Acronym · Lithography & Materials
RET (Resolution Enhancement Technique)
The collective term for methods that print below the naive optical limit without changing wavelength or NA, including optical proximity correction, phase shift masks, assist features and custom illumination.
- Also known as
- Reticle enhancement technology
- Type
- Acronym
- Domain
- Lithography & Materials, Mask data prep and RET
Notes
RET complexity is a large share of why mask costs rose faster than wafer costs from 90 nm onward.
Overview
RET (resolution enhancement technique) is the umbrella term for the methods that let a lithography scanner print features smaller or sharper than its wavelength and lens would otherwise allow, by changing the mask pattern or the way light illuminates it. The family includes optical proximity correction, phase-shift masks, sub-resolution assist features and shaped or off-axis illumination, increasingly optimised together by software. In the terms of the Rayleigh resolution equation, RETs are the main way of lowering the k1 factor once wavelength and numerical aperture are fixed.
How it works
ASML summarises printable feature size as CD = k1 x wavelength / NA, where k1 gathers everything about the process other than the light and the lens, with a physical limit of 0.25. RETs attack k1 from several directions. Off-axis illumination shines light onto the mask at an angle, so more of the diffracted light that carries fine detail passes through the lens; annular, quadrupole and dipole illuminator shapes each suit particular pitches and orientations. Phase-shift masks change the phase of light through parts of the mask so that neighbouring openings interfere in a way that sharpens edges, in alternating or attenuated forms. Sub-resolution assist features are extra shapes too small to print themselves that make isolated features behave more like dense ones.
Optical proximity correction then reshapes every mask polygon so that the printed result matches the intended layout. Trouiller's review of OPC from the 120 nm to 32 nm nodes describes how RET and OPC became inseparable: as illumination became more strongly off-axis, OPC had to become more aggressive.
The techniques now run as computational lithography flows. Siemens' Calibre tools, for example, include separate steps for assist feature insertion, model-based and inverse pixel-based OPC, and source mask optimisation, which tunes the illumination shape and the mask together. The cost is heavy computation, larger mask data and stricter mask rules.
Examples
- Annular or quadrupole illumination chosen in the scanner's illuminator to suit the dense pitches of a memory array.
- Sub-resolution assist features inserted beside main features so that they print with a wider process window.
- Alternating phase-shift masks, which Trouiller's review links to microprocessor gate layers and which require the layout to avoid phase conflicts.
- Source mask optimisation, as in Siemens' Calibre pxSMO, to co-optimise the illumination pupil and the mask for a difficult layer.
Related terms
Part of the Lithography & Materials glossary, with every term in the topic and the lifecycles, tools and standards that use them.
Where it appears on the Hub
Used on 5 pages of the Hub. In each group, the pages that mention it most come first.
Skills
Suggest an improvement
Suggest an improvement to this entry
A clearer definition, an example, a correction or a source an editor can check. Editors review every suggestion before the entry changes.
Last reviewed . Written by The Semiconductor Hub editorial team. The full glossary