CROSS-REFERENCE TO RELATED APPLICATION(S)

[0001] The present application claims priority to and the benefit of U.S. Provisional Application No. 63/380,361 , filed October 20, 2022, entitled "METHOD OF MICROTRANSFER PRINTING", the entire content of which is incorporated herein by reference.

FIELD

[0002] One or more aspects of embodiments according to the present disclosure relate to micro-transfer printing, and more particularly to a system and method for alignment in micro-transfer printing.

BACKGROUND

[0003] Micro-transfer printing may be employed to place and bond one or more semiconductor chips of a first kind onto, or into cavities in, a semiconductor chip of a second kind. For example, a lll-V semiconductor chip including a waveguide with a gain medium may be placed and bonded into a cavity in a silicon photonic integrated circuit, such that light generated by the gain medium is coupled into a silicon waveguide on the silicon photonic integrated circuit.

[0004] It is with respect to this general technical environment that aspects of the present disclosure are related.

SUMMARY

[0005] According to an embodiment of the present disclosure, there is provided a method of micro-transfer printing, including steps of: optically aligning a transfer stamp with a platform wafer, the transfer stamp including a device coupon for bonding to the platform wafer, the device coupon including a device coupon photonic device; applying a lateral force to the transfer stamp, such that a lateral surface of the device coupon engages with a mechanical alignment feature located in the platform wafer and such that the device coupon is aligned relative to the platform wafer; and applying a bonding force to the transfer stamp, such that the device coupon bonds to the platform wafer. [0006] In some embodiments, the method includes a step, after optically aligning the transfer stamp and before applying the lateral force, of moving the transfer stamp relative to the platform wafer so as to reduce a distance therebetween.

[0007] In some embodiments, the distance is reduced such that the device coupon contacts the platform wafer.

[0008] In some embodiments, the distance is reduced such that a gap exists between the device coupon and the platform wafer.

[0009] In some embodiments, the device coupon has a first axis and a second axis, and when applying the lateral force the device coupon engages two mechanical alignment features along the first axis and one mechanical alignment feature along the second axis.

[0010] In some embodiments, wherein the platform wafer includes a trench in a surface thereof, and after applying the bonding force to the transfer stamp, the device coupon overhangs the trench.

[0011] In some embodiments: the platform wafer includes two mechanical alignment features, each having an angled lateral surface relative to the other; and the device coupon is positioned between the two mechanical alignment features, and the lateral force is applied such that two angled lateral surfaces of the device coupon contact respective lateral surfaces of the mechanical alignment features.

[0012] In some embodiments, the mechanical alignment feature is a previously bonded device coupon, such that the device coupon contained on the transfer stamp is engaged with a lateral surface of the previously bonded device coupon.

[0013] According to an embodiment of the present disclosure, there is provided a platform wafer, the platform wafer being suitable for use in a micro-transfer printing process, the platform wafer including: a bonding site for a device coupon; and a mechanical alignment feature, suitable for alignment between the device coupon and the respective bonding site by contact between one or more mechanical alignment features of the device coupon and the mechanical alignment feature.

[0014] In some embodiments, the platform wafer further includes an optical waveguide, wherein the contact between the mechanical alignment feature and the one or more mechanical alignment features of the device coupon causes alignment between the optical waveguide and a corresponding optical waveguide on the device coupon.

[0015] In some embodiments, the bonding site is a cavity with a bed surrounded by one or more sidewalls, and the mechanical alignment feature is a protrusion protruding into the cavity.

[0016] In some embodiments, the protrusion extends from one of the sidewalls of the cavity laterally into the cavity.

[0017] In some embodiments: the mechanical alignment feature is a first mechanical alignment feature; the first mechanical alignment feature is a first protrusion extending from a first sidewall of the cavity laterally into the cavity; the platform wafer further includes a second mechanical alignment feature; and the second mechanical alignment feature is a second protrusion extending from a second sidewall of the cavity laterally into the cavity.

[0018] In some embodiments, the protrusion extends from the bed of the cavity away from the bed of the cavity so as to be engageable with an internal or external mechanical alignment feature of the device coupon.

[0019] In some embodiments, the platform wafer has at least two mechanical alignment features, each having an angled lateral side relative to the other.

[0020] According to an embodiment of the present disclosure, there is provided a device coupon, containing a photonic device, and for use in a micro-transfer printing process, the device coupon containing: a bonding surface, for bonding to a corresponding bonding surface on a platform wafer; and a mechanical alignment feature, for engagement with a corresponding mechanical alignment feature on the platform wafer, so as to allow alignment between the device coupon and the platform wafer.

[0021] In some embodiments, the mechanical alignment feature is a cavity extending at least partially, or entirely, through the device coupon, wherein the cavity extends from a bonding surface of the device coupon towards a surface distalmost from the bonding surface.

