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K. Overhage, Q. Tao, G. M. Jursich, C. G. TakoudisAdvanced Materials Research LaboratoryUniversity of Illinois at Chicago

Atomic Layer Deposition of TiO2 on Silicon and Copper Substrates:Investigation of the Initial Growth

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Acknowledgements REU 2010 at UIC, sponsored by the National Science

Foundation and the Department of Defense

EEC-NSF Grant # 0755115

CMMI-NSF Grant # 1016002

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What is ALD?The Atomic Layer Deposition (ALD) process is used to deposit thin films layer by layer until a desired thickness is achieved.

Introduce one precursor, purge, then the other precursor, purge and repeat many times in the gas phase to deposit films on a substrate

Useful because ALD can deposit very thin films with uniform, conformal coverage

The focus of this study is deposition of TiO2

Photo from Barrier Layers Technology by Prof. Yosi Shacham-Diamand, Tel-Aviv University, 2000

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Substrate with active sites

Chemisorption of source A and saturation mechanism.

Purge

Chemical reaction between source A and source B and saturation mechanism

Purge

Source B (H2O)

Source A (TDEAT)ST

EP

1ST

EP

2STEP

3STEP

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Reaction Mechanism of typical ALD cycleALD is a surface-saturation reaction that deposits each monolayer of film, allowing for precise thickness control.

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Example application of ALDAn example application of an ALD process is the construction of the copper barrier layer in a chip.

The copper barrier layer prevents Cu from reacting with other chip materials, particularly silicon

Diagram from http://www.tms.org/pubs/journals/JOM/9903/Frear-9903.fig.5.lg.gif

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Objectives

Study TiO2 deposition on silicon and copper with different surface chemistries, with the goal of achieving selective deposition

Temperature-independent window Early growth / nucleation period Late growth / constant growth region

Findings can be used in future work to further promote selective deposition of TiO2 on Silicon

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SubstratesDeposition was performed on substrates with different surface chemistries.

Silicon with native oxide (approximately 1.5 nm-thick)

Silicon with reduced oxide (less than 1 nm-thick, 2% HF etching treatment)

Copper with native oxide (approximately 2 nm-thick)

ALD is surface reaction driven – therefore, the surface chemistry of the substrate is critical. Careful preparation steps were taken to properly prepare the substrates.

Optical test, measures film thickness

Light source shines on film, detector measures reflected light

Computer models calculate thickness based on reflective index of material

SE TheorySpectral Ellipsometry – measures film thickness

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Temperature-independent window

TiO2 deposition on silicon is independent of temperature between 150 and 200 °C.

Silicon with native oxide:Slope 1.2 A / cycle

100 125 150 175 200 225 25055

60

65

70

75

TiO2 on Si - Temperature Study

Temperature (oC)

Thic

knes

s (A

ngst

rom

s)

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Late GrowthDeposition from 50 to 150 cycles on silicon with native oxide

Once the early growth phase is complete, TiO2 deposition proceeds at 1.3 Å / cycle. This is in agreement with current literature values.

0 50 100 150 2000

50

100

150

200

250

TiO2 on Silicon with Native Oxide

Number of Cycles

Thic

knes

s (A

ngst

rom

s)

Slope = 1.3 Å / cycle

0 10 20 30 40 50 600

10203040506070

TiO2 Growth on Si(100)

Number of Cycles

Thic

knes

s (A

ngst

rom

s)

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Early GrowthDeposition from 0 to 50 cycles on the two kinds of silicon surfaces

Here we see a negligible nucleation time on both substrate surfaces. Growth rates are equal to the slope of the best fit line.

Silicon with native oxide:Growth rate 1.2 Å / cycle

Silicon with reduced oxide: Growth rate 1.0 Å / cycle

Silicon with < 1 nm oxide

Silicon with 1.5 nm native oxide

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X-rays penetrate sample surface, knocking out core electrons of the film atoms

Detector records energy signal from electrons emitted

Each element has signature peak pattern

Inte

nsity

(Cou

nts)

Binding Energy (eV)

Sample Spectrum

Stronger signal = XPS detects more

atoms

XPS TheoryX-ray Photoelectron Spectroscopy – used to analyze film composition

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Early GrowthXPS results – TiO2 signal on silicon substrate

The TiO2 signal gets stronger as the number of cycles increases, indicating growth of the TiO2 film on the silicon substrate.

445 450 455 460 465 470 4750

500

1000

1500

2000

2500

3000

3500

XPS of Ti 2p, TiO2 on Si (with native oxide less than 1 nm)

30 cycles TiO

15 cycles TiO

10 cycles TiO

5 cycles TiO

Binding Energy (eV)

Inte

nsity

(Cou

nts)

4.2 nm

2.5 nm

2.3 nm

0.8 nm

452 454 456 458 460 462 464 466 468 4700

500

1000

1500

2000

2500

3000

3500

XPS of Ti 2p on Copper, 175oC

30 cycles on Si30 cycles on cu

25 cycles on cu

Binding Energy (eV)

Inte

nsity

(Cou

nts)

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The TiO2 signal is weak, but present after 15 cycles and it does not increase by 20 cycles. The effective nucleation time of TiO2 on copper is about 15 cycles.

0.3 nm

Thickness can’t be determined by SE, should be less than 2 monolayers (<0.3 nm)

4.2 nm (Si)

CopperXPS results – TiO2 signal on copper substrate

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Discussion No nucleation period on silicon

Considerably delayed formation of TiO2 on copper

Selective deposition is achieved at the conditions used in this study

Nucleation period enables selective growth, for thicknesses up to 2.5 nm - could satisfy the requirement for copper barrier application1

1International Technology Roadmap for Semiconductors (Semiconductor Industry Association, San Jose, CA, 2001).

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Future Work

Before I leave … SEM (scanning electron microscopy) will be applied to

probe the early TiO2 film nucleation on both silicon and copper substrates from 5 to 30 cycles of ALD

Later work … Other surface treatments are still in progress to promote

the growing selectivity, such as complete removal of native oxide without immediate reoxidation

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Conclusions

TiO2 nucleation time on silicon substrate is negligible, and the initial growth rate is 1.0 to 1.2 Å / cycle, depending on surface chemistry

Temperature-independent window for TiO2 deposition on silicon is 150 to 200 °C

Nucleation time on copper substrate is found to be ~ 15 - 20 cycles

The potential to achieve greater selective deposition of TiO2 with further research appears to be high

Questions?