Lithography is the most important processing technology for integrated circuits, and its role is similar to that of a lathe in a metalworking workshop. In the entire chip manufacturing process, almost every implementation of the process relies on lithography technology. Lithography is also the most critical technology for manufacturing chips, accounting for over 35% of chip manufacturing costs. In today's technological and social development, the growth of lithography technology is directly related to high-tech fields such as the operation of large-scale computers.
Lithography technology
Lithography technology is closely related to our lives, and the chip production in various electronic products such as mobile phones and computers cannot be separated from lithography technology. The world today is an information society, with various information flows flowing around the world. And lithography technology is the carrier that ensures manufacturing carries information. Having an irreplaceable role in society.
The principles of lithography technology
Lithography is the process of creating the circuits and functional areas required for chip fabrication. The light emitted by the lithography machine is exposed to the thin film coated with photoresist through a patterned mask. The photoresist undergoes property changes when exposed to light, allowing the patterns on the mask to be copied onto the thin film, thereby giving the film the function of an electronic circuit diagram. This is the function of lithography, similar to camera photography. The photos taken by the camera are printed on the film, while the ones engraved by light are not photos, but circuit diagrams and other electronic components.
Lithography technology
Lithography technology is a precision micro machining technique. Conventional lithography technology uses ultraviolet light with wavelengths ranging from 2000 to 4500 angstroms as the image information carrier, and uses photoresist lithography etchants as the intermediate (image recording) medium to achieve pattern transformation, transfer, and processing, ultimately transferring image information to the chip (mainly referring to silicon wafer) or dielectric layer.
In a broad sense, photolithography includes two main aspects: photocopies and etching processes:
1. Photocopying process: The device or circuit pattern prefabricated on the mask is accurately transferred to a thin layer of photoresist pre coated on the surface or dielectric layer of the chip in the required position through an exposure system.
2. Etching process: By using chemical or physical methods, the unshielded surface or dielectric layer of the resist layer is removed from the chip surface or dielectric layer, in order to obtain a pattern that is completely consistent with the resist layer on the chip surface or dielectric layer. The functional layers of integrated circuits overlap in a three-dimensional manner, so the lithography process is always repeated multiple times. For example, large-scale integrated circuits require about 10 photolithography cycles to complete the complete transfer of graphics at each layer.
In a narrow sense, photolithography technology only refers to the photocopier process.
Lithography technology
The Development of Lithography Technology
In the 1970s, GCA developed the first distributed repetitive projection exposure machine, with integrated circuit graphics linewidth ranging from 1.5 μ Reduce m to 0.5 μ Node m.
In the 1980s, SVGL in the United States developed the first generation of stepper scanning projection exposure machines, with integrated circuit graphics linewidth ranging from 0.5 μ Reduce m to 0.35 μ Node m.
In the 1990s and 1995, Cano began working on 300mm wafer exposure machines and launched EX3L and 5L stepper machines; ASML launches FPA2500193nm wavelength stepper scanning exposure machine. The optical lithography resolution has reached the "limit" of 70nm.
Lithography technology
Since 2000, while striving to break through the resolution limit of optical lithography technology, NGL has been researching technologies such as extreme ultraviolet lithography, electron beam lithography, X-ray lithography, and nanoimprinting.
optical lithography
Optical lithography is the process of drawing the structural patterns of large-scale integrated circuit devices on a mask onto a silicon wafer coated with photoresist using the projection method of Guangde irradiation. Through the irradiation of light, the composition of the photoresist undergoes a chemical reaction, thereby generating a circuit diagram. The minimum size that can be obtained by limiting the finished product is directly related to the resolution that can be obtained by the lithography system, and reducing the wavelength of the illumination light source is the most effective way to improve resolution. For this reason, the development of new short wavelength light source lithography machines has always been a research hotspot in various countries.
In addition, optimizing process parameters using various wavefront techniques based on the interference characteristics of light is also an important means to improve resolution. These technologies are breakthroughs achieved through in-depth analysis of exposure imaging using electromagnetic theory combined with lithography practice. Among them are phase-shifting masks, off-axis lighting technology, proximity effect correction, etc. By utilizing these technologies, higher resolution lithography patterns can be obtained at the current level of technology.
In the 1970s and 1980s, lithography equipment mainly used ordinary light sources and mercury lamps as exposure light sources, with characteristic sizes above the micrometer level. Since the 1990s, in order to meet the increasing requirements of IC integration, g-line, h-line, I-line light sources, as well as KrF, ArF and other excimer laser light sources have emerged successively. At present, the development direction of optical lithography technology is mainly manifested in shortening the wavelength of the exposure light source, increasing the numerical aperture, and improving the exposure method.
