Development of a higher Si3N4 selectivity slurry · Ref) M. Pourbaix. Atlas of Electrochemical Equilibria in Aqueous Solutions. National Association of Corrosion p462 (1974) Polishing

1Doc.No. 780-1504-011FUJIMI 2015 - CMP Users Group Meeting Apr-2015

Development of

a higher Si3N4 selectivity slurry

Hisashi Takeda1, Koichi Sakabe2, Yukinobu Yoshizaki2, Tomohiko

Akatsuka2, and Kazumi Sugai2

1 Fujimi Corporation, 11200 SW Leveton Drive, OR 97062 USA2 Fujimi Incorporated, 1-8, Techno Plaza, Kakamigahara, Gifu 509-0109 Japan

2015 CMP Users Group Meeting on Apr 16th, 2015


Outline

1. Introduction

2. Key challenge for Si3N4 CMP

3. Polishing mechanism

4. Approach for improvement

5. Results

6. Summary


Introduction - Parameters for CMP slurry

Category Chemical Interaction Mechanical

ForceChemical reaction

(polished material-

chemical)

Solid-Liquid

Solid-Solid

Physical energy

（friction）

Strength Reaction potential

Hydrophobicity/

Hydrophilicity, electric

charge

Size, Shape, and

Material of abrasive

FrequencyChemical

concentrationSurface area

Number of abrasive

(surface area)

Interaction MechanicalChemical

The effects of chemical, mechanical, and those interactions are

considered in design for CMP slurry.


Key challenge for Si3N4 CMP

Si3N4 patterned wafer Ideal

Issue

Si3N4

Oxide or Poly-Si

Si3N4 polish

by Si3N4slurry

Loss of oxide or Poly-Si, known as erosion, is caused in finer L/S area after Si3N4

clearing. Oxide or Poly-Si removal rates have to be suppressed in a patterned wafer

maintaining Si3N4 removal rate, and edge over erosion (EOE) needs improved as well.

EOE Oxide or Poly-Si loss

L/S area

- Planarization

- No loss of Oxide or Poly-Si


Polishing mechanism for Si3N4

pH vs. zeta potential

Abrasive

- - -

- -

+ + + + + + +

Si3N4

Attraction force

- Abrasive is negatively charged pH-

independently while Si3N4 is positively

charged in a solution below pH 5.

- Si3N4 removal rate can be enhanced by

attraction force between Si3N4 and

abrasive.

Attraction

force

FUJIMI Si3N4 slurry is designed in acidic pH to enhance Si3N4 removal rate.

0

Negative

Positive


Silicon etching in wet system is driven by hydrolysis.

Since hydroxide ion (OH-) accelerates these reactions as a

catalyst, higher pH (higher OH- concentration) can enhance Si

etching rate, resulting in higher Si removal rate.Ref) M. Elwenspoek, and H. V. Jansen. Silicon

Micromachining. Cambridge University Press (2004)

Silicon wet etching can be described as following reactions.

H

H

SiSi

Si+ OH2

H

OH

SiSi

SiH2+

OH-

H

OH

SiSi

Si+ OH2

OH

OH

SiSi

SiH2+

OH-

OH

OH

SiSi

Si+ OH2

OH

OH

OH

SiSi

HSiOH

-

Soluble in water

-( i )

-( ii )

-( iii )

-( iv )

0 5 10 15pH

Etc

hin

g R

ate

(A

/min

)

0 5 10 15pH

Po

lish

ing

Ra

te (

A/m

in)

pH vs. Si etching rate

pH vs. Si polishing rate+ OH2

OH

OH

OHSi

OHOH

OH

OH

SiSi

HSi H

Si

HSi

OH-

Lower pH is better to reduce both Si etching and Si removal rate.

Polishing mechanism for Si

Perhydroxylated silanol


Higher solubility of SiO2 is observed in a solution above pH 10, contributing to

Oxide removal rate.

Ref) M. Pourbaix. Atlas of Electrochemical Equilibria in Aqueous Solutions.

National Association of Corrosion p462 (1974)

Polishing mechanism for Oxide

Lower pH helps minimize Oxide removal rate.

Influence of pH on the solubility of SiO2 at 25˚C


Approach for improvement - Poly-Si removal rate reduction

Design for Poly-Si removal rate reduction

- Acidic pH

- Poly-Si suppressor to make a protection layer on Poly-Si surface to

decrease mechanical force

Hydrophobic surfacePoly-Si

Chemical

Hydrophobic interaction

Hydrophobic surfacePoly-Si

Protection layer

Abrasive

A model for Poly-Si removal rate reduction

- Poly-Si film has hydrophobic surface.

