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The interaction between different barrier metals and the copper surface during the chemical-mechanical polishing
ELS EVIER
Microelectronic Engineering 37 / 38 (1997) 237-243
,
The interaction between different barrier metals and the copper surface during the chemical-mechanical polishing D. Zeidler*, Z. Stavreva, M. P16tner, K. Drescher Dresden University of Technology, Semiconductor and Mikrosystems Technology Laboratory, 01062 Dresden, Germany Abstract The copper polishing rate in a weak acid slurry with H20 2 as a oxidizing agent is determined by a two-step process of the formation of copper oxides followed by its mechanical abrasion. The electrochemical potential is a criterion for the driving force of oxidation-reduction reactions that occur during the Cu dissolution process. Using the experiment a corrosion potential (Ecorr) was determined. It depends on the potential of the cathodic reaction, the Tafel slopes of each reaction and the control of the reaction by Tafel kinetics or concentration polarisation. Potentiodynamic measurements were performed with different H202 contents in the slurry. The dissolution current density (i .... ) is correlated to the oxidation-reduction reaction rate. In the presence of a barrier metal, such as WTi, the dissolution rates were influenced by galvanic effects. W and Ti and their compounds were dissolved in the slurry by the complexing ability of H202. There is an increase in the barrier wet etch rate when copper is present. In the case of polishing both T i - W and Cu simultaneously a dramatic increase of the barrier polish rate was observed. The acceleration of the W or Ti polish rate can be explained by the galvanic coupling between Cu and the barrier metal. By the use of TiN as a barrier metal no galvanic interaction has been observed. The removal rates of TiN have not been influenced by the presence of Cu.
Keywords: Chemical mechanical polishing; Copper; Barrier metal; Potentiodynamic measurements
1. Indroduction
The chemical mechanical polishing of Cu with certain slurries can be considered as a subsequential chemical formation of a passivating layer and its removal by mechanical abrasion [1,2]. The effectiveness of that process is correlated to balanced etch and passivation reactions. The interaction between the copper surface and the polishing slurry can be studied by electrochemical measurements of the polarization behaviour applied during the polishing process as well as under static conditions. The chemical components within the slurry can be characterized by its oxidizing strength, the ability to form the passivating layer and the dissolution of the mechanically abraded metal compounds from the surface. The electrochemical potential is a measure of the driving force of the oxidation-reduction reactions that occur during the metal dissolution. The potential we measured is the mixed corrosion potential Ecorr. It depends on the potential of cathodic and anodic *Corresponding author. Fax: + 4 9 351 463 7172; e-mail: [email protected] 0167-9317/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 9 3 1 7 ( 9 7 ) 0 0 1 1 7 - 2
238
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 2 3 7 - 2 4 3
reactions, the tafel slopes of each reaction and the kind of control of the reactions by either Tafel kinetics or by concentration polarization. In presence of a barrier metal, as W-Ti, TiN or Ti, the dissolution rates were influenced by galvanic effects. W and Ti and their compounds were dissolved in the slurry by the complexing ability of H202.
The corrosion potential and corrosion current density from electrochemical potentiodynamic measurements were used to give some insights into the polishing process itself and to predict the CMP performance.
2. Experimental CMP was carried out using a PRESI MECAPOL E460 polishing tool with a perforated RODEL IC 1000/Suba IV stacked polishing pad. Alumina based RODEL QCTT 1010 was chosen as slurry. Both the wafer carrier and the polishing table were rotated at 40 rpm, and a down force of 21.9 kPa was applied to the carrier. The surface of the pad was conditioned after polishing each wafer using a diamond conditioner. Silicon wafers 4" were metallized by PVD with 100 nm W-Ti or 100 nm TiN as the barrier/ adhesion layer and 1000 nm Cu. The surface topography before, during and after CMP was investigated using a Tencor profilometer. In order to determine polish rates of blanket metal films, sheet resistances with an ASM 4-point prober were measured for all wafers before and after polishing. The electrochemical measurements were carded out with a SENSORTECHNIK PS 5 potentiostat. Wafer samples with Cu, W-Ti, TiN or Cu/barrier metal (working electrode) of approximately 4 cm 2
600 5O0 400
f tu O
. . . .
O-
" .
.
.
.
.
.
"
---
•
300 ".--W-Ti
200 E
/
100 o u
ILl 0 -100
.t"
-- e-- Cu/W-TI --.:.~0
u
I
-200 -2
0
2
4
6
8
10
12
14
16
Concentration o f H20 2 [%] Fig. 1. Corrosion potential of Cu, W - T i and C u / W - T i (4:1) as a function of H202 - concentration.
