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Part 3 of crystal growth sums up the properties of Silicon, describes the identity marks of Silicon Wafers in terms of Semiconductor Type and in terms of Crystal Orientation and this part defines the crystal orientation and its relevance in IC fabrication.

CRYSTAL GROWTH

Silicon, (Si) :The most common semiconductor, atomic number 14,

energy gap Eg= 1.12 eV- indirect bandgap;

crystal structure- diamond, lattice constant 0.543 nm,

atomic concentration 5 x 10 22 atoms/cm -3 ,

index of refraction 3.42, density 2.33 g/cm 3 , dielectric constant 11.7,

intrinsic carrier concentration 1.02 x 10 10 cm -3 ,

bulk mobility of electrons and holes at 300°K: 1450 and 500 cm 2 /V-s,

thermal conductivity 1.31 W/cm°C,

thermal expansion coefficient 2.6 x 10 -6 °C -1 ,

melting point 1414°C; excellent mechanical properties (MEMS applications);

single crystal Si can be processed into wafers up to 300mm in diameter.

In future this diameter will be 450mm.

P type= Always Boron (B) Doped N type= Dopant typically as follows:Res: .001-.005 Arsenic (As)Res: .005-.025 Antimony (Sb)Res:>.1 Phosphorous (P)

EPITAXIAL CRYSTAL GROWTH

The substrate or the wafer only constitutes the strong base of the integrated circuit. The actual active and passive components fabrication and there integration are carried out in overlay films which are grown by epitaxial technique.

EPITAXY is a Greek word meaning : ‘epi’ (upon)&‘taxy’ (ordered). That is an epitaxial film, a few μm thick, is an orderly continuation of the substrate crystal. It grows very slowly layer by layer. Hence the dimension , defects and doping magnitude as well as uniformity can be precisely and accurately controlled in the crystal growth direction.

This precise control is obtained in Molecular Beam Epitaxy (MBE) but not in Liquid Phase Epitaxy(LPE) or in Chemical Vapour Phase Epitaxy (CVPE). The thickness accuracy is within ±3À which is essential for growing Quantum Photonic Devices namely Quantum Dots, Quantum Wells and Super-lattices

Table 2. Identity marks of the Wafer to identify its orientation and semiconductor type.

α (angle between primary and secondary flats as indicated in Figure 9) Type Orientation
45º N <111>
90º P <100>
180º N <100>
P <111>

The normal to the plane along which crystals cleaves is the cleavage plane orientation. Suppose the cleavage plane orientation is<111>. Miller Index is being used to define the planes and their normal. Figure 10 illustrates the Plane’s Miller Index and how the normal to the plane is represented. If the exposed surface of the Si wafer, which is known as major flat, is parallel to cleavage plane then the given wafer has a crystal orientation<111>.

If the cleavage plane orientation is<100>and the wafer major flat is parallel to YZ plane then the crystal orientation is<100>. In this case cleavage plane lies in YZ plane i.e. [100] plane and its orientation is perpendicular to YZ plane i.e. x-axis. Hence Wafer Crystal Plane orientation is<100>

Scribing the wafer along cleavage planes allows it to be easily diced into individual chips (‘die”) so that billions of individual circuits or systems on an average wafer can be separated into individual dies. Each individual die is eutectic ally bonded on ceramic substrate. The substrate is bonded to the header.The gold wire is connected to the bonding pads of the die on one end and to the chip terminals on the header by Thermo-compression bonding or by Ultra-sonic bonding. Next the die is hermetically sealed into Dual-in-Line(DIP) package or TO5 package

In<100>crystal orientation, scribed pieces form rectangle whereas in<111>crystal orientation, scribed pieces form triangles. Here we have to scribe from the base of the triangle to the apex.

For MOS fabrications, wafers with crystal orientation<100>are used. This helps achieve a lower threshold voltage. For BJT and other applications wafers with orientation<111>are preferred.

Silicon Crystal Bulk is isotropic to diffusion of dopents and to etchents used for etching the oxide layer. This is because of the symmetric property of Cubic Structure of Si. But real devices are built near the surface hence the orientation of the crystal does matter.

In 111 crystal terminates on 111 plane and in 100 it terminates on 100 plane. 111 plane has largest number of Si atoms per cm2 whereas 100 has the least number of atoms per cm2. Because of this difference 111 planes oxidize much faster because the oxidation rate is proportional to the Silicon atoms available for reaction.

But because the atom surface density is the highest the dangling bond surface density is also the highest in 111 hence Si/SiO2 has superior electrical properties in terms of interface states in 100. Interface states give rise to 1/f noise or flicker noise. Because of this superiority all MOS devices use 100 crystal orientation. But historically BJT have used 111 because 111 crystal growth is easier to grow by Czochralski method. But as we move to sub-micron and deep and ultra-deep sub-micron BJT, 100 crystal orientation seems to be the crystal orientation of choice for BJT also.

Questions & Answers

A golfer on a fairway is 70 m away from the green, which sits below the level of the fairway by 20 m. If the golfer hits the ball at an angle of 40° with an initial speed of 20 m/s, how close to the green does she come?
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cm
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A mouse of mass 200 g falls 100 m down a vertical mine shaft and lands at the bottom with a speed of 8.0 m/s. During its fall, how much work is done on the mouse by air resistance
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what is inorganic
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Chemistry is a branch of science that deals with the study of matter,it composition,it structure and the changes it undergoes
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A ball is thrown straight up.it passes a 2.0m high window 7.50 m off the ground on it path up and takes 1.30 s to go past the window.what was the ball initial velocity
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2. A sled plus passenger with total mass 50 kg is pulled 20 m across the snow (0.20) at constant velocity by a force directed 25° above the horizontal. Calculate (a) the work of the applied force, (b) the work of friction, and (c) the total work.
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Samuel Reply
can someone explain to me, an ignorant high school student, why the trend of the graph doesn't follow the fact that the higher frequency a sound wave is, the more power it is, hence, making me think the phons output would follow this general trend?
Joseph Reply
Nevermind i just realied that the graph is the phons output for a person with normal hearing and not just the phons output of the sound waves power, I should read the entire thing next time
Joseph
Follow up question, does anyone know where I can find a graph that accuretly depicts the actual relative "power" output of sound over its frequency instead of just humans hearing
Joseph
"Generation of electrical energy from sound energy | IEEE Conference Publication | IEEE Xplore" ***ieeexplore.ieee.org/document/7150687?reload=true
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progressive wave
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A string is 3.00 m long with a mass of 5.00 g. The string is held taut with a tension of 500.00 N applied to the string. A pulse is sent down the string. How long does it take the pulse to travel the 3.00 m of the string?
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Source:  OpenStax, Electrical and electronic materials science. OpenStax CNX. May 01, 2014 Download for free at http://cnx.org/content/col11615/1.14
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