Precision Displacement Measurement via a Distance Measuring Interferometer (DMI)

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Precision Displacement Measurement via a Distance
Measuring Interferometer (DMI)
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Why DMI Is Needed
Important characteristics of International
Technology Roadmap for Semiconductors 2001''
published by the SIA.
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General System

• DMI system is comprised of three parts
• IF
• Laser
• Electronics

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DMI History
Year Description DMI Resolution
1887 Michelson-Morley Experiment. Leads to Michelson interferometer. NA
1902 Pieter Zeeman wins Nobel Prize for effects of magnetic fields on atomic spectra. Leads to Zeeman split laser.
1960 Bell Laboratories develop first HeNe laser
1964 Airborne Instrument Labs, Division Cutler Hammer, first commercial displacement IF
1965 Zeeman HeNe Laser (HP)
1968 Perkin-Elmer Lasergage homodyne IF
1970 Zeeman laser IF (HP) l/16
1987 20 MHz Heterodyne, with 2-pass (Zygo) l/512
1996 Current electronics (HP/Zygo). l/2048
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Michelson Interferometer

• Michelson-Morley experiment (c.a. 1887).
• Typical use of the Michelson interferometer is to
  compare a test optical surface against a known
  high quality reference flat. The output of the
  measurement is a light fringe pattern viewed from
  a diffuse surface. These fringes are spatial
  fringes.

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Michelson Interferometer, Using Polarized Light
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Single Axis Interferometer10706B Plane Mirror IF
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Multiaxis IF10735A
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Design ConsiderationsCNC Protective Covers
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Design ConsiderationsSplit Frequency Limit on
Velocity
For a four pass plane mirror IF.
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Design ConsiderationsMiscellaneous

• Vacuum compatibility.
• Low adjustability.
• Beam size.
• Metric vs. U.S. Customary.
• DCTE between parent structure and IF parts.
• Peak-to-Valley (PV) wavefront per optic.
• Remote Receiver fibers (bend radius).

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DMI System ErrorsDeadpath Environment

• Deadpath Difference in physical optical path
  between Reference and Measure.

Therefore, in this example the deadpath is L4L0.
This is assuming that the air space between the
PBS and the two quarter wave plates are equal.
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DMI System ErrorsDeadpath Environment

• Edlens Equation
• Metrologia, Vol. 2, No. 2, Pg. 71, 1966
• Air Temperature, T (C)
• Barometric pressure, P (mmHg)
• Relative humidity, H ()
• Gas composition. Typically not measured.

or
, therefore
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DMI System ErrorsAbbe Error
Abbe error can be eliminated through with a q
measurement, and by knowing L.
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DMI System ErrorsCosine Error

• Reduced through proper alignment.
• Part of the accuracy budget, and not the
  repeatability budget.
• As an example, you can expect a 10706B to have a
  cosine error of 0.05 ppm (50 nm for a 1 m travel).

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DMI System ErrorsErrors Summary

1. Make the ambient environment tightly-controlled
   and stable, and apply atmospheric compensation
   tools.
2. Minimize deadpath distances and Abbe offsets, and
   subtract in the processing.
3. Properly align the optics.

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Summary

• DMI is currently the most accurate and
  sensitive linear translation measurement scheme.
  Additionally, it has a near limitless translation
  measurement bandwidth.
• Relative, not absolute.
• Noncontact.
• Near coaxial measurement of translation axis.

• Resolution (Agilent 10897B electronics and
  two-pass IF) 1.2 nm.
• Accuracy 2-3 nm.
• Max. range gt 10 meters.
• All 6 DOFs of a rigid body, are indirectly
  measurable.
• Max. velocity (two-pass IF) 2 m/s.
• Typical beam diameters 3, 6 and 9 mm. 9mm is
  preferred.