- Email: [email protected]
ex situ X-ray analysis system for thin sputtered films
Surface and Coatings Technology 110 (1998) 105–110
An in situ/ex situ X-ray analysis system for thin sputtered films A.K. Malhotra 1, J.F. Whitacre *, Z.B. Zhao, J. Hershberger, S.M. Yalisove, J.C. Bilello The University of Michigan, Dept. of Materials Science and Engineering, 2300 Hayward Street, Ann Arbor, MI 48109-2136, USA Received 2 January 1998; received in revised form 24 July 1998; accepted 28 July 1998
Abstract An in situ X-ray analysis system for thin sputtered films was developed. This set-up was designed to perform reflectivity, grazing incidence X-ray scattering, and texture analysis experiments on films as they are deposited. Beam alignment was facilitated by mounting a rotating anode tube tower on a goniometer which allowed both translation and rotation. The X-ray source could then be aligned with either a standard four-circle goniometer (ex situ) or a diffractometer designed around a high-vacuum sputter deposition system (in situ). Custom designed UHV beryllium windows allowed X-rays to enter and exit the sputter chamber. An Inel@ curvilinear position sensitive X-ray detector capable of rapidly and simultaneously collecting diffracted intensities through a 90° range with a resolution of ~0.02° was used. To test system performance, Mo sputter depositions were studied. Diffraction patterns were collected in as little as 2 s. This data can provide information concerning grain size, texture, and strain evolution which are free of the distortions associated with deposition interruption, thermal fluctuation, and surface oxide formation. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Molybdenum films; Sputter deposition; Texture; X-ray diffraction
1. Introduction In situ analysis techniques are common for films grown using ultra-high vacuum systems such as molecular beam epitaxy [1–3], but are rare for sputter deposited films due to probe absorption and scattering by the sputtering gas and plasma environment. X-ray radiation is a logical choice for in situ analysis in such a system, since photon scattering is relatively small in a gaseous atmosphere (compared to that of electron beam probes) and photons are not sensitive to charged particles or magnetic fields. Several systems have been reported which have the capability of performing in situ X-ray analyses of sputter deposited films during growth [4,5]. To minimize data collection times, high intensity X-rays generated from synchrotron light sources were used in previous sputter growth studies. These experiments used the ‘‘z-axis’’ geometry where the incident angle of the X-ray beam with respect to the sample was varied by rotating the entire deposition chamber. This geometry required the use of a multiple axis diffractometer and * Corresponding author. Fax: +1 313 647 4802; e-mail: [email protected] 1Present address: IBM Microelectronics, IBM, 1580 Rte 52, Z/20A, Hopewell Junction, NY 12533, USA.
had data acquisition speeds limited by the X-ray detector scanning velocity. A high-speed in situ X-ray analysis system which uses a laboratory based Rigaku@ 18 kW rotating anode X-ray generator is described in this paper. Rapid data acquisition is made possible through the use of a curvilinear position sensitive (CPS ) detector. Alignment of the incident X-ray beam is achieved by moving the X-ray source, not the sample or the deposition chamber. By virtue of having a design which provides a variable position X-ray source, multiple diffraction set-ups can be constructed without being constrained to a specific tube tower position.
2. System description Fig. 1 depicts a schematic (top view) of the complete system, showing the ex situ four-circle diffractometer system, X-ray source assembly, sputter chamber, in situ sample goniometer, tube tower goniometer, CPS detector, multichannel analyzer and other electronics used to monitor and control the X-ray analysis and sputtering processes. Two PC-type computers independently control the deposition chamber and the X-ray data collec-
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Fig. 1. Schematic top view of the thin film growth and X-ray analysis system showing the major components.
