Making Cryogen Free Measurements: Challenges and Solutions

An interview with Dr Michael Cuthbert, conducted by Alina Shrourou, BSc.

Please outline the term “cryofree”.

Cryofree™ is the Oxford Instruments trademark term used to describe all of our products that function without the need for liquid cryogens. These are more generally referred to as “cryogen free”.

Historically, liquid cryogens were used to achieve low temperatures for research and industrial purposes, with a variety of liquefied gases and a range of applications. Today, the most commonly used cryogens are liquid nitrogen – with a boiling point temperature of 77K (-196 C) – and liquid helium – with a boiling point temperature of 4.2K (-269 C).

The issue with liquid cryogens is that they are costly, potentially hazardous and are in short supply in many regions of the world. Thus, being able to offer cryogen free systems where the cooling stages that would have traditionally relied on liquid cryogens have been replaced by a closed cycle refrigerator that is powered by electricity - much like a domestic fridge – has enabled customers from all over the world identify new applications that were previously not possible.

What challenges do researchers face when making cryogenic observations?

Making sensitive measurements at cryogenic temperatures can be challenging for a variety of reasons:

Typically, secondary calibrated sensors for measuring temperature are used either as an indicator or as part of a control and feedback loop. Examples of this include Si diodes, thermocouples and RuO2 semiconductor chips.

Primary thermometers can sometimes be used when accurate measurements are needed at ultra-low temperatures. Here, the calibration of the sensor is inherent to the physical property being measured, such as radioactive decay or electrical noise.

Controlling the temperature of the measurement is a balancing act between the applied cooling power available from the cryostat, the internal cooling process, and the external passive heat sources from the cryostat, as well as the radiation and external active heat sources from the measurement itself.

A well-designed experiment will take into consideration the self-heating impact of applied electrical, thermal, magnetic and optical power. This is typically done by making temperature measurements and applying heater power as part of a controlled feedback loop. As such, Oxford Instruments have range of temperature sensors and controllers to suit different temperature regimes and magnetic field environments. [see figure 1 below]

Nano-devices are very sensitive to damage through electrostatic discharge (ESD). As feature sizes become smaller, this becomes an ever greater challenge. Sample handling is therefore critical for making good low temperature measurements, both at room temperature, when bonding into the chip carrier, when inserting into the cryostat, thermally cycling and when connecting external measurement equipment.

To help protect sensitive devices, we offer our SampleProtect™ signal biasing & ground switching system. This system works both at room temperature – when handling and storing samples – as well as at low temperatures; before, during and after measurement. We also have a variety of sample loading and unloading options to suit our customers’ applications. [see figure 2 below]

Once in the cryostat, the device is isolated from the outside world by the very nature of solving some of the technical issues described above. This makes manipulating the position or angle of the device relative to the measurement, applied magnetic field or incident radiation a key part of the experimental set-up. At Oxford Instruments, we offer our customers a range of mechanisms for positioning and then re-positioning the sample, between measurements.

As research has migrated from bulk materials with crystallographic orientation specific attributes to thin films and nanowires with highly anisotropic properties, being able to align the sample is increasingly important. Sample alignment is done either by mechanical rotation or by incorporating piezo stages that can be driven electronically. Piezo stages can be used to focus local optics close to the sample or rotate the device in multiple orientations relative to the applied magnetic field.

Finally, where the measurement is complex (such as in scanning probe microscopy) or the signal chain to the device is fixed (such as with GHz frequency coaxial cables), we can couple 2 or more superconducting magnet coils together to rotate or tilt the applied magnetic field away from the normal vertical orientation and into the plane, in 2 or 3 dimensions. These are called vector rotate magnets and are increasingly popular coil structures for qubit and spin physics.

Making the measurement requires the probing signal being delivered to the device and getting the measured response back. There are a host of different techniques for this, but the most common methods we offer are:

• Optical access in free space; through light pipes or windows (typically coated quartz or sapphire)
• Optical access via optical fibre
• Beamline access for neutrons, muons and X-Rays via split coil arrangements
• Metalized windows
• Low frequency electrical access; via constantan or copper twisted pair wiring looms
• MHz radio frequency signals using stainless steel flexible coaxial cables
• GHz microwave signals using stainless steel, copper nickel or niobium titanium semi-rigid coaxial cables.

These services need to balance the ‘connectivity’ of the outside world with the heat load they introduce into the cryogenic system, whilst reducing any sources of noise and interference.

Protect your nano-structure devices from ESD damage

Ensuring good grounding and filtering is a must, and for most applications, this requires low temperature amplification of the measured signal, in order to boost faint measurement signals amid a background of heavily attenuated noise.

Finding a signal in the noise is the perennial task of the modern researcher and eliminating noise sources can be as much of an experiment as the measurement itself. An example (from a very long list of techniques) would be; a neutron-absorbent material plated around the beam to minimise spurious reflections through the cryostat structure.

The other big issue for users looking to make sensitive measurements at low temperatures is vibration and the influence of external and internal mechanical noise sources on their results. For some applications this is a critical consideration, particularly in cryogen free applications.

However, this is less of an issue for systems with good mechanical design, smart material choice and good isolation. Part of the challenge for our designers is generating a clear definition of what constitutes ‘low vibration’ as a specification and over what frequency range  the researchers are interested in. Low frequency solutions are often quite different from high frequency implementations.

