Wet Etching and Cleaning - Surface

Raghavan

1NSF/SRC Engineering Research Center for Environmentally Benign Semiconductor Manufacturing

Wet Etching and Cleaning: Surface Considerations and Process Issues

Dr. Srini Raghavan

Dept. of Chemical and Environmental Engineering

University of Arizona

1999 Arizona Board of Regents for The University of Arizona

Raghavan


Outline

• Etching and cleaning solutions/processes

• Particle adhesion theory

• Surface charge and chemistry

• Contamination

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Etching and Cleaning Solutions

• HF Solutions

– Dilute HF (DHF) solutions - prepared by diluting 49% HF with dionized water

– Buffered HF solutions - prepared by mixing 49% HF and 40% NH4F in various proportions

• example: Buffered Oxide Etch (BOE) - patented form of buffered HF solution

– May contain surfactants for improving wettability of silicon and penetration of trenches containing hydrophobic base

• nonionic or anionic

• hydrocarbon or fluorocarbon

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Etch Rate of SiO2E

tch

Rat

e (Å

/min

)

at c

onst

ant t

emp.

Weight % HF0 100

Etc

h R

ate

(Å/m

in)

NH4F/HF Ratios

Tem

pera

ture

Etch rate of SiO2 increases with increasing weight % of HF in the etch solution, as well as higher ratios of NH4F buffer in BHF solutions. Etch rate also directly increases with increasing temperature.

More NH4F Less NH4F

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Etching and Cleaning Solutions (cont’d)

• Piranha

– H2SO4 (98%) and H2O2 (30%) in different ratios

– Used for removing organic contaminants and stripping photoresists

• Phosphoric acid (80%)

– Silicon nitride etch

• Nitric acid and HF

– Silicon etch

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Etching and Cleaning Solutions (cont’d)

• SC-2 (Standard Clean 2)

– HCl (73%), H2O2 (30%), dionized water

– Originally developed at a ratio of 1:1:5

– Used for removing metallic contaminants

– Dilute chemistries (compositions with less HCl and H2O2) are being actively considered

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Alkaline Cleaning Solutions

• SC-1 (Standard Clean 1)– NH4OH (28%), H2O2 (30%) and dionized water– Classic formulation is 1:1:5– Typically used at 70 C– Dilute formulations are becoming more popular

• Tetramethyl Ammonium Hydroxide (TMAH)– Example: Baker Clean

• TMAH (<10%), nonionic surfactant (<2%), pH regulators for a range of 8-10, and chelating/complexing agents

• Could possibly be used with H2O2 to replace SC1 and SC2 sequence

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Surfactants

• Alkyl phenoxy polyethylene oxide alcohol– Nonionic compounds– Alkyl group: 8 - 9 carbons– 9 - 10 ethylene oxide groups– Examples: NCW 601A (Wako Chemicals), Triton X-100 (Union Carbide)

• Alkyl phenoxy polyglycidols– Nonionic surfactants– Example: Olin Hunt Surfactant (OHSR)

• Fluorinated alkyl sulfonates – Anionic surfactants– Typically 8 carbon chain– Example: Fluorad FC-93 (3M)

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Surfactants (cont’d)

• Acetylenic alcohols

– Unsaturated triple bond in the structure

– Nonionic

– Example: Surfynol 61 (APCI)

• Betaines

– Zwitterionic in nature

– Used mostly in alkaline clean

– Example: Cocoamidopropyl betaine

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RCA Cleaning

Two-step wet cleaning process involving SC-1 and SC-2:

1) 1:1:5 NH4OH-H2O2-H2O at ~70 C

• Oxidizing ammoniacal solution

• Ammonia complexes many multivalent metal ions (e.g. CU++)

• Treatment leaves a thin “chemical” oxide

• Without H2O2, Si will suffer strong attach by NH4OH

2) 1:1:5 HCl-H2O2-H2O at ~70 C

• HCl removes alkali and transition metals (e.g. Fe)

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Problems with SC1 Clean

• Some metals (e.g. Al) are insoluble in this oxidizing, highly basic solution and tend to precipitate on the surface of Si wafers

• High Fe contamination of the wafer surface after a SC1 clean

• Rough surface after cleaning

– SC1 solutions with lower ammonia content (X:1:5, X<1) are being actively investigated

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Particle Removal During SC1 Clean

• H2O2 promotes the formation of an oxide

• NH4OH slowly etches the oxide– In a 1:1:5 SC1, the oxide etch rate is ~0.3 nm/min at 70 ºC.

At the alkaline pH value of SC1 solution, most surfaces are negatively charged. Hence, electrostatic repulsion between the removed particle and the oxide surface will prevent particle redeposition.

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Particle Removal Efficiency vs. Immersion TimeSC1 solutions w/ varying NH4OH concentration

Par

ticl

e R

emov

al E

ffic

ienc

y

0

1.0

Immersion Time

The efficiency curve is steeper with a higher concentration of NH4OH in the SC1 solution.

