- Email: [email protected]
Characterization and production monitoring of thin films using soft X-ray spectroscopy
Surface and Coatings Technology, 43/44 (1990) 1015—1023
1015
CHARACTERIZATION AND PRODUCTION MONITORING OF THIN FILMS USING SOFT X-RAY SPECTROSCOPY* J. NORDGREN and G. BRAY Uppsala University, Department of Physics, Box 530, S-75121 Upp8ala (Sweden) Y. CLAESSON, M. GEORGSON and C.-G. RIBBING Uppsala University, Department of Technology, Box 534, S-75121 Uppsala (Sweden) N. WASSDAHL Uppsala University, Department of Physics, Box 530, S-75121 Uppsala (Sweden)
Abstract The feasibility of using soft X-ray spectroscopy for in situ on-line analysis of thin films produced by reactive magnetron sputtering has been investigated. A high performance grazing incidence instrument of novel design was used to record spectra in the 380—550 eV range from thin films of titanium, TiN and Ti02. The growth of the film could be monitored and impurities or unbalanced gas flows detected. Small amounts of oxidation, such as that corresponding to 90 mm exposure of a titanium metal film to a background pressure of below 6 x iO~mbar, was clearly observed. The results show that a continuous in situ monitoring with respect to both elemental analysis and chemical characterization is feasible on a time scale of technological interest.
1. Introduction The increasing use of various film deposition techniques, in particular reactive cathode sputtering, has created an interest in methods for in situ monitoring in order to control the process of film growth. Parameters to be controlled in reactive sputtering are for example partial gas pressures and/or flows, potentials and plasma current (see, for example, ref. 1). Prevailing monitoring methods are generally based on either spectroscopic recording of optical emission or absorption in the sputtering plasma [2], or mass spectrometric analysis of the sputtering atmosphere [3]. These two methods provide continuous information about the process but no direct information about the growing film. Commonly used spectroscopic methods to extract chemical *Winner of a 1990 ICMC/ICTF Bunshah Award. 0257.8972/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1016
information, such as photoemission or Auger spectroscopy, are not usable as on-line monitoring techniques owing to the charged particles and strong fields associated with the sputtering plasma. We have investigated the feasibility of using soft X-ray emission spectroscopy (SXES) to provide such an in situ and on-line monitoring method, since the X-rays can freely penetrate the sputtering atmosphere. Radiative transitions in the soft X-ray range occur as a result of electric dipole interaction between the two outermost main shells of an atom or molecule, i.e. between valence electrons and localized inner vacancies (Fig. 1). This condition forms a basis for the use of soft X-ray spectra in the study of the electronic structure of matter (see, for example, ref. 4). The energy separation of the inner levels of different atomic species is sufficiently large to provide elemental selectivity, i.e. different atoms of a molecule or compound produce different spectra appearing at well-separated wavelengths. Also, the condition that the transitions directly involve valence electrons makes detailed studies of chemical bonding and valence electronic structure feasible. Furthermore, the selective nature of the dipole transitions gives rise to an atomic state selectivity that greatly facilitates the interpretation of finer details in the spectra. In addition, the use of soft X-rays permits studies of the lighter elements, such as carbon, nitrogen and oxygen, and for heavier elements one avoids problems due to large lifetime broadening. The use of SXES for monitoring film sputtering offers both elemental and chemical bonding information, and also a means to obtain a depth profile analysis since the excitation depth can be varied by varying the energy of the excitation agent. The depth profiling capability is illustrated in Fig. 2, where Al I emission
~,
•
eJectron excitation
—~
V~ence~ Ti3s
200 eV
T1L(=45OeV)
Ti 2p-3s ( 395 eV)
Nis
~
-
~NK(=390eV)
Ti2p~~
:: :
400 eV
80
70
I
I
60
(e V)
Fig. 1. Diagram of soft X-ray transitions in TiN. Fig. 2. Al L emission spectra of an aluminium sample with a very thin oxide layer, excited with electrons of kinetic energy between 200 and 2000 eV.
