Wafer Thickness Measurement Error Reduction via Cavity Calibration

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Solution Overview

Problem

Existing methods for measuring wafer shape and thickness are unable to effectively reduce errors caused by higher order cavity shape changes, such as those due to temperature or stress variations, which affect the accuracy of measurements.

Innovation Solution

The method involves calibrating the cavity characteristics between reference flats in interferometer channels, synchronizing interferograms using a single wavelength tunable laser, and using high-order polynomials to estimate cavity tilt and determine wafer thickness variations, thereby accounting for cavity shape changes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If conventional interferometer methods are used to measure wafer shape and thickness, then measurement capability is provided, but measurement precision deteriorates due to higher order cavity shape changes caused by temperature or stress variations

Engineering Contradiction:

Improvewafer thickness measurement accuracyVSAvoidmeasurement reliability under temperature and stress variations

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The system performs preliminary calibration of the cavity characteristics between reference flats before actual wafer measurement. This preliminary action captures the cavity shape including higher order deformations, which are then used to compensate for measurement errors during subsequent wafer measurements, thereby improving measurement precision under varying temperature and stress conditions

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The invention changes the measurement approach by using high-order polynomial fitting to model cavity shape changes as a function of position and environmental parameters. By parameterizing the cavity deformation and compensating for it mathematically, the system maintains measurement reliability despite temperature and stress variations

Inventive Principle:

Principle #35Parameter changes

2Productivity

If two phase-shifting Fizeau interferometers are combined to measure both sides of wafer simultaneously, then measurement efficiency is improved, but measurement precision deteriorates due to inability to correct higher order cavity shape errors

Engineering Contradiction:

Improvemeasurement throughputVSAvoidthickness variation accuracy

Core Design Contradiction:

ProductivityVSMeasurement precision

Solution Approach 1:

The system performs preliminary measurement and characterization of the cavity shape using the same dual interferometer setup before wafer measurement. This preliminary action enables the system to capture higher order cavity deformations and use them for error compensation during actual production measurements, maintaining both high throughput and precision

Inventive Principle:

Principle #10Preliminary action

Solution Approach 2:

The invention implements a feedback mechanism where cavity characteristics measured during or before wafer measurement are used to correct the thickness variation data. The system continuously monitors and compensates for cavity shape changes, ensuring measurement precision is maintained even with high-speed dual-side measurement capability

Inventive Principle:

Principle #23Feedback

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 significantly increases the accuracy of wafer shape and thickness measurements by considering high-order cavity shape information, reducing measurement errors caused by temperature and stress variations.

Implementation Method 1

synchronizing the interferograms in the two interferometer channels by supplying the light from a single wavelength tunable laser light source

Methodology Applied

Scientific EffectLaser: Laser

Implementation Method 2

combines two phase-shifting Fizeau interferometers to simultaneously obtain two single-sided distance map

Methodology Applied

Scientific EffectInterference: Interference

Implementation Method 3

measuring the surface height on both sides and thickness variation of a wafer

Methodology Applied

Scientific EffectReflection: Reflection

Data Source

PatentUS9121684B2Method for reducing wafer shape and thickness measurement errors resulted from cavity shape changes

Publication Date: 2015.09.01 KLA CORP

AI summary

Methods and systems for reducing wafer shape and thickness measurement errors resulted from cavity shape changes are disclosed. Cavity calibration process is performed immediately before the wafer measurement. Calibrating the cavity characteristics every time the method is executed reduces wafer shape and thickness measurement errors resulted from cavity shape changes. Additionally or alternatively, a polynomial fitting process utilizing a polynomial of at least a second order is utilized for cavity tilt estimation. High order cavity shape information generated using high order polynomials takes into consideration cavity shape changes due to temperature variations, stress or the like, effectively increases accuracy of the wafer shape and thickness information computed.