Metal fusion bonding – Process – With condition responsive – program – or timing control
Reexamination Certificate
1999-12-22
2001-01-30
Ryan, Patrick (Department: 1725)
Metal fusion bonding
Process
With condition responsive, program, or timing control
C228S004500, C228S180500
Reexamination Certificate
active
06179197
ABSTRACT:
BACKGROUND OF THE INVENTION
This invention relates generally to fine wire bonders employed to make electrical connections between electrodes or pads on semiconductor devices and lead out pads. More specifically, the present invention relates to a novel apparatus for detecting the presence of a bonding wire between the bonding tool and the electrode or pad of the semiconductor device during the bonding process.
DESCRIPTION OF THE RELATED ART
Automatic wire bonders are known in the semiconductor manufacturing industry. A commercially available processor controlled automatic wire bonder is made by Kulicke and Soffa Industries, Inc. and is shown and described in U.S. Pat. No. 4,266,710. The wedge bonding mechanism for an automatic wedge wire bonder also commercially available from Kulicke and Soffa Industries, Inc. is shown and described in U.S. Pat. No. 4,239,144.
Heretofore, it was common practice to assign an attendant to one or more automatic wire bonders. These high-speed wire bonders complete an interconnection of fine wire between a first and second bond position in approximately 250 milliseconds. If the fine wire breaks and/or is not properly fed from the wire feed to the bonding tool, a tail of proper length is not positioned below the working face of the bonding tool to permit a proper bonding operation on the next bond. Numerous problems occur which can cause a wire to be missing from the working face of a bonding tool. In addition, other problems occur which cause the first or second bond to be made improperly or to become disconnected from the pad or terminal on which it is made. If the attendant responsible for an automatic wire bonder is responsible for detecting the numerous errors which can and do occur, a large number of semiconductor devices could be processed or operated on by the automatic wire bonder before the attendant could become aware of the problem and shut the machine down. After making the first error, the bonding tool can continue to attempt to make bonds at the first and second bond position without making a good wire interconnection. If wire is missing under the working face of a bonding tool when the bonder attempts to make a subsequent first and second bond, the bonding tool can crash into the terminal pads on the semiconductor device and/or destroy the lead out pads especially when ultrasonic scrubbing is employed. Ultrasonic scrubbing of pads and terminals with a bare bonding tool will damage the electrodes.
Attempts have been made to monitor the condition of a bond at the time it is being made by an ultrasonically driven bonding tool. The prior art devices have monitored the drive current as well as the impedance of the bonding transducer to determine if the bonds being made by the bonding tool are properly attached to the terminals or pads. Through a complex analysis of the changes in impedance relative to the bonding time, such prior art systems have been able to determine with some accuracy whether the first and/or second bond was properly made. The prior art devices required a first subsystem to detect the presence of the bonding wire and a second subsystem to supply ultrasonic energy to the transducer and the wedge. U.S. Pat. Nos. 4,341,574 and 3,852,999 are typical of systems which measure the impedance of the transducer to determine whether the bonds are properly attached to the terminals or pads.
Heretofore, the conductance of a fine wire interconnection has been measured during a complete bonding operation. The prior art systems employed for monitoring a bonding operation, however, have employed a DC voltage source which has been applied to the fine wire. These prior art systems included some means for measuring current changes in the fine wire. This required that the fine wire be insulated or isolated at the wire feed and that the fine wire be grounded at the semiconductor or the work station. The bonding tool and wire feed were insulated so that the current path from the voltage source was directed through the fine wire to the pad or electrode on the device to ground. Some of these prior art devices required that the conductive path be reversed so that current would flow through the semiconductor device being tested. If the voltage source of the prior art conductive devices could be made stable, changes in the current observed were proportional to the impedance of the fine wire plus the impedance of the device being bonded which includes capacitive and resistive components. These prior art systems would ground the fine wire by making contact to the semiconductor device and by providing a path to ground.
In these prior art systems in which the first bond and the second bond must be grounded. Problems occur in that sensing the interconnection after the first bond becomes extremely difficult using conductive impedance measuring instruments or current sensing devices. Attempts to raise the voltage or current in the fine wire to generate larger current flow so as to provide larger values for detection quite often become harmful to the device and can cause destruction of more sensitive devices. Since some electrodes or pads on the devices are more sensitive than others, it was necessary to shut off or disconnect the monitoring system to prevent damage to these devices. For example, the gates of field effect transistors (FETs) have very high impedance and can be easily destroyed with small amounts of current. Other devices having high impedance also cannot be tested properly due to the very small current changes induced into the prior art conductive monitoring systems. CMOS devices have a very small input capacitance and are easily destroyed if current and voltage sources are raised too high. Other types of devices cannot be tested without reversing the polarity of the current flow. When the device under test presents a high capacitive time constant, the device can continue to charge after the first bond is made and create false indications of bad bonds due to decaying current flow. Any speed-up of detection before the device and circuit is fully charged is a compromise which could easily fail to detect some types of improper fine wire interconnections.
SUMMARY OF THE INVENTION
In view of the shortcomings of the prior art, it is an object of the present invention to monitor the presence of a fine wire at the bond point without the need for a DC voltage source or separate detection and excitation subsystems. Such a monitor would be commercially desirable if it would work with various types of semiconductor devices without the need to change the voltage source for the ultrasonic transducer and it may not be necessary to avoid certain terminals on the semiconductor device. Such a wire bond monitor would be extremely desirable if it was inexpensive, simple and structured so as to be incorporated into existing automatic wire bonders without requiring modifications which would change the mode of operation on automatic wire bonders. Further, it would be desirable if the wire bond monitor would provide a miniaturized structure which could be easily incorporated into the existing automatic wire bonders and would not be affected by thermal changes and vibrations of the bonding transducer or rapid movement of the bonding head.
The present invention is a device for use with a bonding tool for monitoring the presence of a bonding wire, the device comprises generator means for generating an AC excitation signal, coupling means for coupling the AC excitation signal to the bonding tool, sensing means for sensing a first current of the AC excitation signal and a second current from the coupling means, comparing means for comparing the first current and the second current, and generating an output signal, and determining means for determining the presence of bonding wire based on the output signal of the comparing means.
The present invention also relates to a method for detecting the presence of a bonding wire in the bonding tool comprising the steps of generating an AC excitation signal, coupling the AC excitation signal to the bonding tool, sensing a fi
Kulicke & Soffa Investments Inc.
Ratner & Prestia
Ryan Patrick
Stoner Kiley
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