Fabrication of High-Aspect-Ratio Structures Using SU-8 Photoresist and UV Lithography

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Aug 20, 2026

SU-8 is an epoxy-based, negative-tone photoresist widely used in microfabrication for producing thick, high-aspect ratio 3D microstructures. Its name derives from the presence of eight epoxy groups per monomer unit, which enables extensive crosslinking upon UV exposure and thermal activation . SU-8 has become the material of choice for high-aspect-ratio (HAR) microstructures because it combines high chemical and mechanical stability, biocompatibility, optical transparency, and low fabrication cost .

Traditional methods like LIGA require expensive synchrotron X-ray sources. SU-8, however, achieves comparable HAR structures (up to 100:1) using conventional UV lithography at 365 nm (i-line), eliminating the need for X-rays . This makes SU-8 UV lithography a cost-effective alternative for fabricating structures in the 1 µm to >500 µm thickness range.

Chemistry of SU-8: Chemically Amplified Crosslinking

SU-8 is a chemically amplified resist. It is composed of:

Bisphenol A Novolac epoxy resin (the polymer backbone)
An organic solvent (gamma-butyrolactone (GBL) or cyclopentanone)
Up to 10 wt% triarylsulfonium hexafluoroantimonate salt acting as the photoacid generator (PAG)

The reaction proceeds as follows:

UV Absorption → photons at 365 nm decompose the PAG, generating hexafluoroantimonic acid ( $HSbF_6$ ).
Acid Catalysis → the acid protonates epoxy groups, initiating cationic ring-opening polymerization.
Thermal Activation (PEB) → crosslinking accelerates at elevated temperature ( $\sim$ 95 °C), forming a dense, insoluble epoxy network.

$\text{PAG} \xrightarrow{h\nu \, (365 \, \text{nm})} HSbF_6$

$HSbF_6 + \text{SU-8 epoxy} \xrightarrow{\Delta T \, (\sim 95°C)} \text{Crosslinked Network}$

A single photon can trigger multiple polymerization events, which is the hallmark of chemical amplification .

Applications of SU-8 HAR structures include:

Microfluidic channel molds (soft lithography masters)
MEMS mechanical components
Bio-MEMS and biosensors
Micro-optical components
Carbon microstructures via pyrolysis

SU-8 photoresist — Wikipedia — Composition, chemistry, and processing overview of SU-8. ↩ ↩²
Fabrication of High Aspect Ratio SU-8 Structures Using UV Lithography and Megasonic-Enhanced Development — EV Group, ECS Transactions, 2010. ↩
Innovative SU-8 Lithography Techniques and Their Applications — Micromachines, 2015. ↩ ↩²

Photoresist (SU-8) and Soft Lithography

Substrate Preparation

Step 1

Clean and dehydrate the wafer (e.g., Piranha clean, DI rinse, 200°C bake) to remove contaminants and surface moisture; this ensures adhesion."

Spin Coating

Step 2

Dispense SU-8 and spin at controlled rpm to achieve target thickness. Thickness depends on viscosity grade and spin speed: h ∝ ω^(−1/2)."

Soft Bake (Pre-Bake)

Step 3

Ramp from 65°C to 95°C on a hotplate to remove solvent. Duration scales with thickness (hours for >200 µm films)."

UV Exposure

Step 4

Expose through a photomask at 365 nm. Use optical filters to cut <350 nm. Dose scales with thickness; hard contact mode for dense arrays."

Post-Exposure Bake (PEB)

Step 5

Ramp 65°C → 95°C to drive acid-catalyzed crosslinking. Gradual ramping minimizes thermal stress and cracking."

Development

Step 6

Immerse in SU-8 Developer (PGMEA) with agitation. Thick films (>200 µm) require extended development (30 min–4 hr)."

Rinse & Hard Bake (Optional)

Step 7

Rinse with IPA, dry. Optional hard bake at 150–200°C for improved mechanical and thermal stability."

1
Step 1
showNumbering: true

steps:
2
Step 2
Begin by cleaning silicon, glass, or other substrates using standard cleanroom procedures (e.g., Piranha solution: $H_2SO_4 : H_2O_2 = 3:1$ ), followed by DI water rinse and dehydration bake at 200°C for 5–30 minutes. Surface cleanliness is critical for adhesion, as any residue or moisture can cause delamination of thick SU-8 films . For improved adhesion, an adhesion promoter (e.g., OmniCoat) may be spin-coated before SU-8 application.

