2003 Preparation of Ultralow Dielectric-Constant Porous Silica Films-to wq

Preparation of Ultralow Dielectric-Constant Porous Silica Films Using Tween80as a Template

Chih-Yuan Ting,De-Fa Ouyan,and Ben-Zu Wan z

Department of Chemical Engineering,National Taiwan University,Taipei106,Taiwan

Porous silica?lms with ultralow-k?1.47-2.36?and low leakage current densities(10?7A/cm2or lower in an electric?eld of1 MV/cm?were prepared by a surfactant-templating process.The nonionic surfactant Tween80?also known as Polyoxyethylene?20?sorbitan monooleate?was used as a template in the spin-on?lm preparation process.The effects from the parameters for preparing coating solution were carefully investigated.Due to its simplicity,the reported process may be a promising one for the preparation

of ultralow-k dielectrics in the future.

?2003The Electrochemical Society.?DOI:10.1149/1.1592521?All rights reserved.

Manuscript submitted April29,2002;revised manuscript received October17,2002.Available electronically July1,2003.

As feature sizes in integrated circuits approach0.13?m?transis-tor gate length?and below,problems with propagation delay, crosstalk noise,and power dissipation due to resistance-capacitance ?RC?coupling become signi?cant.In order to solve these problems,

it is hoped to introduce low dielectric-constant(k?2.0)materials as the interlayer dielectric in future advanced microprocessors.Cur-rently in the progress of low-k materials,one of the approaches is to develop nanoporous silica?lms with k in the range of1.3to3.0. Porous silica?lms can be prepared either by an aerogel or a xerogel process1-3or by a surfactant-templating process.4-10However,?lms prepared by aerogel or xerogel process usually have a random pore-size distribution,which is not favored in the semiconductor industry. On the other hand,the surfactant-templating process usually guar-antees a much more ordered pore arrangement7and a more rational pore size??60??.These advantages make?lms prepared by surfactant-templating process suitable for low-k dielectric applica-tion.

Recently two research groups reported the preparation processes for making mesoporous low-k silica?lms by the templating method.7-9Both used tetraethylorthosilicate?TEOS?as the silica source and ethanol as the solvent.Baskaran et al.?rst applied poly-oxyethylene ether surfactant as the template.7,8They used a20h period of time for aging the coating solution.The resultant k values of the?lms were in the range of1.80to2.50,and the leakage current density of the?lm was not mentioned in their published papers.However,it should be noted that,because the dielectric serves as an insulator between metal lines,low leakage current den-sity of the?lm is also important for a https://www.wendangku.net/doc/806945873.html,ter,Yang et al. applied P123tri-block copolymer as the template.9Additional pro-cedures such as a TEOS re?uxing step and a trimethylchlorosilane ?TMCS?mixing step were adopted for the preparation of the coating solution.However,although the resultant k values of the?lms were in the range of1.42to2.50,there may have been some problem from the solubility of P123in the ethanol solution,which may cause cracks on the?lms when a high concentration of P123is used.10 The templating method for making the low-k?lms,nevertheless has not been systematically presented regarding the effects of the coating solution composition on the low-k values and the leakage current densities of the?lms,although these are important for the future low-k?lm processing.Therefore in this research,we report the effects of the preparation conditions?i.e.,solution composition and mixing time?on the properties of ultralow dielectric constant ?lms by a surfactant-templating method.Polyoxyethylene?20?sor-bitan monooleate?also known as Tween80?was used as the surfac-tant.It was found in this research that?lms prepared with Tween80 surfactant possessed good properties such as ultralow dielectric con-stant?1.47?and low leakage current density(10?7A/cm2or lower?. Furthermore,the surface of the?lm prepared in this research was crack-free,as shown in Fig.1a.The thickness of the?lm was in the range of4000to6000?,depending on the composition of the coating solution.The surface was quite smooth.The roughness around10?over25?m2was measured by an atomic force micros-copy?AFM,Fig.1b?.The superior properties of our low-k?lms made in this research suggest that the Tween series surfactants are one of the potential templates for the preparation of nanoporous silica?lms in the future.

