Polishing of YBa2Cu3O7−y by Heion etchingNoriyuki Inoue, Yasuo Takahashi, Takashi Sudo, Kageyoshi Sakamoto, Toshio Shima, and YoshitakeNishi Citation: Journal of Applied Physics 71, 347 (1992); doi: 10.1063/1.350713 View online: http://dx.doi.org/10.1063/1.350713 View Table of Contents: http://scitation.aip.org/content/aip/journal/jap/71/1?ver=pdfcov Published by the AIP Publishing Articles you may be interested in YBa2Cu3O7−δ Josephson junctions on directionally ion beam etched MgO substrates Appl. Phys. Lett. 63, 2141 (1993); 10.1063/1.110566 Cation termination at ionpolished and chemically etched (001)YBa2Cu3O7 crystal surfaces: An ionchanneling study Appl. Phys. Lett. 58, 777 (1991); 10.1063/1.104515 Growth of YBa2Cu3O7−δ on vicinally polished MgO substrates Appl. Phys. Lett. 57, 2501 (1990); 10.1063/1.103840 Ion beam thinning and polishing of YBa2Cu3O7 films Appl. Phys. Lett. 55, 1915 (1989); 10.1063/1.102331 A nonaqueous chemical etch for YBa2Cu3O7−x AIP Conf. Proc. 182, 376 (1989); 10.1063/1.37930

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Polishing of YBa2Cu307-, by He-ion etching Noriyuki Inoue, Yasuo Takahashi, Takashi Sudo, Kageyoshi Sakamoto, Toshio Shima, and Yoshitake Nishi Department of Materials Science, Tokai University, III 7 Kitakaname, Hiratsuka, Kanagawa 259-12, Japan

(Received 26 June 1991; accepted for publication 24 September 1991)

Clean polishing is performed by helium-ion etching on a surface of high-T, YBa$usO, --y Based on a knock-on cascade model, the decrease of surface roughness is discussed. A rate process of etching is applied for the surface roughness of the high-T, YBa,Cu,O, -,,

I. INTRODUCTION

Ion etching is able to form fragile materials without fracture. l-3 From an engineering point of view, it is impor- tant to know an ion etching rate (R,) and a surface rough- ness (R,) of high-lir, YBa,C!usO, -,,. A rapid etching rate was found when an argon ion was used.4 The ion-beam- etching process has been studied for high-T, superconductors.5*6 However, argon-ion etching alters the roughness of the surface.7 The surface roughness is one of the important factors that control mechanical and electri- cal properties. However, the polishing processes are not, in general, performed under clean atmosphere. Thus, we have started the present study to observe the effect of helium-ion etching under a clean atmosphere on the surface roughness (R,) for the high-T, YBa,Cu30, -,,

II. EXPERIMENTAL PROCEDURE

Samples with a nominal composition YBa,Cu,O, --y were prepared from high-purity powders of CuO, BaC03, and Y203.’ The powders were mixed and reacted at 1210 K for 2 h and then air cooled under pure oxygen. After crushing, a pelletized tablet, 1 mm thick and 13 mm in diameter, was sintered under pure oxygen at 1210 K for 8 h and furnace cooled. The cooling rate was 4.2 K min - ’ at 973 K. The orthorhombic structure of the sample was shown by means of x-ray diffraction. The etching depth and the surface roughness were measured with a surface roughness tester. The resolution was below 10 nm.

Ion etching was carried out with an ion-etching appa- ratus with a Kaufman-type ion source (ISM-S, Elionix, Tokyo). 9-11 The ion-beam energy and ion current density were 1.0 keV and 0.6 mA/cm”, respectively. The ion beam was perpendicular to the sample surface. The ion etching was performed under a helium atmosphere with a pressure of 4x 10 - 4 Torr just above the sample. The sample was placed on a water-cooled sample holder. The rate of He-ion etching was about 1.0 rim/s..

HI. RESULTS

Figure 1 shows changes in the surface roughness (R,) versus the etching time (ii). This figure shows that the helium-ion etching decreases the surface roughness. Re- markably, the polishing is achieved under clean atmo- sphere. The average, maximum, and minimum values of the surface roughness (R$ REaX, and R$“) decrease with

the etching time, respectively. The change in Rr is larger than that in Rmin m . The polishing rate of a rough surface is higher than that of a smooth surface. The maximum values (RF) of the surface roughness also decrease.

