Low-Temperature One-Step Growth of AlON Thin Films with

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Low temperature one-step growth of AlON thin films with homogenous nitrogen doping profile by plasma enhanced atomic layer deposition Hong-Yan Chen, Hong-Liang Lu, Jin-Xin Chen, Feng Zhang, XinMing Ji, Wen-Jun Liu, Xiao-Feng Yang, and David Wei Zhang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b12262 • Publication Date (Web): 17 Oct 2017 Downloaded from http://pubs.acs.org on October 21, 2017

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ACS Applied Materials & Interfaces

Low Temperature One-Step Growth of AlON Thin Films with Homogenous Nitrogen Doping Profile by Plasma Enhanced Atomic Layer Deposition

† † Hong-Yan Chen†, Hong-Liang Lu* , Jin-Xin Chen , Feng Zhang ‡, Xin-Ming Ji†,

Wen-Jun Liu†, Xiao-Feng Yang†, and David Wei Zhang†

†

State Key Laboratory of ASIC and System, Shanghai Institute of Intelligent

Electronics & Systems, School of Microelectronics, Fudan University, Shanghai 200433, China ‡

Key

Laboratory

of

Semiconductor

Material

Sciences,

Semiconductors, Chinese Academy of Sciences, Beijing 100083, China

*Corresponding author. Email address: [email protected] Tel.:+86-21-65642457

KEYWORDS: Aluminum oxynitride (AlON), Plasma enhanced atomic layer deposition (PEALD), Doping profile, X-ray photoelectron spectroscopy, Interface

1

ACS Paragon Plus Environment

Institute

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Abstract The AlON film with homogenous nitrogen doping profile was grown by plasma enhanced atomic layer deposition (PEALD) at low temperature. In this work, the precursors of the NH3 and the O2 were simultaneously introduced into the chamber during the PEALD growth at a relatively low temperature of 185 oC. It is found that the composition of the obtained film quickly changes from AlN to Al2O3 when a small amount of O2 is added. Thus, the NH3:O2 ratio should be maintained at a relatively high level (>85 %) for realizing the AlON growth. Benefited from the growth method, the nitrogen can be doped evenly in the entire film. Moreover, the AlON films exhibit a lower surface roughness than the AlN as well as the Al2O3 ones. The Al 2p and N 1s x-ray photoelectron spectra show that the AlON film is composed of Al-N, Al-O, and N-Al-O bonds. Moreover, a three-layer construction of the AlON film is proposed through the Si 2p spectra analyzation and reconfirmed by the transmission electron microscopy characterization. At last, the electrical and optical tests indicate that the AlON films prepared in this work can be employed as the gate dielectric in transistor application as well as the anti-reflection layer in photovoltaic application.

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1. Introduction In recent years, the aluminum oxynitride (AlON) films have aroused increasing attention in the applications of high-k1-6 and photovoltaic7-10 fields due to their superior characteristics. In high-k application, the metal-oxide-semiconductor field effect transistors (MOSFETs) with AlON gate dielectric exhibit lower leakage current,1-3 higher break down voltage,1, 6 more stable threshold voltage3, 5 and thinner interfacial layer4 than the MOSFETs with Al2O3 ones. Meanwhile, the AlON film shows nearly the same dielectric value as the Al2O3 film (εr~8.9). In photovoltaic application, the AlON films exhibit better anti-reflection ability than that of the Al2O3 ones owing to the higher index of reflection (n) value.9 In addition, the passivation quality of the two films are identical.7, 9 The growth methods of the AlON films can be classified as both direct3-5, 7-9 and indirect1,

2

ways. For the direct growth routine, the most common method is RF

sputtering.7-9 But the obtained films are always in micro-crystalline phase,7 which will greatly increase the leakage current in high-k application. Fortunately, this obstacle can be conquered by replacing the growth method with chemical vapor deposition (CVD)3, 5. The chemical solution deposition is also an option.4 But this coating technology is not commonly used by the industry. The indirect routine of AlON growth includes the oxidation of the AlN film1 and the nitridation of the Al2O3 film.2 Although the indirect ways are relatively facile, the doping profile is inhomogeneous. The dopants will mainly concentrate on the surface of the film, which can hardly reach the interface between the film and the substrate.6 Mover, both of the direct and 3



