Chapter #2.3 - CMOS process [en]
Summary
TLDRThis lecture explores the n-well CMOS process, a key method in integrated circuit manufacturing. It builds upon previous topics like photolithography and masking techniques, explaining how CMOS circuits, particularly nMOS and pMOS transistors, are created on silicon substrates. The process uses complementary masks to define active regions, gate formation, and metallization steps, ensuring precise alignment and connectivity. The lecture highlights the significance of CMOS technology in modern circuit design, emphasizing its efficiency and scalability, and concludes by discussing design rules and future practical applications.
Takeaways
- 🔍 The lecture focuses on the n-well CMOS process, an essential technique in integrated circuit manufacturing, particularly for creating complementary MOS (CMOS) circuits.
- 🖼️ CMOS (Complementary Metal-Oxide-Semiconductor) technology is dominant in the IC industry due to its efficiency and the ability to implement logic with both nMOS and pMOS transistors.
- ⚙️ CMOS inverters are created by pairing an nMOS and a pMOS transistor, where the pMOS is connected to the voltage supply (VDD) and the nMOS to the ground, providing complementary logic.
- 🔄 The n-well CMOS process involves starting with a p-type silicon substrate and creating n-well regions for the pMOS transistors, enabling a complementary layout for the circuit.
- 🔬 Oxide layers are crucial in CMOS fabrication, serving various purposes, including gate dielectric, transistor isolation, and mechanical support for interconnects.
- 💡 The n-well is formed through processes like photolithography, ion implantation, and etching, allowing precise definition of active regions on the substrate.
- 🎛️ In CMOS design, self-alignment of the gate oxide ensures accuracy in channel length, which defines the transistor's minimum feature size.
- 🔗 Metallic contacts and interconnects are added through successive layers of metals, using masks and photolithography to connect and route the circuit components.
- 🛡️ A passivation layer, often opaque, is applied to protect the IC from damage and reverse engineering, followed by wire bonding to connect the IC to external pins.
- 🔄 The lecture emphasizes the significance of design rules in CMOS IC design, which abstracts the manufacturing process for efficient circuit layout and simulation.
Q & A
What does CMOS stand for, and why is it important in integrated circuit design?
-CMOS stands for Complementary Metal-Oxide-Semiconductor. It is important in integrated circuit design because it uses complementary networks of n-type and p-type transistors, which offer advantages in power efficiency and scalability. CMOS is the dominant technique in the industry for building logic gates and circuits.
What is an 'n-well' in the context of the CMOS process?
-An n-well is a region within the p-type substrate where p-type transistors are formed. It provides a substrate for the p-transistors in CMOS circuits. The creation of the n-well is one of the first steps in the CMOS process.
Why is silicon widely used as the base material in CMOS processes?
-Silicon is widely used because it allows the growth of silicon dioxide, which plays multiple roles in the CMOS process, such as serving as a gate dielectric, providing isolation between transistors, and supporting interconnects. Silicon's ability to form this stable oxide makes it the ideal material for creating integrated circuits.
What role does photolithography play in the CMOS process?
-Photolithography is a crucial step in the CMOS process that allows precise patterning of the different layers of the circuit. A mask is used to expose specific areas of a photosensitive material to light, after which certain areas are etched or doped, creating the features of the circuit.
What are the differences between ion implantation and diffusion doping?
-Ion implantation is a more controlled process that allows for precise placement of dopants and creates anisotropic doping profiles. Diffusion doping, on the other hand, is isotropic and less controlled, meaning the dopants spread out in all directions, making it less suitable for precise applications in modern CMOS processes.
What is the purpose of the oxide trench created during the CMOS process?
-The oxide trench isolates active regions of the transistors, preventing unwanted electrical interactions between them. This isolation is crucial to avoid parasitic diodes and maintain the integrity of the individual transistors within the circuit.
What is a 'self-aligned' gate in CMOS technology?
-A self-aligned gate refers to the process where the gate of the transistor is formed before the source and drain doping steps. The gate itself acts as a mask, ensuring that the channel between the source and drain is perfectly aligned with the gate. This improves the precision and performance of the transistors.
How does the CMOS inverter circuit function?