Fig. 2. Schematic front view of the in situ thin film growth and in situ analysis system showing the major components with the CPS detector in the asymmetric scattering geometry.
tion system. The main components of the in situ analysis system are shown in Figs. 1 and 2. In Fig. 1, the CPS detector is shown in the symmetric scattering geometry. Rotating the CPS detector 90° positions it for use in the asymmetric scattering geometry (Fig. 2), where the scattering vector is in the plane defined by the sample normal and incident beam. Fig. 3(a) and 3(b) are schematics of these two geometries (see refs. [6–8] for further details). 2.1. Sputter deposition system The sputter deposition system is a modified version of a high-vacuum ‘‘turn-key’’ model produced by Denton Vacuum Incorporated. The system has three 100 mm planar magnetron sources, which can be driven using either DC or RF power generators. Depositions can be performed while limiting cathode voltage, current, or power. A filament heater is present which can heat the chamber to about 300 °C. The temperature inside is monitored using a sheath thermocouple which provides reliable readings in the presence of an ionized ambient. The pumping system consists of a mechanical roughing pump and a cryopump. The latter is separated from the chamber by a throttle gate valve. Gases are introduced into the chamber by means of two precision bleed valves which are controlled by a flow regulator, allowing for reactive sputter deposition by bleeding reactive gas through one of the valves. Sputtering pres-
(a)
(b) Fig. 3. Schematics of the two scattering geometries available for in situ X-ray analysis of sputter grown thin films: (a) grazing incidence X-ray scattering (GIXS) geometry and (b) asymmetric low angle X-ray scattering geometry.
A.K. Malhotra et al. / Surface and Coatings Technology 110 (1998) 105–110
sure is continuously monitored during deposition by means of a capacitance manometer. A PC-type computer controls the entire deposition system including the pumping sequence, initiation and completion of deposition. The planar magnetron sources in the sputter chamber can be mounted either in parallel or confocal deposition geometry. Fig. 4(a) and 4(b) show schematic views of these two configurations. In the parallel deposition geometry, substrates are mounted on a 12 in platen which rotates during deposition. In the confocal deposition geometry, which is used for in situ X-ray analysis,
107
a single substrate is mounted on a sample holder which has only an azimuthal (w, about its surface normal ) degree of freedom. Beryllium windows are used as X-ray entry and exit ports to the sputter chamber. These windows were custom manufactured by Thermionics Northwest Incorporated. The X-ray entrance window is a 110 mm diameter viewport for which a beryllium foil is brazed onto the stainless steel flange to obtain a 55 mm diameter X-ray transparent port. The exit window is in the sputter chamber door, as shown in Fig. 5. The door was made of a standard NW400 stainless steel flange (~432 mm diameter). A stainless steel strip with a rectangular slot of 400 mm×20 mm houses a 0.13 mm thick beryllium foil. This elongated window is sealed into a slot cut in the door using bolts and an o-ring. The door may be mounted with its beryllium viewport parallel to the sample plane, creating the symmetric grazing incidence scattering geometry [Fig. 3(a)], or normal to the sample plane, giving the asymmetric scattering geometry [Fig. 3(b)]. 2.2. The curvilinear position sensitive detector
(a)
Data acquisition time is minimized through the use of a CPS detector, manufactured by Inel Incorporated (model CPS 590). Similar detectors have been used successfully elsewhere [9,10]. The equipment described here is capable of collecting 90° of diffraction data simultaneously, has a 500 mm radius of curvature and is connected (via electronics which convert detector position to energy level ) to a 4092 channel multichannel analyzer (MCA), also supplied by Inel Inc. The detector offers an angular resolution of ~0.02° (degrees per channel ), which is sufficient to distinguish all of the Mo and W peaks (both BCC structures with a lattice param-
(b) Fig. 4. Schematic views of the two sputtering geometries available in the chamber used for thin film growth and in situ X-ray analysis: (a) confocal deposition geometry and (b) parallel deposition geometry.
Fig. 5. Schematic of the chamber door made out of an NW400 stainless steel flange with rectangular beryllium window which is used as the X-ray exit port out of the sputter chamber to the detector.