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indirect ways mentioned above have difficulties in eliminating pinholes or cracks inside the obtained films. Furthermore, these methods also meet problems in achieving the precise thickness control for ultra-thin films (75:25) for achieving the AlON growth. Fig. 2a shows the measured XRR curve of the AlN, AlON and Al2O3 films with 200 ALD growth cycles. It can be seen that all samples exhibit clear Kiessig fringes even at 2θ=4o except for the AlN (NH3:O2=100:0) film. It indicates that the surface of the films is very smooth. Meanwhile, the period of the Kiessig fringes of the samples decreases from ~0.5 o to ~0.3o as the NH3:O2 ratio decreases from 100:0 to 0:100. This phenomenon reveals that the film thickness increases with the decreased NH3:O2 ratio under the same growth cycles. This trend agrees with the SE data shown in Fig. 1d. Moreover, the total external reflection angles of the AlON and the Al2O3 samples are identical (2θ=0.51±0.01). By contrast, the AlN sample exhibits a low total external reflection angle at 2θ=0.44o. It reveals that the mass density of the AlON and the Al2O3 films prepared in this work are similar. Meanwhile, the mass density of the AlN film is relatively lower. The obtained surface roughness using from XRR and AFM are shown in Fig. 2b, 8


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which is crucial for the electrical properties of the devices.5 The corresponding AFM mapping images of the AlN, AlON, and Al2O3 films with 200 ALD cycles are shown in Fig. 3. It is found that the surface roughness of all the films prepared in this work is less than 0.8 nm. Obviously, the AFM and XRR results have the same trend. As seen from the RMS values obtained by AFM, the AlN film (NH3:O2 ratio: 100:0) exhibits the highest RMS roughness value of 0.27 nm. It is owing to the AlN films prepared by the PE-ALD are mainly in crystallite phase.18, 21 Then, the RMS roughness of the obtained films drops evidently within the NH3:O2 ratio range from 95:5 to 75:25. This is probably due to the oxygen doping in the AlN film turns the crystallization orientation of the obtained film form crystallite phase (AlN) to amorphous phase (AlON). After that, as the NH3:O2 ratio further decreased from 25:75 (AlON) to 0:100 (Al2O3), the corresponding RMS roughness exhibit a slight raise. A possible reason is that the films grown in such low NH3:O2 ratio are mainly composed by Al2O3. Meanwhile, the lattice constant of the Al2O3 (a=5.14 Å) much larger than that of the AlN (a=3.11 Å). Results show that the surface roughness of the AlON films prepared this is lower than that of the AlN and the Al2O3 ones, which is suitable for gate dielectric application. 3.2 Composition and interface analysis To evaluate whether the nitrogen is uniformly doped in the entire AlON film prepared in this work, a representative AlON film prepared under 95:5 NH3:O2 ratio is analyzed by GDOES firstly. The AlN and Al2O3 films are also prepared for comparison. All of the films grown with 200 growth cycles. As shown in Fig. 4a-c, 9



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the time needed for etching off the three films is 1.3 s, 2.2 s, and 2.5 s, respectively. It indicates the film thickness increases with the decreased NH3:O2 ratio, which reconfirms the relationship between the GPC value and the NH3:O2 ratio shown in Fig. 1d. The aluminum concentration (40~50 at.%) in the bulk area of all three films is nearly constant. In contrast, the nitrogen concentration of the AlN, the AlON, and the Al2O3 films are ~40 at.%, ~15 at.%, and ~1 at.%, respectively.

The compositional

profile shows that the nitrogen content in the prepared films decreases dramatically with the decreased NH3:O2 ratio during the films growth. Encouragingly, a homogenous N doping profile can be observed in the entire AlON film (Fig. 4b). It is known that this kind of nitrogen distribution can hardly be achieved by the conventional growth method including annealing the ALD grown Al2O3 or AlN film in NH3 or O2 environment.6, 14 In addition, it is noted that the obtained AlON film is oxygen rich although the NH3:O2 ratio is as high as 95:5 during the film growth. This is mainly due to much higher reactivity of oxygen plasma than that of the ammonia one. In order to investigate the chemical bondings of the AlON film as well as the interfacial properties, the Al 2p, N 1s, and Si 2p spectra of the AlON/Si sample are examined by high resolution XPS, as shown in Fig. 5. The AlN/Si and the Al2O3/Si sample are also analyzed for comparison. From the Al 2p spectrum of the AlN film (Fig. 5a), two peaks are observed at 74.5 and 73.5 eV, corresponding the Al-O bond4, 16, 22

and Al-N bond4, 16-18, 22, respectively. The Al-N bond belongs to the wurtzite

phase AlN. The existence of the Al-O bond is ascribed to the oxidation effect after the 10


