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sapphire submicron-scale structures with nanometre scale resolution
Materials Science and Engineering B59 (1999) 335 – 339
Fabrication and photoluminescence of GaN/sapphire submicron-scale structures with nanometre scale resolution A. Ribayrol a,*, D. Coquillat b, R.M. De La Rue a, S.K. Murad a, C.D.W. Wilkinson a, P. Girard c, O. Briot b, R.L. Aulombard b a Department of Electronics and Electrical Engineering, Uni6ersity of Glasgow, Rankine Building, Glasgow G12 8LT, Scotland, UK Groupe d’Etude des Semiconducteurs, UMR 5650 CNRS-Uni6ersite´ Montpellier 2, Place Euge`ne Bataillon, 34095 Montpellier cedex 05, France c Laboratoire d’Analyse des Interfaces et de Nanophysique, Uni6ersite´ Montpellier 2, Place Euge`ne Bataillon, 34095 Montpellier cedex 05, France b
Abstract We report the fabrication of submicon-scale structures using high resolution etching to transfer patterns from PMMA into GaN with an intermediate mask consisting of a bilayer of titanium and SiNx. Atomic force microscopy measurements showed the high quality of the structures etched in CH4/H2 as well as an erosion of the mask. The low temperature photoluminescence measured on the etched structures was almost as strong as that from the unetched surface. © 1999 Elsevier Science S.A. All rights reserved. Keywords: GaN; Reactive ion etching; CH4/H2; Microstructures; Atomic force microscopy; Photoluminescence
1. Introduction
2. Experimental details
Future optoelectronic devices based on heterostructures in InGaN/AlGaN/GaN will require increasingly complex structures in order to realise the full potential of this material system. In particular, grating-type feedback structures based on photonic microstructure principles will enable better spectral control, greater efficiency and compactness to be achieved. Both one-dimensional and two-dimensional patterns will be used. Nanometre-scale resolution in deeply etched structures with good dimensional control is required, placing severe demands (because of the short wavelengths involved) on both lithography and etching processes [1]. It is also important to establish the extent to which these fabrication processes affect the luminescence properties of the resulting device structures. This paper describes work on developing the processes required for future GaN-based optoelectronic devices and shows the high quality of the submicron-scale structures already fabricated.
The GaN epilayers under study were grown on sapphire by low-pressure MOVPE (ASM OMR 12 reactor) at 76 Torr. The precursors used were triethylgallium and ammonia [2]. The thickness of the epilayers, which were not intentionally doped, was 1.6–1.8 mm. Residual doping levels were low (5 5× 1016 cm − 3), as shown by charging effect problems under scanning electron microscopy investigation. GaN samples were then patterned by electron beam lithography. Polymethylmethacrylate (PMMA) is widely used as a resist in electron beam lithography. However, it provides a weak mask in most chemistries. A new process was therefore developed in order to transfer patterns from PMMA into GaN using an intermediate mask which consisted of a 300 nm thick SiNx layer and a 50 nm thick Ti layer evaporated on top of the SiNx. Reactive ion etching of the mask was performed in an Oxford Plasma Technology System 100 RIE machine (S100). Etching was monitored using a real-time interferometry system from Sofie Instruments with a laser wavelength of 679 nm. The 50 nm Ti layer was etched in a SiCl4 plasma using a 200 nm thick PMMA mask layer. The Ti layer was subsequently used as a mask in a CHF3 plasma to etch the 300 nm thick SiNx layer, which in turn was used to transfer the
* Corresponding author. Fax: +44-141-330-4907. E-mail address: [email protected] (A. Ribayrol)
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 3 7 8 - X
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patterns into the GaN epilayers. Details of these processes are given in Ref. [3]. GaN epilayers were etched using CH4/H2 in an ElectroTech ET340 etching system at a typical dc bias of 900 V [3]. Various patterns including octagonal pillars and octagonal holes arranged in a triangular lattice with diameters ranging from 0.3 to 1 mm have been produced. Fig. 1 shows a scanning electron micrograph of pillars fabricated using a gas flow rate of CH4/H2 = 5/25 sccm, at 200 W and 40 mTorr. In this case, the etching rate was 43 nm min − 1, which is faster than the 30 nm min − 1 etch rate reported in Ref. [4] by CH4/H2/ Ar RIE. Photoluminescence (PL) experiments were carried out on the different patterns at low temperature in an optical cryostat. The 325 nm line of a HeCd laser (10 mW) was used to excite the sample perpendicular to the surface. The beam spot diameter was focused on the sample to about 50 mm. The photoluminescence emission was collected in an optical multichannel analyser (OMA) mounted on a 500 mm focal length Chromex ˚ for monochromator. The spectral resolution is 1.7 A the 70 mm-slit-width used. The atomic force microscopy
Fig. 2. Atomic force microscopy scan of a 0.90 mm hole. The erosion of the titanium layer and that of the SiNx mask are visible at the top left corner of the hole.
