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Impedance characteristics of carbon nitride films for humidity sensors
Sensors and Actuators B 117 (2006) 437–441
Impedance characteristics of carbon nitride films for humidity sensors Ji Gong Lee ∗ , Sung Pil Lee Department of Electronic and Electrical Engineering, Kyungnam University, 449 Masan, Kyungnam 631-701, Republic of Korea Received 9 June 2005; accepted 15 December 2005 Available online 20 February 2006
Abstract Carbon nitride films were deposited on silicon substrate for humidity sensors with meshed electrodes by reactive RF magnetron sputtering system with DC bias. The surface of carbon nitride films had a good uniformity with the grain size of about 30 nm. The EDS analysis revealed that the chemical formula of the carbon nitride film could be expressed between C7 N4 and C3 N and these results also revealed those quite agree with XPS results. The films have very high resistivity, the characteristic of which can be expected to use for insulating films on the Si fabrication. The impedance of the sample deposited on the Si-wafer decreased from 118 k to 4 k in the relative humidity range of 5–95%. Hysteresis of the film deposited on the Si-wafer was about 4.2% of full scale output. © 2006 Published by Elsevier B.V. Keywords: Humidity sensors; Carbon nitride films; Reactive RF magnetron sputter; Surface analysis
1. Introduction Sung and Sung [1] predicted the possible existence of carbon nitride, i.e. C3 N4 material in his unpublished patent disclosure letter at Diamond Technology Centre of Norton Company. Because of their short inter-atomic distances, these hypothetical materials were suspected to be very hard. This speculation initiated by Liu and Cohen in 1989 [2] to develop an empirical model and ab initio calculation of the bulk modulus for covalent solid formed between carbon and nitrogen. On the basis of this model, properties like bond strength of hypothetical C3 N4 compound were assessed. They suggested theoretically that these materials could have bulk modulus comparable to or greater than diamond. After carbon nitride was suggested as a super hard material, considerable interest has arisen in synthesizing the stoichiometric -C3 N4 phase which might have the strongest bulk modulus in carbon nitride phases theoretically. There have been many reports of amorphous carbon nitride films of uncertain composition, but they found only a few observations of crystalline -C3 N4 produced by reactive sputtering, laser abla-
∗
Corresponding author. E-mail address: [email protected] (J.G. Lee).
0925-4005/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.snb.2005.12.049
tion and hot filament CVD [3–6]. Most of deposited films have been discontinuous with isolated crystals [6] or showing only a few isolated grains in an amorphous matrix [5], and high substrate temperatures (600–950 ◦ C) were required for film deposition. According to report of DeVries [7], although over 400 papers have been published on crystalline C3 N4 by 1997, no new super hard materials have come out over a decade. One of the most significant problems degrading the quality of carbon nitride films is to be existence of N H and C H bonds mostly found in sputtering systems. However, the possibility of these reactions with hydroxyl group of carbon nitride films, caused by a hydrogen attack, was suggested in our previous reports and proved that these undesired effect could be applied for fabricating micro-humidity sensors [8]. The hydrogen attack in carbon nitride film could easily break or change the C N and C N bonds to form C H and N H bonds. This was strongly undesirable behavior for the composition of crystalline -C3 N4 . However, if one can use hydrogen’s attractive characteristics due to some hydrogen defects which were made intentionally and bonded weakly, carbon nitride could be a candidate for new materials for humidity sensor applications [8,9]. The similar view mentioned by Abedinov et al. [10] also enhances humidity sensing properties of carbon nitride films. They believed the molecular absorption of the water into the layer changed effec-
438
J.G. Lee, S.P. Lee / Sensors and Actuators B 117 (2006) 437–441
