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Process diagnostics during the deposition of cubic boron nitride
'UgMCE
COATIN68
ELSEVIER
Surface and Coatings Technology90 (I997) 275-284
Hg#NOLOM
Process diagnostics during the deposition of cubic boron nitride R. Pintaske *, Th. Welzel, N. Kahl, M. Schaller, J. Hahn, F. Richter Teehnische Universitiit Chemnitz-Zwiclcau, Institut ffir Physilc, D-09107 Chemnitz, Germany Received 27 August 1996; accepted 30 October 1996
Abstract
Using spatially resolved optical emission spectroscopy and Langmuir double probe teclmiqt/e, the magnetron deposition process of cubic boron nitride thin films has been investigated. The ion current to the r.f.-biased substrate electrode was estimated by means of Bohm's sheath criterion. In order to deposit the cubic boron nitride phase, a much higher ion energy is required in the d.c. magnetron in comparison to the r.f. sputtering magnetron mode at usually applied target power. Furthermore, there is a significant phase in_homogeneity across the substrate holder. Both facts have been explained in terms of the total momentum per deposited boron atom. The plasma excitation degree (vibrational and excitation temperatures) determined by emission spectroscopy was found to be higher in the r.f. sputtering mode. It has been shown that both in situ techniques applied can supply reliable information on the reactive magnetron deposition process. © 1997 Elsevier Science S.A. Keywords: process diagnostics; magnetron deposition; cubic boron nitride
1. Introduction Due to its interesting mechanical, electronic, and optical properties cubic boron nitride (c-BN) is regarded as a new material for many potential applications. Several ion- and plasma-assisted techniques (for reviews see Refs. [1,2]) have been used to deposit boron nitride thin films containing a considerable amount of the cubic phase. In order to understand relevant mechanisms leading to the formation of c-BN and to control the deposition process it is necessary to study the magnetron discharge with respect to the plasma properties. Recent models of the plasma- or ion-assisted c-BN film growth have focused on the flux ratio of bombarding ions and film forming particles and on the ion energy. Using optical emission spectroscopy and electrical probes, more information on the plasma state can be obtained, e.g., on excitation temperatures of relevant particles, electron temperature, and relative particle densities. With the help of these results, current ideas on the c-BN formation process can be improved. Furthermore, on the basis of in situ plasma measurements, various deposition techniques can be better understood and compared with each other. There are only a few published results on the characterisation of the c-BN deposition by Lang-muir probes * Correspondingauthor. 0257-8972/97/$17.00© 1997ElsevierScienceS.A. All rights reserved _PII S0257-8972(96) 03149-0
and emission spectroscopy [3-8]. The determination of plasma parameters by electrical probes is a well-known technique and is believed to be quite simple. However, the application of Langmuir probes to reactive glow discharges is highly problematic. In the case of r.f. plasma excitation fluctuations of the plasma potential have to be taken into account. The contamination of electrical probes with non-conductive films has to be prevented. In the case of magnetron sputter processes the discharges are strongly inhomogeneous and nonisotropic. To obtain information from emission spectra many assumptions have to be made concerning the excitation mechanisms and emission cross-sections. A main goal of the current study was to check to what extent Langmuir probes and optical emission spectroscopy can be applied to investigate the c-BN magnetron deposition process. 2. Experimental The experiments were carried out in a modified vacuum assembly Balzers PLS 570 (see Fig. 1). The vacuum chamber was evacuated by a turbomolecular pump. The magnetron sputtering source could be driven by either an r.f. (13.56 MHz) or a d.c. power supply. Hexagonal boron nitride and pure boron disks (100 m m in diameter, thickness 5 mm) were used as sputtering
276
~ P#ztaske et aL / Surface and Coatings Technology 90 (1997) 275-284
]
Optica_______~l E_mis2ionSpectroscopy
Gas Inlet DC (High Voltage) oo
Magnetron
-
~j
n
Substrate
Thermoregulation
I
Water Cooling
Matching Network
Matching
RF (13.56 MHz)
Network
RF (13.56MI-Iz)
_2-
Langmuir Probe
Turbopump
Fig. i. Schematic of the experimental arrangement for magnetron sputtering and process diagnostics.
targets. By heating the boron target its electrical conductivity becomes high enough to sustain a stable sputtering discharge in the d.c. mode. The substrate holder (150 mm in diameter) was separately r.f. powered, yielding a d.c. component of the substrate bias voltage of up to -400 V. The typical distance between target and substrate was 100 mm. Argon and nitrogen were used as sputter gases. The experiments were performed in the pressure range from 0.1 to 4 Pa. Using three different types of sputtering process (r.f. sputtering of a boron target, r.f. sputtering of an h-BN target, d.c. sputtering of a boron target) it was possible to deposit boron nitride films containing a large amount of the cubic phase. The deposition processes have been investigated by spatially resolved optical emission spectroscopy (OES) and Langmuir probe technique. The Langmuir probe and the optical fibre were mounted on a combined rotary-linear feedthrough which allows to move both components along the axis of symmetry of the arrangement by 100 nun. In addition, the cylindrical Langmuir probe could be moved in the radial direction. The observation axis of the emission spectroscopy was parallel to the electrode surfaces. The spatial resolution of the OES was about 3mm in the axial direction. Langmuir probe measurements were taken with an axial resolution of 1 mm and a radial resolution not better than 5 mm. The latter is due to the distance between both probe tips of 5 ram. The light emitted by the plasma was coupled through a quartz fibre optics into the f = 4 6 0 mm spectrograph (Jobin Yvon HR460). Using various gratings (2400 and
1200 mm -1, respectively) and a LN2 cooled CCD array (1024 x 256 pixels) as a detector a spectral resolution of better than 0.1 nm could be achieved. The spectral range between 200 and 900 nm has been investigated. In Fig. 2 a part of a typical spectrum is presented. The whole detection system was calibrated for its relative spectral sensitivity in the range from 300 to 800 nm by means of a tungsten ribbon lamp. To rationalise the data analysis a computer code was created. Electron temperatures and charge carrier densities were determined with a cylindrical double probe (probe material, platinum; typical probe diameter and length; 100 grn and 10 mm, respectively). With 2/rp>> 1 (2, mean free path; rp, probe radius) the probe worked as a classical Langmuir probe in the collisionless case [9]. Assuming a Maxwellian electron energy distribution function, the electron temperature has been calculated after Johnson and Malter [10,11]. The charge carrier density was computed according to the method of Sonin [12], with the assumption that the electron density n~ is equal to the ion density n+. The computer-controlled acquisition of individual current-voltage characteristics took only a few seconds depending on the voltage range covered and the number of data points. The plasma parameters were typically obtained as averaged values from three probe characteristics taken at each particular set of discharge conditions. Comparing the present curve with the previously acquired one, and by checking the symmetry of the probe characteristic, contamination effects could easily be recognised. The special design of the probe tip (Fig. 3) avoids
R. Pintaske et al. / Surface and Coatings Technology 90 (1997) 275-284
277
4500
IN
4000
B
3500 0
I Nl~
410
3000
/
248
9 A 250
2500 "~ 2000
251
1 4 1 2 14t4
416
418
N2
252
Ar/Ar ÷
1500 t000 500 0
~
-
240
-
- -
290
~
340 Wavelength [nm]
r
390
440
Fig. 2. Part of a typical optical emission spectrum of an r.f. magnetron sputter discharge (boron target; target power, Yr=500 W; y=0.2 Pa; 90% At, I0% N2).
Platinum wire
3. Results and discussion
3.1. Spatial structure of the discharge Glass tube
~-
Copper wire
Fig. 3. Design of the probe tip (active Iength of the probe, 10ram; probe diameter, 100 grn). tracking and ensures a constant probe area in the case of electrically conductive contamination. In order to prevent perturbations of the probe characteristics the probe tips were cleaned by applying a negative d.c. voltage (-280 V) between both tips if necessary. During the cleaning process, an ion current of about 5 mA flew for some seconds leading to a reddish glow of the probe tip. It is worth noting that the time interval between consecutive cleaning steps mainly depends on the discharge conditions, especially on the target material. In the case of an h-BN target it was necessary to clean the probe quite frequently. Using pure boron as target material the probe worked very stable and allows to acquire a series of characteristics until the next cleaning step. The probe has also been applied in mag-netron discharges with carbon targets. Here, due to the conductivity of the deposited carbon the contamination of the probe did not significantly affect the characteristics.
