Electron Beam Focus Adjustment via Electrostatic Field
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Solution Overview
Problem
Current electron beam defect inspection tools face challenges in achieving fast and precise focus control due to the hysteresis effect in magnetic lenses and stage motion-induced vibrations, which result in blurred images during semiconductor inspection.
Innovation Solution
The implementation of a dynamic focus adjustment system using a height detection sub-system that alters the electrostatic field strength between the objective lens and sample stage or applies a bias voltage to the specimen surface, allowing for rapid and precise adjustment of the electron beam focus without stage motion, utilizing an optical illuminating source, detecting unit, and control signals to calculate and apply necessary corrections.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Speed
If magnetic lens current is varied to adjust focus, then focus control is achieved, but responding time is too slow due to hysteresis effect
Solution Approach 1:
The patent replaces the magnetic lens current adjustment mechanism with an electrostatic field adjustment mechanism. By applying voltage to the specimen surface or control electrodes, the electrostatic field strength is altered to adjust the electron beam focus rapidly without the hysteresis effects inherent in magnetic systems. This substitution enables fast focus response while maintaining precision through direct voltage control.
Solution Approach 2:
The patent changes the physical parameter used for focus control from magnetic field strength (current) to electrostatic field strength (voltage). This parameter change allows for rapid focus adjustment because voltage can be changed almost instantaneously compared to magnetic field changes, which are constrained by inductance and hysteresis. The voltage parameter directly controls the electrostatic potential that influences electron beam focusing.
2Speed
If stage motion is used to adjust focus, then focus control is achieved, but vibrations are introduced that blur the image
Solution Approach 1:
The patent replaces the mechanical stage motion mechanism with an electrostatic field adjustment mechanism. Instead of physically moving the stage or specimen to change focus, the system adjusts the electrostatic field between the objective lens and specimen or applies voltage to the specimen surface. This non-mechanical approach achieves focus control without introducing vibrations that would blur the image.
Solution Approach 2:
The patent introduces an electrostatic field as an intermediary between the objective lens and specimen to control focus. Rather than directly moving the specimen or lens mechanically, the electrostatic field acts as a mediator that can rapidly adjust the electron beam focusing without physical contact or motion, thereby avoiding vibration-induced image blur.
3Measurement precision
If higher resolution is achieved, then inspection quality improves, but depth of focus becomes smaller requiring better focus control
Solution Approach 1:
The patent implements a dynamic focus control system that continuously adjusts the electrostatic field strength based on real-time height detection. The system uses an optical height detection sub-system to monitor specimen surface height variations and dynamically modifies the voltage applied to control electrodes or specimen surface bias. This dynamic adjustment maintains the electron beam focus within the acceptable depth of focus range even when operating at high resolution, where the depth of focus is minimal.
Solution Approach 2:
The patent employs a feedback control mechanism where the optical height detection sub-system continuously measures the specimen surface height and feeds this information back to the focus control system. The control system processes this height information and adjusts the electrostatic field strength accordingly to maintain optimal focus. This closed-loop feedback ensures that focus control remains precise and simple even at high resolution settings.
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 enables faster and more accurate focus control, maintaining a sharp image during continuous inspection by compensating for surface topology variations within the acceptable depth of focus, thereby improving the resolution and inspection speed of electron beam systems.
Implementation Method 1
an optical illuminating source for emitting light on the specimen
Implementation Method 2
a detecting unit for receiving image of grating reflected from the specimen
Implementation Method 3
alters the electrostatic field strength between the objective lens and sample stage
Implementation Method 4
applies a bias voltage to the specimen surface, allowing for rapid and precise adjustment of the electron beam focus
Data Source
AI summary
The present invention generally relates to dynamic focus adjustment for an image system. With the assistance of a height detection sub-system, present invention provides an apparatus and methods for micro adjusting an image focusing according the specimen surface height variation by altering the field strength of an electrostatic lens between objective lens and sample stage/or a bias voltage applied to the sample surface. Merely by way of example, the invention has been applied to a scanning electron inspection system. But it would be recognized that the invention could apply to other system using charged particle beam as observation tool with a height detection apparatus.