RU2530203C2 - Method of creating interconnections in semiconductor lasers - Google Patents

Abstract

FIELD: physics, optics.

SUBSTANCE: invention relates to the technology of producing semiconductor lasers. The method of assembling semiconductor lasers includes gripping a line of laser diodes with a vacuum tool, fixing the line on the contact area of a heatsink, pressing, heating in an inert reducing gas, holding at a temperature higher than the temperature of formation of a multiphase eutectic interconnection and cooling the obtained unit. In the process of cooling the unit, a temperature gradient forms between the connected surfaces in the volume of the liquid eutectic interconnection, where the vector of the temperature gradient is directed perpendicular to the connected planes and parallel to the direction of growth of the crystallising phases.

EFFECT: reduced tension and high heat conductivity in the laser line.

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Description

The invention relates to semiconductor electronics, in particular to the production of microwave high-power gallium arsenide discrete devices and integrated circuits, hybrid power modules, computer microcircuits and boards, and can also be used in optoelectronics for research, development and production of high-power semiconductor lasers, laser semiconductor arrays and laser systems based on them.

Powerful semiconductor lasers based on A3 B5 heterostructures and solid solutions based on them are most effective for operating in the wavelength range of 0.5-1.8 microns; the coefficient of conversion of electric energy into a directed light wave reaches 75%. Such lasers are used to pump fiber amplifiers and solid-state lasers, to record information, in the manufacture of printers, navigation systems, medical equipment and powerful technological processing devices.

Particularly promising may be the use of laser matrices for the creation of heavy-duty space emitters using solar energy, since the operation of semiconductor lasers requires a low-voltage direct current generated by solar panels.

An experimental comparative analysis of semiconductor lasers from a number of leading companies and analysis of patent literature showed that all firms use the classic version of creating interconnects in the system (LDB-LDC), using either a gasket from indium, Fig. 1, or gaskets from eutectic alloys based on indium, or classic eutectics - (IN-AG; Sn-Au; Sn-Pb).

In work 1, a diagram of a semiconductor laser is shown, Fig. 1, and a laser manufacturing technology is described, in particular, a method for connecting a crystal with a temperature compensator is described: a crystal 1 is attached to the electrodes 2 through spacers 3, which are made of indium based multicomponent eutectic (soft solder) eutectic soldering method. The technological scheme for creating interconnects in the system (LDB-LDC), below, can be clearly described using the symbols of chemical elements as follows: interconnection from pure indium (In) - 1, eutectic soldering with a gasket from the eutectic Ag-In - 2, contact-reactive eutectic soldering (CREP), in the process of which is formed (irregular eutectic structure) - 3 (prototype), Fig.3.1. And finally, the vector gradient reactive eutectic soldering (HREP) - 4, leading to the formation of a regular eutectic structure in the interconnect in the system (LDB-LDC), Fig.3.2:

A significant difference in KTP between semiconductor laser crystals (LDB), a heat sink (LDC) made of copper (Cu), and interconnects characterized by low thermal conductivity is the reason for the reduced laser radiation power, crystal distortion (Smile) and the decrease in the mean time between failures of the semiconductor laser.

The aim of the invention is to increase the thermal conductivity in the interconnects of a semiconductor laser and to reduce the voltage in the line of laser diodes.

A known method of eutectic soldering (2). This method uses an In-Ag alloy of eutectic composition. The irregular structure of the intermediate layer is the low thermal conductivity of the interconnect. An increase in the composition of indium-based solder with a slight increase in the thermal conductivity of the intermediate layer reduces its ductility and, in general, is not effective.

