Self-Controlled Cleaving Method for Silicon DRIE Process Cross-Section Characterization

At a Glance

Metadata	Details
Publication Date	2021-05-08
Journal	Micromachines
Authors	Dmitry A. Baklykov, Mihail Andronic, Olga S. Sorokina, Sergey S. Avdeev, Kirill A. Buzaverov
Institutions	Institute of Theoretical and Applied Electrodynamics, Bauman Moscow State Technical University
Citations	16
Analysis	Full AI Review Included

Technical Documentation & Analysis: Self-Controlled Cleaving for Silicon DRIE

This document analyzes the research findings regarding optimized cross-section characterization for Deep Reactive Ion Etching (DRIE) of silicon, focusing on how 6CCVD’s advanced MPCVD diamond materials (SCD, PCD, BDD) can enhance and scale this critical MEMS fabrication process.

Executive Summary

The research addresses the critical challenge of obtaining accurate cross-section metrology for high-aspect ratio (AR) silicon microstructures fabricated via the Bosch DRIE process.

Problem Identified: Standard cleaving methods (diamond scribing) cause significant damage and metrology errors, especially in high-AR structures (AR > 10).
Core Achievement: Development of a highly controllable, defect-free cross-section cleaving method utilizing etched auxiliary lines as stress concentrators.
Key Finding (Metrology Error): Transverse crossing auxiliary lines cause severe “silicon build-up” defects (polymerization) at intersections, leading to profile narrowing (up to 85% deviation) and inaccurate etching rate measurements.
Optimal Solution: Dashed auxiliary lines with sharp end-forming stress concentrators provide the best cleaving control without intersecting the target microstructures, preserving the true etching profile.
Process Performance: The experiments successfully achieved high AR structures, reaching a maximum aspect ratio greater than 50.
6CCVD Value Proposition: Diamond hard masks (SCD/PCD) offer vastly superior selectivity (>10,000:1) compared to the SiO₂ mask (243:1) used in this study, enabling the fabrication of even higher AR structures and deeper trenches required for advanced MEMS and optoelectronic devices.

Technical Specifications

The following hard data points were extracted from the experimental results of the Bosch DRIE process characterization:

Parameter	Value	Unit	Context
Maximum Aspect Ratio Achieved	>50	N/A	Observed for 50 µm target lines.
Minimum Aspect Ratio Tested	>10	N/A	Minimum AR for all tested structures.
Target Line Width (W) Range	2 to 50	µm	Range of microstructures analyzed.
Hard Mask Material Used	Thermal SiO₂	N/A	Protective layer for silicon etching.
Hard Mask Thickness	4	µm	Thickness of the SiO₂ layer.
Selectivity (Si:SiO₂) Max	243	N/A	Highest selectivity achieved (Reference, W=50 µm, W:D=1:1).
Etch Rate Max (Reference)	0.527	µm/cycle	Dashed auxiliary line reference (W=50 µm, W:D=1:1).
Profile Angle (A) Range	88.41 to 90.45	°	Measured profile angle variation.
DRIE Process Temperature	5	°C	Operating temperature for deep anisotropic etching.
ICP Power Range	1200 - 1500	W	Inductively Coupled Plasma power.
Cleaving Line Width (S) Range	5, 20, 50, 100	µm	Widths of transverse auxiliary lines tested.
Profile Narrowing Max	85	%	Observed for sub-20 µm trenches with transverse crossing lines.

Key Methodologies

The experiment focused on comparing the influence of two auxiliary cleaving line methods on the resulting Bosch process metrology.

Substrate Preparation:
- Used 25 x 25 mm² substrates diced from 100 mm p-type silicon wafers.
- Crystal Orientation: <100>.
- Hard Mask: 4 µm thick Thermal SiO₂ deposited on the silicon.
Patterning and Lithography:
- Photoresist: 4 µm thick Megaposit SPR220.
- Pattern Transfer: µPG101 laser lithography system used to define target structures (2 to 50 µm width, 1000 µm length) and auxiliary lines.
Mask Etching:
- Reactive Ion Etching (RIE) used CHF₃/Ar gases to transfer the pattern into the SiO₂ hard mask.
Deep Reactive Ion Etching (DRIE) - Bosch Process:
- Three-stage process: Passivation, Breakthrough, and Etching.
- Gases: C₄F₈ (Passivation), SF₆ (Etching), O₂ (added for polymer removal).
- Key Parameters: ICP Power (1200-1500 W), RF Power (5-50 W), Pressure (20-40 mTorr), Temperature (5 °C).
Cleaving Methods Tested:
- Reference Method: Etched dashed auxiliary lines (S = 20 µm) with sharp end-forming stress concentrators (providing up to 5 times higher maximum stress).
- Comparative Method: Etched transverse crossing auxiliary lines (S = 5, 50, 100 µm width), intersecting the target microstructures.
Characterization:
- Optical Microscopy and Field Emission Scanning Electron Microscopy (FE-SEM) used to measure Critical Dimensions (CDs) and cross-section profiles.

