Design, Simulation and Modeling of a Micromachined High Temperature Microhotplate for Application in Trace Gas Detection

A microhotplate (MHP) is a basic Microelectromechanical System (MEMS) structure
that is used in many applications such as a platform for metal oxide gas sensors,
microfluidics and infrared emission. Semiconductor gas sensors usually require high
power because of their elevated operating temperatures. The uniformity of the
temperature distribution over the sensing area is an important factor in gas detection.
There are several silicon micromachined MHP that can easily withstand temperatures
between 200°C and 500°C for long periods. However there is no systematic study on
the effect of the thickness of the various layers of the MHP on its characteristics at
high operating temperatures of up to 700oC with lower power dissipation, lower
mechanical displacement and good uniformity of the temperature distribution on the
MHP. The MHP for the present study consists of a 100 μm × 100 μm membrane
supported by four microbridges of length 113 μm and width 20 μm designed and
simulated using CoventorWare. Tetrahedron mesh with 80μm element size is applied
to the solid model, while the membrane area is meshed with 5μm element size to
obtain accurate FEM simulation results. In the characterization of the MHP, the
length and width of the various layers (membrane, heat distributor and sensing film)
are fixed while their thicknesses are varied from 0.3 μm to 3 μm to investigate the
effect of thickness on the MHP characteristics. At the fixed operation temperature of
700°C, it is shown that as membrane thickness increases, power dissipation, current
density, time constant and heat transfer to the silicon substrate increases, while
mechanical displacement of the membrane remains constant. When the SiC heat
distributor thickness increases, a small increase in power dissipation is observed
while the displacement decreases. The temperature gradient on the MHP is found to
decrease with increasing thickness of the SiC and is a minimum with a value of
0.005°C/μm for a thickness of 2 μm and above. An optimized MHP device at an
operating temperature of 700°C was found to have a low power dissipation of about
9.25 mW, maximum mechanical displacement of 1.2 μm, a temperature gradient of
0.005°C/μm and a short time constant of 0.17 ms.