Vol. 34, No. 5
Journal of Semiconductors
May 2013
Self-adaptive phosphor coating technology for wafer-level scale chip packaging
Zhou Linsong(周琳淞)Ž , Rao Haibo(饶海波), Wang Wei(王伟), Wan Xianlong(万贤龙),
Liao Junyuan(廖骏源), Wang Xuemei(王雪梅), Zhou Da(周炟), and Lei Qiaolin(雷巧林)
School of Opto-Electronic Information, University of Electronic Science and Technology of China, Chengdu 610054, China
Abstract: A new self-adaptive phosphor coating technology has been successfully developed, which adopted a
slurry method combined with a self-exposure process. A phosphor suspension in the water-soluble photoresist
was applied and exposed to LED blue light itself and developed to form a conformal phosphor coating with selfadaptability to the angular distribution of intensity of blue light and better-performing spatial color uniformity. The
self-adaptive phosphor coating technology had been successfully adopted in the wafer surface to realize a waferlevel scale phosphor conformal coating. The first-stage experiments show satisfying results and give an adequate
demonstration of the flexibility of self-adaptive coating technology on application of WLSCP.
Key words: white light-emitting diodes; self-adaptive conformal coating; wafer level encapsulation technology;
multi-chip packaging
DOI: 10.1088/1674-4926/34/5/054010
EEACC: 2520
1. Introduction
Phosphor-converted light-emitting diodes (PC-LEDs),
which employ blue LEDs with yellow phosphors to generate
white light emission, are widely used in solid-state lighting.
There is an inherent structural defect, i.e., inhomogeneous distribution of phosphor due to deficiency of precise control in
thickness and shape of coating layer with the traditional dispensing phosphor coating processŒ1 . With the rapid development of the semiconductor lighting industry and increasing
demand of light efficacy, color uniformity and production
efficiency improvements, wafer level scale chip packaging
(WLSCP) technology has therefore attracted increasing attention among both researchers and manufacturers, but the traditional phosphor coating process cannot fulfill a high quality (homogeneous and maneuverable) coating layer at a wafer
levelŒ2 . In this paper, a new wafer-level packaging technology is introduced. Self-adaptive phosphor coating technology
is successfully adopted in the wafer surface to realize a waferlevel scale phosphor coating. The self-adaptive phosphor coating technology for a multi-chip white LED package is also presented; the first-stage measurement revealed good results.
sired. The coating is then developed with hot water and dried. A
phosphor layer remains on the LED surface only where it was
exposed and insolubilizedŒ5 . The self-exposure slurry method
is shown schematically in Fig. 1.
The quality of the phosphor layer around an LED chip
depends upon the photochemical reaction of the photoresist,
particle size distribution, surface condition of the phosphors,
added dispersants, emission intensity and profile of blue LED
chip, and the exposure and development conditions of the photoresist.
Because of the photosensitivity of the PVA slurry to blue
light, printing (cross-linking of PVA) also occurs with the photoreaction by absorption of the 450–465 nm light from a blue
LED, while traditional slurry printing technology uses 365 nm
light from a mercury UV lamp.
As an alternative to conventional UV light exposure, the
self-exposure process with blue emission of an LED chip was
employed to realize a more convenient and self-aligned method
2. Self-adaptive phosphor coating technology
The slurry method developed for conformal coating purposes in our lab is based on a phosphor suspension in a watersoluble photoresist, which consists of polyvinyl alcohol (PVA)
and a dichromate or diazo compoundŒ3; 4 .
The emission surface of the LED chip is coated with the
slurry and dried. The slurry is distributed onto the surface by
one of several methods: flowing, dipping, or spinning. The
coating is then dried and exposed to UV light outside through a
mask or to blue emission inside from LED chip (known as selfexposure) itself to determine the areas where phosphor is de-
Fig. 1. The schematic diagram of slurry coating process with selfexposure.
* Project supported by the Guangdong Province Scientific Research Program (Nos. 2011B090400083, 2011A081301019).
† Corresponding author. Email: zhou_linsong@sina.cn
Received 27 November 2012
© 2013 Chinese Institute of Electronics
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with conventional coating. The self-adaptive coating is even
more advanced than current conformal coatings, which show
a discriminable color deviation, i.e., bluish, in center point 5#
compared with the other 8 points off the center section across
the light spotŒ6 .
The slurry deposition is actually a method of photolithography. The PVA-based photoresist adopted in this paper is a
negative one. As mentioned above, the phosphor layer coated
with by using the self-exposure process is the remaining part
where it is exposed to the blue light of the LED chip. The thickness and shape of the phosphor layer around the LED chip with
the self-exposure process are therefore controlled by the light
intensity and profile of the LED chip. The configuration of selfexposure coating layer is the mapping of the intensity distribution of the LED chip itself.
