Surface & Coatings Technology 331 (2017) 85–89 Contents lists available at ScienceDirect Surface & Coatings Technology journal homepage: www.elsevier.com/locate/surfcoat Comparative surfacing studies of sputtered and thermal annealed ZnO ﬁlms wet etched with NH4Cl and acidic etchants MARK Liang-Kui Liu, Cheng Shi, Guan-Qing Wang, Yi Wang⁎, Lei Sun⁎ Institute of Microelectronics, Peking University, Beijing 100871, PR China A R T I C L E I N F O A B S T R A C T Keywords: Wet etching ZnO ﬁlms Annealing The wet etchings of as-grown and thermal annealed ZnO ﬁlms were carried out by using NH4Cl and acidic etchants respectively. The lateral proﬁles and side faces of the etched ZnO samples were exhibited by proﬁler and SEM measurements. NH4Cl etchant can avoid W-like lateral proﬁle and eave-like slope at the side face of the etched ZnO patterns for both as-grown and post-annealed ZnO samples. The etching rate has been compared between NH4Cl etchant and acidic etchants for both as-grown and post-annealed ZnO samples. NH4Cl etchant can maintain slower etching rate with higher NH4Cl concentration. The thermal annealing procedure can be helpful to achieving uniform etching surface, and the etching rates of the annealed ZnO samples are even slower, which can be explained by the better crystallinity and lower density of oxygen vacancy. 1. Introduction As one kind of wide bandgap metal oxide materials, zinc oxide plays an important role in microelectronic ﬁeld. Wang et al. reported the application of ZnO nanowire arrays for powering nano-devices, considering the outstanding piezoelectric properties of ZnO . Hoﬀman et al. applied ZnO as transparent channel layer for thin ﬁlm transistors, which can be used in ﬂat panel display applications . Minami et al. used doped ZnO thin ﬁlm as transparent electrode for optical electronic application . The growth, patterning and etching of ZnO ﬁlms always constitute the major steps of ZnO devices fabrication, among which the ﬁlm etching technology can directly aﬀect the manufacture eﬃciency and device reliability. Comparing with dry etching for ZnO ﬁlms [4–6], wet etching has shown the advantages of high selectivity, high etching rate and lower cost. A large amount of researches studied on acid etchants or their mixture, such as HCl [7–9], H3PO4 [8,10], HNO3 , H2SO4 , formic acid , acetic acid [9,11], oxalic acid , citric acid  and hydrogen peroxide . Aqueous acidic salts, such as FeCl3 [11,14] and NH4Cl [15–17], were also proposed. NH4Cl will be a better choice, because it is free of Fe contamination. Up to now, most of the researches focused on the etching rate of the individual etchant, and the lateral proﬁle and the side face morphology of the etched ZnO patterns have been scarcely reported. On the other hand, thermal annealing procedures have been widely used to improve crystallinity and ﬁlm properties of ZnO [2,18,19], and the etching properties of the annealed ZnO samples, especially by wet etching, still need further researches. ⁎ In this article, we've compared the surfacing morphology and etching rate by using NH4Cl and acidic etchants for both as-grown ZnO ﬁlms and post-annealed ZnO samples. The etching surfacing proﬁles and etching rates have been measured by proﬁler, Scanning Electron Microscope (SEM), and Atomic Force Microscope (AFM) measurements. The crystallinity of the annealed samples were measured by X-ray Diﬀraction (XRD). And we've found that the annealing condition will signiﬁcantly inﬂuence the etching properties of the samples. And the Wshape lateral proﬁles eave-like side face can be avoid by using NH4Cl etchant for both as-grown and post-annealed ZnO samples. 2. Experimental details N-type (100) silicon substrates were used during the experiments. Before the depositions of ZnO ﬁlms, SiO2 buﬀer layer was ﬁrstly grown by thermal oxidation. ZnO ﬁlms were then deposited by radio frequency magnetron controlled sputtering (Kurt J. Lesker, PVD-75). During the depositions, ceramic ZnO target (with purity of 99.99%) was used. The sputtering power was 70 W, and the gas pressure was kept at 1 Pa. After the growth of ZnO ﬁlm, about 2 μm-thick positive photoresist (Suzhou Ruihong Electronic Chemicals Co., Ltd., RZJ 304) was spun on ZnO ﬁlm surface. Lithography and development were then performed (Suzhou Ruihong Electronic Chemicals Co., Ltd., RZX 3038). The common baking steps after spin, exposure and development lasted 50 s, 60 s and 65 s respectively, and the baking temperatures were kept at 120 °C. After the patterning, the prepared samples were etched with NH4Cl aqueous solution etchant (with concentration ranged from Corresponding authors. E-mail addresses: email@example.com (Y. Wang), firstname.lastname@example.org (L. Sun). http://dx.doi.org/10.1016/j.surfcoat.2017.10.041 Received 7 April 2017; Received in revised form 29 September 2017; Accepted 12 October 2017 Available online 12 October 2017 0257-8972/ © 2017 Published by Elsevier B.V. Surface & Coatings Technology 331 (2017) 85–89 L.