Introduction to VLSI - scholar.cu.edu.eg Introduction to VLSI ITI Ismailia Si/SiO 2 interface Easily grown thermally on Silicon Block dopants diffusion and other unwanted impurities

Introduction to VLSI ITI Ismailia

Introduction to VLSI Dr. Hassan Mostafa

حسن مصطفى. د

[email protected]


Contents

CMOS Processing

Scaling

SoC Subsystems

Digital Implementation Strategies: ASIC/FPGA

Layout and Design Rules

Controller: Finite-State Machines

Data-path: Shifters, Adders, Multipliers

Memory Design: RAM/ROM


CMOS PROCESSING

(continued)


CMOS PROCESSING

Ion Implantation and

Dopant Diffusion


Ion energy

Ion current and implant time



Example: m = 3 Mask effectiveness = 99.87%


Dopant Diffusion




Implantation Versus Diffusion

Diffusion Ion Implantation

High temperature, hard mask Low temperature, photoresist mask

Isotropic dopant profile Anisotropic dopant profile

Cannot independently control of the dopant

concentration and junction depth

Can independently control of the dopant

concentration and junction depth


CMOS PROCESSING

Thermal Oxidation


Si/SiO2 interface

Easily grown thermally on Silicon

Block dopants diffusion and other unwanted impurities

Resistant to most chemicals used in CMOS processing

Easily patterned and etched with specific chemicals

Excellent insulators with few interface defects

All other materials interfaces have significant problems compared to the Si/SiO2 Interface which limits their explicabilities.


CVD = Chemical

Vapor Deposition

LPCVD = Low

Pressure CVD



1.3*1.3*1.3 = 2.2



Volume = weight / density







CMOS PROCESSING

Etching


Every time produces different etching depth



Vertical etching



Etch rate is controlled by the chemical reaction rate





CMOS PROCESSING

Thin Film Deposition



CVD



PVD









Contents

CMOS Processing

Scaling

SoC Subsystems

Digital Implementation Strategies: ASIC/FPGA

Layout and Design Rules

Controller: Finite-State Machines

Data-path: Shifters, Adders, Multipliers

Memory Design: RAM/ROM


CMOS Scaling


Contents

The MOS Capacitor

MOS transistor : modes of operation

Short channel transistors

Dynamic models

Technology scaling


The MOS Capacitor

MOS = Metal – Oxide – Semiconductor

Metal = polycrystaline Silicon

Oxide = SiO2

semiconductor substrate = monocrystal Si


MOS transistor, adding the terminals

The two terminals necessary to move

from a MOS cap to a MOSFET are the

dopant heavy drain and source

The drain and source in an NMOS are

n+, the substrate is p

The drain and source form diodes with

the substrate

The substrate terminal is connected to

the lowest circuit potential for NMOS

Parasitic junctions reverse


MOSFET as a circuit component

The MOSFET is a four terminal device:

Body (substrate)

Source

Drain

Gate

Body is a signal ground and has a secondary effect

in most designs MOSFET is 3 terminal

The principal transistor property is trans-impedance: current between two terminals (source and drain) is controlled by the voltage on a third terminal (Gate)


Geometry of MOSFET and basic parameters

L is channel length, current reducing

W is channel width, current aiding

tox is oxide thickness, thinner means more charge accumulation

The gate capacitance per unit area can be calculated as: oxox

ox

Ct


Threshold Voltage: Concept

n+n+

p-substrate

DSG

B

VGS

+

-

Depletion

Region

n-channel


Threshold voltage

Threshold voltage is the point at which strong inversion occurs at the channel below the gate

0

0

{ | 2 | | 2 |}

= 0 body source potential threshold

= Body effect coefficient

= Fermi potential ln

th th F SB F

th

AF T

i

V V V

V

N

n


Notes on threshold voltage

The expression of threshold voltage shows it is not a stable constant

There is a decent Source-Body potential dependence, this is known as body effect and is crucial to power control

There is a first order temperature dependence through Fermi potential

Sensitive to substrate doping


Transistor in Linear

n+n+

p-substrate

D

S

G

B

VGS

xL

V(x)+–

VDS

ID

MOS transistor and its bias conditions


Transistor in Saturation

n+n+

S

G

VGS

D

VDS > VGS - VT

VGS - VT+-

Pinch-off


I-V Relations

Quadratic Relationship With VGS

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


I-V Relations (Long-Channel Device)