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eter difference of 0.58%) from a Mo/W multilayer film when Mo Ka radiation is used. Due to geometrical constraints in the present set-up, 2h is limited to a range between 0° and 42° for a fixed horizontal sample position. To minimize systematic peak broadening, two slits are used to define the beam focus and divergence (see Fig. 2). The first is a standard Hu¨ber slit attached to the monochromator housing, and the second slit is a custom designed apparatus located inside the growth chamber. This slit apparatus was made of precision-cut stainless steel and was created to withstand the rigors of a high vacuum and sputter deposition environment. When both slits are narrowed to a width of 0.1 mm, the beam incident upon the substrate has a width of ~0.2 mm and a divergence of a=~0.6°. Fig. 6 shows a schematic of the system geometry [11]. The slits provide a focusing condition and the peak broadening (D2h) due to defocalisation may be calculated using the equation D2h=a sin(h−Q)cos h. From this, D2h= ~0.1° when 2h=20°, a=~0.6° and Q=2°. Presently, with the 18 kW X-ray source, the CPS detector is capable of acquiring diffraction spectra for thin film samples, with an acceptable signal/noise ratio, in as little as 1 s, thereby providing the ability to perform real-time in situ X-ray analysis.
2.3. Tube tower An 18 kW vertical direct drive rotating anode (model RU300), supplied by Rigaku Inc., is used to produce X-rays. Its tube tower has two active ports which provide beams for both the in situ and ex situ experimental set-ups. This particular model has a fixed X-ray maximum intensity ‘‘take-off ’’ angle of 6° above the horizontal plane from all ports. To align the X-ray beam, then, the system requires variation of the entire tube tower angle with respect to the horizontal through a range of up to 90°. In order to achieve this, a custombuilt manipulation system is used (designed by American Design Inc.). The tube tower is provided with one translational (z, vertical ) and one rotational degree of freedom (using Hu¨ber goniometer model 420). Lead weights are used to counter-balance and stabilize the tube tower assembly as it is tilted to various positions. The system uses a Varian turbomolecular pump which can operate regardless of its tilt orientation. The monochromator housings for the in situ diffractometer system, which currently contains a single-crystal, non-focusing graphite monochromator, are mounted on the tube tower and move in concert with the X-ray source. The line source is directed into the sputter deposition chamber. A beam ~3 cm in width passes through the entrance window and onto the center of the sample holder. The four-circle diffractometer is oriented to use the point source from the tube tower. This requires precision alignment and maximizes incident beam intensity. 2.4. Ex situ X-ray analysis The ex situ X-ray analysis system consists of a conventional four-circle Hu¨ber diffractometer system (base model 424, Eulerian cradle model 511) supplied by Blake Industries. A collimator assembly is used to optimize the X-ray beam for thin film analyses. The standard design is modified by the addition of a detector arm which can accommodate the CPS detector. Thin film analyses including reflectivity and texture (pole figure) studies are performed using a specialized software package which simultaneously controls diffractometer motion and data acquisition. With this equipment, pole figures of multiple reflections may be collected at the same time. A standard scintillation counter may also be mounted on the 2h arm and can be used for typical data acquisition. 2.5. Further modifications
Fig. 6. Schematic of the focusing X-ray geometry used. The positioning and width of the two slits defines the defocalisation of the diffracted beam. (Figure modified from Inel@ CPS 90 Directions for Use Manual [11].)
Future modifications to the system include a goniometer for the in situ sample holder which consists of vertical (z) translation, azimuthal (w) rotation, and limited tilt motion. The additional degrees of movement
A.K. Malhotra et al. / Surface and Coatings Technology 110 (1998) 105–110
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Fig. 7. X-ray diffraction spectrum of a NIST LaB powder standard. 6 This data was collected using the asymmetric GIXS geometry with an incident angle of 2°. The predicted peak locations and intensities are indicated by dashed lines.
(a)
Fig. 8. Time resolution of the Inel CPS 590 detector. Patterns collected from an ~2.4 mm thick film housed in the evacuated deposition chamber.
will facilitate both alignment and texturing studies. A heating system capable of heating the sample to about 1000 °C will also be included in the new sample manipulator. This will allow for deposition and in situ analysis of thin films as a function of temperature.
(b) Fig. 9. Real-time diffraction data of a sputtered Mo film. Deposition conditions: base pressure=10−6 Torr, Ar pressure=10 mTorr, power=250 W, time=60 min, deposition rate=12 nm/min. Data acquisition times shown are 5 s (a) and 50 s (b). The data are offset vertically from each other to allow for easy viewing; each scan’s actual baseline is zero.
