Monitoring system for dicing saws

A method and apparatus for accumulating dicing data for process analysis, monitoring process stability and cut quality in a substrate. The apparatus has a spindle motor with a blade attached to the spindle motor. A spindle driver is coupled the spindle to drive the spindle at a predetermined rotation rate. A sensor is connected to the spindle motor to determine the rotation rate of the spindle. A controller is coupled to the monitor in order to control the spindle driver responsive to the load induced on the blade by the substrate.

FIELD OF THE INVENTION 
This invention relates generally to saws of the type used in the 
semiconductor and electronics industry for cutting hard and brittle 
objects. More specifically, the present invention relates to a system for 
monitoring the performance and parameters of a high speed dicing saw 
during cutting operations. 
BACKGROUND OF THE INVENTION 
Die separation, or dicing, by sawing is the process of cutting a 
microelectronic substrate into its individual circuit die with a rotating 
circular abrasive saw blade. This process has proven to be the most 
efficient and economical method in use today. It provides versatility in 
selection of depth and width (kerf) of cut, as well as selection of 
surface finish, and can be used to saw either partially or completely 
through a wafer or substrate. 
Wafer dicing technology has progressed rapidly, and dicing is now a 
mandatory procedure in most front-end semiconductor packaging operations. 
It is used extensively for separation of die on silicon integrated circuit 
wafers. 
Increasing use of microelectronic technology in microwave and hybrid 
circuits, memories, computers, defense and medical electronics has created 
an array of new and difficult problems for the industry. More expensive 
and exotic materials, such as sapphire, garnet, alumina, ceramic, glass, 
quartz, ferrite, and other hard, brittle substrates, are being used. They 
are often combined to produce multiple layers of dissimilar materials, 
thus adding further to the dicing problems. The high cost of these 
substrates, together with the value of the circuits fabricated on them, 
makes it difficult to accept anything less than high yield at the 
die-separation phase. 
Dicing is the mechanical process of machining with abrasive particles. It 
is assumed that this process mechanism is similar to creep grinding. As 
such, a similarity may be found in material removal behavior between 
dicing and grinding. The theory of brittle material grinding predicts 
linear proportionality between material removal rate and power input to 
the grinding wheel. The size of the dicing blades used for die separation, 
however, makes the process unique. Typically, the blade thickness ranges 
from 0.6 mils to 50 mils (0.015 mm to 1.27 mm), and diamond particles (the 
hardest known material) are used as the abrasive material ingredient. 
Because of the diamond dicing blade's extreme fineness, compliance with a 
strict set of parameters is imperative, and even the slightest deviation 
from the norm could result in complete failure. 
FIG. 1 is an isometric view of a semiconductor wafer 100 during the 
fabrication of semiconductor devices. A conventional semiconductor wafer 
100 may have a plurality of chips, or dies, 100a, 100b, . . . formed on 
its top surface. In order to separate the chips 100a, 100b, . . . from one 
another and the wafer 100, a series of orthogonal lines or "streets" 102, 
104 are cut into the wafer 100. This process is also known as dicing the 
wafer. 
Dicing saw blades are made in the form of an annular disc that is either 
clamped between the flanges of a hub or built on a hub that accurately 
positions the thin flexible saw blade. As mentioned above, the saw blade 
employs a fine powder of diamond particles that are held entrapped in the 
saw blade as the hard agent for cutting semiconductor wafers. The blade is 
rotated by an integrated DC spindle-motor to cut into the semiconductor 
material. 
Optimizing the cut quality and reducing process variation requires an 
understanding of the interaction between the dicing tool and the material 
(substrate) to be cut. The most accepted model for material removal by 
abrasion is described in Wear Mechanisms in Ceramics, A. G Evans and D. B 
Marshal, ASME Press 1981, pp. 439-452. This model predicts the threshold 
load that must be applied by the abrasive grain to cause fracture of the 
brittle ceramic. The cracks create localized fracture in the material in 
predicted directions. Material is removed as particles when some of the 
cracks join in three dimensions. The Evans and Marshall model predicts the 
linear relation between the volume of material removed by an abrasive 
particle and the load exerted by this particle according to the following 
equation. 
