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Polyoxide edge tunneling current reduction by top corner rounding
Microelectronic Engineering 19 (1992) 721-724 Elsevier
721
Polyoxide edge tunnelling current reduction by top corner rounding.+ L. Haspeslagh, G. Vanhorebeek and L. Deferm IMEC, Kapeldreef 75, B-3001 Leuven, Belgium Abstract The scaleability of polyoxide as interpoly dielectric is limited by the large electric fields which occur at the edges of the polysilicon layers. Due to this edge effect large tunnelling currents exist at moderate electric fields. Calculation of the electric field distribution at these edges shows the importance of the corner radius. It is also demonstrated that this edge field enhancement can be reduced by performing a rounding etch.
1. INTRODUCTION. The scaleability of the thickness of polyoxide as interpoly dielectric for submicron non volatile memory processes is known to be limited. One of the major factors for this limitation is the increase of tunnelling current which is mainly observed at the edges of patterned polysilicon layers [1]. In this work the influence of the process optimizations for interpoly dielectrics, on the edge leakage current is investigated. Additionally the electric field enhancement at these edges is calculated for different corner curvatures. Finally a means for top corner rounding, making use of a NF3 isotropic plasma etch is presented. This will result in a reduction of the electrical field peaks. 2. PROCESS OPTIMIZATIONS REDUCING THE ASPERITY ENHANCEMENT. It is generally accepted that asperities in polysilicon layers have a strong influence on the tunnelling current of interpoly oxides [2]. By increasing the oxidation temperature from 900°C to 1000°C for a 30 nm interpoly oxide, the polysilicon surface roughness decreases resulting in a lower current for the same voltage, measured on non overlapping poly2 to polyl structures [3], as shown in figure 1. However when the poly 2 is overlapping the polyl structure, the influence of the asperities is less pronounced due to edge effects (figure 2). These edge effects cause an increase in tunnelling current by 2 to 4 orders of magnitude. It will be proven that the shape of the poly edge is determining the interpoly tunnelling current. + P a r t of this work h a s been carried out u n d er the E S P R I T - A P B B project 2039
0167-9317/92/$05.00
© 1992 - Elsevier Science Publishers B.V. All rights reserved.
10 -2 10-3 lo-4 10-5 10-6 10-7 950°C
-
10-8 10-9 0
1
2
3
4
5
6
0
7
1
Electric field LMV/cml
Fig. 1 : Tunnelling current measured on non-overlapping capacitors
3. ELECTRK
2
6 3 4 5 Electric field [MV/cm]
Fig. 2 : Tunnelling on overlapping
7
current measured capacitors
FIELD CALCULATIONS.
The tunnelling current of interpoly oxides is strongly related to the local electric field. At the top corner of a polysilicon structure, the amplitude of the electric field is determined by the shape of that corner. The local electric field enhancement is calculated by solving the 2-dimensional Laplace equation VW=O. A large increase in local electric field is noticed at the poly edge (figure 3), explaining the strong increase in tunnelling current for overlapping structures.
50 nm oxide
-
I
1
Fig. 3 : Electric field distribution around top corner of polysilicon layer
0
10
20
30
40
Radius of curvature of top corner [nml
Fig. 4: Influence of the radius of curvature on the max. electric field
53
L. Haspeslagh et al. / Polyoxide edge tunnelling current reduction
723
By c h a n g i n g t h e r a d i u s of t h e top corner, t h e p e a k electric field c a n be s t r o n g l y r e d u c e d as s h o w n by t h e c a l c u l a t e d field v a l u e s at t h e poly edge (figure 4). Top c o r n e r radii l a r g e r t h a n 10 n m r e s u l t in a electric field r e d u c t i o n by m o r e t h a n a factor of 2. D u e to t h e e x p o n e n t i a l d e p e n d e n c e of t u n n e l l i n g c u r r e n t on t h e electric field, t h e s h a p e of the poly edge can be t h e d o m i n a n t factor for t h e t u n n e l l i n g c u r r e n t . 4. T O P C O R N E R R O U N D I N G . T h e top corner r a d i u s can be increased by u s i n g an isotropic NF3 polysilicon etching, w h i c h is p e r f o r m e d a f t e r t h e n o r m a l polysilicon d r y e t c h a n d r e s i s t strip. T h i s isotropic e t c h r e m o v e s a b o u t 20 n m of polysilicon f r o m t h e surface a n d sidewalls of p a t t e r n e d layer. As a r e s u l t of t h i s e t c h a s l i g h t l y r o u n d e d c o r n e r is o b t a i n e d . A l a r g e r e d u c t i o n of t u n n e l l i n g c u r r e n t is o b s e r v e d , a l t h o u g h t h e influence of edge effects are not fully r e m o v e d (figure 5). T h i s is in a g r e e m e n t w i t h t h e c a l c u l a t i o n s since the r a d i u s o b t a i n e d w i t h a 20 n m e t c h (which m u s t be s m a l l e r t h a n 20 nm) is still r e s u l t i n g in a field e n h a n c e m e n t equal or l a r g e r t h a n 2.
