Overview

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

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)

Need for CMP

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

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+ Gate(P+) (Drain)P+

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+ Gate(P+) (Drain)P+

Sample Image

Table Quill (Wafer Carrier) Animation

Schematic

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

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

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

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

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

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

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

CMP: Consumable

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

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

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

CMP: Removal Rate

 First order approximation  Preston Equation 

p

 Kp Preston Constant, depends on slurry,pad, temp, material to be removed etc  Correction for chemical dissolution

RR



K

0 

K V

1 

K PV

2  For the same system, different materials will be removed at different rates

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

Dishing Erosion Losses

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

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

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

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

CMP: Dummy, Slotting

Cu Cu

Oxide

Effective density of copper = 20% ( Oxide density = 80%) Cu Cu

Oxide

Cu Cu Effective density of copper = 40% ( Oxide density = 60%)

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

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

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

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

CMP: Clean

 Particles to be removed  Brush Scrub  Boundary Layer  Mega Sonic/ ultra sonic  Copper sometimes cleaned in dark!

CMP: Clean

CMP: New Nodes

  Abrasive Free, Fixed abrasive, Micelle Electro polish