Grayscale lithography is becoming essential in microfabrication, particularly for industries demanding complex 2.5D microstructures. From micro-optical elements like refractive and diffractive lenses to biomedical applications such as organ-on-a-chip devices, the need for deeper, more precise grayscale patterns is growing. However, achieving high-quality, deep grayscale structures is a significant challenge, especially when using highly sensitive photoresists.
The height of structures in grayscale lithography is constrained by the thickness of the photoresist film. Utilizing commercially available DNQ-based positive photoresists allows the fabrication of structures up to 80 μm in height. However, when aiming for greater pattern depths, nitrogen which is formed during photolysis induces pronounced bubble formation within the structures.
The study on which this contribution is based explored the difficulties of creating grayscale patterns exceeding 100 µm in depth using a novel positive photoresist developed by micro resist technology, mr-P 22G_XP. This diazoquinone/novolak-based resist was specifically designed for very deep structures or grayscale applications in very thick films. It addresses the common nitrogen bubble formation issue which can deform ultra-thick layers. The study revealed the challenges encountered when adapting this resist for photomask-based grayscale lithography, providing insights into resist behavior, mask requirements, and pattern transfer techniques.
Photomask vs. Direct Writing: A Balancing Act
Both photomask-based and direct laser writing offer distinct advantages. Photomask lithography enables fast exposure of large substrates, making it appealing for high-volume industrial applications. However, grayscale masks need careful optimization to address the resist’s sensitivity, preventing undesired pattern deviations. Conversely, direct laser writing provides superior control over exposure levels—especially when employing controlled multiple low-dose exposures via the “N-Over” mode and CI-Over (Continuous Intensity Overlap) techniques —enabling the correction of nonlinear resist responses and batch-to-batch variations, a feat difficult to achieve with masks.
The Role of the DWL 66+
In their experiments, the authors used the Heidelberg Instruments DWL 66+, a laser direct writer known for its precise control over grayscale exposure. By utilizing multiple overlapping exposures (N-Over and CI-Over techniques), they achieved exceptionally smooth patterns with depths surpassing 160 µm. The inherent bleaching of the resist during exposure further facilitates deeper penetration of the exposure light, which is crucial for high-quality, deep grayscale patterns. The ability to fine-tune the gray value distribution (GVD) proved particularly useful in compensating for resist variations—a challenge that grayscale masks cannot easily address.
Key Challenges in Mask-Based Grayscale Lithography
The mr-P 22G_XP resist, while designed for deep structures, presents unique challenges in mask lithography due to its high sensitivity, especially at low doses. This sensitivity, while crucial for achieving >100 µm depths, makes it prone to overexposure in areas receiving even minimal light, leading to several key issues:
- Resist Sensitivity: Even minimal exposure can result in significant film thickness reduction, affecting pattern accuracy. This means that areas of the mask intended to be “dark” may still allow enough light through to cause unwanted resist development.
- Mask Design:
- Pixelated Masks: Pixelated masks, where grayscale levels are created by sub-resolution features, can introduce visible graininess in the resist surface. The resist’s high sensitivity may resolve individual mask pixels, creating roughness if the resolution is insufficient.
- HEBS Glass Masks: While HEBS (High Energy Beam Sensitive) glass masks offer smoother transitions, they also have limitations. They don’t achieve a perfect 0% transmission; even the dark areas allow some light through, leading to unwanted exposure and film loss in those regions. Furthermore, the mask design must adapt to the non-linear resist response, a feature not readily available in current HEBS masks.
- Pattern Transfer: Since the resist is not chemically or thermally stable, transferring the patterned structures into permanent materials is essential.
- Shape Optimization and Process Stability: The high sensitivity of the thick positive photoresist demands tight environmental control and process stability. Achieving optimal shape control is crucial, as even slight deviations in exposure or environment can lead to insufficient pattern fidelity or variations in the final structure, which compromises the quality of the broader fabrication process.
Key Findings
- Photomask Lithography: Photomask lithography offers speed and efficiency, but the mr-P 22G_XP resist’s extreme sensitivity makes maintaining precise pattern depths extremely challenging with current mask technology.
- Direct Laser Writing: Direct Writing, especially with the DWL 66+, offers superior control, resulting in deeper and smoother patterns (up to 165 µm with a surface roughness < 10 nm, measured on top and next to the bottom of a Fresnel lens structure) but at the cost of longer exposure times.
- Crucial Pattern Transfer: Pattern transfer is a critical step. UV molding with OrmoComp® (an inorganic-organic hybrid polymer suitable for optical applications) and PDMS replication (a non-destructive method due to PDMS’s flexibility) proved effective for creating permanent structures.
The Future of Deep Grayscale Lithography
While grayscale masks offer efficiency for high-throughput production, direct laser writing remains the gold standard for applications demanding high precision and flexibility. With tools like the DWL 66+, researchers and engineers can push the limits of grayscale lithography, achieving deeper, more refined microstructures.
As these findings translate into improved manufacturing processes, they hold the potential to directly impact practical applications across optics, microfluidics, and biomedical devices, fostering greater industry collaboration. Continued development of optimized resists, exposure strategies (including controlled low-dose multiple exposures), and pattern transfer techniques will be key to unlocking the full potential of grayscale lithography.
Future research should focus on:
- Resist Improvements: Developing resists with lower, more controlled sensitivity (while still enabling deep structures) or resists less susceptible to thickness loss during exposure.
- Advanced Mask Technology: Creating masks with significantly higher resolution for effective sub-resolution techniques and improved contrast (darker dark areas) to better control highly sensitive resists. This includes exploring new materials and fabrication methods.
Continued development in these areas will be key to fully realizing the potential of grayscale lithography for a wide range of cutting-edge technologies.
