CAVITY-MOUNTED CHIPS WITH MULTIPLE ADHESIVES

Structures for a cavity-mounted chip and methods of fabricating a structure for a cavity-mounted chip. The structure comprises a laser chip including a body attached to a substrate. The laser chip has an output, and the body of the laser chip has a bottom surface spaced from the substrate by a gap. The structure further comprises a first adhesive in the first gap and a second adhesive positioned in the first gap between the first adhesive and the output of the laser chip. The first adhesive has a first thermal conductivity, the second adhesive has a second thermal conductivity, and the first thermal conductivity of the first adhesive is greater than the second thermal conductivity of the second adhesive.

BACKGROUND

This disclosure relates to photonics chips and, more specifically, to structures for a cavity-mounted chip and methods of fabricating a structure for a cavity-mounted chip.

Photonics chips are used in many applications and systems including, but not limited to, data communication systems and data computation systems. A photonics chip integrates optical components and electronic components into a unified platform. Among other factors, layout area, cost, and operational overhead may be reduced by the integration of both types of components on the same chip.

A laser source may be integrated on the photonics chip. In that regard, a cavity may be formed in the substrate, and the laser source may be inserted into the cavity and attached to the substrate. A laser source generates significant amount of heat during operation. The performance and reliability life of the laser source is tied to effective thermal management. In conventional photonics chips, laser-generated heat is primarily dissipated to the substrate through solder contact at the attachment locations and an optical coupling adhesive.

Improved structures for a cavity-mounted chip and methods of fabricating a structure for a cavity-mounted chip are needed.

SUMMARY

In an embodiment of the invention, a structure comprises a laser chip including a body attached to a substrate. The laser chip has an output, and the body of the laser chip has a bottom surface spaced from the substrate by a gap. The structure further comprises a first adhesive in the first gap and a second adhesive positioned in the first gap between the first adhesive and the output of the laser chip. The first adhesive has a first thermal conductivity, the second adhesive has a second thermal conductivity, and the first thermal conductivity of the first adhesive is greater than the second thermal conductivity of the second adhesive.

In an embodiment of the invention, a method comprises attaching a body of a laser chip to a substrate. The body of the laser chip has a bottom surface spaced from the substrate by a gap. The method further comprises forming a first adhesive in the gap, and forming a second adhesive positioned in the gap between the first adhesive and an output of the laser chip. The first adhesive has a first thermal conductivity, the second adhesive has a second thermal conductivity, and the first thermal conductivity of the first adhesive is greater than the second thermal conductivity of the second adhesive.

DETAILED DESCRIPTION

With reference toFIGS.1,2and in accordance with embodiments of the invention, a structure10for a photonics chip includes a waveguide core12that is positioned on, and over, a dielectric layer14and a substrate16. In an embodiment, the dielectric layer14may be comprised of a dielectric material, such as silicon dioxide, and the substrate16may be comprised of a semiconductor material, such as single-crystal silicon. In an embodiment, the dielectric layer14may be a buried oxide layer of a silicon-on-insulator substrate, and the dielectric layer14may separate the waveguide core12from the substrate16. In an alternative embodiment, one or more additional dielectric layers comprised of, for example, silicon dioxide may be positioned between the waveguide core12and the dielectric layer14.

In an embodiment, the waveguide core12may be comprised of a material having a refractive index that is greater than the refractive index of silicon dioxide. In an embodiment, the waveguide core12may be comprised of a semiconductor material, such as single-crystal silicon or polysilicon. In an alternative embodiment, the waveguide core12may be comprised of a dielectric material, such as silicon nitride, silicon oxynitride, or aluminum nitride. In alternative embodiments, other materials, such as a polymer or a III-V compound semiconductor, may be used to form the waveguide core12.

In an embodiment, the waveguide core12may be formed by patterning a layer of constituent material with lithography and etching processes. In an embodiment, an etch mask may be formed by a lithography process over the layer, and unmasked sections of the deposited layer may be etched and removed by an etching process. The shape of the etch mask may determine the patterned shape of the waveguide core12. In an embodiment, the waveguide core12may be formed by patterning the semiconductor material (e.g., single-crystal silicon) of a device layer of a silicon-on-insulator substrate.

The waveguide core12may include a tapered section18that defines a spot-size converter arranged to receive light of a given mode from a light source, such as a laser. The tapered section18may have a narrow end15defining a facet that is eventually arranged proximate to the light source and a wide end that is connected to another section of the waveguide core12used to route the light to functional circuits on the photonics chip. The gradually-varying cross-section area of the tapered section18may support mode transformation and mode size variation associated with mode conversion when receiving light from the light source.

