Reduction of etching plasma damage on low dielectric constant fluorinated amorphous carbon films by multiple H-2 plasma treatment

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Reduction of etching plasma damage on low dielectric constant fluorinated amorphous

carbon films by multiple H 2 plasma treatment

Jia-Min Shieh, Kou-Chiang Tsai, Bau-Tong Dai, Yew-Chung Wu, and Yu-Hen Wu

Citation: Journal of Vacuum Science & Technology B 20, 1476 (2002); doi: 10.1116/1.1494067

View online: http://dx.doi.org/10.1116/1.1494067

View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/20/4?ver=pdfcov

Published by the AVS: Science & Technology of Materials, Interfaces, and Processing

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amorphous carbon films by multiple H

₂

plasma treatment

Jia-Min Shieh,a)Kou-Chiang Tsai, and Bau-Tong Dai

National Nano Device Laboratories, Hsinchu 30050, Taiwan

Yew-Chung Wu and Yu-Hen Wu

Institute of Materials Science and Engineering, National Chiao Tung University, Hsinchu 30050, Taiwan 共Received 20 March 2002; accepted 20 May 2002兲

Two-step hydrogen plasma treatment on low dielectric constant (low-k) fluorinated amorphous carbon films (a-C:F) was conducted to improve their thermal stability and reduce the damage caused by the patterning processes. First, hydrogen plasma treatment repairs imperfect bonds of as-deposited a-C:F films, stabilizing their chemical structures and increasing their resistance against elevated thermal stresses. After this passivation process, an additional hydrogen plasma treatment was applied to a-C:F films that had been etched using a mixture of N2⫹O2⫹CHF3, enabling

I. INTRODUCTION

Low dielectric constant (low-k) materials as interlayer dielectrics face similar difficulties, whether deposited by chemical vapor deposition共CVD兲 or by spin-on techniques.1 These difficulties include integration issues,2weak resistance against copper diffusion,3and the avoidance of damage dur-ing patterndur-ing.4 Low-k materials such as black diamond SiOC:H films5 and fluorinated amorphous carbon (a-C:F) films6 are increasingly attracting interest since they can be deposited by conventional plasma-enhanced CVD共PECVD兲. The gaseous mixture, containing fluorocarbon and hydrocar-bon, is a precursor in the deposition of a-C:F films,6making the deposition of a-C:F films as easy as preparing fluori-nated SiO2共SiOF兲 films. Several attempts have been made to

address and improve critical areas of material properties of a-C:F films in interconnect processes.6 –11 However, Han and Bae reported that the electrical characteristics of a-C:F films become degraded after annealing at several hundred degrees, typically about 400 °C.7

Previous studies have demonstrated that plasma posttreat-ment is an effective means of modifying as-deposited dielec-tric films.12–15Hydrogen plasma treatment can normally re-pair unstable bonds, or defect sites, into stronger chemical structures.12–14 The treated films were more resistant to moisture uptake and copper diffusion than untreated films.13,14Furthermore, the patterning process for low-k ma-terials typically includes oxygen in the etching gases. How-ever, oxygen gas attacks the low-k materials by decompos-ing chemical structures and generatdecompos-ing unstable bonds in the films, increasing the probability of moisture uptake,15 and thereby degrading the performance of low-k materials, such as a-C:F films.

This study demonstrates, for the first time, a multiple plasma treatment on a-C:F films. In the first step, a hydro-gen plasma treatment converts dangling bonds of as-deposited a-C:F films into chemical structures which are more stable under higher thermal stresses. After the hydro-gen plasma-treated a-C:F films are etched, an additional hy-drogen plasma treatment was again employed, leading to the repair of the bonds damaged by the patterning processes. Hydrogen atoms are known to be able to penetrate deeply into dielectric films and mend imperfect bonds. Conse-quently, hydrogen plasma pretreatment combined with post-treatment can protect low-k materials such as a-C:F films from thermal stresses, and protect against damage due to etching.

II. EXPERIMENT

As-deposited a-C:F films were deposited by PECVD. The basic precursor gas was a mixture of CH4, C4F8. In this

experiment, another class of samples was the H2

plasma-treated a-C:F films. Table I-a lists the deposition10and post-deposition parameters. For simplicity, STD represents the as-deposited a-C:F films, whereas H-3, H-6, and H-9 represent as-deposited a-C:F films with 3, 6, and 9 min of H2plasma

treatment, respectively. Thermal stability was then examined by curing those a-C:F films in a furnace at elevated tempera-tures for 30 min in N2ambient at a flow rate of 10 liter/min.

