고밀도 플라즈마를 이용한 Y-Ba-Cu-O 초전도체 식각 및 소자 제작

임연호1, 한윤봉1, 강형곤2, 한병선2

1. 전북대학교 화학공학부, 2. 전북대학교 전자정보공학부

High-Density Plasma Etching of Y-Ba-Cu-O Superconductor and Device Application

Y. H. Im1, Y. B. Hahn1, H.-G. Kang2, B.-S. Han2

1. School of Chemical Engineering and Technology,

2. Division of Electronics and Information Engineering, Chonbuk Natinal University

INTRODUCTION

Bednorz and Muller reported compound oxides with very high superconducting transition temperature Tc.1 Subsequently, the Tc value was raised to 90 K using the Y-Ba-Cu-O. Potential applications for these high-temperature superconducting thin films include electronic devices such as Josephson junctions, radiation detectors, magnetometers, superconducting flux flow transister (SFFT), wire interconnects for semiconductor circuits and superconducting quantum interference devices (SQUIDs). In order to fabricate these microelectronic devices, microfabrication techniques for patterning thin films into devices must be developed. Many different candidate techniques for superconductor patterning have been studied, among them : wet chemical etching, laser ablation, direct-laser writing, laser-assisted chemical etching, ion beam patterning and reactive ion etching.2-5 Another candidate technology for patterning high-temperature superconductors which have many potential advantages over those mentioned above is a high density plasma etching.

By analogy with semiconductor fabrication, plasma etching of superconductors can be expected to lead to very sharp pattern definition in superconducting thin films. Some limited study of plasma-based etching focused mainly on reactive ion etching (RIE) techniques for patterning high Tc superconducting films has been reported.3 These studies show that reactive etching using Cl2 and Ar ions impinging on a superconducting substrate is feasible, but little attempt has been made to understand the etching process and to characterize the chemical etching and physical sputtering effects.

In this work, the results for inductively coupled plasma etching of Y-Ba-Cu-Oxide thin films are reported. Anisotropic etching of these films using ICP will offer an attractive alternative dry etching technique for patterning ultimate feature sizes that can be obtained.

The ICP etching of Y-Ba-Cu-O films and fabricating of a superconducting flux flow transister(SFFT) has been carried out. For pattern transfer process of the superconductors, a parametric study has been carried out for achieving practical etch rates for the Y-Ba-Cu-O films in a planar-type ICP etcher. The vapor pressures in Cl compounds for Y, Ba, and Cu are higher than those in F compounds for Y, Ba, and Cu.  YCl3, BaCl2 and CuCl2 as volatile compounds have boiling points of 1507, 1560, and 655 □. Therefore, Cl2/Ar mixing gases were selected as an etching gas for Y-Ba-Cu-O. The Langmuir probe was used to study the characteristics of the inductive coupled plasma. The effects of etch gas concentration, ICP source power, rf

* Author for all correspondence. Email : ybhahn@moak.chonbuk.ac.kr

chuck power, and reactor pressure on etch rates have been investigated. Based on the etch characteristics of the Y-Ba-Cu-O films, a SFFT was fabricated.

EXPERIMENTAL

A 0.5 □m thick YBaCuO films were prepared on LaAlO3 substrates using laser ablation method. Etching was performed in a Vacuum Science ICP system (VSICP-1250A), in which the ICP source operated at 13.56 MHz. The temperature of the backside-cooled sample chuck (or the bottom electrode) was held at about 55 □. The ion energy was controlled by the applied rf chuck power at 13.56 MHz. Ion density was measured by the Langmuir probe. For the etch rate experiments, samples (4mm x 4mm) were masked with Apiezon wax. Etch depths were obtained from stylus profilometry measurements of the etched samples after removal of the wax. The error of these measurements was approximately □5%. The samples were patterned with 7 □m thick photoresist (AZ4620) or 0.5 □m SiO2 dielectric masks for scanning electron microscopy (SEM) studies of films etched. The electrical characterization of the patterned films was characterized by measurements of the transition temperature Tc. Variations in films stoichiometry were measured using an energy dispersive spectroscopy (EDS) system to determine the film composition both before and after etching. Also, in order to compare the structural properties of both before and after etching, x-ray diffraction (XRD) measurement was used.

RESULTS AND DISCUSSION

Figure 1 shows the effect of ICP source power on the etch rates and dc bias. During these experiments the reactor pressure, rf chuck power and gas feed rate were kept constant at 5 mTorr, 150 W and 10 sccm Cl2 / 10 sccm Ar, respectively. The etch rates increased up to 700W and remained relatively constant beyond 700W. The dc bias voltage ranged from 70V to 170V when the ICP source power was decreased from 800 to 400W. The plasma density measured by the Langmuir probe show a good agreement with the trend of dc bias in Fig. 1 (i.e. a reverse relationship between the plasma density and the dc bias). The decrease in dc bias with the ICP source power was mainly due to increased ion density at higher ICP source powers.