tive mass and induce tensile of compressive stress and supposed that forming hydrogen bonds of the polar H2 O with the N atoms was the main mechanism. The common requirements for humidity sensors are a good sensitivity, fast response, low hysteresis, high reproducibility, chemical and physical stability, and low cost. Although a large number of materials and compounds have been investigated for humidity sensors, it is still not easy to identify the best humidity sensing material. One material has some merits and it has other demerits as well. Therefore, many different humidity sensors exist and they are only used for a specific application. Recent achievements in miniaturized sensor, especially silicon based MEMs techniques, strongly ask for a new material that can be applied for micro-sensors or multi-sensors. In this respect, carbon nitride film can be a good candidate for new humidity sensing material in terms of having some advantages; facility compatibility with Si fabrication, fast response time over a wide humidity range, long-term stability against chemical contaminations, and low cost. In this paper, we studied the humidity sensing properties of carbon nitride films as function of nitrogen contents and deposition times to get the optimized growth conditions for humidity sensors. The crystalline properties of the carbon nitride films are analyzed by scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS), and humidity characteristics of carbon nitride films are investigated with LCR meter. 2. Experimental The humidity sensor was designed as a capacitance type with micro-hole on the upper plate electrode. Lower electrode (Al) was deposited on a cleaned p-type Si(1 0 0) substrate by a thermal evaporator and patterned by lift-off technique. After the lower electrode had deposited, the samples were heat-treated at 450 ◦ C for 5 min and cleaned by the traditional cleaning process. RF magnetron sputtering system was applied for deposition of sensing materials, carbon nitride film, on the lower electrode [11]. The carbon nitride film was formed by reaction of nitrogen gas (99.999%) and carbon target (99.99%) in the sputtering chamber for 1 h. A mixture of nitrogen and argon was introduced into the chamber for reaction gas and sputtering gas, respectively. The ratio of nitrogen to argon was changed from 0% to 100% in order to make different samples to investigate the effects of nitrogen content in the film and controlled precisely by mass flow controller (MFC). To reduce influence of contamination from chamber wall and undesirable gases, whole inner chamber wall was coiled by heating wire, and thermocouple was attached on the back of the substrate holder in order to control precisely substrate temperature typically at 200 ◦ C. This method might provide more accurate temperature control at the point of a reaction substrate than the conventional heating chamber. Upper electrode has square meshed micro-hole to enhance the absorption and desorption response of the sensing material. Each electrode was formed on the carbon nitride films by lift-off technique. The detail view of the sensors was illustrated in our previous works [7,8]. MFCs controlled humidity through regulating the flow rate of the dry air and the vapor-saturated air in the
measuring chamber. To avoid the condensation of water vapor in the inner wall of the chamber or pipelines caused by different temperature between introducing vapor and atmosphere inside, all measurement objects were installed together in the constanttemperature chamber controlled by PID controller [7,8]. The structure and surface morphology of the films were analyzed by SEM (ABT-32, TOPCON) and EDS (Sigma MS2, Kevex). Current–voltage characteristics were investigated with semiconductor test and analyzer (CATS CA-EDA, Korea). The sensor impedance was measured with a LCR meter (HP 4263B) linked to a computer. To verify the real relative humidity, commercial humidity sensor (Control Company, USA) which has ±0.5% accuracy was used. 3. Results and discussion When the RF power is applied, energetic carbon atoms due to the high plasma energy are sputtered from the target and deposited on the substrate, reacting with nitrogen. Fig. 1 shows the film thickness dependence on RF power between 100 W and 400 W. The deposition time was for 1 h. The deposition rate of carbon nitride film at 300 W is about 3.5 m/h, which is the highest value. The film thickness increases as RF power increases to 300 W. As RF power increases above 300 W, however, the thickness is decreased. If the RF power is raised above 300 W, the self-bias voltage and the ion bombardment energy normally increase. Then, the increase of ion bombardment energy will reduce the film deposition rate through the sputtering of the physisorbed precursors by N2 + ion bombardment [8]. The film deposited at high input power, however, is more dense and harder because of the dehydrogenation of a-CNx :H film and low stress with the substrate. The surface morphology and the cross-section were observed by SEM with accelerating voltages of 10 kV for surface and 15 kV for cross-section. The surface of carbon nitride film has a good uniformity which RMS roughness is ca. 1.21 nm [3] and the grain size of the sample deposited with 50% N2 is about 30 nm (Fig. 2(a)). Al electrode on the top layer and carbon nitride film beneath the aluminum layer (ca. 2 m) are shown in Fig. 2(b).
Fig. 1. Thickness changes as a function of RF power.
