By Dominic Siriani
Taking a look around an airport, bus station, or waiting room, you’re likely to notice a few differences between the scenes now as compared to those of ten or so years ago. One thing that might strike you is that the papery time killers that filled lounging areas from the past are now replaced with personal electronic devices. Thanks to a great number of innovations, including the sophistication of LCDs, LEDs, electronic ink, and microprocessors, portable devices for reading purposes and beyond are becoming ubiquitous.
However, you might notice another striking difference between then and now with respect to these typeface transporters. It’s probably a tiny daily burden you’ve learned to cope with over the years: you can’t exactly roll up your electronic device like your morning newspaper. And I wouldn’t recommend trying it, but go for it if you don’t believe me (disclaimer: I’m not buying you a new phone, tablet, e-reader, or whatever you just broke).
Luckily, there are always smart folks who see these issues, as well as a handful of others: Why can’t I wear my electronics and photonics on my skin? Why can’t I cram my devices into any arbitrary space I want? Why should I have to look at these things but not through them?
Thanks to many forward-thinking materials researchers, the answer to the above questions seems to be, “No reason, let’s make it happen!” So here we are in the age of developing flexible electronics and photonics. The difficulty in making devices flexible is probably pretty obvious: most materials we conventionally use are not flexible! However, that doesn’t mean we can’t make a materials change for some things. For example, polymers are very flexible, so any pieces we can make out of those types of materials could be very helpful. And it turns out for photonics, you can make emitters, modulators, filters, and waveguides out of such materials as well as other organic and inorganic materials.
Nevertheless, if you’re like me, you have a hard time parting with semiconductors. They’re just so good at what they do and doing it efficiently. Well, there’s not necessarily any reason to abandon our band-gap-having, crystalline friends. It’s just that they need to trim down a little, to relieve a bit of the strain. The amount the most stressed crystalline layer needs to stretch or compress upon a deformation is related to how many layers away it is from the neutral plane, where there is no strain. Translation: make the layer thin.
So now you can bend your semiconductor, and you have an efficient source flapping in the breeze. Unfortunately, if you left it just like this, it would fall apart in an instant, its thickness being measured in units of nanometers (hence, nanomembranes). That’s why techniques have been developed to put these membranes on more stable yet flexible substrates. Enter again polymers. Two methods dominate at this time: transfer printing and direct patterning. Transfer printing is a process in which devices are fabricated and then bonded to the flexible substrate. This allows one to put a multitude of different devices, possibly made of different materials, on a single substrate. On the other hand, direct patterning utilizes deposition of a material on the substrate and then etching steps to define the devices. Although often less versatile than transfer printing, direct patterning is another robust method of making this flexible hybrid platform.
![General process illustration for crystalline semiconductor membrane release, transfer and stacking. (a) Begin with source material (e.g., SOI, GeOI, III-V multi layers with a sacrificial layer). Metallization can be applied here, if needed. (b) Pattern top layer into membrane (or strip forms) down to the sacrificial layer. (c) Release membrane by undercutting the sacrificial layer. (d) Fully released membrane settles down on the handling substrate via van der Waals force (“in-place bonding”). Direct flip transfer: (e1). Apply glue on host (e.g., flexible) substrate and attach it to the handling substrate. (f1) Lift-up the host substrate and flip to complete the transfer. Glue can be dissolved if needed. Stamp-assisted transfer: (e2) Bring a stamp (e.g., Polydimethylsiloxane, or PDMS) toward the handling substrate, press and lift-up. (f2) Apply the stamp with membrane attached to a new host substrate (which can be coated with glue, but not necessary). (g2) Slowly peel off the stamp or remove the stamp with shear force, leaving the membrane to stay on the new host substrate. Multiple layers can be applied by repeating (a)-(f1) or (a)-(g2). (Juejun Hu, Lan Li, Hongtao Lin, Ping Zhang, Weidong Zhou, and Zhenqiang Ma, "Flexible integrated photonics: where materials, mechanics and optics meet [Invited]," Opt. Mater. Express 3, 1313-1331 (2013))
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-9-1313)](/library/images/cleo/blog/2014/Blog140318-300x202.png)
General process illustration for crystalline semiconductor membrane release, transfer and stacking. (a) Begin with source material (e.g., SOI, GeOI, III-V multi layers with a sacrificial layer). Metallization can be applied here, if needed. (b) Pattern top layer into membrane (or strip forms) down to the sacrificial layer. (c) Release membrane by undercutting the sacrificial layer. (d) Fully released membrane settles down on the handling substrate via van der Waals force (“in-place bonding”). Direct flip transfer: (e1). Apply glue on host (e.g., flexible) substrate and attach it to the handling substrate. (f1) Lift-up the host substrate and flip to complete the transfer. Glue can be dissolved if needed. Stamp-assisted transfer: (e2) Bring a stamp (e.g., Polydimethylsiloxane, or PDMS) toward the handling substrate, press and lift-up. (f2) Apply the stamp with membrane attached to a new host substrate (which can be coated with glue, but not necessary). (g2) Slowly peel off the stamp or remove the stamp with shear force, leaving the membrane to stay on the new host substrate. Multiple layers can be applied by repeating (a)-(f1) or (a)-(g2). (Juejun Hu, Lan Li, Hongtao Lin, Ping Zhang, Weidong Zhou, and Zhenqiang Ma, “Flexible integrated photonics: where materials, mechanics and optics meet [Invited],” Opt. Mater. Express 3, 1313-1331 (2013))
http://www.opticsinfobase.org/ome/abstract.cfm?URI=ome-3-9-1313)
With these methods, one can make a number of photonic (and electronic devices). And there are some interesting avenues for exploration. For example, despite the strain-mitigation provided by thinning semiconductor membranes, it does not provide strain-elimination. The presence of strain alters the electronic and photonic properties of semiconductors, and therefore one can make tunable devices through flexing the material. However, this isn’t always a good thing; it creates a tough problem to solve when you want an extremely stable device under bending stress.
As hopefully you can see, this is a very exciting and active area of research. There are many open research questions and progress is continuing. If this topic catches your interest and you’re attending CLEO 2014, a great opportunity to learn more is from an expert! John Rogers from the University of Illinois at Urbana-Champaign will be giving a tutorial on flexible photonic devices. So be sure to check it out!
Disclaimer: Opinions, interpretations, conclusions, and recommendations are those of the author and are not necessarily endorsed by the United States Government and MIT Lincoln Laboratory.
Posted: 26 March 2014 by
Dominic Siriani
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