Research Academics  
 
   
 

MEMS Surface Fence Probe for Wall Shear Stress Measurements
 
Contact Person : Dipl.-Ing. Michael Schiffer

Introduction

Objects which are brought into viscious fluids receive a shear stress on their surface which is called wall shear stress. Conventional methods to detect the wall shear stress are mostly insufficient in respect to their sensor-dimensions, their handling, their feedback on the flow, their dynamic behaviour, their fabrication and their low temporal and local resolution.MEMS sensors seem to have advantages for instance concerning their small size which leads to a very little feedback on the flow. The most widespread MEMS wall shear stress sensors are hot wire anemometers but these are unable to detect the flow direction. With the use of pulsed wire anemometers the detection of the flow direction is possible subsequently but at the expense of a continuous measurement signal and the dynamic behaviour. Floating element shear stress sensors (surface balances) – which directly measure the wall shear stress - suffer from their susceptibility to contamination and their low local resolution which leads to a disqualification for an array alignment. The micro fence probe can directly detect the flow direction and has a continuous measurement signal. The probes can easily be arranged in an array. The high sensitivity resp. resolution of the sensor enables the survey of wall shear stress in a range from 10-3 Pa to 102 Pa. To compensate temperature influences on the offset signal and the sensitivity the probe has an on-chip-temperature-sensor (pn-diode). Measurements with a 600µm high micro fence probe (calibrated against a preston tube) show a average sensitivity of 3,38 mV / (V Pa). The developed probes seem to be a very powerful measuring instrument for an active flow control.

Device design and Fabrication

The surface micro fence probe was developed in a cooperation with a subprojekt of the “Sonderforschungsbereich (SFB)” of the “Deutsche Forschungsgemeinschaft (DFG)”. Between two sides of a very small obstacle on the wall a pressure difference between the fence-front and -back is produced due to the approaching flow. The micro fence probe measures this pressure difference directly by sensing the deflection of the micro fence with integrated piezoresistors which occurs as a result of the approaching flow (fig. 1).

     

Figure. 1: Micro fence probe mounted in wall (left).Deflection of the micro fence due to the approaching flow (the flow is adumbrated by red arrows)(right)

The micro fence probe consists of two different parts, the sensing element and the sensor body. The sensing element is a flexible silicon beam with four integrated piezoresistors, connected to a Wheatstone bridge. The deflection of the micro fence induces mechanical stress in the suspension of the fence. This mechanical stress at last changes the electrical resistance of the piezoresistors. The sensing element rests on the sensor body which carries the conductors, the bondpads and a pn-diode for temperature sensing which is integrated in a temperature compensation circuit. The sensor is fabricated with SOI-material in bulk-micromachining whereby the overlayer thickness determines the fence thickness, so it is possible to get the highest efficiency in respect to reproducibility. Figure 2 shows a schematic draw of the Sensor with detailed views of the pn-diode and the folded piezoresistors.

Figure 2 :
Schematic drawing of the Sensor with detailed views of the pn-diode and the folded piezoresistors.
Lower left:view on the pn-diode (upper left: surface; lower left: view on the implanted areas).
Lower right:view on the folded piezoresistors (upper left: surface; lower left: view on the implanted areas).

The Substrate is a double-sided polished SOI-wafer with (100)-orientation, n doped between 1 Wcm – 10 Wcm) and an overall thickness of 358µm. The handlewafer thickness is 350µm, the buried oxide is 500nm thick and the Si-overlayer is 7µm ± 0,5µm thick.

  1. First a thin oxide is grown on the wafer (45nm) which prevents channelling. Then follows the implantation of the piezoresistors and the pn-diode. A structured resist mask protects the regions where no implantation is needed.
  2. Thermal oxidation in dry oxygen atmosphere (1000°C, 100nm): This is needed to anneal the silicon crystalline structure and to activate the dopants. Furthermore the oxide is needed to insulate the conductors from the substrate and it is one component of the passivation which protects the piezoresistors.
  3. LPCVD-Si3N4-deposition (780°C, 40nm): This one is the second component of the passivation.
  4. Deposition and structuring of the AlSi1-metallization: A 1µm thick aluminium film is deposited with a sputter process (the film consists 1% Si to prevent spiking), afterwards a structured resist mask is created and the conductors are structured through a wet-chemical etch process.
  5. The three dimensional structuring of the Si-substrate to create the micro fence is achieved with anisotropic etching in an aqueous KOH-solution. Therby the substrate material of the handle-wafer under the later micro fence is etched down to the buried oxide. The buried oxide serves as an etch stop, the Si3N4-film on the backside as an etch mask.
  6. The structure of the fence is generated through RIE. The resist on the front side is structured so the desired geometry can be uncovered with RIE. The buried oxide is used as an etch stop again.
  7. Wet chemical etching of the buried oxide on the backside of the fence in an aqueous NH4F-solution.
  8. Separation and release of the sensors.

Pictures of two realized sensor designs are shown in figure 3.

       

Figure3 : Pictures of two realized sensor designs.
Left:     sensor design with a 600µm high micro fence (AS 600).
Right:  sensor design with a 300µm high micro fence (AS 300).

Wind tunnel measurement

For the measurements a highly amplifying signal selection and a packaging for the probe was developed (fig. 4). Figure 5 shows calibration curves of two Sensors (with a 600µm high micro fence (AS600)) measured in a wind tunnel. One is realized in SOI-technology (7µm fence thickness), the other is fabricated in standard Si-technology (10µm fence thickness). Due to the smaller fence thickness of the SOI fabricated sensor the sensitivity is nearly doubled.

Figure 4 :Picture of the packaging of the micro fence probe.

Calibration curves of two Sensors (with a 600µm high micro fence (AS600)) measured in a wind tunnel (supply voltage: 1V).
Red :     sensor design, using standard silicon technology
Black :   sensor design, using SOI- technology.

Conclusions

A new designed micro fence probe has been demonstrated and it is up to now worldwide the only one of its kind. It was shown that with standard bulk-micromachining fabrication technique it is possible to create such a sensor and by using SOI-material it could be achieved to realize a very sensitive probe to measure wall shear stress. The micro fence probe can directly detect the flow direction and has a continuous measurement signal. The probes can easily be arranged in an array. The high sensitivity resp. resolution of the sensor enables the survey of wall shear stress in a range from 10-3 Pa to 102 Pa. To compensate temperature influences on the offset signal and the sensitivity the probe has an on-chip-temperature-sensor (pn-diode). Measurements with a 600µm high micro fence probe (calibrated against a preston tube) show a average sensitivity of 3,38 mV / (V Pa). The developed probes seem to be a very powerful measuring instrument for an active flow control.