Biomedical Engineering Reference
In-Depth Information
6
350 nm
70 nm
5
4
3
2
1
0
1
10
100
1000
Shear Rate (S
-1
)
FIGURE 12.6
Shear-thinning behavior of silica-ETPTA
suspensions using 70- and 350-nm particles at a volume
fraction of 0.2. Adapted from Ref.
106
.
rheology measurements (
Figure 12.6
) show that
much higher shear rate is required to achieve low
relative viscosity for 70-nm particles compared
to 350-nm spheres with the same particle volume
fraction (0.20), indicating higher spin speed is
necessary to crystallize smaller particles. The
shear-thinning behavior exhibited by the colloi-
dal suspensions is due to the formation of hex-
agonally packed colloidal layers caused by the
coupling of the centrifugal and viscous forces
[91, 117, 120-124]
and the reduced resistance
when layers of ordered spheres glide over one
another
[120, 124]
.
Besides energy-favorable hexagonally
ordered colloidal crystals, the spin-coating tech-
nology also enables the formation of wafer-
scale, non-close-packed crystals with unusual
square ordering (
Figure 12.7
)
[110]
that are con-
sistent with the industry-standard rectilinear
coordinate system for simplified addressing and
circuit interconnection
[125]
. The alternate for-
mation of hexagonal and square diffraction pat-
terns when the spin speed is higher than 6,000
rpm is observed. The spin-coating process can
be stopped, once a strong four-arm diffraction
pattern is formed on the wafer surface.
FIGURE 12.7
(a) Photograph of a spin-coated monolayer,
non-close-packed colloidal crystal with metastable square
lattice. (b) SEM image of the sample. The inset shows a Fourier
transform of a low-magnification image. Reprinted with per-
mission from
J Vac Sci Technol B
27
(2009), 1043-1047. Copy-
right 2009, American Institute of Physics.
Figure 12.7
a shows a photograph of a 4-in.
colloidal monolayer sample consisting of 380 nm
silica spheres and spin-coated at 8,000 rpm for
150 s. The sample exhibits a distinctive four-arm
diffraction pattern under white-light illumina-
tion, and the angles between the neighboring dif-
fraction arms are 90°. This pattern is characteristic
of long-range square ordering. This is confirmed
by the SEM image in
Figure 12.7
b and is further
evidenced by the squarely arranged peaks in the
Fourier transform of a low-magnification SEM
image. The interparticle distance of the squarely