Vacuum encapsulation for surface acoustic wave (SAW) devices

• US 4,213,104 A
• Filed: 09/25/1978
• Issued: 07/15/1980
• Est. Priority Date: 09/25/1978
• Status: Expired due to Term

• Current Assignee: United Technologies Corporation (Raytheon Technologies Corporation d/b/a RTX)
• Original Assignee: United Technologies Corporation (Raytheon Technologies Corporation d/b/a RTX)
• Inventors: Cullen, Donald E., Wagner, Robert A.
• Primary Examiner(s): Smith, Alfred E.
• Assistant Examiner(s): Nussbaum, Marvin
• Application Number:
  US05/945,359
• Time in Patent Office: 659 Days
• Field of Search: 333/150-155, 333/193-196, 310/313, 310/340, 310/341, 310/344, 29/25, 29/35, 174/52 S
• US Class Current: 333/150
• CPC Class Codes: G10K 11/36 Devices for manipulating acoustic surface waves electro-acoustic amplifiers H03F13/00; networks comprising electro-acoustic elements H03H9/00
• CPC Class Codes: H03H 9/1092 the enclosure being defined by a cover cap mounted on an element forming part of the surface acoustic wave [SAW] device on the side of the IDT's

Abstract

A surface acoustic wave (SAW) device having a substrate of desired material is vacuum encapsulated in a structure fabricated from the same material as that comprising the substrate.

Claims

1. A vacuum encapsulated surface acoustic wave (SAW) structure, comprising:

a SAW device including a substrate of desired material having a major surface on which electro-acoustic transducers and associated electrical conductors are disposed along an acoustic wave propagation path on said substrate, said transducers and propagation path defining an active signal region on said major surface, said electrical conductors providing electrical connections for external signals presented to and received from the active signal region; and

a cover, formed of material having substantially the same thermal characteristics and the same crystallographic orientation as the material comprising said substrate, said cover being bonded through a vacuum tight seal to said major surface of said substrate to enclose said active signal region in a vacuum environment.

2. The structure of claim 1, wherein said cover is orientated on said major surface such that the crystallographic axes of said cover material is in alignment with that of said substrate material.

3. The structure of claim 2, wherein said cover is formed of the same material as that comprising said substrate.

4. The structure of claim 1, wherein said cover is bonded to said major surface with a vacuum tight glass seal.

5. The structure of claim 4, wherein said vacuum tight glass seal is a glass frit seal.

6. A vacuum encapsulated surface acoustic wave (SAW) structure comprising:

a SAW device including a substrate of desired material having a major surface on which electro-acoustic transducers and associated electrical conductors are disposed along an acoustic wave propagation path on said substrate, said transducers and propagation path defining the surface area of an active signal region on said major surface, said electrical conductors providing electrical connections for external signals presented to and received from the active signal region; and

a cover, including a spacer assembly portion and a cap assembly portion each formed of material having substantially the same thermal characteristics and crystallographic orientation as the material comprising said substrate material, said spacer portion having a central aperture extending between an upper surface and a bottom surface thereof, said aperture having a minimum cross section area equal to the surface area of said active signal region, said spacer portion being positioned on said major surface to enclose said active signal region within said central aperture, and being bonded along its bottom surface to said major surface with a vacuum tight glass seal, said cap portion being bonded in position to said upper surface of said spacer with a vacuum tight metal seal to enclose said active signal region in a vacuum environment within said central aperture.

7. The structure of claim 6, wherein said spacer and said cap portions are further positioned to provide orientation of the material crystallographic axes of each with that of said substrate material.

8. The structure of claim 7, wherein said spacer and said cap assembly portions are formed of the same material as that comprising said substrate.

9. The structure of claim 6, wherein said transducers and said associated electrical conductors comprise metallized aluminum deposited on said major surface, wherein said electrical conductors are deposited at a greater coating thickness than said transducers along the portion of the conductors extending through said glass seal.

10. In the vacuum encapsulation of a surface acoustic wave (SAW) device having a substrate of desired material, the method of minimizing strain produced in the substrate material by thermal expansion and contraction of a vacuum encapsulating structure, comprising the steps of:

forming the vacuum encapsulating structure from material having substantially the same thermal characteristics and substantially the same crystallographic orientation as that of the substrate material;

positioning the encapsulating structure on the substrate to provide alignment of the crystallographic axes of each; and

bonding the encapsulating structure to the substrate in a vacuum environment.

11. The method of claim 10, wherein the vacuum encapsulating structure is formed from material identical to that of the substrate material.

12. The method of vacuum encapsulating a SAW device of the type which includes a substrate material having a major surface on which electro-acoustic transducers are disposed within an active signal region thereof, and which further includes electrical conductors disposed on said major surface to provide electrical connections for external signals presented to and received from the transducers within the active signal region, comprising the steps of:

forming a cover assembly which includes a spacer portion and a cap portion each fabricated with material having substantially the same thermal characteristics and substantially the same crystallographic orientation as that material comprising the substrate, the spacer being shaped to provide a central aperture extending between an upper surface and a bottom surface thereof and having a minimum cross sectional area equal to that of the active signal region;

orientating the spacer portion over the active signal region on the major surface, and aligning the crystallographic axes of the spacer material with that of the substrate material;

bonding the bottom surface of the spacer portion to the major surface with a vacuum tight glass seal; and

sealing the cap portion to the upper surface of the spacer with a metal seal in a vacuum environment following alignment of the crystallographic axes of the cap with that of the spacer and substrate.

13. The method of claim 12, wherein the spacer and cap portions of the cover assembly are fabricated from the same material as that comprising the substrate.

14. The method of claim 12, wherein the step of bonding is provided by forming the glass seal using a glass frit.

15. The method of claim 12, further comprising between the steps of bonding and sealing, the step of:

baking the SAW device with the bonded spacer, and the cap portion at an accelerated temperature in a vacuum environment to eliminate out-gassing from each.

16. The method of claim 15, further comprising between the steps of bonding and baking, the step of:

cleaning the SAW device with the bonded spacer in an ultra-violet ozone cleaning tank to remove organic impurities.

17. The method of claim 12, further comprising prior to the step of forming, the step of:

depositing the electrical conductors on the major surface such that the coating thickness of the conductors along that portion which passes through the glass seal is at a greater value than that of the transducers.