Advanced XRD techniques and applications on Bruker D8D plus diffractometer

The nanoFAB is pleased to announce that advanced In-Plane Diffraction (Non-Coplanar Scan), Pole Figure Measurement, 2D Stress Measurement, and Multi-Angle Scattering (SAXS/WAXS) are fully commissioned on the Bruker D8 DISCOVER Plus X-Ray Diffractometer .

IN-PLANE DIFFRACTION (NON-COPLANAR SCAN): With standard diffraction geometries (coplanar scan), such as the Bragg-Brentano geometry, X-rays penetrate to a certain depth into the sample, and the diffraction from lattice planes parallel to the sample surface is measured. In case of ultra-thin films, X-rays completely transmit and no diffraction is observed from the thin film with the standard coplanar scan. In these circumstances, in-plane diffraction enabled by non-coplanar scan provides effective and efficient analysis, in which both the incident and diffracted beams are nearly parallel to the sample surface, as the detector scans in the plane of the horizontally positioned sample. In-plane diffraction has two major features:

(1) The penetration depth of the X-ray is limited to the top surface of the sample. By setting the X-ray incidence at the critical angle or slightly higher, ultra–thin films and the texture of surface layers can be analyzed.

(2) The technique measures lattice planes that are (nearly) perpendicular to the sample surface, which are inaccessible by other techniques. This second feature is a key difference between the in-plane grazing incidence diffraction (IP-GID) vs. the conventional GID with coplanar scan. Figure 1 shows the setup of in-plane diffraction (non-coplanar scan) on the Bruker D8D plus XRD, as compared to coplanar symmetric 2theta/theta scan for powder diffraction on the Bruker D8 XRD and coplanar GID for thin-film applications on the Rigaku XRD.

QUANTITATIVE TEXTURE ANALYSIS: The presence of crystallographic texture (preferred orientation) in polycrystalline materials has a significant effect on the anisotropy of the materials properties. Therefore, it is critically important to obtain a qualitative/quantitative description of the orientation distribution of crystallites, or the orientation distribution function (ODF) in order to characterize and predict their properties. While direct measurement of the ODF is very challenging, pole figures (PF) can be measured to reconstruct the ODF experimentally, where the diffraction angle is fixed and the diffracted intensity of a certain lattice plane is record by varying two geometrical parameters, such as the alpha angle (tilt angle of the scattering vector from the surface normal direction of a sample) and the beta angle (rotation angle of the scattering vector around the surface normal direction of a sample). The sensitive and large size 2D detector available on the Bruker D8D plus provides large coverage in Gamma and 2Theta, thus high speed and high throughput PF measurement (compared to measurement using a 0D or 1D detector), by taking 2D frames as continuous scans in PHI (the azimuthal angle) at successive values of Psi (the tilt angle). 

RESIDUAL STRESS ANALYSIS: Residual stress, created during the materials manufacturing process or accumulated during operation, can have serious negative effects on a product's quality, performance, and durability, as it may result in cracks and delamination of the films, as well as deformation of the substrate depending on the adhesion strength between the film and the substrate. Residual stress X-ray diffraction is one of the techniques for evaluating the near-surface residual stress with high accuracy, which is non-destructive and is applicable to polycrystalline materials with moderate to fine grain sizes. In X-ray diffraction residual stress measurement, the strain in the crystal lattice is measured using the changes in the d-spacing of the crystal lattice planes as the strain gauge, and the residual stress producing the strain is calculated assuming a linear elastic distortion of the crystal lattice. The most commonly used method for XRD stress determination is the sin2(Psi) method. By measuring the change in the d-spacing of a suitable lattice plane for at least two different tilting angles (Psi), the stress present in the plane of sample surface can be calculated from the slope of the d vs. sin2(Psi) plot. Our Bruker D8D plus XRD provides 2D stress measurement with the side inclination method, enabled by Psi tilt and PHI rotation of the Eulerian cradle, together with the large Gamma coverage of the 2D detector.  2D stress analysis measures not only the peak position but also the shape of the diffraction Debye rings. Essentially, it describes the curvature of the diffraction rings as the scattering vector tilted away from the surface normal, providing a more generalization solution than the simple sin2(Psi) peak position analysis, which is a special condition of the 2D stress analysis. The procedure of 2D stress measurement is similar to that of 2D pole figure measurement, except that a higher-angle diffraction peak needs to be used.

