Abrading – Precision device or process - or with condition responsive... – With indicating
Utility Patent
1999-11-12
2001-01-02
Eley, Timothy V. (Department: 3723)
Abrading
Precision device or process - or with condition responsive...
With indicating
C451S005000
Utility Patent
active
06168500
ABSTRACT:
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 use today. It provides versatility in selection of depth and width (kerf) of cut, as we 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 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,
100
a
,
100
b
, . . . formed on its top surface. In order to separate the chips
100
a
,
100
b
, . . . from one another and the wafer
100
, a series of orthogonal lines or “streets”
102
,
104
are cut into he 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.
V
=
α
*
Pn
K
*
l
Eq
.
(
1
)
where, V is the volume of material removed, Pn is the Peak Normal Load, &agr; is a material independent constant, K is a material constant, and l is the cut length. The value of &agr;/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 av 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 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, tie 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 o blade.
According to yet another aspect of the present invention, the current 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.
REFERENCES:
patent: 4942795 (1990-07-01), Linke et al.
patent: 5029418 (1991-07-01), B
Licht Oded Yehoshua
Weisshaus Ilan
Eley Timothy V.
Kulick & Soffa Investments, Inc.
Nguyen Dung Van
Ratner & Prestia
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