Thermal Modeling and Analysis of High Power Semiconductor laser Arrays

Zhiyong Zhang!, Pu Zhang!, Xiaoning Li!,2, Lingling Xiong!, Hui Liu!, Zhiqiang Nie!, Zhenfu Wang!, and Xingsheng Liu!,3

lState Key Laboratory of Transient Optics and Photonics, Xi'an Institute of Optics and Precision Mechanics, Chinese

Academy of Sciences

No. 17 Xinxi Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shanxi, 710119, P.R. China popohopo@163.com, Tel. 8629-88880786, fax 8629-88887075

2Key Laboratory for Physical Electronics and Devices of the Ministry of Education & Shaanxi Key Lab ofInformation Photonic Technique, Xi'an Jiaotong University

No.28, Xianning West Road, Xi'an, Shanxi, 71 0049, P.R. China 3Xi'an Focuslight Technologies Co., LTD

No. 17 Xinxi Road, New Industrial Park, Xi'an Hi-Tech Industrial Development Zone, Xi'an, Shanxi, 710119, P.R. China

Abstract

With the continuous increase of the output power of semiconductor laser array, the heat generation in the active region also increases continuously, which influences the performances and lifetime of semiconductor laser array seriously. In order to improve the performances and lifetime, understanding of the thermal behavior of high power semiconductor laser array packages and optimizing the thermal performance are crucial. By means of numerical analysis, a three-dimensional thermal model has been established, and the static and transient thermal characteristics in continuous-wave (CW) and quasi-continuous-wave (QCW) modes also have been studied systematically for a hard solder, conduction-cooled-packaged 808nm semiconductor laser array. Based on the thermal modeling and analysis, the approaches to reduce thermal resistance have been proposed. The results show that: compared with copper-tungsten (CuW), adopting the copper-diamond composite material as the submount can decrease the thermal crosstalk behavior between emitters, and reduce the thermal resistance by about 30%. In addition, a novel thermal design for the packaging structure of the mounting heat-sink is demonstrated, which has the ability of reducing the thermal resistance of the devices significantly.

Introduction

High power semiconductor laser arrays (sometimes called laser bars), operated at CW or QCW mode, have been widely used in pumping of solid-state laser systems for industrial, science and technology research, military and bio­medical systems, as well as direct applications in material processing (welding, cutting, and surface treatment, etc.) [1-2]. With the increasing development of new application fields, the requirements for the lifetime and reliability of devices are also increasing continuously [3]. For the conventional single-bar CS-packaged high power semiconductor laser arrays, the indium has been adopted generally as the solder joint medium, by which the laser bar is attached to a mounting heat-sink directly, due to its excellent material properties in ductility, thermal conductivity and wettability. But it is also the intrinsic material properties of indium solder that makes the conventional CS-packaging process has several drawbacks, which have been extensively studied, including various failure mechanisms associated with the electro-migration and the electro-thermal migration of the

solder layer under the operation of high current, the thermal resistance of the device increasing due to indium is easily oxidized in high- and low-temperature, damp environment, and other harsh environment [7-9, 11]. All of these influence the lifetime and reliability of devices seriously. Therefore, the indium-free packaging process has drawn significant attention in recent years. A die/bar bonding process using gold-tin solder and CTE matched CuW submount, generally called hard solder technology, has emerged an alternative packaging technology for conduction-cooled semiconductor laser arrays using. Previous study results show that the indium solder bonded lasers have a lower reliability than gold-tin (AuSn) solder bonded devices [7]. The technology of using the AuSn as the solder joint medium can overcome the issues of the electro-migration and the electro-thermal migration of the indium solder layer. Moreover, this hard solder process has the advantages of long-term preservation, high temperature and stable performance, all of which improve the lifetime and reliability of devices greatly.

