The etching of CHF3 plasma polymer in fluorinecontaining dischargesAnand J. Bariya, Hongqing Shan, Curtis W. Frank, Sidney A. Self, and James P. McVittie Citation: Journal of Vacuum Science & Technology B 9, 1 (1991); doi: 10.1116/1.585784 View online: http://dx.doi.org/10.1116/1.585784 View Table of Contents: http://scitation.aip.org/content/avs/journal/jvstb/9/1?ver=pdfcov Published by the AVS: Science & Technology of Materials, Interfaces, and Processing Articles you may be interested in Etch characteristics of iridium in chlorine-containing and fluorine-containing gas plasmas J. Vac. Sci. Technol. A 19, 2400 (2001); 10.1116/1.1385912 Reactive ion etching of SiGe alloys using fluorinecontaining plasmas* J. Vac. Sci. Technol. A 11, 2492 (1993); 10.1116/1.578598 Reactive ion etching of polyimidesiloxanes in fluorinecontaining discharges J. Vac. Sci. Technol. A 10, 1124 (1992); 10.1116/1.578213 Plasma etching of Ti in fluorinecontaining feeds J. Appl. Phys. 71, 462 (1992); 10.1063/1.350679 Effects of doping on polysilicon etch rate in a fluorinecontaining plasma J. Appl. Phys. 57, 2221 (1985); 10.1063/1.334366

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The etching of CHF3 plasma polymer in fluorine .. containing discharges Anand J. Bariyaa) Department of Chemical Engineering, Stanford University, Stanford, California 94305

Hongqing Shan Department of Mechanical Engineering. Stanford University, Stanford. California 94305

Curtis W. Frank Department of Chemical Engineering, Stanford University, Stanford, California 94305

Sidney A Self Department of Mechanical Engineering, Stanford University, Stanford, California 94305

James P. McVittie Department of Electrical Engineering, Stanford University, Stanford, California 94305

(Received 29 May 1990; accepted 1 November 1990)

The etching of CHF3 plasma polymer in fluorine containing electrical discharges was studied. The fluorine sources were SF 6' CF4 , and mixtures of the two. For discharges in SF 6 and mixtures of SF6 and CF4 , a good correlation was obtained between the etch rate and the atomic fluorine concentration measured using actinometry. For CF4 , the etch rate was found to be much higher than that predicted from this correlation. This is attributed to the energetic ion bombardment of the polymer surface in the CF4 discharge. X-ray photoelectron spectroscopy analysis of the etched polymer surface shows an increased fluorine content, but the F:C ratio was independent of the etching conditions. The implications of the results for the kinetics of fluorocarbon plasma polymerization are discussed.

I. INTRODUCTION

Glow discharges are widely used in materials processing and in the fabrication of integrated circuits. These applications generally rely on the complex interactions between the glow discharge (plasma) and exposed solid surfaces. Depending on the glow discharge and the nature of the surface, such interactions can lead to etching or thin film deposition.

When a plasma is struck in certain organic gases, a thin polymeric film is found to be deposited on exposed surfaces. This phenomenon is caned "plasma polymerization." Plas­ma polymerization of fluorocarbons has been the focus of a great deal of interest. This is not only because of the stability (thermal as wen as chemical) and other attractive features of the resulting polymer films, but also because fluorocar­bons are routinely employed in plasma etching, and the inad­vertent formation of plasma polymer plays an important role in the etching process. For instance, by forming selectively on the side-wans of etch features-formation on the bottom being prevented by the directional energetic ion flux incident from the discharge-the plasma polymer films block lateral etching and thereby help prevent undercut. In modeling etching processes, the effect of such "side-wall passivation" must be taken into account. This requires an understanding of the kinetics of plasma polymer deposition.

