Current Zyvex customers are using the ZyVector control system for dopant placement for atomic-scale quantum computing devices using STM Lithography. We believe the market for STM Lithography systems can be expanded to anyone wanting to do atomic-scale patterning and will expand over time.
- Non-QIP dopant placement applications
- Atomic scale 3D structures – Patterned oxide films as hard etch masks
- Nanoscale metrology standards
- Molecular stamps
- NEMS devices
- Metal deposition
- Positive/negative tone etch masks
- Nano electrodes for molecular electronics, nanoplasmonics etc.
- Molecular templates for single-molecule chemistry
Other Dopant-based Nanoelectronic Devices
In addition to quantum computing, there are other applications for precisely-placed dopants. The nanoelectronic devices described above are formed in a single 2D layer with extremely high dopant densities and show metallic conduction as low as 1.5 K. Furthermore, they are buried within the silicon wafer, far from any oxide interfaces, and show extremely low 1/f noise, a useful property for analogue electronics applications. Different types of dopants can also be placed together; for example, phosphorus and boron make atomically precise p-n junctions.
Arrays of single dopants exhibit different interactions depending on the distance between them. If close enough, their electron wavefunctions overlap, giving metallic behavior. If the distance increases so the wavefunctions do not overlap, insulating behavior is displayed.
e.g. Enrico Prati et al. Nature Nanotech. 7, 443–447 (2012) 10.1038/nnano.2012.94
A plasmon is an electromagnetic wave traveling down a conductor. As the wave travels, all electrons in the conductor move sideways, in a concerted fashion, away from the atomic ions. Where two plasmonic conductors are in close proximity, huge electric fields are generated in the nano gap between them. As the gap gets smaller, the size of these electric fields increases dramatically. For the strongest plasmonic effects, precision in the width of a nanogap is crucial. Typically, plasmonic devices are made using metal such as gold, but they can also be made by dense patterns of dopants in a semiconducting material. Unlike in a metal, the wavelength of the plasmons can be controlled by varying the density of the dopants, and the dimensions of the electrodes can be controlled with atomic precision.
e.g. J. Am. Chem. Soc. 135, 3688-3695 (2013)
Atomic Scale 3D Structures
Aside from 2D dopant placement, the patterns formed by HDL can be used for patterned growth of other materials. Using disilane or digermane as gas sources, it is possible to grow defined islands of silicon or germanium on a surface; however, this is a very slow process, as the tip must return to remove hydrogen from the disilane over and over again. A more efficient method of creating defined islands of silicon is to grow a hard material, which acts as an etch mask, and then etch down around the mask, creating a pillar.
We developed the patterned growth of titanium dioxide (TiO2) by atomic layer deposition (ALD). A layer of TiO2 ~2 nm thick can be grown and will act as an etch mask, allowing for pillars up to ~60 nm high to be grown. The pillars can then be used as a template for nanoimprint lithography, in which a structure is pressed into a layer of resist, copying the structure exactly. This method allows one master to stamp out many copies of a structure.
Nanoscale Metrology Standards
One potential application for a 3D nanostructure is as a nano metrology standard. A standard needs to be a structure which is an exact known dimension to be used to calibrate other devices, including an SEM or an AFM. In the case of structures grown from STM lithography, it is possible to make a pattern and then, with atomic resolution, return to image the area that has been patterned. In this way, the actual dimensions of the structure written can be determined as a certain number of silicon dimer rows, allowing the dimensions of the final standard structure to be determined exactly.
Another method of pattern transfer relies on the termination of the hard mask material itself. TiO2 has hydroxyl, -OH termination, which binds to long-chain molecules with the correct termination group. In a method called chemical liftoff lithography, a stamp attaches to surface molecules on a gold sample and removes them, thus exposing gold atoms. By scaling this process down, it should be possible to pattern structures as small as a single molecule. See Science 337 1517 (2012) 10.1126/science.1221774 for more information on Chemical Liftoff Lithography.