利用結晶型沸石及非晶型氧化矽奈米顆粒製備中孔洞型低介電常數薄膜

Abstract

本研究利用水熱程序在鹼性環境下製備出含純矽MFI沸石或是非結晶型奈米顆粒之鍍膜溶液，以製備中孔洞型低介電常數薄膜(mesoporous low-k films)，研究之目的為製備具有介電常數小於2且高機械強度(硬度大於1 GPa且彈性係數大於10GPa)之低介電常數薄膜。本研究利用XRD及FTIR分析鍍膜溶液中顆粒之結晶性，用以探討顆粒結晶性對薄膜性質之影響，並利用動態雷射光粒徑分析儀分析鍍膜溶液中之顆粒的大小。本研究也使用固態NMR分析顆粒所含Si-O-Si及Si-OH基團的量;使用氮氣吸脫附之孔洞分析儀測量樣品的孔洞分佈範圍以及孔洞體積。研究中並使用SEM及光學顯微鏡觀察含不同結晶性顆粒之鍍膜溶液所形成薄膜的表面型態。薄膜的介電常數及漏電流則利用電性分析儀器量測，而機械強度則使用奈米壓痕機(nanoidenter)進行分析。 由本研究結果得知，各鍍膜液中是否含有界面活性劑，對所鍍之薄膜性質有關鍵性的影響。若僅使用不含界面活性劑的純矽MFI沸石或是非結晶型顆粒溶液進行旋轉塗佈程序製膜，所鍍出之薄膜不是破裂就是表面粗糙。以文獻中兩階段水熱程序(60℃48小時，100℃36到48小時)所製備之純矽MFI沸石溶液為例，添加Tween 80界面活性劑後，可以成功鍍膜並能獲得均勻之薄膜，且於最適化的條件下，薄膜介電常數為1.83、硬度1.39 GPa、彈性係數12.3 GPa以及漏電流密度為1.35 × 10-7 A/cm2，這些性質已經符合未來積體電路工業的需求。 為了節省兩階段水熱程序冗長的製程(約需要156到168個小時)，本研究已研發出一系列單階段的水熱製程(僅需27到51個小時)。該製程在100℃水熱溫度及24小時反應時間可製備出含平均粒徑為4.6奈米的非結晶型顆粒之鍍膜液。當水熱反應時間增長為36小時以上時，XRD已可明顯偵測出MFI沸石結晶型顆粒，其粒徑及孔洞體積都隨反應時間增加而增加。固態NMR分析則顯示顆粒上Si-OH基團(silanol groups)濃度隨反應時間增加而減少，換句話說，非結晶型顆粒具最高之SiOH基團(silanol groups)濃度。由研究結果也發現，薄膜之機械強度隨溶液水熱反應時間增長而減少，而利用非結晶型顆粒鍍膜液能鍍出機械強度最高之薄膜。但由於此非結晶性顆粒表面具有很多的SiOH基團(silanol groups)，使用該顆粒所製備的薄膜太厚時會增加疏水性修飾的難度，因此須將薄膜厚度降低。如此可製備出同時也具有小於2的低介電常數、低漏電流密度(為數量級10-7A/cm2)及高機械強度(硬度1.73 GPa、彈性係數17.0 GPa)之中孔洞型低介電常數薄膜。 本研究發現若要製備出機械性質佳的中孔洞型低介電常數薄膜，膜中所含之奈米顆粒之光密度比值(ratio of optical density，FTIR測得，550cm-1波峰強度與450cm-1波峰強度的比值)必須小於10 %。單階段24小時水熱所成之非結晶型顆粒，其光密度比值為0 %，因此機械強度高。而水熱時間大於42小時所成之純矽沸石顆粒，其光密度比值會大於10 % (界於15 %到42 %之間)，因此必須藉由離心程序移除溶液中的大結晶顆粒，才使溶液中的顆粒光密度比值小於10 %，因而成功製備出介電常數小於2、低漏電流密度(為數量級10-8A/cm2)以及高機械強度的中孔洞型低介電常數薄膜。

In this research, mesoporous low dielectric constant (low-k) films were prepared from coating solutions composed of surfactant Tween 80 and nanoparticles of different crystallinity synthesized through hydrothermal processes. The propose is to prepare mesoporous low-k films with dielectric constants of < 2 and high mechanical strength, hardness of > 1 GPa and elastic modulus of > 10 GPa. According to the results obtained in this research, for the films prepared from coating solutions containing PSZ nanocrystal only (no Tween 80 involved), the films showed serious cracking or were very rough. As Tween 80 was added into the solutions containing the PSZ nanocrystals prepared through a two-stage hydrothermal process, films with uniform surface morphology can be obtained. A film spin-coated from coating solution prepared under optimal experimental conditions possessed an ultra low k value of 1.83, a hardness of 1.39 GPa, an elastic module of 12.3 GPa, and a leakage current density of 1.35 × 10-7 A/cm2, all of which met the needs of the integrated circuits (IC) industry. However, the two-stage hydrothermal process took 156 to 168 h to synthesize the PSZ nanocrystals that was too long and took too much energy. In order to address the concern, a faster one-stage hydrothermal processes which took only 27 to 51 h to prepare the coating solutions were developed in this research. Coating solutions containing noncrystalline silica particles with an average size of 4.6 nm were synthesized through the one-stage hydrothermal process that took only 24 h to synthesize the noncrystalline silica particles. As the hydrothermal time was increased to 36 h or higher, MFI-type structure of zeolite nanocrystals could be easily observed. The amount of silanol groups on the particle surface, measured through solid-state NMR spectra, was decreased with the increase of hydrothermal time, indicating that the amount of the silanol groups in the noncrystalline silica particles was the most of all samples. The mechanical strength of the films was also decreased with the increase of hydrothermal time, and the film prepared with the noncrystalline silica particles possessed the strongest mechanical strength. Due to that there were many silanol groups on the surface of the noncrystalline silica particles, the modification of the silanol groups to become hydrophobic might not be complete enough when the film was too thick. Therefore, the film thickness was reduced, and then a mesoporous low-k film possessed k values of smaller than 2, low leakage current densities (of order of 10-8 A/cm2) and high mechanical strength (hardness of 1.73 GPa and elastic modulus of 17.0 GPa) was obtained. It was found that for preparing mesoporous low-k films with high mechanical strength and low k values, the ratio of optical density of < 10 % (ratio of optical density, measured from FTIR spectra, which was the ratio of intensity for band 550cm-1 to band 450cm-1) of the nanoparticles is necessary to prepare the mesoporous low-k films.When the hydrothermal time was higher than 42 h in the one-stage hydrothermal process, the ratio of optical density of the so-obtained nanoparticles was higher than 10 % (between 15 % and 42 %); therefore, a centrifugation step should be applied to remove big nanoparticles, resulting in that the ratio of optical density of the nanocrystal in the resulting centrifuged coating solution could be smaller than 10 %. As a result, mesoporous low-k films which possessed k values of < 2, low leakage current densities of order of 10-8 A/cm2, and high mechanical strength (hardness of > 1 GPa and elastic modulus of > 10 GPa) were successfully prepared from the centrifuged coating solutions.