6 nm half - pitch lines and 0 . 04 μ m 2 static random access memory patterns by nanoimprint lithography

A key issue in nanoimprint lithography (NIL) is determining the ultimate pitch resolution achievable for various pattern shapes and their critical dimensional control. To this end, we demonstrated the fabrication of 6 nm half-pitch gratings and 0.04 μm2 cell area SRAM metal interconnects with 20 nm line half-pitch in resist by NIL. The mould for the 6 nm half-pitch grating was fabricated by cleaving a GaAs/Al0.7Ga0.3As superlattice grown on GaAs with molecular beam epitaxy, and selectively etching away the Al0.7Ga0.3As layers in dilute hydrofluoric acid. The mould for the 0.04 μm2 SRAM metal interconnects was fabricated in silicon dioxide using 35 kV electron beam lithography with polystyrene as a negative resist and a reactive ion etch with the resist as mask. Imprints from both moulds showed excellent fidelity and critical dimension control. Advances in microelectronics and nanotechnology increasingly demand lithography to have a smaller pitch (the centreto-centre distance between two features). Unlike the feature size, which can be further reduced after lithography by ‘overetching’, the pitch is limited by the physics of a lithography method and is fixed once patterned. Today’s lithography for semiconductor integrated circuits (ICs) manufacturing at the ‘90 nm node’ has a pitch of 240 nm. How much further the pitch can be reduced without replacing photolithography is a hot topic for debate. The smallest pitch demonstrated by other forms of lithographies, such as electron beam lithography (EBL) [1–3], scanning probe lithography [4], extreme ultra-violet (EUV) [5], and ion beam [6], is ∼15 nm. However, these patterning techniques are too slow or too costly for manufacturing requirements, and the quality of the features degrades substantially with decreasing pitch. Here, we report the fabrication of 6 nm half-pitch (HP) gratings in a polymer resist using nanoimprint lithography (NIL), a high-throughput and low-cost process, well suited for mass production. This pitch is, to the best of our knowledge, 1 Authors associated with the Nanostructure Laboratory. the smallest created by lithography using a mask. In addition, we demonstrate the fabrication of complex patterns for the metal interconnects of static random access memory (SRAM) cells of area 0.04 μm2 and 20 nm HP. Fundamentally different from conventional lithography, nanoimprint lithography (NIL) patterns nanostructures in a resist by physical deformation of the resist using a mould. NIL is immune to many factors that limit conventional photolithography resolution, such as diffraction, scattering and interference in the resist, backscattering from a substrate, and the developer chemistry. Due to these advantages, various ‘printing’ lithographies with a mould have been developed [7–9], and sub-10 nm feature size has been demonstrated [10–13]. To explore the limits of NIL pitch resolution, we used an unconventional method to create a NIL mould: a series of GaAs/Al0.7Ga0.3As superlattices were grown on a GaAs substrate using molecular beam epitaxy (MBE) and the Al0.7Ga0.3As was selectively etched away from a cleaved edge in a dilute solution of hydrofluoric acid (HF), creating a GaAs periodic grating [11, 14, 15]. The periodicity of the grown superlattices ranged from 80 nm down to 2 nm, although our scanning electron microscope (SEM) analysis could only 0957-4484/05/081058+04$30.00 © 2005 IOP Publishing Ltd Printed in the UK 1058 6 nm half-pitch lines and 0.04 μm2 SRAM patterns by nanoimprint lithography Figure 1. Schematic diagram of the UV curable nanoimprint lithography (UV-NIL) process. (i) A special NIL mould was made by selectively etching Al0.7Ga0.3As from the cleaved edge of a GaAs/Al0.7Ga0.3As superlattice grown by molecular beam epitaxy (MBE). (ii) A liquid resist film spun on a quartz substrate was imprinted by the mould. (iii) The resist was cured (polymerized) by back-face exposure to UV light. (iv) The mould was released from the substrate, leaving the resist patterned on the quartz substrate. (This figure is in colour only in the electronic version) confirm the presence of gratings on the mould with a HP of 4 nm or greater. The edge of the mould was pressed into a thin