Dielectrically isolated resonant microsensors

• US 5,417,115 A
• Filed: 07/23/1993
• Issued: 05/23/1995
• Est. Priority Date: 07/23/1993
• Status: Expired due to Term

• Current Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Original Assignee: Honeywell Incorporated (Honeywell International Inc.)
• Inventors: Burns, David W.
• Primary Examiner(s): Chilcot, Jr., Richard E.
• Assistant Examiner(s): Olsen, James M.
• Application Number:
  US08/095,343
• Time in Patent Office: 669 Days
• Field of Search: 73/778, 73/862.59, 73/702, 73/774
• US Class Current: 73/778
• CPC Class Codes: G01L 1/183 by measuring variations of frequency of vibrating piezo-resistive material
• CPC Class Codes: G01L 9/0019 of a semiconductive element
• CPC Class Codes: G01P 15/0802 Details
• CPC Class Codes: G01P 15/097 by vibratory elements
• CPC Class Codes: G01P 2015/0828 the mass being of the paddle type being suspended at one of its longitudinal ends
• CPC Class Codes: H04R 17/025 using a piezoelectric polymer

Abstract

A resonant strain gauge includes a silicon substrate, a polysilicon flexure beam fixed at both ends relative to the substrate, and a polysilicon rigid cover cooperating with the substrate to enclose the flexure beam within a sealed vacuum chamber. An upper bias electrode is formed on the cover, and a lower bias electrode is formed at the bottom of a trough in the substrate directly beneath the flexure beam. A drive electrode and a piezoresistive element are supported by the beam, formed over a silicon nitride thin film layer deposited onto the top surface of the flexure beam. A second silicon nitride layer covers the drive electrode and piezoresistor, cooperating with the first silicon nitride layer to dielectrically encapsulate the drive electrode and piezoresistor. The silicon nitride further extends outwardly of the beam to a location between the polysilicon layer that contains the beam, and the cover, to isolate the cover from the flexure beam. A polysilicon film is applied over the upper silicon nitride layer as a passivation layer to protect the silicon nitride during gauge fabrication. The process for fabricating the gauge includes a sequence of applying the various polysilicon and silicon nitride layers by low pressure chemical vapor deposition, in combination with selective etching to define the flexure beam, electric circuit components and vacuum chamber.

Claims

1. A strain responsive measuring device, including:

a substrate elastically deformable in response to variations in a selected parameter;

an elongate flexure element formed of polysilicon deposited onto and etched from the substrate to define a beam, and having a first region comprising opposite end portions of the flexure element fixed with respect to the substrate to position the flexure element for lengthwise extension and contraction of the flexure element responsive to the elastic deformation of the substrate, said flexure element further including a second region comprising a medial portion of the flexure element free to oscillate relative to the substrate at a natural resonant frequency that varies with said extension and contraction;

a position sensing means including a first electrical circuit component along and integral with the flexure element, for sensing the position of the flexure element relative to the substrate and generating a position signal to indicate said position;

a second electrical circuit component comprising a drive electrode spaced apart from and electrically isolated from the first circuit component; and

a dielectric encapsulation means surrounding and dielectrically isolating said first and second electrical circuit components, comprising a first dielectric thin film layer between the beam and the first and second electric circuit components and a second dielectric thin film layer cooperating with the first dielectric thin film layer to surround each of the first and second electrical circuit components; and

further comprising a passivation layer covering the second dielectric layer, with the passivation layer and the electric circuit components being on opposite sides of the second dielectric layer.

2. An apparatus for sensing variations in strain, including:

a substrate, and a first bias electrode fixed with respect to a substrate surface portion of the substrate;

a flexure element formed of polysilicon deposited onto and etched from the substrate to define a beam elongated in a longitudinal direction, having a first region comprising opposite ends of the flexure element fixed with respect to the substrate and a second region comprising a medial portion of the flexure element free to oscillate at a resonant frequency, said resonant frequency varying with changes in strain due to external forces acting upon the flexure element, said flexure element being transversely spaced apart from the first bias electrode;

a substantially rigid cover fixed with respect to the substrate and having a cover surface portion transversely spaced apart from the flexure element and disposed on the opposite side of the flexure element from the substrate surface portion, and a second bias electrode fixed with respect to the cover at a cover surface portion;

a biasing means for biasing the first bias electrode and the second bias electrode at respective different voltage levels, to generate a substantially constant electrical field in the region about the flexure element;