Figure 2 shows the effect of rf chuck power on the etch rate and dc bias. The etch rate and dc bias voltage increased as ion bombarding energy increased, typical results in the ICP etching systems. The increase in etch rate with the chuck power might be explained by enhanced sputter desorption of etch products as the ion energy increases.

The transient temperature of the etched sample was compared to that of control sample in Fig. 3. The Y-Ba-Cu-O films showed almost the same Tc of 87 K before and after etching, indicating that the superconducting property of the film was not deteriorated during the ICP etch process. Hence, it is concluded that the ICP etching is a promising technique for patterning high Tc superconducting films into microelectronic devices.

Figure 4 shows the effect of Cl2 concentration in the etch gas on the surface composition of the Y-Ba-Cu-O films etched at 5 mTorr, 150 W rf and 700 W ICP power. Atomic percentages were measured by analyzing the EDS results as a function of Cl2 concentration. At higher Cl2 concentrations (> 50% Cl2), all elements showed relatively constant atomic ratios, but they showed substantial difference at lower Cl2 concentration, especially at 25% Cl2. Cl showed a decrease in atomic percentage up to 75% Cl2 and remained constant beyond 75%. This is believed because of the lower neutral or ionic chlorine densities at higher Cl2 concentrations. By contrast, Cu showed the lowest atomic rate at 25 % Cl2, increased and remained relatively constant beyond 50 % Cl2. Y and Ba showed a less dependency on Cl2 concentration. This is mainly due to the difference in boiling points of their etch products (i.e, the boiling points of YCl3, BaCl2 and CuCl2 are 1507, 1560 and 655 □, respectively). These results also indicate that slower etch rates of the Y-Ba-Cu-O films are obtained at higher Cl2 concentrations (>50% Cl2), mainly because of ion-assisted chemical etching mechanism. Based on the etch characteristics of the Y-Ba-Cu-O films, a superconducting flux flow transister (SFFT) was fabricated. Figure 5 shows the processing sequence for the SFFT fabrication using lithography (b) and Cl2/Ar ICP etching(a,c). The link patterned by width 3 □m has on etch depth of ~500□ as shown in Fig. 5 (b). The characteristics of SFFT  were investigated by examining the current-voltage (I-V) characteristics of YBCO channel under various applied voltages.

SUMMARY AND CONCLUSIONS

Y-Ba-Cu-O films were etched for achieving practical etch rates in a planar type inductively coupled plasma etcher. A parametric study has been carried out in terms of etch characteristics, transition temperature, and morphology of the etched Y-Ba-Cu-O films. It was found that the ion density and the ion energy were dominant factors of etching mechanism of the Y-Ba-Cu-O films. Etch rates of 2500-3500 □/min. were obtained at moderate ICP conditions. The superconducting property of YBCO in terms of the transition temperature was not deteriorated during the ICP etch process and showed Tc of 87 K. SEM micrographs showed a well-defined anisotropic profile. XRD pattern analysis showed that crystalline structure of the etched Y-Ba-Cu-O film is the same as that of unetched sample. Finally, SFFT was fabricated by Lithography and ICP etching process and showed excellent I-V characterics. It is concluded that the ICP etching is overall a promising technique for patterning the high-Tc superconducting films into devices such as SFFTs and SQUIDs.

REFERENCE

1) J. G. Bednorz, and K. A. Muller, J. Phys., B64, 189 (1996).

2) S. Wunderlich, F. Schmidl, L. Dorrer, H. Schneidewind, and P. Seidel, IEEE Trans. Appl. Superconduct., 9, 71 (1999).

3) S. Matsui, N. Takado, H. Tsuge, and K. Asakawa, Appl. Phys. Lett., 52, 69 (1988).

4) A. J. M. and der Harg, E. van der Drift, and P. Hadley, IEEE Trans. Appl. Superconduct., 5, 1448 (1995).

5) M. R. Poor and C. B.Fledderman, J. Appl. Phys., 70, 7640 (1991).

Fig.1 Effect of ICP power on the

etch rate of Y-Ba-Cu-O films

Fig.2 Effect of rf power on the

etch rate of Y-Ba-Cu-O films

Fig.3 Resistance vs temperature of

Y-Ba-Cu-O films.

Fig. 4 Variations of film stoichiometry

as a function of Cl2

concentration in etching process.

(a)                    (b)                      (c)

Fig. 5 Processing sequence for SFFT devices using lithography

and Cl2/Ar ICP etching.