Jan 17, 2015 - 20, L847. (1981). 2 C. Mo Knoedler and T. F. Kuech, J. Vac. Sci. Techno!. B 4, 1573 (1986). l A. Seabaugh, J. Vac. Sci. Techno!. B 6, 77 (1988).
Selective metalorganic reactive ion etching of molecularbeam epitaxy GaAs/Al x Ga1−x As V. J. Law, G. A. C. Jones, D. A. Ritchie, D. C. Peacock, and J. E. F. Frost Citation: Journal of Vacuum Science & Technology B 7, 1479 (1989); doi: 10.1116/1.584516 View online: http://dx.doi.org/10.1116/1.584516 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/7/6?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Molecularbeam epitaxial growth of In x Al1−x As on GaAs Appl. Phys. Lett. 65, 699 (1994); 10.1063/1.112273 Performance of Ga x In1−x P/GaAs heterojunctions grown by metalorganic molecularbeam epitaxy and metal organic vaporphase epitaxy J. Appl. Phys. 75, 2980 (1994); 10.1063/1.356995 Selective regrowth of InP and GaAs by organometallic vapor phase epitaxy and metalorganic molecular beam epitaxy around dry etched features J. Vac. Sci. Technol. B 11, 536 (1993); 10.1116/1.586796 The growth of highquality AlGaAs by metalorganic molecularbeam epitaxy J. Appl. Phys. 70, 973 (1991); 10.1063/1.349608 Selectively Sedoped AlGaAs/GaAs heterostructures with reduced D Xcenter concentrations grown by molecular beam epitaxy J. Appl. Phys. 68, 3343 (1990); 10.1063/1.346387
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Selective metalorganic reactive ion etching of molecular-beam epitaxy GaAs/ A1xG3i _xAs V. J. Law, G. A. C. Jones, D. A. Ritchie, D. C. Peacock,a) and J. E. F. Frost Cambridge University, Cavendish Laboratory, Department o/Physics, Cambridge, CBl Ollb~ England. United Kingdom
(Received 31 May 1989; accepted 7 July 1989) We report on a metalorganic reactive-ion etch (MORIE) methane and hydrogen process for the selective etching of GaAs, AlxGa t ,As and GaAs/AlxGa l xAs heterostructures, grown by molecular-beam epitaxy (MBE). Etch rates, degree of selectivity, and loading effects are examined as a function of aluminium mole fraction (x = 0.0-1.0), gas flow rates, pressure, and applied rfpower. The etch process employs a specially prepared positive photoresist which can withstand high rf powers (1.1 W cm 2) with plasma potentials of - 600 Vdc at 1.3 Pa and prolonged etch times (> 200 min) with little mask erosion, while maintaining a high degree of anisotropy in the etch process. The etch rate ratio of GaAs over Alx Gal -.x As is shown to be a strong function of the methane molecular concentration and by selecting a high methane flow rate, i.e., a low chamber residence time, favorable etch rate ratios are obtained. AI,Ga l "As layers can either be etched through or an etch-stop achieved, before the onset of plasma polymerisation. This process has been successfully used for etching micron and submicron size mesas and GaAs/Alx Gar x-4.S heterostructures, with linear etch rates (Ra) of up to 45 nm min - 1 to a depth of7 f.lm for GaAs and 0-10 nm min 1 for A103 Ga07As, depending upon the plasma conditions used.
