Influence of an r.f. plasma on the optical and structural properties of vacuum-deposited dielectric coatings

Thin Solid Films, 127 (1985) 351 364 PREPARATION AND CHARACTERIZATION

351

I N F L U E N C E OF AN R.F. P L A S M A ON T H E OPTICAL A N D STRUCTURAL PROPERTIES OF VACUUM-DEPOSITED DIELECTRIC COATINGS R. HERRMANN, H.-G. LOTZ, J. Mf.)LLER, W.-D. MLINZ AND H. VOGT Research and Development Department for Coatings, Leybold-Heraeus G.m.b.H., Wilhelm-Rohn-Strasse 25, D-6450 Hanau 1 (F.R.G.) (Received September 24, 1984; accepted January 15, 1985)

SiO and TiO films, deposited at different substrate temperatures, were compared using the following coating processes: reactive evaporation of SiO and TiO from a boat with the assistance of an r.f.-induced plasma; r.f. magnetron sputtering of a quartz target; conventional reactive and non-reactive evaporation of SiO and TiO from a boat and quartz from an electron beam evaporator. The stoichiometry of the SiO films was qualitatively determined by studying the IR absorption bands of SiO films deposited onto germanium substrates. The relative water content inside the films was measured using the water absorption band at 2.96 ~tm. The packing density was calculated from the change in refractive index due to water sorption into the film after exposure to air through venting. Corrosion tests were made with SiO protective layers on aluminium films. The spectral shift of narrow-band-pass filters consisting of SiO/TiO layers was compared with that of conventionally evaporated narrow-bandpass filters.

1. INTRODUCTION The tendency of modern thin film deposition technologies for optical coatings is directed towards higher demands in film quality. The main properties of modern layer systems are high durability, excellent adhesion, high density and insensitivity to environmental influences (e.g. temperature and corrosion). Additional requirements for optical coatings are low absorption and scattering values. Only a part of these demands can be fulfilled with conventional evaporation techniques. The recent investigations of several research groups ~-5 show that ionand plasma-assisted deposition processes are very promising regarding better film properties. The benefits of these techniques are, among others, good adhesion, high density and low absorption. The aim of this work was to compare the stoichiometry, density, corrosion resistance and spectral shift of films deposited by the following methods: conventional reactive evaporation, reactive evaporation with the assistance of an r.f.induced plasma and magnetron sputtering. For our investigations we used the thin 0040-6090/85/$3.30

© ElsevierSequoia/Printedin The Netherlands

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film materials quartz and titanium oxide, which are prominent materials for multilayer applications in the visible and near-IR ranges. 2. EXPERIMENTAL DETAILS

2.1. Film deposition A schematic diagram of the A700Q vacuum coater is shown in Fig. 1. The chamber is evacuated with a turbomolecular p u m p and a Meissner trap. The coater is equipped with two thermal evaporation sources and two ESV 6 electron beam evaporators.

R.F.Power Supply

Power Supply

ESV 6 Vacuum EB-Evap. Pumps

Fig. 1. The vacuum coater A700Q equipped with an r.f. coil for plasma-assisted deposition.

The thicknesses of the layers were determined optically with a photometer and the deposition rate was controlled with a quartz crystal deposition controller. The plasma was generated using the method of Kausche 6 and M u r a y a m a ~. A nine-turn r.f. coil with a diameter of 460 m m was operated at a frequency of approximately 13 M H z at various power levels. The distance between the evaporation sources and the rotating substrates was typically 500 mm. The evaporation was started at a pressure of 2 × 1 0 - 4 P a . The oxygen was introduced through a control valve whereby a constant oxygen pressure of approximately 2 x 10- 2 Pa was maintained during deposition. A schematic diagram of the Z550 sputtering plant for optical coatings is shown in Fig. 2. A cylindrical high rate magnetron cathode with a fused silica target was used. The distance between the target and the rotating substrate holder was approximately 50 m m (Fig. 2). The thicknesses were obtained optically by measuring the transmission through the surface of a test slide. The normal power loads of the cathode were 2 kW at an argon pressure of 0.2 Pa with the addition of 10% oxygen. For all deposition methods a coating rate of approximately 3 A s - 1 was used.

