materials
Jun-Fu Huang, Tain-Cih Bo, Wei-Yuan Chang, Yu-Min Chang, Jihperng Leu, and Yi-Lung Cheng
Citation: Journal of Vacuum Science & Technology A 32, 031505 (2014); doi: 10.1116/1.4868631 View online: http://dx.doi.org/10.1116/1.4868631
View Table of Contents: http://scitation.aip.org/content/avs/journal/jvsta/32/3?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing
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SiCOH materials
Jun-Fu Huang and Tain-Cih Bo
Department of Electrical Engineering, National Chi-Nan University, Nan-Tou, Taiwan, 54561, Republic of China
Wei-Yuan Chang, Yu-Min Chang, and Jihperng Leu
Department of Materials Science and Engineering, National Chiao-Tung University, Hsin-Chu, Taiwan, 30050, Republic of China
Yi-Lung Chenga)
Department of Electrical Engineering, National Chi-Nan University, Nan-Tou, Taiwan, 54561, Republic of China
(Received 6 December 2013; accepted 3 March 2014; published 20 March 2014)
This study investigates the effect of the NH3/N2 ratio in plasma treatment on the physical
and electrical properties as well as the reliability characteristics of porous low-k films. All of the plasma treatments resulted in the formation of a thin and modified layer on the surface of porous low-k films, and the properties of this modified layer were influenced by the NH3/N2ratio in
the plasma. Experimental results indicated that pure N2 gas plasma treatment formed an
amide-like/ nitride-like layer on the surface, which apparently leads to a higher increase in the dielectric constant. Plasma treatment with a mixture of NH3/N2gas induced more moisture uptake
on the surface of the low-k dielectric, degrading the electrical performance and reliability. Among all plasma treatment with NH3/N2mixed gas, that with pure NH3 gas yielded low-k dielectrics
with the worse electrical and reliability characteristics. VC 2014 American Vacuum Society.
[http://dx.doi.org/10.1116/1.4868631]
I. INTRODUCTION
As feature sizes of integrated circuits continuously shrink to submicro, interconnect resistance–capacitance (RC) delay begins to dominate overall device speed as compared to gate delay.1–3To further decrease RC delay time, low resistivity copper (Cu) layer and ultralow dielectric constant materials (k < 2.6) are widely used as conductor and insulator, respec-tively, in the backend interconnect system.3–6To obtain the ultralow dielectric constant materials, introducing porosity into a dielectric film has become an attractive method to produce porous low-k film.6–8However, compared with tra-ditional SiO2 dielectric film, porous low-k films with a
decreasing dielectric constant value have unstable thermal, mechanical, and electrical properties. Therefore, integration of porous low-k films would induce some problems, such as high leakage current, low breakdown voltage, high moisture uptake, and weak tolerance against the chemical mechanical polishing process.8–10
During Cu/low-k interconnect fabrication manufacturing, plasma treatments are indispensible steps, such as etching, resist stripping, Cu barrier film deposition, and Cu oxide (CuOx) reduction.
11–13
During the plasma treatment process, low-k films surrounded with the Cu interconnects was also exposed in the plasma condition, resulting in the modifica-tion of low-k dielectrics. Moreover, the low-k surface may suffer from damage in this plasma process, resulting in a degrading electrical and reliability performance. Therefore, understanding of the plasma damage mechanism on the low-k materials is one of the key factors for successful
integration. In these plasma treatment processes, hydrogen (H2), nitrogen (N2) ammonia (NH3), and oxygen (O2) gases
were commonly used and had been reported.12–16However, these studies only focused on the film properties, rarely on the electrical performance and reliability. Consequently, we investigate the effects of the plasma treatment using mixture of NH3 and N2gas on porous low-k film properties in this
paper. The ratio of the NH3/N2gas in the plasma treatment
was the main parameter, and correlation of electrical proper-ties and reliability characteristics of the porous low-k film with the chemical structure of the film was studied.
