Design of a precision chemical mechanical planarization research system

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Design of a Precision Chemical Mechanical Planarization Research System

By

Fardad All Hashemi B.S., Mechanical Engineering University of California at Berkeley, 1998

SUBMITTED TO THE DEPATMENT OF MECHANICAL ENGINEERING IN PARTIAL FULFILMENT OF THE REQUIREMETS FOR THE DEGREE OF

MASTERS OF SCIENCE IN MECHANICAL ENGINEERING AT THE

MASSACHUSETTS INSTITUTE OF TECHNOLOGY JUNE, 2000

Signature of Author:

Certified by:

Department of Mechaini'cal Engi-neering 9 May, 2000

esto E. Blanco Professor of Mechanical Engineering _mb jgsis Supervisor Accepted by: MASSACHUSETTS INSTITUTE OF TECHNOLOGY SE

P

2 0

2000

AmM A.ooni Professor of Mechanical Engineering Chairman, Committee for Graduate Students

ENG

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-Design of a Precision Chemical Mechanical Planarization Research

System

By

Fardad Ali Hashemi

Submitted to the Department of Mechanical Engineering 9 May, 2000, in Partial Fulfillment of the degree of

Master of Science in Mechanical Engineering

ABSTRACT

The main focus of this study was to design the platen spindle, supporting machine structure, and a novel gimbal mechanism for use in a precision CMP system. Principles of kinematic coupling and precision machine design were used to insure easy and precise assembly of the machine. Many design types were studied to achieve compact yet practical final designs. Stresses arising from the deformations of the machine due to changes in temperature were evaluated and resolved via type synthesis. Three novel designs for a gimbal mechanism with an offset center of rotation were proposed and evaluated.

Thesis Supervisor: Ernesto E. Blanco Title: Professor of Mechanical Engineering

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In memory of Morteza Hashemi September, 1908 to April, 2000

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Reflection

It seems only appropriate, to take some time to reflect on this stage of my life before finishing off the completing piece. I came here two years ago, with hope and expectations regarding the important new path that I was about to embark on. In most ways, right from the beginning MIT surpassed those expectations beyond anything that I could have imagined. In others, it has taken a concentrated and often challenging effort to shape my path towards what I hoped at the onset. In both ways, this has been a most valuable learning experience.

A very important part of this experience has been the people that I have been in contact with. Perhaps, the most important of those people is my advisor Prof. Ernesto Blanco. I met him at the end of my first semester and ever since then I have enjoyed the benefit of his advice in both engineering and life matters. I could not have completed this thesis without him. He has been a mentor to me in the truest sense of the word. I shall forever be grateful.

It would be far from a complete reflection, if I did not reflect on the influence that my friends Amir, Jamie, and Farid have had on my experience here at MIT. Amir and Jamie made my time in the lab so much more interesting. I have never met anyone as well intentioned and kind hearted as Jamie. He is a person of true character. I was once told that if you work hard you must party even harder on your spare time to keep you sanity. I have a feeling Amir and Farid must be very, very hard working people. I would like to furthermore thank Farid for our detailed discussions regarding dynamics and kinematics on the steps of the student union. I admire his passion and detailed understanding of the science of engineering.

The recent passing away of my grandfather has also made me reflect deeply on the role that he has had in helping me get to this stage. I guess the path did not begin two years ago or when I was born but long before then. It has been a cumulative process that began long time ago. I firmly believe that each generation endeavors to build a foundation for the next generations to continue to build upon. Their accomplishments and sacrifices are the bricks that make up this foundation. The more I grow the more I realize how much the success of each new generation depends on the foundation that has been laid beneath them. In that sense I am most lucky. I could never possibly thank my parents enough for the sacrifices that they made so that my brother and I could have a better education.

In fact, as I sit here at brink of a new dawn, both figuratively and literally, seeking to finish laying this very important brick in my layer, I can only hope to be so lucky as to lay a layer as thick and as solid as the two that I stand upon. To my parents and my brother I am eternally grateful for their teaching and continuing support. Their influence on me is the inner silent voice that guides me even when they are not there. I couldn't have done it without you.

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ACHNOWLEDGEMENTS

I would like to take this opportunity to acknowledge and thank Silicon Valley Group for their help and support, which funded the primary stage of this research. In addition, to their financial support, their advice and comments at the review meetings were very critical in guiding me in the right direction. In particular, I would like to thank Mr. Jim Kenon whose insight helped me reduce the height of platen assembly by almost half.

I would also like to take the opportunity to thank Prof. Chun and Prof. Suh for giving me the benefit of their advice. I am very grateful for their understanding and patience during difficult times in this project.

Most of all I would like to thank Prof. Ernesto Blanco who is my immediate advisor for his advice in engineering and life matters. I have learned a lot from him and feel very privileged to have had him as my advisor. I admire his dedication to teaching and the integrity and engineering insight with which he carries out the task.

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Table of Figures

FIGURE 1. SIDE VIEW SCHEMATIC OF ROTARY CONFIGURATION. ... 17

FIGURE 2. TOP VIEW SCHEMATIC OF ROTARY CONFIGURATION. ... 18

FIGURE 3. SIDE VIEWSCHEMATICOFLINEAR CONFIGURATION. ... 19

FIGURE 4. TOP VIEW SCHEMATIC OF PAD-BELTALIGNMENT ... 23

FIGURE 5. SCHEMATIC OF A POSSIBLE TWO TYPE PAD PLATEN. ... 25

FIGURE 6. LAYOUT VIEW OF CONCEPT . ... 27

FIGURE 7. OVERVIEW OF CONCEPT#] ... 28

FIGURE 8. LAYOUT OF CONCEPT #2 . ... 29

FIGURE 9. OVERVIEW OF CONCEPT #2. ... 30

FIGURE 10. LA YOUT OF CONCEPT #3. ... 31

FIGURE 11. GANTRY STRUCTURE FOR CONCEPT #3 ... 32

FIGURE 12. GANTRY STRUCTURE FOR CONCEPT #3...33

FIGURE 13. THE VALUE OF THE MINIMUM WAFER OFFSET DISTANCE AS FUNCTION OF POLISH VELOCITYAND PLATEN ROTATIONAL VELOCITY LIMITS. ... 36

FIGURE 14. MACHINE FOOTPRINTAS FUNCTION OF POLISH VELOCITYAND PLATEN ROTATIONAL VELOCITY LIMITS. ... 3 6 FIGURE 15. THE MAGNITUDE SQUARED OF THE GRADIENT OF FOOTPRINTAS FUNCTION OF POLISH VELOCITYAND PLATEN ROTATIONAL VELOCITY LIMITS. ... 37

