Introduction of Microfabrication techniques
Different techniques have reached an adequate level of maturity for various commercial MEMS-based products, such as pressure sensors, accelerometers, gyroscopes, etc. Recently nanometric structures have attracted enormous interest for their unique electrical properties, magnetic, optical, thermal, and mechanical. These could lead to a variety of electronic, photonic, and sensor devices that outperform the corresponding macroscopic devices. Although electron beam lithography and other lithographic techniques can be used to manufacture nanometric structures, their serial and/or high nature cost precludes their wide-ranging application. This led researchers to explore alternative and potentially superior techniques such as "strain engineering," "self-assembly," and "nano-imprinting" lithography. Among these technologies, self-assembly seems to be the most promising method due to its low cost and its ability to produce nanostructures with different length scales. Finally, some of the analysis techniques used to verify the quality of the manufacturing steps will be briefly illustrated.
Basic microfabrication techniques
Most micro/nanofabrication techniques have their roots in standard manufacturing methods developed for the semiconductor industry. Therefore, a clear understanding of these techniques is necessary for anyone who undertakes a research and development path in the area of micro/nanofabrication. Here, we will present the main manufacturing techniques most frequently used in the production of micro/nanostructures. Some of these techniques, such as thin film deposition and etching, are also common in VLSI microchip manufacturing processes.
Lithography is the process used to transfer designs (patterns) of geometric shapes, generated by a computer, to a substrate (silicon, glass, GaAs, etc.) by using a mask of radiation-sensitive material (resist) covering the surface of the wafer. To define the various regions of the structure to be made, these drawings must then be transferred to the underlying layers. This transfer is accomplished with an "etching" process that selectively removes unmasked portions of a layer. Although photolithography, i.e., lithography using a UV light source, is by far the most used lithographic technique in microelectronics manufacturing, X-ray lithography, and electronic beam lithography (e-beam) are two alternative techniques. They have attracted considerable attention in the manufacturing areas of MEMS and nanodevices.
Each lithographic process must be carried out in a "clean room," that is, an environment in which the area is maintained at well-controlled values of temperature and humidity and is continuously filtered and recycled. The need for this type of environment arises from the fact that the deposition of particles on wafers and lithographic masks can cause an incorrect definition of the geometries or the creation of dislocations or defective points that can induce failures. In a cleanroom, the number of dust particles per unit of volume must be checked together with temperature and humidity.
After depositing the desired material on the substrate, the photolithographic process starts by spinning the substrate with a photoresist. This is a photosensitive polymeric material that can be deposited on the wafer in liquid form (usually before the application of the resist, a material that facilitates its adhesion is used). In the positive resist, the areas exposed to UV rays will be dissolved in the next development step. Therefore it is removed with the result that the drawing transferred to the resist (positive) is the same as the one on the mask. In the negative photoresist, the exposed areas will remain intact after development, and the design formed in the resist is opposite to that of the mask. Due to its better control characteristics in the process of defining small geometries, the positive resist is the most extensively used photoresist in VLSI processes despite the fact that it requires higher exposure energies and longer exposure times. Subsequently, the mask is aligned with the wafer, and the photoresist is exposed to a UV source. In relation to the separation between the mask and the wafer, three different optical exposure systems can be considered:
Although contact printing provides a better resolution compared to the proximity technique, continuous contact of the mask with the photoresist can cause dust particles to remain embedded in the mask causing permanent damage and, therefore, the presence of defects in subsequent exposures. Proximity printing is similar to the previous one except for the presence of a gap of a few microns between the wafer and the mask during exposure. Projection printing is certainly the most widely used system in microfabrication and can ensure higher resolutions compared to contact and proximity methods.
X-ray lithography uses optical sources that emit light with wavelengths less than 10 nm. For these wavelengths, it is not possible to use the principles of optics by reflection or refraction to carry out the transfer of the pattern. Also, there are no materials that are sufficiently transparent to create lenses or masks. For these reasons, it is not possible to make a projection lithograph but a proximity lithograph.
Another method of obtaining resolutions that are lower than 100 nm is to change the type of radiation. It is possible to obtain illumination of the resist by using charged particles such as electrons that can be easily generated both by thermionic effect and by emission due to field effect and can be focused in a beam with a size of a few nm. This electron beam can be used to write the design of the desired structure directly on the resist. In direct electron-beam writing, the electrons form a beam and are accelerated to a specific point on the surface of the wafer. Then electron-sensitive resists such as poly (methyl methacrylate) (PMMA) dissolved in trichlorobenzene (positive) or poly chloromethyl styrene (negative) is exposed by defining the pattern. An electronic beam system consists of electronic cannons, an electronic-optical system (the electronic column), a mechanical stage for positioning the wafer, and a control system.
The two types of an electron guns that are commonly used are the thermionic sources and the field emission sources. In the former, electrons are emitted by heating the source material, for example, tungsten (W) or lanthanum hexaboride (LaB6). In the latter, electrons are extracted from a pointed end due to the strong electric field. They require very high voids and are somewhat unstable unlike the thermionic ones based on LaB6 which have become the most common sources for electron beam lithography systems.