Control of Internal Stress for High Quality Nickel Large Area Mold Inserts

Metallic large area mold inserts (LAMIs) are essential for the replication of polymer microfluidic devices. Successful molding of micro- or nanoscale features over large areas is dependent on improving the dimensional control of the mold inserts, particularly those fabricated by electrodeposition using the LIGA or UV-LIGA processes. A systematic approach to controlling the internal stress of the nickel deposits, which was essential for predicting the final flatness of the LAMIs prior to electroplating, was carried out. The internal stress of the nickel deposits from a nickel sulfamate solution was estimated using a bent strip stress measurement method after maintaining electroplating chemicals and conditions and reducing contamination. Over-electroplating of the nickel LAMIs was performed on SU-8 electroplating molds on 150 mm diameter Si wafers. Detailed characterization of the nickel LAMIs to determine the relationship between the overall flatness of the LAMIs and the internal stress identified a suitable process window in terms of the current densities (10–20 mA/cm2) and the internal stress (−8.3 ∼ −3.0 MPa) for the high quality nickel LAMIs with an overall flatness of 100 μm.