ASTM E2244 Thin Film Length Measurement by Optical Interferometry
ASTM E2244 test method determines the in-plane length and deflection of the films present in microelectromechanical systems by an optical interferometer or interferometric microscope. Design dimensions are also used for precise measurements. The results are expressed in SI units as per the 3-D data received.
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- Overview
- Scope, Applications, and Benefits
- Test Process
- Specifications
- Instrumentation
- Results and Deliverables
Overview
ASTM E2244 provides the standard guide for measuring in-plane length dimensions — line widths, step heights, and lateral feature dimensions — of reflecting thin films and surfaces using an optical interferometric microscope. The technique uses the interference of coherent light to extract sub-nanometer height and in-plane dimensional data from reflective surfaces.
This measurement capability is critical in semiconductor lithography metrology, MEMS fabrication, optical coating characterization, and precision manufacturing, where feature dimensions at the nanometer- to micrometer-scale must be verified with traceable accuracy.
Scope, Applications, and Benefits
Scope
ASTM E2244 covers:
- In-plane length and width measurements of thin film features
- Step height and step width characterization
- Lateral resolution and calibration of interferometric objectives
- Measurement uncertainty estimation for interferometric length data
Applications
- Semiconductor wafer patterning and lithography metrology
- MEMS device feature dimension verification
- Optical coating lateral feature characterization
- Precision mechanical component surface feature measurement
- Thin film deposition process control
Benefits
- Non-contact, sub-nanometer vertical resolution
- Traceable length metrology without contact damage
- Full-field imaging provides rapid areal measurement
- Compatible with reflective metals, semiconductors, and oxide films
- Supports process control for critical dimension (CD) applications
Test Process
Instrument Calibration
The interferometric microscope is calibrated laterally using a NIST-traceable stage micrometer or calibration grating; the pixel-to-length conversion factor is verified for each objective.
1Sample Positioning
The sample is leveled and positioned under the objective; the measurement area is selected using the live interferogram and best-focus image.
2Interferogram Acquisition
Phase-shifting or white-light interferometry is used to acquire height/phase maps; multiple frames are averaged to reduce noise in the lateral measurement.
3Dimensional Analysis
Edge detection algorithms or profile extraction methods are applied to the interferometric image to measure line widths, gaps, and feature lengths;
4Technical Specifications
| Parameter | Details |
|---|---|
| Applicable Materials | Reflecting thin films, metals, semiconductors, oxides |
| Vertical Resolution | <0.1 nm (phase shifting) |
| Lateral Resolution | 0.5–5 µm (objective dependent) |
| Measurement Range | 0.1 µm to several mm in-plane |
Instrumentation Used for Testing
- White-light or phase-shifting interferometric microscope
- NIST-traceable lateral calibration artifact (stage micrometer/grating)
- Anti-vibration optical table
- Surface analysis and dimensional measurement software
- Environmental temperature control (23 °C ± 1 °C)
Results and Deliverables
- In-plane length measurements with uncertainty estimates
- Surface height and step width profiles
- 2D and 3D surface maps of the measured area
- Calibration verification data and traceability documentation
- Full measurement report per ASTM E2244
Frequently Asked Questions
Phase-shifting interferometers (PSI) provide high precision for smooth surfaces with height variations less than λ/4. White-light interferometers (CSI/VSI) accommodate larger height variations and rough surfaces. Both types are applicable per ASTM E2244, with PSI preferred for the highest lateral precision on smooth films.
A certified calibration grating or stage micrometer with NIST-traceable pitch and feature dimensions is measured; the pixel-to-length conversion factor is calculated and applied as a calibration coefficient. Periodic re-calibration ensures measurement traceability.
Lateral resolution is diffraction-limited by the objective NA and wavelength: approximately 0.61λ/NA. For a 100× objective with NA = 0.9 and λ = 550 nm, the lateral resolution is approximately 0.37 µm — sufficient for most thin film and MEMS feature measurement at the micron scale.
ASTM E2244 is specifically for reflective thin films and surfaces that produce adequate interferometric fringe contrast. Non-reflective surfaces (polymers, paper, low-reflectance oxides) may require surface enhancement (thin metal coating) to achieve sufficient reflectance for interferometric measurement.
Mechanical vibration introduces fringe motion that degrades measurement quality, particularly for phase-shifting interferometry which acquires sequential frames over several seconds. Anti-vibration tables, short acquisition times, and vibration-robust algorithms (e.g., Carré method) minimize vibration effects.
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