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|Contact Angle for Measuring Cleanliness|
Contact Angle has long been known as a superior direct method for quantifying surface cleanliness. Indirect methods include Gravimetric Analysis, Ultraviolet Spectroscopy, and Optical Particle Counter. These methods typically require environmentally hostile solvents and are ideal for small parts only due to the large amounts of solvents that would be required for larger samples. While newer indirect methods such as total organic analysis are more environmentally friendly, these technologies are still being developed and have yet to gain widespread acceptance.
Direct methods are more frequently used for measuring cleanliness and include the following:
1. Magnified Visual Examination - The part is inspected under a microscope for contamination. This method is adequate only when gross contamination needs to be removed. The method is low cost but does require personnel who are trained and detail-oriented.
2. Black Light Test - This method requires a dark room, black light, and inspector. It works on the principle that contaminants will fluoresce under black light. This method requires a special room and does not work on parts which themselves fluoresce. Like the magnified inspection method, it is valuable only for gross contamination inspection and works well only on small parts.
3. Water Flow Test - This method requires that a stream of water is flowed over the part. If the water sheets evenly, thus indicating high wettability, the part is considered freer of contaminates. Contrariwise, if the water beads up or channels, this would indicate the presence of contaminants and the part is rejected for further cleaning. This method can be ineffective if there are surfactants or other contaminants which could promote sheeting and this indicate a false negative. Additionally, this method does not work on parts that are so small that water cannot be flowed across them.
4. Gravimetric Method - This method requires the part to be weighed before being contaminated and then after contamination. After cleaning, the part is dried and weighed. The difference between the first and second weights represents the amount of residual contamination. This method requires an extremely accurate scale and is of value only for gross analysis.
5. Optically Stimulate Electron Emission (OSEE) - With this test, a light is shined on the surface in question with ultraviolet light of a fixed wavelength. The light stimulates the emission of electrons from the test surface which, in turn, are measured as current. Any contamination will lower the flow of electrons and thus the current measured. This method requires a calibration standard for each part and does not work with contaminants which do not fluoresce.
6. Direct Oxidation Carbon Coulometry (DOCC) - This method employs a combustion chamber set at a high fixed temperature (above 700° C). Oxygen is introduced to combust carbon-based contaminants into carbon dioxide - which in turn is then measured by CO2 coulometric detection. The method can be very sensitive but also very expensive. It works only on small parts and can cause damage to parts and plating that are sensitive to high temperatures. The method can only detect carbon-based contaminants.
7. X-Ray Photoelectron Spectroscopy (XPS) - This method fires x-rays on a surface under a vacuum. The electrons which are released are quantified by element type. This process is very slow and requires sophisticated costly equipment making it impractical for most applications.
8. Contact Angle (CA) - This method works on the principle that most contaminants will cause water droplets to bead up. It requires a CA goniometer and a relatively clean work area. Based on the tool being used, this method is ideal for small parts as well as large parts. The test liquid (deionized water) must be pure and contaminant-free. After a water droplet is dispensed, the goniometer is used to measure the contact angle. A low angle (say 20°) indicates high wettability and thus high cleanliness while a high angle, say 90° or more, indicates the presence of contaminants. There are some materials, such as PTFE, which exhibit high hydrophobicity even when fully clean. For these materials, the CA method may not be well-suited. For most metals and polymers and silica-based materials, however, CA testing is fast, efficient, and highly accurate.
In the manufacturing of semiconductors, each subsequent layer of film stack on a wafer must be superclean in order for the layer to adhere properly. Likewise, the top surfaces of wire bonding lands and passivating film on semiconductor dies must be ultraclean in order to provide good current flow and reliable encapsulation. On some surfaces, such as silicon dioxide on semiconductor wafers, the surface can be made so clean that the resulting contact angle is 1°.
The CA method has improved greatly in recent decades. Our classic ramé-hart goniometer, for example, (first manufactured in the 1960's) has evolved from a microscope-based system with an 1° resolution to a highly sophisticated camera based system employing high speed CCD cameras and processing hardware to capture the drop profile in less than 0.02 seconds. Our DROPimage software, in real-time, locates the CA tangent and measures the contact angle at a 0.1 resolution. If you upgrade the manual dispensing to an automated dispensing system, you greatly increase the repeatability and add the capacity to perform dynamic as well as static measurements. Additionally, our current-generation systems are so automated that operator subjectivity - a common concern with our legacy tools - is virtually extinct.