What Kinds, Sources, Thicknesses, Sample Spectra
Surface Contamination from the Air, Sample Handling, or Sample Storage
Surface Contamination from Gloves, Oils, Shields, Adhesives…
Two Types of Contamination: Visible and Invisible to Human Eye
Amount / Thickness of Adventitious Carbon Contamination
What is Adventitious Carbon ?
by B. Vincent Crist, PhD (Sr Editor at The XPS Library, USA)
The chemical composition of “adventitious carbon”, which has been widely used for charge referencing of insulators, has never been definitively determined by any analytical method, even though ToF-SIMS and GC–MS could be used to reveal more about this type of carbon.
It is important to note that the ratio of the different types of adventitious carbon chemical state moieties (hydrocarbon, alcohol, ether, ketone, ester, acid, carbonate) changes in accordance with the basic chemical nature of the substrate (e.g. metal, glass, ceramic, oxide, polymer) and the origins of the adventitious carbon.
Adventitious hydrocarbons are convenient as a rough or crude reference energy because it and various types of oxygen appear on the surface of almost every material or product, but there are many variables that you must accept or deal with if you try to use adventitious hydrocarbons as a reference energy.
Adventitious carbon will contain different types of carbon depending on the contaminations that exist in the “production” room, the “packaging” room, the “storage” room, the contamination inside the plastic bag or plastic box used to store the material.
The type and nature of adventitious carbon also depends on your location around the world. Are you near a forest that releases all sorts of pine oils or other tree products? Are you located in a major city that has all sorts of air pollution that will eventually attach itself to a freshly made surface which is your product. Are you located near the ocean or a big lake? These are also sources of airborne organic matter that can attach itself to your surfaces. Is your company making chemicals? Some of the gases from chemicals will move around. You may not smell them, but they might be there in the PPM or PPM level.
It is interesting to note that the ratio of the different types of adventitious carbon chemical state moieties (hydrocarbon, alcohol, ether, ketone, ester, acid, carbonate) changes in accordance with the basic chemical nature of the substrate (e.g. metal, glass, ceramic, oxide, polymer). The as-received fully passivated surface of a metal often has 40–60 at.% of adventitious carbon on it, whereas polished glasses and ceramics have 20–40 at.% of adventitious carbon and polymers have only 1–10 at.%. The C (1s) spectra shown below reveal the various levels and types of adventitious carbon that collect on different materials. The Magnesium and Lead native oxides collect CO2 gas from the air and form Carbonates. The formation can be direct or the native oxide may form a Hydroxide that then adsorbs the CO2 from the air.
This hydrocarbon moiety (component) is the dominant form of adventitious carbon on all materials if the sample has not been recently ion etched, fractured in vacuum, or specially treated to remove the adventitious carbon.
Adventitious Carbon C (1s) Spectra on As-Received Native Oxides, Fresh Exposed Bulk
and Semiconductor Wafers – Examples
by Rajiv Kohli, in Developments in Surface Contamination and Cleaning: Applications of Cleaning Techniques, 2019
Surface contamination can be in many forms and may be present in a variety of states on the surface. The most common categories include:
- Particles such as dust, metals, ceramics, glass, and plastics.
- Thin film or molecular contamination that can be organic or inorganic.
- Ionic contamination such as Na+ and K+ cations and Cl−, F−, SO3−, BO33− and PO43− anions.
- Metallic contaminants in the form of discrete particles on the surface or as trace impurities in the matrix.
- Microbial contamination such as bacteria, fungi, algae, and biofilms.
Other contaminant categories include toxic and hazardous chemicals, radioactive materials, and biological substances, that are identified for surfaces employed in specific industries, such as semiconductors, metals processing, chemical production, nuclear industry, pharmaceutical manufacture, and food processing, handling, and delivery. Common contamination sources include machining oils and greases, hydraulic and cleaning fluids, adhesives, waxes, human contamination, and particulates. Furthermore, a whole host of other chemical contaminants from a variety of sources can also soil a surface.
Typically, surface cleanliness levels are based on the amount of specific or characteristic contaminant remaining on the surface after it has been cleaned. In precision technology applications, cleanliness levels are specified for particles by size (in the micrometer (μm) size range) and number of particles, as well as for hydrocarbon contamination represented by nonvolatile residue (NVR) in mass per unit area for surfaces or mass per unit volume for liquids . The cleanliness levels are based on contamination levels established in a revised industry standard IEST-STD-CC1246E for particles from Level 1 to Level 1000 and for NVR from Level R1E-5 (10 ng/0.1 m2) to Level R25 (25 mg/0.1 m2).
In many commercial applications, the precision cleanliness level is defined as an organic contaminant level less than 10 μg/cm2, although for many applications the requirement is set at 1 μg/cm2. This cleanliness level is either very desirable or required by the function of parts such as metal devices, machined parts, electronic assemblies, optical and laser components, precision mechanical parts, and computer parts. A new standard ISO 14644-13 has been published that gives guidelines for cleaning of surfaces in cleanrooms to achieve defined levels of cleanliness in terms of particles and chemical classifications .
Many of the products and manufacturing processes are also sensitive to, or they can even be destroyed by, airborne molecular contaminants (AMCs) that are present due to external, process or otherwise generated sources, making it essential to monitor and control AMCs. AMC is chemical contamination in the form of vapors or aerosols that can be organic or inorganic, and it includes everything from acids and bases to organometallic compounds and dopants . A new standard ISO 14644-10, “Cleanrooms and associated controlled environments – Part 10: Classification of surface cleanliness by chemical concentration” is now available as an international standard that defines the classification system for cleanliness of surfaces in cleanrooms with respect to the presence of chemical compounds or elements (including molecules, ions, atoms, and particles).