Polysilicon resonating beam transducers

• US 5,090,254 A
• Filed: 04/11/1990
• Issued: 02/25/1992
• Est. Priority Date: 04/11/1990
• Status: Expired due to Term

• Current Assignee: Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Original Assignee: Wisconsin Alumni Research Foundation (University of Wisconsin System)
• Inventors: Sniegowski, Jeffry, Guckel, Henry
• Primary Examiner(s): Ruehl, Charles A.
• Application Number:
  US07/508,001
• Time in Patent Office: 685 Days
• Field of Search: 73/862.59, 73/517 AV, 73/778, 73/702, 73/704, 73/653
• US Class Current: 73/862.59
• CPC Class Codes: G01L 1/183 by measuring variations of frequency of vibrating piezo-resistive material
• CPC Class Codes: G01L 1/186 optical excitation or measuring of vibrations
• CPC Class Codes: G01L 9/0019 of a semiconductive element
• CPC Class Codes: G01L 9/002 Optical excitation or measuring
• CPC Class Codes: H03H 2009/02511 Vertical, i.e. perpendicular to the substrate plane
• CPC Class Codes: H03H 3/0072 of microelectro-mechanical resonators or networks micromembranes or microbeams B81B2203/01; manufacture of microstructural devices in general B81C
• CPC Class Codes: H03H 9/02259 Driving or detection means

Abstract

Force transducers are formed of a beam of polysilicon which is mounted at its ends to a silicon substrate and is encapsulated within a polysilicon shell which defines, with the substrate, a cavity around the resonating beam. The cavity is sealed off from the atmosphere and evacuated to maximize the Q of the resonating beam. The beam is produced by deposition of polysilicon in such a way that, combined with subsequent annealing steps, the beam is in zero or low tensile strain. Resonant excitation of the beam may be accomplished in various ways, including capacitive excitation, and the vibratory motion of the beam may be detected utilizing an implanted resistor which is piezoresistive. Formation of the beam is carried out by depositing the beam on a sacrificial layer and surrounding it in a second sacrificial layer before the encapsulating polysilicon shell is formed. The sacrificial layers are etched out with liquid etchant which passes through channels in the periphery of the shell. Following etching, the interior of the cavity surrounding the beam is maintained in a wash liquid so that the beam is not deflected toward any of the adjacent surfaces, and the wash liquid is removed by freezing and sublimation. The interior surfaces of the cavity and the outer surfaces of the beam are passivated and the channels leading into the cavity may be sealed by oxidation in an oxidizing atmosphere, which also results in consumption of oxygen within the cavity.

Claims

1. A resonant beam type force transducer comprising:

(a) a substrate;

(b) a polysilicon beam firmly mounted to the substrate at its ends and spaced from the substrate between its ends to allow free vibration of the beam, the polysilicon beam comprising polysilicon treated to have built-in zero or low tensile strain;

(c) an encapsulating shell formed of polysilicon deposited by chemical vapor-deposition surrounding the beam and affixed to the substrate as deposited to define a cavity with the substrate enclosing the beam which is sealed off from the surrounding atmosphere, and wherein the interior of the cavity is substantially evacuated;

(d) means for exciting mechanical vibration in the beam including a conducting electrode formed on the beam and another closely spaced plate electrode such that there is capacitive coupling between the two electrodes; and

(e) means for providing an electrical signal corresponding to mechanical vibration in the beam, the frequency of vibration of the beam being affected by external forces applied to the substrate to strain the substrate and hence being applied to the beam.

2. The transducer of claim 1 wherein the polysilicon beam has a layer of material deposited thereon selected from the group consisting of silicon and silicon dioxide.

3. The transducer of claim 1 wherein the polysilicon beam is approximately 200 microns long and is approximately 2 microns thick.

4. The transducer of claim 1 wherein the spacing between the shell and the beam and between the substrate and the beam is about two microns of less.

5. The transducer of claim 1 wherein the encapsulating shell has a layer of silicon nitride deposited on the outer surface thereof.

6. The transducer of claim 1 wherein a well is formed in the substrate under the beam.

7. The transducer of claim 1 wherein the dimensions of and the built-in strain in the beam are selected such that the resonant frequency of the beam with no force applied to it is at least 400 KHz.