##EQU1## 
where, V is the volume of material removed, Pn is the Peak Normal Load, 
.alpha. is a material independent constant, K is a material constant, and 
1 is the cut length. The value of .alpha./K is in the range of 0.1 to 1.0. 
Assuming formula reciprocity, it follows that the measured load should have 
a linear relationship to the material removed. In other words, if a known 
volume of material is removed, then the abrasive cutting wheel has exerted 
a known load on the substrate. 
According to Grinding Technology, S. Malkin, Ellis Horwood Ltd., 1989, pp. 
129-139, a high percentage of mechanical energy input turns into heat 
during the abrasive process. Excessive heat generation due to friction, 
which may be observed as deviation from the linear relationship between 
material removal and load, can cause damage to the workpiece and/or dicing 
blade, possibly resulting in destruction of one or both. 
Prior art systems for monitoring dicing operations rely on visual means for 
determining the quality of the cut in the substrate. These prior art 
systems have the drawback that the cutting process must be interrupted in 
order to visually inspect the kerfs. Furthermore, only short sections of 
the cut are evaluated in order to avoid the excessive time requirements 
for a 100% inspection. The results of the short section inspection must be 
extrapolated in order to provide full evaluation. In addition, these 
visual systems only allow for the inspection of the top surface even 
though the bottom surface is also subject to chipping. Therefore, 
evaluation of the bottom of the semiconductor wafer must be performed 
off-line. That is, by stopping the process and removing the wafer from the 
dicing saw to inspect the bottom surface of the wafer. 
There is a need to monitor blade load during wafer or substrate dicing for 
optimizing the dicing process and maintaining a high cut quality so as not 
to damage the substrate, often containing electronic chips valued in the 
many thousands of dollars. There is also a need to perform monitoring over 
the entire length of the cut and to avoid the need for interrupting the 
process during the monitoring. 
SUMMARY OF THE INVENTION 
In view of the shortcomings of the prior art, it is an object of the 
present invention to help optimize the dicing process and monitor the 
quality of the kerfs placed in a substrate by non-visual means. 
The present invention is a dicing saw monitor for optimizing the dicing 
process and monitoring the quality of kerfs cuts into a substrate. The 
monitor has a spindle motor with a blade attached to the spindle motor. A 
spindle driver is coupled the spindle motor to drive the spindle at a 
predetermined rotation rate. A sensor is connected to the spindle motor to 
determine the rotation rate of the spindle. A controller is coupled to the 
monitor in order to control the spindle driver responsive to the load 
induced on the blade by the substrate. 
According to another aspect of the invention, the controller automatically 
controls at least one of the speed of the spindle, the feed rate of the 
substrate, the cutting depth and a coolant feed rate in response to the 
load placed on the blade. 
According to still another aspect of the invention, the load on the blade 
is measured based on the current required to maintain a predetermined 
rotation rate of the blade. 
According to yet another aspect of the present invention, the current or 
voltage of the spindle motor is measured periodically. 
According to a further aspect of the present invention, a display is used 
to display a variety of conditions of the dicing saw in real-time. 
These and other aspects of the invention are set forth below with reference 
to the drawings and the description of exemplary embodiments of the 
invention.

DETAILED DESCRIPTION 
In the manufacture of semiconductor devices, individual chips are cut from 
a large wafer using a very high speed rotating saw blade. In essence, the 
saw blade grinds away a portion of the wafer along linear streets or kerfs 
(102, 104 as shown in FIG. 1) in one direction followed by a second 
operation in an orthogonal direction. 
The quality of the chips is directly related to the minimization of 
chipping during the dicing operation. The inventors have determined that 
changes in the load on the saw blade-driving spindle cause predictable 
correlated changes in the electrical current to the motor. These changes 
may be displayed in real-time to the operator such that required 
adjustments can be made without interrupting the dicing process. 
