200 Philips tech. Rev. 38, 200-210, 1978/79, No. 7/8

Plasma etching in IC technology

H. Kalter and E. P. G. T. van de Ven

When an electrical discharge is set up in a gas containing halogens or oxygen, particlesare produced that are chemically very reactive. These can be used to etch solids that areconverted into volatile compounds by the particles. The method is known as plasmaetching, and provides a valuable alternative to wet etching in IC technology. Plasmaetching has been increasingly used in IC technology in recent years, largely because themethod is highly compatible with the trend towards further miniaturization. The authorsdescribe the three main systems for plasma etching and give results of their investigationson the control of the etching process by variation of the parameters.

Introduction

Until recently the only etchants. used for etchingsmall patterns in the production of ICs were liquidones, such as solutions containing HF. With the con-tinued miniaturization of ICs [11,it is now necessaryto etch patterns whose smallest details have dimensionsmuch less than 5 (.Lm. Furthermore, the results mustbe reproducible. Even after improvement, this is notusually possible with wet etching methods [21.It canhowever be done with a number of 'dry' methods:details of 1 (.Lmor smaller can for example be made by'physical ion etching', a method in which the atoms ofa solid are physically removed by bombarding thesurface with inert-gas ions [al.Another dry etching method, plasma etching, is the

subject of this article. In this method gas molecules ina gas discharge are fragmented into reactive particles:electrons, ions and neutral radicals, which are allowedto react chemically with the surface to be etched. Asuitable choice ofthe reagents ensures that the reactionproducts are sufficiently volatile for the surface toremain free. In the silicon technology a discharge intetrafl.uoromethane (CF4) is widely used; the productsof the discharge include atomic fluorine and theradical CFa, and also the radicals CF2 and CF, whichare formed by further fragmentation [41.Table I givesa number of equations' for fragmentation and recom-bination reactions that can occur in such a plasma.In etching silicon, silicon dioxide (Si02) and siliconnitride (SiaN4) in a CF4 plasma it is mainly atomicfluorine and sometimes the radical CFa that gives .the

Drs H. Kalter is with Philips Research Laboratories, Eindhoven;Ir E. P. G. T. van de Ven is with the Philips Electronic Com-ponents and Materials Division (Elcoma) in Nijmegen.

conversion into the volatile SiF4. The equations forthe most likely etching reactions are also shown inTable I.As an iIIustrationfig. 1shows photographs of apattern of polycrystaIIine silicon on silicon dioxidethat was obtained after etching with a plasma of CF4andoxygen.In addition to silicon and its compounds, other

materials that form volatile .fluorides, such as themetals tungsten, titanium, molybdenum and tantalum,can be etched by fluorine-containing plasmas. Chlorine-containing plasmas can also be used. Aluminium canbe etched in discharges of CCl4 or BCla [51,silicon ina discharge of CCI4, and chromium (for the fabricationof chromium masks) in a discharge of chlorine andoxygen [61. Volatile chlorides and oxychlorides are

Table J. A number of equations for possible fragmentation andrecombination reactions in a CF4 plasma and for etching silicon,silicon dioxide and silicon nitride with a CF4 plasma.

Type of reaction Equation

e+CF4 _.. CFa+ + F + 2ee+CF4 _.. CFa+F+ee +CFa _.. CF2 +F +e

CFa+F _.. CF4

CFa + CFa _.. C2Fo

F+F _.. F2

Si+4F _.. SiF43Si + 4CFa _.. 4C + 3SiF4

Fragmentation

Si02 + 4F _.. SiF4 + 023Si02 + 4CFs _.. 3SiF4 + 2C02 + 2COSiaN4 + 12F _.. 3SiF4 + 2N2

Recombination

Etching

Philips tech. Rev. 38, No. 7/8 PLASMA ETCHING 201

~------------------------------------------------------------------------------ ---

formed in these processes. Some other metals such asnickel, iron and cobalt form no sufficiently volatilecompound with fluorine or chlorine'at the tempera-tures normally used for plasma etching (~200 "C)and cannot therefore be etched in a plasma containingfluorine or chlorine. Gold occupies an intermediateposition in this respect. Oxygen plasmas are widelyused for removing organic substances such as photo-resist masks [41.

Plasma etching offers a number of advantages overwet etching. The most important is that it can be usedfor etching smaller patterns. Detail dimensions can besmaller than I um, while in wet etching the practicallimit is about 4 [Lm. We have to remember here thatwith modern photolithography images of details ofabout 2 [Lmcan be produced in routine work, whereasin electron and X-ray lithography the details can besmaller than I urn. However, there is little point inusing these methods except in combination with anequally refined etching method.

Another advantage of plasma etching is that theprocess is relatively simple and can be fairly easily

Fig. 1. Photographs made with a scanning electron microscope(SEM) of a pattern of a 0.3 f.Lmpolycrystalline silicon layer on a0.1 [.Lm layer of silicon dioxide after etching with a plasma ofCF4 and oxygen. The upper photograph is a plan view. Thelower photograph is a side view; the polycrystalline siliconlayer was coated with silicon dioxide beforehand to give bettercontrast. On the left, from bottom to top, there are the layers ofsingle-crystal silicon, silicon dioxide, polycrystalline silicon andsilicon dioxide again.

automated. The method is also generally more reliablethan wet methods, partly because there are no dif-ficulties from capillary and galvanic effects. Finally,there is no need for large quantities of corrosive acids,so that plasma etching is safer and less harmful to theenvironment than wet etching.

