Silicon Nitride Cryogenic ALE for Self-Limiting Low-Pressure Etching
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Solution Overview
Problem
Conventional atomic layer etching (ALE) methods for silicon nitride, particularly using fluorocarbon chemistry and hydrogen/fluorine plasma, face challenges in achieving precise etch control and reducing surface roughness, especially in modern ICP RIE plasma systems operating at lower pressures, which affects the performance of silicon nitride waveguides and other nanoscale devices.
Innovation Solution
A cryogenic ALE recipe is developed, utilizing a H2 plasma to modify the silicon nitride surface followed by an SF6 etch step, which reduces spontaneous etching and enhances self-limiting behavior, improving sidewall homogeneity and reducing surface roughness at low wafer temperatures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional ALE methods using fluorocarbon chemistry and hydrogen/fluorine plasma are used, then etching capability is achieved, but surface roughness increases and etch control precision deteriorates
Solution Approach 1:
The patent changes the chemical parameters of the etching process by replacing conventional fluorocarbon chemistry with sulfur hexafluoride (SF6) plasma, and adjusts physical parameters including pressure (20-100 mT) and temperature conditions to achieve reduced surface roughness and improved etch control precision
Solution Approach 2:
The patent introduces a two-step process with an intermediary modification step using hydrogen plasma or oxygen exposure before the main SF6 etching step, which prepares the silicon nitride surface to control the etching reaction and reduce surface roughness
2Productivity
If fluorinated plasma is used at high pressure (>1000 mT) for ALE of silicon nitride, then etching rate is improved, but compatibility with modern ICP RIE plasma systems deteriorates
Solution Approach 1:
The patent changes the pressure parameter from conventional high pressure (>1000 mT) to low pressure (20-100 mT) range, making the process compatible with modern ICP RIE plasma systems while maintaining effective etching capability through optimized SF6 plasma chemistry
3Productivity
If hydrogen plasma surface modification is performed to create chemically modified layer, then etch-per-cycle is increased, but spontaneous isotropic etching increases which reduces sidewall homogeneity
Solution Approach 1:
The patent uses a sequential two-step approach where the first step (hydrogen plasma or oxygen exposure) creates a modified surface layer, and the second step (SF6 plasma) selectively removes this modified layer, copying the self-limiting behavior of atomic layer etching to achieve both high etch-per-cycle and good sidewall homogeneity
Solution Approach 2:
The process incorporates self-limiting feedback where the etching rate is controlled by the thickness of the chemically modified surface layer, which forms and removes in a cyclic manner, automatically regulating the etch depth and preserving sidewall homogeneity
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 cryogenic ALE process achieves improved etch control, reduced surface roughness, and decreased optical loss in silicon nitride waveguides by minimizing isotropic etching and enhancing sidewall uniformity, making it suitable for advanced nanofabrication applications.
Implementation Method 1
the silicon nitride is first exposed to a hydrogen plasma, where the surface is modified by hydrogen implantation in the nitride, creating a chemically modified-damaged layer
Implementation Method 2
This chemically modified layer is then removed by the fluorinated plasma
Implementation Method 3
Atomic layer etching is a cyclic, self-limiting dry etching process, involving alternating steps of surface modification and removal
Data Source
AI summary
Illustrative embodiments describe a recipe using a low pressure (e.g., 20 mT SF6) etch step, which can be achieved by most modern ICP Plasma systems. We find that in our modified etch recipe, the chemically modified layer can reach up to 20 nm in thickness, leading to an etch per cycle rate that is doubled compared to previous reports. We observed that at this lower pressure and at 10° C. wafer temperature, the SF6 modified surface removal step loses its self-limiting aspect and etches material beyond the surface modified layer. However, we find that the self-limiting etch can be recovered by reducing the wafer temperature during the etch, with an added benefit of improved sidewall homogeneity and decreased surface roughness.