Comparison of porous silicon, porous polysilicon and porous silicon carbide as materials for humidity sensing applications

https://doi.org/10.1016/S0924-4247(01)00885-8Get rights and content

Abstract

This paper investigates the suitability of porous polysilicon and porous SiC as materials for sensing humidity. The investigation is a continuation of earlier work on porous single-crystalline silicon, where it was shown that this material was appropriate for humidity sensing, and could be easily integrated with standard Si processing. It was also shown that membrane structures enable the integration of a heating device to ‘reset’ the system. The best microstructure for humidity sensing was obtained for low-doped p-type silicon. The advantage of using polysilicon is that it is possible to tune its response (by doping) so that it has only a very small temperature coefficient of resistance. The idea is that a humidity sensor with a very small temperature dependence could be realised. The advantage of using SiC is that it offers the possibility of a humidity sensor that could withstand very harsh chemical environments.

Introduction

The sensitivity of (single-crystal) porous silicon to humidity depends on porosity, pore structure and size distribution of the pores. These are controlled by electrolyte concentration, current density, anodisation time, and also on doping levels in the substrate. The best microstructure for humidity sensing was obtained for low-doped p-type silicon [1], [2], [3]. It was established that for effective humidity sensing, the pore size distribution should include pore sizes in the range 2–10 nm for low RH values, and also larger (20–100 nm) pores for high RH values [1], [2], [3], [10].

Polycrystalline silicon or polysilicon, differs from single-crystal silicon in that it is composed of many grains of (single-crystal) silicon with different crystallographic orientations. In between grains, the grain boundaries (GBs) are amorphous regions, which are predicted to be depleted of charge carriers [6], [7], [8]. As a result, the current distribution (compared to single-crystal silicon) will be changed. It is not known how this different current distribution will affect porous formation, but it is expected that—since the GBs are depleted regions—most of the current will be tunnelling through GBs, so that normal current flow only occurs within the grains. Therefore, it is expected that the grains themselves will become porous, with the GB regions remaining unaffected.

The advantage of using polysilicon is that, since its electrical conductivity is influenced by GBs, it is possible to tune its response (by doping), so that it has a very small temperature coefficient of resistance [6], [7], [8]. If this feature can be incorporated into a humidity sensor we should be able to reduce the effect of temperature on RH measurements—a well-known problem with all existing humidity sensors.

Some work has been done already on making porous polysilicon [4], [5], but nothing has been found in the literature (so far) regarding using it as a ‘temperature insensitive’ material for RH sensing. On the other hand, the advantage of using SiC is that it offers the possibility of a humidity sensor that could withstand very harsh chemical environments [9]. Another advantage of SiC is that it is not etched by KOH, so that it is an automatic etch stop when making membrane structures. The SiC we are using in this work is deposited as a thin, amorphous film on a standard silicon wafer, using PECVD. Being an amorphous film, we are unable to predict the current flow during porous formation. Therefore, the porous formation parameters are probably not yet fully optimised for this material.

It was also shown that membrane structures enable the integration of a heating device to ‘reset’ the system [1]. Using thin films has the advantage as regards ease of integration with standard processing, due to greater flexibility in choice of doping type and concentration.

Section snippets

Experimental

Thin films of p-type polysilicon and SiC were deposited on standard Si wafers. Polysilicon was deposited using LPCVD and SiC using PECVD, and doped in situ. The thickness of the films were ∼4000 Å (polysilicon) and ∼5000 Å (SiC), and both were doped with boron.

After the thin films were deposited, Al was evaporated on the backside of the wafer, and then the wafers were diced into 10mm×10 mm samples. The samples were then mounted on specially prepared holders for porous formation.

We made porous

Results

Preliminary results on polysilicon and SiC show that they can also be made porous in HF, and that their electrical properties are sensitive to humidity.

Discussion

For porous polysilicon samples, the capacitance increased by up to ∼2500% with humidity increasing from 10 to 90% RH, and at least for this sample, with good linearity between 30 and 90% RH (see Fig. 4). For RH approximately <30%, it seemed that the sensitivity was very large, and also quiet linear. The effect of temperature changes was found to be significantly smaller for porous polysilicon compared to porous single-crystal silicon. However, the response time was much larger than that of

Conclusions

These initial experiments have shown that both LPCVD polysilicon and PECVD SiC are suitable materials for porous formation for humidity sensing applications. The responses (sensitivity, response times, hysterisis, temperature effects, etc.) of the porous polysilicon and porous SiC devices will have to be optimised before a proper comparison with the single-crystal porous silicon devices can be made. However, we have shown that, at least for polysilicon, sensitivities close to the best achieved

Acknowledgements

E.J.C. acknowledges support from the Dutch Technology Foundation STW, Project DEL 4694, and DIMES for sample preparation.

Eamon Connolly was born in Cork, Ireland, in 1969. He obtained the National Diploma in Applied Physics & Instrumentation from Cork Institute of Technology in 1990. Following this, he spent 1 year working on MOCVD in the III–V group at the National Microelectronics Research Centre, Ireland. In 1992, he obtained a BSc in Physical Optoelectronics from the University of Essex, England. In 1996 he obtained a PhD from Cardiff University, Wales, studying the relationship between the

References (10)

  • F.D. King et al.

    Polycrystalline silicon resistors for integrated circuits

    Solid State Electron.

    (1973)
  • T. Seiyama et al.

    Ceramic humidity sensors

    Sens. Actuators

    (1983)
  • G.M. O’Halloran, Capacitive humidity sensor based on porous silicon, Ph.D. thesis, TU Delft,...
  • M.I.J. Beale et al.

