Contact mask processing is a technique by which power density on-target is controlled by simple beam-shaping optics and the feature shape is determined by a blocker mask in contact with the workpiece. The contact mask exposure is performed with a stationary part or a part scanned under the beam. With using the scanning technique, laser firing must be interpolated with the table feed rate to ensure uniform exposure. Contact mask scanning allows very large areas of material to be processed.
Contact mask imaging
The blocker mask material is selected so that the beam does not damage the mask as it ablates the material below. Plastics are commonly machined by using a molybdenum or copper contact mask. Aluminum is also a good choice. On-target fluence is limited by the contact mask material damage threshold.
Side wall quality using contact mask processing is quite good. Periodically, the mask must be cleaned due to ablation byproduct build-up. Feature resolution is limited by the limits of the size of the feature that can be put into the contact mask. Feature sizes down to 25µm have been achieved this way. An additional technique is to laminate an expendable contact mask to the workpiece as an integral component of the final product. This approach has been used for via formation in microelectronics packaging.
Advantages of Contact Mask Imaging |
Disadvantages of Contact Mask Imaging |
Simple optical setup |
Mask contact with work piece often undesirable |
Good beam utilization possible |
Feature resolution limited by mask resolution |
Easily programmed for operator level, computer controlled production |
New masks are needed for damage design |
Most masks easily made by mechanical etch methods |
Heat absorbed by contact mask material may cause thermal damage to work piece |
Usually no need for cameras or sophisticated motion control |
Contact mask requires regular cleaning |