Hybrid integrated optical device with passively aligned laser chips having submicrometer alignment accuracy

A hybrid optical device includes an optical bench chip, a laser chip with a laser waveguide flip-chip bonded onto the optical bench chip, and an optical waveguide chip with an optical device waveguide disposed adjacent the optical bench chip. The optical bench chip has multiple “U” shaped alignment optical waveguides and the optical waveguide chip has multiple alignment optical waveguides, and the pitches of the various sets of alignment waveguides are different. A misalignment between the laser waveguide and the optical bench chip is compensated for by aligning the optical waveguide chip to different positions of the optical bench chip using the multiple alignment optical waveguides on the optical bench chip and the optical waveguide chip, without turning on the laser, so that the laser waveguide of the laser chip is aligned with the optical device waveguide of the optical device chip.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The invention relates to an optical device with hybrid integrated laser chips and optical waveguide chip. In particular, the invention relates to an optical device using flip-chip method to hybrid integrate one or more laser chips with one optical waveguide chip with the help of optical bench chips with “U” shaped alignment optical waveguides having various waveguide distances.

2. Description of the Related Art

The internet based data applications such as social networks, cloud service, big data analysis and high performance computing have been the driving force to boost the bandwidth requirement to an unprecedented level. With the increase of bandwidth and transmission reach, optical interconnects have become the number one choice in data communication systems. Unlike traditional telecommunication systems, lower cost, more compact and more power efficient optical transceivers or engines are highly demanded in data communications. Integrating multiple optical components or chips such as lasers, modulators, photodetectors etc. to form a hybrid integrated optical device is a promising way to reduce assembling cost and footprint.

Laser chip hybridization usually requires turning on the laser chip so that the device chip can be actively aligned to the laser chip. To build the electric connection, the laser chip has to be fixed in a substrate first and connected to the power supply through wire bonding process. However, it becomes extremely challenging when dealing with multiple laser chips aligning with one optical device chip. In this scenario, the optical device chip needs to be fixed on a substrate first, and then the laser chips have to be actively aligned with the optical device chip one by one. It is very difficult to construct the electrical connections when the laser chips are floating for active alignment process.

Passively placing and bonding laser chips with optical waveguide device chips is highly desirable in such hybrid integrated optical devices for its potentials of low cost assembling for massive volume production. Unlike the mature integrated circuit (IC) fully automated packaging processes, optical integration requires very precise alignment in the range of micrometers or less to form an optical transmission path. It becomes very crucial when dealing with the integration of a laser chip with an optical waveguide chip, where two small waveguides need to be aligned with micrometer or sub-micrometer accuracy.

The alignment accuracy in the direction perpendicular to the chip surface (out-plane) can be controlled well by using a flip-chip bonding process, where one chip is placed upside down onto another chip. However, it is very challenging to achieve higher alignment accuracy in the directions parallel to the surface (in plane). A modern top-of-the-line flip-chip bonder can achieve a +/−0.5 micrometer alignment accuracy, however, in practice, the bonding involving processes such as thin metal solder melting, adhesive curing and etc. inevitably contribute to final alignment error due to physical movement of the chip under temperature, stress and/or phase changes. The final alignment error (3σ confidence interval) is usually +/−2 micrometers or worse from the statistics data. The alignment in in-plane waveguide propagation direction is relatively tolerant and satisfied with this alignment error while the in-plane direction perpendicular to waveguide propagation requires very accurate alignment, for example a sub-micrometer accuracy for small optical waveguides such as those in lasers. To increase the alignment tolerance in this direction, many approaches have been attempted. However, none of them is being adopted in mass production due to their limitations.

SUMMARY OF THE INVENTION

Optical devices according to embodiments of the present invention significantly increase the alignment tolerance to sub-micrometer range in the in-plane direction perpendicular to waveguide propagation during flip-chip bonding process.

