Chapter 4 Results and Discussion
4.2 Analysis and Discussion
4.2.1 Parameters affecting BOAC mechanical strength
4.2.1.4 Low-k materials
We also addressed how the low-k materials affected the mechanical strength of BOAC structures using 65LKBOAC and 65ULKBOAC listed in Table 3.2 and shown in Figure 4.19. These two structures were almost the same except the low-k materials in ILD layer M1-5 as shown in Figure 4.20. From Table 4.2, the Ecs of 65LKBOAC and 65ULKBOAC were 95 GPa and 71 GPa, respectively. The difference in Ecs (24 GPa) was large, which cannot be sorely attributed to the decrease in ILD’s mechanical strength from low-k (15 GPa) in 65 nm to ultra-low-k (9 GPa) in 65 nm parts. Weaker interfacial adhesion in ULK/metal due to the existence of porosity [22, 37] may be the culprit of the much lower Ecs. To prove this hypothesis, we used FIB to examine the
cross- ures
4.21~4.24. For the low-k sample (LK), there was no crack found under the indentation and the ILD seemed flat except top copper layer. However, in ultra low-k sample (ULK), there were many cracks observed under the indentation and the ILD bended under the impact force. These observations indicated that the ultra low-k
section of BOAC structures after nanoindentation test as shown in Fig
researcher [37] had shown that the weak adhesion would lower the modulus during nanoindentation. In this study, low modulus and weak adhesion were believed to be the main reasons affecting the mechanical strength of BOAC structures.
0 500 1000 1500 2000
20
Figure 4.19 Nanoindentation results of 65LKBOAC vs. 65ULKBOAC
Figure 4.20 Structures of different low-k materials in 65LKBOAC and 65ULKBOAC
Figure 4.21 FIB cross-sectional view of 65LKBOAC (8,000 X)
No cracks happened
.22 FIB cross
Figure 4 -sectional view of 65LKBOAC (25,000 X)
Figure 4.23 FIB cross-sectional view of 65ULKBOAC (8,000 X)
Lots of cracks
Figure 4.24 FIB cross-sectional view of 65ULKBOAC (25,000 X)
So far
rength of various BOAC structures in a soft film/ hard substrate system. However, the accuracy of these quantified values warranted verification. As a result, BOAC structures in a hard film/soft substrate system will be further analyzed to check if the same mechanical strength of BOAC structures can be obtained. In addition, the applicability and issues of these quantified values will be addressed.
, there is no problem using the quantified values to analyze the mechanical st
4.2.2 H
was the same as that with the original BOAC structures Al film on the top.
Curve fitting was also used to characterize the mechanical strength of various
f for the oxide film was measured to able 4.1. We also changed Ecs and used “try and error” to fit ost matched curve. The efficient fitting range was from beginning to the lowest because the fitting curve didn’t rise up in the end of
cs (Modulus of composite BOAC substrate) for various BOAC structures rve fitting as illustrated in Figures Table 4.3.