Fig. 3. Height profile of a hole (AFM measurement) showing quasi vertical profile. The first step (from the right) corresponds to the extent of the titanium layer, the second step to the SiNx layer.
(AFM) measurements presented in this paper were performed on a Park Scientific instrument type M5 (PSI Geneve), in resonant mode. The sample consisted of a number of 100 mm-large square areas of linear gratings and two dimensional arrays of holes or pillars — sizes ranging from 0.3 mm to about 1 mm — with different periodicities. Additionally some 200 mm squares totally cleared of resist were used for the real-time interferometry measurements, as well as a reference in photoluminescence experiments.
3. Results and discussions
Fig. 1. Scanning electron micrograph of the etched pillars P3 showing smooth etched surfaces and quasi-vertical sidewalls. The height of the GaN pillars was 1.3 mm after 30 min of etching. The distance between two pillars is 2 mm. The diameter of the pillars is 0.60 mm at the top and 0.90 mm at the bottom.
Etch rates for the GaN layers were found to vary from 12 to 40 nm min − 1, depending on the composition of the gases, rf power and pressure. Comparison between different chemistries and subsequent etch rates has been reported before [3]. Methane-based chemistry was found to give the smoothest surfaces and best sidewall verticality among the chemistries we investigated. The etching of GaN in a CH4/H2 mixture is likely to arise from the formation of NH3 and volatile metal-organic compounds containing Ga. Pillars and lines of 1.3 mm depth were etched in under 30 min, an example of which is given in Fig. 1. The 1.8 mm thick GaN epilayer was completely removed after a 45 min
A. Ribayrol et al. / Materials Science and Engineering B59 (1999) 335–339
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Table 1 Description of the different patterns transferred into the GaN epilayers Pattern
Description
Hole/pillar diameter (mm)
Periodicity (mm)
GaN fraction
P1 P2 P3 P4 P5 P6 H1 H2 H3 H4 H5 H6 SQ1 SQ2 X1 X2
Triangular array of pillars Triangular array of pillars Triangular array of pillars Triangular array of pillars Triangular array of pillars Triangular array of pillars Triangular array of holes Triangular array of holes Triangular array of holes Triangular array of holes Triangular array of holes Triangular array of holes 200 mm×200 mm clear square 200 mm×200 mm clear square Unetched GaN Unetched GaN
1.06 9 0.03 0.94 9 0.03 0.60 9 0.03 0.66 9 0.03 0.48 9 0.03 B0.209 0.03 0.98 9 0.03 1.02 9 0.03 1.06 9 0.03 0.46 9 0.03 0.48 9 0.03 0.49 9 0.03 — — — —
2 2 2 1 1 1 2 2 2 1 1 1 — — — —
0.25 0.20 0.09 0.40 0.21 B0.05 0.78 0.76 0.75 0.81 0.80 0.78 — — — —
etch. The etch rate was found to decrease when lateral sizes were reduced, such as in holes and gratings. It is evident in Fig. 1 that a truncated cone shape rather an octagonal rod shape was obtained. This effect was particularly evident and critical on the smaller pillars where the diameter at the top was significantly smaller than that of the bottom on the pillar. This is due to a mask erosion problem during etching in CH4/ H2 in such conditions. The sample is bombarded with reactive species under a high bias (around 900 V), resulting in partial erosion of the Ti and SiNx layers. The edge of the Ti layer is clearly visible in the top left corner of the hole on the AFM image in Fig. 2. Its limit is situated between 60 and 100 nm away from the edge of the underlying SiNx mask. The SiNx itself is observed to be eroded by ca. 30 nm from the edge of the GaN. The verticality of the sidewall is nevertheless quite good, as shown in the height profile of a hole in Fig. 3. It is expected to improve with the use of a stronger masking technology. The PL results at low temperature (5 K) and room temperature show that the signal from the etched structures was almost as strong as that from the unetched surface. Emission at room temperature is promising for the prospect of optoelectronic devices. A short description of the different patterns investigated is given in Table 1. The PL emission of different unetched areas all over the sample showed that the GaN epilayer was of a very good quality and highly uniform. There was a single peak at 3.475 eV, with a width at half maximum of 8 meV. The intensity of this peak was very weak due to the absorption of the titanium layer still covering most of the sample surface.
