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Lecture 3,vlsi,design3_240418_074703.pdf

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The document provides an extensive overview of various deposition techniques used in CMOS process fabrication, including physical vapor deposition (PVD), sputtering, pulsed laser deposition (PLD), and chemical vapor deposition (CVD). It elaborates on the mechanisms, advantages, and disadvantages of each method, highlighting factors like substrate temperature and pressure, which influence film quality and properties. The document also discusses the importance of film composition, nucleation processes, and the growth mechanisms relevant to achieving desired thin film characteristics.

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‘- 1 Khulna University of Engineering & Technology Presented by Dr. Md Nur Kutubul Alam VLSI DESIGN AND TECHNOLOGY EE4121
‘- 2 DEPOSITION
‘- 3 Thin film deposition A large variety of films must be deposited during the CMOS process fabrication. This can be done by physical or chemical deposition methods. In some cases these deposition methods are assisted by a plasma. Four types of deposition technique
‘- 4 Physical vapor deposition Physical vapor deposition (PVD) methods rely on the physical transfer of metal atoms from a metal source to the wafer substrate, unlike chemical methods, which employ a chemical reaction. Typical PVD conditions Low pressure • Base pressure: 10-8 Torr • Deposition pressure: mTorr • Target diameter: >40cm • Water cooled target • Target-wafer distance: 5-30cm • Gas: Argon, nitrogen • Typical materials:Al, Cu, Ta, Ti, Co • Substrate: cooling, heating • RF-bias possible PVD: Deposition based on vaporizing solid materials by heating or sputtering, followed by condensing the vapor on the substrate surface.
‘- 5 A typical system comprises a process chamber, a vacuum system, and a metal heating system. Wafers are usually mounted upside down on a hemispherical chamber ceiling, which may include a planetary system to rotate the wafers for improved uniformity. The metal to be deposited is placed in metal “boat” or ceramic crucible. The metal is heated usually to 500–2500ºC (depending on the metal) to increase the vapor pressure. After a warm-up period, a physical shutter is used to precisely start and end the deposition onto the wafers. A quartz crystal microbalance (QCM) mounted inside the chamber monitors the deposition, and can provide feedback signals for automated control. Simplest possible technique you can do. Here Substrate remains at room temperature therefore it’s a low temperature technique. PVD: evaporation Evaporative deposition, or more commonly just “evaporation”, is a fairly straightforward method for metal deposition. The basic concept is to heat a metal sufficiently to create a vapor, which diffuses and re-condenses in solid form on other surfaces. This process is usually performed in high-vacuum conditions (below 10−5torr) so as to limit gaseous molecular scattering and to create a high- purity process environment. Note that, although the metal to be evaporated is obviously very hot, the wafer substrate usually remains at room temperature, unless intentionally heated or cooled. Also, because of the very low chamber pressures, the metal vapor tends to follow a straight path, leading to very directional deposition and poor sidewall coverage.
‘- 6 Mechanical shutter: – Evaporation rate is set by temperature of source, but this cannot be turned on and off rapidly. A mechanical shutter allows evaporant flux to be rapidly modulated. PVD: evaporation Material is heated to attain gaseous state – Resistive heating – Electron beam evaporation – Ion beam evaporation – Inductive heating evaporation Carried out under high-vacuum conditions (~5x10-7 torr) Advantages – Films can be deposited at high rates – Low energy atoms (~0.1 eV) leave little surface damage – Little residual gas and impurity incorporation due to high vacuum conditions – No substrate heating
‘- 7 PVD: evaporation Thermal Evaporation The simplest evaporation systems use joule heating to heat the metal charge. The dissipative heat can be created by direct conduction currents or magnetic- field-induced eddy currents. In the simpler conductively heated systems, high currents are passed through wound coils or a small metal boat (usually tungsten), inside of which sits the charge. The resistive heating of the boat facilitates deposition of relatively low-melting-point metals such as Ag, Al, and Au. Evaporation of higher-melting-point refractory metals such as Ta, W, Mo, and Ti is challenging because these require very high temperatures to achieve reasonable vapor pressures and deposition rates. Because of this, the use of metal boats and direct conductive heating may not be permissible. Instead inductive heating can be used where the metal sits in a ceramic crucible that is surrounded by a coil. RF excitation of the coil is used to induce eddy currents in the metal. This approach permits a wider range of metals, but the crucible itself may become very hot, which can result in contamination.
‘- 8 PVD: evaporation E-Beam Evaporation Another configuration for evaporation uses a directed electron beam to bombard the metal charge. The electron beam source is usually underneath the metal charge. Strong magnetic fields are used to steer the electron beam in a 270◦ circular arc to impinge on the charge. Although more complicated, the advantage of this approach is that the electron beam heats a central portion of the charge: the outer area of the charge and crucible remain at lower temperatures, so as to minimize contamination. Typical emission voltage: 8-10 kV. Exposes substrates to secondary electron radiation. – X-rays can also be generated by high voltage electron beam. 270° bent beam electron gun is most preferred: – Filament is out of direct exposure from evaporant flux. – Magnetic field can be used for beam focusing and beam positioning. – Additional lateral magnetic field can be used produce X-Y sweep Here we apply magnetic field to bend e-. So if we have any contaminating ion, that bends differently. Thereby it doesn't reach the crucible. Therefore it is much more pure
‘- 9 Condensation control of the evaporant is achieved through control of the substrate temperature Ts. • Higher substrate temperatures: – Increase thermal energy of adsorbed molecules. – Increase surface diffusivity of adsorbed molecules. – Performs annealing of deposited film. End point control: • Quartz crystal thickness monitors (QCMs) – Based on a quartz crystal oscillator where the quartz crystal is exposed to the evaporant flux. – An increase in mass on the surface of the crystal will shift the oscillation frequency in a predictable manner, from which the deposited thickness can be determined from the change in the oscillation frequency. • Optical interference, absorption, or reflectivity – A more sensitive technique better suited for optical coatings. Deposition control Quartz crystal Epi-layer
‘- 10 Like evaporation, sputtering is a physical process, but the transport of the source material to the substrate is produced by an energized plasma. A plasma is used to create a source of impinging ions which are accelerated into a target which is the source material. – Argon is the most common gas used for sputtering. The impinging ions knock loose atoms of the target by physical sputtering. The sputtered atoms travel back through the plasma and are deposited on the substrate. Advantages • Sputtering does not require the source or target material to be heated to its evaporation temperature. • Sputtering provides better utilization of the source or target material. • Sputtering does not cause much compositional change in the source material. – Alloys and mixed composition targets can be directly deposited. Disadvantages • An entire target must be obtained before any sputtering can be done. • Uniformity requires effort. Sputtering
‘- 11 PVD: sputtering Accelerated ions, usually Ar+, knock out atoms in a solid target by bombardment in a potential gradient environment. During the bombardment, momentum exchange occurs between the ions and the atoms of the surface of the target. The energized atoms are volatile and spread out as a vapor to land on the vicinity surface and the sample substrate. The sputtering process requires a vacuum environment, which is prepared by pumping out a stainless steel chamber enclosing the anode, the cathode, the target, the substrate, and so on. The chamber is evacuated to a base pressure of 10−6torr or lower. Then a bombardment gas, usually Ar, is introduced to the chamber and maintained around 1–10 mtorr level. The Ar gas is ionized into Ar+ by applying bias voltage between the anode and the cathode. To obtain uniform thickness of a thin-film metal layer, mechanical movement such as rotation of the substrate holder can be used during the sputtering process. The rotational speed of the stage ranges from 10 to 30 rpm. The deposition rate is a function of many parameters including target-to-substrate distance, ion energy, the mass of the ion species, the mass of the target material, and the like.
