JPH09172221A - Mounting structure of optical semiconductor device - Google Patents

Abstract

PROBLEM TO BE SOLVED: To enable stable operation when a high frequency signal is inputted, by connecting a transmission board and an optical semiconductor device, through a flexible board provided with a transmission line whose impedance is set as a specified value wherein impedances of the transmission board and the optical semiconductor device are matched with each other. SOLUTION: A first board 20 constituting a second transmission line 21 is connected with a transmission board forming a first transmission line on a base, through a second signal line 21a, as a means for electrically connecting an optical semiconductor device, which is formed on an insulating layer 20a composed of polyimide or the like by metallizing or the like, and ground conducting layers 21b which are arranged on both sides of the second signal line 21a at specified intervals. Thereby parasitic components such as parasitic inductance and parasitic capacitance are not practically generated in electrically connected parts. Hence, deterioration of waveform is not caused when a high frequency signal is inputted, and mounting structure of an optical semiconductor element having excellent high frequency characteristics is realized.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a mounting structure for an optical semiconductor device.

[0002]

2. Description of the Related Art As an example of a mounting structure of an optical semiconductor element having excellent high-speed response characteristics, there is one disclosed in Japanese Patent Laid-Open No. 4-349686 (hereinafter referred to as "publication"). The mounting structure (chip carrier) disclosed in this publication has a base on which an optical semiconductor element is installed, a flexible substrate mounted on the base, and means for electrically connecting the optical semiconductor element and the flexible substrate. It is basically equipped with and. This flexible substrate is composed of a polyimide insulating layer and metal conductor layers formed on both sides of this insulating layer, and these forms an impedance-matched transmission line. In this way, by using a flexible substrate on which an impedance-matched transmission line is formed, instead of the electrode pad on which terminals for connection with an external circuit are formed, it has been conventionally generated in terminals and pads. It is described that the parasitic components such as the parasitic inductance and the parasitic capacitance are substantially eliminated and the high frequency response characteristics are improved.
Also, by using a flexible substrate that is known to have high thermal resistance and flexibility, it is possible to reduce the heat transferred to the optical semiconductor element from the outside and to absorb the stress transferred to the element from the outside. Has been done.

[0003]

However, the mounting structure of the optical semiconductor element disclosed in the above publication has the following problems.

Usually, in such a mounting structure, the characteristic impedance of the transmission line for connection to an external circuit is set, and the resistance value of the terminating resistor electrically connected to the optical semiconductor element is set as the input signal. It is known to set a value that can absorb energy (for example, a value obtained by subtracting the resistance value of the optical semiconductor element from the value of this characteristic impedance) to achieve impedance matching in the entire circuit. There is.

However, according to the mounting structure disclosed in the above publication, metal wires are used for electrical connection between the optical semiconductor element and the flexible substrate, the optical semiconductor element and the terminating resistor. Therefore, there is a problem that sufficient high frequency characteristics cannot be obtained due to the parasitic inductance generated in the metal wire. That is, in the above high-frequency band number GH _Z, since the wire length of the metal wire will affect the operation of the circuit as it is as a parasitic inductance, when the high frequency input, there is a possibility that the waveform deterioration due to impedance mismatching occurs. Although this parasitic component has almost no influence on the operation of the circuit from a low frequency of about the audio frequency to a high frequency of a certain level, it is several G.
When it reaches a high frequency of H _Z or higher, it has an influence that cannot be ignored. That is, mismatch reflection due to this parasitic component occurs, the waveform of the input signal deteriorates, and accurate reception becomes impossible.

Therefore, there is a demand for an optical semiconductor element mounting structure that enables stable operation without deterioration of the signal waveform even when a high frequency signal is input.

[0007]

Therefore, according to the mounting structure of the optical semiconductor element of the present invention, the transmission substrate having the first transmission line formed on the base, the optical semiconductor element, and the transmission of these elements. In a mounting structure of an optical semiconductor element including a substrate and a connecting means for electrically connecting the optical semiconductor element, the connecting means is a flexible substrate on which an impedance-matched second transmission line is formed. And
That is, the connection between the transmission substrate and the optical semiconductor element is not a metal wire, but a flexible substrate provided with a transmission line whose impedance is set to a predetermined value so that impedance matching between the transmission substrate and the optical semiconductor element can be achieved. This makes it possible to achieve a circuit in which a parasitic component is not substantially generated in the electrically connected portion, and therefore there is no fear of waveform deterioration even when a high frequency signal is input.

[0008]

BEST MODE FOR CARRYING OUT THE INVENTION Embodiments of the present invention will be described below with reference to the drawings. Each drawing merely schematically shows the size, shape, positional relationship, etc. of each constituent component to the extent that the invention can be understood, and is therefore not limited to the illustrated examples. Further, the same constituent components are designated by the same reference numerals. In the plan view, the first signal line 13a and the ground conductor layers 13b, 19b, 21
b, 23b and the resistor layer 19a are highlighted by hatching, and these components 13 are shown in the front view.
a, 13b, 19a, 19b, 21b and 23b are omitted.