Fig. 4 shows the photoluminescence of the series of hole arrays, denoted H1–H6, as well as that of an adjacent clear square SQ1 and unetched area X1. The PL signal from the unetched area X1 is very weak. The PL signal SQ1 recorded on the large clear square used for the interferometry is also significantly broader than that of adjacent unetched material. This can be explained by the defects introduced during the dry-etching stage and the quality of the GaN which becomes poorer closer to the buffer layer. The geometry of the hole arrays, where most of the GaN material is still present, makes it more difficult for strain relaxation to take place. In the case of small holes, H4 to H6, the peak energy is the same as in the case of unetched GaN, but the peak becomes slightly asymmetric. However in the case of large holes, H1 to H3, it seems that some form of relaxation takes place with a slight shift of the peak position (up to 5 meV) towards lower energies and a broadening of the peak. Fig. 5 shows the photoluminescence of the series of pillar arrays as well as that of an adjacent clear square SQ2, and unetched area X2. The peaks observed in this case are broader than in the case of hole arrays, due to the distribution of the strain relaxation taking place in the pillars along their height. As expected the PL signal is shifted towards lower energies as the pillar size decreases (P4, P3, P5), the shift being as much as 11 meV (P5) as compared to unetched GaN. In the case of pattern P6, the mask erosion was such that most of the pillars have been etched away and only a small cone remains, which explains the low intensity of the related peak.
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A. Ribayrol et al. / Materials Science and Engineering B59 (1999) 335–339
Fig. 4. Photoluminescence spectra of a series of triangular arrays of holes measured at 5 K. See Table 1 for a description of the annotation used.
Fig. 5. Photoluminescence spectra of a series of triangular arrays of pillars measured at 5 K. See Table 1 for a description of the annotation used.
4. Conclusions The results obtained indicate that the dry-etching processes required for future GaN-based advanced optoelectronic devices are becoming available. The room temperature photoluminescence observed after dryetching is encouraging for the prospect of optoelectronic devices or photonic bandgap structures. Extension of the present work to the quantum-well heterostructures required for injection-luminescent devices will be an important further step, but the processes now developed should already be sufficient for
the implementation of purely passive device structures. Acknowledgements The authors would like to thank W. Ward, D. Clifton, S. Ferguson and H. McLelland for their technical assistance in this work. D. Coquillat was supported by a Royal Society/CNRS fellowship during her stay at the University of Glasgow. The work was supported by the European Union through a Marie Curie postdoctoral fellowship (A. Ribayrol) and the EPSRC.
A. Ribayrol et al. / Materials Science and Engineering B59 (1999) 335–339
References [1] S.J. Pearton, R.J. Shul, Semiconductors Semimetals 50 (1997) 103. [2] O. Briot, J.P. Alexis, M. Tchounkeu, B. Gil, R.L. Aulombard, Mater. Sci. Eng. B 43 (1997) 147.
.
339
[3] D. Coquillat, S.K. Murad, A. Ribayrol, C.J.M. Smith, R.M. De La Rue, C.D.W. Wilkinson, O. Briot, R.L. Aulombard, Mater. Sci. Forum 264 – 268 (1998) 1403. [4] C.B. Vartuli, J.D. MacKenzie, J.W. Lee, C.R. Abernathy, S.R. Pearton, J. Appl. Phys. 80 (1996) 3705.
