Hybrid materials: a bottom-up approach for nanotechnology applications

Fabricating small-scale devices, such as sensors and high index waveguides, requires patterning of a substrate surface with micro- and nanostructures. To achieve this, lithographers generally use indirect techniques. For example, to form structures in an organic or inorganic material, the developer may pattern and deposit a sacrificial resist and then transfer the image of that layer to the functional material. However, such multi-step processes often cause a deterioration in lithographic performance and are time-consuming and complicated.

An alternative approach, taken to pattern inorganic films, attempts to simplify the process using an organic resist pattern as a mold. This technique involves depositing the film on the mold and then removing the resist by a lift-off procedure, leaving the patterned structures on the substrate.

The quality of the resist materials is, therefore, essential to the performance of the main lithographic tools in a standard process. Furthermore, it can be strategically relevant for nanotechnology applications in the final device materials, for example, in the optically active nanostructures integrated in microfluidic chips.¹

One emerging alternative to organic polymers for micro- and nanolithography is the use of organic–inorganic hybrid materials. These are cost-effective and enable improved lithographic performance. They offer stability and a wider choice of properties, including thermal and mechanical resistance and chemical endurance. Moreover, it is possible to tune the characteristics of these materials, such as porosity as well as optical and electrical properties, and we can achieve specific functions by embedding nanoparticles, dyes, or other active molecules in the final integrated devices—sensors, lasers, and waveguides being examples. Finally, we can build hybrid materials using a simple, low-cost, bottom-up sol-gel approach (using a solution to form a solid-liquid state), typically carried out at low process temperatures.

Using these techniques, we developed a simple and highly versatile synthesis platform, enabling the preparation of organic–inorganic hybrid spin-on materials for micro- and nanofabrication.² The basis for our lithographic tool is organic ‘ building blocks,’ and we selected specific molecules (such as metal alkoxides) for the addressed applications. These enabled the preparation of thermal, pressure, and radiation-sensitive resists in a wide range of ceramic compositions, including silicon dioxide, germanium dioxide, titanium dioxide, zirconium dioxide, hafnium(IV) oxide, aluminum oxide, and lead zirconium titanate. We coupled them with organic functionalities (epoxy, acrylate, phenyl) with a high degree of control over composition, structure, and processability.

Figure 1 shows the main building blocks of our resist structure, including metal organic precursors such as metal alkoxides. Others examples are organically modified silicon alkoxides, whose organic function can be polymerized, acting as a network modifier (enabling atoms or molecules to form glass materials) or as bridging groups and functional species at the same time. We can use these with radiation-assisted lithography to produce completely inorganic, high-quality glassy hybrid patterns. Organic monomers, whose role is to tailor the rheology (the flow of liquid matter) of the film during the imprinting process, harden the pattern thermally or by radiation. As an alternative building block, we could use organic molecules for patterning by photodegradation to produce completely inorganic patterned structures.

Figure 1. Main building blocks of the hybrid material resist structure. (a) Metal organic precursors, examples of organically modified silicon alkoxide with (b) organic polymerizable function, or (c) organic modifier function. (d) Example of organic monomer and (e) organic molecules added to pattern the film by photopolymerization or photodegradation.

The amount and type of building blocks determined the outcome of our approach. The results enabled direct patterning using different lithographic tools with high performance and the presence of both positive and negative tones. Combined with our knowledge and control of material interactions with radiation or thermal/pressure-driven processes, we reduced the number of steps in the lithographic method as well as the costs. We achieved a variety of compositions, from both organic-inorganic hybrid to totally inorganic, and chemical-physical properties such as transparency, refractive index, stiffness, porosity, and sensing functionality, offering a broad range of possible applications for final devices.

The spin-on materials with ceramic and hybrid compositions described above have enabled a one-step development of plasmonic or fluorescent sensing devices (see Figure 2),^3,4 high-resolution patterns,⁵ dry-etching masks with outstanding resistance (see Figure 3),⁶ and optically active micro and nanostructured platforms.

Figure 2. Microscope images of a micropattern created by x-ray lithography on a thin film prepared from a phenyl bridged polysilsesquioxane matrix doped with a covalently linked quinolinium derivative.³(Left) A fluorescence microscope image. (Center) Optical image. (Right) Scanning electron microscope (SEM) image.

Figure 3. SEM images of the developed resist. (a) The etched silicon structures obtained using the electron beam. (b) Structures obtained using x-rays. (c) Alumina patterns as etching masks.⁶

Our future work will concentrate on engineering functional spin-on systems for optoelectronic devices and integrated sensors. We will also focus on developing advanced amphiphilic materials to realize innovative, durable antifouling coatings, resists, and etching masks for extreme UV lithography and 3D nanofabrication.