* nicholas.randall@csm-instruments.com; phone 781-444-2250; fax 781-444-2251; www.csm-instruments.com

Characterisation of the mechanical properties of MEMS devices using nanoscale techniques

Nicholas X. Randall*

CSM Instruments Inc., 197 First Avenue, Needham MA 02494 USA

ABSTRACT This paper focuses on recent developments in the localised characterisation of the mechanical properties of Microsystems and MEMS devices and structures. Conventional indentation techniques provide a highly powerful method for measuring the load and depth response of bulk and coated materials, but can also be used to measure the mechanical properties of very small micro-machined silicon structures. Beam structures, such as are used for accelerometers, need to be characterised in terms of the number of cycles to failure, the spring constant or the energy required to bend the beam by a required amount. Such localised testing needs to be adapted to work at various distances from the origin of the beam with a positioning accuracy of less than a micron. Initial studies have proved to be highly repeatable. A range of examples is presented which covers a range of application areas, including accelerometer beam structures, microswitches and printer head structures. The basic instrumental concepts are explained together with the modifications required for testing small structures in a localised way. In addition, the localised testing of friction and wear in MEMS devices will be covered with some examples of the technology available and how it may be applied to such small contact areas in an accurate and reproducible way. Keywords: MEMS, mechanical properties, indentation, scratch, friction, wear, tensile, stress, strain.

INTRODUCTION The development of Micro Electro Mechanical Systems (MEMS) has, in recent years, been more focussed on the fabrication process than on issues concerning reliability and the link to mechanical properties. Many issues related to tribology, mechanics, surface chemistry and materials science should be considered in the operation and manufacture of many MEMS devices. At present, very few analytical instruments are available for accurate characterisation of localised properties and specific zones of interest. The indentation method for assessing mechanical properties at low loads and shallow depths is already well established for the characterisation of thin films as well as bulk materials. The depth-sensing indentation method produces a load-displacement curve from which quantitative property values can be calculated using a variety of approaches1-3. Additionally, the indenter (of defined geometry) can be accurately controlled in terms of load, depth and speed of application. Provided that the indenter can be accurately positioned on a site of interest, then the technique can be used to probe the local mechanical properties of different structures, either with single or more complicated loading cycles. Previous work has already shown the application of indentation at the nanometer scale for characterising the properties of extremely small structures such as integrated circuit bonding pads4. In addition to the quasi-static indentation method, where an indenter is moving solely in the vertical direction, the simple addition of an accurate displacement stage enables the sample to be moved horizontally, thus enabling a scratching function. In conventional scratch testing, a diamond stylus is drawn across the sample under either constant or progressively increasing load. Elastic and/or plastic deformation occurs at specific points along the scratch path, such critical points being observed by optical microscopy or by variations in acoustic emission and frictional force5-7. In many cases, the scratch test has now become accepted as a versatile tool for assessing the mechanical integrity of a surface, whether bulk or coated, and has found application in many different fields of materials research8-9. The driving forces for the failure of a coating-substrate system in the scratch test are a combination of elastic-plastic indentation stresses, frictional stresses and the residual internal stress present in the coating. The normal load at which failure occurs is called the critical load, Lc. When a progressive load scratch is performed, a number of consecutive coating failure events may

be observed as the load is increased, the final event corresponding usually to total delamination. The critical load depends on coating adhesion, but also on several other parameters; some are directly related to the test itself (intrinsic parameters) whereas others are related to the coating-substrate combination (extrinsic parameters). A large proportion of modern MEMS devices are made of silicon using the manufacturing processes developed for integrated circuit production. As the applications of such devices increase, it is anticipated that all classes of materials (metals, ceramics, polymers, composites) will become important. There are certain unique features of the mechanical properties of each class that require the testing procedures to vary from one to another. The properties considered to be important for the design of MEMS devices include the strength (yield, ultimate, fatigue), moduli, toughness, friction, adhesion, damping, Poisson’s ratio and residual stresses. From a manufacturing point-of-view, mechanical properties such as strength, hardness, scratch resistance and chemical-mechanical behaviour are important. The effect of the manufacturing procedure on the device properties also needs to be addressed. Properties such as crack propagation, friction, wear, fatigue, adhesion and property changes due to corrosion/oxidation/erosion during service will determine the lifetime of a device. Since many microsystems are subjected to high frequency vibrations, damping and other dynamic properties need to be accurately measured in order to simulate in-service conditions. This paper summarises initial work on characterising the mechanical properties of several different micro-devices using a combination of different methods and equipment. The examples are presented as case studies and cover a range of modern structures. The main objective of such work is to develop a mechanical test platform dedicated to the analysis and characterisation of MEMS devices, systems and components.

