MICROELECTRONIC ENGINEEFIING

Microelectronic Engineering 25 (1994) 287-292

DRY ETCHING TECHNIQUES FOR GaAs ULTRA-HIGH VACUUM CHAMBER INTEGRATED PROCESSING

Duncan Marshall and Richard B. Jackman

Electronic and Electrical Engineering, University College London, Torrington Place, London, WClE 7JE, UK.

Abstract

In-situ processing requires totally dry approaches to pattern generation to be developed. Of particular interest are beam driven reactions that can be local&d without prior masking. This paper considers types of beams that may be utilised and the means by which these beams may drive chemical etching reactions. An ideal form of beam assisted reaction is proposed which requires an etching precursor which displays very different adsorption - desorption characteristics from the most commonly encountered etchant, chlorine. In an attempt to identify improved precursor compounds, the adsorption characteristics of dichloroethane and sulphuryl dichloride on GaAs(lOO) are reported and discussed; these are considered with regard to their use as chemically assisted ion beam and laser photothermal etching gases.

1. Introduction

Diversification of integrated circuit (IC) designs, the demand for responsive and flexible manufacturing of ICs and the high capital cost of a modem fully automated “batch” production facility for ICs can all be cited as reasons for the emergence of single wafer processing [l]. Once the practice of completing each processing step on single wafers has been established (as opposed to completing each of the stages of each process on whole batches of wafers) then the physical location of the IC processing tools becomes important. It is clearly more efficient to cluster the tools required to complete each processing step together to reduce wafer transportation time and minim& external contamination. Such cluster tools, which are often housed on a single vacuum chamber, are increasingly used in Si IC manufacture [3]. In the limit, a single multi-chamber vacuum system equipped with all of the processing tools may be used to completely fabricate devices on a single wafer; this would further reduce the processing time required by eradicating the cleaning steps required as the wafer is passed between processing cluster tools [2]. For III-V device fabrication, the case for such an approach based around an ultra-high vacuum (UHV) system is particularly attractive. The potential level of performance from GaAs devices is often not real&d due to interfacial contamination arising from current processing methods and as device dimensions are reduced the effect of a surface “dead layer” becomes increasingly more important. Furthermore, many modem III-V growth methods (such as MBE, MOMBE and CBE) already utilise UHV conditions; maintaining this environment throughout post growth processing should enable the device engineer to fabricate well character&d high performance devices.

Patterning the UHV grown semiconductor layers is an essential step in device manufacture which conventionally entails the use of photolithography and “wet” chemistry; this approach is precluded by the use of a UHV environment and either dry lithography or maskless pattern generation must be used. In the former category, progress in depositing a dry layer which is modified by focused electron or ion beam exposure is encouraging [4]. However, the removal of the processing steps associated with masking would be desirable and in this context the direct patterning of the surface through the use of projected or focused beams is of interest. An ideal beam assisted etching reaction will offer high spatial resolution whilst not imparting significant damage to the selvedge of the semiconductor; nor should it lead to interfacial contamination. The smaller the device feature to be formed the more harmful such effects will be. Thus, it is important to consider which types of beams may be deployed to best effect.

Elsevier Science B.V. SSDI 0167-9317(94)00027-R

2. Choosing a beam for maskless in-vacua etching

Beams consisting of ionic or neutral particles, photons, electrons and atomic or molecular fluxes, covering a wide range of energies, can all be considered as candidates for directly patterning our III-V semiconductor. Whilst ionic or neutral particle beams typically have enough momentum to cause direct sputtering, incorporating chemically reactive species to assist the sputter removal process can considerably enhance the etch rate and reduce the damage that is implanted in the selvedge of the III-V material [5]. The use of highly focused ion beam (FIB) sources (<50nm) can then enable localised pattern generation to be achieved [6]. Since currently available FIB sources produce essentially inert ionic species, the chemically active species must be simultaneously provided to the surface to be etched; this experimental arrangement is typically known as chemically assisted ion beam etching (CAIBE) [7]. Since the FIB has directed momentum, excellent anisotropic control over the features generated can be achieved. However, the realisation of a fully patterned wafer entails scanning the FIB over comparatively large areas. This slow process will limit the usefulness of the FIB to processing a particular feature that is required in small numbers on the wafer, or to prototyping and customising operations. Furthermore many studies reveal that a material such as GaAs is heavily damaged by the impact of an FIB that typically operates at energies up to SOkeV and that much of this damage cannot be removed by annealing [8]. The development of ultra-low energy FIB sources (<lOeV, the sputter threshold for GaAs) and sources that can be projection patterned would be highly desirable.

