Imaging Cross Section of X-ray Multilayer Mirror by Scanning Tunneling Microscope
- PDF / 241,272 Bytes
- 4 Pages / 414.72 x 648 pts Page_size
- 19 Downloads / 154 Views
INTRODUCTION Multilayers are used in x-ray optics as mirrors.' They usually consist of a stack of bi-layers of two different materials. For x-ray optics, the thickness of the bi-layer, or the d-spacing of the multilayer, ranges from 2 nm to 20 nm. Since x-ray multilayers are usually made by evaporation or sputtering technique, most of materials in multilayers are amorphous or polycrystalline. Characterization is a very important part in the study and fabrication of x-ray multilayers. The main parameters that need to be measured are dspacing and -y which relates to the ratio of the thicknesses of the two layers in the bi-layer. Interfacial properties, such as roughness and interdiffusion, are also important but more difficult to detect. X-ray diffraction (XRD) is a commonly used method to characterize x-ray multilayers. 2 The diffraction pattern can be used to calculate dspacing and give information about -y and the number of layers for the multilayer. The XRD measurement gives integrated information from the whole illuminated area. To get a localized, direct image of the multilayer sample, transmission electron microscopy (TEM) has been employed. 3 The TEM picture of multilayer cross section can reveal atomically resolved microstructure. Since the image is formed by electrons going through the sample, it is not exactly a surface picture of the sample. The sample preparation for TEM is a complex and time consuming process. As an alternative and complementary method, we want to explore the application of scanning tunneling microscopy (STM) in the characterization of x-ray multilayers. STM is a surface-sensitive technique with the potential ability to obtain pictures with atomic lateral resolution. Several 213 Mat. Res. Soc. Symp. Proc. Vol. 382 ©1995 Materials Research Society
groups have obtained atomically resolved cross section images from samples with layered structures by STM. 4 The materials in their samples are in crystal form. For samples that consist of amorphous materials, sample preparation becomes a more difficult issue and we will discuss it in detail in the next section. So far, we have obtained STM pictures with distinguishable layered structures from different kinds of multilayers. The d-spacings of the multilayers measured by STM agree well with those measured by x-ray diffraction.
SAMPLE PREPARATION To image the cross section of the multilayer we want the surface of the cross section sample to be normal to the multilayer surface and to be flat and smooth within the area to be imaged. Generally speaking, the smoother the surface the higher the resolution of the picture. For a multilayer made of crystalline materials, cleaving would be an proper method to obtain the required cross section surface. But for the multilayers made of amorphous materials, such as those in which we are interested, cleaving could not guarantee the multilayer potion of the cross section surface to be smooth and normal to the multilayer surface. Thus we decided to use a polishing method. The multilayers are usually deposited
Data Loading...
