Surface Engineering of Metals - Principles, Equipment and Technologies Part 9 potx

4.3 Depth of implantation: a schematic portraying the range of ion implanta-tion; b dependence of ion implantation range R p and standard deviation ∆Rp on energy of implantation of nitr

Fig 4.3 Depth of implantation: a) schematic portraying the range of ion

implanta-tion; b) dependence of ion implantation range R p and standard deviation ∆Rp on

energy of implantation of nitrogen ions into iron; c) Gaussian implantation profile for nitrogen ion implantation in iron; d) distribution of concentration of ions implanted

in an amorphous body and distribution of defects caused by them; e) defect formation

in substrate C by an incident ion (several thousand atom translocations in the lattice) and the formation of sputter cascades A and B (several hundred vacancies and several hundred interstitial atoms); 1 - implantation profile; 2 - defects profile; 3 - implanted atoms diffusion; 4 - sputtered atoms.

effect is insignificant and then there may occur an additional maximum

of concentration of implanted ions in the vicinity of R max

The curve which determines the distribution of implanted ions at

dif-ferent depths in the host material is known as the implantation profile.

Maximum concentration of implanted ions, especially those of light ments, is not at the surface of the host material but at a distance of some

ele-tenths of a micrometer from it, due to backscatter and the non-elastic acter of interaction of the ions with electrons of the host material Implantedions, colliding with atoms of the host material, causes their displacementwhich, in turn, causes the formation of radiation defects Since the energy

char-of the ion (several dozen keV) is several thousand times greater than theenergy of atom bonds in the lattice (in metals it is approximately 25 eV),one ion is capable of knocking even several thousand atoms out of theirnodular positions In this way, the implanted ion generates along its path

a strongly defected zone, known as a cascade, of several to several tens

nanometers, which propagates laterally to the direction of the ion ment, due to the secondary interaction of atoms knocked out from theirpositions The number of defects exceeds the number of implanted ions by

move-a fmove-actor of 2 to 3 move-and usumove-ally is so big thmove-at there is move-a defect smove-aturmove-ation [14].Because the probability of defect formation depends on the cross-section ofnuclear deceleration, the distribution profile of atom displacement of theimplanted material (radiation defects) is similar to the profile of implanta-tion, with the defect maximum, however, always occurring closer to thesurface [5, 15] (Fig 4.3d) When the ion dose exceeds 1014 ions per cm2,separate disturbed zones superimpose and the enhanced density of pointdefects may cause the formation of amorphous zones, dislocations andmicroporosity, through the coagulation of point defects The boundary value

of the dose necessary to amorphize the substrate decreases with a rise inthe mass of ions and with a drop in substrate temperature The formation

of microblisters may occur during implantation of metals by ions of inertgases [1]

The implanted ions may be located at dislocation boundaries, take upsubstitution positions or form inclusions of a new phase [14] Many defectsformed during the implantation suffer atrophy already at room temperature,while the distribution of the remaining ones is similar to that of an alloyingadditive The number and range of atom displacement in the implanted

material depend strongly on the ion dose, i.e., on the number of implanted

ions per unit surface [3, 6, 9]

In the majority of cases of practical utilization of ion implantation, it isimportant to obtain in the surface layer of a concentration of the alloyingadditive from several to several tens per cent The corresponding doses should

be contained within the range of 1016 to 1018 ions per cm2 Such doses aredelivered within tens and hundreds of seconds [1]

For small doses, the implantation profile corresponds well to the cal Gaussian distribution (see Fig 4.3c and Fig 4.4 - curve 1’).

theoreti-An increase of the ion dose causes an increase in the number andrange of atom displacements which are connected with significant defects inthe crystalline lattice Deepest penetrating are those ions, the incidence direc-tion of which is in agreement with the direction of “empty channels” in thelattice (stemming from the spatial distribution of nodes) and with the crystal-lographic axis of the implanted material This is the tunneling effect, men-tioned earlier Partially decelerated ions also migrate athermically

Fig 4.5 Dependence of profile for nitrogen ions, implanted at 75 keV energy into

pure iron on ion dose: 1 - dose: 3·1017 N + /cm 2; 2 - dose: 6·1017 N + /cm 2; 3 - dose:

1·10 18 N + /cm 2 (From Iwaki, M [20] With permission.)