‘- 12 PVD: sputtering Depending on the voltage waveforms used to generate the plasma, the sputtering process is categorized as either direct current (DC) sputtering or radio frequency (RF) sputtering. Schematics of (a) DC and (b) RF sputtering systems Most often DC plasma is used. But if you need to deposit a non-conductive material you have to use RF sputtering. Otherwise your film will charge up that will change the local electric field. That would spoil or even stop your deposition.
‘- 13 Step coverage Sputtered metal deposition in a densely placed high-aspect-ratio structure. Sputtering can provide reasonably conformal coatings on uneven surfaces, but conformal deposition on high aspect ratio structures remains an issue. The step-coverage of a sputtered thin film in a via hole has been calculated, where the profile shows a high deposition rate on the top surface and a low deposition rate on the sidewall. As a result, the sidewall thickness tapers down toward the bottom. For a very high aspect ratio, the bottom portion may not have sufficient metal coverage due to limited mass transfer into the narrow entrance of the via hole and the higher pressure environment in the chamber. This kind of poor coverage is more significant in high-aspect-ratio vias or trenches as compared to high-aspect-ratio pillars or walls.
‘- 14 Pulsed laser deposition (PLD) Pulsed laser deposition (PLD) is another method for depositing materials, especially metals. The system uses a high-energy laser beam (typically 108 W/cm2) to strike a metal target within a vacuum chamber. The laser beam melts, evaporates, and ionizes a region of the target. This ablation process creates a vapor plume that transfers material to the sample wafer. One major advantage of PLD is the precise stoichiometry/composition control and relatively fast deposition rates. Ideally the deposited material possesses the same chemical composition as the metal target. High quality crystalline deposits are also possible with substrate heating. The biggest drawback is that most PLD systems can only provide uniform deposition over a small surface area, sometimes only about one square centimeter. This decreases the utility of PLD for volume manufacturing. Schematic of a pulsed laser deposition tool.
‘- 15 Electrochemical deposition Electrochemical deposition involves the reduction of metal ions from aqueous, organic, or fused-salt electrolytes. The reduction of metal ions Mz+ in aqueous solution is represented by: Mz+ (metal ion in solution) + z e− (electrons) → M (metal deposit) Two processes can be used to provide the electrons for the reduction reaction: (1) electroplating (or electrodeposition), where an external power supply provides the electrons, (2) electroless deposition, where a reducing agent provides the electrons. Schematic of a general electrochemical deposition cell (using soluble anode).
‘- 16 Chemical vapor deposition Chemical vapor deposition or CVD is a generic name for a group of processes that involve depositing a solid material from a gaseous phase. Continuous film Substrate 2) Dissociation of reactants by electric fields 3) Film precursors are formed 4) Adsorption of precursors 5) Precursor diffusion into substrate 6) Surface reactions by-products 8) By-product removal Exhaust 1) Reactants enter chamber Gas delivery By-products Electrode Electrode RF field Parameters: • Gas composition • Gas flow • Temperature • Pressure • Frequency • RF power This is a plasma enhanced CVD. Using plasma allows to reduce the substrate temperature. Because you can supply energy into the reactants from your plasma. But there is a risk, the plasma can damage the film you deposited. So you have to balance properly Plasma energy can be used to accelerate the deposition process by activating the precursors. This results in higher deposition rates at lower temperatures and better film quality. The charged byproducts deposit anisotropically. 7) Desorption of RF generator
‘- 17 Fundamental processes during CVD Fundamental processes during CVD • Reactant gases (precursors) are pumped in to a reaction chamber (reactor). • Under the right conditions (T, P), they undergo a reaction at the substrate. • One of the products of the reaction gets deposited on the substrate. • The by-products are pumped out. • The key parameters are chemical (reaction rates, gas transport, diffusion). Deposition takes place due to a chemical reaction between some reactants on the substrate.
‘- 18 Mean free path of gas molecules CVD processes can be performed at a range of pressures. In low pressure CVD the mean free path of the active species is longer than the typical topography dimensions, resulting in a more uniform coverage. Kamins 2000 Typical pressure of CVD processes : ] Mean Free Path is larger ] than typical topography dimension – 0.01 Torr (HDP) – 0.1-0.5 Torr (LPCVD) – 760 torr (APCVD) Most important thing for CVD is pressure. Cheapest way is to deposit at room temperature. But at low pressure you get larger mean free path. If you make the mean free path of the molecule larger than your structure, you can deposit very high quality, uniform film.
‘- 19 Chemical vapor deposition The choice of chemical reactions is determined by various parameters: • The precursors have to be volatile (gaseous). • The chemical reactions need to be thermodynamically predicted to result in a solid film. This means that there should be an energy advantage for the desired reaction to occur, meaning the Gibbs Free Energy (GFE) has to decrease. Temperature and pressure can be adjusted forΔG< 0. • The by-products need to be volatile (gaseous). Importance of the Gibbs Free Energy (GFE) in CVD processes: • GFE is a measure of the total available energy in a system. • If the overall GFE of the reactants is greater than the overall GFE of the products, that reaction is thermodynamically favorable. • The equations relating the GFEs also determine the reaction rates. • All of these quantities are affected by temperature and pressure.
‘- 20 CVD reactions types - I
‘- 21 CVD reactions types - II
‘- 22 CVD reactions types - III
‘- 23 Deposition process Precursor arrives surface Migrate on the surface React on the surface Nucleation: Island formation Islands grow Islands grow, cross-section Islands merge Continuous thin film Deposition occurs initially through nucleation on the surface, followed by island growth. These islands merge into continuous films. The mobility of the atoms on the surface is an important parameter in the CVD process and will have a large effect on the final film properties.
‘- 24 CVD nucleation First the barrier to nucleation (creating a nucleus increases surface energy) has to be overcome. Two types of nucleation exist: - Homogenous: Nuclei are formed in vapor form before being deposited and do not incorporate into the crystal structure of the film. - Heterogeneous: Nuclei are formed on the substrate and incorporate into the film structure more easily.
‘- 25 Surface interactions Distance from surface Bonding energy Surface adsorption Interaction between an adsorbing molecule (precursor) and a solid surface Determines precursor surface mobility ▪ Physical adsorption (physisorption) – Weak bond between surface and precursor – Bonding energy usually less than 0.5 eV • Hydrogen bonding, Van der Waals forces – Ion bombardment and thermal energy at 400C can cause migration of physisorbed precursors – High surface mobility ▪ Chemical adsorption (chemisorption) – Actual chemical bonds between surface atom and the adsorbed precursor molecule – Bonding energy usually exceeding 2 eV – Low surface mobility – Ion bombardment with 10 to 20 eV energy in PECVD processes can cause some surface migration of chemisorbed precursors ▪ Self-limiting chemisorption – Precursor does not deposit on other, already adsorbed, precursor molecules, only on the free surface Sticking coefficient: The probability that precursor atom forms chemical bond with surface atom in one collision Van der wall or H bonds. They are relatively mobile Physisorbed precursor Chemisorbed precursor Substrate surface Chemisorbed. Very sticky with the surface
‘- 26 Modes of thin film growth: 3 growth mechanisms Substrate with very low surface energy: Energetically it is more favorable for the reactants to bond with each other than to bond with the substrate. Then you get island growth.
