1. Introduction
The integrated circuit industry is known as the modern "industrial food", leading the development of the future scientific and technological industrial revolution and driving the innovation and development of the world's cutting-edge technologies. The integrated circuit industry is the leading industry in social development, mainly including important links such as semiconductor materials, equipment, chip manufacturing, packaging and testing, and circuit design. With the rapid development of industrial intelligent manufacturing and electronic information technology, the importance of integrated circuits, especially integrated circuit manufacturing, continues to increase. One of the characteristics of integrated circuit manufacturing is the intersection of disciplines. It is a collection of intersections of electronic science, computer science, communication technology, machinery manufacturing and other disciplines. The lithography machine is a typical representative of interdisciplinary characteristics.
The integrated circuit industry mainly includes important processes such as semiconductor materials, equipment, chip manufacturing, packaging and testing, and circuit design. Among them, semiconductor materials involve the growth of wafer materials, special gases, and special chemicals design and manufacturing; semiconductor equipment includes photolithography machines, etching machines, cleaning machines and other processing and production equipment; chip manufacturing refers to the combination of photolithography, etching, and thin film growth. Semiconductor processing techniques are organically combined to realize circuit design on the wafer; packaging testing refers to the electrical connection and protection of the manufactured chips. At the same time, test whether the function of the chip is normal and meets the design requirements. In addition, circuit design implements certain functions through the interconnection between components. In modern large-scale integrated circuit design, device design has been implemented through EDA tools.
2. Photolithography
Photolithography technology is not only one of the most important technologies in chip production, but also the most efficient high-precision micro-machining technology in the world today, supporting the realization and development of very large-scale integrated circuits.
Photolithography technology includes three important elements: photolithography process, photoresist and photolithography machine. During chip production, wafers are covered with photoresist and exposed to a specific light source. Then through the steps of development and etching, the complex pattern on the photolithography mask is copied to the wafer to selectively block precipitation or etching.
After exposure, a chemical reaction occurs in the exposed area, causing the developer to dissolve part of the photoresist, exposing the surface of the wafer, and obtaining a specific photoresist pattern. According to the change of the dissolution of the film in the developer, photoresist can be divided into positive photoresist and negative photoresist. The positive photoresist is soluble in the developer after exposure, while the unexposed positive photoresist is insoluble. In contrast, negative-tone photoresists are insoluble in the developer due to polymerization under UV exposure, while the unexposed parts are soluble in the developer.
3. Photolithography machine
Photolithography machines are known as the "crown jewels of the semiconductor industry" and are the most complex, sophisticated and expensive equipment among modern industrial masterpieces. Advanced lithography machines are generally divided into two types: EUV lithography machines and DUV lithography machines. DUV lithography machines are generally divided into two types: immersion and dry. The principle of lithographic printing technology originates from photolithography in printing technology, which is processed on a plane to form micrographics. In fact, the photolithography machine is the key equipment to realize the photolithography process, which is used to copy the circuit structure on the mask plate to the silicon wafer.
With the development of science and technology, integrated circuits are developing towards high integration and excellent performance by reducing chip size as much as possible. Therefore, improving lithographic resolution has become a core challenge in integrated circuit manufacturing. The theoretical resolution can be calculated using the Rayleigh equation:
In the formula, R parameter is the theoretical resolution; λ is the exposure wavelength; NA is the numerical aperture; k is the process combination parameter; n is the refractive index of the medium; θ is the minimum resolution angle.
It can be seen from this equation that improving the theoretical resolution R of lithography can be achieved by shortening the light source wavelength R, increasing the numerical aperture NA and reducing the process combination parameter k1.
4. Main components of photolithography machine
The lithography machine is a complex structure of equipment. Its main components include laser light source, objective lens system, workbench system, mask table system, mask transfer system, silicon wafer transfer system and other major components.
system and exposure system. The exposure system consists of a projection objective lens and an illumination system, and is the core of the lithography machine. There are several parts in the lithography machine, including workbench, mask table, mask transfer, and wafer transfer. As the execution system of the lithography machine, it is responsible for tracking the entire measurement and calibration process to achieve switching and accuracy. Positioning of silicon wafers and mask plates at different stations.
The whole machine measurement and calibration process relies on the measurement system of the lithography machine to achieve accurate measurement of each position of the wafer and mask. The measurement system includes focusing, wafer alignment, mask alignment, interferometer, zeroing sensor, etc. In addition, Equipment used to realize the calibration function of the entire system includes beam correctors, energy controllers, beam shape controllers, etc.
5. Main exposure methods of photolithography machines
According to different application scenarios, mainstream lithography machines can be divided into contact type, proximity type and projection type. These lithography machines have different working principles and are suitable for different processing scenarios.
1. Contact lithography machine
During the production process, the masks with micro-nano pattern structures in the contact lithography machine are in contact with each other. Under the irradiation of ultraviolet light, the pattern on the mask is transferred to the surface of the substrate to be processed. The characteristics of this type of lithography machine are relatively simple equipment structure, high processing efficiency, low cost, easy to achieve large chip area exposure, easy to use, long focal depth, and strong process adaptability. However, problems such as direct contact with the mask, contamination of the mask photoresist, and mask wear will affect process accuracy. In addition, this kind of photolithography machine also has shortcomings such as low resolution, poor exposure pattern quality, and poor process consistency. It can only reproduce the mask pattern at the same proportion, but cannot reduce the reproduction pattern.
