Optical materials in the deep ultraviolet band

Common optical materials in the deep UV band include fused silica, calcium fluoride(CaF2) and sapphire. The selection of optical materials for the deep UV band depends on the specific requirements of the application, such as the desired transmittance, refractive index, and cost considerations.

 

In recent years, with the demand driving force of the semiconductor industry, short-wave optics has experienced rapid development. However, in the short-wave region, the decrease in the transmittance of optical materials is a prominent issue, making the choices of materials for optical designers very limited. Taking the lithography system as an example, in the i-line (365nm) and g-line (436nm) bands, the transmittance of most heavy flint glasses has already decreased, and there is energy absorption phenomenon, causing significant thermal effects on the lithography lens and affecting the system's performance. In the deep ultraviolet bands of 248nm and 193nm, fused silica and calcium fluoride (CaF2) are the most commonly used materials. Under the 157nm wavelength, only the transmittance of calcium fluoride material meets the usage requirements. In this article, we will provide a specific introduction to these optical materials used in the deep ultraviolet bands to deepen everyone's understanding of this field.

 

Material transmittance in deep ultraviolet band

For the lithography projection system operating at 193nm, it can be divided into two types: dry lithography and immersion lithography. Dry lithography is a traditional lithography technique, where the medium between the projection lens and the photoresist is air. On the other hand, in immersion lithography, water is filled between the projection lens and the photoresist. The refractive index of water is greater than that of air, so the NA value of the immersion system is increased, thereby improving the resolution of the lithography lens. The thickness of the immersion layer is typically only a few millimeters thick, so the loss of beam energy can be negligible. In the illustration below, a comparison is made of the transmittance in the deep ultraviolet band of 10mm thick BK7 material, water, fused silica, and calcium fluoride materials, as shown below：

From the above figure, we can see that for the common BK7 glass, the transmittance significantly decreases at 350nm and quickly diminishes to a small value before 300nm. This phenomenon indicates that it cannot be used in the ultraviolet band and is only used here for comparison purposes. The transmittance of water medium is still acceptable at 193nm, making it suitable for use as an immersion layer. Fused silica material (SiO2) has good transmittance at 193nm, but it rapidly decreases as the wavelength extends to lower bands. Calcium fluoride material can still maintain over 92% transmittance at 150nm. As a crystalline material, the light absorption characteristics of calcium fluoride depend on its purity and processing properties. Different purities of calcium fluoride or calcium fluoride grown using different processes will exhibit significant differences in their absorption characteristics.

 

Refractive index of materials in deep ultraviolet

At different wavelengths, the refractive index of materials varies. Taking fused silica material as an example, as the wavelength decreases, its refractive index significantly increases due to dispersion. The refractive index curve of fused silica material in the deep ultraviolet band is shown in the figure below:

We know that the greater the refractive index of a material, the stronger its ability to refract light beams. Therefore, the high refractive index of fused silica material in the deep ultraviolet band helps correct various aberrations in the system. Compared to fused silica, calcium fluoride material has a relatively slightly lower refractive index in the deep ultraviolet band, as shown in the refractive index curve in the figure below:

From the above figure, it can be seen that as the wavelength decreases, the refractive index of calcium fluoride material tends to increase. Additionally, calcium fluoride is the only material that can be used in the 150nm wavelength band. However, as a crystalline material, calcium fluoride exhibits birefringence. This phenomenon needs attention at 193nm but is not critical. By the time we reach the 157nm wavelength band, birefringence becomes very pronounced and significantly affects the system's performance. This is one of the reasons why the 157nm lithography system was not successfully implemented. By introducing immersion lithography, the system's numerical aperture (NA) value is further increased, extending Moore's law. Water is typically chosen as the immersion liquid, and the refractive index curve of water in the deep ultraviolet band is shown in the figure below:

It can be seen that the refractive index of water in the deep ultraviolet band increases significantly compared to when it is in the visible band, thus helping to reduce the spherical aberration on the last lens surface of the lithography objective lens and improving system performance. In the focal plane region of the lithography objective, because of the high light energy density and concentrated heat, there will be a breakdown effect on the air, and some of the air molecules will dissociate into atoms, thus interfering with the precision of lithography. Therefore, deep ultraviolet lithography objective lens is usually filled with protective gas to avoid the above air breakdown effect, commonly used gas for nitrogen or helium. As we know, the refractive index of air is usually 1. The refractive indices of these protective gases also need to be taken into account when filling the lenses with these gases, which usually deviate slightly from the refractive index. In the table below, the values of the refractive indices of the materials are given at several major UV wavelengths:

It is worth pointing out that the table lists only the approximate refractive index of each material at the corresponding wavelength, which is for reference only. The actual refractive index of a piece of material depends on its purity, process, and environmental parameters such as the temperature and air pressure where it is located, and the refractive index of the material needs to be actually tested for practical applications.

 

Conclusion

In this paper, we have made an introduction to the characteristics of optical materials in the deep ultraviolet band. In the deep ultraviolet band, because of the transmittance, the types of available materials are very limited, and in the paper, we have introduced the characteristics of the transmittance and refractive index parameters of fused silica, calcium fluoride and aqueous media in the short-wave, which will help us to have an intuitive understanding of their performances. Currently, the optical materials used in the deep ultraviolet band are the products of Corning, Schott, Nikon and other companies, and the fused silica materials produced in China are also widely used.