Off-Axis Distance Measurement Optics for Accurate Surface Height
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Solution Overview
Problem
The astigmatism method for distance measurement struggles to accurately measure the height of a surface due to superimposed reflected light from multiple surfaces, leading to inaccurate measurements, particularly when measuring a wafer's front-side main surface.
Innovation Solution
A distance measurement unit with an optical path adjustment mechanism, including a reflective grating and light detection channels, adjusts the optical path of reflected light to ensure it is imaged at a predetermined plane, allowing for accurate measurement by spacing the distance measurement light's path apart from the objective lens' central axis and using a light detection unit with channels aligned to detect the reflected light's incidence position.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If the distance measurement light is transmitted through the central axis of the objective lens, then the measurement setup is simple, but reflected light from multiple surfaces superimposes and measurement accuracy deteriorates
Solution Approach 1:
The patent shifts the distance measurement light from the central axis (one-dimensional alignment) to an off-axis position in the radial direction of the objective lens. This dimensional change in light path positioning enables spatial separation of reflected light from different surfaces, resolving the superposition problem while maintaining optical efficiency.
Solution Approach 2:
The patent segments the optical paths by spatially separating the distance measurement light from the central axis. This segmentation creates distinct detection zones for different surface reflections, allowing the front surface reflected light to be detected separately from rear surface reflected light, thereby improving measurement accuracy.
2Device complexity
If the imaging position of reflected light is not adjusted, then the optical path adjustment mechanism is simple, but measurement accuracy varies with surface height
Solution Approach 1:
The patent implements an optical path adjustment mechanism that responds to surface height variations by adjusting the imaging position of reflected light. This feedback mechanism ensures that the light reception surface maintains optimal detection conditions across different height ranges, preserving measurement accuracy uniformity.
Solution Approach 2:
The patent introduces dynamic adjustment capability to the optical path system, allowing the imaging position to adapt to varying surface heights. This dynamic adjustment ensures consistent measurement accuracy across different object heights by optimizing the light reception geometry for each measurement condition.
3Quantity of substance
If multiple surfaces reflect the distance measurement light, then more light is available for detection, but superimposed reflected light causes measurement errors
Solution Approach 1:
The patent extracts and isolates the reflected light from the front surface by positioning the distance measurement light off-axis. This extraction separates the desired signal (front surface reflection) from unwanted signals (rear surface reflections), enabling accurate measurement while maintaining sufficient light quantity for detection.
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 enables precise measurement of surface height by preventing superimposition of unnecessary reflected light and maintaining measurement accuracy across varying surface heights, ensuring accurate detection and positioning of the condensing point for both distance measurement and irradiation light.
Implementation Method 1
an objective lens 205 that transmits distance measurement light RL1 to object 1 to be processed side
Implementation Method 2
the reflected light is imaged in at least one direction orthogonal to an incidence direction of the reflected light incident to the light detection unit
Implementation Method 3
an optical path adjustment unit 106 that adjusts an optical path of the reflected light RL2; the optical path adjustment unit adjusts the optical path of the reflected light so that the imaging position of the reflected light that is imaged in at least one direction orthogonal to an incidence direction of the reflected light incident to the light detection unit approaches a predetermined plane
Implementation Method 4
a light detection unit 107 that detects the reflected light RL2; A light reception surface 107a of the light detection unit 107 is located to follow along the predetermined plane S
Data Source
AI summary
A distance measurement unit includes: a distance measurement light source that outputs distance measurement light; an objective lens through which the distance measurement light and reflected light are transmitted; an imaging lens through which the reflected light is transmitted and which forms an image at an imaging position; an optical path adjustment unit that adjusts an optical path of the reflected light; and a light detection unit that detects the reflected light. The objective lens allows the distance measurement light to be transmitted therethrough in a state in which an optical path of the distance measurement light is spaced apart from a central axis of the objective lens. The optical path adjustment unit adjusts the optical path so that the imaging position of the reflected light approaches a predetermined plane. A light reception surface of the light detection unit is located to follow along the predetermined plane.