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While there are many ways to create microwave circuits, the two most common involve the removal of excess material in order to reveal a specific pattern. In both cases, the desired circuit design is protected by a photoresist and the entire substrate exposed to an etching process. The unprotected material is simply etched away.
The first method of etching employs a wet or chemical process in which the substrate is submerged in an etching solution. This process is isotropic. The sides of the lines as well as the top surface are exposed to the etching solution, which often results in the sides of the lines being eaten away (undercut) by the etching solution. Since it is impossible to control the rate of undercut, circuit to circuit repeatability suffers.
An alternative approach is a dry ion etching method such as our own Ion Beam Milling process. In simple terms ion beam milling can be viewed as an atomic sand blaster. In place of actual grains of sand, submicron ion particles are accelerated and bombard the surface of the target work while it is mounted on a rotating table inside a vacuum chamber. The target work is typically a wafer, substrate, or element that requires material removal by atomic sandblasting or dry ion etching.
As with any etching process, a selectively applied protectant – a photo sensitive resist (photoresist), is applied to the work element prior to its introduction into the ion miller. The resist protects the underlying material during the etching process (which may be up to four hours or longer, depending upon the amount of material to be removed and the etch rate of the materials). Everything that is exposed to the collimated 15 inch diameter ion beam etches during the process cycle, even the photoresist. The key however is that the photoresist’s etch rate is lower than that of the material that is being etched (generally the target metal etches at a rate 3 to 10 times faster than the photoresist), so while everything etches to some degree, when the process is complete, the metallization that defines the circuit remains. Different formulations of photoresist can be used depending on the type of metal and the amount of material to be removed.
Argon ions strike the target materials while they rotate. This ensures uniform removal of waste material resulting in straight side walls in all features with zero undercutting. This leads to a perfectly repeatable circuit time after time.
This precision and its attendant repeatability is ultimately the key strength of the wide collimated ion beam milling process. Other methods of etching or cutting such as the chemical process or laser simply do not deliver the same level of precision that an ion beam etch can. Furthermore, some materials such as Platinum cannot be etched effectively using a chemical process while other materials are not as suited to other methods of etching or cutting. The Ion Beam Milling process comes as close as possible to a universal etching solution.
The diagram below shows a simplified view of the function of the ion beam miller:
Argon ions contained within plasma formed by an electrical discharge are accelerated by a pair of optically aligned grids. The highly collimated beam is focused on a tilted work plate that rotates during the milling operation. A neutralization filament prevents the buildup of positive charge on the work plate.
As noted in the picture, the work plate is cooled and rotates so as to ensure even uniformity of the ion beam bombardment. The work plate can be angled to address specific requirements, but it usually sits at an 8° to 10° angle to the ion beam.