WO2022200183A1

WO2022200183A1 - Laser diode assembly and a method of assembling such a laser diode assembly

Info

Publication number: WO2022200183A1
Application number: PCT/EP2022/057061
Authority: WO
Inventors: Firooz Nasser-Faili; Daniel James Twitchen
Original assignee: Element Six Technologies Limited
Priority date: 2021-03-24
Filing date: 2022-03-17
Publication date: 2022-09-29

Abstract

There is described a laser diode assembly comprising one or more laser diodes, a carrier, the carrier including a heat spreader comprising Chemical Vapour Deposition, CVD, diamond, and a layer of solder interposed between the one or more laser diodes and the carrier, the layer of solder comprising silver. A method of making such a laser diode assembly is also described.

Description

LASER DIODE ASSEMBLY AND A METHOD OF ASSEMBLING SUCH A LASER DIODE

ASSEMBLY

FIELD OF THE INVENTION

This disclosure relates to a laser diode assembly and a method of assembling such a laser diode assembly. In particular, it relates to high-power laser diodes.

BACKGROUND

A laser diode is a semiconductor device that converts electrical energy into light, similar to a light emitting diode (LED). Laser diodes make powerful beams of light like ordinary lasers, yet they are compact and rugged, and about the same size as a simple LED. Construction of a very early laser diode is described in US 3,245,002, which describes a basic P-N semiconductor laser diode. The most commonly used construction of laser diode these days is a double heterostructure. Applications of laser diodes include telecommunications, industrial machining and heat treatments.

In general, the power density of a laser helps to determine its potential application, as illustrated by the following examples: power density will determine how fast a laser will cut through sheet metal; a narrow power density range is required when performing laser surgery; for safety testing, power density must be below a specific threshold value to avoid eye damage. The power density reflects the power output of the laser diode and its size (cm ²).

Laser diodes are generally available in two categories: lower power and high power. Low powder laser diodes are widely available with a power of below 1 mW up to 10 Watts, and at wavelengths from 375 nm to 1550 nm. By contrast, high-power laser diodes are commercially available with a power ranging from 20 W to multiple kW, and at a variety of wavelengths, e.g. 640 nm, 760 nm, 808 nm, 1060 nm.

Laser diodes typically have an electrical-to-optical efficiency of 50 to 70%, the rest of the energy being dissipated as heat. As such, heat sinks are commonly used in order to transport heat away, thereby prolonging the life and performance of the diode.

In order to mount or assemble a laser diode assembly, it is known to solder a laser diode to a carrier or submount using a soft solder. The carrier usually includes a heat spreader such as aluminium nitride (AIN), copper-tungsten (Cu-W) or a heat sink such as copper (Cu). A soft solder is one that has high ductility at room temperature. Examples of soft solder include indium-containing solder or tin-lead (Sn-Pb) solder. By contrast, a hard solder is one that has low ductility at room temperature. A commonly used hard solder, particularly for laser diode attachment applications, is gold-tin (Au-Sn).

A problem with these solders is that they can lead to high void density in the case of soft solders, and potentially catastrophic Coefficient of Thermal Expansion (CTE) induced stress in the case of hard solders. This stress is typically found in the laser diode cavity.

It is therefore an aim of the invention to provide a solution that addresses the above-mentioned problems.

SUMMARY OF THE INVENTION

In a first aspect of the invention, there is provided a laser diode assembly comprising one or more laser diodes, a carrier, the carrier including a heat spreader comprising Chemical Vapour Deposition, CVD, diamond, and a layer of solder interposed between the one or more laser diodes and the carrier, the layer of solder comprising silver.

Optional and/or preferable features of the first aspect of the invention are provided in the dependent claims.

In a second aspect of the invention, there is provided a method of assembling a laser diode assembly, the method comprising providing one or more laser diodes; providing a carrier; the carrier including a heat spreader comprising diamond; and attaching the or each laser diode to the carrier using a solder, the solder comprising silver.

The method may comprise assembling the laser diode assembly in accordance with the first aspect of the invention.

Optional and/or preferable features of the first aspect of the invention are provided in the dependent claims BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be more particularly described, by way of example only, with reference to the accompanying drawings, in which:

Figure 1 is a perspective view of a laser diode assembly in a first embodiment of the invention;

Figure 2 is a lateral cross-sectional diagram of the laser diode assembly of Figure 1 ;

Figure 3 is a graph showing the thermal stresses experienced in the laser cavity of the laser diode assembly of Figure 1;

Figure 4 is a perspective view of a laser diode in a second embodiment of the invention;

Figure 5 is a graph showing modelled thermal stresses experienced in the laser cavity of various laser diode assemblies; and

Figure 6 is a flow diagram indicating an example method of assembling a laser diode assembly.

