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Thin Solid Films 473

Effect of oxygen plasma treatment on low dielectric constant

carbon-doped silicon oxide thin films

Y.H. Wanga,*, R. Kumara, X. Zhoub, J.S. Panb, J.W. Chaib

aInstitute of Microelectronics, 11 Science Park Road, Singapore 117685bInstitute of Materials Research and Engineering, 3 Research Link, Singapore 117602

Received 29 January 2004; received in revised form 19 July 2004; accepted 22 July 2004

Available online 12 September 2004

Abstract

Low dielectric constant (low k) carbon-doped silicon oxide (CDO) films are obtained by plasma-enhanced chemical vapor deposition.

The k value of the as-deposited CDO film is less than 2.9. However, the k value may be changed during the integration process. In integration

process, photoresist removal is commonly implemented with oxygen plasma ashing or by wet chemical stripping. In this work, the impact of

oxygen plasma treatment has been investigated on the quality of the low-k CDO films. Different plasma treatment conditions, including

variable pressure, r.f. power, and treatment time were employed. A variety of techniques, including X-ray photoelectron spectroscopy (XPS),

Fourier transform infrared (FTIR) spectroscopy, time-of-flight secondary ion mass spectrometry (TOF-SIMS), atomic force microscopy

(AFM), and scanning electron microscope (SEM) were used to analyze the effect of the oxygen plasma post-treatment on the low-k CDO

films. The result indicates that oxygen plasma will damage the CDO film by removing the entire carbon content in the upper part of the film

with increasing treatment time, which results in an increase in the k value and film thickness loss. Our result also confirms that with low r.f.

power and low pressure, the damage will be less.

D 2004 Elsevier B.V. All rights reserved.

Keywords: Low dielectric constant; PE-CVD; Oxygen plasma; Thin film; Dielectrics

1. Introduction

As very large scale integrated circuits continue to shrink,

the reduction in delay time requires a low dielectric constant

(low k) material that can reduce the parasitic capacitance of

multilevel interconnections [1–3]. In the recently years,

many studies on organic and inorganic films for interlayer

dielectrics, as well as the use of porosity and air gaps, have

been reported [4–8]. The carbon-doped silicon oxide (CDO)

is highly suitable for ultra large scale integrated applications

because of the k value less than 2.9, and compatibility with

current integration process. Therefore, the integration of the

CDO films as an interlayer dielectric into multilevel

interconnects has received much attention in recent years

[9–13]. In integration process, photoresist stripping is an

0040-6090/$ - see front matter D 2004 Elsevier B.V. All rights reserved.

doi:10.1016/j.tsf.2004.07.076

* Corresponding author. Tel.: +65 67705797; fax: +65 67731914.

E-mail address: yihua@ime.a-star.edu.sg (Y.H. Wang).

indispensable step. Photoresist removal is commonly

implemented with O2 plasma treatment or by wet chemical

stripping. The dielectric properties of the low-k films can be

degraded during photoresist stripping processes [12].

In this work, different oxygen plasma treatment con-

ditions, including variable pressure, r.f. power, and treat-

ment time were employed. A variety of techniques,

including X-ray photoelectron spectroscopy (XPS), Fourier

transform infrared (FTIR) spectroscopy, Time-of-flight

secondary ion mass spectrometry (TOF-SIMS), atomic

force microscopy (AFM), and scanning electron microscope

(SEM) were used to analyze the effect of the oxygen plasma

post-treatment on the low-k CDO films.

2. Experimental details

Low k CDO films were prepared by a Novellus

Concept Two SEQUEL Express plasma-enhanced chem-

(2005) 132–136

Table 1

Oxygen plasma treatment conditions, thickness, and dielectric constants of

the CDO films

Plasma treatment Thickness (nm) Dielectric

constantPressure (Torr) r.f. power (W) Time (s)

As-deposited 516 2.9

2 500 60 499 (115/384) 3.1

3 500 60 470 (218/252) 3.4

4 500 60 458 (274/184) 3.6

3 200 60 476 (126/370) 3.2

Fig. 1. FTIR spectra of the films: (a) as-deposited; (b) with O2 plasma

treatment at a pressure of 2 Torr and r.f. power of 500 W for 1 min; (c) with

O2 plasma treatment at a pressure of 3 Torr and r.f. power of 500 W for 1

min; (d) with O2 plasma treatment at a pressure of 4 Torr and r.f. power of

500 W for 1 min; (e) with O2 plasma treatment at a pressure of 3 Torr and

r.f. power of 200 W for 1 min.