10-2 10 -3
1o[] : u
"~ lO ~ lO
A__
10 0
1
2 3 4 5 Electric Field [MV/cm]
Non overlapping Overlapping O v e r l a p p i n g w i t h top corner r o u n d i n g
6
Fig. 5 : Effect of top c o r n e r etch on t u n n e l l i n g c u r r e n t m e a s u r e d on polyoxide capacitors F i g u r e 6 shows t h e i n f l u e n c e of t h e oxidation t e m p e r a t u r e on t u n n e l l i n g c u r r e n t m e a s u r e d on c a p a c i t o r s which received a c o r n e r r o u n d i n g etch. T h e t u n n e l l i n g c u r r e n t is for all cases h i g h e r w i t h r e s p e c t to n o n - o v e r l a p p i n g capacitors, w h i c h m e a n s t h a t t u n n e l l i n g at the edge is still d o m i n a n t . H o w e v e r t h e fact t h a t a t e m p e r a t u r e effect is seen, m e a n s t h a t t h e s h a p e of t h e polysilicon c o r n e r is i n f l u e n c e d by t h e o x i d a t i o n t e m p e r a t u r e , r e s u l t i n g in s m a l l e r c o r n e r radii at lower oxidation t e m p e r a t u r e s , w h i c h is s i m i l a r to t h e
ilO-2 10-3
10-4 10.5 C~
,.~
10 .6
¢)
10 -7
®:c o
10-8 10 -9 0
I _fi~lJI
I
1
2
I
I
'
I
.
950 C 1000oc i
.
i
.
3 4 5 6 E l e c t r i c F i e l d [MV/cm]
7
Fig. 6 • T u n n e l l i n g c u r r e n t m e a s u r e d on rounded overlapping capacitors a s p e r i t y e n h a n c e m e n t s e e n on n o n o v e r l a p p i n g capacitors. An oxidation a t low t e m p e r a t u r e is t h u s seen to r e d u c e the effect of t h e corner r o u n d i n g etch. 5. C O N C L U S I O N . I n t h i s w o r k it is s h o w n t h a t process o p t i m i z a t i o n s w h i c h r e d u c e t h e p o l y s i l i c o n a s p e r i t y field e n h a n c e m e n t h a v e n e a r l y no i n f l u e n c e on t h e t u n n e l l i n g c u r r e n t in case of a n o v e r l a p p i n g polyoxide c a p a c i t o r d u e to edge effects. It is c a l c u l a t e d t h a t a s l i g h t top c o r n e r r o u n d i n g r e s u l t s in a n a p p r e c i a b l e local electric field r e d u c t i o n . T h e r o u n d i n g of t h e c o r n e r s w a s r e a l i z e d by u s i n g a s h o r t i s o t r o p i c NF3 p l a s m a , r e s u l t i n g in a t u n n e l l i n g c u r r e n t r e d u c t i o n by 1 decade. References 1 Seiichi Mori et al, I E E E Trans. Electron Dev., ED-38, pp. 270-277, 1991. 2 M. H e n d r i c k s a n d C. Mavero, J. E l e c t r o c h e m . Soc., vol 138, pp. 1466-1474, 1991 3 Kiyonori O h y u et al, J. Electrochem. Soc., vol 137, pp. 2261-2265, 1990
721
Polyoxide edge tunnelling current reduction by top corner rounding.+ L. Haspeslagh, G. Vanhorebeek and L. Deferm IMEC, Kapeldreef 75, B-3001 Leuven, Belgium Abstract The scaleability of polyoxide as interpoly dielectric is limited by the large electric fields which occur at the edges of the polysilicon layers. Due to this edge effect large tunnelling currents exist at moderate electric fields. Calculation of the electric field distribution at these edges shows the importance of the corner radius. It is also demonstrated that this edge field enhancement can be reduced by performing a rounding etch.