To achieve these and other advantages and in accordance with the purpose of the present invention, as embodied and broadly described, the present invention provides a hybrid integrated optical device which includes: at least one optical bench chip having a first side and a second side opposite to the first side and a set of U shaped alignment optical waveguides formed at the first side of the optical bench chip, wherein each of the set of U shaped alignment optical waveguides has a first end and a second end exposed on one end face of the optical bench chip; at least one laser chip having a first side and a second side opposite to the first side and at least one laser waveguide formed at the first side of the laser chip; wherein the laser chip is approximately aligned and flip-chip bonded onto the optical bench chip with the first side of the laser chip facing the first side of the optical bench chip; an optical waveguide chip disposed adjacent the optical bench chip, the optical waveguide chip having a first side and a second side opposite to the first side and at least one optical device waveguide and a first set and a second set of alignment optical waveguides formed at the first side of the optical waveguide chip, wherein each of the alignment optical waveguides has an end exposed on one end face of the optical waveguide chip which faces the one end face of the optical bench chip, wherein the laser waveguide of the laser chip is aligned with the optical device waveguide of the optical waveguide chip, and wherein the end of at least one of the first set of alignment optical waveguides of the optical waveguide chip is aligned with the first end of one of the U shaped alignment optical waveguides of the optical bench chip, the end of at least one of the second set of alignment optical waveguides of the optical waveguide chip is aligned with the second end of the one of the U shaped alignment optical waveguides of the optical bench chip, and the end of at least another one of the first set of alignment optical waveguides of the optical waveguide chip is misaligned with the first end of another one of the U shaped alignment optical waveguides of the optical bench chip, and the end of at least another one of the second set of alignment optical waveguides of the optical waveguide chip is misaligned with the second end of the other one of the U shaped alignment optical waveguides of the optical bench chip.

In another aspect, the present invention provides a method for making a hybrid integrated optical device, which includes: providing at least optical bench chip having a first side and a second side opposite to the first side and a set of U shaped alignment optical waveguides formed at the first side of the optical bench chip, wherein each of the set of U shaped alignment optical waveguides has a first end and a second end exposed on one end face of the optical bench chip; providing at least one laser chip having a first side and a second side opposite to the first side and at least one laser waveguide formed at the first side of the optical chip; providing an optical waveguide chip having a first side and a second side opposite to the first side and at least one optical device waveguide and a first set and a second set of alignment optical waveguides formed at the first side of the optical waveguide chip, wherein each of the alignment optical waveguides has an end exposed on one end face of the optical waveguide chip; approximately aligning and flip-chip bonding the laser chip onto the optical bench chip with the first side of the laser chip facing the first side of the optical bench chip; determining an amount of misalignment of the laser waveguide to a designated location on the first side of the optical bench chip in a direction parallel to the first side of the optical bench chip and perpendicular to a waveguide propagation direction; based on the determined amount of misalignment, selecting one of the set of U shaped alignment optical waveguides on the optical bench chip, a corresponding one of the first set of alignment optical waveguides of the optical waveguide chip and a corresponding one of the second set of alignment optical waveguides of the optical waveguide chip; placing the optical waveguide chip adjacent the optical bench chip wherein the one end of the one end face of the optical waveguide chip faces the one end face of the optical bench chip; aligning the selected one of the set of U shaped alignment optical waveguide on the optical bench with the corresponding selected ones of the first and second sets of alignment optical waveguides on the optical waveguide chip; and fixing the optical bench chip and the optical waveguide chip to each other to form the hybrid integrated optical device.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

The invention relates to an optical device with hybrid integrated laser chips and optical waveguide chip. In particular, the invention relates to an optical device using flip-chip method to hybrid integrate one or more laser chips with one optical waveguide chip with the help of optical bench chips with “U” shaped alignment optical waveguides having various waveguide distances.

An embodiment of the present invention is described with reference toFIGS. 1A-1B. The hybrid integrated optical device comprises an optical bench with multiple “U” shaped alignment optical waveguides, an etched trench, etched spacers, etched alignment structures, metal traces and micro solders, a flip-chip bonded laser chip with at least one laser waveguide, and an optical chip with at least one optical device waveguide and multiple alignment optical waveguides.

