The growth mechanism of SiC film from polyimide LB film

Bangkun Jina, Pingsheng Hea,*, Yongning Shengb, Beifang Yangb

aDepartment of Polymer Science and Engineering, University of Science and Technology of China, Hefei 230026, Anhui,

People’s Republic of ChinabDepartment of Materials Science and Engineering University of Science and Technology of China, Hefei 230026, Anhui,

People’s Republic of China

Received 5 February 2002; revised 11 June 2002; accepted 3 July 2002

Abstract

The growth mechanism of SiC film produced by pyrolysis polyimide LB film was discussed. AES result showed that the

atoms of C and Si form a gradient distribution in the pyrolyzed SiC films. Parameters including the diffusion co-efficiency of

carbon atoms, growth rate of SiC film, and the activation energy were estimated. We suggest that the SiC formation process is

controlled by the diffusion of silicon atoms, and the whole growth rate depends on the reaction of Si and C.

q 2002 Elsevier Science Ltd. All rights reserved.

Keywords: Polyimide; Langmiur–Blodgett film; SiC film; Growth mechanism

Producing inorganic films or ceramic films from organic

precursor films is a feasible way and has drawn much

attention in these days. Various functional inorganic films,

such as films of Nd doped Y2O3 [1], CuO [2], Ni oxide [3],

Pb(Zr,Ti)O3 [4], have been made using this method. In the

earlier work, we have successfully prepared SiC ultrathin

film—a very potential semiconductor material in electronic

industry due to its superior properties: high temperature

resist, high thermal conductivity and a wide band gap—by

pyrolyzing in vacuum the polyimide (PI) LB film deposited

on single crystal Si substrates and have characterized it to be

quasi-single-crystal b-SiC film with various analytical

methods [5–7]. The pyrolyzing method undergoes a process

apparently different from other filmmaking methods such as

chemical vapor deposition (CVD) since they have different

chemical reactions. So, it is important to make clear that

the microcosmic process and the film growth mechanism in

the pyrolyzing process, are helpful for the further study on

this topic. In this paper, we make some fundamental

discussion on the growth mechanism based on the

estimation of the diffusion rate of atoms and the activation

energy of the reaction.

1. Experimental details

1.1. Deposition of PI LB film

PI LB film was deposited using the precursor method [8].

Polyameric acid was synthesized from pyromellitic dianhy-

dride (PMDA) and 4,40-oxydianiline (ODA); N,N-dimethyl-

octadecylamine was obtained from the reaction of

octadecylamine with formal acid and formaldehyde.

Polyameric acid salt, the precursor, was obtained by mixing

the above two compounds at the mole ratio of 1:1. The

precursor was diluted to the concentration of 1 £ 1023 M

with N,N-dimethylformamide (DMF) before use. A home-

made computer-controlled HL-1 Langmuir balance with a

Wilhelmy plate was used to spread the monolayers and

deposit the LB films. The precursor LB film was deposited

to Si(111) wafers at a constant surface pressure of 25 mN/m

on 1 £ 1023 M CdCl2 subphase and the dipping speed was

6 mm/min. The water used here was doubly distilled in a

quartz instrument. The PI LB film was obtained by

calcinating the precursor LB film at 300 8C in vacuum.

1.2. Pyrolysis of PI LB film

Pyrolysis of PI LB film was conducted in the furnace of

the Union Optical HM-4 metallic microscope (made in

0022-3697/03/$ - see front matter q 2002 Elsevier Science Ltd. All rights reserved.

PII: S0 02 2 -3 69 7 (0 2) 00 3 37 -2

Journal of Physics and Chemistry of Solids 64 (2003) 339–342

www.elsevier.com/locate/jpcs

* Corresponding author.

E-mail address: hpsm@ustc.edu.cn (P.S.He).

Japan) with the vacuum of 1 £ 1025 mbar, just after the

imidization process. FT-IR and XPS results showed that SiC

film began to form between 750 and 800 8C and that after

being calcinated at 1000 8C for 1 h, PI LB film was

converted to SiC film completely.

1.3. Instrument analysis

FT-IR spectroscopy was measured with NICOLET-

170SX FT-IR spectroscope; the fractural SEM photograph

was carried out on X-650 scanning electron microscope; and

XPS probe was operated on ESCALB MK-II X-ray

photoelectron spectroscope with Mg Ka (hg ¼ 1253.6 kV)

at the vacuum of 1 £ 1029 mbar.

