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New position sensor is fabricated which is suitable for detecting the accurate straightness reference of a laser beam [2]. This function is realized by fabricating the four-cell type photodiode inside the Si micromesh structure. The sensor does not disturb the direction or the wave front of the reference laser beam absorbing only a part of incident photon and transmitting the rest to the down stream. Figure 2 shows the straightness measurement proposed here. The straightness of 1m size large structure can be measured with 1ƒÊm accuracy at many points by placing sensors in series.

Figure 2: Alignment using transmission-type position sensors.

The position sensor is based on the four cell type photodiode. By comparing the magnitude of four signals obtained from each cell, the relative position between the sensor and the beam spot is obtained.

Figure 3: Fabrication process

Figure 3 is the fabrication process. Si substrate is n-type (100), 5-8 ƒ¶-cm, and 200ƒÊm thick. First, the substrate is oxidized, and 40ƒÊm thick Si diaphragm is fabricated by etching the wafer backside anisotropically. The front side oxide film is then patterned to open the window through which the boron is implanted (100 keV as BF2, 2.0 k~1014atoms/cm2) to make the four cell type photodiode.@The front side oxide film is then completely removed. After the annealing and oxidization, the photodiode diaphragm is patterned as the mesh and the through-holes of the mesh are made by the reactive ion etching. One more oxidization process is carried out for reducing the leak current due to the surface state at the side wall of the through-hole. Followed by the etching of the contact holes for the electrical interconnections, an aluminum layer is deposited, patterned, and sintered.

Figure 4 shows the fabricated photodiode having honeycomb mesh structure. Letters printed on the underlying paper is clearly seen through the mesh. Within this Si mesh, four square photodiode cells are fabricated. There is no breaking point in the Si mesh constituting 1360 through-holes. The mesh dimensions are 150 ƒÊm in pitch, and 20 ƒÊm in beam width.@The honeycomb mesh is 5 ~5mm2 in area.

Figure 4: Fabricated transmission-type position sensor

To demonstrate the function of the multi-point position detection maintaining a straightness of the laser beam, the transmission type position sensors were placed on the translation stage of a machining machine and the deviation from the straight movement was measured. Two transmission-type position sensors are placed as

Figure 5: Translation measurement of table.

shown in Figure 5. Since two sensors were placed at the two ends of the stage, the values of yawing and pitching were calculated from the displacements. Figure 6 shows those values measured in the experiment. The two values are measured simultaneously as a function of the stage displacement. Due to the simple optical configuration, the deviations in 6 degree of freedom were obtained easily.

Figure 6: Angular deviation as function of position