Apparatus and method for wire cutting glass-ceramic wafers

The present invention relates to an apparatus for cutting a mass of material which includes an elongate wire, a first drive for moving the cutting wire along its length through a cutting location, a rotating mass holder for holding and rotating a longitudinally-extending mass of material about its longitudinal axis, and a second drive. The second drive advances the mass holder and/or the wire toward one another to orient the longitudinal axis of the mass of material at the cutting location in a position perpendicular to the wire to cut the mass of material. Other aspects of the invention include a method of cutting, the mass holder per se.

FIELD OF INVENTION 
The present invention relates to a method and apparatus for slicing a mass 
of material into wafers. 
BACKGROUND OF THE INVENTION 
Generally, a magnetic memory storage device includes the following two 
component parts: a head pad and a rigid information disk. The head pad 
supports an element capable of reading or writing data magnetically on the 
information disk. The information disk itself embodies two basic 
components--specifically, a rigid substrate with a coating of magnetic 
media on its surface. 
Today's market for rigid magnetic storage is well established and growing, 
with even greater advances being foreseen through the utilization of thin 
film media technology. Increased information densities, higher disk 
rotation speeds, and lower head flying heights not only afford greater 
efficiencies in data storage and retrieval, but also demand extremely 
tight tolerances to be held in the substrate specifications for flatness, 
rigidity at high rotational velocities, and surface texture. Therefore, 
the substrate must be produced with sufficient surface flatness and 
smoothness so that it can cope with the recent requirement for high 
density recording necessitated by the desire for increased information per 
unit of surface area. Where the product is designed for the high 
performance market, high capacity and rapid access characteristics are key 
requirements. Moreover, the current trend toward small disk drives and 
less powerful motors, particularly for the rapidly developing markets for 
slimline and portable drives, calls for thin, lightweight, rugged disks 
that have high functional densities and area capable of withstanding 
frequent takeoffs and landings with no deterioration in performance. 
Research has been ongoing to discover materials which would satisfy these 
enhanced requirements. Glass substrates, specifically chemically tempered 
glass, have been used in the art. However, this material possesses a 
number of shortcomings which limit its utility. Recently, research has led 
to the development of glass-ceramic materials suitable for use as 
substrates in magnetic memory devices. These suitable glass-ceramic 
materials include glass-ceramics containing lithium disilicate, canasite, 
or fine grained spinel-type crystals. 
An important step in the production of low cost substrates suitable for 
magnetic memory disks is cutting the materials into the desired shapes. As 
discussed above, it is necessary for the cutting method to produce a 
product which is flat and smooth. As is well known, the glass-ceramic 
substrates are used in the form of a thin wafer. The glass-ceramic wafers 
are obtained by slicing a mass of material with a cutting device. 
A variety of saws for slicing brittle materials into wafers have been 
developed. For example, in the semi-conductor industry, it is well-known 
to use annular saws, i.e. I.D. saws; to produce silicon wafers. In this 
method, an internally bladed slicing machine is equipped with a wheel, 
which is a thin plate of stainless steel in annular form and having a 
thickness of a few hundred micrometers. Fine diamond particles of 40 to 60 
.mu.m diameter are electrodeposited on and imbedded in the internal 
periphery of the annular plate to form a cutting blade. A single crystal 
of silicon is cut by putting it under adequate contacting force at the 
diamond blade of the annular plate, which is rotating at a high velocity, 
under tension in the radial direction. As a result, the diamond particles 
grind off the single crystal material to produce a wafer. 
This method has several disadvantages. Firstly, there is likely to be a 
loss of material corresponding to the thickness of the cutting blade. 
Secondly, such a mechanical cutter is likely to form a warped wafer 
surface. Accordingly, subsequent finishing or lapping is required to 
improve the flatness of the wafer. Wire saws were also developed for 
cutting brittle materials. U.S. Pat. Nos. 3,831,576 and 3,841,297 to Mech 
disclose a wire saw for cutting brittle materials such as quartz and 
ceramics. The machine includes a web of wires defining a cutting area 
formed by winding a continuous strand of wire around a number of elongated 
spaced-apart pulleys. The material to be cut is mounted in a fixed 
position on a mounting apparatus and is moved by the mounting apparatus 
into the cutting area for engagement with the wires. A cutting mixture, 
i.e., a slurry containing fine particles of cutting material and a viscous 
carrier, is applied to the cutting area. Similar wire saws are also 
disclosed in U.S. Pat. No. 3,942,508 to Shimizu and U.S. Pat. No. 
