New Horizons for Hall Sensors

The Hall Effect was first discovered by Dr. Edwin Hall in 1879 and is used in everyday applications such as proximity switches, positioning, speed detection and current sensing. But now Hall Cross sensor technology innovation is making a comeback with miniature, printable Hall sensor arrays that offer concentrated sensitivity and are opening up new application areas.

What makes the design of Hall Cross sensors from Micromem Inc. unique is an ability to provide very high magnetic sensitivity while at the same time increasing device density, reducing cost and simplifying manufacturing complexity. The design features a Gallium Arsenide (GaAs) substrate that optimizes sensitivity with cost and overall device size, and provides much higher electron mobility (sensitivity) than silicon and resistance to temperature drift due to a higher energy band gap.

"Magnetic Hall Sensor principles are essentially a physical effect that happens in materials that are carrying current in a magnetic field," says Steve Van Fleet, president of Micromem. "If you have a magnet and a Hall Sensor, and pass bias current into a magnetic field, it will deflect the signal depending on the direction of the magnet. It is very sensitive to a magnetic field, and basically the current output can be used to determine the strength of the magnetic field."

A key innovation in the Micromem technology is an ability to array the Hall sensors to significantly reduce the signal-to-noise ratio.

"While everything in the past has been integrated circuits and chips put onto a PCB board, we now have custom arraying and are actually dissolving them into gallium arsenide ink and using three-dimensional statistical manufacturing," Van Fleet says.

"We have made a very, very small sensor and are using proprietary epilayer gallium arsenide to create a Hall Sensor with concentrated sensitivity," Van Fleet says. He says Micromem is partnering with Nano Opto and using a process called atomic layer deposition (ALD) to basically lay down a molecule at a time to design a concentrator. Because the ALD process is extremely smooth, they have been able to do magnetic modeling and develop an optimum shape to collect magnetic energy in very weak magnetic fields.

"Our focus is to go after new and unmet needs. Our model is interesting because we are a small company focusing on the sensor itself and are partnering with people who have the main expertise in different fields," says Van Fleet. "What we have found is that if we can build cost efficient sensors because, on a 6 inch wafer we get several hundred thousand of these, the cost of the sensors is micro pennies. Arrays open up a whole new opportunity for sensing technology."

One client in Norway is charting minute changes in the earth's magnetic field and is planning to map the magnetic signature of the North Sea to predict the location of mining and drilling reserves.

Another interesting development is a prototype mining core sensor. Using an annulus or basically a head with an array of sensors and a database of 600,000 pre-assayed core samples, they are working to train the device to look for the frequency specifications and the magnetic signature of gold, cadmium, aluminum and uranium. Van Fleet says they have found the signature is repetitive, repeatable and accurate.

Unotron, a customer in the UK, makes high-end, washable keyboards that can be submersed and sterilized. A read switch under each key in the current design has been replaced with a Hall sensor printed on a PEP substrate.

Van Fleet says that because the sensors are printed, there are no traditional integrated circuits, pieces, components or dies. Mean time to failure has been extended from 5 to 20 million hours. Using the Hall Sensor, a magnet in the top of the key creates a magnetic field when the user pushes a key down. Current flowing through the Hall Sensor in quiescent mode deflects the direction of the current and creates the Hall Effect. The XY coordinate is communicated to the keyboard controller and recognizes someone pushed the letter "q."

"It's not a real high tech use of the technology, but it has a good business case and allowed us to get some running experience on printed electronics," Van Fleet says.

Micromem also recently announced a major contract with a medical company on a product called Breast Aware that is designed for early detection of lumps in a woman's breast.

Using a printable array of Hall sensors, the concept is to create a wearable device that the woman puts on her breasts and communicates to a handheld device. FDA testing indicates that a 2 degree temperature differential between a specific zone on one breast and the other is an early indication of a lump. The rationale is that as the lump grows there are more veins, higher density and higher temperature in the location.

Using the Hall Sensor as a temperature sensor is pretty trivial but Van Fleet says they are also working with oncologists to be able to identify the magnetic signature of a healthy breast versus one that is potentially cancerous. This can be done now with an MRI but it is much more expensive.