Wafer Measurement Using Optical Coherence Tomography
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Solution Overview
Problem
Current methods for measuring geometric variables like TTV, bow, and warp in wafers are slow, expensive, and prone to measurement errors due to field curvature and the need for precise positioning of sensors, especially when dealing with larger wafers.
Innovation Solution
A device using an optical coherence tomograph with a scanning system that scans the wafer surface with a measuring light beam in the near-infrared range, where the wafer is partially transparent, allowing simultaneous measurement of both surfaces without moving the wafer, and employs a single scan mirror or reduced distance between dual scan mirrors to minimize field curvature, along with calibration to correct for optical path differences.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If capacitive or optical methods are used to measure wafer surfaces, then measurement precision is improved, but measurement speed deteriorates and device complexity increases due to the need for turning the wafer or using multiple sensors with precise positioning
Solution Approach 1:
Instead of measuring both surfaces simultaneously with two sensors as in conventional methods, the patent inverts the approach by using a single sensor to measure one surface while the wafer rotates, allowing the same sensor to sequentially measure both surfaces. This inversion simplifies the measurement system while maintaining precision and improving speed.
Solution Approach 2:
The patent introduces dynamic rotation of the wafer during measurement, transforming a static two-sensor measurement system into a dynamic single-sensor system. The wafer rotation enables sequential measurement of both surfaces with one sensor, eliminating the need for complex positioning mechanisms and improving measurement throughput.
2Measurement precision
If two sensors are used to measure both wafer surfaces simultaneously, then measurement precision is improved, but device complexity and positioning requirements worsen
Solution Approach 1:
The patent inverts the conventional two-sensor simultaneous measurement approach by using a single sensor with wafer rotation. This reduces device complexity from multiple precision-positioned sensors to one sensor that simply tracks the rotating wafer surface, eliminating complex positioning mechanisms.
Solution Approach 2:
A single sensor is made universal by measuring both wafer surfaces through sequential scanning during wafer rotation. The same sensor that measures the first surface also measures the second surface, eliminating the need for separate dedicated sensors for each surface and reducing overall system complexity.
3Device complexity
If a single sensor measures both wafer surfaces sequentially with wafer rotation, then device complexity is reduced, but measurement precision deteriorates due to wafer movement
Solution Approach 1:
The patent embraces the dynamic nature of wafer rotation rather than treating it as a source of error. By synchronizing the single-sensor measurements with the known rotational position, the system transforms potential precision losses from movement into accurate measurements through coordinate transformation and timing synchronization.
Solution Approach 2:
The system incorporates feedback from the wafer rotation system to compensate for movement during measurement. By monitoring the rotational position and speed, the measurement data is corrected in real-time to account for wafer movement, maintaining precision despite the dynamic measurement conditions.
4Productivity
If conventional optical systems are used for scanning, then measurement speed is improved, but measurement precision deteriorates due to field curvature
Solution Approach 1:
The patent changes the optical parameters by selecting specific wavelength ranges (near-infrared for silicon wafers, visible for glass wafers) where the wafer material is transparent. This parameter change enables optical coherence tomography to penetrate the wafer and measure internal geometry without being distorted by surface reflections or field curvature effects.
Solution Approach 2:
The patent replaces conventional mechanical focusing systems that suffer from field curvature with optical coherence tomography based on light interference. This substitution eliminates mechanical adjustment mechanisms and their associated field curvature problems, providing consistent measurement precision across the entire scanning area.
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
Enables quick, precise, and cost-effective measurement of wafer characteristics with reduced measurement errors, suitable for larger wafers by eliminating field curvature and improving scanning efficiency.
Implementation Method 1
an optical coherence tomograph configured to generate a measuring light beam and to direct it onto the wafer via an optical system
Implementation Method 2
a wavelength range of the measuring light beam is selected in such a way that the wafer is at least partially transparent to it
Implementation Method 3
a scanning device configured to deflect the measuring light beam in two spatial directions
Data Source
AI summary
A device for measuring wafers includes an optical coherence tomograph, which generates a measuring light beam and directs it onto the wafer via an optical system. A scanning device deflects the measuring light beam in two spatial directions. A control unit controls the scanning device so that the measuring light beam scans the surface of the wafer successively at several measuring points. Two measuring points have a distance dmax of 140 mm≤dmax≤600 mm. An evaluation unit calculates distance values and/or thickness values from the interference signals provided by the optical coherence tomograph and, based on the distance values and/or thickness values, at least one characteristic quantity of the wafer such as TTV, warp or bow.