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439
Fig. 2. SEM photographs of carbon nitride films deposited on Si-wafer: (a) surface and (b) cross-section. Table 1 The atomic ratio of carbon nitride film by EDS results Component
C N O Si
N2 ratio (%) 0
30
50
70
100
64.93 4.7 2.64 25.9
55.14 31.59 4.60 8.66
51.97 36.11 6.23 5.69
71.68 23.63 3.67 1.03
69.41 24.44 4.81 1.34
The composition rate of deposited films was investigated by EDS (Table 1 and Fig. 3). Fig. 3 shows the normalized EDS peaks of the samples. The highest rate and lowest rate of carbon/nitrogen are 64.93:4.7 at 0% N2 ratio and 51.97:36.11 at 50% N2 ratio, respectively. We can see that even the sample which is deposited under 0% nitrogen has small amount of N contents. It can be inferred that nitrogen source may be provide from contaminant of chamber wall which were deposited by prior works. The chemical formula is represented as C1−x Nx , in carbon nitride formation. The range of x is 0.25–0.36 in EDS spectrum. Therefore, it can be roughly expressed as C7 N4 and C3 N. These results coincide with XPS spectrum analysis previously reported [3]. The detail component is shown in Table 1. As the N2 ratio increases, the nitrogen incorporation in the film also increases under the condition that nitrogen gas ratio is lower than 50%. However, when N2 ratio is higher than 50%, the nitro-
Fig. 3. Normalized EDS spectrum of carbon nitride films with different N2 ratio.
gen incorporation of the film is even decreased. This is probably due to increased sputtering rate of C species from the target as the steady state N concentration increases on the target surface. Energetic or neutral Ar species probably enhance the nitrogen species’ mobility and increase N sticking at the growth surface. There is indeed an effect of Ar mixture on the nitrogen incorporation in the film. We can see that sputtering rate of carbon and nitrogen is maximum at the growth surface when N2 /Ar ratio is 30–50%. If Ar increases further in the sputtering gas mixture, two things can happen: (1) the sputtering rate of carbon increases with respect to ionized nitrogen species, thus the film contains less nitrogen and (2) chemically enhanced preferential sputtering of nitrogen from the film surface can occur by the energetic Ar species. Increase in Ar gas in the sputter gas mixture may also disrupt the film structure. The momentum of the Ar+ ions is higher than that of either N2 + or N+ ions, and then the increased momentum transfer into the growing film causes disruption of the bonding structure of the film producing amorphous material. We can find that if N2 increases more than 70% in the sputtering gas mixture, sputtering yield and N sticking effect are decreased due to deficiency of energetic Ar species. It can be seen that with higher than 70% N2 sputtered film, nitrogen incorporation is lower than the 50% N2 gas sputtered film. It can be considered that a small amount of oxygen (less than 6.23 at.%) is due to contamination from the chamber wall and the Si peak is caused by the Si substrate. Fig. 4 shows the current–voltage curve of Al/CNx /Al structure by semiconductor test and analyzer. As the supply voltage increases, the current also increases following Ohm’s law, when the film is deposited with nitrogen fraction of 30%, 50%, and 70%. The sample which is deposited in 0% nitrogen, however, shows the non-ohmic (Schottky) characteristics of metal semiconductor contact. The film sputtered in mostly Ar atmosphere has lots of defects and dangling sites. It is supposed that if small amount of nitrogen is out-diffused from chamber wall, it can be diffused to the surface of carbon film. The film should act as n-type semiconductor that donor impurities are doped. Nitrogen plays role of dopant in this process due to limited plasma energy and enhances the electrical resistivity of the film. Therefore, the rectifier contact is formed from the work function difference between Al and nitrogen doped n-type semiconductor. The electrical resistance can be calculated in the linear region of I–V
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Fig. 4. Current–voltage curves of carbon nitride films as a function of N2 /Ar ratio.
curves. As expected by theoretical prediction, the film has high resistivity as shown in Fig. 5. When the nitrogen fraction is 70%, the film resistivity is about 4.5 × 109 cm. The film deposited in 100% N2 has a relatively low resistivity, 2.4 × 108 cm. It is caused that the sputtering yield is decreased due to the deficiency of energetic Ar as mentioned, then carbon and/or nitrogen vacancies exist in the carbon nitride film. Carriers can be hopped easily to vacancies and the film conductivity should be increased. Under the condition exposed to humid atmosphere, it is hard to make the high resistive film for insulating layer because of the hydrophilic properties of carbon nitride. If the carbon nitride can be used for an under layer or a buried layer, it could have much higher resistivity. Fig. 6 shows the impedance characteristics to relative humidity according to different nitrogen ratio. LCR meter with 250 mV and 100 kHz AC sources is applied for measurement. The chamber temperature is fixed at 25 ◦ C. The impedance of sample with 30% N2 sputtering gas changed from 118 k to 4 k in the relative humidity range of 5–95% RH. Then, the sensor sensitivity, which is defined by the ratio of the difference of resistance in 0% RH and resistance in 50% RH to resistance in 0% RH ((R0 − R50 )/R0 ), is 0.75. This value is relatively low compared
Fig. 5. Resistivity changes of the film at the room temperature and 40% RH.