1A and 1B are schematic explanatory views showing a mounting structure (hereinafter, also simply referred to as a mounting structure) 10 of an optical semiconductor element according to an embodiment of the present invention. In particular, FIG. 1A is a plan view of the mounting structure 10, and FIG. 1B is a front view.

The present invention basically comprises a transmission board having a first transmission line formed on a base, an optical semiconductor element, and connecting means for electrically connecting the transmission board and the optical semiconductor element. It is applied to the mounting structure of the optical semiconductor element. Here, the mounting structure 10 has a ceramic base board 11, a ceramic transmission board 12, a laser diode 15 as an optical semiconductor element, a heat sink 17, and a terminating resistor 19. The first transmission line 13 is composed of a coplanar line composed of the first signal line 13a on the transmission substrate 12 and the ground conductor layers 13b provided on both sides thereof with a predetermined distance. The heat sink 17 is provided adjacent to the transmission substrate 12, and the laser diode 15 is placed on the heat sink 17. The heat sink 17 efficiently radiates the heat generated by the laser diode 15 to the base 11, and at the same time functions as a buffer member that absorbs a difference in thermal expansion between the laser diode 15 and the base 11. As a material for the heat sink 17, SiC, diamond, or the like having an insulating property and a high thermal conductivity is suitable. Further, a terminating resistor 19 for impedance matching is provided adjacent to the heat sink 17.

According to the optical semiconductor element mounting structure of the present invention, the second transmission line 21 whose impedance is matched to the connecting means for electrically connecting the transmission substrate and the optical semiconductor element.
And the flexible substrate 20 is formed.

FIG. 2A is a plan view showing a state of the surface of the flexible substrate on the side where the transmission line is provided. The flexible substrate 20 shown in FIG. 1 and a second flexible substrate 22 described later. The back side of the same is shown in the same figure. First, the flexible substrate 20 will be described. This flexible substrate 20 is particularly referred to as a first flexible substrate (hereinafter, first substrate) 20 in order to distinguish it from a second flexible substrate 22 described later. In this figure, a region surrounded by a dotted line on the right side of the first substrate 20 is a side connected to the laser diode 15 (a junction), and a region surrounded by a dotted line on the left side is the transmission substrate 1.
2 and the side to be connected (to be a joint). Also,
2B is a side view of FIG. 2A.

Here, the connection means for electrically connecting the transmission substrate 12 and the laser diode 15 are the second signal line 21a formed by metallization or the like on the insulating layer 20a made of polyimide or the like, and the second signal line 21a. The first substrate 20 that constitutes the second transmission line 21 with the ground conductor layers 21b provided on both sides of the two signal lines 21a with a predetermined space therebetween.
And Here, at the bonding portion on the first substrate 20, one bump 30 for bonding to the signal input portion (not shown) of the laser diode 15 is formed only on the second signal line 21a. Further, in the joint portion, a total of three bumps 30 are formed, one on each of the second signal line 21a and on the ground conductor layers 21b on both sides thereof ((A) and (B) of FIG. 2). . Bump 30
For example, it is made of gold (Au) or solder. This first substrate 20
With the surface on which these 21a, 21b, 30 are provided facing downward, the joint portion of the first substrate 20 is made to face the first signal line 13a and the ground conductor layer 13b at one end on the transmission substrate 12, and The signal input part of the laser diode 15 is installed so as to face the junction part. And then
The bumps 30 are melted by a suitable heat treatment or pressure bonding to bond the bonding portion and. At this time, the first signal line 13a and the second signal line 21a are connected to each other via the bump 30 at the joint portion, and the ground conductor layer 13b,
21b are connected to each other via bumps 30. At the same time, the second signal line 21a,
The signal input portion of the laser diode 15 is connected via the bump 30 at the joint portion.

Further, here, the second flexible substrate (second substrate) 22 is used as a means for electrically connecting the laser diode 15 and the terminating resistor 19. Second
The substrate 22 includes a third signal line 23a provided on the insulating layer 22a and a ground conductor layer 23b formed on both sides of the third signal line 23a with a predetermined space therebetween.
The transmission line 23 is configured. Further, four bumps 30 are provided at predetermined positions ((A) and (B) of FIG. 2). In FIG. 2A, the region surrounded by the dotted line on the right side of the second substrate 22 is the side connected to the laser diode 15 (referred to as a junction), and the region surrounded by the dotted line on the left side is the termination. This is the side connected to the resistor 19 (referred to as the junction). Here, one bump 30 is formed only on the third signal line 23a at the joint portion. Also, at the junction, the third signal line 23a
A total of three bumps 30 are formed on the upper side and on the ground conductor layers 23b on both sides thereof, one for each.

