THE EVALUATION OF β-Si3N4 MICROSTRUCTURES USING PLASMA-ETCHING AS A PREPARATIVE TECHNIQUE

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THE EVALUATION OF β-Si3N4

MICROSTRUCTURES USING PLASMA-ETCHING AS A PREPARATIVE TECHNIQUE

C. O’Meara, P. Nilsson, G. Dunlop

To cite this version:

C. O’Meara, P. Nilsson, G. Dunlop. THE EVALUATION OF β -Si3N4 MICROSTRUCTURES USING

PLASMA-ETCHING AS A PREPARATIVE TECHNIQUE. Journal de Physique Colloques, 1986, 47

(C1), pp.C1-297-C1-301. �10.1051/jphyscol:1986143�. �jpa-00225573�

JOURNAL DE PHYSIQUE

Colloque Cl, suppl6ment au n02, Tome 47, fgvrier 1986 page c1-297

THE EVALUATION OF (3-Si,N, MICROSTRUCTURES USING PLASMA-ETCHING AS A PREPARATIVE TECHNIQUE

C. O'MEARA, P. NILSSON and G.L. DUNLOP

Department

of

Physics, Chalmers University o f Technology, S-412 96 Gtiteborg, Sweden

R6sum6

-

Le decapage au plasma a 6 t 6 u t i l i s e avec succes pour 1 'etude micro- s t r u c t u r e l l e de deux 6-Si N4 materiaux pour f a c i l i t e r l e u r E v a l u a t i o n avec l e s methodes d'analyse: S ~ M e t TEM. L ' u t i l i s a t i o n de c e t t e technique permet une c a r a c t g r i s a t i o n p l u s r e p r k e n t a t i v e des m i c r o s t r u c t u r e s .

A b s t r a c t

-

Plasma-etching has been used s u c c e s s f u l l y t o d e l i n e a t e t h e micro- s t r u c t u r e s o f two 6-Si N m a t e r i a l s i n o r d e r t o f a c i l i t a t e t h e i r e v a l u a t i o n by e l e c t r o n

micros copy^

+he use o f t h i s technique r e s u l t s i n a more repre- s e n t a t i v e e v a l u a t i o n o f the m i c r o s t r u c t u r e s .

I

-

INTRODUCTION

Plasma-etching, a technique which has been w i d e l y used i n t h e p r o d u c t i o n o f semi- conductor devices /1,2/, was f i r s t a p p l i e d t o n i t r o g e n ceramics by C h a t f i e l d /3/ t o e t c h s i a l o n m a t e r i a l . I n p r i n c i p l e the e t c h a n t i s obtained through t h e decomposition o f a r e l a t i v e l y i n e r t gas i n a glow discharge. The gas i s chosen so t h a t t h e r e - s u l t i n g r e a c t i v e species i n t e r a c t w i t h the m a t e r i a l ' s surface t o g i v e a v o l a t i l e product. The p h y s i c a l v a r i a b l e s o f t h e plasma-etchi ng apparatus /I/ i n f l u e n c e the ion-energy o f the plasma and several degrees o f e t c h i n g a r e p o s s i b l e between t h a t o f a p u r e l y chemical "gas-surface" r e a c t i o n and ion-bombardment. For n i t r o g e n ceramics a halo-carbon gas, commonly CF4, i s used s i n c e F atoms r e a c t spontaneously w i t h a l l forms o f S i , Si02 and Si3N t o g i v e a v o l a t i l e product. The e t c h i n g time i s extremely s h o r t and the 8-Si N 8hase i s removed more r a p i d l y than any o t h e r phase i n the m i c r o s t u r c t u r e . 1n3tflis work plasma-etching was used i n t h e micro- s t r u c t u r a l e v a l u a t i o n o f two 8-Si N m a t e r i a l s i n o r d e r t o examine i t s p o t e n t i a l use i n f a c i l i t a t i n g m i c r o s t r u c t u r a l c 2 a t a c t e r i z a t i o n . The technique was applied: ( i ) t o specimens f o r scanning e l e c t r o n microscopy (SEM) f o r q u a l i t a t i v e a n a l y s i s of b u l k m i c r o s t r u c t u r e and t o e x p l o r e i f any d i f f e r e n c e c o u l d be detected i n the m i c r o s t r u c - t u r e s a f t e r h i g h temperature o x i d a t i o n ; and ( i i ) t o transmission e l e c t r o n micro- scopy (TEN) specimens t o t e s t if p a r t i a l removal o f the 8-Si3N4 g r a i n s allowed improved examination o f secondary phases.

