Electronic Materials Engineering (EME) Facilities

back

Fabrication/Processing

Thin film deposition

a.     Pulsed Laser Deposition System-Pascal chamber and coherent Lambda Physik excimer laser:  PLD is a thin film deposition (specifically a physical vapor deposition, PVD) technique where a high power pulsed laser beam is focused inside a vacuum chamber to strike a target of the desired composition. Material is then vaporized from the target and deposited as a thin film on a substrate. This process can occur in ultra high vacuum or in the presence of a background gas, such as oxygen which is commonly used when depositing oxides to fully oxygenate the deposited films. This technique is suitable for depositions on small areas-typically 1cm × 1cm.

Schematic of a PLD system . Source: Wikipedia.

The process of PLD can generally be divided into four stages: Laser ablation of the target material and creation of a plasma; Dynamics of the plasma; Deposition of the ablation material on the substrate; Nucleation and growth of the film on the substrate surface. Each of these steps is crucial for the crystallinity, uniformity and stoichiometry of the resulting film.

The intended use of the system in EME is for deposition of 2 element oxides like ZnO, MgO and CdO or their combinations.

b.     Sputter Deposition System-AJA, multi targets, DC and RF magnetron sources-DC source is used for creating plasmas using metal targets and RF is used for creating plasmas using dielectric targets. The magnetic field confines the plasma and thus improves the uniformity and purity of deposition. Sputtering is usually done in Ar(inert) ambient. But we can also do reactive ion sputtering using oxygen or nitrogen ambient. O and N radicals in the plasma can be used to deposit SiO2 or Si3N4 using a Si target. There are about 30 different targets and 10 shields (to reduce cross contamination) available with EME for current use.

c.     MOCVD reactor- Aixtron 200/4 (for 1X2”, 1X3”, 1X4”, 3X2”)-Used for epitaxial growth of III-V structures. The sources available for the EME reactor are - TMGa, TMIn, TMAl, AsH3, PH3, DEZn, CCl4 and SiH3. The reactor has susceptors that can hold a single 2” (1X2”) wafer or 3 of them (3X2”) at one time. The 1X2” susceptor is used for optimizing growth or growing small sized samples. The reactor uses larger amounts of gases on 3X2” susceptor and is suitable for commercial sample growth. Growth rates of few ML/sec are easily achieved and growth times can be controlled to an accuracy of 0.1 sec, making it suitable for growth of epitaxial nanostructures like QDs. 

d.     MOCVD reactor-Hybrid MR Semicon/Thomas Swan (1X2”) – This is an old reactor and has got extra Antimony and DimethylHydrazene sources. The uniformity across a 2” wafer grown in this reactor is not as good as in the new reactor. This reactor is essentially used for studies on new materials systems.

e.     Plasma enhanced Chemical Vapour Deposition system-Oxford Plasmalab 80+ - This machine is currently in operation and a new system is being ordered (see below).

f.      PECVD-dual frequency (stress control), high T stage – The old PECVD system can only be used for deposition of oxides and nitrides of Si, the new system has the capability for deposition of amorphous Si. The new system has dual frequency sources, one at a frequency of 30-50 MHz for creating the plasma and another source at a frequency of ~100 kHz for biasing the substrate. By varying the ratio of the two frequencies, the strain in the deposited film can be varied and after optimization, stress free films can be deposited. Stress free films are important for MEMS applications and with the new system, films with stress of the order of few MPascals (considered low stress for MEMS applications) can be deposited.

g.     Electron Beam evaporator-Temescal

h.     Thermal evaporator-Edwards – Used for evaporation of metals onto devices to make contacts. Thermal evaporator uses resistive heating for creating metal vapours and thus can’t be used for high melting point metals like Pt, Ti. The electron beam evaporator uses a beam of electrons to produce localized heat. The electron beam is focused on to the target using a magnetic field. The beam locally melts the target, which is deposited on the sample. There is less contamination in metals deposited using e-beam than using the thermal evaporator because of localized heating. High melting point metals like Ti and Pt can also be deposited using e-beam evaporator. Cross contamination in both items of equipment is minimized by proper use of shields.

i.      Atomic Layer Deposition (ALD) system. (on order) Cambridge nanotech-Sivanna 100 (TiO2, HfO2, ZnO, Al2O3, SiO2, Si3N4) – This is a growth technique that results in pin-hole free depositions. The sample surfaces to be coated are alternately saturated with the growth species and result in ML by ML deposition on the surfaces. The grown layers are free of any defects and coat the surfaces uniformly. The surface texture is completely preserved after the growth. Even powders and surfaces with deep trenches can be coated using this technique.

