Research Projects

X-ray Imaging and X-ray Microscopy

X-rays, that is electromagnetic radiation of wavelength below 10 nanometers, are an amazing probe for investigating and learning about the microscopic world. To gain access to extremely intense X-rays beams large particle accelerator laboratories, know as Synchrotrons, have been and are being built all round the the world. Among other experimental stations that use the X-ray beam in a synchrotron, there are the transmission X-ray microscopes which allow for imaging of inorganic and biological specimens at the nanometer resolution scale. With these setups one can observe features in the sample which about 200 to 500 times smaller than those that can be seen in conventional visible light microscope. In addition, the penetration power of the X-rays enables tomographic capabilities and very often the energy of the X-ray beam can be tuned to gain elemental and chemical sensitivity.

Typically, there are two different configurations of X-ray microscopes which are commonly used:

(1) Scanning Transmission X-ray Microscopy (STXM)

In this configuration a focusing lens is used to create a very small spot at the sample position. The sample is then raster scanned while recording the transmitted intensity.

(2) Full-field Transmission X-ray Microscopy (TXM)

In this case the setup is similar to that of a conventional visible light microscope. One condenser lens is used to deliver the illumination into the sample. Then, an objective lens is used to project a magnified image of the sample into a spatially resolving detector.

Currently a new full-field transmission hard X-ray microscope is under design and construction at the Advanced Photon Source, Argonne National Laboratory.

Diffractive X-ray Optics and Fresnel Zone Plates

Since the discovery of X-ray radiation in 1895 by Wilhelm Conrad Röntgen, scientists have developed several types optical elements to focus and manipulate X-ray beams such as mirrors, compound refractive lenses and diffractive X-ray optics.

I am mostly interested in diffractive X-ray optics, and in particular in Fresnel zone plates (FZP) that consist of set of concentric rings whose width decreases as function of their radius.

For large number of rings (>100), the FZP interaction with X-ray is analogous to that of thin lens with visible light. The two key parameters in FZP are its outermost zone width dr which is limits the spatial resolution in the microscope and its zone height h that determines the diffraction efficiency, that is the intensity that is used for focusing or imaging in respect the incoming intensity. Ideal performance of the FZP requires a very small outermost zone width (<50 nm) to achieve very high resolution and at the same time high heights (>200 nm) to achieve sufficient diffraction efficiency. As a result large aspect ration (AR = h/dr) structures need to be produced.

Nanofabrication and Nanostructuring

X-ray diffractive optics and FZPs consist of structures with dimensions in the nanometer scale. To fabricate such tiny devices, one used very advanced electron beam lithography systems on thin layer of resist. Then the pattern created in the resist is transferred to other materials such as silicon, nickel, gold or iridium. Two nanofabrication processes used to produce high quality Fresnel zone plates for X-rays are the following:

(1) e-beam lithography on PMMA + gold electroplating

In this case the e-beam lithography is performed on a thick layer of PMMA (>500 nm to 1.5 um). After the development, gold electroplating is used to grow the FZP structure. The process was optimized by my collegue S. Gorelick.

(2) Zone-doubling approach: e-beam lithography on HSQ + iridium atomic layer deposition

In this case FZP structure is produced by generating a template structure on inorganic resist named HSQ. Later the template is coated with iridium by atomic layer deposition. This approach allows for the fabrication of 20 nm lines and spaces with an aspect ratio above 25. The zone doubling technique was developed by K. Jefimovs, myself and C. David at the Paul Scherrer Institut.