ard film/soft substrate system
When the Al pad was etched from BOAC structures, the stacking of BOAC structures became oxide film/composite substrate structure, which was a hard film/soft substrate system. The nanoindentation results, Efnanoindentationof BOAC structure with oxide as the top layer shown in Figure 4.25, decreased with increasing indenter depth, indicating that the moduli of composite BOAC substrates were smaller than the modulus of oxide film. The mechanical strength of composite BOAC substrates in decreasing order
with
BOAC structures with oxide layer on the top. E be 79.5 GPa as listed in T
with oxide on the top can be obtained by cu 4.26-4.32 and summarized in
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30
Figure 4.25 Nanoindentation results of BOAC structures with oxide film on the top
Table 4.3 Modulus of composite BOAC substrate with oxide film on the top in a strate model
two-layered oxide/composite sub
Samples Ecs (oxide film/composite substrate), GPa
90LKBOAC A 45
90LKBOAC B 41
90LKNormal 63 65LKBOAC 54 65LKNormal 80 65ULKBOAC 42 65ULKNormal 74
0 500 1000 1500 2000
40 60 80 100 120 180 200
140 160
displacement(nm)
90LKBOAC A_oxide 45GPa
Figure 4.26 King’s model fitting result for 90LKBOAC A with oxide film as the top layer
0 500 1000 1500 2000 40
60 80 100 120 140 160 180 200 220
modulus(GPa)
displacement(nm)
BOAC 90 B_Etch 41GPa
Figure 4.27 King’s model fitting result for 90LKBOAC B with oxide film as the top layer
0 500 1000 1500 2000
60 80 100 120 140 160 180 200 220
modulus(GPa)
displacment(nm)
Normal Pad 90_Etch 63GPa
Figure 4.28 King’s model fitting result for 90LKNormal with oxide film as the top layer
260 280
0 500 1000 1500 2000
BOAC 65LK_Etch
Figu s model fitting result for 65LKBOAC with oxide film as the top layer
re 4.29 King’
0 500 1000 1500 2000
60
Normal Pad 65LK_Etch 80GPa
Figure 4.30 King’s model fitting result for 65LKNormal with oxide film as the top layer
0 500 1000 1500 2000 40
60 80 100 120 140 160 180 200 220
modulus(GPa)
displacement(nm)
BOAC 65ULK_Etch 42GPa
Figure 4.31 King’s model fitting result for 65ULKBOAC with oxide film as the top layer
0 500 1000 1500 2000
40 60 80 100 120 140 160 180 200
modulus(GPa)
displacement(nm)
Normal Pad 65ULK_Etch 74GPa
Figure 4.32 King’s model fitting result for 65ULKNormal with oxide film as the top layer
Judging from the curve fitting in Figures 4.26-4.32, it was found that significant deviations from were observed for displacement greater than
700-800 nm. In general, of BOAC structure with oxide as the top layer, rose up at the disp 700-800 nm due to (1) copper/low-k patterned layer underneath oxide layer and (2) the strain-hardening effect. Next we examined the data between 65 nm parts and 90 nm parts as shown in Figure
4.33. of 65 nm samples rose up at shorter displacement compared to 90 nm e (1) the pitch of each metal layer in Cu/low-k for 65 nm is
(2) the thickne 65 nm samples were thinner than that of 90 nm; thus its affected region touched the hard oxide la er earlier. These two reasons contributed the of 65 nm parts to rise up earlier than that of 90 nm at the displacement near 700-800 nm.
ation
smaller (0.16 μm vs. 0.32 μm in 90 nm parts) providing more of strain hardening and sses of the
y
ation nanoindent
E
f0 500 1000 1500 2000
30
Figure 4.33 Nanoindentation results of BOAC 90A vs. BOAC 65LK with oxide
4.2.3 The difference in Ecs between Al film/composite substrate and oxide film/composite substrates
We combined Table 4.2 and Table 4.3 into Table 4.4 for comparison.
Table 4.4 Modulus of composite BOAC substrate (Al vs. oxide)
From Table 4.4, the Ecs of oxide film/composite substrate were lower than those of Al film/composite substrates for the same BOAC design. The reasons for such discrepancy may be generalized as follow.
1. The indentation affected region within the composite substrates was not the same
For Al film/composite substrate system, the indentation affected region within composite substrates included oxide layer and Cu/low-k layers to a less degree as illustrated in Figure 4.34. In contrast, for oxide film/composite substrate, the indentation affected region within composite substrates included primarily Cu/low-k
Ecs (Modulus of composite BOAC substrate), GPa Samples
Al film/composite substrate Oxide film/composite substrate
90LKBOAC A 75.5 45
90LKBOAC B 70 41
90LKNormal 109 63
65LKBOAC 95 54
65LKNormal 125 80
65ULKBOAC 71 42
65ULKNormal 119 74
layers and to a less degree of Si substrate. Since the modulus of oxide and low-k were 79.5 GPa and 15.4 GPa, respectively, it could be easily understood that the Ecs would be larger in the Al film/composite substrate system than that in the oxide film/composite substrate system.