II. EXPERIMENT
The porous low-k dielectric films were deposited on a p-type (100) silicon substrates by plasma-enhanced chemical vapor deposition. The porous low-k films were deposited from diethoxymethylsilane and alpha-terpiene as a matrix and porogen precursor, respectively. A small amount of oxygen was also introduced as an oxidant. The deposition temperature, pressure, and power were 300C, 7.5 Torr, and 600 W, respectively. After deposition, UV curing with 200–450 nm wavelength was performed to remove the or-ganic porogen to form porous low-k dielectric film with pore size and porosity of 1.4 nm and 15%, respectively. The resulting thickness and dielectric constant are about 300 nm and 2.56, respectively. Then, the porous low-k film (blanket wafer) was tested by NH3/N2plasma treatments with various
ratios, ranging from 0 to 1, in an inductive-coupled plasma chamber. The temperature, pressure, RF power, and treat-ment time in NH3/N2 plasma treatment were 300C,
2.0 Torr, 30 W, and 30 s, respectively. After plasma treat-ment, regular pattern of Al metallization with 100 nm thick a)
were thermally evaporated to form ohmic contact on the dielectric films through the shadow mask. All processes and analyses were conducted in a clean room with dry air atmos-phere. All analyses were performed within 1 day after per-forming NH3/N2 plasma treatment to reduce air exposure
effect.
The thickness and refractive index (at a 633 nm wavelength) of as-deposited films were analyzed on an optical-probe system with an ellipsometer (Film TekTM3000SE). The water contact angle (WCA) was determined as the average of five measure-ments (Reme Hardt, Mode 100-00-230). The reflectance of the low-k films was measured using ultraviolet-near-infrared spec-trometer (UV-NIR; Hitachi-U3900H). Chemical bonding of the film was investigated using Fourier transform infrared spectros-copy (FT-IR; Bio-Rad Win-IR PRO]. The compositions of the film surfaces were identified using x-ray photoelectron spectroscopy (XPS; VG Scientific Microlab 350). The electrical characteristics of low-k films were examined by capacitance– voltage measurements at 1 MHz using a semiconductor param-eter analyzer (HP4280A). Leakage and breakdown time meas-urements were done at room temperature (25C) on the metal–insulator–silicon (MIS) structure. The breakdown time is defined as the stressing time at a sudden rise of at least three decades of the leakage current.
III. RESULTS AND DISCUSSION
Figure 1 presents the change in the thickness of porous low-k films after plasma treatment as a function of the NH3/N2gas ratio in the plasma. We used a bilayer model of
ellipsometry measurement to measure the thickness of the top modified layer and the bottom bulk low-k film. As shown, a thin modification layer was formed on the top of the porous low-k films after plasma treatment. The variation in the thick-nesses of the top modification layer is less than 2 nm among all NH3/N2plasma treatment conditions. This value is within
the experimental error, revealing that the penetration depth that is caused by the plasma treatment depends less on the NH3/N2gas ratio than on the plasma treatment time, power,
and method.17Additionally, the total thicknesses shrink after
performing the plasma treatment, suggesting that the porous low-k films were densified by vacuum ultraviolet radiation, radical etching, and ion bombardment in the plasma with NH3/N2mixed gas. Furthermore, the sample was treated with
pure NH3 gas exhibited slightly higher thickness shrinkage
than the others.
The reflectance of the pristine and plasma-treated low-k films was measured using UV-NIR, as displayed in Fig. 2. As indicated, the wavelength of maximum reflectance was shifted from 500 nm to 450 nm upon NH3/N2 plasma
treat-ment. Moreover, all NH3/N2plasma-treated samples could
be divided into two groups based on the reflectance behavior. The reflectance of low-k samples that were treated with NH3
gas remained the same as that of the pristine low-k film. However, the reflectance of pure N2 gas plasma-treated
low-k films was reduced by approximately 20% relative to that of the plasma-treated sample with NH3 gas, revealing
that NH3gas in the plasma process differently modified the
top surface of the porous low-k films.
To investigate further the properties of the plasma-modified layer that were induced using various NH3/N2
ratios, diluted HF solution (1% volume) was used to etch the plasma-treated low-k films with various times. The etching rates of bulk films in all plasma-treated low-k samples were similar to that of the pristine low-k film, indicating that the bulk film in the plasma-treated low-k material was not affected by the plasma treatment. However, the etching rates of the top modified layer of the plasma-treated low-k films were higher than that of the pristine low-k film, as shown in Fig.3. Moreover, the etching rates of the modified top layer depend on the NH3/N2ratio in the plasma. The etching rate
has its minimum value at the NH3/N2ratio of 1.
FT-IR spectra of plasma-treated porous low-k films were analyzed to study the change of bonding structures upon plasma treatment. The obvious peaks in the FT-IR spectra of the porous low-k films are in the regions:1020–1050 cm1, 1250 cm1, 2200–2250 cm1, 2850–3100 cm1, and 3200–3500 cm1, corresponding to Si-O-Si bridging, Si-CH3
stretching, Si-H bending, C-HXstretching, and Si–OH/H-OH
peak, respectively.18 Figure 4plots the absorbance of these peaks relative to those of the pristine low-k film as a function
FIG. 1. (Color online) Thickness variation of porous low-k films after various NH3/N2plasma treatments.