Footnotes

SU-8 Photoresist for Microfabrication: A Complete Guide — Darwin Microfluidics blog. ↩
3
Step 3
Select the appropriate SU-8 viscosity grade based on the desired film thickness. Film thickness $h$ approximately follows: $h \propto \omega^{-1/2}$ where $\omega$ is the spin speed in rpm. Typical spin programs: 500 rpm for 5–10 s (spread), then 1000–3000 rpm for 30–45 s (define thickness). For very thick layers (>200 µm), multiple coating passes or high-viscosity formulations like SU-8 2100 (>400 µm at 1000 rpm) are used . Edge bead removal (EBR) is essential for uniform mask contact.

Footnotes

SU-8 2000 Processing Guidelines — MicroChem/Kayaku datasheet. ↩
4
Step 4
Perform a two-stage hotplate bake: first at 65°C (ramp at ~5°C/min), then at 95°C. The 65°C stage prevents skin formation by allowing gradual solvent release; the 95°C stage drives off residual solvent. Bake times scale with thickness: for 100 µm films, ~10–15 min at 95°C; for 500 µm films, several hours . After baking, cool gradually back to 65°C on the hotplate, then to room temperature on a level non-metal surface to avoid warping and stress cracking.

Footnotes

Optimized SU-8 Processing for Low-Cost Microstructures — Micromachines, 2014. ↩
5
Step 5
Expose through a photomask using a mask aligner at the i-line (365 nm). Use an optical filter (e.g., Hoya UV-34) to block wavelengths below 350 nm, preventing T-topping — a defect where the top surface over-crosslinks due to high short-wavelength absorption . The exposure dose $D$ scales with film thickness: approximately 80–150 mJ/cm² for 100 µm, and higher for thicker films. For doses >250 mJ/cm², split exposure into multiple steps with cooling intervals . Hard contact mode is preferred for dense HAR arrays to minimize air gaps and diffraction.

Footnotes

MEMScyclopedia — SU-8 — Community-contributed characterization of SU-8 photosresist for MEMS. ↩

SU-8 Photolithography as a Toolbox for Carbon MEMS — Micromachines, 2014. ↩
6
Step 6
Ramp the wafer from 65°C to 95°C on a hotplate. The photo-generated acid ( $HSbF_6$ ) catalyzes crosslinking at elevated temperature. Typical PEB: 5 min at 65°C, then 5–30 min at 95°C depending on thickness. Gradual ramping (≤5°C/min) is critical: rapid heating generates thermal gradients and bulk stress, causing microcracking and delamination . After PEB, cool slowly back to 65°C then to room temperature.

Footnotes

SU-8 Photoresist for Microfabrication: A Complete Guide — Darwin Microfluidics blog. ↩
7
Step 7
Immerse in PGMEA-based SU-8 developer with gentle to strong agitation. Development removes unexposed (uncrosslinked) SU-8. Development time depends on film thickness and feature density: thin films (<50 µm) require 1–5 min; thick films (>200 µm) require 30 min to 4 hours . For HAR structures, strong agitation or megasonic-enhanced development can reduce time dramatically — from 240 min down to 10 min . Rinse with IPA; a white residue indicates incomplete development (re-immersing in developer resolves this).

Footnotes

Fabrication of High Aspect Ratio SU-8 Structures Using UV Lithography and Megasonic-Enhanced Development — EV Group, ECS Transactions, 2010. ↩ ↩²
8
Step 8
Rinse thoroughly with IPA followed by DI water, then dry gently with nitrogen. An optional hard bake at 150–200°C further crosslinks the SU-8 network, improving mechanical strength, chemical resistance, and thermal stability (glass transition temperature $T_g > 200°C$ for fully cured SU-8). This step is recommended for permanent structures or molds subjected to repeated PDMS casting.

SU-8 Formulations and Thickness Ranges

The SU-8 2000 series offers twelve viscosity grades, each designed for a specific thickness range. Film thickness depends on both the viscosity grade and the spin speed, following the approximate relation $h = k \cdot \nu^{0.5} \cdot \omega^{-0.5}$ , where $\nu$ is kinematic viscosity and $\omega$ is spin speed .

SU-8 Grade	Viscosity (cSt)	Single-Coat Thickness (µm)	Spin Speed (rpm)
SU-8 2002	~4	1.5–2	3000
SU-8 2005	~8	5–7	3000
SU-8 2015	~60	15–25	3000
SU-8 2035	~160	30–50	3000
SU-8 2075	~2,500	50–100	2000–3000
SU-8 2100	~45,000	100–250	1000–3000
SU-8 2150	~80,000	200–550+	1000–2000

Note: Final thickness is sensitive to ambient temperature. For SU-8 2150, the same spin recipe yields different thicknesses at different room temperatures (e.g., 550 µm at 1000 rpm and 19–23°C vs. 370 µm at 24–27°C) .

Aspect Ratio Capability: SU-8 can achieve aspect ratios exceeding 20:1 with standard formulations and over 40:1 with dry film resist . Advanced techniques like megasonic-enhanced development have demonstrated aspect ratios of 23:1 at 470 µm height , while drawing lithography has produced needle-like structures with aspect ratios >100:1 .