layer of photo-curable resist liquid spin-coated on a quartz substrate, and the resist was cured with UV light (figure 1). The imprints were performed with the UV-NIL operation mode of the NX2000 tool and NXR-2010 resists from Nanonex Corporation. Critical to achieving the high-quality imprints, and thereby improving on our previously published results [11] in both resolution and area, was the mould preparation and imprint process. Briefly: the GaAs sample was cleaved into 2 mm × 2 mm samples so that one side of the sample was free of damage from the cleave. The sample was then washed in acetone with ultrasonic agitation to remove dust particles and inspected under a microscope for defects. Mould samples with defect-free edges were then etched in HF:DI at 1:600 with ultrasonic agitation for 1 min, rinsed in DI, dried with N2 gas, and stored individually in anhydrous ethanol. The samples were then put inside a glove-box with less than 1 ppm H2O and O2 content for surface treatment with 17 mM octanethiol solution for several (3–7) days, until ready for use. Before imprinting, the moulds were rinsed in pure ethanol, acetone, methanol, and finally isopropanol, dried with N2 gas, and baked at 80 ◦C for 5 min. The quartz wafers were prepared by removing dust with a CO2 ‘snow’ cleaning system (Applied Surface Technologies), rinsed with acetone, methanol, isopropanol, dried with N2 gas, baked at 80 ◦C for 5 min, and finally spin-coated with imprint resist. Inspection with a SEM verified that gratings down to 6 nm HP were successfully imprinted into the resist (figure 2). Although not the narrowest lines so far imprinted [12], these lines were densely packed and the GaAs grating mould ensured that the linewidth and half-pitch were constricted to 6 nm. The imprinted gratings extended several hundreds of microns Figure 2. SEM image of polymer resist patterned with 6, 8.5, and 17 nm half-pitch gratings fabricated by imprinting with a cleaved GaAs MBE mould. without defects. Considering that each monomer in the resist has a length of 1.4 nm, and the radius of gyration of the cured polymer chain is half the imprinted linewidth, there were on average nine monomers across the 6 nm resist line, demonstrating that polymerization of the resist can occur even under extremely restricted spatial dimensions. Currently, our limited SEM resolution and thermal damage to the polymer during inspection prevented us from observing any gratings with a HP less than 6 nm. However, it may be possible to observe sub-6 nm HP with improvements in microscopy and the minimization of resist melting. Furthermore, improved SEM image contrast could be obtained by increasing the sub-6 nm half-pitch grating mould depth—though doing this without mechanically damaging the mould and/or polymer during separation would be challenging. Previously [11], we reported the fabrication of 5 nm features with NIL. However, these features were sparsely patterned on the mould, and critical dimension (CD) control was not investigated. Here we explore the smallest pitch of complex patterns typically used by the semiconductor industry and their CD control during NIL. To this end, we tested NIL in the patterning of the metal interconnects of SRAM cells of 0.04 μm2 area and 20 nm line HP. Fabrication of the NIL mould for the SRAM metal interconnects is a challenge, since the complex shape of these patterns requires the use of conventional programmable nanolithography, and the pitch of these patterns is close to the limits of current EBL capabilities. To produce a NIL mould with 20 nm HP and excellent CD control, we used polystyrene (PS) as a negative EBL resist [16] and then as an etch mask for RIE of the mould. PS was selected over (PMMA) poly(methyl methacrylate), a positive EBL resist commonly used for NIL mould fabrication [10, 11], as PMMA is a poor etch mask and requires further processing of pattern transfer to Cr with lift-off. We found that the proximity effect made dense EBL patterning with a PMMA lift-off step increasingly difficult without degrading the line-width control. A thin film of PS was prepared on a silicon substrate with 200 nm thermally grown silicon dioxide. The PS of 2000 Mw molecular weight and polydispersity index of 1.06 was dissolved in chlorobenzene at 1% by weight and spun onto the