Lithography technology
Phase-shifting mask
The photolithography resolution depends on the partial coherence of the lighting system, the spatial frequency and contrast of the mask pattern, and the numerical aperture of the imaging system. The application of phase-shifting mask technology may use traditional lithography techniques and i-line lithography machines to carve patterns with half the size of traditional methods under optimal illumination, and have a larger depth of focus and exposure range. The phase shift mask method may overcome the limitations of traditional lithography methods for line/interval patterns.
With the development of phase-shifting mask technology, numerous types have emerged, which can be broadly divided into alternating phase-shifting mask technology and attenuation phase-shifting mask technology; Edge enhanced phase shift masks, including sub resolution phase shift masks and self aligned phase shift masks; There are several types of chromium free fully transparent phase shifting masks and composite phase shifting methods (alternating phase shifting+fully transparent phase shifting+attenuated phase shifting+binary chromium masks). Especially with alternating and fully transparent phase-shifting masks, the resolution improvement is most significant, creating favorable conditions for achieving sub wavelength lithography.
The characteristic of a fully transparent phase shifting mask is to use a transparent phase shifter with a pattern edge light phase that is larger than a certain width to suddenly undergo a 180 degree change. The interference effect of the diffraction field on both sides of the phase shifter edge produces a "blade" light intensity distribution, and forms a dark area with zero light intensity on all boundary lines of the phase shifter. It has a splitting effect of fine lines dividing into two, which improves the imaging resolution by nearly 1 times.
The potential of optical exposure technology is astonishing both in theory and practice, and cannot be ignored. The resolution enhancement techniques represented by wavefront engineering, which partially counteracts the diffraction effect that limits the resolution of optical systems by controlling the optical phase parameters during the optical exposure process, play an important role, including phase shifting mask technology, optical proximity effect correction technology, off-axis illumination technology, pupil spatial filtering technology, standing wave effect correction technology, defocus overlay enhanced exposure technology, surface imaging technology, and multi-level gel structure process technology. The most remarkable progress in practicality has been made in phase-shifting mask technology, optical proximity correction technology, and off-axis lighting technology, especially in breakthroughs in immersion lens exposure technology and the application of double exposure technology, which have created favorable conditions for the application of resolution enhancement technology.
Electron beam lithography
Electron beam lithography technology is a key technology for the development of micro technology processing, and it plays an irreplaceable role in the field of nanomanufacturing. Electron beam lithography is mainly used to depict tiny circuit diagrams, which are usually measured in nanomicrounits. Electron beam lithography technology does not require a mask and directly projects the converging electron beam spot onto a substrate coated with photoresist on the surface.
In order to apply electron beam lithography technology to the processing of nanoscale microstructures and lithography of integrated circuits, several key technical problems must be solved: high precision scanning imaging and low exposure efficiency of electron beam; The proximity effect caused by the scattering and backscattering of electrons in corrosion inhibitors and substrates; Technical issues related to electronic etchants, electron beam exposure, development, and etching in achieving nanoscale processing.
Lithography technology
Practice has proven that the application of electron beam proximity correction technology, electron beam exposure and optical exposure system matching, mixed lithography technology, and resist exposure process optimization technology is a very effective way to improve the actual lithography resolution of electron beam lithography systems. The main process of electron beam lithography is metalization and delamination. The first step is to scan the desired pattern on the surface of the photoresist. The second step is to develop the exposed graphics and remove the unexposed parts. The third step is to deposit metal on the formed graphics. The fourth step is to remove the photoresist. In the process of metal peeling, the key lies in the control of the photoresist shape in the lithography process. It is best to use thick adhesive, which is conducive to the penetration of the adhesive and forms a clear morphology.
Focused Particle Beam Lithography
The Focused Ion Beam (FIB) system uses an electric lens to focus the ion beam into a very small size micro cutting instrument. Its principle is similar to that of electron beam lithography, but it converts electrons into ions. At present, the ion beam used in commercial systems is a liquid metal ion source made of gallium, which has a low melting point, low vapor pressure, and good oxidation resistance; A typical ion beam microscope includes a liquid-phase metal ion source, an electric lens, a scanning electrode, a secondary particle detector, a 5-6 axis moving specimen base, a vacuum system, anti vibration and magnetic field devices, electronic control panels, and computers and other hard equipment. An external electric field applied to the liquid-phase metal ion source can cause liquid gallium to form small tips, and the negative electric field (Extractor) pulls the gallium at the tip to generate a gallium ion beam. At a normal working voltage, the tip current density is about 1 angstrom 10-8 Amp/cm2. Focused with an electric lens, the size of the ion beam can be determined by a series of Automatic Variable Apertures (AVA), and then subjected to two Focus on the surface of the test piece and use physical collisions to achieve the purpose of cutting.