I, INTRODUCTION
The ability io selectively etch GaAs down to a predetermined underlying Alx Gal _xAs etch-stop layer, has wide applicatiOIls in GaAsl Alx Gal _ xAs heterostructure device fabrication such as high electron mobility transistors (HEMT's), laser diodes, and various millimeter-wave mesa diodes. Current selective reactive ion etching (RIE) of GaAs over AtGa l xAs uses mixtures of CC1 2 F2 and He, sometimes with an addition of oxygen to produce the desired etch-stop, I 4 or SF6' SiCI4 , and He. 5,6 Here the etch rate selectivity of GaAs over Alx Gal x As is achieved by the formation of a nonvolatile AlF3 film 011 the AtGa l _ xAs layer. This is analogous to the selective etching process ofSi over S102 , by controlling the C:H ratio of CF 4 and H2 ctchant gases. 7 •S In this paper we report on the use of a CH 4 and Hz metalorganic reactive-ion etching (MORIE) processes for selective etching of GaAs over Al",Ga,_ xAs, as an alternative to the established gas mixtures listed above. We use the term MORIE 9 ,10 in this paper to distinguish the process from the chlorine and fluorine RIE processing, as there are a number of major differences between the two processes. First, the CH4 and H2 precursors do not generate the corrosive byproducts normally associated with chlorine and fluorine chemistry. It seems that they do form volatile Group III metalorganic compounds, although only Group V halide (PH 3 )11 byproducts have been reported so far. Second, CH 4 and Hz etched GaAs exhibits low electrical damage as measured by the Schottky diode ideality factor. 12 A summary of published MORIE processes are given in Table I, including C 2 Ho and Hz. 14.15 Unfortunately etch rates have mostly been reported as functions of CH4 percentages in H2 and applied rf power densities, without due regard to process flow rates and de self bias potentials. This makes eompati1479
J. Vac. ScI. Technol. B 7 (6), NovlDec 1989
sons between etching systems difficult, as plasma etch rates are related to the concentration of active species. 16.17 In this work, the etching parameters of an RIE machine have been extensively characterized with regard to GaAs etch rate, Al mole fraction selectivity, anisotropy, and loading effect for a series of CH4 and H2 flow rates (0-60 standard cubic centimeters per minute (seem)], pressures ( 1-3.5 Pa), rf power densities (0.2-1.1 W em - 2 ), and associated self bias potential (Vdc )' Etch rates are related to the amount of material consumed and the etch rate notation Ro for 1.0 em - 2 samples is adopted. 18
It EXPERIMENT Etching experiments were carried out in a Plasma Technology RIE 80 system with a 0.110 m 3 s 1 mechanical roots/rotary pump combination giving a base pressure of < 0.1 Pa, with a corresponding maximum flm:v rate of 60 sccm at 1.3 Pa. The 7 liter chamber has an anode to cathode plate area ratio of;::::; 1.8:1 with an aluminium anode and an anodised cathode driven by a 13.56 MHz power supply through a manually operated LC matching network. The 1. Comparison of published maximum etch rates (nm min I) for lII-V materials.
TABLE
Ref
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20
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8 10
10
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20 50
30
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25 70 70
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0.4 1.0 0.6 1.1
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Bias measurements 1I0t previously published.
0734-211X/89/061479-04$01.00
@ 1989 American Vacuum SOCiety
1479
Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 150.140.192.177 On: Sat, 17 Jan 2015 17:48:04
1480
Law et al 1.3 Pa for a series ofH 2 flow rate (25, 30, and 35 seem).
2,
(1.3-3.5 Pa) and that the CH4 flow rate has to be reduced so as not to initiate carbon film deposition on the GaAs substrate (GaAs polymer point). This effect can be best illustrated if R" is replotted against the CH4 molecular concentration, as shown in Fig. 30 Here it can be clearly seen that Ro is dependent upon the CH 4 molecular concentration, or simply the CH4 flux within the chamber for a given rf power and associated dc bias. Under these conditions we have been able to etch GaAs mesa to a depth of 7 ,urn with {) = - 0.1.16 A further series of etching experiments was performed to investigate the etch rate as a function of the amount of material consumed, i.eo, area loading effect. GaAs samples of 1.0 cm 2 were placed on various sizes of bulk GaAs wafers of known wafer area. These were then etched for 20 minutes each, using CH 4 flows of 10, 20, and 30 scem at 1.3 Pa, 1.1 W cm - 2, - 600 V de' Results obtained (not shown) show that the reciprocal etch rate increases linearly with exposed wafer area and becoming more severe with increasing CH 4 flow rate.
B. AI"Ga 1 _ x As Figures 4 and 5 show the effect of applied rf power upon Alx Gal _ x As, with x ranging from 0.0-1.0, for plasma con-
III. EXPERIMENTAL RESULTS A.GaAs Figure 1 shows GaAs etch rate (Ro) as a function ofCH4 flow rate (sccm) for a series of fixed hydrogen flow rates at a constant rf power (1.1 W em - 2, - 600 V de) and pressure (1.3 Pa). Under these conditions it can be seen that Ro is dependent on the CH4 flow rate as expected. However with increasing CH 4 ft.ow there is also an optimal dependence on the II, flow. Furthermore, the degree of anisotropy improved with Ro reaching a value of /) = - 0.1 (where denotes an under cut) at a maximum R" of 40 nm min 1, corresponding to a CH4 flow rate of 30 sccm (CH 4 =50%). Figure 2 shows the dependence of Ro on CH 4 flow rate at the optimized H2 flow rate (30 sccm) with total chamber pressure. Here it is seen that R" increases with total pressure
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J. 'lac. Sci. TechnoL S, Vol. 7, No.6, Nov/Dec 1989 Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 150.140.192.177 On: Sat, 17 Jan 2015 17:48:04
1481
Law et sl.: Selective MORIE of MOE GaAsl Al xGa1_x As
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