A difficulty with plasma etching is that the methodis not yet sufficiently selective for some ofthe combina-tions of materials used in IC technology. Under cer-tain conditions the wafers to be etched can also bedamaged electrically ('radiation damage') by incidentions, electrons and photons from the discharge.

There are three main systems used for plasmaetching, which differ in the shape of the reactor, theelectrode connections and the gas pressure. The re-search described in this article was concerned with themost common of these systems, plasma etching in abarrel reactor.

Studies were made of the variation of the etch rateas a function of gas cornposition, gas flow rate, pres-sure and ternperature. Special attention was paid tothe radiation damage that can affect wafers whenphotoresist is removed by an oxygen plasma, whichappears to be largely caused by the penetration ofsodium ions.

Before examining the results, we shall discuss thethree main plasma-etching systems and indicate someof their typical applications and limitations.

Systems

Diagrams of the reactors used in the three mainplasma-etching systems are shown in jig. 2. In systema the reactor is tubular. The wafers with the materialto be etched are screened from the discharge by a per-forated metal cylinder (the 'etch tunnel'). The etchingaction arises because the neutral radicals formed inthe discharge can reach the wafers by diffusion intothe tunnel, provided their lifetime is sufficiently long.

[1] See for example the LSI issue of Philips Technical Review:Vo!. 37, 265-356, 1977 (No. 11/12).

[2] See for example R. Kilian and M. Liehr, Philips tech. Rev.38,51, 1978/79, and J. J. Kelly and G. J. Koel, Philips tech.Rev.38, 149, 1978/79.

[3] Research on ion etching was the subject of an earlier articlein this journal: H. Dimigen and H. Lüthje, Philips tech.Rev. 35, 199, 1975.

(4] J. R. Hollahan and A. T. Bell (editors), Techniques andapplications of plasma chemistry, Wiley, New York 1974.

(5] R. Reichelderfer, D. Vogel and R. L. Bersin, ExtendedAbstracts Electrochem. Soc. 77-2 (Fall Meeting, Atlanta1977), p. 414.T. O. Hordon and R. L. Burke, Proc. Kodak Microelec-tronics Seminar, Monterey 1977.

l6] C. B. Zarowin and E. I. Alessandrini, Extended AbstractsElectrochem. Soc. 77-2 (Fall Meeting, Atlanta 1977), p. 395.K. Nishioka, H. Abe and T. Itasaka, ibid. p. 406.H. Abe, K. Nishioka, S. Tamura and A. Nishimoto, Proc.7th Conf. on Solid state devices, Tokyo 1975 (Jap. J. app!.Phys. 15, Supp!. 15-1, 1976), p. 25.

202 H. KALTER and E. P. G. T. VAN DE VEN Philips tech. Rev. 38, No. 7/8

Ions are excluded from the etching process by thepresence of the tunnel. Fig. 3 is a diagram of thearrangement for plasma etching in such a reactor. Avacuum pump and a gas-control system provide thedesired gas pressure, usually between 30 and 300 Pa.The energy necessary for the discharge is supplied bythe radio-frequency generator RF. The frequency isusually set to the value of 13.56MHz, which is a per-mitted frequency for industrial operations. At such ahigh frequency there is no need for direct contact be-tween electrodes and plasma. The energy is coupledto the plasma through an impedance-matching net-work and a capacitive coupling with two electrodes thatenclose the reactor chamber.

T ERF/'

cause the density of the gas is about one thousandth ofits density at 105 Pa (1 atmosphere), while the viscosityis about the same. Reactive particles are thereforetransported to the wafers mainly by diffusion. In theseconditions the diffusion coefficient D and the lifetime 't'

of the different particles are of importance. The dif-fusion coefficient is inversely proportional to the gaspressure. For atomic fluorine and oxygen the value ofD is about 500 cm2/s at 100 Pa and 150oe, while ,thevalue of r is about 0.3 s. The distance the particles cantravel is about 2 Vlh' and in this case is equal to about25 cm. This is larger than the diameter of the tunnel,which implies that the reactive particles can easilycover the distance to the wafers; see jig. 4.

RF

IJ

RF

. ' ... '

:...': : ti. .;-.: '.

...

RF

Fig. 2. Diagram of the reactors used in the three main systems for plasma etching.a) Barrel reactor, containing a perforated aluminium tunnel Twith the wafers to be etched W.ERF electrode connected to the radio-frequency generator RF. Eg grounded electrode. Sincethe wafers are not in direct contact with the discharge D, the main contribution to the etchingprocess comes from the long-lived radicals.b) Planar reactor with the wafers on the grounded electrode. Since the wafers are now indirect contact with the discharge, short-lived radicals now contribute to the etching process.Ions also make a limited contribution.c) Reactor for reactive ion etching. The wafers are located on the electrode connected to ther.f. generator; this electrode is much smaller than the grounded electrode here, The ion con-tribution to the etching is generally much larger than in (b).

