The term Micro-Transfer Printing (μ-TP) designates the placement of small-scale semiconductor devices, typically with lateral dimensions below 100 microns and thicknesses of few microns called “platelets”, on a wide range of receiver substrates with or without integrated circuits. This transfer exercise relies on a mechanism called “reversible” or “kinetically controlled” adhesion, using a deformable viscoelastic elastomeric stamp (polydimethylsiloxane, PDMS) [1]. The adhesion of solid platelets, resting on a surface, to the stamp is dependent on the rate of retraction of the latter. It allows a strong adhesion with the platelets when retracted quickly and, after controlled relaxation, releases them on the desired receiver surface slowly. This technique has the potential to become a real “game-changer” for the semiconductor industry as it possesses the ability to combine different semiconductor materials together to realize real hybrid devices on a chip. Here resides a tremendous opportunity because this assembly at the micron-scale can also be realized between a variety of existing technologies.
In particular, µ-TP is very suitable to tackle the challenges of the Silicon photonic-electronic convergence which aims to integrate silicon electronic and optical circuits together on the same chip for high-speed and large capacity communications. Another application of μ-TP is in the realm of mechanically flexible and conformable electronics, displays, RFIDs or other wearable electronics consumer products. These applications are primarily envisioned with organic light-emitting diodes (OLEDs) with their attractive plastic and color rendering capabilities. Rather than a replacement, the well-established inorganic semiconductors could be integrated with OLEDs to harness the qualities of both technologies. Industry is recognizing the opportunity and, the recent acquisition by Apple of LuxVue Technology Corporation, a company in the US with a large patent portfolio in μ-TP, or Samsung with the first quantum dot full-color display made by transfer printing reported in 2011 [2], illustrate the importance and possible deployment of μ-TP.
The technique itself originates from “soft-lithography” used to print a large area of soft materials for flexible devices at low temperature and Prof. John Rogers from the University of Illinois at Urbana-Champaign in the US, pioneered its applicability to inorganic semiconductors and metals. A list of related publications is accessible on the research group’s home page [3] with distinctive demonstrations of flexible displays, state-of-the-art bendable biophotonic devices and broad exploration of this technology. The group also helped establish several companies such as Semprius in the US (solar cells) and more recently X-Celeprint Ltd. in Cork (Republic of Ireland) in collaboration with the Tyndall National Institute. With Seagate, they have demonstrated μ-TP of group III-V lasers on Silicon [4]. This phasing to industrial-scale has placement accuracy within ± 1.5 μm (3-sigma deviation) and with yields over 99% (application-specific). Now a number of university research groups and companies are also investigating this fabrication approach. For example, state-of-the-art positioning was demonstrated recently down to the nano-scale with side-by-side placements with spacing between gallium nitride light-emitting diodes of 150 nm ± 14 nm [5].
μ-TP demonstrates the required qualities to help address some of our present challenges in energy, communications and health. One can imagine its generalization as an assembly tool which in a near future could manipulate objects down to the nanometer scale.
Further reading
[1] Proceedings of the National Academy of Sciences 107, 17095-17100 (2010)
[2] Nature Photonics 5, 176–182 (2011)
[3] http://rogers.matse.illinois.edu
[4] Nature Photonics 6, 610–614 (2012)
[5] Applied Physics Letters 103, 253302 (2013)
Benoit Guilhabert is a Senior Research Fellow at the University of Strathclyde, Glasgow, UK
Antonio Trindade is a Ph. D student at the University of Strathclyde, Glasgow, UK