CNF film and its characteristics

Figure 1a presents a likely life cycle of the CNF film where the film is first made from CNFs extracted from the woods, degraded via a fungal biodegradation process on disposal and sent back to the woods without adverse environmental effects. Electronic systems based on such material could significantly facilitate recycling and management of waste streams. Thus, the ecofriendly wood-based CNF substrate is clearly an ideal substitution for electronics that exist today. However, pure CNF film is vulnerable to water33. To address this issue, we coated the pure CNF film with a bisphenol A-based epoxy resin. As shown in Supplementary Fig. 1, the epoxy coating increased the contact angle of the CNF film from 28.4° to 74.6°, thereby making the CNF film much more hydrophobic. This treatment allowed for easier handling of the CNF substrate and offered better manufacturing capabilities. Epoxy is a type of thermoset plastic commonly used in electronics packaging materials (for example, electronic molding compounds, as well as underfills) due to its ease of handling, desirable materials properties and relatively low cost. The epoxy coating can also enhance the mechanical properties of the CNF film. Figure 1b–e introduces the unique material properties of epoxy-coated CNF films. As shown in Fig. 1b, the CNF film was transparent, thus making it ideal for certain applications. The transmittance was over 80% for an 80-μm thick CNF film and 60% for a 220-μm thick CNF film over the visible spectrum. Figure 1c presents thermogravimetric analysis (TGA) data showing the weight loss of the epoxy-coated CNF film as a function of temperature as well as the first derivative of the TGA curve. There were three peaks in the differential TGA curve, with the first (213 °C) and third (270 °C) peak corresponding to the decomposition of the CNF and epoxy process, respectively. The middle peak observed at 310 °C was attributed to the overlapping of the CNF and epoxy decomposition peaks. The glass transition temperature (T g ) of the film was measured at 72.8 °C (Supplementary Fig. 2), which was similar to that of polyethylene terephthalate (PET) film, a commonly used substrate for flexible electronics. In addition, the CNF film was strong and flexible enough to allow reversible bending as shown in Supplementary Fig. 3. The flexural modulus of the epoxy-coated CNF film was calculated to be 2.5 GPa, which is comparable to that of PET (1.5 to 2.8 GPa)34,35,36. The electrical properties of the CNF film were also appealing for use with electronics. As presented in Fig. 1d, the CNF film did not undergo an electrical breakdown, even at very high voltages (for example, 1,100 V), which is far beyond the requirement for consumer electronics. Furthermore, because the dielectric and RF properties of the substrate are major aspects to be considered in designing a RF circuit, the RF loss and dielectric constant were extracted using a microstrip waveguide and analysed at high frequencies. In the frequency range from 0 to 10 GHz, the dielectric constant ranged from 2.58 to 2.69, and the loss tangent ranged from 0.0302 to 0.0415, as presented in Fig. 1e. The above characterizations for the first time unveiled the suitability of CNF for high-frequency microwave applications. While the dielectric constant and RF loss values were comparable to those of PET film, the biodegradability property of CNF makes it a superior candidate over PET for addressing the abovementioned environmental impact.

Figure 1: Introduction to cellulose nanofibril (CNF) paper and its basic characteristics as a substrate for electronics. (a) An illustration of a likely life cycle of the biobased and biodegradable CNF paper. First, cellulose nanofibrils (CNFs) extracted from the woods is made into CNF paper. The CNF paper can be degraded via fungal biodegradation and sent back to the woods without adverse environmental effects. (b) The transmittance curve over a visible spectrum. Blue and red curves show the transmittance of 80-μm and 200-μm thick CNF films, respectively. (c) A thermogravimetric (TGA) plot showing the weight change of the CNF film as a function of temperature, along with the first derivative of the curve. The film remains stable up to 213 °C. (d) The electrical breakdown characteristics of CNF film. Current is measured while high voltage is applied on both sides of the film. (e) Radio frequency characteristics of the CNF film. Dielectric constant (red) and loss tangent (blue) are measured in the frequency range of 0 to 10 GHz using a microstrip waveguide. Full size image