As well as types with capacitive coupling ofthe r.f. energy thereare also types in which the r.f. energy is coupled by a coil. Al-though these are said to be 'inductively coupled', this is notusually strictly correct. Because of the self-inductance such highvoltages are produced across the coil that its elements start tobehave like capacitive electrodes. The inductive energy transferis only dominant at relatively low pressures (.::;;1 Pa), since onlyelectrons in circular orbits describe a path long enough for them, to collide with molecules and not with the wall.

The reactor chamber is usually made of silica andthe etch tunnel of aluminium. The wafers are placedin the tunnel in batches in a carrier. The gas flow goesmainly around the tunnel and is laminar. This is be-

To obtain uniform etching the etching reaction musttake place at the same rate at the centre of the wafersas at the outer regions. The concentration of thereactive particles must therefore be as constant aspossible between the wafers. This implies that thereshould be little recombination of the reactive particlesduring the diffusion between the wafers, and that theparticles should not start to attack the surface of thewafers too quickly. If not, there would be non-uniformities from the outer regions to the middle ofthe wafers as a result of the decreasing concentrationof reactive particles (fig. 4). In practice it is found thatthe reaction probability at each collision between a

Philips tech. Rev. 38, No. 7/8 PLASMA ETCHING 203

vFM

p

Fig. 3. Diagram of the arrangement for plasma etching in abarrel reactor. The reactor R is enclosed by the grounded elec-trode Eg and the electrode ERF, which is connected to thegenerator RFvia a matching network M. The reactor is connectedto a gas supply G and a vacuum pump P. Gas pressure and therate of flow can be controlled by the valves V, a pressure gaugePG and a flow meter FM.

T "'..._~_I_I__/

15cm

A

T7-j-,-,-"

n

t

Fig.4. Diagrammatic representation of plasma etching in abarrel reactor with etch tunnel T (above) and the number ofreactive particles n, as a function of position x (below). Oncethey have entered the tunnel via the perforations, neutralreactive particles with a sufficiently long lifetime arrive betweenthe wafers W. Because of recombinations and the etching reac-tions there are always fewer reactive particles near the centre ofa wafer than near the edges A and B. In the situation corre-sponding to curve a the variation of n is very small, so that thewafer is uniformly etched. In the situation corresponding tocurve b, on the other hand, the value of n decreases so muchfrom the edges to the centre that the wafer is not uniformlyetched.

reactive particle and a wafer must be less than 10-3 foruniform etching.Aluminium is used for the metal etch tunnel in the

barrel reactor, since it becomes coated with a pro-tective layer of aluminium fluoride in a fluorine-containing plasma. A problem here is that this layerbecomes partly hydrolysed in air, so that a porousfilm of aluminium oxide and aluminium fluoride

gradually forms on. the tunnel. A thick porous layerassists various undesirable recombination processes,and these lead to irreproducible results. It is thereforeimportant to clean the etch tunnel regularly, e.g. witha solution of HF or H3P04.In system b the reactor consists of two flat electrodes

forming parallel capacitor plates at a spacing of 1 to5 cm. The wafers for etching are placed on the lowerelectrode and are in direct contact with the discharge.The gas pressure has a value between 10 and 100 Pa.The lower electrode is grounded; the upper one isconnected to the r.f. generator. In system c the elec-trical connections are reversed. The r.f. electrode withthe wafers for etching is small in relation to thegrounded electrode and the gas pressure is often lowerthan in b (I-I0 Pa).During the discharge the electrodes assume a neg-

ative potential with respect to the plasma [71. This isbecause the electrons have a much higher mobilitythan the ions, and are therefore able to reach theelectrodes more easily. The magnitude of the negativepotential depends on the discharge parameters and thegeometry. The contribution of the ions to the etchingprocess depends on the magnitude of the negativepotentialof the electrode with the wafers and on thegas pressure. In system b, where the wafers are locatedon the grounded electrode and the pressure is fairlyhigh, the ions receive relatively little kinetic energy, sothat the ion contribution. to the etching process islimited. In system c, where the negative potentialofthe electrode with the wafers is relatively high, the ionscan make a considerable contribution to the.etchingprocess, particularly when the gas pressure is relativelylow (~ 1Pa). This arrangement for plasma etching istherefore sometimes called 'reactive ion etching' rsi,In band c there is a contribution from neutral radicalsas well as from the ions. Because of the free contactbetween the wafers and the discharge this contributionis not limited to the long-lived radicals, but those withrelatively short lifetimes also take part in the process.It seems likely that at the same time the reactions ofthe neutral radicals are catalysed by electrons and ions.The same kind of arrangement as in fig. 3 can be

used for systems band c, but with the different shapesfor the reactors and different connections and elec-trode geometry. The r.f. frequency can vary veryconsiderably, from 30 kHz to 30 MHz. In both sys-tems the relative concentration of the various kinds ofreactive particles, and hence the etching process, canbe greatly affected by making variations in the elec-trode geometry, the gas composition and the pressure.

(7) J. L. Vossen and J. J. O'Neill, Jr., RCA Rev. 29, 149, 1968.[8) J. A. Bondur. J. Vac. Sei. Techno). 13, 1023, 1976.

