Wet Etching Selectivity for Polysilicon Over Low-Temperature Oxide
Find Innovative SolutionsGenerate Solutions
Solution Overview
Problem
Conventional wet etching compositions lose selectivity when used with low-temperature silicon dioxide and heavily doped polycrystalline silicon, resulting in reduced etch rates and selectivity, making it difficult to selectively remove polysilicon relative to silicon dioxide.
Innovation Solution
A liquid composition comprising 0.05-0.4% hydrofluoric acid, 15-40% nitric acid, 55-85% sulfuric acid, and 2-20% water is used for wet etching, which maintains high selectivity by retarding nitric acid dissociation and retaining hydrofluoric acid in close proximity to oxidized polysilicon, enhancing etching selectivity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If conventional wet etching compositions (mixtures of HNO3 and HF) are used, then etching of polysilicon can be achieved, but etch selectivity drops from 80 to as low as 2 when low-temperature silicon dioxide and heavily doped polysilicon are used in combination
Solution Approach 1:
The patent changes the chemical composition parameters of the etching solution by introducing sulfuric acid (H2SO4) as a major component (55-85% by weight) and adjusting the ratios of nitric acid (15-40%) and hydrofluoric acid (0.05-0.4%). This parameter change transforms the etching mechanism to achieve high selectivity (up to 80:1) for polysilicon over low-temperature silicon dioxide, resolving the contradiction between maintaining etch selectivity and adapting to modified materials.
Solution Approach 2:
The patent creates a composite etching solution containing three different acids (sulfuric acid, nitric acid, and hydrofluoric acid) in specific proportions. This composite composition leverages the synergistic effects of each acid: sulfuric acid provides high viscosity to retard nitric acid dissociation, nitric acid oxidizes polysilicon, and hydrofluoric acid etches the oxidized polysilicon. This composite approach achieves both high etch selectivity and compatibility with modified materials.
2Temperature
If low-temperature silicon dioxide is used, then deposition temperature is reduced, but etch rates with conventional HNO3/HF mixtures increase up to four times greater than high-temperature oxides
Solution Approach 1:
The patent changes the composition parameters by making sulfuric acid the dominant component (55-85% by weight) and limiting water content (2-20%). This creates a high-viscosity solution that retards the dissociation of nitric acid and controls the diffusion of hydrofluoric acid, thereby controlling the etch rate of low-temperature silicon dioxide while maintaining high polysilicon etch selectivity.
Solution Approach 2:
Sulfuric acid acts as an intermediary substance in the etching solution. Its high viscosity retards the dissociation of nitric acid and controls the release of etching species, serving as a mediator that modulates the interaction between the etchant and low-temperature silicon dioxide. This intermediary effect enables controlled etching of LTO while maintaining high selectivity for polysilicon.
3Reliability
If highly doped polycrystalline silicon is used, then electrical properties are improved, but etch rates with conventional HNO3/HF mixtures decrease significantly
Solution Approach 1:
The patent adjusts the chemical composition parameters by increasing nitric acid content (15-40%) to provide sufficient oxidation capability for heavily doped polysilicon, while maintaining high sulfuric acid content (55-85%) to ensure high solution viscosity. This parameter adjustment restores etch rates for highly doped polysilicon while maintaining the electrical properties benefits of heavy doping.
Solution Approach 2:
The patent utilizes nitric acid as a strong oxidant in the etching solution. The high nitric acid content (15-40%) provides accelerated oxidation of heavily doped polysilicon, creating a sufficient oxide layer that can then be etched by hydrofluoric acid. This strong oxidant mechanism overcomes the reduced etch rates typically observed with highly doped polysilicon in conventional etchants.
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 composition achieves improved etching selectivity for polysilicon relative to silicon dioxide, even with low-temperature oxides and heavily doped polysilicon, forming undercut regions and maintaining high etching performance.
Implementation Method 1
mixtures of HNO3 and HF will selectively etch polysilicon relative to SiO2
Implementation Method 2
HF etches SiO2
Implementation Method 3
The usage of these modified materials, especially in combination, degrades the etch selectivity of conventional wet etchants
Data Source
AI summary
A liquid composition for wet etching has improved selectivity for polysilicon over silicon dioxide, even when the polysilicon is heavily doped and/or the silicon dioxide is a low temperature oxide. The composition comprises 0.05-0.4 percent by weight hydrofluoric acid, 15-40 percent by weight nitric acid, 55-85 percent by weight sulfuric acid and 2-20 percent by weight water. A method and apparatus for wet etching using the composition are also disclosed.