A dielectric layer44may be formed over the waveguide core12and dielectric layer14adjacent to the cavity20. The dielectric layer44may be comprised of a dielectric material, such as silicon dioxide. The dielectric layer44may replace a removed section of a back-end-of-line stack.

A cavity20is formed that penetrates through the dielectric layer14and into the substrate16. The cavity20may be formed by one or more lithography and etching processes. The portion of the cavity20in the substrate16includes sidewalls21,22and sidewalls24,25that are arranged about, and surround, the cavity20. The narrow end15of the tapered section18of the waveguide core12is positioned adjacent to the sidewall22. Conductive traces26may be formed that lead from a bottom or floor23of the cavity20up the sidewall21and onto a surface adjacent to the cavity20. The cavity20extends through the dielectric layer44to the substrate16, and the conductive traces26may extend onto a surface of the dielectric layer44. Mechanical stops28, which may be patterned portions of the dielectric layer14, are arranged adjacent to the opposite sidewalls24,25of the cavity20.

With reference toFIGS.3,4,4Ain which like reference numerals refer to like features inFIGS.1,2and at a subsequent fabrication stage, a laser chip30is positioned in the cavity20such that an output32for laser light is aligned with the narrow end15of the tapered section18of the waveguide core12. The shape and dimensions of the cavity20may be correlated with the shape and dimensions of the laser chip30such that the laser chip30can be inserted into the cavity20. In that regard, the dimensions of the cavity20may be slightly greater than the dimensions of the laser chip30to provide clearance for insertion into the cavity20. In an embodiment, the output32of the laser chip30may be butt coupled with, and adjacent to, the narrow end15of the tapered section18of the waveguide core12.

In an embodiment, the laser chip30may be configured to emit laser light of a given wavelength, intensity, mode shape, and mode size. In an embodiment, the laser chip30may include a laser comprised of III-V compound semiconductor materials. In an embodiment, the laser chip30may include an indium phosphide/indium-gallium-arsenic phosphide laser that is configured to generate continuous laser light in an infrared wavelength range for emission from the output32. For example, the laser included in the laser chip30may generate and output laser light at a nominal peak wavelength of 1310 nm or at a nominal peak wavelength of 1550 nm.

In an alternative embodiment, the laser chip30may include a semiconductor optical amplifier that is configured to amplify the optical power of laser light. An additional waveguide core similar to the waveguide core12may be provided to function as an input to the semiconductor optical amplifier, and the waveguide core12may provide an output from the semiconductor optical amplifier.

The laser chip30has a body31with a surface48that is spaced from the floor23of the cavity20by a gap38. The output32of the laser chip30may be spaced from the tapered section18of the waveguide core12by a gap40. The body31of the laser chip30also has a side surface46and a side surface47opposite to the side surface46. The side surface46of the body31of the laser chip30is adjacent to the sidewall22of the cavity20and is spaced from the sidewall22of the cavity20by a gap, and the side surface47of the body31of the laser chip30is adjacent to the sidewall21of the cavity20and spaced from the sidewall21of the cavity20by a gap.

The laser chip30may have the form of a flip-chip package that includes pads34that are exposed at the surface48and connected to conductive paths inside the body31of the laser chip30. The pads34may be attached to the conductive traces26by solder balls or solder bumps36and the conductive paths may be used to power the laser of the laser chip30. The attachment process may include inserting the laser chip30into the cavity20and reflowing the solder in the balls or solder bumps36such that the laser chip30is mechanically and electrically connected to the conductive traces26.

The laser chip30may contact with the mechanical stops28after attachment. As a result, the mechanical stops28provide passive alignment of the attached laser chip30in a vertical direction, and the height of the mechanical stops28may define the vertical dimension of the gap38after attachment. In an embodiment, the tapered section18of the waveguide core12may be longitudinally aligned substantially collinear with the light emitted from the output32from the laser chip30. In an alternative embodiment, the tapered section18of the waveguide core12may be angled relative to the output32from the laser chip30to provide non-normal incidence, which may reduce back reflection of light from the waveguide core12to the laser chip30. In alternative embodiments, additional waveguide cores may be arranged adjacent to the tapered section18of the waveguide core12, such as a trident arrangement with the tapered section18of the waveguide core12positioned between a pair of added waveguide cores. In an alternative embodiment, the tapered section18of the waveguide core12may be segmented to define a subwavelength metamaterial structure and may optionally include a central rib overlaid on the segments.