The molecular breakdown of a fluorocarbon in a plasma leads to the formation of, among other species, CF x free radicals that are among the primary precursors of the plas­ma polymer, and free F atoms that can etch the polymer as it deposits. The overall kinetics of polymer deposition then re­flects the net balance between the association of the CF, radicals to form the polymer, and the attack by the free F

atoms to etch and remove the polymer. This balance is best illustrated by a comparison between the rate of plasma poly­merization in perfluoroalkanes members of the homologous seriesCnFzn f-2 andinCHF3 ("Freon 23"). The rate of pI as­rna polymerization in perfluoroalkanes increases with in­creasing molecular weight, a trend that goes hand in hand with an increase in the precursor concentration in the dis­charge.1.2 the rate of plasma polymerization in CHF3 is con­siderably higher than that in even the higher perfiuoroal­kanes like C3F Il under similar discharge conditions. 2 Though the concentration of precursor radicals is slightly higher in CHF 3 than in C3F g, the difference is not so great as to explain the vastly greater deposition rate in the former. 2 The reason for the difference lies in the fact that very little free fluorine is generated in the CHF3 discharge, whereas the perfluoroal­kanes, including C}Fs, generate much free fluorine which can then etch the polymer as it deposits.

The coexistence of polymerizing species (CFx radicals) and etching species (F atoms) in the discharge complicates the overall mechanism of polymer deposition. One way to deal with this complexity is to isolate individual reactions, study them independently, and build up the overall picture step by step. In a previous paper,3 there was a focus on the kinetics of CHF 3 plasma polymer deposition. In this paper, we focus on the etching ofCHF3 plasma polymer in fluorine containing discharges. The essential results may be expected to be valid for fluorocarbon plasma polymers in general.

From a broader perspective, the plasma etching of poly­meric materials is of general interest for materials processing and microelectronics applications. For instance, in integrat­ed circuit fabrication, exposure of polymeric materials to plasmas is routine. Two examples are (1) Photoresists,

1 J. Vac. Sci. Techno!. B 9 (1), Jan/Feb 1991 0734-211X/911010001·07$01.00 @) 1991 American Vacuum Society

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2 Bariya et sl.: CHF3 plasma polymer in fluorine-containing discharges 2

which are organic polymers, are commonly used as masks during plasma etching. Photoresist etching leads to loss of control of line width and has an important effect on the shape of the etch profile. (2) Plasma etching of polymeric materials is important in multi-level lithography.

In an earlier study, 4 the etch characteristics of a variety of conventional polymer films in CF4 and CF4 + O2 discharges were investigated, with a stress on the relationship between the etch rate and the chemical structure of the polymers. It was found that polymers containing aromatic groups etch at significantly lower rates than nonaromatic polymers, re­gardless of whether the aromatic moiety is in a side chain or on the backbone. Further, the etch rates are not directly related to the thermal stability of the polymer, but do corre­late to the susceptibility of the polymer to chain scission by high energy irradiation.

In another study, 5 a linear correlation between the fluoro­carbon plasma polymer etch rate in a fluorine-containing discharge and the bulk fluorine atom concentration was re­ported. In this study, using plasma polymerized CHF 3 films, we focus on the etch kinetics and try to elucidate the etch mechanism.

II. EXPERIMENTAL

CHF 3 plasma polymer films were deposited on previously weighed 3 in. diam Si wafers in a commercial reactive ion etching (RIE) system, the AME8110, produced by Applied Materials, Inc. The reactor is schematically illustrated in Fig. 1. Substrates can be placed on either electrode. Those placed on the powered electrode are subjected to a flux of energetic positive ions. The energy of these ions depends, among other things, on the dc self-bias that develops be­tween the powered and grounded electrodes as a result of the difference in their areas.6 The dc self-bias between the elec­trodes Vdc was maintained constant during deposition. Films were deposited at various values of dc bias in order to compare their etch characteristics. During deposition, the pressure and CHF3 flow rate were maintained at 50 mTorr and 85 sccm, respectively.

After film deposition, the Si wafers were weighed again, and the amount of polymer deposited determined. All

weighings were done on an electronic weighing balance sen­sitive to 10 - 5 grams.

