Piezoelectric Layer Etching via Oxygen Plasma Oxidation
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Solution Overview
Problem
Microelectronic devices with piezoelectric elements face challenges in etching piezoelectric materials due to etch residue buildup and mask erosion during dry etching, and uncontrollable undercutting during wet etching, leading to undesirable profiles.
Innovation Solution
A three-step etching process involving an oxygen plasma oxidation of the top surface, followed by a photoresist etch mask formation with UV baking, and a specific wet etching solution (5% NH4F, 1.2% HF, 18% HCl) with controlled ratios and agitated rinses to maintain adhesion and prevent mask separation.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If dry etching is used on piezoelectric layers over a micron thick, then etching can be performed, but etch residue buildup and mask erosion occur
Solution Approach 1:
The etching process is divided into three sequential steps: a first wet etch step that removes the majority of the piezoelectric layer, followed by an agitated rinse, and then a short second wet etch step that completes the etching. This segmentation allows each step to be optimized independently, achieving complete etching without the residue and mask erosion problems of dry etching.
Solution Approach 2:
An intermediary oxidized surface layer is created on the piezoelectric material before etching. This oxidized layer serves as a mediator that improves mask adhesion and enables controlled wet etching, preventing the mask lifting and undercutting issues that would otherwise occur.
2Productivity
If wet etching is used on piezoelectric material, then etching can be performed, but the mask undercuts in an uncontrollable manner and lifts from the piezoelectric material
Solution Approach 1:
The top surface of the piezoelectric layer is oxidized with oxygen plasma before the etch mask is formed. This preliminary oxidation creates a surface that enhances mask adhesion, preventing the mask from lifting during wet etching and enabling controlled, vertical etch profiles.
Solution Approach 2:
The physical and chemical parameters of the piezoelectric surface are changed through oxidation, transforming it from a non-adhesive surface to one that strongly bonds with the photoresist mask. This parameter change enables the wet etching process to proceed without mask lifting or uncontrollable undercutting.
3Device complexity
If a simple etch mask formation is used, then the process is simple, but adhesion is insufficient and mask separation occurs
Solution Approach 1:
The piezoelectric layer surface is oxidized with oxygen plasma before the photoresist is applied. This preliminary oxidation step creates a surface that strongly adheres to the photoresist mask, preventing mask separation during the wet etching process while adding only one additional step to the overall process.
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 method effectively removes the piezoelectric layer with controlled profiles, minimizing etch residue and mask separation, resulting in a desired vertical side profile and improved adhesion of the piezoelectric element mask.
Implementation Method 1
oxidizing a top surface of a piezoelectric layer with an oxygen plasma
Implementation Method 2
oxidizing a top surface of a piezoelectric layer with an oxygen plasma
Implementation Method 3
forming an etch mask containing photoresist on the oxidized top surface
Implementation Method 4
The piezoelectric layer is etched using a three step process: a first step includes a wet etch of an aqueous solution of about 5% NH4F, about 1.2% HF, and about 18% HCl
Data Source
AI summary
A microelectronic device containing a piezoelectric thin film element is formed by oxidizing a top surface of a piezoelectric layer with an oxygen plasma, and subsequently forming an etch mask containing photoresist on the oxidized top surface. The etch mask is conditioned with an oven bake followed by a UV bake. The piezoelectric layer is etched using a three step process: a first step includes a wet etch of an aqueous solution of about 5% NH4F, about 1.2% HF, and about 18% HCl, maintaining a ratio of the HCl to the HF of about 15.0, which removes a majority of the piezoelectric layer. A second step includes an agitated rinse. A third step includes a short etch in the aqueous solution of NH4F, HF, and HCl.