novel precursors for chemically assisted ion beam etching : reactions of dichloroethane on gaas...

Vacuum/volume 44/numbers 3/4/pages 249 to 256/1993 0042-207X/9356.00+.00 Printed in Great Britain O 1993 Pergamon Press Ltd

Novel precursors for chemical ly assisted ion beam etching: reactions of d ichloroethane on GaAs(100) D u n c a n Marshal l and Richard B Jackman, University College London, Electronic and Electrical Engineering, Torrington Place, London WCIE 7JE, UK

The potential advantages of chemically assisted ion beam etching (CAIBE) over reactive ion beam etching (RIBE) are considered for GaAs. The ideal form of etching precursor that CAIBE requires is discussed, with particular reference to use of the technique for in situ etching in an ultra-high vacuum environment. To investigate various candidates for this role requires a detailed insight into the microscopic surface reactivity of the molecular species concerned on GaAs and the effect of an impinging inert gas ion beam. A systematic study into the reactivity of alkyl halides on GaAs( l O0) has been undertaken through the use of in situ surface spectroscopic probes and this paper reports the first insight into the adsorption and beam induced behaviour of 1,2 dichloroethane. Dissociative chemisorption is seen to occur initially giving a bound CI layer and by-products which can be desorbed without leaving surface carbon. Higher exposures give rise to molecular physisorption. This form of behaviour is ideal for CAIBE purposes. However, argon beam bombardment can lead to some fragmentation of the molecular species. This effect is discussed in terms of the contamination that may arise during GaAs etching.

1. Introduction

Chemically assisted ion beam etching (CAIBE) offers distinct advantages over other forms of 'dry' etching techniques based upon beams. This form of etching relies on an ion beam of an essentially inert nature impinging on an adsorbate covered surface ; the adsorbate phase is 'activated' in some way by the ion beam, gives rise to the formation of volatile products containing atoms from the solid concerned and hence etching is achieved. This activation may take the form of electronic/vibrational exci- tation of surface species or momentum transfer giving rise to mixing of the adsorbate and substrate species. Any of these processes may lead to enhanced chemistry ~. Simple physical sputtering of the products of a spontaneous chemical reaction between the adsorbate and the substrate material can also occur. The adsorbate is replenished by a second, molecular beam of the appropriate gas. The chemical component of a CAIBE experiment is vital if the highly damaging effects, both structural and electrical, of a purely physical sputtering process are to be avoided 2. Reactive ion beam etching (RIBE) combines the chemical and physical components by employing a fast ion beam of a reactive nature. This method, however, suffers from the direct flow of reactive species through the ion beam source concerned.

This paper considers the potential advantages of CIABE over RIBE for in situ GaAs etching in an ultra-high vacuum (uhv) environment and the ideal form of etching precursor that CAIBE requires. To investigate various candidates for this role requires a detailed insight into the microscopic surface reactivity of the molecular species concerned on GaAs and the effect of an impinging inert gas ion beam. However, the surface chemistry of alkyl halide species, that are of particular interest, has not been previously studied in any detail. We have undertaken a systematic study into the reactivity of this class of compounds on GaAs(100)

through the use of in situ surface spectroscopic probes. This paper reports the first insight into the adsorption and argon beam induced behaviour of 1,2 dichloroethane.

2. CAIBE and etching precursor selection

The primary advantages that may be envisaged for CAIBE are :

(i) the ion beam source lifetime may be considerably extended over RIBE, since exposure to reactive species is avoided ;

(ii) the ion beam may be highly focused if liquid metal ion source technology is employed, enabling a maskless etching reaction to be envisaged 3-6 ; and

(iii) 'layer-by-layer' etching control may be envisaged through the synchronized modulation of reactant gas and ion beam SOUrCes 7.

All of these advantages are particularly valuable in any in situ device fabrication environment where a material that is vulnerable to damage and environmental contamination (such as GaAs) is grown and processed completely within an ultra- high vacuum (uhv) system 8.