Fig. 6. Impedance vs. relative humidity of carbon nitride films with the different N2 ratio.
to humidity sensors based on organic materials [12–14]. However, organic humidity sensors have some disadvantages such as low operating temperature, low long-term stability and swelling. Carbon nitride humidity sensor has higher linearity than other humidity sensors based on inorganic materials which have a high sensitivity at low humidity range and a low sensitivity at high humidity [15,16]. When water molecules adsorb on carbon nitride film, the hydrogen attack in carbon nitride film could easily break the C N and C N bonds and form C H and N H bonds [9]. The sample sputtered in 100% Ar atmosphere has the highest sensitivity, but it has low reversibility. Fig. 7 shows hysteresis characteristics in adsorption and desorption process of the carbon nitride humidity sensor, which is deposited on different substrates such as Si-wafer, alumina, and quartz. Even though the impedance changes of films deposited on alumina and quartz have a wider span, the hysteresis is larger than that of the film deposited on Si-wafer. The hysteresis of humidity sensors is caused by ink-bottle shaped pores that are wider in the interior that at surface. The condensed water vapor in pores cannot easily come out. We can see that the film deposited on alumina has much ink-bottle shaped pores because of high roughness. The hysteresis of the film deposited on the Si-wafer
Fig. 7. Hysteresis curves of carbon nitride humidity sensors with 50% N2 ratio.
J.G. Lee, S.P. Lee / Sensors and Actuators B 117 (2006) 437–441
is about 4.2% FSO at the 50% RH. The film has low surface roughness so that the adsorbed water molecular can be easily desorbed.
[3] [4] [5] [6]
4. Conclusions Carbon nitride films, a new humidity sensing material, were deposited on the silicon substrate by RF magnetron sputtering system. When N2 /Ar ratio was 30–50%, sputtering rate of carbon and nitrogen sticking effect were maximum. The deposited films had the chemical formula of C7 N4 –C3 N and grain size was about 30 nm. When the nitrogen fraction of the films was 70%, the film revealed very high resistivity of about 4.5 × 109 cm, even though it was exposed at 40% RH. In the humidity range of 5–95% RH, the sensor impedance changed from 118 k to 4 k at 25 ◦ C and it revealed better linearity than inorganic humidity sensors. Hysteresis of the carbon nitride humidity sensor which was deposited on the Si-wafer with the 50% N2 ratio is about 4.2% FSO at 50% RH. As far as these results are concerned, carbon nitride films can be used for dew point sensors or wide range fast humidity sensors. Acknowledgment The authors wish to acknowledge the financial support of Kyungnam University made in the program year of 2005. References [1] C.M. Sung, M. Sung, Mater. Chem. Phys. 43 (1996) 1–18. [2] A.Y. Liu, M.L. Cohen, Science 245 (1989) 841–842.
[7] [8] [9] [10] [11] [12] [13] [14] [15] [16]
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J.G. Lee, S.P. Lee, J. Electroceram. 13 (2004) 321–326. S.P. Lee, J.B. Kang, Microchem. J. 70 (2001) 239–246. H. Han, B.J. Feldman, Solid State Commun. 65 (1988) 921–923. P. Gonzalez, R. Soto, E.G. Parada, X. Redondas, S. Chiussi, J. Serra, J. Pou, B. Leon, M. Perez-Amor, Appl. Surf. Sci. 109–110 (1997) 380–383. R.C. DeVries, Mater. Res. Innovations 1 (1997) 161–162. S.G. Ha, J.G. Lee, S.P. Lee, J. Korean Phys. Soc. 45 (2004) 1268–1271. J.G. Lee, S.P. Lee, Sens. Actuators B 108 (2005) 450–454. N. Abedinov, C. Popov, Z. Yordanov, I.W. Rangelow, W. Kulisch, Appl. Phys. A 79 (2004) 531–536. J.G. Lee, S.P. Lee, Trans. EEM 5 (2004) 98–103. Y. Sakai, M. Matsuguchi, T. Hurukawa, Sens. Actuators B 66 (2000) 135–138. M. Gong, M. Lee, H. Rhee, Sens. Actuators B 73 (2001) 185–191. S.T. McGovern, G.M. Spinks, G.G. Wallace, Sens. Actuators B 107 (2005) 657–665. G. Neri, A. Bonavita, S. Galvagno, N. Donato, A. Caddemi, Sens. Actuators B 111–112 (2005) 71–77. D. Saha, R. Giri, K. Mistry, K. Sengupta, Sens. Actuators B 107 (2005) 323–331.