IV. DISCUSSIONS

A. Model of surface roughness change mechanism

To explain the polishing mechanism, the difference in etching rates between convex and concave parts is dis- cussed on the basis of two reasons. Based on the ion sput- tering mechanism of Sigmund (see Ref. 12)) the sputtering takes place when a knock-on cascade arrives easily to the surface. The first reason is a difference of length of the knock-on cascade. The length at the convex part is shorter than that at the concave part. Thus, the knock-on cascade arrives easily at the surface at the convex part, whereas it is difficult at the concave part (see Fig. 2). Another reason is caused by a difference in the bonding energy. The bond- ing energy (Eb) of the surface atom is one of the dominant factors. Since the coordination number of the convex part is smaller than that of the concave part, & of the convex part is smaller than that of the concave part (see Fig. 3). Therefore, the etching rate of the convex part becomes

0 1 2 3 5 ti ( 103s4

FIG. 1. Changes in surface roughness (R,) against etching time (ti). Solid line (0 ), broken line (A), and dotted line (V) are for average value R$ maximum value RF, and minimum value Rp of He-ion etching, respectively. Solid line ( 0 ) is for the average value of Ar-ion etching.

347 J. Appl. Phys. 71 (l), 1 January 1992 0021-8979/92/l 30347-03$04.00 @I 1992 American Institute of Physics 347 [This article is copyrighted as indicated in the article. Reuse of AIP content is subject to the terms at: http://scitation.aip.org/termsconditions. Downloaded to ] IP:

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E 6- cr concave part 4

FIG. 2. Knock-on cascade models of convex and concave parts. 2

OL 2 3 4

ti (103s)

larger than that of the concave part. Thus, the surface roughness decreases.

B. Effect of helium-ion etching

Argon is, in general, used for the ion etching. It does not remarkably decrease the surface roughness (see Fig. 1). The argon ion etches not only the concave part but also the convex part, because the length of the knock-on cas- cade is too long. If the length of the knock-on cascade becomes small, the etching rate of the convex part de- creases. Since an energy transfer efficiency of the helium ion is smaller than that of the argon ion,12 the helium etching decreases the length of the knock-on cascade. Thus, polishing by the effective knock-on cascade occurs when using helium ions.

FTG. 4. Rate processes applied for surface roughness (R,). Solid line (0 ), broken line (A), and dotted line (V) are for average value R$ maximum value Rz”, and minimum value Rm,‘” of He-ion etching, re- spectively.

From these results, X is expressed by the following equation (see Fig. 6) :

loglo[ - ln( 1 -X)] =n log,, tf + loglo k. (3)

The values of n and log,, k are 1.2 and - 4.14, respec- tively. This linear plot of Eq. (3) confirms the assumption of the rate process [see Eq. ( 1) J.

If n is assumed to be 1.2 for RF and R$, the broken (RE*) and dotted (RE’“) curves are shown in Figs. 1, 2, and 4. mR, values of Es. (2) are 2.7 for Ry(F = 0.9990) and 2.1 for Rp(F = 0.9092).

These linear plots in Fig. 6 also confirm that the rate process does not depend on the surface roughness. The difference between Rz” and REi" is that of kinetic constant between the convex and concave parts. Namely, these re- sults agree with our discussions based on the knock-on cascade model.

C. Rate process

If the energy is involved in the etching in a collision of the knock-on cascade, a rate process can be applied. Based on the rate process,‘3-15 the R, change X is assumed to be expressed by the following equation in relation to the etch- ing time (ti):

X=1 -exp( -kc). (1)

Here, k and n are constant. X is assumed to express

X= [ (R, - oR,>/(mR, - ‘R,)], (2) where mR, and OR, are the R, values of the high-T, oxide (YBa2Cu307 -J etched for infinite time and before the etching, respectively. R, of the etched sample approaches mRm in Fig. 4. “R, of Eq. (2) is 2.42 pm for Rk when the correlation coefficient I; of EQ. ( 1) is maximum (F = 0.9999) as shown in Fig. 5.

IA

concave part convex part FIG. 5. Changes in correlation coefficient (F) with mR,. Here, mR, is the surface roughness (R,) value of the high-T, oxide (YBa,Cu,O,...J etched to infinity. The solid line is for the average value of the He ion. FIG. 3. Bonding energy model of convex and concave parts.

348 J. Appl. Phys., Vol. 71, No. 1, 1 January 1992 inoue et al. 348

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FIG. 61 Linear plots between log,, k and etching time (ri). Solid line (0), broken line (A), and dotted line (V) are for average value R$ maximum value Rm,P and minimum value R”,‘” of He-ion etching, re- spectively.

V. CONCLUSION

The helium-ion etching polishes the surface of YBa,Cn,O, -,, under clean atmosphere. Using a knock-on cascade model, the polishing model is discussed. Namely,

the surface roughness change is caused by the difference of etching rate between the convex and concave parts on sur- face.

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