Stage Metrology Concepts: Application Specific ... - Owens Design
Stage Metrology Concepts: Application Specific ... - Owens Design
Stage Metrology Concepts: Application Specific ... - Owens Design
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<strong>Stage</strong> <strong>Metrology</strong> <strong>Concepts</strong>:<br />
<strong>Application</strong> <strong>Specific</strong> Testing<br />
Hans Hansen<br />
Sr. Systems <strong>Design</strong> Engineer<br />
BSME, MSE<br />
2/28/2008
Topics<br />
• Review of <strong>Stage</strong> Error Definitions<br />
• Setting <strong>Stage</strong> <strong>Specific</strong>ations for <strong>Application</strong>s<br />
• <strong>Application</strong>-<strong>Specific</strong> Ways of Providing <strong>Stage</strong><br />
<strong>Specific</strong>ations
Basic Definitions<br />
• Accuracy- the degree to which a system produces the<br />
correct result compared to a standard reference<br />
• Repeatability- the degree to which a system produces<br />
the same result from run to run<br />
By definition, accuracy can not be better than<br />
repeatability, but repeatability can be better than<br />
accuracy.<br />
• Precision- the resolution/”fineness” to which a system<br />
can position.
<strong>Stage</strong> Errors – Single Axis<br />
• Degrees of Freedom<br />
– Translation <strong>Stage</strong><br />
• 3 Translations : 2 fixed, 1 free<br />
• 3 Rotations : All fixed<br />
– Rotary <strong>Stage</strong><br />
• 3 Translations : All fixed<br />
• 3 Rotations : 2 fixed, 1 free<br />
• Errors<br />
– Motions that are supposed to be fixed, but aren’t<br />
– Motions that are supposed to be free but are inaccurate<br />
– Both have repeatable and non-repeatable terms
Accuracy, Repeatability & Resolution<br />
Low Accuracy<br />
Low Repeatability<br />
Low Accuracy<br />
High Repeatability<br />
High Accuracy<br />
High Repeatability<br />
Fine Resolution<br />
Coarse Resolution
Linear <strong>Stage</strong> Definitions<br />
• X Direction – Free: Positioning Accuracy &<br />
Repeatability<br />
• Y Direction – Fixed: Horizontal Straightness Error,<br />
Repeatable and Non-repeatable<br />
• Z Direction – Fixed: Vertical Straightness, Repeatable<br />
and Non-repeatable<br />
• Ө x – Fixed: Roll Error<br />
• Ө y – Fixed: Pitch Error<br />
• Ө z – Fixed: Yaw Error
<strong>Stage</strong> Errors - Linear
<strong>Stage</strong> Errors - Angular
Abbe’ Error<br />
• Linear Errors are affected by rotation errors an<br />
amount that depends upon where the error is<br />
measured.<br />
• Horizontal Straightness Measured at A Given<br />
Height is the sum of the straightness at the center<br />
of roll + product of the roll and the height from the<br />
roll center to the point of measurement<br />
• Height– Abbe’ distance<br />
• Error Component from roll – Abbe’ error
Abbe’ Error
Bidirectional vs. Unidirectional<br />
Positioning Repeatability<br />
• Unidirectional- target always approached from<br />
one direction<br />
• Bidirectional- target approached from both<br />
directions.<br />
• Applies to both linear and rotary systems
Unidirectional Repeatability
Bidirectional Repeatability
Squareness<br />
• Arises when we combine axes<br />
• Example: Squareness of XY <strong>Stage</strong> System<br />
– Measured using a square and indicator<br />
– Need to take measure at appropriate height<br />
– Combines rotation and linear errors
Fixed vs. Moving Sensitive Direction<br />
• Affects appropriate orientation of indicator and<br />
reference surface when measuring straightness<br />
• Example- Single axis microscope<br />
– Sample Fixed<br />
– Sample Moving<br />
– Indicator should always be mounted where the<br />
objective is mounted.
