Novolak Resin and DNQ: How the Photoresist Pair Actually Works
DNQ novolak resist mechanism — dissolution inhibition, Wolff rearrangement, positive-tone development. Established chemistry for g-line/i-line photoresists.
Positive photoresists built on diazonaphthoquinone (DNQ) and novolak resin have patterned silicon wafers since the 1970s. The chemistry is well characterised in the published literature — yet procurement teams and new formulators often receive only a product data sheet without understanding why the two components must work together.
This article explains the DNQ-novolak resist mechanism: what each component contributes, what happens on exposure and in developer, and why formulation ratios matter. The science here is established public knowledge. Orion-specific performance claims are avoided.
The two components
A DNQ-novolak positive photoresist contains:
Novolak resin — a phenol-formaldehyde condensation polymer (typically cresol novolak). Novolak alone dissolves readily in aqueous alkaline developer (TMAH). It provides the film-forming matrix, adhesion to the silicon wafer, and mechanical integrity after development.
DNQ photoactive compound (PAC) — a diazonaphthoquinone derivative (typically esterified with THBP, cresol, or other phenolic groups). DNQ alone does not form a useful resist film. Its role is to modify novolak dissolution behaviour on exposure.
Neither component works alone as a photoresist. Together they create a system where unexposed areas resist developer and exposed areas dissolve — the definition of positive-tone behaviour.
Dissolution inhibition in unexposed areas
In the unexposed resist film, DNQ PAC molecules interact with novolak resin through dissolution inhibition:
- DNQ PAC associates with novolak polymer chains
- This association reduces the rate at which novolak dissolves in alkaline developer
- The unexposed film therefore remains intact during development
The degree of inhibition depends on:
- PAC loading relative to novolak (standard ratio: 3.0–3.5 parts novolak to 1 part PAC)
- PAC ester structure (THBP esters vs cresol esters vs CNR systems)
- Novolak molecular weight and acid-dissolution rate (ADR)
Higher PAC loading increases photospeed but can reduce unexposed-film resistance to developer (dark erosion). Lower PAC loading improves dark erosion resistance but requires more exposure energy.
What happens on UV exposure: the Wolff rearrangement
When UV photons (g-line at 436 nm or i-line at 365 nm) strike DNQ PAC in the resist film:
- Photoabsorption — DNQ diazo ketone absorbs UV energy (n→π* transition)
- Nitrogen loss — the diazo group releases N₂ gas
- Wolff rearrangement — the molecule rearranges to form a ketene intermediate
- Hydrolysis — ketene reacts with trace water in the film to form an indene carboxylic acid
The carboxylic acid product is more soluble in alkaline developer than the parent DNQ PAC. In exposed areas, the dissolution inhibitor is destroyed and replaced by a dissolution promoter.
This is the fundamental photoswitch: DNQ converts from an inhibitor to a promoter on exposure.
Development: the solubility contrast
After exposure, the wafer is immersed in aqueous alkaline developer (typically tetramethylammonium hydroxide, TMAH):
The contrast between exposed and unexposed dissolution rates defines the resist's gamma (contrast curve). High-contrast resists show a sharp transition between soluble and insoluble at a narrow exposure dose range — critical for submicron feature fidelity.
| Film area | DNQ state | Novolak dissolution | Result |
|---|---|---|---|
| Unexposed | DNQ PAC intact | Inhibited — slow dissolution | Remains on wafer |
| Exposed | DNQ → carboxylic acid | Uninhibited — fast dissolution | Removes from wafer |
Post-exposure bake (PEB)
Many DNQ-novolak processes include a post-exposure bake (typically ~120 °C) between exposure and development. PEB:
- Completes the DNQ-to-acid conversion reaction
- Stabilises the resist profile
- Reduces standing-wave effects from reflective substrate
I-line processes are more sensitive to PEB temperature and time than g-line processes because submicron features demand steeper sidewalls.
