Cl 2 -Based Dry Etching of GaN and InGaN Using Inductively Coupled Plasma

Transcription

1 Journal of The Electrochemical Society, 147 (5) (2000) 1859 Cl 2 -Based Dry Etching of GaN and InGaN Using Inductively Coupled Plasma The Effects of Gas Additives Ji-Myon Lee, Ki-Myung Chang, In-Hwan Lee, and Seong-Ju Park z Department of Materials Science and Engineering and Center for Optoelectronic Materials Research, Kwangju Institute of Science and Technology, Kwangju , Korea The effects of added H 2, Ar, and CH 4 gases on the etch characteristics of GaN and InGaN were studied using an inductively coupled Cl 2 -based plasma. Each added gas had a unique effect on the etch rate, anisotropy, surface roughness, and sidewall morphology. The most anisotropic etch profile was obtained using Cl 2, but the etched surface showed the roughest morphology and was covered with etch residues, the origins of which were the micromasking of the sputtered dielectric. When H 2 gas was added to the Cl 2 plasma, the etch residues were removed and the surface roughness was decreased, even though the etch rate was slightly decreased. The etch rate of GaN by Cl 2 /H 2 /Ar plasmas was saturated above an Ar flow rate of 16 sccm and the surface roughness of the etched GaN was lower, compared with Cl 2 /H 2 plasmas at low source power. Finally, it was found that the In compound was etched as a result of reaction with CH The Electrochemical Society. S (99) All rights reserved. Manuscript submitted September 30, 1999; revised manuscript received January 21, Cl 2 -based plasmas are in extensive use in the etching of Si, Al, and III-nitrides in a variety of processes. The highest dry etch rates for III-nitride materials have been achieved with high density plasmas, produced via electron cyclotron resonance (ECR) or inductively coupled plasma (ICP). 1,2 The majority of previous studies have been directed toward mesa formation in UV/blue/green photonic devices, where the etch depths are relatively large and the final surface morphology is not that important. Currently, attention is being directed to the development of GaN-based electronic devices for high power switching and transmission applications. 3,4 The etching requirements are quite different for these high power devices and care must be taken to retain smooth surface morphologies, in order to obtain ohmic contacts with low contact resistivity. To achieve this requirement, some researchers have examined the use of alternative plasma chemistries, such as those based on bromine and iodine. 5 Because chlorine-based gas chemistry is widely used in the processing of semiconductor devices, and can be easily accommodated in nitride-based device processing, it would be desirable to use a Cl 2 - based plasma to etch GaN, to give a smooth etched surface by applying suitable gas additives. Cho et al. 2 reported binary gas chemistries such as Cl 2 /Ar, Cl 2 /N 2, and Cl 2 /H 2, and concluded that ICP discharges are well-suited to achieving smooth etched surfaces when appropriate plasma conditions are used. Shul et al. 6 studied the Cl 2 - based ECR etching of GaN as a function of the percent H 2 and SF 6. Smith et al. 1 and Shul et al. 7 reported a high etch rate for GaN using inductively coupled Cl 2 /Ar and Cl 2 /H 2 /Ar plasmas, respectively. However, to date, only a few reports on the effect of gas additives to Cl 2 plasmas have appeared. In this paper, we report on the effects of gas additives such as H 2, Ar, and CH 4 on the etch characteristics of GaN and InGaN and, in addition, on the plasma characteristics of an inductively coupled Cl 2 -based plasma. Each gas additive had unique effects on the etch rate, etch anisotropy, surface roughness, and sidewall morphology, all of which require reliable control for device fabrication. Experimental The n-type GaN and In 0.12 Ga 0.88 N epi layers with thickness of 1.5 and 0.3 m, respectively, were grown on a c plane sapphire substrate by metallorganic chemical vapor deposition (MOCVD). The GaN layer showed strong band-edge photoluminescence (PL) at room temperature. The mole fraction of In in the InGaN was obtained from PL measurement. All samples were cut into m squares, except for the sample etched with pure Cl 2 plasma. In this z sjpark@kjist.ac.kr case, a whole wafer with a diameter of 2 in. was used, in order to increase the optical emission spectroscopy (OES) signal and to identify the etch product. The etch reactor was equipped with a 3 kw ICP power supply and connected to a load lock chamber. Superimposing an rf power (13.56 MHz) of between 100 and 250 W on the sample provided the dc-bias voltages for the control of the impinging ion energy. All samples were mounted on an anodized Al carrier which was clamped to a cathode and back-side cooled with He gas. SiO 2 was deposited