Due to their extremely low working force range, mechanical microsystems are sensitive to phenomena which can usually be ignored in macroscopic mechanical systems. Stiction refers to the adhesion originating from the tribological contact of moving parts, which can result in immobilisation of the device. In a typical microscopic contact, stiction forces in the nano- and micro-Newton range can prove severely detrimental to the microsystem functionality. Different force types can be responsible for stiction, although capillary forces are believed to be predominant in mechanical microsystems.
Tribological Functionality of a Microshutter
This article describes the use of the Nano-Scratch Tester (NST) from CSM Instruments for investigating the tribological functionality of a microsystem known as a microshutter. This micromachined system consists of a shutter blade, a suspension beam, two fixed electrodes and two stoppers at either side of each electrode to prevent short circuits. The hole under the blade is positioned so that it is either open or closed, depending on where the blade is positioned. A typical microshutter is shown in Fig. 1. With this kind of microsystem, stiction can occur between the sides of the blade and their respective stopper surfaces. Recent efforts have been made to lower the surface energy of polysilicon microstructures by depositing extremely thin coatings by plasma polymerisation. However, it is very difficult to deposit sufficient material on vertical sidewall structures.
Figure 1. SEM micrograph of a typical polysilicon microshutter.
The plasma polymerisation process is in this case applied with hexafluoropropene as a precursor molecule in order to grow a Teflon-like coating. The samples are placed under a Faraday cage (i.e., a field free zone) within the reactor in order to promote a conformal growth of the polymer film, this being necessary in order to cover all faces of the microstructures.
The Scanning Electron Microscope (SEM) images shown in Fig. 2 confirm that the deposition process is sufficiently isotropic to coat the lateral surfaces of the electrodes, stoppers and blade over their whole height.
Figure 2. SEM micrographs showing the original polysilicon surface texture (a) of the microshutter sidewall and the Tefl on-like coated microstructure (b) which can signifi cantly reduce stiction in this type of microsystem.
Thin Film Quality Control
Quality control of such thin films is very difficult in-situ owing to the complexity of the micro-machined structure and the difficulties in positioning a probe tip onto a sidewall. For this reason, different types of Teflon-like coatings are deposited onto wafers of mono- and polysilicon material similar to that of the microshutter itself. Subsequent NST measurements are then performed in order to characterise the adhesion and frictional properties of the coatings.
To give an idea of the dimensional limitations involved with the microshutter, some explanation of its operation are needed: electrostatic forces are only used to keep the shutter in the open or closed position, whilst the driving force to switch the shutter from one state to the other is only delivered by the spring energy stored in the suspension beam.
The electrostatic voltage is only active over the narrow gap (0.5 - 1 μm) between the shutter blade and one of the electrodes. The shutters are brought to resonance by applying an alternating excitation voltage on one electrode and a direct attraction voltage on the other.
The results presented in Fig. 3 show three-dimensional SFM images of nano-scratches made on the polymer coating deposited on two different substrates. The plastic deformation seems to be more marked for the polysilicon substrate than for the polished Si. The surface roughness is also higher in the case of the former. This factor could well influence the tribological characteristics of the microshutter contacts and so needs to be optimised.
Figure 3. Scanning Force Microscopy (SFM) images showing the difference in plastic deformation of the deposited polymer coating for a standard polished Si substrate (a) and a polysilicon substrate (b). The images were taken at the main critical point along the scratch path where delamination begins. Note the difference in surface roughness between the two types of sample.
Nano scratch tests were performed on the different sample types with an applied load in the range 0 to 1 mN, using a standard diamond tip of radius 2 μm. A typical result is shown in Fig. 4 for the case of the polysilicon substrate and a distinct change in both the penetration depth and frictional force signals corresponds to the measured critical point. A standard cantilever was used, having a maximum load of 100 mN and a resolution of 1.5 μN. The maximum measured penetration depth was around 0.9 μm and scratch speed was maintained at 2 mm/min.
The Micro Electro Mechanical Systems (MEMS) industry already accounts for over a billion dollars a year and is growing rapidly. Much of the research and development effort is presently concentrated on improving the fabrication processes for various devices. However, there are still many serious issues related to tribology, mechanics, surface chemistry and materials science in the operation and manufacture of MEMS devices. Such issues are currently preventing rapid commercialisation of MEMS systems from taking place.
The solutions lie in a fundamental understanding of friction/stiction, wear and the role of surface contamination and environmental debris in microscale devices. Very little is understood about the tribology of bulk silicon and polysilicon films used in the construction of these devices.
Additional problems lie in the characterisation of components in-situ within their normal operating environment. Dimensional limitations make it very difficult to place a measuring probe exactly at the interface where tribological modifications take place in service.
Scratch Resistance, Adhesion, Wear, Roughness and Deformation
The Nano Scratch Tester is a pioneering instrument in the field of surface mechanical properties characterisation. Combined with a scanning force microscope it becomes a very powerful tool for investigating scratch resistance, adhesion, wear, roughness and deformation modes (both elastic and plastic) in both bulk and coated materials.
The example featured in this application note is only one of many systems which can be better understood by adequate characterisation methods. Microelectronic 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. The Nano Scratch Tester, with its versatile operation modes and high resolution, should prove to be a useful tool in the investigation of many such parameters and their effects on service operation and lifetime.
Figure 4. Typical NST results showing the critical point at which the indenter reaches the Si substrate. The upper curve corresponds to the penetration depth and the lower curve to the frictional force, for the measurement shown in Figure 3(b).