Wire bonding method and apparatus and semiconductor device

A wire bonding method and apparatus implement the flatly thinner plastic deformation for the joint section of a wire, which has a diameter ranging 100-600 .mu.m, on its feed side, feed out and position the flatly deformed wire joint section to a target joint surface, and join the wire to it by pressing the positioned wire joint section, with vibration being applied, onto the joint surface with a ultrasonic wire bonder. A high-power semiconductor device fabricated based on this scheme has a long life of wire joints.

BACKGROUND OF THE INVENTION
 1. Field of the Invention
 The present invention relates to a wire bonding method and apparatus for
 interconnecting electronic parts by using electrically conductive wires,
 and to a semiconductor device. Particularly, the inventive method and
 apparatus are applied suitably to semiconductor devices which are intended
 for high-speed switching of large currents in automobile equipment
 controllers, electric-car drive controllers and other vehicle-installed
 motor controllers.
 2. Description of the Prior Art
 In the manufacturing process of semiconductor devices which include
 multiple semiconductor chips and electronic parts, a scheme of wire
 bonding is used for the electrical connection between the electrodes of
 semiconductor chips and electronic parts and between the terminals of
 electronic parts.
 A typical conventional wire bonding apparatus will be explained first with
 reference to FIG. 14. FIG. 14 is a side view of the conventional wire
 bonding apparatus. This wire bonding apparatus is designed to feed a wire
 101, which is supplied from a bobbin (not shown), to the groove of wire
 press section 112 of a bonding tool 111 by way of a through-hole 115
 formed in a horn 110 and a gap of clamp section of a wire clamp mechanism
 120.
 With ultrasonic vibration being applied to the bonding tool 111 which is
 fixed to the tip section of the horn 110, the wire 101 is pressed onto the
 electrode of a semiconductor chip 102 as one part of connection so that
 the wire 101 is joined to it, and next the wire 101 is fed and brought by
 the bonding tool 111 to the terminal 104 of another electronic part, e.g.,
 a resistor, as another part of connection and joined to it in the same
 manner.
 The wire clamp mechanism 120 is located between the wire press section 112
 of the bonding tool 111 and the through-hole 115 of the horn 110, and it
 serves to hold and guide the wire 101 when it is fed out. The bonding tool
 111 and horn 110 are supported on a vertical moving mechanism and
 horizontal moving table so that they can move vertically and horizontally
 relative to the semiconductor chip 102 and electronic part.
 Automobile equipment controllers and electric-car drive controllers are
 required to be made much smaller in size and weight. The drive controller
 incorporates semiconductor devices which implements high-speed switching
 of large currents for producing a.c. power for driving motors by being
 supplied with power from such a d.c. power source as battery.
 Electronic components have their operating currents increasing to match
 with the trend of higher-power drive controllers, and therefore wires of
 large diameters are used for the electrical connection between
 semiconductor chips and between semiconductor chips and electronic parts
 of semiconductor devices. For wires of large diameters, aluminum wires
 which are inexpensive and light are used, instead of wires having higher
 electrical conductivity that mainly consist of expensive gold. Aluminum
 wires are thicker due to the lower electrical conductivity than gold
 wires, and aluminum wires with diameters of 100-600 .mu.m are necessary
 for high-power semiconductor devices.
 Semiconductor devices used in automobile equipment controllers and
 electric-car drive controllers are required to be durable against severe
 heat cycles and power cycles thereby to last long, in addition to the
 demand of compactness and light weight. In order to meet these
 requirements, it is necessary to improve the strength of wire joints.
 There is a limit in widening the joint area based merely on pressing the
 wire 101 having a circular cross section onto the planar target joint
 surface, and there is also a limit in improving the strength and life of
 joints based merely on the application of ultrasonic vibration to the
 limited joint area. Specifically, the conventional wire bonding scheme
 works for joining by pressing the wire 101 having a circular cross section
 onto a planar target joint surface so that the wire is deformed, and the
 pressing force needs to be increased progressively to overcome the
 increasing resistance of deformation.
 Accordingly, in order for the conventional wire bonding scheme to improve
 the strength and life of wire joints by raising the degree of deformation
 of the wire 101 while retaining the mechanical strength of the deformed
 section of the wire 101, it is necessary to increase the ultrasonic output
 for the metallic joint process thereby to increase the pressing force of
 the wire 101. However, an excessive pressing force by the increased
 ultrasonic output can result in the breakage of the electronic part or
 semiconductor chip 102 having the target joint surface.
 On this account, conventionally, there is a limit in widening the joint
 area, and thus there is a limit in improving the strength and life of wire
 joints.