on the sample as a mask layer by plasma enhanced chemical vapor deposition (PECVD), and patterned by means of carbon-based photoresist (PR). After PR lithography by a conventional flood exposure, the SiO 2 mask was patterned with a buffered oxide etchant (BOE). The SiO 2 mask was then stripped away with a buffered-hf solution after the plasma etching process was completed. The etch conditions used in this study were 30 sccm Cl 2, 8 sccm H 2,8 30 sccm Ar, 10 mtorr pressure, W ICP power, 20 C table temperature, and W rf power (dc bias between 90 and 400 V). All experiments were conducted after the background pressure had been reduced to less than mbar. During the etching process, the plasmas were monitored by OES. Etch rates were estimated from time-averaged depth profiles, measured with a surface profilometer after removal of the SiO 2 mask. Etch anisotropy and etched surface morphology were examined by scanning electron microscopy (SEM) and atomic force microscopy (AFM). Results and Discussion Figure 1 shows a typical optical emission spectrum from a Cl 2 plasma. The emission bands from Cl 2 (306 nm), Cl 2 ( nm), and atomic Cl ( nm) 8 are prominent in the spectrum. Atomic Ga (403, 417 nm) and GaCl x ( nm) 9 were observed in the spectrum as an etch product. The Ga signal at 403 nm was also visible when GaN was etched in a pure Ar plasma, in which only physical sputtering exist, as shown in the inset of Fig. 1. These results indicate that both physical and chemical processes take place during the etching of GaN by a Cl 2 plasma. Figure 2 shows the etch rate of GaN with the Cl 2 plasma as a function of ICP power at 10 mtorr. During the experiment, the rf power was maintained at a constant level of 100 W. With increasing ICP power, the induced dc bias voltage decreases, due to the increased plasma density at higher ICP power. 9 However, the GaN etch rate increases with increasing ICP power, as the result of the higher concentration of reactive species in the plasma, indicating that the etching proceeds in a reactant-limited regime 9 under the etching conditions employed in this study. An anisotropic feature,

2 1860 Journal of The Electrochemical Society, 147 (5) (2000) Figure 1. Typical OES from a Cl 2 plasma during etching of GaN at 1000 W of ICP power and 100 W of rf power. The inset shows the Ga signal at 403 nm when GaN is etched with pure Ar plasma. which is the result of a physical process, was clearly visible on the etched surface as shown in Fig. 3. At a low ICP power of 500 W, needles were formed on the etched surface (Fig. 3a), which might be caused by the micromasking effect of the Si-based dielectric which was sputtered by the impinging energetic ions. 10 Although, the needles could be partially removed by increasing the ICP power as shown in Fig. 3b due to the increase in the chemical process by the increasing plasma flux, the rms roughness value measured by AFM was still in excess of 500 Å. Figure 4 shows the OES of a Cl 2 /H 2 plasma. The Balmer series of atomic H spectral lines are easily resolved at 434, 486, and 656 nm. The intensities of Cl radicals and the Cl 2 spectral band, however, were considerably decreased, in contrast to those of the Cl 2 plasma in Fig. 1 probably because H is capable of scavenging reactive species such as Cl, Cl, and Cl 2. These decreased emission intensities of the reactive chlorine species will have an influence on the etch rate, since Cl radicals and ions are mainly responsible for the chemical and physical etch process. Some researchers 2,11 proposed that hydrogen might cause a decrease in the density of neutral atomic chlorine and the formation of HCl. The data herein clearly confirm this proposal. Figure 5a shows the etch rate of GaN etched in the Cl 2 /H 2 (30/8 sccm) plasma as a function of ICP power. The etch rate of GaN by the Cl 2 /H 2 plasma increases with increasing ICP power. However, the etch rate of GaN in the Cl 2 /H 2 plasma is slightly lower than that in the Cl 2 plasma under the same conditions. Smith et al. 1 also Figure 3. SEM image of a Cl 2 plasma etched surface at (a) 500 W of ICP power and (b) 1000 W of ICP power. reported a similar result, that is, the etch rate of GaN in the Cl 2 /Ar plasma increases with increasing ICP power. However, they did not examine the effects of the reactive chlorine and ion densities on the etch rate. To investigate the relation of plasma species to the etch rate of GaN, the rare-gas actinometric method 12,13 was used in this study. Figure 5b shows the relative OES intensity ratios of chlorine radicals (754.7 nm) and chlorine ions (455 nm) 14 normalized with respect to Figure 2. Etch rate of GaN vs. ICP power in a Cl 2 plasma at 100 W of rf power. Figure 4. OES from Cl 2 /H 2 (30/8 sccm) plasma at 1000 W of ICP power and 100 W of rf power.

