Patent Application
20170003463
Embodiments presented herein generally relate to waveguides in a photonic device, and more specifically, to accurate placement of a laser.
Silicon-on-Insulator (SOI) optical devices may include an active surface layer that includes waveguides, optical modulators, detectors, complementary metal-oxide-semiconductor (CMOS) circuitry, metal leads for interfacing with external semiconductor chips, and the like. Transmitting optical signals from and to this active surface layer introduces many challenges. In some optical devices, lenses are used to focus the light from an external fiber optic cable or a laser source into the waveguides, thereby shrinking the mode or adjusting the numerical aperture such that the optical signal can be efficiently transferred into the sub-micron waveguides.
So that the manner in which the above-recited features of the present disclosure can be understood in detail, a more particular description of the disclosure, briefly summarized above, may be had by reference to embodiments, some of which are illustrated in the appended drawings. It is to be noted, however, that the appended drawings illustrate only typical embodiments of this disclosure and are therefore not to be considered limiting of its scope, for the disclosure may admit to other equally effective embodiments.
To facilitate understanding, identical reference numerals have been used, where possible, to designate identical elements that are common to the figures. It is contemplated that elements disclosed in one embodiment may be beneficially utilized on other embodiments without specific recitation.
One embodiment presented in this disclosure is a method. The method generally includes disposing a bottom surface of a laser on a support member, wherein the support member is formed on a substrate and extends in a direction perpendicular to a base plane of the substrate, wherein the bottom surface of the laser is in a facing relationship with the base plane, and wherein solder is disposed on the base plane such that a height of the solder in the direction perpendicular to the base plane is less than a height of the support member so that a gap is created between the solder and the laser; aligning the laser with an optical waveguide; and heating the solder, after the alignment of the laser with the optical waveguide, so that the solder contacts the laser.
Another embodiment presented herein is a photonic chip. The photonic chip generally includes a substrate defining a base plane, a support member extending from the base plane wherein a height of the support member relative to the base plane is selected to provide alignment of a laser when mounted on the chip in at least one direction. The photonic chip further includes solder disposed on the base plane, wherein a height of the solder relative to the base plane is less than the height of the support member relative to the base plane; and an optical waveguide configured to receive an optical signal from the laser when mounted on the photonic chip.
Another embodiment presented herein is a method. The method generally includes mating a first surface of a laser with a second surface of a support member, wherein the support member extends from a base plane of a substrate and wherein the first surface and the base plane are in a spaced, facing relationship to one another to form an interstitial gap, and wherein solder is disposed in the interstitial gap, and wherein the solder has a thickness that is less than the interstitial gap; aligning the laser with an optical waveguide; and after the alignment, heating the solder so that the solder is reflowed into contact with one of the first surface of the laser and the base plane of the substrate.
The cost of optical transceivers is greatly influenced by the cost of packaging. The packaging cost is often driven by the process used to attach and actively align a semiconductor laser with a waveguide to achieve high precision and tight tolerances. Because precise placement and alignment of a laser with a waveguide may be difficult, alignment may be achieved using micro-optics (e.g., silicon lenses on the order of 100 to 200 microns in diameter) and an active alignment technique. However, using lenses increases the cost and complexity of the optical device. Moreover, the lenses need to be aligned to ensure the signal from the light-carrying medium or from a light generating device focuses onto the waveguide. As such, not only do the lenses add cost to an optical system, but coupling efficiency suffers if the lenses are not aligned correctly.
The approach used to attach and align the laser can greatly influence the overall cost of optical transceivers as well as the cost of manufacturing equipment, overall quality, yield, and manufacturability. There is also rising demand for cheaper and more compact solution which is primarily driven by the growth of data centers. Therefore, there is a need for a packaging scheme which allows for precise passive placement and alignment of a laser resulting in good coupling efficiency, even in the absence of micro-optics.