 SUMMARY OF THE INVENTION
 The present invention is intended to overcome the foregoing prior art
 deficiency, and its prime object is to provide a wire bonding method and
 apparatus capable of accomplishing wire joints which are durable against
 severe heat cycles and power cycles thereby to have a long life, and are
 useful for semiconductor devices which implement high-speed switching of
 large currents.
 Another object of the present invention is to provide a high-power
 semiconductor device which is smaller in size and weight and durable
 against severe heat cycles and power cycles thereby to have a long life
 based on the enhanced strength of wire joints.
 In order to achieve the above objective, the inventive wire bonding method
 comprises a step of implementing the flatly thinner plastic deformation
 for the joint section of a wire on the feed side thereof, and a step of
 joining the wire to a target joint surface by feeding out and positioning
 the flatly deformed wire joint section processed by the wire deforming
 step to the target joint surface, and pressing the positioned wire joint
 section, with vibration being applied, onto the target joint surface with
 a ultrasonic wire bonder. Preferred wire diameters range from 100 to 600
 .mu.m.
 Alternatively, the inventive wire bonding method comprises a step of
 implementing the flatly thinner plastic deformation for the joint section
 of a wire on the feed side thereof to match with the intended length of
 wire loop, and a step of joining the wire to a target joint surface by
 feeding out and positioning the flatly deformed wire joint section
 processed by the wire deforming step to the target joint surface, and
 pressing the positioned wire joint section, with vibration being applied,
 onto the target joint surface with a ultrasonic wire bonder. Preferred
 wire diameters range from 100 to 600 .mu.m.
 Wires used for the inventive wire bonding method are preferably made of
 aluminum or aluminum alloy.
 Preferably, the wire deforming step of the inventive wire bonding method
 implements the flatly thinner plastic deformation for the joint section of
 the wire at a ratio of 2 or more in terms of deformation factor W/D, where
 W is the width of deformed wire at the joint section and D is the original
 wire diameter.
 Preferably, the wire joining step of the inventive wire bonding method
 joins the joint section of the wire to the target joint surface at a
 deformation factor W/D of a ratio 2 or more, where W is the width of
 deformed wire at the joint section and D is the original wire diameter.
 More preferably, the wire joining step of the inventive wire bonding method
 joins the joint section of the wire to the target joint surface at a
 deformation factor W/D of a ratio in the range from 4 to 6, where W is the
 width of deformed wire at the joint section and D is the original wire
 diameter.
 In order to achieve the above objective, the inventive wire bonding
 apparatus comprises means of implementing the flatly thinner plastic
 deformation for the joint section of a wire on the feed side thereof, and
 means of joining the wire to a target joint surface by feeding out and
 positioning the flatly deformed wire joint section processed by the wire
 deforming means to the target joint surface, and pressing the positioned
 wire joint section, with vibration being applied, onto the target joint
 surface with a ultrasonic wire bonder.
 The wire deforming means of the inventive wire bonding apparatus includes
 an upper mold and a lower mold, with one mold being moved to another mold
 by means of a driving device so that the wire is deformed. Preferably, the
 one mold has the formation of a V-shaped groove and has slope sections at
 its wire inlet and outlet. Preferably, the one mold has the formation of a
 flat groove and has slope sections at its wire inlet and outlet.
 Preferably, the joining means includes a wire press section having a
 V-shaped groove or a flat groove.
 The inventive wire bonding apparatus comprises means of implementing the
 flatly thinner plastic deformation for the joint section of a wire, which
 has a diameter in the range from 100 to 600 .mu.m, on the feed side
 thereof, and means of joining the wire to a target joint surface by
 feeding out and positioning the flatly deformed wire joint section
 processed by the wire deforming means to the target joint surface, and
 pressing the positioned wire joint section, with vibration being applied,
 onto the target joint surface with a ultrasonic wire bonder.
 In order to achieve the above objective, the inventive semiconductor device
 has a wire joint surface of semiconductor chip, to which is joined a wire
 by ultrasonic wire bonding with the rendition of flatly thinner plastic
 deformation for the joint section of the wire at a ratio of 2 or more in
 terms of deformation factor W/D, where W is the width of deformed wire at
 the joint section and D is the original wire diameter.
 Alternatively, the inventive semiconductor device has a wire joint surface
 of semiconductor chip, to which is joined a wire by ultrasonic wire
 bonding with the rendition of flatly thinner plastic deformation for the
 joint section of the wire at a ratio in the range from 4 to 6 in terms of
 deformation factor W/D, where W is the width of deformed wire at the joint
 section and D is the original wire diameter.
 Preferred wire diameters D for these semiconductor devices range from 100
 to 600 .mu.m. Wires used for these semiconductor devices are preferably
 made of aluminum or aluminum alloy.
