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Trimethylindium transport studies: the effect of different bubbler designs
ARTICLE IN PRESS
Journal of Crystal Growth 272 (2004) 37–41 www.elsevier.com/locate/jcrysgro
Trimethylindium transport studies: the effect of different bubbler designs L.M. Smitha,, R. Odedraa, A.J. Kingsleya, K.M. Cowarda, S.A. Rushwortha, G. Williamsa, T.A. Leesea, A.J. Purdiea, R.K. Kanjoliab a
Epichem Ltd., Power Road, Bromborough, Wirral, Merseyside, CH62 3QF, UK b Epichem Inc., 1429 Hilldale Avenue, Haverhill, MA 01832, USA Available online 12 October 2004
Abstract The transport of trimethylindium (TMI) must be highly reliable throughout the lifetime of a bubbler. This study reports stability data and transport rates obtained for a number of alternative bubbler designs when tested under different conditions of use. The best results, with exceptionally stable flux output across a wide range of operating parameters throughout the bubbler lifetime, were obtained using a novel perforated disc design. This design is an extension of the cross dipleg concept which provides multiple gas pathways to minimise channelling effects and allow uniform and efficient depletion of the bubbler contents. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.G Keywords: A1. Organometallic precursors; A3. Organometallic vapour-phase epitaxy; A3. Metalorganic vapour phase epitaxy; B1. Precursors; B1. Trimethylindium; B2. Semiconducting III–V materials
1. Introduction In today’s metalorganic vapour-phase epitaxy (MOVPE) processes a steady, controllable flux of precursor into the reaction chamber is a key factor especially when fabricating highly complex device Corresponding author. Tel.: +44-151-334-2774; fax: +44151-334-6422. E-mail addresses: [email protected], [email protected] (L.M. Smith).
structures employing ternary and quaternary layers. For solid precursors, e.g. Trimethylindium (TMI) the simple diptube bubbler design has been demonstrated to have a performance range that does not meet all customer requirements, especially at high material transport rates [1,2]. Channelling effects are deemed the main cause of pick up variations and different container geometries have been proposed to achieve improved gas-phase saturation and transport efficiency [1–4].
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.108
ARTICLE IN PRESS 38
L.M. Smith et al. / Journal of Crystal Growth 272 (2004) 37–41
In this study, data was obtained for a number of alternative bubbler designs when tested under different conditions of use. The differences observed between bubblers with different internal designs highlight the improvements in TMI vapour concentration and stability that can be achieved using new concepts. 2. Experimental procedure The different bubbler concepts were fabricated and filled with solid TMI prior to connection to equipment described previously [2] (see Fig. 1). An accurately controlled temperature bath was used to maintain the bubbler temperature while carrier gas (high-purity nitrogen) was bubbled through the material at controlled rates at a set pressure. The gas stream was then passed, undiluted, through an ultrasonic analyser, an Epison III (Thomas Swan & Co. Ltd.) held at the calibration temperature of 65 1C, to monitor and record the concentration of source material in the gas phase. The range of conditions available for operation are listed below and comparable data was obtained for each variety of dipleg design: Flow rates: 100–1000 sccm Bath temperature: 17–40 1C Pressure: 300–1000 m bar
3. Results The output flux of TMI from the different bubbler designs is summarised in Table 1. An
Fig. 1. Schematic representation of equipment used to assess TMI transport.
