1. (Thermal Evaporation)
Sputtering
Sputtering is a form of PVD
PVD
Resistance-Heated
(Thermal Evaporation)
Sputtering
E-beam Evaporation

2. Sputtering References
R.A. Powell and S. Rossnagel, “PVD for Microelectronics: Sputter Deposition Applied to Semiconductor Manufacturing” (Academic Press, 1999)
D.M. Manos and D.L. Flamm, “Plasma Etching: An Introduction” (Academic Press, 1989)
W.N.G. Hitchon, “Plasma Processes for Semiconductor Fabrication” (Cambridge University Press, 1999)
J.L. Vossen and W. Kern, “Thin Film Processes” (Academic Press, 1991)
M. Konuma, “Film Deposition by Plasma Techniques” (Springer-Verlag, 1992)
D.M. Dobkin and M.K. Zuraw, “Principles of Chemical Vapour Deposition” (Kluwer Academic Publishers, 2003)
J.E. Mahan, “Physical Vapour Deposition of Thin Films” (John-Wiley & Sons, 2000)
M. Ohring, “The Materials Science of Thin Films” (Academic Press, 1992)

3. A target is bombarded with inert energetic ions, typically Ar+
Sputtering Process
A target is bombarded with inert energetic ions, typically Ar+
Atoms at the surface of the target are knocked loose by a collision cascade process analogous to atomic billiards
sputtered atom
incident ion

4. Sputtering Yield
Atoms are sputtered from the target with a certain probability, Y, called the sputtering yield
Y = # sputtered (ejected) target atoms
# incident ions
[Y] = atoms/ion

5. Typical sputtering yields are between 0.1 and 4
From Ohring, Table 3-4, p. 113

6. Sputtering for Film Deposition
The sputtered atoms may be deposited (condensed) on a substrate surface for film deposition
Target = source material
Substrate for film deposition

7. Sputtering for Etching
A sample can be placed on the target for etching (plasma-etching, dry-etching, reactive ion etching)
Target = sample to be etched

8. A plasma is used as the source of ions
Sputtering
A plasma is used as the source of ions
Other plasma-related processes: PE-CVD, SIMS
modified from Mahan, Fig. VII.1, p. 200

9. Sputtering
There exist different means of creating the plasma
Sputtering DC RF
Magnetron
Sputtering
Microwave
(ECR)

10. DC Sputtering: Gas Conditions
A gas is admitted into a chamber filling the space between two electrodes
Typically an inert gas is used like Ar, Ne, Kr, and Xe
Ar is most commonly used
The gas pressure ~ 0.1 – 1 Torr
from Mahan, Fig. VI.2, p. 155

11. DC Sputtering: Anode/Cathode
To create a plasma, a dc voltage
(~ 100’s to 1000’s Volts) is applied between two electrodes
Cathode-anode separation ~ few cm’s)
The cathode is negatively biased and attracts positive ions from the plasma
from Mahan, Fig. VI.2, p. 155

12. DC Sputtering
etching
deposition
from Vossen (1991), Fig. 7, p. 24

13. Plasma Creation
electrons
ions
anode
cathode - +
photoemission
ionization
Cosmic rays or uv light causes photoemission from the cathode and ionization of the neutral gas atoms

14. Electrons are accelerated toward the anode
Plasma Creation
anode
cathode - +
Electrons are accelerated toward the anode
Ions are accelerated toward the cathode
 current flow

15. Electrons may collide with neutral gas atoms causing ionization
Plasma Creation
anode
cathode - +
Electrons may collide with neutral gas atoms causing ionization

16. Ions striking the cathode produce secondary electrons
Plasma Creation
anode
cathode - +
Ions striking the cathode produce secondary electrons

17. Plasma Creation
anode
cathode - +
Secondary electrons accelerate toward anode and collide with gas atoms causing ionization
e- + Ar  Ar+ + 2e-

18. A multiplication process occurs forming a plasma
Plasma Creation
anode
cathode - +
A multiplication process occurs forming a plasma
This is known as breakdown

19. I-V Characteristic Ohmic Region
from Mahan, Fig. VI.14, p. 185
Ohmic Region
Cosmic rays or uv light causes photoemission of the cathode or ionization of the neutral gas atoms