[0022] In some embodiments, the mechanical alignment feature is a protrusion.

[0023] In some embodiments, the device coupon includes a plurality of tethers arranged around a periphery of the device coupon, and wherein the mechanical alignment feature is provided by a gap between two tethers which is larger than a gap between two other tethers.

[0024] In some embodiments, the device coupon has: a first flat mechanical alignment feature, and a second flat mechanical alignment feature, oblique to the first flat mechanical alignment feature.

[0025] According to an embodiment of the present disclosure, there is provided a method of micro-transfer printing, including steps of: optically aligning a transfer stamp with a platform wafer, the transfer stamp including one or more device coupons for bonding to the platform wafer, the or each device coupon including a device coupon photonic device; applying a lateral force to the transfer stamp, such that one or more lateral surfaces of the device coupon(s) engage with one or more mechanical alignment features located in the platform wafer so as to align the device coupon relative to the platform wafer; and applying a bonding force to the transfer stamp, such that the device coupon(s) bond to the platform wafer.

[0026] In some embodiments, the method includes a step, after optically aligning the transfer stamp and before applying the lateral force, of moving the transfer stamp relative to the platform wafer so as to reduce a distance therebetween.

[0027] In some embodiments, the distance is reduced such that the one or each device coupon contacts the platform wafer.

[0028] In some embodiments, the distance is reduced such that a gap exists between the one or each device coupon and the platform wafer.

[0029] In some embodiments, the platform wafer includes a platform photonic device, and applying the lateral force to the transfer stamp causes alignment between the platform photonic device and the device coupon photonic device.

[0030] In some embodiments, the platform photonic device and the device coupon photonic device each include a waveguide.

[0031] In some embodiments, the device coupon has a first axis and a second axis, and when applying the lateral force the device coupon engages two mechanical alignment features along its first axis and one mechanical alignment feature along its second axis. [0032] In some embodiments, the platform wafer includes a trench etched into a surface thereof, and after applying the bonding force to the transfer stamp, the device coupon(s) overhang the trench.

[0033] In some embodiments, there are at least two mechanical alignment features, each having an angled lateral surface relative to the other, for example both having a triangular shape, and at least a part of the device coupon has a trapezoidal shape.

[0034] In some embodiments, the device coupon is positioned between the two mechanical alignment features, and the lateral force is applied such that two lateral sides of the device coupon contact respective lateral sides of the mechanical alignment features.

[0035] In some embodiments, one or more of the mechanical alignment features is a previously bonded device coupon, such that the device coupon contained on the transfer stamp is engaged with one or more lateral surfaces of the previously bonded device coupon.

[0036] In some embodiments, the device coupon contains a plurality of waveguides, and wherein applying the lateral force to align the device coupon aligns each of the plurality of waveguides relative to the platform wafer.

[0037] According to an embodiment of the present disclosure, there is provided a platform wafer, the platform wafer suitable for use in a micro-transfer printing process, the platform wafer including: one or more bonding sites for respective device coupons; and one or more mechanical alignment features, suitable for alignment between the device coupons and the respective bonding site by contact between one or more mechanical alignment features of the device coupon and the mechanical alignment feature(s).

[0038] In some embodiments, the bonding site is a cavity with a bed surrounded by one or more sidewalls, and the or each mechanical alignment feature is a protrusion protruding into the cavity.

[0039] In some embodiments, the protrusions extend from one or more of the sidewalls of the cavity laterally into the cavity. [0040] In some embodiments, the protrusions extend from the bed of the cavity away from the bed of the cavity so as to be engageable with one or more internal or external mechanical alignment features of the device coupon.

[0041] In some embodiments, the bonding site is a cavity with a bed surrounded by one or more sidewalls, and the or each mechanical alignment feature is a recess recessed from the cavity.

[0042] In some embodiments, the or each recess is provided in one or more of the sidewalls of the cavity extending away from the cavity.

[0043] In some embodiments, the or each recess is provided in the bed of the cavity away from the bed of the cavity.

[0044] In some embodiments, a bonding site of the one or more bonding sites is a cavity with a bed surrounded by one or more sidewalls, and the platform wafer includes a trench etched into a part of the bonding site adjacent to a sidewall of the one or more sidewalls.

[0045] In some embodiments, the platform wafer has at least two mechanical alignment features, each having an angled lateral surface relative to the other, for example each having a triangular shape, wherein optionally the space between the two triangular shaped mechanical alignment features provides a trapezoidal cavity into which a trapezoidal shaped portion of a device coupon can be placed.