Referring to FIG. 2, an exemplary embodiment of the present invention is 
shown. In FIG. 2, monitor 200 includes spindle motor 202 coupled to saw 
blade 204 through shaft 203. Current provided by spindle driver 206 drives 
spindle motor 202 at a rate of between about 2,000 RPM and about 80,000 
RPM. The rotation of the spindle motor 202 is monitored by RPM sensor 208 
which, in turn, generates an output 209 representative of the rotation 
rate of spindle motor 202 to summing node 218. In turn, the summing node 
218 provides a control signal 219 to spindle driver 206 to control the 
rotation of spindle motor 202 such that the spindle motor rotates at a 
substantially constant speed. 
Spindle motor 202 generates feedback current 211 which is monitored by load 
monitor 210. The load monitor 210 periodically determines the feedback 
current at a rate of between about 10 Hz and 2500 Hz, as desired. The 
output 213 of load monitor 210 is connected to control logic 212. Control 
logic 212 also receives process parameters 214. These process parameters 
214 may be based on historical data gathered from similar dicing 
processes, for example. Optionally, the control logic 212 generates 
control signals 215 which are combined with output 209 of RPM sensor 208 
at summing node 218. Summing node 218 operates on these signals and 
provides signal 219 to control spindle motor 202 based on the process 
parameters 214, the real-time information from load monitor 210 and the 
rotation rate of spindle motor 202 as defined by output 209 of RPM sensor 
208. 
Control logic 212 may also include a filter to determine an RMS value for 
each of the cuts produced by the blade in the substrate. In addition, 
control logic 212 may also generate signals for display on display monitor 
216. The displayed information may include several parameters, such as 
present spindle motor speed, cutting depth, blade load, substrate feed 
rate, coolant feed rate, and the process parameters 214. The display may 
also provide information related to processes to follow, such as 
information received from other process stations which may be connected to 
the dicing saw monitor via a network, for example. The displayed 
information and process parameters may be retained in a memory as part of 
control logic 212 or in a external memory, such as a magnetic or optical 
media (not shown). 
Referring to FIG. 3, the exemplary load monitoring principle is shown. In 
FIG. 3, blade 204 rotates at a rate Vs while substrate 300 is feed into 
blade 204 at a rate Vw. A cutting force (F) 302 is exerted by the blade 
204 on substrate 300. Cutting force 302 is proportional to the load on the 
spindle 203 (shown in FIG. 2) which, in turn, is proportional to the 
current consumption of spindle motor 202 required to maintain the 
rotational rate Vs. 
Using this model the inventors have determined through simulations that the 
load on the blade 204 is related to the feedback control current 211 
according to the following equation: 
##EQU2## 
where, Load is measured in grams, FB is the feedback control current in 
amps, VS is the spindle speed in KRPM, Lsim is the simulator disk radius, 
and Lblade is the blade radius. As one of ordinary skill in the art 
understands, FB may also be measured in volts as current and voltage are 
proportional to one another according to Ohm's law. 
The amount of material removed M from the wafer during dicing operations is 
measured according to the following equation: 
EQU M=D*W*FR (3) 
Where, D is the blade cut depth, W is the kerf width, and FR is the feed 
rate of the wafer into the blade. 
To test the material removal rate, the inventors performed a series of 
experiments according to Table 1. 
TABLE 1 
______________________________________ 
Limits Cut Depth Blade Thickness 
Feed Rate 
______________________________________ 
Low 0.002 in. 0.001 in. 2.0 in./sec. 
(0.05 mm) (0.025 mm) (50.8 mm/sec) 
High 0.020 in. 0.002 in. 3.0 in./sec. 
(0.5 mm) (0.05 mm) (76.2 mm/sec) 
______________________________________ 
The tests were performed eight times using silicon wafers. During the 
tests, one factor (D, W, or FR) was kept constant while the other factors 
varied. For example, the spindle speed was kept constant and the cut depth 
was changed at increments of 0.002 in. The results of the tests are shown 
in FIG. 4. As shown in FIG. 4, the test points 402 are plotted for the 
various series of tests. The different symbols shown 
(.tangle-solidup.,.box-solid.,.smallcircle.,.quadrature.,etc.) each 
illustrate a separate test run. The result of these test runs is an 
essentially straight-line plot supporting the hypothesis presented above 
in Eq. 3. Although the tests were performed as outlined above in Table 1, 
in normal process operations, the cutting depth may as deep as about 0.5 
in. (12.7 mm) or more depending on the particular process. 
