STRUCTURES, AND TEST SYSTEMS FOR DETERMINING A SURFACE CHARACTERISTIC OF A CHIP FACET
A test system for determining a surface characteristic of a chip facet comprises a chip, which has a facet and includes a waveguide, a detector, and a processor. The on-chip waveguide is configured to direct test light towards the facet, where a portion of the test light is reflected and a portion of the test light is transmitted. The detector is configured to measure an amount of the reflected portion or the transmitted portion, and the processor is configured to determine a surface characteristic of the facet, such as a facet angle, a facet curvature, and/or a facet roughness, on the basis of the measured amount.
This application is a continuation of U.S. patent application Ser. No. 16/546,179, filed Aug. 20, 2019, now allowed, which is a continuation of U.S. patent application Ser. No. 15/427,185, filed Feb. 8, 2017, now U.S. Pat. No. 10,429,313, which is hereby incorporated by reference in its entirety.
FIELD OF THE INVENTIONThe present invention relates to methods, test structures, and test systems for determining a surface characteristic of a chip facet. More particularly, the present invention relates to methods, test structures, and test systems for determining a surface characteristic of a facet of a photonic chip including a waveguide.
BACKGROUND OF THE INVENTIONIn devices and systems incorporating photonic chips, it is often necessary to couple light from an on-chip waveguide into an off-chip optical fiber. With reference to
Conventionally, scanning electron microscopic (SEM) techniques are used to determine the facet angle. In one technique, the facet angle is determined from top-view or perspective-view SEM images of chips on a wafer. Unfortunately, the accuracy and throughput of this technique may be inadequate, and the number of measurement sites that can be accommodated by the technique may be limited. In another technique, the facet angle is determined from cross-sectional SEM images of chips on a wafer. Although this technique provides a higher accuracy, the technique often requires additional and/or destructive processing steps.
Accordingly, there is a need for new methods, test structures, and test systems for determining the facet angle and other surface characteristics of a chip facet, particularly at the wafer scale.
SUMMARY OF THE INVENTIONAccordingly, an aspect of the present invention relates to a test system for determining a surface characteristic of a chip facet, the test system comprising: a chip having a facet, the chip including a waveguide configured to direct test light towards the facet; a detector configured to measure an amount of a portion of the test light reflected by the facet or a portion of the test light transmitted by the facet; and a processor configured to determine a surface characteristic of the facet on the basis of the measured amount.
Another aspect of the present invention relates to a method of determining a surface characteristic of a chip facet, the method comprising: providing a chip having a facet, the chip including a waveguide; directing test light towards the facet via the waveguide; measuring an amount of a portion of the test light reflected by the facet or a portion of the test light transmitted by the facet; and determining a surface characteristic of the facet on the basis of the measured amount.
Numerous exemplary embodiments of the present invention will now be described in greater detail with reference to the accompanying drawings wherein:
The present invention provides methods, test structures, and test systems for determining a surface characteristic of a chip facet, typically a facet of a photonic chip. Advantageously, the methods, test structure, and test systems may be used to determine the surface characteristic of the chip facet at the wafer scale, i.e., while the photonic chip is on a wafer, prior to dicing or packaging. The methods, test structures, and test systems may be implemented with any suitable type of integrated photonics platform, for example, a silicon-on-insulator (SOI) platform, e.g., with silicon and/or silicon nitride waveguides, a silica-on-silicon platform, or a III-V material platform.
In general, the photonic chip includes a facet and at least one waveguide. The facet is typically an edge facet disposed at an edge of the photonic chip, and a top edge of the facet typically forms the chip edge. The facet is typically formed as a sidewall of a trench in the wafer, e.g., by etching. The facet may be substantially perpendicular to a plane of the photonic chip, and typically to a plane of the wafer, or may be inclined relative to the chip plane. The facet may be coated with an antireflective coating.
The waveguide may be any suitable type of waveguide, for example, a ridge waveguide, e.g., a narrow single-mode ridge waveguide or a wide multi-mode ridge waveguide, a strip waveguide, or a slot waveguide. The waveguide is typically formed in the photonic chip, i.e., formed on or in a substrate or layer of the photonic chip. Accordingly, the waveguide is typically disposed substantially parallel to the chip plane. In some embodiments, the waveguide or a segment thereof is disposed substantially perpendicular to the top edge of the facet, and typically to the chip edge. In other embodiments, the waveguide or a segment thereof is disposed at an oblique angle to the top edge of the facet. In yet other embodiments, the waveguide or a segment thereof is disposed substantially parallel to the top edge of the facet.
