Photolithography machines, its past and present are exciting, and its future is even more worth looking forward to.
Recently, the topic of lithography machines has been extremely hot, and it seems that the whole world is paying attention to the development of China’s local related industries.
To build a commercially available high-end photolithography machine (which can manufacture 7nm and more advanced process chips) is a complex project, because high-end photolithography machines require too many precision components, each of which The technical content is very high, and combining these parts into a usable machine requires long-term accumulation of technology and practice.
The concept of lithography can be divided into broad and narrow senses. In a narrow sense, it is to use light to “copy” integrated circuit patterns. This is also what we often call optical lithography technology, especially ultraviolet lithography technology (DUV and EUV). At a broad level, lithography refers to various integrated circuit “copying” and “printing” technologies, some of which use light and some that do not (such as electron beam and nanoimprint lithography).
Divided by application, semiconductor lithography technology is mainly used in three areas: integrated circuit manufacturing in the front-end process, chip packaging in the back-end process, and display panel manufacturing. Among them, the one with the highest technical content and the most attention is the front-end process photolithography process. The DUV and EUV we often talk about are this part of the application. This article mainly discusses lithography technology in this area.
Overview of Lithography Technology
First, let’s take a look at various photolithography technologies from a broad level.
In the semiconductor industry, the development of lithography technology has gone through multiple stages. Contact/proximity lithography, optical projection lithography, and step-by-step (repeated) projection lithography appeared earlier. At present, integrated circuit manufacturing mainly uses optical lithography technology, including scanning lithography, immersion scanning lithography, and extreme ultraviolet lithography processes. In addition, there are X-ray, electron beam lithography, focused particle beam lithography, nanoimprint, and laser direct writing technologies.
Optical lithography uses irradiation and projection to “draw” the large-scale integrated circuit structure pattern on the mask on the silicon wafer coated with photoresist. Through the irradiation of light, the components of the photoresist undergo a chemical reaction to form Circuit diagram, optical lithography requires mask. The minimum feature size of an integrated circuit is directly related to the resolution of the lithography system, and reducing the wavelength of the irradiation light source is an effective way to improve the resolution. Therefore, the development of new short-wavelength light source lithography machines has always been a research hotspot in the industry.
Electron beam lithography, this technology does not require a mask and directly hits the focused electron beam spot on a substrate coated with photoresist. There are some problems in electron beam lithography that hinder the popularization of this technology, such as: the low exposure efficiency of high-precision scanning imaging of electron beams; the proximity effect caused by the scattering and backscattering phenomena of electrons in the resist and substrate; in the realization of nanometer In scale processing, there are process technical issues such as electronic resist, electron beam exposure, development, and etching.
Focused ion beam (FIB) lithography is a microdissection instrument that uses an electric lens to focus an ion beam into a small size. Its working principle is similar to that of electron beam lithography. Currently, commercial ion beams are liquid metal ion sources, and the metal material is gallium. A typical ion beam microscope includes a liquid metal ion source, an electric lens, a scanning electrode, a secondary particle detector, a 5-6 axis moving specimen base, a vacuum system, a device that is resistant to vibration and magnetic fields, and an electronic control panel. , as well as computers and other equipment. By applying an external electric field to the liquid metal ion source, the liquid gallium can form a small tip. Coupled with the negative electric field, the gallium at the tip is pulled, and the gallium ion beam is derived. It is focused through an electric lens and undergoes a series of automatic variable apertures (AVA). Adjust the size of the ion beam, then focus it twice on the surface of the test piece, and use physical collision to achieve the purpose of cutting.
Nanoimprint lithography uses technologies such as electron beams to engrave circuit patterns on a mask, then deforms the polymer on the object through the mask, and then uses a certain method to solidify the polymer to complete the transfer of the pattern. Nanoimprinting has high resolution and low cost, but has the disadvantages of large engraving errors, high defect rate, and easy contamination of the mask.
Mainstream optical lithography process
As mentioned above, the basic principle of photolithography in a narrow sense, which is the current mainstream photolithography technology, is to use light to expose a wafer coated with photoresist through a photomask (mask) with patterns. Properties will change after exposure to light, causing the circuit diagram on the photomask to be copied onto the wafer to form an electronic circuit diagram.