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3. Preliminary results
4. Conclusions
3.1. In situ real-time studies
This paper has described a set-up designed to allow for both real-time in situ as well as traditional ex situ X-ray analysis of thin sputtered films. The unique ability to rotate and translate the entire X-ray tube tower allows for alignment without a sample goniometer (in situ configuration), as well as the ability to switch between multiple diffractometers. It has been shown that meaningful in situ data can be collected in very short time periods and that the use of a CPS detector is a practical solution to the problems encountered when performing real-time X-ray analysis of sputtered films as they are grown. This system will be a useful tool in probing the evolution of thin film microstructure, residual stress, and texture. The in situ observation of other phenomena (i.e. phase relationships, transformations, etc.) will also be possible.
The growth of a sputter deposited Mo film was monitored during deposition. The system was aligned such that the line source X-ray beam was incident onto the substrate at an angle of a=2° in the asymmetric GIXS geometry. Detector 2h calibration was achieved using a powdered LaB specimen as a standard (NIST 6 standard reference material 660). Fig. 7 shows the collected LaB spectrum with the predicted JCPDS peaks 6 overlaid. To study the time resolution of the system, the diffracted signal from a highly textured ~2.4 mm thick Mo film was measured. Fig. 8 shows typical data. A well-defined (110) peak may be discerned in as little as 2 s. After 22 s the (211) peak is also well defined. No other peaks are observed in this geometry due to the high degree of both out-of-plane and in-plane texturing in this film. These effects have been observed elsewhere and are consistent, in this case, with a film having a strong (110) out-of-plane texture [12–14]. As an in situ study, Mo was deposited onto a Si(100) test grade substrate under the following conditions: base pressure=10−6 Torr, Ar pressure=10 mTorr, power= 250 W, time=60 min, deposition rate=12 nm/min, and substrate rotation speed (through w)=20 rpm. The ˚ ) was operated X-ray generator (Mo target, l=0.7093 A at 56 kV×250 mA=14 kW. Diffraction spectra from the film were collected every 3 min with acquisition times of 5 and 50 s. Fig. 9(a) and 9(b) show some of the resulting data (background from the bare substrate/sputter ambient has been subtracted ). The diffracted intensities increase with film thickness, as expected. For Mo, the data shows that the signal-to-noise ratio is too small for film thicknesses less than 90 nm for useful diffraction data to be extracted with an acquisition time of 50 s or less. To acquire meaningful data for very thin films, then, it is necessary to interrupt the deposition (either by closing a shutter or turning off the cathode power) and collect a spectrum for a longer period of time. Using this method, very thin films may be studied and compared. As films of decreasing thickness are observed, however, a decrease in the signal-to-noise ratio makes the observation of well-defined peaks increasingly difficult. Thus far, it has been possible to collect usable X-ray data from Ta films which are as thin as 40 nm [14].
Acknowledgement This work was supported by the USARO and ARPA under Contract DAAH04-95-1-020.
References [1] P. Claverie, J. Massies, R. Pinchaux, M. Sauvage-Simkin, J. Frouin, J. Bonnet, N. Jedrecy, Rev. Sci. Instrum. 60 (7) (1989) 2369. [2] S. Brennan, P.H. Fuoss, J.L. Kahn, D.W. Kisker, Nucl. Instrum. Meth. Phys. Res. A291 (1990) 86. [3] K. Akimoto, J. Mizuki, I. Hirosawa, J. Matsui, Rev. Sci. Instrum. 60 (7) (1989) 2362. [4] A.P. Payne, B.M. Clemens, S. Brennan, Rev. Sci. Instrum. 63 (1) (1992) 1147. [5] J.H. Je, D.Y. Noh, H.K. Kim, K.S. Liang, J. Appl. Phys. 81 (9) (1997) 6126. [6 ] P.H. Fouss, S. Brennan, Ann. Rev. Mater. Sci. 20 (1990) 365. [7] W.C. Marra, P. Eisenberger, A.Y. Cho, J. Appl. Phys. 50 (1979) 6927. [8] P. Eisenberger, W.C. Marra, Phys. Rev. Lett. 46 (1981) 1081. [9] J. Angilillo, R.D. Thompson, K.N. Tu, J. Appl. Crystallogr. 22 (1989) 523. [10] R.D. Thompson, J. Angilillo, Mater. Res. Soc. Symp. Proc. 208 (1991) 291. [11] Inel Electronic Instrumentation Manual: CPS 90, Directions for Use, 1997. [12] O.P. Karpenko, J.C. Bilello, S.M. Yalisove, J. Appl. Phys. 76 (8) (1994) 4610. [13] R.M. Bradley, J.M.E. Harper, D.A. Smith, J. Appl. Phys. 60 (12) (1986) 4160. [14] J.F. Whitacre, S.M. Yalisove, J.C. Bilello, in preparation.