FIG. 2. (Color online) Reflectance of porous low-k films after various NH3/N2plasma treatments.
of NH3/ (N2þ NH3) ratio in the plasma. The intensities of
Si-O-Si bridging in the NH3/N2 plasma-treated low-k films
remain unchanged for all conditions. However, thee peak intensities of Si-CH3 stretching and C-HX stretching are
slightly lower than those of the pristine low-k film, whereas the Si–OH/H-OH (3200–3500 cm1) peak intensity is higher. The maximum change is observed in the plasma-treated low-k film with pure NH3gas, indicting that pure NH3gas in the
plasma induces more replacement of Si-CH3 bonds by
Si–OH/H-OH bonds.
The surface roughness of the treated porous low-k films was examined using AFM. Figure5shows the AFM images of pristine and NH3/N2treated porous low-k films. No surface
defect or damage was observed in the plasma treated low-k films. The average root-mean square (rms) value is also pre-sented in Fig.5. The pristine low-k film had an rms value of 0.34, while the plasma-treated low-k films had higher values, and the rms value also increased as the NH3/N2 ratio
decreased. The low-k films that were plasma-treated with pure N2gas had the highest rms value of 0.41, possibly because
this plasma condition produces the most nitrogen active spe-cies in the plasma, which heavily bombarded the surface of the film.
XPS was used to investigate the changes in the surface composition of the porous low-k films following NH3/N2
plasma treatment. The three elements of Si, O, and O were observed in the as-deposited low-k film. Another element of N was detected in the low-k films that were treated by a mix-ture of NH3and N2gases. Additionally, the plasma-treated
low-k film with pure N2gas yielded the strongest N1s signal.
The bonding energy of the N1s peak is399.0 eV, which is attributed to Si-N and sp3C-N bonds.16,19,20Moreover, C 1 s XPS spectrum shifts to a higher binding energy at 289.0 eV, which is assigned to N-C¼O structures. The rel-ative atomic percentages of these elements in the porous low-k films were calculated using the relative elemental sensitivity factor method. Figure6plots the C and N atomic percentages relative to the Si atomic percentage as functions of the NH3/N2gas ratio. The figure reveals that plasma
treat-ment with a higher NH3/N2ratio caused severe carbon
deple-tion on the porous low-k film surface, whereas treatment with pure N2gas resulted in a much N incorporation.
One of the most important concerns associated with plasma treatment is surface hydrophobicity. Measurements of the WCA are commonly used to evaluate the hydropho-bicity of low-k films. Measurements were made at five sites on every sample, and Fig. 7plots the averaged results. The untreated as-deposited porous low-k film had a WCA of 90, larger than those of the treated films, indicating that
this as-deposited porous low-k film seems to be hydrophobic.
FIG. 3. Wet etching rate of various NH3/N2plasma-treated low-k films.
FIG. 4. (Color online) Relative absorbance of Si-O, Si-CH3, CHx, and
Si-OH/H-OH bonds of various NH3/N2plasma-treated low-k films.
FIG. 5. (Color online) AFM images and rms values of various NH3/N2
plasma-treated low-k films.
FIG. 6. (Color online) C/Si and N/Si atomic percentages of various NH3/N2
Upon NH3/N2 plasma treatment, the treated porous low-k
film had a lower WCA value, indicating that this treated po-rous low-k film was susceptible to the absorption of mois-ture. Moreover, the decrease in this value increases as the NH3/N2 ratio increases, suggesting that the porous low-k
film is more hydrophilic following the NH3/N2plasma
treat-ment with a higher NH3/N2 ratio. This trend is consistent
with the results of FT-IR and XPS analyses, which reveal that the porous low-k films that were plasma-treated with pure NH3 gas resulted in the formation of more Si–OH/
H-OH bonds and a larger subtraction of C atoms.
