Failed challenger to EUV, NIL finds footing

Nanoimprint lithography, which has lagged behind traditional optical lithography for decades, is becoming the technology of choice in the fast-growing photonics and biotechnology chip markets.

纳米压印光刻 (NIL) 于 20 世纪 90 年代中期首次推出，一直被吹捧为传统光学光刻的低成本替代品。即使在今天，NIL 也有可能使用更少的工艺步骤和显著降低的资本设备成本来匹配当前的 EUV 尺寸、产量和吞吐量。

NIL differs from optical lithography in that NIL uses a copy of a master stamp patterned by an electron beam system to transfer the image directly onto silicon wafers and other substrates. The low-viscosity photoresist is deposited onto the substrate by jetting, similar to how an inkjet printer works. A patterned stamp (mask) is then pressed into the photoresist surface, and fluid flows into the pattern through capillary action. UV radiation cross-links the thermoset material, removing the mask and leaving a patterned photoresist on the substrate.

The disadvantage is alignment on multiple metal layers, which is the main advantage of photolithography. The process of pressing the mold used in NIL into the resist can cause twisting or deformation, causing misalignment between different layers. Cutting-edge semiconductors can have more than two dozen layers, each precisely aligned with the layers below to ensure accurate and reliable chip performance. This is particularly problematic for advanced semiconductor nodes where feature sizes shrink below 10 nanometers. Coverage alignment tolerances for these dimensions are very tight.

"Nanoimprint is the ideal lithography tool for nanostructure definition, which requires no alignment, or more precisely no multi-layer alignment," said Theodor Nielson, CEO of NIL Technology. "NIL is efficient, fast, and requires The capital expenditure is significantly lower than that required for using a stepper. However, when many mutually registered lithography steps are required, steppers are superior.”

This feature uniformity in sub-10nm processes is a major advantage of photonics. Another is pattern flexibility. Photonic devices rely on nanoscale manipulation of light through patterns and frequencies of surface structures on substrates. NIL can be used to create a variety of three-dimensional (3D) nanostructures from a single impression, providing unique optical properties for applications in advanced photonic devices.

Figure 1: Schematic diagram of EVG’s SmartNIL process, including two steps – working stamp manufacturing and imprinting. Both steps are performed in the same tool.

NIL offers many advantages over traditional lithography, including EUV. Among them:

• It can reproduce feature sizes below 5nm with higher resolution and lower line edge roughness (LER);

• 由于整个过程避免了对透镜阵列的需要和光源所需的极端功率，因此 NIL 的运行成本显著降低；

• It requires fewer process steps, and it is much more compact than EUV systems, so multiple machines can be clustered together to increase throughput.

However, NIL is yet to find its way into semiconductor manufacturing lines due to various technical, financial and logistical hurdles. As early as 2008, researchers demonstrated cost-effective NIL production below 45 nanometers, and current NIL technology can print dimensions below 10 nanometers with alignment accuracy as low as 2 nanometers.

Part of the reason is the cost of adding another lithography technology to the fab. Existing lithography equipment requires huge investment, and industry standardization of optical scanners makes it more difficult to replace. While it may be cheaper to pattern certain layers using NIL, it is a technique that uses a different process on additional equipment and uses different materials than those used in optical systems. Any new process or material added to a workflow adds complexity, time, and resources, thereby increasing costs and reducing throughput. It’s not just the cost of the process. This is all the associated costs of adding additional process steps.

“If you can already do something with standard lithography, and there’s a lot of capacity there, then these lines will run at these resolutions,” said Thomas Urhmann, director of business development at EV Group. “To further promote nanoimprint lithography, There is a need to adopt new applications where manufacturing processes have not yet been established, and technology enables applications.”

Photonics is an emerging industry driven by growing global demand for light energy systems. Photonic components use fewer layers than traditional chips, yet they are critical to a variety of products and services, including telecommunications, data networks, biophotonics, consumer electronics, automotive and more. These vertical markets rely heavily on optical and photonic components such as LED and laser chips, optical glass, detectors and image sensors, lenses, prisms, filters, gratings, optical fibers, and more.