[0046] According to an embodiment of the present disclosure, there is provided a device coupon, including a photonic device, and for use in a micro-transfer printing process, the device coupon including: a bonding surface, for bonding to a corresponding bonding surface on a platform wafer; and one or more mechanical alignment features, for engagement with corresponding mechanical alignment features on the platform wafer, so as to allow alignment between the device coupon and the platform wafer.

[0047] In some embodiments, the mechanical alignment features are one or more cavities extending at least partially, or entirely, through the device coupon, wherein the cavities optionally extend from a bonding surface of the device coupon towards a surface distalmost from the bonding surface. [0048] In some embodiments, the mechanical alignment features are one or more protrusions which optionally extend from a bonding surface of the device coupon away from the device coupon.

[0049] In some embodiments, the device coupon includes a plurality of tethers arranged around a periphery of the device coupon, and wherein the mechanical alignment feature is provided by a gap between two tethers which is larger than a corresponding gap between two other tethers.

[0050] In some embodiments, at least a part of the device coupon has a trapezoidal geometry, for example a trapezoidal footprint or periphery.

[0051] In some embodiments, the device coupon includes one or more lll-V semiconductor-based devices, for example one or more waveguides.

[0052] According to an embodiment of the present disclosure, there is provided a device, including a device coupon bonded to a platform wafer.

[0053] According to an embodiment of the present disclosure, there is provided a method of micro-transfer printing, including steps of: optically aligning a transfer stamp with a platform, the transfer stamp including a device coupon; applying a lateral force to the transfer stamp, such that one or more lateral surfaces of the device coupon(s) engage with one or more mechanical alignment features located in the platform wafer so as to align the device coupon relative to the platform wafer; and applying a bonding force to the transfer stamp, such that the device coupon(s) bond to the platform wafer.

[0054] According to an embodiment of the present disclosure, there is provided a device, including: a platform wafer, including a first photonic device and a device coupon, including a second photonic device coupled to the first photonic device; wherein the device coupon is bonded to a bonding surface of the platform wafer, and wherein one or both of the platform wafer and the device coupon contains a mechanical alignment feature.

[0055] In some embodiments, the device coupon and platform wafer contain respective mechanical alignment features.

[0056] In some embodiments, the device coupon mechanical alignment feature is a cavity extending at least partially through the device coupon, and the platform wafer mechanical alignment feature is a protrusion extending from the bonding surface of the platform wafer at least partially through the cavity of the device coupon.

[0057] In some embodiments, the protrusion is shorter in height, as measured perpendicularly to the bonding surface, than the cavity into which it protrudes.

[0058] In some embodiments, the platform wafer mechanical alignment feature is a cavity in a bed of the platform wafer extending away from the device coupon, and the device coupon locking feature is a protrusion extending from the device coupon into the cavity of the bed.

[0059] In some embodiments, the platform wafer mechanical alignment feature provides a pair of angled lateral surfaces relative to one another, for example by including a pair of triangular regions, and at least a part of the device coupon is trapezoidal in shape.

[0060] In some embodiments, the first photonic device and second photonic device are lll-V semiconductor-based devices, for example a first waveguide and a second waveguide couple to one another.

[0061] In some embodiments, the bonding surface of the platform wafer is provided by a bed of a cavity, the cavity having one or more sidewalls, wherein there is a trench etched int the bed of the cavity proximal or adjacent to the sidewall, and the device coupon overhangs the trench.

BRIEF DESCRIPTION OF THE DRAWINGS

[0062] These and other features and advantages of the present disclosure will be appreciated and understood with reference to the specification, claims, and appended drawings wherein:

[0063] FIG. 1 is a schematic top view of a platform wafer and a plurality of device coupons, according to an embodiment of the present disclosure;

[0064] FIG. 2A is a schematic top view of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0065] FIG. 2B is a schematic top view of a platform wafer and a device coupon, according to an embodiment of the present disclosure; [0066] FIG. 3A is a schematic cross-sectional side view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0067] FIG. 3B is a schematic cross-sectional side view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0068] FIG. 3C is a schematic cross-sectional side view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0069] FIG. 4A is a schematic cross-sectional side view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0070] FIG. 4B is a schematic cross-sectional side view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0071] FIG. 5A is an electron micrograph of a section of a portion of a platform wafer, according to an embodiment of the present disclosure;

[0072] FIG. 5B is an electron micrograph of a section of a portion of a platform wafer, with a schematic drawing of a device coupon superimposed on it, according to an embodiment of the present disclosure;

[0073] FIG. 6A is a schematic cross-sectional side view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0074] FIG. 6B is a schematic top view of a portion of a platform wafer and two device coupons, according to an embodiment of the present disclosure;

[0075] FIG. 6C is a schematic top view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0076] FIG. 6D is a schematic top view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0077] FIG. 6E is a schematic top view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0078] FIG. 6F is a schematic top view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0079] FIG. 6G is a schematic top view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure;

[0080] FIG. 6H is a schematic cross-sectional top view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure; and [0081] FIG. 6I is a schematic cross-sectional side view of a portion of a platform wafer and a device coupon, according to an embodiment of the present disclosure.