FIGURE 16. WELDED STAINLESS STEEL LOWER STRUCTURE (CONCEPT #]A). ... 39

FIGURE 17. WELDED STAINLESS SIDE STRUCTURE . ... 40

FIGURE 18. WELDED STAINLESS STEEL LOWER BASE. ... 41

FIGURE 19. A CAST POLYMER COMPOSITE LOWER STRUCTURE (CONCEPT#1B). ... 42

FIGURE 20. CONCEPT #JB DESIGN WITH THE THRU HOLES FOR THE RAIL BOLTS...43

FIGURE 21. CONCEPT #1 C DESIGN WITH THE THRU HOLES FOR THE RAIL BOLTS...44

FIGURE 22. METHOD OF ASSEMBLY FOR A CONCEPT#JC DESIGN...45

FIGURE 23. RENDERED VIEW DIAGRAM OF THE TOP TABLE SHOWING THE LARGE PLATEN HOLES...49

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FIGURE 25. RENDERED VIEW DIAGRAM SHOWING THE BOLT CLEARANCE HOLES FOR THE PLATEN ASSEMBLY. ... 51

FIGURE 26. RENDERED VIEW DIAGRAM SHOWING MOUNTING FEATURE FOR THE GANTRY LINEAR ENCODER STRIP. ... 5 2 FIGURE 27. RENDERED VIEW DIAGRAM SHOWING THE MOUNTING FEATURES FOR THE GANTRY RAILS AND MOTOR, AND BALLSCREW BRACKETS. ... 53

FIGURE 28. RENDERED VIEW DIAGRAM SHOWING CONDUITS IN THE TOP TABLE. ... 54

FIGURE 29. RENDERED VIEW DIAGRAM SHOWING CLEARANCE HOLES FOR METAL INSETS. ... 55

FIGURE 30. RENDERED VIEW OF THE LAPPING CONFIGURATION FOR THE TOP TABLE ... 56

FIGURE 31. SCHEMATIC DIAGRAM OFA BOLT BEING SUBJECTED TO SHEAR BETWEEN TWO MATERIALS WITH DIFFERENTAMOUNTS OF EXPANSION. ... 57

FIGURE 32. SCHEMATIC OF A MATERIAL SPECIMEN UNDERGOING THE UNIAXIAL TENSION TEST. ... 60

FIGURE 33. RENDERED SCHEMATIC OF THRU HOLE BOLT DESIGN. ... 61

FIGURE 34. 2-D SCHEMATICS THE BOLT BEING MODELED AS A FIXED/FIXED BEAM IN BOTH THE NEUTRAL AND STRAINED STATES. ... . ... ... 62

FIGURE 35. SCHEMATIC OF AX LONG SECTION OF THE BOLTAT LOCATION X. ... 63

FIGURE 36. SCHEMATIC OF AX LONG SECTION OF THE BOLT UNDER ANGULAR DEFLECTION. ... 65

FIGURE 37. SCHEMATIC OF AX LONG SECTION OF THE BOLT UNDER TRANSLATION DEFLECTION. ... 67

FIGURE 38. RENDERED VIEW DIAGRAM OF THE LOWER FRAME CAST IRON PLATES...71

FIGURE 39. RENDERED VIEW DIAGRAM SHOWING ACCESS PORTS IN THE LOWER FRAME ... 72

FIGURE 40. RENDERED VIEW DIAGRAM SHOWING THE BOLT PATTERNS REQUIRED FOR THE ASSEMBLY OF THE LO WER F RAM E. ... ... --- . .---.--73

FIGURE 41. RENDERED VIEW DIAGRAM SHOWING NECESSARY FINISHED SURFACES FOR THE LOWER FRAME. ... 74

FIGURE 42. RENDERED VIEW DIAGRAM SHOWING THE LOWER FRAME ASSEMBLY READY FOR LAPPING. ... 75

FIGURE 43. RENDERED VIEW DIAGRAM OF COMPLETED LOWER FRAME . ... 76

FIGURE 44. RENDERED VIEW DIAGRAM OF THE ASSEMBLY OF THE TOP TABLE ONTO THE LOWER FRAME...77

FIGURE 45. COEFFICIENT OF KINETIC FRICTION AS A FUNCTION OF THE RATIO OF POLISH PRESSURE OVER POLISH VELOCITY FOR 7PSI PRESSURE. ... ... 81

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FIGURE 47. REQUIRED MOTOR TORQUE AS A FUNCTION OF POLISH PRESSURE AND VELOCITY...83

FIGURE 48. REQUIRED MOTOR POWER AS A FUNCTION OF POLISH PRESSURE AND VELOCITY...84

FIGURE 49. SCHEMATIC OF ROTOR AND STATOR CLAMPED FITS. ... 87

FIGURE 50. SCHEMATIC OF ROTOR AND STATOR FITS ... 88

FIGURE 51. SCHEMATIC OFA SHAFT SUPPORTED BYA RADIAL BALL BEARING...93

FIGURE 52. SCHEMATIC OFA SHAFT SUPPORTED BY ONE RADIAL AND ONE AXIAL BALL BEARING CONFIGURATION. ... 9 4 FIGURE 53. SCHEMATIC OF A SHAFT SUPPORTED BY TWO RADIAL AND ONE AXIAL BALL BEARING CONFIGURATION. ... 9 5 FIGURE 54. SCHEMATIC OF A SINGLE DIRECTION AXIAL HYDROSTATIC BEARING. ... 96

FIGURE 55. SCHEMATIC OF SINGLE DIRECTIONAXIAL, RADIAL, AND MOMENT SUPPORTING HYDROSTATIC BEARING CONFIG URATION. ... ... ----.. ...- 97

FIGURE 56. SCHEMATIC OF AN AXIAL SELF-COMPENSATING HYDROSTATIC BEARING. ... 98

FIGURE 57. MAGNIFIED SECTION OF AN AXIAL SELF-COMPENSATING HYDROSTATIC BEARING. ... 99

FIGURE 58. RENDERED CUT-AWAY VIEW OF A CROSS ROLLER BEARING. ... 100

FIGURE 59. SCHEMATIC OF THE ROLLER ELEMENT ALIGNMENT IN A CROSS ROLLER BEARING...101

FIGURE 60. SCHEMATIC SHOWING THE RESULTING PLATEN DIAMETER ... 105

FIGURE 61. RENDERED VIEW OF THE TOP PLATEN SECTION... 106

FIGURE 62. RENDERED VIEW OF THE PLATEN ROTOR SHAFT ... 107

FIGURE 63. RENDERED VIEW OF THE POSITIONING STEP ON THE ROTOR SHAFT ... 108

FIGURE 64. RENDERED VIEW OF BEARING DISPLACEMENT FEATURE ... 109

FIGURE 65. RENDERED VIEW OF BEARING LOCATION FEATURE...110

FIGURE 66. RENDERED VIEW SHOWING THE ASSEMBLY OF THE BEARING AND LOWER RETAINING RING...111

FIGURE 67. RENDERED SECTION VIEW OF PLATEN CAVITY FEATURES. ... 112

FIGURE 68. RENDERED VIEW OF STIFFENING RIB STRUCTURES. ... 113

FIGURE 69. DiSPLACEMENT FEA RESULTS FOR PLATEN STRUCTURE WITH THICK TOP PLATE UNDER POLISH LOAD. FIGURE 70. DISPLACEMENT FEA RESULTS OF PLATEN STRUCTURE WITH RIB SECTION UNDER POLISH LOAD. ... 115