a position sensing means including a first electrical circuit component along and integral with the flexure element, for sensing the position of the flexure element relative to the substrate and generating a position signal to indicate said position;

a second electrical circuit component comprising a drive electrode along and integral with the flexure element;

a drive means for generating a periodically varying drive signal and providing the drive signal to one of the drive electrode, the first bias electrode and the second bias electrode, thus to drive the flexure element in a periodic mechanical oscillation relative to the substrate, said drive means receiving the position signal and controllably altering the frequency of said mechanical oscillation toward coincidence with the natural resonant frequency, in response to changes in the position signal frequency; and

a dielectric encapsulation means, surrounding and dielectrically isolating the first and second electrical circuit components and comprising a first dielectric thin film layer between the beam and the electric circuit components, and a second dielectric thin film layer cooperating with the first dielectric layer to surround each of the electric circuit components wherein the encapsulation means is formed between the cover and the beam, to dielectrically isolate the cover from the beam and from the substrate; and

,

a passivation layer covering the second dielectric layer, wherein the passivation layer and the electrical components are on opposite sides of the second dielectric layer.

3. A process for fabricating a strain responsive electrostatically oscillated resonant beam sensor, including the steps of:

treating a semiconductor substrate by local oxidation to form a trough and an etch channel of a sacrificial material in the semiconductor substrate, with the trough and channel being open to a substantially planar surface of the substrate;

depositing semiconductor material over the semiconductor substrate and first sacrificial layer to form a first semiconductor layer;

depositing dielectric material over the first semiconductor layer to form a first dielectric thin film layer;

depositing semiconductor material over the first dielectric thin film layer to form a second semiconductor layer, and doping the second semiconductor layer to enhance its electrical conductivity;

defining at least one electric circuit element in the second semiconductor layer by selectively etching the second semiconductor layer;

depositing dielectric material over the electric circuit elements and the first dielectric thin film layer to form a second dielectric thin film layer that cooperates with the first dielectric thin film layer to dielectrically encapsulate the at least one electrical circuit element;

defining a resonant flexure beam, and selectively etching the first and second dielectric thin film layers, and the first semiconductor layer to form the resonant beam as fixed at at least on end thereof with respect to the semiconductor substrate and having a region supporting the at least one electrical circuit element, said selective etching further removing the semiconductor material from about the etch channel;

depositing sacrificial material over the second dielectric thin film layer to form a second sacrificial layer, and selectively etching the second sacrificial layer to define an upper cavity volume;

depositing semiconductor material over the second sacrificial layer, to form a third semiconductor layer, and selectively etching the third semiconductor layer to form a cover; and

removing the first and second sacrificial layers by etching, and sealing the etch channel, whereby the cover and the substrate cooperate to form an enclosed chamber containing the beam, and the beam is free to oscillate relative to the semiconductor substrate and the cover.

4. The process of claim 3 including the further step of:

depositing the dielectric material over the planar surface of the semiconductor substrate and the first sacrificial layer, before the step of forming the first semiconductor layer, to form a third dielectric thin film layer between the substrate and the first semiconductor layer.

5. The process of claim 4 including the step of:

depositing at least one of the first and second dielectric layers between the first semiconductor layer and the third semiconductor layer to isolate the cover from the first semiconductor layer.

6. The process of claim 3 further including the step of:

depositing a semiconductor material over the second dielectric thin film layer, after the step of depositing the second dielectric thin film layer, to form a passivation layer over the second dielectric thin film layer.

7. The process of claim 6 wherein:

the passivation layer is formed to a thickness and a range of 400-2000 angstroms.

8. The process of claim 3 including the further step of:

after the step of forming the trough and etch channel and before filling the trough and etch channel with the sacrificial material, defining a first bias electrode in the trough and forming the first bias electrode by ion implantation.

9. The process of claim 8 including the further step of:

after the step of etching the third semiconductor layer to form the cover, defining a second bias electrode in the cover and forming the second bias electrode by ion implantation.

10. The process of claim 3 wherein:

the first and second dielectric thin film layers are each formed to a thickness of about 800 angstroms, and the second semiconductor layer is formed to a thickness of about 4400 angstroms.

11. The process of claim 3 wherein:

the first and second dielectric thin film layers are each formed to a thickness of about 400 angstroms, and the second semiconductor layer is formed to a thickness of about 4400 angstroms.