1. INTRODUCTION. The scaleability of the thickness of polyoxide as interpoly dielectric for submicron non volatile memory processes is known to be limited. One of the major factors for this limitation is the increase of tunnelling current which is mainly observed at the edges of patterned polysilicon layers [1]. In this work the influence of the process optimizations for interpoly dielectrics, on the edge leakage current is investigated. Additionally the electric field enhancement at these edges is calculated for different corner curvatures. Finally a means for top corner rounding, making use of a NF3 isotropic plasma etch is presented. This will result in a reduction of the electrical field peaks. 2. PROCESS OPTIMIZATIONS REDUCING THE ASPERITY ENHANCEMENT. It is generally accepted that asperities in polysilicon layers have a strong influence on the tunnelling current of interpoly oxides [2]. By increasing the oxidation temperature from 900°C to 1000°C for a 30 nm interpoly oxide, the polysilicon surface roughness decreases resulting in a lower current for the same voltage, measured on non overlapping poly2 to polyl structures [3], as shown in figure 1. However when the poly 2 is overlapping the polyl structure, the influence of the asperities is less pronounced due to edge effects (figure 2). These edge effects cause an increase in tunnelling current by 2 to 4 orders of magnitude. It will be proven that the shape of the poly edge is determining the interpoly tunnelling current. + P a r t of this work h a s been carried out u n d er the E S P R I T - A P B B project 2039
0167-9317/92/$05.00
© 1992 - Elsevier Science Publishers B.V. All rights reserved.
10 -2 10-3 lo-4 10-5 10-6 10-7 950°C
-
10-8 10-9 0
1
2
3
4
5
6
0
7
1
Electric field LMV/cml
Fig. 1 : Tunnelling current measured on non-overlapping capacitors
3. ELECTRK
2
6 3 4 5 Electric field [MV/cm]
Fig. 2 : Tunnelling on overlapping
7
current measured capacitors
FIELD CALCULATIONS.
The tunnelling current of interpoly oxides is strongly related to the local electric field. At the top corner of a polysilicon structure, the amplitude of the electric field is determined by the shape of that corner. The local electric field enhancement is calculated by solving the 2-dimensional Laplace equation VW=O. A large increase in local electric field is noticed at the poly edge (figure 3), explaining the strong increase in tunnelling current for overlapping structures.
50 nm oxide
-
I
1
Fig. 3 : Electric field distribution around top corner of polysilicon layer
0
10
20
30
40
Radius of curvature of top corner [nml
Fig. 4: Influence of the radius of curvature on the max. electric field
53
L. Haspeslagh et al. / Polyoxide edge tunnelling current reduction
723
By c h a n g i n g t h e r a d i u s of t h e top corner, t h e p e a k electric field c a n be s t r o n g l y r e d u c e d as s h o w n by t h e c a l c u l a t e d field v a l u e s at t h e poly edge (figure 4). Top c o r n e r radii l a r g e r t h a n 10 n m r e s u l t in a electric field r e d u c t i o n by m o r e t h a n a factor of 2. D u e to t h e e x p o n e n t i a l d e p e n d e n c e of t u n n e l l i n g c u r r e n t on t h e electric field, t h e s h a p e of the poly edge can be t h e d o m i n a n t factor for t h e t u n n e l l i n g c u r r e n t . 4. T O P C O R N E R R O U N D I N G . T h e top corner r a d i u s can be increased by u s i n g an isotropic NF3 polysilicon etching, w h i c h is p e r f o r m e d a f t e r t h e n o r m a l polysilicon d r y e t c h a n d r e s i s t strip. T h i s isotropic e t c h r e m o v e s a b o u t 20 n m of polysilicon f r o m t h e surface a n d sidewalls of p a t t e r n e d layer. As a r e s u l t of t h i s e t c h a s l i g h t l y r o u n d e d c o r n e r is o b t a i n e d . A l a r g e r e d u c t i o n of t u n n e l l i n g c u r r e n t is o b s e r v e d , a l t h o u g h t h e influence of edge effects are not fully r e m o v e d (figure 5). T h i s is in a g r e e m e n t w i t h t h e c a l c u l a t i o n s since the r a d i u s o b t a i n e d w i t h a 20 n m e t c h (which m u s t be s m a l l e r t h a n 20 nm) is still r e s u l t i n g in a field e n h a n c e m e n t equal or l a r g e r t h a n 2.
