Abstract:
A method and apparatus for monitoring the presence of a bonding wire in a wire bonding machine. The apparatus generates an AC signal which is coupled to the bonding tool. A sensor senses a current of the AC signal flowing to the bonding tool and a current of the AC signal returned from the bonding tool. The current flows are compared to one another to determine the presence or absence of the bonding wire in the bonding tool.

Description:
This application is a division of U.S. patent application Ser. No. 09/097,792, filed on Jun. 16, 1998, now U.S. Pat No. 6,039,234. 
    
    
     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 first current representative of the AC excitation signal and a second current representative of a return signal from the coupling means, and determining the presence of the bonding wire based on the first and second currents. 
     According to another aspect of the invention, the comparison of the first current and second current generates a warning signal indicating that the bonding wire is absent from the bonding tool. 
     According to still another aspect of the invention, the comparison of the first current and second current automatically halts the operation of the bonding tool when the bonding wire is absent from the bonding tool. 
     According to yet another aspect of the present invention, the AC excitation signal is used as the ultrasonic bonding signal for the ultrasonic transducer and as the reference signal for determining the presence of the bonding wire in the bonding tool. 
    
    
     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. 
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The invention is best understood from the following detailed description when read in connection with the accompanying drawing. It is emphasized that, according to common practice, the various features of the drawing are not to scale. On the contrary, the dimensions of the various features are arbitrarily expanded or reduced for clarity. Included in the drawing are the following Figures: 
     FIG. 1 is a block diagram of an exemplary embodiment of the present invention; 
     FIG. 2 is a partial block diagram of the exemplary embodiment of the present invention; 
     FIG. 3 is a partial schematic diagram of a second exemplary embodiment of the present invention; and 
     FIG. 4 is a flow chart of an exemplary embodiment of the present invention. 
    