Two significant disadvantages exist for CAIBE. The first is the additional experimental complexity of two separate gas fluxes on the processing chamber. To minimize the effect this has on the etching reproducibility would suggest that a precursor should be used whose extent of reaction with the surface self limits; precise control of the flux ratio of the two beams will then not be necessary. The second problem is the inherent reactivity required of the etchant gas. In a RIBE experiment the etchant (which itself can be inert) is fragmented in the ion source and it is the resultant fast ionic and radical species which are believed to be responsible for the surface reaction which occurs 9. No such fragmentation occurs in CAIBE; the etchant gas must display spontaneous

249

D Marshal l and R B Jackman : Novel precursors for CAI B E

reactivity with the substrate concerned. This has, to date, meant that highly corrosive gases such as chlorine have been almost exclusively utilized in CAIBE reactions ~° ~2. Such gases are highly antagonistic to vacuum chambers and gas handling sys- tems and the GaAs itself.

The reactivity of the GaAs/chlorine system has been widely studied ~ k ~ 3 ~ 7. Tyrrell et al showed that spontaneous reactivity in this system gives rise to the formation of strongly chemisorbed phases both in the sub-monolayer and multilayer adsorption regimes ~3. Figure 1 shows a schematic representation of the adsorption of chlorine on GaAs(100) (4x 1), along with the thermally promoted desorption products that occur at three surface coverages, sub-monolayer, post-monolayer and multilayer. At the lowest of these coverages, a strongly held chlorine phase is formed which is bound to the terminating Ga species. Thermal desorption of this phase gives rise to Ga monochloride. Thermal etching can only be driven at this stage by the loss of As2 from the GaAs lattice to maintain surface stoichiometry. As the coverage increases, a mixed Ga/As/C1 surface phase forms which thermally desorbs as GaCI3 and As2. At higher coverages, diffusion of chlorine within this 'corrosion' phase is hindered and the effective C1 abundance increases; the thermally promoted desorption products now become AsCI3, GaCI3 and Clz. The AsC13 species desorbs at temperatures below room temperature. With regard to the choice of chlorine as a CAIBE precursor for in situ processing, it is apparent that spontaneous surface reactivity will give rise to non-stoichiometric material in the outermost few GaAs layers in the absence of any beam effects. Devices which rely upon the electronic quality of any interfaces subsequently fabricated will therefore be degraded. Furthermore, the efficiency of any inert gas ion induced etching reaction will be highly dependent upon the exact surface coverage which exists, since the nature of the adsorbed layer is so strongly coverage sensitive. This, in

(a) sub-monolayer

',b) post monolayer

C1 GaCl 3 As2

\ / /

(c) mu l t i l aye r

Clo GaC1 C1 AsC13

Figure 1. Schematic representation of adsorbtion of chlorine on GaAs(lO0), along with the thermally promoted desorption products, at three surface coverages (a) sub-monolayer, (b) post-monolayer and (c) multilayer.

250

turn, implies that the reaction will be highly sensitive to the ion beam/molecular beam flux ratio being used. It is likely that this effect is the origin of the controversy in the literature ~ o.~ 7 over the products of a chlorine CAIBE reaction and the GaAs etching rate.

The ideal precursor for a CAIBE process for in situ processing may therefore be one which displays spontaneous reactivity with the semiconductor surface which is limited to a monolayer, further exposure giving rise to the simple physisorption of molecular species. It may be envisaged that such a reactivity pattern will severely limit the etching rate achieved. However, it has been shown that under typical CAIBE conditions, the locality in which the inert beam is incident upon displays a steady-state con- centration of reactive species of the order of a monolayer ' 3 and hence this restriction may not give rise to a reduction in the observed etching rate.