Fabrication process of GaAs devices on CNF substrates

Compared with devices operating at low frequencies (∼MHz) or direct current (DC) levels, microwave (∼GHz) devices are especially difficult to fabricate on foreign substrates, due to the small feature sizes and high temperature processes required for high performance. Here we present, for the first time, methods to fabricate microwave GaAs-based devices on foreign substrates, namely the CNF substrate in this case. It should be noted that today’s majority portable gadgets (>85% in cell phones) with wireless communication functions employ GaAs-based microwave devices for their superior high-frequency operation and power handling capabilities. Figure 2a outlines the procedure for manufacturing GaInP/GaAs heterojunction bipolar transistors (HBTs) on a CNF substrate via schematic illustrations. Thin heterojunction epitaxial layers in stacks of n-cap layer (GaAs:Si)/n-emitter layer (GaInP:Si)/p-base layer (GaAs:C)/n-collector layer (GaAs:Si)/n-sub-collector layer (GaAs:Si) were grown on a 500-nm thick sacrificial layer (Al 0.96 Ga 0.04 As) on a GaAs wafer. The fabrication process began by following conventional procedures to fabricate the HBTs (Supplementary Fig. 4), followed by protective anchor patterning using a photoresist (PR). This will protect the devices and allow the devices to be tethered to the substrate after etching away the underlying sacrificial layer using a diluted hydrofluoric acid (HF) solution. Van der Waals contact with a soft elastomer stamp made of polydimethylsiloxane (PDMS) to the device breaks the anchors on all four sides and easily picks up a single device. The devices are transfer printed in deterministic assembly onto a temporary Si substrate using ultrathin polyimide (PI, ∼1 μm) as an adhesive, followed by ground–signal–ground (G–S–G) RF interconnect metallization. PI is an excellent material for GaAs-based devices not only as an adhesive, but also as a passivating material that can suppress the high surface states of GaAs and prevent leakage current37. Devices are then released from the temporary substrate and printed onto a CNF substrate using a PDMS stamp (Supplementary Fig. 5). Figure 2b–d presents optical microscopy images of fully formed HBTs on a GaAs substrate that are ready to be picked up. As shown in Fig. 2b, an array of 1,500 releasable HBTs on a 5 × 6 mm2 GaAs substrate can be fabricated. The image in Fig. 2e presents an array of HBTs on a CNF substrate wrapped around a tree stick, demonstrating the high flexibility of these electronics.

Figure 2: The fabrication process for deterministic assembly of GaAs devices on CNF paper and quantitative analysis on the influence of GaAs to the environment. (a) Schematic illustration of the fabrication process of GaInP/GaAs HBTs on a CNF substrate. The HBTs are fabricated on a sacrificial layer grown on a GaAs substrate and released with protective anchors made of photoresists. Each HBT is picked up using a PDMS micro-stamp and printed onto a temporary Si substrate with polyimide as the adhesive. After RF interconnect metallization, the devices are released from the temporary substrate and printed onto a CNF substrate using a PDMS stamp. (b) An optical microscopy image showing 1,500 releasable HBTs in a dense array format on a 5 × 6 mm2 size GaAs substrate. Scale bar, 2 mm. (c) A magnified image of the array. Scale bar, 200 μm. (d) An optical image showing a single releasable HBT that is tethered to the substrate with photoresist anchors. Scale bar, 30 μm. (e) A photograph of an array of HBTs on a CNF substrate wrapped around a tree stick with a ∼3 mm radius. (f) Comparison chart showing the amount of the arsenic corresponding to each type of device/transistor listed as well as the amount of water calculated according to the EPA standard based on the quantity of the arsenic present in these devices/transistors. For a single conventional cell phone, ∼138 l of water is required to satisfy the EPA standard, whereas only 0.32 l is required using our approach. In addition, 23 l is required for a single conventional chip with 40 HBTs, while only 0.054 l is required for the same number of HBTs with our approach. One HBT only requires 0.0013, l of water. Full size image

Analysis of the influence of GaAs on the environment

The Environmental Protection Agency (EPA) has set the arsenic standard for drinking water at 10 p.p.b.38, that is, 10 μg l−1. Compared with a typical GaAs MMIC, which only consists of a few HBTs on a large substrate, this pick-and-place method greatly reduces the usage of expensive and hazardous semiconductor materials. Figure 2f presents a quantitative analysis on the amount of arsenic present in the corresponding device/transistor due to the usage of GaAs that may lead to adverse environmental impact. Also shown in Fig. 2f is the amount of water calculated according to the EPA standard based on the amount of arsenic present in these devices/transistors. This analysis shows that a significant amount of clean water can be saved or preserved using our deterministic assembly approach in making the GaAs-based electronics. The weight of arsenic was obtained by converting the measured volume of either conventional GaAs chip or our printed HBTs to weight. As an example, a conventional miniature GaAs HBT-based MMIC with 40 HBTs on a 1.15 × 0.75 mm2 large and 100-μm thick substrate39 was used as a reference for the comparison. Moreover, our single GaAs HBT with a volume of 5.04 × 10−6 mm3 was used. Assuming that there are six GaAs HBT-based MMIC chips in a typical cell phone, ∼138 l of water is required at minimum to meet the standards, whereas the same cell phone using our approach only requires 0.32 l of water. For a single conventional chip with 40 HBTs, 22.9 l of water is required, whereas only 0.054 l is required for the 40 HBTs fabricated using our method. This approach is even more advantageous where only a few HBTs are required. For instance, a single conventional chip with 40 HBTs and 20 HBTs would have similar weight because they are typically built on a similarly sized substrate; however, 20 HBTs printed using our approach would weigh exactly half of the 40 HBTs. In fact, a single printed HBT only requires 0.0013, l of water to meet the EPA standard for drinking water.

