Micro-mechanical capacitive inductive sensor for wireless detection of relative or absolute pressure

• US 20040057589A1
• Filed: 06/17/2003
• Published: 03/25/2004
• Est. Priority Date: 06/18/2002
• Status: Active Grant

• Current Assignee: Corporation for National Research Initiatives
• Original Assignee: Corporation for National Research Initiatives
• Inventors: Pedersen, Michael, Huff, Michael A., Ozgur, Mehmet
• Granted Patent: US 7,024,936 B2
• Time in Patent Office: Days
• US Class Current: 381/152
• CPC Class Codes: G01L 9/0073 using a semiconductive diaphragm

Abstract

A micro-mechanical pressure transducer is disclosed in which a capacitive transducer structure is integrated with an inductor coil to form a LC tank circuit, resonance frequency of which may be detected remotely by imposing an electromagnetic field on the transducer. The capacitive transducer structure comprises a conductive movable diaphragm, a fixed counter electrode, and a predetermined air gap between said diaphragm and electrode. The diaphragm deflects in response to an applied pressure differential, leading to a change of capacitance in the structure and hence a shift of resonance frequency of the LC tank circuit. The resonance frequency of the LC circuit can be remotely detected by measuring and determining the corresponding peak in electromagnetic impedance of the transducer.

Claims

1. A micro-mechanical pressure transducer comprising:

a capacitive transducer structure comprising;

a pressure sensitive diaphragm formed from a first substrate,

a conductive layer formed on the diaphragm, and

an electrode formed on a second substrate,

an inductor coil formed within a plurality of layers forming the second substrate,

the first substrate being bonded to the second substrate whereby a pre-determined air gap is formed between the diaphragm and the electrode and the capacitive transducer structure is integrated with the inductor coil to form a LC tank circuit, the resonant frequency of which may be detected by imposing an electromagnetic field on the capacitive transducer.

2. The micro-mechanical pressure sensor of claim 1, wherein the conductive layer is formed using at least one metal selected from the group consisting of aluminum, silver, tin, lead, copper, gold, platinum, palladium, nickel, chromium, titanium and alloys thereof.

3. The micro-mechanical pressure sensor of claim 1, wherein the electrode is formed using at least one metal selected from the group consisting of aluminum, silver, tin, lead, copper, gold, platinum, palladium, nickel, chromium, titanium and alloys thereof.

4. The micro-mechanical pressure sensor of claim 1, wherein the first substrate is single crystal silicon.

5. The micro-mechanical pressure transducer of claim 1, wherein the first substrate is silicon on insulator (SOI).

6. The micro-mechanical pressure sensor of claim 1, wherein the second substrate is a low-temperature co-fired ceramic (LTCC), and wherein the first substrate is hermetically bonded to the second substrate.

7. The micro-mechanical pressure sensor of claim 1, wherein the second substrate is a high-temperature co-fired ceramic (HTCC), and wherein the first substrate is hermetically bonded to the second substrate.

8. The micro-mechanical pressure sensor of claim 1, wherein the second substrate is sequentially-build-up (SBU) multi-layer printed circuit boards.

51. The micro-mechanical pressure sensor recited in claim 1 wherein the inductor coil is formed within a plurality of layers forming the second substrate and under the electrode formed on the second substrate, whereby the inductance of the coil is scaleable according to the number of layers used to form the coil.

52. The micro-mechanical pressure sensor recited in claim 1 wherein the inductor coil is implemented in several layers in the second substrate, directly under the fixed counter electrode and the diaphragm to reduce the overall size of the device and increase the inductance of the coil.

