Semiconductor device manufacturing: process – Packaging or treatment of packaged semiconductor – Metallic housing or support
Reexamination Certificate
2001-03-21
2002-11-12
Niebling, John F. (Department: 2812)
Semiconductor device manufacturing: process
Packaging or treatment of packaged semiconductor
Metallic housing or support
C257S067000
Reexamination Certificate
active
06479327
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
The present invention relates to a semiconductor device manufacturing technique and, more particularly, to a technique which is advantageous when used for manufacturing semiconductor devices incorporating power supply transistors such as power MOSFETs, IGBTs (insulated gate bipolar transistors) and bipolar power transistors, i.e., for manufacturing low voltage drive power transistors through reduction of electrical resistance used in the power supplies of portable apparatuses and the like, power transistors having low thermal resistance used in the power supplies of high output apparatuses such as laser beam printers and the like and power transistors for high current used in automobile electronics.
2. Description of the Related Art
Known power supply transistors incorporated in chargers for portable telephones, video cameras and the like and in power supply circuits of office automation (OA) apparatuses and the like include low voltage drive power transistors through reduction of an on resistance (Ron). For example, a power transistor for driving at a low voltage is described in “Hitachi Databook: Hitachi Semiconductor Package” issued in July, 1997 by Semiconductor Division, Hitachi, Ltd., page 329.
This low voltage drive power transistor comprises a power MOSFET. A power MOSFET in that article has a structure in which a semiconductor chip (chip) incorporating the power MOSFET is secured on a support substrate made of metal referred to as “header”; ends of a gate lead and a source lead extending diagonally upward on the header are connected to electrodes (a gate electrode and a source electrode) on the upper surface of the chip with conductive wires; and the upper surface of the header is coated with an encapsulation element made of insulating resin to cover the chip, wires and the ends of the leads.
In such a power MOSFET, the lower surface of the header is exposed to define a radiating surface, and three leads are exposed at one side of said encapsulation element. Two of the leads are the gate and source leads, and the remaining one lead is a drain lead connected to the header. Aluminum is used for the wire connected to the source electrode, and a connection structure having two wires is used to accommodate an increased amount of current.
Further, “a power MOSFET for power management” is described in pp. 19-20 of “Gain” issued on Sep. 2, 1996 by Semiconductor Division, Hitachi, Ltd. This power MOSFET is described as being primarily used for chargers of portable telephones, video cameras and the like and for power management during the charging and discharging of lithium ion secondary batteries such as power supplies of OA apparatuses and notebook type personal computers.
Furthermore, Japanese Patent Laid-Open No. 307103/1997 (Japanese Patent Application No. 120211/1996) discloses a technique for a composite power MOSFET incorporating a negative voltage protection circuit for preventing any breakdown of the element attributable to a negative voltage applied to the drain.
As described in “Hybrid Packaging Technique” issued on May 15, 1988 by Industrial Research Institute, page 25, power transistors are widely used in power supplies for driving motors to operate devices in various parts of an automobile. A hydraulic pump system driven by an electric motor and power steering driven by an electric motor itself are described in “Electronic Systems of Automobiles” issued on Aug. 5, 1992 by Rikogakusha, pp. 110-112.
SUMMARY OF THE INVENTION
For example, a power MOSFET is used by incorporating it in a rectifier circuit of a power supply of an OA apparatus. While rectifier circuits have conventionally employed diodes, power MOSFETs have recently come into use because of their low on resistance.
The reduced on resistance has resulted in a trend toward power MOSFETs having higher output. Further, the progress of fine processing techniques in the manufacture of semiconductor devices has resulted in power MOSFETs having higher characteristics and, for example, MOSFETs having an on resistance of about 0.34 m&OHgr; (when they are in the form of semiconductor chips) have been developed.
The inventors have developed this time a semiconductor device (resin-encapsulated semiconductor device) in a configuration of a power MOSFET having an output as high as 500 W (5V, 100 A) and have found that following problems can occur as a result of an examination of conventional structures including configurations of encapsulation elements (packages).
In conventional resin-encapsulated semiconductor devices, wires having large diameters are used to provide high output, and two wires are used. While gold wires are desirable because of their low resistance, aluminum is used because of the high cost of gold. Aluminum is connected to electrodes and leads by means of wire bonding utilizing ultrasonic oscillation (USWB) and is formed to have a maximum diameter of 500 &mgr;m. This is the maximum dimension of aluminum wires available on the market which is used here because custom-made parts are expensive.
When aluminum wires having a diameter of about 500 &mgr;m or more are used, a wire bonding apparatus utilizing ultrasonic oscillation can damage semiconductor chips formed from fragile semiconductors such as silicon, and it is therefore limited to the use of aluminum wires having a diameter of about 500 &mgr;m. Further, aluminum wires having a thickness in the excess of 500 &mgr;m are unsuitable for use because they can be cracked or cut when wound around a spool. This problem becomes more serious, the higher the purity of aluminum. Aluminum having high purity is used for wire bonding.
In addition, the output of conventional semiconductor devices in the form of power MOSFETs has been in the range from about 200 to 300 W at the maximum, which is quite small in comparison to 500 W which is achieved this time.
When two aluminum wires having a diameter of 500 &mgr;m are used, a great amount of heat is generated at the region of the wires, which can deteriorate resin (epoxy resin) having a glass transition temperature (Tg) in the range from about 155 to 170° C.
In consideration to this, the inventors examined increase of the number of wires used. According to a study carried out by the inventors, in conventional power MOSFET encapsulating structures, no consideration is paid on heat radiation from a source lead.
In the field of automobiles, while a compressor for power steering has conventionally been driven by a fan belt, there is a trend toward motor-driven systems (hydraulic pump type electric power steering) to reduce the weight of vehicle bodies and fuel consumption.
Further, systems for driving steering directly without a pump (directly driven electric power steering) have come in use in small cars to achieve a further reduction of weight.
Both of the systems described above employ a high current transistor (semiconductor device). For example, hydraulic pump type electric power steering and directly driven electric power steering require currents of 120 A and 70 A, respectively.
Especially, in Europe where regulations exist to prevent electric wave interference, motors must be of the brushless type, and the maximum torque of directly driven electric power steering is determined by the current which is allowed to flow through the transistor, e.g., MOSFET; incorporated in the driving system for the same. This factor consequently determines the total stroke of volume of the cars in which the steering system can be used.
It is assumed that existing transistors in TO220 type packages is limited to use in cars having total stroke volumes up to about 1500 cc because they can only carry a current of about 75 A.
Furthermore, transistors are incorporated in the engine room in an automobile where they can be subjected to a high temperature and are thus used in a severe environment in terms of temperature. Referring to a value as a result of an experiment by the inventors, when a current of 110 A is applied to two wires having a diameter of 500 &mgr;m used in a package
Hirashima Toshinori
Takahashi Yasushi
Hitachi , Ltd.
Lattin Christopher
Mattingly Stanger & Malur, P.C.
Niebling John F.
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