Tests At The Operating Wavelength

The gain of the CCD camera, measured with the photon-transfer technique, was measured to be 3.95 e-/ADU. Figure 5 shows a 5 sec dark image of the integrated detector. The bright disk is the image of the MCP intensifier's background irradiance, which is an average of 800 ADU higher than the CCD.

Figure 5. A 5 sec dark image. Figure 6. Image of the beamed radiation at 17.1 nm

Figure 6 is an image of the 17.1 nm radiation of the beam used for the experiments. To evaluate the spatial resolution, a grid was used (see Fig. 7). The grid is divided into 4 areas with the opaque and void strips with widths of 9, 13, 19 and 23 ^m respectively. Since the radiation beam was well collimated, the grid was installed directly in front of the phosphor screen of the detector with no additional optical devices to project the image of the grate on the detector. Figure 7 (right) shows the image of the grid acquired with the detector. The grid plate was not thick enough for the blocking strips to be completely EUV opaque and the darker area in the center results from a lack of thickness uniformity of the grid plate. In the image acquired, the 19 ^m grid can be resolved. Therefore the detector resolution is better than 26 Lp/mm. Figure 8 is the intensity distribution of the 19 ^m grid image.

The cross section of the EUV light beam is small enough to be confined within the sensitive area of the EUV photo-diode installed in the beam or within the bounds of the EUV detector. Therefore, the total photon flux Fphoton of the radiation beam can be acquired with the diode by

photon qe where Ephoton is the photon energy in eV, I is the current of the diode in amps and qe is the charge of one electron in coulombs. We define the response of the detector as the ratio of output electrons in the CCD readout node to the number of the incident EUV photons. Figures 9 and 10 show this ratio at 17.1 nm and 19.5 nm. The response of the detector is measured to be 18 e-/photon and 21 e-/photon at wavelengths of 17.1 nm and 19.5 nm respectively.

immiituii i mil ttiii m im>-miim ml,; imii

I III ha I .11111 till III IIM II I IIIIIIIIHIIII M11 II IM II III 11 III M11

iiiiiiiiniiii

IIIIIIIIHIIII iiiiiitiiiiiii 1111 II H II I II t iiiiihihiiip

IIIIIIIIHIIII llllltllllllll

Figure 7. (left) Spatial resolution test grid and (right) its corresponding EUV image acquired with 17.1 nm wavelength illumination.

19 miron grid

19 miron grid

—'—i—'—i—'—i—r—i—»—T—i—i—— 1 ! 1—i

1140 1144 1145 1152 1156 1160 1164 1168 1172 1176 1180

Pixel NO.

Figure 8. Intensity distribution of 19 ^m grid image.

—'—i—'—i—'—i—r—i—»—T—i—i—— 1 ! 1—i

1140 1144 1145 1152 1156 1160 1164 1168 1172 1176 1180

Pixel NO.

Figure 8. Intensity distribution of 19 ^m grid image.

2.00E+010 1.80E+010 ■ 1.60E+010 ■ i 1.40E+010 I 1.20E+010 ! 1.00E+010 ■

G: measurements on 17.1nm - Linear Fit of DATA171_G

Incident photon number (10 photons)

Figure 9. Response of the detector at a wavelength of 17.1nm.

1.80E+010-1.60E+010-1.40E+010-■ 1.20E+010-; 1.00E+010-> 8.00E+009-6.00E+009-4.00E+009-

H: measurements on 195nm - Linear Fit of DATA195_H

Incident photon number (1C8 photons)

Figure 10. Response of the detector at a wavelength of 19.5nm.

R: 21 e/photon

4.00E+009

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