From：
What are the steps involved in taking apart a photolithography machine? First, remove the precision air cushioned vibration isolation platform underneath, then remove the dozen-ton steel structure, and you will see inside, a light source, a bunch of lenses and a floating platform, which are the three core parts of a lithography machine，
The light source and illumination system, which emits uniform and adjustable decoherent UV light onto the photomask; the objective lens and imaging system, which projects the image on the photomask onto the wafer as losslessly as possible; the workpiece table movement and measurement and control system, which is responsible for aligning and levelling, precision positioning and transporting the wafer. The most difficult to manufacture?
Lithography accuracy：
Let’s take a look at what parts are in a lithography machine and how they work, and why is it so difficult to manufacture? Since no manufacturer is sponsoring my machine, I will try to understand the three most core systems in a photolithography machine in three videos, using no physical disassembly.
Do not underestimate this pile of lenses, they can account for 40% of the total price of the machine, why the lens how important, because the photolithography machine is simply a giant camera to make chips, it is through the exposure of the development, the integrated circuit graphics on the photomask, replicated on the silicon coated with photoresist, a good photolithography machine, to carve out more precise chips, into the language of engineers, refers to one of the key parameters of the photolithography machine, the resolution (Resolution), here the resolution, not refers to the display resolution on the screen, but refers to the optical diffraction limit resolution.
Simply put, it is the smallest size that can be engraved by a single exposure of the photolithography machine, and in layman’s terms, it is the light of the knife, how fine it can be, the smallest line width engraved on the chip is 90 nanometers, or 45 nanometers, so the resolution of the photolithography machine, the smaller the better, because it represents the precision of photolithography, how to reduce the resolution of a photolithography machine? Here it is important to mention the Rayleigh criterion, which has dominated the development of lithography since the last century.
Lithography resolution, or Critical Dimension (CD), is equal to the wavelength of the light source, compared to the numerical aperture of the lens, multiplied by a process factor, where the process factor refers to environmental factors and engineering techniques outside the lithography machine, such as temperature and humidity, the use of Phase Shifting Mask, high sensitivity photoresist (chemical amplification), etc. For a lithography machine to be more accurate and to have a smaller resolution, it means either reducing the wavelength of the light source or increasing the numerical aperture of the lens.
The evolutionary path of light sources is clear, early G-line lithography, using high-pressure mercury lamps as the light source, wavelength 436 nm, corresponding to the minimum resolution, about 500 nm, later DUV (deep ultraviolet) lithography, the light source changed to krypton fluoride (KrF) excimer laser, wavelength 248 nm, the resolution can be reduced to 180 nm, and to the recent years of EUV lithography, the use of wavelength only 13.5 nm, we will talk more about light sources and illumination systems later.
Numerical Aperture：
Today’s lithography lens is another parameter that determines the reduction in resolution, the numerical aperture (Numerical Aperture), or NA for short. in optics, the numerical aperture of a lens depends on its ability to refract light and the refractive index of the medium, as expressed in the following formula:
However, for numerical apertures, the angle here refers to the maximum light gathering angle of the lens, the greater the angle, the greater the light gathering capacity of the lens and the larger the numerical aperture, how can the light gathering capacity of the lens be enhanced? Without changing the material properties, either by increasing the diameter and making the lens bigger, or by increasing the curvature (i.e. reducing the radius of curvature) and bending the lens.
Since high-precision lithography requires a large numerical aperture, can’t we just build a large curvature lens with a diameter of two meters and put it in the lithography machine? Unfortunately, no, because we have to solve the problem of chromatic aberration and aberration of a single spherical mirror, which means that different colours of light are refracted differently in the lens and cannot converge at the same distance，
There are many different types of aberrations, such as spherical aberration (spherical aberration), which refers to the difference in refractive power between the edge of a spherical mirror, and the central part of a convex lens, which has a large refractive amplitude at the edge, and the corresponding focal length will be shortened and the imaging distance will be shifted forward; while the case of a concave lens is just the opposite, so sticking two suitable concave and convex lenses (positive and negative lenses) together can be used to offset the chromatic and spherical aberration of a single lens.