Fabrication and photoluminescence of GaN/sapphire submicron-scale structures with nanometre scale resolution A. Ribayrol a,*, D. Coquillat b, R.M. De La Rue a, S.K. Murad a, C.D.W. Wilkinson a, P. Girard c, O. Briot b, R.L. Aulombard b a Department of Electronics and Electrical Engineering, Uni6ersity of Glasgow, Rankine Building, Glasgow G12 8LT, Scotland, UK Groupe d’Etude des Semiconducteurs, UMR 5650 CNRS-Uni6ersite´ Montpellier 2, Place Euge`ne Bataillon, 34095 Montpellier cedex 05, France c Laboratoire d’Analyse des Interfaces et de Nanophysique, Uni6ersite´ Montpellier 2, Place Euge`ne Bataillon, 34095 Montpellier cedex 05, France b
Abstract We report the fabrication of submicon-scale structures using high resolution etching to transfer patterns from PMMA into GaN with an intermediate mask consisting of a bilayer of titanium and SiNx. Atomic force microscopy measurements showed the high quality of the structures etched in CH4/H2 as well as an erosion of the mask. The low temperature photoluminescence measured on the etched structures was almost as strong as that from the unetched surface. © 1999 Elsevier Science S.A. All rights reserved. Keywords: GaN; Reactive ion etching; CH4/H2; Microstructures; Atomic force microscopy; Photoluminescence
1. Introduction
2. Experimental details
Future optoelectronic devices based on heterostructures in InGaN/AlGaN/GaN will require increasingly complex structures in order to realise the full potential of this material system. In particular, grating-type feedback structures based on photonic microstructure principles will enable better spectral control, greater efficiency and compactness to be achieved. Both one-dimensional and two-dimensional patterns will be used. Nanometre-scale resolution in deeply etched structures with good dimensional control is required, placing severe demands (because of the short wavelengths involved) on both lithography and etching processes [1]. It is also important to establish the extent to which these fabrication processes affect the luminescence properties of the resulting device structures. This paper describes work on developing the processes required for future GaN-based optoelectronic devices and shows the high quality of the submicron-scale structures already fabricated.
The GaN epilayers under study were grown on sapphire by low-pressure MOVPE (ASM OMR 12 reactor) at 76 Torr. The precursors used were triethylgallium and ammonia [2]. The thickness of the epilayers, which were not intentionally doped, was 1.6–1.8 mm. Residual doping levels were low (5 5× 1016 cm − 3), as shown by charging effect problems under scanning electron microscopy investigation. GaN samples were then patterned by electron beam lithography. Polymethylmethacrylate (PMMA) is widely used as a resist in electron beam lithography. However, it provides a weak mask in most chemistries. A new process was therefore developed in order to transfer patterns from PMMA into GaN using an intermediate mask which consisted of a 300 nm thick SiNx layer and a 50 nm thick Ti layer evaporated on top of the SiNx. Reactive ion etching of the mask was performed in an Oxford Plasma Technology System 100 RIE machine (S100). Etching was monitored using a real-time interferometry system from Sofie Instruments with a laser wavelength of 679 nm. The 50 nm Ti layer was etched in a SiCl4 plasma using a 200 nm thick PMMA mask layer. The Ti layer was subsequently used as a mask in a CHF3 plasma to etch the 300 nm thick SiNx layer, which in turn was used to transfer the
* Corresponding author. Fax: +44-141-330-4907. E-mail address: [email protected] (A. Ribayrol)
0921-5107/99/$ - see front matter © 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 1 - 5 1 0 7 ( 9 8 ) 0 0 3 7 8 - X
336
A. Ribayrol et al. / Materials Science and Engineering B59 (1999) 335–339
patterns into the GaN epilayers. Details of these processes are given in Ref. [3]. GaN epilayers were etched using CH4/H2 in an ElectroTech ET340 etching system at a typical dc bias of 900 V [3]. Various patterns including octagonal pillars and octagonal holes arranged in a triangular lattice with diameters ranging from 0.3 to 1 mm have been produced. Fig. 1 shows a scanning electron micrograph of pillars fabricated using a gas flow rate of CH4/H2 = 5/25 sccm, at 200 W and 40 mTorr. In this case, the etching rate was 43 nm min − 1, which is faster than the 30 nm min − 1 etch rate reported in Ref. [4] by CH4/H2/ Ar RIE. Photoluminescence (PL) experiments were carried out on the different patterns at low temperature in an optical cryostat. The 325 nm line of a HeCd laser (10 mW) was used to excite the sample perpendicular to the surface. The beam spot diameter was focused on the sample to about 50 mm. The photoluminescence emission was collected in an optical multichannel analyser (OMA) mounted on a 500 mm focal length Chromex ˚ for monochromator. The spectral resolution is 1.7 A the 70 mm-slit-width used. The atomic force microscopy
Fig. 2. Atomic force microscopy scan of a 0.90 mm hole. The erosion of the titanium layer and that of the SiNx mask are visible at the top left corner of the hole.
