The deposition and wet etching of Mg-doped ZnO films and their applications for solidly mounted resonators

In this report, a solidly mounted resonator (SMR), consisting of an Au electrode, Mg-doped ZnO (MgXZn1−XO) piezoelectric film and Bragg acoustic reflector, was fabricated on a Si substrate by radio frequency (RF) magnetron sputtering. As a key processing step for the SMR, MgXZn1−XO films with high c-axis orientation were fabricated and the crystalline structure, surface morphology and roughness of the films were investigated. The surface morphology, optical transmittance and shape control of MgXZn1−XO films were investigated by the chemical wet-etching method with various etchants. The profiles and line patterns of MgXZn1−XO films etched with FeCl3·6H2O solutions are satisfactory and fully meet the industrial requirements. The Bragg acoustic reflector, made entirely of metal, has small internal stress and good heat conduction. An SMR based on a MgXZn1−XO piezoelectric film shows a resonant frequency of 2.402 GHz, and the keff2, QS and QP of the SMR are 3.07%, 415 and 546, respectively.


Introduction
With the advancement in micro/nano fabrication, lm bulk acoustic resonators (FBARs) have been proposed as typical micro-electromechanical system (MEMS) piezoelectrical devices. 1-4 As bulk acoustic wave (BAW) devices operating in the GHz range, FBARs have attracted much attention due to their small size, high operating frequency and potential applications in high-frequency communication and mass-sensitive sensor areas. [5][6][7][8] As a kind of FBAR, solidly mounted resonators (SMRs) are composed of a piezoelectric layer sandwiched between electrodes and Bragg reector consisting of alternating high and low acoustic impedance quarter-wavelength thick dielectric or metallic layers. 9,10 The SMR, with good mechanical strength and excellent acoustic properties, and being closer to CMOS integration, was therefore chosen in this work. 11,12 In recent decades, owing to excellent piezoelectric property, better quality factor, and high electromechanical coupling coefficient, ZnO is becoming a very promising candidate for FBAR devices as a piezoelectric material. [13][14][15][16] However, ZnO have the drawback of low longitudinal acoustic wave velocity and low resistance, which limits its application to high sensitivity acoustic sensors. 17 Mg X Zn 1ÀX O, a ternary compound formed by alloying ZnO and MgO, has attracted more and more attention due to its special properties such as higher acoustic velocity and resistance than that of ZnO. [18][19][20] Studies have shown Mg 2+ is doped into the ZnO crystal by substituting the Zn 2+ position and Mg X Zn 1ÀX O still maintains the wurtzite crystal structure when the percentage of Mg atoms is less than 33%. 21 By controlling the percentage of Mg-doped in the material, the Mg X Zn 1ÀX O lms with satisfactory sound velocity and electromechanical coupling coefficient can be tailored. 22 Mg X Zn 1ÀX O lms have been fabricated by a variety of techniques such as magnetron sputtering, 23,24 atomic layer deposition, 25,26 spray pyrolysis, 27,28 pulsed laser deposition (PLD), 29,30 and sol-gel coating. 31,32 Among these deposition techniques above, magnetron sputtering might be a most practical method because of its high deposition rate, good adhesion and good uniformity, which is suitable for commercialization. 33,34 In generally, ZnO thin lms can be easily and fast etched by various common etchants such as HCl, HNO 3 , H 2 SO 4 , H 3 PO 4 and so on. 35,36 However, anisotropic etching proles could be generally obtained and the surface shapes are too difficult to control in the acid solutions through a number of experiments, which has a negative impact on etching patterning. Recently, ferric chloride (FeCl 3 $6H 2 O) as a kind of representative etchants, has been carried out to solve this problem. 37,38 In this paper, we fabricated Mg X Zn 1ÀX O lms with high c-axis orientation by RF magnetron sputtering and characterized structure and surface morphology of Mg X Zn 1ÀX O lms. In addition, we explored the effects of FeCl 3 $6H 2 O as a novel etchant on surface morphology, optical transmittance and shape control of Mg X Zn 1ÀX O lms. The novel Bragg acoustic reector, made entirely of metal, has small internal stress and 2. Experiment The Mg X Zn 1ÀX O thin lms were deposited on (100) oriented silicon (Si) substrates in the RF magnetron sputtering system with a base pressure of 5 Â 10 À5 Pa. Mg X Zn 1ÀX O (X ¼ 10%) was used as a target source material and a mixture of argon (Ar) (99.999%) and oxygen (O 2 ) (99.999%) was used as sputtering gas. The distance target-to-substrate is about 70 mm and the target diameter is 80 mm.

