Thermopile Pixel Release via Combined Anisotropic and Isotropic Etching
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Solution Overview
Problem
Existing manufacturing techniques for infrared imagers with thermopile sensing structures face challenges in producing smaller pixels due to imprecision in cavity shape and alignment, with methods like DRIE being too imprecise and wet anisotropic etching being unsuitable for integration with CMOS due to aggressive etchants, and isotropic etching struggling with deep cavity formation.
Innovation Solution
A method combining anisotropic and isotropic etching to precisely release thermopile pixels, involving anisotropic etching from the backside of the substrate and subsequent isotropic etching through membrane openings to form and extend cavities, using agents like xenon fluoride to ensure precise alignment and shape control while avoiding damage to CMOS structures.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Manufacturing precision
If DRIE is used for releasing the membrane, then the membrane can be released, but the positioning precision of the cavity under the membrane is insufficient (+/−5 μm)
Solution Approach 1:
The cavity formation process is segmented into two distinct steps: first, anisotropic etching from the backside creates a preliminary cavity with good vertical profile; second, isotropic etching from the frontside through membrane openings completes the cavity formation and achieves precise alignment. This segmentation allows each step to optimize for its specific function, resolving the precision limitation of single-step DRIE.
Solution Approach 2:
The conventional approach of etching only from the backside is inverted by adding a frontside etching step through membrane openings. This reverse approach allows the cavity to be formed from both directions, enabling precise alignment with the membrane position while maintaining the manufacturing capability of anisotropic etching for the main cavity volume.
2Manufacturing precision
If wet anisotropic etching is used, then good control of cavity position and shape is achieved, but aggressive etchants attack other CMOS structures on the wafer
Solution Approach 1:
The membrane openings serve as intermediaries that guide the isotropic etchant to the substrate only in the desired cavity regions. This intermediary structure allows precise control of cavity position and shape while protecting other CMOS structures on the wafer from exposure to aggressive etchants, as the etchant can only access the substrate through the controlled openings in the membrane.
Solution Approach 2:
The etching process applies different qualities to different locations: isotropic etching is applied locally only where membrane openings expose the substrate, while the rest of the wafer remains protected. This local application of etching ensures precise cavity formation without causing harmful attacks on other CMOS structures throughout the wafer.
3Reliability
If isotropic etching is used, then selectivity and protection of other structures is achieved, but deep cavities cannot be formed with square/rectangular shapes
Solution Approach 1:
The cavity formation is segmented into two functional parts: the main deep cavity volume is created by anisotropic etching from the backside, which provides the necessary depth and vertical profile; the cavity shape and precise boundaries are defined by isotropic etching from the frontside through membrane openings, which provides selectivity and protection. This segmentation allows each etching method to contribute its strengths to the final cavity structure.
Solution Approach 2:
The cavity formation transitions from single-dimensional backside etching to two-dimensional etching by adding frontside etching through membrane openings. This dimensional change allows the isotropic etchant to access the substrate from the front, enabling precise shape control and square/rectangular cavity formation while maintaining the depth capability of backside anisotropic etching.
4Productivity
If previous methods are used for membrane release, then the process is simpler, but the release speed is slow and C4F8 consumption is high
Solution Approach 1:
Anisotropic etching from the backside is performed as a preliminary action to create a cavity that releases the membrane more efficiently. This preliminary cavity formation reduces the amount of material that needs to be removed in subsequent steps, thereby increasing overall release speed and reducing C4F8 consumption compared to using isotropic etching alone for the entire 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 achieves higher precision and faster membrane release, reducing C4F8 consumption and enabling the production of smaller thermopile pixels with improved cavity alignment and shape, addressing the limitations of previous techniques.
Implementation Method 1
anisotropic etching of a portion of the second layer back surface beneath a central part of the first layer
Implementation Method 2
performing a first isotropic etching through a third layer opening of a first portion of the second layer front surface through a first layer first opening
Implementation Method 3
Dry isotropic etchants on the other hand, for example, xenon fluoride (XeF2) is highly selective and will not attack other structures present on the wafer
Data Source
AI summary
A method for manufacturing an imaging device is presented. The method starts with providing a wafer having a membrane with an opening bonded to a substrate. A photoresist layer is deposited over the membrane and wafer surface. A portion of the substrate back surface under a central part of the membrane is etched anisotropicly. A first region of the photoresist layer is removed, exposing an opening in the membrane, so that a first isotropic etching of the substrate is performed through the membrane opening. A second region of the photoresist layer is stripped, exposing a second membrane opening, providing access for a second isotropic etching of the substrate through the first and/or second membrane opening.