- Email: [email protected]
Rapid high-resolution X-ray diffraction measurement and analysis of MOVPE pHEMT structures using a high-brilliance X-ray source and automatic pattern fitting
Journal of Crystal Growth 221 (2000) 520}524
Rapid high-resolution X-ray di!raction measurement and analysis of MOVPE pHEMT structures using a high-brilliance X-ray source and automatic pattern "tting Tamzin La!ord*, Mark Taylor, John Wall, Neil Loxley Bede Scientixc Instruments Ltd., Bowburn South Industrial Estate, Durham DH6 5AD, UK
Abstract A novel micro-focus X-ray tube in combination with a focusing optic that uses total external re#ection has been used to enhance the di!racted intensity in a double-crystal experiment, whilst simultaneously reducing the beam footprint on the sample. The increased intensity allows data to be collected more quickly. Advances in auto-"tting using the full dynamical theory of X-ray di!raction mean that sample material parameters can be extracted quickly and objectively, opening the way to automatic data analysis. Both features are attractive for non-destructive quality control of semiconductor device structures, as well as for process development and research purposes. 2000 Elsevier Science B.V. All rights reserved. PACS: 61.10; 07.85.J Keywords: X-ray di!raction; Brightness; Data "tting; Quality control
1. Introduction High-resolution X-ray di!raction is a tool widely used for the non-destructive characterisation of semiconductor device structures, such as pseudomorphic HEMT structures (pHEMTs). Parameters such as layer thickness and alloy composition in epitaxial structures are obtained by "tting data using the dynamical theory of X-ray di!raction [1}3]. In order to obtain a suitable incident beam, conditioned in angle and wavelength, two main experimental con"gurations are used. * Corresponding author. Tel.: #44-191-377-2476; fax: #44191-377-9952. E-mail address: [email protected] (T. La!ord).
In the "rst, the double-crystal di!raction technique, the beam from the X-ray source is di!racted once from a reference crystal, and slits or a channel are used to restrict the beam size. This technique gives a beam where all the wavelengths present are di!racted at their respective Bragg angles, and all wavelengths that can get through the slits are incident on the sample. This is an e$cient use of X-ray power, but, in order to achieve narrow peak widths and high intensities, it is necessary to match the reference crystal type to the sample substrate material. The second technique involves crystal monochromation of the beam, which results in a low but "xed angular divergence. This puts a lower limit on the peak FWHM that can be measured. Since
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 6 2 - 4
T. Laword et al. / Journal of Crystal Growth 221 (2000) 520}524
much of the X-ray intensity is thrown away, a higher power is required in order to achieve the same di!racted peak intensities as are seen in the double-crystal technique. The sample can be of any material, regardless of the crystals used in the beam conditioning. Demand is growing for the capability to probe just a restricted area of the wafer. This capability can be important in examining device structures on patterned wafers, where small regions on the sample may be left for this purpose, next to each die. When using a conventional sealed tube source, examining such a small area is only possible by slitting down the X-ray beam, which signi"cantly reduces the X-ray intensity. This increases the count times required to achieve the same quality of data, and can cause the loss of some important but weak features if there is not su$cient intensity overall. The solution we describe is to use a novel highbrilliance laboratory X-ray source. This new source enables high-resolution X-ray di!raction (HRXRD) data to be collected rapidly from small sample areas with enhanced signal intensity. To extract sample material parameters from diffraction data of complex structures requires "tting by dynamical simulation. Auto-"tting of the data gives the parameters quickly and with an objective measure of the goodness of "t, with the possibility of trying many thousands of simulations. A new genetic algorithm has been applied to the "tting process, which is fast and reliable on a desktop PC, and, in combination with a suitable choice of error function, less likely than many methods to fall into false minima [4]. The combination of the new source for data collection, with rapid auto-"tting to extract structural parameters, makes quality control HRXRD analysis of device structures more widely viable.
2. Instrumentation Fig. 1 shows a schematic arrangement of the QC200 X-ray di!raction system used for this work. Sample (u) and detector (2h) axes can be scanned while the other elements are set up and then "xed.
521
Fig. 1. Instrument con"guration of the new Bede QC200. M } Microsource; R } re#ex micro-mirror; RC } reference crystal; A } variable aperture; S } sample; D } Bede EDRa detector. The sample sits horizontally without any need of adhesive or clips to hold it in place.
