Maskless lithography, is there a chance?
Latest update time：2022-05-06 12:26
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At present, affected by the continued spread of the global chip shortage, the chip issue has attracted widespread attention. As the core equipment in the chip manufacturing industry, the lithography machine has become the focus of industry attention.
In chip manufacturing, photolithography is a crucial link. Photolithography technology refers to the technology of transferring the pattern on the mask (also called photomask) to the wafer with the help of photoresist under the action of light. This is a bit like the process of using the negative film to develop photos after the film in a film camera is exposed, but this process is much more complicated and tedious than developing photos.
Since the level of photolithography technology directly determines the process level and performance level of the chip, photolithography has become the most complex and critical process step in chip manufacturing. There is a saying in the industry that "integrated circuits are the crown of the information age, and photolithography technology is the jewel in the crown."
It can be said that photolithography is the foundation of modern semiconductor, microelectronics, and information industries, and it directly determines the development level of these technologies. In the nearly 60 years since the successful invention of integrated circuits in 1959, the line width of its graphics has been reduced by about four orders of magnitude, and the circuit integration has been improved by more than six orders of magnitude. The rapid progress of these technologies is inseparable from the credit of photolithography.
Mask prices are rising
The photolithography process is the most critical step in the manufacturing process. Photolithography determines the key dimensions of the chip and accounts for approximately 35% of the overall manufacturing cost of the entire chip manufacturing process.
Generally speaking, the equipment and materials required for a chip production line are similar. However, with the continuous improvement of chip manufacturing technology, the required equipment precision is higher and the material purity is also higher. These will lead to the continuous increase in production costs. However, the cost of photolithography masks has increased the fastest.
The mask, also known as a photomask or photomask, is a graphic transfer tool or master in the microelectronics manufacturing process. Its function is similar to the "film" of a traditional camera. According to the graphics required by the customer, fine patterns at the micron and nanometer levels are engraved on the mask substrate through the photolithography process. It is a carrier that carries graphic design and process technology.
In the chip manufacturing process, it is not completed in one exposure. It has to go through multiple exposures during the manufacturing process, which means that multiple alignment operations have to be performed during the chip manufacturing process (different masks have to be replaced for each exposure, and the mask and silicon wafer have to be aligned each time). As the number of photolithography mask layers increases, the cost naturally increases, and it also drives up the cost of photoresist, photolithography, etching and other accompanying process materials.
At the current process stage, the use of photolithography masks has become a key technology that determines the application prospects of various photolithography methods, but at the same time, the share of mask costs in the entire photolithography cost has continued to rise. According to IBS data, in the 16/14nm process, the cost of the mask used is about US$5 million, and by the 7nm process, the mask cost quickly rose to US$15 million.
In the 7nm process, the mask cost is about $15 million
(Photo source: IBS)
According to simulation studies by IMEC, as the process technology gradually improves, the emergence of EUV and high-NA EUV lithography machines has brought new challenges to masks. The impact of mask defects will become increasingly greater, indicating that mask design rules need to become more stringent.
In addition, the precise positioning problem in the photolithography process is also very difficult to control. How to position so many masks? The higher the technical difficulty, the more difficult it is to control the cost, which is also a major problem that leads to rising costs.
Overall, mask production is complex, time-consuming, and expensive, and once completed it cannot be modified. While masks help the industry evolve rapidly, these defects also severely limit its development.
Xiang Yujian from the University of the Chinese Academy of Sciences pointed out that the use of traditional photolithography methods to process microlens arrays requires the use of grayscale masks, but the cost of grayscale masks is relatively high. In addition, since there is often a "one-to-one correspondence" between the mask and the processed microlens, this also leads to a serious lack of flexibility in processing microlens arrays.
Therefore, in order to achieve a more flexible lithography process, the industry began to study replacing physical masks with other things, or even simply not using masks for processing. This method is called "maskless lithography."
What is "maskless lithography"?
In the pan-semiconductor field, depending on whether a mask is used, lithography technology is mainly divided into mask lithography and direct write lithography.
Schematic diagram of direct write lithography and mask lithography
Direct-write lithography, also known as maskless lithography, refers to the process of focusing and projecting a high-precision light beam controlled by a computer onto the surface of a substrate coated with a photosensitive material, and directly performing scanning and exposure without a mask.
Direct write lithography can be further divided into two main types according to the different radiation sources: one is
charged particle based direct write lithography (CPML)
, including electron beam direct write, ion beam direct write, etc.; the other is
optical based direct write lithography (OML)
, including interference lithography, laser direct write lithography, and lithography technology based on spatial light modulators, etc.