2. Results and discussion

2.1. Auger electron spectroscopy (AES) probe of SiC film

Fig. 1 shows the distribution of Si and C atoms in a so-

called 101(111), 1000 8C/1 h SiC film (the SiC film was

prepared by pyrolyzing 101 layers PI LB film deposited on

Si(111) wafer at 1000 8C for 1 h, similarly hereinafter),

measured using AES probe with Mg anion and Ar61 ion

beam sputtering. The horizontal coordinate is the etching

time, which can represent the sputtering depth in film. The

longer the time, the deeper would be the depth. Before

sputtering, XPS result has shown that the ratio of the Si, C

atoms in SiC film is Si/C ¼ 1.0335:1.000, confirming the

formation of SiC film. At the beginning of sputtering, the

concentration of C atoms is slightly greater than that of Si

atoms, meaning that there were more C atoms on the surface

of the SiC film. The reason is that Si atoms are prone to

sublime under high vacuum and thus leave more C on

the surface [9]. With the depth goes deeper, the Ar61 ion

beam began to sputter on the SiC layer and the ratio of Si to

C became nearly 1:1. The two concentration lines extend

almost parallel only with a slight separation. At the end, the

ion beam sputters on the Si wafer and the concentration of Si

atoms becomes much higher than that of C atoms. The

shapes of the lines suggested that the concentrations of C

and Si distributed gradiently in the SiC film with the

concentration of C atoms becoming less and that of Si atoms

becoming more gradually as the depth extended. Thus it can

be assumed that Si and C atoms have diffused mutually

during the SiC formation process and reacted with each

other to form an SiC film.

2.2. Kinetics of SiC film growth

While calcinating PI LB film in vacuum, the polyimide

decomposes at about 540 8C to leave C skeleton [5] and the

remainder C reacts with Si wafer at the interface to form

SiC. The later process may include two cases: (1) C atoms

diffuse through SiC–Si interface to react with Si wafer; (2)

Si atoms diffuse through SiC–Si interface to react with C

skeleton. The whole growth rate of the SiC film is

determined by two factors: the diffusion rate of C or Si

atoms and the reaction rate of C and Si at the interface. The

slower one controls the whole process (Fig. 2).

The diffusion rates of C and Si atoms are different and

the dominant one can be judged by estimating the order of

the diffusion coefficient. Usually there are three main

diffusion mechanisms in crystal: exchange mechanism, gap

mechanism and hole mechanism. They all obey the Fick’s

first law although they have different activation energies.

Here we will judge the SiC film growth mechanism by

estimating the diffusion rate of C or Si atoms and the

apparent activation energy of the reaction with FT-IR

spectrum.

First, we estimate the order of the diffusion coefficient in

the diffusion process. Supposing the case here is that the C

atoms diffuse through interface to react with Si wafer (if the

case is true, the calculated results should be in accordance

with that in literature; if not, the case should be that the

diffusion of Si atoms is dominant) and define CC, CSiC, CSi

as the concentrations of C atoms in C skeleton, SiC layer

Fig. 2. Schematic diagram of PI LB film to SiC film.

Fig. 1. AES of SiC film. It shows the distribution of C and Si atoms

in the SiC film. The two lines represent the concentrations of C and

Si, respectively. The shapes of the lines suggest the gradient

distribution of C and SiC film and indicate the mutual diffusion of

the atoms.

B.K. Jin et al. / Journal of Physics and Chemistry of Solids 64 (2003) 339–342340

and Si wafer, respectively. Then according to Fick’s law

JC ¼ 2DCðdc=dxÞ ð1Þ

where JC is the number of C atoms diffusing through unit

area of SiC layer in unit time, DC the diffusion coefficient of

the C atoms, x the thickness of SiC layer. At a short time

interval, 2(dc/dx ) can be expressed as (CC 2 CSi)/x then

JC ¼ DCðCC 2 CSiÞ=x ð2Þ

According to the definition of JC

JC ¼ dN=dA dt ¼ ðdN=dA dxÞðdx=dtÞ

¼ ðdN=dVÞðdx=dtÞ ð3Þ

where N is the number of C atoms, dA the fractural area of

interface, dt the unit time, dV the unit area, and dN/dV means

that the C atoms required to grow per unit volume of SiC at

the interface, so that

dN=dV ¼ dC ¼ CC 2 CSi ð4Þ

Combining formulae (3) and (4) into Eq. (2)

xðdx=dtÞ ¼ DCðCC 2 CSiÞ=ðCC 2 CSiÞ ¼ DC ð5Þ

Integrating the above formula from 0 to time t (at time t ¼ 0,

the thickness x ¼ 0)

x2 ¼ 2DCt ð6Þ

It is possible to estimate the diffusion coefficient of C atoms

in the growth process of the SiC layer from formula (6). In

order to obtain the growth rate, we put a 40(111) sample in

the vacuum furnace of the metallic microscope, and raise the

furnace temperature step by step: first calcinating the sample

at 700 8C for 0.5 h, then 800 8C/0.5 h, 850 8C/0.5 h, 900 8C/

0.5 h, 950 8C/0.5 h, 1000 8C/0.5 h, respectively. After each

heating step, the sample was cooled to room temperature

and the measured FT-IR spectrum with blank Si(111) wafer

as a reference. The results are shown in Fig. 3. According to

the Beer’ Law, the strength of absorption peak should be

proportional to the thickness of the film, so that the relative

thickness of SiC film pyrolyzed at different temperature and

further the growth rate of the film can be estimated by

calculating the areas of the absorption peaks.