In terms of imaging, the principles of focused ion beam microscopy and scanning electron microscopy are relatively similar. Among them, the surface of the sample in the ion beam microscope is excited by the scanning impact of gallium ions, and the secondary electrons and ions are the source of the image. The resolution of the image is determined by the size of the ion beam, the acceleration voltage of charged ions, the strength of the secondary ion signal, the grounding condition of the sample, and the resistance to vibration and magnetic field of the instrument. Currently, the highest image resolution of commercial models has reached 4nm. Although its resolution is not as good as that of scanning electron microscopy and penetrating electron microscopy, it does not have the problem of sample preparation for the analysis of fixed-point structures and is more economical in terms of working time.
Focused ion beam projection exposure not only has extremely high exposure sensitivity and no proximity effects as mentioned earlier, but also includes the ability to control a depth of focus greater than the exposure depth. The ion beam emitted by the ion source has excellent parallelism, and the numerical aperture of the ion beam projection lens is only 0.001, with a focal depth of up to 100 μ m. That is to say, any fluctuation on the surface of the silicon wafer within 100 μ Within m, the resolution of the ion beam remains basically unchanged. And the depth of focus of optical exposure is only 1-2 μ M is. Her main role is to repair the circuit, perform abnormal analysis on the production line, or perform photoresist cutting.
EUV lithography technology
In the development process of microelectronics technology, people have been researching and developing new IC manufacturing technologies to reduce linewidth and increase chip capacity. We also commonly refer to soft X-ray projection lithography as extreme ultraviolet projection lithography. In the field of lithography technology, our scientists have conducted the most in-depth research on extreme ultraviolet projection lithography (EUV) technology and made breakthrough progress, making it the most promising technology to be widely used in future integrated circuit production. It supports the production and use of integrated circuits with 22nm and smaller line widths.
Lithography technology
EUV is currently the closest practical deep submicron lithography technology. Excimer laser lithography technology with a wavelength of 157nm will also be applied in the near future. If an EUV with a wavelength of 13nm is used, a thin strip of 0.1um can be obtained.
Around 1985, predecessors had already conducted theoretical discussions on EUV technology and conducted many related experiments. In the past decade, the development of the microelectronics industry has been hindered by numerous obstacles, which has led people to have a sense of crisis. And from the development process of microelectronics technology, it can be judged that if extreme ultraviolet lithography technology is not introduced soon to make comprehensive improvements to the current chip manufacturing methods, the entire chip industry will be in a precarious situation.
The EUV system mainly consists of four parts: extreme ultraviolet light source; Reflective projection system; Lithography template (mask); Can be used for extreme ultraviolet photoresist coatings.
Lithography technology
The alignment and alignment accuracy of the lithography machine used in extreme ultraviolet lithography technology should reach 10nm, and its development and manufacturing principles are actually very similar to traditional optical lithography in principle. The research focus on lithography machines is to require extremely fast and precise positioning, as well as field by field leveling and focusing technology, because lithography machines require a lot of times to stitch patterns and step by step scanning exposure during operation. Moreover, the acquisition and processing of incident aligned light wave signals also need to be addressed.
Current status of EUV technology
The progress of EUV technology is still relatively slow and will consume a large amount of funds. Although few manufacturers currently apply this technology to production, extreme ultraviolet lithography technology has always been a research hotspot in recent years. All manufacturers are also full of expectations for this technology, hoping that it can make greater progress and be put into large-scale use as soon as possible.
All manufacturers are aware that the use of EUV technology is necessary for semiconductor processes that aspire to the next generation. The shorter the wavelength, the higher the frequency, and the energy of light is proportional to the frequency and inversely proportional to the wavelength. However, due to the high frequency, traditional aerosols are directly penetrated. Nowadays, the development of semiconductor technology has been constrained by many physical disciplines from various aspects.
Lithography technology
In terms of etching in the 45nm process, EUV technology has already shown some characteristics, so now EVU technology needs to break through. From external support, we need to replace the sol, but a suitable one has not been found yet. From the perspective of EUV technology itself, while also trying to find ways to reduce output energy as much as possible.
