Reactive ion beam etching of silicon with a new plasma ion source operated with CF4 : SiO2 over Si selectivity and Si surface modification

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Reactive ion beam etching of silicon with a new plasma ion source operated with CF4 : SiO2 over Si selectivity

and Si surface modification

C. Lejeune, J.P. Grandchamp, J.P. Gilles, E. Collard, P. Scheiblin

To cite this version:

C. Lejeune, J.P. Grandchamp, J.P. Gilles, E. Collard, P. Scheiblin. Reactive ion beam etching of

silicon with a new plasma ion source operated with CF4 : SiO2 over Si selectivity and Si surface

modification. Revue de Physique Appliquée, Société française de physique / EDP, 1989, 24 (3),

pp.295-308. �10.1051/rphysap:01989002403029500�. �jpa-00246051�

Reactive ion beam etching of silicon with a new plasma ^{ion source} operated ^with CF4 : SiO2 ^{over Si} selectivity ^{and Si surface} modification

C. Lejeune, ^{J. P.} Grandchamp, ^{J. P. Gilles, E.} Collard and P. Scheiblin

Institut d’Electronique Fondamentale, Université Paris XI et C.N.R.S. (Unité ^{associée N°} 22), ^Bâtiment 220, 91405 Orsay, Cedex, ^France

(Reçu ^{le 23} juin 1988, révisé le 21 juillet ^1988, accepté ^{le 26} septembre 1988)

Résumé.

Nous présentons des résultats de gravure

faisceau d’ions réactifs délivré par

type de canon à ions

la Source d’Ions Reflex Electrostatique Maxi-SIRE - alimenté en gaz

CF4 pur. Ils concernent la silice et le silicium monocristallin et démontrent que les conditions d’irradiation peuvent ^être optimisées ^de façon à ^{définir un} procédé ^de gravure à la fois très sélectif et anisotrope de la silice vis-à-vis du silicium et qui n’entraîne pas d’altérations irrémédiables du silicium sous-jacent ; les facteurs de

qualité ^{en font une} alternative très valable aux procédés ^{actuels sous} plasmas CHF3 ^ou CF4/H2, ^pour lesquels

les dommages induits par l’hydrogène ^{sont bien connus. Pour le} procédé proposé, ^avec des ions de 500 eV

incidence normale, les faits essentiels sont : i)

sélectivité SiO2/Si ^{de 19 est} obtenue pour l’opération ^{de la} décharge ^{de source}

voisinage ^de

pression minimale de fonctionnement, ^ce qui ^{entraîne une très forte} fragmentation ^{des neutres} injectés ; ii) les vitesses de gravure associées à ^cette sélectivité sont respectivement

de 130 nm/min et 7 nm/min pour SiO2 ^{et Si,} résultats normalisés à une densité de courant d’ions de 1 mA cm-2 ; iii) ^{la couche de} blocage fluorocarbonée qui ^{se forme sur} le silicium et assure l’atténuation de son

attaque, peut être enlevée par

simple bain de 60 s dans l’acide fluorhydrique ^concentré (50 %) ; iv) ^ce

traitement laisse un silicium propre dont les qualités électriques

^sont que faiblement altérées vis-à-vis de celles d’un échantillon témoin ; ^la procédure standard de guérison ^des dommages, c’est-à-dire

traitement

plasma oxygène suivi d’un recuit lent sous azote, semble donc pouvoir ^dans

conditions conduire à de très bons résultats. Des informations concernant les cinétiques ^et les mécanismes de croissance de la couche de résidu et d’évolution des dégâts superficiels du silicium ont été obtenues grace à ^{des mesures} ellipsométriques,

des mesures de caractéristiques électriques ^{de contacts} métal-silicium et des spectres d’analyse Auger (en

surface et en profondeur). Les résultats sont rapportés ^{et discutés en mettant en avant} les effets associés à la dose d’irradiation par les ions et à la pression de fonctionnement du canon à ions.

Abstract.

Reactive Ion Beam Etching is obtained from

specific ion gun, the Electrostatic Reflex Ion Source (Maxi-ERIS), ^{which is} operated with pure CF4 gas. The reported ^{results concern both} silicon dioxide and single-crystal ^silicon. They ^{show that the} operation ^{of the source} discharge ^{down to} its minimum pressure which implies

^extensive fragmentation ^{of the} injected neutrals, provides

very convenient process for selective etching ^of SiO2

Si,

^basic problem in semiconductor technology. ^{From the} characteristic

performances ^{which are achieved,} this process appears ^as

fair alternative solution to the standard reactive ion etching process with CF4/H2 ^or CHF3 (in

plasma environment). It is known that these latter ones lead to

deep lying modifications of the Si single-crystal, ^{which are} attributed to hydrogen-induced extended defects.

For the proposed RIBE process with

500 eV beam at normal incidence the main features are : i) selectivity SiO2/Si : ^{19/1 ;} ii) ^{etch rates :} 130 nm/min and 7 nm/min, respectively ^for SiO2 ^and Si, ^data normalized to a

1 mA cm-2 current density ; iii) ^the blocking carbonaceous film which is formed over the silicon and insures the slow-down of the etch rate may be removed by

simple dip ^{for 60}

in concentrated hydrofluoric ^acid (50 %) ; iv) ^{such a} post-etching ^treatment

without further plasma ^{oxidation or thermal} annealing ^{- leaves}

a

clean Si substrate, ^the electrical properties ^{of which}

only slightly ^altered

compared ^{to a control} sample.

Informations about the kinetics and mechanisms of the formation of both the overlayer ^{and the} near-surface

damage ^are obtained from ellipsometry, Auger ^electron spectroscopy, Auger sputter profiling ^{and metal-}

silicon contact electrical measurements. They

reported ^and discussed with

special emphasis

^{the effect}

of both the ion exposure dose and the operation pressure of the ion gun.