Biographies Ji Gong Lee received his M.S. degree in electronic engineering from Kyungnam University, Korea (2003). He is currently pursuing the Ph.D. at the same university. He is particularly interested in chemical sensors and new sensing materials. Sung Pil Lee received his Ph.D. in electronic engineering from Kyungpook National University, Korea (1989). Since 1988, he has been a Professor of Electronic and Electrical Engineering Department at Kyungnam University, Korea, heading the Sensor Development Program in Research Institute of Engineering and Technology. The research activities of Dr. S. Lee are in various fields of smart sensor development, sensing materials for humidity sensor and taste sensors.
Impedance characteristics of carbon nitride films for humidity sensors Ji Gong Lee ∗ , Sung Pil Lee Department of Electronic and Electrical Engineering, Kyungnam University, 449 Masan, Kyungnam 631-701, Republic of Korea Received 9 June 2005; accepted 15 December 2005 Available online 20 February 2006
Abstract Carbon nitride films were deposited on silicon substrate for humidity sensors with meshed electrodes by reactive RF magnetron sputtering system with DC bias. The surface of carbon nitride films had a good uniformity with the grain size of about 30 nm. The EDS analysis revealed that the chemical formula of the carbon nitride film could be expressed between C7 N4 and C3 N and these results also revealed those quite agree with XPS results. The films have very high resistivity, the characteristic of which can be expected to use for insulating films on the Si fabrication. The impedance of the sample deposited on the Si-wafer decreased from 118 k to 4 k in the relative humidity range of 5–95%. Hysteresis of the film deposited on the Si-wafer was about 4.2% of full scale output. © 2006 Published by Elsevier B.V. Keywords: Humidity sensors; Carbon nitride films; Reactive RF magnetron sputter; Surface analysis
1. Introduction Sung and Sung [1] predicted the possible existence of carbon nitride, i.e. C3 N4 material in his unpublished patent disclosure letter at Diamond Technology Centre of Norton Company. Because of their short inter-atomic distances, these hypothetical materials were suspected to be very hard. This speculation initiated by Liu and Cohen in 1989 [2] to develop an empirical model and ab initio calculation of the bulk modulus for covalent solid formed between carbon and nitrogen. On the basis of this model, properties like bond strength of hypothetical C3 N4 compound were assessed. They suggested theoretically that these materials could have bulk modulus comparable to or greater than diamond. After carbon nitride was suggested as a super hard material, considerable interest has arisen in synthesizing the stoichiometric -C3 N4 phase which might have the strongest bulk modulus in carbon nitride phases theoretically. There have been many reports of amorphous carbon nitride films of uncertain composition, but they found only a few observations of crystalline -C3 N4 produced by reactive sputtering, laser abla-
∗
Corresponding author. E-mail address: [email protected] (J.G. Lee).
0925-4005/$ – see front matter © 2006 Published by Elsevier B.V. doi:10.1016/j.snb.2005.12.049
tion and hot filament CVD [3–6]. Most of deposited films have been discontinuous with isolated crystals [6] or showing only a few isolated grains in an amorphous matrix [5], and high substrate temperatures (600–950 ◦ C) were required for film deposition. According to report of DeVries [7], although over 400 papers have been published on crystalline C3 N4 by 1997, no new super hard materials have come out over a decade. One of the most significant problems degrading the quality of carbon nitride films is to be existence of N H and C H bonds mostly found in sputtering systems. However, the possibility of these reactions with hydroxyl group of carbon nitride films, caused by a hydrogen attack, was suggested in our previous reports and proved that these undesired effect could be applied for fabricating micro-humidity sensors [8]. The hydrogen attack in carbon nitride film could easily break or change the C N and C N bonds to form C H and N H bonds. This was strongly undesirable behavior for the composition of crystalline -C3 N4 . However, if one can use hydrogen’s attractive characteristics due to some hydrogen defects which were made intentionally and bonded weakly, carbon nitride could be a candidate for new materials for humidity sensor applications [8,9]. The similar view mentioned by Abedinov et al. [10] also enhances humidity sensing properties of carbon nitride films. They believed the molecular absorption of the water into the layer changed effec-
438
J.G. Lee, S.P. Lee / Sensors and Actuators B 117 (2006) 437–441