PROPERTIES OF VACUUM-DEPOSITED DIELECTRICS

Match box

..~

i

Magnetron cathode PK~O0 I I

Inle~

Ar+02~ll ~ - - - - 5 1 5 ~

353

i ~

~ I1-~

Rotary feed through with Vacuum pumps substrateholder Fig. 2. The sputtering plant Z550 with a cylindrical magnetron cathode.

2.2. Deposition of linefilters Line filters of the F a b r y - P e r o t filter type were evaporated onto unheated substrates with and without plasma assistance and onto heated substrates (260 °C) without plasma assistance. SiO and TiO which were both reactively evaporated from a boat source were used as starting materials. Using the conventional notation we utilized the following filter design at a centre wavelength of about 600 nm: substrate/(HL) 32H(LH)3/air At different substrate temperatures the spectral shift of the peak wavelength was measured using a Perkin-Elmer spectrophotometer equipped with a heatable sample holder.

2,3. Sample analysis The stoichiometry of the SiO films was qualitatively determined by studying the IR absorption bands in the wavelength region between 2.5 and 25 lain. The absorption bands are listed in Table 18. The most important bands lie between 7 and 16 lain. TABLE I CHARACTERIZATION OF S i O FILMS BY IR SPECTROSCOPY

Wavelength (~tm) Average 2.76 2.96 4.41 9.35 9.60 10.20 10.64 11.30 12.42 22.22

Indication Range 2.74-2.77 2.95-2.99 4.39-4.43 9.26-9.62 9.40-9.70 10.52-10.75 11.29-11.36 12.27-12.50 22.22-22.47

Si--OH H20, Si--OH Si--H SiO2 Si20 3 SiO SiOH SizO 3 SiO 2 SiO 2

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R. HERRMANNet al.

The relative water content inside the films was measured using the water absorption band at 2.96 pm. The packing densities were computed by the method of ref. 9 utilizing the refractive indices of the films measured in vacuum and water-saturated air. The morphology was investigated from electron micrographs of the surface. For testing the protective properties of SiO films glass slides were coated with an aluminium layer of approximately 0.1 ~tm thickness with an SiO layer of 1 ~tm thickness on top of the aluminium. These samples were dipped into an alkaline N a O H solution (0.2 wt.~o), by which method the destruction of the aluminium layers was observed. 3. RESULTS 3.1. Stoichiometry Figure 3 shows a transmission spectrum which is characteristic for reactively evaporated SiO films with a thickness of aproximately 1 ~tm. A strong absorption band occurs at a wavelength of 9.43 rtm which is located between the absorption bands of S i O 2 (9.35 lam), Si20 3 (9.6 tam) and SiO (10.2 I~m). The weak absorption band at 11.3 ~tm is typical of Si20 3 . Thus it can be concluded that this film consists of a mixture of SiO, S i z O 3 and SiO 2. As can be seen from Fig. 4 the absorption band of Si20 3 at 11.3 ~tm becomes smaller at higher substrate temperatures ( T - - 260 °C).

Wavelength (pro) 7

8

9

12

10

14

16

I

/ J'5,\,I

/

e.

.o_

\

I

\ \

\

e.

\

\\

I--

11 3 um

/ I /

/J

9 43 um

L 1400

I 1200

I

I 1000

I 800

i 600

Wavenumber (crn 1) Fig. 3. 1R spectrum of reactively evaporated SiO without a plasma at low substrate temperature (80 °C).

In contrast with Fig. 3, Fig. 5 shows an IR spectrum of a sputtered SiO2 film. Here a strong absorption band can be observed near 9.3 p.m and a weak absorption band at 12.2 pro. Both bands indicate that the stoichiometry of the sputtered S i O 2 film is very good in contrast with the poor stoichiometry of the reactively evaporated SiO film of Fig. 3.

