Chemical vapour deposition

Chemical Vapour Deposition is a Chemical Synthesis route of Nanomaterials. Specially thin films like Graphene and Carbon NanoTubes are grown by this method.

1. Malaviya National Institute Of Technology, Jaipur CHEMICAL VAPOUR DEPOSITION Presented by: Monika Shrivastav (2019RPY9541) Submitted to: Dr. Srinivasa Rao Assistant Professor,MNITJ

2. I. Chemical Vapor Deposition CVD

3. Chemical Vapour Deposition (CVD) It has it's name because during this method the chemical reactions take place between the substrate molecules and precursor molecules on the surface of the substrate. Substrate: One on which surface reaction takes place or on that surface change occurs.Ex: For CNT Powder activated carbon (PAC) Catalyst: To enhance the reaction rate. Wev have to impregnatehe catalyst molecules into the substrate. Ex: Ni, Fe etc. Precursor gas: The gas contains required element to synthesis of particular substance.Ex: For CNT Acetylene, Methane, Ethanol, Ethylene. Career gas or Forced Gas: Put a force on precursor gas to adsorbed on substrate surface. secondlyprovide essential energy to react the precursor gas with substrate molecules. it also removes the byproducts (usually the gases) from the chamber.Ex: Ammonia, Hydrogen and Nitrogen.

4. Chemical vapor deposition (CVD) systems Atmospheric cold-wall system used for deposition of epitaxial silicon. (SiCl 4 + 2H 2  Si + 4HCl ) Low pressure hot-wall system used for deposition of polycrystalline and amorphous films, such as poly-silicon and silicon dioxide.

5. Types of CVD APCVD (Atmospheric Pressure CVD), mass transport limited growth rate, leading to non-uniform film thickness. LPCVD (Low Pressure CVD) Low deposition rate limited by surface reaction, so uniform film thickness ( many wafers stacked vertically facing each other; in APCVD, wafers have to be laid horizontally side by side. Gas pressures around 1-1000mTorr (lower P => higher diffusivity of gas to substrate) Better film uniformity & step coverage and fewer defects Process temperature 500 °C PECVD (Plasma Enhanced CVD) Plasma helps to break up gas molecules: high reactivity, able to process at lower temperature and lower pressure (good for electronics on plastics). Pressure higher than in sputter deposition: more collision in gas phase, less ion bombardment on substrate Can run in RF plasma mode: avoid charge buildup for insulators Film quality is poorer than LPCVD. Process temperature around 100 - 400°C. MOCVD (Metal-organic CVD, also called OMVPE - organo metallic VPE), epitaxial growth for many optoelectronic devices with III-V compounds for solar cells, lasers, LEDs, photo-cathodes and quantum wells.

6. CVD sources and substrates Types of sources Gasses (easiest) Volatile liquids Sublimable solids Combination Source materials should be Stable at room temperature Sufficiently volatile High enough partial pressure to get good growth rates Reaction temperature < melting point of substrate Produce desired element on substrate with easily removable by-products Low toxicity Substrates Need to consider adsorption and surface reactions Cu substrate , Silicon waffer , Si/SiO x etc.

7. Thermal - CVD When a conventional heat source (e.g., a furnace) is used, the technique is called thermal CVD. It consists of a quartz tube inserted into a tube furnace and has a gas inlet on one side and a gas outlet on the other side. The sample is placed onto a quartz boat inside the tube. Example Carbon Nanotubes: Hydrocarbons or CO are used as precursor. A typical growth process involves: 1 st : purge reactor with inert gas; 2 nd : gas flow is switched for specified growth period; 3 rd : gas flow is switched back to inert gas while the reactor cools down. For growth on substrates, catalysts need to be applied on substrate before loading it inside reactor. Typical temperatures for catalytic CVD in CNT growth are in the range of 800–1500 K. SOURCE: http://www.azonano.com/images/Article_Images/ImageForArticle_3423(1).jpg

8. Reaction Process in CVD Mass transport of the reactant Gas-phase reactions Mass transport to the surface Adsorption on the surface Surface reactions Surface migration Incorporation of film constituents, island formation Desorption of by-products Mass transport of by-products

9. Reaction Process in CVD

10. a) Epitaxial Growth The term epitaxy describes an ordered crystalline growth on a monocrystalline substrate . Because the substrate acts as a seed crystal, the deposited film takes on a lattice structure and orientation identical to those of the substrate . Homoepitaxy : a crystalline film is grown on a substrate or film of the same material. This technique can grow more purified films than the substrate, can fabricate layers with different doping levels and layers of different isotopes. Heteroepitaxy : a crystalline film is grown on a substrate or film, but the materials are different from each other. This technique is used to grow e.g. GaN on Sapphire or AlGaInP on GaAs

11. Homoepitaxy : When the thin crystal layer lattice is the same as that of the substrate (e.g. Si film on Si substrate). Heteroepitaxy : When the thin crystal layer lattice is different from that of the substrate (e.g. GaAs film on Si ). Terminology (Epitaxial Growth)

