2.1 Photolithograpy v - Ogrc

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II.I PHOTOLITHOGRAPHY

210/7/2015 Dr. Zeki KOCABIYIKOĞLU

PHOTOLITHOGRAPHY

Photolithography or optical lithography, literally means light-stone-writing in

Greek.

Why Photolithography is important and it is included in these lectures ?

Because, without photolithography

– TV, DVD,

– Computers/Internet,

– Cell Phones and other electronic equipment

that have become an essential part of our lives, could

not have reached the technology state of today.


PHOTOLITHOGRAPHY

Photolithography is deeply positioned in the center of digital revolution that

we are living today


Electronic technologies are, to a much extent effected by the developments in

integrated circuits and hence photolithography.

PHOTOLITHOGRAPHY

Photolithography is a process used in Manufacturing of

– Semiconductor Devices / Integrated Circuits

and

– Printed Circuit Boards

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The importance of lithography can be appreciated in two ways.

First, due to the large number of lithography steps needed in IC

manufacturing, lithography typically accounts for about 30 percent of the cost

of manufacturing.

Second, lithography tends to be the technical limiter for further advances in

feature size reduction and thus transistor speed and silicon area.


PHOTOLITHOGRAPHY


A modern integrated circuit contains millions of individual elements, typically a

few microns across.

PHOTOLITHOGRAPHY

No phyisical tool except optical techniques is

adequate for fabrication on these small scales.


The image resolution is dependent on the wavelengths of the ligth used.

PHOTOLITHOGRAPHY

The wavelength of radiation must be at least an order of magnitude smaller

than the items to be imaged.


This can be achieved by using uItraviolet light or X-rays with wavelength in the

order of 10-7 m .

An electron beam can have a wavelength in the order of 10-12 m and is capable of

achieving, very fine resolutions.

PHOTOLITHOGRAPHY


PHOTOLITHOGRAPHY

In photolithography, a patterned image is projected through a mask / reticle to

the surface of the base material which is coated with a photosensitive material

called photoresist.

Photoresist is a chemical that 'hardens' (negative resist) or ‘softens’ (positive

resist) when exposed to light.


Base material used in semiconductor or integrated circuit fabrication, is a

crystalline silicon wafer on which an insulating layer of silicon dioxide (glass) is

formed by an oxidation process at 750-1100ºC.

In the case of Printed Circuit Board (PCB) manufacturing, the base material is

copper clad boards which are (usually) :

glass reinforced epoxy resin with copper lamination on the top and if necessary

on the bottom as well.

PHOTOLITHOGRAPHY


In brief, with photolithography,

1. A layer of photoresist, is applied on top of the base material (silicone dioxide or

copper in PCB’s).

2. A mask with transparent and opaque areas printed on it (also called a photomask

/artwork mask) is placed between a source of illumination and the base material.

Photomask is produced as a result of the design process and defines :

“the shape of the desired pattern for the integrated circuit or the printed circuit

board”.

PHOTOLITHOGRAPHY


3. Base material is exposed to light through the transparent parts of the mask.

The areas exposed to radiation are changed in solubility, whereas the areas in

the mask’s shadow are unaffected.*

PHOTOLITHOGRAPHY


4. Then the photoresist is developed (washed with a solvent), in which areas of

unhardened photoresist undergo a chemical change and gets washed away.

This leaves selected areas of the underlying base material exposed.

The hardened resist material that remains on the base plate protects the

underlying base material from the later processes which are applied to the

exposed areas.

PHOTOLITHOGRAPHY


5. After development process the substrate is exposed to an etchant* which

removes the portions of the silicone dioxide (for IC’s) or copper (in PCB’s)

which are not covered by a layer of resist.

6. The final process in each layer’s photolithography is the

– removal of the remaining resist layer.

In this way the small-scale features of integrated circuits and Printed Wiring

Boards are created.

PHOTOLITHOGRAPHY


photolithography

essentials

(with +ve resist)

PHOTOLITHOGRAPHY

* Photolithograpy can only be

used on flat surfaces.


A

D

Processes of photolithography :

Photolithographic processes show some minor differences in manufacturing of

integrated circuits and printed wiring boards.