The lithography system is very complex. The entire equipment consists of light source, projection objective, workpiece stage, mask stage, alignment and measurement, mask transmission, wafer transmission and other parts. In addition, environmental and electrical systems, photolithography calculation (OPC) and mask optimization (SMO) software, development, and glue coating equipment are also required to provide support.
With the evolution of process technology, various systems of lithography machines are constantly being optimized and upgraded, and dual workpiece stage technology and immersion technology are successively adopted.
Currently, in the front-end process of integrated circuit manufacturing, ultraviolet lithography processes are mainly used, including deep ultraviolet DUV and extreme ultraviolet EUV. In the early years, the semiconductor process technology had not yet evolved to the 180nm node. The lithography accuracy at that time was not as high as it is now, and there was no need to use DUV and EUV. Contact/proximity lithography machines (Aligner) were used. Scanning projection/repetitive stepper.
When the process technology developed to 0.25 micron, the scanning exposure field size and exposure uniformity of the stepper scanning lithography machine (Scanner) became more advantageous, and gradually became the mainstream lithography equipment (DUV and EUV). It uses a 26mm x 8mm slit and adopts a dynamic scanning method (the mask and the wafer move synchronously) to achieve an exposure field of 26mm x 33mm. After the current exposure field is scanned, it moves to the next exposure field until the entire wafer is exposed.
In order to meet the ever-increasing performance index requirements, each component system of the lithography machine continues to break through technical bottlenecks in the fields of optics, precision machinery, materials and other fields, and achieves the integration of multiple high-precision technologies. In recent years, in EUV lithography systems, the importance of light sources seems to have become more prominent and has received more attention.
By configuring different types of light sources (i-line, KrF, ArF, EUV), stepper scanning lithography machines can support all integrated circuit process nodes. However, in order to meet the requirements of the most advanced processes, each generation of stepper scanning lithography machines has gone through Major technological upgrades have been made. For example: the static exposure field of 26mm x 8mm of the stepper scanning lithography machine is relatively small, which reduces the difficulty of manufacturing the objective lens system; but its dynamic scanning method of reverse movement of the workpiece stage and mask stage improves the performance requirements for the motion system.
From DUV to EUV
Since SVGL launched the Micrascan I stepper scanning lithography machine in 1990, the lithography machine industry has entered the DUV era. Until the mass production of 7nm chips, DUV has been the market leader. In this process, DUV technology is also constantly evolving to meet the development requirements of process technology. For example, the more advanced the process, the smaller the line width, which requires the lithography machine to have a higher exposure resolution. In order to improve the resolution, the numerical aperture (NA) of the lithography machine objective lens must be continuously increased, and wavelength changes must be adopted. Short light source. In addition, the immersion lithography system is also a great invention. It increases the refractive index by adding deionized water between the objective lens and the wafer, thereby achieving the effect of improving resolution.
When the process technology developed to 22nm, new methods had to be introduced to further improve the resolution of photolithography, and multiple exposure technology was born. There are many types of multiple exposure techniques, including: double exposure (DE), exposure-curing-exposure-etch (LFLE), double exposure (LELE), triple exposure (LELELE), self-aligned multiple exposure (SAMP).
Multiple exposure is to split the original layer of lithography patterns into two or more masks to achieve the superposition of image density, thus achieving a pattern smaller than the limit resolution of the lithography machine. For example, using DUV plus quadruple exposure technology (SAQP) for multiple exposure processing can improve the process technology level from 40nm of double exposure (SADP) to 20nm.
When the process node evolves to 5nm, the combination of DUV and multiple exposure technologies will no longer be able to meet mass production needs. EUV lithography machines will become a necessity in the front-end process. Without it, it will be difficult to manufacture 5nm chips that meet application requirements. Even if some 5nm chips can be manufactured without EUV, the yield rate of the entire production line is very low and large-scale commercial production cannot be achieved.
As process nodes continue to evolve, 3nm and 2nm chips have been or are about to be released. The industry has higher and higher requirements for EUV lithography machines, and has also raised more expectations for its development prospects and development paths.
Although each major component of the EUV lithography system requires high-precision technology, the importance of the light source is even more prominent. Especially in recent years, China has continued to accumulate strength in the development of EUV lithography machines, and the light source is the top priority. .