Lasers can provide power densities that are high enough to “ablate” a semiconductor. Whilst the rate of material removal through this process can be very high, material redeposition, selvedge damage and poor material selectivity all prevent the III-V device engineer from employing such an approach. Photochemical dissociation of a chemical species by the laser light can lead to an essentially damage free etching process. The use of a focused or a projection patterned light source can then lead to single step patterning of the semiconductor [9]. However, gas phase diffusion of the activated species away from the irradiated region leads to isotropic etching. More useful is the promotion of a photothermal effect in the III-V by the laser light whilst in the presence of a chemically reactive species. If a short pulse laser is used (such as an excimer laser operating at 248nm with 15ns pulses) then a very shallow region of the surface can be heated for long enough to promote the desired chemical etching reaction without widespread degradation of the GaAs wafer; high etching rates can be achieved with precise depth control (- one monolayer per laser pulse) and extremely low levels of selvedge damage [lo]. In the case of a projection patterned excimer laser beam, sub-micron features can be formed over macroscopic areas (- lcm2) in a single exposure; complete wafer patterning is now viable. Geometric control over the feature shape is less easy to exercise than in the case of a FIB, with the profile of the etched structures often being “U” shaped. Future developments in the field of laser optics and phase shift masking may offer improved geometric control with a resolution capability of around 0.2nm. [lo]

Electron beams can be created at significantly lower cost than FIB and laser sources. They do not possess adequate momentum to cause direct atomic ejection from a III-V compound. In the presence of a chemically reactive species, electron stimulated desorption of reaction products can occur and etching can result [ 111. However, the absence of directed momentum and the requirement for scanning a focused beam to achieve patterning would appear to combine the weaknesses of FIB and laser sources for in-vacua III-V maskless etching. Electron impact can also lead to significant electrically active defect creation in the III-V material. “Hot” molecular or atomic gas fluxes can be used to cause III-V semiconductor etching where the flux strikes the surface. Ono et al [ 121 have etched GaAs with a hot chlorine beam and achieved an aspect ratio of 1O:l when an Si02 mask layer was used. However, in a maskless etching environment scattering of the reactive flux is likely to lead to a loss of anisotropic capability. The use of chlorine, which displays spontaneous reactivity with GaAs (although not spontaneous etching), is also undesirable. This is discussed further below.

D. Marshall, R.B. Jackman I Dry etching techniques 289

3. Novel Precursors for maskless in-vacua GaAs etching

The “chemical assistance” referred to in section 2 is often achieved by introducing chlorine gas to the beam etching chamber. CAIBE and photothermal laser etching require a chemical precursor which spontaneously adsorbs on the surface to be etched. However, in a maskless in- situ etching environment, the extent to which the etchant gas reacts with the ti GaAs surface beyond the region to be etched will degrade any interfaces subsequently formed in this region. It is well documented that molecular chlorine reacts with GaAs at elevated temperatures to form volatile Ga and As chlorides [13,14]. We have studied the temperature dependent reactions of chlorine on G&s at a molecular level with various surface spectroscopic probes [ 151; it is apparent that clean GaAs surfaces at room temperature will be considerably perturbed by the presence of a multi-layer “corrosion” phase, consisting of Cl, Ga and As species when chlorine is admitted during etching. An etching gas which displays reduced reactivity is therefore required. The reactivity of the halogen gas can be reduced by use of an alkyl halide; the presence of a single carbon group in a species such as CC14, however, may give rise to a considerable surface carbon residue [ 161. We have therefore undertaken a systematic study of the surface chemistry of a range of halogens and halogenated compounds in order to assess the extent of the problem and to attempt to propose a solution.