20. Saturation Region (Region A)
I-V Characteristic
from Mahan, Fig. VI.14, p. 185
Saturation Region (Region A)
All the available free electrons are collected at the anode as quickly as they are created
I = constant

21. Townsend Discharge (Region B)
I-V Characteristic
Townsend Discharge (Region B)
The electrons are accelerated to sufficient energy to cause ionization of the neutral gas atoms
from Mahan, Fig. VI.14, p. 185

22. Secondary electron emission produces multiplication process
I-V Characteristic
Breakdown (Region C)
Secondary electron emission produces multiplication process
from Mahan, Fig. VI.14, p. 185

23. Plasma is created near edges of cathode where E-field is highest
I-V Characteristic
Normal Glow (Region D)
Plasma is created near edges of cathode where E-field is highest
The current increases at constant voltage as the plasma extends over the entire cathode surface
from Mahan, Fig. VI.14, p. 185

24. Abnormal Glow (Region E)
I-V Characteristic
Abnormal Glow (Region E)
Plasma is now extended across entire cathode surface
For further increases in current, the dc applied voltage must increase
This is the region where most sputtering processes occur since it gives the highest sputtering rate
from Mahan, Fig. VI.14, p. 185

25. I-V Characteristic Arc (Region F)
If the current is increased further, the cathode becomes heated which will either melt or, if the cathode material is refractory, will result in thermionic emission of electrons
from Mahan, Fig. VI.14, p. 185

26. Collisions
2 ways for plasma particles (electrons, ions) to interact and lose energy
Collisions
Elastic
Inelastic

27. Elastic Collisions M0, E0 Mr Mr, Er Recoil angle, q M0, E0
Conserves energy and momentum
Does not result in any internal excitations of the gas atoms or molecules (e.g., vibration, rotation, electronic)
M0, E0
Before
collision Mr
Mr, Er
After
collision
Recoil angle, q
M0, E0

28. Elastic Collisions
Mr, Er
M0, E0
Recoil angle, q Mr
Conservation of energy and momentum
 derive Er (M0, Mr, E0, q)

29. Elastic Collisions
Mr, Er
M0, E0
Recoil angle, q Mr
Er = 4 E0 M0Mrcos2q/(M0+Mr)2
M0 = mass of incident particle
Mr = mass of struck particle initially
assumed to be stationary
E0 = energy of incident particle
Er = energy of recoiling particle (Mr) initially assumed to be stationary
q = recoil angle

30. Heavy Particle Collisions
Er = 4 E0 M0Mrcos2q/(M0+Mr)2
For collisions among ions and neutrals in the plasma, M0 ~ Mr
Er = E0 cos2q
Energy transfer is efficient among ions and neutrals
Ions and neutrals will “thermalize” to the same temperature

31. Elastic Collisions
The ions do not acquire kinetic energy from the applied field as readily as do the electrons
v = mE
mobility, m = et/m
mi >> me
Typical ion or neutral atom energies are 0.03 – 0.1 eV ( K)
Ions & neutrals have insufficient energy to cause ionization in the gas
So where do the ions come from ?

32. Light-Heavy Collisions
electron
M0, E0
Mr, Er q
neutral gas particle Mr
For an electron-gas atom collision:
M0 << Mr
Therefore,
Er ~ 4 E0 (M0/Mr) cos2q

33. Elastic Collisions
Er ~ 4 E0 (M0/Mr) cos2q
The electron will transfer a maximum energy of
Er = 4 (M0/Mr) E0
e.g., for an electron colliding with an Ar atom, Er/E0 < 1.4 x 10-4
Very little energy is transferred in an elastic collision from the electron to the ions or neutrals in the plasma

34. Elastic Collisions
But electrons acquire a much higher kinetic energy (and temperature) from the applied field compared to the ions or neutrals due to their much smaller mass
Typical electron energy is 1-10 eV ( K)
Recall that typical ion or neutral atom energies are 0.03 – 0.1 eV ( K)
Electron and ion temperatures are not equal (not in thermodynamic equilibrium)