Fig 4.6 Dependence of profile and depth of implantation on atomic number of

ions implanted into gold, and on ion energy (From Deicher, M et al [21] With

permission from Elsevier Science.)

Usually, deviations are observed at the surface in the direction of increasedconcentration, which is the result of ion etching of the implanted mate-

rial, or under the surface of the material (for x > 2∆R p), where ions may occurdue to random tunneling (Fig 4.6)

Besides the above, ion mixing of materials in the deceleration zone, aswell as possible diffusion processes, causes deviations from the Gaussiandistribution curve [14]

The depth of penetration of ions into the solid is relatively small - itexceeds 1 µm only exceptionally and drops rapidly with a rise of ion mass(Fig 4.6) For ions of the same elements it rises with a rise of the accelerat-ing voltage (Fig 4.7a) and ion energy (Fig 4.7b) In order to obtain animplantation profile without a sharp maximum peak it is possible to raisethe energy of the ions during the implantation process The final profile will

be an algebraic sum of implantation profiles obtained at different energylevels (Fig 4.7b) Another method is to increase the ion dose [8]

Since implantation involves the introduction of ions to the host materialbut basically without arise in volume, ion implantation is accompanied bythe formation of compressive stresses and a local rise in the surface tem-perature of the implanted material The bombarding ion may bring about,

in the cascade zone, local temperature of approximately 1000ϒC in a timeshorter than 10-11 s Heating up of the implanted material depends predomi-nantly on energy and dose of ions and is described by the density of powersupplied At power density of 10 kW/m2 the surface of the material heats

up to approximately 100ϒC, while at 100 kW/m2, it heats up to 350 to 500ϒCwithin several minutes At the maximum achievable power density of 6000kW/m2, the material may melt or even vaporize [8, 9, 24, 25] Usually, theimplantation process is conducted in such a way as not to allow the tem-perature of the implanted material to exceed 200ϒC, thus minimizing oreliminating changes of properties and deformation of the implanted mate-rial [14]

4.3.2 Pulse ion beam implantation

On account of the non-stationary character of interaction of the pulse ionbeam with the solid, due to the sufficiently short ion pulse (ns, µs) of veryhigh energy, the beam causes melting of the thin surface layer of the solid(which is not observed with the application of the continuous beam) andthe introduction of a foreign component (beam ions) into the molten liq-uid

In the case of metals and alloys, modification of surface propertiestakes place as the result of the coexistence of three processes [26]:

– thermal processes: melting, recrystallization, rapid cooling (at rates

of 107 to 1011 K/s);

– stress processes, caused by the propagation of stresses formed by theshock wave (ablative or dilatation) connected with extremely rapid vaporiza-tion of a portion of the molten material;

– physico-chemical processes, connected with the supply of a foreigncomponent to the molten zone in the form of ions or material coating thesubstrate, which are sputtered out by the ions of the beam, combined withthermal processes; these are, naturally, alloying processes

The relative participation of these processes depends on the parameters ofion beams, their mean energy and thermal properties of the material sub-jected to implantation From the scientific point of view, these processes havenot been thoroughly researched to date

4.4 Ion beam implantation equipment

To carry out ion beam implantation, special ion accelerators are used,

called ion beam implanters [27].

Ion implanters, in the most general sense, may be divided into two groups:– with continuous ion beam, traditionally called simply ion implanters,

– with pulse ion beam, which are versions of high voltage ion diodes orionotrons.

