67021-ultraviolet-led-multi-chip-module-based-on-ceramic-substrate.pdf

Ultraviolet LED Mu lti-Chip Module Ba sed on Ceram ic Substrate Burkhardt, T. 1,2 , Hornaff, M. 1 , Acker, A. 1 , Peschel, T. 1 , Beckert, E. 1 , Suphan, K.-H. 3 , Mensel, K. 4 , Jirak, S. 4 , Eberha rdt, R. 1 , Tünnerma…

Ultraviolet LED Multi-Chip Module Based on Ceramic Substrate

Burkhardt, T.

1,2

, Hornaff, M.

, Acker, A.

, Peschel, T.

, Beckert, E.

, Suphan, K.-H.

Mensel, K.

, Jirak, S.

, Eberhardt, R.

, Tünnermann, A.

1,2

Fraunhofer Institute for Applied Optics and Precision Engineering IOF, Albert-Einstein-

Str. 7, 07745 Jena, Germany

Institute of Applied Physics, Abbe School of Photonics, Friedrich-Schiller-University Jena,

Albert-Einstein-Str. 15, 07745 Jena, Germany

Micro-Hybrid Electronic GmbH, Heinrich-Hertz-Str. 8, 07629 Hermsdorf, Germany

Lastronics GmbH, Winzerlaer Str. 2, 07745 Jena, Germany

*Phone: +493641807339, E-mail: Thomas.Burkhardt@iof.fraunhofer.de

Abstract

A high power ultraviolet (UV) light emitting diode (LED) multi-chip module package based on aluminum

nitride (AlN) and alumina (Al

) is presented. The AlN substrate with a high thermal conductivity of up to

180 W/(m·K) and LED chips based on a copper alloy provide superb thermal management and heat extraction.

Efficient cooling is an important prerequisite for the increase of extractable optical power and decrease of ther-

mally induced wavelength shift. A design of a stackable module featuring arrays of 7×2 indium-gallium-alumi-

num-nitride UV LED chips at 395 nm is developed. This configuration of sub-modules allows for the scalable

assembly of line sources with different lengths. Applications using UV LEDs cover market segments such as

curing of adhesives, inks and coatings, sterilization of medical equipment and treatment of potable water, as well

as various uses in chemical detection, biochemical analytics and spectroscopy.

Thermal and thermo-mechanical modelling of the sub-mount is conducted using finite elements analysis. Die

attach using eutectic gold-tin solder, lower melting tin-lead solder and silver-filled adhesive are compared with

respect to optical output power and wavelength drift. Mechanical strength and structure of the resulting joints are

investigated using shear force measurements, cross-sectioning and micro-tomography. An optical output power

of 7.7 W is achieved using a cluster of 14 LED chips at 1050 mA resulting in a peak irradiance of 30.8 W/cm² at

the LED surface with respect to the footprint and pitch of the attached chips.

Key words: solid state lighting, ceramic packaging, ultraviolet light emitting diode, ceramics

Introduction

UV-sources are used in a wide field of applications

such as the curing of adhesives, inks, coatings and

industrial paints, the sterilization of medical equip-

ment and the treatment of potable water. Further-

more such light sources are needed for a various

uses in chemical detection, biochemical analytics

and spectroscopy. Currently high power mercury

gas-discharge lamps are used to cover this wave-

length range. Due to the high temperature of the

discharge tubes and the infrared (IR) radiation

characteristics such lamps find limited use in the

processing of temperature sensitive materials, e. g.

the curing of paints on plastics. Solid state light

sources based on UV-LED however offer a solution

for the abandonment of toxic mercury lamps, while

providing additional benefits as smaller form factor,

increased lifetime and ruggedness, an application

tailored emission spectrum with reduced IR

emission and added flexibility to UV applications

[1], [2]. High power density UV-LED modules are

currently under scientific investigation and com-

mercial systems are entering the market. An Al

based assembly using 98 densely packed LED chips

is shown in [3]. Ceramic materials, such as alu-

mina, aluminum nitride and even low temperature

co-fired ceramics (LTCC) can be used in high

power packaging due to their high thermal con-

ductivity [4]. Especially applications like water and

air disinfection need high power sources in the

UVC range (280 nm to 100 nm) [5]. The develop-

ment of solid state deep UV sources aim at the

decrease of emitted wavelength and the improve-

ment of output power and external quantum effi-

ciency [6], [7]. Highly efficient light sources pro-

mise to deliver a significant contribution to a global

sustainable development.

