Mask Cleaning Processes and Challenges

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Mask Cleaning Processes and

Challenges

Brian J. Grenon

Grenon Consulting, Inc.

92 Dunlop Way Colchester, VT 05446

Phone: 802-862-4551 Fax: 802-658-8952

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Mask Cleaning Processes

• Overview

• What are we trying to clean?

• Mask Cleaning Processes

• Possible Contamination Sources

• Analysis of Mask Cleaning Processes

• What is Required?

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Overview

• New mask cleaning processes are required to

remove and eliminate particles on photomasks

and to remove contaminants that can cause defect

formation at 248, 193, 157nm and EUV

lithography wavelengths.

• Current cleaning processes do not provide mask

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S ilic a G e l S u r f a c e

C h e m ic a lly A d s o r b e d W a t e r P h y s ic a lly A d s o r b e d W a t e r H S i H H H H H H H H O O O O O O O O O O O O O O S i S i S i S i S i S i H H O

What are we trying to clean? Quartz

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What are we trying to clean? Chromium

The Mask Substrate - Cr

• The ESCA analyses indicate that the sputter process deposits

chromium ions below the surface of the quartz.

• The non-conductive chromium presents a “grounding” challenge

from an ESD perspective - particularly in a “dry-environment”.

• Contrary to common belief the “chrome film” is neither

– Conductive

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What are we trying to clean? Chromium

1200 angstroms

Carbon contamination

Quartz Chromium film

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What are we trying to clean? Chromium

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Chrome MoSiON Quartz ion species

Depth

profile of

MoSiON

Substrate

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Mask Cleaning Processes

Mask Cleaning -The Reality

• All currently used mask cleaning processes leave

residues.

• The contaminants are either sulfates, ammonium

ions, organic compounds or digestive products of the

cleaning tool.

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Mask Cleaning Processes

Mask Cleaning Process Overview

There are essentially two fundamental mask cleaning processes in use:

– sulfuric acid/hydrogen peroxide (heated), usually followed by dilute ammonium hydroxide rinse

– Brush clean with detergents

All reticles are exposed to sulfuric acid/hydrogen peroxide either as resist

stripping process or final cleaning process. Whether ammonium hydroxide or detergents are used is often optional or arbitrary.

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Mask Cleaning Processes

Mask Cleaning Process Overview

What happens during these processes?

• Rinsing with Ammonium Hydroxide forms a surface layer of

ammonium silicate

O-_NH 4+

can reduce transmission

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Mask Cleaning Processes

Mask Cleaning Process Overview

What happens during these processes?

Cleaning with detergents leaves a thin mono-molecular film of the detergent on the mask surfaces. (Langmuir-Blodgett Film)

F H F

O _ + + +

CH₃(CH₂)_x - - SO₃- _Na+ _OR __{C - C - C - S - O K, Na, NH} 4

O or Aliphatic sodium sulfonate H F H

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Mask Cleaning Processes

Langmuir-Blodgett Films (detergents for contaminants on quartz or

chromium surface)

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Mask Cleaning Processes

Langmuir-Blodgett Films (detergents for contaminants on quartz or

chromium surface)

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Possible Contamination Sources

1. Chemical reactions of cleaning chemicals with quartz and absorber surfaces

(particularly chromium film).

2. Degradation products of cleaning tool materials, tank liners, tubing, filters….

3. Out-gassing from mask carriers and packaging materials.

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Possible Contamination Sources

Resist is Bulk Surface

Classical Organo-silylation of Resist

Hydroxy-silicon compound

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Possible Contamination Sources

Classical Organo-silylation of Resist

Resist is Bulk Surface

Hydroxy-silicon compound

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Possible Contamination Sources

O O O O H H H H H H H H H HH H O O O O H H H H H H H H H HH H

O- H O -H O -H O - H

In the case of optics contamination, the quartz becomes

the bulk surface. Silylation occurs.

Bulk Surface

Poly

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Plate #5, back,

negative ions

Plate #5, backside

positive ions

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Analysis of Mask Cleaning Processes

Plate #5, chrome,

negative ions

Plate #5, chrome

positive ions

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Analysis of Mask Cleaning Processes

Backside Contamination

20 40 60 80 100 120 140 160 180 200 # URS A De fe ct s

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Quartz - Clean Process vs. Residue (Positive Ions) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 NH_ 4 _Na _Si 118 149 Ca _K Si2C 5h15 O (PDM S) SiC3 H9 (P DMS) C_6H _5 C_4H _9 C_5H _11 C_2H_ 5 112 B Al 118.1 2 Mg % co n cen tr at io n

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Quartz - Clean Process vs. Residue (Negative Ions) 0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 NH _3 S F Cl O r Si % C onc e n tr at io n

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What is Required ?

Mask Cleaning Requirements

• All reticle surfaces must be free of environmentally-deposited contaminants

therefore, mask cleaning processes must have the capability to REMOVE:

– hydrocarbons

– detergents

– carbonized stains from repair processes

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What is Required ?

Mask Cleaning Requirements

• The cleaning processes must prevent the formation of surface modifying

contaminants:

– Silicic acid

– Langmuir-Blodgett films from detergents

– Ammonium silicates

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Summary and Conclusions

• Current mask cleaning processes are inadequate at current and future lithography

wavelengths.

• Detergents and wetting agents contaminate optical surfaces, sulfuric acid/peroxide chemistries react with current films.

• A clear understanding of the specific customers process, mask absorber and mask

manufacturing conditions are required in order to meet customer needs.

• Mask cleanliness is a systemic challenge not just cleaning, but requires handling, storage, packaging and environmental considerations.

• The current chromium film is the biggest problem with mask cleaning, it is not

stable to cleaning chemistries. Any new film should consider “cleanability” as a requirement.