-A CMOS inverter consists of an n-type MOSFET (NMOS) and a p-type MOSFET (PMOS) connected in series. When the input is '1' (high voltage), the NMOS conducts and pulls the output to '0' (low voltage). When the input is '0', the PMOS conducts, pulling the output to '1'. This complementary behavior creates the inversion effect.
Why is the gate oxide thickness critical in the CMOS process?
-The gate oxide thickness is critical because it directly affects the capacitance of the gate, which in turn influences the transistor's switching speed and power consumption. A thinner oxide layer allows for faster switching and lower power use, but it must be precisely controlled to avoid leakage and reliability issues.
What is a 'passivation layer' and why is it important?
-The passivation layer is the final protective coating applied to an integrated circuit. It protects the circuit from environmental factors such as moisture and contaminants, and prevents damage during handling. In modern chips, passivation layers are often opaque to prevent reverse engineering of the circuit.
How is metallization used to complete the CMOS process?
-Metallization involves adding conductive metal layers, such as aluminum, to connect various parts of the transistor (source, drain, gate) to each other and to external circuitry. Multiple metal layers, separated by insulating oxide layers, are often used to create complex interconnections within the chip.
What is the purpose of the 'pad mask' in the CMOS process?
-The pad mask defines the areas where metal contacts, or pads, are exposed to allow external connections between the integrated circuit and the package it is placed in. These connections are essential for integrating the chip into larger systems and making it operational in devices.
Outlines
🔬 Introduction to CMOS Process
This section introduces the CMOS process by briefly reviewing previous discussions on photolithography, epitaxial growth, doping, and etching. It explains how these processes combine to form integrated circuits on a silicon substrate, laying the groundwork for understanding the end-well CMOS process, which dominates modern integrated circuit design due to its efficiency and widespread adoption. The basic structure of a CMOS inverter is explained, highlighting the role of NMOS and PMOS transistors and the complementary design logic behind CMOS circuits.
🔄 CMOS Manufacturing Process Overview
This paragraph delves into how CMOS circuits are built using complementary masks to simultaneously process NMOS and PMOS transistors. It starts with a P-type substrate where N-wells are created to house the P-type transistors. The complementary nature of the process is highlighted, as each step is done for both NMOS and PMOS transistors using specific masks. The importance of CMOS logic's energy efficiency and its scalability for large-scale manufacturing is emphasized, setting the stage for the detailed manufacturing steps.
⚙️ Defining Active Zones in the CMOS Process
This section explains the role of the active mask in defining regions where transistors will be formed, such as the drain, channel, and source. The active regions are those where electrons or holes will flow, essentially determining where the electrical connections within the transistor occur. The summary explains how oxide layers and molecules are used to separate these regions and prevent unintended connections, preparing the substrate for the next steps in the transistor formation.
📏 Gate Formation and Alignment
Here, the script describes how the gate oxide is grown and the importance of its precise thickness, as it affects the gate's capacitance. Polysilicon is deposited to form the conductive gate, with masks defining the channel length. The term 'self-aligned gate' is introduced, explaining how the gate structure ensures proper alignment between the source and drain. This step is crucial for defining the transistor's size and performance, particularly in modern 22-nanometer transistors.
🔗 Drain, Source, and Doping Alignment
This section covers the creation of the transistor’s drain and source regions using ion implantation. The gate structure acts as a protective barrier, ensuring that the doping is precisely aligned with the channel. The 'self-aligned gate' technique ensures that the transistor’s components are well-positioned without overlap, which is crucial for reliable transistor function. The steps for doping both NMOS and PMOS transistors are discussed, as well as the role of the polysilicon gate in protecting specific regions during this process.
🔧 Interconnects and Metallization
In this part, the metallization step is explained, where metal layers are added to connect the different parts of the transistors (such as the drain, source, and bulk). A highly conductive material like titanium is used to form ohmic contacts, ensuring efficient current flow between the metal and the semiconductor. Oxide layers are added to provide isolation between the components, and photolithography is applied to remove unwanted metal, leaving only the desired connections.
