Lithography

PREPARED BY K.S.VAGHOSI M.E.PART II ( PRODUCTION ) ROLL NO 139

GUIDED BY PROF. G.D.KARADKAR

MECH. DEPTT.

LITHOGRAPHY

The regions of IC is fabricated by a sequence of steps, each step adding an another layer to selected areas of the surface. The form of the each layer is determined by a geometric pattern representing circuit design information that is transferred to the wafer surface by a procedure known as LITHOGRAPHY.

LITHOGRAPHY

1. 2. 3. 4.

Lithography process is classified as under Photo lithography Electron lithography x-ray lithography ion lithography

PHOTO LITHOGRAPHY

Photo lithography is also known as optical lithography, uses light radiation to expose a coating of photo resist on the surface of the silicon wafer, a mask containing the required geometric pattern for each layer separate the light sourced from the wafer, so that only the portions of the photo resist not blocked by the mask are exposed.

MASK

The MASK consists of a flat plate of transparent glass onto which a thin film of an opaque substance has been deposited in certain areas of desired patterns. Thickness of the glass plate is about 2 mm. while the deposited film is only few microns thick. The itself is fabricated by lithography, the pattern being based on circuit design data, usually in the form of digital output from the CAD system used by the circuit designer

PHOTO RESIST

A photo resist is an organic polymer that is sensitive to light radiation in a certain range; the sensitivity causes either an increase or decrease in the polymer to certain chemicals. Typical practice in the semiconductor processing is to use the photo resists that are sensitive to ultraviolet light. UV light has a short wavelength compared to visible light, permitting sharper imaging of microscopic circuit details on the wafer surface. It also permits the fabrication and photo resists areas in the plant to be illuminated at low levels outside the UV band.

PHOTO RESIST
1. 2. 3.

4.

The performance of the photo resist is characterized by following measures Adhesion to the wafer surface Etch resistance how much the resists itself stands up to the etchant Resolution a term used to describe the minimum feature width and spacing that can be transferred from the mask to the wafer surface. Photosensitivity a measure of the response to increasing light intensities.

TYPES OF PHOTO RESIST

Two types of photo resists are available 1. Positive 2. Negative A positive resists becomes more soluble in developing solutions after exposure to light. A negative resist becomes less soluble ( the polymer cross links and hardens ) when exposed to light. Figure -1 illustrates the operation of both resist type.

Resist Tone
Negative: Positive: Prints a pattern that is opposite of the pattern that is on the mask. Prints a pattern that is the same as the pattern on the mask.

Resist Tone
Areas exposed to light become polymerized and resist the develop chemical.
Ultraviolet Light Chrome island on glass mask Island Exposed area of photoresist
photoresist

Window

Shadow on photoresist
photoresist oxide silicon substrate

oxide

silicon substrate

Negative Lithography

Resulting pattern after the resist is developed.

Resist Tone
Ultraviolet Light

Areas exposed to light become photosoluble.

Chrome island on glass mask

Shadow on photoresist

Island

Window
photoresist

Exposed area of photoresist

photoresist oxide silicon substrate

oxide

silicon substrate

Positive Lithography

Resulting pattern after the resist is developed.

EXPOSURE TECHNIQUE

The resists are expose through the mask by one of three exposure techniques : 1. Contact printing 2. Proximity printing 3. Projection printing

CONTACT PRINTING

In contact printing, the mask is pressed against the resist coating during exposure. This results in high resolution of the pattern onto the wafer surface. An important disadvantage is that physical contact with the wafer gradually wears out the mask.

PROXIMITY PRINTING

In proximity printing, the mask is separated from the resist coating by a distance of 10 to 25 microns. This eliminates mask wear but resolution of the image is slightly reduced.

PROJECTION PRINTING

In projection printing the use of high quality lens system to project the image through the mask onto the wafer. This has become the preferred technique because it is non contact and the mask pattern can be reduced through optical projection to obtain high resolution.

PROCESSING SEQUENCE IN LITHOGRAPHY

REQUIREMENT OF A LITHOGRAPHY SYSTEM

(1) Small dimensions (line width) (2) Small variations in dimensions (line width control) (3) Large depth of focus (tolerance of non-planar substrates and thick resists) (4) Accurate alignment of subsequent patterns (registration) (5) Low distortion of image and sample (high quality masks, projection systems) (6) Low cost (high throughput) (7) High reliability (high yield) (8) Tolerance of contamination particles on mask and sample (clean room requirements) (9) Uniformity over large areas (large wafers)

LIMITATIONS OF OPTICAL LITHOGRAPHY

Minimum feature size = k/NA where k = proportionality factor (typically 0.5 for diffraction limited systems) = wavelength NA = numerical aperture = sin (2 = acceptance angle of lens at point of focus) measure of light gathering power of lens
However, depth of focus = /(NA)2 important because wafers are not flat Increasing NA is not the answer reduce to reduce feature size

OTHER LITHOGRAPHY TECHNIQUES

EXTREME ULTRAVIOLET LITHOGRAPHY

Extreme ultraviolet lithography represents a refinement of current UV lithography through the use of shorter wavelength during exposure. The ultraviolet wavelength spectrum ranges from about 10 nanometer to 380 nm, the upper end of which is near the visible light range approximately 400 to 700 nm wavelength. EUV technology permits the feature size of an integrated circuit to be reduced to at least 0.03 m, compared to about 0.1 m with conventional UV exposure.

b) Extreme Ultraviolet Lithography

Small wavelengh Better resolution No lenses: mirrors Laser plasma sources 10 nm

ELECTRON BEAM LITHOGRAPHY

Electron beam lithography has the advantage of shorter wavelength compared to UV photography thus virtually eliminating diffraction during exposure of the resist and permitting higher resolution of the image. Another potential advantage is that a scanning E-beam can be directed to expose only certain regions of the wafer surface, thus eliminating need of a mask. Unfortunately, high quality electron beam systems are expensive. Also due to the time consuming sequential nature of the exposure method production rate are low compared to mask technique of optical lithography. Accordingly use of E- lithography tends to be limited to small production quantities. E beam techniques are widely used for making mask of the optical lithography.