Rotary <strong>Stage</strong> Definitions<br />
• X Direction – Fixed: Radial Motion, Repeatable and<br />
Non-repeatable<br />
• Y Direction – Fixed: Radial Motion, Repeatable and<br />
Non-repeatable<br />
• Z Direction – Fixed: Axial, Repeatable and Nonrepeatable<br />
• Ө x – Fixed: Tilt Error in XZ plane<br />
• Ө y – Fixed: Tilt Error in YZ plane<br />
• Ө z – Free: Positioning Accuracy and Repeatability
Rotary <strong>Stage</strong> Errors<br />
Axial Radial Tilt/Wobble
Error Correction- Mapping<br />
• Errors measured and stored in computer as a<br />
lookup table or function<br />
• Mapping data must travel with stage<br />
• Only as accurate as repeatability of system as<br />
measured and in use.<br />
• Meaningful only if done at appropriate Abbe’<br />
height
2D Accuracy<br />
• Renishaw lasers used with an<br />
optical square to calibrate out<br />
accuracy, straightness, yaw,<br />
and orthogonality in one test<br />
• Laser outputs are read into<br />
two free encoder inputs of an<br />
A3200 system. Need to do<br />
this to synchronize the<br />
encoder and laser information<br />
• Matlab is used to post process<br />
the results and generate 2D<br />
calibration files<br />
Courtesy of Aerotech
Difficulty in Specifying <strong>Stage</strong>s Given<br />
All The Error Terms<br />
• <strong>Stage</strong>s typically specified in terms of the<br />
classical individual stage errors.<br />
• Performance we are interested in result from the<br />
combination of these errors
Proposal<br />
• Define performance in terms of applicationspecific<br />
tests<br />
• Specify individual errors only as necessary
Example – Simple 1 Axis System<br />
• Requirement to Position Sample Under<br />
Objective in X,Y & Z to <strong>Specific</strong> Accuracy &<br />
Repeatability<br />
• Specify X Positioning<br />
• Specify Y&Z Straightness<br />
• Both need to be at height of sample and take<br />
into consideration moving vs. stationary<br />
objective
Example – 2 Axis XY System<br />
• Requirement to Position Sample Under<br />
Objective in X,Y & Z to <strong>Specific</strong> Accuracy &<br />
Repeatability<br />
• Specify X&Y Positioning Test<br />
• Specify Combined Z Straightness Test<br />
• Both need to be at height of sample and take<br />
into consideration moving vs. stationary<br />
objective
4 Axis System<br />
Brushless DC Drives, 20 nm Resolution
Example – 3 Axis RӨZ System<br />
• Requirement to Position Sample Under<br />
Objective in R,Ө & Z to <strong>Specific</strong> Accuracy &<br />
Repeatability<br />
• Specify R,Ө Positioning Test<br />
• Specify Combined Z Straightness Test<br />
• Both need to be at height of sample and take<br />
into consideration moving vs. stationary<br />
objective
3 Axis RӨZ System (nanomotion motors)
Conclusion<br />
• Defining stage specifications in terms of<br />
individual stage errors can be cumbersome.<br />
• Consider defining your own specifications that<br />
are unique to your application
Backup
Linear vs. Mechanical Bearings<br />
Parameter<br />
Units<br />
Linear Motors<br />
Linear Bearings<br />
Linear Motors<br />
Air Bearings<br />
accuracy<br />
microns<br />
±5<br />
±0.5<br />
accuracy, mapped<br />
microns<br />
±1<br />
±0.25<br />
repeatibility, bi-directional<br />
microns<br />
±0.5<br />
±0.2<br />
positional stability<br />
microns<br />
+/-0.04<br />
+/-0.04<br />
encoder resolution<br />
microns<br />
0.01-1<br />
0.01-1<br />
straightness and flatness<br />
microns<br />
6-12<br />
2<br />
roll,pitch,yaw<br />
arc-sec<br />
10<br />
2<br />
speed, no load<br />
mm/sec<br />
1000-2000<br />
500<br />
acceleration, no load<br />
mm/sec^2<br />
30000<br />
10000<br />
settling time<br />
sec<br />
Measurement Techniques<br />
Straightedge Reversal Technique<br />
• Place straightedge on the carriage, parallel to the axis that is being tested<br />
• Mount gage on a stationary part of the tool, with the sensitive contact<br />
positioned against the straightedge and scan axis of interest<br />
• Repeat scan with the straightedge flipped 180 degrees around its long axis,<br />
and the gage head repositioned on the opposite side of the machine scanning<br />
the same surface.<br />
• The gage head's direction of motion is reversed, so that a "bump" on the<br />
straightedge should read as a positive number on both trials.<br />
• The results of one trace are subtracted from the other. Because the gage head<br />
was reversed, carriage errors cancel each other out, so any remaining<br />
deviation from zero reflects error in the straightedge. (assumes that carriage<br />
errors are repeatable.)<br />
• The results can be used as correction factors for all future uses of the<br />
straightedge.
Reversal- Setup 1
Reversal – Setup 2
Reversal- Math<br />
T 1 (Z) = P(Z) + S(Z)<br />
T 2 (Z) = P(Z) - S(Z)<br />
T= Indicator Reading<br />
P= <strong>Stage</strong> Straightness Error<br />
S= Straightedge Straightness Error