The atomic layer epitaxy (ALE) growth system consists of 3 ultra high vacuum (UHV) chambers, equipped with multi-growth techniques such as e-beam evaporation, Silicon sublimation and gas sources and reflection high-energy electron diffraction (RHEED). The analysis chamber has multi-characterisation techniques such as scanning tunnelling microscopy (STM), low energy electron diffraction (LEED), photoelectron spectroscopy (both XPS & UPS) and magneto-optic Kerr effect (MOKE). The wide variety of in-situ characterisation tools in this system allows us to derive important electronic and magnetic properties of surfaces, thin films and nanostructures.
The system is a modified electron microscope for real-time in-situ observations at the nanoscale. It comprises a 200kV Transmission Electron Microscope (TEM) equipped with in-situ e-beam evaporation, gas injection and energy filtered TEM (EFTEM). The system permits real-time imaging of design & growth and reaction phenomena on a sub-nanometre length scale. The ability to directly observe design & growth and reaction phenomena in-situ enables the mechanisms of microstructural evolution to be elucidated more efficiently than conventional post-mortem examination. Using this system, IMRE is able to study nucleation, growth and coalescence/reaction pathways through diffraction patterns, bright-field, dark field, imaging at near atomic resolution. The system is also able to use the energy filter (EF) to conduct EF-TEM and electron energy loss spectroscopy analysis at nanometer resolution.
Contact: Dr Lin Ming
Metal Organic Chemical Vapour Deposition (MOCVD - Arsenides and Phosphides)
An Aixtrion 200/4 system using tertiary-butyl arsine and tertiary-butyl phosphine as the group V precursors are used for the growth of III-Arsenides and Phosphides such as InGaAlP and InGaAsP. The n-type and p-type dopants are Si and Zn respectively. The focus of these materials are for applications in Vertical Cavity Surface Emitting lasers, Quantum dot lasers, tunable lasers, integrated optics and optical MEMS. The system is capable of growing three 2-inch wafers at a time.
Metal Organic Chemical Vapour Deposition (MOCVD - Nitrides)
An EMCORE D180 low pressure system is used for the growth of III-Nitrides for the fabrication of ultra-violet, blue, green and white LEDs and semiconductor lasers emitting in the UV, violet, blue and green wavelengths. The system is capable of growing six 2-inch wafers with substrate rotation. The substrates used are sapphire, silicon and free standing GaN. Materials studies include growth of quantum dots, piezoelectric effects on the optical emission in quantum wells and microcavities. The system is equipped with an Epimetric monitor for in-situ reflectivity measurement. The n-type and p-type dopants are Si and Mg/Zn respectively.
The inductively coupled plasma (ICP) system is typified by a high density plasma discharge. The system is used to etch semiconductor materials such as GaAs /InP /GaN and dielectrics (SiO2/Si3N4) during various device fabrication steps. The system consists of two chambers, namely, process and load lock, capable of evacuating to base pressures of 10-6 Torr. Chlorine/Fluorine based gases are introduced to etch the materials in a plasma, generated by an RF power operating at 13.56 MHz. An additional 2 MHz ICP RF source is superimposed to create high-density plasma with minimal damage on the sample. Anisotropic etch profiles and variation in etch rates can be achieved by optimising various process parameters.
The 700m2 cleanroom facility in IMRE has capabilities in nanofabrication for non-silicon materials and characterisation, with tools like nanoimprinter, UBM sputtering system and an MOCVD for gallium nitride growth.
The system is equipped with four process chambers that can fabricate organic electroluminescence devices, such as for pre-treatment, organic film deposition, metal and transparent conducting oxide electrodes modification and deposition and passivation. Each process chamber can be operated independently to prevent cross contamination and the specimens can be transferred from one chamber to another in the vacuum. The system is also connected to a glove box purged with high purity nitrogen gas with oxygen and moisture levels below 1 ppm. The system enables the fabrication, characterisation and testing of OLEDs in a well controlled environment.
Wafer Bonding System, Karl Suss BA6, is a tool designed for wafer level bonding. This system consists of two parts: One is used for pattern alignment with which ~1m m alignment accuracy can be reached. The other one is the bonding machine, also called substrate bonder. The bonding process is done in a vacuum chamber with tunable temperature (up to 400deg C), voltage (up to 2000V) and pressure (up to 4atm). All parameters can be programmed. The machine is useful for silicon-to-glass anodic bonding, silicon-to-silicon prebonding and etc.
Facilities for Materials Characterisation
i) Mechanical Properties
Nano-indenter with Atomic Force Microscope
The Nano-indentation/AFM system is a unique instrument for characterising the elastic, plastic, stress-strain, hardness, creep, fracture, residual stresses and other mechanical properties of coatings, thin films, interface, bulk materials and the near surface region of materials. The force resolution is 0.75 mN; displacement resolution is 0.05 nm.
The MicroTest systems are designed to meet the requirements of a broad variety of sub-miniature testing applications. It has high precision position control, displacement measurement resolution better than 50nm, high stiffness load frame, testing in environmental chamber, light weight pneumatic micro tensile grips, micro bend fixture, micro indentation tools, die shear fixture, ball shear fixture, variable angle micro peel fixture, constant 90 degree angle peel fixture, XY-Theta positioning stage, vacuum stage, vision system using microscope of video camera.