With reference toFIG.5in which like reference numerals refer to like features inFIG.4and at a subsequent fabrication stage, an adhesive50may be formed that is positioned in a portion of the gap38between the surface48of the body31of the laser chip30and the floor23of the cavity20. The adhesive50is also positioned between in the gap40between the output32of the laser chip30and the tapered section18of the waveguide core12, as well as in the gap between the side surface46of the laser chip30and the sidewall22of the cavity20. The adhesive52, which may be highly flowable before curing, may be dispensed adjacent to the laser chip30and allowed to wick, via capillary action, between the laser chip30and the cavity20. The adhesive50may be concurrently or almost concurrently cured during dispensing such that the flow of the adhesive50across the gap38is limited. For example, the adhesive50may be cured using ultraviolet light, during dispensing, to limit the extent of the flow across the gap38.

The adhesive50in the gap40may be positioned between the output32of the laser chip30and the tapered section18of the waveguide core12. In an embodiment, the adhesive50may contact the surface48of the body31of the laser chip30. In an embodiment, the adhesive50may directly contact the surface48of the body31of the laser chip30. In an embodiment, the adhesive50may contact the floor23of the cavity20. In an embodiment, the adhesive50may directly contact the floor23of the cavity20. In an embodiment, the adhesive50may contact the surface48of the body31of the laser chip30and the floor23of the cavity20such that the gap38is bridged. In an embodiment, the adhesive50may directly contact the surface48of the body31of the laser chip30and the floor23of the cavity20such that the gap38is bridged.

The adhesive50may be characterized by optical properties that are optimized to promote light coupling between the laser chip30and the tapered section18of the waveguide core12and to provide a refractive index match between the laser chip30and the tapered section18of the waveguide core12. The optical properties of the adhesive50may also be optimized to reduce back reflection from the waveguide core12to the output32of the laser chip30.

With reference toFIG.6in which like reference numerals refer to like features inFIG.5and at a subsequent fabrication stage, an adhesive52is formed in the portion of the gap38that is unfilled by the adhesive50. The adhesive52is also positioned in the gap between the side surface47of the laser chip30and the sidewall21of the cavity20. The adhesive52, which may be highly flowable before curing, may be dispensed adjacent to the laser chip30and allowed to wick, via capillary action, between the laser chip30and the cavity20, followed by a thermal cure of both adhesives50,52.

In an embodiment, the adhesive52may extend fully from the floor23of the cavity20to the surface48of the body31of the laser chip30such that the gap38is bridged. In an embodiment, the adhesive52may contact the surface48of the body31of the laser chip30. In an embodiment, the adhesive52may directly contact the surface48of the body31of the laser chip30. In an embodiment, the adhesive52may contact the adhesive50inside the gap38. In an embodiment, the adhesive52may directly contact the adhesive50inside the gap38.

The adhesive52has a different composition than the adhesive50. In an embodiment, the adhesive52may include an organic binder and a particle filler distributed in the organic binder. For example, the adhesive52may be a composite including an organic binder and particles of an inorganic particle filler dispersed in the organic binder. The organic binder may be comprised of a polymer, an epoxy, or a resin. The particle filler in the adhesive52may include particles comprised of silicon dioxide, boron oxide, calcium oxide, carbon, aluminum nitride, or boron nitride. In an embodiment, the particle filler in the adhesive52may have a mean particle size of about one micron. In an embodiment, the adhesive52may have a higher thermal conductivity than the adhesive50, which leads to a reduction in thermal impedance during operation. In an embodiment, the adhesive52may have a thermal conductivity that is greater than or equal to 0.2 W/m-K. The thermal conductivity of the adhesive52may be tailored through properties such as the particle size and the filler content of the particles (i.e., particle concentration in weight percent) in the binder. The adhesive52may be chosen to have a coefficient of thermal expansion and modulus that is independent of the adhesive50, which may permit an improvement in the mechanical properties in comparison with a full fill by an adhesive lacking a particle filler.

The combination of adhesives50,52may permit both optical requirements and thermal requirements to be satisfied for a given application of the structure10. The adhesive50preserves the improvement in optical coupling arising from index matching, and the adhesive52improves the ability to conduct heat generated by the laser chip30from the laser chip30to the substrate16. The particle concentration in the adhesive52may also improve the thermo-mechanical reliability of the structure10.

In an alternative embodiment, the laser chip30may be attached to a top surface of either the dielectric layer14or a top surface of the substrate16instead of being attached inside the cavity20. The gap38would then exist between the top surface and the body31of the laser chip30.

