Pattern Transfer: Additive techniques-PhysicalVapor Deposition and Chemical Vapor Deposition Dr. Marc Madou, Winter 2011 UCI Class 6 Content Physical vapor deposition (PVD) – Thermal evaporation – Sputtering – Evaporation and sputtering compared – MBE – Laser sputtering – Ion Plating – Cluster-Beam Chemical vapor deposition (CVD) – Reaction mechanisms – Step coverage – CVD overview Epitaxy Electrochemical Deposition Physical vapor deposition (PVD) The physical vapor deposition technique is based on the formation of vapor of the material to be deposited as a thin film. The material in solid form is either heated until evaporation (thermal evaporation) or sputtered by ions (sputtering). In the last case, ions are generated by a plasma discharge usually within an inert gas (argon). It is also possible to bombard the sample with an ion beam from an external ion source. This allows to vary the energy and intensity of ions reaching the target surface. Physical vapor deposition (PVD): thermal evaporation Heat Sources Advantages Disadvantages Resistance No radiation Contamination e-beam Low contamination Radiation RF No radiation Contamination Laser No radiation, low contamination Expensive N = N o exp- u e kT 6 The number of molecules leaving a unit area of evaporant per second Physical vapor deposition (PVD): thermal evaporation K n = ì/D > 1 A ~ cos| cos u/d 2 N (molecules/unit area/unit time) = 3. 513. 10 22 P v (T)/ (MT) 1/2 The cosine law This is the relation between vapor pressure of the evaporant and the evaporation rate. If a high vacuum is established, most molecules/atoms will reach the substrate without intervening collisions. Atoms and molecules flow through the orifice in a single straight track,or we have free molecular flow : The fraction of particles scattered by collisions with atoms of residual gas is proportional to: The source-to-wafer distance must be smaler than the mean free path (e.g, 25 to 70 cm) Physical vapor deposition (PVD): thermal evaporation t 1 /t 2 =cos| 1 /cos| 2 ì = (tRT/2M) 1/2 q/P T From kinetic theory the mean free path relates to the total pressure as: Since the thickness of the deposited film, t, is proportional to the cos |, the ratio of the film thickness shown in the figure on the right with u = 0° is given as: Physical vapor deposition (PVD): sputtering W= kV i P T d -V working voltage - i discharge current - d, anode-cathode distance - P T , gas pressure - k proportionality constant Momentum transfer Evaporation and sputtering: comparison Evaporation Sputtering Rate Thousand atomic layers per second (e.g. 0.5 µm/min for Al) One atomic layer per second Choice of materials Limited Almost unlimited Purity Better (no gas inclusions, very high vacuum) Possibility of incorporating impurities (low-medium vacuum range) Substrate heating Very low Unless magnetron is used substrate heating can be substantial Surface damage Very low, with e-beam x-ray damage is possible Ionic bombardment damage In-situ cleaning Not an option Easily done with a sputter etch Alloy compositions, stochiometry Little or no control Alloy composition can be tightly controlled X-ray damage Only with e-beam evaporation Radiation and particle damage is possible Changes in source material Easy Expensive Decomposition of material High Low Scaling-up Difficult Good Uniformity Difficult Easy over large areas Capital Equipment Low cost More expensive Number of depositions Only one deposition per charge Many depositions can be carried out per target Thickness control Not easy to control Several controls possible Adhesion Often poor Excellent Shadowing effect Large Small Film properties (e. g. grain size and step coverage) Difficult to control Control by bias, pressure, substrate heat Physical vapor deposition (PVD): MBE, Laser Ablation - MBE – Epitaxy: homo-epitaxy hetero-epitaxy – Very slow: 1µm/hr – Very low pressure: 10 -11 Torr Laser sputter deposition – Complex compounds (e.g. HTSC, biocompatible ceramics) Physical vapor deposition (PVD): Ion cluster plating Ionized cluster: it is possible to ionize atom clusters that are being evaporated leading to a higher energy and a film with better properties (adherence, density, etc.). – From 100 mbar (heater cell) to 10 -5 to 10 -7 mbar (vacuum)-- sudden cooling – Deposits nanoparticles Combines evaporation with a plasma » faster than sputtering » complex compositions » good adhesion Gas cluster ions consist of many atoms or molecules weakly bound to each other and sharing a common electrical charge. As in the case of monomer ions, beams of cluster ions can propagate under vacuum and the energies of the ions can be controlled using acceleration voltages. A cluster ion has much larger mass and momentum with