The Figures are not drawn to scale.

Throughout the description, similar parts have been assigned the same reference numerals, and a detailed description is omitted for brevity.

DETAILED DESCTIPION

Diamond has the highest thermal conductivity of all materials, and therefore has excellent heat spreading properties. It has a low CTE of around 1x1 O ⁶ K ¹ at room temperature, compared to around 5 to 6 x10⁶ K ¹ for GaAs. This means that when a device such as a GaAs laser diode is soldered to a diamond heat spreader, considerable stresses can be introduced, which can lead to cracking in the solder (which reduces the heat transfer during use) or damage to the device or the heat spreader. It is therefore desirable during the soldering process to keep the soldering temperature as low as possible. Hard solders such as Au-Sn typically require high soldering temperatures, but give a good thermal contact, whereas soft solders such as indium can give a contact with a high degree of voids, leading to poor thermal contact. The inventors have surprisingly found that a silver based solder, in combination with a diamond heat spreader, offers superb thermal management and provides the low void advantages of hard solders with the low soldering temperature of soft solders. Previous attempts at using a diamond heat spreader with a Au-Sn hard solder have caused residual stress in the laser diode, which has had a negative effect on the performance of the laser diode, impacting properties such as frequency. With a silver based solder, the effect of the CTE mismatch between the diamond and the device is significantly reduced because the soldering temperature is lower. In particular, with a nano-silver solder (i.e. one that contains nano-particles of silver dispersed in a solvent), the effect of the CTE mismatch is further reduced as the melting point of the solder can be lowered compared to that of a solder that uses coarser particles of silver material.

Referring to Figures 1 and 2, a laser diode assembly in one embodiment of the invention is indicated generally at 10. The laser diode assembly comprises a single laser diode 12 mounted on a carrier 14, with a layer of solder 16 interposed between the laser diode 12 and the carrier 14. The laser diode assembly 10 is configured to have a power density of no less than 1 kWcrrr², no less than 2 kWcrrr², no less than 3 kWcrrr², no less than 5 kWcrrr², no less than 8 kWcrrr², and typically between 4 kWcrrr² and 10 kWcrrr².

The laser diode 12 is a high-power laser diode comprising gallium arsenide (GaAs). However, laser diodes comprising alternative semiconductor materials such as gallium phosphide (GaP), gallium nitride (GaN), indium phosphide (InP) or other lll-V semiconductor crystals could equally be used instead. The laser diode is optionally a long cavity, broad area, single edge type, with a double heterostructure construction.

In alternative embodiments, the laser diode assembly 10 may comprise a plurality of laser diodes (not shown). The plurality of laser diodes may be interconnected and arranged in series. Alternatively, the plurality of laser diodes may be arranged individually, separated from one another by a gap.

The carrier 14 comprises a heat spreader 14a and a heat sink 14b. The heat spreader 14a comprises polycrystalline CVD (chemical vapour deposition) synthetic diamond.

The thickness of the polycrystalline CVD synthetic diamond heat spreader 14a may be selected from any of at least 50 pm, 100 pm, 150 pm, 200 pm, 250 pm, 300 pm, 400 pm, 500 pm, 600 pm, 800 pm and 1000 pm. The polycrystalline CVD synthetic diamond 14a has a thermal conductivity at 300 K, selected from any of at least 500 W rrr¹ K ¹, 700 W rrr¹ K ¹, 1000 W rrr¹ K ¹, 1300 W rrr¹ K¹, 1800 W rrr ¹ K ¹, 2000 W m ¹ K ¹and 2200 W m ¹ K ¹.

The flatness and surface finish of the heat spreader 14a affects conductor definition and adhesion and also the quality of the laser diode 12 attachment. The surface flatness may be selected from any of no more than 50 pm, no more than 30 pm, no more than 20 pm, no more than 10 pm, no more than 5 pm, no more than 1 pm, and no more than 0.1 pm. Flatness values are given per 10 mm length. Flatness may be measured by using a dial-gauge thickness measurement tool. The thickness at one side of a diamond part is measured, and then the thickness at an opposite side of the diamond part is measured. The difference between the two measurements is nominally the flatness.

The surface finish of the diamond typically has a roughness Ra value of less than 50 nm, less than 25 nm, less than 10 nm or as low as 1 nm.

In the embodiment shown in Figure 1, the heat spreader 14a is provided as a single, discrete unit. In testing, this combination of laser diode, polycrystalline CVD synthetic diamond and silver based solder results in lower residual Tresca stresses in the laser diode cavity, compared to a laser diode attached to a polycrystalline CVD synthetic diamond heat spreader using a Au-Sn based solder - see graph in Figure 3, in which TM200 refers to a grade of commercially available polycrystalline CVD synthetic diamond having a nominal thermal conductivity of 2000 W nr¹ K ¹.

