Directional SiO2 Etch via Low-Temperature NH4F Deposition
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Solution Overview
Problem
Current precleaning techniques for semiconductor devices, such as conformal etching and sputter etching, often lead to excessive enlargement of vias and trenches, causing potential leakage and device failure, while methods like sputter etching result in redeposition of oxides that hinder subsequent deposition in vias and trenches.
Innovation Solution
A method involving positioning a silicon substrate in a processing chamber, cooling it to a low temperature, exposing it to ammonium fluoride (NH4F), and using a low energy inert plasma to selectively etch native silicon oxides from the bottom surfaces of features while preserving the cross-sectional dimensions by controlling the etching process through temperature and pressure adjustments.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If conformal etching is used to remove native oxides, then oxide removal is achieved, but excessive cross-sectional enlargement of vias and trenches occurs causing leakage and device failure
Solution Approach 1:
The patent applies local quality by making the etching process directional rather than uniform. The biased plasma delivery system creates different etching rates at different locations: aggressive etching at the bottom of vias/trenches where oxide removal is needed, and minimal etching on sidewalls where dimension control is critical. This spatial variation in etching intensity resolves the contradiction between complete oxide removal and dimension preservation.
Solution Approach 2:
The patent uses preliminary action by first depositing a conformal layer of ammonium fluoride etchant across all surfaces before applying the biased plasma. This pre-coating ensures that oxide surfaces are already saturated with etchant, so when the plasma is activated, etching begins immediately and uniformly across all surfaces simultaneously. This preliminary saturation prevents the need for prolonged plasma exposure that would cause excessive sidewall etching.
2Reliability
If sputter etching is used to remove native oxides, then oxide removal from upper surfaces is achieved, but redeposition of oxides occurs at via or trench openings preventing subsequent deposition
Solution Approach 1:
The patent replaces the mechanical sputtering process with a chemical etching mechanism using ammonium fluoride. Instead of physically bombarding surfaces with ions to remove oxides (which causes redeposition), the chemical etchant reacts with and dissolves oxides in situ. This chemical mechanism eliminates the momentum transfer that causes redeposition, allowing oxide-free surfaces without contaminating the via/trench openings.
Solution Approach 2:
The ammonium fluoride serves as an intermediary substance that mediates the oxide removal process. Rather than directly sputtering oxides off the surface (which causes redeposition), the NH4F chemically reacts with the oxides to form soluble products that can be removed without redeposition. This intermediary chemical reaction step prevents the harmful redeposition effect while achieving complete oxide removal.
3Reliability
If standard etching is used to remove native oxides, then oxide removal is achieved, but damage to nearby features occurs due to lack of selectivity
Solution Approach 1:
The patent applies parameter changes by controlling the substrate bias voltage and plasma power levels to achieve selective etching. By adjusting these electrical parameters, the etching becomes highly directional with ions preferentially arriving at surfaces perpendicular to the plasma flow (bottom of vias/trenches) rather than sidewalls. This parameter control provides the selectivity needed to protect nearby features while removing oxides from target areas.
Solution Approach 2:
The patent introduces dimensionality by using biased plasma delivery to create directional etching. Instead of isotropic etching that affects all surfaces equally, the electrical field adds a directional component that makes etching rate dependent on surface orientation. This dimensional control through field directionality allows selective removal of oxides from horizontal surfaces while protecting vertical sidewalls and nearby features.
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
This approach allows for preferential etching of native oxides from the bottom surfaces of vias and trenches, preventing damage to nearby features and ensuring proper deposition, thereby enhancing the reliability of semiconductor devices by maintaining the original dimensions and preventing leakage.
Implementation Method 1
exposing the surface of the substrate to ammonium fluoride (NH4F)
Implementation Method 2
exposing the substrate to a low energy inert plasma to selectively form one or more volatile products on the top and bottom surfaces of the features
Implementation Method 3
exposing the substrate to low pressure at a second temperature to sublimate the non-reacted NH4F from the surface of the substrate; and heating the substrate to a third temperature, which is higher than the first and second temperature, to sublimate the one or more volatile products
Implementation Method 4
cooling the substrate to a first temperature; exposing the surface of the substrate to ammonium fluoride (NH4F) at the first temperature
Data Source
AI summary
Methods for processing a substrate are described herein. Methods can include positioning a substrate comprising silicon in a processing chamber, delivering a plasma to the surface of the substrate while biasing the substrate, exposing the surface of the substrate to ammonium fluoride (NH4F), and annealing the substrate to a first temperature to sublimate one or more volatile byproducts.