FEM analysis of multilayered MEMS device under thermal and residual stress

It is essential to ensure reliability to produce a Micro-Electro Mechanical Systems(MEMS) on commercial scale. Reliability problem in inkjet printhead, one of MEMS, is also very important. To eject an ink drop, temperature of heater must be high so that ink contacting with surface reaches above 280° C on the instant. Its heater is embedded in the thin multi-layer in which several materials are deposited. MEMS processes are the main sources of residual stresses development. Residual stress is one of the factors reducing the reliability of MEMS devices. We measure residual stresses of single layers that consist of multilayer. FE analysis is performed using design of experiment. Transient analysis for heat transfer is performed to get a temperature distribution. And then static analysis is performed with the temperature distribution obtained by heat transfer analysis and the measured residual stresses to get a stress distribution in the structure. Although the residual stress is bigger than thermal stress, thermal stress is more influential on fatigue life.     

Determination of tensile properties and residual stresses of Ni-Co thin films

This paper reports tensile properties and residual stresses of Ni-Co thin films. To measure elastic (and plastic) properties, direct tensile tests using dog-bone type specimens are performed first. Assuming that residual stresses vary linearly through the film thickness, bending and membrane residual stress components are measured using cantilever beam and T-structure beam specimens, respectively. Averaged values of Young’s modulus, yield strength and tensile strength are found to be about 163GPa, 1,700MPa and 2,000MPa, respectively. The membrane and bending residual stress components are found to be about 825MPa and 47MPa, respectively.     

Fatigue test of MEMS device : a monolithic inkjet print

A testing system was developed to improve the reliability of printhead and several printheads were tested. We developed a thermally driven monolithic inkjet printhead comprising dome-shaped ink chambers, thin film nozzle guides, and omega-shaped heaters integrated on the top surface of each chamber. To perform a fatigue test of an inkjet printhead, the testing system automatically detects a heating failure using a Wheatstone bridge circuit. Various models were designed and tested to develop a more reliable printhead. Two design parameters of the width of reinforcing layer and heater were investigated in the test. Specially, the reinforcing layer was introduced to improve the fatigue life of printhead. The life-span of heater with a reinforcing layer was longer than that without a reinforcing layer. The wider the heater was, the longer the life of printhead was.     

Tensile and high cycle fatigue test of Al–3% Ti thin films

This paper presents the results of tensile and high cycle fatigue tests with stress ratio R = 0.1 for an Al–3% Ti thin film of 1 μm thickness in atmospheric air at room temperature. Specimens with three different widths (50, 100, and 150 μm) were fabricated to study the width effects of each sample. Test results show that tensile and fatigue properties for the Al–3% Ti thin film with different widths are very close, and, thus, width effects were found to be minimal. The elastic moduli ranged from 80 to 82 GPa, and the tensile strengths ranged from 369 to 379 MPa. Fatigue strength coefficients of the specimens with 50, 100, and 150 μm width were 193, 181, and 164 MPa, respectively. In addition, fatigue strength exponents of the specimens with 50, 100, and 150 μm width were −0.023, −0.020, and −0.013, respectively. When present test results are compared with typical properties of bulk aluminium, the Al–3% Ti thin film is found to have longer life at the same stress, but it is more sensitive to the stress level.     

An apparatus for performing microtensile tests at elevated temperatures inside a scanning electron microscope

In this paper, we introduce an apparatus to perform microtensile tests at elevated temperatures inside a scanning electron microscope. The apparatus has a stroke of 250 μm with a displacement resolution of 10 nm and a load resolution of 9.7 μN. Measurements at elevated temperatures are performed through use of two silicon-based micromachined heaters that support the sample. Each heater consists of a tungsten heating element that also serves as a temperature gauge. To demonstrate the testing capabilities, tensile tests were performed on submicron Cu films at various temperatures up to 430 °C. Stress–strain curves show a significant decrease in yield strength and initial slope for the samples tested at elevated temperature, which we attribute to diffusion-facilitated grain boundary sliding and dislocation climb.     

Residual strain measurement of piezoelectric multilayers by spiral structure

A new structure is described to measure the residual strain of thin film of piezoelectric multi-layers. The spiral shaped structure consists of the four fixed-guided beams. Piezoelectric multilayers consisting of SiOx/Pt/PZT/Pt on SiNx substrate are used to evaluate the suggested structure. Finite element analysis predicts that the out-of-plane displacement of the spiral structure by residual stress depends linearly on the beam length, but there is little difference depending on the beam width. PZT is prepared by sol-gel method and multilayered spiral structures are released by microfabrication technique. Sensitivity analysis of the spiral structure with various layer stack shows that the high displacement of piezoelectric multilayers can be decreased by the application of SiOx layer with compressive stress over the piezoelectric multilayers.     

Study on effect of mean stress on fatigue life prediction of thin film structure

This paper describes the effect of mean stress on fatigue life prediction of structure made with thin film. It is well known that the mean stress influences fatigue life prediction of mechanical structure. We investigated a reasonable method for considering mean stress when fatigue strength assessment of micro structure of thin film should be performed. Fatigue tests of smooth specimen of beryllium-copper (BeCu) thin film were performed in ambient air at R = 0.1 with 5 Hz. A micro probe was designed and made with BeCu thin film by the precision press process. Fatigue tests of micro structure were performed with 5 Hz frequency, in ambient air to verify the fatigue life predicted by computer simulation through FE analysis. The fatigue life predicted by the S _a -N curve modified by Goodman method with principal stress through FE analysis shows a more reasonable result than other methods.