JPH02298109A - Integrated type piezoelectric thin film functional element - Google Patents

Abstract

PURPOSE:To prevent the reduction of the insulation resistance of a piezoelectric thin film resonator by capacitively coupling a piezoelectric thin film resonator and an integrated circuit with a coupling capacitor formed by using a passivation film as a dielectric layer to suppress the diffusion of electrode metallic atoms to the resonator. CONSTITUTION:A piezoelectric thin film resonator 2 made of a ZnO piezoelectric film on a semiconductor substrate 1 and an integrated circuit circuit 3 forming an oscillation circuit are formed integrally. Then connection pads 6a, 6b are provided to the part opposite wiring patterns 5a, 5b of the integrated circuit 3 and wiring patterns 4a, 4b of the piezoelectric thin film resonator 2. The piezoelectric thin film resonator 2 and the integrated circuit 3 are capacitively coupled via coupling capacitors Ca, Cb formed by using the connection pads 6a, 6b whose pattern faces are opposite to each other as electrodes and the passivation film 7 to protect the pattern face as the dielectric layer. Since the DC potential fed to the piezoelectric thin film resonator is suppressed lower, the characteristic deterioration of the resonator caused by the deterioration of the insulation resistance due to the state of DC potential is prevented.

Description

[Detailed Description of the Invention] [Object of the Invention] (Industrial Application Field) This invention relates to an integrated piezoelectric device in which a resonator using a piezoelectric thin film as a vibrating membrane and a semiconductor integrated circuit are integrated on the same substrate. The present invention relates to thin film functional elements.

(Prior art) A piezoelectric thin film resonator formed by thin film processing, an amplifier, a filter, etc. are integrally formed on a semiconductor substrate, and VHF band U
It is possible to realize an ultra-small, one-chip, monolithic type high-frequency circuit that can be used in a high-frequency band such as the HF band.

The inventors have already submitted a technical research report to the Institute of Electronics, Information and Communication Engineers.
US86-71 (January 30, 1987), Japanese Patent Publication No. 6
No. 3-138808 and the like disclose an example of fabricating a one-chip oscillator that integrates a piezoelectric thin film resonator and an integrated circuit.

(Problem to be solved by the invention) A test sample of a one-chip oscillator configured as a circuit according to the disclosed technology was tested at a DC power supply voltage of 6 V and a temperature of 8 V.
When testing the reliability of continuous operation under the condition of 5°C, the oscillation frequency fluctuated or stopped oscillating after 500 hours, and all 10 test samples stopped oscillating after 1000 hours.

In order to investigate the cause of this, we measured and analyzed the characteristics of the piezoelectric thin film resonator in detail, and found that the resonance loss increased, the resonance frequency fluctuated, and especially the anti-resonance level deteriorated significantly. This phenomenon is electrically equivalent to adding a resistance in parallel with the resonator, and means that the insulation resistance of the ZnO piezoelectric film has decreased significantly.Such a resonator deterioration phenomenon occurs when no DC voltage is applied. This deterioration phenomenon is considered to be due to the influence of the electric field of the applied DC power source, since it does not occur in the case of the wafer.

The ZnO piezoelectric film that makes up the resonator has a fibrous polycrystalline crystal structure, with many grain boundaries running through it, so it has a structure in which metal atoms easily move along these grain boundaries. Therefore, due to the electric field of the applied DC voltage, ZnO
It is thought that the metal atoms of the upper and lower electrodes sandwiching the piezoelectric film diffuse into the ZnO piezoelectric film, and are further activated at high temperatures to promote this diffusion, thereby lowering the insulation resistance of the resonator.

The present invention was made to solve the above-mentioned problems, and an object of the present invention is to provide an integrated piezoelectric thin film functional element that suppresses characteristic deterioration due to DC voltage.

[Structure of the Invention] (Means for Solving the Problem) The present invention provides an integrated piezoelectric thin film functional element in which a semiconductor integrated circuit and a piezoelectric thin film resonator are integrally formed on a semiconductor substrate. The semiconductor integrated circuit and the piezoelectric thin film resonator are connected by capacitive coupling via a passivation film formed on at least a portion of the semiconductor substrate.