tive mass and induce tensile of compressive stress and supposed that forming hydrogen bonds of the polar H2 O with the N atoms was the main mechanism. The common requirements for humidity sensors are a good sensitivity, fast response, low hysteresis, high reproducibility, chemical and physical stability, and low cost. Although a large number of materials and compounds have been investigated for humidity sensors, it is still not easy to identify the best humidity sensing material. One material has some merits and it has other demerits as well. Therefore, many different humidity sensors exist and they are only used for a specific application. Recent achievements in miniaturized sensor, especially silicon based MEMs techniques, strongly ask for a new material that can be applied for micro-sensors or multi-sensors. In this respect, carbon nitride film can be a good candidate for new humidity sensing material in terms of having some advantages; facility compatibility with Si fabrication, fast response time over a wide humidity range, long-term stability against chemical contaminations, and low cost. In this paper, we studied the humidity sensing properties of carbon nitride films as function of nitrogen contents and deposition times to get the optimized growth conditions for humidity sensors. The crystalline properties of the carbon nitride films are analyzed by scanning electron microscope (SEM) and energy dispersive X-ray spectrometer (EDS), and humidity characteristics of carbon nitride films are investigated with LCR meter. 2. Experimental The humidity sensor was designed as a capacitance type with micro-hole on the upper plate electrode. Lower electrode (Al) was deposited on a cleaned p-type Si(1 0 0) substrate by a thermal evaporator and patterned by lift-off technique. After the lower electrode had deposited, the samples were heat-treated at 450 ◦ C for 5 min and cleaned by the traditional cleaning process. RF magnetron sputtering system was applied for deposition of sensing materials, carbon nitride film, on the lower electrode [11]. The carbon nitride film was formed by reaction of nitrogen gas (99.999%) and carbon target (99.99%) in the sputtering chamber for 1 h. A mixture of nitrogen and argon was introduced into the chamber for reaction gas and sputtering gas, respectively. The ratio of nitrogen to argon was changed from 0% to 100% in order to make different samples to investigate the effects of nitrogen content in the film and controlled precisely by mass flow controller (MFC). To reduce influence of contamination from chamber wall and undesirable gases, whole inner chamber wall was coiled by heating wire, and thermocouple was attached on the back of the substrate holder in order to control precisely substrate temperature typically at 200 ◦ C. This method might provide more accurate temperature control at the point of a reaction substrate than the conventional heating chamber. Upper electrode has square meshed micro-hole to enhance the absorption and desorption response of the sensing material. Each electrode was formed on the carbon nitride films by lift-off technique. The detail view of the sensors was illustrated in our previous works [7,8]. MFCs controlled humidity through regulating the flow rate of the dry air and the vapor-saturated air in the
measuring chamber. To avoid the condensation of water vapor in the inner wall of the chamber or pipelines caused by different temperature between introducing vapor and atmosphere inside, all measurement objects were installed together in the constanttemperature chamber controlled by PID controller [7,8]. The structure and surface morphology of the films were analyzed by SEM (ABT-32, TOPCON) and EDS (Sigma MS2, Kevex). Current–voltage characteristics were investigated with semiconductor test and analyzer (CATS CA-EDA, Korea). The sensor impedance was measured with a LCR meter (HP 4263B) linked to a computer. To verify the real relative humidity, commercial humidity sensor (Control Company, USA) which has ±0.5% accuracy was used. 3. Results and discussion When the RF power is applied, energetic carbon atoms due to the high plasma energy are sputtered from the target and deposited on the substrate, reacting with nitrogen. Fig. 1 shows the film thickness dependence on RF power between 100 W and 400 W. The deposition time was for 1 h. The deposition rate of carbon nitride film at 300 W is about 3.5 m/h, which is the highest value. The film thickness increases as RF power increases to 300 W. As RF power increases above 300 W, however, the thickness is decreased. If the RF power is raised above 300 W, the self-bias voltage and the ion bombardment energy normally increase. Then, the increase of ion bombardment energy will reduce the film deposition rate through the sputtering of the physisorbed precursors by N2 + ion bombardment [8]. The film deposited at high input power, however, is more dense and harder because of the dehydrogenation of a-CNx :H film and low stress with the substrate. The surface morphology and the cross-section were observed by SEM with accelerating voltages of 10 kV for surface and 15 kV for cross-section. The surface of carbon nitride film has a good uniformity which RMS roughness is ca. 1.21 nm [3] and the grain size of the sample deposited with 50% N2 is about 30 nm (Fig. 2(a)). Al electrode on the top layer and carbon nitride film beneath the aluminum layer (ca. 2 m) are shown in Fig. 2(b).