In general terms these processes are :

1. mask manufacture

2. priming of the substrate

3. application of photoresist

4. soft bake*

5. alignment and exposure

6. post exposure bake*

7. develope

8. hard bake and inspect*

9. etching

* (usually not applicable to PCB’s)

PHOTOLITHOGRAPHY


1. Mask Manufacture

Before a pattern can be lithographed onto a base plate it is necessary to produce

a mask through which exposure occurs.

Photomask is used just like negatives of photography films that capture images

which could be reproduced later.

PHOTOLITHOGRAPHY

The quality of the photomask will determine the

ultimate quality of semiconductor chips and LCD

panels.


In semiconductor / PCB manufacturing, by using the photomask, specific images

of detailed device design are transferred onto the surface of

– Semiconductor Silicon Wafers

or

– PCB’s

by means of photolithography.

PHOTOLITHOGRAPHY


The mask used in PCB manufacturing take the form of a positive or negative

photographic film of the required track layout.

The original digital files of these Track Layout Films are obtained from the “CAD

Systems” .

The masks used for printed circuit board manufacture are often full sized and

hence require no photograhic enlargement or reduction.

PHOTOLITHOGRAPHY


A Full sized mask used for integrated circuit manufacturing that require no

photograhic enlargement or reduction is called a MASK*,

whereas a mask which contains a part of the wafer is called a RETICLE**.

Reticles must be stepped and repeated accross the entire substrate.

PHOTOLITHOGRAPHY


Mask Substrates:

Commonly used substrates for mask making are

• soda-lime glass

or

• quartz

both of which are transparent to optical wavelentghs.

Quartz masks have a very similar coefficient of thermal expansion to silicon, but

are somewhat more expensive than soda-lime masks

PHOTOLITHOGRAPHY


Mask Coatings:

Coating on the surface of the mask substrate must be opaque to optical

wavelengths, and easily patterned with high definition.

The two most common mask coating materials are

• emulsion

and

• chrome

Emulsion is much cheaper; however, the coating is not of as high a quality as

chrome.

The glass substrate in this condition is called a “photomask blank”

PHOTOLITHOGRAPHY


The mask used in integrated circuit manufacturing is produced by a

photolithography process as well.

In recent years the process has been automated by using pattern generators.

This device derives the optical pattern image file (as picture elements of pixels)

from the “Computer Aided Design System”.

PHOTOLITHOGRAPHY


For mask or reticle manufacturing, photoresist is applied on top of “photomask

blank”, and it is scanned on a high precision x-y table by an electron beam of light

controlled by the pixel/position information provided by the pattern generator.

The resist must be electron beam resist not an optical beam resist.

The process is slow.

E-beam replaced optical beams because it has a shorter wavelegth and higher

exposure speed compared to UV sources.

PHOTOLITHOGRAPHY


To make a complete mask, very often we need to repeat the mask-making scan

operation several hundreds of times to make a complete wafer mask.

As the process is slow, this would be very time consuming and costly.

PHOTOLITHOGRAPHY


It is common practice in wafer production to use

– pattern generation with electron beam (e-beam) lithography for a reticle

and then

– to reproduce the pattern as an image by step and repeat processes

to form the two dimensional array of devices on the wafer.

A step-and-repeat camera is used to perform this operation.

PHOTOLITHOGRAPHY


The photomasks are directly exposed with electron beams under hard vacuum

After exposing and developing the resist, the final patterned surface is etched into

the chrome film or emulsion with a wet or dry etch process .

Dry etch is used for advanced reticle process.

Masks used for high-resolution lithography consist of opaque chromium lines on

transparent glass substrates;

PHOTOLITHOGRAPHY

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2. Priming of the substrate

This involves

– wet cleaning to remove any traces of contamination,

– dehydrating to get rid of the adsorbed water,

– priming the surface to improve adhesion of photoresist.

PHOTOLITHOGRAPHY

Hot Plate Dehydration Bake and

HMDS (hexamethyldisilazane) Vapor

Prime (which acts as an adhesion

promoter) in IC fabrication


3. Application of photoresist and soft bake

There are two kinds of photosensitive resist material in use :

• Resist materials which become hard on exposure to radiation

(Negative resists).

PHOTOLITHOGRAPHY


– Resist materials which become more soluble on exposure to

radiation (positive resists).

