Download Fabrication of microchannels Using Layer – by – Layer Machining in

Fabrication of microchannels Using Layer – by – Layer
Machining in Micro USM
1
Jain, V., 2Kumar, P. , 3Sharma, A.K.
1
Mechanical Engineering Department
Maharishi Markandeshwar Engineering College, Mullana
2,3
E-mail: [email protected]
Mechanical and Industrial Engineering Department
Indian Institute of Technology Roorkee
Roorkee, Uttarakhand – 247 667, India.
E-mail: [email protected], [email protected]
Abstract: - Microchannel is the basic structure in any microfluidic device to control, deliver,
manipulate and store the liquid. These microchannels are typically made of silicon, metal,
or glass and often feature circular, rectangular or trapezoidal cross sections, ranging in
terms of the hydraulic diameter from 1 µm to 1000 μm. Various lithographic
micromachining techniques are widely used to fabricate the microchannels on such type of
substrates. Additive manufacturing is one of the silicon micromachining techniques of
producing parts by successive deposition of layers of material as in the case with rapid
prototyping. In this paper an inverse approach is used where material is removed layer-bylayer using micro ultrasonic machining to fabricate the microchannels. The microchannels
fabricated in the present study with this technique, are V shaped open glass and silicon
microchannels. The machined surfaces were characterised by scanning electron
microscope and atomic force microscope (AFM). The width and depth of the channel is
approximated as 420 μm and 150 μm respectively. Some drawbacks of using this
technique are also highlighted.
Keywords: Microchannel, Micro USM, Glass, Micromachining, Layer-by-layer machining
1. INTRODUCTION
In any microfluidic device microchannels are
the only via for the flow of liquid. They control,
deliver, manipulate and sometimes used to store the
liquid. In the previous literature, various types
(shapes) of microchannels made of different
materials for different applications were reported
by many authors. The microchannels, which are
typically used in quartz or silicon substrates with
the channel dimension of a few microns to a few
hundred microns, are currently fabricated by
conventional
technologies
such
as
Photolithography,
Additive
Techniques,
Subtractive Techniques, and Pattern Transfer
Techniques. The technologies have been developed
for about three decades and have become one of the
biggest branches in the development of IC and
MEMS [1-4]. Lee et al. [5] suggested a low-cost
microfabrication
method
for
making
microchannels, which are suitable for high-volume
production, particularly in the field of life sciences.
The authors used polymer as a substrate material
and hot embossing for the component fabrication.
Microchannels made of glass or polysilicon was
successfully designed, fabricated and tested by Lee
and Lin [6]. The fabrication process used timed
wet-chemical etching to selectively etch sacrificial
materials with the assistance of etch holes. These
microchannels have the potential to be integrated
with other micromachined microfluidic systems,
including DNA chip and lab-on-a-chip. Recently a
microfluidic device is fabricated [7] from glass
substrates and polymer sheets in microscope-slide
format using low-cost, additive manufacturing
techniques i.e. rapid-prototyping, for separating
particles and transferring blood cells from
undiluted whole human blood.
Owing to high heat transfer rate and low
temperature rise, a microchannel heat sink is
commonly demanded for a wide variety of
applications. First demonstration of microchannel
heat sinks in InP (substrate material of laser diodes)
was conducted by Phillips [8] using precision
sawing and orientation dependent etching. With the
development of micromachining technologies,
some other methods were suggested such as laser
machining, numerical controlled milling and
extrusion for cutting microchannels in aluminium.
A detailed review has been done [9,10] for types of
manufacturing processes used in the fabrication of
micro heat exchangers with the main focus on
passages with hydraulic diameter of less than 200
μm. In the present study an inverse approach is
used where material is removed layer-by-layer
using micro ultrasonic machining (micro USM) to
fabricate the microchannels.