However, the thermal management challenges still remain with the hard solder technology and what's more, it even becomes serious compared to the single-bar, conduction­cooled-packaged semiconductor laser arrays with indium solder. The reasons are mainly that the thermal conductivity of Cu W submount is poorer than that of copper, and the route of dissipation heat becomes longer due to the introduction of submount. On the other hand, it is generally know that the performances and lifetime of high power semiconductor laser arrays are negatively affected by junction temperature. The threshold current increases, the output power and the efficiency both reduce with the temperature increasing of the active region [12]. In addition, the junction temperature rise could also cause spectral broadening [10] and wavelength shift as well as sacrificing the lifetime and reliability of the devices [3].

Therefore, it is very important to understand the thermal properties and to optimize the thermal design for devices packaged with the hard solder packaging process. In this paper, the static and transient thermal behavior of a hard solder, conduction-cooled-packaged 808nm semiconductor laser array, have been studied systematically and some approaches to reduce thermal resistance of the package are presented.

Device Structure and Modeling Conditions

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As a typical example of a high power passively cooled semiconductor laser array, the configuration of the device is shown in Fig. 1. It consists of four parts: the cathode, the laser bar, the submount and the mounting heat-sink. The bar is bonded to a CuW sub mount with AuSn solder firstly, and then the CuW submount is attached to the copper heat-sink using the indium solder or other solder such as SnAgCu. Due to the coefficient of thermal expansion of the Cu W submount is close to that of the laser bar, the mismatch of coefficient of thermal expansion (CTE) between the laser bar and Cu anode are solved successfully.

N·side(Cathode) I

I Mounting Heat Sink (p·side)

Fig. 1: A sample of a hard solder, conduction-cooled­packaged 808nm semiconductor laser array and the schematic

diagram of the front view of device.

GaAs Substrate

Fig. 2: Schematic diagram of a semiconductor laser array.

Taking a laser array containing two emitters for example, the detailed schematic structure is illustrated in Fig. 2. In actual manufacturing process, according to the fill factor of the array, a laser diode arrays may contains several to tens individual emitters, and the width of emitters is also variable. The quantum well (QW) is the active region, and it is cladded by the cladding layers. The heat is primarily generated inside the emitters in the active region, and the heat generation rate of each emitter is assumed to be the same. The electrical­optical conversion efficiency is assumed to be 50%. In other words, if the output power of a device containing 19 emitters is 40W, the heat generation of each emitter is 2.1 W. In the actual laser diode operation, the bottom of mounting heat-sink is fixed on a thermal-electric cooler (TEC), and the upper surface of TEC is kept at 25°C, so the bottom of mounting

heat-sink has a fixed temperature of 25 °C is assumed in this study. In addition, in order to decrease the computational time and improve the analysis efficiency, half of package structure is used in the modeling due to the symmetrical design of the device.

Based on the above modeling conditions, the steady-state thermal characteristics of a hard solder, conduction-cooled­packaged 40W 808nm semiconductor laser array with 30% filling factor in CW mode and the transient thermal characteristics of a hard solder, conduction-cooled-packaged 250W 808nm semiconductor laser array with 75% filling factor in QCW mode are investigated respectively.

Steady-state Thermal Characteristics

Fig. 3 depicts the steady temperature distribution of quantum well in the laser bar. As shown in Fig. 3, for edge­emitting semiconductor laser array, the peak temperature of the active region is located at the central emitter of the semiconductor laser array and close to the frontal cavity surface. The temperature of frontal cavity surface of each emitter is higher than that of rear cavity surface of each emitter. The lateral cycle variations of the temperature in the active region are consistent with the lateral cycle variations of the structure of the active region. Corresponding to each emitter in the quantum well, there is a sudden change in the temperature. When the output power of the device operated in continuous wave mode is 40W, the peak temperature of the active region is 48.23 °C, by calculation, the thermal resistance of the device is about 0.581KlW.

50

<? 1:0 -1000 CqliOlJ(i -2000 -5000 -4000 -3000 -2000

� X Location (lim) o

Fig. 3: The temperature profile of the active region at the steady state.

