Embedding the LED with the waveguide

Abstract

Micro-LEDs can be used to generate light for intra-chip or inter-chip communication. The micro-LEDs or the active layers of the micro-LEDs can be embedded in a waveguide. The waveguide can include a lens.

Description

TECHNICAL FIELD

[0001] The present disclosure relates generally to optical communications using micro-LEDs, and more particularly, to optical communication systems with micro-LEDs embedded in waveguide material. BACKGROUND

[0002] The demand for high performance computing and networking is ubiquitous and seems to be ever present. Prominent applications include data center servers, high performance computing clusters, artificial neural networks, and network switches.

[0003] For decades, significant integrated circuit (IC) performance and cost improvements have been driven by shrinking transistor size and increasing die size, outlined in the famous Moore's Law. Billions of transistors are allowed to integrate functionality previously segmented across multiple ICs onto a single system on chip (SoC).

[0004] However, as the marginal performance benefit decreases and yield decreases and cost per transistor increases, further shrinking transistors yields significantly less benefit. Independent of these limitations, a single IC can contain so much functionality and the functionality is limited because the program of the IC cannot be optimized for different functionality, such as logic, DRAM, and I / O at the same time.

[0005] In fact, there are significant benefits to "de-integrating" a SoC into smaller "chiplets," including: the program of each chiplet can be optimized for its functionality, such as logic, DRAM, high speed I / O; chiplets are well suited for reuse in multiple designs; the design cost of a chiplet is lower; and the chiplet has higher yield because it is smaller with fewer devices.

[0006] However, compared to a SoC, the main disadvantage of a chiplet is that the use of a chiplet typically requires more chip-to-chip connections. Chip-to-chip connections are typically less dense and require much more power (e.g., normalized to energy per bit) than on-chip connections between functional blocks in a SoC. SUMMARY

[0007] Some embodiments provide optical interconnects (e.g., between micro-LED source based chips and / or chiplets). Micro-LEDs can be generally defined as LEDs with diameters < 100 pm, in some embodiments < 20 pm, in some embodiments < 4 pm, and in some embodiments < 1 pm, and can be fabricated with diameters < 1 pm. In some embodiments, micro-LED sources can support optical links with lengths > 1 m, speeds > 1 Gbps, with low driving power and extremely high density.

[0008] In some embodiments, the micro-LED is coupled to an optical communication channel with high efficiency. In some embodiments, the optical communication channel comprises, or in some embodiments consists of, a waveguide, free space, or some combination of the two. Embodiments for coupling a micro-LED to an optical communication channel are discussed herein.

[0009] Some aspects of the invention provide an optical coupling device for an optical communication system, comprising: a micro-LED on a substrate, the micro-LED having an active layer; a waveguide on the substrate, the micro-LED being at least partially within the waveguide; a reflector on at least a portion of the waveguide, the reflector being positioned to reflect light from the micro-LED down an axis of the waveguide; a lens within the waveguide, the lens being in an optical path of the light from the micro-LED. In some aspects, the active layer of the micro-LED is within the waveguide.

[0010] In some aspects, the lens is in the optical path between the micro-LED and the reflector. In some aspects, the lens is a convex lens. In some such aspects, the lens is of a material having a higher refractive index than the material of the waveguide. In some aspects, the lens is a concave lens. In some such aspects, the lens is of a material having a lower refractive index than the material of the waveguide. In some aspects, the reflector forms a 45 degree turning mirror.

[0011] In some aspects, the reflector is in the optical path between the micro-LED and the lens. In some aspects, the lens is a convex lens. In some such aspects, the lens is of a material having a higher refractive index than the material of the waveguide. In some aspects, the lens is a concave lens. In some such aspects, the lens is of a material having a lower refractive index than the material of the waveguide.

[0012] In some aspects, the reflector has a curved shape. In some such aspects, the reflector has a parabolic shape.

[0013] In some aspects, the reflector is at one end of the waveguide.

[0014] In some aspects, the micro-LED includes a curved surface.

[0015] In some aspects, the active layer of the micro-LED does not extend to a side edge of the micro-LED. In some such aspects, the micro-LED includes a curved surface.

