Electronic and mechanical systems printed onto flexible substrates

Abstract

Printing methodologies that combine capabilities from both nanoimprint lithography (NIL) and transfer printing (TP) have been utilized to fabricate electronic and mechanical device components onto plastic substrates. A wide variety of materials have been utilized in the fabrication of thin-film transistors (TFT) and mechanical resonators. Thin-film transistors with active layers of Pentacene (Pn), poly(3-hexylthiophene) (P3HT), carbon nanotube mats (CNTM), graphene and silicon (Si) nano-ribbons have all been fabricated with a polymethylmethacrylate (PMMA) dielectric layer on a polyethylene terephthalate (PET) substrate [1, 2, 3, 4]. Figure 1 shows optical micrographs of (a) CNTM, (b) graphene and (c) Si nano-ribbon devices. The performance of each of these devices on plastic is as good as that for non-printed devices fabricated using a SiO 2 dielectric layer on a Si substrate. The most recently fabricate Si nano-ribbon TFT devices have been doped n-type and exhibit an electron mobility of 144 cm2/Vs. Mechanical resonators constructed from both Si features and thin Au-coated polymer films are shown in figure 2. These structures were aligned and printed over a previously printed cavity on a polycarbonate (PC) substrate. The mechanical response of these structures was measured optically in air using an AC voltage applied to a probe tip to actuate the resonator. For a Si resonator of thickness 70 nm and area 10,000 µm2 a resonance frequency of approximately 480 KHz was measured (Fig. 2a). For a 200 nm thick PC film coated with 35 nm of Au and area 10,000µm2, a resonance frequency of approximately 520 KHz was measured for the highly strained (note wrinkle pattern in Au layer) device shown in Fig. 2b and of approximately 35 KHz for the relatively unstrained device shown in Fig. 2c. Preliminary calculations suggest a fundamental resonant frequency of 42 KHz for the Si resonator (Fig. 2a) and 80 KHz for the (unstrained) Au coated PC film (Fig. 2c). Differences between measured and calculated frequencies are most likely related to strain in the printed structures. Similar printing methods have been used to fabricated transistors, resistors, capacitors, inductors and mechanical resonators onto plastic substrates in an additive process that does not require the use of chemical processing on the device substrate. The transfer printing process can thus be used to build circuits based on dissimilar materials in ways not possible with standard lithographic methods. For example, an array containing both inorganic, metallic and polymeric resonators could be printed onto a common substrate. Details of the printing methods and characteristics of the resulting devices will be presented as a function of membrane material, thickness, printing conditions and cavity dimensions.