1. Introduction

Due to their distinctive structural characteristics, micrometer-scale HMS patterns play a pivotal role in a multitude of scientific and industrial applications. These patterns, which exhibit uniform pore size and a high specific surface area, are essential for a number of applications, including catalysis, drug delivery, and adsorption processes. The precise control of particle size and shape during synthesis allows for the tailoring of mesoporous silica with specific functionalities, thereby improving its efficacy in supporting catalysts or delivering therapeutic drugs, for example. In current fabrication methods, the most significant challenge is the control of the uniformity and size of the micrometer-scale patterns of mesoporous silica. This is a critical factor that can significantly impact the suitability of mesoporous silica. This paper provides an in-depth discussion on the fabrication of micrometer-scale HMS patterns with orthogonal alignment using soft lithography. This approach addresses the challenges of achieving uniformity and micrometer-scale accuracy in HMS patterning, which is crucial for the aforementioned applications. The method employs soft lithography, including the design of the master mold, the stamping of PDMS, and the sol-gel process. In conclusion, this study examines the degree of precision and customization of HMS materials, which is important for the development of innovative applications of nanotechnology.

2. Technical Points in the Preparation Process

Soft lithography assumes a pivotal function in the meticulous preparation of micrometer-scale HMS patterns, characterized by their orthogonal alignment to a polymer dielectric template. In this section, the key steps and technical points of this preparation method will be summarized, including the key processes such as template design, photoresist coating, exposure, development, etching, etc., and the effects of different parameters on the pattern morphology and performance will be explored. Through the systematic summary and analysis, it aims to provide researchers with clear preparation guidelines and optimization paths, and to promote the further development and application of micron-scale HMS patterning technology.

2.1. Master Mold and Stamp Preparation

First and foremost, the design and fabrication of the master mold are crucial aspects that directly impact the quality and performance of the final HMS pattern. A suitable master mold design should consider the following respects:

The process should begin with the design and fabrication of a master mold with the desired hexagonal pattern, which is typically accomplished by lithography on a silicon wafer or other suitable substrate. The master mold must have features orthogonal to the substrate to ensure proper alignment of the HMS pattern. The next step should be flexible impression preparation. Typically, PDMS is used to make the elastomeric stamp, and the PDMS prepolymer is poured onto the master mold and allowed to cure. The PDMS is then carefully peeled off to produce a stamp that is the opposite of the master mold. Finally, the silica ink should be formulated. Silica inks are formulated, which include silica precursors such as tetraethyl orthosilicate (TEOS) and structure guides to form mesoporous structures. The ink may also contain other components that control the size and properties of the HMS. By reasonably setting the master mold, the morphology and properties of the HMS pattern can be effectively controlled, thus improving the success rate of the preparation process good quality of the HMS pattern.

2.2. Soft Lithography Patterning

The cost-effectiveness of soft lithography, especially for large-scale production, positions it as an advantageous choice within the manufacturing industry [5]. Additionally, the technology also excels in patterning large or non-planar surfaces, providing more pattern-transferring methods than conventional techniques. Moreover, soft lithography can achieve finer details in laboratory settings, with resolutions that can be significantly smaller than those of photolithography [6].

The soft lithography patterning consists of two main aspects. Micro contact printing is one facet. After applying a silica solution to the PDMS stamp, it should be pushed onto a silicon substrate. In order to create a pattern that complements the relief elements of the stamp, the stamp transfers the silica ink. Conversely, the capillary force lithography technique is used. The silica ink is pulled into the stamp's features via capillary action when the PDMS stamp is in contact with the silicon substrate. It is possible to make more intricate patterns with this technique.

2.3. Pattern Transfer and Solidification

In the sol-gel stage, the inking pattern on the silica substrate undergoes a sol-gel transformation where silica precursors polymerize and form a solid silica network, whereas in the hardening and calcination stage, the substrate that has formed the sol-gel film pattern should be heated to remove the organic components and harden the HMS structure [7]. This step is critical for the formation of the mesoporous structure.

The removal of organic components during the hardening and baking stages is critical, as the presence of these components may lead to defects or inhomogeneities in the final HMS structure. By heating the substrate, the organic components are broken down and volatilized, leaving behind the silicon skeleton from which the pore structure is formed. The formation and stability of the silicon skeleton directly affect the quality and properties of the final HMS structure. In addition, during the roasting process, crystalline phase transformation and further optimization of the pore structure occur. By appropriately controlling the roasting conditions, such as temperature and time, the pore size and distribution of the HMS structure can be regulated to optimize its performance and application.

2.4. Verification and Characterization

Verification of orthogonal alignment ensures the accuracy of the HMS pattern alignment with the silicon substrate [8]. Techniques such as scanning electron microscopy (SEM) or atomic force microscopy (AFM) are employed to verify the alignment. The precise orientation of the feature with respect to the substrate lattice confirms the orthogonality of the channel, guaranteeing accurate alignment and homogeneity of the HMS pattern. The orthogonality of the channel can be confirmed by the specific orientation of the feature relative to the substrate lattice, ensuring precise alignment and uniformity of the HMS pattern.

