Materials Science and Engineering, B15 (1992) 229-236 229

A1N thin films prepared by plasma-enhanced chemical vapour deposition

C. H. L e e , C. T. W u a n d C. Y. P u e n g

Graduate Institute of Materials Engineering, Cheng Kung University, Tainan (Taiwan)

(Received January 18, 1992; in revised form May 15, 1992)

Abstract

The process of plasma-enhanced chemical vapour deposition of AIN thin films on oxidized silicon substrates is charac- terized by using NH 3 and AI(CH3) 3 as reactants and argon as a carrier gas. Mirror-smooth thin films of A1N are deposited under a wide variety of growth conditions. Growth characteristics are determined as functions of process parameters including substrate temperature, plasma power, reaction pressure, reactant ratio and flow rate of reactants. The refractive indices of the films vary between 1.8 and 2.0 depending on the process conditions, which is consistent with the change in the film density. X-ray diffraction and transmission electron micrograph analyses indicate that the deposits prepared below 200 °C are amorphous while growth above 400 *C produces polycrystalline material.

1. Introduction

Because it is a physically stable, chemically inert, electrically semiconducting and thermally refractory compound A1N has been subjected to numerous studies for different applications [1-16]. As a III-V compound semiconductor, for example, AIN was highly regarded for application to optical devices operating in the UV region, because it has a wide band energy gap of 6 eV. With its lack of an inversion centre of crystal symmetry, AIN is strongly piezoelectronic with a high acoustic velocity. Thus it is popularly used for fabrication of surface acoustic wave (SAW) devices [1-3]. Because it is chemically resistive to oxidation and thermally resistive to heat wave shocks at high temperatures, AIN was also studied as a prospective coating material for application under different en- vironmental conditions [4]. Since AIN is a wide band gap material, both crystalline and amorphous AIN are electrically highly resistive, and have been considered as a useful insulator because they possess good physi- cal stability. However, because A1N has a high thermal conductivity and a low thermal expansion coefficient which is comparable with those of silicon and GaAs, it was also studied as a potential insulating material for application to substrates for electronic packaging [5]. Finally, because it has good mechanical and thermal properties, which leave no residual stress after heat treatment, it has been investigated for passivation application for semiconductor processing [6-10]. In

particular, it was found to be exceptional as a cap insu- lator for ion implantation in GaAs [6-8].

As GaAs integrated circuit (IC) technology con- tinues to develop, the search for a mechanically com- patible insulator for passivation becomes important. This work was therefore started to investigate A1N as a common insulating material for GaAs IC processing applications in a similar manner to Si3N 4 and SiO2 for silicon IC engineering.

A1N can be prepared with a variety of properties by means of different techniques depending on the appli- cation. For SAW and semiconductor device applica- tions, epitaxial growth of AIN films on silicon and sapphire is generally carried out at high temperatures because of the need for good crystalline material. However, for protective coatings and device passiva- tion amorphous AIN films that can be prepared at low temperatures are favourable. Since low temperature processes are required for GaAs IC engineering, recent development of AIN thin film preparation for GaAs work has been aimed at the low temperature tech- niques. These include r.f. and d.c. sputtering [1, 11-13], molecular beam epitaxy [15], evaporation [16, 17], r.f. plasma-assisted chemical vapour deposition (CVD) [4, 18] and microwave conventional and downstream plasma CVD [19, 20]. Because of the nature of the reactants and availability of the equipment this work on A1N thin film preparation is carried out by means of r.f. plasma-assisted CVD. Although similar work has been reported in the literature [18], for the future needs of

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230 (; It, Lee et al. / Pl:'CVD AlN thin fihns

our laboratory we investigated in this work the process with different sets of parameters which lead to similar but different results. For example, in this work we used argon as the main carrier gas in an attempt to see ionization enhancement for plasma generation. In addi- tion, in this work we studied the effect of heat treat- ment on the etching rate of phosphoric acid, because of the need of subsequent process steps for IC engineer- ing. Finally, for economic reasons, we used silicon wafers as substrates which are expected to be equally effective for characterization of the film deposition process under low temperature conditions. As the first part of the investigation the results from the point of view of the process are reported in this paper.

thickness and refractive index. The microstructure and surface morphology are determined by X-ray, trans- mission electron microscopy (TEM) and scanning electron microscopy analysis. The chemical composi- tion and completeness of reaction are characterized by Auger electron spectroscopy and Fourier transform IR (FTIR) analysis. The ratio of aluminium to nitrogen of the films is determined by measurement of the electron diffraction X-ray fluorescence. The etching rate in phosphoric acid is determined by measurement of sample thickness before and after etching at different temperatures.