 The inventive semiconductor device comprises a high-power semiconductor
 device, which includes a positive terminal and an output terminal which
 are fixed on an insulation substrate, a first power element and a second
 diode which are joined to the positive terminal, and a second power
 element and a first diode which are joined to the output terminal, and a
 negative terminal which is fitted on the insulation substrate through an
 insulator, with the first power element having its emitter electrode
 connected to the output terminal by wire bonding, the second diode having
 its anode electrode connected to the output terminal by wire bonding, the
 second power element having its emitter electrode connected to the
 negative terminal by wire bonding, and the first diode having its anode
 electrode connected to the negative terminal by wire bonding, wherein
 wires to be joined by ultrasonic wire bonding to the joint surfaces of the
 first and second power elements and the first and second diodes are
 rendered at the joint sections thereof with flatly thinner plastic
 deformation at a ratio of 2 or more in terms of deformation factor W/D,
 where W is the width of deformed wire at the joint section and D is the
 original wire diameter.
 Alternatively, the inventive semiconductor device comprises a high-power
 semiconductor device, which includes a positive terminal and an output
 terminal which are fixed on an insulation substrate, a first power element
 and a second diode which are joined to the positive terminal, and a second
 power element and a first diode which are joined to the output terminal,
 and a negative terminal which is fitted on the insulation substrate
 through an insulator, with the first power element having its emitter
 electrode connected to the output terminal by wire bonding, the second
 diode having its anode electrode connected to the output terminal by wire
 bonding, the second power element having its emitter electrode connected
 to the negative terminal by wire bonding, and the first diode having its
 anode electrode connected to the negative terminal by wire bonding,
 wherein wires to be joined by ultrasonic wire bonding to the joint
 surfaces of the first and second power elements and the first and second
 diodes are rendered at the joint sections thereof with flatly thinner
 plastic deformation at a ratio in the range from 4 to 6 in terms of
 deformation factor W/D, where W is the width of deformed wire at the joint
 section and D is the original wire diameter.
 Preferred wire diameters D for these high-power semiconductor devices range
 from 100 to 600 .mu.m. Wires used for these high-power semiconductor
 devices are preferably made of aluminum or aluminum alloy.
 According to the inventive wire bonding method and apparatus, it becomes
 possible to increase the joint area between the wire and the target joint
 surface without imposing an excessive ultrasonic output, pressing force
 and their application time length at the wire joining process, whereby it
 is possible to manufacture electronic components and semiconductor devices
 which are enhanced in the strength of wire joints and durable against
 severe heat cycles and power cycles thereby to have a long life.
 The inventive wire bonding method and apparatus implement the prior wire
 deformation, so that the ultrasonic output, pressing force and their
 application time length can be reduced at the wire joining process,
 whereby it becomes possible to prevent the breakage of electronic parts
 including semiconductor chips and eventually manufacture reliable
 electronic components and semiconductor devices.
 These and other features and advantages of the present description of
 preferred embodiments taken in conjunction with the accompanying drawings.

DESCRIPTION OF THE PREFERRED EMBODIMENTS
 Embodiments of the inventive wire bonding method and apparatus and
 semiconductor device will be explained with reference to the drawings.
 FIG. 1 is a plan view of a high-power semiconductor device based on an
 embodiment of this invention, FIG. 2 is a cross-sectional view taken along
 the line X--X of FIG. 1, and FIG. 3 is a schematic diagram showing the
 principal portion of this semiconductor device.
 This semiconductor device is intended for high-speed switching of large
 currents for producing a.c. power for driving a motor by being supplied
 with power from such a d.c. power source as battery, as disclosed in
 Japanese Published Unexamined Patent Application No. Hei 9-102578.
 In FIG. 1 through FIG. 3, a frame-shaped case 12 is fixed to an insulation
 substrate 10 which is made of aluminum nitride or the like. The insulation
 substrate 10 has on its rear side the attachment of a heat sink plate 90.
 A positive terminal (connecting conductor) 20, output terminal 22 and
 negative terminal 24 are made of a material having a high electrical
 conductivity, such as copper or aluminum, and dimensioned so as to conduct
 a certain value of current at a low power loss. The positive terminal 20
 and output terminal 22 are fixed to the insulation substrate 10 of
 aluminum nitride or the like by means of soldering of good heat
 dissipation or silver brazing.
 First power switching elements 30A and 30B such as IGBT (Insulated Gate
 Bipolar Transistor) chips and second diodes 42A and 42B are soldered to
 the positive terminal 20, and second power switching elements 32A and 32B
 such as IGBT chips and first diodes 40A and 40B are soldered to the output
 terminal 22. The first power switching elements 30A and 30B have their
 emitter electrodes bonded to the output terminal 22 through wires 50A1 and
 50A2, and the second diodes 42A and 42B have their anode electrodes bonded
 to the output terminal 22 through wires 50A3 and 50A4.
