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PHOTOMASK

PHOTOMASK 8.3

8.2.4 Develop Process
The exposed resist is removed by dissolution in an appropriate solvent. The solvent most frequently
used is tetramethylammonium hydroxide (TMAH) in an aqueous solution. Surfactants are also added
to improve the wetting of the resist. Develop tools have typically utilized one of two approaches to
dispensing developer onto the resist surface. Puddle processes dispense a fixed amount of develop-
er. The developer is left on the surface for some period of time and then rinsed off. The second
approach is to spray developer on the resist for a fixed period of time. Either method has drawbacks.
Evaporation rates across the surface cause temperature gradients that affect develop rates. The hoses,
tubes, or nozzles used to deliver the developer to the resist are potential defect sources. Surface ten-
sion will induce forces that can impact some line width geometries.

8.2.5 Etch Process

Once the exposed resist is removed, the underlying chrome is etched using the unexposed photo
resist as a masking agent. Both wet and dry etch processes are in common use. Wet processes involve
the use of powerful acids. Dry etch processes utilize etchant gases such as chlorine in a plasma
induced by radio frequency energy.

8.2.6 Resist Removal and Cleaning

After the pattern is etched into the chrome, or other opaque layer, of the photomask, the unexposed
resist needs to be removed. Typically this is accomplished in a wet process involving the use of
strong acids and/or acid cocktails.

8.2.7 Inspections

At this point, the photomask has completed the manufacturing process and is ready for quality assur-
ance. Critical dimensions are measured to ensure that the mask features are printed at the proper size.
In addition, the critical dimensions must meet range criteria to achieve a given uniformity across the
printed field. Since semiconductor devices are built layer by layer, the image fields of the photomasks
used for each layer must “stack” upon each other within some tolerance. Typically, there is a design
grid against which the placement of the field is measured. This measurement step is called registra-
tion. A defect inspection must be performed. This inspection must ensure there are no reticle defects
larger than a given size. If defects are found, they must be repaired or determined to be within the
specification required for printing. In some cases, optical measurements subjected to simulations may
show a given defect to fall outside the print window of the wafer stepper/scanner. When the photomask
is a phase shifting mask (PSM), measurements of the optical transmission and phase are required.

8.2.8 Final Cleaning and Pellicle Mounting

After all inspections are complete, the photomask must receive a final cleaning. While this involves
the use of strong acids, it is not typically as vigorous as the resist strip/clean process. Since the pho-
tomask has passed through the inspection steps at this point, the primary goal is not to damage it.
Also, excessive cleaning can alter the phase and transmission characteristics of phase shift masks.
Once cleaned, a pellicle is installed over the printable field to protect it from contamination at the
wafer fab. The pellicle is a rectangular anodized metal frame with an optically transparent film on it.
The frame height is such that any airborne or other contaminant that should fall on the film will lie
outside the focus plane of the reticle image when printing the wafer.

8.2.9 Final Shipment
The photomask is now ready for shipment to the wafer fab.

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