/
Photolithography Identify the basic steps of a photolithographic process Photolithography Identify the basic steps of a photolithographic process

Photolithography Identify the basic steps of a photolithographic process - PowerPoint Presentation

kittie-lecroy
kittie-lecroy . @kittie-lecroy
Follow
369 views
Uploaded On 2018-10-29

Photolithography Identify the basic steps of a photolithographic process - PPT Presentation

Describe the differences between positive and negative photoresist Explain why photolithography requires a clean environment Classify cleanrooms using both ISO and US FED standards Describe the process of a ID: 701877

mask resist dose printing resist mask printing dose pattern positive photoresist resolution light negative projection wafer clean exposure particles

Share:

Link:

Embed:

Download Presentation from below link

Download Presentation The PPT/PDF document "Photolithography Identify the basic step..." is the property of its rightful owner. Permission is granted to download and print the materials on this web site for personal, non-commercial use only, and to display it on your personal computer provided you do not modify the materials and that you retain all copyright notices contained in the materials. By downloading content from our website, you accept the terms of this agreement.


Presentation Transcript

Slide1

Photolithography

Identify the basic steps of a photolithographic process

Describe the differences between

positive

and

negative

photoresist

Explain why photolithography requires a clean environment

Classify

cleanrooms

using both ISO and US FED standards

Describe the process of a

RCA clean

Describe the process of applying resist via

spinning

and calculate the resulting resist thicknesses

Describe, compare and contrast the exposure processes of

Contact printing

Proximity printing

Projection printing

Calculate the

resolutions

of the above processes and explain what they depend on and why

Define and calculate

depth of focus

and explain how it is related to resolution in projection printing

Define the terms

Dose

and

dose to clear

,

Light intensity

,

Resist sensitivity

, and

Contrast

Calculate sensitivity and contrast for both positive and negative resist

Explain the shape of

resist profiles

and calculate the slopes of resist layer

Explain the need for and use of

alignment marks

Explain how

photo masks

are

producedSlide2

Reminder of the photolithography steps in the μ-machining process

Silicon wafer

SiO

2

Spin on photoresist

Glass plate

Opaque region

Unexposed photoresist removed by developer

Mask

Exposed

photoresist

becomes less

soluableSlide3

Reminder of the photolithography steps in the μ-machining process

Note the pattern is the opposite of that on the mask. This is true for

negative

photoresist.Slide4

Keeping it clean

Photolithography

can be the “bottle neck” in terms of how small you can make a MEMS structure.

Dust particles on masks behave as extra opaque regions (

lugares

oscuros) and transfer unwanted patterns.  Photolithography must be done is a very clean environment.Slide5

Clean rooms

Clean rooms

are classified based on how many particles of a certain six exist within a certain volume:

In

the

EE. UU.Class 1

 less than 1 particle > 0.5 μm/ft3Class 10  less than 10 particles > 0.5 μm/ft3Class 100

 less than 100 particles > 0.5 μm/ft3Class 1000  less than 1000 particles > 0.5 μm/ft3

Outside the US (ISO 14644-1)ISO Class 1  less than 10 particles > 0.1 μm/m3ISO Class

2  less than 100 particles > 0.1 μm/m3ISO Class 3  less than 1000 particles > 0.1 μm/m3Slide6

Comparison of cleanroom standards

Slide7

Comparison of cleanroom standards

Equivalency of cleanroom classifications

C

n

 

= 10

N (0.1 / D)2.08ISO 14644-1US FED STD 209E

ISO 1-ISO 2-ISO 3Class 1ISO 4Class 10ISO 5Class 100ISO 6Class 1000ISO 7Class 10,000

ISO = log(US) + 3US

 = 10(ISO - 3)Slide8

Clean room etiquette and requirements

Bunny suits

required (main source of airborne dust is human skin)Not constructed near sources of pollutionFloors are conductive for electrostatic

discharge.Only certain types furniture are allowedSpecially designed paper (pens no pencils)No eating and drinkingPerfume, cologne and makeup are

discouraged.Rose-Hulman students in the clean room facility at RHIT¿Por qué parece tan amalliro

?Slide9

Wafer cleaning

RCA clean

Developed

by

Werner Kern

in 1965 while working at RCA Laboratories1:1:5 to 1:1:7 by volume solution of NH4OH : H

