Pad Conditioning Effects in Chemical Mechanical Polishingcmpconsulting.org/cariboost_files/pad_cond_20effets_20in_20cmp.pdf · AS Lawing NCCAVS CMPUG 5/5/2004 Pad Conditioning Effects

AS Lawing NCCAVS CMPUG 5/5/2004

Pad Conditioning Effects in Chemical Mechanical Polishing

Pad Conditioning Effects in Chemical Mechanical Polishing

A. Scott LawingRohm and Haas Electronic Materials CMP Technologies

Phoenix, AZ


IntroductionIntroduction

BackgroundPad Conditioning and Pad Surface Structure

Competing effects of wear and restoration Conditioner design and cutting behaviorVoid-filled vs. solid padsIntrinsic vs. induced structureContact areaProcess effects

Pad Conditioning and Polish RateEx situ responseIn situ responseRate maximization behaviorHydrodynamic effects

Pad Conditioning and PlanarizationPlanarization efficiencyDishing

Conclusions


BackgroundBackground

Polishing PadWafer

ConditionerSlurry

Pad

AsperityPad Pore

Diamond Crystal

Pad conditioning is the process of “dressing” the polishing padPad is contacted with an abrasive medium, typically a diamond abrasive discThe conditioning process involves the removal of a thin layer of pad material

Conditioning determines the intrinsic asperity structure of the padConditioning acts to maintain surface stability through the removal of worn surface material and restoration of the intrinsic structure


Extremes of Pad SurfacesExtremes of Pad Surfaces

Conditioning Dominated

Wafer Dominated

Abrasive wear results in truncation of asperity tips

Pad conditioning restores asperities to their intrinsic structure

-40 -20 0 20Pad Height (um)

Frequency (a.u.)

-40 -20 0 20Pad Height (um)

Frequency (a.u.)

Conditioning dominated surface exhibits the intrinsic structure imparted by a medium aggressive conditionerWafer dominated surfaces exhibits the truncated asperity structure (red component in the distribution at lower center) induced by polishing many wafers in the absence of conditioning

Intrinsic contribution to distribution in blue at lower center


Competing EffectsCompeting Effects

Pad Wear Rate

Con

ditio

ner C

ut R

ate

Over-conditioning

Severe GlazingIntrin

sic structure restored above horizontalMore stable

Less stable

Cut Rate = Wear Rate

The pad surface structure is determined by the balance between the competing effects of pad wear due to pad-wafer contact and pad surface restoration due to conditioning

The pad wear rate is a function of process conditions and the consumable setThe conditioner cut rate is a function of the conditioner design and the process conditions

· The intrinsic structure of the pad surface is also dependent on the conditioner cutting characteristics



In this example, conditioner cut rate is varied by manipulating the intrinsic aggressiveness of the pad conditioner through changes in diamond configuration

Pad wear rate is held constant by using an identical process condition and consumable set

The effect of increasing cut rate is to reduce the amount of steady-state glazing on the pad surface

-40 -30 -20 -10 0 10 20 30

-30 -20 -10 0 10 20

Pad Wear Rate

Con

ditio

ner C

ut R

ate

Increasing Conditioner Aggressiveness



In this example, pad wear rate is varied by manipulating wafer down-force

The point at far left represents a pad surface after break-in (i.e. zero pad wear)

The effect of increasing wear rate is to increase the amount of steady state glazing on the pad surface

Pad Wear Rate

Con

ditio

ner C

ut R

ate

Increasing Wafer

Down-force


Conditioner DesignConditioner Design

Pad surface structure is the product of many individual pad-diamond crystal interactions

Structure is driven by the cutting characteristics of diamond crystalsIntrinsic pad surface structure can be manipulated through conditioner design

Size, shape and density of diamonds on conditioner surface drive cutting characteristics and intrinsic surface structureMore aggressive diamonds tend to remove more pad per unit interaction and result in more “rough” surface

Diamond Crystal TypesSolid Pad Surface

DiamondFurrows


Conditioning and Intrinsic StructureConditioning and Intrinsic Structure

-50 -30 -10 10 30

Height (µm)

Freq

uenc

y (a

rb. u

nits

) HighMediumLow

50 -30 -10 10 30

Height (µm)

-50-40-30-20-10

010203040

Hei

ght (

µm)

-50-40-30-20-10

010203040

1 1.2 1.4 1.6 1.8 2Lateral Dimension (mm)

Hei

ght (

µm)

Aggressiveness Void-filled SolidVoid-filled

Solid

Pad surface height statistics are significantly affected by conditioning aggressiveness

Aggressiveness adjusted through changes in conditioning designIn solid pad, aggressiveness affects roughness

Solid pad has no inherent textureIntrinsic solid pad surface height distribution nearly Gaussian

In void-filled pad aggressiveness affects near surface regionRoughness of void-filled pad determined by filler materialConditioning superimposes additional roughness frequency


Intrinsic Structure vs. Pad WearIntrinsic Structure vs. Pad Wear

0102030405060708090

100 c) High

0102030405060708090

100 b) Medium

0102030405060708090

100 a) Low

D/h

(Dim

ensi

onle

ss)

Pad Height PDF Asperity Aspect Ratio (D/h)

-30 -20 -10 0 10

Freq

uenc

y (a

.u.)