204 H. KALTER and E. P. G. T. VAN DE VEN Philips tech. Rev. 38, No. 7/8

The etching profile

The profile obtained with plasma etching dependsstrongly on the relative contributions from the variouskinds of reactive particles and therefore on the system;see fig. 5. Let us look first at etching in the barrelreactor. At a pressure of about 100 Pa the mean freepath of the reactive particles in the reactor is of theorder of 50 [Lm. This distance is much greater than thewidth of the etch grooves and the thickness of thelayer to be etched. Since for each collision the prob-ability of a reaction with a wafer is certainly no greaterthan 10-2, the particles are incident from arbitrarydirections and easily reach the sides ofthe etch grooves.The etching process is therefore isotropic and there isundercutting. The lateral dimensions of the patternare thus to some extent determined by the thicknessof the layer and the uniformity of the etching process.Fig.6 shows an example of the undercutting thatoccurs in isotropic plasma etching. Some of thepolycrystalline silicon layer has been etched awayeven though it was covered with photoresist. When thedimensions become smaller than about 2 [Lm under-cutting of this nature can reduce the accuracy toomuch and is no longer permissible.

In this respect plasma etching in a planar reactorcan give better results because ofthe contribution fromshort-lived radicals and from ions. The short-livedradicals have a greater reaction probability on col-lision with a wafer than the long-lived ones. This meansthat they have a smaller probability of arriving at thesides of the etch grooves after collisions. As the relativecontribution from short-lived radicals increases inrelation to that from long-lived radicals, etching in thedepth direction (anisotropic) increases and the under-cutting decreases. The same is true, only more so, forthe relative contribution from ions, since these areincident at right angles to the wafer and do not there-fore cause undercutting. The situation is similar whenthe ions catalyse the reactions of neutral radicals, asprobably happens in etching aluminium and poly-crystalline silicon with a CCl4 plasma or silicondioxide with a CHF3 plasma. Fig. 7 gives an exampleof plasma etching in a planar reactor with hardly anyundercutting.

To conclude, reactive ion etching is mainly aniso-tropic because of the relatively high ion contribution.In this respect this system is rather like the physical ionetching with accelerated inert-gas ions that we men-tioned earlier [9l. In comparison with that system,however, reactive ion etching has the advantage thatthe etching action is mainly chemical, so that a lowerion energy is sufficient. This means that there are noproblems with 'ditches', which are formed at the resistedges in physical ion etching (fig. 8). Another advan-

Fig. 5. Illustrating the effect of the different types of reactiveparticles on the etching profile. M photoresist masking. S spec-imen to be etched. Long-lived neutral radicals RI can causeundercutting if they reach the sides of the etch groove directlyor via collisions with the surface. Short-lived neutral radicalsRs give less undercutting since they can only arrive at the sidesdirectly, not via collisions. Ions I are incident at right angles anddo not give any undercutting.

Fig. 6. SEM photograph taken after plasma-etching a poly-crystalline silicon layer in a barrel reactor. Some of the siliconbenea th the photoresist has also been etched away.

1

Fig.7. SEM photograph taken after plasma-etching a poly-crystalline silicon layer in a planar reactor. The silicon beneaththe photoresist is almost unaffected by the etching.

tage over physical ion etching is that the reactionproducts are volatile, so that there is no undesiredredeposition of material that has been removed. Thediagram of fig. 9 gives a schematic representation ofthree different redeposition processes that adverselyaffect the final result in physical ion etching.

Philips tech. Rev. 38, No. 7/8 PLASMA ETCHING 205

Applications and limitations

Because of the differences in relative contributionsfrom the reactive particles the typical applications andthe limitations of the three systems are rather different.Plasma etching in a barrel reactor is widely used foretching silicon nitride and for removing photoresists.This system is unsuitable for dimensions smaller than2 [Lmbecause of the isotropic etching profile. Anotherlimitation is that it is not very easy to etch silicondioxide on a layer of silicon without removing siliconas well. This happens because the etch rate of theoxide is usually much less than that of the silicon. Itis also not possible to etch aluminium in a barrelreactor, since the aluminium is coated with a protectiveskin of oxide.

Aluminium can however be etched successfully witha CCl4 plasma in a planar reactor and by reactive ionetching. The oxide skin is then removed by the bom-bardment of the charged particles or by reactions with

I

Fig. 8. Illustrating the formation of ditches in 'physical ionetching'. M photoresist masking. S specimen to be etched. Inthis form of etching there is some etching of the mask as well.The mask is etched most rapidly in the neighbourhood of thetransitions between the upper surface of the mask and the sides.The slope of the sides of the mask with respect to the surface tobe etched then becomes smaller, and ions are no longer incidentparallel to the sides. Some of them collide with the sides of themask before they arrive at the etch surface. There is consequentlyan increase in the number of incident ions close to the edges.This gives a local increase in the etch rate, so that ditches areformed.

A

c!\

/ \

Fig. 9. Diagrammatic representation of some undesirable rede-position processes that can arise in physical ion etching. Inprocess A particles from the photoresist masking M are scatteredon to the surface of the specimen S; this delays the etching pro-cess. In process B particles from the specimen are sputtered on tothe sides of the mask, which can cause the formation of sharpedges after the resist is removed. In process C particles. that wereoriginally sputtered from the specimen arrive back at the etchsurface after colliding with gas atoms. This also delays theetching process. The effect increases in magnitude with increas-ing gas pressure.

the short-lived radical CCh. Plasma etching in a planarreactor and reactive ion etching are also suitable foretching polycrystalline silicon or silicon dioxide onsilicon. Since the etching can be anisotropic the detaildimensions can be very small (:(; I [Lm). A difficultywith anisotropic etching in a planar reactor is that theetch rates of different materials are not very differentin the gases commonly used in this system, so that theetching is not particularly selective. Disadvantages ofreactive ion etching and, to a lesser extent, of plasmaetching in a planar reactor are the erosion of photo-resists and the radiation damage caused by ions andelectrons. The radiation damage in reactive ion etchingcan be reduced by using a higher gas pressure andlarger electrodes. Measures such as these can make thesystem comparable with plasma etching in a planarreactor.