 A negative terminal 24 is fixed on column-shaped insulators 60A-60F over
 the first power switching elements 30A and 30B and second diodes 42A and
 42B in parallel to the positive terminal 20. The second power switching
 elements 32A and 32B have their emitter electrodes bonded to the negative
 terminal 24 through wires 50B1 and 50B2, and the first diodes 40A and 40B
 have their cathode electrodes bonded to the negative terminal 24 through
 wires 50B3 and 50B4.
 On the insulation substrate 10, there is attached a terminal pad 70, on
 which gate resistors 74A and 74B are fixed. The gate resistors 74A and 74B
 are bonded to the base electrodes (formed of a material such as silicon)
 of the first power switching elements 30A and 30B of IGBTs or the like
 through wires 78A1 and 78B1, respectively. Further attached on the
 insulation substrate 10 is another terminal pad 72, on which gate
 resistors 76A and 76B are fixed. The gate resistors 76A and 76B are bonded
 to the base electrodes of the second power switching elements 32A and 32B
 of IGBTs or the like through wires 78A2 and 78B2, respectively.
 In FIG. 2, indicated by 80 and 82 are solder for connecting the first power
 switching element 30A to the positive terminal 20 and connecting the
 second power switching element 32A to the output terminal 22,
 respectively.
 The semiconductor device shown in FIG. 1 and FIG. 2 has a circuit
 arrangement as shown in FIG. 3. Common reference numerals are used
 throughout these figures. Specifically, the second diode 42A is connected
 between the positive terminal 20 and the output terminal 22 by the wire
 50A3, and further connected between these terminals are the first power
 switching elements 30A and 30B by the wires 50A1 and 50A2, respectively,
 and the second diode 42B by the wire 50A4. The first diode 40A is
 connected between the output terminal 22 and the negative terminal 24 by
 the wire 50B3, and further connected between these terminals are the
 second power switching elements 32A and 32B by the wires 50B1 and 50B2,
 respectively, and the first diode 40B by the wire 50B4.
 The circuit receives d.c. power on the positive terminal 20 and negative
 terminal 24, and produces a.c. power on the output terminal 22.
 The wires 50A1-50A4, 50B1-50B4, etc. are connected by wire bonding. Because
 of the limited current capacity of a single wire for the connection
 between a power switching element or diode and other electronic part,
 multiple wires are generally used in parallel depending on the value of
 current flowing out of the power switching element or diode.
 The foregoing circuit arrangement constitutes a semiconductor device which
 implements high-speed switching of large currents for driving a motor or
 the like by being supplied with power from a d.c. power source.
 The wires 50A1-50A4, 50B1-50B4, 78A1, 78A2, 78B1 and 78B2 which conduct
 large currents are made of aluminum or aluminum alloy which includes
 silicon, nickel, etc. In case the electrical conductivity is major
 concern, wires of gold, silver or their alloys may be used.
 In semiconductor devices in which electrodes of semiconductor chips and
 electronic parts are connected by wire bonding, the current flowing
 through each wire brings about the peel-off of joint between the wire and
 electrode, resulting in the heat concentration. The concentrated heat
 aggravates the peel-off of the joint. Therefore, it is required to prolong
 the device life attributable to power cycles which is dependent on the
 peel-off life of wire joints.
 Next, the breakage of wire joint caused by power cycles will be explained
 with reference to FIGS. 4A-4C. FIG. 4A is a plan view of a wire, FIG. 4B
 is a side view of the wire and electrode, and FIG. 4C is a cross-sectional
 view taken along the line A--A of FIG. 4B.
 The wire 101 is rendered the flatly thinner plastic deformation in its
 portion to be joined to the electrode 102A so that it has an increased
 joint area, as shown in FIGS. 4A and 4B. With a ultrasonic vibration being
 applied, for example, the deformed joint section of the wire 101 is joined
 by wire bonding to the electrode 102A of a semiconductor chip or
 electronic part.
 When a large current flows through the joint repeatedly, there emerges a
 crack 103 around the joint 100 between the wire 101 (50A1-50A4, 50B1-50B4,
 78A1, 78A2, 78B1 and 78B2) and the electrode 102A as shown in FIG. 4B. The
 crack 103 progresses to reduce the connection area, resulting in an
 increased resistance of the joint 100 and eventually in the defective
 connection. Provided that the speed of progress of crack (the amount of
 progress of crack in a power cycle: da/dn) is virtually constant
 throughout the life, the life N of the joint 100 is formulated as follows.
EQU N=(bo-bf)/(da/dn)/2 (1)
 where bo and bf are the original and final widths of the joint 100 as shown
 in FIG. 4C, and da/dn is the rate of progress of the amount of crack a
 with respect to the number of times of current conduction n.