3. Results and discussion

2. Experimental details

Sample preparation for this study of A1N plasma- enhanced CVD (PECVD) was carried out in a home- made reaction system which is schematically shown in Fig. 1. Briefly it consists of four sections of apparatus including a quartz reaction chamber with an insulated resistance-heated pedestal, a 500W, 13.6 MHz, plasma generator with a pack of r.f. matching circuits and two semicircular plate electrodes around the reac- tion tube on top of the pedestal, a mechanical pump with a pressure sensor, and a reactant gas manifold with mass flowmeters and a hydrogen purifier. Tri- methylaluminium (TMA) and ammonia are used as reactants. Hydrogen is used as the carrier gas for TMA which is kept at 25 °C. n-type silicon of (100) orienta- tion is used as the substrate. The deposition tempera- ture is set between 200 °C and 400 °C. The reaction chamber pressure is kept constant between 1 and 5 Torr, while the total gas flow rate through the reaction chamber is fixed between 200 and 500 c m 3 m i n - 1. The r.f. power for plasma generation is set between 25 and 100 W.

The growth characteristics of the AIN thin films are analysed by means of ellipsometric measurement of the

~Vacuurn NH 3

Pump J Jl~scrubber

Ar

I I Radio Frequency I

Generator

Fig. 1. Schematic diagram of the PECVD apparatus.

3.1. Growth rates and process parameters PE CVD has been developed mainly for low tem-

perature deposition of thin films of various kinds by application to semiconductor processing. The principle of the process is to use r.f. microwave radiation to pro- mote a chemical reaction which would otherwise not occur at the same temperature. The work is carried out by introducing the reactants into the reaction zone in the reaction chamber while microwave radiation of fre- quency 13.6 MHz is applied between two electrodes of a plasma generator outside the chamber. The mole- cules of the reactants are ionized and accelerated toward the substrate sitting on a pedestal which is grounded. Chemical reaction between aluminium ions and NH radicals takes place on the substrate surface resulting in A1N thin film deposition, The deposition rate is system dependent. In general, the plasma condi- tion dominates the deposition rate when the substrate temperature is low, while the plasma structure and field intensity are controlled by process parameters.

Figure 2 shows the experimental data of substrate temperature effects on growth rate and refractive index for the temperature range between 200 °C and 400 °C. The growth rate data should be explained by reasoning that r.f. radiation activates NH 3 and AI2(CH 3)3 to result in the deposition of AIN thin film on the substrate and no chemical reaction takes place between the two source chemicals at temperatures below 400°C, Therefore, under the experimental conditions, growth rates are independent of substrate temperature. As to the temperature effect on reaction kinetics, it is appa- rent that the rates are controlled by ion diffusion through the plasma sheath near the substrate which is insensitive to temperature. However, because ion mobility increases with growth temperature, which yields deposits of greater density and better quality at higher temperatures, the increase in refractive index with growth temperature can be attributed to the increase in film density.

C. H. Lee et al. / PECl/D AIN thin films 231

c E120 .<E

0

8 I I I I I .,.

>w 1.9

1.7 200 250 500 350 400

q) rY

Ternperoture ( ' C )

Fig. 2. Dependence of growth rate and refractive index on sub- strate temperature (gas pressure, 1 Tort; NH3 flow rate, 50 cm 3 rain- l; T M A flow rate, 0.5 cm 3 rain- 1; argon flow rate, 50 cm 3 min- l; total f low rate, 300 cm 3 min- 1; r.f. power, 25 W).

_e E~

v 75

o 60

0 L. 50

(3

x 2.1 q) "1o c 2,0

. ~ 1 .9 13

I I I I I

I I I I I

1 2 3 4 5

Pressure ( Tort )

Fig. 4. Dependence of growth rate and refractive index on reac- tion pressure (r.f. power, 25 W; NH3 flow rate, 50 cm 3 min-1; TMA flow rate, 0.5 cm 3 min-~; argon flow rate, 50 cm 3 min-z; total flow rate, 300 c m 3 min- l; substrate temperature, 300 °C).

• <C 90

so

O n," 70

so o

(3 I I I I

IP

_ c •

>~ .