In addition to the method of eutectic soldering with solders having a eutectic structure (In-Ag; Sn-Au; Sn-Pb; Ag-Cu), semiconductor electronics widely use the KREP method - Contact-Reactive Eutectic Soldering. This method (3) is mainly used for mounting silicon crystals on a gold-coated heat sink; It has many advantages over the classic ways to create interconnects:

1 - the use of solders is not required;

2 - eliminated the production of solder foils, gaskets, wires, disks, washers, rods, beads ...;

3 - the use of fluxes is not required;

4 - the process of precision (accurate) alignment of the connected elements is simplified, since solder profiles are excluded, which is extremely important when creating optoelectronic devices;

5 - improves the quality of interconnects and dramatically increases the performance of assembly equipment.

The KREP method as applied to laser semiconductor crystals (A ₃ B ₅ , A ₂ B ₆ , A ₄ B ₆ ) has two significant limitations:

1 - the laser structures of the series (A ₃ B ₅ , A ₂ B ₆ , A ₄ B ₆ ) do not have the property to enter into a low-temperature eutectic reaction with high-heat-conducting metals of the first group of the system of elements of DI Mendeleev, and, therefore, the EPR method (Contact Reactive Eutectic Soldering) for mounting these crystals on a heat sink is not possible;

2 - interconnect (an intermediate layer between the crystal and the heat sink), made by the method of EEC, has a low thermal conductivity; the coefficient of thermal expansion (CTE) differs sharply from the CTE of laser crystals.

An irregular two-phase eutectic structure is not acceptable for system interconnection (LDB-LDC); a laser interconnect requires that a phase having a KTP equal to the KTP of semiconductor laser crystals be the “foundation” for the LDB, and a phase having a thermal conductivity equal to the thermal conductivity of the LDC should be part of an efficient heat sink.

One of the most important tasks in the development of laser technologies is the study and finding of the most general laws and methods for the formation of eutectic low-temperature interconnects with crystals (A ₃ B ₃ , A ₂ B ₆ , A ₄ B ₆ ) and solid solutions based on them in contact with highly heat-conducting metals 1 -th group of the system of elements Mendeleev D.I. in order to use the KREP method in the manufacture of semiconductor lasers.

In new technologies, it is supposed to exclude elements of the classical way of creating interconnects that are widely used in the manufacture of optoelectronic products (solders, gaskets, balls, indium, etc.). For gallium arsenide crystals, the difficulty noted in paragraph 1 on the previous page is eliminated by using the technology presented in the patent “Laser diode line” (4), FIG. 2.

In patent 4, the line of laser diodes (consisting of parallel-connected semiconductor laser diodes based on semiconductor A ₃ B ₅ laser heterostructures, multilayer oxides (rear and front mirrors), as well as multilayer contact metallization on the upper and lower planes of the line of laser diodes) differs in that the outer conductive layers made of elements that are similar (by type, the lattice parameter and thermal expansion coefficient) with a ₃ a _{5-heterostructure} laser and solid solutions of Nove A ₃ B _5; one of these elements is germanium. The correct ratio of the thicknesses between the outer Ge layers on the upper and lower planes of the LDB (line) makes it possible to obtain zero line curvature (Smile = 0) and the minimum voltage in laser crystals. Besides:

1. Germanium is not oxidized in the operating temperature range;

2. The solid surface of germanium (unlike gold) is easy

it is polished that allows to receive an ideal plane

without any roughness;

3. The use of germanium does not introduce dislocations in the LDB;

4. Germanium does not adsorb elements from the environment;

5. The technology for the production of semiconductor lasers using rulers according to application No. 201010430/28 acquires a new quality and provides a high yield.

6. No company in the world produces a line of laser diodes without gold coating the outer planes of the line;

7. No company in the world produces a line of laser diodes coated with the outer planes of the germanium line.

This is the advantage of the KREP method when using the structure in Fig. 2 over the technologies presented in (4, 5, 6). For crystals A ₃ B ₃ (GaAs) used in different companies, the element base of the laser unit (electrode - crystal - electrode) is as follows:

EP 0905838 A1, 03/31/1999 [Au - GaAs - Au],

RU 2007804 C1, 02.15.1994 [Au - GaAs - Au],

EP 1519458 A2, 02.30.2005 [Au - GaAs - Au],

Pat. RF №2455739, 2012 [Ge - A ₃ B ₅ - Ge].