This creates huge opportunities for NIL. According to McKinsey, the global light-enabled systems market is currently approximately US$1.4 trillion and is expected to reach nearly US$2 trillion by 2025. While photonic components account for about 9% of this total, or about $120 billion, the component market is growing much faster than the overall system itself, with compound annual growth rates of 10% and 6% respectively. This is due to the increase in applications and the proliferation of photonic components in these systems.

It also leverages the strengths of NIL, namely its ability to create high-resolution nanostructures on different substrates with excellent reproducibility and scalability. NIL provides a cost-effective method to fabricate complex nanostructures below 10 nanometers, which is critical for fabricating small photonic devices such as photonic crystals, waveguides, and grating couplers. The technology also enables the fabrication of photonic components with highly uniform and detailed subwavelength features, thereby enhancing light-matter interactions and improving device performance.

“Wavelengths are very unforgiving,” Urhmann said. "Small changes in the photonics can have a huge impact on their performance, especially when you look at line edge roughness on the structure. With NIL, once you have a proven template, and you replicate that template, then the entire wafer It's going to be 100% the same specs, which is a huge asset for applications like augmented reality."

Figure 2: Examples of NIL photonics applications demonstrating process capabilities for nano and micro structures as well as complex shape structures

"In photonics, often with these small feature sizes, if you use optical lithography to produce these features, the cost will be significantly higher than NIL," said Obducat Group CEO Patrik Lundström. "The cost-effectiveness of NIL technology is that of photonics One of the key advantages. Additionally, NIL is easier to use with photoresist and the actual formation of structures in the photoresist material, as well as substrate-to-substrate repeatability.”

The "actual formation" of the structure is an important distinction in NIL. Unlike optical lithography, which patterns a resist into the silicon pattern in the application, NIL creates structures directly on the substrate material without etching. This enables the imprinting of extremely fine circuits on a variety of surfaces that may not be suitable for optical systems.

“NIL has a very strong advantage in terms of flexibility in choice of imprint material,” said Eleonora Storace, project manager for nanoimprint lithography at imec. “It is substrate agnostic. You can basically imprint on any type of substrate. "

NIL also has no mode field restrictions, making it highly adaptable to the diverse and less standardized optoelectronics market. In particular, full-field UV-NIL allows patterns to be printed on large areas without stitching errors. The technology supports a variety of structure sizes and shapes, including 3D, and can even be used on high-topography surfaces, a key requirement for many photonic devices.

The lack of diversity and standardization in this relatively new and rapidly growing market can also be a significant challenge for companies looking to adopt NIL technology to frame their new photonics applications - especially as NIL does not yet have a mature technology In the case of ecosystems.

To help meet the optoelectronics industry's growing demand for NIL devices, NIL Technologies is forming alliances with materials suppliers to help incubate new ideas. For example, EV Group (EVG) created a Photonics Competence Center to support new solutions in the industry and announced multiple agreements with material suppliers such as Toppan Photomask and Taramount to provide master templates and new packaging solutions. Just this month, EVG announced a new agreement with Notion Systems to develop inkjet coating capabilities. These collaborations aim to establish NIL as an industry standard production process for optoelectronics manufacturing.

NIL still faces many challenges in the photonics market, including the lack of a mature materials ecosystem. While the availability of materials and consumables is improving, there are still gaps that need to be addressed.

“The ecosystem has improved tremendously over the past decade,” said imec’s Storace. “There’s a lot of maturity for those suppliers that can deliver high volume to support the fabs, and they’re getting there, but those two things go hand in hand. As long as there’s not a critical mass of customers placing orders, supply The chain will not develop on its own.”

However, the situation is improving. “Extensive progress has been made in materials over the past two years, with many new materials being introduced, and we know there are many more in development,” Lundström added. “We are also seeing good development in the master template supply chain, with many well-established companies in the semiconductor space entering this space, which will bring benefits in terms of availability of reliable suppliers.”

The success of NIL in the photonics market has reignited interest in its potential applications in silicon photonics manufacturing in semiconductor foundries. Silicon photonic devices require precise and complex optical structures that are often challenging to fabricate using traditional photolithography techniques, especially at the smallest nodes. The large numerical aperture of EUV reduces its depth of field to only a few hundred nanometers. But NIL, with its high-resolution patterning at the nanometer scale, enables the fabrication of complex and miniaturized optical structures that are critical for silicon photonic devices. NIL can also be integrated with existing semiconductor manufacturing processes.