To reduce the surface reactivity of the etchant with GaAs, the alkyl halide and chlorofluorocarbon (CFC) families of compounds can be considered. Whilst CFCs are widely used in plasma, RIE and RIBE they are known to display negligible sticking probability at room temperature as molecular species ~ 9 On clean surfaces (as will be present in an uhv in situ etching application) carbon tetra-chloride has been observed to display weak dissociative adsorption on silicon 2°, but significant quan- tities of carbon are left on the surface following thermal etching. In addition, all of these compounds are due to be phased out of commercial use by the year 2000 under the revised Montreal protocol to limit the environmental damage they are known to cause 2~. 1,2 Dichloroethane (CH2C1CH2CI) is not thought to be as environmentally destructive. Adsorption studies of metal alkyls which contain a C C bond have revealed 'elimination' reactions taking place and the desorption of unsaturated hydro- carbons 22. It may therefore be supposed that the presence of a C C bond in an etching precursor may enable the removal of surface carbon to take place. The only previous study of 1,2 dichloroethane adsorption 23 involved multilayer condensation on liquid nitrogen cooled Ge. Monolayer coverages were not investigated. For these reasons, the surface reactions of dichloroethane on GaAs have been studied in some detail using the in situ surface spectroscopic methods of thermal desorption spectroscopy (TDS), Auger electron spectroscopy (AES) and low energy electron diffraction (LEED) ; the results are presented and discussed below.

3. The reactions of 1,2 dichloroethane of GaAs(100)

3.1. Experimental. All experiments described here were carried out in a stainless steel uhv system consisting of an analytical chamber equipped with electron optics (for AES and LEED measurements) and a second chamber for adsorption/desorption experiments; this region could be operated between uhv and several mtorr lbr simulation of real etching conditions. The system was equipped with a 1-300 amu mass spectrometer, allowing thermal desorption spectroscopy (TDS) to be carried out in addition residual gas analysis. The GaAs sample received no treatment prior to insertion into the vacuum chamber and system bakeout. Before experiments commenced, the GaAs was argon bombarded to remove surface oxides and then annealed using slow heating cycles to 780 K. AES and LEED analysis showed this treatment gave a contamination free reconstructed GaAs(100) (4x 1) surface. 1,2 Dichloroethane (BDH Ltd, 99.8%) was purified with several freeze-pump-thaw cycles and

D Marshal l and R B Jackman : N o v e l p r e c u r s o r s f o r C A I B E

was introduced into the vacuum chamber as a vapour by means of a directed dosing tube. Argon bombardment was by means of an Ion Tech fast a tom beam source operating at an energy of 2.5 keV, with a neutral beam flux of around 1 mA c m - 2.

3.2. Low temperature adsorption. Clean GaAs(100) was exposed between 0.1-200 L (1 L = l0 -6 torr s) of dichloroethane at a surface temperature of 100 K. Thermal desorption spectra were then moni tored for mass values corresponding to CxHyCI~ (x,z = 0-2, y = 0-4), GaClx (x = 0-3), AsCl~ (x = 0-3), AsH~ (x = 0-3), CHx (x = 1-3) and Ga(CxHy)z (x = 0-2, y = 0-3, z = 0-3). Figure 2 reveals T D spectra recorded for C2H4C12, plotted as a function of increasing exposure of dichloroethane to the GaAs surface. Two peaks are apparent ; the higher temperature peak (160 K) is seen to populate initially with the second, lower temperature peak (120 K) occurring as the exposure increases. The area under a peak in a TDS experiment can be used as an indication of the relative populat ion of that state ; uptake curves derived on this basis for these two peaks are presented in Figure 3(a). The higher temperature,/3, peak is seen to saturate at exposures of around 3 L ; the lower temperature, c~, peak continues to grow in intensity without apparent limit once it begins to populate. This behaviour is typical for a system displaying two distinct adsorption regimes 24, and the/3 state can be ascribed as a monolayer phase, whilst the ct state indicates the presence of a multilayer phase.

