Previous Lecture. Electron beam lithoghraphy e - Electrons are generated in vacuum. Electron beams propagate in vacuum

Transcription

1 Previous Lecture Electron beam lithoghraphy e - Electrons are generated in vacuum Electron beams propagate in vacuum

2 Lecture 6: Vacuum & plasmas Objectives From this vacuum lecture you will learn: What vacuum is and what it is used for Basic vacuum theory Basic parts of a vacuum system Generation of vacuum: Pumps Measuring vacuum: Gauges

3 What is vacuum? General definition vacuum = empty space, from vacuus = [Latin] empty Scientific definitions A gas pressure lower than atmospheric. A space where the pressure is significantly lower than atmospheric. A condition in which the quantity of atmospheric gas present is reduced to the degree that, for the process involved, its effect can be considered negligible.

4 Ideal gas law Experimentally found by Robert Boyle and published pv nrt p = pressure V = volume n = number of gas molecules R = universal gas constant T = temperature Works well for sub atmospheric pressure and normal temperature.

5 Ideal gas law Experimentally found by Robert Boyle and published pv qnrt p = pressure V = volume n = number of gas molecules R = universal gas constant T = temperature Works well for sub atmospheric pressure and normal temperature. For better accuracy use a correction factor q(p,t). (gas specific)

6 Kinetic gas theory Daniel Bernoulli explained (1738) gas pressure from a molecular point of view. The gas molecules are treated as hard spheres. are many, small, and far apart compared to their size. collide elastically with walls and each other. move randomly with constant speed between collisions. obey Newton s laws of motion. Collisions Pressure

7 Gas molecule speed distribution Derived from kinetic gas theory mv 2 2kT P v m 4 2 kt v e v = gas molecule speed m = gas molecule mass k = Boltzmann s constant

8 Gas molecule speed & mean free path Derived from kinetic gas theory 3kT v rms m l kt 2 d 2 p v rms = rms velocity l = mean free path d = gas molecule diameter N 2 at room temperature ~ 500 m/s ~ 7 cm between 10-3 mbar

9 Why use vacuum? 500 km altitude mean-free ~1 m 100 km altitude Space begins Scientific instruments e.g. Electron microscope, Mass spectrometer TV-tubes, etc. Thin-film coating Freeze drying Light bulbs / tubes Plasma processes Force O 2 free Evaporation Long mean free path 335 mbar Mt. Everest Sea level mbar Suction pads Plastic forming Packing Food preservation

10 Vacuum quality Ultra-high vacuum High vacuum Fore vacuum Low vacuum mbar N 2 mean free path 1 km 100 m 10 m mbar 1 dm 1 cm 1 mm

11 m 1 D Gas flow regimes Mean free path << wall-to-wall distance Flow limited by molecule-molecule collisions Gas flows like virtually weightless liquid 10-1 Viscous flow Molecular flow P mbar Mean free path >> wall-to-wall distance Flow limited by molecule-wall collisions High conductance requires free line-of-sight over large solid angle

12 Gas flow rates Q 60 sccm = 1 mbar l/s Q = Gas flow P = Pressure P P p = Pump inlet pressure C = Conductance S p = Pumping speed C = Q (P-P p ) Q Commonly: S p = Q P p 1 l/s = 3.6 m 3 /h P p Q Q S p Process gas flow [sccm] Gas leaks [mbar l/s] Fore vacuum pumps [m 3 /h] High vacuum pumps [l/s]

13 Electrical feedthrough Ceramics Chamber wall Stainless steel Aluminum Ceramics Vacuum system Motion feedthrough Metal bellows Magnetically coupled Elastomer O-ring Ferro-fluidic Window Borosilicate glass Quartz Sapphire MgF Flange seal Elastomer O-ring Metal seal Pump Gauge

14 Generation of vacuum Atmospheric pressure High vacuum 100 kn/m 2 <10-5 mbar High vacuum pump 10 ton/m 2 Large absolute pressure difference Small absolute pressure difference 1 N/m 2 ~ 10-2 mbar Fore vacuum pump Exhausts

