Selective etching of silicon nitride over silicon oxide using ClF3/H2 remote plasma

Precise and selective removal of silicon nitride (SiNx) over silicon oxide (SiOy) in a oxide/nitride stack is crucial for a current three dimensional NOT-AND type flash memory fabrication process. In this study, fast and selective isotropic etching of SiNx over SiOy has been investigated using a ClF3/H2 remote plasma in an inductively coupled plasma system. The SiNx etch rate over 80 nm/min with the etch selectivity (SiNx over SiOy) of ~ 130 was observed under a ClF3 remote plasma at a room temperature. Furthermore, the addition of H2 to the ClF3 resulted in an increase of etching selectivity over 200 while lowering the etch rate of both oxide and nitride due to the reduction of F radicals in the plasma. The time dependent-etch characteristics of ClF3, ClF3 & H2 remote plasma showed little loading effect during the etching of silicon nitride on oxide/nitride stack wafer with similar etch rate with that of blank nitride wafer.

As the semiconductor device size is decreased to sub-nanoscale and the device integration is changed from two dimensional to three dimensional structure, more precise and selective etch technology is required for the semiconductor device fabrication 1 . In the various semiconductor devices, silicon nitride has been widely used as a barrier layer for dopant diffusion, a gate sidewall spacer layer, a buffer layer, etc. due to high insulating characteristics, high thermal and mechanical stability, etc. and selective etching of silicon nitride over silicon and/or silicon oxide is important for various microelectronic applications 2 .
These days, in the three dimensional NOT-AND type flash memory fabrication, the number of silicon nitride/ silicon oxide (SiN x /SiO y ) stack is increasing and the thickness of repeating SiN x /SiO y layer is decreasing continuously for higher memory density in the vertical direction. Therefore, the etching of SiN x layers uniformly and ultra-high selectively to SiO y layers in the SiN x /SiO y stack is becoming more challenging process. Until now, the selective etching of SiN x in SiN x /SiO y stacks is achieved by wet etching using a hot phosphoric acid (H 3 PO 4 ) 3-6 . In case of the wet etching, however, the penetration of an etch solution into holes is getting more challenging as the thickness of the SiN x /SiO y layer is decreased and the remaining SiO y layers can be collapsed due to the surface tension. Moreover, several additives for increasing the etch selectivity of SiN x /SiO y are found to cause oxide regrowth problems after etching unless its process condition is not carefully controlled 5 . To solve these problems, a dry process for isotropic and selective etching of SiN x needs to be developed as an alternative technology for three dimensional NOT-AND type flash memory fabrication.
Various studies have been reported for selective etching of SiN x over SiO y using dry etch processes. For example, an ultra-high selective etching of SiN x over SiO y was reported using CF 4 -based (CF 4 /O 2 /N 2 , CF 4 /CH 4 / Ar) gases with a microwave chemical downstream etcher and an inductively coupled plasma (ICP) etcher [7][8][9] . In addition, NF 3 -based (NF 3 /O 2 /NH 3 , NF 3 /O 2 /N 2 ) gases were also used to ultra-high selective etching of silicon nitride over silicon oxide with downstream etchers based on ICP or capacitively coupled plasma (CCP) [9][10][11][12][13] . However, the etch selectivity of nitride over oxide still needs to be increased further for the application of current semiconductor process due to the thin thickness of oxide. Moreover, the use of fluorocarbon (CF x ) etch gases has contamination issues by carbon or deposition of CF x (CH x ) polymers on the surface of the film, and which is a detrimental factor for a device fabrication. Even though these limits for engineering aspects are excluded,  (17,200)] arouse the needs for the alternative etch gases for environmental aspects in the near future 14 . ClF 3 with the GWP of ~ 0 has been used primarily as an in-situ cleaning gas for chemical vapor deposition (CVD) chambers in replacement of perfluorocarbon compounds (PFC), which have high GWP values or as an etch gas for silicon etching by heating, neutral cluster beam etching, reactive ion beam etching, etc. [15][16][17][18][19] . In addition, the ClF 3 have been investigated for etching of SiGe in an ICP system 20 , SiC etching with ultra-high etch rate over 10 µm/min 21 , selective etching of transition metals and metal nitrides such as tantalum (tantalum nitride) over metal oxide (Ta 2 O 5 ) with low pressure gaseous etching method 22 . In this study, ClF 3 remote plasma was applied for a fast and ultra-high selective etching of silicon nitride (SiN x ) over silicon oxide (SiO y ) applicable for current and next-generation semiconductor device fabrication including three dimensional NOT-AND type flash memory. The etching of SiN x using ClF 3 showed high etch rate over 80 nm/min and the etch selectivity of SiN x over SiO y of ~ 130. The etch selectivity of SiN x was further increased with H 2 addition in the ClF 3 plasma. The effect of Cl, F, and H radicals on the selective etching of SiN x was investigated using plasma and surface analysis tools, and its etch mechanism was suggested.