( a ) 500

~:~ 4 0 0

C u) a) .~- 3 0 0 . ~ t I

¢1 "~ 2 0 0

L. 1 0 0

Q- 0 0

o

co.

i 2 3 Exposure, L

o {I

(b) loo-

> , ~ . 8 0 "~ ~

~ " I 6 0

4 0 • p

- - . ~ 20 ~

0 1 0 0 2 0 0

Exposure, L

Figure 3. Uptake curves derived on the basis of the area under TDS peaks for (a) 98 amu (C2HaClf) fragment (the higher temperature of the two peaks has been labelled fl, whilst the lower temperature peak has been labelled ~) ; and (b) 104 amu (GaC1 ÷) fragment (labelled fl).

98 amu

I (1.6L) (1.OL) (0.8L)

i

(0.6L)

(0.4L)

(0.3L)

;300K -SQQK 730K TEMPERATURE

Figure 2. Thermal desorption spectra for detection of 98 amu (C2H4C12 +) plotted as a function of increasing exposure of GaAs(100) to 1,2 dichloroethane (expressed in Langmuirs).

All other CxHyClz (x,z = 0-2, y = 0-4) species detected showed qualitatively similar behaviour. Cracking pattern analysis for the CxHyCI= (~) state shows a good match of all species to the expected ratio due to ion source fragmentation of the parent molecule in the mass spectrometer. A reasonable fit is also apparent for the CxHy C1 z (fl) state, but the intensity for the C2H4C1, C 2H 3C1 and CH 2C1 appear rather high. This effect is highlighted in Table 1. This suggests the desorption products from the CxHy Clz (/3) state are predominantly C 2H 4C12, but contain some intensity due to fragmented species prior to entry into the analysing mass spectrometer. These may be a mixture of C2H4C1, C2H3C1 and CH2C1. However, the additional C2H3C1 and CH2C1 intensity may arise as a consequence as ion source fragmentation of the C2H4C1 species in the mass spectrometer.

Intriguingly, distinctly different TD spectra were obtained for the mass relating to GaCI. Typical spectra, plotted as a function of increasing exposure of the GaAs surface to dichloroethane, are presented in Figure 4, with Figure 3(b) showing the uptake

Table 1. The relative intensities of the desorbing components of 1,2 dichloroethane from GaAs. The alpha and beta states correspond to the physisorbed and chemisorbed states, respectively. The values for the unreacted molecule were derived experimentally from residual gas analysis measurements

Mass fragment (amu) 98 62 49 35 27 14 13

Relative Intensity Unreacted molecule 1.0 25.2 10.1 2.6 35.0 2.9 1.2 ct state 1.0 25.7 9.9 2.7 35.1 2.8 1.1 fl state 1.0 52.3 26.8 7.3 97 7.4 2.8

251

D Marsha l landR BJackman : Novel precursors for CAIBE

1 0 4 a m u

(80L) J~ ( 1 5 L )

( 3 , 2 L )

Z:~ (OIBL) j ~IiI[Ii ~

(0.3L) ~,~~ I100K 300K 500K OOK

I

TEMPERATURE

Figure 4. Thermal desorption spectra for detection of 104 amu (GaCI +) plotted as a function of increasing exposure of GaAs(100) to 1,2 dichloroethane (expressed in Langmuirs).

curve derived on the basis of the areas of the peaks seen. It is apparent that GaC1 shows a single peak at very high temperatures (620 K) whose intensity saturates at around 3 L. The coincident nature of the saturation exposures between the CzHyC1 z fl peak and the GaCI peak indicates that they arise from a monolayer phase on the GaAs surface. No TD spectra could be recorded at mass units responding to GaClx (x = 2, 3), AsHy (x = 2, 6) and Ga(C~Hy)z (x = 0-2, y = 0-3, z = 0-3). Extremely weak spectra were detected at masses 75 and 76; these could relate to As and AsH. The peak temperature of both masses was coincident at 230 K and peak saturation occurs at around 3 L. AES analysis of the surface region following desorption of the and # CzHxCly phases revealed only peaks at 31 and 55 eV, indicative of As(MNN) and Ga(MVV) Auger transitions. No carbon (KLL) signal was seen, indicating the presence of a clean, carbon free, surface within the detection limits of the AES technique (around 1% of a monolayer).

