It gives a basic idea about masking in fabrication and lithography techniques

1. BY
Nivetha B

2.  Introduction to Masking
 Method of operation
 Parameters
 Masking types
 Exposure technique
 Application
 Introduction to Lithography
 Lithography techniques

3.  Masks are used to produce a pattern on a substrate, normally a thin slice
of silicon known as a wafer in the case of chip manufacturing.
 Several masks are used in turn, each one reproducing a layer of the
completed design, and together they are known as a mask set.
 Mask making is a fabrication process where a computer-aided design
(CAD) is transferred to a thin (80-100 nm) layer of metal in a glass or
fused silica substrate, known as mask or photomask.
 The metal works as an absorption layer for light at different wavelengths

4. Masks are generally written on either of two classes of write tools.
 Electron beam (e-beam) writers precisely direct a focused beam of
electrons onto the mask substrate while controlling the position of those
electrons through the use of an interferometer controlled stage.
 Laser/LED writers essentially perform in the same fashion but use photon
energy as opposed to charged electrons.
 The energy delivered by the mask maker to the substrate surface is
intended to react with the resist coating on the chrome film.
 The locally cross linked molecules of the resist become either sensitive or
insensitive to developers. When the develop step removes exposed resist,
the process is referred to as positive working, and when leaving behind
exposed resist, negative working.

5.  After development it moves to etch step.
 In this part of the process, the surface of the mask that has been left
uncovered by resist becomes exposed to the etching medium.
 The resists are engineered to withstand the etching process and at the very
least stand up to the etch chemistry with a removal rate that is slower than
the removal rate of the underlying chrome.
 Etching can be accomplished by using liquid (wet) or plasma (dry) etch
chemistry. After complete removal of unwanted chrome, the mask is
stripped of all remaining resists, typically in Nanostrip heated to 60 °C.

6.  Exposure time
The energy needed to expose the photoresist on the mask plate
depends on the type and the thickness of the photoresist used.
 Defocus
The best focus position for an exposure can depend on e.g., the resist
thickness, or reflectivity. This parameter can be adjusted using defocus value.
 Developer
The type of developer affects sensitivity, resolution, and development
window.
 Development time
Development time affects sensitivity, resolution, and exposure
window.

7. Based on the operation principle the masks are divided into two broad
categories:
 Conventional Binary mask
 Advanced phase-shifting mask
Binary masks
 A binary mask consists of a transparent plate called blank, covered with a
patterned film of opaque material.
 The transmission characteristic is a binary one, i.e., ``1'' for transparent and
``0'' for opaque.
 The blank is made of soda lime, borosilicate glass, or fused quartz.
 High quality photomask must meet stringent requirements in flatness,
accuracy of pattern placement, minimum feature size, line width control
over the entire mask area, and defect density.

8.  Adding a phase-shifting function to the on-off property of binary masks
yields a higher resolution at the same or even larger amount of depth of
focus.
 Both amplitude and phase are used to store the information about the image
on the mask.
 Recently enormous efforts for industrial application have been made, and
pilot production has already started.

10. Contact printing
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

11. The mask is above the wafer surface.
Advantages
 Mask damage is minimal
 Good registration possible
Disadvantages
 Poorer resolution due to distance from the surface
 Defects from contaminants on mask or wafer due to contacting surfaces
 Diffraction errors
Projection printing
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

12. Disadvantages
 Errors due to focus of lens system may occur
 Limiting factor in resolution can be due to optical system

13. Photomask are used for the fabrication of devices including but not
limited to:
 Integrated circuits and systems
 MEMS and Bio-MEMS
 Transport phenomena in materials and engineering systems
 Micro- and Nano- technology to tissue repair and regeneration
 Optics
 Photonics
 Quantum devices
 Thin film devices and solar cells

15. A lithography system undertakes a process whereby highly
complex circuit patterns drawn on a photomask made of a large glass
plate are reduced using ultra-high-performance lenses and exposed
onto a silicon substrate known as a wafer.
Techniques used in lithography
 Cleaning
 Surface Preparation
 Photoresist application
 Etching
 Photoresist removal
 Optical lithography
 Electron beam lithography
 Photoresist coaters

