NbC/Si multilayer mirror for next generation EUV ... - OSA Publishing

NbC/Si multilayer mirror for next generation EUV light sources Mohammed H. Modi,1, * S. K. Rai,1 Mourad Idir,2 F. Schaefers,3 and G. S. Lodha1 1

X-ray Optics Section, Raja Ramanna Centre for Advanced Technology Indore 452013, India Brookhaven National Laboratory – NSLS II, 50 Rutherford Dr. Upton 11973-5000, NY, USA 3 Helmholtz-Zentrum Berlin (HZB BESSY-II), Institute for Nanometre Optics and Technology, 12489 Berlin, Germany * [email protected] 2

Abstract: In the present study we report a new multilayer combination comprised of refracting layers of niobium carbide and spacer layers of silicon as a more stable and high reflecting combination for the 10 - 20 nm wavelength region. The reflectivity of the new combination is comparable to Mo/Si conventional mirrors. Annealing experiments carried out with NbC/Si multilayer at 600°C temperature showed a ~2.5% drop in the soft xray reflectivity along with a marginal contraction in the multilayer period length. The multilayer structure is found stable after the heat treatment. Crystallization of the niobium carbide and silicon layers is responsible for the compaction in the period length as revealed by the grazing incidence xray diffraction measurements. No signature of silicide formation or any other chemical species could be detected. The multilayer structures were grown by ion beam sputtering technique using a compound target of niobium carbide. Soft x-ray reflectivity measurements performed at the Indus-1 and BESSY-II synchrotron radiation sources are found in good agreement with the simulations. ©2012 Optical Society of America OCIS codes: (230.4170) Multilayers; (340.7470) X-ray mirrors; (310.4165) Multilayer design; (310.1860) Deposition and fabrication.

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Received 16 May 2012; accepted 30 May 2012; published 20 Jun 2012

2 July 2012 / Vol. 20, No. 14 / OPTICS EXPRESS 15114

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1. Introduction Mo/Si multilayers are currently the most promising reflective coating for extreme ultraviolet (EUV) lithography activity operating at 13.6 nm wavelength. It is a known combination for best reflectivity performance in the spectral region close to the Si L-edge. However, it has a severe drawback of poor thermal stability due to negative heat of mixing between the Mo and Si. Resultantly this structure undergoes interface degradation right after the deposition process. Different approaches have been adopted in the past to overcome the interdiffusion and thermal stability problem by inserting a barrier layer of boron carbide or pure carbon in between Mo and Si layers [1]. However, all these efforts lead to significant compromise in the reflectivity performance. Rapid development of free electron laser (FEL) sources [2, 3] generating ultra short EUV pulses have posed a new challenge for the optics. X-ray pulses of very high intensity induce radiation damage in optical components [4, 5]. The emerging technology requires improved optical components. Numerous research work is being carried out to find a high stability and high reflectivity multilayer mirrors [6, 7]. Carbides of refractory metals (Mo, Nb) are known to exhibit excellent physical properties like high melting point, good electrical conductivity and extreme hardness. In the 10 - 20 nm wavelength region, the refractive index properties of these metal carbides are almost identical to their metallic constituents. Therefore, a multilayer comprised of a refracting layer of metal carbide with a spacer layer of Si should exhibit a similar high reflectivity performance. Previous studies on carbide layers suggest that these carbides are non-stoichiometric and contain vacancies in the carbon lattice sites [8, 9]. In multilayer case, the presence of unsaturated carbon in the near vicinity of metal species (Nb, Mo) may act as a barrier for the Si to prevent the chemical mixing. Earlier, a barrier layer of B4C between the Mo and Si was used for this purpose. However, inserting an extra layer leads to a phase variation in the

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waves reflecting from different interfaces at the Bragg condition and thereby resultant phase mismatch reduces the reflectivity performance. Moreover, the barrier layer should be ultrathin in order to minimize the phase mismatch which requires a stringent deposition control. Beta

Delta

0.20

0.10 Mo

Mo

NbC

0.01

β

NbC

δ

0.10

Si

Si 0.00

8

10

12

14

Wavelength (nm)

16

18 8

10

12

14

16

18

0.00

Wavelength (nm)

Fig. 1. A comparison of the optical constants of NbC, Mo and Si in the10-20 nm wavelength region.

