FR2868877A1 - Semiconductor laser emitter has laser diode in stack with two heat dissipators for improved thermal properties - Google Patents

Abstract

Assembly comprises a laser diode emitting at a wave length of 0.6 - 2 mcm and first and second (109) heat dissipators, stacked and fixed together by micro-ball (107) welding. One of the dissipators is applied directly to the back of the laser diode, and one or both dissipators can form part of the assembly's housing. One or both dissipators can also incorporate a current generator or modulator and/or an electronic component that controls the operation of the laser diode. The first heat dissipator is made from a semiconductor material such as silicon, silicon carbide, aluminium nitride or gallium nitride; the second one is of a metal element e.g. copper or aluminium or a semiconductor.

Description

Description

  The present invention relates to improving the thermal behavior of a semiconductor laser emitter.

  Generally, the optical transmission subassembly integrates the control electronics necessary for supplying the polarization currents and voltages of the laser component and the laser itself. Such an assembly can take several different forms: support silicon or other semiconductor integrating the interconnection lines between the different elements, support silicon or other structured semiconductors or interconnections are made from son. In normal use, the laser operating in such a subset sees its temperature increase and vary according to the external temperature conditions but also according to the conditions and the level of electrical injection.

  The operation at a high temperature entails the following disadvantages: - a shorter life of the laser - a variation of the emission wavelength detrimental to the operation of the systems in which the lasers are incorporated - A saturation then a decrease of the emitted power which is no longer compensable by an increase in the injection current.

  Thus, the vast majority of laser components, in particular edge-emitting lasers of the Fabry-Perot or Distributed Feedback (DFB) type, incorporate a Peltier-type temperature regulator allowing both compensate for temperature fluctuations and maintain an operating temperature close to room temperature (25 C). This solution introduces a significant cost and additional complexity into the assembly of the laser components. In addition, these temperature regulators are relatively large and hamper the integration and miniaturization of transmitters today engaged in the telecommunications industry.

  In the case of surface-emitting lasers (in English Vertical Cavity Surface Emitting Laser (VCSEL)), the same types of problem mentioned above are encountered and are often amplified, particularly in the case of lasers with vertical emission from materials. quaternary on InP substrate for long emission wavelengths (1.25} pm at 1.65pm). Indeed, so-called Bragg mirrors consisting of periodic stacks of GaInAsP materials and InP are poor thermal conductors and do not allow temperature operation of this type of laser.

  The present invention aims to avoid or at least significantly reduce thermal effects for lasers side and vertical emission. For this purpose, it is proposed different types of microassembly according to the nature of the laser (lateral or vertical emission) using the report by reversal of the laser component on a semiconductor with high thermal conductivity to position thus: 1) to locate the active transmission area closer to a first support acting as a heat sink and 2) to release the rear face of the component for the integration of a second heat sink. Unlike soldering techniques called solders widely used in the context of very high power lasers, the proposed assembly is made from so-called flip chip technologies from ball seals. These techniques are documented in various articles and patents, for example: "Hybridization and positioning method of an optoelectronic component and application of this method to the positioning of this component with respect to an integrated waveguide (FR9210040)" or This process from ball-jointing thus allows a better management of the thermal of the components but also the simultaneous realization of the interconnections for the direct current and current arrivals. modulation of the laser diode. The use of ball connections makes it possible to minimize parasitic elements because of the small size of the balls with respect to the use of wires and thus also to ensure better integrity of the microwave signals. The rear face of the component is then accessible it is proposed a method of removing the substrate and deposition or transfer of a metal element with high heat dissipation to improve the temperature behavior of the laser device. The invention applies similarly to laser components with lateral and vertical emission.

  The invention will be better understood and the various other innovative points will become apparent with the aid of the following description and the related figures which represent: FIG. 1 (a): sectional view of a laser emission by the conventional wafer having a contact on the front face and a contact on the rear face Figure 1 (b): diagram of a laser emission by the conventional wafer having the two contacts on the front face Figure 2: Schematic diagram of the invention and example 25 of laser assembly and heat sink as proposed in the invention (a) Schematic diagram of the invention (b) with N and P contact for the electrical injection of the modulation current on the rear face and the front face respectively (c) with N and P contacts for electrical injection reported on the front panel.

