Resonator with seed layer

Details Resonator with seed layer Resonator with seed layer

2002-07-30

2004-12-07

Dougherty, Thomas M. (Department: 2834)

Electrical generator or motor structure

Non-dynamoelectric

Piezoelectric elements and devices

C310S311000, C310S364000, C029S025350, C333S187000

Reexamination Certificate

active

06828713

ABSTRACT:

BACKGROUND
The present invention relates to acoustic resonators, and more particularly, to resonators that may be used as filters for electronic circuits.
The need to reduce the cost and size of electronic equipment has led to a continuing need for ever-smaller electronic filter elements. Consumer electronics such as cellular telephones and miniature radios place severe limitations on both the size and cost of the components contained therein. Further, many such devices utilize electronic filters that must be tuned to precise frequencies. Filters select those frequency components of electrical signals that lie within a desired frequency range to pass while eliminating or attenuating those frequency components that lie outside the desired frequency range.
One class of electronic filters that has the potential for meeting these needs is constructed from thin film bulk acoustic resonators (FBARs). These devices use bulk longitudinal acoustic waves in thin film piezoelectric (PZ) material. In one simple configuration, a layer of PZ material is sandwiched between two metal electrodes. The sandwich structure is preferably suspended in air. A sample configuration of an apparatus
10
having a resonator
12
(for example, an FBAR) is illustrated in
FIGS. 1A and 1B
.
FIG. 1A
illustrates a top view of the apparatus
10
while
FIG. 1B
illustrates a side view of the apparatus
10
along line A—A of FIG.
1
A. The resonator
12
is fabricated above a substrate
14
. Deposited and etched on the substrate
14
are, in order, a bottom electrode layer
15
, piezoelectric layer
17
, and a top electrode layer
19
. Portions (as indicated by brackets
12
) of these layers—
15
,
17
, and
19
—that overlap and are fabricated over a cavity
22
constitute the resonator
12
. These portions are referred to as a bottom electrode
16
, piezoelectric portion
18
, and a top electrode
20
. In the resonator
12
, the bottom electrode
16
and the top electrode
20
sandwiches the PZ portion
18
. The electrodes
14
and
20
are conductors while the PZ portion
18
is typically crystal such as Aluminum Nitride (AlN).
When an electric field is applied between the metal electrodes
16
and
20
, the PZ portion
18
converts some of the electrical energy into mechanical energy in the form of mechanical waves. The mechanical waves propagate in the same direction as the electric field and reflect off of the electrode/air interface.
At a resonant frequency, the resonator
12
acts as an electronic resonator. The resonant frequency is the frequency for which the half wavelength of the mechanical waves propagating in the device is determined by many factors including the total thickness of the resonator
12
for a given phase velocity of the mechanical wave in the material. Since the velocity of the mechanical wave is four orders of magnitude smaller than the velocity of light, the resulting resonator can be quite compact. Resonators for applications in the GHz range may be constructed with physical dimensions on the order of less than 100 microns in lateral extent and a few microns in total thickness. In implementation, for example, the resonator
12
is fabricated using known semiconductor fabrication processes and is combined with electronic components and other resonators to form electronic filters for electrical signals.
The use and the fabrication technologies for various designs of FBARs for electronic filters are known in the art and a number of patents have been granted. For example, U.S. Pat. No. 6,262,637 granted to Paul D. Bradley et al. discloses a duplexer incorporating thin-film bulk acoustic resonators (FBARs). Various methods for fabricating FBARs also have been patented, for example, U.S. Pat. No. 6,060,818 granted to Richard C. Ruby et al. discloses various structures and methods of fabricating resonators, and U.S. Pat. No. 6,239,536 granted to Kenneth M. Lakin discloses method for fabricating enclosed thin-film resonators.
However, the continuing drive to increase the quality and reliability of the FBARs presents challenges requiring even better resonator quality, designs, and methods of fabrication. For example, one such challenge is to eliminate or alleviate susceptibility of the FBARs from damages from electrostatic discharges and voltage spikes from surrounding circuits. Another challenge is to eliminate or alleviate susceptibility of the resonator from frequency drifts due to interaction with its environment such as air or moisture.
SUMMARY
These and other technological challenges are met by the present invention. According to one aspect of the present invention, a resonator fabricated on a substrate has a seed layer on which a bottom electrode, piezoelectric portion, and a top electrode are fabricated. The seed layer allows the piezoelectric portion to be fabricated even at higher quality.
According to another aspect of the present invention, an electronic filter has a resonator fabricated on a substrate. The resonator includes a seed layer portion made of Aluminum Nitride having a thickness ranging from about 10 Angstroms (one nanometer) to about 10,000 Angstroms (one micron). In experiments, the seed layer having thickness within a range from 100 Angstroms to 400 Angstroms have been successfully implemented. The resonator further includes a bottom electrode on the seed layer portion, the bottom layer made of Molybdenum. Piezoelectric portion is on the bottom electrode, the piezoelectric portion being made of Aluminum Nitride. Finally, a top electrode is on the piezoelectric portion, the top electrode made of Molybdenum.
According to yet another aspect of the present invention, a method of fabricating a resonator is disclosed. First, a seed layer is fabricated on a substrate. Next, a bottom electrode is fabricated on the seed layer. Then, piezoelectric portion is fabricated on the bottom electrode. Finally, a top electrode is fabricated on the piezoelectric portion.
Other aspects and advantages of the present invention will become apparent from the following detailed description, taken in combination with the accompanying drawings, illustrating by way of example the principles of the invention.


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patent: 2003/0112095 (2003-06-01), Wang et al.
Beck et al., Microwave Filters and Switches Produced using Micro-Machining Techniques. No known date.
Lakin et al., Thin Film Bulk Acoustic Wave esonator and Filter Technology, Aug. 21, 2001.
Lakin et al., Improved Bulk Wave Resonator Coupling Coefficient For Wide Bandwidth Filters, IEEE Ultrasonic Symposium Paper 3E-5, Oct. 9, 2001.
Larson III, et al.A BAW Antenna Duplexer for the 1900 MHz PCS Band, undated.
Telschow et al., UHF Acoustic Microsopic Imaging of Resonator Motion, IEEE-Ultrasonics Symposium, Oct. 22-25, 2000, paper 31-3.
Nguyen, Clark T.C., Microelectromechanical Devices for Wirel ss Communications, Proce dings, 1998 IEEE International Micro Electro Mechanical System Workshop, Jan. 25-29, 1998, pp. 1-7.

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