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FIGURE 7 1. RENDERED VIEW OF SPLASHGUARD AND PLATEN INTER FACE. ... 117

FIGURE 72. RENDERED VIEW OF THE BEARING SEAT ON THE CAPSULE... 118

FIGURE 73. RENDERED VIEW OF THE BEARING ASSEMBLY INSIDE THE CAPSULE ... 119

FIGURE 74. RENDERED VIEW OF BEARING LUBRICATION HOLES...120

FIGURE 75. RENDERED VIEW OF HALF RING RETAINING RINGS...121

FIGURE 76. RENDERED VIEW SHO WING RETAINING RING INTERFACE FEATURE ... 121

FIGURE 77. RENDERED VIEW SHO WING HOW THE HALF RING RETAINING RINGS CLAMP DOWN ON THE OUTER RACE OF THE BEARING. ... - ... 122

FIGURE 78. SCHEMATIC BEARING RETAINING RINGS. ... 123

FIGURE 79. RENDERED SECTION ViEW OF THE PLATEN ASSEMBLY. ... 124

FIGURE 80. RENDERED VIEW OF SPLASHGUARD IN THE PLATEN ASSEMBLY...125

FIGURE 81. RENDERED VIEW PLATEN AND CAPSULE SUBASSEMBLIES AS THEY ARE ASSEMBLED TOGETHER. ... 126

FIGURE 82. RENDERED SECTION VIEW OF THE PLATEN ASSEMBLY WITH THE ALL RETAINING RINGS ATTACHED.. 127

FIGURE 83. RENDERED VIEW OF THE ROTARY ELECTRICAL COUPLING IN THE ASSEMBLY. ... 128

FIGURE 84. RENDERED VIEW OF THE ROTARY FLUID UNION IN THE ASSEMBLY... 128

FIGURE 85. RENDERED VIEW SHOWING THE BRACKET IN THE PLATEN ASSEMBLY. ... 130

FIGURE 86. SCHEMATIC SHOWING THE RELATION BETWEEN THE PINS AND THE UNDERSIDE GROOVES OF THE DETACHABLE SECTION (BOTTOM VIEW)... 132

FIGURE 87. RENDERED SECTIONED VIEW OF THE ENDPOINT DETECTION SENSOR IN THE ASSEMBLY ... 134

FIGURE 88. RENDERED VIEW OF THE AMPLIFIER AND ADAPTER PLATE. ... 135

FIGURE 89. RENDERED VIEW OF AMPLIFIER AND ADAPTER PLATE IN ASSEMBLY...136

FIGURE 90. RENDERED VIEW OF THE OVERALL PLATEN ASSEMBLY ... 137

FIGURE 91. RENDERED SECTION VIEW OF THE OVERALL PLATEN ASSEMBLY. ... 138

FIGURE 92. RENDERED VIEW OF THE OVERALL PLATEN ASSEMBLY SHOWING THE COMPONENTS INSIDE THE PLATEN ROTOR SHAFT ... 139

FIGURE 93. SCHEMATIC OF AN EXAGGERATED WAFER VS. PAD SPINDLE AXES MISALIGNMENT. ... 140

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FIGURE 95. SCHEMATIC SHOWING THE MOMENT EXERTED ABOUT THE CENTER OF ROTATION OF THE WAFER BY

THE FRICTION FORCE. ... 143

FIGURE 96. SIMPLE FOUR-BAR LINKAGE...145

FIGURE 97. SCHEMATIC OF A FOUR-BAR LINKAGE SHOWING THE LOCATION OF THE INSTANT CENTER OF THE CO UPLER LINK. ... ... 146

FIGURE 98. RENDERED VIEW OF A FOUR-BAR GIMBALED MECHANISM DESIGN. ... 147

FIGURE 99. ALTERNATIVE FOUR-BAR GIMBA LED MECHANISM DESIGN. ... 148

FIGURE 100. SCHEMATIC OF A FOUR BAR LINKAGE SYSTEM WITH A DESIRED INSTANT CENTER FOR THE COUPLER LIN K . ... . --- .... .. -.. --- ... .- 14 9 FIGURE 101. SCHEMATIC OF A FOUR BAR LINKAGE SYSTEM SHOWING THE LOCATION OF THE INSTANT CENTER OF THE COUPLER LINK. ... ... 153

FIGURE 102. SCHEMATIC OF A FOUR BAR LINKAGE SYSTEM SHOWING THE LOCATION OF THE ACTUAL INSTANT CENTER OF THE COUPLER LINK... ... 156

FIGURE 103. SCHEMATIC OF THE SECONDARY LINKAGE SYSTEM SHOWING THE LOCATION OF THE ACTUAL INSTANT CENTER OF THE COUPLER LINK... 160

FIGURE 104. SCHEMATIC OF THE PRIMARY LINKAGE SYSTEM SHOWING THE LOCATION OF THE ACTUAL INSTANT CENTER OF THE SECONDARY COUPLER LINK... 162

FIGURE 105. MAGNITUDE OF MOMENT ARM FOR MOMENTS ABOUT THE PRIMARY INSTANT CENTER OF ROTATION. ... ... 16 5 FIGURE 106. MAGNITUDE OF MOMENT ARM FOR MOMENTS ABOUT THE SECONDARY INSTANT CENTER OF ROTATION... 166

FIGURE 107. RENDERED VIEW SCHEMATIC OF A SIMPLE SPHERICAL JOINT. ... 168

FIGURE 108. SCHEMATIC OF THE TARGET BODIES TO BE COUPLED FOR THE TRANSMISSION OF TORQUE. ... 169

FIGURE 109. SCHEMATIC SHOWING THE TWO TARGET BODIES CONNECTED BY A TELESCOPING JOINT. ... 170

FIGURE 110. SCHEMATIC SHOWING THE UNIVERSAL JOINT CONFIGURATION ON THE MOVING BODY...171

FIGURE 111. SCHEMATIC SHOWING THE COMPLETED COUPLING OF THE TARGET BODIES TO EACH OTHER USING A TELESCOPING CONSTANT VELOCITY JOINT... 172

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FIGURE 112. SCHEMATIC SHOWING THE TWO TARGET BODIES AND COUPLING INAN ARBITRARY CONFIGURATION.

