Measuring and testing – Vibration – By mechanical waves
Reexamination Certificate
2001-06-19
2003-07-22
Kwok, Helen (Department: 2856)
Measuring and testing
Vibration
By mechanical waves
C073S651000, C073S662000, C310S334000
Reexamination Certificate
active
06595058
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to a method for determining a dynamic response of a microstructure, and more particularly to a method for determining a dynamic response of a microstructure by using a pulsed broad bandwidth ultrasonic transducer as a bulk acoustic wave (BAW) hammer to excite the microstructure. The present invention also relates to an apparatus for determining a dynamic response of a microstructure.
BACKGROUND OF THE INVENTION
Micro-sensors and micro-actuators are the key components in a micro electrical mechanical system (MEMS). The performance of a micro-sensor or micro-actuator correlates closely with the dynamic mechanical properties thereof. For instance, the bandwidth, resolution, and response time of some micro-sensors are determined by their mechanical resonance. The output characteristics of micro-actuators such as the force amplitude and the operating frequency thereof are also determined by their dynamic behaviors. Therefore, the testing method for evaluating the dynamic behaviors of the microstructures is very important. Several excitation and detection approaches have been developed to characterize the dynamic responses, vibration characteristics such as the natural frequencies and the mode shapes of the microstructures. Moreover, the material properties, e.g. residual stress, Young's modulus and fatigue properties, can also be determined.
The measured dynamic response of a microstructure will be affected by the excitation technique. Please refer to
FIG. 1
which schematically shows a conventional excitation device which drive the microstructure through built in electrostatic electrodes. The microstructure
10
formed on a silicon substrate
11
by a semiconductor manufacturing process is an insulator cantilever, for example, made of silicon oxide. In order to allow the cantilever
10
to be excited, a conductive film
12
such as a chromium film is applied over the insulator cantilever
10
. Then, a variable-frequency sinusoidal voltage could be applied between the silicon substrate
11
and the metallized line
12
leading to the cantilever
10
by way of a variable frequency oscillator
13
. Accordingly, the cantilever
10
with the chromium film
12
can be electrostatically attracted toward the substrate with either voltage polarity so as to excite the mechanical motion of the cantilever
10
. In this approach, an additional conductive film which does not belong to the microstructure is deposited. Therefore, this test method is a destructive one. On the other hand, the presence of the additional film
12
may influence the dynamic behavior of the original cantilever
10
.
FIG. 2
schematically shows another conventional excitation device which mechanically excites a microstructure. As shown, a test chip
20
with a microstructure (not shown) is attached onto a piezotransducer
21
, and a voltage
22
is applied for driving the piezotransducer
21
so as to mechanically excite the test chip
20
. The piezotransducer
21
is made of PZT. The natural frequencies of a PZT disc are strongly dependent on the ratio of diameter/thickness (D/T), and a PZT disc with finite dimension has complex mode distribution in the frequency domain. Accordingly, when a PZT transducer acts as the based excitation source applied to a microstructure, it is likely to be strongly coupled with the dynamic responses of the microstructure in the frequency range of a spurious mode of the PZT transducer. In brief, a dynamic coupling effect will exist between the piezotransducer
21
and the test chip
20
so as to interfere with the dynamic responses of the microstructure.
Further refer to
FIG. 3
which shows a further conventional excitation device which uses a swept-sine signal to drive a microstructure. As shown, a specimen
31
with a microstructure (not shown) is attached onto a PZT transducer
30
. By providing a dynamic signal analyzer
32
, a swept-sine signal is generated to drive the PZT transducer
30
and further the specimen
31
. As known, a swept-sine signal generated by a dynamic signal analyzer typically has frequencies under 50 kHz so as to be suitable for a millimeter dimensional microstructure. As for a micron dimensional microstructure with higher natural frequencies, higher exciting frequencies will be required.
FIG. 4
shows a still further conventional excitation device which uses acoustic waves to excite a microstructure. As shown, a small loudspeaker
41
is mounted above a cantilever
40
to be excited. By providing a power for the loudspeaker
41
, the acoustic waves
43
propagate via the air to the cantilever
40
, thereby forcing the cantilever
40
to vibrate. In this approach, the acoustic waves
43
have to be transmitted to the cantilever
40
via air so that the cantilever
40
cannot be excited in a vacuum environment where micron dimensional microstructures are possibly located.
SUMMARY OF THE INVENTION
Therefore, an object of the present invention is to provide a method and/or apparatus for determining a dynamic response of a microstructure, in which no additional film is deposited on the microstructure to be excited.
Another object of the present invention is to provide a method and/or apparatus for determining a dynamic response of a microstructure, in which the dynamic coupling effect between the transducer and the microstructure is minimized.
A further object of the present invention is to provide a method and/or apparatus for determining a dynamic response of a microstructure, which can be used for a micron dimensional microstructure.
A still further object of the present invention is to provide a method and/or apparatus for determining a dynamic response of a microstructure, which can be operated in a vacuum environment.
According to a first aspect of the present invention, a method for determining a dynamic response of a microstructure includes steps of attaching the microstructure to an ultrasonic transducer device; providing a pulse voltage to excite the ultrasonic transducer device so as to generate a pulsed bulk acoustic wave which has a bandwidth of at least 20%; and utilizing free vibration of the microstructure resulting from the pulsed bulk acoustic wave to determine the dynamic response of the microstructure. Preferably, the bandwidth is no less than 30%.
The microstructure is preferably attached to the ultrasonic transducer device by adhering a substrate of the microstructure to the ultrasonic transducer device in a nondestructive manner. For example, the substrate is adhered to the ultrasonic transducer device by wax or a sticky tape.
Preferably, the ultrasonic transducer device is a piezocomposite ultrasonic transducer. More preferably, the ultrasonic transducer device includes a piezoelectric portion and a polymer portion around the piezoelectric portion.
A second aspect of the present invention relates to a method for determining a dynamic response of a microstructure includes steps of attaching the microstructure to a piezocomposite ultrasonic transducer device formed of a piezoelectric material and a polymer material around said piezoelectric ceramic material; providing a pulse voltage for the piezocomposite layer to excite the ultrasonic transducer device so as to generate a pulsed bulk acoustic wave; and utilizing free vibration of the microstructure resulting from the pulsed bulk acoustic wave to determine the dynamic response of the microstructure.
Preferably, the piezoelectric material is PZT, and the polymer is epoxy resin or silicone.
Preferably, the pulsed bulk acoustic wave has a bandwidth of at least 20%, and more preferably, at least 30%, and a central frequency of hundreds of kHz to tens of MHz.
A third aspect of the present invention relates to an apparatus for determining a dynamic response of a microstructure includes a pulse generator for providing a pulse voltage; a piezocomposite ultrasonic transducer device including a plurality of piezoelectric ceramic rods filled with a polymer therebetween, and connected to said pulse generator for generating a pulsed bulk acoustic
Fang Weileun
Lai Wen Pin
Alston & Bird LLP
Computed Ultrasound Global Inc.
Kwok Helen
LandOfFree
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