3. Wafer-level scale chip packaging (WLSCP)
technology
Fig. 2. The chromaticity distribution of white LEDs with different
coating layers. a: With self-exposure conformal coating. b: With selfexposure remote phosphor coating. c: With conventional phosphor
coating. d: A commercial conformal coating product.
to make the photoresist cross-linked, i.e., the coating is exposed
to the emission of the blue LED chip itself through “back exposure”; it means that the coating is exposed from the LED surface instead of from an outside UV light source through “forward exposure”. With this self-exposed process, the desired
coating patterns and the thickness distribution of the remaining
phosphor layer are determined simultaneously according to the
blue light intensity and profile emitted from the LED chip.
The thickness of the phosphor layer was controlled easily
by altering the time of exposure (i.e., time in operation) or the
light intensity with adjustment of the current in the LED chip.
The illuminated area at 15 cm distance from each white
LED bead with different coating technology was divided into
nine sections of 15 15 cm2 . The chromaticity coordinates of
the center point of each section were measured, which gave the
nine-point chromaticity distribution results shown in Fig. 2.
The standard deviation (SD) of the chromaticity coordinates for both self-exposure examples is 0.003 in x and 0.004 in
y, comparatively, the data for a commercial conformal coating
example is 0.008 in x and 0.013 in y, those for a conventional
coating product is 0.010 in x and 0.023 in y.
It is clearly shown that the uniformity of light output with
self-adaptive coating and conformal coating is superior to that
The main processes include: wafer washing (cleaning) !
slurry (Phosphor suspension) coating ! self-exposure ! developing ! cutting (dicing) ! testing (measuring). The processes are as same the self-adaptive phosphor coating technology described above as applied for a single LED chip. In this
phosphor coating process, typically, the slurry was prepared
by suspending 180 mg phosphor particles into 1 mL solution
of PVA based water-soluble photoresist, which is composed
of 2.5% PVA and 0.025% ammonium dichromate (ADC, or
diazo resins)Œ7 . Forward exposure with outside UV source
or self-exposure with LED itself in the exposure process of
WLSCP can be applied, according to the different situation.
Self-exposure was used in our present research.
The time of exposure has a great influence on the thickness of the phosphor layer. After electric connection via gold
wire welding on the wafer, the wafer was coated with phosphor
slurry. 20 mA current from a DC power supply was applied to
the wafer for exposure, the exposure time was chosen from 2 s
to 4 s in order to realize different phosphor layer thicknesses.
After exposure, the wafer was developed in 70 ıC water for
about 1 min, and then placed in 80 ıC oven for 1 h drying, as
shown in Fig. 3.
Figure 3(a) shows the wafer with electric series connection. Figure 3(b) shows the wafer of the thinner phosphor layer
with 2 s exposure, and Figure 3(c) shows the wafer of thicker
phosphor layer with 4 s exposure.
As the self-exposure processes were driven by a DC power
supplier and the chips were series connected electrically, with
the same current flow and period of time, the exposure for each
section was determined totally by the emission intensity of corresponding area on wafer, the variation of light intensity of different emission areas results in the fluctuation of the thickness
of phosphor layer over the wafer surface, as shown in Fig. 3(b).
That means, the thickness of phosphor layer with selfexposure process was varied according to the profile of emission efficacy of the epitaxial LED layer on the wafer, and this is
main difference between self-exposure technology and current
conformal coating of uniform thickness.
The self-adaptive phosphor coating technology was successfully adopted in the multi-chip white LED packaging, as
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Fig. 4. Series and parallel connection structure.
Ra of 77 for this module.
4. Conclusion
Fig. 3. (a) The wafer with electric series connection. (b) The wafer of
a thinner phosphor layer with 2 s exposure. (c) The wafer of a thicker
phosphor layer with 4 s exposure.
shown in Fig. 4. Figure 4(a) shows the 10 W LED module with
a self-adaptive phosphor coating layer of series and parallel
connection structure, a total of 9 (3 3) chips. The experimental processes were the same as those for WLSCP. Figure 4(b)
shows the 50 W LED module with a self-adaptive phosphor
coating layer. Clear phosphor patterns corresponding to the
LED matrix can be observed; the measurements show a result
of luminous efficiency over 70 lm/W with CCT of 5263 K and
The slurry method with a self-exposure process was first
successfully applied over the wafer level scale. A phosphor
suspension in the water-soluble photoresist was applied and
exposed to the blue light of LEDs themselves over the wafer
and developed to form a conformal phosphor coating with selfadaptability to the angular distribution of intensity of blue light
over the wafer surfaceŒ8 10 . The thickness and shape of the
phosphor layer is mapped to the light intensity of the LED epilayer over the wafer, which is helpful for making light output
more uniform across the whole illumination area.
Acknowledgment
We thank AquaLite (Wuhan, China) for providing us with
the wafer. We also thank the University of Electronic Science
and Technology of Optoelectronic Technology Center for providing us with scientific equipment and technical support.
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