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Photolithography

Microwave and Millimeter wave circuit fabrication begins with the creation of a circuit design, often using a Computer Aided Design (CAD) program such as AutoCAD.  The design file is converted into either a raster or vector image which is used to drive an optical pattern generator or laser writer to create an AR chrome or ion oxide image on a glass photomask.  The AR chrome photomask has about 950 Angstroms of chrome with about 150 Angstroms of chrome oxide. The chrome glass plate is coated with photoresist which when exposed to either a laser or optical light source will render an image that matches the original design.

The purpose of the photolithography process is to create a mask on a metalized substrate that matches the intended circuit design.  The substrate is then exposed to the Ion Milling process, which removes material from areas not covered by the photoresist.

The Masking ProcessTo apply the photoresist layer, the substrate is initially heated to a temperature sufficient to drive off any moisture that may be present on the substrate surface. A liquid or gaseous "adhesion promoter" is applied to promote adhesion of the photoresist to the substrate.

The substrate is then covered with raw photoresist by spin coating. A viscous, liquid solution of photo resist is dispensed onto the wafer, and the wafer is spun rapidly to produce a uniformly thick layer. The spin coating process results in a uniform thin layer, generally between ½ and 8 microns thick. The photo resist-coated wafer is then prebaked to drive off excessive solvents, typically at 90 to 100 °C for 30 to 60 seconds on a hotplate.

Applying the PhotoresistAfter prebaking, the substrate is covered by the photomask that was created from the original circuit design and exposed to intense ultraviolet light.  In much the same manner as photographic film, the parts of the photoresist that are exposed to the light react.  Depending on the needs of the application, there are two types of photoresists used; the more common Positive photoresist, which becomes soluble in a developer solution when exposed and the Negative photoresist, which becomes insoluble in a developer solution. This chemical change allows the technician to remove the photoresist from areas to be milled and to expose the underlying metallization.  Areas not covered by photoresist will be milled away.

The resulting substrate is then "hard-baked", typically at 100 °C to 125 °C for 20 to 30 minutes. The hard bake solidifies the remaining photoresist, to increase its durability when exposed to the ion milling process.

Photolithography is as much an art as it is a science.  Ion Beam Milling’s team of photolithography technicians has been developing precision artwork to meet the custom requirements of our customers for more than twenty years.  Our experience means that we can deliver a higher quality product at a lower cost than the competition.

While customers generally use our photolithography service as the precursor to our Ion Beam Milling services, our micro fabrication facilities are available to assist with purely photolithography work that is application specific. 

Ion Beam Milling technicians are capable of producing features with very tight tolerances.  Lines and spaces as small as 10 microns can be realized using our in house equipment.

Our engineers and technicians welcome the opportunity to discuss how Ion Beam Milling’s photolithography services can meet your specific needs.