35. Elastic Collisions
Electrons and ions/neutrals may each be described by a separate M-B distribution each with their own temperatures, Te and Ti
from Manos, Fig. 13, p. 206

36. Inelastic Collisions
Ions have insufficient energy to cause ionization in the gas
Very little energy is transferred in an elastic collision from electrons to the ions or neutrals in the plasma
Inelastic collisions must be responsible for producing the plasma

37. Inelastic Collisions
Mr, Er, DU
M0, E0 q Mr
For inelastic collisions :
DU = E0Mrcos2q / (M0 + Mr)
DU = change in internal energy of the struck particle (vibrational, rotational, electronic excitations)

38. Inelastic Collisions
DU / E0 = Mrcos2q / (M0 + Mr)
For an inelastic collision between an electron and a neutral, M0 << Mr:
DU = E0cos2q

39. Inelastic Collisions
DU = E0cos2q
DU ~ E0 for forward scattering (q = 0)
Practically all of the electron energy can be imparted to the atom or ion in an inelastic collision

40. Inelastic Collisions
The energy transfer may vary from less than 0.1 eV (for rotational excitation of molecules) to more than 10 eV (for ionization)
from Dunlap, Fig. 8.3, p. 194

41. Townsend Discharge
As the voltage is increased, electrons may gain sufficient energy from the applied field to ionize a gas atom in an inelastic collision :
e- + Ar  Ar+ + 2e-
At this point, ions are created for the first time (Townsend discharge)
from Mahan, Fig. VI.14, p. 185

42. Ionization Potential (eV)
Townsend Discharge
The electron energy must exceed the ionization energy of the gas atoms
Neutral
Ion
Ionization Potential (eV) Ar
Ar+
15.8
Ar++
27.6 F F+
17.4 H H+
13.6 He
He+
24.6 N N+
14.5 O O+ Si
Si+
8.1 N2
N2+
15.6 O2
O2+
12.2
SiH4
SiH4+

43. Townsend Discharge
Typical ionization energies (10-15 eV) are greater than the mean electron energy (1-10 eV)
Therefore, only electrons in the high energy tail of the M-B distribution will contribute to ionization
from Manos, Fig. 13, p. 206

44. The minimum in the Paschen curve is around 1 Torr-cm
The breakdown voltage required to form the plasma is described by the Paschen curve
The minimum in the Paschen curve is around 1 Torr-cm
from Powell, Fig. 3.2, p. 53

45. Paschen Curve
Low pressure or small anode-cathode spacing: electrons can undergo only a small number of collisions in traversing the applied field; not enough ionizing collisions take place to sustain the plasma; a larger voltage is required to sustain the plasma
from Powell, Fig. 3.2, p. 53

46. Paschen Curve
High pressure: mean free path of electrons is reduced; electrons cannot gain sufficient acceleration (i.e., sufficient energy) between collisions to cause ionization
from Powell, Fig. 3.2, p. 53

47. Paschen Curve
Due to the differences in ionization energy and ionization cross-sections for different gases, the Paschen curve will have slightly different shapes for different gases
from Konuma, Fig. 3.1, p. 50

48. Typical dc Plasma Characteristics
Plasma species
Neutral atoms (or molecules depending on the gas), ions, electrons, radicals, and excited atoms
Plasma density
ni = ne ~ cm-3
nn ~ 3x1015 cm-3
Degree of ionization
ni, ne << nn
ne / nn ~ 10-5
Plasma temperature
Te ~ K (1 – 10 eV)
Ti , Tn ~ K (0.03 – 0.1 eV)

49. Typical Plasma CharacteristicsLD
from Manos, Fig. 3, p. 191

50. “Cold” Plasma
Plasma temperature
Te ~ K (1 – 10 eV)
Ti , Tn ~ K (0.03 – 0.1 eV)
Te >> Ti, Tn but ne, ni << nn
The plasma is essentially at the neutral gas temperature which is quite low
“cold” plasma

51. “Glow” Discharge
Within the plasma, excited atoms can relax to lower energy states causing the emission of light with a wavelength that is characteristic of the gas used
“glow discharge”
from Dunlap, Table 8.4, p. 195

52. “Glow” Discharge
from Mahan, colorplate VI.18