Bilayer Resist Structure for Minimizing Etch Undercut
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Solution Overview
Problem
Existing substrate etching methods face limitations in achieving clean metal liftoff with minimal etch undercuts, particularly when trying to deposit metal layers thicker than the etch depth of the underlying substrate, and struggle to provide sufficient step coverage on non-planar surfaces while maintaining pattern resolution.
Innovation Solution
A bilayer resist structure comprising a thick top photoresist sensitive to near UV or violet light and a thin bottom resist sensitive to deep UV light is used, where the bottom resist layer forms an overhang resist edge profile after etching, allowing for clean metal liftoff and minimizing etch undercuts by using a dielectric layer for improved adhesion and reducing intermixing between resist layers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If a single layer of positive photoresist is used to create an overhang profile for metal liftoff, then metal can be deposited on the substrate, but etch undercuts are created around the photoresist opening that connect metal on the substrate to metal on sidewalls when metal thickness exceeds etch depth
Solution Approach 1:
The patent divides the single photoresist layer into two separate resist layers: a bottom resist layer that defines the pattern and an overhang resist layer that creates the overhang profile. This segmentation allows each layer to perform its specific function independently, enabling clean metal liftoff while preventing etch undercut connections.
Solution Approach 2:
The patent introduces a vertical dimension differentiation by creating an overhang structure where the top resist layer extends beyond the bottom resist layer edges. This dimensional change in the resist profile allows the metal to be deposited on the substrate without connecting to sidewalls, solving the etch undercut problem.
2Ease of operation
If negative photoresist is used to provide natural negative slope for overhang and clean metal liftoff, then metal liftoff is facilitated, but resist swelling around the development pattern occurs making dimension control difficult
Solution Approach 1:
The patent changes the material parameters by selecting a positive photoresist composition and processing conditions that prevent swelling. By using a two-layer system with controlled development, the patent achieves the desired overhang profile without the dimensional instability associated with negative photoresist swelling.
3Quantity of substance
If isotropic etching is used to remove exposed substrate material, then complete removal is achieved, but etch undercuts are created around the photoresist opening
Solution Approach 1:
The patent segments the etching function by using the two-layer resist structure where the overhang resist layer protects the sidewalls during etching. This allows isotropic etching to completely remove substrate material while the overhang structure prevents undercut formation that would connect metal regions.
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 enables clean metal liftoff with minimized etch undercuts and adequate step coverage on non-planar surfaces, allowing for thicker metal deposition without connecting to sidewalls, and effectively seals etch undercut regions from environmental attacks.
Implementation Method 1
a thick top photoresist sensitive to a second range of energy such as for example near UV or violet light
Implementation Method 2
a thin bottom resist sensitive to a first range of energy such as for example deep UV light
Implementation Method 3
subsequent deposition of metal by sputter deposition or evaporation method
Implementation Method 4
subsequent deposition of metal by sputter deposition or evaporation method
Data Source
AI summary
A method of minimizing etch undercut and providing clean metal liftoff in subsequent metal deposition is provided. In one embodiment a bilayer resist mask is employed and used for etching of underlying substrate material and subsequent metal liftoff. In one embodiment, the top layer resist such as positive photoresist which is sensitive to selected range of energy, such as near UV or violet light, is first patterned by standard photolithography techniques and resist development in a first developer to expose portion of a bottom resist layer which is sensitive to a different selected range of energy, such as deep UV light. The exposed portion of the bottom layer resist is then removed by anisotropic etching such as oxygen reactive ion etching using the top layer resist as the etch mask to expose portion of the underlying substrate. This minimizes the undercut in the bottom resist around the top photoresist opening. The resultant patterned bilayer resist stack is then used as the etch mask for the subsequent etching of the exposed portion of the underlying substrate material. Because there is no undercut in the bottom resist layer, the etch undercut in the substrate material is also minimized relative to the edges of the top photoresist opening.