Jackson M.J. Micro and Nanomanufacturing

Подождите немного. Документ загружается.

Microgrinding 301

least five main players that develop such machines in the world

market, i.e., Moore Nanotechnology Systems, Precitech, Toyoda,

Nachi Fujikoshi, and Toshiba [83]. These machines are available in

2 to 5 axes configurations as shown in Fig. 6.32. Usually, grinding

wheel is attached on a vertical spindle, y-axis, to perform grinding

operation on the workpiece where it is vacuum chucked on the main

horizontal spindle. Depending on the number of axes, this kind of

machine can produce various types of surfaces such as piano, cylin-

ders,

spheres, aspheres and conic sections, Fresnel and diffractives,

freeform and microstructures (Figure 6.33). Applications of these

surfaces include hard disks, photocopier drum, night vision devices

(Figure 6.34), lenses (for camera, charged-couple-device (CCD), CD

and DVD pick up, freeform optics (for laser printers, scanners and

conformal military optics radar system (Figure 6.35) and display for

notebooks/mobile phone, street sign reflectors [83].

-ill

ULG-100A(H)(X,Z)

ti....

ULG-100A(HC){X.Z.C)

ULG.100C{H^){X,Y,2,C)

ULC*100A(HB)(X.Z.B)

1—

[T.i'l.

[15—

—rll

ULG-IOOA!H!(X,Z)

iis—2ii

ULG-lOOAiHY) iX, Z. Y) ULG-IOOCiH-] (X, Y.Z, C.AJ

f G

Fig. 6.32. A wide range of Toshiba models from 2 to 5 axes ultraprecision turn-

ing and grinding machines [86]

302 Micro- and Nanomanufacturing

Fig. 6.33. A variety of optical products made on Toshiba machine (a) spheric and

aspheric lenses (plastic and glass) (b) turning and grinding of molds for spheric

and aspheric lenses (c) toric and free form lens machine [86]

Fig. 6.34. Grinding of a large diameter aspheric lens glass on a Toshiba ULG

lOOA, 2 axis ultraprecision turning and grinding machine

Microgrinding 303

Grinding of mold

for cemented carbide aspherlc lens

Acctffwy (*

form;

maw urn

•Workpiece : Csmpntfld Mfbd^

BProfile ;

i^frB

the fifj.ire,

•Machining conditions

Wnee*

sp;ndi3 spe«J -32000 min

WorK spindis sp*ed 800 min •

Feedrate- 1 mm.'min

Depth

cut "0.5 ii m

Wneei SD25O0,15 mm-co.

Cocian!

• - Yusriifoken CM50

•Machine : ULG-lOOAtHC)

•Machining accuracy

Surface nHcnmeftft; 0,044 am Ry

ifMjift^a)

J^titnBl U.;^ l.J^HMj i~ i.J

-.•fiw -j

fe^

Tikllt • .tfl*

ISiKriS) ''--^'l

Fig. 6.35. A typical operation of Toshiba's

ULG 100 [86]

(a) grinding of tungsten carbide mold for

glass lens

(b) Form accuracy of an aspheric lens on

Taylor Hobson

Precitech's OPTIMUM 2800 (Fig. 6.36) is a high performance,

2-axis, computer controlled, ultra-precision, contouring machine

specifically designed for single-point diamond turning and grinding

of ultraprecision optical components. The machine is built on a natu-

ral granite base and utilizes a pneumatic vibration isolation system.

The hydrostatic oil bearing slideways are constructed in an offset

"T"

configuration where the x-axis (spindle) slide represents the

cross-arm of the "T", and the z-axis (tool holding) slide represents

the stem of the "T"'. Both the x- and z-axes have 200mm of travel.

The work holding spindle is a pneumostatic air bearing design. The

spindle is powered through the use of a brushless DC motor and will

run up to a maximum speed of 3,000 rpm.

