WO1990010313A1

WO1990010313A1 - Vibration-isolated mount for gt-cut crystals

Info

Publication number: WO1990010313A1
Application number: PCT/US1990/000604
Authority: WO
Inventors: Robert G. Kinsman; J. Earl Foster; Michael J. Onystok
Original assignee: Motorola Inc
Current assignee: Motorola Solutions Inc
Priority date: 1989-02-27
Filing date: 1990-02-06
Publication date: 1990-09-07
Anticipated expiration: 1991-08-27
Also published as: KR920700481A

Abstract

Vibration dampening caused by a mounting structure attached to a vibration node (15), is reduced by a vibration isolation region (12). The vibration isolation region (12) is a void region (14) cut through the crystal plate (10) and surrounding the area to which the mounting structure is attached. Support members (16) are left intact, bridging the void region (14) and supporting the vibrating plate (10). The support members (16) and void region (14) isolate vibrating surfaces from the mounting structure.

Description

VIBRATION -ISOLATED MOUNT FOR ffT-CTT CRYSTALS

BACKGROUND OF THE INVENTION

This invention relates to a mounting system for piezoelectric quartz crystals. In particular this invention relates to mounting piezoelectric quartz crystals that have non-vibrational nodes, such as GT-cut crystals. A GT-cut crystal maintains a nearly constant natural frequency over a relatively wide ambient temperature variation. The mode of vibration of the GT-cut crystal is longitudinal extension in the plane of the wafer. The crystal plate vibrates with a vibration null or nodal points substantially in the geometric center of the plate. Prior art mounting schemes attach wires to the center of the GT-cut crystal blank.

These prior art crystals were quite large, however, typically vibrating at or near 100 kilohertz or below. At higher frequencies where the crystal plate is smaller wire mounting techniques become increasingly more impractical. Crystal plates intended to vibrate at frequencies over 1 megahertz are too small to accommodate a mounting wire.

U.S. patent number 4,447,753, disclosed a method for mounting a miniature GT-cut quartz resonator. The GT-cut quartz resonator is held between two vibration-isolation structures that suspend the vibrating plate. The vibration-isolation structure, as well as the quartz resonator plate, is etched from a single piece of quartz and can be manufactured to resonate in the 1 to 2 megahertz range. There are some difficulties in manufacturing such devices however. A need exists for alternate ways to mount GT quartz crystal resonators economically without substantially affecting vibration.

SUMMARY OF THE INVENTION

There is provided a GT-cut piezoelectric quartz crystal resonator, which has a piezoelectric quartz crystal plate with planar opposing sides and a substantially non-vibrating node in the geometric center of the plate. The crystal in the preferred embodiment mounts upon a pedestal that attaches to the crystal plate at the node. A vibration isolation region minimizes vibration damping attributable to the mounting pedestal. This region comprises a slot cut through the crystal plate and substantially surrounding the non-vibrating node. Supporting sections of the crystal plate are left in place. The slot effectively decouples the mounting region of the crystal from the remaining area of the crystal plate. The region of the crystal plate outside of the isolation region is free to vibrate at its natural frequency.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 shows a top perspective view of the CT-cut resonator.

Figure 2 shows an elevation view of the GT-cut crystal resonator on a mounting pedestal.

Figure 3 shows, in greater detail, the mounting area and vibration isolation regions.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT Referring to figure 1 there is shown a rectangularly shaped piezoelectric crystal plate (10). The crystal plate (10) has two substantially planar and opposed faces (6) and (8) that resonate at at least one natural frequency when subjected ton external electric signal applied to the two electrodes (23) and (25) deposited on both sides (6) and (8).

A node (15) exists in the approximate geometric center of the crystal plate (10) where vibration is essentially zero. Immediately surrounding this node (15) is a mounting region (12), which surrounds the geometric center of the plate (10). The non-vibrating node (15) of a GT-cut crystal plate (10), is essentially a single point. Regions of the crystal plate distant from the node become increasingly more active with vibration as the distance from the center nodal point increases.

Attaching a mounting pedestal anywhere about the nodal point will dampen crystal vibration. Damping of the vibration of the plate (10) caused by a mounting pedestal attached to region (12) is reduced by inclusion of a void region (14) and support regions (16). In this embodiment there are four, substantially partial annular regions cut completely through the plate (10), leaving four, radially disposed support portions (16) as shown. The void regions (14) extend through the thickness of the plate (10) whereas the support regions (16) support the remainder of the plate (10).

Referring to figure 3, there is shown in greater detail the construction and geometry of the mounting region (12), void regions (14) and support members (16) as used in the preferred embodiment. The extension of the void regions completely through the plate (10) is more clearly seen in this figure.

