High frequency semiconductor device

A high frequency semiconductor device comprising metal electrode leads formed on one surface of a flexible film, a plurality of bumps formed on selected portions of the electrode patterns, a recessed portion formed on the flexible film within a region bounded by the plurality of bumps and a plurality of electrode pads of a high frequency semiconductor element respectively bonded to the bumps in alignment with each other.

BACKGROUND OF THE INVENTION 
The present invention relates to a high frequency semiconductor device, and 
more particularly to packaging which can provide the semiconductor device 
with an excellent high frequency characteristic. 
Referring to FIGS. 6A and 6B, the previous method for packaging a high 
frequency transistor (3GHz or higher), particularly a GaAs-FET (field 
effect transistor) will be explained. As shown in FIGS. 6A and 6B, a 
GaAs-FET chip 100 is sealed within a package consisting of a base 101 and 
a cap 107 both of which are made of alumina ceramic. In FIGS. 6A and 6B, 
102 are bonding wires, and 103A, 103B and 103C are Au-plated layers with 
which a drain electrode lead 104, a source electrode lead 105 and a gate 
electrode lead 106 are connected, respectively. 
The schematic process for packaging such a GaAs-FET is shown in FIG. 7 of 
the flow chart. In Step 1, the back surface of a GaAs wafer, which has 
been ground and shaped to a prescribed thickness, is metallized with e.g. 
Au on which solder for dice bonding is to be applied. In Step 2, the wafer 
is scribed and broken into a number of chips. In Step 3, each of the chips 
is dice-bonded on the Au-plated layer 103B using e.g. an Au/Sn solder. In 
Step 4, wire-bonding is made for each of the chips by using bonding wires 
102. In this step, it is important to make the source inductance as small 
as possible to improve the high frequency characteristic, particularly, 
the noise factor (F) and the gain (Ga). For this purpose, the length of 
bonding wires 102s is made as short as possible, and the number of the 
wire bondings is increased (4 in FIG. 6A). Finally, in Step 4, the cap 107 
is bonded to the base 101. 
Meanwhile, reduce fabricating costs for microwave semiconductor packages 
have been eagerly demanded; this demand is so great that it cannot be 
satisfied only by reducing the cost for semiconductor chips themselves. As 
a result, cost reductions with respect to assembling the semiconductor 
chip or packaging it have been eagerly demanded. Some microwave packages 
occupy in their assembling and mounting cost almost half the entire cost 
of the semiconductor device. However, the reduction in cost is limited as 
long as the conventional ceramic package is used. Further, the high 
performance of the semiconductor devices has been further required; this 
requirement also cannot be satisfied merely by improving the chips 
themselves, and so must be satisfied in the viewpoint of packaging. For 
example, in order to shorten the length of the above-described source 
wire, the "flip-chip bonding" technique has been proposed for a power FET; 
in the flip-chip bonding technique, bumps formed on the electrodes of a 
ceramic package are bonded with the pads on a chip which are provided in 
opposition to the bumps. However, this technique, which improves the 
performance but uses the ceramic package, is still expensive since it 
requires a step of forming bumps on the expensive ceramic body and cannot 
satisfy the requirement of low cost. 
SUMMARY OF THE INVENTION 
In view of such inconvenience, an object of the present invention is to 
provide a package which can realize the high frequency characteristic of a 
semiconductor device by low cost. 
In order to attain this object, in accordance with the present invention, 
there is provided a high frequency semiconductor device comprising metal 
electrode patterns formed on one surface of a flexible film, a plurality 
of bumps partially formed on the electrode pattern, and a plurality of 
electrode pads of the high frequency semiconductor device bonded to the 
bumps in opposition to each other. In this case, a recessed portion is 
formed on the flexible film among the plurality of bumps. Preferably, the 
surface of a recessed portion formed on the flexible film is partially 
covered with the metal electrode pattern to be connected with the 
electrode pads. 
