Introduction
 Fig. 1 |
For the professional involved in the production of optical surfaces, both new and traditional methods are used to assess the quality of output from flat lapping systems such as the MULTIPOL machines supplied by ULTRA TEC Mfg., Inc. This note defines four parameters, which are used to evaluate polished surfaces, and describes how they are measured. The four parameters, as illustrated in fig 1, are: · Texture · Flatness · Parallelism · Surface Finish A lapped (gray) surface will have a mean roughness in the region of 0.1 to 0.01 microns (100 to 10 nanometers); a polished surface will have a mean roughness of below 1nm. |
Flatness is the deviation of the surface from the best fitting plane i.e. the macro surface topography. It can be defined as an absolute total value; for example - a 50mm diameter disc is required to be flat to 0.003mm (i.e. 3 microns). However it is more frequently specified as deviation per unit length; i.e. the disc above would be specified to be flat to 0.0006mm per cm. Flatness can also be defined in terms of wavelengths of light (see measurement of flatness).
Parallelism defines the angle between two surfaces of a sample. It can be specified as a thickness difference per unit length or as an angular deviation e.g. a thickness difference of 1 micron per cm is equivalent to 20 seconds of arc, or 100 micron radians angle.
Surface finish is the measure of residual defects in the surface i.e. scratches, digs or pits etc. after polishing is complete. The specification for quality depends upon the final use of the surface but the defects can be defined as total number or number per unit area. It is also usual to put limits on the length, width and depth of any defects. Examples are the SEMI specifications for semiconductor wafers and MIL specifications for optical components.
Measuring Surfaces: Techniques
Flatness:
Large variations of several microns can be measured using conventional electro-mechanical gauges, preferably of the non-contact type for polished surfaces. The ULTRA TEC Precision Gauge Micromount UMI245 holds a gauge of this type and can be used to measure samples mounted on a precision jig. (Fig. 2)
Fig. 2
| Small variations of less than one or two microns are measured using interference fringes produced between the surface and an optical flat illuminated by monochromatic light. (Monochromatic light is used because the fringes then have more contrast and are more sharply defined). Like Newton’s rings, the fringes may be regarded as contours of equal distance from the surface of the flat; the separation between each fringe of the same color represents a height difference of half a wavelength of the light used (Fig. 3). The optical flat method has the disadvantage that the surfaces of the flat and specimen must be in close contact leading to scratching of both. The ULTRA TEC Fizeau Interferometer UMI 5000 shows the fringes using a non-contact method, where the sample is separated by several mm from the optical reference flat. The fringes are produced by a telescope/eye safe laser system and are viewed through the telescope eyepiece. |  Fig. 3 |
They can also be photographed or displayed on a CCTV system. Samples can be measured whilst they remain in position on a precision polishing jig.
The fringes follow the direction of the arrows when the optical flat is pressed in closer contact with the surface of the sample.
| Parallelism: For large deviations from parallel, surfaces can be measured mechanically i.e. 10 microns per cm is equivalent to 1 milliradian or approximately 3.5 minutes of arc. The sample is supported on a three-ball plane with the measuring device above one ball. Rotation of the sample about the axis at right angles to the three-ball plane allows differences in height to be measured. The sample surfaces must, of course, be flat to a finer limit than the out-of-parallelism (fig. 4). |
Fig. 4 |
Fig. 5 |
Smaller values can be measured using the ULTRA TEC Autocollimator (UMI183), which allows differences as small as a few seconds of arc to be measured on polished surfaces. The Autocollimator consists of a reflecting telescope with a calibrated cross wire eyepiece. Using an accurately parallel reference disc, a three-ball plane under the telescope is set precisely at right angles to the optical axis. The reference disc is then replaced by the sample. If surfaces of the sample are not parallel, the reflected cross wire image from its upper surface will be displaced when viewed in the eyepiece. Samples can be assessed in position on the precision polishing jig (Fig. 5) and the out-of-flatness corrected using the micrometer tilt screws on the precision jig. |
Texture and surface finish:
A qualitative assessment can be made using grazing incidence light from a point or strip source and examining the surface using a magnifying eyepiece (fig 6). With experience, very small defects can be recognized.
Fig. 6
Texture can be measured quantitatively using a contact surface profile instrument, although polished surfaces tend to be below the sensitivity of many machines. The instrument measures the vertical movement of a fine stylus (of contact or non-contact type) as it is drawn for a short distance over the surface, which is of course scratched. Parameters such as mean displacement etc. are calculated and displayed.
Surface quality can be observed using incident light microscopy. Fitting a Nomarski differential interference contrast attachment can enhance surface relief. Manual measurements of defects larger than the resolution of the microscope system can be measured using a graticule or a measuring eyepiece.
Non-contact image analysis instruments, which use a variety of techniques such as laser triangulation, Nomarski interference etc., are also available. These can give three-dimensional plots of the surface and can calculate mean texture, number and size of defects etc. and are useful for controlling large through puts of high quality.
Further Reading
1. Cutting and polishing optical and electronic materials, by G.W. Fynn and W.J. Powell, Published by Adam Hilger Ltd. (2nd ed.)
2. ULTRA TEC Application Note. Back-Lapping Semiconductor Wafers
