EFFECT OF CRYSTALORIENTATIONIN OXIDATION PROCESS OF VLSI FABRICATION

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1 International Journal of Research in Engineering, Technology and Science, Volume VII, Special Issue, Feb ISSN EFFECT OF CRYSTALORIENTATIONIN OXIDATION PROCESS OF VLSI FABRICATION Shahina Firdoush, Avinow Raj Dept. of electrical and electronics engineering RVS College of engineering and technology Jamshedpur, India ABSTRACT: Fabrication process is used to create the integrated circuits that are basic functioning unit of all electrical and electronic devices. In VLSI fabrication process of semiconductor manufacturing the oxidation refers to the conversion of silicon to silicon dioxide. The function of a layer of oxide on silicon on chip is multipurpose. The oxidation process is very much affected by the temperature, pressure, dry and wet oxidation and crystal orientation process. This paper discusses about the effect of crystal orientation in oxidation process while going through fabrication of integrated circuits. Keywords vlsi fabrication; oxidation; parameters effecting oxidation ;crystal orientation process. 1. INTRODUCTION In the manufacturing of semiconductor devices and IC s the term oxidation is used to refer the conversion of silicon to silicon dioxide. This process can be done by different methods. a. Tharmal oxidation or b. Electrochemical oxidation. In IC technology that is manufacturing of ICs, mainly thermal oxidation is used. Detail of thermal oxidation and briefly about electrochemical oxidation is discussed here. It is important to note that the deposition of silicon dioxide is not called oxidation. Thus the term oxidation is used only for converting existing layer of silicon to silicon dioxide. Thus it includes modification process. Layer of Silicon taken and modified to get silicon dioxide. It may be considered similar to doping w here p- type or n-type material is added to silicon and modified. Silicon dioxide is often referred to as oxide and occasionally as silica. The importance of the oxide can be understood as in the CMOS transistor, the oxide is an essential and important component in the gate. It is reffered as gate oxide. Thus oxide is a essential part of the structure. Secondly it is used for protecting surface of silicon layer. Thus it acts as passivation layer. Silicon dioxide is actually glass. The only difference in the normal glass and silicon dioxide glass is the normal glass (e.g. Bottles or glass beakers) have lot of other material contaminants in it to make it suitable as required whereas the pure silicon dioxide used Shahina Firdoush, Avinow Raj 1

2 EFFECT OF CRYSTALORIENTATIONIN OXIDATION PROCESS OF VLSI FABRICATION in the field of semiconductors is of high purity. It acts as good insulator and thus it is used also as an insulator between transistors and also in between wires. Thus it is also an electrically essential part of this chip manufacturing. In some processes it is used as a sacrificial layer. Whenever silicon is exposed to air or water, it forms automatically a native oxide or a thin oxide layer. It is very thin i.e. 2 nm s thick (20Å). Since it forms on the surface all the times, the formation process of oxide is needed to be understood. 2.TYPES OF OXIDATION Thermal Oxidation The oxidation process can be divided into thermal oxidation or electro chemical oxidation. Thermal oxidation that is heating effect can be further divided into dry and wet oxidation. Dry oxidation means, it is oxidized with oxygen i.e. in absence of water vapour. Wet oxidation means the silicon is oxidized with stream or water vapor to form silicon dioxide. The overall reaction can be written as For dry oxidation: For wet oxidation: 3. OXIDATION PARAMETERS: The requirement of fabricated oxide and desired characteristics for the fabrication can be mainly influenced by the used oxidant species. It gets affected by change in temperature. Additionally it is possible to vary hydrostatic pressure in the reaction chamber. Furthermore the oxidation process is affected by crystal orientation of the used silicon substrate in the process. Generally wet oxidation grows slowly when compared to dry oxidation but it becomes advantageous. The reason is that the higher growth rate is the oxidant solubility limit for, which is much better than dry oxidation process. Effect of temperature Shahina Firdoush, Avinow Raj 2

3 International Journal of Research in Engineering, Technology and Science, Volume VII, Special Issue, Feb ISSN The oxidation rate increases significantly with the temperature in the furnace for wet as well as dry oxidation. Oxide thickness versus oxidation time for (100) oriented silicon by dry oxidation for various temperatures Oxide thickness versus oxidation time for (100) oriented silicon by wet oxidation for various temperature. Effect of pressure The oxidation rate changes with the change in hydrostatic pressure in the furnace for dry and wet oxidation in nearly the same manner. Basically it increases with the increase in hydrostatic pressure. The major advantages of higher pressure oxidation process over conventional atmospheric oxidation are the lower processing temperature generally employed for the process and the higher oxidation rate. Both these lead to less impurity diffusion in the silicon layer and minimum junction movement during the several oxidation steps which are necessary in the manufacturing of high-density multilayer IC devices. The integrity and quality of oxide layer formed in higher pressure have been found to be comparable to atmospheric oxides. Oxidation- Shahina Firdoush, Avinow Raj 3

