Process Development for Yellow Phosphor Coating on Blue Chip

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Process Development for Yellow Phosphor Coating on Blue Light Emitting Diodes (LEDs) for

White Light Illumination

K. H. LEE1and S. W. Ricky LEE

1,2,3

1Department of Mechanical Engineering2Electronic Packaging Laboratory/Center for Advanced Microsystems Packaging (CAMP)

3Nano and Advanced Materials Institute (NAMI)Hong Kong University of Science & Technology

Clear Water Bay, Kowloon, Hong Kong

Abstract

There are several methods to produce a white lightemitting diode (LED). The most common commerciallyavailable white LED is made by mixing the blue light from aGaN chip and the yellow light from the emission of aYttrium Aluminum Garnet:Cerium (YAG:Ce) yellow

phosphor coating. The quality of white LED using thisapproach heavily depends on the optimization of packingdensity, thickness and uniformity of the phosphor layer. The

present study is intended to develop a new methodology forcoating a uniform yellow phosphor layer. This newlydeveloped coating method is based on the concept of screen-

printing. A matrix array of LEDs is firstly mounted on asilicon substrate with the flip chip configuration. A siliconmold plate is fabricated with etched cavities that match withthe dimensions and pattern of LED array. The silicon mold

plate is then placed over the substrate that carries the LEDarray and serves as a printing mask. The yellow phosphor

powder is pushed into the apertures by a squeegee blade andbonded to the LED with UV curable epoxy. The siliconmold plate is released after curing. Compared with othercoating approaches, this yellow phosphor printing method is

relatively simple and can make good quality white LEDs.

1. Introduction

In the past, LEDs are mainly used for signals, decorationand message display. Due to the improvement of brightness

and efficiency in recent years, LEDs have been used invarious lighting applications such as personal flashlight and

backlighting for flat panel display [1-2]. Compared with thetraditional incandescent light bulb and fluorescent light tube,LEDs have the following advantages: long life, low powerconsumption, compact design and high reliability [3-5].LEDs are also regarded as enviromental friendly prodcuts asthey contain no mercury and no UV light is generated when

blue LEDs are used for white light illumination. In a recentstudy, it has been demonstrated that the a single high-

brightness LED (HB-LED) may have the capacity to reachthe same efficacy (~100 lm/w) as, or even higher than, the

conventional fluorescent light tube [6].

The most straightforward method to generate white lightis by combining the lights with three fundermental colors,

namely, red, green and blue (RGB). White LEDs may beproduced in this way as well [7-9]. However, this methodrequires a more complicated electrical design for the controlof light intensity and uniformity. A white LED may also be

produced by coating downconverting phosphor layers ontothe surface of a LED. The principle is based on theabsorption and re-emission of light [10]. One example is to

coat the red and green phosphor layers on a blue LED chip[11-12]. With the excitation of blue light, the phosphorlayers can emit red light and green light, respectively. Theun-absorbed blue light, combined with the excited red andgreen lights, can result in white light. Similarly, an UV LEDchip coated with red, green and blue phosphor layers canalso generate white light [13]. These methods require a

precise weighting of each kind of phosphor layer in order togive desired white light. There is another way to generatewhite light using yellow phosphor (YAG:Ce) coating fordownconverting. When a blue LED is coated with a yellow

phosphor layer, the phosphor layer will absorb part of the

blue light and re-emits yellow light (see Figure 1). The re-emitted yellow light will then combine with the un-absorbedblue light, as illustrated in Figures 1 and 2, to give an

illumination of white light [14-18].

Figure 1: Configuration of a blue LED with yellow phosphorcoating for white light generation

Figure 2: Principle of white light generation from the

mixing of blue luminescence and yellow phosphorescence

In the industry, the yellow phosphor layer is usuallyfabricated using one of the following three methods, namely,slurry coating method, powder settling method and electro-

phoretic deposition (EPD). The mechanism of slurry coatingis based on the photo-development of phosphor suspensionin a photoresist. This requires a precise mixing ratio of the

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slurry and the yellow phosphor [19]. It also needs a strictcontrol of coating parameters such as the spinning speed andthe flow rate of the slurry. The settling method is based onthe sedimentation motion of phosphor powder suspended inan aqueous solution (potassium silicate and barium acetate).This method also requires an exact mixing ratio of theaqueous solution and the phosphor powder [20]. The EPDmethod is implemented by depositing charged particles

under a designated electric field [21]. Several parameterssuch as the mixing ratio of electrolyte/phosphor powder andthe applied voltage need to be considered during the process.Among the aforementioned methods, EPD is considered the

best in terms of packing density, layer thickness and

uniformity of the yellow phosphor coating.

In this paper, a newly developed printing method forcoating the yellow phosphor layer on a blue LED chip isintroduced. The process of this printing method is simplerand the control of chromaticity is also more straightforward.The implementation of this new method will be illustratedand discussed in subsequent sections.

