Surface metrology

Some important types of instrumentation for measuring surfaces both past and present are reviewed. Exhaustive lists of instruments and performance are not presented; rather more emphasis is placed on the philosophy of measurement. An attempt is made to classify the surface features and also the function of surfaces as a pre-requisite to measurement. It is revealed that, as the push towards miniaturization is being taken beyond the nanometrology scale, some theoretical restrictions are likely to be encountered. 1. Why measure surfaces? In recent years surface texture has been recognized as being significant in many fields. In particular the surface roughness is an important factor in determining the satisfactory performance of the workpiece, in tribology for example or in coatings. Also in engineering applications the surface roughness has been found useful in machinetool monitoring. These aspects will be discussed presently. It is, however, pertinent to consider how the importance of surface roughness is changing with the passage of time and how the importance of roughness depends on the actual scale of size of the workpiece and the process used to make it. In very general terms the requirements for energy transfer and information transfer and storage have dominated the development of technology. This will no doubt also be true in the future but the factors governing such transfer depend themselves on size. As objects get smaller changes emphasizing the importance of the surface take place. The energy and force equations experience a change of balance, figure 1(a), as the scale of size of the moving object decreases. Information storage in 3D is possible but still very difficult to achieve; also, data retrieval is a major problem. On the other hand, storage of data on surfaces is still actively being extended. This capability is obviously a function of the surface area. There is no problem with accessibility of the stored data like there is with volume storage. Notice that information storage trends tend to have the opposite trend to the energy equations with respect to the effect of the scale of size, figure 1(b). In both situations the critical regime is that of area. Consider figure 1(a). In the force diagram momentum effects quickly decrease as the size is reduced, in fact by a factor of L3. Damping is proportional to the area and decreases as L2. Elastic forces only decrease as a linear factor of L so that they become progressively more important than the others as the scale of Figure 1. The importance of surface properties with scales of size. size decreases. In rotational situations the dependence on scale is even more pronounced. For example the moment of inertia decreases by a factor of L5. Of the above factors, the elastic properties can be controlled by varying the properties of materials, which are well understood, and inertial forces can be controlled by design. This leaves areal or damping forces which cannot be controlled easily. Energy losses and transfer and information storage and transfer are all sensitive to uncontrolled areal properties and the areal property which is most likely to be influential is the surface roughness. 0957-0233/97/090955+18$19.50 c © 1997 IOP Publishing Ltd 955