photolithography
some details
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Photolithography is the most widely used method of "drawing" microcircuits. At first glance, everything is very simple: made a mask and use reducing optics to print as many images as needed. Repeat all if necessary.
It's like that. But there are nuances. Real devices are much more complex than it shown schematically in Fig.1. But more on that complexity below. The size of microcircuit elements, their density (the degree of integration) are determined by the capabilities of lithography.
Fig.1. Scheme of exposure system
Size matters
Size matters
The wavelength of light is the main "limiter" of resolution in photolithography. Just as a wave on the surface of water "does not notice" a thin reed, so light "does not notice" an obstacle if the size of the obstacle is much smaller than the wavelength. In the case of photolithography, it is necessary not just to “notice” (to fix presence of some darkening), but to accurately reproduce the shape of the mask elements (“print” it in the resist). At the same time, the smaller the size of the “printed” elements, the better.
Well, then it is necessary to use radiation (light) with a short wavelength, - the clever one will say. Indeed, at the beginning, in the production of microcircuits with element sizes of more than 1 micrometer, radiation with a wavelength of about 430 nm (0.43 micrometers) was used. With the development of technology and increasing requirements (reduction in size), it was replaced to 365nm, then to 248nm, and 193nm radiation. The gradual transition to shorter wavelength radiation is due to the gradual development of technology, physical limitations and economic factors.
The main problems / technical difficulties that must be taken into account / solved when "choosing" the wavelength of radiation for photolithography are the following:
It is almost impossible (very difficult) to create a sufficiently powerful and stable radiation source with an arbitrary wavelength.
The transparency and refraction of the lenses of the optical system, as well as the photosensitivity of the resist depend on radiation. So during the transition to the "new" wavelength in photolithography, it is necessary to change/replace the optics, and masks, and resist (and therefore solvents, and - possibly - etching technology ... etc.) - wow! not easy!
The restructuring of the technological cycle means high costs and the need to create almost new production.
So not everything is so easy with the transition to a new wavelength. And this means that it needs to try to achieve maximum efficiency without changing the radiation.
Optics and sharpness of image
Optics and sharpness of image
When printing an image, light from a special source passes through the mask. Then the light is focused by a special optical system on the surface of the resist. Part of the light is scattered and does not reach the resist. That is, it does not take part in the formation of the image.
It's like receiving incomplete information. Incomplete means distorted. So, the image of the mask on the surface of the resist will be distorted. The transparency of the mask changes in steps (1 or 0), and the illumination in the image is a smooth curve (Fig.2.).
Fig.2. Mask transparency and intensity distribution.
The clarity/sharpness of the printed image depends on the amount of light from the mask collected by the lens and transferred to the resist. That is, it depends on the diameter of the lens. Or, to be precise, depends on the angle that the lens covers as viewed from the surface of the resist (or - on the maximum angle of incidence of light on the resist). The larger the angle, the sharper the image!
Fig.3. Incident angle and defocus
But… on the other hand, as this angle increases, the slightest shift / deviation / inaccuracy in the position of the resist surface leads to image defocusing (Fig.3). Converging rays instead of a point form a blurred spot. Thus, the requirements for the optical system are pushing very high requirements for the substrate positioning system. When moving horizontally (for step-by-step printing of one mask pattern on different parts of the substrate), not the slightest vertical shift is allowed. Otherwise, the image will "blur".
Here we do not even mention the requirements for the purity and uniformity of the lens material, for the accuracy of surface treatment. This is a separate large task that only a few manufacturers of lithographic equipment can handle.
That is: of course, to ensure a high-quality image, high-quality optics are needed; but, for photolithography, this is not enough. Dimensions and distances are very critical here, which means that oh-so-so-precise mechanics are also important here.
By the way, in the figure (at the beginning of the page) the reducing optical system is shown as a single lens (in front of the resist). In reality, everything is somewhat more complicated. The optical system of the stepper consists of dozens of lenses. Moreover, each lens treated individually. Therefore: “Wafer steppers are the most expensive pieces of equipment in the lithography tool set. Their prices have increased by an average of 17% per year since they were introduced in the late 1970s, to the point where the prices for leading-edge step-and-scan systems now exceed $50M”. Harry J. Levinson. Principles of Lithography, Third Edition. Published: 2011)»
Nikon lithographic reduction lens circuits of different years (for different wavelengths).
Immersion lithography
Immersion lithography
As predicted by classical optics, the resolution of an optical system (image clarity) can be increased by filling the space between the lens and the resist with water (a liquid with a refractive index greater than one). By the way, this approach is used in immersion microscopy. But it is one thing when the liquid is placed between the lens and the fixed slide, and another is when the plate must move step by step, "substituting" the areas for exposure. And productivity can't be lost!
... And the engineers resolved this technical task.
Optimization
Optimization
The work of engineers and scientists from many countries and companies to optimize photolithography has led to remarkable results. In less than 40 years from 1980 to 2015, the minimum size of elements printed in photolithography has decreased by more than 100 times (more than a micron in 1980, 10nm in 2015). While the wavelength decreased about twice (2.26x - from 436 to 193 nm).
Based on: Jaione Tirapu Azpiroz, Alan E. Rosenbluth. Impact of Sub-Wavelength Electromagnetic Diffraction in Optical Lithography for Semiconductor Chip Manufacturing. 2013 SBMO/IEEE MTT-S International Microwave & Optoelectronics Conference (IMOC).
Every element of the photolithographic system has been optimized to achieve this. There were developed the following:
algorithms/computer programs for selecting the optimal "shape" and polarization of the radiation source for each(!) mask (so called source-mask optimization);
algorithms/programs for calculating the correction (distortion) of mask elements to obtain the required pattern in the recite (Optical Proximity Correction, Inverce Lithography, etc.);
technologies of double exposure, double patterning ("splitting" the mask), etc. for printing "dense" patterns of the mask in several passes.
A new industry called Electronic Design Automation (EDA) was born and computational lithography (Computational Lithography) grew out of it.
In the early 2020s,the EDA market was between $11 billion and $12 billion. By 2030, the volume is projected to be up to 25 billion dollars or more.
By the way, masks for the production of modern microcircuits (with minimal element sizes) are optimized for a specific manufacturer. And the cost of such a set of masks exceeds a million dollars.
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