Studies on Electrical Properties of RF Sputtered Deposited Boron Carbon Nitride Thin Films

Boron carbon nitride (BCN) ﬁlms were prepared by reactive magnetron sputtering from a B 4 C target and deposited to make metal-insulator-metal (MIM) sandwich structures using aluminum as the top and bottom electrodes. BCN ﬁlms were deposited at various N 2 /Ar gas ﬂow ratios, substrate temperatures. The electrical characterization of the MIM devices includes capacitance vs. voltage (C-V), current vs voltage, and breakdown voltage characteristics. The above characterizations were performed as a function of deposition parameters. By varying the nitrogen concentration in the deposited ﬁlms and substrate deposition temperatures, the electrical properties of BCN can be tuned accordingly. BCN ﬁlms having dielectric constant as low as 2.13 with high dielectric breakdown strength of 3.4 MV/cm and resistivity of 3 × 10 12 (cid:2) .cm were achieved.

Inter layer dielectric (ILD) materials play the role of isolating electrically conducting and semiconducting features and films. 1 The interconnect delay (τ = R × C) due to constant shrinking of device size can be reduced not only by the resistance of the conductor but also by decreasing the capacitance of ILD. A wiring metal with low resistivity and a high quality insulating film with a low dielectric constant may lead to a reduction of the wiring capacitance. There exist atomic bonding similarities amongst boron, carbon, and nitrogen that allow formation of compounds with a wide compositional range. ILD is made up of smaller atoms/ions of C, B which has comparatively lower polarizability and thus has lower dielectric constant. 2 BCN compounds have been expected to combine the excellent properties of B 4 C, BN and C 3 N 4 , with their properties adjustable, depending on composition and structure. 3 The electrical properties of BCN compounds are thought to be intermediate between those of semi metallic graphite and insulating hexagonal-BN (h-BN). 4,5 The boron-carbonnitrogen phase diagram contains interesting phases, such as diamond, graphite, fullerene, cubic-BN, B 4 C and there is also a hypothetical C 3 N 4 . This ternary alloy is expected to combine properties of diamond, cubic boron nitride, tetragonal amorphous carbon and boron carbide. 6 Also, the ability to control the bandgap (Eg) by changing the atomic composition and structure makes them suitable for the application in electronic and photonic devices. 7,8 As graphite is semi metallic and h-BN is insulating (bandgap >3.8 eV), it is expected that such a hybrid between semi metallic graphite and insulating h-BN may show adjusted semiconductor properties. 9 Dense (pore-free) BCN films with a low dielectric constant in the range of 1.9 to 2.1 have been successfully demonstrated. 10 Several methods of preparing boron carbon nitride films have been reported, such as chemical vapor deposition (CVD), 11,12 plasma enhanced CVD, 13 ion beam deposition, 14,15 cathodic arc plasma deposition 16 pulsed laser deposition 17,18 and DC and RF magnetron sputtering. 19,20 Also, shock wave compression or high pressure/high temperature techniques (HP/HT) have been used. The BCN films with different compositions can be obtained by varying the power to the composite target in dual target sputtering. 11 Pan et al. prepared BCN films by PLD from a sintered B 4 C target. The films prepared were found to be smooth and adhered well to the substrate. They also observed that with the help of reactive nitrogen plasma they can incorporate more amounts of nitrogen in the films. 21 The sputtering technique provides unique advantages over other techniques such as freedom to choose the substrate material and a uniform deposition over relatively large area. Furthermore, it is possible to control the deposition parameters to prepare BCN films of various compositions. According to the chemical properties of the B, C and N during the depositing process of the BCN films, B and C can exist both as elements and as compounds reacting with N or each other, while N could be existed in the films only as compounds by reacting with B or C. Therefore, the process of N deposited into the films and the reciprocity among B, C, and N during the deposition of films becomes the main factors, which affect the composition and properties of films. There is an increasing trend of nitrogen content in the films with respect to increasing N 2 /A r flow ratio but it gets saturated at higher N 2 /A r gas ratios. 22 Increasing the carbon content to the composition of BC 4 N, the sample exhibits an amorphous structure. 23 Although the dielectric breakdown strength (DBS) and resistivity of B 4 C is less compared to the ULSI standards, addition of N 2 will increase its DBS and resistivity by shifting its properties toward BN. The DBS of amorphous BN was found to be around 2 MV/cm by Zedlitz et al. 24 Therefore the DBS of the BCN films can be controlled accordingly. Although there have been many past researches on the electrical properties of BCN films, there seems to be a lack of systematic studies performed on the electrical properties of rf sputtered BCN films specifically. So far the lowest value of dielectric constant of BCN is found to be 2.2 by PACVD process. 25 The focus of this work is to synthesize the boron carbon nitrogen (BCN) thin films through rf sputtering and investigate the electrical properties and assess its feasibility as a low-κ material in the CMOS back end technology.