The current problems with EUV lithography technology:
1. The cost is too high, up to 65 million US dollars, which is more expensive than the 193nm ArF immersion lithography machine;
2. No suitable light source was found;
3. No defect free mask;
4. No suitable photoresist has been developed;
5. Lack of human resources;
6. Can be used for early development work of 22nm technology.
Prospects of EUV lithography technology
Under the laws of Moore's Law and in the rapidly developing information age of science and technology, the new generation of lithography technology should be selected and studied. It is currently the most concerned technology in the microelectronics industry, and among these high-tech technologies, extreme ultraviolet lithography has obvious advantages compared to other technologies. The resolution of extreme ultraviolet lithography can reach at least 30nm or less, and it is more likely to be favored by various integrated circuit manufacturers because extreme ultraviolet lithography is an extension of traditional lithography technology, and integrated circuit designers also prefer to choose this lithography technology that fully complies with design rules. The manufacturing difficulty of masks in extreme ultraviolet lithography technology is not high, and it has a certain yield advantage.
The manufacturing cost of EUV lithography technology equipment is very high, and many aspects, including masks and processes, cost a lot of money. At the same time, the design and manufacturing of extreme ultraviolet lithography optical systems are extremely complex, with many unresolved technical problems. However, solutions to these difficulties are currently being studied. Once these problems are solved, there will be no fundamental technical difficulties in the application of extreme ultraviolet lithography technology in large-scale integrated circuit production.
X-ray lithography technology
In 1895, German physicist Roentgen was the first to discover X-rays and thus won the Nobel Prize in Physics. X-rays are electromagnetic waves with wave particle duality like other particles, which can be the product of energy level transitions of heavy atoms or accelerated electron electromagnetic coupling radiation. The wavelength of X-rays is extremely short. In 1972, X-rays were first proposed for lithography technology. When used for lithography, the wavelength of X-rays is usually between 0.7 and 0.12nm. Its strong penetrability determines that it can also define high-resolution patterns on thick materials.
Basic X-ray Lithography Process
The wavelength of X-rays is extremely short, which prevents serious diffraction phenomena from occurring. When we use X-ray for exposure, the selection of wavelength is limited by certain factors. During the exposure process, the photoresist absorbs X-ray photons and produces photoelectrons whose range changes with the X-ray wavelength. These photoelectrons reduce the lithography resolution. The shorter the wavelength of X-ray, the farther the range of photoelectrons, which is more unfavorable for lithography. Therefore, increasing the wavelength of X-rays helps to improve lithography resolution. However, long wavelength X-rays can widen the linewidth of the pattern, and considering various factors, the wavelength of X-rays can usually only be chosen as a compromise.
This year's research has found that when the line width of a shape is small to a certain extent (usually 0.01) μ Under the influence of waveguide effects, the resulting pattern linewidth is smaller than the actual mask pattern, so the X-ray lithography resolution is also affected by the distance between the mask and the wafer.
In addition, a large amount of experimental research is needed to address the many factors that affect the quality of X-ray lithography patterns during micro processing.
Lithography technology
X-ray lithography mask
In post optical lithography technology, the most important and challenging one is mask manufacturing technology, among which 1:1 lithography is very difficult and one of the challenges that hinders technological development. So, we believe that mask development is an important link for its application in industrial development, and it is also the key to determining success or failure. In the past, scientists have made tremendous progress in its development, as well as the discovery and application of some new materials. Some have been put into practice in the laboratory, but there have been no significant achievements in industrial development.
The basic structure of X-ray masks includes thin films, absorbers, frameworks, and substrates, among which Si, SiC, and diamond are generally used as the substrate materials for thin films. The absorber mainly uses materials such as gold and tungsten, and its structural diagram is shown in the figure:
Lithography technology
The performance requirements for masks are as follows:
1. To ensure the effective transmission of X-rays and other light rays, with sufficient mechanical strength, high X-ray absorption, and sufficient thickness.
2. Ensure the quantity of its aspect ratio, and have a high resolution and contrast.
3. The size of its mask should ensure its accuracy, with no or fewer defects.
Low pressure CVD is often used for substrates such as Si3N4 films, while methods such as evaporation sputtering electroplating are often used to manufacture absorbers. To improve the quality of X-ray masks, it is necessary to choose the right materials and optimize the process.
X-ray lithography technology not only has the advantages of high resolution, but also high yield. Based on the current application status of X-ray lithography technology, it is urgent to put it into mass production to play a more important role in the production of large-scale or ultra large scale IC circuits, and to overcome the difficulties of high-precision graphic mask technology.
