VLSI Design Lecture PPTs PPT...JFET (Junction Field-Effect Transistor) Finds application especially...

VLSI Design Lecture PPTs

INSTITUTE OF AERONAUTICAL ENGINEERING

Dundigal, Hyderabad -500 043

1 6/3/2015

Department : ELECTRONICS AND COMMUNICATION ENGINEERING

Course Code : 57035

Course Title : VLSI DESIGN

Course Coordinator : VR. Sheshagiri Rao, Professor

Team of Instructors B. Kiran Kumar, Assistant Professor

Course Structure : Lectures Tutorials Practicals Credits

4 1 - 4

Unit I Introduction to IC technology

Topics

• MOS, PMOS, NMOS, CMOS and BiCMOS Technologies:

• Oxidation • Lithography • Diffusion • Ion implantation • Metallization • Encapsulation • Probe testing • Integrated Resistors and Capacitors

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Acronym of VLSI

• V -> Very • L -> Large • S -> Scale • I -> Integration

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Types of Field Effect Transistors (The Classification)

» JFET

MOSFET (IGFET)

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n-Channel JFET

p-Channel JFET

n-Channel EMOSFET

p-Channel EMOSFET

Enhancement MOSFET

Depletion MOSFET

n-Channel DMOSFET

p-Channel DMOSFET

FET

MOSFET (Metal-Oxide Semiconductor Field-Effect Transistor) Primary component in high-density VLSI chips such as memories and microprocessors

JFET (Junction Field-Effect Transistor) Finds application especially in analog and RF circuit design

Metal Oxide Semiconductor(MOS)

• Advantages of FET over conventional Transistors

• Unipolar device i. e. operation depends on only one type of charge carriers (h or e)

• Voltage controlled Device (gate voltage controls drain current)

• Very high input impedance (109-1012 )

• Source and drain are interchangeable in most Low-frequency applications

• Low Voltage Low Current Operation is possible (Low-power consumption)

• Less Noisy as Compared to BJT

• No minority carrier storage (Turn off is faster)

• Very small in size, occupies very small space in ICs

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Switch Model of NMOS Transistor

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Gate

Source (of carriers)

Drain (of carriers)

| VGS |

| VGS | < | VT | | VGS | > | VT |

Open (off) (Gate = Closed (on) (Gate = )

Ron

Switch Model of PMOS Transistor

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Gate

Source (of carriers)

Drain (of carriers)

| VGS |

| VGS | > | VDD – | VT | | | VGS | < | VDD – |VT| |

Open (off) (Gate = ) Closed (on) (Gate = ) Ron

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MOS transistors Symbols

D

S

G

D

S

G

G

S

D D

S

G

NMOS Enhancement NMOS

PMOS

Depletion

Enhancement

B

NMOS with Bulk Contact

Channel

MOSFET Circuit Symbols

• (g) and (i) are the most commonly used symbols in VLSI logic design.

• MOS devices are symmetric.

• In NMOS, n+ region at higher voltage is the drain.

• In PMOS p+ region at lower voltage is the drain

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The NMOS Transistor Cross Section

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n areas have been doped with donor ions (arsenic) of concentration ND - electrons are the majority carriers

p areas have been doped with acceptor ions (boron) of concentration NA - holes are the majority carriers

Gate oxide

n+

Source Drain

p substrate

Bulk (Body)

p+ stopper

Field-Oxide (SiO2) n+

Polysilicon Gate

L

W

Carriers and Current

• Carriers always flow from the Source to Drain • NMOS: Free electrons move from Source to

Drain.

– Current direction is from Drain to Source. • PMOS: Free holes move from Source to Drain.

– Current direction is from Source to Drain. –

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The MOSFET Channel

• Under certain conditions, a thin channel can be formed right underneath the Silicon-Dioxide insulating layer, electrically connecting the Drain to the Source. The depth of the channel (and hence its resistance) can be o t olled the Gate s oltage. The le gth of

the channel (shown in the figures above as L) a d the ha el s idth W, a e i po ta t design parameters.

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REGION OF OPERATION CASE-1 (No Gate Voltage)

• Two diodes back to back exist in series. • One diode is formed by the pn junction

between the n+ drain region and the p-type substrate

• Second is formed by the pn junction between the n+ source region and the p-type substrate

• These diodes prevent any flow of the current. • There exist a very high resistance.

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NMos Cut View

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REGION OF OPERATION Creating a channel

• Apply some positive voltage on the gate terminal.

• This positive voltage pushes the holes downward in the substrate region.

• This causes the electrons to accumulate under the gate terminal.

• At the same time the positive voltage on the gate also attracts the electrons from the n+ region to accumulate under the gate terminal.

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REGION OF OPERATION Creating a channel

• When sufficient electrons are accumulated under the gate an n-region is created, connecting the drain and the source

• This causes the current to flow from the drain to source

• The channel is formed by inverting the substrate surface from p to n, thus induced channel is also called as the inversion layer.

• The voltage between gate and source called vgs at which there are sufficient electron under the gate to form a conducting channel is called threshold voltage Vth.

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Formation of Channel

• First, the holes are repelled by the positive gate voltage, leaving behind negative ions and forming a depletion region. Next, electrons are attracted to the interface, creating a channel

i e sio la e .

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MOS Transistor Current direction

• The source terminal of an n-channel(p-channel) transistor is defined as whichever of the two terminals has a lower(higher) voltage.

• When a transistor is turned ON, current flows from the drain to source in an n-channel device and from source to drain in a p-channel transistor.

• In both cases, the actual carriers travel from the source to drain.

• The current directions are different because n-channel carriers are negative, whereas p-channel carriers are positive.

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MOS I/V

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For a NMOS, a necessary condition for the channel to exist is:

THGS VV

REGION OF OPERATION Applying small Vds

• Now we applying some small voltage between source and drain

• The voltage Vds causes a current to flow from drain to gate.

• Now as we increase the gate voltage, more current will flow.

• Increasing the gate voltage above the threshold voltage enhances the channel, hence this mode is called as enhancement mode operation.

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Operation – nMOS Transistor

• Accumulation Mode - If Vgs < 0, then an electric field is established across the substrate.

• Depletion Mode -If 0 Vtn, the region below the gate will be inverted.

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V =0


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Voltage-Dependent Resistor

• The inversion channel of a MOSFET can be seen as a resistor.

• Since the charge density inside the channel depends on the gate voltage, this resistance is also voltage-dependent.

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Channel Potential Variation

• “i e the e s a channel resistance between drain and source, and if drain is biased higher than the source, then the potential between gate and channel will decrease from source to drain.

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Channel Pinch-Off

• As the potential difference between drain and gate becomes more positive, the inversion layer beneath the interface starts to pinch off around drain.

• When VD s> VGs - Vth, the channel at drain totally pinches off, and when VD s< VGs - Vth, the channel length starts to decrease.

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Transistor in Saturation Mode

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S

D

B

G VGS VDS > VGS - VT

ID

VGS - VT - + n+ n+

Pinch-off

Assuming VGS > VT

VDS

The current remains constant (saturates).

Du i g pi hoff

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Does this mean that the current i =0 ? Actually, it does not. A MOSFET that is pinched off at the drain end of the channel still conducts current:

The large E in the depletion region surrounding the drain will sweep electrons across the end of the pinched off channel to the drain.

This is very similar to the operation of the BJT. For an npn BJT, the electric field of the reversed biased CBJ swept electrons from the base to the

collector regions.

N-Channel MOSFET characteristics

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Enhancement-Mode PMOS Transistors: Structure

• p-type source and drain regions in n-type substrate.

• vGS < 0 required to create p-type inversion layer in channel region

• For current flow, vGS < vTP • To maintain reverse bias on

source-substrate and drain-substrate junctions, vSB < 0 and vDB < 0

• Positive bulk-source potential causes VTP to become more negative

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P-channel MOSFET characteristics

linear saturation

p transistor

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Depletion-Mode MOSFETS

• NMOS transistors with • Ion implantation process

is used to form a built-in n-type channel in the device to connect source and drain by a resistive channel

• Non-zero drain current for vGS = 0; negative vGS required to turn device off.

