ISSUES AND CHALLENGES IN THE APPLICATION OF MICRO-BALL BEARING FOR SILICON BASED MICROSYSTEMS ROBIN PANG SUI TING NATIONAL UNIVERSITY OF SINGAPORE 2008 i ISSUES AND CHALLENGES IN THE APPLICATION OF MICRO-BALL BEARING FOR SILICON BASED MICROSYSTEMS ROBIN PANG SUI TING (B Eng (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING DEPARTMENT OF MECHANICAL ENGINEERING NATIONAL UNIVERSITY OF SINGAPORE ii PREFACE This thesis is submitted for the degree of Master of Engineering in the Department of Mechanical Engineering, National University of Singapore, under the supervision of Dr Sujeet Kumar Sinha No part of this has been submitted for any degree or diploma at any other University or Institution As far as this candidate is aware, all work in this is original unless reference is made to other work Part of thesis have been submitted to an international journal below: Journals publications: R Pang, S K Sinha and X Tang, “Applications of surface micro-bearings on Si for high wear life” submitted to Journal of Micromechanics and Microengineering iii ACKNOWLEDGEMENTS I would like to express my gratitude for Dr Sujeet Kumar Sinha for his patience, guidance, vision, and advice for making this project a success I would also like to express my sincere gratitude to the help of many people, without which this project will not be as successful They are 1) Satyanarayana Nalam for his words of encouragement and motivation, 2) Nitya Nand Gosvami, for his early help with the strain gauge, 3) Eric Tang Xiao Song for his help in designing the channels for the silicon plates, which was a pivitol part of the experiment I would also like to express my sincere gratitude to the staff of the Material Science Lab and Fabrication Support Center for their great help and support, namely; Mr Thomas Tan, Madam Zhong Xiang Li, Mr Abdul Khalim Bin Abdul ,Mr Juraimi, Mr Maung Aye Thein, Mr Ng Hong Wei, Mr Tan Wee Khiang, Mr Lam Kim Song, Mr Chi Kiang, Mr Tay Peng Yeow, Mr Tan Hui Meng, Mr Nadarajah, Mr Rajamohan, Mr Rajendran, and last but not least, Mr T.Rajah Lastly, I would like to thank by mother, my girlfriend, and my beloved father for their patience and support through these times I am also grateful to all the unexpected events and circumstances which have happened to me in my life during this period iv CONTENTS PAGE Acknowledgments ii Contents iii Summary vi List of Symbols ix List of Figures ix List of Tables xiv CHAPTER DESCRIPTION CHAPTER SCOPE AND OBJECTIVES OF THE PROJECT 1.1 Introduction to the friction challenges of PAGE Micro Electro Mechanical Systems 1.2 Focus and objectives of the Project 1.3 Structure of the thesis CHAPTER LITERATURE REVIEW 2.1 Introduction to MEMS 2.1.1 Tribological Challenges 2.1.2 Mico and nano rolling elements 12 2.1.3 Progress by researchers 14 2.1.4 Adhesion of microspheres onto surface 14 2.2 Theory 16 2.2.1 Critical Radius for Surface bearings 17 2.2.2 Critical Rotational Speed for Surface Bearings 22 v CHAPTER EXPERIMENTAL DETAILS 3.1 Experimental Setup 27 3.1.1 Micro ball bearings 27 3.1.2 Flat silicon plates 28 3.1.3 Rotary Lifecycle Tribometer 28 3.2 Experimental Procedure 32 3.2.1 Preparation of Silicon Plates 32 3.2.2 Fabrication of the main Channel 33 3.2.3 Transference of glass/polymer microspheres to flat bottom silicon plate 35 Transfer of glass/polymer microspheres on channeled silicon plate 37 3.3 Humidity Control 38 3.4 Recording and determination of failure of run 40 Chapter MICRO-BEARING LIFECYCLE RESULTS 4.1 Effects of humidity 42 4.2 Lifecycle runs 43 4.3 Description of Lifecycle runs 46 Chapter DISCUSSION 5.1 Factors affecting the lifecycle for glass microspheres 51 5.1.1 Alignment and stability of test plates 51 5.1.2 Dispersion factor 52 5.1.3 Channel 53 5.1.4 Wear of micro-balls 53 5.1.5 Melting of Glass Microspheres 54 5.1.6 Lubricating effects 54 3.2.4 vi 5.2 Factors affecting the lifecycle for runs of polymer microspheres 58 5.2.1 Deformation of Polymer microspheres 58 Chapter CONCLUSION Chapter RECOMMENDATIONS 7.1 Incorporating the ball bearings with journal bearings 62 7.2 Depositing of micro or nano bearings onto MEMS Interfaces 63 References 65 Appendix 1-1 Appendix 2-1 vii SUMMARY Recently, there has been a constant effort to make machines of smaller sizes down to micron and nanometer scales Apart from difficulties in manufacturing, one great challenge facing these machines is that surface plays a major role in its functioning The related effects are stiction, adhesion, friction and wear (tribology) Most of the micron-sized