9. A micro-mechanical pressure sensor comprising:

a first substrate including a diaphragm,

a conductive layer formed on the diaphragm,

a second substrate including a plurality of layers,

an inductor formed within the plurality of layers,

an electrode formed on the second substrate,

the first and second substrates being bonded together to form a sealed cavity between the electrode and the conductive layer formed on the diaphragm,

a first via connecting the electrical inductor to the fixed electrode, and

a second via connecting the electrical inductor to the conductive layer on the diaphragm,

wherein deflections of the diaphragm in response to pressure differentials between the sealed cavity and the exterior atmosphere result in changes of capacitance between the electrode and the conductive layer on the diaphragm.

10. The micro-mechanical pressure sensor of claim 9, wherein the second hybrid substrate is formed using low-temperature co-fired ceramics (LTCC), in which a plurality of conductive layers are formed.

11. The micro-mechanical pressure sensor of claim 9, wherein the hybrid substrate is comprised of a first plurality of layers with spiraling conductors and a second plurality of insulating layers.

12. The micro-mechanical pressure sensor of claim 9, wherein a plurality of first and second vias are placed in each insulating layer to connect the spiral conductors.

13. The micro-mechanical pressure sensor of claim 9, wherein the conductive layer is formed from at least one metal selected from the group consisting of aluminum, silver, tin, lead, copper, gold, platinum, palladium, nickel, chromium, titanium and alloys thereof.

14. The micro-mechanical pressure sensor of claim 9, wherein the first substrate is a single crystal silicon.

15. The micro-mechanical pressure transducer of claim 9, wherein the first substrate is silicon on insulator (SOI).

16. The micro-mechanical pressure sensor of claim 9, wherein the second substrate is a low-temperature co-fired ceramic (LTCC), and wherein the first substrate is hermetically bonded to the second substrate.

17. The micro-mechanical pressure sensor of claim 9, wherein the second substrate is a high-temperature co-fired ceramic (HTCC), and wherein the first substrate is hermetically bonded to the second substrate.

18. The micro-mechanical pressure sensor of claim 9, wherein the second substrate is sequentially-build-up (SBU) multi-layer printed circuit boards.

19. A micro-mechanical pressure sensor comprising:

a first substrate including a pressure sensitive diaphragm,

a conductive layer formed on the diaphragm,

a second hybrid substrate including a plurality of layers,

an electrical inductor formed within the plurality of layers,

a fixed electrode formed on top of the second substrate,

the first and second substrates being bonded together and hermetically sealed to form a cavity between the fixed counter electrode and the conductive layer formed on the diaphragm, whereby the counter electrode and the conductive layer formed on the diaphragm form a capacitive structure,

at least one first via connecting the electrical inductor to the fixed electrode, and

at least one second via connecting the electrical inductor to the conductive layer on the diaphragm,

wherein deflections of the diaphragm in response to a pressure differential between the sealed cavity and the exterior atmosphere results in a change of capacitance between the fixed counter electrode and the conductive layer on the diaphragm, and

wherein the capacitive structure and the electrical inductor form an LC circuit.

20. The micro-mechanical pressure sensor of claim 19, wherein the second hybrid substrate is formed using low-temperature co-fired ceramics (LTCC), in which a plurality of conductive layers are formed.

21. The micro-mechanical pressure sensor of claim 20, wherein at least twenty conductive layers are formed in the LTCC second substrate.

22. The micro-mechanical pressure sensor of claim 19, wherein the inductor has a large inductance value and a large quality factor (Q).

23. The micro-mechanical pressure sensor of claim 19, wherein the hybrid substrate is comprised of a plurality of layers with spiraling conductors and a plurality of insulating layers.

24. The micro-mechanical pressure sensor of claim 19, wherein a plurality of first and second vias are placed in each insulating layer to connect the spiral conductors.

25. The micro-mechanical pressure sensor of claim 19, wherein the conductive layer is selected from the group consisting of aluminum, silver, tin, lead, copper, gold, platinum, palladium, nickel, chromium, titanium and alloys thereof.

26. The micro-mechanical pressure sensor of claim 19, wherein the first substrate is single crystal silicon.

27. The micro-mechanical pressure sensor of claim 19, wherein the second substrate is a low-temperature co-fired ceramic (LTCC).