MULTI-ANGLE SCATTERING (SAXS/WAXS) ANALYSIS: Small-Angle X-ray Scattering (SAXS) analyzes the elastic scattering behavior of X-rays when traveling through the material, recording their scattering at small angles (typically 0.1 - 10°). SAXS quantifies nano-scale density differences in a sample, enabling characterization of particle sizes, shapes, distribution, pore sizes, characteristic distances of partially ordered materials, and much more. Wide-Angle X-ray Scattering (WAXS) is similar to SAXS, except that the distance from sample to the detector is shorter and thus diffraction maxima at larger angles are observed. WAXS usually covers 2theta 5-60 degree, revealing scattering/diffraction at sub-nanometer scale such as crystalline lattice planes. Multi-angle scattering analysis utilizes both SAXS and WAXS in conjunction to probe a wide range of length scale from an angstrom to a micrometer, revealing a full picture of the materials structure. 

In-plane diffraction, pole figure, and 2D stress measurements are now available for user training and staff analysis. Please see the application examples and discussions below for more details. If you are interested in utilizing these techniques for your materials characterization, please submit a training request on LMACS. If you have any questions, please feel free to contact Dr. Xuehai Tan (xtan@ualberta.ca) and Peng Li (peng.Li@ualberta.ca) – the Characterization Group Manager.

Figure 1: Comparison of non-coplanar scan, coplanar symmetric scan, and coplanar GID scan setups on nanoFAB XRD systems
Figure 1 - Comparison of non-coplanar scan, coplanar symmetric scan, and coplanar GID scan setups on nanoFAB XRD systems.


Application Examples

Sample: Epitaxially grown Ga2O3 thin film on sapphire substrate
Analysis: In-plane diffraction (Non-coplanar scan)
Application: Coupling non-coplanar 2theta-nc / PHI scan and coplanar 2theta/theta symmetric scan to characterize epitaxially grown Ga2O3 thin film on sapphire substrate.

Sample: Single crystal sapphire α-Al2O3
Analysis: In-plane diffraction (Non-coplanar scan)
Application: Coupling different non-coplanar scans and coplanar symmetric scan to characterize single crystalline materials. An example is given on how in-plane diffraction could be used to guide PFIB-TEM lamella preparation.

Sample: Soda can Al sheet produced through cold-rolling
Analysis: Fast pole figure measurement enabled by 2D detector
Application: Texture analysis of the orientation distribution of crystallites from pole figure measurements.

Sample: Fiber-textured In-Ga-Zn-O (CAAC-IGZO) film on Si substrate for 3D glasses-free holographic displays
Analysis: Pole figure, coplanar symmetric scan, non-coplanar in-plane GID, PIB-STEM
Application: A correlative study of fiber-textured IGZO film using pole figure, coplanar and non-coplanar XRD (In-plane GID), and PFIB-STEM microscopy of the film cross section.
Sample Courtesy: Avalon Holographics Inc., Edmonton

Sample: Fe film with compressive stress
Analysis: 2D stress analysis
Application: A more generalization residual stress analysis solution than the simple sin2(Psi) peak position analysis

Sample: A hard carbon with rich closed pores.
Analysis: Multi-angle X-ray scattering
Application: Coupling SAXS and WAXS to investigate the internal pore sizes and the short-range ordered carbon structure.

Optical Emission Interferometry (OEI) in the Plasma-Therm Versaline PECVD

Overview

In plasma-enhanced chemical vapour deposition (PECVD) tools, the deposition rate of deposited films can often fluctuate with relative gas concentrations, material build-up on the chamber walls, and other mechanisms that make timed depositions inconsistent. To combat this problem, a simple technique called Optical Emission Interferometry (OEI) can be used. OEI provides a more robust form of deposition control based on the interference of light reflected off of the growing film to precisely hit target film thicknesses.