1017
L emission spectra of an aluminium sample with a thin (few nanometres) oxide layer, excited with electrons of various energies, have been plotted. One notices that the pure Al L spectrum of aluminium metal is obtained when an excitation energy of 2 keV is used, whereas spectral features characteristic of Al2 03 grow on lowering the electron energy. At 200 eV almost only the Al L emission of Al2 03 is present. The range of 200 eV electrons is only about 1 nm, while 2 eV electrons penetrate some 100 nm [5]. For the present investigation we have chosen titanium as a suitable system for reactive sputtering, and in particular we have studied the sputtering of TiN. Titanium is reactive and exhibits interesting stoichiometric variations in the formation of different compounds. Also, it has considerable technical interest. The soft X-ray emission of titanium is found in the 395—460 eV range. It has been shown previously [6, 7] that the sputtering process itself gives rise to characteristic X-rays from the substrate excited by high energy secondary electrons. This spontaneous radiation allows spectral analysis to be made. However, for rapid data collection, as in process control, we have found that auxiliary excitation by electron bombardment greatly facilitates the recording of spectra. In a previous report, we presented results from a feasibility study of the use of SXES for film production monitoring [7], suggesting that the method has a potential for such applications. In the present paper we disclose new results from further investigations showing more definitely the applicability of the method. Further development of the technique is underway and a dedicated instrument is being constructed, which is expected to provide sufficient sensitivity for real-time process control.
2. Experimental details The film deposition equipment used in the present work was a d.c. magnetron sputtering system described previously [8]. The sputtering chamber was pumped with a 13 in oilwith diffusion pump, which cold provided a base 7mbar the liquid nitrogen trap in use. pressuresputtering of below the 3 x current 10During was kept at 4 A and the voltage around 450 V, giving a deposition rate for TiN of the order of 1 nm s’. Target-to-substrate distance was 120 mm and the target size 206 mm x 86 mm. At an argon gas flow of 100 standard cm3 min ‘, an operating pressure of 4 x iO~mbar was maintained. Connected to the sputtering chamber was a grazing incidence grating spectrometer constructed primarily for soft X-ray spectroscopy using synchrotron radiation [9]. The spectrometer was designed according to a new concept which allows a large spectral range to be covered in a small size instrument, while still providing both high resolution and luminosity. The instrument employs three spherical gratings, two of radius 5 m and line frequencies 400 and 1200 lines mm 1, and one grating of radius 3 m and line frequency 300 lines mm ‘. The gratings are fixed on a precision ground
1018
reference block in order to accept radiation through the common entrance slit at angles of incidence appropriate for the various wavelength ranges covered by the different gratings. Selection of the grating in use is made simply by operating a movable aperture between the gratings and the entrance slit so that the appropriate grating is being illuminated (Fig. 3). The fact that the three gratings all define a focal curve of their own calls for a detector that can be moved in a three-axis coordinate system covering these curves. This is accomplished by means of three high precision translational tables, driven by computer-controlled stepper motors, and a flexible welded bellows between the detector house and the grating compartment. The diffracted X-rays are detected by a 40 mm diameter detector based on multichannel plates (MCPs) and a resistive anode read-out system, the detector was a custom version of a detector produced by Surface Science Ltd., Mountain View, CA. In order to enhance the efficiency of the detector the front MCP was coated with CsI and an electron capturing electrode was mounted outside the detector face. The detector was operated in two-dimensional mode in order to allow a large solid angle to be accepted without losing resolution as a result of the curvature of the spectral lines caused by the concave grating. Also, the two-dimensional detection allows distortions in the detector itself to be accounted for. The ultimate energy resolution of the instrument depends on photon energy and amounts to 0.02 eV at 50 eV and 0.5 eV at 1000 eV. In the present work the resolution was set to 2 eV at 450 eV. Excitation of soft X-ray emission was made by means of an electron gun mounted on a side flange of the sputtering chamber. The gun was operated at typically a few kilovolts and focusing and steering of the electron beam were accomplished by a quacirupole doublet magnetic lens.