Figure 4.34 The affected regions within the composite substrates
2. The impact force was different in two different film/substrate systems
When the indenter traveled through the film, it needed higher impact force in the oxide film/composite substrate system than that in the Al film/composite substrate system because the oxide film was harder than the Al film. Moreover, high impact force would damage the structures under the impact of indentation; thereby the modulus would be underestimated. As a result, the oxide film/composite substrate
Al (1.2 μm) Oxide (1.2 μm)
Si substrate
Copper/low-k
would be damaged by impact force more easily than Al film/composite substrate, (3 μm)
(0.9 μm) Copper /FSG Oxide (1.2 μm)
Si substrate
Affected
Al system Oxide system
(0.9 μm)
region within composite Copper /FSG
Copper/low-k (3 μm)
However, there are two conflicting observations in these two film/substrate sy
1. There was large discrepancy of the calculated Ecs of 65LKBOAC between the
Al/su GPa as
listed in Table 4.4. Figure 4.35 showed the relation between these two
film/substrate system odul a
and the Ecs was 95 GPa, indicating that the modulus of the substrate ide layer should be an 79.5 GPa, or even 95 GPa. But in the ate system, the modulus of the composite substrate under oxide layer, GPa, which was not le.
l/substrate system, Ef t increased with inc g indenter the indenter penetra ide layer at the displacement of ≧
shown in Figure 4.3. It was thought that composite substrate under , the should decreased stems.
bstrate system and the oxide/substrate system, namely 95 GPa vs. 54
s. In the Al/substrate, the m us of oxide was 79.5 GP composite
under the ox larger th
oxide/substr
Ecs, was 54 reasonab
2. In the A nanoindenation reasin
depth after ted into the ox
1200 nm as
oxide layer was weaker than the oxide layer Efnanoindentation
after the indenter penetrated into the oxide layer. Instead, Ef
increased after the displacement ≧ 1200 nm as shown in Figure 4.3, which was not reasonable.
Figure 4.35 Comparison of Ecs between different film/substrate systems The reason for such discrepancies was the contact-area effect induced by the difference in the hardness of materials in the BOAC stack. The contact-area effect will be further discussed in the following section.
38]
w
hardness and modulus would be affected by the contact area (A). For Al
film/com cs )
ated. For the oxide film/composite substrate, the Ecs would be
effect and m
4.2.4 The contact area effect
The hardness would affect the modulus during nanoindentation. Researchers [36, found that the hardness would affect the contact area (A) during nanoindentation.
Soft films on hard substrates tended to pile-up when indented, while hard films on soft substrates tended to sink-in. Hence the true contact depth was underestimated in the case of a soft film on a hard substrate and overestimated in the case of a hard film on a soft substrate, when compared to the calculated contact depth using the Oliver–
Pharr method. Since contact area was a function of contact depth, contact area (A) as underestimated in the case of a soft film on a hard substrate and overestimated in the case of a hard film on a soft substrate. From Equation 3.1 and Equation 3.2, the
posite substrate, the E would be overestimated while the contact area (A was underestim
underestimated while the contact area (A) was overestimated. The contact area (A) ethod to eliminate the contact area factor will be discussed in the following section.
4.2.4.1 Eliminating the contact area factor
A method [36, 38] based on P/S2 was employed to eliminate the contact area by combining Equation 3.1 and Equation 3.2 into Equation 4.1.
2
as directly proportional to hardness H and inversely proportional to the
exp
hig ples were lower than those of 90 nm
ement depth,
were higher than those of 90 nm e
previous section. The P/S2
1100 nm depth. This was caused by the changing Ecs as the affected region moving from stiff oxide layer down to more of weak low-k layers with the increasing indenter dep
diff
continually changing substrate with increasing indenter depth through different layer in the stack. For BOAC structures using 90 nm technology such as 90LKBOAC A and 90LKBOA B which had the same layout in the copper/low-k layers, the increasing slopes between 200 nm to 1100 nm depth were almost the same. This result was the same for normal pad structures with different low-k materials such as 65LKNormal and 65ULKNormal because bond pad strength was primarily supported by the Cu/dielectric layer with 90 % copper density even though the low-k material was very different. Toward the end of 1000 nm depth, the low-k material had little
2 w
square of the reduced modulus Er. And the contact area (A) was eliminated in the ression of P/S2. Since Es (Ecs) was proportional to Er according to Equation 3.5, would use this relation to analyze the true modulus during nanoindentation. Figure and 4.37 showed the P/S2 as a function of displacement in the Al film/composite strate and oxide film/composite substrate samples, respectively.
