The object of the test method is to evaluate the mechanical fatigue properties of microscale materials in a short time by applying high load and high cyclic frequency bending stress usin
General
The test equipment must generate resonant vibrations with a consistent amplitude and stable frequency for the test structure As illustrated in Figure 1, the system includes an actuator for oscillation, a sensor for amplitude detection, a controller to ensure constant amplitude during resonant vibration, and a recorder for monitoring purposes.
The amplitude control method is classified as follows a) Constant strain control
Applied strain in the test part is maintained at constant It can be applied for elastic or inductile materials
Applied stress in the test part is maintained at constant Load monitoring and closed loop control is crucial for the method.
Actuator
The actuator must generate oscillation forces with the required amplitude and frequencies in the specified direction Different types of actuators, such as electrostatic, piezoelectric, thermal, and electromagnetic, can be utilized Installation of the actuator within the test structure is addressed in section 5.1.
Sensor
The sensor must accurately measure the specimen's movement to assess either the stress amplitude during constant stress amplitude testing or the strain amplitude during constant strain amplitude testing.
The sensor and its associated electronics shall be accurate to within 1 % of the range of the stress or strain amplitude
To ensure effective failure detection and maintain constant vibration, the sensor must continuously measure movement In cases where the specimen is an elastic material that does not exhibit changes in vibrating properties, it is acceptable to measure movement at regular time intervals.
Movement detection in test structures can be achieved through various methods, including measuring displacement, stress, or strain Clause A.2 outlines a technique for detecting rotational displacement based on changes in capacitance Clause B.2 describes the use of an integrated strain gauge within the specimen for measurement Additionally, Clause C.2 presents a non-contact displacement gauge method for assessing mass displacement.
Controller
The controller must generate an oscillation signal for the actuator based on the sensor's movement signal to sustain the desired resonant vibration During testing, it is essential to keep the amplitude and frequency of the specimen constant Depending on the vibration characteristics, one of the following methods should be utilized for the specimen: a) Closed loop method.
To ensure accurate measurement, the frequency and amplitude of the oscillation signal applied to the specimen must be adjusted to align with changes in the resonant frequency Typically, the actuator's signal is derived from the specimen's movement Utilizing a self-excited oscillation circuit or a phase-locked loop circuit can effectively maintain the resonant frequency Additionally, an automatic gain control circuit (AGC) can regulate the amplitude by adjusting the oscillation signal based on the detected amplitude.
Elastic or inductile materials that exhibit a linear response without plastic deformation can be evaluated using an open loop method This testing involves either pausing at regular intervals to assess the resonant characteristics or continuously actuating the test structure from beginning to end at a specified resonance frequency and oscillation signal amplitude.
The stability of the frequency and amplitude shall be maintained throughout the test to within ± 3 % of the desired value.
Recorder
The test equipment shall include a recorder for collecting the “record data” indicated in 8.6.
Parallel testing
When conducting tests with multiple equipment units simultaneously, it is essential to implement measures that prevent any electrical or mechanical interference between the units.
General
The specimen shall be capable of applying a constant and high-load amplitude to the test part via resonant vibration Examples of specific structures are shown in the Clauses A.1, B.1, and C.1
Integrating mechanisms for actuation and movement sensing in specimens is allowed, as demonstrated in Annex A.1 Additionally, Annex B.1 presents a structure that incorporates a mechanism solely for amplitude detection.
Resonant properties
The specimen must possess resonance characteristics that facilitate the application of the required deformation at its specific resonance frequency, ideally exceeding 1,000 Hz to achieve a high number of cycles in a short duration Additionally, the quality factor should be greater than 100 to ensure a large amplitude It is crucial to prevent the specimen from vibrating in any mode other than the one designated for testing, which necessitates avoiding the presence of other resonant modes near the testing mode.
Test part
The specimen must include a test section where stress levels are high enough to cause failure During reliability testing of the actual device, the deformation in the test section under resonant vibration—both in-plane and out-of-plane bending—should match that of the real device If the structure can only withstand low stress, a notch or similar method can be used to focus stress on the specific area of the test section.
Specimen fabrication
When manufacturing the test part of the specimen, it is essential to refer to Clause 5 of IEC 62047-3 The specimen must be produced using the same method as the target MEMS device to ensure reliable evaluation Additionally, it is crucial to maintain identical shapes, dimensions, and multilayer film structures.
Test amplitude
The test amplitude must be defined based on the specimen's reference strength, which should be established using the methods outlined in section 7.1 Depending on the reference strength, one of the following procedures should be selected for determining the test amplitude: a) maintain a constant amplitude at 100% of the reference strength to assess fatigue life at a specific amplitude; b) gradually decrease the amplitude from a high level to quickly obtain an S-N curve; or c) gradually increase the amplitude from a low level to generate an S-N curve when the number of test parts is limited.
