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NANO FINISHING PROCESS

The electronics and computer industries are always in demand of

higher and higher precision for large devices and high data packing
densities. The ultimate precision obtainable through finishing is
when chip size approaches atomic size (~ 0.3nm)

To finish surfaces in nanometer range, it is required to remove

material in the form of atoms or molecules individually or in the
groups.

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Abrasive Flow Machining (AFM)

• Abrasive Flow Machining (AFM) was identified in 1960s as a method to
deburr, polish, and radius difficult to reach surfaces and edges by flowing an
abrasive laden viscoplastic polymer over them.
• It uses two vertically opposed cylinders, which extrude an abrasive medium
back and forth through passage formed by the workpiece and tooling.
• Abrasion occurs wherever the medium passes through the restrictive
• passages.

The three major elements of the process are:

(a) The Tooling, which confines and directs the abrasive medium flow to the areas where
deburring, radiusing and surface improvements are desired.
(b) The Machine to control the process variables like extrusion pressure, medium flow
volume, and flow rate.
(c) The abrasive laden Polymeric Medium whose rheological properties determine the
pattern and aggressiveness of the abrasive action. To formulate the AFM medium, the
abrasive particles are blended into special viscoelastic polymer, which show change in
viscosity when forced to flow through restrictive passages.
AFM can process many selected passages on a single workpiece or multiple parts
simultaneously. It reduces surface roughness by 75 to 90% on cast, machined or EDM'd
surfaces. its application in wide range of industries. Aerospace, aircraft, medical
components, electronics, automotive parts, and precision dies and moulds manufacturing
industries

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Magnetic Abrasive Finishing (MAF)

• Magnetic abrasives are emerging as important finishing methods for metals and
ceramics.
• It produces efficiently and economically good quality finish on the internal and
external surfaces of tubes as well as flat surfaces made of magnetic or non-magnetic
materials.
• In this process, usually ferromagnetic particles are sintered with fine abrasive
particles (Al2O3, SiC, CBN or diamond) and such particles are called ferromagnetic
abrasive particles (or magnetic abrasive particles).
• MAF process in which finishing action is generated by the application of magnetic
field across the gap between workpiece surface and rotating electromagnet pole.

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Magnetorheological Finishing (MRF)

•The MRF process relies on a unique "smart fluid", known as Magnetorheological
(MR) fluid.
•MR-Fluids are suspensions of micron sized magnetizable particles such as carbonyl
iron, dispersed in a non-magnetic carrier medium like silicone oil, mineral oil or
water.
On the application of an external magnetic field to a MR-suspension, a
phenomenon known as Magnetorheological Electromagnet Magnetic abrasive
brush Cylindrical workpiece effect, shown in Fig. is observed

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The MR-polishing fluid lap has following merits over traditional lap:-
1. Its compliance is adjustable through the magnetic field.
2. It carries heat and debris away from the polishing zone.
3. It does not load up as in grinding wheel.
4. It is flexible and adapts the shape of the part of the workpiece which is in
its contact.

The computer controlled Magnetorheological finishing process has

demonstrated the ability to produce the surface accuracy of order 10-100 nm
peak to valley by overcoming many fundamental limitations inherent to
traditional finishing techniques

Applications that use high precision lenses include medical equipment such as
endoscopes, collision-avoidance devices for the transportation industry,
scientific testing devices and military's night vision equipment like infrared
binoculars. Missiles are equipped with a wide variety of high precision lenses
for navigation, target location, and other functions. The nano diamond doped
MR fluid removes edge chips, cracks, and scratches in sapphire bend bars.

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Magnetorheological Abrasive Flow Finishing

(MRAFF)

The use of magnetorheological polishing fluid with cerium oxide abrasives for
finishing optical lenses up to the level of 0.8 nm

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•In MRAFF process, a magnetically stiffened slug of magnetorheological

polishing fluid is extruded back and forth through or across the passage
formed by workpiece and fixture.
• Abrasion occurs selectively only where the magnetic field is applied across
the workpiece surface, keeping the other areas unaffected.
•The rheological behaviour of polishing fluid changes from nearly Newtonian to
Bingham plastic upon entering and Bingham to Newtonian upon exiting the
finishing zone. The abrasive (cutting edges) held by carbonyl iron chains rub
the workpiece surface and shear the peaks from it

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Comparison of surface before and after MRAFF, (a) Initial surface before
MRAFF, (b) Final surface after MRAFF for 200 cycles at B = 0.574 T

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Magnetic Float Polishing (MFP)

LEVITATE:- Magnetic Float Polishing (MFP). The magnetic float polishing technique is based on the
rise or cause to
rise and hover ferro-hydrodynamic behaviour of a magnetic fluid that can levitate a non-magnetic float
in the air and abrasives suspended in it by magnetic field. The levitation force applied by the
abrasives is proportional to the field gradients and are extremely small and highly
controllable.

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MRF

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Elastic Emission Machining (EEM)

Though this process was developed as early as in 1976 [32], it
attracts the attention because of its ability to remove material at the
atomic level by mechanical methods and to give completely
mirrored, crystallographically and physically undisturbed finished
surface.
The material removal in conventional machining is in part due to the
deformation or fracture based on migration or multiplication of pre-
existing dislocations, or by the enlargement of cracks originating
from pre-existing micro cracks.