The high-speed aspheric grinding system is designed and manu-

factured for use on Precitech OPTIMUM machining systems. This

compact aspheric grinding system utilizes a high-speed, turbine

driven, air-bearing spindle. The air-bearing spindle is mounted on a

304 Micro- and Nanomanufacturing

manually positioned mechanical slide assembly. The slide is

mounted on a fabricated steel column such that the grinding spindle

is positioned in the vertical direction.

The grinding spindle operates over a speed range of 10,000 -

70,000 rpm. The turbine drive provides extremely smooth friction

free spindle rotation. The grinding system accommodates grinding

wheels from 3 mm - 15 mm in diameter. The system has been de-

signed primarily for small aspheric components, particularly lenses

and lens molds up to 30 mm in diameter [87].

Fig. 6.36. (a) Precitech's OPTIMUM 2800 ultraprecision turning and grinding

machine (b) Close view of grinding machine used to study ductile streak forma-

tion [88].

Semi-ductile grinding followed by simple mechanical polishing

is an economical process for producing a mirror-like surface for hard

and brittle aluminoborosilicate glass. A fine grit resinoid-bond

grinding wheel was used by Ong and Venkatesh [88] to generate

large amounts of ductile streaks to improve surface finish and to re-

duce polishing time. The ground samples were polished with differ-

ent slurries on Precitech's OPTIMUM 2800.

Microgrinding 305

Table 6.6. Polishing parameters for polishing aluminoborosilicate glass [88]

Prion's

cocJiicicDl

for

piano

giass

and the

lisihtness

inda of

each pa&lied satnpic

Polishing reagent Thickness removed during

Pofehinj

Wishing

lime,

Rclalivc

ifxjcd, Prcsion Ligtei

index,

(IpgriUizc)

polishing,

dr(tiiri)

pressuic,

(}:Pa)

d/iniini d,t;d!{nir']

cociTicicnl ,<

Cenum oxide

ii23

l^uM AO

Polycrystafce

3M MW 45 im

mylT''

tiiamond

MmMm

m mim 6,5 182/43

1.012x10-^-

li]\

\Mm 3JI3XIO-^^ 25,9!

Fig. 6.37. Moore's Nanotech 500FG ultraprecision machine

306 Micro- and Nanomanufacturing

Fig. 6.38. Close-up of grinding and turning of aspheric lens glass on Moore's

nanotech 500FG ultraprecision turning and grinding machine [89]

Nanotech 500FG (Fig. 6.37), developed by Moore Nanotech-

nogy Systems adopts the microgrinding technique. The machine is

capable of generating arbitrary confocal shapes on materials ranging

from optical glass and infrared materials to non-ferrous metals, crys-

tals,

polymers, and ceramics. The microground surface typically re-

quires little or no post polishing. The machine temperature is main-

tained stable to less than ±0.5^ C. Grinding is done in a flood-cooled

environment.

Namba et al. [8] developed an ultraprecision surface grinder and

succeeded in stabilizing their grinding process by using a spindle ro-

tor made from a low-thermal expansion material. This machine has

two vertical spindles with hydrostatic bearing of high precision and

rigidity. It can machine at extremely fine depth of cut (0.1 |Lim) and

capable of producing at submicron flatness and nanometer surface

Microgrinding 307

roughness (5 nm

Rmax)

on optical glass (NbF), Mn-Zn ferrite and

electronic materials using diamond abrasive grinding wheels.

McKeown et al. [90] of Cranfield Unit for Precision Engi-

neering (CUPE) also developed a 3-axis ultraprecision grinding ma-

chine (Nanocenter) which can perform diamond turning, grinding,

polishing, and capable of measuring complex machined profiles

through 1.25 nm resolution interferometry. They also suggested that

as a rule of thumb, the machine must have static "loop-stiffness" be-

tween tool and workpiece of at least 300 N/|um in order to establish

safe conditions for ductile grinding.

