Bio-inspired fabrication of flexible ceramics
The microfabrication of optoelectronic components that are made of ceramic and attached to a mechanically flexible polymer sheet, has recently attracted attention. This is due to their varied practical applications including flexible flat panel and wearable displays, radio-frequency identification tags, and electronic papers. Such high-technology potential is attributed to polymers' distinctive properties such as flexibility, light weight, ease of design and coloring, and low cost.
The bio-inspired approach is based on the site-selective deposition of metal oxides, at low temperature, using an aqueous media containing an organometallic precursor. This method takes advantage of the large difference in chemical reactivity between the monolayer-covered (masked) and unmasked regions.1,2
To fabricate a flexible device on which a variety of optoelectronic microelements are arranged, at least two requirements must be satisfied. First, ceramic microelements, i.e. metal oxides, must be strongly adhered to the polymer surface. In most cases, this surface is treated with an ultraviolet light or oxygen plasma through microprocessing, but the resultant heterogeneous polymeric surface cannot serve as an ideal support for immobilizing ceramic films due to its variety of randomly distributed polar-functional groups.3 Second, due to extremely high chemical durability and physical stability, metal oxides are difficult to microstrucuture with typical microfabrication processes using chemical and physical etching. Thus, microprocessing of metal oxides currently depends on photolithographic technology using a resist.
The bio-inspired processing shown in Figure 1 overcomes these obstacles. First, we deposit an extremely thin silica layer on the polymer sheet (step I) by means of a simple chemical vapor deposition of tetraethoxysilane and then subsequent photocalcination. As illustrated in Figure 2, the actual thickness of the silica layer is only 1nm. Significantly, this nanolayer drastically changes the chemical properties of the polymer from a heterogeneous surface into one that is homogeneous and resembles glass. In other words, a sandwiched silica nanolayer serves as an adhesive nanoscale material, which strongly attaches the metal oxide film to the heterogeneous polymer.3 Next, using an organosilane self-assembled monolayer (SAM), the silica/polymer sheet is covered with a methyl- or fluoroalkylsilane SAM.
The resulting highly hydrophobic surface is then photolithographically micropatterned to fabricate a molecular template (step II). When looked at under ultraviolet light the irradiated and unmasked areas—which are made of the bottom part of the SAM (a 0.2∼0.27-nm-thick siloxane monolayer)—remain on the buffered silica surface (step III).
In our process we use tin oxide film, a typical wide-bandgap transparent semiconductor (Eg=3.8eV). We deposit it on a polyimide sheet covered with a molecular template from an aqueous solution of tin chloride that is then dehydrated. Initially, the film forms uniformly over the entire surface of the template (step IV), but the deposition mechanisms on each region are significantly different. Tin oxide film binds covalently to the siloxane group of the template. In contrast, due to an inert trifluorocarbon group reaction against the tin chloride molecule, the deposited film adsorbs physically onto the SAM-covered region. Thus, the absorbed deposits on the SAM-covered surface can be readily eliminated by a sonication with absolute toluene, resulting in a highly resolved micropattern of tin oxide: see step V in Figure 3(b).
X-ray photoelectron microscopy confirms the perfect site-selectivity of this micropattern. Such area-selective elimination is possible due to the different interfacial wetting properties in each region. To be more precise, absolute toluene easily enters the interface between the film and the SAM-covered surface by virtue of its wetting property which is very similar to trifluorocarbon. However, the toulene has difficulty entering the hydrophilic interface between the film and the siloxane-covered surface. This simple chemical lithography technique is referred to as ‘solution lithography’.4 The edge feature of the resulting micropattern looks relatively sharp. Actually, its variation is estimated to be less than 5%, which satisfies the pattern-resolution requirements of current microelectronics design rules. Furthermore, due to covalent linkage between the films and the buffer nanolayer, the mechanical flexibility remains as shown in Figure 3(a). The resultant tin oxide film shows a high sensitivity against hydrogen gas and a faster recovery time.
Using an appropriate precursor solution, the microfabrication method we propose here can be applied to various metal oxides other than tin oxide, and additionally, is readily and widely applicable to the production of novel polymer-based flexible microsensors and other devices using metal oxides.
This study was performed by the author, Dr. Nato Shirahata, in collaboration with Dr. Atsushi Hozumi of the National Institute of Advanced Industrial Science and Technology (AIST).