3. Results and discussion The soft X-ray emission spectra of titanium, TiN and Ti02 have all been studied previously using different means of excitation [10, 11]. The Ti L spectra exhibit distinct differences owing to the differences in the bonding and, obviously, there are differences with respect to the N K and 0 K spectra. Figure 4 shows the emission spectra of these three materials in the 380— 550 eV range, i.e. the N K spectrum at 390 eV, the Ti L23 spectrum at 395 eV and 450 eV and the 0 K spectrum at 525 eV. The spectra were recorded in situ after typically 2—4 mm of sputtering using the power settings and gas flows listed in Table 1. It should be noted that the N K emission line accidentally almost coincides with the Ti 2p—3s transition. The spectra of Fig. 4 were used in the present experiments as reference spectra in the investigation of the monitoring capability of the soft X-ray analysis method. The present method of in situ analysis provides a depth profiling capability since the range of the exciting electrons is dependent on the kinetic energy [5]. This means that variation of acceleration voltage allows the excitation depth to be varied down to a depth corresponding to the lowest
1019
~
~
I
I~
©L1I~/\ \
I~I~
\\
/1111
II I II
~//
~
~ IT{L~~ I!u~II ~ r~® - III I!11L~ r2r1
k~
/
_~_~//~‘
/
~J~I1 ~ I~I ~
I ir~ IR ~7~1 ~ o___~._r~i~_~ ~
Ji
I ~ ~ ~ ~
~
0
~
-Q
‘~
.c ~ ‘-~
/~
/
EE
I
in
/
•1
~
LJ..J
/
-~
o
©(_@
©
©
©
© ©
0
H
1020
xl 0~
2-
1JUL90mmn
-
0.5
400
450
500
550
500
4O~J
Emission Energy (eV)
Emission Energy (eV)
Fig. 4. Soft X-ray emission spectra of, from top, TiN, titanium, and Ti02. The spectrum at 400eV for TiN has been divided by 3. Fig. 5. Soft X-ray emission spectra of a sputtered titanium film recorded immediately after sputtering (bottom) and after 90 mm (top) at a background pressure of below 6 x iO~mbar. The rise in the oxygen signal indicating oxidation of the film should be noted.
TABLE 1 Power settings and gas flows used in the different experiments (for Ti02 the pumping speed was somewhat reduced) 3 mm Figure Material Current (A) Voltage (V) Gas flow (standard cm Ar N 2 02 4
5 6 7
TiN Ti Ti02 Ti Ti TiN TiN
4 4 4 4 4 4 4
450 440 440 440 440 450 440-460
100 100 —
iOO 100 100 100
15
—
—
—
—
100
—
—
—
—
15 0-15
— —
cx cx
1021
energy sufficient for X-ray excitation. In the present case the threshold energies range from about 400 eV for N K emission to 530 eV for 0 K emission. From that value the excitation cross-section rises to a maximum at about 3—5 times the threshold value [12]. The mean free path for 400 eV electrons is of the order of a few nanometres and increases for increasing electron energy. 5 keV electrons penetrate a few hundred nanometres into a material such as titanium. By directing the electron beam onto the substrate at a grazing angle the surface sensitivity is further enhanced. The sensitivity of the soft X-ray probe is demonstrated in Fig. 5. A titanium film was sputtered and a first recording of the soft X-ray spectrum was immediately obtained. This spectrum shows a faint oxygen signal due to the oxygen content in the residual gas atmosphere. The second spectrum was recorded after 90 mm at a background pressure below 6 x i0~ mbar. One notices a significant increase in the oxygen signal, indicating that the film has been subject to oxidation during that time. In many applications it is important to control the oxidation of a sputtered film. For example, it has been suggested that thin TiN films could replace noble metal films in heat-mirror multilayers [13, 14], resulting in improved stability. In this application it is of crucial importance to control the stoichiometry and to avoid oxidation to benefit from the metallic character of TiN. Figure 6 shows a series of SXES spectra recorded after successive short periods of TiN sputtering starting with a titanium metal film. Each sputtering period was 15 s and each spectrum was recorded during approximately 30 s with the plasma shut off. One notices a gradual change in the spectral features as the titanium metal is buried under the TiN film. The Ti L spectrum changes shape from the typical titanium metal two-peak spectrum to the three-peak spectrum of TiN. Also, the nitrogen signal grows on the low energy side of the Ti 2p—3s line, reaching a state where the titanium line only causes a shoulder on the high energy side of the N K line. The accumulation time of 30 s to obtain sufficient spectral quality obviously requires the sputtering to be shut off during recording. We expect these time periods to be considerably reduced when a new dedicated instrument for monitoring sputtered films which is at present being constructed in our laboratory is introduced. Then spectrum recording during sputtering will be the normal condition and we shall need to reduce background signals due to UV light and charged particles. We have investigated this problem and found that filtering the radiation using for example a 100 nm foil of indium, silver or copper reduces the background from the sputtering plasma to a negligible level. Other means of background reduction would be proper masking and introduction of apertures. The effect of varying the flow of N2 in TiN sputtering is shown in Fig.
7. Here a series of SXES spectra were recorded after typically 4 mm of TiN sputtering at various N2 gas flows, giving a range of understoichiometric films. One can clearly see the gradual change from titanium metal film to TiN film, in both the Ti L spectrum and the N K—Ti 2p—3s spectrum. Also, one observes that the rates of change of the spectral features are different for the
cx
1022 4
x10
-
1.8
A
120 S
1.6 1.4 1.2
j~
———-—~
-
X4
8
~
105
J
x104
i~ sccm/min N
2
~ /L ~
7.
