第39卷增刊2稀有金属材料与工程
get throughV ol.39,Suppl.22010年8月
RARE METAL MA TERIALS AND ENGINEERING
August 2010
Received date:2010-04-01
Foundation item:Chine National Foundation of High Technology (2008AA8041606);Shanghai Key Laboratory of Special Artificial Microstructure Materials and Technology (07DZ22302);Shanghai Educational Development Foundation (2007CG26)and Program for Y oung x T T j U y (6K 5)
S ,D ,f ,I f S S y ,T j U y ,S ,R ,T F x 6656,j 6@j Low Dielectr ic Constant Nanopor ous Silica Films
Shen Jun 1,Zhu Yumei 1,Lin Xuejing 1,Wu Guangming 1,Zhou Bin 1,Ni Xingyuan 1,Yao Lanfang 2,Wang Guoqing 3,Wang Peiqing 3,Wang Qingfeng 3,Niu Xixian 4
(1.T ongji University,Shanghai 200092,China)
(2.University of Shanghai for Science and Technology,Shanghai 200093,China)(3.Shanghai Sunvea Chemical Material Co.Ltd.,Shanghai 200235,China)(4.Zhongxing Electric Automobile Corporation,Shanghai 200433,China)
Abstr act:A novel route to prepare low-dielectric constant mesoporous SiO 2films was reported.Silicate sols were prepared with the precursor of TEOS and template of CTAB catalyzed by hydrochloric acid.The films were prepared by dip-coating process.FTIR,XRD and AFM were employed to characterize the films.The dielectric constants were measured by impedance analysis apparatus.The films with dielectric constants smaller than 2.2could be acquired by adjusting the concentration of CTAB and aging time.
Key wor ds:low dielectric constant;thin films;sol-gel process CLC number :O78
Document code:A
Ar ticle ID:1002-185X(2010)S2-031-05
Multilevel interconnect technology beyond the 0.13micron generation requires interlayer dielectric materials with much lower dielectric constant (k)than the traditional SiO 2(k=4.0)to reduce the capaci
tance between the metal wires,and thus reduce the RC delay and cross-talk noi of the circuits [1].A great deal of intensive rearch has been conducted in this area.The materials currently studied include non-fluorinated polymers,inorganic-organic hybrids [2],porous polymer [3]and porous silica [4].Among them porous films with the possibility to achieve ultra-low-k are considered to be the most promising materials for the deep submicron technology and future integrated circuits (IC)process technology.Silicon bad porous films are especially attractive becau they usually have good chemical and thermal stability;furthermore,they are compatible with the silicon wafer and related materials ud in existing IC technology.The porous xerogel films can be prepared by traditional sol-gel process and k-values in the range of 1.5-2.5have been obtained.However,low mechanical strength and hydrophilicity are significant concerns with respect to the materials becau of their amorphous nature.
In this paper,a novel route to prepare low-dielectric
constant nanoporous SiO 2films is reported.Silicate sols are prepared with the precursor Si(OC 2H 5)4(TEOS)and surfactant cetyltrimethyl ammonium bromide (CTAB)catalyzed by hydrochloric acid.The films are prepared by dip-coating process.
The films with dielectric constant smaller than 2.2(1MHz),can be acquired by adjusting the concentration of CTAB and aging time.A number of treatments to develop the mechanical strength,scratch-resist and hydrophobic properties of the films are also introduced.
1Exper imental
1.1Synt hesis of m esoporous silica films
The mesoporous silica films were prepared from sol-gel process via dip coating.A prehydrolyzed solution was firstly prepared by hydrolysis and condensation of tetraethoxysilane (TEOS:Si(C 2H 5O)4).TEOS,ethanol,deionized water and hydrochloric acid were mixed at room temperature with a mole ratio of 1:25:1.2:0.06and refluxed at 70°C for 1h.Then additional water and HCl was added into the refluxed sol at room temperature.After being aged for 3h,another solution obtained by dissolution of CTAB in ethanol was added into the mixture,and the CTAB/TEOS molar ratio was varied
E cellent alents in on g i niv ersit 200J02Co rr esp on din g au th or :hen Jun Ph..Pro essor Po hl n stitute o olid tate Ph sics on g i niversit hanghai 200092P..China el/a :008-21-98071E-mail:shen u i.ed u
·32·稀有金属材料与工程第39卷
from0.05to0.17.Clear and stable sols without any
precipitation were obtained.The templated films were
prepared by dipping silicon wafers and glass in the
solution previously aged for1d at ambient temperature.
