The field of the invention is vibration testing of articles of manufacture, and particularly, systems for vibrating very small items and observing the vibrational modes and resonances.
Non-contact vibration measurement using laser doppler vibrometry is a well-established technique. The laser doppler vibrometry method uses an interferometer to measure the doppler frequency shift induced by the vibration of the object. In order to measure the vibration of the object, the laser measurement beam strikes the object to be measured and interferes with a reference beam. The resulting frequency shift induced in the interference beam is the vibration of the target surface.
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 is determined in large part by 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 and the mode shapes of 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 technique used to vibrate the microstructure. FIG. 1 shows a conventional excitation device which drives 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 is 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. With this electrostatic approach, an additional conductive film which does not belong to the microstructure is deposited. Therefore, this test method is a destructive one. In addition, 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. 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 physically attached 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, and a PZT disc with finite dimensions has complex mode distributions in the frequency domain. As a result, when a PZT transducer acts as the excitation source applied to a microstructure, the spurious vibrational modes of the PZT transducer is strongly coupled with the dynamic responses of the microstructure. This dynamic coupling effect will interfere with the dynamic responses of the microstructure.
FIG. 3 shows a further conventional excitation device which uses a swept-sine signal to drive a microstructure. A specimen 31 with a microstructure (not shown) is attached to a PZT transducer 30. By providing a dynamic signal analyzer 32, a swept-sine signal is generated to drive the PZT transducer 30 and excite the specimen 31. 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. A small loudspeaker 41 is mounted above a cantilever 40 to be excited. By providing AC power to the loudspeaker 41, the resulting acoustic waves 43 propagate via the air to the cantilever 40, causing the cantilever 40 to vibrate. The testing is limited to the frequency range of the loudspeaker 41 which can be much narrower than the desired vibrational frequencies.