Plasma Etching Ion Bombardment Tilt Control for Wafer Edge Symmetry
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Solution Overview
Problem
Current plasma etching methods for silicon wafers face challenges in maintaining uniformity, particularly at the wafer edge, due to variations in gas flow, temperature, and plasma density, leading to asymmetries and tapered profiles, which result in defective die.
Innovation Solution
A method involving a cyclical etch process with multiple over etch steps of different inclinations, controlled by electrical bias power, to correct asymmetries introduced during the main etch, ensuring symmetrical features by adjusting the ion bombardment inclination and plasma sheath tilt.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a cyclical Bosch process is used for plasma etching, then the etching capability is improved, but the uniformity of the etch profile deteriorates due to discontinuities at the wafer edge
Solution Approach 1:
The overetch process is segmented into multiple sequential steps (first overetch with outward tilt, second overetch with reduced or zero tilt, third overetch with inward tilt) to address different regions of the wafer. This segmentation allows each step to target specific asymmetry issues at the wafer edge while maintaining overall etch uniformity.
Solution Approach 2:
The plasma sheath tilt angle is dynamically adjusted between different overetch steps. The system transitions from an outward tilt during the first overetch step to reduced/zero tilt in the second step, and to an inward tilt in the third step. This dynamic control of plasma sheath geometry enables correction of edge effects while maintaining etch uniformity across the wafer surface.
2Manufacturing precision
If focus rings and electrostatic chucks are used to control plasma uniformity, then the plasma uniformity is improved, but the complexity of the device increases
Solution Approach 1:
The invention controls plasma uniformity by changing process parameters (RF power, gas flow rates, pressure) and plasma sheath tilt angle between different etch steps rather than adding complex hardware. The plasma sheath tilt is controlled by adjusting the potential difference between the plasma generator and substrate, allowing uniformity control through parameter optimization.
3Manufacturing precision
If the plasma sheath tilt is optimized during the over etch cycle, then the alignment with the main etch is improved, but the process complexity increases
Solution Approach 1:
The overetch cycle is segmented into multiple steps with progressively optimized plasma sheath tilt angles. The first step uses outward tilt to address edge effects, the second step uses reduced or zero tilt for alignment, and the third step uses inward tilt to correct remaining asymmetries. This segmentation of the overetch process into targeted steps improves alignment precision while managing process complexity through systematic optimization.
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 significantly improves the symmetry of features near the wafer edge, particularly for high aspect ratio features, by selectively controlling the ion bombardment direction, thereby reducing asymmetries and defects.
Implementation Method 1
a negative polarisation of the wafer surface is typically achieved such that positive ions from the plasma are attracted to the surface of the wafer. The region in the plasma from where the positive ions have been extracted is known as the plasma sheath.
Implementation Method 2
The positive ions bombard the wafer surface in a direction generally perpendicular from the plasma sheath.
Implementation Method 3
The thickness and tilt of the plasma sheath is dependent upon plasma density, local potential and topology of the wafer. These parameters vary throughout the process chamber and lead to variation in plasma sheath thickness across the surface of the wafer.
Implementation Method 4
The tilt of the plasma sheath corresponds to the incident angle at which ions bombard the surface of the substrate. The incident angle of ion bombardment at the edge of the wafer may be tilted with respect to the incident angle of ion bombardment at the centre of the wafer.
Data Source
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AI summary
According to the invention there is provided a method of plasma etching one or more features in a silicon substrate, the method comprising the steps of: performing a main etch using a cyclical etch process in which a deposition step and an etch step are alternately repeated; and performing an over etch to complete the plasma etching of the features; in which: the over etch comprises one or more etch steps of a first kind and one or more etch steps of a second kind, each of the etch steps of the first and second kind comprising etching by ion bombardment of the silicon substrate; and ion bombardment during the one or more etch steps of the second kind has an inward inclination with respect to ion bombardment during the one or more etch steps of the first kind.