3.3. Argon bombardment of dichloroethane/GaAs(100). Fol- lowing surface preparation and adsorption at 100 K, described above, a series of experiments was performed where various degrees of argon neutral beam bombardment were carried out prior to recording TD spectra. The effect of this bombardment on the GaC1 species seen during a TDS experiment is revealed in Figure 5 where the GaCI TDS yield is plotted as a function of increasing exposure to the argon beam for a fixed exposure of 1,2 dichloroethane to the GaAs sample. No new peaks arise in the spectra, but the GaC1 peak temperature decreases to around 510 K. Intriguingly, the peak intensity initially rises with bombardment, followed by a gradual reduction. This is clearly shown in the plot shown in Figure 6(a). This is in sharp contrast to the effect of bombardment on the dichloroethane signal due to both

and # derived from a TDS experiment which is rapidly lost

TEMPERATURE

Figure 5. Thermal desorption spectra relating to 104 amu (GaCI +) for a GaAs(100) surface exposed to a constant amount of 1,2 dichloroethane followed by varying exposures to a 2.5 keV argon beam (exposures given as atoms per m2).

after a bombardment time equivalent to only the first trace for GaCI in Figure 5; this is revealed in Figure 7 and plotted in Figure 6(b). TD spectra monitoring mass units 75 and 76 following argon bombardment also show this rapid loss. A new signal is seen to arise, however, following bombardment, at a mass 13, which relates to CH. Figure 8 reveals TD spectra for this species plotted as a function of increasing argon bombardment, again for a fixed prior exposure of dichloroethane to the GaAs. Figure 6(c) reveals the area under the high temperature this peak plotted against the argon bombardment carried out prior to the TDS experiment. In a similar manner to the GaC1 signal, the CH peak area is seen to initially rise prior to slowly decreasing to zero.

3.4. Discussion

3.4.1. Adsorption/desporption processes. The interaction of dichlorocthane with a GaAs(100) (4x 1) surface gives rise to chemisorption; the TDS peaks apparent for GaCl and the C~H,.Clz (fl) phases are broad, symmetric and display a slight shift to lower temperatures as coverage increases. Such behaviour is indicative of a second order desorption reaction 24. Application of the Redhead equation 25, assuming a value ofv of 1013 s l (ref 24), suggests binding energies for these adsorbed states leading to these desorbing products as 160 and 40 kJ mol 1 respectively. The GaC1 peak energy is coincident with that observed in the CI2/GaAs(100) system ~3. By contrast the C2H4C12 (~) peak is

252

O Marshall and R B Jackman : Novel precursors for CAI B E

(a) 2oo~

~- .'~'- 100

,4

~ ' ~ 0 a . 0 e + 0 l e + 1 6 2 e + 1 6

Ar exposure (atoms/cm 2j

• p

(b) 200-

~ . . ~ 150: '~ ~

e- 10o-

i

,4 s0-

0.Oe+0 1 . 0 e + 1 5

Ar exposure

2 . 0 e + 1 5

( a t o m s / c m 2)

¢+p

(e) so-

6 0

e -~_ e- 4 0

• p i

,4 20.

a. 0- 0 . 0 e + 0 2 . 0 e + 1 5 4 . 0 e + 1 5

2

Ar exposure ( a t o m s / c m ) Figure 6. Desorption yield, based on the area under the TDS peaks achieved, plotted as a function of increasing exposure of an adsorbed layer to a 2.5 keV argon beam (a) 104 amu (GaCI+); (b) 98 amu (C2H4Cl~) ; and (c) 13 amu (CH+).

<

F--

Z

z_

OOK

98 amu

I.OL) O.OE !

1. O.L). O~2EJ

300K TEMPERATURE

500K 7 )OK

Figure 7. Thermal desorption spectra relating to 98 amu (C2H4C1~-) for a GaAs(100) surface exposed to a constant amount of 1,2 dichloroethane followed by varying exposures to a 2.5 keV argon beam (exposures given as atoms per m2).

