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Reactive ion etching of novel materials—GaN and SiC
Vacuum 70 (2003) 249–254
Reactive ion etching of novel materials—GaN and SiC ! A. Szcze)sny*, P. Sniecikowski, J. Szmidt, A. Werbowy Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-062 Warsaw, Poland
Abstract The results of the reactive ion etching of GaN films on sapphire and bulk 4H–SiC in the fluorine containing plasmas are presented. CF4/Ar chemistry for etching of GaN as well as CF4/Ar and CF4/O2 chemistries for etching of 4H–SiC ( ( are used. Etch rates up to B500 A/min for GaN and B1800 A/min for 4H–SiC were obtained. The influence of pressure and gas flow rates on the etch rate and surface morphology were investigated. Stylus profilometry was used to measure the etch profiles, whereas AFM and SEM measurements provided us with information on the quality of the etched surfaces. Additionally, SIMS measurements were carried out to determine the influence of ion bombardment on the exposed surfaces and the possibility of their implantation. r 2003 Published by Elsevier Science Ltd. Keywords: Gallium nitride (GaN); Silicon carbide SiC; Reactive ion etching (RIE)
1. Introduction Gallium nitride (GaN) and silicon carbide (SiC) are very promising wide bandgap, compound semiconductors with a vast range of possible applications. GaN has a great potential for the fabrication of lasers and light-emitting diodes for UV/blue/green light emission, whereas SiC is an excellent material for high-power, high-frequency, high-temperature applications. Both materials can also be applied in active electronics, optoelectronics and sensors. The necessity of such process steps of technology like deep mesa isolation, formation of laser facets and others has focused attention on high-speed and high-quality etch techniques. Chemically GaN and SiC are very stable materials with high bond strength, which makes them insoluble in all mineral acids and *Corresponding author. E-mail address: [email protected] (A. Szcze)sny).
bases at room temperature. Therefore, a significant amount of effort has been devoted to the development of their dry etch processing. Two general classes of plasma techniques have been reported for pattern transfer to GaN and SiC. The first is the conventional reactive ion etching (RIE). RIE of GaN was performed by Heon Lee et al. [1] in CHF3/Ar and C2ClF5/Ar plasmas, while Lin et al. [2] reported on a BCl3 based plasma in the same type of process. One of the most important parameters of this technique is the high ion energy of >200 eV which is useful in breaking bonds in the materials like GaN and SiC. The second class of methods are those involving application of high density plasma sources, including electron cyclotron resonance (ECR) [3,4] and inductively coupled plasma (ICP) [3,5–8]. These tools operate at lower pressure (0.133–0.266 Pa) and lower ion energy but with higher ion fluxes. A major advantage of these reactors is the ability to control ion flux and ion energy separately.
0042-207X/03/$ - see front matter r 2003 Published by Elsevier Science Ltd. doi:10.1016/S0042-207X(02)00651-6
A. Szcze)sny et al. / Vacuum 70 (2003) 249–254
250
In this work we report on the reactive ion etching of GaN and 4H–SiC in CF4/Ar and CF4/ Ar, CF4/O2 plasmas, respectively.
surements (Cameca IMS 6F) were used to investigate the possibility of an ion implantation during the etch process.
2. Experiment
3. Results and discussion
We used a 2.8 mm thick, n-type (ND =6 1017 cm 3) GaN film, grown on a sapphire (Al2O3) substrate and bulk 4H–SiC substrates, doped with nitrogen (ND =5 1018 cm 3). The samples were patterned with evaporated ( aluminium (B4000 A), which was chemically removed after the etch process. We used a conventional Oxford Plasmalab 80 Plus RIE system. The plasma was generated by a radio frequency (13.56 MHz) glow discharge. Electronic grade CF4, Ar and O2 gases were injected into the chamber through mass flow controllers with the total flow rates of 25 standard cubic centimetres per minute (sccm) during etching of GaN in CF4/ Ar plasma and 30 sccm during etching of SiC in both, CF4/Ar and CF4/O2 plasmas. The step heights were obtained from profilometry measurements with Tencor Instruments Alpha Step 200. The surface morphology of selected GaN samples was examined with scanning electron microscopy (SEM), whereas the SiC samples were measured using atomic force microscopy (AFM). Secondary ion mass spectroscopy (SIMS) mea-
Several parameters were changed during GaN etching. The rf plasma power varied between 150 and 290 W, the pressure ranged from 4 to 20 Pa and the gas flow ratios were set at 4:1 and 3:2. The ( etch rate under these conditions ranged from 91 A/ ( min (CF4/Ar) up to 497 A/min (Ar). As shown in Fig. 1, the etch rate of GaN was decreasing with the increasing pressure. This was due to shorter mean free paths in plasmas of higher pressure, which resulted in an increased number of collisions in the gas, thus decreasing the number of reactive species reaching the surface. The etch rate also depended on the gas flow ratios. The decrease in the etch rate was less significant for the plasma containing more argon. This was due to the higher contribution of the physical sputtering to the etch process. Fig. 2 shows the influence of the chamber pressure on the etch rate of 4H–SiC for two different plasma chemistries. We found that the optimum range of pressure was between 5.33 and 8 Pa for both plasmas. Slightly higher etch rates were received in the argon containing plasma. For
300 290 W, 20 sccm CF4, 5 sccm Ar 290 W, 15 sccm CF4, 10 sccm Ar
Etch Rate [A/min]
250
200
150
100
50
0 2
4
6
8
10
12
14
16
18
Chamber Pressure [Pa]
Fig. 1. Etch rate of GaN vs. chamber pressure for two different gas ratios.
