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BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates in general to a single chip with a
piezoelectric crystal device integrated on a monolithic integrated
circuit, and more particularly to a single-chip radio structure with a
piezoelectric crystal device such as a surface acoustic wave resonator,
precise reference frequency generator and other passive devices integrated
on a monolithic integrated circuit and a method of fabricating the same,
in which a silicon substrate, a thick or thin piezoelectric crystal on
silicon (TPCS) and a metal layer are provided to implement a single-chip
transmission/reception system for a portable terminal.
2. Description of the Prior Art
In portable communication systems which have recently been studied, there
has been of more importance a technique for fabricating devices which are
low in power consumption, small in size and low in cost under the
condition that they satisfy all specifications on reception sensitivity,
channel selection, etc. Due to the recent trend where devices become
smaller in size through developments in microelectronic fabrication
technology, the above aims have been accomplished to a certain degree with
respect to intergrated circuits composed of a number of transistors.
However, passive devices such as a crystal resonator, filter, inductor,
etc. are discrete components which are still large in volume. As a result,
such devices are the main obstacles in the miniaturation of a transceiver.
Generally, a clock generator or a reference frequency generator for
communication equipment comprises a hybrid circuit consisting of a crystal
resonator, oscillator and control circuit, which may be constructed in any
one of various manners such as Colpitts, Hartley and Clapp. A typical
Colpitts-type oscillator is shown in FIG. 1, herein.
In a conventional oscillator shown in FIG. 1, a crystal resonator X10 is
operated based on a characteristic of a quartz crystal which is very
stable in mechanical resonance frequency. The quartz crystal is of a thin
membrane shape and the resonance frequency thereof is proportional to
inverse of the thickness thereof. Noticeably, because a resonance
frequency of 100 MHZ corresponds to a very thin thickness of about several
tens .mu.m, it is impossible to obtain the higher resonance frequency.
The low resonance frequency of the crystal may be made higher by using a
frequency multiplier composed of an inductor L19, resistor R20 and
capacitor C21 in FIG. 1 or a so-called phase locked loop (PLL).
Alternatively, a surface acoustic wave resonator shown in FIG. 2 may be
replaced for the conventional crystal to raise a basic mode resonance
frequency. However, such devices are conventionally used as external
devices because they are difficult to be implemented on silicon integrated
circuits. In particular, the surface acoustic wave resonator is difficult
to be integrated. Many studies have been made since the 1970's of using,
for a UHF-band reference signal generator, a surface acoustic wave
resonator which is capable of obtaining a high frequency and has an
advantage of ease in mass production, instead of conventional means based
on a high-frequency resonance mode of a crystal and a frequency multiplier
(see: Acoustic Surface Wave Resonator Devices, U.S. Pat. No. 3,886,504,
May 20, 1974).
FIGS. 2 and 3 show a conventional 2-terminal surface acoustic wave
resonator. This surface acoustic wave resonator is to use a phenomenon
where a resonance occurs as an elastic wave or acoustic wave is produced
on a piezoelectric substrate 51 and then blocked by acoustic wave
reflectors 56 and 57. In this resonator, the basic principle of the
resonace and equivalent circuit model are the same as those of a
conventional quartz crystal resonator.
But, the surface acoustic wave resonator utilizes a surface acoustic wave
component which mechanically resonates along the surface of a crystal on
which various electrodes 52-57 are laid, whereas the conventional crystal
resonator utilizes a resonance mode of a bulk acoustic wave which
mechanically resonates in the direction of a thickness of a thin crystal.
In the 2-terminal surface acoustic wave resonator shown in FIGS. 2 and 3,
when a variation of a voltage signal across lead wires 62 and 64 in FIG. 2
is applied to transducer electrodes 52 and 54 formed on the surface of a
crystal, a fine mechanical displacement is formed on and propagated along
the crystal surface due to properties of a piezoelectric material to
produce a surface acoustic wave. The produced surface acoustic wave is
detected by a transducer composed of electrodes 53 and 55, which
transduces the mechanical resonance into an electrical signal in the
reverse procedure of the generation of surface waves.
In such a surface acoustic wave device, a bulk acoustic wave component may
be generated, reflect from the bottom of the crystal bordered on a metal
plate 60 and return to the transducer electrodes 53 and 55 formed on the
surface of the crystal. This bulk acoustic wave component becomes a factor
of deteriorating characteristics of the device. In order to suppress such
an effect of the bulk acoustic wave component on the characteristics of
the device, an acoustic absorption material such as epoxy may be applied
on the bottom of the crystal to absorb a part of the wave component
arriving at the bottom and irregularly reflect the remainder.
A resonance frequency of the surface acoustic wave resonator is determined
depending on a distance or pitch between two electrodes just adjacent
respectively to the reflectors 56 and 57, twice which is a wavelength of a
resonance mode.
