Multilayer Laboratory

Introduction

 The ESRF Multilayer Laboratory is in charge of the development and the production of multilayer coatings for X-ray optical devices. It is open to any ESRF beamline that needs advice concerning existing or future multilayer-based optical elements. Being part of the ESRF Optics Group, the laboratory works in close collaboration with the Metrology Laboratory, the Crystal Laboratory, and the Beamline BM05. It is located in Sector 03-7.

The core of the laboratory is a sputter deposition facility where substrates up to 100 cm long and 15 cm wide can be coated with multilayers of desired repetition period and lateral thickness gradient. If necessary, depth-graded layered structures can be deposited to tailor the obtained reflectivity profile.

All multilayers as well as the required substrates are routinely characterised using an in-house laboratory X-ray reflectometer. More demanding performance tests are usually carried out on the Optics Beamline BM05. Computational algorithms are used to retrieve the structure of deposited multilayers as well as to simulate the optical properties of those to be designed.

Multilayers

 A multilayer is a stack of (usually two) alternating thin layers (A,B) deposited N times on each other (Figure 1). The resulting structure has a repetition period d = t_A + t_B where t_i is the single layer thickness. Due to different optical indices n_i of the two layers, the whole stack acts like a one-dimensional artificial Bragg reflector when exposed to an X-ray beam.

Fig. 1: A schematic multilayer structure and a typical measured reflectivity spectrum. Layer A usually consists of a strongly absorbing material (metal). Layer B is a spacer made of a low-density material.

The properties of multilayers can be described in analogy to those of single crystals. Wavelength l, periodicity d, and Bragg angle theta are related by the modified Bragg equation

taking into account refraction effects. Since the multilayer period d ranges typically between 2.0 nm and 10.0 nm, the Bragg angle theta for hard X-rays (E = 5 to 100 keV) is rather small (0.2 to 2.0 deg). The scattering power increases with increasing optical contrast between the layer materials. The reflectivity is strongly affected by interface roughness between adjacent layers. The penetration depth is limited due to strong absorption and extinction effects leading to a typical spectral resolution of about 1-2%. This offers a relatively high photon flux as compared to single crystals. By variation of the thickness ratio gamma = t_A / d (the equivalent of the structure factor of single crystals) one can strongly reduce undesired harmonics in the reflected spectrum. More recently, the available range of energy resolution has been extended from 0.25% to 20% using low-contrast and depth-graded multilayers, respectively.

Fig. 2: Focusing geometry using a multilayer mirror

Multilayers are often used as focusing elements on beamlines (Figure 2). For this purpose, they have to be bent and then require a lateral thickness gradient along the beam footprint. The depth gradient can generally be neglected. Using simulation algorithms we are able to design the optimum multilayer structure for the desired application and photon energy. For each case, the theoretical optical performance has to be reconsidered in the light of realistic thin film growth and chemical stability.