Selective Etching of Silicon Wafers Using HF-HNO3-Acetic Acid
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Solution Overview
Problem
Current wet etching processes for semiconductor wafer thinning, such as those using TMAH, KOH, EDP, HF—HNO3—H2SO4 solutions, face issues like anisotropic etching, lack of doping selectivity, and low etch rates, which are inadequate for wafer-scale silicon removal and control in 3-D semiconductor device integration.
Innovation Solution
A method involving a mixture of hydrofluoric acid, nitric acid, and acetic acid in a 1:3:5 ratio, with boron-doped silicon wafers, where the reaction mixture is re-circulated and added back to create a seasoned solution that enhances etching efficiency by increasing nitride-oxide concentration, allowing for isotropic etching with high selectivity and rate.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If traditional wet etching processes (TMAH, KOH, EDP) are used, then anisotropic etching is achieved, but the etch profile depends on wafer crystallographic orientation making it unsuitable for wafer scale silicon removal
Solution Approach 1:
The patent changes the chemical composition parameters of the etching solution by adding nitric acid to the HF-based solution, which fundamentally alters the etching mechanism from anisotropic to isotropic while maintaining high etch rates. This parameter change enables uniform etching across the entire wafer surface regardless of crystallographic orientation.
2Productivity
If HF-HNO3-H2SO4 mixture is used, then isotropic etching with high etch rate is achieved, but no doping selectivity is provided preventing adequate etch stop mechanism
Solution Approach 1:
The patent modifies the chemical composition by using HF-HNO3-acetic acid instead of HF-HNO3-H2SO4, and optimizes the concentration ratios. This parameter change provides doping selectivity where heavily boron-doped regions etch at different rates than lightly-doped regions, enabling etch stop mechanisms while maintaining high etch rates.
Solution Approach 2:
The etching process exhibits local quality differences based on doping concentration. Heavily doped regions (≥1×10^19 atoms/cm³) show different etching behavior compared to lightly doped regions, allowing selective etching and etch stop mechanisms to function effectively.
3Manufacturing precision
If traditional wet etching processes are used, then process control is achieved, but etch rate is very low
Solution Approach 1:
The patent dramatically changes the etch rate parameter by using HF-based solutions with nitric acid and acetic acid in optimized ratios, achieving etch rates up to 5 μm/min compared to traditional low etch rates. This parameter change maintains process control through doping selectivity while dramatically improving productivity.
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 solution achieves a significantly higher etch rate, up to 5 μm/min, which is more than six times faster than traditional methods, effectively thinning heavily doped silicon wafers while maintaining control and precision, suitable for 3-D semiconductor device integration.
Implementation Method 1
causing the mixture to react with portions of one or more silicon wafers, the portions of the one or more silicon wafers are doped with boron
Implementation Method 2
where the reaction mixture is re-circulated and added back to create a seasoned solution that enhances etching efficiency by increasing nitride-oxide concentration
Data Source
AI summary
A method of preparing an etch solution and thinning semiconductor wafers using the etch solution is proposed. The method includes steps of creating a mixture of hydrofluoric acid, nitric acid, and acetic acid in a solution container in an approximate 1:3:5 ratio; causing the mixture to react with portions of one or more silicon wafers, the portions of the one or more silicon wafers are doped with boron in a level no less than 1×1019 atoms/cm3; collecting the mixture after reacting with the boron doped portions of the one or more silicon wafers; and adding collected mixture back into the solution container to create the etch solution.