4,494,523 to Wells. 
Conventional wire saws suffer from a number of disadvantages. The use of a 
web of wires can produced sliced wafers of variable and inconsistent 
thickness due to wandering of the individual wires as they proceed through 
the material. In addition, such wire movement produces wafers with an 
unsuitable curvature. Further, changes in the tension and speed of the 
wire produce inconsistencies in the surface of the wafer. Accordingly, 
expensive subsequent finishing work, such as lapping, is required to 
improve the flatness and smoothness of the wafer surface. This greatly 
increases the cost of the parts being produced and results in material 
loss. In addition, some waists produced may be curved to such a degree 
that lapping is unable to produce flat surfaces. Another disadvantage of 
wire saws is that low cutting speeds must be used for brittle materials. 
Lastly, wire saws have high operating costs, with the wire and slurry 
generally being the largest cost components. 
The present invention is directed to overcoming these deficiencies. 
SUMMARY OF THE INVENTION 
The present invention relates to an apparatus for cutting a mass of 
material which includes an elongate cutting wire. A first drive moves the 
cutting wire along its length through a cutting location. A rotating mass 
holder holds and rotates a longitudinally-extending mass of material which 
rotates about its longitudinal axis. A second drive advances either the 
mass holder toward the wire, the wire toward the mass holder, or both the 
mass holder and the wire toward one another. This orients the longitudinal 
axis of the mass of material at the cutting location in a position 
perpendicular to the wire to cut the mass of material. 
Another aspect of the present invention relates to a method for cutting a 
mass of material having a longitudinal axis. An elongate cutting wire is 
moved along its length through a cutting location. The method then 
involves advancing either the mass of material toward the wire, the wire 
toward the mass of material, or both the wire and the mass of material 
toward one another. This orients the longitudinal axis of the mass of 
material at the cutting location in a position perpendicular to the wire 
for cutting the mass of material. The mass of material is rotated around 
its longitudinal axis so that it is cut. 
Another aspect of the present invention relates to a work holder for 
rigidly holding material to be cut. The work holder includes a 
longitudinally-extending arbor which has an exterior surface with a 
plurality of pins extending outwardly from the exterior surface. There is 
also a sleeve configured to surround and slidably receive the arbor, and 
having a longitudinally-extending space to receive the plurality of pins. 
The space and pins interact to prevent relative rotation between the arbor 
and the sleeve.

DETAILED DESCRIPTION OF THE DRAWINGS AND THE INVENTION 
The present invention relates to an apparatus for cutting a mass of 
material which includes an elongate cutting wire. A first drive moves the 
cutting wire along its length through a curing location. A rotating mass 
holder holds and rotates a longitudinally-extending mass of material which 
rotates about its longitudinal axis. A second drive advances either the 
mass holder toward the wire, the wire toward the mass holder, or both the 
mass holder and the wire toward one another. This orients the longitudinal 
axis of the mass of material at the curing location in a position 
perpendicular to the wire to cut the mass of material. 
Another aspect of the present invention relates to a method for cutting a 
mass of material having a longitudinal axis. An elongate cutting wire is 
moved along its length through a cutting location. The method then 
involves advancing either the mass of material toward the wire, the wire 
toward the mass of material, or both the wire and the mass of material 
toward one another. This orients the longitudinal axis of the mass of 
material at the cutting location in a position perpendicular to the wire 
for cutting the mass of material. The mass of material is rotated around 
its longitudinal axis so that it is cut. 
Another aspect of the present invention relates to a work holder for 
rigidly holding material to be cut. The work holder includes a 
longitudinally-extending arbor which has an exterior surface with a 
plurality of pins extending outward from the exterior surface. There is 
also a sleeve, configured to surround and slidably receive the arbor, and 
having a longitudinally-extending space to receive the plurality of pins. 
The space and pins interact to prevent relative rotation between the arbor 
and the sleeve. 
