Negative-tone photoresist is a type of photosensitive material used in photolithography and microfabrication. It undergoes a chemical change when exposed to ultraviolet (UV) light, electron beams, or other radiation sources. The exposed regions harden and become insoluble, while the unexposed regions remain soluble and can be washed away during development.
How It Works
- Coating: The negative-tone photoresist is applied as a thin film onto a substrate.
- Exposure: The photoresist is exposed to UV light or another radiation source through a photomask, which defines the pattern.
- Development: The unexposed areas are dissolved in a developer solution, leaving behind the hardened (exposed) regions.
Key Characteristics
- Pattern Formation: The final pattern on the substrate corresponds to the exposed areas, making it “negative” compared to the mask.
- High Resolution: Used in applications requiring fine features with smooth sidewalls.
- Common Materials: SU-8, KMPR, AZ 15nXT are some examples of negative-tone resists.
Applications
- 3D Nanofabrication & Microfabrication (e.g., MEMS, photonic devices, optical elements).
- Micro-optics & Photonic Integration (e.g., spot size converters, waveguides, and lenses).
- Biomedical Devices (e.g., lab-on-chip systems).
Difference from Positive-Tone Photoresist
| Feature | Negative-Tone Photoresist | Positive-Tone Photoresist |
|---|---|---|
| Exposure Effect | Exposed areas harden and remain | Exposed areas become soluble |
| Pattern Formation | Inverse of the photomask | Same as the photomask |
| Resolution | Better for thicker layers | Generally higher for ultra-fine features |
| Common Use Cases | Micro-optics, 3D printing, MEMS | IC lithography, semiconductor fabs |
Refractive Index: The refractive index of negative-tone photoresists typically ranges from 1.49 to 1.57. For instance, a study measured the refractive indices of various photoresists used in three-dimensional direct laser writing and found values within this range.
Abbe Number: The Abbe number, which indicates the material’s dispersion (variation of refractive index with wavelength), for these photoresists ranges between 35 and 51. Higher Abbe numbers correspond to lower dispersion.
Optical Characteristics:
- Transparency: Negative-tone photoresists are generally transparent in the visible spectrum, making them suitable for optical applications.
- Absorption: The absorption properties can vary based on the specific formulation and exposure wavelength.
- Thermo-Optic Coefficient: Some negative-tone photoresists exhibit a thermo-optic coefficient (dn/dT) of approximately −6.8 × 10⁻⁵ K⁻¹, indicating a decrease in refractive index with increasing temperature. if.t.u-tokyo.ac.jp
These optical properties are essential for designing and optimizing processes in microfabrication and photonic device manufacturing.
