高介電常數射頻金屬-絕緣層-金屬電容及其電容值變動之研究

Abstract

隨著互補式金氧半電晶體元件的特徵尺寸持續地微縮，閘極介電質（目前主流為二氧化矽）的厚度也隨之減少以保持住電容值，使得電晶體元件的驅動電流保持在可接受的大小。半導體工會組織所訂定的半導體技術未來指標中指出，在2003至2005年時，等效閘極介電質的厚度將會減少至將近1.5奈米。然而降低二氧化矽的厚度至如此小的尺寸時，將會導致量子穿遂漏電流的發生，且此漏電流的大小會隨著厚度的減少而成指數性地增加。因此，高介電常數介電質為一個解決此問題的適當替代物，除了可避免介電質厚度的降低以減少漏電流外，且可和互補式金氧半電晶體的製程整合在一起，並可達到與二氧化矽等效的電容值。 在此，我們研究使用高介電質『氧化鋁鉭』於金氧金（MIM）電容且整合在400oC的後段製程上。在高頻時，為了達到高電容密度、微小的電容值下降率和低漏電流，所以使用此高介電質『氧化鋁鉭』。此高電容密度的電容可以有效地減少在射頻積體電路上晶片的大小。除了高電容值密度外，我們還測得了氧化鋁鉭金氧金電容的電壓係數（VCC）與溫度係數（TCC）隨著頻率的增加而迅速遞減。而當操作頻率到達1 GHz時，與電壓相關的電容值變動率將小於萬分之二，此一性質可保證於精確的射頻電容電路應用中。 我們同時也解釋了此電壓相關的電容變化值與介電質厚度及頻率的相依性，並且也說明了氧化鋁鉭金半金電容的電容值與溫度的相關性。以『自由載子注入模型』為基礎，可通盤地瞭解 (1) 介電質厚度t與電容值變動的相依性，此為電場極化的本質特性所導致的。(2)頻率與溫度對電容值變動的相關，這些均是由於絕緣層內電荷的遷移率改變造成了偶極的鬆弛時間不同所導致的。 除此之外，我們已量測與分析了從10 KHz到10 GHz的電容值對於電壓與頻率的變化率，並且對於鬆弛時間加入了一項與頻率相關的因子來解釋電容值變動的降低，此模型也可被應用於預測之後的電容值變化率。

As the feature sizes of complementary metal-oxide-semiconductor (CMOS) devices are scaled downward, the gate dielectric thickness must also decrease to maintain a value of capacitance to keep device drive current at an acceptable level. The Semiconductor Industry Association’s (SIA) International Technology Roadmap for Semiconductors (ITRS) indicates that by the year 2003~2005, the equivalent thickness of the gate dielectric will need to be approximately 1.5 nm. Reducing the thickness of SiO2 to these dimensions will result in an exponential increase of direct tunneling leakage current. A suitable replacement dielectric with a high relative permittivity □ (or dielectric constant) must exhibit low leakage current, have the ability to be integrated into a CMOS process flow, and exhibit at least the same equivalent capacitance of SiO2. We have studied the metal-insulator-metal (MIM) capacitors integrity using high-□□Al doped TaOx dielectrics formed under 400oC backend process. Using high-□□Al doped TaOx dielectric, we have obtained record high MIM capacitance density of 17 fF/□m2 at 100 KHz, small 5% capacitance reduction to radio frequency (RF) range, and low leakage current density of 8.9×10-7 A/cm2. This very high capacitance density with good MIM capacitor characteristics can significantly reduce the chip size of radio frequency integrated circuits (RF ICs). In additional to the very high capacitor density of 17 fF/□m2 at 100 kHz, the measured voltage-coefficient of capacitor (VCC) and temperature-coefficient of capacitor (TCC) of Al doped TaOx MIM capacitors decrease rapidly with increasing frequency. Excellent normalized voltage-dependent capacitor variation (□C/C) □ 200 ppm is obtained at these capacitors as frequency reaching 1 GHz, which can ensure precision capacitor circuit application at RF regime with drastically reduced capacitor size. We also explain the dependence of normalized voltage-dependent capacitor variation □C/C-V on dielectric thickness and frequency, below 1 MHz, as well as temperature dependence of capacitance of Al doped TaOx MIM capacitors. Based on free carrier injection model, a unified understanding is achieved: (1) the dielectric thickness (t) dependence □C/C is an intrinsic property due to electric field polarization, (2) the frequency dependence of □C/C and temperature dependence of capacitance are all due to change of relaxation time with different carrier mobility in insulator. We have measured and analyzed the normalized frequency-dependent voltage-dependence of capacitance (□C/C-V), from 10 KHz to 10 GHz. We modified a free carrier injection model having a frequency dependent [1+(f/f0)2]-1/2 pre-factor for relaxation time (□□, to explain the reduction of □C/C in the GHz range. This model is also applied to predict the □C/C for future applications.