高功率發光二極體效能之提升: 1.雷射剝離成長在圖案化藍寶石基板上之氮化鎵磊晶層 2.以添加鑽石的Sn-3wt.%Ag-0.5wt.%Cu銲錫降低LED晶片接著材料的熱阻

Abstract

以GaN為材料的高功率發光二極體(LED)因為有高亮度、高效率、壽命長、尺寸小、環保等優點，有潛力取代傳統燈源成為下一世代固態照明的燈源。為了達到更高的亮度表現，必須用更高的電流驅動LED。受限於 LED 的轉換效率，更高的驅動電流表示會產生更多的熱。產生的熱使 LED 的溫度上升，降低 LED 的表現。隨著溫度的上升，LED 的光電轉換效率、可靠度會下降。因此，高功率 LED的熱管理非常重要。要提升 LED 的散熱表現必須降低熱阻。傳統的 GaN 藍光 LED 成長在藍寶石基板上，但是藍寶石基板不佳的導電與導熱限制了 LED 的表現，並使得 LED 晶片的熱阻上升。傳統使用銀膠作為 LED 晶片接著材料，但是銀膠的熱傳導係數很低，提高了晶片接著材料的熱阻，使 LED 晶片的散熱產生問題。在這篇論文中，將對提高 LED 晶片基板導熱進行探討以及改善晶片接著材料的熱傳導效果。 圖案化藍寶石基板(PSS)可以提升 GaN LED 的內部量子效率與光取出效率。藉由晶圓接合與雷射剝離技術(LLO)製作的薄型 GaN LED 可以將 GaN LED 磊晶層轉移到導熱導電較佳的基板。結合 PSS 與 LLO 的優點可以提升 LED 的表現。但是以圖案畫藍寶石成長氮化鎵製作的薄型 GaN LED 卻有需要較高雷射分離能量，產出率不佳，漏電流提高的缺點。本研究藉由 SEM 與 TEM 觀察剝離後的 PSS 與 GaN表面形貌和組成，並計算在 GaN/sapphire 介面雷射能量的分布探討 PT-LED 的雷 射剝離機制。發現當雷射能量低於 920mJ/cm2 時，PSS 側面的雷射能量不足以使GaN 分離。當雷射能量達 920mJ/cm2 時，PSS 側面的 GaN 開始分離。此時頂部因PSS 斜面形狀與雷射能量反射聚集非常高的能量，高能量分離產生的 H 2 穿透至側面 GaN/sapphire 介面，幫助 GaN 分離。高能量和 H2 穿透產生的”機械應力”在 GaN產生缺陷，使 PT-LED 的漏電流上升。PSS 頂部因高能量發生 sapphire 融化的現象， 融化的 sapphire 附著在 GaN 和 PSS 上，使得 PT-LED 產出率不佳。 晶片接著(DA)材料扮演著將 LED 晶片固定在封裝基板上的角色。傳統使用銀膠作為 DA 材料，但銀膠的熱傳導係數很低，只有(4W/mK)，使 DA 成為熱傳導的阻礙，讓 LED 散熱發生問題。使用添加鑽石的 Sn-3wt.%Ag-0.5wt.%Cu 銲錫(SAC305D) 來 降 低 DA 材 料 的 熱阻。在(SAC305D)中發現，鍍鎳層的鑽石(SAC305DN)和鍍鈦層的鑽石(SAC305DT)以及裸鑽(SAC305DB)相較之下，不會在鑽石/銲錫介面產生孔隙，可以讓熱阻降低。單位長度面積熱阻(R A/h )的結果顯示，SAC305DN 和銀膠(Ag-P)、燒結銀(Ag-P)、SAC305 相較之下，分別有 92%、73%、52%的下降。DA 層的厚度和鑽石/金屬的介面熱阻對於 SAC305DN DA 層熱阻的影響非常重要。

High brightness GaN-based Light-emitting diodes (LEDs) have a great potential become the next generation solid state lighting source due to their advantages of high illumination, high efficiency, long life, small size, environmental protection. To achieve better light output performance, it is necessary to drive the LEDs at a higher current. More heat will be generated when the current is increase because the restriction of LED’s photoelectric conversion efficiency. Such heating caused degradation of LEDs. With increase in operation temperature, both luminous efficiency and reliability of LEDs decreased. As a result, thermal management is very important in High power LEDs. In order to achieve better thermal dissipation performance, thermal resistance of LEDs must reduce. Conventional GaN-LED are usually grown on sapphire substrate. The poor electrical and thermal conductivities of sapphire substrate have a negative effect on the LED performance, and increase the thermal resistance of LED chip. A conventional die-attach material is silver paste. But the very low thermal conductivity (4W/mK) of silver paste increase the thermal resistance of die-attach material. Therefore, improve the thermal dissipation performance of LED substrate and die-attach material were investigated in this dissertation. Pattern sapphire substrate (PSS) has been employed to improve both internal quantum efficiency (IQE) and light-extraction efficiency (LEE). Thin-Gan LED process can solve thermal dissipation problem, in which GaN LED epi-layer was stripped off and transferred to conductive substrates by wafer bonding and laser lift-off (LLO) technologies. It is obvious that a combination of the advantage of PSS-LED with the thin-GaN LED process will improve the LED performance. However, the critical laser energy of pattern substrate thin-GaN LED (PT-LED) was high, the yield rate was very low, and the leakage current was high. The requirement of high laser energy and the root cause of low yield and high leakage current were investigated by SEM, TEM and the calculation of laser energy on Gan/sapphire interface. When laser power was below critical laser power (920mJ/cm2), the laser energy density was not high enough to separate GaN from the sidewall of pattern. When laser power reached 920mJ/cm2, GaN was lifted from the sapphire substrate. The pattern shape and reflection of laser induce a extreme high energy area at the top of PSS. Thermal decomposition of GaN at this area yielding H 2 gas, theses gas penetrated/separate GaN/sapphire interface of the sidewalls of PSS. The high laser energy and “mechanical stress” by H 2 penetrated cause the deformation and an increase in leakage current. The melted sapphire adhesive to GaN and sapphire lead to low yield of PT-LED. Die-attached (DA) material bonds LED chips onto a packaging board. Conventional DA material is silver paste, but the very low thermal conductivity (4W/mK) make DA a heat dissipation barrier, posing problem to removal of heat generated by LED. To reduce thermal resistance of DA material, diamond add Sn-3wt.%Ag-0.5wt.%Cu solder (SAC305D) was used. In SAC305D DA material, gaps were found at diamond/solder interface in bare diamond particle (SAC305DB) and Ti-coated diamond particle (SAC305DT). No gaps were found in Ni-coated diamond particle (SAC305DN) and the thermal resistance was reduced. The reduction of area thermal resistance per unit thickness ( RA/h ) of SAC305DN to silver-paste (Ag-P), sinter-silver (Ag-S) and SAC305 were 92%, 73% and 52%. The thickness of DA material and interfacial thermal resistance of diamond/metal play an important role in thermal resistance of SAC305DN.