28. The micro-mechanical pressure sensor of claim 19, wherein the first and second substrates are bonded together using thermo-compression bonding.

29. The micro-mechanical pressure sensor of claim 19, wherein the first and second substrates are bonded together using eutectic bonding.

30. A method of forming a micro-mechanical pressure sensor comprising the steps of:

providing a first substrate,

depositing a first masking layer on the first substrate,

patterning the first masking layer on a front side of the first substrate,

etching a cavity on the front side of the first substrate,

removing the first masking layer from the first substrate,

forming a bulk layer in the front side of the first substrate,

depositing a second masking layer on the bulk layer,

patterning the second masking layer on the back side of the first substrate to form an opening,

etching a second cavity on the back side of the first substrate, thereby forming a diaphragm in the first substrate,

removing the second masking layer from the first substrate,

depositing a conductive layer on the front side of the first substrate to provide a highly conductive diaphragm,

providing a second substrate formed from a plurality of dielectric layers and a plurality of conductive layers forming an inductive coil,

polishing the front side of the second substrate to achieve a smooth surface,

depositing a second conductive layer on the front side of the second substrate,

patterning the second conductive layer to form a counter electrode and a bonding area, and

bonding the first and second substrates together to form an air gap between the conductive diaphragm and the counter electrode.

31. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the first substrate is single crystal silicon.

32. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the first substrate is silicon on insulator (SOI).

33. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the second substrate is a low-temperature co-fired ceramic (LTCC).

34. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the second substrate is a high-temperature co-fired ceramic (HTCC).

35. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the second substrate is sequentially-build-up (SBU) multi-layer printed circuit boards.

36. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the diaphragm is formed in the first substrate by chemical etching.

37. The method of forming a micro-mechanical pressure sensor as recited in claim 36, wherein the chemical etching is performed in a solution of water and potassium hydroxide (KOH).

38. The method of forming a micro-mechanical pressure sensor as recited in claim 36, wherein the chemical etching is performed by deep reactive ion etching (DRIE).

39. The method of forming a micro-mechanical pressure sensor as recited in claim 36, wherein the bulk layer is formed in the first substrate to provide a natural termination for the chemical etching.

40. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the bulk layer is formed by the diffusion of boron in the first substrate at an elevated temperature.

41. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the conductive layer on the diaphragm is formed from at least one metal selected from the group consisting of aluminum, silver, tin, lead, copper, gold, platinum, palladium, nickel, chromium, titanium, and alloys thereof.

42. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the first and second substrates are bonded together using thermo-compression bonding.

43. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the first and second substrates are bonded together using eutectic bonding.

44. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein a top surface of the second substrate is polished in preparation for the formation of the counter electrode.

45. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the polishing of the top surface of the second electrode is mechanical polishing.

46. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the counter electrode is formed from at least one metal selected from the group consisting of aluminum, silver, tin, lead, copper, gold, platinum, palladium, nickel, chromium, titanium, and alloys thereof.

47. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the first masking layer is formed from a material selected from the group consisting of silicon dioxide, silicon nitride, and photoresist.

48. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the step of etching the cavity on the front side of the first substrate is performed using an etchant selected from the group consisting of potassium hydroxide (KOH), tetramethyl ammonium hydroxide (TMAH), cesium hydroxide (CsOH), ethylenediamene pyrocatecol (EDP), and deep reactive ion etching (DRIE).

49. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the step of etching the cavity on the back side of the first substrate is performed using an etchant selected from the group consisting of potassium hydroxide (KOH) and isoprophyalcohol (IPA), tetramethyl ammonium hydroxide (TMAH), cesium hydroxide (CsOH), ethylenediamene pyrocatecol (EDP), and deep reactive ion etching (DRIE).

50. The method of forming a micro-mechanical pressure sensor as recited in claim 30, wherein the first substrate is hermetically bonded to the second substrate