Large area Mo/Si multilayer (ML) mirrors with high reflectivity are fabricated using magnetron sputtering deposition system. Thin film growth is optimized for film roughness, density, and interface quality by changing process parameters through fabrication of thin films. Mo/Si MLs are fabricated with varying thickness ratio, number of layer pairs, and periodicity from 0.3 to 0.45, 5 to 65, and 40 to 100 Å, respectively. The samples are characterized using hard X-ray reflectivity and transmission electron microscopy. Soft X-ray performance tests of MLs are done by soft X-ray reflectivity using Indus-1 synchrotron radiation. ML coating with thickness errors of
X-ray multilayer (ML) mirror is a one dimensional artificial Bragg reflector [
Large area MLs have different potential applications such as soft X-ray/extreme ultra violet (EUV) lithography [
Using magnetron sputtering, Kortright et al. [
To meet these requirements, recently we have commissioned a specially designed magnetron sputtering system. In this paper we report fabrication and evaluation of high reflectivity Mo/Si MLs on 300
ML samples are fabricated using magnetron sputtering system which has both DC and RF compatibility [
The performance of Mo/Si ML is tested using Indus-1 reflectivity beamline [
Fabrication of high-reflectivity MLs with good energy band pass depends strongly on nature of interface, density contrast, and thickness error. While thickness error from layer to layer in the multilayer stacks depend on the stability of the process parameters, the interface characteristics and density contrast depends on kinetic energy (KE) of the add-atoms. KE of condensed particles is optimized by adjusting flux and energies of sputtered atoms through a systematic variation of process parameters during growth of thin films, bilayers, and finally MLs. Desired film quality is obtained after several iterations, by optimizing gas flow rate, pressure, power, target- substrate distance, and substrate speed. Some of these results are discussed below.
A typical example of the influence of argon pressure and film thickness on the quality of Mo single film is shown in Figure
Mo film quality interms of thickness (
Film |
|
|
|
|
---|---|---|---|---|
(mbar) | (Å) | (Å) | (gm/cc) | |
Mo-1 | 4.0 |
87.5 | 4 | 7.07 |
Mo-2 | 4.0 |
116 | 4.5 | 10.06 |
Mo-3 | 4.0 |
180 | 6 | 10.1 |
Mo-4 | 3 |
155 | 3 | 10.2 |
M0-5 | 5 |
160 | 6.5 | 9.79 |
Mo-6 | 8 |
175 | 8 | 8.86 |
Measured and fitted XRR profile at Cu K
A challenging task during fabrication of ML stacks is to achieve a stable deposition condition for precise control of thickness in atomic scale from layer to layer. This needs that discharge plasma and its distribution over the target should be stable during fabrication of ML, which takes approximately few hours. The key parameters for stable and uniform plasma depend on purity of Ar gas, uniform gas flow over the target, conditioning of vacuum chamber (degassing and quality of vacuum), and a stable plasma power supply. Quality of vacuum in the process chamber is maintained by constantly pumping without breaking vacuum in the chamber. The out-gassing from inner wall of process chamber was pre-conditioned by fabricating test samples for many runs before final fabrication of actual samples. For stable plasma and to avoid contamination from target surface during film growth before fabrications of actual ML samples, the target is presputtered for about 30 minutes. The stabilization of gas flow rate, power, and vacuum is maintained within
Thickness and roughness of the various layers of Mo/Si MLs deduced from the fit of XRR results of Figures
Sample | Total thickness (Å) | Mo thickness (Å) |
|
Mo roughness (Å) |
---|---|---|---|---|
IL1 thickness (Å) |
|
IL1 roughness (Å) | ||
Si thickness (Å) |
|
Si roughness (Å) | ||
IL2 thickness (Å) |
|
IL2 roughness (Å) | ||
ML-1 | 90.3 | 28.0 | 26.0 | 4.7 |
8.9 | 9.5 | 3 | ||
47.4 | 6.04 | 4 | ||
6.0 | 22.8 | 2 | ||
| ||||
ML-2 | 66.0 | 16.7 | 26.0 | 4.7 |
8.0 | 9.54 | 3 | ||
35.0 | 6.0 | 4 | ||
6.3 | 23.8 | 2 | ||
| ||||
ML-3 | 68 | 21.0 | 28.0 | 5 |
9.0 | 9.5 | 4 | ||
31.0 | 7.6 | 4 | ||
7.0 | 18.5 | 2 | ||
| ||||
ML-4 | 93.4 | 30.4 | 28.02 | 5 |
10.0 | 6.55 | 4 | ||
44.5 | 8.0 | 4 | ||
8.5 | 14.5 | 3 |
Measured and fitted XRR profile at Cu K
After optimizing process parameters by fabricating smaller number of layer pairs, MLs are fabricated with larger number of layer pairs. Figure
Measured and fitted XRR profile at Cu K
Cross-sectional electron microscopy studies are undertaken to study the interfaces and periodicity of Mo/Si ML. Figure
Cross-sectional TEM image of sample ML-3. Dark lines correspond to Mo and bright lines to Si.
Desired lateral thickness profile of the coating on large area substrate is obtained by iterative control of pumping port opening, argon flow from two sides of chamber, substrate motion, installation of masking arrangements, and optimization of target-substrate distance. Uniform flow of argon across the target is realized by flowing unequal gas flow rate on two sides of targets (along length) and adjusting opening of pumping port. Gas flow rate is higher on the pumping side than the other side, to compensate direct pumping effect. After assuring uniform gas flow, the spatial distribution of sputtered atoms depends upon the geometry of target and target-substrate distance. A planner rectangular magnetron source produces a rectangular-shaped sputtering track. More material is sputtered out from the target where magnetic field is more. The spatial distribution of deposition rate profile over a planner rectangular cathode is cosine like structure [
XRR profile of Mo/Si MLs with
Measured lateral variation of periodicity along the length obtained from best-fit results of Figure
In order to further minimize spatially variation of deposition rate along the length, we figured the mask, with tapering towards centre. More flux of deposited material at the centre compared to both sides is compensated by adjusting amount of tapering of mask towards centre. Figure
XRR spectra of Mo/Si MLs with
Measured lateral variation of periodicity along the length obtained from best-fit results of Figure
Reproducibility is the crucial issue for larger-area multilayer for device applications. Our preconditioned system ensures reproducibility of system in terms of density contrast, interface quality, and thickness control from run to run. A typical example of reproducibility of fabricated multilayers from run to run is shown in Figure
XRR spectra of Mo/Si MLs with
The actual performance of MLs is tested using reflectivity beam line on Indus-1 SR source. Figure
Measured and fitted soft X-ray reflectivity (angle scan) profile of Mo/Si ML (sample no. ML-3) at the wavelength
Measured and fitted of soft X-ray reflectivity profile of Mo/Si ML (sample no. ML-4) at the wavelength
Large area, high-reflectivity Mo/Si MLs are developed using specially designed magnetron sputtering system. The process parameters are optimized for growth of thin film suitable for X-ray ML fabrication. Mo/Si MLs are fabricated with rms interface roughness in the range of 2 to 5 Å. The lateral variation in periodicity of ML is minimized to 0.5 Å over 300