Experimental
Thin films of BCN were deposited by reactive RF magnetron sputtering in an Ultra High Vacuum system. Powder pressed, B 4 C target (3 inch diameter) with a purity of 99.5% was used. The system base pressure was approximately 4 × 10 −7 torr. Reactive sputtering was used wherein the composition of the film was controlled by changing gas flow ratios of Ar and N 2 gases to achieve reactive process. The N 2 to Ar ratio was varied from 0.25 to 1, in steps of 0.25 by changing the individual gas flow rate, while total gas flow was kept constant at 20 sccm and the deposition pressure was 5 mTorr. For each gas flow ratio, the deposition temperatures used were room temperature, 200 • C, 300 • C, 400 • C and 500 • C. The R.F power to the B 4 C target was kept constant at 200 W for all depositions.
The glass substrates were used to fabricate Metal-Insulator-Metal (MIM) structures. The substrates were cleaned by rinsing with acetone, methanol, deionized water (DI) and dried with nitrogen. The three aluminum electrode strips (3 mm wide) were deposited using a mechanical mask in a thermal evaporating, cryo-pumped vacuum system. The control substrate along with the aluminum striped substrate were loaded in the sputtering system to deposit BCN films. To measure the film thickness, a glass slide was used as a control substrate. A black marker line of 2 mm wide was drawn on this control substrate. After depositing the required film, the black marker line was etched away with acetone dip to leave a step in the middle. The target is placed facing the substrate holder at an angle of 45 • . The rotation speed of the substrate was set around 20 rpm to get uniform films. The sputtering duration was kept 1 hour for all the samples. The thickness measurements of BCN films were done by Veeco Dektak 150 Profilometer on the step in the control substrate. The BCN thicknesses ranged from 900 Å-2000 Å. Finally, five aluminum strips of 3 mm width each running perpendicular to the back electrode were deposited on the BCN layer to form the top electrode of the MIM structures.
Current-Voltage (I-V) measurement was performed on MIM devices by using HP 4145B semiconductor parameter analyzer. This was also used to measure the resistivity and breakdown voltage of the BCN films. The surge of 1 mA current is considered as the electrical breakdown condition for the films. The dielectric breakdown strength was calculated by knowing the thickness. The capacitance was measured by using HP 4275A Multi-Frequency LCR meter at 1 MHz frequency. The dimension of overlapping electrodes was found to be 3 mm × 3 mm and hence its effective area was 9 mm 2 and the film thickness gives the distance between the electrodes of MIM capacitor. By knowing the area of the overlapping electrodes and the film thickness, the dielectric constant was calculated.
Parasitic compensation was taken care before each capacitance measurement. A self-test was performed before every running to ascertain the instrument's proper running. Measurement of HP test cable compensated for electrical length that is 1 m length to minimize the parasitic capacitance. The switch was set to facilitate the measuring bridge circuit to its optimum balance and for minimizing incremental measurement errors when standard test leads were used. 1 m position was used as this was the standard leads. When the switch is set to its 1 m position, compensation is appropriately made for high frequency measurements for any propagation loss and phase errors in test leads. Atomic compositions of the BCN films were found by XPS analysis. XPS was performed using a PHI 5400 ESCA system. The base pressure during analysis was 10 −9 Torr. The Mg K-α X-ray source (hν = 1253.6 eV) at a power of 350 watts was used for the analysis. Although care was taken to minimize the exposure time in air, the atmospheric exposure could not be completely avoided. Both the survey and the high-resolution narrow spectra were recorded with electron pass energy of 44.75 eV and 35.75 eV, respectively, to achieve the maximum spectral resolution. Any charging shift produced by the samples was carefully removed by using a B.E. scale referred to C (1s) B.E. of the hydrocarbon part of the adventitious carbon line at 284.6 eV. 26 The C 1s peaks located at 286-286.9 eV are due to the presence of C-N bonds. 34,35 Any surface contamination from ex-situ transfer was removed by Ar+ ion beam sputtering to minimize the adventitious carbon and oxidation. The concentration of oxygen was found to be less than 5% due to ex-situ XPS measurement and hence the atomic concentration of B, C and N are normalized with respect to oxygen. Non-linear least square curve fitting was performed using a Gaussian/Lorentzian peak shape after the background removal.

