High Aspect Ratio Capacitive Transducer Fabrication

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Solution Overview

Problem

Existing methods for fabricating high aspect-ratio capacitive transducers are limited by device height due to Deep Reactive Ion Etching (DRIE) limitations, resulting in restricted gap heights and sensing/actuation areas, which are not suitable for 3-D, small-footprint, thick devices and integration with CMOS circuits.

Innovation Solution

A method involving forming bottom and top trenches in substrates to create a channel around a transducer structure, allowing for high aspect ratio capacitive transducers with independently defined gap dimensions, enabling increased device height and sensitivity through deep reactive ion etching and bonding techniques.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Manufacturing precision

If Deep Reactive Ion Etching (DRIE) is used to fabricate capacitive gaps, then gap precision can be achieved, but device height is limited due to etching lag

Engineering Contradiction:

Improvegap dimension precisionVSAvoiddevice height

Core Design Contradiction:

Manufacturing precisionVSLength of stationary object

Solution Approach 1:

The device is segmented into two separate substrates (first substrate containing the proof mass and spring, second substrate containing the electrode) that are bonded together. This allows the capacitive gap to be formed by bonding interfaces rather than etching through a single thick substrate, enabling greater device height while maintaining precise gap control through the bonding process.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The fabrication approach transitions from vertical etching through a single substrate to horizontal bonding between two substrates. By moving the gap formation process to a different dimensional approach (bonding interface rather than etched through-thickness), the patent overcomes the height limitation imposed by vertical DRIE etching lag.

Inventive Principle:

Principle #17Another dimension (Dimensionality change)

2Reliability

If device height is increased to achieve high aspect ratio, then sensitivity improves, but gap height is compromised due to DRIE limitations

Engineering Contradiction:

Improvetransducer sensitivityVSAvoidgap height

Core Design Contradiction:

ReliabilityVSLength of stationary object

Solution Approach 1:

By dividing the device into two bonded substrates, the proof mass can be positioned at greater heights while the gap is precisely controlled at the bonding interface. This segmentation allows independent optimization of device height for sensitivity and gap height for transduction precision.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The bonding interface acts as an intermediary that precisely defines the gap dimension. Rather than relying on DRIE etching to create the gap, the bonding process itself establishes a controlled interface that separates the proof mass from the electrode, enabling precise gap control independent of device height.

Inventive Principle:

Principle #24Intermediary (Mediator)

3Manufacturing precision

If sacrificial material is used to define high aspect ratio gaps, then gap precision is achieved, but total chip area increases to ensure structural integrity

Engineering Contradiction:

Improvegap definition precisionVSAvoidtotal chip area

Core Design Contradiction:

Manufacturing precisionVSArea of stationary object

Solution Approach 1:

The patent extracts the gap definition function from the bulk substrate etching process and relocates it to the bonding interface between two substrates. This eliminates the need for sacrificial materials and large chip areas, as the gap is naturally defined by the bonding contact between substrates rather than requiring additional structural support.

Inventive Principle:

Principle #2Taking out (Extraction)

4Length of stationary object

If two-sided surface micromachining and bulk micromachining are combined, then full wafer thickness is achieved, but process complexity increases

Engineering Contradiction:

Improvewafer thicknessVSAvoidfabrication process complexity

Core Design Contradiction:

Length of stationary objectVSDevice complexity

Solution Approach 1:

The device is fabricated as two separate substrates using independent micromachining processes, then bonded together. This segmentation allows each substrate to be optimized independently and simplifies the overall process by avoiding the complexity of coordinating two-sided processing through a single thick substrate.

Inventive Principle:

Principle #1Segmentation

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 transducers with higher aspect ratios and improved sensitivity by allowing greater device height and flexible gap definitions, facilitating integration with CMOS circuits and enhancing capacitive transduction performance.

Implementation Method 1

deep reactive ion etching

Methodology Applied

Scientific EffectDeep Reactive Ion Etching:

Implementation Method 2

bonding techniques

Methodology Applied

Scientific EffectBonding:

Data Source

PatentUS10524059B2Capacitive-based transducer with high aspect ratio

Publication Date: 2019.12.31 THE RGT UNIV OF MICHIGAN

AI summary

Techniques are presented for fabricating transducers and other types of microstructures having high aspect ratios. To achieve high aspect ratios, wafers are etched from both sides for example using deep reactive ion etching. The three-dimensional structure is designed to have an overall footprint less than four hundred micrometers with a thickness on the order of 0.5-2 millimeters as compared to conventional planar devices.