Dec 30, 2013

We demonstrated advanced techniques for control of gas flow in a reactor in order to achieve good etch depth uniformity for large area GaAs etching. It was found that a finite difference numerical method was quite useful for simulation of gas flow distribution in the reactor for dry etching of GaAs. The experimental results in BCl3/N2/SF6/He ICP plasmas confirmed that the simulated data fitted very well with real data. It is noted that a focus ring could help improve both gas flow and etch uniformity for large area GaAs plasma etch processing. The simulation results showed that optimization of clamp configuration could decrease gas flow uniformity as low as ±1.5% on a 100 mm (4 in.) GaAs wafer and ±3% for a 150 mm (6 in.) wafer with the fixed reactor and electrode, respectively. Comparison between simulated gas flow uniformity and real etch depth distribution data confirmed that control of gas flow distribution in the chamber would be significantly important in order to achieve excellent dry etch uniformity of large area GaAs wafers.

Source:Solid-State Electronics

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Dec 24, 2013

Photoelastic characterization of residual stress in GaAs-wafers

Residual stress in GaAs-wafers was investigated on different length scales by rapid full wafer imaging and by microscopic imaging of dislocation cells, using photoelastic homodyne techniques. These non-contact and non-destructive defect and stress imaging methods will be described in detail. In connection with in situ calibration of the photoelastic SIRD™ measurement system absolute shear stress values can be extracted. Local stress fields imaged in μm-scale by the photoelastic instrument SIREX™ show the defect arrangement in cell patterns and allow to characterize the local stress enhancement. The advantages and limits of the vertically integrated photoelasticity measurement employed in both systems will be discussed.

Source: Materials Science in Semiconductor Processing

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Dec 22, 2013

Ultra-high vacuum direct bonding of a p–n junction GaAs wafer using low-energy hydrogen ion beam surface cleaning

Low-energy hydrogen ion bombardment is used to clean GaAs surfaces. The hydrogen ions produce contamination-free surfaces without changes in surface composition (stoichiometry) and surface roughness. The wafers were brought into contact at room temperature after cleaning under ultra-high vacuum (UHV), and bonded over the whole area (2 inches) without application of external mechanical pressure. After bonding, the p-GaAs/n-GaAs wafer pair was annealed at 200 °C for 30 min under UHV conditions (<5×10−10 mbar) to improve the interface bonding strength and to achieve a full-area wafer bonding.
Infrared (IR) imaging of the as-bonded wafers directly reveal the real bonding behaviour. High-resolution transmission electron microscopy images reveal that the wafers have been directly bonded without damage of the crystal lattice or intermediate layer and the interface is smooth. Current–voltage characterization shows near-ideal forward characteristics and the recombination in p–n junction space charge region.

Source: Vacuum

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Dec 17, 2013

Large-depth defect profiling in GaAs wafers after saw cutting

Positron lifetime measurements and Doppler-broadening spectroscopy using slow positrons were combined to investigate open-volume defects created by sawing wafers from GaAs ingots introduced by a diamond saw cutter. The depth distribution represents a large-depth (up to 9.5 μm), wedge-like profile. This was found during step-by-step etching and assembling the respective individual S(E) curves. The depth and the concentration of the defects introduced by the diamond saw depend on the advance of the saw blade. The thermal stability of the detected defects was studied by an isochronal annealing experiment. It was concluded from the positron lifetime measurements and from the Doppler-broadening parameters as well as from the annealing behavior that small vacancy aggregates consisting of at least two vacancies are created by the sawing procedure. More extended defects such as microcracks were analyzed by scanning electron microscopy (SEM). Rutherford-backscattering spectroscopy shows that there is no amorphous material in the near-surface region.

Source: Applied Surface Science

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GaAs wafer mapping by microwave-detected photoconductivity

An improved highly sensitive method of detecting photo conductivity by microwave absorption (MDP) is described. The method is applicable to semi-insulating and low resistivity marterial as well. It is non-destructive and can be applied to epi-layers. MDP topograms are compared with those for photo luminescence, point contact, and EL2 using the same sample. The MDP-contrast is mainly due to a so far unknown recombination centre determining the carrier lifetime under non-equilibrium. The annealing behaviour of MDP is very similar to that of the point-contact method. In general, MDP has the potential to be used as a new tool for material quality inspection with direct evidence for device properties.

Source: Materials Science and Engineering: B

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Dec 12, 2013

Optimisation of the epi-ready semi-insulating GaAs wafer preparation procedure

The surface quality is crucial for growth of epitaxial layers on III–V semiconductor substrates. In this work the procedures of epi-ready semi-insulating (SI) GaAs wafer preparation were developed. The atomic force microscopy (AFM), triple crystal X-ray diffraction (TCD) and X-ray photoelectron spectroscopy (XPS) were used to monitor morphology and composition of substrates with different chemical treatment history. We propose an optimised epi-ready SI GaAs wafer preparation procedure involving NH4OH:H2O2:H2O/NaOCl:H2O2:H2O etching/polishing.

Source: Vacuum

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Dec 10, 2013

Photoelastic characterization of residual strain in GaAs wafers annealed in holders of different geometry

The spatial distribution of residual strain in undoped 2 inch GaAs wafers multi-step annealed in holders of different geometry was characterized by the scanning infrared polariscope (SIRP) method. The SIRP maps reveal that the distribution of strain is significantly influenced by the symmetry of annealing, in particular by the points of contact between wafer and holder. In contrast to the as-grown state, the annealed wafers show fine patterns of slip lines. The lowest level and the most homogeneous distribution of residual strain were achieved by annealing in a vertically positioned holder of graphite rings. The radial temperature differences in the wafers caused by heating and cooling were checked by means of thermocouples on dummies of graphite. Temperature gradients up to 30 K cm−1 were measured depending upon the rates of cooling and heating.

Source:Materials Science and Engineering: B

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