By extracting the components of thermal flux of one emitter and its adjacent pitches in the quantum wells from the simulation results, the characteristics of changes of the components of thermal flux versus lateral position are described in Fig. 4. The sign of value represent the direction of thermal flux in Fig. 4. In the emitter regions, the changes of the x-component (perpendicular to the PN junction) value of thermal flux follow a tangent function law approximately, and that of the y-component (parallel to the PN junction) value of thermal flux follow a quadratic function law approximately. At the middle of the emitters, the x-component values of thermal flux are almost zero and the y-component absolute values of thermal flux reach the maximum. When the observation points are in the intersection of emitters and

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pitches, the x-component reach the maximum value, while the y-component drop drastically and its absolute value almost reach the minimum. Furthermore, it is also noticed that, in the pitch regions, the characteristics of changes of the x­component value follow a cotangent function law approximately and the y-component values of thermal flux are not zero, its absolute values could be as high as 5.4E+5 W/um2·K, about a seventh of that in the emitter regions.

With the aforementioned characteristics of thermal flux in the quantum well, some facts could be illustrated. under the extraction of highly thermally conductive copper-heat-sink, most of the waste heat produced in the quantum well mainly dissipated into heat-sink with the "window", of which the sizes are consistent with that of the emitters in the quantum well. For an epi-down bonded laser diode, though the active region is very close to the submount and the thermal conductivity of the insulation layer (Si02) in the bar is poor, there is still a small part of the waste heat that spread in the lateral direction. More importantly, the lateral heat-spreading could cause thermal crosstalk between emitters, thereby leading to the accumulation of a large amount of waste heat in the active region and the poor consistency of temperature of emitters, which may contribute to the spectrum broadening and the poor beam quality.

;:; , � -5 .0,10· .,. � . '; - 1.0 , 1 0

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� 0

- 2.5, I II'

"" . E - 3 . 0xI0 '"

\.J ,. -3.5, I O'

0

,

0

200 400 600 800 1000 6.0,10'·

'" �

..;: �.o.\ 10· �

� � , ,

2.0.<10' ..: , ,

i , \ , , , <=? 0.0

i , , , , i= , -2 .0 .\ 10' Q , , , , � , -4.0.\ 10' � i i

Emiller � Pi,d, J -6.0xI0· e 200 400 600 800 1000 �

Latc .. 1 Position (urn)

Fig. 4: The characteristics of changes of the components of thermal flux in the quantum wells.

The vertical temperature profile of the device at steady state is shown in Fig. 5. The copper heat-sink, the indium solder layer and the CuW submount & AuSn solder each

contribute about 62.7%, l.72% and 3l.51%, respectively, to

the total effective thermal resistance of the device. This fact highlight the significance of the CuW submount & AuSn solder to the total effective thermal resistance of the device.

50 C",,, .\'"II",Oj/1II Chip SubSffllfC 4� & A�� . -.

45 P .. :=.

t I • :: '-' � .. 40 ... 42 ""

= -; 35 ... 39 .. Q. S 30 .. �

25

o 2000 4000 6000 8000 Vertical position (11m)

Fig. 5: The vertical temperature profile of a hard solder, conduction-cooled-packaged 40W 808nm semiconductor

laser array at steady state.

The vector plot of thermal flux distribution of device from the side is shown in Fig. 6. The result indicates that a large amount of heat generated in the laser bar is mainly dissipated from P-side, flowing through the AuSn solder layer, the CuW submount, the indium solder layer, and then injecting into copper heat-sink. According to the direction of thermal flux vector and the divergent angles of thermal flux, it is obvious that the heat dispersing capability of the area near the front cavity surface of resonant cavity is worse, compared to that of the area near the rear cavity surface of resonant cavity. This is mainly because the bar is attached to the edge of the heat-sink for edge-emitting semiconductor laser array .

Frontal Cavity Surface

J\N

Cavity Surface

Fig. 6: The vector plot of thermal flux distribution of device from the side.

Transient Thermal Characteristics

High power QCW semiconductor laser arrays operating at a nominal wavelength of 808 nm with pulse durations of at least one millisecond are required in some actual applications. These relatively long pulse durations could cause the active region of laser diode to accumulate excessive heat, and then the active region experience high peak temperature and drastic thermal cycling, which are considered the primary contributing factors for both gradual and catastrophic degradation of high power semiconductor laser arrays.

