Just as manufacturing technology plays a crucial role in various fields today, nanofabrication technology occupies a key position in the domain of nanotechnology. Nanofabrication technology encompasses several methods, including mechanical processing, chemical etching, power beam machining, and electric field engineering on aluminum surfaces using scanning tunneling microscopy (STM).
There is still no unified definition for nanofabrication technology; Generally, the processing of materials with dimensions below 100 nm is called nanofabrication, as is processing with nanometer-level surface roughness. Nanofabrication refers to the processing of parts where size accuracy, shape accuracy and surface roughness are all at the nanometer level.
The following machining technologies can achieve nanoscale processing:
Nanoscale mechanical processing technology
Nanoscale mechanical processing methods include ultra-precision cutting with single-point tools made of single-crystalline diamond and CBN, ultra-precision multi-point abrasive processing with grinding tools made of diamond and CBN abrasives, and free abrasive processing or mechanical-chemical processing of composites , such as grinding, polishing and elastic emission machining.
Currently, ultra-precision cutting with single-edge diamond tools has produced chips as thin as 3 nm in laboratories, and nanoscale grinding has been achieved using ductile grinding technology. Sub-nanoscale removal can be achieved through processes such as elastic emission machining, resulting in Angstrom-level surface roughness.
Energy Beam Processing Technology
Energy beam processing is a special machining method that uses high-density energy beams, such as laser beams, electron beams, or ion beams, to remove materials from the part. It mainly includes ion beam processing, electron beam processing and light beam processing.
Electrolytic jet machining, electrical discharge machining, electrochemical machining, molecular beam epitaxy, and physical and chemical vapor deposition also fall under power beam processing. Sputter removal, precipitation, and surface treatment with ion beam processing, as well as ion beam-assisted etching, are also research and development directions for nanoscale machining.
Compared with solid tool cutting, the position and machining rate of ion beam machining are difficult to determine. To achieve nanoscale machining precision, a sub-nanoscale sensing system and a closed-loop adjustment system for the machining position are required.
Electron beam machining removes atoms from the surface of the penetration layer in the form of thermal energy, which can be used for etching, photolithography exposure, welding, micromachining, and nanoscale drilling and milling.
In early 1999, deep ultraviolet (DUV) lithography machines for 0.18 μm processes were successively launched. The so-called next generation lithography (NGL) technologies used to replace optical lithography after 0.1 μm mainly include extreme ultraviolet, X-ray, electron beam and ion beam lithography. A brief introduction to the progress of various lithography technologies is provided below.
1. Optical Lithography
Optical lithography projects the structural diagrams of large-scale integrated circuit devices onto the mask on a photoresist-coated silicon wafer via an optical system. The minimum feature size that optical lithography can achieve is directly related to the resolution that the optical lithography system can achieve, and reducing the wavelength of the light source is the most effective way to improve resolution.
Therefore, the development of new short-wavelength light source lithography machines has always been an important research topic internationally.
Currently, the wavelength of the light source of commercial lithography machines has moved from the ultraviolet band of the mercury lamp light sources in the past to the deep ultraviolet (DUV) band such as the KrF excimer laser (wavelength 248 nm) used for 0.25μm technology and the ArF excimer laser (wavelength 193nm) used for 0.18μm technology.
Furthermore, using the interference characteristics of light and optimizing process parameters with various wavefront technologies is also an important way to improve lithography resolution. These technologies are advancements made through in-depth analysis of exposure images based on electromagnetic theory and the practice of lithography, including phase shift masks, off-axis lighting technology, and proximity effect correction.
Using these technologies, higher resolution lithographic patterns can be achieved at the current level of technology. For example, in early 1999, Canon released the FPA-1000ASI scan scaler, which uses a 193nm ArF light source.
With wavefront technology, it can achieve a lithographic linewidth of 0.13μm on a 300mm silicon wafer. Optical lithography technology includes lithography machines, masks, photoresists and a series of technologies, involving optics, mechanics, electricity, physics, chemistry, materials and other fields of research.
Currently, scientists are exploring F2 laser lithography (wavelength 157 nm) with shorter wavelength. Due to the high level of light absorption, obtaining new optical substrate and mask materials for lithography systems is the main difficulty of this wavelength technology.
2. Extreme Ultraviolet Lithography
Extreme Ultraviolet Lithography (EUVL) employs extreme ultraviolet light with a wavelength of 10-14 nm as a light source. Although initially referred to as soft X-ray lithography, it is more similar to optical lithography. The difference is that due to the strong absorption of the material, its optical system must be in reflective form.
