Characterization of complex inter-layer dielectric (ILD) stack by spectroscopic ellipsometry in nanoelectronic manufacturing

Dmitriy V. Likhachev

GLOBALFOUNDRIES, Dresden

Agenda • Interlayer dielectrics (ILDs) in an integrated circuit

• Anatomy of the ILD stack

– Interfaces between dielectrics

– Test pad film stack

• Current problem: inter-layer correlation effects

• Experimental details

– Sample preparation

– Determination of the thin films optical properties

• Determination of the films thicknesses in multi-layer ILD film stack

• Calculated uncertainty of ellipsometric analysis for all dielectric films from complete ILD stack

• Maximum differences for the TEOS and BLOK average thicknesses

• Conclusions

GLOBALFOUNDRIES Confidential 2

Interlayer dielectrics (ILDs) in an integrated circuit

3

Cross-section of 64-bit high-performance microprocessor chip (Source: IBM, http://www-

03.ibm.com/press/us/en/attachment/19014.wss?fileId=ATTACH_FILE2&fileName=1057.jpg)

Cu wiring

Interlayer

dielectric

Two types of the interlayer

dielectrics:

• An intermetal dielectric – the

isolation between metal

lines in the same level;

• An interlevel dielectric – the

isolation between two metal

levels.

Anatomy of the ILD stack

4

Interfaces between dielectrics

• Interlayer dielectric: low-κ dielectric

materials with dielectric constant κ in the

range of 2.5–3.0 (ordinary SiO2 has the κ-

value of ~4).

• Barrier/etch stop: low-κ dielectric barrier/

etch stop film (BLOk, Barrier LOw κ): lower

dielectric constant κ (~3–5 for SiCxNyHz

vs. ~6–9 for SiNxHy).

Measurements of inter-layer dielectric films

thicknesses and optical properties in the

back-end-of-line (BEOL) process always

have been an important task for interconnect

metrology in semiconductor development and

manufacturing since those properties directly

influence device performance. (Source: C.K. Goldberg and V.S. Wang, Compatibilities of dielectric films, in:

Interlayer Dielectrics for Semiconductor Technologies (Elsevier, 2003), p.79)

Anatomy of the ILD stack

5

Test pad film stack

Process engineers face many challenges in BEOL in-line process monitoring

and control. Multi-layer stacks of dielectric films cannot be characterized by

single-wavelength ellipsometry (even with MAI). Even SE still might not

provide sufficient information for unambiguous multi-layer optical analysis.

Current problem: inter-layer correlation effects

6

0

2

4

6

8

10

12

14

16

18

20

1 2 3 4 5

Max d

iffe

ren

ce,

Å

Wafer #

TEOS BLOK

Experimental details

7

Sample preparation

LDRTEOS ~200Å

LDRTEOS ~200Å UV-cured ULK ~1800Å

UV-cured ULK ~1800Å BLOK ~250Å

BLOK ~250Å Ta ~160Å

TEOS ~4000Å TEOS ~4000Å

Si Substrate Si Substrate

Two sets of five unpatterned test wafers, each with typical ILD structure on

standard 300 mm Si(100) wafers, were prepared using an Applied Materials

Producer™ SE tool for dielectric chemical vapor deposition and an Applied

Materials Endura™ tool for metal physical vapor deposition. One set of four-

layer test wafers had the following structure: LDR TEOS/UV-cured ULK/

BLOK/TEOS/Si substrate, another set of five-layer test wafers had an

additional thin Ta metal layer between the BLOK and TEOS films.

Determination of the thin films optical properties

8

Spectroscopic ellipsometry:

Rotating-compensator spectroscopic ellipsometer (KLA-Tencor, former

Therma-Wave, Opti-Probe® OP9000 metrology tool)

Spectral range: 190–800 nm

AOI: ~65°

Modeling

Critical point with parity (CPP) model for the complex dielectric function

(included in the GO™ (Global Optimizer) software, ver. 1.2a)

− Ta film: 3-oscillator CPP model; ~160 Å layer is optically transparent

− BLOK and UV-cured ULK films: 2-oscillator CPP models

Crystalline Si substrate, Si–SiO2 interface and TEOS: dispersion tables

from Herzinger et al.

Optical characterization of the films by means of an “additive” thin-film

stack approach where the next film optical properties are extracted after

complete characterization of previously deposited films in the stack.

C.M. Herzinger, B. Johs, W.A. McGahan, J.A. Woollam, W. Paulson, Ellipsometric determination of optical

constants for silicon and thermally grown silicon dioxide via a multi-sample, multi-wavelength, multi-angle

investigation, J. Appl. Phys., 83, 3323–3336 (1998).

