The introduction and experimental validation of EUV proximity lithography technology have enhanced the lithographic resolution of mask aligner equipment to the nanometer scale, which has provided new insights for holographic mask technology. Compared with EUV projection lithography equipment, EUV proximity lithography technology does not require a complex reflective projection system or complex multilayer mask structures, which results in relatively lower costs. Experimental results have demonstrated that HL based on extreme ultraviolet light can obtain resist profile distributions that are consistent with those obtained via simulation, and they exhibit high fidelity in terms of light intensity and resist profile, compared with the target patterns. Holographic mask technology based on extreme ultraviolet light imposes higher demands on lithography equipment and mask manufacturing technology, and it shows significant potential in advanced node manufacturing.

Progress The key to HL technology lies in the design and fabrication of holographic masks. Initially, holographic masks were created via actual optical interference, which allowed for a continuous phase distribution. Subsequently, it evolved into a process of synthesizing holographic masks through computer algorithms, and this was followed by the fabrication of the phase and amplitude layers via mask manufacturing techniques.

There are various methods for implementing interference in actual optical setups. Total internal reflection (TIR) holography uses evanescent waves (total internal reflection light) to record holographic mask information (Figs. 4 and 6). However, owing to the instability of holographic mask materials at that time, the shape of the obtained mask itself could change, resulting in low resolution and contrast in the reconstructed images. As a result, the early development of TIR holographic lithography progressed slowly. The photosensitive material HRF35 introduced by DuPont, which could maintain chemical stability during the exposure process, was suitable for HL mask fabrication and led to further advancements in TIR holographic lithography. Ross et al. applied excimer lasers to holographic lithography and achieved sub-micron resolution and mitigated the impact of speckle effects. Holtronic Technologies is dedicated to the research and commercialization of TIR holographic lithography equipment. Its products undergo continuous updates and iterations to improve performance and enhance capabilities and efficiency for large-area micro-nano manufacturing. The TIR holographic mask aligner from Holtronic Technologies can reduce the linewidth of resist to the sub-micron level (Fig. 11).

However, the photosensitive polymer used in TIR holographic masks exhibits chemical instability and is prone to degradation under multiple exposures to ultraviolet light, which results in changes to the phase information carried by the mask and hence makes it unsuitable for long-term use. Synthetic holographic mask technology avoids this issue by utilizing the physical characteristics of the masks to generate phase delay, which thereby improves the stability of the mask. The design of synthetic holographic masks essentially involves phase retrieval and optimization problems that can be solved by using iterative algorithms, such as the Gerchberg-Saxton (GS) algorithm, to compute the phase and amplitude distributions of the synthetic holographic mask. By adjusting the thickness of the mask substrate, the phase of the transmitted light field can be controlled. The wave-optical method is used to design masks and fabricate non-periodic patterns with a resolution of 3 μm and a proximity distance of 50 μm using illumination at a wavelength of 365 nm (Fig. 12). Additionally, phase modulation can be achieved through sub-micron-sized structures on the mask (Fig. 21). This method exhibits a high tolerance to local defects in the mask, misalignment errors, and disturbances during the exposure process while also eliminating phase noise. Moreover, holographic lithography can be applied to the fabrication of interconnects on nonplanar substrates (Fig. 41).

Researchers combined holographic mask technology with EUV proximity lithography by introducing the concept of “computational proximity lithography”. These researchers proposed an iterative design algorithm for EUV holographic masks based on the GS algorithm, designed corresponding holographic masks for elbow test patterns with different periods and linewidths (Fig. 29), and provided mask fabrication solutions. By comparing simulation results with the obtained resist profiles, they demonstrated the feasibility of applying EUV synthetic holographic masks to arbitrary mask patterns. Although EUV holographic lithography significantly improves lithographic resolution, compared with other synthetic HL techniques, its industrial implementation still faces challenges owing to the limited availability of commercial EUV proximity lithography equipment.

Conclusions and Prospects Holographic lithography essentially focuses on enhancing lithographic resolution. Therefore, it can also be combined with computational lithography to optimize and synergistically improve various process conditions, such as the light source, exposure mode, initial mask distribution, and other process parameters. Some challenges still exist regarding the application of HL. For example, the computational efficiency of designing holographic masks for large-area full-chip patterns is relatively low, and it has not been extensively adopted for mass production. Overall, HL is a promising lithographic technique with great development prospects, especially when combined with EUV technology, and it has the potential to reduce manufacturing costs for advanced node processes.

Holographic lithography (HL) is an advanced mask aligner lithography technique that confers the advantages of traditional proximity lithography while enhancing lithographic resolution. HL is an extension of proximity lithography technology, and it provides a new approach for large-area pattern fabrication, integrated circuit interconnection packaging, and micro/nanostructure manufacturing. With the continuous iteration of lithography technology, mainstream projection lithography is developing toward the higher resolution and smaller feature size. However, the sophisticated system and complex manufacturing processes have led to an explosive growth in lithography costs. In particular, with the introduction of extreme ultraviolet (EUV) reflective lithography machines, lithography costs have increased several hundred times, compared with early lithography equipment costs. However, unlike projection lithography, synthetic HL does not require complex optical systems, which results in lower costs and easier maintenance. Compared with contact lithography, synthetic HL does not involve hard contact between the mask and the silicon wafer, which reduces mask contamination and shortens the mask cleaning cycle, thus extending its lifespan. By replacing binary masks with holographic masks that contain phase information, synthetic holographic lithography can achieve higher lithographic resolution and imaging contrast, compared with proximity lithography. Compared with traditional interferometric lithography, synthetic HL enables the transfer of non-periodic patterns and can be used for arbitrarily-shaped mask patterns. Additionally, it is either not affected by speckle effects or can reduce and eliminate the impact of speckle effects on imaging. Moreover, domestic mask aligner technology in China is also relatively mature, meaning that a foundation for the development of HL exists.