Friction and wear in mems

Friction and Wear in MEMS: technology issues
and new potential lubrication methods
Friction and wear are two key factors that are detrimental at the nanoscale, because they are
linked with surface forces. It is important to understand their mechanisms and to find ways to
overcome them. At the macroscale lubricant are classically used to do so and are well understood.
However, those lubricants cannot be readily used at the nanoscale and other lubrication methods
must be found. In this paper I will focus on two new potential lubricants that were tested in
Microelectromechanical Systems (MEMS): ionic liquid films and alcohol vapor. They are both
very different and are not used in the same situations but they both show great potential. In fact,
thanks to the alcohol vapor lubricant a device did not show signs of wear after 11 days of operation,
whereas it failed within minutes in the absence of alcohol. No signs of wear were seen after 100
cycles of operation of a device with ionic liquid film but were observed in an uncoated device.
Introduction
From smartphones to cars to smart drugs, nanotechnology and microelectromechanical systems
(MEMS) have a wide range of applications and opportunities, such as accelerometers in airbags,
sensors and actuators. Yet we can only see the tip of the technology that is currently revolutionizing
our everyday life. Feynman’s visions1 are coming true but although we might swallow a camera,
we cannot swallow a nano surgeon robot yet. While there is no lack of ideas, there are a lot of
technological constraints to overcome to make them into a device.
1See R.P. Feynman, There’s plenty of room at the bottom, Caltech Engineering and Science, Vol. 23:5, pp.22-36,
1960

Friction and Wear in MEMS: technology issues and new potential lubrication methods 2
Both fabrication and reliable operations are limited by our current technology. Nanotribology
plays a major role in constraining MEMS: adhesion, friction and wear are three issues impacting
the industry. In this review, I focus on friction and wear during the operation of MEMS and on the
failure mechanisms they lead to. Even in devices designed and built with low-friction materials,
this issue still persists: we might need a higher voltage source than allowed by a design to overcome
static friction [1]. However, friction is also source of abrasive wear.
Even in MEMS with no contacting surfaces, wear (ﬂuid erosion for example) limits the lifetime
of a device and thus renders a lot of technologies non-viable. Since, we cannot afford to change
MEMS in a complete device every other week, commercialized MEMS devices have non-moving
parts or very restricted motion. The study of those nanotribology issues can lead to the development
of more complex technologies such as nano/micro motors, and gear systems.
1 Current technology: limits and opportunities
MEMS are divided into four classes: class I MEMS without moving or impacting parts, class
II MEMS with moving parts but without rubbing or impacting surfaces, class III MEMS with
impacting parts but without rubbing parts, and ﬁnally class IV MEMS with rubbing parts. The
latter class of MEMS experiences the most friction and wear because of the rubbing natures of
those devices. When scaling down structures, the size dependence of forces renders surface forces
predominant over volume forces. This scaling effect leads to an aggravated importance of friction
and wear which drastically reduces performances and lifetime of MEMS [2].
During operation, MEMS can be exposed to different environments and conditions: some will
be used in a relatively humid environment, others will experience large accelerations or will be

exposed to chemicals. It is thus important to understand the failure mechanisms and their origins.
However, should we signiﬁcantly reduce wear and friction, we would be able to build viable class
IV MEMS such as micromotors, microactuators and optical switches.
2 MEMS tribology
As discussed previously, MEMS applications are limited today by operation failure. The failure
mechanisms are scale dependent [3] and include friction and wear. As the dimensions of a device
reduce from 1 mm to 1 micron, the area gets a million times smaller and the volume a billion
times smaller. This implies that friction and surface tension are about a thousand times larger than
inertial or electromagnetic forces [4]. Friction, wear or contamination are known to alter MEMS
device performances or lead to their irreversible failure.
2.1 Friction
Several models have been developed to describe friction at the nanoscale for a single asperity on
a ﬂat [1]. Each has its own domain of validity and limitations, but they show that friction is de-
pendent on the real area of contact and that, at the nanoscale, friction does not follow Coulomb’s
law and instead is proportional to velocity [3]. Adhesion is considered as a major contributor to
friction.
In Tambe and Bushan [3], dry, lubricated, hydrophilic and hydrophobic surfaces are studied
with different rest times, sliding velocities and environment conditions (relative humidity and tem-
perature). From the results presented in Figure 1, the authors infer that the scan size dependence of
friction is independent of velocity and that friction is proportional to the scan size. This property

