1. CMP C M P
HEMICAL
ECHANICAL
LANARIZATION

2. Overview Need for CMP Process Detailed process steps
Production: Relevant information
Dishing and Erosion
Integration issues
Post-CMP Clean
Review

3. Need for CMP Where is CMP used? To planarize oxide, tungsten or copper
Why planarize?
Oxide: need planar surface for photolithography
Copper: no other way to remove excess copper and form the wires (shape)
W: relatively easy method
Details: (next page)

4. Need for CMP Oxide: (or any other insulator/ dielectric)
Consider the following example: Process steps for ‘tapping’ the ‘gate’ and ‘source/ drains’
(Source)P+
(Drain)P+
Gate(P+)

5. Need for CMP 1. Deposit oxide (insulator/dielectric)
2. Coat photo resist and .... bring the mask to focus
Focus on which plane?
Top Plane
Bottom plane
(Source)P+
(Drain)P+
Gate(P+)

6. Need for CMP Solution: Deposit oxide and then PLANARIZE it
Planarization removes or decreases ‘topography’
It also removes some material from the ‘lower’ regions
(Source)P+
(Drain)P+
Gate(P+)

7. Sample Image
Table
Quill (Wafer Carrier)
Animation

8. Schematic

9. Background Used to polish ‘bare wafer’ IBM pioneered
Uses particles. Class 100 or Class 1000 clean room
Pad
soft pad, hard pad, stacked pad
hard pad with perforations, grooves
Quill
surface tension, vacuum
Slurry
abrasive, chemicals
Animation

10. Removal Mechanism
Copper
No Removal in this region
Removal in these regions
Pad
==> non planar surface --> planar surface

11. CMP: Pad Pad is softer than wafer whether it is Cu or oxide or W
Pure chemical dissolution is rare
A complexing or oxidizing film usually forms
Pure mechanical removal is possible, but not preferred
Scratches
==> shorts
Pad has asperities
helps in holding particles

12. CMP: Pad Common pads from Rodel Inc soft pads: SUBA IV
hard pads: IC1000
stacked pad: IC1400, IC1000/SUBA IV
Slurry Transportation
soft pad doesn’t have grooves
has ‘hair like’ surface
hard pads have grooves/perforations

13. CMP: Pad Grooves add mechanical flexibility
Soft pad, tend to planarize less
Tend to polish more
Scratch less
Pad conditioning
loss of asperities, roughness
use diamond particle conditioner between polishes
initial conditioning with few blank wafers
Sample conditioner surface
©Rodel Inc

14. CMP: Consumable Pad can be used for 150 polishes (for example)
Polish rate and within wafer non uniformity will go worse (WIWNU)
PETEOS- Plasma Enhanced TEOS based oxide

15. CMP: Consumable Mostly patented slurries (expensive)
Oxide polish with KOH or Ammonia slurry
silica (and sometimes ceria) abrasives
ceria has very high polish rate (considering its hardness)
Copper polish with acid based or neutral slurries
Peroxide, glycine, BTA (benzo triazole) inhibitor
silica or alumina abrasives
W polish with Ferric nitrate, peroxide, iodate slurries
alumina abrasives
In general, 1 to 3 min polish, 100 to 300 ml slurry used

16. CMP: Consumable Silica particles are small Removal rate is low
lower scratches
fumed or colloidal
mono dispersed or poly dispersed
surfactants (general)
Alumina
harder, more removal, more likelyhood of scratches
ceria
soft, some scratch, more removal (for silica polish)
Usually, removal rates in few hundred nm/min

17. CMP: Quill (Head) control
Pressure application is expected to be uniform on the wafer
To ensure it, ‘back pressure’ is applied on the wafer
vacuum used mainly in ‘picking up’ the wafer from the load station and after polish

18. CMP: Removal Rate
Both pad and quill (wafer holder) rotating at the same rate
Quill off-center
Relative velocity at all the locations will be same
Practically
quill will be ‘swept’ to the front and back (i.e. Edge and center of the pad)
quill usually rotates at a slightly different rpm
based on experimental optimization
Done to ensure uniformity
and to utilize maximum pad area

19. CMP: Removal Rate First order approximation Preston Equation
Kp Preston Constant, depends on slurry,pad, temp, material to be removed etc
Correction for chemical dissolution
For the same system, different materials will be removed at different rates

20. CMP: Removal Rate Proposed Mechanisms Fluid layer holds the pressure
Model not able to explain observation
good to explain within wafer non uniformity
Particles hold the wafer
Increase in pressure increases number of particles in contact
and not the friction coefficient
and not the indentation depth
Removal rate == # particles, indentation depth, shear length

21. Dishing Erosion Losses
Soft pad ==> more dishing. Erosion
Dishing/Erosion ==> higher resistance, more variation in resistance
Dishing/Erosion ==> DOF in the next level!

22. Dishing Erosion Losses
eg. Copper polish
Liner (barrier), oxide and copper
‘bulk’ removal sees only copper
liner and oxide ‘touch up’ CMP needed to remove liner
usually 2 step CMP
Selectivity
too high a selectivity leads to drastic dishing/erosion
controllable selectivity is the key
sometimes 1:1:1 slurry used

23. CMP: Dummy, Slotting
Copper CMP: Very wide line ==> very high dishing
Resistance increase too much
Due to erosion, some lines may disappear!
Simple schematics below

24. CMP: Dummy, Slotting Aerial Density important
Max density and min density in a ‘window’ is specified
Window size depends on characteristic planarization length
function of material, slurry, pad
can be mm for oxide, 100 um for Cu, W
M1 layout

25. CMP: Dummy, Slotting
Effective density of copper = 20% ( Oxide density = 80%)
Effective density of copper = 40% ( Oxide density = 60%)

26. CMP: Dummy, Slotting Some areas may not have any metal
Will cause non uniformity
“stretch” the process too much
Use “dummy fill” to bring the density to minimum level
Some lines will be too wide
slot them , for better manufacturability
These are examples are “design for manufacturability” or DFM
Both dummy and slotting are used in copper
Characterization with MIT / Sematech mask Set

27. CMP: Copper Single step vs Multi step (cost of equipment, slurry)
Barrier CMP
Ta harder, less reactive
Traces of barrier ==> short
More mechanical removal ==> scratches ==> short
More chemical removal ==> Cu loss
Need to strike a balance

28. CMP: Oxide Blank Oxide vs STI LOCOS vs STI
LOCOS: semi recessed/ fully recessed
Issues:
Dummy Fill / Reverse Active Mask
Dishing
Notch
Stress
Nitride Polish

29. CMP: W Well established Liner removal is easier (Ti/TiN)
lingering liner can be removed (for example) with plasma etch / RIE
for a short time (touch up RIE)
Need to have seemless dep (to avoid corrosion issues)
Good selectivity is achievable
W plug is never large
==> dishing not an issue
Erosion still an issue
not for resistance
however for DOF

30. CMP: Clean Particles to be removed Brush Scrub Boundary Layer
Mega Sonic/ ultra sonic
Copper sometimes cleaned in dark!

31. CMP: Clean

32. CMP: New Nodes
Abrasive Free, Fixed abrasive, Micelle
Electro polish