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The effects of junction temperature on the semiconductor laser arrays' lifetime can be expressed by using an Arrhenius equation, given by [3]:

Lifetime(,)= Iffie(EalkT)

Where Ea is the empirical activation energy of the device, T is junction temperature in degrees Kelvin, k is Boltzman's constant, I is the drive current and m is a current acceleration factor. Though the theories about the effect of junction temperature and thermal cycling on the semiconductor laser arrays' lifetime and reliability are not mature, the Arrhenius equation could still give us some guidance. According to the equation, the most direct and effective approach to increase the lifetime and improve the reliability of devices is to decrease the junction temperature. Therefore, for the purpose of the effective thermal management of high power semiconductor laser arrays assemblies, analyzing and understanding the transient thermal behavior in the QCW mode become important and critical.

In this paper, the discussions on transient thermal characteristics are carried out from two aspects: firstly, the transient thermal responses of high power semiconductor laser arrays operating in single-pulse condition are investigated, so that we could obtain the overall transient thermal properties of the devices; secondly, the heat accumulation effect is analyzed under multiple pulses at a certain frequency. Understanding these two aspects will help us evaluate the pulse width and operation frequency (or duty cycle) effects on the diode laser array operation in QCW mode.

� Single-pulse Thermal Responses

As illustrated in Figure 7, the transient thermal responses of device in single-pulse (under same pulse repeating frequency and different pulse width) condition are depicted.

In the process of simulation, the temperature sampling point is the node whose temperature is highest in the quantum well. When the high power semiconductor laser array operates in QCW mode, it is obvious that the process of transient thermal responses in single-pulse condition could be divided into two phrases, one is heating up, and the other is cooling down. If enlarging selected region, we may found that the peak temperature of the active region has a exponential relationship with pulse duration; the time required for going back to original state is also relevant to pulse duration. In other words, the longer the pulse width is, the more time is required for going back to original state of the device; the rate of temperature rising reduces gradually with the temperature rises, and if the pulse duration is enough long, the peak temperature would tend to hit the temperature of device at the steady state.

If the device operates at the pulse repetition frequency of 200 Hz, the period of the pulse is 0.005s. From Fig. 7 we can see that the peak temperature of the active region at the time of completion of one pulse cycle goes up with the increasing of pulse width. That is to say, the duty cycle determines the duration of thermal relaxation in the process of cooling down and the peak temperature of the active region. Therefore, the temperature in the active region would rise from a higher value at the starting of next pulse cycle, when the pulse

repetition frequency maintains unchanged and the duty cycle exceeds a certain "threshold" value.

45

f-) 40 � = ... 35 � ... OJ Q. e u 30 I-

· l� : 36

' / 30 --

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� �

- l S0us - 200us - 250us - 350us - 400us - 450us - 500us

0.00 0.0 I 0.02 0.03 0.04 0.05 Time ( s)

Fig. 7: transient thermal responses of the device operating in single-pulse condition, under same pulse repeating frequency

and different pulse width.

� Heat Accumulation Effect

From the above analysis, the temperature of the active region in the beginning of the next cycle cannot go back to the initial temperature, and then the peak temperature of the active region at the end of the next cycle would increase, when the devices operate in a condition, of which the pulse width remains constant, and the duty cycle is higher than a certain value, or the pulse repetition frequency is fixed, and the pulse duration is greater than a certain value. This thermal phenomenon is referred to as the heat accumulation effect [6]. The existence of the heat accumulation effect shows that the device is in the unsteady state. With the increasing of operating time, the semiconductor laser array would reach the dynamic thermal balance state finally, and the heat accumulation effect disappears.

---P '-' 1: = ... � I-.. Q. I: .. f-

45

40

35

30 t tATS

25��--���----.-�-.--�.-� 0.000

Time (s) Fig. 8: transient thermal responses of the device operating at

QCW mode, the schematic diagram of the process of heat accumulation effect.