‘- 27 Volmer - Weber (island growth) • Smallest stable clusters nucleate and grow in three dimensions • Atoms in the deposit are more strongly bound to each other than to the substrate • Low surface mobility • Typical for many metals on insulators Island growth Frank - van der Merwe (layer-by-layer growth) • Smallest stable clusters nucleate and grow in two dimensions. • Atoms of the deposit are more strongly bound to (steps of) the substrate than to each other. • Monotonic decrease of bonding energy down to the bulk crystal value • High surface mobility • Single-crystal epitaxial growth Layer growth Stranski - Krastanov • Layer growth merges into island growth after deposition of a few monolayers. • Transition caused by disturbances of the monotonic decrease of bonding energy (lattice misfit, relaxation of internal stress) • Metal - metal and metal - semiconductor systems Island growth Layer growth Basic modes of thin film growth
‘- 28 Film deposition In a simplified model, as gas flows over the substrate film growth is determined by adsorption and reaction rates. The deposition rate depends on the process conditions.
‘- 29 CVD deposition regimes Two limiting cases can be distinguished: • hG >> kS : reaction limited growth • Chemical reaction rate can’t match precursor diffusion and adsorption rates. The precursors pile up on the substrate surface and wait their turn to react. • Growth controlled by processes on surface: adsorption, decomposition, surface migration, chemical reaction, desorption of products • kS is highly temperature dependent (increases with T ) • This is the common limit at lower temperatures • Temperature and reactant choices are important •kS >> hG : transport limited growth • When surface chemical reaction rate is high enough, the chemical precursors react immediately when the adsorb on the substrate surface. • Growth controlled by transfer to substrate • hG is not very temperature dependent • This is common limit at higher temperatures • Gas dynamics and reactor design are important to obtain uniform film growth
‘- 30 CVD deposition regimes Single wafer RTCVD Batch LPCVD Most batch LPCVD systems operate in the surface reaction rate limited regime as temperature can be precisely controlled and stabilized in these systems. Single wafer RTCVD systems usually operate in mass transport limited regime as precise temperature control is more difficult to achieve in rapid thermal processing tools but, on the other hand, gas flows can be controlled very well.
‘- 31 Typical single wafer CVD module used in a cluster tool. Single wafer CVD in a cluster tool Single wafer PECVD
‘- 32 CVD deposition reactants
‘- 33 Silane Based LPCVD or APCVD (~ 400°C) or HDP SiH4 + 2 O2 → SiO2 + 2 H2O PECVD (~ 400°C) SiH4 + N2O DCS based → SiO2 + Products HTO (~ 800°C) SiH2Cl2 + N2O → SiO2 + Products TEOS based LPCVD TEOS (~ 650°C) TEOS (+ O2) → SiO2 + Products TEOS/Ozone (400-600°C) TEOS + O3 → SiO2 + Products PECVD (~ 400°C) TEOS + O2 → SiO2 + Products C2H5 O C2H5 O Si O C2H5 O C2H5 For safety issue, people changed into this one. Just oxidize this structure, you oxidize away your hydrocarbon, and you expose the SiO2 Tetra Ethyl Ortho Silicate Oxide deposition processes Silane is dangerous. It spontenously catches fire if it comes in contact with the O2 and ambient. So from gas leak or whatever, you can have fire in your clean room. DCS has Cl2 in it, which is very corrosive. Dangerous for health.
‘- 34 The step coverage is a measure of the deposited film reproducing the slope of a step on the substrate surface. It is one of the most important specifications in CVD processes has a very good step coverage: – Sidewall step coverage – Bottom step coverage – Conformality – Overhang Step coverage depends on surface mobility. Larger surface mobility gives better step coverage. Step coverage of CVD
‘- 35 Polycrystaline silicon Poly- Si ▪ Applications • Gate electrodes, local interconnect, capacitor plate, TFT, Microsystems, … ▪ Physical Structure: • Crystalline grains, separated by grain boundaries • Under certain conditions, Si is deposited in amorphous state (not crystallized) Deposition ▪ High temperature (~ 600-700˚C) furnace LPCVD processes. ▪ Silane (SiH4) or DCS (SiH2Cl2) as silicon source gases. • Nitrogen as purge gas • Arsine (AsH3), Phosphine (PH3), and Diborane (B2H6) used as dopant gases. ➔ Silane process SiH4 Silane Heat → Si + Poly-Si H2 Hydrogen ➔ DCS process Heat SiH2Cl2 → Si Dichlorosilane Poly-Si + 2HCl Hydrochloride This is the same chemistry. Just you dont start from a epitaxial surface, you get a poly Si. Also depending on temperature, you can get polycrystalline Si
‘- 36 T<550˚C Amorphous Si T > 900˚C Single Crystal Si 550˚C <T< 900˚C Polysilicon Grain Boundary Grain Crystal structure of Silicon deposited with silane as source gas on single crystal silicon substrate depends on temperature: • T > 900 ˚C deposit single crystal silicon • 900 ˚C > T > 550 ˚C deposit polysilicon • T < 550 ˚C deposit amorphous silicon Microstructure of Si CVD films
‘- 37 In MBE, the constituent elements of a semiconductor in the form of ‘molecular beams’ are deposited onto a heated crystalline substrate to form thin epitaxial layers. An important feature is that growth rates are typically on the order of a few Å/s and the beams can be shuttered in a fraction of a second, allowing for nearly atomically abrupt transitions from one material to another. The ‘molecular beams’ are typically from thermally evaporated elemental sources, but other sources include metal-organic group III precursors (MOMBE), gaseous group V hydride or organic precursors (gas-source MBE), or some combination (chemical beam epitaxy or CBE). T o obtain high-purity layers, it is critical that the material sources be extremely pure and that the entire process be done in an ultra-high vacuum environment. Molecular Beam Epitaxy (MBE)
‘- 38 Effusion: the process where individual molecules flow through a hole without collisions. - The source material is heated to vapor phase. - Ultra-low pressure in UHV leads to molecules with mean free paths of hundreds of meters. - Opening in effusion cell is small – molecules travel straight out of it with no collisions, forming a beam. A typical MBE system may feature 8 effusion cells. The Crucible is constructed of pyrolytic boron nitride (PBN) to withstand temperatures up to 1400°C Effusion sources
‘- 39 MBE characteristics • “Low” growth rates: VG can vary from 0.01ML/s to 1.5 ML/s (max) • Smooth growth surface with steps of atomic height and large flat terraces • Precise control of surface composition and morphology • Abrupt variation of chemical composition at interfaces • in-situ control of crystal growth at the atomic level • Wide range of materials can be deposited: Semiconductors IV-IV: microelectronic (MOSFET, HBT, …): Si, Ge, & C III-V: optoelectronic (lasers, LED, HEMT, HBT, …) : Arsenides (GaAs, AlAs, InAs), Antimonides (GaSb, InSb), Phosphides (InP, GaP), Nitrides (GaN, InN, …) II-VI: spintronic, optoelectronic (lasers, …), ZnO, ZnSe, CdS, HgTe Functional oxides SrTiO3, BaTiO3, … Al2O3, LaAlO3, Gd2O3, …
