the physical structure (nmos)users.encs.concordia.ca/~asim/coen 451/lectures/l2.1...concordia vlsi...

CONCORDIAVLSI DESIGN LAB

1

The Physical Structure

(NMOS)

Field Oxide

SiO2

Gate oxide

Field Oxiden+n+

AlAlSiO2SiO2

Polysilicon Gate

channel

L

P Substrate

DS

L

W

(D)(S)

Metal

n+n+

(G)

Poly

contact


2

3D Perspective


3


4

Fabrication Process

•Crystal Growth

•Doping / Diffusion

•Deposition

•Patterning

•Lithography

•Oxidation

•Ion Implementation


5

Fabrication- CMOS Process

Starting Material Preparation

1. Produce Metallurgical Grade Silicon (MGS)SiO2 (sand) + C in Arc FurnaceSi- liquid 98% pure

2. Produce Electronic Grade Silicon (EGS)HCl + Si (MGS) Successive purification by distillationChemical Vapor Deposition (CVD)


6

Fabrication: Crystal Growth

Czochralski Method

Basic idea: dip seed crystal into liquid pool

Slowly pull out at a rate of 0.5mm/min

controlled amount of impurities added to melt

Speed of rotation and pulling rate determine diameter of the ingot

Ingot- 1to 2 meter longDiameter: 4”, 6”, 8”


7

Fabrication: Wafering

Finish ingot to precise diameter

Mill “ flats”

Cut wafers by diamond saw: Typical

thickness 0.5mm

Polish to give optically flat surface


8

Fabrication: Oxidation

Silicon Dioxide has several uses:- mask against implant or diffusion- device isolation- gate oxide-isolation between -layers

SiO2 could be thermallygenerated or through CVDOxidation consumes siliconWet or dry oxidation

Quartz Tube

Wafers

Quartz Carrier

Resistance Heater

O2 or Water

VaporPump


9

Fabrication: Diffusion

Simultaneous creation of p-n junction over the entire surface of wafer

Doesn’t offer precise control Good for heavy doping, deep junctions Two steps:

Pre-depositionDopant mixed with inert gas introduced in to a furnace at 1000 oC.Atoms diffuse in a thin layer of Si surfaceDrive-inWafers heated without dopant

Resistance Heater

wafers

Temp: 1000

Dopant Gas


10

Fabrication: Ion

Implantation

Precise control of dopant Good for shallow junctions and threshold adjust Dopant gas ionized and accelerated Ions strike silicon surface at high speed Depth of lodging is determined by accelerating field


11

Fabrication: Deposition

Reactant

0.1 -1 Torr

Loader

Pump

Used to form thin film of Polysilicon, Silicon dioxide, Silicon Nitride, Al.

Applications: Polysilicon, interlayer oxide, LOCOS, metal.

Common technique: Low Pressure Chemical Vapor Deposition (CVD).

SiO2 and Polysilicon deposition at 300 to 1000 oC.

Aluminum deposition at lower temperature- different technique


12

Fabrication: Metallization

Standard material is Aluminum

Low contact resistance to p-type and n-type

When deposited on SiO2, Al2O3 is formed: good adhesive

All wafer covered with Al

Deposition techniques:Vacuum EvaporationElectron Beam EvaporationRF Sputtering

Other materials used in conjunction with or replacement to AlIn today’s technology are cupper and its alloys.


13

Fabrication: Etching

Wet Etching Etchants: hydrofluoric acid (HF), mixture of nitric acid and HF Good selectivity Problem:

- under cut- acid waste disposal

Dry Etching Physical bombardment with atoms or ions good for small geometries. Various types exists such as:

Planar Plasma Etching Reactive Ion Etching

Plasma Reactive species

RF


14

Fabrication: Lithography

Mask making

Most critical part of lithography is conversion from layout to master mask

Masking plate has opaque geometrical shapes corresponding to the area on the wafer surface where certain photochemical reactions have to be prevented or taken place.

Masks uses photographic emulsion or hard surface

Two types: dark field or clear field

Maskmaking: optical or e-beam


15

Lithography: Mask making

Optical Mask Technique

1. Prepare Reticle Use projection like system:

-Precise movable stage-Aperture of precisely rectangular size and angular orientation-Computer controlled UV light source directed to photographic plate

After flashing, plate is developed yielding reticle


16

Fabrication: Lithography

Step & Repeat

Printing

Printing


17

Lithography: Mask

making

Electron Beam Technique

Main problem with optical technique: light diffraction

System resembles a scanning electron microscope + beam blanking and computer controlled deflection


18

Patterning/ Printing

Process of transferring mask features to surface of the silicon wafer.

Optical or Electron-beam

Photo-resist material (negative or positive):synthetic rubber or polymer upon exposure to light becomes insoluble ( negative ) or volatile (positive)

Developer: typically organic solvant- e.g. Xylen

A common step in many processes is the creation and selective removal of Silicon Dioxide


19

Patterning: Pwell mask


20

Patterning/ Printing

substrate

SiO2


21

Fabrication Steps

Apply PR

Pre-bake

Printer align expose

mask

Develop, rinse, dry

Post bake

Inspect, measure

Etch

Strip resist

Deposit or grow layer


22

Fabrication Steps


23

3D Perspective


24

The Physical Structure

(NMOS)

Field Oxide

SiO2

Gate oxide

Field Oxiden+n+

AlAlSiO2SiO2

Polysilicon Gate

channel

L

P Substrate

DS

L

W

(D)(S)

Metal

n+n+

(G)

Poly

contact


Videos for Fabrication

A very clear site showing each fabrication step http://www.virlab.virginia.edu/VL/MOS_kit.htm/state/related

4 min wafer production https://www.youtube.com/watch?v=AMgQ1-

HdElM&list=PL8InEUrivGYt2Fze1vXsdkHDPWBP7NTXw&index=

9 min video showing IC fabrication process

https://www.youtube.com/watch?v=i8kxymmjdoM

A 10 minute presentation of Global Foundries IC manufacturing process. https://www.youtube.com/watch?v=qm67wbB5GmI&index=13&list=PL8InEUrivGYt2Fze1vXsdkHDPWBP7N

TXw

3 min animation of IC fabrication

https://www.youtube.com/watch?v=d9SWNLZvA8g

A 4 min very nice presentation with animation of 3D IC manufacturing https://www.youtube.com/watch?v=YIkMaQJSyP8

25
http://www.virlab.virginia.edu/VL/MOS_kit.htm/state/relatedhttps://www.youtube.com/watch?v=qm67wbB5GmI&index=13&list=PL8InEUrivGYt2Fze1vXsdkHDPWBP7NTXwhttps://www.youtube.com/watch?v=d9SWNLZvA8ghttps://www.youtube.com/watch?v=YIkMaQJSyP8

the physical structure (nmos)users.encs.concordia.ca/~asim/coen 451/lectures/l2.1...concordia vlsi...

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