新型微機械式可變電容之設計與製作
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Date
2005
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Abstract
本研究主要是設計與製作一新型微機械式可變電容，以改善傳統電子式變容器，其可調範圍較小、品質因子低、整合性不佳與寄生效應嚴重等缺點。然而，一般以靜電驅動之間距調整微機械式可變電容，其可動結構是以電鑄或沉積金屬薄膜方式製作，製程較為複雜，而本研究直接以黃光微影的方式，定義SU-8負型光阻以製作電容之可動結構，可簡化製程，此SU-8介電層也可使上電極板與基材有效絕緣以減少寄生效應，進而提升品質因子。本研究之新型微機械式可變電容，具有製程簡單、生產成本低、良率高等特點，除了可取代電子式變容器外，其整套製程也能與標準積體電路製程相容，因此有能力達成將微機械式可變電容與其他電子電路整合，實現系統單晶片(system on a chip, SOC)的目標。
本研究主要分為元件製程與訊號量測兩大部分：
1. 在元件製程上，成功地運用光阻犧牲層技術與犧牲層保護膜，製作出SU-8光阻之懸浮微結構。整個微機械式可變電容元件包括：下電極、光阻犧牲層、SU-8可動結構層與上電極，最後再將犧牲層移除，以完成元件製作。
2. 在訊號量測上，由電容電壓關係曲線得知，當頻率固定在2000 Hz時，此電容值變化是在外加偏壓為40伏特與無外加偏壓下的電容差值，則此元件具有0.3 pF的電容變化量。由史密斯圖得知，頻率從45 MHz至1.079 GHz的範圍內，此可變電容是電容性元件，其具有理想的品質因子，且等效電路為良好的開路。當頻率固定在1.079 GHz時，其輸入反射係數表示為S(1,1)=0.540/-176.102。
In this study, a novel design and fabrication of micromachined tunable capacitor can be completed, such that the drawbacks of traditional electrical varactor, like small tuning range, low quality factor, severe parasitic effect and poor integration, can be significantly improved. However, the fabrication process of general gap-tuning capacitors with electrostatic drive are complicated, because their movable structure are formed by electroforming or deposition procedure. In this study, the tunable capacitor and its movable structure can be realized by SU-8 lithographic technique, this SU-8 dielectric structure can decrease parasitic effect due to electrical insulation between upper and lower electrode plates and increase quality factor further. In this study, the novel micromachined tunable capacitor has the merits of simple fabrication process, low cost and high yield rate, which can replace the traditional electrical varactor and all of the fabrication process is fully compatible with CMOS process and able to integrate micromachined tunable capacitor and other electric circuit, which can be realize the goal of system on a chip. In this study, it is mainly including of two parts-device fabrication and signal measurement: 1. In device fabrication, it is successfully using photoresistor sacrificial layer and sacrificial protection film to fabricate the suspension microstructures of SU-8 photoresistor. The entire micromachined tunable capacitor includes of lower electrode, photoresistor sacrificial layer, movable structure of SU-8 and upper electrode. The final step is to remove the sacrificial layer. 2. In signal measurement, according to the C-V curve, when the frequency is fixed to 2000 Hz, the capacitance differential is the subtraction of applied DC bias at 40 V and zero bias. And it comes to 0.3 pF of the capacitance differential. As Smith chart shows we can know that the tunable capacitor is capacitive device with good quality factor as well as the well open of equivalent circuit when the frequency is between the range from 45 MHz to 1.079 GHz. When the frequency is fixed to 1.079 GHz, the input reflection coefficient shows as S(1,1)=0.540/-176.102.
In this study, a novel design and fabrication of micromachined tunable capacitor can be completed, such that the drawbacks of traditional electrical varactor, like small tuning range, low quality factor, severe parasitic effect and poor integration, can be significantly improved. However, the fabrication process of general gap-tuning capacitors with electrostatic drive are complicated, because their movable structure are formed by electroforming or deposition procedure. In this study, the tunable capacitor and its movable structure can be realized by SU-8 lithographic technique, this SU-8 dielectric structure can decrease parasitic effect due to electrical insulation between upper and lower electrode plates and increase quality factor further. In this study, the novel micromachined tunable capacitor has the merits of simple fabrication process, low cost and high yield rate, which can replace the traditional electrical varactor and all of the fabrication process is fully compatible with CMOS process and able to integrate micromachined tunable capacitor and other electric circuit, which can be realize the goal of system on a chip. In this study, it is mainly including of two parts-device fabrication and signal measurement: 1. In device fabrication, it is successfully using photoresistor sacrificial layer and sacrificial protection film to fabricate the suspension microstructures of SU-8 photoresistor. The entire micromachined tunable capacitor includes of lower electrode, photoresistor sacrificial layer, movable structure of SU-8 and upper electrode. The final step is to remove the sacrificial layer. 2. In signal measurement, according to the C-V curve, when the frequency is fixed to 2000 Hz, the capacitance differential is the subtraction of applied DC bias at 40 V and zero bias. And it comes to 0.3 pF of the capacitance differential. As Smith chart shows we can know that the tunable capacitor is capacitive device with good quality factor as well as the well open of equivalent circuit when the frequency is between the range from 45 MHz to 1.079 GHz. When the frequency is fixed to 1.079 GHz, the input reflection coefficient shows as S(1,1)=0.540/-176.102.
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Keywords
可變電容, SU-8光阻, 面型微加工, 品質因子, 射頻微機電系統, tunable capacitor, SU-8 photoresistor, surface micromachining, quality factor, RF MEMS