Classification

Physics ^Abstracts

61.80J

81.60

81.60C

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/rphysap:01989002403029500

1. Introduction.

Selective etching ^of Si02 ^{over Si is a basic} problem ⁱⁿ

semiconductor technology. ^{In a} pure CF4 discharge

excited with RF power, Si and Si02 ^{are etched at}

very similar rates. Therefore anisotropic ^{and selec-}

tive etching ^of silicon dioxide over silicon is generally

achieved by ^the combination of an ion activated process (Reactive ^Ion Etching - RIE) ^{in a} plasma

environment and the presence of hydrogen ^{either in}

the etchant molecule or in the gas mixture [1, 2].

Unfortunately, hydrogen ^{atoms and ions have been}

shown to be responsible ^of damage and contami- nation of the silicon crystal ^{down to} depths ^as large

as 30-50 nm [3-5]. They ^are very difficult ^{to cure} and alternative processes ^are required. Reactive Ion Beam Etching (RIBE) using pure halocarbon _gases may provide ^{a solution} [6-10]. However, ^{the selec-} tivity ^{has to} be further increased and, furthermore the compatibility of the process with the VLSI circuit

requirements ^{has to be} investigated. Therefore the contamination and damage of the silicon near-sur-

face have to be analyzed ^{first to} determine their effects and their origin and second in order to find a

solution which may ^restore the surface to a device-

quality ^state.

In section 2, ^the experimental apparatus and pro- cedures are described. A specific ^ion gun has been

developed ^{for RIBE which can be} operated ^with

fluorocarbon gases without lifetime problems ^and

with reduced contamination. It has a new ionization chamber : the Electrostatic Reflex Ion Source

(ERIS) ^{and a} plasma bridge neutralizer. For the RIBE procedure, the neutrals are injected ^{into the}

source discharge ^{chamber. For the} present ion gun the pressure within this latter chamber may be ^as low

as 1 x 10-4 mbar whereas the corresponding pres-

sure within the interaction chamber is 1 x

10- 5 mbar. The composition of the ion beam is on-

line recorded ; it will be hereafter referred to as a

« CF+x ^{ion beam}

The gun also delivers a flow of reactive neutrals and radicals which _may affect the ion beam-sample interactions.

In section 3, ^{we first} report the variations of the etch rates and the resulting Si02 ^{over Si} selectivity ^as

functions of the gun operation pressure ; they ^are

discussed briefly ^{from a} comparison ^{to the associated}

variation of the ion beam composition. ^{Then we} report ^{results of} measurements concerning ^{the ir-}

radiation of unmasked Si single-crystal ; they ^were

devoted to improve ^the understanding ^{of the}

mechanisms which insure : 1) ^the growth ^{and thick-}

ness limitation of the CF-carbonaceous overlayer ; 2) the Si etch rate slow-down, ^and 3) ^{the Si near-}

surface damage ^and contamination. Both the tran- sient and steady ^{states have been} analyzed using ^the following complementary diagnostics : 1) ^on-line

variation of the SiF4 partial pressure ; 2) ellipsomet-

ric parameter (0394 2013 03C8) variations ; 3) Auger sputter

profiles (ASP), (100 ^{eV Ar+} sputter beam) ; 4) ^elec-

trical evaluation of Metal-Si contacts (Mercury-Sili-

con probe diode). Both as-etched and wet-cleaned

samples ^as obtained after a 60 s dip ^{in HF 50 %,}

have been compared. ^{Such a} simple post-etching

treatment has been chosen

instead of the more

standard oxygen plasma ^{treatment followed} by ^a

concentrated HF dip

^{in order to} minimize the

consumption ^{of the} underlying ^{silicon substrate.}

The influence of the CFx ^ion beam energy and incidence has been studied, ^{but in this} paper ^we report and discuss mainly ^the influence of _{the gun}

operation pressure and the exposure dose effects.

2. Expérimental set-up ^and procedures.

A schematic diagram ^{of the RIBE} apparatus ^is shown in figure ^1.

2.1 THE ION GUN : ELECTROSTATIC REFLEX ION SOURCE (MAXI-ERIS).

^A specific ion gun is

used, the Maxi-ERIS [11]. ^{The source} discharge ^{is a}

three-electrode structure, ^{with a} hot tantalum cathode and a small graphite anode which are both located within the cylindrical ^{source chamber} (250 ^mm diameter). ^{This latter is} negatively ^biased

with respect ^to the hot cathode and thereby ^insures

the electrostatic containment (reflex effect) ^{of the} primary ionizing electrons which have been initially

accelerated from the cathode. This gun has been

especially developed ^{in order to} satisfy the various

requirements of ion-beam assisted _processes

either deposition ^or etching. ^In particular ^a steady-

state operation may be reached for the operation

with fluorocarbon _gases which generally ^{leads to the} deposition ^of insulating ^{films on the chamber walls}

or electrodes [12]. Because of the small anode the temperature ^{of which} may be far above room

temperature ^no deposition ^{occurs on the anode}

which conversely ^is slowly ^etched. Its material must be chosen according ^{to the} chemically ^reactive plasma environment. It has to be pointed ^{out that}

standard ion sources using ^a magnetic confinement and a DC potential excitation have a very short lifetime in relation with the extinction of the dis-

charge [8, 13]. ^With fluorocarbon gases such ^as

CF4, ^{the hot cathode tantalum wire has been shown}

to be passivated ^{after about one hour of} operation

due to the formation of tantalum carbide. The beam contamination is then strongly ^{reduced and for} typical operation conditions as such reported ^{in this}

paper the cathode lifetime is about 100 h [14]. ^The

extraction optics ^{is a three} grid system constructed of stainless steel as the ionization chamber. _{It may} deliver a 7 cm diameter beam in the energy range 0.2-2 keV at current densities of up ^to 1 mA cm- 2,

as measured on the sample holder located 15 cm

Fig. ^1. - Schematic diagram of the RIBE apparatus. ^For

RIBE process the neutral gas is injected ^{into the}

discharge chamber. For

CAIBE process, gases

also injected directly into the interaction chamber.

