EUV Resist Chemistry and Underlayers

Abstract

The resist is where an optical intensity field becomes a material boundary. At 13.5 nanometers, photons are absorbed discretely and generate energetic electrons that initiate chemical events over finite distances. Conventional chemically amplified resists use photoacid generation, protected polymer, quencher, post-exposure diffusion, and development to amplify sensitivity. Metal-containing and more molecularly uniform materials seek greater EUV absorption or reduced chemical variability. High NA requires thinner films because depth of focus and aspect-ratio collapse become tighter, but thin films provide less etch budget. Underlayers must therefore manage adhesion, wetting, reflectivity where relevant, chemical interaction, mechanical behavior, and pattern transfer. I evaluate materials through resolution, roughness, sensitivity, stochastic failure, outgassing, contamination, collapse, shelf life, process latitude, and etched result. No resist can be optimized in isolation. The useful object is a stack and process that converts an aerial image into a transferable feature population at manufacturing dose.

The resist records energy through chemistry

An EUV photon at wavelength $\lambda$ has energy

$$E_\gamma=\frac{hc}{\lambda}.$$

At 13.5 nanometers this is about 92 electronvolts, far above ordinary chemical-bond energies. Absorption can eject photoelectrons and generate secondary electrons. Their transport spreads energy beyond the initial absorption site and creates ions, radicals, acids, or other active species depending on material.

The optical image is therefore not copied directly. It is convolved with absorption, electron transport, chemical generation, diffusion, reaction, dissolution, and mechanical survival.

For resist absorption coefficient $\alpha$ and film thickness $t$, absorbed fraction is

$$F_{abs}=1-e^{-\alpha t}.$$

Thinner High NA films absorb fewer photons unless material absorption rises. Increasing absorber content can change electron yield, chemistry, film properties, contamination, and etch behavior.

Chemical amplification trades dose for blur and variability

In a chemically amplified positive-tone resist, exposure creates photoacid. Post-exposure bake allows acid to catalyze deprotection reactions, changing developer solubility. One generated acid can cause several reactions, increasing sensitivity.

Amplification introduces distributions. Photoacid generators and quenchers occupy discrete locations. Acid diffuses and may be neutralized. Polymer composition and free volume vary. Bake temperature and time change reaction length. Excess diffusion blurs the edge. Too little can leave unreacted islands and defects.

A characteristic diffusion length can be screened as

$$L_d\approx\sqrt{2D_at},$$

where $D_a$ is effective acid diffusivity and $t$ is bake time. The model is simple because diffusivity and reaction depend on local chemistry, but it shows why bake control and molecular design affect resolution.

Resist architecture	Possible advantage	Central risk
Chemically amplified polymer	Mature process and high sensitivity	Component distribution, diffusion, blur
Metal-containing resist	Higher EUV absorption and etch resistance	Contamination, residue, chemistry, integration
Molecular or single-component	Reduced component segregation	Synthesis, glass formation, sensitivity, process robustness
Dry-deposited resist	Thin conformal films and material efficiency	Deposition control, defectivity, new process ecosystem

Imec notes development of metal-containing and single-component materials to address chemical stochasticity. None is automatically superior. Each changes the full stack.

Development turns chemistry into topology

Developer removes one chemical state preferentially. Dissolution contrast, swelling, surface inhibition, developer transport, rinse, and drying determine the final profile. Aerial image and latent chemistry can be acceptable while development creates scum, footing, top loss, roughness, or missing features.

For dense contacts, developer must enter and clear small volumes. For narrow lines, capillary forces during drying can collapse the pattern. Collapse pressure grows with surface tension and geometry while mechanical resistance falls rapidly with smaller width and taller aspect ratio.

Thinner resist reduces aspect ratio and helps collapse. It also reduces transfer margin and signal for metrology. Rinse chemistry, surface treatment, mechanical modulus, and critical-point or alternative drying approaches can change the outcome.

AbsorbDeposit discrete photon energy AmplifyGenerate and transport chemistry DifferentiateCreate dissolution contrast DevelopForm free-standing topology TransferMove edge into hard material

Underlayers are active interfaces

The underlayer beneath resist can promote adhesion, control surface energy, prevent intermixing, tune acid or radical behavior, reduce poisoning, provide etch selectivity, and act as a hard mask. Its composition can also generate secondary electrons or change dose response.

Surface energy must support uniform coating without dewetting while preserving later development. Chemical groups at the interface can quench active species and create footing or residue. Roughness can print through a very thin film. Outgassing can contaminate scanner optics.

I treat the resist-underlayer boundary as a designed reaction region. Measurements should include thickness, roughness, composition, interdiffusion, adhesion, latent-image depth, dissolution profile, collapse, and etched edge. A stack that looks excellent on a silicon coupon may behave differently over real device topography and materials.

High NA makes stack thickness a focus contributor. Uniformity across wafer and local topography must be held within the focus and etch budget. Coat and bake equipment, dispense, solvent, filtration, exhaust, and wafer history all matter.

ImageEUV photon and electron field

Sets discrete energy deposition with finite lateral and depth spread.

RecordReactive resist film

Creates chemical contrast through molecularly distributed events.

SupportEngineered underlayer

Controls interface chemistry, adhesion, mechanics, and transfer access.

CarryHard mask and target layer

Receives the edge through selective etch and final material processing.

The RLS triangle needs a fourth axis

Resist development has long traded resolution, line-edge roughness, and sensitivity. Lower dose improves throughput and worsens photon statistics. More chemical blur can improve sensitivity and roughness at some scales while reducing resolution. Higher contrast may demand less soluble or more brittle materials.

Stochastic failure adds a fourth axis. Average roughness can look acceptable while rare missing contacts or bridges destroy yield. Imec explicitly identifies failure as an additional performance dimension.

I evaluate a material with:

• dense and isolated resolution across orientations;
• line-width and edge-roughness spectrum;
• local critical-dimension uniformity;
• missing, merged, bridged, and collapsed feature probability;
• dose-to-size and process window;
• outgassing and contamination;
• film uniformity, shelf life, and track compatibility;
• underlayer adhesion and chemical interaction;
• etch selectivity and post-etch edge;
• inspection and rework behavior.

A single best-dose image cannot represent that population.

Material screening needs mechanism

High-throughput screening can identify promising formulations. Mechanistic measurement explains why they work and whether improvement will transfer to other patterns and tools. NIST work on next-generation lithography polymers emphasizes reaction-diffusion profiles and latent-image behavior because buried chemistry governs edges.

I want calibrated absorption, electron yield and transport proxies, chemical-generation efficiency, quencher distribution, diffusion, glass transition, dissolution kinetics, mechanical modulus, surface energy, outgassing species, and etch response. Models should predict changes before recipes are retuned.

The resist frontier is not the material with the lowest dose or finest isolated feature. It is the stack whose physical and chemical transformations are uniform enough that trillions of features survive exposure, development, and transfer. That requires materials science connected continuously to statistics and device yield.