[0016] Some aspects further provide an electrical connection from the top of the waveguide through a via in the waveguide to a contact on the top of the LED.

[0017] These and other aspects of the invention will be more fully understood upon consideration of the following BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1A cross-sectional view showing a simplified structure of a micro-LED embedded in a waveguide.

[0019] Figure 2 A LED sealed by a seal in a waveguide medium is shown.

[0020] Figure 3a A LED with a curved top surface is shown.

[0021] Figure 3b An embodiment is shown in which the active layer of a LED does not always extend to the side of the LED and in which the side is curved.

[0022] Figure 4 A LED in an insertion hole of a waveguide is illustrated, as well as an example dipole radiation pattern for light emitted by the LED.

[0023] Figure 5a And 5b A side cross-sectional view and a top cross-sectional view, respectively, of an embodiment of a waveguide with one end in the form of a curved reflector that reflects light emitted from the top of the LED down the axis of the waveguide is shown.

[0024] Figure 6a An embodiment of a convex lens in a waveguide medium in the optical path of light from a LED is illustrated.

[0025] Figure 6b An embodiment of a concave lens in a waveguide medium in the optical path of light from a LED is illustrated.

[0026] Figure 7a Another embodiment of a convex lens in a waveguide medium in the optical path of light from a LED is illustrated.

[0027] Figure 7b Another embodiment of a concave lens in a waveguide medium in the optical path of light from a LED is illustrated.

[0028] Figures 8a to 8e illustrate process steps in forming a curved waveguide sidewall structure.

[0029] Figure 9 An example electrical connection to a LED in a waveguide medium is shown. DETAILED DESCRIPTION

[0030] Micro-LEDs are made from p-n junctions of direct bandgap semiconductor materials. Micro-LEDs are distinguished from semiconductor lasers (SLs) in that: (1) micro-LEDs do not have an optical resonator structure; (2) the optical output from micro-LEDs is almost entirely spontaneously emitted, whereas the output from SLs is primarily stimulated emission; (3) the optical output from micro-LEDs is not temporally and spatially coherent, whereas the output from SLs has considerable temporal and spatial coherence; (4) micro-LEDs are typically designed to operate with minimal current falling to zero, whereas SLs are designed to operate above a minimum threshold current, which is typically at least 1 mA.

[0031] Micro-LEDs can be distinguished from standard LEDs by having an emission area of less than 100 μm x 100 μm. Micro-LEDs have a small etendue, allowing them to be efficiently coupled into small waveguides and / or imaged onto small photodetectors. For convenience, the following discussion can generally refer to LEDs. However, it should be recognized that the discussion relates to micro-LEDs, which can be considered a particular type of LED.

[0032] Figure 1 A cross-sectional view showing a simplified structure of a micro-LED embedded in a waveguide. Light is emitted from a micro-LED in a thin active layer 111 via radiative recombination of charge carriers. In some embodiments, this radiative recombination occurs primarily in one or more quantum wells (QWs) (not shown in Figure 1 The active layer is bounded above and below by p and n layers 113 and 115. The active layer is shown in Figure 1 The waveguide core is bounded by waveguide cladding 118.

[0033] The active layer can be thought of as comprising a large number of infinitesimal emitters, where Figure 1 An example infinitesimal emissive element 119 is shown. Each emitter is much smaller than an optical wavelength, and emits light over a large range of angles. The emission pattern from each of the LED surfaces (i.e., the top and bottom) is approximately Lambertian, with light emitted into a hemisphere of 2π steradians.

[0034] The LED can also emit a significant amount of light from the edges of the LED adjacent to the active layer (as opposed to from the top and bottom surfaces), especially in the case of small LEDs, such as micro-LEDs. Because the active layer is typically very thin, this edge emission occurs over a small range in the vertical direction (y direction in Figure 1

[0035] LEDs are made from semiconductors with a high refractive index (typically > 2.5). If the LED is interfaced to a medium with a lower refractive index, then light is emitted at angles exceeding the critical angle θ c ​All light rays incident on the interface (relative to the normal to the interface) experience total internal reflection (TIR). Light is extracted from the LED (and some light is also reflected) at angles less than θ c Some portion of the light incident on the interface; this portion is called the light extraction efficiency (LEE).