2O2 : H2O is used to remove organic contaminants and heavy metalsHCl : H2O2 : H2O in a 1:1:5 to 1:2:8 volume ratio is used to remove aluminum, magnesium, and light alkali

ionsBoth steps approximately 20 minutes while gently heating to 75-85°C on a hot plateThere are other cleaning techniques, such as “piranha clean”Slide10

Photoresist

Often called “resist”

Three (3) components

:

a

base resin, which is a polymer: gives the resist structure

photoactive compound (PAC): The light-sensitive componentsolvent.Comes in two varietiesPositive resistNegative resist

Photoresist is the “stuff” of photolithographyUsually sensitive to a narrow band of ultraviolet (UV) lightSlide11

Positive versus negative resist

Positive

resist

Exposure

degrades the PAC

Becomes

more soluble to the developer after exposureUnexposed regions of the resist are left behind after developmentDeveloped resist pattern is identical to the mask pattern. Alkalis such as NaOH or KOH used as developers

Very sensitive to UV light with wavelength of 365 nm, called the I-line of the mercury spectrumNegative resist Exposure increases MW of resist or creates new insoluble products

Becomes less soluble to the developer after exposureUnexposed regions of the resist are removed after developmentDeveloped resist pattern is the opposite of the mask pattern. Organic solvents such as benzene used as developersVery sensitive to UV light with wavelength of 405

nm, called the H-line of the mercury spectrum~ 10 times more sensitive than positive resistSlide12

Positive versus negative resist

Slide13

Te

toca

a ti

The mask shown in the figure is used to transfer a pattern to a silicon wafer. Sketch the resulting pattern on the wafer after exposure and development for both positive and negative resist.. Also sketch the profile from the side of the wafer.

Positive resist

Negative resist

Mask

Side view of waferSlide14

Applying resist

Pour it onto wafer

Spin wafer to

distribute the solution across

surface

Photoresist

can also be sprayed (which may or may not get around uniformity issues)

Three steps

A

pre-bake to reduce water (water can

the reduce adhesion of resist)Spin on the resistA post-bake to remove the solventSlide15

Applying resist

T

is the film

thickness

C

is polymer concentration,η is viscosity, ω is rotational spin speed, and

K, α, β, and γ are experimentally determined constants that vary from system to system.

Coating step (proceso de cubrirlo) poses the largest danger from a dust contamination.

An empirical relation often used to predict the film thickness is given by

Spinning creates a slight vacuum along the plane of the wafer entraining airborne dust particlesSlide16

Te

toca

a ti

With a given spinner and photoresist formulation, a spin speed

of 4000

rpm gives a resist thickness of 0.7μm. How would you create a resist layer

a. 0.8 μm thick?b. 1.0 μm?Slide17

Exposure and pattern transfer

Two

basic

types of machinesContact

aligner from University of Alabama Center for Materials for Information Technology

Projection “stepper” from Cornell NanoScale Science and Technology Facility¿Por qué parece tan

amalliro?Projection printerContact alignerSlide18

Ultraviolet light spectrum

Ultraviolet light

regions

Emission

spectrum for an Hg-

Xe lamp

Photoresist made to be sensitive to ultraviolet (UV) light.Hg-Xe lamps often used as light source.Slide19

Contact and proximity printing

Contact printing

Proximity printing

Contact printing and proximity printing create features in the photoresist that more or less look just like the mask.Slide20

Projection printing

In projection printing the size of the features on the resist can be different that on the mask itself.

A generic lithographic projection systemSlide21

Projection printing

Photolithography

is

an optical

printing process

f

f

s

s'Slide22

Te

toca

a ti

A projection printing system is to reduce the size of mask features by a facture of 5. If the objective lens is kept 60 mm from the surface of the wafer, what should the focal length of the objective be?

f

f

s

s'

Respuesta

: 50 mmSlide23

Dose, dose to clear, and

arial

image

latent image

Dose

to clear

Dose required

to expose

the resist

so that it completely

develops

D

p

for positive

resist (completely develops away)

D

g

0

for negative resist (completely remains)

D

Dose

Optical energy

per

unit surface

area

D

=

It

exposure

I

Light intensity

Optical power

per

unit surface

area

[W/m

2

]Slide24

Development and post-treatment

Positive

resist

Exposure degrades the PAC

Becomes

more soluble

to the developer after exposureUnexposed regions of the resist are left behind after developmentDeveloped resist pattern is identical to the mask pattern. Alkalis such as NaOH or KOH used as developers

Negative resist Exposure increases MW of resist or creates new insoluble productsBecomes less soluble to the developer after exposureUnexposed regions of the resist are removed after developmentDeveloped

resist pattern is the opposite of the mask pattern. Organic solvents such as benzene used as developers

DeveloperSlide25

Development and post-treatment

After exposure, a mild oxygen

plasma

can be used to remove leftover exposed/unexposed resist.

