EP0981842A1

EP0981842A1 - High frequency semiconductor laser module

Info

Publication number: EP0981842A1
Application number: EP98924030A
Authority: EP
Inventors: Heiner Hauer; Albrecht Kuke; Eberhard Moess
Original assignee: Robert Bosch GmbH
Current assignee: Robert Bosch GmbH
Priority date: 1997-05-12
Filing date: 1998-03-21
Publication date: 2000-03-01
Also published as: DE19719853A1; US6456641B1; WO1998052255A1

Abstract

Disclosed is a high frequency semiconductor laser module with a silicone substrate, especially a low resistance siliconized substrate, a laser diode mounted thereon and at least two H-F feeds, one of which is insulated from said siliconized substrate by a dielectric layer. According to the invention, the laser diode is laid on the siliconized substrate by means of a metallic assembly layer, and the H-F layer is moved away so as to be in the vincinity of the laser diode on the dielectric layer.

Description

-High frequency -semiconductor laser module-

The invention relates to a high-frequency semiconductor laser module according to the preamble of claim 1.

State of the art

It is known to use semiconductor laser modules for optical communications in the high-frequency range of several GHz or Gbit / s. Driver circuits for the HF modulation of the semiconductor laser are usually designed in such a way that the hot HF connection is negative and the ground connection has a positive polarity. An RF modulation current must be conducted to the laser diode with as little loss as possible from the driver circuit. For this purpose, microstrip lines, coplanar lines or grounded coplanar lines are used. These lines require a dielectric carrier which, depending on the frequency to be transmitted and on the dielectric constant of the carrier, must have a certain thickness and insulation capability. In the case of microstrip lines and grounded or grounded coplanar lines, the underside of the dielectric carrier is metallized over a large area, at least in the region below the line, and is connected to ground potential. Microstrip lines have a single hot trace on the top of the carrier. at Coplanar lines is the hot one. Conductors on the upper side of the carrier are surrounded on both sides at a defined distance by ground lines. To avoid losses, an RF line must be terminated with its complex wave resistance as precisely as possible. HF cables with a characteristic impedance of 25 Ω, 50 Ω or 75 Ω are usually used. A laser diode has a complex resistance in the size of 3 Ω. To suitably adjust the termination, an ohmic resistor of the appropriate size is connected in series. It must be taken into account that parasitic imaginary resistance components due to the assembly also contribute to the complex overall resistance. Because of the low resistance of the laser diode, bond wires are particularly harmful due to their inductance, while capacitive resistance components make only a very small contribution. Another aspect that must be considered when installing a laser diode is adequate dissipation of heat loss. For this reason, a laser diode must be mounted on a carrier that conducts heat as well as possible.

Taking these technological boundary conditions into account, several construction concepts for high-frequency semiconductor laser modules have been developed according to the prior art. If you build a laser diode on a highly insulating and at the same time good heat-conducting substrate such as aluminum oxide ceramic, almost all of the above-mentioned conditions can be met. Figure 1 shows such a conventional structure. The laser chip 1 is on a substrate or substrate 2 which is metallized and structured on its surface and which is made of a insulating and good heat-conducting material such as aluminum oxide ceramic. This carrier 2 simultaneously carries the electrical HF leads 41 and 43. The hot conductor of these leads is formed as a microstrip line with an upper 41 and lower 42 metallization and the carrier 2 as a dielectric intermediate layer. The ground line 43 is at the same potential as the rear-side metallization 42. A disadvantage here is that no high-precision guide grooves for the optical components required for light coupling, such as microlenses or optical waveguides, can be structured by micromechanical methods on a ceramic substrate. A micromechanical structuring for guide grooves, on the other hand, is possible without problems using anisotropic etching technology on single-crystalline silicon substrates. An example of this is described in DE 3809396 AI.

