Soft X-Ray Contact Printing Of Periodic Structures For Distributed Feedback (DFB) Semiconductor Lasers And Integrated Optics.

Abstract

ABSTRACT. This paper describes work that has been done to develop X-ray contact printing of submicron gratings for integrated optical devices in our department. Previously, integrated optical devices, such as waveguide filters, have been made here using holographic exposure directly onto waveguide or semiconductor substrates. X-ray contact printing of holographically generated masks should make the fabrication of such devices simpler. The paper describes the work to date and gives fabrication details for the X-ray masks.X-ray contact printing is comparatively simple to perform, once the mask fabrication process is worked out. The paper describes our first results from the contact printer. In addition some results are presented on the manufacture of an X-ray mask for Quantum Dot structures. 1. INTRODUCTION. Many devices, such as Distributed Feedback (DFB) semiconductor lasers, Lateral Surface Superlattice devices and Quantum Dot arrays incorporate submicron grating structures with periods below 400 nm. First order DFB lasers in the GalnAsP semiconductor system with emission wavelengths between 1.3 and 1.55 um require etched gratings with periods between 0.19 Jim and 0.23 Jim to meet the Bragg condition. Wavelength filters on ion exchanged waveguides require periods of the order o!210 nm (for 632.8 nm HeNe laser light) or greater. The gratings are usually produced either by holographic exposure directly onto a sample which has been previously coated with photoresist or has been immersed in a photochemical etch, (1) or by electron beam lithography. Holographic lithography is the only technique which can produce large uniform areas of grating rapidly (potentially several centimetres in diameter). However, this technique suffers from two disadvantages; firstly, it is complicated to perform, especially on high reflectivitysemiconductor surfaces, and secondly, it is not always reliable. Electron beam lithography can produce very complex grating structures, in particular a X / 2 shift in period is easy to implement, but the throughput is low and the capital cost is high.Using holographic lithography (and also electron beam lithography) to define masks for X-ray contact printing potentially provides a high throughput, simple route for submicron pattern replication.Much of the current interest in X-ray printing employs elaborate and costly X-ray sources, such as synchotrons or laser induced plasmas( A simpler and cheaper alternative source, such as a modified e-beam evaporator demonstrates the advantages of X-ray printing, such as parallel processing, feature reproduction down to 100 nm linewidths, sample exposure independent of substrate type and of contamination by low atomic number material, repeatability and a lower technology operating procedure, at the expense of longer exposure times due to the low brightness of the source.