I 1

-

EXPERIMENTAL

Two 8-Si N m a t e r i a l s were examined: a hot-pressed 6 - 9 N w i t h Y O3 as s i n t e r i n g a d d i t i v e 3 ( f l ~ 209) and pressureless s i n t e r e d 6-51 N w i t 4

%

^{w/o Y}

a3,

2 w/o A1 0 s i n t e r i n g a d d i t i v e s (RP

I )

141. Oxidized samples30f b o t h materia?s were prepaqei by heat treatment a t 1350'~ i n an a i r atmosphere f o r 200 hours. SEM specimens were pre- pared f o r plasma-etching by diamond g r i n d i n g and p o l i s h i n g and TEM speciniens were prepared by diamond g r i n d i n g f o l l o w e d by i o n - m i l l i n g from one s i d e and plasma e t c h i n g from the opposite s i d e .

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986143

JOURNAL DE PHYSIQUE

A Technics I1 plasma-etching apparatus was used and t h e physical v a r i a b l e s con- t r o l l e d i n o r d e r t o ensure t h a t a predominantly chemical e t c h was obtained thus avoiding a d d i t i o n a l ion-bombardment. The material was etched with CF4 ( 8 vol% 0 2 ) gas using t h e following conditions: power 300 W; pressure 0.3 t o r r ; and frequency 30 KHz. Etching times were 2-3 minutes f o r SEM specimens and 30 seconds f o r t h i n f o i l s .

111

-

RESULTS AND DISCUSSION

Scanning microscopy. The scanning micrographs of Fig. l ( a ) and (b) show t h e etched s u r f a c e s of as-received m a t e r i a l , NH-209 and RP I r e s p e c t i v e l y . The 6-Si N g r a i n s (dark regions) were r a p i d l y removed during t h e e t c h i n g process thus enabliflg t h i s major c r y s t a l 1 i n e phase t o be d i s t i n g u i s h e d from t h e secondary phase(s) ( l i g h t net- work). Rapid estimation of grain s i z e d i s t r i b u t i o n , a s p e c t r a t i o and d i s t r i b u t i o n of secondary phase(s) can be made from the micrographs. These images c o n t r a s t s t r o n g l y with the t y p i c a l SEM image obtained from unetched 6-Si N4 where no micro- s t r u c t u r a l d e t a i l can be i d e n t i f i e d , a s i s shown i n Fig. 2 ( a ) f a r unetched NH 209.

I t i s n o t p o s s i b l e t o a s s e s s t h e n a t u r e ( g l a s s o r c r y s t a l 1 in e ) of t h e secondary phase(s) v i s i b l e i n t h e s e SEM images, but EDAX a n a l y s i s showed these regions t o contain ( i n a d d i t i o n t o S i ) up t o 80 w/o

Y

i n both m a t e r i a l s with varying small q u a n t i t i e s of Mg, Fe and Ca i n NH 209, and A1 and Fe i n RP I .

S p e c i f i c d i f f e r e n c e s i n e t c h i n g behaviour occured a f t e r heat-treatment. (For purpo- s e s o f comparison oxidized specimens were always etched simultaneously with

un-

oxidized material .) A region of poor microstructural d e f i n i t i o n up t o 100

um

wide was observed around t h e edge of specimens of oxidised NH 209. ( F i g .

2(b).)

No such zone was observed i n oxidised RP I but etching d e f i n i t i o n was g e n e r a l l y lacking compared t o simultaneously etched unoxi dized material

.

These e t c h i n g d i f f e r e n c e s must be due t o changes .which occur i n t h e chemical composition of the secondary phase system during oxidation and can t e n t a t i v e l y be explained i n terms of the chemical processes involved i n e t c h i n g . In CF / O plasmas t h e b a s i c r e a c t i o n taking p l a c e i n t h e e t c h i n g of Si o r

i t s

comp%un?is Si02 and Si N

is

t h e chemi- s o r b t i o n of atomic F and i t s desorbtion as SiF o r SiF

.

~ l t h o a g f l t h e r e l a t i v e s e l e c t i v i t y f o r Si

,

Si02 o r Si N compared t o

t

s i l ica?e of mixed elemental compo- s i t i o n v a r i e s according t o t h e 3 p i r t i c u l a r conditions of e t c h i n g /5/, i t can be assumed t h a t mixed phases w i l l e t c h more slowly. This i s because mixed s i l i c a t e s contain elementswhich a r e not as r e a c t i v e with F a s i s S i , and metals such a s A1 and Y whose f l u o r i d e s a r e i n v o l a t i l e . I f oxidation r e s u l t s i n outward metal c a t i o n movement t o t h e oxide s c a l e /5/ then t h e zones depleted i n metal c a t i o n c o n t e n t may be expected t o etch a t a comparable r a t e t o 6-Si3N4.

Fig. 1

.

