Nondestructive Microscopy Technique Offers a Path Toward In-Silicon Quantum Computers

Illustration: Science Advances

An international team of researchers has developed a nondestructive imaging technique that can peer deep inside of silicon to locate and characterize various structures. While this should be a boon for testing and measuring conventional silicon chips currently used in information processing, it could have its greatest impact by enabling the next generation of devices for quantum information processing.

In research described in the journal Science Advances, researchers from the University of Linz in Austria, University College London, ETH Zurich, and École Polytechnique Fédérale de Lausanne in Switzerland have adapted the well-established microscopy technique known as Scanning Microwave Microscopy (SMM) to identify dopants deep inside the silicon without causing any damage to the material. (Dopants are atoms that are added to a semiconductor to change its electrical and optical properties.)

SMM is used for a wide range of applications from characterizing biological cells to new materials, such as graphene, or standard semiconductor samples. It’s done by combing an Atomic Force Microscope (AFM)—which has a nanoscopic probe that’s scanned over the sample of interest—with a Vector Network Analyzer (VNA) that sends out a microwave signal from the AFM probe. The signal is reflected within the volume of the sample and measured by the VNA, which gives information about the three-dimensional structure and electrical properties of the sample.

The researchers used this SMM technique to image the electrical properties of a patterned layer of phosphorus atoms under a silicon surface. By using this technique, the researchers were able to image 1,900 to 4,200 densely packed atoms buried four to 15 nanometers below the surface.

Of course, there other microscopy techniques like Secondary Ion Mass Spectrometry (SIMS) that can image these dopants. However, the main advantage of SMM is that it doesn’t modify or damage the sample.

“We see potentially a global impact with our technique for standard silicon chips, which are becoming so sophisticated and complex that taking snapshots of their smallest working parts is extremely difficult and time-consuming, and currently involves destroying the chip,” explained Georg Gramse, a post-doc at the University of Linz who led the research, in an e-mail interview with IEEE Spectrum.

Gramse also notes that non-destructive imaging technologies are also becoming important for governments who are interested in knowing what is inside the foreign-made electronics they are using.

While SMM’s non-destructive scanning is sure to be of assistance to those making silicon chips for classical information processing, Gramse believes it will likely have an enormous impact on the fabrication of phosphorus-in-silicon quantum computers.

Quantum computers operate quite differently from classical computers that switch transistors either on or off to represent data as ones and zeroes. Instead, quantum computers use quantum bits that, because of the laws of quantum mechanics, can be in a state of superposition where they simultaneously act as both 1 and 0.

Four years ago, an initial step was taken towards making it possible to fabricate such quantum computers by using the same silicon used in today’s computers. The trick was to implant a phosphorus atom in the silicon. This approach manages to use the nuclear spin of the phosphorus atom that is embedded in silicon as a quantum bit, or qubit.

This latest research offers an important advance towards realizing these phosphorous-in-silicon devices since the SMM can be integrated into the same scanning probe instrument used to pattern the silicon devices. This would dramatically increase the speed of producing three-dimensional patterned structures. This is due to the fact that the same scanning probe instrument used to pattern the device also allows in situ and iterative control during the entire lithography molecular beam epitaxy (MBE) process for atomic-scale doping.

Gramse adds: “Currently we are studying the physical behavior of phosphorus layers on powered devices, which is the next step on the way towards in-silicon quantum computers.”