Pore Sealing Studies of Porous Silica for Novel Low-K Underfill Materials
Jihperng (Jim) Leu
|摘要:||底部填膠(underfill)材料已經廣泛的應用在覆晶封裝中，其作用為填滿晶片與基板間的間隙以避免凸塊(solder bumps)失效的產生，然而在射頻元件的應用中，底部填膠材料除了要有良好的熱機械性質，也要具備低介電常數以降低在高使用頻率下能量的耗損。本研究利用多孔性二氧化矽填充物的孔洞(介電常數為1)發展出具備低介電常數的底部填膠材料。吾人使用一種無機的材料(hexamethylcyclotrisiloxane，D3)先在低溫下封住多孔性二氧化碳的孔洞，然後在底部填膠的膠聯反應過程中加熱移除。首先，吾人利用示差掃描熱量計(DSC)和流變儀(Rheometer)去探討硬烤(curing)和粘度的行為，然後設計出封孔預處理所需的溫度流程，在95℃下，D3可以封住部分通孔，甚至全部的通孔，在3:1的D3對多孔性二氧化矽重量分率封孔處理後，孔洞體積和表面積降低了約50%。然後，在膠聯反應過程中，D3於125℃ ~ 165℃汽化由孔洞跑出，阻礙環氧樹脂灌入孔洞，因而成功的留下孔洞，此外，吾人利用氮氣吸脫附儀(BET)和掃瞄式電子顯微鏡(SEM)去研究探討D3和環氧樹脂(epoxy resins)之間的表面形貌與封孔機制。吾人成功的利用封孔材料對15 wt%孔洞性的二氧化矽填充物進行封孔預處理，將介電常數由3.2降低至2.86(10.6%)。然而，由於(1)多孔性二氧化矽的孔洞結構,(2)和硬烤過程中，D3汽化而破壞環氧樹脂和二氧化矽間的界面接著，最後使得底部填膠的機械性質由3 GPa降低至1.5GPa。|
Underfill materials had been widely employed in the flip-chip packaging to fill the gap of solder bumps connecting IC chip and organic substrate in order to prevent failure of the solder joints. For radio-frequency (RF) device applications, underfill materials shall possess low dielectric constant to alleviate power loss at high-frequency, in addition to good thermal and mechanical properties. In the thesis, a novel approach by incorporating porosity through porous silica filler was attempted to develop low-k underfill materials. An inorganic, sacrificial material, hexamethylcyclotrisiloxane (D3), was employed to temporarily seal the interconnected pores in the porous silica at low temperature, and was later removed thermally during the crosslinking reaction. The viscosity and curing behavior of epoxy resins was first investigated by differential scanning calorimetry (DSC) and rheometer to design temperature profile for pore-sealing pretreatment and outgassing of sacrificial material. We found that D3 either completely or partially sealed the connected pores at 95 ℃ with volume and surface area reduction up to ~50% at the 3:1 weight ratio of D3 to porous silica. D3 outgassed during crosslinking reaction (125 ~ 165 ℃), and successfully leaved porosity within the interconnect pores without the back-flow of epoxy resins. In addition, the morphology of D3/porous silica and pore sealing mechanism were studied by scanning electronic microscopy (SEM) and Brunauer-Emmett-Teller method (BET). We successfully developed a pore-sealing pretreatment of porous silica filler by a sacrificial material to reduce dielectric constant from 3.2 to 2.86 or 10.6% with 15% filler. However, resulting from that (1) porous silica possessed porous structure, and (2) D3 vaporized and diffused out of pores during curing reaction, which destroyed the adhesion between epoxy resins and silica fillers, the mechanical strength was also reduced from 3.0 GPa to 1.5 GPa.
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