Some samples were introduced into a glass vesl
containing10mL of hexamethy-disilazane(HMDS).
The vesl was heated from25to80C at a heating rate
of1°C/min and kept at80C for12h.The obtained
films were calcined at450C for4h with a heating rate
of1C/min to eliminate the surfactant.
1.2Char acterizat ion
Fourier-transform infrared spectroscopy(FTIR) spectra of the films were recorded in the500-3800cm-1 range using BRUKER TENS0R-27spectrometer.The films were also analyzed using a thermogravimetric analyzer(TGA).The mesostructure of the films were characterized by X-ray diffraction(XRD,Rigaku D/max 2550V)with Cu Kαradiation(40kV,100mA).An atomic force microscope was ud to obrve surface structures of the mesoporous silica films.The mesostructure was identified by transmission electron microscopy(TEM)using a JEOL JEM-2100F with an accelerating voltage of200kV.For the TEM obr-vations,the samples were peeled off from the substrate. The refractive index and thickness of the films were measured and calculated by ellipsometry(EIJJP-A)with 632.8nm light at an angle of incidence of70°.
For the dielectric constant measurement,metal dots were deposited on the film using thermal evaporation through a contact mask.The backside of the silicon wafer also had a platinum layer deposited on it.The capacitance of the aforementioned metal-insulator-metal structure was measured using a HP4284LCR at a frequency of1MHz.The dielectric constant was calculated from the capacitance,the film thickness and the area of the metal electrode.
2Results and Discussions
2.1Ther ma l pr oper ties of m esoporous silica film s
The as-synthesized films were expected to have both solvent and CTAB inside.In Fig.1,TGA results of the as-synthesized films show the mass loss at about60, 260and400 C.The60and260C mass loss peaks indicate desorption of solvent and CTAB respectively. Between260and400C there is a steep mass loss, which is due to a drastic decomposition of the residual CTAB and the removal of the surface methyl groups.
Fig.2shows the FTIR spectra of the sample A,B and C(all CTAB/TEOS m olar rati o was0.10).The
566-1
f
Fig.1Thermogravimetric analysis(TGA)of mesoporous silica film
Fig.2FTIR spectra of mesoporous f ilms:(A)as-synthe sized,(B) 450C directly calcined,(C)treated by HMDS vapor
and calcined at450C
surfactant in curve A of Fig.2.But the abnce of the C-H stretching vibration modes in the trace B and C indicate the complete removal of the surfactant via the decomposition with450C calcination[5].
A broad and complex band around3300-3700cm-1 is due to the interaction of water molecules via hydrogen bonds with Si-OH groups inside the pores.This absorption band is greatly reduced after HMDS vapor treatment,suggesting the hydrophobicity of the film is improved,so the stability of the film in the air is also improved.The increa in absorbance of Si-O-Si antisymmetric stretching mode around791cm-1and the disappearance of Si-OH stretching vibrations around950 cm-1in trace C indicates that more Si-O-Si bonds are formed and the silanol groups are reduced.Therefore, the silica network was strengthened,and thus the mechanical strength of the films incread.
2.2Refr active index and dielectr ic consta nt of
m esopor ous silica films
Mesoporous films posss a high specific surface area,in most cas,open pores.The contaminations
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增刊2Shen Jun et al:Low Dielectric Constant Nanoporous Silica Films·33·
refractive index drift(shown in Table1).The initial
refractive index of the film with HMDS vapor treatment
is1.19,which is a little higher than that without surface
modification(1.18),yet still interesting as low-indexes.
However,the refractive index of unmodified film is not
quite stabilized.It was found that the refractive index of
the directly calcined film was incread from1.18to
1.34after5months of storage under ambient condition,
while the modified film was incread from1.19only to
1.23.The treated film shows a good stability over a long
should是什么意思
time period becau of the reduced silanol quantity after
the treatment.The HMDS vapor treatment is considered
to be efficient to increa the hydrophobicity and the
stabilization of the films.