Nanoimprint lithography technology
Nanoimprinting technology was first proposed by Chinese American scientist Zhou Yu from Princeton University in 1995. This technology has the advantages of high production efficiency, low cost, and simple process, and has been proven to be one of the most promising next-generation lithography technologies for replicating nanoscale large-area structures. At present, this technology can achieve a resolution of less than 5 nm. Nanoimprinting technology mainly includes thermal imprinting, ultraviolet imprinting, and micro contact printing.
Nanoimprinting technology is the most commonly used method for processing polymer structures. It uses high-resolution electron beams and other methods to create complex nanostructure patterns on stamps, and then uses pre patterned stamps to deform the polymer material and form structural patterns on the polymer.
Lithography technology
1. Hot embossing technology
Nanothermal imprinting technology is a low-cost and fast method for obtaining parallel replicated structures at the micro nano scale. This technology can replicate the structure on the seal onto a large surface as needed under high temperature conditions, and is widely used for micro and nano structure processing. The entire hot stamping process must be carried out in a vacuum environment with a pressure of less than 1Pa to avoid distortion of the stamping pattern caused by the presence of air bubbles. The hot stamping seal is made of SiC material, which is very hard and reduces the possibility of fracture or deformation during the stamping process.
In addition, SiC has stable chemical properties and does not react with most chemicals, making it easy to clean the seal with different chemicals after stamping. In the process of making the seal, a layer of chromium film with high selectivity (38&1) is first plated on the surface of SiC as the etching mask for the subsequent process reaction ion etching. Then, ZEP resist is uniformly coated on the chromium film, and then electron beam lithography is used to etch nano patterns on the ZEP resist. In order to break the chemical bonds of SiC, a high voltage must be applied to SiC. Finally, at a DC voltage of 350V, nano patterns with smooth etched surfaces and vertical planes were obtained on the SiC surface by reactive ion etching.
The entire hot stamping process can be divided into three steps:
(1) The polymer is heated above its glass transition temperature. This can reduce the viscosity of the polymer during the embossing process, increase its fluidity, and rapidly deform under a certain pressure. But it is not necessary to keep the temperature too high, as it increases the time for heating and cooling, thereby affecting production efficiency. However, it does not significantly improve the molding structure, and may even cause polymer bending and damage to the mold. To ensure that the polymer maintains the same viscosity throughout the entire embossing process, it is necessary to control the heating temperature through a heater.
(2) Apply mechanical pressure on the seal, approximately 500~1000KPa. Increasing pressure between the seal and the polymer can fill the cavity in the mold.
(3) After the embossing process is completed, the entire stack is cooled below the glass transition temperature of the polymer to solidify the pattern, providing sufficient mechanical strength for easy demolding. Then, residual polymer (PMMA) is removed by reactive ion etching, and the nano patterns on the template are completely transferred to the polymer on the surface of the silicon substrate. Combined with etching technology, the patterns are transferred to the silicon substrate.
2. UV imprinting lithography technology
The UV imprinting process involves loading a substrate coated with monomers and a transparent seal into an alignment machine, which is fixed on their respective chucks in a vacuum environment. After the optical alignment of the substrate and seal is completed, contact imprinting begins. The UV exposure of the seal promotes the polymerization and solidification of the polymer in the imprinting area.
Compared with thermal imprinting technology, ultraviolet imprinting has lower environmental requirements and can only be performed at room temperature and low pressure. Therefore, using this technology for production can greatly shorten the production cycle and reduce seal wear. Due to the needs of the manufacturing process, the production of UV imprinted seals requires the use of materials that can be penetrated by ultraviolet light.
Lithography technology
In the past, seals were made by coating PDMS material on a quartz substrate in UV embossing technology. PDMS is an elastic material with a very small Young's modulus, and the soft seal made with it can achieve high resolution. However, in subsequent experiments, it was found that due to the physical softness of PDMS itself, it is also prone to deformation during the imprinting process under low external pressure. Recently, the French National Nanostructure Laboratory proposed the use of a three-layer structure soft seal to reduce the deformation of UV imprinted seals.
The seal uses a 2mm thick quartz substrate, with a 5mm thick PDMS buffer layer in the middle and a top layer composed of PMMA. The specific steps for making a seal are to first evenly coat PMMA on ion activated PDMS material, and then deposit a layer of 30nm thick germanium film on PMMA as an etching mask in the subsequent process. Then, coat the germanium film with a high sensitivity to electron beam etching agent. Then, electron beam lithography and reactive ion etching can be used to obtain high aspect ratio patterns on the top layer of the seal PMMA. Finally, the residual germanium film can be removed. This method can greatly improve the hardness of the seal while maintaining high resolution, and reduce the deformation of the seal imprint.
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