A model for manual analysis


In a digital circuit…

In steady state we need the MOSFET in one of two

modes: OFF or ohmic. In transition we depend on

saturation to supply switch currents

OFF mode is a high impedance mode, the MOS

accepts whatever voltage is imposed by the rest of the

circuit, ideally no current flows

Ohmic mode is a low impedance mode, there is little

drop on MOS and it can drive a stable node

Saturation supports maximum current, reducing the

switching time by charging/discharging capacitors


Transistor History (5/5)

Scaling challenges

61

(2000) 180nm transistor (2014) 14nm transistor

Transistor scaling does not come for free

Several challenges are standing against this scaling

The strongest challenge is process variations


Deep Sub-Microm (DSM) effects

Velocity Saturation

Process Variations

Drain Induced Barrier Lowering (DIBL)

Hot Carrier Injection (HCI)

Negative Bias Temperature Instability (NBTI)

Sub-threshold Conduction


Velocity Saturation

x (V/µm) x c = 1.5

u n

( m

/ s )

u sat = 10 5

Constant mobility (slope = µ)

Constant velocity


Something else saturates

The new velocity model says for certain drain voltages, the field is so high, drift actually slows down its growth due to random collisions

Thus at some field carriers will not move any faster, and any additional VDS has no effect

This is the definition of a saturation process

This is what is termed velocity saturation


I-V Relations (Short Channel Transistor)

-4

V DS (V)

0 0.5 1 1.5 2 2.5 0

0.5

1

1.5

2

2.5 x 10

I D (

A)

VGS= 2.5 V

VGS= 2.0 V

VGS= 1.5 V

VGS= 1.0 V

Early Saturation


Which happens?

The saturation mechanism depends on what happens first

If drain voltage causes critical field before pinchoff, there will be velocity saturation, otherwise pinchoff saturation

A short channel causes higher fields, thus short channels suffer from velocity saturation more often


Comparing short and long channel

For the same transistor operating in pinchoff or velocity saturation, there is a distinction:

For the same VGS, the pinchoff transistor supplies higher current

This gets much worse at higher values


A unified model for manual analysis

S D

G

B


Process Variations

Random Dopant Fluctuations (RDF)

Channel Length Variations


DIBL (Drain Induced Barrier Lowering)

The drain can be at a high potential

Drain can act to aid in creating the depletion region

It can also attract carriers into the channel

As drain potential increases, it becomes easier to create the channel

Threshold drops with drain potential

Low V DS threshold

Drain-induced barrier lowering (for low L )

VDS


Hot Carriers Injection (HCI)

The hot carrier effect is a temporal effect

The hot carrier effect occurs due to the accumulation of rare high energy charges to be trapped in the oxide of the MOSFET

As time passes these electrons create an increased barrier to channel formation

With time threshold voltage rises


Negative Bias Temperature Instability (NBTI)

NBTI is the generation of interface traps under negative bias conditions (i.e., VGS = - VDD) at elevated temperatures in PMOS transistors


Subthreshold conduction

We always assume that below threshold voltage drain current drops to zero

Below threshold, there is weak inversion and some charge

In this mode, the MOSFET behaves as a parasitic BJT

An exponential current flows

This current was normally considered a secondary effect, in sub-micron it is considered a major factor

0 0.5 1 1.5 2 2.5 10

-12

10 -10

10 -8

10 -6

10 -4

10 -2

V GS (V)

I D (A

)

VT

Linear

Exponential

Quadratic

kT

qV

nkT

qV

D

DSGS

eeII 10


Technology scaling – general considerations

With each technology node, features get smaller, this leads to some positive and some negative effects

The main question is what happens to voltages as dimensions shrink?

Full scaling: Voltage scales the same as dimensions (by S in the tables), excellent results, but unrealistic because of loss of noise margin and compatibility with pins

Constant voltage: Don’t scale voltages at all, leads to extreme fields and heat

General scaling: Voltage scales differently from dimensions (by U in the tables), typical nowadays

Results in new fab techniques such as high-k materials.


Technology scaling – short channel

3 2 1/ 1/ 1/Power delay PDelay S SU S

Battery life

Heat

Current

drive


MEMS


What are MEMS/NEMS?

MEMS/NEMS = Micro/Nano-Electro-Mechanical Systems

Tiny machines (micro and nano scale)

Not just micro/nano-fabrication

Enabling technology to augment as they are fabricated for a specific application (non-standardization)

Miniaturization for performance enhancement

MEMS/NEMS Cairo University 77


RF Switch from

MEMtronics

Energy harvester from

Perpetuum


Examples


Optical Switch from

Lucent

Lab-chips from

Agilent


Examples


Microphone from

Knowles Digital Micro-mirrors

Device (DMD) from TI


Examples


Inkjet Nozzle from

HP Accelerometer from

Analog Devices


Examples


Micro-scale gear chains

Micro-scale guitar


Examples


Needle without pain

Roboroach


Examples


History

There’s plenty of room

at the bottom

1959


Richard Feynman

What I want to talk about is the problem of manipulating and controlling things on a

small scale. It is a staggeringly small world that is below. In the year 2000, when they look

back at this age, they will wonder why it

was not until the year 1960 that anybody

began seriously to move in this direction. Why cannot we write the entire 24 volumes of the Encyclopedia Brittanica on

the head of a pin?


