室溫液相沉積法沉積氧化矽薄膜之研究

Abstract

本論文研究之主旨在探討利用一種可以在室溫暨液相狀況下，在矽基板上沉積氧化矽薄膜之新技術。此技術只需將高純度的矽酸粉末加入六氟矽酸溶液中，予以攪拌、溶解、並過濾而形成飽和浸泡溶液，因為此溶解過程為放熱反應，故只需升高飽和溶液之溫度，此溶液即處於過飽和狀態，如此即可在矽基板上沈積一含微量氟之氧化矽薄膜。此氧化矽薄膜之化學分子式可定義為SiO2-xFx。 在本論文中，我們分別於浸泡溶液中加入水、雙氧水及硼酸等添加劑加速並控制氧化矽薄膜之沈積速率。我們發現在反應溶液中所加入之水量多寡除了可以控制薄膜之沈積速率外，並存在一極大沈積速率。我們首度提出水在此反應中可為反應物及溶劑之論點來解釋此現象。 添加雙氧水除可增快沈積速率外，於添加適當水量的沈積溶液中，更可因催化效應加速沈積速率。於沈積溶液中添加雙氧水，溶液顏色由無色轉變成淡黃色，隨雙氧水添加量增加而轉變成金黃色。若此時再加入氫氟酸，則溶液恢復無色狀態。此顏色變化可用於判別浸泡溶液之飽和程度。 在添加硼酸之情況下，氧化矽薄膜之沈積速率可控制在80~3620埃/小時，為目前已知的最大沈積速率區間。當硼酸濃度大於一臨界值時，氧化矽薄膜之品質不良且沈積速率無法控制。由沈積速率及折射率之量測結果，可以定義出二個薄膜沈積反應區間：異相反應區（heterogeneous reaction range）及均相反應區（homogeneous reaction range）。氧化矽之沈積速率嚴重影響薄膜的品質及折射率。我們發現折射率隨沈積速率增加而近似線性減小，且下降速率隨硼酸濃度之增加而增加，於異相反應區，薄膜折射率介於1.39~1.44之間。於均相反應區內，因氧化矽之自發性凝核沉積，導致薄膜的品質及沈積速率無法控制。 我們亦利用紫外線照射添加硼酸之溶液，結果顯示薄膜沈積速率髓紫外光強度之增強而有明顯的增加。當硼酸濃度升高時，光輔助效果更顯著。若硼酸濃度維持一定值，我們發現沈積速率隨紫外光強度之增強而增加。 藉由霍氏轉換紅外線光譜(FTIR)與歐傑電子光譜(AES)，確知氟元素是以Si-F之形式存在，其含量可由水添加量、硼酸濃度及沈積溫度控制而與沈積速度無關。化學蝕刻速率之分析顯示沈積速率越快則結構密緻性，蝕刻速率亦越快。因此我們可以結論以液相沉積法沉積之氧化矽薄膜，其折射率主要由沈積速率所控制而非薄膜之氟元素濃度。

In this thesis, a novel technology named liquid-phase deposition (LPD) method for fluorinated silicon dioxide formation on Si substrates at room temperature was studied. The SiO2-saturated immersing solution was prepared by dissolving silicic acid in hydrofluosilicic acid solution with stirring and then being filtered. Because this dissolving process is an exothermic reaction, oxide can be formed on Si substrates by increasing the temperature of immersing solution. The composition of LPD-oxide can be represented as SiO2-xFx. In this study, water, hydrogen peroxide, and boric acid were used as additives to enhance and control the deposition rate of LPD-oxide. The deposition rate as a function of water content has been observed to have a maximum, and the role of water playing in the deposition reaction, reactant or solvent, is clarified. It is also found that hydrogen peroxide additive in the solution increases the SiO2 deposition rate proportionally. Additionally, at moderate quantity of water added, high peroxide concentration can further enhance the reaction catalytically. The colorless SiO2-saturated hydrofluosilicic acid solution turns into a yellowish color, and with more peroxide the color is enhanced into golden. This coloring phenomenon can be used to justify the growth solution whether saturated or not. For boric acid additive, the oxide deposition rate can be controlled in the range of 8~362 nm /h which is larger than that reported by other investigators. The cut-off boric acid concentration that the spontaneous nucleation of oxide sets in has been determined. From the results of film quality and deposition rate, we can define two deposition reaction ranges: heterogeneous reaction range and homogeneous reaction range. In heterogeneous reaction range, the refractive index of oxide film approximately linearly decreases with the increasing of deposition rate, and the decreasing rate of refractive index is larger for higher boric acid concentration. The refractive index of oxide film is about 1.39~1.44. In homogeneous reaction range, the quality of film is too bad to determine the deposition rate. With the assistance of 254 nm ultraviolet (UV) illumination, the deposition rate of oxide is significantly increased, and this photo-assisted effect is much stronger at higher boric acid concentration. At a constant boric acid concentration the deposition rate of oxide is found to increase linearly with UV light intensity. However, within the UV intensity range studied, the photoeffect on the oxide growth rate is not as strong as boric acid addition. The UV spectrophotometric results show that the strong UV absorption of the growth solution is associated with SiF6-2 but not with H2O, H3BO3, or BF4- species in the solution. FTIR spectra and AES depth profiles indicate that a small amount of fluorine was naturally incorporated into the oxide. The atomic concentration of fluorine in the oxide film can be controlled by the quantity of water added, the deposition temperature, and the boric acid concentration, but is independent of the growth rate. The P-etch rate indicates that the higher the deposition rate, the faster the etch rate, and the lower the refractive index. The refractive index of oxide is dominated by the growth rate but not by the incorporated fluorine concentration.