Recently, in the seminar "Advanced Photolithography Technology and Layout Design Optimization for Integrated Circuits" at the School of Microelectronics of the University of Chinese Academy of Sciences, Niu Yujiang, Wu Haowen, Xiang Yujian and other students conducted in-depth research on the photolithography process and introduced the characteristics and classification of maskless photolithography technology.
Electron beam direct writing lithography
:
a technique that uses an electron beam to draw patterns on a sample coated with electron beam glue.
Electron beam light column structure diagram
The advantage of this technology is that the wavelength of high-energy electrons in the electron beam is extremely short, so the resolution is high and it can process nano-level microstructures. At the same time, it uses a direct writing method to expose the graphics, and the processing method is very flexible.
The disadvantages are: in stereoscopic light, the etching depth is related to many factors in the process, and it is difficult to accurately control the etching depth; equipment such as electron beam emitters have complex structures and are expensive; this technology uses a single-point direct writing method, which requires a long time to accumulate high-energy electrons for depth exposure, so it is not suitable for batch and large-area production.
Ion beam direct writing lithography technology
:
Due to the large mass of ions, after accelerated focusing, they can be used for micro-nano processing such as etching, deposition, and ion implantation on materials and devices. This is called focused ion beam direct writing technology.
In the ion beam direct writing processing system, the ion beam from the liquid metal ion source is accelerated, mass analyzed, shaped, and then gathered on the sample surface. The high-energy ion beam is gathered on the sample surface and bombarded point by point. The computer can control the beam scanner and blanking assembly to process a specific pattern without the need for a mask.
In comparison, ions are much more massive than electrons, and can largely overcome the scattering problem of electron beams in resists. However, field ions have a large mass, which results in a small penetration depth in photoresists; the energy of the ion beam formed by field ions is relatively dispersed, and the focal depth is not large, resulting in low resolution; in addition, the equipment for ion beam lithography is complex, resulting in high production costs and insufficient development potential.
Interference lithography
:
Laser interference lithography technology refers to the use of the interference and diffraction characteristics of light to regulate the light intensity distribution in the interference field through a specific light beam combination, and record it with photosensitive materials to produce a lithographic pattern.
Laser interference lithography technology can be divided into double-beam, triple-beam, quad-beam and five-beam interference lithography according to the number of light beams involved in the interference.
Interference lithography is a simple and low-cost process that can easily expose large areas with high resolution. However, it can only be used to expose periodic patterns, which are synthesized from a finite number of sinusoidal series and are only approximate results, so further improvement in resolution is limited.
Spatial light modulator-based lithography technology (SLM maskless lithography technology)
:
Use a programmable SLM device to directly modulate the illumination beam to form different patterns that are directly projected onto the substrate to complete exposure, which is equivalent to digitizing the physical mask plate and is called SLM-based digital lithography technology.
SLM is a micro device that can modulate the spatial distribution of light. It is composed of many tiny units arranged in a linear or square array. These units are controlled by computer programming, which conveniently digitizes the graphic mask and changes the mask shape flexibly through programming, replacing the "physical mask" used in traditional lithography, thus avoiding the problems of complex and expensive mask manufacturing and poor flexibility of traditional lithography systems.
In the industrial production of pan-semiconductors, there is a significant difference in the lithography accuracy (minimum line width) required by the market segments of direct-write lithography and mask lithography applications.
It is understood that in lithography application fields with substrate warping and substrate deformation, the adaptive adjustment capability of direct-write lithography gives it the advantages of high yield and good consistency. It also has technical characteristics that projection lithography does not have, such as high flexibility, low cost and shortened process flow. It is mainly used in mask manufacturing, IC back-end packaging, low-generation FPD manufacturing, and some low-end IC front-end manufacturing.
Comparison of direct write lithography and mask lithography in different semiconductor market segments
However, direct writing lithography is also limited by factors such as production efficiency and lithography accuracy, and currently cannot meet the needs of large-scale manufacturing in the semiconductor industry. On the one hand, the production efficiency of charged particle direct writing lithography is low, and it will produce a serious proximity effect in large-scale production, which seriously affects the resolution and accuracy of the graphics; on the other hand, laser direct writing lithography is limited by the laser wavelength, and its lithography accuracy is not as good as that of electron beam, ion beam and other charged particle direct writing lithography technologies, and it cannot meet the needs of high-end semiconductor device manufacturing.
The general view of the industry on maskless lithography is that it is a potential solution to reduce the ever-increasing number of photomasks and is a promising candidate for lithography technology. However, it is still in an early stage of development and there are still many technical problems to be solved. In the near future, it may only be a niche lithography technology and cannot replace the mainstream DUV and EUV lithography technologies, but its lower cost will continue to attract attention in the future.
Maskless lithography technology "emerges"
Not long ago, Russia also revealed new plans for maskless lithography.