The first group has been used for ion implantation since the advent ofimplantation technology, while the second group is in existence since the1980s and used only for laboratory purposes [9]

4.4.1 Continuous ion beam implanters

The main component systems of the implanter are (Fig 4.8): the ion source,separator, focusing and accelerating system, deflecting system vacuumsystem and the working (implantation) chamber [28 −35]

Ion source This is the most significant functional element of the

im-planter, serving to produce the initially formed beam of positive ions of agiven type It comprises a discharge chamber where ionization takes place

of gas, vapour or gaseous compounds of solids, and an extractor, used forextracting ions from the ionization zone, their initial formation into abeam and directing it to the focusing-accelerating system Properties ofthe source are determined by technological possibilities and the effective-ness of the implanter [28 −35]

Depending on the mechanism of ionization of the ion-forming substance,ion sources are divided into three groups [33 −37]:

1 With discharge in the gas phase, often referred to as plasma

im-planters:

– with extraction of ions from the discharge plasma: spark (very seldom

used in implanters), with capillary arc discharge (also seldom used);

– with low voltage arc discharge in magnetic field or without a

mag-netic field; the best known and used in implanters are the Bernas, Nielsen

and Freeman sources The Bernas source operates with an arc discharge atseveral tens to several hundred volts and a current of several tens amperes

in a gas under pressure of approximately 1.33 Pa It operates in a magneticfield generated by an electromagnet with controlled induction up to severalhundred Gauss, perpendicular to the direction of ion extraction The fieldserves to limit the escape of ions from the central portion of the dischargechamber and to enhance the effectiveness of ionization by extending thepath of moving electrons (Fig 4.9a) Discharge in the source takes placebetween the hot cathode and the anode (anti-cathode) In order to avoidcondensation of the ionized material, the discharge chamber may be heated

by a resistance heater and screened by foil, e.g., tungsten or molybdenum

At the same time it must be intensively water cooled The ionized materialcan be a pure element or a chemical compound It is supplied to the dis-charge chamber in the form of vapour under appropriate pressure in quan-tities ranging from fractions of a milligram to several hundred milligrams,depending on the dose of implanted ions and on the type of chemical sub-stance used for the formation of ions of the given element Vapours of theimplanted elements or chemical compounds are formed in separate heatingchambers and supplied in a controlled manner to the discharge cham-

the elements which, in turn, allows high energy of ions without the need toraise the extraction voltage [28−35].

In the Nielsen source (Fig 4.9b) the cathode, powered by direct current inthe form of a coil, supplies electrons to the discharge zone and the anode is agraphite cylinder The source serves to produce ions of both gaseous andsolid elements

The above-described ion sources belong to the most versatile and allowthe production of the majority of ions within the range of atomic masses from

1 to 240 Their characteristic feature is the hot cathode and the heater whichenables vaporization of solid materials Such sources are used mostly inlaboratory implantators [30]

In industrial implantators there are usually applied specialized sources

of ions of one element, often with a cold cathode which, although ing long service life, allow the obtaining of relatively small currents of theion beam and can operate with gases only [31] An example of an ioniza-tion source, especially for the generation of strong fluxes of nitrogen ions, isthe J.H Freeman slot source (Fig 4.9c) in which the cathode, made of tung-sten wire, is located very close to the extraction opening in the shape of a slot.This, in conjunction with the strong cathode glow current (up to approxi-mately 100 A) and the effect of the magnetic field, perpendicular to the direc-tion of ion extraction, allows the obtaining of maximum plasma density op-posite the extraction slot [31];

featur-– with electron oscillation (so-called F.M Penning sources), also ferred to as sources with cold or hot cathode The best known are G.

