factors affecting the sealing efficiency of low-k dielectric surface pores using successive he and...

FACTORS AFFECTING THE SEALING EFFICIENCY OF LOW-k DIELECTRIC SURFACE PORES USING

SUCCESSIVE He AND Ar/NH3 PLASMA TREATMENTS

Juline Shoeba) and Mark J. Kushnerb)

a) Department of Electrical and Computer EngineeringIowa State University, Ames, IA 50011

[email protected]

b) Department of Electrical Engineering and Computer ScienceUniversity of Michigan Ann Arbor, Ann Arbor, MI 48109

[email protected]

http://uigelz.eecs.umich.edu

October 2009*Work supported by Semiconductor Research Corporation JULINE_GEC09

Low-k Dielectrics

Modeling of Porous Low-k Sealing

Sealing Mechanism

Surface Site Activation by He plasma pre-treatment

Sealing by Ar/NH3 Treatment

Sealing Efficiency Dependence

Porosity and Interconnectivity

Treatment time and Pore Radius

Concluding Remarks

AGENDA

University of MichiganInstitute for Plasma Science & Engr.JULINE_GEC09

POROUS LOW-k DIELECTRIC

Metal interconnect lines in ICs run through dielectric insulators.

The capacitance of the insulator contributes to RC delays.

Porous oxides, such as C doped SiO2 (with CHn

lining pores) have a lower dielectric constant which reduces the RC delay.

Porosity is 0.5. Inter-connected pores open to surface may degrade k-value by reactions with plasma species.

Ref: http://www.necel.com/process/en/images/porous_low-k_e.gif


GOALS AND PREMISES OF SEALING MECHANISM To prevent the degradation of low-k

materials pores open to the surface should be sealed.

He plasma followed by NH3 plasma treatment has been shown to seal pores in SiCOH.

He+ and photons break Si-O bonds and remove H atom from CHn lining pores

Ref: A. M. Urbanowicz, M. R. Baklanov, J. Heijlen, Y. Travaly, and A. Cockburn, Electrochem. Solid-State Lett. 10, G76 (2007).

Plasma Treatment

Time (s)

Function

He 200 Surface

Activation

NH3 30

( Post-He)

Sealing

Subsequent NH3 exposure seals the pores by adsorption reactions forming C-N and Si-N bonds.

Reaction mechanisms and scaling laws for He-NH3 plasma sealing of porous SiCOH have been developed based on results from a computational investigation.

Experimental results by others were used to build the mechanism.


MODELING : LOW-k PORE SEALING

Hybrid Plasma Equipment Model (HPEM)

Plasma Chemistry Monte Carlo Module (PCMCM)

Monte Carlo Feature Profile Model (MCFPM)

Energy and angular

distributions for ions and

neutrals

He PLASMA

Porous Low-k

Coils

WaferSubstrate

Metal

Plasma

Ar/NH3 PLASMAS


MONTE CARLO FEATURE PROFILE MODEL (MCFPM)

The MCFPM resolves the surface topology on a 2D Cartesian mesh to predict etch profiles.

Each cell in the mesh has a material identity. (Cells are 4 x 4 ).

Gas phase species are represented by Monte Carlo pseuodoparticles.

Pseuodoparticles are launched towards the wafer with energies and angles sampled from the distributions obtained from the PCMCM.

Cells identities changed, removed, added for reactions, etching deposition.

PCMCM

Energy and angular distributions for ions

and neutrals

HPEM

MCFPM

Provides etch rateAnd predicts etch

profile


80 nm wide, 80 nm thick porous SiCOH.

Average pore radius: 0.8-1.1 nm

Process integration steps:

1. Ar/C4F8/O2 CCP: Etch 10:1 Trench

2. O2 ICP: Remove CFx polymer and PR.

3. He ICP: Activate Sites

4. NH3 ICP: Seal Pores

INITIAL LOW-k PROFILE, PROCESS INTEGRATION

University of MichiganInstitute for Plasma Science & Engr.

Hard Mask

Porous Low-kSiCOH

Si

JULINE_GEC09

AR/O2 PLASMAS: POLYMER REMOVAL AND CH3

GROUP ETCHING

Polymer Sputtering Ar+(g) + P(s) P*(s) + Ar(g)

Ar+(g) + P*(s) CF2(g) + Ar(g)

O+(g) + P(s) CF2(g) + O(g)

O2+(g) + P(s) CF2(g) + O2(g)

Polymer Etching O (g) + P(s) P*(s) + O (g)

O+ (g) + P*(s) COF(g) + O (g)

O2+ (g) + P*(s) COF(g) + O2(g)

CHn Group Etching M+(g) + CHn(s) CHn-1(s) + H(g) + M(g)

O(g) + CHn-1(s) CO(g) + Hn-1(g)

O+(g) + CHn-1(s) CO(g) + Hn-1(g)

O2+ (g) + CHn-1(s) CO(g) + Hn-1O(g)


SURFACE ACTIVATION IN He PLASMA

He+ and photons break Si-O bonds and remove H from CH3 groups.

Bond Breaking He+(g) + SiO2(s) SiO(s) + O(s) + He(g)

He+(g) + SiO(s) Si(s) + O(s) + He(g)

He+(g) + P(s) P*(s) + He(g)

hν + SiO2(s) SiO(s) + O(s)

hν + SiO(s) Si(s) + O(s)

Activation He+(g) + CHn(s) CHn-1(s) + H(g) + He(g)

hν + CHn-1(s) CHn-2(s) + H(g)

He+(g) + CHn(s) CHn-1(s) + H(g) + He(g)

hν + CHn-1(s) CHn-2(s) + H(g)

Reactive sites assist sealing in the subsequent Ar/NH3 treatment.


