2. Photomask Fabrication
A mask, or photomask, is a glass or quartz plate coated on
one side with chrome.
What is Conventional Mask??
Conventional Mask pattern generator uses step-and -repeat
and contact printing photolithography method to transfer the
circuit layout designs from CAD into the photomask for circuit
requiring minimum line width of < 1µm.
What is Modern mask??
Modern mask pattern generation systems use precision lasers
or electron beams to image the design of one layer of an
integrated circuit (IC), or chip, onto the mask especially for
circuit requiring minimum line width of <1µm. After the design
has been exposed on the mask, the pattern is etched into the
chrome, and the mask is inspected.
3. Mask-making process Flow
CIRCUIT Layout
Digitizing
Flow A Pattern
Generation (reticle)
CIRCUIT Layout
Flow B Reticle Flow C Master plate
Master Plate
processing
Contact print
substrate
Working plate
processing
Wafer exposure
(Contact, proximity, projection)
Reticle Processing
Wafer exposure
(stepper)
Mask Processing
Wafer exposure
(proximity, Projection)
Conventional Modern
Laser/ E-beam direct write
4. Conventional Photomask
Fabrication
1. Circuit Layout
2. Data Digitising
3. Photomask Coat Materials and Sizes
4. Pattern Generation
5. Step & repeat reduction into master copy
mask plate
6. Contact print working mask plate
5. Circuit Layout
In the circuit design process,
patterns which represent the
circuits are created by the chip
designer. These patterns are
then sent via magnetic media or
electronically to the mask shop
where the pattern data is
prepared for mask
manufacturing.
6. Data Digitising
Early photomasks were cut by hand in a material called
rubylith, a sandwich of a clear backing layer and a thin
red layer of Mylar. The red layer was cut with a stylus
and peeled off, leaving the desired pattern in red. The
original rubylith copy of the mask was 100 to 1000 times
larger than the final integrated circuit and was
photographically reduced to form a reticle for use in a
step-and-repeat camera. Today, computer graphics
systems and optical pattern have largely supplanted the
use of rubylith. An image of the desired mask is created
on a computer graphics system.
7. Photomask Coat Materials and
Sizes
The primary material used to make a mask is a
quartz substrate that has a layer of chrome on one
side. The chrome layer is covered with an anti-
reflective coating and a photosensitive resist. Mask
sizes range from three to nine inches square, but
most masks produced today are five or six inches
square.
8. Pattern Generation
Once the image is complete, files containing the
commands needed to drive a photolithography
pattern generator are created on magnetic tape or
disks. A pattern generator consists of a light source
and a series of motor-driven shutters. The pattern
generator uses 4 flash lamps to expose the series of
rectangles composing the mask image directly onto a
blank photographic plate called the reticle.
The chrome-covered mask or reticle coated with a
layer of photoresist is moved under the light source
as the shutters are moved and opened to allow
precisely shaped patterns of light to shine onto the
resist creating the desired pattern.
9. Pattern Generation (Continued)
The reticle or mask is processed through the
development, develop inspect, chrome etch, resist
stripping, and final inspection steps that transfer
the pattern permanently into the chrome layer as
follow:
(a) Etch and resist stripping
(b) Defect Inspection
(c) Repair
(d) Metrology Inspection
(e) Cleaning
(f) Final Inspection
10. Step & Repeat reduction
After the creation of the reticle of the circuit image,the
reticle pattern is transferred to a new resist-coated
mask blank by a step-&-repeat camera in order to
reduce the reticle images into the final size on a master
copy mask plate. After each of the exposure steps, the
reticle or mask is processed through the development,
develop inspect, chrome etch, resist striping, &final
inspection that transfer the pattern permanently into the
chrome layer of the master copy mask plate. This
process gives a master copy of the actual circuit
features.
11. Contact print working mask plate
The master copy mask plate is then used to create
multiple working mask plates in a contact printer
using photolithography. This tool brings the master
copy mask into contact with a resist-coated mask
blank and has a UV light source for transferring the
image from the master copy plate into the working
copy plate. After the exposure steps the reticle or
mask is processed through the development,
develop inspect, chrome etch, resist stripping, &
final inspection that transfer the pattern permanently
into the chrome layer of the working mask plate.
Inspections are again very critical since any
undetected mistake or defect has the potential of
creating thousands of scrap wafers.
12. Advanced Photomask Fabrication
For ULSI fabrication, the minimum features or line
widths of the devices in the circuits are reduced from
1µm to approaching 0.1 µm or lower. This requires
modern mask pattern generation systems that use
precision lasers or electron beams or writers to image
the design from CAD of an integrated circuit (IC), or
chip, onto the mask. This method is fast, direct and
requires fewer processing steps, but the image writer
systems used are much more expensive.
14. X-ray mask fabrication
Target parameter: <0.15µm minimum feature size; then the width of the
absorber pattern should be controlled within less than 10nm
Conventional method:
(a) A silicon wafer is usually used as a starting material,
(b) The silicon wafer is coated with layer of the membrane material such as
silicon nitride or silicon carbide.
(c) The back of the silicon wafer is patterned lithographically to protect the outer
ring. The membrane material at the backside of the silicon wafer is then etched
away,
(d) The silicon wafer is etched through backside in a long silicon etch that
remove most of the silicon wafer with the remaining wafer material forming a
ring beneath the membrane.
(e) The ring is then bonded onto a glass ring for additional strength and
mechanical stability.
15. X-ray Mask
Fabrication(Continued)
(f) The next process is to deposit the absorber layer on the membrane.
A thin gold or tantalum is deposited by electroplating on the
completed mask blank, followed by a thick stencil resist layer which
is baked. Thin layers of chromium and imaging resist are deposited
subsequently on the stencil resist.
(g) The top imaging resist layer is exposed, developed and used to
pattern the chromium in chromium etch solution.
(h) The patterned chromium acts as hard etch mask during the RIE
etch of the thick stencil resist.
(i) The chromium is then stripped and a layer of gold is then deposited
on the resist trenches by electroplating.
(j) Finally the resist is stripped and the gold/tantalum base is removed
leaving the patterned gold absorber layer on the membrane.
16. Problems in X-ray mask
fabrication
Distortion of mask (membrane and absorber) patterns
a) placement errors associated with e-beam writing on
the resist patterning that is transferred to the mask
membrane and absorber patterns.
b) non-uniform clamping of the mask
c) different thermal expansion of the different layers that
may change the stress on the mask materials during
processing and thus different degree of relaxation.
