Repetition: Physical Deposition Processes

Transcription

1 Repetition: Physical Deposition Processes PVD (Physical Vapour Deposition) Evaporation Sputtering Diode-system Triode-system Magnetron-system ("balanced/unbalanced") Ion beam-system Ionplating DC-glow-discharge RF-glow-discharge Magnetron- discharge Arc-discharge Ion-Cluster-beam Reactive versions of the above processes

2 Repetition: Rates and Cooling Rates PVD Effusion cell, Ion Gun Sputtering Evaporation High rate magnetron High rate electron gun R[nm/s] R T[K/s] E particle 0.1eV R T[K/s] E particle 1eV These extremely high cooling rates show, that PVD processes (apart from being a direct transition from vapor! solid state) always have to be considered as non equilibrium processes. 7

3 Vacuum Physics Central Termini: Mean free path: the way a gas particle (or a film particle) can travel without a collision with another particle. Impingement rate: number of particles which hit a surface per second and unit area at constant pressure. Coverage time: time needed for the formation of a full monolayer.

4 Mean Free Path I Collision of two particles, 1 and with radius r R/: R σ 1 r If both particles are considered as points then a collision happens, if particle 1 is located within a disk of the area σ π R. σ is called the collision cross section.

5 Mean Free Path II Aparticle moves straight for a distance l through a gas. Within a cylinder of the volume V l σ it will collide with each particle located in V. σ l The cylinder contains N n V particles. For straight movement this is the collision number.

6 Mean Free Path III One collision happens if N 1. This yields the mean free path λ as: N 1 n V n λ σ λ n σ π n R 4 π n r Macroscopic information: Particle density n, from the ideal gas equation. Microscoic information: Collision cross section σ, contains energy dependent atom/molecule radii or the general interaction cross sections of the colliding particles.

7 Mean Free Path IV State of movement of the background gas: Energetic coating particle: relative movement may be neglected Gas particle: relative movement may not be neglected λ 4 1 π n r λ 1 4π n r

8 Mean Free Path - Example λ 1 4 πn r p 0.1 Pa k 1, J/K T 300K r m λ k T 4 π p r 14.6 cm p V N k T N V [J / K] 300[K] π0.1[j m ] [m n ] k p T

9 Mean Free Path - Rough Estimate λp5mmpa p 1 Pa!λ 5 mm p 10-4 Pa!λ 50 m He Mittlere Freie Weglänge [m] 1 0,1 0,01 1E-3 H O CO ; Ar; N 1E Druck [Pa]

10 Mean Free Path: Scale Consideration CERN LHC: U 4.3 π 7 km p[pa] λp5mmpa λ[mm] p[pa] 5 λ[mm] 7 Pa mbar Within LHC a pressure of approx mbar has to be maintained, to exclude interparticle collisions.

11 Gas Phase Transport Clausius' law of distance: N(x) N(0) exp x λ This means: A significant number of collisions happens before the mean free path is reached. Only approx. 37% of the particles reach λ without a collision. The mean free path is only a statistical measure.

12 Gas Phase Transport - Statistics λ λ λ 0 0 x exp x exp x d d 1 d d n Consider large ensemble of single situations: Calculate the expectation value of free throw distances:

13 Areal Impingement Rate I Initial situation: Gas molecules hit surface Wanted: number of gas molecules, which hit the unit surface within 1 second.

14 Areal Impingement Rate II Approach: cylinder with unit top areas, heigth u. Only particles with a velocity component u in the direction of e 1, which trespass the top cylinder surface reach the surface within unit time. u e 1 1 Differential areal impingement rate: dz u u{ 1 n{ Φ(u) du cylinder volume particle density

15 Areal Impingement Rate III Differential areal impingement rate: dz u u{ 1 n{ Φ(u) du cylinder volume particle density Total areal impingement rate: Z 0 dz u n 0 u Φ(u) du Maxwell-distribution of one velocitycomponent: Φ(u) m π m k T e mu k T

16 Areal impingement rate IV Calculation of the total areal impingement rate: T k m m p T k p n V N m T k T k m n du e u T k m n du (u) u n dz Z m T k 0 T k m u 0 0 u π π π Φ

17 Areal Impingement Rate V Z Z(p,T,m) p m m π k T p 0.1 Pa Z s -1 cm - etwa 70 ML/s m kg (O ) k 1, J/K T 300K p 10-4 Pa Z s -1 cm - etwa 0. ML/s

18 Areal Impingement Rate - Graphic

19 Types of Vacuum Name Pressure [Pa] Mean free Coverage time path [mm] O, 300K [ML/s] Rough vacuum Atm! ! ! 700 Fine vacuum 1!0.1 5!50 700! 70 High vacuum (HV) 0.1! ! ! 0.07 Ultra high vacuum (UHV) 10-5! ! ! Extreme UHV (XHV) < ! ! mm 500 m mm km (!)

20 Types of Pumps Gas transporting: + Rotary pump Rough vacuum/fine vacuum + Diffusion pump High vacuum + Turbomolecular pump High vacuum Gas adsorbing: + Cold traps Fine vacuum + Cryo pumps High vacuum/uhv + Sublimation pumps UHV + Getter pumps UHV reactive gases + Ione getter pumps UHV inert molecules (activation)

21 Flow Types Flow through a pipe, diameter d: Laminar/turbulent: Rough vacuum/fine vacuum λ << d d λ Particle collisions probable, global flow Molecular: High vacuum, UHV d λ λ >> d Wall collisions probable, no flow

22 Flow Types and Pumping Systems Efficient in the laminar region: + Gas transporting pumps: Rotary pump Ejector pump + Rotary pumps, except Turbomolecular pumps Efficient in the molecular region : + Gas transporting pumps: Diffusion pump Turbomolecular pump + Gas adsorbing pumps

23 Design Criteria for Vacuum Systems Mean free path λ: + Choice of pump type + Pump velocity + Dimension of pipe diameters Areal impingement rate Z: + Coverage times (e. g. surface analytics) + Impurity content in coatings (ratio of impingement rate of the coating particles and of the background gas particles)

24 Impurities Sticking coefficient α: ZDes α 1 Z Z... Z Des... Ipingement rate Desorption rate High sticking coefficient α 1 (Z Des 0): Reactive gases: O H 0 Complex carbohydrates (pump oil) Low sticking coefficient α << 1 (Z Des Z): Inert gases: Noble gases N CH 4 Carbohydrates without reactive groups

25 Impurities: Example Coating material: Al, m kg Rate Al: 10 nm/s At/(m s -1 ) Impurity: O, m kg Sticking coefficient α: approx. 1 für Al und O Temperture: 300K Wanted: ackground gas pressure, at which 1% Oxygen is incorporated unto the coating Z Z O Al p m 19 O p m O π k m m π k O T O T Pa

26 Design Criteria: Summary Mean free path λ: Influences gas dynamics. Even at rather high pressures (10 - Pa) the mean free path reaches the dimensions of average deposition chambers. Areal impingement rate Z: Crucial for coating purity. The pressure of the background gas has to be at least in the medium high vacuum to guarantee sufficient film purity.