Fig. 2. Schematic representation of the principle of operation of the ellipsoidal micro-mirror in focusing an X-ray beam. Mirror aperture 0.5}1 mm. Focal distance 150}600 mm, depending on the mirror.
A high brightness X-ray beam with a small footprint is obtained by using the microsource威 } a micro-focus X-ray tube } in conjunction with the unique Re#ex威 micro-mirrors. The microsource uses a unique design of electron optics, which incorporates electron beam alignment and focussing and complete, accurate control over the "nal electron beam shape and position on the target. In order to increase the permissible power loading on the copper target, the electron optics stretch the electron beam laterally from a spot to a line. The target is angled to the electron beam to maximise the power loading and minimise self-(target) absorption of the X-rays produced. By matching electron beam aspect ratio to target angle and viewing at 903 to the electron beam, the X-rays are seen to be produced from a circular projected spot with a diameter (FWHM) down to 15 lm [5]. In the work we report in this paper, the X-rays were collected and focused by an ellipsoidal mirror [6] positioned close (+12 mm) to the target. A schematic ray-tracing diagram is given in Fig. 2 to illustrate the principle of operation. The mirror works by means of total external re#ection of the X-rays from an internal gold surface. The re#ectivity
522
T. Laword et al. / Journal of Crystal Growth 221 (2000) 520}524
of X-rays from most surfaces is low, however, for metals at low-glancing angles ((13) near 100% re#ection e$ciency is achieved, provided the roughness of the surface is low, i.e. (+1 nm r.m.s. roughness. Rays pass into the mirror through the circular entrance aperture (+0.5 mm diameter) and are re#ected from the curved gold surface and out of the exit aperture (+1.0 mm diameter). Note that, since there is no aperture stop, some unre#ected beam passes directly through the two mirror apertures, but it is of low intensity in comparison with the re#ected X-rays. In Fig. 2, the curved pro"le shown is part of an ellipse (with foci at the X-ray source and X-ray beam focus). An alternative pro"le is a section of a parabola (with the X-ray source at the parabola focus) in which case a parallel beam of X-rays is re#ected. For the ellipsoidal mirrors, careful control of the internal shape of the mirror allows the focal length and beam convergence angle to be varied. For HRXRD, a paraboloidal mirror should give the highest e$ciency of coupling into the beam conditioner by giving the smallest beam divergence. However, a smaller footprint at the sample is obtained by compromising between angular divergence and the focus diameter, hence the use of the ellipsoidal micro-mirror in these experiments. Since more of the rays from the source are focused in the forward direction, a higher incident intensity at the sample is achieved after di!raction from the reference crystal. For the work reported here, the mirror used had a divergence of approximately 2.7 mrad with a nominal focal length of 300 mm, giving a focused X-ray spot size at the sample of 1 mm (FWHM). The spot size was reduced further for some experiments by means of a slit. The X-ray source, mirror and reference crystal are mounted in a brass block with an external shutter and variable aperture for "ne control of the beam size. This block is held "xed with respect to the horizontal sample wafer such that the beam makes the Bragg angle with the sample wafer. Rotation of the source assembly is possible to allow for di!erent reference crystals with di!erent Bragg angles. Di!racted X-rays were detected using a scintillation counter (Bede EDRa) mounted on an arc. Any part of the wafer could be translated into
the X-ray beam; the area of the sample under test could be visually inspected remotely.
3. Data 5tting Having collected a high-resolution di!raction scan from a device structure, the data were "tted using Bede RADS Mercury software utilising the dynamical theory of X-ray di!raction [1}3]. The di!erential evolution genetic algorithm gives fast convergence on the "t with the minimum error [4]. Thousands of simulations can be tested in the time it would take a human operator to try just a few. The use of the least absolute log deviation (LALD) as the error function ensures that weak intensity features, such as those arising from the mismatched layer in a pHEMT, and from thickness fringes, are given importance, and the much more intense substrate peak does not dominate the process. LALD also gives sharp, deep true minima with less pronounced false minima. The algorithm is also set such that, when in a minimum, some combination of values well away from that condition will be tried in order to test whether that minimum is a false one. The value of the minimised error function is given as the "t progresses. Also given are a standardised s parameter and a residuals parameter, which can be used to compare the goodness of "t for di!erent data sets. Since the process is quick and objective, some of the previously required skill is removed and automated analysis is possible. This is particularly of interest in production-line quality control environments.