~J~L ~ ~ ~
0.8 ~
A
-
A
6-
~
~
-
4
0.6 ~ ~
I ~
~
-
2~* 0
o 400
450
Emission Energy (eV)
400
450
Emission Energy (eV)
Fig. 6. Series of soft X-ray emission spectra recorded after successive 15 s periods of TiN sputtering starting with a pure titanium film (bottom). Each spectrum was recorded for approximately 30s. The Ti L spectrum at 450 eV has been multiplied by 4. Fig. 7. Series of soft X-ray emission spectra recorded after typically 4 mm of TiN sputtering at various N2 gas flows. The Ti L spectrum at 450 eV has been multiplied by 4.
Ti L and the N K spectra. On going down from high flows (full stoichiometry), the Ti L spectrum exhibits the most rapid change; on going up from low flows the N K—Ti 2p—3s spectrum changes the fastest. Several interesting observations were made during the present experiments. For example, sputtering a Ti02 film on TiN required longer time than on titanium metal in order to obtain a pure film. Still, after 8 mm of sputtering (some 80 nm thickness) on top of TiN, there was a distinct nitrogen signal and a small but significant difference in the Ti L spectrum compared with the reference spectrum of TiO2. In contrast to this, a spectrum identical to the Ti02 reference spectrum was obtained after only 2 mm when sputtered on top of a titanium metal film. This is somewhat surprising in view of the high cohesive energy of TiN, and it demonstrates the sensitivity of the monitoring method in detecting small variations in the sputtered films. Another observation, which has already been touched on, is the high sensitivity of the film to the
: cx cx cx cx cx cx cx
1023
cleanliness of the sputtering atmosphere. The oxygen signal could only be completely removed at fast sputtering, and small instantaneous leakages, such as those induced by operating an 0-ring-sealed feedthrough during sputtering, were immediately detected in the X-ray spectrum.
4. Conclusions We have shown that SXES can be used for in situ monitoring of thin films deposited by reactive magnetron sputtering. Auxiliary excitation by means of an electron beam allows recording of spectra in less than 30 s of sufficient quality to allow both elemental analysis and detailed chemical bonding information to be gained. We have demonstrated that the method has high sensitivity for impurities in the sputtering atmosphere and that it can be used for monitoring and control of film growth. The results obtained so far have been acquired with an instrument which, although of very high performance, is not intended for the purpose. We are at present constructing a dedicated instrument for process control, which is expected to offer considerably higher sensitivity. With this instrument we anticipate that process control at time constants of the order of 1 is feasible.
Acknowledgments This project has been supported by the Swedish Board for Technical Developments (STUF) and the Swedish Natural Research Council (NFR).
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
B. Chapman, Glow Discharge Processes, Wiley.Interscience, New York, 1980. J. E. Greene, J. Vac. Sci. Technol., 15(1978)1718. W. Sprout and J. Tomashek, U.S. Patent 4,428,811, January 1984. J. Nordgren and C. Nordling, Commun. At. Mol. Phys., 13 (1983) 229. J. C. Ashley, C. J. Tung, R. H. Ritchie, V. E. Anderson, IEEE Trans. Nuci. Sci., 23(1976) 1833. M. Hecq, Appl. Phys. Lett., 49 (1986) 445. Y. Claesson, N. Wassdahl, M. Georgson, G. Bray, C.-G. Ribbing and J. Nordgren, Vacuum, 41 (1990)1275. A. G. Spencer, M. Georgson, C. A. Bishop, E. Stenberg and R. P. Howson, Sol. Energy Mater., 18(1988) 87. J. Nordgren and R. Nyholm, NucI. Instrum. Methods, 246 (1986) 242. D. W. Fisher and W. Baun, J. Appl. Phys., 39(1968) 4757. J.-E. Rubensson, N. Wassdahl, 0. Bray, J. Rindstedt, R. Nyholm, S. Cramm, N. Mártensson and J. Nordgren, Phys. Rev. Lett., 60(1988) 1759. M. Gryzinski, Phys. Rev. A, 138 (1965) 336. B. Karlsson and C.-G. Ribbing, P~oc.Soc. Photo-Opt. Instrum. Eng., 324 (1982) 52. Y. Claesson, M. Georgson, A. Roos and C.-G. Ribbing, Sol. Energy Mater., 20(1990) 455.
