1994_uv-assisted deposition of teos sio2 films using

ELSEVIER Applied Surface Science 79/81)(1994) 332 337

applied surface science

UV-assisted deposition of TEOS SiO 2 films using the spin-coating method

K o j i K i n a s h i a, M i c h i o N i w a n o *'~', N o b u o M i y a m o t o ~l, K o j i H o n m a b

'~ Research Institute o f Electrical Communication, Tohoku Unicersity, Sendai 980, Japan t, Chemitronics ('o., Ltd., Tatsuno 2-7-3, Higashiyarnato, Tokyo 189, Japan

(Received 13 October 1993; accepted for publication I December 1993)

Abstract

We have previously proposed a method of depositing silicon dioxide films on Si from tctraethoxysilanc Si (OCzHs) 4 (TEOS) using ultraviolet (UV) light from a low-pressure mercury lamp. In the method, an organic solution which contains TEOS is spin-coated onto a Si wafer surface to form a thin organic film which is then exposed to UV light to synthesize silicon dioxide. In this study, the photochemical reactions in the oxide formation process have been investigated using infrared (IR) and UV absorption spectroscopy. The IR and UV absorption data confirm that the UV light decomposes the organic compounds in the spin-coated organic film to convert the film into a silicon dioxide film. We also demonstrate with thermal desorption spectroscopy (TDS) measurements that the deposited film is stable with respect to substrate heating to approximately 400°C.

1. Introduction

Dielectric films such as silicon dioxide are widely used in integrated circuit (IC) technology. Deposition of these films is commonly carried out by means of chemical-vapor-deposition (CVD). In general, this technique requires relatively high substrate temperatures for film deposition; the substrate temperature ranges from 400°C to 1000°C, depending on the chemical system [1]. Such high substrate temperatures are often unde- sirable because they may enhance the rates of unwanted side processes such as dopant diffu- sion. It therefore is quite important for fabricat- ing highly-reliable, high-density ICs to perform

* Corresponding author. Fax: (+ 81) 22 266 6295.

dielectric film deposition at lower temperatures. One promising technique to do this is photon-induced CVD (photo-CVD). In this technique, relatively high-energy photons are utilized as the exciting light source to decompose the source materials. For example, in the photo-CVD of silicon dioxide film, ultraviolet (UV) light has been utilized to decompose source gases such as silane (Sill4), N20 and O 2 [2-5].

We have previously proposed a simple method of depositing a silicon dioxide film on Si at low temperature by photolysis of tetraethoxysilane (TEOS), Si(OC2Hs) 4 [6]. This compound seems to be a promising class of silicon sources for silicon oxide deposition because of its atomic composition and molecular structure. In our deposition method, a spin-coated organic film which contains TEOS is irradiated by UV light from a

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K. Kinashi et al. /Applied Surface Science 79/80 (1994) 332-337 333

low-pressure mercury lamp to generate silicon oxide. The major advantage of this method is that the oxide film formation can be accomplished at low temperatures, that is, below 100°C, and moreover, neither a specialized reaction chamber nor a gas doser system is necessary. Additionally, this method makes it possible to perform selec- tive deposition of a silicon oxide film through projection of a photomask. In a previous paper [6], we demonstrated that some of the physical properties of these deposited films are compara- ble to those of oxide films obtained with conven- tional methods.

In this study, we investigate the photochemical reactions which take place in the spin-coated organic film when exposed to UV light, using infrared (IR) and UV absorption spectroscopy. Detailed knowledge of the deposition process is inevitably necessary in order to improve and con- trol the film quality and to develop deposition sources. We also examine the thermal properties of the deposited films using thermal desorption spectroscopy (TDS).