Experimental

The coating solution in this research was prepared by mixing TEOS?1M?,deionized water?6-12M?,ethanol,HCl?hydrochloric acid0.1-0.4M?and Tween80(Mw?1310)at30°C for3h.TEOS was from Merck,and the other chemicals were from Acros.The coating solution was spin-coated on a precleaned silicon wafer?4or 6in.?at a speed of2600rpm for30s.Then the wafer was baked at 106°C for3h and was calcined at475°C for5h in air?ow.Finally, the?lm surface was modi?ed to be hydrophobic by immersing it in a hexamethyldisilazane?HMDS?/tolune solution at80°C and dried at100°C for3min.

For capacitance and leakage current measurements of the?lms, an array of alumina dots of thickness about5000?were formed on the topside of the?lm by condensation of aluminum vapor through an aluminum shadow mask.The back side of the silicon wafer was etched by HF to remove the native oxide and then thermally coated with aluminum.Capacitance measurements were performed with a Keithley model82CV meter.The frequency and the oscillation level were1M Hz and100mV,respectively.The dielectric constant was calculated from the capacitance,the?lm thickness,and the area of electrode.The leakage current density of the?lm was determined from the current-voltage(I-V)characteristics measured by a HP4156semiconductor parameter analyzer.The?lm thickness was measured from cross-sectional scanning by electron microscopy ?SEM?taken on an S-800?Hitachi?.The surface roughness was examined by an atomic force microscope?AFM,Digital Instrument, Nanoscope III a?.

Results and Discussion

Effects of surfactant amount in the coating solutions.—The di-electric constants and the leakage current densities of the?lms pre-pared with different amounts of Tween80are shown in Fig.2.In the coating solutions,the molar ratios of TEOS/HCl/H2O/ethanol were maintained at1/0.25/4.2/8.6.Because Tween80acts as a template in the?lm and is removed after calcination,if the amount of Tween80 is increased in the solution,the pore volume in the?lm is expected to increase and the dielectric constant of the?lm is expected to be lower.Nevertheless,from the data shown in Fig.2,it can be found that dielectric constants are around1.9and are not changed signi?-cantly with the amount of Tween80.In order to investigate what caused this outcome,bulk silica samples were prepared.Instead of spin-coating,a thick layer of solution in a?at-bottomed culture dish was quickly dried in a vacuum oven.After baking at106°C for3h, the samples were scratched from the container and calcined at

z E-mail:benzuwan@https://www.wendangku.net/doc/806945873.html,.tw 0013-4651/2003/150?8?/F164/4/$7.00?The Electrochemical Society,Inc.

475°C for 5h.Nitrogen adsorption-desorption experiments over these samples were conducted for the surface area and the pore size distribution.The results from the detailed analysis of the isotherms are summarized in Table I.It is found that the major pore sizes of the bulk samples are about 35?,which correspond to the mesopo-res.Because approximately 80%of the pore volume is contributed

by these mesopores,it suggests that the ?lms are composed of pores of uniform size distribution.Besides,it is interesting to ?nd out that the surface areas were increased,however,the pore volumes were nearly the same when the amount of Tween 80was increased.Therefore,it is proposed that pores in the samples templated by too much Tween 80collapsed during the calcination process.This may explain why the dielectric constants in the ?lms,shown in Fig.2,were not changed with the amount of Tween 80added in the solution.

In order to verify more about the pore collapse in the ?lm,the theoretical k values of the ?lms prepared using different amounts of surfactants were calculated and compared with the experimental k values.The following assumptions were made for the calculation.First,the pore volume in the ?lm was totally from the volume oc-cupied by the surfactants.Second,the dielectric constant of the ?lm is a linear combination of those of air (k ?1)and silica (k ?4).From assumption 1,the porosity of the ?lm can be evaluated,and the theoretical k values can be calculated through assumption 2.The results are shown in Fig.2as triangles.It can be found when the molar ratio of Tween 80to TEOS was as small as 0.06,the experi-mental k value ?1.8?was found to be close to the calculated one ?1.75?.However,when the ratio of Tween to TEOS was increased from 0.06,a greater difference between the experimental and the calculated k values gradually developed.These results indicate the collapse of pores in the silica ?lm,when the ratio of Tween to TEOS is larger than 0.06.The collapse is more pronounced when more Tween is in the ?lm.

