Optical Position-Measuring Device Cross Grating Diffraction Suppression
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Solution Overview
Problem
Optical position-measuring devices with crossed scales face significant position errors, known as interpolation errors, which affect measurement accuracy, particularly in high-precision applications like semiconductor manufacturing.
Innovation Solution
The optical position-measuring device employs a light source and two scales with periodic grating regions arranged in crossed relation, where the illumination beam is split into sub-beams that impinge on and are reflected back from a second scale, recombining to generate position-dependent signals. The measuring graduation is configured as a two-dimensional cross grating with different optical properties, specifically designed to suppress higher diffraction orders, reducing position errors.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional linear gratings are used in optical position-measuring devices, then the device structure is simple, but interpolation errors occur due to disturbing higher diffraction orders
Solution Approach 1:
The patent uses a composite grating structure combining linear grating regions with two-dimensional cross grating regions. The cross grating regions have a checkerboard-like pattern of transparent and opaque areas that selectively suppress higher diffraction orders while maintaining measurement functionality. This composite approach eliminates interpolation errors without requiring complete structural redesign.
Solution Approach 2:
The grating structure is designed with different local properties: linear grating regions for measurement and cross grating regions for filtering. The cross grating regions are positioned at specific locations where they suppress disturbing diffraction orders locally, while other regions maintain their original measurement function. This localized approach improves precision without increasing overall device complexity.
2Measurement precision
If two-dimensional cross grating is used to suppress higher diffraction orders, then position measurement accuracy improves, but the grating manufacturing complexity increases
Solution Approach 1:
The grating is manufactured as a composite structure combining simple linear grating lines with two-dimensional cross grating regions. The cross grating regions feature a checkerboard pattern of transparent and opaque areas that can be fabricated using standard photolithography techniques. This composite design achieves superior measurement accuracy while remaining compatible with conventional manufacturing processes.
Solution Approach 2:
The grating design modifies specific parameters of the grating structure (introducing two-dimensional patterns in certain regions) to achieve the desired diffraction suppression. By carefully controlling the size, shape, and arrangement of the cross grating regions, the patent optimizes the balance between measurement precision and manufacturing feasibility using established fabrication methods.
3Adaptability or versatility
If crossed scales are used for two-directional measurement, then measurement versatility improves, but interpolation errors increase due to interfering diffraction orders
Solution Approach 1:
The crossed scale configuration incorporates two-dimensional cross grating regions that act as optical filters. These cross grating regions are integrated into both scales and selectively suppress higher diffraction orders that would otherwise cause interpolation errors. This allows the system to maintain its two-directional measurement versatility while eliminating the precision-degrading effects of disturbing diffraction orders.
Solution Approach 2:
The patent converts the potentially harmful effect of higher diffraction orders into a beneficial filtering mechanism. The cross grating regions are designed to diffract and suppress the disturbing higher orders while allowing the useful measurement signals to pass through. This transforms what would be measurement errors into a useful optical filtering function that improves overall system performance.
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
This configuration significantly reduces position errors, enabling high-accuracy position measurement along two orthogonal directions, suitable for precise translational and rotational movements in semiconductor manufacturing applications.
Implementation Method 1
an illumination beam is split by diffraction at a measuring graduation of a scale into different sub-beams
Implementation Method 2
the at least two sub-beams subsequently impinge on the second scale and are reflected back toward the first scale
Implementation Method 3
Upon recombination of suitable sub-beams, periodic signals are generated in a detection unit in response to a displacement of the scale relative to another scale due to the interference of the two sub-beams
Implementation Method 4
The at least one measuring graduation of at least one of the scales is configured as a two-dimensional cross grating which has a filtering effect that suppresses disturbing higher diffraction orders at the cross grating
Data Source
AI summary
An optical position-measuring device for determining a relative position of scales includes a light source, the scales and a detector. The scales are movable relative to each other along measurement directions and disposed in different planes in crossed relation to each other, and each have a graduation having grating regions which are arranged periodically and have different optical properties. At the first scale, the illumination beam is split into sub-beams, the sub-beams subsequently impinge on the second scale and are reflected back toward the first scale, and the reflected-back sub-beams strike the first scale again, where they are recombined, so that a resulting signal beam subsequently propagates toward the detector. The measuring graduation of one or more of the scales is configured as a two-dimensional cross grating which has a filtering effect that suppresses disturbing higher diffraction orders.