In present study, the NbC/Si multilayers are deposited on Si substrate to carry out high temperature annealing studies. The carbide layers of niobium are deposited using a commercial sputtering target of NbC in an ion beam sputtering system. Both soft x-ray and grazing incidence x-ray reflectivity (GIXR) measurements performed after the annealing experiments suggest that the multilayer structure is highly stable. The soft x-ray reflectivity measurements carried out at the Indus-I and BESSY-II synchrotron sources are found in good agreement with the simulations based on structural parameters derived from the GIXR analysis. 2. Experimental The NbC/Si multilayers were deposited on silicon (100) substrate using an ion beam sputtering system. A base pressure of ~1 × 10−7 mbar was created before purging the high purity Argon gas at constant 4.5 SCCM flow rate. High purity commercial targets were used to deposit the NbC and Si layers. The film thickness was controlled by keeping the deposition time fixed for each layer. To study the thermal stability of the multilayers, one sample was annealed up to 700°C in steps of 100°C for 40 minutes at each step. Other sample was directly annealed at 600°C for 1h in a vacuum of < 1 × 10−6 mbar. GIXR measurements were performed using Cu Kα (λ = 0.154 nm) radiation at a homemade reflectometer in 0 to 3° angular range with step resolution of 0.005°. The reflected beam was analyzed by a multilayer monochromator followed by a NaI scintillation detector over a six order of dynamic range. Grazing incidence x-ray diffraction (GIXRD) spectra were recorded using a Philips X’Pert Pro diffractometer. GIXRD measurements were performed at a fixed glancing angle of 0.27° to limit the penetration of the beam into the film. GIXRD data were recorder in 30 - 75° range in step of 0.05° and 10 sec acquisition time. The soft x-ray reflectivity measurements were performed at the Indus-1 and BESSY-II storage ring facilities. Normal incidence reflectivity near 87° incidence angle was measured at the BESSY-II optics beamline. At Indus-1, wavelength v/s reflectivity measurements of as prepared and annealed sample were carried out at 70° incidence angle using the reflectivity beamline [10]. Details of the experimental station on Indus-1 reflectivity beamline are given in Ref [11]. The reflectivity data were analyzed using the Parratt formalism [12]. The reflected field intensity was calculated with a grazing incidence angle θ as an independent parameter. The effect of interfacial roughness was considered using the Nevot-Croce model [13]. In a least

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square refinement procedure, thicknesses, densities and interface roughness were optimized using a LabVIEW simulator [14]. 3. Results and discussion In order to find a stable multilayer structure for 10-20 nm wavelength range we tried a new multilayer combination comprised of refracting layers of niobium carbide with spacer layers of silicon. Using the atomic scattering database of Henke et al. [15] it is found that the NbC compound gives a similar refractive index contrast with Si as one obtains using the Mo in the 10 - 20 nm wavelength region. Optical constants of NbC, Mo and Si are plotted in the Fig. 1. The reflectivity calculations carried out for NbC/Si and Mo/Si using identical structural parameters (d = 6.3 nm, N = 51 layer lairs, Γ = 0.428, roughness σ = 0.3 nm for all interfaces) shows that the calculated reflectivity of Mo/Si (73.1%) and NbC/Si (70.6%) are very close. The results of calculations are plotted in the Fig. 2. It is found that the theoretical reflectivity for any other identical structure of Mo/Si and NbC/Si remains close with each other. 80 85.0 degree

Mo/Si NbC/Si

Reflectivity (%)

60

40

20

0 11.5

12.0

12.5

13.0

13.5

14.0

Wavelength (nm) Fig. 2. Calculated soft x-ray reflectivity profile of Mo/Si and NbC/Si multilayers with identical structural parameters (d = 6.3 nm, Γ = 0.428, σ = 0.3 nm, N = 51 layer pairs). At 85.0° incidence angle, the peak reflectivity of two multilayers is slightly different by ~2.5%.