D. Zeidler et al. I Microelectronic Engineering 37/38 (1997) 237-243
t --
I m-- W - T i
-
~
! ~
E 2
mqhn"=u=,= "
t
Cu/W-Ti
•" ' ~ " C
<
239
u
41-.
mO
E
0u
..--- a-
0
I 0
4
2
6
8
10
12
14
16
C o n c e n t r a t i o n o f H20 2 [%]
Fig. 2. Corrosion current density as a function of H 2 0 : concentration for Cu, W - T i and C u / W - T i (4:1).
in area were exposed to solutions in a flat cell. The three-electrode configuration cell consisted of a saturated calomel electrode as reference (Ere f = 242 mV versus SHE), and platinum as counterelectrode. Test solutions contained ca. 5% complexing and buffering substances and 4% of Alumina abrasives. The effect of the oxidant concentration in the slurry on the removal rate was investigated within a range of 1.5 ..... 20% H202. The pH was 3.8+0.2. 0,20
400
0,18
E corr qr T i N
300
0,18
/
T
0,14
l.u 0
0,12
200 E
0 o
o
0,10
--4-.-i
100
0,08
- of T i N
~,,u !
• 0,06
u.l
20,04
= . , ~ r'=`'="
0
20,02
i..f -2
0
L0,00 2
4
6
8
H202 - concentration
10
12
14
16
[%]
Fig. 3. Wet etch rate for Cu in slurry with different H202 concentrations.
a3
240
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 237-243
Electrochemical studies were carried out on metal films subjected to the following pretreatment: precleaned in 40% KOH for 4 min, DI rinsed and for 30 s 10% H 2 S O 4 and DI rinsed.
3. Result and discussion The shift of the corrosion potential of Cu towards the anodic direction (relative to the pure metal) with increasing H202 concentration in the slurry is an indication of the formation of a passivating layer on the surface. The corrosion current densities measured decrease with increased H202 content which has to be explained by an increased passivation behaviour of the films formed (Figs. 1 and 2). The wet etch rates of Cu in the same slurries confirm the tendency of stronger passivation behaviour with increased H202 content; the wet etch decreases (Fig. 3). The polish rate has a maximum between 5 and 10% H 2 0 2 (Fig. 4) [3]. The formation and dissolution of the passivating layer is described by the following reactions: 2 C u +½ 02 + H 2 0 ~ 2 C u
+ + 2 OH-
2 Cu~ + 2 O H - --->Cu20~s ) + H20 Cu20(s ) .~_1 02 ~_ 2 H 2 0 --->2 Cu 2+ + 4 OH C u 2 0 ( s ) ..[_ 1 0 2 ~
2 CuO
The oxidised layer of W-Ti(10% Ti) in the slurry caused by a pH value of 4 is very thin. Addition 60
50
c
40
E C
~
30
lU
¢Y
,-
u U.I
20
T
10
0
2
4
6
-_
8
10
.J_
12
14
16
H20 2- concentration [%] Fig. 4. Polish rate of Cu in slurry with different H202 concentrations (pH of the slurry 4.0).
241
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 237-243
of H:O 2 resulted a increase in W-Ti dissolution. W + 4 O H - --~WO 2 + 2 H 2 0 + 4 e WO 2 + OH- --->WO 3 + H + + 2 eWO 3 + O H - ~ HWO 4 Ti + 02 ~ TiO 2 TiO 2 + O H - + xH20 --->[TiO2(OH)(H20)~ ]In order to understand the behaviour of W under solution conditions, thermodynamic data on tungsten species were taken from the E - p H diagram (Pourbaix) [4]. This diagram was drawn assuming an activity of 10 -4 M for all dissolved W species. From the Pourbaix diagram WO 3 is a stable phase only at pH values less than 2, while WO 2 exists over a wide range of pH. Under oxidizing conditions in H20 2 based CMP slurries, the aqueous polytungstate species is the most stable phase, but the formation of this phase is proceeded by the WO 2 phase. If the dissolution of WO 2 is slow, then the presence of WO 2 may be expected on the W surface during polishing at pH values of 4. When Cu is present there is an dramatic increase in the W - T i etch rate. Similar to this behaviour during polishing both W - T i and Cu simultaneously there is a dramatic increase in the W - T i polish rate (Figs. 5 and 6). This acceleration of the W - T i rate can be explained by the galvanic interaction between Cu and W-Ti.
40-
I
[
---o-- W-Ti 7.