PROPERTIES OF VACUUM-DEPOSITED

Wavelength (pm) 8 9 10

7

cO o'J ¢/)

DIELECTRICS

12

14

355

16

11.3 p m

E

\

e-

'\

l-

\

/ J

I

/ I

I

1200 1000 Wavenumber (cm ')

1400

800

600

Fig. 4. IR spectrum of reactively evaporated SiO without a plasma at high substrate temperature (260°C). Wavelength (Fro) 8 9 10 I

I

J

12

14

I

[

16

12.2 m

g

f ,/

e /

I

1400

I

9.3 ,am I'~" 1

1200 1000 Wavenumber (cm ' )

I

i

800

600

Fig. 5. IR spectrum ofmagnetron-sputtered SiO2

Figures 6 and 7 were obtained from SiO 2 films reactively evaporated from SiO under the influence of the r.f.-induced plasma. Figure 7 is the spectrum for an r.f, power input of 500 W. One can observe a gradual transition of the IR spectra with increasing r.f. power from the very poor stoichiometry of Fig. 3 to the very good stoichiometry of Fig. 5. The influence of substrate temperature on the spectrum of a plasma-assisted SiOz film (r.f. power, 500 W) is demonstrated in Fig. 8. In contrast

356

R. HERRMANN et al. Wavelength (pm) 8 9 10

12

14

i

[

16 :-

o

E:

I

¢;

9 4 3 t~m

t 1400

I

L

]

L

1200 1000 Wavenumber (cm-')

]

800

600

Fig. 6. IR spectrum of reactively evaporated SiO with plasma assistance (r.f. power input, 100 W) at low substrate temperature (80 °C). Wavelength (?m) 10 8 9

7

12

,

14

16

I

12.2/~m

C

.9 E t-t~

I

1400

I

1200 Wavenurnber

[

L

1000 (cm-')

L 800

l 600

Fig. 7. IR spectrum of reactively evaporated SiO with plasma assistance (r.f. power input, 500W) at low substrate temperature (80 °C). with Fig. 7 there is a small absorption peak at 10.64 ~tm indicating the presence of SiOH. Figures 9 and 10 show the spectra of SiO2 films deposited non-reactively by electron b e a m evaporation of fused silica at substrate temperatures of 80 and 260 °C. The absorption peaks at 9.3 and 12.351xm indicate that the films consist p r e d o m i n a n t l y of SiO2. In addition, however, we find two absorption peaks at 10.64

PROPERTIES OF VACUUM-DEPOSITED DIELECTRICS

357

Wavelength (pm) 7

8

9

10

12

14

16

#-

1400

1200 1000 Wavenumber (cm 1)

800

600

Fig. 8. IR spectrum of reactively evaporated SiO with plasma assistance (r.f. power input, 500 W) at high substrate temperature (260 °C).

Wavelength (pm) 7

8

9

-

J

[

10

12 I

um

14

16

r

J-

12.35 ,um

C

.o ._~

E

¢/) C

1400

1200

1000

800

600

Wavenumber(cm-1) Fig. 9. IR spectrum of electron-beam-gun-evaporatedfused silica at high substrate temperature (260°C). and 11.3 I~m which are characteristic for S i m O H bands and S i 2 0 3 molecules. By increasing the substrate temperature from 80 to 260 °C the S i - - O H and Si20 3 peaks can be reduced, but the stoichiometry of these films is still worse than the stoichiometry of the r.f.-plasma-assisted film of Fig. 7 deposited by boat evaporation of SiO onto unheated substrates. Even the addition of oxygen during the electron beam evaporator deposition does not change the situation.

R. HERRMANN et al.

358 Wavelength (pm) 8 9 10

7

]2

,%

\

1064 ,urn

14

16

J'-~

1235 urn

tO

\

(/)

.£=

\

(/} t-" t~

\ \,

I--

9 3 i~rn

1400

1200

1000

800

600

Wavenumber (cm-') Fig. 10. IR spectrum of electron-beam-gun-evaporated fused silica at low substrate temperature (80 :C).