8. The transducer of claim 1 wherein the substrate comprises single crystal silicon.

9. A resonant beam type force transducer comprising:

(a) a substrate;

(b) a polysilicon beam firmly mounted to the substrate at its ends and spaced from the substrate between its ends to allow free vibration of the beam, the polysilicon beam comprising polysilicon treated to have built-in zero or low tensile strain;

(c) an encapsulating shell of polysilicon surrounding the beam and affixed to the substrate to define a cavity enclosing the beam which is sealed off from the surrounding atmosphere, and wherein the interior of the cavity is substantially evacuated;

(d) means for exciting mechanical vibration in the beam including a conducting electrode formed on the beam and another closely spaced plate electrode such that there is capacitive coupling between the two electrodes, wherein the electrode on the beam in the means for exciting mechanical vibration in the beam is formed in the polysilicon beam by implantation of a dopant in a pattern in the surface of the polysilicon beam which is connected to a conductive pad at a position outside the beam; and

(e) means for providing an electrical signal corresponding to mechanical vibration in the beam, the frequency of vibration of the beam being affected by external forces applied to the substrate to strain the substrate and hence being applied to the beam.

10. A resonant beam type force transducer comprising:

(a) a substrate;

(b) a polysilicon beam firmly mounted to the substrate at its ends and spaced from the substrate between its ends to allow free vibration of the beam, the polysilicon beam comprising polysilicon treated to have built-in zero or low tensile strain;

(c) an encapsulating shell of polysilicon surrounding the beam and affixed to the substrate to define a cavity enclosing the beam which is sealed off from the surrounding atmosphere, and wherein the interior of the cavity is substantially evacuated;

(d) means for exciting mechanical vibration in the beam including a conducting electrode formed on the beam and another closely spaced plate electrode such that there is capacitive coupling between the two electrodes; and

(e) means for providing an electrical signal corresponding to mechanical vibration in the beam, the frequency of vibration of the beam being affected by external forces applied to the beam, wherein the means for providing an electrical signal corresponding to the vibration in the beam includes an electrode formed in the polysilicon beam by implantation of a dopant in a pattern in the surface of the polysilicon beam to form a piezoresistive conductor in the beam which is connected to a conductive pad at a position outside the beam.

11. A resonant beam transducer structure comprising:

(a) a substrate;

(b) a polysilicon beam mounted to the substrate at at least one of its ends and spaced from the substrate between its ends to allow free vibration of the beam, the polysilicon beam comprising substantially undoped polysilicon treated to have built-in zero or low tensile strain;

(c) an encapsulating shell formed of polysilicon deposited by chemical vapor deposition surrounding the beam and affixed to the substrate as deposited to define a cavity with the substrate which surrounds the beam and which is sealed off from the atmosphere, the cavity being substantially evacuated.

12. The transducer structure of claim 11 wherein the interior surfaces of the shell and substrate defining the cavity and the outer surface of the beam having a layer of silicon dioxide formed thereon.

13. The transducer structure of claim 11 wherein the beam has a length of about 200 microns and a thickness of about 2 microns.

14. The transducer structure of claim 11 wherein the spacing between the shell and the beam and between the substrate and the beam is about two microns or less.

15. The transducer structure of claim 11 wherein the substrate comprises single crystal silicon.

16. The transducer structure of claim 11 wherein the substrate includes a well formed therein under the free portion of the beam.

17. The transducer structure of claim 11 wherein the beam is mounted at both of its ends to the substrate to suspend the beam above the substrate.

18. A resonant beam transducer comprising:

(a) a substrate;

(b) a polysilicon beam mounted to the substrate at at least one of its ends and spaced from the substrate between its ends to allow free vibration of the beam, the polysilicon beam comprising substantially undoped polysilicon treated to have built-in zero or low tensile strain;

(c) an encapsulating shell of polysilicon surrounding the beam and affixed to the substrate to define a cavity with the substrate which surrounds the beam and which is sealed off from the atmosphere, the cavity being substantially evacuated; and

(d) a conducting electrode formed on the polysilicon beam to which voltage can be provided to excite vibration of the beam, and a piezoresistive electrode formed on the polysilicon beam through which and electrical current can be passed to provide an electrical signal corresponding to the vibration of the beam.

19. The transducer structure of claim 18 wherein the conductive electrode for exciting the beam is formed by implantation of a dopant in the surface of the polysilicon beam in a pattern, and further including a conductive plate formed on the polysilicon shell above the corresponding excitation electrode on the beam so that excitation of the beam can be obtained by providing an alternating current between the plate on the polysilicon shell and the underlying electrode on the beam.