 In the context of fracture mechanics, the term da/dn is dependent on the
 fracture mechanics parameter .DELTA.J as follows.
EQU da/dn=C.sub.1.multidot.(.DELTA.J.sup.m) (2)
 where C.sub.1 and m are constants.
 When the wire 101 in its portion of the joint 100 is modeled to be a film,
 the .DELTA.J is proportional to the wire thickness H (shown in FIG. 4C) at
 the joint 100 as follows.
EQU .DELTA.J=(.DELTA..alpha..multidot..DELTA.T).sup.2.multidot.E.multidot.H/
 (2-2.nu.) (3)
 where .DELTA..alpha. is the difference of linear expansion coefficients
 between the wire 101 and the power switching element 102 (30A, 30B, 32A,
 32B, 40A, 40B, 42A and 42B), .DELTA.T is the width of temperature
 variation, E is the Young's modulus of the wire 101, and .nu. is the
 Poisson's ratio of the wire 101.
 Based on the formulas (1) and (3), the influence of H on N is assessed by
 the following formula.
EQU N=C.sub.2 /H.sup.m (4)
 where C.sub.2 is a constant.
 In the case of the wire 101 of pure aluminum, m takes a value of around
 1.4.
 Accordingly, in order to enhance the strength and life of the joint 100, it
 is suggested to reduce the wire thickness H, or, in other words, increase
 the deformation factor W/D, where W is the width of deformed wire and D is
 the original wire diameter which is determined from the current capacity.
 The wire 101 of pure aluminum, which is inferior to gold in electrical
 conductivity, needs to be thicker than a gold wire, and its diameter D
 ranges from 100 to 600 .mu.m for the foregoing high-power semiconductor
 device, for example.
 Next, embodiments of the inventive wire bonding method and apparatus will
 be explained with reference to FIG. 5 through FIG. 7. FIG. 5 is a brief
 side view of the wire bonding apparatus based on the first embodiment of
 this invention.
 This wire bonding apparatus includes a horn 110, on which is attached a
 bonding tool 111 having a wire press section 112 and functioning to press
 a wire 101 with a diameter of the 100-600 .mu.m range, with vibration
 being applied, onto a target joint surface, e.g., the electrode 102A of
 semiconductor device, a wire pre-forming mechanism 140 which is located
 between the wire press section 112 of the bonding tool 111 and the
 through-hole 115 formed in the horn 110 and adapted to act the flatly
 thinner deformation on the wire 101 which is supplied from a supply reel
 (not shown) and fed through the through-hole 115 of the horn 110, a wire
 clamp mechanism 120 which clamps the wire 101, and a stage 160 which
 mounts an object of bonding 105, i.e., the semiconductor device.
 On the figure of bonding object 105, indicated by 102 is a semiconductor
 chip which represents the power switching elements 30A, 30B, 32A, 32B,
 40A, 40B, 42A and 42B, and 104 is a terminal which represents the output
 terminal 22 and negative terminal 24.
 This wire bonding apparatus is characterized in the disposition, on the
 wire feed-through path, of the wire pre-forming mechanism 140 which
 deforms in advance the wire 101 of aluminum or aluminum alloy which
 includes silicon, nickel, etc. and with a diameter of the 100-600 .mu.m
 range at a ratio of deformation factor W/D of around 2.0 or more
 (preferably 2.5 or more, or more preferably in the range from 4 to 6).
 FIG. 6A is a cross-sectional view of the apparatus used to explain the wire
 pre-forming mechanism 140 shown in FIG. 5, and FIG. 6B is a
 cross-sectional view taken along the line C--C and seen along the
 direction D of FIG. 6A.
 This wire pre-forming mechanism 140 includes molds 141 and 142 located
 below and above the wire 101, as shown in FIGS. 6A and 6B. The lower mold
 141 and upper mold 142 are supported at the positions on both sides of the
 horn 110 so that they can move independently of the horn 110, as shown in
 FIG. 7 of another wire pre-forming mechanism 140A which will be explained
 later.
 A cam 145, which is movable to the right and left on the drawing by a
 formation driving device 144, is disposed in contact with the upper mold
 142 as shown in FIG. 6A. The driving device 144 moves the cam 145 to the
 left on the drawing so that the upper mold 142 is pushed down toward the
 fixed lower mold 141, thereby pressing the wire 101, which is then
 deformed in its cross section in accordance with the shape of the mold.
 The wire 101 is deformed to meet a deformation factor W/D of a ratio of
 around 2.0 or more, or preferably 2.5 or more, or more preferably in the
 range from 4 to 6.