or Al film/composite substrate samples, P/S2 was inversely proportional to the are of the reduced modulus Es (Ecs), implying that lower values of P/S2 meant her Ecs. From Figure 4.36, P/S2 of 65 nm sam
samples at about 200 nm displac indicating that Ecs of 65 nm samples samples at the initial stage as described in th increased as the depth increased from about 200 nm to
th. Since composite substrate was composed of oxide/M1-6 layers with low-k and erent pattern density, such composite substrate was not uniform, but exhibited a
influence on P/S2 that 65ULKNormal sample increased a little for lower modulus of ultra low-k. For the 65LKBOAC and 65ULKBOAC, obvious difference in the postive slopes was observed, which was caused by the large difference in modulus between ULD and LK materials while the Cu/low-k layers were mainly supported by the low-k materials in these two BOAC samples. The P/S2 of 65ULKBOAC increased faster than 65LKBOAC because 65ULKBOAC had lower modulus in Cu/ULK layers. After 1100 nm depth, positive slopes were also observed for all samples, presumably affected by the hardness of the stiffer substrates. From Table 4.1, the hardness of oxide and Al were 7.8 GPa and 0.25 GPa.
much harder
2
The oxide layer under the Al film was than the Al film. As the indenter penetrated into to the oxide layer after about 1200 nm depth, the P/S increased in different slope due to the hardness of stiffer, oxide layer.
0 500 1000 1500 2000
0
Figure 4.36 P/S as a function of displacement for Al/substrate BOAC structures
square of the reduced modulus Es (Ecs). The higher values of P/S2 meant the lower Ecs. From Figure 4.37, the P/S2 of normal pad samples were lower than those of BOAC samples at about 500 nm depth, implying that Ecs of normal pad samples were higher than those of BOAC samples around 500 nm depth, which had been discussed the previous sections. The P/S2 values then decreased at depth＞500 nm because of
n the indenter traveled downward. For the 65LKBOAC and 65ULKBOAC, the P/S2 values became the same after 500 nm displacem
trate system was in
the following two reasons.
1. Ecs changed to larger value as the affected region expanded to hard oxide layer and Si substrate beneath the Cu/low-k layers whe
ent depth because the contribution from low-k decreased while the affected region included more of the stiffer, oxide layer and Si substrate.
2. Second reason was the hardness effect. In Al film/composite substrate, the hardness effect took place when the indenter penetrated into the substrate. But in oxide film/composite substrate, the hardness effect happened before the indenter penetrated into the substrate. Al film/composite subs
characterized as a soft film/hard substrate system where the hard substrate was not affected by the deformation behavior and the soft Al film is accommodating all the plastic deformation until the indenter is close to the film/substrate interface. From Table 4.1, the hardness of oxide and copper (copper was under the oxide) was 7.8 GPa and 1.13 GPa, respectively.
Therefore, the oxide film/composite substrate was a hard film/soft substrate system. When the soft substrate yielded at about 500 nm depth, the effective hardness decreased; thus P/S2 decreased.
Here, 90LKBOAC A and 90LKBOAC B in Al film/composite system were used to illustrate such trend. These two samples did not show significant differences because
in their BOAC structures were almost the same except the top copper metal layer. As shown in Figure 4.38, the top copper metal was a block copper in 90LKBOACA and a copper ring in 90LKBOACA. In oxide film/composite substrate system, the materials under the oxide layer were copper and FSG for 90LKBOACA and 90LKBOAC B samples. From Table 4.1, the hardness of copper and FSG were 1.31 GPa and 6.68 GPa, respectively. As a result, the effective hardness was higher in 90LKBOACB (FSG) than in 90LKBOAC A (copper); thereby P/S2 of 90LKBOACB (FSG) was higher at about 500 nm depth of displacement.