20. A resonant beam transducer structure comprising:

(a) a substrate;

(b) a polysilicon beam mounted to the substrate at at least one of its ends and spaced from the substrate between its ends to allow free vibration of the beam, the polysilicon beam comprising substantially undoped polysilicon treated to have built-in zero or low tensile strain;

(c) an encapsulating shell of polysilicon surrounding the beam and affixed to the substrate to define a cavity with the substrate which surrounding the beam and which is sealed off from the atmosphere, the cavity being substantially evacuated, wherein the polysilicon shell has peripheral portions bonded to the substrate with channels extending through the peripheral portions from the cavity to the perimeter of the polysilicon shell and further including material selected from the group consisting of silicon dioxide, polysilicon, silicon nitride, and combinations thereof formed in the channels sealing off the channels so that atmospheric gases cannot enter or exit the cavity through the channels.

21. A resonant beam structure comprising:

(a) a substrate;

(b) a polysilicon beam mounted to the substrate at at least one of its ends and spaced from the substrate between its ends to allow free vibration of the beam, the polysilicon beam comprising substantially undoped polysilicon treated to have built-in zero or low tensile strain, wherein the beam includes a lower layer of polysilicon, an upper layer of polysilicon, and a layer of silicon nitride between the two layers of polysilicon to electrically isolate the lower layer from the upper layer;

(c) an encapsulating shell of polysilicon surrounding the beam and affixed to the substrate to define a cavity with the substrate which surrounds the beam and which is sealed off from the atmosphere, the cavity being substantially evacuated.

22. A resonant beam transducer comprising:

(a) a substrate;

(b) a polysilicon beam mounted to the substrate at at least one of its ends and spaced from the substrate between its ends to allow free vibration of the beam;

(c) an encapsulating shell of polysilicon surrounding the beam and affixed to the substrate to define a cavity with the substrate which surrounds the beam and which is sealed off from the atmosphere, the cavity being substantially evacuated;

(d) means for exciting mechanical vibration in the beam; and

(e) means for projecting light through the polysilicon shell onto the beam and means for receiving the projected light reflected from the beam passed back through the polysilicon shell and for converting the received light into an electrical signal corresponding to the received light which is indicative of the vibration of the beam.

23. The transducer of claim 22 wherein the means for projecting light includes a light source and an optical fiber to direct light from the light source to the polysilicon shell.

24. The transducer of claim 22 wherein the means for receiving the light reflected from the beam includes an optical fiber to direct light reflected from the beam to a remote position and a photodetector for detecting the light at the remote position.

25. The transducer of claim 22 wherein the means for exciting mechanical vibration in the beam includes a conducting electrode formed on the polysilicon beam to which voltage can be provided and another closely spaced plate electrode such that there is capacitive coupling between the two electrodes.

26. The transducer of claim 22 wherein the interior surfaces of the shell and substrate defining the cavity and the outer surface of the beam have a layer of silicon dioxide formed thereon.

27. The transducer of claim 22 wherein the beam has a length of about 200 microns and a thickness of about 2 microns.

28. The transducer of claim 22 wherein the beam is mounted at both of its ends to the substrate to suspend the beam above the substrate.

29. The transducer of claim 22 wherein the means for projecting light includes a laser providing a substantially monochromatic light output.

30. A resonant beam transducer comprising:

(a) a substrate;

(b) a polysilicon beam mounted to the substrate at at least one of its ends and spaced from the substrate between its ends to allow free vibration of the beam;

(c) an encapsulating shell of polysilicon surrounding the beam and affixed to the substrate to define a cavity with the substrate which surrounds the beam and which is sealed off from the atmosphere, the cavity being substantially evacuated;

(d) means for exciting mechanical vibration in the beam including a conducting electrode formed on the polysilicon beam to which voltage can be provided and another closely spaced plate electrode such that there is capacitive coupling between the two electrodes, and wherein the conductive electrode for exciting the beam is formed by implantation of a dopant in the surface of the polysilicon beam in a pattern, and further including a conductive plate formed on the polysilicon shell above the corresponding excitation electrode on the beam so that the excitation of the beam can be obtained by providing an alternating current between the plate on the polysilicon shell and the underlying electrode on the beam; and

(e) means for projecting light through the polysilicon shell onto the beam and means for receiving projected light reflected from the beam and for converting the received light into an electrical signal corresponding to the received light which is indicative of the vibration of the beam.