However, all of the listed and analyzed technologies have a significant and unacceptable disadvantage in semiconductor optoelectronics - interconnections in the laser blocks of semiconductor laser devices have an irregular, uncontrolled structure, Fig. 4. The irregular structure of the most common interconnect made of a tin-lead system (Sn-Pb) eutectic alloy obtained by eutectic soldering and KREP method (ΔT = 0) is shown in Fig. 3.1. In Fig.3.2 - the same interconnect, but obtained by the method of HREP.

The aim of the invention is the creation of a controlled regular eutectic structure in the interconnect by the method of vector Gradient Eutectic Soldering (HREP).

In this method, the structure of one phase is coordinated by KTP with a laser crystal and is a continuation of this crystal in the intermediate layer, and the other phase, having high thermal conductivity, is a continuation of the heat sink and, filling the space between the phases, is in contact with the LDB plane; both phases are parallel with respect to each other and perpendicular to the planes LDC and LDB. A model of a given structure is shown in FIG. 5. The model is based on the fact that the equations of heat conduction and eutectic crystallization are identical during the formation of an intermediate layer in the interconnect (LDC and LDB). The heat flux passing through the interconnect (LDB-LDC) per unit time from the lower plane of the crystal having a temperature T ₂ to the upper plane of the heat sink with a lower temperature T ₁ can be expressed as

$$J=\kappa \left(T_2-T_{\mathrm{one}}\right)\frac{A}{h},$$

where κ is the thermal conductivity (proportionality coefficient), J is the heat flux, A is the area of the end face of the interconnect, the thickness of the interconnect, h (Δs) is the thickness of the interconnect. If the area is ΔA, then

$$\Delta J=\kappa \Delta T\frac{\Delta A}{\Delta s}.$$

But ΔJ / ΔA is the absolute value of h - the vector directed to where the heat flows, i.e. from T_one+ ΔT to T_one; the vector is perpendicular to the isotherms of two planes, a. ΔT / Δs is the maximum rate of change equal to ∇T; the directions h and delta T are opposite, and the differential heat equation (heat flux) for the interconnect formed by a rectangular crystal can be represented as a vector equation

h = -κ∇T,

where κ is the coefficient of proportionality (thermal conductivity). This equation was obtained in the analysis of a laser operating in a continuous mode (CW). In the manufacture of a laser unit during crystallization of a eutectic liquid formed in the volume of the interconnect at a temperature of (Te + ΔT) by the reaction (A + B) = Le under conditions corresponding to (T2-T1) = - ΔT, that is, when the temperature is T ₂ the lower plane of the crystal is less than the temperature T _{1 of the} upper plane of the heat exchanger, (HREP method), a solid intermediate layer having a regular structure is formed, figv; the distance between the plates (rods) of this structure X and their growth rate (the growth rate of the solid phase R in the liquid volume) correspond to relations (7), therefore:

$$\begin{array}{l}\lambda =A{R}^{-0.5}\text{,}\Delta \text{t}={\text{<figure-callout id="5" label="BR" filenames="00000008.png,00000009.png" state="{{state}}">BR</figure-callout>}}^{\text{0}\text{,5}},\\ \lambda =-c\Delta T,\text{here s}=\text{A}x\text{B}\\ \text{h}=-\text{to}\nabla \text{T}\text{.}\end{array}$$