SU-8 2000 Processing Guidelines — MicroChem/Kayaku datasheet. ↩
Optimized SU-8 Processing for Low-Cost Microstructures — Micromachines, 2014. ↩
Innovative SU-8 Lithography Techniques and Their Applications — Micromachines, 2015. ↩ ↩²
Fabrication of High Aspect Ratio SU-8 Structures Using UV Lithography and Megasonic-Enhanced Development — EV Group, ECS Transactions, 2010. ↩

type: "warning" title: "Critical: Thermal Stress Management" content: "The most common failure mode in thick SU-8 processing is cracking and delamination caused by thermal stress during baking steps. Always use gradual temperature ramps (≤5°C/min) and allow the wafer to cool slowly from 95°C → 65°C → room temperature on the hotplate before removing. Rapid cooling causes internal stress due to the CTE (coefficient of thermal expansion) mismatch between SU-8 (~52 ppm/K) and silicon (~3 ppm/K) ."

SU-8 Photoresist for Microfabrication: A Complete Guide — Darwin Microfluidics blog. ↩

type: "tip" title: "Optimization Tip: Reducing T-Topping" content: "T-topping is a well-known defect in SU-8 where the top of the structure broadens due to over-exposure at short UV wavelengths (<350 nm). Use an optical filter (e.g., Hoya UV-34, 2.5 mm thick) to block wavelengths below 350 nm and transmit only i-line (365 nm) radiation. This ensures uniform acid generation through the film depth and produces near-vertical sidewalls. For dense arrays, also use hard contact exposure mode and lower-than-recommended doses (~180 mJ/cm² for films up to 200 µm) to prevent crosslinking in narrow gaps due to diffraction 2."

MEMScyclopedia — SU-8 — Community-contributed characterization of SU-8 photosresist for MEMS. ↩
SU-8 Photolithography as a Toolbox for Carbon MEMS — Micromachines, 2014. ↩

Key Process Challenges and Optimization Strategies for HAR Structures

Challenge	Root Cause	Mitigation Strategy
T-topping	High absorption at <350 nm overexposes the top layer	Use 365 nm bandpass filter; reduce dose for dense arrays
Cracking	Thermal stress from rapid heating/cooling; insufficient exposure	Gradual ramp ≤5°C/min; optimize exposure dose
Delamination	Poor adhesion; CTE mismatch with substrate	Use adhesion promoter (OmniCoat); proper dehydration
Incomplete development	Thick films (>200 µm) trap unexposed resist	Megasonic-enhanced development; extended time
Mask sticking	Residual solvent from insufficient soft bake	Ensure complete solvent removal in soft bake
Sidewall undercut	UV absorption depth limit in very thick films	Keep thickness ≤500 µm for standard contact lithography

Exposure Dose Considerations: The correct dose is one of the most critical parameters. Insufficient exposure leads to structures detaching during development, while excessive dose causes feature broadening and T-topping. The relationship between dose and thickness is approximately:

$D_{optimal} \approx D_0 \cdot \left(\frac{t}{t_0}\right)^{\alpha}$

where $D_0$ is a base dose (e.g., 80 mJ/cm² for 100 µm), $t$ is film thickness, and $\alpha \approx 1$ – $1.5$ .

MEMScyclopedia — SU-8 — Community-contributed characterization of SU-8 photosresist for MEMS. ↩ ↩²
SU-8 Photoresist for Microfabrication: A Complete Guide — Darwin Microfluidics blog. ↩ ↩²
Fabrication of High Aspect Ratio SU-8 Structures Using UV Lithography and Megasonic-Enhanced Development — EV Group, ECS Transactions, 2010. ↩
Innovative SU-8 Lithography Techniques and Their Applications — Micromachines, 2015. ↩
SU-8 Photolithography as a Toolbox for Carbon MEMS — Micromachines, 2014. ↩

Advanced Lithography Techniques for 3D SU-8 Structures

Beyond standard front-side UV exposure, several innovative techniques exploit SU-8's unique properties for 3D structuring:

Technique	Principle	Typical Application
Inclined UV exposure	Tilted substrate during exposure; angled light path	Microfluidic mixers, microneedles
Back-side UV exposure	UV enters through transparent substrate from below	Embedded microchannels, 3D interconnects
Moving-mask UV lithography	Mask translates during exposure; dose varies spatially	Multi-height structures from single resist layer
Multi-photon polymerization	Femtosecond laser; two-photon absorption at focal point	Arbitrary 3D structures, photonic crystals
Drawing lithography	Mechanical pulling of soft-baked SU-8	Microneedles; aspect ratio >100:1
Maskless lithography	SLM-based projection; no physical mask	Rapid prototyping, micropillar arrays