VTN0

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pMOS are 2.5 time slower than

nMOS due to electron and hole

mobilities

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Basic processes involved in fabricating Monolithic ICs

1. Silicon wafer (substrate) preparation

2. Epitaxial growth

3. Oxidation

4. Photolithography

5. Diffusion

6. Ion implantation

7. Metallization

8. Testing

9. Assembly processing & packaging

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Oxidation

1. SiO2 is an extremely hard protective coating & 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 through windows in the SiO2 can be accomplished to fabricate various components.

Formation of silicon dioxide layer on the surface of Si wafer 1. protects surface from contaminants 2. forms insulating layer between conductors 3. form barrier to dopants during diffusion or ion implantation 4. grows above and into silicon surface

Dry oxidation: lower rate and higher quality Wet oxidation: higher rate and lower quality

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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-----------> Si O2+ 2H2 (Wet )

Si + O2 -------------> SiO2 (Dry )

54

Photolithography

• Coat wafer with photoresist (PR)

• Shine UV light through mask to selectively expose PR

• Use acid to dissolve exposed PR

• Now use exposed areas for – Selective doping – Selective removal of material

under exposed PR

Wafer

Photoresist

Mask

UV Light

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Adding Materials

• Add materials on top of silicon – Polysilicon – Metal – Oxide (SiO2) - Insulator

• Methods – Chemical deposition – Sputtering (Metal ions) – Oxidation

Silicon

Added Material (e.g. Polysilicon)

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Oxide (Si02) - The Key Insulator

• Thin Oxide – Add using chemical deposition – Used to form gate insulator & block active areas

• Field Oxide (FOX) - formed by oxidation – Wet (H20 at 900oC - 1000oC) or Dry (O2 at 1200oC) – Used to insulate non-active areas

Silicon Wafer Silicon Wafer

SiN / SiO2 FOX FOX SiO2 Thin Oxide

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Patterning Materials using Photolithography

• Add material to wafer • Coat with photoresist • Selectively remove

photoresist

• Remove exposed material • Remove remaining PR

Silicon

Added Material (e.g. Polysilicon)

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Diffusion

• Introduce dopant via epitaxy or ion implant e.g. Arsenic (N), Boron (P)

• Allow dopants to diffuse at high temperature

• Block diffusion in selective areas using oxide or PR

• Diffusion spreads both vertically, horizontally Silicon

Diffusion

Blocking Material (Oxide)

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Ion Implantation

180 kV

Resolving Aperture

Ion Source

Equipment Ground

Acceleration Tube

90° Analyzing Magnet

Terminal Ground

20 kV

Focus Neutral beam and beam path gated

Beam trap and gate plate

Wafer in wafer process chamber

X - axis scanner

Y - axis scanner

Neutral beam trap and beam gate

Gases

Ar

AsH3

B11F3 *

He

N2

PH3

SiH4

SiF4 GeH4

Solids

Ga

In

Sb

Liquids

Al(CH3)3

Process Conditions

Flow Rate: 5 sccm

Pressure: 10-5 Torr

Accelerating Voltage: 5

to 200 keV

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Metallization

• Sputter on aluminum over whole wafer • Pattern to remove excess metal, leaving wires

p substrate

Metal

Thick field oxide

n well

n+n+ n+p+p+p+

Metal

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nMOS fabrication steps 1. Processing is carried out on a thin wafer cut from a single crystal of silicon of high purity into which the required p-impurities are introduced as the crystal is grown.

2. A layer of silicon dioxide (Si02) is grown all over the surface of the wafer to protect the surface, act as a barrier to dopants during processing, and provide a generally insulating substrate on to which other layers may be deposited and patterned.

3. The surface is now covered with a photoresist which is deposited onto the wafer and spun to achieve an even distribution of the required thickness.

4. The photoresist layer is then exposed to ultraviolet light through a mask which defines those regions into which diffusion is to take place together with transistor channels.

5. These areas are subsequently readily etched away together with the underlying silicon dioxide so that the wafer surface is exposed in the window defined by the mask.

6. The remaining photoresist is removed and a thin layer of Si02 is grown over the entire chip surface and then polysilicon is deposited on top of this to form the gate structure.

7. Further photoresist coating and masking allows the polysilicon to be patterned (as shown in Step 6) and then the thin oxide is removed to expose areas into which

8. Thick oxide (Si02) is grown over all again and is then masked with photoresist and etched to expose selected areas of the polysilicon gate and the drain and source areas where connections (i.e. contact cuts) are to be made.

9. The whole chip then has metal (aluminium) deposited over its surface. This metal layer is then masked and etched to form the required interconnection pattern.

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1.

2.

3.

……………………………………… ………………………………………

……………………………………… ………………………………………

Substrate

Thick oxide

(1m)

Photoresist

p

p

p

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4.

5. ……………………………………… ………………………………………

……………………………………… ………………………………………

Window in

oxide

Mask

UV light

p

p

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6.

7. ……………………………………… ………………………………………

……………………………………… ……………………………………… Patterned Poly. (1-2 m)

On thin oxide

( 800-1000A0 )

……………………………………… ………………………………………

…… ……

…… ……

………… ………… n+ diffusion (1 m deep)

p

p

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8.

……………………………………… ……………………………………… …… …… ……. …… …… ………… ………… Contact holes

(cuts)

………… ………… ………… ………… ………… ………… ……

……

……

……

9.

……………………………………… ……………………………………… …… …… ……. …… …… ………… …………

Patterned

Metallization

(aluminum

1 m)

………… ………… ………… ………… ………… ………… ……

……

……

……

p

p

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CMOS FABRICATION

• There are a number of approaches to CMOS fabrication, including the p-well, the n-well, the twin-tub, and the silicon-on-insulator processes.

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The p-well Process

In primitive terms, the structure consists of an n-type substrate in which p-devices may

be formed by suitable masking and diffusion and, in order to accommodate n-type devices,

a deep p-well is diffused into the n-type substrate as shown.

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The p-well CMOS fabrication

In all other respects-masking, patterning, and diffusion-the process is similar to nMOS fabrication. In summary, typical processing steps are: • Mask 1 - defines the areas in which the deep p-well diffusions are to take place. • Mask 2 - defines the thinox regions, namely those areas where the thick oxide is to be stripped and thin oxide grown to accommodate p- and n-transistors and wires. • Mask 3 - used to pattern the polysilicon layer which is deposited after the thin oxide. • Mask 4 - A p-plus mask is now used (to be in effect "Anded" with Mask 2) to define all areas where p-diffusion is to take place. • Mask 5 - This is usually performed using the negative form of the p-plus mask and defines those areas where n-type diffusion is to take place. • Mask 6 - Contact cuts are now defined. • Mask 7 - The metal layer pattern is defined by this mask. Mask 8 - An overall passivation (overglass) layer is now applied and Mask 8 ts needed to define the openings for access to bonding pads.

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……………………………………… … … 1. p-well

(4-5 m)

SiO2

………… …… …… 2. Thin oxide

and

polysilicon

Polysilicon

p

p

n

n

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………… …… …… 3.

p-diffusion

P+ mask

(positive)

………… …… …… 4.

n-diffusion P+ mask

(negative)

p

n

p

n

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Vin

n

p

Vout VSS VDD

CMOS p-well inverter showing VDD and VSS substrate connections

Polysilicon

Oxide

n-diffusion

P-diffusion

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Vin

p

n

Vout VSS VDD

CMOS n-well inverter showing VDD and VSS substrate connections

Polysilicon

Oxide

n-diffusion

P-diffusion

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The n-well Process • As indicated earlier, although the p-well process is widely used, n-well fabrication

has also gained wide acceptance, initially as a retrofit to nMOS lines.

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The twin-tub-Tub Process

A logical extension of the p-well and n-well approaches is the twin-tub fabrication process.

Here we start with a substrate of high resistivity n-type material and then create both .. n-well and p-well regions. Through this process it is possible to preserve the performance of n-transistors without compromising the p-transistors. Doping control is more readily achieved

and some relaxation in manufacturing tolerances results. This is particularly important as far

as latch-up is concerned.

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Vin

n

Vout VSS VDD

Twin-tub structure

( A logical extension of the p-well and n-well)

Polysilicon

Oxide

n-diffusion

P-diffusion

p

n substrate

well well

Epitaxial

layer

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Bipolar compatible CMOS(Bi-CMOS) technology:

Introduced in early 1980s

Combines Bipolar and CMOS logic

Low power dissipation High speed High packing density High output

drive

High Noise Margin High transconductance

(gm)

High input impedance

Bi-CMOS

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The objective of the Bi-CMOS is to combine bipolar and CMOS so as to exploit the advantages of both the technologies. Today Bi-CMOS has become one of the dominant technologies used for high speed, low power and highly functional VLSI circuits.