The use of negative photoresist is more wide spread .

PHOTOLITHOGRAPHY


A good photoresist should have the following properties :

1. It should undergo a large change in solubility on exposure to radiation

2. The boundaries between exposed and unexposed areas should form vertical

walls after etching;

3. The less soluble form of the resist should be easily removable after processing.

PHOTOLITHOGRAPHY


Negative resists have a crucial weakness of poor resolution for fine lines and have

a practical resolution limit of 2-3 µm.

PHOTOLITHOGRAPHY


.

Positive resists can produce good resolutions but the etching process needs to be

carefully monitored as the etchant slowly removes the unexposed resist areas.

PHOTOLITHOGRAPHY


In the manufacturing of PCB’s the photoresists are

– in liquid form

or

– in the form of a dry film.

The liquid form is the oldest and becoming obsolete, difficult to control.

Dry film is applied to the board by removing a cover film and allowing the

photosensitive polymer to adhere directly to the board’s surface.

The second cover film may be removed later during processing.

PHOTOLITHOGRAPHY


In IC manufacturing resist is applied to the

surface using a spin-coating machine*.

The image shows a wafer of 125 mm diameter

that has been mounted on the vacuum chuck

and is ready for spinning.

The user dispenses a drop of photoresist in the

center of the wafer and the centrifugal force

(3000-6000 rpm) spreads the fluid into an even

film, eliminating excess photoresist at the edge

of the wafer in the process.

PHOTOLITHOGRAPHY

http://en.wikipedia.org/wiki/centrifugal

../../../../../../../wiki/Image:Spinner.jpg

../../../../../../../wiki/Image:Spinner.jpg


In IC photolithography application of the resist is concluded by a soft bake.

The wafer is gently heated in a convection oven and then a hotplate

– to evaporate the resist solvent

and

– to partially solidify the resist.

The photoresist coatings become

photosensitive

only after softbaking.*

PHOTOLITHOGRAPHY

Soft Bake on Vacuum Hot Plate in IC

fabrication


4. alignment and exposure

Before any resist exposure takes place it is important to ensure that the image will

be correctly aligned with the substrate.

Industrial photolithography machines use automatic pattern recognition to

achieve the alignment registration.

For complex devices special features are built into masks called alignment marks.

PHOTOLITHOGRAPHY


exposure

In photolithography there are three methods which are commonly used to

exposure photoresist layers to radiation through the desired mask pattern. These are

– direct through mask, (contact or proximity)

– projection (enlarged mask image)

– direct write

PHOTOLITHOGRAPHY


a schematic diagram of exposure methods.

PHOTOLITHOGRAPHY

direct through mask

projection


Contact and proximity lithography are the simplest methods of exposing a

photoresist through a photomask.

These direct through mask methods use a mask, which is the same size as the

devices to be fabricated.

In PCB manufacturing generally contact lithography is sufficient.

PHOTOLITHOGRAPHY


In IC fabrication, contact lithography offers

• high resolution

but

• practical problems such as mask damage make this process unusable in

most production environments.

PHOTOLITHOGRAPHY


Proximity printing

• reduces mask damage by keeping the mask a set distance above the wafer

but

• the ligth scattering impaires the resolution capability, making proximity

printing insufficient for today’s technology.

Approximately 2- to 4-micron resolution is possible with proximity printing.

PHOTOLITHOGRAPHY


The most common method of exposure in IC photolithography is projection

printing.

Projection lithography became a viable alternative to contact/proximity printing in

the mid 1970s.

In projection lithography an image of the mask is projected onto the wafer.

PHOTOLITHOGRAPHY


There are two major classes of projection lithography tools:

– scanning

and

– step-and-repeat systems

PHOTOLITHOGRAPHY


The first systems were scanning projection aligners.

Because at that time the production of high-quality quartz lenses was hardly

feasible and extremely expensive.

PHOTOLITHOGRAPHY


Scanning projection printing, employs mirrors (rather than lenses) to project a slit

of light from the mask onto the wafer as the 1:1 mask and wafer are moved

simultaneously by the slit.

As submicron feature sizes were introduced this technique became insufficient.