moving machines are currently made of Si or some polymers which are tribologically inferior Due to large surface to volume ratios experienced by small objects, many devices fail due to the strong adhesion and friction forces acting at the surfaces, limiting their long-term operational cycles and commercial feasibility Rotary microelectro mechanical systems experience significant rubbing, friction and wear at the interface Self-assembled monolayers, widely used against stiction, are unable to withstand long-term shear conditions, and remove easily under high pressure surface contact Liquid lubricants actually increases stiction Solid lubricants at contacting interface and magnetic levitation techniques are being studied, but challenges are the cost of fabrication as well as the ability to withstand long operating cycles Micro-ball bearings, which are robust and possess low rolling friction, have shown great promise for reducing friction and increasing the lives of moving MEMS devices Yet, this area requires further research in conducting long-term life cycle tests for smaller-sized (< 100 µm) balls Materials selection for the ball is also a major challenge A rotary tribometer with bottom plate rotating and top plate attached to a low friction bearing support and with a constant dead weight of 235 µN, was designed and built to quantify the lifecycle of these micro-ball bearings Micro-ball bearings made of glass (borosilicate; Ø 53 ± 3.7 µm) and polymer (polystyrene; Ø 50.7 ± 2.0 µm) viii materials were tested on circular silicon plates (diameter 15 mm) The tests were carried out both with and without channels on the bottom disk to compare the differences The channel is 2.5mm wide with internal diameter of 6mm, 28 microns in depth The aim was to investigate the possibility of the latter (balls rolling without channel) to deposit balls directly onto MEMS surfaces without the need for creating channel which complicates Numerical analysis was first carried out to estimate the critical rolling condition (rpm of the disk) and humidity that could allow the balls to stay on the surface due to surface forces without rolling off due to centrifugal force Subsequently, in the lifecycle test, the glass ball bearings rolling in the channels gave the most promising results of extremely low friction and life cycle exceeding million cycles of rotation (500 RPM) Minimal wear, occasional fracture and melting of the glass micro-balls were found on the Si surfaces Tests on the polymer balls were not encouraging because polymer balls plastically deformed, changing from spherical to cylindrical This was not conducive for rolling in a circular path Finally, the major challenges in the experiments were found to be proper alignment of the rotating surfaces to avoid them contacting at the edges during the lifecycle tests ix LIST OF SYMBOLS AND ABBREVIATIONS COF MEMS RH DF TP BP Si RMS Rc wc F Mr Md g Coefficient of friction Microelectromechanical systems Relative humidity Dispersion Factor Top plate Bottom plate Silicon Root mean square roughness Critical microball radius Critical rotational speed Force of adhesion Rolling resistance Moment of dislodge Gravitational constant x CHAPTER CONCLUSION Surface force enabled micro-ball bearing technology has been tested with balls rotating between two 15 mm circular Si plates(wafer), with and without a channel on the surface of the plate 53 µm diameter glass and 51 µm diameter polymer microspheres rolling on rotating silicon substrates reduces friction and wear Both materials of micro bearings exhibit extremely low friction properties due to rolling as well as the ability to adhere onto the substrate due to surface forces alone However, the glass micro bearing rolling in channel shows superior lifecycle performance because they have proved to be more robust due to high stiffness and softening/melting temperatures Polymer microspheres did not perform as well under the current conditions using the rotary tribometer tester built in this research Many important observations are made Micro-ball bearings reduce the friction in humid conditions (R.H above 55%) effectively, and the glass microspheres rolling in a large channel increases the lifecycle substantially A simple model is used to determine the theoretical critical radius for different materials, below which, adherence of the microsphere onto the silicon surface under stationary conditions will occur That is, the surface forces can overcome the gravitational force if the radius of the micro-sphere is equal