With reference toFIG.7in which like reference numerals refer to like features inFIG.2and at a subsequent fabrication stage in accordance with alternative embodiments, the adhesive52may be applied in a portion of the gap38before attaching the laser chip30and before applying the adhesive50. The adhesive52may be comprised of a material that exhibits negligible flow due to a high viscosity. The laser chip30is inserted into the cavity20and attached to the substrate16after the adhesive52is applied. In an embodiment, the adhesive52may be comprised of filler particles (i.e., nanoparticles) with a sub-micron particle size.

With reference toFIG.8in which like reference numerals refer to like features inFIG.7and at a subsequent fabrication stage, the adhesive50may be applied after attaching the laser chip30to the substrate16. The adhesive50may fill the portions of the gap38unfilled by the adhesive52, the gap40, the gap between the side surface46of the body31of the laser chip30and the sidewall22of the cavity20, and the gap between the side surface47of the body31of the laser chip30and the sidewall21of the cavity20.

With reference toFIG.9and in accordance with alternative embodiments, an adhesive54may be applied in the gap38before applying either the adhesive50or the adhesive52. In an embodiment, the adhesive54may be a composite including an organic binder and particles of an inorganic particle filler dispersed in the organic binder. The organic binder may be comprised of a polymer, an epoxy, or a resin. The particle filler in the adhesive54may include particles comprised of silicon dioxide, boron oxide, calcium oxide, carbon, aluminum nitride, or boron nitride. The adhesive54has a different composition than either the adhesive50or the adhesive52.

The adhesive54may function as a dam during the application of the adhesive50for controlling the flow of the adhesive50in the gap38, which may eliminate the requirement for concurrently curing the adhesive50during its dispensing. In an embodiment, the adhesive54may contact the surface48of the body31of the laser chip30. In an embodiment, the adhesive54may directly contact the surface48of the body31of the laser chip30. In an embodiment, the adhesive54may contact the adhesive50inside the gap38. In an embodiment, the adhesive54may contact the adhesive52inside the gap38. In an embodiment, the adhesive54may contact the adhesive50and the adhesive52inside the gap38. In an embodiment, the adhesive54may directly contact the adhesive50inside the gap38. In an embodiment, the adhesive54may directly contact the adhesive52inside the gap38. In an embodiment, the adhesive54may directly contact the adhesive50and the adhesive52inside the gap38.

In an embodiment, the adhesive54before curing may have a higher viscosity than the adhesive50before curing. In an embodiment, the adhesive54before curing may have a higher viscosity than the adhesive52before curing. In an embodiment, the adhesive54before curing may have a higher viscosity before curing than either the adhesive50before curing or the adhesive52before curing.

The adhesive54may have a thermal conductivity that may be tailored through properties the particle size and the filler content of the particles (i.e., particle concentration in weight percent) in the binder. In an embodiment, the particle concentration of the filler in the adhesive54may be greater than the particle concentration of the filler in the adhesive52. The higher particle concentration may also provide the enhanced viscosity of the adhesive54in comparison with the adhesive52. In an embodiment, the adhesive54may have a thermal conductivity of greater than or equal to 1 W/m-K. In an embodiment, the adhesive54may be characterized as a thermal interface material. In an embodiment, the adhesive54may have a higher thermal conductivity than the adhesive50, which may be due at least in part to the presence of the particle concentration. In an embodiment, the adhesive54may have a higher thermal conductivity than the adhesive52, which may be due at least in part to the higher particle concentration. In an embodiment, the adhesive54may have a higher thermal conductivity than either the adhesive50or the adhesive52.

With reference toFIG.10and in accordance with alternative embodiments, the laser chip30be attached to the conductive traces26by a single solder pad56. The solder pad56may function as a dam during the application of the adhesive50to restrict flow of the adhesive50across the gap38and during the application of the adhesive52to restrict flow of the adhesive52across the gap38. A portion of the adhesive50, a portion of the adhesive52, and the solder pad56are positioned within the gap38with the solder pad56is positioned between the adhesive50and the adhesive52.

References herein to terms such as “vertical”, “horizontal”, etc. are made by way of example, and not by way of limitation, to establish a frame of reference. The term “horizontal” as used herein is defined as a plane parallel to a conventional plane of a semiconductor substrate, regardless of its actual three-dimensional spatial orientation. The terms “vertical” and “normal” refer to a direction in the frame of reference perpendicular to the horizontal, as just defined. The term “lateral” refers to a direction in the frame of reference within the horizontal plane.