For the dielectric constant measurement,the Au
jerrelectrode was sputtered on the film.The area of the Au
electrode is0.25mm2.The dielectric constant was
calculated using the standard expression k=Cd/ε
A,
where C is the capacitance,d is the thickness of the film,
ε0is the permittivity(8.854×10-12F·m-1),and A is the
electrode area.The thickness of the untreated and
HMDS-treated films were estimated as181and190nm,
respectively.The dielectric constant k of the untreated
film was2.14,and that of HMDS-treated film was2.21,
which are approximately the same as that calculated
from the correlation[6,7]:
1 6.33(1)
k n
=+-(1)
The results are shown in Table1.
Table1Dielectr ic consta nt a nd r efr active index of
m esopor ous films
Sample Refractive index(after5months
storage under ambient condition)
Porosity/%
Dielectric
constant
B 1.18(1.34)61 2.14
C 1.19(1.23)59 2.21
2.3Effect of surfact ant concentr ation on mesost ru-
ct ur e and dielectr ic proper ties of t he films
The mesostructures of the films change with the ratios of surfactant/TEOS.Fig.3shows the typical XRD patterns of the calcined films with different mole ratios of CTAB/TEOS as the structure directing agents.In Fig.3, strong and narrow different peaks of film B and D are obrved in the2θrange of1°-5°,which are the typical X-ray diffraction peaks of well-ordered mesoporous silica films.On the other hand,there is a little difference between the positions of the diffraction peaks in the two samples.Strong and narrow(100)Bragg diffraction peak associated with a one-dimensional hexagonal mesostruc-
y B f F3T ff f f()()
Fig.3XRD patterns of the calcined ordered nanoporous SiO2 films with different mole ratios of CTAB/TEOS:
(B)0.10,(D)0.15and(E)0.17
becau the axis of CTAB hexagonal tubes tend to parallel the surface of the substrates.In spectrum D of Fig.3,three diffraction peaks were obrved in the2θrange of0.5°-2°,which can be indexed as(200),(210) and(211)reflections of a cubic mesostructure,and the results are in good agreement with that reported by Kresge[7].When the mole ratio of CTAB/TEOS incread to0.17,the films have no typical diffraction peaks as shown in spectrum E of Fig.3,which indicate it has no mesoporous structure.According to the rearch by Kresge[7],with the increa of CTAB concentration, the hexagonal structures change to cubic structures and then to lamellar structures during the synthesis procedure of mesoporous powder.Therefore,it is reasonably considered that the films with the0.17mole ratio of CTAB/TEOS have a lamellar mesoporous structure after drying.However,the lamellar mesoporous structures of the films are easy to collap in the process of burning out of the CTAB,which result in the disappearance of the mesoporous structure.In contrast,the mesoporous structure of hexagonal and cubic pha can be maintained during the procedure of thermal treatment as shown in Fig.3.Fig.4shows TEM morphology of sample B which have ordered hexagon
al mesoporous structure.
The above estimation is supported by AFM obrva-tion.Fig.5gives the AFM morphology of the silica films
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2
·34·稀有金属材料与工程第39卷
formed at the0.10and0.15CTAB concentrations.It can
be found that there is a distinct difference between the
surface morphologies of the films with different CTAB
concentration.Fig.5a shows that the micro units of the
films surface prent a strip-shaped character.The
rearch by Kresge[7]indicated that the strip-shaped units
should be tubiform pores,which is the typical character of
the hexagonal films.It can also be en in Fig.5a that the
array of most tubiform pores are parallel with the surface
of the substrate,which result in the lack of(110)and(210)
diffraction peaks in spectrum B of Fig.3.In Fig.5b there
are many hatch of the pores which can be en on the film
surface becau the pores are three-dimensional cubic
食堂英文formation.Thus,AFM obrvations for the films show the
good agreement with the XRD results as shown in Fig.3.
It has been found that as the concentration of CTAB
incread,the array of mesoporous structure change
continuously.If the concentration of surfactant is not
proper,the surfactant will not act as template.The
refractive index and dielectric constant of the film are strongly dependent on film mesoporous structure.As a result,the surfactant concentration governs the properties of the films.The dielectric constant and refractive index of the calcined films are given in Table 2.From the table it can be en that compared with acid catalyzed silica films(n=1.4,k=3.8),the dielectric constant and the refractive index of the mesoporous films decrea largely,which indicate that the films have high porosity.Additionally,the concentration of CTAB can have big influences on the dielectric constant and refractive index of the films.When the concentration is low,the film will have high dielectric constant for low porosity in the film after thermal treatment.The dielectric constant of the film decreas with the increa of CTAB concentration.However,the dielectric constant of the calcined films will increa if the CTAB concentration is too high becau the excessively high concentration will lead to the collap of mesoporous structure in the process of CTAB removal.