Accordingly, the maximum possible resonance frequency of the surface
acoustic wave resonator is determined according to a line width in a
semiconductor manufacturing process. Typically, a lithography process with
a resolution of up to 1 .mu.m is used to obtain a basic mode resonance
frequency higher than 1 GHz.
Noticeably, the above-mentioned surface acoustic wave resonator or
conventional crystal resonator is provided with one type of substrate 51
composed of only a piezoelectric crystal. For this reason, a hybrid
circuit must be provided on a printed circuit board (PCB) to connect the
surface acoustic wave resonator to other integrated circuits formed on
silicon or GaAs substrates. In this case, signals must be passed through a
bonding wire 61 and lead wires 62, 63, 64 and 65 for connection with the
chip circuits, resulting in increases in occupied area and PCB
manufacturing cost as compared with a single-chip integrated circuit
structure.
As a result, studies have recently been made of integrating surface
acoustic wave devices with silicon or gallium-arsenide circuits on a
single chip. Such studies may generally be classified into a method of
forming a surface acoustic wave device by depositing a piezoelectric
material such as ZnO or AlN on a silicon substrate (see: ZnO Films on
{110}-Cut <100>-Propagating GaAs Substrates for Surface Acoustic Wave
Device Applications, IEEE Trans. Ultrason. Ferroelec. Freq. Cont., vol.
42. pp. 351-361, 1995) and a method of providing a surface acoustic wave
device in an integrated circuit by bonding a thick crystal or LiNbO.sub.3
crystal on silicon (see: Integrated Circuit including a Surface Acoustic
Wave Transformer and a Balanced Mixer, U.S. Pat. No. 5,265,267, 1993).
However, in the deposition method, a frequency stability of the deposited
ZnO or AlN with respect to variations in temperature and time is inferior
to that of a piezoelectric single crystal. For this reason, the deposition
method is not suitable for a reference frequency generator of a
communication system with a strict standard.
Generally, in a piezoelectric crystal, the speed of a surface acoustic wave
varies on the order of 20 to 30 ppm with respect to a temperature
variation of 0.degree. to 50.degree.. Further, the piezoelectric crystal
has a long-term stability wherein a resonance frequency varies on the
order of 1 ppm for one year, which is on an improved trend. In this
connection, in the case where the piezoelectric crystal is used as a
substrate of a surface acoustic wave resonator, it will offer sufficient
frequency stability and precision, as is well known in the art.
Hence, in order to maintain a frequency stability at a degree satisfying
the communication system standard and increase an integration level of a
transceiver circuit without depending on a frequency multiplier generating
undesirable higher harmonics, there is required a method of bonding a
piezoelectric crystal on silicon to make a piezoelectric crystal/silicon
structure and forming a surface acoustic wave device and other passive
devices on the piezoelectric crystal/silicon structure.
On the other hand, in the second method of bonding a thick crystal or
LiNbO.sub.3 crystal on silicon, the associated fabrication method is not
specified and the entire device is large in thickness because the
piezoelectric crystal is not adjusted in thickness. Further, in the case
where a surface acoustic wave device is formed on the piezoelectric
crystal, it is difficult to be connected to an integrated circuit.
Moreover, it is hard to provide a single-chip transmitter/receiver or
radio structure with even passive devices such as an inductor,
transmission line, etc., integrated therein.
SUMMARY OF THE INVENTION
Therefore, the present invention has been made in view of the above
problems, and it is an object of the present invention to provide a
single-chip radio structure with a piezoelectric crystal device for
precise frequency control, such as a surface acoustic wave resonator, and
other passive devices integrated on a monolithic integrated circuit and a
method of fabricating the same, in which a piezoelectric crystal wafer
with a thickness adjusted by mechanical grinding and polishing processes
is bonded on a silicon substrate to implement a single-chip
transmission/reception system.
In accordance with the present invention, the above and other objects can
be accomplished by a provision of a single-chip transmitter/receiver
structure with a piezoelectric crystal device for the generation of a
stable reference frequency, such as a surface acoustic wave resonator, and
other passive devices integrated on a silicon integrated circuit.
Piezoelectric crystal wafer bonded on a finished silicon wafer and the
surface of piezoelectric wafer is subjected to mechanical grinding and
polishing processes to adjust a thickness of the crystal wafer. Formed on
the surface of the resultant piezoelectric crystal wafer is a metal layer
for forming transducer electrodes and acoustic wave reflectors and passive
devices such as an inductor. An existing semiconductor manufacturing
process is directly utilized to provide a device using piezoelectric
material such as quartz crystal and other passive devices, such as an
inductor, in an integrated circuit while satisfying specifications on a
frequency stability required by communication equipment. Preferably, the
piezoelectric crystal device may be a surface acoustic wave resonator.
