Semiconductor Plasma Etching and Corrosion Testing: A Complete Technical Guide

Semiconductor plasma etching and corrosion testing is essential for validating dry patterning processes, qualifying plasma-resistant chamber materials, and preventing plasma-induced degradation of metal interconnects and bond pads. This guide explains the typical test samples, key measurement parameters, evaluation methods, and international standards used by accredited third-party laboratories and in-house fab laboratories. Read on to learn how etch rate, uniformity, selectivity, and corrosion behavior are characterized and controlled in modern semiconductor manufacturing.

Technical Overview

Plasma etching, often described as plasma corrosion in semiconductor process terminology, is a dry removal technique in which an ionized gas mixture generates reactive radicals and energetic ions that remove material from a wafer surface through a combination of chemical reaction and physical ion bombardment. Because the ion flux is directional, plasma etching achieves the anisotropic, high-fidelity pattern transfer required for sub-micron features that wet etching cannot provide.

In practice, the term covers two related phenomena that both require systematic testing. The first is controlled material removal during device fabrication, where parameters such as etch rate, uniformity, selectivity, and sidewall profile must be quantified and controlled. The second is unintended degradation, known as plasma-induced corrosion, in which chlorine- or fluorine-based chemistries leave reactive residues on exposed aluminum bond pads, copper interconnects, or mixed-metal structures that subsequently corrode when exposed to ambient moisture, often through galvanic mechanisms.

A complete plasma etching and corrosion testing program therefore spans three areas: process characterization of the etch step itself, evaluation of the plasma erosion resistance of chamber materials and consumable components, and reliability assessment of finished wafers and packaged devices that have undergone plasma processing.

Testing Samples

Blanket and Patterned Wafers

Unpatterned blanket films of silicon dioxide, silicon nitride, polycrystalline silicon, low-k dielectrics, and photoresist are used to determine intrinsic etch rates. Patterned wafers with trenches, vias, and gate structures are then used to evaluate realistic etch performance, including profile control, critical dimension (CD) bias, microloading, and aspect-ratio-dependent etching.

Metal Films and Interconnect Structures

Aluminum metallization with TiN barrier layers, damascene copper, tungsten plugs, and cobalt or ruthenium liners are tested for etch behavior and for susceptibility to post-etch corrosion. Bond pads on product wafers are a particular focus because residual chlorides from Cl2/BCl3-based etch chemistries can initiate pitting and galvanic corrosion in humid environments.

Chamber Hardware and Material Coupons

Etch chamber components are continuously eroded by the plasma, so candidate materials are qualified before use. Typical coupons include quartz, sintered alumina, silicon carbide, silicon, yttria and other oxide spray coatings, anodized aluminum, and elastomeric seals. Testing measures erosion rate, weight loss, surface roughening, and particle generation under representative fluorine, chlorine, and oxygen plasma chemistries.

Packaged Devices and Test Vehicles

Finished integrated circuits, MEMS devices, and advanced packaging structures such as through-silicon vias and redistribution layers are tested after plasma processing to confirm that no latent corrosion sites, residues, or moisture-sensitive defects were introduced during fabrication.

Testing Items

Core measurement items for plasma etch process characterization include the following:

• Etol rate and etch depth for each film stack and chemistry
• Within-wafer, wafer-to-wafer, and lot-to-lot etch uniformity
• Selectivity to the masking layer and to the underlying film
• Anisotropy, sidewall angle, bowing, footing, and notching in cross-section
• Critical dimension uniformity and CD bias after etch and after resist strip
• Aspect-ratio-dependent etching and microloading effects
• Post-etch surface roughness and trench bottom morphology
• Residual chlorine, fluorine, and polymer residue levels on exposed surfaces

For plasma-induced corrosion and material erosion evaluation, typical test items include:

• Corrosion incubation time and time-to-failure under humidity exposure
• Corrosion rate, pit density, and pit depth on metallization
• Weight loss and dimensional change of material coupons after plasma exposure
• Surface roughness evolution measured before and after plasma erosion
• Electrochemical parameters such as open circuit potential and polarization resistance
• Particle and contamination release from eroded chamber components
• Ionic contamination levels extracted from device surfaces and packages

Testing Methods

Plasma Exposure and Etch Characterization

Samples are processed in reactive ion etching or inductively coupled plasma tools using documented recipes with defined gas chemistries (for example SF6, CF4, CHF3, Cl2, BCl3, or HBr mixtures), radio-frequency power, pressure, and electrode temperature. Deep reactive ion etching with a Bosch-type process cycle is used for high-aspect-ratio silicon structures. Etch depth is measured step-wise using predefined exposure times to establish etch rate curves.

