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Single step electrochemical etching in ammonium fluoride
Sensors and Actuators 74 Ž1999. 109–112
Single step electrochemical etching in ammonium fluoride H. Ohji ) , P.J. French Delft UniÕersity of Technology, DIMES, Faculty of Information Technology and Systems, Postbus 5031, 2600GA Delft, Netherlands
Abstract This paper presents a new technique of micromachining using single step electrochemical etching in an ammonium fluoride based etchant. This etching technology is to fabricate 3-D structures in single crystal silicon by a combination of anisotropic and isotropic etching mode. The etch rate and morphology of the etched surface are investigated for the etch parameters Žetchant concentration, current density.. Optimization of these parameters makes it possible to make free standing beams. Using the ammonium fluoride as the etchant, aluminium survives the etching. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Electrochemical etching; Free standing structure; Porous silicon; Wet etching; Ammonium fluoride
1. Introduction Electrochemical etching in hydrofluoric acid ŽHF. is known as a technique of porous silicon formation which has been studied since the 1950s w1,2x and much attention has been paid to the formation mechanism w3x. Recently, porous silicon has been applied to micromachining and micro devices because of its interesting properties. For example, porous silicon has been used for a sacrificial layer of surface micromachining because the porous silicon can be removed easily with 1% KOH at room temperature w4x. In addition to this application, visible photoluminescence ŽPL. and electroluminescence can be observed from porous silicon and several studies related with the source of the PL have been performed w5x. In the above mentioned applications and studies, the pore diameter is smaller than 50 nm. We focus on the macro porous silicon which has a pore diameter larger than 50 nm. Using this macro pore formation technology, 3-D single crystal silicon structures have been fabricated w6–9x. However, aluminium which is used for interconnect layer is attacked by hydrogen ions during the electrochemical etching. To solve this problem, ammonium fluoride is used as an etchant instead of HF etchant because of the lower aluminium etch rate. In this paper, the morphology of the etched surface is
investigated for etch parameters. Optimization of the parameters makes it possible to fabricate free standing beams.
2. Experimental 2.1. Set-up The sample is etched in an ammonium fluoride etch mixture ŽAFEM. under anodic bias. AFEM is composed of ammonium fluoride Ž13–14.5 wt.%., acetic acid Ž32.6–33.8 wt.%. and the rest is water. The ammonium fluoride concentration is changed by diluting AFEM with water. A sketch of the electrochemical etching set-up is shown in Fig. 1. The set-up consists of outer etchant bath and inner sample holder. The inner sample holder has a hole to illuminate the back side of the sample with a white light of which the intensity can be varied. Copper strip is put around the hole to make a electrical connection to the sample. The sample is fixed on the copper strip by silver epoxy. A platinum grid is used for a counter electrode. Positive voltage is applied between the sample and the counter electrode. Current is monitored by ampere meter and adjusted by the light intensity. 2.2. Fabrication process
)
Corresponding author. Tel.: q31-15-278-4729; Fax.: q31-15-2785755; E-mail:[email protected]
Process flow for electrochemical etching is shown in Fig. 2. Start material was n-type Ž100. silicon, 10 14 cmy3 phosphorus doped. Silicon nitride was deposited and pat-
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 8 . 0 0 3 3 4 - 3
H. Ohji, P.J. Frenchr Sensors and Actuators 74 (1999) 109–112
110
minium which is used for an interconnect layer in sensors is attacked by the hydrogen ions during the etching. To solve this problem, AFEM is used as an etchant because the aluminium etch rate is lower than that of HF. The aluminium etch rate in AFEM as a function of ammonium fluoride concentration is shown in Fig. 3. In case of the 5% HF etchant Žwhich is suitable to fabricate mechanical structures w8x., the aluminium etch rate is 50 nmrmin, so the aluminium interconnect layer is etched away within a few minutes. On the other hand, aluminium etch rate in AFEM is much lower making it very useful for porous formation when aluminium structures are present. Fig. 1. Electrochemical etching set-up.
terned. Initial pits were then made by KOH w10x. Vertical walls were formed by electrochemical etching using the set-up shown in Fig. 1. After desired depth was obtained, current density was increased by increasing the light intensity. The width of the trenches increased and trenches were connected under the structures. Thus free standing structures made of single crystal silicon were fabricated with one mask.