FIG. 1Ais a perspective view illustrating a hybrid integrated optical device according to an embodiment of the present.FIG. 1Bis an exploded perspective view of such hybrid optical device. The components are drawn in a way as if they were transparent for the purpose of easy observation of structures behind (structures behind other structures are shown in dashed lines). The hybrid integrated optical device comprises at least one optical bench100, an optical waveguide device chip110and at least one flip-chip bonded laser chip120.

The optical bench100can be made of any semiconductor or insulating materials including, but not limited to, silicon, silica, and indium phosphide. The optical bench100has a first side and a second side generally opposite to the first side. The optical bench100includes multiple “U” shaped alignment optical waveguides102A-102C on the first side formed by etching or deposition technologies. The two end sections of the “U” shaped alignment optical waveguides102A-102C are denoted102C1-102A2and102A2-102C2, which are preferably parallel to each other. The waveguides102A-102C are made of optical transparent materials including, but not limited to, silicon, silicon nitride, and indium gallium arsenide phosphide. The optical bench100further has a trench103and multiple spacers104formed by etching technologies. The depth of both the trench103and spacers104are precisely controlled using semiconductor processing techniques. The optical bench100further comprises includes metal traces105and micro solders106in the trench103for the purpose of electrically connecting to the flip-chip bonded laser chip120. The optical bench100further includes alignment mark structures107. The end facets of the waveguides102A-102C, i.e. the facets located at the near end of the optical bench, are coated with anti-reflection coating to reduce light reflection.

The optical waveguide chip110can be made of any semiconductor or insulating materials including, but not limited to, silicon, silica, and indium phosphide. The optical waveguide chip110has a first side and a second side generally opposite to the first side. The optical waveguide chip110further includes at least one optical device waveguide111and multiple alignment optical waveguides112A1-112C1and112C2-112A2on the two sides of the optical device waveguide111on the first side. The alignment optical waveguides112A1-112C1and112C2-112A2are preferably parallel to each other. The end facets of the waveguides of the optical waveguide chip110are coated with anti-reflection coating to reduce light reflection.

The laser chip120has a first side and a second side generally opposite to the first side. The laser chip120further includes at least one laser waveguides121on the first side. The laser chip120further includes electrodes122to receive external electrical power.

FIG. 2Aillustrates a front view of the scenario of a laser chip220flip-chip bonded to an optical bench chip200. Note that inFIGS. 2A and 2B, as compared toFIGS. 1A and 1B, like structure are labeled with like reference numerals (for example, the laser chip220inFIG. 2Agenerally corresponds to the laser chip120inFIG. 1A, etc.). To clearly illustrate all the structures and relative positions of these structures, the laser chip220is intentionally lifted up in the figure. The laser chip220is bonded on to the optical bench200though a flip-chip process in which the laser chip220is flipped thus its first side faces the first side of the optical bench200. The laser chip220is then aligned to the optical bench200by comparing center position of the laser waveguide221on the first side of the laser chip and alignment mark structures207on the first side of the optical bench. The laser chip220is then push on to the optical bench200while heating up either or both the laser chip220and optical bench chip200. The laser chip220is stopped by the spacers204from further descending. The micro solders206are melted to form electrical connection and mechanical bonding between the metal trace205and the electrode222.

FIG. 2Billustrates a cross-sectional view of the bonded structure in the direction parallel to the x-z plane. As shown inFIGS. 1A and 1B, the z direction is perpendicular to the first and second surfaces of the chips100and110; the x direction is in-plane and parallel to the waveguide propagation direction; and the y direction is in-plane and perpendicular to the waveguide propagation direction. The height of the spacers204is designed and fabricated to guarantee the mode centers of the laser waveguide221and the alignment optical waveguides202are in the same horizontal plane (x-y plane). The alignment optical waveguides202are designed to have similar optical mode size as the laser waveguide (221). In other words, this approach transfers the challenge of directly aligning the laser waveguide121with optical waveguide111to the alignment of the alignment optical waveguides102and112. The latter is less challenging because no laser chip needs to turn on, especially in the scenario when multiple laser chips need to align with the optical waveguide chip110.

