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|標題:||Improved performance of flip-chip white light emitting diodes with conformal phosphor layer prepared by model conformal coating technique|
|關鍵字:||White light emitting diode|
model conformal coating
correlated color temperature uniformity
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|摘要:||In this study, the white light emitting diodes (LEDs) were fabricated by combining the flip-chip device structure and conformal phosphor layer. To improve the angular correlated color temperature (CCT) uniformity of white LEDs, the model conformal coating (MCC) technique has been proposed to prepare the phosphor layer on the flip-chip LED. Firstly, by designing the model shapes, four phosphor layers with various geometries can be formed. These four geometries of the MCC-prepared phosphor layers consisted of conventional cubic, convex-surface cubic, conventional cylinder, and convex-surface cylinder shapes. Through the design of the geometry of the MCC-prepared phosphor layer to the conventional cubic shape, the fabricated white LED would have the lowest CCT deviation than those of the other devices. This is attributed to the relatively low difference of optical paths between the lights radiated from normal and lateral directions of the LED chip to the conventional cubic phosphor layer. Therefore, this white LED possesses higher CCT uniformity.
Subsequently, the conventional pulsed spray coating (PSC) and the MCC methods were both used for the preparation of phosphor layers on the LED chips. These two devices possessed similar CCT deviation. Obviously, these two white LEDs both has good CCT uniformity at a lower CCT (5100 K). On the other hand, when the CCT was near 5800 K, the CCT deviations of these two devices were measured to be 460 and 800 K. From the cross-sectional image of white LED by scanning electron microscopy, it can be found that the MCC-prepared phosphor layer has higher thickness uniformity than that of PSC-prepared phosphor layer. This is the reason why the white LED contained with the MCC-prepared phosphor layer possesses better CCT uniformity. In other words, the smoother surface of the MCC-prepared phosphor layer can cause more uniform phosphor conversion efficiency at various light extraction angles, leading to better CCT uniformity of white LED. Especially at higher CCT value (5800 K), i.e. the lower phosphor concentration in the phosphor layer, the surface roughness would apparently affect the CCT deviation.
In addition to the CCT characteristics, the light emission and thermal propertiesof the white LEDs were also investigated in this research. In this part, the phosphor layers were formed by the MCC, PSC, and dispensing methods. According to the measured results, the white LED with the dispensing-prepared phosphor layer has higher light output power than those with the MCC- and the PSC-prepared phosphor layers. Because the phosphor powders of MCC- and PSC-prepared phosphor layers were both conformally coated on LED chips, the back-scattered lights generated from the device were easily absorbed by the chip, leading to the increment in the junction temperature and the reduction in the light emission efficiency. On the other hand, as the dispensing-prepared phosphor layer was formed on the LED chip, the phosphor powders were uniformly distributed in the glue. In this case, the back-scattered lights sourced from the device were less absorbed by the chip, i.e., the heat created from the phosphor powders would induce less effect on the LED chip, resulting in higher light emission efficiency in this device. Even though the white LED with the dispensing-prepared phosphor layer has better light emission characteristic than those of the other two samples, its inferior CCT uniformity would lower the possibility of optoelectronic applications.
Moreover, the effect of encapsulated material for packaging process on the device characteristics of white LEDs was also analyzed. It is well known that the silicone is a common material for LED package. Nevertheless, there is still room for improvement in the heat dissipation of the device packaged with silicone. Thus, in this work, a novel inorganic encapsulated material has been presented for the LED package. Apparently, the inorganic glue indeed has better heat dissipation, inducing higher light emission in the device.