3. Results and discussion 3.1. Aluminium etch rate in AFEM Free standing structures fabricated by electrochemical etching in HF have been demonstrated w9x. However, alu-
Fig. 2. Process flow for electrochemical etching.
3.2. Effects of the AFEM concentration In order to obtain smooth etched surface and uniformity, it is important to optimize the concentration of ammonium fluoride. To investigate the effects of the ammonium fluoride concentration, three ammonium fluoride concentration are used as an etchant and the morphology of the etched surface was observed by SEM in Fig. 4a, b, c. In Fig. 4a, smooth etched surface can be seen in right side due to electropolishing. Critical current density decreases with the decreasing ammonium fluoride concentration. If the current density is larger than the critical current density, electropolishing occurs. In this figure, electropolishing has occurred only right hand side because the current is concentrated on the mask edge w4x. In Fig. 4c, lateral branches can be seen along the main pores. In case of 4.5% ammonium fluoride in Fig. 4b, straight pores are observed. A more detailed study of the effects of ammonium fluoride concentration on the formation of lateral pores is presently underway. However, it should be noted that the formation of lateral branching increases with applied voltage. The higher electric field around the vertical pores results in the generation of electronic holes which are available for the formation of lateral pores. The experiments on etchant concentration showed that 4.5% ammonium fluoride was most suitable for electrochemical
Fig. 3. Aluminium etch-rate.
H. Ohji, P.J. Frenchr Sensors and Actuators 74 (1999) 109–112
111
Fig. 4. Morphology of etched surface as a function of aluminium fluoride concentration Ž3 V, 30 min etching..
etching for micromachining. In the subsequent experiments, 4.5% ammonium fluoride was used. 3.3. Single step electrochemical etching 3.3.1. Anisotropic etching mode The etching mechanism of this process is the same as that of macro porous silicon formation w11,12x. An n-type silicon electronic hole is a minority carrier so the electronic holes which are consumed by the etching are supplied with the illumination from the back side of the sample. The electronic holes generated at the back side of the sample diffuse to the front side of the sample and reach the pore tip, therefore, straight pores can be achieved. It has been shown that a KOH pit can be used as a starting point for macro pore w10x. Using the V-shaped groove as an initiator, we have demonstrated how to make trench structures w8x. 3.3.2. Isotropic etching mode In this electrochemical etching, the trench width is controlled by adjusting the light intensity during the etching. Fig. 5 shows etch rate and trench width as a function of current density. The etch rate stays constant at around 1 mmrmin against the current density. On the other hand, the trench width is increased with the increase of the current density and a good relationship can be seen between the trench width and current density. When the light intensity is increased, more electronic holes are generated. Additional holes are used, not to increase the vertical etch rate, but to enlarge the trench width. This means the trench width is controlled by the current density without affecting the width of existing trenches which is an important advantage of this etching technique. 3.3.3. Combination of anisotropic and isotropic etching mode Using this advantage, free standing structures are fabricated by single step etching. First, the trenches are formed. After the desired depth is obtained, the current density is increased by increasing the light intensity. The width of
the trenches is increased under the structures without affecting the width of existing trenches. The connection of the trenches under the structures can be achieved and free standing structures are obtained with one mask. If the current density is larger than the critical current density, electropolishing occurs at the silicon surface under the structures. Thus the desired gap length and smooth surface are also obtained. 3.4. Free standing beams Free standing beams are shown in Fig. 6a and a close up view of the beam tip is shown in Fig. 6b. In the first step, the current density was 16 mArcm2 for 20 min which was used to form the trenches by anisotropic etching mode. In the following step, the current density was raised to 38 mArcm2 for 10 min to connect the trenches under the beams by isotropic etching mode. Thus free standing beams with height, width and length of 20 mm, 2 mm and 250 mm, respectively, were made of single crystal silicon. Clear gap between the beams and substrate can be seen in Fig. 6b. The maximum width of the structure which can be fabricated is determined by the trench width which can be achieved, which is in turn determined by the
Fig. 5. Silicon etch rate and trench width as a function of current density.
