Material Science and Technology 2 HACETTEPE UNIVERSITY DEPATMENT OF CHEMICAL ENGINEERING Material Science and Technology 2 NECDET DALGIÇ YILMAZ ŞAHAN 20519627 20520003 gökay savan şaban gündoğmuş 20519962 20419911 22.04.2017

SEMICONDUCTORS 22.04.2017

GOALS information about electrical characteristics of materials Discussing how p-n junctions work Describing the processing and properties of semiconductors 22.04.2017

OUTLINE Semiconductor basics Definition of the semiconductor Common semiconductor material Semiconductor crystal structure How silicon crystal conducts electiricity? P-type and N-type silicon The PN Junction Basic semiconductor devices Semiconductor Manufacturing Process Summary References 22.04.2017

Semiconductor basics resistance resistivity ,f(T) conductor insulator 22.04.2017 FIGURE 1 : Resistivity chart

WHAT IS SEMICONDUCTORS have electrical properties between a conductor and insulator do not conduct as well as conductors do not insulate as well as insulators their atoms are closely , have a crystal structure electronic devices, computers, telephones made from semiconductors conductivity can be controlled by doping 22.04.2017

Most common semiconductor material four valence electron SILICON Most common semiconductor material four valence electron covalent bond ,very stable FIGURE 2: structure and lattice of pure crystal of Silicon 22.04.2017

How silicon crystal conducts electricity? pure silicon electrical conductor by adding impurity introduce an impurity such as phosphorus, arsenic, aluminum,gallium can be exist unbounded electron can exist a hole 22.04.2017

P-TYPE SILICON With elements that have three valence electrons Such as B, Al and Ga Fourth bond cannot be formed Positive charge Not a complete connection There is a empty place ‘hole’ Hole movement will exist By diffusion of B2H6 22.04.2017

Illustration of the hole movement Electron leave the covalent bond and move into hole Therefore, another hole will be existed Holes will move to 22.04.2017

N-TYPE SILICON These elements have five valence electrons to share with other atoms Such as Sb, P and As Fifth electron cannot form a bond Negative charge Greatly increasing the conductivity By diffusion of PH3 22.04.2017

The PN Junction A single type (p or n ) semiconductor material is not very useful For useful applications, semiconductor crystal must contain both P-type and N-type PN junction can only be created by inserting different impurities into different parts of a single crystal 22.04.2017

N region will has positive charge after missing some electrons Those electrons will fill the holes in the P region, which will therefore has a negative charge. So there will be potential difference and electrical current 22.04.2017

Forming a PN Junction p-type n-type 22.04.2017

n-type p-type Semiconductors p-type and n-type are brought together 22.04.2017

n-type p-type Electrons and holes migrate across the junction 22.04.2017

n-type p-type The depletion layer is formed 22.04.2017

Electric field n-type p-type A potential difference is set up across the depletion layer 22.04.2017

Impurity(dopant) concentration Higher dopant concentration, more carriers (electrons or holes) Higher conductivity, lower resistivity Electrons move faster than holes N-type silicon has lower resistivity than P-type silicon at the same dopant concentration 22.04.2017

Basic semiconductor devices Resistor Transistors Capacitors Diode www.made-in-china.com/.../Resistor-Capacitor.jpg 22.04.2017

resistor Resistors are made by doped silicon or polysilicon on an IC chip. Resistance is determined by length, line width,height, ana dopant concentration V=I*R ( Ohm law) 22.04.2017

capasitors Charge storage device The capacitor's function is to store electricity, or electrical energy. The capacitor also functions as a filter, passing alternating current (AC), and blocking direct current (DC). 22.04.2017 blogs.courant.com/.../capacitor.jpg

diode P-N Junction(transistor),electron tube with anode ana cathode Allows electric current go through only in one direction Used commonly in bridge form to convert AC to DC voltage. “Rectification” 22.04.2017 Various types of Bridge Rectifiers

transistors PNP or NPN Switch Amplifier Anolog,digital circuit Fast, high power device www.ehobbycorner.com/images/trn_type.jpg injection diffusion collection 22.04.2017

MOS transistors Metal-oxide semiconductor Also called MOSFET( MOS Field Effect Transistor) Simple, symmetric structure Switch, good for digital, logic circuit Most commonly used devices in the semiconductor industry 22.04.2017 www.doitpoms.ac.uk/.../images/mosfet.jpg

Bipolar transistors NPN-PNP configuration Advantages Market share reducing rapidly Still used for anolog systems power devices TV,Cellar phone, etc. Advantages Switch signal high speed Handle large circuit(high power amplifier) Wireless transmitter Not effective weak signaal amplification A standard bipolar transistor static.howstuffworks.com/gif/amplifier-transi... 22.04.2017

Semiconductor Manufacturing Process Fundamental Processing Steps 1.Silicon Manufacturing a) Wafer Manufacturing b) Crystal structure 2.Photolithography a) Photoresists b) Photomask and Reticles c) Patterning 22.04.2017