31. A method of detecting the vibrations of a microbeam encapsulated within a shell which will pass light of selected wavelengths, comprising the steps of:

(a) directing substantially monochromatic light through the encapsulating shell onto the vibrating beam, the spacing between the shell and the beam being close to the wavelength of the light such that interference patterns are formed as the beam vibrates;

(b) detecting light reflected from the beam in the interference pattern and providing an electrical signal corresponding thereto which is indicative of vibrations of the beam.

32. A resonant beam type force transducer comprising:

(a) a substrate;

(b) a polysilicon beam firmly mounted to the substrate at its ends and spaced from the substrate between its ends to allow free vibration of the beam, the polysilicon beam comprising substantially undoped polysilicon treated to have built-in low tensile strain selected to provide a resonant frequency of the beam which is greater that the resonant frequency of the beam with zero built-in strain;

(c) an encapsulating shell of polysilicon surrounding the beam and affixed to the substrate to define a cavity enclosing the beam which is sealed off from the surrounding atmosphere, and wherein the interior of the cavity is substantially evacuated;

(d) means for exciting mechanical vibration in the beam including a conducting electrode formed on the beam and another closely spaced plate electrode such that there is capacitive coupling between the two electrodes; and

(e) means for providing an electrical signal corresponding to mechanical vibration in the beam, the frequency of vibration of the beam being affected by external forces applied to the substrate to strain the substrate and hence being applied to the beam.

33. The transducer of claim 32 wherein the polysilicon beam has a layer of material deposited thereon selected from the group consisting of silicon nitride and silicon dioxide.

34. The transducer of claim 32 wherein the dimensions of and the built-in strain in the beam are selected such that the resonant frequency of the beam with not force applied to it is at least 400 kHz.

35. The transducer of claim 32 wherein the substrate comprises single crystal silicon.

36. A method of detecting the vibrations of a beam encapsulated within a shell formed of silicon comprising the steps of:

(a) projecting deep red or infrared light through the encapsulating silicon shell onto the vibrating beam from a position outside the shell;

(b) reflecting the light from the beam back through the silicon shell;

(c) detecting the light reflected from the beam and passed through the silicon shell and providing an electrical signal corresponding thereto which is indicative of vibrations of the beam.

37. The method of claim 36 wherein the shell in which the beam is encapsulated is formed of polysilicon.

38. A resonant beam transducer comprising:

(a) a substrate;

(b) a beam mounted to the substrate at at least one of its ends and spaced from the substrate between its ends to allow free vibration of the beam;

(c) an encapsulating shell of silicon surrounding the beam and affixed to the substrate to define a cavity with the substrate which surrounds the beam and which is sealed off from the atmosphere, the cavity being substantially evacuated;

(d) means for exciting mechanical vibration in the beam;

(e) means for projecting deep red or infrared light through the silicon shell onto the beam and means for receiving the projected light reflected from the beam passed back through the silicon shell and for converting the received light into an electrical signal corresponding to the received light which is indicative of the vibration of the beam.

39. The transducer of claim 38 wherein the encapsulating shell is formed of polysilicon.

40. A resonant beam type force transducer comprising:

(a) a substrate;

(b) a beam firmly mounted to the substrate at at least one of its ends and spaced from the substrate between its ends to allow free vibration of the beam;

(c) means for exciting mechanical vibration in the beam including;

a conducting electrode formed on the beam and another closely spaced plate electrode such that there is capacitive coupling between the two electrodes, means for providing a photovoltaic voltage in response to light which is electrically connected to the conducting electrode formed on the beam and the plate electrode to apply voltage thereto, and a light source providing modulated light to the means for providing a photovoltaic voltage; and

(d) means for providing an electrical signal corresponding to mechanical vibration in the beam.

41. The transducer of claim 40 wherein the means for providing photovoltaic voltage comprises a photodiode.

42. The transducer of claim 40 wherein the light source includes an LED and a function generator connected to drive the LED at the resonant frequency of the beam.

43. The transducer of claim 40 including an encapsulating shell surrounding the beam and affixed to the substrate to define a cavity with the substrate which is sealed off from the surrounding atmosphere, and wherein the interior of the cavity is substantially evacuated.