Influences of deposition temperature on thermal stability and moisture resistance of chemical vapor deposited fluorinated silicon oxide by using indirect fluorinating precursor

Influences of deposition temperature on thermal stability and moisture resistance of

chemical vapor deposited fluorinated silicon oxide by using indirect fluorinating

precursor

Kow Ming Chang, Shih Wei Wang, Chin Jen Wu, Ta Hsun Yeh, Chii Horng Li, and Ji Yi Yang

Citation: Applied Physics Letters 69, 1238 (1996); doi: 10.1063/1.117423 View online: http://dx.doi.org/10.1063/1.117423

View Table of Contents: http://scitation.aip.org/content/aip/journal/apl/69/9?ver=pdfcov

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Influences of deposition temperature on thermal stability and moisture

resistance of chemical vapor deposited fluorinated silicon oxide

by using indirect fluorinating precursor

Kow Ming Chang, Shih Wei Wang, Chin Jen Wu, Ta Hsun Yeh, Chii Horng Li, and Ji Yi Yang

Department of Electronic Engineering and Institute of Electronics, National Chiao Tung University, National Nano Device Laboratory, Hsinchu, Taiwan

~Received 15 April 1996; accepted for publication 13 June 1996!

In this letter, fluorinated silicon oxide (FxSiOy) films were deposited in the electron cyclotron resonance~ECR! chemical vapor deposition system with SiH4, O2, and CF4as the reaction gases.

The CF4, in contrast to SiF4or FSi~OC2H5!3used in other reports, is an indirect fluorinating source.

The fluorinating mechanism is similar to that of the etching of oxide by fluorocarbon plasma, therefore, the thermal stability of the incorporated fluorine must strongly depend on the deposition temperature. It is found that the thermal stability and moisture resistance are greatly improved by increasing the deposition temperature. However, the higher deposition temperature also results in a higher compressed stress and dielectric constant. Besides, to get the moisture resistance, the deposition temperature must be above 300 °C. On the other hand, ECR-SiO2~without fluorination!,

even deposited at room temperature, is shown to have a good water resistance. Therefore, by choosing deposition temperature for FxSiOy to have enough thermal tolerance and capping with ECR-SiO2, the moisture resistor is suggested for the inter metal dielectric applications.

@S0003-6951~96!01035-2#

There has been an increased interest in low dielectric constant ~;3.0! fluorinated silicon oxide (FxSiOy), with various inexpensive precursors and its easy integrated prop-erty for intermetal dielectric ~IMD! applications.1 Concur-rently, how to maintain other desired properties for IMD applications ~such as low mechanical stress, high thermal stability, and low moisture absorption!2becomes another im-portant subject of research. In our study, FxSiOy films were deposited in the electron cyclotron resonance ~ECR! chemi-cal vapor deposition system with SiH4, O2, and CF4 as

re-action gases.3 The CF4, in contrast to SiF4 or FSi~OC2H5!3

used in other reports,1is an indirect fluorinating source. The incorporation of fluorine comes from the unvolatile silicon fluoride, which has a similar formation mechanism as the etching of oxide by fluorocarbon plasma, therefore, the con-centration and stability of fluorine in F_xSiO_y must strongly relate to the process temperature. In this letter, we clarify the influences of deposition temperature on thermal stability, moisture resistance, and other properties of FxSiOy film.

F_xSiO_y films (SiH₄/O₂/CF₄58/85/10 sccm, MW 300 W, 3 mTorr! were deposited on N-type Si ~100! 4 in. wafers to a thickness around 240 nm. With temperature ~T! at 25, 100, 200, and 300 °C. After they were annealed in nitrogen ambient at various temperatures ~400, 500, 600, 700, and 800 °C! for half an hour, the thermal stability of these films was analyzed by Fourier transform infrared spectroscopy

~FTIR!. From FTIR spectrum, variation of the Si–OH peak

around 3650 cm21with time was monitored as the moisture absorption. The variation of film stress with time was mea-sured, which also reflects the moisture absorption in the film. The refractive index was investigated by ellipsometry and the dielectric constant ~k! was measured using metal-insulator-semiconductor ~MIS! structure at 1 MHz.

Figure 1 shows the thermal stability of FxSiOy films.

Note that for all the films deposited at various temperatures, the ratio of Si–F (;930 cm21) over Si–O ~stretching mode, ;1080 cm21) peak intensity does not degrade at 400 °C after half an hour annealing. However, for the 25 °C deposited film, the ratio starts to decay at 500 °C. On the other hand, the film deposited at 100 °C can stand T above 500 °C and only slightly degrade during 600 °C annealing. The film deposited at 200 °C can stand T above 600 °C and the film deposited at 300 °C even can stand T above 700 °C, without outgassing of fluorine. Therefore, thermal stability of fluorine is shown to strongly depend on the deposition tem-perature.

It is assumed that the formation of Si–F bonds in FxSiOy film, with the addition of CF4, is mainly by two

reactions:3~1! homogeneous reaction in the plasma; the ac-tive F and O atoms react with SiH4, which results in the

FIG. 1. Variations of Si–F/Si–O peak intensity from FTIR spectrum with different annealing temperatures, for FxSiOyfilms deposited at 25, 100, 200, and 300 °C.