An in situ/ex situ X-ray analysis system for thin sputtered films A.K. Malhotra 1, J.F. Whitacre *, Z.B. Zhao, J. Hershberger, S.M. Yalisove, J.C. Bilello The University of Michigan, Dept. of Materials Science and Engineering, 2300 Hayward Street, Ann Arbor, MI 48109-2136, USA Received 2 January 1998; received in revised form 24 July 1998; accepted 28 July 1998
Abstract An in situ X-ray analysis system for thin sputtered films was developed. This set-up was designed to perform reflectivity, grazing incidence X-ray scattering, and texture analysis experiments on films as they are deposited. Beam alignment was facilitated by mounting a rotating anode tube tower on a goniometer which allowed both translation and rotation. The X-ray source could then be aligned with either a standard four-circle goniometer (ex situ) or a diffractometer designed around a high-vacuum sputter deposition system (in situ). Custom designed UHV beryllium windows allowed X-rays to enter and exit the sputter chamber. An Inel@ curvilinear position sensitive X-ray detector capable of rapidly and simultaneously collecting diffracted intensities through a 90° range with a resolution of ~0.02° was used. To test system performance, Mo sputter depositions were studied. Diffraction patterns were collected in as little as 2 s. This data can provide information concerning grain size, texture, and strain evolution which are free of the distortions associated with deposition interruption, thermal fluctuation, and surface oxide formation. © 1998 Elsevier Science S.A. All rights reserved. Keywords: Molybdenum films; Sputter deposition; Texture; X-ray diffraction
1. Introduction In situ analysis techniques are common for films grown using ultra-high vacuum systems such as molecular beam epitaxy [1–3], but are rare for sputter deposited films due to probe absorption and scattering by the sputtering gas and plasma environment. X-ray radiation is a logical choice for in situ analysis in such a system, since photon scattering is relatively small in a gaseous atmosphere (compared to that of electron beam probes) and photons are not sensitive to charged particles or magnetic fields. Several systems have been reported which have the capability of performing in situ X-ray analyses of sputter deposited films during growth [4,5]. To minimize data collection times, high intensity X-rays generated from synchrotron light sources were used in previous sputter growth studies. These experiments used the ‘‘z-axis’’ geometry where the incident angle of the X-ray beam with respect to the sample was varied by rotating the entire deposition chamber. This geometry required the use of a multiple axis diffractometer and * Corresponding author. Fax: +1 313 647 4802; e-mail: [email protected] 1Present address: IBM Microelectronics, IBM, 1580 Rte 52, Z/20A, Hopewell Junction, NY 12533, USA.
had data acquisition speeds limited by the X-ray detector scanning velocity. A high-speed in situ X-ray analysis system which uses a laboratory based Rigaku@ 18 kW rotating anode X-ray generator is described in this paper. Rapid data acquisition is made possible through the use of a curvilinear position sensitive (CPS ) detector. Alignment of the incident X-ray beam is achieved by moving the X-ray source, not the sample or the deposition chamber. By virtue of having a design which provides a variable position X-ray source, multiple diffraction set-ups can be constructed without being constrained to a specific tube tower position.
