Self-Assembled Monolayer used in Micro-motors

ME395 Term Project

Yong Zhu, Joel Gregie & Prad Prabhumirashi

June 5th, 2000

Chapter I - Introduction

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Micromachined devices, such as microelectromechanical systems (MEMS), high-density sensor arrays, and microfluidics, are poised to bring the next technology revolution. At this time, many of these, particularly MEMS, are fabricated from silicon using lithographic techniques developed in the microelectronic industry. Surface microstructures typically range from 0.1 to several m in thickness with lateral dimension of 10-500 m. A polysilicon- based device is shown in Fig. 1.1 Due to the fact that surface area to volume ratio scales with 1/L (L = dimension), surface forces, such as adhesion and friction, often play a detrimental role in the fabrication and efficient operation of MEMS devices.

Surface engineering is one of the major issues of concern in MEMS.2 For example, surface modification, may reduce stiction during the fabrication of MEMS devices. If a device has moving components, the surface modification may be necessary to reduce friction by forming lubrication and passivation layers between moving parts. Surface modification is also responsible for compatibility in chemical or bio-medical devices. One can modify the surface either by altering its topography or by altering it’s chemistry. A good example of topographic modification is intentionally roughening one of the contacting surfaces so that the actual contact occurs at the sharp points of the textured surface, thereby reducing the actual contact area. In this paper, however, we focus on chemical modification of the surface. 1 Roya Maboudian & Roger T. Howe, J. Vac. Sci. Technol. B, 15 (1), Jan/Feb 1997, p. 1-20.2 X. –Y. Zhu & J. E. Houston, Tribology Letters, 7 (1999), p. 87-90.

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Chemical modification of the surface can be done in numerous ways. Hydrogen terminated surfaces have been studied for several years in the integrated circuit industry, where they have been used to obtain low temperature epitaxial films, and low-defect gate oxides. These applications rely on hydrogen to passivate the silicon surface and still allow easy access to the underlying crystalline substrate for the future growth of films. In another approach recently developed, Man et al. have shown that fluorocarbon films formed by a plasma polymerization reaction are promising in reducing adhesion.3

Diamond-like Carbon (DLC) coatings provide another means to reduce stiction. These films are hard, hydrophobic, exhibit relatively low surface energies, and can also be doped to enhance their electrical conductivity. Other properties like thermal stability, chemical inertness, and the ability to be deposited near room temperatures makes DLC an attractive choice. Last but not the least, chemical modification of the surface can also be done by using Self-Assembled Monolayers (SAMs) coating. Our term paper focuses on mechanical aspects of employing SAM coating on a particular device, which will be introduced subsequently. Our paper out-line is as follows –

Description of our device What are SAMs? How are they formed (self assembly)? Different types of SAMs Fabrication of the device Friction studies – with and without SAM coating on the

device Mechanical testing – Nano-indenter test Conclusions

Chapter II - DeviceMicro-motor is one of the important applications about MEMS devices. It can be

used in many areas, e.g. for optical switching (putting a micro-mirror on the rotor). Figure 2 shows a SEM micrograph from Sandia National Lab.

3 P. A. Man et al., Proceedings of the 9th IEEE International Workshop on Solid-State Sensors and Actuators, Transducers ’93, Yokohama, Japan, 1993, p. 55-59.

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As same as other MEMS devices, due to the scaling law, there is a large surface area to volume ration in the microscopic area. So the surface forces, such as adhesion and friction, are often detrimental to the fabrication and operation of MEMS devices. Thus, one of the key issues in MEMS is surface engineering to reduce the adhesion and friction.

In micromotors, Studies indicated that bearing friction consumes a significant portion of the motive torque. Surface treatments using monolayer coating have been investigated for reducing the friction problems in polysilicon surface microstuctures, in particular, octadecyltrichlorosilane (OTS) has shown the ability to reduce friction. This project reports an investigation of siloxane-anchored self-assembled monolayers (SAM) for reducing stiction, friction and wear in Polysilicon Micromotors. Figure 3 shows a schematic diagram of our device. It has been reported that some SAMs have very small coefficient of friction and are therefore useful as solid lubricants.

