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. 2014 Oct 6;11(99):20140627.
doi: 10.1098/rsif.2014.0627.

Geckoprinting: Assembly of Microelectronic Devices on Unconventional Surfaces by Transfer Printing With Isolated Gecko Setal Arrays

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Geckoprinting: Assembly of Microelectronic Devices on Unconventional Surfaces by Transfer Printing With Isolated Gecko Setal Arrays

Jaeyoung Jeong et al. J R Soc Interface. .
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Abstract

Developing electronics in unconventional forms provides opportunities to expand the use of electronics in diverse applications including bio-integrated or implanted electronics. One of the key challenges lies in integrating semiconductor microdevices onto unconventional substrates without glue, high pressure or temperature that may cause damage to microdevices, substrates or interfaces. This paper describes a solution based on natural gecko setal arrays that switch adhesion mechanically on and off, enabling pick and place manipulation of thin microscale semiconductor materials onto diverse surfaces including plants and insects whose surfaces are usually rough and irregular. A demonstration of functional 'geckoprinted' microelectronic devices provides a proof of concept of our results in practical applications.

Keywords: adhesive; flexible electronics; gecko; seta; solar microcell; transfer printing.

Figures

Figure 1.
Figure 1.
Optical images, optical microscopy and SEM of a tokay gecko, (Gekko gecko) toes and isolated setal stamp. (a) Optical image of a tokay gecko holding its body weight (58 g) with only two legs, hanging suspended from a smooth glass surface. (b) Magnified view of the underside of the gecko's toes, attached to an optically transparent glass surface. Uncurling the toes makes contact and engages adhesion. (c) Curling mechanism of toes when the gecko detaches its adhesive toes from the glass surface. (d) Optical microscopy image of the setal array glued on transparent glass permanently with cyanoacrylate superglue. The inset shows the SEM image of the setal array. (Online version in colour.)
Figure 2.
Figure 2.
Schematic illustration and SEM images of transfer printing procedures using the natural setal stamp. (a) The setal stamp approaches a thin micro silicon (Si) plate which is temporarily anchored on the original substrate with a PR after selectively removing underlying oxide (SiO2) layers. (b) Upon contacting the Si plates, slight dragging the setal stamp horizontally engages adhesion. (c) Retracting the setal stamp vertically during dragging breaks the temporary anchors and picks up the Si plate. (d) Bringing the setal stamp holding the Si plate into contact with the receiver substrate and (e) dragging in the distal direction, opposite to that used to engage the array, places the micro Si plate on the receiver substrate. (f) Retracting the setal stamp while dragging leaves the micro Si plate in place. (g) SEM image of the micro Si plate (outer radius approx. 75 μm, inner radius approx. 20 μm, thickness approx. 10 μm) that anchored to the original substrate. The anchors (size approx. 12 × 60 μm, thickness approx. 1.8 μm) temporarily hold the Si plate whose underlying oxide layer (thickness approx. 1 μm) is chemically removed with HF acid. (hi) SEM images of the Si plates (h) retrieved by the natural setal stamp and (i) placed onto a smooth silicon substrate, respectively. (Online version in colour.)
Figure 3.
Figure 3.
Adhesive forces at the interfaces of the Si plates, measured separately for each interface. (a) Schematic illustration of measurement of the forces formula image between the Si plate and setal stamp without removing the sacrificial layer and (b) the forces formula image between the Si plate and anchors after removing the sacrificial layer. The other force acting downward, formula image, is the force between the micro Si plate and the anchor. (c) Time dependence of the normal (solid lines) and shear (dotted lines) forces on the micro Si plate. The black solid line represents the normal force between the setal stamp and the micro Si plate, formula image, whose maximum is 3.24 mN. The red solid line indicates the normal force between the micro Si plate and the anchors, formula image, which is less than 0.63 mN. The separate measurements indicate that formula image is greater than formula image, enabling picking up the Si plate from the original substrate. (d) Schematic illustration of the force measurement formula image during the approach–release–retract procedure. (e) Measurement of the lower limit of the force formula image between the Si plate and receiver substrate (Si). (f) Normal (solid lines) and shear (dotted lines) forces on the micro Si plate during the releasing procedure. The normal force formula image between the micro Si plate and the Si receiver substrate is greater than the force formula image between the setal stamp and micro Si plate. (Online version in colour.)
Figure 4.
Figure 4.
SEM images of ‘geckoprinted’ micro Si plates on various surfaces. (a) SEM image of stacked Si plates, printed vertically. (b) Magnified image of the triple layers of micro Si plates aligned and printed vertically using the setal stamps. (c) Optical microscope image of 55 micro Si plates aligned laterally and printed to write letters of ‘GIST’. (d) Micro Si plate transfer printed on the rough surface (roughness approx. 0.34 μm RMS) back side of the silicon wafer. (e) Micro Si plate integrated on (f) the plant (F. japonicum). (g) Micro Si plate integrated on (h) the back of an insect (Z. morio). The inset shows the magnified image of the surface. (i) Micro Si plate integrated on (j) the wing of the same insect (Z. morio). (k) Results of durability experiments. During one cycle, we brought the setal stamp in contact with the smooth Si substrate, then, dragged horizontally, and retracted from the smooth Si substrate, as we did when picking up the micro Si plate. The measurement results indicate that there is no continuous decrease of the forces over 400 cycles. A tear occurred in the thin backing of the setal array after 400 cycles. More durable bonding of the backing of the setal array may enable greater longevity of the setal stamp. (Online version in colour.)
Figure 5.
Figure 5.
Transfer printing of functioning microelectronic devices on unconventional substrates. (a) Colorized SEM image of the thin (thickness approx. 6.7 μm) 2J solar microcell (size approx. 760 × 760 μm) transfer-printed (b) on a leaf (K. japonica). The inset in (a), shows the magnified image of the surface. (c) Current–voltage (I–V) characteristics of the solar microcells on the original substrate (GaAs wafer) and on the leaf. The electrical performances of the solar microcells on both surfaces are almost identical (efficiency approx. 19.3%, fill factor approx. 0.80). (d) Schematic illustration of vertical-type solar microcells whose p-contact faces down, printed onto the dry metal line (Au) for direct electrical connection without aids of any gluing adhesives. (e) Colorized SEM image of vertical-type 2J solar microcells integrated onto dry metal line (Au, thickness: 60 nm, Ti, thickness: 20 nm) making electrical contact. (f) I–V characteristics of the vertical-type 2J solar microcells on the Au film, measured by probing the top contact and the Au line on the substrate (efficiency approx. 19.7%, fill factor approx. 0.82) confirming electrical contact at the interface. (Online version in colour.)

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