Al/HfO2/Si電容應用於電壓控制振盪器電路之研究

Abstract

在最近這幾年，射頻（Radio Frequency, RF）電路所使用的電壓控制振盪器（Voltage Controlled Oscillator, VCO）電路，是需要較大電壓變化量來改變可變電容的電容值，並藉此來改變VCO電路的振盪頻率。因為VCO電路在CMOS製程中所使用的可變電容是金氧半導體（Metal-Oxide-Semiconductor, MOS）電容，然在其閘極（Gate）都使用氧化矽（SiO2），二氧化矽的介電常數約為3.9。在本論文實驗的VCO電路中之可變電容有2種：一種是飛利浦（Philips）廠商所提供型號為BB135之P-N接面電容；第二種是自製的Al/HfO2/Si電容是使用二氧化鉿（HfO2）來做為電容的介電材料，因為HfO2介電常數比氧化矽（SiO2）大，目的是要在相同厚度以及面積下，就可以有較大的電容值，希望能藉此有較大的單位電壓改變電容值的比值。我們在頻率100 kHz下量測Al/HfO2/Si電容並做調變電壓與所量測到的電容值之關係表，所量測的可變電容隨電壓改變的電容值在單位電壓下，可發現其最大與最小之電容值比例約為4.25，並用此電容做為VCO電路中的可變電容。

In recently years, the voltage controlled oscillator (VCO) in the radio frequency (RF) circuit needs a larger voltage variation for adjusting the value of the varactor and changing the operating frequency of the VCO. The varactor used in the VCO is also called MOS capacitor since it is implemented in the metal-oxide-semiconductor (MOS). However, the material of the gate of CMOS is SiO2. The dielectric constant of SiO2 is about 3.9. In this thesis, there are two types of varactors used in the VCO: one is the Philips’ BB135 p-n junction capacitor; the other is Al/HfO2/Si capacitor with HfO2 dielectric. Because the dielectric constant of HfO2 is larger than that of SiO2, the capacitance of Al/HfO2/Si capacitor will be larger than that of MOS capacitor with the same thickness and area. Therefore, there will be larger unit voltage for adjusting the ratio of the capacitance values of the capacitors. The Al/HfO2/Si capacitance of capacitors was measured at 100 kHz. The maximum capacitance ratio, that is, the largest capacitance divided by the smallest capacitance, is about 4.25. The Al/HfO2/Si capacitors are then implemented as the varactor in the VCO circuit.