Thin film voltage-tuned semiconductor bulk acoustic resonator (SBAR)

Disclosed is a thin film voltage-tuned semiconductor bulk acoustic resonator (SBAR). A piezoelectric film is positioned between a first electrode and a second electrode and positioned adjacent a semiconductor substrate containing a via hole. A variable voltage source applies a DC bias voltage to the electrodes which causes an electric field to be created between the electrodes within the piezoelectric film. The resulting electric field causes the piezoelectric film to resonate at a frequency different than its unbiased resonant frequency. By adjusting the variable voltage source to change the DC bias voltage, the resonant frequency from the thin film semiconductor bulk acoustic resonator (SBAR) can be varied.

BACKGROUND OF THE INVENTION 
1. Field of the Invention 
This invention relates generally to a semiconductor bulk acoustic resonator 
(SBAR) and, more particularly, to a thin film voltage-tuned semiconductor 
bulk acoustic resonator. 
2. Discussion of the Related Art 
In many electrical applications, it is desirable to utilize high frequency 
(i.e., greater than 1 GHz) voltage-controlled oscillators (VCOs). As is 
well understood, high frequency voltage-controlled oscillators typically 
have a poorer phase noise ratio than single-frequency oscillators which 
are stabilized by acoustic resonators. This is because the 
voltage-controlled oscillators contain high 1/f noise components, such as 
varactors, which help determine the output frequency. To eliminate the 
noise problems inherent in varactors, the varactors can be combined with 
high-Q (quality factor) acoustic resonators to create narrowband VCOs with 
relatively low phase noise. However, the tuning range of these VCOs are 
typically only a few hundred parts per million (ppm) and the frequency 
control linearity is not good. 
In order to eliminate this phase noise problem and the narrow tuning range, 
it is possible to vary the frequency of crystal-controlled oscillators by 
applying and varying a voltage potential between the oscillator's 
electrodes. This results in creating a VCO that has phase noise ratios as 
low as an equivalent single-frequency oscillator with highly linear 
control characteristics. Unfortunately, this type of VCO is not practical 
because an extremely high voltage is required to vary the frequency. For 
example, a quartz crystal resonator having a thickness of one millimeter 
requires 1,000 volts to produce an electric field of one million volts per 
meter (V/m). As such, this technique is not practical, nor utilized for 
crystal-controlled oscillators since an unreasonably high voltage is 
required to make any useful frequency change. 
Each of the above-mentioned techniques produces voltage-controlled 
oscillators having low phase noise and relatively linear characteristics. 
However, each of these techniques have several drawbacks associated with 
their use that effect their practicality, cost, size, complexity and power 
consumption. What is needed then is a voltage-tuned acoustic resonator 
that has low phase noise, highly linear characteristics and can be varied 
over a broad frequency range by utilizing practical DC bias voltages. This 
will, in turn, reduce the cost, size, complexity and power consumption of 
voltage-controlled oscillators as well as tunable filters. It is, 
therefore, an object of the present invention to provide such a device. 
SUMMARY OF THE INVENTION 
In accordance with the teachings of the present invention, a thin film 
voltage-tuned semiconductor bulk acoustic resonator (SBAR) is disclosed as 
a variable frequency resonator having relatively low phase noise with 
highly linear characteristics. This is basically achieved by utilizing a 
thin (about 1 micrometer) piezoelectric film positioned between a first 
electrode and a second electrode and placed atop a semiconductor substrate 
having a via hole passing therethrough. A DC bias voltage is then applied 
to the first and second electrodes to create a strong electric field 
between the first electrode and the second electrode within the thin 
piezoelectric film. This allows the electric field to be varied 
substantially by adjusting the DC bias voltage over moderate levels which, 
in turn, varies the resonant frequency of the resonating piezoelectric 
film. 
In one preferred embodiment, a semiconductor substrate having a via hole 
passing through the semiconductor substrate is provided. A piezoelectric 
film is positioned between a first electrode and a second electrode 
positioned adjacent to the semiconductor substrate and passing over the 
via hole. A DC bias voltage is then applied from the first electrode to 
the second electrode to create an electric field between the electrodes 
within the piezoelectric film. This electric field causes the 
piezoelectric film to resonate at a frequency slightly different than its 
unbiased resonant frequency. By adjusting the DC bias voltage, the 
electric field varies which causes the resonant frequency to be similarly 
adjusted. 
Use of the present invention results in producing a thin film voltage-tuned 
semiconductor bulk acoustic resonator (SBAR) having low phase noise and 
highly linear characteristics. As a result, the aforementioned problems 
associated with the current approaches of making low phase noise and 
highly linear VCOs have been substantially eliminated.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT 
The following description of a thin film voltage-tuned semiconductor bulk 
acoustic resonator (SBAR) is merely exemplary in nature and is in no way 
intended to limit the invention or its application or uses. 