Experimental section
Etching of silicon nitride. Figure 1 is a schematic drawing of a remote type inductively coupled plasma (ICP) etching system used in this study. The inside of process chamber was coated with an aluminum oxide layer by anodizing. The base pressure of the process chamber measured with a convection gauge was maintained at 3 × 10 -3 Torr and the operating pressure monitored by capacitance manometer (Baratron gauge) was maintained at 200 mTorr. 13.56 MHz RF power was applied to the planar type ICP coil at upper side of a chamber. For the isotropic etching of SiN x , double grids with multiple holes with 1.5 mm radius were arranged at the center of ICP reactor to prevent an ion bombardment effect and deliver radicals on the substrate. The substrate temperature was measured at the sample stage below the sample, which was monitored by a thermocouple and adjusted from 25 to 500 °C by a silicon carbide (SiC) heater connected to an external power supply. The chlorine trifluoride (ClF 3 , > 99.9%, 200 sccm), H 2 (> 99.999%), and Argon (> 99.999% Ar, 200 sccm) were flown through a circular shape gas distributor to the process chamber.
Sample preparation. Blank 1.5 µm thick SiN x thin films, blank 300 nm thick SiO y thin films, and multilayer stacks composed of repeating SiO y (27 nm) and SiN x (27 nm) thin films were deposited by a plasma enhanced chemical vapor deposition (PECVD) process (supplied by WONIK IPS Inc.).
Characterization. The etch rate of SiN x and SiO y were measured by a step profilometer (Tencor, Alpha-step 500) and with Scanning Emission Microscopy (SEM, Hitachi S-4700) after patterning with photoresist (PR, AZ 5214E) as an etch mask. Also, the etch profiles of the multilayer thin films composed of SiN x /SiO y stacks were observed by the SEM. The surface roughness of films after the etching was measured by atomic force microscope (AFM, XE-100, Park System) with a non-contact measurement mode. The characteristics of ClF 3 /H 2 plasma were analyzed with Optical Emission Spectrometry (OES, Avaspec-3648). Byproduct gases during etching process were monitored with Fourier-Transform Infrared Spectroscopy (FT-IR, MIDAC 12,000). The binding state and atomic composition of SiN x and SiO y (thin films of initial thickness of 500, 300 nm, respectively) before and after the etching were analyzed by X-ray Photoelectron Spectroscopy (XPS, MXP10, ThermoFisher Scientific) with a monochromated Al Kα source (1,486.6 eV) with spot size of 400 µm. The expected energy resolution of XPS is below 0.5 eV FWHM. The Avantage 5.0 software was used for curve fittings and the areas of each peak Figure 1. Schematic drawing of a remote-type inductively coupled plasma (ICP) etcher. At the center of the chamber, double grids having multiple holes are installed to prevent an ion bombardment and to deliver radicals only to the substrate. During the process, the substrate temperature was controlled (RT ~ 500 °C) by a silicon carbide (SiC) heater located below the substrate. www.nature.com/scientificreports/ were calculated with shirley background. The incident angle of X-ray to the sample was 50° and a hemispherical sector energy analyzer was positioned perpendicular to the sample stage.
Results and discussion Figure 2 shows etch characteristics of SiN x and SiO y with ClF 3 gas only and ClF 3 remote plasmas. For ClF 3 remote plasmas, 200 sccm of Ar was added to 200 sccm of ClF 3 for the plasma stability. As shown in Fig. 2a, the etch rates of SiN x and SiO y were increased gradually with increasing rf power due to the enhanced dissociation of ClF 3 reaching the maximum etch rates of SiN x and SiO y at ~ 90 and ~ 0.8 nm/min, respectively. Note that, the etch selectivity of SiN x over SiO y didn't vary significantly (~ 120) over rf powers of 100 ~ 400 W. As shown in Fig. 2b, the SiN x and SiO y could be also etched just by flowing ClF 3 gas only without dissociating ClF 3 by rf plasmas and the increase of substrate temperature increased the etch rates of both films. However, the overall SiN x etch rates by ClF 3 gas flow only were much lower compared to etching with ClF 3 remote plasmas, and which demonstrates that ClF 3 remote plasma etching is much more effective method for SiN x etching compared with that by thermal etching without plasma. Meanwhile, even though etch rates of both materials were increased with increasing the substrate temperature, the etch selectivity of SiN x over SiO y was decreased. Same trend was observed for remote plasma etching. As shown in Fig. 2c, the increase of substrate temperature to ~ 500 °C at a fixed rf power of 300 W showed a gradual decreases of etch selectivity below 40 while showing increased SiN x etch rates over 600 nm/min. The effect of process temperature on the etching of SiN x and SiO y can be understood by plotting the etch rates of SiN x and SiO y logarithmically as a function of inverse temperature (1/T) for ClF 3 remote plasma etching as shown in Fig. 2d. For the chemically activated etching, the etch rates can be described as a following Arrhenius equation.
