Low-Pressure Plasma Chlorine Corrosion Testing: Principles, Methods, and Applications

Low-pressure plasma chlorine corrosion testing is a specialized reliability evaluation technique widely used in the semiconductor and microelectronics industries to assess the corrosion resistance of devices and materials exposed to chlorine-based plasma environments. This test simulates the aggressive conditions of reactive ion etching (RIE) processes, where chlorine radicals attack exposed metallization and form hygroscopic corrosion products. This article provides a comprehensive overview of the testing principles, sample types, test items, methods, and practical applications of low-pressure plasma chlorine corrosion testing.

Technical Overview

Low-pressure plasma chlorine corrosion testing is an accelerated reliability evaluation method designed to reproduce the chlorine-containing plasma conditions that occur during dry etch steps in semiconductor fabrication, such as reactive ion etching, inductively coupled plasma (ICP) etching, and downstream plasma processing. During these processes, chlorine-based chemistries including Cl2, BCl3, and CCl4 generate highly reactive chlorine radicals that can attack exposed metal surfaces, most notably aluminum metallization, bond pads, and interconnect structures.

The underlying corrosion mechanism involves the reaction of chlorine radicals with aluminum to form aluminum chloride (AlCl3), a hygroscopic and volatile compound. When the treated sample is subsequently exposed to ambient air or moisture, AlCl3 undergoes hydrolysis to produce aluminum hydroxide and hydrochloric acid. The released hydrochloric acid further attacks the metal surface, initiating an autocatalytic corrosion cycle that can propagate rapidly along interconnect lines, bond pads, and layered metal stacks. This mechanism is a leading cause of post-etch corrosion defects, latent field failures, and yield loss in wafer fabrication and advanced packaging.

By subjecting test samples to a controlled low-pressure chlorine plasma followed by controlled humidity exposure, the test accelerates these corrosion mechanisms in a laboratory setting, enabling manufacturers to qualify processes, compare material stack robustness, and validate post-etch cleaning or passivation steps before mass production.

Testing Samples

Semiconductor Wafers and Test Structures

Patterned wafers, comb and serpentine test structures, Kelvin resistance monitors, and dedicated corrosion test vehicles are the most common sample types. These structures allow quantitative assessment of chloride-induced corrosion through electrical parametric shifts and visual defect inspection.

Exposed Metallization and Bond Pads

Aluminum, aluminum-copper alloys, aluminum-silicon alloys, copper metallization, and multi-layer stacks such as Ti/Al/TiN or Ni/Au bond pads are tested to evaluate their susceptibility to chlorine plasma attack after etch or cleaning steps.

Packaged Devices and Components

Decapsulated integrated circuits, discrete devices, power semiconductors, and LED chips may be evaluated to determine whether residual chlorine species from packaging or etch processes pose a corrosion risk to exposed metal regions.

Metal Films, Coatings, and MEMS Structures

Blanket metal films, barrier layers, anti-corrosion coatings, released MEMS structures, and precision micro-machined components can be tested to characterize material compatibility with chlorine plasma environments.

Testing Items

• Corrosion morphology and defect classification of metal lines, pads, and vias
• Chloride residue detection and quantification on exposed surfaces
• Electrical characterization including leakage current, continuity, and resistance shift
• Metallization integrity such as pitting, lateral attack, and undercut evaluation
• Post-etch corrosion susceptibility after controlled humidity exposure
• Effectiveness comparison of post-etch passivation, cleaning, and rinse chemistries
• Cross-sectional analysis of corrosion depth and propagation paths

Testing Methods

Test Equipment

Testing is performed in a dedicated plasma chamber or reactive ion etcher equipped with chlorine gas delivery, a radio frequency (RF) plasma source, a vacuum pumping system with corrosion-resistant hardware, and precise pressure, gas flow, and power control. Downstream or asher-type plasma systems may also be used for purely chemical exposure without ion bombardment.