Before and after each treatment, the Fourier-transform infra-red共FTIR兲 absorption spectra of each film were examined to monitor structural changes. Secondary ion mass spectros-copy 共SIMS兲 was performed to analyze hydrogen distribu-tions in the a-C:F films. Thermal desorption spectroscopy

共TDS兲 was used to measure the gas release 共such as H2O兲

from the a-C:F films, to examine the effect of plasma treat-ment on resistance against moisture uptake. The electrical measurements were taken by metal–insulator– semiconductor capacitors. Two types of wafers were used to

a兲_{Author to whom all correspondence should be addressed; electronic mail:} [email protected]

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evaluate etchingcharacteristics such as damage due to etch-ing. These wafers were blanket a-C:F films, and a-C:F films covered with 50 nm of SiO₂for patterning experiments. A 50 nm silicon oxide mask was patterned by electron-beam li-thography共smallest feature resolution ⬃50 nm兲. Both types of wafers were etched in a helicon-wave plasma etching pro-cess. Etching gases, N2 and O2were used at a total flow rate

of 100 sccm, with the addition of CHF3 to control the

etch-ing profiles. Table I共b兲 lists other etching parameters.

III. RESULTS AND DISCUSSION

A. Fundamental characteristics and thermal stabilities of plasma-modified low-k a-C:F films

Figure 1共a兲 shows that the dielectric constant of a-C:F films increases from 2.35 to 3.7 as the annealing temperature increases to 450 °C. Consequently, a modification, such as plasma treatments of as-deposited a-C:F films, was required in addition to the optimization of deposition conditions. NH₃ plasma treatment is not applicable to a-C:F films since its refractive index and thickness changed by more than 9% in a process time of 1.5 min, although NH₃ plasma effectively modified many materials.15 Therefore, hydrogen plasma treatment was adopted herein. Figures 1共a兲, and 1 共b兲 show the dielectric constants and the leakage currents of a-C:F films treated by H2 plasma remain relatively unchanged,

be-cause the hydrogen plasma repairs the unstable bonds or de-fect sites into stable chemical bonds. The leakage currents and dielectric constants of H2-treated films can be improved

or at least maintained. Notably, stresses of the H2-treated

a-C:F films are maintained at the same level (⫺10 MPa) as those of untreated films, and FTIR spectra for the chemical structures of such films are also the same as those of the as-deposited samples, as shown in curve a and curve b of Fig. 2共a兲, implying that hydrogen plasma bombardment nei-ther introduced a densification effect nor caused reconstruc-tion in the a-C:F films. In general, some dangling bonds caused by deposition, thermal stresses, or patterning pro-cesses may temporarily exist on the surface of the film. Those unstable sites tend to react with other species

共mois-ture兲 from the environment, degrading the electrical charac-teristics 共by forming CvO bonds兲. However, the degrada-tion of electrical characteristics due to imperfect bonds can be decreased by forming stable bonding structures共hydrogen passivated bonds兲.

FIG. 1. 共a兲 Dielectric constants of H2-treated a-C:F films as functions of

annealing temperature. 共b兲 Leakage currents of STD samples, H2-treated

a-C:F films before, and after annealing for 30 min at 450 °C.

TABLEI.共a兲 Deposition and postdeposition parameters. 共b兲 Etching parameters.

共a兲 Deposition parameter Postdeposition parameter

Reactant gases CxFy/CH4⬃10 NH3, N2, and H2

rf plasma power 200 W 200 W

Process pressure 550 mTorr 100 mTorr

Temperature 250 °C 250 °C

Flow rate 300 sccm 200 sccm

共b兲 Etching parameters for a-C:F Etching parameters for SiO2

Reactant gases N2/O2⬃80/20 or 共65/35兲 CHF3/CF4⬃5/95

rf plasma power 2000 W 2000 W

Bias power 60–120 W 60 W

Process pressure 5 mTorr 5 mTorr

Temperature 60 °C 60 °C

Flow rate 100 sccm 100 sccm

1477 Shiehet al.: Reduction of etching plasma damage 1477

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Figure 1共a兲 displays the dielectric constants of H2-treated

a-C:F films as a function of annealing temperature and illus-trates that its dielectric constant rises from 2.35 to 2.8 as the annealing temperature increases to 450 °C. In comparison, the untreated sample abruptly changes its electrical charac-teristics, and its dielectric constant rises to 3.7 such that the thermal stability of the dielectric constants of a-C:F films is significantly improved after treatment with H2 plasma.