DETAILED DESCRIPTION

[0082] The detailed description set forth below in connection with the appended drawings is intended as a description of exemplary embodiments of a system and method for assembly provided in accordance with the present disclosure and is not intended to represent the only forms in which the present disclosure may be constructed or utilized. The description sets forth the features of the present disclosure in connection with the illustrated embodiments. It is to be understood, however, that the same or equivalent functions and structures may be accomplished by different embodiments that are also intended to be encompassed within the scope of the disclosure. As denoted elsewhere herein, like element numbers are intended to indicate like elements or features.

[0083] Micro-transfer printing is a process that may be employed to construct silicon photonic devices. For example, a silicon photonic integrated circuit (PIC) may be fabricated by forming waveguides or other passive structures (e.g., optical filters, or multiplexers, such as arrayed waveguide gratings, or echelle gratings) on a silicon-on- insulator (SOI) wafer. The silicon photonic integrated circuit may also include active devices, such as photodiodes. Certain active devices, such as optical amplifiers, may be more readily fabricated on lll-V semiconductor wafers than on silicon wafers. In some embodiments, one or more such lll-V devices are fabricated on lll-V chips referred to as device coupons and then bonded into cavities (referred to as bonding sites) on a silicon photonic integrated circuit. The device coupon, once bonded into the bonding site, may provide an optical function that may not otherwise be readily available on a silicon photonic integrated circuit. For example, the device coupon may include a device coupon photonic device such as an amplifying waveguide (e.g., a semiconductor optical amplifier (SOA), or a reflective semiconductor optical amplifier (RSOA), or a laser); in operation the light from the amplifying waveguide may be processed on the silicon photonic integrated circuit (e.g., the light from the amplifying waveguide may be combined, in an echelle grating on the silicon photonic integrated circuit, with light from other amplifying waveguides, which may be on the same device coupon or on a separate device coupon (bonded into a separate bonding site). If the amplifying waveguide is a semiconductor optical amplifier (SOA), or a reflective semiconductor optical amplifier (RSOA) then a laser may be formed that has the gain medium on the device coupon and (in the case of the amplifying waveguide being a reflective semiconductor optical amplifier) one reflector or (in the case of the amplifying waveguide being a (non-reflective) semiconductor optical amplifier) both reflectors on the silicon photonic integrated circuit.

[0084] The device coupons may be fabricated in a batch on a suitable wafer (e.g., a lll-V wafer), which may be referred to as the donor wafer. After a release layer is etched away from under each device coupon, each device coupon may be held in position by a plurality of tethers. One or more of the device coupons may be picked up by a transfer stamp; during this process the tethers securing the device coupons to the donor wafer may be broken. The transfer stamp may then carry the device coupons to the silicon photonic integrated circuit (which may, in the context of micro-transfer printing, be referred to as the platform wafer), and insert them (e.g., simultaneously or concurrently) into the respective bonding sites. An alignment process (e.g., an optical alignment process) may be used to align the device coupons to the bonding sites, and each device coupon may be bonded into a respective bonding site. The alignment may have the effect, for example, of aligning each of one or more waveguide facets on each of the device coupons with a respective waveguide facet on the silicon photonic integrated circuit.

[0085] Micro-transfer printing (MTP), like flip-chip bonding, may rely on visual markers (“fiducials”) on the target chip (e.g., the platform wafer) and on the device coupons being bonded. Image processing may be used to identify the relative position of the fiducials to determine how to adjust the position of the device coupons relative to the target, in order to place the device coupons being bonded accurately on the platform wafer. In a well optimized system, relative alignment may be the part of the micro-transfer printing print cycle that takes the longest, and therefore the one where there is the most potential to improve the cycle time.

[0086] FIG. 1 shows a platform wafer 105 with a plurality of bonding sites 110, and a plurality of amplifying waveguides 115, in groups of eight. Each group of eight amplifying waveguides 115 may be on a respective one of a plurality of device coupons (e.g., 12 device coupons, as illustrated in FIG. 1 (the outline of each device coupon is not shown, in FIG. 1 , for ease of illustration)). When optical alignment is used, the limitation of the camera’s field of view may result in a certain level of uncertainty in the rotational alignment, which may, as illustrated in FIG. 1 , translate into positioning errors that are greater for device coupons that are more distant from the rotation reference point 120. This component of the alignment error may be a significant issue for large arrays. The use of large arrays may be advantageous, however, for achieving high micro-transfer printing throughput.