Etching

a.     Reactive Ion Etching System-Oxford Plasmalab 80+ (old RIE system)

b.     ICPRIE Cl/Br chemistry (new machine on order)

c.     ICP etching-Oxford Plasmalab system100 (F-Chemistry)—(system owned by LPC) - The old RIE system has provision for Cl2/SiCl4/CHF3/CH4/H2/O2 and Ar. CHF3 is used to etch oxides and nitrides of Si, but not Si. So plan is to upgrade the F-chemistry system (system owned by LPC) to have SF6, so it can be used to etch Si. The new system (machine on order) will be based on Cl/Br chemistry for etching III-Vs. It cann’t be used for Si. It will have inductively coupled plasma, which means denser plasma is created to get faster etch rates. With the old RIE system, InP was etched using CH4/H2. This etch forms a polymer on the surface. To avoid this, Cl chemistry is used and requires temperatures in excess of 200oC as InCl is a solid below this temperature. The new system has this provision.

Annealing/thermal

a.     Rapid Thermal Annealer – AET - For general use

b.     Rapid Thermal Annealer-JetFirst – specifically for optoelectronic device processing - One RTA is intended to be used just for device processing to avoid issues with contamination. The other system is for general use.

c.     Tube furnaces – EME has 3 tube furnaces, the largest one with a diameter of 3”. One of them is used for wet oxidation by injecting water vapour into it. Maximum temperature obtainable is 1500oC. N2, O2, Ar and forming gas ambients can be used for annealing.

Ion-Implantation

a.     High Energy Ion Implanter-NEC 1.7 MV Tandem accelerator

b.     Low Energy Ion Implanter-150kV accelerator

Device fabrication

a.     Wire Bonder-MEC, Wedge type-Used for bonding ~25 micron metal wires (Au, Al) to samples for making electrical contacts. The wedge is brought into contact with the metal wire and sample and an ultrasonic pulse, heat and pressure is generated to locally melt the wire and make contact with the sample. Then the wire is cut off using a clip.

b.     Nanoimprint lithography – Nanoimprint lithography can be used for serial production of wafers. It requires a mask that can be prepared on a quartz plate using the EBL system. The mask pattern is etched into the quartz and the patterns etched should at least have vertical dimension ratio of 1:5 with respect to unetched regions (refer to the figure below). The substrate to be patterned is covered with a layer of resist; the mask is pressed against this resist surface, compressing regions of the resist in accordance to the mask pattern. With the mask still on the resist, the resist is exposed to UV light, which cures it and the resist retains its shape after the mask has been removed. Now the substrate is etched (ICPRIE could be used) through the resist to transfer the pattern.

c.     Mask aligner-Karl Suss MJB3

d.     Mask Aligner-Karl Suss MA6 with near field holography and backside alignment (the mask can be aligned to features on the rear side of the wafer) -200 nm resolution – used for UV lithography.

e.     Raith 150- Electron Beam Lithography System – The electron beam lithography is a slow and tedious process, suitable mostly for small area patterning. Resolution of <20 nm can be obtained. The beam and stage can both be moved with high precision to repeat/ extend the patterns to different areas of the sample. High precision in stage movement is achieved using a laser interferometer for accurately detecting the movement.

Characterization

Electrical characterisation

a.     Temperature dependent I-V/C-V

b.     Deep Level Transient Spectrometer (DLTS) (from 10K to 800 K)-This system is specially designed for wide bandgap semiconductors and is suitable for probing impurity levels close to the mid-bandgap region.

c.     Electro-chemical voltage (ECV) profiler with photovoltaic spectroscopy (PVS) system-BioRad-This equipment is used for measuring the dopant concentration (≥1014/cm3) in a sample. Dopant concentration as a function of depth can be measured (resolution of few 10s of nms is possible). The technique requires minimal sample preparation.

d.     Hall Effect system – The present system can only be used at RT and the magnet is very small. The new one (not bought yet) would have the capability of temperature dependent measurements.