downstream the optics. Graphite may also be used

as a base material for the grids, ^{but not} molybdenum

which is etched and thus leads to beam and substrate contamination. The positive ^{ion beam current is}

electron compensated ^{on the target in order to allow}

the irradiation of insulating ^material (Si02 ). ^The

electrons are delivered by ^an auxiliary discharge

which has the same structure but a much smaller chamber : the Medium-ERIS [15]. ^{It is fed with}

argon ; ^a plasma bridge insures the coupling ^between

the neutralizer discharge plasma ^{and the ion-beam} plasma.

The interaction chamber is pumped by ^a liquid- nitrogen-trapped diffusion pump (7001/s ^for Argon).

The base pressure (pi) ^{in this chamber is 5 x} 10-7 mbar and the operation pressure range is 1 x 10- 5 _{mbar up} ^{to 10 x} 10- 5 mbar during typical

RIBE experiments, the neutrals being injected ^into

the source chamber. This pressure, recorded ^both by

a capacitance ^{manometer and a} Penning ^ionization

gauge, will be used as a characteristic parameter ^of

the RIBE process. The pressure (ps ) within the

source chamber - also recorded by ^a capacitance

manometer - is an order of magnitude higher. ^The

REVUE DE PHYSIQUE

APPLIQUÉE. -

ion beam composition ^{is on-line} recorded with a

magnetic-mass-spectrometer (MMS). ^The compo- sition of the stable neutrals present within the interaction chamber is recorded with a quadrupole

mass spectrometer (QMS) ; this latter is located within an independent ^chamber differentially pumped ^{and connected to} the main chamber

through ^{a 2 mm diameter} hole. Because of the DC excitation (80-130 V) of the hot cathode discharge

linked to the efficient electrostatic containment, ^the primary electrons have very large dissociation and ionization yields. Therefore, ^the injected ^neutrals

may be extensively fragmented ^{both as} CFy ^radicals

and CF+x ^ions through stepwise processes. The

fragmentation ^{is more extensive as the} discharge voltage ^{and current increase, and as the} injected

neutral flow decreases. The variation of this latter

implies the variation of the pressures in both the ionization and interaction chambers. In figure ^{2 are}

shown typical ion beam and neutral phase compo- sition spectra ^{for two values of the} CF4 ^flow rate, which correspond ^{to the extreme} values of the pressure range of interest for RIBE with the present ion gun. The spectra of the neutral phase (Figs. ^2a,

21

Fig. ^2.

Ion beam and neutral composition ^{for two} values of the CF4 gas flow rate, ^as injected ^{into the}

discharge ^chamber. a) ^and b) : neutrals and ions for the resulting highest pressure HP

9

10- 5 mbar within the interaction chamber. c) ^and d) : ^the corresponding spectra for the lowest pressure LP

1

10- 5 mbar. The beam current density

respectively 0.5 and 0.25 mA cm- 2 for HP and LP.

2c), clearly ^{shows that} heavy fluorocarbon molecules such as C2F6, C3F6, C4F6, C3F8 ^and C4F8 ^{are also} synthetized. ^The existence of these species may be

imputed ^to the presence of the large ^{amount of}

unsaturated radicals such as CF and CF2 ^{both within}

the source chamber and the ion beam chamber. A discussion about the plasma chemistry which governs the overall behaviour of the ion gun would be

interesting ^but is nevertheless out of the _scope of the

present paper. It ^must be pointed ^out that these spectra ^are associated to the steady-state operation

of the gun, that ^means conditions for which both the thermal equilibrium and the chamber conditioning steady ^{state are reached.} Starting ^{from a clean vessel}

the latter requires ^{about 2 h} [14]. ^From figure ^{2 it is} clearly ^{seen how the} pressure is ^a sensitive parameter in order to modify ^and/or control the ion beam and neutral flow composition, although this latter is rather difficult to be determined with the present

experimental set-up.

2.2 SAMPLE EXPOSURE AND POST-ETCHING TREAT- MENT.

The samples ^{to be} exposed ^to the ion beam

were introduced via a load-lock system. They ^were

stuck with a carbon paste ^on the water-cooled substrate holder ; this latter is driven by ^{a motor} system which allows the choice of both the sample

position ^and orientation within the beam cross-sec-

tion. A small Faraday cup is included in the sample

holder to measure the irradiation current density.

The emission or the subsequent ^{formation of stable}

neutrals associated to the beam sample exposure ^are on-line recorded by ^{the QMS - in} particular ^the SiF4 partial pressure.

Masked samples ^{with HPR 204} resist, post-backed

at 110 °C, ^{were used for etch} rate measurements.

They ^were exposed ^{for a} given period ^{of time so that}

a step of about 150-200 nm is obtained, ^{in order to}

reduce the uncertainties associated to the profilome-

ter measurement and to the transient variation of the silicon etch rate (Sect. 3.2a). Thermal silicon dioxide

(600 nm) grown ^on silicon substrate and Si single- crystal ^{were used for the} present results. The Si substrates were n-type, (100)-oriented crystals ^with resistivity ranging ^between 4.0 and 6.0 ilcm. For the

analysis ^{of the} CF+x/Si interaction kinetics and

mechanisms, ^unmasked samples ^were considered. A standard organic cleaning ^{was the} only pre-exposure treatment.