It is known that in the sol-gel process under acidic environment,water is the reactant for the hydrolysis of TEOS to form https://www.wendangku.net/doc/806945873.html,ter silanol is polycondensed to form an SiO 2framework and wa-ter molecules.Acid is the catalyst for both hydrolysis and polycon-densation reactions.Therefore,for preventing the pores in the ?lm from collapsing and for the improvement of k values of the ?lm further,the effects of the amount of water and acid in the coating solution and the effect of solution mixing time were examined.In these later experiments,the molar ratio of Tween to TEOS at 0.13was used.The theoretical k value was calculated as 1.42,and it was our goal to prepare a porous silica ?lm with a k value close to this number.

Effects of the amount of water in the coating solution .—The effect of the amount of water in the solution on the ?lm properties is shown in Fig.3.The molar ratios of TEOS/Tween 80/HCl/ethanol in the coating solution were maintained at 1/0.13/0.25/8.6,and the so-lution mixing time was maintained for 3h.It was found that the leakage current density was too large to be measured,when a low molar ratio of H 2O/TEOS at 0.86was used.The dielectric

constants

Figure 1.?a ?.Cross-sectional SEM image of the surfactant-templating me-soporous ?lm prepared from the colloid with Tween 80as the template;?b ?.Top view AFM

image.

Figure 2.Variation of dielectric constant ???and leakage current ???at an electric ?eld of 1MV/cm for ?lms prepared from solutions with different amounts of Tween 80.In the solutions,the molar ratios of TEOS/HCl/H 2O/ethanol ?1/0.25/4.2/8.6.

Table I.Surface area and pore volume of porous silica prepared from colloids a with different amounts of Tween 80as the tem-plate.

Tween 80/TEOS ?molar ratio ?Brunauer-Emmett-Teller surface area ?m 2/g ?Total pore volume b ?cm 3/g ?Mesopore size ???Percentage

of mesopore volume c to total

pore volume ?%?0.063220.2373592.80.094260.3053888.30.115130.3273488.20.13

5320.3123481.1

a The molar ratios of TEOS:HCl:H 2O:ethanol ?1:0.25:4.2:8.6.b

Total pore volume at P /P 0?0.95single point ?cm 3/g ?.c

Mesopore volume,measured from pore sizes between 20and 50?of isotherm desorption curve,divided by the total pore volume.

of the ?lms were decreased from a high value of 7.12,when the molar ratios of H 2O/TEOS were increased from a low value of 0.86.In fact,when the ratios were higher than 2.5?in the range of 2.5to 8.8?,the dielectric constants were around 2.They were not signi?-cantly changed with the amount of water in the coating solution.It is seen that for the hydrolysis of TEOS,four molecules of water are required to have a complete conversion of a molecule of TEOS to a molecule of silanol.In the later polycondensation from silanols,an average of two molecules of water is produced with the formation of one of SiO 2.Therefore,for the formation of a SiO 2from a TEOS,at least two water molecules are needed.In comparison to the experi-mental data,it is apparent that there was not suf?cient water in the coating solution with a low molar ratio of H 2O/TEOS at 0.86.The high k value of the ?lm from this solution was mainly due to the incomplete hydrolysis of TEOS in the coating solution.However,the hydrolysis reaction should be close to complete when the molar ratios of H 2O/TEOS were larger than 2.5.This is because,when the molar ratio of H 2O/TEOS was larger than 2.5,the amount of H 2O in the solution for hydrolysis reaction was no longer a signi?cant factor for improving the dielectric constant of the ?lms.Therefore,the collapse of pores or the higher k values than that calculated should be from incomplete polycondensation rather than from hydrolysis.Effects of mixing time on the coating solution .—The longer mix-ing time for the solution is one of the ways to make the polycon-densation reaction more complete.In order to understand the effect of mixing time,a series of experiments was carried out.In the so-lution the molar ratios of TEOS/Tween 80/HCl/H 2O/ethanol at 1/0.13/0.25/4.2/8.6were used.The results are shown in Fig.4.It can be found that k values of the ?lms were reduced from 1.98,1.81to 1.74,when the mixing time were increased from 3,6,to 9h.There-fore,if a shorter mixing time,i.e.,3h,was used,it is apparent that there would be a problem from incomplete polycondensation reac-

tions in the solution,which cause a higher k value of the ?lm.However,even a longer mixing time,i.e.,9h,was used in this research,the measured k value from the ?lm was still higher than the one from the theoretical calculation.This suggests that a longer mixing time is not suf?cient for a complete polycondensation under the reaction conditions designed in this research.