20
A. Szcze)sny et al. / Vacuum 70 (2003) 249–254
251
1400 CF4/Ar CF4/O2
1200
Etch Rate [A/min]
1000 800 600 400 200 0 2.66
3.66
4.66
5.66
6.66
7.66
Chamber Pressure [Pa]
Fig. 2. Etch rate of 4H–SiC vs. chamber pressure for two different plasma chemistries (50% CF4 at the total gas flow rate of 30 sccm, rf power 270 W).
1800 SiC CF4/Ar
1600
SiC CF4/O2 GaN CF4/Ar
Etch Rate [A/min]
1400 1200 1000 800 600 400 200 0 0
20
40
60
80
100
% of CF4 in CF4/Ar and CF4/O2
Fig. 3. Etch rate of 4H–SiC and GaN as a function of plasma composition; parameters were: 6.66 Pa, 270 W, total gas flow rate 30 sccm for SiC; 20 Pa, 290 W, total gas flow rate 25 sccm for GaN.
the pressures between 2.66 and 6.66 Pa the etch rate rose, despite an increasing mean free path. This showed that the amount of reactive species reaching the surface was vital for the etch process. Fig. 3 presents the dependence of the etch rate of GaN and 4H–SiC on the content of CF4 in ( plasma. The highest etch rate of GaN (497 A/min) was achieved in the plasma containing argon only.
An addition of CF4 decreased the efficiency of the ( etch process (etch rates of 123 and 94 A/min). This was caused by the formation of inhibitors on the surface of GaN which were blocking the etching process. In the case of 4H–SiC, the etch rate increased with the increasing concentration of ( CF4, reaching a maximum of 1000 A/min for the ( CF4/O2 mixture and 1600 A/min for the CF4/Ar
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mixture. Products of chemical reactions in the CF4 containing plasma during those processes were more volatile than those resulting from the reactions with GaN which is responsible for the observed difference. The etch rate of 4H–SiC increased monotonically with the growth of CF4 content, reaching a maximum at 90%, which indicates that again the limiting step is the supply of atomic fluorine to the surface.
The surface morphology of the GaN film etched in argon is presented in Fig. 4. The surface is very rough and damaged, which was caused by a bombardment of energetic ions. Their energies were so high, that the erosion of the mask was observed. The same situation was observed for the SiC sample etched in pure argon. Its rough surface is shown in Fig. 5. Changes in composition near the surface of the etched material following its
Fig. 4. SEM micrograph of GaN etched in Ar plasma (25 sccm, 20 Pa, 290 W).
Fig. 5. AFM micrograph of 4H–SiC etched in Ar plasma (30 sccm, 4 Pa, 300 W).
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case of GaN, the surface is smooth (Fig. 7) and less damaged (no oxygen near the surface was observed during SIMS measurements). The edge of etched mesa is sharp with no sign of erosion of the mask. SIMS measurement indicated that no ion implantation took place. This was caused both by a nature of etched materials and too low ion energies required for implantation in such films.
exposure to air are generally considered to be a good indirect indicator of the surface damage caused by the RIE process. As the amount of damage in the film increases, it becomes more reactive. In this case the damaged surface layer of GaN and SiC readily reacted with oxygen, which was observed during SIMS measurements (Fig. 6). The addition of CF4 to plasma changed the situation (chemical reactions took place). In the
1.0E+07 1.0E+06
counts/sec
1.0E+05 C O F Si GaN
1.0E+04 1.0E+03 1.0E+02 1.0E+01 1.0E+00 0
50
100
150
200
250
300
350
Time [s]
Fig. 6. SIMS profile (Cs sputtering) of GaN after etching in Ar plasma (25 sccm, 20 Pa, 290 W).