increase in the measured reading indicates an increased gas-phase concentration of TMI in the carrier gas due to an increased pick-up efficiency in the bubbler. As can be seen marked differences are recorded when the bubbler design is altered. The first results obtained related to the standard dip tube design and this data is used as a baseline against the other readings may be judged. Improved results were noted for a dual-chamber bubbler design, which mimics the commonly used arrangement of connecting two bubblers in series to achieve reliable pick-up [1]. Superior results were also shown for a cross-dipleg design. This can be attributed to distribution of the carrier gas to several points within the bubbler to optimise carrier gas/product contact area and residence time. An extension of this design to provide a plethora of gas pathways into the bubbler via a perforated disc was found to yield the best results for gas saturation of all bubblers tested. Figs. 2–6 illustrate further the development of bubbler geometry. Fig. 2(a) shows the conventional bubbler concept with a gas flow down the diptube. Fig. 2(b) highlights the depletion around the dipleg that arises from using this configuration. Clearly, as the TMI is removed preferentially from around the diptube area the remaining TMI is not contacted by the carrier gas to the same degree resulting in a reduced pick-up efficiency as the bubbler is used [1]. Two bubblers in series and with the carrier gas in the reverse flow direction have been studied to discourage non-uniform depletion, see Fig. 3. In particular, the combination of this concept into a single unit with two chambers has been investigated to minimise the source container footprint required (see Figs. 4(a) and (b)). As demonstrated in Table 1, whilst these alterations have provided an improved pick-up efficiency further gains were necessary to be found. Issues with channelling around the dip tube led to a design using an increased number of gas outlet points to mimic multiple diptubes and distribute the carrier gas to several areas in the bubbler to avoid localised depletion (see Figs. 5(a) and (b)). A cross-dipleg radiates 3–4 limbs (each with several outlet holes) to the edge of the bubbler effectively and improves the pick-up efficiency by a similar
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Table 1 Epison readings and TMI transport rates for different bubbler styles Bubbler style
Temperature (1C)
Flow (sccm)
Pressure (mbar)
Epison reading
TMI transport rate (g/h)
Normal flow
10 10 10 10 17 17 40 10 10 17 25
300 300 300 300 800 450 1000 300 300 800 900
500 1000 1000 1000 300 300 1000 500 1000 300 330
0.13 0.06 0.08 0.08 0.50 0.42 0.60 0.15 0.09 0.51 0.85
0.156 0.072 0.096 0.096 1.599 0.756 2.399 0.180 0.108 1.631 3.058
2 normal flow bubblers in series Cross-dipleg Dual chamber (reverse flow) Perforated disc
Fig. 2. Conventional bubbler design showing (a) gas flow direction and (b) depletion of TMI as used. Fig. 3. Bubbler in reverse flow configuration.
factor [3,5]. The use of more limbs was not found to increase the efficiency significantly. The ultimate design for distributing the inlet carrier gas over the base of the bubbler was a perforated disc (see Fig. 6). Small holes in the disc effectively act as individual diplegs and increase efficiency of the pick up accordingly. Interestingly, the number of holes in the disc affects the pick-up only to a finite density. Further increases in the number of holes after this point has no effect or in extreme cases decreases
efficiency [6]. This surprising observation is not fully understood but is thought to relate to interference channelling between carrier gas pathways formed in the TMI from adjacent holes. If holes are insufficiently spaced their benefit is reduced as the effective number of gas inlet points is no longer proportional to the number of holes. Further investigations are required to define the relationship between hole density and performance.
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L.M. Smith et al. / Journal of Crystal Growth 272 (2004) 37–41
Fig. 4. Dual chamber design in (a) standard and (b) reverse flow.
Fig. 6. Perforated disc design.
Fig. 7. Disc bubbler output flux with time solid TMI,100% fill, 800 sccm, 17 1C, 318 m bar. Fig. 5. Cross dipleg design in (a) standard and (b) reverse flow.
Having established the optimum design for efficient saturation of the vapour exiting the bubbler the stability of this flux was studied. First, a short-term stability was measured to ensure for any given fill level the output remained steady for the period required for a growth run. A second experimental series was then performed to further assess the efficiency of pick-up from the perforated disc design from initial use to full depletion. Prolonged monitoring of output was recorded and exceptional stability of the measured readings to low fill levels was found as illustrated in Fig. 7.
