Structure and depth profile composition analysis of Cr/(B4C)/V/(B4C) multilayer for water window application
Introduction
Based on the natural optical contrast between carbon and water in the water window region (λ = 2.3–4.4 nm), soft X-ray (SXR) microscopy has permitted the imaging of the hydrated biological samples at the nanoscale level [1], [2]. It has been tremendously developed in both the lab-based facilities and synchrotron [3], [4]. The multilayer mirror is an important component of the SXR microscope system. It often serves as the normal incidence collector, or high resolution objective mirrors. Sc-, Ti-, and V-based multilayers are commonly used in the water window region given to their 2p absorption edges located at λ = 3.11 nm, 2.73 nm, and 2.42 nm, respectively. Despite the high theoretical reflectance of these multilayers, the experimentally achieved reflectance is rather low, as reported for Cr/Sc [5], [6], [7], [8], [9], Ni/Ti [10], [11], and Cr/V [12], [13]. This is caused by the strong effect of the interface roughness and diffusion on the reflectance of ultrathin multilayers with a layer thickness smaller than 1 nm. To improve the layer structure and the SXR reflectance, interface engineering methods using B4C barrier layer [14] or nitridation [15] have been developed. A maximum reflectance of 32% was achieved at λ = 3.11 nm using the Cr/Sc multilayer with B4C barrier layer [13]. At the short wavelength range of the water window, λ = 2.42–2.73 nm (near the V-L edge), Sc- and Ti- based multilayers cannot be used due to their low reflectance. Instead, V-based multilayers are used for the SXR mirror [12], [16], [17]. Cr/V is one of the promising candidates as it can provide both a high theoretical reflectance, 60% near the V-L edge at near normal incidence, and relatively sharp interfaces. Nevertheless, the development of V-based multilayer mirror is difficult due to the even smaller period than required for the Sc- and Ti-based multilayers. The Cr/V multilayer is also less studied compared to Cr/Sc, while the internal structure and chemical composition of this multilayer system is not well understood. This prevents the further improvement of the SXR reflectance of the multilayer at this region.
Interface barrier layers have been used to improve the Cr/V multilayer structure and a 9% reflectivity was obtained at near normal incidence [13]. To further explore this method and study the physical mechanism of the improvement, we have made a series of works to characterize and develop the interface engineered Cr/V multilayer mirrors using B4C as the barrier layers. Theoretical calculations displayed that the introduction of 0.1 nm B4C barrier layer at both interfaces will only decreases the absolute reflectance by 1% at both near normal incidence and the Brewster angle (45°). The experimental results show that the interface widths of the 1.8 nm-period Cr/V multilayer were significantly reduced after introducing 0.1 nm barrier layers, and a high reflectance of 24.3% was achieved at λ = 2.441 nm under the grazing incidence of 42° as reported in Ref. [18]. A suppressed crystallization of the layer structure after introducing barrier layers was also observed. However, the chemical changes induced by the ultrathin B4C barrier layer and the possible compound formation at the interfaces were unknown. Thus, the improving mechanism of the barrier layers is still not fully understood. In this paper, we will mainly focus on the composition analysis of the Cr/V multilayer structure without and with B4C barrier layers, using X-ray photoelectron spectroscopy (XPS). Vanadium boride, extra vanadium carbide and B4C compound were found in the multilayer with barrier layers, which is considered as the main reason for the reduced interface width of the structure.
Section snippets
Experimental details
The multilayers were fabricated by direct current (DC) magnetron sputtering. The base pressure before deposition is 3.0 × 10−4 Pa. High purity argon gas (99.999%) is supplied as the working gas, and the pressure during deposition is 0.2 Pa. The substrate are superpolished single-crystal silicon wafers with the root-mean-squared (RMS) roughness of ∼0.2 nm. The deposition rates of Cr and V are 0.07 nm/s and 0.02 nm/s, respectively.
Three periodic multilayers without barrier layer (sample 1), with
GIXR measurements and fitting results
The GIXR measurements and fitted curves of the Cr/V and Cr/B4C/V/B4C multilayers are displayed in Fig. 1. For the Cr/V multilayer without B4C barrier layer (sample 1), only the 1st Bragg peak is clearly observed with a low peak reflectance of 0.8% (Fig. 1(a)). The 2nd and 3rd Bragg peaks are very weak. After introducing 0.1 nm B4C barrier layer at the interfaces (sample 2), all Bragg peaks are much enhanced while the peak reflectance of the 1st Bragg peak is much increased to 1.9%. It is
Summary
In order to study the composition changes in the SXR Cr/V multilayer with ultrathin B4C barrier layers, different Cr/V multilayers with and without B4C barrier are studied by using GIXR and XPS depth profile measurements. The GIXR results show that the 0.1 nm B4C significantly improve the layer structure of the thick Cr/V multilayer with a period of ∼4.0 nm which is similar to the case of the 1.8 nm-period multilayer [18]. The XPS depth profile displays an enhanced elemental contrast of Cr and
Acknowledgments
This work is supported by National Natural Science Foundation of China (No. 11443007, No. 11505129, No. 11575127), National Key Scientific Instrument and Equipment Development Project (No. 2012YQ13012505), and Shanghai Pujiang Program (No. 15PJ1408000).
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