PhotoLithography

• Photo-litho-graphy: latin: light-stone-writing

• Photolithography: an optical means for transferring patterns onto a substrate.

• Patterns are first transferred to an imagable photoresist layer.

• Photoresist is a liquid film that is spread out onto a substrate, exposed with a desired pattern, and developed into a selectively placed layer for subsequent processing.

• Photolithography is a binary pattern transfer: there is no gray-scale, color, nor depth to the image.

Photolithography

Ten Basic Steps of

Photolithography

1. Surface Preparation

2. Photoresist Application

3. Soft Bake

4. Align & Expose*

5. Develop

6. Hard Bake

7. Inspection

8. Etch

9. Resist Strip

10. Final Inspection

Introduction to the Lithography Process

Basics of Photolithography for Processing• Microfabrication processes:

– Additive ® deposition

– Subtractive ® etching

– Modifying ® doping, annealing, or curing

• Two primary techniques for patterning additive and subtractive processes:

– Etch-back:

• photoresist is applied overtop of the layer to be patterned

• unwanted material is etched away

– Lift-off:

• patterned layer is deposited over top of the photoresist

• unwanted material is lifted off when resist is removed

Surface Preparation:1. Wafer Cleaning

• Typical contaminants that must be removed prior to photoresist coating are:

• atmospheric dust (minimized by good clean room practice)

• abrasive particles

• lint from wipers

• photoresist residue from previous photolithography

• bacteria (minimized by good DI water system)

• films from other sources:– solvent residue

– H 2O residue

– photoresist or developer residue

– oil

– silicone

2. Wafer Priming• Adhesion promoters are used to assist resist coating.

The factors to limit adhesion are:

• moisture content on surface

• wetting characteristics of resist

• type of primer

• delay in exposure and prebake

• resist chemistry

• surface smoothness

• stress from coating process

• surface contamination

Ideally want no H 2O on wafer surface

– Wafers are given a “singe” step prior to priming and coating 15 minutes in 80-90°C convection oven

2. Photoresist Spin Coating• Wafer is held on a spinner chuck by vacuum and resist is coated to uniform thickness by spin coating.

• Typically 3000-6000 rpm for 15-30 seconds.

• Resist thickness is set by:

– primarily resist viscosity

– secondarily spinner rotational speed

• Resist thickness is given by t = kp 2 /w 1/2 , where

– k = spinner constant, typically 80-100

– p = resist solids content in percent

– w = spinner rotational speed in rpm/1000

• Most resist thicknesses are 1-2 mm for commercial Si processes.

Spin Coater

Spinning Artifact• Edge Bead

– residual ridge in resist at edge of wafer

– can be up to 20-30 times the nominal thickness of the resist

– radius of wafer greatly reduces the edge bead height

– non-circular wafers greatly increase the edge bead height

– edge bead removers are solvents that are spun on after resist coating and which partially dissolve away the edge bead

There are two types of photoresist: 1. Positive Photresist2. Negative Photoresist

Positive Photoresist: For positive resists, the resist is exposed with UV light wherever the underlying material is to be removed. In these resists, exposure to the UV light changes the chemical structure of the resist so that it becomes more soluble in the developer. The exposed resist is then washed away by the developer solution, leaving windows of the bare underlying material. In other words, "whatever shows, goes." The mask, therefore, contains an exact copy of the pattern which is to remain on the wafer.

Negative Photoresist:

Negative resists behave in just the opposite manner. Exposure to the UV light causes the negative resist to become polymerized, and more difficult to dissolve. Therefore, the negative resist remains on the surface wherever it is exposed, and the developer solution removes only the unexposed portions. Masks used for negative photoresists, therefore, contain the inverse (or photographic "negative") of the pattern to be transferred

Types of photoresists:

● Positive resists:

There are two kinds of positive resists:(1) the PMMA (polymethymethacrylate) resists,(2) the two-component DQN resist involving diazoquinone ester (DQ) andphenolic novolak resin (N).In the latter kind, the first component takes about 20-50% by weight in thecompound.

Positive resists are sensitive to UV lights with the maximum sensitivity at awavelength of 220 nm.

The PMMA resists are also used in photolithography involving electron beam,ion beam and x-ray. Most positive resists can be developed in alkaline solventssuch as KOH (potassium peroxide), TMAH (tetramethylammonium hydroxide),ketones or acetates.

● Negative resists:(1) Two-component bis (aryl) azide rubber resists, and(2) Kodak KTFR (azide-sensitized polyisotroprene rubber).Negative resists are less sensitive to optical and x-ray exposures but more sensitive to electron beams.

Xylene is the most commonly used solvent for developing negative resists.

In general, positive resists provide more clear edge definitions than thenegative resists. So, it is a better option for high resolution patterns for micro devices.Line definitions of photoresists:

3.Prebake (Soft Bake) • Used to evaporate the coating solvent and to densify the resist after spin coating.

• Typical thermal cycles:

– 90-100°C for 20 min. in a convection oven

– 75-85°C for 45 sec. on a hot plate

• Commercially, microwave heating or IR lamps are also used in production lines.

• Hot plating the resist is usually faster, more controllable, and does not trap solvent like convection oven baking.

3. Soft Bake

• Partial evaporation of photo-resist solvents

• Improves adhesion• Improves uniformity• Improves etch resistance• Improves linewidth

control• Optimizes light

absorbance characteristics of photoresist

4. Alignment and Exposure

• Transfers the mask image to the resist-coated wafer

• Activates photo-sensitive components of photoresist.

UV Light Source

Mask

ResistResist

l

5. Develop

• Soluble areas of photoresist are dissolved by developer chemical

• Visible patterns appear on wafer– windows– islands

to vacuum pump

vacuum chuck

spindle

developerdispenser

6. Hard Bake Evaporate remaining photoresist

Improve adhesion

Higher temperature than soft bake

Used to stabilize and harden the developed photoresist prior to processing steps that the resist will mask.

Postbake removes any remaining traces of the coating solvent or developer.

• Postbake introduces some stress into the photoresist.

• Some shrinkage of the photoresist may occur.

• Longer or hotter postbake makes resist removal much more difficult.

7. Development Inspection

• Optical metrology• Quality issues:

– particles– defects– critical dimensions– linewidth resolution– overlay accuracy

8. Plasma Etch-Or Add Layer

• Selective removal of upper layer of wafer through windows in photoresist: subtractive

• Two basic methods:– wet acid etch– dry plasma etch

• Quality measures:– defects and particles– step height– selectivity– critical dimensions

• Adding materials (additive)• Two main techniques:

– Sputtering– evaporation

PlasmaPlasma

CF4CF4

8. Plasma Etch-Or Add Layer

9. Photoresist Removal (strip)

• No need for photoresist following etch process

• Two common methods:– wet acid strip– dry plasma strip

• Followed by wet clean to remove remaining resist and strip byproducts

O2O2

PlasmaPlasma

10. Final Inspection

• Photoresist has been completely removed• Pattern on wafer matches mask pattern (positive resist)• Quality issues:

– defects– particles– step height– critical dimensions