page_title: Photolithography
section_title: Stochastic effects
hierarchical_section_title: Photolithography / Stochastic effects
caption_attribution_description: Each set of diffractive orders from the same source points are of the same color, and are separated by a distance of wavelength/pitch. Refs.: J. Finders et al., SEMI Technology Symposium, Semicon Japan 1999; N. Zeggaoui et al., Proc. SPIE 7823, 78233Y (2010).
context_page_description: Photolithography, also called optical lithography or UV lithography, is a process used in microfabrication to pattern parts on a thin film or the bulk of a substrate. It uses light to transfer a geometric pattern from a photomask to a photosensitive chemical photoresist on the substrate. A series of chemical treatments then either etches the exposure pattern into the material or enables deposition of a new material in the desired pattern upon the material underneath the photoresist. In complex integrated circuits, a CMOS wafer may go through the photolithographic cycle as many as 50 times.
  Photolithography shares some fundamental principles with photography in that the pattern in the photoresist etching is created by exposing it to light, either directly or with a projected image using a photomask. This procedure is comparable to a high precision version of the method used to make printed circuit boards. Subsequent stages in the process have more in common with etching than with lithographic printing. This method can create extremely small patterns, down to a few tens of nanometers in size.
context_section_description: As light consists of photons, at low doses the image quality ultimately depends on the photon number. This affects the use of extreme ultraviolet lithography or EUVL, which is limited to the use of low doses on the order of 20 photons/nm².
  This is due to fewer photons for the same energy dose for a shorter wavelength (higher energy per photon).
  The stochastic effects would become more complicated with larger pitch patterns with more diffraction orders and using more illumination source points.
caption: Photons are divided among multiple source points. The photons making up the exposure dose are divided equally among the source points (two are shown here) which are positioned within the pupil.
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