Atmospheric Pressure, Low cost, Surface Micromachined Pirani Pressure Sensor kourosh khosraviani, Simon Fraser University, School of Engineering Science

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Atmospheric Pressure, Low cost, Surface
Micromachined Pirani Pressure Sensorkourosh
khosraviani, Simon Fraser University, School of
Engineering Science
Operation and Structure principal
Structure and design
Experimental result and discussion

• 3 masks, CMOS compatible
• Heaters material is Polysilicon, Polysilicon
  echant TMAH
• LPCVD oxide as mask for Polysilicon, Thermal
  oxide as sacrificial layer
• Two released and two unreleased heaters per
  device in order to build a Wheatstone bridge and
  improve performance

• Applied pressure between 100kPa to 800kPa with
  heaters constant power density of 0.01 mW/µm2
• Minimum and maximum output variation, 2mv and
  19mv respectively
• Device transition pressure corresponds to 50nm
  gap size
• theoretical thermal conductance results are
  consistent with experimental data from sensors 1,
  3, and 4
• Only 1 to 5 percent of heater's area has solid
  contact
• Sensors 2 and 5 data correspond to 50 of solid
  contact

• Takes advantage of thermal conductivity of gases
• Gap is smaller than Mean Free Pass of gas
  molecules
• Thermal conductivity is a function of pressure
• When pressure changes heater temperature changes
• Heater temperature becomes measure for pressure
• Gap size defines the pressure measurement range
• Nanometer gap size leads to atmospheric range

Fabrication steps
(a)
Bridge output voltage versus applied pressure
(b)
Fabrication challenge
(c)

• Building gaps smaller than 1?m is very difficult
• Waters surface tension pulls down the heater
  membrane
• Membrane remains there because of different
  existing forces
• Avoiding is complicated and expensive
• Have to use Freeze-drying technique

(d)
(e)
(f)
Our sensor
Actual device

• Takes advantage of heaters material surface
  irregularities
• Only 1 to 5 percent of total contact area has
  solid contact
• Heater material is Polysilicon with 50nm surface
  roughness
• We allowed the heaters membrane collapse to
  substrate
• Nanometer gap size is built between heater and
  heatsink

• Starting from wafer with Polysilicon sandwich
• Patterning the Polysilicon, LPCVD dioxide as a
  mask, and TMAH as etchant
• Patterning the LPCVD dioxide for bonding pads,
  and protecting the unreleased heaters
• Depositing and pattering the aluminum in order to
  make bonding pads
• Sacrificially etching the thermal oxide and
  releasing the heaters
• collapsing the heater membrane during final
  drying step by water surface tension

• Heaters dimension 6µm x 50µm, 0.55µm thick
  Polysilicon
• Aluminum patches protect unreleased heaters