1. Lecture 6
Metallization

2. Two Types of Thin Film Dielectric Film (CVD Process) Oxide Nitride
Epitaxial silicon
Conducting Film (PVD Process)
Aluminum alloy
Ti, TiN
Silicides
Copper (CVD or electroplating)
Tungsten (Metal CVD)
Polysilicon (LPCVD)

3. Conducting Thin Film Applications
Front-End-Of-Line (FEOL)
Gate and electrodes
Polysilicon
Polycide
Back-End-Of-Line (BEOL)
Interconnection
Silicides
Barrier
ARC

4. Interconnection
Al-Cu alloy is commonly used material
Deep sub-micron metallization …. Copper

5. Interconnection
Copper Metallization

6. Silicides
To reduce contact resistance of metal / semiconductor
interface.
TiSi2, WSi2 and CoSi2 are commonly used materials
Self-aligned-silicide-process (SALICIDE)

7. Barrier Layer
To prevent aluminum diffusion into silicon (junction-spiking)
TiN is widely used barrier material

8. Barrier Layer
To prevent aluminum diffusion into silicon
TiN is widely used barrier material

9. ARC (anti reflective coating) to reduce “notching” during
photolithography process.
TiN is widely used material

10. Physical Vapor Deposition (PVD) Process
PVD works by vaporizing the solid materials, either by
heating or by sputtering, and re-condensing the vapor on
the substrate to form the solid thin film.

11. Physical Vapor Deposition (PVD) Process
Evaporation
Thermal Evaporation
Electron Beam Evaporation
Sputtering
Simple DC Sputtering
DC Magnetron Sputtering
DC Triode
RF Diode
RF Triode
RF / DC magnetron

12. Thermal Evaporation
In the early years of IC manufacturing, thermal evaporation
was widely used for aluminum deposition.
Aluminum is relatively easy to vaporized due to low melting
point (6600C).

13. System needs to be under high vacuum (~ 10-6 Torr)
Flowing large electric current through aluminum charge
heats it up by resistive heating.
Aluminum starts to vaporized
When aluminum vapor reaches the wafer surface, it
re-condenses and forms a thin aluminum film.

14. The deposition rate is mainly related to the heating power,
which controlled by the electric current.
The higher the current, the higher the deposition rate.
A significant trace amount of sodium, low deposition rate
and poor step coverage.
Difficult to precisely control the proper proportions for the
alloy films such as Al:Si, Al:Cu and Al:Cu:Si.
No longer used for metallization processes in VLSI and ULSI

15. Electron Beam Evaporation
A beam of electrons, typically with the energy about
10 keV and current up to several amperes, is directed at the
metal in a water-cooled crucible in vacuum chamber.
This process heats the metal to the evaporation temperature.
IR lamp is used to heat the wafer (improve step coverage).

16. Better step coverage (higher surface mobility due to
lamp heating)
Less sodium contamination (only part of aluminum charge is
vaporized.
Cannot match the quality of sputtering deposition, therefore
very rarely used in advanced semiconductor fab.

17. Sputtering The most commonly used PVD process for metallization.
Involves energetic ion bombardment, which physically
dislodge atoms or molecules from the solid metal surface,
and redeposit them on the substrate as thin metal film.
Argon is normally used as sputtering atom.

18. When power is applied between two electrodes under
low pressure, a free electron is accelerated by the electric field.
When it collides with Ar, another free electron is generated
(ionization collision). Ar becomes positively charged.
The free electron repeat this process to generate more free
electrons.

19. The positively charged Ar ions are accelerated toward a negatively
biased cathode, usually called target. The target plate is normally
made from the same metal that to be deposited on wafer.
When these energetic argon ions hit the target surface, atoms of the
target material are physically removed from the surface by the
momentum transfer of the impacting ions.

20. Sputtered-off atoms leave the target and travel inside the vacuum
chamber in the form of metal vapor.
Eventually, some of them reach the wafer surface, adsorb and become
so-called adatoms.
The adatoms migrate on the surface until they found nucleation sites
and rest there.
Other adatoms re-condense around the nucleation sites to form grain.
When the grains grow and meet with other grains, they form a
continuous poly-crystalline metal thin film on the wafer surface.

21. Simple DC Sputtering The simplest sputtering system.
Wafer is placed on on the grounded electrode and the target is the
negatively biased electrode, the cathode.
When a high-power DC voltage (several hundred volts) is applied,
the argon atoms are ionized by electric field.