CASE STUDIES 1 Mechanical properties of accelerometer structures Microfabricated accelerometer structures are now commonly used for instrumentation, tilt control, shock detection and seismic activity monitoring applications. The main component of the capacitive sensor is a silicon beam structure which is modified in order to be able to measure different inertial ranges. Such structures need to be characterised in terms of the number of cycles to failure, the spring constant or the energy required to bend the beam a required amount. Nanoindentation has already been applied to the measurement of scanning probe microscopy (SPM) cantilever spring constants10 and so seemed the logical choice for investigating accelerometer beams. Spring constants can be measured by dynamic methods (cantilever resonance), deflection methods (static loading of the beam) or by measuring the physical dimensions of the beam (provided that the material properties are known). Standard indentation equipment utilises either pyramidal or spherical indenter geometries, depending on the application. For measuring accelerometer beams, a tungsten carbide spherical indenter of tip radius 10 µm was chosen and its shank diameter reduced to approximately 1 mm in order to be able to access a single beam without contacting the sidewalls of the microstructure.

Figure 1: Continuous multicycles (100 cycles) at two different distances from the origin of the beam. Note the varied hysteresis depending on position.

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Continuous multicycles (total 100 cycles) were performed at different distances from the origin of the beam, positioning accuracy being guaranteed by a displacement stage with lateral resolution of 1 µm. A typical beam is of length 3800 µm and width 2100 µm. Two examples are shown in Fig. 1 for which hysteric variations are evident. Fig. 1 (a) was measured with a maximum load of 180 mN and loading rate 400 mN/min. whereas Fig. 1 (b) was measured with a maximum load of 270 mN and loading rate 500 mN/min. Higher frequency multicyclic testing could also be considered for simulating in-service conditions but was beyond the range of the instrument in its present configuration. The non-linear behaviour of the beams is thought to be due to a structure present beneath the beam after fabrication. At a certain deflection, the beam makes contact with this structure and therefore deviates from linear load-depth behaviour. Fatigue and failure properties of the accelerometer beams were investigated by using progressive load multicycles to measure the failure load and/or the number of cycles to failure. An example is shown in Fig. 2; the plotted curves in (a) and (b) clearly show the point at which catastrophic structural damage occurs. The photograph of the indenter tip in contact with an accelerometer beam has been included to demonstrate the positioning accuracy required for such testing. It should be noted that the detection load for all tests (the resistance required for contact) was 1 mN and that subsequent analysis of failed beams showed that the indenter had not significantly indented the Si material.

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2 Deflection properties of microbeam switches The microbeam switch, or MicroReed11, is an ultra-small passive MEMS magnetic proximity sensor designed for portable applications. The fabrication technology is based on selective electrodeposition of metals which allows the size of the sensor to be drastically reduced (actual packaged volume is 2 mm3) and improves the uniformity of its characteristics. The reduction in size inevitably leads to a reduction of the contact force that results in an increase of contact resistance but also to a high sensitivity to surface contamination. The contact characteristics are therefore critically dependent on process design and environment. The microbeam is built up from a borosilicate glass substrate which is first metallised with Cr-Au and structured to provide two contacts. The three-dimensional microstructure is then constructed step by step on the metallised substrate by selective electrodeposition of metals. The magnetic beams are

Figure 2: Load-depth progressive multicycle resulting in failure of the accelerometer structure. Upper plot (a) shows load-depth curve whereas lower plot (b) shows load and depth cycles as a function of time. The photograph (c) shows the indenter in contact with an accelerometer structure.

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made of low stress 80Ni – 20Fe alloy and the contact surfaces are made of rhodium to prevent sticking problems. The suspended beam and the air gap are achieved using the sacrificial layer technique. The microbeam switch is typically expected to survive 200,000 operation cycles if packaged with epoxy sealing. However, it can survive up to 100 x 106 operation cycles if hermetically packaged to maintain the device in a controlled atmosphere.