3 Journal of The Electrochemical Society, 147 (5) (2000) 1861 the Ar (750.4 nm) emission intensity as a function of ICP power. At 150 W of rf power, the etch rate is dependent on the relative intensity of chlorine atoms, while it is dependent on the relative intensity of chlorine ions at 250 W of rf power. Figure 6 shows the rms roughness of etched samples which were measured using AFM, which was normalized to the rms roughness of the as-grown samples. The normalized roughnesses of all samples etched at high rf power are larger than those at low rf power and these are independent of ICP power, as shown in Fig. 6. These results clearly indicate that the physical process is dominant in the etching of GaN using chlorine plasma at high ion energy. Figure 7 shows the SEM images of a GaN surface etched with (a) Cl 2 and (b) Cl 2 /H 2 plasma at a high ICP power (2000 W). The GaN surface etched with a Cl 2 /H 2 plasma is smoother than that with a Cl 2 plasma due to the removal of the needles by suppressing the scattering of mask materials by hydrogen. 10 Moreover, the anisotropy in the etch profile of the sidewall was decreased slightly as shown in Fig. 7b, indicating that a more isotropic etching process took place, probably due to a decrease in the impinging ions. These results demonstrate that the addition of H 2 to the chlorine plasma enhances the chemical etching process, resulting in a more isotropic profile and smoother etched surfaces. In Fig. 8a, the OES of a Cl 2 /H 2 /Ar mixed gas plasma shows the spectral lines of Ar at 415 and 420 nm, in addition to those of Cl 2 and H 2 (see Fig. 4). Atomic spectra with stronger intensity are also seen between 700 and 900 nm and these overlap the atomic spectra of Cl. The addition of Ar to the Cl 2 /H 2 mixed gas plasma considerably increased the intensities of all reactants, especially atomic Cl radicals which were decreased by the addition of H 2 to the Cl 2 gas (see Fig. 1 and 4). The effect of the Ar additive gas can be clearly seen in the etch rate of GaN as shown in Fig. 8b. The etch rate of GaN increases up to 50% at 16 sccm of Ar. However, the induced dc bias voltage decreases with added Ar, suggesting that the etch process is largely dominated by the reactive chemical species in the Figure 5. Etch rate of GaN vs. ICP power with (a) Cl 2 /H 2 (30/8 sccm) plasma at 1000 W ICP power and 100 W rf power, and (b) rare-earth actinometry of chlorine and chlorine ion. Figure 6. RMS roughness of GaN etched with Cl 2 /H 2 (30/8 sccm) plasma. The rms values were normalized to those of an as-grown sample. Figure 7. SEM image of etched GaN surface with (a) Cl 2 and (b) Cl 2 /H 2 plasmas.