Figure 2 – SEM Micrograph of a Micro-motor

Synchronous motor

To design synchronous motors for micropositioning, large starting torque, small frictional force, and fine angle resolution are desirable. Means to optimize these characteristics in micro-motors will be discussed here.

Starting torque – The design of a rotating motor begins with an estimation of the torque exerted by the electric field. This can be expressed in terms of the derivative of the

stored energy which, for a given bias V between rotor and stator, is conveniently

represented as , where C represents the capacitance across the driving electrodes

that have voltage V across them. To find the rotor torque T, we take the derivative with respect to the rotor angle . Torque values are of the order of pNm for voltages of order 100V and typical micro-motor dimensions. The angular dependence of this torque, which must overcome frictional restraint in order to cause the motor to rotate, is being studied in terms of electrostatic field.

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Equation 1

Figure 3 - Top view of a synchronous motor

Friction reduction – A technique to reduce the friction is to provide bushings to support the rotor. Friction-reducing bushings have been incorporated in several of the motors. In one design, hemispherical bushings extend from the under-surface of the rotor to provide a small-area contact with the substrate as shown in figure 2. To make the bushings, circular holes are patterned in the resist covering the sacrificial oxide layer and this layer is then isotropically etched before depositing polysilicon to form the rotor. The resultant extended hemispheres of polysilicon reduce friction between the two surfaces. In this project, we put SAM layer below the bushings to reduce the coefficient of friction, then reduce the friction.

Chapter III - Self-Assembled Monolayers (SAMs) – Novel Nanostructures

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GroundPlane

Stator

Hub

Rotor

Self-assembled monolayers (SAMs) are ordered molecular assemblies formed by the adsorption of an active surfactant on a solid surface as shown in Figure 5.4 The field of SAMs has witnessed a terrific growth in synthetic sophistication and depth of characterization over the past decade. In 1946 Zisman published the preparation of a monomolecular layer by adsorption of a surfactant onto a clean metal surface.5 The formation of monolayers by self-assembly of surfactant molecules at surfaces is one example of the general phenomena of self-assembly. SAMs offer opportunities to increase fundamental understanding of self-organization, structure-property relationships, and, interfacial phenomena. It is possible to tailor both head and tail groups of constituent molecules in SAMs. This enables us to further study complex intermolecular, molecular-substrates and molecule-solvent interactions like ordering and growth, wetting, adhesion, lubrication, and corrosion. SAMs are also structurally well defined thus making studies of physical chemistry and statistical physics in 2 dimensions a possibility.

Figure 5 – Self-assembled monolayers are formed by simply immersing a substrate into a solution of the surface-active material. The driving force for the spontaneous formation of the 2D assembly includes chemical bond formation of molecules with the surfaces and intermolecular ineractions.4

SAMs, in contrast to ultrathin films made by molecular beam epitaxy (MBE) or chemical vapor deposition (CVD), are highly ordered and can incorporate a wide range of groups both in the alkyl chain and at the chain termina. Therefore, a variety of surfaces with specific interactions can be produced with a fine chemical control. Due to their dense and stable structure, SAMs have potential applications in wear protection and corrosion prevention. Kumar et al. have exploited the high molecular order parameter property of SAMs in electro-optic devices.6

While the majority of papers in recent years deal with thiols on gold, this is by no means the only system to consider. Some of the systems of SAMs are given below.

1. Monolayers of Fatty Acids – Spontaneous adsorption of long-chain n-alkanoic acids (CnH2n+1COOH) has been studied in the past few years. This is an acid-base

4 A. Ulman, Chem. Rev., 96 (1996), p. 1533-54.5 W. C. Bigelow, D. L. Pickett, W. A. Zisman, J. Colloid Interface Sci., 1 (1946), p. 5136 A. Kumar, H. A. Biebuyck, G. M. Whitesides, Langmuir, 10 (1994), p. 1498.