For guidance on test amplitude determination, refer to the experimental data and analysis of fatigue testing for silicon in Annex D For comprehensive information on metal material testing, consult ISO 12107.
The decrease and increase step of the test amplitude for the procedures b) and c) should be selected preferably close to the standard deviation of measured reference strength.
Load ratio
The load ratio for the test method is set at -1, given that the quality factor (Q) of the resonant vibration is sufficiently high, reaching 10 or more This high Q factor results in an amplitude that exceeds the limits of (quasi-)static testing methods.
Vibration frequency
The frequency shall be the resonant mode at which the test part is in the required stress state specified in 5.3, or a frequency close to it.
Waveform
The waveform of the displacement of the specimen and the stress and strain of the test part can be regarded as sinusoidal, irrespective of the actuating waveform
Test time
The test time is defined as the moment the test concludes, regardless of whether the specimen has failed It can be calculated based on the number of test cycles corresponding to the vibration frequency For materials with lifetime characteristics that are independent of frequency, like silicon, the test cycles are selected based on the stress cycles experienced by the actual devices throughout their lifetimes Refer to Annex D for more details.
Test environment
The test environment should be maintained at a constant temperature and humidity
Frequency response test
Before conducting the fatigue test, it is essential to measure the resonant properties of the specimens If there are variations in the resonant properties among the specimens, the controller will require tuning, necessitating the measurement of the resonant properties for all specimens.
The frequency response test measures resonant properties by applying an oscillation signal from a function generator, sweeping the frequency around the expected resonant frequency To ensure accurate measurements for the fatigue test, the load applied during this response test must be minimal If the load's effect is significant, the number of load cycles from the response test should be incorporated into the fatigue test data.
General
The fatigue test involves applying resonant oscillation at a specified amplitude to the specimen, concluding either upon specimen failure or when the predetermined test duration is achieved.
Initial load application
Proper specification of the amplitude increase rate at the beginning of a fatigue test is crucial, as the test is conducted in resonance with a high quality factor Rapid amplitude increases can lead to overshooting and unexpected failures, while excessive delays can compromise test results Therefore, careful control of the initial load is essential to ensure accurate lifetime test measurements Any potential effects should be documented in the test report, detailing the procedures for amplitude increase.
Monitoring
Continuous monitoring of the specimen's vibration during testing is essential for detecting failures This can be achieved by tracking the vibration frequency and amplitude, with changes recorded at appropriate time intervals In the absence of a monitoring system, the fatigue test can be paused at specific intervals to perform a frequency response test as outlined in section 7.2.
Counting the number of cycles
The fatigue test cycles can be tracked using a counter, or alternatively, they can be calculated by multiplying the vibration frequency by the duration of the testing period.
End of the test
The test shall end at the point of specimen failure, or when a predetermined loading time or a predetermined number of cycles has elapsed
Specimen failure is defined as the following: a) fracture of the test part; b) a certain percentage change in the amplitude; c) a certain change in the oscillating frequency.
Recorded data
The specimen's failure must be documented, and measurements of the oscillation amplitude, frequency, temperature, and humidity of the testing environment should be taken at regular intervals throughout the test.
The test report shall include the following information
• Mandatory a) reference to this International Standard, i.e IEC 62047-12 b) test piece material
− in the case of a single crystal: crystallographic orientation c) method and details of test piece fabrication
− method of thin film deposition
− heat treatment (annealing) conditions d) shape and dimensions of test piece e) test equipment
− oscillation method (self-oscillation, external oscillation)
− test monitoring method (amplitude, frequency, number) f) fatigue test conditions
− reference strength, and its measurement method
− mean stress (in the case of displacement control, mean displacement)
− stress amplitude (in the case of displacement control, displacement amplitude)
− testing environment (temperature and relative humidity)
The number of applied cycles to failure is crucial in testing; if the test piece does not fracture within a predetermined number of cycles, it is important to record both the cycle count and the notation "no failure."