6CCVD Solutions & Capabilities

The research highlights the critical need for robust hard masks and precise material processing to achieve reliable metrology in high-AR DRIE. 6CCVD’s MPCVD diamond materials are uniquely positioned to meet and exceed the requirements of this advanced MEMS fabrication research.

Applicable Materials

To replicate or extend this research to higher aspect ratios and more complex topologies, 6CCVD recommends the following materials:

Research Requirement	6CCVD Material Solution	Technical Advantage
Ultra-High Selectivity Hard Mask	Optical Grade Single Crystal Diamond (SCD)	Diamond offers selectivity >10,000:1 in the Bosch process, significantly surpassing the 243:1 achieved with SiO₂. This enables etching trenches >500 µm deep with minimal mask erosion.
Large-Area, High-Uniformity Masking	High Purity Polycrystalline Diamond (PCD) Plates	Available in custom dimensions up to 125 mm diameter, PCD provides exceptional uniformity and robustness for scaling research from 25x25 mm² samples to full production wafers.
Integrated Functional Devices	Boron-Doped Diamond (BDD) Substrates	For the integrated optoelectronic and energy harvesting devices mentioned in the abstract, BDD provides a conductive, chemically inert, and mechanically superior substrate compatible with deep etching.

Customization Potential

The success of the defect-free cleaving method relies entirely on the precise geometry of the auxiliary lines and stress concentrators. 6CCVD offers specialized services to ensure perfect integration of these features:

Custom Dimensions: We supply SCD and PCD plates in custom thicknesses (0.1 µm to 500 µm) and substrate sizes (up to 10 mm thick) to match specific DRIE chamber requirements.
Ultra-Low Roughness Polishing: Our SCD materials are polished to Ra < 1 nm, and inch-size PCD to Ra < 5 nm. This ultra-smooth surface is critical for high-resolution lithography, ensuring the sharp, defect-free transfer of the complex dashed auxiliary line patterns and stress concentrators required for controlled cleaving.
Advanced Patterning & Shaping: 6CCVD provides in-house laser cutting and shaping services to define complex geometries, including the sharp end-forming stress concentrators, directly into the diamond hard mask material with high precision.
Metalization Services: Should the research extend to integrated devices requiring electrical contacts or bonding layers (e.g., for TSVs), we offer internal metalization capabilities including Au, Pt, Pd, Ti, W, and Cu deposition.

Engineering Support

The observed “build-up” defects caused by intersecting auxiliary lines demonstrate the complex interplay between topology and plasma chemistry. 6CCVD’s in-house PhD engineering team specializes in material selection and process compatibility for extreme environments. We can assist researchers in optimizing diamond hard mask thickness and material grade for similar Bosch DRIE Process Characterization projects, ensuring that material properties do not introduce secondary metrology errors.

Call to Action: For custom specifications or material consultation, visit 6ccvd.com or contact our engineering team directly. We ship globally (DDU default, DDP available) to support your most demanding MEMS research needs.

View Original Abstract

Advanced microsystems widely used in integrated optoelectronic devices, energy harvesting components, and microfluidic lab-on-chips require high-aspect silicon microstructures with a precisely controlled profile. Such microstructures can be fabricated using the Bosch process, which is a key process for the mass production of micro-electro-mechanical systems (MEMS) devices. One can measure the etching profile at a cross-section to characterize the Bosch process quality by cleaving the substrate into two pieces. However, the cleaving process of several neighboring deeply etched microstructures is a very challenging and uncontrollable task. The cleaving method affects both the cleaving efficiency and the metrology quality of the resulting etched microstructures. The standard cleaving technique using a diamond scriber does not solve this issue. Herein, we suggest a highly controllable cross-section cleaving method, which minimizes the effect on the resulting deep etching profile. We experimentally compare two cleaving methods based on various auxiliary microstructures: (1) etched transverse auxiliary lines of various widths (from 5 to 100 μm) and positions; and (2) etched dashed auxiliary lines. The interplay between the auxiliary lines and the etching process is analyzed for dense periodic and isolated trenches sized from 2 to 50 μm with an aspect ratio of more than 10. We experimentally showed that an incorrect choice of auxiliary line parameters leads to silicon “build-up” defects at target microstructures intersections, which significantly affects the cross-section profile metrology. Finally, we suggest a highly controllable defect-free cross-section cleaving method utilizing dashed auxiliary lines with the stress concentrators.

Tech Support

Original Source

DOI: https://doi.org/10.3390/mi12050534

References

2003 - Critical tasks in high aspect ratio silicon dry etching for microelectromechanical systems [Crossref]
2019 - MEMS at Bosch-Si plasma etch success story, history, applications, and products [Crossref]
2018 - DREM2: A facile fabrication strategy for freestanding three dimensional silicon micro-and nanostructures by a modified Bosch etch process [Crossref]
2014 - Deep silicon etching: Current capabilities and future directions
2005 - Advanced etching of silicon based on deep reactive ion etching for silicon high aspect ratio microstructures and three-dimensional micro-and nanostructures [Crossref]