2. Experiment

The deposition was performed in a simple aluminum box equipped with a low-pressure mercury lamp. Since TEOS is volatile, collodion, which is nitrocellulose dissolved in ethylalcohol and ethylether, was used as the carrier which prevents TEOS from evaporating from the substrate when exposed to UV light. Note that nitrocellulose is sensitive to UV light; under UV irradiation, this material is readily decomposed into volatile products such as CO, CO 2, H20 , N2, and NO [7]. The deposition procedure was as follows. First, TEOS, collodion and ethylalcohol were mixed with a volume ratio of 4 :2 :1 . The resul- tant mixture was spin-coated onto a silicon wafer with a spinner. The wafer was then placed 2 cm from the UV lamp in the deposition box. Prior to UV irradiation, the deposition box was purged of air by flowing dry N 2 gas. The optical power density on the wafer surface was about 3 m W / c m 2 at 254 nm. The temperature of the wafer surface under UV irradiation was approximately 60°C, as

measured by a thermocouple attached to the sample surface.

The initial decomposition of organic compounds in the spin-coated organic film caused by UV irradiation was monitored by Fourier-trans- form infrared (FT-IR) spectroscopy. After exposure to UV, the sample was placed in a dry N 2 environment to record infrared absorption spectra. UV absorption measurements were performed using a Seya-Namioka monochromator and a deuterium discharge lamp. For this mea- surement, the TEOS-containing organic solution was coated onto a LiF crystal plate which is transparent above 105 nm, and the absorption spectra of the film were measured before and after exposure to UV light. Thermal desorption experiments were carried out using an ultra-high vacuum chamber equipped with a quadrupole mass spectrometer (QMS). The base pressure of the chamber was in the order of 10 -8 Torr. The Si substrate was heated directly with a Ta resis- tive heater. The substrate temperature of the sample was monitored by an Alumel /Cromel thermocouple which was attached to the sample surface. The linear heating rate was set at 10 degree /min . Outgas species desorbing from the surface during ramped heating were detected by the QMS.

3. Results and discussion

3.1. IR absorption

Fig. 1 shows infrared absorption spectra of the TEOS-containing organic film spin-coated on a Si wafer surface, taken for different UV exposure times. The thickness of the spin-coated film prior to UV exposure was approximately 0.1 /xm as measured by a profilometer. As shown in the top of Fig. 1, the film prior to UV exposure exhibits five pronounced peaks at 840, 1080, 1280, and 1660, and 2980 cm-~, which can be attributed to the C-N, Si-O, C-O, N=O, and C - H stretching vibrations, respectively. A broad peak at 3400 cm-~ is attributed to the stretching vibration of O - H . TEOS does not contain a hydroxyl group, but collodion and ethylalcohol do. The observed

334 K. Kinashi et aL /Applied Surface Science 79 /80 (1994) 332 337

O - H vibration peak, therefore, is attributed to the collodion and ethylalcohol which remained in the spin-coated film. It is probable that hydrolysis of TEOS partly occurred during spin coating to produce OH-subst i tuted oligonomers. Such OH- containing products are also the origin of the O - H vibration peak. The intense peaks at 840, 1280 and 1660 cm -~ are most probably due to fibrous nitrocellulose.

As can be seen from Fig. 1, while the intensity of the S i -O peak did not decrease considerably with increasing UV exposure, those of all other peaks did. This confirms that UV light decomposes organic compounds in the spin-coated film to generate a silicon oxide-like film. In particular, the peaks at 840, 1280 and 1660 c m - ~ are significantly decreased in intensity and vanish for 40 min UV exposure. This shows that nitrocellulose, which was used as the carrier for TEOS, is readily dissociated by UV, which is consistent with the results of a previous study [7].

Upon close inspection of Fig. 1, we notice that the O - H vibration peak at 3400 c m - ] initially increases and then drops with increasing UV exposure. Additionally, for a 5 min exposure a new peak appears at 1740 cm ~. This peak is due

p I i r ] d c 'b

F T - I R I

T E O S SiO 2

f e " ~ g r JI

o r I., L i

/

4000 3000 2000 1000 WAVENUMBER (cm -1)

Fig. 1. Infrared absorption spectra of the TEOS-containing organic film taken for different UV exposure times. The figure at tached to each spectrum is the exposure time in minutes,

z LL I I-'- Z L U > t-. 5 LLI n"

E z ,

F T - I R

T E O S S i O 2

\

~ ~ _ b

0 10 20 30 40

i

'\ j

' I '%-__° do! 1 e i

/ /

t

0

0 10 20 30 40

EXPOSURE TIME (min)