Table 1I gives a summary of special features, thematerials most commonly etched and limitations forthe three main plasma-etching systems.

The etch rate in a barrel reactor

As in other etching methods, the etch rate in plasmacan be varied by changing the conditions. In our in-vestigations of plasma etching in a barrel reactor wehave found that the etch rate in this system is stronglydependent on the composition and flow rate of the gas,and on the temperature.

Gas composition

We have studied the effect ofthe gas composition onthe etching of silicon and its compounds by CF4 plas-mas. The etch rate with a pure CF4 plasma is relativelylow. Adding oxygen increases the etch rate consid-erably, since more atomic fluorine is formed. Fig. JOshows the etch rate as a function of the oxygen contentof the plasma for two kinds of silicon dioxide and forsilicon nitride. The etch rate increases exponentiallyat first with the oxygen content and reaches a maximumat 2.5 to 3%. A further increase to 15% can be seen tohave very little effect on the etch rate.

At low oxygen contents the effect of the gas com-position on the etch rate depends on the material to beetched. The selectivity of the etching process cantherefore be changed by changing to another oxygencontent. This is illustrated in jig. 11, where the ratioRN/ Ro of the etch rate of silicon nitride to that ofsilicon dioxide is plotted as a function of the oxygencontent.

A marked effect on the selectivity has also beenfound in etching silicon and silicon dioxide with a

[9) See the article by Dimigen and Lüthje quoted in [3].

206 H. KALTER and E. P. G. T. VAN DE VEN Philips tech. Rev. 38, No. 7/8

plasma of CF4 and oxygen. With a plasma of CF4with 80% of oxygenthese materials are etched at aboutthe same rate, whereas with a plasma of CF4 with 8%of oxygen the etch rate for silicon is many times largerthan that for silicon dioxide [lOl.

Table U~ Some special. features, the most widely etched materials and some limitations, forthe three main systems for plasma etching.

Gas flow rate

The effect of the gas flow rate for etching silicondioxide and silicon nitride with a plasma of CF4 with4% of oxygen is shown in fig. 12. The etch rate R isfound to be inversely proportional to the gas flow

Plasma etching in abarrel reactor with

etch tunnel

Plasma etching in aplanar reactor

Reactive ion etching

Location of thewafers

Gas pressure

Reactive particles

Etching profile

Materials mostwidely etched

Limitations

outside discharge

30-300 Pa

long-livedradicals

isotropic

silicon nitridepolycrystallinesiliconphotoresist

undercutting

will not etchSi02 on siliconwill not etchaluminium

WOr-------------------------~run/min

c._x-x-x-x-x-x_"x.---- SiO;

r ··· · .. · ·Si3~lol

V

R

1

---co~Fig. 10. The etch rate R plotted on a logarithmic scale againstthe oxygen content C02 for the etching of three silicon compoundsin a barrel reactor with a plasma of CF4 and oxygen. The labelSi02* refers to silicon dioxide that has not been prepared bythermal oxidation of silicon, but by a gas-phase reaction be-tween silane (SiH4) and oxygen at a relatively low temperature(400-450 °C). The experiments were performed at a gas pressureof 133 Pa (1 torr), an r.f.· power of 100W, a temperature of100°C and a gas flow rate of 300 cm3/minute. Increasing cO2from 0 to 2 or 3% gives a large increase in R. At still highervalues of C02 the value of R remains constant.

inside discharge inside discharge

10-100 Pa 1-10 Pa

long- and short- long- and short-lived radicals lived radicalsions (small ions (considerablecontribution) contribution)

isotropic or anisotropicanisotropic

aluminium aluminiumpolycrystalline polycrystallinesilicon siliconSi02 on silicon Si02 on silicon

undercutting erosion ofsometimes photoresistsome erosion of radiationphotoresist damagesome radiationdamage

rate q [11]. A relation of this kind between I/R and qwould also be expected on theoretical grounds. In afluorine-containing plasma" the value of R is deter-mined by the concentration CF of atomic fluorine andthe reaction-rate constant (X for the chemical reactionthat produces the etching:

(1)

The fluorine concentration is determined by the life-time 1: in the reactor and the generation rate g:

CF = 1:g. (2)

The generation rate is assumed to be constant, sincethe r.f. power used in our experiments was always thesame. The lifetime depends on the rate r of the hetero-geneous recombination reactions at the reactor walls,the gas flow q and the rate of the homogeneous re-combination reactions in the gas, which is a functionof the pressure p. The reciprocal of the lifetime 1: isgiven by:

1/7: = Clr + C2q + csf(p), (3)

where Cl, C2 and Cs are constants. On combining the

----------------------------~-~.--------~

Philips tech. Rev. 38, No. 7/8 PLASMA. ETCHING 207

5~-------------------------~

1

°0~------5~--~--~W~----~5~~%Oo~~-C02

Fig. 11. The ratio RN/Ro calculated from fig. 10 of the etch rateof silicon nitride and silicon dioxide, as a function of the oxygencontent C02' When C02 is increased to about 2% the value ofRN/Ro decreases and so therefore does the selectivity for bothmaterials. At-higher oxygen contents RN/Ro is virtually constant.