is attributed to surface roughness.
Figure 1: Velocity dependence of the friction force for different scan sizes. Friction appears to be higher with
larger scan size for all velocities and the trend lines are parallel.[3]
It also appears that higher rest time leads to higher adhesion, especially in soft materials that
can conform to the asperities and thus have larger contact area. Higher relative humidity and lower
temperatures enables the condensation of water molecules on hydrophilic surfaces and thus leads
to higher adhesion, which causes more difficulty in sliding and thus higher friction. No significant
changes are observed on hydrophobic surfaces.
As presented in Figure 2, the velocity dependence of friction is more complex. Some samples,
silicon (100), PDMS and Z-15 (perfluoropolyether lubricant) exhibit a decrease of the friction
force with higher velocities in the microscale but an increase in the nanoscale! Diamond-like
carbon (DLC), self-assembled monolayer hexadecanethiol (HDT) and Z-DOL (another perfluo-
ropolyether lubricant) are fairly independent of velocity in the microscale but HDT and Z-DOL
have an increasing friction with velocity whereas DLC’s friction force decreases with higher ve-
locity in the nanoscale. However, friction is significantly higher in the microscale than in the

nanoscale [3]. Friction is a source of wear, but friction can also result from wear [5]. As debris are
formed due to wear, third body interactions start appearing, leading to higher friction.
Figure 2: Evolution of the friction force for the different samples in the nano- and microscale as a function of
velocity. The different graphs show no consistent behavior from one material to another and from one scale
to another. [2]
Friction is often linked with stick-slip behavior, preventing smooth sliding. This property and
high values of the static coefﬁcient of friction mean that of higher power or forces are required
to achieve steady state motion [1]. Friction by its nature leads to abrasive wear and to plastic
deformation, both leading to failure of a device.
2.2 Wear
Wear has multiple origins and is mostly attributed to adhesion and friction. It can be useful in
the macroscale, when polishing a surface with sand paper for example, but like fatigue it can

lead to catastrophic failure over time. This is especially true in the microscale, where, as already
discussed, surface energies and friction are dominant. Wear is linked with the smoothing out of
materials, the removal of materials and generally the alteration of the shape and properties of the
system.
Wear is the main reason why a lot of devices are not viable; without proper protection, some
will fail within minutes of operation [5]. This fact shows why wear needs to be assessed and il-
lustrates why today only MEMS with no rubbing surfaces are commercialized. Wear can lead to
different phenomena: one of the most common is the creation of debris [2, 5]. Debris will act as a
third body and will increase adhesion and friction. Debris are formed as asperities rub against the
opposing surface and are indicators of the local removal of a layer.
As layers are locally removed, other mechanisms can take place. For example, if the natural
silicon dioxide is locally peeled off [1, 5], the bare silicon surface will be exposed. Silicon sur-
faces have dangling bonds and if the passivation layer is not replenished fast enough, tribological
reactions will lead to chemical junctions between two silicon surfaces. Those junctions are strong
enough to pull out other silicon grains, further wearing out the device. Any factor that increases
friction or adhesion will also increase wear. For example, high relative humidity can lead to high
wear in silicon and silicon dioxide [1].
Although lubricants can be used to reduce wear, they are also subject to this triblogical phe-
nomenon. If the lubricant layer is not constantly replenished it will get worn out and rendered
useless over time [2, 5].