The substrates coated with the CHF3 plasma polymer films were then placed on the powered electrode of a parallel plate etching reactor (Drytek 184) and etched in fluorine containing discharges. These were discharges in SF 6' CF4 or mixtures of the two. During etching, the fluorine atom con­centration atthe center of the discharge, [F), was monitored using actinometry,7-11 necessitating the addition of a small amount of Ar to the discharge. Actinometry involves mea­suring the 750.9 nm Ar emission line intensity (I Ar ) and the 703.4 nm F emission line intensity (IF) in the plasma emis­sion spectrum. For a constant Ar mole fraction in the feed gases, the atomic fluorine concentration is proportional to the product of the pressure P and the ratio of the F line intensity to the Ar line intensity:

We have avoided using a proportionality constant, and have dealt instead with relative fluorine atom concentrations. Therefore, [F] has arbitrary units throughout this paper.

The etching time was limited to ensure that the polymer films were not etched through. The pressure and power were varied, but the total flow of the fluorine-bearing gases was maintained at 200 sccm and that of Ar at 10 sccm. The dc self-bias was monitored. After etching, the substrates were weighed yet again, and the mass of polymer etched away determined. The time average etch rate was then obtained by dividing the amount of polymer etched away by the etch time. Note that the etch rate so obtained is also an average for the area of the silicon wafer. Polymer films deposited on both electrodes at various levels of dc bias were subjected to this process to investigate the dependence of the etching characteristics on deposition conditions.

Modification of the surface chemical structure of the plas­ma polymer films by exposure to the fluorine containing dis­charges was investigated with x-ray photoelectron spectro­scopy (XPS). The XPS was performed on a V G ESCALAB system with Al Ka x rays. The samples were exposed to air during their transfer from the etching system to the spectrometer.

hexode (powered electrode) l-::::>--"Powered" substrates

FIG. I. Schematic of deposition reactor. The bell-jar, and the base-plate on which it rests, con­stitute the grounded electrode. The bell jar encloses the powered electrode. Substrates can be ver­tically mounted on the powered electrode, or can He horizontally on the base plate.

bell-Jar --....... ooo! (grounded)

"Grounded" substrate

base-Plate ____ ....... ~::::::::::::::~~::~------_1 (grounded)

J, Vac. Sci. Techno!. B, Vol, 9, No.1, Jan/Feb 1991

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3 Bariya eta/": CHF3 plasma polymer in fluorine·containing discharges 3

III. RESULTS A. Etch rate studies

Figure 2 shows the etch rate, in ,ug/min cm2, of CHF3

plasma polymer films deposited under a variety of different conditions as a function of the relative fluorine atom concen­tration measured using actinometry. The pressure and pow­er of the etching discharge were varied in order to vary the atomic fluorine concentration. Figure 2 contains etch rate data obtained using SF 6 alone and mixtures of SF 6 and CF4

as the fluorine sources. Two mixtures of SF 6 and CF4 were investigated, one with SF6 and CF4 flow rates of 125 and 75 sccm, respectively, and the other with flow rates of 160 and 40 seem, respectively.

The most striking aspect of Fig. 2 is that there is a clear correlation between the etch rate and the F atom concentra­tion, Fitting a power-law curve to the data in Fig. 2 (shown as the solid curve) yields the following relationship between the etch rate Re , and the F atom concentration in the bulk plasma, [F]

(1)

where ke is a proportionality constant. Equation (1) was obtained using a least-square fit to the

data in Fig. 2. The equation holds over a wide range of pres-

400

-N

E u c: "e 300 0

--C) .=;, Q) -co a: C» 200 c: :c ~ UJ

100

100 200 300 400

F-Concentration (arb. unit)

FIG, 2. CHF, plasma polymer etch rate R, in,ug/(min cm l), as a function

the relative atomic fluorine concentration, [F], measured with actino­metry. Data are presented for plasma polymer deposited on both the powered and grounded electrode of the system illustrated in Fig. I at var­ious levels of de bias. [FJ was varied by changing the pressure, power and composition (SFo,CF. ratio) of the discharge. The curve is a least-squares fit to the data which yields Rc (5 [F] 1.0.