Here, a conductor pattern extending from the laser diode 15 is formed on the heat sink 17 near the terminating resistor 19 by metallization or the like (not shown).

FIG. 3 is a perspective view showing a pattern for electrical connection on the terminating resistor 19. Here, except for one side of the upper surface of the terminating resistor 19, it is composed of a ground conductor layer 19b shaped like a U-shape and a resistor 19a provided at the center of the U-shape.

Each component 23a, 23b, 3 of the second substrate 22
0 face down, heat sink 1
A portion near the end of the conductor pattern on 7 and the junction are opposed to each other, and the resistor layer 19a on the terminating resistor 19 and the ground conductor layers 19b on both sides thereof are disposed to oppose each other. After that, the bumps 30 are melted by suitable heat treatment, pressure bonding, or the like, and the bonding portions and are bonded to each other to form the mounting structure 10 shown in FIG.

Next, the impedance setting of the flexible substrate will be described. FIG. 4 is an explanatory view of impedance setting of the flexible board, and is a side view of the flexible board. The characteristic impedance of the transmission line includes the dielectric constant ε _r of the insulating layer of the flexible substrate, the gap (gap) G between the signal line and the ground conductor layer, the thickness H of the insulating layer, the thickness t of the signal line, and the width of the signal line. Since it is determined based on the relationship such as w, a transmission line having a desired characteristic impedance can be obtained by setting these ε, G, H, t, and w to appropriate values. For example, when the characteristic impedance of the transmission board 12 is set to 50Ω,
The characteristic impedance of the first substrate 20 of the mounting structure 10 is also 5
Set to 0Ω. If the laser diode 15 has a resistance of about 5Ω, the terminating resistor 19 and the second resistor
The substrate 22 may be set to have 45Ω.

FIG. 5 is a diagram showing an equivalent circuit in the mounting structure 10. 8A to 8C are plan views (FIG. 8A) and front views (FIG. 8B) of a conventional mounting structure 100 using metal wires for electrical connection. ) And an equivalent circuit diagram ((C) of FIG. 8) at this time. The conventional structure 100 will be briefly described. A heat sink 17 having a flexible substrate having a transmission line whose impedance is matched on a base 11 and a laser diode 15 is mounted.
And a terminating resistor 19, a metal wire electrically connecting the flexible substrate and the laser diode 15, and a metal wire electrically connecting the laser diode 15 and the terminating resistor 19.

The mounting structure 10 shown in FIGS. 5 and 8C.
Comparing the equivalent circuit diagram of the conventional structure 100 with that of the conventional structure 100, it can be seen that the conventional transmission line can be set so that the parasitic inductance is substantially generated in the metal wire, but the parasitic component is not substantially generated when the flexible substrate is used.

FIG. 6 shows a mounting structure 10 and a conventional structure 100.
At wire length 0.5mm, 1mm, 2m respectively
3 is a graph showing the results of simulating frequency response characteristics for Samples 1, 2, and 3 where m is m, where the vertical axis represents relative current value (unit dB) and the horizontal axis represents frequency (unit GH _Z ). It is shown. Here, mounting structure 1
The laser diode 15 of 0 was a parallel connection of a resistor (5Ω) and a capacitor (5 pF) (see FIG. 5). The metal wires in Samples 1 to 3 are equivalent to the coils L1 and L2, and the inductance value is 1n per 1 mm of wire length.
H. Further, similarly to the mounting structure 10, the laser diode is a parallel connection of a resistor (5Ω) and a capacitor (5 pF) (see (C) of FIG. 8). These mounting structures 10,
The value of the current flowing through each circuit when a signal with a changed frequency was input to Samples 1 to 3 was calculated as a relative current value (d
B). In the figure, curve I shows the result of the mounting structure 10, curve II shows the result of sample 1, curve III shows the result of sample 2, and curve IV shows the result of sample 3.

Usually, the evaluation of the frequency characteristic is made by judging the -3 dB band (the frequency band when the relative current value shows -3 dB). Looking at the -3 dB band, in sample 1 (wire length 0.5 mm; 0.5 nH), about 6 GH from curve II
_{For Z} and sample 2 (wire length 1 mm; 1 nH), curve III
From about 4.5GH _Z, Sample 3 (wire length 2 mm; 2n
It Waru in H) the curve IV is about 3GH _Z. As described above, the longer the wire length is, the narrower the -3 dB band is, and it can be understood that the wire length directly affects the circuit as a parasitic inductance. On the other hand, in the case of the mounting structure 10 in which the electrical connecting means is a flexible substrate, the frequency band when the relative current value is -3 dB is
The curve I, about 7GH _Z, is wider than the band of the sample 1-3. This is because the impedance-matched ideal transmission line is formed, so that a parasitic inductance component does not occur, and theoretically the sample length is equal to the case where the wire length is 0 (no wire is used). is there. For this reason, the frequency band did not depend on the length of the flexible substrate, and both the first substrates 20 and 22 showed the same results as the curve I in the graph. Therefore, according to the mounting structure of the present invention, even when a high frequency signal is input, waveform deterioration due to impedance mismatch is less likely to occur, and good high frequency characteristics can be obtained.