16.  Cleaning
If organic or inorganic contaminations are present on the wafer
surface, they are usually removed by wet chemical treatment, e.g. solutions
containing hydrogen peroxide.
Other solutions made with trichloroethylene, acetone or methanol can
also be used to clean.
 Surface preparation
The wafer is initially heated to a temperature sufficient to drive off
any moisture that may be present on the wafer surface, 150 °C for ten minutes
is sufficient. Wafers that have been in storage must be chemically cleaned to
remove contamination.
HMDS is applied to promote adhesion of the photoresist to the wafer.
The surface layer of silicon dioxide on the wafer reacts with HMDS to form
tri-methylated silicon-dioxide.
In order to ensure the development of the image, it is best covered
and placed over a hot plate and let it dry while stabilizing the temperature at
120 °C.

17.  Photoresist application
• The wafer is covered with photoresist by spin coating. A viscous, liquid
solution of photoresist is dispensed onto the wafer, and the wafer is spun
rapidly to produce a uniformly thick layer.
• The top layer of resist is quickly ejected from the wafer's edge while the
bottom layer still creeps slowly radially along the wafer leaving a very flat
layer.
• The photo resist-coated wafer is then prebaked to drive off excess
photoresist solvent, typically at 90 to 100 °C for 30 to 60 seconds on a
hotplate.
• Exposure and developing After prebaking, the photoresist is exposed to a
pattern of intense light.
• Positive photoresist, the most common type, becomes soluble in the
developer when exposed; with negative photoresist, unexposed regions are
soluble in the developer.
• The resulting wafer is then "hard-baked" if a non-chemically amplified
resist was used, typically at 120 to 180 °C for 20 to 30 minutes.

18.  Etching
• In etching, a liquid ("wet") or plasma ("dry") chemical agent removes
the uppermost layer of the substrate in the areas that are not protected by
photoresist.
• Dry etching techniques are generally used to avoid significant
undercutting of the photoresist pattern. This is essential when the width
of the features to be defined is similar to or less than the thickness of the
material being etched.
• Wet etch processes are generally isotropic in nature, which is often
indispensable for microelectromechanical systems, where suspended
structures must be "released" from the underlying layer


19.  Photoresist removal
When photoresist is no longer needed, it should be removed. This
requires a liquid "resist stripper", which chemically alters the resist so that it
no longer adheres to the substrate.
Alternatively, photoresist may be removed by a plasma
containing oxygen, which oxidizes it. This process is called ashing, and
resembles dry etching.
When the resist has been dissolved, the solvent can be removed by
heating to 80 °C without leaving any residue.

20.  Optical lithography
• Optical Lithography refers to a lithographic process that uses visible or
ultraviolet light to form patterns on the photoresist through
printing. Printing is the process of projecting the image of the patterns onto
the wafer surface using a light source and a photo mask.
• There are basically two optical exposure methods: shadow printing and
projection printing.
• In shadow printing, the mask and wafer may be in direct contact, as in
contact printing, or in close proximity.
• Contact printing yields very high resolution (~ 1 μm), but suffers from
drawback caused by dust particles or silicon causing permanent damage to
the mask and defects in the wafers.
• Proximity printing is not as prone to particle damage. However, the small
gap between the mask and wafer (typically 10 μm to 50 μm) introduces
optical diffraction at the feature edges on the photomasks and the
resolution is typically degraded to the 2 to 5 μm regime.

22.  Electron Beam lithography
Electron-beam lithography (often abbreviated as e-beam
lithography, EBL) is the practice of scanning a focused beam of electrons to
draw custom shapes on a surface covered with an electron-sensitive film
called a resist (exposing).
The electron beam changes the solubility of the resist, enabling
selective removal of either the exposed or non-exposed regions of the resist
by immersing it in a solvent
The primary advantage of electron-beam lithography is that it can
draw custom patterns (direct-write) with sub-10 nm resolution.
This form of maskless lithography has high resolution and low
throughput, limiting its usage to photomask fabrication, low-volume
production of semiconductor devices, and research and development.