Download Thermal Analysis of High Power Pulse Laser Module

Thermal Analysis of High Power Pulse Laser Module
JinHan Ju
PerkinElmer Optoelectronics
Salem MA 01970
Abstract
Thermal management is very critical in laser diode packaging, especially for a high power laser module,
because the excessive heat generated in the laser diode can result in thermally induced optical failures, and
mechanical failures like micro crack in tunnel junction due to high thermal stress, and also lead to long
term reliability problems due to laser chip degradation and joint fatigue. In this paper, thermal analysis has
been conducted on a 3-D finite element model of a high power pulse laser module to evaluate the thermal
performance of different chip bonding media, different carrier and main substrate materials, for both steady
state of the module and transient analysis of pulses of nanoseconds. Thermal stress is also calculated for
different bonding media for comparison. A program using APDL of ANSYS has been developed for the
analysis that helps the laser module packaging material selection and process evaluation.
Introduction
Laser diode packages have been widely used in various applications including medical and biomedical
instruments, telecommunications, consumer products, military and space industries. The performance and
reliability of the laser diode module is highly dependent on how it is packaged. Thermal management is
very critical in laser diode packaging, especially for high power laser module, because the excessive heat
generated in the laser diode can result in thermally induced optical failures, and mechanical failures like
micro crack in tunnel junction due to high thermal stress, and also lead to long term reliability problems
due to laser chip degradation and joint fatigue.
The semiconductor Gallium Arsenide (GaAs) laser chip is bonded onto a ceramic substrate (carrier). The
Chip on Carrier (CoC) is attached onto a main ceramic board and then using thermally conductive epoxy
glued to a heat sink. The heat sink is eventually mounted on the Kovar package body. The laser chip
bonding material can be hard solder, soft solder or thermal and electrical conductive epoxy in different
applications. Hard solder results in high thermal stress because of its high melting temperature, but it yields
high bonding strength and long term joint reliability; soft solder has lower melting temperature, but it
incurs time and temperature dependent creep deformation at low stress level; while epoxy has easier
manufacturing process, but it has much lower thermal conductivity and also other electrical and material
stability issues.
In this paper, thermal analysis is conducted on a 3-D finite element model of high power pulse laser module
to evaluate the thermal performance of different chip bonding materials, different carrier materials and
main board materials. A set of materials is defined for the laser module, based on the steady state tunnel
junction temperature from a designed experiment matrix. Thermal transient analysis of pulses of
nanoseconds is calculated, as well as the thermal stress in the laser chip. A program using APDL of
ANSYS has been developed for the analysis that helps the laser module packaging material selection and
process evaluation. The bonding material and substrate material with higher thermal conductivity are
preferable to achieve lower laser chip working temperature and the bonding material with lower melting
temperature is preferable to achieve lower thermal stress in the laser chip. To determine an optimum set of
material and an optimum laser module operating temperature, not only should the relationship of laser
performance and life span with respect to temperature, but also the thermal stress and bonding strength be
considered and evaluated according to different laser chips and their different applications.
Analysis
The FEA model of the laser module is built in ANSYS as shown in Figure 1, which is half of the laser
module due to its structural symmetry. Laser chip (100 microns thick) material is Gallium Arsenide (GaAs)
and its Quantum Well and active Tunnel Junction (5 microns thick) are AlGaAs and InGaAs. The carrier
and main board are ceramic substrates made of Aluminium Nitride (AlN) or Alumina (Al2O3). The heat
sink is either the same as the package body material Kovar, or some other material, for example Aluminium
alloy, or copper.
Figure 1. FEA Model of Half Laser Module
In this study, Au80Sn20 is selected as hard solder, Sn60Pb40 as soft solder to be the laser chip bonding
materials, and AlN or Al2O3 as carrier and main board, Kovar or Aluminium alloy as heat sink for laser
packaging material evaluation.
Semiconductor material and ceramic material have strong temperature dependent thermal and mechanical
properties, like GaAs in the laser operating range, its thermal conductivity K=56968.5T^(-1.23) W/m-°K
being used in the simulation. Here in Table 1 gives the thermal conductivity K and Coefficient of Thermal
Expansion (CTE) at temperature of 25°C [1] [2] [3]. Orthotropic material properties are used for laser chip
and others are isotropic.
Table 1. Some Material Properties
Material
GaAs
Junction
AlN
Al2O3 Kovar
K (W/m/K)
51
10
215
15
CTE (ppm)
5.0
4.8
4.9
5.0
Al
AuSn
SnPb
Epoxy
15
200
251
57.5
2.5
5.8
24.0
16.0
21.0
28.0
First of all, the tunnel junction temperature Tj at steady state is taken as the target and a lower laser chip
working temperature is preferred, therefore, a 2-Level 4-Factor Design of Experiment matrix [4] as shown
in Table 2 is worked out to evaluate the sensitivity of each variable.
Table 2. Design of Experiment Matrix
Run
Chip Bond
Carrier
Main Board
Heat Sink
Tj (°C)
1
SnPb
Al2O3
Al2O3
Kovar
70.129
2
SnPb
Al2O3
AlN
Al
57.590
3
SnPb
AlN
Al2O3
Al
47.884
4
SnPb
AlN
AlN
Kovar
54.742
5
AuSn
Al2O3
Al2O3
Al
59.499
6
AuSn
Al2O3
AlN
Kovar
65.284
7
AuSn
AlN
Al2O3
Kovar
56.352
8
AuSn
AlN
AlN
Al
46.003
The base is controlled at testing environment temperature 25°C. The power applied on the active Tunnel
Junction is pulse of 1760W for 6.5 nanoseconds at the frequency of 22KHz, so for steady state analysis, the
average power Pa is applied on the tunnel junction, where Pa = 1760*6.5E-9*22E3 = 0.25 W.