Characterization of the structural properties of the HMS patterns is crucial for assessing their quality and performance. Techniques such as SEM, TEM, and X-ray diffraction (XRD) are used to characterize the pore size, surface area, and mesostructural orientation of the fabricated HMS patterns. These characterization techniques provide valuable insights into the structural integrity and functionality of the HMS patterns, facilitating their optimization for specific applications.

3. Comparison between Soft Lithography and Conventional Methods

Highly ordered mesoporous structures with regulated pore sizes and orientations can be produced via soft lithography to create micron-scale HMS patterns on silicon substrates [9]. The resulting HMS patterns also exhibit high surface areas, which is beneficial for catalysis and sensor applications [10]. The use of silicon substrates provides excellent compatibility with existing semiconductor processes, facilitating the integration of these mesoporous materials into electronic devices [11-12].

3.1. Advantages of Soft Lithography

Typically, soft lithography offers the following main advantages over conventional methods:

Firstly, soft lithography can produce patterns with micrometer resolution and precision, which has advantages over traditional methods such as embossing or etching, especially where complex or small features are required. Secondly, soft lithography is compatible with a wide range of materials, including organic and inorganic substances, which is an advantage over methods that are limited to specific materials. Thirdly, compared to more conventional microfabrication methods like photolithography or electron beam lithography, which necessitate costly machinery and cleanroom spaces, soft lithography is sometimes thought of as a less expensive approach. Fourthly, although soft lithography can produce complex patterns, the complexity of the process may be less than that of conventional methods involving multiple steps, such as deposition, etching, and lifting processes in microfabrication. Fifthly, soft lithography is highly customizable because the pattern can be easily changed by modifying the elastic stamp. In contrast, conventional methods may require extensive retooling or the design of new masks for each new pattern. Lastly, soft lithography is more environmentally friendly than some traditional methods because it typically requires fewer harmful chemicals and can be performed at lower temperatures.

3.2. Applications of Soft Lithography

Soft lithography has a wide range of applications including catalyst carriers, sensors, controlled drug release, photonic crystals i.e. optical components, tissue regeneration and energy storage. HMS modes with high surface area and tunable pore sizes can be used as catalyst carriers to enhance the efficiency of catalytic reactions. Ordered porous structures are ideal for sensing applications, and pores can be functionalized with specific receptors for the detection of target molecules. HMS can be used as a platform for controlled drug release, where a variety of drugs can be added to the pores for slow, sustained release. Regular patterns can be used to fabricate photonic crystals or other optical components with tailored optical properties. HMS patterns can provide structured scaffolds for cell growth, potentially guiding tissue formation in regenerative medicine. In addition, the porous structure can provide channels for ion transport, favoring energy storage applications such as batteries or supercapacitors.

3.3. Present Limitations

Conventional microfabrication techniques are often designed for scalability and can produce large numbers of identical structures. Soft lithography, while adaptable, may face challenges in scaling up for mass production compared to these methods [13]. One of the primary concerns is scalability. It is possible that soft lithography may not match the mass production capabilities of traditional photolithography. Furthermore, despite its versatility, soft lithography may encounter difficulties in achieving the same level of precision and uniformity across larger surfaces or more complex three-dimensional structures. Additionally, despite its versatility, soft lithography might face challenges in achieving the same level of precision and uniformity across larger surfaces or more complex three-dimensional structures. The technique's reliance on the quality of the elastomeric stamps can also introduce variability, as the stamps' deformation or wear over time may affect pattern fidelity. Furthermore, the soft lithography process is sensitive to environmental factors such as temperature and humidity, which can influence the curing process and material properties. Addressing these limitations is crucial for the broader adoption of soft lithography in industrial applications.

4. Conclusion

This paper demonstrates the feasibility of fabricating micrometer-scale, orthogonally aligned HMS patterns using soft lithography, addressing the key challenges presented previously, and providing a controllable and unified approach to fabricate HMS structures with potential applications in catalysis, drug delivery and adsorption processes. Soft lithography has proven to be versatile, cost-effective, highly accurate and customizable, which is essential for the development of nanotechnology and materials science. The results demonstrate the hypothesis that soft lithography can achieve the pattern alignments and structural features required for high-performance applications, and also confirm the importance of optimizing each step in the soft lithography process, from master mold design to sol-gel transition, to achieve the best possible results.

In the future, further research will be conducted with the objective of enhancing the scalability of soft lithography in order to meet the demands of the industrial sector. Future research will aim to enhance processes in order to reduce variability and extend the operational lifespan of flexible stamps. Furthermore, strategies to mitigate the impact of environmental factors on the lithography process will be developed. Moreover, the integration of soft lithography with other nanotechnologies will be a significant area of investigation. This will entail interdisciplinary collaborations to expand the scope of materials applications, with the potential to transform fields such as energy storage, regenerative medicine, and photonics.