4 EFFECT OF CRYSTALORIENTATIONIN OXIDATION PROCESS OF VLSI FABRICATION induced stacking faults are significantly reduced with higher pressure oxidation, which leads to improved device performance and thus becomes beneficial for IC manufacturing process. Oxide thickness versus oxidation time for (110) oriented silicon by dry oxidation at 1000 C for various pressures. Effect of Crystal Orientation The oxide thickness is an important parameter for the oxidation process. Oxide growth rate is much affected by pressure, time and temperature. Oxide growth is accelerated by an increase in oxidation time, oxidation pressure or oxidation temperature. Other factors that affect thermal oxidation growth rate for SiO2 are: the wafer's doping level, the presence of halogen impurities in the gas phase, the crystallographic orientation of the silicon wafer, the presence of plasma during growth of oxide layer and the presence of a photon flux during oxide layer growth. Crystal states of substance influence its structural elements atoms of crystal lattice. A crystal is defined as an array of atoms, repeated in three dimensions in regular manner. Crystal lattice is supposed to possess a cubic symmetry, when each edge of crystal lattice is of the same length. Every direction in a crystal can be characterized using three dimensional coordinate system: x, y, z plane. In a cubic crystal formed crystallographic plane exists, that is perpendicular to the vector which is going along the direction of three dimensional coordinate system: x, y, z planes. Crystallographic planes orientation is described using numbers x, y, z, which are known as Miller indices. Silicon has the basic diamond crystal structure two merged face centered cube (FCC) cells offset by a/4 in x, y, and z. The studies of oxidation have shown that the oxidation rate also depends on the crystal orientation of the silicon substrate. Experiments have demonstrated many times that the oxide growth is faster on (111) oriented surfaces than on (100) oriented at any temperature for dry as well as wet oxidation. For wet oxidation, it was found that the (111) and (100) orientation Shahina Firdoush, Avinow Raj 4

5 International Journal of Research in Engineering, Technology and Science, Volume VII, Special Issue, Feb ISSN represent the upper and the lower bound for oxidation rates, respectively. Therefore, the growth rate for all other orientations lies between these two extremal values. Oxide thickness versus oxidation time for (100), (110), and (111) oriented silicon by wet oxidation 4. CONCLUSION (H O). It is important to understand orientation effects on oxidation more necessarily because many structures actually use etched trenches and other shaped silicon regions as part of their structure. Ligenza suggested from his studies that the crystal orientation effect might be caused by differences in the surface density of silicon atoms on the various crystal faces which very much affect oxidation rate. He argued that since silicon atoms are required for the oxidation process, crystal planes having higher densities of atoms should oxidize much faster compared to others. Density of Si atoms is characterized by crystallographic planes orientation. The thickness of the growing oxide depends on crystallographic planes orientation and time. Shahina Firdoush, Avinow Raj 5

6 EFFECT OF CRYSTALORIENTATIONIN OXIDATION PROCESS OF VLSI FABRICATION Furthermore, he added that not only the number of silicon atoms per cm is important, but also the number of bonds matter, since it is necessary to break Si-Si bonds for proceeding the oxidation. License calculated the available bonds per cm on the various silicon surfaces and give conclusion regarding the oxidation that oxidation rates in H2O ambient should be in the order (111)>(100), which was observed experimentally too. REFERENCES: 1. Oxidation Process and Different Crystallographic Plane Orientation Dependence Simulation in Micro and Nano Scale Structures By-Darius Andriukaitis, Roulade s Aniline s, Department of Electronics Engineering, Kaunas University of Technology 2. Oxidtion Parameters Www i.iue.tuwien.ac 3. NPTEL-oxidatio 4. INTEGRATED CIRCUIT FABRICATION (OXIDATION) E5163 IC DESIGN 5. Thermal oxidation en.m.wikipedia.org/wiki/thermal oxidation. 6. CMOS IC LAYOUT concepts, methodologies, and tools. 7. Introduction to CMOS VLSI design by- N.Wesley, D. Harris, 3/e, 2004 Shahina Firdoush, Avinow Raj 6