NANO MANUFACTURING TECHNIQUES

RAMACHANDRAN . M(2009206031)M.E CIM (III Sem)

CHEMICAL ETCHINGTo remove complete layers or to transfer patterns from a mask to the underlying layer.Types of Etching:Dry Chemical EtchingWet chemical EtchingIn Wet Chemical Etching which only requires a liquid solution dissolving a specific selected material.Mask material was used for selectively etch the material.While mask material must not dissolve or at least much slower than the material which is to be machined.

CHEMICAL ETCHING Contd.,During etching materials exhibit isotropic or anisotropic etching properties.In isotropic process, undercutting of the mask layer by the same distance as the etch depth.Anisotropic process allow the etching to stop on certain crystal planes in the substrate.For e.g single crystal materials(silicon) exhibit anisotropic etching properties.

CHEMICAL ETCHING Contd.,Wet etching is well suited forEtching thin films on substrates.Etch the substrate itself.

REACTIVE ION ETCHING (RIE)Applications: Used in the Micro electronics industryFor highly anisotropic etching of micron- or nano scaled structures.Reactive Ion Etching (RIE):Plasma supported gas chemical technique.The gas pressure maintained in the range between 0.001and1 Mbar.A Radio Frequency (RF)13.56MHz at a few hundred watts, applied to the wafer platter is used to initiate a plasma.

REACTIVE ION ETCHING (RIE)The parameters affect the RIEPressureGas flowsRF power.DEEP REACTIVE ION ETCHING (DRIE):Etch depths of hundreds of microns can be achieved with nearly vertical side walls. This process is also called as Bosch process.Bosch process has two different compositions of gases are alternatively used in the reactor.The first gas composition creates a polymer on the surface of the substrate.The second gas composition etches the substrate.Etch rates are 3 to 4 times higher than Wet etching.

MAGNETICALLY ENHANCED REACTIVE ION ETCHING (MERIE)In this technique a magnetic field is added whose field direction is aligned perpendicular to the electric field direction.At the given pressure the magnetic field increases the probability of ionization.(nearly two orders of magnitude)The magnetic field enables a reduction of the working pressure which enhances the anisotropy of the etch profile.

ION BEAM ETCHING (IBE)The IBE etch tool consits of vaccum chamber with an ion source with filament cathode, an anode the wafer to be etched mounted on a table with an adjustable tilt angle.A low temperature and low pressure plasma is maintained inside the ion source.The interface between ion source and chamber there are extraction grids and a neutralizer.The neutralizer consist of a tungsten filament, which emits electrons serving to neutralize positive charges on the wafer surface.

ION BEAM ETCHING (IBE) CONTD.,Applying a voltage to the extraction grid a current of (positively) charged ions is extracted from the plasma which is accelerated into the vacuum chamfer towards the wafer substrate.TypesReactive ion beam etching (RIBE)Chemically Assisted Ion beam etching (CAIBE)In RIBE , additionally a reactive gas is introduced to the ion source and reactive ions are formed due to the collisons together with neutral particles.In CAIBE additonally , Argon gas was introduced into the chamber.

ION BEAM ETCHING (IBE) CONTD.,Advantages of RIBE and CAIBE techniques,Kinetic energy of the ions and the current density can be controlled independently.The setup provides high flexibility in determining the anisotropic etching profiles due to the adjustability of the sample surface.The chamber may also used to perform RIE and simple sputter etching.DIS ADVANTAGES OF IBE:The sensitivity of the filament cathode when facing reactive gases.Ablation of the cathode due to the chemical reactions and sputtering because it may lead to contamination.Lower etching rates in parallel-plate RIEFor larger wafer plates the rates may be inhomogeneous.

OTHER ETCHING TECHNIQUESOther Dry Etching methods are.,Sputter etchingVapor phase etching.Sputter Etching essentially RIE without using reactive ions. The systems are based on sputter deposition systems, where ions bombard the substrate instead of the material target as it happens in sputter deposition.

OTHER ETCHING TECHNIQUESVapor phase etchingThe wafer is placed inside a reaction chamber, to which one or more gases introduced.The wafer surface is chemically dissolved by the gas molecules.Types: Silicon dioxide etching using hydrogen fluoride (HF).Silicon etching using Xenon di flouride (XeF2)BothA vapor phase process has to be designed care fully to avoid by product.

QUANTUM-CONFINED MATERIALIf the total size of the semiconductor is reduced to a dimension well this excitation Bohr radius(quantum confinement), the forced deviation from the preferred distance will be compensated by both a spilt of the continuous bands into closely neigh bouring but discrete energy levels and a higher energetic separation of the semiconductors valance and conduction band i.e the ground and excited state.The shape of possible quantum confined materials areRod-like, ellipsoidal, cubic, rectangular, spherical, tetrapodes, toroides and bandstuctures.

