This invention relates to measurements of material properties, and, more particularly, to an instrument for use in determining surface characteristics of materials by measuring a current of photoelectronics emitted from a surface.
The quality and condition of its surfaces often determine the suitability of a material for use in an industrial process. To cite some examples, if a material is to be painted or bonded to another material, the presence, type, and amount of contaminants such as grease or dirt often determine the success of the subsequent operation. Coatings or oxides are placed onto the surfaces of semiconductor wafers during the fabrication of microelectronic components, and the thickness of the coating or oxide must be established within critical limits for the success of subsequent operations. The cleanliness of memory disk drive heads must be checked periodically to be certain that contamination does not interfere with read/write functions.
In another setting, the condition of a surface may be a measure of the previous use of the material, and may provide an indicator of its potential for further use. Scratches and abrasion marks on a surface used in bearings and fatigue striations on the surface of a part subjected to cyclical loadings are examples.
In recognition of the critical importance of surfaces, a number of techniques have been developed to observe and measure surface characteristics. Surfaces may be viewed either directly or with magnification. They may be measured by devices such as profilometers which give a quantitative measurement of roughness. More recently, sophisticated techniques such as ellipsometry, low energy electron diffraction, Auger electron spectroscopy and scanning electron microscopy have been employed for characterizing surfaces. However, when used in an industrial environment all of these techniques have serious drawbacks. Most require the use of bulky, expensive instruments. Most of the advanced techniques require that the piece under study be placed in a high-vacuum chamber, thereby effectively limiting the size of the parts and the rate at which they may be examined.
Perhaps most critically, however, these techniques are not readily adapted to acceptance testing on an industrial scale by relatively unskilled personnel. For example, ellipsometry may be used to determine the thickness of an oxide coating formed on a surface by a particular heat treatment, but it is typically not practical to check by ellipsometry every piece subsequently given the same heat treatment. Instead, it must be assumed that uncontrolled variables do not enter the process and that each heat treatment is successful in producing the desired surface. In some industrial operations such as coating a surface with adhesives, there simply is no way to predict when problems such as isolated patches of grease on a previously cleaned surface may arise. There is no readily available means to discriminate between such patches of grease that will not interfere with painting of the surface, and those which might prevent application of an acceptable coating.
Additionally, when utilizing many highly advanced surface characterization techniques it is difficult to know whether the characteristic measured is really important and determinative of acceptability of the surface. That is, measurements may be made of surface characteristics, but one is then faced with the problem of deciding whether that measurement is at all relevant to the planned usage of the material and, if so, what the limits of acceptable variation of the measured characteristic might be in order to ensure success of the subsequent processing.
There has therefore existed a continuing need for a surface monitoring apparatus and associated monitoring technique which is sensitive to microscopic surface charateristics, is measurable by relatively inexpensive and compact apparatus, does not require that the surface to be studied be placed in a vacuum chamber, and, most importantly, allows the development of acceptability and quality control tests which may be utilized by relatively unskilled personnel for testing large surfaces of numbers of parts. The present invention fulfills this need, and further provides related advantages.