In a feature of the present invention, there is provided a single-chip
radio structure with a piezoelectric crystal device integrated on a
monolithic integrated circuit, comprising a silicon substrate; a
piezoelectric crystal wafer bonded on the silicon substrate, the
piezoelectric crystal wafer being adjusted in thickness by mechanical
grinding and polishing processes; the piezoelectric crystal device formed
on the piezoelectric crystal wafer, the piezoelectric crystal device
including at least one surface acoustic wave resonator for selecting a
specific frequency; a plurality of passive circuit devices selectively
formed on the piezoelectric crystal wafer; the monolithic integrated
circuit formed on the silicon substrate, the monolithic integrated circuit
including partial or all components of a radio transceiver; and a metal
wire for connecting the piezoelectric crystal device and passive circuit
devices formed on the piezoelectric crystal wafer directly to the
monolithic integrated circuit formed on the silicon substrate.
In another feature of the present invention, there is provided a method of
fabricating a single-chip radio structure with a piezoelectric crystal
device integrated on a monolithic integrated circuit, comprising the first
step of bonding a piezoelectric crystal wafer on a silicon substrate; the
second step of adjusting a thickness of the piezoelectric crystal wafer by
using mechanical grinding and polishing processes; and the third step of
forming the piezoelectric crystal device and other passive devices on the
piezoelectric crystal wafer, the piezoelectric crystal device including at
least one surface acoustic wave resonator for selecting a specific
frequency.
BRIEF DESCRIPTION OF THE DRAWINGS
The above and other objects, features and advantages of the present
invention will be more clearly understood from the following detailed
description taken in conjunction with the accompanying drawings, in which:
FIG. 1 is a circuit diagram showing the construction of a conventional
fixed frequency oscillator using a crystal resonator;
FIG. 2 is a perspective view of a conventional 2-terminal surface acoustic
wave resonator formed on a piezoelectric crystal wafer;
FIG. 3 is a plan view of the conventional 2-terminal surface acoustic wave
resonator in FIG. 2;
FIG. 4 is cross-sectional view of a surface acoustic wave resonator formed
on a silicon substrate in accordance with the present invention;
FIG. 5 is cross-sectional view of a bulk acoustic wave resonator in
accordance with the present invention; and
FIG. 6 is a block diagram of a radio frequency receiver connected to the
surface acoustic wave resonator in FIG. 4.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
FIG. 4 is a sectional view of a surface acoustic wave resonator with a thin
or thick crystal/silicon structure in accordance with the present
invention. As shown in this drawing, the crystal/silicon structure
comprises a silicon substrate 101, a thin piezoelectric crystal wafer 104
and an epoxy bonding layer 103.
The piezoelectric crystal wafer 104 is subjected to mechanical grinding and
polishing processes in such a manner that it can have a thickness smaller
than 50 .mu.m. The epoxy bonding layer 103 has a thickness smaller than 3
.mu.m. As a result, a thickness from the surface of the silicon substrate
101 to the surface of the piezoelectric crystal wafer 104 is about 55
.mu.m at the maximum, so that a surface acoustic wave device formed on the
piezoelectric crystal wafer 104 can be connected with a circuitry 107 on
the silicon substrate 101 through a metal inter connection layer 106, or
wire-bonding.
The surface acoustic wave resonator comprises a metal pattern 105 including
a transducer electrode 203 and reflection gratings 201 and 204 disposed as
shown in FIG. 6.
In each of the reflection gratings 201 and 204 and interdigital transducer
electrode 203, two adjacent metal lines must have a distance therebetween
which is equal to half a wavelength of a surface acoustic wave causing a
resonance, as indicated in the reflection grating 57 of FIG. 3. Each of
the electrodes must have a sufficient width to prevent a characteristic
degradation resulting from a diffraction of the acoustic wave.
In designing the metal electrodes 201, 203 and 204 of the surface acoustic
wave resonator, an input impedance at input terminals 106 and 202 with
respect to the resonator, a resonance frequency and a resonance Q value
must be mainly considered. Further, determination has to be made about
whether the resonator will be constructed in a 1-terminal type as shown in
FIG. 6 or a 2-terminal type as shown in FIG. 3. Moreover, it must be
determined whether an impedance matching circuit will be used for the
resonator.
Formed on the piezoelectric crystal wafer 104 of FIGS. 4 and 6 may be other
well-known passive circuit elements such as a high-Q inductor,
electromagnetic delay line, filter, etc., as well as the surface acoustic
wave resonator. The epoxy bonding layer 103 and metal layer 102 are
applied between the silicon substrate 101 and the piezoelectric crystal
wafer 104 to bond them to each other. The epoxy bonding layer 103 and
metal layer 102 further function to absorb and remove an undesired bulk
acoustic wave component generated during the operation of the surface
acoustic wave device. Furthermore, the epoxy bonding layer 103 and metal
layer 102 act to prevent electrostatic or electromagnetic interactions
between the electrodes of the surface acoustic wave device.