The process step required for both CMOS and bipolar are almost similar The primary approach to realize high performance Bi-CMOS devices is the addition of bipolar process steps to a baseline CMOS process. The Bi-CMOS gates could be used as an effective way of speeding up the VLSI circuits. The applications of Bi-CMOS are vast. Advantages of bipolar and CMOS circuits can be retained in Bi-CMOS chips.

Bi-CMOS technology enables high performance integrated circuits IC’s but increases process complexity.

Features

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Higher switching speed Higher current drive per unit area, higher gain

Generally better noise performance and better high frequency characteristics Improved I/O speed (particularly significant with the growing importance of package limitations in high speed systems). high power dissipation

lower input impedance (high drive current) low packing density low delay sensitivity to load It is essentially unidirectional.

Characteristics of Bipolar Technology

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Lower static power dissipation

Higher noise margins

Higher packing density

High yield with large integrated complex functions

High input impedance (low drive current)

Scalable threshold voltage

High delay load sensitivity

Low output drive current (issue when driving large capacitive loads)

Bi-directional capability (drain & source are interchangeable)

A near ideal switching device,

Low gain

Characteristics of CMOS

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CMOS process BI-POLAR process 1 . N-well 1. n+ sub-collector

2. P base doping(extra step)

3. PMOS source and drain 3. p+ base contact

4. NMOS source and drain 4. n+ emitter

Bi-CMOS FABRICATION PROCESS

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npn-BJT Fabrication

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BJT Processing

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1. Implantation of the buried n+ layer

2. Growth of the epitaxial layer

3. p+ isolation diffusion

4. Base p-type diffusion

5. Emitter n+ diffusion

8. Metal deposition and etching

6. p+ ohmic contact

7. Contact etching

9. Passivation and bond pad opening

p-substrate

n+ buried layer

p-base layer

n+ layer n+ layer p+ layer

BJT Processing

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6. p+ ohmic contact

7. Contact etching

9. Passivation and bond pad opening p-substrate

BJT Processing

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6. p+ ohmic contact

7. Contact etching


n+ buried layer

n epi layer

BJT Processing

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6. p+ ohmic contact

7. Contact etching


n+ buried layer

p+ isola

tion layer

p+ isola

tion layer

BJT Processing

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6. p+ ohmic contact

7. Contact etching


n+ buried layer

p+ isola

tion layer

p+ isola

tion layer

p-base layer

BJT Processing

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6. p+ ohmic contact

7. Contact etching


n+ buried layer

p+ isola

tion layer

p+ isola

tion layer

p-base layer

n+ layer n+ layer

BJT Processing

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6. p+ ohmic contact

7. Contact etching


n+ buried layer

p+ isola

tion layer

p+ isola

tion layer

p-base layer


BJT Processing

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6. p+ ohmic contact

7. Contact etching


n+ buried layer

p+ isolation layer

p-base layer


BJT Processing

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6. p+ ohmic contact

7. Contact etching


n+ buried layer

p+ isolation layer

p-base layer


BJT Processing

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6. p+ ohmic contact

7. Contact etching


n+ buried layer

p+ isolation layer

p-base layer


Lateral view of npn BJT

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Lateral PNP BJT

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Doping Profiles in a BJT

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NMOS PMOS

G

D S G

D S

NPN-BJT

C B E

N Plus Buried Layer

N-Well (Collector)

N-

Diff

N-

Diff

P-Diff P-Diff N-Plus

Emitter

P-SUBSTRATE

P-EPITAXY

BICMOS STRUCTURE

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P-SUBSTRATE

P-SUBSTRATE

P-SUBSTRATE IS TAKEN

P-TYPE SUBSTRATE IS COVERED WITH OXIDE LAYER

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N Plus Buried Layer

P-SUBSTRATE

P-SUBSTRATE

A WINDOW IS OPENED THROUGH OXIDE LAYER

THROUGH THE WINDOW N TYPE IMPURITIES IS HEAVILY DOPED

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N Plus Buried Layer P-SUBSTRATE

P-EPITAXY

P-EPITAXY LAYER IS GROWN ON THE ENTIRE SURFACE

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N Plus Buried Layer P-SUBSTRATE

P-EPITAXY

THE ENTIRE SURFACE IS COVERED WITH OXIDE LAYER AND TWO WINDOWS

ARE OPENED THROUGH THE OXIDE LAYER

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N Plus Buried Layer

N-Well (Collector)

N-Well

P-SUBSTRATE

P-EPITAXY

THROUGH THE TWO WINDOWS N-TYPE IMPURITIES ARE DIFFUSED TO

FORM N-WELLS

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N Plus Buried Layer

N-Well (Collector)

N-Well

P-SUBSTRATE

P-EPITAXY

THREE WINDOWS ARE OPENED THROUGH THE OXIDE LAYER , IN THESE

THREE WINDOWS THREE ACTIVE DEVICES NMOS,PMOS AND NPN BJT

ARE FORMED

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N Plus Buried Layer

N-Well (Collector)

N-Well

P-SUBSTRATE

P-EPITAXY

THE ENTIRE SURFACE IS COVERED WITH THINOX AND POLYSILICON

AND ARE PATTERNED TO FORM THE GATE TERMINALS OF THE NMOS

AND PMOS

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N Plus Buried Layer

N-Well (Collector)

N-Well P-Base

P-SUBSTRATE

P-EPITAXY

THROUGH THE 3RD WINDOW THE P-IMPURITIES ARE MODERATELY

DOPED TO FORM THE BASE TERMINAL OF BJT

N-WELL ACTS LIKE THE COLLECTOR TERMINAL

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N Plus Buried Layer

N-Well (Collector)

N-Well P-Base

P-SUBSTRATE

P-EPITAXY

N-

Diff

N-

Diff

N-Plus

Emitter

THE N-TYPE IMPURITES ARE HEAVILY DOPED TO FORM

1.SOURCE AND DRAIN REGION OF NMOS

2.EMITTER TERMINAL OF BJT

3.AND INTO NWELL COLLECTOR REGION FOR CONTACT PURPOSE

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N Plus Buried Layer

N-Well (Collector)

N-Well

N-

Diff

N-

Diff

P-Diff P-Diff

P-Base

N-Plus

Emitter

P-SUBSTRATE

P-EPITAXY

THE P-TYPE IMPURITES ARE HEAVILY DOPED TO FORM

1.SOURCE AND DRAIN REGION OF PMOS

2.AND INTO P-BASE REGION FOR CONTACT PURPOSE

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N Plus Buried Layer

N-Well (Collector)

N-Well

N-

Diff

N-

Diff

P-Diff P-Diff

P-Base

N-Plus

Emitter

P-SUBSTRATE

P-EPITAXY

THE ENTIRE SURFACE IS COVERED WITH THICK OXIDE LAYER

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N Plus Buried Layer

N-Well (Collector)

N-Well

N-

Diff

N-

Diff

P-Diff P-Diff

P-Base

N-Plus

Emitter

P-SUBSTRATE

P-EPITAXY

THE ENTIRE SURFACE IS COVERED WITH THICK OXIDE LAYER AND IS

PATTERNED FOR CONTACT CUTS

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NMOS PMOS

G

D S G

D S

NPN-BJT

C B E

N Plus Buried Layer

N-Well (Collector)

N-

Diff

N-

Diff

P-Diff P-Diff N-Plus

Emitter

P-SUBSTRATE

P-EPITAXY

METAL CONTACTS ARE FORMED

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Resistors & Capacitors fabrication

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Oh s La • Current I in terms of Jn • Voltage V in terms of electric field

– Result for R

IRV

JtWJAI

LVE /

EWtJtWJAI

VL

WtJtWJAI

tW

LR

1

tW

LR

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Sheet Resistance (Rs)

• IC resistors have a specified thickness – not under the control of the circuit designer

• Eliminate t by absorbing it into a new parameter: the sheet resistance (Rs)

W

LR

W

L

tWt

LR sq

Nu e of “ ua es

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ELECTRON AND HOLE MOBILITY

Carrier Mobilities versus Doping Concentration

0.00E+00

2.00E+02

4.00E+02

6.00E+02

8.00E+02

1.00E+03

1.20E+03

1.40E+03

1.60E+03

1.00E+14 1.00E+15 1.00E+16 1.00E+17 1.00E+18 1.00E+19

Doping Concentration (Na or Nd)

Carr

ier

Mo

bilit

y (

cm

2/V

-sec)

mu_n

mu_p

6/3/2015 114

DIFFUSED RESISTOR

Aluminum contacts

The n-type wafer is always biased positive with respect to the p-type diffused region. This ensures that the pn junction that is formed is in reverse bias, and there is no current leaking to the substrate. Current will flow through the diffused resistor from one contact to the other. The I-V characteristic follows Ohm’s Law: I = V/R

n-wafer Diffused p-type region

Silicon dioxide

6/3/2015 115

Layout/Mask Layer 1 - Diffusion (green)

L

W

L/W is the number of ‘squares’ long the resistor is said to be.