4. Results That the presence of the micro-mirror has no adverse e!ect on resolution is shown in Fig. 3, where the FWHM of a Si 0 0 4 re#ection is 5.1 arcsec (Si reference crystal). The theoretically achievable FWHM is 4.3 arcsec. Fig. 4 shows double-crystal di!raction data for a MOVPE-grown GaAs-based pHEMT. The data were collected using a Bede QC200 double-crystal di!ractometer "tted with a micro-focus source,
T. Laword et al. / Journal of Crystal Growth 221 (2000) 520}524
Fig. 3. Double-crystal di!raction scan of Si 0 0 4. Si 0 0 4 reference crystal, 0.25 mm di!racting area, Microsource at 50 W. FWHM 5.1 arcsec.
Fig. 4. Double-crystal di!raction scans of a pHEMT. Dashed line: standard generator at 50 W with a beam footprint of approximately 3;2 mm, Ka #Ka , total scan time 53 min. Solid line: using the new Bede QC200 with a Microsource at 50 W and a re#ex micro-mirror, beam footprint 1 mm diameter, Ka only, total scan time 22 min.
micro-mirror optics and a Ge reference crystal. The di!racted intensity has been increased by a factor of 10, while the beam footprint has been simultaneously reduced by more than 85%, compared with
523
Fig. 5. Bede RADS mercury auto-"t (solid line) to the QC200 experimental data (dotted line) using full dynamical theory. In addition to the layer parameters, a low Lorentzian curve was included to model the di!use scatter. A standardised s and R factor are given as measures of the goodness of "t.
a standard compact 50 W tube with a 0.25 mm diameter focal spot and no mirror optics. The intensity gain allows data to be collected more rapidly. The auto-"t is shown in Fig. 5. The LALD error function is chosen to re#ect the fact that the data are spread over a wide intensity range, with important signals which are also weak (Table 1). Fitting took only a few minutes for this structure, with "ve free parameters. Less time is taken when the starting model is closer to the "t model. Another example of a RADS mercury auto-"t is shown in Fig. 6. This structure is more complex, including graded Si}Ge layers. The non-linear variation of Si}Ge lattice parameter with alloy composition is taken into account. These data were collected on a Bede D1 system di!ractometer using a monochromated beam and a 2 kW Cu-sealed tube source.
Table 1 Fit parameters to the pHEMT data (Fig. 5) Layer
Material
Thickness/As
Composition fraction, x
3 2 1 Substrate
GaAs Al Ga As V \V In Ga As V \V GaAs
350.7 (#5.2, !3.0) 415.7 (#5.2, !3.4) 157.5 (#4.2, !1.6) *
* 0.286 (#0.024, !0.037) 0.117 (#0.000, !0.000) *
524
T. Laword et al. / Journal of Crystal Growth 221 (2000) 520}524
Fig. 6. Auto-"t (solid line) to 0 0 4 di!raction data (dotted line) of a complex Si}Ge structure. Sample courtesy Dr. Detlev GruK tzmacher, Paul Scherrer Institute, Switzerland. A schematic diagram of the "t structure is given on the right.
5. Conclusion
References
A double-crystal instrument giving high intensity in a small beam footprint is obtained by using a unique micro-focus tube and micro-mirror combination. Together with fast auto-"tting software, device structures can be quickly analysed and material parameters can be rapidly and objectively deduced. In this way, the process is suitable for quality control applications.
[1] S. Takagi, Acta Crystallogr. 15 (1962) 1311. [2] S. Takagi, J. Phys. Soc. Jpn. 26 (1969) 1239. [3] D. Taupin, Bull. Soc. Fr. Mineral. Crystallogr. 87 (1964) 469. [4] M. Wormington, C. Panaccione, K.M. Matney, D.K. Bowen, Philos. Trans. Roy. Soc. Lond. 357 (1999) 2827. [5] U.W. Arndt, J.V.P. Long, P. Duncumb, J. Appl. Crystallogr. 31 (1998) 936. [6] U.W. Arndt, P. Duncumb, J.V.P. Long, L. Pina, A. Inneman, J. Appl. Crystallogr. 31 (1998) 733.