The transient analysis is done for 100 seconds using the average power and the first 5 pulses using pulse
power input, followed by the thermal stress analysis on the laser chip based on its working temperature.
Analysis Results & Discussion
Steady State Thermal Analysis
The steady state thermal analysis is conducted at testing temperature of 25°C for the DOE matrix and the
tunnel junction temperatures calculated are shown in Table 2 whereby we can get the average Tj for each
material candidate from all the combinations and then the sensitivity of the junction temperature to
component material selection, or the impact of the material selection from Tj difference of its change, as
listed in Table 3. So, the biggest impact on the junction temperature is the carrier material, 11.88°C and
followed by the heat sink material, 8.88°C.
Table 3. Sensitivity of Junction Temperature to Component Material
Component
Material
Average Tj (°C)
Tj Difference (°C)
Chip Bond
SnPb
57.59
0.80
AuSn
56.78
Al2O3
63.13
AlN
51.25
Al2O3
58.47
AlN
55.90
Kovar
61.63
Al
52.74
Carrier
Main Board
Heat Sink
11.88
2.56
8.88
Of course, there are other factors that we have to take into account when we determine the material set,
besides the thermal effect, such as the material cost of AlN being higher than Al2O3, and AuSn solder
having higher reflow temperature to induce higher thermal stress in the chip on carrier, and other
manufacturing process related factors. Therefore, for example here in this study we would choose SnPb for
chip bonding material, AlN for carrier, Al2O3 for main board, and Al for heat sink as the selected material
set for the laser chip packaging.
Transient Thermal Analysis
The transient of first 100 seconds are calculated for the laser chip module by using the average power, as
shown in Figure 2. The tunnel junction temperature reaches 47.76°C, which is about 99.5% of the total
temperature rise from testing temperature 25°C to steady state temperature 47.88°C.
Figure 2. Laser Chip Transient in First 100 Seconds
Thermal transient of the laser module by applying pulse power is also simulated and the first 5 pulses are
shown in Figure 3 that the laser module temperature is climbing up pulse after pulse, and only the area near
the junction follows the pulse temperature fluctuation. The tunnel junction temperature at pulse peak is
about 10°C higher than chip bottom.
Figure 3. First 5 Pulse Cycles of Laser Chip
Thermal Stress Analysis
Thermal stress is calculated at steady state by converting thermal element to structural element and
transferring thermal results to structural analysis, for both AuSn and SnPb solder materials, based on the
assumption of stress free at solder solidifying temperature.
The feature of element birth and death is used in this study to simulate the laser module packaging process
flow at different temperatures, from chip bonding at solder melting temperature to CoC attachment to main
board and the main board attachment to package body at epoxy curing temperature.
Table 4 gives the maximum von Mises stress of the laser chip operating at 25°C. The stress contour of the
module using SnPb solder is shown in Figure 4.
Figure 4. Stress Distribution of Laser Module
Table 4. Laser Chip Stress for Different Solders
Material
AuSn
SnPb
Laser Chip
106.66
30.12
Tunnel Junction
21.38
4.67
Some laser modules operate at very low temperature, down to minus 40°C or minus 55°C. It should be
noted that the thermal stress in the chip and solder joint would increase accordingly, as the operating
temperature decreasing. For example, the chip stresses are as listed below in Table 5 for operating
temperature of minus 40°C.
Table 5. Stress in Laser Chip Operating at - 40°C
Material
AuSn
SnPb
Laser Chip
132.61
47.17
Tunnel Junction
22.37
5.23
Here in this study, the analysis is only focused on the laser chip, and further analysis is needed for the
bonding material layer on its stress and strength, time and temperature related stability, according to laser
module design, packaging materials and application.
Conclusion
The thermal analysis is conducted on the high power pulse laser module for different laser chip packaging
materials, which is very helpful to evaluate various situations to eliminate numerous expensive prototypes
and difficult measurements. The packaging material set should be evaluated to cost-effectively achieve
lower laser chip working temperature. To determine an optimum laser module operating temperature, not
only should the relationship of laser performance and life span with temperature, but also the thermal stress
in the chip and joints be considered and evaluated according to different laser chips and their different
applications.
References
1. R.O.Carlson, G.A.Slack and S.J.Silverman, “Thermal Conductivity of GaAs and GaAsP Laser Semiconductors”, Journal of Applied Physics, Vol. 36 No.2 pp505-506, Feb 1965
2. “Thermal of Gallium Arsenide (GaAs)”, http://www.ioffe.ru/SVA/NSM/Semicond/GaAs/thermal.html
(Current Oct 1, 2003)
3. F.G.Yost, M.M.Karnowasky, W.D.Drotning and J.H.Gieske, “Thermal Expansion and Elastic Properties
of High Gold-Tin Alloys”, Metallurgical Transactions A, 21A, pp.1885-1889, July 1990
4. Stephen R. Schmidt, Robert G.Launsby, Understanding Industrial Designed Experiments, 4th Edition, Air
Academy Press & Associates, 2000, p.2-28