OEI uses similar principles to other interferometric tools like the Filmetrics F50-UV; the intensity of light at a single wavelength reflected off of a thin film will vary depending on the wavelength of light and the optical thickness of the film. This is due to the relative phase difference between light reflected off the top surface of the film and at the film-substrate interface, i.e., conditions of constructive and destructive interference (shown in Fig. 1). Across a broad spectrum of wavelengths, this can be used to fit to both refractive index and film thickness robustly (as Filmetrics does). However, if the intensity of light reflected off of a growing film is recorded continuously, the intensity at a "single" wavelength will oscillate approximately sinusoidally as the thickness increases (satisfying constructive and destructive interference conditions at specific thicknesses)—this is the fundamental premise of OEI. By noting when the extrema occur during film growth, the period of this oscillation (T) can be used to measure the deposition rate of the film, since T is set by the phase conditions for constructive/destructive interference. T = λ/(2n); where λ is the wavelength of interest and n is the refractive index of the material at this wavelength.

Diagram illustrating (a) destructive and (b) constructive interference, incorporating the principles of Optical Emission Interferometry. Two waves reflect off surfaces, with destructive showing out-of-phase and constructive in-phase interactions.
Figure 1. Phase conditions for destructive and constructive interference of light reflected off of a thin film, from H. G. Tompkins, and J. N. Hilfiker, Spectroscopic Ellipsometry: Practical Application to Thin Film Characterization (Momentum Press, 2016).

To use OEI in the Plasma-Therm Versaline PECVD, light produced by the plasma is reflected off of the growing film and collected by a spectrometer centred above the wafer for plasma monitoring. The Plasma-Therm EndpointWorks software package is then used to monitor the evolution in time of the spectral intensity emitted from the plasma within a narrow range of wavelengths. Choosing a range that encompasses an emission line of a typical gas/radical species within the plasma produces the oscillatory data shown in Fig. 2. Values of T are calculated based on known values of n and λ to give an estimate of the deposition rate based on the spacing of the extrema. Thus, in order to function correctly, each OEI recipe requires the user to have knowledge of both the growing film (single wavelength refractive index) and of typical optical emission lines (usable wavelengths) within the plasma.

Graph titled "Endpoint Data" shows an oscillating white line with red and green highlights, likely depicting Optical Emission Interferometry data. The X-axis is time and the Y-axis is smoothed data. A red notation reads "T = λ/(2n)" with a horizontal bracket, possibly showcasing Plasma-Therm analysis.
Figure 2. EndpointWorks Thickness Detector data for a growing SiO2 film.

Wavelength choice

The choice of what wavelength to use for OEI depends on the chemistry of the plasma in question, since each gas/radical species emits light at different wavelengths (depending on its electronic transitions), and thus the available wavelengths will differ for each recipe. Potential emission lines are further limited if the gas species characteristic of an emission line is consumed during the reaction. The intensity of these reactant emission lines will fluctuate as the film grows and as the local gas concentration changes. In contrast, the intensity of an emission line of an inert gas (He, Ar, or N2 are often used to dilute the reactants) will be much more stable, and will provide a reduction in noise in the reflected light. This leads to a more clearly defined periodic signal, where the extrema can be easily identified in software. Additionally, since variation in T is linear with the wavelength of light used, the choice of shorter wavelengths will reduce the period of oscillation, lending the OEI method more sensitivity and flexibility to accurately measure thinner films. Note, for films that are optically absorbing (e.g. a-Si and Si-rich nitrides), the film will absorb more strongly at shorter wavelengths, decreasing the amplitude of the oscillation as the film grows. Ultimately, this decay reaches a point where extrema cannot be identified past a certain thickness. A general rule of thumb is to choose the shortest wavelength inert gas emission line that is not strongly absorbed by the depositing film. A typical spectrum collected during deposition is shown in Fig. 3, where a N2 emission line at ~333 nm is highlighted, being suitable for the gas chemistry involved. For more details on choosing an emission line appropriate for your deposition, please see the Plasma-Therm Versaline PECVD OEI recipe development How-To on the nanoFAB's Confluence Knowledge Base.

Graph showing Si-rich nitride spectra with wavelength (nm) on the x-axis and counts on the y-axis, using Plasma-Therm's Optical Emission Interferometry. Main peak at 333 nm. Inset shows detailed view of 300-400 nm range with multiple peaks.
Figure 3. Typical plasma emission spectrum (as measured on the Versaline's integrated Ocean Optics spectrometer) for the deposition of Si-rich nitride. Inset: full-width at half-max (dashed lines) of the N2 emission line centred at ~333 nm.