12. Epitaxial Growth Homoepitaxy of Si on a Si substrate SiCl 4(g) +2H 2(g = Si (s) +4HCl (g) at approx. 1000-1200 °C

13. Epitaxial films can be grown from solid, liquid, or gas phases. It is easier to control the growth rate in gas phase epitaxy by controlling the flow of gases. In CVD, gases containing the required chemical elements are made to react in the vicinity of the substrate inside the reactor. Chemical Vapor Deposition (CVD)

14. b) Vapor-Liquid-Solid (VLS) growth Catalytic nanodots on substrate (e.g. UTAM technique) Equilibrium vapor pressure of the catalyst must be small so that the droplet does not vaporize Catalyst must be inert

15. Nanostructures by CVD Chang et al. Chem . Mater., Vol. 16, No. 24, 2004 1D Zinc oxide ( ZnO ) nanowires and nanorods fabricated by CVD. diameters from 20 to 300 nm Length 20 µm

16. CVD advantages and disadvantages (as compared to physical vapor deposition) Advantages: High growth rates possible, good reproducibility. Can deposit materials which are hard to evaporate. Can grow epitaxial films. In this case also termed as “vapor phase epitaxy (VPE)”. For instance, MOCVD (metal-organic CVD) is also called OMVPE ( organo -metallic VPE ). Generally better film quality, more conformal step coverage (see image below). Disadvantages: High process temperatures. Complex processes, toxic and corrosive gasses. Film may not be pure (hydrogen incorporation…). All substrates can't used to make thin films.

17. Doping in CVD films Doping is usually done for epitaxial (thus single crystal) film during film growth. Dopant will be grown directly onto crystalline site (no need of dopant activation). Doping is realized by adding gas containing the dopant. Such as PH 3 , B 2 H 6 , AsH 3 (all gas phase at room temperature); or PCl 3 , BCl 3 , AsCl 3 (all liquid at RT). They will go through: dissociation, lattice site incorporation, and burying of dopants by other atoms in the film. The dopant concentration C: (P is partial pressure of he dopant species, and v growth rate) However, there is also unintentional doping process: Out-diffusion of dopant from heavily doped substrate into the epi -layer. Auto-doping – dopant from substrate diffuses into gas stream first, then back into epi -layer.

18. Types of CVD For R&D, PECVD is most popular, followed by LPCVD. (can be higher) and epitaxy Si… (can have high deposition rate)

19. Further Examples of CVD Dielectrics : silicon dioxide , silicon nitride … Metal : tungsten , copper , titanium , aluminium … Semiconductors: epitaxial silicon , germanium … Nitrides : TiN , TaN Many other nanostructures ,such as nanobelts , nanotube , SnO 2 nanoboxes …. SnO 2 nanoboxe Carbon Nanotube (SEM) Carbon Nanotube (TEM)

20. 1. Metalorganic CVD

21. Metal-Organic Chemical Vapor Deposition (MOCVD) Example Reaction : Ga(CH 3 ) 3 + AsH 3  3CH 4 + GaAs Trimethal Gallium Gas Arsene Gas Methane Gas on the substrate The reaction occurs in a sealed container (reactor) NOTE !! Arsene gas is highly toxic & highly flammable ! Trimethal gallium gas is highly toxic !! Methane gas is highly explosive ! If you are British, MOCVD  OMCVD!

22. MOCVD: Dopants can be introduced in precisely controlled amounts! Dopants are introduced in precisely controlled amounts!

23. 2. Plasma Enhanced CVD (PECVD) PECVD – plasma-enhanced CVD: – glow-discharge plasmas (usually RF field: 100 kHz – 40 MHz ), or MW – 2.54 GHz plazma at reduced gas pressure between 50 mtorr and 5 torr ) are sustained within chambers where simultaneous vapor-phase chemical reactions and film depositions occur – plasma activation of reactions (average electron energies range from 1 to 10 eV ) ! chemical reactions occur at much lower temperatures than in thermal CVD M ain applications of PECVD: - microelectronics (DRAM cells) - plasma modification of metal surfaces ( nitriding , carburizing): the atoms of nitrogen and carbon that deposit on metal surfaces modify them by diffusing into the underlying matrix - diamond films

24. Plasma Enhanced CVD (PECVD) Use RF-induced plasma to transfer energy into the reactant gases, forming radicals that is very reactive. (RF: radio-frequency, typically 13.56MHz for PECVD) Low temperature process (<300 o C), as thermal energy is less critical when RF energy exists. Used for depositing film on metals (Al…) and other materials that cannot sustain high temperatures. (APCVD/LPCVD at such low temperatures gives increased porosity and poor step coverage) Surface reaction limited deposition, thus substrate temperature control is important to ensure uniformity. At low T, surface diffusion is slow, so one must supply kinetic energy for surface diffusion – plasma (ion bombardment) provides that energy and enhances step coverage. Disadvantages: plasma damage, not pure film (often lots of H incorporated into film). “Good” quality films (though generally not as good as LP or APCVD films deposited at much higher T ): energy supplied by plasma (i.e. ion bombardment of film) increases film density, composition, and step coverage. Cold-wall