 In this embodiment, the lower mold 141 has a planar wire contact surface
 and the upper mold 142 has its wire contact surface formed to include a
 V-shaped profile, so that an excessive tensile stress which can break the
 wire 101 does not emerge in the wire center. This V-shaped profile of the
 upper mold 142 also serves to guide the wire 101 in its longitudinal and
 lateral directions. In addition, the upper mold 142 has its wire inlet and
 outlet formed to have rounded slope sections 149, so that the shearing
 stress acting on the ends of deformation section of the wire 101 is
 alleviated.
 A variety of variant versions of the wire pre-forming mechanism 140 are
 conceivable to carry out the flatly thinner deformation of the wire 101
 while avoiding a concentrated stress. Shown in FIG. 7 is a variant
 mechanism as an example.
 Using the wire pre-forming mechanism 140, when the wire 101 having a
 diameter of 300 .mu.m, for example, is deformed at a ratio of a
 deformation factor W/D of 2.0, it will become to have a thickness H of
 around 120 .mu.m. The wire pre-forming mechanism 140 of this embodiment
 can be adjusted independently of the horn 110, so that the deformation
 factor W/D can be set arbitrarily. Specifically, when the wire 101 having
 a diameter of 300 .mu.m, the thickness H will be around 100 .mu.m by the
 process at W/D=2.5, it will be around 80 .mu.m by the process at W/D=3.0,
 it will be around 70 .mu.m by the process at W/D=3.5, it will be around 60
 .mu.m by the process at W/D=4.0, it will be around 50 .mu.m by the process
 at W/D=5.0, and it will be around 40 .mu.m at W/D=6.0.
 For a wire diameter of 300 .mu.m, when the ratio of the deformation factor
 W/D is set in the range from 4 to 6 so that the resulting thickness H will
 be around 40 to 60 .mu.m, the above formula (4) suggests the significant
 improvement of the strength and life of the joint 100.
 Although the wire pre-forming mechanism 140 of the foregoing embodiment
 uses the cam 145 to move the upper mold 142, variant versions include a
 swing motion mechanism as shown in FIG. 7 and an up/down motion mechanism.
 FIG. 7 is a brief perspective view of the wire bonding apparatus based on
 the second embodiment of this invention. The wire pre-forming mechanism
 140A of this embodiment includes a lower mold 141A which is fixed
 obliquely to a side frame 161, an upper mold 142A which is fixed on a
 shaft 162 pivoted on the side frame 161 so that the upper mold 142A can
 have a swing motion, and a swing drive device 144A including a servo motor
 for driving the upper mold 142A to swing about the shaft 162.
 The lower mold 141A and upper mold 142A have the same formation on their
 wire contact surfaces as those of the lower mold 141 and upper mold 142 of
 the previous embodiment shown in FIGS. 6A and 6B.
 In operation, in contrast to the embodiment shown in FIG. 6A in which the
 driving device 144 moves the cam 145 straight so that the upper mold 142
 is pushed down thereby to press the wire 101 to deform, the embodiment
 shown in FIG. 7 is designed such that the swing drive device 144A directly
 drives the upper mold 142A to swing thereby to press the wire 101 to
 deform.
 According to the foregoing embodiments, in which the wire pre-forming
 mechanisms 140 and 140A are employed for implementing the flatly thinner
 deformation of the wire 101, it is possible to shape the wire press
 section 112 of the bonding tool 111 to the upper molds 142 and 142A, so
 that the joint area becomes wide enough for stable wire bonding to take
 place.
 FIGS. 8A and 8C are cross-sectional views of two embodiments of wire
 pre-forming mechanism, and FIGS. 8B and 8D are side views of the wire
 press section located at the tip of the wire bonding tool. Specifically,
 FIGS. 8A and 8C show a V-shaped groove 142a and flat groove 142b formed on
 the upper mold 142 (142A) of the pre-forming mechanism 140 (140A), and
 FIGS. 8B and 8D show a V-shaped groove 112a and flat groove 112b formed on
 the wire press section 112 of the bonding tool 111. Namely, the bonding
 tool 111 which presses the wire 101 (not shown) has its wire press section
 112 at the tip rendered the virtually same formation of the V-shaped
 groove 112a or flat groove 112b as the groove 142a or 142b of the upper
 mold 142 (142A) of the pre-forming mechanism 140 (140A), so that the joint
 area becomes wide enough for stable wire bonding to take place.
 Next, the operation of the inventive wire bonding apparatus equipped with
 the wire pre-forming mechanism 140 (140A) will be explained with reference
 to FIGS. 9A-9C and FIGS. 10A and 10B.
 FIGS. 9A-9C are partially cross-sectional side views of the inventive wire
 bonding apparatus used to explain the first operation. The wire clamp
 mechanism 120 is adapted to hold the wire 101 and allow it to run in its
 longitudinal direction. The wire pre-forming mechanism 140 (140A) is
 located on the wire feed path by being aligned to the wire clamp mechanism
 120.