Figure 3: The three stages of the test: approach (a), first contact (b) and final contact (c). Note that the detection load is 10 µN and the tip radius is 20 µm. Microbeam switches need to be characterised in terms of the total deflection possible between the rest position and the point at which the beam makes contact with the electrode underneath. A simple load-depth cycle can be utilised to measure the total deflection as well as the load required to reach that point. An indentation instrument is the logical choice for this type of testing and a spherical diamond tip of radius 20 µm was used and positioned accurately at a predefined position on the upper electrode (beam) of the switch. The tip approaches the surface and begins a load-depth cycle once it detects a certain resistance (detection load of 10 µN). The tip then pushes the beam downwards until it contacts the electrode underneath. The load required and the measured displacement are recorded simultaneously during the cycle. Fig. 3 shows the three stages of a typical test as seen through a side-mounted monocular lens and Fig. 4 presents a result for which a total deflection of 7 µm can be measured for an applied load of 60 µN. A scanning electron microscope image shows a microbeam switch viewed from the side.

Figure 4: Typical load versus depth plot (a) giving a total deflection of 7 µm for an applied load of 60 µN. The scanning electron micrograph (b) shows a sideview of a microswitch.

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3 Failure of microfabricated printer heads A typical printing head chip12-14 consists of 480 polarisation diodes and micro-electromagnets (480 dpi) which allow dots to be magnetised on a magnetic drum; magnetic ink (powder) is then transferred onto the drum and finally onto the paper. With 18 chips mounted on a printer, outputs of up to 700 pages per minute are possible. The electromagnets are made up of multi-level copper coils and electroplated Ni-Fe poles which are integrated onto a Ni-Fe magnetic structure. In service, the head is in contact with a printer drum rotating at 2 ms-1 and must therefore retain a certain amount of flexibility. The failure load at various points along the head membrane can be measured using a specially adapted, flat geometry indenter and a standard load-depth cycle. Several tests over the load range 0 – 10 N have confirmed that the structure fails at two distinct points; the first critical load (Lc1) corresponds to first cracking of the Fe-Si substrate, after which the second critical load (Lc2) corresponds to catastrophic failure of the structure to support further load. An example is shown in Fig. 5 for which the two micrographs correspond to the two critical failure points labelled on the load-depth curve. The final spike on the curve corresponds to the failed structure being compressed between the advancing indenter and the underlying support structure. For an indenter with a flat contact radius of 500 µm, the critical failure loads, Lc1 and Lc2, are 5.2 N and 6.3 N respectively.

Figure 5: Optical micrographs (a) and (b) showing the effect of critical failure loads (Lc1 and Lc2 respectively) measured from the load-depth curve in (c). The positioning of the indenter on the printer head structure is shown in (d).

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4 Scratch resistance of Micro-Slit reflective coating The variable-entrance slit system (or Micro-Slit) is now commonly used as a critical diaphragm component in many spectrophotometers. This micromachined structure (Fig. 6 (a)) consists of a central aperture plate supported by a pair of flexible beams which allow light of different wavelengths to pass through the slit. The aperture plate is coated with a thin (500 nm) aluminium coating which serves the function of masking any light around the aperture.

Figure 6: Optical micrograph (a) of a typical Microslit structure showing the central aperture plate supported by a pair of flexible beams of thickness 80 µm. The zoomed image (b) shows two scratches made on each side of the central slit (scratch direction from top to bottom) and corresponds to the arrowed area in (a). Characterisation of the adhesion of this Al coating to its Si substrate is difficult owing to the small size of the aperture plate. Scratch testing was used to accurately measure the adhesion and scratch resistance by making progressive load scratches over the load range 0 – 10 mN with a 5 µm radius spherical diamond tip. Fig. 6 shows two such scratches made on each side of the central slit. Subsequent optical microscopy along the scratch paths allows the critical failure points to be observed: first failure consists of cracking at the sides of the path, whereas final failure is seen as delamination of the coating from the substrate, as shown in Fig. 7. Scanning Force Microscopy (SFM) as an integral part of the testing system provides significant additional information, especially regarding deformation modes.

Figure 7: Optical micrograph (a) of failure where the coating delaminates from the substrate. The SFM image in (b) shows the deformation along the sides of the scratch track.

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5. Tensile properties of microfabricated silicon beams Modern MEMS devices are fabricated from very small volumes of material and it is very difficult to measure the tensile properties of such small structures by conventional methods. It is important to measure the actual properties of microfabricated structures as opposed to measuring the properties of larger material volumes and then extrapolating down. One way is to actually fabricate tensile test structures on the same wafer on which actual devices have been manufactured. In this way, the tensile beams can be made with the same dimensions as those of interest and by the same techniques.