In an attempt to modify the reactivity of chlorine with GaAs, whilst offering a stable carbon containing by-product, a compound containing a C2 group was chosen; 1,Zdichloroethane (CH2ClCH2Cl) [17]. Dramatically different adsorption characteristics were observed; dissociative adsorption at sub-monolayer coverages was followed by molecular physisorption in the multilayer regime. The first monolayer may be desorbed as GaCl (with similar energetics to the case of molecular chlorine adsorption) and various C2HxC1 species. Importantly, the C- C bond in the dichloroethane is not cleaved and no residual carbon is found following repeated adsorption-desorption cycles. It is clear that the extent to which a clean GaAs surface is disrupted by the presence of the etching gas has now been considerably reduced. The effect of inert ion bombardment on the adsorbed phases present was also determined [17]. Additional molecular fragmentation occurred leading to enhanced Ga and As chloride formation, but C-C bond scission led to the incorporation of unwanted C species over the mixing depth of the inert gas ion beam. To reduce the likelihood of such contamination an etching gas which is likely to decompose to give rise to a more volatile by-product was sought.

The adsorption of sulphuryl dichloride (SO2C12) on GaAs has recently been studied. As an inorganic analogue of dichloroethane the possibility of carbon contamination is eliminated. Clean GaAs (100) (4x1) Ga terminated surfaces were prepared in a UHV system and the adsorption characteristics of SO2C12 were monitored by thermal desorption spectroscopy (TDS), low energy electron diffraction (LEED) and Auger electron spectroscopy (AES). Thermal desorption curves revealing the detection of GaCl and SO2 arc shown in figures 1 and 2 respectively. The mass spectrometer signal for each fragment is plotted against the GaAs temperature for a series of adsorption experiments as the SO2Cl2 exposure is increased. In the case of GaCl (figure 1) a single high temperature peak is seen which saturates in intensity at relatively low gas exposures. The energetics and uptake characteristics of this peak are similar to those for molecular chlorine adsorption and thus the presence of a strongly bound Cl species is implied. In contrast the SO2 spectra are more complex; three peaks are apparent at low temperatures with the highest temperature peak showing saturation in intensity at low gas exposures.

A detailed comparison of this data with spectra collected for the thermal desorption of SO2C12 reveals that the lower temperature peaks arise from the desorption of molecular SO2C12 whilst the higher temperature peak is due to SO2 itself. Thus, the SO2 which desorbs at the highest temperature, 370K, (Edes = 93 k.I/mol) can be described as the by-product of the dissociative chemisorption reaction which gives rise to the Cl (detected here as GaCl). The intermediate energy peak (Edes = 64kI/mol) derives from the first layer of physisorbed SO2C12 parent

290 D. Marshall, R.B. Jackmatl I Dry etching tcdwiq~ces

molecule on the previously reacted monolayer with the lowest energy peak (Edes = 39kJ/mol) being due to SO2C12 physisorbed multilayers. The measured adsorption behaviour of SO2C12 on GaAs( 100) is represented schematically in figure 3.

The effect of an argon ion beam (5OOeV) on adsorbed multilayers of SO2C12 highlights interesting differences in precursor behaviour that may be attributed to their surface reactions. Both Cl2 and C2H4Cl2 show a significant degree of ion beam induced reaction enhancement on GaAs; however, no similar effects can be discerned from analogous experiments using SO2C12 as a precursor. The primary effect of the argon beam appears to be the near instant sputter removal of physisorbed SO2C12 followed by the gradual erosion of the surface bound SO2 and GaCl layers. Considering the chlorinating nature often associated with SO2C12 this is indeed intruiging; however, in contrast with Cl2 and C2H4Cl2 the complete and self limiting reaction of SO2C12 on GaAs implies the presence of a ‘buffer layer’ between any new source of chlorine and unreacted GaAs. The cross section for SO2C12 removal by 5OOeV argon ions is

high (o = 9.10-l%m%on-1) and experimental data indicates that it is simply eroded in preference to fragmentation or mixing within the selvedge.