... 17 3

FIGURE 113. RENDERED VIEW DIAGRAM OF THE HEAD SPINDLE ASSEMBLY... 174

FIGURE 114. RENDERED VIEW DIAGRAM OF THE MAIN BRACKET...175

FIGURE 115. RENDERED VIEW DIAGRAM SHO WING THE HEAD CAPSULE AND THE SPHERICAL SECTIONS. ... 176

FIGURE 116. RENDERED VIEW DIAGRAM SHO WING SPHERICAL JOINTASSEMBLY. ... 177

FIGURE 117. RENDERED VIEW DIAGRAM SHOWING THE INITIAL COMPONENTS OF THE TELESCOPING CONSTANT VELOCITY JOINTASSEMBLY. ... ---....-.-..----.. 178

FIGURE 118. RENDERED VIEW DIAGRAM SHOWING THE SECONDARY RING OF THE MOVING UNIVERSAL JOINT. .. 179

FIGURE 119. RENDERED VIEW DIAGRAM SHO WING THE PRIMARY RING OF MOVING UNIVERSAL JOINT. ... 180

FIGURE 120. RENDERED VIEW DIAGRAM OF THE COMPLETED TELESCOPING JOINT IMPLEMENTATION. ... 181

FIGURE 121. RENDERED VIEW DIAGRAM OF THE COMPLETED TELESCOPING CONSTANT VELOCITY JOINT ASSEM BLY... .. ... ... ---... . . . 182

FIGURE 122. RENDERED SECTION VIEW OF A BELLOW. ... 183

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1 Introduction

The trend in the semiconductor industry has always been to continuously reduce the size of solid-state chips. To this end, the industry has employed multiple layered circuit designs with increasingly smaller line widths. To achieve small line widths within the range of 5pm a small depth of focus is required during the lithography steps. This in turn requires the topography of the surface that the pattern is projected on to, to be very flat. In recent years, semiconductor manufacturers have used different techniques to planarize the wafer surface before each lithography step. One of these techniques is Chemical Mechanical Planarization (CMP). CMP is a method of producing extremely flat surfaces on materials such as tungsten, aluminum or silicon using slurries such as aluminum oxide, silica, or cerium oxide abrasives. CMP is a lapping process by which two-body and three-body abrasion is used with the assistance of a chemical agent to globally planarize the surface of silicon wafers. Compared to other semiconductor technologies, CMP is still at an early stage of development. As such, there is still much to study and learn about the process, such as the wafer/pad contact characteristic and the role of chemistry in the polishing process. There is a collaborating research group at MIT that is trying to study the underlying science behind the CMP process. They require a flexible and easily modifiable machine to use in their research. It is the goal of this project to design a machine that can be used as a research tool to learn more about the CMP process and that can serve as stepping stone for the design of competitive production CMP tools in the future. These two goals complement each other since any

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information obtained from the use of the tool in process research will facilitate the understanding that is necessary to design a capable production tool.

2 Design Goals

As a research tool, this machine must first and foremost be as flexible as possible in its design. It should be easily and endlessly modifiable after its production to allow for the changes and additions that may be deemed necessary after initial research results. It should also allow multiple CMP process recipes (pad texture and slurry combinations) to be run on the machine with the minimum required changeover effort and expense. To be a good research tool, it must produce as little vibration as possible and have high damping as not to introduce extra variables into the process parameters. Finally, at the request of the collaborating research group, the machine must also allow two separate recipes to be run, consecutively and without machine downtime, on the same wafer.

As a predecessor of a production CMP tool, this machine must take into account potential end-customer needs. It must have the minimum amount of footprint possible while still maintaining its design flexibility. It must be easily transportable with a minimum amount of preparation. It must also have easy and repeatable assembly. Furthermore, it must withstand the temperature ranges present in it operating environment and during its transport, without sustaining any damage or misalignment. Finally, it must not generate any particles during its operation since this would

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3 Choosing Polish Configuration

3.1 CMP Configurations

As mentioned before CMP is a lapping process in which the wafer surface is pressed against a pad surface that polishes it. In most configurations the wafer is held upside down and lowered down onto the pad. The pad is usually loaded with slurry that contains abrasive particles and a fresh supply of slurry is delivered to the contact surface over time. It is the relative motion of the pad with respect to the wafer that polishes the wafer surface via two-body and three-body abrasion. In two-body abrasion, abrasive particles, embedded and stuck to the top surface of the pad, are dragged over the wafer surface under pressure and hence remove material. In three-body abrasion, abrasive particles that are within the pad-wafer contact area hit the wafer surface with an impingement angle and velocity and remove material as a result. In both cases, the amount of contact pressure and the relative velocity of the pad on the wafer influence the amount of material that is removed. As such, in order to achieve uniform, global material removal rate, the contact pressure on every point of the wafer must be the same. In addition, the relative velocity of the pad with respect to any point on the wafer must also be the same (uniform across the wafer surface).

There are three main configurations used in commercial CMP tools that try to achieve these conditions. The first and most widely used configuration is a rotary configuration. In this configuration the wafer is lowered down onto a pad of larger area as shown below.

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Application of Polishing Force Op Wafer Carrier Wafer Platen

Figure 1. Side view schematic of rotary configuration.

A force exerting mechanism then presses the wafer surface against the pad surface in order to generate the necessary polishing pressure. In most cases, the wafer spindle has a gimbaled mechanism to insure uniform pressure distribution at the contact surface. The design of the gimbaled mechanism is critical and several options are discussed in section 15. Both the wafer and pad are attached to spindles that rotate them to generate the relative motions necessary for polishing to occur. The wafer is held inside a wafer carrier that is then attached to a spindle via a gimbal and the pad is bonded onto a platen that is attached to another spindle. The wafer center of rotation is separated from the pad center of rotation by the distance s denoted in the following figure.

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PLATEN

WAFER

Figure 2. Top view schematic of rotary configuration.

In this configuration, the velocity of the pad relative to any point on the wafer, at a location (r, 0) in the reference frame of the wafer, is given by Eq. 1 in the coordinates of the ground reference frame. A detailed derivation of this equation is included in the Appendix section.

(Eq. 1) -=VH (aH P )r sin0 ( H rc oPS]

As can be seen from Eq. 1, if op = oH= og then = -wes

j

regardless of rand 0. The

relative velocity i in the coordinates of the rotating wafer reference frame is given by

Eq. 2 below (assuming that the two reference frames are initially coincident at t=O). (Eq. 2) =cos- sin(wJO, 0t)I -wOjos -COS(wot)

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Another type of configuration that is used widely in commercial CMP tools is the linear configuration. In this configuration the wafer is lowered upside down upon a pad that is attached to moving linear belt. The linear belt is usually made of stainless steel and raps around two large drums. The motion of the belt is supplied by the rotation of these drums. In most commercial CMP tools, only one of the drums is driven by a motor and the other is allowed to rotate freely. As with the rotary configuration, there is a force exerting system that provides the necessary polishing pressure and a gimbaled mechanism to distribute the pressure evenly over the contact surface. A schematic of this configuration is shown in the following figure.