After the RIBE exposure, the wafers ^were dipped

into absolute ethanol as soon as they ^{were taken out}

of the load-lock chamber. This procedure ^{has been}

shown ^to reduce the sample contamination and

oxidation in particular ^{for those} having ^{the thinnest}

carbonaceous overlayer. They ^{will be} subsequently

called as-etched samples. ^{It has} already ^{been shown}

that 02 plasma post-etching treatments are efficient for the removal of C, F-films grown ^on silicon [4, 16]. ^{However a} silicon dioxide layer ^{is also} produced, thereby consuming the underneath silicon substrate up ^to 2-3 nm depth. Thus after a HF dip, ^{the near-}

surface of the silicon substrate is also removed and it is no longer possible ^to evaluate the damage ^and

contamination of the very interesting interfacial

layer. ^{In order to} preserve the integrity ^{of this} layer,

different post-RIBE ^wet procedures ^{were evaluated} concerning ^their ability ^{to remove the} carbonaceous

overlayer. ^{A 60 s} dip ⁱⁿ concentrated hydrofluoric

acid (50 %) ^{was shown to} be efficient from ellip- sometric, Auger ^and electrical characterizations and for standard exposure conditions, ^{as discussed in}

section 3. One _may expect ^{that such a wet} procedure

did not remove the underneath silicon as far as this latter had not suffered important oxidation. They

will be called wet-cleaned samples. They ^{were also}

maintained in absolute ethanol until they ^were

characterized. For samples ^{which were left at room}

air after etching, ^{the time} for the HF dip ^{which was} required ^{to remove the} overlayer decreases ; ^{after a} week, ^{30 s were effective.}

2.3 SAMPLE CHARACTERIZATIONS.

2.3.1 Ellipsometric measurements.

A manual

Rudolph ^{Research T436 was used to} determine the

ellipsometric angles. ^The polarizer-compensator- sample-analyzer configuration (PCSA) ^{was chosen.}

d and w ^{were measured in all four zones at a 70°}

angle ^of incidence and at a 546.1 nm wavelength

[17]..

2.3.2 Auger ^Electron Spectroscopy (AES) ; Auger Sputter Profiling (ASP).

Auger ^Electron Spec- trometry ^was performed from nonderivative spectra collected in the EN(E) ^mode. They ^were quantitat- ively exploited by ^{the use of the} peak ^to background

ratio (Px/B), hereafter referred to as the Auger

ratio. The advantages ^of the method have already

been discussed [18]. ^{The electron gun delivered a}

3 keV, ^0.5 J..LA beam, ^{and the CMA} aperture angle

was 42.18°. For ASP, ^{a low} energy ion _{gun has} been

developed ^{in order to reduce the knock-on} spreading effect ; it delivered a 100 eV-50 03BCA ^{Ar+ ion beam in}

a 7 mm diameter spot.

2.3.3 Electrical evaluation : Mercury Probe-Silicon Diode.

A mercury-silicon contacting ^{device was}

used in order to analyze ^the Schottky-barrier ^diode

as established between mercury and silicon : it is the

Mercury Probe-Silicon Diode [19]. ^The Hg-Si ^con-

tact to be tested had a 1 mm2 area ; ^the samples ^were

stuck backside on a metal electrode with a silver paste which insured a large ^{area contact} (100 mm2).

I-V ; C-V ; 1/C2-V ^and G-V characteristics were

recorded with this device which can be used locally

and instantaneously ^{to form a diode.}

3. Expérimental ^results.

3.1 ETCH RATES AND Si02/Si SELECTIVITY. - The

dependence of the etch rates and Si02/Si selectivity

versus the operation pressure

pi

is shown in

figure ^{3. The etch rate} values have been normalized to a 1 mA cm- 2 ^current density, ^{as it is usual in}

RIBE data in order to make easier the comparison

between various experiments. ^{Of course} this pres- entation of the results has only ^a practical meaning ^if

the ion gun ^is able to deliver a current density up ^to this value and if the beam composition ^{is not}

affected by ^the necessarily linked modification of the

source parameters. Generally, ^{for a low} operation

pressure, the ^beam composition ^is strongly ^modified

as the discharge ^{current is} increased and a nonlinear behaviour of the Si etch rate is observed, ^{with a} deficit as compared ^{to a linear} extrapolation [10].

Our own experiments ^confirm this feature. Con-

versely ^the Si02 ^{etch rate is not as much affected, so}

that higher selectivity might ^{be achieved with a}

further increase of the beam current density.

For the present ^{work the Si etch rate of}

7 nmlmin/mAlcm 2 corresponds ^more physically ^to

an effective sputtering yield ^{of 0.1 Si} atoms/imping- ing CFx ^ion (500 ^{eV ions at normal} incidence). This

value is smaller than the 0.2-0.3 values reported ⁱⁿ

the literature for beams extracted at about the same

energy from more standard source discharges oper- ated with CF4 [6-8]. ^{However it must be} pointed ^out that, ^{for these latter} experiments, ^the operation

pressures within ^{the ion sources} and process cham- bers (respectively ^{about 10-3 and 10-4} mbar) ^are higher than those of the present device. The 0.1

Fig. ^3.