3.2. Absorbed water and packing density T h e I R analysis, so far, o n l y gives i n f o r m a t i o n a b o u t t h e c h e m i c a l c o m p o s i t i o n of the S i O 2 films a n d n o t a b o u t the s t r u c t u r a l p r o p e r t i e s of the films. A q u a l i t a t i v e m e a s u r e for the p a c k i n g d e n s i t y of the S i O z films w a s o b t a i n e d by m e a s u r i n g the r e l a t i v e w a t e r c o n t e n t inside the films u s i n g the w a t e r a b s o r p t i o n b a n d at 2.96 pm. T h e r e l a t i v e w a t e r a b s o r p t i o n is listed in T a b l e II for different d e p o s i t i o n c o n d i t i o n s . W i t h i n c r e a s i n g r.f. p o w e r a d e c r e a s e in a b s o r b e d a n d c h e m i s o r b e d w a t e r is o b s e r v e d . T h e w a t e r c o n t e n t of the S i O 2 films d e p o s i t e d by e l e c t r o n b e a m g u n e v a p o r a t i o n of fused silica d e c r e a s e s w i t h i n c r e a s i n g s u b s t r a t e t e m p e r a t u r e . A t TABLE II I N F L U E N C E OF T H E D E P O S I T I O N PROCESS O N THE RELATIVE W A T E R A B S O R P T I O N OF S i O FILMS C O M P U T E D BY U S I N G T H E W A T E R A B S O R P T I O N B A N D AT

2.96 pm

Deposition process

Relative water absorption

SiO~ with boat; T~ = 80 °C

1

SiO~ with boat; plasma assisted (Nf = 100 W); T~= 80 °C

0.79

SiO~ with boat; T~= 260 °C

0.53

SiO x with boat; plasma assisted (Nf = 500 W); T~= 80 °C

0.47

SiO~ with boat; plasma assisted (Nr = 500 W); T~= 260°C

0.43

SiO 2 with electron beam evaporator; T~= 80 °C

0.73

SiO 2 with electron beam evaporator; T~= 260 ~'C

0.54

SiO z with electron beam evaporator; T~= 295 °C •iO 2

with magnetron sputtering; T~= 150 °C

0.30 ~- 0

PROPERTIES OF VACUUM-DEPOSITED

359

DIELECTRICS

substrate temperatures below 260°C the relative water content of the electronbeam-evaporator-deposited films is larger than the water content of the best plasma-assisted film deposited onto unheated substrates. The magnetron-sputtered films again have outstanding properties. N o water could be detected inside these films. The observations of the water content of the films are supported by the computation of the packing densities from refractive index measurements in vacuum and water-saturated air. These measurements show that the packing densities of the plasma-assisted films are still relatively low (approximately 80~o for the best films) compared with the packing density of the sputtered coatings which reaches more than 97~o. 3.3. C o r r o s i o n t e s t s

The results of the corrosion tests are listed in Table III. Without plasma and at low substrate temperatures the protective properties of the SiO film are very poor. After 30 min all aluminium layers peel off(Fig. 11). It should be emphasized that the worst result was obtained with electron-beamevaporated SiO2 layers which were destroyed after only 4 min. At high substrate temperatures (T = 160 °C) the protective behaviour of the SiO films is better. As can be seen from Table III there is no difference between the resistance properties of SiO layers coated onto cold and hot substrates with the assistance of an oxygen plasma and a layer of SiO 2 coated by electron beam evaporation onto hot substrates. Transmission pictures, taken with a light microscope after corrosion times of 30min and 8 h, show small pinholes with light shining through them (Fig. 1 l(b)). After 8 h the diameters of the spots have increased and the light coming out is brighter (Fig. 1 l(c)). The same is true for plasma-assisted SiO layers which are deposited onto hot substrates (160 °C). Comparison of Figs. 1 l(c) and l l(d) shows that the number and the size of the pinholes are slightly smaller than on unheated substrates.

TABLE III CORROSION RESISTANCE OF SiO-PROTECTED ALUMINIUM LAYERS AGAINST ATTACK OF ALKALINE

NaOH

SOLUTION

Source

Boat Boat Electron beam evaporation

Evaporation material

SiOx SiOx SiO2

Plasma assistance

No Yes No

Corrosion resistancefor the following substrate temperatures during .deposition 70 ~C

160 °C

+ - -

0 + +

complete destruction of the aluminium layer after 4 min; - , complete destruction of the aluminium layer after 30min; 0, aluminium layer begins to peel off after 30min; +, no peeling off after 8 h and increased size of pinholes.

--,

R. HERRMANNet al.

360

J

(a)

(b)

(c) (d) Fig. 11. Light transmission photographs of SiO-protected aluminium layers after corrosion in NaOH solution (0.2wt.%):(a) SiO without plasma on cold substrate after 25 rain; (b) SiO with plasma on cold substrate after 25 min; (c) SiO without plasma on cold substrate after 8 h; (d) SiO with plasma on hot substrate (T ~ 160°C) after 8 h.

