CMP C M P HEMICAL ECHANICAL LANARIZATION

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+ (Drain)P+ Gate(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+ (Drain)P+ Gate(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+ (Drain)P+ Gate(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 Removal in these regions Pad ==> non planar surface --> planar surface

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 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

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 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

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