Wafer Thickness Measurement Using Near-Infrared Optical Reflection
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Solution Overview
Problem
Current methods for measuring the thickness and flatness of silicon wafers and the depth of trenches etched on them are inadequate, particularly for thin wafers and high aspect ratio trenches, as they lack accuracy and precision, and often require destructive processes or are unable to handle steep sidewalls and rounded bottoms effectively.
Innovation Solution
A non-contact optical measurement system using a chromatic confocal sensor that positions itself to receive reflected light from both the upper and lower surfaces of the wafer, utilizing the index of refraction and wavelength range transparency to determine trench depth and wafer thickness, allowing for precise measurement of localized thickness and shape without damaging the wafer.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If capacitance sensors are used to measure wafer thickness, then measurement can be performed, but the resolution and accuracy are insufficient for thin wafers requiring 0.1 micron or smaller measurement accuracy
Solution Approach 1:
The patent replaces the mechanical/electrical capacitance sensing system with an optical measurement system. The optical stylus uses light reflection principles to measure wafer thickness, achieving superior resolution (better than 0.1 micron) compared to capacitance sensors. This substitution enables accurate measurement of thin wafers while maintaining non-contact operation.
Solution Approach 2:
The patent changes the measurement parameter from electrical capacitance to optical reflection properties. By measuring the reflection of light at different wavelengths and analyzing the interference patterns, the system achieves higher precision in thickness measurement. The optical system measures surface height by detecting the wavelength of reflected light, providing sub-micron resolution.
2Quantity of substance
If capacitance sensors with large area are used, then measurement coverage is limited to about 10 locations for 4-inch wafer, but detailed height and thickness information over the entire wafer is needed
Solution Approach 1:
The patent segments the measurement process into multiple discrete measurement locations across the wafer surface. The optical stylus can be positioned at numerous different locations (x, y coordinates) to measure thickness and flatness point-by-point. This segmentation allows comprehensive coverage of the entire wafer surface while maintaining high precision at each measurement point, generating detailed height maps and thickness profiles.
Solution Approach 2:
The patent creates a digital copy or map of the wafer surface topography by measuring at multiple locations. The optical stylus measures the height at each position, and these measurements are compiled into a comprehensive digital representation of the wafer surface. This copying approach allows full surface characterization without requiring physical contact across the entire surface simultaneously.
3Reliability
If non-contact optical measurement is used for trench depth, then destructive processes are avoided, but current methods cannot effectively handle steep sidewalls and rounded bottoms of high aspect ratio trenches
Solution Approach 1:
The patent replaces mechanical stylus profiling or cross-sectional physical sectioning with an optical measurement system. The optical stylus uses light reflection and interference to measure trench depth non-destructively. By analyzing the reflection patterns of light at different wavelengths, the system can determine the depth of high aspect ratio trenches with steep sidewalls and rounded bottoms without physical contact or destruction of the sample.
4Measurement precision
If capacitance measurement technique is used, then wafer thickness can be measured, but detailed information about wafer material properties such as relative permittivity is required which becomes problematic for multi-material wafers or those with solder bumps
Solution Approach 1:
The patent replaces the electrical capacitance measurement system with an optical measurement system. The optical stylus measures thickness based on light reflection and interference principles, which are independent of the material's electrical properties. This substitution makes the measurement system universally applicable to wafers with different materials, multi-layer structures, solder bumps, or other features that would complicate capacitance-based measurements.
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 approach provides accurate and non-destructive measurement of wafer thickness, flatness, and trench depth, enabling precise process control and characterization, even for thin wafers and high aspect ratio trenches, with improved resolution and coverage compared to existing methods.
Implementation Method 1
positioning a non-contact optical height measurement instrument so that the instrument receives reflected light from both the upper and lower surfaces
Implementation Method 2
A non-contact optical measurement system using a chromatic confocal sensor
Implementation Method 3
utilizing the index of refraction and wavelength range transparency to determine trench depth and wafer thickness
Implementation Method 4
utilizing the index of refraction and wavelength range transparency
Data Source
AI summary
A method and apparatus for the measurement of wafer thickness, flatness and the trench depth of any trenches etched thereon using the back surface of the wafer to accurately measure the back side of a trench, rendering the trench an effective bump, capable of being measured on the top surface and the bottom surface through a non-contact optical instrument that simultaneously measures the wavelength of the top surface and bottom surface of the wafer, converting the distance between wavelengths to a thickness measurement, using a light source that renders the material of which the wafer is composed transparent in that wavelength range, i.e., using the near infrared region for measuring the thickness and trench depth measurement of wafers made of silicon, which is opaque in the visible region and transparent in the near infrared region. Thickness and flatness, as well as localized shape, can also be measured using a calibration method that utilizes a pair of optical styli.