Several exemplary embodiments of test structures that can be used in methods and test systems for determining a surface characteristic of a chip facet are described hereafter. It should be understood that individual elements of any of the exemplary embodiments may be combined as appropriate to arrive at further embodiments.
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The test structure may be included in a test system that also includes a light source, a detector, and/or a processor. The light source, e.g., a laser or a broadband light source, is configured to provide the test light. The detector, e.g., a photodetector, is configured to measure an amount, e.g., an optical power, of the reflected portion of the test light or the transmitted portion of the test light. Typically, the detector detects the reflected portion of the test light or the transmitted portion of the test light and provides an output signal representative of the amount of the reflected portion of the test light or the transmitted portion of the test light to the processor. The processor, e.g., a general-purpose processor or a special-purpose processor, is configured, e.g., programmed with instructions, to determine a surface characteristic of the facet 230, such as a facet angle, a facet curvature, and/or a facet roughness, on the basis of the measured amount. Typically, the surface characteristic is determined by using a empirical model, such as a calibration curve, relating the measured amount to the surface characteristic.
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Advantageously, the use of a Michelson interferometer may enhance the signal of the reflected portion of the test light, providing a larger dynamic range and improved measurement sensitivity. To illustrate how the signal of the reflected portion of the test light may be enhanced, a Michelson interferometer including a 2×2 MMI coupler may be considered. If two signals with respective optical powers of P1 and P2 enter the two input ports of the 2×2 MMI coupler, the output optical power Pout from each of the output ports of the 2×2 MMI coupler will be 1/2[P1+P2+2√{square root over (P1P2)} cos(Δϕ)], where Δϕ is the relative phase shift between the electric fields of the two signals. If the output optical power from each of the two paths is input to a balanced photodetector 550, then the output photocurrent from the balanced photodetector 550 will be proportional to 2√{square root over (P1P2)} cos(Δϕ). If we assume that P1 is the optical power of the portion of the test light reflected by the facet 230, and that P2 is the optical power of reference light reflected from a mirror 544 with a reflectivity of 100%, then P1 is likely to be smaller than P2. If the relative phase shift between the two signals is modulated through a by the phase shifter 543, then the interference signal at the balanced photodetector 550 will have a power greater than P1 alone. In this way, the amount of the reflected portion of the test light can be more easily measured, even when the amount is relatively small.
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In some embodiments, the facet 230 includes a lithographically defined feature, e.g., a lens, near the first waveguide 220, that is configured to facilitate reflection of the reflected portion of the test light or to facilitate transmission of the transmitted portion of the test light. In a non-illustrated embodiment, the lithographically defined feature is configured to facilitate coupling of the reflected portion of the test light into the first waveguide 220, and the detector is configured to measure an amount of the reflected portion of the test light.
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The test structure may be included in a test system that also includes a light source, a detector, and/or a processor. The light source, e.g., a laser or a broadband light source, is configured to provide the test light. The detector, e.g., a photodetector, is configured to measure an amount, e.g., an optical power, of the reflected portion of the test light or the transmitted portion of the test light. Typically, the detector detects the reflected portion of the test light or the transmitted portion of the test light and provides an output signal representative of the amount of the reflected portion of the test light or the transmitted portion of the test light to the processor. The processor, e.g., a general-purpose processor or a special-purpose processor, is configured, e.g., programmed with instructions, to determine a surface characteristic of the facet 1430, such as a facet angle, a facet curvature, and/or a facet roughness, on the basis of the measured amount.
Typically, the measured amount is compared to a threshold amount to determine whether the surface characteristic, typically the facet angle, is within a range corresponding to total internal reflection. For example, if a measured optical power of the reflected portion of the test light is greater than a threshold optical power, then the facet angle is determined to be within a facet-angle range providing total internal reflection. For another example, if a measured optical power of the transmitted portion of the test light is greater than a threshold optical power, then the facet angle is determined to be outside of a facet-angle range providing total internal reflection. In some instances, a plurality of first waveguides 1420, each having output segments disposed at different first angles relative to the top edge of the facet 1430, may be used to determine the surface characteristic, typically the facet angle, more precisely. By determining which of the first angles result in the test light being substantially totally internally reflected by the facet 1430, both a lower bound and an upper bound for the facet angle may be determined.
