Positive vs Negative Photoresist: Differences Explained

A photoresist is the light-sensitive coating that carries the circuit pattern. With a positive resist, the areas exposed to light become soluble and wash away in the developer, so the pattern matches the mask. With a negative resist, the exposed areas harden and stay, so the pattern is the reverse of the mask. Positive resists dominate fine patterning in chip making, while negative resists are used where thick films or special profiles are needed.

AspectPositive resistNegative resist
Effect of lightExposed areas become soluble and are removed by the developerExposed areas become insoluble (cross-linked) and remain
Pattern compared with the maskSame polarity: the resist pattern copies the maskReversed polarity: the resist remains where the mask is clear
ResolutionHigh, which suits fine patternsTraditionally lower, because the resist can swell in the developer
DeveloperUsually an aqueous alkaline solution (such as TMAH)Traditionally an organic solvent; negative-tone development of modern resists also uses solvents
Sidewall profileSlightly sloped to nearly verticalCan be made with an undercut, which helps lift-off processes
Typical materialsNovolac with a diazonaphthoquinone (DNQ) inhibitor; chemically amplified polymers for KrF, ArF and EUVCyclized rubber with bis-azide (older); epoxy-based SU-8 for thick MEMS structures
Thick filmsLess commonWell suited: SU-8 makes tall, high-aspect-ratio structures
Typical usesNearly all modern IC lithographyMEMS, lift-off patterning, printed circuit boards, thick layers for packaging, and some special layers in advanced nodes

Figures are typical orders of magnitude and vary by process generation and product.

When to use which

Positive
The default for IC manufacturing, where resolution and well-controlled pattern transfer matter most.
Negative
When you need very thick, tall structures, a lift-off profile, or a specific mask polarity.

FAQ

What happens to a positive resist when it is exposed?

The exposed area becomes soluble and is dissolved by the developer, so the resist pattern copies the mask.

Why is positive resist preferred for fine patterns?

It offers higher resolution, because negative resists traditionally swell in the developer.

When is a negative resist used?

For thick, tall structures (such as SU-8 in MEMS), for lift-off processes that need an undercut profile, and for some special layers.

What is a chemically amplified resist?

A resist in which exposure creates an acid that triggers many chemical reactions during baking, so a small light dose is enough. It is used for KrF, ArF and EUV lithography.

Check your understanding

Which correctly describes positive photoresist?

  1. AExposed areas remain after development
  2. BExposed areas dissolve and are removed during development
  3. CEverything dissolves during development regardless of exposure
Show answer and explanation

Answer: B. Exposed areas dissolve and are removed during development

In a positive resist, exposed areas become soluble in developer and are removed, leaving the pattern where the mask blocked light. Representative types are novolak resins for i-line and chemically amplified resists for KrF and ArF, developed in alkaline TMAH solution. Their superior resolution makes them the mainstream. Negative resists keep the exposed areas, analogous to photographic negatives and positives.

Which correctly describes photoresist?

  1. AAn adhesive for holding wafers in place
  2. BA metal used for wiring
  3. CA photosensitive material whose properties change when exposed to light, becoming more or less soluble in developer
Show answer and explanation

Answer: C. A photosensitive material whose properties change when exposed to light, becoming more or less soluble in developer

Photoresist is a photosensitive polymer, coated tens of nm to several µm thick, whose chemistry changes on exposure: in positive resists exposed areas dissolve in developer, and in negative resists they remain. Beyond capturing the pattern, it masks the substrate during subsequent etching or implantation, so formulations must balance resolution, sensitivity and etch resistance—a field in which Japanese companies hold a large share.

Which is a characteristic of chemically amplified resists?

  1. AThey dissolve during development even without exposure
  2. BThey react only to heat, not to light
  3. CAcid generated by exposure acts as a catalyst during the post-exposure bake (PEB), driving the reaction with a small exposure dose
Show answer and explanation

Answer: C. Acid generated by exposure acts as a catalyst during the post-exposure bake (PEB), driving the reaction with a small exposure dose

In a chemically amplified resist, exposure generates acid from a photoacid generator, and during the post-exposure bake (PEB, about 90–130 °C) that acid catalytically removes protecting groups from the polymer, repeating the reaction tens to hundreds of times. One photon drives many reactions, giving the high sensitivity required by dim KrF, ArF and EUV sources; IBM developed the concept in the 1980s. Excess acid diffusion blurs the image, so PEB temperature and time are tightly controlled.

  1. Which method develops a positive chemically amplified resist with an organic solvent, dissolving the unexposed areas?
  2. What is the main purpose of the bottom anti-reflective coating (BARC) applied under resist?
  3. What is an advantage of metal-oxide-containing resists being developed for EUV lithography?

Practice Lithography questions

Key terms explained

Photoresist
A light-sensitive material whose solubility in developer changes when exposed. In positive resist the exposed areas dissolve; in negative resist they remain.
Positive photoresist
A photoresist whose exposed areas dissolve and are removed during development. One whose exposed areas remain is a negative resist.
Chemically amplified resist
A resist in which acid generated by exposure catalyzes a chain of reactions during the post-exposure bake (PEB). It needs little exposure dose and is widely used from KrF onward, but is sensitive to basic contamination.

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