Page 229 - MEMS Mechanical Sensors

218 Flow Sensors

and silicon-carbide [49] have been used. Also, thermistors made of germanium (ther-
mistor: an electrical resistor making use of a semiconductor whose resistance varies
sharply in a known manner with the temperature) were employed [47, 50, 51].
Thermocouples for temperature detection have been made out of aluminum/polysili-
con [52], platinum/high boron doped silicon [53], n-polysilicon/p-polysilicon [54],
+
gold/polysilicon [41], and aluminum/p -doped silicon [28]. The thermocouple uses a
self-generating effect due to temperature to measure the flow rate. When there is a
temperature difference between two contacts of two materials, a voltage propor-
tional to the temperature difference is generated. This effect is known as the Seebeck
effect. The effect is expressed as ∆V =⋅α ∆T, where α is the Seebeck coefficient. A
thermopile is realized by connecting several thermocouples together.
As a general rule, the lower the mass of the sensing element (resistor, thermistor,
thermocouple, and their support structure) and the higher the thermal isolation from
the carrier chip, the faster is the sensor in responding to changes in fluid flow and the
higher is the sensitivity [49]. Therefore, the sensing elements, including the heater, are
suspended on a cantilever to stand free into the flow [55], are placed on very thin
membranes [41, 42, 50, 51, 53, 54, 56], or on bridges crossing the flow path [43, 48,
49]. Often a thin-film of silicon nitride is used as membrane or bridge material. An
excellent paper on how to obtain low-stress LPCVD silicon nitride was published by
Gardeniers et al. [57]. PECVD mixed frequency silicon nitride or oxi-nitride is also
an option. It is important that the supporting material has small thermal conductivity
or that a thermal barrier is implemented [55]. Using too thin a support for the resis-
tors means that the sensor becomes less robust and is prone to damage.
For the design of a thermal flow sensor, the hydrodynamic boundary layer and
the thermal boundary layer need to be taken into account. For pressure-driven flows,
large flow velocity gradients occur close to walls. For a detailed explanation and for
calculating the thickness of the boundary layers, see [58]. The thickness of the bound-
ary layer is dependent on the thermal conductivity and on the viscosity of the fluid
[41]. An analytical model for a calorimetric flow sensor consisting of a heater plus an
up- and downstream temperature sensor is given by Lammerink et al. [43]. A similar
structure was simulated in SPICE by Rasmussen et al. [59]. The model can be used for
electrical, thermal, and fluidic simulations. Ashauer et al. [41] presented a numerical
simulation describing the propagation of a heat pulse. Damean et al. [60] modeled
the heat transfer in a microfluidic channel with one resistive line across it. The model
was used to determine fluid and flow characteristics.
Some thermal flow sensors can also be used as a pressure difference sensor. The
differential pressure is indirectly measured with the mass flow, which is generated
through the differential pressure. With the sensor from HSG-IMIT (Germany) the
sensitivity can be chosen to be between 0.5 mbar up to 5 mbar [61]. For the sensor
from Sensirion AG (Switzerland) the measurement range is ±100 Pa with a lowest
2
detectable pressure of ±0.002 Pa, which corresponds to a force of 0.00002 g/cm or
a geographic height difference of 0.16 mm [62]. With this setup, a pressure equaliza-
tion occurs and so it is not suitable for absolute pressure measurement.
Each specific category of thermal flow sensors is discussed below, and examples
of MEMS devices are given. The section of thermal flow sensors is spilt into research
and commercial devices. So far, commercial devices are using only the thermal meas-
urement principle.