Printed Touch Sensors Using Carbon NanoBud Material
A new carbon nanomaterial is enabling touch sensors with high contrast for outdoor readability and flexibility that allows folding and sharp-angle 3-D formability.
by Anton S. Anisimov, David P. Brown, Bjørn F. Mikladal, Liam Ó Súilleabháin, Kunjal Parikh, Erkki Soininen, Martti Sonninen, Dewei Tian, Ilkka Varjos, and Risto Vuohelainen
THE user experience of current consumer electronics, and of touch displays in particular, is limited in large part by the properties of existing conductive materials used as transparent electrodes in capacitive touch sensors. In this article, we discuss recent progress in achieving advanced conductor material characteristics based on Canatu’s Carbon NanoBud (CNB) technologies that allow improved optical and mechanical performance, novel flexible and three-dimensional form factors, and reduced cost. These advances are significant because they will enable flexible or 3-D-shaped touch sensors, high-optical-quality touch displays with almost no reflectionsand high outdoors contrast, and cost-effective manufacturing through dry roll-to-roll processing.
Specifically, we describe how we created a CNB-based GFF (two-layer Glass-Film-Film structure) demonstrator with only 2.2% reflection and 40% better contrast in bright ambient light compared to a comparable structure using ITO. We also show that CNB films can repeatedly be folded 140,000 times at a 2-mm radius. For 3-D-shaped rigid touch, we achieved formability with 1-mm-radius edges and 120% stretching.
Limitations of ITO
Indium tin oxide (ITO) is the current industry- standard transparent conductor material. However, ITO is a brittle ceramic that has presented many challenges to developers of flexible electronics. In addition to its physical-property limitations, ITO poses optical-performance challenges due to a high index of refraction. Touch screens made with ITO layers tend to reflect relatively high amounts of incident light, and this may cause display images to become washed out in bright indoor and normal outdoor conditions. The effect can be reduced by using index-matching layers in ITO sensors, but these add cost and complexity.
Recently, silver nanowires and different forms of metal-mesh alternatives have emerged as ITO replacement materials. These have benefits for large-area touch displays because they often provide higher conductivity values for comparable light-transmission performance compared to ITO. However, silver nanowires and metal meshes are metallic based and hence also have relatively high-ambient-light reflectance, which can result in a washout effect.
To address the shortcomings of the above existing solutions, we developed a new carbon nanomaterial, the Carbon NanoBud; a hybrid of carbon nanotubes and fullerenes.1 Hybrid-ization is achieved directly in the material synthesis process and the resulting material combines the high functionality of fullerenes with the high conductivity and robustness of nanotubes. The aerosol synthesis of carbon nanotubes has been demonstrated by Nasibulin et al.2 This method has been modified and scaled to produce commercial quantities of clean, lightly bundled, high-crystallinity CNBs directly in the gas phase, thus eliminating the need for liquid processing.
We have also developed a new thin-film manufacturing method called Direct Dry Printing (DDP), based on the work described in Kaskela et al.3 and the thermophoretic technique described in Gonzales et al.4 that allows direct synthesis and patterned deposition of CNBs by aerosol deposition. The combination of aerosol synthesis and DDP allows homogeneous or patterned deposition on any substrate at room temperature and pressure, resulting in a simple, scalable, one-step, low-cost, and environmentally friendly thin-film manufacturing process that improves the quality and performance of final products. Unlike conventional methods, no material degrading or hazardous acid treatments and no sonication, surfactants, or functionalizations for dispersion, purification, and deposition are required. DDP is applicable to both sheet and roll-to-roll implementations and can be combined with conventional screen, gravure, and flexo printing to allow the production of continuous rolls of complex, multi-layered components.
CNB Film and Touch-Sensor Manufacturing
For this work, we have made homogeneous and patterned depositions of CNB films on A4- and A3-sized sheets by combining the aerosol synthesis method with room-temperature deposition based on a modification of the above described filter transfer technique. Consequently, high purity, low bundling, and low concentration of catalyst material have been achieved.5
Fine patterning and conductive traces. Most projected-capacitive touch sensors require fine patterning with minimum feature sizes of 25–50 μm. We have achieved this via laser ablation, which maintains the dry manufacturing process with no liquid handling and hence a lower environmental footprint. Because no masks are required in laser ablation, the lead time for pattern changes is short. Only one process step is required, as opposed to eight steps in photolithography. Laser patterning is therefore more cost competitive than the more commonly used photolithography (see Table 1), and the low incremental capital expenditure for laser equipment as opposed to a photolithography line makes it more flexible for demand fluctuations and enables better line utilization.
Canatu uses CNB patterning with 30-μm gaps in its production, enabling fully invisible conducting patterns on touch screens.
To complete the touch sensor, a conductive silver (Ag) layer is printed using a Microtec MTP-1100 TVC screen-printing machine. Ag traces are fine patterned, either by laser ablation in the same process step as the CNB patterning or, for even better production efficiency, by direct screen printing. We can now produce silver patterns down to 30 μm/30 μm lines and gaps with both laser ablation and screen printing.
