微污染與元件特性之評測

Abstract

隨著半導體製程對空氣中微污染的敏感度日與俱增，關於各種無塵室內污染行為及污染對元件特性的衝擊開始被廣泛的研究。本論文主要偵測現行無塵室空氣中大部份的微污染（airborne molecular contamination） 並且研究各種污染對元件的特性及參數的影響。 我們首先利用將無塵室空氣通過超純水或通過吸附劑的方式對無塵室作空氣採樣，再將這些採樣後的樣品送至各種精密的儀器作分析，以得到定性及定量的分析資料。我們利用氣相層析質譜儀（GC/MS）來分析樣品中所含之有機污染，離子層析儀（ION CHROMATOGRAPH）分析無機離子污染及感應耦合電漿質譜儀（ICP MS）分析金屬污染。此外我們也特別設計製造了一個迴風槽（CLEAN BENCH）來模擬無塵室內的迴風系統，在槽內裝置了可更換的濾網模組，在調整槽內各項參數至和無塵室相同後，我們也對槽內空氣作採樣分析，希望利用不同的濾網組合來得到對空氣污染最佳的控制能力。 其次經過標準RCA清洗的6吋晶圓，也被放置到無塵室及迴風槽內作暴露實驗，藉由熱脫附大氣壓質譜儀（TDS-APIMS）分析晶圓表面附著的有機化合物，全反射X螢光分析儀（TRXRF）及二次離子質譜儀（SIMS）來分析表面附著金屬污染及硼污染，由分析後結果來研究空氣中污染含量及其於晶圓表面的附著行為之間相互關係。 最後，我們實際在晶圓上製作MOS元件，並在匣極氧化製程前於無塵室及迴風槽內作暴露，經由氧化層的漏電流密度，崩潰電場強度及崩潰電荷等來得知微污染對氧化層可靠度的影響，利用偏壓加溫方式（Bias Temperature Stress）來量測氧化層內可移動金屬離子的含量及利用C－V量測來觀察元件主動區內硼摻雜濃度的變動量。 由實驗結果我們發現，新型鐵氟龍(PTFE)材質的濾網，除了在塵粒（particle）及金屬的過濾表現出極優越的能力，濾網本身也不會釋出多餘有機污染及摻雜。相較之下現行一般無塵室所用的玻璃纖維超高效能濾網（Glass fiber ULPA filter）雖然和前者一樣有良好的塵粒（particle）及金屬的過濾表現，但是因為它使用了乳膠來固定濾網內纖維，使大氣下採樣的結果含有較高濃度的D6有機化合物，又製造者為了強化玻璃纖維的塑性，故在其中多加入了硼元素，也在實驗中觀察到有微量硼釋出的現象。當元件暴露在新型鐵氟龍(PTFE)材質的濾網下，氧化層崩潰電埸只有些微的退化由14MV/cm變成集中在13MV/cm，漏電流密度只增加了幾個毫安培(nA)，並且也量測不到移動金屬離子污染。相同的暴露時間下，玻璃纖維超高效能濾網下的元件則顯示出相當明顯的退化，氧化層崩潰電埸變成分散在9∼12MV/cm之間，由C－V量測也得到主動區有明顯的摻雜變動量。 由各種實驗結果，我們認為新型鐵氟龍(PTFE)材質的濾網，和化學濾網的組合將可以提供完美的低塵粒，低金屬，低有機及無機污染的控制能力，在未來製造超大型積體電路的無塵室中提供高效能，高良率的製程環境。

As the increased sensitivity of the semiconductor manufacturing process to airborne molecular contamination (AMC), the researches on cleanroom contamination behavior and their impact on device performance are broadly pursued. In this thesis, we detected most airborne molecular contaminations in present cleanroom, and took an inquiry into the influence on device parameter and performance. First, we sampled cleanroom atmosphere by passing the air through ultra clean water or absorbent, and then these samples were taken into qualitative and quantitative analysis by precision instruments. Here, we use GC/MS for organic compounds contamination analysis, ion chromatograph (IC) for inorganic ion contamination and ICP-MS for metal contamination analysis. In addition, we specially designed and fabricated a clean bench (CB) to imitate the clean room recycling air- flow system. The filter module in the CB is changeable. After adjusting all parameter of CB as well as cleanroom airflow condition, the same sampling procedures were pursued in CB. Secondly, 6-inch wafers were also exposed in actual cleanroom and CB for 24~72 hrs after RCA cleaning. The organic compound absorbing on wafer surface was analyzed by TDS-APIMS, metal contaminating behavior was analyzed by TRXRF, and SIMS analyzed excess boron impurity. From comparing with these experimental results, we can comprehend the relation between the contamination quantities containing in cleanroom atmosphere and the quantities absorbing on wafer surface. Finally, we manufactured MOS capacitors on wafers that exposed in cleanroom and CB before gate oxidation process. The effects on gate oxide reliability were estimated by leakage current density, breakdown field and charge to breakdown (Qbd) characterized by J-E and V-ramp test. BTS (bias-temperature stress) method and C-V measurement were used to realize mobile ion in oxidation film and boron doping variation in active region. From the experiment result, PTFE filter shows excellent particle and metal isolating ability and operates without outgassing extra impurity such as organic and trace boron. Although commercial ULPA filter reveals the same control ability at particle and metal contamination control as well as PTFE filter. But it shows high D6 organic compound concentration either at air sampling or wafer exposing experimental analysis result as operating. And further, trace boron impurity was observed exhaling. This is because filter fabricator adds latex and boron into media of filter for strength enhancement and fixation. Therefore, as filter is corroded by acid such like HF vapor in the cleanroom, organic contamination and trace boron will come out. Devices exposing under PTFE filter show just a little degradation. Breakdown field of gate oxide changes from 14 MV/cm to converging at 13 MV/cm, and leakage current density adds just several nA. Moreover, there is no obvious mobile ion monitored by BTS test. For the same exposing time, devices exposed under ULPA filter show incontestably degradation. Oxide breakdown field change from converging at 13 MV/cm to scattering around at 9~12 MV/cm. And from C-V test, it gets obviously doping concentration variation in active region. We consider new PTFE filter combining with chemical will provide excellent particle, metal, organic and inorganic contamination control ability. And in the future, achieve high performance and high yield ULSI manufacturing environment.