to or lower than the critical value The next important parameter is the critical rotational RPM of the silicon disk, above which the micro-ball bearings will be dislodged outwards due to the centrifugal force 58 Important factors such as critical ball radius ( Rc ), presence of a channel, dispersion factor, critical rotational speed ( wc ), wobble, and number of balls, and ball material, are all crucial factors affecting the lifecycle of the micro-ball bearing Rolling cycles exceeding million is consistently obtained for the glass microspheres when the balls are rotated within a 28 micron deep, 2.5mm wide (3mm internal diameter) channel fabricated on the bottom Si plate The runs for polystyrene (polymer) balls under similar conditions not show promising results because polymer balls are less robust, and hence deform plastically These preliminary results with micro-ball bearings on silicon shows that glass ball bearings rolling in a channel is a promising technology for increasing lifecycle of micro machines This is especially so since real MEMS rotating components wear fast [63,64] Hence, this work shows the potential of incorporating micro-balls into MEMS devices for increasing lifecycle The results also show that a main channel alone is enough to encapsulate and provide ideal rotation of the micro-ball, and achieve an impressive lifecycle of million or more It also shows the importance of the material of the microball due to a possible boundary lubrication mechanism phenomenon associated with the surface melting of glass micro-ball due to pressure Glass microspheres and a wide main channel can be suitably and easily implemented into actual MEMS devices since it shows the most promising way of solving the tribological problems of stiction and low life cycles of MEMS device due to wear 59 CHAPTER RECOMMENDATIONS These preliminary results calls for further investigation of glass and polymer microspheres rolling in conditions listed below flat silicon with i reduced load ii increased rotational speed iii smaller circular silicon diameter 7.1 Incorporating The Ball Bearings With Journal Bearings The micro ball bearings can serve first to prevent stiction During rotary motion, the ball bearings can serve as lubrication and friction reducers Once the rotary component have increased in rotational speed sufficiently, the journal bearing can take over Figure 7.1: Schematic of the MIT microengine, showing the air path through the compressor, combustor and turbine Forward and aft thrust located on the centerline hold the rotor in axial equilibrium, while a journal bearing around the rotor periphery holds the rotor in radial equilibrium.( Diagram courtesy of [65] with permission from the publisher) 60 7.2 Depositing Of Micro Or Nano Bearings Onto MEMS Interfaces MEMS technology has also allowed the building of micro/nano probes[72] These probes can also be successfully used to carry MEMS ball bearings, positioned, and deposited onto MEMS interfaces Figure 7.2: A Microfabricated silicon neural probe arrays used in neuroscience for probing (Source: Kewley et al [73] with permission from the publisher) In summary, the other possible ball deposition techniques which need to be considered are mentioned below: i Pushing the microspheres onto the silicon surface via an opening in the surface itself, with the aid of a microtube ii Using prearranged beads in an array pattern to press deposit the microspheres onto the silicon plates In this way, quantifying of the microspheres can be more easily done iii Incorporation of some or all the above techniques for experimentation with real MEMS devices 61 References Roya Maboudian, W Robert Ashurst, Carlo Carraro, “Tribological challenges in micromechanical systems”, Tribology Letters, Vol 12, No 2, February 2002 Danelle M Tanner et al., “MEMS Reliability, 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William M.; Peterson, Ken A.; Dugger, Michael T.; Eaton, William P.; Irwin, Lloyd W.; Senft, Donna C.; Smith, Norman F.; Tangyunyong, Paiboon; Miller, Samuel L Source: Microelectronics and Reliability, v 39, n 3, 1999, p 401-414 72 Jack W Judy, “Microelectromechanical systems (MEMS): fabrication, design and Applications” Smart Mater Struct 10 (2001) p1115–1134 73 Kewley D T, Hills M D, Borkholder D A, Opris I E, Maluf N I, Storment C W, Bower J M, Kovacs G T A, “Plasma-etched neural probes” Sensors Actuators A Vol 58, No 1, p27–35, 1997 74 N Satyanarayana, Sujeet K Sinha, “Tribology of PFPE overcoated self-assembled monolayers deposited