Fabrication of SU-8 Polymer Waveguides Using Focused Ion Beam Lithography

Samanta, S. , Banerji, P. and Ganguly, P. (2016) Fabrication of SU-8 Polymer Waveguides Using Focused Ion Beam Lithography. In: 2016 MRS Fall Meeting, Boston, MA, USA, 27 Nov - 02 Dec 2016,

Abstract

An attempt is made to fabricate air-cladded SU-8 waveguides of different widths on an oxidized Si substrate using focused ion beam lithography with varying ion doses;and this report discusses limitations & advantages of applying the method on SU-8 for waveguide fabrication purpose. Our starting material is Si (100) substrate.The sample is cleaned by Piranha cleaning;H2O2 is poured in a quartz beaker & conc. H2SO4 is slowly dropped into it (volume ratio of the mixture is 1:1),and left undisturbed for 25mins.Next it is thoroughly rinsed with DI water & is blown with dry nitrogen & is put in oxidation furnace.After reaching a temperature of 10500C,it is oxidized in a sequence of dry-wet-dry for 30mins-2hrs-30mins respectively to obtain an oxide thickness of 1µm,which is uniform throughout the wafer surface.Next it has undergone a general heating for 30mins at 1500C.Oxygen Plasma treatment is carried out at a power of 40W for 45secs.It is then spin coated with SU-8 5 polymer at 500rpm for 10secs and ramped to 6000rpm for next 20secs to achieve a resist thickness of ~1.5μm (measured by Dektak surface profiler).It is then baked on a hotplate at 650C and 950C for 1min and 3mins respectively,and cooled at room temperature for 10mins for thermal relaxation.It is then placed on Au coater to obtain a layer of gold before proceeding to FIB lithography for pattern generation. The width of the waveguides as inspected under FESEM is 0.6,0.8,1.0,1.5 and 2.0μm.The depth as obtained from AFM for 1,15&40 doses is 0.195,0.34&0.57μm respectively.The rms roughness values are 0.118,0.415&0.512μm for 1,15&40 doses respectively.However,to operate in single-mode region,waveguide should have film thickness within 0.43&1.72μm as obtained from our indigenously developed modeling software EIMM©.Thus we have chosen 0.57μm etch depth which is obtained on applying dose 40 for trial purpose.However,as thickness is near the cut-off region,it is obvious there will be error & will prone to huge loss;also as dose increases,the milling depth increases,thereby making the surface rougher;and rougher means more prone to optical loss.FIB lithography has also a limitation of scanning long distances.As the current is low here it takes a lot of time to scan the whole waveguide length.We know that for optical characterization,the waveguides should be fabricated edge to edge of the substrate.So our sample is cleaved in such a way that it can be scanned within feasible time.Beam current is kept fixed at 10nA & area dose is 0.01C/cm2.It is observed from SEM view of waveguide edge (where dose for FIB lithography is taken 15) that the portion of the wafer where there is no SU-8 coating on it,the waveguide pattern is fair,thus we can say FIB lithography is favorable on Si rather than SU-8 for higher doses.But FIB has been successfully applied to make any micro-changes in the waveguide structures generated by optical lithography,& creating photonic-crystal waveguide structures made of SU-8 polymer.

• Item Type: Conference Proceedings
• Status: Published
• Refereed: Yes
• Glasgow Author(s) Enlighten ID: Samanta, Dr Swagata
• Authors: Samanta, S., Banerji, P., and Ganguly, P.
• College/School: College of Science and Engineering > School of Engineering > Electronics and Nanoscale Engineering