W-Ti with C u
//
30-
--
/
I= c 20
f
w'
"8 i .m I--
10
l
2
4
6 8 10 H202-ConcentmUon[%]
12
Fig. 5. Wet etch rate of W-Ti and the influenceof Cu.
14
16
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 237-243
242 60,
T
50 r
I
I
--
Cu Cu with TiN
r
40 r
E
c o_
30 r
20 ~
10-
_J_
0.
2
0
4
6
8
10
12
14
16
H202-Concentration [%] Fig. 6. Polish rates of W - T i as a function of different H202 concentrations and the influence of Cu.
This indicates that H202 acts as a lixiviant for W - T i and causes dissolution rather than passivation of the metal surface. TiN passivated with increasing of H202 content and the wet etch rate is very slow (Figs. 7 and 8). In the presence of Cu, TiN formed no local elements and the dissolution rate is not increased.
500,
Z
400. C
E E
300,
/
j_
\j\
I
~'= 0°" ~ 200,/ 100,
o;/ 0
|
4
8
12
16
20
H2OfConcentration [%] Fig. 7. Corrosion potential and corrosion current density of TiN layers in slurry solution with different H : O : concentrations.
243
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 237-243 500
W-Ti + Cu 400
E E
300. i:
"6 @
200.
.~_ 0 O_
W-Ti 100'
0.
i....,,=11
f
J 0
2
I....-.=
4
~
~mll
-m
6
8
10
12
14
16
H202-eoneentration [%] Fig. 8. Wet etch rate of Cu and the influence of the TiN barrier.
Acknowledgements T h e financial support for this w o r k was p r o v i d e d by the G e r m a n D e p a r t m e n t o f E d u c a t i o n and R e s e a r c h u n d e r contract Nr. 01 M 2933 D 1.
References [1] [2] [3] [4]
D. Zeidler, Z. Stavreva, M. P16tner and K. Drescher, Microelectronic Engineering 33 (1997) 259. J.M. Steigerwald, R. Zirpoli, S.P. Murarka, D. Price and G.J. Gutmann, J. Electrochem. Soc. 141 (1994) 2842. Z. Stavreva, D. Zeidler, M. P16tner, G. Grasshoff and K. Drescher, Microelectronic Engineering 33 (1997) 249. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE (Houston, TX, 1974).
Microelectronic Engineering 37 / 38 (1997) 237-243
,
The interaction between different barrier metals and the copper surface during the chemical-mechanical polishing D. Zeidler*, Z. Stavreva, M. P16tner, K. Drescher Dresden University of Technology, Semiconductor and Mikrosystems Technology Laboratory, 01062 Dresden, Germany Abstract The copper polishing rate in a weak acid slurry with H20 2 as a oxidizing agent is determined by a two-step process of the formation of copper oxides followed by its mechanical abrasion. The electrochemical potential is a criterion for the driving force of oxidation-reduction reactions that occur during the Cu dissolution process. Using the experiment a corrosion potential (Ecorr) was determined. It depends on the potential of the cathodic reaction, the Tafel slopes of each reaction and the control of the reaction by Tafel kinetics or concentration polarisation. Potentiodynamic measurements were performed with different H202 contents in the slurry. The dissolution current density (i .... ) is correlated to the oxidation-reduction reaction rate. In the presence of a barrier metal, such as WTi, the dissolution rates were influenced by galvanic effects. W and Ti and their compounds were dissolved in the slurry by the complexing ability of H202. There is an increase in the barrier wet etch rate when copper is present. In the case of polishing both T i - W and Cu simultaneously a dramatic increase of the barrier polish rate was observed. The acceleration of the W or Ti polish rate can be explained by the galvanic coupling between Cu and the barrier metal. By the use of TiN as a barrier metal no galvanic interaction has been observed. The removal rates of TiN have not been influenced by the presence of Cu.
Keywords: Chemical mechanical polishing; Copper; Barrier metal; Potentiodynamic measurements
1. Indroduction
The chemical mechanical polishing of Cu with certain slurries can be considered as a subsequential chemical formation of a passivating layer and its removal by mechanical abrasion [1,2]. The effectiveness of that process is correlated to balanced etch and passivation reactions. The interaction between the copper surface and the polishing slurry can be studied by electrochemical measurements of the polarization behaviour applied during the polishing process as well as under static conditions. The chemical components within the slurry can be characterized by its oxidizing strength, the ability to form the passivating layer and the dissolution of the mechanically abraded metal compounds from the surface. The electrochemical potential is a measure of the driving force of the oxidation-reduction reactions that occur during the metal dissolution. The potential we measured is the mixed corrosion potential Ecorr. It depends on the potential of cathodic and anodic *Corresponding author. Fax: + 4 9 351 463 7172; e-mail: [email protected] 0167-9317/97/$17.00 Copyright © 1997 Elsevier Science B.V. All rights reserved. PII S 0 1 6 7 - 9 3 1 7 ( 9 7 ) 0 0 1 1 7 - 2
238
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 2 3 7 - 2 4 3
reactions, the tafel slopes of each reaction and the kind of control of the reactions by either Tafel kinetics or by concentration polarization. In presence of a barrier metal, as W-Ti, TiN or Ti, the dissolution rates were influenced by galvanic effects. W and Ti and their compounds were dissolved in the slurry by the complexing ability of H202.