Fig. 2, Intensities of the infrared absorption peaks as a function of the UV exposure time. (a) 840 cm I, (b) 1080 cm l , (c) 1 2 8 0 c m - I (d) 1660cm t , ( e ) 1740cm i ,(f) 298/) cm I and (g) 3400 cm t

to the stretching vibration of the carbonyl group, C=O. Upon further exposure to UV this peak also decreases in intensity. These trends can be clearly seen in Fig. 2, in which the intensities of the peaks observed in the spectra of Fig. 1 are plotted as a function of exposure time. From these observations, we determine that some photochemical reactions proceed through an inter- mediate chemical state.

We discuss the time evolution of the O - H peak intensity. The hydrolysis and condensation of TEOS is known to be a reaction route to the production of glasses [8]. It therefore is most probable that initially a hydrolysis reaction takes place in which O C 2 H 5 groups in TEOS are re- placed with a hydroxyl group to generate OH- substituted monomers [8,9]; that is,

t H s C 2 0 - - S i - - O C 2 H 5 + H 2 0

I f

H s C 2 0 - - S i - - O H + C 2 H s O H . (1) J

The origin of the water consumed in this reaction process would be water present in air or ethylal-

I~ Kinashi et al. /Applied Surface Science 79/80 (1994) 332-337 335

cohol incorporated in the film. Note that ethylalcohol is decomposed by UV to generate water and volatile hydrocarbons [10]. Water is also decomposed by UV light to produce hydrogen and OH radicals [10]. These radicals will promote the hydrolysis reaction. The OH-substituted monomers generated by the hydrolysis then react with each other or with TEOS to generate oli- goethoxysilanes such as Si20(OC2H5) 6 which contain the S i - O - S i bridging bond:

I I H s C 2 0 - - S i - - O H + H O - - S i - - O C 2 H 5

I I

I I -~ H s C 2 0 - - S i - - O - - S i - - O C 2 H 5 + H 2 0 ,

I I (2)

I I H s C z O - - S i - - O C 2 H 5 + H O - - S i - - O C 2 H 5

I I

I I H 5 C 2 0 - - S i - - O - - S i - - O C 2 H 5

I [

+ C2HsOH. (3)

cm -] appears accompanied with a decrease in intensity of the N=O peak at 1660 cm -1. The carbonyl groups thus generated would be decomposed upon further exposure to UV. These decomposition processes can explain the observed evolution of the C=O peak intensity.

3.2. UV absorption

In order to examine the photosensitivity of the spin-coated film in the UV region, we have performed optical-absorption measurements in the wavelength region 100-220 nm. In Fig. 3 we show the absorption spectra of the spin-coated film before and after 40 min UV exposure. For com- parison, we also show the absorption spectrum of synthetic quartz in Fig. 3. We can see that upon UV exposure the absorbance of the film is enhanced in the vicinity of 150 nm and the spectrum of the UV exposed film shows a close re- semblance to that of synthetic quartz. This indicates that UV irradiation converts the spin-coated organic film into an inorganic silicon oxide film.

The extent to which photochemical reactions, important for oxide formation, occur in the film is

These processes can be referred to as condensation or polymerization. Water and ethylalcohoi are released by the condensation. We can therefore say that at the early stages of polymerization, hydroxyl-containing compounds such as OH-substituted monomers, water and ethylalcohol are increasingly generated, leading to an increase in the OH peak intensity. However, such hydroxyl- containing compounds will be gradually consumed through repeated cycles of hydrolysis and polymerization, leading to a decrease in the intensity of the O - H vibration peak.

One possible process which produces the carbonyl group is the decomposition of the nitro group in nitrocellulose; that is, - C - O - N O - , -C=O + NO T. This is consistent with the obser- vation that upon UV exposure the N=O peak at 1660 cm -1 is reduced in intensity. Indeed, we observed that when a thin collodion film formed on Si is exposed to UV, the C=O peak at 1740

I . . . . I . . . . I

UV absorption TEOS SiO 2

== Before UV exposure .{5

a J

z ~ re m n" 0 b r/?