3. X-ray lithography
X-ray lithography (XRL) features a light source wavelength of approximately 1 nm. For providing high-resolution exposure, XRL has been widely recognized since its invention in the 1970s. Countries with synchrotron radiation devices, such as those in Europe, the United States, Japan and China, have successively conducted related research.
XRL is the most mature among all next-generation lithography technologies. The main difficulty of XRL lies in obtaining a mask substrate with good mechanical and physical properties. In recent years, significant progress has been made in mask technology. Silicon carbide (SiC) is the most suitable substrate material.
Although XRL is no longer the only candidate for future technologies due to in-depth research into XRL-related issues, the development of optical lithography, and new advances in other lithography technologies, the United States has recently reduced its investment in XRL. However, XRL remains one of the indispensable candidate technologies.
4. Electron beam lithography
Electron beam lithography (EBL) uses a high-energy electron beam to expose the photoresist and obtain structural graphics. With its de Broglie wavelength around 0.004 nm, EBL is unaffected by diffraction limits, achieving near-atomic scale resolution. EBL can achieve extremely high resolution and generate graphics directly.
It is not only an indispensable mask preparation tool in large-scale integrated circuit (VLSI) production, but also the main method for processing special-purpose devices and structures. The resolution of current electron beam exposure machines has reached less than 0.1 µm. The main disadvantage of EBL is its low productivity, only 5 to 10 wafers per hour, much less than the current optical lithography level of 50 to 100 wafers per hour.
The SCALPEL technology developed by Lucent Technologies in the United States stands out. This technology reduces mask graphics like optical lithography and uses special filtering techniques to remove stray electrons generated by mask absorbers, thereby improving output efficiency and ensuring resolution.
It should be noted that regardless of the lithography technology used in the future, EBL will be an indispensable infrastructure for the research and production of integrated circuits.
5. Ion beam lithography
Ion beam lithography (IBL) uses ions formed by the ionization of liquid or solid-state atoms, accelerated and focused or collimated by an electromagnetic field, to expose the photoresist. The principle is similar to EBL, but the De Broglie wavelength is shorter (less than 0.0001 nm) and has advantages such as a small proximity effect and a large exposure field. IBL mainly includes Focused Ion Beam Lithography (FIBL) and Ion Projection Lithography (IPL).
FIBL was developed first, and recent experimental research has achieved 10 nm resolution. Due to its low efficiency, it is difficult to apply as an exposure tool in production and is currently mainly used as a mask repair tool and special device cutting tool in VLSI. To solve the shortcomings of FIBL, people have developed IPL technology with higher exposure efficiency, and considerable progress has been made.
Lithography Galvanoformung Abformung Technology
The Galvanoformung Abformung (LIGA) lithography process is a comprehensive technology composed of deep synchrotron radiation X-ray lithography, electroforming and plastic molding. The most basic and central process is deep synchrotron radiation lithography, while electroforming and plastic molding are fundamental to the practical application of LIGA products.
Compared with traditional semiconductor processes, LIGA technology has many unique advantages, such as a wide range of materials that can be used, including metals and their alloys, ceramics, polymers and glass; can produce three-dimensional microstructures with heights of several hundred micrometers to one millimeter and aspect ratios greater than 200; the lateral dimensions can be as small as 0.5μm and the machining accuracy can reach 0.1μm; can realize replication and mass production at low cost.
Various microdevices and microdevices can be produced using LIGA technology. Successful or ongoing LIGA products include microsensors, micromotors, micromechanical parts, integrated optical and micro-optical components, microwave components, vacuum electronic components, miniature medical instruments, nanotechnology components and systems, etc.
The application of LIGA products covers a wide range, such as machining technology, measuring technology, automation technology, automotive and transportation technology, power and energy technology, aviation and aerospace technology, textile technology, precision engineering and optics, microelectronics, biomedicine, environmental science and chemical engineering, etc.
Scanning Tunneling Microscope Technology
The Scanning Tunnel Microscope (STM), invented by Binning and Bobrer, not only allows people to observe the surface structure of objects with single-atom resolution, but also provides an ideal route for nanoscale machining based on atomic units. Atomic level operation, assembly and remodeling can be carried out using STM technology.
The STM brings a very sharp metal needle (probe) closer to the sample surface at about 1 nm. When voltage is applied, a tunneling current is generated. The tunneling current changes an order of magnitude every 0.1 nm. By keeping the current constant and examining the surface of the sample, the surface structure can be discerned.
The tunneling current generally passes through a single atom at the tip of the probe, so its lateral resolution is at the atomic level. Scan tunneling micromachining technology can not only remove, add and move single atoms, but can also perform STM lithography, precipitation and electron beam induced recording at the probe tip and more.
