Issued Date: 2018/01/25
Issued By: iST
Detecting crystal structure stacking shall damage samples under test, is there any non-destructive and easy alternative?
What’s the purpose of identifying crystal structure stacking and film characteristics? It’s essential to ensure well organized stacking of crystal structure by measuring the thickness and density between layers at the R&D stage of the wafer / LED process. Otherwise, the strength of the structure, electric properties, and transparency of the crystal may get hampered, which, in turn, lead to poor product quality.
A transmission electron microscope (TEM) is most likely the instrument employed for crystal structure and film characteristics tests. As TEM is designed for micro (nanometer/micron) grade structure testing, the size of samples for testing has to be 0.1mm or smaller. That is, you have to break the sample to get the layer required for analysis.
To detect crystal structure stacking and film characteristics of samples with macroscopic scale, a size greater than 0.1mm, and without damaging them, you need the new X-ray diffraction (XRD) analysis tool to meet your requirements.
The iST Tech Classroom this month shall share this XRD with you. It is good for analyzing Non-destructive crystal structure stacking and measuring the roughness of surfaces between layers and the thickness and density between multiple films. (Note 1: Other roughness measurement tools).
This material crystal structure analysis method pairs the X-ray diffraction pattern of crystal against databases to identify material texture, crystal pattern, and grain size of nanometer crystals in a non-destructive manner.
Can XRD be useful to you?
Learn more about the functions of XRD
The XRD deployed by iST comes with XRR (thin film X-ray reflectivity) functions. The XRD tool acquires the diffraction pattern according to diffraction principles and pairs it with pattern databases to derive the crystal structure. Being the reflection pattern of XRD, the XRR is aimed at measuring film thickness (up to 0.1nm thick), roughness of the sample surface and layers interfaces, electronic density of film, and even perform multi-layer film analysis for a total thickness below 500nm. This makes it ideal for analyzing non-destructive film material characteristics.
Pattern database? What do patterns look like? How do you pair them?
The XRD analyzer employs X-ray diffraction to get spectrum data instead of an image of the sample availed by TEM. You have to pair the XRD data diagram against crystal diffraction databases to identify crystal stacking. You must be wondering: what do patterns look like and how is iST going to help in the pairing operation?
The red colored diffraction pattern in the diagram displays 3 diffraction peaks. This suggests a polycrystalline structure. Further database pairing tells us this is an α-Fe BCC crystal structure.
Diffraction pattern of polycrystalline α-Fe BCC crystals
Minimum limit of analysis sample size
XRD is designed for macro crystal structures and is unable to run at nanometer level like TEM does. However, iST is capable of precision analyzing much smaller graphic samples (up to 0.3mm) with a more powerful X-ray source (6kW) along with laser focusing technology to get a higher resolution at a more powerful signal. In addition, the analysis size of the sample may go up to the size of 12″ wafers.
What does the pattern generated look like? How do we pair them against databases to validate crystal structure?
By fitting the black XRR pattern and green pattern of a simulation structure shown in the signal using software, the data of film roughness, thickness and density can be unveiled.
Case study for different samples
Case 1: XRR analysis over bilayer nanometer film Al2O3
- Analysis sample: Sample for analysis: Al2O3/Si
- see figure below for results of fitting the XRR pattern of aluminum oxide films on silicon substrate. The table below suggests there are two layers of aluminum oxide films at different densities (the thickness is 0.94 nm and 6.69 nm for upper and lower layer respectively); roughness of aluminum oxide surface and the silicon substrate interface is 0.49 nm and 0.23 nm odd respectively.
Case 2: LED epitaxy HR XRD analysis
- Analysis sample: InxGa1-xN/GaN superlattice
- See below for an HD XRD pattern for the epitaxy of LED wafer. Fitting of superlattice InxGa1-xN/GaN suggests thickness and composition at 2.4nm/14.35nm and x=16.27%.
Note 1: AFM, another roughness measurement device
Even a layman over material analysis would have known that an atomic force microscope (AFM) is good at measuring surface roughness of samples. However, this is not the case with thickness, density, and interface roughness between sample layers.
Note 2: See table below for functions of material analysis items
Surface roughness XRD AFM
Interface roughness XRD /
Multilayer film thickness XRD TEM
Multilayer film density XRD /
Structure / TEM
Composition XRD TEM-EDS
Lattice defect / crystallinity XRD TEM
This article is aimed to share our detection and verification experiences with honorable clients like you. Should there be any sample abnormalities requiring determination or inspection, or you need to know more about the aforementioned techniques, please ring Mr. Johnson Chang at +886-3-579-9909 Ext. 6613 or email him at firstname.lastname@example.org.