The optical alignment in x-y plane is explained in the top views inFIG. 3A-3D. The alignment in the waveguide propagation direction (x direction) is relatively tolerant while the in-plane direction perpendicular to waveguide propagation (y direction) requires very accurate alignment, for example a sub-micrometer accuracy for small optical waveguides such as those in lasers.FIG. 3B-3Dillustrates the enlarged areas of E1and E2marked inFIG. 3Awith the perfect in-plane alignment between the laser waveguide321and the optical device waveguide311with the help of an optical bench chip300. Note that inFIGS. 3A-3D, as compared toFIGS. 1A and 1BandFIGS. 2A and 2B, like structure are labeled with like reference numerals (for example, the laser waveguide321inFIGS. 3A-3Dgenerally corresponds to the laser waveguide121inFIG. 1A and 221inFIG. 2A, etc.).

FIG. 3Ais a top view illustrating the perfect in-plane (y direction) alignment between the laser waveguide321and the optical device waveguide311with the help of an optical bench chip300in the scenario when the laser chip320is perfectly aligned with the alignment mark structures307on the optical bench chip300. To clearly show the relative positions of all related structures (alignment marks and waveguides), the enlarged areas of E1and E2marked inFIG. 3Aare illustrated. When the laser chip320is perfectly aligned with the optical bench chip300, the laser waveguide321is also perfectly aligned with the alignment mark structures307(in the middle of the alignment structure) as illustrated in E1enlarged drawing inFIG. 3B. In such a situation, the center alignment optical waveguide302B (with end sections302B1and302B2) on the optical bench chip300and the alignment optical waveguide pair312B1/312B2on the optical waveguide chip310are chosen for the final alignment process according to the design.FIG. 3Aillustrates the final optical alignment process. Following the arrows, light is input from the right end of the alignment optical waveguide312B1, passes through the interface of optical bench chip300and the optical waveguide chip310and enters the section302B1of the alignment optical waveguide302B, and makes a U-turn to the other section302B2, then passes through the interface again and enters the alignment optical waveguide312B2and is finally received by a light detective device at the right end of the waveguide312B2. By this process, the alignment optical waveguides312B1-302B-312B2on the optical bench chip300and optical waveguide chip31form perfectly aligned a big “U” shaped loop by fine tuning the relative position of the optical bench chip300(together with laser chip320) and the optical waveguide chip310. The enlarged drawing of E2inFIG. 3Billustrated the perfectly aligned scenario when the final optical alignment process is done. When the alignment optical waveguide set312B1-302B-312B2is aligned, i.e.302B1is aligned with312B1and302B2is aligned with312B2, the laser waveguide321is also aligned with the optical device waveguide311. This optical alignment process is with sub-micrometer accuracy.

FIG. 3CandFIG. 3Dare enlarged top views E1and E2illustrating the perfect in-plane (y direction) alignment between the laser waveguide321and the optical device waveguide311with the help of an optical bench chip300in scenarios when the laser chip320(for example the center of the laser waveguide321) is misaligned with the alignment mark structures307on the optical bench chip. After the flip-chip bonding process, the misalignment shift between the laser chip320and the optical bench chip300is read by the relative shift of between the laser waveguide321and the alignment mark structure307on the optical bench chip300. The misalignment shift is converted approximately to a digitized value of n×Δ, where n is an integer number and Δ is the minimum resolution of the alignment marks and equal to the pitch difference between the different sets of alignment optical waveguides as will be explained in more detail later. In the scenarios illustrated inFIG. 3CandFIG. 3D, the laser chips320shifted down and up by a Δ distance (i.e. n=1), respectively. Correspondingly, alignment optical waveguide sets312C1-302C-312C2or312A1-302A-312A2are chosen, respectively, for the alignment process similar to the one in the scenario illustrated inFIG. 3A.