Y.H. Wang et al. / Thin Solid Films 473 (2005) 132–136 133

ical vapor deposition (PE-CVD) system. The plasma was

sustained with two r.f. generators at 13.56 MHz and 400

kHz. The Si (100) substrates (200 mm p-type single

crystal wafers) were heated at 400 8C during the

deposition. The precursors used were liquid tetramethyl-

cyclotetrasiloxane (C4H16O4Si4, Schumacher), O2, and

CO2 gases.

The O2 plasma treatments were carried out in a PE-CVD

chamber. O2 gas flow rate was 200 sccm, and the substrate

temperature was kept at 350 8C. The r.f. (13.56 MHz)

powers were 200 and 500 W. The chamber pressure was

maintained at 2, 3, or 4 Torr. The treatment time ranged

from 10 to 60 s. Table 1 lists the O2 plasma treatment

condition for each sample.

The film thickness and refractive index (at 632.8 nm

wavelength) were measured on an Opti-probe system

from Therma Wave. The thickness of the films was also

confirmed by SEM cross-sectional measurements using a

JEOL JSM-6700F system. TOF-SIMS experiment was

performed using a CAMECA IONTOF-SIMS IV system,

which was operated in the dual beam interlaced mode.

Depth profiling was achieved by employing low energy

Ar+ beam to raster and sputter a surface area of 200�200

Am2, while high-energy Ga+ beam was applied to analyze

an area of 75.2�75.2 Am2 in the center of the sputtered

crater simultaneously. Both sputter and analysis ion

beams were incident at 458 to the sample surface normal.

The base pressure of the TOF-SIMS chamber was

1�10�9 Torr. The mass resolution of 5000 (M/DM)

was achieved at 29 atomic mass unit (amu). The film’s

chemical bonding and structure were characterized by

FTIR spectroscopy using a Bio-Rad QS 2200 with 4

cm�1 resolution. The chemical compositions of the films

were obtained from XPS measurements. The XPS

measurements were performed ex situ in a VG ESCA-

LAB 220i-XL system utilizing an Mg Ka X-ray source.

The XPS depth profiles were carried out using an Ar+

sputtering at 3 keV and 1.0 AA/cm2. The surface

roughness was measured by AFM (Digital Instrument,

Dimension 3000 series) system in terms of standard

deviation of the measured heights within a surface area

of 5�5 Am2. Tapping mode was used in AFM analysis.

Dielectric constant and leakage current were measured by

an SSM Mercury Probe Cyclic Voltammetry system

(SSM 495) on a metal–insulator–semiconductor structure

at 1 MHz. The average k value was obtained from the

measurement of 49 sites.

3. Results and discussion

Fig. 1 shows the FTIR spectra of the as-deposited

CDO film and after different treatments (the post-

deposition O2 plasma treatment conditions are described

in Table 1). Fig. 1(a) is the infrared absorption spectrum

of the as-deposited CDO film, which reveals the SiUC

stretching mode at ~800 cm�1, SiUCH3 bending mode at

~1270 cm�1, CUH stretching at ~2900 cm�1, and SiUH

stretching in the range of 2100–2300 cm�1. The shoulder

at a higher wavenumber of the 1040 cm�1 absorption

peak (Si–O), around 1130 cm�1, is an indication of some

degree of a cage SiUO bond structure [7,9]. As shown in

Fig. 1(b), there is no obvious change of the FTIR

spectrum for the CDO film after O2 plasma treatment

with a pressure of 2 Torr and r.f. power of 500 W for 1

min. Similar result is found for the sample after O2

plasma treatment with a pressure of 3 Torr and r.f. power

of 200 W for 1 min, as shown in Fig. 1(e). However,

with the plasma treatment pressure increase from 2 to 3

and 4 Torr, the intensities of SiUH and SiUCH3 peaks

are decreased, a small peak related to SiUOH at 950

cm�1 appears, and the SiUO peak shifts from 1040 to

1080 cm�1, as shown in Fig. 1(c–d). The result shows

that lower r.f. power and pressure of the O2 plasma

Fig. 2. SEM cross-sectional images: (a) as-deposited CDO film on Si

substrate; (b) after O2 plasma treatment at a pressure of 2 Torr and r.f. power

of 500 W for 1 min, showing a double-layered structure on Si substrate.

