Engrave hundreds of "ice sculptures" at the same time at the end of the fiber with the thickness of one-eighth hair
Remember the ice sculpture exhibition you saw as a child? Palaces, animals, and jungles make people have to admire the craftsman’s superb skills.
If such an ice sculpture is on the end of an optical fiber that is only one-eighth the thickness of a hair, and hundreds of them are carved at the same time, what kind of scenery is it?
In 2020, the Qiu Min research team of West Lake University has successively published a series of research results. They are comfortable with "ice sculptures" as small as micrometers or even nanometers, from precise positioning to precise control of the intensity of engraving, to the use of "ice sculptures" as molds to make structures and processing. Devices, a set of "Ice Carving 2.0" three-dimensional micro-nano processing system with the goal of "raw materials in and finished products out" has emerged.
"In fact, we just replaced the "photoresist" in the traditional electron beam lithography technology with ice," said Qiu Min, professor of optical engineering and vice president of West Lake University. But this change has created a whole new imagination.
What is "ice carving"
How to use chocolate powder to sprinkle the words "Happy Birthday" on the surface of the cream cake? You need a mold with the words "Happy Birthday" hollowed out on the mold. The chocolate powder was sprinkled onto the cake through the mold, and the words "Happy Birthday" appeared.
Similar principles are also applied to traditional electron beam lithography (one of the core technologies of micro-nano processing).
Assuming that we want to process nano-scale metal characters on a silicon wafer, we first need to apply a material called "photoresist" evenly on the surface of the wafer; using electron beams is equivalent to an invisible "carving knife". Write the metal word on the photoresist in a vacuum environment, the properties of the photoresist at the corresponding position will change; then use a chemical reagent to wash away the modified part of the glue, and a "hollowed out" photoresist mold is ready The next step is to "fill" the metal into the hollow position to make it "long" on the surface of the wafer; finally, all the photoresist is cleaned with chemical reagents, and only the metal words are left after the waste is removed.
It can be seen that photoresist is a very critical material in the process of micro-nano processing. So some people say that if China wants to make chips, it is not enough to have a photoetching machine, and it has to break the foreign monopoly on "photoresist".
But such "photoresist" has limitations.
"Applying photoresist to the sample is the first step in traditional photolithography. This action is a bit like spreading an egg cake. If the iron plate is not flat, the cake will not be spread well. At the same time, the area where the glue is applied cannot be Too small, otherwise the glue will not be easy to spread out; the material should not be too brittle, otherwise it will be easy to crack." said Zhao Ding, assistant researcher in Qiu Min's laboratory.
So, what about turning photoresist into water ice? The vacuum environment of minus 140 degrees can make the water vapor condensate into amorphous ice.
"After we put the sample into the vacuum equipment, we first cooled the sample and then injected water vapor, and the water vapor would condense on the sample into a thin layer of ice." Zhao Ding said, the shortness of photoresist is precisely that of water. Director. The "impermanent" water vapor can wrap the surface of any shape, even the smallest sample is no problem; the lightness of the water vapor also makes it possible to process on fragile materials. Corresponding to "photoresist", they named this layer of water ice "ice gel", and named the electron beam lithography technology involved in ice gel "ice carving".
In fact, once the photoresist is replaced with ice gel, the processing flow can be greatly simplified.
"When the electron beam hits the ice layer, the hit ice will disappear on its own, because the electron beam decomposes and vaporizes the water, so that the ice template can be carved directly. It does not need to be cleaned with chemical reagents like traditional photolithography. Mold, thereby avoiding the pollution from the tape washing, and the photoresist residue that is difficult to clean, resulting in low yield." Zhao Ding explained.
In the same way, the last step of "lithography" requires the use of chemical reagents to wash the glue again, while "ice carving" only needs to melt or sublimate the ice into water vapor, as if this layer of gelatin had never existed.
"Raw materials in, finished products out" are just six words, vividly depicting their ambitious goals for "Ice Carving 2.0"-one in and one out, the raw materials are sent in, and the finished devices are brought out. Qiu Min said that in essence, "ice carving" still belongs to electron beam lithography. But as a green and "gentle" processing method, it is especially suitable for non-planar substrates or fragile flexible materials, and even biological materials.
Professor Zhou Lei, head of the Department of Physics of Fudan University and an expert on metamaterials and metasurfaces, said that this work has important practical significance for the development of more integrated and functional optoelectronic devices. "Ice engraving can organically combine the optical frontier metasurface with the widely used optical fiber, which not only finds a suitable landing platform for the former, but also gives the latter a new lease of life."
A lonely "dancer" who is painful and happy
This is a group of lonely "ice dancers". Qiu Min's team has been cultivating in the experimental field of "ice carving" for 8 years.
At first, he learned that a research team from Harvard University demonstrated the prototype of "ice engraving" processing for life sciences, which inspired him and let him see the huge potential of this technology in the field of micro-nano processing.
This is a no-man's land. After returning to China from the Royal Swedish Institute of Technology, Qiu Min and Zhao Ding, one of the first batch of recruited doctoral students, came together to challenge this subject. "Don't be a follower on the broad road, but break a new path in a unique way. I think this is the choice most scientific researchers are more willing to make." Zhao Ding said.
The principle of "ice carving" is simple and clear, but the realization of the instrument is extremely difficult. The team needs to make a lot of changes to the original electron beam lithography equipment. Zhao Ding fought for it for 5 years. "A lot of work starts from scratch, such as the injection of water vapor. It is very simple to say. In fact, after repeated experiments, how low the temperature is, how far is the distance between the injection port and the sample, how much injection volume and rate are... All have to be verified one by one."
After Zhao Ding graduated, his younger brother Hong Yu relayed and drew dozens of design drawings for the research and development of the ice carving system. Because there is no ready-made one to buy, in most cases you have to do it yourself. He has a lot of knowledge in vacuum technology and heat. Now, after completing two years of post-doctoral research abroad, Zhao Ding has returned to Qiu Min's laboratory to continue this "ice carving" long-distance race.
In fact, there are only two laboratories doing ice carving in the world, one in China and one in Denmark. Obviously, this is not a popular research direction, and the research and development cycle is very long. It is difficult to publish articles on this topic quickly and obtain high citations.
"But this is an exciting new technology," Qiu Min said. "This kind of exploration may bring great breakthroughs, or there may be nothing, but this is the meaning and fun of basic research. "And when we enlarge our perspective to the background of Made in China, in the transition from a big manufacturing country to a strong manufacturing country, the exploration and innovation of ultra-precision machining represented by micro-nano processing is the future of China's manufacturing. .
At the end of the newly published article, Qiu Min’s team looked forward to the future of "ice carving" in a very sci-fi way-there is no doubt that future research on "ice carving" will focus on those traditional "lithography" capabilities Unattainable areas. Benefiting from the unique biocompatibility of water, it is hoped that "ice-carving" photonic waveguides or electronic circuits on biological samples can be realized. This will unprecedentedly improve the ability of human intervention in biological samples, and at the same time open up new interdisciplinary and research directions.
