Advancements in Packaging of Kilo-Watts Level High Power Diode Lasers for DPSSLs Applications
High power diode lasers with high electro-optical conversion efficiency, compact size, high reliability and long lifetime, have found increased applications in pumping sources for traditional diode-pumped solid-state lasers (DPSSLs) and fiber lasers, as well as direct applications such as material surface processing, illumination, medical treatment and display etc. [1–3]. Pumping of solid state lasers is one of the most important applications for high power diode lasers. The performances of DPSSL are significantly influenced by the main parameters of diode laser pumping sources, such as output power, wavelength, spectral width and beam quality etc. The key merits for diode laser pumping of solid state lasers are 1) efficient transport of pump power to the gain medium; 2) efficient absorption of pump radiation; 3) high uniformity of absorbed pump power density and 4) high durability under various working conditions [4–5]. Therefore, selection of diode laser pumping sources with the right wavelength, narrow spectral width, proper beam distribution, and high reliability needs to be taken into account. Kilowatt level diode lasers are used for pumping high power solid state lasers and they are made of multiple diode laser arrays. They are typically in the form of a vertical stack, a horizontal array or an area array. The thermal management, packaging process and optical effects influence the performance of the kW-level diode lasers significantly. With the increase of high current and output power, the reliability and lifetime of high power diode lasers becomes a challenge, especially for the harsh working conditions such as the repeated on-off power-cycling with long pulse duration and high duty cycling. As a result, packaging technologies for high power diode laser arrays becomes critical.
PACKAGING TECHNOLOGIES FOR HIGH POWER DIODE LASER
The basic packaging structure of a diode laser device consists of laser chip, bonding layer and mounting substrate. The packaging is to bond the diode laser bar on a mounting substrate or a submount with metallization deposited on it to provide mechanical support, thermal management and functional connections. The quality of the packaging critically influences major characteristics of the diode laser, such as thermal behavior, achievable output power, wavelength, spectrum, lifetime, and even the polarization properties etc. [10–12]. The thermal management, structure design and materials selection are very important in heat dissipation, stress reduction and performance improvement. The metallization is critical in achieving high quality packaging and long term stability. The suitable solder layer and mounting substrate should be selected and optimized to meet the requirement of the application.
Package structure of a diode laser influences the major characteristics, such as thermal behavior, output power and polarization . Mounting material, metallization, bonding layer and bonding process should be taken into account in designing package structure. A typical package structure of single-bar diode laser, in which a diode laser bar is mounted on a heat-sink, is shown in Figure 1. The heatsink could be a conduction cooled material such as copper or a liquid cooled structure such as a micro-channel cooled cooler. The laser bar is epi-down mounted on the heat sink. For indium bar bonding, the laser diode bar is commonly directly bonded to the mounting heat sink with indium solder, as shown in Figure 1 (a). To improve the reliability and lifetime, hard solder bonding technology has been developed in recent years. In this package structure, high power diode laser bars can be bonded on a CTE-matched submount such as Copper-Tungsten (CuW) or ceramic. Subsequently, the resulting subassembly is soldered to a high thermal conductive heatsink using other solders, such as SnAgCu or AuSn, as shown in Figure 1 (b). Single-bar diode laser is the basic structure to extend to different categories of diode laser arrays, such as horizontal arrays and vertical stacks. Horizontal array is packaged with the single-bar diode laser horizontally arranged along the cooling plated, while the vertical stack is assembled with the single-bar diode laser vertically stacked by clamping fixture.
Package structure of the conduction cooled diode laser stack (G-Stack) is designed in different concept as show in Figure 2 . Diode laser bar is sandwiched in the CTE-matched submount (e. g. CuW) with hard solder layer (AuSn) to achieve the performances of anti-thermal fatigue, higher reliability and longer lifetime. The diode laser bars are isolated from the cooling plate (or heatsink) with the insulator. The ceramic material with high thermal conductivity is chosen as the insulator. Compared with the conventional indium bonding process, the application of hard solder for bonding the diode laser stack will significantly increase the reliability and lifetime of the devices. In this configuration, double side cooling is achieved as the heat can be dissipated through both the anode and cathode of the mounting submount. G-Stack structure can be directly used as the single diode laser device, or it can be applied as the basic unit for packaging the horizontal arrays and area arrays.
