Reactive ion etching of gallium nitride in silicon ...

Reactive

ion etching

of gallium nitride

in silicon tetrachloride

plasmasa)

I. Adesida, A. Mahajan, and E. Andidehb) Center& Compound Semiconductor Microelectronics and Department of Electrical and Computer Engineering, University of Illinois, Urbana-Champaign, Illinois 61801

M. Asif Khan, D. T. Olsen, and J. N. Kuznia APA Optics, Inc., Minneapolis, Minnesota 55434

(Received 31 March 1993; accepted for publication

13 September 1993)

The reactive ion etching characteristics of gallium nitride (GaN) in silicon tetrachloride plasmas (SiCld, l:l/SiClb:Ar, and l:1/SiCld:SiF4). in the pressure range between 20 and 80 mTorr have been investigated. For the pressure range investigated, etch rates are found to be essentially identical for the different gas mixtures and also invariant with pressure. However, for all gas mixtures, etch rates increased monotonically with increasing plasma self-bias voltage exceeding 50 nm/min at 400 V. This is one of the highest etch rate ever reported for GaN. Smooth and anisotropic etch profiles are demonstrated for structures of submicrometer dimensions. The slight overcut observed in the etch profiles is attributed to the significant role of physical ion bombardment in the etching mechanism. Auger electron spectroscopy show that a wet etch in dilute HF is needed to clear the Si (in the form of SiO,) embedded in the near surface of GaN during etching thereby restoring etched surfaces to their virgin state. Gallium nitride (GaN) and aluminum gallium nitride (AlGaN) material system are wide, direct band gap semiconductors with band gap varying from 3.4 eV for GaN to 6.2 eV for aluminum nitride (AlN) at room temperature. The range of band gap exhibited by these materials make them very suitable for applications in optoelectronics. Ultraviolet (W) emission as well as luminescence throughout the visible spectrum have been demonstrated in these materials. For example, p-n junction light emitting diodes which emit in the UV and blue wavelengths have been recently reported by Akasaki et al. ’ and Nakamura et a1.2 Littlejohn et aL3 have shown that the peak electron drift velocity in GaN exceeds 1 X lo7 cm/s. This and the theoretical work by Das and Ferry4 have established GaN as a suitable material for high speed device applications. Experimental work related to achieving high quality GaN for electronic devices has been initiated. For example, Khan et a1.5 have reported the growth of AlGaN/GaN heterostructures with enhanced mobilities. Other applications of GaN include its use as an insulating layer in a metalinsulator-semiconductor (MIS ) or a semiconductorinsulator-semiconductor (SIS) structure on GaAs.6 These wide range of potential applications have prompted extensive research on the growth of GaN and other III-V nitrides. Excellent reviews on these materials have recently been published.‘.* What is obvious from these reviews is that progress in growth is not matched by progress in the processing techniques required to turn these materials into devices. High quality GaN is chemically stable and very inert resisting all mineral acids and bases at room temperature. GaN etches slowly in hot alkalis and can also be etched electrolytically in NaOH.9,‘0 With such difficulty in ‘)Part of this letter was presented orally at the Workshop of Wide Band Gap Nitrides held in St. Louis, MO, April 1992. “Present address: Intel Corporation, Hillsboro, OR 97124. 2777

Appl. Phys. Lett. 63 (20), 15 November

obtaining reliable wet etching processes, it is therefore imperative to investigate the dry etching characteristics of these materials. In this letter, we present our work on the reactive ion etching (RIE) characteristics of GaN in SiC14 plasmas. The influence of the plasma self-bias voltage, chamber pressure, and different gas mixtures on etch rates are presented. Etch profiles obtained for submicrometer geometries under various plasma conditions are also presented. Auger electron spectroscopy of etched GaN surfaces is discussed. The GaN samples used in this study were epitaxially grown by metalorganic chemical-vapor deposition on sapphire substrates. The surfaces of the samples were very smooth and the thicknesses of the layers ranged from 2.0 to 5 ,wm. All reactive ion etching processes were performed in a Plasma Technology RIE system.” The RIE chamber is enclosed in a nitrogen-purged glove box to hinder atmospheric contamination. The rf-driven cathode is made of aluminum coated with alumina and is covered with a quartz plate. Both electrodes are water cooled to maintain a nominal temperature of 20 “C. For all processes, the chamber was first evacuated to 100 mTorr by a Roots blower pump, then pumped to a base pressure of 5X 10m6 Torr using a turbomolecular pump. During etching, only the mechanical pump was used to pump the process gases. For etch rate determination, the samples were patterned with a 2-pm-thick photoresist mask: and for etch profiles of submicrometer lines, the samples were patterned with lifted-off 50-nm-thick NiCr masks. Etched depths were measured using a Tencor profilometer while etch profiles were inspected in a scanning electron microscope. Surfaces of processed samples were probed by Auger electron spectroscopy in a Perkin-Elmer system equipped with an argon sputter gun. Figure 1 shows the results of the etch rate of GaN in SiCl, plotted as a function of plasma self-bias voltage obtained at various chamber pressures between 20 and 80

1993 0003-6951/93/63(20)/2777/3/$6.00

@ 1993 American Institute of Physics

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FIG. 3. Etch profile of 4CO-nm-period gratings in GaN etched in SiCl, at 400 V, 20 mTorr using NiCr mask. Note the smoothness of the etched surfaces.