“These technologies are very complementary and they can coexist very smoothly,” Storace said. “From a processing perspective, the challenge is to bring these two worlds together. That’s what we do at imec. We have a CMOS fab in which we embed NIL tools, so we can take advantage of All the expertise of the technical staff is used to come up with new processes and thus be able to create a complete product.”

Another opportunity for NIL in semiconductor manufacturing is 3D NAND flash memory chips. NAND flash memory consists of a series of memory cells that can be arranged into a two-dimensional array. Each memory cell consists of a transistor and a floating gate, which stores data as 0 or 1. Transistors control the flow of current between the memory cell and the rest of the circuit. The simplicity of NAND flash memory structure makes it a good candidate for NIL manufacturing.

Canon Nanotechnologies is betting big on 3D NAND flash memory with its NIL manufacturing technology. The company currently has test equipment at SK Hynix and KIOXIA (formerly Toshiba) manufacturing plants and plans to begin mass production of 3D NAND flash memory using NIL by 2025. Canon is also building a new $357 million factory in Utsunomiya, north of Tokyo, to double the output of its lithography equipment, including NIL.

The main challenge for Canon's targets remains alignment, especially near the wafer edge, although the company believes it has largely solved the alignment issue with its TTM alignment system and its High-Order Distortion Correction (HODC) system .

Canon's method uses moiré patterns with proprietary control technology to measure nanometer-scale deviations between the wafer and mask in real time (Figure 3). This is a common approach used by most NIL tool manufacturers, but the process of physically pressing the master onto the substrate and heating the resist can cause micro-deformations in the wafer that affect the alignment of subsequent layers. Canon's HODC technology doesn't try to avoid these distortions, but rather corrects them using laser illumination modulated by a Digital Mirror Device (DMD). The laser thermally deforms the wafer and mask (Figure 4), and distortion correction can be performed due to differences in thermal expansion coefficients.

Figure 3: TTM oscilloscope can measure positional deviation between mask and wafer in real time

Figure 4: Proprietary matching system

“We can now meet all the requirements for overlay accuracy in 3D NAND flash memory,” said Doug Resnick, vice president of marketing and business at Canon Nanotech. “We have achieved overlay accuracy of 1.8 nanometers on closed systems and 2.3 nanometers on mix-and-match overlays.”

In addition to photonics and semiconductors, the application of NIL in the broader field of materials science is growing rapidly. NIL has expanded to include the actuation of smart materials, enhancement of filtration membrane performance, augmented reality, sensor technology, biomedical products and genome sequencing.

Augmented reality and 3D sensing are definitely hot topics at NIL right now,” says Uhrmann. “For applications like fingerprint sensors or spectral sensors, you need tiny optics. Other applications include metallic lenses and metallic optics, but where it's really shining now is in genome sequencing. "

The genome sequencing process involves using capacitance changes from an external voltage to move nucleotides through nanoscale nanopores. Each genomic type of nucleotide generates a hindered ion current with a unique magnitude, and the electrostatic charge distribution of each type can be measured to determine their sequence on the chain.

Making these nanopores was initially accomplished by growing them organically on a substrate, but making their sizes consistent was a challenge. NIL solves this challenge by printing consistent, evenly distributed nanopores in the material at high speed, thereby significantly reducing the costs associated with genome sequencing. This has quickly become the technology of choice for genomic testing companies and laboratories.

Figure 5: Process for producing perforated nanopores in free-standing polymer membranes via NIL and polymer reflow

Although nanoimprint lithography has been around for decades, it is only now that it has become widely used as a production-level manufacturing tool. Initially targeted at semiconductor manufacturing, its adoption has been limited by challenges with coverage alignment, throughput, and defect rates. Instead, NIL has been adopted by other industries where single- or limited-layer imprinting is an advantage rather than a hindrance.

Photonic components in particular are taking advantage of the nanoscale capabilities of NILs without the random or line edge roughness challenges of optical lithography. Other applications, such as biomedical and genome sequencing, are also adopting NIL manufacturing to bring their products to market at a much lower cost than other manufacturing technologies.