Learn about Lithography Imaging Systems and Optical Coating Technologies

0040-09963 PEDESTAL,150MM FLAT,IS,NI LIFT2,HVCEN

0021-02395 REV.B INSERT RING,ALUMINUM DxZ SACVD

 

The research and development of high-end lithography machine is a systematic project, involving the continuous improvement and breakthrough of all aspects of technology, involving the development of low-absorption loss quartz materials and high-purity thin film materials in material science, precision optical processing technology, coating technology, optical integrated assembly technology in the field of precision optics, and nano-precision displacement control technology in precision machinery. One of the most sophisticated machines in the history of mankind.

The Development History of Lithography Machine

The process flow of semiconductor integrated circuit manufacturing is established by the preparation method of countertop transistor developed by Fairchild, a famous semiconductor manufacturer in the United States: the whole process is to make a mask according to the structure that needs to be made on the silicon substrate, and then the structure is reduced and developed on the surface of the silicon wafer by the method of photoplate making, so as to realize the transfer of the device structure from the mask to the silicon wafer (photolithography).
With the development of semiconductor integrated circuits, the size of semiconductor devices is getting smaller and smaller, and the number of devices accommodating on the finite-scale silicon wafer surface is increasing, and the requirements for the lens resolution of the lithography machine are also increasing.

The continuous improvement of the resolution of the lithography machine is the key to promoting the development of semiconductor integrated circuits in accordance with Moore's law, so the research and development of higher resolution lithography machines has become the continuous pursuit of all lithography machine suppliers. Lithography machine resolution and operating wavelength and numerical aperture determination of the system:

where k1 is the process factor, λ is the exposure wavelength, and NA is the numerical aperture of the objective. According to the formula, reducing the exposure wavelength of the lithography machine is an important way to improve the resolution of the lithography machine.
So far, lithography machines have gone through a total of 5 generations of products based on the exposure wavelength of the lithography machine. Among them, the first-generation and second-generation lithography machines use 436nm G lines and 365nm I lines generated by mercury lamps as lithography light sources, respectively, which can meet the chip production of 0.8μm to 0.35μm process. These two generations of lithography machines generally use the contact/proximity exposure method, which is simple in structure and cheap in price.
Generations 3 to 5 lithography use projection lithography to accurately scale down the circuit diagram on the mask onto a silicon wafer using projection imaging. Projection lithography machines typically use 4:1 or 5:1 reduced imaging exposure, and their resolution is related to the numerical aperture and wavelength of the lens. The fifth-generation lithography machine uses EUV light as the light source, and the price of a single lithography machine is as high as 100 million US dollars.

The process of 193nm dry lithography machine reaches 65nm, using the immersion lithography method, using high refractive index liquid H2O instead of air as the exit medium, the process of 193nm lithography machine can be increased to 22 nm, so 193nm lithography machine will be the core equipment of high-end IC processing at present and for a long time in the future.


The core structure of the 193nm lithography machine is shown in the figure, and the illumination mode setting system and projection imaging system contained in it contain more than 20 optical lenses respectively, and the performance of the anti-reflection coating seriously affects the overall transmittance of the optical system of the lithography machine, for example, assuming that the light reflection and absorption loss of each surface is 0.5%, the light energy loss caused by specular reflection and thin film absorption of the system can reach 40%, so improving the performance of the lens anti-reflection coating of the lithography machine is a key technology in the research and development process of high-precision lithography machine. However, 193nm lithography coatings present some unique technical difficulties compared to conventional imaging systems.

Imaging optics use a large number of spherical optics to adjust the direction of beam propagation. As the system has higher and higher requirements for imaging quality, the reflection of light on the surface of the element will produce a large amount of stray light, which significantly reduces the imaging quality, so the coating of optical films with various properties on the surface of the lens has become a technical way to ensure the performance of high-precision imaging systems.