2. Proximity lithography machine
Proximity lithography machines are developed from contact lithography machines. In proximity lithography machines, there is a tiny gap between the mask and the silicon wafer. This way, the mask is not in direct contact with the photoresist. Due to the separation, problems such as photoresist contamination of the mask plate can be effectively avoided, and the chip production qualification rate and production efficiency can be improved. At the same time, the service life of the mask plate is also extended.
However, in a proximity lithography machine, the image received on the silicon wafer is not a geometric projection in the mask, but the diffraction of the mask due to the diffraction effect produced by the gap between the silicon wafer and the mask. image. This directly affects the processing quality of the photolithography process, causing the edges of the mask pattern to be distorted.
3. Projection lithography machine
The working principle of a projection lithography machine is similar to that of a camera. It uses optical patterns to achieve exposure between the mask and the photoresist. Since the projection lithography machine adopts the projection process, the reticle and the substrate are no longer in contact with each other, thereby reducing the influence of reticle defects. The item shrinkage of the optical system allows the processing of finer structures than the mask pattern, greatly improving the exposure resolution of the lithography process. Typically, the image of the mask is projected through an optical system and scaled to a quarter of its original size, allowing for fine etching on the silicon wafer covered with photoresist. In addition, the projection lithography machine has the exposure capability of scanning imaging, which improves the productivity of the lithography process to a certain extent.
6. Development of light sources
During the development of photolithography technology, people have been exploring light sources with shorter exposure wavelengths. From the 1980s to the early 1990s, the photolithography process mainly used UV light sources with wavelengths of 436nm (G line) and 365nm (I line) generated by high-pressure discharge mercury lamps as the exposure light source for photolithography. The process node range was about 250- 800nm. In the early 2000s, researchers discovered that by refracting ArF light into ultrapure water, a light source with a wavelength of 134nm could be obtained. The final photolithography process reaches 7-45 nm. With the increasing improvement of light source technology, the wavelength of the exposure light source has now developed to the extreme ultraviolet light source spectrum, with a wavelength reaching an astonishing 13.5 nanometers, which can realize photolithography processes in the 3-7 nanometer range.
EUV lithography technology has been studied since the late 1980s. However, EUV light is easily absorbed due to its short wavelength and high energy, which has always been the bottleneck of EUV lithography technology. By continuously improving the EUV light collection device, technicians finally made the UV light source meet the needs of lithography.
There are currently four main ways to obtain EUV light sources: synchrotron radiation source (SRS), laser-produced plasma (LPP), discharge-produced plasma (DPP) and laser-assisted discharge plasma (DPP). LDP). Synchrotron radiation sources can produce high-power EUV sources without debris contaminating optics. It can also continuously output EUV light. LPP, DPP and LDP all use high-energy beams to generate high temperature on the target, thereby generating high-temperature, high-density plasma and emitting EUV light. Among them, LPP technology uses high-intensity pulse laser as driving energy to irradiate the target, causing the target to generate high-temperature plasma and radiate EUV light; DPP covers the target between the anode and cathode, and the two electrodes generate strong Discharge causes the target to generate plasma, and the plasma is heated to generate EUV light; LDP is a combination of LPP and DPP technology. First, the target is irradiated with pulse laser to refine the target, and then DPP technology is used to discharge the target to generate EUV light.
7. Conclusion
With the development of integrated circuits, photolithography machines have become one of the most sophisticated equipment in the world, and a variety of photolithography machines are used in chip production. In this review, typical lithography machines such as contact, proximity or projection lithography machines are introduced. Major components and core systems such as the objective lens system and mask transfer system were also reviewed. This review also covers key elements of photolithography technology, including light sources and photoresists.
For the next generation of lithography machines, lithography technology will move towards higher exposure resolution, which can be achieved by continuously reducing the light source wavelength, process parameters and increasing the numerical aperture according to the Rayleigh equation. However, the difficulty of upgrading photolithography technology and manufacturing will also increase, and the cost will also increase significantly. In fact, EUV lithography has shortened the light source wavelength to 13.5 nm, which makes the opportunity to improve resolution through the light source very limited. In these cases, increasing the numerical aperture of the projection objective will provide an alternative.
The larger the numerical aperture (NA) of the lithography machine, the stronger the system's ability to receive diffracted light and its ability to receive higher-order diffraction levels. When higher order diffracted light is received, the image will gain more detail and higher resolution. The NA of the optical system currently used in EUV lithography is 0.33NA, and ASML is developing a next-generation EUV lithography machine with 0.55NA. If the technical bottleneck can be overcome, the process can be expanded to 3 nanometers.
In addition, in order to further improve the exposure resolution, there are some other lithography technologies, such as multi-photon lithography, surface plasmon lithography, nanoimprint, etc., which may also be good candidates for next-generation lithography.