Thermal annealing effect of tetrahedral amorphous carbon films deposited by filtered vacuum arc

Youngkwang Lee *†,Tae-Young Kim*†, Kyu Hwan Oh†, Kwang-Ryeol Lee**Future Technology Research Division, Korea Institute of Science and Technology

†School of Materials Science & Engineering, Seoul National University

Thermal annealing of tetrahedral amorphous carbon (ta-C) film has been widely used to reduce its high level of compressive stress. In the present work, we investigated the changes in atomic bond structure and the mechanical properties caused by the thermal annealing. The ta-C films were prepared by a filtered vacuum arc process. In order to obtain ta-C films of various atomic bond structures, negative bias voltage of the substrate were varied from 0 to 650V. The specimens were annealed in vacuum at 600 using rapid thermal annealing (RTA) system. The residual stress of the anneal℃ed ta-C film changed in contrasting manner depending on the initial atomic bond structure or negative bias voltage applied to the substrate during deposition. At the lower bias voltage ranging from 0 to 150V, residual compressive stress of the film significantly decreased by the annealing. In contrast, the residual compressive stress increased by the thermal annealing when the applied negative bias voltage was larger than 150V. However, no significant change in the hardness was observed after the thermal annealing in this range of the negative bias voltage. This contrasting behavior will be discussed in terms of the changes in atomic bond structure analyzed by Raman spectroscopy.

I. Abstract

II. Introduction and Motivation

• High Hardness• Smooth Surface• Chemical Inertness

High Compressive Residual Stress

ta-C deposition

Moon et al. Acta Materialia 50 (2002) p.1219

III. Experimental Procedure

Rapid Thermal AnnealingVacuum : below 8 mTorr

Temperature : 600 ºCTime : 7 min

Filtered Vacuum Arc

Carbon target

H.F. bias voltage : 600V

Ar environment (10-4 Torr)

Film thickness : ~5nm

Residual Stress Hardness

As-deposited Annealed

Inte

nsit

y (a

.u.

)

Inte

nsit

y (a

.u.

)

0 100 200 300 400 500 600 700

1540

1550

1560

1570

1580

1590

ta-C (As-deposited)

ta-C (Annealed)

G-p

eak

posi

tion

(cm

-1)

Bias voltage (V)

Bias Voltage : 0V ~ 50V

Summary of Changes by Thermal Annealing

Compressive residual stress is relieved as the distortion of atomic bond is relaxed.

Six fold rings increase with increasing sp2 sites by thermal annealing

Six fold rings can be highly distorted in the amorphous carbon matrix as the size of the ring increases.

Bias Voltage : 200V ~ 500V

Compressive residual stress increases as the distortion of the large six fold ring becomes significant.

• Thermal annealing affects the structure and the residual stress of ta-C film in a different manner depending on the initial structure of the film. – For ta-C of higher sp3 fraction, the residual compressive stress decreased

as the distortion of atomic bond is relaxed with increasing sp2 hybrid bond.

– For ta-C of smaller sp3 fraction, the residual compressive stress increased as the distortion of the large six fold ring becomes significant.

Characteristic of ta-C Experimental Procedure Our Goal

Thermal Annealing of ta-C

Observe Change of 1.Residual Stress

2. Hardness

Change of Bond

Structure

Build Model of Relationship between Change of Bond

Structure and Mechanical Property by Thermal

Annealing

Filtered Vacuum Arc

Carbon target

H.F. bias voltage : 0 V~650 V

Vacuum (10-5 Torr)

Film thickenss : 90~120 nm

Buffer layer deposition ta-C Deposition Thermal Annealing

IV. Result -1. Change of Mechanical Properties by Thermal Annealing

Change of Mechanical

Property

Different Behavior with Bias Voltage0V~50V : Residual Stress Decreased

200V~500V : Residual Stress Increased

No Significant Change by Thermal Annealing

IV.Result -2. Raman Spectrum Analysis

Different Behavior with Bias Voltage

0~50V : No significant change>200V : D-peak increases significant increase in 6 fold ring

D-Peak G-PeakG-Peak

G-peak Position of Annealed ta-CShow Higher Shift

in All Range of Bias VoltageIncrease of sp2 Contents

* Corrected data considering the stress effect(Shin et al. APL 78, No. 5 (2001) p.631)

V.Discussion

Bias Voltage Stress Hardness sp2 Contents Sixfold Ring

0V~50V

200V~500V

sp2 contents increases by thermal annealing without generating six fold

ring.

VI. Conclusion

Model of Annealing Effect on Residual Stress

This structural change will reduce the

3-dimensional constraint in atomic bond configuration.