Metal-based wafer level packaging
ALIGNMENT ShIFT X-AXIS AND Y-AXIS IN ThREE LOCATIONS
master recipe or path is defined such that
when the bonds are completed in the bond
X1 Y1 X2 Y2 X3 Y3 X Y
module, the wafers can return to the align-
misalign misalign
er, and a post-overlay image is taken. If the
(µm) (µm)
accuracy is within the tolerances set by the
-0.03 0.02 -0.08 0.07 -0.10 0.10 1.25 -1.85 recipe, then the bond chamber is loaded
-0.03 0.02 -0.08 0.07 -0.10 0.10 0.80 -0.90
with the next aligned pair. If the accuracy
is not within specification the operator
-0.03 0.02 -0.08 0.07 -0.10 0.10 0.50 -1.75
is notified to remedy the situation. The
-0.03 0.02 -0.08 0.07 -0.10 0.10 0.95 -1.05
frequency of the checks can be selected to
match the product necessities.
-0.03 0.02 -0.08 0.07 -0.10 0.10 1.70 -2.15
Both metal diffusion and metal
-0.03 0.02 -0.08 0.07 -0.10 0.10 1.85 -2.40 eutectic bonds are capable of achieving
<5 µm post-bond alignment accuracy.
-0.03 0.03 -0.08 0.08 -0.10 0.10 1.60 2.25
The metal diffusion bonds are affected by
-0.02 0.03 -0.07 0.07 -0.10 0.10 -1.20 0.30
thermal expansion difference between top
-0.03 0.02 -0.08 0.07 -0.10 0.10 0.55 -1.75
and bottom wafers if the upper and lower
heat sources are not consistent with one
Average -0.03 0.02 -0.08 0.07 -0.10 0.10 1.25 -1.85
another. In eutectic bonding, the major
Table 4. Au to Au alignment accuracy, post bond.
contributor to post-bond alignment ac-
curacy is the shifting that may occur if the
applied force or other mechanical systems
mended for BSA alignment and usually use of verniers in the alignment key as a
in the bonder do not prevent the two
required for IR alignment. IR alignment method of quantifying lot to lot varia-
wafers from sliding on the liquid interface.
has the additional advantage that for tions. Figure 5 shows an alignment key
Currently, metal diffusion bonding with
samples that are IR transparent (resistivity with graduated lines used to assess the post
Cu has be proven as a 1-2 µm process
levels >0.01 Ohm-cm) and have no metal bond alignment shifts. Fast, calibrated
with ability to achieve <1 µm alignment
surrounding the alignment keys, the IR assessments of final overlay accuracy allow
accuracy in well-defined 3D integration
imaging method can be used for post-bond for rapid feedback and process develop-
applications. MEMS bonding with eutectic
overlay accuracy assessment and inline ment improvements.
bonds is usually a 2-3 µm process.
metrology during full automated produc- One of the key benefits of post-bond
tion
8
. Figure 4 shows the degradation that alignment checks is that the alignment can
equipment
occurs in IR imaging when the wafers are be checked for each wafer pair. Thus when
The sensitivity of the metal bonds to uni-
too highly doped or do not have a polished production tools are running unattended,
form temperature and force applications
backside on at least one of the wafers. the cluster tool can make use of the rapid
has led to improvements in wafer-level
Alignment key recommendations exist speed of the alignment module relative to
bonding equipment. Improvements in
for all the types of imaging conditions. In the rate at which the bonder can complete
materials and design have led to bond
addition, it is very useful to consider the a full metal bond thermal treatment. The
Sample # POST BOND ALIGNMENT DATA
LX LY RX RY Acc
Post
1
-3.0 0.0 -1.5 0.0 2.3
2
-0.5 -2.0 0.0 2.0 2.0
3
-3.5 -0.5 1.5 0.0 1.1
4
0.0 0.5 0.0 0.5 0.5
5
-1.0 -1.0 -0.5 -1.0 1.3
6
-0.5 0.0 0.0 0.0 0.3
7
0.0 -2.0 0.5 0.0 2.0
8
-0.5 -2.0 -0.5 2.0 2.1
9
0.0 0.0 -0.5 -2.0 2.0
10
0.0 -1.5 -0.5 0.0 1.5
Max 0.0 0.5 1.5 2.0 2.3
Min -3.5 -2.0 -1.5 -2.0 0.3
Avg -0.9 -0.9 -0.3 0.2 1.5
St Dev 1.3 1.0 0.8 1.2 0.7
Figure 7. SUSS MicroTec CB8 wafer bonder with WEC (wedge error compensation)
Table 5. Cu to Cu diffusion bonding, post bond alignment results. to ensure parallelism between pressure plates for all bonds.
www.globalsmt.net Global SMT & Packaging – June 2009 – 13
Page 1  |  Page 2  |  Page 3  |  Page 4  |  Page 5  |  Page 6  |  Page 7  |  Page 8  |  Page 9  |  Page 10  |  Page 11  |  Page 12  |  Page 13  |  Page 14  |  Page 15  |  Page 16  |  Page 17  |  Page 18  |  Page 19  |  Page 20  |  Page 21  |  Page 22  |  Page 23  |  Page 24  |  Page 25  |  Page 26  |  Page 27  |  Page 28  |  Page 29  |  Page 30  |  Page 31  |  Page 32  |  Page 33  |  Page 34  |  Page 35  |  Page 36  |  Page 37  |  Page 38  |  Page 39  |  Page 40  |  Page 41  |  Page 42  |  Page 43  |  Page 44  |  Page 45  |  Page 46  |  Page 47  |  Page 48  |  Page 49  |  Page 50  |  Page 51  |  Page 52