Light-Based Lithography in Microchip Miniaturization

By Ibtisam AbbasiReviewed by Lexie CornerOct 4 2024

Significant progress has been made in producing thinner microchips with extraordinary computational capabilities. Light-based lithography has been instrumental in advancing semiconductor technology for over five decades, enabling precise printing of electronic components, circuits, and ultra-thin semiconductor chips. This method represents the pinnacle of precision manufacturing.¹

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The latest development in this field, extreme ultraviolet lithography (EUVL), plays a key role in producing fast and efficient computer chips, which support advancements in artificial intelligence, modern electronics, 5G technology, and quantum machine learning.² This article discusses the importance of light-based lithography in enabling the production of thinner, more efficient microchips.

The Importance of Light-Based Lithography in Semiconductor Industry

Photolithography, or light-based lithography, is the most widely used technique in semiconductor manufacturing. These chips contain miniaturized transistors—tiny silicon switches that process and store data.

As the number of transistors on a chip increases, so does its ability to compute and process data rapidly. Over the years, advancements in lithography have allowed the use of shorter wavelengths of light, enabling even smaller transistors and resulting in thinner chips with much faster processing times.³

At its core, light-based lithography relies on projecting and recording optical images onto a photoresist, a light-sensitive material, to create precise patterns on a chip. The quality of these recorded images directly impacts the performance of the electronic components produced during the subsequent fabrication stages.

Currently, chemically amplified resists (CARs) are the foundation of this technology. These photoresists consist of a polymeric core attached to photoacid generators (PAGs) along with other protected groups and quenchers.

During the lithography process, the interaction between light and the PAGs leads to the emission of protons. These protons diffuse through the material, causing significant changes in the solubility of functional groups, leading to the formation of hydrophilic de-protected groups.

During the development phase, these hydrophilic groups dissolve, leaving behind the unexposed areas of the photoresist, which feature characteristic undulations known as line-width roughness (LWR).⁴ This process is crucial in creating the intricate patterns that define modern microchips.

One common technique, projection printing, involves coating a semiconductor wafer with a polymer and exposing it to ultraviolet light as the light passes through a mask. Optical elements focus the mask's image onto the wafer surface, ensuring the production of high-resolution patterns.⁵

Impact on Microchip Miniaturization

The Integrated Chip (IC) fabrication industry has transformed significantly, enabling the development of highly efficient ICs with reduced transistor feature sizes. Lithography has been instrumental in producing micro and nanoscale semiconductor chips.

Traditional lithography methods used light sources emitting wavelengths between 193 nm and 248 nm. For even shorter wavelengths, EUVL was introduced, utilizing 13.5 nm photons from tin (Sn) plasma and a network of mirrors and optics to project patterns onto silicon wafers.⁶

According to Rayleigh's criterion, shorter wavelength light sources are used to fabricate nanostructures with smaller feature sizes. EUVL has enabled a further reduction in feature size, improving microchip efficiency and reducing manufacturing times.⁷

The evolution of EUVL, from fabricating micrometer-sized structures to today’s high-resolution nano-patterns, has been driven by advancements in semiconductor materials, light sources, and photomasks. With ongoing research into advanced photoresist materials, more efficient semiconductor chips and smaller ICs are expected in the future.

Technological Advancements

EUV-Interference Lithography (EUV-IL) with 5nm Resolution

Researchers at the Paul Scherrer Institute recently utilized EUV-IL to produce more efficient and dense microchips using a novel mirror-based technology. This breakthrough allowed for line patterning of photoresist material with a resolution of 5 nm, a significant achievement in semiconductor manufacturing. Achieving patterning below 10 nm resolution was previously considered nearly impossible.⁸

The novel EUV mirror interference lithography (MIL) method involves reflecting two coherent beams by identical mirrors, creating an interference pattern where the pitch is controlled by adjusting the grazing angles and light source wavelength.

To achieve these results, researchers employed the EUV-IL tool at the XIL-II beamline at the Paul Scherrer Institute, setting a new standard for both scientific and industrial applications, particularly in the development of advanced photoresist and under-layer materials.

