Development of semiconductor technology over the last five decades has led to aggressive scaling down of integrated circuit (IC) device dimensions. ICs have become faster, denser and more power-efficient by continuous shrinking down of the metal-oxide-semiconductor field-effect transistor (MOSFET) size and implementation of complex integration schemes using novel materials. We are steadily approaching the physical limits of scaling and along the way more and more obstacles appear that need to be overcome in order to continue further. Traditional process control and device characterization techniques are becoming insufficient for addressing these problems. Novel techniques must be implemented for obtaining information about structural and electrical properties on materials and geometries with nanometer resolution. This is particularly relevant at the present transition from silicon dioxide gate dielectrics to ones with higher dielectric permittivity – high-K dielectrics.
The present work is a contribution to this search for novel suitable analytical techniques and their implementation in semiconductor technology. It exploits extensively the high resolution imaging possibilities of atomic force microscopy (AFM) as a key support technique from the selection of prospective high-K candidates to their integration into a suitable MOSFET fabrication process. Particular attention is paid to conductive atomic force microscopy (C-AFM) which offers the possibility of mapping simultaneously topography dimensions and electrical conductivity.
Initially, AFM and C-AFM are used for the development and optimization of a device isolation technology that is relevant in the context of high-K dielectrics in ultra large scale integration (ULSI) ICs – shallow trench isolation (STI). For the first time, reliable detection is obtained of the common problem related to STI – nitride erosion after the chemical planarization (CMP) step. Again with the help of C-AFM, two different techniques for planarity optimization are developed and evaluated – oxide etchback and reverse nitride masking.
Next, C-AFM supports the investigation of two principally different types of prospective high-K dielectric materials. First generation dual-stack dielectrics that consist of a high-K material on top of a thin interfacial silicon dioxide layer are the easier but less effective solution. C-AFM reveals imperfections in the investigated titanium oxide – silicon dioxide stacks related to the insufficient stability of such bilayer structures. Second generation high-K dielectrics in the face of epitaxial rare-earth metal oxides possess key advantages such as higher thermal stability and the possibility for engineered interface with silicon. C-AFM investigates their properties and proves the superiority of these materials. Imperfections are observed as well that show the need for growth and processing optimizations. For the first time, charge trapping is observed on the nanoscale directly on the high-K dielectric surface. Nonuniform leakages in rare-earth metal oxides grown under insufficiently optimized conditions presumably related to grain boundaries are discovered in some samples. Based on AFM measurements, predictions are made about the expected behavior of MOS devices incorporating these materials.
The compatibility of epitaxial rare-earth metal oxides with standard complementary metal-oxide-semiconductor (CMOS) processing is investigated next. Incompatibility with some steps such as for example cleaning with acid-containing solutions is determined and suitable replacement steps are chosen. Changes in film properties are determined during key steps that could indicate incompatibility of the dielectrics with the standard gate-first integration scheme.
In order to determine to what extent the observed microscopic changes affect macroscopic device behavior, epitaxial dielectric layers are integrated for the first time into complete devices. Rare earth metal oxide MOSFETs are fabricated into a modified gate-first process using different gate dielectrics. C-AFM is used for process control in critical steps. Electrical evaluation of the functional devices featuring praseodymium oxide (Pr2O3), including charge pumping, reveals that at this initial stage of development the high-K gate dielectric devices suffer from degraded performance when compared to SiO2 reference devices. Imperfections such as high density of interface states, susceptibility to charge trapping and gate leakages for large area devices are observed. Neodymium oxide (Nd2O3) integration after further optimization of the gate-first process fails to produce functional devices due to substantial degradation of the gate dielectric and excessive gate leakages. The MOSFET behavior for both materials as determined by macroscopic electrical characterization results is compared to AFM predictions and they coincide very well.
It is concluded that the imperfections of the gate dielectrics are at least partially a result of the integration process. Analysis is carried out and critical performance-reducing steps are identified. The gate structuring by reactive ion etch (RIE), the source/drain ion implantation and the high temperature source/drain activation anneal are responsible for the dielectric degradation to the largest extent. The inseparable link between these steps and conventional processing leads to the idea of implementing an entirely different approach for gentle integration of high-K dielectrics.
Once again with the help of AFM and C-AFM, a replacement gate technology (RGT) is developed and implemented for high-K gate dielectric MOS devices in order to prove this concept. By positioning the gate dielectric growth module after the source/drain implantation and anneal and avoiding the aggressive RIE through indirect gate patterning with CMP, the integration process is adapted to the sensitive high-K materials in order to preserve their as-grown state. Electrical evaluation of devices with Gd2O3 produced using RGT proves the advantage of RGT. The first integration attempt is compared to conventional fabrication technology and there are definite improvements in terms of threshold voltage stability and interface state distribution. The first RGT high-K devices still do not exhibit the mobility and low defect density of equivalent state-of-the-art SiO2 devices but this is expected considering the 40-year-long optimization history behind silicon dioxide. Further optimization is needed for epitaxial rare-earth metal oxides as well, both in terms of growth conditions and process integration