J. Vac, Sci. Technol. S, Vol. 9, No. t, Jan/Feb 1991

sure and power. This is illustrated in Fig. 3, which is the same as Fig. 2 except that ( 1 ) only those data points in Fig. 2 that pertain to plasma polymer deposited on the powered electrode at a de bias of - 200 V are shown, and (2) of these data points, those obtained at various pressures are now dis­tinguished by different symbols. Thus the data points shown in Fig. 3 are a subset of those shown in Fig. 2. The solid curve represents Eq. (I). Clearly, there is no significant pressure­related deviation from Eq. (1).

Interestingly, there are no significant differences between th etch characteristics ofthe plasma polymer films deposited under different conditions. Thus films deposited on the powered or the grounded electrode, and at various values of de bias, aU etch at about the same rate for the same atomic fluorine concentration in the bulk plasma. This is illustrated in Fig. 4, again a plot of Re versus [FJ, in which different symbols are used for data points obtained for plasma poly­mer films deposited under different conditions. Again note that the data points shown in Fig. 4 are a subset of those shown in Fig. 2.

The effect of adding CF4 to the discharge is shown in Fig. 5. This shows the etch rate of plasma polymer deposited on the powered electrode at a de bias of - 200 V as a function of [F]. The etching was done with mixtures of SF 6 and CF4 , or only CF4 , as the fluorine sources. Two SF 6 + CF4 mixtures were used, as described above. The solid curve represents Eq. (1). The etch rates obtained with the two SF 6 + CF4

mixtures do not deviate significantly from this curve. How-

500

Etching: 0

.: 150 mtorr

400 II : 200 mtorr v: 250 mtorr 0

It. : 300 mtorr c:;--0: 350 mtorr E

(.) '0/ : 400 mtorr C 8: sao mtorr 300 'e ..... Ol ;;1. -(l) -«S 0: Ch 200 c:

~ W

100 200 300 400

F~Concentration (arb. unit)

FIG. 3" Rc as a function of [FJ showing the effect of pressure. The plasma polymer was deposited on the powered electrode at a de bias of -- 200 V. The curve represents Eq. (l).

... -.. .-.' •.•.• -•.•.•.•.•.• ,' ......... -.:.:.~.:.:.:.: •.•• " .•••.•••••• ' ••••• :.:<.:.;.;.:.;0;.;.;.;.:.; ............. ; ..••.• : •.• :.;.; ••.•...•.••.•••• ; •••••• :.:":.~.:.:.:-:-:-.;.;.;.:.: •.•••.•.•.•••..•...•...•..• -.-.-.,.-.-£ •••••.• '." •• ....... - ...... ,.' ... -............. :.;.;.;.; ............ ' ..•............................•....•........ , .. Redistribution subject to AVS license or copyright; see http://scitation.aip.org/termsconditions. Download to IP: 130.209.6.50 On: Sat, 20 Dec 2014 13:43:54

4 8ariya et sl.: CHF. plasma polymer in fluorlne-containing discharges 4

500

o : - 200 V dc' Powered \l : - 300 V dc' Powered

400 I/),: -400 V dc' Powered . .: - 200 V dc' Grounded

N \'9' : -300 V dc' Grounded

E A : - 400 V dc' Grounded 0

c: 's 300 -..

~ ~ a: C) 200 c: "f' :.c $:a UJ

100

100 200 300 400

F-Concentration (arb. unit)

FIG. 4. Dependence of CHF, plasma polymer etch characteristics on depo­sition conditions. Deposition conditions are specified in terms of the de bias and the placement (powered or grounded electrode) or the substrate in the deposition reactor. Open symbols: powered electrode. Filled symbols: grounded electrode. The curve represents Eq. (1).

ever, the etch rates obtained with CF4 alone as the fluorine source deviate considerably, being always much higher than predicted by equation (1).