OEI controlled deposition recipes are currently qualified for SiO2 Dep ("OEI SiO2 Dep") and Stoichiometric Nitride ("OEI Stoichiometric Nitride") recipes, with an OEI recipe in progress for the Si-Rich Nitride. For user-developed films that differ from these established recipes, users are welcome to qualify their own OEI recipes following the Step-by-step instructions in the linked OEI recipe development How-To. If you encounter problems setting up your OEI recipe on EndpointWorks, please contact Tim Harrison (tr1@ualberta.ca).

AML Wafer Bonder now available for training

The nanoFAB is pleased to announce an upgrade to our wafer bonding and packaging area, with the recent installation of a wafer bonder from Applied Microengineering Ltd (AML).  The AWB-04 wafer aligner-bonder from AML offers a wide range of bonding techniques and process flexibility. Of particular note is its in situ alignment capability, allowing users to verify correct alignment as the wafers are being brought into contact.

Bonding techniques include:

Technical features:

Application Examples

Anodic Bond: borosilicate glass to Si

Cross-sectional SEM (left) and TEM (right) images of bonding interface.
Cross-sectional SEM (left) and TEM (right) images of bonding interface.

Si–Si Direct Bond: in situ RAD + low-temperature anneal

TEM images of the bonding interfaces of silicon wafers after a low-temperature (250 °C) anneal performed in the AWB-04 chamber without breaking vacuum. The thin amorphous native SiO2 layer can be seen along the interface.
TEM images of the bonding interfaces of silicon wafers after a low-temperature (250 °C) anneal performed in the AWB-04 chamber without breaking vacuum. The thin amorphous native SiO2 layer can be seen along the interface.

Si–Si Direct Bond: in situ RAD + external high-temperature anneal

TEM images of the bonding interfaces of silicon wafers after a high-temperature (1100 °C) anneal performed in Tystar General Anneal (Tube 6). The thin amorphous native SiO2 layer can be seen along the interface, but has diffused into the bulk Si layer.
TEM images of the bonding interfaces of silicon wafers after a high-temperature (1100 °C) anneal performed in Tystar General Anneal (Tube 6). The thin amorphous native SiO2 layer can be seen along the interface, but has diffused into the bulk Si layer.

Au–Au Thermocompression Bond: Si to Si with Au interlayers

SEM images of silicon wafers bonded via Au–Au thermocompression bonding: full wafer cross-section (left) and close-up of bond interface (right). Patterned Au films are deposited on Si wafers and bonded using moderate temperatures (~300 °C) and high pressures (~7.5 MPa). Images courtesy of Jones Microwave Inc. and RM3.
SEM images of silicon wafers bonded via Au–Au thermocompression bonding: full wafer cross-section (left) and close-up of bond interface (right). Patterned Au films are deposited on Si wafers and bonded using moderate temperatures (~300 °C) and high pressures (~7.5 MPa). Images courtesy of Jones Microwave Inc. and RM3.

If you are interested in using the AML wafer bonder in your work, please submit a request through LMACS to receive training. If you have any questions, please feel free to contact Scott Munro or Aaron Hryciw.

New Spray Coating Tool Available

We are happy to announce a new addition to our fabrication tool lineup: a Sono-Tek ExactaCoat Spray Coater. This tool is located in our recently renovated 10k Lithography area (ECERF W1-031); please stay tuned over the coming months as we add more photolithography capabilities to this labspace.

The ExactaCoat, an ultrasonic spray coating system, can accommodate specimens up to 300 mm in diameter. This tool is primarily used for photoresist coating in situations where conventional spin coating is impractical or yields poor results: specimens that are too fragile, exhibit large topographical variations, or possess irregular shapes.

The spray coater offers a particular advantage for specimens where topography presents a challenge to achieving good film uniformity during spin coating: examples include MEMS or microfluidic devices with deep (tens to hundreds of µm) features etched via DRIE or KOH/TMAH. Process development is currently underway to formulate recipes suitable for coating deep and/or high aspect ratio topography wafers. Additionally, spray coating is a valuable technique for those looking to apply photoresist as a dicing protect layer on extremely fragile or through-etched substrates that are not compatible with spin coating.