IMAPS/ACerS 8th International CICMT Conference and Exhibition (2012) | April 16-19, 2012 | Erfurt, Germany

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As efficient cooling is important for the increase of

extractable optical power and a minimized ther-

mally induced wavelength shift, thin layer bonding

techniques as well as reduced chip-die substrate

thickness gain relevance [8]. Soldering processes

can provide thin metallic bond layers with good

thermal conductivity and high mechanical strength.

Gold-tin solder alloys are used in opto-electronic

packaging, die-attach and for the assembly of

micro-optical systems [9], [10]. High temperature

tin-based solder alloys with a high lead content and

lead-free drop-in replacements such as Bi-Ag are

proposed for the assembly of power circuits [11].

Another die-attach technology is the use of micro

and nano-scale silver sinter materials to provide

high strength joints with outstanding electrical and

thermal conductivity. Silver-sintering die-attach

provides interconnects of semiconductor devices

that are operable at high temperatures up to 350 °C

[12]. The reduction of bond pressure down to

2 MPa and temperatures down to 200 °C, necessary

for the processing of sensitive opto-electronic

components, is reported [13]. Thermo compression

provides a simple yet high temperature process for

bonding of components. Thin and therefore

thermally high conductive bond layers are feasible

[14]. A new approach of Die-attach is reactive

multilayer bonding using nano-scaled alternating

layers of two materials. Foils or deposited films of

nano-engineered materials generate heat by a self-

propagating exothermic reaction allowing for a

localized reflow of solder. Small and temperature-

sensitive components can be joined without thermal

or thermo-mechanical damage [15]. The high pro-

cessing temperatures also allow for welding and

thus produce high shear strength joints [16]. An-

other state-of-the-art bonding technology for die-

attach is using silver-filled epoxy resins with the

filler particles providing thermal conduction while

the epoxy is generating adhesion bonding [17].

Current developments include nano-scaled filler

materials to enhance the thermal conductivity of

adhesives for packaging of high brightness LED

[18]. Die-attach technologies for high temperature

applications are an issue beyond solid state lighting.

A review on materials for device interconnection

technologies for high power electronics is given in

[19].

The system presented utilizes a chip-on-board de-

sign to provide a multi-chip module package of

high power UV-LED on ceramic substrates. LED

chips in the UVA range (400 nm to 300 nm) are

mounted on stackable aluminum nitride and alu-

mina substrates. Goal of the project is the increase

of optical output power by efficient cooling. The

scalable design using sub-modules is of advantage

to a flexible and customer specific use. Die-attach

is conducted using eutectic gold-tin solder in a flux-

free processing, tin-lead solder and two silver-

loaded adhesives. The thermal behavior is com-

pared by measurement of optical power and wave-

length drift. Mechanical strength and micro-

structure are analyzed by shear force measure-

ments, cross-sectioning and micro-tomography.

Design

Based on the targeted areas of application a design

for a stackable multi-chip module is developed.

This approach allows for the scalable assembly of

line sources with different lengths as well as for the

use of single modules for lighting systems. The

suggested design offers flexible use in various

applications. An array of 7×2 LED-chips per sub-

module is designed to provide a line shaped light

source for homogenous illumination of a larger

two-dimensional surface without complex beam

shaping optics. Addressed applications are the

curing and polymerization of adhesives, inks, paints

and coatings in continuous processes. An alternate

design of 3×2 LED-chips per sub-module is pro-

posed for the use as an UV light source for the

homogeneous illumination of a digital mirror de-

vice (DMD). Such devices find applications in the

exposure of offset plates for offset printing. DMD

are capable of a fast and cost efficient manufac-

turing of offset plates in Computer-to-plate (CTP)

or Computer-to-conventional-plate (CTCP) proces-

ses [20]. These techniques lead to reduced repress

time and improved print quality.

A ceramic substrate (either alumina or aluminum

nitride) is used as a mechanical system carrier, to

provide electrical contact and allow for proper ther-

mal management and cooling of the LED-chips.

Substrate dimensions are 12×10 mm², with a thick-

ness of 1 mm for aluminum nitride and 0.38 mm for

alumina. The electrical wiring and solder pads are

manufactured directly onto the surface of the sub-

strates. The wiring structures are manufactured of

W/Ni/Pd/Au layers. Solder land pads with di-

mensions of 1100×1100 µm² with a spacing of

300 µm are structured on the substrate surfaces

(Figure 1). The pads are made of sputtered eutectic

gold-tin for soldering with gold-tin solder alloy or

thin film gold for the use of adhesives and soldering

with a tin-lead solder alloy [10].