Effects of the amount of acid in the coating solution .—Although a longer stirring time can improve the extent of polycondensation in the solution and reduce the dielectric constant of the ?lm,a shorter process is preferred for industrial application.For more complete polycondensation in the colloid while maintaining a short mixing time,i.e.,3h,the addition of a larger quantity of acid catalyst in the solution may be the way.Therefore,the effects of the HCl amount in the colloid on the k values of the ?lms were investigated.The molar ratios of TEOS/Tween 80/H 2O/ethanol were maintained at 1/0.13/4.2/8.6.Figure 5shows the dielectric constants and the leakage cur-rent densities of the ?lms prepared from solutions with different amount of HCl.It can be observed when the amount of HCl was increased,the dielectric constants of the ?lm dropped signi?cantly from 1.90to 1.47,and the leakage current densities dropped from 5?10?7to 1.2?10?7A/cm 2.Moreover,the lowest dielectric constant of the ?lm ?1.47?measured in these experiments has been close to the theoretical one ?1.42?.These results fully demonstrate that the addition of more acid catalyst,i.e.,HCl,is an effective way to enhance the polycondensation reaction rates in the solution.Therefore,silica precursors adsorbed around micelles formed from surfactants of Tween 80would condense more completely.As the silanols condense more completely,there may be two impacts.First,the polar groups ?i.e.,silanols ?in the structure are replaced by non-polar SiO 2groups.Second,the structure of colloid silica encapsu-lated with micelles would be mechanically stronger and the collapse of structure during high-temperature calcination process may be avoided.Both of these two impacts help to reduce the dielectric constant and the leakage current density of the ?lm.

Conclusions

Smooth and crack-free nanoporous silica ?lms can be prepared by using Tween 80?a nonionic surfactant ?as the template.The superior properties of the ?lm suggest that Tween series surfactants are potential candidates for the surfactant-templating process.

Another remarkable result is that the ultralow dielectric constants exhibited by the nanoporous ?lms can be prepared in this research.By tuning the parameters such as colloid composition and mixing time of the coating solution,the dielectric constants can be con-trolled in the range of 1.47-2.36.In addition,the leakage current densities can be of 10?7A/cm 2order or lower in an electric ?eld of 1MV/cm.Moreover,dielectric constants of the ?lms were

almost

Figure 3.Variation of dielectric constant ???and leakage current ???at an electric ?eld of 1MV/cm for ?lms prepared from solutions with different amounts of water.In the solution,the molar ratios of TEOS/Tween 80/HCl/ethanol ?

1/0.13/0.25/8.6.

Figure 4.Variation of dielectric constant ???and leakage current ???at an electric ?eld of 1MV/cm for ?lms prepared with different stirring time.In the solution,the molar ratios of TEOS/Tween 80/H 2O/ethanol/HCl ?

1/0.13/4.2/8.6/0.24.

Figure 5.Variation of dielectric constant ???and leakage current ???at an electric ?eld of 1MV/cm for ?lms prepared from solutions with different amounts of HCl.In the solutions,the molar ratios of TEOS/Tween 80/H 2O/ethanol ?1/0.13/4.2/8.6.

the same,even after the?lms were exposed in the atmosphere for one month.

In summary,low dielectric constant?lms were prepared by using Tween80as the templates.The preparation process of the coating solution developed in this research is simple and shows a great potential in advanced semiconductor application.

Acknowledgments

The measurements of low-k and leakage current density in Na-tional Nano Device Laboratory of National Science Council in Tai-wan and assisted by Dr.Wen-Fa Wu of National Nano Device Labo-ratory are appreciated.The?nancial support from China Petroleum Company in Taiwan is also appreciated.

National Taiwan University assisted in meeting the publication costs of this article.

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