[0036] A multimode optical waveguide supports propagation of a finite light cone angle. This light cone has an angular range θ WG (see Figure 1 ), which is often expressed as the numerical aperture (NA) of the waveguide, where NA = sin(θ WG ). A challenge for LED-based optical communication based on waveguides is to maximize the light coupling efficiency from the LED to the waveguide.

[0037] There can be two major challenges to efficiently couple light from the LED to the waveguide: extracting light from the LED structure, and efficiently coupling the extracted light to the waveguide.

[0038] LEE can be increased by reducing the reflection at the LED-external medium (L-E) interface. This can be achieved by (1) reducing the refractive index step at the interface; (2) illuminating the interface with light at angles closer to normal incidence. Techniques for achieving both are discussed below.

[0039] Figure 2 An LED is shown sealed by an encapsulant. In Figure 2 , the LED 211 has a back contact 213 (or bottom contact, as viewed in Figure 2 ) of a reflective material. The back contact is on a substrate 215. The LED is shown as having a width D. The LED, including its active layer 217, is sealed by an encapsulant 219 at least over the back contact. In Figure 2 , the encapsulant is also shown on the substrate, with the encapsulant extending generally a radius R from the center of the active layer. The encapsulant is within a waveguide medium 221, providing a propagation medium, with the waveguide medium also on the substrate.

[0040] The index step at the L-E interface can be reduced by sealing the LED in an "encapsulant" medium that more closely matches the index of the LED, with the ideal case being that the encapsulant index matches the index of the LED. In some embodiments, the encapsulant and / or the propagation medium can be an inorganic substance, such as SiO2, SiO x N y , Si3N4, Al2O3, indium tin oxide (ITO), TiO2, or stoichiometric variants thereof. In some embodiments, the encapsulant and / or the propagation medium can be an organic substance, such as a polymer.

[0041] Further, there is an encapsulant-propagation medium (E-P) interface, where light travels through the propagation medium to the receiver end of the optical link. If the LED is directly encapsulated in the propagation medium, there is no distinct encapsulant-propagation medium interface. In some embodiments, a combination of thin films of the materials mentioned above are alternated to act as an anti-reflective coating on the surface of the LED or encapsulant.

[0042] Similar to the L-E interface, there is also the possibility of reflection at the E-P interface. In fact, if the E-P interface is parallel to the L-E interface through which light is transmitted, the encapsulant does not improve the LEE, because the net reflection will be the same whether the encapsulant is present or not. Ideally, light should impinge on the E-P interface at normal incidence, in which case there is no TIR (although there is still Fresnel reflection due to the index discontinuity). If the E-P interface is a spherical surface centered on the center of the LED's emission area, all light rays are close to this normal condition, with the radius R of the sphere being significantly larger than the emission diameter D of the LED. Thus, in some embodiments, the E-P interface is a spherical surface centered on the center of the LED's emission area, and in some such embodiments, the spherical surface has a radius R that is significantly larger than the emission diameter D of the LED. In some embodiments, R is significantly larger than D if R is 5 or more times larger than D. In some embodiments, R is significantly larger than D if R is 10 or more times larger than D. In some embodiments, R is significantly larger than D if R is 100 or more times larger than D. Due to the very small emission diameter of micro-LEDs, a moderate value of R can provide a significant LEE improvement.

[0043] The surface of the encapsulant can be curved by a variety of methods, such as: (1) thermal reflow, e.g., heating a polymer above its glass transition temperature; (2) photolithographic methods, e.g., overexposure of a positive tone photoresist; or (3) etching processes, e.g., oxygen plasma etching of organic materials, or phosphoric acid based etching of silicon nitride.

[0044] In some embodiments, the LED surface is roughened to increase the LEE. If the surface is rough on the scale of the optical wavelength, the LEE at the interface between the LED and the low-index medium can be significantly increased compared to a smooth interface.