The higher-than-expected etch rates observed with CF4

discharges result from the energetic ion bombardment inci­dent onto the surface of the plasma polymer film in the CF4

discharge. To understand why energetic ion bombardment is important in CF4 discharges and not in SF6 discharges, one should consider the electric potential in the plasma. Figure 6 shows the spatial variation of the time average electrical po­tential in a plasma maintained between paraIIel plate elec­trodes. One of the electrodes is grounded, and thus has po­tential V = O. The potential in the bulk of the plasma is approximately constant at a value known as the "plasma potential," Vp. Vp is always higher than the potential of ei­ther electrode. Adjacent to the electrodes are regions across which the potential varies considerably. These regions con­stitute the plasma "sheaths." A dc bias Vdc is established between the two electrodes when their areas are not equal (the "area of the grounded electrode" includes the areas of an grounded surfaces in contact with the discharge such as the walls ofthe chamber, and is therefore usually larger than the area of the other, "powered," electrode; hence, Vdc < 0). From Fig. 6 it is clear that the potential across the sheath associated with the powered electrode V'h is given by

(2)

J. \lac. Sci. Technol. S, Vol. 9, No.1, Jan/Feb 1991

500

Etching: \l: SFslCF JAr (160/40110)

400 b..: SFtCF JAr (125175110) • : CFiAr (200110)

N E Q

c: 300 'E -~ Q)

ca ex: C) 200 c: :c $:a W

100

F-Concentration (arb. unit)

FIG. 5. R" as a function of [FJ for CF4 containing discharges. The solid line

represents equation ( I ). When CF4 is the sole fluorine source, the etch rates arc considerably higher than predicted from Eq. (1).

Now, discharges in SF 6 are electronegative, 12 which means that they have a high negative ion concentration, al­most equal to the positive ion concentration. As a result, the value of v~ is relatively low. In addition, the magnitude of Vdc was measured to be low: it never exceeded 30 V and had

v

···{····f········· ." J~ ..... .

~----------------------------~--~ x

o Small. or powered, electrode

• Large. or grounded. electrode

FIG. 6. Time average electric potential in a parallel-plate discharge. Y:h = potential drop across the sheath associated with the powered electrode, Vdc = de bias, and Vp = plasma potential.

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5 Bariya et at.: CHF~ plasma polymer in fluorine-containing discharges 5

TABLE 1. Surface analysis of etched CHF, plasma polymer: etch conditions for the various samples.

Sample Pressure Power Etch time [FJ no. (mTorr) (W) (min) (arb. units)

3S0 300 1.0 260.6 2 350 150 2.5 127.2 3 350 150 1.0 l24.4 4 350 500 0.5 356.6 5 350 400 0.5 362.0

a mean value of 9 V (this includes the I Vdc I measurements for the SF 6 + CF4 mixtures). Thus, as per Eg. (2), the value of V,h was low.

CF4 discharges, on the other hand, behave e1ectropositive­ly. Both Vp (not measured) and, Vel" I (measured) are high­er than in the SF 6 discharge. Indeed, the magnitude of Vdc

was found to exceed 200 V for all the runs with CF4 as the sole fluorine source. The mean value was 300 V. Hence, as indicated by Eq. (2), the value of V,h is high. Thus, ( V,h )SF" ~ ( V,h )C'F,·

Positive ions generated in the discharge are accelerated across the sheaths and strike the electrodes with consider­able kinetic energy. The kinetic energy is a monotonically increasing function of the potential drop across the sheath (it also depends on the ratio of sheath thickness to collision mean free path). Thus the substrates, which are placed on the powered electrode, experience ion bombardment with ion energies dependent primarily on : V'h I. Since ( V,h ) SF" ~ ( V,h ) CF4 , the energy of the bombarding ions is much lower in the SF6 discharge than in the CF4 discharge. As a result, the etch rate for the same atomic fluorine con­centration is higher in the CF4 discharge. Such enhancement of surface chemical processes by energetic ion bombardment is a well recognized phenomenon in plasma processing.