Fig. 3. Height profile of a hole (AFM measurement) showing quasi vertical profile. The first step (from the right) corresponds to the extent of the titanium layer, the second step to the SiNx layer.
(AFM) measurements presented in this paper were performed on a Park Scientific instrument type M5 (PSI Geneve), in resonant mode. The sample consisted of a number of 100 mm-large square areas of linear gratings and two dimensional arrays of holes or pillars — sizes ranging from 0.3 mm to about 1 mm — with different periodicities. Additionally some 200 mm squares totally cleared of resist were used for the real-time interferometry measurements, as well as a reference in photoluminescence experiments.
3. Results and discussions
Fig. 1. Scanning electron micrograph of the etched pillars P3 showing smooth etched surfaces and quasi-vertical sidewalls. The height of the GaN pillars was 1.3 mm after 30 min of etching. The distance between two pillars is 2 mm. The diameter of the pillars is 0.60 mm at the top and 0.90 mm at the bottom.
Etch rates for the GaN layers were found to vary from 12 to 40 nm min − 1, depending on the composition of the gases, rf power and pressure. Comparison between different chemistries and subsequent etch rates has been reported before [3]. Methane-based chemistry was found to give the smoothest surfaces and best sidewall verticality among the chemistries we investigated. The etching of GaN in a CH4/H2 mixture is likely to arise from the formation of NH3 and volatile metal-organic compounds containing Ga. Pillars and lines of 1.3 mm depth were etched in under 30 min, an example of which is given in Fig. 1. The 1.8 mm thick GaN epilayer was completely removed after a 45 min
A. Ribayrol et al. / Materials Science and Engineering B59 (1999) 335–339
337
Table 1 Description of the different patterns transferred into the GaN epilayers Pattern
Description
Hole/pillar diameter (mm)
Periodicity (mm)
GaN fraction
P1 P2 P3 P4 P5 P6 H1 H2 H3 H4 H5 H6 SQ1 SQ2 X1 X2
Triangular array of pillars Triangular array of pillars Triangular array of pillars Triangular array of pillars Triangular array of pillars Triangular array of pillars Triangular array of holes Triangular array of holes Triangular array of holes Triangular array of holes Triangular array of holes Triangular array of holes 200 mm×200 mm clear square 200 mm×200 mm clear square Unetched GaN Unetched GaN
1.06 9 0.03 0.94 9 0.03 0.60 9 0.03 0.66 9 0.03 0.48 9 0.03 B0.209 0.03 0.98 9 0.03 1.02 9 0.03 1.06 9 0.03 0.46 9 0.03 0.48 9 0.03 0.49 9 0.03 — — — —
2 2 2 1 1 1 2 2 2 1 1 1 — — — —
0.25 0.20 0.09 0.40 0.21 B0.05 0.78 0.76 0.75 0.81 0.80 0.78 — — — —
etch. The etch rate was found to decrease when lateral sizes were reduced, such as in holes and gratings. It is evident in Fig. 1 that a truncated cone shape rather an octagonal rod shape was obtained. This effect was particularly evident and critical on the smaller pillars where the diameter at the top was significantly smaller than that of the bottom on the pillar. This is due to a mask erosion problem during etching in CH4/ H2 in such conditions. The sample is bombarded with reactive species under a high bias (around 900 V), resulting in partial erosion of the Ti and SiNx layers. The edge of the Ti layer is clearly visible in the top left corner of the hole on the AFM image in Fig. 2. Its limit is situated between 60 and 100 nm away from the edge of the underlying SiNx mask. The SiNx itself is observed to be eroded by ca. 30 nm from the edge of the GaN. The verticality of the sidewall is nevertheless quite good, as shown in the height profile of a hole in Fig. 3. It is expected to improve with the use of a stronger masking technology. The PL results at low temperature (5 K) and room temperature show that the signal from the etched structures was almost as strong as that from the unetched surface. Emission at room temperature is promising for the prospect of optoelectronic devices. A short description of the different patterns investigated is given in Table 1. The PL emission of different unetched areas all over the sample showed that the GaN epilayer was of a very good quality and highly uniform. There was a single peak at 3.475 eV, with a width at half maximum of 8 meV. The intensity of this peak was very weak due to the absorption of the titanium layer still covering most of the sample surface.
