Timo Kaufmann

Fully symmetric vertical hall devices in CMOS technology

We present a novel CMOS-integrated, vertical Hall sensor (VHS) with optimized symmetry for the measurement of in-plane magnetic field components. Due to the junction field effect, conventional five-contact VHS (5CVHS) suffer from considerable offsets caused by their inherent electrical asymmetry under contact permutations. The novel device achieves a higher degree of symmetry by the appropriate connection of four identical three-contact elements. As a result, with a bias voltage of 3.5 V and after current switching a mean residual offset of 2.5 μV with a standard deviation of 33.8 μV is achieved among 45 samples on an 8-inch wafer. This represents an improvement by a factor of more than 4 over 5CVHS fabricated on the same wafer. In addition, the power consumption is reduced by 47%.

Novel coupling concept for five-contact vertical hall devices

This paper reports on an improved coupling concept of CMOS-based five-contact (5C) vertical Hall sensors (VHS) for measuring in-plane magnetic field components. In contrast to existing sensor approaches using four 5C-VHS connected in parallel, only two 5C-VHS coupled in series are required. The coupling concept thus minimizes sensor footprint as well as power consumption. Furthermore, the 5C-VHS coupled in series exhibit a Hall sensitivity increased by 34 % and an inherent offset signal reduced by up to 80 % compared to individual 5C-VHS. The paper presents the novel coupling concept, finite element simulations describing the influence of the coupling scheme on the magnetic field sensitivity and experimental results comparing single 5C-VHS and arrangements of two and four coupled 5C-VHS.

Piezo-hall effect in CMOS-based vertical hall devices

This paper reports on the piezo-Hall effect in CMOS-based five-contact vertical Hall sensors (VHS) able to measure in-plane magnetic field components. The geometry of such devices and the characteristic current flow in the deep n-wells strongly differ from those of structures used so far to characterize the piezo-Hall response. In contrast to standard planar Hall plates, homogeneous mechanical in-plane stress was found to cause a weak change in the magnetic sensitivity of the VHS. The paper presents the custom-made measurement setup and its detailed characterization as well as experimental results acquired using single VHS and coupled sensor systems comprising four VHS connected in parallel. The experimental results are supported by finite element simulations. It is concluded, that the low sensitivity change is due to the vertical current density changes induced by the applied mechanical stress.

Piezoresistive Response of Vertical Hall Devices

This paper reports on the piezoresistive response of CMOS-based five-contact vertical Hall sensors (VHS) under mechanical stress. Single sensor elements and coupled sensor systems comprising four individual VHS were exposed to normal in-plane stress σxx along the sensor axis using a four-point bending bridge setup. The resulting stress sensitivity of the offset signals at an input voltage of Vin = 3 V was below 2 μV/MPa. In addition, an inhomogeneous stress distribution was generated by applying vertical forces close to the sensor systems using an SU-8 pillar. A resulting maximum offset increase by 109 μV/N at Vin = 3 V was measured. In conclusion, the influence of mechanical stress on the offset behavior of VHS is small compared to other offset sources such as the junction field effect and variations in sensor geometry.

Piezoresistive response of five-contact vertical Hall devices

This paper reports on the piezoresistive effect and resulting offset behavior of CMOS-based five-contact vertical Hall sensors (VHS) under mechanical stress. Single sensor elements and coupled sensor systems comprising four individual VHS were exposed to normal in-plane stress σxx along the sensor axis using a four-point bending bridge setup. The resulting stress sensitivity of the offset signal at a bias voltage of bias = 3 V was below 2 µV/MPa. In addition, an inhomogeneous stress distribution was generated applying vertical forces close to a four-sensor system using a polymer cylinder. A resulting maximum offset increase by 20 µV/N at Vbias = 3 V was achieved. In conclusion, the influence of mechanical stress on the offset behavior of VHS is small compared to other offset sources such as the junction field effect and variations in sensor geometry.