The spatial structure of the magnetron discharge has been investigated at typical deposition conditions. As can be seen from Fig. 4, the discharge is extremely inhomogeneous with respect to the charge carrier density and electron temperature. The toroidal race track structure of the magnetron source is clearly observable. The occurrence of the distinct density maximum at the discharge centre does not depend on the particular discharge condition, it was found to be a typical feature of our magnetron device. Bradley [13] observed a similar behaviour for a d.c. magnetron. Obviously, the values obtained in the plasma torus do not represent the electron temperature since the electron energy distribution is not Maxwellian, and the strong magnetic field (125 mTorr) could affect the probe characteristic. This experimental finding can be explained by the magnetic field structure of the magnetron used, In this so-called unbalanced magnetron configuration [14,15], some magnetic field lines do not close at the target surface, but extend into the bulk region and close somewhere at the target's rear side. Fast electrons can escape from the race track guided by the magnetic field which forms a kind of magnetic bottle. Electrons captured in this region suffer many inelastic collisions and cool down rapidly as was found from temperature measurements (Fig. 4(b)). In the case of a negatively biased substrate electrode, electrons are repelled which increases their density in this region. If the substrate is held at floating potential while the target is d.c. powered, the above density maximum can
278
t?. Pintaske et aL / Surface and Coatings Technology 90 (7997) 275-284
30
r.f. power supply, the density maximum is significantly enhanced. This observed maximum of charged particle density is assumed to be important for deposition experiments using the above arrangements• It is a zone of enhanced ionisation and excitation by electron collisions. Such reactive species can reach the substrate. Furthermore, particles released at the target are involved in inelastic collisions with electrons in this region• The described spatial structure of the discharge is also reflected in the inhomogeneity of the charge carrier density in front of the substrate. The radial distribution, which is strongly dependent on the distance between the target and the substrate (Fig. 6), is of importance for the homogeneous deposition of c-BN as will be discussed below•
g.
2s 20
3.2. Charge carrier density and electron temperature in dependence on the target power and substrate bias voltage
g
l0
[]
5
(a) 4
5 ,a=
/ ~
/
4
S
0 23
Radial probe position [ram]
90 80 70 60 50 40 30 20 l0 68 Distance from probe to substrate [mm] (b)
68 Radial probe position [mm]
0 +v au 40 30 20 10 ++ Distance from probe to substrate [mm]
The ion bombardment of the growing boron nitride film is important for the formation of the cubic phase. Since there is no net current to an r.f.-biased electrode, the ion flux cannot be measured directly. For this reason Bohm's sheath criterion [16] v+ =
Fig. 4. Spatial distribution of the charge carrier density (a) and the electron temperature (b) (boron target; d.c. target power, _Pz=200 W; substrate bias voltage, UBs= - 2 0 0 V; p = 1 Pa; 90% Am, 10% N2; target-substrate distance, dr4 = I00 nan). For a discussion of the term "electron temperature" see text.
,/<
(1)
m+
is applied from which the ion flux to a negatively biased electrode can be estimated. Here, v+ is the speed of ions entering the sheath edge, k is Boltzmann's constant,
6-
5-
•
op
+
9
~ ~"" ,a=
o~
i
68 23
1
0-
23
Radial probe position [mm]
90 80 70 60 50 40 30 20 I0 68 Distance from probe to substrate [ram] Fig. 5. Spatial distribution of the charge carrier density (process parameters as in Fig. 4, but the substrate was held at floating potential and p = 0.2 Pa).
also be detected (Fig. 5). On the other hand, the r.f. substrate discharge (without target discharge) causes a spatial distribution of the charge carriers which is nearly constant (about 109cm -s) and therefore not shown here. But, if the d.c. magnetron discharge is sustained, whereas the substrate is negatively biased by a separate
•
•' 5-
'A
dT.S= 60 mm
[] dz.s= 8 0 m m
:"
•
"-~ 42
d'r-s= IOOmm
" A',
a
'~ o
i ,, ..d
-
,,
2-
"
"
""
•
~o ..
'm
L
A.'(.".*'"
1-
,,',A
"",,
.
"..
o+.-60
•
-
'. ..... • -40
-20
0
0
0
0
Radial probe position[mm] Fig. 6. Radial distribution of the charge carrier density in front of the snbstrate (h-BN target; r.f. target power, PT----1000 W; substrate bias voltage, UBs = - 150 V; p =0.2 Pa; 97% Ar, 3% N £ distance between probe and substrate, 10 ram). With decreasing target-substrate distance dr..s the inhomogeneity increases drastically. The typical distance for deposition experiments is 100 ram. Since the substrates are located within a radius of 20 mm around the centre of the substrate holder, the corresponding ion flux varies by less than 20%.
279
tL Pintaske et al. / Surface and Coatings Technology 90 (1997) 275-284
T~ is the electron temperature, and m+ is the mass of ions. In the present study, the ion density was determined by Langmuir probes. The dimension of the plasma sheath was estimated visually to be less than 5 mm. Therefore the probe was located in the undisturbed bulk plasma at a distance of not less than 10 m m from the substrate electrode. Fig. 7 shows the ion density n + in front of the centre of the substrate as a function of the r.f. target power and the substrate self bias voltage. It is mainly determined by the r.f. target power, however, it does not significantly depend on the substrate bias voltage. A similar behaviour was found in the case of the d.c.powered magnetron (Fig. 8). Assuming a collisionless sheath, the ion energy at the substrate is determined by the difference between plasma potential Up~ and the substrate self bias potential UBS. Since Up1 cannot be determined using double probes, it
500
)
r.£ substrate bias voltage IV]
-150
..... arget power [W]
Fig. 7. Charge carrier density in front of the substrate as a function of the r.f. target power and substrate bias voltage (boron target; p=0.2 Pa; 90% Ar, 10% N2; distance between probe and substrate, 15 ram).
was estimated from the floating potential Um of the substrate holder, with the assumption that Up1 is somewhat positive with respect to UF~ [16]. For our magnetron configuration we found that Up~ is slightly positive (about + 2 0 V) in the case of an r.f. discharge. But, Up1 was negative (about - 3 0 V ) for a d.c.-powered magnetron. In both sputtering modes (r.f. and d.c., respectively) the plasma potential is nearly independent of the target power PT. F r o m this, it was concluded that the ion energy is not affected by PT. Due to big uncertainties in the determination of Um as well as the fact that for most of our deposition experiments [gBsl >>lgpll, for further discussions Up1 is neglected. For the reported deposition parameters we obtained almost constant electron temperatures T~ o f about 5 eV. At high nitrogen partial pressure, T~ was found to be slightly increased (about 7 eV). It should be noted that the obtained value for T~ is obviously too high. Such an overestimation can be caused by the fact that double probes only collect high energetic electrons• In the case of a deviation from the Maxwellian distribution this can lead to errors in temperature measurement. F r o m the above experimental results it was concluded that, for the present magnetron configuration, the ion flux to the substrate and the ion energy can be practically separated from each other• In addition, it was found that the whole radial distribution of the charge carrier density in front of the substrate holder is not affected by the substrate bias voltage (Fig. 9). 3.3• N2 vibrational temperature The relative population density of the vibrational levels of the N2(C3Hu) electronic state was determined from the second positive system (SPS, electronic 2.0
E
1.6
o
..'.,', - " *"w-, ,, ',
.a..,.' ,'.,'
1.2
'+.', '.'::,.
..y ..[ ../
~ O 0.8
~, ,'~
-
300 /d
o 0.4 -
r.f. substrate bias voltage [V]
• UBS=+I1 V(fl.) • UBS= -100V o UBS=-150V
-150
rget power [W]
Fig. 8. Charge carrier density in front of the substrate as a function of the d.c. target power and substrate bias voltage (for parameters see Fig• 7).
Y: '%
,;;+
0.0- --------~------~ 0 50 -I00 -50 Radial probe position [mm]
I00
Fig. 9. Radial distribution of the charge carrier density in front of the substrate with the substrate bias voltage as the curve parameter (discharge conditions as in Fig. 6, but target-substrate distance dr-s= 100 mm; 100% N2).
R. Pintaske et aL / Surface and Coatings Technology 90 (1997) 275-284
280
transition C3_/-/u--+B3//g) of According to
•
=
LSrel(.4 ) • q(v'---+ v i)
- -
molecular
nitrogen.