lower energy per atom than a monomer ion carrying the same total energy. Upon impact on solid surfaces, cluster ions depart all their energy to an extremely shallow region of the surface. Cluster plating material is forced sideways and produces highly smooth surfaces. Also individual atoms can be ionized and lead to ion plating (see figure on the right, example coating : very hard TiN) Physical vapor deposition (PVD):Ion cluster plating and ion plating Chemical vapor deposition (CVD): reaction mechanisms Mass transport of the reactant in the bulk Gas-phase reactions (homogeneous) Mass transport to the surface Adsorption on the surface Surface reactions (heterogeneous) Surface migration Incorporation of film constituents, island formation Desorption of by-products Mass transport of by-produccts in bulk CVD: Diffusive-convective transport of depositing species to a substrate with many intermolecular collisions-driven by a concentration gradient SiH4 SiH 4 Si Chemical vapor deposition (CVD): reaction mechanisms Fl = D Ac Ax o(x) = qx µU | \ | . | 1 2 o = 1 L o(x)dX = 2 3 0 L } L q µUL | \ | . | 1 2 Re L = µUL q o = 2L 3 Re L Energy sources for deposition: – Thermal – Plasma – Laser – Photons Deposition rate or film growth rate (Fick’s first law) (gas viscosity q, gas density µ, gas stream velocity U) (Dimensionless Reynolds number) Laminar flow L o(x) dx (U) (Boundary layer thickness) Fl = D Ac 2L 3 Re L (by substitution in Fick’s first law and Ax=o) Mass flow controlled regime (square root of gas velocity)(e.g. AP CVD~ 100-10 kPa) : FASTER Thermally activated regime: rate limiting step is surface reaction (e.g. LP CVD ~ 100 Pa----D is very large) : SLOWER Chemical vapor deposition (CVD) : reaction mechanisms Fl = D Ac 2L 3 Re L R = R o e - E a kT Chemical vapor deposition (CVD): step coverage Fl = D Ac 2L 3 Re L R = R o e - E a kT Step coverage, two factors are important – Mean free path and surface migration i.e. P and T – Mean free path: ì = o w z u=180 0 u=270 0 u=90 0 u is angle of arrival kT 2 1 2 P T ta 2 > o Fldu } u = arctan w z Chemical vapor deposition (CVD) : overview CVD (thermal) – APCVD (atmospheric) – LPCVD (<10 Pa) – VLPCVD (<1.3 Pa) PE CVD (plasma enhanced) Photon-assisted CVD Laser-assisted CVD MOCVD The L-CVD method is able to fabricate continuous thin rods and fibres by pulling the substrate away from the stationary laser focus at the linear growth speed of the material while keeping the laser focus on the rod tip, as shown in the Figure . LCVD was first demonstrated for carbon and silicon rods. However, fibers were grown from other substrates including silicon, carbon, boron, oxides, nitrides, carbides, borides, and metals such as aluminium. The L-CVD process can operate at low and high chamber pressures. The growth rate is normally less than 100 µm/s at low chamber pressure (<<1 bar). At high chamber pressure (>1 bar), high growth rate (>1.1 mm/s) has been achieved for small-diameter (< 20 µm) amorphous boron fibers. Chemical vapor deposition (CVD) : L-CVD Epitaxy VPE: – MBE (PVD) (see above) – MOCVD (CVD) i.e.organo-metallic CVD(e.g. trimethyl aluminum to deposit Al) (see above) Liquid phase epitaxy Solid epitaxy: recrystallization of amorphous material (e.g. poly-Si) Liquid phase epitaxy Epitaxy Selective epitaxy Epi-layer thickness: – IR – Capacitance,Voltage – Profilometry – Tapered groove – Angle-lap and stain – Weighing Selective epitaxy Electrochemical deposition: electroless Electroless metal displacement Electroless sustainable oxidation of a reductant – Metal salt (e.g.NiCl 2 ) – Reductant (e.g.hypophosphite) – Stabilizer:bath is thermodynamically unstable needs catalytic poison (e.g. thiourea) – Complexing agent : prevent too much free metal – Buffer: keep the pH range narrow – Accelerators: increase deposition rate without causing bath instability (e.g. pyridine) Deposition on insulators (e.g. plastics): seed surface with SnCl 2 /HCl 1. Zn(s) + Cu 2+ (aq) ------> Zn 2+ (aq) + Cu(s) 2. Reduction (cathode reaction) : Ni +2 + 2e - —> Ni Oxidation (anode reaction): H 2 PO 2- + H 2 O—> H 2 PO 3 - +2H + +2e - ------------- ----------------------------- Ni +2 + H 2 PO 2 - + H 2 O —> Ni + H 2 PO 3 - + 2H + e.g. electroless Cu: 40 µmhr -1 Cu Electrochemical deposition: electroless Evan’s diagram: electroless deposition is the combined result of two independent electrode reactions (anodic and cathodic partial reactions) Mixed potential (E M ): reactions belong to different systems i deposition = i a = i c and I=A x i deposition Total amount deposited: m max = I t M/Fz (t is deposition time, Molecular weight, F is the Faraday constant, z is the charge on the ion) CMOS compatible: no leads required Evan’s diagram F= 96,500 coulombs=1, 6 10 -19 (electron charge) x 6. 