‘- 40 III-V growth techniques: MBE The main benefit of using the MBE growth technique resides in the possibility to use a wide range of in-situ diagnostic tools such as ion gauge, quartz crystal microbalance, mass spectrometer and characterization techniques like pyrometry, ellipsometry spectroscopy or normal incidence reflectometry (thickness measurement) and Reflection High Energy Electrons Diffraction (RHEED). Electrons gun 30kV incident beam Sample CCD camera Phosphorescent screen Incidence angle θ Diffracted beams a 1/a Specular spot Volumes' diffraction lines RHEED setup The possibility to use RHEED is by far the key advantage of the MBE growth approach as we can study in real time the crystalline properties of the layer (amorphous, polycrystalline, monocrystalline, textured), the surface morphology and roughness (2D, 3D) the growth kinetics (growth rate), the native oxide removal from substrate surface (surface deoxidation) and investigate the atomic arrangement of the surface (surface reconstruction). After C.Merckling, imec
(0,0) (0,1) Si(100) - 1x1 [110] [110] [110] [001] Si(100) - 2x1 2x1 surface periodicity : Si-Si dimer rows (0,-1) (0,0) (0,1) (0,-½) (0,½) 1x1 unreconstructed Si surface (0,-1) RHEED: surface reconstructions
‘- 42 III-V growth techniques: MBE and MOVPE MBE crystal growth technique is particularly appreciated by scientists in the area of surface, interface and growth science. However, for production purposes, due to the need for ultra- high vacuum, the complex maintenance of the phosphide reactors and the low growth rates, MBE technology is only preferred for low volume production of very specific heterostructures where a perfect control of the interfaces is necessary for the final devices like for example Heterojunction Bipolar Transistors (HBT), High Electron Mobility Transistors (HEMT) or Quantum Cascade Lasers (QCL). Metal Organic Vapor Phase Epitaxy (MOVPE) is far more suited for large volume production. MOVPE is a chemical vapor phase deposition technique based on chemical reactions of mixed gases onto a heated substrate. For MOVPE, the element sources are not metals but precursors with high vapor pressure around room temperature. These precursors can be either hydrides or metal organics in which the metal is bonded to organic radicals. After C.Merckling, imec
‘- 43 III-V growth techniques: MOVPE An overview of the most commonly used MOVPE precursors for group-III, group-V and dopant elements for III-V epitaxy is given in the table below. After C.Merckling, imec List of the different hydrides and metal organic precursors used for III-V epitaxy by MOVPE. If the hydride sources are packaged in pressurized bottles, the metal organic sources, that can be either liquid or solid, are contained in a bubbler in which the ultra-pure carrier gases, N2 or H2, can be introduced to carry the precursor molecules to the reactor.
‘- 44 III-V growth techniques: MOVPE The bubbler is put in a thermal bath to accurately control the precursor temperature and together with a set of mass flow controllers at the inlet of the bubbler and a pressure controller at the outlet, an excellent control of the precursor concentration held in the carrier gas is obtained. Schematic diagram of a MOVPE system including the gas mixing system of the different elements, the reactor area and the exhaust region The carrier gas containing the different precursors is injected into the reactor at reduced pressure, between 50 mbar and 500 mbar, on a rotating and heated substrate where the epitaxy process will take place. Due to the high pressure during the growth process, much higher growth rates can be achieved while maintaining a high epitaxial quality. However, for more complex heterostructures, lower growth rates, comparable to MBE technology, need to be used to obtain abrupt interfaces leading to an equivalent layer quality for both growth techniques.
‘- 45 Atomic Layer Deposition (ALD) is a deposition method that uses a cyclic process of self-limiting chemisorption reactions of gas phase precursors to deposit a thin film on a substrate. It is ideally suited to deposit thin films on structured surfaces and has become the major method to deposit high- dielectrics. Atomic Layer Deposition (ALD) Phase 1 : Precursor pulse Phase 2 : Precursor purge Phase 3 : Water pulse Phase 4 : Water purge (…) Major principles of ALD: • Precursor gases or vapors are alternately pulsed on to the substrate surface. • Precursor gases introduced on to the substrate surface will chemisorb or surface reaction takes place at the surface. Phase 2: Precursor Purge Phase 3: Water pulse Phase 4: Water pulse In MOCVD two precursors are always present in the reaction chamber. But in ALD only one is present at a time. ALD is a surface driven technique.
‘- 46 Self-limiting chemisorption Association Dissociation Chemisorption mechanisms Surface reactions in ALD are all self-limiting. This self-limiting characteristics of the process steps is the foundation of ALD. Most ALD processes are based on a chemisorption saturation process followed by an exchange reaction. → → → Ligand exchange • limited number of reactive surface sites You start with an -OH group. Then supply HfCl4 or ZrCl4. The Chlorides are relatively big atoms. Once one of the Chlorine is replaced and Hf/Zr bonds with the -OH group, the layer of -OH is completely covered. Therefore no further reaction is possible.
‘- 47 ALD of HfO2 using HfCl4 and H2O HfCl4(g) + 2 H2O(g) → HfO2 + 4 HCl(g) HfCl4 H2O -OH Hf O Cl H -Hf-Cl + H2O(g) → -Hf-OH + HCl(g) x -OH + HfCl4(g) → -O-HfCl(4-x) + x HCl(g) purge purge 1 2 3 HfO2 can be deposited by ALD using subsequent pulses of HfCl4 and H2O. Phase 1 & 2: Movement of precursor in the reaction chamber, self limiting reaction Phase 3 & 4: Water pulse completes the reaction of the first layer. Also it creates more –OH group at the surface that allows reaction of the next layer.
‘- 48 Atomic Layer Deposition Desorption Decomposition Condensation Incomplete reaction < 1 monolayer per cycle more less PROCESS WINDOW Temperature ALD precursor requirements: Sufficient volatility (high VP) Fast and complete chemisorption reaction Avoid gas phase reactions: - stable against decomposition at ALD temperature - no reaction with reaction products No etching or dissolution in the film or substrate For ALD the temperature window must be in-between condensation/incomplete reaction and decomposition/desorption of the precursors.
ALD films and applications Currently a whole series of ALD processes are available for various applications, including metal deposition.
‘- 50 Illustration of an ALD system based on laminar flow. ALD tools ALD tools have to be optimized to handle ‘pulses’ of gases. This can for example be achieved with a simple laminar flow reactor design. Other designs are based on the typical ‘showerhead’design as used in regular CVD tools.
‘- 51 Atomic dimensions are now routine Some high points of Intel's 45nm HKMG technology are: high- first, metal-gate-last integration; optimized interfacial SiO2 layer (not shown on the picture); hafnium oxide (HfO2) gate dielectric (1nm EOT); and dual band-edge work function metal gates (TiN for PMOS; TiAlN for NMOS). K. Kuhn – INTEL - 2009 2nd International CMOS Variability Conference - London Still, the advantages outweigh the disadvantages and high-/metal gates went into full production in the 45nm technology node, first introduced by Intel.