3.4. Spectral shift ofline filters SiO and TiO are standard materials for optical interference filters. Using the conventional evaporation technique the substrates must be heated to high temperatures (250-350 °C) during deposition in most cases. SiO/TiO layer systems deposited at low substrate temperatures exhibit absorption, low packing density and poor durability. It is well known 1° that evaporated filter systems containing SiO/TiO are sensitive to humidity which penetrates into the porous layers from the air. The humidity inside the layers causes an increase in the effective refractive index and the optical thickness which results in an optical shift of the filter system. This effect can easily be observed in line filter systems. In our investigations we measured the spectral shift of line filters at different substrate temperatures. The result is shown in Table IV. With increasing substrate temperature the centre wavelength is shifted towards shorter wavelengths. In Fig. 12 the relative shift of the peak wavelength versus substrate temperature is plotted. Without plasma assistance the strongest shift occurs at a low substrate temperature, With plasma assistance the drift is smaller but still worse than the drift of SiO/TiO filters which are deposited at high substrate temperatures (260 °C). For comparison the spectral shift of magnetron-sputtered line filters consisting of SiO and T a / O s layers is shown in Fig. 12.

361

PROPERTIES OF VACUUM-DEPOSITED DIELECTRICS

TABLE IV RELATIVE S H I F T O F P E A K P O S I T I O N F O R F A B R Y - - P E R O T I N T E R F E R E N C E FILTERS A T D I F F E R E N T M E A S U R I N G TEMPERATURES

Measuring temperature offilter (°C)

23 50 80

A l / l o f o r the following SiO/TiO without plasma deposition; temperature, 80 °C

SiO/TiO with plasma deposition; temperature, 80 °C

SiO 2/TiO 2 without plasma deposition; temperature 260 °C

Si02/Ta20 a magnetron sputtered

0 0,05 0,06

0 0.015 0.022

0 0.007 0.014

0 0.003 0.009

006 f

005

/

00/,

F~

003

/

/

/

/

/

/

f /

I

002

/

/ /

/

001

20

~0 60 MEASURING TEMPERATURE (°C)

80

Fig. 12. Relative shift in peak position vs. measuring temperature for Fabry-Perot interferencefilters for the layer systemglass/(HL)32I-I (LH)a/air (L, SiO; H, TiO; 10 = 600 nm): +, without plasma (deposition temperature, 80°C); O, with plasma (deposition temperature, 80°C); A, without plasma (deposition temperature, 260 °C); Q, magnetron-sputtered Fabry-Perot filter with SiO2/Ta205 .

4. CONCLUSIONS S i O 2 films reactively deposited by boat evaporation of SiO under the influence of an r.f.-induced plasma in an oxygen atmosphere were compared with SiO2 films deposited by sputtering and electron beam evaporation. The aim of this was to develop an r.f.-plasma-assisted process which would be able to produce SiO2 films on unheated substrates with good stoichiometry and high packing density. The investigation of the IR spectra reveals that the stoichiometry of the films can be improved considerably when the oxygen plasma is interacting with the condensing film. A broad spectrum of reactions have been observed to take place in oxygen plasmas 11. These include reactions between

362

R. HERRMANNe t al.

electrons and molecules, ions and molecules, ions and ions, and electrons and ions. In a high frequency discharge the dissociation of molecular oxygen by electron collision can occur by the following reactions: e + 0 2 --*O2*(A3Yu+)-*O(3p)+o(3p)+e -

(1)

e - + O 2 --* O2*(B 3Y~u) --~ O(3p)+ O ( 3 D ) + e

(2)

The excited states of oxygen are designated by the spectroscopic symbols A 3Eu+ and B 3E u. The production of atomic oxygen by dissociative attachment, i.e. e +O2~O

+O

(3)

can be neglected since the reverse of reaction (3) is very fast (k = 3.0 x 10- lo cm ~ s - 1). Negative and positiv.e oxygen ions are produced by dissociative attachment: e +02--*0

+O

(4)

e +O2~O

+O++e

(5)