[0087] As such, in some embodiments, mechanical alignment features are employed to reduce the time spent aligning the device coupons to the target, and to increase the resilience of large arrays to rotational errors. Such mechanical alignment features may be suitable for removing the rotational error with the level of accuracy desired for the largest arrays. The use of mechanical alignment features may also result in a reduction of 30% in the bonder cycle time (compared to a cycle time achievable using optical alignment) and therefore be a net contributor to throughput increase.

[0088] FIGs. 2A and 2B illustrate the use of mechanical alignment features 205, in some embodiments. In FIG. 2A, the device coupon 200 is substantially rectangular, with a long axis (parallel to the Z axis of the coordinate system shown) and a short axis (parallel to the X axis of the coordinate system shown). The device coupon 200 has two mechanical alignment features 205 along its longitudinal axis and one mechanical alignment feature along its short axis. One of these two mechanical alignment features 205 is a portion of a lateral surface (or “lateral side”, or edge) of the device coupon 200, the portion being devoid of tethers 210 (i.e., there being a gap between the tethers) which otherwise might interfere with (e.g., make contact with) the corresponding mechanical alignment feature 205 on the platform wafer 105. Each of the mechanical alignment features 205 of the platform wafer 105 may be a protrusion extending horizontally into the cavity that defines the bonding site 110 (where the vertical direction is the Y direction illustrated). These protrusions may (i) have upper surfaces that are flush with the upper surface of the surrounding portion of the platform wafer 105, and may (ii) have been formed by having been masked during an etch step that formed the cavity of the bonding site 110 in the platform wafer 105.

[0089] The bonding site 110 may also include relief, or “undercut”, features, including (i) an inside corner relief 215 (to avoid corner contact which might otherwise occur, e.g., if the radius of the inside corner of the cavity that forms the bonding site 110 is greater than the radius of the corresponding outside corner of the device coupon 200) and (ii) a tether relief 220 (to avoid contact which might otherwise occur between a wall of the cavity that forms the bonding site 110 and some of the tethers 210).

[0090] FIG. 2A shows the position of the device coupon 200 in the bonding site 110 before it is aligned (having been coarsely positioned, e.g., using optical alignment). After the coarse alignment is performed, the transfer stamp may be moved (e.g., vertically) so as to reduce a distance between the device coupon 200 and the platform wafer 105 (e.g., so as to reduce the height of a gap between the bottom surface of the device coupon 200 and the bottom (or “bed”) of the cavity that forms the bonding site 110. In some embodiments, this distance is reduced to zero, e.g., the bottom surface of the device coupon 200 is brought into contact with the bed of the cavity that forms the bonding site 110; in other embodiments a small gap remains. A lateral (e.g., horizontal) force may then be applied to the device coupon 200 to move it horizontally (e.g., sliding the device coupon 200 along the bed of the cavity that forms the bonding site 110, if the device coupon 200 is in contact with the bed of the cavity). FIG. 2B shows the position of the device coupon 200 in the bonding site 110 after it has been aligned, by being displaced horizontally (as shown by the arrows) until each of the three mechanical alignment features 205 of the device coupon 200 abuts against a corresponding one of the three mechanical alignment features 205 of the platform wafer 105. In the aligned position, the waveguide of the device coupon 200 (e.g., an amplifying waveguide 115) is aligned with a corresponding waveguide 225 of the platform wafer 105 (e.g., a facet of the waveguide 115 of the device coupon 200 is aligned with a facet of the waveguide 225 of the platform wafer 105, which may be a silicon photonic integrated circuit). As such, the contact between each of the mechanical alignment features 205 of the platform wafer 105 and a corresponding mechanical alignment feature 205 of the device coupon 200 causes alignment between the waveguides 115, 225. As used herein, each of the mechanical alignment features 205 may be said to “cause” such alignment even if it is not the sole cause, e.g., if one or more other mechanical alignment features 205 also participate in achieving the desired alignment.

[0091] Once the device coupon 200 has been aligned (e.g., by the application of a lateral (e.g., horizontal) force that causes the mechanical alignment features 205 of the device coupon 200 to abut against the corresponding mechanical alignment features 205 of the platform wafer 105), a bonding force may be applied to the transfer stamp and (as a result) to the device coupon 200 (e.g., a vertical force, pushing the device coupon 200 against the bed of the cavity that forms the bonding site 110, to cause the device coupon 200 to bond to the platform wafer 105).