13 amu

/\]~ak, 4.8L O.OE15

=<

3.2E15

100K 300K 500K -OOK TEMPERATURE

Figure 8. Thermal desorption spectra relating to 13 amu (CH +) for a GaAs(100) surface exposed to a constant amount of 1,2 dichloroethane followed by varying exposures to a 2.5 keV argon beam (exposures given as atoms per m2).

sharp with a rapid drop to zero after the peak temperature is reached ; such behaviour is indicative of zero or fractional order desorption kinetics. The binding energy for this phase is around 30 kJ mo1-1. This is close to the energy for sublimation of 1,2 dichloroethane [33 kJ mol -~ (ref 23)] and hence this peak rep- resents a weakly held molecular, physisorbed state. The uptake characteristics are commensurate with the presence of a chemisorbed monolayer phase at low exposures (< 3 L) and a molecular multilayer phase physisorbed upon this at higher exposures ; the initial sticking coefficient at 100 K is calculated to be unity. The most likely desorption products from the CxHyCI: (fl) state are C2H4C12, C2H4C1 and C2H3C1. GaC1 is also seen to desorb from this monolayer phase, but no GaClx (x > 1) species are found. The coincident nature of the desorption temperature of this GaC1 phase with that observed following sub-monolayer adsorption of chlorine on GaAs(100) (4× 1) ~ indicates that dissociative adsorption of dichloroethane gives rise to free Cl species which then react with the terminating Ga layer. Thermal etching may therefore proceed in this system in an analogous way to the chlorine/GaAs system if the coverage was always less than a monolayer. The peaks at 75 and 76 may be due to AsH. However, they are extremely weak and organic impurities in the dichloroethane dosing gas (such as benzene) may give rise to signals at these mass units. It is therefore not possible to unam- biguously assign them from the data presented here. A schematic representation of the likely adsorption processes as a function of coverage is shown in Figure 9.

Of considerable importance is the complete removal o f all surface carbon through the desorption processes which occur, as indicated by the AES measurements performed. Murrell et a122 have previously observed fl-hydride elimination following the adsorption of metal alkyls containing C-C bonds. Such a process may be occurring here. Dissociative adsorption occurs on the

253

D Marshal l and R B Jackman : Novel precursors for CAI B E

(a)

C2H4C12

C2HxCly GaC1 ( 1 6 O K ) ( 6 2 0 K )

/ /

(b)

C2H4Cl 2

\ / C2H4C 12 (12OK)

Figure 9. Schematic representation of adsorption of 1,2 dichloroethane on GaAs(100), along with the thermally promoted desorption products, at two surface coverages (a) sub-monolayer and (b) multilayer.

clean GaAs surface giving rise to CI and C2H4C1 bound species ; the latter then undergoes fl-hydride elimination and C2H3C1 (chloroethene) is desorbed. Alternatively, the second C1 moiety may break from the molecular adsorbate and bond to the surface; ethene (C2H4) would now be the desorption product. The former process is favoured since no signal directly attributable to C 2 H 4 w a s observed. The C2H4CI formed could also desorb without further loss; it is a mixture of this and the process described above that is most likely to be the origin of the mixture of products ascribed to the CxHyCI~ (fl) phase. However, the data presented here are difficult to assign with total confidence since the measured desorption yield at a given mass contains con- tributions from all of these possible sources.

3.4.2. Argon beam induced processes. The most immediate effect of an impacting 2.5 keV argon beam is the removal of all weakly held, physisorbed, species. The cross-section for this process, which is greater than 1 × 10-14 cm 2 indicates the rapid nature of this erosion. Thus, the C2H4C12 (c~) signal is lost under the shortest bombardment carried out. The effect of the beam on the GaC1 signal is rather different; the initial increase in signal intensity must indicate that more free C1 is available for bonding with Ga species (and hence TDS reveals more GaC1 desorbing). This must occur through the argon beam induced decomposition of some of the CxHyClz (fl) species prior to the loss of all of this phase from the surface. Coincident with this increase is the appearance, and growth of, a TDS signal due to CH. Intriguingly, no CHx (x > 1) products were observed. This indicates that the beam is causing further disintegration of some of the CxHyClz (fi) species prior to loss. Any stable fragments from this decomposition (such as CH4 etc.) would be expected to physisorb ; further bombardment from the argon beam would lead to their rapid loss. However, any free C, H and CI moieties created will be mixed into the selvedge by the energetic beam. The mixing depth expected for a 2.5 keV energy beam is around 100 nm. The