• ~ i I I l

O 25 50 75 100

n~ R.F. Power (Wott)

Fig. 3. Dependence of growth rate and refractive index on r.f. power (gas pressure, 1 Torr; NH 3 flow rate, 50 c m 3 m i l l - l ; TMA flow rate, 0.5 cm 3 min-]; argon flow rate, 50 cm 3 min-1; total flow rate, 300 cm 3 min- ]; substrate temperature, 300 *C).

Figure 3 shows the effect of r.f. power in the range 10-100 W on growth rate and refractive index. The data indicate that, in the low power range, both growth rate and refractive index increase with plasma power. In the higher power range, on the contrary, growth rates decrease with the increase in r3. power while the refractive index continues to increase with r.f. power to reach a maximum value of about 2 and then decreases as the plasma power further increases. These data indicate that plasma ion bombardment etches the

deposit, preventing fast growth when the r.f. power is high. For this work an r.f. power of more than 25 W is apparently needed for deposition enhancement. With regard to the refractive index data, it is apparent that the maximum value of the index must occur for the r.f. power which results in a high film density and an opti- mum growth rate.

The effect of reaction pressure is shown in Fig. 4, which indicates the trend of decrease in growth rate and increase in refractive index with the increase in pressure. The growth rate data mean that, at a lower reaction pressure, plasma ionization is more complete and the plasma sheath effect which prevents ions from reaching the substrate is reduced, which yields higher deposition rates. On the contrary, when the plasma power increases, the trend is reversed for both ioniza- tion and sheath effects. However, the refractive index increases gradually with the pressure because the opti- mum growth condition usually comes with a lower growth rate for thin film CVD.

The effect of the total gas flow rate on growth rate at a fixed temperature, r.f. power and reaction pressure is investigated. The results are shown in Fig. 5. The gross trend of the growth rate data indicates that it increases with the total gas flow rate because of the increase in the total number of reactive ions for reaction. How- ever, when the total flow rate is low, the rate of increase is lower, possibly because of fast recombination of ions before they reach the substrate for deposition. Of course, it can also be explained in terms of structural change of the plasma zone; the optimum deposition rate may occur only when the total flow of reactant is

232 ('. tt. l ,ee e/al. / PE( 'V / ) A IN thin,filtns

400

I ~ 100

O~ X

2.0 c -

~ 1.9 .m_

~ 1.8 I I I I

200 300 400 500

7ota! 'iow rate (c .c . /min)

Fig. 5. Dependence of growth rate and refractive index on total gas flow rate (gas pressure, 1 iorr; r.f. power, 25 W; substrate temperature, 300 °C NH 3 :Ar :TMA= 50%: 49.5%:0.5%).

120

• -~ 110

~ 9o

1~ 8o

• 70 8

6 O X

¢..

- - 2.0

I I I I I I

:~ 1.9 0 o I I I I I I

25 50 75 100 125 150 DC

NH= / TMA

Fig. 7. Dependence of growth rate and refractive index on NH3:TMA ratio (r.f. power, 25 W; gas pressure, 1 Torr; TMA flow rate, 0.5 cm 3 rain-i; argon flow rate, 50cm 3 min-t; total flow rate, 300 cm 3 min- ~; substrate temperature, 300 °C).

60C

C

~ 40O

n,' 200

100 £

~ 1.9

~ 1.8 0 . 3 3 0 . 5 0 . 7 5 1 2

~" TMA Flow Rate ( c.c./min )

Fig. 6. Dependence of film growth rate and refractive index on TMA flow rate (r.f. power, 25 W; gas pressure, 1 Torr; NH 3 flow rate, 50 cm 3 min-1; argon flow rate, 50 cm 3 min-l; total flow rate, 300 cm 3 min- ~; substrate temperature, 300 °C).

sufficiently high, resulting in greater efficiency for CVD. Therefore , the rate of increase is faster for higher flow rates in this work. T h e decrease in refrac- tive index with the increase in the deposition rate means that the film density is lower for the film of higher growth rate because the ions have tess chance to move to be properly situated before reaction for AIN formation.