02 10 23 (Avogadro’s number) + - Electrochemical deposition :electrodeposition- thermodynamics Electrolytic cell – Au cathode (inert surface for Ni deposition) – Graphite anode (not attacked by Cl 2 ) Two electrode cells (anode, cathode, working and reference or counter electrode) e.g. for potentiometric measurements (voltage measurements) Three electrode cells (working, reference and counter electrode) e.g. for amperometric measurements (current measurements) Electrochemical deposition :electrodeposition-thermodynamics (E) E = E 0 + RT zF ln a M z + ² G=² G 2 -² G 1 ² G=-(E 2 -E 1 )zF=-E cell zF ² G=² G 0 -RT ln a M z+ =² G 0 -RT ln C M z+ ¸ M z+ ² G= - EzF E 2 > E 1 : - battery E 2 < E 1 : + E ext > E cell to afford deposition (Nernst equation) 1. Free energy change for ion in the solution to atom in the metal (cathodic reaction): or also 2. The electrical work, w, performed in electrodeposition at constant pressure and constant temperature: and since AV =0 AG =G m (free energy pure metal)- G e (free energy of ion in the electrolyte) AG = - w +PAV 3. Substituting Equation (2) in (1) one gets (1) (2) 4. Repeat (1) and (2) for anodic reaction: or Electrochemical deposition :electrodeposition-thermodynamics (q) A thermodynamic possible reaction may not occur if the kinetics are not favorable Kinetics express themselves through all types of overpotentials E -E o = q ( + anodic and - is cathodic) ² G* = ² G # +|uA| k c ÷ = kT h e ÷ AG # _ RT k ÷ = k ÷ c kT h e ÷ |FA| RT i ÷ = k ÷ z F = k ÷ c z F kT h e ÷ |FA| RT i ÷ = k ÷ zF = k c ÷ z F kT h e (1 ÷ |)FA| RT Electrochemical deposition :electrodeposition- kinetics-activation control Understanding of polarization curves: consider a positive ion transported from solution to the electrode Successful ion jump frequency is given by the Boltzmann distribution theory (h is Planck constant): (without field) (with field) Electrochemical deposition :electrodeposition- kinetics-activation control i e = i ÷ = k ÷ c zF kT h e (1 ÷ |)FA| e RT = i ÷ = i ÷ c zF kT h e ÷|FA| e RT q=A|÷A| e i = i ÷ ÷ i ÷ i = i e (e (1÷|)Fq RT ÷ e ÷|Fq RT ) q= a + blog(i) (Butler-Volmer) (Tafel law) At equilibrium the exchange current density is given by: The reaction polarization is then given by: The measurable current density is then given by: For large enough overpotential: Electrochemical deposition :electrodeposition- kinetics-diffusion control dC dX = C x=· 0 ÷ C x=0 o q c = RT nF ln C x=0 C · 0 i = nFD 0 C · 0 ÷ C x=0 o I l = nFAD 0 C · 0 o i = i l (1 ÷e nFq c RT ) From activation control to diffusion control: Concentration difference leads to another overpotential i.e. concentration polarization: Using Faraday’s law we may write also: At a certain potential C x=0 =0 and then: C x=0 C · 0 = 1- i i l we get : Electrochemical deposition :electrodeposition- non-linear diffusion effects o = tD 0 t ( ) 1 2 I l = nFAC · 0 D 0 tt | \ | . | 1 2 I l = nFAC · 0 D 0 tt | \ | . | 1 2 + AnFD 0 C · 0 r Nonlinear diffusion and the advantages of using micro-electrodes: An electrode with a size comparable to the thickness of the diffusion layer The Cottrell equation is the current-vs.-time on an electrode after a potential step: For micro-electrodes it needs correction : I l = nFAD 0 C · 0 o Electrochemical deposition :electrodeposition- non-linear diffusion effects I l,m = trnFD 0 C · 0 (disc) I l,m = 2trnFD 0 C · 0 (hemisphere) I l,m = 4trnFD 0 C · 0 (sphere) I l,m = AnFD 0 C · 0 r + L The diffusion limited currents for some different electrode shapes are given as (at longer times after bias application and for small electrodes): If the electrodes are recessed another correction term must be introduced: Homework Homework: demonstrate equality of ì = (tRT/2M) 1/2 q/P T and ì = kT/2 1/2 a 2 t P T (where a is the molecular diameter) What is the mean free path (MFP)? How can you increase the MFP in a vacuum chamber? For metal deposition in an evaporation system, compare the distance between target and evaporation source with working MFP. Which one has the smaller dimension? 1 atmosphere pressure = ____ mm Hg =___ torr. What are the physical dimensions of impingement rate? Why is sputter deposition so much slower than evaporation deposition? Make a detailed comparison of the two deposition methods. Develop the principal equation for the material flux to a substrate in a CVD process, and indicate how one moves from a mass transport limited to reaction-rate limited regime. Explain why in one case wafers can be stacked close and vertically while in the other a horizontal stacking is preferred. Describe step coverage with CVD processes. Explain how gas pressure and surface temperature may influence these different profiles.