Fully automated hot embossing processes utilizing high resolution working stamps

Abstract

Nanoimprint lithography (NIL) is a fast replication technology for structures with sizes ranging from micrometer down to few nanometers range. This paper describes the technology for imprinting of polymer substrates as well as spin-on polymers by using soft working stamp materials. A fully automated hot embossing system, the EVG®750 was built to use this rapid replication processes. By utilizing soft working stamps, we demonstrate the possibility to replicate, in fully automated mode, both high-aspect ratio features in thermoplastic materials as needed for microfluidic lab-on-chip applications as well as high resolution features down to 50 nm in polymer that can be used as templates for pattern transfer in the fabrication of plasmonic substrates for bio-sensing applications.

Introduction

Hot embossing is a patterning technique for thermoplastic polymer sheets and spin-on polymers on substrates. It enables the creation of two-dimensional and three-dimensional structures by applying elevated temperatures and high contact forces. The similarities in regards to process parameters and equipment specifications between wafer bonding and hot embossing have led to the modifications of wafer bonder systems used for thermo-compression bonding processes to accommodate embossing processes some years back [1]. The low cost fabrication of disposable, polymer based devices needed for emerging point-of-care diagnostic or bio-sensing devices can be realized by hot embossing processes at low cost. Hot embossing processes are generally used to address different applications ranging from polymer based lab-on-chip systems, where imprinting is performed on thick polymers sheets, to the fabrication of sub 100 nm features for bio-sensing or data recording applications, which requires imprinting into thin (less than 200 nm) spin-on layers. The use of microfluidic devices for medical or environmental diagnostics requires the fabrication of a large number of devices that is beyond the replication throughput of the exiting R&D embossing systems. The injection molding technologies are not well adapted to cover this transition from the R&D to prototyping series because of the costly initial investment in molds and processing equipment. Consequently, the development of commercial applications based on hot embossing processes requires fabrication systems and processes with throughputs that are beyond the existing R&D tools. The need for fully automated processes implies an automated de-embossing process of stamp and imprinted substrate. While a range of micro fluidic devices are still fabricated by using Si wafers, there is a demand for cheap, disposable and thus polymer based devices [2], [4]. A cheap manufacturing method is required to fabricate those devices, which also demands an inexpensive fabrication method for stamps. Soft working stamps provide a cost effective alternative to Si or Ni based masters since and can be replicated many times from those masters and the original masters are not used for imprinting directly. The soft stamp materials have been used for UV-based nanoimprint lithography processes where no high temperature and force is required to transfer the structures into the polymer [3]. However, the use of such working stamps for hot embossing processes has not been reported so far and is described in the current work. The use of the soft working stamps materials allow a reliable and residue-free separation of stamp and imprinted substrate after imprinting, which is a key issue for automated process runs by using an anti-adhesive monolayer on the stamp surface [3], [5]. Another important aspect is the use of compliant layers that ensure imprint uniformity over large areas. This paper demonstrates the realization of a fully automated hot embossing process for spin-on polymers as well as thick (0.5–3 mm) polymer substrates, which includes handling as well as automatic embossing and de-embossing using soft stamps materials. The experiments were performed on the first fully automated hot embossing system initially developed for hard disc double side patterning using Ni-stamps [4], [6].