3 Ionic liquid films
Classically, lubricants have been used to reduce friction and wear. However lubricants must comply
with several requirements. Indeed, a lubricant must chemically bond to the device, be long-lasting
and withstand the same environments and conditions the device is exposed to. Two examples
of common lubricant are the perfluoropolyether (PEFE) Z-TETRAOL and PEFE Z-DOL which
are widely used for their excellent properties, in particular in the hard disk drive industry [3, 2].
However those lubricants are insulators and may not be suitable in every devices.
3.1 Principle
Like the two aforementioned lubricants, ionic liquid films are specifically designed to resist to
the regular operation of MEMS devices but are about twice as thin, from 0.5 nm to 2.5 nm dep-
dending on the treatment vs 1 nm to 7nm for the Z-TETRAOL [2]. They are strongly bonded to
the substrate by electrostatic interactions which offers good lubrication property in comparison to
classically covalently-bonded fluids. They show no volatility and a high decomposition tempera-
ture, which enables their use in a wide range of temperatures. Moreover, they exhibit great heat
transfer properties and, thanks to their ionic nature, they are conductors. This conducting property
has two major advantages: it enables electrical applications and also prevents charge build-up in
the devices, reducing the risk of electrical breakdown and arcing, further reducing wear.
At least one of the two ions is organic and one or both ions have a delocalized charge to pre-
vent the formation of a crystal lattice. When using imidazolium cations, the coefficient of friction
is reduced by the presence of long organic side chains. Oxygen-rich anions reduce wear.
Two ionic liquid films have been evaluated and compared to each other: 1-butyl-3-methylimidazolium

hexafluorophosphate (BMIM-PF6) and 1-butyl-3-methylimidazolium octyl sulfate (BMIM-OctSO4).
3.2 Results
The two ionic lubricants have been tested in different conditions and the results have been com-
pared to similar experiments on PEFE Z-TETRAOL. The two ionic lubricants have been either un-
treated, thermally treated or thermally treated and rinsed. The untreated sample is completely un-
bonded and the rinsed one is completely bonded whereas the not-rinsed sample is partially bonded.
For each of the six samples, surface potential has been measured using a Kelvin probe atomic
atomic force microscope (AFM) and adhesion, friction and wear have been studied using an AFM
[2]. The results are presented in Figure 3. The two ionic liquid films tested present approximately
the same behavior as the Z-TETRAOL. From this result, we can infer that the two ionic liquids are
fairly good lubricants.
However, the results show that the untreated lubricant in both cases leads to a worse behav-
ior than the bare silicon or silicon natural oxide (higher coefficient of friction). Indeed, as the
molecules are not bonded they are moving freely and are likely to form a meniscus on the AFM
tip and drastically increase adhesion and friction. Although the treated and rinsed samples exhibit
a better behavior regarding friction, they have the worst surface potential and thus the most sur-
face rearrangement (and wear), yet they have better results than the Z-TETRAOL. This can be
explained by the lack of mobile lubricant to replenish worn out immobile lubricant.
On the other hand, the partially bonded coatings show both the lowest coefficient of friction
and the lowest surface potential. They also have the lowest number of cycle dependency. These
coatings have similar results and appear to be very good lubricants.

Figure 3: Summary of the results: a. Comparison of the coefficient of friction of the different samples and
silicon. b. Comparison of the average surface potential of the different samples and silicon. c. Comparison
of the evolution of the coefficient of friction with the number of cycles for the different samples and silicon. [2]
The difference in surface potential between the two ionic liquid films and the Z-TETRAOL is
attributed to the build-up of charges in the Z-TETRAOL whereas charges are immediately dissi-

pated in the conducting ionic liquid films. Based on these findings, ionic liquid films show strong
potential as MEMS lubricants.
3.3 Limits
The ionic liquids show potential but they also have some shortcomings. In the experiments, only
a hundred cycles were taken into account for measuring wear and the BMIM-OctSO4 starts to
show wear after only 60 cycles in the best scenario of the partially bonded lubricant. However the
coefficients of friction of the Z-TETRAOl and the BMIM-PF6 are only studied over 100 cycles
and did not show signs of wear when they stopped the experiment [2]. Although Z-TETRAOL
has been extensively studied, the behavior of both ionic liquids has not been reported for longer
use. I believe they have a real potential, but 100 cycles might only represent a few minutes of
operation. Hence, wear of the ionic lubricants needs to be analyzed further to check if they really
meet anti-wear behavior criteria.
As mentioned, one interesting feature of the ionic liquids is their conducting property that en-
ables the use of lubricants in MEMS where insulating lubricants cannot be used. However and
conversely, ionic lubricants cannot be used in MEMS devices that require an insulator: a capaci-
tance for instance. Thus Z-TETRAOL remains a lubricant of choice in such cases and the industry
will have to carefully assess their choice of lubricants, especially in a MEMS device that requires
both behaviors.
However further work show that those lubricants have tremendous potential. As discussed
in this review, the BMIM-PF6 seems a good lubricant. Other studies have thus been conducted
with other fluorine containing anions and other cations. It appear that a combination of perfluo-