Y.H. Wang et al. / Thin Solid Films 473 (2005) 132–136134

treatment, the damage on the CDO film will be less.

Moreover, the SiUOH bond and HUOH bond signals

(broad band in the range of 3000–3700 cm�1 and around

950 cm�1 shown in Fig. 1(c–d)) appear in the FTIR

spectrum. The result indicates that during O2 plasma

treatment, oxygen radicals react with the functional

groups of CDO films, breaking SiUCH3 and SiUH

bonds. This causes the CDO films to generate dangling

bonds. The dangling bonds can easily react with

hydroxide ions in the environment and form SiUOH

bonds. The contribution of the highly polarized SiUOH

components to the orientation polarization will increase to

the k value of the film. Furthermore, the SiUOH bonds

in the CDO films lead to moisture uptake, which is

Fig. 3. Typical TOF-SIMS depth profiles of the CDO film after O2 plasma

responsible for the increase of k value and leakage

current [14].

The k values of the CDO films after different plasma

treatments are listed in Table 1. As seen from Table 1, the k

value of the as-deposited CDO film is 2.9. The O2 plasma

treatments change the k value of the CDO films. For the

CDO film after O2 plasma treatment with a pressure of 2

Torr and r.f. power of 500 W, the k value increases to 3.1.

With the treatment pressure increase to 3 and 4 Torr, the k

values of the two samples increase to 3.4 and 3.6,

respectively. The CDO film after O2 plasma treatment (at

a pressure of 3 Torr and r.f. power of 200 W), the k value is

3.2. Comparing the two samples (treated with a same

pressure of 3 Torr, but with different r.f. power, 200 and 500

W, respectively), the result shows a treatment with lower r.f.

power supply resulting in less damage on the low-k film,

which is consistent with the FTIR result.

The cross-sectional SEM images of the two samples,

as-deposited CDO film on Si substrate and after O2

plasma treatment at a pressure of 2 Torr and r.f. power of

500 W for 1 min, are shown in Fig. 2. Compared with

the homogeneous structure of as-deposited CDO film on

Si substrate in Fig. 2(a), a double-layered structure can

be seen from Fig. 2(b), which clearly shows the changes

in the CDO film after the oxygen plasma treatment.

Moreover, with the increase of treatment pressure and r.f.

power, the upper layer thickness increases and the total

film thickness decreases, as shown in Table 1. The

following TOF-SIMS and XPS depth profile analysis

confirm that the upper layer is silicon oxide and the

lower layer is still CDO film.

Fig. 3 is a typical TOF-SIMS result of the CDO film

after O2 plasma treatment (at a pressure of 2 Torr and r.f.

power of 500 W for 1 min), which directly shows the

treatment at a pressure of 2 Torr and r.f. power of 500 W for 1 min.

Fig. 5. Typical XPS spectra of the CDO film after O2 plasma treatment (at a

pressure of 3 Torr and r.f. power of 200 W for 1 min) at different depth.

Y.H. Wang et al. / Thin Solid Films 473 (2005) 132–136 135

effect on low-k CDO films. The main negative secondary

ion species detected are C�, CH�, O�, Si�, SiH2�, SiO�,

SiOH�. As can be seen in the left-hand plot in Fig. 3,

the intensities of C� and CH� are very low within the

first 110 s, while the intensities exhibit a rapid increase

by about 2 orders of magnitude at the equilibrium region.

On the contrary, the intensities of O�, SiH2�, SiO� and

SiOH� drop obviously in the same time. The results

indicate that after O2 plasma treatment, the C and CH

contents are lost significantly and this has resulted in

serious damage of the upper portion of the CDO film.

Thus, the upper layer in Fig. 2(b) is actually SiOx:H, and

the lower layer is still low-k CDO film. The C� depth

profiles of the as-deposited CDO film and the films after

different O2 plasma treatment conditions are shown in

Fig. 4. The C� depth profiles suggest that the damage of

the upper portion of the CDO film depends on the O2

plasma treatment condition. As the treatment pressure

increases from 2 to 3 and 4 Torr, the thickness of the

upper layer SiOx:H increases as shown in Fig. 4(b–d). In

addition, as the r.f. power increases from 200 to 500 W,

the upper layer SiOx:H thickness also increases, which

are consistent with the SEM and FTIR investigations.