-K. Liu et al. PR Pattern 200 Etched after: (a) 0 10s 20s Profile (angstrom) -200 -400 ZnO -600 Oxide -800 100nm -1000 -1200 (a) -1400 0 50 100 150 200 Lateral Position (um) PR Pattern ZnO 500 Etched after: (b) 10s 20s Profile (angstrom) 0 Oxide -500 100nm -1000 (b) -1500 -2000 -2500 0 50 100 150 200 Lateral Position (um) ZnO PR Pattern Oxide 200nm Profile (angstrom) 0 Etched after: (c) 40s 80s (c) Fig. 2. SEM photographs of as-grown ZnO ﬁlm's cross sections: (a) etched with HCl etchant, (b) etched with H3PO4 etchant, (c) etched with NH4Cl etchant. -500 10 kV to 15 kV) and AFM (SII NanoTechnology Inc., SPI3800N/SPA400, at contact mode with Si3N4 tip). The etchings with acidic etchants (HCl and H3PO4 are selected as control groups) have also been carried out. The acid concentration is set as 6.7 mmol/L, 10.0 mmol/L, 12.5 mmol/L, 15.0 mmol/L and 16.7 mmol/L for both HCl etchant and H3PO4 etchant. The etching morphology and etching rate with NH4Cl etchant were then compared with the control groups. To study the inﬂuence of anneal on the etching results, some ZnO samples were annealed in O2 and N2 ambience in tube-type anneal furnace (Nabertherm, C250) respectively. All annealing temperature was controlled at 400 °C, and annealing time was kept for 1 h. Same as the treatments above, the annealed ZnO ﬁlms were patterned and etched with diﬀerent etchants. Measurements of the annealed ZnO ﬁlms after the etchings were also the same as the case for as-grown samples. -1000 -1500 -2000 0 50 100 150 200 Lateral Position (um) Fig. 1. Etching proﬁles of as-grown ZnO ﬁlm: (a) etched with HCl etchant (12.5 mmol/L), (b) etched with H3PO4 etchant (12.5 mmol/L), (c) etched with NH4Cl etchant (3.0 mol/ L). 1.0 mol/L to 4.0 mol/L) at room temperature. The morphologies of the lateral and vertical etching surfaces were detected by Proﬁler (AMBiOS, XP-1), SEM (FEI, Quanta 600, with operating voltage ranging from 86 Surface & Coatings Technology 331 (2017) 85–89 L.-K. Liu et al. Fig. 3. SEM photographs of ZnO ﬁlm's cross sections: (a) annealed in N2 and etched with HCl etchant (12.5 mmol/ L), (b) annealed in N2 and etched with NH4Cl etchant (1 mol/L), (c) annealed in O2 and etched with HCl etchant (12.5 mmol/L), (d) annealed in O2 and etched with NH4Cl etchant (1 mol/L). All samples were over-etched for measurements. ZnO Oxide ZnO Oxide Substrate Substrate 1um 1um (b) (a) ZnO ZnO Oxide Oxide Substrate Substrate 1um 1um (c) (d) etched with HCl, H3PO4 and NH4Cl etchants respectively. The samples were over etched until the SiO2 buﬀer layers were revealed in the unprotected regions. SEM measurements have shown the presence of eave-like vertical side faces in the samples etched with HCl and H3PO4 etchants, which would bring diﬃculty to patterning control (see Fig. 2a and b). In the contrary, the vertical side face of the sample etched with NH4Cl etchant is nearly perpendicular to the substrate. We've not found signiﬁcant eave-like proﬁle even after longer over etching time. The uniform etching proﬁle in Fig. 2c are consistent with the absence of Wlike etching proﬁle in Fig. 1c. It's because there are little porous structures (holes) or other structural defects formed during the etching with NH4Cl etchant, however, such structural defects might appear when acid etchant is used [7,12]. The relationship between surface roughness of etched ZnO ﬁlm and etching time (with NH4Cl etchant) has been studied by AFM measurements. The root mean squares (RMS) of the surface roughness are 4.492 nm (etched after 20 s), 17.62 nm (etched after 40 s), 27.46 nm (etched after 80 s) and 31.48 nm (etched after 100 s) respectively. The RMS of surface roughness increases when etching time becomes longer. Such diﬀerence might be induced by ZnO ﬁlm itself rather than by the etching procedure. ZnO ﬁlms were grown by sputtering, and the growth procedure follows the island-growth principle (Volmer-Weber growth model) [20–22]. The islands expand and connect together, ﬁnally ﬁll in the whole surface of the substrate and construct the continuous ﬁlms. The etching of ZnO likes a converse procedure of growing. When ZnO ﬁlm is etched, the island structure would appear again, therefore ZnO ﬁlm surface becomes rougher. Fig. 3 shows the