In addition to the micromechanical structurability, a carrier made of silicon has the advantage that it has a very good thermal conductivity of 151 W / (m * K). The heat loss is therefore very well dissipated from a laser diode soldered directly onto the silicon carrier. However, the low specific electrical resistance of the silicon substrate of approx. 700 Ωcm is disadvantageous. There is also high-resistance silicon with a specific electrical resistance of 6 kΩcm. However, wafers made of this silicon are about 100 times more expensive than normal low-resistance wafers due to the more complex manufacturing process and are therefore out of the question for mass production. If the electrical lines are to be routed on the silicon substrate, a short circuit must be avoided at least one of the feed lines must be isolated from the substrate. Polyimide (PI), for example, is well suited as an insulation layer for the electrical HF lines. The PI layer thickness must be so great that in the case of coplanar lines, the HF field between the lines attached to the PI top side does not reach into the silicon underneath. In the case of grounded coplanar lines or microstrip lines, the silicon substrate is shielded by a metallic base layer to ground potential between the PI layer and the silicon substrate. These cables require PI as a dielectric, which must have a layer thickness of 10 to 20 μm for a frequency range around 5 - 10 GHz.

Combining the possibilities of laser diode mounting on the substrate and the HF line routing known from the prior art, the following two mounting concepts are available on the basis of silicon as a microstructurable substrate.

For example, the laser diode 1 can be attached on top of the PI layer 14 (FIG. 2). This would have the advantage that the RF lines can be led to the laser diode with little loss. Due to the fact that the thermal conductivity of PI compared to silicon is several orders of magnitude, sufficient dissipation of the heat loss of the laser diode through such a thick PI layer would not be possible.

If the laser diode is placed on a ceramic intermediate carrier 4 which is mounted directly on the silicon substrate 10 (FIG. 3), sufficient heat dissipation is admittedly ensures that the additional thickness tolerances of the intermediate carrier 4 and its mounting layers negate the advantage of the high positioning accuracy of the micro-optical components mounted in silicon holding structures with respect to the light exit surface of the laser diode.

Advantages of the invention

The high-frequency semiconductor laser module with the features of the main claim has the advantage that good heat dissipation from the laser diode is guaranteed and an additional height tolerance during laser assembly is avoided, the RF line preferably with PI as a dielectric, preferably as a microstrip line, Co - The planar line or grounded coplanar line is guided close to the laser diode and the laser diode itself is mounted on a metallic laser mounting layer on the silicon substrate.

Further advantageous embodiments of the invention can be found in the subclaims.

In some cases, an insulation layer is required between the metal layer on which the laser diode is mounted and the silicon substrate, as explained below. This insulation layer does not have to have the thickness required for the HF waveguide, since it is only in the immediate vicinity of the laser diode, so that the line losses are only very small. On the other hand, the insulation layer between the laser mounting layer and the silicon substrate must have good heat conduction. The Insulation layer advantageously consists of silicon nitride and has a thickness between 0.2 μm and 2 μm, preferably 1 μm. Silicon nitride is particularly well suited because its thermal expansion (2.8 * 10 ^{~ 6} K ^"1 ) is well matched to the silicon substrate and also has sufficient thermal conductivity at 25 W / (K * m). To improve adhesion a further layer of less than 0.2 μm thickness can be applied under this silicon nitride layer. This layer preferably consists of silicon dioxide. Because of the low thermal conductivity of only 1.4 W / (m * K), the silicon dioxide layer must not be thicker.

drawing

The invention is illustrated by the following figures.

Show it:

Figures 1 to 3 high-frequency semiconductor laser modules according to the prior art;

Figures 4a to 4d schematically first to fourth embodiments of the invention

High frequency semiconductor laser module;

Figures 5a to 5d fifth to eighth embodiments of the high-frequency semiconductor laser module according to the invention; Figures 6a to 6c ninth to eleventh embodiments of the high-frequency semiconductor laser module according to the invention; and

FIGS. 7a to 7c show a twelfth embodiment of the high-frequency semiconductor laser module according to the invention;

Description of the embodiments

For the manufacture of laser diodes, n- and p-doped semiconductor substrates are used. Here, n-doped semiconductor substrates are preferred because they can be produced with a lower dislocation density, so that the laser diodes produced thereon have a better yield and a better aging behavior than in the case of p-doped semiconductor substrates. The laser diode chips can either be mounted so that the epitaxial layers with the active laser layer are on top (Epi-up) or below (Epi-down). The position of the epitaxial layer and the type of doping of the semiconductor substrate determine the position of the cathode and the anode of a laser diode. Since the driver circuits for the laser modulation current are usually designed such that the hot conductor is at a negative potential, the cathode of the laser diode must be connected to the hot conductor. In contrast, the anode of the laser diode can also be at ground potential.