+ in
(2)
kB Tvib
the logarithm of the normalised relative population density was plotted against the vibrational energy E ~' of the upper electronic state. Here, I ~''~" (2) is the emission intensity of the transition at the wavelength 2, v' and v" are the vibrational quantum number of the upper and lower electronic state, respectively, T,~bis the vibrational temperature, no is the density of all molecules in the upper electronic state, Z is the partition function of the vibrational levels in the upper electronic state, q(v'~v') is the Franck-Condon factor, are 1 is the relative spectral sensitivity, C is a geometrical constant, and kB is Boltzmann's constant. The last term of Eq. (2) is constant for a particular electronic transition. If the population distribution of the vibrational levels of the upper electronic state obeys Boltzmann's law the plot will give a straight line. From its slope the vibrational temperature can be deduced. Under the given discharge conditions we found the levels to be typically occupied according to a Boltzmarm distribution (see Fig. 10). In the observed spectral range, 25 bands of the SPS were identified. Most of the band areas were disturbed mainly by argon lines. In order to minimise the perturbation effects the band head was taken for the intensity in Eq. (2). This was possible because we verified experimentally that the ratio of the band area and the height of the band head remained constant. However, only 11 bands could be used for temperature calculation because even some band heads were superimposed. In the discharge power range typical for our depos-
ition experiments, the vibrational temperature is remarkably higher for r.f. excitation than for the d.c. sputtering mode (cf. Fig. 11). These results suggest that the dissociation of N2 via the vibrationally excited C state could be an efficient path to create atomic nitrogen in r.f. discharges which plays an important role in nitride film formation. The Na(C3Hu) vibrational temperature mainly depends on the target power (Fig. 11), whereas it does not significantly correspond to any other discharge parameter (total pressure, gas composition, measuring position), not shown here. Both effects are obviously caused by different excitation mechanisms, but there is no explanation for that until now.
3.4. Excitation temperatures Emission lines can be used to check whether the discharge is in equilibrium, and if so, to determine the excitation temperature. There are two simple models, the local thermal equilibrium (LTE) approximation [17] and the corona model [18]. If the LTE approach is valid, a linear dependence of the logarithm of the normalised relative population density on the excitation energy Eo×o can be found (Boltzmarm plot): =
LGo~(2)' A(Xi + x ) ) 'gi
lob T
+
(3)
here/u(2) is the optical emission intensity of the transition i--+jwhich is proportional to the population density of the upper level i, C' is a constant for a particular species X, A is the transition probability, g~ is the statistical weight, and T is the equilibrium temperature. Furthermore, if the respective upper levels are populated only by inelastic electron collisions the measured temperature can be considered as the electron temperature.
Upper vibrational energy E v' [eV]
0.0 -0.5
0.0
0.2 , • .:1
+ -
,
0.4
0.6
I
I
0.8
1.0
I
+
,
t
1.2
7000
,
+.
":::
[] •
....
g
6000.
r.fi magne~on d.c. magne~on
-I.5 -2.0-
"'",,.,,
,
"'"'"'-,.
-2.5 -
e~
"N ".,..
-3.0-
-3.5 -4.0 -
.-.D''"'"
"'" '%,
--
5000-
5170K
[] r.f. magnetron(500 W) • d.c. magnetron(200 W)
o.....~.....~.. .=...... [].....=.... ...%-"
.....~..-"~'"'~'"
4000,
O
',, "',, 3170 I~ • ""-...
-4.5 Fig. 10. Typical Boltzmann plot of the N2(C3/7~) vibrational levels (boron target; r.f. target power, 500 W; d.c. target power, 200 W). The relative population density was normalised to a m a x i m u m value of 1. The straight lines indicate a population according to a Boltzmann distribution.
>
3000,
2000 0
• • •..o'~ "~0"0~.
I
t
I
t
I
I
100
200
300
400
500
600
Target power [W] Fig. 11. N2(Ca/7~) vibrational temperature vs. target power during magnetron sputtering (boron target; p =0.2 Pa; 50% Ar, 5 0 5 Na).
12. l~intaske et al. / Swface and Coatings Technology 90 (1997) 275-284
The LTE method was applied to about 100 lines of Ar and A_r+. Several discharge conditions have been studied. It was found that, in general, the respective excited levels are not occupied according to a Boltzmann distribution [ 19]. The corona model is a simple collision-radiation model based on the assumptions that electron impact is the only source of excitation, and relaxation exclusively occurs via spontaneous emission. By using this approach we obtained information on the electron temperature from the intensity ratio of emission lines [19]. Varying the electron density in a wide range between 6 x l0 s and 5 x 1011 cm -3, from the behaviour of the emission intensities it was observed that the excitation mechanisms do not remain constant. At low electron densities, single-step excitation is the dominant mechanism, while at higher densities and lower temperatures excitation by two successive electron collisions becomes more important. Taking this result into account, it was concluded that only a few intensity ratios of Ar/N2 lines and bands give reliable results with respect to the excitation or electron temperature. The pressure dependence of a selected intensity ratio as given in Fig. 12 was verified by means of Langmuir probe measurements (Fig. 13). The intensity ratios of selected Ar and A_r÷ lines as well as N2 and N~- bands did not reflect changes in electron temperature as measured by electrostatic probes [19]. F r o m boron (249.7 ram) and nitrogen atoms (411.0 and 415.2 nm) only a few very weak emission signals could be detected. For the time being we are not able to obtain information from these line intensities with respect to ground state densities of both species, since the respective excitation mechanisms are not clear.
0.45 t
0.401 2" 0.35~
0 : '.
"7-
~6¢D
o
5-
[]
4-
.,.a o(D
0:0
0:4
0:8
I:2
1:6
Total pressure [Pa] Fig. 13. Electron temperature vs. total pressure (Langmuir probe measurements; distance between probe and substrate, 10 mm; for other parameters see Fig. 12).
3.5. Deposition experiments Boron nltride thin films have been deposited by magnetron sputtering of a pure boron target. Remarkable differences in the substrate bias voltage required to grow cubic BN were observed. As can be seen from Fig. 14 [20], there is a threshold value of the substrate bias voltage. Obviously a certain ion energy is necessary to initiate the c-BN formation. These threshold energies as well as optimum energies at which maximum c-BN contents are obtained are much higher in the case of d.c. sputtering compared to those of the r.f. mode [21]. To explain the observed differences between d.c. and
0.8.
+o 0.6-
0.25-
._.9 -~ 0.4-
i
6"... "°}'""oD...
~a 0.2.
0.x5 0.I0 0.05
>2
t ,
%.•
N
t:a r.f. magnetron (500 W) • d.c. magnetron (160 Vv)
1.0
O
0.20
: d ,.. '.
8-
rn r.f. magnetron (500 W) • d.c. magnetron (200 W)
lm
0.30<
°
281
• ...............
Q ...............
11~
0.0.
~
I
i
i
t
t
I
2
3
4
0
Total pressure[Pa] Fig. 12. Intensity ratio of optical emission of argon (420.07 rim) and nitrogen (337.13nm) in dependence on the total pressure (boron target; r.f. target power, 500 W; d.c. target power, 160 W; 50% Ar, 50% Nz; substrate at floating potential). According to the corona model this intensity ratio was taken as a measure for the electron temperature [ 17].
~
[ I
----o--r.f. magnetr0n (500 W) I
l
100 200 300 Substrate bias voltage [-V]
I
400
Fig. 14. Relative c-BN content of deposited films in dependence on the substrate bias voltage during r.f. and d.c. sputtering of a pure boron target ( p = 0 . 2 Pa; 90% Ar, 10% N2). The intensity ratio [o/(Ih+I¢) represents the relative c-BN content of the films. Here, _Tois the intensity of the IR-Reststrahlenbande of the c-BN at 1055cm -1, and Ih corresponds to non-cubic absorption at 1390 cm -t.