4 1862 Journal of The Electrochemical Society, 147 (5) (2000) Figure 8. (a) OES from Cl 2 /H 2 /Ar (30/8/23 sccm) plasma at 1000 W of ICP power and 100 W of rf power. (b) Etch rate of GaN vs. Ar flow rate in Cl 2 /H 2 /Ar gas mixture. Figure 9. RMS roughness of GaN etched with Cl 2 /H 2 /Ar and Cl 2 /H 2. plasma. It has also been reported that the etch rate of III-V semiconductors is increased by Ar addition either via an increase in radical density (e.g., by Penning ionization) or the enhanced ion-sputter removal of the etch product by the Ar. 2,15 Figure 9 shows the rms roughness of GaN etched with Cl 2 /H 2 /Ar and Cl 2 /H 2 as a function of ICP power. The rms roughness was measured by AFM in a contact mode. When Ar is not added, the surface roughness is relatively independent of ICP power. However, the surface roughness sharply increases with increasing the ICP power when Ar is added to the plasma. This is probably due to the generation of a large amount of Ar ion flux. Low rms roughnesses can be seen at lower ICP power (<1500 W), which might be caused by the enhancement of sputter removal of etch product by Ar. 15 However, the anisotropy was not influenced by added Ar up to 32 sccm of Ar flow rate. These results show that the addition of Ar to the Cl 2 -based plasma is necessary in order to achieve a high etch rate and a smooth surface morphology which are, in turn, required in the fabrication of device structures. In the fabrication of blue light emitting diodes and laser diodes where an In-containing compound is used as an active layer, the etching of In becomes very important. When a Cl 2 -based plasma is used, the presence of In leads to a roughening of the etched surface and sidewall, 16 resulting in poor optical performance of the device. 17 Since the In-containing compound is believed to be etched by a reaction with CH radicals to form volatile metallorganic indium compound [In(CH x ) y ], 18 we investigated the etch characteristics of InGaN by adding CH 4 to the Cl 2 plasma. Figure 10 shows the OES of mixtures of (a) Cl 2 /H 2 /Ar and (b) Cl 2 /CH 4 /H 2 /Ar gas plasmas at 1000 W of ICP power and 100 W of rf table power. In the case of Cl 2 /CH 4 /H 2 /Ar plasma, the CH spectral line at nm, which is formed by dissociative excitation of CH 4 (CH 4 e r CH* H 2 H e) can be seen and the intensity of the H spectrum is enhanced by the dissociation of CH 4. However, the intensities of the Cl and Figure 10. OES of (a) Cl 2 /H 2 /Ar and (b) Cl 2 /CH 4 /H 2 /Ar at 200 W of rf powers.

5 Journal of The Electrochemical Society, 147 (5) (2000) 1863 is etched by reaction with CH 4. Therefore, the Cl 2 /CH 4 /H 2 /Ar plasma chemistry is recommended for the etching of In-containing GaN structures. Conclusions The effects of gas additives H 2, Ar, and CH 4 on the etch characteristics of GaN and InGaN have been examined using an inductively coupled Cl 2 -based plasma. In pure Cl 2 plasmas, the etch profile was the most anisotropic, but the etched surface was very rough. The etched surface was covered with some etch residues which originated from the micromasking of a sputtered dielectric. When H 2 gas was added to the Cl 2 plasma, the OES intensity of all the chlorine species was decreased due to H scavenging and the etch rate was slightly decreased. However, the etch residues were removed and the surface roughness was decreased. When Ar gas was added to the Cl 2 /H 2 plasma, the OES intensity of all the species in the plasma was increased due to Penning ionization. The etch rate of GaN by Cl 2 /H 2 /Ar plasma was sharply increased but saturated above an Ar flow rate of 16 sccm. The surface roughness of the etched GaN was small with Cl 2 /H 2 plasmas at low source power. Finally, it was found that the In compound was etched by reaction with CH 4. Acknowledgment This work was supported by the Korea Research Foundation through grant no E Kwangju Institute of Science and Technology assisted in meeting the publication costs of this article. Figure 11. Emission intensity of (a) Cl and (b) CH, and (c) etch rate of InGaN as a function of CH 4 flow rate. Cl 2 emission bands are decreased due to the H scavenging. Figure 11 shows the plasma emission intensity of (a) atomic Cl, (b) CH, and (c) the etch rate of InGaN as a function of CH 4 concentration at 1000 W of ICP power and 100 W of rf power. Although the quantitative measurement of radical density requires the use of actinometry, actinometers which are suitable for the study of CH 4 -containing plasma have not yet been devised to our knowledge. Furthermore, Pearton et al. 19 reported that the correlation of the emission intensity of active etchant species produced by a high density plasma and the etch rate of semiconductor was a more convenient method for technological purposes. Therefore plasma emission peak intensities were used for determining the effect of CH 4 additive gas in the Cl 2 plasma chemistry in this work. Figure 11a and b shows that the Cl emission intensity is decreased, due to H, which is derived from CH 4 while CH emission intensity increases up to 20% of CH 4 and then decreases. The etch rate of InGaN in Fig. 11c is closely related to the CH intensity, showing an increase in etch rate of up to 20% of CH 4. 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