JULINE_GEC09

SEALING MECHANISM IN Ar/NH3 PLASMA

N/NHx species are adsorbed by activated sites forming Si-N and C-N bonds to seal pores.

Further Bond Breaking M+(g) + SiO2(s) SiO(s) + O(s) + M(g)

M+(g) + SiO(s) Si(s) + O(s) + M(g)

N/NHx Adsorption NHx(g) + SiOn(s) SiOnNHx(s)

NHx(g) + Si(s) SiNHx(s)

NHx(g) + CHn-1 (s) CHn-1NHx(s)

NHx(g) + P*(s) P(s) + NHx(s)

SiNHx-NHy/CNHx-NHy compounds seal the pores where end N are bonded to Si or C by Si-C/Si-N

NHy(g) + SiNHx(s) SiNHx-NHy(s)

NHy(g) + CHn-1NHx(s) CHn-1NHx-NHy(s)


JULINE_GEC09

CCP for trench etch.

Ar/C4F8/O2 = 80/15/5 40 mTorr, 300 sccm 10 MHz 5 kW

CFx polymers deposited on side-walls efficiently seal the open pores.

Depending on the application polymers:

Removed by O2 plasmas (followed by other pore sealing process)

Maintained for pore sealing

Ar/C4F8/O2 CCP TRENCH ETCH

Animation Slide-GIF

Hard Mask

Porous Low-kSiCOH

Si


He PLASMA PRE-TREATMENT

Ion density: 3.8 x 1010 cm-3.

Porous low-k exposed for up to 300 s to the plasma.

250V substrate bias assisted ablating H and Si-O bond breaking.

Conditions: He, 10 mTorr, 300 W ICP, 250V Bias


PORE-SEALING BY SUCCESSIVE He AND NH3/Ar TREATMENT

Surface pore sites are activated by 200s He plasma treatment.

Successive 30 s NH3 treatment seals the pores forming Si-N and Si-C bonds.

Initial Surface Pores


Animation Slide-GIF

He Plasma Site Activation

Ar/NH3 Plasma Pore Sealing

JULINE_GEC09

POLYMER REMOVAL: Ar/O2 PLASMA TREATMENT

Oxygen Ion density: 3.8 x 1010 cm-3.

56 s exposure to plasma.

250V substrate bias assisted in removal of polymer deep in trench.

Conditions: Ar/O2, 10 mTorr, 300 W ICP, 250V Bias


POLYMER REMOVAL FROM SIDEWALLS

Keeping the mask, the trench was exposed to Ar/O2 plasmas for polymer removal.

Ion bombardment activates/removes the polymers. CHn groups are also etched during this process due to the presence of oxygen species.

Neutral oxygen and Ar+ activate polymer while ions remove it by etching or sputtering reactions

Ar+(g) + P(s) P*(s) + Ar(g)

Ar+(g) + P*(s) CF2 + Ar(g)

O(g) + P(s) P*(s) + O (g)

O+(g) + P(s) CF2 + O (g)

O+(g) + P*(s) COF + O(g)


Animation Slide-GIF

Hard Mask

Porous Low-kSiCOH

Si

JULINE_GEC09

POLYMER REMOVAL & CH3 DEPLETION Polymer removal in

Ar/O2 plasmas may also remove CH3 groups as O diffuses into the porous network.

Ar/O2 plasma exposure sufficient to remove all polymer also results in CH3 removal.

An optimum strategy may be to leave traces of polymer in recesses (that is, shorter Ar/O2 plasma exposure) to prevent CH3 removal.


Hard Mask

Porous Low-kSiCOH

Si

JULINE_GEC09

SEALING: WITH POLYMER REMOVAL

Mask was removed to expose tops of the low-k material.

He plasma activated the tops and the polymer free side-walls of the trench.

Successive NH3 plasma exposure seals the top and side-walls through C-N and Si-N bonding.


Si

Animation Slide-GIF

Activation

Sealing

Activation Sealing

JULINE_GEC09

SEALING EFFICIENCY: PORE RADIUS

Sealing efficiency decreases with increasing pore size dropping below 75% > 1.0 nm.

Sidewall efficiency is less sensitive to pore radius due to some unremoved polymer.

C-N and Si-N are “first bonds.”

Sealing requires N-N bonding, which has limited extent.

Too large a gap prevents sealing.


SEALING: TREATMENT TIME DEPENDENCE

Without He plasma treatment, Ar/NH3 plasmas seal only 40% of pores.

Sealing efficiency increases with He treatment time for 200s, then saturates.

200 s treatment breaks nearly all side-wall/surface Si-O bonds and activates most CH3 groups.

Sealing efficiency of pores increases for 20s of Ar/NH3

treatment, then saturates – all dangling bonds on the surface are passivated.


SEALING EFFICIENCY: ASPECT RATIO

Sealing efficiency on sidewalls decreases with increasing aspect ratio.

Ability for ions to penetrate into features to activate sites decreases with larger aspect ratio.

Sealing improves with longer treatment but some sites are shadowed and will not be sealed.


CONCLUDING REMARKS

Integrated porous low-k material sealing was computationally investigated

Ar/C4F8/O2 Etch Ar/O2 Clean He Pretreatment Ar/NH3 sealing

Si-N and C-N bonds formed by adsorption on active sites followed by one N-N bond linking C or Si atoms from opposite pore walls.

Pore sealing efficiency is independent of porosity and interconnectivity, while dependent on both He and NH3 plasma treatment time.

The sealing efficiency degrades when with pore radius > 1 nm and with increasing aspect ratio.


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