FIG. 1 is a schematic view of one embodiment of an apparatus in accordance 
with the present invention. As shown in FIG. 1, wire 100 is discharged 
from feed spool 110, winds around a plurality of guide rollers 130A-D, and 
is received by take-up spool 120. A mass of material M to be cut is 
positioned against and above wire 100 between guide rollers 130C and 130D 
at cutting location J. This is shown in more detail in FIG. 2 which is a 
perspective view of a portion of the apparatus of FIG. 1. In addition, 
FIG. 2 shows that wire 100 is wound around rollers 130A-D a plurality of 
times between the point at which the wire is discharged from feed spool 
110 and received by take-up spool 120. Each point at which wire 100 
contacts/cuts mass M is a cutting location J. The cutting locations in 
FIG. 2 range from J.sub.1 to J.sub.n with n corresponding to the number of 
cutting locations and n+1 corresponding to the number of pieces mass M is 
cut into. It should be noted, however, that the pieces of mass M formed 
outside of locations J.sub.1 and J.sub.n, as a result of cutting, are not 
useful and are discarded. As shown in both FIGS. 1 and 2, mass of material 
M is mounted on mass holder 300 for cutting. Further details concerning 
the mass holder are depicted in FIGS. 5 and 6, discussed infra. 
As shown in FIGS. 1 and 2, mass M and wire 100 have a number of components 
of movement. Mass M can be transversely advanced toward and retracted from 
wire 100, at cutting location J, along arrow A to move the mass into a 
position to be cut or to retract it from such a cutting position, 
respectively. In addition, mass M is mounted for rotation about the 
longitudinal extent of mass holder 300. Preferably, such rotation is in 
the direction shown by arrow B. Alternatively, mass M can be rotated in a 
direction opposite that of arrow B. However, to facilitate cutting, it is 
desirable for the sense and/or magnitude of rotation of mass M, at cutting 
location J, to differ from that of wire 100 at cutting location J. 
Preferably, this is achieved by rotating mass M in a direction defined by 
arrow B, at cutting location J, which is opposite to the direction of 
movement of wire 100 defined by arrow C at cutting junction J. 
As also shown in FIG. 1, it is desirable to apply a slurry S of cutting 
material to wire 100 to facilitate cutting of mass M. The slurry is 
withdrawn from reservoir 150 through feed conduits 160 by pumps 170. Pumps 
170 convey the slurry from discharge conduits 180 to fluid discharge 
manifolds 190. Fluid discharge manifolds 190 apply slurry S to wire 100 
with excess slurry S collecting in reservoir 150 for subsequent use. The 
slurry includes a cutting material made up of relatively fine abrasive 
particles, such as silicon carbide or the like, and a fluid carrier, such 
as glycerine, water, or oil. 
FIG. 3 is a schematic view of another embodiment of an apparatus in 
accordance with the present invention. FIG. 4 is a perspective view of a 
portion of the apparatus of FIG. 3. FIGS. 3 and 4 show sufficient details 
of this embodiment to distinguish it from the embodiment of FIGS. 1 and 2. 
In all other respects, these embodiments are the same and FIGS. 1 and 2 
should be referred to for further details of the apparatus of FIGS. 3 and 
4. 
As shown in FIG. 3, wire 100 is wound around a plurality of guide rollers 
130 A-D. Mass of material M to be cut is positioned against and below wire 
100 between guide roller 130C and 130D. This is contrary to the embodiment 
of FIGS. 1 and 2 where mass M is positioned above wire 100. As shown in 
FIG. 4, wire 100 is wound around rollers 130 A-D a plurality of times. 
Each point at which wire 100 contacts/cuts mass M is a cutting location J. 
The cutting locations in FIG. 4 range from J.sub.1 to J.sub.n with n 
corresponding to the number of cutting locations and n+1 corresponding to 
the number of pieces mass M is cut into. 
As shown in FIGS. 3 and 4, mass M and wire 100 have a number of components 
of movement. Mass M can be transversely advanced toward and retracted from 
wire 100, along the path defined by arrow A, to move the mass into a 
position to be cut or to retract it from such a cutting position, 
respectively. In addition, mass M is mounted for rotation about the 
longitudinal extent of work holder 300 in the direction shown by arrow B. 
Alternatively, mass M can be rotated in a direction opposite that of arrow 
B. However, to facilitate cutting, it is desirable for the sense and/or 
magnitude of rotation of mass M at cutting location J, corresponding to 
arrow B, to differ from that of wire 100 at cutting junction J, defined by 
arrow C. 