282
P~ Pintaske et aL / Surface and Coatings Technology 90 (1997) 275-284
r.f. sputtering we discuss the c-BN formation in terms of the total momentum per deposited atom Ptot/a following an idea of Mirkarimi et al. [22]: /)tot _ F+ 2m~/~+E+ a
(4)
&
where F+ and F~ are the flux of the ions and deposited atoms, respectively, E+ is the ion energy, and m+ is their mass. It should be noted that we consider the flux of atoms being deposited to be the relevant parameter, whereas in Ref. [22] the flux of atoms arriving at the substrate was used. The ion flux onto the substrate was assessed using Bohrn's criterion [16]: F+ ~ 0 . 6 . n + • ,,/kTe
x ¢ rr/+
(5)
The flux of deposited atoms was estimated from the deposition rate r and the film composition:
NA .r.p
F~ = 2 - M
(6)
where NA is Avogadro's number, and p is the film density. Measurements of the film composition by elastic recoil detection analysis (ERDA) revealed that the films are stoichiometric BN. Therefore a molecular weight of 25 gmo1-1 was taken for M. The film density was estimated on the basis of the relative c-BN content measured by FT-IR and the assumption of a void-free structure. As determined by Langmuir probes, the ion density in front of the substrate holder for d.c. conditions is about haft of the value measured under r.f. sputtering conditions. According to Eq. (5) this leads to a lower ion flux to the substrate. The deposition rate was nearly the same resulting in similar F~ in both cases. Thus, to yield a comparable total momentum per deposited atom a higher ion energy is required during d.c. sputtering. Our third deposition technique (r.f. sputtering of an h-BN target) can also be included in this interpretation of the c-BN formation. The optimum ion energy is the same for both r.f. sputtering methods (h-BN target and boron target, respectively). During sputtering of the h-BN target the ion density in front of the substrate was measured to be higher. However, Ptot/a is nearly equal since the higher ion flux is compensated by a higher deposition rate. For all three sputtering modes we obtained optimum values of Ptot/a in the same range where films with maximum c-BN content could be deposited: 240 (eV. amu) 1/2 for d.c. sputtering of a boron target, 340 (eV' amu) 1/2 for r.f. sputtering of a boron target, and 390 (eV. amu) 1/2 for r.f. sputtering of an h-BN target. This consideration is also useful to compare our methods with other PVD or even CVD techniques in a
semi-quantitative manner. But, great care has to be taken since there are big uncertainties (at least 40%) in the experimental determination of the values required to calculate Ptoja. For a more detailed discussion not only the ion momentum transfer but also the degree of plasma excitation should be taken into account. As can be seen from Fig. 11, the vibrational temperature of the electronically excited molecular state N2(C3f/~) is much higher in the r.f. sputtering mode. Such a remarkably high vibrational excitation in the r.f. mode can easily lead to the formation of nitrogen atoms via dissociation of the C state. It has been demonstrated by Lu et al. [23] that atomic nitrogen plays an important role during c-BN deposition. Our Langmuir probe measurements showed that an increase in total pressure from 0.2 to 1 Pa led to an increase in the ion density by a factor of 2 under d.c. conditions. One should assume that increasing n+ in this way it would be possible to deposit c-BN at lowered E+. However, it was difficult to deposit the cubic BN phase at a pressure of 1 Pa. As demonstrated in Figs. 12 and 13, a pressure increase is equivalent to a decrease in both the degree of plasma excitation and the electron temperature. This is another indication of the importance of the degree of plasma excitation. Deposition experiments were carried out using a multi-step process [24]. Here, after nucleation of the cubic phase at optimum conditions the c-BN growth could be continued at drastically reduced ion impact (r.f. sputtering, h-BN target, 100% N2, ]UBsl< 150 V). We found that c-BN, once nucleated, continued to grow down to UBs values of - 6 0 V. However, for UBs = - 6 0 and - 8 0 V c-BN is deposited only on the inner part of the substrates (20 cmZ), whereas on the outer region of the substrates h-BN deposition occurred (Fig. 15). The FT-IR measurements presented in this figure also reveal that the spatial transition between the c-BN and h-BN region is very sharp. No mixture of the two phases was observed. In order to explain the content of Fig. 15 we consider the observed inhomogeneity of n+ (cf. Figs. 6 and 9) which, according to Eq. (5), results in an analogous inhomogeneity of F+. F~ is nearly homogeneous in the range of E+_< 100 eV as confirmed by our measurements. The energy of the bombarding ions is also uniform across the substrate area. Because the distribution of n÷ is practically independent on UBs (cf. Fig. 9), it does not vary if different ion energies are applied. Therefore, the radial distribution of Ptoja is very similar to that of n+ in the whole E+ range investigated. From this we draw the following conclusion: Ptot/a or (cf. Eq. (4) and Eq. (5)) n+. E~ 2, respectively, govern whether c-BN, once nucleated, continues to grow or the growth turns over into h-BN. Above a certain threshold value (Ptot/a)crit the growth of c-BN can be maintained. This
R. Pintaske et al. / Surface and Coatings Technology 90 (1997) 275-284
a
c
f
283
the discharge, the charge carrier density, and the electron temperature were obtained. Using OES the plasma excitation degree was determined. Based on these results, differences between r.f. and d.c. magnetron sputtering processes were revealed. Our results indicate that the formation of c-BN does not solely depend on ioninduced phenomena, but also on the excitation conditions in the discharge. In order to discuss the c-BN deposition process in more detail, better knowledge on the ion energy at the substrate is required. This can be obtained by direct access using ion energy analysers or by measuring the plasma potential. To improve the reliability of OES measurements regarding relative particle densities (boron, nitrogen), other active techniques, e.g., laserinduced fluorescence, will be applied.
Acknowledgement
40 mm
600
8;0 ' 1 0 ; 0 ' 12'00' 14'00 16'00 '18;0 '2000 Wave number [cm "1]
Fig. 15. FT-IR transmission spectra demonstrating the phase inhomogeneity of deposited boron nitride films at the substrate. Only within the inner substrate region c-BN could be grown. The IR beam with a diameter of about t mm was moved from the outer region towards the centre of the substrate in l-ram steps (spectra a-g). The absorption features at about 800 and 1390 cm -x correspond to non-cubic phases, whereas the peak at 1055 cm -~ is caused by c-BN. explanation is confirmed by the results obtained at the other UBs values investigated (for a detailed discussion see Ref. [24]): at UBs= --40 V, (])tot~a) is lower than the threshold value across the whole substrate. Thus only h - B N is deposited. At [UBs]- 100 V c-BN is grown also at the edge of the substrate. However, (/)tot/a) increases to such a high value that - - starting f r o m the centre of the substrate - - the resputtering limit is exceeded. We found an operating point of UBS= -- 100 V at which the whole substrate area could be covered homogeneously with c-BN.
4. Conclusions
The magnetron deposition process of boron nitride films has been characterised by Langmuir probes and optical emission spectroscopy. It could be demonstrated that both in situ diagnostic techniques can be operated under extremely severe discharge conditions. The combined application of both methods has supplied information on relevant process parameters. F r o m double probe measurements information on the spatial structure of
This work has been financially supported by the G e r m a n Bundesministerium for Bildung, Wissenschaft, Forschung und Technologie ( B M B F ) under G r a n t No. 03M2101F0. The authors are indebted to E. Schneider for F T - I R measurements. They wish to thank S. Prause and Th. Weber for technical assistance.
References
[1] H. Hofs/iss and C. Ronning, Proceedings of the International Conference on Beam Processing of Advanced Materials, Cieveland, OH, USA, 1995. [2] K. Bewilogua, A. Schatze, S. Kouptsidis and H. L~ithje, in A. Feldmann, W.A. Yarbrough et al. (eds.), Proceedings of the Applied Diamond Conference 1995 (Application of Diamond Films and Related Materials: Third International Conference), National
Institute of Standards and Technology Gaithersburg, MD, USA, I995, p. 831. [3] M. Kuhr, S. Reinke and W. Kniisch, Surf Coat. Technol., 74/75 (1995) 806. [4] Y. Ito, S. Kuriki, M. Saidoh and M. Niskikawa, Jpn. J. Appl. Phys., 33 (I994) 5959, [5] T. Ichiki, T. Momose and T. Yoshida, Appl. Phys., 75 (1994) 1330. [6] H. Yokoyama, M. Okamoto and Y. Osaka, Jpn. 3". Appl. phys., 30 (1991) 344. [7] M. Mieno and T. Yoshida, Surf Coat. Technol., 52 (1992) 87. [8] W. Dworschak, K. Jung and H. Ehrhardt, Thin Solid Films, 254 (I995) 65. [9] P.M. Chung, L. Talbot and K.J. Touryan, Electric Probes in Stationary and Flowing Plasmas: Theory and Application, Springer Verlag Berlin, Heidelberg, New York, 1975, ch. 1. [I0] E.O. Johnson and L. Maker, Phys. Rev., 80 (1950) 58. [11] S. Klagge and M. Tichy, Czech. J, Phys., B35 (1985) 988. [12] A.A. Sonin, A[AA J., 4 (1966) 1588. [ 13] J.W. Bradley, private communication. [I4] J. Musil, S. KadIec and W.-D. M~inz, J. Vac. Sci. Technol., A 9 (1991) 1171. [15] R.D. Ameli, Surf Coat. Technol., 59 (1993) 105. [16] B. Chapman, Glow Discharge Processes, J. Wiley & Sons, New York, 1980, ch. 3.