Figure 3: Flip chip LED assembled on a silicon substrate

Figure 4: Phosphor coating by screen-printing method

2. Mask Fabrication and Printing Process

The coating method developed in the present study isbased on the screen-printing process. The targeted LEDshould be a flip chip assembly as shown in Figure 3, whichis a popular configuration for HB-LEDs. A silicon mold ismade to serve as the printing mask. The yellow phosphor

powder is pushed into the aperture of the mask by asqueegee blade and bonded to the surface of LED with UVcurable epoxy. A schematic diagram is given in Figure 4 toillustrate this configuration. The detailed procedure is

described in detail as follows.

(a) Silicon wafer with thermal oxide layers

(b) Oxide etching to open window on one side of the wafer

(c) Wafer thinning by dry etching

(d) Oxide etching for window patterning

(e) Create apertures by through silicon etching

Figure 5: Process flow of silicon mold fabrication

In the present study, the printing mask was made of a 4-inch double-side silicon wafer. The fabrication process ofthe silicon mold plate is illustrated in Figure 5. The siliconwafer was originally coated with 3 m thick thermal oxidelayers on both sides. After creating a ring mask, the wafer

was thinned from 400 m to a range between 120m and

200 m, depending on the desired thickness of the printing

mask (related to the thickness of the yellow phosphor layer).The next step was to pattern the thermal oxide on another

side of the wafer. Subsequently, through silicon apertures ina matrix array pattern were formed by deep reactive ionetching (DRIE). In the present study, the size of the blueLED chip was 1.0 mm x 1.0 mm. The size of apertures onthe silicon mold plate was determined as 1.4 mm x 1.4 mm,allowing sufficient tolerance between the LED chip and the

side walls of the aperture.

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Figure 6: Silicon mold as the printing mask

Figure 7: Close-up view showing flip chip LEDs in the

cavities of silicon mold

For batch mode production, a number of flip chip LEDsas shown in Figure 3 were assembled on a silicon substrate.The silicon mold plate was then placed over the substratethat carried the LEDs and served as a printing mask. Thewhole setup was fixed on a plastic platform by three screwswhich were 120

o apart (see Figure 6). From the close-up

view in Figure 7, it can be seen that the pattern anddimensions of apertures on the silicon mold plate matchedwith the matrix array of LEDs. It should be noted that,

during the process tuning stage, dummy flip chips wereemployed for exercise. After the process was optimised, real

blue LED chips were used for the yellow phosphor coating

process.

Right before the printing process, a small volume of UVcurable transparent epoxy was dropped on the top of eachLED chip inside the cavity of the silicon mold plate. Thevolume of the dispensed epoxy was controlled by amicrometer. Afterwards, the yellow phosphor powder wasscreen-printed on the top of the silicon mold plate using asqueegee blade. Subsequently, the epoxy was partially curedusing a UV lamp of 75 mW/cm

2for 1 minute (5 minutes for

full curing) to bind the phosphor power in place. It should benoted that, if the epoxy was fully cured at this stage, thesilicon mold plate would be very difficult to be released. In

order to release the printing mask without lifting the printedyellow phosphor layer, a pin stamp as shown in Figure 8should be used. During this release process, the pins on thestamp were aligned with the apertures of the silicon mold

plate. These pins would serve for two functions; i.e., tocompact the yellow phosphor powder and to shear the

phosphor layers off the side walls of the apertures. It shouldbe noted that there must be some tolerance between each pin

and each aperture. The current design made the pin size 10%smaller than the silicon mold aperture size. For a steadystamping process, the stamp was clamped on an Instronmachine as shown in Figure 9 and then lowered down. Oncethe push pins were put in contact with the top of the printedyellow phosphor layers, the silicon mold plate was lifted andremoved manually. After the removal of the mold plate, theepoxy was fully cured by the UV lamp and then the whole

process was completed.

Figure 8: Pin stamp for the release of silicon mold plate

Figure 9: Printing mask release process

3. Results and Discussion

By implementing the aforementioned printing method, auniform yellow phosphor coating can be achieved. Figures10 and 11 show the top view and the cross-sectional view,respectively, of a yellow phosphor coating with a LED chipunderneath. In these two figures, the thickness of the yellow

phosphor layer was around 100 m. From Figure 11, itcould be seen that the four sides and the top surface of theLED chip were coated with yellow phosphor powder. It wasalso found that some of the epoxy leaked into the gap

between the silicon mold and the substrate. Therefore, a

slight amount of phosphor powder residue can be seen onthe substrate surface around the LED chip. In the presentstudy, this bleeding area occupied very limited space. As

long as the bleeding area does not affect the subsequent wirebonding process for interconnection, it is not considered a

threat to the performance of the HB-LED assembly.