Wire 100 typically has a diameter of 0.150 to 0.175 mm. The material used 
to form the wire is typically high carbon steel. 
Wire 100 is wound around the plurality of guide rollers 130A-D a plurality 
of times, up to several hundred times depending on part thickness. Each of 
guide rollers 130A-D has a plurality of grooves in its outer surface for 
aligning the wire 100. 
Both feed spool 110 and take-up spool 120 include a drive shaft (not shown) 
connected to a drive motor (not shown) for rotation thereof. The drive 
motors are generally energized and controlled to maintain a substantially 
constant movement of the wire 100. Generally, such movement is at a wire 
speed of 3 to 10 m/sec. Further, it is desirable to maintain a 
substantially constant tension on the wire between the spools 110 and 120. 
Typically, a wire tension of 20 to 35 Newtons is used with 25 to 30 
Newtons being particularly desirable. 
It may also be desirable periodically to reverse the direction of movement 
of the wire at cutting location J to produce a sawing action. In this mode 
of operation, wire 100 moves continuously from teed spool 110 to take-up 
spool 120 at a substantially constant rate even though the direction of 
movement of the wire 100 is periodically reversed. Thus, new wire is 
continually supplied to the first cutting junction. 
Alternatively, wire 100 can be directed in a one way wire management scheme 
where the wire's direction of movement at the cutting location is not 
reversed. Instead, wire 100 is moved around guide rollers 130A-D in a 
constant direction, and new wire is supplied to the cutting location at a 
constant rate. 
Further details regarding wire saws, albeit having stationary work as 
opposed to oscillating or rotating work, are well known to those of 
ordinary skill in the art. For example, see U.S. Pat. Nos. 3,831,576 and 
3,841,297 to Mech, U.S. Pat. No. 3,942,508 to Shimizu, and U.S. Pat. No. 
4,494,523 to Wells, which are hereby incorporated by reference. 
A preferred embodiment of work holder 300 is shown in FIG. 5. As shown, 
mass holder 300 includes a longitudinally-extending arbor 305 having an 
exterior surface. A plurality of pins 315 extend outwardly from the 
exterior surface of arbor 305. Sleeve 310 has a hollow interior 330 
configured to receive and to surround the exterior surface of arbor 305. 
Sleeve 310 has longitudinally-extending space 320 to receive the plurality 
of pins 315 which extend outwardly from the exterior surface of arbor 305. 
As a result, when arbor 305 is fitted within hollow interior 330 of sleeve 
310, pins 315 prevent relative rotation between arbor 305 and sleeve 310. 
Mass of material M is somewhat rigidly fixed to arbor 305 for rotation by 
use of sleeve 310. Some axial and longitudinal play is present between 
mass of material M and arbor 305. More particularly, mass M has a hollow 
longitudinally-extending shaft H to receive sleeve 310. Mass M is attached 
to sleeve 310 with an adhesive. It is particularly preferred to use a 
two-part epoxy which extends throughout the surface of mass M defining 
shaft H. The diameter of sleeve 310 is less than the diameter of shaft H 
of mass M. 
An example of a system for rotating mass holder 300 is illustrated in FIG. 
6. Mass holder 300 is mounted for rotation by mounting one end of the work 
holder against stationary center 660 by collar 620. Mass holder 300 is 
positioned at stationary center 660. When the embodiment of mass holder 
300 is as shown in FIG. 5, arbor 305 is positioned at stationary center 
660 by collar 620, and sleeve 310 is held in position by set screws 630. 
The opposite end of mass holder 300 is equipped with flange 640 complete 
with plurality of alignment pins 650. A plurality of alignment pins 650 
position mass holder 300 in rotating center 660. Rotating center 610 is 
rotated by a drive (not shown) which can be directly connected to rotating 
center 610. Alternatively, the drive can be connected to rotating center 
through a number of intermeshing gears (not shown). 