Determination of the thin films optical properties: Ta/TEOS/c-Si

9

Comparison of Ta film optical properties from different studies

10

H.G. Tompkins, T. Zhu, and E. Chen, Determining thickness of thin metal films with spectroscopic ellipsometry for

applications in magnetic random-access memory, J. Vac. Sci. Technol. A 16, 1297–1302 (1998).

T. Waechtler, B. Gruska, S. Zimmermann, S.E. Schulz, T. Gessner, Characterization of sputtered Ta and TaN films by

spectroscopic ellipsometry, in: T.-A. Tang, G.-P. Ru, Y.-L. Jiang (Eds.), ICSICT-2006 - 2006 8th International Conference

on Solid-State and Integrated Circuit Technology Proceedings, Shanghai, China, October 23–26, 2006, IEEE Conf. Proc.,

pp. 2184–2186 (2006).

Determination of the films thicknesses in multi-layer ILD film stack

11

No Ta layer

No Ta layer

Determination of the films thicknesses in multi-layer ILD film stack

12

Normalized residual uniqueness test from thin LDR

TEOS layer deposited on the multi-layer stack with

and without intermediate thin Ta metal layer. The

presence of the Ta layer reduces the uniqueness

range (usually, the parameter range that keeps the

residual values within 10% of the minimum value).

0.90

1.00

1.10

1.20

1.30

1.40

1.50

1.60

1.70

1.80

0.5 0.6 0.7 0.8 0.9 1.0 1.1 1.2 1.3 1.4 1.5

No

rma

lize

d R

esi

du

al

Normalized Film Thickness

no Ta layer

with Ta layer

Uniqueness

range

No Ta layer

Calculated uncertainty of ellipsometric analysis for all dielectric films from complete ILD stack

13

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

4.0

4.5

5.0

LDR TEOS ULK BLOK TEOS

Un

cert

ain

ty (

Å)

no Ta layer

with Ta layer

In modern semiconductor manufacturing the global trend of miniaturization dictates

continuous decrease in uncertainty for all thickness measurements and the

measurements should match stricter requirements for 2X and 1X process nodes.

Currently, one of the most important problems is to achieve required tool-to-tool

matching for routine process control in semiconductor processing. For instance,

current specification for the matching of ellipsometric tools on the ONO stack is 0.3%

for the bottom oxide thickness which results in a value of 3 Å for 1000 Å oxide layer.

Strong correlation between

the model fit parameters is

synonymous with large

parameter uncertainties. As

can be seen, the uncertainties

for all dielectric films were

greatly reduced by inclusion

of the thin metal layer into the

multi-layer film stack, which

indicates a decline in inter-

layer correlation effects.

Maximum differences for the TEOS and BLOK average thicknesses

14

0

2

4

6

8

10

12

14

16

18

20

1 2 3 4 5

Ma

x d

iffe

ren

ce,

Å

Wafer #

no Ta layer

with Ta layer

0

2

4

6

8

10

12

14

16

18

20

1 2 3 4 5

Ma

x d

iffe

ren

ce,

ÅWafer #

no Ta layer

with Ta layer

Maximum differences for the TEOS (a) and BLOK (b) average thicknesses

obtained during “additive” thin-film stack characterization (with and without

Ta thin metal layer).

a b

Conclusions

15

A method to reduce thicknesses correlations between the dielectric films in

the multi-layer ILD film stack has been presented. The method is based on

inclusion of additional thin absorbing metal layer into the multi-layer structure

in order to suppress the inter-layer correlations, thereby allowing an

accurate determination of the thicknesses of individual films in the stack.

We reported the characterization of the film stack with up to five-layer film

structure in which four of the films are dielectric by an ex situ single-angle

spectroscopic ellipsometer. The optical characterization of all layers in the

spectral range of 200–800 nm (6.20–1.55 eV) has been performed using an

“additive” thin-film stack approach.

The uniqueness of obtained ellipsometric solutions and accuracy of

established optical models for the multi-layer thin-film stack have been also

investigated. The reduction in uncertainties for all dielectric films clearly

indicates a decrease in cross-correlations of the model parameters.

Publication: D.V. Likhachev, Characterization of complex inter-layer dielectric

stack by spectroscopic ellipsometry: A simple method to reduce parameters

correlations// Thin Solid Films 550, 305–311 (2014)

doi: 10.1016/j.tsf.2013.11.082

Acknowledgements

16

Author wish to thank

Dr. Denis Shamiryan

(MAPPER Lithography,

The Netherlands) and

Mr. Steffen Brunner

(GLOBALFOUNDRIES,

Germany) for their

technical advice and

support.

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