The Moscow Institute of Electronic Technology (MIET) of Russia recently announced that it will start manufacturing lithography machines, and claimed that the technology of this new lithography machine can be comparable to EUV.
It is understood that Russia's technical direction is completely different from the EUV produced by ASML. ASML uses extreme ultraviolet light EUV with a wavelength of 13.5nm, which can be used to manufacture advanced processes of 7nm and below. Russia has developed an X-ray lithography machine based on synchrotron radiation accelerators and plasma sources, which uses X-rays with a wavelength between 0.01nm and 10nm, which is shorter than EUV extreme ultraviolet light.
At the same time, another advantage of X-ray lithography is that it does not require a photomask and can perform direct writing lithography, which can significantly reduce the cost of chip manufacturing while also providing higher precision.
According to the existing common thinking, the photolithography machine uses a mask for magnification to transmit light of a specific wavelength, and then uses a lens to shrink it, accurately "projecting" the integrated circuit pattern onto a silicon wafer.
In order to accurately "project", the lithography machine needs to achieve extremely high exposure resolution and extremely high repeat positioning accuracy. There are three main factors that determine the resolution of the lithography machine, namely the constant K, the wavelength of the light source and the numerical aperture of the objective lens. The shorter the wavelength, the higher the resolution. Therefore, in theory and from the historical development of lithography machines (the wavelength is getting shorter and shorter), the X-ray lithography machine used by Russia is obviously better and more advanced than the EUV lithography machine.
However, maskless X-ray lithography also faces the above-mentioned dilemma of low efficiency. Maskless lithography itself is not difficult, but the difficulty lies in the improvement of X-ray process and efficiency. Currently, no organization in the world can solve this problem. This is also the problem and challenge that Russia will face next.
According to the plan, Russia will complete the verification of the main technical solutions - the manufacture of dynamic mask models and two control experimental studies. The technology and model of dynamic masks will be developed as early as November this year, as well as the technical specifications and feasibility studies of prototype lithography machines, with a process of 28nm and above.
In fact, as early as 20 years ago, the Moscow Institute of Electronic Technology in Russia started research in the field of maskless EUV. In 2002, it launched a project called "Development of soft X-ray source based on micro-focusing X-ray tube array, suitable for 10nm maskless lithography machine", and the progress is relatively smooth so far.
Therefore, the industry has high expectations that Russia may make breakthroughs in this area.
In addition, maskless lithography technology has now achieved a certain degree of industrial application in specific fields such as scientific research, medical treatment, military industry, and special devices.
Professor Wei Dacheng's research group at Fudan University has developed a new type of fast and ultra-sensitive COVID-19 detection chip using a small desktop maskless lithography machine - MicroWriter ML3. MicroWriter ML3 can adjust the lithography pattern at will, helping users to quickly develop prototype chips. The molecular micro-nano electromechanical chip based on graphene field-effect transistors manufactured using this device has the characteristics of fast and ultra-high sensitivity in detecting ions, biomolecules and COVID-19, which to a certain extent helps research in the field of biomedical detection.
Last year, the "Core Image Vision" project, led by Liu Guanpeng, a doctoral student at the School of Materials Science and Engineering of Nanchang University, was also committed to the research and implementation of MicroLED maskless lithography technology, helping my country's research and product development of semiconductor and maskless lithography related technologies.
From market applications and technological trends, we can see that maskless lithography technology has been in a continuous process of evolution and exploration.
Final Thoughts
In general, each lithography technology has its own characteristics and advantages and disadvantages. The advantage of mask lithography technology lies in its accuracy and reliability in the process of transferring circuit patterns. However, with the increasing cost, the large-scale use of maskless technology has become a potential alternative risk in the future.
However, at present, maskless technology still has many problems to be solved. It can only meet the lithography needs of industries with relatively low precision requirements, and its efficiency is low. It cannot meet the needs of industries with high requirements for pattern transfer accuracy and production efficiency. Therefore, mask lithography may not be at risk of rapid iteration in the short term.
However, there may not be a perfect technical solution in the world. All of us must repeatedly weigh multiple factors such as cost, efficiency, and performance, striving to find the optimal solution in the dynamic changes of the industry.
For a long time, mask lithography technology has been the best choice in the lithography process route; in the future, maskless lithography technology may gradually attract industry attention due to its cost advantages and industry layout.
Everything is possible, but nothing is easy.
*Disclaimer: This article is originally written by the author. The content of the article is the author's personal opinion. Semiconductor Industry Observer reprints it only to convey a different point of view. It does not mean that Semiconductor Industry Observer agrees or supports this point of view. If you have any objections, please contact Semiconductor Industry Observer.
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