re-Sidenius sources and their different modifications Fig 4.10 shows theprinciple of operation of such a source The ion emitter here is plasma oflow pressure discharge, ignited in the electrode system, in which the cath-ode has the shape of a hollow cylinder (either a wire coil [Fig 4.10b] orsolid [Fig 4.10c]), forming a niche The glow of the hot hollow cathode isfrom direct current and the cathode is heated to a temperature at which ahigh electron emission current is obtained (Fig 4.10b) When appropriatepressure conditions are met, arc discharge is activated in the dischargechamber and plasma is formed, screened from the hot cathode by a bipo-lar layer of spatial electrical charge Practically the entire usedinterelectrode voltage is situated in this layer, and in this zone electrons,emitted by the cathode, achieve sufficient energy for the ionization of thegas In the discharge chamber there may or may not exist a magnetic field

In the second case, the magnetic field created by the flow of current throughthe cathode may be compensated by an external field In the absence of amagnetic field, electrons move perpendicularly to the axis of the sourceand after passing through plasma they are decelerated by the electricfield in the layer situated opposite the spot where they are emitted by thecathode The electrons are slowed down, their direction is reversed andthey are accelerated in the direction of the discharge plasma; next, decel-erated at the cathode, and thus the cycle repeats itself Electrons oscillateinside the hollow cathode Maximum ionization occurs near the

– very high frequency (seldom used in implanters);

– duoplasmotrons, i.e., sources with extraction of ions from plasma which

diffused from the discharge chamber to the zone of beam formation Ratherseldom used in implanters; characterized by high values of ion currents

2 Thermoemissive - with thermal emission of ions from the

sur-faces of solids (Fig 4.11) This source operates on the principle of zation of ionization of atoms colliding with the surfaces of metals The

utili-h o l l o w t u n g s t e n c y l i n d e r w i t utili-h t utili-h e e x t r a c t i o n o r i f i c e o f 0 2 m mdiameter, together with rhenium foil is heated to approximately 2500 to3000°C by electrons emitted by glowing external cathodes Atoms ofthe substance designated for implantation pass from the vaporizer tothe inside of the hollow cylinder where, after making contact with thesurface of the rhenium foil they undergo thermoionization Such sourcesallow the obtaining of ions not only of single atom elements but also ofmolecules [31]

Fig 4.11 Schematic of thermoemissive ion source: 1 - tungsten cylinder; 2 - rhenium

foil; 3 - cathode I; 4 - cathode II; 5 - evaporator; 6 - extraction aperture (From M˙czka,

D., et al [31] With permission.)

3 Field - with surface ionization in which the difference between the

work of exit of electrons from the metal and the ionization potential of ments for ionization is utilized

ele-4 With bombardment by accelerated electrons (not used in

implant-ers)

Extraction systems These systems are built of one, two or three flat,

cylindrical or conico-cylindrical electrodes, the first of which serves toextract ions and the remaining two to the formation of the beam(Fig 4.12) To the electrodes, voltages are applied of several tens kV Con-trol of the extraction system involves either a change of value of theapplied voltage, or change of location of electrodes relative to ion source[8, 9, 28, 30−37]

In the case of single electrode extraction systems, the extraction andacceleration of ions occur with the aid of a conico-cylindrical electrode,placed near the extraction orifice with a negative potential relative to

Fig 4.13 Principle of action of sector magnetic field on a divergent beam of monoenergy

ions: m 1 , m 2 - ion masses; Φm - angle of divergence of magnetic field; U - potential accelerating ions; r m1 , r m2 - curvature radii of ion paths with masses m 1 , and m 2 ; B -

vector of induction of magnetic field.

Mass separators Separators are used for precise selection of ions in

the beam, based on an analysis of mass of the beam ions Separators allow

through only ions with a given e/m (charge to mass) ratio, utilizing the

effect of the homogenous magnetic field, limited by two planes (so-calledmagnetic lens) This field affects the beam of single energy ions but ofdifferent masses in two different ways: it focuses the divergent beam of ions

of same masses but the sites of focusing depend on masses (Fig 4.13) inaccordance with the equation [31]:

Most often used separators are electromagnetic, as well as separators erating on the principle of crossed electrical and magnetic fields [16]

op-Scanning systems To scan the treated material with an ion beam in

order to ensure the required homogeneity of the implantation process, thefollowing systems are used:

1 System for deflecting the ion beam in the x-z axis (scanner):

a) mechanical - in the form of a rotating and possibly laterally movingshield with openings, modulating the ion beam mechanically [39];

Fig 4.14 Schematic of scanning system (collector) of Polish-built UNIMAS-79

im-planter: 1 - ion beam; 2 - probe; 3 - implanted material (From M˙czka, D et al [31].

