SEM Cold Cathode Current Stability via Dual Vacuum Pumping

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Solution Overview

Problem

Combining a cold field-emission (CFE) electron source with a boosting process in scanning electron microscopes (SEMs) results in unstable current due to residual gas generation, particularly from insulators holding anode electrodes, which deteriorates resolution characteristics at low accelerating voltages.

Innovation Solution

Implementing a configuration with a CFE electron source, an anode electrode at positive potential, and an insulator separated from ground potential within a single vacuum chamber, connected to both an ion pump and a non-evaporable getter (NEG) pump to effectively exhaust residual gases, improving current stability.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Measurement precision

If a CFE electron source is combined with a boosting process, then resolution characteristics at low accelerating voltage are improved, but current stability deteriorates due to residual gas generation

Engineering Contradiction:

Improveresolution characteristicsVSAvoidcurrent stability

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent combines both ion pump and NEG pump in the vacuum exhaust system to simultaneously remove different types of residual gases. The ion pump removes chemically active gases while the NEG pump removes chemically stable gases, achieving comprehensive residual gas removal that stabilizes the CFE electron source current when combined with boosting process

Inventive Principle:

Principle #5Merging (Combining)

Solution Approach 2:

The patent uses a composite vacuum pumping system combining two different pump technologies (ion pump and NEG pump) with complementary characteristics. This composite approach leverages the strengths of each pump type to achieve the required vacuum quality for stable CFE electron source operation during boosting process

Inventive Principle:

Principle #40Composite materials

2Measurement precision

If a CFE electron source is used instead of SE electron source, then energy width is reduced to 0.3 eV improving resolution, but current stability becomes extremely worse (10%/min)

Engineering Contradiction:

Improveenergy widthVSAvoidcurrent stability

Core Design Contradiction:

Measurement precisionVSReliability

Solution Approach 1:

The patent merges ion pump and NEG pump operations to maintain extremely low residual gas pressure in the electron gun chamber. This combined pumping system prevents gas adsorption/desorption cycles that cause current instability, enabling CFE electron source to maintain both narrow energy width (0.3 eV) and stable current output

Inventive Principle:

Principle #5Merging (Combining)

3Measurement precision

If boosting process is implemented with CFE electron source, then resolution at low accelerating voltage is enhanced, but residual gas amount increases causing current instability

Engineering Contradiction:

Improveresolution at low accelerating voltageVSAvoidresidual gas amount

Core Design Contradiction:

Measurement precisionVSObject-generated harmful factors

Solution Approach 1:

The patent converts the harmful effect of residual gas generation during boosting process into a manageable condition by using both ion pump and NEG pump. The system actively removes the harmful residual gases that would otherwise cause current instability, allowing the boosting process to proceed with stable CFE electron source operation and achieve high resolution at low accelerating voltage

Inventive Principle:

Principle #22Blessing in disguise (Convert harm into benefit)

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 configuration enables high-resolution imaging at low accelerating voltages with increased current stability, addressing the unique challenge of residual gas impact on CFE electron sources when combined with boosting processes.

Implementation Method 1

an ion pump and a NEG pump are connected to the vacuum chamber

Methodology Applied

Scientific EffectIon pumping:

Implementation Method 2

an ion pump and a NEG pump are connected to the vacuum chamber

Methodology Applied

Scientific EffectAdsorption: Adsorption

Implementation Method 3

a CFE (cold field-emission) electron source

Methodology Applied

Scientific EffectField emission:

Implementation Method 4

a process in which primary electron beam is caused to pass in a SEM column with high accelerating voltage

Methodology Applied

Scientific EffectElectron acceleration:

Data Source

PatentUS10297416B2Scanning electron microscope

Publication Date: 2019.05.21 HITACHI HIGH TECH CORP

AI summary

The purpose of the present invention is to be able to acquire high-resolution images in a scanning electron microscope using a combination of a cold cathode (CFE) electron source and a boosting process, even at low accelerating voltage enhancing the current stability of the CFE electron source. A configuration in which a CFE electron source (101), an anode electrode (103) at positive (+) potential, and an insulator (104) for isolating the anode electrode (103) from ground potential are accommodated within a single vacuum chamber (105), and an ion pump (106) and a non-evaporable getter (NEG) pump (107) are connected to the vacuum chamber (105), is employed.