Moreover, strongly H2-passivated samples 共H-6 and H-9兲

were more stable than other samples at all annealing tem-peratures. After thermal annealing at 450 °C for 30 min, the leakage currents of H-6 and H-9 samples were also much lower than those of H-3 and STD samples, as depicted in Fig. 1共b兲. For example, the leakage current of H-6 and H-9 samples only increases from Id⬇6.0⫻10⫺9 A/cm2 to Id

⬇1.28⫻10⫺8_A/cm2 _(@E

bias⫽1.0 MV/cm) after thermal

annealing at 450 °C. The values for annealed H-3 and STD

samples are Id⬇5.8⫻10⫺8 A/cm2 and Id⬇5.0

⫻10⫺7 _A/cm2_{, respectively.}

Therefore, hydrogen plasma treatment helps to stabilize the chemical structures of a-C:F films because of the pres-ence of hydrogen passivated chemical structures in the films, which modification for H2-treated a-C:F films can suppress

the uptake of moisture and reduce thermal decomposition during high-temperature annealing. Figure 3 displays TDS spectra that verify the presence of more hydrophobic sur-faces in the H2-treated samples than in the other samples,

and that the desorption amount of H₂O of the H-9 sample is the least among all samples. In those TDS spectra, H₂O physically absorbing on the surface of the film dominates the desorption in the lower temperature below 200 °C and H₂O chemically bonding with films contributes the desorption in the higher temperature above 400 °C. Curve c in Fig. 2共b兲 also shows that spectral peaks of CFx(980– 1350 cm⫺1) and CuH (2873– 2954 cm⫺1)11 and the obtuse shape of the CvO bond in the FTIR spectrum of the annealed H-6 sample, are the same as those of the STD sample. However, Figs. 1共a兲, 1 共b兲, and 3 reveal that the thermal stability of the H-9 sample was similar to that of the H-6 sample. An excess of hydrogen atoms in the H-9 sample might break the CFx bonds and actively react with fluorine species before outgas-sing via HF structures at elevated thermal stresses. This ef-fect was the dominant reason for the saturation of treatment times. Considering the dielectric constants and leakage cur-rents together, the H2 treatment time of 6 min is enough to

inhibit the degradation caused by thermal stresses.

B. Hydrogen plasma treatments on etcheda-C:F films

Our previous work10 demonstrated that a sidewall passi-vation, which is provided by nitrogen gas forming a CxNy layer, can help the N2/O2etching process to pattern a perfect

150 nm damascene structure on the a-C:F films. This study tries to improve the aspect ratios of the damascene trenches by increasing the bias power. Figure 4共a兲 displays scanning

FIG. 2. 共a兲 FTIR spectra of STD, H-6, and STD samples etched by N2,

CHF3.共b兲 FTIR spectra of a-C:F films etched by N2⫹O2⫹CHF3; etched

samples with post H2treatment, and annealed H-6 sample.

FIG. 3. TDS spectra of H2O desorption as a function of temperature for

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electron microscopy共SEM兲 micrographs for patterned a-C:F films capped with 50 nm SiO2 as a hard mask, for various

bias powers from 60 to 120 W. The process time was 60 s; the photoresist sizes defined by electron-beam lithography was 130 nm, and N₂/O₂ 共80/20兲 etching gases for a-C:F films were used. An CHF₃/CF₄ 共5/95兲 etching gas was used to etch the SiO₂hard mask and all other parameters were the same as those used for etching the a-C:F films. Although the aspect ratio of the patterned damascene trenches increased from 2.3 to 2.7 as the bias power increased from 60 W to 120 W, the width of the patterned damascene trenches also in-creased from 160 nm to 230 nm because the inin-creased en-ergy of ion bombardment enen-ergy provided insufficient an-isotropy of the etching rate in the narrowed trenches/vias and a vertical etching profiles could thus not be maintained. Ac-cording to this result, an additional etching gas, CHF3, was

added to control the etching profiles. Its effect was examined by comparing the etching pattern关left-hand side micrograph of Fig. 4共b兲兴 in the N2/O2 process with that关central

micro-graph of Fig. 4共b兲兴 using the N2/O2/CHF3 recipe. The flow

rate of the additional CHF3gas was 10 sccm. A larger pattern

of around 400 nm and an enhanced isotropic etching rate, obtained with a lower ratio of N2/O2 共65/35兲, were

em-ployed to examine the contribution of CHF3 to the

passiva-tion of the sidewalls. The central micrograph of Fig. 4共b兲 shows protection of the sidewall of the pattern was observed, and an excellent profile of 130 nm damascene trenches was patterned by the N2/O2/CHF3 etching gases, as shown in

the right-hand side micrograph of Fig. 4共b兲.