The working principle of micro ultrasonic
machining is same as with macro USM which is a
well established process. The material removal
takes place by the action of abrasives. A slurry
contains abrasives is being fed in between the tool
and workpiece. The vibrating tool strikes the
abrasive particle which further hits the workpiece,
results in material removal. The basic mechanisms
involved are abrasion, erosion and hammering.
Micro USM is generally meant for drilling in hard
and brittle materials such as glass, silicon and
ceramics. Even, for generating a complex structure
using micro USM, the basic principle of drilling is
exploited. In a pattern tool, micro features are
fabricated on the tool bottom. The tool functions as
a pattern and is travelled vertically toward
workpiece, and, therefore, micro features can be
“replicated” onto the workpiece in one sinking
operation [11]. Sun et al. [12] reported the contour
machining using micro USM in which a spiral
groove on low melting glass was generated using
layer-by-layer machining. From the literature it is
evident that silicon and glass are the most
prominent materials for microfluidic applications
and ultrasonic machining is perfectly suitable for
such high hardness and high impact brittle
materials [13]. The microchannels fabricated in the
present study with this technique, are V shaped
open microchannels on glass and silicon. The
machined surfaces were characterised by scanning
electron microscope and atomic force microscope.
2. EXPERIMENTAL SET UP
The micro USM consists of the five basic
elements viz. high frequency oscillating current
generator, the acoustic head, the micro tool, the
abrasive slurry and the workpiece. Experiments
were conducted on an AP-500 (240-volt) model
Sonic-Mill Ultrasonic Machine, with the maximum
power output of 500 Watts. Figure 1 illustrates the
experimental set up of micro USM for making the
microchannels. The solid cylindrical tool was
applied for making the channels. In case of micro
USM, the shape and dimensions of the final feature
solely depend on those of the tool shape and
dimensions. The tool should be so designed to
provide the maximum amplitude of vibration at the
free end at a given frequency. The fabrication of
microtool and fixing it to the tool holder/horn is a
real challenge in micro USM. A misalignment
resulted in machining inaccuracies and even a
minute crack in the tool causing the machine to
stop. The micro tool was attached to the horn by
soft soldering. Thickness of the tool was kept at
300 μm. The slurry was prepared with silicon
carbide abrasive material with water as a slurry
media. Commercial borosilicate glass with 2 mm
thickness and a <1 1 1> p-type single crystal silicon
wafer with 0.5 mm thickness was employed as
workpiece material for the experimentation.
Fig. 1 A Micro USM Set-Up
2.1 Layer-By-Layer Machining
For holding the samples, the bottom of the
workpieces was fixed on a Perspex sheet using
commercial adhesive, and then Perspex sheet is
further fastened on a Bakelite board. In the initial
stage, tool was made to maintain a gap of 10-20 μm
with the workpiece surface. At this position the zaxis movement (downward movement of the tool)
was locked and machine starts while table was
moved from left to right direction. The machining
was done for a specific length of the channel at this
setting. Time was noted down using a stop watch.
After achieving the desired length, the tool was
allowed to move downward by 50 μm using z-axis
control. The precise control of depth is ensured by
the dial gauge mounted on the machine. The tool is
again locked and work table movement is reversed.
The whole cycle was repeated for number of times
to achieve the designed channel dimension. Figure
2 demonstrate the cutting of straight glass
microchannels using layer-by-layer technique.
After every trial, the Perspex sheet was detached
from the board and workpiece was removed from
the sheet by heating it to a nominal temperature in
a domestic oven. After the ultrasonic drilling, the
specimens are cleaned on ultrasonic bath.
Fig. 2 Cutting of Straight Open Microchennel
Fig. 4 A Glass Microchannel Fabricated using
Layer-By Layer Machining
3. RESULTS AND DISCUSSION
Figure 3 and 4 show the machined glass
microchannels. A zoomed view of the channel can
also be seen in inset (Fig. 3) which shows the
waviness in the channel. A x-section is taken along
plane A-A and the SEM micrographs are shown in
Figures 4 & 5. As the table is moving in x
direction, because of the bottom edge of the tool, a
deep cavity is formed in the middle of the channel.