Si02 and Si etch rates

functions of the neutral pressure in the interaction chamber and resulting ^the Si02/Si ^etch selectivity (steady-state values). ^Data

normalized to

ion-current density ^{of 1 mA} cm - 2. Beam

energy W+

500 eV, and incidence 0

0°.

sputtering yield value compares ^to the values re-

ported ^{for the} operation ^{of such} standard ion sources

but operated ^with CHF3 [16]. This different be- haviour may ^be attributed to the extensive fragmen-

tation of the injected ^{neutrals and/or to the low} operation pressure which both may be achieved with the Maxi-ERIS ion gun. The pressure variation

implies ^the variation of the composition ^{of the} beam,

but also the variation of both the composition ^and

flux of the neutrals and radicals which are emitted

out of the ionization chamber, ^as already ^mentioned.

As far as the ion beam is concerned, ^its composition dependence ^versus pressure is shown in figure 4,

where are plotted ^the variations of the peak height

ratio of the main ions. The reference is CF3’ , ^the

most abundant ion in the usual RIE plasma ^environ-

ment. The high selectivity values, ^{in the} range 10-20,

are achieved for the lowest part of the pressure variation range. The comparison ^between figure ²

and figure 4 shows that such high ^{values are associat-}

ed to the more extensive fragmentation ^{of the}

neutrals and to a large ^increase of both the C+ and F+ monoatomic ions, the dominant ion

being ^{then CF+ . Values} reported ^for CFx ^bombard-

ment of silicon show that the etch yield ^{decreases as}

far as the F/C ^{ratio decreases} [20]. ^{A value of 0.1 at}

500 eV was reported ^{for the} sputtering yield ^of

silicon by ^{CF+ ions ; it is in} agreement with the value derived from this work. However as shown in

figure 2, ^{the ion} _spectrum ^{is so rich that this} compari-

son has only ^a qualitative meaning.

Fig. ^4.

Dependence ^{of the} CFx ^{ion beam} composition

versus

pressure pi ^to be associated to the data in figure ^3.

The source discharge voltage is 120 V. The discharge

current is adjusted (about ² A) ^{in order to} provide

constant beam current density (j+

^{0.5 mA} cm-2). ^Yet

for the lowest pressure value j + ^{decreases to}

0.25 mA cm- 2 because of the pressure limitation of the

discharge ^current.

What about the contribution of the neutrals and radicals in the present ^RIBE process ? ^As already known, ^{the slow-down of the Si etch rate can be} attributed to the formation of a C, F-blocking overlayer [2, 5]. ^{This latter can} develop ^{in RIBE}

processes with CF4, ^{as will be discussed later} on, but not in the more classical Reactive Ion Etching (RIE)

with pure CF4 ^{in a} plasma environment because of the role played by ^{the dense} population ^{of reactive}

neutrals. In typical ^RIE conditions the neutral to ion flow density ^ratio range is 10- 2-10- 3, the ^{ion flow} density having ^{about the same value as in the} present RIBE experiment (0.25-0.5 ^mA cm- 2). ^{On the other}

hand, for this latter this ratio has a much smaller value. If we consider

as a first estimation

only

the stable neutrals which are thermalized within the interaction chamber (as they ^{are recorded} by ^the QMS), ^the following ^{values are obtained :}

Low pressure operation :

High pressure operation :

Two comments must be added. Firstly, ^{as shown}

in figure ^{2a-c, a} large variety ^of fluorocarbon mol- ecules are present ^{in the} interaction chamber and their respective sticking coefficient and further on

influence _{may be} quite different. Secondly, ^radicals

and heavy ^{molecules are} ejected ^{from the ion source}

and impinge straight ^{on the} sample. They ^contribute

to an increase of the above values of the ratio. A

more accurate estimation would require ^the analysis

of the nature and flow of the neutral species ^which

are emitted outward the ion source. As far as concerns the RIBE _process by itself, experiments

have been performed ^{in order to estimate the effects}

of these particles : 1) ^Similar measurements as those

reported ^{above but done in a farther cross section of}

the beam, ^{located 37 cm} downstream the grids [21] ; 2) Influence of the electrons delivered by ^{the neut-}

ralizer discharge ^and 3) ^{Exposure of the} samples ^to

the ion source plasma ^{when the} optics ^bias potentials

were turned off. Data will be reported ^{in a further}

paper.

The present results show that a Si02/Si ^{etch rate} selectivity up ^to 20 may be achieved for the RIBE mode and the operation ^of the gun ^at the lowest pressure (1 ^{x 10- 5} mbar). ^{In a} practical process

higher ^values might ^{also be} expected ^{with the}

present apparatus ^{from the use of two} modifications

to the present primitive ^RIBE experimental process.

First, ^{if the} operation pressure is varied du ring ^the etching ^{of the silicon dioxide film of} given thickness ;

it is possible ^to begin ^{at the} highest ^{value and then}

lower the pressure ^as far as the etching proceeds

towards the silicon substrate. The mean value of the silicon dioxide etch rate will then be higher, ^because

for the pressure range which is considered in figure 3,

it varies by ^a factor of 2. The etch time of the oxide film will then reduced, ^{and if the mean} value of the silicon dioxide etch rate is used to define a concept of

effective selectivity », ^a higher ^{value of this}

latter might ^be expected ^{from a} convenient monitor-

ing ^{of the} RIBE pressure. It ^must be noticed that whatever is the pressure in this range the etching

remains anisotropic and that the operation pressure of an ion source is easily monitored. Second, ^from the direct injection of fluorocarbon _gases within the interaction chamber known as the CAIBE mode for

Chemically ^{Aided Ion Beam} Etching. ^{Our measure-}

ments show that starting ^from the lowest pressure value as reported ⁱⁿ figure ³ (1 ^{x 10-5} mbar) ^without

additional _gases (RIBE procedure), ^the injection ^of CF4 up ^to 10 x 10- 5 _mbar, does not modify ^{the Si}

etch rate but conversely increases that of Si02 by ^a

factor of 1.3, ^and thereby ^{involves a} selectivity ^value

of about 25.