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[17] M. Schaller, Diploma Thesis, Technische Universit~it ChemnitzZwickau, Institut fttr Physik, 1996. [18] A. Anders, A Formulary for Plasma Physics, Akademie Verlag Berlin, 1990, ch. 4. [19] R. Pintaske, M. SchaIier, Th. Welzel, N. Kahl, J. Hahn and F. Richter, submitted to J. Phys. D: Appl. Phys. [20] J. Hahn, M. Friedrich, R. Pintaske, M. Schaller, N. Kahl, D.R.T. Zahn and F. Richter, Surf` Coat. Technol., 5 (1996) 1103. [2I] Since the plasma potentiaI is not exactly known the c-BN content
is given versus the substrate bias voltage (see discussion in Section 3.3). [22] P.B. Mirkarimi, K.F. McCarty, D.L. Medlin, W.G. Wolfer, T.A. Fried.mann, G.F. Cardinale, D.G. Howitt, J. Mater. Res., 9 (1994) 2925. [23] M. Lu, A. Bousetta, R. Sukach, A. Bensaoula, K. Walters, K. Eipers-Smith and A. Schultz, Appl. Phys. Lett., 64 (1994) 1514. [24] J. Hahn, F. Richter, R. Pintaske, M. R6der, E. Schneider and Th. Welzel, submitted to Surf. Coat. Technol.
COATIN68
ELSEVIER
Surface and Coatings Technology90 (I997) 275-284
Hg#NOLOM
Process diagnostics during the deposition of cubic boron nitride R. Pintaske *, Th. Welzel, N. Kahl, M. Schaller, J. Hahn, F. Richter Teehnische Universitiit Chemnitz-Zwiclcau, Institut ffir Physilc, D-09107 Chemnitz, Germany Received 27 August 1996; accepted 30 October 1996
Abstract
Using spatially resolved optical emission spectroscopy and Langmuir double probe teclmiqt/e, the magnetron deposition process of cubic boron nitride thin films has been investigated. The ion current to the r.f.-biased substrate electrode was estimated by means of Bohm's sheath criterion. In order to deposit the cubic boron nitride phase, a much higher ion energy is required in the d.c. magnetron in comparison to the r.f. sputtering magnetron mode at usually applied target power. Furthermore, there is a significant phase in_homogeneity across the substrate holder. Both facts have been explained in terms of the total momentum per deposited boron atom. The plasma excitation degree (vibrational and excitation temperatures) determined by emission spectroscopy was found to be higher in the r.f. sputtering mode. It has been shown that both in situ techniques applied can supply reliable information on the reactive magnetron deposition process. © 1997 Elsevier Science S.A. Keywords: process diagnostics; magnetron deposition; cubic boron nitride
1. Introduction Due to its interesting mechanical, electronic, and optical properties cubic boron nitride (c-BN) is regarded as a new material for many potential applications. Several ion- and plasma-assisted techniques (for reviews see Refs. [1,2]) have been used to deposit boron nitride thin films containing a considerable amount of the cubic phase. In order to understand relevant mechanisms leading to the formation of c-BN and to control the deposition process it is necessary to study the magnetron discharge with respect to the plasma properties. Recent models of the plasma- or ion-assisted c-BN film growth have focused on the flux ratio of bombarding ions and film forming particles and on the ion energy. Using optical emission spectroscopy and electrical probes, more information on the plasma state can be obtained, e.g., on excitation temperatures of relevant particles, electron temperature, and relative particle densities. With the help of these results, current ideas on the c-BN formation process can be improved. Furthermore, on the basis of in situ plasma measurements, various deposition techniques can be better understood and compared with each other. There are only a few published results on the characterisation of the c-BN deposition by Lang-muir probes * Correspondingauthor. 0257-8972/97/$17.00© 1997ElsevierScienceS.A. All rights reserved _PII S0257-8972(96) 03149-0
and emission spectroscopy [3-8]. The determination of plasma parameters by electrical probes is a well-known technique and is believed to be quite simple. However, the application of Langmuir probes to reactive glow discharges is highly problematic. In the case of r.f. plasma excitation fluctuations of the plasma potential have to be taken into account. The contamination of electrical probes with non-conductive films has to be prevented. In the case of magnetron sputter processes the discharges are strongly inhomogeneous and nonisotropic. To obtain information from emission spectra many assumptions have to be made concerning the excitation mechanisms and emission cross-sections. A main goal of the current study was to check to what extent Langmuir probes and optical emission spectroscopy can be applied to investigate the c-BN magnetron deposition process. 2. Experimental The experiments were carried out in a modified vacuum assembly Balzers PLS 570 (see Fig. 1). The vacuum chamber was evacuated by a turbomolecular pump. The magnetron sputtering source could be driven by either an r.f. (13.56 MHz) or a d.c. power supply. Hexagonal boron nitride and pure boron disks (100 m m in diameter, thickness 5 mm) were used as sputtering
276
~ P#ztaske et aL / Surface and Coatings Technology 90 (1997) 275-284
]
Optica_______~l E_mis2ionSpectroscopy
Gas Inlet DC (High Voltage) oo
Magnetron
-
~j
n
Substrate
Thermoregulation
I
Water Cooling
Matching Network
Matching
RF (13.56 MHz)
Network
RF (13.56MI-Iz)
_2-
Langmuir Probe
Turbopump
Fig. i. Schematic of the experimental arrangement for magnetron sputtering and process diagnostics.
targets. By heating the boron target its electrical conductivity becomes high enough to sustain a stable sputtering discharge in the d.c. mode. The substrate holder (150 mm in diameter) was separately r.f. powered, yielding a d.c. component of the substrate bias voltage of up to -400 V. The typical distance between target and substrate was 100 mm. Argon and nitrogen were used as sputter gases. The experiments were performed in the pressure range from 0.1 to 4 Pa. Using three different types of sputtering process (r.f. sputtering of a boron target, r.f. sputtering of an h-BN target, d.c. sputtering of a boron target) it was possible to deposit boron nitride films containing a large amount of the cubic phase. The deposition processes have been investigated by spatially resolved optical emission spectroscopy (OES) and Langmuir probe technique. The Langmuir probe and the optical fibre were mounted on a combined rotary-linear feedthrough which allows to move both components along the axis of symmetry of the arrangement by 100 nun. In addition, the cylindrical Langmuir probe could be moved in the radial direction. The observation axis of the emission spectroscopy was parallel to the electrode surfaces. The spatial resolution of the OES was about 3mm in the axial direction. Langmuir probe measurements were taken with an axial resolution of 1 mm and a radial resolution not better than 5 mm. The latter is due to the distance between both probe tips of 5 ram. The light emitted by the plasma was coupled through a quartz fibre optics into the f = 4 6 0 mm spectrograph (Jobin Yvon HR460). Using various gratings (2400 and
1200 mm -1, respectively) and a LN2 cooled CCD array (1024 x 256 pixels) as a detector a spectral resolution of better than 0.1 nm could be achieved. The spectral range between 200 and 900 nm has been investigated. In Fig. 2 a part of a typical spectrum is presented. The whole detection system was calibrated for its relative spectral sensitivity in the range from 300 to 800 nm by means of a tungsten ribbon lamp. To rationalise the data analysis a computer code was created. Electron temperatures and charge carrier densities were determined with a cylindrical double probe (probe material, platinum; typical probe diameter and length; 100 grn and 10 mm, respectively). With 2/rp>> 1 (2, mean free path; rp, probe radius) the probe worked as a classical Langmuir probe in the collisionless case [9]. Assuming a Maxwellian electron energy distribution function, the electron temperature has been calculated after Johnson and Malter [10,11]. The charge carrier density was computed according to the method of Sonin [12], with the assumption that the electron density n~ is equal to the ion density n+. The computer-controlled acquisition of individual current-voltage characteristics took only a few seconds depending on the voltage range covered and the number of data points. The plasma parameters were typically obtained as averaged values from three probe characteristics taken at each particular set of discharge conditions. Comparing the present curve with the previously acquired one, and by checking the symmetry of the probe characteristic, contamination effects could easily be recognised. The special design of the probe tip (Fig. 3) avoids
R. Pintaske et al. / Surface and Coatings Technology 90 (1997) 275-284
277
4500
IN
4000
B
3500 0
I Nl~
410
3000
/
248
9 A 250
2500 "~ 2000
251
1 4 1 2 14t4
416
418
N2
252
Ar/Ar ÷
1500 t000 500 0
~
-
240
-
- -
290
~
340 Wavelength [nm]
r
390
440
Fig. 2. Part of a typical optical emission spectrum of an r.f. magnetron sputter discharge (boron target; target power, Yr=500 W; y=0.2 Pa; 90% At, I0% N2).