15 Pump types Positive displacement Momentum transfer Entrapment

16 Generation of vacuum Atmospheric pressure High vacuum pump Turbo Diffusion Momentum transfer High vacuum <10-5 mbar ~ 10-2 mbar Fore vacuum pump Rotary vane Scroll Multiple stage roots Diaphragm Positive displacement Exhausts

17 Generation of vacuum Atmospheric pressure High vacuum High vacuum pump Cryo Ion Entrapment <10-5 mbar ~ 10-2 mbar Fore vacuum pump Rotary vane Scroll Multiple stage roots Diaphragm Exhausts

18 Rotary vane pump Very common fore vacuum- and general vacuum pump. Typically 1 or 2 stage configuration. Gas is moved by rotating vanes. Oil is used as seal, lubricant, and coolant. B A B B A A B A

19 Rotary vane pump + High capacity - Potential back streaming of oil into vacuum chamber. Atm - ~10-3 mbar

20 Scroll pump Moving scroll orbiting a fixed scroll. Compressed gas volume pushed towards center outlet. Gas outlet Gas inlet

21 Scroll pump + Oil free + Reliable, low maintenance. - Low to medium capacity Atm - ~10-2 mbar

22 Diaphragm pump + Oil free + Reliable, low maintenance. - Low capacity Atm - ~1 mbar

23 Roots pump - Single stage boaster Counter rotating blades moves gas volume. No contact between surfaces oil free operation.

24 Roots pump - Single stage boaster + High capacity from 10 mbar to ~10-4 mbar. + Oil free - Works well only together with fore vacuum pump. Roots pump. Fore vaccum pump

25 Roots pump - Multiple stage Multiple stage counter-rotating blades. No contact between surfaces oil free operation.

26 Roots pump - Multiple stage + Medium capacity + Oil free - Moving parts don t seal higher ultimate pressure Atm - ~ mbar ~ mbar

27 Turbo pump Fast moving rotor (30k to 90k rpm) with several stages and many blades per stage. High efficiency in the molecular regime where gas molecules collide with rotor blade and not each other. Some pumps have magnetic, noncontact, bearings. Best pump capacity for heavy (slow) gas molecules. Stator blade Rotor blade

28 Turbo pump + High capacity + Low maintenance - Sudden large gas loads may cause severe, expensive damage mbar - ~10-8 mbar

29 Turbo pump Tool #404 September 2012

30 Diffusion pump Hot dense oil vapor is forced through central jets angled downward to form a conical curtain of vapor. Gas molecules are knocked downwards and eventually reach the fore vacuum pump.

31 Diffusion pump + Simple pump without moving parts. + High capacity + Low maintenance - Needs cooled baffle to reduce oil contamination of vacuum chamber mbar - ~10-8 mbar

32 Cryo pump Cool head with several plates (stages). The metal top side of the cool (12K) plates traps gas molecules by cryocondensation. The bottom side of the plates are coated with active charcoal and traps gas molecules by cryoadsorption. The cooling is done with a Helium filled refrigerator loop. Helium gas expender Helium gas compressor

33 Cryo pump + Very High capacity down to ~10-9 mbar. + No contamination. - Pump saturates fast if exposed to high pressure or continuous high gas flow. - Need periodic regeneration (heating) of cool head. Gas Pumping speed (Ø20cm pump) [ l/s ] Water vapor 4000 Air 1500 Hydrogen 2500 Argon 1200

34 Ion pump Array of steel tubes Titanium plate Magnet B Ti Free electrons move in helical trajectories towards the anode, ionizing gas molecules upon collisions. U Gas ions strike the Ti cathodes and some gets buried. Sputtered Ti deposits inside the tubes and getters gas molecules through chemical reactions.

35 Ion pump + Simple pump without moving parts. + Can work at very low pressure ~10-11 mbar. + Oil free. - Not suitable for gas loads.

