1 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman1 Microelectronics Processing Physical Vapor Deposition

2 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman2 Issues  Vacuum basics  Vacuum Technology basics  Some vacuum systems  Evaporation  Sputter deposition  Metallization

3 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman3  Kinetic theory of gases  At relatively low pressures  Not too low temperatures  Molecules described as rigid balls  Constant velocity  Elastic collisions  Redistribution of kinetic energy Vacuum basics

4 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman4 Vacuum basics Ideal gas law PV = nRT (1) P - pressure V - volume T - temperature n - number of moles R - gas constant

5 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman5 Vacuum basics Ideal gas law PV=nRT (1) P - pressure V - volume T - temperature N - number of moles R - gas constant R=k B N AV k B = 1.38x J/molec K N AV = 6x10 23 molec/mol 1

6 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman6 Vacuum basics The average molecular speed v av = (8k B T/  M) 1/2 (2) M – molecular weight The average time between collisions t av = 1/(2 1/2  Nd 2 v av ) (3) d – molecular diameter N – number density of molecules (per unit volume)

7 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman7 Vacuum basics The average distance between collisions (the Mean Free Path - ) = 1/(2 1/2  Nd 2 ) (4) Thus, if is larger than the dimension of the chamber, the particles will travel without collisions! Thus, if is larger than the dimension of the chamber, the particles will travel without collisions!

8 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman8 Vacuum basics In order to increase the mean free path we have to reduce N reduce P

9 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman9 Vacuum basics

10 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman10 Vacuum basics

11 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman11 Vacuum basics

12 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman12 Vacuum basics - Conclusions - I When the gas flows through a system with Dimensions V 1/3 >>, flow is viscous. When P is lowered, increases If exceeds system dimensions, the flow becomes molecular. Reducing P Molecular flow No collision between source and target (less contamination)

13 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman13 Vacuum basics - Conclusions - II Surface and film contamination is determined by the background pressure of the chamber, or by the ratio between the partial pressure of the desired species and that of the background molecules in the chamber. Example: what is the value of P for flow through a tube 5 cm in diameter to be molecular?

14 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman14 Vacuum Technology basics S – Pumping speed (volume per unit time, l/sec) Q – Flow to the pump (mass per unit time, ) Q = PS The lowest pressure achievable by a given pump: P = Q/S

15 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman15 Vacuum Technology basics P = Q/S This equation holds when gas is inserted to the chamber A leak: Gas flows intentionally into the chamber Imperfect seal (air leaks in from the outside) Outgasing from chamber walls

16 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman16 Vacuum Technology basics The effect of tubes, orifices, restrictions are measured in terms of the conductance C = Q/  P  P = P 1 –P 2 Two obstacles in series, C 1 and C 2 : C -1 = C C 2 -1 Tubes, constrictions, valves, and other components reduce system conductance.

17 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman17 Typical Vacuum System

18 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman18 Rotary pumps

19 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman19 Rotary pump with gas ballast

20 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman20 Root pump

21 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman21 Sorption pumps ~800m 2 /cm 3

22 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman22 High vacuum pumps: Diffusion pump Gaede 1913

23 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman23 Diffusion pump disadvantage Diffusion pumps are based on boiling oil. Oil can be transported to the chamber by back-streaming, causing contamination to the processed semiconductor. Partial solutions : 1.Use low vapor pressure oil 2.Insert a cold trap over the pump

24 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman24 High vacuum pumps: cryogenic pump Cryogenic Pump Turbo Molecular Pump High pumping speed High throughput for H 2 O Momentary power loss is detrimental. Large capacity Need regeneration

25 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman25 High vacuum pumps: turbo-molecular pump Cryogenic Pump Turbo Molecular Pump High speed rotation blades. Require backing pump. Produces vibration.

26 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman26 Speed versus pressure comparison – clarify pump choice Pumping speed(l/sec) Pressure (torr)

27 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman27 Evaluation of vacuum pumps

28 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman28 Measurement of Pressure and Leak Detection

29 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman29 Pressure Gauges

30 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman30 Mechanical Gauges Bourdon Gauge Diaphragm Gauge

31 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman31 Reference point in measuring Pressure

32 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman32 A mechanical Gauge Capacitance gauge: the deflection of a membrane Is measured as a change in capacitance

33 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman33 Thermocouple Gauge  A filament is heated by a constant DC current (20 to 200mA)  The filament is exposed to the gas  The heat in the filament is transported to the gas  As the pressure decreases, the temperature increases  We measure the temperature of the filament (by a thermocouple)  The pressure is obtained by an output voltage (V<20 mV DC)  Pressure range: 2 Torr – Torr  With an industrial D/A converter, range extended to 10 3 – Torr  Pirani sensor: the filament is a thermal resistor

