Lithography Instructor Abu Syed Md. Jannatul Islam 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

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

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

Basic Things 4 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. Similarly, p+ and p− designations refer to the heavily doped and lightly doped p-type regions, respectively. 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−.

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

Why Lithography? 6

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

“ writing a pattern on stone” What is Lithography ? 8 Lithography comes from two Greek words, “lithos” which means stone and “graphein” which means write. “ writing a pattern on stone” Lithography is the transfer of geometric shapes on a mask to a smooth surface 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. Typically 8-25 lithography steps and several hundred processing steps between exposure are required to fabricate a packed IC.

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

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

Photolithography Photolithography is the process 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. A light sensitive photoresist is spun onto the wafer forming a thin layer on the surface. The resist is then selectively exposed by shining light through a mask which contains the pattern information for the particular being fabricated. The resist is then developed which completes the pattern transfer from the mask to the wafer.

Photolithography is an optical means for transferring patterns 12 Photolithography is an optical means for transferring patterns onto a substrate Overview of the Photolithography Process Surface Preparation (Get rid of H2O, RCA clean, apply adhesion promoter Deposit (Photoresist Coating by Spin Casting) Soft Bake (90 – 120°C for 60 –120 sec to remove solvent from liquid photoresist Photo Mask Alignment Exposure (Pattern transfer) Development (Remove soluble photoresist) Hard Bake (100 – 180°C) to increase adhesion Etching (Remove oxide) Stripping (Photoresist removal) Post Processing/Cleaning (Ashing)

Photolithography 13 Grow Oxide Layer

Photolithography 14 Add Photoresist

Photolithography 15 Photo-Mask

UV Exposed to Photomask to transfer pattern Photolithography 16 UV Exposed to Photomask to transfer pattern

Photolithography 17 Remove Photoresist

Remove the oxide using Etching Photolithography 18 Remove the oxide using Etching

Now remove the photo resist by ashing Photolithography 19 Now remove the photo resist by ashing

ion implantation or diffusion Photolithography 20 Diffuse new region by ion implantation or diffusion

Photolithography 21

Photolithography 22

The exposed areas will become softened (for positive photoresist) 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 After the etching step(s), the photoresist is stripped away, leaving behind a permanent pattern of the photomask on the wafer surface

Photolithography 24

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

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 etc.)

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

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 photoresist

What Is a Photomask? Material Used to make Photomasks: 30 Photomasks are high precision plates containing microscopic images of electronic circuits. Material Used to make Photomasks: There are four types of material used to make photomasks; 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 Increasing Cost 35 Source λ Resolution Hg lamp(g-line) 436 nm 400 nm Hg lamp (i-line) 365 nm 350 nm KrF 248 nm 150 nm ArF 193 nm 80 nm F2 157 nm Research Increasing Cost Difficulties lie in sources, and materials for optics and masks Extreme UV, X-ray lithography reasearch topics

Wafer Exposure Systems 36

Wafer Exposure Systems 37

Contact Printing The mask is directly in contact with the wafer 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 The mask is above the wafer surface Advantages 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 islands vacuum chuck spindle developer dispenser to vacuum pump

Hard Bake Evaporate remaining photoresist Improve adhesion 42 Evaporate remaining photoresist 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 Etch oxide with hydrofluoric acid (HF) 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 ethyl ketone (MEK), CH3COC2H5 • methyl isobutyl ketone (MIBK), CH3COC4H9 Plasma etching with O2 (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 Electron gun generates beam of electrons Condenser lenses focus the e-beam 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 (4X) Optical tricks seen before all apply: off axis illumination (OAI), phase shift masks and OPC

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

X-ray Lithography Advantages: Low diffraction Shorter exposure time 52 Advantages: Low diffraction Shorter exposure time Scattering is minimum X –rays pass through spots 1nm 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

X-ray Lithography 53