  Figure 3: manufacturing method of the invention in the case of a side emission laser or the two contacts are located on the same side.

  Figure 4: Application of the invention to the particular case of a vertical emission laser.

  Figure 1 shows the conventional stack of semiconductor laser diodes well known to those skilled in the art. Thus, the optical and electrical confinement layers (101), the confinement layers (105) of the optical cavity and the emission layer (104) are distinguished or the laser effect is realized. The injection of the carriers is obtained by the production of two contacts (100), one of which is in front face and the other via the rear face, generally these two contacts inject holes and electrons respectively which will recombine in the active emission area (104) for emitting light. The variant shown in FIG. 1b relates to the same conventional laser structure where the back-face contact is brought opposite by additional etching steps. These two structures are made in the form of manipulable chips.

  FIG. 2 proposes the block diagram of the invention (FIG. 2a) and its application to the case of a lateral emission laser diode where the contacts are front and rear faces (FIG. 2b) and only on the front face (FIG. 2c) The block diagram (Fig 2a) of the invention is shown in its final form. It is clear that the production process itself contributes to the original character of the set and will be described below. The invention consists in hybridizing by reversal with solder balls (107) (so-called flip chip technique) an optical or electronic component (108) on a semiconductor substrate (106) for example made of silicon or are previously integrated with interconnection lines necessary for the operation of the component (108). Other types of support are possible such as: silicon carbide, aluminum nitride, gallium nitride. In a higher complexity level, the current and modulation generator of the laser diode can be integrated on the only heat sink reported.

  In the same way, at least one of the dissipating supports may integrate the electronics allowing operation and / or control of the laser diode.

  The solder balls (107) provide the electrical connections and drain the heat of the operating component to the semiconductor substrate which is a good heat sink. It is clear to those skilled in the art that the greater the number of balls (107) the larger the contact area will be and the greater the thermal drainage to the substrate 106 will be effective. A coating method with a thermal conductive but electrically insulating polymer element makes it possible to improve the thermal resistance and to solidify the assembly. The rear face of the component (108) is then used to integrate either by deposition or also by transfer a second heat sink (109) made of metallic materials (gold, copper, for example) or known for their great thermal conduction properties. (silicon carbide, aluminum nitride, gallium nitride). The electrical connection of the lasers is thus performed at the same time that the heat sinks are directly on the back of the laser or by transfer of a material with high thermal conductivity.

  This arrangement and method can be directly applied to the assembly of laser diodes covering the range of wavelengths ranging from 0.6 pm to 2 pm according to the two configurations (front and rear contacts (fig la) and contacts on the front face (FIG. lb)). Figure 2b shows an example configuration. In this particular case, the second heat sink is carried forward and soldered by balls (107). It also makes it possible to provide the electrical connection on the rear face of the laser diode (112) via balls (109) connected to the power supply lines. component integrated on the semiconductor substrate (106). A second coating (110) makes it possible to solidify the assembly and also to improve the thermal contact between 109 and 112.

  In the particular case of a laser diode having the two contacts in front of the assembly is then very close to the schematic diagram. The transfer of the second heat sink can be in this case achieved either - by direct electrolytic growth of the metal on the back of the component on thicknesses greater than 20 50 - im - by transfer of a block of metal glued by a glue to good thermal conduction. The heatsink can then be completely integrated into the housing or be the housing which significantly improves the heat exchange surface.

  It is this second particular case which is represented in FIG. 2c.

  Figure 3 proposes and details an example of modular manufacturing method according to the geometric and topological characteristics of the laser used. It applies to side-emitting lasers with n and p contacts on the front panel as well as vertical emission lasers called VCSEL. The starting substrate 106 (FIG. 3a) integrates the power supply lines of the laser and the bonding studs of the solder balls for the connection. It may also include other components necessary for the proper functioning of the laser transmitter in question (capacitive passive elements, inductive, resistors, photodiodes, control circuit ...).

  This substrate may be made of silicon, the most commonly used but also various semiconductor materials such as aluminum nitride (A1N), gallium nitride (GaN), diamond.