where k is a rate constant, R is the gas constant (1.987 cal K -1 mol -1 ), T is the process temperature (K), and E a is the activation energy. The calculated activation energies (E a ) of SiN x and SiO y were 1.93 and 3.18 kcal/mole, respectively. The higher activation energy of SiO y means that the etch rate of SiO y rises faster than that of SiN x with the increase of temperature, and which leads to the decreases in etch selectivity of SiN x over SiO y even www.nature.com/scientificreports/ though the etch rates of both materials are increased exponentially with increasing substrate temperature. The root mean square (RMS) surface roughness of SiN x and SiO y after the etching with each process condition (remote plasma-and thermally-etching) showed no significant differences in the RMS surface roughness among the samples for different etch methods ( Figure S1, supplementary information).
To improve the etch selectivity of SiN x over SiO y , H 2 was added to ClF 3 in addition to Ar (Ar was also added to ClF 3 /H 2 for plasma stability) and, the effect of H 2 addition to ClF 3 on the etch characteristics of SiN x and SiO y was investigated as a function of H 2 percentage in ClF 3 /H 2 (ClF 3 /H 2 /Ar plasma) and the results are shown in Fig. 3a. To increase the H 2 percentage in ClF 3 /H 2 , H 2 flow rate was increased while keeping the substrate temperature at 25 °C, operating pressure at 200 mTorr, the ClF 3 flow rate at 200 sccm, Ar flow rate at 200 sccm, and the rf power at 300 W. The etch rates of both SiN x and SiO y were decreased with the increase of H 2 percentage, however, the etch selectivity of SiN x over SiO y was increased with the increase of H 2 percentage in ClF 3 /H 2 . To study the mechanism of selectivity SiN x etching over SiO y , the dissociated species in the plasmas was investigated by OES at the center of chamber and the byproducts during the process was monitored using FTIR at the pumping site. Figure 3b,c shows optical emission spectra and the relative emission peak intensities of Cl, F, and H normalized by the intensity of Ar as a function of H 2 percentage in ClF 3 /H 2 , respectively. In Fig. 3b, the optical emission peak intensities related to Cl, H, F, and Ar could be measured at 280, 656, 704, and 750 nm, respectively. In Fig. 3c, the optical emission intensities of Cl, F, and H were normalized by the optical emission intensity of Ar (750 nm) to minimize the effect of electron density on the estimation of radical density from the emission intensity. As shown in Fig. 3c, the increase of H 2 percentage did not change the intensity of Cl, however, it decreased F intensity while increasing H intensity. Figure 3 shows the FTIR data of the byproduct gases such as SiF 4 and HF measured at the pumping site for different H 2 percentage in ClF 3 /H 2 . As the flow rate of H 2 is increased, the concentration of SiF 4 was decreased, and which means that the etching of SiN x was suppressed while increasing HF concentration due to the reaction of hydrogen (H) with fluorine (F) radical in the plasma. Usually, the addition of hydrogen to fluorine based plasma leads to scavenging of F radicals by forming gaseous HF molecules 23,24 which have negligible effects on the etching of SiN x (and SiO y ) unlike their aqueous (ionized) state 25,26 .