<

Quartz c ~

I , , B , I d , , , / ,

100 150 200 WAVELENGTH (nm)

Fig. 3. UV absorption spectra of the TEOS-containing organic film before and after UV exposure, as well as of synthetic quartz.

336 1( Kinashi et al. /Applied SurJace Science 79 /80 (1994) 332-337

determined from a compromise between the absorption of UV light by the film and the strength of the interaction of UV light with the film. The UV absorption spectra of Fig. 3 demonstrate that the wavelengths of the emission lines from a low-pressure mercury lamp, 184 and 254 nm, are out of the region where the absorbance of the film is high. This means that the UV light may pass through the film; if otherwise, UV light would not penetrate the film so that UV-induced reactions occur only in the vicinity of the film surface. On the other hand, if absorption of UV light were quite weak, UV-induced reactions would not occur enough to convert the organic film into an inorganic silicon oxide film. As can be seen from Fig. 3, a weak absorption band is visible around 190 nm. We therefore speculate that the 184 nm light from a low-pressure mercury lamp predominantly interacts with the TEOS-containing organic film to generate silicon oxide. As mentioned above, upon UV irradiation the absorbance of the film was enhanced around 150 nm. This enhancement was interpreted as a result of oxide formation. We note that the emission lines from the lamp lie outside this wavelength region. This means that the silicon oxide generated by UV irradiation does not absorb the UV light significantly so that UV-induced reactions proceed efficiently in the entire portion of the film. Hence, we can say that the UV light used here is suited to the conversion of the TEOS-containing organic film into silicon oxide film.

3.3. TDS

Fig. 4 shows the intensities of mass fragments desorbing from the spin-coated film before and after UV exposure, plotted as a function of the substrate temperature. Mass fragments of m / e = 12 and 14 are due to carbon and hydrocarbon CH 2, respectively, and those of rn/e = 28 origi- nate from C 2 H 4 , C O , and N 2. m / e = 30 is most likely due to NO. It should be noted that while the unexposed film exhibits an intense desorption peak around 170°C, the film following 40 min UV exposure shows no desorption peak below ap-

TEOS SiO 2

' x 1 / 40 .

Before UV exposure _ _ ~ ; - - . . . . .

~" M/e= 18 . . . . ~ - ~ After UV exposure

"', x 1/2

L L ! I - z ,/ i,

28 / . . . . . . . . ] / ,\.

L . ~ E , I _ . , k 6 . L • • 0 200 4 0 600

T E M P E R A T U R E (°C)

Fig. 4. hUensitics of mass fragments desorbing from the TEOS-containing organic film before and after UV exposure as a function of the substrate temperature.

proximately 400°C. This indicates that due to UV exposure the spin-coated organic film is converted to the inorganic oxide film which is ther- mally stable up to at least 400°C.

4. Summary

We have investigated the method of depositing a silicon dioxide film on Si from tetraethoxysilane Si(OC2Hs) 4 (TEOS) using UV light from a low- pressure mercury lamp. In the method, an organic solution which contains TEOS and nitrocellulose is spin-coated onto a Si wafer surface to form an organic film which is then exposed to UV light to generate an inorganic silicon oxide film. IR and UV absorption data confirmed that the spin-coated organic film is converted by UV irradiation to inorganic silicon oxide. IR data also demonstrated that some organic compounds in the spin-coated film are decomposed through photochemical intermediates. TDS data showed that the deposited film is stable with respect to substrate heating to about 400°C.

K. Kinashi et aL /Applied Surface Science 79/80 (1994) 332-337 337

Acknowledgments

T h e a u t h o r s wish to t h a n k Dr . M. Y a n a g i h a r a

and A. A r a i for t h e i r a s s i s t ance in t he U V ab-

s o r p t i o n m e a s u r e m e n t s . P a r t o f this w o r k was

s u p p o r t e d by a G r a n t - i n - A i d for G e n e r a l P ro j ec t

R e s e a r c h f r o m t h e Min i s t ry o f E d u c a t i o n , Sci-

e n c e and C u l t u r e o f J a p a n .

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1994_uv-assisted deposition of teos sio2 films using

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