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Photolithography Process

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Photolithography Process

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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 Photolithography1.Surface Preparation2.Photoresist Application3.Soft Bake4.Align & Expose*5.Develop6.Hard Bake7.Inspection8.Etch9.Resist Strip10.Final InspectionIntroduction to the Lithography Process

141414131313This following slides addresses Engineering Technology Content Standard 3 and 4 on Manufacturing and Materials.

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 80-90C convection oven

10Wafer Priming Adhesion promoters are used to assist resist coating. Resist adhesion factors: 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 2 O on wafer surface Wafers are given a singe step prior to priming and coating 15 minutes in 80-90C convection oven2. 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 beadThere 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: 90-100C for 20 min. in a convection oven 75-85C 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 BakePartial evaporation of photo-resist solventsImproves adhesionImproves uniformityImproves etch resistanceImproves linewidth controlOptimizes light absorbance characteristics of photoresist17171716161616154.Alignment and ExposureTransfers the mask image to the resist-coated waferActivates photo-sensitive components of photoresist.UV Light SourceMaskResistl1818181717171716

5.DevelopSoluble areas of photoresist are dissolved by developer chemicalVisible patterns appear on waferwindowsislandsto vacuum pumpvacuum chuckspindledeveloperdispenser19191918181818176.Hard BakeEvaporate remaining photoresistImprove adhesionHigher temperature than soft bakeUsed 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.

20202019191919187.Development InspectionOptical metrologyQuality issues:particlesdefectscritical dimensionslinewidth resolutionoverlay accuracy

21212120202020198.Plasma Etch-Or Add LayerSelective removal of upper layer of wafer through windows in photoresist: subtractiveTwo basic methods:wet acid etchdry plasma etchQuality measures:defects and particlesstep heightselectivitycritical dimensionsAdding materials (additive)Two main techniques:SputteringevaporationPlasmaCF422222221212121208.Plasma Etch-Or Add Layer

9.Photoresist Removal (strip)No need for photoresist following etch processTwo common methods:wet acid stripdry plasma stripFollowed by wet clean to remove remaining resist and strip byproductsO2Plasma232323222222222110.Final InspectionPhotoresist has been completely removedPattern on wafer matches mask pattern (positive resist)Quality issues:defectsparticlesstep heightcritical dimensions