4. Diffusion & Ion Implantation 3.Oxide Growth & Removal a) Oxide Growth & Deposition b) Oxide Removal c) Other effects d) Local Oxidation 4. Diffusion & Ion Implantation a) Diffusion b) Other effects c) Ion Implantation 22.04.2017

CRYSTAL GROWTH Czochralski Process is a Technique in Making Single-Crystal Silicon A Solid Seed Crystal is Rotated and Slowly Extracted from a Pool of Molten Si Requires Careful Control to Give Crystals Desired Purity and Dimensions 22.04.2017

WAFER MANUFACTURING The Silicon Crystal is Sliced by Using a Diamond-Tipped Saw into Thin Wafers Sorted by Thickness Damaged Wafers Removed During Lapping Etch Wafers in Chemical to Remove any Remaining Crystal Damage Polishing Smoothes Uneven Surface Left by Sawing Process 22.04.2017

Photolithography Photolithography is a technique that is used to define the shape of micro-machined structures on a wafer. 22.04.2017

Photolithography and Photoresist The first step in the photolithography process is to develop a mask, which will be typically be a chromium pattern on a glass plate. Next, the wafer is then coated with a polymer which is sensitive to ultraviolet light called a photoresist. Afterward, the photoresist is then developed which transfers the pattern on the mask to the photoresist layer. 22.04.2017

There are two basic types of Photoresists Positive and Negative. Positive resists Positive resists decomposes ultraviolet light. The resist is exposed with UV light wherever the underlying material is to be removed. Negative resists Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve. Therefore, the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions. 22.04.2017

Photolithography Photomasks and Reticles This is a square glass plate with a patterned emulsion of metal film on one side. The mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface. Each mask after the first one must be aligned to the previous pattern. 22.04.2017

A common reticle is the 5X When a image on the photomask is projected several time side by side onto the wafer, this is known as stepping and the photomask is called a reticle. A common reticle is the 5X The patterns on the 5X reticle are reduced 5 times when projected onto the wafer. This means the dies on the photomask are 5 times larger than they are on the final product. There are other kinds of reduction reticles (2X, 4X, and 10X), but the 5X is the most commonly used. Reduction reticles are used on a variety of steppers, the most common being ASM, Canon, Nikon. 22.04.2017

Examples of 5X Reticles 22.04.2017

Photolithography and Patterning The last stage of Photolithography is a process called ashing. This process has the exposed wafers sprayed with a mixture of organic solvents that dissolves portions of the photoresist . Conventional methods of ashing require an oxygen-plasma ash, often in combination with halogen gases, to penetrate the crust and remove the photoresist.  Usually, the plasma ashing process also requires a follow-up cleaning with wet-chemicals and acids to remove the residues and non-volatile contaminants that remain after ashing.  Despite this treatment, it is not unusual to repeat the "ash plus wet-clean" cycle in order to completely remove all photoresist and residues. 22.04.2017

Oxidation of Silicon Dry oxide - Pure dry oxygen is employed Disadvantage - Dry oxide grows very slowly. Advantage - Oxide layers are very uniform. - It has especially low surface state charges and thus make ideal dielectrics for MOS transistors. Wet oxide - In the same way as dry oxides, but steam is injected Disadvantage - Hydrogen atoms liberated by the decomposition of the water molecules produce imperfections that may degrade the oxide quality. Advantage - Wet oxide grows fast. - Useful to grow a thick layer of field oxide 22.04.2017

Methods of planar process Diffusion --A uniformly doped ingot is sliced into wafers. --An oxide film is then grown on the wafers. --The film is patterned and etched using photolithography exposing specific sections of the silicon. Diffusion is a cheaper and more simplistic method, but can only be performed from the surface of the wafers. Dopants also diffuse unevenly, and interact with each other altering the diffusion rate. Ion Implantation --A particle accelerator is used to accelerate a doping atom so that it can penetrate a silicon crystal to a depth of several micron Ion implantation is more expensive and complex. It does not require high temperatures and also allows for greater control of dopant concentration and profile. 22.04.2017

Summary Semiconductors; materials, conductivity between conductor and insulator Its conductivity can be controlled by dopant concentration and applied voltage Silicon, germanium, and gallium arsenate Silicon most popular: abundant and stable oxide Boron doped semiconductors is p-type, majority carriers are holes P, As or Sb doped semiconductor is P-type, the majority carriers are electrons Higher dopant concentration, lower resistivity 22.04.2017

Summary R=g *l / A C=KA/d Capacitors are mainly used in DRAM Bipolar transistors can amplify electric signal, mainly used for analog systems MOSFET electric controlled switch, mainly used for digital systems 22.04.2017

References http://www.play-hookey.com/semiconductors/pn_junction.html http://rel.intersil.com/docs/lexicon/manufacture.html 22.04.2017