cross-section of the vertical etching proﬁles of the annealing samples. The samples were annealed in N2 and O2 ambience, and etched respectively. In the case with HCl etchant, eave-like vertical side face has been suppressed (see Fig. 3a), and the abrupt vertical side face is kept for the one etched with NH4Cl etchant (see Fig. 3b). Annealing in N2 ambience also conducts smoother side face. The etching morphology with both HCl and NH4Cl etchants becomes acceptable after annealing. Such phenomenon can be explained by the enhancement of ZnO ﬁlm crystallinity. In Fig. 4, the inﬂuence of annealing Intensity (arb. units) ZnO annealed in N2 ZnO annealed in O2 as-grown ZnO 20 25 30 35 40 45 50 2Theta (Degree) Fig. 4. XRD measurement proﬁles of as-grown ZnO ﬁlm (black line), annealed in O2 at 400 °C (blue line), and annealed one in N2 at 400 °C (red line). (For interpretation of the references to color in this ﬁgure legend, the reader is referred to the web version of this article.) 3. Results and discussions Fig. 1 shows the etching proﬁles of as-grown ZnO ﬁlms etched with the three diﬀerent etchants. For HCl and H3PO4 etchants, and we can observe obvious over-etching near the dual boundary side of the photoresist pattern region, which turns out to form a “W”-like etching trench proﬁle before the whole ZnO ﬁlm is completely removed. In the case with NH4Cl etchant, etching rate is 1.5 nm/s, which is much slower than those with acidic etchants, and therefore W-like proﬁle doesn't appear even during a longer etch time (see Fig. 1c). Fig. 2 shows the cross-section morphology of as-grown ZnO ﬁlms 87 Surface & Coatings Technology 331 (2017) 85–89 L.-K. Liu et al. Etching depth (nm) absorbed by ZnO during the annealing, remove extra hanging bonds which are induced by the oxygen vacancy, and construct a more stable state for ZnO [23–25]. In Fig. 5, we can see the etching depth owns a linear relationship with etching time, while the slope value represents the etching rate. Clearly displayed in Fig. 5, etching rate of the as-grown ZnO ﬁlm is faster than that of the annealed ZnO ﬁlm, because the ZnO ﬁlm becomes denser and more stable bind connection appeared in crystal after annealing. As shown in Fig. 6, we also obtained much slower etching rate with NH4Cl etchant than with acidic etchants, while the concentration of NH4Cl is about one order magnitude larger than that of acidic etchants. All of them own a positive correspondence relationship between the etchant concentration and the etching rate. The following equations describe the etching mechanism of NH4Cl: (a) 200 Conc. of 1mol/L Conc. of 4mol/L 150 100 50 Etching of as-grown ZnO 0 20 40 60 80 100 120 140 160 H2 O ↔ H+ + OH− NH 4+ + OH− ↔ NH 4 OH 2H+ + ZnO(s) → Zn2 + + H2 O(l) Etching time (s) (b) 160 Etching depth (nm) NH4Cl will partially ionize in water which makes its etching rate lower than that of acidic ones which is totally ionized. Meanwhile, partial ionization of NH4Cl is also helpful for keeping the pH value stable during the etching procedure. In our experiments, we observed that the ﬂuctuation of NH4Cl solution's pH value was slightly changed while acid solution's pH value had an obvious raise when etching time extended. And the gradient of etching rate with NH4Cl etchant is also smoother than the cases with HCl and H3PO4 etchants. With relative higher applicable concentration and smoother etching rate gradient, it would bring us a wider design window and lower precision requirement to control the etchant concentration. Circle: annealed in O2 Triangle: annealed in N2 Red: Conc. of 1mol/L Blue: Conc. of 4mol/L 120 80 40 4. Conclusions Etching of annealed ZnO 0 0 50 100 150 200 250 The etching proﬁles with NH4Cl and acidic etchants were compared, and NH4Cl etchant has shown better etching controllability for the asgrown ZnO samples. Both W-like shape on horizontal etching surface and eave-like slope at vertical etching side face can be avoided with NH4Cl etchant. After the thermal annealing of ZnO samples, the crystallinity has been improved and the eave-like slope side face has also been suppressed with acidic etchant. NH4Cl etchant shows a better applicable design window for industrial applications, because of the optimized size control and stable etching rate. And the thermal annealing procedure is also recommended to obtain uniform etching proﬁles for ZnO ﬁlms. 300 Etching time (s) Fig. 5. Relationship between etching depth and etching time with NH4Cl etchant for: (a) as-grown ZnO ﬁlms, (b) ZnO ﬁlms annealed in O2 and N2 ambient respectively. 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