Hidden defects in polymer-based laser welds visualized by Terahertz microprobes

by M. Nagel, S. Sawallich, C. Matheisen and M. Brosda

Fig. 1

Fig. 1: Laser transmission welded polypropylene polymer parts with different pigments concentrations and visually invisible internal welds [Polymer Parts Treffert GmbH & Co. KG]

In the last decade, laser transmission welding of polymers has become the method of choice for joining complex three-dimensional parts with highest accuracy. Laser welding of polymers is broadly used for example for the fabrication of components from automotive or medical sectors. The welding process is based on a focused laser beam which is used to melt the polymer at the interface between the two parts to be joined. Usually one part is chosen to be transparent and the other is chosen to be absorbing to assist the welding process. Parts with identical optical properties (such as transparent/transparent or white/white) may also be joined – however the control of the melting process is much more demanding in this case (Fig.1). Process monitoring and non-destructive testing techniques are playing a crucial role in polymer laser welding especially in industrial application cases, such as the sealing of enclosures or sensors with internal electronics, where the laser welding is often the last step in the production chain of the final product. At this point, however, where the product value chain is almost complete any process failure is especially costly.
Particularly in safety-critical components there is an ongoing trend to hundred percent complete inspections which can only be achieved through non-destructive testing methods. So far, a final inspection of the weld is often associated with the costly destruction of the product since current non-destructive testing methods such as optical microscopy (based on light from the visible or infrared region) are restrained by the large absorption or scattering found in many plastics. Hence, for certain visually non-transparent materials such as polypropylene, polyamide and fiber reinforced polymers the currently available non-destructive testing methods are unable to uncover defects and inhomogeneity in laser welds. Accordingly, process and quality control are impaired due to insufficient feedback information.

Terahertz based non-destructive testing of polymers

Radiation from the Terahertz (THz) frequency range is very promising for the non-destructive testing of polymer components because of its high transmissivity through most polymers – essentially regardless of color or composition. Consequently, THz imaging has already been tested for polymer sorting applications [1] and the analysis of large-scale polymer weld joints [2] using classic diffraction-limited far-field transmission schemes. Typical air void defects in polymer laser welds have diameters in the range of ca. 50-100 µm. Unfortunately, due to the large wavelength of THz rays the spatial resolution which can be achieved through far-field approaches is insufficient to recognize such microscopic defects or inhomogeneities found at laser-generated welds.

Fig. 2

Fig. 2: The process of scattering light generation at a small air void caused by a pulsed plane wave. (a) Plane wave propagating towards the void, (b) plane wave/void interaction and (c) propagation of the incident and the scattered waves.

THz light scattering from defects

Using microprobes as a key component for THz light detection close to the sample surface instead of far-field detection enables the monitoring of micron-scale buried air voids and inhomogeneity in laser welds, as demonstrated now in a study undertaken by Protemics GmbH [3] and the Fraunhofer ILT [4], both located in Aachen, Germany. The reason for the much better recognition performance of the microprobe-based detection in contrast to earlier approaches is given by the ability to measure very efficiently the light scattering generated by the local inhomogeneities in the welds. The process of scattered light generation at a buried air void in a polymer bulk material is sketched in Fig. 2 (a)-(c). The structure under test is shown in cross-sections at different times of plane wave excitation. A THz plane wave pulse is incident from the bottom side and propagating towards the air void (Fig. 2 (a)). The scattering interaction between the incident plane wave and the air void is generating a second spherical wave propagating away from the defect into every room direction. Due to the decreased phase retardation within the air void in comparison to the bulk material the undeflected forward propagating part of the scattered wave is running in front of the plane wave. Both waves are subject to interference effects which is also visible in the more detailed field simulation shown in Fig. 3. As in Fig. 2 the incident plane is propagating into the upper direction, too. The shown THz field amplitude refers to the field vector component in transversal direction.
The reason, why the surface-near detection is superior to standard far-field detection becomes very apparent from these simulation results. The main signal contribution caused by the presence of the void is the scattering light, however, because of the radial divergence of the scattered wave this information is almost completely lost at larger distances from the sample where the far-field is detected. Likewise, interference effects are much more pronounced at closer distances. As described by theory for Mie scattering [5] the extinction efficiency is maximum for particle sizes close to the wavelength of the incident light, which is giving a further reason why the THz light is especially attractive for this application.

Fig. 3

Fig. 3: Snap-shot of a time-domain field simulation showing the scattering light generation from an incident THz pulse plane wave at an air void.

The samples investigated in this work have been processed at the Fraunhofer – Institute for Laser Technology ILT in Aachen, Germany. They are based on a polypropylene opaque to visible light with a material thickness of 1000 µm per joining partner. A buried laser weld with a lateral width of ca. 380 µm has been generated between both parts.

