2.2 Photo-Lithography
Photo lithography is a non-contact process which deploys the power of light
exposure to print extremely small features (up to sub micron levels) into photo-
chemicals and resists. Major steps in optical lithography are pattern transfer,
alignment and exposure; which are explained in the forthcoming sections in details.
2.2.1 Pattern Transfer
In lithography processes a pattern is transferred to a photosensitive material by
selective exposure to a radiation source (UV source in photolithography, X-Ray
source in X-Ray lithography, electron beam in e-beam lithography etc.) (Fig.2.3).
Physical properties of the photosensitive chemical change when it is exposed to such
radiation source. The changed properties of the photo-chemical render the exposed
regions to be constitutionally different from the unexposed regions and this different
created by light is utilized to print features and structures on the surface of the
photochemical [ 17 ]. The changed properties are different with positive and negative
tone photochemicals. In a positive tone resist the exposed regions are debonded and
in the negative tone resist these regions are cross-bonded (Fig.2.4a, b).
If the resist is exposed to a specific wavelength of light the chemical resistance of
the resist to developer solution differs. If the resist is placed in a developer solution
after selective exposure to a light source, one of the two regions will be etched
(exposed or unexposed). If the exposed material is etched away by the developer,
the material is positive resist as shown in Fig.2.4a. If and the unexposed region is
etched away, it is considered to be a negative resist as shown in Fig.2.4b.
Fig. 2.2 Schematic of the
demagnification lens
placement after shadow
mask [ 16 ]
38 G. Bhatt et al.