SEM micrographs of plasma-etched s u r f a c e s of as-received material ( a ) NH 209 and ( b ) RP I . The B-Si N g r a i n s (dark r e g i o n s ) a r e p r e f e r e n t i a l l y removed i n the e t c h i n g procesg

t h u s

d e l i n e a t i n g t h i s major c r y s t a l l i n e phase from t h e secondary phase network (arrowed).

F i g . 2. (a) A t y p i c a l secondary e l e c t r o n image of unetched B-Si N m a t e r i a l (SEM).

( b ) An edge zone o f p o o r l y d e f i n e d m i c r o s t r u c t u r e i s obsetved i n plasma- etched o x i d i z e d specimens o f NH 209 (SEM).

Transmission microscopy. Examination o f plasma-etched t h i n f o i l 8 of NH 209 revealed t h a t m i c r o s t r u c t u r e c o n s i s t e d o f 8-Si N w i t h a s t r o n g l y v a r y i n g g r a i n s i z e and s i g n i f i c a n t q u a n t i t i e s o f a secondary3c$ystal 1 in e phase which formed an almost continuous network around t h e n i t r i d e g r a i n s ( F i g . 3 ( a ) ) . The secondary c r y s t a l 1 in e phase has been i d e n t i f i e d by e l e c t r o n d i f f r a c t i o n as Y-N a p a t i t e . Dark f i e l d micro- scopy using d i f f u s e s c a t t e r e d e l e c t r o n s has shown t h a t the c r y s t a l l i n e phases are surrounded by t h i n glass f i l m s (Fig. 3 ( b ) ) . Plasma-etched RP I was found t o c o n s i s t of r e l a t i v e l y uniformly s i z e d 6-Si N g r a i n s w i t h small q u a n t i t i e s of a secondary c r y s t a l l i n e phase which was a l s o i d e i t i f i e d as Y-N a p a t i t e . This phase was n o t continuous and occurred a t mu1 ti -g r a i n j u n c t i o n s ( F i g . 4 ( a ) ) . A t h i n i n t e r g r a n u l a r glass f i l m surrounded both the B-Si3N4 and Y-N a p a t i t e phases.

As i s summarized i n Table I , e v a l u a t i o n s o f these two m i c r o s t r u c t u r e s a f t e r plasma- e t c h i n g d i f f e r e d i n terms o f t h e e s t i m a t i o n o f secondary phase content from

previous m i c r o s t r u c t u r a l e v a l u a t i o n s c a r r i e d o u t i n our l a b o r a t o r i e s on p l a i n ion-beam t h i n n e d specimens. I n the e a r l i e r . i n v e s t i g a t i o n s NH 209 was found t o c o n t a i n o n l y small q u a n t i t i e s o f Y-N a p a t i t e a t m u l t i g r a i n j u n c t i o n s (Fig. 4 ( b ) ) , ' ~ h i l e i n RP

I

no secondary c r y s t a l l i n e phase was detected. A p o s s i b l e explanation f o r these d i f f e r i n g e v a l u a t i o n s under1 i e s our attempt t o p l asma-etch the f o i 1s. The i o n - t h i n n i n g process used i n ceramic t h i n - f o i l p r e p a r a t i o n has the disadvantage o f being slow and o f i n t r o d u c i n g a r t i f a c t s i n t o the m a t e r i a l due t o r a d i a t i o n damage and p r e f e r e n t i a l s p u t t e r i n g / 6 / . I n t h i n n i n g ceramic f o i l s a compromise has t o be made between the q u a l i t y o f the f o i l and t o t a l time t o p e r f o r a t i o n . M i l l i n g a t high-angles (>ZOO) can r e s u l t i n f a s t b u t p r e f e r e n t i a l s p u t t e r i n g of polyphase m a t e r i a l , w h i l e a t low i n c i d e n t angles, a l l o f the m i c r o s t r u c t u r a l components t h i n a t v i r t u a l l y the same r a t e b u t t h i n n i n g times can be very long.

I n our l a b o r a t o r y ceramic f o i l s have u s u a l l y Been thinned from approximately 30 pm t o p e r f o r a t i o n a t m i l l i n g angles o f 25-35 i n o r d e r t o a v o i d excessively l o n g t h i n n i n g times. I t was f e l t t h a t even i f some p r e f e r e n t i a l s p u t t e r i n g occured, t h e r e should e v e n t u a l l y be s u f f i c i e n t shadowing o f the secondary phase by the more s l o w l y s p u t t e r e d B-Si N t o preserve the b u l k m i c r o s t r u c t u r e i n the f i n a l specimen.