[0045] In some embodiments, the shape of the LED's surface can be altered to improve the LEE. In this case, the LED outside the emission area can be considered to be the encapsulant. Figure 3a An LED is shown with a curved top surface 313, where the radius of curvature R x is centered on the center of the emission area 311, which has a size D x in the relevant dimensions. This causes light to impinge on the L-E interface closer to normal incidence, increasing the LEE. As Rx becomes significantly larger than D x , the impact becomes significant.

[0046] In some embodiments, the active layer of the LED can not always extend to the side of the LED. In these embodiments, curving the surface of the LED can more effectively reduce reflections because the emission diameter can be made small relative to the radius of curvature of the surface.

[0047] Figure 3b An embodiment is shown in which the active layer 311 of the LED does not always extend to the side of the LED and in which the side surfaces 315a, 315b are curved with a radius R y For edge emission, the vertical emission dimension D y is very small. For moderate R y values, this achieves a significant LEE improvement.

[0048] In some embodiments, the LED side surfaces can not be at right angles to the active layer; for example, the side surfaces can be radiused or at non-normal angles. In some embodiments, the edges are straight and undercut, so the shape of the LED resembles an inverted pyramid. In such embodiments, less LED light can be reflected or refracted downward toward the substrate to which the LED is attached, which will reduce loss of light into the substrate.

[0049] In some methods, the angled and / or radiused LED side surfaces can be fabricated by utilizing halogen-based dry etching and varying plasma pressure and power, for example, as discussed in IEEE Photonics Technology Letters, 17-1, 2005 DOI: 10.1109 / LPT.2004.837480, the disclosure of which is incorporated by reference herein, and by using KOH anisotropic etching.

[0050] In some embodiments, the dipole radiation pattern of the LED active layer is modified by exploiting the optical cavity effect (especially in the case of quantum well (QW) devices) and by employing a highly reflective layer (e.g., the p-contact) very close to the QW, so that the light emission is primarily coupled into the lateral modes. In some embodiments, this is achieved by "tuning" the total thickness of the epitaxial layers above the QW, and by employing a highly reflective p-side contact, such as an Ag-based contact, to increase effectiveness. In this embodiment with proper cavity tuning, light is directed within the GaN epitaxial layers at an angle close to the normal of the side surface toward the LED mesa edge, so that only a small fraction of the light suffers TIR at the L-E interface.

[0051] In addition, such light can be coupled into the waveguide at angles within its numerical aperture (NA), thereby enabling efficient coupling of light from the LED active layer into the waveguide. In some embodiments, to direct light in one direction, one end of the waveguide can be terminated by a reflective metallization 439 (e.g., processed onto a Si interconnect chip), for example, as described in Figure 4 In Figure 4 , an insertion hole 437 is provided or formed so that a mesa containing an LED chip can be inserted into the insertion hole, thereby effectively inserting the active layer 419 of the micro-LED inside the waveguide in some embodiments. The dipole radiation pattern 411 is generally oriented slightly laterally with respect to the normal of the substrate 433, 435 on which the waveguide 431 is present, so that light from the active layer can be preferentially directed or emitted into the waveguide. To direct light in one direction of the waveguide, one end of the waveguide can be terminated by a reflective metallization 439, for example, as described in Figure 4 .

[0052] In one set of embodiments, a reflector is positioned above the active area at a distance from the active area so that edge emission is maximized. In another set of embodiments, the sides of the LED are angled so that a reverse pyramid is formed so that the central angle of the emitted distribution approximately coincides with the optical propagation axis of the waveguide, thereby maximizing coupling into the waveguide. In another set of embodiments, the edges of the LED are radiused, where the center of the curvature coincides with the center of the surface of the LED.

[0053] In another set of embodiments, a reflector is positioned below the active area at a distance from the active area so that edge emission is maximized. In another set of embodiments, the sides of the LED are angled so that a pyramid is formed so that the central angle of the emitted distribution approximately coincides with the optical propagation axis of the waveguide, thereby maximizing coupling into the waveguide.

[0054] In different sets of embodiments, a reflector is positioned at a distance from the active area so that surface emission is maximized. In another set of embodiments, the top surface of the LED is radiused, where the radius of curvature is centered on the center of the surface of the active area.