Intrinsic ComponentWear ComponentOverall FitDepth of ContactData

-40 -30 -20 -10 0 10 20 30-30 -20 -10 0 10 20

a) Low b) Medium c) High

In steady state polishing, surface structure is determined by balance between restorative effect of conditioning and destructive effect of wear due to pad-wafer contact

Conditioner acts to restore intrinsic structure by removing worn surface materialLess aggressive conditioner characterized by lower cut-rate such that more wear is evident on the steady-state surface as aggressiveness is reduced

Asperity structure becomes more truncated (lower Diameter/heightratio) as aggressiveness is decreasedEstimated contact area increases as aggressiveness is decreased

Contact area estimated at 11.3 %, 7.7 % and 2.2 % for the low, medium and high aggressive conditioner respectively


Process EffectsProcess Effects

Increased conditioning down-force results in the removal of more pad material shifting the balance between wear and restoration

Below a critical down-force, polish rate will drop with decreasing down-forceAbove critical down-force rate saturates with increasing conditioner down-force

Intrinsic structure does not change significantly with increasing conditioner down-force

Once intrinsic structure is restored (wear component is accounted for) additional conditioning does not change the structure of the pad surface

0 5 10 15 20Conditioning Load (lb-hours)

Rou

ghne

ss

R = 2"R = 5"R = 8"

30 min @ 5 lb

+30 min @ 8 lb

+30 min @ 11 lb

+30 min @ 14 lb

"Broken-In"

20002200240026002800300032003400360038004000

0 5 10 15

Conditioning Down Force (lbs)

Pol

ish

Rat

e (Å

/min

)

Fumed, HighFumed, HighColloidal, MediumColloidal, Medium

Solid Pad Texture with Break-inConditioning DF Effect on Rate


Ex SituEx Situ Rate DecayRate Decay

1400

1600

1800

2000

2200

2400

2600

2800

3000

3200

0 5 10 15 20 25 30Time Without Conditioning(min)

Pol

ish

Rat

e (Å

/min

)

Medium Aggressive

High AggressiveFumed

Colloidal

Pad Height (tick = 10 mm)

Freq

uenc

y (a

rb.u

nits

)

Pad Height (tick = 10 µm)

Fumed - Medium Colloidal - Medium

Break-In 0 min3 min

15 min

31 min

Break-In0 min 31 min

Rate Data Pad Surface Data

Typical “logarithmic” decay of rate after conditioning is suspendedIncrease in asperity wear with time corresponds to decrease in polish rateColloidal slurries induce less significant asperity wear and result in less significant rate decayConditioning sets initial rate but has little influence after conditioning is suspended


Polish Rate, Slurry and Conditioning Polish Rate, Slurry and Conditioning

Steady-State Polishing with 100% In Situ Conditioning

0500

1000150020002500300035004000

0 5 10 15Wafer Pressure (psi)

Polis

h R

ate

(Å/m

in)

Modified Preston fitHigh fumedMedium fumedLow fumedMedium colloidalLow colloidal

0

500

1000

1500

2000

2500

3000

3500

0 20 40 60Linear Velocity (m/min)

Polis

h R

ate

(Å/m

in)

V = 30 m/min DF = 6 psi

Less aggressive conditioners exhibit more significant deviation from maximum polish rateColloidal slurries exhibit less significant deviation from maximum rate compared to fumed silica slurriesMore significant deviation from maximum rate with increased velocity as opposed to increased down-force

Low aggressive-fumed silica data set exhibits rate maximization with respect tovelocity


Polish Rate, Slurry and Conditioning Polish Rate, Slurry and Conditioning Steady State Pad Surfaces after In Situ Conditioning with Fumed Slurry

Z (tick mark = 10 µm)

Freq

uenc

y (a

.u.) Break-in

2 psi 10 psi 14 psi6 psi

Pad Height (tick = 10 µm)

Break-in 2 psi6 psi

10 psi14 psi

Z (tick mark = 10 µm)

Break-in

15 rpm 30 rpm 45 rpm 60 rpm

High Aggressive Low Aggressive

High aggressive conditioner exhibits the least surface wear and the maximum polish rateLow aggressive conditioner exhibits the most surface wear and the most significant polish rate deviationMore significant wear as a result of increase in down-force compared to increase in velocity

Increasing down-force driving abrasive wearThis contradicts trend of more significant rate reduction with increase in velocity compared to increase in rate

Colloidal slurries induce less significant asperity wear (not shown) corresponding to less significant deviation from maximum rate


Polish Rate and Hydrodynamics Polish Rate and Hydrodynamics

02468

10121416

1.E-05 1.E-04 1.E-03Sommerfeld Number So

Red

uced

Pol

ish

Rat

e(Å

/min

)/(ps

i*m/m

in)

