The Critical Role of Outgassing Control for Vacuum Environments

Outgassing control is therefore critical for any application where vacuum environments are used, such as in diverse fields including semiconductor manufacturing, accelerator physics, space simulation, metallurgy, and scientific instrumentation. This blog post will examine what outgassing is, the harm it causes, and the stringent contamination control measures required to minimize outgassing. We will explore how quantitative cleanliness standards are set and verified, strategies to reduce outgassing risks during manufacturing, effective cleaning techniques, protocols to avoid recontamination after cleaning, and the challenges of achieving the necessary cleanliness levels.

What is Outgassing and Why is it Harmful in Vacuum Systems?

Outgassing refers to the release or evolution of gases either absorbed on the surfaces of, or trapped within materials used inside vacuum environments. When these contaminated materials are exposed to the vacuum, the gases can escape and emanate into the vacuum chamber or components.

Types of Gases Released Through Outgassing

There are two primary categories of gases that outgas from materials in a vacuum:

Volatile Hydrocarbons: These include light gases such as water, methane, carbon dioxide, and other low molecular weight hydrocarbons. These types of volatiles tend to outgas relatively quickly and easily from materials.

Non-Volatile Hydrocarbons: These are heavier hydrocarbons that have higher molecular weights. They are more tightly bound in materials and outgas more slowly over longer periods. But they can still gradually contaminate vacuum systems.

Negative Effects of Outgassing in Vacuum Systems

Outgassing causes various seriously detrimental effects for the performance of vacuum systems across a range of applications:

The Critical Need to Control Outgassing

Based on these highly deleterious effects, controlling outgassing is essential for:

In summary, outgassing can severely impair vacuum system function and must be minimized as much as possible.

Setting Quantitative Standards for Vacuum Cleanliness

To manage and control outgassing, stringent quantitative cleanliness standards must be set based on the maximum allowable quantities of outgassed contaminants.

Key Parameters for Vacuum Cleanliness Specifications

Vacuum cleanliness is governed by specifications for maximum permissible outgassing rates of relevant gases. Key parameters include:

These outgassing rates are typically defined in pressure x volume units per unit time, such as Pa x m3/second. Very low magnitudes are usually required.

Verifying Cleanliness Through Residual Gas Analysis

The primary technique used to verify whether vacuum cleanliness standards are met is residual gas analysis (RGA). This works by sampling gases present in the vacuum environment and using mass spectrometry to identify and quantify the amounts and types of gases making up the residual gas.

By periodically performing RGA on a vacuum system, any contamination issues can be detected. RGA allows verifying that outgassing specifications are continually satisfied over the operating lifetime of the vacuum equipment. If standards are exceeded, additional cleaning is generally required to reduce outgassing levels.

Strategies for Minimizing Outgassing During Manufacturing

To achieve stringent vacuum cleanliness requirements, extraordinary care must be taken during the manufacturing, assembly, and testing of components to minimize contamination that could lead to outgassing.

Material Selection to Minimize Outgassing Risk

Proper selection of materials is critical to avoid those prone to outgassing:

Critical Process Control Measures

In addition to rigorous material selection, stringent control of manufacturing processes is vital:

Personnel Training

Training personnel on cleanliness protocols and avoiding contamination is vital for success.

Effective Bake-Out Cleaning Techniques

Bake-out vacuum heating is one of the most common and effective cleaning methods used to remove outgassed contaminants prior to vacuum service.

Typical Bake-Out Process Steps

In a bake-out process, parts are heated to around 120°C or higher for a minimum of 24 hours. This is done under vacuum conditions below 10^-5 mbar. Sometimes a nitrogen purge is also applied to aid in removing outgassed molecules.

How Bake-Out Cleaning Works

Heating accelerates outgassing from materials and allows those contaminants to be removed from the system rather than redeposited.

Benefits of Bake-Out Cleaning

When properly performed, bake-out provides multiple benefits:

Optimizing Bake-Out Cleaning

The bake-out process can be supplemented and optimized by using:

Analyzing the quantity and types of residues collected during bake-out using RGA allows dialing in process parameters like temperature, pressure, and duration to maximize cleaning effectiveness.

Preventing Recontamination After Cleaning

After parts undergo extensive cleaning procedures, diligent precautions must be taken to prevent recontamination prior to installation in vacuum systems.

Packaging Solutions

Packaging is the first line of defense against recontamination:

Controlled Environments

Cleaning, assembly, and testing should take place in controlled cleanroom environments optimized to avoid adding contamination.

Transportation Methods

Transportation methods must protect parts from mechanical shock, vibration, impacts, and temperature swings that could reintroduce contamination.

Handling Procedures

Strict adherence to cleanroom garment protocols is mandatory whenever handling cleaned parts. Parts must only contact cleaned surfaces and gloves. Special handling fixtures may be utilized.

Training Personnel on Protocols

Comprehensive training to personnel on recontamination avoidance protocols is equally as important as the cleaning itself. Cleanliness can easily be ruined by simple mishandling.

Challenges of Achieving Stringent Outgassing Standards

While critical, achieving highly stringent outgassing standards poses formidable challenges:

Tight Control Over All Factors

Maintaining cleanliness specifications requires extremely tight control over materials, processes, cleaning methods, assembly environments, and personnel training. A single weak link can ruin all other efforts.

Constant Verification Testing

Standards must be verified continuously via RGA and other methods over the entire operating lifetime of the vacuum equipment. This requires diligence.

Avoiding Human Errors

Humans can easily make errors that introduce recontamination. Personnel need excellent training on contamination control disciplines.

In summary, stringent outgassing standards demand near perfection at all steps. Personnel must uphold consistent procedures to prevent contamination and ensure high vacuum performance over the entire product lifetime.


Preventing detrimental outgassing is imperative when working with contamination-sensitive vacuum processes, as found widely in fields from semiconductor manufacturing to space simulation. But achieving sufficiently low outgassing levels poses many difficulties. Careful material selection, tightly controlled manufacturing, extensive precision cleaning, and rigorous contamination control after cleaning are all essential. Outgassing standards need constant verification via residual gas analysis. There are significant technology and human factors challenges to overcome. However, the rewards of minimizing harmful outgassing are reliable vacuum systems with extended operational lifetimes.

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