Taking the transient thermal responses of the device operated at the pulse width of 500us and the duty cycle of 10% for example, as depicted in Fig. 8, we could intuitively comprehend the heat accumulation effect. In Fig. 8, �T denotes the variances in temperature of the active region caused by a single pulse incentive, and �Ti (i=l, 2, 3, 4, 5) denotes the temperature accumulation of the active region

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caused by the part heat being fail to extract from the active region timely in a single cycle. After analyzing the curves, we find that the variances in temperature of the active region modulated by the injection current remain unchanged, aproximatelyand �T is about 17.8 °C; �Ti reduces gradually as the number of pulse periods increases.

Through the curve fitting, it is further found that the temperature accumulation decreases exponentially with the increasing of number of pulse periods, it is shown in Fig. 9, and the relationship follows the following formula:

� Tj=5 .626*exp( -316*t)+ 1.063*exp( -50.83 *t) At the same time, basing on the above relation, if we order t=0.005*n, (n=l, 2, 3, ... ) in sequence , the temperature accumulation per pulse �Tj can be calculated. Moreover, the peak temperature of the active region Tmax, which the devices experience after reaching the state of the dynamic thermal balance, can be also calculated by the following formula:

Tmax=L�Ti+�T

For the operating condition of pulse durations of 500us and pulse repetition frequency of 10%, if assuming that the devices reach the state of the dynamic thermal balance when �Tds less than 0.01 °C, by making some simple calculations, we can know that after the operating time of devices is more than 0.0918s (about 19 pulses), the heat generated in each pulse cycles can be dissipated timely and the heat accumulation effect disappears, and at this moment the peak temperature of the active region Tmax is about 45.93 °C.

It is obvious that those parameters above mentioned could characterize the capacity of dissipating heat of semiconductor laser array in the other aspect. In order to improve the electrical-optical property, increase the lifetime and improve the reliability of devices in QCW mode effectively, we must decrease the peak temperature of the active region Tmax and the temperature accumulation per pulse �Ti modulated by the injection current by reasonable thermal management and effective thermal optimization design.

00 0.005 0.01 0.015 0.02 0.025 003 0035 0.04 0.045 0.05

Time Is

Fig. 9: relationship between the temperature accumulation of the active region and the operating time of devices.

Thermal Optimization

Since the thermal resistance of submount dominates the total effective thermal resistance except the mounting heat­sink, as elaborated as in the sections of steady-state thermal characteristics, the natural approach of improving thermal performance is through the change of material of the

submount. It has been found that the steady state temperature of device reduces obviously when the material of submount is copper-diamond composite material in Fig. 10. By calculations, the thermal resistance could be reduced by about 30%.

50 -P 45 -t:: .: 40 <': � �35 4.1 E- 30

1 E-7

_0_ Cl/W -,,- CII-Di(fmond

1 E-5 1 E-3 0.1 Time (s)

Fig. 10: The curves of the peak temperature of active region versus time of semiconductor laser arrays fabricated from

different submount materials.

Replacing the CuW with Cu-diamond material also could decrease the thermal crosstalk between emitters. Fig. 11 describes the influence of the choice of materials of submount on the variation trend of the self-thermal resistance and interactive thermal resistance of the emitter at the edge of the laser bar. Here, due to the power dissipation Pm of emitter m (the source emitter), the temperature of emitter n (the measure laser) increases from Tn to Tn+�Tn. we define the thermal crosstalk Rnm to emitter n from emitter m as the interactive thermal resistance given by:

Rnm=�Tn/ Pm

When n=m, Rnm is referred to as self-thermal resistance of emitter n. As the Fig. 1 I suggests, for the marginal emitter, the temperature rising depends largely on the self-thermal resistance, the contribution of the adjacent emitter and sub­adjacent emitter to the thermal crosstalk behavior is the most prominent, accounting for about 30.5% and 17.5% of the total interactive thermal resistance respectively. When the copper-diamond composites is adopted as the material of submount, the self-thermal resistance of the marginal emitter and the interactive thermal resistance between adjacent emitters in the semiconductor laser array decrease by 50% and 29.5% respectively.

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5�-----------------�-o-_�C�u��7V� ----' -Cu-Diamond

, Self-thermal Resistance

o 2 4 6 8 10

Serial number of the emitters Fig. 11: the variation trend of the self-thermal resistance and

interactive thermal resistance of the 1 st emitter, when the different submount materials are adopted.