H. Ohji, P.J. Frenchr Sensors and Actuators 74 (1999) 109–112
112
Fig. 6. Free standing beams by single step electrochemical etching in AFEM Ž4.5% ammonium fluoride, 1.5 V..
current density. More experiments will have to be performed to establish the practical limitations.
4. Conclusions In this paper, the single step electrochemical etching in AFEM by way of new technique for micromachining has been presented. Morphology of the etched surface has been investigated. The combination of anisotropic and isotropic etching mode made it possible to fabricate free standing beams with single step. Aluminium survived the etching because the aluminium etch rate in 4.5% ammonium fluoride is five times lower than that of 5% HF. Investigations are continuing to further improve the morphology.
Acknowledgements The authors would like to thank the staff and processing crew of DIMES for assistance in preparing the samples. In addition we wish to thank T. Ozeki and Dr. K. Tsutsumi of Mitsubishi Electric for their encouragement.
References w1x A. Uhlir, Electrolytic shaping of germanium and silicon, Bell Tech. J. 35 Ž1956. 333–347. w2x D.R. Turner, Electropolishing silicon in HF acid solutions, J. Electrochem. Soc. 105 Ž1958. 402–408. w3x R.L. Smith, S.D. Collins, Porous silicon formation mechanisms, J. Appl. Phys. 71 Ž8. Ž1992. R1–R22. w4x P. Steiner, A. Richter, W. Lang, Using porous silicon as a sacrificial layer, J. Micromech. Microeng. 3 Ž1993. 32–36.
w5x L.T. Canham, Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers, Appl. Phys. Lett. 57 Ž1990. 1046–1048. w6x V. Lehmann, Porous silicon—a new material for MEMS, IEEE MEMS Workshop ’96, San Diego, USA, pp. 1–6, 1996. w7x S. Ottow, V. Lehmann, H. Foll, Processing of three-dimensional microstructures using macroporous n-type silicon, J. Electrochem. Soc. 143 Ž1996. 385–390. w8x H. Ohji, S. Lahteenmaki, P.J. French, Macro porous silicon formation for micromachining, Proceedings SPIE, Micromachining and Microfabrication Process Technology III, Austin, TX, USA, September 1997, pp. 189–197. w9x H. Ohji, P.J. Trimp, P.J. French, Fabrication of free standing structure using single step electrochemical etching in hydrofluoric acid, IEEE MEMS Workshop ’98, Heidelberg, Germany, pp. 246– 250, 1998. w10x V. Lehmann, H. Foll, Formation mechanism and properties of electrochemically etched trenches in n-type silicon, J. Electrochem. Soc. 137 Ž1990. 653–659. w11x V. Lehmann, A. Gosele, Porous silicon formation: a quantum wire effect, Appl. Phys. Lett. 58 Ž1991. 856–858. w12x V. Lehmann, The physics of macropore formation in low doped n-type silicon, J. Electrochem. Soc. 140 Ž1993. 2836–2843. Hiroshi Ohji received his BSc and MSc in precision engineering from Osaka University, Japan in 1986 and 1988 respectively. In the same year he joined Mitsubishi Electric, Japan. Since 1996, he has been at the Laboratory for Electronic Instrumentation, Delft University of Technology as a visiting scientist in the field of micro-fabrication technologies and their applications. Paddy French received his BSc in mathematics and MSc in electronics from Southampton University, UK, in 1981 and 1982, respectively. In 1986 he obtained his PhD, also from Southampton University, which was a study of the piezoresistive effect in polysilicon. After 18 months as a post-doc at Delft University, The Netherlands, he moved to Japan in 1988. For 3 years he worked on sensors for automotive applications at the Central Engineering Laboratories of Nissan Motor. He returned to Delft University in May 1991 and is now a staff member of the Laboratory for Electronic Instrumentation with interests in micromachining and process optimisation related to sensors.
