Electromagnetic Compound Lens for Charged Particle Beam Aberration Correction
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Solution Overview
Problem
Charged particle optical systems face challenges in achieving high resolution and low aberration imaging due to dispersion and aberration introduced by beam separators and lenses, which degrade imaging quality and reduce throughput in inspecting micro and nano-sized defects on ICs.
Innovation Solution
An electromagnetic compound lens comprising a magnetic lens with a permanent magnet and an electrostatic lens, where the focusing power is adjustable by varying the electrostatic lens, is used to minimize dispersion and aberration, improving imaging resolution in charged particle optical systems.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If a beam separator comprising a magnetic deflector is used to separate secondary electron beams from primary electron beams, then beam separation is achieved, but dispersion is generated in the beams
Solution Approach 1:
A beam separator is introduced as an intermediary component between the primary electron beam path and the secondary electron beam detection path. The beam separator uses a magnetic deflector to separate the two beam types spatially, allowing them to be processed independently. This mediator approach enables beam separation while managing the dispersion effect through proper optical design.
Solution Approach 2:
The patent adjusts optical parameters including the separation angle between primary and secondary beam paths, and modifies lens parameters (focal length, aperture) to compensate for dispersion. By changing these parameters, the system optimizes the trade-off between effective beam separation and minimizing dispersion-induced degradation of beam quality.
2Measurement precision
If lenses are used in the optical system, then beam focusing is achieved, but aberration is generated affecting resolution
Solution Approach 1:
The optical system is divided into multiple lens stages rather than using a single lens. By segmenting the focusing function across multiple lenses with different characteristics, the system can correct aberrations introduced by each individual lens through the combined action of the lens group, thereby maintaining focusing capability while reducing overall aberration.
Solution Approach 2:
The patent employs composite lens designs combining different lens materials and types (e.g., electrostatic lenses, magnetic lenses, or lenses with different glass compositions) to achieve complementary optical properties. This composite approach allows each lens material to contribute its strengths while compensating for the weaknesses of others, reducing overall system aberration.
3Productivity
If multiple primary beams or multiple secondary beams are used, then inspection throughput is improved, but beam separation complexity increases
Solution Approach 1:
Multiple primary electron beams and their corresponding secondary electron beams are merged into a unified optical pathway after separation. The beam separator handles all beams through a common magnetic deflection field, and the subsequent optical system processes them through shared lenses and detectors. This merging approach maintains high throughput by processing multiple beams simultaneously while using a relatively simple common beam separation mechanism.
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
The solution effectively reduces dispersion and aberration, enhancing the imaging resolution and throughput of charged particle optical systems, particularly in inspecting ICs, by using a compact and adjustable electromagnetic compound lens.
Implementation Method 1
an electrostatic lens and a magnetic lens. A focusing power of the compound lens may be changed by varying a focusing power of the electrostatic lens
Implementation Method 2
The magnetic lens may include a permanent magnet
Implementation Method 3
a beam separator may comprise a magnetic deflector that is used to deflect electrons using electromagnetism
Data Source
AI summary
An electromagnetic compound lens may be configured to focus a charged particle beam. The compound lens may include an electrostatic lens provided on a secondary optical axis and a magnetic lens also provided on the secondary optical axis. The magnetic lens may include a permanent magnet. A charged particle optical system may include a beam separator configured to separate a plurality of beamlets of a primary charged particle beam generated by a source along a primary optical axis from secondary beams of secondary charged particles. The system may include a secondary imaging system configured to focus the secondary beams onto a detector along the secondary optical axis. The secondary imaging system may include the compound lens.