The polarities for a laser diode on an n- or p-doped semiconductor substrate with epi-up or epi-down mounting are shown in FIGS. 4a to 4d. Figure 4a shows a laser diode whose anode is on the top of the chip. Only laser diodes in which the anode is arranged on the underside of the chip can be mounted directly on a conductor layer applied to the silicon substrate. FIG. 4b shows such a laser diode with epi-down mounting on an n-type semiconductor substrate and FIG. 4c with epi-up mounting on a p-type semiconductor substrate. In the case of FIGS. 4a (n-doped semiconductor substrate, epi-down) and 4c (p-doped semiconductor substrate, epi-down), the insulating intermediate layer according to the invention is used.

FIGS. 5a to 5d show the exemplary embodiments according to the invention for the layer structure and the HF supply with the possible chip / substrate polarity shown in FIGS. 4a to 4d in the case of a microstrip HF line. Alternatively, the two other line types known per se, coplanar line and grounded coplanar line, can be used as HF lines.

FIG. 5a shows an exemplary embodiment according to the invention for a laser diode LD with an n-doped semiconductor substrate and top-lying epitaxial layers, and FIG. 5d for a p-doped semiconductor substrate with bottom-lying epitaxial layers. In both cases, a thin, good heat-conducting insulation layer 11 made of approximately 1 μm thick silicon nitride is applied to a silicon substrate 10. Since the silicon substrate 10 is to be micromechanically structured to produce mounting grooves, the insulation layer 11 can also be used as an etching mask for the micromechanical structuring. A first conductor layer lies on this insulation layer 11 12, which is structured in areas 12a and 12k. The region 12a of this conductor layer 12 is at ground potential and is connected to the anode A of the laser diode LD located above via a bonding wire 13a. The laser diode LD with its cathode K located at the bottom is conductively mounted on the region 12k of the conductor layer 12. A structured PI layer 14, which serves as a dielectric for an HF waveguide and typically has a thickness of 10 μm to 20 μm, is arranged over part of the region 12a of the conductor layer 12. The hot conductor 15 of the RF waveguide is structured on this PI layer 14 in a second conductor layer. This is connected to the conductor layer 12k, on which the cathode K of the laser diode is mounted, by means of bonding wires 17k and 18k via the terminating resistor 16 which is arranged in series.

FIGS. 5b and 5c each show an exemplary embodiment for a laser diode with an n-doped semiconductor substrate and epi-down assembly or for a p-doped laser diode and epi-up assembly. An insulating intermediate layer is not necessary here, since the anode of the laser diode LD is at the bottom and is mounted directly on the lower conductor layer 12, which is at ground potential. The conductor layer 12 need not be structured in these exemplary embodiments. The RF waveguide, consisting of a lower conductor layer 12, a dielectric 14 made of PI with a thickness of 10 to 20 μm and a structured upper hot conductor track 5, is brought close to the laser diode LD. The hot conductor 15 is connected via the terminating resistor 16 which is connected in series with the laser diode and via the bonding wires 17k and 18k to the cathode K of the laser diode LD located above. In the above-mentioned exemplary embodiments according to FIGS. 5a to 5d, the RF waveguide can be brought close to the laser diode with little loss and terminated with a terminating resistor 16, which together with the series resistance of the laser diode LD must be as large as the impedance of the RF waveguide . In the above exemplary embodiments, LD bonding wires are used to connect the RF waveguide to the laser diode. The inductive resistance component of these bond wires cannot be compensated for by the capacitive resistance components of the laser diode and is therefore disruptive. In order to remedy this disadvantage, two further exemplary embodiments according to the invention for laser diodes with n-doped or p-doped semiconductor substrates and epi-down mounting are proposed below. Laser diodes are used for this, which have both contacts on the epitaxial side. The assembly and electrical contacting takes place via solder bumps.

