Design and Performance of Thin Metal Film Interconnects for Skin-Like Electronic Circuits
Title: Design and Performance of Thin Metal Film Interconnects for Skin-Like Electronic Circuits
Authors: Stéphanie P. Lacour, Joyelle Jones, Z. Suo, and Sigurd Wagner
Source: IEEE Electron Device Letters 2004, 25, 179-181 
Keywords: thin film
In this paper, the authors design and produce a stretchable electrical connection by depositing a thin film of gold on a polymer substrate. The film is electrically conductive at high strains and can be cycled 100 times without a significant change in its electrical properties. The authors hope to use the device in areas such as robotics, medical devices, or conformal displays, where parts need to be flexible, stretchable, or curved.
The device is produced using lithographic fabrication methods. First, a block of polydimethylsiloxane (PDMS) is stretched to a specified length by a custom-built fixture (Figure 1 a,b). The top surface of the block is coated with Riston photoresist (c). The photoresist can be patterned into stripes by exposing it to light and washing it. A thin layer (about 5 nm) of chromium is deposited on the PDMS substrate, followed by a 25 nm thick gold layer (d). The purpose of the chromium layer is to allow the gold to adhere to the PDMS.
Once both metal layers have been deposited, the remaining photoresist can be dissolved in a potassium hydroxide solution, washing away the metal above the photoresist as well. This leaves behind stripes of gold on the substrate (e). Finally, the PDMS substrate is released from the fixture which has been holding it in a strained position. The PDMS relaxes, causing a compressive stress to be applied to the thin film of metal atop the PDMS. This forces the metal to buckle (f).
Experimental methods and results
The authors use a combination of a camera, a multimeter, and a stepper motor to examine the buckled surface of the film and its electrical and mechanical properties. The buckling of the film can be seen in the image in Figure 2.
The authors then perform two simple experiments to investigate the electrical properties of the film under mechanical deformation. First, the researchers stretched the film while measuring the electrical resistance of the film, and stopped when electrical contact was broken. They also captured images of the film at different levels of strain.
One issue with the gold strips that the authors immediately discovered was that they formed cracks in the direction parallel to the direction of the applied strain. These cracks were formed upon releasing the PDMS substrate from its stretched state during production of the device. When the PDMS was stretched in one direction, it was compressed in the orthogonal directions, due to Poisson effects. Upon release of the initial strain, the PDMS stretched outward in the orthogonal directions, tearing cracks open in the gold film. The authors postulate that smaller stripes of gold will prevent cracks from occurring.
Despite the cracking, the film remained electrically conductive up to a strain of about 25%, based on the results shown in Figure 3. This is a significant improvement over a gold film by itself, which generally stops conducting at strains of 1-2%.
The second experiment the authors performed was to measure the electrical resistance of the film while cyclically loading and unloading the film in one direction. Studying the device under cyclic loading is important, as it demonstrates how much wear and tear the device can handle before it breaks. The authors are able to cycle the loading on their device up to 100 times without causing it to fail, at a strain rate of 1% per minute. This shows that the device can withstand repeated stretching for a time, but in some applications, such as an actuator or some rapidly moving, high usage device, a more robust solution would be required.