Once mass holder 300, including sleeve 310 and arbor 305, are inserted into 
shaft H of mass M, mass M is fixed to mass holder 300 for rotation. It is 
contemplated that this arrangement would allow some relative axial and 
longitudinal movement between sleeve 310 and arbor 305. This is depicted 
in the cross-sectional illustration of FIG. 7. Following cutting of mass 
M, the resulting wafers remain fixed to sleeve 310 by the 
previously-applied adhesive. To remove the wafers, arbor 305 is removed 
from sleeve 310, as illustrated in FIG. 8. The disks are then removed from 
sleeve 310 by pressing toward one another the longitudinally-extending 
surfaces defining space 320 to narrow the circumferential extent of space 
320 (i.e., sleeve 310 is crimped), as illustrated in FIG. 9. By so 
reducing the circumference of sleeve 310, the wafer disks can easily be 
slipped off of sleeve 310. The longitudinally-extending surfaces defining 
space 320 are normally from 0.2 to 0.6 cm apart, preferably 0.3 cm, absent 
crimping. Sleeve 310 must be thin enough so that it can effectively be 
crimped, must not break wire 100 once it cuts through mass of material M 
and contacts the sleeve, and must be thick enough so that wire 100 will 
not cut through the sleeve and reach arbor 305. Sleeve 310 has a thickness 
from 1 to 1.6 mm, preferably 1.5 mm. 
In operation, as shown in FIGS. 1 and 2, mass M is mounted for rotation 
about the longitudinal extent of mass holder 300. Mass M can be of any 
shape; however, it is desirable for mass M to be a cylindrical boule. Mass 
M is moved against wire 100 along the path defined by arrow A, to form a 
cutting location J where wire 100 contacts mass M. Mass M is then cut by 
rotating it along the path defined by arrow B while transversely moving 
wire 100 along the path defined by arrow C. During such cutting, slurry S 
of curing material is withdrawn by pumps 170 from reservoir 150, passed 
through feed conduits 160 and discharge conduits 180, and applied to wire 
100 by spray nozzle 190. Once cutting is completed, mass M can be 
retracted from the cutting position by moving it along the path defined by 
arrow A away from wire 100. The resulting wafers can then be removed from 
sleeve 310. The embodiment of FIGS. 3 and 4 can be operated in a 
substantially similar fashion. 
The apparatus of the present invention is particularly suitable for slicing 
glass-ceramic materials, specifically those glass-ceramics containing 
spinel, lithium disilicate or sappharine crystals, as well as any other 
glass-ceramic material suitable for use as magnetic memory disk 
substrates. 
Glass-ceramics comprised predominately of generally uniformly-sized 
spinel-type crystals uniformly dispersed within a highly siliceous 
residual glass matrix phase are disclosed in greater detail in U.S. patent 
application Ser. No. 08/332,703 to Beall et al., which is hereby 
incorporated by reference. The composition of these glass-ceramic articles 
expressed in terms of weight percent on the oxide basis, includes: 35-60% 
SiO.sub.2, 20-35% Al.sub.2 O.sub.3, 0-25% MgO, 0-25% ZnO, at least about 
10% MgO+ZnO, 0-20% TiO.sub.2, 0-10% ZrO.sub.2, 0-2% Li.sub.2 O, and 0-8% 
NiO. However, if the Al.sub.2 O.sub.3 constituent is present in an amount 
less than about 25%, the TiO.sub.2 +ZrO.sub.2 +NiO amount should be at 
least 5%. The composition may also contain up to 5% of optional 
constituents, such as BaO, CaO, PbO, SrO, P.sub.2 O.sub.5, B.sub.2 
O.sub.5, Ga.sub.2 O.sub.3, 0-5% R.sub.2 O, or 0-8% of a transition metal 
oxide; the R.sub.2 O can be Na.sub.2 O, K.sub.2 O, Rb.sub.2 O or Cs.sub.2 
O. 
Glass ceramics exhibiting a crystal phase assemblage comprised 
predominately of a mixture of lithium disilicate and tridymite uniformly 
interspersed with a residual glass phase and forming an interlocked 
microstructure with the glass are disclosed in U.S. patent application 
Ser. No. 08/265,192 to Beall et al., which is hereby incorporated by 
reference. The composition, expressed in terms of weight percent on the 
oxide basis, includes 75-95% SiO.sub.2, 3-15% Li.sub.2 O, 0-6% Al.sub.2 
O.sub.3, and 0-6% K.sub.2 O. The nucleating agent for this glass-ceramic 
can be 0-0.1% Pd or 0-5% P.sub.2 O.sub.5 ; however, if Pd is absent, the 
P.sub.2 O.sub.5 amount is at least 0.5% and, if P.sub.2 O.sub.5 is absent, 
then the Pd amount is at least 0.005%. Additionally, up to 15% of optional 
ingredients may be added, including B.sub.2 O.sub.3, Na.sub.2 O, ZnO, MgO, 
CaO, SrO, ZrO.sub.2, TiO.sub.2, F, Sb.sub.2 O.sub.3, As.sub.2 O.sub.3, 
PbO, and BaO. 