10-2 10 -3
1o[] : u
"~ lO ~ lO
A__
10 0
1
2 3 4 5 Electric Field [MV/cm]
Non overlapping Overlapping O v e r l a p p i n g w i t h top corner r o u n d i n g
6
Fig. 5 : Effect of top c o r n e r etch on t u n n e l l i n g c u r r e n t m e a s u r e d on polyoxide capacitors F i g u r e 6 shows t h e i n f l u e n c e of t h e oxidation t e m p e r a t u r e on t u n n e l l i n g c u r r e n t m e a s u r e d on c a p a c i t o r s which received a c o r n e r r o u n d i n g etch. T h e t u n n e l l i n g c u r r e n t is for all cases h i g h e r w i t h r e s p e c t to n o n - o v e r l a p p i n g capacitors, w h i c h m e a n s t h a t t u n n e l l i n g at the edge is still d o m i n a n t . H o w e v e r t h e fact t h a t a t e m p e r a t u r e effect is seen, m e a n s t h a t t h e s h a p e of t h e polysilicon c o r n e r is i n f l u e n c e d by t h e o x i d a t i o n t e m p e r a t u r e , r e s u l t i n g in s m a l l e r c o r n e r radii at lower oxidation t e m p e r a t u r e s , w h i c h is s i m i l a r to t h e
ilO-2 10-3
10-4 10.5 C~
,.~
10 .6
¢)
10 -7
®:c o
10-8 10 -9 0
I _fi~lJI
I
1
2
I
I
'
I
.
950 C 1000oc i
.
i
.
3 4 5 6 E l e c t r i c F i e l d [MV/cm]
7
Fig. 6 • T u n n e l l i n g c u r r e n t m e a s u r e d on rounded overlapping capacitors a s p e r i t y e n h a n c e m e n t s e e n on n o n o v e r l a p p i n g capacitors. An oxidation a t low t e m p e r a t u r e is t h u s seen to r e d u c e the effect of t h e corner r o u n d i n g etch. 5. C O N C L U S I O N . I n t h i s w o r k it is s h o w n t h a t process o p t i m i z a t i o n s w h i c h r e d u c e t h e p o l y s i l i c o n a s p e r i t y field e n h a n c e m e n t h a v e n e a r l y no i n f l u e n c e on t h e t u n n e l l i n g c u r r e n t in case of a n o v e r l a p p i n g polyoxide c a p a c i t o r d u e to edge effects. It is c a l c u l a t e d t h a t a s l i g h t top c o r n e r r o u n d i n g r e s u l t s in a n a p p r e c i a b l e local electric field r e d u c t i o n . T h e r o u n d i n g of t h e c o r n e r s w a s r e a l i z e d by u s i n g a s h o r t i s o t r o p i c NF3 p l a s m a , r e s u l t i n g in a t u n n e l l i n g c u r r e n t r e d u c t i o n by 1 decade. References 1 Seiichi Mori et al, I E E E Trans. Electron Dev., ED-38, pp. 270-277, 1991. 2 M. H e n d r i c k s a n d C. Mavero, J. E l e c t r o c h e m . Soc., vol 138, pp. 1466-1474, 1991 3 Kiyonori O h y u et al, J. Electrochem. Soc., vol 137, pp. 2261-2265, 1990