Grey-Scale Photolithography for 3D Wafer Height Profiles
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Solution Overview
Problem
Conventional semiconductor fabrication techniques are limited in creating gradient height profiles and three-dimensional structures on silicon wafers, as they are primarily suited for planar structures, and there is a need for batch fabrication techniques that can achieve varying heights effectively.
Innovation Solution
The method involves using grey-scale photolithography to form three-dimensional structures by applying a photoresist layer on a substrate, employing a photomask with varying pattern density and pitch below the resolution of the photolithography system, followed by development and isotropic etching to transfer the three-dimensional profile to the substrate, such as silicon dioxide, to achieve structures like wedge-shaped layers that can be used in light sensing devices and Fabry-Perot interferometers.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If conventional photolithography techniques are used, then planar structures can be formed, but three-dimensional structures and gradient height profiles cannot be achieved
Solution Approach 1:
The photomask is designed with pre-calculated varying pattern densities that correspond to the desired three-dimensional profile. By preparing the mask with predetermined density gradients before the lithography process, the system can achieve complex 3D shapes that would otherwise require multiple fabrication steps
Solution Approach 2:
Different regions of the photomask are assigned different pattern densities to create local variations in light transmission. This allows each region of the photoresist to receive a customized exposure dose, resulting in different etch depths and ultimately forming gradient height profiles and three-dimensional structures
2Shape
If a photomask with pitch below resolution is used, then three-dimensional profiles can be formed, but the pattern density must vary continuously
Solution Approach 1:
The photomask design utilizes continuous variation of pattern density as a controllable parameter. By systematically adjusting the density of sub-resolution features across the mask surface, the invention transforms a complex 3D shaping problem into a manageable parameter optimization problem, where density gradients directly correspond to desired height profiles
3Manufacturing precision
If isotropic etching is used to transfer the profile, then the photoresist three-dimensional profile is accurately transferred, but the etch rate must match between photoresist and SiO2
Solution Approach 1:
The invention treats etch rate as a critical parameter that must be controlled and matched. By selecting and optimizing isotropic etchants with specific chemical compositions and concentrations, the process achieves matched etch rates between photoresist and SiO2, enabling faithful profile transfer without requiring complex rate control mechanisms
4Productivity
If grey-scale photolithography is used for batch fabrication, then gradient height profiles can be obtained, but the process complexity increases compared to conventional methods
Solution Approach 1:
The invention combines multiple functions into a single photomask and lithography step. The photomask simultaneously encodes both the lateral pattern definition and the vertical height profile information through varying pattern densities. This merging of pattern generation and height profiling into one step enables batch fabrication of three-dimensional structures without requiring sequential processing operations
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 the fabrication of three-dimensional features with improved optical properties, such as enhanced optical responsivity in photodiodes and reduced manufacturing complexity and costs, by creating gradient height profiles that average out interference effects and facilitate the deposition of optical filter layers.
Implementation Method 1
applying photoresist on a layer and using a photolithography system to expose the photoresist
Implementation Method 2
An isotropic etchant is used to etch the layer such that the three-dimensional profile of the photoresist is transferred to the layer
Data Source
AI summary
Forming a three-dimensional structure includes applying photoresist on a layer and using a photolithography system to expose the photoresist. The photolithography system includes a photomask having a pattern thereon, where the pattern provides varying pattern density across a surface of the photomask and has a pitch that is less than a resolution of the photolithography system. The method includes subsequently developing the photoresist such that photoresist remaining on the layer has a three-dimensional profile defined by the photomask. An isotropic etchant is used to etch the layer such that the three-dimensional profile of the photoresist is transferred to the layer.