Mg X Zn 1ÀX O lms wet etching
The sputtered Mg X Zn 1ÀX O lms were standard photolithography processed by a designed pattern mask. About 1 mm thickness positive photoresist (AZ 4620) was coated on the surface of the Mg X Zn 1ÀX O lms by three-step spin coating. Aer prebaked at 100 C for 1 min, a mask aligner was used to transfer the pattern design on the mask to the photoresist. Aer that, the Mg X Zn 1ÀX O lms with patterned photoresist was dipped in the developer solution (AZ 400K) to remove the exposed photoresist, washed and post-baked at 100 C for 5 min.
To acquire patterned Mg X Zn 1ÀX O lms, the wet etching was carried out in dilute HCl, C 2 H 4 O 2 and FeCl 3 $6H 2 O aqueous solution prepared at the concentration values of 0.001 mol l À1 , 0.01 mol l À1 , and 0.1 mol l À1 , respectively. All wet etchings were carried out in an ambient temperature kept at 25 C and etching time was precisely controlled. Aer the wet etching process, Mg X Zn 1ÀX O lms were immediately washed by deionized water and dried by nitrogen.

SMR fabrication
The procedures of the SMR fabrication were be divided into three steps: Bragg reector deposition, Mg X Zn 1ÀX O lms fabrication, and top electrode deposition. The Bragg reector, consisting of Ti and W layers, was rst deposited on p-type 3 inch Si (100) substrate with 1-10 U cm resistivity at 25 C, and then Mg X Zn 1ÀX O lms were deposited on the top of the Bragg reector at 300 C. Lastly, a Au lm was deposited on top of Mg X Zn 1ÀX O lms for top electrode. Similar to Mg X Zn 1ÀX O lms, Ti, W, and Au lm were also obtained by the RF magnetron sputtering system using Ti, W, and Au targets in a pure Ar atmosphere, respectively. The distance target-to-substrate is about 70 mm and the diameters of all targets are 80 mm. Finally, a thermal annealing process at 300 C was also performed to relieve the stress in multilayer lms to improve the performance of the SMR.

Characterizations
The crystalline structure of Mg X Zn 1ÀX O lms was investigated by X-ray diffraction (XRD, Bruker Advanced D8) using a Cu-Ka radiation (l ¼ 1.54187 A) in a q-2q scanning mode. The surface morphology and cross-sectional morphology of Mg X Zn 1ÀX O lms were observed by a eld emission-scanning electron microscope (FE-SEM, Carl Zeiss Ultra55). The surface roughness (RMS) of Mg X Zn 1ÀX O lms was investigated by atomic force microscopy (AFM, Nanorst 3000). The optical transmission of Mg X Zn 1ÀX O lms was measured by UV spectrophotometer (PerkinElmer Lambda 750). The cross-sectional morphology of SMR was observed using a FE-SEM. Finally, the frequency response of SMR was measured by S-scattering parameters with a probe station (Cascade EPS 150 RF) and a network analyzer (HP 8712E). All measurements of SMR were carried out in an ambient temperature kept at 25 C.

Results and discussions
The sputtering parameters have an important effect on the crystalline structure and crystalline grain size of Mg X Zn 1ÀX O lms, which determine the quality of lms. Aer many experiments and measurements, the optimal sputtering parameters of Mg X Zn 1ÀX O lms with high c-axis orientation and all metal materials in experiments were summarized in Table 1. Fig. 1 shows the XRD patterns of Mg X Zn 1ÀX O lms grown on silicon substrates. For the Mg X Zn 1ÀX O lms, a very strong diffraction peak was observed at 34.4 with a full width at half-maximum (FWHM) of 0.30 , which corresponds to the diffraction from the Mg X Zn 1ÀX O (002) plane. This indicates that the preferential Mg X Zn 1ÀX O growth orientation is along the wurtzite c-axis and perpendicular to the surface of substrate. Fig. 2 shows the SEM surface and cross-sectional images of Mg X Zn 1ÀX O lms deposited on silicon substrate. The Mg X Zn 1ÀX O lms demonstrate clearly a hummock-like surface morphology Paper with smooth, homogeneous, uncracked, compact and dense. The surface morphology consists of a large number of hexagonal crystalline grains with no visible pores and defects over the lms in Fig. 2(a). The Mg X Zn 1ÀX O lms perpendicular to the surface of silicon substrate exhibit highly oriented and compact columnar structure. Besides, the grain boundaries among columns in lms and the good cohesion between the Mg X Zn 1ÀX O lms and silicon substrate are observed in Fig. 2(b), obviously. Fig. 3 shows a three-dimensional AFM image of the Mg X Zn 1ÀX O lms deposited on silicon substrate. The surface roughness of the Mg X Zn 1ÀX O lms is 3.37 nm. The rounded and homogeneous grain shape can be observed from the picture, which reveals the fact that Mg X Zn 1ÀX O lms with a homogeneous smooth surface over the whole wafer. Low surface roughness is connected with low acoustic loss in Mg X Zn 1ÀX O lms, which is suitable for SMR.