Results and Discussion
The normalized atomic concentrations of boron, carbon and nitrogen were calculated. Table I shows the variation of boron, carbon and nitrogen concentrations in the film with different deposition temperatures. Table II shows the variation of boron, carbon and nitrogen concentrations with respect to the increase in N 2 /Ar gas flow ratios.
Deposition rates vs. N 2 /Ar gas flow ratios.- Figure 1 shows the variation of deposition rate as a function of N 2 /Ar gas flow ratios at different deposition temperatures. The deposition rate decreases with increase in N 2 /Ar gas flow ratios from N 2 /Ar = 0.25 to can be attributed to the fact that the increase in the nitrogen gas in the sputtering ambience decreases the effective sputtering yield and hence results in a lesser deposition rate. When the N 2 /Ar flow ratio increases above 0.75, the deposition rate slightly increases because, the rate of formation of BCN is favored more due to chemical reaction than the decreasing sputtering yield due to nitridation. As we can see in the Table II, the nitrogen concentration is at its highest for R = 1 and hence nitrogen is favorably bonding with carbon and hence explains slight increase in deposition rate. Figure 2 shows the variation of dielectric constant as a function of substrate deposition temperatures at various gas flow ratios. The dielectric constants of the samples tend to decrease with increasing temperature. It is reported in the literature that the increase in deposition temperature decreases the concentration of carbon in the BCN film. 26 According to the values given in the Table I, the concentration of nitrogen is highest and the concentration of carbon is lowest around 500 • C. It has been reported that dielectric constant of B 4 C is around 4.8 to 8. 27 The lowest value of dielectric constant of BN was found to be 2.2 by Plasma Assisted Chemical vapor deposition (PACVD). 28 Hence the BCN thin film shows more of BN characteristics at higher temperatures and B 4 C characteristics at lower temperatures respectively, thereby having  lower dielectric constant at higher deposition temperatures and higher dielectric constant at lower deposition temperatures.

Dielectric constant vs. deposition temperatures.-
From the optical transmission studies on the film, the refractive indexes were calculated. The refractive index of the films deposited at different substrate deposition temperatures was found to decrease with increase in substrate deposition temperature as shown in Figure 3. Figure 4 shows the plot of dielectric constant vs. N 2 /Ar gas flow ratios at different deposition temperatures. The plot shows a decreasing trend of dielectric constant with the N 2 /Ar gas flow ratio till N 2 /Ar = 0.75 and then shows an increasing trend at the end. This shows an increase in concentration of nitrogen for gas ratio from N 2 /Ar = 0.25 till N 2 /Ar = 0.75 and there is a negligible change after that as per the values given in the Table II. The increase in the dielectric constant at N 2 /Ar = 1 can be attributed to the fact that there is an increase in the deposition rate at N 2 /Ar = 1 and therefore there is an increase in the film thickness. This increase in the film thickness gives rise to increase in the dielectric constant may be due to the formation of low dielectric constant interface layers  adjacent to the electrode in the thinner films. This dominates the capacitance of the film. It can also be due to the degradation of the film quality when the film becomes thinner. Hence thinner films have lower dielectric constants. 29,30 I-V characterization.- Figure 5 shows the I-V characteristics at different substrate deposition temperatures per unit area. The leakage current density increases with increase in substrate deposition temperatures. Figure 6 shows the I-V characteristics at different N 2 /Ar gas flow ratios per unit area. The leakage current density decreases with increase in N 2 /Ar gas flow ratios. This implies that incorporation of nitrogen in the film increases the resistance to leakage current. Figure 7 shows the plot of resistivity vs. deposition temperatures at different N 2 /Ar gas flow ratios. From the figure below, we can see that the resistivity values tend to increase with increase in temperature and then start to decrease at 400 • C. According to Table I, the nitrogen and carbon concentrations are increasing and decreasing respectively with increase in deposition temperatures. The electrical resistivity is found to increase with decrease in the carbon concentration in the film. 31 Beyond 400 • C temperature there is decreasing trend in resistivity for all the gas ratios. This may be due to the possible diffusion of aluminum into the BCN film, thereby decreasing the resistivity of the film.

