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Synthesis and Fabrication of Electronic Materials Prof Dr Nguyen Van Hieu International Training Institute for Materials Science (ITIMS) Lab Of Nanosensors Lecture Content 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors (“refreshing”) 5.1.2.History of semiconductor technology 5.2 Growth and process of semiconductor materials 5.2.1 Types of semiconductors 5.2.2 Crystal growth and wafer fabrication 5.2.3 Physical and chemical vapor deposition (PVD&CVD) 5.3 Synthesis of one-dimensional nanostructures 5.3.1 Synthesis of carbon nanotubes 5.3.2 Synthesis of metal oxide nanowires (NWs) 5.4 Nanostructures fabricated by physical methods 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors Metallic conductor:  typically or freely moving electrons per atom Semiconductor:  typically freely moving electron per 109-1017 atoms What is the result on the properties of such a material? Tai ngay!!! Ban co the xoa dong chu nay!!! 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors Semiconductors in the periodic table II III IV V VI Be B C N O Mg Al Si P S Elemental semiconductors: C, Si, Ge (all group IV) Compound semiconductors: III-V: GaAs, GaN… II-VI: ZnO, ZnS,… Group-III and group-V atoms are “dopants” Zn Ga Ge As Se 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors  Small impurities can dramatically change conductivity: slight phosphorous contamination in silicon gives many extra free electrons in the material (one per P atom!)  slight aluminum contamination gives many extra holes (one per Al atom)  P Al 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors - Atomic radius: 0.117nm, or 0.234nm -Lattice constant: 0.5nm -Atomic radius ~ As, In (0.121, 0.166) 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors Silicon dopants II III IV V VI Be B C N O Mg Al Si P S Zn Ga Ge As Se In Boron most widely used as p-type dopant; Aluminum in old processes (Indium (In) seldom used) Phosphorous and arsenic both used widely as n-type dopant (Antimony (Sb) seldom used) P: higher diffusion, better activation than As 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors p-n junction (diode) n-type doped semiconductor p-type doped semiconductor e.g silicon with phosphorus impurity e.g silicon with Al impurity electrons determine conductivity holes determine conductivity p-n junction: current can only flow one way! Semiconductor diode 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors The field effect accumulation depletion inversion ++++++++ - - - - 10 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology Over 35 years The chip contains 04 bipolar transistors The chip contains over a million MOS transistors 11 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology IC Minimum Feature Size 12 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology IC Minimum Feature Size 13 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology Ultimate Small Scale Structure 14 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology 15 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology 1950 Junction Transistor 16 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology 1950 - Alloy Junction Transistor 17 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology 1958 – First Planar Transistor 18 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology Basic Bipolar Paired Transistors Bias “Resistor” NPN Bipolar Device Bias “Resistor” 19 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology Modern Integrated Circuit Section 20 5.1 Introduction to semiconductors 5.1.2.History of semiconductor technology SEM Cross-Section of Integrated Circuit Wiring Layers Wiring Layers Wiring Layers Vias through Passivating Layers CMOS Devices 21 5.2 Growth and process of semiconductor materials 5.2.1 Types of semiconductors 1) Elemental semiconductors 2) Binary compounds 3) Oxide semiconductors 4) Layered semiconductors 5) Magnetic semiconductors 6) Amorphous semiconductors 7) Organic semiconductors 22 Lecture Content 5.1 Introduction to semiconductors 5.1.1.Essentials on semiconductors (“refreshing”) 5.1.2.History of semiconductor technology 5.2 Growth and process of semiconductor materials 5.2.1 Types of semiconductors 5.2.2 Crystal growth and wafer fabrication 5.2.3 Physical and chemical vapor deposition (PVD&CVD) 5.3 Synthesis of one-dimensional nanostructures 5.3.1 Synthesis of carbon nanotubes 5.3.2 Synthesis of metal oxide nanowires (NWs) 5.4 Nanostructures fabricated by physical methods 5.5 Practice on the synthesis of ZnO and SnO2 NWs 23 5.2.1 Types of semiconductors 1) Elemental semiconductors  The elements Si and Ge are well-kwon semiconductors  Their crystal structures are the same as diamonds  Some elements from the group V and VI of the periodical