Fig. 7. SEM micrograph of GaN etched in CF4/Ar plasma (20 sccm CF4, 5 sccm Ar, 20 Pa, 290 W).
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4. Summary
References
We have found that CF4 based plasmas are useful in etching of both GaN and SiC. However, better results can be achieved for SiC, due to a higher volatility of the reaction products. Moreover, the pressure range between 5.33 and 8 Pa seems to be optimal for such processes. GaN was etched at much lower rates, but the surfaces achieved were of a good quality. Ion implantation was not observed.
[1] Lee H, Oberman DB, Harris Jr. JS. Appl Phys Lett 1995;67:1754–6. [2] Lin ME, Fan ZF, Ma Z, Allen LH, Morkoc H. Appl Phys Lett 1994;64:887–8. [3] Hong J, Shul RJ, Zhang L, Lester LF, Cho H, Hahn YB, Hays DC, Jung KB, Pearton SJ, Ostling M, Zetterling CM. J Electr Mater 1999;3:196. [4] Pearton SJ, Zolper JC, Shul RJ, Ren FJ. Appl Phys 1999;86:1–78. [5] Wang JJ, Lambers ES, Pearton SJ, Ostling M, Zetterling CM, Grow JM, Ren F, Shul R. Vac Sci Technol A 1998;16:2204. [6] Wang JJ, Lambers ES, Pearton SJ, Ostling M, Zetterling CM, Grow JM, Ren F. Solid-State Electron 1998;5:743. [7] Cho H, Vartuli CB, Donovan SM, Abernathy CR, Pearton SJ, Shul RJ, Constantine C. Vac Sci Technol A 1998;16:1631–5. [8] Cho H, Hahn Y, Hays DC, Abernathy CR, Donovan SM, MacKenzie JD, Pearton SJ, Han J, Shul RJJ. Vac Sci Technol A 1999;17:2202–8.
Acknowledgements We would like to thank Prof. A. Olszyna for SEM micrographs and Prof. A. Barcz for SIMS profiles. This work was supported by grant no. 503/285/1 from the Warsaw University of Technology.
Reactive ion etching of novel materials—GaN and SiC ! A. Szcze)sny*, P. Sniecikowski, J. Szmidt, A. Werbowy Institute of Microelectronics and Optoelectronics, Warsaw University of Technology, Koszykowa 75, 00-062 Warsaw, Poland
Abstract The results of the reactive ion etching of GaN films on sapphire and bulk 4H–SiC in the fluorine containing plasmas are presented. CF4/Ar chemistry for etching of GaN as well as CF4/Ar and CF4/O2 chemistries for etching of 4H–SiC ( ( are used. Etch rates up to B500 A/min for GaN and B1800 A/min for 4H–SiC were obtained. The influence of pressure and gas flow rates on the etch rate and surface morphology were investigated. Stylus profilometry was used to measure the etch profiles, whereas AFM and SEM measurements provided us with information on the quality of the etched surfaces. Additionally, SIMS measurements were carried out to determine the influence of ion bombardment on the exposed surfaces and the possibility of their implantation. r 2003 Published by Elsevier Science Ltd. Keywords: Gallium nitride (GaN); Silicon carbide SiC; Reactive ion etching (RIE)
1. Introduction Gallium nitride (GaN) and silicon carbide (SiC) are very promising wide bandgap, compound semiconductors with a vast range of possible applications. GaN has a great potential for the fabrication of lasers and light-emitting diodes for UV/blue/green light emission, whereas SiC is an excellent material for high-power, high-frequency, high-temperature applications. Both materials can also be applied in active electronics, optoelectronics and sensors. The necessity of such process steps of technology like deep mesa isolation, formation of laser facets and others has focused attention on high-speed and high-quality etch techniques. Chemically GaN and SiC are very stable materials with high bond strength, which makes them insoluble in all mineral acids and *Corresponding author. E-mail address: [email protected] (A. Szcze)sny).
bases at room temperature. Therefore, a significant amount of effort has been devoted to the development of their dry etch processing. Two general classes of plasma techniques have been reported for pattern transfer to GaN and SiC. The first is the conventional reactive ion etching (RIE). RIE of GaN was performed by Heon Lee et al. [1] in CHF3/Ar and C2ClF5/Ar plasmas, while Lin et al. [2] reported on a BCl3 based plasma in the same type of process. One of the most important parameters of this technique is the high ion energy of >200 eV which is useful in breaking bonds in the materials like GaN and SiC. The second class of methods are those involving application of high density plasma sources, including electron cyclotron resonance (ECR) [3,4] and inductively coupled plasma (ICP) [3,5–8]. These tools operate at lower pressure (0.133–0.266 Pa) and lower ion energy but with higher ion fluxes. A major advantage of these reactors is the ability to control ion flux and ion energy separately.