The sheet resistance rhos, is the resistance of each square

5 squares in this case

If rhos is 100 ohms per square, R = 500 ohms

The resistance, R = rhos (L/W)

Top View

Side View

P type Diffusion N wafer

Resistor termination

6/3/2015 116

IC Fabrication: Ion Implantation

• Si substrate (p-type) • Grow oxide (thermally) • Add photoresist • Expose (visible or UV source) • Etch (chemical such as HF) • Ion implantation (inject dopants) • Diffuse (increase temperature and allow

dopants to diffuse)

P-type Si Substrate

oxide

N-type diffusion region

6/3/2015 117

Using Sheet Resistance (Rs)

• Ion-i pla ted o diffused IC esisto

P-type Si Substrate

6/3/2015 118

Poly Film Resistor

• To lower the capacitive parasitics, we should build the resistor further away from substrate

• We a deposit a thi fil of pol “i hea il doped) material on top of the oxide

• The poly will have a certain resistance (say 10 Ohms/sq)

Oxide Polysilicon Film (N+ or P+ type)

6/3/2015 119

Diffused Resistor -Dope a region of the silicon (n-type or p-type) to an acceptable NA or ND. -Then place a contact at each end of the diffusion region. -The diffusion region will have a given resistivity specified in "Oh s / “ ua e -Then alter the geometry (L/W ration) to get the desired resistance - typically these have a sheet resistance between 100 to 200 ohms/sq - to save space -These are laid out using a serpentine geometry

-The interesting thing about the l/W ratio is that if l=W, then the shape is a square and R=Rs, this is true no matter how big the square is. -In fact, the l/W ratio is actually the number of squares in a given trace geometry - We typically just count the squares and use:

R= Rs*(no.of.squares)

6/3/2015 120

6/3/2015 121

Polysilicon Resistor -Another way to fabricate a resistor is to use Polysilicon. -Polysilicon has a high resistivity prior to Ion Implantation -Use undoped Polysilicon to create a high value resistor Before Ion Implantation : Rs = 10M Ohms/Square After Ion Implantation : Rs = 20 to 40 Ohms/Square - Typically don't even need 1 square to get our resistively so we don't need to do a serpentine layout - One drawback is that the resistance can vary widely with process when using less than 1 square to get a resistor in the k-Ohms range. - These are typically used when we just want a BIG resistor and don't care about the exact value

6/3/2015 122

Metal Resistor -Metal can also be used for very small resistors

-The M1 layer typically has sheet resistance on the order of mohms/sq. - Use a serpentine layout to get a small resistor (1-10 ohms)

Cross sections of resistors of various types available from a typical n-well CMOS process.

-n-well process is used for medium value of resistors, while the n+ and p+ diffusions are useful for low value resistors.

-The resistance value depends on the length and width of the diffused regions, the tolerance of the resistor value is very poor (i.e., 20 to 50 %)

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Capacitors

• Composed of two conductive plates separated by an insulator (or dielectric). – Commonly illustrated as two parallel metal plates

separated by a distance, d.

– C = e A/d – where e = er eo – er is the relative dielectric constant – eo is the vacuum permittivity

6/3/2015 125

CMOS Capacitors -There are 3 common ways to make a capacitor

1) MOS Capacitor: -simply create a MOS structure where the Gate (Metal) terminal is one terminal and the Body (Semiconductor) terminal is Ground - while this is easy to implement, the capacitance changes with the bias voltage (i.e., VG) due to the depletion and inversion which occurs

6/3/2015 126

MIM Capacitor -"Metal Insulator Metal"-this is simply a parallel plate capacitor using two metals and an insulator -This type of capacitor is created using an extra process step that puts in an additional metal layer that can be very close to one of the other metal layers to get a smaller plate-to-plate separation -Since the plates are made of metal, the capacitance doesn't change with bias voltage-these capacitors are not as large as MOS capacitors

Interpoly and MOS capacitors in an n-well CMOS process.

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6/3/2015 128

UNIT 2 BASIC ELECTRICAL PROPERTIES

Topics • Basic electrical properties of MOS and BiCMOS

circuits: • Ids-Vds relationships • MOS transistor threshold voltage, gm, gds • figure of merit wo • pass transistor • NMOS inverter • Various pull-ups • CMOS inverter analysis and design • BiCMOS inverters

6/3/2015 129

MOSFET I-V Characteristics I-V Plots, Channel Length Modulation

– Saturation equation yields curves independent of VDS. Not sure! So we consider the effect of channel length modulation.

130

Quadratic Relationship

0 0.5 1 1.5 2 2.5 0

1

2

3

4

5

6 x 10

-4

V DS (V)

I D (A

)

VGS= 2.5 V

VGS= 2.0 V

VGS= 1.5 V

VGS= 1.0 V

Resistive Saturation

VDS = VGS - VT

6/3/2015

MOSFET I-V Characteristics Channel Length Modulation

• Channel Length Modulation

– With pinch-off the channel at the point y such that Vc(y)=VGS - VT0, The effective channel length is e ual to L = L – ΔL

– ΔL is the length of channel segment over which QI=0.

– Pla e L i the ID(SAT) equation:

20)( )(

2VV

L

WCI TGS

oxnSATD

ΔL Drain

n+ Source

n+

Substrate (p-Si)

(p+) (p+)

Oxide 0 y L’ L

Channel Pinch-off point (QI=0)

Depletion region

VB=0

VS=0 VGS>VT0

VDS>VDSAT

6/3/2015 131

MOSFET I-V Characteristics Channel Length Modulation

– ΔL increases with an increase in VDS. We can use

– λ: channel length modulation coefficient

– ID(SAT) can be rewritten as

– The above form produces a discontinuity of current at VDS=VGS-VT0. We can include the term in ID(lin) with little error since λ is typically less than 0.1. We will usually ignore λ in manual calculations.

VLVL

L

ΔLLL

ΔLLLΔLLL DSDS

1

1

1

11

1

11111

'

1

)1()(2

20)( VVV

L

WCI DSTGS

oxnSATD

6/3/2015 132

MOSFET I-V Characteristics Substrate Bias Effect

– So far, VSB=0 and thus VT0 used in the equations. – Clearly not always true – must consider body effect – Two MOSFETs in series:

– – – V“B M = VD“ M ≠ . Thus, VT i the M e uatio is epla ed

by VT = VT(VSB) as developed in the threshold voltage section.

D

S G

D

G

M1

M2 VSB

S

6/3/2015 133

MOSFET I-V Characteristics Substrate Bias Effect (Cont.)

• The general form of ID can be written as • ID = f (VGS,VDS,VSB) • which due to the body effect term is non-

linear and more difficult to handle in manual calculations

6/3/2015 134

MOSFET I-V Characteristics Summary of Analytical Equations

– The voltage directions and relationships for the three modes of pMOS are in contrast to those of nMOS.

nMOS

Mode ID Voltage Range

Cut-off 0 VGSVT

Linear ( nCox/2)(W/L)[2(VGS-

VT)VDS-VDS2]

VGS VT，VDS> VGS -VT

Saturatio

n

( nCox/2)(W/L)(VGS-

VT)2(1+ VDS)

VGS VT，VDS VGS -VT

G

D

B

S

VDS VSB VGS ID

G

S

B

D

VDS

VSB VGS

ID

6/3/2015 135

6/3/2015 136

Pass-Transistor Logic Circuits (1)

A simple approach for implementing logic functions utilizes series and parallel combinations of switches that are controlled by input variables to

connect the input and output nodes.