This is also the principle of the Gauss-type lens, born in 1817, yes, the genius pupil who speed calculates 1+2+3 all the way up to 100, the mathematician, physicist and astronomer Gauss who often appears in advanced mathematics textbooks, and who, during his part-time job as director of the Göttingen Observatory, incidentally made a little improvement to the aberration of the telescope, but in addition to the aberration of the sphere, there is also the field area, that is, the bending of the image field, which refers to the best imaging surface of a spherical mirror, not a plane, but a curved surface, in addition to image dispersion, aberration, coma, and various higher-order aberrations, all of which need to be solved.
Lens construction:
Isn’t the human eye a simple spherical mirror? Why do we see almost no aberration in the image? Firstly, the lens is so intelligent that it not only focuses itself, but also has a refractive index that decreases in a gradient from the centre to the edge, which compensates for the spherical aberration, secondly, the retina, which receives the projected image, is itself a curved surface that adapts to the field, and most importantly, we have a retouching brain that automatically compensates for defective visual information.
The photolithography lenses that do not know how to brainstorm, will have to rely on the number of lenses to fix the picture. For example, the aforementioned inclusion of negative lenses to compensate, followed by the inclusion of aspheric mirrors, but the manufacturing of irregular mirrors is difficult and costly, so the more common method is to split the spherical mirror, such as a lens, split into two pieces of curvature reduced by half, you can maintain the same numerical aperture premise, the spherical aberration wise-difference and image dispersion, down to 1/4 of the original, so that by constantly increasing the number of lenses, share the curvature By increasing the number of lenses, sharing the curvature and improving the symmetry of the overall structure, the optical error of the lens can be continuously reduced.
So from the 19th century, with the development of camera technology, the number of lenses in the lens, increasing, from two Gauss type, 3 Kirk type, 4 double Gauss type, to 6 Prana type, for ordinary camera lenses, 6 lenses is good enough structure, you can take the lens on your own mobile phone, take it apart and see (don’t), the number of lenses in it is usually 5-6, but for photolithography lenses, 6 is not enough.
In order to catch up with Moore’s Law and meet the nano-scale chip process, since the 1980s, the number of lenses in the lithography lens, based on the double Gaussian structure, has skyrocketed, and the imaging lens in the current mainstream lithography machine, which usually has more than 20 lenses, many bigger than your home pot, has to be installed inside a one-metre diameter, more than one metre high mirror cylinder, inflatable protection, with a total weight from several hundred kilograms to A tonne, for example, a certain 193nm deep ultraviolet lithography, which has a double telecentric design of lenses inside it, an engineering euphoria. It features three groups of positive and negative lenses, symmetrical in structure, to share the spherical aberration, balance the rotation and offset the distortion..
Lenses subjected to heat：
But are there any disadvantages to using so many lenses? Of course there is, light loses some of its energy each time it is transmitted or reflected, the more lenses in the lens, the greater the energy loss to the light source, especially in the extreme ultraviolet, which is very easily absorbed and cannot even penetrate the air, so the lens for EUV lithography is even more special and must use a total reflector, and (objective) a maximum of 6-8 lenses to minimise the total reflection loss，
EUV lithography will be explained separately later, here first not to expand, back to the lens-based lithography lens, light in each projection or reflection, the loss of energy will not disappear, but into the heat of the lens, which is often concentrated in the relatively thin waist, and this part of the lens set, is mainly responsible for balancing the field curve and adjust the focal length, so in the chip production, the lithography lens is heated, usually resulting in changes in the exposure focal length and field curve, affecting the yield of lithography.
Therefore, to ensure that the heat generated by the lens does not affect production, the lithography lens also needs to be equipped with thermal effect simulation and real time calibration functions to calibrate and compensate for the position and deformation of certain lenses by adjusting them in time according to the illumination conditions, exposure dose, and the light transmission of the photomask. This requires that certain lenses in the lithography lens be made into movable parts that can move in the axial direction, or that certain lenses undergo controlled physical deformation, or even in some cases, that specific lenses be actively heated by infrared light to deliberately produce aberrations to compensate for other imaging errors, but of course even if the effects of thermal effects on lithography can be corrected, lenses made of quartz should not be overheated for long periods of time in production, as this can lead to changes in lens density and lens transmission. as this can lead to changes in the density of the lens and premature ageing of the lens.
So the lithography machine, but also a variety of means of cooling, such as adding air cooling kit in the lens, in the mirror barrel plus water cooling kit, while in the chip production, the light transmission of the photomask to properly reduce, such as in the mask template design, add some redundant (Dummy) graphics, as a way to reduce the amount of light into the lens, there is a silicon wafer coated photoresist, but also try to choose low outgassing type, to prevent the gel in the exposure, the release of special gas, corrosion of the bottom layer of the lens, these are to protect the lithography machine, the most expensive lens, to reduce the frequency of maintenance and extend the service life.