🔒 Finalizing the CMOS Circuit: Passivation and Packaging
This final section explains the last steps in the CMOS manufacturing process, where additional metal layers are applied to create external connections. A passivation layer is added to protect the integrated circuit from damage and reverse engineering. The script concludes by describing how wire bonding is used to connect the metal contacts to the external pins of the circuit’s package, finalizing the CMOS inverter for use in electronic devices.
Mindmap
Keywords
💡CMOS Process
💡N-Well
💡Photolithography
💡Polysilicon Gate
💡Doping
💡Oxide Layer
💡Trench Isolation
💡Active Region
💡Self-Aligned Gate
💡Ohmic Contact
Highlights
Introduction to CMOS process and review of photolithography, epitaxial growth, doping, ionic implantation, and etching as methods to create integrated circuits on silicon substrates.
Explanation of why CMOS (Complementary Metal-Oxide-Semiconductor) is the dominant process in integrated circuit design, especially over NMOS.
Overview of the CMOS inverter circuit using NMOS and PMOS transistors, demonstrating how complementary networks of transistors implement logic and its complement.
Detailed explanation of the CMOS NAND gate, where the NMOS and PMOS transistors are arranged in series and parallel to create complementary logic.
Introduction to the N-well CMOS process, explaining the creation of an N-well in a P-substrate as a template for manufacturing CMOS circuits.
Importance of silicon oxide in the CMOS process as a dielectric material, trench isolation, and a foundation for interconnects, enhancing circuit performance and reliability.
Step-by-step breakdown of the N-well CMOS process, beginning with the P-substrate, oxide growth, and the formation of the N-well through ionic implantation.
Explanation of self-aligned gates and the precision offered by ionic implantation in aligning the channel, drain, and source regions of the transistor.
The role of the gate oxide in transistor function, where its thickness determines the capacitance and behavior of the transistor gate.
Description of how the gate and source/drain regions are doped in CMOS transistors, and the creation of contacts between metal layers and the semiconductor regions.
Details on the metallization process, including the use of titanium for ohmic contacts and multiple layers of aluminum interconnects to link different parts of the circuit.
Introduction of the passivation layer, its role in protecting the circuit, and the final pad mask step to connect the circuit with external packaging pins.
Brief overview of design rules abstraction, which will be covered in the next lecture, emphasizing how the process is linked to CAD tools like Virtuoso.
The importance of controlling the feature size of transistors in defining the performance of integrated circuits, highlighting the challenges of scaling CMOS technology.
Final remarks on how the CMOS process is well-suited for large-scale manufacturing, with a focus on power efficiency and the scalability of the technology.
Transcripts
hi so in this section we talk about
uh the end well cmos process
so last time we discussed
photolithography
and advanced technique of the mask
exposition
and the time before that we talk about
um
all the different operation that we can
apply
to the mask including
epitaxial growth so growing
something on the substrate and oxide for
example
doping and ionic implant
and etching and we have seen that
combining these three steps
step by step we can create an integrated
circuit
by connecting different layer of
semiconductors and metals and we create
a circuit
on a silicon substrate this way
in this lecture we're going to see the
details
of such a process and we will see a
specific one
the end well cmos process now before
i dive into the subject i'd like to
explain
quickly why we talk about cmos
cmos means complementary mass mass comes
from the transistor mass
and so explain why we explain this
process and why this process is actually
the the main the main one used in this
in the integrated circuit industry
and the reason is uh we will see that in
the next lecture the reason is
[Music]
that the main circuit design technique
is the cmos technique we will see that
there are other types of
circuit that we can that that was used
actually
for example nmos but now the dominating
one
is the cmos uh circuit design technique
and so the cmos
process and in particular the annual
cmos process is dominating so we will
see
uh in this lecture what that is but uh
what is a cmos circuit
so that we're on the same page we can
take the example of
the inverter for example so let's draw a
cmos inverter
it's like that so we have the ground
here
the first transistor is the nmos and the
second transistor is the pmos and the
pmos
is connected to vdd and we create the
cmos inverter by connecting the two
gates together
and we have here the output and here the
input
now the principle of a complementary
mos circuit is that you have
a network of n-transistor implementing
the logic
um and implementing uh yeah
the the network of nmos transistor
implements the logic
and you have a complementary network
of p transistor which implements the
complement of the logic
so for example with the the simple
example of the inverter
if you have the input that is connected
to a one
uh then you will activate the nmos
transistor and you will
drive a zero to the output so a one
gives a zero
and then when uh the the inputs change
from one to zero
the n-mos transistor is deactivated and
so instead of having
a high impedance output
it's the p transistor that is activated
and drives a 1.