Except in the case of the control of the etching profile by etching recipes, attack by oxygen from the etching plasmas or photoresist stripping processes is a critical issue for low-k materials. The FTIR spectra in curve c and curve d of Fig. 2共a兲, shows that the N2 and CHF3 gases do not observably

affect the chemical structures of a-C:F films. In comparison, oxygen gas apparently alters the characteristics of the film, as concluded from the appearance of enlarged CvO bonds

共1650 and 1850 cm⫺1_{兲 in curve a and curve b of Fig. 2 共b兲.} FIG. 4. 共a兲 SEM profiles for patterned a-C:F films capped with 50 nm SiO2as a hard mask for a range of bias power from 60 to 120 W.共b兲 left-hand side

micrograph refers to the a-C:F film etched by the N2/O2 recipe; central micrograph refers to the a-C:F films etched by the N2/O2/CHF3 recipe, and

right-hand side micrograph refers to 130 nm damascene trenches patterned by N2/O2/CHF3etching gases.

FIG. 5. Leakage currents of STD samples; STD samples etched for different process times; etched STD samples with post H2treatment, and etched H-6

samples with post H2treatment.

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These bonds differ from the obtuse shapes for STD or an-nealed H-6 samples. Damage caused by etching plasmas fi-nally increased the leakage currents and dielectric constants of a-C:F films, as shown in Figs. 5 and 6共a兲. The mean leakage current increased from Id⬇6.0⫻10⫺9 A/cm2 to Id

⬇7.0⫻10⫺8 _A/cm2 _(@E

bias⫽1.0 MV/cm) for a-C:F films

after the etching processes, and the dielectric constant in-creased to ␬⬇2.55 共␬⬇2.35 for as-deposited a-C:F films兲. Figure 6共b兲 also showed that the addition of CHF3 etching

gas did not obviously change the dielectric constants of the etched a-C:F films. The dielectric constants of a-C:F films etched with different bias powers were␬⬇2.55, as shown in Fig. 6共c兲. Thus, increasing the power seems not further to damage a-C:F films. Therefore, attack by oxygen is a major factor that determines the damage caused by etching. The oxygen plasma causes several functional groups (C–Fx) to breakdown, not only replacing them with hydroxyl groups or absorbed water, but also leaving many dangling bonds in a-C:F films. Curve a and curve b in Fig. 2共b兲 show the

enlarged CvO and CuH bonds in the FTIR spectra of etched a-C:F films, indicating an uptake of moisture and chemical reconstruction in such films.

Therefore, in this study, posthydrogen plasma treatment was applied to the etched a-C:F films, and was expected to convert the damaged bonds into more stable chemical bonds. Curve d and curve e of Fig. 2共b兲 reveal that weaker CvO bonds were actually measured for etched a-C:F films with H2 posttreatments. The leakage currents of etched a-C:F

samples with posttreatment were also found to be reduced to those of the STD samples, as shown in Fig. 5. Moreover, the etched samples with the two-step plasma treatments exhib-ited passivation superior to that of the etched STD sample with only a posttreatment, as determined by comparing their corresponding FTIR spectra and leakage currents. Oxygen plasma etches a-C:F films by chemical reactions, not only breaking the chemical bonds 共i.e., C–Fx兲 but also replacing them with CvO bonds. These reactions occur on the sur-faces and interiors of the film by the diffusion of oxygen radicals into the porous inner structure of a-C:F films to attack weakly bonded structures. The hydrogen in the H-6 sample penetrated deeply into a-C:F films as shown in SIMS analyses of Fig. 7, causing initial passivation of a-C:F films against sequential damage from etching plasmas, even when the outer portions of a-C:F films were removed during pat-terning processes. The electrical characteristics of a-C:F films with multiple H2 treatments improved after patterning.

IV. CONCLUSION

A novel two-step hydrogen plasma treatment was for the first time applied to low-k a-C:F films, to passivate the chemical structures against thermal stress, and attack by oxy-gen during patterning processes. First, the hydrooxy-gen plasma treatment increased the thermal stability of a-C:F films. Hy-drogen elements penetrated deeply into a-C:F films, provid-ing an initial passivation against sequential damage caused by etching plasmas. Further hydrogen treatment could repair the damage caused by the etching of H2-treated a-C:F films.

FIG. 6. 共a兲 Dielectric constants for a-C:F films etched by N2⫹O2⫹CHF3

gases as functions of etching process times.共b兲 Dependence of the concen-tration of the CHF3etching gas on dielectric constants of a-C:F films.共c兲

Dependence of the rf power on dielectric constants of a-C:F films etched by N2⫹O2⫹CHF3gases.

FIG. 7. Distribution profiles of hydrogen elements in H2-treated a-C:F films

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ACKNOWLEDGMENTS

The authors would like to thank the National Science Council of the Republic of China for financially supporting this research under Contract No. NSC91-2721-2317-200.

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