What is Nanolithography?
Nanolithography is the art of making structures on the nanometre scale. This might be used for creating integrated circuits and parts for semiconductor technology, where being able to manufacture the smallest possible transistors and circuits not only allows for creation of smaller devices but can help increase the power efficiency and performance of the components.1
Advances in lithography methods have also made it possible to construct complex structures that can be used for microelectromechanical or nanoelectromechanicalsystems (MEMS or NEMS) devices. Such miniature machines have already found use as pH sensors and transistors but there is a wealth of future possible development for such technology, such as using the devices for drug delivery.2
How does nanolithography work?
There are a variety of different techniques for performing nanolithography machining depending on the types of materials being worked with and the specifications of the final structure. Generally, most nanolithography techniques make use of the properties of light or electrons to create patterns in a substrate.
This patterning can be targeted via the use of masks being added to the photoresist in order to protect areas from the incoming light. The pattern is then etched onto the uncovered areas and, if necessary, the previously masked areas can be removed.
Electron beam lithography (EBL or e-beam lithography) is the technique that can be used to create the smallest features (as small as 5 nm).3 Rather than using light to illuminate the surface, a tightly focused beam of electrons is scanned over the surface. The electron beam exposes the pattern and then the resist can be developed. After this, the pattern transfer can be completed either by etching and resist removal or evaporating a metal onto the resist and dissolving the remaining unwanted metal and resist.
The Technical Challenges
While impressively minute and complex devices can be shaped with nanolithography, the smaller the scale of the manufacturing, the more precision in the pattern transfer becomes of paramount importance. Issues with precision can lead to manufacturing mistakes with wasted materials and their associated costs. However, there are a number of technical challenges that need to be overcome to make very high precision pattern transfer possible.
In order for the electron beam or light source to be able to correctly trace a pattern over a resist, it needs feedback on its relative position, both in terms of the relative height of the source and its position in the horizontal plane of the resist.1
Another related issue in electron beam lithography is the ‘stitching error’. When transferring larger patterns, the total pattern is broken down into smaller writing areas that are joined by movements of a translation stage. Here, the movements of the translation stages must be absolutely precise to preserve the full pattern over its constituent areas.4
Thankfully though, there are several options available to improving the stability and precision of nanolithography fabrication. Raith are market-leaders in offering technologies for nanofabrication that overcome many of the technical challenges associated with nanolithography fabrication.
Raith offers a broad range of electron and ion beam technologies from 20 eV to 100 kV for e-beam lithography. In the 100 kV range, Raith has the EBPG5200, which has an overlay accuracy of ≤ 5 nm for extreme direct write precision. Where 50 kV would be more appropriate, and users need a machine with a small footprint, the VOYAGER includes the Raith’s patented eWRITE system to allow for quick processing (up to 1 cm²/h) for samples of up to 8 inches.
Raith can achieve this incredible precision over even large write areas through two different lasers, one for working distance and another for staging distance. Working distance is controlled by using a laser source and measuring the reflected signal from the resist. Relative high-precision positioning of the electron beam to the surface is obtained through measuring interference pattern with a mirror mounted on the stage. This technology allows the mapping of errors in the field of view to an accuracy of 1 nm. The benefits of the XY Staging is not just the enhanced precision though but that it can be maintained even over complex surface shapes such as an inclined surface or one with strong curvature.
Given that stitching areas in nanolithography is a source of additional errors and imprecision, Raith have developed the traxx and periodixx technologies, which allow for stitch-free lithography. By using their exclusive Fixed-Beam-Moving Stage (FBMS) technology to translate the sample, it is possible to draw patterns up to centimetres in length in a continuous writing mode. This removes any of the stitching errors associated with writing region by region and is particularly beneficial in high-precision manufacturing applications such as making waveguides and X-ray optics.
In conjunction with traxx, peridoixx uses a similar principle, the Modulated-Beam-Moving-Mode (MBMS) to combine the advances of FBMS with some beam movement as required for drawing repetitive patterns. Both write modes can be combined with the Laser Interferometric Stage and are available on many of the Raith lithography instruments including VOYAGER, RAITH150 Two, eLINE Plus and VELION.
The large range of instruments offered by Raith in combination with their very high precision and capabilities of very small scale machining make them ideal for nearly any nanolithography application.
1) Pimpin and W. Srituravanich, Eng. J., 2012, 16, 37–55
2) Fujita and Y. Mita, Nanofabrication Handb., 2012, 353–378.
3) Chen, Microelectron. Eng., 2015, 135, 57–72.
4) K. Dey and B. Cui, J. Vac. Sci. Technol. B, Nanotechnol. Microelectron. Mater. Process. Meas. Phenom., 2013, 31, 06F409