Top technique for surface inspection

Measuring the cleanliness of surfaces in a cleanroom is an important part of any contamination control programme as it allows contamination control personnel to qualify the effectiveness of cleaning procedures for parts, work surfaces, part handling components, transport equipment, incoming materials and new tools.

One common piece of equipment used to measure surface cleanliness is a surface scanner particle counter, a large, specialised device that utilises a laser to scan the surface under test for particles. These machines are used in the semiconductor, disk drive and flat panel display industries to detect yield-damaging particles directly on the surface of the product. Though widely used in these industries, these machines can only be used on the wafers, panels or disks they are designed for. A common practice of particle counting using this method is called the "Witness Wafer" programme. In this programme, non-product wafers or disks are utilised and placed at strategic locations throughout a facility. Particle levels are recorded on each wafer before they are placed in their specific locations. They are left undisturbed in the cleanroom for a set amount of time – usually 24 hours – and then collected and scanned again. The pre-test particle count is then subtracted from the post-test particle count and the number of "adders" is an indication of cleanliness levels in that particular area of the cleanroom. This method cannot directly test any surface other than the product for which the instrument was designed. The cleanliness of wafer/disk transport boxes or cassettes cannot be directly measured using a surface scanner particle counter, nor can wafer or disk handling equipment be tested. Another popular method is optical particle counting which utilises a medium – either water or air – as well as some type of energy to extract particles from a surface and then transport them back to an optical particle counter. The most common method involves the use of ultra-pure water, an ultrasonic transducer, a liquid particle counter and a tank big enough to hold the part or parts to be tested. The water cleanliness is verified and the part is immersed in the tank. The ultrasonic transducer is activated for a predetermined period of time at a specific frequency. The water is then passed through the liquid particle counter. The pre- and post-particle data are compared to determine the cleanliness of the part(s). The advantage of this method is that a complete part can be tested to determine cleanliness levels. The disadvantage is that it is an indirect method subject to particle transport issues or the potential alteration of the physical and chemical properties of the particle either by the submersion in water or by the ultrasonic energy.

Airborne particle counter The alternative is to use an airborne particle counter with a specialised head. Pressurised air jets on the outside of the head blow inwardly over the surface. At the centre of the head is a vacuum inlet. Particles are essentially sucked off the surface and transported to an aerosol particle counter. This method has the advantage that the device is extremely portable and any flat surface can be measured. An obvious disadvantage is that the accuracy of the measurement is affected by the environment around the object under test. Another distinct factor that can affect the measurement concerns the bonding forces of the particles to the surface. Particles below 1.0 micron are already difficult to remove and when electrostatic charge is present, few, if any, particles from the surface will be removed and counted. A third factor to consider is operator technique. As the probe is hand held, the speed at which the surface is scanned, as well as the planarity of the probe to the surface, will affect the particle count levels. Another traditional method is magnified visual inspection (optical microscopy). This involves using a microscope and counting particles under the field of view and is limited to small parts that can be moved to the microscope and measured, or to the use of specialised microscopes that can be attached to tools or fixtures to view these surfaces. Advantages are that it is a direct measurement, it is relatively low cost and it is easy to verify readings. Disadvantages are that it is labour intensive and it is limited to what can fit under the microscope. A slight variation to magnified visual inspection utilises tape or contact surfaces to contact the surface under test and then the tape or contact surface is viewed under the microscope. The latest method, which has been developed by the Fraunhofer Institute along with Advanced Clean Production (ACP) in Germany, is a unique portable surface particle counter designed to directly count and size particles on any flat surface. Utilising glancing light to illuminate the surface under test, an electronic camera and image system are able to size and count the particles on the surface. An image of the surface under test is captured and processed in less then 0.5 of a second. The screen updates to show the particle count, as well as the location of each individual particle under the test head. Particle sizing is from 0.5 microns upwards. The Particle Guard is also able to store an electronic image of the surface under test as well as the particle size and count numbers which enables comparisons of surfaces before and after cleaning to be made. This method allows for adjustment to compensate for varying surface roughness on materials as well as different materials' light reflection index. Surfaces that had been difficult to accurately measure before are now much easier to measure. Cleanroom walls, work tables, SMIF pods and thin plastic films can all be measured for cleanliness levels using this method.

No environmental impact Another unique aspect of this device is that particle measurements on surfaces are not impacted by the general room environment. In the higher level cleanrooms (ISO 8, ISO 9) or even unclassified rooms, particle levels can be tested on a variety of surfaces. The software is capable of passing data directly into Excel to allow for further analysis and graphical representation. Settings for distinct surfaces can be stored and recalled to facilitate re-testing of surfaces as part of an ongoing surface contamination inspection system. The disadvantages of this system are that multiple scans are required for larger surfaces and only surfaces large enough to support the head can be easily scanned. However, special fixtures can be adapted to accommodate smaller objects. For example, one has been made to allow the testing of magnetic recording heads for data storage. Particle Guard can also provide information useful in diagnosing the nature of the particle itself. As it utilises a digital camera, the image of the area under test can be seen on the screen and the shape of particles can be determined using this image.