1:1:5 NH4OH:H2O2:H2O

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Standard Clean for Silicon

• Step 1 - Piranha/SPM– 4:1 H2SO4 (40%):H2O2 (30%) @ 90 C for 15 min

– Removes organic contaminants

• Step 2 - DI water rinse

• Step 3 - DHF– HF (2%) for 30 sec


• Step 5 (SC-1/APM)– 1:1:5 NH4OH (29%):H2O2 (30%) H2O at 70 C for 10 min

– removes particulate contaminants

– desorbs trace metals (Au, Ag, Cu, Ni, etc.)

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Standard Clean for Silicon (cont’d)


• Step 7 - SC-2– 1:1:5 HCl (30%):H2O2 (30%):H2O at 70 C for 10 min

– dissolves alkali ions and hydroxides of Al3+, Fe3+, Mg3+

– desorbs by complexing residual metals


• Step 9 - Spin rinse dry

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Adhesion of Particles to Surfaces

• Attractive Forces (AF)– van der Waals forces (short range)– Electrostatic (if the charge on the particles is opposite to the charge

on the surface (typically longer range)

• Repulsive Forces (RF)– Electrostatic (charge on the particle has the same sign as that on the

surface)– Steric forces (due to absorbed polymer layers on the surface of the

particles and wafer) (short range)

When AF > RF, particle deposition is favorable

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Particle Deposition Model

• Parameters controlling deposition

– zeta potential of wafers

– size and zeta potential of particles

– ionic strength and temperature of solution

• Transport of particles towards the wafer requires diffusion through a surface boundary layer (particles move along the flow in the solution and deposit by diffusion).

Along the flow

Diffusion layer

Sub

stra

te

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Surface Charge and Surface Electricity

• Development of surface charge

– Adsorption of H+ and OH- ions (oxides)

– Selective adsorption of positive or negative ions (hydrophobic materials)

– Ionization of surface groups (polymers such as nylon)

– Fixed charges in the matrix structure exposed due to counter ion release

• example: positively charged modified filters used in DI water purification

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Surface Charge Development on SiO2

Immersed in Aqueous Solutions

-O-Si...

-Si-O...

-Si-O...

-O-Si...

-O-Si-OH2+

-O-Si-OH2+

-O-Si-OH

-O-Si-O-

-O-Si-O-

-O-Si-OH

Bulk Solid Solution

Bulk Solid Solution

H+

OH-

Bulk SiO2

Aqueous Solution

Acidic Solutions (low pH)

Basic Solutions (high pH)H+ OH-

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Point of Zero Charge (PZC) of Materials

• PZC = the solution pH value at which the surface bears no net charge; i.e. surf = 0

0su

rf

(mic

roco

ulom

bs/c

m2 )

pH

20

-20

PZCMaterial pHPZC

SiO2 2-2.5

TiO2 5.5-6

Al2O3 ~9

Si ~4

Ny lon ~6

Development of + or - charge at a given pH depends on the nature of the metal-oxygen bond and the acid/base character of the surface MOH groups. Acidic oxides have a lower PZC than basic oxides.

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Surface Potential (o) and Zeta Potential ()

Solid Liquid

+ - - +

+ - + -

+ - - -

+ -

0

o

Surface Potential (o ):

• Not experimentally measurable

• Oxides immersed in aqueous soln’s, o = 0.059 (PZC-pH) volts

Zeta Potential ( ):

• Potential in the double layer at a short distance (typically the diameter of a hydrated counter ion) from the solid surface

• Experimentally measurable through electrokinetic techniques

• Decreases (more negative) with increasing pH

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Zeta Potential Electrophoretic Method

E K

= dielectric constant of liquid

= viscosity of liquid

K = constant dependent on particle size >> 1/ or << 1/

(1/ is the electrical double layer thickness)

• Technique useful for particles suspended in aqueous or non-aqueous media

E

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Zeta Potential from Streaming Potential

V

(+) and (-) chargesLIQUID IN

P

LIQUID OUT

• Generation of an electrical potential due to the flow of liquid past a charged surface

• Potential generated = streaming potential (Estr), which is related to zeta potential

4k EP

s

, , and k are viscosity, dielectric constant, and conductivity of solution; Es/P is the slope of the streaming potential vs. pressure drop.

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Streaming Potential CellSchematic Sketch - 6” wafers

BlockCell

LIQ OUTLIQ IN

Channel

Electrode LIQ IN LIQ OUT Electrode

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Zeta Potential vs. pHOxide Wafer - Activation Etch

0

(-)

Zet

a P

oten

tial

, mV

pH

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Contamination Mechanisms

• Liquid film draining (liquid/air interface)

• Bulk deposition from liquids

• Contaminant pick-up from air

A

L

Hydrophilic Hydrophobic

A

L(OR)

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Wet Etching and Cleaning - Surface