Application of Polishing Force Wafer Carrier Wafer VB B H (0B Pad Steel Belt

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Rotating Drums

Figure 3. Side view schematic of linear configuration.

In addition to the motion of the belt, the wafer rotates with a speed ft . The velocity of the linear pad relative to any point on the wafer, at location (r, 0) in the reference frame of the wafer, is given by Eq. 3 in the coordinates of the ground reference frame. The relative

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velocity in the coordinates of the rotating wafer reference frame is given by the Eq. 4 (assuming that the two reference frames are initially coincident at t=O).

(Eq. 3) V =VB +rWHsin0 rWHCosOj

(Eq. 4) H= [y + rOJF sin(Ht + )Ios(y )- rH cos(Ht+O)sin

t)i - rWH cos(wHt +O)0os(Hot)- [yB + rojH sin (wHt +0)]sin

(CHt)

J

3.2 Advantages and Disadvantages of each Configuration

One advantage of the rotary configuration is that it provides a uniform velocity profile at the contact surface when op = wH= oN. In addition, as can be seen from Eq. 2, the direction of the relative velocity profile over the wafer surface changes continuously with time. This way any scratch patterns left on wafer surface from the polishing action are not always in one direction. Instead, any such patterns will superimpose on each other from all directions and average out during each rotation. In effect, the net distance traveled by the pad over any point on the wafer is zero for each rotation. This can be seen by taking the time integral of Eq. 2 over one rotation as shown below.

21r

(Eq. 5) Dnet = coos sin(wt) -oos -cos(wot)i dt = coos - cos(wot)-o - [sin(wot)to =0

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However, under the necessary condition that o>H=W)p the wafer and pad will always rotate with the same phase that they started out with at the beginning of the process. In this manner, any given point on the pad will always follow the same path of travel on the wafer surface at each rotation. This can be a disadvantage if the polishing properties of the pad are not uniform over its entire area. For example, if a small region on the pad contains a higher concentration of abrasive particles than another, then this region will polish the areas of the wafer that it is contact with faster. Since the pad and the wafer are in phase, such a region will contact the same area on the wafer at each rotation and selectively polish that area of the wafer faster than others. The effect is not distributed over the entire surface of the wafer.

As was already mentioned above, the rotational velocities of both the platen and the wafer have to be the same in order to achieve a uniform velocity profile across the wafer surface. It is clear that if the wafer rotates faster than the platen the edges of the wafer will be polished faster. If the platen rotational speed is faster than that of the wafer, then again the edges of the wafer will polish faster. Therefore it is very critical to maintain the two speeds the same, as the effects of any random variations will superimpose instead of cancel (even if the average or mean of the variation is zero). With the linear system, however, small errors in the wafer rotation velocity or the belt linear velocity will not significantly affect the uniformity of the velocity profile across the wafer. This is an advantage of the linear system that allows for more design flexibility and reduced cost concerning the selection and implementation of the wafer and belt spindle drive systems.

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One disadvantage of the linear configuration is that the relative velocity profile is not uniform across the wafer surface. As can be seen from Eq. 3 the velocity of the pad relative to a point on the wafer is a function of the location of that point. If (% were equal to zero, then the velocity profile would be uniform. However, in that case, the direction of the relative velocity profile would always remain the same. In this manner any scratch patterns left on the wafer surface from the polishing action does not average out. With the linear configuration there is a compromise between maintaining a uniform velocity profile and averaging out the effects of the direction of that profile. Most commercial CMP tools optimize between the two limitations by using low values of (% compared to vo. In this way, they achieve some direction averaging with a minimum effect on the uniformity of the profile. However, the rate at which the direction is averaged is not as fast as that of the rotary configuration.

In practice, the linear configuration has some additional disadvantages to the rotary configuration that are not obvious from the kinematic analysis. The collaborating research team here at MIT polished many wafers using a test bed that contains both configurations. Experience shows that it is much easier to replace the pad on rotary configuration tool than the linear one. In the rotary configuration the self-adhesive circular pad is simply placed over the platen and then gradually pressed into it from one side to another, thus releasing the trapped air bubbles. Since the pad area is larger than the platen it does not have to be accurately centered with the platen and the excess pad material can be trimmed off at the end. This job takes one person roughly five minutes to complete.

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The linear pad, on the other hand, is a long rectangular self-adhesive pad that needs to be rapped around the metal belt from one end to another. As with the rotary pad, the linear pad is gradually pressed onto the metal belt in a single direction to expel any air bubbles trapped between the pad/belt contact surfaces. In this case, however, the pad/metal belt alignment is very critical. As the pad is gradually attached to the belt, great care must be taken to insure that the edge of the pad is aligned with that of the belt. Otherwise, the pad will travel off of the belt or wrinkle to maintain its path along it as shown below.

Belt

Pad rums

Figure 4. Top view schematic of Pad-Belt alignment.

Any small deviations at the start will amplify during the rest of the installation process in the same manner that the separation between two intersecting lines increases with distance from the intersection point. As a result, it takes two to three people ten minutes to rotate the belt, release all the air bubble, and maintain pad-belt alignment in order to install the linear pad.

The linear configuration is also harder to maintain than the rotary one. As the pad and belt stretch, the tension on the metal stainless steel belt needs to be adjusted.

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However, if at any time the stainless steel belt is stretched too much and then relaxed, the pad, which sustains permanent deformation when stretched, will not relax as much as the flexible metal belt that it is glued on to. As a result, after relaxation the pad has been observed to crawl up on the belt during the next polishing run. The latter in turn generates bumps on the pad that affect the pressure distribution within the pad-wafer contact surface.

3.3 Reasons for Choosing the Rotary Configuration

As was mentioned earlier, the kinematics of the rotary system are such that it produces the desired uniform relative velocity profile with a time variant direction when

wp = oH = ot. The variation of direction with time is constant and continuous and

averages out to be zero over each rotation as can be seen from Eq. 5. In addition the rotary configuration is easier to maintain in practice than the linear one. Furthermore, the rotary configuration requires two spindles instead of three which makes it easier to design and manufacture. Finally, since there are more rotary configured commercial CMP tools than linear ones, there are a wider variety of pads available for that design. As a result, the decision was made to use the rotary configuration in the design of this machine.

4 Two Platen Machine

As was mentioned earlier, one of the design goals of this machine is to allow two separate recipes to be run consecutively and without machine down time on the same wafer. Such flexibility will allow one process recipe to be used to quickly remove material and another to be used to more accurately planarize the surface of the wafer.

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Two separate recipes may involve two different pads and two different slurries in addition to different polish pressure and relative velocity parameters. A single and very large platen can be provided with two different pad materials in concentric rings, as shown in the figure below.