The electron beam lithography

Types of EBL
1. 2.

Electron Beam Direct Write Electron Projection Lithography

Bragg-Fresnel lens for x-rays Paul Scherrer Institute

Electron Beam Direct Write

Types of electron guns Thermoionic Field emission
Write-field (WF)

Scanning methods Raster scan Vector scan

Specifications, a real example

Raith150

Beam size 2nm @ 20 keV Beam energy 100eV - 30 keV Minimum line width 20 nm Import file format GDSII, DXF, CIF, ASCII, BMP

Resist limitations

Tendency of the resist to swell in the developer solution. Electron scattering within the resist.

Broadens the diameter of the incident electron beam. Gives the resist unintended extra doses of electron exposure .

Applications of Electron Beam Lithography

Research
- Nanopatterning on Nanoparticles - Nanowires - Nanopillars - Gratings - Micro Ring Resonators - Nanofluidic Channels

Industrial / Commercial
- Exposure Masks for Optical Lithography - Writing features

Nanopatterning on nanoparticles

Significance
- Photonic Crystals - Quantum Dots - Waveguides

Electron Beam Lithography

- Fine writing at moderate electron energies - 37nm thick lines with 90nm periodicity - 50nm diameter dots with 140nm periodicity

Nanowires

Applications
- High-Density Electronics (Sensors, Gates in FETs) - Molecular Electronics & Medical/Biological Applications

EBL with Electrochemical size reduction

- High-Resolution Controlled Fabrication - Widths approaching 10nm regime

Patterning of Films of Gold Nanoclusters with Electron Beam Direct Write Lithography
- Sub 50nm wide Nanowires - Controlled thickness at single particle level

Controlled Fabrication of Silicon Nanowires by Electron beam lithography and Electro- chemical size reduction (2005), Robert Juhasz, Niklas Elfstrom and Jan Linnros Nanometer Scale Petterinng of Langmuir-Blodgett Films of Gold Nanoparticles by Electron Beam Lithography (2001), Martinus H.V Werts, Mathieu Lambert, Jean-Philippe Bourgoin and Mathias Brush

Nanopillars

Significance
- Quantum Confinement Effects - Photoconductive response in Nanopillar arrays

EBL and Reactive Ion Etching

- Etched Pillars with 20nm diameter

Nanotechnology using Electron Beam Lithography, Center for Quantum Devices

Gratings

Applications
- Distributed Feedback Lasers - Vertical Cavity Surface Emitting Lasers

Continuous Path Control Writing using EBL

- Avoids stitching errors

Nanotechnology using Electron Beam Lithography, Center for Quantum Devices

Micro Ring Resonators

Applciations
- Optical Telecommunication and Networks

EBL and Dry Etching

- 105 devices/cm2 density

Nanotechnology using Electron Beam Lithography, Center for Quantum Devices

Nanofluidic Channels

Significance
- Laboratory on a chip - Single Molecule Detection

Electron Beam Lithography

- Single step planar process - Tubes with inner dimension of 80nm

(2005) A single-step process for making nanofluidic channels using electron beam lithography, J. L. Pearson and D. R. S. Cumming

X RAY LITHOGRAPHY

X ray lithography has been under development since 1972. As in E-beam lithography, the wavelengths of x rays are much shorter than UV light. Thus they hold the promise of sharper imaging during exposure of the resist. X ray are difficult to focus during lithography. Consequently contact or proximity must be used, and a small X ray source must be used at a relatively large distance from the wafer surface in order to achieve good image resolution through mask.

Types of x-ray sources:

Electron Impact X-ray source Plasma heated X-ray source Laser heated E-beam heated Synchrotron X-ray source

Mask: Needs a combination of materials that are opaque (heavy element, e.g. Au) and transparent (low atomic mass membrane, e.g. BN or S3N4) to x-rays Mask written by e-beam Diffraction is not an issue (shadowing is, see next viewgraph) Masks difficult to make due to need to manage stress Dust less of a problem because they are transparent to x-rays

ION LITHOGRAPHY
Ion lithography system divide into two categories
1.

2.

Focused ion beam system Its operation is similar to a scanning E beam system and avoids the need for a mask. Masked ion beam system It expose the resist through a mask by proximity printing. As with E beam and x ray systems, ion lithography produces higher image resolution than conventional UV photolithography.

Ion source

Ions scatter much less than electrons so a higher resolution is feasible Problems:

Ion beam Mask

Ion Beam source (e.g. Gallium) Mask Beam forming Not as mature as EPL

Reference plate

Electrostatic lens system (4:1 reduction)

Step-andscan wafer stage Vacuum chamber

Thank You