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AlN film is exposed in the atmosphere.17, 18, 21, 23, 24 The other part of the Al-O bond comes from the residual oxygen inside the film.21 In contrast, the Al 2p spectra of the AlON sample is dominated by the Al-O bond. This is due to the much higher reactivity of the oxygen plasma than that of the ammonia plasma21 as discussed in the results from GDOES. It is encouraged that a small portion of Al-N bond (7.3 % of the total N 1s intensity) is observed inside the AlON sample (Fig. 5b), indicating the nitrogen doping in the AlON film is realized by forming chemical bonds, rather than physically “add” the ammonia molecules inside the film. The existence of the Al-N bond can also prove that the oxygen plasma didn’t saturate all the Al-CH3 bonds on the film surface. Some of the Al-CH3 bonds will react with the ammonia plasma during the film growth. Obviously, the Al2O3 sample (Fig. 5c) exhibit only the Al-O bonds in the Al 2p spectrum due to without nitrogen source during the film growth. By comparing the N 1s spectra of the AlON sample (Fig. 5e) with that of the AlN sample (Fig. 5d), a more accurate bonding state information of the nitrogen inside the AlON film can be obtained. Both of the AlN and AlON films can be fitted by two subpeaks located at ~397.4 eV and ~396.4 eV, corresponding to N-Al-O (bulk) and N-Al bonds, respectively.4, 16-18, 22, 25 For the N 1s spectra of the AlON sample, the existence of the N-Al bond (Fig. 5e) corresponds with the Al-N bond of the same sample found in Al 2p (Fig. 5b). Moreover, the existence of the N-Al-O bond confirms the formation of ternary compound of AlOxNy in the AlON sample.4, 5 It reveals that the prepared AlON sample is not a “mixture” of AlN and Al2O3. Normally, the ALD growth of ternary compound is realized by alternately growing 11



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two kinds of binary compounds.26-28 In addition, there is no obvious subpeak shown in >400 eV range, which is commonly observed by the AlON films grown with high temperature method.25 The absence of the N-O bond in the AlON film is probably due to the relatively low growth temperature and plasma power. Based on this reason, it is proposed that the ammonia plasma only reacts with the Al-CH3 bonds during the film growth. The Si 2p spectra of the AlON sample is also investigated to gain insight into the interfacial chemical states. As shown in Fig. 5f, the AlN sample have four subpeaks centered at 102.2, 101, 99.8 and 99.2 eV, corresponding to the Si-N (Si3N4),4,29 the Si-N (SiNx), Si-Si (Si 2p3/2) and Si-Si (Si 2p1/2) bonds,30-33 respectively. It indicates that the interfacial layer of the AlN sample contains silicon nitride in both stoichiometric and non-stoichiometric phase.30 Meanwhile, the absence of the Si-O related bonds (centered within 102.2~103.2 eV range)29,30 indicates no oxygen exists at the AlN/Si interface. This finding reveals that the oxygen contamination during the film growth is maintained at a very low level, nearly all of the dangling bonds on the Si surface are saturated by nitrogen at initial stage of the growth. Based on this finding, it can also be deduced that the oxygen related bonds observed in the Al 2p and N 1s spectra of the AlN sample mainly comes from the oxidization effect in the atmosphere. For the AlON sample, three subpeaks centered at 103.4, 102.1, and 101.2eV are observed except for the Si substrate. They are assigned to be the Si-O (SiO2), Si-N (Si3N4), and Si-N (SiNx) bonds, respectively. It reveals that the oxygen participated in the growth process of the AlON film. Because the oxygen is also 12


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pulsed into the chamber during the AlON growth. Although the oxygen related bonds is observed in both of the Al 2p and N 1s spectrum of the AlN and the AlON films, the oxygen in the two films does not share the same source. The oxygen in the AlON mainly comes from the residual oxygen during the film growth. While for the AlN film, the oxygen mainly comes from the oxidation effect. For the Al2O3 sample, only the Si-O (SiO2) and the Si-Si (Si substrate) bonds are observed, which agree well with other literatures.4 Moreover, the TEM test is used to verify the surface roughness and the interfacial composition analyzation of the films mentioned above. The surface morphology of the three representative samples can be observed by the low-resolution TEM images shown in Fig. 6a1 to c1. It can be seen from Fig. 6a1 that the surface of the AlN sample is a little rough. By contrast, the AlON and Al2O3 surface is atomically smooth (Fig. 6b1 and c1). This finding agrees with the surface roughness data shown in Fig. 3. The high-resolution TEM images shown in Fig. 6a2-c2 exhibits the interfacial composition of the three films. As shown in Fig. 6a2, the AlN film composes of two layers. As discussed in the Al 2p and Si 2p spectra, the Al-N bonds on the surface of the AlN film are replaced with Al-O bonds owning to the oxidization effect. Thus, the Al2O3 forms on the top of the AlN film. As a result, the layers 1 and 2 are assigned to Al2O3 and AlN, respectively. Moreover, although the Si-N bonds is detected in the Si 2p spectra of the AlN sample (Fig. 5f), the AlN sample shows a sharp interface between layer 2 and the Si substrate in TEM image. The probable reason is that the interfacial layer is relatively thin. It is 13



