## 3 Progress in Grating Fabrication Technology
### Electron Beam Exposure  
At present, lithography technology has undergone several generations of development, and the exposure mode has also changed from the initial-contact type to the projection type, and then to the electronic-scanning type. Exposure patterns are also gradually developing towards smaller line widths and higher precision, and electron-beam lithography technology has emerged accordingly. Electron-beam lithography is an important patterning method for nanosystems and devices, especially in mask fabrication. Electron-beam lithography is a relatively old patterning method, but it is still the most useful patterning technique in nanofabrication. In the 1980s, more and more nanolithography technologies flourished. Electron-beam lithography offers the unique advantages of high-resolution feature size, high-reliability processing, high-precision alignment, and flexibility for high pattern replication. In the early 1990s, [PERSON] and [PERSON] of the Russian Institute of Physics and Technology used the ZBA-20 variable-shape electron-beam lithography system to fabricate an artificial crystal refractive-index grating with a feature size much smaller than the wavelength of the incident light (period 2-3 \(\upmu\)m, variable duty cycles, grating area 20 \(\times\) 20 mm\({}^{2}\), and two-dimensional zero-order grating). Its principle provides an opportunity to generate a medium with a desired refractive-index profile and design an ideal refractive-index distribution by calculating the duty cycle of each period of the optical element, providing a new degree of freedom for the fabrication of optical elements. After photolithography, plasma is used to coat and etch a metal layer, and finally the CO\({}_{2}\) laser beam is focused into a line using the fabricated grating structure [72]. In 2000, [PERSON], [PERSON], et al. from Nanyang Technological University in Singapore proposed a simple UV holographic interference lithography technique for fabricating surface-relief gratings on hybrid Si-Ti glass with a laser wavelength of 325 nm. The properties of the sol-gel glass, such as refractive index and film thickness, were discussed. The exposure time was 2 min and the exposure dose was 3.5 mW/cm\({}^{2}\). In contrast to traditional photoresist-based microfabrication techniques, this is a single-step fabrication process that does not involve any etching steps [69]. In 2007, [PERSON] and [PERSON] of Japan used the transparency of inorganic resists to solve the problem of exposure blur. By changing the accelerating voltage from 1 to 4 kV at a holographic exposure dose of 250 \(\upmu\)C/cm\({}^{2}\), a blade-shaped mold for binary optics was fabricated. As shown in Figure 36, the design value of the pattern line width was 200 nm. It can be seen from the figure that the width of the line expanded at low voltage [73]. In 2011, [PERSON] et al. of LLNL in Berkeley, California, USA, successfully fabricated a 10,000 lines/mm ultradense sawtooth substrate by electron-beam lithography, then tested it. The fabrication process for the high-quality sawtooth substrates was successfully extended to ultrashort grating periods of 100 nm. The gratings had a near-perfect triangular-groove profile and a very smooth blazed surface [74]. In 2018, [PERSON] and [PERSON] in Finland proposed a method based on the composition of atomic layers and fabrication of buried-mask gratings. The grating mask was fabricated by electron-beam lithography, and the bottom grating profile was fabricated  
Figure 36: Three-dimensional spin on glass mold diagram. in fused silica by reactive-ion etching. Atomic layer deposition was used to prepare an amorphous TiO\({}_{2}\) thin film to bury the SiO\({}_{2}\) grating, which improved the utilization rate of the grating and solved the problem of damage to the grating structure [75]. In electron-beam exposure, the positioning accuracy of the workpiece table and the distortion of the exposure field need to be guaranteed in a very harsh environment.