and so this is the principle for any
gate designed with the cmos
design technique you will have a network
of transistor
of n transistor implementing the the
logic and you will have a symmetrical
network of transistor
implementing the complementary logic for
example
we can quickly draw a nand gate
cmos nand gate this way so we have two
n transistor in series
and this would be the output and then
you will have
a complementary network of p
transistor and since it's complementary
they are in parallel
so we will see the details of all that
in the next lecture
so yeah so here we have vdd and then we
connect the gates
together for example this way oops
here and so here it's a nand gate and we
can connect
two gates together for example uh this
way and then we get an
end gate so yeah so we will
talk about that in the next lecture here
the
what we are interested in is that we are
going to see how we
manufacture such circuits and so you
have to imagine that
the idea behind a complementary moss
is that we can build the two
networks of transistors using
complementary sets
of masks so we will for each
step of the way we will do the step for
the
n mos and the pmos at the same time
using complementary mask
so we start with a p substrate
here
and and we will see the first step that
we do
is that we create because inside the p
substrate we can
easily create an nmos and then inside
the p substrate we create
what's called n well
and this n well will be uh the
the substrate for the p transistor and
so uh
this is the template to design any uh
any cmos circuit we have a p substrate
an n
well and then we apply all the masks and
all the steps
in a complementary way you will see that
okay and yeah just to finish on that
subject
the the reason why we use cmos is
because of the property of cmos logic
which we will see in the next lecture
and
because the nrcmos process is well
well adapted to very large scale and now
is
i mean we know how to do that well
okay so uh let's start with the process
so we start with a p
substrate on which we can grow uh
oxides so we have already discussed
about that the
the reason we use silicon instead of
other semiconductors
is mainly the ability to grow an oxide
and the oxide will be used
you'll see in every step of the process
because we can use the oxide
as a dielectric for the gate as a
capacitance
we can use it as protective layers we
will use that a lot
we can also use the oxide as isolation
between transistors
for example between the p and the n
transistor between the n well and the p
substrate we can use a
trench isolation we call that field
oxide
and we also use the oxide as foundations
for the interconnect because the
interconnects are
heavy compared to the rest and so we can
isolate the transistors from the
interconnect and
use this isolation as a foundation to
support
the interconnect so really the oxide is
used a lot
in the cmos process
so we start with our substrate on which
we have grown
an oxide and then we apply
our p negative rp
to begin with and we apply the n-well
mask
to create as i said here uh the
substrate for the p
transistor that's the first step to do
so uh
we apply the rp we put the mask on and
then we apply
the light and since it's a negative
rp only the zone that
was exposed is not removed by the
solvent
okay so the zone that was not exposed
that was protected by the mask
is removed by the solvent and
then we can etch the rest of
oxide and we can do a ionic implant
remember that we also
could uh and we can and we did
use uh diffusion doping but diffusion
duping is isotropic
while ionic implant is anisotropic so we
use that
to create the the n-well
which gives its name to the process and
the n-well will be the substrate for the
p-transistor so here we will have
the end transistor here and the
p-transistor here
you'll see how it's all symmetrical here
next step cleaning so we remove the rp
we remove
the oxide and we start the next step
oh i forgot to mention that since we
used ionic implantation instead of
diffusion
we need a reheat to clean the surface
because ionic implantation is a bit
can damage the surface so next step
we grow another layer of oxide
to protect
this time we're going to design the
active zone so you'll see
when when you do the lab
we have to select what are the active
zone and sometimes
students are confused about what is the
active zone
and the mask active that comes that
define this zone
the active region are simply where the
transistors
are going to be positioned on the onto
the substrate
including the drain the channel and the
source
and it's uh the active region also
include the bulk and the tap to the
substrate so the connection the
the the connection to the substrate so
um
the active mask is really where um
electrons are or
holes are going to flow okay you can see
that that way or you can just see that
it's just where the transistor will be
the whole thing
so yeah so we use that for n plus and p
plus region to tap the substrates
p and n and a drain and source
and end the channel so we first
grow an oxide and then we deposit a thin
layer of
these chemicals here these molecules
because you'll see that it will you'll