Fig. 4 shows the photoluminescence of the series of hole arrays, denoted H1–H6, as well as that of an adjacent clear square SQ1 and unetched area X1. The PL signal from the unetched area X1 is very weak. The PL signal SQ1 recorded on the large clear square used for the interferometry is also significantly broader than that of adjacent unetched material. This can be explained by the defects introduced during the dry-etching stage and the quality of the GaN which becomes poorer closer to the buffer layer. The geometry of the hole arrays, where most of the GaN material is still present, makes it more difficult for strain relaxation to take place. In the case of small holes, H4 to H6, the peak energy is the same as in the case of unetched GaN, but the peak becomes slightly asymmetric. However in the case of large holes, H1 to H3, it seems that some form of relaxation takes place with a slight shift of the peak position (up to 5 meV) towards lower energies and a broadening of the peak. Fig. 5 shows the photoluminescence of the series of pillar arrays as well as that of an adjacent clear square SQ2, and unetched area X2. The peaks observed in this case are broader than in the case of hole arrays, due to the distribution of the strain relaxation taking place in the pillars along their height. As expected the PL signal is shifted towards lower energies as the pillar size decreases (P4, P3, P5), the shift being as much as 11 meV (P5) as compared to unetched GaN. In the case of pattern P6, the mask erosion was such that most of the pillars have been etched away and only a small cone remains, which explains the low intensity of the related peak.
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Fig. 4. Photoluminescence spectra of a series of triangular arrays of holes measured at 5 K. See Table 1 for a description of the annotation used.
Fig. 5. Photoluminescence spectra of a series of triangular arrays of pillars measured at 5 K. See Table 1 for a description of the annotation used.
4. Conclusions The results obtained indicate that the dry-etching processes required for future GaN-based advanced optoelectronic devices are becoming available. The room temperature photoluminescence observed after dryetching is encouraging for the prospect of optoelectronic devices or photonic bandgap structures. Extension of the present work to the quantum-well heterostructures required for injection-luminescent devices will be an important further step, but the processes now developed should already be sufficient for
the implementation of purely passive device structures. Acknowledgements The authors would like to thank W. Ward, D. Clifton, S. Ferguson and H. McLelland for their technical assistance in this work. D. Coquillat was supported by a Royal Society/CNRS fellowship during her stay at the University of Glasgow. The work was supported by the European Union through a Marie Curie postdoctoral fellowship (A. Ribayrol) and the EPSRC.
A. Ribayrol et al. / Materials Science and Engineering B59 (1999) 335–339
References [1] S.J. Pearton, R.J. Shul, Semiconductors Semimetals 50 (1997) 103. [2] O. Briot, J.P. Alexis, M. Tchounkeu, B. Gil, R.L. Aulombard, Mater. Sci. Eng. B 43 (1997) 147.
.
339
[3] D. Coquillat, S.K. Murad, A. Ribayrol, C.J.M. Smith, R.M. De La Rue, C.D.W. Wilkinson, O. Briot, R.L. Aulombard, Mater. Sci. Forum 264 – 268 (1998) 1403. [4] C.B. Vartuli, J.D. MacKenzie, J.W. Lee, C.R. Abernathy, S.R. Pearton, J. Appl. Phys. 80 (1996) 3705.