02468

10121416

0.000 0.002 0.004 0.006Frictional Loading Factor µV/Pc (µm)

Red

uced

Pol

ish

Rat

e(Å

/min

)/(ps

i*m/m

in)

High fumed Series6Medium fumed Medium ColloidalLow fumed Low Colloidal σ

µ

co P

VS =Boundary

Lubrication PartialLubrication

Influence of partial lubrication with increasing Sommerfeld number explains the more significant rate decrease with increasing relative velocity

Hydrodynamic effects become more significant at low contact pressure (high contact area) and high velocity

Aggressively conditioned surfaces are dominated by solid contact


Estimation of PadEstimation of Pad--Wafer Contact AreaWafer Contact Area

-30 -20 -10 0 10Pad Height (µm)

Pad

Hei

ght F

requ

ency

1.E+00

1.E+01

1.E+02

1.E+03

1.E+04

1.E+05

1.E+06

1.E+07

1.E+08

Vol

ume

(µm

3 )

Pad Height in Contact

Pad Height not in Contact

Cumulative Volume

-30-25-20-15-10

-505

1015

0 0.5 1 1.5Scan Length (mm)

Pad

Hei

ght (

µm)

Using simple elastic model, equate volumetric displacement of ideal bulk material with the pad height probability distribution function (pdf) at a given applied pressure

Effective spring constant used was obtained by fitting patterned wafer polish data to elastic pad model (Lawing & Merchant ECS 2000)Similar length scale for pad compression

“Asperities” are defined as the portion of the pad lying above the contact plane (dotted line in the figures above)Estimate of contact coincides with maximum in the component of the distribution due to abrasive wear


Polish Rate and Contact AreaPolish Rate and Contact Area

1500

1700

1900

2100

2300

2500

2700

2900

3100

0 10 20 30

Time w/o conditioning (minutes)

Rat

e (Ǻ

/min

ute)

Colloidal in situ #1

Colloidal in situ #2

Fumed in situ

Colloidal ex situ

Polish rate is maximized with respect to estimated contact areaIncreasing ex situ rate trends support this observation

· Wear model demands that surface area increases as a function of time without conditioning

Above a critical point rate is proportional to contact area (applied pressure)Working model is that below a critical contact area rate is limited by absolute contact area and not contact pressure

1000

1500

2000

2500

3000

3500

Model Fractional Contact Area (Relative)

Pol

ish

Rat

e (Å

/min

)

Fumed #190% Colloidal/10% FumedColloidalFumed #2Asst. Conditioners in situ

Rate as a Function Estimated Contact Area Rate Trends with Various Initial Conditions

Increasing AggressivenessIncreasing Contact Area


-0.10

0.10.20.30.40.50.60.70.80.9

11.1

0 2000 4000 6000 8000 10000 12000

Amount Removed from Hi Area (Å)

Pla

nariz

atio

n E

ffici

ency

Low AggressiveHigh Aggressive

0

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 2000 4000 6000 8000 10000 12000Amount Removed from High Area (Å)

Ste

p H

eigh

t Rem

oved

(Å)

Low AggressiveHigh Aggressive

Conditioning and Planarization Conditioning and Planarization

Low aggressive conditioning results in more efficient planarization

More truncated asperity structure results in preferential polishing on high areas and more efficient step removal

RRhi

RRlo

Plo < Phi

Planarization Efficiency of 10% Density Features

( )Hi

LoHi

RRRRRRPE −

=

Step Height of 10% Density Features

Dotted lines indicate ideal planarization


Conditioning and DishingConditioning and Dishing

-0.5

0

0.5

1

0 0.3 0.6 0.9 1.2 1.5

Fractional Polish Time

Nor

mal

ized

Ste

p H

eigh

t

0 0.3 0.6 0.9 1.2 1.5

Fractional Polish Time

90%

10%

50% 5 µm

50% 50 µm

50% 500 µm

Over-polish

Dishing

Medium/High Aggressive Low Aggressive

Low aggressive conditioning results in less dishingMore truncated asperity structure penetrates less deeply into low lying areas


ConclusionsConclusions

Pad conditioning defines the intrinsic structure of the pad surface

Intrinsic pad structure can be adjusted through changes in conditioner design

Process and consumable variables adjust the balance between pad surface restoration due to conditioning and asperity wear due to pad-wafer contact

Conditioner design, conditioner and wafer down-force, relative linear velocity, conditioning time and slurry type are all significant factors in defining equilibrium pad surface structure

Pad surface structure and conditioning have a significant effect on CMP process response

Polish rate, planarization performance and hydrodynamic effects are heavily influenced by pad conditioning


Documents

Pad Conditioning Effects in Chemical Mechanical Polishingcmpconsulting.org/cariboost_files/pad_cond_20effets_20in_20cmp.pdf · AS Lawing NCCAVS CMPUG 5/5/2004 Pad Conditioning Effects