[0092] FIGs. 3A - 3C show an assembly process in some embodiments. FIG. 3A shows the device coupon 200 in the cavity that forms the bonding site 110, in the position that it may be in after having been coarsely positioned using, e.g., optical alignment, and moved (e.g., lowered) into the cavity. The device coupon 200 includes an amplifying waveguide 115 including a multiple quantum well (MQW) gain medium and a waveguide ridge. It also includes contacts 305 through which electric current may be supplied (e.g., via wire bond connections made to the contacts 305) to the gain medium. The bed of the cavity that forms the bonding site 110 includes a layer of adhesive 310 for bonding the device coupon 200 into the bonding site 110 once it has been aligned. The adhesive 310 does not extend all the way to the edge of the cavity that forms the bonding site 110; the gap between the edge of the adhesive 310 and the edge of the cavity forms a relief channel 315 to avoid contact between the device coupon 200 and, e.g., a meniscus that may be present in the inside corner of the cavity or a meniscus formed by the adhesive 310.

[0093] FIG. 3B shows the position of the device coupon 200 after it has been moved laterally until one or more mechanical alignment features 205 of the device coupon 200 abut against one or more mechanical alignment features 205 of the platform wafer 105, and FIG. 3C shows the position of the device coupon 200 after the application of a (vertical) bonding force causing the device coupon 200 to bond to the bonding site 110 of the platform wafer 105. In the view of FIGs. 3A - 3C, the platform wafer 105 is a silicon on insulator wafer including a silicon substrate, a buried oxide (e.g., SiO2) (BOX) layer on the silicon substrate, and a silicon (e.g., crystalline silicon) device layer (or “silicon on insulator layer” (SOI layer)) on the buried oxide layer. The amplifying waveguide 115 of the device coupon 200 may emit light in the Z direction. The platform wafer 105 may include a waveguide in the device layer, also extending along the Z direction (not visible in the cross-sectional views of FIGs. 3A - 3C because it does not extend through the cutting plane) that may, after alignment and bonding of the device coupon 200, be aligned with the amplifying waveguide 115 of the device coupon 200.

[0094] FIG. 4A shows an embodiment in which the device coupon 200 includes a vertical cavity surface emitting laser (VCSEL). FIG. 4B shows a similar embodiment, with a relief channel 315 to avoid contact, between the device coupon 200 and the platform wafer 105 that in the absence of the relief channel 315 might occur at the edge at which the wall of the cavity meets the bed of the cavity (e.g., if a meniscus is present that causes the radius of this edge of the cavity to be greater than the radius of the corresponding edge of the device coupon 200).

[0095] FIG. 5A is an electron micrograph of a sectioned platform wafer 105, showing a cavity with a relief channel 315 including a shallow portion 505 adjacent to the wall of the cavity and a deeper portion 510 set back about 4 microns from the wall of the cavity. The bed of the cavity may be composed of benzocyclobutene (BCB), which may be observed, in FIGs. 5A and 5B, to form a meniscus in the deeper portion 510 of the relief channel 315. The deeper portion 510 of the relief channel 315 may have a depth of between 2 microns and 4 microns, and a width of between 5 microns and 8 microns. FIG. 5B is the electron micrograph of FIG. 5A with a drawing of the device coupon 200 superimposed on it, showing that when the device coupon 200 is installed in the platform wafer 105, the bottom surface of the device coupon 200 is above both the bottom of the shallow portion 505 of the relief channel 315 and the bottom of the deeper portion 510 of the relief channel 315. FIGs. 5A and 5B also show cleaving debris that is not relevant for the present disclosure.

[0096] The use of mechanical alignment features 205 may be versatile with respect to the characteristics of the device coupons 200 and of the platform wafer 105. A method using mechanical alignment features 205 may be used to align and bond device coupons 200 including edge-coupled devices or surface-coupled devices. The performance of an alignment method using mechanical alignment features 205 may be independent of die size and therefore independent of the number of (e.g., optical) functions on each device coupon 200; for example, each device coupon 200 may include a single amplifying waveguide 115, or “laser ridge” (e.g., a single reflective semiconductor optical amplifier (RSOA)) or, e.g., eight laser ridges (as illustrated in FIG. 1 ).

[0097] The mechanical alignment features 205 may connect to inner surfaces of the device coupon 200. The mechanical alignment features 205 may be or include recesses, e.g., as illustrated in FIG. 6A, trenches 605 in the platform wafer 105, and one or more protrusions 610 at the bottom of each device coupon 200 that fit into the trenches. Each bottom protrusion 610 may be formed on the bottom of the device coupon 200, portions of which may be covered, in the donor wafer, by the release material that is etched away upon release of the device coupon 200 from the donor wafer. As such, each protrusion 610 may be composed, or composed in part, of a different material, such as BCB, that is not etched away during release, to ensure the survival of the protrusion 610 through the release process. In some embodiments the survival of the protrusion may instead be arranged by walling off this area of the device coupon 200 so that the etchant does not reach the protrusion. In other embodiments a mechanical alignment feature 205 may be a recess in a side wall of the cavity that forms the bonding site 110, and the corresponding mechanical alignment feature 205 on the device coupon 200 may be a protrusion extending from a lateral surface of the device coupon 200.