MIS structures were constructed herein to evaluate the dielectric property of porous low-k films under various NH3/N2 plasma treatment conditions. The change in the
dielectric constants of porous low-k films following plasma treatment at various NH3/N2gas ratios was calculated from
the measured accumulation capacitance of the MIS structure. The changes in the dielectric constants of NH3/N2
plasma-treated low-k films upon O2 plasma treatment for
1 min were also evaluated, as presented in Fig.8. The dielec-tric constant of the as-deposited pristine low-k films is 2.56. After NH3/N2 plasma treatment, the dielectric constant of
the plasma-treated low-k films increases. Under pure NH3or
pure N2 gas plasma treatment conditions, the increase is
larger, being 0.55 and 0.71, respectively. This can be
attributed to more formation of Si-OH bonds or Si-N/C-N bonds on the surface layer for pure NH3 or pure N2 gas
plasma treatment, respectively. Treatment with O2 plasma
increases the dielectric constants of all NH3/N2
plasma-treated low-k films by the replacement of Si–CH3
and Si-H bonds with Si-O bonds.12However, the magnitude of the increase depends on the NH3/N2 gas ratio in the
plasma. The increase in the dielectric constant becomes larger with the NH3/N2 gas ratio. The pure N2 gas
plasma-treated sample exhibits a smaller increase in the dielectric constant owing to the formation of protective Si-N/C-N layer. This layer suppresses the penetration of ox-ygen radical into the low-k film.
Figure9plots the leakage current densities at 1 MV/cm and 2 MV/cm for the porous low-k films treated with various NH3/N2gas ratios. The leakage current densities of the
pris-tine low-k films are presented for reference. The leakage cur-rent densities of all plasma-treated samples are higher than those of the pristine low-k film, suggesting that the top modi-fied layer induced by NH3/N2plasma treatment has a poor
insulating property, leading to an increase in the leakage cur-rent. In an electrical field of 1 MV/cm, all plasma-treated low-k films, except that treated with pure NH3gas, exhibit
the similar increases in the leakage current. In the elevated electrical field of 2 MV/cm, pure NH3 or pure N2 gas
plasma-treated low-k films exhibited the larger increase in the leakage current; the former exhibited the largest. The largest increase in the leakage current for pure NH3 gas
plasma-treated low-k film can be attributable to more absorp-tion of moisture, which provides ionic conducabsorp-tion pathways by releasing mobile ions (Hþ, OH).21
Based on the results of the above analyses, the reaction mechanism in the NH3/N2plasma can be described as
fol-lows. In pure N2gas plasma, only N, N2, and N2* active
spe-cies are generated, no hydrogen spespe-cies is produced. Physical bombardment by N radicals is favorable, roughing the film’s surface. Moreover, the weak bonds in the low-k dielectric film, such as Si-H, Si-CH3, and C-Hx bonds, can
be broken by these active species in the plasma, forming Si-N and C-N bonds. As NH3gas was added into the plasma,
other active species in addition to the N, N2, and N2* active
FIG. 7. Water contact angle of porous low-k films after various NH3/N2
plasma treatments.
FIG. 8. (Color online) Change in dielectric constant of porous low-k films after NH3/N2and O2plasma treatments.
FIG. 9. (Color online) Increase in leakage current density at 1 MV/cm and 2 MV/cm of porous low-k films after various NH3/N2plasma treatments in
MIS test structures.
species, such as H, NH2, NH4, and N2H, may be generated.
The Si-CH3 group in the low-k film is broken to form
Si-dangling bonds. This Si-dangling bond easily absorbs H or NH2
species to form Si-H or Si-NH2bonds due to a lower
reac-tion energy, which is thermodynamically favorable.22–25The Si-H and Si-NH2bonds are not stable in air and easily react
with ambient air to form Si-OH, which is more hydrophobic and has a higher dielectric constant. As the portion of NH3
in the plasma increases, the number of H and NH2 active
species increases accordingly. At the same time, the amount of the generated N, N2, and N2* active species is limited
because more energy is required to generate these active spe-cies due to a fixed plasma power of 30 W. These changes in the plasma result in the significant replacement of CH3
groups by H and NH2active species, the formation of more
Si-OH bonds, and the reduction of Si-N and C-N bonds. To understand further the dielectric reliability, voltage ramping-up to dielectric breakdown of the porous low-k films following plasma treatment under various NH3/N2
con-ditions was measured using MIS test structures. The dielec-tric breakdown voltage is defined as the voltage at which the leakage current suddenly increases. The dielectric break-down electric-field was calculated as the measured dielectric breakdown voltage divided by the film thickness. Figure10
plots the calculated dielectric breakdown electric-fields from 20 sites. As shown in the figure, all NH3/N2 mixture gas
plasma-treated samples had a poorer dielectric breakdown performance than the pristine low-k film. Moreover, the breakdown electric-field of the NH3/N2 mixture gas
plasma-treated low-k films decreases as the NH3/(N2þ NH3)
ratio increases. The sample that was plasma-treated with pure NH3gas has the lowest breakdown voltage and the highest
leakage current.