Manufacturing difficulty：
At present, the main manufacturers who can produce photolithography are three German and Japanese companies, Zeiss, Nikon and Canon, of which Zeiss in Germany is mainly deeply bound to Asmac, while the two Japanese, itself is to do photolithography, so Nikon Canon’s best lenses, are not in the camera, but used in their own photolithography，
Why are lithography lenses so difficult to build? Firstly, because of the complex structure of the lens, both to use a large number of lenses to solve the aberration, but also to find ways to improve the numerical aperture, Zeiss to the DUV lithography machine to make the lens, the numerical aperture can reach 0.93, already close to the limit of dry lithography value 1, this is because the refractive index of air, approximately equal to 1, and the closing angle of the lens theoretically does not exceed 90 degrees. So the upper limit of NA is 1.
The only way to get a lens with a numerical aperture greater than 1 is to dope it with water or oil. This is the principle of immersion lithography, by changing the medium between the lens and the silicon wafer from air to ultra-pure water, using the refractive index of water greater than air, thus further increasing the numerical aperture of the system, in order to make the resolution of the lithography smaller, can the numerical aperture be increased infinitely? No, because there is a conflict between numerical aperture (NA) and depth of field (DoF). Depth of field is the depth of focus, which is the range of distances that the pattern projected on the wafer surface can be allowed to move vertically while remaining sharp and out of focus.
This example is to illustrate that the design and manufacture of lithography lenses is a complex system engineering that requires a strong combination of various disciplines, including optics, mechanics, materials, thermal simulation, etc. Secondly, like all high-end manufacturing industries, lithography lenses require sufficient talent pool and technology accumulation.
If you take Zeiss as an example, their lenses for EUV lithography can achieve a surface undulation deviation of less than 0.05 nanometres, 0.05 nanometres you may have no idea, this is smaller than the diameter of many atoms, if this lens is enlarged to the size of Hainan Island, corresponding to the local undulation of the surface, no more than 1 millimetre.
The manufacturing of this super-smooth industrial product involves cutting, grinding, polishing, centring, cleaning, gluing, anti-reflective layer deposition, etc., requiring extremely precise machine tools, grinding machines, and various chemical-mechanical grinding and polishing methods, and even if a single lens can be made, it must be assembled and centred precisely, as any horizontal error or tilt in one piece will affect the entire optical system.
Because the lithography lens on the production of assembly and measurement of the precision requirements are extremely high, so good at precision manufacturing Germany and Japan, occupy the first-mover advantage, compared to the domestic optical lens manufacturers started late, the accumulation of technology is still shallow, the current application is mainly concentrated in security monitoring, car cameras, and mobile phone lenses and other low-end areas, high-end optical lenses and foreign still have a large gap, now undertake China Lithography lens is the main research and development task of Changchun Institute of Optics, Beijing Guowang optical and other enterprise units, 16 years of the projection objective developed by Changguang, numerical aperture 0.75, imaging performance basically reached the level of foreign objectives of the same level, they are also in the immersion and EUV lens research and development work, hope to see an early breakthrough in the success of high-end domestic lenses, but also hope that we have some more domestic lithography Patience, the lens is only one of the links.
Asmac’s most advanced lithography machine, is the product of the industry’s best overseas countries, if you disassemble the Dutch machine, inside is the German lens, the United States of America’s light source, Japan’s materials, and China wants to make up all the shortcomings of the industry, to achieve the lithography machine or even the entire semiconductor industry, the road is long and difficult, but this is not bad news, because other countries that do not have a complete industry chain, simply will not have the idea, so you can not be arrogant, nor should you be arrogant, in the face of science and technology, the only way to be practical and realistic.
Conclusion：
The lithography lens we are talking about today refers to the projection imaging lens, there is also a set of relatively simple structure in the lithography illumination lens, its function and purpose is completely different, in addition to the lithography machine, the workpiece table to carry silicon wafers, why can float in the air to move at high speed, accurate positioning, and how the Japanese lithography machine duo rose to power, and why the Dutch manufacturers are later to take the lead, later in the lithography machine series, we will continue to Talk.
