Back injection molding of polymer films withcapillary filled sub-micron silver wires
Sijia Xie1, Barbara Horváth1, Jerome Werder2, Helmut Schift*1
Paul Scherrer Institut (PSI) 1, Lab. for Micro- and Nanotechnology, 5232 Villigen PSI, Switzerland
FHNW University of Applied Sciences and Arts Northwestern Switzerland 2, 5210 WindischE-mail: helmut.schift@psi.ch

Monolithic integration of integrated circuits onto macroscopic polymer parts can be done byadding individual electronic elements after the polymer part has been molded or by adding afilm with the entire circuit prepared in advance. In the first case, the patterning of non-flat
surfaces requires a patterning method with tools that can follow the outline of a 3D topography,
in the second case a flexible film that needs to be deformed. This approach has been used forthermoforming and is now also favored for printed electronics [1,2]. This way, fabrication
methods for low-cost flexible electronics can be used to prepare the polymer films.
We placed a 175 µm-thick polymeric film into the crowned mold of an injection-molding toolthat constitutes a concave cylinder lens with a radius of 30 mm and a depth of 10 mm, with aconvex cylinder at the opposite side. The film (PMMA: Evonik Plexiglas film 99524, Tg113°C) was patterned by thermal nanoimprint (T-NIL) at 180°C and was pressed into thiscrowned surface by the viscous polymer (Evonik Plexiglas 7N, Tg 110°C) injected onto thebackside of the film. The tool temperature (40°C) was low enough that the 260°C hot melt wascooled down to ~100°C when it reaches the film upon entering the mold cavity, thus it was ableto deform the thermoplastic film without erasing its topography. The resulting back-injectedcylinder-like element consists of the film that is covering the top of a 3 mm thick convex
polymer body. Two cases were implemented: When the film is patterned towards the moldsurface, the resulting cylinder has a surface topography constituted by the film. When it ispatterned towards the injecting polymer, the surface structures are embedded. The latter is aninteresting alternative if not only surface structures are used, but films that are alreadyprocessed, e.g., covered with electrodes.
We used both cases for the fabrication of components covered with a metal wire array (Fig. 1).
Our process has, in comparison to inkjet printing used for printed electronics, resolutions down
to sub-µm and relies on nanoparticle self-confinement in V-grooves [3]. Similarly, films with
open microchannels were back-injection molded (Fig. 2). The shape of the 3.1 µm wide Vgrooves is slightly flattened by the process, thus enabling capillary filling in an open
microfluidic approach [4]. In both cases, the wires are electrically conducting.
Combining nanoimprint lithography and additive patterning techniques may be a viablestrategy to create low-cost devices on large areas or even free-form components. Suchstrategies can be applied to various designs of electrical circuits that enable to build complex
devices as planned within the SFA project FOXIP [5]. Applications would be LEDs and sensors
embedded in polymer components.
References:
[1]
[2]
[3]
[4]
[5]

H. Schift, J. Appl. Phys. 121(2) (2015) 415-435
T. Senn, et al., Microelectron. Eng. 88 (2011) 11.
B. Horváth, et al., Adv. Mat. Technol. 4(3) (2019) 1800652.
B. Horváth, B. Křivová, and H. Schift, Micro and Nano Eng. 3 (2019) 22.
FOXIP Project, https://www.sfa-am.ch/foxip.html.

Figure 1. Fabrication of films with self-confined metal line pattern. Left side: Schematics of process(a) Dispensing of silver ink, (b) solvent evaporation, (c) drying, (d) densification, (e) simultaneous
sintering of metal lines and thermal reflow of substrate. Right side: SEM micrograph of V-grooves
cross-section with particles (substrate: spincoated PMMA on Si-wafer) [3].

Figure 2. Back-injection molded film on polymer cylinder component. Left side: A patterned PMMAfilm (175µm) is placed into the crowned side of an injection molding tool. Center: Cylinder elementwith surface silver wires. Right side: SEM micrograph with 4-µm period V-grooves in PMMA film
(original: groove width/depth 3.1/2.2 µm, depth after sintering and back injection molding: 1.2 µm),with Ag wires in the vertices (light grey). In comparison to Fig. 1, V-grooves did not reflow due to alow sintering temperature, and not because of the back-injection molding process, too.