Plasma Etch Endpoint Detection via Backside Optical Interference
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Solution Overview
Problem
The fabrication of photolithographic masks for ultra large scale integrated semiconductor wafers faces challenges in achieving uniform etch rates across the mask surface due to RF electrical non-uniformities in the plasma reactor's support pedestal, leading to non-uniform etch rate distribution and difficulties in precise etch depth control, which are exacerbated by the sensitivity of etch rate distribution to consumable component variations and contamination risks.
Innovation Solution
The introduction of a continuous titanium ring around the cathode's perimeter, nickel plating on the facilities plate and cathode to enhance RF electrical uniformity, and the use of backside optical measurement for real-time etch rate monitoring and end-point detection, allowing for continuous etch process control without interrupting the process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If periodic interruptions are introduced to measure etch depth and control the process, then etch depth precision is improved, but productivity decreases and contamination risk increases
Solution Approach 1:
The patent replaces mechanical interruption-based measurement with optical field-based real-time monitoring. Laser light penetrates the transparent quartz mask substrate to detect etch depth continuously through optical interference patterns, eliminating the need to stop the etch process for measurements.
Solution Approach 2:
The patent enables continuous etch process operation by implementing real-time optical monitoring that does not require process interruption. The laser-based detection system operates continuously throughout the etch process, maintaining uninterrupted material removal while providing constant depth feedback.
2Manufacturing precision
If periodic interruptions are introduced to measure etch depth, then etch depth control is improved, but contamination risk increases
Solution Approach 1:
The patent substitutes mechanical removal and reassembly operations with non-contact optical measurement. The laser-based detection system monitors etch depth through the transparent mask without requiring physical access to the etch chamber or mask, eliminating contamination sources from mask handling.
Solution Approach 2:
The patent uses the transparent quartz mask substrate itself as an optical intermediary. The laser light passes through the transparent mask to reach the etch front and reflect back, using the mask's optical transparency to enable measurement without physical intervention or additional access paths that could introduce contamination.
3Device complexity
If RF electrical non-uniformities in the support pedestal are present, then device complexity is reduced, but etch rate uniformity deteriorates
Solution Approach 1:
The patent applies localized RF power adjustment zones across the support pedestal surface. By creating spatially varying RF electrical properties in different regions of the pedestal, the system compensates for local etch rate non-uniformities without requiring complete system redesign.
Solution Approach 2:
The patent dynamically adjusts RF electrical parameters (power, frequency, phase) across the support pedestal to optimize etch rate uniformity. By changing these electrical parameters in response to real-time etch depth feedback, the system maintains uniform etching despite variations in mask properties or chamber conditions.
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 solution achieves a more uniform etch rate distribution and reduces the need for frequent process interruptions, enhancing productivity and reducing contamination risks while maintaining precise control over etch depth, thus meeting stringent requirements for mask fabrication.
Implementation Method 1
transmitting light through the pedestal and through the bottom surface of the mask, while viewing through the support pedestal light reflected from the periodic structure and detecting an interference pattern in the reflected light
Data Source
AI summary
A method is provided for defining a pattern on a workpiece such as a transparent substrate or mask or a workpiece that is at least transparent within a range of optical wavelengths. The method includes defining a photoresist pattern on the top surface of the mask, the pattern including a periodic structure having a periodic spacing between elements of the structure. The method further includes placing the mask on a support pedestal in a plasma reactor chamber and generating a plasma in the chamber to etch the top surface of the mask through openings in the photoresist pattern. The method also includes transmitting light through the pedestal and through the bottom surface of the mask, while viewing through the support pedestal light reflected from the periodic structure and detecting an interference pattern in the reflected light. The method further includes determining from the interference pattern a depth to which periodic structure has been etched in the top surface.