Using microprobes to pick up the scattering light

The measurement system used for the tests in this work has been described in detail in an earlier publication [6]. A THz plane wave pulse is transmitted through the sample under test (SUT). In order to measure the transmitted field in the time-domain the tip of the microprobe is scanned across a virtual plane in a distance of a few tens of micrometers above the SUT. Fig. 4 (a) is showing the recorded THz field image at the time when the peak amplitude of the plane wave has just reached the microprobe. The image exhibits a large reddish colored background area (corresponding to the peak of the THz plane wave) including an L-shaped lighter area, which refers to the laser-welded area. Within this area there are 6 prominent spots generated by air voids showing strongly decreased THz field amplitudes reaching even negative values. The measured differences between the spots are attributed to size differences of the air voids. Fig. 4 (b) is showing a further image taken at a time-delay of 230 fs later than Fig. 4 (a). Here, the propagation of the spherical waves scattered from the air voids is clearly visible.

Fig. 4

Fig. 4: Transmitted THz plane wave and scattering light measured in close distance to a polymer laser weld using THz microprobes. (a) Measurement on plane-wave peak and (b) 230 fs later. (c) Sample configuration.

Conclusion

The Terahertz microprobing technique is a highly attractive novel approach for the inspection of laser welds in polymers. The new method is paving the way for the non-destructive inspection of critical types of polymers such as polypropylene or fiber reinforced polymers which can only be analyzed by destructive methods, so far.

References:

[1] A. Maul, M. Nagel, “Polymer identification with terahertz technology,” OCM 2013-Optical Characterization of Materials-conference proceedings, 265 (2013).
[2] S. Wietzke, C. Jördens, N. Krumbholz, et al. „Terahertz imaging: a new non-destructive technique for the quality control of plastic weld joints,” Journal Of The European Optical Society – Rapid Publications, 2 (2007).
[3] www.protemics.com
[4] www.ilt.fraunhofer.de
[5] H. C. van de Hulst, Light Scattering by Small Particles (John Wiley & Sons, Inc., 1957).
[6] M. Nagel, A. Safiei, S. Sawallich, C. Matheisen, T. M. Pletzer, A. A. Mewe, N. J. C. M. van der Borg, I. Cesar, H. Kurz „THz microprobe system for contact-free high-resolution sheet resistance imaging,” 28th European Photovoltaic Solar Energy Conference and Exhibition, pp. 856-860 (2013).

BIMETALLIC GRATING STRUCTURES – A new concept for large-scale bias-free terahertz emitters

Fig. 1

Fig. 1: (top) Top-view showing both investigated lateral schemes of the fabricated radial mode Terahertz emitter structures featuring complementary bimetal/semiconductor sequences. (bottom) Cross-section view of device including the principle THz field distribution after optical excitation.

A new concept for the optical generation of THz radiation has been introduced by AMO GmbH, Germany. The approach – filed for patent application [1] by the company – uses grating structures made of two different metal materials which are configured on a semiconducting substrate. THz pulse generation is triggered by optical excitation of this structure through femtosecond near-infrared pulses and subsequent acceleration of the photo-induced charge-carriers. Scaling of the actively emitting areas into a range of square-mm sizes (helpful to avoid conversion efficiency degradation through pump saturation effects) is straightforward.
Earlier large-scale THz emitter concepts used voltage-biased metal-semiconductor-metal (MSM)-structures [2,3]. For these structures increasing emitter area means an increasing probability for device fade-out through a single short-cut defect. Now, the emitter is operated bias-free because inherent Schottky-fields present at the bimetallic/semiconductor interfaces are used for charge-carrier acceleration. As a result, the emitter stays functionally unimpaired even in case of short-cut defects.

Bias-free Terahertz emission – Schottky-field vs. photo-Dember

The introduced emitter is sharing this robustness with an earlier bias-free THz emitter concept based on lateral photo-Dember field induction [4], but it features the important further advantages of simple monolithic fabrication and higher efficiency. In order to generate lateral photo-Dember fields the fabrication of (mono-) metallic gratings with three-dimensional wedged profiles is required. These profiles are difficult to realize for arbitrary shaped gratings like radial or curved ones instead of shown linear gratings. More important, lateral charge carrier acceleration through Schottky-fields is not limited to semiconductor materials with pronounces differences in electron and hole mobilities and diffusion processes (as used for photo-Dember field induction). Consequently, the new approach enables bias-free Terahertz emitters with improved efficiency. A demonstration device in terms of a radial mode THz emitter has now been presented using the new bimetal grating concept.

No bimetal, no Terahertz emission

The key feature enabling Terahertz generation within the novel large-scale grating structure is based on the application of two different metal materials: While the lateral Schottky-fields in a monometallic grating always cancel out over a full grating area, there is a net lateral field at bimetallic gratings resulting from the Schottky-field difference between both metals applied. Fig. 1 (top) is showing the principle configuration of the investigated radial grating emitter. By choosing the medial sequential order of the applied metals (e.g. metal 1/metal 2/semiconductor instead of metal 2/metal 1/semiconductor) it is possible to flip the direction of the generated THz field by 180° as shown in the cross-section view at the bottom of Fig. 1. A representative surface-profile is shown in Fig. 2. Three height levels visible in this plot correspond to plateau areas representing the bare semiconductor surface (dark green), metal 1 or metal 2 (light green) and metal 1/metal 2 one above the other (yellow). No wedged structures have been formed in this case.