    An experimental and theoretical study of the formation and microstructure of porous silicon

    J. Crystal Growth

    (1985)
  • R. Herino et al.

    Porosity and pore size distributions of porous silicon

    J. Electrochem. Soc.

    (1987)
There are more references available in the full text version of this article.

Cited by (126)

  • Polymer composites for humidity sensors

    2022, Polymeric Nanocomposite Materials for Sensor Applications
  • Electrical and hysteric properties of organic compound-based humidity sensor and its dualistic interactive approach to H<inf>2</inf>O molecules

    2021, Materials Today Communications
    Citation Excerpt :

    The comparison of the present data is made with previously reported data in Table 1. The response and recovery data in the present study is better than the response/recovery time of SnO2 [33], Zn2SiO4/Si-NPA [34], Porous SiC [20], PSDA-b-PEG and PSDA-b-PEG:PEG= 1:1 [35], and PANI-TiO2 composite [36] materials based sensors. However, IESM-1 & 2 [37], CeO2 [38], Bi2S3 nanobelts [39], ZnO/PVP-RGO [40], ZnO/Si [41] materials based sensors showed comparatively good response and recovery time than the present data.

  • Ethanol gas sensing performance of electrochemically anodized freestanding porous SiC

    2019, Diamond and Related Materials
    Citation Excerpt :

    In comparison with porous silicon, porous silicon carbide (PSiC) demonstrates high thermal conductivity, chemical stability and large energy band; it is able to withstand harsh chemical environments [3]. Furthermore, PSiC demonstrated good sensing properties toward Н2O, NH3 and H2 [3–6]. The operational principle of chemical sensors on PSiC is based on change of PSiC electrical conductivity at the adsorption of molecules on its surface.

  • High sensitivity and low hysteresis of humidity sensor based on imidazole derivative

    2023, Journal of Materials Science: Materials in Electronics
View all citing articles on Scopus

Eamon Connolly was born in Cork, Ireland, in 1969. He obtained the National Diploma in Applied Physics & Instrumentation from Cork Institute of Technology in 1990. Following this, he spent 1 year working on MOCVD in the III–V group at the National Microelectronics Research Centre, Ireland. In 1992, he obtained a BSc in Physical Optoelectronics from the University of Essex, England. In 1996 he obtained a PhD from Cardiff University, Wales, studying the relationship between the electrical/magnetic properties and dimensionality of layered-perovskites including High-Tc superconductors. During this period he also worked on the application of percolation theory to model doping in YBCO and BSCCO HTS materials. Following this he did postdoctoral work at the University of Limerick, lreland, on thick-film NH3 gas sensors. Subsequently, he was an EU-TMR postdoctoral fellow at the National Centre for High Resolution Electron Microscopy, Delft University of Technology, The Netherlands, investigating microstructural pinning mechanisms in HTS materials. Currently, he is working on the development of a humidity sensors based on porous Si and related materials at the Laboratory for Electronic Instrumentation, Delft University of Technology.

Orla O’Halloran received a MSc from University of Limerick in lreland in 1993. In 1994 she joined the Electronic Instrumentation Laboratory TU Delft where she carried out research work on the material porous silicon, in particular its application to humidity sensing. In 1999 she was awarded a PhD degree for this research. At present she is a process analist at Philips Semiconductors Nijmegen.

Hoa T.M. Pham received a BSc degree in Chemistry in 1994 and a MSc degree in Material Science in 1996. From 1996 to 2000, she worked on Porous silicon for Humidity Sensors in National Centre for Natural Science and Technology in Vietnam. Since September 2000, she is a PhD student in the ECTM laboratory at DIMES, Delft University of Technology, The Netherlands, working on materials for post-processing surface micromachining.

Pasqualina M. Sarro received the Laurea degree in solid-state physics from the University of Naples, Italy, in 1980. From 1981 to 1983, she was a post-doctoral fellow in the Photovoltaic Research Group of the Division of Engineering, Brown University, Rhode Island, USA, where she worked on thin-film photovoltaic cell fabrication by chemical spray pyrolysis. In 1987, she received the PhD degree in Electrical Engineering at the Delft University of Technology, The Netherlands, her thesis dealing with infrared sensors based on integrated silicon thermopiles. Since then, she has been with the Delft Institute of Microelectronics and Submicron Technology (DIMES), at the Delft University, where she is responsible for research on integrated silicon sensors and microsystems technology. In April 1996 she became Associate Professor in the Electronic Components, Materials and Technology Laboratory of the Delft University and in December 2001 A. van Leeuwenhoek Professor in the same department. She is an IEEE member since 1984 and a Senior member since 1997. She has authored and co-authored more than 200 journal and conference papers. She acts as reviewer for several technical journals and she has served as technical program committee member of the ESSDERC Conferences (since ’95), and EUROSENSORS Conferences (since ’99) and she is co-chair for Europe and Africa for the IEEE Sensors 2002 Conference.

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 postdoc at Delft University, The Netherlands, he moved to Japan in 1988. For 3 years he worked on sensors for automotives at the Central Engineering Laboratories of Nissan Motor Company. He returned to Delft University in May 1991 and is now a staff member of the Laboratory for Electronics Instrumentation with interests in micromachining and process optimisation related to sensors. In 1999 he was awarded the Antoni van Leeuwenhoek chair.

1

Present address: FA Group, QRI Department of Consumer Systems, Nijmegen Building, FB0.096 Philips Semiconductors, Gerstweg 2, 6534AE Nijmegen, The Netherlands.

View full text