The concentrations of ionic and neutral species for an r.f. oxygen plasma have been discussed by Bell and K w o n g 12. The primary reactive species have been determined to be 02(1/~g) molecules and O(3p) atoms, their individual concentrations ranging from 10~o to 20~o, depending on the experimental conditions 13 and wall and other catalytic effects. Both the atoms and the excited molecules react with the evaporation material and the substrate during deposition. From the above relations it is assumed that the oxidation of SiO t o SiO 2 occurs at the substrate surface according to the following reaction mechanisms: SiO + O(3p) -* SiO 2

(6)

2SiO + O2(a lag) -* 2SiO/

(7)

SiO + + O ( 3 p ) + e

IS) (9)

--, SiO 2

2SiO + + O 2 ( a ~Ag)+e- --, 2SiO 2

Compared with the normal oxidation reaction under the influence of molecular oxygen the plasma reactions (6)-(8) are highly favoured at low substrate temperatures. This conclusion, confirmed by the investigations of stoichiometry, shows that the SiOz content increases at higher r.f. power levels. The relatively high corrosion resistance of plasma-assisted SiO films can be explained by the formation of aluminium silicates (sillimanite, disthene, andalusite and mullite) between the aluminium and the SiO layer. The chemical attack starts within small pinholes which allow a lateral penetration of the N a O H solution to the aluminium surface. The influence of the oxygen plasma on the packing de/asity of the deposited SiO and TiO layers is relatively small. This can be concluded from the measured water content of SiO layers and from the spectral shift of SiO/TiO line filters. There is practically no difference between conventionally evaporated and plasma-assisted layers in that respect. Figure 13 shows scanning electron micrographs of SiO films on germanium substrates evaporated conventionally, with

PROPERTIES

(a)

OF VACUUM-DEPOSITED

!

79 nm

363

DIELECTRICS

1

(b)

! 78 nm I

(c) 78 nm Fig. 13. Scanning electron micrographs of SiO layers (thickness, 1 ~tm) deposited on germanium substrates by (a) conventional evaporation from a boat, (b)plasma-assisted evaporation and (c) magnetron sputtering of fused silica. (Photographic angles, 60°.)

plasma assistance and with m a g n e t r o n sputtering. The surfaces of the e v a p o r a t e d films look gritty whereas the a p p e a r a n c e of the sputtered surface is smooth. F u r t h e r i m p r o v e m e n t s in the deposition process have to be m a d e regarding the packing density of the films and the related water absorption. A higher packing density can be obtained in two ways: (i) by increasing the power density of the r.f. plasma to produce m o r e ions; (ii) by combining the r.f. plasma with ion accelerating

364

R. HERRMANNet al.

grids to enhance the kinetic energy of the ions, which enables them to resputter loosely bonded particles from the surface. A future improvement in film quality at low substrate temperatures could be achieved by using the ionized-cluster beam or the secondary ion beam sputtering technique. REFERENCES

W. Heitmann, Appl. Opt., 10 (1971) 2414. J. Ebert, Opt. Thin Films, SPIE Trans., 325 (1982) 29. F.H. Allen, Opt. ThinFilms, SPIETrans.,325(1982)93. R. Netterfield, Appl. Opt., 22 (1) (1983) 178. R. Netterfield and P. J. Martin, Proc. ISA T83 andlPA T83 Conf., Kyoto, 1983, p. 909. H. Kausche, Vorrichtung zur Katodenzerst~iubung, Ger. Patent 1,515,311, 1965. Y. Murayama, J. Vac. Sci. Technol., 12 (4) (1975) 818. W.A. Pliskin and H. S. Lehman, J. Electrochem. Soc., 112 (10) (1965) 1013-1019. S. Ogura, Thin Solid Films, 30 (1975) 3-10. T.M. Christmas and D. Richmond, Opt. Laser Technol., (June 1977) 109. A.T. Bell, in J. R. Hollahan and A. T. Bell (eds.), Techniques and Applications of Plasma Chemistry, Wiley, Toronto, 1974, p. 26. 12 A.T. Bell and K. Kwong, AIChE J., 18 (1972) 990. 13 F. Kaufman and J. R. Kelso, J. Chem. Phys., 32 (1960) 301. 1 2 3 4 5 6 7 8 9 10 11