Metrology and Surface Analysis

Etch depth and uniformity are measured by stylus profilometry, spectroscopic ellipsometry, and atomic force microscopy in accordance with surface texture standards such as ISO 4287 and ISO 25178. Cross-sectional scanning electron microscopy and transmission electron microscopy characterize sidewall profiles and CD values, while CD-SEM metrology maps feature uniformity across the wafer. Chemical residues and post-etch surface states are analyzed by X-ray photoelectron spectroscopy, energy-dispersive spectroscopy, and time-of-flight secondary ion mass spectrometry. Trace metal contamination can be quantified by total reflection X-ray fluorescence or inductively coupled plasma mass spectrometry.

Corrosion Evaluation Methods

Material coupons exposed to plasma are cleaned and weighed following practices aligned with ASTM G1, and gravimetric corrosion rates are calculated per ASTM G31 where immersion-based comparisons are needed. Electrochemical testing, including potentiodynamic polarization and electrochemical impedance spectroscopy, quantifies the corrosion behavior of metal films and galvanic couples. Surface inspection with optical microscopy and SEM, supplemented by pit statistics, documents corrosion morphology and severity.

Environmental Reliability Testing

Post-plasma corrosion susceptibility is verified through accelerated environmental tests, including steady-state temperature-humidity bias testing, highly accelerated stress testing, and damp heat cycling aligned with IEC 60068-2-78 and the IEC 60749 series for semiconductor devices. Salt mist exposure aligned with ISO 9227 or ASTM B117 may be applied to evaluate harsh-environment robustness of packaged parts. Acceptance criteria typically combine electrical parametric measurements, leakage checks, and visual failure analysis after exposure.

Applications

Plasma etching and corrosion testing supports every major stage of semiconductor product and equipment development:

• Process development and release of etch recipes for logic, memory, power, and image sensor technologies
• Qualification of new chamber materials, coatings, and consumable parts for etch equipment
• Root cause analysis of bond pad corrosion, interconnect pitting, and yield loss after plasma steps
• Reliability qualification of devices prior to mass production, including moisture sensitivity assessment
• Vendor qualification of wafers, metal targets, coatings, and sealing materials
• MEMS deep silicon etch development and release process optimization
• Advanced packaging development, including through-silicon via reveal and redistribution layer patterning

Accredited third-party laboratories are frequently engaged for independent verification, particularly when customers require data traceable to international standards or when failure analysis demands specialized analytical instrumentation beyond in-house capability.

FAQ

What is the difference between plasma etching and plasma corrosion?

Plasma etching refers to the intentional, controlled removal of material to form device patterns. Plasma corrosion refers to unintended material degradation, either the erosion of chamber components by the plasma itself or the post-process corrosion of metallization caused by reactive residues left on the wafer after etching.

Why do aluminum bond pads corrode after chlorine-based plasma etching?

Chlorine-based chemistries leave chlorinated residues on aluminum surfaces. When the wafer is later exposed to ambient moisture, these residues hydrolyze to form hydrochloric acid, which attacks the aluminum and can produce severe pitting, especially where dissimilar metals create a galvanic cell. Post-etch passivation steps, deionized water rinses, and controlled storage environments are used to mitigate this risk.

How is etch uniformity measured?

Etch depth is mapped at multiple sites across the wafer, typically center, mid-radius, and edge locations, using profilometry, ellipsometry, or AFM. The uniformity is reported as a percentage range or standard deviation of etch depth across all measured points, and it is tracked across wafers and lots to confirm process stability.

What does selectivity mean in plasma etch testing?

Selectivity is the ratio of the etch rate of the target film to the etch rate of the mask or underlying layer. High selectivity to the mask preserves pattern fidelity, while high selectivity to the underlayer protects devices during overetch. It is measured by etching stacked film samples and comparing the removal rates of each layer.

Which standards are commonly referenced?

Surface texture measurements reference ISO 4287 and ISO 25178, corrosion preparation and rate determination reference ASTM G1 and ASTM G31, salt mist exposure references ISO 9227 and ASTM B117, and environmental testing of semiconductor devices references the IEC 60068 and IEC 60749 series. National standards such as the GB/T 2423 environmental testing series are also widely applied.

How can plasma-induced corrosion be prevented?

Effective countermeasures include adding in-situ post-etch passivation steps using fluorine or hydrogen chemistries, performing prompt deionized water or solvent rinses after chlorine etching, controlling cleanroom humidity and wafer queue times, using corrosion-resistant chamber coatings, and verifying robustness through accelerated humidity and bias stress testing before production release.