How Do X-Rays Work?

X-rays have electromagnetic radiation between ultraviolet and gamma rays. X-rays are a type of electromagnetic radiation with a short wavelength. The wavelength is between (0.01 to 100) Angstroms. Discovered by German physicist WK Roentgen in 1895, it is also called Roentgen ray. Roentgen rays have a high ability to penetrate, and can penetrate many substances that are opaque to visible light, such as ink paper and wood. This kind of invisible rays can cause many solid materials to generate visible fluorescence, which can make photographic films sensitive and air ionization. The shorter the wavelength, the greater the energy of X-rays, called hard X-rays. , Called soft X-ray. Ultra-hard X-rays with a wavelength less than 0.1 angstrom are called hard X-rays within the range of 0.1 to 1 angstrom and soft X-rays within the range of 1 to 10 angstrom.

Soft X-ray projection lithography is an extension of the existing visible-near-ultraviolet projection lithography to the soft X-ray wave band (1-30 nm). However, because the refractive index of any material in this band is close to 1, and the absorption is large, the reflection system must be used for the miniature projection optical system, and the reflectivity of single-layer film mirrors to normal incident soft X-rays is almost zero, which cannot be used It forms a normal incidence system. After the 1970s, with the continuous improvement of ultra-smooth surface processing technology and ultra-thin film preparation technology, the reflectance of 13nm Mo / Si multilayer films prepared by people has approached 70%, which has led people to integrate soft X with multilayer film mirrors. Ray projection lithography systems are possible.

As shown in the figure, the future soft X-ray projection lithography equipment is composed of a laser plasma light source, an illumination optical system, a miniature projection optical system, a mask and a silicon wafer precision workbench, a shock absorption system and a corresponding vacuum chamber. The miniature projection optical system is a reflective optical system composed of two or three aspherical mirrors. At this time, the minimum aberration area is a circular ring with the optical axis as the center. In order to obtain a sufficient lithographic range, the reflective mask and the silicon wafer must be scanned synchronously. Structurally, the miniature projection optical system is a telecentric optical path on the image side to prevent magnification changes in the focal depth range. In order to improve the light energy efficiency of the system, the bandwidth of each multilayer film system must be strictly matched.

Due to the short working wavelength of soft X-ray projection lithography, there are still many key technical problems to be solved.

Research on soft X-ray optics in China, which began in the late 1970s, involves soft X-ray light sources, soft X-ray radiation metrology, ultra-precision optical processing / detection, and soft X-ray multilayer film technology. In terms of soft X-ray imaging optics, which is the main technical basis, Changchun Opto-Mechanics ranks first in China. In recent years, important progress has been made in key technologies and system integration of projection lithography.

X Soft X-ray soft X-ray laser plasma light source

Changchun Institute of Opto-Mechanics has carried out research on laser plasma light source technology for pollution-free soft X-ray projection lithography. CO ² frozen target and gas target laser plasma soft X-ray light source have been developed, and strong radiation was measured in the soft X-ray projection lithography working band (13nm).

X Soft X-Ray Soft X-Ray Multilayer Film Technology

In the development of soft X-ray multilayer film elements, the optical constant of the coating material is the basis of all calculations and designs. When the film thickness is on the order of nanometers, the optical constant of the coating material is both a function of wavelength and a function of film thickness. The optical constants calculated by BLHenke et al. Using photometric absorption to determine the atomic scattering factor and the optical constants measured by DL Winddt et al. Using the reflectance method correspond to bulk materials or thicker films (d100nm), while soft X-ray multilayer films The film thickness is generally less than 10nm. The accumulation of optical constants in soft X-ray bands in the state of ultra-thin films has become an urgent need for research. The Changchun Institute of Optics and Mechanics cooperated with the Tohoku University Scientific Research Institute to accurately determine the optical constants of Au, C, Mo, Rh, Ru, Pt, W, Si and other substances between 60 and 900 eV.

Soft X-ray float polishing super smooth surface processing technology

Ultra-smooth substrates are the basis for preparing soft X-ray multilayers, but conventional optical processing is difficult to achieve surface roughness of less than 0.5 nm. Since 1992, Changchun Institute of Light Machinery has started to study the float polishing technology. After more than a year of hard work, a new prototype of the float polishing machine has been developed with a new process of immersion polishing of tin grinding discs. The surface roughness of the flat sample after the process experiment is less than 0.3nm.

X Soft X-ray soft X-ray normal incidence imaging system

Began to develop 18.2nm Schwartzschild type soft X-ray microscope from 1991. It consists of a soft X-ray laser plasma light source, a spherical condenser with a multilayer coating, a 10 × Schwartzschild microscope objective, and an Al / C soft X-ray filter, achieving a spatial resolution of less than 1 m. Based on the research of the soft X-ray normal incidence microscopic imaging system, a soft X-ray projection lithography principle working device with a working wavelength of 13 nm is designed. It consists of a soft X-ray laser plasma light source, an ellipsoid condenser, a transmission mask, and a 0.1 × Schwartzschild miniature imaging system, mask and silicon wafer precision stage. The multilayer optical substrate is now ready for installation and commissioning. The resolution is expected to reach less than 0.25 m.

At present, soft X-ray projection lithography still needs to solve many key technologies, especially deep sub-nanometer mirror processing and multilayer film preparation technology. However, the technical route and the way to solve the problem are also basically clear.

Therefore, at the beginning of this century, it is entirely expected that soft X-ray projection lithography technology will be used to mass-produce integrated circuits with feature line widths less than 0.1 m. ^[3]