Enter the quantum world! First U.S. Commercialization of Atomic-Scale Precision Lithography
We often refer to chips, or integrated circuits: components (e.g., transistors) are "integrated" in a semiconductor chip through a planar manufacturing process and encapsulated in a protective housing to perform a specific function. In order to make the transistors smaller and increase the transistor density, it naturally requires a more precise etching knife - photolithography.
Just when domestic lithography is actively catching up with AMSL, there is another major news in the field of lithography. on September 19, the American company Zyvex Laboratories announced the world's highest resolution lithography system - ZyvexLitho1TM [1]. The tool uses quantum physics techniques to achieve atomic-level precision patterning and sub-nanometer resolution at 768 picometers (i.e., 0.768 nanometers). This advancement enables quantum computers to provide unbreakable encryption for truly secure communications, as well as to discover drugs faster and predict the weather more accurately.
01ZyvexLitho1™: the first commercialized atomic-level precision lithography
The ZyvexLitho1TM is based on the scanning tunneling microscope (STM) instrument that Zyvex Labs has been refining since 2007, and incorporates many automated features and functions not found in any commercial scanning tunneling microscope. Zyvex Laboratories is now taking orders for the ZvyvexLitho1TM system with a lead time of approximately 6 months ARO (after receipt of order).
Left: Michelle Simmons; Right: Joe Lyding
Professor Michelle Simmons, winner of the 2015 Feynman Prize, CEO of Silicon Quantum Computing and Director of the Centre for Quantum Computing and Communications Technologies at the University of New South Wales, said, "There are many challenges to building a scalable quantum computer. We strongly believe that high precision manufacturing is required to realize the full potential of quantum computing. We are excited about ZyvexLitho1, the first commercially available tool to provide atomic-level precision patterning."
Joe Lyding, inventor of STM lithography, 2014 Feynman Prize winner and professor at the University of Illinois, said, "To date, Zyvex Labs' technology is the most advanced and the only commercial implementation of this atomic-level precision lithography."
Zyvex Labs is a US-based nanotechnology company dedicated to producing atomic-level precision manufacturing tools. This product was made with the support of DARPA (Defense Advanced Research Projects Agency), the Army Research Office, the Department of Energy's Office of Advanced Manufacturing and Professor Reza Moheimani of the University of Texas at Dallas, who was recently awarded the Industrial Achievement Award by the International Federation of Automatic Control "for supporting the control of single-atom scale quantum silicon device manufacturing development."
02Hydrogen Depassivation Lithography (HDL): Achieving Higher Resolution and Precision
Hydrogen depassivation lithography (HDL) is a form of electron beam lithography (EBL) that achieves atomic resolution with very simple instrumentation and uses electrons of very low energy [2]. It uses quantum physics to efficiently focus low-energy electrons and vibrational heating methods to produce a highly nonlinear (multi-electron) exposure regime.HDL uses a monolayer of H atoms attached to the silicon surface as a very thin resist layer and uses electron-stimulated desorption to create patterns in the resist.
Left: 0.768 nm (2 atoms wide) line; Right: -0.384 nm exposure of an array of H-atoms (white spots). HDL exposure of a single H-atom, with other H-atoms spaced 0.384 nm apart on average can be exposed by this technique.
Traditional EBL uses large, expensive electron optical systems and very high energies (200 Kev) to achieve small spot sizes; however, the high-energy electrons (necessary to obtain small spot sizes) are dispersed in the polymer resist used in traditional EBL and disperse the deposited energy, resulting in larger structures. HDL achieves higher resolution and precision than traditional EBL.
The data show that the deposition energy in the photoresist does not drop to 10% of the center of the beam until the radial distance is about 4 nm.
Using HDL, the experimental team was able to expose individual atoms >10 times smaller than the 10% threshold radius of EBL. This much smaller exposure area is surprising because HDL does not use optics, but simply places the tungsten tip about 1 nm above the H-passivated silicon sample. One would expect the exposure region to be much larger if there were no optics to focus the electrons from the tip.
(to scale) shows the W scanning tunneling microscope (STM) tip approximately 1 nm from the H-passivated silicon surface.
It seems unlikely that the electrons follow only the solid arrow path required to expose individual H atoms. To solve this mystery, we must understand that electrons are not actually emitted from the tip (in imaging and atomic precision lithography modes), but rather from the sample to the tip (in imaging mode) or from the tip to the sample (in lithography mode) modes. Using a simple model with an infinitely flat and conducting substrate, the emission of a single W atom at the STM tip apex, and a simplified model of the tunneling current, we will see that the current decreases exponentially with tunneling distance.
A simple HDL model is shown below. The model assumes an infinitely flat conductor as the substrate and a sphere of radius Rt as the apex of the STM tip. The tunnelling current from the STM tip can be modeled by the simplified expression
where i is the tunnelling current, K is a constant, V is the tip sample bias, Td is the tunnelling gap, ɸ is the local barrier height, me is the electron mass, and ħ is Planck's constant/2π. Using the equations V = 4 V (common HDL bias), ɸ = 4 eV (~ the work function of the tungsten tip), and Td = 1 nm, one can set constant K set to 0.194, which will produce a current of 1 nA, which is typical of HDL exposure currents. a rough rule of thumb for STM tunneling current as a function of tip height is that changing the tip height by ±0.1 nm will decrease/increase the tunneling current by an order of magnitude. Ultimately, this model predicts a 7.75-fold decrease/increase, providing some credibility to the model.