[0055] Once light is extracted from the LED, it generally has a very wide angular spectrum that significantly exceeds the acceptance angle range of the waveguide (often expressed as the "numerical aperture" of the waveguide). In addition, light emitted by the LED can propagate in directions away from the acceptance cone of the waveguide.

[0056] The product of the spatial and angular apertures of an LED is expressed by its etendue. It is generally not possible to reduce the etendue of an LED; it can only be conserved or increased. Mirrors can be used to confine the LED emission to a hemisphere. In some embodiments, these mirrors can be fabricated as part of the structure of the LED itself, as described in Figure 5a and 5bas shown in the middle.

[0057] By using a curved optical surface external to the LED, either refractive (e.g., a lens) or reflective (e.g., a curved mirror), both the spatial spread width and the angular spread width of the LED's light can be traded off. If the light from the LED is spatially spread by a factor M in one dimension, the angular width of the LED's light in that dimension can be reduced to 1 / M of its previous extent. The ability to reduce the angular width by increasing the spatial width can be especially powerful for very small micro-LEDs, and this reduction in angular width allows the LED's light to be efficiently coupled to a waveguide with a limited NA. For example, a 1 um x 1 um micro-LED can be efficiently coupled to a 4 um x 4 um waveguide with an NA of 0.25 if appropriate curved optical elements are used (this is quite practical for a multi-mode waveguide). This is discussed below.

[0058] Figure 5a and 5b show a side cross-sectional view and a top cross-sectional view, respectively, of an embodiment of a waveguide with one end in the form of a curved reflector that reflects light emitted from the top of the LED down the axis of the waveguide. In Figure 5a and 5b In some embodiments, the reflector is metalized to increase the reflectivity from the curved waveguide surface. In some embodiments, the shape of the reflector is approximately parabolic, with the center of the top surface of the LED at the focus of the parabola to minimize the angular spread of the reflected light.

[0059] In some embodiments, a convex lens with a higher index than the waveguide medium is introduced into the path of the vertical light along with a 45° turning mirror. Figure 6a An embodiment of a convex lens in the waveguide medium in the optical path of the light from the LED is illustrated. In Figure 6a In some embodiments, the angled end is angled at approximately 45 degrees to the substrate, with the angled end tilted over the LED. A reflector 617 is over the angled end of the waveguide medium, with the reflector forming a 45° turning mirror. A lens 651 is in the waveguide medium, between the LED and the reflector. In Figure 6a Figure 6a ​In some embodiments, a convex lens with a higher index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer.

[0060] In some embodiments, a concave lens with a lower index than the waveguide medium is introduced into the path of vertical light along with a 45° turning mirror. Figure 6b An embodiment of a concave lens in a waveguide medium in the optical path of light from an LED is illustrated. As in Figure 6a In some embodiments, a convex lens with a higher index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer. Figure 6b In some embodiments, a convex lens with a higher index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer. Figure 6a In some embodiments, a convex lens with a higher index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer. Figure 6b In some embodiments, a convex lens with a higher index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer. Figure 6b In some embodiments, a convex lens with a higher index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer.

[0061] In some embodiments, a convex lens with a higher index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer. Figure 7a Another embodiment of a convex lens in a waveguide medium in the optical path of light from an LED is illustrated. In Figure 7a In some embodiments, a convex lens with a higher index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer. Figure 7a The lens of FIG. 7 can be, for example, a convex lens with a higher refractive index than the waveguide medium.

[0062] In some embodiments, a concave lens with a lower index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer. Figure 7b Another embodiment of a concave lens in a waveguide medium in the optical path of light from an LED is illustrated. In Figure 7b In some embodiments, a concave lens with a lower index than the waveguide medium is introduced into the path of horizontal light emitted from the edge of the LED's active layer.Figure 7a In some embodiments, an LED 711 with a back reflector / contact 712 is on a substrate 715. The LED, including its active layer 713, is in a waveguide medium 719. The waveguide medium has a parabolic end in which the LED is located. A parabolic reflector 717 is over the parabolic end of the waveguide medium. A lens 753 is located approximately one focal length from the side of the active layer of the LED, allowing the light to spread a distance before reducing its angular extent. Figure 7b The lens of 7a, 7b can be, for example, a concave lens with a lower refractive index than the waveguide medium.

