A NEW ELLIPSOMETRIC PROGRAMME APPLIED TO THE CHARACTERIZATION OF TRANSPARENT CONDUCTING TITANIUM NITRIDE FILMS

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A NEW ELLIPSOMETRIC PROGRAMME APPLIED TO THE CHARACTERIZATION OF TRANSPARENT

CONDUCTING TITANIUM NITRIDE FILMS

G. Celotti, R. Rosa, C. Summonte, G. Martinelli

To cite this version:

G. Celotti, R. Rosa, C. Summonte, G. Martinelli. A NEW ELLIPSOMETRIC PRO- GRAMME APPLIED TO THE CHARACTERIZATION OF TRANSPARENT CONDUCTING TI- TANIUM NITRIDE FILMS. Journal de Physique Colloques, 1983, 44 (C10), pp.C10-273-C10-276.

�10.1051/jphyscol:19831057�. �jpa-00223515�

JOURNAL DE PHYSIQUE

Colloque CIO, supplément au n°12, Tome M, décembre 1983 page C10-273

A NEW ELLIPSOMETRIC PROGRAMME APPLIED TO THE CHARACTERIZATION OF TRANSPARENT CONDUCTING TITANIUM NITRIDE FILMS

G. Celotti, R. Rosa, C. Summonte and G. Martinelli

CNR, Istituto LAMEL, Via Castagnoli 1, 40126 Bologna, Italy

*Iat-Ltuto di Física dell'Univevsitá, Via Paradiso 12, 44100 Fervora, Italy

Résumé - La caractérisation de couches de TiN sur Si monocristallin, obtenues par implantation ionique, a été effectuée en traitant les mesures ellipsométriques par un programme très souple qui s'adapte bien aux conditions expérimentales. Les ré- sultats obtenus sur les couches implantées et recuites par faisceaux d'énergie (faisceau d'électrons, lumière incohérente) sont examinés en vue d'applications tech- nologiques possibles.

Abstract - The optical characterization of TiN films produced on Si substrates by ion-implantation was performed by handling the ellipsometric measurements through a flexible program which well conforms to the various experimental situations. The results obtained on as-implanted and transient thermally annealed films are discus- sed in relation to different possible technological applications.

1. INTRODUCTION

Bulk Titanium nitride (TiN) is a very stable compound (M.P. 2930°C), characte- rized -by an electrical resistivity (22^til-cm) lower than that of metallic Ti. The employment of TiN in thin film form as conductive layer and diffusion barrier in semiconductor technology was described as early as in 1969 /l/: since then, much work was performed in this field, with a number of interesting achievements /2-6/.

However, little work has been so far reported on the feasibility of TiN-based transparent conductive films for silicon solar cells

11/:

this is due to the funda- mental difficulty of obtaining a good compromise between good optical and electri- cal properties, which are differently related to stoichiometric deviations and impurities content. We adopted a preparative technique based on ion-implantation of Nitrogen onto a Silicon wafer coated with a layer of metallic Ti / 8 / .

For optical characterization we exploited the ellipsometric technique, suppor- ted by a proper computer program , able to deal with considerably absorbing films:

in this work the strategies of such a program are described in detail, together with some applications to ion-implanted TiN films, subsequently annealed by diffe- rent transient thermal procedures.

2. COMPUTER MODEL

A brief account of the model has been given in /9/. Here we recall the main features of the code as well as the improvements with respect to the previous ver- sion described in details in /10/. The program has been named ROSALBA; this name will be used in the following as a shorthand. ROSALBA is able to deal with systems containing one or more thin absorbing layers deposited on a substrate, however the inversion procedure may be applied only to one film at a time, all the others parameters being held constant. The code determines simultaneously the unknown parameters n, k, d, in a variety of experimental situations, which can be grouped into two categories : two ( * t f &) measurements sets or one (i^^) measurement set plus the experimental reflect- ance. The adopted numerical procedure is based on a series of successive interpola- tions in the three-dimensional (n,k,d) space. Such a strategy may appear as quite naive, however it allows one to control step by step the search for the solution, so that problems arising from multiple and/or physically meaningless solutions are avoided. Moreover it should be noted that different initial estimates of the range in which the unknown parameters are supposed to lie, as well as the increments

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19831057

JOURNAL

DE

PHYSIQUE

1 . READ I N P U T F A T A I

1

^{2 . COMPUTE (+,A]}

5 . REAP

I

( # m , ~ m ~ b l

Fig.

1

- Schematic flow-

HEAI'

chart of the ell~psome-

tric programme ROSALBA.

