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Report
 
EUV Reflectivity measurements

on Acktar Sample Magic Black

S. Doring, Dr. K. Mann

 
Laser-Laboratorium G¨ottingen e.V.

October 28, 2011
 
 
Measurement report
 
1. Introduction

 

Using the EUV source of LLG, reflectivity measurements  have been conducted  on Magic Black Sample (Ref.  #333) at  a center  wavelength  of 12.98nm.  The measurement proce- dures and results  are described  in the following.  All angles described in this report  are grazing angles (relative  to sample surface).

 

 

2. Setup

 

Fig.1 shows the  experimetal  setup.   The  EUV radiation  is generated  in the  chamber on the left (source chamber)  using a laser produced  plasma  from a gas puff target.   The spectral characteristics of the source depends on the employed target gas as shown in fig.2 (left).  For the measurements  Oxygen was used as target  material.

 

 

Figure 1:  Left:  Schematic  of the  measurement setup.   Right:   Measurement   beam  profile at sample  position  recorded  with  a  EUV  sensitive  CCD  camera.    The  size  is  roughly  300µm  x

500µm.

 

 

The light emitted  by the  source passes through a 150µm slit aperture placed approx- imately  2mm behind  the  plasma  and  a Zirconium filter which blocks higher wavelength components of the spectra.  In the optics chamber an optics in Kirkpatrick-Baez  arrange- ment images the  slit onto the  sample placed in the  middle of the  experimental  chamber via a Mo/Si multilayer  mirror.  This mirror works as a spectral filter in the EUV spectral region (blocking out-of-band  radiation)  and is adapted  to the emission line of Oxygen at 12.98nm (cf fig. 2, right).  The reflectometer placed in the experimental chamber consists of two independent rotary  stages for sample holder and detector  diode, respectively. The beam diameter  on the sample (perpendicular  to the  incident radiation) is appr.   300µm in horizontal  and 500µm in vertical direction  (cf fig. 1, right).

For monitoring  of intensity  fluctuations  of the plasma source, a second EUV photodiode is used as a  reference monitor.   It is illuminated  by a second filtering multilayer  mirror with the same specifications as the main mirror.

Figure 2:  Left:  Oxygen spectrum  (red)  together  with the spectral  reflectivity  characteristics of the Mo/Si  filter mirror  (black).  Right:  spectrum  of the measurement beam calculated  from raw spectra  and  filter mirror  reflectivity  (left).   About 84% of the beams overall intensity  is in the Oxygen 12.98nm emission  line.

 

 

3. Measurements

See tables  below for setup  specifications and measurement parameters.

 

Parameters for  measurements

 

 

geometry for measurements

 

detector   scans  at   5/10 /45

sample angle

angular range

-10     to  +10     relative   to   specular   angle

(2*sample angle)

angular detector  step size

0.1

averaging

25 EUV pulses/measurement point

 

Setup specifications

angular accuracy

<1

angular repeatability

<0.1

absolute accuracy of reflectivity

<5%

absolute repeatability of reflectivity

<0.5%

 

Three measurements  have been conducted to analyse EUV light scattering  properties of the sample. For these the angle of incidence of the light onto the sample was set to 5 , 10and 5  degrees grazing angle (angle to sample surface), respectively.  For each of these positions  a detector  scan has been made,  centered  on the  angle of specular reflectance with a range of -10to +10. All the measurements  are background  corrected. 

 

Figure 3: Results of the conducted measurements. Top: Summary of all three detector scans plotted against the detector angle relative to the angle of specular reflectance of each measurement (10◦, 20◦ and 90◦). Below, for clarification of the angular ranges probed, the measurements are plotted against the absolute detector angles.

 

4. Conclusion

The data recorded for 45◦ sample angle represents the noise level of the detector. For this sample angle either all radiation impinging on the sample is absorbed or the re- flected/scattered light is too weak to be detected with our equipment. Under 5◦ and 10◦ sample angle the specular reflectance of the measurement beam can be seen as peaks in the middle of the respective datagrams. These peaks have a reflectivity of approximately 1% and 0.1% respectively. The flanks of these peaks correspond to the scattered light. Their asymetry stems from the fact that under small angles of incidence the scattered light is blocked by the sample itself.

 

 

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