Ultra-Thin Pyrocarbon Films as a Versatile Coating Material
© The Author(s). 2017
Received: 22 September 2016
Accepted: 3 February 2017
Published: 16 February 2017
The properties and synthesis procedures of the nanometrically thin pyrolyzed photoresist films (PPF) and the pyrolytic carbon films (PCF) were compared, and a number of similarities were found. Closer examination showed that the optical properties of these films are almost identical; however, the DC resistance of PPF is about three times higher than that of PCF. Moreover, we observed that the wettability of amorphous PPF and PCF was almost comparable to crystalline graphite. Potential applications executed by utilizing the small difference in the synthesis procedure of these two materials are suggested.
KeywordsPyrolytic carbon Pyrolyzed photoresist Nanographite film Chemical vapor deposition
Graphitic carbon is a versatile material that has been widely studied throughout the centuries. Especially, fullerenes, carbon nanotubes, and recently graphene materials have received wide attention. At the same time, amorphous carbon remains somewhat overshadowed by these crystalline materials. One can attribute this to the incomparability of the vast majority of electrical and optical properties of amorphous carbon films and their crystalline counterpart (e.g., graphene) [1, 2]. However, amorphous carbon films have one significant advantage over crystalline graphitic carbon, that is, the production cost and simplicity. These properties are themselves a reasonable justification for the preference of utilization of these materials in the industrial sector.
Two well-known nano-graphitic carbon materials are a pyrolyzed photoresist film (PPF) [3, 4] and a pyrolytic carbon film (PCF) [1, 5]. Although in contrast to graphite and graphene the crystallization of these materials is very low [1, 3, 5, 6], the simplicity of the deposition of these films on various surfaces makes them a good competitor for graphene.
As the name describes, the PPF is a carbon-based photoresist film that has been pyrolyzed at high temperature [3, 4]. During the pyrolysis, the resist solvents are evaporated and remaining carbon atoms are hybridized by sp2 and sp3 bonds . The noteworthy factor here is that the PPF is fabricated from photoresist, while PCF is conventionally fabricated by chemical vapor deposition (CVD) via gaseous hydrocarbon pyrolysis in the temperature range of 1000 °C and above [6, 7]. In the lower temperature regime, i.e., at around 1000 °C, the nano-graphitic carbon is typically very amorphous. However, by increasing the temperature, the degree of crystallization increases as well [8, 9]. Eventually above 2000 °C, the CVD procedure results in the highly oriented pyrolytic graphite [8, 9]. However, since only a few substrates can hold such an extreme synthesis temperature, the deposition temperature of PCF rarely exceeds 1200 °C.
The interesting aspect for both PCF and PPF is that they can be deposited as an ultra-thin film on a dielectric and a semiconducting substrate [1, 4]. Although PCF and PPF are seemingly very similar materials, there are still some small differences present. In this paper, we give an insight to the synthesis and properties of these two ultra-thin carbon materials. A remarkable fact is the influence of the small difference in the synthesis procedure of these two materials (PPF is fabricated by photoresist precursor and PCF in homogeneous hydrocarbon CVD) on the range of their applicability. As part of the discussion of the properties of these materials, recommendations for their potential use are given. Learning the small differences between these films can easily help one to find the most suitable technique to fabricate carbon layers to suite the desired purpose.
Experimentally, a PPF and a PCF can be synthetized on any substrate that withstands the 1100 °C processing temperature. For this particular experiment, we used the silica substrates (later, for demonstration purposes in the “Discussion” section, also silicon and oxidized silicon substrates were also used).
A PPF was fabricated by spin coating the substrate by the carbon-based photoresist (nLOF-AZ2070 diluted with AZ ebr solvent with a 1:4 ratio). After the substrate is coated by a resist layer, it is baked on a hot plate (110 °C/1 min) and then pyrolyzed in a CVD system. The thickness ratio of the original photoresist film and the PPF is 1:10, i.e., the thickness of a 300-nm-thick photoresist film resulted in a 30 (±3)-nm-thick PPF. The thickness of the PPF film can be varied by changing the thickness of the resist film . In contrast to the PPF, a PCF is fabricated by using gaseous CH4 as a carbon precursor. Thickness of the PCF is controlled by methane concentration in the process (see, e.g., ). For our experiment, we used a CH4:H2 ratio of 4:1 at about 32 mBar which in turn resulted in a PCF with thickness of 30 (±3) nm.
