Feel it in the air

Druck Limited has utilised silicon’s mechanical properties in the RPT 410, its latest barometric pressure transducer.

Solid state sensors have been produced since the 1970’s but manufacturers have used silicon in integrated circuit technology to utilise its properties as a semiconductor whilst ignoring silicon’s mechanical properties.

Druck Limited, the specialist British based manufacturer of precision measuring and test equipment, has utilised silicon’s mechanical properties in the RPT 410, its latest barometric pressure transducer employing Resonant Silicon Pressure Transducer technology. Where did it come from?

The design of the sensor assembly was brought to Druck by John Greenwood from Nortel Networks in 1990 and the company identified the technology as an opportunity to replace competitor sensors within its own instruments.

The design brief for the complete transducer was one that gave an accuracy and stability unachievable with Druck’s standard piezo resistive devices as a barometric reference. The resonator structure was designed in the mid ’80s. The structure is micro-machined from silicon using Boron diffusion to define the pattern. Difficulties, however, were encountered in the initial stages of achieving the 3D structure without the aid of the design tools which are available today.

The resonant structure requires a vacuum in which to operate, which left the question of how to seal the device whilst maintaining a connection to the electrodes mounted inside the vacuum. The problem was remedied by bonding the structure to the substrate chip using a glass frit thermally matched to silicon.

The main challenges in producing the devices were equipment for assembling the sensors and finding test equipment accurate enough to measure the performance. The processing equipment was designed by Druck specifically for the product and the RPTs are calibrated against a Ruska 7000 Series, which offers 30ppm accuracy.The development of an optimised measurement circuit took several years. The second-generation design now in use copes with a wider range of sensor parameters and gives lower jitter on the output frequency.

The resonating structure is etched from single crystal silicon, giving it high stability and is housed within a vacuum, isolating it from the applied media making it insensitive to the effects of humidity.

How Does it Work?

Resonant silicon technology uses the mechanical properties of the silicon as opposed to the electrical properties used in piezo resistive technology, which is generally acknowledged as cheap whilst delivering a good dynamic. However, they do have drawbacks, such as significant power requirements, low output signal, large offset, and temperature dependence.

A 3D structure is micro-machined from single crystal silicon and forced into resonance using electrostatics. A second silicon plate is bonded to the resonator providing a vacuum environment for the resonating element. This plate houses the drive and pick up electrodes which both maintain the resonator structure in resonance and detect any change to the frequency of resonance.

Applied pressure – from the reverse side of the resonator silicon diaphragm – deflects the diaphragm and induces stress into the structure resulting in a change to the resonant frequency that is proportional to applied pressure.

The RPT 410 detects the change in resonant frequency and also measures the temperature of the silicon by recording the voltage drop across a diode.These two measurements are used to calculate the pressure value in engineering units.

Manufactured in Druck’s class 100 clean room, the multi-layer sensor structure comprises a resonator and pressure sensitive diaphragm micro-machined from a single piece of silicon.

The resonator is isolated from the pressure medium which ensures performance is maintained regardless of variations in pressure media density. The sensor is available with either a frequency or voltage output corresponding to the barometric pressure range. Current consumption is conserved by an external trigger shutdown function.

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