Lecture 3,vlsi,design3_240418_074703.pdf

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Lecture 3,vlsi,design3_240418_074703.pdf

  • 1. ‘- 1 Khulna University of Engineering & Technology Presented by Dr. Md Nur Kutubul Alam VLSI DESIGN AND TECHNOLOGY EE4121
  • 3. ‘- 3 Thin film deposition A large variety of films must be deposited during the CMOS process fabrication. This can be done by physical or chemical deposition methods. In some cases these deposition methods are assisted by a plasma. Four types of deposition technique
  • 4. ‘- 4 Physical vapor deposition Physical vapor deposition (PVD) methods rely on the physical transfer of metal atoms from a metal source to the wafer substrate, unlike chemical methods, which employ a chemical reaction. Typical PVD conditions Low pressure • Base pressure: 10-8 Torr • Deposition pressure: mTorr • Target diameter: >40cm • Water cooled target • Target-wafer distance: 5-30cm • Gas: Argon, nitrogen • Typical materials:Al, Cu, Ta, Ti, Co • Substrate: cooling, heating • RF-bias possible PVD: Deposition based on vaporizing solid materials by heating or sputtering, followed by condensing the vapor on the substrate surface.
  • 5. ‘- 5 A typical system comprises a process chamber, a vacuum system, and a metal heating system. Wafers are usually mounted upside down on a hemispherical chamber ceiling, which may include a planetary system to rotate the wafers for improved uniformity. The metal to be deposited is placed in metal “boat” or ceramic crucible. The metal is heated usually to 500–2500ºC (depending on the metal) to increase the vapor pressure. After a warm-up period, a physical shutter is used to precisely start and end the deposition onto the wafers. A quartz crystal microbalance (QCM) mounted inside the chamber monitors the deposition, and can provide feedback signals for automated control. Simplest possible technique you can do. Here Substrate remains at room temperature therefore it’s a low temperature technique. PVD: evaporation Evaporative deposition, or more commonly just “evaporation”, is a fairly straightforward method for metal deposition. The basic concept is to heat a metal sufficiently to create a vapor, which diffuses and re-condenses in solid form on other surfaces. This process is usually performed in high-vacuum conditions (below 10−5torr) so as to limit gaseous molecular scattering and to create a high- purity process environment. Note that, although the metal to be evaporated is obviously very hot, the wafer substrate usually remains at room temperature, unless intentionally heated or cooled. Also, because of the very low chamber pressures, the metal vapor tends to follow a straight path, leading to very directional deposition and poor sidewall coverage.
  • 6. ‘- 6 Mechanical shutter: – Evaporation rate is set by temperature of source, but this cannot be turned on and off rapidly. A mechanical shutter allows evaporant flux to be rapidly modulated. PVD: evaporation Material is heated to attain gaseous state – Resistive heating – Electron beam evaporation – Ion beam evaporation – Inductive heating evaporation Carried out under high-vacuum conditions (~5x10-7 torr) Advantages – Films can be deposited at high rates – Low energy atoms (~0.1 eV) leave little surface damage – Little residual gas and impurity incorporation due to high vacuum conditions – No substrate heating
  • 7. ‘- 7 PVD: evaporation Thermal Evaporation The simplest evaporation systems use joule heating to heat the metal charge. The dissipative heat can be created by direct conduction currents or magnetic- field-induced eddy currents. In the simpler conductively heated systems, high currents are passed through wound coils or a small metal boat (usually tungsten), inside of which sits the charge. The resistive heating of the boat facilitates deposition of relatively low-melting-point metals such as Ag, Al, and Au. Evaporation of higher-melting-point refractory metals such as Ta, W, Mo, and Ti is challenging because these require very high temperatures to achieve reasonable vapor pressures and deposition rates. Because of this, the use of metal boats and direct conductive heating may not be permissible. Instead inductive heating can be used where the metal sits in a ceramic crucible that is surrounded by a coil. RF excitation of the coil is used to induce eddy currents in the metal. This approach permits a wider range of metals, but the crucible itself may become very hot, which can result in contamination.
  • 8. ‘- 8 PVD: evaporation E-Beam Evaporation Another configuration for evaporation uses a directed electron beam to bombard the metal charge. The electron beam source is usually underneath the metal charge. Strong magnetic fields are used to steer the electron beam in a 270◦ circular arc to impinge on the charge. Although more complicated, the advantage of this approach is that the electron beam heats a central portion of the charge: the outer area of the charge and crucible remain at lower temperatures, so as to minimize contamination. Typical emission voltage: 8-10 kV. Exposes substrates to secondary electron radiation. – X-rays can also be generated by high voltage electron beam. 270° bent beam electron gun is most preferred: – Filament is out of direct exposure from evaporant flux. – Magnetic field can be used for beam focusing and beam positioning. – Additional lateral magnetic field can be used produce X-Y sweep Here we apply magnetic field to bend e-. So if we have any contaminating ion, that bends differently. Thereby it doesn't reach the crucible. Therefore it is much more pure
  • 9. ‘- 9 Condensation control of the evaporant is achieved through control of the substrate temperature Ts. • Higher substrate temperatures: – Increase thermal energy of adsorbed molecules. – Increase surface diffusivity of adsorbed molecules. – Performs annealing of deposited film. End point control: • Quartz crystal thickness monitors (QCMs) – Based on a quartz crystal oscillator where the quartz crystal is exposed to the evaporant flux. – An increase in mass on the surface of the crystal will shift the oscillation frequency in a predictable manner, from which the deposited thickness can be determined from the change in the oscillation frequency. • Optical interference, absorption, or reflectivity – A more sensitive technique better suited for optical coatings. Deposition control Quartz crystal Epi-layer
  • 10. ‘- 10 Like evaporation, sputtering is a physical process, but the transport of the source material to the substrate is produced by an energized plasma. A plasma is used to create a source of impinging ions which are accelerated into a target which is the source material. – Argon is the most common gas used for sputtering. The impinging ions knock loose atoms of the target by physical sputtering. The sputtered atoms travel back through the plasma and are deposited on the substrate. Advantages • Sputtering does not require the source or target material to be heated to its evaporation temperature. • Sputtering provides better utilization of the source or target material. • Sputtering does not cause much compositional change in the source material. – Alloys and mixed composition targets can be directly deposited. Disadvantages • An entire target must be obtained before any sputtering can be done. • Uniformity requires effort. Sputtering
  • 11. ‘- 11 PVD: sputtering Accelerated ions, usually Ar+, knock out atoms in a solid target by bombardment in a potential gradient environment. During the bombardment, momentum exchange occurs between the ions and the atoms of the surface of the target. The energized atoms are volatile and spread out as a vapor to land on the vicinity surface and the sample substrate. The sputtering process requires a vacuum environment, which is prepared by pumping out a stainless steel chamber enclosing the anode, the cathode, the target, the substrate, and so on. The chamber is evacuated to a base pressure of 10−6torr or lower. Then a bombardment gas, usually Ar, is introduced to the chamber and maintained around 1–10 mtorr level. The Ar gas is ionized into Ar+ by applying bias voltage between the anode and the cathode. To obtain uniform thickness of a thin-film metal layer, mechanical movement such as rotation of the substrate holder can be used during the sputtering process. The rotational speed of the stage ranges from 10 to 30 rpm. The deposition rate is a function of many parameters including target-to-substrate distance, ion energy, the mass of the ion species, the mass of the target material, and the like.
