Custom Motion Control

DN Staff

June 27, 2005

3 Min Read
Custom Motion Control

In the demanding field of photolithography, engineers face some of their biggest hurdles in aligning the photomask with the wafer. Actuators must maintain wafers in a fixed position relative to the lithography mask.

That's a tough challenge, considering that a typical holographic lithography process involves six or more systems or stages. These include loading substrate, pre-alignment, auto focus, auto alignment, scanning stages, microscope alignment, and unloading substrate.

For precise, auto alignment of the wafer, engineers must rely on a real-time control system to fix and maintain the position between the wafer and the auto focusing system. To perform the final alignment, designers use microscopes in conjunction with a vision system to match alignment marks in the hologram with those on the substrate.

Motion controllers with digital signal processors (DSP) are suitable for many applications, but they often fall short when it comes to high-precision motion control in lithography. To provide update rates as fast as 200 kHz, machine builders increasingly are designing their own motion controllers on a custom PCB. That approach, however, can mean major expense in time and money. Even worse, the fixed personality of the motion controller makes the system inflexible for future redesigns or for accommodating variations in the motion control algorithms at run time.

To overcome these problems, more engineers are turning to reconfigurable I/O (RIO) and SoftMotion technologies from National Instruments as effective tools for lithography and other applications where they want high-precision, customized motion control with the complete flexibility of an FPGA. The NI SoftMotion Development Module, which is an add-on to the LabVIEW graphical development environment, provides all the functions that typically reside on a motion controller's DSP.

The Right Tools for Customizing

Consider a typical application, where there are three main elements of a motion controller: supervisory control loop, trajectory generator, and control loop. Supervisory control serves as the main loop of the motion control system, intercepting commands from the user and signaling the trajectory generator to start/stop moves. The trajectory generator acts as a path planner that creates set points for the control loop. You can perform 2D, 3D (roll, yaw, pitch), and helical circular interpolation with contouring and registration moves. The spline engine function uses a cubic spline algorithm, and four set points to calculate interpolated positions between two positions from the trajectory generator. Finally, the control loop creates the command signal based on the set point provided by the trajectory generator.

The SoftMotion tool allows engineers to design custom control schemes for all elements of the motion controller: supervisory loop, trajectory generator, and control loop.

Using LabVIEW, you can handle path planning, trajectory generation, and position and velocity loop control. You can then deploy the code on any LabVIEW-based programmable automation controller (PAC). National Instruments recommend high-speed PXI or FPGA-based CompactRIO as preferred PAC platforms to deploy custom motion control systems for semiconductor lithography.

Besides lithography, engineers can benefit from using NI SoftMotion technology in many other semiconductor wafer processing applications, such as etching, metrology, dicing, and bonding. For example, as part of dicing, the wafer is cracked along scribe lines at a distance of the order of 100 micrometers, requiring high-precision motion control. Still another prime application area that requires the same submicron-scale precision is the manufacture of microelectromechanical (MEMS) systems. Here, too, engineers need custom control algorithms to address fixturing and structural resonance and vibration excited by process motion.

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