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Growth of crystalline LiF on CF4 plasma etched LiNbO3 substrates
Journal of Crystal Growth 187 (1998) 573—576
Letter to the Editors
Growth of crystalline LiF on CF plasma etched 4 LiNbO substrates 3 H. Nagata!,*, N. Mitsugi!, K. Shima", M. Tamai", E.M. Haga" ! Optoelectronics Research Division, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274-8601, Japan " Advanced Materials Research Division, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274-8601, Japan Received 20 December 1997
Abstract The LiNbO (LN) substrates were etched by electron cyclotron resonance (ECR) plasma of CF gas and analyzed by 3 4 a surface sensitive method. After the etching process, crystalline LiF particles were found on the surface. These LiF particles were removed from the surface by an HNO rinse or a heat treatment in an O atmosphere and a LiNb O 3 2 3 8 phase appeared. The surface morphology was rough in spite of the removal of the LiF layer. Such chemical deterioration of the LN surface due to the CF ECR plasma etching treatment is not desirable for LN optoelectronic devices. ( 1998 4 Elsevier Science B.V. All rights reserved.
A reactive dry etching technique has been applied to the fabrication of LiNbO (LN)-based op3 toelectronic devices especially for broadband use. For instance, an electron cyclotron resonance (ECR) plasma of fluorocarbons is considered to be suitable for etching the LN substrate because of its high chemical reactivity [1]. Noguchi et al. reported high-speed LN optical modulators modified by the ECR etching to have 3—4 lm deep trenches on both sides of the optical waveguide [2]. Similar investigations have been reported and mainly de-
* Corresponding author.
scribe the performance of the devices [3,4]. However, to our knowledge, there was almost no report focusing on the etching process of the LN with fluorocarbons. Previously, we reported the formation of a LiF layer on the LN surface etched by CHF ECR plasma [5]. Now, in this letter, the 3 structure of the LiF layer was found to change depending on the etching gas from amorphous for CHF to crystalline for CF . Further, the occurrence 3 4 of a significant roughening and a possible deficiency in Li were revealed for the etched surface. Fig. 1a—Fig. 1c show results of an atomic force microscopic (AFM) observation, a grazing incidence X-ray diffractometry (XRD), and a transmission electron microscopic (TEM) observation,
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 0 0 9 - 8
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H. Nagata et al. / Journal of Crystal Growth 187 (1998) 573—576
respectively, for the z-cut LN single crystal surfaces after the ECR etching treatments with CHF . The 3 etching was carried out using an ANELVA L-310R ECR plasma etcher under the following conditions; about 400 W microwave power, 80 V magnet voltage, 500 V ion acceleration voltage, 20 cm gap between the sample and the acceleration electrode, about 0.01 Pa of CHF (3 sccm), 500—600 nm/h 3 etching rate, and 1—2 h etching duration. As is seen in Fig. 1a, the etched surfaces were covered with fine particles, and the peak-to-peak surface roughness deteriorated to be 43 nm (2 lm]2 lm area) from 2.2 nm before the etching. These particles were thought to be amorphous judging from the absence of a specific diffraction peak from the etched surface in Fig. 1b and Fig. 1c. Further, the presence of LiF on the same etched surface was found previously by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) [5], suggesting the growth of amorphous LiF particles by the CHF ECR plasma etching. 3 On the other hand, for the CF ECR plasma 4 etched z-cut LN surface, the existence of crystalline LiF was confirmed as shown in the grazing incidence XRD results of Fig. 2a. The etching conditions had been set at almost the same as those for the CHF , while a faster etching rate was obtained 3 (about 800 nm/h). Fig. 3a is the AFM image of this surface, showing a larger peak-to-peak surface roughness of 210 nm for the 2 lm]2 lm area. As the reason for such structural difference of the etched LN surface, the difference in the plasma species, e.g. existence of HF species in the CHF 3 plasma which might react with LiF particles, was considered. Further, there was a possibility that a temperature increase on the substrate surface caused by the incidence of ionic species (and electrons) was larger for the CF plasma leading to 4 grain growth and crystallization of the LiF layer. In the fabrication of high-speed LN devices having trenches on both sides of the waveguides, a metal pattern mask is commonly used because a resin mask could not endure the hours long etching process, for a 3—4 lm deep etching. We used a Ni pattern mask and after the ECR etching, the remaining mask was removed by dilute HNO 3 (0.5—1 N). Further, this HNO treatment was found 3 to also remove the LiF layer formed on the etched
Fig. 1. AFM image (a), grazing incidence XRD profiles (b), and TEM image (c) of the LN surfaces after ECR plasma etching with CHF . The grazing incidence XRD was performed using 3 a Cu K beam with incidence beam angles of 1.5°, 1.0°, 0.5°, 0.2°, a and 0.1° with respect to the sample surface.
