noise cikk1
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88 L . K . J . Vandamme and A . J. van Kem enade
of change in mi c r o - s t r uc t u r e a r ound t he ho l es o r t o d i s cus s t he i n f l uence o f ma t e r i a l
pa r amet e r s . Her e , t h e m i n i mum i nc r ease in no i se and r e s i s tance i s ca l cu l a t ed t ha t can be
expec t ed due t o a l oca l inc r ease i n cur r en t dens i t y aRer degr ada t ion . Ca l cu l a t i ons a r e checked
by expe r i ment a l r e su l ts .
M o d e l
W e c o n s i d e re d l o n g n a r r o w f i lm r e si s to r s , d e g r a d e d b y h o l e s o r k i n k s , s h o w n
schemat i ca l ly i n F i g . 1 . T he gen e r a l equa t i ons f o r r e s i s tance R and l / f no i se i n r e s i s tance SR
due t o condu c t i v i t y f l uc t ua t ions a r e [ 1 ,2 ]
R = [ 1 / 1 2 ] f p j 2 d A ( l )
S R = [ 1 / I 4 ] / [ u p 2 / n f ] j 4 d A = [ 1 / I 4 f ] / C u s p 2 j 4 d A ( 2 )
wher e 10(~ "~) i s the shee t r e s i s t ance ; J ( A / m) t he t wo- d i m ens i ona l cu r r en t den s i t y ; dA ( m 2) an
a r ea e l ement ; n ( m 2) t he t wo- d i m ens i ona l fr ee cha r ge c a r r i e r concen t r a t i on ; c t a d i mens i on l es s
l / f n o i s e p a r a m e t e r a n d C , , ( m 2) t he ch a r ac t e r i s ti c l / f no i se f o r a un i t a r ea [ 3 ,4 ] . T h e p r ob l em
can a l so be so l ved i n t h r ee d i mens i ons w i t h p ( f ~ cm) , J ( A /m2) , and d A ( m 3 ) b e c o m e s a v o l u m e
elemen t , and n (m 3) c a r r i e r concen t r a t i on pe r un i t vo l um e [ 1 ] .
W e appr o x i ma t e the i nc r eased l oca l cu r r en t dens i t y a r ound a ho l e o r k i nk by J = I / W ,~ ove r a
l eng t h equa l t o t he w i d t h W wi t h W o~ = W - 2a , wh er e 2a i s t he d i ame t e r o f t he ho l e . T h e
cur r en t dens i ty i n t he undam aged pa r t s i s J = I / W . T he nu mber o f ho l e s o r k i nks i s k. T o
s i mpl i fy he ca l cu l a t i ons we a s sume a l l ho l e s o r k i nks have t he s ame d i men s i ons . T he r e s i s t ance
o f th e k d a m a g e d p a r t s t h e n b e c o m e s R ~ = k p W / W c f r. T h e t o t a l r e si s ta n c e o f t h e u n d a m a g e d
par t s i s R~ = p ( L - kW ) / W . T he r e s i s t ance o f t he en t i re s ampl e becom es
R = R u + R d = 1 + \ W e ft - 1 ( 3 )
h ,
' I '
hole kink
~ L " , ~ _ l
2 a t
F ig . 1 . G eom et r y o f a r e s i s t o r w i t h a ho l e and a k i nk .
B
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Resistance noise measurement
Th e r e s i s t a n c e 1% o f t h e u n d a m a g e d sa m p le i s 1% = p L/W , h e n c e t h e r a t i o R /Ro b e c o m e s
R / R 0 = 1 + \ ~ - (4 )
Th e sa m e a p p ro x im a t io n s a p p l i ed t o e q . (2 ) l e a d s fo r t h e r e s i s t a n c e n o i se i n t h e k d e g ra d e d
p a r t s t o
= R d C u s / f k W W e fr (5)R d 2
a n d in t h e u n d a m a g e d p a r t s t o
2S Ru = R u C u s / f W ( L - k W ) (6 )
respec t ive ly . Resis tance noise in the d i f fe ren t par ts i s uncorre la ted hence the to ta l resis tance
noise densi ty SR is the su m ofe qs. (5) and (6) .