Based on the aforementioned conclusion of the temperature distribution of quantum well at the steady state, we have known that the packaging structure design plays a crucial role in the temperature distribution of quantum well.

In addition, from consideration of the basic theories and knowledge of heat transferring, different materials have different angles of thermal divergence. In other words, the divergent angles of thermal flux in the heat-sink are different due to the thermal conductivities of materials are different. The relation of the divergent angles of thermal flux to the thermal conductivities of materials is given by [6]:

81=90tanh{0.355(nkI180t6}

For the above reasons, in order to enhance the heat dispersing capability of the area near the frontal cavity surface of resonant cavity, improve the threshold of catastrophic optical mirror damage (COMD) and cut down the thermal resistance, the packaging structure design adopts the improvement as shown in Fig. 12. As what the below picture implies, the improving measure is mainly aimed at the structure design of heat-sink. Firstly, moving the semiconductor laser array towards the central area of heat­sink, and denoting by d the removing distance. Secondly, cutting away the part of heat-sink handicapping the output of light near the frontal cavity surface, and the cutting angle 8 depends on the fast axis divergence angle.

J\N CuW Submount

1 Rear Cavity

L Surface

Surface Cu Heat-sink

YSIS OF A SINGLI BAP.

(a) The initial packaging structure.

J\N

THE STATION Ii TP..ANSIENT-STATE THEP.llAL ANALY IS 0' A SIRGLI B.U

(b) The optimized packaging structure.

Fig. 12: schematic of optimized measures for heat sink.

The relation of the divergent angles of thermal flux 8) to the cutting angle 8 is given by:

8)+8=nI2

Keeping the cutting angle at 50 degrees, the relationship between the backward distance and the thermal resistance is shown in Fig. 13. As illustrated in Fig. 13, when the cutting angle is 50 degrees constantly, the thermal resistance of the device decreases gradually and the decreasing rate also shows a downward trend with the backward distance d increasing. It is worthwhile to note that the thermal resistance of the device is 0.506 KIW, at 8 = 50 degrees, d = 1000 um. Comparing to the case of the traditional aligned packaging structure design, the thermal resistance decreases by 12.9%.

;;- 0.58 � --� 0.56 c .;: '"

.� 0.54 ... os E 0.52 ...

..c �

o 200 400 600 800 1000 Distance (urn)

Fig. 13: relation of the backward distance to the thermal resistance.

On the other hand, when d = 1000 um, as the cutting angle reduces gradually, the trend of thermal resistance is discussed in Fig. 14. It is found that the thermal resistance decreases slightly with the cutting angle changing from 50 degrees to 25 degrees. When the cutting angle is less than 25 degrees, the thermal resistance is almost a constant number. Through analysis, the primary cause of the above phenomena is that the divergent angle of thermal flux of copper is about 75 degrees.

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0.58 ;;: 0.56 " � "-' ... 0.54 y = eo: 0.52 -'" 'r;; ... 0.50 I. ----------

� = 0.48 '-...

-= """ 0.46

0 10 20 30 40 50 utting angle (degree)

Fig. 14: relation of the cutting angle to the thermal resistance.

Conclusions

Static and transient thermal behaviors are investigated for the single-bar, hard solder, conduction-cooled-packaged 808nm laser in CW and QCW mode respectively. The laser array's thermal property not only affects electrical and optical characteristics, it is also one of the determining factors to lasers long-term reliability. For the improvement of the overall property of devices, the packaging structure design and optimization from thermal management point of view is necessary. When the copper-diamond composite is adopted as the material of submount, the thermal crosstalk behavior between emitters can be decreased. At the same time, the thermal resistance could be reduced by about 30%. In addition, an approach to reduce overall thermal resistance and facet facet temperature by moving the semiconductor laser array towards the central area of heat-sink and cutting away the part of heat-sink handicapping the output of light near the frontal cavity surface is demonstrated. It can decrease the thermal resistance of devices by at least 12.5%.

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