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Vapor etching

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Vapor etching refers to a process used in the fabrication of Microelectromechanical systems (MEMS) and Nanoelectromechanical systems (NEMS). Sacrificial layers are isotropically etched using gaseous acids such as Hydrogen fluoride and Xenon difluoride to release the free standing components of the device.

Economic advantages and novel technological possibilities result from micron to nano scale size reductions. (MEMS to NEMS) The small dimensions make the use of isotropic wet etch processes traditionally used in micro fabrication suffer from stiction, the permanent adherence of the free standing structure to the underlying substrate due to the scaling of surface effects occurring during the drying of the acid.[1] Vapor etching overcomes stiction because no liquids are used during the etch process. Commonly, hydrogen fluoride and xenon difluorides are used to etch silicon dioxide and silicon sacrificial layers respectively.

HF vapor etching

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The wet etching of SiO2 in buffered hydrogen fluoride solutions is a common and well understood process in micro fabrication. In 1966, Holmes and Snell found that SiO2 can be etched in hydrogen fluoride vapor.[2] Initially the interest in this finding was low, because wet etch processes have higher etch rates and did not require sophisticated equipment. During the advent of MEMS technology and the consecutive reduction of size however, stiction started to have a significant impact on the production yields. Therefore HF vapor etching became an interesting commercial fabrication technology. Water or an alcohol catalysts are required, because anhydrous HF does not etch SiO2.

Etch chemistry

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The etch chemistry depends on the catalyst used.

Etch reaction with water catalyst

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If water is used, the H2O is adsorbed at the SiO2 surface and forms silanol groups can never form.[3]

SiO2 + 2 H2O → Si(OH)4

The HF reacts with the silanol groups and forms SiF4 and H2O according to the following reaction.

Si(OH)4 + 4 HF → SiF4 + 4 H2O

The etch process commonly takes place at reduced pressures, to promote the desorption of the reaction products. Water is formed during the etch reaction. The efficient H2O removal is critical to prevent the formation of a liquid layer.

Etch reaction with alcohol catalyst

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Alternatively, different alcohols such as methanol, ethanol, 1-propanol or IPA can be used to initiate the reaction.[4] An example for this reaction, using methanol (CH3OH) is given below.[5] Firstly the HF and the methanol are absorbed to the surface.

CH3OH (g) ↔ CH3OH (ads) HF (g) ↔ HF (ads)

is formed by an ionization reaction of the absorbed HF and absorbed CH3OH

The ionized HF then reacts with the SiO2 according to the following reaction.

Finally the reaction products are removed from the surface by desorption.

XeF2 vapor etching

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Xenon difluoride, bromine trifluoride, chlorine trifluoride and fluorine can be used for gaseous silicon etching.[6][7] Xenon difluoride is most commonly used to etch silicon in academia and industry, because it has a high selectivity towards other semiconductor materials, allows high process control and is easy to use at room temperature.[8][9]

Etch systems

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The synthesis of XeF2 is an endothermic process which results in a white powder which sublimes at low pressures.[10] (P < 4 Torr)[11] The low vapor pressure allowed early researchers and engineers to use it in comparatively simple set ups. Modern vapour etch tools are more sophisticated and are characterized by the way the gas is feed into the etch chamber. In pulsed systems the etchant is expanded, feed into the reaction chamber and remains there until it has been consumed by the reaction. Then the chamber is evacuated and this process is repeated for multiple cycles.[12][13] In contrast to that, a carrier gas flows through a bubbler to continuously supply xenon difluoride into the etch chamber in continuous flow systems.[14]

Etch chemistry

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The general etch reaction is summarized by the following equation.

2 XeF2 + Si → SiF4 + 2 Xe

The detailed etch kinetic is more complex reaction consisting of four steps.[15][16] After the etchant has been mass transported to the silicon surface, the xenon difluoride is absorbed on the silicon surface.

2 XeF2 (gas) + Si (s) → 2 XeF2 (abs) + Si (s)

The XeF2 disassociates into absorbed fluorine and gaseous Xe.

2 XeF2 (abs) + Si (s) → 2 Xe (g) + 2 F (abs) + Si (s)

The fluorine bonds with the surface silicon to form silicon tetra fluoride.

2 Xe (g) + 2 F (abs) + Si (s) → 2 Xe (g) + SiF4 (ads)

The reaction product is desorpted from the silicon surface.

2 Xe (g) + SiF4 (ads) → " Xe (g) + SiF4 (g)

The reaction products are mass transferred from the surface to the etch chamber, and ejected from there by a vacuum pump.

