Magnetic field sensors are employed in down-hole oil, gas, and geothermal well-drilling applications for azimuth sensing, orientation/rotation sensing, and magnetic anomaly detection. Key requirements of these applications include high measurement accuracy in the near-DC frequency regime, high-operating temperatures, high mechanical shock and vibration, and severe size constraints. Silicon manufacturing processes enable the development of rugged components with small size compatible with assembly processes used for adjacent electronics in hermetically sealed hybrid and/or ceramic packages.
Silicon-based magnetic sensors include Anistotropic Magnetoresistive (AMR), Giant Magnetoresistive (GMR) and Tunnelling Magnetoresistive (TMR) sensors. Commercially available GMR and TMR sensors generally cannot be operated much above 150°C. While GMR and TMR have enabled great areal density growth for magnetic recording industry over the past two decades, AMR sensors provide high accuracy measurements in the near-DC regime above 150°C. This is in part due to simplicity of their construction, but also due to their low noise characteristics at low frequencies compared to GMR and TMR. This paper will describe the extension of Honeywell's low noise AMR sensors into high temperature regime up to 225°C.
Sensors being reported have room temperature bridge resistance of ~700 Ω, open loop sensitivity of ~2.5 mV/V-Gauss, with a temperature coefficient of sensitivity of −2500 ppm/°C. The low-frequency minimum detectable field monotonically increases with increasing temperature. At room temperature it is ~2.2 μG/√Hz@1 Hz and reaches a value of ~26μG/√Hz@1 Hz at 225°C. Signal and noise density both increase with increasing sensor bias voltage such that low-frequency signal-to-noise ratio does not vary in the bias voltage range of 2.5 V to 10V.
These sensors have also been configured in a closed loop format using low noise electronics. Measurements of closed loop transfer function in the range of ±0.8 Gauss were made. The sensor was placed in a thermal chamber while the feedback electronics were placed outside at room temperature. The linearity of the transfer function is quite excellent; deviation from linearity increases monotonically with increasing temperature reaching < 0.002% of full scale or 29 μGauss at 225°C. Closed loop operation of a typical sensor shows 1-σ measurement variability of 21 μGauss at 220°C. By a combination of averaging and closed loop operation an input step from 0 to 75 μGauss is replicated at the output to within 0.1 μG at 225°C.
Creep and Material Design