Microwave GaAs electronic devices on CNF substrates

Figure 3c–e shows the electrical properties of a single finger (2 × 20 μm2) non-self-aligned HBT on a CNF substrate, which is optically shown in Fig. 3a,b. The inset image of Fig. 3c shows an optical microscopy image of the device that was measured. The Gummel plot presented in Fig. 3c reflects collector and base electric current, I C and I B , against base-emitter voltage, V BE with zero V BC bias. The common-emitter current gain curve under zero V BC bias is shown in green in Fig. 3c, which indicates that the β had its maximum value of 14.49 at a V BE of 1.86 V. Under an extreme bending condition (that is, at a bending radius of 2.5 mm), the maximum β value decreased slightly to 13.64 (Supplementary Fig. 6). The I C versus V CE curve is presented in Fig. 3d. The positive V CEOFFSET value of 0.14 V is due to the single heterojunction structure of our HBT where the offset comes from the difference in bandgap between the emitter (GaInP) and the base (GaAs). The decaying collector current observed as V CE was increased at high base current is attributed to poor thermal dissipation as the thermal conductivity of the underlying CNF substrate (κ=1.0 W m−1 K−1)40 was lower than that of a typical GaAs substrate (κ=56 W m−1 K−1)41. The RF performances of the HBT were analysed from the measured scattering (S) parameters from 0.045 to 50 GHz (Supplementary Fig. 7). Open and short patterns of the probing pads on the CNF substrate were used to subtract the effect of parasitic inductances and capacitances of the pad. Figure 3e presents the current gain (H 21 ) and power gain (G MAX ) against frequency for the device under a bias of V C =2 V and I B =2 mA. A high cut-off frequency (f T ) of 37.5 GHz and a maximum oscillation frequency (f max ) of 6.9 GHz were obtained. The relatively low f max of 6.9 GHz was attributed to the non-self-aligned structure of the HBT where large emitter-base spacing (2 μm) introduced high base resistance, causing the f max to drop42. These outstanding RF results further prove the suitability of CNF for microwave applications. Although we observed a decay of current at increasing voltages due to the relatively low thermal conductivity of the CNF film, the frequency responses of the HBT were sufficiently high to be used as practical amplifiers in mobile devices where the cellular frequency is in the range of 800 to 2,500 MHz. By incorporating materials with high thermal conductivities, such as boron nitride or diamond nanoparticles into the CNF film, the device performance can be further improved.

Figure 3: Microwave active GaAs electronic devices on CNF paper. (a) Photograph of an array of HBTs on a CNF substrate put on a tree leaf. (b) Magnified photograph of the array. (c) Gummel plot and β plot showing the maximum DC gain of the HBT. The maximum β is 14.49. The inset optical image shows one of the HBTs in the array that was measured and characterized. (d) I C versus V CE plot of the HBT plotted at 0.5 mA steps of I B . (e) Current gain (H 21 ) and power gain (G MAX ) as a function of frequency, with a collector voltage bias of 2 V and a base current bias of 2 mA. (f) Current versus voltage plot of the Schottky diode on a CNF substrate. The red curve shows the logarithmic scale and the blue curve shows the linear scale. The inset optical image shows the diode transferred onto a CNF substrate with G–S–G interconnects. (g) Measured S 11 (red) and S 21 (blue) plotted against frequency under a forward current bias of 10 mA. (h) Measured S 11 (red) and S 21 (blue) plotted against frequency under a reverse voltage bias of −0.5 V. Full size image