[0098] Referring to FIG. 6B, a mechanical alignment features 205 for a device coupon 200 may be another device coupon 615, when a method that may be referred to as cascaded alignment is employed. Such a configuration may be employed, for example, to construct a system including a distributed feedback (DFB) laser coupled to an electroabsorption modulator (EAM), which is in turn coupled to a waveguide 225 on a silicon photonic integrated circuit. In such a system, the compression of the electro-absorption modulator may help to ensure a zero-width or near zero-width gap between the different interfaces, improving the optical coupling efficiency.

[0099] As illustrated in FIG. 6C, angled mechanical alignment features 205 in the platform wafer 105 may be used to align a device coupon 200 by wedging the device coupon 200 between the angled mechanical alignment features 205. Such a configuration may be advantageous for aligning a device coupon 200 that includes one or more linear, two-port optical components (e.g., optical amplifiers, optical modulators, or photodetectors). The geometry of the device coupon 200 and of the platform wafer 105 may be designed so that the final position of the device coupon 200 enables coupling to ports (e.g., waveguide facets) that are on different sides of the device coupon 200. [00100] FIG. 6D shows a similar configuration with two mechanical alignment features 205 each having an angled lateral side relative to the other. In FIG. 6D, one of the mechanical alignment features 205 is perpendicular to the amplifying waveguide 115 and to the corresponding waveguides 225 of the platform wafer 105, and the other mechanical alignment feature 205 is oblique to the amplifying waveguide 115 and to the corresponding waveguides 225 of the platform wafer 105. FIG. 6E shows another embodiment in which one mechanical alignment feature 205 is perpendicular to the amplifying waveguide 115 and to the corresponding waveguides 225 of the platform wafer 105 (this mechanical alignment feature 205 may be positioned such that when the device coupon 200 makes contact with it, a small gap remains between the waveguides 115, 225, as shown), and the other mechanical alignment feature 205 is oblique to the amplifying waveguide 115 and to the corresponding waveguides 225 of the platform wafer 105. In FIG. 6E (and in several of the other drawings), several elements are labeled with the reference symbol 105; these elements (which may be, e.g., protrusions within, or side walls of, the cavity that forms the bonding site 110) may all be parts of a single platform wafer 105, which appear disconnected in some of the drawings (e.g., in FIG. 6E) because not all of the platform wafer 105 is illustrated.

[00101] A device coupon 200 may contain several optical functions. For example, a device coupon 200 may include nine lasers (or eight lasers, as illustrated in FIG. 6F, or in FIG. 6G), each connected, after installation of the device coupon 200, to a different waveguide of the platform wafer 105.

[00102] In some embodiments (as mentioned above), instead of fully relying on fiducials for alignment, only a coarse alignment is performed using visual processing. After that, the device coupons 200 are lowered down to hover over, or lightly touch, the floors of the respective cavities in the platform wafer 105. A lateral movement is then performed that brings the device coupons 200 into contact with physical features (mechanical alignment features 205) of the platform wafer 105 that block the device coupon 200 from moving further in the lateral direction. There may be multiple cavities and multiple device coupons on a single MTP stamp, and these multiple device coupons may be aligned simultaneously using mechanical stops that are in each cavity and protrude in orthogonal directions so as to simultaneously align all of the device coupons in one motion even if rotation and position errors result in increasingly larger misalignment errors for sites farther out radially from the center of the array of cavities and corresponding device coupons.

[00103] The elasticity of the stamp allows the stamp to deform while the device coupon 200 stays at its target spot. By doing so, the combination of the lateral movement and the stamp elasticity compensate for alignment uncertainty: every device coupon 200 may be pushed as far as it can go against the mechanical alignment features 205 and stays in this position until the stamp is displaced laterally to the extent that each device coupon 200 of the array of device coupons 200 is abutting against one or more respective mechanical alignment features 205. The device coupons 200 and the stamp may then be pressed down to bond the device coupons 200 to the platform wafer 105.

[00104] Both on the bonded device coupon 200 and on the platform wafer 105, the mechanical alignment features 205 on the platform wafer 105 and their counterpart surfaces on the device coupon 200 are positioned so that when in contact in the final position, the active function of the device coupon 200 is in (or near) the correct position for interfacing with the platform wafer 105; for example, in this position, a facet of a laser on the device coupon 200 may be aligned with a waveguide on the platform wafer 105. Mechanical alignment features may be advantageous when a large number of devices are to be printed simultaneously, be it arrays of devices side by side in the same print cavity, or multiple dies across one - or several - reticles. In such a scenario placement accuracy may be at significant risk when optical alignment is used, particularly as a result of any alignment error in rotation.