0042-207X/03/$ - see front matter r 2003 Published by Elsevier Science Ltd. doi:10.1016/S0042-207X(02)00651-6
A. Szcze)sny et al. / Vacuum 70 (2003) 249–254
250
In this work we report on the reactive ion etching of GaN and 4H–SiC in CF4/Ar and CF4/ Ar, CF4/O2 plasmas, respectively.
surements (Cameca IMS 6F) were used to investigate the possibility of an ion implantation during the etch process.
2. Experiment
3. Results and discussion
We used a 2.8 mm thick, n-type (ND =6 1017 cm 3) GaN film, grown on a sapphire (Al2O3) substrate and bulk 4H–SiC substrates, doped with nitrogen (ND =5 1018 cm 3). The samples were patterned with evaporated ( aluminium (B4000 A), which was chemically removed after the etch process. We used a conventional Oxford Plasmalab 80 Plus RIE system. The plasma was generated by a radio frequency (13.56 MHz) glow discharge. Electronic grade CF4, Ar and O2 gases were injected into the chamber through mass flow controllers with the total flow rates of 25 standard cubic centimetres per minute (sccm) during etching of GaN in CF4/ Ar plasma and 30 sccm during etching of SiC in both, CF4/Ar and CF4/O2 plasmas. The step heights were obtained from profilometry measurements with Tencor Instruments Alpha Step 200. The surface morphology of selected GaN samples was examined with scanning electron microscopy (SEM), whereas the SiC samples were measured using atomic force microscopy (AFM). Secondary ion mass spectroscopy (SIMS) mea-
Several parameters were changed during GaN etching. The rf plasma power varied between 150 and 290 W, the pressure ranged from 4 to 20 Pa and the gas flow ratios were set at 4:1 and 3:2. The ( etch rate under these conditions ranged from 91 A/ ( min (CF4/Ar) up to 497 A/min (Ar). As shown in Fig. 1, the etch rate of GaN was decreasing with the increasing pressure. This was due to shorter mean free paths in plasmas of higher pressure, which resulted in an increased number of collisions in the gas, thus decreasing the number of reactive species reaching the surface. The etch rate also depended on the gas flow ratios. The decrease in the etch rate was less significant for the plasma containing more argon. This was due to the higher contribution of the physical sputtering to the etch process. Fig. 2 shows the influence of the chamber pressure on the etch rate of 4H–SiC for two different plasma chemistries. We found that the optimum range of pressure was between 5.33 and 8 Pa for both plasmas. Slightly higher etch rates were received in the argon containing plasma. For
300 290 W, 20 sccm CF4, 5 sccm Ar 290 W, 15 sccm CF4, 10 sccm Ar
Etch Rate [A/min]
250
200
150
100
50
0 2
4
6
8
10
12
14
16
18
Chamber Pressure [Pa]
Fig. 1. Etch rate of GaN vs. chamber pressure for two different gas ratios.
20
A. Szcze)sny et al. / Vacuum 70 (2003) 249–254
251
1400 CF4/Ar CF4/O2
1200
Etch Rate [A/min]
1000 800 600 400 200 0 2.66
3.66
4.66
5.66
6.66
7.66
Chamber Pressure [Pa]
Fig. 2. Etch rate of 4H–SiC vs. chamber pressure for two different plasma chemistries (50% CF4 at the total gas flow rate of 30 sccm, rf power 270 W).
1800 SiC CF4/Ar
1600
SiC CF4/O2 GaN CF4/Ar
Etch Rate [A/min]
1400 1200 1000 800 600 400 200 0 0
20
40
60
80
100
% of CF4 in CF4/Ar and CF4/O2
Fig. 3. Etch rate of 4H–SiC and GaN as a function of plasma composition; parameters were: 6.66 Pa, 270 W, total gas flow rate 30 sccm for SiC; 20 Pa, 290 W, total gas flow rate 25 sccm for GaN.
the pressures between 2.66 and 6.66 Pa the etch rate rose, despite an increasing mean free path. This showed that the amount of reactive species reaching the surface was vital for the etch process. Fig. 3 presents the dependence of the etch rate of GaN and 4H–SiC on the content of CF4 in ( plasma. The highest etch rate of GaN (497 A/min) was achieved in the plasma containing argon only.