The shorter the wavelength of the light source, the higher the resolution of the lithography machine and the more advanced the process technology. Different from the excimer laser light source used by DUV, the EUV lithography machine uses a 13.5nm wavelength ion light source. This light source uses a carbon dioxide laser to bombard atomized tin (Sn) metal droplets and evaporate them into plasma (laser Plasma (LPP) is obtained through the transition between energy levels of high-valent tin ions.
The future development path of EUV
At present, 3nm process chips have been mass-produced. In the next three years, there is little doubt that 2nm will be mass-produced. In the foreseeable next few years, 1nm and even more advanced process chips will also be mass-produced. In this industry context, the importance of EUV lithography machines has become increasingly prominent.
At present, EUV lithography machines must continue to evolve to keep up with the development of process technology. To improve lithography accuracy, in addition to improving the numerical aperture NA of the objective lens, people focus mainly on improving the resolution of the light source, increasing the Light source power is an important development path.
Currently, the most advanced EUV lithography machines are produced by ASML. The highest NA of commercial EUV has reached 0.33, and EUV products with an NA value of 0.55 will also be available in 2024 and are expected to be commercially available in 2025.
In terms of light sources, there are several development paths to choose from to increase power.
1. The traditional LPP light source system can continuously increase power on the existing basis.
The advantage of LPP light source is its high conversion rate. Major manufacturers hope that the power can reach the industrial application standard of more than 200W, which requires a huge carbon dioxide laser device. In practical applications, the laser of a high-level EUV LPP light source needs to reach a power of 20kW. However, after such emission power is repeatedly reflected, the power reaching the focus is only about 350W.
Lower power does not mean that it cannot operate normally, but for a lithography machine that sells for hundreds of millions of dollars, such power is not enough to maximize utilization, especially after reaching the 3nm and 2nm process nodes, in order to maximize To optimize the scanning speed, the 3nm node requires a focus power of 1500W, and the 2nm node requires a focus power of 2800W. Such power cannot be achieved by existing LPP EUV. Currently, ASML is leading the way in this regard and is stepping up research.
Second, time-sharing high-power fiber laser can be used to shoot liquid tin targets. The light source power produced by this method is expected to be several times higher than that of traditional LPP.
3. The FEL (free electron laser) solution using an energy recovery linear accelerator (ERL). The ultimate power of this light source is also very high, up to 10kW. According to data from the Japanese High Energy Accelerator Research Institute, FEL can achieve nearly one-seventh of the power consumption cost of the LPP solution. However, this light source has many technical difficulties that need to be overcome, and the cost is high.
4. Particle accelerator light source based on Steady-state microbunching (SSMB) technology. The SSMB concept was proposed in 2010 by Zhao Wu, a professor at Stanford University and a visiting professor at Tsinghua University, and his doctoral student Daniel Ratner.
Based on the SSMB principle, high-power, high-repetition frequency, narrow-bandwidth coherent radiation can be obtained, and the wavelength can cover the terahertz to extreme ultraviolet bands. The figure below shows the schematic diagram of the SSMB principle verification experiment.
EUV light sources based on SSMB are expected to achieve large average power and have the potential to expand to shorter wavelengths, providing new ideas for breakthroughs in high-power EUV light sources.
Currently, Tsinghua University is actively supporting and promoting the project establishment of SSMB EUV light source at the national level. The Tsinghua SSMB research group has submitted a project proposal for the “Steady-state Micro-bunching Extreme Ultraviolet Light Source Research Device” to the National Development and Reform Commission, applying for the “14th Five-Year Plan” “National major scientific and technological infrastructure.
From its birth to the present, lithography machines have gone through many iterations and developed a variety of application technologies. In order to cope with the evolving application needs, new technological peaks and problems are also waiting for the industry to climb and overcome.
For China’s semiconductor industry, in the face of external pressure, it is necessary to continuously strengthen its self-research capabilities while maintaining access to the international supply chain. Lithography machines, especially EUV lithography machines, are an important part. At present, Chinese local companies have been able to achieve independence in lithography machines for packaging and display panels, but there is still a long way to go in advanced process chip manufacturing.
In terms of improving the power level of EUV light sources, many development paths have emerged. Whether it is to follow the traditional technical route or to find a new way to find better solutions to catch up with the international advanced lithography technology level, it is necessary to carry out technology research and development and engineering in a down-to-earth manner. Verification work is definitely not something that can be achieved in the short term. It requires long-term persistence and unremitting efforts.