MODELO DE GESTIÓN Y ADMINISTRACIÓN DEL TALENTO HUMANO:

Ideally, the straightness o f the reference system can be checked by measuring a perfectly flat surface. Any deviation from the flatness would be due to the imperfection o f the reference system. However, it was not practical to obtain an adequate flatness standard o f reasonable size (more than 100 mm). Instead, the method used was to compare the measurement results on a nominal flat from two instruments, one was SPLOT and the other was a high quality commercial product: the WYKO 6000 phase shifting interferometer.

The sample was a 150 mm diameter and 25 mm thickness Zerodur nominal flat. It had been polished to 400 nm P-V nominal flat, plus some subsequent polishing experiments on this surface by D. Kim in OSL. Figure 7.1 shows the contour o f the resulting deformed surface, measured with the WYKO 6000. Since the interferogram in the Fizeau configuration is formed by the inclination o f the target surface with respect to the reference one, the contour o f the surface can be obtained by removing the tilting term from the original OPD data. Several signatures o f the polishing experiments could easily be seen. Overall P-V error o f the surface was shown as about 700 nm. Figure 7.2 shows a two-dimensional profile, along a centre line selected for the probe tip o f SPLOT to trace. Both ends o f this line were marked on the sides o f the sample. This profile showed the particular features o f the polished surface.

7.2.2 Measurements

Figure 7.3 illustrates a series o f measurements on the sample flat with the laser reference system enabled. Two scans were made along the selected centre line. Each scan consisted o f 22 points at 7 mm intervals. Each graph was generated by subtracting a least square linear fitting from the original measurements to remove the tilt term. The solid line was the result from the WYKO 6000.

SATREC1 OP D Title: Note: Contour Date. *06^4/99 ^ Time. 17:32.-05 WL: 632.800 nm W ed^. 0.50 wv/fr Size: 368 X 240 Pupil: 97.0%

Surface Stats

Terms Removed:

Figure 7.1: The picture o f surface contour o f sample flat using WYKO 6000.

0. 4 - 0.3 - =L O) -0.3 - - 0 . 5 - -0. 6 0 2 0 0 0 0 4 0 0 0 0 6 0 0 0 0 8 0 0 0 0 1 0 0 0 0 0 1 2 0 0 0 0 1 4 0 0 0 0 D istance ( ^ )

Figure 7.2: Two-dimensional profile o f the sample flat along the line which SPLOT traces to measure.

t en 0) X 0,6 0.4 0. 2 0 .0 0 .2 0.4 -0 .6 1 0 0 0 0 0 1 2 0 0 0 0 1 4 0 0 0 0 0 2 0 0 0 0 4 0 0 0 0 6 0 0 0 0 8 0 0 0 0 Horizontal position (/xm)

Figure 7.3: Profile measurements of the sample flat with the reference system enabled. Square and circle points are results by SPLOT and the solid line is the profile produced by the WYKO 6000.

The results of the two scans by SPLOT satisfactorily replicated the features of the surface observed by the WYKO 6000. The 0.5 pm o f valleys at the sides were clearly seen, and the slopes at marginal areas were similar between the two types of instrument. However, due to the sampling distance, the detailed description o f the complex central part was simplified.

The two SPLOT scans were within ±0.2 pm. This difference is quite similar to the P-V error of the repeatability at the single point measurement due to the air turbulence, as shown in the chapter 6. Without more suppression of the air turbulence, this difference

is unlikely to be reduced further. However, the differences between the average of two SPLOT scans and one WYKO 6000 scan are the systematic measurement errors. One o f the clear example of systematic errors was shown in the region of 90-120 mm, where the successive errors of -0 . 2 pm were visible.

E ■g) ■<D 0,6 0.4 0 .2 0.0 0.2 0.4 -0. 6 2 0 0 0 0 4 0 0 0 0 6 0 0 0 0 8 0 0 0 0 0 1 0 0 0 0 0 1 2 0 0 0 0 1 4 0 0 0 0

Horizontal position (}im)

Figure 7.4: Profiles o f two scans of the sample flat without the reference system enabled. The solid line is the profile produced from the WYKO 6000.

The systematic measurement errors may come from the reference system, or other sources of error. The distinction of the effect of the reference system from other source o f error can be possible by comparing the two measurements, one with the reference system enabled and the other without the reference system. For this purpose, the sample flat was measured again but without the reference system enabled (i.e. with open loop operation). Figure 7.4 illustrates a series o f measurements on the same sample flat in this situation. The two scans were made along the same line as for the measurements with the reference system. Each graph was generated by removing the tilt error from the original data.

These measurements also showed features of the sample surface similar to measurements with the reference system enabled. However, successive errors o f -0.2 pm were not visible in the region o f 90-120 mm. Therefore, the systematic errors occurred in this region are due to the reference system.

Except the region o f 90-120 mm, the measurements without the reference system enabled were generally worse than with the reference system enabled. For example.

the errors without the reference system enabled were -0 . 1 |im in the range o f 0 -6 0 mm

and maximum +0.2 p.m in the region o f 80 m m -100 mm and -0.3 |j,m in the region o f 130 mm - end. On the other hand, the errors with the reference system enabled remained within ± 0.1 fim in the same regions. These findings imply that the reference system compensated systematic errors from the air bearing carriage efficiently although not completely.

As a result, the RMS errors o f a SPLOT scan with respect to the WYKO 6000 scan were average 0 . 1 0 pm and 0 . 1 2 pm for the measurements w ith and without the

reference system enabled respectively. The reason why RMS errors o f both measurements are similar is due to the random error and incomplete error compensation o f the reference system.

The reasons for the imperfection o f the reference system are investigated with regard to the motion errors o f the air bearing carriage in the following section.

7.3 Motion errors of the air bearing carriage