Type 1 Pad

Type 1 Pad

Figure 5. Schematic of a possible two type pad platen.

In addition, each of the two slurries can be separately supplied to the platen during its respective run. However, some chemical slurries are acidic while others are basic. If two slurries of varying pH are used on the same platen, then the pad needs to be flushed with plenty of water to neutralize the pH back to seven before the application of the second slurry. Experience with the test bed machine shows that once a pad is loaded with an acidic or basic slurry, it acts like a sponge and soaks up the slurry. Although conditioning can help the situation, it is still very difficult and time consuming to flush out the old slurry and neutralize the pH on the pad. Furthermore, a platen large enough to contain two separate pads, as shown in Figure 5, would have four times the area of the two smaller platens large enough to contain one pad. As a result the decision was made to

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design the machine with two single pad platens in order to satisfy the requirement for a two-recipe step process.

5 Machine Configuration

5.1 Conceptual Designs for Two Platen Machine Configuration

Four conceptual designs were generated for the two platen machine layout. The key goal in all of the concepts was to increase the throughput/footprint ratio while still meeting process requirements and not compromising the accessibility that is required for maintenance. All four concepts also include a washing station to clean off the wafer in between the two recipe steps and after the completion of the second step.

The first and simplest of these concepts is to lay the platens and washing station side by side along the length of the machine. This concept is shown in the figure below where the two large blue disks represent the platens and the small silver disk represents the washing station.

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Platen 1

Figure 6. Layout view of Concept #1.

Concept #1 can be implemented with a different motor for each platen or using one motor and center shaft to drive both platens as shown in Figure 6. With this design a gantry structure holding the wafer carrier and force application mechanisms can be mounted as shown in the following figure. Such a structure would ride on rails and transfer the wafer between the two platens and washing station.

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Wafer Motor

- Wafer Carrier

Figure 7. Overview of Concept #1

In its most compact form, a Concept #1 machine will be 6ft long, 2.5ft wide, and roughly 4.5-5ft high for an 8in-diameter wafer and 27in-diameter platens. It requires 2 linear axis of motion: one for lowering and raising the wafer onto the platens and the cleaning station; and another along the length of the machine for wafer sweep and transportation. As expected, this concept also requires three axis of rotation, one for each platen and one for the wafer.

A second concept reduces the footprint of the machine even more but adds extra complexity to the design. The layout of this design can be seen in the following figure.

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Washing

-jjtton

P

e

Center .SaftLC

Figure 8. Layout of Concept #2.

Since the wafer diameter (8in) is much smaller than that of the platen (27in), it does not cover the entire area of the platen during the lapping process. In Concept #2, the area of platen 1 that is not covered by the wafer carrier is tucked under the cleaning station. Similarly, the uncovered area of platen 2 is tucked under platen 1. As with Concept #1, this concept also allows for the use of one motor and a center shaft (as shown), or the use of two motors to drive each platen. In order to prevent slurry from splashing from one platen onto another the design of the splashguards around the platens will be critical with this concept. The overview of this concept is shown in the figure below.

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Figure 9. Overview of Concept #2.

In its most compact form, a Concept #2 machine will be 4ft long, 2.5ft wide, and roughly 6ft high. This machine is slightly taller than a Concept #1 machine in order to have the same amount of space available for the placement of the different components such as slurry pumps under the two platens. Concept #2 has the same axis of motion as Concept #1.

An even more complex concept is to stack the two platens and the cleaning station one on top of another in order to save space. After all, the footprint of this machine is more critical in a fab environment than its height. Thus it would make sense to reduce the footprint of the machine at the expense of increased height. To this futile end, Concept #3, which is shown in the figure below, was generated.

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Platen 1

Wafer Carrier

Platen 2

Figure 10. Layout of Concept #3.

In this concept the wafer carrier fits in between the two platens during the polishing process. To achieve this, the structure that is holding the wafer carrier has to be cantilevered and a belt has to be used to transfer the torque from the head motor to the wafer carrier as shown below.

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Wafer Motor

Cantilevered Structure

Pulley System

Wafer Carrier

Figure 11. Gantry structure for Concept #3.

At first look it might seem that such a design saves a lot of footprint at the expense of increased height, complexity, and an undesirable cantilevered force application structure. However, a closer look at Figure 10 will reveal that the gantry structure needs to travel back in order to clear the cantilevered wafer carrier structure from the platens, before it can raise and lower the wafer carrier to the other platen or the cleaning station. In fact, in its most compact form, a Concept #3 machine is still 4ft long, 2.5ft wide. Although it is

9ft high and very complex it offers no reduction in footprint over Concept #2.

In fact, the only way to reduce footprint by stacking the machine components vertically is to stack the platens, the cleaning station, and the gantry structure one on top of another as shown below.

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Wasing: Wafer Motor Platen 1 Wafer Carrier Platen 2 Center Shaft Motor

Figure 12. Gantry structure for Concept #3.

In the figure above the gantry is shown in its resting position. This is also the position that the gantry will be at in order to feed the wafer to the cleaning station. In order to polish the wafer on platen 1, both the gantry and platen 1 move forward and the wafer is lowered onto platen 1. Similarly in order to polish the wafer on platen 2, platen 1 moves back in and platen 2 moves forward. In its most compact form, Concept #4 is only 3.5ft long, 2.5ft wide, and 9ft high. Concept #4 provides the smallest footprint of all four concepts. However, two additional linear axis of motion have been introduced in order to achieve this. Concept #4 is the most complex of all four concepts as well and saves only 12.5% of foot print over Concept #2.

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It is clear that there is no advantage in choosing Concept #3. Among the remaining choices, Concetp#4 offers the smallest footprint. However, the complexity that is gained is not worth the small 12.5% reduction that it offers over Concept #2. Thus the choices are limited to either of the first two concepts. Concept #2 also offers a small 11.1% reduction in footprint over the first concept. However, the extra complexity gained with Concept #2 over Concept #1 is no as much that of Concept #4 over Concept #2. If this machine were to be developed purely a commercial tool, Concept #2 would be the best of all. However, considering that this machine's is a first attempt at a prototype design and will also be primarily used in a research environment where it needs to be endlessly modifiable, it is best to choose Concept #1 for the sake of the simplicity and future flexibility that it offers.