see that in the next step it will help
in the next step and then we apply of
course
the rp we apply the active
mask and we apply photolithography and
you see here
those are the region all the active
region
in yellow here where we will have a
connection so for example this one here
will be the top
the bulk the connection to the p
substrate here will be the connection to
the end substrate
here we will have the p transistor and
here we will have
the n transistor also note that
the mask represented here are its
cumulative so we have here the n-wall
mask
and on top of it we have the active mask
so once we have selected this zone and
we have applied the light
here we have a positive rp so only the
regions that were not exposed
or removed by the solvent and
and then we can grow in these zones in
the zones that are
left open and protected by the rp by the
resin
we can grow an oxide which will
determine where the transistors and
which
will separate all the transistors and
the all the active zone
they will be separated and that's why we
have included this
molecules here because it will prevent
the growth
of the the oxide where it is
placed and so the result is that we have
here trench
of a big trench of oxide in between each
active zone so that the bulk is
separated from the end transistor uh
and the end transistor is separated from
the p transistor and so on
which prevents parasitic diodes and and
and such um so yeah so now we have to
remove these
molecules we have to remove the rest of
the oxide and we keep only the trench
so now we have our two substrates and we
have our
active zones that are separated by oxide
trench
and we can start to design the
transistors
the first step is to design and to draw
the gates
so the way we do that again we grow an
oxide of the silicon
oxide but this time this oxide
will be used as the gate oxide so
compared to the other steps
where the oxide was simply a protective
layer
this layer will be the the size and the
thickness
of this oxide has to be controlled
because it will
affect the capacity the capacitance of
the gate
uh because it's the gate oxide so we use
a cvd technique or the vpe technique
the same to grow this oxide precisely
and uniformly then we deposit
on top of it we deposit the polysilicon
the conductive
gate using the
poly mask and then we apply drp
and we apply the poly mask so
the poly mask is the mask
here that will define the length
of the channel or the
the length of the gate here these widths
here
represents uh the the size the feature
the minimum feature of the transistor so
when you say
when we say that we have a 22 nanometer
transistors
this 22 nanometer refer to the length of
the channel which itself
in the process refer to um
the width of this trace here
and this is this is the the most
critical
uh part of the process because it
defines the minimum
uh length of the transistor so when
uh so this is what limits actually the
size of the transistors
this step the minimum width that we can
achieve with this pulley including
diffraction and all we discussed in the
previous
lecture okay and since we are designing
an inverter don't forget that
we draw the gate for the end transistor
here
we draw the gate for the p transistor
and
we link them together so the mask
has represented here the the two gates
are linked together
and we do that using the poly layer
and so the mask represents this uh
connection between the two
transistors okay so again
rp photolithography solvent we remove
the drp and we can apply
etching and we only keep
the polysilicon gate now we only have
applied the gate and so the
we only have the polysilicon now we need
to create
the drain and the source uh using uh n
plus um doping for the p
substrate to create the end transistor
and
using p plus doping on the n well to
create the p transistor
so same step we apply an rp we apply
the mask now the interesting thing is
that uh
we use uh in this process the reason we
uh position the gate first is because
during the doping of the drain and the
source
we say that the gate is self-aligned
meaning that the the channel of the
transistor will be
uh well aligned between the drain and
the source there is no
small step in between it's really well
aligned
why because we first put
the oxide then we put the poly
layer and then we used the poly layer
and the oxide
as a protective layer for the doping so
you see for example here we have the
p plus mask and the p plus mask
covers the whole active region for
the p transistor here but the doping of
course
will because it's ionic implant it's
precise
it will not go under the
the gate because it's protected by the
gate oxide and the policy and the
polysilicon
so that's why we say that the gates are
self-aligned
only the region not protected by the
oxide
will will have diffusions that will be
used as drainage source
and so here you see we have two steps
one for
the p plus mask which is used for the p
transistors
and the p tap to the substrate
and then we do the next step the the
same step sorry
for the end transistor and the end tap
to the n1
same thing
now we have our transistors formed
we have our connection to uh the p
