多孔性及兩相式(MSQ/高溫起孔洞劑)低介電材料之吸水性與擴散行為研究

Abstract

本研究利用Solid-FirstTM製程以Methylsilsesquioxane (MSQ)為介電材料的基材、poly (styrene-block-4-vinylpyridine) (PS-P4VP)以及 poly (styrene-block- butadiene-block- styrene) (PS-PB-PS)為高溫起孔洞劑。利用自行組裝之石英微量天坪探討下列三種低介電薄膜之吸水性及水氣擴散行為(1)不同製程溫度的MSQ膜(2) 製程溫度為250 oC 之MSQ/Porogen混參膜(3)多孔性薄膜。 研究結果顯示孔隙率較大之多孔性薄膜有較高的吸水量乃是因為表面積與氫氧基較多之故。以HMDS改質孔隙率19.3 %的多孔性薄膜則可降低約17 %的吸水量。MSQ/PS-P4VP混合膜的吸水性較多孔性薄膜高乃因PS-P4VP有較高吸水量之故且高溫起孔洞劑之砒啶與氫氧基之作用使得MSQ/PS-P4VP有較多之殘餘氫氧基。MSQ/PS-PB-PS之吸水性比MSQ/PS-4VP差因為PS-PB-PS之吸水性較差之原故。 綜合QCM、C-V、吸附脫附行為顯示水吸附於MSQ或MSQ/Porogen的方式以物理吸附為主。在本研究顯示水氣下列兩種方式吸附(1)以凡得瓦耳力附著於MSQ或起孔洞劑中、MSQ與基材之介面以及起孔洞劑與基材之介面而此吸附方式佔總吸水量的80 % (2)以氫鍵(Si-O……H2O)附著此吸附方式佔總吸水量的20 %。 水氣在薄膜中擴散行為大至符合Fickian擴散方式，但是各種薄膜有幾乎相同的擴散係數除了高含量高溫起孔洞的薄膜乃是因為孔洞型態改變之故。擴散係數在高溫起孔洞劑含量20 wt%下保持不變是因為有一層緻密薄膜在表面形成且為速率控制。而此緻密薄膜可為擴散阻障層，保護CVD、ALD之先驅物擴散進入介電材料中。但是PS-PB-PS系統則無此緻密薄膜，本論文將提出MSQ/PS-P4VP表面緻密薄膜形成方式。

Incorporation of porosity into dielectric materials is a viable method to reduce k-value down to < 2.5. A Solid-FirstTM scheme based on high-temperature porogen, poly (styrene-block-4-vinylpyridine) (PS-P4VP), poly(styrene-block- butadiene-block -styrene) (PS-PB-PS) and methyl-silsesquioxane (MSQ) as the matrix have been employed to prepare porous low-k dielectric in order to circumvent the reliability issues encountered in the integration of as-deposited porous dielectric. The impact of high-temperature porogens, their loadings, and porosities on the moisture uptake and diffusion behavior was investigated using a home-built quartz crystal microbalance (QCM). Three low-k dielectric systems were employed in this thesis to simulate interlayer dielectrics (ILD) at different stages of Solid-FirstTM integration scheme; namely: (1) MSQ films cured at different temperatures up to 400 oC, (2) MSQ/porogen hybrid films cured at 250 oC with various porogen loadings, and (3) their corresponding porous films burned out at 400 oC. The moisture uptake of porous films cured at 400 oC increased with porosity due to an increase of pore surface area and residual silanol, Si-OH groups. Further surface treatment of porous MSQ by hexamethlydisilazane (HMDS) eliminated surface Si-OH groups and led to a 17% reduction of moisture uptake for porosity at 19.3%. In addition, moisture absorption of MSQ/PS-P4VP hybrid films cured at 250 oC were greater than porous MSQ ones because of the high water uptake in PS-P4VP (6.7 wt%) and the increased Si-OH concentration arisen from the interaction between the polar pyridine moiety of porogen with Si-OH in the MSQ matrix In contrast, moisture uptake of MSQ/PS-PB-PS hybrid films was much less than MSQ/PS-P4VP films because of the hydrophobic characteristic of PS-PB-PS. Based on moisture uptake, sorption/desorption behavior, and HMDS pretreatment by using QCM and CV measurements, we concluded that the moisture uptake in porous MSQ films or MSQ/high-temperature porogens hybrid films in this thesis is solely physical sorption. The physical sorption of moisture uptake underwent by Van der Waals long range force with (1) available surface area within the MSQ or porogen matrix, inside the pores, and at the MSQ/substrate and porogen/substrate interfaces, which contributed < 80% of moisture uptake, and (2) available hydrogen-bonded Si-OH--H2O sites, which were formed immediately after sample preparation, accounted for < 20% of moisture uptake. Finally, the moisture diffusion in MSQ/PS-P4VP films and their corresponding porous MSQ films followed Fickian diffusion behavior with almost the same diffusion constant except in the high porogen loading region where pore morphology has changed. The constant diffusion constant at porogen loading below 20% could be attributed to the formation of a rate controlling, dense layer on the top of porous or hybrid low-k films. Such thin but dense layer could serve as a diffusion barrier layer beneficial in certain processing modules. In contrast, no skin layer was observed for MSQ/PS-PB-PS system. The formation mechanism of such a dense layer in MSQ/PS-P4VP system was proposed in the thesis.