Single step electrochemical etching in ammonium fluoride H. Ohji ) , P.J. French Delft UniÕersity of Technology, DIMES, Faculty of Information Technology and Systems, Postbus 5031, 2600GA Delft, Netherlands
Abstract This paper presents a new technique of micromachining using single step electrochemical etching in an ammonium fluoride based etchant. This etching technology is to fabricate 3-D structures in single crystal silicon by a combination of anisotropic and isotropic etching mode. The etch rate and morphology of the etched surface are investigated for the etch parameters Žetchant concentration, current density.. Optimization of these parameters makes it possible to make free standing beams. Using the ammonium fluoride as the etchant, aluminium survives the etching. q 1999 Elsevier Science S.A. All rights reserved. Keywords: Electrochemical etching; Free standing structure; Porous silicon; Wet etching; Ammonium fluoride
1. Introduction Electrochemical etching in hydrofluoric acid ŽHF. is known as a technique of porous silicon formation which has been studied since the 1950s w1,2x and much attention has been paid to the formation mechanism w3x. Recently, porous silicon has been applied to micromachining and micro devices because of its interesting properties. For example, porous silicon has been used for a sacrificial layer of surface micromachining because the porous silicon can be removed easily with 1% KOH at room temperature w4x. In addition to this application, visible photoluminescence ŽPL. and electroluminescence can be observed from porous silicon and several studies related with the source of the PL have been performed w5x. In the above mentioned applications and studies, the pore diameter is smaller than 50 nm. We focus on the macro porous silicon which has a pore diameter larger than 50 nm. Using this macro pore formation technology, 3-D single crystal silicon structures have been fabricated w6–9x. However, aluminium which is used for interconnect layer is attacked by hydrogen ions during the electrochemical etching. To solve this problem, ammonium fluoride is used as an etchant instead of HF etchant because of the lower aluminium etch rate. In this paper, the morphology of the etched surface is
investigated for etch parameters. Optimization of the parameters makes it possible to fabricate free standing beams.
2. Experimental 2.1. Set-up The sample is etched in an ammonium fluoride etch mixture ŽAFEM. under anodic bias. AFEM is composed of ammonium fluoride Ž13–14.5 wt.%., acetic acid Ž32.6–33.8 wt.%. and the rest is water. The ammonium fluoride concentration is changed by diluting AFEM with water. A sketch of the electrochemical etching set-up is shown in Fig. 1. The set-up consists of outer etchant bath and inner sample holder. The inner sample holder has a hole to illuminate the back side of the sample with a white light of which the intensity can be varied. Copper strip is put around the hole to make a electrical connection to the sample. The sample is fixed on the copper strip by silver epoxy. A platinum grid is used for a counter electrode. Positive voltage is applied between the sample and the counter electrode. Current is monitored by ampere meter and adjusted by the light intensity. 2.2. Fabrication process
)
Corresponding author. Tel.: q31-15-278-4729; Fax.: q31-15-2785755; E-mail:[email protected]
Process flow for electrochemical etching is shown in Fig. 2. Start material was n-type Ž100. silicon, 10 14 cmy3 phosphorus doped. Silicon nitride was deposited and pat-
0924-4247r99r$ - see front matter q 1999 Elsevier Science S.A. All rights reserved. PII: S 0 9 2 4 - 4 2 4 7 Ž 9 8 . 0 0 3 3 4 - 3
H. Ohji, P.J. Frenchr Sensors and Actuators 74 (1999) 109–112
110
minium which is used for an interconnect layer in sensors is attacked by the hydrogen ions during the etching. To solve this problem, AFEM is used as an etchant because the aluminium etch rate is lower than that of HF. The aluminium etch rate in AFEM as a function of ammonium fluoride concentration is shown in Fig. 3. In case of the 5% HF etchant Žwhich is suitable to fabricate mechanical structures w8x., the aluminium etch rate is 50 nmrmin, so the aluminium interconnect layer is etched away within a few minutes. On the other hand, aluminium etch rate in AFEM is much lower making it very useful for porous formation when aluminium structures are present. Fig. 1. Electrochemical etching set-up.
terned. Initial pits were then made by KOH w10x. Vertical walls were formed by electrochemical etching using the set-up shown in Fig. 1. After desired depth was obtained, current density was increased by increasing the light intensity. The width of the trenches increased and trenches were connected under the structures. Thus free standing structures made of single crystal silicon were fabricated with one mask.