Fig. 1. Thickness changes as a function of RF power.
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439
Fig. 2. SEM photographs of carbon nitride films deposited on Si-wafer: (a) surface and (b) cross-section. Table 1 The atomic ratio of carbon nitride film by EDS results Component
C N O Si
N2 ratio (%) 0
30
50
70
100
64.93 4.7 2.64 25.9
55.14 31.59 4.60 8.66
51.97 36.11 6.23 5.69
71.68 23.63 3.67 1.03
69.41 24.44 4.81 1.34
The composition rate of deposited films was investigated by EDS (Table 1 and Fig. 3). Fig. 3 shows the normalized EDS peaks of the samples. The highest rate and lowest rate of carbon/nitrogen are 64.93:4.7 at 0% N2 ratio and 51.97:36.11 at 50% N2 ratio, respectively. We can see that even the sample which is deposited under 0% nitrogen has small amount of N contents. It can be inferred that nitrogen source may be provide from contaminant of chamber wall which were deposited by prior works. The chemical formula is represented as C1−x Nx , in carbon nitride formation. The range of x is 0.25–0.36 in EDS spectrum. Therefore, it can be roughly expressed as C7 N4 and C3 N. These results coincide with XPS spectrum analysis previously reported [3]. The detail component is shown in Table 1. As the N2 ratio increases, the nitrogen incorporation in the film also increases under the condition that nitrogen gas ratio is lower than 50%. However, when N2 ratio is higher than 50%, the nitro-
Fig. 3. Normalized EDS spectrum of carbon nitride films with different N2 ratio.
gen incorporation of the film is even decreased. This is probably due to increased sputtering rate of C species from the target as the steady state N concentration increases on the target surface. Energetic or neutral Ar species probably enhance the nitrogen species’ mobility and increase N sticking at the growth surface. There is indeed an effect of Ar mixture on the nitrogen incorporation in the film. We can see that sputtering rate of carbon and nitrogen is maximum at the growth surface when N2 /Ar ratio is 30–50%. If Ar increases further in the sputtering gas mixture, two things can happen: (1) the sputtering rate of carbon increases with respect to ionized nitrogen species, thus the film contains less nitrogen and (2) chemically enhanced preferential sputtering of nitrogen from the film surface can occur by the energetic Ar species. Increase in Ar gas in the sputter gas mixture may also disrupt the film structure. The momentum of the Ar+ ions is higher than that of either N2 + or N+ ions, and then the increased momentum transfer into the growing film causes disruption of the bonding structure of the film producing amorphous material. We can find that if N2 increases more than 70% in the sputtering gas mixture, sputtering yield and N sticking effect are decreased due to deficiency of energetic Ar species. It can be seen that with higher than 70% N2 sputtered film, nitrogen incorporation is lower than the 50% N2 gas sputtered film. It can be considered that a small amount of oxygen (less than 6.23 at.%) is due to contamination from the chamber wall and the Si peak is caused by the Si substrate. Fig. 4 shows the current–voltage curve of Al/CNx /Al structure by semiconductor test and analyzer. As the supply voltage increases, the current also increases following Ohm’s law, when the film is deposited with nitrogen fraction of 30%, 50%, and 70%. The sample which is deposited in 0% nitrogen, however, shows the non-ohmic (Schottky) characteristics of metal semiconductor contact. The film sputtered in mostly Ar atmosphere has lots of defects and dangling sites. It is supposed that if small amount of nitrogen is out-diffused from chamber wall, it can be diffused to the surface of carbon film. The film should act as n-type semiconductor that donor impurities are doped. Nitrogen plays role of dopant in this process due to limited plasma energy and enhances the electrical resistivity of the film. Therefore, the rectifier contact is formed from the work function difference between Al and nitrogen doped n-type semiconductor. The electrical resistance can be calculated in the linear region of I–V
440
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Fig. 4. Current–voltage curves of carbon nitride films as a function of N2 /Ar ratio.