Mg X Zn 1ÀX O lms etching characteristics
The functions of etching rates and concentrations of different etchant aqueous solutions were shown in Fig. 4     electrolytes, so only a part of C 2 H 4 O 2 can be ionized to generate less H + ions compared to HCl at the same mole concentration. As a result, the concentration of H + ions is the lowest among the three solutions and the etching rate is lowest.
The SEM micrographs of surface texture Mg X Zn 1ÀX O lms etched in different etchants were illustrated in Fig. 5. The concentration of all etchants was controlled to be 0.01 mol l À1 and the etching time was controlled in 1 min. It is observed clearly that the etched samples show distinctive rough surface morphologies due to different etching rates of different etchants. The sample as-grown Mg X Zn 1ÀX O lms without etched consists of close-packed hummock-like crystals surface morphology with smooth, homogeneous and uncracked. By compared, hummocklike crystals are becoming gradually disappeared and the surfaces are becoming gradually rough and cracked with the increase of etching rate in different etchant solutions. It can be proved that surface texture of Mg X Zn 1ÀX O lms is the most severely damaged in HCl solution, the least damaged in C 2 H 4 O 2 solution, and the moderate in FeCl 3 $6H 2 O solution. Fig. 6 shows the optical transmittance of Mg X Zn 1ÀX O lms etched in different etchants. The thicknesses of Mg X Zn 1ÀX O lms deposited on glass were 1 mm and the etching time was controlled in 20 s. It was observed clearly that the optical transmittance decrease with the etching rates increase. The etchants destroy the surface morphologies of Mg X Zn 1ÀX O lms and lead to the changes of optical transmittance. The etching rates are faster, the morphologies of destruction are more serious, and the optical transmittances are lower. The results above show that the optical properties can be controlled by varying either etchants or etchants concentration. Fig. 7 shows etched proles of Mg X Zn 1ÀX O lms by different etchants at the same time. The concentration of etchants was controlled to be 0.01 mol l À1 and the etching time was controlled in 3 min. A rough and high in the middle and low on both sides etching prole is observed at the bottom of the step from Fig. 8(a), which results from the faster etching rate near the mask edge than that in the center in HCl solutions. Such kind of etching prole has an adverse effect on the shape and size of the devices and deteriorate devices performance. In contrast, a very smooth etching prole is observed clearly from Fig. 8 Fig. 8(a). But the right angles of the line patterns were destroyed seriously, which indicates that the etching rates along the different directions are not homogeneous on the sample surface. As shown in Fig. 8(b), the problem was settled satisfactorily by FeCl 3 $6H 2 O solutions. The right angles of the line patterns etched in FeCl 3 $6H 2 O solutions are pointed and integrated, which indicates the etching rates along the different directions are homogeneous on the sample surface and fully meets the requirement of shape and size of the devices. Fig. 9(a) shows a 3D-view schematic illustration of SMR and Fig. 9(b) shows the cross-section view morphologies of   This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 9672-9677 | 9675 integrated SMR with Au/Mg X Zn 1ÀX O/Ti/W multilayer lms structure. The Mg X Zn 1ÀX O lms perpendicular to Bragg reector exhibit highly oriented and compact columnar structure. The interfaces between the Mg X Zn 1ÀX O lms and Bragg reector are clearly visible and distinct, verifying that the different membrane layers are not diffusive with each other. As the Bragg reector was made entirely of metal, it had small internal stress and good heat conduction. Note that top W layer of the Bragg reector on the top of Si also served as the electrode for frequency measurements.

SMR characterization
The reection coefficient S (1,1), impedance and phase response of SMR were measured with probe station and network analyzer and displayed in Fig. 10. The frequency response of SMR based on Mg X Zn 1ÀX O lms we fabricated is approximately around 2.402 GHz with a return loss of À24.57 dB, and the series resonant frequency (f s ) and parallel resonant frequency (f p ) appeared at 2.383 GHz and 2.414 GHz, respectively. Both the corresponding coupling coefficient k eff 2 and quality factor Q values of SMR can be derived easily based upon the formula as follows: 40 where Q S and Q P are parallel and series quality factors and Z is the input electrical impedance. According to calculation above, the k eff 2 , Q S and Q P of SMR we fabricated are 3.07%, 415, and 546, respectively.

Conclusions
In this paper, we fabricated Mg X Zn 1ÀX O lms with high c-axis orientation by RF magnetron sputtering and investigated the crystalline structure, surface morphology and roughness of lms.

Conflicts of interest
There are no conicts to declare.