This study deals with different methods to improve the adhesion between sputtered aluminium and positive photoresist. Factors controlling the adhesion and different ways to measure the adhesion have been investigated. Different surface treatments prior to resist disposition have been investigated as well.
The investigated surface treatments and adhesion measurements are compatible with the available equipment and the existing process cycle at Strand Interconnect AB. All tests were made in class 1000 clean room. All tests in this study were performed with MICROPOSIT S1818 SP16, which is a commercial and commonly used positive resist manufactured by Shipley.
To provide sufficient adhesion on the aluminium surface some kind of surface treatment must be used. Today a wet chemical treatment is used at Strand Interconnect. In this report methods to modify the surface properties and to measure the adhesion have been investigated. The three methods to modify the aluminium surface were oxygen plasma, wet chemicals and primers and were used in this thesis. The RF power and time duration of the oxygen plasma were varied, while the temperature, gas flow and pressure were fixed. The adhesion was determined indirectly from measuring contact angles of 50 µl DI water droplets on sputtered aluminium in the wettability test as well as directly from the undercut caused by the etch fluid at the interface between the photoresist and the aluminium surface.
An oxygen plasma with 200 W power for 30 s resulted in the lowest measured contact angle, which means that the resist adheres well on the surface. The angle was 2.99 degrees compared to 6.34 degrees for the wet chemical treatment used today. The same treatment also resulted in the lowest undercut, which correlates well with the result from the contact angle measurements. The measured undercut for a 25µm wide conductor was 1.41 µm, corresponding to an undercutting constant (ku) of 1410. The wet chemical surface treatment used today resulted in an undercut of 1.60 µm, equivalent to a ku of 1233. Similar results were obtained for a 15 µm wide conductor.
Source: Linköping University
Author: Tomicic, Daniel