Photolithography

Fundamental Principle

Photolithography is a process that involves the selective exposure of photoresponsive polymers. Photoresist is put down onto a silicon substrate, and this mixture is exposed to UV light through a patterned mask. In development, exposed areas in positive resist dissolve, and exposed areas in negative resist harden and become permanent. This is a patterned resist that creates a protective stencil that is subjected to additional etching or implantation. The entire process is carried out in ultra-clean conditions, whereby the patterns of circuits are not destroyed by minute contamination.

Process Sequence

The general steps include 1) wafer cleaning, which involves chemical bath of the wafer to remove the contaminants; 2) resist coating, which involves spin coating the wafer to form uniform thin films; 3) soft bake, which involves curing events; 4) Alignment and Exposure, which involves projecting the mask pattern onto the wafer using advanced stepper/scanner machines; 5) Development, which involves chemical developers to remove the soluble areas of the resist; and 6) Hard Bake and Inspection, which involves strengthening the mask pattern and makes immersion technology excimer lasers of 193 nm ArF adopted in the modern exposure tools.

Resolution Enhancement

The minimum feature size is determined by the equation of Rayleigh: CD = k1 x (wavelength/NA), where wavelength is the wavelength, and numeric aperture is the numeric aperture. Some of the solutions to the problem of diffraction limits being pursued by the industry include Phase-Shift Masks, where an interference is applied on sharper edges, Optical Proximity Correction, where mask patterns are pre-distorted to offset the optical effects, Off-Axis Illumination, where water is placed between the lens and the wafer to improve effective NA, and Immersion Lithography. In multiple patterning, multiple exposures In Multiple Patterning, multiple patterning techniques, including SADP (self-aligned double patterning), are used to divide the dense pattern into multiple exposures.

EUV Lithography

The use of the light of 13.5 nm as opposed to the usual 193 nm systems in extreme UV lithography (EUV) is a radical change. EUV requires a completely new technology: light sources in the shape of droplets of tina based on plasma, reflective, numerous-layered mirrors instead of lenses (all materials absorb EUV), beam pathways in vacuums, and reflective masks. The 5nm nodes and smaller are already underway in the production of the EUV, simplifying production tremendously by reducing the number of patterning steps necessary in the 193nm lithography to 5nm lithography, but they are staggeringly complex and costly.

Applications

Besides semiconductors, photolithography is applied to create MEMS devices, including accelerometers and micro-mirrors; microfluidic chips to analyze biomedical samples; advanced chip packages, including through-silicon vias; and photonic circuits in optical communications and biotechnology platforms, including cell culture substrates and DNA microarrays. Its patterning functions allow centrality to modern micro- and nanofabrication.

Equipment & Materials

Stepper/Scanner exposure equipment, which moves patterns and aligns them, Coater/Developer equipment, which processes resist orders without human intervention, Photo masks (quartz plates with a pattern of chromium), which serve as a template, Photoresists (compound formulations that are sensitive to wavelengths) and metrology instruments such as CD-SEMs, which can measure feature sizes and detect defects with atomic precision of a nanometre.

Challenges

Astronomical cost Photon tools are expensive (greater than 150M), Stochastic effects Photon and molecular disorder cause faults on the lowest atomic scales. Overlay accuracy is only available in the range of better than 2 nm between layers. In particular, EUV has a poor light source power and a basic resolution vs. depth of focus trade-off. Higher resolution in a technology maximises faults on the lowest atomic levels and decreases the best overlay accuracy to a matter of hundreds of nanometres.

Outlook

The next generation of high-NA EUV systems will be anamorphic optics of 8 nm resolution for 2 nm node chips. It may be extended to very short wavelengths or other techniques, including Directed Self-Assembly (to add block copolymer chemistry to patterning), and Multi-Beam Mask Writing can be much quicker and much more complicated to make photomask fabrication. The technology of the information age is still photolithography, which continues to sustain further advancement of computing, communications, and a host of other technologies.