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Bombardment angle dependence of reactive ion-bean etching of GaAs with CH,/H, J M Villalvilla, C Santos and J A Vallh-Abarca, Apartado 99, E-03080 Alicante, Spain

Departamento

de Fisica Aplicada,

Universidad

de Alicante,

The angular dependence of the etch depth has been studied for GaAs using a Kaufman-type ion-beam sours and methane f 13%1-hydrogen as the processing gas mixture. The experimental results show that the etch depth increases with the sample inclination in the range O-50“. To explain these results we have developed model based on a reaction between a polymer carbonaceous film and the substrate where the desorption t the products is considered.

1. Introduction The use of hydrocarbon/hydrogen mixtures has been proposed’ and used for RIE of InP2, GaAs3 and ternary III-V semiconductors4 as an alternative to chlorine-containing etchants, more toxic and corrosive. It has been postulated that the group III element is removed as a volatile organometallic compound and the group V element as a hydride. Reactive ion-beam etching (RIBE) has been demonstrated to be an appropriate method for characterizing process parameters. In this technique, a collimated beam of reactive ions is extracted from a discharge and bombards the sample. Independent control over ion energy, beam-current density and sample environment is allowed, while in other techniques these parameters are interdependent. These characteristics are very convenient for basic studies. This paper reports the angular dependence of the etch depth of GaAs reactively ion-beam etched with a CH4/H2 mixture. The experimental results are analysed using a model based on an etching mechanism by a surface film reaction.

Figure 1 shows the dependence of the etch depth on time fc two different sample inclinations. Here we can see the existent of a linear time dependence for these conditions of etching. 1 Figure 2, we present the angular dependence of the etch depth GaAs for a 40 min etching time. This picture shows that the etc depth increases with the sample inclination. These result haI same tendency as those reported by Barklund and Blom6 fi CHF, + O2 RIBE of Si or Okano and Horiike7 for Ar + Cl* RIB system of Si. 3. Model and discussion A simple model of the etching dependence on the sample inc nation may be obtained by considering the deposition of the fil and its reaction with the substrate. It is likely that the surface

2. Experimental and results The experiments were carried out in a RIBE apparatus. This experimental system is composed of two major sections : an ion source and an etching chamber. The ion source is based on Kaufman design as we have described in ref 5. A sample, undoped GaAs (p = 1 x lo7 R cm, orientation (100)) is located on the holder in the etching chamber and we can control its inclination through a manipulator. Measurement of the etch depth is made by a surface profiler (Planer SF220), after the removal of the mask from the GaAs bombarded sample. We have kept fixed both the gas feed and the operational variables of the ion source, anode-cathode voltage 5&60 V, arc current 1.5 A, and solenoid current 0.3 A. The total pressure in the etching chamber is 0.06 Pa with 13% methane partial pressure. The high voltage of the beam supply is 500 V and the current density at the centre of the beam is approximately 0.9 mA cm-‘.

ETCHING

TIME (min)

Figure 1. Etch depth of GaAs reactively ion beam etched as a function etching time for 13% CH, at 0” (0) and 40” (A) sample inclination. 11

J M Vi//e/vi//a

et al: Angle dependence

of ion-beam

etching Table 1. Parameters of the etching model

J = 0.9 mA cm-’

A = 0.15 a = 0.8 K.. = 6.31 x 10-‘6cm2sm’ KE*= 1.89 x LO-’ cm2 s-’ b = 1.67 x 1O-3 cm-’

n = 1 x 10’5cm~* nfs = 2.2 x lOI4 cm-’ n, = 1 x 102’cmnGa = 2.2 x 10z2 cm-’

where nc is the atom density of carbonaceous

dzc

3

0

10

20

INCLINATION

Figure 2. GaAs etch depth as function etching

30

I

I

40

50

ANGLE,

nc __ dt

I I

Equation

60

(4) can be solved with the initial conditions

angle for 40 min

time and 13% CH4 ion beam.

t = n,

:+-ln

1 CB

completely covered by a carbonaceous layer deposited by the beam and then a carbonaceous film-substrate reaction occurs. The group V is probably removed as metal organic compounds, (CH,),,Ga, and the group V element as the hydride. These arguments can be related to the information obtained by Ugolini et al8 from XPS spectra where the core level binding energy shifts observed upon deposition of amorphous hydrogenated carbon films on GaAs represent an indication that a chemical reaction takes place at the interface. In this case, we assume that an initial process step is the incorporation of the incoming carbonaceous particle before the chemical reaction. Depending on the carbonaceous thickness over GaAs the etch products may be desorbed. We have assumed that the carbonaceous ion flux of the beam sticks with a constant value on the surface and we have neglected the sticking coefficient of the non-energetic species. We are also neglecting other processes such as the contribution of chemical and physical sputtering. We assume that the limiting step is the extraction of Ga. The deposition rate of the polymeric carbonaceous material on surface will be : Tc = JAa cos 8,

(1)

where JA is the organic ion flux that arrives to the sample, J is the total ion-beam flux, A the organic ion fraction and e is the sample inclination angle. The organic-ion sticking coefficient on polymeric carbonaceous film is CL The rate of extraction of carbonaceous material from the surface, owing to chemical reaction with the target, has been assumed as

Rc = Kcnsnf e-“+

(2)

where Kc is the reaction constant, n, and n, are the surface densities of the GaAs substrate and carbonaceous species near the substrate respectively, zc is the thickness of polymer carbonaceous film and /3 is the attenuation coefficient. The net flux of carbonaceous material over the surface will be : dz,

nc

--= dt

1114

Fc =Tc-Rc

ep8’c.

(4) of zc = 0

att=O:

$(‘I

of inclination

= Fc = JAcr cos B- Kcnsnf

film. Then

(3)

c+ue-pzc c+U

(5)

where

a = - Kg,n,

(6)

c = J cos 8Aa.

(7)

The flux of Ga atoms that react and leave the solid will be :

hia

~ = KGan,nf ecBzc lZGa dt

(8)

where nGa is the GaAs atomic density, dz,,/dt the GaAs etching rate and KGa is the reaction constant. This equation has been solved numerically using the time dependence of z,-, equation (5). The difference between Ga etch depth and thickness carbonaceous overlayer is plotted in Figure 2 with the parameters that are shown in Table 1, where it is compared with the experimental results.

Acknowledgments This work was partially supported by the Commisibn Interministerial de Ciencia y Technologia (Programa National de Microelectr6nica). We wish to thank J A Quintana for photoresist patterning of samples and Centro National de Microelectrbnica de Madrid for providing the GaAs samples.

References ’ U Niggebrugge, M Klug and G Garus, Znst Phys Conf Ser, 79, 367 (198% ‘T ‘_ R--’Hayes, M A Dreisbach, P P Thomas and W C Dautremont, .I Vat Sci Technol, B7, 1130 (1989). ‘V J Law, M Tewordt, S Ingram and G A C Jones, J Vat Sci Technol, B9, 1449 (1991). “S J Pearton, IJ K Chakrabarti, A Katz, A P Perley and W S Hobson, J Vat Sci Technol, B9, 1421 (1991). 5J M Villalvilla, C Santos and J A VallCs-Abarca, Vacuum, 39,683 (1989). 6A M Barklund and H 0 Blom, J Vat Sci Technol, AlO, 1212 (1992). ‘H Okano and Y Horiike, Jap J Appl Phys, 20,2429 (1981). “D Ugolini, J Etle, P Oelhafen and M Witlmer, Appl Phys, A48, 549 (1989).