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reaction, and the driving force is the formation of a surface salt between the carboxylate anion and a surface metal cation. Figure 6 shows a schematic of fatty acid monolayers on AgO and Al2O3.6

Figure 6 – a schematic of fatty acid monolayers on AgO and Al2O3.6

2. Monolayers of Organosilicon Derivatives – SAMs of alkylchlorosilanes, alkylalkoxysilanes, and alkylaminosilanes are few of the examples. These SAMs require hydroxylated substrates for their formation. The driving force for this self-assembly is the in situ formation of polysiloxane, which is connected to the surface silanol groups (-SiOH) via Si-O-Si bonds. Substrates on which these monolayers have been successfully prepared include silicon dioxide, aluminum oxide, quartz, glass, mica, zinc selenide, germanium oxide, and gold. A schematic description is given by Figure 7.6

Figure 7 – A schematic description of a polysiloxane at the monolayer-substrate surface.6

3. Organosulfur Adsorbates on Metal and Semiconductor Surfaces – The most studied example is alkanethiolates on Au(111) surfaces. Other examples include thiophenols, thiocarbaminates and mercaptopyridines. Figure 8 shows some examples.

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Figure 8 – Surface-active organosulfur compounds that form monolayers on gold.6

4. Alkyl Monolayers on Silicon – In 1996, Linford and Chidsey demonstrated for the first time that robust monolayers can be prepared where the alkyl chains are covalently bonded to a silicon substrate mainly by C-Si bonds.7 This gives us another example of SAMs.

5. Multilayers of Diphosphates – This uses a reaction, which results in an insoluble salt. This is the case for phosphate monolayers. In these salths, the phosphates form layer structures, with one OH group sticking to either side. Thus, when an alkyl group replaces OH group, forming alkyl phosphonic acid, a bilayer structure is formed with alkyl chains extending from both sides of the metal phosphate sheet.

Mechanical properties of monolayer lubricants are being studied by a number of groups. The common techniques used for this purpose are nanoindentation8, cantilever beam array technique1 and interfacial force microscopy8. Mechanical behavior of SAMs in our device will be discussed in subsequent sections.

7 M. R. Linford and C. E. D. Chidsey, J. Am. Chem. Soc., 115 (1993), p. 12631.8 J. D. Kiely and J. E. Houston, Langmuir, 15 (1999), p. 4513-19.

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Chapter IV – Device Fabrication ProcessThe fabrication process of a synchronous motor is shown in Fig. 9.9 To fabricate

this motor, the Si wafer is first coated with two insulating layers. The first is a thermally grown oxide, which is followed by a layer of Si3N4 deposited by CVD. The polysilicon grounding plate is then deposited and patterned (Fig. 9a) In this design, phosphosilicate glass (PSG) is used as the sacrificial spacing layer (Fig. 9b). A second layer of polysilicon is then deposited and patterened to form the rotors and stators of the micro-motor (Fig. 9c). It is then necessary to deposit an additional PSG layer to allow for spacing between the hub and the rotors (Fig 9d). Next, isotropic and anisotropic etching are used to open the hole for the hub in the PSG (Fig 9e). Then a third polysilicon deposition forms the hub (Fig. 9f). Finally, the sacrificial PSG is etched away with BHF, and the exposed polysilicon surfaces are coated with SAMs (Fig. 9g).

Coating the silicon or polysilicon surfaces of micromotors with SAMs is a fairly easy process to integrate into the fabrication recipe. During processing, all exposed silicon surfaces develop a thin, native oxide. If this surface is hydrated, it will react with the appropriate SAM precursor to covalently bond the SAM to the surface. The oxide layer can be thickened with thermal annealing in an oxygen or wet atmosphere, but a layer of 15 Å, has been found to be suitably thick. This makes a high temperature anneal unnecessary. To hydrate the surface, the wafer is soaked in a solution such as 70:30 H2SO4:H2O2 at 80 oC. The wafer is then rinsed with DI water and dried. The samples are then immersed in a solution with the SAM precursors, and the SAMs attach to the surface in the following manner. A diagram of the process is shown in Fig. 10. The precursors are alkyl-siloxanes such as octadecyltrichlorosilane (OTS), but different alkyl-siloxanes can be used to control the thickness of the monolayers, by changing the alkyl chain length.10,11,12, The trichlorolsilane end of the molecule hydrolyzes in solution where the Cl atoms attached to the silicon are replaced by OH ions. This is the end that forms the Si-O-Si bonds to the substrate. The alkyl chains then stick up from the surface at a slight angle which is dependent on the head group and the chain The chains can cross-link, which further stabilizes the film. The wafers are then rinsed and dried again. Drying is fairly easy in during this step, as the SAM acts as anti-stiction coating. It has been found that the initial hydrolisis of the surface can be done in the same solution that contains the SAM precursors, which further simplifies this process. These coatings have been found to be resistive to hot organic solvents, acids, and boiling water, so they should hold well up during subsequent processing.

