What Is Optical Lithography?

Photolithography is a major process in the production of planar transistors and integrated circuits. It is a processing technique to open a mask (such as silicon dioxide) on the surface of a semiconductor wafer to perform localized diffusion of impurities.

As the current mainstream lithography technology, excimer lithography mainly includes: 248 nm KrF excimer laser technology with a feature size of 0.1 m; 193 nm ArF excimer laser technology with a feature size of 90 nm; and a feature size of 65 nm 193 nm ArF immersion technology (Immersion, 193i). Among them, 193 nm immersion lithography is the longest and most competitive of all lithography technologies, and it is also a research hotspot on how to further realize its potential. Conventional lithography technology uses air as the medium between the photoresist and the exposure lens, while immersion technology replaces air with a liquid medium. In fact, because the refractive index of the liquid medium is closer to the refractive index of the lens material of the exposure lens than the air medium, the lens aperture size and numerical aperture (NA) are equivalently increased, and the depth of focus (DOF) and exposure can be significantly improved. Process tolerance (EL), immersion lithography uses this principle to improve its resolution.

The first generation immersion lithography machine prototypes of the world's three largest lithography machine manufacturers ASML, Nikon and Cannon were all developed and improved on the basis of the original 193nm dry lithography machine, which greatly reduced the development cost and risk. Because the principle of immersion lithography system is clear and does not change much with the existing lithography technology, the current 193nm ArF excimer laser lithography technology has been widely used in mass production of semiconductors below 65nm; ArF immersion lithography technology is at 45nm Node is the mainstream technology for mass production.

In order to further advance the 193i technology to the 32 and 22nm technology nodes, lithography experts have been looking for new technologies. Before there is no better new lithography technology, double exposure technology (or double molding technology, DPT) ) Has become a hot spot of concern. ArF immersion double exposure technology has been considered by the industry as the most competitive technology for 32nm nodes; among lower 22nm nodes and even 16nm node technologies, immersion lithography technology also has considerable advantages.

The main challenges facing immersion lithography are: how to solve the problems of defects such as bubbles and pollution during exposure; research and development of photoresist with good compatibility with water and refractive index greater than 1.8; research and development refraction Optical lens materials and immersed liquid materials with large rates; and the expansion of effective numerical aperture NA. In response to these difficult challenges, scholars at home and abroad, as well as companies such as ASML, Nikon and IBM have done related research and put forward corresponding countermeasures. Immersion lithography machines will be developed towards higher numerical apertures to meet the requirements of smaller lithography line widths. ^[1]

The traditional method to improve the resolution of lithography technology is to increase the NA of the lens or shorten the wavelength. Usually, the first method is to shorten the wavelength. As early as the 1980s, extreme ultraviolet lithography has begun theoretical research and preliminary experiments. The technology's light source is extreme far ultraviolet light with a wavelength of 11 to 14 nm. Its principle is mainly achieved by the extremely short wavelength of the exposure light source. The purpose of improving the resolution of lithography. Because all optical materials have a strong absorption of light at this wavelength, they can only take a reflective optical path. The EUV system is mainly composed of four parts, namely a reflective projection exposure system, a reflective lithographic mask, an extreme ultraviolet light source system, and a lithographic coating that can be used for extreme ultraviolet. The main imaging principle is that extreme far ultraviolet light waves with a wavelength of 10 to 14 nm are projected on a reflective mask through a periodic multilayer film mirror, and the extreme ultraviolet light reflected by the reflective mask is passed through a polygon mirror. The reduced projection system images the integrated circuit geometry on the reflective reticle into a photoresist on the surface of the silicon wafer to form a lithographic pattern required for integrated circuit manufacturing.

The current EUV technology uses an exposure wavelength of 13.5nm. Because of its short wavelength, all optical lithography does not need to use optical proximity correction (OPC) technology, so it can extend the lithography technology to technology nodes below 32nm . In September 2009, Intel demonstrated the 22 nm process wafer to the world for the first time, saying that it will continue to use 193nm immersion lithography and plan to cooperate with EUV and EBL exposure technology to extend the 193nm immersion lithography technology to 11nm process node. ^[1]

Electron beam lithography uses the electron beam generated by the electron gun to focus, center, and correct various aberrations through the electromagnetic lenses of the electron beam column, electron beam spot adjustment, electron beam flow adjustment, and electron beam exposure alignment marks. A series of adjustments such as detection, electron beam deflection correction, and electronic scanning field distortion correction. Finally, the scanning lens is used to write out on the surface of the substrate coated with an electronic resist (photoresist) according to the arrangement of the electron beam exposure program Desired graphics.

Electron beam lithography is basically divided into two categories, one is an electron beam exposure system manufactured by a large-scale production photomask, and the other is an electron beam lithography system that directly writes nanoscale patterns on a substrate. Electron beam lithography originated from scanning electron microscopy, and was first developed by G. Dupingen University of the Federal Republic of Germany. Mollenstedt et al. Proposed in the 1960s. The wavelength of the electron beam exposure depends on the electron energy. The higher the electron energy, the shorter the exposure wavelength, which is generally on the order of 10-6 nm. Therefore, the electron beam lithography is not affected by the diffraction limit, so the electron beam lithography can get close to Resolution for atomic size.

However, when the electron beam is incident on the resist and the substrate, the electrons will collide with the atoms of the solid material to generate the electron scattering phenomenon, including front scattering and back scattering electrons. These scattered electrons also participate in the "exposure". Scattered electrons can reach tens of nanometers, and backscattered electrons can return to tens of microns from the substrate back to the resist. The edges of all actual electron beams exposed and developed have to be expanded outward. This is the so-called "electron beam proximity effect. At the same time, if there is an insulating dielectric film on a semiconductor substrate, electrons will also generate a certain amount of charge accumulation when passing through it. These accumulated charges will also repel the electrons that are subsequently exposed, causing offset, and non-conductive insulators (such as glass sheets) must not be exposed with electron beams. Also, spatial alternating magnetic fields and laboratory temperature changes will cause electron beam exposures. "Drift" phenomenon occurs. Therefore, even with an exposure system with a 2nm electron beam spot, it is not easy to expose a pattern structure below 50nm. The resolution of the electron beam lithography technology already adopted by the Massachusetts Institute of Technology (MIT) will be advanced to 9nm. Electron beam direct writing lithography may not be needed

Making masks is more flexible. However, due to its low exposure efficiency, it is mainly used for nano-manufacturing of small samples in the laboratory. The electron beam exposure has to adapt to mass production, and how to further increase the exposure speed is a difficult problem. In order to solve the efficiency problem of electron beam lithography, it is usually used in conjunction with other lithography technologies. For example, in order to solve the problem of exposure efficiency, electron beam lithography and optical lithography are usually used to match and mix lithography. That is, most of the exposure processes are still prepared by existing mature semiconductor optical lithography processes, and only nano-scale patterns are available. Or the process layer is realized by electron beam lithography.

Under the circumstance that the traditional optical lithography technology is approaching the limit of the process, the electron beam lithography technology may appear in hybrid lithography that matches the optical exposure technology and EUV technology represented by the current 193i. Play an important role. It should be mentioned that the electron beam exposure technology is the key technology to promote the further development of microelectronic technology and microfabrication technology. It has a broad application prospect in the microsystem microfabrication fields such as microelectronics, micro-optics and micromechanics. Except for the writing lithography technology, almost all new-generation lithography technology requires mask making without the electron beam exposure technology. ^[1]