Lithography and Photoresists: Types, Processes, and Applications in Microelectronics - Pro, Study notes of Electrical and Electronics Engineering

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ECE 6450 - Dr. Alan Doolittle

Georgia Tech

Lecture 7

Lithography and Pattern Transfer

Reading:Chapter 7

ECE 6450 - Dr. Alan Doolittle

Used for Pattern transfer into oxides, metals, semiconductors. 3 types of Photoresists (PR):1.) Positive Georgia Tech

: PR pattern is same as mask. On exposure to light, light degrades the polymers (described in more detail later) resulting in the photoresist being more soluble in developers. The PR can be removed in inexpensive solvents such as acetone. 2.) Negative

: PR pattern is the inverse of the mask. On exposure to light, light polymerizes the rubbers in the photoresist to strengthen it’s resistance to dissolution in the developer. The resist has to be removed in special stripping chemicals. Theseresists tend to be extremely moisture sensitive. 3.) Combination

: Same photoresist can be used for both negative and positive pattern transfer. Can be removed in inexpensive

solvents.

Lithography and Photoresists

MaskPatternPatterntransferred to

the Photoresiston the wafer

Light Positive PR

Light Negative PR

ECE 6450 - Dr. Alan Doolittle

Georgia Tech

1.) Etching Processes: open windows in oxides for diffusion, masks for ion implantation, etching,metal contact to the semiconductor, or interconnect.2.) Lift off Processes: Metalization (more common in III-V).

Uses of Lithography:

Wafer

PhotoresistMetal, Oxide, etc…

Wafer

Metal, Oxide, etc…

Wafer

Wafer

Spin PR

Lithography

Etch Layer usingPR as Mask

Remove PR

PhotoresistWafer

Wafer

Metal Wafer

Wafer

Metal

Spin PR

Lithography

Evaporate Metal

Lift Off excessmetal with PR

ECE 6450 - Dr. Alan Doolittle

1.) Resolution Georgia Tech

: How small of features can you make. (Current production state of the art is ~

um)

2.) Registration

: Can you repeatability align one layer to another. (~1/3 of resolution or 0.06 um)

3.) Throughput

: Can these be done in a cost effective time. (50-100 wafers an hour, down to 1 chip per

hour).At this point, CMOS example will be given in class using supplemental lecture 7b:

Issues with Photolithography

ECE 6450 - Dr. Alan Doolittle

1.) Resolution: Resolution is “diffraction limited”. As patterns approach thesame order of magnitude as the wavelength of light, onemust be concerned with the wavelike nature of light. Georgia Tech

2 2 2

r

g

W^

Issues with Photolithography

Square Mask in the Near Field (Mask close to Wafer) The mask can be placed in close proximity or directly in contactwith the wafer (contact or proximity printing). We define thiscase, known as the near field or Fresnel diffraction limit, by theexpression:

gW D

W^ =∆

r

g

W D

W W+ ∆ W

Definitions used for Resolution Equations

ECE 6450 - Dr. Alan Doolittle

Georgia Tech

(Contd…) Square Mask in the Near Field (Mask close to Wafer) Effect of increasing mask-wafergap spacing

Assuming:

λ λ

(^2) W g^ <<

Then the minimum feature sizethat can be resolved is:

g k W

λ ≈min

where k is a constant, normallyclose to 1, that depends on thephotoresist and the developmentprocedures Example: For a k=1, and

λ=0.365 (I-line) 1 um

0.^

um

5 um

1.35 um

10 um

1.^

um

20 um

2.^

um

g (gap)

Wmin

ECE 6450 - Dr. Alan Doolittle

(cont’d…) Square Mask in the Far Field (Mask far away from the wafer)Georgia Tech

k^ NA W

λ (^ )^ ≈min (^

)^

(^ )^

NA

n f fn

f d R

λ λ^ α

λ α

λ^

(^61). 0 (^61). 0 sin

(^22). 1 sin 2

(^22). 1

=

=

Where NA is the numerical aperture of the focusing optics. The Numerical Aperture describesthe focusing strength of the projection system:However, all our derivation is based on a “point source” which is not ever possible, thus, we cangeneralize using a constant k (normally ~0.75) the result as:Briefly discuss immersion lithography.

ECE 6450 - Dr. Alan Doolittle

Depth of Focus: Georgia Tech

While increasing the NA will result in smaller patterns, it also effects the depth

of focus (range of lengths for which the image is in focus on the wafer).

2 NA

focus

of

Depth

Large NA results in small Depth of Focus

Small NA results in large Depth of Focus

Depth of Focus:

MetalLineWafer

Oxide

Variations in surface heights of a processed wafer must be less than the optical Depth ofFocus. Thus, for high resolution lithography the surface must be planar (flat).

MetalLineWafer

Oxide

High resolution (small depth of field) lithography can focuson point A or B but not A and B simultaneously

A

B

ECE 6450 - Dr. Alan Doolittle

Georgia Tech

min max

min max

I

I

I

I

MTF

Consider a diffraction grating instead of a single squareaperture, the Fraunhofer limited (far field) intensitypattern (non-normalized intensity in W/cm

2 ) is shown.

We can define a measure of the contrast in the arealimage (image on the wafer) by the Modulation TransferFunction, MTF is a measure of an exposure tool’s ability tomodulate the intensity of light at the wafer

surface and

decreases with decreasing diffraction grating period (moredestructive interference).

Diffraction Gratings

Light DiffractionGrating Mask IntensityImage onWafer (Arealimage)

I^ max

I^ min^

(does not have to =0)

ECE 6450 - Dr. Alan Doolittle

The MTF uses the power density (W/cm Georgia Tech

2 or (J/sec)/cm

The resist responds to the total amount of energy absorbed.Thus, we need to

define the Dose, with units of energy

density (mJ/cm

2 ), as the Intensity (or power density) times

the exposure time.

•We can also define

D= the minimum dose for^100

which the photoresist will completely dissolve whendeveloped

•We define

Das the maximum energy density for^0 which the photoresist will not dissolve at all whendeveloped

•Between these values, the photoresist will partiallydissolve. In many cases, we want very high contrast, producing sharplines. In very few cases, improving step coverage fordeposited layers, or even in an image reversal process, onemay desire moderately low contrast.

Optical Dose Supplied to PR

Photoresist Pattern after development

Position

PR Thickness

ECE 6450 - Dr. Alan Doolittle

Georgia Tech

Steppers and Scanners can have a reductionbuilt in. Thus, a 5X reduction means toproduce 0.5 um lines, the mask must have2.5 um features. Also, dirt or particles onthe mask are much smaller on the wafer.Most importantly, defects are consistentfrom exposure to exposure. Steppers caneasily incorporate lasers instead of Hg-vaporbulbs, increasing resolution dramatically.Where are we today:Pentium II was a 0.25 um technology andwas produced exclusively with excimersteppers.Current and future generations ofmicroprocessors will be 0.18, 0.15 and 0.13um technology.See Predictions from Solid StateTechnology Table I. Update: 2008 node is actually 0.065 nm Read sections 7.7, 7.8, 7.9 in your text.

Final Points