Platinum wire
3. Results and discussion
3.1. Spatial structure of the discharge Glass tube
~-
Copper wire
Fig. 3. Design of the probe tip (active Iength of the probe, 10ram; probe diameter, 100 grn). tracking and ensures a constant probe area in the case of electrically conductive contamination. In order to prevent perturbations of the probe characteristics the probe tips were cleaned by applying a negative d.c. voltage (-280 V) between both tips if necessary. During the cleaning process, an ion current of about 5 mA flew for some seconds leading to a reddish glow of the probe tip. It is worth noting that the time interval between consecutive cleaning steps mainly depends on the discharge conditions, especially on the target material. In the case of an h-BN target it was necessary to clean the probe quite frequently. Using pure boron as target material the probe worked very stable and allows to acquire a series of characteristics until the next cleaning step. The probe has also been applied in mag-netron discharges with carbon targets. Here, due to the conductivity of the deposited carbon the contamination of the probe did not significantly affect the characteristics.
The spatial structure of the magnetron discharge has been investigated at typical deposition conditions. As can be seen from Fig. 4, the discharge is extremely inhomogeneous with respect to the charge carrier density and electron temperature. The toroidal race track structure of the magnetron source is clearly observable. The occurrence of the distinct density maximum at the discharge centre does not depend on the particular discharge condition, it was found to be a typical feature of our magnetron device. Bradley [13] observed a similar behaviour for a d.c. magnetron. Obviously, the values obtained in the plasma torus do not represent the electron temperature since the electron energy distribution is not Maxwellian, and the strong magnetic field (125 mTorr) could affect the probe characteristic. This experimental finding can be explained by the magnetic field structure of the magnetron used, In this so-called unbalanced magnetron configuration [14,15], some magnetic field lines do not close at the target surface, but extend into the bulk region and close somewhere at the target's rear side. Fast electrons can escape from the race track guided by the magnetic field which forms a kind of magnetic bottle. Electrons captured in this region suffer many inelastic collisions and cool down rapidly as was found from temperature measurements (Fig. 4(b)). In the case of a negatively biased substrate electrode, electrons are repelled which increases their density in this region. If the substrate is held at floating potential while the target is d.c. powered, the above density maximum can
278
t?. Pintaske et aL / Surface and Coatings Technology 90 (7997) 275-284
30
r.f. power supply, the density maximum is significantly enhanced. This observed maximum of charged particle density is assumed to be important for deposition experiments using the above arrangements• It is a zone of enhanced ionisation and excitation by electron collisions. Such reactive species can reach the substrate. Furthermore, particles released at the target are involved in inelastic collisions with electrons in this region• The described spatial structure of the discharge is also reflected in the inhomogeneity of the charge carrier density in front of the substrate. The radial distribution, which is strongly dependent on the distance between the target and the substrate (Fig. 6), is of importance for the homogeneous deposition of c-BN as will be discussed below•
g.
2s 20
3.2. Charge carrier density and electron temperature in dependence on the target power and substrate bias voltage
g
l0
[]
5
(a) 4
5 ,a=
/ ~
/
4
S
0 23
Radial probe position [ram]
90 80 70 60 50 40 30 20 l0 68 Distance from probe to substrate [mm] (b)
68 Radial probe position [mm]
0 +v au 40 30 20 10 ++ Distance from probe to substrate [mm]
The ion bombardment of the growing boron nitride film is important for the formation of the cubic phase. Since there is no net current to an r.f.-biased electrode, the ion flux cannot be measured directly. For this reason Bohm's sheath criterion [16] v+ =
Fig. 4. Spatial distribution of the charge carrier density (a) and the electron temperature (b) (boron target; d.c. target power, _Pz=200 W; substrate bias voltage, UBs= - 2 0 0 V; p = 1 Pa; 90% Am, 10% N2; target-substrate distance, dr4 = I00 nan). For a discussion of the term "electron temperature" see text.
,/<
(1)
m+
is applied from which the ion flux to a negatively biased electrode can be estimated. Here, v+ is the speed of ions entering the sheath edge, k is Boltzmann's constant,
6-
5-
•
op
+
9
~ ~"" ,a=
o~
i
68 23
1
0-
23
Radial probe position [mm]
90 80 70 60 50 40 30 20 I0 68 Distance from probe to substrate [ram] Fig. 5. Spatial distribution of the charge carrier density (process parameters as in Fig. 4, but the substrate was held at floating potential and p = 0.2 Pa).
also be detected (Fig. 5). On the other hand, the r.f. substrate discharge (without target discharge) causes a spatial distribution of the charge carriers which is nearly constant (about 109cm -s) and therefore not shown here. But, if the d.c. magnetron discharge is sustained, whereas the substrate is negatively biased by a separate
•
•' 5-
'A
dT.S= 60 mm
[] dz.s= 8 0 m m
:"
•
"-~ 42
d'r-s= IOOmm
" A',
a
'~ o
i ,, ..d
-
,,
2-
"
"
""
•
~o ..
'm
L
A.'(.".*'"
1-
,,',A
"",,
.
"..
o+.-60
•
-
'. ..... • -40
-20
0
0
0
0
Radial probe position[mm] Fig. 6. Radial distribution of the charge carrier density in front of the snbstrate (h-BN target; r.f. target power, PT----1000 W; substrate bias voltage, UBs = - 150 V; p =0.2 Pa; 97% Ar, 3% N £ distance between probe and substrate, 10 ram). With decreasing target-substrate distance dr..s the inhomogeneity increases drastically. The typical distance for deposition experiments is 100 ram. Since the substrates are located within a radius of 20 mm around the centre of the substrate holder, the corresponding ion flux varies by less than 20%.
279
tL Pintaske et al. / Surface and Coatings Technology 90 (1997) 275-284
T~ is the electron temperature, and m+ is the mass of ions. In the present study, the ion density was determined by Langmuir probes. The dimension of the plasma sheath was estimated visually to be less than 5 mm. Therefore the probe was located in the undisturbed bulk plasma at a distance of not less than 10 m m from the substrate electrode. Fig. 7 shows the ion density n + in front of the centre of the substrate as a function of the r.f. target power and the substrate self bias voltage. It is mainly determined by the r.f. target power, however, it does not significantly depend on the substrate bias voltage. A similar behaviour was found in the case of the d.c.powered magnetron (Fig. 8). Assuming a collisionless sheath, the ion energy at the substrate is determined by the difference between plasma potential Up~ and the substrate self bias potential UBS. Since Up1 cannot be determined using double probes, it
500
)
r.£ substrate bias voltage IV]
-150
..... arget power [W]
Fig. 7. Charge carrier density in front of the substrate as a function of the r.f. target power and substrate bias voltage (boron target; p=0.2 Pa; 90% Ar, 10% N2; distance between probe and substrate, 15 ram).
was estimated from the floating potential Um of the substrate holder, with the assumption that Up1 is somewhat positive with respect to UF~ [16]. For our magnetron configuration we found that Up~ is slightly positive (about + 2 0 V) in the case of an r.f. discharge. But, Up1 was negative (about - 3 0 V ) for a d.c.-powered magnetron. In both sputtering modes (r.f. and d.c., respectively) the plasma potential is nearly independent of the target power PT. F r o m this, it was concluded that the ion energy is not affected by PT. Due to big uncertainties in the determination of Um as well as the fact that for most of our deposition experiments [gBsl >>lgpll, for further discussions Up1 is neglected. For the reported deposition parameters we obtained almost constant electron temperatures T~ o f about 5 eV. At high nitrogen partial pressure, T~ was found to be slightly increased (about 7 eV). It should be noted that the obtained value for T~ is obviously too high. Such an overestimation can be caused by the fact that double probes only collect high energetic electrons• In the case of a deviation from the Maxwellian distribution this can lead to errors in temperature measurement. F r o m the above experimental results it was concluded that, for the present magnetron configuration, the ion flux to the substrate and the ion energy can be practically separated from each other• In addition, it was found that the whole radial distribution of the charge carrier density in front of the substrate holder is not affected by the substrate bias voltage (Fig. 9). 3.3• N2 vibrational temperature The relative population density of the vibrational levels of the N2(C3Hu) electronic state was determined from the second positive system (SPS, electronic 2.0
E
1.6
o
..'.,', - " *"w-, ,, ',
.a..,.' ,'.,'
1.2
'+.', '.'::,.
..y ..[ ../
~ O 0.8
~, ,'~
-
300 /d
o 0.4 -
r.f. substrate bias voltage [V]
• UBS=+I1 V(fl.) • UBS= -100V o UBS=-150V
-150
rget power [W]
Fig. 8. Charge carrier density in front of the substrate as a function of the d.c. target power and substrate bias voltage (for parameters see Fig• 7).