1015
CHARACTERIZATION AND PRODUCTION MONITORING OF THIN FILMS USING SOFT X-RAY SPECTROSCOPY* J. NORDGREN and G. BRAY Uppsala University, Department of Physics, Box 530, S-75121 Upp8ala (Sweden) Y. CLAESSON, M. GEORGSON and C.-G. RIBBING Uppsala University, Department of Technology, Box 534, S-75121 Uppsala (Sweden) N. WASSDAHL Uppsala University, Department of Physics, Box 530, S-75121 Uppsala (Sweden)
Abstract The feasibility of using soft X-ray spectroscopy for in situ on-line analysis of thin films produced by reactive magnetron sputtering has been investigated. A high performance grazing incidence instrument of novel design was used to record spectra in the 380—550 eV range from thin films of titanium, TiN and Ti02. The growth of the film could be monitored and impurities or unbalanced gas flows detected. Small amounts of oxidation, such as that corresponding to 90 mm exposure of a titanium metal film to a background pressure of below 6 x iO~mbar, was clearly observed. The results show that a continuous in situ monitoring with respect to both elemental analysis and chemical characterization is feasible on a time scale of technological interest.
1. Introduction The increasing use of various film deposition techniques, in particular reactive cathode sputtering, has created an interest in methods for in situ monitoring in order to control the process of film growth. Parameters to be controlled in reactive sputtering are for example partial gas pressures and/or flows, potentials and plasma current (see, for example, ref. 1). Prevailing monitoring methods are generally based on either spectroscopic recording of optical emission or absorption in the sputtering plasma [2], or mass spectrometric analysis of the sputtering atmosphere [3]. These two methods provide continuous information about the process but no direct information about the growing film. Commonly used spectroscopic methods to extract chemical *Winner of a 1990 ICMC/ICTF Bunshah Award. 0257.8972/90/$3.50
© Elsevier Sequoia/Printed in The Netherlands
1016
information, such as photoemission or Auger spectroscopy, are not usable as on-line monitoring techniques owing to the charged particles and strong fields associated with the sputtering plasma. We have investigated the feasibility of using soft X-ray emission spectroscopy (SXES) to provide such an in situ and on-line monitoring method, since the X-rays can freely penetrate the sputtering atmosphere. Radiative transitions in the soft X-ray range occur as a result of electric dipole interaction between the two outermost main shells of an atom or molecule, i.e. between valence electrons and localized inner vacancies (Fig. 1). This condition forms a basis for the use of soft X-ray spectra in the study of the electronic structure of matter (see, for example, ref. 4). The energy separation of the inner levels of different atomic species is sufficiently large to provide elemental selectivity, i.e. different atoms of a molecule or compound produce different spectra appearing at well-separated wavelengths. Also, the condition that the transitions directly involve valence electrons makes detailed studies of chemical bonding and valence electronic structure feasible. Furthermore, the selective nature of the dipole transitions gives rise to an atomic state selectivity that greatly facilitates the interpretation of finer details in the spectra. In addition, the use of soft X-rays permits studies of the lighter elements, such as carbon, nitrogen and oxygen, and for heavier elements one avoids problems due to large lifetime broadening. The use of SXES for monitoring film sputtering offers both elemental and chemical bonding information, and also a means to obtain a depth profile analysis since the excitation depth can be varied by varying the energy of the excitation agent. The depth profiling capability is illustrated in Fig. 2, where Al I emission
~,
•
eJectron excitation
—~
V~ence~ Ti3s
200 eV
T1L(=45OeV)
Ti 2p-3s ( 395 eV)
Nis
~
-
~NK(=390eV)
Ti2p~~
:: :
400 eV
80
70
I
I
60
(e V)
Fig. 1. Diagram of soft X-ray transitions in TiN. Fig. 2. Al L emission spectra of an aluminium sample with a very thin oxide layer, excited with electrons of kinetic energy between 200 and 2000 eV.