Further, in accordance with the present invention, there is provided a high 
frequency semiconductor device comprising a plurality of lead frames, a 
plurality of bumps partially formed on a surface of the lead frames one of 
which has a recessed portion among the bumps. Moreover, a plurality of 
electrode pads of electrode pads of the high frequency semiconductor 
device are bonded to the bumps in correspondence each other. 
In accordance with the present invention, the above flip-chip bonding 
technique, in which a high frequency semiconductor chip is bonded onto a 
metal pattern in the condition laid facedown which is opposite to the 
ordinary manner, can be carried out on a film carrier. 
Further, use of metallic bumps permits not only the ordinary wire bonding 
step to be omitted but also the source inductance, which deteriorates the 
high frequency characteristic of the semiconductor device, to be 
restrained. Furthermore, use of the film carrier and provision of the 
bumps thereon permits the semiconductor device to be completed at a cost 
very much lower than that of the ceramic package, and also the floating 
capacitance to be restrained as compared with use of the ceramic carrier; 
this is very advantageous to realize the high performance of the 
semiconductor device. 
Further, in accordance with the present invention, air isolation using a 
recessed portion (gap) is made for electrical isolation between the input 
and output of the semiconductor device, which is particularly essential 
for the high frequency semiconductor device, so that the electrical 
isolation performance can be greatly improved. Further, in accordance with 
the present invention, bumps are formed on a film carrier lead or a lead 
frame and thereafter the semiconductor chip is sealed to provide leads, so 
that the package can be provided at low cost.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
Now referring to the drawings, several embodiments of the present invention 
will be explained. 
Embodiment 1 
As shown in FIGS. 1A and 1B, electrode leads 32 of metal patterns are 
formed on one main surface of a flexible film 30 of polyimide. Bumps 34 of 
gold are formed on the required positions on the electrode leads 32 by a 
technique such as duplicating bumping. For example, in this embodiment, 
the bumps 34 are formed on the leads 32 in alignment with the bonding pads 
of a GaAs-FET 31 chip as a high frequency semiconductor device for 
processing a signal at 3 GHz or higher. In the duplicating bumping 
technique, the bumps 34 can be formed by first forming bumps on a glass 
plate (not shown), bonding the bumps on the metal leads 32 and detaching 
the glass plate from the bumps so that the bumps are duplicated onto the 
metal leads 32. The bumps 34 may be also formed on the leads 32 by a 
technique other than the duplicating bumping. The GaAs-FET 31, which is a 
semiconductor chip, is positioned under recognition of the bumps 34 of the 
electrode leads 32 and the electrode pads on FET 31 for single bonding 
thereto. Additionally, 35 is a gap or recessed portion formed on the 
flexible film 30 between the bumps 34. A cap 36 of ceramic as shown in 
FIG. 1B is not shown in FIG. 1A. In the leads 32 shown in FIG. 1A, S is a 
source lead; D is a drain (output) lead; and C is a gate (input) lead. 
FIG. 2 is an enlarged perspective view in the neighborhood of the bumps 34. 
In this embodiment, the height and diameter of the gold bumps 34 are set 
for 50 to 100 .mu.m and 500 .mu.m.phi., respectively. The height of the 
bumps 34 must be set for a value larger than a prescribed value; if not, 
the parasitic capacitance is increased to deteriorate the characteristic 
of the semiconductor device. 
In this embodiment, it is not necessary to form bumps on the expensive high 
frequency semiconductor device, and the polyimide film as a flexible film, 
which is less expensive and has a lower permitivity than alumina ceramic 
used in the ordinary ceramic package, is used so that the semiconductor 
device can be realized with a low floating capacitance at very low cost. 