Y: '%
,;;+
0.0- --------~------~ 0 50 -I00 -50 Radial probe position [mm]
I00
Fig. 9. Radial distribution of the charge carrier density in front of the substrate with the substrate bias voltage as the curve parameter (discharge conditions as in Fig. 6, but target-substrate distance dr-s= 100 mm; 100% N2).
R. Pintaske et aL / Surface and Coatings Technology 90 (1997) 275-284
280
transition C3_/-/u--+B3//g) of According to
•
=
LSrel(.4 ) • q(v'---+ v i)
- -
molecular
nitrogen.
+ in
(2)
kB Tvib
the logarithm of the normalised relative population density was plotted against the vibrational energy E ~' of the upper electronic state. Here, I ~''~" (2) is the emission intensity of the transition at the wavelength 2, v' and v" are the vibrational quantum number of the upper and lower electronic state, respectively, T,~bis the vibrational temperature, no is the density of all molecules in the upper electronic state, Z is the partition function of the vibrational levels in the upper electronic state, q(v'~v') is the Franck-Condon factor, are 1 is the relative spectral sensitivity, C is a geometrical constant, and kB is Boltzmann's constant. The last term of Eq. (2) is constant for a particular electronic transition. If the population distribution of the vibrational levels of the upper electronic state obeys Boltzmann's law the plot will give a straight line. From its slope the vibrational temperature can be deduced. Under the given discharge conditions we found the levels to be typically occupied according to a Boltzmarm distribution (see Fig. 10). In the observed spectral range, 25 bands of the SPS were identified. Most of the band areas were disturbed mainly by argon lines. In order to minimise the perturbation effects the band head was taken for the intensity in Eq. (2). This was possible because we verified experimentally that the ratio of the band area and the height of the band head remained constant. However, only 11 bands could be used for temperature calculation because even some band heads were superimposed. In the discharge power range typical for our depos-
ition experiments, the vibrational temperature is remarkably higher for r.f. excitation than for the d.c. sputtering mode (cf. Fig. 11). These results suggest that the dissociation of N2 via the vibrationally excited C state could be an efficient path to create atomic nitrogen in r.f. discharges which plays an important role in nitride film formation. The Na(C3Hu) vibrational temperature mainly depends on the target power (Fig. 11), whereas it does not significantly correspond to any other discharge parameter (total pressure, gas composition, measuring position), not shown here. Both effects are obviously caused by different excitation mechanisms, but there is no explanation for that until now.
3.4. Excitation temperatures Emission lines can be used to check whether the discharge is in equilibrium, and if so, to determine the excitation temperature. There are two simple models, the local thermal equilibrium (LTE) approximation [17] and the corona model [18]. If the LTE approach is valid, a linear dependence of the logarithm of the normalised relative population density on the excitation energy Eo×o can be found (Boltzmarm plot): =
LGo~(2)' A(Xi + x ) ) 'gi
lob T
+
(3)
here/u(2) is the optical emission intensity of the transition i--+jwhich is proportional to the population density of the upper level i, C' is a constant for a particular species X, A is the transition probability, g~ is the statistical weight, and T is the equilibrium temperature. Furthermore, if the respective upper levels are populated only by inelastic electron collisions the measured temperature can be considered as the electron temperature.
Upper vibrational energy E v' [eV]
0.0 -0.5
0.0
0.2 , • .:1
+ -
,
0.4
0.6
I
I
0.8
1.0
I
+
,
t
1.2
7000
,
+.
":::
[] •
....
g
6000.
r.fi magne~on d.c. magne~on
-I.5 -2.0-
"'",,.,,
,
"'"'"'-,.
-2.5 -
e~
"N ".,..
-3.0-
-3.5 -4.0 -
.-.D''"'"
"'" '%,
--
5000-
5170K
[] r.f. magnetron(500 W) • d.c. magnetron(200 W)
o.....~.....~.. .=...... [].....=.... ...%-"
.....~..-"~'"'~'"
4000,
O
',, "',, 3170 I~ • ""-...
-4.5 Fig. 10. Typical Boltzmann plot of the N2(C3/7~) vibrational levels (boron target; r.f. target power, 500 W; d.c. target power, 200 W). The relative population density was normalised to a m a x i m u m value of 1. The straight lines indicate a population according to a Boltzmann distribution.
>
3000,
2000 0
• • •..o'~ "~0"0~.
I
t
I
t
I
I
100
200
300
400
500
600
Target power [W] Fig. 11. N2(Ca/7~) vibrational temperature vs. target power during magnetron sputtering (boron target; p =0.2 Pa; 50% Ar, 5 0 5 Na).
12. l~intaske et al. / Swface and Coatings Technology 90 (1997) 275-284
The LTE method was applied to about 100 lines of Ar and A_r+. Several discharge conditions have been studied. It was found that, in general, the respective excited levels are not occupied according to a Boltzmann distribution [ 19]. The corona model is a simple collision-radiation model based on the assumptions that electron impact is the only source of excitation, and relaxation exclusively occurs via spontaneous emission. By using this approach we obtained information on the electron temperature from the intensity ratio of emission lines [19]. Varying the electron density in a wide range between 6 x l0 s and 5 x 1011 cm -3, from the behaviour of the emission intensities it was observed that the excitation mechanisms do not remain constant. At low electron densities, single-step excitation is the dominant mechanism, while at higher densities and lower temperatures excitation by two successive electron collisions becomes more important. Taking this result into account, it was concluded that only a few intensity ratios of Ar/N2 lines and bands give reliable results with respect to the excitation or electron temperature. The pressure dependence of a selected intensity ratio as given in Fig. 12 was verified by means of Langmuir probe measurements (Fig. 13). The intensity ratios of selected Ar and A_r÷ lines as well as N2 and N~- bands did not reflect changes in electron temperature as measured by electrostatic probes [19]. F r o m boron (249.7 ram) and nitrogen atoms (411.0 and 415.2 nm) only a few very weak emission signals could be detected. For the time being we are not able to obtain information from these line intensities with respect to ground state densities of both species, since the respective excitation mechanisms are not clear.
0.45 t
0.401 2" 0.35~
0 : '.
"7-
~6¢D
o
5-
[]
4-
.,.a o(D
0:0
0:4
0:8
I:2
1:6
Total pressure [Pa] Fig. 13. Electron temperature vs. total pressure (Langmuir probe measurements; distance between probe and substrate, 10 mm; for other parameters see Fig. 12).
3.5. Deposition experiments Boron nltride thin films have been deposited by magnetron sputtering of a pure boron target. Remarkable differences in the substrate bias voltage required to grow cubic BN were observed. As can be seen from Fig. 14 [20], there is a threshold value of the substrate bias voltage. Obviously a certain ion energy is necessary to initiate the c-BN formation. These threshold energies as well as optimum energies at which maximum c-BN contents are obtained are much higher in the case of d.c. sputtering compared to those of the r.f. mode [21]. To explain the observed differences between d.c. and
0.8.
+o 0.6-
0.25-
._.9 -~ 0.4-
i
6"... "°}'""oD...
~a 0.2.
0.x5 0.I0 0.05
>2
t ,
%.•
N
t:a r.f. magnetron (500 W) • d.c. magnetron (160 Vv)
1.0
O
0.20
: d ,.. '.
8-
rn r.f. magnetron (500 W) • d.c. magnetron (200 W)
lm
0.30<
°
281
• ...............
Q ...............
11~
0.0.
~
I
i
i
t
t
I
2
3
4
0
Total pressure[Pa] Fig. 12. Intensity ratio of optical emission of argon (420.07 rim) and nitrogen (337.13nm) in dependence on the total pressure (boron target; r.f. target power, 500 W; d.c. target power, 160 W; 50% Ar, 50% Nz; substrate at floating potential). According to the corona model this intensity ratio was taken as a measure for the electron temperature [ 17].
~
[ I
----o--r.f. magnetr0n (500 W) I
l
100 200 300 Substrate bias voltage [-V]
I
400
Fig. 14. Relative c-BN content of deposited films in dependence on the substrate bias voltage during r.f. and d.c. sputtering of a pure boron target ( p = 0 . 2 Pa; 90% Ar, 10% N2). The intensity ratio [o/(Ih+I¢) represents the relative c-BN content of the films. Here, _Tois the intensity of the IR-Reststrahlenbande of the c-BN at 1055cm -1, and Ih corresponds to non-cubic absorption at 1390 cm -t.