H. Nagata et al. / Journal of Crystal Growth 187 (1998) 573—576
575
Fig. 2. Grazing incidence XRD profiles for the CF ECR 4 plasma etched z-cut LN surface (a), for the surface after a succeeding HNO treatment (b), and for the surface after a succeed3 ing O annealing at 600°C (c). The XRD measurements were 2 performed using a Cu K beam with incidence beam angles of a 2.0°, 1.5°, and 1.0° with respect to the sample surface.
LN surface. For instance, a 600 nm deep trench etched by the CF ECR plasma was deepened addi4 tionally by 80 nm after the 0.5 N HNO etching for 3 30 min at room temperature. The AES analyses proved that the fluorine existing in the CF etched 4 surface disappeared completely by the HNO treat3 ment. Similar results were obtained also by the O annealing of the CF etched samples at 600°C 2 4 for 1 h. Fig. 2b and Fig. 2c show the grazing incidence XRD results for the HNO etched and the O an3 2 nealed samples, respectively. The crystalline LiF disappeared and the other crystalline phase
Fig. 3. AFM images for the CF ECR plasma etched z-cut LN 4 surface (a), for the surface after succeeding HNO treatment (b), 3 and for the surface after succeeding O annealing at 600°C (c). 2
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H. Nagata et al. / Journal of Crystal Growth 187 (1998) 573—576
appeared. Some of the XRD peaks were assigned to the LiNb O phase, and others could not be as3 8 signed within LN relating materials such as Nb O , etc. Although the LiNbO did not decom2 5 3 pose to LiNb O at 600°C [6,7], the XRD peaks 3 8 from LiNb O were observed for the O annealed 3 8 2 sample. The results suggested that the formation of the LiF layer was due to the CF ECR plasma 4 etching causing a partial deficiency in Li on the LN surface. Fig. 3b and Fig. 3c are the AFM images corresponding to the samples in Fig. 2b and Fig. 2c, respectively. Both images showed similar surface morphology and peak-to-peak roughness of 500—600 nm for the 2 lm]2 lm area. Although the AES analyses for these surfaces revealed the O/Nb atomic ratio to be about 3 and the probable existence of a LiNbO phase at the surface, in addition 3 to the LiNb O , the origin for the observed surface 3 8 texture has not been found yet. At least, the morphology of both the thermally etched and the HF/HNO etched z-cut LN surfaces were signifi3 cantly different from Fig. 3b and Fig. 3c [6,7]. The LN surface etched by the CF ECR plasma 4 was mainly investigated. Due to the etching, the crystalline LiF layer grew on the surface accom-
panying a Li deficit and a surface roughening. These facts should be taken into account in the fabrication process of LN-based devices to prevent the deterioration of the device performance. This study was supported by Special Coordination Funds for Promoting Science and Technology, “Fundamental research on new materials of function-harmonized oxides,” from the Japanese Science and Technology Agency, to whom we are deeply indebted. References [1] J.L. Jackel, R.E. Howard, E.L. Hu, S.P. Lyman, Appl. Phys. Lett. 38 (1981) 907. [2] K. Noguchi, O. Miyomi, H. Nakazawa, S. Seki, J. Lightwave Technol. 13 (1995) 1164. [3] K. Miura, M. Minakata, S. Kawakami, Technical Digest of Conf. Lasers Electro-Opt. (1987) 288. [4] K. Noguchi, O. Mitomi, H. Miyazawa, Technical Digest of Optical Fiber Commun. (1996) 205. [5] K. Shima, N. Mitsugi, H. Nagata, J. Mater. Res. (1998), to be published. [6] H. Nagata, T. Sakamoto, H. Honda, J. Ichikawa, E.M. Haga, K. Shima, H. Haga, J. Mater. Res. 11 (1996) 2085. [7] H. Nagata, K. Shima, J. Ichikawa, J. Am. Ceram. Soc. 80 (1997) 1203.