S R _ p 2 C u s [ kW L - kW q_ - - ~ + (7 )
f W;tt VVq J
2Re s i s t a n ce n o i se o f a fa i l u re f l e e sa m p le i s SRO = R o C u s / f W L . Th e r a t i o SR/SRo SHOWS
the increase in resis tance noise
S R / S R 0 = 1 L [ \W e f fJ - 1 ( 8 )
The noise increase in Equa t ion (8) can a lso be w ri t ten as a func t ion of resis tance increase wi th
R - 1% = A R and SR - SRO = A SR with eqs. (4) and (8) f ollo ws
A . I ( ,o ,SR0 [kWeffJ \W eft)
Eq u a t io n s (4 ) a n d (8 ) a re fu n c t io n s o f W/L , W /Wa r an d k , a n d t h e t h i rd p o w e r i n (8 ) SHo ws th e
st ronger increase in noise than in resis tance due to holes or k inks. Fro m eq . (9) i t i s a lso c lea r
tha t for W /W ar> 1 the increase in noise i s s t ronger than in the resis tance . For k i s la rge r than 1 ,
t h e r e s i s t o r c a n b e c o n s id e re d a s k r e s i s t o r s i n se r i es o f l e n g th L /k a n d d e g ra d e d b y o n e h o l e o r
k ink , each having the same R/1% and SR/SR0 as the to ta l resis tor . O nly when degrad a t ion i s du e
to a n i n c re ase i n t h e n u m b e r o f h o l es o r k in k s a l l ha v in g t h e sa m e W/W ,n wi l l we f i n d a l i n e a r
re la t ion be tween ASR/S~o and AR /Ro as g iven by eq . (9) and SR/S~o and R/1% as g iven by
S R / S R o = R / R o ~ - ~ - + 1 - ~ % ( io )
89
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90 L. K. J. Vandamrne and A. J. van Kemenade
EXPERIMENTS AND RESULTS
The purpose of our experiments is not to study the degradation process in film
resistors, but the effect of a degradation type (holes and kinks) on the increase in resistance
and the noise.
To verify equations (4) and (8), experiments with decreasing W~ as well as experiments with
increasing k were performed. Carbon paper film resistors with various W/L ratios were used.
The result o f a degradation is realised by punching holes in the carbon paper resistors at the
same spot with increased diameter or by punching an increasing number of holes at regular
distances, all with the same diameter. Before degradation we started by measuring the
reference values R~ and SR0 of the undamaged resistors. First, experiments on samples
degraded by one hole or kink were performed. After the first damage was made, R and SR
were measured. This process was repeated after each subsequent enlargening of the hole or
kink by using punches with a larger diameter ranging We~W from 0.1 to 1.
The calculated and experimentally observed relative increase in noise versus a relative
increase in resistance for decreasing W~ is presented in Fig. 2. The full lines were calculated
for W/L values of 10/100 and 10/300 from eqs. (4) and (8) with k = 1. The squares represent
the experimental results obtained from 2 resistors with different W/L ratios with one hole with
increasing diameter. The triangles stem from a third sample degraded by an increasing kink.
Second, the experiment with increasing k was performed on a carbon film resistor with
a W/L ratio of 5/563. Again Ro and SR0 were measured first and this was repeated for each
additional hole all with the same diameter. Measurements were taken for one up to ten holes.
All holes had the same diameter and were distributed equidistantly across the length of the
resistor.
Fig. 3 shows the relative increase in noise versus the relative increase in resistance for
increasing numbers of holes k. The full line was calculated with eqs. (4) and (8), W/Wc~ being
constant.
DISCUSSION AND CONCLUSION
Although carbon filmq are of no interest in microelectronics they can be used to verify
the calculations. The correlation for the effect of current crowding between a simulation with a
carbon film and A1 filing is perfect.
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3 0 - , [ , , ' -
1 0 - W / 0 1 3 0 , 0
5W / l = 1 0 / I 0 0 _
3
2I
~ R / R oI I I I I
Ii
1 1 . 2 1 . 4 1 . 6F i g . 2 . R e l a t i v e i n c r e a s e i n l / f n o i s e v e r s u s r e l a t iv e i n c r e a s e i n r e s i s t a n c e d u e t o o n e h o l e o r
k i n k . F u l l li n e s a r e c a l c u l a t e d f r o m e q s . ( 4 ) a n d ( 8 ) . E x p e r i n a e n t a l r e s u l t s w i t h a h o l e :
W / L = 1 0 / 1 0 0 U ; W / L = 1 0 / 3 0 0 ~ ; a n d w i t h a k i n k : W / L = 1 0 / 3 0 0 A .
2. 0
1. 8
T s / s 0
1. 5
1 . 3 I " / •
R/Ro4F-
1 0 ' " ' 'IIF
1 . 0 0 1 . 0 5 1 . 1 0 1 . 1 5
F i g . 3 . R e l a t i v e i n c r e a s e i n l / f n o i s e v e r s u s r e l a t i v e i n c r e a s e i n r e s i s t a n c e d u e t o a n
i n c r e a si n g n u m b e r o f h o l e s f o r W / L = 5 / 5 6 3 .
o.