Our conventional hot wall CVD consists of a vacuum chamber, computerized mass flow controllers, and gas lines for hydrogen, methane, and nitrogen. Before each process, the chamber is pumped down for 1 h to ensure proper vacuum. In order to make the comparison of the PPF and PCF more transparent, the processing temperatures were the same for PPF and PCF. More precisely, the chamber is heated to 700 °C at 20 °C/min and then to 1100 °C at 10 °C/min. The maximum temperature of 1100 °C is kept for 5 min, and then, the chamber is cooled down to 700 °C in 1.5 h; the rest of the cooling is done overnight. Since the photoresist acts as the carbon precursor in PPF, the sample is heated and cooled in the hydrogen atmosphere. Heating is done in 5 sccm H2 flow (0.2 mBar) and cooling, in contrary, in static H2 atmosphere of 5 mBar. Alternatively, the PCF is done on a substrate by first heating the chamber to 700 °C in H2 flow (5 sccm) and then injecting the CH4:H2 gas mixture into the chamber. Thereafter, at 700 °C, the CH4:H2 gas mixture is replaced by H2 (static 5 mBar) in which the rest of the cooling took place. Since PCF is grown on both sides of the sample, the back side carbon is removed by the oxygen plasma (100 W/20 sccm/2 min).
More careful analysis reveals that there are practically no differences in the Raman spectrum of the PPF and the PCF. However, although the films are very amorphous, the existence of D and G peaks reveal that the material is a nano-graphitic material [9, 13, 15].
Optical absorption in graphitic carbon is governed by the pi-electrons , and thus, it is expected that the optical absorption is similar in all graphitic carbon materials with dominating sp2 hybridization. In crystalline graphene, the linear electron band structure can be well described as a constant at NIR but slightly changes due to the M-saddle point absorption at UV (the absorption peak maximum is located at around 260 nm) . In that sense, both of the carbon films are somewhat close to the optical properties of graphene/graphite [16, 17]. However, despite the absorption spectra of the PCF and the PPF somewhat resembling the graphene’s absorption spectrum, it is reasonable to remember that due to the amorphous nature of the PPF and the PCF, those films have no well-specified electron band structure.
Since the PCF grows in the homogenous CVD process, the film appears as a uniform coating throughout the surface despite the surface texture. More precisely, the PCF can be easily grown on an arbitrarily shaped substrate/structure where the sample size is restricted only by the CVD chamber. In Fig. 5, one can observe about 10-nm-thick PCF grown on a patterned silicon substrate (see detailed growth parameters in ). In spite of the grating structure, the carbon film is continuous and homogeneous. Since carbon is often considered as a bio-compatible material, such a hydrophilic coating could offer a nice platform, e.g., for biological studies. Also, since PCF is a conductive material, this film could be used as a conductive layer when electroplating metals (this will be reported elsewhere) or a contact material for semiconductors . Furthermore, because of strong optical absorption in PCF, an ultra-thin PCF was recently used to increase the absorption of black silicon .
Moreover, the robustness of the PCF is indicated by a recent explosive electron emission study where the PCF was suggested to be a coating material for the development of a hybrid cathode, which was characterized by low plasma expansion velocity . The PCF coating allowed generation of powerful microwave pulses of long duration, up to microseconds along with high efficiency (due to the low velocity scatter electrons emitted from the cathode) .
Physical properties of two slightly different nanoscale carbon films were compared. The PPF and the PCF were grown on a silica substrate in very similar processes, which allowed us to compare differences of these films in an unbiased manner. Overall, the Raman spectra and linear optical absorption of PPF and PCF were almost identical for both of the films, while the electrical DC resistivity was observed to be higher for the PPF then for PCF. Moreover, as an alternative material for graphene, either film could be applied in experiments where very thin carbon layer is found to be beneficial.
Chemical vapor deposition
Pyrolytic carbon film
Pyrolyzed photoresist film
This research was funded by Ministry of Education and Science of Russian Federation, project ID RFMEFI57714X0092.
TK prepared the samples and made the measurements. TK and PK made the data analysis and wrote the manuscript. Both authors read and approved the final manuscript.
The authors declare that they have no competing interests.
Open AccessThis article is distributed under the terms of the Creative Commons Attribution 4.0 International License (http://creativecommons.org/licenses/by/4.0/), which permits unrestricted use, distribution, and reproduction in any medium, provided you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made.
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