3. Results and discussion 3.1. Aluminium etch rate in AFEM Free standing structures fabricated by electrochemical etching in HF have been demonstrated w9x. However, alu-
Fig. 2. Process flow for electrochemical etching.
3.2. Effects of the AFEM concentration In order to obtain smooth etched surface and uniformity, it is important to optimize the concentration of ammonium fluoride. To investigate the effects of the ammonium fluoride concentration, three ammonium fluoride concentration are used as an etchant and the morphology of the etched surface was observed by SEM in Fig. 4a, b, c. In Fig. 4a, smooth etched surface can be seen in right side due to electropolishing. Critical current density decreases with the decreasing ammonium fluoride concentration. If the current density is larger than the critical current density, electropolishing occurs. In this figure, electropolishing has occurred only right hand side because the current is concentrated on the mask edge w4x. In Fig. 4c, lateral branches can be seen along the main pores. In case of 4.5% ammonium fluoride in Fig. 4b, straight pores are observed. A more detailed study of the effects of ammonium fluoride concentration on the formation of lateral pores is presently underway. However, it should be noted that the formation of lateral branching increases with applied voltage. The higher electric field around the vertical pores results in the generation of electronic holes which are available for the formation of lateral pores. The experiments on etchant concentration showed that 4.5% ammonium fluoride was most suitable for electrochemical
Fig. 3. Aluminium etch-rate.
H. Ohji, P.J. Frenchr Sensors and Actuators 74 (1999) 109–112
111
Fig. 4. Morphology of etched surface as a function of aluminium fluoride concentration Ž3 V, 30 min etching..
etching for micromachining. In the subsequent experiments, 4.5% ammonium fluoride was used. 3.3. Single step electrochemical etching 3.3.1. Anisotropic etching mode The etching mechanism of this process is the same as that of macro porous silicon formation w11,12x. An n-type silicon electronic hole is a minority carrier so the electronic holes which are consumed by the etching are supplied with the illumination from the back side of the sample. The electronic holes generated at the back side of the sample diffuse to the front side of the sample and reach the pore tip, therefore, straight pores can be achieved. It has been shown that a KOH pit can be used as a starting point for macro pore w10x. Using the V-shaped groove as an initiator, we have demonstrated how to make trench structures w8x. 3.3.2. Isotropic etching mode In this electrochemical etching, the trench width is controlled by adjusting the light intensity during the etching. Fig. 5 shows etch rate and trench width as a function of current density. The etch rate stays constant at around 1 mmrmin against the current density. On the other hand, the trench width is increased with the increase of the current density and a good relationship can be seen between the trench width and current density. When the light intensity is increased, more electronic holes are generated. Additional holes are used, not to increase the vertical etch rate, but to enlarge the trench width. This means the trench width is controlled by the current density without affecting the width of existing trenches which is an important advantage of this etching technique. 3.3.3. Combination of anisotropic and isotropic etching mode Using this advantage, free standing structures are fabricated by single step etching. First, the trenches are formed. After the desired depth is obtained, the current density is increased by increasing the light intensity. The width of
the trenches is increased under the structures without affecting the width of existing trenches. The connection of the trenches under the structures can be achieved and free standing structures are obtained with one mask. If the current density is larger than the critical current density, electropolishing occurs at the silicon surface under the structures. Thus the desired gap length and smooth surface are also obtained. 3.4. Free standing beams Free standing beams are shown in Fig. 6a and a close up view of the beam tip is shown in Fig. 6b. In the first step, the current density was 16 mArcm2 for 20 min which was used to form the trenches by anisotropic etching mode. In the following step, the current density was raised to 38 mArcm2 for 10 min to connect the trenches under the beams by isotropic etching mode. Thus free standing beams with height, width and length of 20 mm, 2 mm and 250 mm, respectively, were made of single crystal silicon. Clear gap between the beams and substrate can be seen in Fig. 6b. The maximum width of the structure which can be fabricated is determined by the trench width which can be achieved, which is in turn determined by the
Fig. 5. Silicon etch rate and trench width as a function of current density.