roalkylphosphate (FAP) as the anion with an imidazolium cation show even more potential and is
a more stable lubricant [6].
4 Alcohol adsorption
A less traditional way to reduce wear and friction is to use a vapor-phase lubrication. Depending
on the saturation pressure of lubricant, the coating replenishes itself over time until the system runs
out of vapor.
4.1 Principle
Alcohol adsorption lubrication is a potential new vapor-phase lubricant. The problem with classi-
cal lubricant is that they need to be conformally deposited on the substrate and chemical or physical
vapor deposition techniques cannot comply with this requirement. However, in vapor-phase lubri-
cation (VPL), such a coating is achieved by the adsorption of the gas molecule on the MEMS
surfaces [5].
Molecules can desorb from the surface, but as long as equilibrium is reached and the pressure
is maintained, another one will adsorb automatically and replenish the coating. One of the main
issues with VPL is the need for high temperatures or catalytic precursors to enable the coating.
However, alcohol presents interesting vapor pressure at ambient temperatures and can thus cir-
cumvent this issue.
Although the mechanism is not known, when using alcohol adsorption, high molecular weight
oligomers (a polymer comprised of only a few monomers) form during operation and further dras-
tically reduce friction and wear [5]. The wear protection mechanism is simple: alcohol molecules

adsorb on the surface and disrupt the equilibrium of strongly adsorbed water molecules, driving
their removal. Water is known to increase wear and friction in silicon and silicon dioxide. The
polymer formation is referred to as an in-situ lubrication method.
4.2 Results
The lubrication properties of alcohol adsorption were evaluated at the macroscale with a ball-on-
flat tribometer, at the microscale with a sidewall MEMS tribometer and at the nanoscale with an
AFM [5]. Although, the lubricant exhibited excellent properties in the three scales, I will focus
on the microscale. The different experiments were conducted with a constant alcohol-vapor pres-
sure and thus concentration in a stainless steel chamber. The MEMS device was coated with a
chemisorbed organic monolayer, rendering the surfaces hydrophobic.
The alcohol used in the experiments is pentan-1-ol (or 1-pentanol). To assess its influence on
Figure 4: isotherm thickness for pentan-1-ol. It appears that coverage is obtained at P/Psat=10%. [5]
the tribology, the authors determined the isotherm thickness. It appears that coverage is reached
when P/Psat = 10%, where P is the pressure and Psat is the vapor saturation pressure, and higher
ratios do not yield significantly better results, which are presented in Figure 4. The spectrum shows
that the water adsorption equilibrium is shifted and that water molecules are desorbing from the

surfaces whereas alcohol molecules are being adsorbed [5].
Then lifetime, defined as the number of cycles the device oscillates under a given load before
failure, was assessed by measuring the friction coefficient over time. Without the alcohol vapor,
the device fails in less than two minutes of operation ( 104 cycles), which reveals high wear and
friction. However at 95% of Psat, the device was turned off by the operator and did not show
wear. At 15% a device did not fail and had no sign of wear even after eleven days of operation,
but a liquid build-up was observed. Those results are presented in Figure 5. The liquid build-up is
Figure 5: a: Friction coefficient over time for 0%, 15% and 95% Psat. b: On the left a SEM image of the
device at 15% Psat after 108 cycles and on the right an unused device. In a, we can see that 15% is enough
to prevent failure. In b, we can observe a liquid build-up circled in white. [5]
circled in white in Figure 5.b and is attributed to high molecular weight oligomeric species. The
liquid deposits are observed near the tall asperities and do not dissolve in the alcohol. The same
formation is observed in the macroscale experiment and the authors have been able to determine
it was oligomeric species using TOF-SIMS spectroscopy and analyzing the peaks. The reaction
mechanism is not known, but the material is only formed in the contact region during sliding and
appears to further reduce friction and wear [5].
When alcohol vapor is no longer fed to the system, friction and wear revert back to their usual

