Choices for Cleanliness Verification

If cleaning is a step in your manufacturing process, cleanliness verification should be a part of your day-to-day operations. Why? Without a means to measure cleanliness you cannot assess if your cleaning process is performing as it should. You may be producing products that may fail later on down the line. Or you could be expending too much effort and money on cleaning.

The level of cleaning obtained by a given process is determined at the substrate surface, upon completion of all cleaning, rinsing, and drying procedures. The cleanliness verification system used is determined by the quality objectives for the method and the finished product.

Cleanliness verification techniques can be classified into three main categories: gross verification, semi-precise verification, and precise verification. (They can also be more broadly classified as direct and indirect methods.) As some overlap may exist among the techniques in these categories, the test of choice is dependent upon the needs of the specific application.

Gross verification means that there is no visible contamination, but this is not quantified. These techniques are totally qualitative (you can almost see the dirt – 0.1 gram/square inch and above). Among the applications that use gross verification are auto parts/engine cleaning, basic metal cleaning, firearms, and heavy equipment.

Examples of techniques used for gross verification include the dissecting microscope, nonvolatile residue (nvr), Scotch tape test, ultraviolet (uv) fluorescence, water break test, and white glove test.

Semi-precise verification is highly qualitative or quantitative to a moderate level of precision (0.001 – 0.1 gram/square inch). Semi-precise verification is used for high-quality finishing/plating, subcritical electronics assembly, avionics, some instrumentation, electrical, subcritical aerospace, and automotive control parts (fuel injectors, etc.).

Contact angle measurement, the Millipore test, and the optical microscope are examples of semi-precise verification tests.

Precise verification is quantitative to extremely low levels of measure or ultra qualitative (0.001 gram/square inch down to absolute zero). Semiconductors, disk drives, critical aerospace, oxygen lines, and medical devices use precise verification.

Tests that fall under this category include Auger electron spectroscopy (AES), carbon coulometry, electron spectroscopy for chemical analysis (ESCA), Fourier transform infrared (FTIR), fluorometer, gas chromatography/mass spectrophotometry (GC/MS), ion chromatography, optically stimulated electron emission (OSEE), particle counting, scanning electron microscope (SEM), and secondary ion mass spectroscopy (SIMS).

A description of some of the tests noted for the three main categories of cleanliness verification follows.

Gross Verification Tests
Nonvolatile Residue (NVR)
The NVR test requires extraction of contamination from a dirty part into a volatile solvent, evaporating off the solvent, and measuring the weight of the remaining residue using an analytical balance. Almost any clean volatile solvent can be used.

Scotch Tape Test
For polished or lapped parts, a common test is known as the Scotch tape test. A strip of transparent tape is affixed to the surface in question with firm pressure. The tape is removed and placed on a clean, white sheet of paper. The surface should appear as white as the original sheet of paper.

Ultraviolet (UV) Fluorescence
Fluorescence can provide a visual indication of where contamination remains on a surface. (Contaminants will fluoresce in the presence of UV light.) The intensity of the radiation can also be measured via a registered signal on an instrument, which dictates the degree of contamination on a surface. This form of analysis is useful for locating contamination, but it does not identify it.

Water Break Test
In the water break test, if water beads, the surface is considered to be contaminated with a hydrophobic substance (oil/grease). If the water breaks or sheets off, the surface is considered clean.

Semi-precise Verification Tests

Contact Angle Measurement
See article on page S-8.

Millipore Test
The patch test, also known as the Millipore test, consists of spraying a representative number of parts with filtered hexane, isopropyl alcohol, or trichlorethylene at a pressure of 60 to 80 psi through a filter jet nozzle with a 1.2-micron membrane filter. The spray is collected and vacuum filtered onto a clean filter membrane, and the membrane is inspected for contaminants (placed under a microscope to measure — in microns — and count the number of dirt particles remaining). Weighing the membrane pad determines the total contaminant (in milligrams) that has been left behind

Optical Microscope
Optical microscopes use a beam of light and lenses to magnify objects. There are simple, high-power, and optical microscopes. Optical methods are an excellent way to perform simple quality control checks or verify certain types of cleanliness quickly and efficiently.