I N T E R S E C T I N G P O I N T

I 1 . I N T E K P O L A T E I n , k , J I

i

14. P R I N T T H E S O L U T I O N ( n . k . d )

chosen of n, k, d (see below) do not affect the uniqueness of the solution.

The flowchart scheme of ROSALBA is shown in Fig. 1, which must be read as follows.

Box 1. The program reads the input data: i) number of phases of the system;

ii) n,k,d of all but one phase; iii) the range of values of the unknown n,k,d for the layer under investigation; iv) the wavelength A and the incident angle I?.

Box

2.

Via the 2x2-matrix method for multilayered structures /11/ a set of

(?,A) as a function of n, k, d is obtained for further use. n, k, d range in the in), { k ] , {dl arrays respectively, in which the unknown n,

k,

d are supposed to lie.

Box

3.

If the inversion process is required the control is transmitted to Box

5,

otherwise to Box -

4.

Box 4. ROSALBA displays y and A as a function of the input data. Such a displaymay be useful for: i) determining more exactly the in], {k), {dl arrays; ii) choosing the best working conditions. In fact, by treating the simulated

Y,

and A

values as experimentally observed values, it is possible to evaluate the sensi- bility of different experimental arrangements.

Box 5. If the inversion process is needed, the code reads the set of measure- ments

( "

, ^{d m )}

A.

Box

6.

For each value of k in {k) the program interpolates in the set $f computed ( y , A ) every possible solution corresponding to the experimental ( y ,

nm)_. So that at the end of this part of computation we find a set of solutions A,, A2,

A

...

^{A .}

₃ .. ^{call it}

^{A)

^{, with:}

A.

=

(nA3, kA3, dA3)- 3

If d is known, by an independent measurement, the final solution is determined by computing the interpolated values of n and k corresponding to d.

Box

7.

If d is unknown, to determine the three unknown parameters simultaneo-

usly it is necessary at least another ellipsometric measurement (Box

8)

or a

p h o t o m e t r i c measurement of t h e r e f l e c t a n c e (Box 11).

Box8.

With a second s e t o f e x p e r i m e n t a l d a t a ( yl m

, A m ) B

t h e above c a l c u l a - t i o n i s r e p e a t e d . The n u m e r i c a l p r o c e d u r e f o r d e t e r m i n i n g t h e s o l u t i o n i s indepen- d e n t of t h e employed method: i ) m u l t i p l e - a n g l e - o f - i n c i d e n c e (MAI), ii) m u l t i p l e - f i l m - t h i c k n e s s (MF'T) / 1 2 / .

Box 9. The { B ] s o l u t i o n s a r e d e t e r m i n e d .

Eo.

I n t h e ( n , k , d ) s p a c e {A] and { B ) s o l u t i o n s g e n e r a t e two c u r v e s and t h e program s e a r c h e s f o r t h e i r i n t e r s e c t i o n which y i e l d s t h e v a l u e o f t h e unknown p a r a m e t e r s .

Box 11. The program r e a d s t h e e x p e r i m e n t a l r e f l e c t a n c e

Em

⁽ O R ) . The a n g l e

irR

a t which t h e r e f l e c t a n c e i s measured may be d i f f e r e n t from t h e a n g l e o f i n c i d e n - c e w i t h r e s p e c t t o t h e geometry o f a n a l y s e d systems.

Box 12. F o r e a c h A. t h e r e f l e c t a n c e

[R

⁽ 7PR)

1, .

i s c a l c u l a t e d .

Box 13. The s o l u t i b n i s d e t e r m i n e d by comput=r?g t h e i n t e r p o l a t e d v a l u e s o f n , k , d c o r r e s p o n d i n g t o

Em

(

19'

)

.

Box 1 4 . The s o l u t i o n d A e r m i n e d a c c o r d i n g t o one o f t h e d e s c r i b e d p r o c e d u r e s i s p r i n t e d .

Box 15. Whichever way i s f o l l o w e d , t h e o b t a i n e d v a l u e s o f n , k , d a r e r e i n s e r - t e d i n i n u t t o c a l c u l a t e t h e c o r r e s p o n d i n g Y, and

A ,

c a l l them ( Y C , A'. The d i f f e - r e n c e s

lye

- 'yml and

inC - ~ ' 1

r e p r e s e n t t h e e r r o r s due t o t h e whole m a t h e m a t i c a l p r o c e d u r e . These e r r o r s must b e s m a l l e r t h a n t h e e x p e r i m e n t a l e r r o r s i n

ym

a n d h m . ROSALBA n e e d s a q u i t e c o n t a i n e d computer t i m e . I f one u s e s 20 s t e p s i n e a c h o f t h e i n ] , { k ) , { d ) a r r a y s , i t i s n e c e s s a r y -3 s e c of CPU t o f i n d t h e s o l u t i o n by a p p l y i n g MA1 o r MFT method w i t h t h e Cyber 70/76 CDC System.