behavior, indicating that the lubricant must be continuously replenished during operation. Unlike
typical solid coating, the alcohol lubricant does not change the device functionality, which is also
a great property.
4.3 Limits
Although alcohol adsorption shows tremendous potential in MEMS devices applications, some
points must be assessed. The liquid build-up in the microscale experiment has not been identiﬁed.
The authors believe it is the same oligomeric species as observed in the macroscale experiment, but
they are not certain of it [5]. This lubrication requires constant vapor feeding to the system during
operation. How can that be accomplished in common operation, by a customer for example? It also
means that it cannot be used in a MEMS device that requires a vacuum or a very speciﬁc pressure
to operate. Moreover, the vapor may affect the insulator permittivity of a capacitance. Hence, the
lifetime of MEMS is improved by four orders of magnitude with alcohol adsorption lubrication,
but this is not suited for all applications. Furthermore, the experiments were only conducted with
hydrophobic surface coatings. The surface energies of the hydrophobic surfaces may help the
alcohol vapor shift the water adsorption equilibrium and such good results may not be obtained
with hydrophilic surfaces. Further studies by the Dugger, Kim et al show that relative humidity
above 25-30% can drastically reduce the performance of the lubrication method in MEMS.
In a patent the authors obtained in 2011, they claim that a small pump could be built-in with
a supply that should last a long time [7], but they do not specify that time. They also claim that
polymers saturated with alcohol could provide a long lasting pentan-1-ol source. This has been
assessed in Dugger et al [8]. They found that such polymers can be thermally activated to deliver

the alcohol vapor in the system. Future work is needed to evaluate the integration of those polymers
in MEMS package and their lifetime.
Conclusion
Friction and wear are two very limiting factors in today’s technology. A lot of MEMS applications
are rendered non-viable because they involve sliding surfaces and might fail within minutes of
normal operation. Sliding, velocity, temperature, relative humidity and surface roughness are key
parameters that influence the tribology of MEMS. For these reasons, it is important to study the
failure mechanisms of such devices and to find ways to reduce them drastically. Usually friction
increases with velocity at the small scale, but it appears that this behavior is dependent on the
material and whether the system is at the nano- or the microscale.
Two new lubrication methods, ionic liquids films and alcohol adsorption, show great potential.
Both lubricants reduce friction and wear. The latter method even led to no wear after eleven days
of operation. Ionic liquid films appear as a good alternative to the classic Z-TETRAOL when
an insulating lubricant cannot be used, ionic liquids are conductors by nature. Moreover, those
lubricants can potentially reduce air emissions associated with hydrocarbon oxidation observed
in hydrocarbon based oils. They are considered ”green lubricants” [2]. However, the lifetime of
devices protected against friction and wear using this method must be studied further and over
longer periods of time.
Alcohol adsorption shifts the water adsorption equilibrium and tend to drive the removal of
water molecules, hence reducing wear in silicon and silicon dioxide for example. It also leads to

the formation of an olygomer build-up which further decreases wear. However, the behavior of
this lubrication method must be studied in the case of hydrophilic surfaces. For both lubricants, a
question must also be answered: how will the alcohol vapor be fed to the MEMS device or how
will the ionic liquid be replaced when completely worn out? The two proposed methods show
nonetheless incredible potential and might very well lead the way to viable class IV MEMS.

References
[1] S. H. Kim, D. B. Asay, and M. T. Dugger, “Nanotribology and mems,” Nano Today, vol. 2,
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[2] M. Palacio and B. Bhushan, “Ultrathin wear-resistant ionic liquid ﬁlms for novel mems/nems
applications,” Advanced Materials, vol. 20, no. 3, pp. 1194–1198, 2008.
[3] B. B. Nikhil S Tambe and, “Scale dependence of micro/nano-friction and adhesion of
mems/nems materials, coatings and lubricants,” Nanotechnology, vol. 15, no. 2, p. 1561, 2004.
[4] W. Zhang, G. Meng, and H. Li, “Electrostatic micromotor and its reliability,” Microelectronics
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[6] A. E. Somers, P. C. Howlett, D. R. MacFarlane, and M. Forsyth, “A review of ionic liquid
lubricants,” Lubricants, vol. 1, no. 1, pp. 3–21, 2013.
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Dec. 6 2011. US Patent 8,071,164.
[8] R. S. Johnson, C. M. Washburn, A. W. Staton, M. W. Moorman, R. P. Manginell, M. T.
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