    
     DETAILED DESCRIPTION 
     In the manufacture of semiconductor devices, individual circuits or die are interconnected with a substrate, such as a chip package, using very thin wires. One method of attaching these wires to the pads of the die and the pads of the chip package is by using an ultrasonic bonder. The ultrasonic bonder uses high frequency energy to scrub the bonding wire on the pad which effectively “welds” the wire to the pad. If the bonding wire is absent when the pad is “scrubbed” the pad my be irreparably damaged. 
     Referring to FIG. 1, an exemplary embodiment of the present invention is shown. In FIG. 1, semiconductor device  100  is placed on an isolated workstation  102 . As part of the manufacturing process, bonding wire  108  is to be connected between pad  104  of semiconductor device  100  and a pad of a package or substrate (not shown). To this end, bonding wire  108  is supplied by supply reel  110  and bonded to pad  104  by wedge  106  of ultrasonic transducer  124 . Workstation  102  is isolated from ground or any other reference so that electrical energy present at wedge  106  of the bond head (not shown) is not conducted through semiconductor device  100 . 
     Ultrasonic transducer  124  has an ultrasonic element  125 , such as a piezoelectric element for example, contained therein. An AC excitation signal is supplied by ultrasonic generator  116  to input  126  of ultrasonic element  125 . The excitation signal has a frequency of between 50 to 200 KHz, and more preferably about 60 KHz or about 120 KHz. The excitation signal extends and contracts ultrasonic transducer  124  in the longitudinal direction through ultrasonic element  125  which in turn moves wedge  106  across the surface of pad  104  with bonding wire  108  between pad  104  and wedge  106 . The frequency an which the wedge  106  is scrubbed across pad  104  creates localized heating which in turn transfers molecules between the pad and bonding wire  108  resulting in an effective bond. 
     The excitation signal  117  from ultrasonic generator  116  is amplified by amplifier  118  and provided to input  126  of ultrasonic element  125  via resistor  120  as excitation signal  136 . Voltage  122  across resistor  120  represents the current drawn from amplifier  118  of excitation signal  136 . The value of resistor  120  is selected such that an accurate measurement of current is achieved without unnecessarily reducing the level of the excitation signal. It is contemplated that the value of resistor  120  may be between about 0.1 ohms and 10 ohms with a preferable value of about 2 ohms. Excitation signal  136  is conducted to both ultrasonic element  125  and wedge  106 . A portion of excitation  136  is provided at output  128  of ultrasonic element  125  and coupled to one end of resistor  130  through return path  138 . The other end of resistor  130  is connected to ground reference  132 . The voltage across resistor  130  represents the current returned from ultrasonic element  125  based on excitation signal  136 . The value of resistor  130  may be selected to be about the same value of resistor  120 . Ultrasonic transducer  124  is also isolated from any ground reference so that current from excitation signal  136  is not drained which may result in erroneous measurements through return path  138 . 
     As mentioned above, excitation signal  136  is also conducted to wedge  106 . As the ultrasonic transducer  124  starts the bonding process, wedge  106  is lowered to contact bonding wire  108  and pad  104 . If bonding wire  108  is present, a conductive path exists between wedge  106  and ground reference  114  through bonding wire  108  and clamp  112 . This conductive path reduces the portion of excitation signal  136  returned via return path  138 . This reduction in the current of the returned excitation signal is measured as voltage  134  across resistor  130 . If the voltage  134  is less that voltage  120  by about 15% the presence of bonding wire  108  is confirmed. This percentage may vary based on factors such as signal to noise ratio. 
     On the other hand, if bonding wire  108  is not present, the conductive path to ground reference  114  is interrupted. In this case nearly all of excitation signal  136  (other than that dissipated by ultrasonic element  125 ) is returned to ground reference  132  through return path  138 . This increased signal is measured as voltage  134 . The absence of bonding wire is determined if voltage  134  is within about 15% of the voltage  120 . In this way the current inherent in excitation signal  136  is used to determine the presence of bonding wire  108 . 
     Referring now to FIG. 2, a block diagram of a control circuit of the present invention is shown. As shown in FIG. 2, ultrasonic generator  116  generates a control signal  202  to digital signal processor (DSP)  204 . Voltage  120  and voltage  134 , which represent the source current and return current of excitation signal  136 , respectively, are input to analog to digital converter (ADC)  206  through inputs  210  and  212 , respectively. ADC  206  converts the analog voltage signals  120  and  134  into digital representations and provides these digital representations to DSP  204 . DSP  204  determines the difference in the source and return currents and if the difference between theses currents is less than about 15%, DSP  206  generates warning signal  208  as an alert that bonding wire  108  is absent. If the difference between the currents is greater than about 15% DSP  204  determines that bonding wire  108  is present and warning signal  208  is not generated. 
     Referring now to FIG. 3, a block diagram of a second exemplary embodiment of the present invention is shown. As shown in FIG. 3, ultrasonic generator  116  is coupled to amplifier  118  and processor  300 . Ultrasonic generator  116  provides signal  302  to processor  300  to indicate that a bonding operation is in progress. Processor  300  is coupled to resistors  120  and  130  to determine if bonding wire  108  (shown as resistive element  308  in FIG. 3) is present. If processor  300  determines that bonding wire  108  is not present (based on the above discussed criteria) processor  300  generates halt signal  304  to ultrasonic transducer  116  to terminate the generation of excitation signal  136 . In addition, processor  300  may also generate a further signal  306  which may be a warning signal, another process termination signal or status signal for use with other processing equipment (not shown). 
     Referring to FIG. 4, a flow chart of an exemplary process of the present invention is shown. In FIG. 4, at Step  400 , the bonding process is started. At Step  405 , the excitation signal is generated by the ultrasonic generator. At Step  410 , the source current of the excitation signal is measured. At Step  415 , the return current of the excitation signal is measured. 
     At Step  420 , the source current and return current are compared to one another. If the source current is greater than the return current by a predetermined value Step  425  is entered, otherwise Step  435  is entered. At Step  425 , an alert is issued indicting the absence of the bonding wire and at Step  430 , the bonding process is halted. Alternatively, at Step  435  the bond is formed and at Step  440 , the process continues. 
     In addition to, or in place of the above mentioned alert, a signal may be generated to control the bonder based on a determination that the bonding wire is missing. The signal may halt the operation of the bonder in a non-destructive, restartable manner, for example. In addition, the alert generated at Step  425  may take the form of a visual alert and/or an audible alert. 
     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.