1017
L emission spectra of an aluminium sample with a thin (few nanometres) oxide layer, excited with electrons of various energies, have been plotted. One notices that the pure Al L spectrum of aluminium metal is obtained when an excitation energy of 2 keV is used, whereas spectral features characteristic of Al2 03 grow on lowering the electron energy. At 200 eV almost only the Al L emission of Al2 03 is present. The range of 200 eV electrons is only about 1 nm, while 2 eV electrons penetrate some 100 nm [5]. For the present investigation we have chosen titanium as a suitable system for reactive sputtering, and in particular we have studied the sputtering of TiN. Titanium is reactive and exhibits interesting stoichiometric variations in the formation of different compounds. Also, it has considerable technical interest. The soft X-ray emission of titanium is found in the 395—460 eV range. It has been shown previously [6, 7] that the sputtering process itself gives rise to characteristic X-rays from the substrate excited by high energy secondary electrons. This spontaneous radiation allows spectral analysis to be made. However, for rapid data collection, as in process control, we have found that auxiliary excitation by electron bombardment greatly facilitates the recording of spectra. In a previous report, we presented results from a feasibility study of the use of SXES for film production monitoring [7], suggesting that the method has a potential for such applications. In the present paper we disclose new results from further investigations showing more definitely the applicability of the method. Further development of the technique is underway and a dedicated instrument is being constructed, which is expected to provide sufficient sensitivity for real-time process control.
2. Experimental details The film deposition equipment used in the present work was a d.c. magnetron sputtering system described previously [8]. The sputtering chamber was pumped with a 13 in oilwith diffusion pump, which cold provided a base 7mbar the liquid nitrogen trap in use. pressuresputtering of below the 3 x current 10During was kept at 4 A and the voltage around 450 V, giving a deposition rate for TiN of the order of 1 nm s’. Target-to-substrate distance was 120 mm and the target size 206 mm x 86 mm. At an argon gas flow of 100 standard cm3 min ‘, an operating pressure of 4 x iO~mbar was maintained. Connected to the sputtering chamber was a grazing incidence grating spectrometer constructed primarily for soft X-ray spectroscopy using synchrotron radiation [9]. The spectrometer was designed according to a new concept which allows a large spectral range to be covered in a small size instrument, while still providing both high resolution and luminosity. The instrument employs three spherical gratings, two of radius 5 m and line frequencies 400 and 1200 lines mm 1, and one grating of radius 3 m and line frequency 300 lines mm ‘. The gratings are fixed on a precision ground
1018
reference block in order to accept radiation through the common entrance slit at angles of incidence appropriate for the various wavelength ranges covered by the different gratings. Selection of the grating in use is made simply by operating a movable aperture between the gratings and the entrance slit so that the appropriate grating is being illuminated (Fig. 3). The fact that the three gratings all define a focal curve of their own calls for a detector that can be moved in a three-axis coordinate system covering these curves. This is accomplished by means of three high precision translational tables, driven by computer-controlled stepper motors, and a flexible welded bellows between the detector house and the grating compartment. The diffracted X-rays are detected by a 40 mm diameter detector based on multichannel plates (MCPs) and a resistive anode read-out system, the detector was a custom version of a detector produced by Surface Science Ltd., Mountain View, CA. In order to enhance the efficiency of the detector the front MCP was coated with CsI and an electron capturing electrode was mounted outside the detector face. The detector was operated in two-dimensional mode in order to allow a large solid angle to be accepted without losing resolution as a result of the curvature of the spectral lines caused by the concave grating. Also, the two-dimensional detection allows distortions in the detector itself to be accounted for. The ultimate energy resolution of the instrument depends on photon energy and amounts to 0.02 eV at 50 eV and 0.5 eV at 1000 eV. In the present work the resolution was set to 2 eV at 450 eV. Excitation of soft X-ray emission was made by means of an electron gun mounted on a side flange of the sputtering chamber. The gun was operated at typically a few kilovolts and focusing and steering of the electron beam were accomplished by a quacirupole doublet magnetic lens.