2. System description Fig. 1 depicts a schematic (top view) of the complete system, showing the ex situ four-circle diffractometer system, X-ray source assembly, sputter chamber, in situ sample goniometer, tube tower goniometer, CPS detector, multichannel analyzer and other electronics used to monitor and control the X-ray analysis and sputtering processes. Two PC-type computers independently control the deposition chamber and the X-ray data collec-
0257-8972/98/$ – see front matter © 1998 Elsevier Science S.A. All rights reserved. PII S 02 5 7 -8 9 7 2 ( 9 8 ) 0 0 67 8 - 1
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Fig. 1. Schematic top view of the thin film growth and X-ray analysis system showing the major components.
Fig. 2. Schematic front view of the in situ thin film growth and in situ analysis system showing the major components with the CPS detector in the asymmetric scattering geometry.
tion system. The main components of the in situ analysis system are shown in Figs. 1 and 2. In Fig. 1, the CPS detector is shown in the symmetric scattering geometry. Rotating the CPS detector 90° positions it for use in the asymmetric scattering geometry (Fig. 2), where the scattering vector is in the plane defined by the sample normal and incident beam. Fig. 3(a) and 3(b) are schematics of these two geometries (see refs. [6–8] for further details). 2.1. Sputter deposition system The sputter deposition system is a modified version of a high-vacuum ‘‘turn-key’’ model produced by Denton Vacuum Incorporated. The system has three 100 mm planar magnetron sources, which can be driven using either DC or RF power generators. Depositions can be performed while limiting cathode voltage, current, or power. A filament heater is present which can heat the chamber to about 300 °C. The temperature inside is monitored using a sheath thermocouple which provides reliable readings in the presence of an ionized ambient. The pumping system consists of a mechanical roughing pump and a cryopump. The latter is separated from the chamber by a throttle gate valve. Gases are introduced into the chamber by means of two precision bleed valves which are controlled by a flow regulator, allowing for reactive sputter deposition by bleeding reactive gas through one of the valves. Sputtering pres-
(a)
(b) Fig. 3. Schematics of the two scattering geometries available for in situ X-ray analysis of sputter grown thin films: (a) grazing incidence X-ray scattering (GIXS) geometry and (b) asymmetric low angle X-ray scattering geometry.
A.K. Malhotra et al. / Surface and Coatings Technology 110 (1998) 105–110
sure is continuously monitored during deposition by means of a capacitance manometer. A PC-type computer controls the entire deposition system including the pumping sequence, initiation and completion of deposition. The planar magnetron sources in the sputter chamber can be mounted either in parallel or confocal deposition geometry. Fig. 4(a) and 4(b) show schematic views of these two configurations. In the parallel deposition geometry, substrates are mounted on a 12 in platen which rotates during deposition. In the confocal deposition geometry, which is used for in situ X-ray analysis,
107
a single substrate is mounted on a sample holder which has only an azimuthal (w, about its surface normal ) degree of freedom. Beryllium windows are used as X-ray entry and exit ports to the sputter chamber. These windows were custom manufactured by Thermionics Northwest Incorporated. The X-ray entrance window is a 110 mm diameter viewport for which a beryllium foil is brazed onto the stainless steel flange to obtain a 55 mm diameter X-ray transparent port. The exit window is in the sputter chamber door, as shown in Fig. 5. The door was made of a standard NW400 stainless steel flange (~432 mm diameter). A stainless steel strip with a rectangular slot of 400 mm×20 mm houses a 0.13 mm thick beryllium foil. This elongated window is sealed into a slot cut in the door using bolts and an o-ring. The door may be mounted with its beryllium viewport parallel to the sample plane, creating the symmetric grazing incidence scattering geometry [Fig. 3(a)], or normal to the sample plane, giving the asymmetric scattering geometry [Fig. 3(b)]. 2.2. The curvilinear position sensitive detector
(a)
Data acquisition time is minimized through the use of a CPS detector, manufactured by Inel Incorporated (model CPS 590). Similar detectors have been used successfully elsewhere [9,10]. The equipment described here is capable of collecting 90° of diffraction data simultaneously, has a 500 mm radius of curvature and is connected (via electronics which convert detector position to energy level ) to a 4092 channel multichannel analyzer (MCA), also supplied by Inel Inc. The detector offers an angular resolution of ~0.02° (degrees per channel ), which is sufficient to distinguish all of the Mo and W peaks (both BCC structures with a lattice param-
(b) Fig. 4. Schematic views of the two sputtering geometries available in the chamber used for thin film growth and in situ X-ray analysis: (a) confocal deposition geometry and (b) parallel deposition geometry.
