Fabrication Of Circular Gratings On Lnp By E-beam Lithography And Reactive Ion Etching

Abstract

We demonstrate the possibility of fabricating second and first-order circular gratings on InGaAsPDnP waveguides by electron beam (E-beam) lithography of a single PMMA resist layer and two-step reactive ion etching (RE). Recently, there has been a growing interest in the study of circular grating resonators. Circular gratings have potential applications in low-divergence surface-emitting DFB lasers and in multi-port couplers[ 11. In spite of some theoretical works, it seems that no report on the fabriation of circular gratings on semiconductor materials has been published. This is probably due to the technical challenge in making circular gratings by conventioanl holographic techniques. The electron beam @-beam) lithography [2] offers high definition and reproducibility at small dimensions( nanometer grating lines), as well as flexibility in defining complex structures. We report here our results in fabricating first-order and second-order circular gratings on InP by E-beam writing and RIE etching. Our electron beam lithography system is a modified JEOL JSM-840A Scanning Electron Microscope (SEM). A commercial CAD package, DesignCAD, is used for the programmable pattern layout. A pattern generator driven by the CAD output package controls the electron beam of the SEM. The samples used in our experiment were grown by MOCVD on InP substrates. A double heterostructure planar waveguide was formed by the growth of a bottom cladding layer of InP (d =lo00 nm), followed by a guiding layer of InGaAsP ( hg =1.3 pm, d =170 nm) and a top cladding layer of InGaAsP (hg=1.0 pm, d= 200 nm). In the fabrication of circular gratings, a thin layer of Si02 (100nm) was deposited on the sample. Its purpose is to avoid erosion of the polymethyl methacrylate (PMMA) resist during the reactive ion etch (RE) process of InGaAsP. Then, a layer of PMMA resist of thickness 250 nm was spun on top of the sample and baked for 90 minutes at 165O C. Patterns of circular gratings were defined on the PMMA by using an electron beam exposure at 40 kev. The beam current was 150 pA and the line dose was 1.35 nc/cm. The resist was then developed in methyl isobutyl ketone (MIBK) at 21°C for 1 minute and rinsed in isopropanol. The circular gratings were then transferred into the Si02 layer by RIE, using CF4/H2, with the resist as a mask. After stripping the resist, the gratings were etched into the InGaAsP by R E using C W A r RE. Finally, the residual Si02 layer was removed. Figs.1-2 show the SEM photographs of a second-order circular grating and a secondorder+ first-order combination produced by the above techniques. The period of the second-order grating period is 0.398 pm and that of the first-order grating is 0.199 pm for an optical wavelength of 1.3 pmThe second grating has 350 circles while the combination