Lithography 1 Instructor Abu Syed Md Jannatul Islam

Lithography 1 Instructor Abu Syed Md. Jannatul Islam Lecturer, Dept. of EEE, KUET, BD Department of Electrical and Electronic Engineering Khulna University of Engineering & Technology Khulna-9203

Basic Things 2 Very-high-purity, single-crystal silicon ingot Solid cylinder 10 cm to 30 cm in diameter and can be 1 to 2 m in length. Crystal is then sawed (like a loaf of bread) to produce circular wafers that are 400μm to 600μm thick The surface of the wafer is then polished to a mirror finish using chemical and mechanical polishing (CMP) techniques. Electrical and mechanical properties of the wafer depend on the orientation of the crystalline structure, the impurity concentrations, and the type of impurities present.

Basic Things 3 These variables are strictly controlled during crystal growth A specific amount of impurities known as doping allows the alteration of the electrical properties of the silicon, in particular its resistivity. Depending on the types of impurity, either holes (in p-type silicon) or electrons (in n-type silicon) can be responsible for electrical conduction. If a large number of impurity atoms is added, the silicon will be heavily doped (e. g. , concentration > ∼ 10^18 atoms/cm− 3). When designating the relative doping concentrations in semiconductor material, it is common to use the + and – symbols.

Basic Things 4 A heavily doped (low-resistivity) n-type silicon wafer is referred to as n+ material, while a lightly doped material (e. g. , concentration < ∼ 1016 atoms/cm− 3) is referred to as n−. Similarly, p+ and p− designations refer to the heavily doped and lightly doped p-type regions, respectively. The ability to control the type of impurities and the doping concentration in the silicon permits the formation of diodes, transistors, and resistors in integrated circuits.

Why Lithography? 5 Ø Simple layers of thin films do not make a Device.

Why Lithography? 6

History of Lithography 7 Ø It was invented in 1796 by German author and actor Alois Senefelder as a cheap method of publishing theatrical works.

What is Lithography ? 8 Lithography comes from two Greek words, “lithos” which means stone and “graphein” which means write. “ writing a pattern on stone” Ø Lithography is the transfer of geometric shapes on a mask to a smooth surface Ø It uses light or other forms of radiant energy to change the chemical properties of thin layers of films that have been coated on a substrate. Ø Typically 8 -25 lithography steps and several hundred processing steps between exposure are required to fabricate a packed IC.

What is lithography ? 9 Lithography is one of the 4 major processes in the top-down model Ø Lithography Ø Etching Ø Deposition Ø Doping In order to perform the other 3 processes, we must precisely define where to do them Lithography Does

Types of Lithography 10 Ø Ø Ø Photolithography E-beam lithography X-ray lithography. Interference lithography. Scanning Probe lithography

Photolithography 11 Photolithography is the process of transferring patterns of geometric shapes on a mask to a thin layer of photosensitive material (called photoresist) covering the surface of a semiconductor wafer. Ø A light sensitive photoresist is spun onto the wafer forming a thin layer on the surface. Ø The resist is then selectively exposed by shining light through a mask which contains the pattern information for the particular being fabricated. Ø The resist is then developed which completes the pattern transfer from the mask to the wafer.

Photolithography 12 Photolithography is an optical means for transferring patterns onto a substrate Overview of the Photolithography Process 1. Surface Preparation (Get rid of H 2 O, RCA clean, apply adhesion promoter 2. Deposit (Photoresist Coating by Spin Casting) 3. Soft Bake (90 – 120°C for 60 – 120 sec to remove solvent from liquid photoresist 4. Photo Mask Alignment 5. Exposure (Pattern transfer) 6. Development (Remove soluble photoresist) 7. Hard Bake (100 – 180°C) to increase adhesion 8. Etching (Remove oxide) 9. Stripping (Photoresist removal) 10. Post Processing/Cleaning (Ashing)

Photolithography 13 Grow Oxide Layer

Photolithography 14 Add Photoresist

Photolithography 15 Photo-Mask

Photolithography 16 V Exposed to Photomask to transfer pattern

Photolithography 17 Remove Photoresist

Photolithography 18 Remove the oxide using Etching

Photolithography 19 Now remove the photo resist by ashing

Photolithography 20 Diffuse new region by ion implantation or diffusion

Photolithography 21

Photolithography 22

Photolithography 23 The surface patterns of the various integrated-circuit components can be defined repeatedly using photolithography Here, a photographic plate with drawn patterns will be used to selectively expose the photoresist under a deep ultraviolet illumination (UV) The exposed areas will become softened (for positive photoresist) The exposed layer can then be removed using a chemical developer, causing the mask pattern to be duplicated on the wafer Silicon dioxide, silicon nitride, polysilicon, and metal layers can be selectively removed using the appropriate etching methods

Photolithography 24

Photoresist Coating 25 The wafer surface is coated with a photosensitive layer called photoresist, using a spin-on

Photoresist Coating 26

Photoresist Composition 27 The most commonly used positive resist consist of diazonaphtoquinone (DQ), which is the photoactive compound (PAC), and novolac (N), a matrix material called resin. Upon absorption of UV light, the PAC undergoes a structural transformation which is followed by reaction with water to form a base soluble carboxylic acid, which is readily soluble in basic developer (KOH, NAOH, TMAH

