CMOS Processing and Well Isolation

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Questions and Answers

What is the purpose of the deposition of a conductive barrier layer in the dual Damascene process?

To increase the etch selectivity of materials

To prevent the diffusion of copper (correct)

To act as a hardmask for etching

To enhance the topography of the dielectric layer

In the dual Damascene process, which steps follow the deposition of the Cu seed layer?

CMP to stop on hardmask 4

Deposition of passivation layers

Cu electroplating to deposit a thick Cu layer (correct)

Etching of the metal layers

Which hardmask is not removed during the etching of the remaining hardmasks in the dual Damascene process?

Hardmask 3

Hardmask 2

Hardmask 4

Hardmask 5 (correct)

What is typically the last step in the fabrication of bondpads in a CMOS process?

Lithography and etch of Aluminum Signup and view all the answers

In the CMOS BEOL process, how are successive layers fabricated?

By alternating between dual and single damascene processes Signup and view all the answers

What is a primary advantage of super steep retrograde well (SSRW) technology in CMOS fabrication?

Higher carrier mobility in the channel Signup and view all the answers

In the process of gate oxide engineering for smaller CMOS technologies, what is the purpose of incorporating N2O in SiO2?

As a diffusion barrier Signup and view all the answers

What purpose does low doped drain (LDD) formation serve in CMOS technology?

To limit high local electrical fields Signup and view all the answers

What is the first step in the process of S/D junction fabrication when forming spacers?

Deposition of thin nitride layer Signup and view all the answers

Which step follows the deposition of a thin nitride layer in the spacer formation process?

Deposition of thick oxide layer Signup and view all the answers

Which ion is typically used for n-well formation in SSRW technology?

Arsenic Signup and view all the answers

What is the primary function of rapid thermal annealing (RTA) in poly-Si gate formation?

To promote dopant activation Signup and view all the answers

What characteristic of the gate oxide becomes a concern when its thickness is reduced to below 4-5 nm?

Reduced electrical isolation Signup and view all the answers

What material is primarily used for the barrier layer in the deposition process?

Titanium Signup and view all the answers

Which metallization technology is favored due to the low resistivity of the material?

Copper metallization Signup and view all the answers

What is the main advantage of the Damascene process over conventional metal patterning?

Enables the integration of more metal layers Signup and view all the answers

What is the purpose of the conductive barrier layer in the single Damascene process?

To prevent copper diffusion Signup and view all the answers

In the dual Damascene process, what is the first step after depositing hardmask 3?

Dielectric deposition for via-1 Signup and view all the answers

Why is aluminum not preferred in smaller CMOS technologies?

High resistivity causing RC delays Signup and view all the answers

What is the primary technique used for tungsten removal in the CVD process?

Chemical mechanical planarization (CMP) Signup and view all the answers

What role does the Cu seed layer play in the single Damascene process?

Facilitates electroplating Signup and view all the answers

What type of isolation is typically used in newer CMOS processes with nodes smaller than 250nm?

Shallow trench isolation (STI) Signup and view all the answers

What is the purpose of CMP in the well isolation process?

To planarize the surface Signup and view all the answers

Which of the following steps is NOT part of the well formation process?

Deposition of pad oxide Signup and view all the answers

In well engineering, what is the primary purpose of including a shallow implant to finetune Vt?

To improve transistor Vt characteristics Signup and view all the answers

Which step follows the deposition of the nitride layer during the STI process?

Coating with resist for lithography Signup and view all the answers

What does the 'RIE' acronym stand for in the context of etching processes?

Reactive Ion Etching Signup and view all the answers

What typically characterizes a 'uniformly doped well' in older CMOS technologies?

Simple doping followed by drive-in diffusion Signup and view all the answers

Which of the following is a method for STI filling?

Thermal growth of liner oxide Signup and view all the answers

What is the main purpose of silicidation in CMOS processing?

To create a high conductivity material at the source/drain and gate Signup and view all the answers

Which transition metals are commonly used in the silicidation process?

Titanium, Cobalt, Tungsten, and Nickel Signup and view all the answers

What is the role of titanium nitride (TiN) in the salicide process?

To protect cobalt from oxidation during annealing Signup and view all the answers

What occurs during the annealing step of the silicidation process?

Reaction between silicon and metal to form silicide Signup and view all the answers

What is the primary function of chemical mechanical polishing (CMP) in back-end-of-line processing?

To planarize the surface for smaller CMOS technologies Signup and view all the answers

Which step is NOT part of the contact hole formation process in CMOS?

