Nanotechnology-Enabled Sensors

188 Chapter 4: Nano Fabrication and Patterning Techniques
leaves the substrate with only the film which was deposited directly onto
it.
Limiting factors for achieving high resolution patterning are the wavelength
of the light that is used in the photolithography process, and the diffraction
caused by the mask and photolithography system. At present, deep
ultraviolet (DUV) light with wavelengths of 250 and 190 nm are utilized.
This allows feature sizes down to 50 nm. The current commercial standard
for photolithographic system is 193 nm DUV.
The absolute resolution limit in a photolithographic system is given by a
quarter-wavelength divided by the numerical aperture (NA). 74 The NA is
equal to nsinθ, where n is the refractive index of the media in which the
lens of the photolithographic mask is placed, and θ is the half-angle of the
maximum cone of light that can enter or exit the lens. For a system with
lenses operating in air, NA is equal to 1. This will limit the feature size to
approximately 50 nm for a 200 nm wavelength.
It is possible, to achieve feature sizes less than 50 nm using DUV light
together with liquid immersion techniques, which involve immersing the
optics and samples in a liquid. This enables the use of optics with NAs exceeding
1. In this technique, deionised water is typically used as the photolithographic
media, as it has a refractive index equal to 1.35, which is
higher than that of air (n = 1). This will allow the effective NA to be increased
which enhances the resolution without changing the light source.
Features sizes less than 30 nm have been reported by researchers at IBM
using this technique with a standard wavelength of 193 nm. 75
Another new alternative is extreme ultraviolet lithography (EUV) . EUV
lithography systems are currently under development. 74 They employ
13.5 nm wavelengths, which is approaching X-rays wavelengths of the
electromagnetic spectrum. However, the biggest challenges in lens production
for EUV remains in producing very flat mirrors (aberration free) and
accurate lenses (or magnetic reflectors and lenses) with defect free coatings,
and thermal management systems ensuring accurate imaging.