Sub-Micron Holographic Grating Masks Replicated by X-ray Contact Printing for Integrated Optics

Thomas, Kenneth Maxfield (1989) Sub-Micron Holographic Grating Masks Replicated by X-ray Contact Printing for Integrated Optics. PhD thesis, University of Glasgow.

Abstract

The work presented in this thesis covers a wide spectrum of topics concerned with the development of a lithographic system to print periodic structures. Such structures are incorporated in integrated optical devices such as 1st order Distributed Feedback (DFB) InGaAsP / InP semiconductor lasers and waveguide filters. The gratings were defined by holographic lithography and reproduced by x-ray contact printing. Holographic lithography can be used to produce large area submicron gratings but its success depends upon the reflectivity of the substrate and on the processing of the resist. X-ray lithography, on the other hand is conceptually a much simpler technology and has the advantage that resist exposure is relatively insensitive to the substrate. The technique may provide a simple reliable route to high resolution (<100 nm linewidths) pattern replication with a potentially high throughput. A two mirror symmetric interferometer arrangement was used to define the gratings. Collimating lenses were not used. A theoretical analysis of gaussian beam interference was used to predict the size of variations in period from the plane wave case. Holographic gratings with periods down to 187 nm were exposed in AZ 1350 J photoresist with the UV lines of an argon ion laser. Conventional thin resist holography was used to define thin resist (0.15 mum AZ 1350 J) patterns which were reactive ion etched (RIE) into GaAs wafers (360 nm period) using a CH4:H2 plasma. Thick resist holographic and O2 RIE techniques were developed to produce thick metal absorber patterns (0.3 mum high), lines and metal dot arrays, suitable for a high contrast (10:1) mask for x-ray contact printing. The metal dot arrays were produced by double holographic exposure of resist followed by angled metal evaporation (shadowing) along the lattice planes, O2 RIE and lift-off processes. The use of the metal patterns as masks for subsequent etching or diffusion (ion exchange in glass) into the underlying material was demonstrated. The use of a spin-on anti reflection coating (ARC) under the resist was shown to eliminate coherent reflection effects inside the resist during holographic exposure. Polyimide in solution was spun on to microscope coverslips and cured to form an x-ray mask substrate. Polyimide membrane x-ray masks are cheap to produce and can be conformed into intimate contact with the sample to be printed. After definition of the gold absorber pattern, a 0.3 mum polyimide layer is spun over the metal to protect it during the subsequent etching step and to provide extra strength. After attachment to metal rings the glass was dissolved away in hydrofluoric acid to produce free standing polyimide membranes 1.3 mum thick, 15 mm in diameter. The membranes are coated with a 60 nm layer of gold-palladium alloy, which is optically transparent, to form a conductive layer and to allow registration. A soft x-ray contact printing system was designed and built to transfer the mask patterns into 0.2 mum thick PMMA (BDH) resist layers with copper Lalpha radiation [1.33 nm]. The x-ray source is an electron bombardment type; based on a VG-EG 2 electron evaporator gun. The anode was a copper hearth. A sample stage was designed to allow simple registration between sample and mask. Intimate contact between mask and sample which is necessary for high contrast pattern replication at 100 nm linewidths was achieved by an electrostatic hold-down mechanism. 285 nm period gold grating patterns were transferred into vertical walled resist patterns after 6 hour exposures at 300 W e-gun power with negligible resist shrinkage after development. The resist was developed in 1:3 mixture MIBK : IPA at 23

• Item Type: Thesis (PhD)
• Qualification Level: Doctoral
• Keywords: Electrical engineering, Optics
• Date of Award: 1989
• Depositing User: Enlighten Team
• Unique ID: glathesis:1989-78054
• Copyright: Copyright of this thesis is held by the author.
• Date Deposited: 28 Feb 2020 12:09
• Last Modified: 28 Feb 2020 12:09
• URI: https://theses.gla.ac.uk/id/eprint/78054