Fig. 5. Schematic of the chamber door made out of an NW400 stainless steel flange with rectangular beryllium window which is used as the X-ray exit port out of the sputter chamber to the detector.
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eter difference of 0.58%) from a Mo/W multilayer film when Mo Ka radiation is used. Due to geometrical constraints in the present set-up, 2h is limited to a range between 0° and 42° for a fixed horizontal sample position. To minimize systematic peak broadening, two slits are used to define the beam focus and divergence (see Fig. 2). The first is a standard Hu¨ber slit attached to the monochromator housing, and the second slit is a custom designed apparatus located inside the growth chamber. This slit apparatus was made of precision-cut stainless steel and was created to withstand the rigors of a high vacuum and sputter deposition environment. When both slits are narrowed to a width of 0.1 mm, the beam incident upon the substrate has a width of ~0.2 mm and a divergence of a=~0.6°. Fig. 6 shows a schematic of the system geometry [11]. The slits provide a focusing condition and the peak broadening (D2h) due to defocalisation may be calculated using the equation D2h=a sin(h−Q)cos h. From this, D2h= ~0.1° when 2h=20°, a=~0.6° and Q=2°. Presently, with the 18 kW X-ray source, the CPS detector is capable of acquiring diffraction spectra for thin film samples, with an acceptable signal/noise ratio, in as little as 1 s, thereby providing the ability to perform real-time in situ X-ray analysis.
2.3. Tube tower An 18 kW vertical direct drive rotating anode (model RU300), supplied by Rigaku Inc., is used to produce X-rays. Its tube tower has two active ports which provide beams for both the in situ and ex situ experimental set-ups. This particular model has a fixed X-ray maximum intensity ‘‘take-off ’’ angle of 6° above the horizontal plane from all ports. To align the X-ray beam, then, the system requires variation of the entire tube tower angle with respect to the horizontal through a range of up to 90°. In order to achieve this, a custombuilt manipulation system is used (designed by American Design Inc.). The tube tower is provided with one translational (z, vertical ) and one rotational degree of freedom (using Hu¨ber goniometer model 420). Lead weights are used to counter-balance and stabilize the tube tower assembly as it is tilted to various positions. The system uses a Varian turbomolecular pump which can operate regardless of its tilt orientation. The monochromator housings for the in situ diffractometer system, which currently contains a single-crystal, non-focusing graphite monochromator, are mounted on the tube tower and move in concert with the X-ray source. The line source is directed into the sputter deposition chamber. A beam ~3 cm in width passes through the entrance window and onto the center of the sample holder. The four-circle diffractometer is oriented to use the point source from the tube tower. This requires precision alignment and maximizes incident beam intensity. 2.4. Ex situ X-ray analysis The ex situ X-ray analysis system consists of a conventional four-circle Hu¨ber diffractometer system (base model 424, Eulerian cradle model 511) supplied by Blake Industries. A collimator assembly is used to optimize the X-ray beam for thin film analyses. The standard design is modified by the addition of a detector arm which can accommodate the CPS detector. Thin film analyses including reflectivity and texture (pole figure) studies are performed using a specialized software package which simultaneously controls diffractometer motion and data acquisition. With this equipment, pole figures of multiple reflections may be collected at the same time. A standard scintillation counter may also be mounted on the 2h arm and can be used for typical data acquisition. 2.5. Further modifications
Fig. 6. Schematic of the focusing X-ray geometry used. The positioning and width of the two slits defines the defocalisation of the diffracted beam. (Figure modified from Inel@ CPS 90 Directions for Use Manual [11].)
