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. This article presents some results from measurements made with a Nano Tribometer from CSM Instruments on a selection of commonly used MEMS structural materials.
The tests were performed using a Si (100) ball of radius 500 μm as the spherical partner mounted on a stainless steel cantilever. The three sample materials consisted of a single-crystal Si (100) wafer (phosphorous doped), a diamond-like carbon (DLC) film of thickness 10 nm (deposited on a Si (100) wafer) and a hexadecane thiol (HDT) self-assembled monolayer (SAM) deposited on a Au(111)/Si(100) substrate by immersion.
Force Calibration Plot Method
The adhesive forces were measured in ambient conditions (22°C, relative humidity of 45% - 55%) using a technique very similar to the ‘force calibration plot’ method used in Scanning Force Microscopy (SFM).
This consists of bringing the ball into contact with the sample material in a controlled way and keeping the surfaces in contact for a period of time. The maximum force, needed to pull the upper and lower surfaces apart, is measured as the adhesive force.
A typical example of such an adhesion measurement is shown in Fig. 1 for a Si (100) ball in contact with a flat of the same material. As the ball approaches the fl at sample within a few nanometers (point A), an attractive force exists between the two surfaces. The ball is therefore pulled toward the sample and contact occurs at point B. The adsorption of water molecules on the sample surface can also accelerate this so-called snap-in, due to the formation of a water meniscus. From this point on, the ball is in contact with the sample surface, and as the Z-piezo extends further, the cantilever is further deflected. This is represented by the sloped portion of the curve. The time effects on the adhesive force can be studied by maintaining the Z-piezo at its maximum length for various time periods. As the ball is then retracted from the surface (point C), it goes beyond the zero deflection (flat) line due to the attractive force. This phenomenon can be due to long-range meniscus force, van der Waals force or electrostatic force. At point D, the ball snaps free of the adhesive forces and is again in free air.
Figure 1. Typical adhesion data for a Si (100) ball contacting a Si (100) flat with a rest time of 2 seconds. The cantilever deflection is plotted as a function of time (a) and of displacement (b) as the ball is approached to the surface, contact established and the ball then retracted.
Figure 2. Variation of friction force as a function of applied normal load for measurements made on Si (100), DLC and HDT surfaces with a Si (100) ball of radius 500μm. A sliding speed of 720μms-1 and sliding amplitude of 1000μm were used in the linear reciprocating mode.
Friction Force Measurements
The frictional forces were measured by using the instrument in the linear reciprocating mode (as opposed to the pin-on-disk mode) with applied normal loads in the range 100 to 2500μN. Average values of the coefficient of friction were obtained by measuring the frictional force as a function of normal load and reproducibility was found to be within ± 5%. Some typical results are summarised in Fig. 2 where it can be seen that all three samples exhibit a linear response over the measured load range. The coefficients of friction were calculated and ranked in the following order: μSi (0.47) > μDLC (0.19) > μHDT (0.15). This confirmed that thin layers of DLC and HDT can be used as effective lubricants for Si materials in MEMS devices.
The effects of velocity were investigated by measuring the frictional force with velocities from 50 to 2200μms-1. All tests were carried out in an ambient condition at a normal load of 2000μN. The results are shown in Fig. 3 (a) and indicate that for Si (100), the friction force initially decreases until equilibrium occurs, whilst it seems that the velocity has almost no effect on the friction properties of DLC and HDT.For Si (100), at high velocity, the water meniscus is broken and does not have enough time to rebuild. Tribochemical reactions are also thought to play an important role, as the SiO2 native oxide interacts with water molecules producing Si(OH)4 which is removed and continuously replenished during sliding. This Si(OH) 4 layer is known to be of low shear strength. On the other hand, the DLC and HDT surfaces exhibit hydrophobic properties and can only absorb a few water molecules in ambient conditions so the friction force is not significantly influenced by the sliding velocity.
Figure 3. Experimental results showing the influence of (a) sliding velocity, (b) relative humidity and (c) temperature on the friction force of Si (100), DLC and HDT.
The effects of relative humidity were investigated by introducing a mixture of dry and moist air. The humidity could therefore be varied from 5% to 65% while the temperature, normal load and scanning velocity were maintained at 22°C, 2000μN and 720 μms-1 respectively.
The results are shown in Fig. 3 (b) and it can be seen that for Si (100), the friction force increases with a relative humidity increase up to 45% but then shows a slight decrease with a further increase in the relative humidity. The humidity seemed not to have any influence on the friction properties of DLC or HDT. In the case of Si (100), the initial increase in humidity up to 45% causes more adsorbed water molecules which form a larger water meniscus which leads to an increase in friction. But at very high humidity (65%), large quantities of such molecules can form a continuous water layer which separates the ball and sample surfaces, creating a lubricant layer which causes a decrease in friction.
The temperature of the tribological contact was varied from 25°C up to 125°C whilst maintaining the relative humidity, normal load and scanning velocity at 45%-55%, 2000μN and 720 μms-1 respectively.
The results presented in Fig. 3 (c) show that at temperatures above 50°C, an increase in temperature causes a significant decrease in the friction for Si (100) and a slight decrease in the case of DLC. The HDT seems not to be influenced by changes in temperature over the range tested. At high temperatures, desorption of water and reduction of surface tension lead to the decrease in friction forces of Si (100) and DLC. However, in the case of HDT, only a few water molecules are adsorbed on the surface so the aforementioned mechanisms do not exert a significant influence and thus the HDT seems unaffected by any temperature change.