table such as P, S, Se, Te are also semiconductors 24 5.2.1 Types of semiconductors 2) Binary compounds Compounds formed from elements of the groups: III-V (such as GaAs); II-VI (such as HgTe); I-VII (such as CuCl) 25 5.2.1 Types of semiconductors 3) Oxide semiconductors  CuO and Cu2O are well-known semiconductors SnO2 Eg=3.6 eV ZnO=3.37 eV 26 5.2.1 Types of semiconductors 4) Layer semiconductors •Typical layer semiconductors are PbI2, MoS2, GaSe •The bonding within layers is typically covalent •The behavior of electrons in the layer is quasitwo dimensional •The interaction between layers can be modified by incorporating foreign atoms 27 5.2.1 Types of semiconductors 5) Magnetic semiconductors • Many compound containing magnetic ions such as Eu, Mn, Co have semiconductor and magnetic properties • The magnetic alloy semiconductors containing lower concentrations of magnetic ion are known as dilute magnetic semiconductor • Traditional electronic devices are based on control of electric charge, but magnetic semiconductors allow control of quantum spin-state • •Atoms of Mg can be inserted at desired locations by using STM •MBE is used for doping This would theoretically provide spin •That material is used to fabricate the polarization, which is important storage devices property of SPINTRONIC DEVICES 28 5.2.1 Types of semiconductors 6) Amorphous semiconductors Definition: Amorphous materials are in condensed phase and not possess the long range translational order (periodicity) of atomic sites A glass is an amorphous solid which exhibits a glass transition Usually we are speaking about three different orders (simplest definition): Short range order means the order within the range of 0-10 Å (local order) Medium range order is the order within the range of 10-100 Å Long range order means order over 100 Å a-Si:H is typical amorphous semiconductors 29 5.2.1 Types of semiconductors 7) Organic semiconductors Semiconductor in organic materials-mechanism They have been extensively used for optical devices such as solar-cell, OLED, 30 10 5.4.1 Lithography techniques X-Ray lithography Wavelenghts in the range of 0.04 to 0.5nm (1) A mask consisting of a pattern made with an X-ray absorbing material on Xray transparent membrane     (2) An X-ray source of sufficient brightness in the wavelength range of interest to expose the resist through the mask Diffraction limits lithography resolution to l/2 Obvious solution: use lower wavelengths sources DUV and EUV approaching standardization X-Ray lithography still at “exploratory” stage (3) An X-ray sensitive to resist material 97 5.4.1 Lithography techniques X-Ray lithography Resolution limit: 25nm 35nm Au line (a) and 20nm W dot (b) 98 5.4.1 Lithography techniques X-Ray lithography Structures produced with X-ray litho  Device patterns with feature sizes less than 40 nm achieved by x-ray lithography and by lift off 99 33 5.4.1 Lithography techniques X-Ray lithography Structures produced with X-ray lithography 125 nm feature 100 5.4.1 Lithography techniques Electron beam lithography 1) Casting of thin PMMA film 3) Development of PMMA e 4) Metallization 2) E-beam patterning of PMMA 5) Lift-off Employs a beam of electron instead of photons Advantage: Fast turn-around time Disadvantage: Slow throughput The resolution is limited by: (i) forward scattering of the electrons in the resist; and (ii) back scattering from the underlying substrate It is the most powerful tool for the fabrication of feature size of 3-5nm 101 5.4.1 Lithography techniques Electron beam lithography Applications of electron beam lithography Mainly employed for the fabrication of photomasks Also used to write patterns directly on wafer 102 34 5.4.1 Lithography techniques Electron beam lithography Electron beam lithography system Throughput enhanced by variable beam shaping 103 5.4.1 Lithography techniques Focused Ion Beam Lithography   FIBL components:    Ion source Ion optics column Sample displacement table   Specifications:     Accelerating voltage 3-200 kV Current density up to 10 A/cm2 Beam diameter 0.5-1.0 μm Ions: Ga+ , Au+ ,Si+ ,Be+ etc FIB is a very attractive tool in lithography, etching, deposition, and doping 104 5.4.1 Lithography techniques Focused Ion Beam Lithography FIB fabricated nanostructures 105 35 5.4.2.Nanomanipulation and nanolithography •Scanning tunneling microscopy (STM) •Atomic force microscopy (AFM) •Nanomanipulation •Nanolithography 106 5.4.2.Nanomanipulation and nanolithography Scanning tunneling microscopy (STM) Current Feedback 107 5.4.2.Nanomanipulation and nanolithography Atomic force microscopy (AFM) 108 36 5.4.2.Nanomanipulation and nanolithography Nanomanipulation (1) “The interactions or forces between the tip and the sample surface offer a means to carry out precise and controlled