1.Define an Integrated circuit.An integrated circuit(IC) is a miniature ,low cost electronic circuit consisting of active and passive components fabricated together on a single crystal of silicon.The active components are transistors and diodes and passive components are resistors and capacitors.
2.What are the basic processes involved in fabricating ICs using planar technology?1.Silicon wafer (substrate) preparation
2.Epitaxial growth
3.Oxidation
4.Photolithography
5.Diffusion
6.Ion implantation
7.Isolation technique
8.Metallization
9.Assembly processing & packaging
3.List out the steps used in the preparation of Si – wafers.1.Crystal growth &doping
2.Ingot trimming & grinding
3.Ingot slicing
4.Wafer policing & etching
5.Wafer cleaning
4.Write the basic chemical reaction in the epitaxial growth process of pure silicon.The basic chemical reaction in the epitaxial growth process of pure silicon is the hydrogen reduction of silicon tetrachloride.
SiCl4 + 2H2 <•••••1200oC ••••••> Si + 4 HCl
5.What are the two important properties of SiO2?1.SiO2 is an extremely hard protective coatng & is unaffected by almost all reagents except by hydrochloric acid. Thus it stands against any contamination.
2.By selective etching of SiO2 , diffusion of impurities through carefully defined windows in the SiO2 can be accomplished to fabricate various components.
6.Explain the process of oxidation.The silicon wafers are stacked up in a quartz boat & then inserted into quartz furnace tube. The Si wafers are raised to a high temperature in the range of 950 to 1150oC & at the same time, exposed to a gas containing O2 or H2O or both. The chemical action is
Si + 2H2O •••••••••••> SiO2+ 2H2
7. What are oxidation induced defects in semi conductor?1.Stacking faults
2.Oxide isolation defects
Stacking faults:Structural defects in the silicon lattice is called oxidation induced stacking faults. The growth of stacking faults is a strong function of substrate orientation , conductivity type & defect nuclei present. The stacking faults formation can be suppressed by the addition of HCl.
Oxide isolation defects :The stress along the edges of an oxidised area produce severe damage in the silicon. Such defects results in increased leakage in nearby devices. High temperatures (around 950oC ) will prevent stress induced defect formation.
8.What is lithography?Lithography is a process bywhich the pattern appearing on the mask is transfered to the wafer.It involves two steps: the first step requires applying a few drops of photoresist to the surface of the wafer & the second step is spinning the surface to get an even coating of the photoresist across the surface of the wafer.
9.What are the different types of lithography? What is optical lithography?The different types of lithography are :
1.Photolithography
2.Electron beam lithography
3.X ray beam lithography
4.Ion beam lithography
5.Optical lithography:
10.What are the two processes involved in photolithography?a) Making a photographic mask
b) Photoetching
The development of photographic mask involves the preparation of initial artwork and its reduction , decomposition of initial artwork or layout into several mask layers. Photoetching is used for the removal of SiO2 from desired regions sothat the desired impurities can be diffused.
11.Distinguish between dry etching & wet etching.
Dry etching
1.Gaseous mixture is used as the chemical reagent.
2.Smaller line openings( 1µm) are possible with dry etching
3.It produces straight walled etching process.
Wet etching
Chemical reagents used are in the liquid form.
Line opening are larger.(> 1µm)
It produces patterns with undercutting.
12.What is meant by reactive etching?
The term reactive plasma is meant to describe a discharge in which ionization & fragmentation of gases takesplace & produce chemically active plasma species, frequently oxidizers and reducing agents. Such plasmas are reactive both in the gas phase & with solid surfaces exposed to them. When these interactions are used to form volitile products so that material is removed or etching of material form surfaces that are not masked to form lithographic patterns , the technique is known as reactive plasma etching.
13.What are isotropic & anisotropic etching processes?
Isotropic etching is a wet etching process which involves undercutting.
Aisotropic etching is a dry etching process which provides straight walled patterns.
14.Define diffusion.
The process of introducing impurities into selected regions of a silicon wafer is called diffusion. The rate atwhich various impurities diffuse into the silicon will be of the order of 1µm/hr at the temperature range of 900oC to 1100oC .The impurity atoms have the tendency to move from regions of higher concentrations to lower concentrations.
15.What is dielectric isolation?
In dielectric isolation, a layer of solid dielectric such as SiO2 or ruby completely surrounds each components thereby producing isolation , both electrical & physical. This isolating dielectric layer is thick enough so that its associated capacitance is negligible. Also, it is possible to fabricate both pnp & npn transistors within the same silicon substrate.
16.What are the advantages of ion implantation technique?
1. It is performed at low temperature. Therefore, previously diffused regions have a lesser tendency for lateral spreading.
2. In diffusion process, temperature has to be controlled over a large area inside the oven, whereas in ion implantation process, accelerating potential & beam content are dielectrically controlled from outside.
17.What is metallization?
The process of producing a thin metal film layer that will serve to make interconnection of the various components on the chip is called metallization.