Another example of an ultraprecision grinding machine was de-

veloped by Suzuki and Murakami of Toyoda Machine Works (1995)

for machining non-axisymmetric aspheric mirrors. This is a 5-axis

machine having feedback resolution of 10 nm, and the rotational po-

sitioning is controlled by a laser rotary encoder with resolution of

0.00002^. It is clearly seen that machine rigidity, high dynamic

stiff-

ness,

high thermal stability, precise and smooth feedback resolution

control, ability to achieve fine depth of cut, and the use of special

tooling play a vital role in producing very smooth surfaces under

sub-nanometric level in ultraprecision machining.

6.5.2 Tetrahedral Desktop Machine Tool

The dynamic characteristics of a revolutionary machine tool struc-

ture used for machining engineering components at the micro and

nanoscales are of paramount importance. Minimizing the effects of

vibrations at the micro- and nanoscale is vital because if a machined

workpiece oscillates during the machining process then an increase

in the depth of cut will occur that will reduce the quality of surface

finish and the dimensional accuracy of the machined component.

The stacking of atoms inspired the design of a new platform struc-

ture in order to improve on existing technology, since the goal of the

machine is to allow the manipulation of molecules at the nanoscale.

The idea for the structure comes from the structural stability af-

forded by the tetrahedron. The tetrahedral structure is an extremely

stable structure and it is hypothesized that the shape could minimize

vibrations better than conventional machine tool structures. Vibra-

tions travel through a tetrahedral structure; they are concealed out or

308 Micro- and Nanomanufacturing

minimized due to the interference between the vibrating waves as

they travel through the loops of the structure. The ability to mini-

mize vibrations is needed because if the spindle oscillates during

machining, an increase in the depth of cut will occur, thus reducing

the quality of surface finish, or dimensional accuracy of the ma-

chined part will be reduced significantly. Figure 5.39 shows the tet-

rahedral structure of the machine tool [91].

Fig. 6.39. Tetrahedral machine tool frame [91, 92]

Microgrinding 309

6.5.3 Binderless Wheel

Figure 6.40a indicates the wheel shape, and nominal size of the

wheel. When the wheel is in use it is fitted to a shank as shown in

Fig. 6.40b [57].

' Polycrs'staJUine

diamond laver

5iain

(a) (b)

Fig. 6.40. Binderless diamond grinding wheel shown in (a) wheel shape without

shank and (b) wheel with shank.

The wheel is produced by depositing diamond on a metal, com-

monly carbide substrate. Three types of wheel can be found based

on the type of deposition method applied. Figure (6.41) shows dia-

mond grain structure of each type of wheel. Unlike a bonded wheel,

the diamond layer is only on the top of the substrate surface and can

be very fine grain size whereas the smallest grain in bonded wheel

must be larger than that of

bond.

The maximum grain size can go to

10 |Lim. However, it has a higher density of diamond grains.

The binderless diamond wheel was tested to machine Pyrex

glass,

the silicon die, and packaging chips. Pyrex glass and silicon

die were machined changing feed and depth of cut to investigate

machining mode and surface finish. Ductile streaks were observed

on both surfaces (Fig. 6.42). The achievable surface finish was

found to be 0.1 |Lim. The packaging chip was machined in order to

examine all the six Cu trace layers. Chip packaging machined quite

easily with a binderless wheel revealing Cu layers.

310 Micro- and Nanomanufacturing

Fig. 6.41. SEM micrographs of diamond wheel surface: (a) thermally treated

wheel, (b) chemically treated wheel, (c) chemically treated wheel with diamond

facets [92].

Flatness of the binderless wheel helps planar delayering of the

chip packaging, as seen in Fig. 6.43, without the need for polishing.

Minor defects in horizontal copper traces and major ones in vertical

viaducts that link the Cu traces are evident in these micrographs.

The invention of the binderless diamond-grinding wheel has

made the machining of the silicon die and the chip packing much

easier and more economical (Fig. 6.44). It has the potential of grind-

ing glass and infrared optical materials such as Si and Ge.

(a)

(b)

Fig. 6.42. (a) SEM picture shows elevated partial ductile streaks on optical Pyrex

glass [62], (b) optical micrograph shows abundant ductile streaks on silicon die

[93]