It is important to note that metal-mesh-based touch sensors require very high tolerance and early design know-how of display pixel geometry to reduce the moiré effect between the display and the sensor. Metal-mesh manufacture is also demanding, as the bonding equipment needs to be highly controlled. CNB sensors are display-design agnostic due to the pattern invisibility and random orientation of the CNB deposition.
Manufacturing process and line. In our factory in Helsinki, Finland, we are developing the production capacity for medium-volume manufacturing of CNB films and touch sensors (400 m2 of CNB film/month or 20,000 mobile-phone touch sensors/month). Our business model for high-volume touch-sensor manufacturing is to deliver CNB films for touch module manufacturers (Fig. 1).
The company is currently building a 600-mm-wide roll-to-roll CNB deposition machine. The first unit was scheduled for production in June 2014. The capacity for the machine is 8000 m2/month and the company is planning to have four lines installed by Q4 2014. The current facility allows capacity up to 500,000 m2/month.
Fig. 1: Shown is a CNB touch-sensor manufacturing process and business model for high-volume touch-sensor sales.
CNB Film Properties: Transparency, Reflectivity, and Haze
Since 2007, Canatu has been able to double CNB film conductivity at a given transparency approximately every 12 months. Figure 2 shows CNB film releases since 2011. We now manufacture Gen 5 films with the following properties without substrate: 100 Ω/□ at 94%, 150 Ω /□ at 96%, and 270 Ω /□ at 98%. In the lab, we can make 100 Ω /□ at >95%. CNB film transparencies are on par with ITO in touch devices with stacks such as GF1 or GFF. High transparency is needed for enabling bright display images and pattern invisibility.
Fig. 2: CNB film transmission vs. sheet resistivity is compared for the current Gen 5 to previous results from 2011 to 2013. Transmission is substrate-normalized.
The haze of CNB film is negligible, as measured by the HunterLab Ultra-Scan VIS spectrometer (similar to ASTM D1003-95 standard) (see Fig. 3).
Fig. 3: The haze of substrate-normalized CNB films appears above as a function of sheet resistivity. Haze does not increase at low sheet resistivity as it does with AgNW and metal meshes.
Color neutrality. A transmission spectrum for a CNB film is shown in Fig. 4. The transmission spectrum was first measured by HunterLab from the film on a PET substrate (ASTM E1164 standard). Subsequently, the absorption of the PET was subtracted to obtain substrate normalized data. As can be seen, the optical absorption is uniform over the entire visible spectrum. The CIELAB color coordinates after normalization were measured as L*= 97.9 ± 0.1, a* = 0.0 ± 0.1, and b* = 0.6 ± 0.1, demonstrating that CNB films and sensors have very little color distortion.
Fig. 4: The transmission spectrum of substrate-normalized CNB films is compared to ITO.
Mechanical and environmental performance. CNB films on a 130-μm PET substrate were exposed to severe (180°) bending at a radius of 2mm, with results shown in Fig. 5. Sheet resistance was shown to remain nearly constant over 30,000 bend cycles, after an initial change of a few percent. In another similar test with 140,000 bend cycles, the change in resistivity was less than 7%. This demonstrates the applicability of CNBs for flexible and foldable touch products
Fig. 5: The change in resistivity for a CNB film on a 130-μm PET substrate is shown for repeated bends. The bending radius was 2 mm.
We have applied film insert molding (FIM) (aka in-mold decoration or IMD) as a standard industrial process for rigid 3-D shaped touch devices. Figure 6 shows a demonstrator with 120% stretching and 87° bending at 1 mm radius in an FIM device, demonstrating the high stretchability of CNB films. In collaboration with Bayer MaterialScience, we have made both 1-CNB-layer and 2-CNB-layer test devices for PF1 (Plastic-Film) and PFF (Plastic-Film-Film) type touch stack constructions. CNB layers, applied on polycarbonate Makrofol DE film, were three dimensionally shaped by a high-pressure forming process.6 The resulting inserts were injection back-molded with clear Makrolon polycarbonate resin. In all test devices, CNB layers maintained their conductivity with a linear response to stretching.
Fig. 6: This film-insert-molded demonstrator for CNB films shows the possibility of sharp angles and deep indentations.
We exposed the CNB films to all typical consumer-electronics environmental tests, and these tests were passed, as shown in Table 2.
Touch Sensors
We made 13.3-in.-diagonal CNB projected-capacitive touch sensors with the manufacturing process as described. The touch stack was the Glass-Film-Film (GFF) type with sense and drive electrodes on separate PET sheets, laminated together and to the front glass with optically clear adhesive. The CNB film sheet resistivity was 220 Ω/□. The sensors were bonded with a flexible circuit board to the driving electronics, and the touch module assembly was “plug and play” retrofitted to an existing Intel Ultrabook reference design for comparison with the existing standard commercial ITO One-Glass Sensor (OGS). In this product, there is an air gap between the touch and the display. The assembly was made by SMK Corp. in Japan. An Atmel mXT224 chip was used as the touch controller. No modifications to the touch-sensor chip were required.
