Micromachined Thermal Mass Flow Sensor With Self-Cleaning Capability And Methods Of Making the Same

• US 20090158859A1
• Filed: 12/19/2007
• Published: 06/25/2009
• Est. Priority Date: 12/19/2007
• Status: Active Grant

• Current Assignee: Siargo Limited
• Original Assignee: Siargo Limited
• Inventors: Wang, Gaofeng, Huang, Liji, Yao, Yahong, Chen, Chih-Chang
• Granted Patent: US 7,878,056 B2
• Time in Patent Office: Days
• US Class Current: 73/861.351
• CPC Class Codes: G01F 1/6845 Micromachined devices
• CPC Class Codes: G01F 1/6888 Thermoelectric elements, e.g. thermocouples, thermopiles
• CPC Class Codes: G01F 15/006 characterised by the use of a particular material, e.g. anti-corrosive material G01F15/14 takes precedence

Abstract

The current invention generally relates to Micro Electro Mechanical Systems (MEMS) thermal mass flow sensors for measuring the flow rate of a flowing fluid (gas/liquid) and the methods of manufacturing on single crystal silicon wafers. The said mass flow sensors have self-cleaning capability that is achieved via the modulation of the cavity of which the sensing elements locate on the top of the cavity that is made of a silicon nitride film; alternatively the sensing elements are fabricated on top of a binary silicon nitride/conductive polycrystalline silicon film under which is a porous silicon layer selective formed in a silicon substrate. Using polycrystalline silicon or the sensing elements as electrodes, an acoustic wave can be generated across the porous silicon layer which is also used for the thermal isolation of the sensing elements. The vibration or acoustic energy is effective to remove foreign materials deposited on top surface of the sensing elements that ensure the accuracy and enhance repeatability of the thermal mass flow sensing.

Claims

1. A MEMS thermal mass flow sensor comprising:

a heater thermistor, a reference thermistor and two temperature sensing thermistors disposed on a silicon substrate.

2. The MEMS thermal mass flow sensor of claim 1 wherein:

the sensing elements are fabricated on a membrane under which is a cavity that can be modulated by the capacitive force;

3. The MEMS thermal mass flow sensors of claim 2 wherein:

the membrane providing the house for the sensing elements is made of LPCVD silicon nitride of 0.3˜

1.2 μ

m;

4. The MEMS thermal mass flow sensors of claim 2 wherein:

a self-cleaning capability is achieved via the modulation of the cavity underneath the membrane on which the sensing elements are fabricated. The modulation generates vibrations that are effective to remove foreign materials deposited on the top surfaces of the sensing elements;

5. The MEMS thermal mass flow sensors of claim 2 wherein:

a self-cleaning capability is achieved via the modulation of the cavity underneath the membrane on which the sensing elements are fabricated. The modulation of the cavity can be performed by applying a pulse voltage with a desired frequency or a periodic wave function with desired amplitude.

6. The MEMS thermal mass flow sensor of claim 1 wherein:

a selectively formed porous silicon island is provided on silicon substrate as thermally isolated area.

7. The MEMS thermal mass flow sensor of claim 6 wherein:

a first masking and patterning procedure is performed to define the silicon nitride window for selective formation of porous silicon.

8. The MEMS thermal mass flow sensor of claim 6 wherein:

said the thickness of the porous silicon island be in the range from 100 to 200 μ

m.

9. The MEMS thermal mass flow sensor of claim 6 wherein:

said the porosity of porous silicon island be higher than 50%.

10. The MEMS thermal mass flow sensor of claim 1 wherein:

a thin layer of silicon nitride deposition is performed on top of the porous silicon layer as electrical insulation layer.

11. The MEMS thermal mass flow sensor of claim 1 wherein:

a first high temperature coefficient of resistance (TCR) layer is formed on said silicon substrate

12. The MEMS thermal mass flow sensor of claim 1 wherein:

a second masking and patterning procedure is performed to define thermistors including heater, temperature sensing thermistors on said the top of porous silicon island.

13. The MEMS thermal mass flow sensor of claim 1 wherein:

said reference thermistor is disposed on the area of substrate other than the porous silicon island.

14. The MEMS thermal mass flow sensor of claim 1 wherein:

successively an interconnection layer is formed on said substrate and a third masking and patterning procedure is performed to remove portions of said metal layer to define interconnection and bonding pads region.

15. The MEMS thermal mass flow sensor of claim 1 wherein:

a passivation layer is formed and a fourth masking and patterning procedure is performed to remove portions of said passivation for forming a contact hole on said bonding pads and the open-slots on said the perimeters of porous silicon island.

16. The MEMS thermal mass flow sensor of claim 15 wherein:

said the open slots around the said porous silicon island are to isolate the lateral heat conduction from heater thermistor to sensor thermistors.

17. The process according to claim 11, wherein said the layer is formed of platinum, gold, silicon carbide, tantalum nitride etc.

18. The MEMS thermal mass flow sensor of claim 12 wherein:

said the temperature sensing thermistors further includes an upstream sensing thermistor and a downstream thermistor disposed on upstream and downstream locations relative to said heater on said thermally-isolated porous silicon island.

19. The MEMS thermal mass flow sensor of claim 12 further comprising:

a reference resistor disposed on said substrate wherein said reference thermistor having a resistance ranging from three to twenty-five times a resistance of said sensing thermistor.

20. The MEMS thermal mass flow sensor of claim 18 wherein:

said the temperature sensing thermistors disposed on the upstream and downstream locations of heater thermistor could be arranged in a symmetrical or nonsymmetrical configuration related to the center of heater thermistor.

21. The MEMS thermal mass flow sensor of claim 4 and claim 5 wherein:

said self-cleaning feature is realized by ultrasonic energy agitation generated from a thermally induced ultrasonic acoustic wave emitter.

22. The MEMS thermal mass flow sensor of claim 21 wherein:

a selectively formed porous silicon island is formed by electrochemical anodization as a thermally isolation area.

23. The MEMS thermal mass flow sensor of claim 22 wherein:

said porous silicon with a top dielectric film and the top thermistors formed an ultrasonic acoustic wave emitter.

24. The MEMS thermal mass flow sensor of claim 23 wherein:

said the ultrasonic acoustic wave emitter be used to clean up the sensor surface active region in a periodical cycle base.

25. The MEMS thermal mass flow sensor of claim 24 wherein:

said the thickness of the porous silicon island be in the range from 100 to 200 um.

26. The MEMS thermal mass flow sensor of claim 22 wherein:

said the porosity of porous silicon island be higher than 50%.

27. The MEMS thermal mass flow sensor of claim 1 further comprising:

a thin fluorocarbon passivation coating on top surface of whole device by plasma enhanced deposition process is to eliminate the sticking issue of alien particles and debris, particularly, the mixture of dust and moisture.

28. The process according to claim 27, wherein said the thin passivation coating is formed and preferred to be hydrophobic and low surface energy with materials such as polymer, Teflon or Teflon-like films.