Lithography Reading Assignments: Plummer, Chap 5.1~5.4, 5.6

1

Lithography Is the Designer’s “Brush”

Lithography is indispensible for defining locations and configurations of circuit elements/functions. 2

ITRS 2007

The major challenge in litho: CD, CD control, Overlay, Defect Control, Cost. 3

Critical Layers Passivation 2, nitride Passivation 1, USG

At least the following 4

Metal 4

Tantalum barrier layer

Lead-tin alloy bump

Copper

FSG

• Active Metal 3

• Gate

FSG

Nitride etch stop layer

FSG

• Contact • M1

Copper

Metal 2 Tungsten plug

Nitride seal layer

Copper FSG

M1

Cu

Cu

Tantalum barrier layer

FSG FSG

Tungsten local Interconnection

PSG STI

T/TiN barrier & adhesion layer

Tungsten n+

n+ USG P-well P-epi P-wafer

p+ N-well

p+

PMD nitride barrier layer 4

Photo Lithography

5

Photo Lithography ¾ Photolithography is the patterning process that transfers the designed patterns from the mask to the resist on the wafer surface. It is the core of the manufacturing process flow ¾ Process Sequence: photoresist coating, alignment and exposure, photoresist developing ¾ Requirements • • • •

High resolution High sensitive Precise alignment Low defect density

High yield and good imaging

6

Photolithography Process Light Source Mask

Positive Resist

Photoresist Substrate

Resist Coating

Alignment & Exposure

Developing 7

“Positive” and “Negative” Resist Processes Photoresist Substrate UV light

Mask/reticle Photoresist Substrate Positive Photoresist

After Development

Substrate Dr. Ko, Institute of Nanotechnology, NCTU

Positive PR: from insoluble to soluble Negative PR: from soluble to insoluble

Exposure Negative Photoresist

Substrate 8

Photoresist ¾ Photoresists are the photosensitive material coated temporarily on the wafer surface and used to transfer the image of designed circuit on the mask to the wafer surface ¾ The photosensitive chemical reaction (break bonds) mainly sensitive to the UV light and not sensitive to the visible light. ¾ Since resist is not sensitive to yellow light, all the fabs use yellow to illuminate lithography areas ¾ Ingredients: polymer, solvent, sensitizer, additives

9

Photoresist Material Parameter ¾ Primary two functions of resist • Precise pattern formation • Protection of the substrate during etch

¾ Parameters can be categorized as follows: • Optical properties: resolution, photosensitivity, and index of refraction • Mechanical/chemical properties: solid content, viscosity, adhesion, etch resistance, thermal stability, and sensitivity to ambient • Processing and safety related properties: particle count, metals content, process latitude, shelf life, flash point, and threshold limit value 10

Ingredients in I-line Resist •

Polymer: Novolak (Etch mask)

•

Photoactive compound (PAC, or called sensitizer): diazonaphthoquinone (DNQ) (Control photochemical reaction during exposure)

•

Additive: phenolic materials (Modify photochemical reaction during exposure)

•

Solvent: PGMEA, EL (Liquid suspension) H3C

Source:Prof. Ko in NCTU

H

O CH2

C CH3

H

O O

C

H3C

C

CH3

O C O

OH

C2H5

11

Chemical Reaction in I-line Resist

12

Photo-Resist ¾ Positive PR • Becomes soluble after exposure • When developed, the exposed parts dissolved • Better resolution

¾ Negative PR • Exposed PR becomes crosslinked polymer • Cross-linked polymer has higher chemical etch resistance. • Becomes insoluble after exposure

⎡ ⎛E Contrast ratio : γ ≡ ⎢ln⎜⎜ T ⎣ ⎝ E1

⎞⎤ ⎟⎟⎥ ⎠⎦

−1

• When developed, the unexposed parts dissolved. • Cheaper material • Swelling lead to poor resolution

13

Positive and Negative Resists Azide/isoprene negative resist Swells during develop

Novolak resin resist No swelling during develop

Marginal step coverage

Good step coverage

Organic solvent developer

Aqueous developer

Toxic strippers

Environmentally benign resist stripper

Sensitive to ambient oxygen

Operate well in air 14

DUV Resists • g-line and i-line resists have maximum quantum efficiencies < 1 and are typically ≈ 0.3. • Chemical amplification can improve this substantially. • DUV resists all use this principle. A catalyst is used. • Photo-acid generator (PAG) is converted to an acid by photon exposure. Later, in a post exposure bake, the acid molecule reacts with a “blocking” molecule on a polymer chain, making it soluble in developer AND REGENERATING THE ACID MOLECULE. ∴ catalytic action ∴ sensitivity is enhanced. a)