254

(a)

A? C2H4C ] 2 \ /

(b)

Desorbs as GaCI (620K), CH (70OK)

, m ix ing I depth _T(100nm)

Figure 10. Schematic representation of the argon beam (2.5 keV) induced processes following adsorption of 1,2 dichloroethane on GaAs(100) at multilayer coverages. (a) Initial beam induced desorption of molecular species accompanied by some molecular fragmentation ; and (b) mixing into selvedge of C1 and C.

surface material will therefore become highly dispersed. As the temperature of the GaAs is increased during TDS, some of these free species will migrate and collide with lattice/other con- taminant species ; reaction can occur and the resultant GaCI and CH species are thermally diffused to the surface and lost. This form of reaction explains the decrease in the GaCI peak desorption temperature observed (700-510 K). Prior to exposure energy is required to free the C1-Ga entity from the underlying lattice, following bombardment the mixing of the beam has already provided this; the energy that is required now is for thermal migration through the selvedge. The likelihood of further col- lisions before loss and hence the desorption of CHx, GaClx (x > 1) is small. Thus, argon beam bombardment results in the fragmentation of a proportion of the molecular material present and the dissolution of all atomic species into the material selvedge. The GaAs sub-surface region will thus become con- taminated by exposure to the argon beam. The ultimate loss of GaC1 and CH signals is presumably due to sputter erosion ; the cross-section for this process is 5 × 10 -6 cm 2 These processes are schematically summarized in Figure 10.

4. Implications for etching with dichloroethane

4.1. Thermal etching. The spontaneous reactivity of 1,2 dichloroethane with GaAs(100) conforms to the ideal set out in Section 2 above; dissociative adsorption gives rise to CI species which can actively 'etch' the GaAs. The by-products are all adequately stable such that they desorb at temperatures below room temperature. Thus, in any 'real' etching environment the surface will be covered with a monolayer of C1. Thermal etching could be driven by heating the GaAs and should lead to a con- taminationfree process. The desorption temperature of the GaC1 (700 K) will rate limit the process (AsClx desorbs below room temperature J 3). However, this temperature is high given the total thermal budget permissible for GaAs device processing. Clean thermal etching may be possible with 1,2 dichloroethane if the heat source is highly localized ; such is the situation when a laser is utilized which may be pre-patterned or highly focused 2°. In such a case low contamination, high control GaAs etching may

D Marshall and R B Jackman : Novel precursors for CAI B E

be possible and the use of photolithography for the delineation of surface patterns may be unnecessary ; an excellent method for in situ etching may therefore be achieved.

4.2. CAIBE. The limit to spontaneous reactivity required in Sec- tion 2 above has been achieved. The majority of any physisorbed material initially present during an CAIBE experiment will be desorbed intact by the incoming argon beam. The exact ratio of the inert gas ion beam and the molecular beam of 1,2 dichloroethane in any CAIBE experiment will not critically determine the etching rate and a reproducible process should result. 1,2 Dichloroethane is relatix~ely inert and will be easily handled in an uhv environment. However, sufficient reactivity will be apparent to achieve etching at a reasonable rate given that around a monolayer of C1 is 'donated' by 1,2 dichloroethane to the surface. We have previously identified ~3'26 the primary route to inert beam enhancement of etching rate in the chlorine/GaAs system as due to ion-assisted adsorption and reaction with fur ther impinging reactive gas to create more volatile species which are then desorbed, i.e.