Fig. 2

Fig. 2: Surface-profile measurement of the center region of a radial bimetal grating emitter.

Advanced monitoring of THz near-field emission

The THz emission process has been measured using photoconductive microprobes (from the TeraSpike TD-800 series) developed in-house through AMO. The probes allow the selective time-domain sampling of every THz vector-field component in x-, y- and z-direction in terms of amplitude and phase. Fig. 3 is showing a single snap-shot of the field amplitude distribution in z-direction measured shortly after optical excitation at a pair of radial emitters. The measurement plane is on the emitter backside as sketched by the dashed red line in Fig. 1. As expected for the z-component of a radial mode, the largest field magnitudes are observed at the center of each emitter. Both emitters have been fabricated using the converse bimetal sequences also illustrated in Fig. 1. As a consequence, the field lines on both radial emitters are pointing in opposite direction which confirms that the Schottky-field induced THz generation is working as expected. A movie showing the time evolution of the excitation process can be watched by following the link in the caption of Fig. 3.

Fig. 3: Measurement of the z-component of the Terahertz near-field distribution shortly after optical excitation. To watch a movie showing the full time-domain excitation process click here.

Fig. 3: Measurement of the z-component of the Terahertz near-field distribution shortly after optical excitation. To watch a movie showing the full time-domain excitation process click here.

Conclusion

Bimetallic grating structures are highly attractive for the production of bias-free large-scale THz emitters of arbitrary shape and size. The given example of a radial-mode emitter demonstrates the flexibility of this approach very nicely. Further important attributes are robustness and efficiency. In addition to pulsed generation the concept should also be attractive for continuous wave (cw) Terahertz signal generation [5] using semiconducting materials with sufficiently short carrier lifetimes.

References:

[1] M. Nagel, German patent application, DE 102012010926 A1

[2] A. Dreyhaupt et al., Appl. Phys. Lett. 86, 121114 (2005), http://dx.doi.org/10.1063/1.1891304

[3] M. Awad et al., Appl. Phys. Lett. 91, 181124 (2007); http://dx.doi.org/10.1063/1.2800885

[4] G. Klatt et al., Optics Express, Vol. 18, Issue 5, pp. 4939-4947 (2010), http://dx.doi.org/10.1364/OE.18.004939

[5] Dohler et al., Terahertz Science and Technology, IEEE Transactions on , Vol. 3 , Issue 5, pp. 532 – 544 (2013), http://dx.doi.org/10.1109/TTHZ.2013.2266541

TERAHERTZ MICROPROBES: Efficiently fostering the development of Graphene-based touch-screen displays

 

Touch-screen displays are still among the most expensive parts in a mobile device. The replacement of indium tin oxide (ITO) which is used as optically transparent conduction layer by Graphene is widely considered as a promising route to lower the costs of this component. Being better suited for future flexible touch-screens is a further advantage of Graphene against ITO. However, a hitherto existing problem is given by the lack of suitable measurement tools for the quality and process analysis of large-scale graphene layers. Photoconductive (PC) Terahertz microprobes developed at AMO GmbH (Aachen, Germany) have now proved as a key component for contact-free high-resolution mapping of graphene layer sheet conductance [1]. In comparison to standard contact-based four-point probing up to 1000-fold increased measurement speed (5 ms/pixel) has already been achieved.

THz microprobe and graphene conductivity measurement data

Fig. 1: (a) THz microprobe tip-structure (b) Measurement data example taken from a sheet conductance sample study conducted for Samsung Techwin, South Korea. The plot is showing the sheet conductivity of a structured Graphene layer.

The microprobes are used as THz near-field detectors triggered by femtosecond laser pulses for the spatially resolved mapping of pulsed THz radiation transmitted through the device under test. Absolute sheet conductivity values can be directly determined from the obtained transmission data.

In contrast to earlier diffraction-limited THz transmission systems [2, 3] – the new microprobe-equipped system is able to achieve substantially higher (deep sub-wavelength) resolution as required for the inspection of the typically micro-structured graphene layers in touch screen devices. At the same time the measurement area size can be freely selected from small cut-outs to full display mappings.
In addition to sheet conductivity mappings the microprobes are also used for further analytic applications on active graphene-based devices as they have already been applied for THz emission measurements at optically excited graphite and graphene samples [4].

[1] http://www.amo.de/thz_tip.0.html?&L=1&L=2
[2] J. L. Tomaino et al. , “Terahertz imaging and spectroscopy of large-area single-layer graphene,“ Opt. Express, 19, 1, 141-146 (2011)
[3] J. D. Buron, D. H. Petersen, P. Bøggild, D. G. Cooke, M. Hilke, J. Sun, E. Whiteway, P. F. Nielsen, O. Hansen, A. Yurgens, and P. U. Jepsen, “Graphene Conductance Uniformity Mapping,” Nano Lett. 12 (10), 5074–5081 (2012).
[4] M. Nagel, A. Michalski, T. Botzem, and H. Kurz, “Near-field investigation of THz surface-wave emission from optically excited graphite flakes,“ Opt. Express 19 (5), 4667-4672 (2011).