Ion Source Cathode Equipotentiality to Reduce Sputtering
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Solution Overview
Problem
Ion sources, such as Bernas and indirectly heated cathode (IHC) ion sources, face shortened lifetimes due to sputtering effects on filaments and cathodes, leading to electrical shorts and reduced performance in semiconductor processing.
Innovation Solution
The ion source design includes electrically conductive walls with the cathode or filament connected to the chamber, and strategically placed electrodes with applied bias voltages to reduce sputtering by minimizing positive ion attraction and maintaining components at the same voltage as the chamber, thereby preventing electrical shorts.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If the cathode is left floating or isolated from the chamber walls, then it can maintain a negative bias to repel electrons and direct them toward the chamber center, but it becomes susceptible to sputtering from positive ion bombardment and chemical vapor deposition causing electrical shorts
Solution Approach 1:
The cathode is electrically connected to the chamber walls, maintaining it at the same potential as the grounded chamber. This eliminates the negative bias that would otherwise attract positive ions and cause sputtering, while preventing electrical shorts from chemical vapor deposition.
2Productivity
If the cathode is biased negatively to control electron position and increase ion density, then ionization efficiency improves, but the cathode experiences increased sputtering from positive ion attraction
Solution Approach 1:
By connecting the cathode to the chamber walls, the invention maintains the cathode at ground potential rather than applying a negative bias. This eliminates the electrostatic attraction of positive ions to the cathode surface, preventing sputtering and cathode erosion while still achieving the desired ion density through other means.
3Use of energy by moving object
If the filament is exposed in the chamber to generate electrons, then electron emission efficiency is high, but the filament is subject to sputtering and other phenomena that reduce its life
Solution Approach 1:
The invention introduces a cathode as an intermediary between the filament and the chamber environment. The cathode absorbs the harmful effects of positive ion bombardment and sputtering, protecting the filament while allowing the filament to continue generating electrons efficiently.
Solution Approach 2:
The cathode is designed as a sacrificial component that can be replaced more easily and cheaply than the filament. It absorbs the damage from plasma exposure and sputtering, serving as a protective barrier that extends filament life while being itself replaceable.
4Quantity of substance
If chemical vapor deposition occurs on the negatively charged cathode, then the cathode becomes electrically connected to the grounded walls, but this causes failure of the ion source
Solution Approach 1:
By maintaining the cathode at the same potential as the chamber walls through electrical connection, the invention eliminates the electric field that would drive chemical vapor deposition onto the cathode surface. This prevents the formation of conductive deposits that could cause electrical shorts and ion source failure.
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 design extends the life of ion sources by reducing sputtering and eliminating electrical shorts, while maintaining performance for various ion species, with empirical data showing up to 40% more beam current at 30% less source power.
Implementation Method 1
The filament emits thermionic electrons, which are accelerated toward and heat the cathode, in turn causing the cathode to emit electrons into the chamber of the ion source.
Implementation Method 2
A magnetic field may be used to confine the path of the electrons.
Implementation Method 3
The cathode and repeller may be biased so as to repel the electrons, directing them back toward the center of the chamber.
Implementation Method 4
The cathode is subjected to bombardment from electrons on its back surface, and by positively charged ions on its front surface. The ion bombardment results in sputtering, which causes erosion of the cathode.
Data Source
AI summary
An ion source having improved life is disclosed. In certain embodiments, the ion source is an IHC ion source comprising a chamber, having a plurality of electrically conductive walls, having a cathode which is electrically connected to the walls of the ion source. Electrodes are disposed on one or more walls of the ion source. A bias voltage is applied to at least one of the electrodes, relative to the walls of the chamber. In certain embodiments, fewer positive ions are attracted to the cathode, reducing the amount of sputtering experienced by the cathode. Advantageously, the life of the cathode is improved using this technique. In another embodiment, the ion source comprises a Bernas ion source comprising a chamber having a filament with one lead of the filament connected to the walls of the ion source.