Silicon Concentration Analysis in Phosphoric Acid Etchants
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Solution Overview
Problem
Current methods for determining silicon concentration in silicon nitride etchant solutions are inadequate, particularly in concentrated phosphoric acid solutions, as they are either cumbersome, expensive, or not amenable to automation and on-line use, leading to challenges in controlling the etch rate and selectivity of silicon nitride etching processes.
Innovation Solution
A method involving the addition of a predetermined concentration of fluoride ions to a test solution, measured using a fluoride ion specific electrode, allows for the calculation of silicon concentration by reacting with silicon ions, enabling accurate and rapid determination suitable for on-line process control, using a compact and inexpensive apparatus.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If conventional methods (ammonium molybdate reaction) are used to determine silicon concentration, then the method is simple and inexpensive, but phosphate ions interfere with the reaction making it unusable for concentrated phosphoric acid solutions
Solution Approach 1:
The patent introduces fluoride ions as an intermediary substance that selectively reacts with silicon ions to form volatile SiF4 gas, which is then hydrolyzed to silicic acid for measurement. This intermediary reaction pathway eliminates the interference from phosphate ions that plague direct ammonium molybdate methods, while still enabling simple colorimetric detection of silicon concentration
Solution Approach 2:
The patent changes the chemical parameters of the measurement system by using fluoride ions to transform silicon ions into a different chemical form (SiF4 gas, then silicic acid) that can be measured without phosphate interference. This parameter transformation allows the use of simple colorimetric methods in previously intractable concentrated phosphoric acid environments
2Measurement precision
If sophisticated methods (atomic absorption analysis or inductively coupled plasma-atomic emission spectroscopy) are used, then accurate silicon concentration determination is achieved, but the equipment is large, complex, expensive and costly to maintain
Solution Approach 1:
The patent replaces complex mechanical and electronic measurement systems (atomic absorption spectrometers, plasma emission spectrometers) with a simple chemical transformation followed by colorimetric detection. The fluoride ion reaction and subsequent colorimetric measurement eliminate the need for expensive, complex instrumentation while maintaining accuracy
Solution Approach 2:
The patent employs inexpensive, easily prepared chemical reagents (fluoride ions, hydrolysis agents, colorimetric indicators) that can be readily disposed of after use, replacing expensive, maintenance-intensive sophisticated analytical equipment with simple, disposable chemical systems
3Measurement precision
If the existing method (adding concentrated hydrofluoric acid and detecting SiF4 via conductivity change) is used, then silicon concentration can be determined in concentrated phosphoric acid, but the method is cumbersome, time consuming and involves handling hazardous gas
Solution Approach 1:
The patent converts the hazardous SiF4 gas handling step into a beneficial intermediate by immediately hydrolyzing it to silicic acid in aqueous solution, which is then detected using safe, non-hazardous colorimetric methods. The potentially dangerous gas phase reaction is transformed into a safe liquid phase measurement
Solution Approach 2:
The patent introduces aqueous hydrolysis as an intermediary step that converts volatile, hazardous SiF4 gas into non-volatile, safe silicic acid in solution. This intermediary transformation eliminates the need to handle hazardous gases while maintaining the ability to accurately measure silicon concentration
4Manufacturing precision
If silicon concentration is not accurately controlled, then the etch rate and selectivity cannot be optimized, but available determination methods are inadequate for concentrated phosphoric acid solutions
Solution Approach 1:
The patent establishes a feedback loop where silicon concentration in the phosphoric acid etchant is continuously or periodically measured using the fluoride ion-colorimetric method, and this information feeds back to optimize the etching process parameters. This enables real-time control of etch rate and selectivity based on actual silicon concentration levels
Data Source
AI summary
Low concentrations of silicon in an etchant solution are analyzed by adding a predetermined concentration of fluoride ions to a test solution comprising a predetermined volume of the etchant solution, and measuring the concentration of fluoride ions in the test solution. Reaction with silicon ions in the test solution reduces the concentration of fluoride ions, which are present in stoichiometric excess, so that the silicon concentration of the etchant solution can be calculated from the difference between the predetermined and measured concentrations of fluoride ions in the test solution. The method is especially suited for analysis of silicon nitride etchants comprising a high concentration of phosphoric acid.