Each of the switches can be implemented either by a single NMOS transistor or by a pair of CMOS

transistors connected in CMOS transmission gate

configuration. CMOS transmission gate Y=AC

6/3/2015 137


An essential requirement in the design of pass-transistor logic is ensuring that every circuit node has at all times a low-resistance path to

VDD or to ground.

If B is high, S1 closes and Y=A.

Y will be VDD if A is high or ground if A is low.

If B is low, S1 opens and Y becomes a high-impedance node.

If voltage of Y is initially zero, it will remain so.

If voltage of Y is initially high at VDD, then the inevitable leakage current will

discharge the C and can no longer be considered a static circuit.

A basic design requirement of PTL circuits is that every node have, at all times, a low resistance path to either ground or VDD. Such a path does not exist in (a) when B is low and S1 is open. It is provided

in (b) through switch S2.

6/3/2015 138


The problem can be easily solved by establishing for node Y a low-resistance path that is activated when B goes low.

Another switch, S2, controlled by B is connected between Y and ground. When B goes low, S2 closes and establishes a low-resistance path

between Y and ground.

A basic design requirement of PTL circuits is that every node have, at all times, a low resistance path to either ground or VDD. Such a path does not exist in (a) when B is low and S1 is open. It is provided in

(b) through switch S2.

MOSFET Ids-Vds

6/3/2015 139

Terminal Voltages

• Mode of operation depends on Vg, Vd, Vs – Vgs = Vg – Vs – Vgd = Vg – Vd – Vds = Vd – Vs = Vgs - Vgd

• Source and drain are symmetric diffusion terminals – By convention, source is terminal at lower voltage – Hence Vds 0

• nMOS body is grounded. • Three regions of operation

– Cutoff – Linear – Saturation

Vg

Vs

Vd

Vgd

Vgs

Vds

+-

+

-

+

-

6/3/2015 140

nMOS Cutoff

• No channel • Ids = 0

+-

Vgs

= 0

n+ n+

+-

Vgd

p-type body

b

g

s d

6/3/2015 141

nMOS Linear

• Channel forms • Current flows from d to s

– e- from s to d • Ids increases with Vds • Similar to linear resistor

+-

Vgs

> Vt

n+ n+

+-

Vgd

= Vgs

+-

Vgs

> Vt

n+ n+

+-

Vgs

> Vgd

> Vt

Vds

= 0

0 < Vds

< Vgs

-Vt

p-type body

p-type body

b

g

s d

b

g

s dIds

6/3/2015 142

nMOS Saturation

• Channel pinches off • Ids independent of Vds • We say current saturates • Similar to current source

+-

Vgs

> Vt

n+ n+

+-

Vgd

< Vt

Vds

> Vgs

-Vt

p-type body

b

g

s d Ids

6/3/2015 143

I-V Characteristics

• In Linear region, Ids depends on – How much charge is in the channel? – How fast is the charge moving?

6/3/2015 144

Channel Charge

• MOS structure looks like parallel plate capacitor while operating in inversion

– Gate – oxide – channel • Qchannel =

n+ n+

p-type body

+

Vgd

gate

+ +

source

-

Vgs

-drain

Vds

channel-

Vg

Vs

Vd

Cg

n+ n+

p-type body

W

L

tox

SiO2 gate oxide

(good insulator, ox

= 3.9)

polysilicongate

6/3/2015 145

Channel Charge


– Gate – oxide – channel • Qchannel = CV • C =

n+ n+

p-type body

+

Vgd

gate

+ +

source

-

Vgs

-drain

Vds

channel-

Vg

Vs

Vd

Cg

n+ n+

p-type body

W

L

tox

SiO2 gate oxide

(good insulator, ox

= 3.9)

polysilicongate

6/3/2015 146

Channel Charge


– Gate – oxide – channel • Qchannel = CV • C = Cg = eoxWL/tox = CoxWL • V =

n+ n+

p-type body

+

Vgd

gate

+ +

source

-

Vgs

-drain

Vds

channel-

Vg

Vs

Vd

Cg

n+ n+

p-type body

W

L

tox

SiO2 gate oxide

(good insulator, ox

= 3.9)

polysilicongate

Cox = ox / tox

6/3/2015 147

Channel Charge • MOS structure looks like parallel plate

capacitor while operating in inversion

– Gate – oxide – channel • Qchannel = CV • C = Cg = eoxWL/tox = CoxWL • V = Vgc – Vt = (Vgs – Vds/2) – Vt

n+ n+

p-type body

+

Vgd

gate

+ +

source

-

Vgs

-drain

Vds

channel-

Vg

Vs

Vd

Cg

n+ n+

p-type body

W

L

tox

SiO2 gate oxide

(good insulator, ox

= 3.9)

polysilicongate

Cox = ox / tox

6/3/2015 148

Carrier velocity

• Charge is carried by e- • Carrier velocity v proportional to lateral E-field

between source and drain

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Carrier velocity



• v = mE m called mobility • E =energy

6/3/2015 150

Carrier velocity



• v = mE m called mobility • E = Vds/L • Time for carrier to cross channel:

– t =

6/3/2015 151

Carrier velocity



• v = mE m called mobility • E = Vds/L • Time for carrier to cross channel:

– t = L / v

6/3/2015 152

nMOS Linear I-V

• Now we know – How much charge Qchannel is in the channel – How much time t each carrier takes to cross ds

I

6/3/2015 153

nMOS Linear I-V

• Now we know – How much charge Qchannel is in the channel – How much time t each carrier takes to cross

channelds

QI

t

6/3/2015 154

nMOS Linear I-V

• Now we know – How much charge Qchannel is in the channel – How much time t each carrier takes to cross

channel

ox 2

2

ds

dsgs t ds

dsgs t ds

QI

t

W VC V V V

L

VV V V

ox = W

CL

6/3/2015 155

nMOS Saturation I-V

• If Vgd < Vt, channel pinches off near drain – When Vds > Vdsat = Vgs – Vt

• Now drain voltage no longer increases current

dsI

6/3/2015 156

nMOS Saturation I-V



2dsat

ds gs t dsat

VI V V V

6/3/2015 157

nMOS Saturation I-V



22

2

dsatds gs t dsat

gs t

VI V V V

V V

6/3/2015 158

nMOS I-V Summary

• Shockley 1st order transistor models

2

cutoff

linear

saturatio

0

2

2n

gs t

dsds gs t ds ds dsat

gs t ds dsat

V V

VI V V V V V

V V V V

6/3/2015 159

Example

• Example: a 0.6 mm process from AMI semiconductor

– tox = 100 Å – m = 350 cm2/V*s – Vt = 0.7 V

• Plot Ids vs. Vds – Vgs = 0, 1, 2, 3, 4, 5 – Use W/L = 4/2 l

14

2

8

3.9 8.85 10350 120 /

100 10ox

W W WC A V

L L L

0 1 2 3 4 50

0.5

1

1.5

2

2.5

Vds

I ds (m

A)

Vgs

= 5

Vgs

= 4

Vgs

= 3

Vgs

= 2

Vgs

= 1

6/3/2015 160

MOS Transistor Basics Two Terminal Structure

• Two terminal structure (p-substrate): The MOS capacitor

• Important derived parameters. With VG = VB = 0: – F – Buck Fermi Potential (Substrate) – S – Surface Potential (Substrate)

161

A D I

VG

VB

6/3/2015

MOS Transistor Basics Two Terminal Structure (Continued)

– VSB – Flat Band Voltage (applied external voltage to G-B to flatten bands of substrate – equal to built-in potential difference of MOS – equal to work function difference GB between the substrate (channel) and gate.

• Operation – With VG0, but small, VB=0, Depletion – Holes repelled from substrate-oxide interface due to positive bias leaving negatively charged fixed acceptors ions behind. The result is a region below the interface that is depleted of mobile carriers.

