When it comes to component selection for semiconductor equipment, the name of the game is often cost of ownership. In other words, how long and how well it will perform as designed out-of-the-box and installed into the system, and ultimately, uptime. Semiconductor work is so precise and delicate, that the slightest instability of an image scanner or a compounding delay in move, acquire, measure (MAM) time can require a diagnostic pause and ultimately up drives the cost of ownership.
This makes the ability for a motion control system to maintain high performance over time, while resisting wear caused by the environment and activity, invaluable. The accuracy, resolution and throughput demands also prevent the use of some protections like inductive or sealed encoders. This makes careful component selection crucial. Yes, a component can, indeed, lessen the cost of ownership of expensive machinery used to work on semiconductors. Let’s touch on some of the factors to consider when building motion systems for the semiconductor industry.
Making the most of “useful” time
The time spent on collecting measurement data, acquiring images, patterning, is useful time. The time spent on wafer loading/unloading, pre-alignment and motion between imaging, measurement or inspection sites needs to be minimized to maximize tool productivity.
One example is a linear scale’s interpolation error, which affects the efficiency of linear motors—the lower the error, the better the constant velocity. So, if you’re scanning a camera over a wafer, or wherever you need constant velocity, low error improves the image-taking quality of the machine.
The accuracy of interpolation is ascertained with a serially produced measuring standard and is indicated by a typical maximum value u of the interpolation error. Encoders with an analog interface are tested with a HEIDENHAIN electronic device (e.g., EIB 741). The maximum values do not include position noise and are indicated in the specifications.
The accuracy of the interpolation is mainly influenced by:
- the size of the signal period
- the homogeneity and period definition of the graduation
- the quality of scanning filter structures
- the characteristics of the sensors
- the quality of the signal processing
Interpolation error isn’t the only “naturally” occurring imperfection that must be accounted for to ensure each millisecond is maximized.
The accuracy of the measuring standard is indicated by the uncompensated maximum value of the baseline error. This accuracy is ascertained under ideal conditions via measurement of the position errors with a serially produced scanning head. The distance between the measuring points is equivalent to the integer multiple of the signal period. As a result, interpolation errors have no effect. The accuracy grade a defines the upper limit of the baseline error within any section of up to one meter in length.
Accuracy of the measuring standard
The accuracy of the measuring standard is mainly determined by:
- the homogeneity and period definition of the graduation
- the alignment of the graduation on its carrier
- the stability of the graduation carrier
The amount of position noise depends on the signal processing bandwidths necessary for forming the position values. It is ascertained within a defined time interval and is stated as a product-specific RMS value.
Position noise is a random process leading to unpredictable position errors. The position values are grouped around an expected value in the form of a frequency distribution. In the speed control loop, position noise influences the speed stability at low traversing speeds.
Sustaining high precision over time
Accuracy and resolution specifications of standalone components are one thing, but if the components can’t maintain those performance characteristics within a fully integrated system, they will only add to the cost of ownership of machinery. Each component must be considered within the larger system they’ll be part of. Especially in high-throughput environments, this introduces factors like temperature, contamination and wear.
Part of the resistance to this depends on the engineering of the system, how everything fits and works together. We’ll take a look at that first, through the lens of one of the most important variables: temperature. Then, we’ll explain some of the different motion control technologies to keep an eye out for. Technologies that not only make the engineering process easier, but also deliver ongoing, above-and-beyond performance value.
Engineering for environmental variables (as seen through one variable: temperature)
Say you are imaging an area—point-to-point movement, complete stops and fixed in a position for a certain amount of time with only a small amount of acceptable imposition. The motor may be able to move to a position quickly enough, over and over and over again, but if it’s sized wrong and the machine doesn’t stop, the heat will generate thermal expansion and introduce unacceptable inaccuracies.
That expansion can have a huge impact on encoders. How quickly you’re able to go from place to place involves closed-loop control. If that signal is noisy and the resolution is too low, the electronics can’t perform calculations at the bandwidth or speed that it’s capable of therefore performance inevitably degrades.
The thermal behavior of your encoders are essential criterion for the working accuracy of the machine. As a general rule, the thermal behavior of the linear encoder should match that of the measured object. During temperature changes, encoders should expand or contract in a defined, reproducible manner. The graduation carriers of HEIDENHAIN encoders have differing coefficients of thermal expansion, so you can select the encoder with the thermal behavior best suited to your application.
Finally, imagine bearings as railroad tracks on a hot summer day, where slight deformation can cause a slight meandering of train cars from right to left. The same effect is possible, in this high-throughput world, if bearings aren’t ideally specified. As a result, maybe images you&