Schottky diodes based on GaAs are commonly used in high speed communication systems such as mixers and rectifiers. Same fabrication techniques with minor changes, as described in Fig. 2a can be implemented to fabricate high-performance Schottky diodes. Similar to the HBTs, nearly 1,200 Schottky diodes with high yield can be fabricated on a 5 × 6 mm2 GaAs substrate. Figure 3f (with an inset image showing the measured diode) presents the DC performance of the diode measured on a CNF substrate, where an ideal Schottky behaviour with a low turn-on voltage of 0.7 V was obtained. A logarithmic plot (shown in red) of the data shows a good ideality factor of 1.058. Figure 3g,h presents the measured S-parameters of the diode at forward bias and reverse bias, respectively (polar plots and Smith charts are shown in Supplementary Fig. 8). At a forward current bias of 10 mA (V=740.6 mV), the insertion loss (S 21 ) was only −1 dB at 20 GHz, making it suitable for RF applications. At a reverse voltage bias of −0.5 V (I=−414.1 pA), the insertion loss (S 21 ) reached −2 dB at 4.3 GHz. The low resistance obtained under reverse bias at high frequencies shows that these diodes can perform with high switching speeds in microwave circuits.

Passive elements are crucial components that are used for various purposes, such as RF chokes and impedance matching networks in RF circuits. To demonstrate the full capability of the CNF substrate for microwave circuit application, simple metal–insulator–metal (MIM) capacitors and spiral inductors were fabricated on a CNF substrate. Figure 4a presents the structure of the two passive elements on a CNF substrate with schematic illustrations. Bottom inductor metal and MIM capacitors, with 200 nm of TiO 2 as the dielectric material, were deposited on a releasable thin PI (∼1 μm) sheet spin casted on a temporary Si substrate. Another PI layer served as via holes during the subsequent metallization step for the G–S–G RF interconnects. The finished passive components were then released from the temporary substrate and transfer printed onto the CNF substrate. Figure 4b shows a photograph of the inductors and capacitors on a CNF substrate placed on a tree leaf. Figure 4c,d shows the optical microscopy images of the measured inductor and capacitor, respectively. Measured S-parameters are plotted in Supplementary Fig. 9. The inductance of the 4.5 turn inductor versus frequency is plotted in Fig. 4e. The width of the metal line of this inductor was 10 μm and the spacing between the adjacent metal lines was 5 μm. A constant inductance of ∼6 nH was obtained up to ∼8 GHz, with a self-resonant frequency (f res ) of 15.1 GHz. A peak Q value of ∼20 was obtained at 8 GHz as shown in the inset image of Fig. 4e. Figure 4f plots capacitance against frequency for a 30 × 30 μm2 MIM capacitor with Q factor plotted in the inset image. A constant capacitance of ∼1.3 pF was measured up to 6 GHz, with a f res of 12.1 GHz. Such high Q and f res values obtained at a broad frequency range suggest that these inductors and capacitors are applicable for high speed RF integrated circuits, in conjunction with the microwave devices, on CNF substrates. To evaluate the printed microwave devices on a CNF substrate in an application, four microwave GaAs-based Schottky diodes and an MIM capacitor were combined into a simple integrated circuit to form a full bridge rectifier, as optically shown in Fig. 4g with its circuit diagram shown in Supplementary Fig. 10. The rectification behaviour of RF-to-DC conversion at 5.8 GHz is shown in Fig. 4h. This frequency is one of the popular frequencies in wireless local area network, commonly used in high speed Wi-Fi systems. As shown in the plot, the rectifier can rectify a 21 dBm input signal to an output power of 2.43 mW. The ability to rectify such high-frequency signals can be attributed to the excellent electron mobility of GaAs and the low turn-on voltage of the Schottky diodes. With an appropriate matching network, the rectification ratio is expected to increase drastically by enhancing the reflection loss of the circuit. S 11 of the rectifier is shown in Supplementary Fig. 11.

Figure 4: Microwave passive elements and integrated circuit on CNF paper. (a) An exploded view schematic illustration of the inductor and capacitor on a CNF substrate. (b) Array of inductors and capacitors on a CNF substrate put on a tree leaf. (c) Optical image of the measured 4.5 turn inductor. Scale bar, 100 μm. (d) Optical image of the measured MIM capacitor. Scale bar, 100 μm. (e) Inductance plotted against frequency with an inset plot showing the inductor Q factor as a function of frequency. (f) Capacitance plotted against frequency with an inset plot showing the capacitor Q factor as a function of frequency. (g) An optical microscopy image of a full bridge rectifier built on a CNF paper. Here the microwave Schottky diodes and an MIM capacitor were integrated. Scale bar, 50 μm. (h) Measured rectified DC output power of the rectifier while applying RF input power from 10 to 21 dBm at 5.8 GHz. Full size image

