Quantum at Yale

Instruments and Equipment

Equipment used in quantum research at Yale is housed at multiple Yale core research facilities including the Becton Center Cleanroom, West Campus Cleanroom, and Yale Institute for Nanoscience and Quantum Engineering (YINQE). In this section, you will explore the creation of a typical metal layer of a micro/nano device. This multiple-step process is accomplished using highly specialized systems for metal vapor coating, the spin-coating of protective layers, UV exposure and patterning of the protective layer, and plasma etching of the unprotected metal. These techniques are applied regularly in quantum and non-quantum cleanroom research worldwide. Quantum research frequently depends on niobium due to its superconducting properties, but the full list of research materials used in “small science” spans large portions of the periodic table, with new applications being discovered all the time. Some common example materials are Si, GaAs, SiO₂, Al, Cr, Al₂O₃, AlN, SiN.

A complete list of cleanroom equipment, which is shared among multiple departments and reserved and billed using an online system, is available.

On this page you can learn about:

• E-Beam Evaporation
• Spin-Coating
• Direct-Write Lithography
• Plasma Etching
• Yale Institute for Nanoscience and Quantum Engineering (YINQE)

E-Beam Evaporation

An electron beam evaporator, capable of evaporating solid materials to coat research devices.

Photo credit: Sean Rinehart

When something is measured in daily life, it is typically in inches or centimeters. Quantum devices are designed in terms of microns or a hair’s breadth. Because these devices are so small, they can’t be created using the traditional machining techniques found in factories. Instead, specialized equipment is used to fabricate these devices ultra-thin layer by ultra-thin layer.

Adding metal to a product is commonly done via welding and extrusion however applying these techniques on such a small scale would be impractical to impossible. Most metallization of micro devices is done using vapor deposition, in which the metal in question is evaporated into gas and then redeposited as a solid film. This can be achieved with heat, chemical reactions, or physical disruption. The system pictured here uses a technique known as Electron Beam or E-Beam Evaporation. In E-Beam evaporation, electrons are emitted from a tungsten filament (similar to a light bulb) then concentrated, guided, and accelerated by magnets until reaching a crucible of the desired metal. The energy from the electrons heats the metal in the crucible until it melts and evaporates onto target devices. This entire process happens at vacuum levels of under 1 Torr. This is done because the molecules making up the air we breathe would intercept both the electrons and the metal vapor. In nature, these vacuums occur naturally above the earth’s magnetosphere, or on the surface of Pluto.

Process flow diagram showing metal on a research sample.

Image credit: Sean Rinehart

A 2 inch silicon wafer with silicon oxide glass coating.

Photo credit: Sean Rinehart

The same wafer after being coated with a 40 nm thick layer of gold.

Photo credit: Sean Rinehart

Spin-Coating

A lithography spinner, used to evenly coat research devices with easy to remove masking layers. Photo credit: Sean Rinehart

For large scale devices, metal layers are commonly removed in bulk through physical milling or laser etching. These processes are either too destructive or not precise enough for quantum needs, where devices can be microns down to nanometers in size, and individual features can be hundreds of times smaller.

Process flow diagram showing photoresist spun onto a research sample.

Image credit: Sean Rinehart

A lithography spinner with a wafer loading onto a vacuum chuck.

Photo credit: Sean Rinehart

For these devices, selective chemical etches are used to remove a single material on a device. This provides a new challenge: how do you remove part of a layer, leaving the functional elements in place?

The answer involves a multi-step technique known as photolithography. Like darkroom photography, photolithography takes a light-sensitive film which is exposed then developed. The first step is coating the sample with photoresist, a light-sensitive fluid that resists most chemical etching. This spin-coater has a chuck that holds a wafer in place using light vacuum, while spinning at several thousand RPM, removing any excess photoresist and leaving a microns-thick film left on the surface of the sample.

Direct-Write Lithography

Following spin coating of a sample, the next step is exposure. In a film camera, exposure is determined by shutter speed, which determines how much light is allowed to reach the film Additionally, traditional film is sensitive to multiple colors of light (red, blue, and yellow). Photoresist is primarily sensitive to ultraviolet light and exposure to ultraviolet causes photoresist to become soluble (or insoluble, depending on the type of photoresist. There are dozens.)

To copy a pattern multiple times, it is practical to expose an entire sample all at once with a mercury UV lamp, using an opaque mask to transfer the pattern. With single devices, it can be beneficial to transfer the pattern directly. In direct-write lithography, a focused UV laser or electron beam is, instead, used to expose the film line by line, tracing around (or over) the connections and functional elements that are to be kept. Once finished, the sample can be rinsed in developer, washing away the unexposed resist and leaving protected device features and unprotected areas reading for chemical etching.

Process flow diagram showing photoresist being exposed by UV laser.

Image credit: Sean Rinehart

A laser writer used to create lithography masks or directly expose photoresist.

Photo credit: Sean Rinehart

A mask aligner, for rapid processing of entire wafers.

Photo credit: Sean Rinehart

Process flow diagram showing photoresist post-development, exposing the metal layer for future etching.

Image credit: Sean Rinehart

Plasma Etching

A plasma etching system capable of using a wide variety of ionized gases to etch an even wider variety of materials.

Photo credit: Sean Rinehart

When the functional area of the device is protected, the next step is to remove excess material. Since traditional milling is too destructive, plasma etching is used. Plasma etching uses high power RF energy to ionize gases, breaking down stable molecules into electrons and reactive species like elemental fluorine and chlorine. These reactive species then bond with the unwanted material on the device, forming gases which vaporize off the device. As with metal evaporation, very low pressure is required for this process to work. With careful selection of gas type, pressure, and power, it is possible to perform etches for hundreds of materials without damaging features or underlying materials, thus resulting in a finished layer step. Most complete devices require multiple iterations of this process, from deposition to etching, with different functional layers being implemented each time.

Take a look at the plasma video for deeper insight into this process. Deep reactive ion etching (DRIE) is used to etch silicon trenches 100s of microns deep at near-90 degree angles. It achieves this through alternating steps of SF₆ plasma (purple) to etch the silicon, and C₄F₈ plasma (white) to coat the etch sidewall with a protective teflon-like polymer.

Deep silicon etch requires alternating steps of two different gas types.

Credit: Sean Rinehart

Process flow diagram of a sample after chemical etching of the metal layer.

Image credit: Sean Rinehart

Process flow diagram showing photoresist removed.

Image credit: Sean Rinehart

Yale Institute for Nanoscience and Quantum Engineering (YINQE)

“Image our surprise, then, when it was discovered roughly 20 years ago that there were revolutionary processing methods which relied on precisely the parts of quantum physics which Einstein hated, and which we were told, growing up, were only a concern to philosophers.”

- A. Douglas Stone, Carl A. Morse Professor of Applied Physics and of Physics (Shelton, 2015a)

Yale Institute for Nanoscience and Quantum Engineering (YINQE)

The Yale Institute for Nanoscience and Quantum Engineering is a shared core facility for electron microscopy, atomic force microscopy, and electron-beam lithography. Graduate students, undergraduates, post-docs, and faculty have hands-on access to all instruments.

A concise summary and photos of additional devices can be found on the YINQE site.

A virtual tour of equipment found in the shared laboratory of Michel Devoret and Robert Schoelkopf (Becton) and the laboratory of Jack Harris (Sloane Physics Laboratory) at Yale is also available.

Acknowledgments | Sources