(Function) By capacitively coupling the piezoelectric thin film resonator and the integrated circuit using the coupling capacitance formed using the passivation film as a dielectric layer, the DC voltage from the integrated circuit side is divided by the coupling capacitance, and the piezoelectric thin film resonator and the integrated circuit are capacitively coupled. By lowering the DC voltage applied to the resonator and suppressing the diffusion of electrode metal atoms into the resonator, a decrease in insulation resistance of the piezoelectric thin film resonator is prevented.

(Example) An example of the present invention will be described below with reference to the drawings. Figure 1 (aHb) is a structural diagram of an embodiment in which a one-chip oscillator is constructed on a semiconductor substrate by applying this invention.
The figure is a plan view, and the fb) figure is a BB sectional view of figure (a). In the figure, a piezoelectric thin film resonator 2 made of a ZnO piezoelectric film and an integrated circuit 3 forming an oscillation circuit are mounted on a semiconductor substrate 1.
are integrally formed, and the wiring pattern 4a of the piezoelectric thin film resonator 2flll.

Connecting pads 6a, 6b are provided at opposing portions of wiring patterns 5a, 5b on the integrated circuit 3 side, and the connecting pads 6a, 6b whose pattern surfaces are facing each other are used as electrodes to protect the pattern surfaces. Coupling capacitances Ca and C formed using the passivation film 7 as a dielectric layer for
The piezoelectric thin film resonator 2 and the integrated circuit 3 are capacitively coupled via b.

In the figure, 8a and 8b represent a power supply terminal and a ground terminal, and 9a and 9b represent output terminals, respectively.

FIG. 2 is a circuit diagram of the one-chip oscillator of this embodiment, and the portions indicated by two-dot chain lines are an equivalent circuit of the piezoelectric thin film resonator 2 and a specific circuit example of the integrated circuit 3 constituting the oscillation circuit, respectively.

As shown in the figure, the electrical equivalent circuit of the piezoelectric thin film resonator 2 is expressed in the same way as a normal crystal resonator, and in this equivalent circuit, CO
is called the parallel equivalent capacitance, and is the capacitance when the resonator simply functions as a capacitor, and includes stray capacitance such as lead wires in the capacitance between the electrodes of the resonator. Ll and C1 are called equal series inductance and equal series capacitance, respectively, and are equivalent constants when the piezoelectric thin film resonator 2 operates as an electromechanical vibration system, and are constants determined by the vibration mode, material, and structural dimensions of the electrodes. It is. Note that R1 is an equal series resistance representing the loss of the resonator.

The oscillation circuit of the integrated circuit 3 is a modified Colpitts type oscillation circuit, and in the figure, the collector of the transistor T" constituting the amplification section of the oscillation circuit is connected to the DC power supply E via the power supply terminal 8a, and the emitter is connected to the resistor R2. A bias resistor R3 is connected to the ground through the ground terminal 8b, and is connected to the base between the collector and the base and between the base and the ground.

A DC bias E' is applied by R4.

Load capacitances C2 and C3 are connected in series between the emitter and the ground, and the output terminal 9a is connected between the connection point and the ground.
Output is taken through 9b. Note that C4 connected between the collector and ground is a capacitor for high frequency bypass.

The piezoelectric thin film resonator 2 and the oscillation circuit of the integrated circuit 3 have a coupling capacitance Ca formed by the connection pads 6a and 6b,
Capacitively coupled by Cb, the oscillator is constructed as a whole.

In this embodiment, the dimensions of each of the connection pads 6a and 6b are approximately 100 μm square, and the passivation pads 6a and 6b serve as dielectric materials. As for 7, the dielectric strength is piezoelectric thin film resonator 2.
108V, which is an order of magnitude better than the ZnO film that makes up the
A P-3iN film with high reliability of about /m is used, and the film thickness is 1 μm, which is the commonly used film thickness, and the coupling capacity JiC
a, formed cb.

The capacitance values of the coupling capacitances Ca and Cb can be calculated using the following equation.

Ca, Cb = EO・εr・S/l−・−(1)
However, C0...Dielectric constant of vacuum approximately 8.9 xlo-12
F/nε "... Relative dielectric constant of P-3iN approximately 7.5S・
...area of pad 6ab 1O-8rr? T...
...P-3iN film thickness 1μm The above value is (
1) When calculated by substituting into the formula, Ca and Cb are approximately 0.7p
It becomes F.

On the other hand, the parallel equivalent capacitance CO of the 400MH2 band piezoelectric thin film resonator 2 using the ZnO piezoelectric film is 2.5
μm, the relative permittivity is approximately 8.8, and the electrode size is 100 μm.
When calculated by substituting these values into equation (1) above, the capacitance of CO is approximately 0.3 pF.