Referring to FIG. 1, a schematic block diagram of one preferred embodiment 
of a thin film voltage-tuned semiconductor bulk acoustic resonator (SBAR) 
10, is shown. A more detailed description of a typical SBAR 10 is set 
forth in R. B. Stokes and J. D. Crawford, "X-Band Thin Film Acoustic 
Filters on GaAs," IEEE MTT Symposium 1992, herein incorporated by 
reference. The SBAR 10 includes a semiconductor substrate 12 having a via 
hole 14 extending therethrough. The semiconductor substrate 12 is 
preferably four mils thick and made of gallium arsenide (GaAs), however, 
other semiconductor materials, such as silicon, can also be used. A first 
electrode 16 having a thickness of between about 800 to 2,000 angstroms 
and preferably made of aluminum (Al), is positioned atop or adjacent to 
the semiconductor substrate 12 and the via hole 14. A thin piezoelectric 
film 18 having a thickness of between 4,000 angstroms to 10 micrometers is 
positioned adjacent the first electrode 16, over the via hole 14. This 
allows the piezoelectric film 18 to resonate or vibrate freely. The 
piezoelectric film 18 is preferably made of aluminum nitride (AlN). 
However, the piezoelectric film 18 can also consist of other suitable 
piezoelectric materials, such as zinc oxide. A second electrode 20, also 
having a thickness of between about 800 to 2,000 angstroms and preferably 
made of aluminum (Al), is placed adjacent the piezoelectric film 18 to 
create a parallel plate structure between the first electrode 16 and the 
second electrode 20 which comprises the active region of the SBAR 10. 
A first electrical connector 22 is attached to the first electrode 16 and a 
second electrical connector 24 is attached to the second electrode 20. A 
variable voltage source 26 is connected to the first and the second 
electrical connectors 22 and 24 through an RF choke 28. The variable 
voltage source 26 is capable of applying a variable DC bias voltage, 
preferably between -30 volts and +30 volts, through the first and second 
electrical connectors 22 and 24 to the first electrode 16 and the second 
electrode 20. In addition, since the SBAR 10 is an insulator, essentially 
no power is consumed from the variable voltage source 26. 
In operation, the DC bias voltage applied by the variable voltage source 26 
creates an electric field (V/m) between the first electrode 16 and the 
second electrode 20 within the piezoelectric film 18. The electric field 
is equal to the bias voltage divided by the thickness of the piezoelectric 
film 18 or the distance between the first electrode 16 and the second 
electrode 20. The electric field causes the piezoelectric film 18 which is 
supported by its edge around the via hole 14 to have a mechanical resonance 
at a particular frequency, which is different than the resonant frequency 
without the bias. As a result, by adjusting the variable voltage source 26 
to vary the DC bias voltage, the electric field between the first electrode 
16 and the second electrode 20 will vary and cause the piezoelectric film 
18 to resonate at different frequencies. 
For example, assume the SBAR 10 resonates at 4.8 GHz (fundamental) when a 
DC bias voltage is applied. This frequency can be changed by 1,000 parts 
per million (ppm) if the DC bias voltage is varied between -30 volts and 
+30 volts. This provides an SBAR 10 which has a highly linear frequency 
change versus voltage characteristic, as can clearly be seen in FIG. 2 
which shows the frequency change as the DC bias voltage is varied between 
0 and +30 volts. As seen in FIG. 2, as the DC bias voltage is increased to 
+30 volts, the frequency of the 4.8 GHz (fundamental) SBAR 10 is decreased 
by over 2 MHz. The SBAR 10 can also withstand up to 100 volts DC, thus 
giving it a tuning range of 3,000 parts per million (ppm). In addition, it 
should also be noted that for a piezoelectric film 18 having a thickness of 
one micrometer, an electric field of 1 million V/m can be achieved with a 
DC bias voltage of only 1 volt. 
This highly linear and sensitive frequency change versus voltage 
characteristics of the SBAR 10 can be utilized to construct high frequency 
low phase noise voltage-controlled oscillators or tunable filters. In 
addition, this effect can also be used to correct or compensate for 
temperature sensitivity inherent in the SBAR 10. This can be achieved by 
simply applying a correction voltage corresponding to the operating 
temperature of the SBAR 10 from the first electrode 16 to the second 
electrode 20. 
Turning to FIG. 3, another embodiment of the SBAR 10 is shown. In this 
embodiment, the via hole 14 does not extend through the semiconductor 
substrate 12, but leaves a thinned region 30 of the semiconductor 
substrate 12. This thinned region 30 is preferably one micrometer in 
thickness and makes up the bottom layer of the SBAR 10 active region. 
Since the thinned region 30 is only about one micrometer in thickness, the 
piezoelectric film 18 is not inhibited from being able to resonate or 
vibrate. 
Still another embodiment of the SBAR 10 is shown in FIG. 4. The SBAR 10 in 
FIG. 4 is a two-port resonator which can also serve as a tunable one-pole 
filter. This SBAR 10 comprises two piezoelectric films 18, the first 
electrode 16, the second electrode 20, and a third electrode 32 having a 
third electrical connector 34. Preferably, the third electrode 32 is 
grounded to help shield the first electrode 16 from the second electrode 
20. To operate as a voltage tunable two port resonator or one-pole filter, 
the variable voltage source 26 applies the D.C. bias voltage across at 
least two of the electrodes, such as the first electrode 16 and the second 
electrode 20. It should also be noted that one skilled in the art would 
readily recognize that the SBAR 10 can contain additional piezoelectric 
layers as well as additional electrode layers to construct higher order 
resonators and tunable filters. 
The foregoing discussion discloses and describes merely exemplary 
embodiments of the present invention. One skilled in the art would readily 
recognize from such discussion, and from the accompanying drawings and 
claims, that various changes, modifications and variations can be made 
therein without departing from the spirit and scope of the invention as 
defined by the following claims.