3.2 CARBONACEOUS OVERLAYER GROWTH AND CLEANING.

3.2.1 SiF4 partial pressure.

The on-line variation of the SiF4 partial pressure (SiF’ peak height) ^{as a}

function of the Si irradiation ion dose is shown in

figure ^5. Three values of the _pressure were con-

Fig. ^5.

On-line variation of the SiF4 partial ^pressure (SiF’ peak height ^{in the} QMS)

function of the Si irradiation dose. Parameter : the gas flow ^rate and the

resulting pressure p; within the interaction chamber.

W+

500 eV ; ⁰

0° ; Si(100) ; electron-compensated

ion beam. The given time scale is related to the high ^and

medium pressure (0.5 ^{mA cm-2 ion current} density).

sidered, ^{in order to} demonstrate the influence of both the ion dose and neutral pressure. ^{For these} measurements the characteristic time of the vacuum

equipment ^{was estimated to be about}

^{2 s ; it}

affects mainly ^the signal ^increase which would be very sharp ^{in the limit of a} vanishing characteristic time. The sample introduction speed and the ion beam density profile ^{have also to be} considered for a more accurate quantitative analysis ^{of these} graphs.

Experiment ^{with RCA} clean silicon demonstrates that the effect of the native oxide layer ^{does not}

affect significantly ^the characteristic features of the above graphs, ^as they ^{are discussed now. The}

maximum Ro ^{of each curve} corresponds ^{to the silicon} etching. ^{Then the silicon removal rate is} decreasing

as a result of the carbonaceous overlayer growth ^on

the Si surface. A critical dose Dl does appear which may be attributed to the establishment of the steady

state of the overlayer ^{thickness and} structure, the etch rate (Rs) being ^{then constant. The} resulting

attenuation factor Rs/ Ro ^{of the etch rate is} clearly

seen ; it varies from 0.45 to 0.15, ^{for the considered} pressure variation range. The lower the attenuation

factor, ^the higher ^{the dose} DI, ^{a feature that will be}

further imputed ^to the increase of the overlayer

thickness (see Fig. 9). ^Here again Dl presents ^a sharper increase in the lowest part of the pressure range ; typical ^{values are :}

The steady ^state relative values of the etch rate

(R,) ^{as derived} from these graphs ^{have been com-} pared ^{to the etch rate values as measured from} profilometer ^data. They ^are proportional. ^{From this}

result it may be assumed that when the overlayer steady ^{state is} reached, whatever is the pressure, similar mechanisms are implied in the removal of the underneath silicon. In the lack of a better infor- mation, ^{if we assume that for the transient} period ^of

the overlayer ^{formation the etch rate variation is}

also given by ^the graphs ⁱⁿ figure 5, ^{it is} possible ^to

obtain a crude estimation of the silicon thickness el which is removed for an exposure ^dose Di: ^A

value of about 4 nm is obtained, ^almost independent

of the operation pressure. Similar graphs ^{to those in} figure ^{5 are also obtained} when other RIBE process parameters ^are modified, ^{such as the} incidence, ^the

energy ^or the nature of the injected neutrals. An

important ^{feature which must be} emphasized ^{is that}

for the present ^RIBE experiments ^the overlayer

thickness increases with increasing ^the energy, ⁱⁿ the studied energy range (250-1 500 eV) ; ^{see reference} [22] ^{for the} preliminary results related to the energy influence and for the operation ^with CHF3.

The data shown in figure ⁵ imply ^{a time} depen-

dence of silicon etch rate. Conversely, ^{for the}

present experimental conditions there is no blocking overlayer grown ^on the silicon dioxide, ^{the etch rate} of which does not depend ^on the exposure time, excepted ^{for a} slight initial increase imputed ^{to the}

variation of the surface temperature. ^{In such cases} for which a blocking overlayer formation is required

to insure the etch rate slow-down of the underneath substrate and thereby ^to provide ^{for the film-to-}

substrate selectivity, this latter selectivity concept has to be more precisely ^defined and used. In

particular, the values estimated from long ^time

exposure, i, e. the

steady-state selectivity

^values

as those reported ⁱⁿ figure ^{3, are not} sufficient to

determine the optimum ^{overetch time for a} given

process. The time dependence of the etch rate and the resulting removed thickness during ^{the initial}

transient period ^{of the silicon} exposure ^to reactive

species ^{have to be} considered [2].

3.2.2 Ellipsometric ^{data and} post-RIBE cleaning.

In figure ^{6a is shown} the variation of d as a function of the ion dose for the same three pressures ^as in

figure ^5. The initial decrease of à may be attributed

to the thickness increase of a thin overlayer grown

on the substrate. However the critical dose Dl, ^as

defined previously ⁱⁿ figure 5, ^now corresponds approximately ^{to a} minimum value of d. Beyond Dl ^a slight increase of à is observed up ^{to a} higher

dose D2 beyond ^{which a} plateau ^is reached. The value of D2 is about twice that of Di. ^{Such a}

variation of d versus the ion dose has been attributed

to the final reorganization ^of the near-surface of the silicon substrate, ^{once the} overlayer thickness is

Fig. ^6.

a) Variation of the ellipsometer angle ^{à as}

function of the Si irradiation dose. The parameters ^{are the}

same

in figure ^{5 ;} b) Variation of .L1 for both as-etched and wet-cleaned samples processed ^at the lowest pressure . value LP.

constant and as long ^{as the Si interface regresses,}

because two phenomena interfere for the determi- nation of the ellipsometric parameters : the substrate

damage (crystal ^{defects and atom} incorporation) ^and

the overlayer growth. ^{The first} amorphization step which requires ^{a dose of about 2 x} 1015 cm- 2 [20]

cannot be seen in the present exposure scale. The saturation value of à depends ^{on the} properties ^of

both layers. Yet for low layer thicknesses, ^a quanti-

tative analysis ^is not accurate [17]. Nevertheless the above variations are significative ^{of the} sequential

effects of the reactive ion bombardment as will be discussed later on. In order to investigate ^more precisely ^{for the} contribution of the overlayer ^{in the}

d values, ^the measurements have been done on wet- cleaned samples. ^{The data are} plotted ⁱⁿ figure ^6b

for the samples processed ^at the lowest pressure.