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 In the EEM process a polyurethane ball, 56 mm in diameter, is mounted on a shaft

driven by a variable speed motor The axis of rotation is oriented at an angle of ~ 45º
relative to the surface of the workpiece to be polished.
 The workpiece is submerged in slurry of ZrO2 or Al2O3 abrasive particles and water. The
material removal rate for the workpiece was found to be linear with dwell time at a
particular location, allowing the total thickness of material removed to be controlled to
within 20 nm. The removal rate, however, was found to vary non-linearly with
concentration of abrasives in the slurry.
 The proposed mechanism of material removal due to slurry and workpiece interaction
involves erosion of the surface atoms by the bombardment of abrasive particles without
the introduction of dislocations.
 Surface roughness as low as 0.5 nm rms have been reported on glass and a surface
roughness of ≤ 1nm rms was obtained when polishing single crystal Silicon.
 particle removes a number of atoms after coming into contact with the surface .
 The type of abrasives used has been found to be critical to the removal efficiency.

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If thematerial removal can occur on atomic size units, then the finish generated can be
close to the order of atomic dimensions (∼0.2 nm to 0.4 nm). Using ultra fine particles to
collide with the workpiece surface, it may be possible to finish the surface by the atomic
scale elastic fracture without plastic deformation. This new process is termed as Elastic
Emission Machining (EEM).

Rotating sphere and workpiece interface in EEM

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Schematic of EEM assembly used on NC Machine

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CMP is a process of smoothing surface with the combination of chemical and mechanical forces

CMP is the most preferred process used

in semiconductor industry for silicon wafer finishing and planarization. Since the
material removal in fine abrasive finishing processes is extremely small, they can be
used successfully to obtain nanometer surface finish, and very low value of
dimensional tolerances.

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Micro Metal Forming issues

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COLD HEADED PARTS

Cold headed parts can be formed in same dimensions and with special
machine equipment even down to wires of 0.3mm in diameter

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two reasons :- surface surface and surface tension

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Nano plastic forming

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Metrology for micro machined components

the available The need for dimensional metrology at micro and nano scale is evident both
measurement in terms of quality assurance of components and products and in terms of
technologies
appear not process control. As critical dimensions are scaled down and geometrical
sufficient As critical complexity of objects is increased, the available measurement technologies
dimensions are
scaled down and appear not sufficient.
geometrical
complexity of
objects is increased

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The generic measurement tasks to be performed in dimensional micro

metrology are
Distance as defined between two surfaces oriented in the same direction. Example: distance

between two lines of a line grating or two planes in a microstructure. • Width as defined by the

distance between two opposing surfaces. Example: width of a channel. • Height as defined by the

distance between two surfaces of same orientation but placed in a vertical direction. Example: depth

of microfluidic channel.

• Geometry (or form) as defined by the distance between the surface of the object and a predefined

reference. Example: flatness of wafer.

• Texture and roughness defined as geometries of surface structures whose dimensions are small

compared to the object under investigation. This poses a particular challenge for micro sized objects

because the surface becomes dominant with respect to object volume.

• Thickness of layers and aspect ratio as defined by the depth of a structure divided by its width.

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Instrumentation for dimensional micro metrology.

The available instrumentation for dimensional metrology at micro

scale can be categorised as follows
• Surface topography measuring instruments (including mechanical,
optical and SPM based instruments).
• Scanning electron microscopes (SEM).
• Micro coordinate measuring machines (micro CMMs).

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The principal methods of surface topography measurement are stylus

profilometry, optical scanning techniques, and scanning probe microscopy (SPM)

These methods, based on acquisition of topography data from point by point

scans, give quantitative information of heights with respect to position. Their
interaction with the surface under investigation as well as their range and
resolution differs significantly.

In a stylus profilometer, the pick-up draws a stylus over the surface at a constant
speed, and an electric signal is produced by the transducer. This kind of
instrument covers vertical ranges up to several millimetres with resolutions as
good as nanometric, with lateral scans up to hundreds of millimetres being
possible. The interaction between tip and surface gives limitations as to
detectable features

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Optical scanning techniques:

It encompass most typically optical profilometers, confocal microscopes, and
interferometers. The optical methods are noncontacting which allows measurements
on soft surfaces. However, this kind of instrument is subject to measurement errors
related to achieving a useful reflection signal from surfaces that are shiny or
transparent to the light source.

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Scanning probe microscopy (SPM)

SPM techniques rely on very low (if any) contacting forces between a very small tip
and the sample, resulting in high lateral and vertical resolution. Typically
measurement ranges are relatively small (few hundred μm horizontal, below 50 μm
vertical).

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Scanning electron microscopy (SEM)

Scanning electron microscopy (SEM) is based on scanning an electron beam on the

specimen. The electrons interact with the sample and different detectors can characterise
physical and chemical properties of the sample surface. Magnification levels cover a wide
range with an extremely large depth of field. However standard SEM operates under
vacuum, which might be seen as a limitation.

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SEM provides nice pictures primarily for qualitative evaluation. However, in a metrology
context SEM is most commonly used in a 2D mode for measurement of for example
critical line widths etc. in semiconductor industry. In this case, calibration is crucial since
the measurement of a 40 nm wide transistor gate requires a bias of less than 4 nm and
repeatability in the sub-nm range. Therefore efforts are undertaken to understand
influence parameters such as edge effects due to electron beam-sample interaction,
electron source tip geometry etc.

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