Kalathil C. Eapen

Effect of surface chemistry on the tribological performance of a MEMS electrostatic lateral output motor

The effect of surface chemistry on the tribological performance and reliability of a MEMS lateral output motor is reported. Relative humidity (RH) and octadecyltrichlorosilane (OTS) self-assembled monolayer (SAM) coatings were used to change surface chemistry. Electrical and tribological performance of uncoated and OTS-coated motors were found to be dependent on RH. For uncoated motors, excessive wear of sliding contacts and welding (permanent adhesion) of static contacts were observed at 0.1% RH. Degradation of electrostatic force and high static friction (stiction) forces limited dynamic performance and reliability and caused device sticking at and above 70% RH. Around 50% RH, uncoated motors exhibited negligible wear, low adhesion, and a wear life at least three orders of magnitude longer than in the dry environment (experiments were stopped without failure after about one billion cycles). Water vapor behaved as a gas phase replenishable lubricant by providing a protective adsorbed film. The OTS coating broadened the operating envelope to 30–50% RH and reduced stiction, which allowed better dynamic performance at high RH. The OTS coating improved durability at 0.1% RH, but it was still poor. At high RH, stiction problems reoccurred when the OTS coating was worn away. By controlling and balancing surface chemistry (adsorbed water and OTS), excellent performance, low friction and wear, and excellent durability were attained with the lateral output motor.

Lubrication of Microelectromechanical Systems (MEMS) Using Bound and Mobile Phases of Fomblin Zdol

A lubrication scheme for MEMS electrostatic lateral output motors based on a mixture of bound and mobile lubricant was studied. Lubrication by bound monolayer alone provided some increase in operational life, but after a short time, the film wore away and the device failed in the unlubricated mode. A mobile phase was used to provide lubricant replenishment. Tribological studies were conducted on Si(100) wafers, as well as on MEMS electrostatic lateral output motors, dip-coated with a mixture of bound and mobile phases of Fomblin Zdol. Accelerated screening tests on Si(100) wafers were undertaken using a pin on disk tribometer. However, the optimum balance of bound and mobile phases was determined by studies on the device itself. The fractional surface coverage of lubricant and the ratio of bound to mobile phase was varied through selection of reaction temperature and rinse chemistry. The mobile phase on model surfaces and devices acted as a source of lubricant replenishment, and together with the bound phase provided dramatic improvement in performance. The wide variation seen in the performance of individual devices suggests that dip coating does not provide a uniform coating on the contacting surfaces of these devices.

Effects of adsorbed water and sample aging in air on the μN level adhesion force between Si(100) and silicon nitride

The microscale adhesion force of single crystal silicon (100) to silicon nitride was studied using a newly developed micro/nanoadhesion measurement apparatus. The effect of water adsorption on adhesion between clean surfaces and surfaces aged in air was determined. During experiments, varying relative humidity (RH) controlled water adsorption. Clean and aged samples showed measurably changed water adsorption and adhesion above 60% RH only, and aged samples showed more water adsorption and adhesion than clean ones above 90% RH. A contaminant film grew to a saturated thickness of about 6 A over 50 days of aging, and the film changed surface chemistry which resulted in higher contact angle. The contaminated surface with higher contact angle exhibited more affinity for water and microadhesion on the molecular scale, which is contrary to the common interpretation of contact angle data. The divergence of water adsorption and adhesion force behavior between clean and aged samples above 90% RH may indicate that the Kelvin radius became comparable to a characteristic length on the contaminated surface at high RH. A molecular model is proposed in which the contaminant film provides porosity, which allows capillary condensation and the growth of a multilayer water film.

Ionic-Liquid Lubrication of Sliding MEMS Contacts: Comparison of AFM Liquid Cell and Device-Level Tests

Lubrication of microelectromechanical systems (MEMS) became very critical as the devices became complex and its reliability began to deteriorate. In this paper, ionic liquids (ILs) with low volatility and high environmental stability were investigated as lubricants for sliding MEMS devices. A method that is based on atomic force microscopy (AFM) with a liquid cell was developed to study friction and wear properties of surfaces lubricated with ILs, having a systematic variation in molecular geometry and chemistry. Six-member pyridinium and five-member imidazolium rings are compared as cations in ethyl methyl pyridinium and ethyl methyl imidazolium ethyl sulfate; influence of short and long alkyl chain lengths on lubrication is studied with butyl methyl pyrrolidinium and hexyl methyl pyrrolidinium bis(trifluro methyl sulfonyl) imide. Formation of a surface-screening cation layer was discovered and linked to low friction and wear of IL-lubricated hydrogenated-silicon (H-Si) substrates. Several promising IL lubricants were identified from the AFM study and were tested in real MEMS motor devices. The friction and wear data obtained for these tests showed good correlation with the failure life span of lubricated MEMS motors. This supports a conclusion that the AFM-liquid-cell technique can be used in screening IL lubricants for MEMS devices.