Sono-Tek ExactaCoat Spray Coater
Sono-Tek ExactaCoat Spray Coater
Illustration depicting stages of droplet formation: solvent evaporation in flight, landing on a substrate, and forming a closed resist film, annotated with arrows and text labels. The spray coating process is enhanced with our Spray Coating Tool available for precision applications.
Schematics of the spray coating process
(MicroChemicals)

The spray coating process uses high-frequency (ultrasonic) mechanical vibration that is transferred to the liquid in the nozzle. The standing waves created in the liquid are broken up as the liquid exits the nozzle—this aerosolised liquid is then dispensed onto the surface of the specimen as a fine mist.

 
To coat an extended specimen such as a wafer, the spray of atomised resist is rastered across the work area in overlapping stripes. This is illustrated in the following videos, where a cleanroom wipe is used to show the sprayed resist more clearly.

After spray coating a wafer with resist and softbaking, it may be exposed and developed to complete a photolithography process. The following optical microscope images illustrate some preliminary results of photolithography on Si wafers, showcasing excellent film uniformity and achieving lateral feature sizes as small as 5 µm.

Photolithography using spray coating: Si wafers spray coated with a ~4.5 µm thick film of AZ 1505, exposed in a contact mask aligner, and developed. The smallest feature size pictured in each image is 5 µm.

We are thrilled to introduce this new capability, along with the array of associated applications it makes possible. If spray coating is a technique you feel would be beneficial to your work, please submit a request through LMACS to receive training. If you have any questions, please do not hesitate to contact Breanna Cherkawski or Aaron Hryciw.

Resources

Application note: Spray coating (MicroChemicals GmbH)

Raith_GDSII toolbox documentation now on Read The Docs

Now in its tenth year of existence, the Raith_GDSII MATLAB toolbox makes it easy to generate patterns for Raith electron-beam lithography (EBL) and focused ion beam (FIB) applications using MATLAB. It can also be used to generate “plain” (non-Raith-dialect) GDSII files for non-EBL applications such as printing photomasks or direct-write laser lithography exposures. This open-source project is maintained by the nanoFAB, may be downloaded from GitHub.

As of the most recent update, the technical documentation for the toolbox is now hosted online on Read the Docs, as opposed to the PDF user manual which served as documentation for previous versions.

This switch to online documentation ensures the documentation is always up to date (rebuilt with each GitHub commit), and includes quality of life improvements such as copy-to-clipboard buttons on all code block examples and a “search as you type” feature. This update also includes a small bugfix relating to outputting FBMS path elements with curvature in “plain” GDSII dialect, as well as several typographical edits to the documentation content.

We are excited about the potential of this updated documentation format to simplify and enhance both the initial learning and ongoing utilisation of the Raith_GDSII toolbox, for anyone interested in EBL, FIB, or other lithographic techniques. For more information, please contact Aaron Hryciw.

A screen shot of RAITH GDSII Toolbox application.
A screen shot of RAITH GDSII Toolbox application.

Broad Ion Beam (BIB) polishing for SEM/EDX/EBSD

The RES 102 system features:

An image of a machine with an arrow pointing to it, emphasizing its characterization.
RES102 BIB Ion Milling System equipped with two Ar guns

Among general electron microscopy applications, Electron Backscatter Diffraction (EBSD) is a surface technique – typical probing depth is in the range of a few tens nm for a beam energy of 20 kV. Smooth and damage-free surface is critically important to obtain high quality EBSD data. Compared to conventional mechanical polishing techniques, which can result in very rough surfaces and a thick damaged layer, low energy beam at grazing incident angle and rotating stage on the RES102 can effectively polish the surface to reduce roughness and remove the damaged layer.

Oxford active microscopy on zeiss fems.

With the programmable recipes of flexible parameters (beam energy, beam current, incident angle, stage rotating speed), the RES102 BIB has produced very nice results for our users for EBSD analysis. See recent examples below.

Application Examples (EBSD)

A series of images characterizing different types of granules.
(A) SEM image; (B) Orientation map; (C) Phase maps of Alumina and Aluminium; (D) Grain size distribution

Characterization of a rock sample using microscopy.
(A) SEM image; (B) Orientation map; (C) Grain size distribution
A screen shot of a computer screen displaying nanofabrication images.
(A) Orientation map; (B) Crystal unit cell orientation and (C) Grain Size Distribution

The RES102 ion milling system is now available to general users for both staff analysis and user training. Any users interested in getting trained on this tool or staff analysis should submit a request on LMACS. If you have any questions, please contact the tool managers Drs. Nas Yousefi and Shihong Xu or Peng Li – the Characterization Group Manager