80·r-------------~------~o~----,min/1ffTI

60R-1

1 40

20

O~--_=~--~~--~~--~~~o 100 200 300 400cmfmin---q

Fig. 12. Reciprocal of the etch rate R, plotted against the gasflow rate q, for silicon dioxide and silicon nitride etched in abarrel reactor with a plasma of CF4 containing 4% of oxygen ata gas pressure of 133 Pa, an r.f. power of 100 Wand a tempera-ture of 100°C. The relation between l/R and q is linear for bothmaterials.

30~~-------------'run/min

R

120

10

°0~~m~0~~2~00~~3~0=0--4~0=0~Pa____:_._p

Fig. 13. Etch rate R as a function of the gas pressure p for silicondioxide and silicon nitride etched in a barrel reactor with a plasmaofCF4 containing 4% ofoxygen at an r.f. power of 100 W, a gasflow rate of 300 cma/minute and a temperature of 100 "C, In thepressure range investigated the etch rate was found to be vir-tually independent of the pressure for both materials.

three equations above we obtain the following expres-sion for the reciprocal of the etch rate:

l/R = Clr/ag + c2q/ag + C3!(p)/ag. (4)

In the pressure range that we studied the etch rate of

silicon dioxide and silicon nitride in the CF 4 plasmawith 4% of oxygen is almost independent of the pres-sure; see fig. 13. The homogeneous recombinationreactions do not appear to play any significant parthere. This implies that the constant C3 in fig. 4 can beequated to zero, so that

l/R = clr/ag + c2q/ag. (5)

The parameters in this equation, apart from thereaction constant a, are independent of the material tobe etched. The value that can be derived from thestraight lines of fig. 12 for clr/c2, for example, is there-fore found to be the same, 250 cmê/minute, for the twomaterials considered.Equation (5) also indicates that a change in the gas

flow rate does not affect the selectivity of the etchingprocess. The 'ratio RN/Ro of the etch rates of siliconnitride and silicon dioxide is equal to the ratio of-theirreaction constants for all values of q. The value ofRN/Ro calculated from fig. 12 is 1.77 ± 0.01.As equation (5) shows, the etch rate reaches its

maximum value when the gas flow rate is so low thatthe second term is negligible compared with the first.However, a maximum for the etch rate by no meansguarantees optimum results from the etching process.Since the value of Clr is determined by the interactionbetween the reactive particles and the reactor walls, itis strongly dependent on impurities and on the natureand roughness ofthe wall. These effectsmake it difficultto obtain reproducible results at low flow rates.

Temperature

The etch rate in plasma etching also varies stronglywith the temperature. The magnitude of the tempera-ture dependence is a function of the material to beetched and the gas composition. In all cases we havefound that the effect of the temperature on the etchrate can be described by Arrhenius's equation for reac-tion velocity:

R = C exp (-Ea/kT), (6)

where c is a constant and Ea is the activation energyof the chemical reaction. The measured etch rates forfour materials are plotted on a logarithmic scale infig.14 as a functionrof l/T, for etching with a plasmaof CF4 with 4% of oxygen. Table III shows the valuesof the activation energy determined from the slope ofthe straight lines.Differences in the activation energies of the various

materials offer a means of varying the selectivity of the[la] G. Bell and B. Hasler, Extended Abstracts Electrochem.

Soc. 77-2 (Fall Meeting, Atlanta 1977), p. 411.[U] For a planar reactor, however, the etch rate at constant pres-

sure is almost independent of the gas flow rate. See C. J.Mogab, J. Electrochem. Soc. 124, 1262, 1977.

208 H. KALTER and E. P. G. T. VAN DE VEN Philips tech. Rev. 38, No. 7/8

etching process by changing the temperature. As anexample we show infig. 15 how the ratio RN/Ro oftheetch rate of silicon nitride to that of silicon dioxide in-creases with increasing temperature.The activation energy is determined not only by the

material, but also by the nature of the active particles.The etch rate is plotted on a logarithmic scale as a func-tion of 1fT infig. 16 for the etching of silicon nitride bya CF4 plasma with no oxygen, and by a plasma withan oxygen content of 8%. In spite of the large dif-ference in etch rate there is no difference in slope, sothat the activation energy is the same: 0.17 eV. Wecan therefore assume that the etching particles are thesame in both plasmas. This is in complete agreementwith the effect we discussed earlier of adding oxygenwhen etching with a CF4 plasma.The examples discussed here show that the tempera-

ture is an important factor in plasma etching. Goodtemperature control is therefore necessary for uniform, and reproducible etching. When more than one waferis being etched at the same time the temperatures of the-wafers should be as far as possible kept the same. Itmust also be possible to etch a number of successivebatches of wafers at an almost constant temperature.Good temperature control can be attained in a barrelreactor because of the use of the aluminium etchtunnel and by starting the etching process by pre-heating to a predetermined temperature in an inactivenitrogen plasma.