22. These accelerate and bombard the target, then sputtered-off the
target material from the surface.

23. DC Magnetron Sputtering
The most popular method for PVD metallization process, because
it can achieve high deposition rate, good film uniformity, high film
quality, and easy process control.
High deposition rate allow single-wafer processing, which has
several advantages over batch-processing.

24. A rotating magnet is placed on top of metal target.
In a magnetic field, electrons will be constrained near magnetic
field line.
This gives electrons more chances for ionization collision.
Therefore, the magnetic field serves to increase plasma density and
cause more sputtering near the magnet.

25. CVD vs PVD CVD: Chemical reaction on the surface
PVD: No chemical reaction i.e. purely physical
CVD: Better step coverage (50-100%) and gap-fill capability
PVD: Poor step coverage (<15%) and gap-fill capability
CVD: Impurities in the film, lower conductivity, hard to deposit
alloy.
PVD: Purer deposited film, higher conductivity, easy to deposit

26. Basic Metallization Process
Burn-in Step
To condition the target before processing production wafers.
Native oxide and defects on the target were removed.
De-gas (Orient/Degas Chamber)
To orient the wafer.
Heat the wafer to drive-out gases and moisture.
Prevent out-gassing during the deposition process

27. Titanium Deposition Process
Normally deposited as welding layer prior to aluminum alloy
deposition (reduce contact resistance)
Titanium can trap oxygen and prevent it from bonding with
aluminum to form high resistivity aluminum oxide.
To produce larger grain size, wafer is normally heated to 3500C.
Collimated chamber is normally used in deep submicron IC
fabrication to achieve better titanium step coverage.

28. Collimator allows metal atoms to move in mainly in vertical direction
Significantly improve bottom step coverage

29. Titanium Nitride Deposition Process
TiN is widely used as ARC, glue and barrier layers.
The deposition normally uses a reactive sputtering process.
When nitrogen flows with argon into the process chamber, some
nitrogen molecules dissociate as a result of ionization collision.
Free nitrogen radicals are very reactive. They can react with
sputtered Ti atoms to form TiN and deposit it on the wafer surface.
They can also react with Ti target to form a thin TiN layer on the
target surface.
Argon bombardment could dislodge TiN from the target surface,
redeposited on the wafer surface.

31. Al-Cu alloy Deposition Process
Needs an ultrahigh baseline vacuum to achieve low film resistivity.
Standard process
Depositing aluminum alloy over tungsten plug, after Ti and TiN
wetting layer.
Normally deposited at 200 C, to achieve smaller grain size for
better line patterned etch.

32. Metal Thin Film Measurement
Thickness Measurement
Reflectivity
Sheet Resistance
Deposition Rate
Film Stress
Process Uniformity

33. Thickness Measurement
Metal films such as aluminum, Ti, TiN and copper are opaque films;
therefore, optical-based technique such as reflecto-spectrometry
cannot be used.
A destructive process is normally required to precisely measure
the actual film thickness.

34. Step height measurement (profilometer)
SEM / TEM
Four point probe – indirect measurement

35. Acoustic Measurement
Laser shot on thin film surface
Photo-detector measures reflected intensity
Thermal expansion causes a sound wave
Propagates and reflects at interface of different materials
Acoustic wave echoes back and forth
Film thickness can be calculated by;
d = Vs ∆t / 2
Vs – speed of sound
∆t - time between reflectivity peaks

36. Reflectivity Reflectivity change indicates a process drift.
A function of film grain size and surface smoothness
Larger grain size, lower reflectivity
Can be measured using Reflectometry (intensity of the
reflected beam of light).
Reflectivity measurement results usually use the relative
value to silicon.

37. Sheet Resistance Measurement
Most important characteristics of conducting film.
Widely used to rapidly monitor the deposition process uniformity
by indirectly measure the film thickness.
Four Point Probe is commonly used measurement tool

38. Deposition Rate

39. Film Stress Measurement
Stress is due to the mismatch between different materials
Compressive stress causes hillock, short between metal
Tensile stress causes crack, metal open, peel off
Two types of measurement
Contact – profilometer
Non-contact – capacitance measurement

40. Process Uniformity Max-min uniformity
(Max value – Min value) / 2 x average