They ^{show that :}

i) ^{for the lowest} dose in the scale, say D DI, ^the

HF dip ^{has no effect on 0394. Times} higher ^{than 60 s}

have been tested without effect. The substrate is not able to be cleaned by ^{this wet} procedure. ^{The AES} spectrum ^{of the} wet-cleaned sample surface is shown in figure 7a ; ^it demonstrates the presence of ^a large

amount of both carbon and fluorine, ^{but the} SILVV peak is nevertheless seen. The AES spectrum of the as-etched sample ^{is about the same as that for}

the wet-cleaned sample. It may be assumed that carbon and fluorine are incorporated within the silicon lattice, ^{as discussed} by Chuang et ^al. [23] ;

Fig. ^7.

^AES spectra

peak

background EN(E)

mode - for as-etched and wet-cleaned samples processed

at the lowest pressure : a) Wet-cleaned after

small exposure dose (1.2

1016 cm-2) ; b) ^and c) Wet-cleaned and as-etched after exposure for the critical dose D2

1.5 1017 cm-2, ^as defined in figure ^6.

ii) for D > Dl, ^{the wet} cleaning ^{treatment leads}

to an increase of d which is approximately ^constant

and equal ^{to 17°. In} figure ^{7b and 7c are shown the}

AES spectra of wet-cleaned and as-etched samples

which both have received the same CFx ^{ion dose} D2 (1.5 ^{x 1017} cm- 2). ^{The as-etched} spectrum ^is

typical of Si with its C, F-blocking overlayer. ^The

oxygen peak ^{is not intense} (Po/B

0.02) ^{and the} SiLVV, (89 eV) peak ^{is not seen.} The wet-cleaned

sample spectrum corresponds typically ^{to the case of}

silicon contamined in room air, during the transfer time to the Auger diagnostic chamber ; Po/B

0.08 corresponds approximately ^{to one} Si02 ^mono- layer. ^{The carbon} peak ^is very small (PCIB

0.03), ^{and no} residual fluorine signal ^{is seen : the}

substrate seems really

^clean », from the AES point

of view. The d value of the etched + wet-cleaned silicon sample ^{for D}

D2, ^is approximately equal ^to

that of a silicon with its native oxide. Although ^the ellipsometric measurements were done under a dry nitrogen flow, ^a native oxide overlayer does exist.

Furthermore, ^as shown in reference [24], ^{a silicon} damaged ^{under Ar+} ion bombardment _may have, when the saturation is reached and according ^{to the} light wavelength, ^the same d value as that of a

substrate cleaned in an ultra-high ^{vacuum. Therefore}

a more sensitive characterization must be done in order to further investigate the residual near-surface

damage (Sect. 3.4).

Fig. ^8.

Auger Sputter ^Profiles (peak

background EN(E) mode), through ^the C, F-overlayer ^{and the Si}

near-surface. The parameter is the pressure the values of which

those given ⁱⁿ figure 3 and referred to

HP, MP and LP ; ^Ar+ sputter-beam : ¹⁰⁰ eV ; ^{0.13 mA} cm- 2.

The given overlayer thickness values

estimated from the C- and Si-signal variations.

3.3 OVERLAYER AND NEAR-SURFACE IN-DEPTH ANALYSIS (ASP).

^{ASP was used to} estimate the thickness and the composition profile ^{of the car-}

bonaceous overlayer. In-depth profile ^{scans are}

shown in figure 8, for as-etched samples having

received an exposure dose higher ^than D2, ^{and for}

the three typical pressures already considered. Of

course the method _may involve uncertainties, ⁱⁿ particular concerning the fluorine which desorbs under electron impact [23, 25] ^and may chemically

react under the ion bombardment. The F-profiles ⁱⁿ figure ⁸ correspond ^{to the} steady ^state of the electron induced desorption. ^A further paper ^will report

more details on AES, ^ASP measurements and the

comparison ^{of the results} with XPS data. We com- ment the main features of the in-depth analysis (ASP) ⁱⁿ relation with the present ^address.

As the sputtering ^{of the residue} proceeds ^{the Si-} signal grows and the CKLL signature changes beyond

a critical dose from a graphitic shape ^{to a} shape

which is suggestive of the presence of Si-C. In order to clarify ^the discussion, this dose was chosen to define a conventional C, F-film/Si interface (1). ^A question ^{now arise : do the} Si-C bonds exist within the silicon near-surface or are they ^induced by ^the

Ar+ sputter ion bombardment (100 ^eV ions) ? ^The

answer to this question ^is important ; ^{first in order to}

determine the appropriate post-etching treatment to restore the silicon crystal ^{to a device} quality ^state [4]

and, ^{second because} Si-C bonds have been identified from XPS data on silicon exposed ^to CF4/H2 ^RIE,

for conditions of selective etching ^of Si02 ^{over Si} [2, 3]. As shown in figure 7b the AES spectrum ^{of a} wet-cleaned Si surface does not show incorporated

carbon. Therefore it may be assumed ^that the C, ^F- overlayer/silicon interface is relatively abrupt, ^at

least for the irradiation conditions involving ^the

formation of a rather thick carbonaceous overlayer (low pressure operation). ^The exponential ^decrease

of the carbon signal ^{in the} graphs ^of figure 8, ^as observed for the low pressure values, may be considered as a corroboration of this feature. Such a

variation is in fact predicted ^{when an atomic} layer ^is sputtered, if the removal rate of the monolayer ^is

assumed to be proportional ^{to the surface} coverage

as a consequence of the statistical ^nature of the

sputtering process [26]. Recent XPS data on Si

samples exposed ^{to RIBE with} CHF3 ^{in the present} apparatus ^{have been} reported by Cardinaud et al.