With permission.)

Fig 4.15 Simplified schematic of ion implanter (without separator and beam

deflec-tor) with fixed beam and movable stage (in z-x plane): 1 - discharge chamber with ion source and extracting electrode; 2 - three-electrode beam focusing system;

3 - movable stage; 4 - treated load; 5 - work chamber.

b) electrical - in the form of two generators of saw-tooth shaped age, producing voltage with an amplitude dependent on maximum ionenergy and on size of implanted surface, with a frequency of deflectionvoltage from 1 to 100 kHz (Fig 4.14)

volt-2 System for mechanical feed in the x-z plane of the stage with the

implanted material and a fixed ion beam (Fig 4.15) with possible maskingoff of beam

3 System for mechanical deflection of the ion source together with theextraction and acceleration system, usually in one direction, with mask-ing off of ion beam

The last two systems usually find application in industrial implanters forthe implantation of gases and metals [33-35].

Vacuum systems Systems of vacuum pumps, valves and vacuum gauges

serve to obtain vacuum in the zone of extraction and acceleration within theworking chamber High vacuum is of special significance in systems withlow working voltages, while soft vacuum in the single optical system maycause losses of beam current even up to 90% and deteriorate resolution [8, 9,

28, 30]

Work chamber This chamber is used for placing of the treated load It

should be equipped with mechanical systems for fixing and moving (intwo directions) or rotating the load (of special importance for complexshapes) in such a way that the treated elements do not mutually screen offthe beam incidence line (Fig 4.16) The chamber is, moreover, equippedwith loading/unloading systems which are usually cooled The cooling iseither forced by a water jacket or water canals, or it may be natural,through radiators which give off heat Laboratory implanters are equippedwith more systems than industrial implanters, e.g., heaters, goniometer, re-frigerators, etc [40]

Fig 4.16 Examples of providing movement to load by means of movable z - x stage.

(From Podgórski, A et al [38] With permission.)

Brand (manufacturer) Ion energy[keV]

Beam rent intensity [mA]

cur- ting voltage [kV]

Accelera-Mass separator

Load temperature [ C]

Type of implanted ions

Maximum zone of implantation [mm]

Load mass

smaller than the high current unit High current implanter (ARE,

Used for implantation of large size components Diameter of working chamber: 2.5 m; length: 2.5 m Pressure: 1.3 10 -5 Pa.

nitrogen N + (40%) + + N2 + (60%)

20 20 or

-First serial-produced implanter for tooling and machine components Maximum load size: 300 mm Homogeneity and reproducibility of implantation: –10% Typical dose: 3 10 -17 ions/cm 2 Duration of implantation for a single component: 30 min.; for a multi-component load: 90 min.

TECVAC 221(U.K.) 90 ca 100 10 16.5 For implantation of primary ions and dynamic ionmixing Maximum load size: 1 m; duration of

implantation: 40 min.

Westinghouse

(USA, 1983)

0.5−

Fraunhofer Institute (Germany,

1980)

Ion beam

-Riken (Institute for Physical and

nium, aluminum

-IMTE (Institute for Basic

Technical Problems, Polish

Academy of Sciences, Poland,

1986)

50−100 1 60−100 NO 20−600 nitrogen 15x15 - Implanter designed by Institute for ElectronicMaterials Technology

WiD-63 (Institute of Physics,

-varied 1−200

Technical data relating to some implanters (Data from [8, 17, 33, 34, 42] and various other sources)