This is because the abrasives are more prone to
strike to the surface of the workpiece beneath the
vibrating tool edge. The major drawback of using a
straight cylindrical tool is the precision and
dimension of the channel. Because of the
movement of the table (back and forth) the channel
cannot sustain the straightness and observe some
extent of out of straightness (OOS). At some
position the width of the channel is more than other
locations. The width of a channel is not constant,
but is narrow at the base than at the top. The OOS
can be observed in SEM micrographs (Fig. 4).
There is always a possibility of side
cutting because of the side edge of the tool. The
abrasives are trapped between the side edge of tool
and walls of the channel. The relative contact gives
rise to the taper of walls and results in a V-type
microchannel as shown in figure 5. These 3dimensional pictures show that the channel
configurations were machined sharply. The side
views of the channels show that the angled walls
form channels that are not rectangular, but are
actually V-shaped. Such type of taper
microchannels can be used to trap the liquid e.g., to
form surfaces that are difficult to dry, which could
find use for lubrication, sensing, or micro-heatpipe. From the side view of channels width and
depth of the channel can be approximated as 420
μm and 150 μm respectively. The total time taken
to cut the channel is also high as obtaining higher
depth of the channel involves number of cycles.
The wear of tool is much larger as lateral face is
also involved in cutting. A SEM micrograph
(figure 6) depicts the wear of side edge and bottom
edge of the solid cylindrical micro tool after
machining.
Fig. 5 A V-Type Channel
Fig. 3 Machined Glass Microchannel;
Inset: Zoomed View of Channel
Fig. 6 Worn Out Micro Tool
For measuring surface roughness of the
microchannels atomic force microscope (AFM)
(Model: NT-MDT: NTEGRA) was used. For AFM
measurements, the microchannel was sectioned
along its axis to make its base surface accessible to
an AFM for carrying out the roughness
measurement. The vertical resolution of the AFM
is 0.1 nm and lateral resolution 10 nm. An area of
10 μm × 10 μm was measured. For each
microchannel, three readings of surface roughness
were taken and average value of mean surface
roughness was recorded. Figure 7 (a & b) depicted
the average surface roughness of 735.81 nm and
478.49 nm of glass and silicon microchannel
respectively.
Sometimes during the experiments solid
cylindrical tool tends to create cracks in the
channels (figure 8 (a)). Since, fatigue loading and
pointed edge of the tool is involved such kind of
problem can be predicted. Because of point loading
the x-sectional area of the workpiece is very less
and stress induced is much more for progressive
brittle cracking under repeated alternating or cyclic
stresses of an intensity considerably below the
normal stress. However, such type of cracking was
observed in rare cases only. There are also some
secondary cuttings around the microchannels due
to deflection of some abrasive particles by the
vibrating micro tool near the microchannel wall
and can be termed as stray cutting. A SEM
micrograph of channel with stray cutting is shown
in figure 8 (b). Very low depth microchannels were
demonstrated during layer-by-layer machining and
time taken during machining was too high.
Fig. 8 (a) Crack in the Silicon Microchannel (b)
Stray Cutting on the Walls of a Microchannel
Fig. 7 Typical 3D Topograhs of Micro USM
Machined Surface on (a) Glass (b) Silicon
4. CONCLUSION
The V-type open microchannels were
fabricated on two hard-to-cut materials such as
glass and silicon, with the help of micro USM
using layer-by-layer technique. The fabricated
channels can be used for various types of
microfluidic devices. The relative contact between
the abrasive and side edge of a solid cylindrical
tool gives rise to the taper of walls and results in a
V-type microchannel. As the lateral edge of the
tool is also involved in cutting, the tool wear is
more in this technique. The channels have poor
form accuracy in terms of straightness and a low
aspect ratio but, same can be improved by
maintaining the precision of the equipment. Further
study can be focused on a die-sinking operation for
the fabrication of such microchannels.
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