[22]. ^No Si-C bonds were detected even for 1500 eV ions in the Si2p ^{detailed peak as} obtained from a

monochromatized radiation. _{It may be} expected ^that

the same behaviour is also valid for RIBE with

CF4.

The lower the RIBE processing pressure, the

higher ^the sputter ^dose required ^{to remove the} major fraction of the C, F-overlayer. ^{For the lowest}

pressure ^a plateau ^is clearly ^{seen in} the carbon

profile ^{with about the same} Auger ^ratio PC/B ~

0.75, ^{whatever are} the energy, incidence _{and gas}

(CF4 ^or CHF3). ^It corresponds ^to the bulk of the carbonaceous layer grown under the ion bombard-

ment.

Whatever is the pressure, the fluorine profile

shows two regions ^which correspond ^{either to a}

difference of concentration or to a difference in the bond strength ^{of the fluorine with the} surrounding

atoms. The inner region, ^{near the} film-Si interface may be imputed ^{to the direct} incorporation ^{into the}

carbonaceous overlayer ^{of the fluorine atoms which}

are produced by ^the dissociation of the impinging

fluorocarbon molecular ions such as CF+x. ^Con- versely ^the steep fluorine decrease which is seen at the topmost part ^{of the} carbonaceous film _{may be}

imputed ^{to the formation of a mixed} layer. ^This

latter would be formed under the bombardment of reactive neutrals, neutralizing electrons and ener-

getic ^{ions. The} synergetic ^{effects of the} irradiation of this mixed layer ^with the three types ^of particles

determine the balance between the deposition ^and

the removal of the carbon at the interface between the C, ^F-film and the gas phase. ^{On the other side of}

the C, F-film, ^and beyond ^{the interface} I, ^the fluorine steeply decreases within the silicon. This feature _{may be} partly attributed to the Ar+ activated

etching ^{of the Si} in the presence of fluorine. It has been shown that the chemical enhancement of

sputtering is very high ^{at low} ion energy [27].

Anyhow, ^{no more} fluorine is seen in the AES spectrum ^{of a} wet-cleaned sample which has not been bombarded by ^Ar+ (Fig. 7b). We may ^assume

that, ^{as well as the} carbon atoms, ^the fluorine atoms

are not incorporated deeply ^{into the silicon lattice.}

For the rather low pressure, the Si signal ^{shows an} exponential ^{increase here} again, ^as that which might

be expected ^{from the} existence of a sharp ^{interface I.}

Assuming ^this fact, ^this portion ^{of the} graphs ^has

been used to determine the required ^{dose to} sputter

a thickness equal ^{to the electron} escape depth (SiLVV ^{electrons mean free} path through graphite

was taken as : A e

^3.8 Â). ^The overlayer ^thickness

values given ⁱⁿ figure ^{8 were then} derived, assuming

a constant sputter ^erosion through the entire over-

layer. ^The comparison ^{of the} slopes ^{of the} SiLVV signal increase, ⁱⁿ figure 8, demonstrates that the

sputtering yield ^{of the} blocking overlayer depends

on the _process pressure. ^The associated variation of the composition ^{of the} incoming ion and neutral flows _{may be} supposed ^{to affect the} physico-chemi-

cal nature of the carbonaceous residue layer. ^XPS

measurement would give further information about the chemical bonds and thickness of this film [22].

Whereas ^at the interface 1, ^the (PSi/B)I Auger

ratio is independent ^of the pressure and is equal ^to 0.4, ^the (PC/B)I Auger ^{ratio increases as the}

pressure increases and is respectively 0.2, ^{0.3 and}

0.4, ^{values to be} compared ^{to 0.75} corresponding ^to

the bulk carbonaceous film. This increase of the carbon percentage ^at the interface may be explained

from the decrease of the overlayer thickness which

as a consequence ^{leads to a} deeper incorporation ^of

the carbon atoms into the silicon lattice, ^associated

to a less sharp interface between the overlayer ^and

the silicon. These features are corroborated by ^the following ^results dealing ^{with the} electrical evalu- ation of the residual contamination and damage ^of

the silicon.

3.4 NEAR-SURFACE SILICON DAMAGE : CONTACT

ELECTRICAL EVALUATION. - The current-voltage

characteristics of Hg/Si ^{contacts of} RIBE-exposed

Si are shown in figure 9 for wet-cleaned samples.

Both forward and reverse 1 V characteristics are

given for three exposure doses and compared ^to

those of a control sample which also has received the wet cleaning treatment, but without beam exposure.

The dose effect on the near-surface damage ^is clearly

demonstrated.

*

D D2 : ^{The I-V,} C-V and G-V characteristics of the contact are almost unchanged, ^{in the limit of}

the data uncertainty. ^{A fair} Schottky ^{contact is}

Fig. 9. 2013 I-V reverse and forward characteristics for

Hg/Si ^contacts

wet-cleaned samples ^for increasing

values of the exposure dose. Low pressure RIBE process ; W+

500 eV ; ⁰

0° ; (n-type (100) Si ; 4-6 Hem. Mer-

cury-Probe ^Silicon Diode).