A striking resemblance. That is why the method of creating a controlled regular structure in the laser interconnect is called gradient (vector) eutectic soldering - HREP, ΔТ> 0. Figure 5. KREP method (ΔT = 0). The step of the structure λ is regulated not only by the change in the growth rate of the solid phase R in the process of vector crystallization and the temperature gradient ΔT in the interconnect, but also with the introduction of ultrasonic vibrations in the process of solidification of the liquid. The power of ultrasound P and the step of the structure λ in the interconnect are related by the following relation, (8):

$$P=\Delta t_p\left(\mathrm{one}-\frac{\lambda }{{\lambda }_{\ast }}\right)C\rho \tau ,$$

where Δt is supercooling during crystal growth, C is heat capacity, ρ is density, and τ is time. Effective implementation of a structure with desired properties is possible only with good adhesion in the system (LDB contact layer - LDC contact layer). The strength of the intermediate layer (adhesion) F _A is determined by the method described in (9), by the equation:

$$F_A=\frac{\mathrm{one}}{\pi R}\left(\frac{F_x}{hR}-\frac{F_y}{R^2}h\right),\text{Where:}$$

R is the radius of the shear rod;

F _y - pressure on the rod; F _x is the horizontal shear force.

In order to obtain the required adhesion in the Ni-Au system, which is used to metallize almost all laser parts and is an extremely important element in the production technology of high-power laser devices, it is important to understand the quantitative diffusion pattern in the Ni-Au system studied in Ref. 10.

The hypothesis of the invention is that the developed laser model can be implemented if: 1 - the interconnect is formed as a result of contact of solid surfaces (crystal and substrate) by reaction with a phase transition on solid planes; 2 - the interconnect structure is two-phase and controllable; 3 - heat removal from a laser heterostructure operating in CW mode is carried out on a highly efficient heat sink, for example, in high-power semiconductor lasers that deliver at least 250 watts of optical power per crystal in the upper micron wavelength range.

The aim of the invention is to create a controlled regular eutectic structure of the interconnect in the system (LDB-LDC) to increase the thermal conductivity of the interconnect and reduce the voltage in the line of laser diodes.

The goal is achieved by the fact that:

in a method for assembling semiconductor lasers, including capturing a line of laser diodes (LDB) with a vacuum tool, fixing LDB on a contact pad of an LDC heat sink, compressing, heating in an inert-reducing gas medium, holding at a temperature above the temperature of formation of a multiphase eutectic interconnect, cooling the resulting block, according to the invention, when the unit is cooled between the joined surfaces of the LDB and LDC in the volume of the liquid eutectic interconnect, a temperature gradient is created, the vector of which is flax mating planes perpendicular and parallel to the growth direction crystallizing phases, Figure 5, position 8, 9.

As a result, the minimum voltage in the LDB line is achieved (the smile is close to zero). The total thermal resistance Rt (the contact layer of the laser heterostructure (crystal), the interconnection, the contact layer of the heat sink) in the semiconductor laser is minimal, and, therefore, the transmitted optical power of the laser radiation is maximum. Such is the proposed laser model with maximum efficiency and a short MTBF.

The invention is applicable to all types of semiconductor lasers and laser arrays. Thus, single-chip lasers can be manufactured, that is, lasers consisting of a single laser diode, multi-chip lasers — laser diode arrays and laser arrays, which can consist of dozens of laser diode arrays. In this case, contact layers in arrays or in discrete laser crystals can be performed both on one side (+) and on both sides of heterostructures (A ₃ B ₅ , A ₂ B ₆ , A ₄ B ₆ ) (+ and -) or planes of positive and negative electrodes. The invention can be used for the production of conductive and water-cooled heat exchangers (heat-sink) with a regular structure in Cu-W, CU-Mo, AlN-Ge, SiC-Ge, Ge-Ag systems and in element systems having a eutectic type phase diagram.

Our invention has a significant novelty. No one had ever imagined that interconnects in laser optoelectronics could be performed by vector crystallization.

The invention is aimed at research and development of a technology for the production of high-performance semiconductor lasers based on semiconductor structures again, using two-phase high-heat-conducting interconnects in the system (LDB-LDC); it can be used for all methods of creating interconnects, including for the Au-Sn system.