FIGURE 1. The thermal desorption spectra for detection of 104 amu (GaCl+) plotted as a function of increasing exposure of GaAs( 100) to sulphuryl dichloride.

FIGURE 2. The thermal desorption spectra for detection of 64 amu (SO2+) plotted as a function of increasing exposure of GaAs( 100) to sulphuryl dichloride.

120K

GaCl

520K 920K I 120K 520K 920K

4. Implications for maskless in-vacua GaAs etching

The choice of beam to drive the required patterning step is likely to depend upon the precise requirements for the device being created. For example, at pesent only FIB or electron beam sources offer the spatial resolution that certain GaAs quantum dot or wire devices are likely to demand. However, the projection patterning capability offered by laser sources is currently a powerful advantage for the device engineer wishing to pattern a full wafer during IC

D. Marshall, R.B. Jackman I Dry etching techniques 291

fabrication. The laser also offers the best prospects for ultra-low damage etching and the beam can be readily redirected into another chamber on the vacuum system for other processing steps. It should be noted that both FIB and photo-projection technologies are still demonstrating considerable improvements as time progresses; the issues involved in beam choice need regularly revisiting in the light of the developments that emerge.

FIGURE 3. A schematic representation of the adsorption of sulphuryl dichloride on GaAs(100) at surface cover-ages: (a) sub monolayer, (b) 2-3 monolayers, (c) multilayers

SO2 Ede93KJ/mol(370K)

It is apparent that chlorine is not an ideal gas for ultra-low contamination maskless in-situ beam assisted etching of clean GaAs. Under realistic etching conditions we would expect a corrosion layer to exist on all regions that would need removing before interfaces of high purity, and abruptness, could be fabricated. Furthermore, the corrosive nature of chlorine will lead to a persistent “memory” effect in the vacuum chamber used, limiting the scope for multi-tasking in this region of an in-situ fabrication system.

It is the limited reactivity of dichloroethane and sulphuryl dichloride towards GaAs that should enable them to be exploited in a future in-situ etching environment. Both of these precursors react to give a chlorinated overlayer without the formation of involatile by-products; on this basis a thermally driven etch may be envisaged. A knowledge of the heats of adsorption allows the surface lifetime of the various adsorbed species to be plotted against surface temperature (figure 4). It can be seen that the lifetimes of all species other than GaCl are very short at room temperature whilst elevated temperatures are required to remove the GaCl phase. Thus, at elevated temperatures, in the presence of a background pressure of sulphuryl dichloride or dichloroethane, GaCl desorption will enable further dissociative adsorption to occur and hence etching; this should lead to contamination free laser photothermal etching . Dichloroethane and sulphuryl dichloride are both transparent to excimer laser radiation (248nm) and have good vapour pressures (70 torr and >lOO torr respectively at 300K). They also do not appear on the list of substances due to be phased out by the Montreal protocol in the mid 1990’s for environmental protection [18]. Sulphuryl dichloride is more difficult to handle due to its propensity to react with water. Optimal etch rates may be achieved by using a high pulse repetition rate and ensuring a constant molecular flux in the etch region.

292 D. Marshall, R.B.

FIGURE 4. Mean surface lifetimes of etching precursors and

104

reaction fragments on GaAs, as a 10 2

function of temperature. 10 O

Jackman I Dry etching techniques

10-2

Surface lifetime lo 4

(seconds) 10-6

10-8

10 -lo

10 -12

loo 300 500 700 900 Temperature (K)

It is clear that FIB sources may be utilised with novel precursors to achieve enhanced etch rates; however, current data suggests that high precursor fluxes around the beam irradiated area (such that many adsorbed layers are present) may lead to unacceptable contamination. In a real etching environment these problems may be overcome by modulating precursor/beam fluxes such that preferential surface removal is achieved without causing significant contamination.

ACKNOWLEDGEMENTS

The authors acknowledge the financial assistance of the Science and Engineering Research Council (SERC) and the EC “SCIENCE” initiative. Leybold (UK) Ltd. are also thanked for sponsorship in connection with this work. One of us, (DM) also thanks the (SERC) and DRA, Electronics Division (Malvern) for the award of a CASE studentship.

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