To design and optimize the package structure, thermal design of high power lasers is critical since the junction temperature rise originating from large heat fluxes strongly affects the device characteristics such as wavelength, power, threshold current, efficiency, and reliability . Thermal performances are closely related to the device package structure and solder materials. The ability to design and employ high-quality solder interfaces in high power laser device packages, to understand the physics of the behavior of these packages and interfaces, and to prevent possible functional (optical) and mechanical ("physical") failures is of obvious practical importance. If the accumulated heat cannot be easily dissipated, the raised temperatures at the location of the p-n junction adversely affect the output power, the slope efficiency, the threshold current and the device lifetime. Excessive heat could also cause spectral broadening and wavelength shift. Thermal management of high power diode laser devices becomes a major challenge in laser design, manufacturing and application. In general, the electro-optical conversion efficiency of diode laser is about 55%, and, therefore, 45% of electric energy is converted to heat by non-radiative recombination, Joule heating, optical absorption, etc. In order to decrease the temperature rise in the region of active source, the thermal behaviors of conduction-cooled diode lasers and liquid cooling diode lasers need to be analyzed and the structures of different diode laser can be optimized.
Liquid-cooled single-bar diode lasers: Micro-channel cooler (MCC) has high efficiency of heat dissipation for high power diode lasers, which can operate under CW mode and QCW mode with high duty cycle. MCC is fabricated by very thin copper foils with micro-channel cut outs, and the dimension of micro-channel is about hundreds micrometer width . The structure of micro-channel can be optimized to achieve efficient cooling and more uniform temperature distribution. Figure 3 shows the temperature distribution, pressure drop and flow rate of a high efficiency micro-channel cooler.
Thermal behavior of a single bar 80W hard solder MCC-packaged high power diode laser array in continuous wave (CW) mode is illustrated in Figure 4. The thermal power of simulated device is 80W with the flow of 0.3 l/m. Simulation results shows that the maximum temperature of hard solder MCC packaged high power diode laser array is 45.3 °C.
Figure 5 illustrates the correlation of junction temperature and output power of high power diode lasers at certain thermal resistance. For 808nm diode lasers, a wall plug efficiency of 52% was assumed. The lifetime of high power diode lasers is strongly related to the junction temperature. Small designed thermal resistance and low junction temperature are required for long lifetime of the semiconductor device. If the junction temperature of a diode laser is limited to 40 °C, the reliable output power from a hard soldered CS (HCS) package is 60W and it can reach 140W for a micro-channel cooled device. Because the thermal resistances of HCS is about 0.7 K/W and that of a MCC is normally below 0.3K/W.
Liquid cooled vertical stacks: Figure 6 shows the simulation of the thermal behavior characteristics of a 20bar micro-channel cooled vertical stack for CW applications . The maximum temperature on the cooler of the 20bar MCC stack is about 37 °C at CW 2000W in steady state. It is contributed to the good ability of heat dissipation from the MCC package structure.
Conduction cooled diode laser stack (G-Stack): Conduction-cooled diode laser stacks are mainly operated at low duty cycle. The transient thermal behavior of a G-Stack is important in achieving high performance and reliability. Temperature distribution and heat flux of a 5000W 12bar G-Stack diode laser are shown in Figure 7 (a) and (b) , respectively. The operation parameters, such as pulse width, duty cycle and frequency, have great influences on the performances of high power diode laser. When the pulse duration is less than 300µs, the temperature influence between two neighboring bars is very small and there is no "thermal crosstalk" behavior between neighboring laser diode bars; On the other hand, when the operation time is greater than 300µs, the temperature influence between two neighboring laser bars is significantly increased.
Metallization is required on mounting substrates, heatsink and other parts when soldering process is necessary. Metallization serves as a diffusion barrier and wetting layer between the solder material and the substrate material. It also prevents oxidation or moisture from the atmosphere, decreases the contact resistance, increases the soldering strength and enhances the device reliability . For a mounting substrate on which the diode laser bar is directly bonded, the metallization is generally more sophisticated. Metallic layer coated on the surface of mounting substrate consists of an adhesion layer, a diffusion barrier layer and a wetting and oxidation prevention layer. The function of the adhesion layer is to provide good adhesion to the mounting substrate and to the barrier layer that can withstand high temperatures, low temperatures and temperature cycling.
There are two common metallization structures utilized in manufacturing the high power diode lasers, as shown in Figure 8. One is Ni/Au structure and the other is Ti/Pt/Au structure. Ni/Au metallization layer is deposited on the parts used in high power diode laser packaging, such as mounting substrate, cooler, heatsink, insulator and electrode, as shown in Figure 8 (a). For the latter metallization structure, Titanium (Ti) is widely used as adhesion layer as it has good adhesion to many kinds of metal materials, such as copper and platinum and ceramics. A diffusion barrier layer is deposited on the adhesion layer to prevent or slow down the diffusion between the solder material and the bonding substrate. Good barrier prevents intermetallic formation of the solder material and the mounting substrate materials. Pt, Ni, W and Cr are the typical diffusion barrier layer in diode laser packaging. The wetting and oxidation prevention layer is a sacrificial layer which provides good wetting to the solder material and prevent the barrier layer to be exposed to air and oxidized. The wetting and oxidation prevention layer is generally consumed or partially consumed during die bonding process. Au is the most common wetting and oxidation prevention layer in diode laser packaging. Figure 8 (b) shows the typical three layer metallization structure.
