Spin coating for semiconductor wafers with digital drive

The application challenge is to maximize the uniformity of thin film thickness.

Spin coating is a procedure used to apply uniform thin films to flat substrates. In short, an excess amount of a solution is placed on the substrate, which is then rotated at high speed in order to spread the fluid by centrifugal force. The thickness of the coat is related to the spinning velocity, and for how long it is spinning at different speeds. Acceleration also affects the coated film’s properties as it provides a twisting force to the resin. Spin speeds vary between 500 rpm and 10,000 rpm, depending on the resin used and the required thickness.

The diagram on page two shows an implementation using an ION 500 Digital Drive to control the spin velocity and acceleration. The system host computer controls the ION 500 through a serial RS485 link.

Using the velocity profile mode the system can be set up to change the velocity at predetermined time points that match the specific thin film coating recipe.
In velocity contouring profile mode the motion is controlled by changing the acceleration, velocity, and deceleration parameters while the profile is being executed.

The ION supports up to 2 breakpoints. The trigger condition can be one of 10 parameters. For this application the trigger condition is time. At each trigger point the acceleration and velocity parameters are updated and the next set of parameters is buffered in preparation for the next trigger.

Field oriented control is ideal for this high RPM spinning application. Field Oriented Control provides the smooth motion at slow speeds as well as efficient operation at high speeds. FOC also can improve motor efficiency and can help in motor sizing. Also, the fully digital control loops, and easy to use hardware trace feature for ultra‐accurate tuning can improve the overall motion system.

Pro-Motion® GUI offers a unique wizard that helps programmers quickly configure motion systems for testing which includes auto-tuning for both the current loop and the position loop of servo motors. Pro-Motion® is equipped with an oscilloscope, a very useful tool for detecting the speed profile and for monitoring the effect of changing the system parameters. Programming is supported by C-Motion®, a source code library full of commands for motion control and therefore for the development of C / C ++ programs that run on the host.
The figure below shows the connections from the system board to the ION digital drive, to the motor that moves the support for the wafer. The ION digital drive can be connected via CAN or serial communication channel.

The motion control features and products described in this solution would also apply to many other industrial applications such as spindle control, bar code reader or drum scanners/printers.

The ION Digital Drive is a compact and fully integrated module that provides excellent motion control performance, can be connected to a CAN or serial network, with power drivers for DC, brushless DC, or stepper motors. With the use of technologically advanced MOSFETs, the ION digital drive can develop a high power density with a very compact case.
The features of the ION include: profile generation, servo compensation, stall signaling, Field Oriented Control, digital torque control and many other motion control functions.