In a further embodiment of the laser diode assembly, the heat spreader 14a is segmented, as shown in Figure 4. In such an embodiment, the heat spreader 14a comprises a plurality of elongate heat spreader segments 18. The heat spreader segments 18 are arranged side-by- side, soldered to one another. The resultant heat spreader maintains similar thermal conductivity to that of a discrete single-piece diamond heat spreader, with a potentially better CTE match than that of discrete single-piece diamond. The segmented heat spreader 14a has cross-plane mechanical homogeneity, physical properties and rigidity, and also has in plane mechanical flexibility and zonal consistent and homogeneous physical property. Segmentation allows optimization with respect to the design and location of predicted hot spots.

Figure 5 is a graph showing modelled thermal stresses in the laser cavity of various laser diode assemblies. Modelling was performed based on the mechanics of a multi-layered cantilever using the CTEs of various materials, and assuming symmetric bow of the assembly. Once the bow profile is established it is translated to a stress value, as a given bow profile can be used to derive a force load applied to the part across a surface of the assembly, which in turn can be used to model projected stress across the assembly.

The various assemblies are a laser diode attached using AusoSn2o to CuW, a laser diode attached using AusoSn2o (which has a soldering temperature of around 280°C) to BeO, a laser diode attached using AusoSn2o to TM200, a laser diode attached using indium (which has a soldering temperature of around 150°C) to TM200, a laser diode attached using nano-Ag to TM200, a laser diode attached using indium to segmented TM200 and a laser diode attached using nano-Ag to copper. It can be seen that the lowest stresses when attaching to a diamond heat spreader are achieved using nano-Ag to segmented TM200.

As indicated in Figure 5, a segmented polycrystalline CVD synthetic diamond heat spreader performed even better in testing than the unsegmented component of the same material. Only the AusoSn2o with CuW heat spreader, and AusoSn2o with BeO combinations outperformed the aforementioned segmented laser diode assembly. The residual stresses in the laser cavity experienced by the laser diode assembly comprising the segmented polycrystalline CVD synthetic diamond heat spreader were more than three times less when compared to diamond/Au-Sn combination and similarly facet stress was reduced five times.

As mentioned previously, a heat sink 14b is attached to the heat spreader 14a. The heat spreader 14a is located intermediate the laser diode 12 and the heat sink 14b. The heat sink 14b contains copper, although other suitable materials are envisaged. The heat spreader 14a and heat sink 14b are soldered together, preferably using a second silver based solder, similar to the first said silver based solder 16. It is important to note that the heat sink 14b is not essential to the invention and it may therefore be omitted.

Importantly, the layer of solder 16 comprises silver. In a preferred embodiment, the solder 16 comprises a silver paste containing particles of silver dispersed in a solvent or amalgam. The solder 16 comprises particles of silver having a largest linear dimension of no more than 100 nm (hence ‘nano-particles’, or ‘Ag-nano’). The layer of solder 16 optionally has a thickness of 15 to 30 pm.

For reference, in Figures 3 and 5, all layers of solder tested had the same thickness of 25 pm. For the same of completeness, the heat spreader segments 18 in Figure 4 were soldered together using, for example, 20 pm thick Ag-nano solder. By way of example only, a suitable solder that is commercially available, has the properties listed in Table 1:

Table 1

Due to its very high re-flow temperature (961 °C) after sintering, the solidified solder will behave similarly to hard solders. The solvent is an organic binder formulation.

In use, a current is passed through the laser diode 12. Once the current reaches a certain threshold, the laser diode 12 begins to lase in a known manner. Heat is produced as a by product, which is conducted away quickly and efficiently through the diamond heat spreader 14a thanks to the superior qualities of the polycrystalline CVD synthetic diamond, and onwards towards the heat sink 14, if present.

Referring to Figure 6, to assemble the laser diode assembly, the method comprises the steps below.

Step 1

Provide one or more laser diodes as required. The laser diodes may be as described above.

Step 2 Provide a carrier. Preferably, the carrier includes a heat spreader comprising CVD synthetic diamond. The carrier may additionally include a heat sink, such as copper. Alternatively, the heat spreader may contain copper and the heat sink may contain different materials. Step 3

Attach the or each laser diode to the carrier using a silver solder. The silver solder may be as described above. Preferably, the step of attaching the laser diode to the carrier is carried out at a temperature of no greater than 200°C, 250°C, or 300°C. Optionally, the method further comprises cooling the solder at a rate slower than 40 K min ¹, 60 K min ¹, 80 K min ¹, 100 K min ¹, 120 K min ¹.