Glass-ceramics containing a crystal phase assemblage comprised 
predominantly of a mixture of uniformly distributed sapphirine 
(4MgO-5Al.sub.2 O.sub.3 --SiO.sub.2) and .alpha.-quartz crystals 
exhibiting a crystal/grain size of less than about 1000 A are disclosed in 
U.S. patent application Ser. No. 08/332,704 to Beall et al, which is 
hereby incorporated by reference. The precursor glass exhibits a 
composition, expressed in terms of weight percent on the oxide basis, 
includes: 46-52% SiO.sub.2, 23-28% Al.sub.2 O.sub.3, 12-16% MgO, 0.5-3% 
B.sub.2 O.sub.3, 0-5% ZnO, a nucleating agent which can be 5-12% TiO.sub.2 
or 0-5% ZrO.sub.2, and up to 6% of optional ingredients; no more than 3% 
of the optionals may be alkali oxides. 
The glass-ceramic wafers produced by the cutting apparatus of the present 
invention have an exterior surface with a average flatness of 3 to 10 
microns and an axisymmetric shape, having a thicker middle and thinner 
edges. 
The flatness of these wafers can be further improved to an average flatness 
of 1 to 3 microns by lapping the cut wafers. 
EXAMPLES 
Example 1 
Experiments were undertaken to compare the characteristics of a 
conventional wiresaw cutting process with a stationary workpiece versus a 
wiresaw cutting process with a rotating workpiece. The tests were 
performed using a G300 wiresaw manufactured by HCT Shaping Systems SA of 
Switzerland fitted with a workpiece rotator. 
The mass of material being sliced was a glass-ceramic containing canasite 
crystals ("canasite") of a composition as described above. This mass of 
material was formed by melting the desired batch of glass-ceramic 
precursor glass and thereafter forming the melt into a cylindrical boule. 
It was discovered that the differences between a conventional wiresaw 
cutting process with a stationary workpiece and a wiresaw cutting with 
rotating workpiece manifested themselves in the following areas: a) cut 
rate; b) part dimensional variation; c) part shape and subsequent ability 
to be lapped flat; d) slurry usage/piece; e) wire usage/piece; f) surface 
finish; g) sensitivity to alignment; and h) sensitivity to thermal 
effects. Also, sensitivity to rotational speed and direction is discussed. 
Cut Rate 
In a conventional process, the fastest cut rate for canasite in any 
practical, repeatable sense was 120-135 minutes for a round 65 mm diameter 
workpiece with a 19 mm diameter central bore. This was at a wire tension 
of 30N and a wire speed of 10 m/s. The range of cut times reflect the 
various slurry conditions which were used during the experiment. In all 
cases, the slurry included SiC grit of 13-15 .mu.m mean particle size and 
an oil carrier (as opposed to water). 
In wiresaw cutting with rotating workpiece, under the same process 
conditions and with the same slurry, cut times for the same product were 
consistently 60-70 minutes. 
Part Dimensional Variation 
In a conventional process, dimensions of the sliced wafer, as characterized 
by total thickness variations ("TTV") and flatness of each side, are 
strongly influenced by slurry age and cut rate. At the process parameters 
defined above, TTV and flatness of the wafers showed continual degradation 
with slurry life and the distribution of TTV and flatness for wafers 
created within a run always had "fliers," i.e., wafers which skewed the 
distribution. These fliers strongly influenced the standard deviation 
values which characterize the distributions. The presence of fliers and 
the characteristic shape of the distributions typify cutting canasite with 
the wiresaw process, regardless of general process conditions. Typical 
values of TTV and flatness at various points over the life of a batch of 
slurry are shown in Table 1 below. 
