Why does the vacuum chuck need to be slightly smaller than the substrates? Can the chuck and substrate be the same size?
The chuck should always be slightly smaller than the substrate to avoid damage to the motor assembly as well as to prevent vacuum interlock problems. Substrates are held onto the chuck using vacuum (this does not apply to the non-vacuum, recess chuck line). If your substrate is smaller than the chuck a couple things may happen – 1) Material will surely get underneath the substrate which may,in time, reach the vacuum center hole. Once material reaches the vacuum center hole it will be pulled down into the motor assembly. Although several precautions and guards are in place to protect the motor, the possibility exists that over a period of time the material will reach inside the motor causing it to fail. 2) The substrate may not be large enough to seal the vacuum. The vacuum chucks are designed with a cross design to aid in vacuum distribution. If the substrate is not large enough to cover the entire chuck, vacuum may escape thus not satisfying the vacuum interlock and rendering the spin