How are cryogenic temperatures reached in the 4K FAMILY offered by Oxford Instruments NanoScience?

Our 4K family of cryostats achieve cryogenic temperatures using either liquid helium or closed cycle coolers (for cryogen free systems). [see figure 3 below]

Our liquid helium systems for optical spectroscopy and microscopy run continuously, with helium liquid or gas flowing from a storage vessel to the cryostat through a low loss transfer line, before heat exchange with the sample stage and finally, exhaust.

On the other hand, our cryogen free cryostats operate by circulating compressed helium gas through an external circuit to the cold head, where a series of valves and expansion chambers generate the low temperatures. From the cold head to the sample stage, we rely on conduction cooling by choosing materials such as oxygen free high purity copper for low temperature applications.

What is the difference between a bottom- or top-loading cryostat mechanism?

As described earlier, sample exchange is an important aspect of sample handling and throughput of measurements. We offer customers the ability to load devices into their cryostat by either using a sample probe loaded into the cryostat – typically into exchange gas (known as top loading) – or by using a sample puck or chip carrier onto a cold finger in vacuum (known as bottom loading). The customers’ experiment or laboratory layout usually dictates which method of sample exchange is best suited for their application.

How do researchers know which Cryofree solution offered by Oxford Instruments is most suited to their specific applications?

With many products for a wide range of scientific applications, this can be tricky. That’s why we have recently been putting a lot of effort into technical training and developing product literature that is more application-focused, so that our staff can quickly identify the right product to suit our customers’ needs.

We also recently launched our new website that has application pages sitting alongside our product pages. In addition, we are creating a network of application nodes looking at measurement challenges and describing solutions for customers, based on their application. We want to provide researchers with information about the connectivity of the various techniques, whilst also describing their differences.

This project is still in it’s early stages, but we have branded it “Pathways” and are launching a new node each month, as we make connections between the different considerations that our prospective customers need to be mindful of.

What solutions can Oxford Instruments NanoScience provide for experiments requiring a low temperature in a magnetic environment?

We can adapt our 4K cryostat family to suit the pole pieces of an electromagnet or more typically, integrate our cryogenic capabilities with our superconducting magnet expertise.

We offer these cryo-magnet systems using both liquid helium and cryogen free components, and combine them with the Integra wet systems and Teslatron dry systems, which are our two laboratory workhorse platforms. These flexible, lab-scale magnet systems offer a range of superconducting magnets and a series of probes and inserts for measurements at room temperature all the way down to mK temperatures. [see figure 4 below]

How do you think cryogenics will advance quantum technologies?

Where does one start? This is a huge topic and one that is really exciting right now. We are seeing significant levels of governmental and private investment going into Quantum Technologies all around the world.

Of the high-level themes of quantum research and technology development – clocks, imaging, sensing, cryptography, computing and simulation – I believe that Quantum Computing & Simulation are the areas that most need cryogenic temperatures right now. [see figure 5 below]

There are many candidate qubit structures with superconducting flux qubits, spin qubits and topological qubits, all needing cryogenic temperatures to function. NV centres in diamond and optical ion traps are the main two candidates currently operating at room temperature, albeit within a vacuum or other special conditions.

In the longer term, I foresee signal conditioning and control systems migrating from room temperature to cryogenic temperatures, in order to improve latency and noise from the control circuits. This might extend to the control of other qubit types that do not current require cryogenic systems.

Looking further out, quantum sensing is a growing area of interest with great potential in industry and medicine. There may be requirements for future systems to incorporate cryogenics as new techniques and technologies are developed. Overall, it is an exciting field and we are watching with great interest the amazing developments our customers are making right now on a daily basis.

What do you hope to provide to the materials research community by being at MRS 2018?

MRS is first and foremost an opportunity for us to renew relationships with customers and partners. We also enjoy learning about new developments and updating potential customers on the latest products and services that we can offer to support their research needs.

Where can readers find more information?

Our website has lots of product and application details for gathering more information and specification datasheets and our Pathways application nodes have additional detail, customer stories, application papers and videos. However, we love to talk science and engineering so always appreciate the opportunity to meet researchers in person at meetings such as MRS. Our US headquarters at in Concord MA, so we are conveniently located to the Boston area.

• https://nanoscience.oxinst.com/

About Michael Cuthbert

Following undergraduate studies in Chemical Physics at University of Glasgow and PhD in High Temperature Superconductivity at Imperial College London, Dr Michael Cuthbert pursued research in superconductivity using pulsed magnetic fields up to 60T and ultra low temperatures in the mK regime in the Condensed Matter group at University of Bristol. Upon joining Oxford Instruments in 1998 Dr Cuthbert spent time in Japan and across Asia in technical and commercial roles before taking up sales & marketing responsibility for North America based in Concord MA.

Since 2008, Dr Cuthbert has been based in Oxfordshire, UK having a number of technical and commercial leadership roles within Oxford Instruments. Dr Cuthbert is currently Business Development & Strategic Marketing Director for Oxford Instruments NanoScience and also leads the Quantum Technologies market sector for Oxford Instruments Plc. As a keen advocate of technology and the importance of science within society Dr Cuthbert is a member of the Institute of Physics and sits on a number of advisory panels as well as supporting STEM outreach to local schools in Oxfordshire.