Finally, the 2 × 2 white LED array, which fabricated by adhering the LED chips to the phosphor plate, was also presented in this research. In the white LED array, the distance between LED chips was varied from 1 to 2.5 mm, and the effect of this distance on the light emission characteristic and the CCT uniformity of the white LED array was discussed. It can be found that the optimum distance between LED chips is 1.5 mm. When the distance between LED chips is less than 1.5 mm, this short distance could cause the interactive influence of heats generated from various chips, leading to the increase in the device temperature and the decrease in the light emission efficiency of the white LED array. With increasing the distance between LED chips above 1.5 mm, the white LED array is similar to the combination of four light sources rather than single light source. In this white LED array, the light emission efficiency and the CCT uniformity both become worse.|
本論文結合覆晶式發光二極體(LED)元件製程以及螢光層塗佈技術來製作白光LED，在螢光層製作方面，使用的是敷型塗佈技術(conformal phosphor)。有別於一般敷型塗佈技術所使用的脈衝噴印塗佈(pulsed spray coating, PSC)，本研究利用模具敷型塗佈(model conformal coating, MCC)來製作螢光層。首先，藉由模具的設計，利用MCC方法製作四種不同幾何圖形的螢光層，分別為一般立方體、凸形表面立方體、一般圓柱體及凸形表面圓柱體。可以發現當螢光層的幾何圖形被設計為一般立方體時，將使得白光LED具有最低的色溫偏差值。這是因為當具有一般立方體幾何圖形的螢光層塗佈在方形的LED晶片上時，此時從晶片所發出的正向光與側向光到達螢光層所需要的距離差值相對較小，使得元件擁有較佳的色溫均勻度。 接著將MCC方法以及傳統PSC方法塗佈的螢光層分別製作在LED晶片上，並探討這兩種螢光層塗佈技術對元件色溫均勻性的影響。由掃描式電子顯微鏡影像觀察發現使用MCC方法製作的螢光層之厚度均勻性比PSC方法製作的較佳，而厚度均勻性會影響元件色溫均勻性的優劣。特別在較高色溫下(5800 K)，螢光層中的螢光粉濃度較低，此時螢光粉在螢光層中的分佈均勻性更重要，造成螢光層厚度均勻度對於元件色溫均勻性的影響更加顯著。上述結果顯示，比較MCC與PSC兩種螢光層塗佈技術，前者能夠讓白光LED擁有更高的色溫均勻性。 白光LED的發光特性與熱性質也在本論文中探討，此部分白光LED中的螢光層塗佈方法使用了MCC、PSC以及點膠式(dispensing)三種技術。以點膠式方法塗佈螢光層所製作的元件具有較高的光輸出功率。由於MCC及PSC方式所製備的螢光層中的螢光粉都是直接敷型塗佈在晶片上面，造成元件產生的背散射光容易被晶片吸收，使得元件的接面溫度升高，並且降低元件的發光效率。儘管以點膠式方法塗佈螢光層所製作的白光LED具有較佳的發光特性，然而其低劣的色溫均勻性將大幅降低其產品應用性。 除了螢光層塗佈方式的開發比較之外，本論文也探討不同封裝膠材對白光LED的元件特性影響。傳統LED的元件封裝大都使用矽膠材(silicone)進行填膠製程，本研究使用新穎的無機膠材(inorganic glue)對元件進行封裝。與一般的矽膠材相比，本研究所使用的無機膠材具有較佳的散熱特性，使得封裝後的元件擁有較高的光輸出功率。 本論文同時也開發了白光LED陣列模組(2 × 2陣列)，其製作方式是將藍光LED晶片陣列貼附在螢光貼片上，並探討晶片間距的改變(1~2.5 mm)對元件發光特性及色溫均勻度的影響。實驗結果顯示，在此陣列模組中，晶片與晶片之間的最佳距離為1.5 mm；當晶片間距小於1.5 mm時，由於晶片與晶片的距離太接近會導致熱的交互影響，造成元件溫度的上升以及發光效率的降低。而當晶片間距大於1.5 mm時，因為晶片與晶片之間的距離愈來愈遠，造成元件已經不再以單一光源的形式出光，反而類似於四個獨立光源同時發光，此時白光LED陣列模組的光強度變弱且元件色溫均勻性變差。
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