HIGH-FIELD CORROSION OBSERVED IN A FIELD ION MICROSCOPIC STUDY

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HIGH-FIELD CORROSION OBSERVED IN A FIELD ION MICROSCOPIC STUDY

D. Ramanathan, P. Vijendran

To cite this version:

D. Ramanathan, P. Vijendran. HIGH-FIELD CORROSION OBSERVED IN A FIELD ION MICROSCOPIC STUDY. Journal de Physique Colloques, 1986, 47 (C7), pp.C7-425-C7-428.

�10.1051/jphyscol:1986771�. �jpa-00225966�

HIGH-FIELD CORROSION OBSERVED IN A FIELD ION MICROSCOPIC STUDY

D. RAMANATHAN and P. V I J E N D R A N

Technical Physics & Prototype Engineering Division, Bhabha Atomic Research Centre, Bombay - 400 085, I n d i a

A b s t r a c t r The f i e l d c o r r o s i o n e f f e c t of n i t r o g e n on tungsten, molybdenum and rhenium was s t u d i e d t o e v a l u a t e i t s e f f e c t on t h e t i p shape and s i z e and i t s c r y s t a l l o g r a p h i c and chemical s p e c i f i c i t y . The promotion e f f e c t of hydrogen on t h i s c o r r o s i o n w a s a l s o

s t u d i e d f o r comparision. I n t h e case of molybdenum, hydrogen showed a d i s t i n c t promotion e f f e c t .

1 , I n t r o d u c t i o n r F i e l d promoted c o r r o s i o n h a s befn s t u d i e d f o r oxygen and o t h e r r g a c t i v e g a s e s u s i n g helium-ion imaging and w i t h hydrogen promotion . The r e a c t i v i t y of t h e s u r f a c e was shown t o be dependent on t h e supply o f t h e r e a c t a n t s t o t h e h i g h - f i e l d r e g i o n and i t s e f f e c t on t h e r e a c t i o n r a t e - c o n t r o l l i n g f a c t o r s . A

s i t e - s e l e c t i v e degradation2 of t h e o r i g i n a l s u r f a c e l e a d i n g t o g r o s s s t r u c t u r a l / d i m e n s i o n a l changes have been observed. The p r e s e n t s t u d y of n i t r o g e n a d s o r p t i o n w a s mainly t o a s s e s s macroscopic

e f f e c t s of n i t r o g e n on t h e s t a b i l i t y of t h e f i e l d i o n e m i t t e r s under normal imaging conditions. Hence, t h e a d s o r p t i o n e f f e c t s were s t u d i e d on m a t e r i a l s l i k e t u n g s t e n , molybdenum and rhenium, having d i f f e r e n t c r y s t a l l o g r a p h i c s t r u c t u r e s and m e l t i n g p o i n t s .

2 . E x ~ e r i m e n t a l r Our s t u d i e s were c a r r i e d o u t i n a standard UHV f i e l d i o n microscope w i t h p r o v i s i o n f o r a d m i t t i n g c o n t r o l l e d amounts of helium, hydrogen and nitrogen. The d e t a i l s of t h e

microscope were d e s c r i b e d eleewhere3. A l l i m i n g s were done at 78K u s i n g helium. A uadrupole mess s p e c t r o m e t e r ? ~ ~ w i t h a n a e s range of 1-60 mu? enabled a r e s i d u a l g a s a n a l y s j s of t h e chamber environment with a d e t e c t i o n s e n s i t i v i t y of 1 0 ~ ' ~ t o r r . The c o r r o s i o n process was s t u d i e d a t d i f f e r e n t n i t r o g e n p r e s s u r e s by observing t h e changes i n t h e clean-image p a t t e r n of t h e specimen a 8 a f u n c t i o n of time with t h e f i e l d on. The p a r t i a l p r e s s u r e (p/p) of t h e adsorbing g a s and t h e image g a s were monitored. The a d s o r b a t e w a s allowed t o cover the e n t i r e t i p cap by promoted m i g r a t i o n 2 under t h e h i g h f i e l d and t h e n i t r o g e n was t h e n c u t o f f . The imaging f i e l d was r a i s e d t o desorb t h e a d s o r b a t e and r e s t o r e t h e c l e a n s u r f a c e . The changes i n t h e image symmetry could be d e t e c t e d a t t h e end as w e l l as a t i n t e r m e d i a t e d e s o r p t i o n s t e p s and

p h o t o g r a p h i c a l l y recorded. The new BIV w a s noted. The background spectrum was taken b e f o r e and a f t e r t h e experiment. These

o b s e r v a t i o n s were r e p e a t e d f o r d i f f e r e n t p r e s s a e s of n i t r o g e n and a l s o by changing t h e image g a s t o helium + l o p hydrogen, as

r e q u i r e d f o r hydrogen-promotion.

