Introduction

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Hands-on design training for MultiMEMS
D. Lapadatu and P. Ohlckers

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| 1. Introduction

As part of the microBUILDER project, a technology demonstrator has been fabricated. This design tutorial is based on this demonstrator.
Description of the flow sensor demonstrator
The flow sensor demonstrator combines four of the technologies of the microBUILDER project:

- thinXXS plastic module for ease of fluidics interfacing
- Piezoresistive silicon sensor module from Infineon Sensonor MultiMEMS
- DRIE postprocessing of silicon module
- Standard readout electronics for ease of data collection

The specifications for the demonstrator are as follows:

Plastics module with 2 flow sensors |
Flow range: | 0 – 8 ml/min
| 0 – 30 ml/min
Media: | liquids (water)
Power supply: | 5 V DC
Signal output: | 0 – 2.5 V

The responsibilities for developing the different parts of this mixed technologies module was

Flow sensor chip | SINTEF, SensoNor
Electronics | HSG-IMIT
Module + assembly | thinXXS
Test and characterisation | All above

Silicon sensor module
The silicon flow sensor module has been fabricated in the MultiMEMS MPW service with additional DRIE postprocessing at Sintef MiNaLab to provide 100 micron deep front side recesses for the Venturi channel that generates the pressure drop measured by the pressuresensor with piezoresistive sensing elements embedded.

The top- and bottom side of the silicon module from the flow sensor.

In addition to the pressure sensor, the sensor module contains a diode for monitoring the temperature of the liquid flowing through the sensor. This is useful as the viscosity of most water based liquids is highly nonlinear with temperature, and by monitoring the temperature one can correct for this using the electronics module.
The readout electronics module
The readout electronics module is a printed circuit board with electronics providing a both digital and analogue interface to the silicon sensor element. The electronics has been designed and manufactured at HSG-IMIT in Germany.

The PCB mounted on top of the thinXXS plastic module with flexboards connecting to the two silicon modules. | The PCB with circuits and connectors for interfacing a computer to the sensor.

It provides an interface to two Whetstone bridges and can be run of a 3.3 V power supply. It provides the possibility of individual sensor signal amplification in the range 2 … 1000. It also fits a microcontroller and a12 bit ADC withs adjustable sampling rate up to 4 kHz. In addition microBUILDER provides a LabView interface to the sensor.
The injection moulded plastic module
To simplify the fluidic interface to the sensor module, the sensor and electronics are mounted on a injected moulded plastic module that fits into the thinXXS construction kit. The construction kit provides a range of connectors and standard slides that enable you to easy prototype your system or set up advanced experiments.

The assembled demonstrator in the thinXXS base plate. Also shown is a standard slide from thinXXS.

Measurments from first version
A first version of this flow sensor has been fabricated and tested. The results using a syringe pump and balance as illustrated below shows good correlation between the sensor output and actual flow.

Test setup for the flow sensor. Using a syringe pump and a balance the flow sensor performance was found to be very good as illustrated in the plot below. The deviation was found to be less than 3 %

Using thinXXS micropump in a setup as shown below, the results from the balance and the sensor deviated significantly.

The pulsed pumping of the thinXXS pump slide results in a significant deviation between the sensor and calculated throughput. The deviation was found to be 12.8 %

The problem was identified to be an effect of the pressure pulses and a setup with a setup reducing this effect was also tested, improving the accuracy with pulsed input from 12.8 % to 7.8 %

Above: The setup to correct for effects of the pulsed pumping of the thinXXS pump module. Below: Results from measurements using the thinXXS pump slide, with and without the addition of a fluidic capacitor.

Hands-on design training
In this design hands-on session we will redesign the silicon flow sensor module. To enable submitting the finished design directly to the MultiMEMS MPW service, we will omit the 100 micron add-on recess etch and use the frontside DRIE etch with a nominal depth of 10 micron available.
The tutorial is divided into the following sub- assignments:

- Piezoresistor
  - Gives an introduction to the tool we will use for this session
  - Gives an introduction to the design hand-book and how to use it
  - Introduces key layers for resistors and conductors as well as other layers required.
- Fluid flow through hole
  - Introduce anisotropic etching of silicon
  - Introduce deep reactive ion etching
  - Show how the NOWEL combined with these two layers can be used to make a hole through the whole silicon wafer.
  - Projection layers
- Pressure sensor
  - Illustrates how to make a circular membrane
  - Discuss positioning of piezoresistors to maximize sensitivity
- Venturi channel
  - How to generate a pressure drop from a flow
- Etching of glass
  - Isotropic etching
  - Fluid infrastructure

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Last updated: June. 2009 24.06.2009