FABRICATION OF GLASS AND FILM PHOTOMASKS
The fabrication of a photomask requires several steps. We will describe each of them in the following section.
The pattern information is created by the researcher in a drawing package, often in AutoCAD or other suitable software packages such as L-Edit. The data is sent to the manufacturer by a variety of methods, such as email or ftp. This data is then processed into internal CAD format (Gerber) and transferred to a lithography tool which then exposes the design onto the photomask substrate. We use the same equipment and exposure process for both glass & film photomasks.
Once the manufacturing process is finished, the mask is cleaned and inspected, ready to be shipped to the laboratory for fabrication of your microfluidic mold.
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The following process flow is a simplified description of these tasks :
Step 1 – Data Preparation for photomask fabrication
Mask data preparation (MDP) is the step that translates an intended set of designs and layers into a form that can be physically written by the photomask writer. Usually this involves processing complex polygons into a data format such as Gerber. Typically a design is delivered to photomask data preparation in DXF or GDSii format, and quite often the design needs re-formatting to fix drawing errors that have been created when constructing the design.
Data preparation is a crucial step in making a photomask – and incorrectly formatted designs are the number one factor for delays in manufacturing. Once the design has been formatted, a PDF checkplot is sent to the customer for approval before moving onto the next stage.
Step 2 – The critical fabrication step : Exposure
The imager is a flat bed, granite based system able to expose both glass and film substrates. It has a photohead, mounted onto a Y stage of the coordinate table.
The coordinate table transports the photohead to those areas of the mask where the relevant images are to be placed. A Z-axis controls the photohead depth, thus allowing for different material thicknesses to be used. A laser exposes short, high energy flashes of light, which are directed onto a modulating digital micromirror (DMD).
This matrix consists in a bank of tiny square mirrors that can be tilted electronically into one of two orientations. With the mirrors tilted in one orientation, the light from the mirror goes through the optical system to image the mask, in the other orientation the light is absorbed.
This matrix is then projected through a series of lenses and optically reduced onto the pre-sensitized material, giving a pixel size of 0.8µm. This configuration works like a darkroom enlarger where the matrix replaces the slide and the laser replaces the projection lamp.
The film or chrome plate acts like the photographic film. Each projected picture represents a small area of the whole mask. To get a complete image you have to add as many pictures (frames) as the photomask requires. The Y-motor moves the photohead continuously along the Y-axis of the plotter while a linear encoder tracks the position and triggers the next flash at the correct point.
The encoders then move the head one step over in the X direction, and the imaging process re-starts. Every frames are stitched together to give the overall photomask image.
Step 3 – Chrome Process for glass and quartz photomasks fabrication
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The quartz / glass (substrate) has a layer of chrome on one side. The chrome is covered with an AR (anti-reflective) coating and a photosensitive resist. The photomask substrate with chrome, AR, and resist is known as a blank photomask. |
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The blank photomask is placed on the bed of the photomask-writer and imaged. The light sensitive particles on the resist absorb the light, and this creates a latent image in the photoresist layer. Nothing can yet be seen on the photomask until the development process. |
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The next stage of photomask fabrication is development. The development of the resist makes the unexposed parts of the resist soft, and these dissolve out of the mask layer. This leaves just the pattern of the device, which now acts as a mask itself to stop the etching of the chrome. |
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Etching is a chemical process to selectively remove the chrome parts of the photomask that are not required. It is one of the most critical steps, and requires inspection before and after this fabrication process. The chrome is protected from the etchant where the photo-resist remains. Nevertheless, careful control must be proceeded to ensure that the image is not under/over etched. |
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The final chemical process of the photomask fabrication is to strip the remaining photo-resist. This leaves us with chrome on soda lime/quartz pattern, with a thin Anti reflective coating on top (this coating can further be removed when specifically required). |
Step 4 – Transparent film photomask fabrication Process
The film is fed into the processor after imaging via a series of rollers, and undergoes some chemical changes through the development of the image. Development changes the exposed silver halide crystals into ‘black’ particles in the emulsion layer of the photomask.
The development times and temperatures are critical for maintaining the correct CD of the photomask, and the transportation of the film from one step to another needs to be finely tuned. Fresh chemicals are added to the tanks in pre-defined rates according to the mask size, and the chemistry is pumped and re-circulated through the tank to ensure correct dilution.
Fixation is the next chemical process of the photomask fabrication and is necessary to stop the development reactions and obtain a stable image. At this stage, the unexposed silver halide is washed out of the film photomask and the base dyes are cleared. Just like during development, fixer is consumed during the process, and materials washed out of the film build up in the fixer tank. Fixer replenishment helps to counteract these processes.
Following adequate fixation, it is important to wash the film to remove residual processing chemicals and byproducts. Again, careful replenishment rates are set to ensure that the wash stage is clean and free of contaminates (which are washed to drain). The chemical and wash phases are complete and all that remains is to dry the film.
At this stage the most important factors are adequate drying of the film while maintaining good dimensional stability.
Step 5– Photomask cleaning
The photomasks go through a robust cleaning process before they are sent to final inspection. The benefit of using a glass based photomask is that they are much easier to clean – the glass and chrome surface can be treated with a variety of chemicals, with no detriment of the image or surface quality, whereas film photomasks are much more delicate and have to be treated with care.
Step 6 – The last step of the photomask fabrication : the inspection
Once the photomask has been processed and cleaned, we then have to inspect it for accuracy / quality / defects and flaws that can sometimes affect the final product. The inspection process includes a visual inspection, the measurement of an image coupon or similar, and a surface inspection.
Step 7 – Dispatching
Once an order is finalized, we package the photomask carefully and arrange dispatch directly to the customer. We ship by registered methods throughout the world, and you will be emailed your tracking number on dispatch of the photomask.
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