5.2 Wafer and Platen Center to Center Offset Distance

Once the machine configuration is chosen the wafer and platen center to center offset can be chosen. This offset is the minimum distance between the center of the wafer and the center of the platen. When the wafer is at its inner most position during the wafer sweep, this offset distance corresponds to the s dimension in Figure 2. It is important to note that the center of the platen can be inside the wafer surface. As can be seen form equations 1 and 2 there is nothing that requires the dimension s to be larger than the wafer radius. In fact, as long s is not zero, any other value will achieve the required constant velocity profile. However, as can be seen from equation 2 for smaller values of s the value of at must be larger in order to achieve the same magnitude for relative polish velocity. Thus, larger values of s will increase the machine footprint while smaller values

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of s will increases the required rotational velocities of the motors. The latter extreme will either require larger motors or the need for gear increase mechanisms and can present many other problems associated with wear and tear and harmonic vibrations in the machine. Either extreme will most likely increase the machine's volume and thus present a disadvantage as far as a commercial tools is concerned. The footprint of the machine given a value for s can be approximately calculated, although it will not be accurate since a detailed design for the machine is not yet complete at this stage. In addition, the size of the motors and gear increase mechanisms required to provide the necessary torques and rotational velocities depends, among other things, on the market availability of parts and is thus not a closed form function of s. As a result, choosing the appropriate value for the minimum offset distance is an iterative process. First, after reviewing the types of motors that are available in the market and taking into account wear and tear and vibration considerations, a range is chosen for the upper limit of the rotational velocities that can be required of the motor. This range was chosen to be between 400RPM to 500RPM. Ideally it is best to remain on the lower side of this range, as there are many more practical options available that way. Next, a range is chosen for the upper limit of the desired relative polish velocity. This range was chosen to be between 3.75 m/s and 4 m/s. Ideally, it is best to remain on the upper limit of this range, as this will allow higher polish velocity experiments to be carried out and adds to the flexibility of this machine as a research tool. Next, the values of s are calculated for each of the polish velocity and motor rotational velocity combinations possible within the two ranges. This plot is included in the following figure.

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3.8-3.6 3.4* o~3.2 34 3 -- 3.95 2.8 3.9 50 480 -. 8 460 403.8 420 400 3.75 velocity (m/s) RPM

Figure 13. The value of the minimum wafer offset distance as function of polish velocity and platen rotational velocity limits.

As is expected the value of s is larger for higher polish velocities and lower platen rotational velocities. At this point it is helpful to evaluate the estimated footprint of the entire machine for each of the combinations within the chosen ranges.

4600-4500 6 4400 C- 4300 0 LL 4 4200 - 3.95 4100 -3.9 500 -3.85 480 460 403.8 420 400 3.75 velocity (m/s) RPM

Figure 14. Machine footprint as function of polish velocity and platen rotational velocity limits.

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As is expected, the magnitude of the machine footprint is higher for higher values of polish velocity and lower values of RPM. If this were not the case there would be no compromise. The previous figure also shows that foot print changes less with respect to changes in the required maximum polish velocity, than it does with respect to changes in the maximum allowed motor speed. The following figure shows the magnitude of the gradient of the footprint, squared, as a function of polish velocity and motor speed.

1400 a 1200 ' 1000 800-CM 0 600 _-3.95 400 -3.9 ( 500 -3.85 -8 480 460 3.8 400 3.75 velocity (m/s) RPM

Figure 15. The magnitude squared of the gradient offootprint as function of polish velocity and platen rotational velocity limits.

As can be seen from the previous figure, the evaluated footprint function has a higher gradient at lower motor speed allowances. To reduce the footprint of the machine as much as possible, it is best to choose values for s corresponding to polish velocity requirement of 3.75 m/s and motor speed allowance of 500RPM(See Figure 14). At the same time it is desired to operate at lower motor speed allowances while achieving higher polish velocities. According the previous figures, the change between 3.75 m/s and 4 m/s

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for polish velocity requirements will change the footprint by a relatively small amount. As a result the desired 4 m/s requirement is chosen. However, the change between 400RPM and 500 RPM for motor speed allowances makes relatively larger changes to the machine footprint. In fact, according to the gradient function, larger changes of footprint per change in motor speed allowance can be expected near the 400RPM limit. As a result, 500RPM is chosen as the motor speed allowance. With this combination a value of 3.00in. is obtained for the minimum wafer offset distance from Figure 13.

6 Machine Lower Structure

6.1 Conceptual Designs for Machine Lower Structure

As was mentioned before, the machine Lower Structure is the structure that houses the two platens, the cleaning station, the motors for each of the two platens, all slurry and DI water pumps, and all electronic equipment such as power amplifiers. The gantry structure is mounted on the Lower Structure using rails to allow its motion along the length of the machine. Two motors that drive two ball screws, with ball nuts attached to the gantry, provide the gantry's back and forth motion. These motors and the bearings that support the ball screws on either side are also mounted on the Lower Structure. The Lower Structure also contains conduits for the power and fluid lines that supply the platens, cleaning station, and the gantry structure to pass through. Furthermore, the Lower Structure, like every other component of the machine, must also be able to withstand the expected temperature changes in its operational and transport environments. Finally, the Lower Structure must also be stiff and have high damping.

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With these requirements in mind three conceptual designs were generated for the Lower Structure.

The first concept is to make the entire Lower Structure out of welded Stainless Steel extruded tubes. Stainless Steel is chosen because of its stiffness and ability to resist the corrosive properties of the slurries present. The overall structure for this concept is shown below.

Side Structure

End Structure

Lower Base

Figure 16. Welded Stainless Steel Lower Structure (Concept #1a).

This Stainless Steel structure is made of five sub assemblies that are welded first, and then welded together to form the overall structure. The Side Structure, which is shown

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below, consists of a large 8in by 4in and in thick tubing. The rails for the gantry structure are attached to the top surface of this tube. This large rectangular tube is then supported off of the Lower Base via four lin by lin cross tubes and three 2in by 2in pillar tubes.

Surface for mounting gantry rails

8X4 tubing

Cross Tubes

Pillar Tubes

Figure 17. Welded Stainless Side Structure.

The two End Structures shown in Figure 16 are there to add torsional rigidity to the overall structure. The latter structure is made of 3in by 2in and 3in by lin tubes. The end structures will be welded onto the large 8in by 4in tubes on the Side Structure and onto the Lower Base at the bottom.

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The Lower Base, which is shown in the following figure, is made of 3in by 3in and 3in by 6in rectangular tubes on the sides with 3in by 2in rectangular tubes used as crossbeams.

Crossbeam 3X3 tubing

6X3 tubing

Figure 18. Welded Stainless Steel Lower Base.

The two platens sit in capsules that rest on top of these crossbeams and are bolted onto them. The capsules hold the platen motors and contain the bearings necessary to support each platen. For more detail on the capsule and platen designs refer to section 14.

Another concept for the machine Lower Structure is to make it entirely out of a non-metallic polymer composite casting. The damping properties of this material are very high and complex shapes can be readily cast instead of machined. A compact design using this material follows. Features such as threaded metal inserts for screws can also be cast into the material for attachment purposes.

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Holes for Platen Capsules to sit in.

Holes for metal inserts for the gantry rails to bolt onto.