Innovative SU-8 Lithography Techniques and Their Applications — Micromachines, 2015. ↩
Optimization of maskless SU-8 photolithography for fabrication of dense high-aspect-ratio pyrolytic carbon micropillar arrays — DTU, Results in Chemistry, 2025. ↩

1 / 9

Question · Term

- title: "SU-8 Fabrication Key Concepts"

Click to reveal

Answer · Definition

substrate: "Silicon, 100mm, <100>"
adhesion_promoter: "OmniCoat, 2000rpm, 30s, 200°C, 2min"
su8_grade: "SU-8 2100"
target_thickness: 200 # µm
spin_coating:
  spread: 500 # rpm
  spread_time: 10 # s
  spread_accel: 100 # rpm/s
  spin: 1500 # rpm
  spin_time: 30 # s
  spin_accel: 300 # rpm/s
  edge_bead_removal: true
soft_bake:
  temp_step1: 65 # °C
  time_step1: 10 # min
  ramp_rate: 5 # °C/min
  temp_step2: 95 # °C
  time_step2: 90 # min
  cool_down: "65°C → room temp (gradual)"
exposure:
  wavelength: 365 # nm (i-line)
  filter: "Hoya UV-34 (blocks <350 nm)"
  contact_mode: "hard contact"
  dose: 180 # mJ/cm²
  num_steps: 1
post_exposure_bake:
  temp_step1: 65 # °C
  time_step1: 5 # min
  ramp_rate: 5 # °C/min
  temp_step2: 95 # °C
  time_step2: 30 # min
  cool_down: "65°C → room temp (gradual)"
development:
  developer: "SU-8 Developer (PGMEA)"
  time: 30 # min
  agitation: "strong manual"
  rinse: "IPA → DI water → N₂ dry"
hard_bake:
  enabled: false
  temp: 200 # °C
  time: 30 # min

Knowledge Check

Question 1 of 5

Q1Single choice

Which UV wavelength is optimal for exposing SU-8 photoresist, and why?

436 nm (g-line) — because it provides the deepest penetration

365 nm (i-line) — because shorter wavelengths below 350 nm cause T-topping, and g-line has insufficient energy for efficient crosslinking

254 nm — because shorter wavelengths provide higher resolution

All wavelengths above 300 nm work equally well

Summary and Key Takeaways

The fabrication of high-aspect-ratio microstructures with SU-8 photoresist and UV lithography is a well-established, cost-effective process that follows a systematic workflow of seven key steps: substrate preparation, spin coating, soft bake, UV exposure, post-exposure bake, development, and optional hard bake .

Key success factors:

Control solvent removal during soft bake — too fast causes skinning, too slow wastes time and risks contamination.
Optimize exposure dose for film thickness and pattern density — use 365 nm filtered light, hard contact mode, and consider dose splitting for thick films.
Manage thermal stress through gradual temperature ramping during PEB and controlled cooling.
Ensure complete development using extended times, strong agitation, or megasonic enhancement for thick and dense HAR structures.
Select appropriate SU-8 grade based on target thickness and aspect ratio — higher viscosity grades (SU-8 2100/2150) enable thicker films but require longer processing times.

The unique chemically amplified crosslinking chemistry of SU-8 — in which a single photon triggers cascade reactions — enables high sensitivity and the formation of mechanically robust structures rivaling those produced by the far more expensive LIGA process 2.

SU-8 Photoresist for Microfabrication: A Complete Guide — Darwin Microfluidics blog. ↩
SU-8 photoresist — Wikipedia — Composition, chemistry, and processing overview of SU-8. ↩
Fabrication of High Aspect Ratio SU-8 Structures Using UV Lithography and Megasonic-Enhanced Development — EV Group, ECS Transactions, 2010. ↩

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Browse all research articles

Fabrication of High-Aspect-Ratio Structures Using SU-8 Photoresist and UV Lithography

Chemistry of SU-8: Chemically Amplified Crosslinking

Footnotes

Photoresist (SU-8) and Soft Lithography

Substrate Preparation

Spin Coating

Soft Bake (Pre-Bake)

UV Exposure

Post-Exposure Bake (PEB)

Development

Rinse & Hard Bake (Optional)

Footnotes

Footnotes

Footnotes

Footnotes

Footnotes

Footnotes

SU-8 Formulations and Thickness Ranges

Footnotes

Footnotes

Footnotes

Key Process Challenges and Optimization Strategies for HAR Structures

Footnotes

Advanced Lithography Techniques for 3D SU-8 Structures

Footnotes

- title: "SU-8 Fabrication Key Concepts"

Knowledge Check

Summary and Key Takeaways

Footnotes

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