In order to experimentally test the reflectivity performance of NbC/Si multilayer at elevated temperatures, a sample of d = 8.0 nm, Γ = 0.5, N = 10 layer pairs is annealed up to 700°C in steps of 100°C for 40 minutes at each step. The GIXR results suggest that upon annealing at 700°C the multilayer structure remains intact. A marginal contraction in the period length from as deposited value of 8.14 ± 0.01 nm to 7.93 ± 0.01 nm is found. The reflectivity at λ = 0.154 nm is reduced from 56% to 53%. Results of GIXR measurements along with the best fit are shown in the Fig. 3. In the inset, a comparison of the first Bragg peak of as-prepared, 500°C and 700°C annealed samples indicate that the multilayer performance is marginally changed at high temperatures. The contraction in the multilayer period is probably due to inter atomic rearrangement caused by the annealing as indicated by the change in density of the NbC and Si layers. The GIXR results revealed that the density of the Si is 2.2 ( ± 0.036) g/cm3 for as prepared sample which increases to 2.37 ( ± 0.04) g/cm3 after the annealing at 700°C. Similarly, for as prepared sample the density of the NbC layer is 6.87 ( ± 0.11) g/cm3 which reduces to 6.57 ( ± 0.11) g/cm3. After the annealing, a net increase of 7.7% in density of the Si layer and a net decrease of 4.6% in density of the NbC layer is found. There is a net increase in the multilayer density of 3.3%. This density change should have caused a period contraction from 8.14 nm to 7.87 nm. However the period thickness after 700°C is found to be 7.93 nm instead of 7.87 nm. This difference of ~0.06 nm may be due to roughness convolution effects during the analysis. In the as prepared sample, the roughness of Si-on-NbC interface and NbC-on-Si interface is 0.3 nm and 0.84 nm respectively. After 700°C annealing these two roughness

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5

10

First Bragg peak

Intensity (a.u.)

3

10

1

10

Reflectivity (%)

values are slightly changed to 0.4 nm and 0.82 nm respectively for Si-on-NbC and NbC-on-Si interfaces. Virgin 500C 700C

50 25 0 0.6

0.7

-1

10

700C

-3

10

500C

-5

10

As deposited

0.0

0.5

1.0

1.5

2.0

2.5

3.0

Incidence Angle (deg) Fig. 3. GIXR results of the NbC/Si multilayer annealed up to 700°C for 40 minutes are shown. The open circles represent measured data whereas best fit is shown by the continuous line. After the annealing, Bragg peaks shift to higher angle indicating a contraction in the period length as marked by the vertical dashed line. All features in the reflectivity curve persist after the annealing which suggests the multilayer structure is intact. In the inset, a comparison of the first Bragg peak of as deposited sample with that of 500°C and 700°C annealed sample is shown.

In the present study, the NbC/Si multilayers used were grown by ion beam sputtering process. In these multilayers the density of the NbC layer is found to be lower than its bulk value 7.8 g/cm3. In thin film process it is common to have a reduced density with respect to the bulk value because of variation in the packing density. Presence of voids in between inter atomic spaces is the main cause of the reduced density which has been observed in the simulation of deposition process [16]. In the present case, difference in density of the NbC layer with respect to its bulk value is attributed to variation in stoichiometry and presence of unsaturated carbon atoms in the film [8]. The intra-layer structural changes caused by thermal annealing are investigated by GIXRD measurements and the results are shown in Fig. 4. From the GIXRD data it is evident that both NbC and Si are in the amorphous state before the annealing. After 600°C annealing, the interatomic displacement caused a poly-crystallization in both the materials. The intense peak observed at 34.2° is corresponds to the NbC (111) phase [JCPDS No. 038-1364]. The presence of a diffraction peak at 48.1° corresponds to Si (220). Other phases of niobium carbide could not be detected because of their low diffraction volume. However, from the data it seems that there is a peak near 53.8°, which could be due to asymmetric reflection from the substrate. No signature of the formation of any silicide phases could be detected from the GIXRD data. This suggests that no free volume of niobium atoms is available to react with the silicon even at the interfaces. It seems that all niobium atoms are chemically isolated due to their bonding with carbon or by surrounding free carbon atoms. Earlier it was reported that the stoichiometry of NbC depends on deposition technique and process conditions [8]. In any case the issue of stoichiometry is not supposed to affect the stability of the carbide/silicon multilayer. The stability issue is mainly related with the presence of free niobium atoms and its accessibility to near neighbor silicon.

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NbC (111)

30

Si (220)

Intensity (a.u.)

25 20 15 10 5 0 30

40

50

60

70

80

2-Theta (deg) Fig. 4. GIXRD measurement of NbC/Si multilayer measured at fixed incidence angle of 0.27°. After the annealing, crystallization takes place in both materials as indicated by the presence of diffraction peaks of NbC (111) at 34.2° and Si (220) at 48.1°.