These results were as expected to confirm that the multiple carrier gas pathways provided in this design act to minimise channelling effects and allow uniform depletion of the bubbler contents. A depleted bubbler was studied until pick up characteristics dropped off markedly to establish whether this new perforated disc design was particularly suited to source dosimetry under extreme parameters. With only 15% fill at 0 h Fig. 8 shows the steady output achievable at extreme operation until significantly lower fill levels are reached. In this example the source output begins to fall at around 3 h which equates to a 5% fill level.
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pick-up of TMI to be achieved under extreme conditions and to be maintained throughout the bubbler lifetime. References
Fig. 8. Disc bubbler performance on TMI depletion.
4. Conclusion A transport study has been used to assess different bubbler designs. Significant improvement in the stability of flux exiting the alternative bubblers was noted and a superior design established. The use of a perforated disc allows reliable
[1] N.D. Gerrard, L.M. Smith, A.C. Jones, J. Bosnell, J. Crystal Growth 121 (1992) 500. [2] M.S. Ravetz, L.M. Smith, S.A. Rushworth, A.B. Leese, R.K. Kanjolia, J.I. Davies, R.T. Blunt, J. Electronic Materials 29 (1) (2000) 156. [3] M.S. Ravetz, R. Odedra, L.M. Smith, S.A. Rushworth, A.B. Leese, G. Williams, R.K. Kanjolia, Proceedings of the 13th International Conference on Indium Phosphide and Related Materials, Nara, Japan, 2001, p. 310. [4] M. Timmons, P. Rangarajan, R. Stennick, J. Crystal Growth 221 (2000) 635. [5] G. Williams, R. Odedra, M.S. Ravetz, A. Nelson, R.T. Blunt, H. Williams, European Patent Appl. 2003 EP 1 329 540, A3, 2003. [6] G. Williams et al., UK Patent Appl. 2003 GB 0323388.9, 2003.
Journal of Crystal Growth 272 (2004) 37–41 www.elsevier.com/locate/jcrysgro
Trimethylindium transport studies: the effect of different bubbler designs L.M. Smitha,, R. Odedraa, A.J. Kingsleya, K.M. Cowarda, S.A. Rushwortha, G. Williamsa, T.A. Leesea, A.J. Purdiea, R.K. Kanjoliab a
Epichem Ltd., Power Road, Bromborough, Wirral, Merseyside, CH62 3QF, UK b Epichem Inc., 1429 Hilldale Avenue, Haverhill, MA 01832, USA Available online 12 October 2004
Abstract The transport of trimethylindium (TMI) must be highly reliable throughout the lifetime of a bubbler. This study reports stability data and transport rates obtained for a number of alternative bubbler designs when tested under different conditions of use. The best results, with exceptionally stable flux output across a wide range of operating parameters throughout the bubbler lifetime, were obtained using a novel perforated disc design. This design is an extension of the cross dipleg concept which provides multiple gas pathways to minimise channelling effects and allow uniform and efficient depletion of the bubbler contents. r 2004 Elsevier B.V. All rights reserved. PACS: 81.15.G Keywords: A1. Organometallic precursors; A3. Organometallic vapour-phase epitaxy; A3. Metalorganic vapour phase epitaxy; B1. Precursors; B1. Trimethylindium; B2. Semiconducting III–V materials
1. Introduction In today’s metalorganic vapour-phase epitaxy (MOVPE) processes a steady, controllable flux of precursor into the reaction chamber is a key factor especially when fabricating highly complex device Corresponding author. Tel.: +44-151-334-2774; fax: +44151-334-6422. E-mail addresses: [email protected], [email protected] (L.M. Smith).
structures employing ternary and quaternary layers. For solid precursors, e.g. Trimethylindium (TMI) the simple diptube bubbler design has been demonstrated to have a performance range that does not meet all customer requirements, especially at high material transport rates [1,2]. Channelling effects are deemed the main cause of pick up variations and different container geometries have been proposed to achieve improved gas-phase saturation and transport efficiency [1–4].
