CMOS Topic 2 -manufacturing_process

EE603 – CMOS IC DESIGN

Topic 2Manufacturing Process

Faizah Amir

POLISAS

Lesson Learning Outcome

At the end of this session, you should be able to:

• explain the manufacturing process for CMOS integrated circuit.

• apply the design rules in designing CMOS layout.

Introduction Video

Stages of IC Fabrication

Silicon ingot & Wafer

Preparation of Silicon Wafer

1. Crystal Growth

2. Single Crystal Ingot

3. Crystal Trimming and Diameter Grind

4. Flat Grinding

5. Wafer Slicing

6. Edge Rounding

7. Lapping

8. Wafer Etching

9. Polishing

10. Wafer Inspection

Slurry

Polishing table

Polishing head

Polysilicon Seed crystal

Heater

Crucible

Wafer

A wafer is a thin slice of semiconductor material such as a silicon crystal, used in the fabrication of integrated circuits and other micro-devices.

Wafer is the base material for IC manufacturing process.

Diameter of wafer is typically between 4 and 12 inches (10 and 30 cm, respectively) and a thickness of at most 1 mm.

Wafers are obtained by cutting a single-crystal ingot into thin slices.

Wafer Properties Required in IC Manufacturing

Silicon wafer must be in the form of a single-crystalline, lightly doped wafer.

The surface of the wafer is doped more heavily and a single crystal epitaxial layer of the opposite type is grown over the surface.

Diameter of wafer is typically between 4 and 12 inches (10 and 30 cm, respectively) and a thickness of at most 1 mm.

Silicon Wafer

The wafer size is increasing from time to time to ensure that the die yield is high.

Fundamental of IC Process StepBasic Steps:• Oxide growth• Thermal diffusion• Ion implantation• Deposition• Etching• Planarization

Photolithography

• Photolithography is the means by which the above steps are applied to selected areas of the silicon wafer.

Silicon Wafer

OxidationDescription:• Oxidation is the process by which a layer of

silicon dioxide is grown on the surface of a silicon wafer.

Uses:• Protect the underlying material from contamination• Provide isolation between two layers.Very thin oxides (100Å to 1000Å) are grown using dry oxidation techniques. Thicker oxides (>1000Å) are grown using wet oxidation techniques

Diffusion• Diffusion is the movement of impurity atoms at the surface of

the silicon into the bulk of the silicon.

• Always in the direction from higher concentration to lower concentration. A gas containing the dopant is introduced in the tube.

• Diffusion is typically done at high temperatures: 800 to 1400°C

Diffusion

Diffusion Predeposition

• Dopant source is supplied.• Wafer is heated between 1000°C to 1200°C.• Dopant is deposited by controlling time and

temperature in the specified amount of dopant.• Impurities is deposited on the wafer surface until the

solid solubility level is achieved.

Dopant atoms

Diffusion Drive in

• No more dopant is supplied.• Drive in is done to drive the dopants on the wafer

surface into the substrate according to the specified depth by controlling temperature and time.

• Variable such as time, temperature and ambient gas is controlled to obtain the specified junction depth.

Dopant atoms driven into the wafer

Ion Implantation

Ion implantation method is used widely in large scale IC fabrication.

Dopant ion beam (boron @ phosphorus) is accelerated with high energy(10-1000eV).

Ion implantation causes crystalline damage which must be annealed.

Ion Implantation• Ion implantation is the process by which impurity ions are

accelerated to a high velocity and physically lodged into the target material.• Annealing is required to activate the impurity atoms and

repair the physical damage to the crystal lattice. This step is done at 500 to 800°C.• Ion implantation is a lower temperature process compared to

diffusion.• Can implant through surface layers, thus it is useful for field-

threshold adjustment.• Can achieve unique doping profile such as buried

concentration peak.

Ion Implantation

Deposition Deposition is the means by which various materials are deposited on the silicon wafer.

Examples:• Silicon nitride (Si3N4)• Silicon dioxide (SiO2)• Aluminum• Polysilicon

There are various ways to deposit a material on a substrate:• Chemical-vapour deposition (CVD)• Low-pressure chemical-vapour deposition (LPCVD)• Plasma-assisted chemical-vapour deposition (PECVD)• Sputter deposition

Material that is being deposited using these techniques covers the entire wafer and requires no mask.

Deposition

EtchingEtching is a process of selectively removing the unwanted material from the surface of the wafer.