Letter to the Editors
Growth of crystalline LiF on CF plasma etched 4 LiNbO substrates 3 H. Nagata!,*, N. Mitsugi!, K. Shima", M. Tamai", E.M. Haga" ! Optoelectronics Research Division, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274-8601, Japan " Advanced Materials Research Division, Sumitomo Osaka Cement Co., Ltd., 585 Toyotomi-cho, Funabashi-shi, Chiba 274-8601, Japan Received 20 December 1997
Abstract The LiNbO (LN) substrates were etched by electron cyclotron resonance (ECR) plasma of CF gas and analyzed by 3 4 a surface sensitive method. After the etching process, crystalline LiF particles were found on the surface. These LiF particles were removed from the surface by an HNO rinse or a heat treatment in an O atmosphere and a LiNb O 3 2 3 8 phase appeared. The surface morphology was rough in spite of the removal of the LiF layer. Such chemical deterioration of the LN surface due to the CF ECR plasma etching treatment is not desirable for LN optoelectronic devices. ( 1998 4 Elsevier Science B.V. All rights reserved.
A reactive dry etching technique has been applied to the fabrication of LiNbO (LN)-based op3 toelectronic devices especially for broadband use. For instance, an electron cyclotron resonance (ECR) plasma of fluorocarbons is considered to be suitable for etching the LN substrate because of its high chemical reactivity [1]. Noguchi et al. reported high-speed LN optical modulators modified by the ECR etching to have 3—4 lm deep trenches on both sides of the optical waveguide [2]. Similar investigations have been reported and mainly de-
* Corresponding author.
scribe the performance of the devices [3,4]. However, to our knowledge, there was almost no report focusing on the etching process of the LN with fluorocarbons. Previously, we reported the formation of a LiF layer on the LN surface etched by CHF ECR plasma [5]. Now, in this letter, the 3 structure of the LiF layer was found to change depending on the etching gas from amorphous for CHF to crystalline for CF . Further, the occurrence 3 4 of a significant roughening and a possible deficiency in Li were revealed for the etched surface. Fig. 1a—Fig. 1c show results of an atomic force microscopic (AFM) observation, a grazing incidence X-ray diffractometry (XRD), and a transmission electron microscopic (TEM) observation,
0022-0248/98/$19.00 ( 1998 Elsevier Science B.V. All rights reserved. PII S 0 0 2 2 - 0 2 4 8 ( 9 8 ) 0 0 0 0 9 - 8
574
H. Nagata et al. / Journal of Crystal Growth 187 (1998) 573—576
respectively, for the z-cut LN single crystal surfaces after the ECR etching treatments with CHF . The 3 etching was carried out using an ANELVA L-310R ECR plasma etcher under the following conditions; about 400 W microwave power, 80 V magnet voltage, 500 V ion acceleration voltage, 20 cm gap between the sample and the acceleration electrode, about 0.01 Pa of CHF (3 sccm), 500—600 nm/h 3 etching rate, and 1—2 h etching duration. As is seen in Fig. 1a, the etched surfaces were covered with fine particles, and the peak-to-peak surface roughness deteriorated to be 43 nm (2 lm]2 lm area) from 2.2 nm before the etching. These particles were thought to be amorphous judging from the absence of a specific diffraction peak from the etched surface in Fig. 1b and Fig. 1c. Further, the presence of LiF on the same etched surface was found previously by Auger electron spectroscopy (AES) and X-ray photoelectron spectroscopy (XPS) [5], suggesting the growth of amorphous LiF particles by the CHF ECR plasma etching. 3 On the other hand, for the CF ECR plasma 4 etched z-cut LN surface, the existence of crystalline LiF was confirmed as shown in the grazing incidence XRD results of Fig. 2a. The etching conditions had been set at almost the same as those for the CHF , while a faster etching rate was obtained 3 (about 800 nm/h). Fig. 3a is the AFM image of this surface, showing a larger peak-to-peak surface roughness of 210 nm for the 2 lm]2 lm area. As the reason for such structural difference of the etched LN surface, the difference in the plasma species, e.g. existence of HF species in the CHF 3 plasma which might react with LiF particles, was considered. Further, there was a possibility that a temperature increase on the substrate surface caused by the incidence of ionic species (and electrons) was larger for the CF plasma leading to 4 grain growth and crystallization of the LiF layer. In the fabrication of high-speed LN devices having trenches on both sides of the waveguides, a metal pattern mask is commonly used because a resin mask could not endure the hours long etching process, for a 3—4 lm deep etching. We used a Ni pattern mask and after the ECR etching, the remaining mask was removed by dilute HNO 3 (0.5—1 N). Further, this HNO treatment was found 3 to also remove the LiF layer formed on the etched
Fig. 1. AFM image (a), grazing incidence XRD profiles (b), and TEM image (c) of the LN surfaces after ECR plasma etching with CHF . The grazing incidence XRD was performed using 3 a Cu K beam with incidence beam angles of 1.5°, 1.0°, 0.5°, 0.2°, a and 0.1° with respect to the sample surface.
