2.1

Samples preparation

The resin used in these experiments is Ormostamp® (hybrid photoreactive polymer, Micro Resist Technology GmbH), combined with the photoinitiator (PI) Cabazyl (1,3,5-Tris(2-(9-ethylcabazyl-3)ethylene)benzene, SIGMA-ALDRICH). 2 mg of PI are mixed with 1 ml of DCM (Dichloromethane) by means of a magnetic stirrer for 5 minutes, 1 g of Ormostamp® is then added, and the mixture is stirred for 2 hours. In order to evaporate the rest of the DCM, the mixture is placed in a fume hood for at least 8 hours. The process is performed under red light illumination to avoid undesired polymerization.

The material selected as insert substrate is P20 mold steel, a common material used for plastic injection mold cavities. The inserts, whose dimensions are 150×40×2 mm, are polished previously to their use as substrates for MPP. Two different methods have been tried in order to improve the adhesion of the polymerized microstructures to the inserts, with and without an additional flat Ormostamp® extra layer:

Method 1: (1) polished steel inserts are cleaned with acetone and isopropanol before being used; (2) a thin layer of the prepared resin is deposited on one side of the steel insert, by using a Zehntner ZAA2300 doctor blade automatic coater at 3 mm/s, with 50 μl of the prepared resin, providing a 30 μm thickness film before curing.

Method 2: (1) polished steel inserts are cleaned with acetone and isopropanol before being used; (2) sample is heated at 200ºC for 5 minutes; (3) the sample is cooled to room temperature before depositing a thin layer (30 μm) of pure Ormostamp on one side of the steel insert as in method 1; (4) sample is heated at 80ºC for 2 minutes; (5) the thin layer of the deposited resin is cured with an UV lamp for 1 minute; (6) the sample is heated at 130 ºC for 10 minutes; (7) a thin layer of the prepared resin is deposited on top of the cured layer, following the same procedure as step (2) in method 1.

2.2

Experimental Setup

The system used for the MPP process is an OPTEC micromachining station which integrates a laser source Satsuma HP² (Amplitude Systèmes), emitting at a visible wavelength (515 nm), with micrometric precision XYZ linear stages (Aerotech). The laser beam is focused with an PLAPON60XO Olympus microscope objective with nominal 60X magnification. To obtain adequate beam distribution, a 4x beam expander was used to reach a beam diameter of 4 mm of the collimated beam at the microscope objective entrance. The setup is represented in Figure 1 (left). The process is performed under red light illumination to avoid undesired polymerization of the resin.

Figure 1.

(left): Photopolymerizing system with integrated coaxial camera. (right): confocal profilometer system at AIMEN.

Different geometries were designed in CAD and then fabricated by MPP of the as deposited resin coating the steel insert. The laser beam was focused inside the resin with an energy under 10nJ and at a 500kHz repetition rate, following the path corresponding to the previously prepared CAD geometries. Immediately after the fabrication, the inserts were submerged in a bath of Methyl Isobutyl Ketone (MIBK) for 30 minutes, and then rinsed in an Isopropanol bath for 10 minutes, in order to eliminate the not polymerized resin. The structures were measured before and after the replication process by using a confocal profilometer S-Neox (Sensofar), Figure 1 right.

2.3

Replication process

The injection molding process was performed with an ENGEL injection molding machine, model VICTORY 40. The material chosen for the replication of the microstructures on the insert surface is Zylar® 960, which is a high flow transparent styrene acrylic copolymer that provides toughness equivalent to some grades of polycarbonate, but can be processed at lower temperatures (around 200ºC). Using lower processing temperature extends the life of the inserts, and additionally, avoids accidental damages on the electric components of the optoelectronic devices during the IM. The IM was performed by following the technical recommendations of the material manufacturer.

3.1

Method 1

Results obtained following Method 1 showed poor adhesion of the photopolymerized resin to the polished steel insert. In this method, lines were fabricated at different angles in a star-like geometry. Figure 2 shows the difference between a structure partially attached to the surface and another structure that got completely detached from the insert and collapsing over itself. No replication was done in this case, due to the lack of good adhesion of the polymerized structures.