Plasma self-bias voltage (-V) FIG. 1. Etch rate of GaN in SiCl, plasma vs plasma self-bias voltage for different chamber pressures.

mTorr. The gas flow rate was 10 seem. The range of bias voltage, 100-400 V, used corresponds to a plasma power density of 0.12-0.72 W/cm’ in our RIE system. It is observed from Fig. 1 that at all pressures, etch rates increase monotonically with increasing bias voltage. Etch rates in excess of 50 nm/min are obtained at 400 V. Practically, no etching was obtained below 150 V. This is most likely due to a surface oxide barrier that required significant ion activity for its removal. It is also observed that similar etch rates were obtained at each bias voltage for the different chamber pressures. This and the increase in etch rate with increasing bias voltage suggest that there is a significant physical component in the etching. In terms of the chemical etching component, possible etch products are nitrogen and the chlorides of Ga. Figure 2 compares etch rates of GaN in a SiC14 gas with those in SiCld/Ar and Sic&./

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SiF, gas mixtures at a pressure of 40 mTorr. The flow rate for each gas in the mixture was 10 seem. It is observed that the etch rates for the two gas mixtures are essentially identical to those for SiCl, alone. The additional physical etching expected from Ar ions was not manifested. The addition of SiF, was motivated by possible chemical removal of N via reactions with F, neutrals to form NF, resulting in higher etch rates. However, the etch rate was not enhanced and it is deemed that no chemical etching was detived from the addition of SiF,. Overall, the magnitude of the etch rates obtained are significant, and at 50 nm/min at higher bias voltages, this is sufficient to achieve deep-etched structures in GaN. Etched profiles of submicrometer-wide lines in GaN have been studied using 50-nm-thick NiCr as etch masks. Etch profiles were investigated for a bias voltage of 400 V at pressures between 20 and 80 mTorr. A typical profile for a 400-nm-period gratings etched 300 nm deep into Sic& at 20 mTorr is shown in Fig. 3. A slightly overcut profile is observed which confirms strong physical etching component. Figure 4 shows a GaN structure which is 400 nm in linewidth and 700-nm-deep etched in a l:l/SiCl,:Ar plasma at 20 mTorr. A slight overcut profile is also observed for this structure. This overcut was obtained for all profiles etched in the different gases at the pressure ranges specified above. The etched surfaces and profiles were

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Plasma self-bias voltage (-V) FIG. 2. Etch rate of GaN vs plasma self-bias voltage at 40 mTorr for different gas mixtures. 2778

Appl. Phys. Lett., Vol. 63, No. 20, 15 November 1993

FIG. 4. Etch profiIe of 400-nm-wide line etched 700-nm-deep into GaN in a l:l/SiCl,:Ar plasma at 400 V and 20 mTorr. Adesida et al.

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smooth in all cases which reflects the high quality of the epitaxial layer. The surfaces of virgin and etched GaN have been studied using Auger electron spectroscopy. On the virgin GaN surface, Ga, N, 0, and C peaks were observed. The C and 0 are from atmospheric contamination and the 0 may reflect the oxides that needed to be sputtered off prior to the attainment of significant etch rates as discussed above. In addition to the aforementioned peaks, Si and Cl peaks were obtained on etched GaN surfaces. The 0 peak also increased in intensity. The Si and Cl peaks are from the etchant species. Auger depth profile revealed that these additional peaks were intermixed on the surface to a depth of 2 to 3 nm. A dip of the etched sample in a dilute hydrofluoric (HF) solution removed the Si and Cl peaks and reduced the 0 peak to the level observed on the virgin sample. The removal of the Si peak and the reduction in the 0 peak showed that the Si on the etched surface existed in the form of SiO, once the etched sample was exposed to the ambient. Therefore, it is necessary to perform a postRIE wet process in dilute HF to completely restore the stoichiometry of the GaN surface. The reactive ion etching characteristics of GaN in SiCI, plasmas have been presented. It was shown that similar etch rates were obtained for GaN in SiCI,, a 1:l mixture of SiCg and Ar, and a 1:l mixture of SiCI, and SiF, in the pressure range between 20 and 80 mTorr. The etch rates increased monotonically with increasing plasma self-

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Appl. Phys. Lett., Vol. 63, No. 20, 15 November 1993

bias voltage exceeding 50 nm/min at 400 V. This is one of the highest etch rates ever reported for GaN. Etch profiles of submicrometer geometries were demonstrated. The slight overcut in the profiles is attributed to the significant physical component in the mechanism needed to etch GaN. The relatively high etch rates, anisotropic profiles, and smoothness of etched surfaces make reactive ion etching in SiCl, plasmas useful for practical device fabrication in GaN. The authors gratefully acknowledge the technical assistance of Dr. A. Ketterson, D. Ballegeer, J. Hughes, and N. Finnegan. This work was supported at the University of Illinois by NSF Grant ECD 89-43166. ‘I. Akasaki, M. Amano, M. Kito, and K. Hiramatsu, J. Lumin. 48/49, 666 (1991). *S. Nakamura, M. Senoh, and T. Mukai, Jpn. J. Appl. Phys. 30, L1708 (1991). ‘M. A. Littlejohn, J. R. Hauser, and T. H. Glisson, Appl. Phys. Lett. 26, 625 (1975). 4P. Das and D. K. Ferry, Solid-State Electron. 19, 851 (1976). ‘M. A. Khan, J. M. Van Hove, J. N. Kuznia, and D. T. Olsen, Appl. Phys. Lett. 58, 2408 ( 1991). ‘G. Martin, S. Strite, J. Thornton, and H. Morkop, Appl. Phys. Lett. 58, 2375 (1991). ‘R. F. Davis, Proc. IEEE 79, 702 (1991). *S. Strite and H. Morkoq, J. Vat. Sci. Technol. 10, 1237 (1992). ‘T. L. Chu, J. Electrochem. Sot 118, 1200 (1971). “J. I. Pankove, J. Electrochem. Sot. 119, 1118 (1972). “E. Andideh, I. Adesida, T. Brock, C. Caneau, and V. Keramidas, J. Vat. Sci. Technol. B 7, 1841 (1989).

Adesida et al.

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