  • 12. ‘- 12 PVD: sputtering Depending on the voltage waveforms used to generate the plasma, the sputtering process is categorized as either direct current (DC) sputtering or radio frequency (RF) sputtering. Schematics of (a) DC and (b) RF sputtering systems Most often DC plasma is used. But if you need to deposit a non-conductive material you have to use RF sputtering. Otherwise your film will charge up that will change the local electric field. That would spoil or even stop your deposition.
  • 13. ‘- 13 Step coverage Sputtered metal deposition in a densely placed high-aspect-ratio structure. Sputtering can provide reasonably conformal coatings on uneven surfaces, but conformal deposition on high aspect ratio structures remains an issue. The step-coverage of a sputtered thin film in a via hole has been calculated, where the profile shows a high deposition rate on the top surface and a low deposition rate on the sidewall. As a result, the sidewall thickness tapers down toward the bottom. For a very high aspect ratio, the bottom portion may not have sufficient metal coverage due to limited mass transfer into the narrow entrance of the via hole and the higher pressure environment in the chamber. This kind of poor coverage is more significant in high-aspect-ratio vias or trenches as compared to high-aspect-ratio pillars or walls.
  • 14. ‘- 14 Pulsed laser deposition (PLD) Pulsed laser deposition (PLD) is another method for depositing materials, especially metals. The system uses a high-energy laser beam (typically 108 W/cm2) to strike a metal target within a vacuum chamber. The laser beam melts, evaporates, and ionizes a region of the target. This ablation process creates a vapor plume that transfers material to the sample wafer. One major advantage of PLD is the precise stoichiometry/composition control and relatively fast deposition rates. Ideally the deposited material possesses the same chemical composition as the metal target. High quality crystalline deposits are also possible with substrate heating. The biggest drawback is that most PLD systems can only provide uniform deposition over a small surface area, sometimes only about one square centimeter. This decreases the utility of PLD for volume manufacturing. Schematic of a pulsed laser deposition tool.
  • 15. ‘- 15 Electrochemical deposition Electrochemical deposition involves the reduction of metal ions from aqueous, organic, or fused-salt electrolytes. The reduction of metal ions Mz+ in aqueous solution is represented by: Mz+ (metal ion in solution) + z e− (electrons) → M (metal deposit) Two processes can be used to provide the electrons for the reduction reaction: (1) electroplating (or electrodeposition), where an external power supply provides the electrons, (2) electroless deposition, where a reducing agent provides the electrons. Schematic of a general electrochemical deposition cell (using soluble anode).
  • 16. ‘- 16 Chemical vapor deposition Chemical vapor deposition or CVD is a generic name for a group of processes that involve depositing a solid material from a gaseous phase. Continuous film Substrate 2) Dissociation of reactants by electric fields 3) Film precursors are formed 4) Adsorption of precursors 5) Precursor diffusion into substrate 6) Surface reactions by-products 8) By-product removal Exhaust 1) Reactants enter chamber Gas delivery By-products Electrode Electrode RF field Parameters: • Gas composition • Gas flow • Temperature • Pressure • Frequency • RF power This is a plasma enhanced CVD. Using plasma allows to reduce the substrate temperature. Because you can supply energy into the reactants from your plasma. But there is a risk, the plasma can damage the film you deposited. So you have to balance properly Plasma energy can be used to accelerate the deposition process by activating the precursors. This results in higher deposition rates at lower temperatures and better film quality. The charged byproducts deposit anisotropically. 7) Desorption of RF generator
  • 17. ‘- 17 Fundamental processes during CVD Fundamental processes during CVD • Reactant gases (precursors) are pumped in to a reaction chamber (reactor). • Under the right conditions (T, P), they undergo a reaction at the substrate. • One of the products of the reaction gets deposited on the substrate. • The by-products are pumped out. • The key parameters are chemical (reaction rates, gas transport, diffusion). Deposition takes place due to a chemical reaction between some reactants on the substrate.
  • 18. ‘- 18 Mean free path of gas molecules CVD processes can be performed at a range of pressures. In low pressure CVD the mean free path of the active species is longer than the typical topography dimensions, resulting in a more uniform coverage. Kamins 2000 Typical pressure of CVD processes : ] Mean Free Path is larger ] than typical topography dimension – 0.01 Torr (HDP) – 0.1-0.5 Torr (LPCVD) – 760 torr (APCVD) Most important thing for CVD is pressure. Cheapest way is to deposit at room temperature. But at low pressure you get larger mean free path. If you make the mean free path of the molecule larger than your structure, you can deposit very high quality, uniform film.
  • 19. ‘- 19 Chemical vapor deposition The choice of chemical reactions is determined by various parameters: • The precursors have to be volatile (gaseous). • The chemical reactions need to be thermodynamically predicted to result in a solid film. This means that there should be an energy advantage for the desired reaction to occur, meaning the Gibbs Free Energy (GFE) has to decrease. Temperature and pressure can be adjusted forΔG< 0. • The by-products need to be volatile (gaseous). Importance of the Gibbs Free Energy (GFE) in CVD processes: • GFE is a measure of the total available energy in a system. • If the overall GFE of the reactants is greater than the overall GFE of the products, that reaction is thermodynamically favorable. • The equations relating the GFEs also determine the reaction rates. • All of these quantities are affected by temperature and pressure.
  • 23. ‘- 23 Deposition process Precursor arrives surface Migrate on the surface React on the surface Nucleation: Island formation Islands grow Islands grow, cross-section Islands merge Continuous thin film Deposition occurs initially through nucleation on the surface, followed by island growth. These islands merge into continuous films. The mobility of the atoms on the surface is an important parameter in the CVD process and will have a large effect on the final film properties.
  • 24. ‘- 24 CVD nucleation First the barrier to nucleation (creating a nucleus increases surface energy) has to be overcome. Two types of nucleation exist: - Homogenous: Nuclei are formed in vapor form before being deposited and do not incorporate into the crystal structure of the film. - Heterogeneous: Nuclei are formed on the substrate and incorporate into the film structure more easily.
  • 25. ‘- 25 Surface interactions Distance from surface Bonding energy Surface adsorption Interaction between an adsorbing molecule (precursor) and a solid surface Determines precursor surface mobility ▪ Physical adsorption (physisorption) – Weak bond between surface and precursor – Bonding energy usually less than 0.5 eV • Hydrogen bonding, Van der Waals forces – Ion bombardment and thermal energy at 400C can cause migration of physisorbed precursors – High surface mobility ▪ Chemical adsorption (chemisorption) – Actual chemical bonds between surface atom and the adsorbed precursor molecule – Bonding energy usually exceeding 2 eV – Low surface mobility – Ion bombardment with 10 to 20 eV energy in PECVD processes can cause some surface migration of chemisorbed precursors ▪ Self-limiting chemisorption – Precursor does not deposit on other, already adsorbed, precursor molecules, only on the free surface Sticking coefficient: The probability that precursor atom forms chemical bond with surface atom in one collision Van der wall or H bonds. They are relatively mobile Physisorbed precursor Chemisorbed precursor Substrate surface Chemisorbed. Very sticky with the surface
  • 26. ‘- 26 Modes of thin film growth: 3 growth mechanisms Substrate with very low surface energy: Energetically it is more favorable for the reactants to bond with each other than to bond with the substrate. Then you get island growth.