QUANTUM-CONFINED MATERIAL CONTD.,According to the dimensionality of the quantum confinement, there three base structuresQuantum dots:Semiconductor structure confined in all three spatial directions.Quantum wires:Structures being confined in two spatial directions and non- confined in the third direction.Quantum Wells:Semiconductor structure being confined in one spatial direction and non-confined in other two directions.

QUANTUM-CONFINED MATERIAL CONTD.,Quantum confined materials:From the periodic table II and VI CdS, CdSe, CdTe, ZnS, ZnSe or HgS.From the periodic table III and VInP, InAs, GaAs or GaN. Optical properties:1. Optical transitions.2. Absorption.Inter band: transitions between the valence and conduction bands.Intra band: transitions between the quantized sub-bands or states within the conduction band (transitions of electrons) and the valance band (transition of holes)

OPTICAL PROPERTIES CONTD.,3. Fluorescence/ Luminescene:Photo luminence/ Fluorescence: optically excited emission.Electro lumineescence: emission generated by recombination of electrically injected charges.

Non Linear Optical properties:Electro- optic effectOptically induced change of the refractive index

QUANTUM DOTSAny volume within a semiconductor crystal confines electrons, holes and excitations in all three spatial dimensions to a diameter of several nanometers to several hundred nanometers is called a quantum dot.Structure:It consits of,Core: The key material, defining the principal optical properties of the particles.Shell: needed to protect the core from dissolving and from polluting outside influences and prevents optically excited cores from relaxing radiationless.Coating: To functionalize the particles and to make them accessible for further chemical and biological use.

QUANTUM DOTS CONTD.,

QUANTUM DOTS CONTD.,Cores:The material used for the core is chosen based upon the emission wavelength of interest, For e.g CdS for UV-blue, CdSe for most visible spectrum.The cores can be synthesized as nano metere shapes like spheres, rods, phyramids, or other shapes.The choice of the material determines the coarse wavelength region of the emission.For e.g Spherical CdSe cores of above 6nm dia emit at 655nm, whereas smaller but otherwise equal cores of below 3nm will emit at 525nm.

QUANTUM DOTS CONTD.,Why shells?Their emission is quite weak and often un realiable.To improve radiation of the core material.Un covered cores tend to spontaneously dissolve leading to irreversible decomposition of the nano crystals.To over come the above problem;The shell material is composed of a structurally related material which is non-radiactive, transparent, and can be efficiently affiliated to the core material.

QUANTUM DOTS CONTD.,Coating:To make them accessible for chemical binding reaction.A possible coating e.g consits of organic ligands that are covalently attached to the surface of the shell and enhanced by hydrophobic/ hydrophilic polymers. The hydrophobic part of the polymer binds the organic ligands, while the hydrophilic interacts with the external solvent providing solubility in buffers.E.g like, antibodies, streptavidin, lectins, nucleic acids and related molecules of biological interest may be attached to the surface when configurating these polymers with carboxylic acid derivatives.

FABRICATION OF QUQNTUM DOTSQuantum dots are prepared byElectron beam lithographyMolecular beam epitaxy.Pulsed laser deposition.Self assembly or Colloidal synthesis.They are produced as en capsulated species that can be dispersed in liquid medium, self assembled in forms of super lattices, grown on substrates in manifold geometrical shapes, embedded in wider band gap semiconductors or incorporated in glasses or polymers.

FABRICATION OF QUQNTUM DOTS CONTD.,COLLOIDAL SYNTHESIS:This method produce quasi-spherical and slightly rod like particles from few molecules up to bulky semiconductor material.The size range from 1nm to 6nm, which is hardly achievable by other techniques.The nano particles are produced by precipitation reactions.Metallic and non-metallic compounds are brought to reaction within aqueous or organic solvents resulting in a colloidal solution. In the colloidal phase the particles are endangered to form larger aggregates.

FABRICATION OF QUQNTUM DOTS CONTD.,Dimeric aggregate will form the core of a larger as the process continues.To minimize this aggregations is to prepare the particles in highly charged state.Adding large non interacting organic molecules in a controlled way during the precipitation reaction.These cover the surface of the nano particles and slow down the reaction.The small crystallites size will depend on concentration, temperature and PH value of the solution.

FABRICATION OF QUQNTUM DOTS CONTD.,End of the synthesis reaction the size distribution was narrowed by fractionation.Adding to the colloidal solution a liquid, which is not able to solve the nano particles, leads to successive precipitation of the particles according to decreasing size.The separation from the rest of the particles may be realized by filtration and centrifugation.In this method a very narrow size of particles distribution to produce up to the gram scale.

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