Why ester structure matters
DNQ PACs are not a single molecule — they are a family of esters with different phenolic backbones:
Ester structure controls:
- Absorption spectrum — which wavelength the PAC photobleaches at
- Bleach speed — how quickly the inhibitor is destroyed on exposure
- Inhibition strength — how effectively unexposed areas resist developer
- Thermal stability — resistance to flow during high-temperature bake steps
Orion supplies DNQ-THBP building blocks and grades (P-1403, P-1610, P-3101, P-0711) for g-line/i-line photoresist programmes. For grade selection, see DNQ-THBP Grades Compared.
| PAC type | Ester backbone | Typical use |
|---|---|---|
| DNQ-THBP | Tetrahydroxy benzophenone | G-line and i-line (industry standard) |
| DNQ-cresol ester | Cresol novolak / cresol resin | UV CtCP plate coatings, some photoresists |
| DNQ-CNR | Cresol novolac resin matrix | Advanced i-line (e.g. Orion P-0711) |
Novolak resin selection
Novolak resins differ in:
- Molecular weight distribution — affects film mechanical properties and dissolution rate
- Meta/para cresol ratio — influences ADR and thermal properties
- Acid-dissolution rate (ADR) — the key parameter for developer resistance
Formulators choosing their own novolak (e.g. with Orion P-1403) select resin ADR to match their developer concentration and process time. Formulators using matched-novolak grades (e.g. Orion P-1610) accept Orion's resin selection for faster qualification.
Standard blend ratio: 3.0–3.5 parts novolak to 1 part PAC.
The same chemistry, different working modes
DNQ chemistry serves both photoresists and offset plates — but the working mode inverts:
The Wolff rearrangement is the same photochemical event. Formulation — resin type, DNQ loading, developer chemistry, and coating architecture — determines whether exposed areas stay or go.
See Positive vs Negative Working Plates for the plate-side comparison.
| Application | Working mode | Exposed area behaviour |
|---|---|---|
| Positive photoresist | Positive | Dissolves in developer (removed) |
| Conventional UV CtCP plate | Positive | Diazo coating removed in alkaline developer |
| Developer-free UV CtCP (Orion Next) | Negative | Coating remains; unexposed areas remove in water |
Formulation variables that affect resist performance
These are general formulation principles. Orion-specific performance data for individual P-series grades should be taken from product information PDFs and trial results — not assumed from this mechanism overview.
For wavelength-specific guidance, see g-Line vs i-Line Photoresist.
| Variable | Effect on performance |
|---|---|
| PAC loading | Photospeed vs dark erosion trade-off |
| Novolak ADR | Developer resistance and contrast |
| Resist thickness | Resolution limit and aspect ratio |
| PEB temperature/time | Sidewall angle and standing waves |
| Developer concentration | Dissolution rate and process window |
| Exposure wavelength | G-line (436 nm) vs i-line (365 nm) resolution |
Common questions.
How does DNQ make a novolak resist positive-working?
DNQ PAC inhibits novolak dissolution in unexposed areas. On UV exposure, DNQ undergoes a Wolff rearrangement to form a carboxylic acid that promotes dissolution. Exposed areas therefore dissolve in alkaline developer while unexposed areas remain — positive-tone behaviour.
What is the standard novolak-to-DNQ ratio in photoresists?
3.0–3.5 parts novolak resin to 1 part DNQ PAC. This ratio applies to DNQ-THBP positive-tone systems used in g-line and i-line microlithography.
Why do different DNQ esters exist?
DNQ is esterified with different phenolic backbones (THBP, cresol, novolac) to tune UV absorption wavelength, bleach speed, dissolution inhibition strength, and thermal stability for specific applications — photoresists, plate coatings, or advanced i-line nodes.
Is the DNQ-novolak mechanism the same for plate coatings and photoresists?
The Wolff rearrangement is the same photochemical event. Formulation determines the working mode: positive photoresists remove exposed areas in alkaline developer; conventional CtCP plates work similarly; developer-free CtCP plates (Orion Next) invert the mode — exposed areas remain, unexposed areas remove in water.