Electricity: motive power systems – Linear-movement motors
Reexamination Certificate
2002-02-15
2004-06-29
Lam, Thanh (Department: 2834)
Electricity: motive power systems
Linear-movement motors
C310S012060, C318S119000
Reexamination Certificate
active
06756751
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Technical Field
This application relates to handling substrates, and more particularly to handling of a substrate with control over an operating range spanning a minimum of three and a maximum of six degrees of freedom.
2. Description of Related Art
Substrate handling mechanisms are often used in equipment designed to process semiconductor wafers, flat screen liquid crystal displays, printed circuit boards, and micromachine assemblies. Similar mechanisms are used in failure analysis systems, electrical and functional testing systems, and IC packaging systems.
Modern semiconductor wafers may be cylindrical substrates of silicon, up to 300 mm in diameter, and may be less than 1 mm thick. During many of the manufacturing processes of semiconductor devices, wafers are held on a substrate holder known as a chuck, using vacuum. The chuck is often also used as a substrate handler, to position the substrate at a specified location in up to six dimensions, and to move the substrate from an input section of a processing or testing system, through the process steps, and finally to an output section for removal. A chuck may be machined from aluminum, silicon carbide or other material, having a top surface that is machined to be flat. There may be vacuum outlets in the flat surface of the chuck that may be in the form of connected grooves cut into the flat surface. The vacuum system holds the wafer, or other planar substrates such as liquid crystal display panels, to the flat chuck surface. A non flat substrate or wafer may be made flatter by the action of the vacuum hold down of the chuck. On the other hand, the strain placed upon the chuck by the flattening action of the vacuum may result in warpage of the chuck surface, and consequent loss of planarity of the wafer or substrate. One method for addressing the chuck warpage problem is to make the chuck thicker and more massive, and thus more resistant to the strain of the wafer. However, increasing the mass of the chuck results in increased force necessary to move the chuck, and consequently increases stage mass and motor power levels.
Many of the manufacturing steps used to create integrated circuits, and other small dimension devices on substrates, require that the wafer or substrate position and orientation be precisely controlled. This requirement may be meet using what is known as an X Y stage to manipulate the wafer over a planar region, and what is known as a Z-theta chuck system to raise and lower and rotate the wafer about an axis normal to the nominal XY plane. Certain processes also require active manipulation of the plane of the wafer in order to maintain the wafer surface parallel to the plane of the process tool. This may be necessary if the wafer front surface and the wafer back surface are not exactly parallel. This condition is known as taper. The thickness of the wafer may also vary from place to place, a situation known as bow. Thus, a chuck may be required to rotate the front surface of the wafer in what is known as roll, pitch and yaw. Each of these three motions can be considered to be rotations around the X, Y or Z axis respectively.
Processes that require these sorts of motions include step and repeat camera imaging systems, which need the front surface of the wafer to be flat over a large surface area. If a change in the front surface location with respect to the focal plane of the camera occurs during any of the rotations around the orthogonal axes, then the resulting image will not be in focus at all points.
As semiconductor technology has increased with improved semiconductor performance, the wafer diameters have increased over successive generations of semiconductor manufacturing equipment from less than 100 mm to the current standard of 300 mm. At the same time, the precision requirements of the semiconductor manufacturing equipment has become tighter as the critical line width sizes have become smaller with the increased technological level. The requirement to maintain tighter line widths that accompanies the increase in technological level, also results in increased alignment accuracy and precision requirements, and to a decreased depth of field capability. The depth of field problem requires that the wafer surface be flatter, which consequently requires that the wafer chuck be flatter and strong enough to hold the wafer flat. The increase in required precision thus includes the need for improved capabilities to move the wafer accurately in the horizontal plane, the XY plane, as well as in the vertical direction, i.e. Z. Increase in required precision also requires accurate motion of the wafer in the roll, pitch and yaw directions.
Traditional chuck systems rely on mechanical bearings and machining tolerances to maintain the plane of the wafer attached to the chuck parallel to the XY plane of the stage. Mechanical approaches to a wafer chuck become more difficult as the mass of the chuck increases and the precision requirements become more severe. This is because as the wafer chuck mass increases, the mechanical bearings used to constrain the chuck necessarily become larger. As the precision requirements increase, the mechanical bearings must resort to increased levels of what is known as a preload in order to achieve the necessary stiffness to maintain precision and avoid vibration. As each of the elements becomes more massive and the stiffness increases, the forces required to support the more massive chuck and overcome the friction of bearings and actuators also consequently increases. Typical electromechanical actuators, such as motors, dissipate power in proportion to the square of the force they produce. Thus, as the chuck mass increases and the bearing mass increases, the size of the actuator and the actuator power must also increase, resulting in increased power dissipation and local heating of the wafer. Heating of the wafer may be a problem because expansion of the wafer results in a shifting of the location of different parts of the wafer, and thus loss of precision and repeatability. The different coefficients of thermal expansion of the aluminum (or other material) chuck and the semiconductor wafer may also result in mismatched stress between the chuck and the wafer, and may result in wafer warpage. Thus, power dissipation in the actuators of the chuck may lead to thermal gradients and corresponding changes in the mechanical dimensions of the chuck mechanism, which may be another major impediment to achieving the levels of precision required in many semiconductor processes, liquid crystal display processes, thin film magnetic head processes, and micro-machining processes.
Some of the above described problems have been addressed in prior art chuck mechanisms by restricting the range of Z motion to less than 0.1 mm, and using a flexure suspended chuck driven in the Z direction by piezo actuators. While these devices provide large forces with negligible heat generation, they are unable to provide sufficient range of motion to allow wafer transfers between a transfer robot and the chuck. This is because during the loading and unloading of a wafer onto the chuck, a minimum gap of approximately 6 mm must be established between the bottom of the wafer and the top of the chuck. This spacing is necessary for the robot or operator to insert a vacuum paddle between the chuck and the wafer to move the wafer while only touching the wafer backside, and thus prevent damage to the front surface of the wafer. Contact with the front surface of the wafer may result in physical damage such as scratching, and may also result in contamination of the devices on the front surface. Thus, piezo actuators must have a separate mechanism to provide the chuck with enough separation to allow wafer transfers. This additional requirement of piezo actuators increases the cost, complexity, and the mass of the stage.
Another problem with mechanical methods of moving a wafer chuck around, such as the piezo actuators, is that the physical contact of the piezo actuators with the chuck may represent an
Active Precision, Inc.
Choate Hall & Stewart
Jones Judson H.
Lam Thanh
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