Figure 6 shows that the growth rate increases linearly with the flow rate of T M A under the condition

of a sufficiently large flow rate of N H 3 of 55 cm -~ min-~, with the rest of the parameters fixed as indi- cated. Th e data mean that aluminium ions dominate the rate of reaction on the surface of the substrate. Again, the refractive index decreases with the increase in growth rate because of the change in film density. The data of growth rate vs. N H 3 : T M A ratio are plot- ted in Fig. 7 which indicate an increasing trend. The results are consistent with those of Fig. 5, which were attributed to the change in the nature of the plasma under the conditions of this set of experiments, namely both the rates of ion recombination and the structure change of the plasma zone, e.g. sheath thickness varia- tion with the reactant ratio and the total flow rate. which results in a more complex nature of the increas- ing trend of growth rates. Of course, the corresponding change in the refractive index data can be similarly explained as described for those in Fig. 5 previously. Th e thicknesses of P ECV D AIN films are linearly related to deposition time as shown by the data in Fig. 8. This means that the plasma conditions and the flow parameters are stable during A1N deposition in this work.

3.2. M i c r o s t r u c t u r e s X-ray diffraction and transmission electron micro-

scopic techniques are used for analysing the micro- structure of the AIN thin films deposited on silicon substrates. Figure 9 shows a series of X-ray dhffraction data for the samples grown at ~ r e n t temperatures, T h e spectra indicate that AIN and silicon have the same diffraction peak at 2 0 = 33.260 which becomes stronger for films grown at higher temperatures. How-

5OOO

4OOO

. <

- '3ooo

c 2ooo .2

1000

0 0

I I I !

10 20 30 40 50

C. H. Lee et al. / PECVD AIN thin films 233

Time ( min. )

Fig. 8. Dependence of film thickness on deposition time (r.f. power, 25 W; substrate temperature, 300"C; gas pressure, 1 Torr; NH 3 :TMA= 100; argon flow rate, 50 cm 3 min-~; total flow rate, 300 cm 3 min- l).

-- ~ 400"C 300"C

200 *C

. . . . . . . . I . . . . . . . . . . . A - - ~ . . . . . 1 . I SI PJbetrat. 8 2 8 401 60 65

Fig. 9. X-ray diffraction patterns of AIN films grown at different substrate temperatures on silicon (1 Torr, 25 W).

ever, they cannot be resolved to reveal the maximum growth temperature below which the AIN growth is amorphous. However, by subtracting a common value of the (100) diffraction signal of the growth at 200 °C, we obtained a spectrum after growth at 400 °C as shown in Fig. 10. The result indicates that along the Si(100) peak there is a weak peak of AIN which is due to the temperature effect on the growth of crystalline AIN thin fdms at 400 °C.

Since X-ray diffraction data do not reveal a clear-cut temperature limit for the deposition of amorphous A1N, electron diffraction analysis and TEM micro- graphs are relied on for microstructural study. Figure 11 shows three sets of electron micrographs for AIN growth at 200 °C, 300 °C and 400 °C. As shown by Figs. 11(b) and 11(c), growth at 300 °C and 400 °C results in clearer diffraction ring patterns and the bright field photographic contrast of oriented aggregates although not grain boundaries. This means that the deposits possess preferred orientation or are polycrys-

ALN oo)

30 40 5 0 6 0

Fig. 10. X-ray diffraction pattern of AIN film grown at a 400 °C substrate temperature (1 Torr, 25 W, TMA :NH 3 -- 1:100).

talline in nature. However, the results in Fig. 1 l(a), exhibithag neither a clear diffraction pattern or the photographic contrast of aggregates, indicate that the deposits are amorphous. Other parameters, i.e. reac- tion pressure, flow rates of reactants and plasma power, also influence the microstructure of A1N growths. In general, a low plasma power (less than 50 W), large flow rate and high reaction pressure favour the growth of amorphous A1N films, which is the interest of the present work.

3.3. Compos i t ion The composition of the AIN films is analysed by

means of wave-dispersive spectroscopy (WDS). For this work, all the samples are prepared to have a solid phase composition with AI:N ratio in the range 0.8-1.05. However, the composition is controlled by the feeding rates of the reactants. Figures 12 and 13 respectively show the solid phase AI:N ratio as a function of TMA and NH 3 flow rates. As expected, the AI:N ratio does exhibit a trend to increase and to de- crease with the flow rates of the corresponding reac- tants. WDS analysis also indicates no trace impurity element of concentration greater than 0.01 at.% except for nitrogen in the case of growth under conditions of excess NH 3 flow. The energy-dispersive spectra (energy-dispersive spectroscopy (EDS) or X-ray photoelectron spectroscopy) show that the deposits mainly consist of AI-N bonds which are formed by the AI 2p and N ls electrons. However, some of the sam- ples which are prepared with excess NH 3 do reveal an additional N - N bonding energy peak in the EDS spectra.