Radial distribution of the tunneling current over the sample
However, this alone does not explain the atomic resolution of the HDL. H atoms are only 0.384 nm apart, and even with perfect tip positioning and a random (quantum) exposure mechanism, an error rate (removal of adjacent H atoms) is expected to be seen about 30% of the time. At a bias of 4 V, there is sufficient energy transfer through a single electron impact, and the HDL in atomic resolution mode is a highly nonlinear exposure process with exposure probabilities that vary with the five tunneling currents. When the dependence on the current is applied to the current distribution, the exposure efficiency is significantly increased, as shown in the following figure.
The normalized current distribution due to quantum tunneling effects and normalized exposure efficiency results in the release of hydrogen atoms per electron as a function of the radial distance (in nm) from the peak.
For common patterns using HDL exposure patterns, the lateral distance to unwanted exposures is about 0.47 nm, resulting in an error rate of 10-6. HDL is ideal for solid-state quantum device fabrication where energy levels and tunneling rates are extremely sensitive to distance variations. several examples of HDL lithography are shown below.
02The complete scanning tunneling lithography system: six features
Embedded in the ZyvexLitho1TM is the ZyVector™. This 20-bit digital control system features low noise and low latency, enabling users to produce atomically accurate patterns for solid-state quantum devices and other nanodevices and materials.ZyvexLitho1™ is a complete scanning tunneling lithography system with features not available in any other commercial scanning tunneling lithography system [3]: the ability to achieve distortion-free imaging, adaptive current feedback loops automatic lattice alignment, digital vector lithography, automated scripting, and built-in metrology.
1)Distortion-free imaging. The system's proprietary creep (Creep), hysteresis correction algorithm allows distortion-free imaging, and atomically accurate tip positioning, enabling unprecedented lithographic accuracy.
2)Adaptable current feedback loop. All commercial STMs use the same proportional-integral (PI) loop to raise and lower the STM's tip as it scans to maintain a setpoint current. Unfortunately, tip collapse with this simple control loop is a frequent occurrence. This can be tolerated if the imaging is simple, but is a serious problem when doing lithography. Therefore, the inclusion of a patented adaptive current control loop in the ZyvexLitho1's control system will greatly reduce tip collisions.
The figure shows a straight line scan of a silicon (Si) surface with several perturbations leading to current errors in profile E where the control loop is a standard PI loop [4]. In profile F, the adaptable control loop is opened and the tip of the needle is scanned in the same straight line. The more precise current control provides a more accurate profile and avoids tip collapse when surface perturbations are large.
3) Automatic lattice alignment. Because the lithography and imaging modes are energetically separated, the silicon surface can be imaged both before and after lithography. This non-exposed imaging mode allows automatic identification of the silicon lattice and thus the location of the pixels on the surface. This lattice-locking process automatically maintains the accuracy of tip positioning (and therefore lithography).
4) Digital vector lithography.ZyvexLitho1 uses hydrogen depassivation lithography to remove H atoms from the surface of a Si(100)2×1 reconstruction. This self-developing exposure technique is binary in nature. Either the H-Si bond is broken (sending the H atom into vacuum) or it is not; there are no partial exposures or proximity effects. Using this process and the global reference lattice as a silicon surface lattice, digital lithography can be performed. A sub-nanometer pixel is four surface silicon atoms. The design lattice is the same as the pixel lattice Computer Aided Design (CAD) files can be loaded into ZyvexLitho1 and the pattern can be automatically decomposed into different geometries so that the tip vectors can be used for different lithography patterns; exposures can then be performed automatically.
5) Automation and scripting. Almost all programs can be automated: command line interface for individual commands or scripts, scripting menus for built-in and user-written scripts. And, with multiple pattern input modes: as geometric shapes, vector lists, black/white bitmaps.
6) Built-in metrology. There is a non-destructive imaging mode so that new patterns can be aligned with old ones and the pattern quality can be checked after writing.
Not only that, but the complete ZyvexLitho1TM system includes a ScientaOmicron ultra-high vacuum STM (scanning tunneling microscope) configured for the fabrication of quantum devices. said Dr. Andreas Bettac, SPM product manager at ScientaOmicron, "Here we are bringing The latest ultra-high vacuum system designs and ScientaOmicron's proven SPM are combined with Zyvex's STM lithography-specific, high-precision STM controller. I look forward to a continued fruitful collaboration with Zyvex."
Reference links：
[1]https://www.newswire.com/news/the-worlds-1st-sub-nanometer-resolution-lithography-system-21823319
[2]https://www.zyvexlabs.com/apm/products/zyvex-litho-1/physics-of-zyvex-litho-1/
[3]https://www.zyvexlabs.com/apm/products/zyvex-litho-1/features-of-zyvex-litho-1/
[4]https://doi.org/10.1109/TCST.2018.2844781