Figure 1: Aluminum nitride substrates with

electrical wiring and structured eutectic gold-tin

solder layers in two design variants (7×2 LED-

chips and 3×2 LED-chips).

IMAPS/ACerS 8th International CICMT Conference and Exhibition (2012) | April 16-19, 2012 | Erfurt, Germany

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UVA emitting indium-gallium-aluminum-nitride

LED-chips SL-V-U40AC by Semiled Inc. with a

center wavelength of 395 nm, a chip size of

1070×1070 µm² and a rated typical optical output

power of 210 mW to 250 mW are used. The initial

design was based on NS375C-3SAA LED-chips by

Nitride Semiconductor Co., Ltd. with a chip size of

600×600 µm² but was changed to the former due to

considerations of wavelength, optical output power

and availability. The Semiled chips are available

with a back metal gold layer usable for soldering

and adhesive bonding.

Electrical contacting of the front contact is con-

ducted by ultrasonic assisted wedge-wedge wire

bonding of Au and AlSi wires. Back-side contacts

are connected by the solder or the conductive adhe-

sive to the wiring structures on the substrate. Sub-

modules are mounted to a scalable and liquid-

cooled heat-sink by means of indium foil. The use

of indium allows for a proper thermal contact and

excellent cooling due to its high ductility and its

high thermal conductivity of 82 W/(m·K).

Thermo-mechanical and thermal simulations

Based on the design data and the NS375C-3SAA

LED-chips thermal and thermo-mechanical

analyses are conducted by finite elements si-

mulations (FE). Thermo-mechanical analysis is

used to evaluate the stresses to substrate, LED-

chips and joining material due to different

coefficients of thermal expansion. Thermal analysis

provides insight in the capability of the design and

material selection on efficient heat removal from

the chips. FE analysis is computed using ANSYS

11.0 SP1.

Targeted design goals are 80 mW optical output

power and 100 mW respectively. The LED-chips

achieve 80 mW optical at 200 mA, 4.5 V and about

9.5% efficiency resulting in a thermal load of

840 mW. 100 mW optical are attained at 350 mA,

5 V with less than 6% efficiency and a resulting

thermal load of 1650 mW. A ¼-model with sym-

metric boundary conditions is used for simulation.

Base temperature of the substrate is set to 0°C and a

natural convection with 10 W/(m²·K) is applied to

all outer surfaces. Table 1 shows relevant material

properties used for the simulations. Additionally

temperature dependency for Young’s modulus,

Poisson’s ratio and plasticity of the gold-tin solder

are modeled.

Modeling soldering using eutectic AuSn with a

solder layer of 10 µm is done. Solidification of the

solder alloy from solidus to room temperature leads

to a shrinking of about 5%. Resulting mechanical

stresses within the solder layer are near the yield

strength of the material indicating plastic defor-

mation of the solder. Maximum values of 320 MPa

for AlN and 337 MPa for Al

are calculated.

Stresses within the base materials are computed to

be 93 MPa for AlN and 95 MPa for Al

which is

significantly below the ultimate tensile stresses for

those materials.

Table 1: Material properties for simulation

model (sources: [21], [22], [23] and respective

data sheets,

(1)

┴

C-axis).

Property λ TCE E ρ

Unit W/K·m ppm/K GPa g/m³

24 6.8 340 4.0

AlN 180 4.7 320 3.3

Sapphire 40 5.4

(1)

430 4.0

Au80Sn20 58 16 59 14.7

Adhesive 17 30 1.21 3.2

Thermal analysis is conducted to provide results of

temperature distribution and heat flux within the

sub-assemblies. For the design goal of 100 mW

optical output power per chip and a thermal load of

1.65 W chip temperatures are calculated to be 23 K

above substrate temperature for AlN and 40 K for

using 10 µm AuSn solder layers (Figure 2

and Figure 3).

Figure 2: Temperature distribution within LED-

chips and AlN substrate (thickness 1 mm) for

eutectic AuSn solder (layer thickness 10 µm) and

1.65 W thermal load.

Calculations are repeated with respect to a con-

ductive silver-loaded thermoplastic/thermoset adhe-

sive with a thermal conductivity of 17 W/(m·K).

Assumed thicknesses of the adhesive layers are

10 µm and 30 µm respectively. Substrate material

is Al

with a thickness of 0.38 mm. Thermo-

mechanical strain is mainly influenced by the base

material and not the adhesive. Equivalent stresses

are 43 MPa for both adhesive layer thicknesses.

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