Method for measuring saw blade flexure

A system for measuring the flexure of a saw blade in situ includes in one embodiment a non-contact displacement sensor and means for directing a fluid flow at the blade adjacent the sensor. The deflection of the blade by the fluid flow is detected by the sensor which generates a signal related to blade flexure. The system indicates when a saw blade is correctly tensioned or when retensioning is required.

BACKGROUND OF THE INVENTION 
This invention relates generally to semiconductor wafer processing and more 
particularly to the optimization of wafer sawing using an internal 
diameter saw blade. 
In semiconductor wafer preparation the flatness, depth of damage and edge 
perfection of a wafer are dependent upon the degree to which the internal 
diameter saw blade follows an ideal plane as it traverses the wafer, and 
its freedom from out-of-plane vibration. To meet these requirements the 
saw blade must be kept under a constant, high tension. An improperly 
tensioned blade will produce bowed or otherwise defective wafers and the 
useful life of the blade will be diminished, because the blade becomes 
unstable with respect to deviation under the forces of cutting. Further, 
the blade becomes free to vibrate with a greater amplitude than a properly 
tensioned blade. 
Methods and apparatus are known in the art for monitoring blade deviation 
or displacement from a steady-state position as it moves through a 
crystal. However, these systems do not indicate whether the blade requires 
retensioning or merely needs dressing. If the blade is dressed when 
retensioning was the proper course of action, the result will be lower 
wafer quality and yield. 
It is known in the art to measure blade tension by applying a force to the 
blade rim through a force gauge or a balance arm in contact with the blade 
and then reading deflection with a dial indicator. This method, however, 
requires stopping the saw, assembly and calibration of sensitive, 
precision equipment, and numerous measurements by a skilled operator. This 
is both expensive and time consuming. 
SUMMARY OF THE INVENTION 
The present invention provides a rapid, non-contact system and method for 
measuring blade flexure, and thus blade tension, without demounting the 
blade and while the blade is rotating. One embodiment of the present 
invention comprises a non-contact displacement sensor positioned adjacent 
the cutting rim of the saw blade, and means for directing a fluid flow at 
the blade near the sensor location. The fluid flow, for example, 
pressurized air, causes the blade to deflect. The deflection of the blade 
from its normal or steady-state position is detected by the sensor which 
produces a signal responsive thereto corresponding to blade flexure. The 
measured flexure, when compared to the flexure of a properly tensioned 
blade, is used to indicate when retensioning is required. 
It is therefore an object of the present invention to provide an improved 
system and method for measuring saw blade flexure. 
Another object of this invention is to provide a rapid, non-contact system 
and method for measuring saw blade tension. 
Yet another object of this invention is to provide a system and method for 
in situ measurement of saw blade flexure while the blade is rotating.

DETAILED DESCRIPTION OF THE INVENTION 
Referring now to the drawings, there is shown in FIG. 1 an internal 
diameter saw blade 10 for cutting wafers from a semiconductor crystal 12. 
The core 14 of a typical blade 10 has a thickness of about 4-5 mils. A 
cutting edge 16 on the inner portion of blade 10 has a thickness of about 
10-11 mils and comprises, for example, diamond or other abrasive particles 
bonded to the blade. A properly tensioned and dressed blade 10 will cut a 
kerf slot 18 of about 12 mils as it passes through crystal 12. As blade 10 
loses tension through normal usage, the flexure, i.e., the distance the 
blade deflects under a given force, increases and allows the blade to 
vibrate. It is apparent that as the lateral vibration amplitude increases 
the kerf slot 18 widens, which correspondingly decreases the yield or the 
number of wafers that can be cut from crystal 12. 
FIGS. 2 and 3 show one embodiment of a system according to the present 
invention wherein air (indicated by arrow 19) from a pressurized supply 
(not shown) is controlled by a valve 20 which is conected to a nozzle 22. 
Nozzle 22 is positioned a fixed distance, for example 30-50 mils, from 
blade 10 by a support member 24. A displacement sensor 26 is mounted on 
support member 24 adjacent nozzle 22. The optimum spacing of sensor 26 and 
nozzle 22 from blade 10 varies according to the available air pressure, 
the nozzle 22 orifice, the sensitivity of sensor 26, and the blade 10 
characteristics. 
Sensor 26 is preferably a non-contact sensor that generates an electrical 
signal related to the lateral displacement of blade 10. This signal is 
coupled to a meter 28 or other suitable device to provide an indication of 
blade deflection. Such sensing and indicating devices are known in the 
art, for example, the "Dyna-Track" system manufactured by Silicon 
Technology Corporation. With the sensor 26 of this system spaced about 
30-50 mils from blade 10, it has been found that at a pressure of about 60 
pounds per square inch an air flow through a 60 mil nozzle deflects a 
typical blade approximately 0.5 mil when the blade is properly tensioned. 
This deflection is a measure of blade flexure and is inversely related to 
blade tension. As blade 10 loses tension through usage the flexure, and 
thus the measured deflection, will increase. 
In a manual mode of operation an initial reference measurement is made 
after blade 10 has been tensioned by opening valve 20 and reading the 
deflection on meter 28. Subsequent measurements are made at periodic 
intervals and compared to the reference measurement. Retensioning is 
required when the measured deflection exceeds a predetermined value 
corresponding to the minimum permissible blade tension. The present system 
is also advantageously employed during the retensioning operation to 
determine when the proper tension settings are attained, for example, by 
obtaining a reading equal to the reference value. 
It will be readily apparent to those skilled in the art that above 
described system may be automated by including means to initiate a 
measurement at periodic intervals. For example, valve 20 can be opened by 
a solenoid (not shown) coupled thereto responsive to a signal from a 
counter which increments after each saw operation. Blade 10 flexure would 
thus be automatically measured each time a preselected number of wafers 
are cut. Means may also be included to provide an alarm indication when 
the tension lower limit is exceeded, or to disable the saw until 
corrective action is taken. 
The present invention therefore provides a system for rapidly measuring saw 
blade flexure while the blade is installed and rotating. The system may 
readily be automated for periodically measuring the blade flexure and 
providing an indication when the blade tension falls below acceptable 
limits. 
While the present invention has been described and illustrated with respect 
to specific embodiments, it is to be understood that various modifications 
may be made without departing from the spirit and the scope of the 
invention.