The corrosion potential and corrosion current density from electrochemical potentiodynamic measurements were used to give some insights into the polishing process itself and to predict the CMP performance.
2. Experimental CMP was carried out using a PRESI MECAPOL E460 polishing tool with a perforated RODEL IC 1000/Suba IV stacked polishing pad. Alumina based RODEL QCTT 1010 was chosen as slurry. Both the wafer carrier and the polishing table were rotated at 40 rpm, and a down force of 21.9 kPa was applied to the carrier. The surface of the pad was conditioned after polishing each wafer using a diamond conditioner. Silicon wafers 4" were metallized by PVD with 100 nm W-Ti or 100 nm TiN as the barrier/ adhesion layer and 1000 nm Cu. The surface topography before, during and after CMP was investigated using a Tencor profilometer. In order to determine polish rates of blanket metal films, sheet resistances with an ASM 4-point prober were measured for all wafers before and after polishing. The electrochemical measurements were carded out with a SENSORTECHNIK PS 5 potentiostat. Wafer samples with Cu, W-Ti, TiN or Cu/barrier metal (working electrode) of approximately 4 cm 2
600 5O0 400
f tu O
. . . .
O-
" .
.
.
.
.
.
"
---
•
300 ".--W-Ti
200 E
/
100 o u
ILl 0 -100
.t"
-- e-- Cu/W-TI --.:.~0
u
I
-200 -2
0
2
4
6
8
10
12
14
16
Concentration o f H20 2 [%] Fig. 1. Corrosion potential of Cu, W - T i and C u / W - T i (4:1) as a function of H202 - concentration.
D. Zeidler et al. I Microelectronic Engineering 37/38 (1997) 237-243
t --
I m-- W - T i
-
~
! ~
E 2
mqhn"=u=,= "
t
Cu/W-Ti
•" ' ~ " C
<
239
u
41-.
mO
E
0u
..--- a-
0
I 0
4
2
6
8
10
12
14
16
C o n c e n t r a t i o n o f H20 2 [%]
Fig. 2. Corrosion current density as a function of H 2 0 : concentration for Cu, W - T i and C u / W - T i (4:1).
in area were exposed to solutions in a flat cell. The three-electrode configuration cell consisted of a saturated calomel electrode as reference (Ere f = 242 mV versus SHE), and platinum as counterelectrode. Test solutions contained ca. 5% complexing and buffering substances and 4% of Alumina abrasives. The effect of the oxidant concentration in the slurry on the removal rate was investigated within a range of 1.5 ..... 20% H202. The pH was 3.8+0.2. 0,20
400
0,18
E corr qr T i N
300
0,18
/
T
0,14
l.u 0
0,12
200 E
0 o
o
0,10
--4-.-i
100
0,08
- of T i N
~,,u !
• 0,06
u.l
20,04
= . , ~ r'=`'="
0
20,02
i..f -2
0
L0,00 2
4
6
8
H202 - concentration
10
12
14
16
[%]
Fig. 3. Wet etch rate for Cu in slurry with different H202 concentrations.
a3
240
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 237-243
Electrochemical studies were carried out on metal films subjected to the following pretreatment: precleaned in 40% KOH for 4 min, DI rinsed and for 30 s 10% H 2 S O 4 and DI rinsed.