To investigate the effect of NH3/N2 gas ratio in the
plasma on the low-k dielectric’s long term reliability, time-dependent-dielectric breakdown (TDDB) was per-formed to measure the dielectric breakdown times. Three different electrical-fields were applied to stress Al/porous low-k/Si MIS capacitors. Measurements were made on 20 capacitors for each condition and the measured dielectric breakdown times were expressed as a Weibull distribution.
In a Weibull distribution analysis, two important parame-ters are used to evaluate the reliability of a dielectric film: T63.2% is the characteristic breakdown time for 63.2% of
failure and b is the Weibull slope or shape parameter, which governs the breakdown distribution. Compared to the pristine low-k film, all plasma-treated samples had shorter characteristic lifetimes and a smaller b value, indi-cating that NH3/N2 plasma treatment shortened dielectric
breakdown time and widened the distribution. The reduc-tion of the dielectric breakdown time may be caused by an accumulation of defects owing to plasma-induced damage. Figure 11compares T63.2%values as a function of applied
electric-field for the pristine and plasma-treated samples. All plasma-treated samples exhibited reduced characteristic lifetimes, and these reductions were significant in stronger stressing electric-fields. Additionally, in the same applied electric-field, the T63.2% values of plasma-treated low-k
films decreased as the NH3/(N2 þ NH3) ratio increased.
These results were found to correlate well with the moisture contents in the plasma-treated films. Accordingly, the pure NH3plasma-treated samples with the highest moisture
con-tent show the shortest time-to-failure, indicating that the moisture content in a low-k film plays an important role in reducing dielectric time-to-failure. However, the samples that were plasma-treated with pure N2gas had the longest
time-to-failure, indicating that amide-like or nitride-like layers on the surface retard low-k dielectric breakdown. Furthermore, the log of the characteristic lifetimes of the studied low-k films is proportional to the electric-field; therefore, the lifetimes can be fitted using E-model (ther-mochemical model).26 The lifetime of the pristine low-k film was found to be well fitted with E-model and the electric-field acceleration factor (c) was 11.21. However, the lifetimes of all samples that were plasma-treated with various NH3/N2 gas ratios were found to derivate
signifi-cantly from the E-model, especially for the plasma-treated sample with a higher NH3/N2 gas ratio. These findings
reveal that the plasma-treated samples exhibit an additional failure mechanism in additional to the thermochemical
FIG. 10. Dielectric breakdown field of porous low-k films after various NH3/N2plasma treatments.
FIG. 11. (Color online) Characteristic dielectric breakdown time at 63.2% failure rate of porous low-k films after various NH3/N2plasma treatments as
driven mechanism. The absorption of moisture in the plasma-treated low-k films generates mobile ions that may be responsible for this additional failure mechanism.
The charge-to-breakdown (Qbd) data were calculated by
integrating the leakage current as a function of time obtained from TDDB testing until breakdown, which is plotted in Fig.12. As expected, the pristine samples display the largest Qbd values and remain constant value in the range of the
stressing electric-fields that were analyzed in this study. The NH3/N2plasma-treated low-k films resulted in a reduced Qbd
value and the total Qbdvalue decreased with increasing the
NH3/N2gas ratio. The plasma-treated low-k films with pure
NH3gas had the lowest Qbdvalue, which was approximately
two orders of magnitude lower than that of the pristine sam-ple. This finding also agrees with the likelihood of an addi-tional failure mechanism for NH3/N2 plasma-treated low-k
films. This additional failure mechanism may be associated with the absorption of moisture in plasma-treated samples. Moreover, as the applied electric-field was increased, the total Qbd value observably decreased, revealing that the
released mobile ions from the absorbed moisture caused more TDDB degradation in a stronger electric-field.
IV. CONCLUSIONS
In this study, the impact of various NH3/N2gas mixtures
in the plasma on the porous low-k dielectrics was investi-gated. The physical, electrical, and reliability characteristics were compared. Experimental results indicate that all plasma treatments resulted in the formation of a modified layer on the surface of porous low-k films, and the properties of this modified layer were affected by the NH3/N2 ratio in the
plasma. A plasma treatment with pure N2 gas formed an
amidelike/nitridelike layer on the low-k film’s surface, lead-ing to a higher dielectric constant. Plasma treatment with a mixture of NH3/N2gas induced more moisture uptake on the
surface of the low-k dielectric, degrading the electrical performance and reliability. Among all plasma treatment with NH3/N2 mixed gas, that with pure NH3 gas yielded
low-k dielectrics with the worse electrical and reliability characteristics.
ACKNOWLEDGMENTS
The authors would like to thank the National Science Council of the Republic of China, Taiwan, for financially supporting this research under Contract No. NSC-102-2221-E-260-009.
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