Metal working – Piezoelectric device making
Reexamination Certificate
1999-07-29
2002-02-26
Hall, Carl E. (Department: 3729)
Metal working
Piezoelectric device making
C427S100000
Reexamination Certificate
active
06349454
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of The Invention
This invention relates to thin film resonators (TFRs) and a method of making TFRs.
2. Description of Related Art
TFRs are thin film acoustic devices which can resonate in the radio frequency to microwave range, for example, 0.5 to 5 Gigahertz (GHz), in response to an electrical signal.
FIG. 1
shows a typical TFR
10
with a piezoelectric film
12
between a first electrode
14
and a second electrode
16
which apply an electric field to the piezoelectric film
12
. The film
12
is made of a piezoelectric crystalline material, such as zinc oxide, aluminum nitride (AlN) and other piezoelectric crystalline material, which exhibits a piezoelectric effect. The piezoelectric effect occurs when the piezoelectric material expands or contracts in response to an electric field applied across the piezoelectric material, for example by the first and second electrodes
14
and
16
, or produces an electric field in response to mechanical stress or strain applied to the piezoelectric material. If the electric field across the film
12
is an alternating electric field having frequency components corresponding to resonant frequencies (e.g., a fundamental frequency and harmonics), the fundamental frequency of which is defined for a film of uniform thickness as the acoustic velocity (v) in the film
12
divided by two (2) times the thickness (t) of the film or f
r
=v/2t. The film
12
will mechanically vibrate at the resonant frequencies which in turn produces an alternating electric field at the resonant frequencies.
The first and second electrodes
14
and
16
are typically of metal, such as aluminum. The acoustic impedance mismatch between the first electrode
14
and the air creates a first acoustic reflecting surface
18
at the interface between the top surface of the first electrode
14
and the air. A second acoustic reflecting surface
22
can be established at an interface between the second electrode
16
and a substrate
24
(or air if a portion of the substrate
24
under the film
12
is removed). Alternatively, acoustic reflecting layer(s) can be created between the second electrode
16
and the substrate
24
to suppress unwanted frequencies, such as harmonics of the fundamental frequency. The acoustic reflecting layer(s) can be formed from a material having desired characteristic acoustic impedance(s) and with the proper dimensions to provide desired reflection characteristics for the second reflecting surface at the interface between the second electrode
14
and the acoustic reflecting layers. As such, the acoustic reflecting layers can reflect desired frequencies while suppressing unwanted frequencies. An acoustic cavity created between the first and second reflecting surfaces and with the proper dimensions establishes a standing wave at the resonant frequencies of the piezoelectric film
12
. The dimensions of the acoustic cavity, for example the thickness of the piezoelectric film
12
and the electrodes
14
and
16
, define the operating frequencies for the TFR
10
. Energy outside the operating frequencies of the TFR
10
is lost, while energy within the operating frequencies is preserved.
The TFR
10
structure can be formed on the substrate
24
, such as a silicon (Si), Gallium Arsenide (GaAs) or other semiconductor substrate, for monolithic integration purposes, such as integration with active semiconductor devices. For discrete applications, the TFR
10
is typically formed on other suitable substrates, such as quartz, sapphire, aluminum nitride (AlN), or silicon carbide. If the TFR
10
has acoustic reflecting layer(s), the acoustic reflecting layer(s) are formed on the substrate
24
followed by the second electrode
16
which is formed on the reflecting layer(s). If there are no acoustic reflecting layers, then the second electrode
16
is formed on the substrate
24
, for example using chemical vapor deposition (CVD) or sputtering. See, Kern & Vossen, “Thin Film Processes,” Vols. I and II, Wiley & Sons. The piezoelectric film
12
is then formed on the second electrode
16
, and the first electrode
14
is formed on top of the piezoelectric film
12
, for example using chemical vapor deposition (CVD) or sputtering. To improve the performance of the TFR
10
, a portion of the substrate
24
is removed from under the second reflecting surface
22
. To remove the portion of the substrate
24
, the substrate
24
includes an etch stop
28
, such as a boron doped p+ layer implanted in a silicon (Si) substrate, at the upper surface of the substrate
24
adjacent to the bottom of the second electrode
16
. The etch stop
28
is used to protect the second electrode
16
from a chemical etch removing the portion
30
of the substrate
24
.
By growing the piezoelectric film
12
on the second electrode, the resulting piezoelectric film
12
is polycrystalline in that distinct crystals having different lattice orientations are present throughout the piezoelectric film
12
. Such a non-uniform or irregular crystalline structure with grain boundaries between the differently oriented crystallites or crystal grains reduces the quality of the piezoelectric film
12
.
Two figures of merit are used to measure the quality of piezoelectric films: a quality factor Q and an electro-mechanical coupling coefficient. The quality factor Q for a TFR is a measure of the resonance quality of the acoustic cavity while the coupling coefficient is a measure of the efficiency of conversion between electrical and mechanical energy within the acoustic cavity. Both of these figures of merit are inversely proportional to the acoustic loss introduced by the TFR at the operating frequency band. If the piezoelectric film
10
has a polycrystalline structure with grain boundaries and other defects, such as point imperfections or dislocations in the crystal lattice, or poor reflectivity of the reflecting surfaces
18
and
22
for example due to surface roughness, acoustic losses can result from acoustic scattering within the film
12
and acoustic radiation into the surrounding areas of the device
10
. Thus, if the film
12
is polycrystalline, acoustic losses will be introduced by the film
12
, thereby producing a lower quality TPR.
TFRs can be used at radio frequency (RF) because piezoelectric films can be made thin, for example at higher frequencies, such as 0.5-10 GHz, the piezoelectric film
12
can be between 0.4 and 8 microns in width. Because TFRs produce an alternating electric field at the resonant frequency in response to an alternating electric field having frequency components corresponding to the resonant frequencies, TFRs can be used as radio frequency (RF) filter elements. TFR filters have a distinct size advantage over conventional RF filters, such as those based on ceramics. For example, thin film resonators can have volumes of 1.5 cubic millimeters while ceramic resonators are typically not less than hundreds of cubic millimeters in volume. At the same time, a ceramic element typically introduces more loss to the input signal at the operating frequency band than the TFR. TFR also have higher power handling capabilities than surface acoustic wave (SAW) devices, for example 200 milliwatts vs. 2 watts. As mentioned above, however, TFRs can introduce losses to an electrical signal applied to the TFR in part due to the polycrystalline structure of the film
12
. Typical TFR fabricating methods produce piezoelectric films with on the order of 10
8
distinct crystalline orientations separated by grain boundaries.
Thus, a need exists for a high quality TFR which introduces low loss to the electrical signal applied to the TFR.
SUMMARY OF THE INVENTION
The present invention involves a thin film resonator (TFR) produced with an improved piezoelectric film which is epitaxially grown on a growing surface, resulting in a piezoelectric film with less grain boundaries. Epitaxial growth refers to the piezoelectric film having a crystallographic orientation taken from or emulating the crystallographic orientation of a single cr
Manfra Michael James
Pfeiffer Loren Neil
West Kenneth William
Wong Yiu-Huen
Agere Systems Guardian Corp.
Hall Carl E.
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