0022-0248/$ - see front matter r 2004 Elsevier B.V. All rights reserved. doi:10.1016/j.jcrysgro.2004.08.108
ARTICLE IN PRESS 38
L.M. Smith et al. / Journal of Crystal Growth 272 (2004) 37–41
In this study, data was obtained for a number of alternative bubbler designs when tested under different conditions of use. The differences observed between bubblers with different internal designs highlight the improvements in TMI vapour concentration and stability that can be achieved using new concepts. 2. Experimental procedure The different bubbler concepts were fabricated and filled with solid TMI prior to connection to equipment described previously [2] (see Fig. 1). An accurately controlled temperature bath was used to maintain the bubbler temperature while carrier gas (high-purity nitrogen) was bubbled through the material at controlled rates at a set pressure. The gas stream was then passed, undiluted, through an ultrasonic analyser, an Epison III (Thomas Swan & Co. Ltd.) held at the calibration temperature of 65 1C, to monitor and record the concentration of source material in the gas phase. The range of conditions available for operation are listed below and comparable data was obtained for each variety of dipleg design: Flow rates: 100–1000 sccm Bath temperature: 17–40 1C Pressure: 300–1000 m bar
3. Results The output flux of TMI from the different bubbler designs is summarised in Table 1. An
Fig. 1. Schematic representation of equipment used to assess TMI transport.
increase in the measured reading indicates an increased gas-phase concentration of TMI in the carrier gas due to an increased pick-up efficiency in the bubbler. As can be seen marked differences are recorded when the bubbler design is altered. The first results obtained related to the standard dip tube design and this data is used as a baseline against the other readings may be judged. Improved results were noted for a dual-chamber bubbler design, which mimics the commonly used arrangement of connecting two bubblers in series to achieve reliable pick-up [1]. Superior results were also shown for a cross-dipleg design. This can be attributed to distribution of the carrier gas to several points within the bubbler to optimise carrier gas/product contact area and residence time. An extension of this design to provide a plethora of gas pathways into the bubbler via a perforated disc was found to yield the best results for gas saturation of all bubblers tested. Figs. 2–6 illustrate further the development of bubbler geometry. Fig. 2(a) shows the conventional bubbler concept with a gas flow down the diptube. Fig. 2(b) highlights the depletion around the dipleg that arises from using this configuration. Clearly, as the TMI is removed preferentially from around the diptube area the remaining TMI is not contacted by the carrier gas to the same degree resulting in a reduced pick-up efficiency as the bubbler is used [1]. Two bubblers in series and with the carrier gas in the reverse flow direction have been studied to discourage non-uniform depletion, see Fig. 3. In particular, the combination of this concept into a single unit with two chambers has been investigated to minimise the source container footprint required (see Figs. 4(a) and (b)). As demonstrated in Table 1, whilst these alterations have provided an improved pick-up efficiency further gains were necessary to be found. Issues with channelling around the dip tube led to a design using an increased number of gas outlet points to mimic multiple diptubes and distribute the carrier gas to several areas in the bubbler to avoid localised depletion (see Figs. 5(a) and (b)). A cross-dipleg radiates 3–4 limbs (each with several outlet holes) to the edge of the bubbler effectively and improves the pick-up efficiency by a similar
ARTICLE IN PRESS L.M. Smith et al. / Journal of Crystal Growth 272 (2004) 37–41
39
Table 1 Epison readings and TMI transport rates for different bubbler styles Bubbler style
Temperature (1C)
Flow (sccm)
Pressure (mbar)
Epison reading
TMI transport rate (g/h)
Normal flow
10 10 10 10 17 17 40 10 10 17 25
300 300 300 300 800 450 1000 300 300 800 900
500 1000 1000 1000 300 300 1000 500 1000 300 330
0.13 0.06 0.08 0.08 0.50 0.42 0.60 0.15 0.09 0.51 0.85
0.156 0.072 0.096 0.096 1.599 0.756 2.399 0.180 0.108 1.631 3.058
2 normal flow bubblers in series Cross-dipleg Dual chamber (reverse flow) Perforated disc
Fig. 2. Conventional bubbler design showing (a) gas flow direction and (b) depletion of TMI as used. Fig. 3. Bubbler in reverse flow configuration.
