十二吋矽晶圓在快速熱製程中溫度均勻性之研究

Abstract

自1958年有積體電路工業以來，熱處理製程一直是業界控制改善相關材料結構特質及電子物理性質的最有效方法之一。近幾年，晶圓尺寸加大，微電子元件發展傾向於縮小積體電路線體尺寸，降低製程的加熱預算便成為當務之急。單片快速熱製程（Rapid Thermal Processing--RTP）即因應而生來取代無法滿足目前深次微米元件製程的傳統對流式高溫爐管。快速熱製程的瞬時加熱光源系統之輻射頻譜（波長及強度）若能與晶圓片之輻射吸熱性質相致，將大幅減低熱預算。在過程中，同一片晶圓上溫度是否均勻，一直是業界能否完全接受RTP的重要課題。如何設計出一能減緩晶圓上不均勻昇溫的快速加熱系統，乃是所有RTP設備製造廠的一個最主要研發目標。 本文主要提出一種逆向模式之數值演算法以便能評估計算快速熱製程中使得晶圓上溫度均勻所需的最佳照射熱通量或邊緣的熱補償。本文係以矽為晶圓材料，考慮晶圓熱物理性質隨溫度而變化，並採用含未來(future-time)溫度演算法的逆解熱傳方法。一維模式及二維模式均有分析。一般RTP加熱爐，都先以調整晶圓邊緣的熱補償為主使得晶圓上溫度能夠均勻。因此，本文首先探討了使晶圓上溫度均勻之垂直及側向邊緣的熱補償。數值模擬採用晶圓上下兩邊都受到對稱而均勻的光源照射熱通量20 W/cm2，周圍環境溫度為27oC，晶圓從27oC被快速加熱至1097oC。除了300 mm直徑 (12吋) 厚度為0.725 mm之矽晶圓外，也討論100 mm直徑 (4吋) 厚度0.6 mm，150 mm直徑 (6吋) 厚度0.675 mm和200 mm直徑 (8吋) 厚度0.725 mm的矽晶圓。結果顯示，晶圓上溫度均勻所需的照射熱通量可直覺地、有效地經由此逆向模式評估計算出來。假若快速加熱系統能根據此逆向方法來動態控制晶圓上照射的熱通量，本文顯示晶圓上溫度的不均勻性將大大地降低。在以一維方法分析RTP的強度模式(intensity mode)時，對於100 mm，150 mm，200 mm和300 mm直徑的矽晶圓，在過程中，最大的溫差分別僅有0.184，0.385，0.655和0.132oC；即使加進因溫度控制而產生之 □0.7728oC至 □3.864oC的溫度量測誤差，過程中最大的溫差亦分別僅有0.618，0.776，0.981和0.326oC而已。若以一維方法分析12吋直徑矽晶圓RTP的溫度模式(temperature mode)時，對於溫昇律100，200和300oC/sec，過程中最大的溫差分別為0.152，0.388和0.658oC；而以二維方法分析之，對於溫昇律100，200和300oC/sec，過程中最大的溫差分別亦僅為0.835，1.174和1.516oC；並發現溫度的不均勻隨溫昇律之增大而變大。 由於快速熱製程的加熱光源系統只對矽晶片選擇性吸熱的特殊性，過程中改變加熱光源系統或晶圓片之輻射頻譜，可能在晶圓上產生不均勻的溫度分佈。本文應用傅氏轉換紅外線頻譜儀（FTIR）來量測RTP加熱系統中的加熱燈源的放射頻譜，以及矽晶圓在常溫中的熱輻射性質，此外，並對製程中所使用具高介電係數BST薄膜的矽晶圓，做其輻射性質的量測。 本文以逆向熱傳模式分析晶圓於快速熱製程的溫度均勻性之結果與方式，及輻射性質量測實驗得到之結果，對業界應能提供RTP設備製造最佳化的設計理念，並希望可以提供未來在此方面研究者的一項參考。