282
P~ Pintaske et aL / Surface and Coatings Technology 90 (1997) 275-284
r.f. sputtering we discuss the c-BN formation in terms of the total momentum per deposited atom Ptot/a following an idea of Mirkarimi et al. [22]: /)tot _ F+ 2m~/~+E+ a
(4)
&
where F+ and F~ are the flux of the ions and deposited atoms, respectively, E+ is the ion energy, and m+ is their mass. It should be noted that we consider the flux of atoms being deposited to be the relevant parameter, whereas in Ref. [22] the flux of atoms arriving at the substrate was used. The ion flux onto the substrate was assessed using Bohrn's criterion [16]: F+ ~ 0 . 6 . n + • ,,/kTe
x ¢ rr/+
(5)
The flux of deposited atoms was estimated from the deposition rate r and the film composition:
NA .r.p
F~ = 2 - M
(6)
where NA is Avogadro's number, and p is the film density. Measurements of the film composition by elastic recoil detection analysis (ERDA) revealed that the films are stoichiometric BN. Therefore a molecular weight of 25 gmo1-1 was taken for M. The film density was estimated on the basis of the relative c-BN content measured by FT-IR and the assumption of a void-free structure. As determined by Langmuir probes, the ion density in front of the substrate holder for d.c. conditions is about haft of the value measured under r.f. sputtering conditions. According to Eq. (5) this leads to a lower ion flux to the substrate. The deposition rate was nearly the same resulting in similar F~ in both cases. Thus, to yield a comparable total momentum per deposited atom a higher ion energy is required during d.c. sputtering. Our third deposition technique (r.f. sputtering of an h-BN target) can also be included in this interpretation of the c-BN formation. The optimum ion energy is the same for both r.f. sputtering methods (h-BN target and boron target, respectively). During sputtering of the h-BN target the ion density in front of the substrate was measured to be higher. However, Ptot/a is nearly equal since the higher ion flux is compensated by a higher deposition rate. For all three sputtering modes we obtained optimum values of Ptot/a in the same range where films with maximum c-BN content could be deposited: 240 (eV. amu) 1/2 for d.c. sputtering of a boron target, 340 (eV' amu) 1/2 for r.f. sputtering of a boron target, and 390 (eV. amu) 1/2 for r.f. sputtering of an h-BN target. This consideration is also useful to compare our methods with other PVD or even CVD techniques in a
semi-quantitative manner. But, great care has to be taken since there are big uncertainties (at least 40%) in the experimental determination of the values required to calculate Ptoja. For a more detailed discussion not only the ion momentum transfer but also the degree of plasma excitation should be taken into account. As can be seen from Fig. 11, the vibrational temperature of the electronically excited molecular state N2(C3f/~) is much higher in the r.f. sputtering mode. Such a remarkably high vibrational excitation in the r.f. mode can easily lead to the formation of nitrogen atoms via dissociation of the C state. It has been demonstrated by Lu et al. [23] that atomic nitrogen plays an important role during c-BN deposition. Our Langmuir probe measurements showed that an increase in total pressure from 0.2 to 1 Pa led to an increase in the ion density by a factor of 2 under d.c. conditions. One should assume that increasing n+ in this way it would be possible to deposit c-BN at lowered E+. However, it was difficult to deposit the cubic BN phase at a pressure of 1 Pa. As demonstrated in Figs. 12 and 13, a pressure increase is equivalent to a decrease in both the degree of plasma excitation and the electron temperature. This is another indication of the importance of the degree of plasma excitation. Deposition experiments were carried out using a multi-step process [24]. Here, after nucleation of the cubic phase at optimum conditions the c-BN growth could be continued at drastically reduced ion impact (r.f. sputtering, h-BN target, 100% N2, ]UBsl< 150 V). We found that c-BN, once nucleated, continued to grow down to UBs values of - 6 0 V. However, for UBs = - 6 0 and - 8 0 V c-BN is deposited only on the inner part of the substrates (20 cmZ), whereas on the outer region of the substrates h-BN deposition occurred (Fig. 15). The FT-IR measurements presented in this figure also reveal that the spatial transition between the c-BN and h-BN region is very sharp. No mixture of the two phases was observed. In order to explain the content of Fig. 15 we consider the observed inhomogeneity of n+ (cf. Figs. 6 and 9) which, according to Eq. (5), results in an analogous inhomogeneity of F+. F~ is nearly homogeneous in the range of E+_< 100 eV as confirmed by our measurements. The energy of the bombarding ions is also uniform across the substrate area. Because the distribution of n÷ is practically independent on UBs (cf. Fig. 9), it does not vary if different ion energies are applied. Therefore, the radial distribution of Ptoja is very similar to that of n+ in the whole E+ range investigated. From this we draw the following conclusion: Ptot/a or (cf. Eq. (4) and Eq. (5)) n+. E~ 2, respectively, govern whether c-BN, once nucleated, continues to grow or the growth turns over into h-BN. Above a certain threshold value (Ptot/a)crit the growth of c-BN can be maintained. This
R. Pintaske et al. / Surface and Coatings Technology 90 (1997) 275-284
a
c
f
283
the discharge, the charge carrier density, and the electron temperature were obtained. Using OES the plasma excitation degree was determined. Based on these results, differences between r.f. and d.c. magnetron sputtering processes were revealed. Our results indicate that the formation of c-BN does not solely depend on ioninduced phenomena, but also on the excitation conditions in the discharge. In order to discuss the c-BN deposition process in more detail, better knowledge on the ion energy at the substrate is required. This can be obtained by direct access using ion energy analysers or by measuring the plasma potential. To improve the reliability of OES measurements regarding relative particle densities (boron, nitrogen), other active techniques, e.g., laserinduced fluorescence, will be applied.
Acknowledgement
40 mm
600
8;0 ' 1 0 ; 0 ' 12'00' 14'00 16'00 '18;0 '2000 Wave number [cm "1]
Fig. 15. FT-IR transmission spectra demonstrating the phase inhomogeneity of deposited boron nitride films at the substrate. Only within the inner substrate region c-BN could be grown. The IR beam with a diameter of about t mm was moved from the outer region towards the centre of the substrate in l-ram steps (spectra a-g). The absorption features at about 800 and 1390 cm -x correspond to non-cubic phases, whereas the peak at 1055 cm -~ is caused by c-BN. explanation is confirmed by the results obtained at the other UBs values investigated (for a detailed discussion see Ref. [24]): at UBs= --40 V, (])tot~a) is lower than the threshold value across the whole substrate. Thus only h - B N is deposited. At [UBs]- 100 V c-BN is grown also at the edge of the substrate. However, (/)tot/a) increases to such a high value that - - starting f r o m the centre of the substrate - - the resputtering limit is exceeded. We found an operating point of UBS= -- 100 V at which the whole substrate area could be covered homogeneously with c-BN.
4. Conclusions
The magnetron deposition process of boron nitride films has been characterised by Langmuir probes and optical emission spectroscopy. It could be demonstrated that both in situ diagnostic techniques can be operated under extremely severe discharge conditions. The combined application of both methods has supplied information on relevant process parameters. F r o m double probe measurements information on the spatial structure of
This work has been financially supported by the G e r m a n Bundesministerium for Bildung, Wissenschaft, Forschung und Technologie ( B M B F ) under G r a n t No. 03M2101F0. The authors are indebted to E. Schneider for F T - I R measurements. They wish to thank S. Prause and Th. Weber for technical assistance.
References
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lL Pintaske et al. / Surface and Coatings Technology 90 (1997) 275-284
[17] M. Schaller, Diploma Thesis, Technische Universit~it ChemnitzZwickau, Institut fttr Physik, 1996. [18] A. Anders, A Formulary for Plasma Physics, Akademie Verlag Berlin, 1990, ch. 4. [19] R. Pintaske, M. SchaIier, Th. Welzel, N. Kahl, J. Hahn and F. Richter, submitted to J. Phys. D: Appl. Phys. [20] J. Hahn, M. Friedrich, R. Pintaske, M. Schaller, N. Kahl, D.R.T. Zahn and F. Richter, Surf` Coat. Technol., 5 (1996) 1103. [2I] Since the plasma potentiaI is not exactly known the c-BN content
is given versus the substrate bias voltage (see discussion in Section 3.3). [22] P.B. Mirkarimi, K.F. McCarty, D.L. Medlin, W.G. Wolfer, T.A. Fried.mann, G.F. Cardinale, D.G. Howitt, J. Mater. Res., 9 (1994) 2925. [23] M. Lu, A. Bousetta, R. Sukach, A. Bensaoula, K. Walters, K. Eipers-Smith and A. Schultz, Appl. Phys. Lett., 64 (1994) 1514. [24] J. Hahn, F. Richter, R. Pintaske, M. R6der, E. Schneider and Th. Welzel, submitted to Surf. Coat. Technol.