Embodiments of the present disclosure provide a packaging method involving the use of lithographically defined features and solder to achieve passive, high precision alignment of a laser component on a silicon photonic chip, sub-mount, or wafer. For example, the packaging methods disclosed herein allow for precise alignment of a laser, using alignment markings, prior to the laser making contact with material (e.g., solder) used for bonding the laser to a photonic chip. The laser may be disposed on a support member of the photonic chip, where a height of the support member is finely tuned for precise alignment of the laser in a vertical direction with a waveguide formed in the photonic chip. As used herein, the term disposed does not necessarily mean contacting. That is, the laser may be disposed on the support member, even though other intermediary layers of material may be between the laser and the support member. Alignment markings may be used to laterally align the laser with the waveguide. Once the laser has been properly aligned with the waveguide, solder may be reflowed such that the solder contacts the laser. By precisely aligning the laser with the waveguide prior to the solder making contact with the laser, laser alignment precision is improved.
Precise placement and alignment of the laser allows for butt-coupling of the laser to a silicon photonic system (e.g., a silicon waveguide). That is, the laser may be directly coupled with the waveguide such that light is transferred between the laser and the waveguide without the use of lenses. The technique can be used on a single die or at wafer scale assembly and allows volume manufacturing with significant cost reduction.
The operations 200 generally include, at block 202, disposing a bottom surface of a laser (e.g., laser 104) on a support member such that the laser 104 is disposed on the support member 106. In certain embodiments, the support member may be formed on a substrate. For example, the support member may be part of the substrate to form a single monolithic structure. In other embodiments, the support member and the substrate may be two separate structures that have been attached together. As illustrated in
Generally,
The support members 106 may be lithographically defined using standard complementary metal-oxide-semiconductor (CMOS) and/or silicon micro-electro-mechanical systems (MEMs) processes which allow for precise control of the height of the support members 106. The height of the support members 106 are determined such that, upon placement of the laser on the support members 106, the laser is aligned with the waveguide 102 in the vertical direction. Thus, the support members 106 are accurate reference surfaces of the substrate 302 such that when the bottom surface of the laser is disposed directly on the support members 106, the precise location of the laser relative to the waveguide 102 in the vertical direction (e.g., z direction as illustrated in
The photonic chip 100 also includes a stack of layers 308 (e.g., formed on top of the substrate 302) which includes a silicon layer which contains the waveguide 102. In certain embodiments, the stack of layers 308 may be an inter-layer dielectric (ILD). Solder 110 is disposed on the substrate 302 such that a height (i.e., the z direction) of the solder 110 above the substrate 302 is less than the height of the support members 106 above the substrate 302. Therefore, a gap exists between the top of the solder 110 and the bottom of the laser when the laser is disposed on top of the support members 106. This gap allows for precise alignment of a laser 104 (e.g., to be disposed on the support members 106) with the waveguide 102, as will be described in more detail with respect to
In the embodiment shown, an electrode layer 310 is disposed between the substrate 302 and the solder 110. The electrode layer 310 may be used to make an electrical connection with the solder. That is, when the laser is disposed on the support members 106 and solder 110 is reflowed and makes contact with the laser, the electrode layer 310 may be used for powering the laser via a circuit of the photonic chip 100. For example the circuit of the photonic chip may be wire bonded to the electrode layer 310 using a bond pad on the top surface of the substrate to form an electrical contact with the solder 110.
Moreover, the waveguide 102 includes an interface that is substantially perpendicular to a base plane of the substrate 302 from which the support members 106 extend. As used herein, “substantially perpendicular” means the interface and the base plane may not be precisely perpendicular given the limitations of fabrication techniques used to generate these features. Thus, these surfaces may be up to 5-10 degrees off from being perpendicular.
Returning to the operations 200 of
In certain embodiments, the laser 104 and the silicon photonic substrate both have lithographically defined and etched features (e.g., alignment markings) which allow vision based passive alignment along the X and Y axes. Moreover, as described above, the height of the support members 106 are determined such that, upon placement of the laser 104 on the support members 106, the laser 104 is aligned with the waveguide 102 in the vertical direction (e.g., z direction as illustrated).