[0063] Figure 5a 、 5b The curved waveguide sidewall structure in 7a, 7b can be obtained via various methods. In one set of methods, illustrated in Figures 8a-8e, the process steps to form the curved waveguide sidewall structure are shown. In Figure 8a, a core waveguide layer 813 is deposited on a substrate 811, and the waveguide layer is patterned by some lithography technique to have a sharp end face. A mask 815 is then deposited that protects the waveguide except for a small length at the exposed end, as can be seen in Figure 8b, which shows a view down the axis of the waveguide, and Figure 8c shows a side view. The exposed waveguide is subjected to an etching process that rounds the sharp edge of the waveguide to provide a rounded edge 823, as can be seen in Figure 8d. By controlling the various etching parameters, the profile of the waveguide surface can be made approximately the desired shape, for example, a parabola with the desired measurements. Finally, the mask is removed, leaving a planar waveguide layer 821 and curved end 823 on top of the substrate, as can be seen in Figure 8e.

[0064] In some embodiments of an LED sealed in an optical waveguide, electrical connection is made from the top of the waveguide to a contact on the top of the LED through a via through the waveguide. Figure 9 An example electrical connection to an LED in a waveguide medium is shown. In Figure 9 In some embodiments, an LED includes an active layer 911. The LED, and especially the active layer, is in a waveguide medium 913. The top of the LED includes a conductive contact 915. A portion of the top contact is exposed by a via 917 in the waveguide medium. A metal trace 919 or layer extends down along a portion of the top of the waveguide medium and along one side of the via to contact the top contact. In another set of embodiments, the top contact of the LED is transparent and can comprise a conductive oxide, such as indium tin oxide (ITO). The transparent contact helps to spread the current from the small via contact point, and can protect the semiconductor material during fabrication. In some embodiments, the via is as small as possible and as close to the back of the LED (relative to the direction of propagation of the waveguide) as possible to minimize light loss. In some embodiments, the semiconductor material is GaN, and the top portion of the LED that is contacted by the ITO is p-doped GaN.

[0065] While the application has been discussed with respect to various embodiments, it will be understood that the application includes the novel and nonobvious claims supported by this disclosure.

Claims

1. An optical coupling device for an optical communication system, comprising: a micro-LED on a substrate, the micro-LED having an active layer; a waveguide on the substrate, the micro-LED at least partially within the waveguide; a reflector on at least a portion of the waveguide, the reflector positioned to reflect light from the micro-LED down an axis of the waveguide; and a lens within the waveguide, the lens having a material with a different index of refraction than a material of the waveguide, wherein the reflector is in an optical path between the micro-LED and the lens, the lens being a focal distance from the active layer of the micro-LED.

2. The optical coupling device of claim 1, wherein the active layer of the micro-LED is within the waveguide.

3. The optical coupling device of claim 1, wherein the lens is a convex lens.

4. The optical coupling device of claim 3, wherein the material of the lens has a higher index of refraction than the material of the waveguide.

5. The optical coupling device of claim 1, wherein the lens is a concave lens.

6. The optical coupling device of claim 5, wherein the material of the lens has a lower index of refraction than the material of the waveguide.

7. The optical coupling device of claim 1, wherein the reflector has a curved shape.

8. The optical coupling device of claim 7, wherein the reflector has a parabolic shape.

9. The optical coupling device of claim 7, wherein the reflector is at an end of the waveguide.

10. The optical coupling device of claim 1, wherein the micro-LED includes a curved surface.

11. The optical coupling device of claim 1, wherein the active layer is separated from a side edge of the micro-LED.

12. The optical coupling device of claim 11, wherein the micro-LED includes a curved surface.

13. The optical coupling device of claim 1, further comprising a metal trace from a top of the waveguide through a via in the waveguide to a contact on a top of the micro-LED.

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