Silicon Chunk Etching Sequence for Ultra-Low Surface Metal Content
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Solution Overview
Problem
Existing methods for producing silicon chunks, particularly polysilicon, fail to efficiently remove surface metals such as tungsten, leading to contamination levels that exceed the pptw range, which negatively impact semiconductor applications by affecting minority charge carrier lifetime and micrometal precipitates.
Innovation Solution
A method involving comminution of silicon ingots followed by treatment with a preliminary etching bath containing 6.6% to 12% hydrofluoric acid (HF) and 40% to 65% nitric acid (HNO3), and a main etching bath with 5.3% to 6.5% HF and 40% to 65% HNO3, utilizing controlled temperature and residence times to achieve efficient tungsten removal.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If conventional comminution using tungsten carbide crushing tools is used, then silicon chunks can be produced efficiently, but surface metal contamination (especially tungsten) increases to 400-1000 pptw
Solution Approach 1:
The patent applies preliminary action by introducing a pickling bath treatment before the main etching process. The pickling bath (HF/HCl/H2O2) pre-cleans the silicon chunk surfaces, removing loose contaminants and preparing the surface for more effective tungsten removal in the subsequent etching bath. This preliminary step prevents re-deposition and enhances overall cleaning efficiency.
Solution Approach 2:
The patent employs parameter changes by optimizing the composition and temperature of the etching baths. The etching bath contains specific concentrations of HF (4-10%) and HNO3 (50-70%), with temperature control (20-50°C). These parameter optimizations enable selective removal of tungsten while minimizing damage to the silicon substrate, achieving ultra-low surface metal content.
2Object-affected harmful factors
If multistage wet-chemical etching with multiple corrosive baths is used, then surface metal content can be reduced, but processing time and equipment complexity increase
Solution Approach 1:
The patent merges multiple cleaning functions into a streamlined sequence. The pickling bath combines HF, HCl, and H2O2 to achieve both organic and inorganic contaminant removal in one step. The subsequent etching bath integrates tungsten-specific removal with general metal cleaning, reducing the need for multiple separate treatment stages while maintaining effectiveness.
Solution Approach 2:
The patent applies local quality by using different bath compositions for different cleaning objectives. The pickling bath (HF/HCl/H2O2) targets organic contaminants and loose metals, while the etching bath (HF/HNO3) specifically addresses tungsten removal. This localized approach to each cleaning stage optimizes processing efficiency without requiring excessive numbers of baths.
3Object-affected harmful factors
If low HF content (0.1%-0.5%) is used in etching baths, then equipment costs and safety risks are reduced, but tungsten removal efficiency decreases
Solution Approach 1:
The patent optimizes the HF concentration parameter to 4-10%, which is higher than conventional low-HF processes (0.1%-0.5%) but carefully controlled to balance effectiveness and safety. This parameter optimization, combined with temperature control (20-50°C) and residence time management, achieves superior tungsten removal while maintaining reasonable equipment requirements and operational safety.
Solution Approach 2:
The patent introduces HNO3 as an intermediary substance that enhances the etching process. The combination of HF and HNO3 creates a synergistic effect where HNO3 acts as an oxidizing agent that facilitates tungsten dissolution, allowing effective tungsten removal at moderate HF concentrations. This intermediary role of HNO3 reduces the burden on HF concentration and simplifies equipment requirements.
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 method significantly reduces the total surface metal content to less than 13 pptw, preferably less than 11 pptw, effectively addressing tungsten and other metals, ensuring minimal contamination for semiconductor applications.
Implementation Method 1
contacting the silicon chunks with a preliminary etching bath containing 6.6% to 12% hydrofluoric acid (HF) and 40% to 65% nitric acid (HNO3)
Implementation Method 2
contacting the silicon chunks with at least one main etching bath containing 5.3% to 6.5% HF and 40% to 65% HNO3
Data Source
AI summary
Silicon chunks with reduced surface metal content and methods for producing the same. Where the method for producing the silicon chunks include the steps of (a) comminuting a silicon ingot or block into silicon chunks, (b) contacting the silicon chunks with at least one preliminary etching bath, comprising 6.6% to 12% by weight of hydrofluoric acid and 40% to 65% by weight of nitric acid, and (c) contacting the silicon chunks with at least one main etching bath, comprising 5.3% to 6.5% by weight of hydrofluoric acid and 40% to 65% by weight of nitric acid.