• Optional a) purpose of the test
− in the case of polycrystalline thin film: texture and grain size c) internal stress d) mask design
− resolution of mask drawing and lithography e) surface roughness of test piece
− photographs of the finished test part, along with any surface treatment (cleaning procedure) f) brief description of fracture characteristics g) detailed test results
− S-N curve (S is peak stress or stress amplitude)
− fatigue strength, statistical processing (fatigue probability)
Example of testing using an electrostatic device with an integrated actuation component and displacement detection component
The specimen comprises a flexible beam test part and a fan-shaped mass created through dry etching of a thin film of single crystal silicon, as illustrated in Figure A.1 One end of the flexible beam is anchored to the mass, while the other end is secured to a substrate An electrostatic comb-drive actuator is linked to the mass, allowing for in-plane bending of the flexible beam A comb electrode attached to the mass generates a signal that is proportional to the displacement, and the mass features a deflection scale that can be observed under a microscope The test material, utilized for both the electrostatic actuator and sensor, is a conductive material.
The resonance frequency of the specimen is influenced by the moment of inertia and the flexural rigidity of the beam, with the fundamental frequency of in-plane vibration ranging from 38.75 kHz to 39.71 kHz due to the non-uniform thickness of the thin film The Q value of the oscillator was estimated at 370 based on displacement response measurements from an external signal applied to the actuator To enhance the Q value, the bottom surface of the oscillator substrate was etched, and no additional resonance modes were detected near the first in-plane vibration frequency during testing.
Figure A.1 – Microscope image of the specimen
The test sample consisted of a single crystal silicon with a 10 µm wide, 30 µm long, and 5 µm thick SOI active layer, featuring etched surfaces on the sides In-plane bending deformation introduced stress on the sidewall surfaces, which posed a risk of failure To mitigate the effects of surface roughness on the sidewalls, specific measures were implemented.
The IEC 2065/11 standard emphasizes achieving a smooth side surface through lithography and etching techniques To investigate the potential for failure in beam-structured test parts, a 4 µm deep notch was introduced at the center of one side This notch featured a half-circle tip with a radius of 0.5 µm, and the stress concentration behavior was analyzed using the finite element method.
The test equipment utilized for applying resonant oscillation to the specimen comprised solely an electric circuit, with the actuation and detection mechanisms integrated into the specimen itself As illustrated in Figure A.2, the resonant vibration was generated through a self-oscillation method.
5 Oscillating circuit 6 Automatic gain controller
9 Control PC 10 Oscillating waveform monitor
Figure A.2 – Block diagram of test equipment
Eight sets of test equipment were produced to operate simultaneously In this configuration, sharing the power supply and circuit board can lead to unstable oscillations due to electrical interference To mitigate this issue during parallel testing, specimens with slightly different frequencies were chosen to reduce electrical coupling.
The actuation component enhances a drive signal by adjusting its amplitude and phase based on the displacement signal to achieve feedback oscillation This amplified actuation signal, processed through a biased high voltage amplifier, is then applied to the electrostatic comb actuator During stable oscillation, the drive signal voltage ranges from \$V_{pp} = 20 \, V\$ to \$V_{pp} = 40 \, V\$.
The displacement amplitude of the specimen was determined by measuring the capacitance of the comb electrodes using a charge amplifier This process generated an electrical signal that was proportional to the angular displacement Simultaneously, the amplitude was observed through a microscope by reading the deflection scale, which facilitated the calibration of the electrical signal.
In this technique, the stress on the test part is assessed through finite element analysis, utilizing the measured angular displacement of the mass, as the force applied cannot be directly measured.
The self-excited oscillation circuit enables the specimen to vibrate at its resonance frequency To maintain a constant displacement, an automatic gain controller (AGC) is integrated into the circuit Additionally, the specimen's amplitude can be adjusted through software, as the AGC's reference voltage is regulated by an analog voltage output from a computer.
Rapidly increasing the AGC amplitude in the vibration control circuit can lead to destabilized oscillation due to mechanical resonance and high Q value Unstable vibrations may occur after several dozen milliseconds, resulting in test part failure from vibration overshoot shortly thereafter To mitigate this issue, the AGC amplitude was increased linearly over a 10-second period through computer control, achieving stable vibration without overshoot This method allowed for a controlled increase in oscillation amplitude, and the duration of the amplitude rise was negligible compared to the fatigue life, thus excluded from the test results.
This test method outputs an electric displacement signal, which indicates the stability of the specimen's vibrations A stable sinusoidal signal suggests that the specimen is functioning without failure, while a sudden break in the waveform indicates a failure The displacement signal's waveform is monitored over a short duration using an oscilloscope, and long-term data is collected by recording the detected amplitude of the Automatic Gain Control (AGC) at specified intervals Concurrently, temperature and humidity measurements are taken to ensure comprehensive analysis.
The analog-digital converter circuit recorded the amplitude and frequency signals every second to monitor for potential failures Additionally, in certain tests, a digital oscilloscope captured the displacement signal at the moment of failure, triggered by a drop in the displacement signal.