0 500 1000 1500 2000
0
Figure 4.37 P/S as a function of displacement for BOAC structures with oxide 2
as the top layer
Figure 4.38 The materials and layout under oxide layer for 90LKBOACA and 90LKBOAC B
We then evaluated the Er excluding the hardness effect by using a constant H(E) and substituting Equation 4.1 into Equation 4.2:
)
where H(E) for Al was 0.6 GPa [36] while H(E) for oxide was 7.8 GPa obtained in our Laboratory. The results were showed in Figures 4.39 and 4.40.
For Al film/composite substrate system, Er showed the changing substrate with
the direction of indentation. Comparing to Figure 4.3, was overestimated, while the calculated values listed in table 4.2 were also overestimated.
Th
a three-layered system, i.e. the soft film-hard layer-soft su a
increasing indenter depth, indicating that the composite substrates is not uniform in
ation nanoindent
Ef
is elucidates the problem encountered and mentioned in section 4.2.3.
In oxide film/composite substrate system, Er also seemed different as compared to those shown in Figure 4.25. The contact area effect enlarged the difference among different BOAC structures although the trend of their mechanical strength was still the same.
Even though there was expectation to quantify the Ecs from Figure 4.39, there was no fitting curve method for
bstr te system yet. However, it is still useful to use the overestimated, calculated from Table 4.2 to predict if the BOAC structures can pass the bondability test their trend was the same with the true modulus as shown in Figure 4.39. A ation of King’s model will be needed if the exact modulus of the multi-layered tes is sought after. A model based on multi-layered substrate will be a topic
re study.
0 500 1000
Figure r excluding the hardness effect of BOAC structures with Al film as the top layer
Figure 4.40 Er excluding the hardness effect of BOAC structures with oxide as the top layer
4.3 Model of mechanical strength in BOAC structures
The calculated values of substrate modulus were overestimated when Ecs was used without excluding the contact area effect (hardness effect) because the substrate actually is not a uniform matrix, instead a multilayered metal/low-k structure. Here we attempt to exclude the contact area effect to build a model that can fit the Er’s results as shown in Figures 4.39 and 4.40. We treated a typical BOAC structure as a 3-layered model as schematically illustrated by Figure 4.41. From Figure 4.39 in Al/composite substrate system, the Er first increased to the maximum and decreased
increase of Er to the m mposite substrate underneath the oxide
com
mini
fected ered copper/low-k structure with different layout and varying mechanical characteristics as
the substrates using King’ odel which assumed a homogeneous and uniform substrate. Instead, we attem
with increasing indenter depth. It is believed that the middle hard layer induced the aximum, then the soft co
layer caused the decrease of Er after indentation affected region reaching the soft posite substrate.
From Figure 4.40 in oxide/composite substrate system, Er firstdecreased to the mum and increased with increasing indenter depth increasing, indicating that the soft composite substrate was softer than the oxide layer in the initial indentation stage and later the increase of Er was induced by Si substrate after indentation af region reaching the hard Si layer. Because the composite substrate is a multilay
the indenter depth increases, we could not evaluate the exact mechanical strength of s m
pted to quantify the modulus of the soft composite substrate in experimental observations and theoretical calculation.
Al film-oxide layer-soft substrate
AlOxide Soft composite
substrate
Figure 4.41 Schematic diagram of a 3-layered BOAC model
First we evaluated the modulus of the soft composite substrate from the basic structures using the theoretical calculation from the modulus equation of composite material, Equation 4.3:
n s n
c
x E x E x E
E =
1×
1+
2×
2+ ... + ×
……. (4.3)where
Xn = Volume fraction of n material En = Modulus of n material
Figure 4.42 showed the fraction of each layer in soft composite substrate and the modulus of each layer was summarized in Table 4.1. Table 4.5 showed the results by using the theoretical values from the modulus equation of composite material.
Figure 4.42 showed the fraction of each layer in soft composite substrate and the modulus of each layer was summarized in Table 4.1. Table 4.5 showed the results by using the theoretical values from the modulus equation of composite material.