3ders.org - Chinese researchers taking ice lithography to nano 3D printing | 3D Printer News & 3D Printing News

Jul 27, 2018 | By Thomas

In a vacuum environment, near minus 130 degrees, water vapor may condense into a layer of super smooth, thin ice. Scientists used this special "ice" to replace the photoresist material in traditional electron beam exposure, creating several three-dimensional metal structures on a micro-nano scale: pyramids, mushrooms and bridges. This novel and simple electron beam lithography, utilizing this ice resists (iEBL or Ice EBL) technique, is expected to show potential for three-dimensional micro-nano processing.

On June 25, 2018, researchers at the State Key Laboratory of Modern Optical Instruments of Zhejiang University, led by Professor Qiu Wei, published a paper titled “Three-dimensional in situ electron beam lithography using water ice in the Nano Letters journal, introducing its 3D nanofabrication method, based on electron-beam lithography, using ice resists (iEBL). The paper was co-authored by Yu Hong, a doctoral student at Zhejiang University; Zhao Ding, a postdoctoral fellow; Professor Zhao Ding and Qiu Yu, professors of optical engineering at West Lake University.

iEBL is a development on the standard e-beam lithography (EBL) technique, which is used to design devices, systems and functional materials at the nano scale.

EBL is the practice of scanning a focused beam of electrons to draw custom shapes on a surface covered with an electron-sensitive film called a resist (exposing). Exposure to the electron beam changes the solubility of the resist, enabling selective removal of either the exposed or non-exposed regions of the resist by immersing it in a developer. The primary advantage of electron-beam lithography is that it can draw custom patterns (direct-write) with sub-10 nm resolution.

However, it is difficult in real situations. The slight vibration of the instrument, the interference of external magnetic fields and the experience of the operator will affect the final result of this form of direct writing.

At present, the accuracy of electron beam exposure is about 60-80 nm, which is equivalent to one thousandth of a human hair.

With the increasing demand for miniaturization and refinement of micro-nano devices, scientists are increasingly aware of the limitations of the lithography process. Zhao Ding, the first author of the paper and postdoctoral researcher of Zhejiang University, says, "To make a three-dimensional structure requires very tedious and lengthy steps. Repeating in and out of the vacuum and non-vacuum environment, any dust in the process could destroy the sample."

"Another limitation is that the photoresist is difficult to clean, and residue is almost inevitable, which will affect the accuracy of the product; if it is replaced by ultrasonic cleaning, then there is also the risk of damaging the structure."

“If ice is used in place of the photoresist material, the result will be very different.”

Annotated SEM images of ice patterns and corresponding metal structures.

A few years ago, a Harvard research team proposed the idea of “Ice-assisted electron beam lithography” and the Qiu Wei team hoped to advance the technology in the 3D micro-nano device processing field. “When ice is exposed to the electron beam, it is 'self-disappearing,' leaving a three-dimensional structure template.” This greatly shortens the processing steps. Therefore, they proposed a 3D nanofabrication method, based on electron-beam lithography, using ice resists (iEBL) and fabricated 3D nanostructures by stacking layered structures and those with dose-modulated exposure, respectively.

After 6 years of research and development, Professor Qiu Wei's team built a new scanning electron microscope (SEM) that integrates a liquid-nitrogen dewar, a water vapor injector, an airlock chamber and a metal deposition chamber.

The "iEBL" process requires only five steps: cooling, ice deposition, exposure, material evaporation and peeling. Using their modified Zeiss Sigma field emission scanning microscope, the Qiuwei team succeeded in creating nano-three-dimensional shapes, such as pyramids, mushrooms and bridges. While greatly simplifying the steps, the quality of the work is also impressive: the resolution is up to 20 nm and the positioning accuracy is below 100 nm.

Figure: Basic steps for iEBL: (a) Cooling: The sample is cooled to -130 °C; (b) Ice layer deposition: water vapor is injected into SEM and deposited onto the sample to form a layer of ice resist; (c) Exposure: exposure to electron beam; part of the ice layer is removed directly; (d) Material evaporation: The sample is transferred to the coating chamber in a vacuum environment; (e) Lift-off: The sample is taken out and placed in isopropanol to melt the ice layer.

A standard EBL process requires additional exposing and developing steps for alignment and needs 12 individual steps for 3D nanofabrication. The entire process of iEBL is realized in one vacuum system by skipping the spin-coating and developing steps required for commonly used resists. This needs far fewer processing steps and is contamination-free compared to conventional methods.

The solid state of water, such as snowflakes, frost and ice, which we see in our daily life, is the crystalline state of water. What is needed for "iEBL" is an amorphous ice. "Under the scanning electron microscope, the surface of amorphous ice is very smooth," Hong Yu introduced, “and the microstructure of the crystalline ice surface is uneven, with many angular edges, which is not conducive to fine operation.”

The crystalline state of water

More experiments have found that the vicinity of 130 degrees in the vacuum is just the condition that water vapor condenses into amorphous ice. "We later discovered that this happens to be the climatic environment of the comet, and the ice on the comet is also in this amorphous state," Qiu said.

The smoothness of ice allows the electron beam to engrave more fine three-dimensional structures on the ice. "We also tried to put a nano silver medal on a nano silver wire with a thickness of one thousandth of a human hair," Hong Yu said.

Qiu Wei believes that the iEBL process could be used to create new, complicated optoelectronic devices, which are based on quantum dots, nanotubes, nanowires, graphene, fiber, etc materials. “These unique advantages make it a strong competitor in 3D micro-nano processing technology.”



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