9 Fan LS, Tai YC, Muller RS, IEEE International Electronic Devices Meeting, 666, (Dec. 1988)10 Xia Y., Whitesides GW, Annu. Rev. Mater. Sci., 28, 153, (1998)11 Deng K, Collins RJ, Mehregany M, Sukenik C, IEEE Tans. Elect. Dev., 1995, p. 368.

12 Allara DL, Parikh AN, Rondelez F, Langmuir, 11, 2357, (1995)

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9a

9b

9c

9d

9e

9f

9g

10

Figure 9 – Fabrication Process of a Micromotor with SAM

Silicon/Poly-SiOxideSi3N4

PSGSAM

Figure 10 - Deposition of OTS to oxidized silicon

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CH3

Si

[ ]n

OO

CH3

Si

[]n

O OOOH OH

Cl ClCl

CH3

Si

[]n

CH3

Si

[ ]n

OH OHOH

Oxide

Silicon

Chapter V – Friction studies

Effect of SAM on the coefficient of friction can be seen in two ways. One is indirect, going through Gear Ratio, while another is gotten by a mechanical shown later.

Experimental comparison

A comparative study of electrical operation of micromotors without SAM deposition and those with SAM depositions in room air has been carried out13. The performance of the motors was evaluated by “mechanical probing”, determining minimum operating voltage and gear ratio versus excitation voltage. If “mechanical probing” was required to start a motor, this motor was designated as having a stiction problem. Since the performance of wobble micromotors can be tested with relative simplicity by measuring the gear ratio as a function of excitation voltage, we have focused on studying wobble micromotors for long term operational stability. Quantitative measurements of wear in micromotors can also be studied from the changes in the gear ratio of wobble micromotors under extended operation. Mechanical wear in the bearing of the wobble micromotor results in a corresponding increase in the bearing clearance. Since the gear ratio of the wobble micromotor is the ratio of bearing radius to the bearing clearance, changes in the gear ratio can be used to measure wear in the bearing of the wobble micromotors.

The wobble motors used for the study of carbon chain monolayers have twelve (~30o span) stator poles, but only six stator poles were excited to accommodate our power supply, which provides six independent phases. All wobble motors tested have rotor/stator gaps of 1.5 .

The motors coated with C18 (OTS) showed the most desirable results. These motors were studied over a nice month period. Repeatable results were obtained from motors coated with OTD throughout the study. It's shown the data for the gear ratio and minimum operating voltage as a function of time for motors coated with OTS. Motor 5, which is included as a reference, is a typical sample from Group 1 (no surface treatment) and has a bearing radius of 10 . Motors 6 and 7 are typical samples from Group 2 (coated with OTS). Motor 6 has a bearing radius of 10 , and Motor 7 a bearing radius of 18 . For each session, these motors were operated for 30 minutes with the same excitation speeds and excitation voltages. It is evident that the micromotors with OTS have a relatively stable rotor speed and minimum operating voltage. For Motor 6, the change in gear ratio and minimum operating voltage around 150 days after OTS deposition was due to a minor damage to the bearing during probing for gear ration measurements. NO stiction problems were observed for the OTS coated motors during the experiments. For the micromotors without OTS coating (e.g. Motor 5), the rotor speed decreased and minimum operating voltage increased with time. For these reference micromotors, rotors had some stiction problems, and sometimes mechanical probing of the rotors was required beyond 20 days after release.

To further study the effect of OTS deposition on micromotor operation, gear ration as a function of excitation voltage was measured form Group 1 and 2 (see Figure 13 Deng K, Collins RJ, Mehregany M, Sukenik C, IEEE Tans. Elect. Dev., 1995, p. 368.

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11). For the wobble micromotors, under “ideal” conditions, the gear ratio equals the ratio of bearing radius to the bearing clearance. It is desirable to eliminate rotor slip since it does not produce output torque but dissipates motive torque. In figure 11, the gear ration for the micromotors coated with OTS is very close to the ideal gear ration, and is much smaller that that without OTS. This implies that OTS reduces slip in wobble micromotors. The increase in the gear ratio at the smaller excitation voltages (and hence smaller motive torques) is due to increased rotor slip.

