Off-axis Alignment System Using Cube-Corner Prism
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Solution Overview
Problem
Conventional off-axis alignment systems are vulnerable to alignment mark tilting and defocusing, require high spatial coherence illumination, and are complex and difficult to construct due to the use of optical wedges and other complex components.
Innovation Solution
An off-axis alignment system with a simple optical path design that includes an illumination module, an interference module with a polarization beam splitter and a cube-corner prism, and a detection module, which diffracts light beams twice to generate interference signals immune to tilting and defocusing, and eliminates the need for high spatial coherence illumination.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If a polarization splitting prism based beam-splitting system is used, then two wavelengths can be handled, but alignment signals with more than two wavelengths cannot be handled
Solution Approach 1:
The beam-splitting function is segmented into multiple independent beam splitters, each dedicated to a specific wavelength. This allows the system to handle multiple wavelengths simultaneously while keeping each beam splitter simple and avoiding the complexity of a polarization splitting prism that can only handle two wavelengths.
2Measurement precision
If sub-beam interference occurs at the image plane, then alignment signals can be detected, but large alignment errors occur for alignment marks with non-uniform reflectivity under conditions such as rotation or magnification error
Solution Approach 1:
The interference detection is moved from the image plane to the Fourier plane (pupil plane), which is a different spatial dimension in the optical path. This dimensional change allows the system to detect interference patterns that are immune to alignment mark defects such as non-uniform reflectivity, rotation, and magnification errors, thereby improving measurement precision while reducing sensitivity to these harmful factors.
3Reliability
If an array of optical wedges is used for superimposition and interferometric imaging, then sub-beams can be combined, but stringent manufacturing requirements and high costs are imposed
Solution Approach 1:
The complex optical wedge array is extracted and replaced with simpler optical components including individual beam splitters and a lens. This extraction eliminates the need for precise wedge array manufacturing and assembly while achieving the same beam superimposition and interferometric imaging functions, thereby reducing manufacturing difficulty and cost while maintaining reliability.
4Adaptability or versatility
If a rotating self-referencing interferometer with composite prism is used, then two images of the alignment marker can be produced, but strict manufacturing and assembly requirements are imposed
Solution Approach 1:
Instead of using a complex rotating self-referencing interferometer with composite prism, the system uses stationary beam splitters and a lens to create the necessary image copies and interference patterns. This copying approach achieves the same functionality with simpler components that have less stringent manufacturing and assembly requirements.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The system provides accurate alignment results that are immune to tilting and defocusing of the alignment mark, reduces complexity and cost by avoiding complex optics, and simplifies construction and integration.
Implementation Method 1
a beam splitter configured to split the light beam into a first sub-beam and a second sub-beam, respectively having orthogonal linear polarizations
Implementation Method 2
a first quarter-wave plate disposed in the transmission path of the first sub-beam between the beam splitter and the alignment mark; a second quarter-wave plate disposed in the transmission path of the second sub-beam
Implementation Method 3
the incidence of a single-wavelength or multi-wavelength illumination light beam on a phase grating alignment mark causes the diffraction of the light beam into sub-beams of different diffraction orders
Implementation Method 4
those of the same diffraction order and with different diffraction order signs (e.g., ±1, ±2 and ±3 orders) are then mutually superimposed and interfere in pairs on an image plane or pupil plane of the alignment system, generating interference signals of the respective orders
Implementation Method 5
a photoelectric detector configured to detect the interference signal
Data Source
AI summary
An off-axis alignment system includes, sequentially along a transmission path of a light beam, an illumination module (10), an interference module (20) and a detection module (30). The interference module (20) includes: a polarization beam splitter (21) having four side faces, the illumination module (10) and the detection modules (30) both located on a first side of the polarization beam splitter (21); a first quarter-wave plate (22) and a first reflector (23), sequentially disposed on a second side opposite to the first side; and a second quarter-wave plate (24) and a cube-corner prism (25), sequentially disposed on a third side of the polarization beam splitter (21); and a third quarter-wave plate (26), a second reflector (27) and a lens (28), sequentially disposed on a fourth side of the polarization beam splitter (21). The second reflector (27) is located on a rear focal plane of the lens (28). A center of a bottom of the cube-corner prism (25) is situated on an optical axis of the lens (28). An off-axis alignment method is also disclosed.