  • 27. ‘- 27 Volmer - Weber (island growth) • Smallest stable clusters nucleate and grow in three dimensions • Atoms in the deposit are more strongly bound to each other than to the substrate • Low surface mobility • Typical for many metals on insulators Island growth Frank - van der Merwe (layer-by-layer growth) • Smallest stable clusters nucleate and grow in two dimensions. • Atoms of the deposit are more strongly bound to (steps of) the substrate than to each other. • Monotonic decrease of bonding energy down to the bulk crystal value • High surface mobility • Single-crystal epitaxial growth Layer growth Stranski - Krastanov • Layer growth merges into island growth after deposition of a few monolayers. • Transition caused by disturbances of the monotonic decrease of bonding energy (lattice misfit, relaxation of internal stress) • Metal - metal and metal - semiconductor systems Island growth Layer growth Basic modes of thin film growth
  • 28. ‘- 28 Film deposition In a simplified model, as gas flows over the substrate film growth is determined by adsorption and reaction rates. The deposition rate depends on the process conditions.
  • 29. ‘- 29 CVD deposition regimes Two limiting cases can be distinguished: • hG >> kS : reaction limited growth • Chemical reaction rate can’t match precursor diffusion and adsorption rates. The precursors pile up on the substrate surface and wait their turn to react. • Growth controlled by processes on surface: adsorption, decomposition, surface migration, chemical reaction, desorption of products • kS is highly temperature dependent (increases with T ) • This is the common limit at lower temperatures • Temperature and reactant choices are important •kS >> hG : transport limited growth • When surface chemical reaction rate is high enough, the chemical precursors react immediately when the adsorb on the substrate surface. • Growth controlled by transfer to substrate • hG is not very temperature dependent • This is common limit at higher temperatures • Gas dynamics and reactor design are important to obtain uniform film growth
  • 30. ‘- 30 CVD deposition regimes Single wafer RTCVD Batch LPCVD Most batch LPCVD systems operate in the surface reaction rate limited regime as temperature can be precisely controlled and stabilized in these systems. Single wafer RTCVD systems usually operate in mass transport limited regime as precise temperature control is more difficult to achieve in rapid thermal processing tools but, on the other hand, gas flows can be controlled very well.
  • 31. ‘- 31 Typical single wafer CVD module used in a cluster tool. Single wafer CVD in a cluster tool Single wafer PECVD
  • 33. ‘- 33 Silane Based LPCVD or APCVD (~ 400°C) or HDP SiH4 + 2 O2 → SiO2 + 2 H2O PECVD (~ 400°C) SiH4 + N2O DCS based → SiO2 + Products HTO (~ 800°C) SiH2Cl2 + N2O → SiO2 + Products TEOS based LPCVD TEOS (~ 650°C) TEOS (+ O2) → SiO2 + Products TEOS/Ozone (400-600°C) TEOS + O3 → SiO2 + Products PECVD (~ 400°C) TEOS + O2 → SiO2 + Products C2H5 O C2H5 O Si O C2H5 O C2H5 For safety issue, people changed into this one. Just oxidize this structure, you oxidize away your hydrocarbon, and you expose the SiO2 Tetra Ethyl Ortho Silicate Oxide deposition processes Silane is dangerous. It spontenously catches fire if it comes in contact with the O2 and ambient. So from gas leak or whatever, you can have fire in your clean room. DCS has Cl2 in it, which is very corrosive. Dangerous for health.
  • 34. ‘- 34 The step coverage is a measure of the deposited film reproducing the slope of a step on the substrate surface. It is one of the most important specifications in CVD processes has a very good step coverage: – Sidewall step coverage – Bottom step coverage – Conformality – Overhang Step coverage depends on surface mobility. Larger surface mobility gives better step coverage. Step coverage of CVD
  • 35. ‘- 35 Polycrystaline silicon Poly- Si ▪ Applications • Gate electrodes, local interconnect, capacitor plate, TFT, Microsystems, … ▪ Physical Structure: • Crystalline grains, separated by grain boundaries • Under certain conditions, Si is deposited in amorphous state (not crystallized) Deposition ▪ High temperature (~ 600-700˚C) furnace LPCVD processes. ▪ Silane (SiH4) or DCS (SiH2Cl2) as silicon source gases. • Nitrogen as purge gas • Arsine (AsH3), Phosphine (PH3), and Diborane (B2H6) used as dopant gases. ➔ Silane process SiH4 Silane Heat → Si + Poly-Si H2 Hydrogen ➔ DCS process Heat SiH2Cl2 → Si Dichlorosilane Poly-Si + 2HCl Hydrochloride This is the same chemistry. Just you dont start from a epitaxial surface, you get a poly Si. Also depending on temperature, you can get polycrystalline Si
  • 36. ‘- 36 T<550˚C Amorphous Si T > 900˚C Single Crystal Si 550˚C <T< 900˚C Polysilicon Grain Boundary Grain Crystal structure of Silicon deposited with silane as source gas on single crystal silicon substrate depends on temperature: • T > 900 ˚C deposit single crystal silicon • 900 ˚C > T > 550 ˚C deposit polysilicon • T < 550 ˚C deposit amorphous silicon Microstructure of Si CVD films
  • 37. ‘- 37 In MBE, the constituent elements of a semiconductor in the form of ‘molecular beams’ are deposited onto a heated crystalline substrate to form thin epitaxial layers. An important feature is that growth rates are typically on the order of a few Å/s and the beams can be shuttered in a fraction of a second, allowing for nearly atomically abrupt transitions from one material to another. The ‘molecular beams’ are typically from thermally evaporated elemental sources, but other sources include metal-organic group III precursors (MOMBE), gaseous group V hydride or organic precursors (gas-source MBE), or some combination (chemical beam epitaxy or CBE). T o obtain high-purity layers, it is critical that the material sources be extremely pure and that the entire process be done in an ultra-high vacuum environment. Molecular Beam Epitaxy (MBE)
  • 38. ‘- 38 Effusion: the process where individual molecules flow through a hole without collisions. - The source material is heated to vapor phase. - Ultra-low pressure in UHV leads to molecules with mean free paths of hundreds of meters. - Opening in effusion cell is small – molecules travel straight out of it with no collisions, forming a beam. A typical MBE system may feature 8 effusion cells. The Crucible is constructed of pyrolytic boron nitride (PBN) to withstand temperatures up to 1400°C Effusion sources
  • 39. ‘- 39 MBE characteristics • “Low” growth rates: VG can vary from 0.01ML/s to 1.5 ML/s (max) • Smooth growth surface with steps of atomic height and large flat terraces • Precise control of surface composition and morphology • Abrupt variation of chemical composition at interfaces • in-situ control of crystal growth at the atomic level • Wide range of materials can be deposited: Semiconductors IV-IV: microelectronic (MOSFET, HBT, …): Si, Ge, & C III-V: optoelectronic (lasers, LED, HEMT, HBT, …) : Arsenides (GaAs, AlAs, InAs), Antimonides (GaSb, InSb), Phosphides (InP, GaP), Nitrides (GaN, InN, …) II-VI: spintronic, optoelectronic (lasers, …), ZnO, ZnSe, CdS, HgTe Functional oxides SrTiO3, BaTiO3, … Al2O3, LaAlO3, Gd2O3, …