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Figure 10: Top view of the screen-printed phosphor powder

Figure 11: Cross-sectional view of the screen-printed

phosphor powder

The main objective of the present study is to make whitelight from blue light LEDs with yellow phosphor coating.An equipment of Ocean Optics Inc. (OOIIrrad2), wasemployed to characterize the results of LEDs subject to thecurrent printing method. The photometric and colorometricdata were measured. A test matrix was determined to studythe effect of the thickness of the yellow phosphor coating onthe chromaticity of LEDs. Five different cases wereinvestigated as shown in Table 1. Sample 1 was a bare blue(465 nm) LED chip as a base case for reference. Samples 2,3, 4 and 5 were coated with various thicknesses of yellow

phosphor powder. It should be noted that the thickness ofphosphor layer was controlled by the thickness of the siliconmold plate for the printing process.

Table 1: Test matrix for studying the effect of the thickness ofyellow phosphor coating

Sample

Thickness of the

silicon mold

Corresponding coating

thickness

1 N.A. N.A.

2 120 m 20 m

3 130 m 30 m

4 160 m 60 m

5 200 m 100 m

During the experiment, the applied voltage to the LEDswas approximately 3.5V at a steady current of 50 mA. The

photo-luminescence (PL) spectrum for each of the samplesis shown in Figures 12-16. Since sample 1 was a bare,monochromic blue LED chip, it could be seen that thedominant wavelength in the spectrum was 465 nm, as shownin Figure 12. For a pure white light, the colorimetric datashould be x = 0.333, y = 0.333. In this case, sample 1 had a

colorimetric data of x = 0.151, y = 0.041. In Figure 13, itwas found that the dominant wavelength of sample 2 (with20 m thick of phosphor coating) was still 465 nm, and the

peak corresponding for phosphorecence was rather low. Thisreveals that only a very small portion of blue light wasabsorbed and re-emitted as yellow. The correspondingcolorimetric data was measured as x = 0.264, y = 0.232 andthe LED appeared as bluish-white. This indicates that moreyellow phosphor coating is needed on the LED chip. Figure14 shows the spectrum of sample 3 (with 30 m thick of

phosphor coating). The dominant wavelength was 568 nmand the colorimetric data was x = 0.346, y = 0.366. Thisreveals that the amount of yellow phosphor powder wasalmost optimized and the combination of blue and yellowlights is very close to the white light. The result of sample 4

(with 60 m thick of phosphor coating) is shown in Figure15. It can be seen that the peak corresponding to

phosphorecence became dominant. The colorimetric datawas x = 0.426, y = 0.493, falling between the green and redregion. The light emission of LED appeared as greenish-yellow. Lastly, for sample 5 (with 100 m thick of phosphorcoating), the LED light became yellow. There was only one

peak at 574 nm in the PL spectrum shown in Figure 16. Thecolorimetric data was x = 0.461, y = 0.516. This indicatesthat the yellow phosphor coating was too thick and all bluelight had been turned into yellow. Figures 17 and 18 showthe chromaticity plot and the color, respectively, of all 5samples. The results of all experimental data are

summarized in Table 2.

It should be noted that the packing density is definded as

the ratio of the volume of the substantial solid part (Vs) tothe total volume of the coated powder (V) [22]. This can be

expressed as

Packing density = Vs/ V = W / Ats (1)

where W is the total weight; A and t are the coating area andlayer thickness, respectively; and sis the phosphor powderdensity. By implementing the newly developed printingmethod, the packing density can reach around 70%, which is

similar to that of the EPD method.

Table 2: Experimental results of the test samples

Sample

Colorimetric

data (x, y)

Dom.

wavelength

Color

appearance

1 (0.151, 0.041) 465 nm Blue

2 (0.264, 0.232) 465 nm Bluish-white

3 (0.346, 0.366) 568 nm White

4 (0.426, 0.493) 572 nm Greenish-yellow

5 (0.461, 0.516) 574 nm Yellow



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Figure 12: PL spectrum of sample 1, bare blue LED chip

Figure 13: PL spectrum of sample 2 with 20m thick of

yellow phosphor coating







Figure 17: Chromaticity plot of the test samples

Figure 18: Comparison of LEDs with different thicknesses ofyellow phosphor coating

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4. Conclusions

The present study was to develop a new yellow phosphorcoating method for generating white light from blue LEDs.With this printing approach, various thicknesses of uniformyellow phosphor coating could be deposited on blue LEDs,resulting in different chromaticity data. The key parameterof this method is the thickness of the yellow phosphor layer,which is controlled by the thickness of the silicon mold

plate. The present method does not require the fine-tuning ofthe mixing ratio of electrolyte/phosphor powder and theapplied voltage as in the process of EPD. The newlydeveloped printing technique is considered a moreconvenient method for controlling the chromaticity of blue

LEDs coated with downconverting yellow phosphor powder.

Acknowledgments

This study was sponsored by a research grant ofDAG05/06.EG34 offered by the Hong Kong University ofScience and Technology (HKUST). The blue LED chipsused in this study were provided by Advanced PackagingTechnology (APT) Ltd. The measurement of chromaticitywas done at the Photonics Technology Center of HKUST.

The authors would like to acknowledge all these supports.

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Process Development for Yellow Phosphor Coating on Blue Chip

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