1238 Appl. Phys. Lett. 69 (9), 26 August 1996

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formation of FxSiOy species and deposition and~2! hetero-geneous reaction on film surface; the active fluorine is ab-sorbed~physical and chemical! on the deposited film and the unvolatile fluoride will remain in the film during the subse-quent deposition. At the same time, the volatile fluoride re-sults in the simultaneous etching. This can be realized from the fact that there will be low deposition rate at high CF4

flow rate.3 Therefore, the bonding strength of fluorine in these unvolatile species buried in the film will determine the stability during subsequent thermal cycles. At low deposition temperature, there will be weak bonding or physical ad-sorbed fluorine buried in the film. Besides, low surface mi-gration energy of the deposited species at low temperature also leads to a less dense network. Upon thermal cycles, these easy-going 2F bonds will break and diffuse out. Op-positely, most of the weakly bonded fluorine~fluoride! will be volatile and the film structure will become denser during the high temperature deposition. Therefore, the remaining

2F bonds can stand a harsher thermal stress. This reflects in

the shift of the Si–F absorbance peak position in the FTIR spectrum. The increase of Si–F peak vibration frequency with the increase in deposition temperature is assumed due to the increase of average bonding strength, as shown in Table I. We can provide a desired thermal tolerance ~500–700 °C! for back-end processes simply by adjusting the deposition temperature.

Deposition temperature also affects other properties of FxSiOy film, as summarized in Table I. Both of dielectric constant and compressed stress increase with the increasing process temperature, which will result in a higher transmis-sion delay and metal interconnection cracking problems. Therefore, within allowed tolerance for thermal stability, low deposition temperature is preferred.

To investigate the moisture resistance of FxSiOy, the follow-up recording of moisture absorption by FTIR spec-trum was made, as shown in Fig. 2~a!. Except for the 300 °C deposited film, the peak intensity of Si–OH (;3650 cm21) in the F_xSiO_y films all increases during the 168 h air exposure~25 °C, humidity 42%!. Variation of film stress with air-exposure time more distinctly reflects the ab-sorption of moisture, as shown in Fig. 2~b!. Note that only 300 °C deposited film did not swell~more compressed! with exposure time. Lower deposition temperature and higher fluorine concentration3both lead to a less dense network of oxide, which will enhance the water absorption. Here it is shown that, to get water resistance, the deposition tempera-ture must be greater than 300 °C.

The outgassing of water had been shown to arise reli-ability problems.4–7 However, the high deposition tempera-ture, which results in a higher k and higher stress, is also undesirable. Here we find that ECR-SiO2 ~SiH4/O2

52/85 sccm, 300 W, 3 mTorr! even deposited at room

tem-perature, does not absorb moisture and swelling, as shown in Figs. 2~a! and 2~b!. After capping by a thin ECR-SiO₂layer

~40 nm!, the moisture absorption of FxSiOyfilm deposited at lower temperature can be prevented. Therefore, the necessity of high deposition temperature for moisture resistance

~300 °C! is avoidable.

In summary, using the indirect fluorinating precursor CF4, the concentration and stability of fluorine in FxSiOy

TABLE I. Changes of FxSiOyfilm dielectric constant, refractive index, Si–F/Si–O intensity ratio, Si–F absor-bance peak position, density, and mechanical stress due to deposition temperature.

Deposition temperature ~°C! Dielectric constant ~k! Refractive index ~n! Si–F/Si–O ratio ~FTIR! Si–F peak (cm21) ra (g/cm3₎ Stress ~MPa! 25 3.17 1.375 0.08552 932.48 1.8418 274.2 100 3.35 1.406 0.07329 933.39 1.9763 297.3 200 3.44 1.429 0.06603 934.31 2.0742 2150.4 300 3.51 1.434 0.06336 934.47 2.0953 2203.8 a_{The density of F}

xSiOy ~r! is determined from an n value using Lorentz–Lorentz relationship:

8

r5K(n2_21)/(n2_{12), where K58.046. The density of thermal SiO}

2is 2.212 g/cm3.

FIG. 2. Variations of~a! Si–OH (;3650 cm21) peak intensity from FTIR spectrum and~b! mechanical stress with time, in clean room air ~25 °C, humidity 42%!, for FxSiOy~250 nm! deposited at ~1! 25 °C, ~2! 100 °C, ~3! 200 °C, ~4! 300 °C, ~5! ECR-oxide ~250 nm!, and ~6! ECR-oxide/ FxSiOy/ECR-oxide~40/240/40 nm! deposited at 25 °C.

1239

Appl. Phys. Lett., Vol. 69, No. 9, 26 August 1996 Changet al.

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film will strongly depend on the deposition temperature. Higher temperature deposition can improve thermal stability but result in a higher k and compressed stress. Therefore, within allowed tolerance for thermal stability, low deposition temperature is preferred. The thermal stability~500–600 °C! provided by 100 and 200 °C deposited film is enough for currently used back-end processes. On the other hand, to get a moisture resistance, the deposition temperature must be greater than 300 °C. However, capping a thin ECR-SiO2~40

nm! layer ~even deposited at room temperature! on FxSiOyis shown to prevent water absorption. In this way, high tem-perature deposition can be avoided.

This work is supported under Taiwan R.O.C. National Science Council Contract No. NSC 85-2215-E009-061.

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