The CNB touch sensor passed Windows WHCK tests and is therefore fully certified for Windows 8. As characterized by Atmel, the touch performance was found to be equivalent to commercial ITO sensors. The reflectivity, as measured by HunterLab (ASTM E1164) from the CNB GFF touch display, was significantly lower than that from the comparison ITO OGS touch display. In a bright office on a sunny day (2000 lux, 1000-cd/m2 specular light), the resulting 4.2:1 contrast is 30% better in the CNB-based laptop than in the ITO-based laptop.
To compare CNB to ITO and metal meshes, we performed optical characterization of various touch modules at the Intel laboratory in Santa Clara. Table 3 shows that CNB touch modules have the lowest haze in this test.
To demonstrate touch on 3-D surfaces, we made a 12-cm-diameter dome-shaped PFF touch sensor with a 15-cm radius of curvature (Fig. 7).
Fig. 7: At the top is a 5-in. dome-shaped CNB projected-capacitive multi-touch sensor. On the bottom is a 3-D shaped-CNB FIM demonstrator with touch (right).
The drive and sensor sheets were made using a 500-Ω/□ CNB film. An Atmel mXT768E controller was used and the sensor pattern was “Flooded X” type with 10-finger multi-touch, 254-dpi resolution, and a 12-msec report interval. There is a high transmission of >97% through the active CNB layers, and the patterns are totally invisible.
In collaboration with TactoTek Oy, we also built a highly transparent 3-D shaped demonstrator with slider, wheel, and button touch, applying the FIM process with CNB on thin polycarbonate substrate and clear PMMA overmold (Fig. 8). The radius of curvature was 130 mm. TactoTek did the forming and injection molding with integrated LEDs.
Optical Demonstrators
To demonstrate touch-display contrast in a direct-bonded construction, we built a 10-in. optical demonstrator to compare a TFT-LCD touch panel with an ITO-based 150-Ω/□ GFF stack to one with a 150-Ω/□ CNB GFF stack. The ITO film chosen for this demonstrator was industry state of the art with complex index-matching layers and optically optimized ITO. There was no air gap between the touch and the display. In order to demonstrate the lowest possible reflectivity, we added an AR coating to the front window (glass/air interface). As seen in Table 4, the CNB GFF device has 2.2% total reflectance. The total reflectance from the ITO GFF device is 3.4%.
Table 4 shows the breakdown of the reflection values in the touch-display structure. The CNB sensor stack has no inherent reflections; hence, the 1.8% specular reflections in the GFF stack originate from the glass/AR/air interface and from the display (Fig. 8).
For the ITO sensor stack, despite complex index-matching layers, there are still 1% specular reflections from the ITO layers. By better optimizing the AR coating, using a less reflective display, and optimizing the direct bonding materials, <1% specular reflection is feasible with CNB GFF sensors.
Contrast ratios of the combined TFT-LCD used in this demonstrator (with ON Brightness of 220 cd/m2 and OFF brightness of 0.3 cd/m2) and the GFF touch stack were calculated using the measured reflections from the stacks presented in Fig. 8. The contrast ratio was calculated using the following formula:
Rh – the contrast at high ambient
I0 – white luminance of the display in dark room
Idk – black luminance of the display in dark room
Rd – diffuse reflectivity
Rs – specular reflectivity
Amb – diffused (daylight) ambient illumination
Spec – specular (glare) light source
Fig. 8: A stack diagram of direct-bonded 150-Ω/□ ITO GFF (left) is compared to CNB GFF (right). A photo of the demonstrator is at far right.
Figure 9 shows the contrast ratio plotted as a function of ambient illumination for the direct-bonded GFF devices with AR coating. In this simulation, we considered both diffuse ambient illumination and a bright specular source (i.e., a bright spot/area reflecting directly from the display). This combination is specified, for instance, in Vehicle Display standard J-1757. In a 2000-lux ambient/1000-cd/m2 specular source, the contrast ratio of CNB-based devices is 12:1 (40% higher than the similar ITO device).
Fig. 9: The chart shows the contrast ratio for direct-bonded AR-coated GFF devices with ITO and CNB.
Potential Impact
We have ramped up the production of CNB films and built several products and demonstrators showing the applicability of the CNB material for high-optical-quality, flexible, and 3-D formable touch sensors. A Windows-8-certified 13.3-in. Ultrabook touch module, a 5-in. multi-touch dome-shaped demonstrator, and a 3-D shaped film insert molded touch device have been successfully produced as demonstrators.
CNB films are now a commercially viable option for high-volume applications and for high-quality flat, flexible, and 3-D formed touch sensors. Canatu is now in the prototyping phase, with more than 30 customers worldwide for mobile phones, tablets, phablets, laptops, smart watches, digital cameras, automotive consoles, and white goods.
Acknowledgments
The authors want to acknowledge the funding received from TEKES, the Finnish Funding Agency for Technology and Innovation. We also wish to thank Adi Abileah for guidance and for reviewing our optical calculations for the contrast ratio.
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