Ultimately, by using a silver based solder, a laser diode assembly for high-power density components is now realisable since the longstanding attachment problem has been convincingly solved.

Firstly, the silver-based solder has the advantageous ‘hard’ properties of the known Au-Sn based solder, but with a lower melting point, thereby requiring lower soldering temperatures, and minimising the otherwise problematic CTE mismatch.

Secondly, heat is more effectively transported away from the laser diode than when using soft solders because the silver-based solder does not suffer from voids, as soft solders do. At least, the silver based solder is low void, as voids may be inevitable in practice.

While this invention has been particularly shown and described with reference to embodiments, it will be understood by those skilled in the art that various changes in form and detail may be made without departing from the scope of the invention as defined by the appended claims.

For example, although only a single laser diode has been described above, multiple laser diodes may be mounted onto the carrier.

For example, although the heat spreader has been described above as comprising polycrystalline CVD synthetic diamond, single crystal CVD synthetic diamond may be used instead.

For example, although the laser diode has been described above as being a high-power laser diode, this invention finds utility with low-power laser diodes too, though is less beneficial.

Claims

1. A laser diode assembly comprising one or more laser diodes, a carrier, the carrier including a heat spreader comprising Chemical Vapour Deposition, CVD, diamond, and a layer of solder interposed between the one or more laser diodes and the carrier, the layer of solder comprising silver.

2. A laser diode assembly as claimed in claim 1, wherein the solder comprises a silver paste containing particles of silver dispersed in a solvent.

3. A laser diode assembly as claimed in claim 1 or 2, wherein the solder comprises particles of silver having a largest linear dimension of no more than 100 nm.

4. A laser diode assembly as claimed in any preceding claim, wherein the layer of solder has a thickness of 15 to 30 pm.

5. A laser diode assembly as claimed in any preceding claim, comprising a plurality of laser diodes.

6. A laser diode assembly as claimed in claim 5, wherein the plurality of laser diodes are interconnected and arranged in series.

7. A laser diode assembly as claimed in claim 5, wherein the plurality of laser diodes laser diodes are discrete and arranged individually.

8. A laser diode assembly as claimed in any preceding claim, wherein the or each laser diode comprises gallium arsenide (GaAs), gallium phosphide (GaP) or gallium nitride (GaN).

9. A laser diode assembly as claimed in any preceding claim, wherein the heat spreader is provided as a single, discrete unit.

10. A laser diode assembly as claimed in claim 9, wherein the heat spreader is segmented.

11. A laser diode assembly as claimed in any preceding claim, wherein the heat spreader comprises copper.

12. A laser diode assembly as claimed in claim 11 , the CVD synthetic diamond being polycrystalline CVD synthetic diamond.

13. A laser diode assembly as claimed in claim 12, wherein the polycrystalline CVD synthetic diamond has a thermal conductivity at 300 K, selected from any of at least 500 W nr¹K ¹, 700 W rrr¹ K ¹, 1000 W rrr¹ K ¹, 1300 W rrr¹ K ¹, 1800 W rrr¹ K ¹, 2000 W rrr¹ K ¹and 2200 W m-¹ K ¹.

14. A laser diode assembly as claimed in claim 11 , the CVD synthetic diamond being single crystal CVD synthetic diamond.

15. A laser diode assembly as claimed in any preceding claim, further comprising a heat sink attached to the heat spreader.

16. A laser diode assembly as claimed in claim 15, wherein the heat spreader is located intermediate the or each laser diode and the heat sink.

17. A laser diode assembly as claimed in claim 15 or 16, wherein the heat sink contains copper.

18. A laser diode assembly as claimed in any preceding claim, wherein the laser diode is a high-power laser diode.

19. A laser diode assembly as claimed in claim 18, configured to have a power density of no less than 1 kWcrrr², no less than 2 kWcrrr², no less than 3 kWcrrr², no less than 5 kWcrrr², and no less than 8 kWcrrr².

20. A laser diode assembly as claimed in any preceding claim, wherein the laser diode is a heterostructure or a double heterostructure laser diode.

21. A method of assembling a laser diode assembly, the method comprising providing one or more laser diodes; providing a carrier; the carrier including a heat spreader comprising diamond; and attaching the or each laser diode to the carrier using a solder, the solder comprising silver.

22. The method as claimed in claimed 21 , comprising attaching the or each laser diode at a temperature of no greater than 200°C, 250°C, or 300°C.

23. The method as claimed in claimed 21 or 22, further comprising cooling the solder at a rate slower than 40 K min ¹, 60 K min l, 80 K min-1, 100 K min ¹, 120 K min ¹.

24. The method as claimed in claimed 21 to 23, wherein the laser diode assembly is the laser diode as claimed in any of claims 1 to 20.