Figure 19. A cast polymer composite Lower Structure (Concept #1b).

In the design shown above, two large holes are cast in for the placement of the two platen capsules. The polymer composite can be cast in many complex shapes and offers flexibility on the design form. For example, a Concept #1b Lower Structure can be cast such that the rails bolt onto metal inserts in the structure or it can just easily be cast such that the bolts pass all the way through and fasten into nuts on the other side, as shown below.

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Upside down grooves to hide away rail nuts.

Figure 20. Concept #1b design with the thru holes for the rail bolts.

With the polymer composite material, special feature forms such as the upside down grooves shown in Figure 20 can also be easily cast in, in order to hide away the nuts for the rail bolts. The form flexibility of the cast polymer composite gives the freedom to make a machine that is both aesthetically pleasing, a key advantage for a commercial tool, and also free of sharp protruding features. While such features can be machined into the steel and granite structures as well, they will cost more per unit than that cast polymer structure for high volume production runs. These issues are of concern from a commercial point of view and need to be examined in the design of this machine if it is to

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Another design option that is available is to make the entire Lower Structure out of slabs of granite. As with the cast polymer composite structure, metal inserts would have to be used for the necessary assembly features. The damping of granite, is roughly three times that of stainless steel and one third that of the cast polymer composite material and is thus a compromise between the two. The top of surface of the granite can also be machine to a flatness of 0.0005in over the entire top surface of the Lower Structure. This allows the top surface to be used as a datum plane from which the alignment of the rest of the machine components can be adjusted during assembly. A typical first order design using this concept is shown below.

Hole for Platen Top Table

Capsules.

Pillars--Bottom Base

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As the separation lines in the above figure indicate, the Lower Structure is broken into six different pieces: the Top Table, the four pillars, and the bottom base. Each piece is made entirely of one piece of granite and then assembled together using dowels and epoxy as shown below.

Contact

Mating Piece A _Surface

Mating Piece BStePg

Figure 22. Method of assembly for a Concept #Ic design.

In order to attach to mating pieces, a large hole is drilled into each piece at the contact surfaces. Then epoxy is brushed on the two contacting surfaces and inside each hole. Next, an appropriately sized steel peg, slightly shorter than twice the depth of each hole, is inserted inside one of the holes. Then the two pieces are connected such that the protruding section of the steel peg is inserted into the hole on the other mating piece. In this manner the length of the steel peg is shared between the two holes and offers extra strength to the contact surface bond by increasing the overall surface area that the epoxy can bond to.

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6.2 Choosing a Concept for the Lower Structure

In all of the above concept designs, the machine Lower Structure is made entirely of one material. This was primarily done to insure that the entire Lower Structure would expand and contract uniformly with changes in temperature. Each of the concepts above has its own advantages and disadvantages, some of which are listed below.

1. The Stainless Steel structure is much stiffer than the cast polymer composite and granite ones.

2. For small volume production (in this case a single unit production) the stainless steel and granite designs are less costly to make since they do not incur patterning costs.

3. If extra components need to be attached to the Lower Structure after production, the stainless steel structure can be easily drilled and tapped for the addition of fastener holes. The cast polymer and granite structures, however, both require additional metal inserts to be fixed using epoxy.

4. While the stainless steel structure has the advantage of having the same coefficient of thermal expansion as the Stainless Steel rails that mount on it, the cast and granite structures have a lower coefficient of thermal expansion and will hold the their shape better over time with cyclic changes in temperature.

5. The granite and cast polymer structures also have considerably better damping properties than the steel. In fact, the cast polymer composite,

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which has the best damping properties of all three materials, has ten times the damping of Stainless Steel.

6. The cast polymer material provides a greater flexibility concerning design form and is also less costly for high volume productions.

7. The top surface of the granite structure can be machined to flatness of 0.0005in and serve as a datum plane for the precise assembly of the other components that are attached to it.

After analyzing the three concept designs above, it was difficult to clearly rate them and choose one over the others. In fact, at the end of this particular concept generation phase none of the options presented itself as a viable option to proceed upon. Furthermore, new methods, which will be discussed in detail in later sections, were explored to overcome problems associated with the difference in the thermal expansion coefficient of two mating parts. As a result, it was decided to use the information obtained in the concept generation phase to make the machine Lower Structure out of two different materials, taking advantage of the properties of each. After careful consideration, the Lower Structure was split into two parts: a Top Table were the platens, the cleaning station, and gantry structure are attached; and a supporting lower frame that holds up the Top Table and contains other components such as the slurry pumps and power amps. It was decided to attach all components that require precision assembly or alignment to the Top Table and to place any components whose position is not critical underneath the Top Table. The latter components are to be fixed both directly and via mounting brackets to the Lower Frame. This latter design path decouples structural

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requirements, such as damping and stiffness, from the assembly feature requirements that are necessary for the precision attachment of the platens, the cleaning station, and the gantry components.

7 Detailed Design of the Machine Lower Structure

7.1 Design of the Top Table

Since granite can be lapped to a flatness of 0.0005in over areas as large as 5940in2

and since it can also resist the corrosive properties of the slurries present, it was chosen to be the material for the Top Table. The basic design of the Top Table is a platform large enough to hold all the necessary components and thick enough to withstand the weight of the gantry that rides on top of it. The table is also the central piece, upon which nearly all components of the machine are mounted. The design starts out with a large, thick slab of granite with two large holes to hold the two platen assemblies. The assemblies are designed and built as separate modules that are then assembled onto the table (See section 14). The following figure shows this initial stage of the table design. Although the actual table will be made of granite, it is shown in green metallic color so that the described features can be seen more clearly.

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Figure 23. Rendered view diagram of the Top Table showing the large platen holes.

The two large holes are actually slightly larger in diameter than the component of the platen assembly that will be placed inside them. This is done for two reasons. The first is to allow for the flow of air around the platen assembly for cooling purposes. This is of concern since the large motors used for the platen spindles can generate a substantial amount of heat. The second reason is to allow the fine positioning of the platen assemblies inside the table to be independent of the location of the two large holes. This relaxes the location tolerances for the holes and thus reduces cost. It is still necessary to define the location of the platen by a method that is both repeatable and accurate. To achieve this, a set of two smaller holes is provided on one side of each of the two large

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holes. Steel dowel pins will be inserted into these holes in order to provide a position reference for the placing of the platen assemblies. Once the dowel pins are in place the entire platen assembly is placed inside the large holes and pushed until the outer rim of the capsule (See section 13) sits against the two steel dowel pins. In this manner the location of the assembly is kinemtacially defined in the plane of the table. This method is used in order to satisfy the requirement on the machine to have repeatable assembly with minimum assembly effort. The following figure shows how the capsule sits against the outer edge of the dowel pins in the complete machine assembly.

Figure 24. Rendered view diagram showing the location pins for the platen assembly.