Development of a prototype photoacoustic microscope and spectroscope and advanced semiconductor material characterization
Xu, Lu (2012) Development of a prototype photoacoustic microscope and spectroscope and advanced semiconductor material characterization. PhD thesis, Dublin City University.
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The thesis can be divided into two parts.
In the first part of my thesis, I present the design, construction and test results of a prototype gas-cell Photoacoustic (PA) Spectrometer and Microscope. It is a low
cost, non-contact technique, which can be used to characterize semiconductor band-gap structures and subsurface defects. It requires no liquid coupling and no
sample surface preparation in advance.
The instrument development includes the optical system design, mechanical design of the PA cell using AutoCAD®, pre-amplifier circuit design, system noise analysis, hardware control, data acquisition system and graphical user interface (GUI) development using LabView®. A multiple-microphone detection scheme, helium gas coupling, acoustic resonance and a high power laser light source are
used to enhance the PA signal and to increase the data acquisition speed. The PA system is calibrated to remove the acoustic resonance effect and the background
fingerprint of light source intensity spectrum. The linear relationship between the PA signal and the source intensity is verified. The impacts of the lock-in amplifier
performance, the focus offset and the coupling gas within the cell on the PA signal are discussed.
Various samples are used to verify performance of the developed PA system. These include Silicon wafers, GaAs wafers, multi-layered structures on silicon substrates, carbon-black powder, laser-machined air trenches, bonded silicon wafers and a packaged IC chip. For spectroscopy applications, the PA spectra of two types of GaAs wafers are characterized successfully. For microscopy applications, the PA system is proven to have a vertical resolution of ~ 20 nm and a lateral resolution of ~ sub-100 um. Its probe depth could be as deep as 450 um below the silicon surface. The data acquisition speed of the PA system is improved for industrial applications. Two high-resolution (10,000 pixels) thermal images (one in phase and another in amplitude) of semiconductor devices can be obtained in less than 50 seconds across an area of approx. 9 mm x 9 mm.
In the second part of my thesis, other related non-destructive characterization work on advanced semiconductor materials is presented. In chapter six, Synchrotron X-ray Topography (SXRT) and Micro-Raman Spectroscopy (uRS) are used to study two sets of the femto-second and nano-second laser machined grooves on InP substrates. In chapter seven, other characterization work is presented to study the H2 preconditioning effect on self-assembled Ge-islands on Silicon. Both cases demonstrate the commercialised metrology tools’ capabilies to analyse the distribution profile of the strain and the chemical composition on the top surface. The PA system prototype presented in this thesis can be used as a complementary tool. It provides ultra-deep probe depth for the subsurface defects, when compared to the SXRT and the uRS methods.
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