If the solder 110 has a height above the substrate 302 that is greater than the height of support members 106 above the substrate 302, the laser 104 and the solder 110 may be in physical contact before the solder 110 is reflowed during the heating process. This may result in a gap between support members 106 of the photonic chip and laser (more specifically, the bottom surface of the laser 104 and the top surface of the support members 106) prior to the solder reflow process. As a result, the bonding equipment may first align the components in two axes (X and Y axes) and subsequently allow the laser 104 to travel in the Z direction while the solder is being reflowed (e.g., until the laser makes contact with support members 106). However, as the laser 104 moves in the Z direction to make contact with the support members 106 (e.g., by applying pressure on the laser 104 in the z direction), the alignment of the laser 104 and the waveguide 102 in the X and Y axes may be lost. In other words, it is difficult to maintain laser alignment in the X and Y axes while the laser moves in a downward direction towards the support members 106 and pushes against the reflowed solder 110. To avoid this problem, the embodiments herein form the solder 110 on the substrate 302 such that a gap exists, prior to the solder 110 being reflowed, between the laser 104 and the solder 110 when the vertical stops (e.g., support members 106) are in contact with the laser 104. For example, while resting on the support members 106 (which passively aligns the laser in the Z direction), the laser can be adjusted in the X and Y direction using, e.g., the alignment marks 312. Once aligned in the X, Y, and Z directions, pressure may be applied in the downward direction to maintain the alignment when the solder is reflowed. In this manner, placement of the laser 104 on the support members 106 without making contact with the solder 110 allows for precise alignment in X, Y, and Z directions before the solder 110 is reflowed.
In certain embodiments, once the laser 104 and the waveguide 102 have been aligned, pressure may be applied to the laser 104 towards the support members 106 to prevent misalignment of the laser 104 with the waveguide 102 during the solder reflow process. In certain embodiments, the solder may be heated during the solder reflow process through the substrate 302. That is, heat may be applied (e.g., via another laser not shown) to a bottom portion of the substrate 302 until the heat is transferred to the solder and the solder is reflowed. In this embodiment, the laser that heats the solder emits light that can propagate through the material of the substrate until it strikes the solder. As such, this laser may be disposed beneath the bottom of the photonic chip. However, disposing the laser such that it emits light that strikes the solder in the z direction is not a requirement. In other embodiments, the laser can be located at one of the sides of the photonic chip such that the light strikes the solder in the X or Y direction. Moreover, in other embodiments the solder may be reflown using a heat source other than a laser. For example, an electrical current or heating element disposed proximate to the solder in the photonic chip may be used to reflow the solder.
During the reflow process, the solder 110 is heated by a heat source (e.g., another laser) such that the solder 110 liquefies and domes until the solder 110 makes contact with the laser metal 608, as illustrated in
While examples of the present disclosure have described solder as being disposed on the substrate to facilitate understanding, persons of ordinary skill in the art understand that the solder may be disposed anywhere in a gap between the laser and the substrate so long as a thickness of the solder is less than the gap. For example, solder may be disposed on a bottom surface of the laser, which can be reflowed into contact with the substrate once the laser has been properly aligned.
In the preceding, reference is made to embodiments presented in this disclosure. However, the scope of the present disclosure is not limited to specific described embodiments. Instead, any combination of the described features and elements, whether related to different embodiments or not, is contemplated to implement and practice contemplated embodiments. Furthermore, although embodiments disclosed herein may achieve advantages over other possible solutions or over the prior art, whether or not a particular advantage is achieved by a given embodiment is not limiting of the scope of the present disclosure. Thus, the preceding aspects, features, embodiments and advantages are merely illustrative and are not considered elements or limitations of the appended claims except where explicitly recited in a claim(s).
The flowchart and block diagrams in the Figures illustrate the architecture, functionality and operation of possible implementations of systems or methods. It should also be noted that, in some alternative implementations, the functions noted in the block may occur out of the order noted in the figures. For example, two blocks shown in succession may, in fact, be executed substantially concurrently, or the blocks may sometimes be executed in the reverse order, depending upon the functionality involved.
In view of the foregoing, the scope of the present disclosure is determined by the claims that follow.
