Richard Leach - Fundamental prinsiples of engineering nanometrology (778895), страница 33
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The surface beingmeasured is displaced downward (Z), and the laser beam position on theautofocus sensor changes accordingly (W). Figure 6.19c shows the autofocusstate where the autofocus sensor detects the laser spot displacement andOptical instrumentsFIGURE 6.19 Principle of point autofocus operation.feeds back the information to the autofocus mechanism in order to adjust theobjective back to the in-focus position. The specimen displacement, Z1, isequal to the moving distance of the objective, Z2, and the vertical positionsensor (typically a linear scale is used) obtains the height information of thespecimen [70].The disadvantage of the point autofocus is that it requires a longermeasuring time than other non-contact measuring methods since it mustobtain the coordinate values of each point by moving the mechanism of theinstrument (as with chromatic confocal – see section 6.7.2.2.1).
Also, theaccuracy of the instrument will be determined by the laser spot size (seesection 6.7.1) because of the uneven optical intensity within the laser spot(speckle) that generates focal shift errors [71].Point autofocus instruments can have relatively high resolution. Thelateral resolution is potentially diffraction limited but the axial resolution isdetermined by the resolution of the master scale, which can be down to 1 nm.The range is determined by the xy and z scanner, and can be typically 150 mmby 150 mm by 10 mm. The method is almost immune to the surface141142C H A P T ER 6 : Surface topography measurement instrumentationreflectance properties since the autofocus sensor detects the position of thelaser spot (the limit is typically a reflectivity of 1 %).
The point autofocusinstrument irradiates the laser beam on to a specimen surface that causes thelaser beam to scatter in various directions due to the surface roughness of thespecimen. This enables the measurement of surface slope angles that aregreater than the half aperture angle of the objective (less than 90 ) bycapturing the scattered light that is sent to the autofocus sensor.6.7.3 Areal optical techniques6.7.3.1 Focus variation instrumentsFocus variation combines the small depth of focus of an optical system withvertical scanning to provide topographical and colour information from thevariation of focus [69].
Figure 6.20 shows a schematic diagram of a focusFIGURE 6.20 Schema of a focus variation instrument.Optical instrumentsvariation instrument. The main component of the system is a precisionoptical arrangement that contains various lens systems that can be equippedwith different objectives, allowing measurements with different lateralresolution. With a beam-splitting mirror, light emerging from a white lightsource is inserted into the optical path of the system and focused onto thespecimen via the objective. Depending on the topography of the specimen,the light is reflected into several directions. If the topography shows diffusereflective properties, the light is reflected equally strongly into each direction.In the case of specular reflections, the light is scattered mainly into onedirection.
All rays emerging from the specimen and hitting the objective lensare bundled in the optics and gathered by a light-sensitive sensor behind thebeam-splitting mirror. Due to the small depth of field of the optics, only smallregions of the object are sharply imaged. To perform a complete detection ofthe surface with full depth of field, the precision optical arrangement ismoved vertically along the optical axis while continuously capturing datafrom the surface. This ensures that each region of the object is sharplyfocused. Algorithms convert the acquired sensor data into 3D informationand a true colour image with full depth of field.
This is achieved by analysingthe variation of focus along the vertical axis.Various methods exist to analyse this variation of focus, usually based onthe computation of the sharpness at a specific position. Typically, thesemethods rely on evaluating the sensor data in a small local area. In general, asan object point is focused sharper, the larger the variation of sensor values ina local neighbourhood. As an example, the standard deviation of the sensorvalues could be used as a simple measure for the sharpness.The vertical resolution of a focus variation instrument depends on thechosen objective and can be as low as 10 nm.
The vertical scan rangedepends on the working distance of the objective and ranges from a fewmillimetres to approximately 20 mm or more. The vertical resolution is notdependent upon the scan height, which can lead to a high dynamic range.The xy range is determined by the objective and typically ranges from 0.14mm by 0.1 mm to 5 mm by 4 mm for a single measurement. By usingspecial algorithms and a motorised stage the xy range can be increased toaround 100 mm by 100 mm.In contrast to other optical techniques that are limited to coaxial illumination, the maximum measurable slope angle is not dependent on thenumerical aperture of the objective. Focus variation can be used with a largerange of different illumination sources (such as a ringlight), which allows themeasurement of slope angles exceeding 80 .Focus variation is applicable to surfaces with a large range of differentoptical reflectance values.
Specimens can vary from shiny to diffuse143144C H A P T ER 6 : Surface topography measurement instrumentationreflecting, from homogeneous to compound materials, and from smooth torough surface properties (but see below). Focus variation overcomes theaspect of limited measurement capabilities in terms of reflectance by usinga combination of a modulated illumination source, controlling the sensorparameters and integrated polarization. In addition to the scanned heightdata, focus variation also delivers a colour image with full depth of field thatis registered to the 3D data points.Since focus variation relies on analysing the variation of focus, it isonly applicable to surfaces where the focus varies sufficiently during thevertical scanning process.
Surfaces not fulfilling this requirement, such astransparent specimens or components with only a small local roughness, aredifficult and sometimes impossible to measure. Typically, focus variationgives repeatable measurement results for surfaces with a local Ra of 10 nm orgreater at a lc of 2 mm (see section 8.2.3).6.7.3.2 Phase-shifting interferometryA phase-shifting interferometer (PSI) consists of an interferometer integratedwith a microscope (see Figure 6.21) [72,43]. Within the interferometer,a beam-splitter directs one beam of light down a reference path, which hasa number of optical elements including an ideally flat and smooth mirrorfrom which the light is reflected.
The beam-splitter directs a second beam ofFIGURE 6.21 Schema of a phase-shifting interferometer.Optical instrumentslight to the sample where it is reflected. The two beams of light return to thebeam-splitter and are combined forming an image of the measured surfacesuperimposed by an interference pattern on the image sensor array (camera).Usually a PSI uses a co-axial alignment, i.e. the two beams propagate in thesame direction, but off-axis arrangements can be used [73].
The image of thesurface can be either focused onto the detector or not. In the latter casea digital propagation algorithm is employed allowing numerical focusing[74]. The optical path in the reference arm is adjusted to give the maximuminterference contrast. During measurement, several known shifts betweenthe optical path to the measured surface and the optical path to the referencemirror are introduced and produce changes in the fringe pattern. Phase mapsare then constructed from each shifted interferogram. There are several waysto shift the difference in optical paths. For example, the objective and reference mirror of the system are translated with the use of a piezoelectricactuator.
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