80 Surface analysis

X-ray photoelectron spectroscopy (XPS) analysis of CHF3 plasma polymer films exposed to SFr:. ( + Ar for ac­tinometry) discharges under a variety of conditions was car­ried out to gain additional insight into the surface chemistry resulting from the plasma exposure of the polymer films. Table I shows the conditions of power, pressure and time under which the various samples were etched in the SF(, dis­charge, and the corresponding relative atomic fluorine con­centration determined using actinometry. The etch duration was usually limited by concerns about the polymer being etched all the way through. Thus generally the etch duration was reduced for the higher F concentrations.

method 1:

(;0) ~~~

1

1 ~-~--.-~~~

265 290 294 298 302

(b2)

286 290 294 298 102

286 290 29< 298 302 Binding Energy I eV

(bi)

286 290 294 298 302

(b3)

2116 290 294 2se 3Q2

286 290 294 298 302 !hnding Energy I eV

FIG. 7. XPS C Is peaks of (a) as-deposilt'd. unetehed CHF, plasma polymer (deposition conditions: pressure = 50 mTarr, de bias = .- 200 V and de­position time = 2 h); (bI )-(h5) etched polymer samples corresponding, respectively, to samples 1-5 in Table I.

Figure 7 (a) shows the C ls peak from the XPS spectrum of an as-deposited, unetched CHF, plasma polymer film. Figures 7 (hI )--7 CbS) show the post-etch C Is peaks for the samples listed in Table I. The C Is peaks are resolved into C Is peak can be resolved into individual "sub-peaks" corre­sponding to C atoms bonded to three, two, one, or no flu­orine atoms. 13·15 The last category (no fluorine atoms) ac­tually corresponds to two sub-peaks: a C atom belonging to this category that has at least one neighbor (an adjacent C atom with which there is a chemical bond) that is bonded to one or more fluorine atoms has a different binding energy for its Is electrons than one that has no fluorinated neighbors. Thus up to five sub-peaks may be discerned in the overall C Is peak, and these may be labeled, in descending order of binding energy, CF3 , CFb CF, C-CF, and C. For the C ls peaks presented in Fig. 7 only four sub-peaks are discernible, and these are assigned to CF3, CF2, CF, and C-CF<. In fact, all the samples analyzed had their C Is peaks resolvable into only four sub-peaks and the peak assignments were made in this manner.

The fluorine to carbon ration (F:C) at the surface of the plasma polymer film can be obtained form the XPS spec­trum using either of the following two methods:

F (3 X area ofCF:\ peak) + (2XareaofCF2 peak) + (areaofCFpeak) -=--'-..--:..-----"'-'.--~'--"------=--'~~-'-....:-..-----'.---"-

C total area of C Is peak

method 2:

F Area of Is peak X 0.25. C Area of C Is peak

J. Vac. Sci. Techno!. B, Vol. 9, No.1, Jan/Feb 1991

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6 Bariya et a/.: CHF 3 plasma polymer in fluorine-containing discharges 6

The factor of 0.25 in method 2 is an atomic sensitivity factor (ASF). 3 The ASF takes into account the facts that (1) F atoms and C atoms have different sensitivites to x rays for photoelectron emission and (2) the kinetic energies of the photoelectrons emitted by F atoms and C atoms are differ­ent, leading to a difference in their escape depths.

Now, the F Is photoelectrons, with a kinetic energy of about 793 eV, have an escape depth ( = 3 X electron mean free path) of about 25 A16 while the C Is photoelectrons, with a kinetic energy of about 1195 eV, have an escape depth of about 60 A. 16 Method 1 gives a value for the F:C ratio that is an average over the escape depth of the C is photoelec­trons. If the material is homogeneous, with a constant atom­ic fluorine density, then method 1 and method 2 should give the same value for the F:C ratio. If, however, the fluorine atom density changes over a depth equal to the escape depth of the C 1s photoelectrons, the two methods differ: method 2 gives a higher value ofthe F:C ratio ifthe surface is richer in fluorine, and vice versa.