34 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman34 Hot Cathode ion Gauge Refined by Bayard-Alpert – 1950 Range: – Torr

35 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman35 Cold cathode Vacuum Gauge

36 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman36 Leaks and leak testing

37 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman37 Places likely to leak O-rings seals metal gaskets electrical feedthroughs shut-off valves with through leaks, internal welds/brazes on utility pipes chamber welds

38 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman38 Reasons for poor vacuum high vacuum pump failing high solvent concentration in pump oil poor quality oil in a mechanical pump sample outgassing outgassing from new chamber fixturing increased vapor pressure due to heating venting to air during humid weather helium permeation through rubber or plastic components

39 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman39 Residual Gas Analysis - Principle Mass separation Quadrupole mass spectrometer

40 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman40 Residual Gas Analysis - Principle Lorentz force lawLorentz force law F =q(E+vXB( Newton’s 2 nd law F=ma Mass spectrometry m/q=E+vXB

41 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman41 Physical Vapor Deposition: 1. Evaporation Schematic diagram of evaporation equipment Types of evaporation sources: (a)Filament evaporation; (b)Electron beam source.

42 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman42 Evaporation geometry for system using planetary substrate hoder

43 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman43 Source evaporation rate The evaporation rate in g/sec is estimated to be Where A S the source area in cm 2, m is the gram-molecular mass, T the temperature in K, and P e the vapor pressure in torr. Problem alloying!

44 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman44 Evaporation – Why is not being used in present day Si technology? The Contact Hole Filling Problem: The atoms are coming in straight line from a “point source”, i.e. their incidence on the sample is nearly perpendicular. The hole filling problem looks like this: The actual situation may look slightly better, due to a small sticking coefficient, which is the ratio of the number of atoms that “stick” on the surface relative to the number of incident atoms: S C = F reacted /F incident. But this effect is usually not so dramatic.

45 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman45 DC Sputter deposition Schematic diagram of DC powered sputter deposition equipment (glow discharge) ground -V (DC) Vacuum Cathode shield ~ V

46 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman46 DC Sputter system Plasma structure and voltage distribution Al, W, Ti, silicides, other metals

47 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman47 Basic properties of plasma

48 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman48 Basic properties of plasma

49 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman49 Processes in sputter deposition

50 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman50 Sputtering yield in a DC system

51 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman51 The contact hole filling problem Fort aspect ratios A = w/d smaller than approximately 1, the layer at the edge of the contact hole will become unacceptably thin or will even be non-existing - the contact to the underlying structure is not established.

52 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman52 The contact hole filling problem – the real thing The real thing together with one way to overcome the problem is shown above (the example is from the 4Mbit DRAM generation, around 1988).

53 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman53 Reactive sputter deposition

54 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman54 RF sputter deposition

55 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman55 RF sputter deposition V 1 /V 2 = (A 2 /A 1 ) m where A 1, and A 2 are the respective electrode areas. m was found experimentally to be between 1 and 2.

56 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman56 Magnetron sputter deposition

57 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman57 Deposition methods for thin fims

58 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman58 Comparison of various layer deposition processes

59 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman59 Metals – Can you guess who is the winner? Desired PropertyMaterials not meeting requirement Very good conductivityAll but Ag, Cu High eutectic temperature with Si (> 800 o C would be good) Au, Pd, Al, Mg Low diffusivity in SiCu, Ni, Li Low oxidation rate; stable oxideRefr. Metals, Mg, Fe, Cu, Ag High melting pointAl, Mg, Cu Minimal interaction with Si substratePt, Pd, Rh, V, Ni, Mo, Cr (form silicides easily) Minimal interaction with poly SiSame as above No interaction with SiO 2 Hf, Zr, Ti, Ta, Nb, V. Mg, Al But must stick well to SiO 2 ? Must also comply with other substrates, e.g. TiN ? Chemical stability, especially in HF environments Fe, Co, Ni, Cu, Mg, Al Easy structuringPt, Pd, Ni, Co, Au Electromigration resistantAl, Cu.... and many more,...

60 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman60 Metalization – the winner (so far …)

61 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman61 Metallization

62 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman62 Metallization

63 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman63 Spiking

64 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman64 Electromigration

65 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman65 Deposition processes summary A series of materials are deposited during processing in thin layers to provide wiring, insulation, and contacts. The main techniques used are physical vapor deposition (PVD) and chemical vapor deposition (CVD). More recently, electro-plating and spin-on processes have been developed for special applications. Schematic cross section of a 2 level IC 1.Oxide patterned using hard mask, 2.Polysilicon gate electrode, 3.PMD, 4.Metal 1, 5.IMD, 6.Metal 2, 7.Passivation layer.

66 Microelectronics Processing Course - J. Salzman – Fall 2006 Microelectronics processing - E. Finkman66 PVD cluster tool