From the capacitance values of the coupling capacitances ca and cb and the parallel equivalent capacitance CO, the potential EO applied to the piezoelectric thin film resonator 2 by the DC bias E' is usually determined by the following equation since CG is sufficiently larger than 01.

EO=Ct/Co ・E′・・・・・・(2) However, C
1 is the composite capacitance of ca, cb, and CO, and Manako's composite capacitance Ct is determined by the following equation.

Ct = Ca - Cb - Co / Ca - Cb + Ca = Co + Cb - C0 By substituting the capacitance values of Ca, Cb, and Co obtained by equation (1) into this equation, the combined capacitance C1 becomes approximately 0.169 F, and further Substituting the capacitance value of this composite capacitance Ct into equation (2), EO = 0.1610.3 x E' = 0.53E
', and the potential EO applied to the piezoelectric thin film resonator 2 can be lowered to about 172 of the DC bias E'.

When the one-chip oscillator of this example was subjected to a high-temperature continuous operation test, stable oscillation characteristics were obtained even after 1000 hours, demonstrating that reliability was improved.

In this embodiment, for example, a circuit configuration may be adopted in which only the ground sides of the piezoelectric thin film resonator 2 and the integrated circuit 3 are directly connected.

It should be noted that the present invention is not limited to the above-mentioned embodiments, and can be implemented with various modifications without changing the gist.

For example, by utilizing the technology of the present invention, a filter in which a plurality of upper electrodes are provided opposite to the lower electrode of a piezoelectric thin film resonator, and each electrode opposing portion is elastically coupled, or a common lower electrode A plurality of upper electrodes may be arranged perpendicularly to each other, and the opposing parts of each electrode may be spaced apart to the extent that elastic coupling can be ignored, or grooves or sound absorbing means may be provided between each opposing part of the electrodes. A multi-element resonator including resonators can be configured.

Furthermore, the material of the piezoelectric thin film resonator is not limited to znO, but also AI N
, Ta205, PbTi03, etc. can also be used.

[Effects of the Invention] According to the present invention, by capacitively coupling the piezoelectric thin film resonator and the integrated circuit, the direct current potential applied to the piezoelectric thin film resonator can be suppressed to a low level, thereby preventing a decrease in insulation resistance due to the direct current potential. Deterioration of the characteristics of the underlying resonator can be prevented and high reliability can be obtained.

Another effect is that by providing a coupling capacitance, even if dielectric breakdown occurs in the piezoelectric thin film resonator, the coupling capacitance maintains insulation and prevents damage to the integrated circuit.

In addition, the coupling capacitance for cutting DC can be formed using the passivation film as is, and there is no need for special processes or external capacitors or resistors, so it is possible to reduce costs and make the circuit elements monolithic. It also has the effect of making it easier.

[Brief explanation of the drawing]

FIG. 1 (aHb) is a plan view and a sectional view showing the structure of an integrated piezoelectric thin film functional element according to the present invention, and FIG. 2 is a circuit diagram of the same embodiment. 1...Semiconductor substrate 2...Piezoelectric thin film resonator 3...Integrated circuit 4a, 4b, 5a, 5b...Wiring button 6a, 6b... ... Connection pad 7 ... Passivation film 8a ... Power supply terminal 8b ... Ground terminal 9a, 9b ... Output terminal Ca, Cb ... ...Coupling Capacity Agent Patent Attorney Yuki Komiya Figure 1 Figure 2

Claims (2)

[Claims] (1) In an integrated piezoelectric thin film functional element in which a semiconductor integrated circuit and a piezoelectric thin film resonator are integrally formed on a semiconductor substrate, a passivation film is formed on at least a portion of the semiconductor integrated circuit and the semiconductor substrate. An integrated piezoelectric thin film functional element, wherein the semiconductor integrated circuit and the piezoelectric thin film resonator are connected through capacitive coupling. (2) A piezoelectric thin film resonator and an integrated circuit are integrally formed on a semiconductor substrate, and the piezoelectric thin film resonator and the integrated circuit are connected to connection pads that face each other via a passivation film, and the passivation film is formed on a dielectric layer. An integrated piezoelectric thin film functional element characterized in that it is capacitively coupled by a coupling capacitance formed as an integrated piezoelectric thin film functional element.