ByMadhushankar

10 Step Patterning

Previous Classes

Raw wafer preparation

Wafer Fabrication

Oxidation

Reference

Chapter 8: Microchip Fabrication Peter Van Zant 5th edition

Definition - PatterningPatterning is the series of processes that

establishes the shapes, dimensions, and placement of the required physical “parts” (components) of the IC in and on the wafer surface layers.

Patterning is also called photolithography, photomasking, masking, oxide removal (OR)), metal removal (MR), and microlithography.

At the end, surface layer is left with either a hole or an island

Patterning

It is the process that sets the surface (horizontal) dimensions on the various parts of the devices and circuits.

First is to create, in and on the wafer surface, pattern with the dimensions established in the design phase of the IC or device.referred to as the resolution

Second goal is the correct placement of the circuit pattern on the wafer.called alignment or registration

Five mask set silicon gate transistor.

IC Processing Flow

Materials

IC Design

Masks

IC Fab

Test

Packaging

Final Test

Thermal Processes

Photo-lithography

Etch PR strip

Implant PR strip

Metallization CMPDielectric deposition

Wafers

IC Fabrication

e-Beam or Photo

EDA PR Chip

Photolithography

Ion ImplantMask or Reticle Etch

EDA: Electronic Design Automation

PR: Photoresist

Photolithography

Temporarily coat photoresist on wafer

Transfers designed pattern to photoresist

Most important process in IC fabrication

40 to 50% total wafer process time

Determines the minimum feature size

Photolithography Process

Applications of Photolithography

Main application: IC patterning processOther applications: Printed electronic

board, nameplate, printer plate.

Overview of the Photo masking Process1st stepPattern on the mask is transferred into a

layer of photoresist

polymerization.

Removing the soluble portion with chemical solvents (developers) leaves a hole in the resist layer corresponds to the opaque pattern

Overview of the Photomasking Process2nd stepTransfer takes place from the photo resist layer into

the wafer surface layer transfer occurs when etchants remove the portion

of the wafer’s top layer that is not covered by the photo resist.

Concept of Holes & IslandsPrevious example - > created holes

The hole came about because the pattern in the mask was opaque to the exposing light

A mask whose pattern exists in the opaque regions is called a clear-field mask

clear-field

Concept of Holes & Islands

The pattern could also be coded in the mask in the reverse, in a dark-field mask. If the same steps were followed, the result of the

process would be an island of material left on the wafer surface

Types of photo resists1. -ve: previous example2. +ve:

Within the resists, the light changes the chemical structure from relatively non soluble to much more soluble -> “photosolubilization”

An island is produced when a light-field mask is used with a positive photoresist

Mask and photoresistpolarity results.

10 step process (clear field mask & -ve photo resist)

10 step process

2nd Step10 step process

Assignment:

List the steps and draw the cross sections for Dark field mask & +ve photo resist

Photo resist components and their roles

1. Polymers Commonly used resists are designed to

react to ultraviolet or laser sources -- optical resists

In a negative resist, the polymers change from un-polymerized to polymerized after exposure to a light or energy source (polyisopreme) - polymerization

The basic positive photoresist polymer is the phenol-formaldehyde polymer photo -solubilization

Photo resist components and their roles

2. Solvents Largest ingredient by volume in a

photoresist

Makes the resist a liquid and allows the resist to be applied to the wafer surface as a thin layer

For negative photoresist, the solvent is an aromatic type, xylene.

In positive resist, the solvent is either ethoxyethyl acetate or 2-methoxyethyl.

Photo resist components and their roles

3. Sensitizers.

Chemical sensitizers are added to the resists to cause or control certain reactions of the polymer

Sensitizers are added to either broaden the response range or narrow it to a specific wavelength.

Photo resist components and their roles

4. Additives

Various additives are mixed with resists to achieve particular results.

Some negative resists have dyes intended to absorb and control light rays in the resist film.

Positive resists may have chemical dissolution inhibitor systems.

inhibit the dissolution of non exposed portions of the resist during the development step.

Photoresist Performance Factors

Dimensions required on the wafer surface.

Function as an etch barrier during the etching step, a function that requires a certain thickness for mechanical strength.must be free of pinholes

In addition, it must adhere to the top wafer surface.

1. Resolution capability

The smallest opening or space that can be produced in a particular photoresist is generally referred to as its resolution capability Smaller the opening or space produced, the

better the resolution capability.

Thinner – more pinholesThicker – less pinholes and acts etch barrier –

Trade off

The capability of a particular resist relative to resolution and thickness is measured by its aspect ratio

1. Resolution capability

Positive resists have a higher aspect ratio as compared to negative resistsfor a given image-size opening, the resist

layer can be thicker.

The ability of positive resist to resolve a smaller opening is a result of the smaller size of the polymer.

2. Adhesion capability

A photoresist layer must adhere well to the surface layer to faithfully transfer the resist opening into the layer.Lack of adhesion results in distorted images.

Resists differ in their ability to adhere to the various surfaces used in chip fabrication.

Negative resists generally have a higher adhesion capability than positive resists

3. Photoresist exposure speed, sensitivity, and exposure source

The primary action of a photoresist is a change in structure in response to an exposing light or radiation. The faster the speed, the faster the

wafers can be processed through the masking area.

Negative resists typically require 5 to 15 sec of exposure time, whereas positive resists take three to four times longer.

Photoresist exposure speed, sensitivity, and exposure sourceThe sensitivity of a resist relates to the amount

of energy required to cause the polymerization or photo-solubilization to occur.

Common positive and negative photo-resists respond to energies in the ultraviolet and deep ultraviolet (DUV) X-rays or electron beams.

Resist sensitivity, as a parameter, is measured as the amount of energy required to initiate the basic reaction.