Figure 2.

Photopolymerized resin forming a star-like geometry.

3.2

Method 2

In Method 2, a layer of pure Ormostamp® is applied to the steel insert and cured before applying the resin+photoinitiator mix. This first layer improves drastically the adhesion of the photopolymerized microstructures to the steel insert, as the attachment is not produced directly to the steel insert, but to the pure Ormostamp® layer.

Several groups of parallel lines were photopolymerized using this method (Figure 3), as well as several AIMEN logos. Different groups of lines were used to evaluate the adhesion of the photopolymerized structures and optimize the process parameters.

Figure 3.

Groups of photopolymerized lines varying laser pulse energy, from 6 nJ per pulse to 10 nJ

As it can be seen in Figure 3, the suitable laser pulse energy for photopolymerization was found to be in the range between 6 and 10 nJ. The photopolymerization threshold was found at 6nJ, under the above-mentioned experimental conditions. while pulse energies over 10 nJ resulted in the appearance of undesired bubbles inside the resin, a well-known consequence of excessive radiation doses of Ormocer® resins.

Several AIMEN logos were photopolymerized and replicated by injection molding, with the processing parameters optimized for line writing (8nJ/pulse @ 500kHz), as shown in Figure 4. Writing energy and overlap is high enough to produce lines of 5 microns in width, as can be seen in such figure. The distance between the hatch lines is 2 μm, so there is overlapping between them, and the structure is created as a solid block, as shown in Figure 5, with a maximum roughness around 200 nm. In this way, a compromise between fabrication speed and film roughness was made in order to be able to fabricate the master on large areas with a roughness that could be adequate for the target application.

Figure 4.

Photopolymerized AIMEN logo contour (left) and injection molded replica of the polymerized logo (right).

Figure 5.

3D topography of the photopolymerized AIMEN logo on a steel insert (left), where overlapping of the parallel lines in the hatching can be appreciated (right).

Replication of polymerized structures was made by the injection molding process described in section 2.3, using Zylar® 960 as injection material. The results obtained from these trials show a good quality of replication (Figure 6), with details as small as 0.5 μm perfectly reproduced in lateral resolution. As expected, the maximum roughness of the replicated sample was smoother than the equivalent in the original insert (130 nm instead of 200 nm), indicating that a correction of the insert roughness would not be necessary. However, the pitch between the hatch lines should be optimized when the application needed for lower roughness, with a cost in terms of fabrication speed.

Figure 6.

3D topography of the injection molded replica of an AIMEN logo. The lines overlapping can still be appreciated.

More than 50 replicas were made by injection molding and no damage to the insert is detected, which means that photopolymerized Ormostamp® on tool steel inserts is a good choice for applications that require Injection Molding replication of microstructures with high resolution.

However, even if the injected replicas show very good quality, some defects appear on the edge of the structures (highlighted in light blue, Figure 7), when measured with the optical profilometer, in the form of sharp peaks of about 1 micron in height, suggesting problems with the filling and demolding process during the injection cycle of the Zylar® 960 material. To confirm this hypothesis, a high resolution replication material, 101TH (Thixotropic silicone, Microset), was used to faithfully replicate the polymerized structures (Figure 8). This replica was measured and compared with the injected ones, in order to confirm that the polymerized features were not damaged during the injection process (Figure 7).

Figure 7.

Topographic profile of (a) a photopolymerized insert, (b) an injection molded replica, (c) a 101TH replica.

Figure 8.

AIMEN logo replicated in high resolution replicating silicone 101TH.

Figure 7 shows that the 101TH replica does not possess the same defects as the injection molded replica in the edges, indicating that the origin of the defect is either in injection molding or caused by the demolding. Outside these edge defects, the high resolution replication silicone obtains similar geometries than the injection molding copolymer, which suggests that the injection molding process induces this defect.