TABLE 1 
______________________________________ 
SLURRY AGE 
TTV .+-. .sigma.(.mu.m) 
AVG. FLATNESS .+-. .sigma.(.mu.m) 
______________________________________ 
New 18 .+-. 6 24 .+-. 9 
1000 PCS 31 .+-. 19 42 .+-. 23 
2000 PCS 42 .+-. 31 36 .+-. 13 
______________________________________ 
Numbers were generated using a 12 point Linear Variable Differential 
Transducer ("LVDT") mechanical gauge. 
In wiresaw cutting with a rotating workpiece, sliced wafers have TTV and 
flatness distributions of an entirely different character. The 
distributions were tighter, and fliers were typically not found. The few 
fliers that were observed had an assignable cause. In addition, no 
degradation of TTV and/or flatness was observed with slurry life under any 
conditions tested. Data for TTV and flatness using wiresaw slicing with 
rotation is shown in Table 2 below. 
TABLE 2 
______________________________________ 
SLURRY AGE 
TTV .+-. .sigma.(.mu.m) 
AVG. FLATNESS .+-. .sigma.(.mu.m) 
______________________________________ 
New 8 .+-. 3 8 .+-. 3 
1000 PCS 10 .+-. 2 6 .+-. 2 
2000 PCS 11 .+-. 3 4 .+-. 2 
3000 PCS 11 .+-. 6 6 .+-. 7 
4000 PCS 11 .+-. 5 6 .+-. 2 
5000 PCS 13 .+-. 4 10 .+-. 7 
______________________________________ 
Numbers were generated using a 12 point Linear Variable Differential 
Transducer ("LVDT") mechanical gauge. 
Part Shape and Subsequent Ability to Be Lapped Flat 
In a conventional process, variation of surface topography is attributable 
to wire wander during cutting, as well as to wire wear. Consequently, most 
surfaces produced vary in the direction of cut but not parallel to the 
wires. This produces a "potato chip" shape, often with a dramatic entrance 
or exit effect. This shape gives the part compliance under the pressure of 
lapping and makes it difficult to lap these parts flat without undue stock 
removal. 
In wiresaw cutting with a rotating workpiece, parts are characterized by an 
axisymmetric shape which is to be expected considering the nature of the 
process. Parts are typically thin on the edge and thicker towards the 
middle. FIGS. 10A and B, respectively, illustrate a top and perspective 
view of a Tropel measurement. A Tropel measurement is an interferometric 
scan from which flatness of the wafer can be deduced. The Tropel 
measurement of a wafer produced by this process shows that the wafer has a 
thicker middle and thinner edges. Post-lap measurements with TROPEL for 
conventional and rotated parts produced the following results: 
Post-lapped flatness (conventional): 4-8 .mu.m 
Post lapped flatness (wiresaw cutting with rotation): 1-2 .mu.m 
Slurry Usage 
In a conventional process, as shown above in Table 1, TTV and flatness fall 
off as the slurry ages. The slurry life depends upon part requirements for 
lapping. 
In wiresaw cutting with a rotating workpiece, since it was shown that 
dimensions are not affected by slurry life, the slurry life depends upon 
what can be tolerated relative to cut rate degradation and cleanliness 
requirements. This makes slurry life comparisons between conventional 
wiresaw cutting and wiresaw cutting with a rotating workpiece difficult, 
if not impossible. For particular glass-ceramic products, wiresaw cutting 
with a rotating workpiece produced a product which could be successfully 
lapped to the required flatness of 1-2 .mu.m. The conventionally-sliced 
product could not be lapped to this flatness. 
Wire Usage 
Wire usage for the two processes can be compared directly with reasonable 
precision by running to the same exit wire diameters for each process. 
Observations show the following wire usage per piece for each process. 
Conventional=50 meters 
Wiresaw Cutting with a Rotating piece=35 meters 
Surface Finish 
In a conventional process, the nature of the surface finish depends upon 
the wire management scheme during the cutting process. One way wire 
movement is capable of producing a relatively homogeneous surface but 
shows the effect of wire wander where there are sudden changes in slope on 
the surface. Back and forth wire movement produces a series of discrete 
grooves corresponding to each change in wire direction. The groove depth 
is typically a few microns. A graph depicting the surface smoothness 
across these grooves produced by a conventional process with back and 
forth wire movement is shown in FIG. 11. The graph indicates a surface 
roughness measurement, by measuring between wafer surface peaks and 
valleys, of 4 to 5 micrometers. 