3. Results and discussion The soft X-ray emission spectra of titanium, TiN and Ti02 have all been studied previously using different means of excitation [10, 11]. The Ti L spectra exhibit distinct differences owing to the differences in the bonding and, obviously, there are differences with respect to the N K and 0 K spectra. Figure 4 shows the emission spectra of these three materials in the 380— 550 eV range, i.e. the N K spectrum at 390 eV, the Ti L23 spectrum at 395 eV and 450 eV and the 0 K spectrum at 525 eV. The spectra were recorded in situ after typically 2—4 mm of sputtering using the power settings and gas flows listed in Table 1. It should be noted that the N K emission line accidentally almost coincides with the Ti 2p—3s transition. The spectra of Fig. 4 were used in the present experiments as reference spectra in the investigation of the monitoring capability of the soft X-ray analysis method. The present method of in situ analysis provides a depth profiling capability since the range of the exciting electrons is dependent on the kinetic energy [5]. This means that variation of acceleration voltage allows the excitation depth to be varied down to a depth corresponding to the lowest
1019
~
~
I
I~
©L1I~/\ \
I~I~
\\
/1111
II I II
~//
~
~ IT{L~~ I!u~II ~ r~® - III I!11L~ r2r1
k~
/
_~_~//~‘
/
~J~I1 ~ I~I ~
I ir~ IR ~7~1 ~ o___~._r~i~_~ ~
Ji
I ~ ~ ~ ~
~
0
~
-Q
‘~
.c ~ ‘-~
/~
/
EE
I
in
/
•1
~
LJ..J
/
-~
o
©(_@
©
©
©
© ©
0
H
1020
xl 0~
2-
1JUL90mmn
-
0.5
400
450
500
550
500
4O~J
Emission Energy (eV)
Emission Energy (eV)
Fig. 4. Soft X-ray emission spectra of, from top, TiN, titanium, and Ti02. The spectrum at 400eV for TiN has been divided by 3. Fig. 5. Soft X-ray emission spectra of a sputtered titanium film recorded immediately after sputtering (bottom) and after 90 mm (top) at a background pressure of below 6 x iO~mbar. The rise in the oxygen signal indicating oxidation of the film should be noted.
TABLE 1 Power settings and gas flows used in the different experiments (for Ti02 the pumping speed was somewhat reduced) 3 mm Figure Material Current (A) Voltage (V) Gas flow (standard cm Ar N 2 02 4
5 6 7
TiN Ti Ti02 Ti Ti TiN TiN
4 4 4 4 4 4 4
450 440 440 440 440 450 440-460
100 100 —
iOO 100 100 100
15
—
—
—
—
100
—
—
—
—
15 0-15
— —
cx cx
1021
energy sufficient for X-ray excitation. In the present case the threshold energies range from about 400 eV for N K emission to 530 eV for 0 K emission. From that value the excitation cross-section rises to a maximum at about 3—5 times the threshold value [12]. The mean free path for 400 eV electrons is of the order of a few nanometres and increases for increasing electron energy. 5 keV electrons penetrate a few hundred nanometres into a material such as titanium. By directing the electron beam onto the substrate at a grazing angle the surface sensitivity is further enhanced. The sensitivity of the soft X-ray probe is demonstrated in Fig. 5. A titanium film was sputtered and a first recording of the soft X-ray spectrum was immediately obtained. This spectrum shows a faint oxygen signal due to the oxygen content in the residual gas atmosphere. The second spectrum was recorded after 90 mm at a background pressure below 6 x i0~ mbar. One notices a significant increase in the oxygen signal, indicating that the film has been subject to oxidation during that time. In many applications it is important to control the oxidation of a sputtered film. For example, it has been suggested that thin TiN films could replace noble metal films in heat-mirror multilayers [13, 14], resulting in improved stability. In this application it is of crucial importance to control the stoichiometry and to avoid oxidation to benefit from the metallic character of TiN. Figure 6 shows a series of SXES spectra recorded after successive short periods of TiN sputtering starting with a titanium metal film. Each sputtering period was 15 s and each spectrum was recorded during approximately 30 s with the plasma shut off. One notices a gradual change in the spectral features as the titanium metal is buried under the TiN film. The Ti L spectrum changes shape from the typical titanium metal two-peak spectrum to the three-peak spectrum of TiN. Also, the nitrogen signal grows on the low energy side of the Ti 2p—3s line, reaching a state where the titanium line only causes a shoulder on the high energy side of the N K line. The accumulation time of 30 s to obtain sufficient spectral quality obviously requires the sputtering to be shut off during recording. We expect these time periods to be considerably reduced when a new dedicated instrument for monitoring sputtered films which is at present being constructed in our laboratory is introduced. Then spectrum recording during sputtering will be the normal condition and we shall need to reduce background signals due to UV light and charged particles. We have investigated this problem and found that filtering the radiation using for example a 100 nm foil of indium, silver or copper reduces the background from the sputtering plasma to a negligible level. Other means of background reduction would be proper masking and introduction of apertures. The effect of varying the flow of N2 in TiN sputtering is shown in Fig.
7. Here a series of SXES spectra were recorded after typically 4 mm of TiN sputtering at various N2 gas flows, giving a range of understoichiometric films. One can clearly see the gradual change from titanium metal film to TiN film, in both the Ti L spectrum and the N K—Ti 2p—3s spectrum. Also, one observes that the rates of change of the spectral features are different for the
cx
1022 4
x10
-
1.8
A
120 S
1.6 1.4 1.2
j~
———-—~
-
X4
8
~
105
J
x104
i~ sccm/min N
2
~ /L ~
7.