on Si surface”, J Phys D: Appl Phys No 38 p3512-3522 (2005) 68 Appendix The wobble is defined as the relative angular displacement between the TP and BP When there are balls between the Si plates, the different ball sizes cause the TP to rest at a tilt relative to the BP (see Figure 1.1) A wobble can also come about when the BP is not flat to begin with but has an angle relative to the horizontal plane (see Figure 1.2) If this angle is large enough, it can cause touching of the Si plates near the end, and result in high friction when the BP rotates The configuration for the minimum angle of wobble achievable to not cause the plates to touch is when the TP is resting on a ball at the ends  2R  θ c , is given as tan −1   ,  D where D is the diameter of the Si plate The lowest value of θ c should be used, and of the plates (See Figure 1.3) The critical angle of wobble, occurs when we use the lower limit of the ball radius Since the size of the balls is given as 53 ± 3.7 µm , the lower limit is 49.3µm , which gives a θ c value of approximately θ c will be considered a large wobble, since ideally the wobble must be kept to below this theoretical θ c value 0.19o Any wobble above 2o Figure 1.1 θ Figure 1.2 θo Figure 1.3 1-1 Appendix Dispersion Factor (DF) and mean the effective mass ( x ) are quantities that can be used to describe the quality of the dispersion of deposited bearing balls on the bottom plate (BP) Due to the large surface to volume ratio of the balls, the strong surface force causes agglomeration of the balls To quantize this effect, we introduce the above two quantities The relationship between DF and x is given as by () DF = x −1 x is determine by counting and averaging the mean size of the balls and agglomerates deposited on the Si plate To get DF values for poorly and well-dispersed cases, we −5 emulate actual deposition conditions by weighing 22 ( ±2) x10 g of balls and depositing them on the Si substrate used for a BP using the methods described in the paper 5-7 SEM images are then taken using magnification of about 27 To tabulate the frequency of the various mean masses, we print out the image and strike out the balls/agglomerates as effective masses and its frequencies of occurance is noted The data for the images taken to represent the dispersion of the particular plate is tabulated using an Excel spreadsheet, and the mean effective masses for the images is calculated by the formula, given as N x= ∑ xi f i N ∑ fi , where ≤ i ≤ N Here, x represents the number of balls in a particular agglomerate where the subscript i denotes the number( i =1 means the ball is singly dispersed, and x7 = ), f is the frequency of occurance (eg, f is the number of times counted for the agglomerate of size of balls), and N represents the largest observed agglomerate size For poorlydispersed balls, N is around 12-15 while it is around for well dispersed balls To avoiding double counting, overlap regions of the images are striked out A ballpoint pen is used to dot out locations on the Si plate Deliberately imaging with the pen mark can help to identify regions of overlap Three samples for each dispersion mode are taken The respective effective masses for the plates are averaged to provide an estimate for the DF The DF for poorly-dispersed is 0.64 ± 0.05 , and is higher for well-dispersed, at 0.83 As we approached DF of 1, we start getting more singly dispersed balls A DF of means that there is no agglomeration, and that all the balls are dispersed perfectly 2-1 Appendix Programming code used to calculate the critical radius and critical rotational speed Rc=8.314; V=1.8*10^-5; T=293; Dc=0.0075; crd=3.2*10^-9; density1=1050; density2=2200; L=2*10^-10; Lmax=4*10^-6; COF=0.7; E=60*10^9; v=0.22; g=9.81; Fdry=3*pi*SE*R*(1/(1+58.14*R*RMS/p^2) + 1/(1+1.817*RMS/H)^2); for RH=1:100; Fcap(RH)=4*pi*R*ST*cos(theta)*(1+1.817*RMS*Rc*T*log(RH/100)/(2*V*ST*cos(th eta))); F(RH)=Fdry; end; for RH=1:100; if F(RH)>Fcap(RH) F(RH)=F(RH); else F(RH)=Fcap(RH); end; X(RH)=RH; end; for RH=1:100; Rc1(RH)=((3*F(RH))/(4*pi*density1*g))^(1/3); Rc2(RH)=((3*F(RH))/(4*pi*density2*g))^(1/3); Rc1(RH)=Rc1(RH)*10^6; Rc2(RH)=Rc2(RH)*10^6; end; plot(X, Rc1, X, Rc2); 3-1 for RH=1:100; w1(RH)=((3*F(RH)*crd)/(pi*density1*Dc*R^4))^(1/2); w2(RH)=((3*F(RH)*crd)/(pi*density2*Dc*R^4))^(1/2); w1(RH)=w1(RH)*60/(2*pi); w2(RH)=w2(RH)*60/(2*pi); end; plot(X, w1, X, w2); 3-2 [...]