Micro-transfer printing
Moving finished, pre-tested device coupons (lasers, amplifiers, photodiodes) from their growth wafer onto a photonic wafer with an elastomer stamp, many at a time. A route to heterogeneous integration that places only the III-V material that is needed, where it is needed.
Silicon and silicon nitride circuits cannot generate or amplify light, so lasers and amplifiers made from III-V semiconductors must be joined to them. Heterogeneous integration by die or wafer bonding places an unpatterned III-V layer over the silicon wafer and processes the devices afterward, in the silicon fab. Micro-transfer printing takes the other order: the devices, or thin coupons of the epitaxial material, are defined on their native InP or GaAs wafer, released from it, and only then moved to the target.
The release is done by etching a sacrificial layer grown beneath the device layer, leaving each coupon suspended above the substrate by small tethers. A soft elastomer stamp (PDMS), patterned with posts that match the coupon positions, presses onto the source wafer; adhesion to an elastomer depends on peel speed, so a fast retraction breaks the tethers and lifts the coupons, and a slow retraction onto the target wafer leaves them behind (Meitl et al. 2006). One stamp can move an array of coupons in one step, and the source wafer is populated densely, so expensive III-V material is used far more efficiently than when a whole wafer area is bonded. The coupons are micrometers thick and tens to hundreds of micrometers long, with a thin adhesive or direct bonding to the target.
Two properties distinguish it from bonding and from flip-chip laser attach. Because coupons can be tested, or at least their material qualified, before printing, known-good devices can be selected; and because the target is a finished or nearly finished photonic wafer, the silicon process does not have to accept III-V processing steps. The cost is placement accuracy. Printers align to the order of a micrometer, so the coupling between a printed device and the waveguide below it must tolerate that offset; designs either make the coupon a gain section coupled evanescently through tapers that forgive lateral error, or print pre-made devices next to waveguides and rely on a coupling structure designed for the tolerance.
Demonstrations include lasers, amplifiers and photodiodes printed on silicon-on-insulator and on silicon nitride platforms, and narrow-linewidth tunable lasers built from a printed gain coupon and a nitride external cavity. The technique's case in production rests on wafer-scale throughput and yield, which the reviews treat as the open questions alongside thermal management of printed lasers.
References: M. A. Meitl et al., Nat. Mater. 5, 33 (2006); J. Zhang et al., APL Photonics 4, 110803 (2019); G. Roelkens et al., APL Photonics 9, 010901 (2024).