Simple microscopes, or magnifiers, consist of a single lens or a set of lenses that provide direct magnification.

High-power compound microscopes are used in evaluating the cleanliness of critical components like circuit boards. These instruments are typically delicate and expensive, requiring a variable degree of operator skill, training, and patience in order to maximize their potential.

Optical microscopes are ideal for viewing residual oils and greases, flux residues, certain particles, and surface anomalies.

Precise Verification Tests

Auger Electron Spectroscopy (AES)
AES is used for compositional analysis or determining which atoms are present on a surface. Electrons are directed toward the surface, ionizing surface atoms by causing the removal of an electron from the atom’s inner shell. The atom now becomes excited and must release energy to “relax” and return to its original state. This is done by transferring the extra energy to an electron that can leave the atom. The exiting electron is known as the auger electron. The AES method of analysis measures the energy of the auger electron, which is unique to each particular atom. AES is used in the semiconductor field for corrosion and failure analyses, and thin-film analyses.

Carbon Coulometry
The technique employs in-situ direct oxidation of surface carbon to carbon dioxide (CO2), followed by automatic CO2 coulometric detection.

Electron Spectroscopy for Chemical Analysis (ESCA)
ESCA is a spectrophotometric technique in which X-ray bombardment of a surface results in the emission of an electron from a given atom. Knowing the energy of the X-ray and measuring the energy of the emitted electron can determine the binding energy of the electron. ESCA methods reveal chemical structure, bonding, and oxidation state. ESCA has the potential to be very useful in identifying organic compounds.

Fourier Transform Infrared (FTIR)
See article on page S-10.

Gas Chromatography/Mass Spectrophotometry (GC/MS)
GC/MS is used to identify surface contamination by extracting contaminants into solvent and analyzing them. Organic compounds are separated via GC and are then identified, based on molecular weight, by MS.

Ion Chromatography
Ion chromatography separates, identifies, and quantifies ions. The analysis begins with a sample, typically a water matrix containing ions of interest. A portion is injected into the system and combined with an eluent stream that carries the sample to the analytical column. The analytical column separates the ions of interest in the sample into narrow bands within the stream of the eluent.

The eluent then sweeps these groups of ions into the suppressor device, which electrolytically transforms the eluent into pure water, leaving just the ions of interest in pure water to be swept downstream to the conductivity detector. The detector detects the ions based on their conductivity relative to the water eluent. At this point all interfering ions have been removed and the detector’s sensitivity has been maximized, allowing for detection of very low part-per-billion levels of ions.

Optically Stimulated Electron Emission (OSEE)
When high energy UV light hits a surface, electrons will be emitted, and the reflected current can be measured. A clean surface will give the highest return current, so any drop in current represents contamination. This method is good for seeing low levels of contamination (both ionic and nonionic). It can detect contamination, but it cannot identify it.

Particle Counting
See article on page S-4.

Scanning Electron Microscope (SEM)
SEM utilizes a beam of electrons that is passed over a very small area of a surface. The beam scatters when it strikes the surface, outlining the topography of the surface. The back-scattering carried in by the return beam of electrons is measured by the microscope. The result is a finely detailed, 3-D image of the surface being scanned.

Scanning microscopes are capable of magnifying an image to more than 100,000 times its original size. This method is well suited for identifying particulate and potentially nonuniform or thick films of contaminant.

Secondary Ion Mass Spectroscopy (SIMS)
SIMS identifies elements but does not identify bonding characteristics. An incident high-energy ion beam strikes the surface and blows atoms and molecules off the surface. The atoms and ions flying off the surface enter a mass spectrometer and can be identified by the mass and charge ratios. SIMS is sensitive to all elements and their isotopes and can be used to quantify part-per-billion levels in semiconductors.

Generally, SIMS is considered a surface analysis method with a spatial resolution laterally on the order as small as 1 micron in certain cases. Though SIMS is used most in the semiconductor field, it has made a major impact in other thin-film subjects, and a variation of the process, called Time of Flight SIMS, is now used on many organic bio-molecules and inorganic species.