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The test structure may be included in a test system that also includes a light source, a detector, and/or a processor. The light source, e.g., a laser or a broadband light source, is configured to provide the test light. The detector, e.g., a photodetector, is configured to measure an amount, e.g., an optical power, of the test light transmitted by the waveguide 1820. Typically, the detector detects the transmitted test light and provides an output signal representative of the amount of the transmitted test light to the processor. The processor, e.g., a general-purpose processor or a special-purpose processor, is configured, e.g., programmed with instructions, to determine a surface characteristic of the facet 1830, such as a facet angle, a facet curvature, and/or a facet roughness, on the basis of the measured amount.
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Prior to forming the facet 1830, the test segments are formed near the expected location of the facet 1830, typically so as to bound the expected location. When the facet 1830 is formed, e.g., by etching, some of the test segments may be entirely or partially removed. For example, as shown in
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The present disclosure is not to be limited in scope by the specific embodiments described herein. Indeed, other various embodiments of and modifications to the present disclosure, in addition to those described herein, will be apparent to those of ordinary skill in the art from the foregoing description and accompanying drawings. Thus, such other embodiments and modifications are intended to fall within the scope of the present disclosure. Further, although the present disclosure has been described herein in the context of a particular implementation in a particular environment for a particular purpose, those of ordinary skill in the art will recognize that its usefulness is not limited thereto and that the present disclosure may be beneficially implemented in any number of environments for any number of purposes.
Claims
1. A method for determining a surface characteristic of a facet of a photonic chip, the method comprising:
- using a first waveguide defined in a wafer to direct test light toward a sidewall etched in the wafer, the sidewall forming the facet of the photonic chip;
- measuring at least one of: a reflected portion of the test light that is reflected by the sidewall, or a transmitted portion of the test light that is transmitted through the sidewall; and,
- determining a surface characteristic of the facet based upon at least one of the reflected portion or the transmitted portion.
2. The method of claim 1 comprising selectively etching the wafer to form the sidewall.
3. The method of claim 1 wherein the measuring is performed before the photonic chip is separated from the wafer.
4. The method of claim 1 wherein the measuring is performed after the photonic chip is separated from the wafer.
5. The method of claim 1 comprising coupling the test light into the first waveguide using a first vertical coupler.
6. The method of claim 1 comprising coupling the reflected portion of the test light to a first photodetector (PD).
7. The method of claim 6 wherein the first PD is separate from the photonic chip, and wherein the coupling of the reflected portion of the test light to the first PD comprises using a second vertical coupler.
8. The method of claim 6 wherein the first PD is integrated with the photonic chip.
9. The method of claim 6 comprising coupling the reflected portion of the test light into a second waveguide that is monolithically integrated with the first waveguide, for guiding at least a fraction thereof into the first PD.
10. The method of claim 9 first waveguide, and collecting the reflected portion of the test light upon a reflection thereof from the facet using an end section of the second waveguide that is inclined toward the first waveguide.
11. The method of claim 6 comprising collecting the reflected portion of the test light into an end section of the first waveguide that is proximate to the facet.
12. The method of claim 10 comprising coupling at least a fraction of the reflected portion of the test light from the first waveguide into a second waveguide for guiding to the first PD.
13. The method of claim 12 comprising using an on-chip Michelson interferometer to measure the reflected portion of the test light.
14. The method of claim 13 comprising using the first waveguide as a test arm of the on-chip Michelson interferometer, using the second waveguide as a reference arm of the on-chip Michelson interferometer, using an integrated 2×2 optical coupler to optically couple the first waveguide to the second waveguide, and an integrated reflector disposed at one end of the second waveguide to reflect a fraction of the test light back toward the integrated 2×2 optical coupler as reference light for mixing with the reflected portion of the test light prior to coupling to the first PD.
15. The method of claim 14 comprising using an integrated balanced photodetector comprising the first PD coupled to the second waveguide and a second PD coupled to the first waveguide.
16. The method of claim 14 comprising adjusting an optical phase of the reference light using an integrated phase shifter.
17. The method of claim 8 wherein the first PD is disposed above or below the first waveguide relative to a plane of the wafer.
18. The method of claim 1 wherein the measuring comprises measuring the transmitted portion of the test light.
19. The method of claim 18 wherein the measuring is performed on-wafer using an optical fiber to collect the transmitted portion of the test light into a photodetector (PD).
Type: Application
Filed: Mar 26, 2020
Publication Date: Jul 16, 2020
Inventors: Matthew Akio Streshinsky (New York, NY), Ari Novack (New York, NY), Michael J. Hochberg (New York, NY)
Application Number: 16/830,399