Transcript Light sources for steppers

Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
TALK 3
1
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
• Photo-litho-graphy: latin: light-stone-writing
Photolithography is an optical means for transferring
patterns onto a substrate. It is essentially the same process
that is used in lithographic printing.
• Patterns are first transferred to an imagable photoresist
layer.
• Photoresist is a liquid film that can be spread out onto a
substrate, exposed with a desired pattern, and developed
into a selectively placed layer for subsequent processing.
• Photolithography is a binary pattern transfer: there is no
gray-scale, colour, nor depth to the image.
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Overview of the Photolithography Process
• Surface Preparation
• Coating (Spin Casting)
• Pre-Bake (Soft Bake)
• Alignment
• Exposure
• Development
• Post-Bake (Hard Bake)
• Processing Using the Photoresist as a Masking Film
• Stripping
• Post Processing Cleaning (Ashing)
Equipment and technological processes for manufacturing GaAs MMICs
Cleaning
LITHOGRAPHY ONE
Typical contaminants that must be removed prior to photoresist
coating:
• atmospheric dust (minimized by good clean room practice)
• abrasive particles
• photoresist residue from previous photolithography (minimized by
performing oxygen plasma ashing)
• bacteria (minimized by good DI water system)
• films from other sources:
– solvent residue
– H2O residue
– photoresist or developer residue
– oil
– silicone
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Wafer Priming
• Adhesion promoters are used to assist resist coating.
• Resist adhesion factors are-:
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moisture content on surface
wetting characteristics of resist
type of primer
delay in exposure and prebake
resist chemistry
surface smoothness
stress from coating process
Most common for GaAs use are 1,1,1,3,3,3-hexamethyldisilazane (HMDS) or
monazoline C or trichlorobenzene
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
Soft bake
LITHOGRAPHY ONE
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
Hot plate heating the resist is usually faster, more controllable, and
does not trap solvent like convection oven baking.
The thickness of the resist is usually decreased by 25 % during prebake
for both positive and negative resists
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
Masks
LITHOGRAPHY ONE
For simple contact, proximity, and projection systems, the mask is
the same size and scale as the printed wafer pattern. i.e. the
reproduction ratio is 1:1.
Projection systems give the ability to change the reproduction
ratio. Going to 10:1 reduction allows larger size patterns on the
mask, which is more robust to mask defects. 5:1 is common now
Mask size can get unwieldy for large wafers.
Most wafers contain an array of the same pattern, so only one cell
of the array is needed on the mask – so use a stepper
Equipment and technological processes for manufacturing GaAs MMICs
Postbake (Hard Bake)
LITHOGRAPHY ONE
 Used to stabilize and harden the developed photoresist prior to
processing steps that the resist will mask.
 Main parameter is the plastic flow or glass transition temperature.
 Postbake removes any remaining traces of the coating
solvent or developer.
 This eliminates the solvent burst effects in vacuum processing.
 Postbake introduces some stress into the photoresist.
 Some shrinkage of the photoresist may occur.
 Longer or hotter postbake makes resist removal much more difficult.
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
After process strip
Resist is usually stripped these days in a
plasma etch system
An O2 plasma is created which generates many O
radicals – these efficiently oxidise the resist and
produce CO, CO2 and other gaseous products
which are pumped away
Some systems use RF plasmas – others use
microwaves
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
For GaAs
The main lithography tools for MMIC production
are:1) Proximity contact aligners – non critical layers
2) Optical steppers – medium critical layers
3) Electron beam systems – critical layers (next
talk)
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Proximity contact layers – modern example
Suss Microtec MA6 – common in GaAs fabs,
Other major vendor for GaAs is EVG
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Proximity system - schematic
MA6 stage with main functions shown
1
Mask holder
2
Mask
3
Wafer/substrate
4
Chuck
5
Chuck stage
6
Wedge compensation
pistons (3x)
7
Pneumatic brake
8
Adjustable contact force
piston with pressure
controller
9
Precise ball-bearing guide
10
High precision Z-drive
11,12,13
Manual alignment x-y-Ɵ
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Proximity system
 The proximity system offers non contact exposure
at small mask-wafer gaps.
 The key to uniform exposure results is a constant
exposure gap over the entire surface.
 Three reference balls with precisely matching
diameters are placed near the edges of a wafer or
substrate; together with the compensation
system, they ensure that the mask-to-substrate
gap is defined with high accuracy.
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Proximity system –optics and performance
Wavelength
350450nm
280350nm
240260nm
Hg lamp 1 kW
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Hg lamp
300W
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HgXe lamp
500W
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Resolution
proximity gap
100um
7um
<7um
<7um
Resolution
proximity gap
20um
2.5um
2um
<2um
Equipment and technological processes for manufacturing GaAs MMICs
Steppers -1
LITHOGRAPHY ONE
Steppers, as the name suggest, expose one circuit at once and then jump to
the next one
All steppers have reduction optics so the features written on the mask are
usually 5x bigger than the final features will be on the wafer. This means the
structures on the reticule can be written with greater precision.
Outline of how a stepper covers an
entire wafer
Equipment and technological processes for manufacturing GaAs MMICs
Steppers -2
The main optics of a stepper is shown below
In practice a stepper will have up to 20 lenses in it
Light source, condenser lens, reticule and
projection lens
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
Steppers -3
The light source is one of the major definer of the smallest
feature that can be written using a stepper
For GaAs MMIC applications the most common wavelengths
used are I-line (365nm) or 248nm
Light sources for steppers
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
Steppers -4
Alignment for steppers needs to very accurate if the
reticle is to be produced in the correct position all over
the wafer.
Typical laser interferometer alignment systems like that
shown are typical and are capable of giving a positional
accuracy of <10nm
X-Y stage and optical interferometer
arrangement of typical stepper
LITHOGRAPHY ONE
Equipment and technological processes for manufacturing GaAs MMICs
LITHOGRAPHY ONE
Steppers -5
There are only really 3 suppliers of steppers
worldwide
ASML in Europe and Canon and Nikon in Japan.
Typical data for steppers
used in GaAs fabs
Lens
Field size
Overlay
NA
Resolution
X&Y
2 point global
alignment
0.4 to 0.63
<150nm
22 x 28mm
<25nm