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noteworthy that the sharp interface is beneficial for achieving a higher dielectric constant in high k application.5, 34 For the AlON sample shown in Fig. 6b2, a tri-layer structure can be found in the sample. The layer 1 is assigned to the Al2O3 owning to the oxidation. The layer 2 is the AlON film. The bright atoms at the interface of layer 3 and the Si substrate represents the existence of oxidized Si atoms (SiO2).17, 18 This finding is in agreement with the observation of the Si-O bonds in the Si 2p spectra of the AlON sample shown in Fig. 5g. According to the Si 2p results, the layer 3 is composed of the SiO2, the Si3N4, and the SiNx. As shown in Fig. 6c2, the Al2O3 film exhibits a bilayer structure. Similar to the AlON sample, the SiO2 is also observed at the interface between layer 2 and the substrate. The layers 1 and 2 are assigned to the Al2O3 film and amorphous SiO2, respectively. According to the XPS and TEM results, the multi-layer models are built for the AlN, AlON and Al2O3 films respectively. A suitable model is beneficial for the researchers to obtain more accurate results in SE, XRR and other tests based on simulation or regression. As shown in Fig. 6a3 to c3, an interfacial layer is added for all the samples because the formation of the interfacial layer is inevitable during the film growth.5,

34

According to the Si 2p analyzation results (Fig. 5f to h), the

components of the interfacial layers of the three samples are summarized respectively in Fig. 6a3 to c3. However, according to the TEM results, the interlayer thickness of the three samples are different. The AlN sample has the minimum interlayer thickness. This is mainly due to no O2 is led into the chamber during the AlN growth. By contrast, the interfacial layers of the AlON and the Al2O3 sample (1.2 and 1.9 nm, 14


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respectively) are much thicker. This is because of the existence of O2 during the AlON and the Al2O3 growth. The Si surface can be easily oxidized in O2 environment. Meanwhile, the interfacial layer of the AlON sample is thinner than that of the Al2O3. This is probably because of the Si-N bonds existed in the interfacial layer of the AlON sample block the diffusion of oxygen from the upper AlON layer.3, 6 It is well known that the oxidation of the AlN happens just after the film is exposed into atmosphere. Thus, an Al2O3 oxidization layer is added on both of the AlN and AlON film. According to the TEM images shown in Fig. 6 a2 and b2, the thickness of the oxidization layers in this work is ~1.7 nm. Dalmau et al. reported that the thickness of the oxidization layer increases slowly in the atmosphere and will eventually reaches ~10 nm on the AlN surface.35 3.3 Electrical properties To exam the reliability of the AlN, the AlON, and the Al2O3 films prepared in this work, the J-V tests are performed on the prepared samples. The Mo top electrodes (with thickness of 200 nm, 400 µm in width square shape arrays) are deposited on the three representative samples. As shown in Fig. 7a, all of the three samples exhibit no visible “soft” breakdown (leads by the defects in the dielectric film) point until the final “hard” (intrinsic) breakdown point down point is reached. It indicates that there is no obvious pin-holes or cracks inside the dielectric films. More importantly, the AlON sample exhibits the lowest leakage current. The possible reason is that the nitrogen atoms in the AlON film act as obstacles for the leakage paths and strengthen the oxygen networks of the Al2O3 environment.6 By contrast, the AlN sample exhibits 15



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the highest leakage. This is owing to the AlN film prepared by PE-ALD is in microcrystalline phase.18,

21

The grain boundaries inside the film will provide the

paths for the leakage current. To analyze from the intrinsic (“hard”) break down voltage, the AlON sample exhibit the highest break down voltage. The possible reason is that the nitrogen doping in the AlON film is beneficial for relieving the interfacial strain at the Si/AlON interface.3 Based on this finding, the nitrogen doping plays a crucial role in enhancing the electrical properties of the AlON films prepared in this work. 3.4 Optical properties In addition, the influence of nitrogen doping on the optical properties of the AlON films, the n value of the AlON films are measured by ellipsometer. Fig. 7b shows the n value dependence on the NH3:O2 ratio of the films. Evidently, the AlN film (NH3:O2=100:0) exhibits a high n value of 1.93 at 633 nm. For the Al2O3 film, it exhibits a low n value of 1.61 at 633 nm. These values agree well with the former works.24,

