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What are the origins of photolithography and where is the future heading? What kind of evolution has lithography gone through, how to make smaller chips, the first generation of lithography, than Super Mario Contra has what kind of connection? When did the domestic lithography machine and in which links, pulled the gap, welcome into the history of lithography evolution.
The last video we explained the basic principle and process of photolithography, in a word, is the photomask on the integrated circuit layout copy to the silicon wafer photoresist, so as to carry out subsequent etching, deposition or ion implantation, photolithography is the most core step in the chip process, and photolithography is the most core equipment, so the next, I will use two or three issues of space to explain the past life of photolithography, the rise and fall of photolithography manufacturers. The rise and fall of lithography manufacturers, and the evolution of lithography technology.
Masked lithography：
First, let’s turn the clock back to 1960. That year saw the birth of the clean room, and soon after, the first integrated chip was produced and mankind moved into the era of large-scale integrated circuits. In 1965, the prophet Moore proposed Moore’s Law, which became the roadmap for semiconductor technology for the next 50 years, and unlike today, when computers and EDA software must be used for chip design and manufacturing, in the far-flung days of semiconductors 60 years ago, these hardware and software did not yet exist In the ancient days of semiconductors, neither hardware nor software existed.
So the 60s chip production, to rely on manual hand rubbing, the first generation of chip engineers in the square paper, with colored pencils to draw a good integrated circuit layout, and then a fine blade, in the photomask master (Rubylith Mask), freehand transistor and circuit connections, a little carved out, and finally the template graphics, using a camera to reduce 50-100 times, in order to get a photomask used for lithography, and this manual mask matched the original lithography machine, is the contact lithography machine.
As the name suggests, contact lithography will only simply and brutally cover the photomask on the silicon wafer, the mask and photoresist coating direct contact, and then hit the light irradiation to complete the exposure, however, the failure rate and cost of this light method are very high, because the gel itself and its adhering floating dust particles, not only affect the lithography effect, but also cause pollution and broken ring to the photomask, and the damage effect will accumulate with the number of lithography, which not only makes This not only makes the yield of each lithography low, often engraved 10 chips, only 1 can be used, but also seriously deplete the life of the photomask, resulting in a mask, at most only a dozen times, to solve this problem, lift the photomask a little, do not let it contact with the photoresist on the line？
The early engineers thought so too. So they added to the contact lithography a platform that could be moved horizontally and vertically, and a microscope for measuring the photomask and silicon spacing with overlay, so that the two were as close as possible when lithographing, but not in direct contact – the progressive lithography. It avoids the photoresist from staining the photomask, but introduces a new problem: the accuracy of the lithograph is reduced due to the diffraction effect of light.
Macroscopically we think that light travels in a straight line, but microscopically it doesn’t. Light is volatile and when it passes through small holes and narrow slits or encounters subtle obstacles, it will produce diffraction, or bypass, which deviates from the original straight line and shines where it shouldn’t. The wavelength of the light source, compared to the larger the slit, the more serious the diffraction phenomenon, which is like holding a knife and trying to split a 100nm wide slit in a silicon wafer, only to find that the blade is 400nm wide and can only be used to split a melon.
In progressive lithography, lithographic accuracy, in addition to being limited by wavelength, depends on the distance from the photomask to the silicon wafer; the greater the pitch, the greater the error between the projection on the silicon wafer and the pattern on the mask. Hold the photomask up and the lithography accuracy is not enough; put the photomask down and the lithography cost is too high, this period hovering between contact and progressive lithography is the masked lithography era in the history of semiconductors. These two ancient lithography machines, collectively known as Mask Aligner, used a 1:1 photomask, which was a mask, and the lithography machine only used to shine light on the silicon wafer, the construction is simple and does not require any complex optical system，
During this period, three of the first lithographers, K&S (Kulicke and Soffa), Cobit and Suss, created the conditions for the first chips to be created, and in 1969, just a year after its founding, Intel produced its first product, the 3101 SRAM memory chip, using contact lithography. The reason was that the production yield of contact lithography was too low and the loss of photomasks was too great, resulting in expensive chips that could only be used in scientific research and the military.