Typical Test Conditions

• Base pressure: typically 10^-5 to 10^-3 Torr range before process gas introduction
• Process pressure: approximately 50 to 500 mTorr depending on the simulated process
• Process gas: pure chlorine (Cl2) or chlorine mixtures with BCl3, Ar, or O2
• RF power: commonly 100 W to 1000 W adjusted according to device sensitivity
• Exposure time: from several minutes to several hours based on acceleration requirements
• Substrate temperature: controlled between room temperature and elevated set points
• Post-exposure humidity conditioning: typically 85 degrees Celsius and 85 percent relative humidity or ambient moisture exposure to activate chloride hydrolysis

Test Procedure

• Sample preparation, documentation of baseline electrical parameters, and initial optical inspection
• Sample loading and chamber evacuation to the specified base pressure
• Introduction of chlorine process gas and stabilization of pressure and flow
• Plasma ignition and timed exposure under defined power and temperature conditions
• Chamber purge with inert gas and controlled sample retrieval
• Humidity or moisture conditioning to activate the chloride corrosion mechanism
• Final inspection, electrical measurement, and comparative analysis against baseline data

Evaluation and Analysis Techniques

• Optical microscopy and scanning electron microscopy (SEM) for corrosion morphology and defect mapping
• Energy-dispersive X-ray spectroscopy (EDX) for chlorine residue identification and distribution
• Focused ion beam (FIB) cross-sectioning for corrosion depth and subsurface propagation
• X-ray photoelectron spectroscopy (XPS) for surface chemical state analysis of chloride species
• Electrical parametric testing including line resistance, leakage, and open or short detection
• Mass change measurement or profilometry for quantitative corrosion rate assessment where applicable

Applications

Low-pressure plasma chlorine corrosion testing supports quality assurance and process development across multiple high-technology industries. In wafer fabrication, it is used to qualify aluminum etch processes, validate post-etch treatment chemistries, and monitor corrosion risk during process transfers or equipment changes. In assembly and test facilities, it helps evaluate the robustness of bond pads and exposed metals after decapsulation, die prep, or plasma descum operations.

Additional applications include reliability qualification of power devices and LEDs with exposed metallization, material selection studies for barrier and capping layers, evaluation of MEMS devices processed with chlorine plasma release steps, and failure analysis support where chlorine-induced corrosion is suspected as a root cause. The test also provides valuable data for supplier qualification, incoming inspection criteria, and lifetime modeling of corrosion-sensitive products.

FAQ

What pressure range defines low-pressure plasma chlorine corrosion testing?

Testing is typically conducted at process pressures between 50 and 500 mTorr, which corresponds to the operating regime of most reactive ion etching and plasma processing equipment used in semiconductor manufacturing. The exact pressure is selected to match the process being simulated.

Why is humidity exposure performed after the plasma treatment?

Chlorine residues on metal surfaces form hygroscopic chlorides such as AlCl3 that only convert into visible corrosion products when they react with moisture. Controlled humidity exposure after plasma treatment accelerates and standardizes this hydrolysis reaction, ensuring that latent corrosion risk is revealed during the test rather than appearing later in the field.

How are test results judged as pass or fail?

Acceptance criteria are typically defined by the customer or relevant internal specification and may include limits on visible corrosion defects per unit area, maximum resistance or leakage shift on electrical test structures, allowable chlorine signal intensity from surface analysis, or complete absence of corrosion on critical features such as bond pads.

Can the test be applied to copper metallization?

Yes. Although aluminum is the most corrosion-sensitive common metallization in chlorine plasmas, copper and copper alloy structures can also be evaluated. Copper forms volatile copper chlorides under plasma exposure, and the test can assess barrier layer effectiveness and post-etch residue risk for copper interconnect processes.

How long does a typical test take?

Plasma exposure itself usually ranges from a few minutes to several hours depending on the acceleration factor required, while post-exposure humidity conditioning may add 24 to 96 hours. Including inspection and electrical characterization, most evaluations are completed within one week.