Rapid high-resolution X-ray di!raction measurement and analysis of MOVPE pHEMT structures using a high-brilliance X-ray source and automatic pattern "tting Tamzin La!ord*, Mark Taylor, John Wall, Neil Loxley Bede Scientixc Instruments Ltd., Bowburn South Industrial Estate, Durham DH6 5AD, UK
Abstract A novel micro-focus X-ray tube in combination with a focusing optic that uses total external re#ection has been used to enhance the di!racted intensity in a double-crystal experiment, whilst simultaneously reducing the beam footprint on the sample. The increased intensity allows data to be collected more quickly. Advances in auto-"tting using the full dynamical theory of X-ray di!raction mean that sample material parameters can be extracted quickly and objectively, opening the way to automatic data analysis. Both features are attractive for non-destructive quality control of semiconductor device structures, as well as for process development and research purposes. 2000 Elsevier Science B.V. All rights reserved. PACS: 61.10; 07.85.J Keywords: X-ray di!raction; Brightness; Data "tting; Quality control
1. Introduction High-resolution X-ray di!raction is a tool widely used for the non-destructive characterisation of semiconductor device structures, such as pseudomorphic HEMT structures (pHEMTs). Parameters such as layer thickness and alloy composition in epitaxial structures are obtained by "tting data using the dynamical theory of X-ray di!raction [1}3]. In order to obtain a suitable incident beam, conditioned in angle and wavelength, two main experimental con"gurations are used. * Corresponding author. Tel.: #44-191-377-2476; fax: #44191-377-9952. E-mail address: [email protected] (T. La!ord).
In the "rst, the double-crystal di!raction technique, the beam from the X-ray source is di!racted once from a reference crystal, and slits or a channel are used to restrict the beam size. This technique gives a beam where all the wavelengths present are di!racted at their respective Bragg angles, and all wavelengths that can get through the slits are incident on the sample. This is an e$cient use of X-ray power, but, in order to achieve narrow peak widths and high intensities, it is necessary to match the reference crystal type to the sample substrate material. The second technique involves crystal monochromation of the beam, which results in a low but "xed angular divergence. This puts a lower limit on the peak FWHM that can be measured. Since
0022-0248/00/$ - see front matter 2000 Elsevier Science B.V. All rights reserved. PII: S 0 0 2 2 - 0 2 4 8 ( 0 0 ) 0 0 7 6 2 - 4
T. Laword et al. / Journal of Crystal Growth 221 (2000) 520}524
much of the X-ray intensity is thrown away, a higher power is required in order to achieve the same di!racted peak intensities as are seen in the double-crystal technique. The sample can be of any material, regardless of the crystals used in the beam conditioning. Demand is growing for the capability to probe just a restricted area of the wafer. This capability can be important in examining device structures on patterned wafers, where small regions on the sample may be left for this purpose, next to each die. When using a conventional sealed tube source, examining such a small area is only possible by slitting down the X-ray beam, which signi"cantly reduces the X-ray intensity. This increases the count times required to achieve the same quality of data, and can cause the loss of some important but weak features if there is not su$cient intensity overall. The solution we describe is to use a novel highbrilliance laboratory X-ray source. This new source enables high-resolution X-ray di!raction (HRXRD) data to be collected rapidly from small sample areas with enhanced signal intensity. To extract sample material parameters from diffraction data of complex structures requires "tting by dynamical simulation. Auto-"tting of the data gives the parameters quickly and with an objective measure of the goodness of "t, with the possibility of trying many thousands of simulations. A new genetic algorithm has been applied to the "tting process, which is fast and reliable on a desktop PC, and, in combination with a suitable choice of error function, less likely than many methods to fall into false minima [4]. The combination of the new source for data collection, with rapid auto-"tting to extract structural parameters, makes quality control HRXRD analysis of device structures more widely viable.
2. Instrumentation Fig. 1 shows a schematic arrangement of the QC200 X-ray di!raction system used for this work. Sample (u) and detector (2h) axes can be scanned while the other elements are set up and then "xed.
521
Fig. 1. Instrument con"guration of the new Bede QC200. M } Microsource; R } re#ex micro-mirror; RC } reference crystal; A } variable aperture; S } sample; D } Bede EDRa detector. The sample sits horizontally without any need of adhesive or clips to hold it in place.
