One Stop Micro

Beam drift causing placement errors

Some micromachining projects require the accurate placement of features. However, Micromachining tools can drift over time. For example, if an IR laser heats up, the beam can drift, or effects like thermal lensing can occur. In a gas filled UV laser, the cavity is filled with gas mixtures under high pressure. To prevent leakage of the gas, the cavity mirrors (High Reflector and Output Coupler) are sealed to the cavity using polymer gaskets. To align these cavity mirrors, a set of screws is made available on the outside. If an unusual amount of heating occurs, it will distort the frame of the laser head or move the cavity mirrors since they sit on these relatively softer gaskets. I will go into the details of beam drift from laser cavities in another article, and only discuss errors introduced by the motion system here.

Placement errors caused by the motion system

Placement errors call for correcting the motion system to reduce errors. Although the logical thing would be to use stages that inherently have less errors- like a stage with a linear reference scale placed inside the stage- sometimes we might not have that luxury. Even if we do have accurate linear stages, further correction of the already low level of errors might be required.

Some motion system vendors provide the facility for lookup tables. Local errors can be found out by traversing around a reference piece (like a calibrated and traceable ruler or XY plate) . These errors can then be incorporated into the lookup tables. The motion system uses various mathematical functions to interpolate these discreet error values into a continuous XY area.

The question might then be asked: why go to the trouble of using external reference plates when there is already an accurate ruler inside each of these stages. After all, the ruler is already calibrated to 0.1 microns. The answer to this is:

1. The calibration ruler is one just one axis. For example: take a stage that has a perfectly rigid glass ruler inside it. The ruler has subdivision markers of 0.1 microns. All it promises is that placement precision will be around the 0.1 micron range at the temperature that it was fabricated at, along the perfect axis of the ruler. When the stage moves along its axis, say X, the carriage will also move, ever so slightly, in the Y and Z axis. When a second stage, one that travels in Y axis, is placed on top of the first one, it will move in the Y axis when the X axis is moving, even though Y has not yet been powered.
2. When you fasten two stages, there is a lot of bending, yawing, pitching and other deformations introduced. The reference scale inside the stages lose their relevance to a great degree- particularly for very heavily loaded stages and/or for those with larger travel. Of course, there is always the issue of orthogonality of the fastened stages, which, in an ideal world, can be corrected by simple techniques. The philosophy here should be to physically reduce orthogonal errors to a minimum, and correct whatever remains using software (lookup table). In reality, orthogonal errors may not be linear- they might be wavy. Mechanical correction is not possible in such cases- the software route must be taken.

Value addition through precision in placement

Micromachining projects can involve large parts (like 12 inch wafers), and on it might be features that are in the micron-size range with precision placement requirements. Service providers might have a world-class micromachining capability, but advertising placement precision, accuracy and all those fancy terms can add a lot of value to ordinary micromachining contracts and systems.

The Disadvantages

Another point to remember is the temperature dependence of calibrated systems, as also the overheads in maintaining a steady temperature. You can’t let operators make any adjustments. Assembling and disassembling in a job-shop setting is a pain. Hysteresis is a killer, so you end up having to track a lot of parameters. You cannot rest assured that by bringing back all the original parameters (after an adverse event) your system is back to its original calibration. Imagine what will happen if one your operators is not careful enough with your precision aligned stages- you have to start all over again.

Wasn’t your core-competency laser micromachining in the first place? Why get into these fancy setups if you don’t absolutely need it??

Exotic error correction products

There are some outfits that sell interferometry based stage precision tracking and correcting systems. There are a few others who sell other add-ons that can enhance accuracy of placement. Buyer beware: some of these are indeed exotic, and perform well under a very small operational parameter window.