Figure 21 - Gear Ratio as a function of excitation voltage with and without OTS

In order to investigate the lifetime of OTS coatings, rotor speed was measured periodically after extended operation of the motors coated with OTS. For these measurements, micromotors were operated for long periods of time (e.g. 5 to 12 hours) every seven or ten days for nince months. A 100V excitation at 10,000 rpm frequency, corresponding to 10,000 wobble cycles per minute, was used to test these motors. Fig. 7 presents the data for the gear ration as a OTS coatings. Both motors have bearing radii of 10 . For the motors with OTS coating (Fig. 7(a)), the change in the gear ratio is within 4% over a 9 month period. Note that the first data point (wobble cycles=0) for Motor 9 is the gear ratio measured after release but before OTS coating. For the motor without OTS coating , gear ratio is 51 at the start of the motor operation and decrease to near 30 after continuous operation of 10 million wobble cycles. This motor was then stored in room air for a few days and then operated periodically for te rest of the experiment. The increase in the gear ration during this period is perhaps dure to increased stiction or friction as a result of the storage in room air. The results of this experiment indicate that wear of bearing without OTS coating is significant and results in changes in the gear ration from start of micromotor operation by as much as 40%, while the OTS monolayer is a robust coating.

Since OTS shows promise, many motors from Group 4 were also tested to examine the storage time effects on devices coated with OTS. These motors were not tested after release and immediately after OTS deposition, but were first tested after they were stored for a known period of time (e.g. more than a month, up to nine months). In all cases, there were no indications of stiction problems. The changes in the gear ratio for

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each motor was within 10% in all cases. This implies that extended storage in room air of motors coated with OTS does not significantly affect the operational performance of the motors.

To further study friction in OTS coated motors, a stroboscopic dynamimetry technique has been used to measure the dynamics of salient-pole micromotors. This technique is described. It's shown step transient data for 12:8 micromotors with and without OTS coatings; Motors 12 (with OTS coating) and 13 (without OTS coating) are otherwise identical. The rise time of the step response for a motor coated with OTS is shorter than that without OTS. This result indicates that the motor with OTS coating experiences less friction.

Coefficient of Friction

Figure 12 a derived speed and distance plot vs. time from removal of excitation

Figure 12 shows a typical steady-state speed and deceleration profile as captured by a sampling, network analyzer. From the figure, we can see that the actions of retarding forces of the motion of the devices following removal of excitation. Two possible sources for such retarding forces are friction and viscous drag. Assuming a planar spin (no vertical wobbling along the length of the shaft) in the absence of excitation, frictional forces are assumed to arise from contact of the bushing rim (on the bottom of the gear or turbine) with the substrate surface below it. Viscous drag on a moving object in a fluid is comprised of a pressure drag and a skin friction drag. The relative contributions of pressure drag and skin friction drag to the total viscous drag depend upon the geometry of the body and its orientation with respect to the fluid. For the structures considered in this study, the pressure drag is assumed to be negligible compared to the skin friction drag since the test structures are essentially thin plates with the thin edge perpendicular to the flow stream.

Here the micro-motor I am trying to show is the wobble micro-motor which has two or three bushing under the rotor14. The reason why I choose this kind of motor is that the friction is easy to control. The form of the frictional force is assumed to be independent of contact area and only dependent on the mass of the test structure. Thus, the torque due to the frictional forces is simply

14 Dhuler V, Mehregany M, Phillips S, IEEE Tans. Elect. Dev., Vol. 40, No. 11, 1993

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Equation 2

where Td is the torque due to the frictional forces, cd is the dynamic coefficient of friction, M is the mass of the object, g the gravitational constant and Rc the radius of the bushing rim on the bottom of the gears or turbines.