Results of the XPS analysis of CHF 3 plasma polymer films (deposited on the powered electrode at a dc bias of - 200 V) exposed to SF 6 ( + Ar for actinometry) plasmas

at 350 mTorr pressure and various values of power and time are presented in Table II (a similar analysis of polymer etched in CF4 discharges was not done). Shown in this table are the percentages of the total area of the C Is peak consti­tuted by each sub-peak, as well as the total areas of the CIs, F Is and F 2s peaks. Also shown are the F:C ratios obtained using the two methods described above, and the F Is:F 2s ra­tio. The F 2selectrons, with a kinetic energy of 1448 eV, have an escape depth of around 125 A [Dilks l6 provides a value of 25 A for the mean free path, in plasma poly (vinylidene fluoride), ofF 2s photoelectrons excited with Mg Ka x rays; these photoelectrons have a kinetic energy that is 233 e V less than those excited by the Al Ka x rays used in our investiga­tions J, larger than the escape depth of F Is photoelectrons. Thus F Is:F 2s for an arbitrary sample is less than that of an homogeneous sample if the fluorine atom density increases with depth and vice versa.

The first row contains data for a control sample which is an as-deposited plasma polymer film unexposed to any sub­sequent discharges.

The following points should be noted about the data in Table II.

(1) Exposure of the CHF3 plasma polymer films to flu-

TABLE II. XPS analysis of samples described in Table 1.

C Is peak analysis

Sample CF, CF2 CF C-CF, cont. 17 25 29 29

17 38 23 22 2 20 34 29 17 3 16 39 28 17 4 19 35 24 23 5 16 40 22 22

J. Vac. Sci. Techno!. B, Vol. 9, No.1, Jan/Feb 1991

orine-containing discharges leads to a large increase in the density of CF2 groups in the polymer. The density of CF3

groups is relatively unchanged. This is also clear from com­paring Fig. 7(a) with Figs. 7(bl )-7(b5).

(2) The F:C ratio of the etched plasma polymer films is significantly higher than the F:C ratio of the as-deposited plasma polymer film (the control sample) .

(3) There is no correlation between [FJ and the F:C ratio ofthe etched plasma polymer films. Indeed, the various sam­ples of etched plasma polymer do not differ significantly in their F:C ratio: they all lie within ± 3% of a mean value of 1.524 (calculated using method 1). As mentioned above, this is significantly higher than the value of 1.3 for the con­trol sample.

(4) The F:C ratio of the etched plasma polymer films obtained with method 2 is systematically higher than that obtained with method 1. However, the difference is only slight. The maximum difference is for sample 1, for which method 2 gives a value of the F:C ratio that is 7.33% higher than that calculated using method 1. Overall, method 2 gives an F:C ratio that is, on the average, just 5% higher than that obtained with method 1.

e 5) The various samples of etched plasma polymer films do not differ significantly in the values of their F l \:F 2s ratio. These values all lie within ± 4.5% of a mean value of 26.176. However, this mean value is higher than the F,:F2s ratio for the control by 11.2%, a significant amount.

The fact that the F:C ratios calculated using method 2 are systematically higher than those calculated using method 1 implies that the fluorine density is probably higher at the surface and decreases as one proceeds down to depths of the order of the escape depth of C Is photoelectrons (about 60 A). However, since the discrepancy between the F:C ratios obtained with the two methods is only slight, the gradient in the fluorine density is small, implying that there is signifi­cant fluorine diffusion down to about 60 A.

The fact that the Fh :F2s ratios of the etched polymer films are systematically higher than the corresponding ratio for the control sample implies that there is a significant differ­ence in the fluorine density between near-surface regions and regions at depths of the order of the escape depth of F 2s photoelectrons (about 125 A). The lack of any differences in the F Is :F2s ratios between the various etched samples indi­cates that this gradient in the fluorine density is not depen­dent on the etching conditions.