Wet etch of the source/drain region Signup and view all the answers

What material is primarily used as a pre-metal dielectric in CMOS processing?

Phosphorus-doped oxide (PSG) Signup and view all the answers

What is the purpose of removing unreacted metal after the silicidation process?

To prevent unwanted electrical conductivity Signup and view all the answers

Study Notes

CMOS Processing

CMOS processing involves a sequence of steps, often with variations specific to fabrication companies
A typical 130nm twin-well dual damascene copper-based CMOS process is illustrated
The process starts with a doped silicon wafer
Initial steps include labeling, cleaning, and zero-level alignment mark fabrication for later lithography steps

CMOS: Well Isolation (STI)

Well isolation in newer CMOS (below 250nm nodes) uses shallow trench isolation (STI)
Older processes use LOCOS isolation
STI involves steps like oxide and nitride deposition, RIE (reactive ion etching), and STI filling (with barrier or thick oxide)
A hard mask (nitride) is used during etching of the silicon trenches
Pad oxide and nitride are then removed

CMOS: Well Formation

Real processes often involve complex well compositions and implantations for specific doping profiles (well engineering)
Well formation can sometimes occur before well insulation (e.g. LOCOS, STI)
This process involves sacrificial oxide growth, resist coating, focused ion beam lithography, and precise doping of the well
Phosphorus and Arsenic implantations are used for p-well and n-well doping, respectively

CMOS: Poly-Si Gate Formation

Gate oxide formation, typically using thermal oxidation for thin oxides
High-k gate oxides (e.g., Al2O3, HfO2) are used for 45nm CMOS and below, due to limitations in thermal oxide quality at such thin thicknesses
Poly-silicon deposition by low pressure CVD and subsequent patterning
Post-process annealing (RTA) for activated implants

CMOS: Low Doped Drain (LDD) Formation

Low-doped drain (LDD) implantations are crucial for smaller transistors, to better control the local electric field, reducing hot-carrier generation
Resist coating, focused ion beam lithography, and ion implantation processes to create LDD regions
Light or well countering doping of poly-silicon

CMOS: S/D Junction Fabrication (Spacer Formation):

Spacer formation is crucial to protect LDD implants
PVD of thin nitride + thicker oxide layers
Selective dry etch for controlled spacer thickness
PVD or evaporation with better dry etch control is ideal for precision
Spacer etching is used selectively, for uniform etching of oxide or nitride

CMOS: S/D Junction Fabrication (Implantation):

High-dose implantation for source and drain contacts
Resist coating and lithography defining the S/D implantation regions
Precise doping of poly-Si by ions, to ensure precise contact junctions

Silicidation (Salicide) Process

Silicide formation (e.g., CoSi2 formation) improves electrical conductivity of source and drain and poly gate
Metal (e.g., cobalt) deposition and annealing
Removal of unreacted metal

CMOS: Back-End-of-Line (BEOL) Processing

BEOL processing involves metallization for interconnects
Planarization is necessary for smaller spacing dimensions in later CMOS nodes Methods like reflow of PSG/BPSG, conformal dielectric and/or CMP steps are used for planarization
Multi-metal layers are commonly used to increase connectivity
Cu (copper) is commonly used for better conductivity than aluminum

CMOS: Pre-Metal Dielectric

Deposition of nitride and thick oxide layers
CMP for planarization of pre-metal dielectric layers

CMOS: Contact Hole Formation

Formation of contact holes through the use of hardmasks (e.g. SiC)
Tungsten (or similar low-resistance metal) filling of the contact holes
CMP for planarization of the metal layers

CMOS: Metallization (Damascene) Technology

Damascene process is used to create complex metal patterns
Uses multiple layers of metal
The use of Cu (copper) and new dielectric materials is typical for modern smaller node CMOS fabrication

CMOS: M1(Single Damascene)

Initial Metal layer (M1) deposition includes steps for hardmasks, etching, and resist coating
Cu plating process (physical vapor deposition and electroplating) is used as a common method

CMOS-BEOL: Dual Damascene

Dual Damascene is used for Via-1, and metal-2 fabrication
Multiple dielectric layers are created and etched
This process is often used due to better control over patterns

CMOS: Passivation and Bondpads

Passivation layers are deposited to protect the device
Bond pads are formed for external electrical connection to the chip.
Methods include resist coating, patterning, and aluminum deposition

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Description

Explore the intricate steps involved in CMOS processing, including well isolation techniques like STI and LOCOS. This quiz covers the workflows from doped silicon wafers to well formation and the various methods of fabrication used in modern CMOS technologies.