MEMS Resonator Doping and Beam Layout for Temperature-Stable Frequency

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Solution Overview

Problem

Microelectromechanical systems (MEMS) resonant elements exhibit significant temperature sensitivity, leading to variations in resonant frequency due to thermal changes in Young's modulus, which complicates maintaining stable frequency output across temperature ranges.

Innovation Solution

The implementation of MEMS resonant elements with carefully designed doping concentrations, orientations, and structural configurations, such as rotated or tapered beams, to minimize temperature sensitivity, combined with integrated circuits for temperature measurement and frequency adjustment, allows for stable frequency maintenance across temperature ranges.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Reliability

If MEMS resonant elements are used to provide frequency output, then frequency generation is achieved, but temperature sensitivity causes frequency variation across temperature ranges

Engineering Contradiction:

Improvefrequency stabilityVSAvoidtemperature sensitivity

Core Design Contradiction:

ReliabilityVSObject-affected harmful factors

Solution Approach 1:

The patent changes physical parameters of the resonant element including doping concentration (10^19 to 10^21 atoms/cm³), crystal orientation angles, and geometric dimensions to minimize temperature coefficients of frequency. By adjusting these parameters, the resonant element achieves reduced temperature sensitivity while maintaining reliable frequency generation across temperature ranges.

Inventive Principle:

Principle #35Parameter changes

2Reliability

If doping concentration is increased to 10^19 to 10^21 atoms/cm³ to reduce temperature sensitivity, then frequency stability improves, but manufacturing precision requirements increase

Engineering Contradiction:

Improvetemperature sensitivity reductionVSAvoiddoping concentration control

Core Design Contradiction:

ReliabilityVSManufacturing precision

Solution Approach 1:

The patent specifies a doping concentration range of 10^19 to 10^21 atoms/cm³ that achieves reduced temperature coefficients of frequency. This parameter change balances manufacturing feasibility with performance requirements, allowing sufficient tolerance in doping control while still achieving the desired temperature stability.

Inventive Principle:

Principle #35Parameter changes

Solution Approach 2:

The patent uses heavily doped semiconductor materials (such as silicon with phosphorus or boron doping) that exhibit reduced temperature coefficients of elasticity. This material selection combines specific doping levels with crystal orientation to create a composite structure that inherently resists temperature-induced frequency drift.

Inventive Principle:

Principle #40Composite materials

3Reliability

If multiple process steps are used to minimize temperature coefficients, then frequency stability across temperature improves, but device complexity increases

Engineering Contradiction:

Improvefrequency stabilityVSAvoidmanufacturing process steps

Core Design Contradiction:

ReliabilityVSDevice complexity

Solution Approach 1:

The patent divides the resonant element into multiple functionally distinct layers including a substrate layer, a sacrificial layer, and a resonant structure layer. Each layer is fabricated with specific doping concentrations and orientations to contribute differently to the overall temperature compensation, allowing complex temperature stability to be achieved through modular layer design.

Inventive Principle:

Principle #1Segmentation

Solution Approach 2:

Different regions of the resonant element are doped with different concentrations and orientations. The substrate may have one doping profile while the resonant structure has another, and specific portions of the structure are oriented at different crystallographic angles. This local differentiation allows each region to contribute optimally to temperature compensation.

Inventive Principle:

Principle #3Local quality

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 results in MEMS resonant elements with reduced temperature sensitivity, providing a stable frequency output and improved resistance to fabrication process variations, enabling more precise temperature control and enhanced performance in temperature-varying environments.

Implementation Method 1

thermally induced changes to the Young's modulus (or other variables) tend to change the mechanical stiffness of the structure

Methodology Applied

Scientific EffectThermal expansion: Thermal Expansion

Implementation Method 2

Resonant elements exhibit variation in resonant frequency from, among other things, their temperature

Methodology Applied

Scientific EffectTemperature sensitivity:

Data Source

PatentUS11807518B1Multi-die integrated circuit package

Publication Date: 2023.11.07 SITIME CORP

AI summary

The present inventions, in one aspect, are directed to micromachined resonator comprising: a first resonant structure extending along a first axis, wherein the first axis is different from a crystal axis of silicon, a second resonant structure extending along a second axis, wherein the second axis is different from the first axis and the crystal axis of silicon and wherein the first resonant structure is coupled to the second resonant structure, and wherein the first and second resonant structures are comprised of silicon (for example, substantially monocrystalline) and include an impurity dopant (for example, phosphorus) having a concentrations which is greater than 1019 cm-3, and preferably between 1019 cm-3 and 1021 cm-3.