Meanwhile, in order for the semiconductor device to acquire excellent high 
frequency performance, it is essential to be able to take electrical 
isolation, which is represented by -.vertline.S.sub.12 .vertline., between 
the input and output of the semiconductor device. The degree of isolation 
substantially depends on the coupling capacitance between the input and 
output. Where the input/output electrodes are formed on the polyimide film 
as in this embodiment, the isolation will be deteriorated because of the 
absence of an earth pattern for shielding between the input and output 
unlike the prior art as shown in FIGS. 6A and 6B. In the FET 31 chip 
actually used, its source and drain are very near to each other (the 
distance therebetween is very short as about 3 .mu.m) so that the degree 
of isolation at this portion is critical for the high frequency 
semiconductor device. In this embodiment, in order to obviate this, as 
shown in FIG. 1B, a recessed portion (gap) 35 having a depth of about 200 
.mu.m-300 .mu.m is formed between the input electrode and output electrode 
on the flexible film 30; without the presence of e.g. resin having large 
permittivity the input and output are isolated by an air layer thus formed 
to enhance the degree of isolation. Further, the source electrode lead 32 
(S) is extended on the surface of the gap 35 to provide the shielding 
effect so that the degree of electric isolation can be further enhanced. 
Thus, in an example of packaging a HEMT semiconductor chip which is a kind 
of GaAs FET the degree of isolation can be improved by 5 dB in a Ku band 
as shown in the following table. This result is very advantageous for the 
high frequency semiconductor device for 9 Hz or higher. Also, the noise 
factor NF can be improved by 0.1 dB. 
TABLE 
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ISOLATION NF 
______________________________________ 
THIS EMBODIMENT -25 dB 0.8 dB 
(PRESENCE OF GAP) 
NO GAP -20 dB 0.9 dB 
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(measured at the frequency of 12 GHz) 
The semiconductor chip 31 bonded to the electrode leads 32 on the flexible 
film 30 through the bumps 34 is sealed by the ceramic cap 36. Such sealing 
is incomplete to remove humidity; however, this is not significant as long 
as the semiconductor chip 31 is completely passivated by SiN. The package 
constructed in accordance with this embodiment is not so different from 
the conventional ceramic package in appearance so that the completed 
semiconductor device can be mounted in a circuit in substantially the same 
way as the conventional process. 
Further, the semiconductor chip may be molded by resin as shown in FIGS. 3A 
and 3B or covered with a flexible resin film in place of being sealed by 
the ceramic case 60. 
Embodiment 2 
FIGS. 4A and 4B shows the high frequency semiconductor device according to 
still another embodiment of the present invention; this embodiment 
provides a beam lead type semiconductor device. FIG. 5 shows an enlarged 
view of the semiconductor device in the neighborhood of the bumps 34. As 
seen from FIG. 5, bumps 34 are formed on leads 62 to be constructed by a 
lead frame by e.g. the duplicating bumping technique as mentioned 
previously. The respective leads are made integral at the position not 
shown. 
The process for fabricating the semiconductor device according to this 
embodiment will be explained. First, as seen from FIG. 4B, in order to 
provide the shielding effect between the input and the output of the FET 
chip 31, or between the gate and drain thereof, the source electrode lead 
62 (S) is bent to provide a recessed portion or gap 35. Thereafter, bumps 
are attached on the leads as shown in FIGS. 4A and 5. The source electrode 
lead 62 (S) is not required to be bent to provide a recessed portion as 
long as the bumps 34 are 50 .mu.m or higher. Next, as seen from FIGS. 4A 
and 4B, a ceramic case 60 is attached to sandwich the leads 32. Finally, 
the integral portion (not shown) of the leads, i.e. a part of the lead 
frame is cut to separate the leads individually. In this way, the beam 
lead type semiconductor device is completed. The semiconductor chip, in 
place of being sealed by the ceramic case 60, may be molded by resin as 
shown in FIGS. 3A and 3B or covered with a flexible resin film. The beam 
lead type semiconductor device thus completed has advantages of very small 
size and low cost (about half the conventional device using the ceramic 
package). 
In accordance with the present invention, a superior high frequency 
characteristic and low cost can be realized simultaneously for the high 
frequency semiconductor device operating at 3 GHz or higher. This is 
advantageous for reducing the production cost of an SHF converter used in 
broadcasting-by-satellite or satellite communication. Thus, the present 
invention has high industrial value in fabricating the high frequency 
semiconductor device with high performance.