Solder layer: Different solders, such as indium and gold-tin solder are the commonly used bonding layer materials. The selection of solder material is usually driven by the thermal stress limitations and thermal mechanical performance of the package or substrate. If this is not a factor, then cost and environmental factors come into play. Although indium solder has ductile ability, results in recent years showed indium solder bonded lasers have a lower reliability than AuSn solder bonded devices due to the fast electromigration, oxidation and thermal fatigue. For certain applications, high power laser bars with higher reliability and longer lifetime without decreasing high output power are required . In order to obtain the high optical output power and to enhance the reliability, developing hard solder bonding technology becomes a trend.
AuSn solder has been successfully used for highly reliable die attach and fluxless soldering in the packaging process of high power diode lasers. AuSn solder has good thermal and electrical conductivity, high corrosion resistance, no thermal fatigue and the possibility of fluxless soldering. In general, a composition of AuSn at or close to the eutectic point with 80 wt% Au and 20 wt% Sn is commonly recommended.
Figure 9 illustrates the stress-strain characteristics of indium and gold-tin solder. AuSn solder does not quickly degrade from fatigue damage and generally has no fatigue problem even under thermal cycling. This makes AuSn solder the preferred choice when the diode laser is operated under harsh environmental conditions or pulsed mode.
There are other bonding layers developed recently in bonding high power diode laser bars. The nanosilver paste, a novel die-attached material, was used in packaging high power diode laser bars. The test results show that the nanosilver paste was a promising die-attached material in packaging high power diode laser although it was still in the infant stage.
Mounting substrates: The performances of mounting substrate are different for different materials, and each material has its advantages in certain aspects. The selection of the mounting substrate is driven both by the need of matching the CTE of diode laser chip and by providing high thermal conductivity in order to achieve the ability of highly effective heat dissipation and minimize the junction temperature. Table 1 lists a series of commonly used materials for making mounting substrates [19–21].
Copper is widely used as mounting substrate because of the high thermal conductivity and electric conduction. However, due to the CTE mismatch to GaAs, the application of the copper is still limited in packaging of the high power diode lasers. CuW, which is a metal composite, is also commonly used as mounting substrate due to its CTE closely matched with GaAs-based diode laser chip. The thermal conductivity of CuW is much lower than that of copper. For mounting high power laser bars, CuW is generally only served as a stress relief buffer layer and it is bonded with high thermal conductivity copper in order to increase the heat dissipation. Cu-diamond offers not only higher thermal conductivity but also matched CTE with GaAs-based diode laser chip. However, complex and immature fabrication process and high cost prevent it from being widely used today in packaging of high power semiconductor lasers. Aluminum Nitride (AlN) and BeO are ceramic materials. Their CTE are more closely matched to that of diode laser and the thermal conductivity of the materials is decent for low to medium power applications. The diamond can be used in the high power applications due to the high thermal conductivity and high electric resistivity. These materials are used as a mounting substrate for diode lasers especially when electrical insulation is needed .
There are two steps applied to packaging the hard solder microchannel cooled (MCC) high power laser bar. The first step is bonding the laser bar on a CTE-matched sub-mount using the gold-tin bonding technology. Subsequently, the second step is bonding the completed laser bar unit on MCC heatsink using other soldering process. The anode (p-side metallization) is electrically connected to the MCC heatsink, while the cathode (n-side metallization) is electrically connected to the copper N-foil. Insulator was soldered between the MCC heatsink and copper N-foil, as shown in the Figure 10. The assembly demands precise positioning, superior thermal and electrical contact, and reliable contact junctions among all involved parts.
Besides AuSn and indium bonding technology, new materials and process are being explored in packaging high power diode laser bars. A new non-soldered clamp-mounted diode laser bar structure was reported without applying any solder. The module comprises at least one laser chip clamp-mounting between electrodes as shown in Figure 11 .
The bonding process of the conduction cooled diode laser stack (G-stack) is illustrated in Figure 12. The first step is to bond the diode laser bars and the insulator on CTE-matched submounts using AuSn solder. Sophisticated mounting tool is applied for precisely positioning all components together when soldering the diode laser bars. The second step is to bond the diode laser stack on the heat sink through a second hard soldering process. The insulator should be double-side metalized for both bonding the submount and the heatsink with different solder layer . In this configuration, the heat can be dissipated through both the anode and cathode of the submount.
To be continued