H. Nagata et al. / Journal of Crystal Growth 187 (1998) 573—576
575
Fig. 2. Grazing incidence XRD profiles for the CF ECR 4 plasma etched z-cut LN surface (a), for the surface after a succeeding HNO treatment (b), and for the surface after a succeed3 ing O annealing at 600°C (c). The XRD measurements were 2 performed using a Cu K beam with incidence beam angles of a 2.0°, 1.5°, and 1.0° with respect to the sample surface.
LN surface. For instance, a 600 nm deep trench etched by the CF ECR plasma was deepened addi4 tionally by 80 nm after the 0.5 N HNO etching for 3 30 min at room temperature. The AES analyses proved that the fluorine existing in the CF etched 4 surface disappeared completely by the HNO treat3 ment. Similar results were obtained also by the O annealing of the CF etched samples at 600°C 2 4 for 1 h. Fig. 2b and Fig. 2c show the grazing incidence XRD results for the HNO etched and the O an3 2 nealed samples, respectively. The crystalline LiF disappeared and the other crystalline phase
Fig. 3. AFM images for the CF ECR plasma etched z-cut LN 4 surface (a), for the surface after succeeding HNO treatment (b), 3 and for the surface after succeeding O annealing at 600°C (c). 2
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H. Nagata et al. / Journal of Crystal Growth 187 (1998) 573—576
appeared. Some of the XRD peaks were assigned to the LiNb O phase, and others could not be as3 8 signed within LN relating materials such as Nb O , etc. Although the LiNbO did not decom2 5 3 pose to LiNb O at 600°C [6,7], the XRD peaks 3 8 from LiNb O were observed for the O annealed 3 8 2 sample. The results suggested that the formation of the LiF layer was due to the CF ECR plasma 4 etching causing a partial deficiency in Li on the LN surface. Fig. 3b and Fig. 3c are the AFM images corresponding to the samples in Fig. 2b and Fig. 2c, respectively. Both images showed similar surface morphology and peak-to-peak roughness of 500—600 nm for the 2 lm]2 lm area. Although the AES analyses for these surfaces revealed the O/Nb atomic ratio to be about 3 and the probable existence of a LiNbO phase at the surface, in addition 3 to the LiNb O , the origin for the observed surface 3 8 texture has not been found yet. At least, the morphology of both the thermally etched and the HF/HNO etched z-cut LN surfaces were signifi3 cantly different from Fig. 3b and Fig. 3c [6,7]. The LN surface etched by the CF ECR plasma 4 was mainly investigated. Due to the etching, the crystalline LiF layer grew on the surface accom-
panying a Li deficit and a surface roughening. These facts should be taken into account in the fabrication process of LN-based devices to prevent the deterioration of the device performance. This study was supported by Special Coordination Funds for Promoting Science and Technology, “Fundamental research on new materials of function-harmonized oxides,” from the Japanese Science and Technology Agency, to whom we are deeply indebted. References [1] J.L. Jackel, R.E. Howard, E.L. Hu, S.P. Lyman, Appl. Phys. Lett. 38 (1981) 907. [2] K. Noguchi, O. Miyomi, H. Nakazawa, S. Seki, J. Lightwave Technol. 13 (1995) 1164. [3] K. Miura, M. Minakata, S. Kawakami, Technical Digest of Conf. Lasers Electro-Opt. (1987) 288. [4] K. Noguchi, O. Mitomi, H. Miyazawa, Technical Digest of Optical Fiber Commun. (1996) 205. [5] K. Shima, N. Mitsugi, H. Nagata, J. Mater. Res. (1998), to be published. [6] H. Nagata, T. Sakamoto, H. Honda, J. Ichikawa, E.M. Haga, K. Shima, H. Haga, J. Mater. Res. 11 (1996) 2085. [7] H. Nagata, K. Shima, J. Ichikawa, J. Am. Ceram. Soc. 80 (1997) 1203.