H. Ohji, P.J. Frenchr Sensors and Actuators 74 (1999) 109–112
112
Fig. 6. Free standing beams by single step electrochemical etching in AFEM Ž4.5% ammonium fluoride, 1.5 V..
current density. More experiments will have to be performed to establish the practical limitations.
4. Conclusions In this paper, the single step electrochemical etching in AFEM by way of new technique for micromachining has been presented. Morphology of the etched surface has been investigated. The combination of anisotropic and isotropic etching mode made it possible to fabricate free standing beams with single step. Aluminium survived the etching because the aluminium etch rate in 4.5% ammonium fluoride is five times lower than that of 5% HF. Investigations are continuing to further improve the morphology.
Acknowledgements The authors would like to thank the staff and processing crew of DIMES for assistance in preparing the samples. In addition we wish to thank T. Ozeki and Dr. K. Tsutsumi of Mitsubishi Electric for their encouragement.
References w1x A. Uhlir, Electrolytic shaping of germanium and silicon, Bell Tech. J. 35 Ž1956. 333–347. w2x D.R. Turner, Electropolishing silicon in HF acid solutions, J. Electrochem. Soc. 105 Ž1958. 402–408. w3x R.L. Smith, S.D. Collins, Porous silicon formation mechanisms, J. Appl. Phys. 71 Ž8. Ž1992. R1–R22. w4x P. Steiner, A. Richter, W. Lang, Using porous silicon as a sacrificial layer, J. Micromech. Microeng. 3 Ž1993. 32–36.
w5x L.T. Canham, Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers, Appl. Phys. Lett. 57 Ž1990. 1046–1048. w6x V. Lehmann, Porous silicon—a new material for MEMS, IEEE MEMS Workshop ’96, San Diego, USA, pp. 1–6, 1996. w7x S. Ottow, V. Lehmann, H. Foll, Processing of three-dimensional microstructures using macroporous n-type silicon, J. Electrochem. Soc. 143 Ž1996. 385–390. w8x H. Ohji, S. Lahteenmaki, P.J. French, Macro porous silicon formation for micromachining, Proceedings SPIE, Micromachining and Microfabrication Process Technology III, Austin, TX, USA, September 1997, pp. 189–197. w9x H. Ohji, P.J. Trimp, P.J. French, Fabrication of free standing structure using single step electrochemical etching in hydrofluoric acid, IEEE MEMS Workshop ’98, Heidelberg, Germany, pp. 246– 250, 1998. w10x V. Lehmann, H. Foll, Formation mechanism and properties of electrochemically etched trenches in n-type silicon, J. Electrochem. Soc. 137 Ž1990. 653–659. w11x V. Lehmann, A. Gosele, Porous silicon formation: a quantum wire effect, Appl. Phys. Lett. 58 Ž1991. 856–858. w12x V. Lehmann, The physics of macropore formation in low doped n-type silicon, J. Electrochem. Soc. 140 Ž1993. 2836–2843. Hiroshi Ohji received his BSc and MSc in precision engineering from Osaka University, Japan in 1986 and 1988 respectively. In the same year he joined Mitsubishi Electric, Japan. Since 1996, he has been at the Laboratory for Electronic Instrumentation, Delft University of Technology as a visiting scientist in the field of micro-fabrication technologies and their applications. Paddy French received his BSc in mathematics and MSc in electronics from Southampton University, UK, in 1981 and 1982, respectively. In 1986 he obtained his PhD, also from Southampton University, which was a study of the piezoresistive effect in polysilicon. After 18 months as a post-doc at Delft University, The Netherlands, he moved to Japan in 1988. For 3 years he worked on sensors for automotive applications at the Central Engineering Laboratories of Nissan Motor. He returned to Delft University in May 1991 and is now a staff member of the Laboratory for Electronic Instrumentation with interests in micromachining and process optimisation related to sensors.