The development of photolithography projection objective optical path structures
Semiconductor lithography system objective lenses have evolved significantly as the industry has grown. From simple photographic lens and microscope head combinations to purely reflective structures, advances in objective lens structures have improved the lithographic resolution of the system.
Photolithography system has a very key performance indicators, is the lithographic resolution R, to determine the photolithographic resolution of the main three factors, respectively, k1, λ and NA, k1 is the scale factor, usually determined by the photolithographic process, taking the value of the range of 0.25 ~ 0.5. λ is the wavelength of the light source, the smaller the wavelength, the smaller the resolution, the NA value is the numerical aperture of the projection objective, the larger the object NA value, the smaller the resolution of the system. NA value is the numerical aperture of the projection objective, the larger the NA value of the objective, the smaller the resolution of the system.
Accompanied by the shortening of the wavelength, and the increasing NA value, the optical path structure of the projection objective lens has also undergone a number of adjustments and iterations, the evolution of several structures of the objective lens:
A. Early version-photographic lens plus microscope head
B. introduction of aspherical and immersion pure refraction system
C. Refractive/reflective structures
D. Purely reflective structure
A. Early version-photographic lens plus microscope head
With the early projection objective lens, the idea was relatively simple. At one end of the mask, a photographic lens was chosen to ensure a large enough field of view, while at the wafer end, a microscope head was used to ensure a large NA value and a small field of view, thus realizing a proportional scaling of the pattern. This results in the prototype of a projection objective in the form of a combination of a camera lens and microscope head, as shown in the figure below:
In photolithography applications, because the wafer is flat, so correcting the Petzval field curvature of the objective lens is the first task of the lens design, used to correct the field curvature of the method, usually in the waist of the lens using a negative optical focal length lens, in the coarser part of the lens, the use of positive optical focal length of the lens, so the need to correct the field curvature of the structural form of the objective lens is usually a gourd shape, according to the performance of the different, divided into a single-waist, double-waisted or multiwaisted systems. Early lithography system, the working wavelength is 436nm (g line), 405nm (h line) or 365nm (i line), the material of the objective element is commonly used optical glass, contains a small number of lenses, and all for the spherical lens, the lens NA value is not large, to the single-waisted system is the most common, the early structure of the objective lens is shown in the following figure:
The above objective lens works in the 405nm band, the NA value is 0.36, the scaling rate is 0.2, the element material is mainly glass, containing a total of 18 lenses, of which glued lenses are also used at the rear end.
B. Introduction of aspherical and immersion pure refraction system
With the increase of the NA value of the objective lens, the traditional single-waist system no longer meets the requirements, and double-waist as well as multiple-waist objective structures have appeared. In order to obtain good projection performance, it is necessary to ensure that there are no elements in the lens that cause light to bend suddenly, because such elements introduce higher order aberrations that are difficult to be corrected away, and therefore the propagation of light through the objective lens needs to be very smooth. This also means that the number of lenses in the lens needs to be increased to achieve a smooth folding of the light, and lenses at this stage usually have close to 30 spherical lenses to achieve this.
The picture above shows a double-waisted objective lens operating at 248nm with an NA of 0.75, containing 29 lenses, all made of fused silica. Because of the large NA value, the working distance of the objective is very short, and the system at this point has changed from the earlier single telecentric system at the wafer end to a double telecentric system on the left and right sides. Because the glue of the glued lens will have an effect on the transmittance and image quality of the lens, the glued lens is no longer used at this stage. At this time the lens, the barrel diameter of about 60 ~ 80 centimeters, the length of more than 1 meter, weighing several hundred kilograms, is quite a huge lens. The increasing number of lenses is accompanied by an increase in cost, and the continued increase in design difficulty. With the development of optical manufacturing process, the processing level of aspherical elements has been improved, therefore, aspherical lenses are used in the objective lens system, on the one hand, aspherical lenses can be more effective in correcting the system aberration, on the other hand, it helps to reduce the size of the objective lens system. The figure below compares the structure of an objective lens with an aspherical lens. It can be seen that with the use of an aspherical lens, the aperture of the largest lens in the lens has been reduced by 10%, while the length of the entire lens has been shortened by 15%, which reduces the size of the entire lens by about 40%, which is a very significant figure.
Such lenses are the mainstream of the DUV band lens form, followed by the introduction of immersion technology, the lens of the NA value of the further increase, which makes the device resolution to obtain a significant improvement.
C.Refractive/Reflective Structure
By introducing an aspherical lens in the projection objective, the NA value of 193nm dry etching can be increased to the order of 0.95. If immersion lithography is used, for example, by filling pure water between the objective and the photoresist, the numerical aperture of the above purely refractive lens can be increased to the order of 1.1. The high NA characteristic of the objective lens makes the aperture of the lens in the rear portion of the lens very large, and at this time, the uniformity of the lens material is no longer easy to satisfy the performance requirements of the objective lens, and the aberration correction of the entire lens becomes very difficult. For these reasons, the introduction of a refractive/reflective hybrid objective structure, through the use of mirrors, helps to correct Petzval field curvature, but also effectively reduce the size of the entire system, and the system NA value can be raised to 1.35 orders of magnitude. In addition, foreign giants for 157nm exposure objective lens has also had a long time of research and development, in the previous article we mentioned, 157nm band, only calcium fluoride material to meet the transmittance needs. However, can issue 157nm light F2 laser there is a poor monochromatic problem, its monochromatic far less than the ArF or KrF laser, which leads to the lens system there will be obvious chromatic aberration, purely refractive system has not been able to effectively correct on-axis chromatic aberration, at this time must also be introduced into the folding structure to solve the problem of chromatic aberration.
A representative refractive objective lens shown above, using immersion technology, NA value of 1.3
D. Purely Reflective Structure
The Dutch company ASML, with the help of 193nm immersion lithography, has gained a monopoly in the market in one fell swoop. The company combined with upstream suppliers and downstream equipment customers, spent more than 10 years to demonstrate the feasibility of EUV lithography and completed the development of the principle prototype. EUV refers to extreme ultraviolet light, the wavelength of 13.5nm, at this wavelength, the material to the beam of light absorption is very large, the refractive or refractive system is no longer suitable for the use of the above, you need to use a purely reflective structure. Reflective system to solve the traditional reflective telescope blocking problem, and, due to the reflector in this band of the coating reflectivity of the problem, a single reflector reflectivity in the order of 60% -70%, so the objective system needs to be used as few as possible the number of mirrors to complete the exposure. The earliest EUV principle device, a coaxial reflection system with only two mirrors, the primary and secondary mirrors, similar to the traditional telescope system, called Microfield Exposure Tools MET (Microfield Exposure Tools), whose optical path is shown below:
After the completion of the principle verification, and then developed a four-mirror system, six-mirror system and other objective structures, the following figure is a six-mirror system of the optical path diagram:
The above six-mirror system has a magnification of 0.25 and an NA value of 0.5, and all mirrors are off-axis aspherical elements with sub-nanometer face shape accuracy. Recently, ASML has shipped an EUV lithography machine with an NA of 0.55, and the price of one device is reportedly more than 300 million euros, which shows its high technological content. In addition, the eight-mirror system containing eight mirrors, as well as the ten-mirror system has also been reported in the paper, and I believe that the NA value of the EUV lithography machine will also be improved in the future.
Conclusion
In this paper, we have made a detailed introduction to the development of the projection exposure objective, including the early objective prototype, pure refractive objective, refractive-reflective hybrid objective and purely reflective objective and other parts, along with the development and application of these objectives, photolithography resolution from the beginning of the first few hundred nanometers to the current several nanometers, has made great progress.