Optical Coating Technology and Classification

The three major methods of physical vapor deposition, chemical vapor deposition and liquid deposition are currently the main thin film preparation methods, and each type of preparation method can be subdivided.
Physical vapor deposition is a technology that uses physical methods to vaporize materials into gaseous atoms or molecules under vacuum conditions, and then deposits thin films on the surface of the substrate.
Chemical vapor deposition is the preparation of thin films by chemical reactions on the surface of substrates in high-temperature environments, which are mainly used in semiconductor integrated electronic technology, such as the epitaxial deposition of dielectric films in integrated circuits on silicon substrates.
Liquid deposition is a technology that uses chemical methods such as solution chemical reaction or electrochemical reaction to deposit thin films on the surface of the substrate, which does not require a vacuum environment and has simple equipment, and its applications involve electronic components, surface coating and decoration.
Optical thin films are mainly prepared by physical vapor deposition method, and thermal evaporation, sputtering and ion plating methods are often used at present.
Thermal evaporation is the earliest developed in physical vapor deposition coatings and has been widely used.
Thermal evaporation is carried out in a vacuum, in a vacuum chamber, when the membrane material is heated, its atoms will escape from the surface and then condense on the substrate to form a thin film, which is the simple process of thermal evaporation.
Commonly used thermal evaporation methods include resistance evaporation, electron beam evaporation, and ion beam-assisted evaporation.
For the evaporation of metals with low melting points such as gold, silver, and aluminum, sulfur selenides such as zinc sulfide and zinc selenide, and fluoride such as magnesium fluoride and ytterbium fluoride, resistive thermal evaporation is generally used, because these materials have low melting points and are easy to evaporate. When selecting an evaporation source material, it is necessary to consider its wettability with the film material and whether it will have a chemical reaction with the film material. For the evaporation of oxides such as silicon oxide and refractory metals such as tungsten, electron beam evaporation is generally used. The electron beam directly bombards the film material, which has a high energy density (up to 10E9 W/cm2), which is a good way to evaporate refractory metals and high-melting dielectric materials.
Ion beam-assisted deposition (IAD) is an assisted deposition method in which the ion source generates charged ions in the process of resistive evaporation or electron beam evaporation of the membrane, and the deposited particles obtain greater kinetic energy by colliding with these charged ions, thereby changing the process of film growth. The use of ion beam-assisted deposition technology can improve the properties of optical films, improve the crystallinity, directionality, adhesion strength, compactness and surface morphology of thin films.

Magnetron sputtering coating technology

Magnetron sputtering coating is a vacuum coating technology that uses charged ions to bombard the surface of the target so that the target atoms obtain repulsive energy and leave the surface of the target and are finally deposited on the surface of the substrate. The working principle of magnetron sputtering is shown in the diagram. A negative bias pressure is applied to the magnetron cathode target so that the sputtering gas is broken down and a glow discharge occurs. The sputtering gas ions (generally Ar ions) generated during the discharge process accelerate the bombardment of the target surface under the action of a high-energy electric field in the plasma sheath layer on the surface of the cathode target. On the one hand, the bombardment of high-energy sputtering gas ions on the target surface causes some atoms on the surface of the target to obtain recoil energy and detach from the target surface to become sputtered atoms and finally deposited on the substrate surface. On the other hand, the secondary electrons are emitted from the surface of the target and accelerated into the glow discharge plasma region under the action of the sheath layer of the cathode target surface. The secondary electrons entering the plasma region move under the binding action of the magnetic field of the target surface and collide with the sputtering gas atoms to ionize them, so the secondary electrons are an important energy source for magnetron discharge to sustain itself.

The binding effect of the magnetic field on the surface of the target on the secondary electrons significantly increases the plasma concentration near the target surface, which effectively solves the problem of low deposition rate of ordinary diode sputtering. Therefore, the movement of electrons under the binding action of the magnetic field of the target surface is the key to understanding the principle of magnetron sputtering. The figure below shows the distribution of electric and magnetic fields near the magnetron sputtering target surface.