SOI Substrate Evaluation via Interface State Density
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Solution Overview
Problem
Current methods for evaluating the radio-frequency characteristics of SOI substrates are complex and require actual device production, lacking a direct method to assess the trap layer immediately under the BOX layer.
Innovation Solution
A method involving a measuring SOI substrate with a trap layer, where interface state density is measured using pseudo MOSFET or CV methods, and a relationship between interface state density and leakage power is determined to estimate radio-frequency characteristics without forming a device on the evaluation substrate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If a high-resistance base wafer is used to reduce crosstalk, then crosstalk is reduced, but inversion layer is generated on the BOX layer interface which degrades radio-frequency characteristics
Solution Approach 1:
An intermediate layer (trap layer) is introduced between the BOX layer and the base wafer. This trap layer has higher dielectric constant than the BOX layer and contains trap states that capture carriers, preventing inversion layer formation at the BOX layer-base wafer interface while maintaining the electrical isolation function for crosstalk reduction.
Solution Approach 2:
The dielectric constant parameter of the layer between BOX layer and base wafer is changed from the BOX layer's value to a higher value by introducing a trap layer with higher dielectric constant. This parameter change enables the trap layer to provide both electrical isolation and carrier trapping functions.
2Reliability
If an intermediate layer (trap layer) is introduced to prevent inversion layer formation, then radio-frequency characteristics are improved, but device complexity increases
Solution Approach 1:
The trap layer serves multiple functions simultaneously: it provides electrical isolation for crosstalk reduction, traps carriers to prevent inversion layer formation, and improves radio-frequency characteristics. This multi-functionality reduces the need for additional separate components.
Solution Approach 2:
The electrical isolation function and carrier trapping function are merged into a single trap layer structure, rather than using separate layers for each function. This consolidation simplifies the overall substrate structure while achieving multiple objectives.
3Measurement precision
If actual device production is performed to evaluate radio-frequency characteristics, then accurate evaluation is obtained, but evaluation time and cost increase
Solution Approach 1:
Instead of evaluating actual RF devices, the patent uses a simplified test structure (MOS capacitor or diode) that copies the essential electrical characteristics of the SOI substrate. This test structure evaluates the substrate's intrinsic properties without requiring full device fabrication.
Solution Approach 2:
The substrate evaluation is performed before actual device production using the simplified test structure. This preliminary evaluation determines whether the substrate meets RF characteristics requirements, avoiding time loss from producing devices on unsuitable substrates.
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 simple evaluation of SOI substrates for radio-frequency suitability by converting interface state density to resistance, allowing for accurate assessment of leakage power and radio-frequency characteristics without producing an actual device.
Implementation Method 1
it has been known a technology that introduces an intermediate layer (a trap layer) such as a polysilicon layer or a nitride oxide to an interface between a BOX layer and a base wafer
Implementation Method 2
the signal is propagated through a capacitor between wiring lines of devices or through a wafer
Data Source
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AI summary
The present invention is a method for evaluating an SOI substrate, including the steps of: forming a device onto a measuring SOI substrate, and on the measuring SOI substrate, previously determining a relationship between an interface state density and a leakage power upon application of radio-frequency, or converting the interface state density to a resistance followed by previously determining a relationship between the converted resistance and the leakage power; measuring an interface state density of the evaluation target SOI substrate to determine the interface state density or a resistance converted from the interface state density; evaluating a leakage power of the evaluation target SOI substrate from the measured interface state density of the evaluation target SOI substrate on the basis of the previously determined relationship between the interface state density and the leakage power, or from a resistance converted from the measured interface state density of the evaluation target SOI substrate on the basis of the previously determined relationship between the resistance and the leakage power. This makes it possible to evaluate a substrate suited for radio-frequency in a simpler manner as possible without measuring radio-frequency characteristics actually.