Projection lithography：
But in fact, even the US military, which has always been a landlord, was too expensive for chips, especially the Air Force, because the development of missile systems required a large number of chips, so the US Air Force contacted a veteran optical equipment manufacturer, Perkin Elmer, in 1967, hoping to make a high-precision photolithography machine without pressing the photomask onto the photoresist. After several years of refinement and development, Perkin Elmer launched the groundbreaking Micralign 100 in 1974, which was the first of the mainstream lithography machines.
The Micralign uses two coaxial spherical mirrors to project the pattern on the photomask onto the silicon wafer after three reflections. This symmetrical light path design eliminates most of the aberrations produced by the spherical mirrors, allowing the lithographic pattern to reach the ideal resolution，
The birth of Micralign allowed the yield of chip production to soar from about 10% for contact lithography to 70% overnight, a leap in lithography technology that led to a plunge in chip prices. 1974’s Motorola 6800 microprocessor, priced at $295 per unit, and the following year eight engineers ran away from Motorola to MOS Technology, who used the newly born Not only is the performance stronger, but it is also sold for $25, which is the technology generation difference, bringing down the blow, here mentioned Motorola and MOS Technology, are the chip manufacturers in the ancient times, later one indulged in doing mobile phones, one indulged in game consoles, MOS6502 this chip, may be many young partners, have not heard of, but If we talk about the famous Nintendo Red and White Machines and the Tears of the Times, the Venus, all based on MOS6502 products.
In short, the projection lithography was responsible for bringing chips from an unaffordable luxury to the common man, and Perkin Elmer, too, reaped great commercial success, their sales slogan was very simple, buy my lithography machine and save 100,000 photomasks, which was extremely tempting for semiconductor companies who were buying masks by the truckload at the time. The first Micralign 100 was bought by Texas Instruments for a whopping $98,000, three times the price of the most advanced mask lithography machine of the year, and Intel quickly followed suit afterwards, and although it was expensive, they found they had found a treasure after buying it, because the photomasks saved were small money, and most importantly, the production yield of the chips had doubled several times, making the chip makers smile from ear to ear，
In the second year after the launch of Micralign, orders for lithography machines were in short supply and as sought after as ASML, with a large number of customers lined up until a year later. However, this did not last long, as chip processes shrank rapidly and transistor densities multiplied every 18 months in accordance with Moore’s Law, step lithography, based on lens sets, began to make its mark. period, moving into the period of scaled projection，
The weaknesses of Micralign’s purely reflective projection method are becoming increasingly apparent, such as the reflector, which is difficult to use to completely eliminate spherical aberrations, and the high blocking of the reflected light path, which also results in low numerical aperture and image resolution, all of which prevent Perkin Elmer’s lithography from keeping up with the more sophisticated chip processes.
Optical aberrations and corrections：
What is aberration? Resolution, numerical aperture? These are important parameters to measure an imaging system. Here we have to insert some basic optical knowledge. Remember the convex lens imaging in junior high school physics class? The object placed in front of the concave lens, as long as the distance is appropriate, will be projected on the screen behind the mirror, a perfect inverted image, but perfect is only the ideal situation, in fact, the various parts of the object and the lens distance is not equal, the far part of the imaging near, the near part of the imaging far，
So the most perfect imaging surface is not a flat surface, but a curved surface, which is a type of basic aberration, image field bending, or field curvature for short. Many early affordable cameras, in order to make the image quality better, would be designed to take pictures with the negative bent to complete the exposure to cater for the curved image field, but traditional silicon wafers are brittle and cannot be bent, so the lithographer has to suppress the field curvature through a complex combination of lenses.
Also, I wonder if you ever burned ants with a magnifying glass when you were in primary school? The distance between the magnifying glass and the ant is called the focal length. The ability of a convex lens is to refract light and converge on a focal point, but this is actually only the ideal situation, in fact, the degree of refraction of parallel light at the edge of a spherical lens and the central part is different, the refraction at the edge of a convex lens is large, the corresponding focal length will be shortened, the imaging distance forward, while a concave lens is just the opposite, this kind of imaging blur at the edge of a spherical lens due to focal length deviation, is This is called spherical aberration, or spherical aberration for short, so in lithography, a combination of convex and concave lenses, or aspheric mirrors are used to eliminate spherical aberration，
A similar phenomenon is chromatic aberration. The colours we see depend on the wavelength of visible light, and different wavelengths of light, which are refracted differently in the lens, do not converge at the same focal point, which can also cause imaging problems. Camera lenses therefore also use a combination of concave and convex lenses to counteract chromatic aberration.