Figure 8: SEM micrographs of flat punch indenter machined from a Tungsten Carbide blank at (a) low (b) medium, and (c) high magnification. A typical tensile sample is shown in Fig. 9 (a) and consists of a fixed end (on left side) and a freestanding ring at the other end with an internal diameter of approximately 50 µm. A flat punch indenter of radius 40 µm (shown in Fig. 8) is lowered into the freestanding ring and then displaced to the right at a steady rate. The tensile sample is therefore stretched until failure occurs, whilst recording the lateral force (stress) and percentage elongation (strain). A typical test result is shown in Fig. 9 (b).

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6. Friction and wear properties of MEMS materials and lubricants Measurement of tribology at the micro and nano scales has been limited in recent years by the lack of dedicated instrumentation. The Scanning Force Microscope (SFM) has been the instrument of choice for investigating friction, wear and lubrication at such small scales. However, the SFM often produces contact pressures in the gigapascal range owing to the small dimensions of the tip. The recently developed Nano Tribometer allows a much greater variation in contact conditions, and is therefore more suited to the study of materials and lubricants at very low loads. A recent study15 has been able to show the transition through differing lubrication regimes as the applied load is varied on a lubricated contact. Because of the large surface area to volume ratio in MEMS devices as the size scale decreases, the surface forces such as adhesion and friction become increasingly critical and dominate over inertial and gravitational forces. It is therefore critical to be able to determine friction and wear properties at such small scales.

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Figure 10: Example of a typical Nanotribometer test (a) using a 200 µm diameter silicon ball loaded onto a microfabricated Si device. The measured values of friction coefficient (µ) are plotted in (b) for different applied load values in the range 250 µN - 4 mN. The Nanotribometer can be used in either circular or linear reciprocating modes, depending on the application. An example of the latter is shown in Fig. 10 (a) where a silicon ball can be seen in contact with a microfabricated Si surface. The applied load, velocity, amplitude and contact geometry can be varied in order to simulate a MEMS contact in the most accurate manner. Some examples of friction coefficient results are plotted in Fig. 10 (b) for different applied load values over the range 250 µN - 4 mN. It can be seen that as the load is increased, the interaction of the ball, sample surface and lubricant fall between three distinct regimes. For loads of 250 µN and 500 µN the lubricant inhibits the sliding motion of the ball on the surface leading to higher frictional resistance. At 1 mN, the friction coefficient is still high but the curve gradually decreases over the duration of the test, characteristic of the “running-in” of the surfaces. For loads greater than 2 mN, the curves seem to have a running-in period after which a steady state is reached where the coefficient of friction stabilises to a constant value.

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CONCLUSIONS The case study examples confirm the importance of mechanical properties characterisation with a testing instrument which is capable of indentation, scratching, pushing, pulling, etc. in highly localised locations of a MEMS device. A combined SFM allows high resolution microscopy of tested regions and provides significant additional information concerning deformation modes in both bulk and coated materials. Accurate test equipment for the scales required by MEMS devices is still not fully developed. The components used in such devices are very light (micrograms) and operate under very light loads (micrograms to milligrams). The resultant friction and wear between these components are highly dependent on surface interactions. MEMS devices no longer consist only of silicon: the recent development of lithography and precision engineering techniques has allowed the use of a large variety of materials, e.g., metals, ceramics, glasses and polymers. Stiction problems need to be solved in all types of devices, e.g., relay contacts, motion-stopping structures, valves, etc. Currently, the friction in bearings is the main requirement for tribology research in non-silicon micro actuators. There is still very little known about the effect of processing parameters (such as doping level, annealing temp. and deposition time), microstructure and specimen size on the micro/nano-mechanical behaviour of MEMS and similar device materials. In order to simulate in-service conditions accurately, high frequency testing methods will need to be developed, together with accurately controlled environmental chambers. The initial work presented in this paper is only the beginning of what will undoubtedly become a very important part of future MEMS research and development, encouraged by the next generation of testing equipment.

ACKNOWLEDGEMENTS Mr Francois Gueissaz from Asulab S. A. is gratefully acknowledged for providing the microbeam switch samples. Dr Brad Boyce at Sandia National Laboratories is acknowledged for providing the microfabricated tensile samples.

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