~J~L ~ ~ ~
0.8 ~
A
-
A
6-
~
~
-
4
0.6 ~ ~
I ~
~
-
2~* 0
o 400
450
Emission Energy (eV)
400
450
Emission Energy (eV)
Fig. 6. Series of soft X-ray emission spectra recorded after successive 15 s periods of TiN sputtering starting with a pure titanium film (bottom). Each spectrum was recorded for approximately 30s. The Ti L spectrum at 450 eV has been multiplied by 4. Fig. 7. Series of soft X-ray emission spectra recorded after typically 4 mm of TiN sputtering at various N2 gas flows. The Ti L spectrum at 450 eV has been multiplied by 4.
Ti L and the N K spectra. On going down from high flows (full stoichiometry), the Ti L spectrum exhibits the most rapid change; on going up from low flows the N K—Ti 2p—3s spectrum changes the fastest. Several interesting observations were made during the present experiments. For example, sputtering a Ti02 film on TiN required longer time than on titanium metal in order to obtain a pure film. Still, after 8 mm of sputtering (some 80 nm thickness) on top of TiN, there was a distinct nitrogen signal and a small but significant difference in the Ti L spectrum compared with the reference spectrum of TiO2. In contrast to this, a spectrum identical to the Ti02 reference spectrum was obtained after only 2 mm when sputtered on top of a titanium metal film. This is somewhat surprising in view of the high cohesive energy of TiN, and it demonstrates the sensitivity of the monitoring method in detecting small variations in the sputtered films. Another observation, which has already been touched on, is the high sensitivity of the film to the
: cx cx cx cx cx cx cx
1023
cleanliness of the sputtering atmosphere. The oxygen signal could only be completely removed at fast sputtering, and small instantaneous leakages, such as those induced by operating an 0-ring-sealed feedthrough during sputtering, were immediately detected in the X-ray spectrum.
4. Conclusions We have shown that SXES can be used for in situ monitoring of thin films deposited by reactive magnetron sputtering. Auxiliary excitation by means of an electron beam allows recording of spectra in less than 30 s of sufficient quality to allow both elemental analysis and detailed chemical bonding information to be gained. We have demonstrated that the method has high sensitivity for impurities in the sputtering atmosphere and that it can be used for monitoring and control of film growth. The results obtained so far have been acquired with an instrument which, although of very high performance, is not intended for the purpose. We are at present constructing a dedicated instrument for process control, which is expected to offer considerably higher sensitivity. With this instrument we anticipate that process control at time constants of the order of 1 is feasible.
Acknowledgments This project has been supported by the Swedish Board for Technical Developments (STUF) and the Swedish Natural Research Council (NFR).
References 1 2 3 4 5 6 7 8 9 10 11 12 13 14
B. Chapman, Glow Discharge Processes, Wiley.Interscience, New York, 1980. J. E. Greene, J. Vac. Sci. Technol., 15(1978)1718. W. Sprout and J. Tomashek, U.S. Patent 4,428,811, January 1984. J. Nordgren and C. Nordling, Commun. At. Mol. Phys., 13 (1983) 229. J. C. Ashley, C. J. Tung, R. H. Ritchie, V. E. Anderson, IEEE Trans. Nuci. Sci., 23(1976) 1833. M. Hecq, Appl. Phys. Lett., 49 (1986) 445. Y. Claesson, N. Wassdahl, M. Georgson, G. Bray, C.-G. Ribbing and J. Nordgren, Vacuum, 41 (1990)1275. A. G. Spencer, M. Georgson, C. A. Bishop, E. Stenberg and R. P. Howson, Sol. Energy Mater., 18(1988) 87. J. Nordgren and R. Nyholm, NucI. Instrum. Methods, 246 (1986) 242. D. W. Fisher and W. Baun, J. Appl. Phys., 39(1968) 4757. J.-E. Rubensson, N. Wassdahl, 0. Bray, J. Rindstedt, R. Nyholm, S. Cramm, N. Mártensson and J. Nordgren, Phys. Rev. Lett., 60(1988) 1759. M. Gryzinski, Phys. Rev. A, 138 (1965) 336. B. Karlsson and C.-G. Ribbing, P~oc.Soc. Photo-Opt. Instrum. Eng., 324 (1982) 52. Y. Claesson, M. Georgson, A. Roos and C.-G. Ribbing, Sol. Energy Mater., 20(1990) 455.