Of course, we do not use compound light for lithography, but usually a single wavelength of ultraviolet light, so the problem of chromatic aberration can be ignored. But the aberration problem includes coma, aberration and distortion, in addition to the field curvature and spherical aberration mentioned earlier, in short, the smaller the chip process, the lower the tolerance of lithography to all kinds of aberrations, the more complex optical systems are needed to do correction, so the result is that the number of lenses in lithography is becoming more and more huge, and the combination is becoming more and more complex. The difficulty of coordinating, aligning, measuring, temperature controlling and compensating for errors in the exposure of such large lenses can be imagined.
Here, by the way, the initial start of China’s lithography is not late, in the era of masked lithography half a practical ago, Shanghai Optical Machinery Factory has made a labor brand contact lithography, and into the period of projection lithography, our lithography and R & D lagged behind, of course, the reasons for this are very complex, but an important factor in technology is that we have a gap with the West in the research and manufacture of complex optical systems, this gap, about 28 years ago (1902), when the German Zeiss Paul Rudolph, made 4 pieces of 3 groups of type Tessar lens (Tessar), has begun to form.
To build a complex device like a photolithography machine, mankind needs to light up a lot of antecedent skills, including optics, mechanics, electronics, software, automation, control technology and so on, as long as there is a shortcoming in one link, the photolithography machine will be difficult to produce. As a Dutch company, for example, if there were no German lenses and American light sources, it would not have been able to build a high-end lithography machine, but of course the rise of Asmac (ASML) was a later story, and in the early 80s the lithography stage was about to be staged or the showdown between the American and Japanese manufacturers.
Conclusion:
So far we have reviewed the earliest days of mask lithography and the 1:1 projection lithography that began in the mid-1970s, a period dominated by American manufacturers, especially the Micralign series of lithography, which dominated the market for a few years before being replaced by more advanced stepper lithography.
So how have the lithography machines evolved since the 1980s, what technology lines have emerged, how did the Japanese duo beat the US manufacturers to lead the global lithography market, how did ASML of the Netherlands emerge as the leader, and what is the resolution and numerical aperture? What impact do they have on lithography and the chip process?
Epilogue：
Finally, I would like to say a few words about the end of the Micralign, the first machine for photolithography: not many people in China know about Perkin Elmer, in fact it was their lenses that were used in the famous Hubble telescope, and then Apollo 11 took their instruments to the moon, but apart from optical accessories, the company’s greatest contribution was the Micralign projection It was only after this that the low-cost mass production of chips became possible, and the previously unattainable chips could be made into consumer electronics and flown into the homes of ordinary people. It was only after this that the low-cost mass production of chips became possible.
Unfortunately, however, after taking over the lithography market with Microlign, Perkin Elmer became complacent and stuck to its guns, neither investing heavily in R&D nor listening extensively to the needs of its customers at a time when chip processes were shrinking wildly and the requirements for lithography precision were changing by the day, and the new step mirror lithography, with its smaller resolution and By the time Perkin Elmer woke up, it was already too late to catch up with the advantages of stepper lithography, and not willing to admit defeat, they placed their last bet on extreme ultraviolet light sources. It took nearly 30 years before the EUV lithography was in mass production at ASML, and Perkin Elmer had to accept defeat and finally sold its most prestigious lithography division to SVG (Silicon Valley Group) in 1990. Group),
The latter, in turn, was acquired by ASML in 2001. In other words, 40 years ago, the boss of lithography, through the changes of the semiconductor era, and finally entrusted to the boss of lithography today, as if it is a closed loop, and Perkin Elmer’s projection lithography, started with the reflector, and finally EUV, round and round for 40 years, to the most cutting-edge EUV lithography today and back to the structure of the reflector, which also seems to be a closed loop，
The vast majority of mankind’s scientific research efforts are spent in circles, groping back and forth, and only when a certain level of accumulation is reached will a breakthrough be found, allowing technology to achieve a period of spiral upward. On the road of chasing science and truth, no one knows where the exit is and how many more detours to take, all we can do is remain humble and keep moving forward.