3.Results and Discussions r The i n c r e a s e i n BIV a f t e r an a d s o r p t i o n experiment corresponds t o an i n c r e a s e of t h e t i p r a d i u s and a p o s s i b l e m o d i f i c a t i o n of t h e t i p shape. Both a f f e c t t h e voltage- f i e l d 4 e m p i r i c a l r e l a t i o n s h i p , so t h a t we can w r i t e

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1986771

C7-426 JOURNAL DE PHYSIQUE

where V, and V, are the two BIVa, Ki ^{and K,} are the field-factors, characterising the tip of radii q and r , before and after the adsorption-desorption. The ratio of radii can be calculated independently by measuring their radii ditectly from the images applyirkg the ring-counting technique. Once rz/rs was evaluated, the ratio K2 /Xi could be obtained from eqn, (1). Table 1 gives the data for the different systems studied. In all cases,except for case (9) and (lo), KZ /Kl was

unity, indicating no change

TABrn 1

THE BIV INCREASE OBSERVED AFTER NITROGEN A D S O ~ ~ I O I

I m a g e gas l n i t i a l 1 i n a l N i t r o g e n

System p r e s s u r e 8 f V B I V p r e s s u r e

( ' I o r r l ( k V s ( k V s I ( T o r r f

TADLE 2

RADIUS ANI) SIIAPE-FACTOR CHANGES WITH NLTKtIGEN ADSCIHPTION

S r . n o . s y s t e m

v 2 1 v l r z l r l k2'kl

. . .

1

+ Ile 1 . 2 9 I . z e 1 . 0 1

2 Mo

!lo 1 . 1 2 1 . 3 3 1 . 0 7

3 Mo - ^{Ile 1 . 2 4 1 . 2 5 0 . 9 9}

4 Re + Ils 1 . 3 7 1 . 3 9 0 . 9 9

5 Re + He 1 . 1 1 1 - 0 8 1 . 0 3

6 W + lle 1 . 6 8 1 . 6 6 1 . 0 1

7 W + He + tl 1 . 4 0 1 . 3 7 1 . 0 2

8

+ i l e + 112 1 . 1 0 1 . 1 2 0 . 9 8

9 Mo + He,

l i 2 * 1 . 3 1 1 . 1 3 1 . 1 5

10 Mo + Ile

l l Z * 1 . 2 1 1 .OO 1 . 2 1

2

in the shape-factor. Thus, these results show that field corrosion process by nitrogen at helium imaging fields occured uniformly and isotropicslly on all the three metals studied - resulting only in a nett loss of material, leading to enhanced BIV values. The clean image could, in all casee, be retrieved after the desorption- process at a new (higher) BIV. The ima es in the desorption steps confirmed this conclusion. [Figs (I), $2) and ( 3 ) j . ^{Cnae ( 9 ) and}

(10) in table 1 referred to the molybdenum tip, nitrogen-adsorbed

and imaged with helium + L O P hydrogen. The shape-factor ratios in

these 2 cases were larger than unity, indicating a gross modi-

( a ^%. o) Clean images Clean images a 4 0 ) Clean images b-h a d s o r p t i o n images images b-i a d s o r p t i o n images t i - n l d e s o r p t i o n images images [ j - n l d e s o r p t i o n images

e - h ^{n - t}

a - d

I

&

F i g 4a; Nitrogen and hydrogen on F i g 4b: P i t r o g e n and hydrogen on

l o l y b d enum Molybdenum(contd. ⁾

t a ) Clean: (b) hydrogen promotion: (n-x] desorption: (p-r) c r a t e r :

c-1) a d s o r p t i o n images (r-x c r a t e r f i l l e d images

C7-428 JOURNAL DE PHYSIQUE

f i c a t i o n of the shape-factors. The images a l s o correspondingly showed a marked difference. [Figs 4a and 4bJ. In t h e desorption images, fi&.4b, a l a r g e c r a t e r was mbserved i n t h e (100) plane and i t s v i c i n a l s . A controlled desorption sequence revealed t h a t t h e c r a t e r was extended t o a depth of 3-4 l a y e r s below the surface.

The mass spectra taken before and after the adsorption were compared.

[ ~ i g s . ( 5 ) and ( 6 ) 1.

F i g 5; Spect~um before adsorption Fig 6; Spectrum a f t e r adsorption

crx~eriment experiment

The mass spectra taken before and after the adsorption were compared (Figs.(S) and (6)). As the nitrogen used was of commercial grade, there was a significant amount of oxygen present in the spectra. The intensity of mass (32) seemed to reduce after the adsorption whereas there was an increase in the intensities of masses 17, 18 and 44. As these spectra were taken at times different from the actual experiment, it was difficult to draw up unambiguous, conclusive evidence for any reaction mechanism occuring at the tip under the high-field observa- tions.

In conclusion, corrosive, site-specific adsorption of nitrogen on molybdenum in the presence of hydrogen was observed as a high-field effect. The exact mechanism of the reaction process requires more sophisticated experimental studies for evaluation.

Acknowledgements t The authors are thankful t o P r o f ( s ) J.H. Block a n d t f o r t h e i r valuable comments.

1. G.X.&. Cranstoun and J.S. Anderson, Surf.Sci.32 (1972) 397-421.

2. i b i d , Surf.Sci.35 (1973) 319-335.

3. P. Vijendran, Sunder Das, Devaki Ramanathan and A.K. Gupta, Proc.

7 t h I n t e r n , Vac. Congr, and 3rd I n t e r n , Conf. Solid Surfaces Vienna (1977).

4. E.W. Muller and T.T. Tsong, F i e l d Ion Microscopy (American

E l s e v i e r , 1969) 187.