curves. As expected by theoretical prediction, the film has high resistivity as shown in Fig. 5. When the nitrogen fraction is 70%, the film resistivity is about 4.5 × 109 cm. The film deposited in 100% N2 has a relatively low resistivity, 2.4 × 108 cm. It is caused that the sputtering yield is decreased due to the deficiency of energetic Ar as mentioned, then carbon and/or nitrogen vacancies exist in the carbon nitride film. Carriers can be hopped easily to vacancies and the film conductivity should be increased. Under the condition exposed to humid atmosphere, it is hard to make the high resistive film for insulating layer because of the hydrophilic properties of carbon nitride. If the carbon nitride can be used for an under layer or a buried layer, it could have much higher resistivity. Fig. 6 shows the impedance characteristics to relative humidity according to different nitrogen ratio. LCR meter with 250 mV and 100 kHz AC sources is applied for measurement. The chamber temperature is fixed at 25 ◦ C. The impedance of sample with 30% N2 sputtering gas changed from 118 k to 4 k in the relative humidity range of 5–95% RH. Then, the sensor sensitivity, which is defined by the ratio of the difference of resistance in 0% RH and resistance in 50% RH to resistance in 0% RH ((R0 − R50 )/R0 ), is 0.75. This value is relatively low compared
Fig. 5. Resistivity changes of the film at the room temperature and 40% RH.
Fig. 6. Impedance vs. relative humidity of carbon nitride films with the different N2 ratio.
to humidity sensors based on organic materials [12–14]. However, organic humidity sensors have some disadvantages such as low operating temperature, low long-term stability and swelling. Carbon nitride humidity sensor has higher linearity than other humidity sensors based on inorganic materials which have a high sensitivity at low humidity range and a low sensitivity at high humidity [15,16]. When water molecules adsorb on carbon nitride film, the hydrogen attack in carbon nitride film could easily break the C N and C N bonds and form C H and N H bonds [9]. The sample sputtered in 100% Ar atmosphere has the highest sensitivity, but it has low reversibility. Fig. 7 shows hysteresis characteristics in adsorption and desorption process of the carbon nitride humidity sensor, which is deposited on different substrates such as Si-wafer, alumina, and quartz. Even though the impedance changes of films deposited on alumina and quartz have a wider span, the hysteresis is larger than that of the film deposited on Si-wafer. The hysteresis of humidity sensors is caused by ink-bottle shaped pores that are wider in the interior that at surface. The condensed water vapor in pores cannot easily come out. We can see that the film deposited on alumina has much ink-bottle shaped pores because of high roughness. The hysteresis of the film deposited on the Si-wafer
Fig. 7. Hysteresis curves of carbon nitride humidity sensors with 50% N2 ratio.
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is about 4.2% FSO at the 50% RH. The film has low surface roughness so that the adsorbed water molecular can be easily desorbed.
[3] [4] [5] [6]
4. Conclusions Carbon nitride films, a new humidity sensing material, were deposited on the silicon substrate by RF magnetron sputtering system. When N2 /Ar ratio was 30–50%, sputtering rate of carbon and nitrogen sticking effect were maximum. The deposited films had the chemical formula of C7 N4 –C3 N and grain size was about 30 nm. When the nitrogen fraction of the films was 70%, the film revealed very high resistivity of about 4.5 × 109 cm, even though it was exposed at 40% RH. In the humidity range of 5–95% RH, the sensor impedance changed from 118 k to 4 k at 25 ◦ C and it revealed better linearity than inorganic humidity sensors. Hysteresis of the carbon nitride humidity sensor which was deposited on the Si-wafer with the 50% N2 ratio is about 4.2% FSO at 50% RH. As far as these results are concerned, carbon nitride films can be used for dew point sensors or wide range fast humidity sensors. Acknowledgment The authors wish to acknowledge the financial support of Kyungnam University made in the program year of 2005. References [1] C.M. Sung, M. Sung, Mater. Chem. Phys. 43 (1996) 1–18. [2] A.Y. Liu, M.L. Cohen, Science 245 (1989) 841–842.
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Biographies Ji Gong Lee received his M.S. degree in electronic engineering from Kyungnam University, Korea (2003). He is currently pursuing the Ph.D. at the same university. He is particularly interested in chemical sensors and new sensing materials. Sung Pil Lee received his Ph.D. in electronic engineering from Kyungpook National University, Korea (1989). Since 1988, he has been a Professor of Electronic and Electrical Engineering Department at Kyungnam University, Korea, heading the Sensor Development Program in Research Institute of Engineering and Technology. The research activities of Dr. S. Lee are in various fields of smart sensor development, sensing materials for humidity sensor and taste sensors.