The strength of the absorption peak of SiC sample

remains almost unchanged after pyrolyzed at 950 8C for

0.5 h, indicating that the C skeleton has reacted with the Si

wafer completely, and the thickness of SiC film has reached

its fullest. Scanning electron microscopy photograph has

shown that the fractural thickness of 209 layers pyrolyzed PI

LB film is about 50–60 nm, meaning that the contribution

of each layer PI LB film to the thickness of SiC film is about

2.5–3.0 A, which is in consistent with the layer distance of

closed packed SiC crystal. To facilitate calculation, the

thickness of 2.5 A is adopted (it should have a little

influence on the calculation here and the activation energy,

because the ‘thickness’ here refers to relative thickness, i.e.

the thickness percentage of 40(111), 950 8C/0.5 h, where the

activation energy is a function of temperature only), where

the thickness of 40(111) SiC film should be 100 A. Then the

thickness of the sample pyrolyzed at other conditions and

the differential thickness (growth rate) can both be

calculated based on the relative areas of the absorption

peaks, and the results are shown in Table 1. The growth rate

at 950 8C is 1.325 A/min, so that DC is about 4 £ 10217 cm2/s

according to formula (6). This value is much greater than

the upper limit extrapolated in the case of C atoms diffusion

Fig. 3. FT-IR spectra of 40(111) pyrolyzed at different temperature.

The areas of the absortion peaks increase with the calcination

teamperature. The areas of 950 8C/0.5 h and 1000 8C/0.5 h samples

are nearly the same, suggesting the complete conversion of PI LB

film to SiC film.

Table 1

Growth rate of SiC film at different temperature

Temperature

(8C/0.5 h)

Relative absorption

area

Relative thickness

(A)

Differential thickness

(A)

Growth rate

(A/min)

Reciprocal temperature

(104 K21)

700 0.372 0.735 – – –

800 5.070 10.02 9.290 0.310 9.320

850 14.78 29.21 19.19 0.640 8.905

900 30.49 60.25 31.04 1.035 8.525

950 50.60 100.0 39.75 1.325 8.117

B.K. Jin et al. / Journal of Physics and Chemistry of Solids 64 (2003) 339–342 341

in SiC (10228 cm2/s) [10], indicating that the diffusion of C

atoms is not the main case here. So it can be inferred that in

the pyrolyzing process, the diffusion of Si atoms through

SiC layer is dominant.

The kinetics of the SiC growth process can be expressed

as follows according to the Ahrrenius relations

rðtÞ ¼ r0 expð2Ea=RTÞ or ln r ¼ ln r0 2 ðEa=RTÞ

where Ea is the apparent activation energy of the film growth

reaction, r(t ) the growth rate, R the gas constant, T the

absolute temperature, and r0 constant. The drawing of ln r(t )

versus 1/T with good linearity is shown in Fig. 4. The slope

of the line is about 21.28 £ 104. Thus the apparent

activation energy Ea is 25.4 kcal/mol, which is similar to

what Matsunami [11], Nagasawa [12] measured in the case

of growing b-SiC on Si wafers using CVD method (20–25,

26.6 kcal/mol). These values are all smaller than the

activation energies of diffusion mechanism in crystals

(Table 2). So it can be assumed that the whole growing

process should be controlled by the reaction of Si and C

atoms but not the diffusion mechanism. Also this value is

much greater than that of growing b-SiC film on

6HSiC(0001) substrate using CVD method (15 kcal/mol

[14], 12 kcal/mol [15]). In the latter case, b-SiC is

homogeneously epitaxy-grown on 6HSiC(0001) buffer

layer, and so it has lower activation energy than that in

the case of heterogeneously epitaxy-grown on Si crystal

[(111) or (100) orientation].

In conclusion, it is inferred that during the process of

pyrolyzing PI LB films to SiC film, C and Si atoms react

firstly at the interface to form a thin layer of SiC, then Si and

C atoms diffuse mutually with the diffusion of Si as

dominant, and the whole growth rate is determined by the

reaction of Si and C atoms.

Acknowledgments

This work was supported by the National Natural

Science Foundation of China (29974028).

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Fig. 4. Linear relationship of ln r(t ) to 1/T.

Table 2

Activation energies of some diffusion mechanism [13]

Diffusion

type

Exchange

mechanism (eV)

Gap

mechanism (eV)

Hole

mechanism (eV)

Activation

energy

10.3 9.5 2.0 (1 eV

¼23.05 kcal/mol)

B.K. Jin et al. / Journal of Physics and Chemistry of Solids 64 (2003) 339–342342