PHOTOLITHOGRAPHY


The advent of computer-aided lens design and improved optical materials

allowed the production of lens elements of sufficient quality to meet the

requirements of the semiconductor industry.

By the early 1980s, step and repeat cameras (steppers) using quality lenses to

project the image began to dominate.

Step-and-repeat projection printers are capable of approximately 1-micron

resolution.

PHOTOLITHOGRAPHY


In these step and repeat cameras (steppers) the uniformly illuminated mask is

imaged onto one rectangular section of the wafer at a time with a magnification M

that is 1:1 or reduction typically around 5/1.

PHOTOLITHOGRAPHY


Reduction imaging is required for higher resolutions. Reducing the image size

has the great advantage of easier fabrication of reticles.

By the early 1980s, steppers began to dominate as device designs pushed to 2 mm

and below.

PHOTOLITHOGRAPHY


However, by the early 1990s a hybrid step-and-scan approach was introduced.

A reduction lens is used to scan the image of a large exposure field onto a portion

of the wafer.

Once the scan and pattern transfer is completed, the wafer is stepped to the next

exposure field and the process is repeated.

Step-and-scan technology is the technology of choice today for below 250nm

manufacturing.

PHOTOLITHOGRAPHY


The direct write method*, which is the subject of investigation at present, bypasses

the requirement of a mask.

A design tape, as output from a CAD system, is used to control the positioning of a

light or electron beam directly onto the photoresist material.

This eliminates the need to manufacture masks.

The process, which is similar to the pattern generation process used to produce

mask masters, has the disadvantage of being very time consuming.

PHOTOLITHOGRAPHY


The major advantage of the method is that it eliminates any error sources

associated with masks.

The direct-write method of e-beam lithography finds application for fabrication of

– ASICs

or

– prototype chip designs

as well as

– masks and reticles.

PHOTOLITHOGRAPHY


In a complex integrated circuit, (for example, CMOS) a wafer will go through the

photolithographic cycle up to 50 times.

PHOTOLITHOGRAPHY

http://en.wikipedia.org/wiki/Integrated_circuit

http://en.wikipedia.org/wiki/CMOS


5. Post exposure bake

This is applicable for IC fabrication and it is a post exposure bake on hot plate at

100-110 º C

PHOTOLITHOGRAPHY


6. Develope and hard bake

There are two classes of resist stripping techniques:

In wet stripping, the soluble areas of the photoresist are dissolved by liquid

developper* chemicals and in IC fabrication a post development thermal bake* is

required

– to evoporate the remaining photoresist solvent

and

– to improve the adhesion of the resist to the wafer surface.

Wet stripping has several inherent problems. Plasma stripping has become the

standard in semiconductor processing.

PHOTOLITHOGRAPHY


7. Inspect

For IC fabrication, inspection at this stage is important as there is no further

chance to correct mistakes.

PHOTOLITHOGRAPHY


Etching

Etching processes are concerned with the removal of material from the surface of

a substrate.

The etching is performed selectively ; the areas which are not to be etched are

covered by a masking material.

The masking material can be either

– a photoresist material after exposure,

or

– a layer such as silicon oxide.

PHOTOLITHOGRAPHY


Some of the substances which need to be removed by the etching process are

as follows:

– semiconductor materials such as silicon;

– materials such as silicon oxide or nitride, aluminium metal;

– final removal of photoresist after the substrate has been

processed.

PHOTOLITHOGRAPHY


One of the biggest problems associated with chemical etching is to control the

material removal reaction.

Under-etching results in insufficient material being removed.

Over-etching is where the etchant undercuts the resist

PHOTOLITHOGRAPHY


There are two types of etching methods in common use, namely

– wet etching

– dry etch (plasma etching)

PHOTOLITHOGRAPHY


Dry etch (plasma 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

with the wafer to remove the surface material.

This is the primary etching method for submicron geometries.

PHOTOLITHOGRAPHY


Wet etch :

Wet etching is a chemical process which is commonly performed using acids.

This is the oldest etching tecnique used in photolitography since the beginning of

semiconductor industry.

It has been replaced to a large degree by dry etching.

Wet etching is done by exposing a batch of wafers to an appropriate acid bath,

either by immersion or spray.

PHOTOLITHOGRAPHY

2.1 Photolithograpy v - Ogrc

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