Since the beginnings of the integrated circuits industry in 1958, thermal processing has been one of the most effective ways to control the phase-structure, properties, and electrophysical parameters of materials. As device dimension shrinks to the sub-micrometer range, reduction of thermal budget during microelectronic processing is becoming a crucial issue. Single wafer rapid thermal processing (RTP) has become an alternative to the conventional furnace-based batch processing in many processes. If the spectral absorptive properties of the wafer to be heated are consistent with the emission spectrum of the heating source, the rapid thermal processing will be in good performance efficiency. Temperature uniformity across the wafer during all processing continues to be a main obstacle for full acceptance of RTP into manufacturing. A numerical inverse-modeling algorithm was developed in the present study which was able to calculate optimal incident heat flux and/or edge-heat compensations on wafers sequentially during rapid thermal processing in order to obtain temperature uniformity across wafers. The temperature-dependent thermal properties of silicon and a future-time algorithm of inverse heat-transfer method are used. Both one-dimensional and two-dimensional analyses are presented. The adjustment of the edge-heat compensation in the wafer perimeters in order to achieve temperature uniformity across the wafer is the predominant task that arises for establishing a thermal process for this furnace. Vertical and lateral edge-heat compensations on the perimeter are discussed. Both sides of the wafer were subjected to a uniform heat flux of 20 W/cm2 from 27oC transition to a steady state of 1097oC via simulation with an ambient temperature 27oC. The incident-heat-flux profiles required for temperature uniformity across 100-mm-diameter (0.6-mm-thick), 150-mm-diameter (0.675-mm-thick), 200-mm-diameter (0.725-mm-thick), and 300-mm-diameter (0.775-mm-thick) silicon wafers were intuitively evaluated using inverse modeling. Our numerical results show that temperature uniformity can be efficiently achieved using inverse modeling and reveal that the thermal non-uniformity can be reduced considerably if the incident heat fluxes on the wafer can be dynamically controlled according to the results calculated by the inverse methods. In the case of intensity mode analyzed by one-dimensional thermal model, the maximum temperature differences were 0.184, 0.385, 0.655 and 0.132oC across 100-, 150-, 200- and 300-mm-diameter wafers, respectively. Although taking account for the effect of measurement errors ranging from □0.7728oC to □3.864oC, the resulting maximum temperature differences were 0.618, 0.776, 0.981 and 0.326oC across 100-, 150-, 200- and 300-mm-diameter wafers, respectively. For the case of temperature mode for 12-inch silicon wafer during rapid thermal processing, the results of one-dimensional analysis show that the maximum temperature differences were only 0.152, 0.388 and 0.658oC, respectively, for linear 100, 200 and 300oC/sec ramp-up rates. While, the results of two-dimensional analysis show that the maximum temperature differences were 0.835, 1.174, and 1.516oC, respectively. Thermal non-uniformity occurring during the ramp increased with the ramp-up rate. A change in spectral distribution of the radiative properties (the wafer or the heating source) is a source of temperature non-uniformity for RTP systems. Measurements of emission spectrum for the heating source and spectral emissivity of silicon wafer in RTP furnace by Fourier Transform Infrared (FTIR) spectroscopy are presented. Five quartz-tungsten-halogen lamps of OSRAM at different lamp voltages and two p-type and heavily doped silicon wafers with front side polished as well as three kinds of (Ba,Sr)TiO3 [BST] thin films on silicon wafer have been carried out using Bomem Michelson series MB-154 FTIR spectrometer. The emission spectrum of the heating source was superposed by three spectral curves located in the range of about 1-4.5 um, 4-9 um and 9-20 um, respectively, corresponding to the emission from the tungsten filament, the warm quartz glass tube and the base part. The spectral peak output, corresponding tungsten filament temperatures from about 2000K to 2600K, range from 1.9-2.5 um (4000-5200 cm-1). The reflectance and transmittance of bare silicon wafer are always 40％ and 50％, respectively, smaller than 8000 cm-1, and, the emittance exists only between 10000 and 8500 cm-1. The effects of BST thin film and oxide on silicon wafer are to reduce the transmittance significantly.