Interdisciplinary

Traditional

Above mm: traditional mechanics

Micron to mm: microelectronics and electrical engineering

Nanometer to micron: chemists

Now

Micro

Nano

Engineering

Biology

Chemistry

Feeds each other



MEMS Advantages

MEMS devices integrate multiple functions like sensing, decision making and control functions on a single chip

High Sensitivity

Portability

Batch fabrication reduces manufacturing cost and time

Low power consumption

Easy to maintain and replace



Sensors and Actuators

Sensor

A device that converts an environmental condition into an electrical signal

Actuator

A device that converts a control signal (usually electrical) into mechanical action (motion)

Basic components of a control system

Sensor

Actuator

Power supply

Controller



Home heating example


Sensors

Actuators

(air flow control)


Actuation methods

Electrostatic actuation

It relies on the attractive force between two conductive plates

Electrostatic comb actuators are a variant that includes two comb sets of inter-digitated “teeth” that are offset relative to each other

An applied voltage brings the two combs together such that the teeth become alternating



Actuation methods

Piezoelectric Actuation

Piezoelectricity is the ability of some crystals to create mechanical stress, or motion by expanding or contracting in response to an applied voltage

Piezoelectric actuation can provide significantly large forces, especially if thick piezoelectric films are used



Actuation methods

Thermal Actuation

It consumes more power than electrostatic or piezoelectric actuation

Two-layers

- The difference in the thermal expansion coefficients between two joined layers of dissimilar materials cause bending with temperature

- One layer expands more than the other as temperature increases

- This results in stresses at the interface and consequently bending of the stack

- The amount of bending depends on the difference in coefficients of thermal expansion and absolute temperature



Passive Micro-machined Mechanical Structures

Fluid Nozzles

Nozzles are among the simplest microstructures to fabricate using anisotropic etching of silicon, electroforming, or laser drilling of a metal sheet

Forming nozzles of circular or arbitrary shape in silicon involves additional fabrication steps




Fluid Nozzles

Nozzles types:

- Top shooters: they are oriented perpendicular to the surface of the wafer as in the inkjet field

- Side shooters: they are oriented parallel to the wafer surface as in the fluid flow field




Hinge Mechanisms At the microscopic scale, hinges extend the utility of the 2D surface micromachining

technology into the 3D

The hinge structure is simple, consisting of a plate and a support arm made of a first polysilicon layer

A staple made of a second polysilicon layer captures the plate support arm

The staple is anchored directly to the substrate

The fabrication utilizes the polysilicon surface micro-machining process



Accelerometers

A sensor that detects change in velocity

Most common application for MEMS accelerometers

Air bag deployment.

Cairo University 95 MEMS/NEMS


Accelerometers

Accelerometers widely used for monitoring vibrations in industrial machinery

Automotive applications: Brake sensor and bounce sensor



Accelerometers

The first demonstration of a micromachined accelerometer took place in 1979 at Stanford University

Basic structure

Inertial mass suspended from a spring

They differ in the sensing of the relative position of the inertial mass as it displaces under the effect of an externally applied acceleration

Sensing methods such as capacitive or piezoelectric

Specifications

Full-scale range (in G) <G=9.81 m/sec2>

Sensitivity (in V/G)

Resolution (in G)

Bandwidth ( in Hz) <acceleration reading times/sec>

Cross-axis sensitivity

Immunity to shock



Accelerometers

Accelerometers for airbag crash sensing are rated for a full range of ±50G and a bandwidth of about 1.0 kHz

Accelerometers for engine vibration have a range of ±1G, but must resolve small accelerations (<100 μG) over a large bandwidth (>10 kHz)

Accelerometers for pacemakers

Incorporate multi-axis accelerometers to monitor the level of human activity, and correspondingly adjust the stimulation frequency

- Full scale range of ±2G and a bandwidth of less than 50 Hz, but they require extremely low power consumption for battery longevity

Accelerometers for military applications can exceed a rating of ±1,000G



Accelerometers

Cross-axis sensitivity is the immunity of the sensor to accelerations along directions perpendicular to the main sensing axis