25. PECVD process parameter Substrate temperature (100-300 o C, up to 1000 o C PECVD available) Control by external heater, very little heating from PECVD process Gas flow (10s to 100s sccm – standard cubic centimeter per minute) Higher flow rates can increase deposition rate and uniformity Pressure ( P  50mTorr – 5Torr ) Changes the energy of ions reaching electrodes Can change deposition rate Increases pressure may lead to chemical reactions in the gas Effects also depend on gas concentration Power (10s to 100s watts) Affects the number of electrons available for activation and the energy of those electrons Increased power may lead to chemical reactions in gas Increased power increases deposition rate Frequency (mostly 13.56MHz, same for plasma etching and sputter deposition) Changes plasma characteristics Changes ion bombardment characteristics

26. 3. High Density Plasma (HDP) CVD High density plasma CVD gives dense layers (SiO 2 ) at low T (150 °C) and low P (1- 10 mTorr ); T increases to 400°C by bombardment. Separate RF (gives substrate biasing for bombardment) from plasma generation (electron cyclotron resonance ECR and inductively coupled plasma ICP). S imultaneous deposition and sputtering/ bombardment. I mproved planarization and filling d ue to preferential sputtering of sloped surface. Mostly used for SiO 2 deposition in backend processes.

27. Microwave plasma chemical vapor deposition system for high quality poly-, mono- and nanocrystalline diamond films. MW frequency – 2.5 4 GHz. PECVD (MW) Plasma Enhanced CVD

28. Diamond films Polycrystalline diamond films Nanocrystalline diamond films

29. Application of diamond films - Semiconductor Devices, RF MEMS, Creation of novel surface materials, i.e. super‐hydrophobic surfaces , super‐hydrophilic surfaces (biocompatible surfaces) Fabrication of 3‐D diamond probes and structures for field emission High selectivity and high voltage range electrochemical sensors and electrodes Defects in diamond (famous NV centers) for single photon sources, quantum computers ( qubit ), quantum cryptography,… - Sensors (quantum sensing in biology and medicine) Tribology .

30. LPCVD reactors use: P = 0.25 – 2.0Torr, T = 500 – 900°C. Transport of reactants from gas phase to surface through boundary layer is still not rate limiting (despite the high T ), so wafers can be stacked vertically for high throughput (100-200 wafers per run). Because LPCVD operates in reaction limited regime, it is VERY sensitive to temperature and so temperature needs to be controlled closely (within +/- 1 o C), so use hot walled reactor for this precise control. Again, a 5-25 o C temperature gradient is often created to offset source gas depletion effects (or one can use distributed feeding). Requires no carrier gas , and low gas pressure reduces gas-phase reaction which causes particle cluster that contaminants the wafer and system. Less auto-doping (at lower P), as out-diffused dopant gas pumped away quickly. 4. Low Pressure Chemical Vapor Deposition (LPCVD)

31. Possible disadvantages: For too low temperature, deposition rates may be too low, film quality decreases. Shadowing (less gas-phase collisions) due to directional diffusion to the surface, so deterioration of the step coverage and filling. Low Pressure Chemical Vapor Deposition (LPCVD) Seems cold wall reactors also exist: cold wall reduce deposition on walls, which leads to depletion of deposition species and particle formation that may flake off walls and fall on wafers. Besides poorer temperature control than hot wall, gas convection is another problem. Cold-wall Hot-wall

32. Selective deposition: Especially important in microelectronics, surface patterning and 3D-growth. Reaction rate of precursor is limited on a non-growth surface. E.g. deposition of Cu from ( hfac )Cu(PMe 3 ) occur on Cu, Pt… but not on SiO 2 . Growth surface acts as co-reactant, and is selectively consumed. E.g. Si reacts with WF 6 or MoF 6 , while reaction at SiO 2 or Si 3 N 4 is slower. A chemical reaction of a gaseous co-reactant occur on the growth surface. E.g. H 2 dissociation on a metal surface, but not on SiO 2 or metal oxide surfaces. Miscellaneous: selective deposition and laser CVD Tungsten spring grown by laser CVD. Laser CVD (energy provided by laser)

33. CVD reactor types: quick summary According to the LPCVD slides, APCVD growth rate should be lower, which is not true. Because: (?? I think) In APCVD reactive gas partial pressure could be set much higher than that in LPCVD. Its pressure could be much lower (by 10  ) than 1atm and is still called APCVD. Gas transport actually increases with T as T 3/2 (APCVD is usually done at higher T than LPCVD). When putting wafer side-by-side facing the gas, more exposed to gas, thus faster transport.

34. Atomic Layer deposition (CVD like) A tomic scale deposition. ALD is similar in chemistry to CVD , except that the ALD reaction breaks the CVD reaction into two half- reactions, keeping the precursor materials separate during the reaction.