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The refractive index of the AlON films monotonically increases with

increasing the NH3:O2 ratio. Results show that the NH3:O2 ratio should be maintained at a very high concentration (>90:10) for maintaining the n value of the AlON film in the range of anti-reflection application. Meanwhile, the surface passivation quality of the AlON at this moment will not be sacrificed. Because as shown in Fig. 7b2, a SiO2 passivation layer can also be formed at the AlON/Si interface, similar to the consequence of the Al2O3/Si interface (Fig. 7c2). Furthermore, if the NH3:O2 ratio is lower than 75:25, the obtained film will mostly be Al2O3 from optical aspect. At this 16


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moment, the obtained film is not suitable for anti-reflection application in visible light range. Results show that by controlling the NH3:O2 ratio at a suitable level, the obtained AlON films can achieve a higher n value than the Al2O3 film while maintaining a reasonable passivation quality. These two qualities of the AlON paves a way for its application in optical-electrical devices.

4. Conclusions In this work, a growth method of the AlON film with homogenous nitrogen doping profile is demonstrated through PEALD. The precursors of the NH3 and the O2 are simultaneously introduced into the chamber during the film growth at a relatively low temperature of 185 oC. It is found that the composition of the obtained film quickly changes from AlN to Al2O3 when a small amount of O2 is introduced into the chamber. Thus, the NH3:O2 ratio should be maintained at a relatively high level (>85:15) for realizing the AlON growth. Benefited from the layer-by-layer growth mechanism, the nitrogen can be doped evenly in the entire film. Moreover, the AlON films exhibit lower surface roughness than the AlN as well as the Al2O3 ones. The X-ray photoelectron spectroscopy analyzation on Al 2p and N 1s spectra show that the AlON film is composed of Al-N, Al-O, and N-Al-O bonds. Moreover, a three-layer construction (Al2O3/AlON/SiO2) of the AlON film on Si substrate is proposed and reconfirmed by the transmission electron microscopy. From electrical aspect, the MOS capacitor with AlON dielectric exhibit lower leakage current as well as higher break down voltage than the one with Al2O3 dielectrics. In addition, the 17



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obtained AlON films exhibit higher index of refraction value than Al2O3 one while maintaining an identical passivation quality. The AlON growth method demonstrated in this work can be utilized to fabricate high performance transistors and optical-electrical devices.

Acknowledgments This work is supported by the National Natural Science Foundation of China (no. 61376008, U1632121 and 61774041), MOST (No. 2016YFE0110700), the Innovation Program of Shanghai Municipal Education Commission (14ZZ004).

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Figure Captions Figure 1. (a) The parameters of one ALD cycle of the AlON growth utilized in this work. (b) The growth per cycle (GPC) of the film prepared under different ammonia plasma processing time. (c) The relationship between the film thickness and the ALD growth cycles when the ammonia plasma processing time is fixed at 10 s. (d) GPC of the films prepared under different NH3:O2 ratio. Figure 2. (a) The measured and simulated XRR curves of the AlN, AlON, and Al2O3 films undergo 200 ALD growth cycles. (b) The RMS roughness of the films prepared under different NH3:O2 ratio. Figure 3. AFM mapping images of the films prepared under different NH3:O2 ratio. (a)-(f): NH3:O2 ratio form 100:0 to 0:100. Figure 4. GDOES depth profile of the (a) AlN, (b) AlON (NH3:O2 ratio=95:5), (c) Al2O3 films undergo 200 ALD growth cycles Figure 5. XPS analysis of the AlN, AlON, Al2O3 sample on (a)-(c) Al 2p spectra, (d)-(e) N 1s spectra, and (f)-(h) Si 2p spectra. Figure 6. (a1)-(c2) TEM images of the AlN, AlON and Al2O3 films on the Si substrate. (a3)-(c3) Construction model for the AlN, AlON and Al2O3 samples. Figure 7. (a) J-V curves of the AlN, AlON and the Al2O3 dielectrics with ~5nm thickness on Si substrate. (b) The n value evolution of the AlON films as the NH3:O2 ratio increases from 0 to 100%.

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Figure 1-Lu 156x122mm (300 x 300 DPI)


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Graphic for manusript 91x70mm (300 x 300 DPI)


Low-Temperature One-Step Growth of AlON Thin Films with

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