MEMS Resonator Sidewall Doping for Low Temperature Drift

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Solution Overview

Problem

Microelectromechanical systems (MEMS) devices face challenges in maintaining stable resonance frequency over a range of operating temperatures due to thermally induced changes in mechanical stiffness, leading to temperature drift, which affects their accuracy and reliability.

Innovation Solution

The method involves fabricating MEMS resonators from highly doped semiconductor materials like monocrystalline silicon with specific doping concentrations and angles relative to crystal planes, and employing multiple masking processes to create regions with varying dopant types and levels, allowing for controlled temperature-dependent characteristics and reduced sensitivity to thermal changes.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If MEMS resonators are fabricated from standard doping concentrations, then manufacturing is simpler, but temperature drift increases and resonance frequency stability deteriorates

Engineering Contradiction:

Improveresonance frequency stabilityVSAvoiddoping concentration control

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The patent applies parameter changes by modifying the doping concentration to exceed 10^19 atoms/cm³, which fundamentally alters the temperature coefficient of frequency. This high doping level shifts the resonator's thermal response characteristics, enabling resonance frequency stability across varying temperatures while maintaining manufacturability through established semiconductor doping techniques.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent implements local quality by creating laterally varying doping profiles within the resonator structure. Different regions of the resonator are doped at different concentrations, with guard ring regions having higher doping levels than the active resonating regions. This spatial variation in doping quality allows localized compensation of thermal effects while preserving the overall resonator performance.

Inventive Principle:

Principle #3Local quality

2Stability of the object's composition

If doping concentration is increased to reduce temperature drift, then temperature sensitivity decreases, but manufacturing complexity increases

Engineering Contradiction:

Improvetemperature coefficient of frequencyVSAvoiddoping profile structure

Core Design Contradiction:

Stability of the object's compositionVSDevice complexity

Solution Approach 1:

The patent applies segmentation by dividing the resonator into distinct doped regions, including active resonating regions and guard ring regions with different doping concentrations. This segmentation allows each region to be optimized independently for its specific function, with guard rings providing thermal stabilization and active regions maintaining resonator performance.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

The patent employs inversion by using counter-doping techniques where regions are doped with opposite polarity dopants to cancel out unwanted thermal effects. By introducing compensating dopant types in specific regions, the overall temperature coefficient is reduced while maintaining the desired resonator characteristics.

Inventive Principle:

Principle #13The other way round (Inversion)

3Reliability

If multiple masking processes are used to create varying dopant regions, then temperature compensation improves, but manufacturing steps increase

Engineering Contradiction:

Improvetemperature compensationVSAvoidfabrication process

Core Design Contradiction:

ReliabilityVSEase of manufacture

Solution Approach 1:

The patent applies preliminary action by performing doping operations at strategic points during the fabrication sequence. Masking layers are applied and removed at specific stages to enable selective doping of different regions before subsequent processing steps. This timing of doping actions ensures proper dopant distribution while minimizing disruptions to the overall fabrication flow.

Inventive Principle:

Principle #10Preliminary action

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 MEMS devices to maintain stable resonance frequency and minimize temperature-induced drift, optimizing them for applications as reference devices or temperature sensors by carefully managing the temperature coefficient of frequency and manufacturing tolerances.

Implementation Method 1

The semiconductor is highly doped, for example, monocrystalline silicon having doping concentrations greater than 10^15 cm^-3, and preferably, between 10^15 cm^-3 and 10^21 cm^-3

Methodology Applied

Scientific EffectDoping: Dopants

Implementation Method 2

Such doping concentrations may provide predetermined temperature coefficients of frequency of the moveable or resonating members of the structures

Methodology Applied

Scientific EffectTemperature coefficient of frequency:

Data Source

PatentUS11770112B1MEMS resonator with co-located temperature sensor

Publication Date: 2023.09.26 SITIME CORP

AI summary

A microelectromechanical system (MEMS) resonator includes a substrate having a substantially planar surface and a resonant member having sidewalls disposed in a nominally perpendicular orientation with respect to the planar surface. Impurity dopant is introduced via the sidewalls of the resonant member such that a non-uniform dopant concentration profile is established along axis extending between the sidewalls parallel to the substrate surface and exhibits a relative minimum concentration in a middle region of the axis.