By Vvon Engineering Team | Published | Updated
Mask cost and lead time are charged per design, laser writing time is charged per square millimetre. Which way that arithmetic falls depends on how often your designs change, how large the pattern is and how many students run the same exposure.
Nanotechnology RFQ checklist (TXT)
Frequent design changes favour maskless lithography because a revised CAD file can be exposed without ordering a new mask. Repeated exposures of a stable design can favour an aligner. Compare the actual mask-service lead time and writer throughput rather than assume a fixed delay for Bangladesh.
Both routes end with patterned resist on a substrate. What differs is where the money and the waiting go.
A mask based process pays once per design. The mask is written by a service, shipped, inspected and then used as many times as it survives. Per exposure the marginal cost is close to zero, which can favour masks for repeated production exposures. In Bangladesh the mask is imported, so the cost per design carries shipping, customs clearance and the risk of damage in transit, and the delay is measured in weeks rather than days. A multilayer process may need several masks; coordinate their delivery with design readiness rather than assume every level must be procured sequentially.
A direct write system pays per exposure instead. There is no mask and no lead time; the pattern goes from the CAD file to the substrate in one session. What you pay is machine time, which scales with the area written and the dose, plus the depreciation and service of a machine that has moving optics and a precision stage. Change the design at four in the afternoon and you can expose the corrected version the same evening.
So the comparison is not equipment cost against equipment cost. It is design iterations per year against exposures per design. A group that settles a design and then makes two hundred of them wants a mask. A group that changes the design every fortnight for eighteen months wants a writer. Most university research is firmly in the second category and most teaching is firmly in the first, which is why facilities that serve both keep arguing about it.
Count the calendar, not the process steps. With a mask, an iteration is: revise the layout, submit to the mask house, wait for writing and shipping, clear customs, inspect, then expose. With a writer, an iteration is: revise the layout, load the file, expose. In a research programme where the number of iterations is the thing that limits progress, that difference compounds. Three iterations a year against thirty is a different project.
There is a middle path that is often the correct answer and gets overlooked. A direct write system can write your own masks. Buy the writer, produce masks in house on blank plates for the designs that have stabilised, and expose those on an aligner for volume and for teaching. That can reduce dependence on externally written masks, while mask blanks, resist processing and inspection still need provision and gives the facility both capabilities from one capital purchase plus a relatively cheap aligner. Durham Magneto Optics build direct write tools for exactly this pattern of use, and the MicroWriter ML3 Pro is the model to put in front of a technical committee weighing that route.
Procurement committees compare headline resolution figures because they are the easiest number in the brochure to line up. It is close to the least useful.
A contact aligner with a good mask, a thin resist and genuine contact resolves finer features than most people assume, and its practical limit is set by contact quality, resist thickness and diffraction in any gap between mask and wafer. A direct write tool's limit is set by the write lens, the beam and the stage. In both cases the achievable line width in a real laboratory is dominated by process rather than by the tool: resist selection and thickness, soft bake temperature and time, exposure dose calibration, developer concentration and temperature, substrate flatness and cleanliness, and the humidity of the room.
That last one deserves emphasis in Bangladesh. Resist behaviour is sensitive to humidity, and a coating bench in a room whose relative humidity swings widely between the dry season and the monsoon will not give the same line width in June as in January. If the specification calls for sub micrometre features, the environmental control and the process development effort are larger line items than the difference between two exposure tools.
Specify the feature size you actually need across the whole device, not the smallest feature anyone has ever wanted. A great many microfluidic, sensor, electrode and MEMS designs live comfortably in the several micrometre range, where both routes work and the decision falls back to turnaround and throughput.
A mask aligner exposes the whole field at once. Whatever is on the mask lands on the wafer in a single flash, so a two centimetre long waveguide is one continuous feature with no internal boundaries.
A direct write tool builds a large pattern out of write fields, moving the stage between them. Wherever two fields meet there is a stitch, and a feature crossing that boundary can show a small discontinuity or lateral offset. For a pad, a via or a short electrode, nobody will ever see it. For a long optical waveguide, a precision resistor, a diffraction grating or an interdigitated structure whose period must not vary, a stitch is a defect. Interferometric stage feedback reduces stitching error substantially and is the feature to ask about if the group makes long continuous structures.
Overlay, meaning the alignment of each layer to the one below, is a separate matter and both routes need it. An aligner does it optically through the mask against marks on the wafer, with the operator or an automatic routine doing the alignment. A writer does it by locating marks with its own optics and then transforming the pattern before writing, which means it can correct for wafer rotation, offset and sometimes scale, and that is a genuine advantage on a warped or non standard substrate.
The common failure is neither tool's fault. Layer one is designed without global alignment marks, or with marks that are destroyed by the etch that follows, and layer three then cannot be aligned to anything. Put durable alignment marks in the first mask level, put them outside the die area, and make sure they survive every process step in the flow. Write that into the design rules for the facility and enforce it.
Direct writing is serial. Time on the machine scales with the area to be exposed and inversely with the writing speed, so a full wafer covered in dense pattern is a long session and a small die is quick. Mask exposure is parallel: the exposure time is set by the dose and the lamp, and it is the same whether the mask carries one die or four hundred.
That is why a teaching laboratory should not run thirty students through a direct write tool on the same pattern. One mask, one aligner and a well drilled sequence gets a class through in an afternoon and gives every student the same starting point. Reserve the writer for the research groups, where the pattern is different every time.
The economic crossover depends on pattern area, exposure dose, handling time, mask cost and the number of repeats. Ask both suppliers to estimate cycle time for the same layout before deciding when mask exposure becomes preferable.
Three patterns cover most Bangladeshi facilities.
Whichever route is chosen, the exposure tool is only part of the spend. The supporting chain is what determines whether the facility produces devices: a spin coater with reproducible speed and acceleration, calibrated hotplates rather than an oven, a developer bench with temperature control, deionised water and a clean drying route, a wet bench with the right chemical handling, a metrology microscope with a calibrated stage for checking line width, and resist storage that meets the formulation's specified temperature and fire-safety requirements. All of it belongs in the same specification as the exposure tool, and most of it sits in the nanotechnology equipment range.
Match resist shipping and storage to the manufacturer's instructions. Some formulations require temperature-controlled handling; others have different permitted conditions. Record batch and expiry dates and verify a control process after receipt rather than assume every ambient shipment is unusable.
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