Si-based digital electronics on CNF substrates

In addition to microwave electronics that allow wireless communication for mobile electronic devices, digital circuits are also important components that are dominant in most electronic devices as microprocessors and controllers. Figure 5 summarizes a set of digital logic circuitries on a CNF substrate using Si-based complementary metal–oxide–semiconductor (CMOS) devices. Figure 5a shows the completed digital circuits on a CNF substrate, which includes ‘universal’ logic gates (Inverter, NOR gate and NAND gate) and a full adder. The fabrication was done by separately printing Si nanomembrane-based p-type metal–oxide–semiconductor field-effect transistors (MOSFETs) and n-type MOSFETs onto a PI-coated temporary Si substrate, followed by deposition of gate oxides and metal interconnects for making CMOS-based digital circuits. A sequence of schematic illustrations describing the fabrication process is shown in Supplementary Fig. 12. Figure 5b presents the current–voltage characteristics of the p-type MOSFET (left) and n-type MOSFET (right). Figure 5c shows an optical image of the CMOS inverter. As presented in Fig. 5d, the inverter exhibits a good input and output relationship. A further modelling of these CMOS transistors established NOR and NAND logic gates, which are optically shown in Fig. 5e,g, respectively. The input and output relationships of the NOR and NAND gates are shown in Fig. 5f,h, respectively. The inputs and outputs can be seen as well-defined ‘0’s and ‘1’s. All of these components can be used together to yield a simple integrated circuit on a CNF substrate. As an example, a full adder, which is highly scalable and useful in many cascaded circuits, was designed and fabricated on a CNF substrate, as optically shown in Fig. 5i. This full adder is a mirror full adder, which consisted of 28 transistors with 4 of them used for inverter construction. As presented in Fig. 5j, the two single bit outputs (SUM and Carry Out) had a 0.2 ms switching delay when responding to the three single bit inputs (Input A, Input B and Carry In). This made the full adder work at a frequency of up to 5 kHz.

Figure 5: Digital electronics on CNF paper. (a) A photograph of CNF paper with digital electronics. (b) I D versus V D plot of a p-type MOSFET (left) and an n-type MOSFET (right) at V G steps of 1 V. (c) An optical microscopy image of an inverter. (d) Input–output characteristics of the inverter. (e) An optical microscopy image of a NAND gate. (f) Input–output characteristics of the NAND gate. (g) An optical microscopy image of a NOR gate. (h) Input–output characteristics of the NOR gate. (i) An optical microscopy image of a full adder. The adder consists of 28 transistors. (j) Characteristics of the full adder: Input A, Input B, Carry In, Sum and Carry Out are shown in descending order. Full size image

Fungal biodegradation tests of the CNF-based electronics

As presented in Figs 2, 3, 4, 5, all types of electronic systems required for building an electronic device can be realized on a CNF paper. To prove the concept of biodegrading electronic devices and to close the cycling loop that is shown in Fig. 1a, one of the electronics that we have presented here was subjected to a fungal degradation test. Figure 6 summarizes a sequence of fungal degradation tests on CNF-based electronic devices. First, two different types of decay fungi, brown rot fungus Postia placenta and white rot fungus Phanerochaete chrysosporium, were considered and tested on the pure CNF substrate and on the epoxy-coated CNF substrate, without any electronics printed on them. Figure 6a presents the average weight loss percentages of these CNF-based films after 28 days. For each degradation test, five identical samples were degraded under the same conditions. Pure CNF samples showed a larger average weight loss (Postia placenta: 19.20%, Phanerochaete chrysosporium: 35.20%) compared with the epoxy-coated samples. While P. placenta induced a slower degradation rate for pure CNF film, it caused a faster degradation for the epoxy-coated CNF film (P. placenta: 9.96%, P. chrysosporium: 6.60%) in comparison with P. chrysosporium. Therefore, P. placenta was chosen as the decaying agent for our CNF-based electronics that consisted of the epoxy-coated CNF film. The amount of epoxy in the epoxy-coated CNF film was 9.6% by weight. Figure 6b shows the weight loss result of digital electronics on CNF substrates after P. placenta decaying for 84 days. Four replicas were made, and on average, the weight loss percentage was 12.25%, with a s.d. of 5.43%, suggesting that the CNF film will fully degrade after an extended period of time. Figure 6c–e shows the images of the decaying process of an epoxy-coated CNF substrate with digital electronics against P. placenta. Photos were taken after 6 h, 10 days, 18 days and 60 days as shown in Fig. 6c,d. As presented in Fig. 6e, the fungi started to partially cover the sample after 10 days, and fully covered the sample after 60 days. Once degraded, the leftover electronics portion, which is encapsulated in PI, can be collected to be further decomposed and recycled. Although PI can deteriorate with certain fungi, the degradation process is extremely slow compared with CNF, and because PI is generally non-permeable to water or solvents it can be used to protect against any leakage of materials to the environment43.