Development of Novel Photoresist Materials

Since 2019, EUVL has become the leading technique for high-volume manufacturing (HVM), replacing traditional deep ultraviolet lithography (DUVL). However, advancements in lithography have introduced challenges. As the light source wavelength decreases, photon energy increases, leading to greater photoresist degradation and stochastic effects.

Current EUVL photoresist systems need optimization to achieve ultrahigh-resolution patterning. Therefore, there is a growing need for advanced photoresists that can efficiently absorb EUV photons.⁹

To address this, researchers have developed new molecular glass photoresists based on Pyrene derivatives with t-Boc-protected groups. The pyrene core, connected to several benzene rings, offers etch resistance and thermal stability, while the t-Boc groups function as acid-sensitive elements, enabling a dissolution transition at lower exposure doses.¹⁰

Both materials, Pyr-4Boc and Pyr-8Boc, demonstrated excellent thermal and film-forming properties, making them suitable as positive photoresists. Testing revealed their initial decomposition temperatures to be above 170 °C, confirming their applicability for high-resolution applications.

Both materials also exhibited a root-mean-square (RMS) roughness below 0.4 nm, indicating their potential for forming stable films. Among the two, Pyr-4Boc offered superior performance, achieving a higher resolution (25 nm half-pitch) with greater sensitivity (50 μC/cm²) and contrast (4.9), compared to Pyr-8Boc.

Additionally, the Pyr-Boc photoresists showed excellent etch resistance, with Pyr-4Boc having double the etch selectivity of commercial PMMA photoresists (950k). These qualities make Pyr-Boc photoresists strong candidates for high-resolution lithography applications, offering enhanced performance.

Conclusion: Overcoming Lithography Challenges for Future Microchips

The adoption of shorter wavelength light sources has significantly improved lithography performance, enabling the production of more efficient microchips. While processes like EUVL continue to evolve, challenges remain, particularly with the degradation of photoresist materials.

Addressing these issues requires the development of novel lithography materials. Ongoing research in this area is expected to contribute to further advancements in microchip performance.

Discover More: How to Optimize Semiconductor Lithography with Multi-Sensor Metrology

References and Further Reading

1. Saifullah, M., et al. (2022). Review of metal-containing resists in electron beam lithography: perspectives for extreme ultraviolet patterning. Journal of Micro/Nanopatterning, Materials, and Metrology. Available at: https://doi.org/10.1117/1.JMM.21.4.041402
2. Fu, N., et al. (2019). EUV lithography: state-of-the-art review. J. Microelectron. Manuf. https://doi.org/10.33079/jomm.19020202
3. Duque, T. (2022). Pushing the Boundaries of Moore’s Law: How Can Extreme UV Light Produce Tiny Microchips? Berkeley Lab. (Online). Available at: https://newscenter.lbl.gov/2022/06/03/extreme-uv-light-tiny-microchips/
   [Accessed on: September 10, 2024].
4. Saifullah, M., et al. (2024). Approaching Angstrom-Scale Resolution in Lithography Using Low-Molecular-Mass Resists (< 500 Da). ACS nano. https://doi.org/10.1021/acsnano.4c03939
5. Paik, S., et al. (2020). Near-field sub-diffraction photolithography with an elastomeric photomask. Nat. Commun. https://doi.org/10.1038/s41467-020-14439-1
6. Raza, A. et al. (2024). Advances, Application and Challenges of Lithography Techniques. In 2024 5th International Conference on Advancements in Computational Sciences (ICACS). https://doi.org/10.1109/ICACS60934.2024.10473245
7. Zhang, Y., et al. (2024). Advanced lithography materials: From fundamentals to applications. Advances in Colloid and Interface Science. https://doi.org/10.1016/j.cis.2024.103197
8. Giannopoulos, I., et al. (2024). Extreme ultraviolet lithography reaches 5 nm resolution. Nanoscale. https://doi.org/10.1039/D4NR01332H
9. Hasan M., et al. (2024). Recent Advances in Metal-Oxide-Based Photoresists for EUV Lithography. Micromachines. https://doi.org/10.3390/mi15091122
10. Cong, X., et al. (2024). Novel Etch-Resistant Molecular Glass Photoresist Based on Pyrene Derivatives for Electron Beam Lithography. ACS Omega. https://doi.org/10.1021/acsomega.4c01044

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