factor [3,5]. The use of more limbs was not found to increase the efficiency significantly. The ultimate design for distributing the inlet carrier gas over the base of the bubbler was a perforated disc (see Fig. 6). Small holes in the disc effectively act as individual diplegs and increase efficiency of the pick up accordingly. Interestingly, the number of holes in the disc affects the pick-up only to a finite density. Further increases in the number of holes after this point has no effect or in extreme cases decreases
efficiency [6]. This surprising observation is not fully understood but is thought to relate to interference channelling between carrier gas pathways formed in the TMI from adjacent holes. If holes are insufficiently spaced their benefit is reduced as the effective number of gas inlet points is no longer proportional to the number of holes. Further investigations are required to define the relationship between hole density and performance.
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L.M. Smith et al. / Journal of Crystal Growth 272 (2004) 37–41
Fig. 4. Dual chamber design in (a) standard and (b) reverse flow.
Fig. 6. Perforated disc design.
Fig. 7. Disc bubbler output flux with time solid TMI,100% fill, 800 sccm, 17 1C, 318 m bar. Fig. 5. Cross dipleg design in (a) standard and (b) reverse flow.
Having established the optimum design for efficient saturation of the vapour exiting the bubbler the stability of this flux was studied. First, a short-term stability was measured to ensure for any given fill level the output remained steady for the period required for a growth run. A second experimental series was then performed to further assess the efficiency of pick-up from the perforated disc design from initial use to full depletion. Prolonged monitoring of output was recorded and exceptional stability of the measured readings to low fill levels was found as illustrated in Fig. 7.
These results were as expected to confirm that the multiple carrier gas pathways provided in this design act to minimise channelling effects and allow uniform depletion of the bubbler contents. A depleted bubbler was studied until pick up characteristics dropped off markedly to establish whether this new perforated disc design was particularly suited to source dosimetry under extreme parameters. With only 15% fill at 0 h Fig. 8 shows the steady output achievable at extreme operation until significantly lower fill levels are reached. In this example the source output begins to fall at around 3 h which equates to a 5% fill level.
ARTICLE IN PRESS L.M. Smith et al. / Journal of Crystal Growth 272 (2004) 37–41
41
pick-up of TMI to be achieved under extreme conditions and to be maintained throughout the bubbler lifetime. References
Fig. 8. Disc bubbler performance on TMI depletion.
4. Conclusion A transport study has been used to assess different bubbler designs. Significant improvement in the stability of flux exiting the alternative bubblers was noted and a superior design established. The use of a perforated disc allows reliable
[1] N.D. Gerrard, L.M. Smith, A.C. Jones, J. Bosnell, J. Crystal Growth 121 (1992) 500. [2] M.S. Ravetz, L.M. Smith, S.A. Rushworth, A.B. Leese, R.K. Kanjolia, J.I. Davies, R.T. Blunt, J. Electronic Materials 29 (1) (2000) 156. [3] M.S. Ravetz, R. Odedra, L.M. Smith, S.A. Rushworth, A.B. Leese, G. Williams, R.K. Kanjolia, Proceedings of the 13th International Conference on Indium Phosphide and Related Materials, Nara, Japan, 2001, p. 310. [4] M. Timmons, P. Rangarajan, R. Stennick, J. Crystal Growth 221 (2000) 635. [5] G. Williams, R. Odedra, M.S. Ravetz, A. Nelson, R.T. Blunt, H. Williams, European Patent Appl. 2003 EP 1 329 540, A3, 2003. [6] G. Williams et al., UK Patent Appl. 2003 GB 0323388.9, 2003.