 First, the operation of wire bonding apparatus for joining the wire 101 to
 the first target joint surface based on the inventive wire bonding method
 will be explained with reference to FIGS. 9A-9C.
 The wire 101 which has been deformed by the pre-forming mechanism 140
 (140A) and fed out is positioned at its deformed section to the wire press
 section 112 of the bonding tool 111, and the wire clamp mechanism 120 is
 operated to hold the wire 101 so that the wire movement relative to the
 bonding tool 111 stops, as shown in FIG. 9A. In this state, the
 pre-forming pressure P1 exerted by the pre-forming mechanism 140 (140A) on
 the wire 101 is zero, while the wire clamp mechanism 120 exerts a wire
 clamping pressure P2=P2c on the wire 101 to hold it.
 Subsequently, the bonding tool 111 and the target joint surface of the wire
 bonding object 105, i.e., the electrode 102A of the semiconductor chip
 102, are moved relatively in the vertical and horizontal directions, so
 that deformed section of the wire 101 is positioned to the target joint
 surface.
 Subsequently, the wire press section 112 at the tip of the bonding tool 111
 presses the deformed wire 101 onto the target joint surface, i.e., the
 electrode 102A of the semiconductor chip 102, with vibration being
 applied, so that both members undergo ultrasonic bonding as shown in FIG.
 9B. In this state, the wire 101 is free from the clamping force of the
 wire clamp mechanism 120 of the pre-forming mechanism 140 (140A), i.e.,
 the pressures P1 and P2 are both zero.
 Subsequently, the pre-forming mechanism 140 (140A) is moved along the wire
 feed path 116 to the position which matches with the prescribed wire
 length Lp along the finished wire loop measured from the wire joint, as
 shown in FIG. 9C. The driving device 144 (144A) shown in FIG. 6 (FIG. 7)
 is activated to exert a pressure P2=Pf on the wire 101 between the lower
 mold 141 (141A) and upper mold 142 (142A) shown in FIG. 8A (FIG. 8C),
 thereby deforming the wire 101 in its cross section. At this time, the
 wire clamp mechanism 120 in not holding the wire 101.
 In this case, the pre-forming mechanism 140 (140A) deforms the wire 101 for
 the amount of two joint sections at once, since both ends of each piece of
 wire are always bonded. A groove may be formed at the end or middle of the
 deformed section of the wire 101 so that it can be cut easily.
 Next, the operation of wire bonding apparatus for joining the wire 101 to
 the second target joint surface based on the inventive wire bonding method
 will be explained with reference to FIGS. 10A and 10B. FIGS. 10A and 10B
 are side views of the wire bonding apparatus used to explain the second
 operation of the apparatus based on this invention.
 Shown in FIG. 10A is the state of the apparatus after the wire 101 has been
 joined to the first target joint surface 102A. The wire 101 is released
 from the molds 141 and 142 of the pre-forming mechanism 140 (140A), and
 the bonding tool 111 is moved to the next target joint surface of the
 electrode 104 along the predetermined wire loop, and, as a result, the
 wire 101 is positioned at its deformed section to the wire press section
 112.
 The wire clamp mechanism 120 exerts a pressure P2=P2c on the wire 101 to
 hold it thereby to stop its movement relative to the bonding tool 111. In
 this state, the wire press section 112 presses the deformed wire 101 onto
 the electrode 104, with vibration being applied, as shown in FIG. 10B, so
 that ultrasonic wire bonding takes place in the same manner as the
 previous bonding process. With the wire 101 being clamped, the wire clamp
 mechanism 120 is moved together with the bonding tool 111 to retreat from
 the joint surface, and the wire 101 is cut off.
 Based on the deformation of both ends of the wire 101 to match with the
 wire loop by the pre-forming mechanism 140 (140A) prior to the joining
 process, it becomes possible to accomplish the wire bonding of enhanced
 joint strength and life N resulting from the wider joint area.
 Although in the foregoing embodiments, the wire pre-forming mechanisms 140
 and 140A are equipped independently of the wire clamp mechanism 120, an
 alternative design is to eliminate the wire clamp mechanism 120 and use
 the wire pre-forming mechanism 140 (140A) to clamp and deform the wire
 101. Although in the foregoing embodiments, the wire pre-forming
 mechanisms 140 and 140A are movable relative to the wire clamp mechanism
 120, both devices may be moved as a unitary member.
 Next, the joint surface of the electrode 102A and wire 101 will be
 explained with reference to FIGS. 11A and 11B. FIG. 11A is a partially
 cross-sectional perspective view showing the joint of the wire joint
 section and target joint surface based on this invention, and FIG. 11B is
 a cross-sectional view taken along the line B--B of FIG. 11A.
