Characterization methods for laser lift-off

This section outlines experimental techniques for evaluation of substrate lift-off nitride membranes (e.g. c-plane GaN) that were implemented by LLO. The characterization techniques in this study include the Nomaski image of laser beam spot at nitride material/sapphire interface at an incident laser pulse energy density, cross-section imaging by scanning electron microscope (SEM) and material quality characterization by X-ray diffraction (XRD). In this section, the evaluation of laser lift-off on GaN LED wafer grown on c-plane sapphire was taken as an example.

(a) (b) (c)

GaN

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2.3.1 Nomarski microscopy

An optical microscope is used to capture the laser beam spot produced at the nitride material/sapphire interface. A successful laser induced-thermal decomposition of the nitride material at the interface requires sufficient incident laser pulse energy density of each single laser pulse. It produces a dark color rectangular pattern (i.e. beam spot) at the interface as Ga metallic droplets present, which leaves a separation gap between the nitride material and the sapphire and changes the transparency and reflection at the interface after LLO. Therefore, the minimum energy density required could be indicated to induce a uniform dark beam spot at the interface. Thus, for successful LLO on a nitride material wafer sample, the single laser pulse used in the step-and-repeat laser pulse scanning on the sample, should possess an incident energy density higher than the threshold value. The beam spot area can be estimated by the use of Nomarski optical microscope image presented.

Figure 2.17 Photograph of beam spots at GaN/sapphire interface on GaN wafer after excimer laser pulse irradiation at Ei over threshold energy density.

In Fig. 2.17, a 1 × 1 cm2 GaN/sapphire wafer sample was flip bonded on a glass slide with epoxy. The top surface is the back surface of double polished sapphire substrate. There are four rows of laser beam spots, which an incident laser pulse energy corresponds to a single row of beam spots, as labeled in the figure. The beam spots are smaller, when a higher incident laser pulse energy density required. It was found that the minimum energy density is 400 mJ/cm2 to produce a mark, which was induced by the GaN thermal decomposition.

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2.3.2 Scanning Electron Microscope (SEM)

The scanning electron microscope (SEM) is an important imaging technique and tool that is widely applied in the examination of the surface of a variety of materials. This technique can provide high magnification images with a good depth of field and high resolution. Moreover, it can present dynamic experimental imaging of the sample in heated up, cooling and starching conditions. The basic principle is a beam of electrons is generated by a suitable source, typically a tungsten filament of a field emission gun [58]. The electron beam is accelerated through a high voltage (e.g. 20 kV) and passes through a system of apertures and electromagnetic lenses to focus and then produce a thin beam of electrons. The beam scans the surface of the sample by means of scan coils, and then the secondary electrons and back scattering electrons are emitted from the specimen. All the electrons produced by the interaction between the beam and sample are collected by a suitably-positioned detector, and then converted to electrical signals passing to a display tube. Finally, the magnified image of the sample surface can be displayed on the screen. The plan- view and cross-sectional configurations of the sample can be examined with this technique. Plan-view SEM is used to study the surface morphology of the materials as well as the microstructure or nanostructure in the plane of the devices. On the other hand, the cross-section SEM can be used to measure the depth distribution of features within the sample to be studied.

In this study, SEM in cross-section configuration measurement was carried out to examine the nitride material (e.g. GaN) after LLO. The measurements were taken using a Philips scanning electron microscope operating at 5 kV. Fig. 2.18 (a) shows a cross-sectional SEM image of a GaN LED wafer sample that was flip-bonded to a glass slide with Araldite epoxy before laser pulse irradiation. The sapphire substrate capped the GaN epitaxial layer on the epoxy resin. In contrast, Fig. 2.18 (b) shows the 57o tilted cross-sectional view of the GaN LED wafer sample after LLO and sapphire substrate removal. The thickness of the GaN LED epitaxial layer is around 4-m-thick and it remains on the epoxy. No micro-cracking, buckling or damage on the GaN layer were observed, which indicates the LLO and the transfer of the GaN epitaxial layer was successful.

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Figure 2.18 Cross-sectional SEM micrograph of (a) a GaN LED epitaxial layer grown on sapphire bonded to a glass slide with epoxy, (b) the GaN LED epitaxial layer after LLO bonded to the glass slide with epoxy. A single laser pulse energy density irradiated through the sapphire to separate the GaN at 500 mJ/cm.2

2.3.3 X-ray diffraction (XRD)

X-ray diffraction (XRD) is a useful tool as a non-destructive technique to study the crystal quality of materials, compositional information of alloys and thickness of epitaxial films. This technique relies on the diffraction of incident x-ray radiation from the periodic lattice planes of a crystal. X-rays are generated by bombarding a metal (typically Cu) with electrons in an evacuated tube and monochromatic x-rays are selected. These x-rays are scattered by the electron cloud surrounding each atom in the crystal. The diffraction of plane wave radiation can be described by Bragg’s equation [59]:

n = 2d sin

where n is the order of diffraction, λ is the wavelength of the incident radiation, d is the distance between lattice planes and  is the Bragg angle

Figure 2.19 Schematic diagram of XRD setup.

Epoxy (a) (b) Sapphire 500 m 5 m 3 .8 4  m GaN Epoxy _Epoxy GaN Cross-section Surface Tilt 57o Tilt 57o

Page | 49 In this work, the XRD measurements were carried out using a high-resolution x-ray diffractometer. The x-ray wavelength is close to the crystal lattice spacing. In order to obtain the compositional information of the sample before and after LLO, -2θ measurements were carried out whereby the sample was scanned through the Bragg angle whilst simultaneously scanning the detector through twice the angle of the sample as shown in Fig. 2.19. A sample of GaN MQW LED grown on sapphire substrate was flip-bonded with epoxy adhesive on the glass slide, the LLO was applied on the sample.

Figure 2.20 XRD -2θ sca s of c-plane GaN LED sample before and after lift-off from sapphire substrate. A pulse of energy density 500 mJ/cm2 was used for LLO.

In Fig. 2.20, the -2θ scan curves of the sample as-grown (i.e. before LLO) and after LLO are presented. Compared to the diffraction peaks on the curve of the as-grown sample and after LLO, it showed no sapphire peak of the sample after LLO, which indicated a successful fully removal and only GaN epitaxial layer remains. The successful lift-off of sapphire can be verified by the compared XRD scans before/after LLO.