The invention is simple technical implementation. The invention contributes to the improvement of the technology for the production of semiconductor lasers, since obtaining high-conductivity interconnects and low-voltage laser lines with zero smile in multilayer MOS systems (metal - oxide - semiconductor) is a complex technological task.

The drawings shown in the description show the following:

Figure 1. The scheme of a semiconductor laser (EP 1519458 A2, 02/30/2005). Semiconductor laser: 1 - crystal; 2 - thermal mixer; 3 - gaskets from a soft multicomponent alloy based on indium; thermal mixer; 4 - laying of solder; 5 - elektrod-tube; 6 - conductive heat exchanger; 7 - insulator; 8 - negative electrode; 9 - clip.

Figure 2. The structure of the line of laser diodes (RF Patent No. 2455739, 2012): 1 - Ge GDG, 2 - PAlGaAs, 3 - PAlGaAs, 4 - InGaAs, 5 - n A1GaAs, 6 - n AlGaAs, 7 - n AlGaAs, 8 - Ge HPP.

Figure 3. Eutectic Sn-Pb. 3.1 - irregular; 3.2 - regular.

Figure 4. The algorithm of the EECC method. Prototype. Left pillar: 1 - Cu, 2 - Au (Ag), 3 - Ge GDH (In), 4 - A ₃ B ₅ , 5 - contact plane in the system (LDB-LDC); middle pillar: 6 - liquid eutectic (Ag-In), (Ge-Au); right column: 7 - Ge (Ag) crystals; solid irregular eutectic structure.

Figure 5. The algorithm of the hydraulic fracturing method. Invention. Prototype. Left pillar: 1 - Cu, 2 - Au (Ag), 3 - Ge GDH (In), 4 - A ₃ B ₅ , 5 - contact plane in the system (LDB-LDC); middle pillar: 6 - liquid eutectic (Ag-In), (Ge-Au); right column: 8 - crystals of Ge (Ag), 9 - crystals of Au (In) - a solid regular eutectic structure.

Literature

1. EP 1 519 458 A2, 02.30. 2005 year

2. Apollonov V.V., Derzhavin S.I., Kuzminov V.V., Mashkovsky D.A., Prokhorov A.M., Filonenko V.A. Heat exchanger for high-power semiconductor lasers. RF patent №2169977, 2001

3. Filonenko V.A., Toporkov A.V. Eutectic soldering. Electronic Engineering, 1973, ser. 2, issue 3 (75), 120-124.

4. Filonenko V.A., Apollonov V.V., Derzhavin S.I. The line of laser diodes. RF patent No. 2455739, 2012

5. RU 2007804 C1, 02.15.1994

6. EP 0905838 A1, 03/31/1999

7. Filonenko V.A. Classification of double eutectics. Proceedings of the USSR Academy of Sciences, "Metals", 1971, 6, 154-160.

8. Filonenko V.A., Abramov O.V. Crystallization of eutectics in an ultrasound field. Physics and chemistry of material processing, USSR Academy of Sciences, 1974, 1, 45-47.

9. Filonenko V.A. Adhesion mechanism in metal-dielectric systems. ZhFKh AN SSSR, 1976, 3, 726-730.

10. Filonenko V.A., Sokolov A.K., Budanov H.G. Diffusion in the nickel - gold system. "Electronic Engineering", ser. 2, Semiconductor devices, 1977, issue 5 (115). Pg. 40-42.

Claims (1)

A method of assembling semiconductor lasers, including capturing a line of laser diodes (LDBs) with a vacuum tool, fixing the LDBs on the contact pad of an LDC heat sink, compressing, heating in an inert-reducing gas medium, holding at temperatures above the temperature of formation of a multiphase eutectic interconnect, cooling the resulting block, characterized in that when the unit is cooled between the joined surfaces of the LDB and LDC in the volume of the liquid eutectic interconnect, a temperature gradient is created, the vector of which is directed n perpendicular to the connected planes and parallel to the growth direction of the crystallizing phases.

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