In wiresaw cutting with a rotating workpiece, a homogeneous lapped surface 
can be produced with back and forth wire movement, for boule RPM's of &gt;3. 
A surface roughness of 12.68 microinches (0.3 micrometers) was measured 
canasite (FIG. 12) using the Taylor-Hobson technique. This can be compared 
to the surface roughness depicted in FIG. 11, i.e., the surface roughness 
of a wafer produced by the conventional process, which has a surface 
roughness measurement an order of magnitude greater as compared to wiresaw 
cutting with a rotating workpiece. 
Sensitivity to Boule Alignment 
The conventional process has no particular requirement that the workpiece 
be aligned to the wire. 
For wiresaw cutting with a rotating workpiece, there is a requirement that 
the axis of boule rotation be perpendicular to any given wire in the web 
as well as within a plane parallel to the web as a whole. However, 
misalignment in either of these planes does not show up in part geometry 
in a one to one correspondence. Intentional misalignment of the web 
relative to the boule axis produces a taper in the part approximately an 
order of magnitude less than would be expected by pure geometrical 
considerations. For example, a 1 mm misalignment over the 250 mm spacing 
of the wire guide would be expected to produce a 0.26 mm taper over a 65 
mm diameter part. The observed taper is approximately 0.025-0.030 mm. 
Sensitivity to Thermal Effects 
In conventional wiresaw cutting, if the machine shuts down for any reason 
long enough tier it to cool down and thermally shift, the cut will reflect 
this as a surface step or irregularity, thereby mining the part. Wiresaw 
cutting with a rotating workpiece, however, is forgiving enough that such 
thermal upsets have no effect on the product. In some cases, the wiresaw 
cutting machine was shutdown due to mechanical problems. When the 
workpiece was removed and reinstalled several days later, the cut was 
completed successfully with measured dimensions similar to those observed 
in Table 2. 
Sensitivity to Boule Rotational Speed and Direction 
No differences in process or part quality was observed for boules rotating 
up to 60 RPM and either co- or counter-rotational wire direction; except, 
there was a slight suggestion of concentric rings over the surface when 
the RPM was less than 3 and the wire was moved back and forth. 
Example 2 
Experiments were run cutting various glass-ceramic materials with a wiresaw 
having a rotating workpiece. Data of surface flatness and TTV is shown in 
Table 3 below. 
TABLE 3 
______________________________________ 
MATERIAL TTV .+-. .sigma.(.mu.m) 
FLATNESS .+-. .sigma.(.mu.m) 
______________________________________ 
Composition 1 
10 .+-. 2 4 .+-. 2 
(CERAMMED) 
Composition 2 
6 .+-. 2 4 .+-. 2 
(CERAMMED) 
Composition 3 
20 .+-. 3 12 .+-. 5 
(GLASS) 
Composition 3 
18 .+-. 2 9 .+-. 1 
(CERAMMED) 
Composition 4 
16 .+-. 2 7 .+-. 1 
(CERAMMED) 
Composition 5 
6 .+-. 2 3 .+-. 2 
Composition 6 
8 .+-. 2 4 .+-. 3 
______________________________________ 
Composition 1 is a glass-ceramic containing lithium disilicate. 
Composition 2 is a glass-ceramic containing spinel crystals of the 
composition 46.9% SiO.sub.2, 24.8% Al.sub.2 O.sub.3, 1-8% ZnO, 15% MgO, 
and 11.5% TiO.sub.2. 
Composition 3 is a glass-ceramic containing sappharine crystals of the 
composition 49.4% SiO.sub.2, 24.5% Al.sub.2 O.sub.3, 14% MgO, 7% 
TiO.sub.2, 3.5% ZrO.sub.2, 0.5% As.sub.2 O.sub.5, and 1.4% B.sub.2 
O.sub.3. 
Compositions 4 and 5 are glass-ceramics containing spinel crystals. 
Composition 6 is a glass-ceramic containing 39.4% SiO.sub.2, 30.7% Al.sub.2 
O.sub.3, 9.85 TiO.sub.2, 8.22% MgO, 10.5% BaO, 0.14% SrO, and 1.11% ZnO. 
Although the invention has been described in detail for the purpose of 
illustration, it is understood that such detail is solely for that 
purpose, and variations can be made therein by those skilled in the an 
without departing from the spirit and scope of the invention which is 
defined by the following claims.