Fig. 2. Schematic representation of the principle of operation of the ellipsoidal micro-mirror in focusing an X-ray beam. Mirror aperture 0.5}1 mm. Focal distance 150}600 mm, depending on the mirror.
A high brightness X-ray beam with a small footprint is obtained by using the microsource威 } a micro-focus X-ray tube } in conjunction with the unique Re#ex威 micro-mirrors. The microsource uses a unique design of electron optics, which incorporates electron beam alignment and focussing and complete, accurate control over the "nal electron beam shape and position on the target. In order to increase the permissible power loading on the copper target, the electron optics stretch the electron beam laterally from a spot to a line. The target is angled to the electron beam to maximise the power loading and minimise self-(target) absorption of the X-rays produced. By matching electron beam aspect ratio to target angle and viewing at 903 to the electron beam, the X-rays are seen to be produced from a circular projected spot with a diameter (FWHM) down to 15 lm [5]. In the work we report in this paper, the X-rays were collected and focused by an ellipsoidal mirror [6] positioned close (+12 mm) to the target. A schematic ray-tracing diagram is given in Fig. 2 to illustrate the principle of operation. The mirror works by means of total external re#ection of the X-rays from an internal gold surface. The re#ectivity
522
T. Laword et al. / Journal of Crystal Growth 221 (2000) 520}524
of X-rays from most surfaces is low, however, for metals at low-glancing angles ((13) near 100% re#ection e$ciency is achieved, provided the roughness of the surface is low, i.e. (+1 nm r.m.s. roughness. Rays pass into the mirror through the circular entrance aperture (+0.5 mm diameter) and are re#ected from the curved gold surface and out of the exit aperture (+1.0 mm diameter). Note that, since there is no aperture stop, some unre#ected beam passes directly through the two mirror apertures, but it is of low intensity in comparison with the re#ected X-rays. In Fig. 2, the curved pro"le shown is part of an ellipse (with foci at the X-ray source and X-ray beam focus). An alternative pro"le is a section of a parabola (with the X-ray source at the parabola focus) in which case a parallel beam of X-rays is re#ected. For the ellipsoidal mirrors, careful control of the internal shape of the mirror allows the focal length and beam convergence angle to be varied. For HRXRD, a paraboloidal mirror should give the highest e$ciency of coupling into the beam conditioner by giving the smallest beam divergence. However, a smaller footprint at the sample is obtained by compromising between angular divergence and the focus diameter, hence the use of the ellipsoidal micro-mirror in these experiments. Since more of the rays from the source are focused in the forward direction, a higher incident intensity at the sample is achieved after di!raction from the reference crystal. For the work reported here, the mirror used had a divergence of approximately 2.7 mrad with a nominal focal length of 300 mm, giving a focused X-ray spot size at the sample of 1 mm (FWHM). The spot size was reduced further for some experiments by means of a slit. The X-ray source, mirror and reference crystal are mounted in a brass block with an external shutter and variable aperture for "ne control of the beam size. This block is held "xed with respect to the horizontal sample wafer such that the beam makes the Bragg angle with the sample wafer. Rotation of the source assembly is possible to allow for di!erent reference crystals with di!erent Bragg angles. Di!racted X-rays were detected using a scintillation counter (Bede EDRa) mounted on an arc. Any part of the wafer could be translated into
the X-ray beam; the area of the sample under test could be visually inspected remotely.
3. Data 5tting Having collected a high-resolution di!raction scan from a device structure, the data were "tted using Bede RADS Mercury software utilising the dynamical theory of X-ray di!raction [1}3]. The di!erential evolution genetic algorithm gives fast convergence on the "t with the minimum error [4]. Thousands of simulations can be tested in the time it would take a human operator to try just a few. The use of the least absolute log deviation (LALD) as the error function ensures that weak intensity features, such as those arising from the mismatched layer in a pHEMT, and from thickness fringes, are given importance, and the much more intense substrate peak does not dominate the process. LALD also gives sharp, deep true minima with less pronounced false minima. The algorithm is also set such that, when in a minimum, some combination of values well away from that condition will be tried in order to test whether that minimum is a false one. The value of the minimised error function is given as the "t progresses. Also given are a standardised s parameter and a residuals parameter, which can be used to compare the goodness of "t for di!erent data sets. Since the process is quick and objective, some of the previously required skill is removed and automated analysis is possible. This is particularly of interest in production-line quality control environments.