A range of both home built and commercial scanning probe microscopes is available in the Micro & Nano Systems Cluster. These include two molecular imaging systems for fine scale imaging, a Topometrix instrument for scanning large wafer size samples, a Digital Instruments STM electrochemical STM, an Omicron UHV AFM and a Nanofactory STM/TEM mount for insertion within a TEM. Homebuilt equipment includes a liquid AFM, a BEEM STM and a future cryogenic STM/AFM.
A scanning electron microscope forms an image of the surface of materials by scanning a fine electron beam. Both secondary and backscattered electrons can be collected giving information about the surface topography and atomic weight of the sample. Our field emission SEM has a resolution of 1 nm at 15 kV and above accelerating voltages and 2.5 nm at 1 kV and our tungsten filament SEM has a resolution of 3.5 nm. Both SEMs have an X-ray spectrometer for elemental identification.
Transmission Electron Microscope (TEM)
A transmission electron microscope can be used to image thin (<100 nm) sections of materials at high resolution. Our 300 kV field-emission microscope is a high resolution instrument capable of resolving atom columns separated by only 0.17 nm. It can also be used to determine crystal structure using bright-field images, dark-field images and diffraction patterns. An X-ray spectrometer allows chemical composition to be determined at nanometre resolution.
The ballistic electron emission microscopy (BEEM) technique is a variant of scanning tunnelling microscopy (STM). BEEM is used to determine, with nanometer resolution, the electronic properties of buried metal-semiconductor interfaces. It can work in spectroscopy mode or as an interface imaging tool. The technique can be applied to a wide variety of organic and inorganic semiconductors covered with metals. IMRE is the only institute in Singapore with dedicated facilities and extensive knowledge related to BEEM research.
Monochromated scanning transmission electron microscopy (M-STEM)
M-STEM provides sub-Angstrom resolution images and a wide range of analyses on the physical properties and chemical compositions of materials.
iii) Optical Characterisation
The JYT64000 system equipped with both micro- and macro-chamber is used for Raman scattering measurements. The system is capable of measuring spectra from both solid and liquid samples. Using this system, it is possible to investigate crystal structure, orientation, composition and stress in semiconductors. In-situ temperature dependent measurements in polymers and biomaterials can also be carried out. Simultaneous polarised uv-visible Raman excitation can be performed using a Spectra Physics Ar+ laser (Excitation wavelengths: 514.5 - 457.9 nm, 351.4 nm, 363.8 nm). Room temperature micro-Raman spectra can be recorded from samples with a spatial resolution of 1.0 mm. The spectral resolution of the set up is 0.2 cm-1. Temperature dependent measurements (77 - 500 K) with a lateral resolution of 2.0 mm can be explored using the OXFORD Microstat microscope cryostat.
The scanning near-field optical microscope has been implemented for ultraviolet visible-near infrared imaging in the illumination mode. This technique is useful to study optical properties from materials with sub-wavelength spatial resolution and with AFM capability. The optical properties can be easily correlated to topographic information. The system is equipped with He-Cd (325 nm) and Ar+ (514.5 - 457.9 nm) laser sources. Al/Cr/Au coated optical fibre probe with aperture 50 nm serves as a nanosource through which photoluminescence can be excited from semiconductors and light emitting polymers. The microscope is equipped with various detectors (PMT, APD and InGaAs photodiode) to perform broadband photoluminescence intensity mapping.
Time-resolved photoluminescence spectroscopy is a very important technique used to study the carrier dynamics in semiconductor materials and devices. The system consists of a Femtosecond Ti:sapphire laser (with frequency doubler and tripler) delivering 100 fs pulses from 800 nm to 266 nm, a streak scope with a spectral coverage of 300nm-1100nm and a time-resolution of 15ps and a close-cycle cryogenic system with temperature range from 4 to 300 Kelvin. The system is currently used for the study of spontaneous emission dynamics of InAs/GaAs quantum dots in openspace and in microcavities, recombination dynamics of III-V nitrides and related low-dimensional structures and recombination dynamics of fluorescent organic semiconductors.
Time-of-Flight Secondary Ion Mass Spectroscopy (ToF-SIMS)
ToF-SIMS allows determination of chemical composition of materials and the distribution of chemical species through the depth of the sample. ToF-SIMS uses a pulsed primary ion beam to desorb and ionise atoms and molecules from the surface of a sample. The resulting secondary ions are accelerated into a mass spectrometer, which measures their “time-of-flight” from the sample surface to the detector to determine the mass of the ions. Because each element has its own mass, analysis of the mass spectrum allows us to determine what elements are in the sample. Depth profiles are obtained when the sample is gradually sputtered and mass spectra are measured from deeper and deeper layers.
This is an essential equipment in the study of molecular structure especially in polymer and protein structure elucidation. The NMR is equipped with both liquid and solid-state probes which enable the study of organic or inorganic materials either in liquid or solid state under a 400 MHz (9.4 Tesla of magnetic field strength) spectrometer.
The VG ESCALAB 220I-XL system is an extremely versatile XPS system. It is equipped with two types of X-ray sources: the twin-anode (Mg/Al) and the twin crystal monochromated Al source, producing spectra from areas ranging from 8 mm down to 20 mm in diameter. The use of the X-ray monochromator with small spot size allows selected area analysis. The lens and analyser combination can also be operated at maximum sensitivity. This instrument is also able to produce elemental mapping with a spatial resolution of 1 mm. Other capabilities of the system include depth profiles, line scans, angle-resolved XPS and ion scattering (1 keV He+).