Polymer Chain INSOL

Exposure

Polymer Chain

b)

INSOL

INSOL

INSOL

Acid

PAG

Post Exposure Bake (PEB) d)

Polymer Chain SOL

c)

SOL Acid

Acid

Polymer Chain SOL

INSOL

Acid

Acid

15

Wafer Exposure System 1:1 Exposure Systems

Usually 4X or 5X Reduction

Light Source

Optical System

Mask Photoresist Si Wafer Contact Printing

Gap

Proximity Printing

Projection Printing

16

Projection Printing Scan Light Source

¾ Project an image of the mask pattern onto a resist coated wafer several centimeters away from the mask. • 1:1 projection optical system is easier to design. • M:1 projection mask is easier to fabricate (5:1 most common or 10:1).

¾ Projection method • Scan • Step-and-repeat • Step-and-scan

Slit

Lens Mask

Synchronized mask and wafer movement

Lens Photoresist Wafer

Step and repeat Light Source Projection Lens Reticle Projection Lens Wafer Wafer Stage

17

I-Line Stepper

NA

Lens Resolution

Variable 0.48 to 0.60

< 280 nm

Field Size Max X & Y 22 X 27.4 mm

Overlay 2 pt. Global Alignment < 40 nm

Throughput 200 mm Wafers 70 Exp., 200 mJ/cm2 > 100 wph 18

Diffraction

Light in free space

Light through a small aperture

19

Fresnel Diffraction g

W

Incident Plane Wave

Mask Aperture

Resist

Wafer

Light Intensity at Resist Surface

W2 Fresnel diffraction (near-field diffraction) applies when λ < g < λ Within this range, the minimum resolvable feature size is Wmin ≈ λg

Thus if g = 10 µm and an i-line (365nm) light source is used, Wmin ≈ ?? 20

Shadowing Printing ¾ Contact printing • • •

Resolution ~ 1um Dust on mask will damage PR pattern. Mask pattern may be contaminated.

¾ Proximity printing • • •

A small gap of 10-50 um. Longer mask lifetime. Poorer resolution.

R = λ × gap

21

Fraunhofer Diffraction Entrance Aperture

Image Plane

α Point Sources B A

d

A' B'

R

Fraunhofer diffraction (far-field diffraction) is dominate in the projection system. According to the Rayleigh Criterion:

1.22λ f 1.22λ f 0.61 λ R= = = d n(2 f sin α ) n sin α 22

Resolution K λ R = NA

λ: wavelength, K1 : system constant=0.61 (ideal) NA = n sinα= d/2f

1

NA (numerical aperture): the capability of the lens to collect the diffraction light. NA is proportional to the lens diameter (camera with large lens) and reversely proportional to the distance between the wafer and the lens. Strayed refracted light Lens Diffracted light collected by the lens

D

Mask

ro Less diffraction after focused by the lens Ideal light Intensity pattern 23

Resolution Comparison

24

Immersion Lithography

NA of 1.35 – 1.44

* Source: Renesas 25

Depth of Focus K 2λ DOF = 2 ( N A)

NA : numerical aperture, λ : wavelength, K2 : system constant =0.5 (ideal)

Ex: Point and shot cameras use small lens without focus but the resolution won’t be great! Very flat wafer surface is needed -> CMP Lens

Center of focus DOF

Depth of focus Photoresist Substrate 26

Modulation Transfer Function (MTF) Light Source

Condenser Lens

Aperture Mask

Intensity at Mask

Objective or Projection Lens

Photoresist on Wafer

MTF =

I MAX − I MIN I MAX + I MIN

Intensity on Wafer

1

1 IMAX

I MIN 0

Position

0

Position

The MTF is a measure of the contrast in the aerial image produced by the exposure system. Usually MTF > 0.5 is necessary fir the resist to properly resolve the feature. 27

Exposure Wavelength

Intensity (a.u)

I-line (365)

G-line (436) H-line (405)

•

High pressure Hg or Hg/Se lamp

•

Lamp intensity is too low at λ < 260 nm

•

Rayleigh Equation R=k1λ/NA

Deep UV (