4. Results That the presence of the micro-mirror has no adverse e!ect on resolution is shown in Fig. 3, where the FWHM of a Si 0 0 4 re#ection is 5.1 arcsec (Si reference crystal). The theoretically achievable FWHM is 4.3 arcsec. Fig. 4 shows double-crystal di!raction data for a MOVPE-grown GaAs-based pHEMT. The data were collected using a Bede QC200 double-crystal di!ractometer "tted with a micro-focus source,
T. Laword et al. / Journal of Crystal Growth 221 (2000) 520}524
Fig. 3. Double-crystal di!raction scan of Si 0 0 4. Si 0 0 4 reference crystal, 0.25 mm di!racting area, Microsource at 50 W. FWHM 5.1 arcsec.
Fig. 4. Double-crystal di!raction scans of a pHEMT. Dashed line: standard generator at 50 W with a beam footprint of approximately 3;2 mm, Ka #Ka , total scan time 53 min. Solid line: using the new Bede QC200 with a Microsource at 50 W and a re#ex micro-mirror, beam footprint 1 mm diameter, Ka only, total scan time 22 min.
micro-mirror optics and a Ge reference crystal. The di!racted intensity has been increased by a factor of 10, while the beam footprint has been simultaneously reduced by more than 85%, compared with
523
Fig. 5. Bede RADS mercury auto-"t (solid line) to the QC200 experimental data (dotted line) using full dynamical theory. In addition to the layer parameters, a low Lorentzian curve was included to model the di!use scatter. A standardised s and R factor are given as measures of the goodness of "t.
a standard compact 50 W tube with a 0.25 mm diameter focal spot and no mirror optics. The intensity gain allows data to be collected more rapidly. The auto-"t is shown in Fig. 5. The LALD error function is chosen to re#ect the fact that the data are spread over a wide intensity range, with important signals which are also weak (Table 1). Fitting took only a few minutes for this structure, with "ve free parameters. Less time is taken when the starting model is closer to the "t model. Another example of a RADS mercury auto-"t is shown in Fig. 6. This structure is more complex, including graded Si}Ge layers. The non-linear variation of Si}Ge lattice parameter with alloy composition is taken into account. These data were collected on a Bede D1 system di!ractometer using a monochromated beam and a 2 kW Cu-sealed tube source.
Table 1 Fit parameters to the pHEMT data (Fig. 5) Layer
Material
Thickness/As
Composition fraction, x
3 2 1 Substrate
GaAs Al Ga As V \V In Ga As V \V GaAs
350.7 (#5.2, !3.0) 415.7 (#5.2, !3.4) 157.5 (#4.2, !1.6) *
* 0.286 (#0.024, !0.037) 0.117 (#0.000, !0.000) *
524
T. Laword et al. / Journal of Crystal Growth 221 (2000) 520}524
Fig. 6. Auto-"t (solid line) to 0 0 4 di!raction data (dotted line) of a complex Si}Ge structure. Sample courtesy Dr. Detlev GruK tzmacher, Paul Scherrer Institute, Switzerland. A schematic diagram of the "t structure is given on the right.
5. Conclusion
References
A double-crystal instrument giving high intensity in a small beam footprint is obtained by using a unique micro-focus tube and micro-mirror combination. Together with fast auto-"tting software, device structures can be quickly analysed and material parameters can be rapidly and objectively deduced. In this way, the process is suitable for quality control applications.
[1] S. Takagi, Acta Crystallogr. 15 (1962) 1311. [2] S. Takagi, J. Phys. Soc. Jpn. 26 (1969) 1239. [3] D. Taupin, Bull. Soc. Fr. Mineral. Crystallogr. 87 (1964) 469. [4] M. Wormington, C. Panaccione, K.M. Matney, D.K. Bowen, Philos. Trans. Roy. Soc. Lond. 357 (1999) 2827. [5] U.W. Arndt, J.V.P. Long, P. Duncumb, J. Appl. Crystallogr. 31 (1998) 936. [6] U.W. Arndt, P. Duncumb, J.V.P. Long, L. Pina, A. Inneman, J. Appl. Crystallogr. 31 (1998) 733.