The Si binding states and surface composition of SiN x and SiO y after the ClF 3 /H 2 plasma etching were analyzed using X-ray Photoelectron Spectroscopy (XPS) and the results are shown in Fig. 4 Fig. 4a,d, the reference SiN x and SiO y showed only Si-N at 101.7 eV, Si-O at 103.4 eV, respectively. After the etching with ClF 3 plasma, however, significant Si-F bonding (103.6 eV) was formed on the SiN x surface, presumably due to the bonding of Si with F (Fig. 4b). The Si-F bonding ratio decreases with addition of H 2 (20%) because of the reduction of F in the plasma (Fig. 4c and Table 1). However, no chlorine or Si-Cl bonding (~ 103.3 eV) was observed on the surface of SiN x even though there were enough Cl radicals in the ClF 3/ H 2 plasma as confirmed through OES data in Fig. 3b, presumably, due to the immediate reaction of Si-Cl with F radicals. Meanwhile, as shown in Fig. 4e,f), there was no significant change in F concentration on the SiO y surface during etching with ClF 3 and ClF 3 /H 2 plasma. Also, no noticeable  www.nature.com/scientificreports/ Si-F bonding formation on the SiO y surface during the etching with ClF 3 and ClF 3 /H 2 plasma was observed from the deconvolution of Si narrow scan data (Si 2p) indicating that most of F is adsorbed on the SiO y surface after the etching. Furthermore, the amount of F on the SiO y surface is much lower than that of SiN x because Si-O bonding is less reactive with F radical compared with SiN x . As shown in Fig. 4g,h, no chlorine was observed on the surface of both SiN x and SiO y even though the chlorine was observed in OES (Fig. 3b). The parameters used for curve fitting of SiN x is described in Table 1 and the normalized chi-square value for curve fitting was below 0.01. The compositional information of each element can be found in Table S1, supplementary information. The etching of SiN x and SiO y can be explained through the bonding energies of silicon (Si) compounds. Figure 5 shows the etch mechanism of SiN x and SiO y under Cl, F radicals. As the bonding energy of Si-F (565 kJ/ mol) is higher than those of Si-N (355 kJ/mol) and Si-O (452 kJ/mol) 22 , the SiN x and SiO y can be etched spontaneously under sufficient F radicals in the plasma although the etching is much active for SiN x than SiO y . However, the bonding energy of Si-Cl (381 kJ/mol) is slightly higher than that of Si-N but lower than that of Si-O, and which means the Cl radical can react only with SiN x and forms Si-Cl bonding. Once the Si-N changes to Si-Cl, Si-Cl can be more easily converted to Si-F by F radicals in the plasma (due to the quick conversion of Si-Cl to Si-F as shown in Fig. 5, no chlorine could be observed on the surfaces of SiN x and SiO y during the etching with ClF 3 /H 2 ), then Si-F on SiN x is removed as a volatile SiF 4 compound. Meanwhile, the addition of H 2 in the ClF 3 plasma reduces the density of F radicals by forming HF in the plasma causing the decreases of However, because the concentration of chlorine in the plasma is not significantly affected by the addition of H 2 as confirmed through OES data in Fig. 3c), the etching of SiN x is decreased more slowly compared to that of SiO y with increasing H 2 percentage through the conversion of Si-Cl on the surface of SiN x to Si-F, and which appears to increases the etch selectivity of SiN x over SiO y . Using the etch conditions of ClF 3 and ClF 3 /H 2 (20%), stacked layers of SiN x /SiO y were etched and the results are shown in Fig. 6. Figure 6a is the reference stack of SiN x /SiO y before the etching. Figure 6b,c are the stacked layer of SiN x /SiO y after the etching using ClF 3 and ClF 3 /H 2 (20%) plasmas for 5 min and 10 min, respectively. As shown in Fig. 6b,c, highly selective etching of SiN x over SiO y could be observed for both ClF 3 and ClF 3 /H 2 (20%) by showing no noticeable differences in SiO y thickness along the etch depth. Therefore, it appears that the etch selectivity for the real SiN x /SiO y could be higher than that measured with blank wafers. The etch depth with increasing the etch time was also measured and the results are shown in d) for both ClF 3 and ClF 3 /H 2 (20%). The measured etch rates of SiN x with ClF 3 and ClF 3 /H 2 remote plasma were 80 and 26 nm/min, respectively, which have similar values with blank samples at the same plasma conditions (Fig. 2a, 3a) because of isotropic etch characteristics of reactive radicals. Furthermore, the etch depth with etch time was linear for both conditions, therefore, no aspect ratio dependent etching was observed. (The process time-dependent etch profiles of SiN x / SiO y stacks are shown in figure S2 and S3, supplementary information).

Conclusion
The isotropic and selective etching of SiN x over SiO y was studied using ClF 3 /H 2 remote plasma with an ICP source. The SiN x etching using plasma assisted thermal processes showed the highest etch rate as well as the smoothest surface morphology compared with that etched only with thermal etching or plasma etching. The temperature dependent etch characteristics of SiN x and SiO y demonstrated a higher activation energy of SiO y compared that of SiN x in the ClF 3 remote plasma. Furthermore, the addition of H 2 (20%) to the ClF 3 plasma improved the etch selectivity of SiN x over SiO y from 130 to 200 even though the etch rate of SiN x was decreased from ~ 83 to ~ 23 nm/min. We believe the fast and ultra-high selective SiN x etching technology can be applied not only to next generation three dimensional NOT-AND type flash memory fabrication process but also to various semiconductor processes where precise etching of SiN x is required.