• Depletion region thickness

Nq A

FS

d

Si

x 2

6/3/2015 162

MOS Transistor Basics Two Terminal Structure (Continued)

• Depletion region charge density

– Note that this density is per unit of area. – With VG>0 and larger, VB=0, Inversion –

A n-type inversion layer forms, a condition known as surface inversion. The surface is inverted when the density of electrons at the surface equals the density of holes in the bulk. This implies that s has the same magnitude but opposite sign to F. At the point depletion depth fixed and the maximum depletion region depth is at s = -F. This depth is:

Nq A

FS

dm

i

x 22

FsSAdA iNqxNqQ 2

6/3/2015 163

MOS Transistor Basics Two Terminal Structure (Continued) – The corresponding

depletion charge density (per unit area) at surface inversion is

– The inversion phenomena is the mechanism that forms the n-channel. The depletion depth and the depletion region charge are critical in determining properties of MOSFET.

FSAdA iNqxNqQ 220

6/3/2015 164

MOS Transistor Basics Four Terminal Structure

• p-Substrate • The MOS n-channel

transistor structure:

L

G(ate) D(rain)

n+

B(Body, Bulk or Substrate)

S(ource)

n+

p

6/3/2015 165

MOS Transistor Basics Four Terminal Structure (Continued)

• Symbols: n-channel - p-substrate; p-channel – n-substrate

• N-channel (for P-channel, reverse arrow or add bubbles)

• P-channel

• Enhancement mode: no conducting channel exists at VGS = 0

• Depletion mode: a conducting channel exists at VGS = 0

G

D

B

S

G

D

S

G

D

S

G G

D

S

S

D

6/3/2015 166

MOS Transistor Basics Four Terminal Structure (Continued)

• Source and drain identification

G

D

B

S

VDS

VSB

VGS

6/3/2015 167

Threshold Voltage Components

• Consider the prior 3-D drawing: Set VS=0, VDS=0, and VSB=0. – Increase VGS until the channel is inverted. Then a

conducting channel is formed and the depletion region thickness (depth) is maximum as is the surface potential.

– The value of VGS needed to cause surface inversion (channel creation) is the threshold voltage VT0. The 0 refers to VSB=0.

– VGS< VT0: no channel implies no current flow possible. With VGS> VT0, existence the channel implies possible current flow.

6/3/2015 168

Threshold Voltage Components (Cont.)

• GC work function difference between gate and channel material which is the built-in voltage that must be offset by voltage applied to flatten the bands at the surface.

• Apply voltage to achieve surface inversion -2F • Additional voltage must be applied to offset the

depletion region charge due to the acceptor ions. At inversion, this charge with VSB=0 is QB0= Q0.

• For VSB non-zero,

• The voltage required to offset the depletion region charge is defined by –QB/Cox where Cox = εox/tox with tox, the oxide thickness, and Cox, the gate oxide capacitance per unit area.

• 4) The final component is a fixed positive charge density that appears at the interface between the oxide and the substrate, Qox. The voltage to offset this charge is:

VNqQ SBFSA i 22

C

Q

ox

ox

6/3/2015 169


• These components together give:

• For VSB=0, VT0 has QB replaced by QB0. This gives a relationship between VT and VT0 which is:

• Thus the actual threshold voltage VT differs from VT0 by the term given. Going back to the definition of QB, this term is equal to:

• In which γ is the substrate-bias (or body effect) coefficient.

ox

BBTT

C

QQVV 00

FSBF V 22

ox

ox

ox

BFGCT

C

Q

C

QV 2

6/3/2015 170

Threshold Voltage Components (Cont.) • The final expression for VT0 and VT

are

• and

– The threshold voltage depends on the source-to-bulk voltage which is clearly separated out. The component is referred to as body effect. If the source to body voltage VSB is non-zero, the corrective term must be applied to VT0.

C

Nq

ox

SA i2

FSBFTT VVV 220

ox

ox

ox

BFGCT

C

Q

C

QV 00 2

6/3/2015 171


• • Those parameters in the VT

equation are signed. The following table gives their signs for nMOS and pMOS transistor.

• For real designs, the threshold voltage, due to variation in oxide thickness, impurity concentrations, etc., VT0 and γ should be measured from the actual process.

Parameter nMOS pMOS

F QB, QB0 γ

VSB

6/3/2015 172

R

Vss

R

1 0

0 1

Vo

•Inverter : basic requirement for producing a complete range of Logic circuits

6/3/2015 173

Vdd

Vss

Vo

Vin

R Pull-Up

Pull Down

Basic Inverter: Transistor with source connected to ground and a load resistor connected from the drain to the positive Supply rail

Output is taken from the drain and control input connected between gate and ground

Resistors are not easily formed in silicon - they occupy too much area

Transistors can be used as the pull-up device

6/3/2015 174

Vdd

Vss

Vo

Vin

D

S

D

S

• Pull-Up is always on – Vgs = 0; depletion

• Pull-Down turns on when Vin > Vt

NMOS Depletion Mode Transistor Pull - Up

Vt V0 Vdd

Vi

• With no current drawn from outputs, Ids for both transistors is equal

Non-zero output

6/3/2015 175

Vgs=0.2VDD

Vgs=0.4 VDD

Vgs=0.6 VDD

Vgs=0.8VDD

Vgs=VDD

Ids

Vds VDD

Vo VDD

VDD

Vin

Ids

VDD –Vds

Ids

Vds

Vgs=-0.6VDD

Vgs=-0.4 VDD

Vgs=-0.2 VDD

Vgs=0

Vgs=0.2VDD

6/3/2015 176

Vo VDD

VDD

Vin

Vinv

• Point where Vo = Vin is called Vinv

Decreasing Zpu/Zpd

Increasing Zpu/Zpd

• Transfer Characteristics and Vinv can be shifted by altering ratio of pull-up to Pull down impedances

6/3/2015 177

NMOS Depletion Mode Inverter Characteristics

• Dissipation is high since rail to rail current flows when Vin = Logical 1

• Switching of Output from 1 to 0 begins when Vin exceeds Vt of pull down device

• When switching the output from 1 to 0, the pull up device is non-saturated initially and this presents a lower resistance through which to charge capacitors (Vds < Vgs – Vt)

6/3/2015 178

Vss

Vo

Vin

D

S

D

S

NMOS Enhancement Mode Transistor Pull - Up

Vdd

Vgg

Vt (pull down)

V0 Vdd

Vt (pull up)

Non zero output

Vin

• Dissipation is high since current flows when Vin = 1 • Vout can never reach Vdd (effect of channel) • Vgg can be derived from a switching source (i.e. one phase of a clock, so that dissipation can be significantly reduced • If Vgg is higher than Vdd, and extra supply rail is required

6/3/2015 179

When cascading logic devices care must be taken to preserve integrity of logic levels i.e. design circuit so that Vin = Vout = Vinv

Cascading NMOS Inverters

Determine pull – up to pull-down ratio for driven inverter

6/3/2015 180

Assume equal margins around inverter; Vinv = 0.5 Vdd

Assume both transistors in saturation, therefore: Ids = K (W/L) (Vgs – Vt)2/2

Depletion mode transistor has gate connected to source, i.e. Vgs = 0 Ids = K (Wpu/Lpu) (-Vtd)

2/2

Ids = K (Wpd/Lpd) (Vinv – Vt)2/2 Enhancement mode device Vgs = Vinv, therefore

Assume currents are equal through both channels (no current drawn by load) (Wpd/Lpd) (Vinv – Vt)2 = (Wpu/Lpu) (-Vtd)2 Convention Z = L/W Vinv = Vt – Vtd / (Zpu/Zpd)1/2 Substitute in typical values Vt = 0.2 Vdd ; Vtd = -0.6 Vdd ; Vinv = 0.5 Vdd

This gives Zpu / Zpd = 4:1 for an nmos inverter directly driven by another inverter

6/3/2015 181

Vdd Vdd

A B C

Inverter 1 Inverter 2

Vin1 Vout2

Pull-Up to Pull-Down Ratio for an nMOS inverter driven through 1 or more pass transistors

It is often the case that two inverters are connected via a series of switches (Pass Transistors) We are concerned that connection of transistors in series will degrade the logic levels into Inverter 2. The driven inverter can be designed to deal with this. (Zpu/Zpd >= 8/1)

6/3/2015 182

Complimentary Transistor Pull – Up (CMOS)

Vdd

Vss

Vo Vin

Vout

Vin

Vdd Vss

Vtn Vtp

Logic 0 Logic 1

P on N off

Both On

N on P off

6/3/2015 183

Vout

Vin

Vdd Vss

Vtn Vtp

P on N off

Both On

N on P off

1 2 3 4 5

1: Logic 0 : p on ; n off 5: Logic 1: p off ; n on 2: Vin > Vtn. Vdsn large – n in saturation Vdsp small – p in resistive Small current from Vdd to Vss 4: same as 2 except reversed p and n 3: Both transistors are in saturation Large instantaneous current flows