A method of fabricating the structure shown in FIG. 4 in accordance with
the present invention will hereinafter be described in detail.
First, the layer 102 of metal, such as aluminum or gold, is deposited on an
area of the silicon substrate 101 on which the crystal wafer 104 is to be
laid. Then, the bonding layer 103 of epoxy resin is coated on the metal
layer 102 to a thickness of 1.about.3 .mu.m by using a spin coating
process. The crystal wafer 104 is bonded on the epoxy bonding layer 103
and then ground from 600 .mu.m to 100 .mu.m in thickness by using a
mechanical grinding process.
An SiC paper is typically used in the mechanical grinding process. Because
the crystal wafer 104 must be sufficiently flat in surface, it is polished
from 100 .mu.m to 50 .mu.m in thickness by using a diamond paste. In the
surface acoustic wave device, the speed of the surface acoustic wave is
constant, nearly regardless of the thickness of the piezoelectric crystal
wafer if the thickness of the piezoelectric crystal wafer is greater than
the wavelength of the surface acoustic wave. In this connection,
differently from a crystal resonator using a bulk acoustic wave, there is
no necessity for extremely precisely controlling the thickness of the
piezoelectric crystal wafer. Finally, the metal pattern 105 of the surface
acoustic wave resonator is formed on the piezoelectric crystal wafer 104
by using photolithography and etching processes and then connected with an
oscillator circuit 205 on the silicon substrate 101 by the metal wire
layer 106.
FIG. 5 is a sectional view of a bulk acoustic wave resonator in accordance
with the present invention. As shown in this drawing, laid between
electrodes 123 and 125 is a piezoelectric crystal wafer 124, the thickness
of which is adjusted by a polishing process. An epoxy bonding layer 122 is
applied between the piezoelectric crystal wafer 124 and a silicon
substrate 121 to bond them to each other. The epoxy bonding layer 122
further functions to absorb an acoustic wave to be propagated into the
silicon substrate 121. The respective layers are the same in thickness and
fabrication process as those in the surface acoustic wave resonator of
FIG. 4.
FIG. 6 schematically shows a single-chip radio structure in accordance with
the present invention, in which a transmission/reception circuit is
connected to the surface acoustic wave resonator in FIG. 4. As shown in
this drawing, a radio receiver comprises an oscillator 205, frequency
synthesizer 207, low-noise amplifier or preamplifier 211, down converter
212, demodulator/digital signal processor circuit 213 and audio amplifier
214. A part or all of them are formed on one silicon substrate by using
well-known VLSI fabrication processes such as a CMOS, bipolar, etc. and
then connected to the surface acoustic wave resonator on the piezoelectric
crystal wafer 104 through the metal wires 106 and 202.
The oscillator 205 is a general oscillation circuit using a crystal, which
may be constructed in various manners. One example of the oscillator 205
is the Colpitts-type oscillator shown in FIG. 1.
In the oscillator of FIG. 1, the crystal resonator X10 is preferably
replaced with the surface acoustic wave resonator in FIG. 4. A variable
capacitor C11 may be provided with a varactor diode or a digitally
controlled capacitor bank for performing a frequency trimming or tuning
function. Capacitors C14 and C15 and an inductor L16 may be appropriately
selected in value to satisfy an oscillating condition.
The output 206 of the oscillator 205 is supplied as a reference frequency
signal to the frequency synthesizer 207, the output 208 of which is
applied as a local oscillation signal to the down converter 212.
Signal lines 27 and 28 for connection of the crystal resonator with the
circuitry in the conventional oscillator are preferably replaced with the
metal wires 136 and 202 on the silicon substrate. The metal wires 106 and
202 can be formed by a general photolithography or wire bonding process,
resulting in an advantage in occupying area or mass production.
As apparent from the above description, according to the present invention,
a thin or thick piezoelectric crystal wafer is bonded on a silicon
substrate and adjusted in thickness by mechanical grinding and polishing
processes. Then, a surface acoustic wave device and other passive devices
are formed on the piezoelectric crystal wafer by a standard lithography
process. Therefore, a high-precision oscillator and various passive
devices can be included in a monolithic integrated circuit to implement a
single-chip radio structure. This single-chip radio structure has the
effect of reducing the volume and weight of the entire receiver while
maintaining excellent performances provided by passive devices on a
crystal substrate, such as frequency stability, frequency linearity and
low power consumption, etc., as they are.
Although the preferred embodiments of the present invention have been
disclosed for illustrative purposes, those skilled in the art will
appreciate that various modifications, additions and substitutions are
possible, without departing from the scope and spirit of the invention as
disclosed in the accompanying claims.
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