Examination o f plasma3e?ched b u l k m a t e r i a l by SEN i n d i c a t e d however t h a t more secondary phase(s) were present i n b o t h m a t e r i a l s than had p r e v i o u s l y been i n f e r r e d by TEM examination. I n v e s t i g a t i o n by plasma-etching o f t h i n f o i l s t h a t had been i o n - m i l l e d from one s i d e o n l y i n d i c a t e s t h a t p r e f e r e n t i a l s p u t t e r i n g o f the secon- dary phase does occur r e s u l t i n g i n a s i g n i f i c a n t underestimation of i t s content.

I t

i s appreciated t h a t plasma e t c h i n g may r e s u l t i n an o v e r e s t i m a t i o n o f the secon- dary phase content and f u r t h e r work i s needed to, examine the r e s u l t s o f d i f f e r e n t e t c h i n g times i n o r d e r t o i n t e r p r e t t h e r e s u l t s q u a n t i t a t i v e l y . I t i s concluded t h a t , i f used i n c o n j u n c t i o n w i t h the examination o f p u r e l y ion- m i l l e d specimens,

C1-300 JOURNAL DE PHYSIQUE

examination of plasma-etched thin f o i l s r e s u l t s in a more representative evaluation of tne

I I I ~

crostructure.

IV

- CONCLUSIONS

Plasma-etching i s a rapid and e f f e c t i v e technique f o r revealing polyphase 6-Si3N4 microstructures f o r evaluation by SEM and i s thus a useful tool f o r production control. Due to preferential sputtering, conventional thin f o i l preparation by ion- milling can lead t o an underestimation of the volume f r a c t i o n of secondary phase while plasma-etching, by s e l e c t i v e l y thinning the 6-Si

N

grains, preserves the secondary phase d i s t r i b u t i o n . Ion-mi 11 ing and plasma-e$cfling are thus complementary specimen preparation techniques f o r transmission microscopy and, i f used together, can r e s u l t in a more representative evaluation of a materials microstructure.

Fig. 3.

TEM

micrographs of plasma-etched

NH 209

thin f o i l s : ( a ) Bright f i e l d image

showing p a r t i a l l y removed 6-Si

N4

grains and interconnecting

Y-N

a p a t i t e

regions(Y) . ( b ) Dark f i e l d irnade revealing intergranular glass films.

F i g . 4. Comparison o f plasma-etched and i o n beam t h i n n e d TEM specimens

( a ) Y-N a p a t i t e ( Y ) i s found a t mu1 t i - g r a i n j u n c t i o n s i n plasma-etched RP I t h i n f o i l s . Sub-grain l i k e f e a t u r e s are most l i k e l y a r e s u l t o f e t c h i n g . (b) Only small i s o l a t e d pockets o f Y-N a p a t i t e are found i n unetched NH 209

t h i n f o i l s .

Table I. Comparison of r e s u l t s TEM a n a l y s i s o f i o n - m i l l e d ( I M ) and plasma- etched (PE) t h i n f o i l s .

1

SECONDARY PHASE CONTENT

1

Small pockets o f Y-N a p a t i t e s (0.1-0.4 pm) a t m u l t i - g r a i n j u n c t i o n s . T h i n i n t e r g r a n u l a r glass f i l m s .

Large q u a n t i t i e s o f Y-N a p a t i t e forming i n t e r c o n n e c t i n 9 networks.

T h i n i n t e r a r a n u l a r a l a s s f i l m s .

No c r y s t a l 1 in e secondary phase. Glass pockets and t h i n i n t e r g r a n u l a r IM glass f i l m s .

Rp 1 --- ---

--- - - - --- ---

PE Small q u a n t i t i e s of Y-N a p a t i t e m a i n l y a t m u l t i g r a i n j u n c t i o n s . T h i n i n t e r g r a n u l a r glass f i l m s .

ACKNOWLEDGEMENTS

Dr. Robert Pompe o f the Swedish I n s t i t u t e f o r S i l i c a t e Research i s thanked f o r p r o v i d i n g the m a t e r i a l RPI

.

F i n a n c i a l support from t h e Swedish Natural Science Foundation (NFR) i s g r a t e f u l l y acknowledged.

REFERENCES

/1/ Flamm, D.L. and Donelly, V.M., Plasma Chem. Plasma Processing 1 (1981) 317.

/2/ Winters, H.F., Coburn, J.W. and Chuang, T.J., J. Vac. S c i . T e c h o ? . 8. 1

(1983) 469.

-

/3/ C h a t f i e l d , C., J. Am. Ceram. Soc.

C-168

(1983).

/4/ F a l k , L.K.L., Pompe, R. and Dunlop, G.L., i n "Science of Ceramics"

12

(1983) 293.

/5/ Clarke, D.L. and Lange, F.F., 3 . Am. Ceram. Soc. 63 (1980) 586 /6/ Howitt, D.G., J. E l e c t r o n Microsc. Tech.

1

(1984)-405.