- Hydrophobic chemical adsorbs on surface of Poly-Si film via hydrophobic

interaction to make a protection layer.


Approach for improvement - Oxide removal rate reduction

Design for Oxide removal rate reduction

- Acidic pH

- Smaller size of abrasive to decrease mechanical force

Oxide

Smaller abrasive

Lower mechanical force

A model for Oxide removal rate reduction

- Oxide removal rate depends on mechanical force.

- Smaller size of abrasive with lower mechanical force can decrease Oxide

removal rate.

Larger abrasive

Higher mechanical force


Approach for improvement - EOE

A model for EOE generation

Abrasive

Oxide or Poly-Si Si3N4

- Abrasives stumbling in a boundary accumulate locally.

- Accumulating abrasives give higher mechanical force locally in a boundary,

resulting in generating EOE.

Design for EOE improvement

- Smaller size of abrasive to decrease mechanical force

Higher mechanical force


Summary for our approaches

Item Approaches

Removal rate

reduction

Poly-Si

- Acidic pH

- Poly-Si suppressor to make a protection layer

on Poly-Si surface to decrease mechanical

force

Oxide

- Acidic pH

- Smaller size of abrasive to decrease

mechanical force

EOE improvement


mechanical force



Item Approaches

Removal rate

reduction

Poly-Si

- Acidic pH



force

Oxide

- Acidic pH


mechanical force

EOE improvement


mechanical force


Poly-Si suppressor screening

Chemical F suppressed Poly-Si removal rate keeping Si3N4 removal rate

and gave the highest Si3N4/Poly-Si selectivity.

Hydrophobic polymerPolymer with different molecular weight

(Chemical D ＜ Chemical E ＜ Chemical F)

* Normalized by Poly-Si or Si3N4 removal rate for No additive

61

100 100

122

5869

58

44

101 101

72

98 98 98



Item Approaches

Removal rate

reduction

Poly-Si

- Acidic pH



force

Oxide

- Acidic pH


mechanical force

EOE improvement


mechanical force


Item Slurry A Slurry B

Abrasive

TypeHigh purity

colloidal silica

High purity

colloidal silica

Normalized secondary

particle size1.0 0.6

Concentration Same concentration

pH Acidic pH

Poly-Si suppressor Chemical F

SelectivitySi3N4/Poly-Si 23 24

Si3N4/TEOS 5 25

Slurry B gave higher Si3N4 /TEOS

selectivity than slurry A.

⇒ Smaller size of abrasive

significantly reduced TEOS

removal rate.

Abrasive size effect on removal rates

* Normalized by Si3N4 removal rate for slurry A

105

184

100

7 4



Item Approaches

Removal rate

reduction

Poly-Si

- Acidic pH



force

Oxide

- Acidic pH


mechanical force

EOE improvement


mechanical force


Abrasive size effect on EOE

Evaluation procedure

TEOS

Measure EOE after Si3N4 clearing

(over polish 30%)

Test samples

Poly-Si Si3N4


Abrasive

TypeHigh purity

colloidal silica

High purity

colloidal silica




pH Acidic pH



Slurry B significantly improved

EOE (> 60%) compared to Slurry

A.

⇒ Smaller size of abrasive

reduced EOE, possibly due to

lower mechanical force.


Abrasive

TypeHigh purity

colloidal silica

High purity

colloidal silica




pH Acidic pH


Not observed

* Normalized by EOE amount for slurry A

Abrasive size effect on EOE

90% reduction 60% 80%

Test samples


Summary

◆ A chemical with hydrophobic character, which can form a

protection layer on Poly-Si surface via hydrophobic interaction,

decreased Poly-Si removal rate maintaining Si3N4 removal rate.

◆ Smaller size of abrasive significantly decreased TEOS removal

rate by reducing mechanical force while it maintained Si3N4 removal

rate.

◆ Smaller size of abrasive significantly improved EOE in the

patterned wafer, possibly due to lower mechanical force.

Documents

Development of a higher Si3N4 selectivity slurry · Ref) M. Pourbaix. Atlas of Electrochemical Equilibria in Aqueous Solutions. National Association of Corrosion p462 (1974) Polishing