Micro formed holographic security features in steel

. Counterfeiting, with a global trade volume of approximately 450 billion euro a year compromises business results of companies in all industrial segments. At worst, product plagiarism causes severe damages to the individual brand reputation due to product defects or liability issues. Therefore, product integration of inseparable and unique security features is essential for a preserving market share in all manufacturing branches. This paper presents a novel approach regarding a replication technology for the manufacturing of holographic security features. Using a microstructured aluminium substrate with holographic properties as a base material, a forming die is manufactured by a combination of physical vapour deposition (PVD) and galvanic coating processes. Furthermore, a process adapted hardness progression between individual layers was created. By the use of this die, the forming of a holographic microstructure into high strength aluminium alloy (3.3547) and spring steel (1.1248) could be demonstrated within a preliminary study.


Introduction
Holographic labeles are accepted security features for large-scale consumer goods in the form of adhesive tags. However, these labels exhibit the disadvantage of easy removal and imitation. Other proven security solutions as data matrix codes as well as RFID labels do not show significant advantages and include eligible costs that can easily exceed the selling price of a product. Evolved and improved security features therefore have to meet requirements at both, counterfeit protection and economic viability. Holographic structures are outstandingly suitable for the use as security features because of the machine readability on the one hand and an opportunity for the customer to perform a visual authenticity check on the other hand. In order to use holographic structures as a security feature, however, unresolved questions still need to be answered. On the one hand, current research and development is targeting the cost-efficient production of unique and function-adapted holographic structures. On the other hand, the development of technologies for the indelible integration of holographic structures into products is an aim of current scientific and technological development [4].
Furthermore, the challenging process chains for the manufacturing of holographic structures by ultraprecision machining or lithographic techniques causes a reliable and cost-efficient technology for the application of holograms on product surfaces. These are needed to increase the acceptance of holograms as security features. Hence, the replication of microstructures and holograms by embossing is a promising approach. The replication of microstructures by embossing is an established manufacturing process in the field of plastics processing. In particular, hot-embossing is established for the moulding of microstructures and nanostructures into plastic substrates. However and due to the process forces and the effect on the wear of the stamping die, the replication of microstructures into metallic substrates has not been accepted. For this reasons, design and evaluation of a process-adapted stamping die are the subject of the investigation presented.

Development of a stamping die with a process-adapted hardness profile
The general requirements for a process adapted forming die are a microstructured hard material layer on a hard elastic substratum. The main difficulty in manufacturing a mask following this structure persist in the implementation of the microstructures. This is because hard material layers are not accessible to established methods such as lithography or ultraprecision machining.
For this reason, a completely new approach based on the use of a holographic aluminium shim is presented. The holographic aluminium shim was manufactured by HoloDimensions GmbH by lithography and roll-to-roll replication technology. Starting from this aluminium shim, the layer structure is performed by means of physical vapour deposition process (PVD). As shown in figure 1, the layer systems consists of a titanium nitride (TiN) and a copper (Cu) layer. The titanium nitride layer represents the hard material layer, the surface and also ensures the transmission of the microstructure into the stamping die. The copper layer forms the adhesion promoter for the subsequent chemical and galvanic nickel (Ni) plating processes. In order to implement the previously presented layer system as replication of the holographic aluminium shim, the substrate has to be conditioned for the coating process. This includes a three-stage ultrasonic assisted cleaning process using acetone, isopropanol and deionized water. The deposition of ultra-hard and stoichiometric TiNlayers on the aluminium substrate is performed with an optimised parameter set based on a Ni target, a sputtering power P = 600 W, the use of the radio frequency mode (RF) in addition with argon (Ar) and nitrogen (N) process gases. Furthermore, a PVD coating machine tool Z400, LEYBOLD HERAEUS AG, Cologne was used. In figure 2, the schematic representation of the PVD coating process, the machine tool and the clamping system for the fixture of the aluminium shim are presented.
By the use of optical inspection with a microscope Axio Image 2, CARL ZEISS MICROSCOPY GMBH, Jena, Germany and a magnification between 10x ≤ f ≤ 100x, the replication accuracy of the aluminium microstructure in the TiN layer has been assessed (Fig. 3).
As a result of the investigation it is shown that the replication of the holographic structure could be done while providing excellent replication quality.   For the implementation of the power and path controlled embossing process the materials testing machine tool Z150, ZWICK GMBH, Ulm, Germany shown in figure 6 was used.

Forming of holographic security features
For the determination of the forming force FF, the following formula can be used [2]: For the forming of high-strength aluminum alloy AMP 8000 with a forming efficiency ηf ≈ 0.8, a die surface S = 400 mm 2 , the 0.2 % proof stress of the alloy and a yield strength Rp0.2 ≈ 120 N/mm 2 , a forming force 50 kN ≤ FF ≤ 150 kN was predefined.
As shown in figure 7 even at low forming forces FF a hologram can be identified visually. An increasing forming force FF leads to a reinforced visibility in succession with an improved transmission of the microstructures. An investigation of the processing result by atomic force microscopy (AFM) confirms the result of the visual assessment. The transmission of the majority of the holographic structure is possible even at low forming forces FF. However, the holographic effect and the transmission of the height profile increases with an equally rising forming force FF. Nevertheless, the TiN-stamp is subject to signs of wear whose initial stage can be observed in The hard-elastic structure of the embossing foil prevents breakage at high forming forces FF [3]. As a primary reason for the increased process reliability, the reduction of the compressive stress load by their derivation into the hard-elastic layer structure is presumed [5,6]. In addition, WLI-images show that the structural plateau edges of the imaged hologram are reduced in terms of profiling (figure 8), but due to direction of force application the plane surface sections nearly unaffected by wear.
Furthermore, it was discovered that features with an increasingly smaller structural size tend to have a lower wear tilt. Under the assumption of a difference of the surface roughness Ra between substrate holographic substructures, this fact might preserve the integrity of the holographic structure. The chipping of the ultra-hard TiNlayer, which can be identified as white spots in figures 8 and 9, is evidently caused by the elastic movement and the compression of the whole forming die under the effect of the forming force FF. However, the hard-elastic structure of the forming die prevents the breakage of the whole surface at high forming forces [7].

Outlook
By the use of the presented method, an enabling technology regarding future forgery-proofing was approved. The replication of holographic structures into the surface of a process adapted forming die was established. By the use of this forming die, a manufacturing process for the forming of holographic structures into materials with a certain market relevance was developed and validated. Based on the forming experiments and with a forming force FF ≤ 150 kN, a number of cycles 50 ≤ n ≤ 500 can be assumed, depending on the material to be embossed.
The objective of the further studies is to produce a reasonable understanding about the microscopic, structural processes during the forming process and the derivation of measures to minimise wear processes.