Optical characterisation

a.     Optical microscopy and imaging

b.     Low temperature (10K) photoluminescence (PL/PLE) and other optical measurement facility

c.     Laser diode/Photodetector characterization setup

d.     MicroRaman/PL system Jovin-Yvon (triple spectrometer)-The system is used to characterize bonds in few µm sized regions of a sample. By characterizing areas in different regions of the sample, a mapping of bond characteristics can be obtained. The same system can be used for micro-PL measurements—useful for characterizing samples whose properties vary on scales of few microns.

Physical characterisation

a.     RBS-C-NEC 1.7MV Tandem accelerator- Can be used to get elemental composition as a function of depth. Coupled to channeling techniques, RBS can also be used for damage studies. Principle is to bombard the sample with He and measure the energy spectrum of back-scattered He. This energy spectrum can be used to obtain the elemental composition of the target. Energy of incident He is ~2 MeV and depth profiles of ~1 µm can be obtained in Si/GaAs/InP. The lower limit on quantities that can be determined depends on the type of impurity element and its concentration. For eg., consider the case of Si uniformly doped with Al or B. Since Al is heavy, back scattered He looses less energy and because B is light, back-scattered He looses more energy. The resultant spectra would be something like this---making detection possible for Al but not for B.

b.     Nanoindentation (Ultra-Micro Indentation System (UMIS) and triboindenters)-Indentor tip is used to exert localized high pressure on the sample, which may result in phase transformations. This technique can be used to achieve different phases with quite different electrical/mechanical properties.

c.     Precision Ion Polishing system – Gatan—used for TEM sample preperation

d.     Ion-Mill – Gatan Duomill---used for TEM sample preperation

e.     Powder XRD- Philips

f.      Double Crystal X-ray Diffractometer- Bede QC2a – The resolution in normal X-ray diffraction systems is limited by the divergence of the X-ray beam. In order to detect small variations in lattice constants, such as that of GaAs and AlGaAs, we need extremely high resolution. In order to achieve this, we use a crystal (called conditioner) as a brag grating to collimate the X-ray beam which is then incident on the sample to be studied. With this system, we can just resolve QWs (few 10s of Å resolution).

g.     XRD system (Pananalytic)- The new XRD system has an additional conditioner between the sample and the detector. This allows us to do reciprocal space mapping. This system has better resolution than the old system. We can resolve QDs and nanowires as well. The stage in the new system has much more freedom with movement than the old system.

A hot stage with this system would be very useful to study crystalisation of materials. It costs about $60,000—trying to get money to buy the hot stage.

h.     Thermal analysis (DCS, TGA, etc)

Differential scanning calorimetry or DSC is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference are measured as a function of temperature. The basic principle underlying this technique is that, when the sample undergoes a physical transformation such as phase transitions, more or less heat will need to flow to it than the reference to maintain both at the same temperature. DSC may also be used to observe more subtle phase changes, such as glass transitions. DSC is widely used in industrial settings as a quality control instrument due to its applicability in evaluating sample purity and for studying polymer curing.

Thermal Gravimetric Analysis (TGA) is a simple analytical technique that measures the weight loss (or weight gain) of a material as a function of temperature. As materials are heated, they can loose weight from a simple process such as drying, or from chemical reactions that liberate gasses. Some materials can gain weight by reacting with the atmosphere in the testing environment. Since weight loss and gain are disruptive processes to the sample material or batch, knowledge of the magnitude and temperature range of those reactions are necessary in order to design adequate thermal ramps and holds during those critical reaction periods.---descriptions copied from WEB.

i.      Dual Beam Focussed Ion Beam (FIB) System (FEI/Zeiss) - One electron beam is used to mill the sample, while the sample can be observed live using the second electron beam. The Raith system has got a faster and much complex beam movement system. It also has a gas injection inlet and a nano-manipulator. Gas injection system can be used for faster, local etching or local stitching. For eg., we can put in a substrate with lots of nanowires on it, direct the nano-manipulator towards one nanowire, stitch them together using Pt, then detach the nanowire base from the substrate by injecting Br, transport it to another sample and use it to make contacts to have a single nanowire device. The manipulator is moved with a piezo stage.

j.      Electron microscopy and Atomic Force microscopy (AFM) facilities

Other

a.     Low temperature Cryostat-Oxford Instruments, 1.5K, 15T (Magneto-optical measurements)

b.     Oxygen-free glove box