To allow the above-described alignment procedure for the different scenarios, the locations and spatial relationship of the straight sections of the alignment optical waveguide302A1-C1and302A2-C2on the optical bench chip300and the alignment optical waveguides312A1-C1and312A2-C2on the optical waveguide chip310are such that (1) when waveguide302B1is aligned with waveguide312B1, waveguide302B2is also aligned with waveguide312B2, but waveguides302A1,302C1,302A2and302C2are misaligned with the corresponding waveguides312A1,312C1,312A2and312C2respectively; (2) from such a position, if the optical waveguide chip310is shifted downwards (in the view ofFIGS. 3A-3D) by a predetermined amount, waveguide302C1will be aligned with waveguide312C1and waveguide302C2will be aligned with waveguide312C2, but waveguides302A1,302B1,302A2and302B2will be misaligned with corresponding waveguides312A1,312B1,312A2and312B2respectively; and (3) from the position of (1), if the optical waveguide chip310is shifted upwards by the predetermined amount, waveguide302A1will be aligned with waveguide312A1and waveguide302A2will be aligned with waveguide312A2, but waveguides302B1,302C1,302B2and302C2will be misaligned with corresponding waveguides312B1,312C1,312B2and312C2respectively. Here, “aligned” means aligned within sub-micrometer accuracy (or “perfectly aligned”), and “misaligned” means not aligned to such accuracy.

The above spatial relationship among the alignment optical waveguides can be achieved by adjusting the spacing between the waveguides in each waveguide set. For example, in one implementation, the spacing (pitch) between adjacent waveguides within the waveguide set312A1-C1and312A2-C2is D, the spacing between adjacent waveguides within the waveguide set302A1-C1is D+Δ, and the spacing between adjacent waveguides within the waveguide set302A2-C2is D−Δ, where Δ is the predetermined amount of shift referred to above. This can also be viewed as shifting the U shaped waveguide302A upwards and shifting the U shaped waveguide302C downwards by the amount Δ.

In alternative implementations, the two sets of alignment optical waveguides302A1-C1and302A2-C2on the optical bench chip300may have a pitch D and the two sets of alignment optical waveguides312A1-C1and312A2-C2on the optical waveguide device chip310may have a pitch D+Δ and D−Δ, respectively, to achieve the same goal that one of the sequences of alignment optical waveguide A, B and C are aligned with each other while the others are misaligned. Further, while the U shaped alignment optical waveguides302A-C on the optical bench chip300are nested within each other in the illustrated embodiments, they may have other configurations, such as side-by-side. It is noted that although in the illustrated embodiments each set of alignment optical waveguides has three waveguides, they may contain more than three waveguides. When 2n+1 alignment optical waveguides are used, and the alignment optical waveguides have pitches D (for112X1and112X2) and D±Δ (for102X1,102X2), a misalignment of the laser chip in the amount of nΔ can be accommodated, where n is an integer number.

In any scenarios shown inFIG. 3A-3D, the laser waveguide321is always perfectly aligned with the optical device waveguide311even though the laser waveguide321is only approximately aligned with the alignment mark on the optical bench chip. Since the lasers on the laser chips320do not have to be turned on, the hybrid laser chips (with optical bench chips300) can be moved freely during the alignment process while the optical waveguide device chip310is fixed on a substrate and the process is not limited to a single laser chip hybridization.

To summarize, in embodiments of the present invention, the hybrid laser chip (together with the optical bench chip) is optically aligned (without tuning on the laser) with the optical waveguide chip by choosing the right set of the alignment optical waveguide combinations on the optical bench chip and the optical waveguide chip. This method provides a passive alignment process (no laser turned on) for hybrid integration of multiple laser chips with one optical waveguide device chip with sub-micrometer accuracy by using well-designed optical bench chips.

It will be apparent to those skilled in the art that various modification and variations can be made in the optical system and related fabrication methods of the present invention without departing from the spirit or scope of the invention. Thus, it is intended that the present invention cover modifications and variations that come within the scope of the appended claims and their equivalents.