Interferometry Thickness Measurement for Doped Semiconductors
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Solution Overview
Problem
Existing methods for checking the thickness of highly doped semiconductor materials, such as silicon, are limited by the high absorption of light, making it difficult to accurately measure thicknesses greater than 100 µm using interferometry, especially during the initial machining phase when the thickness is far from the desired value, leading to unreliable and inaccurate checks.
Innovation Solution
A method and apparatus that utilize low coherence interferometry with a processing unit to analyze the spectrum of interference between reflected radiations from the external and internal surfaces of the semiconductor material, employing a virtual reference surface to determine the thickness, allowing for both preliminary and direct thickness measurements, even when the material is highly doped and opaque, by distinguishing between primary and secondary reflected radiations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If optical interferometry is used to check thickness of highly doped semiconductor material, then non-contact measurement is achieved, but measurement accuracy deteriorates due to high light absorption
Solution Approach 1:
The patent changes the wavelength parameter of the light source to the near-infrared region (700-2500 nm), where highly doped semiconductor materials exhibit lower absorption coefficients. This parameter change enables light to penetrate deeper into the material, allowing interferometry to measure thickness accurately even in highly doped regions where visible light would be completely absorbed.
Solution Approach 2:
The system dynamically adapts the coherence length of the light source based on the thickness being measured. By using low-coherence light with adjustable coherence length, the system can optimize the measurement range for different thickness values, maintaining measurement accuracy across varying thickness conditions while enabling non-contact measurement.
2Measurement precision
If conventional interferometry is used for thickness measurement, then measurement capability is limited to thin samples, but measurement range is restricted
Solution Approach 1:
The patent employs a tunable light source with adjustable wavelength and coherence length parameters. By varying these parameters, the system can adapt to measure different thickness ranges - from very thin samples to thicker highly doped materials - thereby expanding the measurement range while maintaining precision through optimal parameter selection for each measurement condition.
Solution Approach 2:
The measurement system is designed with multi-functionality to handle diverse measurement scenarios. The same interferometric apparatus can measure both thin undoped layers and thick highly doped regions by adjusting light source parameters, making it a universal thickness measurement tool that covers a broad range of semiconductor material thicknesses and doping conditions.
3Strength
If support layer is attached to thin semiconductor slice, then mechanical strength is improved, but thickness measurement accuracy deteriorates due to unknown support layer thickness
Solution Approach 1:
The patent utilizes optical contrast differences between the semiconductor slice and the support layer. By analyzing the interference patterns and optical response, the system can distinguish the semiconductor material from the support layer based on their different optical properties, enabling accurate thickness measurement of the semiconductor slice even when mounted on a support layer with unknown thickness.
Solution Approach 2:
The interferometric measurement acts as an intermediary that indirectly determines the semiconductor thickness by measuring the total thickness and subtracting the support layer contribution. The optical interference pattern serves as a mediator that provides information about both layers, allowing the system to extract the semiconductor slice thickness accurately despite the presence of the support layer.
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 accurate and reliable thickness measurement throughout the machining process, including initial phases, by differentiating between primary and secondary reflections, allowing for precise control of the machining speed and ensuring the desired thickness is reached accurately.
Implementation Method 1
analyse the spectrum of the result of interference between radiations reflected by the external surface and by the surfaces of optical discontinuity inside the object being machined
Implementation Method 2
radiations reflected by the external surface and by the surfaces of optical discontinuity
Implementation Method 3
high absorption of the light radiations is implied... the regions of the silicon layers wherein the conductivity is increased as a consequence of the high doping become less transparent to the light radiations in general, and to the infrared radiations in particular
Data Source
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AI summary
A method for optically checking by interferometry the thickness of an object (2) being machined comprises a phase of direct checking wherein the spectrum of the result of interference between primary reflected radiations (R1) generated by reflection of incident radiations (I) on an external surface (16) and secondary reflected radiations (R2) generated by reflection on an internal surface of discontinuity (17) of the object is analysed, and a preliminary phase wherein information relative to the variation of a virtual thickness (D) delimited and defined by a reference surface (18) and the external surface of the object, indicative of thickness variations of the object being machined. In the preliminary phase, the processing is based on the analysis of the spectrum of the result of interference between primary reflected radiations and reference reflected radiations (Rref) generated by the reflection of the incident radiations on the reference surface, that defines the length of a reference optical path. An apparatus that implements such method for optically checking includes an optical probe (6) that receives and detects the primary, secondary and reference reflected radiations, and a spectrometer (5) that analyses the spectrum of the result of interference.