3. Result and discussion The shift of the corrosion potential of Cu towards the anodic direction (relative to the pure metal) with increasing H202 concentration in the slurry is an indication of the formation of a passivating layer on the surface. The corrosion current densities measured decrease with increased H202 content which has to be explained by an increased passivation behaviour of the films formed (Figs. 1 and 2). The wet etch rates of Cu in the same slurries confirm the tendency of stronger passivation behaviour with increased H202 content; the wet etch decreases (Fig. 3). The polish rate has a maximum between 5 and 10% H 2 0 2 (Fig. 4) [3]. The formation and dissolution of the passivating layer is described by the following reactions: 2 C u +½ 02 + H 2 0 ~ 2 C u
+ + 2 OH-
2 Cu~ + 2 O H - --->Cu20~s ) + H20 Cu20(s ) .~_1 02 ~_ 2 H 2 0 --->2 Cu 2+ + 4 OH C u 2 0 ( s ) ..[_ 1 0 2 ~
2 CuO
The oxidised layer of W-Ti(10% Ti) in the slurry caused by a pH value of 4 is very thin. Addition 60
50
c
40
E C
~
30
lU
¢Y
,-
u U.I
20
T
10
0
2
4
6
-_
8
10
.J_
12
14
16
H20 2- concentration [%] Fig. 4. Polish rate of Cu in slurry with different H202 concentrations (pH of the slurry 4.0).
241
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 237-243
of H:O 2 resulted a increase in W-Ti dissolution. W + 4 O H - --~WO 2 + 2 H 2 0 + 4 e WO 2 + OH- --->WO 3 + H + + 2 eWO 3 + O H - ~ HWO 4 Ti + 02 ~ TiO 2 TiO 2 + O H - + xH20 --->[TiO2(OH)(H20)~ ]In order to understand the behaviour of W under solution conditions, thermodynamic data on tungsten species were taken from the E - p H diagram (Pourbaix) [4]. This diagram was drawn assuming an activity of 10 -4 M for all dissolved W species. From the Pourbaix diagram WO 3 is a stable phase only at pH values less than 2, while WO 2 exists over a wide range of pH. Under oxidizing conditions in H20 2 based CMP slurries, the aqueous polytungstate species is the most stable phase, but the formation of this phase is proceeded by the WO 2 phase. If the dissolution of WO 2 is slow, then the presence of WO 2 may be expected on the W surface during polishing at pH values of 4. When Cu is present there is an dramatic increase in the W - T i etch rate. Similar to this behaviour during polishing both W - T i and Cu simultaneously there is a dramatic increase in the W - T i polish rate (Figs. 5 and 6). This acceleration of the W - T i rate can be explained by the galvanic interaction between Cu and W-Ti.
40-
I
[
---o-- W-Ti 7.
W-Ti with C u
//
30-
--
/
I= c 20
f
w'
"8 i .m I--
10
l
2
4
6 8 10 H202-ConcentmUon[%]
12
Fig. 5. Wet etch rate of W-Ti and the influenceof Cu.
14
16
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 237-243
242 60,
T
50 r
I
I
--
Cu Cu with TiN
r
40 r
E
c o_
30 r
20 ~
10-
_J_
0.
2
0
4
6
8
10
12
14
16
H202-Concentration [%] Fig. 6. Polish rates of W - T i as a function of different H202 concentrations and the influence of Cu.
This indicates that H202 acts as a lixiviant for W - T i and causes dissolution rather than passivation of the metal surface. TiN passivated with increasing of H202 content and the wet etch rate is very slow (Figs. 7 and 8). In the presence of Cu, TiN formed no local elements and the dissolution rate is not increased.
500,
Z
400. C
E E
300,
/
j_
\j\
I
~'= 0°" ~ 200,/ 100,
o;/ 0
|
4
8
12
16
20
H2OfConcentration [%] Fig. 7. Corrosion potential and corrosion current density of TiN layers in slurry solution with different H : O : concentrations.
243
D. Zeidler et al. / Microelectronic Engineering 37/38 (1997) 237-243 500
W-Ti + Cu 400
E E
300. i:
"6 @
200.
.~_ 0 O_
W-Ti 100'
0.
i....,,=11
f
J 0
2
I....-.=
4
~
~mll
-m
6
8
10
12
14
16
H202-eoneentration [%] Fig. 8. Wet etch rate of Cu and the influence of the TiN barrier.
Acknowledgements T h e financial support for this w o r k was p r o v i d e d by the G e r m a n D e p a r t m e n t o f E d u c a t i o n and R e s e a r c h u n d e r contract Nr. 01 M 2933 D 1.
References [1] [2] [3] [4]
D. Zeidler, Z. Stavreva, M. P16tner and K. Drescher, Microelectronic Engineering 33 (1997) 259. J.M. Steigerwald, R. Zirpoli, S.P. Murarka, D. Price and G.J. Gutmann, J. Electrochem. Soc. 141 (1994) 2842. Z. Stavreva, D. Zeidler, M. P16tner, G. Grasshoff and K. Drescher, Microelectronic Engineering 33 (1997) 249. M. Pourbaix, Atlas of Electrochemical Equilibria in Aqueous Solutions, NACE (Houston, TX, 1974).