Shock immunity is an important specification for the protection of the devices during handling or operation

The test is performed by dropping the device from a height of one meter over concrete

The shock impact can easily reach a dynamic peak of 10,000G



Accelerometers

Basic structure

Cairo University 100

M

kfresonance

2

1

MEMS/NEMS


Piezoresistive Bulk Accelerometer

It consists of three substrates:

a lower base

a middle core containing a hinge-like spring, the inertial mass, and the sense elements

a top protective lid




The inertial mass sits inside a frame suspended by the spring

Two thin piezoresistive elements in a Wheatstone bridge configuration span the narrow 3.5-μm gap between the outer frame of the middle core and the inertial mass

The piezoresistors are only 0.6 μm-thick and 4.2 μm-long and are very sensitive to displacements of the inertial mass

The output in response to an acceleration equal to 1G in magnitude is 25mV for a Wheatstone bridge excitation of 10V

The thick and narrow hinge structure allows displacement within the plane of the device, but it is very stiff in directions normal to the wafer, resulting in high immunity to off-axis accelerations




The outer frame acts as a stop mechanism that protects the device in the event of excessive acceleration shocks

It takes 6,000G for the inertial mass to touch the frame

The device can survive shocks in excess of 10,000G

Open apertures reduce the weight of the inertial mass and combine with the stiff hinge to provide a rather high resonant frequency of 28 kHz



Capacitive Bulk Accelerometer

It consists of a stack of three bonded silicon wafers, with the hinge spring and inertial mass incorporated in the middle wafer

The inertial mass forms a moveable inner electrode of a variable differential capacitor circuit

The two outer wafers are identical and are simply the fixed electrodes of the two capacitors



Capacitive Bulk Accelerometer

Holes through the inertial mass reduce the damping effect from air trapped in the enclosed cavity, increasing the operating bandwidth of the sensor

Measuring range is from ±0.5G to ±12G

Electronic circuits sense changes in capacitance, then convert them into an output voltage between 0 and 5V

The rated bandwidth is up to 400 Hz for the ±12G accelerometer

The cross-axis sensitivity is less than 5% of output

The shock immunity is 20,000G



Angular Rate Sensors and Gyroscopes

The gyroscope maintains a fixed orientation with great accuracy, regardless of Earth rotation

It consisted of a flywheel mounted in gimbal rings

The large angular momentum of the flywheel counteracts externally applied torques and keeps the orientation of the spin axis unaltered

The gyroscope derives its precision from the large angular momentum that is proportional to the heavy mass of the flywheel, its substantial size, and its high rate of spin







The use of miniature devices is not good to produce useful gyroscopic action because the angular momentum of a miniature flywheel is small

Instead, micro-machined sensors that detect angular rotation utilize the Coriolis effect

These devices are angular-rate or yaw-rate sensors, measuring angular velocity, however, they are incorrectly referred to as gyroscopes




The Coriolis effect is a direct consequence of a body’s motion in a rotating frame of reference




The Coriolis effect



micromachined angular rate sensors

Basic idea

A vibrating element at their core (the moving body)

In a fixed frame of reference, a point on this element oscillates with a velocity vector v

If the frame of reference begins to rotate at a rate Ω, this point is then subject to a Coriolis force and a corresponding acceleration

The vector cross operation implies that the Coriolis acceleration and the resulting displacement at that point are perpendicular to the oscillation



micromachined angular rate sensors

Main Specifications:

Full-scale range (expressed in º/s or º/hr)

sensitivity [V/(º/s)]

Noise, also known as angle random walk[º/(s⋅ (Hz)1/2)]

Bandwidth (Hz)

Resolution (º/s)

Bias (output) drift (expressed in º/s or º/hr)

As is the case for most sensors, angular-rate sensors must withstand shocks



Angular-Rate Sensor from Benz

It is a strict implementation of a tuning

The tines of the silicon tuning fork vibrate out of the plane of the die, driven by a thin-film piezoelectric actuator on top of one of the tines

The Coriolis forces on the tines produce a torque around the stem of the tuning fork, giving rise to shear stresses that can be sensed with piezoresistive elements

The shear stress is maximal on the center line of the stem and corresponds with the optimal location for the piezoresistive sense elements



Angular-Rate Sensor from Benz

The measured frequency of the primary mode (excitation mode) is 32.2 kHz, whereas the torsional secondary mode (sense mode) was 245 Hz lower


Documents

Introduction to VLSI - scholar.cu.edu.eg Introduction to VLSI ITI Ismailia Si/SiO 2 interface Easily grown thermally on Silicon Block dopants diffusion and other unwanted impurities