The radially disposed support regions (16) in combination with the void regions (14), isolate vibration of the plate (10), from region (12). By virtue of the support sections (16) and void regions (14), the outer regions of the plate (10) are relatively free to vibrate at their natural frequency when an excitation signal is applied o the electrodes (23) and (25).

Alternate embodiments of the invention could include fewer or more support members (16). Still other embodiments of the invention might not use partial annuli surrounding the mounting region (12) but would include removing crystalline material about this region (12) in any shape, subject to the limitation that the void regions (14) substantially surround the mounting area (12) with sufficient support members intact to support the crystal (10).

Alternate mounting schemes for the crystal (10) would include support wires attached to both sides of the mounting area (12) and suspending the crystal (10) by means of these support wires. Those skilled in the art will recognize that other support schemes attached to the mounting areas (12) are possible.

Referring to figure 2, the plate (10) is shown mounted on a center mounting pedestal (17). In this view the plate (10) has the electrodes (23) and (25) deposited across the entire planar surfaces (6) and (8) of the plate (10). The mounting pedestal (17) is attached to a substrate (26), which might be a circuit board, ceramic substrate or other appropriate material to which the quartz wafer can be mounted.

An excitation signal is applied to the plate (10) form an external signal source, not shown, by means of a top electrode connection (18) which may be a single wire (19) attached to the top electrode (16) in the mounting region (12). In the preferred embodiment , the bottom electrode (20) is electrically connected to the center mounting pedestal (17) which carries an excitation signal and is electrically conductive. The excitation signal is carried to the pedestal (17) via conductor (20).

The center mounting pedestal (17) as used in the preferred embodiment is substantially columnar and has sufficient height to permit mounting of the quartz wafer plate (10) above the substrate •26) and clearing the maximum height of the conductor (20). The mounting pedestal (17) could be metallic or could also be ceramic coated with a metallic conductor or some other material.

The described mounting of the crystal plate (10) is intended for a GT-cut quartz crystal plate. This mounting could be adapted to other quartz crystal cuts subject to the limitation that the crystal have at least on non-vibrating node to which the mounting pedestal can be attached. Alternate embodiments might include multiple mounting pedestals each attached to a different non-vibrating node providing a stable attachment point for the pedestal.

Claims

WE CLAIM:

1. A quartz crystal resonator comprised of: a quartz crystal thin plate, said plate having first and second, substantially opposed and substantially planar vibrating faces, and at least one, substantially non-vibrating node in said planar faces and a mounting area substantially centered about said non-vibrating node; and a void region through said crystal plate, substantially surrounding said mounting area and located between said mounting area and said vibrating faces, said vacant region extending through said quartz crystal thin plate to said planar faces and being bridged by at least one support section of quartz crystal said vacant region isolating vibration in said vibrating faces from said mounting area.

2. The quartz crystal resonator of claim 1 including means for mounting said quartz crystal thin plate at said mounting area.

3. The quartz crystal resonator of claim 2, wherein said means for mounting said quartz crystal thin plate includes a pedestal attached to said mounting area.

4. The quartz crystal resonator of claim 3 wherein said pedestal is metallic.

5. The quartz crystal resonator of claim 2 wherein said means for mounting said quartz crystal thin plate includes wires attached to said mounting area.

6. The quartz crystal resonator of claim 1 wherein said means for isolating vibration includes a substantially annular shaped void region through said quartz crystal plate surrounding said mounting area, said annular shaped region being located between said mounting area and said vibrating faces and being bridged by at least one quartz crystal support section.

7. The quartz crystal resonator of claim 1 wherein said quartz crystal plate includes electrodes deposited on said faces.

8. The quartz crystal resonator of claim 1 wherein said non-vibrating node is located substantially in the center of said first and second faces.

9. The quartz crystal resonator of claim 1 wherein said quartz crystal plate is a GT-cut quartz crystal plate.

10. In a thin quartz crystal plate, having first and second substantially opposed and substantially planar vibrating faces a substantially non-vibrating stationary node and a mounting area substantially centered about said node, a method of mounting said crystal, said method comprised of the steps of: completely removing crystal material substantially around said mounting area thereby creating a void region in said thin quartz crystal plate between said first and second substantially opposed faces, except for at least one section for supporting said crystal plate extending from said mounting area across said void region to said vibrating faces; attaching a mounting means for supporting said plate to at least first said first face, substantially within said mounting area.

11. The method of claim 10 wherein the step of removing crystal material is comprised of photolithographically defining and chemically etching said crystal.

12. The method of claim 10 wherein the step of removing crystal material comprises removing substantially annular shaped regions of crystal material from around said mounting area, leaving at least on radially oriented crystal support member.

13. The method of claim 10 and further including the step of depositing electrodes on said crystal.