Future modifications to the system include a goniometer for the in situ sample holder which consists of vertical (z) translation, azimuthal (w) rotation, and limited tilt motion. The additional degrees of movement
A.K. Malhotra et al. / Surface and Coatings Technology 110 (1998) 105–110
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Fig. 7. X-ray diffraction spectrum of a NIST LaB powder standard. 6 This data was collected using the asymmetric GIXS geometry with an incident angle of 2°. The predicted peak locations and intensities are indicated by dashed lines.
(a)
Fig. 8. Time resolution of the Inel CPS 590 detector. Patterns collected from an ~2.4 mm thick film housed in the evacuated deposition chamber.
will facilitate both alignment and texturing studies. A heating system capable of heating the sample to about 1000 °C will also be included in the new sample manipulator. This will allow for deposition and in situ analysis of thin films as a function of temperature.
(b) Fig. 9. Real-time diffraction data of a sputtered Mo film. Deposition conditions: base pressure=10−6 Torr, Ar pressure=10 mTorr, power=250 W, time=60 min, deposition rate=12 nm/min. Data acquisition times shown are 5 s (a) and 50 s (b). The data are offset vertically from each other to allow for easy viewing; each scan’s actual baseline is zero.
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3. Preliminary results
4. Conclusions
3.1. In situ real-time studies
This paper has described a set-up designed to allow for both real-time in situ as well as traditional ex situ X-ray analysis of thin sputtered films. The unique ability to rotate and translate the entire X-ray tube tower allows for alignment without a sample goniometer (in situ configuration), as well as the ability to switch between multiple diffractometers. It has been shown that meaningful in situ data can be collected in very short time periods and that the use of a CPS detector is a practical solution to the problems encountered when performing real-time X-ray analysis of sputtered films as they are grown. This system will be a useful tool in probing the evolution of thin film microstructure, residual stress, and texture. The in situ observation of other phenomena (i.e. phase relationships, transformations, etc.) will also be possible.
The growth of a sputter deposited Mo film was monitored during deposition. The system was aligned such that the line source X-ray beam was incident onto the substrate at an angle of a=2° in the asymmetric GIXS geometry. Detector 2h calibration was achieved using a powdered LaB specimen as a standard (NIST 6 standard reference material 660). Fig. 7 shows the collected LaB spectrum with the predicted JCPDS peaks 6 overlaid. To study the time resolution of the system, the diffracted signal from a highly textured ~2.4 mm thick Mo film was measured. Fig. 8 shows typical data. A well-defined (110) peak may be discerned in as little as 2 s. After 22 s the (211) peak is also well defined. No other peaks are observed in this geometry due to the high degree of both out-of-plane and in-plane texturing in this film. These effects have been observed elsewhere and are consistent, in this case, with a film having a strong (110) out-of-plane texture [12–14]. As an in situ study, Mo was deposited onto a Si(100) test grade substrate under the following conditions: base pressure=10−6 Torr, Ar pressure=10 mTorr, power= 250 W, time=60 min, deposition rate=12 nm/min, and substrate rotation speed (through w)=20 rpm. The ˚ ) was operated X-ray generator (Mo target, l=0.7093 A at 56 kV×250 mA=14 kW. Diffraction spectra from the film were collected every 3 min with acquisition times of 5 and 50 s. Fig. 9(a) and 9(b) show some of the resulting data (background from the bare substrate/sputter ambient has been subtracted ). The diffracted intensities increase with film thickness, as expected. For Mo, the data shows that the signal-to-noise ratio is too small for film thicknesses less than 90 nm for useful diffraction data to be extracted with an acquisition time of 50 s or less. To acquire meaningful data for very thin films, then, it is necessary to interrupt the deposition (either by closing a shutter or turning off the cathode power) and collect a spectrum for a longer period of time. Using this method, very thin films may be studied and compared. As films of decreasing thickness are observed, however, a decrease in the signal-to-noise ratio makes the observation of well-defined peaks increasingly difficult. Thus far, it has been possible to collect usable X-ray data from Ta films which are as thin as 40 nm [14].
Acknowledgement This work was supported by the USARO and ARPA under Contract DAAH04-95-1-020.
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