References

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  1. ^ Zhuang, Y.X.; Menon, A. (June 2005). "On the stiction of MEMS materials". Tribology Letters. 19 (2): 111–117. doi:10.1007/s11249-005-5088-1. ISSN 1023-8883. S2CID 49385010.
  2. ^ J. Holmes, P.J Snell (1966). "A vapour etching technique for the photolithography of silicon dioxide". Microelectronics and Reliability. 5 (4): 337–341. doi:10.1016/0026-2714(66)90162-4.
  3. ^ Chu, P.B.; Chen, J.T.; Yeh, R.; Lin, G.; Huang, J.C.P.; Warneke, B.A.; Pister, S.J. (1997). "Controlled pulse-etching with xenon difluoride". Proceedings of International Solid State Sensors and Actuators Conference (Transducers '97). Vol. 1. IEEE. pp. 665–668. doi:10.1109/sensor.1997.613739. ISBN 0780338294. S2CID 14796800.
  4. ^ Handbook of silicon based MEMS : materials & technologies. Lindroos, Veikko (Second ed.). Norwich. 2015-09-02. ISBN 9780323312233. OCLC 932060052.{{cite book}}: CS1 maint: location missing publisher (link) CS1 maint: others (link)
  5. ^ Jang, Won Ick; Choi, Chang Auck; Lee, Myung Lae; Jun, Chi Hoon; Kim, Youn Tae (2002-04-22). "Fabrication of MEMS devices by using anhydrous HF gas-phase etching with alcoholic vapor". Journal of Micromechanics and Microengineering. 12 (3): 297–306. Bibcode:2002JMiMi..12..297I. doi:10.1088/0960-1317/12/3/316. ISSN 0960-1317. S2CID 250849439.
  6. ^ Arana, Leonel R; Mas, Nuria de; Schmidt, Raymond; Franz, Aleksander J; Schmidt, Martin A; Jensen, Klavs F (2007-01-25). "Isotropic etching of silicon in fluorine gas for MEMS micromachining". Journal of Micromechanics and Microengineering. 17 (2): 384–392. Bibcode:2007JMiMi..17..384A. doi:10.1088/0960-1317/17/2/026. ISSN 0960-1317. S2CID 135708022.
  7. ^ Chu, P.B.; Chen, J.T.; Yeh, R.; Lin, G.; Huang, J.C.P.; Warneke, B.A.; Pister, S.J. (1997). "Controlled pulse-etching with xenon difluoride". Proceedings of International Solid State Sensors and Actuators Conference (Transducers '97). Vol. 1. IEEE. pp. 665–668. doi:10.1109/sensor.1997.613739. ISBN 0780338294. S2CID 14796800.
  8. ^ Sugano, K; Tabata, O (2002-10-08). "Reduction of surface roughness and aperture size effect for etching of Si with XeF2". Journal of Micromechanics and Microengineering. 12 (6): 911–916. Bibcode:2002JMiMi..12..911S. doi:10.1088/0960-1317/12/6/323. ISSN 0960-1317. S2CID 250809960.
  9. ^ Su, K.; Tabata, O. (2002-11-01). "Effects of aperture size and pressure on XeF 2 etching of silicon". Microsystem Technologies. 9 (1–2): 11–16. doi:10.1007/s00542-002-0195-5. ISSN 0946-7076. S2CID 135772969.
  10. ^ Easter, C.; O'Neal, C.B. (October 2009). "Characterization of High-Pressure XeF2 Vapor-Phase Silicon Etching for MEMS Processing". Journal of Microelectromechanical Systems. 18 (5): 1054–1061. doi:10.1109/jmems.2009.2029976. ISSN 1057-7157. S2CID 39315299.
  11. ^ Xu, Dehui; Xiong, Bin; Wu, Guoqiang; Wang, Yuchen; Sun, Xiao; Wang, Yuelin (December 2012). "Isotropic Silicon Etching With $\hbox{XeF}_{2}$ Gas for Wafer-Level Micromachining Applications". Journal of Microelectromechanical Systems. 21 (6): 1436–1444. doi:10.1109/jmems.2012.2209403. ISSN 1057-7157. S2CID 40284249.
  12. ^ Ibbotson, Dale E.; Flamm, Daniel L.; Mucha, John A.; Donnelly, Vincent M. (1984-06-15). "Comparison of XeF2 and F-atom reactions with Si and SiO2". Applied Physics Letters. 44 (12): 1129–1131. Bibcode:1984ApPhL..44.1129I. doi:10.1063/1.94665. ISSN 0003-6951.
  13. ^ Winters, H. F.; Coburn, J. W. (January 1979). "The etching of silicon with XeF2vapor". Applied Physics Letters. 34 (1): 70–73. Bibcode:1979ApPhL..34...70W. doi:10.1063/1.90562. ISSN 0003-6951.
  14. ^ Drysdale, Daniel. "The behaviour of continuous flow xenon difluoride etching of silicon". Journal of Institute of Smart Structures and Systems (ISSS). 4: 70–75.
  15. ^ Xu, Dehui; Xiong, Bin; Wu, Guoqiang; Wang, Yuchen; Sun, Xiao; Wang, Yuelin (December 2012). "Isotropic Silicon Etching With $\hbox{XeF}_{2}$ Gas for Wafer-Level Micromachining Applications". Journal of Microelectromechanical Systems. 21 (6): 1436–1444. doi:10.1109/jmems.2012.2209403. ISSN 1057-7157. S2CID 40284249.
  16. ^ Easter, C.; O'Neal, C.B. (October 2009). "Characterization of High-Pressure $\hbox{XeF}_{2}$ Vapor-Phase Silicon Etching for MEMS Processing". Journal of Microelectromechanical Systems. 18 (5): 1054–1061. doi:10.1109/jmems.2009.2029976. ISSN 1057-7157. S2CID 39315299.
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