Haifeng He

X-ray image reconstruction from a diffraction pattern alone

A solution to the inversion problem of scattering would offer aberration-free diffraction-limited three-dimensional images without the resolution and depth-of-field limitations of lens-based tomographic systems. Powerful algorithms are increasingly being used to act as lenses to form such images. Current image reconstruction methods, however, require the knowledge of the shape of the object and the low spatial frequencies unavoidably lost in experiments. Diffractive imaging has thus previously been used to increase the resolution of images obtained by other means. Here we experimentally demonstrate an inversion method, which reconstructs the image of the object without the need for any such prior knowledge.

High-resolution ab initio Three-dimensional X-ray Diffraction Microscopy

Coherent x-ray diffraction microscopy is a method of imaging nonperiodic isolated objects at resolutions limited, in principle, by only the wavelength and largest scattering angles recorded. We demonstrate x-ray diffraction imaging with high resolution in all three dimensions, as determined by a quantitative analysis of the reconstructed volume images. These images are retrieved from the three-dimensional diffraction data using no a priori knowledge about the shape or composition of the object, which has never before been demonstrated on a nonperiodic object. We also construct two-dimensional images of thick objects with greatly increased depth of focus (without loss of transverse spatial resolution). These methods can be used to image biological and materials science samples at high resolution with x-ray undulator radiation and establishes the techniques to be used in atomic-resolution ultrafast imaging at x-ray free-electron laser sources.

Direct strain measurement in a 65 nm node strained silicon transistor by convergent-beam electron diffraction

Using the energy-filtered convergent-beam electron diffraction (CBED) technique in a transmission electron microscope, the authors report here a direct measurement of the lattice parameters of uniaxially strained silicon as close as 25nm below the gate in a 65nm node p-type metal-oxide-semiconductor field-effect transistor with SiGe source and drain. It is found that the dominant strain component (0.58%) is compressive along the source-drain direction. The compressive stress is 1.1GPa along this direction. These findings demonstrate that CBED can serve as a strain metrology technique for the development of strained silicon device technology.

Experimental lensless soft-X-ray imaging using iterative algorithms: phasing diffuse scattering.

Images of randomly placed two-dimensional arrays of gold balls have been reconstructed from their soft-X-ray transmission diffraction patterns. An iterative hybrid input-output (HiO) algorithm was used to solve the phase problem for the continuous distribution of diffuse X-ray scattering. Knowledge of the approximate size of the clusters was required. The images compare well with scanning electron microscope (SEM) images of the same sample. The use of micrometre-sized silicon nitride window supports is suggested, and absorption filters have been used to allow collection of low spatial frequencies often obscured by a beam stop. This method of phasing diffuse scattering may have application to scattering from individual inorganic nanostructures or single macromolecules.

Off-axis zone-plate monochromator for high-power undulator radiation

We report the design and construction of an off-axis zone-plate monochromator for diffraction-imaging experiments at beam line 9.0.1 at the Advanced Light Source (ALS) synchrotron-radiation facility at Berkeley USA. The device is based on an off-axis zone plate which can be conveniently inserted into or retracted from the beam. We discuss design issues such as the efficiency and spectral purity of the system and the technique for designing heat-tolerant windows for soft x-ray undulator beams. The monochromator functions successfully and good-quality diffractions patterns are being made with the beam it delivers.

Simple constraint for phase retrieval with high efficiency.

I propose the use of a simple real space constraint for iterative phase retrieval intended for diffractive imaging. The proposed constraint is a single parameter equal to the number of nonzero pixels in the image. This greatly simplifies the procedure to determine the constraint. A series of algorithms using this constraint can be easily deduced from existing algorithms, such as hybrid-input-output and difference map algorithms. The high efficiency of these algorithms is largely preserved, as confirmed by numerical studies. A concept widely used in control system theory, proportional-integral-derivative control, is shown to increase the execution speed of the proposed constraint significantly.

Phasing diffuse scattering. Application of the SIR2002 algorithm to the non-crystallographic phase problem.

A new phasing algorithm has been used to determine the phases of diffuse elastic X-ray scattering from a non-periodic array of gold balls of 50 nm diameter. Two-dimensional real-space images, showing the charge-density distribution of the balls, have been reconstructed at 50 nm resolution from transmission diffraction patterns recorded at 550 eV energy. The reconstructed image fits well with a scanning-electron-microscope (SEM) image of the same sample. The algorithm, which uses only the density modification portion of the SIR2002 program, is compared with the results obtained via the Gerchberg-Saxton-Fienup HiO algorithm. The new algorithm requires no knowledge of the objects boundary and proceeds from low to high resolution. In this way, the relationship between density modification in crystallography and the HiO algorithm used in signal and image processing is elucidated.

Analysis of nano-scale stress in strained silicon materials and microelectronics devices by energy-filtered convergent beam electron diffraction

The convergent beam electron diffraction (CBED) technique of transmission electron microscopy (TEM) has excellent capabilities for strain detection at high spatial resolution. Here we report strain measurements in bulk e-Si/SiGe/Si and in a strained 35nm PMOS device in which SiGe acts as the source and drain. CBED measurements of the composition of the relaxed SiGe buffer are in quantitative agreement with Raman spectroscopy. For the PMOS device, CBED measured a uniaxial compressive stress of 1.12GPa in the channel. However, it was found that even in the cross-sectional TEM samples with thicknesses greater than 300nm, the intrinsic surface strain relaxation was often so severe that no recognizable high-order Laue zone lines in the CBED patterns could be collected. The amorphorization of both free surfaces of the TEM sample to a range of about 80nm is proposed to minimize the impact of surface strain relaxation for future studies.

Prospects for single-particle imaging at XFELs

X-ray free-electron lasers will produce pulses of X-rays that are 10 orders of magnitude brighter than todays undulator sources at synchrotrons. This may enable atomic resolution imaging of single macromolecules.

Reconstruction of X-ray images from speckle patterns by iteration

9,10-phenanthrenequinone, C 14H8O2, is both polymorphic and polytypic. The beta form allows different stackings of ordered layers perpendicular to c*. These are best understood as different occupancy wave modulations of a 1:1 disordered parent structure in space group P21/c in which most atoms overlap. Ordered layers have C-1 symmetry in a cell with the a and b axes doubled compared to the parent structure. Each layer has two possible origins. Different stacking sequences allow two orientations of a Z = 8, Cc, structure and two orientations of a Z = 16, F-1 structure with the c axis now doubled. It was found that for some crystals it is necessary to model the reflection data assuming uncorrelated mosaic blocks of different space group symmetry as well as orientation. To allow for stacking faults a disordered component with the parent symmetry was included. Data selection allowed reliable geometry for both the Cc and F-1 components to be obtained using standard programs. The alpha form allows different stackings of ordered layers perpendicular to a*. These are best understood as different occupancy wave modulations of a 1:1 disordered parent structure in space group Pnmn in which most atoms overlap. Ordered layers have P2/c symmetry and each layer has two possible origins. Different stacking sequences allow two orientations of a Z = 4, P21cn structure and two orientations of a Z = 8, B2/c structure with double the a axis (a non standard setting of P2/c).