3 . EXPERIMENTAL

M e t a l l i c T i t a n i u m 70 nm t h i c k f i l m 2 were e v a p o r a t e d o n t o (100) s i l i c o n s

ups

s t r a t e s ~ n d t h e n i m p l a n t e d by 40 keV N I o n s , r e a c h i n g a f i n a l d o s e o f 4 . 3 ~ 1 0 atoms/cm

.

Due t o t h e v e r y h i g h implan$ed d o s e of N i t r o g e n t h a t r e q u i r e s a l o n g t i m e even w i t h t h e 4 mA c u r r e n t s u p p l i e d by o u r L i n t o t t I11 i m p l a n t e r , t h e TiN f i l m s t h u s o b t a i n e d were found t o be c o v e r e d by a 30 nm Carbon l a y e r , which was removed by i o n beam e t c h i n g . Thermal t r e a t m e n t s were performed t o r e a c h a good homogeneity o f t h e f i l m s , a l o w e r i n g o f t h e a b s o r p t i o n and a h i g h e r c o n d u c t i v i t y . To k e e p low t h e Oxygen c o n t a m i n a t i o n due t o t h e a n n e a l i n g , t h e s e t r e a t m e n t s were performed v i a t h e u n c o n v e n t i o n a l t r a n s i e n t methods o b t a i n e d by e l e c t r o n gun and i n c o h e r e n t l i g h t f l a s h e s coming from a h i g h power Xe lamp. The t e m p e r a t u r e s r e a c h e d by t h e samples d u r i n g e l e c t r o n beam a n n e a l i n g were 600, 700 and 800°C f o r a b o u t 5 s e c o n d s , w h i l e d u r i n g i n c o h e r e n t l i g h t were n e a r l y 800°C f o r a b o u t 14 s e c o n d s .

The f i l m s were e l e c t r i c a l l y c h a r a c t e r i z e d t h r o u g h r e s i s t i v i t y measurements performed u s i n g t h e f o u r - p o i n t probe and van d e r Pauw methods. The t h i c k n e s s o f some samples were t e s t e d by T a l y s t e p .

The o p t i c a l c h a r a c t e r i z a t i o n was performed by e l l i p s o m e t r y , u s i n g a G a e r t n e r L-119 i n s t r u m e n t , working w i t h t h e g r e e n l i n e o f a Hg lamp (A =546 nm)

.

The whole a l i g n m e n t p r o c e d u r e was performed w i t h a l l o p t i c a l e l e m e n t s i n s e r t e d ( p o l a r i z e r , a n a l y z e r , compensator) t o b y p a s s t h e problem of t h e n o n - p a r a l l e l i s m of t h e f a c e s o f o n e o f them, namely t h e compensator; i n f a c t we judged i n c o r r e c t t o f o l l o w an a l i g n m e n t p r o c e d u r e w i t h o u t t h e compensator, $ h a t m o d i f i e s t h e l a t e r a l and v e r t i c a l d i s p l a c e m e n t o f t h e beam when p o s i t i o n e d i n C o r C /11/. Due

t o

t h i s problem, a l l t h e measurements were "two z o n e s " , w i t h t h e compensator s e t a t C a t t h e b e g i n n n i n g and no more removed. S e v e r a l a n g l e s of i n c i d e n c e were u s e d , v a r y i n g from 6 0 ° t o 78O.

4 . RESULTS AND DISCUSSION

A l l t h e r e s u l t s o b t a i n e d f o r o p t i c a l and e l e c t r i c a l measurements a r e shown i n T a b l e I ; we r e p o r t t h e f o l l o w i n g o b s e r v a t i o n s :

a ) t h e o p t i c a l c o n s t a n t s o f t h e s a m p l e s T I , X 1 and X2 were o b t a i n e d by u s i n g b o t h t h e e l l i p s o m e t r i c measurements and t h e T a l y s t e p e v a l u a t e d t h i c k n e s s , owing t o t h e h i g h i n - d e p t h inhomogeneity o f t h e f i l m s . The o p t i c a l c o n s t a n t s o b t a i n e d a r e t h e r e f o r e t h e i n t e g r a t e d o n e s , i n agreement w i t h t h e r e l a t i o n /13/:

b ) The o p t i c a l p a r a m e t e r s o f t h e o t h e r samples were o b t a i n e d t h r o u g h MA1 measure- ments and e a c h r e p r e s e n t s t h e a v e r a g e ( w i t h r e l a t i v e s t a n d a r d d e v i a t i o n ) o f 8