Types of Photoresist 28 Positive Photoresist Ø Most commonly used in the IC industry. Ø Become soluble after exposure Ø Better resolution Ø Cheaper Negative Photoresist Ø Becomes insoluble after exposure Ø When developed, the unexposed parts dissolved Ø Cheaper

Soft Bake 29 § 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. § Improves adhesion § Improves uniformity § Improves etch resistance § Improves line width control § Optimizes light absorbance characteristics of

What Is a Photomask? 30 Ø Photomasks are high precision plates microscopic images of electronic circuits. containing Material Used to make Photomasks: Ø There are four types of material used to make photomasks; Ø quartz (the most commonly used and most expensive), LE, soda lime, and white crown.

Types of Photomask 31

Different Photomasks 32 15 – 20 different mask levels are typically required for a complete IC process

Defects in Photomask 33

Photomask Aligner 34

Light Sources 35 Source λ Resolution Hg lamp(g-line) 436 nm 400 nm Hg lamp (i-line) 365 nm 350 nm Kr. F 248 nm 150 nm Ar. F 193 nm 80 nm F 2 157 nm Research Difficulties lie in sources, and materials for optics and masks Extreme UV, X-ray lithography reasearch topics Increasin g Cost

Wafer Exposure Systems 36 36

Wafer Exposure Systems 37 37

Contact Printing 38 § The mask is directly in contact with the wafer § Advantages § Simple § Low Cost § Disadvantages § Poor for small features § Mask damage may occur from contact § Defects from contaminants on mask or wafer due to contacting surfaces

Proximity Printing 39 § The mask is above the wafer surface § Advantages § Mask damage is minimal § Good registration possible § Disadvantages § Poorer resolution due to distance from the surface § Diffraction errors

Projection Printing 40 § An optical system focuses the light source and reduces the mask image for exposure on the surface § Advantages § Higher resolution § Lens system reduces diffraction error § Disadvantages § Errors due to focus of lens system may occur § Limiting factor in resolution can be due to optical system

Develop 41 § Soluble areas of photoresist are dissolved by developer chemical § Visible patterns appear on wafer § windows developer § islands dispenser vacuum chuck to vacuum pump spindle

Hard Bake 42 § Evaporate photoresist remaining § Improve adhesion § Used to stabilize and harden the developed photoresist prior to processing steps § Eliminates the solvent burst effects in vacuum processing § Introduces some stress into the photoresist. § Needed for acid etching, e. g. BOE.

Etching 43 § Etch oxide with hydrofluoric acid (HF) § Only attacks oxide where resist has been exposed

Photoresist Removal (Stripping) 44 § Want to remove the photoresist and any of its residues. – Positive photoresists: • acetone • trichloroethylene (TCE) • phenol-based strippers – Negative photoresists: • methyl ketone (MEK), CH 3 COC 2 H 5 • methyl isobutyl ketone (MIBK), CH 3 COC 4 H 9 Plasma etching with O 2 (Ashing) is also effective for removing organic polymer debris.

Performance Metrics 45 § Resolution: minimum feature dimension that can be transferred with high fidelity to a resist film. § Registration: how accurately patterns on successive masks can be aligned (or overlaid) with respect to previously defined patterns. § Throughput: number of wafers that can be exposed/unit time for a given mask level.

Limitations of Optical Lithography 46 § Resolution becoming a challenge for deep-submicron IC process requirements § Complexity of mask production and mask inspection § High cost of masks

Electron Beam Lithography 47 • Involves direct exposure of the resist by a focused electron beam without a mask • Resolution as low as 10 – 25 nm

Electron Beam Lithography 48 n Electron gun generates beam of electrons n Condenser lenses focus the e-beam n Beam-blanking plates turn beam on and off

Electron Beam Lithography 49 Advantages § Generation of submicron resist geometries § Highly automated and precisely controlled operation § Greater depth of focus than that available from optical lithography § Direct patterning on wafer without using a mask Disadvantages § Low throughput § Expensive resists § Proximity effect: backscattering of electrons irradiates adjacent regions and limits minimum spacing between features

Extreme ultraviolet : EUV 50 Next Generation Lithography § § § Vacuum operation Laser plasma source Very expensive system Uses very short 13. 4 nm light Step and scan printing All reflective optics (at this wavelength all materials absorb!) § Uses reduction optics (4 X) § Optical tricks seen before all apply: off axis illumination (OAI), phase shift masks and OPC 50

Extreme ultraviolet : EUV 51 Challenges: • EUV is strongly absorbed in all materials. • Lithography process must be performed in vacuum 51 • Mask blank must also be multilayer coated to minimize its reflection.

X-ray Lithography 52 Advantages: § Low diffraction § Shorter exposure time § Scattering is minimum § X –rays pass through spots Problems: § Masks are the most § Difficult and critical § Element of an XRL system § lacking of photoresist § 1: 1 printing § High energy x-ray destroy conventional optics 1 nm 52

X-ray Lithography 53 53