The present invention relates, in general, to semiconductor device manufacture, and more particularly to attachment of semiconductor die in a package.
The manufacture of high power UHF transistors involves unique requirements due to characteristics of the radio frequency energy. Very small values of inductance and capacitance can have a great effect on the electrical operation of the device. For example, a wire as short as a few millimeters can have significant values of inductance and capacitance in critical parts of the electric circuit. Thus the electrical characteristics of the package which houses the transistor die is itself a critical part of the electric circuit. Furthermore, in order to ensure consistent performance between devices, discrete components such as chip capacitors are typically mounted in the same package as the transistor die. Chip capacitors comprise a single semiconductor die which typically use a metal-oxide semiconductor process to make a high quality capacitor. Furthermore, improper placement of a die can mean that a portion of the die does not completely contact the mounting surface of the package. This lack of contact can severely degrade both the electrical and the thermal characteristics of the resultant product. Exact placement of these components within the package is critical both for electrical and thermal requirements. Finally, the length and routing of the bond wires used to electrically connect these die and the package terminals are very critical. Multiple and parallel bond wires are typically used to control inductive effects and to handle the high current required. A misplaced die can alter the critical routing, spacing and length of the bond wires, at least degrading the electrical performance and potentially allowing bond wires to short to one another causing the transistor to fail.
Typical of such high power UHF transistors is the MRF898 transistor which is manufactured and marketed by Motorola Inc. This transistor can operate with a power level of up to 60 watts, an operating voltage of 24 volts, and a frequency as high as 900 Mhz. Fabrication of the MRF898 requires three chip capacitors bonded to the package in addition to the transistor die. In addition, the MRF898 requires a total of 90 bonding wires, each having a maximum length and height variation of 38.1 micrometers (1.5 mils) from a nominal value. In order to adhere to this tolerance, the placement of each die relative to the other die and to the package terminals is extremely critical. Each die within the package assembly of the MRF898 transistor must be bonded in place within 50.8 micro meters (2 mils) of its desired nominal position. Consistent bonding of these die in the correct positions is one of the most difficult steps of manufacturing the MRF898 transistor.
According to the prior art, most die bonders were only capable of handling one die at a time. Mounting of more than one semiconductor die in a package required that the die bonding step be repeated from start to finish for each die to be mounted. This makes accurate positioning of the die difficult since each alignment step must be performed separately, potentially using a different die bonding machine and operator. This prior art method has been found to have approximately 300,000 parts per million exceed a spacing variation of 50.8 micro-meters from a predetermined nominal position. This requires reworking or scrapping almost one third of the production and is an unacceptably high failure rate.
Repeated heating and subsequent cooling of the package and any semiconductor die already mounted on that package to a temperature of 425 degrees Celsius, required for eutectic bonding, causes extra stress to both the semiconductor die and the package itself. This extra stress can shorten the useful life of the device as well as being wasteful of electrical energy resources. In addition sequentially bonding each semiconductor die requires four separate die bonding operations, effectively using much the same operator and equipment time as would normally be required for four transistors having only a single semiconductor die in each package.
Finally, the chuck used to pick up the individual semiconductor die must be changed to accommodate each type of semiconductor die. Since production batches are often small, significant time is wasted in down time while the chuck and supporting collet are changed and adjusted. In some cases as much as one half of the total time is spent as down time for setup and adjustment even with a single chuck die bonder. A die bonder having multiple chucks requires even more complex setup and adjustment. As a result, a multiple chuck die bonder would require many times the setup time unless some means were provided to pre-align the chucks. Reducing this setup time can significantly increase productivity in all cases but is even more desirable for a die bonder having multiple chucks.
It would be highly desirable to bond a plurality of semiconductor die in place as a single pass operation. This would heat both semiconductor die and package only once, reduce the cost of manufacturing, and would enable more accurate positioning of each semiconductor die within the package. In addition the die bonder must allow for a rapid change from one type of semiconductor die to another.