shows an illustration of the probing system and a photograph of the fiber stylus. The fiber stylus consists of a shaft (optical fiber) and a stylus with diameters of 3 and 5 µm, respectively (Figure 1
b). The probing system consists of the fiber stylus, two laser diodes (LDX, LDY), and two dual-element photodiodes (PX, PY) facing the X and Y directions. The stylus shaft is fabricated using an acid etching technique [28
]. First, the step index multi-mode optical fibers (core diameter: 100 µm, clad diameter: 125 µm) were stripped of their plastic layers. The tips of the fibers were then immersed in a hydrofluoric acid solution, and hydrofluoric acid etching was carried out at room temperature (23 °C). The diameter of the probe shaft was measured with an optical microscope. After this process, the stylus shaft was rinsed with water and acetone. After fabricating the probe shaft, the shaft’s tip was immersed in ultraviolet curing resin and then moved into contact with a glass probe sphere. Next, the probe shaft and sphere were irradiated by ultraviolet rays and glued together. Figure 1
b shows a photograph of a stylus shaft glued to a stylus tip, the two parts having diameters of 3 µm and 5 µm, respectively. The stylus shaft is fixed to a tube-type piezo driver element to perform attitude adjustment of the stylus shaft; the stylus shaft is installed between the laser diodes and the dual-element photodiodes, which are oriented orthogonally (Figure 1
a). The stylus shaft is irradiated by two focused laser beams emitted by the two laser diodes through condenser lenses in the X and Y directions. The dual-element photodiodes, PX and PY, are located opposite to the laser diodes, LDX and LDY, with respect to the stylus shaft, respectively. Each laser beam penetrating the stylus shaft impinges upon the corresponding dual-element photodiodes. The light intensities detected by PX and PY are converted into voltage signals and are represented as IPX1
(V). Figure 2
shows the measurement principle in the case where the diameter of the stylus shaft is larger than approximately 10 µm. Before the stylus tip comes into contact with the measured plane, the light intensity measured by each element of the dual-element photodiode is equal, as shown in Figure 2
a (i.e., IPX1
). When the stylus tip comes into contact with the measured plane in the +X direction, the stylus shaft is displaced and the light intensity of each element of the dual-element photodiode becomes unequal, as shown in Figure 2
b (i.e., IPX1
). When the stylus shaft is displaced in the +X direction, the angle of refraction of the laser beam passing through the stylus shaft in the Y-direction changes owing to a shift in the part of the stylus shaft being irradiated. Additionally, when the stylus tip comes into contact with the measured plane in the +Y direction, the stylus shaft is displaced, and the light intensity of each element of the dual-element photodiode becomes unequal, as shown in Figure 2
c (i.e., IPX1
). On the basis of these changes in light intensities, the contact direction and magnitude of the stylus tip can be ascertained. When the stylus tip comes into contact with the measured plane in the Z-direction, the stylus shaft buckles and deflects. This deflection is also measured using the above-mentioned method.
The displacement of the fiber stylus is magnified by the stylus shaft, which works as a rod lens. The surface of the micro-hole is scanned in the XYZ directions using a precision stage, and the accuracy of the micro-hole is measured by recording the coordinates of the contact points and the displacement of the fiber stylus.
In general, the stylus shaft must be rigid to transmit the measuring force acting on the stylus tip to the force detection mechanism installed at the root of the stylus shaft. However, because the proposed probe measures the deflection amplitude of the stylus shaft using a laser-based non-contact method, the stylus shaft need not be rigid; this principle also applies to styluses with much smaller diameters and longer lengths. This measurement system measures the deflection of the stylus shaft; however, it does not directly measure the displacement of the stylus tip. The noise present in IX and IY is removed via synchronous detection using a lock-in amplifier. The displacement of the stylus is magnified by using it as a rod lens. The surface of the microstructure is measured by recording the displacement of the stylus shaft as well as the coordinates at which the stylus comes into contact with the measured surface.