t-
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9 2 L . K .J . Va n d a m m e a n d A, J . v a n Ke m e n a d e
Ex p e r im e n ta l re su l t s a r e i n g o o d a g re e m e n t wi th m o d e l c a l c u la t io n s . A q u a n t i ta t i v e
e x p la na t io n sh o ws w h y re s i s ta n c e n o i se m e a su re m e n t s a re m o re se n s it i v e t h a n r e s i s ta n c e
measurem ents on ly . The re la t ive increase in 1 / f no ise i s a complem enta ry ra ther than a
c o m p e t i t i v e t e c h n iq u e c o m p a re d t o c l a ssi c fa i lu re a n a ly si s t e s t s a n d i s a p p l i c a b l e t o LSI AI
in te rconnec ts and th in-f i lm resis tors . As ea r ly as 1971, Vo ssen [5] pro po sed the sc reening o f
m e ta l f i lm d e fe c t s b y c u r re n t n o i se m e a su re m e n t s wi th o u t e x p l a in in g wh y n o i se i s m o re
se n s it i ve t h a n r e s i s t a n c e m e a su re m e n t s o n ly . F ro m th e p e r sp e c t i v e o f n o i se a s a d i a g n o s t i c t o o l
" l / f y noise" i s ex t remely usefu l in s tud ies on e lec t rom igra t ion ac t iva t ion energ ies [6 ,7] and re f .
i n [ 7 ] .
We m u s t s t r e ss , h o we v e r , t h a t e q . (8 ) o n ly t a k e s i n to a c c o u n t t h e i n c re a se in n o i se
in t en s i t y d u e t o c u r re n t d e n s i t y in c re a se a ro u n d th e h o l e s o r k in k s . I f t h e m ic ro s t ru c tu re a ro u n d
these dam ages i s a ffec ted , the noise in tensi ty wi l l be h igh er and eq . (8) w i l l no longer be v a l id .
A c a t a s t ro p h ic i n c re a se i n n o i se o v e r m o re t h a n a f e w d e c a d e s fo r o n ly a f e w p e rc e n t
a d d i t io n a l r e s i s t an c e i s d u e t o m ic ro sc o p ic c h a n g e s i n t h e s t ru c tu re a ro u n d h o l e s a n d k in k s a n d
n o t d u e t o a s im p le in c re a se o f c u r re n t d e n s i t y a ro u n d a h o l e i n t h e c o n d u c t in g p a th . Ou r
m o d e l g iv e s t h e m in im u m in c re a se in n o i se t h a t c a n b e e x p e c t e d o n t h e g ro u n d s o f i n c re a se i n
curren t c rowding .
A nois e measurem ent se t -up for s amples wi th a resis tance la rger than 10 f~ i s s tandard
[3]. O ne must rea l ize tha t i f the impedan ce of the sam ple i s a t least 10 C~, bu t the n umb er o f
f lee ca rr ie rs in the samp le i s la rger than roughly 1013, major d i f f icu l t ies wi l l be encoun te red in
o b se rv in g t h e n o i se d u e t o c o n d u c t io n f lu c tu a ti o n s a b o v e th e o m n ip re se n t t h e rm a l n o i se a n d
backg round nois e of the ampl if ie rs. This i s due to the fac t tha t the re la t ive noise i s inverse ly
prop ort iona l to th e num ber of e ffec t ive ca rr ie rs [2] . I f in te rconnec t resis tances a re in se r ies
wi th r e l a t iv e la rg e se r i e s r es i s t an c e s , t h e l / f n o i se o f t h e d e g ra d in g p a r t c a n a g a in b e h id d e n b y
the thermal no ise o f the s e r ies resis tance . The m ethod is no lon ger appl icable in such cases .
R E F E R E N C E S
[1] L .K.J . Va n d a m m e , On th e Ca l c u l a ti o n o f l / f No i se o f Co n ta c t s , Ap p l i e d Ph y s i c s, 1 1 ,
89-96 (1976) .
[2] F .N. Ho o g e , T .G.M. Kle in p en n in g a n d L .K.J . Va n d a m m e , Ex p e r im e n ta l s t u d i e s o n i / f
no ise , Reports on Progress in Physics , 44 , 479-522 (1981) .
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Resistance noise measurement 93
[3] L.K.J. Vandam me, Criteria o f low -noise thick-fihn resistors, Elec troc om pon ent Science
and Technolog y, 4 171-177 (1977 )
[4 ] L K . J . Vandamme, C omm ents on "An exac t fo rmula fo r the e ffect s o f re s is to r
geom etry on current noise" , IEE E Transact ions on Electron Devices , ED -33, 1833-
1834 (1986).
[5] J .L. Vo ssen, Screening of metal film defec ts by current noise measurements, A ppl.
Phys. Lett. 23, 287 (1971).
[6] W. Ya ng and Z. Celik-Butter, A model for e lectromigration and low frequen cy noise in
thin metal fiim~, Solid State Electron. 34, 91 1-91 6 (1991).
[7 ] L . K~ J .Vandamme, No ise as a diagnos tic tool for qual i ty and re l iabi li ty o f e lec tron
devices , IEE E Tran s Electr. Dev. 41, 2176 -2187 (1994).