Further, since the frequency band does not depend on the length of the flexible substrate, there is no limitation on the fixed position (mounting position) of the transmission substrate, the laser diode, and the terminating resistor, and there is an advantage that the mounting can be easily performed.

Obviously, the invention is not limited to the exemplary forms only. For example, the transmission line is not limited to the one formed by metallization. Further, the semiconductor element is not limited to the laser diode and can be applied to other optical semiconductor elements such as an optical modulator. 7A and 7B
[FIG. 6] is a plan view and a front view showing another example (modification) of another mounting mode using an optical modulator, for example. Transmission line 13 for connection to an external circuit (signal line 13a, ground conductor layer 1
3b) are formed on the left and right sides of the optical modulator, and one flexible substrate 20 is provided so as to straddle the optical modulator and the two transmission substrates 13. The connection is made at the same time. here,
No terminating resistor or heat sink is provided on the base 11. The other parts are the same as those of the mounting structure 10, and detailed description thereof will be omitted.

Although bumps are used here to bond the flexible substrates, they may be bonded by suitable heat treatment or pressure bonding, for example, by forming a bonding portion by plating. Further, each transmission line is not limited to the coplanar line, and the transmission substrate, the semiconductor element, etc. do not have to be arranged side by side.

[0026]

As is apparent from the above description, according to the mounting structure of the optical semiconductor element of the present invention,
The connection means for electrically connecting the transmission substrate on which the first transmission line is formed and the optical semiconductor element is a flexible substrate on which the impedance-matched second transmission line is formed. Therefore, substantially no parasitic components such as parasitic inductance and parasitic capacitance are generated in the electrically connected portion. Therefore, there is no possibility of waveform deterioration even when a high frequency signal is input, and a mounting structure of an optical semiconductor element having excellent high frequency characteristics can be achieved.

[Brief description of the drawings]

1A and 1B are a plan view and a front view schematically showing an example of an embodiment of a mounting structure of an optical semiconductor element of the present invention.

2A and 2B are a flexible substrate and a second
It is explanatory drawing of a flexible substrate.

FIG. 3 is a perspective view showing a pattern for electrical connection on a terminating resistor.

FIG. 4 is an explanatory diagram of impedance setting of a flexible substrate.

FIG. 5 is an equivalent circuit in the embodiment.

FIG. 6 is a graph showing a simulation result in the mounting structure of the embodiment.

7A and 7B are a plan view and a front view showing a mounting form of a modified example.

8A to 8C are plan views of a conventional mounting structure,
It is a front view and an equivalent circuit diagram.

[Explanation of symbols]

 11: base 12: transmission substrate 13: first transmission line 13a: first signal line 13b: ground conductor layer 15: laser diode (optical semiconductor element) 17: heat sink 19: termination resistor 19a: resistor layer 19b: ground Conductor layer 20: Flexible substrate (first flexible substrate) 20a: Insulating layer 21: Second transmission line 21a: Second signal line 21b: Ground conductor layer 22: Second flexible substrate 22a: Insulating layer 23 : Third transmission line 23a: third signal line 23b: ground conductor layer 30: bump

Claims (3)

[Claims] 1. An optical semiconductor device including a transmission substrate having a first transmission line formed on a base, an optical semiconductor device, and connecting means for electrically connecting the transmission substrate and the optical semiconductor device. 2. The mounting structure for an optical semiconductor element according to claim 1, wherein the connecting means is a flexible substrate on which an impedance-matched second transmission line is formed. 2. The mounting structure for an optical semiconductor element according to claim 1, wherein the flexible substrate has a ground conductor layer on the same surface as the surface on which the second transmission line is formed, and the transmission substrate includes: A mounting structure for an optical semiconductor element, comprising a ground conductor layer on the same surface as the surface on which the first transmission line is formed. 3. The mounting structure for an optical semiconductor element according to claim 1, wherein the heat sink is provided between the optical semiconductor element and the base, and the heat sink is provided on the base adjacent to the heat sink. An optical semiconductor further comprising a terminating resistor and a second flexible substrate on which an impedance-matched third transmission line for electrically connecting the optical semiconductor element and the terminating resistor is formed. Device mounting structure.