Peak areas F:C ratios F,,:F2 •

Cis F Is F2s Meth. ! Meth.2 ratio 35.13 186.40 7.92 1.3 1.33 23.54

32.11 206.70 7.80 1.50 1.61 26.50 33.67 220.11 8.46 1.57 1.63 26.02 33.60 220.03 8.50 1.54 1.64 27.33 34.65 216.17 8.48 1.51 1.56 25.49 35.32 220.95 8.65 1.50 1.56 25.54

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7 8ariya et 81.: CHF3 plasma polymer in fluorine-containing discharges 7

IV. CONCLUSIONS

The time (and area) average etch rate of fluorocarbon plasma polymer in fluorine-containing discharges is solely a function of the bulk atomic fluorine concentration, [F], pro­vided there is a negligible energy flux onto the surface due to energetic ion bombardment. The etch rate is then propor­tional to [F1 1

.6

<}. This relationship holds for a wide variety of conditions of pressure, power and composition (SF 6:CF 4 ra­tio). Further, the etch characteristics of the plasma polymer, in the absence of ion bombardment, are very insensitive to the deposition conditions. Thus polymer films deposited on the powered and grounded electrodes at various values of dc bias etch at roughly the same rate for the same atomic flu­orine concentration in the etching discharge. When the sur­face is subjected to energetic ion bombardment, the etch rate is considerably increased. This is a fairly typical feature of plasma etching.

Several studies have shown a correlation between the rate of plasma polymerization in fluorocarbon discharges (on the grounded electrode) and the concentration ofCFx radi­cals, mainly CF2• 17-19 Kitamura, et at., 19 for instance, found a relationship of the form

Rp ex [CF2 V5-

3,

whereRp = rate of polymerization and [CF2 J = concentra­tion ofCF2 radicals. These studies have usually failed to take into account the coexistence of depositing and etching spe­cies in the discharge. Based on the results presented above, a more appropriate correlation should take the form

Rp = Rd - Re

=kd [CF2]"'-k e fFjl69, (3)

where Rd is a "deposition rate," Re is an etch rate, and Rp is the net rate of polymerization. R p = Rd if no etching species are present in the discharge. Of course, this equation is only valid when energetic ion bombardment is not important. In the presence of an energetic ion flux, both Rd and Re are strongly affected.

In spite of Eq. (3), a good correlation between Rp and [ CF 2 J alone is observed because [F] and [CF 2 J are not independent variables but are related by the dissociation and loss processes in the discharge. For example, for a C2F/> dis­charge, the proposed dissociation processes are20

:

C2F" + e --.2CF3 + e- ,

CFl + e - -- CF2 + F + e

According to this scheme, the generation rates of CFz radi­cals and F atoms are the same. The relative loss rates would then determine the correlation between [FJ and [CF2 J. If, for instance, the loss rates are very similar, then [FJ;::::: reF2 ]·

J. Vac. Sci. Techno!. S, Vol. 9, No.1, Jan/Feb 1991

XPS analysis shows that the fluorine content of the CHF3

plasma polymer increases on exposure to SF 6 ( + Ar for actinometry) discharges, There is a large increase in the den­sity of CF2 groups in the polymer, The final F:C ratio is relatively independent of the etching conditions, induding the atomic fluorine concentration. Fluorine diffuses into the plasma polymer: the fluorine density varies only slightly for depths of the order of the escape depth of C Is photoelec­trolls (about 60 A.). However, when one considers distances of the order of the escape depth of F 2s photoelectrons (ap­proximately 125 A.), a significant reduction in the fluorine density is dearly discerned.

ACKNOWLEDGMENTS

This work was supported by grants from SRC, the Semi­conductor Research Corporation (Contract No. 88-MC-106), DARPA, the Defense Advanced Research Projects Authority (Contract No, NOO014-87-K0729) and IBM (Contract No. 12600042).

u) Presently at National Semiconductor Corporation. Santa Clara, CA 95052-8090.

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