FIG. 6 a shows in cross section a laser diode 20 with an n-doped semiconductor substrate and a structured epitaxial side 21 directed downwards. The anode A lies on the active zone 22 and is provided with contact spots 23 there. The cathode K is led out from the semiconductor substrate to the epitaxial side 21 laterally from the active zone 22 and is provided with contact spots 24 there. Corresponding to the position of the contact pads 23 and 24 on the surface 21 of the laser diode 20, corresponding connection pads 33 and 34 are arranged at the corresponding locations on the silicon substrate 10. The connection pads intended for anode contacting can be at ground potential - and do not necessarily require an insulation layer against the silicon substrate. The connection spots 34 connected to the hot conductor 15 of the HF waveguide are electrically insulated from the silicon substrate in the region of the laser diode 20 with the thin insulation layer 11 according to the invention described above. The connection pads 33 are connected to the ground metallization 12 on the silicon substrate. The assembly is carried out according to the known flip-chip method via solder bumps 35 and 36, which are used for mechanical fastening, for electrical contacting, for lateral alignment and for heat dissipation. The solder bumps can either be on the laser diode (chip bumping) or on the substrate 10

(Substrate bumping). The lateral structuring of the layers on the silicon substrate 10 is shown in FIG. 6b. Recesses 33 are provided in the insulation layer 11, which allow contact to the underlying ground metallization 12 for the anode contact. The hot waveguide 15 lying over the dielectric layer 14 is led down to the insulation layer 11 at the end of the dielectric and structured in such a way that the connection pads 34 are formed for the cathode contact. The terminating resistor 16 is advantageously integrated here as a sheet resistor. FIG. 6 c shows the patterning of the connection spots on the epitaxial side of the laser diode 20. A row of contact spots 23 for the anode is arranged on the active zone 22 of the laser diode 20. One or two rows of pads 34 for the cathode lie laterally on one or both sides thereof. FIG. 7a shows an exemplary embodiment for a laser diode with a p-doped semiconductor substrate 20 in epi-down mounting. Here the cathode is on the active zone. In this case, the hot conductor must be led to the center of the chip and the contact surfaces for the ground connection of the laser anode are arranged on the side thereof. Here, the conductor geometry of a coplanar waveguide or a grounded coplanar waveguide can be routed to the contact areas directly under the laser diode 20. The terminating resistor 16 is in turn integrated as a sheet resistor.

The two exemplary embodiments of the module according to the invention explained last with reference to FIGS. 6a to 6c and 7a to 7c do completely without bond wires, so that assembly is simplified and an even higher frequency range is developed by avoiding bond wire inductances.

Claims

claims

1. High-frequency semiconductor laser module, with a silicon substrate, in particular made of a low-resistance silicon, a laser diode arranged thereon and at least two lines for HF supply, one of which is insulated from the silicon substrate by a dielectric layer, characterized in that the Laser diode is attached via a metallic mounting layer on the silicon substrate, and that the RF line is laid close to the laser diode on the dielectric layer.

2. High-frequency semiconductor laser module according to claim 1, characterized in that an insulation layer is arranged between the metallic mounting layer for the laser diode and the silicon substrate.

3. High-frequency semiconductor laser module according to claim 2, characterized in that the insulation layer is a silicon nitride layer.

4. High-frequency semiconductor laser module according to claim 3, characterized in that the silicon nitride layer has a thickness between 0.2 microns and 2 microns.

High-frequency semiconductor laser module according to claim 3 or 4, characterized in that the silicon tridschicht is attached to the substrate via an adhesion improvement layer.

6. High-frequency semiconductor laser module according to claim 5, characterized in that the adhesion improvement layer is a silicon dioxide layer which has a smaller thickness than the insulation layer.

7. High-frequency semiconductor laser module according to claim 6, characterized in that the silicon dioxide layer forming the adhesion improvement layer has a thickness> 0.2 μm.

8. High-frequency semiconductor laser module according to one of claims 1 to 7, characterized in that the

Dielectric layer for insulation of the HF line consists of polyimide (PI).

9. High-frequency semiconductor laser module according to one of claims 1 to 8, characterized in that the

RF line is formed as a microstrip line.

10. High-frequency semiconductor laser module according to one of claims 1 to 8, characterized in that the RF line is formed as a coplanar line, especially as a grounded coplanar line.

11. High-frequency semiconductor laser module according to one of claims 1 to 8, characterized in that the laser diode is connected via solder bumps pointing to the substrate with the RF conductors.