... micro and nano balls and also on the progress of the work done on the reduction of friction for MEMS This is followed by a description of the building of a rotary test rig as well as designing the experiments to investigate the lifecycle of rolling the micro- ball bearings in Chapter 3 Chapter 4 covers the experimental results of the lifecycle tests, optical imaging and 4 FE-SEM analysis of the silicon. .. 1.2: The figure shows the 53 micrometer glass microspheres adhering onto the silicon surface, taken at a 90o incline with the SEM 1.2 Focus and objectives of the Project The focus of this project is to study the feasibility, as well as lifecycle, of using microspheres, primarily a glass and a polymer, as ball bearings without the need for building individual channels for each ball These micro- balls... us the range of parameters which we will be operating the experiment under The model also shows us the feasibility of the conditions under which we can use these microspheres for MEMS as micro ball bearings, taking into account the surface forces and the centrifugal force The model developed by Rabinovich is used to determine the critical radius of the ball bearings below which we expect surface forces... effects The Rc calculated here takes into account only the dry forces, namely van dar Waals The equation for Rc is derived by equating the weight of the micro- ball to the surface force Rearranging the terms and making Rc the subject of formula, we get 1/ 3  3F   Rc =   4π ρ g  , Eqn 2.3 where we have equated the weight of the ball to the surface force Here, ρ is the specific density of the ball s... ideal and promising On the other hand, the strong surface forces can serve to our advantage because they keep the micro bearing balls adhered to the surfaces it is supposed to roll on, theoretically negating the need for individual grooves or channels 15 If the idea of using ball bearings, either with channels or without, can be proved to be a feasible solution for solving friction challenges in MEMS... microball will restrain the balls from going out of the interface due to the rotational dynamics of the interface A single channel that can accommodate all micro- balls simplifies the fabrication process of the MEMS components 1.3 Structure of the thesis The structure of the thesis has been organized as follows Chapter 2 covers literature review of related work that has been conducted on the rolling of micro. .. to characterize the failure mechanism of the micro- ball bearings In Chapter 5, we discuss in detail the factors affecting the lifecycle of the various tests In Chapter 6, the thesis ends with some important and specific conclusions drawn from this study, and in Chapter 7, we have recommended some future work that can be carried out for the applications of the ball bearings for existing MEMS devices... forces to hold the bearings to the silicon surfaces We also use the model developed by Dominik and Tielens [58] to determine the critical rotational speed beyond which the centrifugal force will cause the ball bearing to spiral out of the rotating silicon plates To simplify the analysis, the effects of aerodynamics will be ignored, and only the effect of the centrifugal force will be taken into consideration... Schematic of a MIT microengine 63 Figure 7.2: A Microfabricated silicon neural probe 63 xii List of Tables Descriptions Page Table 2.1: Table showing the theoretical radius for adhesion, and the actual ball size used for the experiment 22 Table 4.1: Table for the specifications for the 5 set of runs 44 xiii CHAPTER 1 SCOPE AND OBJECTIVES OF THE PROJECT 1.1 Introduction to the friction challenges of Micro. .. Vector sum of all external forces Figure 2.12: Diagram showing the various forces acting on a microsphere adhered to a rotating silicon plate by the surface forces The expression for the moment of dislodge on the ball, M d , due to Fd is given as M d = Fd R , Eqn (4) by taking moments of Fd about the contact at the bottom of the ball Since Fd is dependent on the substrate’s rotational speed and the ball s .. .ISSUES AND CHALLENGES IN THE APPLICATION OF MICRO-BALL BEARING FOR SILICON BASED MICROSYSTEMS ROBIN PANG SUI TING (B Eng (Hons.), NUS) A THESIS SUBMITTED FOR THE DEGREE OF MASTER OF ENGINEERING... microspheres for MEMS as micro ball bearings, taking into account the surface forces and the centrifugal force The model developed by Rabinovich is used to determine the critical radius of the ball bearings... of the micro-ball) and the operating conditions (RPM of the Si plate) for the groove-less ball bearings setup Thus, a micro-ball can remain at the interface due to various surface forces if the