The experience summarized above provides a soundbasis for highly reproducible plasma etching of siliconand its compounds. The desired etch rate for eachmaterial can be obtained by an appropriate choice ofgas composition, gas flow rate and temperature. Ifnecessary the selectivity for the different materials canbe altered by varying the gas composition or the tem-perature.

Removal of photoresist masks

The plasma method can also be used for removingmasks of organic photoresists [12]. This is done byallowing an oxygen plasma to act upon the photoresistin the barrel reactor; the resist is then converted intoCO, C02 and water. The method is an attractivealternative to the conventional wet stripping in ICmanufacture with reagents such as fuming nitric acid.lts advantages fall into two main groups. The firstincludes the reduction in environmental pollution, theabsence of the need for certain safety precautions andthe associated reduction in the process costs. Thesecond group includes the increased compatibility withmodern processing, the virtual absence of unwanted

m3,--------------- __ -.nm/mln

10

1'=""---:::':::----=-=--:~=____=__:_:_! 11.8 2.2 2.6 3.0 3.4K--1000/T

Fig.14. Etch rate R, plotted on a logarithmic scale as a functionof the reciprocal of the absolute temperature T for four silicon-containing materials etched in a barrel reactor with a plasma ofCF4 containing 4% of oxygen at a gas pressure of 133 Pa, anr.f. power of 100 Wand a flow rate of 300 cm3/minute. In eachcase the plot is a straight line.

4,-----------------~RN/R03

t 2

O~--~~--~~--~~o 100 200 300oe----T

Fig. IS. The ratio RNIRo calculated from the results of fig. 14 ofthe etch rates of silicon nitride and silicon dioxide, as a functionof the temperature T. In the temperature range investigated thevalue of RNIRo increases nearly linearly with temperature.

100r--o.-----------,run/min

17---::':::---~--__=_~~~1.8 2.2 2.6 3.0 34K-1-1000/T

Fig. 16. Etch rate R, plotted on a logarithmic scale as a functionof the reciprocal of the absolute temperature T for silicon nitrideetched in a barrel reactor with a plasma of CF4 containing nooxygen and 8% of oxygen. The gas pressure was 133 Pa, the r.f.power 100 Wand the flow rate 300 cm3/minute. The plot isagain a straight line in each case. The etch rates are very dif-ferent, but the slopes are almost equal.

Philips tech. Rev. 38, No. 7/8 PLASMA ETCHING 209

Table m. Activation energy Ea for the etching of several mat-erials with a plasma of CF4 with 4% of oxygen in a barrel reactorwith an etch tunnel. The values of Ea were derived from theslopes of the straight lines in fig. 14.

Material Ea{eV)

0.120.100.100.17

Polycrystalline siliconSi02 (from thermal oxidation)Si02* (from gas phase)Silicon nitride

f03nm/mi

R

1

n '\sc\\\\

,2 \

\\\

0 -

10

~1.8 2.2 2.6 3.0 3.4K 1

-fOOO/T

Fig.17. Rate of removal R ofa photoresist (Waycoat Ill) in abarrel reactor by an oxygen plasma, plotted on a logarithmicscale as a function of the reciprocal of the temperature T. As inthe plasma etching of silicon-containing materials the plot is astraight line.

effects on the aluminium interconnections, and thereduction of the problems that can arise after ionimplantation with photoresist masking.In the investigation described here our main interest

was in the temperature sensitivity of the process andthe radiation damage in the processed wafers.In a barrel reactor with no tunnel the wafers are

quickly heated up to a temperature of 200 oe or morebecause of the direct contact with the plasma. Withthe tunnel the heating up takes much longer, so thatthe effect of the temperature on the etch rate is muchmore noticeable [13]. Fig: 17 illustrates how the removalof a photoresist from a silicon wafer depends on thetemperature. Just as in the plasma etching of siliconand its compounds (fig. 14) we again have a straightline, corresponding to Arrhenius's equation. Theactivation energy is found to be much higher in thiscase; it is about 0.5 eV.

[12]This is often referred to as plasma stripping of photoresists.[13] E. P. G. T. van de Ven and H. Kalter, Extended Abstracts

Electrochem. Soc. 76-1 (Spring Meeting, Washington O.C.1976), p. 332.

[14] J. A. Appels, H. Kalter and E. Kooi, Philips tech. Rev. 31,225, 1970.

[15]H. Kalter and E. P. G. T. van de Ven, Extended AbstractsElectrochem. Soc. 76-1 (Spring Meeting, Washington O.C.1976), p. 335.

For reasonably rapid removal of photoresists the temperatureof the wafers should be at least 120°C. The wafers can of coursebe heated up by omitting the tunnel. However, this has the dis-advantage that ion and electron bombardment can cause radia-tion damage to the outer edges of the wafers before the resist hasbeen completely removed from the centre. The wafers for sen-sitive circuits are first preheated in a nitrogen atmosphere at arelatively high r.f. power (about 500 to 800 W), with the plasmainside the tunnel. The photoresist, which is almost unaffected bythis, protects the wafers from damage due to ion and electronbombardment. Next, the photoresist is removed in an oxygendischarge at a relatively low r.f. power (e.g. 100W), with theplasma outside the tunnel.