substrate and the nwl
for the for the bias of
of the bulk and now
we're almost done we need to connect uh
the drain and the source and
the tap to uh the outside world and
to themselves so we need to do uh
the metallization step meaning we are
going to
connect the different part together we
already have connected
the gates together using the polysilicon
now we need to connect
the two drain together for example and
we need to connect
we need to connect the bulk
so the substrate of the end transistor
to
the ground and the substrate of the p
transistor to vdd
and to connect the two drain together
so the first thing we do we apply um a
thin layer
of titanium for example a highly
conductive metal
to all the diffusion remember when we
talk about
the pn junction i said uh when you
connect
two um semicon doped semiconductor
material together you get a depletion
region and i said
when you do that with a metal and a
semiconductor you
also get a depletion region now
we called an ohmic contact
uh contact between a metal and a
semiconductor
such that the properties of the metal in
the semiconductor
allows the current to go through without
without a non-linear effect let's say
okay so it behave
as a resistor and so this is the goal of
this step we put a highly conductive
metal such that the contact between
the source and the drain and the tap
with the metal that we will put later
is good enough okay and there is no
big drop of uh of voltage and such
okay so we first do that and uh
and then we also add a sin uh here it's
represented here
a thin layer of oxide to prevent the
contact between
of course the gate and the drain and the
source
okay so those two steps are
very important and then after that
we deposit um we grow
silicon oxide on top a thick layer
it will be used to isolate the
transistors
from the rest of the interconnect and it
will also be used to support
mechanically support the interconnect
and once we have that we apply the
contact mask and we will draw holes
inside
inside the oxide to put so here are the
contact
mask you see on top of the drain the
source and the tap
and then we can make the metal
the aluminium in that case
flow inside those holes and apply on the
hole
on the whole wafer on the whole
inverter and so here everything is
connected of course we don't want to
connect
everything so we apply the metal one
mask
on top of it we apply rp
photolithography and then we etch the we
remove the metal that we don't
want such that we only keep the
connection
between the source of the end transistor
and
uh the substrate of and the pset the
contact of the p substrate
so this connection here same between
the uh p transistor and the contact to
the n well
this connection here and the connection
between the two drain of the two
transistors
this connection here and the rest you
see this connection here
are removed and then
we are going to do the same thing for
the second layer of metal and we could
do the same thing for the third and
fourth etc
and apply the mask which represents the
interconnection that represents your
circuit so we first grow
an oxide layer a thick oxide layer which
will isolate
between interconnect layer and
will support the next layer then we
apply a mask
which is a vaya between the two
interconnects
so we there is a hole
okay and then we include the second
layer
of metal the metal
again aluminium okay and then we apply
the metal to mask
to select which zone we want to keep and
which zone we want to
remove and here in our case all the
contacts
to do the inverter are done we don't
need another layer of metal to con
to do an uh interconnect so we only use
metal two
to uh connect uh the power supply
uh so basically we're done here the
inverter is finished
and the and and the transistor is
protected by
uh oxide and we have the power supply
which
are connected to the metal two now we
add
the last connection the last layer sorry
is a passivation layer it's a protective
layer
which can be transparent it used to be
transparent or can be opaque
right now we it's common to see so when
you open up an
integrated circuit you don't see we used
to see the connection the
layers of metals and etc because the
oxide is transparent as well
but now we don't see anything because
passivation are opaque
which prevents reverse engineering which
yes you can do reverse engineering with
microscope
so anyway so we applied this passivation
layer which
protects the integrated circuit
and then we apply a last mask which is
called the pad
mask which will be used to connect
in the package connect the metal to
to the pin of the package okay and so in
between we
use um for example
uh we use it's called wire bonding and
these wires
uh can be gold for example or
yeah gold usually and here so we have
our
integrated circuit inside the package
and it's ready to be shipped
so basically that's it for the annual
cmos
process i hope it was clear
in the next lecture we're going to
discuss the design
rules which
are abstraction of this process and
which we are going to use when we design
integrated circuits for example
in virtuoso in the in the lab
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