An addition of CF4 decreased the efficiency of the ( etch process (etch rates of 123 and 94 A/min). This was caused by the formation of inhibitors on the surface of GaN which were blocking the etching process. In the case of 4H–SiC, the etch rate increased with the increasing concentration of ( CF4, reaching a maximum of 1000 A/min for the ( CF4/O2 mixture and 1600 A/min for the CF4/Ar
252
A. Szcze)sny et al. / Vacuum 70 (2003) 249–254
mixture. Products of chemical reactions in the CF4 containing plasma during those processes were more volatile than those resulting from the reactions with GaN which is responsible for the observed difference. The etch rate of 4H–SiC increased monotonically with the growth of CF4 content, reaching a maximum at 90%, which indicates that again the limiting step is the supply of atomic fluorine to the surface.
The surface morphology of the GaN film etched in argon is presented in Fig. 4. The surface is very rough and damaged, which was caused by a bombardment of energetic ions. Their energies were so high, that the erosion of the mask was observed. The same situation was observed for the SiC sample etched in pure argon. Its rough surface is shown in Fig. 5. Changes in composition near the surface of the etched material following its
Fig. 4. SEM micrograph of GaN etched in Ar plasma (25 sccm, 20 Pa, 290 W).
Fig. 5. AFM micrograph of 4H–SiC etched in Ar plasma (30 sccm, 4 Pa, 300 W).
A. Szcze)sny et al. / Vacuum 70 (2003) 249–254
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case of GaN, the surface is smooth (Fig. 7) and less damaged (no oxygen near the surface was observed during SIMS measurements). The edge of etched mesa is sharp with no sign of erosion of the mask. SIMS measurement indicated that no ion implantation took place. This was caused both by a nature of etched materials and too low ion energies required for implantation in such films.
exposure to air are generally considered to be a good indirect indicator of the surface damage caused by the RIE process. As the amount of damage in the film increases, it becomes more reactive. In this case the damaged surface layer of GaN and SiC readily reacted with oxygen, which was observed during SIMS measurements (Fig. 6). The addition of CF4 to plasma changed the situation (chemical reactions took place). In the
1.0E+07 1.0E+06
counts/sec
1.0E+05 C O F Si GaN
1.0E+04 1.0E+03 1.0E+02 1.0E+01 1.0E+00 0
50
100
150
200
250
300
350
Time [s]
Fig. 6. SIMS profile (Cs sputtering) of GaN after etching in Ar plasma (25 sccm, 20 Pa, 290 W).
Fig. 7. SEM micrograph of GaN etched in CF4/Ar plasma (20 sccm CF4, 5 sccm Ar, 20 Pa, 290 W).
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A. Szcze)sny et al. / Vacuum 70 (2003) 249–254
4. Summary
References
We have found that CF4 based plasmas are useful in etching of both GaN and SiC. However, better results can be achieved for SiC, due to a higher volatility of the reaction products. Moreover, the pressure range between 5.33 and 8 Pa seems to be optimal for such processes. GaN was etched at much lower rates, but the surfaces achieved were of a good quality. Ion implantation was not observed.
[1] Lee H, Oberman DB, Harris Jr. JS. Appl Phys Lett 1995;67:1754–6. [2] Lin ME, Fan ZF, Ma Z, Allen LH, Morkoc H. Appl Phys Lett 1994;64:887–8. [3] Hong J, Shul RJ, Zhang L, Lester LF, Cho H, Hahn YB, Hays DC, Jung KB, Pearton SJ, Ostling M, Zetterling CM. J Electr Mater 1999;3:196. [4] Pearton SJ, Zolper JC, Shul RJ, Ren FJ. Appl Phys 1999;86:1–78. [5] Wang JJ, Lambers ES, Pearton SJ, Ostling M, Zetterling CM, Grow JM, Ren F, Shul R. Vac Sci Technol A 1998;16:2204. [6] Wang JJ, Lambers ES, Pearton SJ, Ostling M, Zetterling CM, Grow JM, Ren F. Solid-State Electron 1998;5:743. [7] Cho H, Vartuli CB, Donovan SM, Abernathy CR, Pearton SJ, Shul RJ, Constantine C. Vac Sci Technol A 1998;16:1631–5. [8] Cho H, Hahn Y, Hays DC, Abernathy CR, Donovan SM, MacKenzie JD, Pearton SJ, Han J, Shul RJJ. Vac Sci Technol A 1999;17:2202–8.
Acknowledgements We would like to thank Prof. A. Olszyna for SEM micrographs and Prof. A. Barcz for SIMS profiles. This work was supported by grant no. 503/285/1 from the Warsaw University of Technology.