When etching is performed, the etchant may remove portions or all of:• The desired material• The underlying layer• The masking layer

There are basically two types of etches:• Wet etch which uses chemicals• Dry etch which uses chemically active ionized gases/plasma.

Etching

• Wet etching Liquid chemicals such as acids, bases and solvents are used to

chemically remove wafer surface material. Certain etching agent will etch only certain material. Wet etch is generally applicable only for larger geometries

(>3µm). The disadvantage of wet etching is that it can cause undercutting

(the pattern size is not the same as the mask size). Undercutting happens because wet etching is isotropic (etching

happens in all direction).

WET ETCHING

Etching

ETCHANTS ETCHED LAYER

HIDROFLORIC ACID / NITRIC ACID SiO2

HIDROFLORIC ACID SILICON NITRATE

HIDROFLORIC ACID / NITRIC / ACETIC ALUMINIUM

NITRIC ACID + HIDROFLORIC ACID POLYSILICON

SULFURIC ACID + ASETON + TRICHLOROETERINE PHOTO RESIST

ETCHANTS

Etching

• Dry etch exposes the wafer surface to a plasma created in the gaseous state. The plasma passes through the openings in the patterned resist and interacts physically or chemically (or both) with the wafer to remove the surface material.

• Etching will happen only at the targeted area (anisotropic).

• Pattern size produced is the same as the image on mask size.

• Dry etch is generally applicable for smaller geometries (<2µm).

DRY ETCHING/PLASMA ETCHING

EtchingDIFFERENCES BETWEEN WET ETCHING AND DRY ETCHING

Wet Etching Dry Etching

1. Etching is done using liquid chemical.

Etching is done using plasma.

2. Inexpensive. Highly cost.

3. Etching rate is not uniform. Etching rate is uniform.

4. Undercutting will happen that cause the pattern size is larger than the size of image on the mask.

The pattern size is the same as the size of image on the mask.

5. If phosphoric acid is used to etch certain material, the photoresist is also being etched.

Photoresist lifting will not occur.

6. Isotropic Anisotropic

Etching

PlanarizationPlanarization attempts to minimize the variation in surface height of the wafer.

Planarization techniques• Repeated applications of spin-on-glass (SOG).• Resist etch-back – highest areas of oxide are exposed longest to the etchant and therefore erode away the most.

A chemical-mechanical planarization (CMP) step is included before the deposition of an extra metal layer on top of the insulating SiO2 layer.

PlanarizationChemical Mechanical Polishing (CMP)

• CMP produces the required degree of planarization for modern submicron technology.

Manufacturing Process Sequence of N-Dual- Well

CMOS Circuit

Manufacturing Process Sequence of N-Dual- Well CMOS Circuit

1.

Active area

p+ substrate

p - epi


2.

n+ dopants

p+ dopants

p+ substrate

p+ substrate

p - epi

p - epi


3.

p+ substrate

p - epi

p n


4.

p-epi

p+ substrate

Contact


5.

Chip Manufacturing Video

Design Rules Design rule is a set of regulations which

define the acceptable dimensions and electrical characteristics achievable in a fabrication process.

3D Perspective of NMOS Transistor

Design Rules

• Interface between designer and process engineer

• Guidelines for constructing process masks

• Unit dimension: Minimum line width

» scalable design rules: lambda () parameter

» absolute dimensions (micron rules)

Design Rules

Lambda (λ ) parameter can define design rules in terms of lambdas.

can easily be scaled to different fabrication process as fabrication technology advances.

1λ = 1/2 minimum feature size,

e.g, in 0. 6μm process, 1λ=0.3μm

Design Rules Micron rules 1 micron = 1µm = 1x10-6 m

Technology size @ feature size is mentioned in micron unit, e.g. feature size of 0.5 micron implies

that the distance between the source and drain is 0.5µm.

* A single strand of hair usually has a diameter of 20 to 180µm.

CMOS Process Layer

Intra -Layer Design Rules

5

5

2

3 3 3

A few common design rules:

Verifying The Layout

• The layout must be verified to ensure that none of the design rules is violated.

• Failing to obey the design rules will surely lead to a non-functional design.

• Verifying the layout is now done by computers using Computer-aided Design-Rule Checking (called DRC).

SUMMARY

• The manufacturing process of integrated circuits require a large number of steps each of which consists of a sequence of basic operations.

• The design rules set define the constraints in terms of minimum width and separation that the IC design has to adhere to if the resulting circuit is to be fully functional. This design rules acts as the contract between the circuit designer and the process engineer.

Engineering

CMOS Topic 2 -manufacturing_process