manipulation of atoms, molecules and nanostructures on a surface” Eigler and coworkers used STM by applying pulse voltage Ultrahigh vacuum and ultra low temperature Two processes has been identified for the manipulation, namely parallel and Perpendicular processes The tungsten tip was used to position 35 xenon atoms on a nickel surface, “IBM” 109 5.4.2.Nanomanipulation and nanolithography Nanomanipulation (2) Parallel Process characteristics •The STM tip drags the atom (molecule ) along the surface and positions the atom (molecule) at a desired spot •The bond between the manipulated atom (molecule) is never broken •The relevant energy barrier for such a process is the energy required for diffusion across the surface, typically in the range of 1/10 to 1/3 of the adsorption energy Two groups of parallel process: -Field-assisted diffusion -Sliding process 110 5.4.2.Nanomanipulation and nanolithography Nanomanipulation (3) The field-assisted diffusion: •Based on the present of the intense and inhomogeneous electric field between the STM probe tip and surface •Directional diffusion of atom is due to interaction of the field with the dipole moment of the atom The sliding process: •It is based on the force between the STM and atom •The directional motion of atom is achieved by adjusting the position of the tip, so the force between the STM and atom will pull the atom across the surface with the tip 111 37 5.4.2.Nanomanipulation and nanolithography Nanomanipulation (4) STM can be used for chemical manipulation and the ability of single molecule dissociation and construction Iodobenzen (C6H5I) molecule, the C-I bond is break by injection of 1.5eV tunneling electrons 112 5.4.2.Nanomanipulation and nanolithography Nanomanipulation (5) AFM has also been explored for nano manipulation and fabrication Difference with STM, AFM tip is literally dragged across the substrate surface Depending on the nature of the interaction between the tip and atom, three basic manipulation modes is pushing, pulling, and sliding 113 5.4.2.Nanomanipulation and nanolithography Nanomanipulation (6) In comparison with other nanofabrication methods •The SPM tip has a nano-scale sharp and is the best nanomanipulation tool, it offers extremely fine position control in all tree dimensions, It promises manipulating a single atom •It offers the ability of both manipulation and characterization insitu Fe atoms constructed on Cu (111) using STM 114 38 5.4.2.Nanomanipulation and nanolithography Nanolithography(1) SPM-based nanolithogrphy: •Local oxidation and passivation •Localized chemical vapor deposition •Electrodeposition •Mechanical contact of the tip with the surface •Deformation of the surface by electrical pulse • Anodic oxidation of the sample surface and exposure of electron resist Patterns with sizes of 10-20nm or to 1nm (in UHV) have been demonstrated 115 5.4.2.Nanomanipulation and nanolithography Nanolithography(2) Nanometer holes can be formed using low energy electrons from STM tip when a pulsed electric voltage is applied at the presence of surface gas molecule between the substrate and the tip A possible mechanism is that the electric filed induces the ionization of gas molecules near the STM tip, and accelerate the ions towards the substrate Nanostructures can be created using field evaporation by applying bias pulses to the STM tip-sample tunneling junction d is large, the tip-atom and atom-sample interactions Uat and Uas not overlap When d is small, the atom can be either transferred from the tip to the sample or from sample to the tip 116 5.4.2.Nanomanipulation and nanolithography Nanolithography(3) Field-gradient induced surface diffusion When a voltage pulse is applied to either tip or the sample, a field with a larger gradient will be created at the sample surface around the tip => atoms move toward the position directly below the tip (the field is the strongest) Field electron current are emitted either from the tip or the sample can melt the tip => a mount of tip atoms on the sample surface (deposition) 117 39 5.4.2.Nanomanipulation and nanolithography Nanolithography(4) AFM lithography with tunneling currents  Conducting AFM tip  Scan AFM in constant force mode  Develop modified areas of resist Si Substrate 118 5.4.2.Nanomanipulation and nanolithography Nanolithography(5) AFM lithography using anodic oxidation  Biased AFM scanned over H- passivated p+ Si  Oxide ions in field react with substrate Pattern fabricated on Ti substrate with elevated features being TiO2 Resolution is a few nm 119 5.4.2.Nanomanipulation and nanolithography Nanolithography(6) Nanowires fabrication example Cr nanowire using mechanical AFM lithography Cr nanodots using the same process 120 40 5.4.3 Soft lithography •Microcontact printing •Molding •Nanoimprint •Dip-pen nanolithography 121 5.4.3 Soft lithography Microcontact printing (1) Microcontact printing is a technique that uses an elastomeric stamp with relief on its surface to generate patterned SAM (self-assembled monolayer) on the surface 1) Application of ink to stamp 2) Application of stamp to surface 3) Removal of stamp 4) Residues rinsed off 122 5.4.3 Soft lithography Microcontact printing (2) (a) (a) Printing on a planar substrate with a planar stamp (b) Printing on a planar substrate with rolling stamp (c) Printing on a curved substrate with a planar stamp (b) (c) PDMS = Poly(imethylsiloxane) 123 41 5.4.3 Soft lithography Microcontact printing (3) Printing of PDMS 124 5.4.3 Soft lithography Molding Micromolding in capillaries “a liquid precursor wicks spontaneously by capillary action into the network of channels formed by conformal contact between an elastomeric stamp and a substrate”  Microtranfer molding “Recessed regions of a elastomeric mold are filled with a liquid precursor” Replica molding “Micro-nanostructures are directly formed by casting and solidifying a liquid precursor again an electrometric mold” 125 5.4.3 Soft lithography Nanoimprint •It was developed in the middle of 1990’s •It has demonstrated both high resolution and high throughput for making nanometer scale structures Stamp with the desired feature Typically, thermoplastic polymer is the printed materials •Consists of pressing a mold onto the resist above its glass transition temperature Tg 126 42 5.4.3 Soft lithography Nanoimprint -Step size should be controlled to get the parallelity of the substrate and thermal gradient -The flow of the displaced polymer could set a limit to the feature density -Imprint of 50 nm feature separated by 50nm spaces within an area of 200 x 200 mm has been demostrated 127 5.4.3 Soft lithography Nanoimprint   SiO2 pillars with 10 nm diameter, 40 nm spacing, and 60 nm height fabricated by ebeam lithography Master can be used tens of times without degradation Stamp 128 5.4.3 Soft lithography Nanoimprint  Mask is pressed into 80 nm thick layer of PMMA on Si substrate at 175° C (Tg=105 ° C), P= 4.4 MPa  PMMA conforms to master patterng, resulting in ~10 nm range holes pattern in PMMA 129 43 5.4.3 Soft lithography Nanoimprint    Reactive ion etching is used to cut down resist thickness until shallow regions are completely removed Ti/Au is deposited onto resist Resist and metal-coating is removed by solvent leaving behind metal dots where resist had been removed Metal dots 130 5.4.3 Soft lithography Dip-pen nanolithography It is a direct-write method based upon an AFM and works under ambient conditions Chemisorption is acted as a driving force for moving the molecules from the AFM tip to the substrate via the water filled capilary 131 5.3.4 Soft lithography Dip-pen nanolithography A) Ultra-high resolution pattern of mercaptohexadecanoic acid on atomically-flat gold surface B) DPN generated multi-component nanostructure with two aligned alkanethiol patterns C) Richard Feynmann's historic speech written using the DPN nanoplotter 132 44 5.4.4 Seft-assembly of nanoparticles or nanowires •Capillary force induced assembly •Dispersion interaction assisted assembly •Shear force assisted assembly •Electric-field assisted assembly •Covalently linked assembly •Gravitational field assisted assembly •Template assisted assembly 133 Seft-assembly of nanoparticles or nanowires Capillary force induced assembly One of the commoly used strategies of self-assembly of nanoparticles in order 2D array Basing on lateral capillary force due to deformation of liquid surface creating by the particles The capillary interaction between adjacent particles either floating or partially immersed into liquid 134 Seft-assembly of nanoparticles or nanowires Capillary force induced assembly SEM images of 2D structures of nanospheres selfassembled using capillary force 135 45 5.4.4 Seft-assembly of nanoparticles or nanowires Directed self-assembly of nanowire networks  Patterning substrate with adhesive monolayers  Flow of nanowires in microfluidic channels  Hierarchical assembly of nanowires  Crossed n and p type nanowires… 136 5.4.4 Seft-assembly of nanoparticles or nanowires Directed self-assembly of nanowire networks Nanowires aligned in flow direction and without adhesion promoter Nanowires aligned in flow direction with adhesion promoting patterns Multilayer deposition of nanowires in various orientations 137 5.4.4 Seft-assembly of nanoparticles or nanowires Electric-field assisted assembly AC field (0.5V/μm at MHz), The electrodes are used with a gap size of μm 138 46 Seft-assembly of nanoparticles or nanowires Integration with CMOS operating circuitry 139 Seft-assembly of nanoparticles or nanowires Template assisted assembly 140 END LECTURE 141 47