GaCl(ads) --* GaCI* (ads) (1)

GaCl*(ads) + { ( x - 1)/2}C12 ~ GaCl*(ads) (2)

GaCl*(ads) ~ GaClx(g). (3)

It may be supposed that such a reactivity pattern here would lead to a similar enhancement in etching rate. However, the beam does lead to some fragmentation of the molecular species and contamination. In device applications, where low levels of contamination will lead to critical loss of device performance, CAIBE with dichloroethane and a 2.5 keV argon beam will not be appropriate. The effect of this contamination may be minimized if the argon beam energy is considerably reduced. Firstly, reduction of the mixing depth will limit the penetration of the contamination into the GaAs. Secondly, if the beam energy is reduced to tens of electronvolts then the ratio of desorbed to fragmented by- product may be significantly increased since the energy of the chemical bonds present in the molecular forms will be a few electronvolts, whilst the physisorption bond is only a fraction of this energy. A similar effect may be possible by increasing the mismatch of the mass of the incoming ion to the adsorbed phase.

5. Summary

The advantages of CAIBE over RIBE have been considered; these are based upon the inert nature of the ion beam increasing source life and enabling greater source control and focusing. Maskless, highly localized CAIBE can be envisaged through the use of Ga ÷ ion sources. CAIBE has been almost exclusively carried out with chlorine to date. This is related to the spontaneous reactivity that is required of the CAIBE gas used, since no prior activation or fragmentation is employed before the gas collides with the surface to be etched. In the case of uhv based in situ processing, where the presence of atomically cleaned surfaces can be anticipated, this may be achieved with other compounds such as alkyl halides and the corrosive nature of the pure halogen could then be avoided. The reactivity pattern that would be achieved has been investigated using surface spectroscopic probes.

The adsorption of 1,2 dichloroethane on GaAs(100) (4 x 1) at 100 K has been characterized using thermal desorption spec-

troscopy (TDS) and Auger electron spectroscopy (AES). Whilst this liquid can be routinely handled as a relatively inert species, on GaAs dissociative adsorption occurs giving rise to a bound C1 monolayer and C2HyClz (y < 4, z < 2). The C2Hyflz species are readily desorbed below room temperature. The C1 layer desorbs from the Ga terminated GaAs surface as GaCI at 700 K in an analogous way to the desorption of GaC1 from sub- monolayer chlorine coverages on GaAs. Exposure to further 1,2 dichloroethane gives rise to a physisorbed molecular overlayer, which continues to grow without apparent limit and which rep- resents simple condensation. No free carbon species are formed during this process and continued adsorption/desorption cycles can be carried out without creating carbon contamination on the GaAs. This form of behaviour may be desirable for reproducible, clean, thermal GaAs etching, particularly maskless in situ laser photothermal etching.

This spontaneous reactivity pattern is also ideal for a CAIBE process where self-limiting reactivity in the absence of the inert gas ion beam will reduce the effect of fluctuations in the ion beam/molecular beam flux ratio; a more reproducible process should result. The distinct reduction in corrosive action of 1,2 dichloroethane over chlorine is also an important advantage in CAIBE, especially when in situ processing in an uhv environment is being considered. However, the 2.5 keV argon beam utilized here has been found to decompose a fraction of the alkyl halide species prior to desorption; this leads to carbon contamination being 'mixed' into the GaAs sub-surface region. Whilst the full extent of such contamination is not clear from the work carried out here, this effect is likely to limit the usefulness of this precursor for CAIBE. It may be possible to reduce it by manipulating the beam energy and the mass of the inert ion concerned. Both of these effects will vary the extent to which the energy of the beam couples with the adsorbate phase compared with the GaAs selvedge.

Acknowledgements

The authors gratefully acknowledge the Science and Engineering Research Council (SERC) and The Royal Society for partial financial support for this work. One of us (DM) also thanks the SERC and the Royal Signals and Radar Establishment (RSRE), Electronics Division, Defence Research Agency, Malvern for the award of a CASE studentship.

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