 For a high-power semiconductor device, the wire 101 of aluminum or aluminum
 alloy which includes silicon, nickel, etc. and with a diameter D of the
 100-600 .mu.m range, as shown in FIG. 11B, is used. The wire 101 is
 deformed in advance by the wire pre-forming mechanism 140 (140A) at a
 ratio of deformation factor W/D of around 2.0 or more, or preferably 2.5
 or more, or more preferably in the range from 4 to 6. The deformed wire
 101 is brought in contact with the electrode 102A and joined to it by
 ultrasonic wire bonding with no risk of damage to the semiconductor chip
 102.
 As compared with the conventional wire bonding, in which the bonding
 conditions including the ultrasonic output, pressing force and their
 application time length are optimized to such an extent that the
 semiconductor chip 102 is not damaged and a wire thickness H of 200 .mu.m
 is achieved for a wire diameter D of 300 .mu.m at a ratio of a deformation
 factor W/D of about 1.3, the inventive wire bonding method and apparatus
 are capable of reducing drastically the deformed wire thickness in terms
 of H1/H2, where H1 and H2 are the inventive and conventional thickness, to
 around 0.6 or less, or hopefully around 0.5 or less, or more hopefully in
 the range from 0.3 to 0.2. Consequently, based on the formula (4), the
 inventive wire bonding method and apparatus are capable of enhancing the
 strength and life of wire joint attributable to power cycles by 2-fold or
 more, or hopefully 2.5-fold or more, or more hopefully in the range from 5
 to 9-fold.
 FIG. 12 is a characteristic graph showing the relation between the forming
 pressure P1 (kg f/mm.sup.2) exerted on an aluminum wire having a diameter
 D of 300 .mu.m plotted along the vertical axis and the deformation factor
 W/D after deforming process plotted along the horizontal axis. The graph
 suggests that achieving W/D=3 requires a forming pressure of about 50-100
 kg f/mm.sup.2, and exerting such a large pressure during the wire joining
 process will damage a semiconductor chip or electronic part. Whereas, the
 inventive wire bonding method and apparatus can exert the large forming
 pressure on the wire 101 by means of the pre-forming mechanisms 140 and
 140A.
 A consequent large wire deformation factor W/D based on the inventive wire
 bonding method and apparatus reduces the wire thickness to 0.6 or less in
 terms of H1/H2, where H1 and H2 are the inventive and conventional
 thickness, as mentioned previously. The smaller wire thickness ratio H1/H2
 signifies the extension of joint life attributable to power cycles as
 shown in FIG. 13.
 FIG. 13 is a characteristic graph showing the joint life of aluminum wire
 attributable to power cycles and the forming pressure plotted against the
 ratio of the inventive wire thickness to the conventional wire thickness.
 On this graph, the ratio of the inventive and conventional wire thickness
 H1/H2 is plotted along the horizontal axis, the ratio of the inventive and
 conventional joint life (RLF) resulting from cyclic power applications to
 the wire is plotted along the first vertical axis to draw a curve 201, and
 the forming pressure P2 exerted on the aluminum wire is plotted along the
 second vertical axis to draw a curve 202.
 The graph reveals that the smaller the wire thickness ratio H1/H2, the more
 extended is the joint life, and also suggests that an increased forming
 pressure is required to make the wire 101 thinner.
 The inventive wire bonding method and apparatus are capable of reducing the
 wire thickness by the provision of the wire pre-forming mechanism, and
 thus extending the wire joint life attributable to power cycles, and the
 inventive high-power semiconductor device is durable against severe heat
 cycles and power cycles thereby to last long, while yet being compact and
 light-weight.
 Although the present invention has been explained specifically for the case
 of wire-bonding a high-power semiconductor device, it is also applicable
 to wire-bonding of other semiconductor devices and electronic components.
 The inventive wire bonding method and apparatus are capable of increasing
 the joint area between the wire and target joint surface without the
 burden of an excessive ultrasonic output, forming pressure and their
 application time length at the wire joining process. Consequently, it
 becomes possible to enhance the wire joint strength, and accomplish
 semiconductor devices and electronic components having improved life
 against severe heat cycles and power cycles.
 Particularly, the inventive wire bonding method and apparatus are designed
 to deform the wire in advance, so that ultrasonic output, forming pressure
 and their application time length can be reduce at the wire joining
 process, whereby it becomes possible to prevent the breakage of
 semiconductor chips and other electronic parts and accomplish reliable
 semiconductor devices and electronic components.
 The invention may be embodied in other specific forms without departing
 from the spirit or essential characteristics thereof. The present
 embodiment is therefore to be considered in all respects as illustrative
 and not restrictive, the scope of the invention being indicated by the
 appended claims rather than by the foregoing description and all changes
 which come within the meaning and range of equivalency of the claims are
 therefore intended to be embraced therein.