36 Pumping speed diagram At what Argon gas load [sccm] can we maintain a pump inlet pressure of 1x10-4 mbar? 3600

37 Pumping speed diagram At what Argon gas load [sccm] can we maintain a pump inlet pressure of 1x10-4 mbar?

38 Measuring vacuum Bourdon T/C Capacitance manometer McLeod Pirani Schultz-Phelps Ion gauge Penning Bayard-Apert Ion gauge Invert Magnetron Residual Gas Analyzer [mbar]

39 Pirani vacuum gauge A wire resistor in a gauge tube, heated with an electrical current. A second wire resistor in a closed reference tube. The two wire resistors are 2/4 of a Wheatstone bridge. Higher pressure cools the wire and the resistance drops. Filaments The pressure is measured from the unbalanced bridge. Gauge tube Meter Reference tube Pirani gauge works well for pressure 10 1 to ~10-5 mbar.

40 Capacitance manometer (CM) The unknown pressure P x decide the position of the metal membrane electrode relative a fixed second electrode in a closed volume. The electrode capacitance can be converted to pressure. Overlapping CM gauges works well for atmospheric pressures to ~10-5 mbar. Each CM gauge covers a pressure range of 4 orders of magnitude. True reading for all gases. Rugged

41 Penning vacuum gauge Penning gauge often cylindrical in shape. DC discharge generated by ~ 2 kv. Pressure converted from discharge current. Penning gauge works well for pressure 10-2 to ~10-9 mbar. B I U ~ 2kV

42 Ion vacuum gauge Electrons are emitted from a hot filament. Electrons are attracted by the positive grid but pass several times before captured. I Collisions with gas molecules creates ions that are collected on negative pin. Pressure is converted from current I g. Ion gauge works well for pressure 10-4 to ~10-10 mbar. I g

43 Lecture 6: Vacuum & plasmas Objectives From this plasma lecture you will learn: What glow discharge / plasma is What we use glow discharges for Different types of glow discharges: DC, RF High density plasmas: Magnetically confined, ECR, ICP

44 What is glow discharge? Glow discharge is luminous plasma. Plasma is partially ionized gas. The glow is excess electromagnetic energy radiating from excited gas atoms and molecules.

45 How use glow discharge? Neutral particles are difficult to accelerate. Ions and electrons can be extracted from a glow discharge and easily accelerated. Accelerated inert ions are used for: Ion milling Sputter deposition Accelerated reactive ions are used for: Reactive ion beam etching (RIBE) Reactive ion etching (RIE) Accelerated ions can be filtered and counted Residual gas analysis (RGA)

46 How use glow discharge? Radicals from a plasma is used for: Chemical vapor deposition (PECVD) Plasma etching The electromagnetic radiation from a plasma is used for General illumination (light tubes, ) Light sources for optical lithography LASERs

47 Glow discharge processes Dissociation e* + AB A + B + e * exited state Atomic ionization e* + A A + + e + e Molecular ionization e* + AB AB + + e + e Atomic excitation e* + A A* + e Molecular excitation e* + AB AB* + e

48 DC-plasma reactor Electrodes must have electrically conducting surfaces. Pressure 1 mtorr - 1 Torr Gas Pump

49 DC-plasma reactor Anode Ionization Secondary electron emission Cathode

50 Low pressure glow discharge Aston dark space Crooks dark space Faraday dark space Anode dark space Cathode glow Negative glow Positive glow ~1mbar ~ 1kV

51 RF-plasma reactor Electrically isolated electrode surfaces OK. Pressure 1 mtorr - 1 Torr Gas Impedance matching Pump ~13.5 MHz

52 DC-bias Ion surplus V elect. Electron surplus ~ 0 t V DC-bias

53 DC-bias V 1 A 1 > A 2 4 V 1 / V 2 (A 2 / A 1 ) 4 1 A 2 V 2 A 1 Area A 1 Area A 2

54 Magnetically confined plasma E Magnetron, commonly used for sputter deposition sources.

55 Inductively coupled plasma (ICP) Process gas inlet Water Antenna RF-gen Z-match Electrostatic shield Water Exhausts

56 Electron cyclotron resonance (ECR) Microwave power 0 eb m 2.45 GHz B 2 e fm B T 90mT 31 T

57 Next Lecture Vacuum & Plasma systems for Dry etching