[00105] In some embodiments, the mechanical alignment features 205 are purpose- built features of the platform wafer 105. In some embodiments, existing features (features that serve other functions on the device coupon 200 or on the platform wafer 105) are used as mechanical alignment features 205. For example, in the case of edge- coupled devices, a waveguide facet (e.g., a silicon waveguide facet) may be a mechanical alignment feature against which a counterpart waveguide facet (e.g., a lll-V waveguide facet) on the device coupon 200 is pressed.

[00106] As illustrated in FIGs 6H and 6I, the mechanical alignment features 205 on the device coupon 200 need not be on an outer edge of the device coupon 200. For example, a cutout may be formed in the device coupon 200 (for example, in the middle of the device coupon 200) and a mechanical alignment feature of the platform wafer 105 may extend into or through the cutout and, when the device coupon 200 is correctly aligned, the mechanical alignment feature of the platform wafer 105 may abut against one or more sidewalls of the cutout. Such a configuration is space-efficient but relies on sufficient pre-bond accuracy that the mechanical alignment feature extends into or through the cutout and does not collide with the bottom of the device coupon 200.

[00107] As mentioned above, mechanical alignment features 205 may be used for device coupons 200 with edge-coupled devices; such devices may include, e.g., edge- coupled lasers, inline photodiodes, or electro-absorption modulators (EAMs). Mechanical alignment features may also be used, however, for device coupons 200 with other (e.g., surface-coupled) devices for which the array placement may require some level of accuracy, such as top-side emitting devices such as vertical cavity surface emitting lasers (VCSELs) (as illustrated, for example, in FIGs. 4A and 4B), or pixels of sensor arrays.

[00108] As used herein, “wafer” means a full semiconductor wafer or any portion thereof. As such a semiconductor chip may be referred to as a wafer, and a “platform wafer” may be a semiconductor chip.

[00109] As used herein, “a portion of” something means “at least some of” the thing, and as such may mean less than all of, or all of, the thing. As such, “a portion of” a thing includes the entire thing as a special case, i.e., the entire thing is an example of a portion of the thing. As used herein, when a second quantity is “within Y” of a first quantity X, it means that the second quantity is at least X-Y and the second quantity is at most X+Y. As used herein, when a second number is “within Y%” of a first number, it means that the second number is at least (1 -Y/100) times the first number and the second number is at most (1 +Y/100) times the first number. As used herein, the word “or” is inclusive, so that, for example, “A or B” means any one of (i) A, (ii) B, and (iii) A and B.

[00110] As used herein, when a method (e.g., an adjustment) or a first quantity (e.g., a first variable) is referred to as being “based on” a second quantity (e.g., a second variable) it means that the second quantity is an input to the method or influences the first quantity, e.g., the second quantity may be an input (e.g., the only input, or one of several inputs) to a function that calculates the first quantity, or the first quantity may be equal to the second quantity, or the first quantity may be the same as (e.g., stored at the same location or locations in memory as) the second quantity.

[00111] The terminology used herein is for the purpose of describing particular embodiments only and is not intended to be limiting of the inventive concept. As used herein, the terms “substantially,” “about,” and similar terms are used as terms of approximation and not as terms of degree, and are intended to account for the inherent deviations in measured or calculated values that would be recognized by those of ordinary skill in the art.

[00112] Any numerical range recited herein is intended to include all sub-ranges of the same numerical precision subsumed within the recited range. For example, a range of "1.0 to 10.0" or “between 1.0 and 10.0” is intended to include all subranges between (and including) the recited minimum value of 1 .0 and the recited maximum value of 10.0, that is, having a minimum value equal to or greater than 1 .0 and a maximum value equal to or less than 10.0, such as, for example, 2.4 to 7.6. Similarly, a range described as “within 35% of 10” is intended to include all subranges between (and including) the recited minimum value of 6.5 (i.e. , (1 - 35/100) times 10) and the recited maximum value of 13.5 (i.e., (1 + 35/100) times 10), that is, having a minimum value equal to or greater than 6.5 and a maximum value equal to or less than 13.5, such as, for example, 7.4 to 10.6. Any maximum numerical limitation recited herein is intended to include all lower numerical limitations subsumed therein and any minimum numerical limitation recited in this specification is intended to include all higher numerical limitations subsumed therein. [00113] Although exemplary embodiments of a system and method for assembly have been specifically described and illustrated herein, many modifications and variations will be apparent to those skilled in the art. Accordingly, it is to be understood that a system and method for assembly constructed according to principles of this disclosure may be embodied other than as specifically described herein. The invention is also defined in the following claims, and equivalents thereof.