FIG. 5 is a graph of RMS load above baseline vs. Feedrate of the wafer with 
respect to the blade. In FIG. 5, the following parameters were used: 
Spindle speed--30,000 RPM 
Blade thickness--0.002 in. 
Wafer type--6 in. blank 
Coolant flow--main jet 1.6 l/min 
Cleaning--jet 0.8 l/min 
Spray bars--0.8 l/min. 
In FIG. 5, plot 500 is the material removal load versus the feedrate of the 
substrate as measured on the blade. As shown in FIG. 5, it was found that 
as the feedrate exceeded approximately 3.0 in./sec (78.6 mm/sec) there is 
a departure from the expected linear behavior as illustrated by points 
502. Therefore, in order to maintain the desired linear material removal 
rate (which has a direct bearing on chipping at the bottom portion of the 
substrate during dicing operations) one process parameter that may be 
controlled is the feedrate of the wafer. The feed rate may vary, as 
desired, between about 0.05 in/sec (1.27 mm/sec) to about 20.0 in/sec (508 
mm/sec) depending on the type of material being cut and the condition of 
the blade. 
FIG. 6 is a graph illustrating blade load during cutting operations. In 
FIG. 6, graph 600 is a plot of load measured in Volts RMS versus cuts 
placed in the wafer. As shown in FIG. 6, portions 602, 604, 606 of graph 
600 indicate a reduction in blade load as compared to portions 608, 610. 
This is due to the circular nature of the wafer in that the first and last 
few cuts 102, 104 in any given direction of the wafer 100 (shown in FIG. 
1) are short. As a result, the cuts 102, 104 begin and end in the tape 
(not shown) that is used to mount the wafer 100 and the amount of material 
removed from the wafer 100 is low which, in turn, are indicated as a lower 
blade load. 
In FIG. 6, the diameter of the wafer is approximately 6 in. (152.4 mm) and 
the cut index is 0.2 in. (5.08 mm). Therefore, at about cut 30 the end of 
the wafer is reached for the first series of cuts resulting in reduced 
blade load. Similarly, as the second series of cuts are performed in the 
second direction in the wafer (usually orthogonal to the first series of 
cuts), the first cuts and last cuts are detected as reduced blade loads 
604 and 606, respectively. Therefore, the exemplary embodiment may also be 
used to determine when the end of a wafer is reached based on the reduced 
load on the blade when compared to the expected end of the wafer. In 
addition, if the blade load is too low at a point where the end of the 
wafer is not expected, this may indicate a process failure requiring 
attention of the operator. In this case the operator may be alerted to the 
situation by a visual and/or audible annunciator. If desired, the process 
may also be halted automatically. 
FIG. 7 is another graph illustrating blade loading during dicing 
operations. In FIG. 7, the ordinate is a measure of load voltage above a 
predetermined baseline. The baseline may be determined from theoretical, 
historical or experimental data, for example. As shown in FIG. 7, the load 
above baseline is low for the first few cuts 702, and the last few cuts 
704. The load increases as the cuts progress across the wafer to a maximum 
load 706. The exemplary embodiment monitors the feedback voltage (which is 
directly related to current according to Ohm's law) and may alert the 
operator or change a parameter of the operation, such as feed rate or cut 
depth, if the feedback voltage attains or exceeds a predetermined 
threshold 708. The inventors have found that bottom chipping of the wafer 
is directly related to the load exceeding a desired value. Therefore, by 
monitoring the feedback voltage the exemplary embodiment of the present 
invention is also able to determine chipping of the wafer without the 
necessity of stopping the process to remove the wafer so as to perform a 
visual inspection of the bottom of the wafer. Furthermore, excessive load 
may indicate blade damage or wear which may negatively affect the 
substrate. 
Although the invention has been described with reference to exemplary 
embodiments, it is not limited thereto. Rather, the appended claims should 
be construed to include other variants and embodiments of the invention 
which may be made by those skilled in the art without departing from the 
true spirit and scope of the present invention.