A

post-bake follows, hardening resist even more.After the resist has done what it needs to do (act as a mask for doping, or for the etching of the layer below, e.g., ) resist needs to be removed completely 

strippingPositive resist Wet stripping usually usedChemical solvent such as acetone

or methylethylketone (C4H8O)Often requires T ~ 80°CCan ignite with O2

!Safety importantNegative resist Harder to removeUse of acids and/or chlorinated hydrocarbons; e.g., H2SO4

and H2O2 at 150°C (“piranha” clean)Sometimes a plasma ash required.

Plasma ashingSlide26

Resolution

Diffraction

in contact and proximity printing

https://webspace.utexas.edu/cokerwr/www/index.html/double_slit.gif

http://www.colorado.edu/physics/phys2020/phys2020_sum98/lab_manual/Lab5/Image2088.gif

http://www.colorado.edu/physics/phys2020/phys2020_sum98/lab_manual/Lab5/Image2091.gif

Constructive  dsin(θ)

= Destructive  dsin(θ) =

m·λ(m + ½ λ)Slide27

Resolution

Resolution

: smallest

distinguishable feature size of a

transferred pattern

position

intensity

Contact and proximity printing

zSlide28

Resolution

Projection printing

Want a wide lens

close to the image

planeSlide29

Resolution

Relationship of numerical aperture to other quantities

Numerical aperture

NA

=

n

sin(θmax) = D/

2F where F = f/D (effective f number)

0.16 < NA < 0.60.3 < k1

< 1.1Slide30

Resolution

Contact/proximity

◄ How

do you increase resolution

? ►

ProjectionMake s small (s0 is contact printing  short mask life)Make resist layer thin

Too thin and will not serve as good ion implantation mask, etc.develop awayMake λ smallMake numerical aperture bigBig lenses and costly optics

Other trade-offsMake λ smallSlide31

Resolution

Ultraviolet light

regions

Emission

spectrum for an Hg-

Xe lamp

¡Eso es la razón que usamos luz ultravioleta! smaller wavelengths  better resolutionSlide32

Resolution—Depth of focus

Depth of focus: the distance across which the

aerial image

is in focus.

resist

δ

aerial image

δSlide33

Resolution—Depth of focus

structural or sacrificial layer

resist

δ

Variations in

the height of surface features

must be less than the

depth of focus

.

For the best results surfaces should be planar

(flat).Slide34

Sensitivity and resist profiles

Positive resist

Negative resist

sidewalls not verticalSlide35

Sensitivity and resist profiles

Positive resist

0

50

100

Resist remaining (%)

dose,

D

(

mJ

/cm

2

)

10

D

p

100

D

p

0

(+)

Dp

Dose to clearLithographic sensitivity

ContrastHigh γp  sharp images (más

claro)Slide36

Sensitivity and resist profiles

Negative resist

0

50

100

Resist remaining (%)

dose,

D

(

mJ

/cm

2

)

10

D

gx

20

Dg

0Dg

i

For negative resist

dose to clear ≠ sensitivitysensitivity = Dg

xSlide37

Modeling of resist profiles

For positive resist:

λ

, wavelength

a

and

α are related to resist absorbanceDp, dose to cleark

2, process dependent contrast (same as in δ)k, depends on coherence of lightTo get vertical sidewalls:

Small wavelengthSmall absorbanceSmall dose to clearSmall k2 (small δ)Large NASlide38

Mask

alignment

Mask alignment also called registration

alignment mark

alignment mark

Processed wafer

Mask

Good to use asymmetry with alignment marks

Arrows (

flechas

) are

often used to make alignment marks easier to findSlide39

Mask types

Phase shift masks are used to enhance resolution

Binary mask

Phase shift mask

Alternating phase shift

Attenuating phase shift

Make thicker or thinner

Make less transparentSlide40

Mask materials

glass

High resolution emulsion (gelatin)

or

Thin (

l000-2000 Å

) metal, usually Cr Emulsion is photosensitive. Mask produced by photolithography

Glass first coated with metal. Photoresist coated on top and patterned.Metal etched through photoresist mask and resist is stripped.Slide41

Creating masks

Typically both processes use a

projection printing

machine called a

stepper

.