6/3/2015 184

CMOS INVERTER CHARACTERISTICS

Current through n-channel pull-down transistor 2

2tnin

nn VVI

Current through p-channel pull-up transistor

22

tpDDinp

p VVVI

At logic threshold, In = I

p

tpDDtnp

n

p

nin

tpDDintninp

n

tpDDinp

tninn

tpDDinp

tninn

VVVV

VVVVV

VVVVV

VVVVV

1

22

22

22

p

n

p

ntntpDD

in

VVV

V

1

If n =

p and V

tp = –V

tn

2DD

inV

V

n

nn

p

pp

L

W

L

W

Mobilities are unequal : µn = 2.5 µp

Z = L/W

Zpu/Zpd = 2.5:1 for a symmetrical CMOS inverter

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CMOS Inverter Characteristics

• No current flow for either logical 1 or logical 0 inputs

• Full logical 1 and 0 levels are presented at the output

• For devices of similar dimensions the p – channel is slower than the n – channel device

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CMOS Inverter VTC

00.51

1.522.5

0 0.5 1 1.5 2 2.5

Vin (V)

Vo

ut (

V)

NMOS off

PMOS res

NMOS sat

PMOS res

NMOS sat

PMOS sat

NMOS res

PMOS sat NMOS res

PMOS off

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Cutoff Linear Saturation

pMOS Vin -VDD= VGS> VT Vin -VDD=VGS< VT Vin -Vout=VGD< VT

Vin -VDD=VGS> VT

Vin -Vout=VGD>VT

nMOS Vin = VGS< VT

Vin =VGS> VT

Vin -Vout =VGD> VT

Vin =VGS> VT

Vin -Vout =VGD< VT

VDD

Vin Vout

CL

G S

D

D

G

S

Regions of operations For nMOS and pMOS In CMOS inverter

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Impact of Process Variation

00.51

1.522.5

0 0.5 1 1.5 2 2.5

Vin (V)

Vo

ut (

V)

Nominal

Good PMOS

Bad NMOS

Bad PMOS

Good NMOS

Pprocess variations (mostly) cause a shift in the switching threshold

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Cmos Inverter

• Look at why our NMOS and PMOS inverters might not be the best inverter designs

• Introduce the CMOS inverter • Analyze how the CMOS inverter works

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NMOS Inverter

• When VIN changes to logic 0, transistor gets cutoff. ID goes to 0.

• ‘esisto oltage goes to ze o. VOUT pulled up to 5 V. D ID = 5/R

+

VDS _

R

5 V

VOUT

VIN

5 V

0 V

D ID = 0

+ VDS _

R

5 V

VOUT

VIN

0 V

5 V

When VIN is logic 1, VOUT is

logic 0.

Constant nonzero current

flows through transistor.

Power is used even though

no new computation is being

performed.

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PMOS Inverter

• When VIN changes to logic 1, transistor gets cutoff. ID goes to 0.

• ‘esisto oltage goes to ze o. VOUT pulled do to V.

5 V

VOUT

ID = -5/R

- VDS +

R

5 V

When VIN is logic 0, VOUT is

logic 1.

Constant nonzero current

flows through transistor.

Power is used even though

no new computation is being

performed.

VIN

0 V

5 V

VOUT

ID = 0

- VDS +

R

0 V

VIN

5 V

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Analysis of CMOS Inverter

• We can follow the same procedure to solve for currents and voltages in the CMOS inverter as we did for the single NMOS and PMOS circuits.

• Remember, now we have two transistors so we write two I-V relationships and have twice the number of variables.

• We can roughly analyze the CMOS inverter graphically.

D

S

VDD (Logic1)

D

S

VOUT

VIN

NMOS is “pull-down device” PMOS is “pull-up device” Each shuts off when not pulling

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D

S

VDD (Logic 1)

D

S

VOUT

VIN

VGS(n) = VIN

VGS(p) = VIN – VDD

VGS(p) = VDS(n) - VDD

ID(p) = -ID(n)

VDS(n) = VOUT

VDS(p) = VOUT – VDD

VDS(p) = VDS(n) - VDD

+

VDS(n) _

Use these equations

to write both I-V equations in terms of

VDS(n) and ID(n)

Linear KVL and KCL Equations

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VDS(n)

ID(n)

VDD

NMOS I-V curve

PMOS I-V curve (written in terms of NMOS variables)

CMOS Analysis

VIN = VGS(n) =

0.9 V

As VIN goes up, VGS(n) gets bigger and VGS(p) gets less negative.

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VDS(n)

ID(n)

VDD

NMOS I-V curve


CMOS Analysis

VIN = VGS(n) =

1.5 V


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VDS(n)

ID(n)

VDD

NMOS I-V curve


CMOS Analysis

VIN = VGS(n) =

2.0 V


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VDS(n)

ID(n)

VDD

NMOS I-V curve


CMOS Analysis

VIN = VGS(n) =

2.5 V


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VDS(n)

ID(n)

VDD

NMOS I-V curve


CMOS Analysis

VIN = VGS(n) =

3.0 V


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VDS(n)

ID(n)

VDD

NMOS I-V curve


CMOS Analysis

VIN = VGS(n) =

3.5 V


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VDS(n)

ID(n)

VDD

NMOS I-V curve


CMOS Analysis

VIN = VGS(n) =

4.1 V


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VOUT

VIN C B A E D VDD

VDD

CMOS Inverter VOUT vs. VIN

NMOS: cutoff

PMOS: triode

NMOS: saturation

PMOS: triode

NMOS: triode

PMOS: saturation

NMOS: triode

PMOS: cutoff

both sat.

curve very steep here; only in “C” for small interval of VIN

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ID

VIN C B A E D VDD

CMOS Inverter ID

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D

S

VDD

D

S

VOUT1

VIN D

S

VDD

D

S

VOUT2

Important Points

• No ID current flow in Regions A and E if nothing attached to output; current flows only during logic transition

• If another inverter (or other CMOS logic) attached to output, transistor gate terminals of attached stage do not permit current: current still flows only during logic transition

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Impact of Process Variation

00.51

1.522.5

0 0.5 1 1.5 2 2.5

Vin (V)

Vo

ut (

V)

Nominal

Good PMOS

Bad NMOS

Bad PMOS

Good NMOS

Pprocess variations (mostly) cause a shift in the switching threshold

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Beta Ratio

• If bp / bn 1, switching point will move from VDD/2

Vout

0

Vin

VDD

VDD

0.5

12

10p

n

0.1p

n

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Unit III VLSI CIRCUIT DESIGN PROCESSES

Topics • VLSI design flow • MOS layers • Stick diagrams • Design Rules and Layout • 2 um CMOS design rules for wires • Contacts and Transistors • Layout diagrams for NMOS and • CMOS inverters and gates, Scaling of MOS circuits

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VLSI Design of approach of IC

208 6/3/2015

Layer Types

• p-substrate • n-well • n+ • p+ • Gate oxide (thin oxide) • Gate (polycilicon) • Field Oxide

– Insulated glass – Provide electrical isolation

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Stick diagram

Encodings for a simple single metal nMOS process

COLOR STICK ENCODING LAYERS MASK LAYOUT ENCODING CIF LAYER

MONOCROME MONOCROME

GREEN

RED

BLUE

BLACK

GRAY

n-

diffusion

n+active

Thniox

Polysilicon

Metal 1

Contact cut

Overglass NOT

APPLICABLE nMOS

ONLY

YELLO

W

Implant

Buried

contact

nMOS

ONLY

BROWN

N

D

NP

N

M

N

C

N

G

NI

NB

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Stick Diagrams

Metal

poly

ndiff

pdiff

Can also draw

in shades of

gray/line style. Buried Contact

Contact Cut

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Stick Diagrams

• VLSI design aims to translate circuit concepts onto silicon.

• Stick diagrams are a means of capturing topography and layer information using simple diagrams.

• Stick diagrams convey layer information through colour codes (or monochrome encoding).

• Acts as an interface between symbolic circuit and the actual layout.

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Stick Diagrams

• Does show all components/vias. • It shows relative placement of

components.