FTIR spectral analysis is used to check the com- pleteness of the chemical reaction for growth at differ- ent temperatures. The spectra which are shown in Fig. 14 indicate that the bond stretch of AI-N dominates while the N-N, N - H and AI -H bond signals are very

234 ('. II. Leeetal . / I~F( 'VDAlNth in f i lms

F

L

Fig. 11. TEM diffraction patterns and micrographs of AIN films grown at different temperatures: (a) 200 °C; (b) 300 °C; (c) 400 °C.

weak. This means that the chemical reaction is fairly complete, in particular for the samples grown at tem- peratures above 300 °C.

3.4. Etching rate As a dielectric thin film for passivation applications,

the etching characteristics of the AIN deposits are investigated by means of chemical etching in H3PO4 at elevated temperatures. The results for etching rates as a function of temperature are plotted in Fig. 15. The linear relationship of the data indicates that chemical etching of PECVD AIN thin films is a rate-controlling process which can be activated by etching temperature.

The activation energy is estimated as 0.84 eV. The rates are in the range from 50 to 200 min, which is comparable with that of Si3N 4. Since a heating cycle is unavoidable in silicon IC processing, the annealing effect on etch rates of the AIN thin films is also investi- gated. The results for etch rates after a heat treatment process at 900 °C for 1 h in N2 are shown in Fig. 16 (triangles) in comparison with those of the as-grown samples (circles). The data reveal an annealing effect of a 10%-20% reduction in etching rates depending on the growth temperature. This is probably due to the microstructural enhancement from the annealing process.

C. H. Lee et aL / PECVD A l N thin #lms 235

0 1.1 ° - -

o k -

E 1.0 o

o 0.9

Z ~ 0.8 <

I l I l

0.5 1.0 1.5 2,0

TMA Flow Rote ( c .c /min ) Fig. 12. Dependence of A I : N rat io on T M A f low rate ( l Torr , 25 W; N H 3 f low rate, 50 cm 3 m i n - n).

o ° - -

o k . .

E o

o 0.9

Z

._l <

1.1

1.0

I I I I I I

l z ~ 25 37.5 50 8 z 5 75

NH3 Flow Rote ( c .c /m in ) Fig. 13. Dependence of AI:N ratio o n N H 3 flow rate (1 Torr; 25 W; TMA flow rate, 0.5 cm 3 min- ~).

~ ' C

v N ' - H A L - - H

AL-N

4000 3000 2000 1000

Wovenurnber ( em'* ) Fig. 14. FTIR spectra of films about 600 nm thick deposited at different temperatures (1 Torr, 25 W, TMA : N H 3 = 1:100).

10

._c 8 E

"< 6 L d

0 4-, C 0 ~ 4

c- 2 t - O

I,I

Temperature K 350 300

A H=19.37Kcal/mole

i i i i

2.7 2.9 3.1 3.3

I O00/Tem!~ K

Fig. 15. Etching rate vs. etching temperature.

10

c ° ~

E 8

. <

w ID

O r r 4

.¢-

• ~ 2 O

I , I

0

• as-grown

i ~ A after annealing

I I I I I

200 250 300 350 400

Temperature ("C) Fig. 16. Etching rate vs. growth temperature for films as grown and after annealing (900 °C h-l).

4. Conclusion

Th e process characteristics of PE CVD for deposi- tion of AIN thin films on oxidized silicon substrates are investigated by using N H 3 and AI(CH3) 3 as reactants. Films of uniform growth with mirror-smooth surfaces can be grown at temperatures below 400 °C where no AIN films can be deposited by means of normal ther- mal CVD. Growth rates are found to decrease with the increase in reaction pressure and to increase with the increase in the flow rate of TMA, N H 3: T M A ratio and total flow of the reactants and the carrier gas. In addi- tion, they are independent of the substrate temperature for temperatures below 400 °C. As to the effect of plasma power, the growth rate increases with the increase in r.f. power in the range below 25 W while it decreases after reaching a maximum near 30 W when the r.f. power further increases to 100 W. T he corre-

236 ('. tt. Lee et al. / Pt:'('Vl) AIN thin fihns

sponding refractive index data indicate an increasing t rend with the film density. T h e A I N films g rown at t empera tures above 300 °C are polycrystal l ine while those g rown below 200 °C are amorphous .

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