From the sputter time and the thickness result (measured

by cross-sectional SEM) in Table 1 of the samples, the

calculated the sputter rates (sputter depth divided by

time) of CDO film and the upper layer (SiOx:H) of the

CDO films after O2 plasma treatment are in the range of

1.0–1.1 and 0.8–0.9 nm/s, respectively. Comparing with

the sputter rate of the upper layer SiOx:H, the higher

sputter rate of the CDO film may due to its lower

density.

Fig. 4. TOF-SIMS depth profiles of C� ion in the films: (a) as-deposited; (b) with O

(c) with O2 plasma treatment at a pressure of 3 Torr and r.f. power of 500 W for 1

500 W for 1 min; (e) with O2 plasma treatment at a pressure of 3 Torr and r.f. p

Only the peaks of Si, C, and O were observed for the as-

received low-k CDO films after O2 plasma treatment (at a

pressure of 3 Torr and r.f. power of 200 W for 1 min) in the

XPS survey spectrum, as shown in Fig. 5. The XPS depth

profile result clearly reveals the change of the C content of

the CDO film after O2 plasma treatment. Due to the

exposure in the air before XPS experiment, the C 1s peak

at ~285 eVof the as-received surface (0 min Ar+ sputtering)

is detected and assigned to the surface contaminates. After

2 plasma treatment at a pressure of 2 Torr and r.f. power of 500 W for 1 min;

min; (d) with O2 plasma treatment at a pressure of 4 Torr and r.f. power of

ower of 200 W for 1 min.

Fig. 6. AFM images: (a) as-deposited CDO film, and (b) after O2 plasma

treatment at a pressure of 4 Torr and r.f. power of 500 W for 1 min.

Y.H. Wang et al. / Thin Solid Films 473 (2005) 132–136136

sputtering, no C 1s peak is observed in the survey spectra of

the following two levels (after 4 and 8 min Ar+ sputtering),

as shown in Fig. 5. However, after that, the intensity of C 1s

peak increases with the depth. The depth profile of the C

content in this sample directly shows that O2 plasma

treatment results in the loss of the C content in the upper

layer of the CDO film.

The topography of the samples has been observed by

AFM. All the films have a smooth surface with the root

mean square surface roughness in the range of 0.6–0.8 nm.

Also, no evident change of the surface roughness is

observed for the as-deposited sample and the film after O2

plasma treatment at a pressure of 4 Torr and r.f. power of

500 W for 1 min, as shown in Fig. 6.

Based on the above FTIR, SEM, TOF-SIMS, and AFM

observations, it is confirmed that the O2 plasma results in

the loss of C contents, thus changes the chemical bonding,

thickness, composition, and k value of the films. The

measured k values in Table 1 are consistent with the

calculated results from the two-layered structure with the k

values of 4.2 of oxide 4.2 and 2.9 of CDO. Moreover, it is

found that with low r.f. power, low pressure and short

treatment time, the damage will be less.

4. Conclusions

The effect of O2 plasma treatment with variable pressure,

r.f. power, and treatment time on the low-k CDO films has

been investigated. It is found that O2 plasma damages the

CDO film by removing of the entire C content in the upper

layer of the film with increasing plasma treatment time. This

causes the loss of film thickness and increase in the k value

of the film. Moreover, with low r.f. power, low pressure, and

short treatment time, the damage will be less. Our results

indicate that O2 plasma can exchange the upper degraded

CDO layer (SiOx:H) and attack the lower CDO layer.

Comparing with the sputter rate of the upper layer (SiOx:H)

of the CDO films after O2 plasma treatment, lower than the

sputter rate of CDO film, which may due to the different

density. No obvious change in the topography of the low-k

CDO film after O2 plasma treatment has been observed by

AFM technique.

Acknowledgement

The authors are thankful to J. L. Xie, M. R. Wang, and B.

Narayanan for technical support in sample preparations, S. R.

Wang for AFM analysis, and P. Yew for SEM cross-sections.

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