4. Process latitude

Goal of the overall process is a faithful reproduction of the required image size in the wafer layer(s).

Every step has an influence on the final image size, and each of the steps has inherent process variations. Some resists are more tolerant of these variations;

that is, they have a wider process latitude.

The wider the process latitude, the higher the probability that the images on the wafer will meet the required dimensional specifications.

5. PinholesPinholes are microscopically small voids in the resist

layer. allow etchants to seep through the resist layer and etch

small holes in the surface layer.

Due to particulate contamination in the environment, the spin process, and from structural voids in the resist layer. The thinner the resist layer, the more pinholes. Thicker films have fewer pinholes, but they make the less

resolution

Classic trade-offs in determining a process resist thickness.

Positive resists have got higher aspect ratioAllows a thicker resist film and a lower pinhole count for a

given image size.

6.Particle and contamination levels

Resists, like other process chemicals, must meet

stringent standards for particle content, sodium and

trace metal contaminants, and water content.

7. Step coverage

By the time the wafer is ready for the second masking process, the surface has a number of steps.

As the wafer proceeds through the fabrication process, the surface gains more layers.

For the resist to perform its etch barrier role, it must maintain an adequate thickness over these earlier layer steps.

The ability of a resist to cover surface steps with adequate resist is an important parameter.

8. Thermal flow

2 heating steps. soft bake, evaporates solvents from the

resist. hard bake, takes place after the image

has been developed in the resist layer.

The purpose of the hard bake is to increase the adhesion of the resist to the wafer surface. However, the resist, being a plastic-like material,

will soften and flow during the hard-bake step.

8. Thermal flow

The resist has to maintain its shape and structure during the bake, or the process design must account for dimensional changes due to thermal flow.

The goal is to achieve high bake temperature as possible to maximize adhesion.

This temperature is limited by the flow characteristics of the resist. In general, the more stable the thermal flow of

the resist, the better it is in the process.

Comparison of negative and positive results

Surface Preparation

Photo masking » Painting

3 stages:

Particle removal, Dehydration, and Priming.

Particle Removal - Wafer CleanRemove contaminantsRemove particulateReduce pinholes and other defectsImprove photoresist adhesionBasic steps

Chemical cleanRinseDry

Dehydration Baking

Due to exposure to moisture, either from the air or from post-cleaning rinses -> hydrophilic

To maintain hydrophobic surface Keep the room humidity below 50 percentCoat the wafers with photo resist as quickly as

possible

Additional stepsDehydration bake andPriming with a chemical

Pre-bake and Primer Vapor

P-Well

Polysilicon

Primer

Prebake

Dehydration bakeRemove moisture from wafer surface Promote adhesion between PR and surfaceUsually around 100 °CIntegration with primer coating

Primer

Promotes adhesion of PR to wafer surfaceWildly used: Hexamethyldisilazane (HMDS) HMDS vapor coating prior to PR spin coatingUsually performed in-situ with pre-bake

Photoresist Coating

P-Well

Polysilicon

Photoresist

Primer

Spin CoatingGoal - establishment of a thin, uniform,

defect-free film of photo resist on the wafer surface.

Spin Process - designed to prevent or minimize the build up of a bead of resist around the outer edge of the wafer.Edge bead

Static dispenseWafer sit on a vacuum chuckSlow spin ~ 500 rpmLiquid photo resist applied at center of

wafer Ramp up to ~ 3000 - 7000 rpmPhoto resist spread by centrifugal forceEvenly coat on wafer surface

Dynamic dispenseWafer is rotated at a low speed of ~ 500 rpm. Resist is dispensed onto the surface. The action of the rotation assists in the initial

spreading of the resist.Less resist is used, and a more uniform layer is

achieved. After spreading, the spinner is accelerated to a high

speed spread and thin the resist into a uniform film.

Moving-arm dispensingImprovement on the dynamic dispense techniqueMoving-arm resist dispenserThe arm moves in a slow motion from the center

of the wafer toward its edge.This action creates more uniform initial and final

layers. Saves resist material, especially for larger-

diameter wafers

Ready For Soft Bake

Spindle

To vacuum pump

Chuck

Wafer

Soft Bake

P-Well

Polysilicon

Photoresist

Purpose of Soft Bake

Evaporating most of solvents in PR

Solvents help to make a thin PR but absorb radiation

and affect adhesion

Soft baking time and temperature are important

parameters

Over bake: polymerized, less photo-sensitivity

Under bake: affect adhesion and exposure

Methods of Soft Bake

Conduction- Hot platesConvection

forced-air furnaces, hair dryers,air- and nitrogen fed ovens,oxidation furnaces

Radiation

Baking Systems

Heater

Vacuum

Wafer

Heater

Heated N 2

Wafers

MW Source

VacuumWafer

Photoresist

Chuck

Hot plate Convection oven Microwave oven

Alignment and ExposureOne of the basic ten patterning steps, with

two separate actions

positioning or alignment of the required image on the correct location on the wafer surface.

encoding of the image in the photo resist layer from an exposing light or other radiation source.

Alignment

P-Well

Polysilicon

Photoresist

Gate Mask

ExposureGate Mask

P-Well

Polysilicon

Photoresist

Alignment and ExposureMost critical process for IC fabrication

Most expensive tool (stepper) in an IC fab.

Most challenging technology

Determines the minimum feature size

Exposure sources

Chosen to create the required image size in conjunction with a specific photo resist

Dependable exposure source has been the high-pressure mercury lamplight in the UV range

Resists are tailored to respond to only narrow bands (lines) in the mercury lamp spectrumshorter wavelengths of the spectrum - deep

ultraviolet or DUVlasers, X-rays, and electron beams