2.4Mechanical pr oper t ies of m esoporous films
The mechanical properties were tested as following: a drag-wipe test was rubbing the coating100times using a thin blotting paper slightly soaked into ethanol.Visual obrvation was ud
to verify the prence of the coating.Fine obrvation was also carried out under opti cal m ic roscopy using t he100magnifi cat ion. Abrasion tests can give qualitative information on the durability and mechanical properties of the films.Table 2lists a qualitative comparison of different samples.It
f f yz
y f
Fig.5AFM morphology of the calcined films with different mole ratios of CTAB/TEOS:(a)0.10and(b)0.15
Table2Com par ison of dielectr ic consta nt a nd mechanical pr oper ties for the calcined m esopor ous and ba
ca talyzed silica films
Sample Abrasion n k Ba catalyzed film Coating damaged 1.17 2.08 Mesoporous film with
CTAB:TEOS=0.05
Coating unaffected 1.31 2.96 Mesoporous film with
CTAB:TEOS=0.10
Coating unaffected 1.18 2.14 Mesoporous film with
CTAB:TEOS=0.15
Coating unaffected 1.20 2.27 Mesoporous film with
CTAB:TEOS=0.17
Coating unaffected 1.26 2.65
low k.But good abrasion resistance was measured in the films derived with the method ud here,and the k-value can be still kept low.
3Conclusions
In this paper,the film with high mechanical property,low k and low-n could be obtained with a surfactant under the acid catalyzed condition.The concentration of the surfactant played a crucial role in the mesoporous structure and dielectric constant of the films.After HMDS vapor treatment,thei
r stabilization towards moisture incread,despite of the little increa in the refractive index.
Acknowledgements:The authors are very grateful to the program N N S F f (55),N F f T y
a
b
was obrved that the ilms derived rom ba catal ed sols had ver poor abrasion resistance in spite o their supporters:ational atural cience oundation o China 072001Chine ational oundation o High echnolog
增刊2Shen Jun et al:Low Dielectric Constant Nanoporous Silica Films·35·
(2007AA804807,2007AA804137),Shanghai Committee of Science and T echnology(055211010,0652nm044,07JC14052).
R eferences
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[3]Josh H Golden,Craig J Hawker,Paul S Ho.Semiconductor
Inter na tional[J],2001,24(5):79
[4]Fan H Y,Bentley H R,Kathan K R et al.Non-Crysta lline
Solids[J],2001,285(1-3):79
[5]Jung J I,Bae J Y,Bae B S.Journal of Sol-Gel Science and
T echnology[J],2004,31(1-3):179
[6]Jo M H,Park H H,Kim D J et a l.Jour na l of A pplied Physics
[J],1997,82(3):1299吸血鬼日记第四季第五集>biow job
[7]Kresge C T,Leonowicz M E,Roth W J et al.Nature[J],1992,
359(22):710
低介电常数纳米氧化硅薄膜
沈军1，朱玉梅1，林雪晶1，吴广明1，周斌1，倪星元1，
clutch什么意思姚兰芳2，汪国庆3，汪培庆3，汪庆峰3，牛锡贤4
(1.同济大学，上海200092)
(2.上海理工大学，上海200093)
(3.上海暄洋化工材料科技有限公司，上海200235)
(4.中星电动车科技有限公司，上海200433)
摘要：介绍了一种新制备低介电常数SiO2薄膜的方法。以TEOS为前躯体、盐酸为催化剂、CTAB作为模版剂，采用溶胶-凝胶法制备硅溶胶，以浸渍提拉法制备薄膜。采用FITIR、XRD和AFM等方法表征了薄膜，并用阻抗分析仪测量介电常数。结果表明，通过调节CTAB的浓度和老化时间可以制得介电
常数小于2.2的SiO2薄膜，薄膜拥有较好的机械强度和耐刮擦性，通过采用六甲基二硅胶烷(HMDS)对薄膜表面进行修饰，可以提高薄膜的疏水性能从而提高其在空气中的稳定性。
关键词：低介电常数；薄膜；溶胶-凝胶法
作者简介：沈军，男，1967年生，博士，教授，博士生导师，同济大学波耳固体物理研究所，上海200092，电话/传真：************，E-mail:****************