The form of the viscous drag is modeled as Couette flow------a situation which models fluid flow between two rectangular plates under various conditions of plate motion, fluid pressures and plate angle. We argue that a continuum model of fluids at this scale is valid since at standard atmospheric conditions, the mean free path of air is approximately 0.1 ----fully 20 times smaller than gaps between structures in our study. In our case, one plate (the substrate) is always stationary, the top plate is moving at some velocity, U, and the two plates are parallel and horizontal. For laminar flow conditions, the shear stress, , on the bottom of the moving plate is given by

Equation 3

where is the dynamic viscosity of the fluid between the two plates, and Y the separation between the two plates (Fig. ?).

the shear stress due to the fluid on the top of the moving plate can be similarly derived, but since the velocity gradient of the fluid above the plate is determined by the boundary layer thickness (which in this case is approximately 150 ) the shear stress due to the fluid above the plate is approximately 40 times smaller than the shear stress due to the fluid below the bottom plate (eqn. 3) and can effectively be ignored.

As for the frictional force, it is more convenient to calculate torques and hence, the total torque on the structure due to viscous forces is given by

Equation 4

where is the rotational velocity of the top plate, and R1 and R2 are the inner and outer radii for the disk portion of the gear or turbine. Note that the viscous drag is a first-order function of the rotational velocity .

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y U

Y

Fig. 13 Fluid mechanics model used to calculate viscous drag on the test structures

Combining the total torque due to the fluid sheer stress, the torque due to friction, and the rotational inertia, we obtain the dynamic equation governing the rotational velocity of the structures following removal of excitation as

Equation 5

where

Equation 6

and I is the moment of inertia for the structure.

As a specific example, consider the case of 125 -diameter gear, the deceleration profile for which is shown in Fig. ?. Using a value of for

, and from eqns. (1) and (5), we calculate the torque due to the frictional force, Td, to be

Equation 7

the coefficient of viscous torque to be

Equation 8

and the moment of inertia to be

Equation 9

applying a least-squares fitting routine to the deceleration profiles shown in Fig. ?, we obtain an estimate of

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Equation 10

Chapter V – Proposed Microscratch Test

In microscratch studies15, a conical diamond indenter having a tip radius of about 1 and an included angle of 60o, is drawn over the sample surface, and the load is ramped up, until substantial damage occurs. The coefficient of friction is monitored during scratching. In order to obtain scratch depths 15 X. Li, B. Bhushan, wear,220 (1998) p 51-58

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during scratching, the surface profile of the coated surface about 0.2 mN which is insufficient to damage the sample surface. The actual scratches were made by translating the sample while ramping the loads on the conical tip over different loads. Samples were scratched at ramping loads. For a low load scratch test the normal loads were ramped from 0.2 to 3 mN while for a high load scratch test the normal loads were ramping from 1 mN to 25 mN. For a low scratch test the translating speed was 1 mm/s while for a high load scratch test the translating speed was 5 mm/s. The actual depth during scratching in high load tests was obtained by subtracting the initial profile from the scratch depth measured during scratching. In order to measure permanent depth, the scratched surface was profiled at a low load of 0.2mN and was subtracted from the actual surface profile during scratching. A typical scratch experiment in high load tests consisted of seven subsequent steps shown in the figure 15.

Fig. 15 the steps in microscratch test

The scratch at ramping normal load was made during the fourth step, and surface profiles before and after scratch were obtained during the third and sixth steps, respectively. Scratch-induced damage of a coating, specifically fracture of delamination, were monitored by in-situ tangential (friction) force measurements and by AFM and SEM imaging of the scratches after tests. The damage events of fracture or delamination of a sample during the scratching were correlated to gradual or abrupt increase in the coefficient of friction.

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Normal Load

Microscratch Test:1) approaching the surface2) indent into sample surface by loading the tip to 0.2mN3) translating the sample at a constant load of 0.2mN4) translating the sample in the opposite direction at ramping loads5) unloading of the tip to 0.2mN6) translating the sample at constant load of 0.2mN7) final unloading of the tip

Normal Load

The friction tests are conducted using a wedge-tip under reciprocating motion. The sample is mounted on a table, which is placed in reciprocating motion using a DC motor. The sample is positioned using a x-y stage mounted on a motor driven lead-screw type linear stage. The load on the stationary component is applied by lowering the beam against the sample. Normal and frictional forces are measured with semiconductor strain gages mounted on a crossed-I-beam structure.

We supposed to do another Nanoindenter test to examine the interfacial toughness of SAM. But as covered in the previous part, the bonding between the SAM and substrate is covalent bonding, which is stronger than the SAM itself. So it's not useful to do this test.

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