Figure 5 shows a soft x-ray reflectivity spectrum of a NbC/Si multilayer with period d = (6.54 nm, Γ = 0.49) × 10 with an extra NbC layer on top as measured at BESSY-II at 87.0° near-normal incidence angle. The measured reflectivity is 11%, whereas the expected reflectivity was 20% if the NbC layer had a density close to the bulk value. 12

Reflectivity (%)

87.0 degree

9

d = 6.54 nm N = 10 Layer Pairs Γ = 0.49 σ NbC = 0.7 nm σ Si = 0.7 nm

6

3

0

8

10

12

14

16

Wavelength (nm) Fig. 5. Soft x-ray reflectivity at near normal incidence angle is measured at the BESSY-II synchrotron facility. The circles represent measured data whereas the continuous line is a best fit obtained with the parameters shown in the figure.

A 30 layer pair sample with d = 7.0 nm (Γ = 0.3) has given a peak reflectivity of 42.45% at 13.0 nm wavelength as shown in the Fig. 6. In this sample, roughness of the Si and NbC layers are found to be 0.9 nm and 0.8 nm respectively. The higher roughness of this sample is attributed to the lower thickness of the NbC layer. The soft x-ray reflectivity performance of the same multilayer measured after 600°C annealing for 1h is also shown in the Fig. 6. After the annealing the Bragg peak shifts towards the lower wavelength side due to contraction in the multilayer period from 7.0 nm to 6.86 nm. The multilayer reflectivity is reduced from 42.45% to 40%. After annealing, roughness of both Si and NbC layers are found 0.8 nm. The annealing experiments carried out upto 700°C on different samples of NbC/Si multilayer showed a high thermal stability and a marginal change in reflectivity performance. Whereas, in case of Mo/Si multilayer it is earlier reported that the chemical mixing and structural degradation starts from 150°C only [17]. Incorporation of B4C barrier layer in between the Mo and Si has increased its thermal stability up to ~400°C. The present study suggests that, the NbC/Si multilayer has a potential to emerge as a high reflectivity mirror for the 10 - 20 nm wavelength range. The new combination can be used for high thermal load applications. The clean interface profile comprised of two-layer structure with no chemical

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degradation is an important merit function for the NbC/Si structure. NbC/Si multilayer has a potential to replace the conventional Mo/Si mirrors from high reflectivity normal incidence applications. Efforts are underway to produce a high quality NbC/Si multilayer to get best possible reflectivity as predicted from the simulation. 50 600C 1h As deposited Fit

70 degree

Reflectivity (%)

40 30 20 10 0 10

11

12

13

14

15

Wavelength (nm) Fig. 6. Measured and fitted soft x-ray reflectivity spectra of as deposited and 600°C annealed sample. After annealing the Bragg peak shifts towards lower wavelength due to period contraction from 7.0 nm of as deposited value to 6.86 nm after the 1h annealing.

4. Conclusions The present study suggests that the NbC/Si multilayers have a potential to use as a high reflectivity and high stability mirror in the 10 - 20 nm wavelength range. The calculated reflectivity of this new material combination is comparable with that obtained from the conventional Mo/Si multilayer for the identical structural parameters. The NbC/Si multilayer showed a period compaction of 2% and a soft x-ray reflectivity drop of ~2.5% after the annealing at 600°C for 1h. Another multilayer annealed up to 700°C in step of 100°C for 40 minutes showed a period contraction of 2.5%. No interdiffusion or interlayer formation is observed after the heat treatment. However, the density of NbC and silicon layers is changed which is probably due to crystallization of both the materials, as revealed by the GIXRD measurements. No silicide formation or any other chemical species is detected. Test samples measured at the Indus-1 and at the BESSY-II synchrotron have shown a low reflectivity performance because of high roughness values found in both NbC and Si layers. Further studies are required to reduce the roughness to state-of-the-art values comparable to that routinely achievable in Mo/Si structure. Nevertheless, the high thermal stability and excellent re〉ection properties of the NbC/Si multilayers make it a potential candidate for the use under heavy radiation load on the next generation light sources operating in the EUV region (Plasma Sources, Free Electron Lasers, etc.).

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