Resistivity vs. deposition temperatures.-
Resistivity vs. N 2 /Ar gas flow ratios.- Figure 8 shows the plot of resistivity vs N 2 /Ar gas flow ratios at different deposition temperatures. The plot shows an increasing trend of resistivity with the N 2 /Ar gas flow ratio till N 2 /Ar = 0.75 and then shows a decreasing trend at the end. Table II shows the variation of carbon and nitrogen concentrations with respect to the increase in N 2 /Ar gas flow ratio. This shows an increase of nitrogen concentration in the film for gas ratios from N 2 /Ar = 0.25 till N 2 /Ar = 0.75 and there is a decreasing trend afterwards. More the nitrogen content and lesser the carbon content may bring the characteristics of BN at higher N 2 /Ar gas ratios. BN being an insulator has higher resistivity. The decrease in resistivity at N 2 /Ar = 1 can be explained by the complex expression of the relative permittivity. In a lossy medium, a relative permittivity (dielectric constant) can be broken down into real and imaginary part. ε r (ω) = ε r (ω) + iε r (ω) Figure 7. Effect of deposition temperature on Resistivity.
Dielectric conductivity σ (units S/m, siemens per meter), which sums over all the dissipative effects of the material may represent an actual electrical conductivity caused by migrating charge carriers. 32 The dielectric constant is related directly to conductivity. In the Figure 3, as the dielectric constant shows an increasing trend at N 2 /Ar = 1, the corresponding conductivity also increases, hence this shows a decrease of resistivity at N 2 /Ar = 1.
Dielectric breakdown strength vs. N 2 /Ar gas flow ratio.- Figure  9 shows the variation of dielectric breakdown strength (DBS) as a function of N 2 /Ar gas flow ratios at different deposition temperatures. Breakdown Strength shows an increasing trend with the increase in N 2 /Ar gas flow ratios from N 2 /Ar = 0.25 to 0.75, but has a decreasing trend in the end. The DBS of a-BN was found out to be around 2.2 MV/cm 23  values prepared by this technique seem to fall in this range, which denotes that by varying the concentration of nitrogen in the film we can tailor the DBS requirement for a given application. The decreasing trend may be due to the increase in the thickness of the deposited film at N 2 /Ar = 1 as shown in figure 10. The thicker the films, more will be the defects, asperities and lesser will be its thermal conductivity. This might increase the temperature more in thicker dielectric when the electric field is applied. This lowers the field strength eventually lowering the DBS. 33 Dielectric breakdown strength vs. temperature.- Figure 10 shows the plot of dielectric breakdown strength as a function of deposition temperatures for different N 2 /A r gas flow ratios. Dielectric breakdown strength shows an increasing trend with increase in deposition temperature. As observed in Table I, there is an increase in the nitrogen and boron concentrations leading to more BN phases in the film giving higher DBS.

Conclusions
BCN thin films were deposited successfully by RF magnetron sputtering from B 4 C target in argon and nitrogen ambient. Electrical characteristics are strongly dependent on the N 2 /Ar flow ratios and deposition temperatures. The dielectric constant decreases with increase in nitrogen in the BCN film and it also decreases with the increase in substrate temperatures. The characteristics of BCN film tend more toward BN phases when the nitrogen concentration increase. The resistivity of the film and the dielectric breakdown strength increases with increase in N 2 /Ar flow ratio and also with the substrate temperature. By having the required N 2 /Ar flow ratios and deposition temperatures it is possible to achieve the required BCN films for various applications. Studies conducted revealed that BCN films can be potential low-k materials for inter dielectric layers for ULSI applications.