Introduction to Photomasks in Microfluidics

A photolithography mask is an opaque plate or film with transparent areas which allows light to shine through a defined pattern. They are commonly used in photolithography processes, but are also used in many other applications by a wide range of industries and technologies, notably microfluidics.

The photolithography mask acts as a template, and is designed to optically transfer patterns onto wafers or other substrates in order to fabricate devices of all types.

There are three types of basic material used to make photolithography masks:

Soda Lime (SL), Quartz, and polyester film.

Soda Lime and quartz are the most common substrates for photolithography masks, and typical glass mask sizes can range from 3 square inches to 7 square inches.

Film photolithography masks have less constraints on size, and can be made on sheets from 25 cm x 30 cm (10” x 12”) , up to a huge 3 m long x 1 m wide (120” x 40”). The polyester base on the film is 0.18 mm thick.

Using negative photoresist, the unexposed, or masked, portion of this material is then removed. So, the remaining photoresist can form channels mold for microfluidic applications or protect the underneath substrate so the non-covered substrate parts can be etched.

Photolithography masks are used in wafer fabrication, microfluidics, strain gauges, MEMS, Optics, flat panel displays, BioMed, PC boards…. but are also used in many other applications by a wide range of industries and technologies.

Where are photolithography masks used?

In microfluidic research laboratories, photolithography masks are mainly used to fabricate microfluidic molds or to pattern electrodes on your substrate. Photolithography masks play the role of a photographic negative film in device manufacturing such as chips, microfluidic mold or microelectronic manufacturing.

Current photolithographic tools project light through a photolithography mask and a high aperture lens. The intensity of the light projects an image of the device’s design (the pattern on the photolithography mask) onto a substrate; such as a silicon wafer coated with a light sensitive material called photoresist. Since the precision and accuracy of the pattern geometry on the photolithography mask affects the device quality, the photomask is considered a key part of overall semiconductor technologies.

How are photolithography masks used?

Photolithography masks are used in various applications and processes, but one of the most common procedures is to use the mask in a ‘mask aligner’.

One of the most important step in the photolithography process is the mask alignment. The photolithography mask is aligned with the wafer, so that the pattern can be transferred onto the wafer surface. Many MEMS devices are made with multiple materials and multiple layers, so in that case each mask after the first one must be aligned to the previous pattern for the device to work correctly.

Sometimes the alignment between these layers is highly critical and a complex expensive machinery is required.

Once the photolithography mask has been accurately aligned with the pattern on the wafer’s surface, the photoresist is exposed through the pattern on the mask with a high intensity ultraviolet light.

There are three primary exposure methods: contact, proximity, and projection.

Contact Printing

In contact printing, the resist-coated silicon wafer (or other substrate) is brought into physical contact with the glass photolithography mask. The wafer is held on a vacuum chuck, and the whole assembly rises until the wafer and mask contact each other.

The photoresist is exposed to UV light while the wafer is in contact position with the photolithography mask. Because of the contact between the resist and mask, very high resolution is possible in contact printing (e.g. 1-micron features in 0.5 microns of positive resist).

The problem with contact printing is that debris, trapped between the resist and the photolithography mask can damage the mask and cause defects in the pattern.

Proximity Printing

The proximity exposure method is similar to contact printing except that a small gap, 10 to 25 microns wide, is maintained between the wafer and the mask during the exposure. This gap minimizes (but may not eliminate) wafer damage. Approximately 2- to 4-micron resolution is possible with proximity printing.

Projection Printing

Projection printing avoids mask damage entirely. An image of the patterns on the photolithography mask is projected onto the resist-coated wafer, which is many centimeters away. In order to achieve high resolution, only a small portion of the photolithography mask is imaged. This small image field is scanned or stepped over the surface of the wafer.

Projection printers that step the mask image over the wafer surface are called step-and-repeat systems. Step-and-repeat projection printers are capable of approximately 1-micron resolution.