• Goes one step closer to the layout • Helps plan the layout and routing

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Stick Diagrams

• Does not show – Exact placement of components – Transistor sizes – Wire lengths, wire widths, tub

boundaries.

– Any other low level details such as parasitics..

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Stick Diagrams – Some rules

• Rule 1. • Whe t o o o e sti ks of the sa e t pe

cross or touch each other that represents electrical contact.

Stick Diagrams

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• Rule 2. • Whe t o o o e sti ks of diffe e t t pe

cross or touch each other there is no electrical contact. (If electrical contact is needed we have to show the connection explicitly).

Stick Diagrams

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• Rule 3. • When a poly crosses diffusion it represents

a transistor.

Note: If a contact is shown then it is not a transistor.

Stick Diagrams

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• Rule 4. • In CMOS a demarcation line is drawn to

avoid touching of p-diff with n-diff. All pMOS must lie on one side of the line and all nMOS will have to be on the other side.

Stick Diagrams

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5 V

Dep

Vout

Enh

0V

Vin

5 v

0 V

Vin

5 v

NMOS INVERTER

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NMOS-NAND

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NMOS-NOR

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NMOS EX-OR

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NMOS EX-NOR

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PMOS-INVERTER

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PMOS NAND

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PMOS-NOR

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Sticks design CMOS NAND:

• Start with NAND gate:

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NAND sticks

VDD

a

VSS

out

b

Stick Diagram - Example

NOR Gate

OUT

B

A

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Stick Diagram - Example

Power

Ground

B

C

Out A

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2 I/P OR GATE

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2 I/P AND

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Y= AB+CD

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Y= AB+CD “TICK

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Design Rules

• Design rules are a set of geometrical specifications that dictate the design of the layout masks

• A design rule set provides numerical values – For minimum dimensions – For minimum line spacings

• Design rules must be followed to insure functional structures on the fabricated chip

• Design rules change with technological advances (www.mosis.org)

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Silicon Foundry

• A standard • A foundry allows designers to submit designs

using a state-of-the-art process

• Each foundry state simpler set of design rules called lambda design rules

• All widths, spacings, and distances are written in the form – Value = – TSMC (Thailand Semiconductor Manufacturing

Corporation)

m

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Design Rules Classification

• Minimum width • Minimum spacing • Surround • Extension

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Physical Limitations

• Line width limitation of an imaging system – The reticle shadow projected on the photoresist

does not have sharp edges due to optical diffraction

• Etching process problem – Undercutting of the resist due to lateral etching

decreases the resolution

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Etching Process Problem

Substrate

Vertical etching

Lateral

Etching

Substrate

Isotropic etch

Depletion Region

• If depletion regions of adjacent pn junctions touch, then

– The current blocking characteristics are altered – Current can flow between the two

Substrate

n+ n+

Limits on n+ spacing

Spacing

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Electrical Capacitive Coupling

• This occurs between closely spaced conducting lines

• This leads to a problem called crosstalk – A portion of the electrical energy is coupled to

another causing noise

– This is a major problem in high-density design

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Electrical Rules

• An example of an electrical rule is the allowed width of a metal interconnect line

– To avoid electromigration effects – The design rule set will stipulate the maximum

current flow level permitted

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3D Perspective

Polysilicon Aluminum

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Design rules and Layout

• Why we use design rules? – Interface between designer and process engineer – Guidelines for constructing process masks

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Design Rules

Minimum length or width of a feature on a layer is 2

Why?

To allow for shape contraction

Minimum separation of features on a layer is 2

Why?

To ensure adequate continuity of the intervening

materials.

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Design Rules

Minimum width of PolySi and diffusion line 2 Minimum width of Metal line 3 as metal lines run over a more uneven

surface than other conducting layers to ensure their continuity

2

Metal

Diffusion

Polysilicon

3

2

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Design Rules

PolySi – PolySi space 2 Metal - Metal space 2 Diffusion – Diffusion 3 To avoid the possibility of their associated

regions overlapping and conducting current

2

Metal

Diffusion

Polysilicon

2

3

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Design Rules

Diffusion – PolySi To prevent the lines overlapping to form unwanted capacitor

Metal lines can pass over both diffusion and polySi without

electrical effect. Where no separation is specified, metal lines

can overlap or cross

Metal

Diffusion

Polysilicon

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Metal Vs PolySi/Diffusion

• Metal lines can pass over both diffusion and polySi without electrical effect

• It is recommended practice to leave between a metal edge and a polySi or diffusion line to which it is not electrically connected

Metal

Polysilicon

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poly-poly spacing 2

diff-diff spacing 3 (depletion regions tend to spread outward)

metal-metal spacing 2

diff-poly spacing

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Review:

Note

• Two Features on different mask layers can be misaligned by a maximum of 2l on the wafer.

• If the overlap of these two different mask layers can be catastrophic to the design, they must be separated by at least 2l

• If the overlap is just undesirable, they must be separated by at least l

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When a transistor is formed?

Gate is formed where polySi crosses diffusion with

thin oxide between these layers.

Design rules

min. line width of polySi and diffusion 2

drain and source have min. length and width of 2

And

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The polySi of the gate extends 2 beyond the gate area on to the field oxide to prevent the drain and source from shorting.

no overlap overlap

diffusion short

• Diffusion Problems

PolySi extends in the gate region…

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Depletion Transistor We need depletion implant

An implant surrounding the Transistor by 2

Ensures that no part of the transistor remains

in the enhancement mode

A separation of 2 from the gate of an

enhancement transistor avoids affecting

the device.

2

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Depletion Transistor

Implants are separated by 2to prevent them from merging

2

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Butting Contact

The gate and source of a depletion device can be connected

by a method known as butting contact. Here metal makes

contact to both the diffusion forming the source of the

depletion transistor and to the polySi forming this device’s gate.

Advantage:

No buried contact mask required and avoids

associated processing.

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Butting Contact

n+ n+

Insulating

Oxide

Metal

Gate Oxide PolySi

Problem: Metal descending the hole has a tendency to

fracture at the polySi corner, causing an open circuit.

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Buried Contact

It is a preferred method. The buried contact window

defines the area where oxide is to be removed so that

polySi connects directly to diffusion.

Contact Area must be a min. of 2*2to ensure adequate contact area.

2

2

Contact Area

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Buried Contact

The buried contact window surrounds this contact by in all directions to avoid any part of this area forming a transistor.

Separated from its related transistor gate by to prevent gate

area from being reduced.

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Buried Contact

Here gate length is depend upon the alignment of the

buried contact mask relative to the polySi and therefore

vary by .

2

2

Channel length PolySi

Buried contact

Diffusion 6/3/2015 263

Contact Cut Metal connects to polySi/diffusion by contact cut.

Contact area: 2*2

Metal and polySi or diffusion must overlap this contact area

by so that the two desired conductors encompass the contact area despite any mis-alignment between conducting layers

and the contact hole

4

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Contact Cut Contact cut – any gate: 2apart

Why? No contact to any part of the gate.

4

2

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Contact Cut

Contact cut – contact cut: 2apart

Why? To prevent holes from merging.

2

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Rules for CMOS layout

Similar to those for NMOS except No

1. Depletion implant

2. Buried contact

Additional rules

1. Definition of n-well area

2. Threshold implant of two types of transistor

3. Definition of source and drains regions for the

NMOS and PMOS.

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To ensure the separation of the PMOS and NMOS devices,

n-well supporting PMOS is 6away from the active area of NMOS transistor.

Why?

Avoids overlap

of the associated

regions

n-well n+ 6

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2

2

N-well must completely

surround the PMOS

device’s active area by 2

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2

2

The threshold implant

mask covers all n-well

and surrounds the n-well

by

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2

2

The p+ diffusion mask

defines the areas to

receive a p+ diffusion.

It is coincident with the

threshold mask

surrounding the PMOS

transistor but excludes

the n-well region to be

connected to the supply.

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Rules for CMOS layout A p+ diffusion is required to effect the ground connection

to the substrate. Thus mask also defines this substrate

region. It surrounds the conducting material of this

contact by .

4

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Total contact area = 2*4

Neither NMOS nor CMOS usually allow contact cuts

to the gate of a transistor, because of the danger of

etching away part of the gate

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UNIT-IV GATE LEVEL DESIGN

Topics

• Logic gates and other complex g

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