Micro–nano behaviour of DMBI-PF6 ionic liquid nanocrystals: large and small-scale interfaces

Microelectrical mechanical systems (MEMS) devices coated with a thin film of ionic liquid showed significant improvement in wear life. These promising tribological findings led to the current study of the micro?nano behaviour of an ionic liquid, 1,2-dimethyl?3-butyl imidazolium hexafluorophosphate (DMBI-PF6), when applied as a thin film on a polished silicon surface. The films remain as microdroplets in ambient conditions but undergo drastic changes when agitated by an atomic force microscope (AFM) contact scan or touched with a biased AFM tip. The nanotribological characteristics including mobility, diffusion and scratch resistance are studied. In the case of DMBI-PF6, it was observed that the imidazolium ion is a mobile phase while PF6 is attached to the silicon surface. Crystallites are formed as a result of tip contact and depend on the tip bias and environmental conditions. Crystallization of DMBI-PF6 from the liquid phase with no tip bias showed substantial variation in crystal shape compared to those formed under the influence of an electric bias.

Lubrication of microelectromechanical systems radio frequency switch contacts using self-assembled monolayers

Contact failures in microelectromechanical systems (MEMS) switches prevent widespread use of MEMS technology for current handling in miniature devices. A self-assembled monolayer (SAM) lubricant was applied to MEMS switch surfaces in this paper as a possible approach for preventing contact failure. Chemical and physical processes on SAM lubricated contact surfaces were investigated at low (10 μA) and high (1 mA) current using a micro/nanoadhesion apparatus as a switch simulator with in situ monitoring of contact resistance and adhesion force. This was coupled with ex situ analytical analyses of the contacts using x-ray photoelectron spectroscopy (XPS) and micro-Raman techniques. Diphenyl disulfide was chosen as a lubricant due to its thermal stability, enhanced conductivity, and its ability to form a 3.4 A thick SAM on the gold electrode surface. Hot switching experiments were conducted in humid air (45% RH) and dry nitrogen using a MEMS-scale contact force of 200 μN and 5 Hz frequency. At low current, lu...

Perfluoroalkylations and perfluorooxaalkylations. Part 2. Copper-mediated cross-coupling of secondary perfluorooxaalkyl iodides and aryl halides [1]

Abstract The first successful application of the copper-mediated cross-coupling reaction using secondary perfluorooxaalkyl iodides and iodoaromatic substrates is described. The yields of the cross-coupled products are optimized by careful choice and control of the experimental conditions. The by-products formed are identified and their probable mode of formation is discussed.

Reactions of 1,2-bis(trifluoroacetyl)benzene with nucleophiles leading to heterocyclic compounds

Abstract The reaction of 1,2-bis(trifluoroacetyl)benzene (II) with ammonia, hydrazine, hydroxylamine and L-alanine has been investigated and the resulting heterocyclic compounds fully characterized on the basis of spectral data. The reaction of II with o -aminophenol and o -phenylenediamine led to the formation of tetracyclic compounds XI and XII respectively. The structure of compound XII has been further supported by single crystal X-ray crystallography. Our studies reveal the potential of the diketone II as a valuable synthon for the synthesis of a variety of heterocyclic systems with trifluoromethyl substituents.

MEMS lubricants based on bound and mobile phases of hydrocarbon compounds: film deposition and performance evaluation

The concept of a thin layer of mobile hydrocarbon-based lubricant providing protection by replenishment to a surface already protected by a chemically bound material has been explored for the first time, for application to silicon-based microelectromechanical (MEMS) systems. Several bound/mobile pairs of lubricants were evaluated to study the effects of bound phase end group and mobile phase chemical functionality on wettability and performance. The bound species studied were derived from 1-decanol and 1,7-heptanediol. Mobile phases investigated were, a pentaerythritol ester, a multiply alkylated cyclopentane, Pennzane, and a polysilane developed for spacecraft application. Hydrocarbon lubricant performance was evaluated using electrostatic motors run in dry air, and was compared with that provided by Fomblin Zdol under identical conditions. Motors with hydrocarbon coatings showed substantial improvement in performance over uncoated motors, and for some bound/mobile pairs, was equal to Zdol within experimental error. We believe that for silicon-based devices, hydrocarbon coatings will be preferable at least for some applications, as the degradation observed due to aging of a fluorinated lubricant in direct contact with a silicon surface is absent.

Liquid alkoxides of a perfluorotertiary alcohol and their derivatives

Abstract Sodium and lithium alkoxides of (CF 3 ) 2 CFOCF 2 CF 2 C(CF 3 ) 2 OH have been prepared in high yields by the addition of sodium hydride to the alcohol or in the case of the lithium salt through the addition of (CF 3 ) 2 C=O to the organolithium compound (CF 3 ) 3 CFOCF 2 CF 2 Li. These salts react with active halides e.g. PCl 3 or cyanuric chloride to yield completely substituted products.