  • 40. ‘- 40 III-V growth techniques: MBE The main benefit of using the MBE growth technique resides in the possibility to use a wide range of in-situ diagnostic tools such as ion gauge, quartz crystal microbalance, mass spectrometer and characterization techniques like pyrometry, ellipsometry spectroscopy or normal incidence reflectometry (thickness measurement) and Reflection High Energy Electrons Diffraction (RHEED). Electrons gun 30kV incident beam Sample CCD camera Phosphorescent screen Incidence angle θ Diffracted beams a 1/a Specular spot Volumes' diffraction lines RHEED setup The possibility to use RHEED is by far the key advantage of the MBE growth approach as we can study in real time the crystalline properties of the layer (amorphous, polycrystalline, monocrystalline, textured), the surface morphology and roughness (2D, 3D) the growth kinetics (growth rate), the native oxide removal from substrate surface (surface deoxidation) and investigate the atomic arrangement of the surface (surface reconstruction). After C.Merckling, imec
  • 41. (0,0) (0,1) Si(100) - 1x1 [110] [110] [110] [001] Si(100) - 2x1 2x1 surface periodicity : Si-Si dimer rows (0,-1) (0,0) (0,1) (0,-½) (0,½) 1x1 unreconstructed Si surface (0,-1) RHEED: surface reconstructions
  • 42. ‘- 42 III-V growth techniques: MBE and MOVPE MBE crystal growth technique is particularly appreciated by scientists in the area of surface, interface and growth science. However, for production purposes, due to the need for ultra- high vacuum, the complex maintenance of the phosphide reactors and the low growth rates, MBE technology is only preferred for low volume production of very specific heterostructures where a perfect control of the interfaces is necessary for the final devices like for example Heterojunction Bipolar Transistors (HBT), High Electron Mobility Transistors (HEMT) or Quantum Cascade Lasers (QCL). Metal Organic Vapor Phase Epitaxy (MOVPE) is far more suited for large volume production. MOVPE is a chemical vapor phase deposition technique based on chemical reactions of mixed gases onto a heated substrate. For MOVPE, the element sources are not metals but precursors with high vapor pressure around room temperature. These precursors can be either hydrides or metal organics in which the metal is bonded to organic radicals. After C.Merckling, imec
  • 43. ‘- 43 III-V growth techniques: MOVPE An overview of the most commonly used MOVPE precursors for group-III, group-V and dopant elements for III-V epitaxy is given in the table below. After C.Merckling, imec List of the different hydrides and metal organic precursors used for III-V epitaxy by MOVPE. If the hydride sources are packaged in pressurized bottles, the metal organic sources, that can be either liquid or solid, are contained in a bubbler in which the ultra-pure carrier gases, N2 or H2, can be introduced to carry the precursor molecules to the reactor.
  • 44. ‘- 44 III-V growth techniques: MOVPE The bubbler is put in a thermal bath to accurately control the precursor temperature and together with a set of mass flow controllers at the inlet of the bubbler and a pressure controller at the outlet, an excellent control of the precursor concentration held in the carrier gas is obtained. Schematic diagram of a MOVPE system including the gas mixing system of the different elements, the reactor area and the exhaust region The carrier gas containing the different precursors is injected into the reactor at reduced pressure, between 50 mbar and 500 mbar, on a rotating and heated substrate where the epitaxy process will take place. Due to the high pressure during the growth process, much higher growth rates can be achieved while maintaining a high epitaxial quality. However, for more complex heterostructures, lower growth rates, comparable to MBE technology, need to be used to obtain abrupt interfaces leading to an equivalent layer quality for both growth techniques.
  • 45. ‘- 45 Atomic Layer Deposition (ALD) is a deposition method that uses a cyclic process of self-limiting chemisorption reactions of gas phase precursors to deposit a thin film on a substrate. It is ideally suited to deposit thin films on structured surfaces and has become the major method to deposit high- dielectrics. Atomic Layer Deposition (ALD) Phase 1 : Precursor pulse Phase 2 : Precursor purge Phase 3 : Water pulse Phase 4 : Water purge (…) Major principles of ALD: • Precursor gases or vapors are alternately pulsed on to the substrate surface. • Precursor gases introduced on to the substrate surface will chemisorb or surface reaction takes place at the surface. Phase 2: Precursor Purge Phase 3: Water pulse Phase 4: Water pulse In MOCVD two precursors are always present in the reaction chamber. But in ALD only one is present at a time. ALD is a surface driven technique.
  • 46. ‘- 46 Self-limiting chemisorption Association Dissociation Chemisorption mechanisms Surface reactions in ALD are all self-limiting. This self-limiting characteristics of the process steps is the foundation of ALD. Most ALD processes are based on a chemisorption saturation process followed by an exchange reaction. → → → Ligand exchange • limited number of reactive surface sites You start with an -OH group. Then supply HfCl4 or ZrCl4. The Chlorides are relatively big atoms. Once one of the Chlorine is replaced and Hf/Zr bonds with the -OH group, the layer of -OH is completely covered. Therefore no further reaction is possible.
  • 47. ‘- 47 ALD of HfO2 using HfCl4 and H2O HfCl4(g) + 2 H2O(g) → HfO2 + 4 HCl(g) HfCl4 H2O -OH Hf O Cl H -Hf-Cl + H2O(g) → -Hf-OH + HCl(g) x -OH + HfCl4(g) → -O-HfCl(4-x) + x HCl(g) purge purge 1 2 3 HfO2 can be deposited by ALD using subsequent pulses of HfCl4 and H2O. Phase 1 & 2: Movement of precursor in the reaction chamber, self limiting reaction Phase 3 & 4: Water pulse completes the reaction of the first layer. Also it creates more –OH group at the surface that allows reaction of the next layer.
  • 48. ‘- 48 Atomic Layer Deposition Desorption Decomposition Condensation Incomplete reaction < 1 monolayer per cycle more less PROCESS WINDOW Temperature ALD precursor requirements: Sufficient volatility (high VP) Fast and complete chemisorption reaction Avoid gas phase reactions: - stable against decomposition at ALD temperature - no reaction with reaction products No etching or dissolution in the film or substrate For ALD the temperature window must be in-between condensation/incomplete reaction and decomposition/desorption of the precursors.
  • 49. ALD films and applications Currently a whole series of ALD processes are available for various applications, including metal deposition.
  • 50. ‘- 50 Illustration of an ALD system based on laminar flow. ALD tools ALD tools have to be optimized to handle ‘pulses’ of gases. This can for example be achieved with a simple laminar flow reactor design. Other designs are based on the typical ‘showerhead’design as used in regular CVD tools.
  • 51. ‘- 51 Atomic dimensions are now routine Some high points of Intel's 45nm HKMG technology are: high- first, metal-gate-last integration; optimized interfacial SiO2 layer (not shown on the picture); hafnium oxide (HfO2) gate dielectric (1nm EOT); and dual band-edge work function metal gates (TiN for PMOS; TiAlN for NMOS). K. Kuhn – INTEL - 2009 2nd International CMOS Variability Conference - London Still, the advantages outweigh the disadvantages and high-/metal gates went into full production in the 45nm technology node, first introduced by Intel.