Increasing the gas pressure from about 100 Pa toabout 400 Pa has hardly any effect on the rate ofremoval of the photoresist. Changing the gas flow hasa considerable effect, however. If the flow rate is nottoo low the rate of removal is inversely proportional toit. The results obtained are completely analogous tothose obtained in etching silicon and its compounds(figs 12 and 13).

Radiation damageWhen photoresists are removed by an oxygen plasma

there may be some radiation damage to the wafers byions, electrons and ultraviolet radiation. This can leadto instabilities in the finished circuits, as in an MOStransistor [14]. The seriousness of the damage is foundto depend on the number of sodium ions that penetrate

Table IV. Concentrations (wt ppm) of a number of elements inthree types of photoresist. The concentrations were determinedby radiochemical analysis after excitation by a neutron flux of2.2 x lOll cm-2 S-1 for three hours.

~t Na K Cu Zn As Br AgR

Waycoat III 2.8 <0.2 0.40 5.6 0.03 0.17 0.4Shipley AZ1350H 7.8 0.54 0.32 0.013 0.14 <0.4Shipley AZl11 13 0.65 0.30 11 0.02 0.15 <0.4

into the silicon-dioxide layer from the photoresist. during the discharge [15]. It has been found fromradiochemical analysis that the sodium content of thevarious types of photoresist is relatively high. Photo-resists also contain a number of other elements thatcause less damage and usually occur in considerablylower concentrations; see Table IV.The sodium-ion transport can be traced with the

radioactive isotope 24Na. When the photoresist is.removed without making use of the etch tunnel it isfound that more than half of the sodium is located inthe silicon-dioxide layer afterwards, and can no longerbe removed by rinsing with water or concentratednitric acid or by etching in a 1% HF solution; see

210 PLASMA ETCHING Philips tech. Rev. 38, No. 7/8

Table V. Residue of 24Na on and in Si02 and Si02 coated with30 nm of phosphosilicate glass (PSG), after removal of thephotoresist (Waycoat Ill) in an oxygen plasma, both with andwithout tunnel. The figures quoted are percentages of the amountof 24Na (about 10 ppm) initially used for doping the resist.

Percentage of 24Na

Material withouttunnel

withtunnel

after etching for 30 seconds in 52l%HF

Si02

without rinsing

after rinsing in H20 or HN03

10055

1002

Si02-PSG

without rinsing

after rinsing in H20 or HN03

after etching for 30 seconds inl%HF

100452

10020

o 0.2 0.4---d

Fig. 18. Schematic diagram of the variation in the concentrationCNa of the radioactive isotope 24Na in a silicon-dioxide layer asa function of the depth below the surface. The 24Na originatedin the photoresist (Waycoat Il l), which was first doped with the24Na, then removed from the oxide layer by an oxygen plasmain a barrel reactor with no etch tunnel. A considerable number ofsodium ions are concentrated close to the boundary (dashed line)with the silicon layer beneath the oxide.

[16] R. A. Kushner, D. V. McCaughan, V. T. Murphy andJ. A. Heilig, Phys. Rev. BlO, 2632, 1974.

[17] This glass is generally used in IC technology to give protec-tion from external contamination.

Table V. The diagram ofjig. 18 shows how the sodiumhas become distributed in the oxide layer. A signif-icantly large number of the sodium ions are found tohave travelled right through the oxide layer during thedischarge, finally arriving close to the boundary withthe silicon layer beneath the oxide. The driving forcefor this migration is a potential gradient in the oxidelayer resulting from various interactions between theplasma and the wafer [161. In an MOS transistor sucha migration causes instabilities because of the occur-rence of charge variations in the oxide layer [141.

When the etch tunnel is used the situation is muchbetter. After the surface of the silicon-dioxide layer hasbeen cleaned with a I % HF solution, it is found thatonly I % of the original amount of 24Na is left in theoxide layer.

A considerable improvement can also be attainedby coating the silicon-dioxide layer beforehand with athin layer of phosphosilicate glass (PSG), which hasthe property of combining chemically with sodiumions [171. This makes it almost impossible for thesodium ions to penetrate into the oxide layer. Evenwhen the etch tunnel is not used most of the sodiumions do not travel further than the upper part of thePSG upper layer, as can be shown by etching away apart of this layer with a 1 % HF solution (Table V).

Summary. In plasma etching solid materials are converted intovolatile compounds by chemical reactions with reactive particlesthat are formed in a gas discharge. This method of etching isnow more and more widely used in IC technology because ofthe trend towards further miniaturization. The extent to whichthe various kinds of reactive particles contribute to the etchingprocess mainly depends on the particular system used for theplasma etching, which therefore has a considerable influence onthe results. In the investigations described here, a study wasmade with one particular system - plasma etching in a barrelreactor - to find out which parameters were important inetching silicon-containing materials with CF4 plasmas andphotoresist masks with oxygen plasmas. The etch rate was foundto be dependent on the material, the gas composition and thetemperature, strongly dependent on the gas flow, but almostindependent of the gas pressure. The electrical damage that oftenoccurs to wafers when photoresist is removed is found to be theresult of the migration of sodium ions originating in the photo-resist. This damage can be prevented by choosing the conditionsappropriately.