J O U R N A L D E PHYSIQUE

c o u p l e s of v a l u e s t a k e n a t d i f f e r e n t a n g l e s of i n c i d e n c e . We n o t e t h a t , due t o t h e low dependence of e l l i p s o m e t r i c measurements from t h e a n g l e of i n c i d e n c e , t h e ( n , k ) c u r v e s o b t a i n e d by M A 1 e v a l u a t i o n s , a s i t ' s w e l l known, may be n o t enough r e s o l v e d . Anyway t h e computer program can f i n d a mathematical i n t e r s e c t i o n , but t h e experimental e r r o r s have a very l a r g e i n f l u e n c e on it. However, i f one performs s e v e r a l e l l i p s o m e t r i c measurements a t more t h a n two a n g l e s of i n c i d e n c e , t h e average v a l u e s of a l l i n t e r s e c t i o n s may g i v e , a l s o i n t h i s case, a c c e p t a b l e r e - s u l t s .

TABLE I

O p t i c a l and e l e c t r i c a l p r o p e r t i e s of ion-implanted TiN f i l m s on S i .

Sample Annealing Temper. ("C) n An. k Ak. loy3 d(nm)Ad

~(pJ2.

cm)

T1

-- ^--

1.77 ( 7 ) 0.640 (30) 105 ( 5 ) 284

G 1 e l . beam 600 1.72 ( 1 ) 0.529 ( 4 ) 100 (1) 177

G2 e l . beam 700 1.69 ( 2 ) 0.517 (6) 102 ( 2 ) 119

G3 e l . beam 800 1.58 ( 2 ) 0.519 (6) 101 ( 2 ) 32

X I i n c . l i g h t 800 2.15 (5) 0.140 (20) 82 ( 5 ) 254

X2 i n c . l i g h t 800 2.16 ( 5 ) 0.070 (20) 82 ( 5 ) 205

An; Ak, Ad a r e s t a n d a r d d e v i a t i o n s 5. CONCLUSIONS

By means of a f l e x i b l e computer program

,

we were a b l e t o t r e a t s u c c e s s f u l l y v a r i o u s a s p e c t of t h e complex problem t o e l a b o r a t e e l l i p s o m e t r i c measurements of even n o t t r a n s p a r e n t f i l m s d e p o s i t e d on a b s o r b i n g s u b s t r a t e s . Moreover, i t i s worthwhile t o p o i n t o u t t h a t t h i s program can handle d a t a coming from samples n o t e s p e c i a l l y p r e p a r e d , b u t i n any c a s e encountered i n experimental p r a c t i c e , whose o p t i c a l parameters and t h i c k n e s s do n o t t h e r e f o r e maximize t h e s e n s i t i v i t y of t h e procedure.

A s r e g a r d s t e c h n o l o g i c a l a p p l i c a t i o n s , a f t e r i n c o h e r e n t l i g h t a n n e a l i n g performed i n a i r , t h e o p t i c a l p r o p e r t i e s become i n t e r e s t i n g , a c c o u n t i n g a l s o f o r t h e f a i r l y low r e s i s t i v i t y , f o r p h o t o v o l t a i c a p p l i c a t i o n s on S i l i c o n s o l a r c e l l s . On t h e o t h e r hand, a f t e r e l e c t r o n beam a n n e a l i n g , t h e o p t i c a l parameters a r e n o t s a t i s f a c t o r y f o r t h i s purpose, b u t , owing t o t h e very low r e s i s t i v i t i e s s o o b t a i - ned, t h i s t r e a t m e n t appears s u i t a b l e f o r f i l m s t o be employed a s i n t e r c o n n e c t i o n s and/or d i f f u s i o n b a r r i e r s i n m i c r o e l e c t r o n i c s .

REFERENCES

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/ l o /

ROSA R., "Rosalba o d e l l ' e l l i s s o m e t r i a " , Lo Scarabeo, Bologna, 1980.

/ll/ AZZAM R.M.A., BASHARA N.M., "Ellipsometry and p o l a r i z e d l i g h t " , North Holland, Amsterdam, 1977.

/12/ Mc CRACKIN F.L., COLSON J.P., i n " E l l i p s o m e t r y i n t h e measurement of s u r f a c e s and t h i n f i l m s " E. P a s s a g l i a , R.R. Stromberg and J . Kruger (eds. )

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NBS Misc.

Publ. 256, 1964, p.61.

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