  First, all the solder balls (107) will be made (FIG. 3b) by conventional techniques.

  These beads will preferably be small around 20pm in diameter so as to position as much as possible. These techniques involve different technological steps of resin, lithography, opening of the contact zones, deposition of the fusible material (indium for example), formatting of the balls. The lateral or vertical emission laser component is then positioned and welded by heat treatment to the array of balls which thus ensures the electrical connection and the thermal contact between the laser and the semiconductor dissipating medium (FIG. 3c). The component is then coated with a thermally conductive and electrically insulating polymer (111) to ensure a mechanical hold and a thermal contact as wide as possible of the laser chip. Note that following the welding process we can have a phenomenon of self alignment of the chip. The production of solder balls as well as the coating methods are described in the processes relating to US Pat. Nos. 5,494,669 and 3,915,348. Resin (114) is then carried out, a conventional method in microelectronics. Polishing the assembly allows to planarize and release the rear face of the component (Fig3e). The polishing also makes it possible to eliminate all or part of the substrate on which the laser has been made and which generally is of GaAs or InP material considered to be poor thermal conductors.

  This is particularly recommended for vertical emitting lasers. From this step, a metal bonding deposit (115) is formed on the entire substrate (FIG. 3f), which subsequently allows electrolytic deposition of the same metal or another metal. At this stage, the size of the dissipator can be defined by conventional resin and photolithography techniques (FIG. 3g). The deposit is then produced over thicknesses ranging from a few tens of microns to 100 μm (FIG. 3h). The steps shown in FIGS. 3i, 3j and 3k consist in eliminating the different resin layers by solvents, thus releasing the entire stack of the device. These steps are technological steps known to those skilled in the art.

  The manufacturing method in the case of a vertical emission laser is similar. It nevertheless presents the difference that we will specify. It is thus necessary to make a hole in the starting substrate allowing the emission of light down without absorption in the substrate 106 The final assembly is shown in Figure 4. This hole can serve as a guide for the fiber. This feature can be avoided if the starting substrate is transparent to the emission wavelength of the laser.

  This type of assembly is particularly interesting for laser applications requiring the generation of high power but also for operations with high intensity modulation (for example, operation of laser diodes with a high modulation frequency of 10 GHz and above) without have to use an external cooler Peltier type. Moreover in this flip chip configuration of the component, the substrate can be completely removed and replaced by a high dissipation material. Thus the heat is evacuated closer to the semiconductor junction. Indeed conventional thinning can hardly be pushed beyond a thickness of 100pm of substrate without posing handling problems.

  The invention is directly applicable also to power electronic devices such as bipolar transistors, field effect transistors which are components having two contacts on the front face and whose architecture and materials are compatible with the method described.

Claims (10)

claims   An assembly comprising a first heat sink, a laser diode and a second heat sink, the laser diode and the first and second dissipators being stacked and assembled by the ball soldering technique.   2. The assembly of claim 1 wherein one of the two dissipators is directly deposited on the rear face of the laser diode.   3. The assembly of claim 1 wherein one of the two dissipater is integrated in the housing.   4. The assembly of claim 3 wherein one of the two dissipators is also part of the housing of the assembly.   5. Assembly according to claim 2, characterized in that at least one dissipator integrates a current generator and modulation and / or electronics for the operation and / or control of the laser diode.   6. The assembly of claim 1, wherein the first dissipator is made from a semiconductor material such as silicon, silicon carbide, aluminum nitride, gallium nitride and the second heat sink is either a metal element deposited in copper (Cu), gold (Au) or a metallic element (copper (Cu), gold (Au)) or semiconductor reported (silicon carbide, aluminum nitride, gallium nitride).   The assembly of claims 1 and 2 wherein the laser diode emits in a wavelength range of from 0.6 pm to 2 pm.   8. Use of the assembly according to claims 1 to 7 wherein the laser diode is more specifically a vertical emission laser from the front or rear face.   9. Use of the assembly according to claims 1 to 4 and 6 wherein the laser diode is replaced by a bipolar transistor.   10. Use of the assembly according to claims 1 to 4 and 6 wherein the laser diode is replaced by a field effect transistor.