Fabrication of high aspect ratio silicon microstructures by anodic by dfsiopmhy6


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       Fabrication of high aspect ratio
       silicon microstructures by anodic

M D B Charltont             and G J Parker

Department of Electronics and Computer             Science, Mountbatten      Building,
University of Southampton, Southampton,             SO17 1 BJ, England

Presented   on 21 October      1996, accepted       for publication   on 9 April 1997

Abstract.   We describe a refinement of the anodization process commonly used for
the formation of porous silicon, which allows the fabrication of arrays of very high
aspect ratio sub-micron pores and free-standing pillars. These structures are
shown to possess a wide photonic band gap in the near infra red.

1. Introduction                                                               at the pore tips where the vast majority of holes will be
                                                                              consumed. Few holes are then available for dissolution
The discovery of room-temperature photoluminescence        of                 of the side walls, and so they become passivated against
porous silicon [l] and its suitability for the fabrication of                 dissolution.    By pre-patterning the wafer surface with
photonic bands structures has stimulated a large interest in                  defect sites we pre-determine where macropores will
the development of silicon as an optical material. A photo-                   form. We do this using a KOH etch in conjunction with
assisted electrochemical etching process has recently been                    standard photolithography to create pyramidal notches in
developed for the fabrication of highly ansitropic trenches                   the required positions.
in silicon [24]. We have refined the anodic etching process                        The formation of the pores is affected by several
for the fabrication of arrays of very high aspect ratio sub-                  experimental parameters including bias voltage, wafer
micron pores and free-standing pillars in silicon [5]. Free-                  resistivity, electrolyte concentration, and photocurrent
standing pillars over 20 pm in height have been fabricated                    density.     The diameter of the pores is found to be
with diameters as small as 150 nm. Arrays of parallel-sided                   directly dependent upon the photocurrent density.        To
pores have been fabricated to depths of over 150 pm with                      ensure the formation of parallel-sided pores, an electrical
diameters as small as 200 mn. These structures are shown                      control system was used to regulate the illumination
to be suitable for applications as photonic band structures                   intensity throughout the etch. However, by adjusting the
and quantum wires.                                                            photocurrent during the duration of the etch we can also
                                                                              fabricate unusually shaped structures.

2. Formation      of macroporous             silicon
                                                                              3. Wafer preparation
Porous silicon can be formed by etching crystalline silicon
                                                                              3-5 S2 cm-’ n-type silicon wafers were used as the
under electrically biased conditions in a weak solution of
                                                                              basic substrate for our microstructure, allowing sub-micron
hydrofluoric (HF) acid. Holes are required to enable the
                                                                              macropore. formation. A thin oxide layer was grown by
dissolution of the silicon in the electrolyte. For n-type
                                                                              oxidizing the wafers in a dry oxygen environment, and the
silicon, the majority charge carriers are electrons. By
                                                                              surface coated in photoresist. This was patterned with a
illuminating the rear surface of the wafer with sufficiently
                                                                              regular lattice pattern using direct write e-beam lithography.
energetic photons, mobile holes can be generated by a
                                                                              The resist was developed and hard baked leaving circular
process of photon absorption. Under anodically biased
                                                                              windows at the lattice sites. The exposed silicon dioxide
conditions, the photogenerated holes are electrostatically
                                                                              was then removed by plasma etching and the resist stripped
attracted to the regions of high electric field strength
                                                                              to leave a fully patterned silicon dioxide mask. Pyramidal
causing rapid localized dissolution at these points. Initially,
                                                                              notches were created in the exposed silicon windows by a
the electric field will tend to be concentrated at sharp
                                                                              KOH etch. A weak solution of HF acid in ethanol was used
discontinuities on the wafer surface.        Surface defects
                                                                              as the electrolyte for the anodization process, the ethanol
therefore act as seeding points for macropore formation. As
                                                                              acting as a wetting agent.
the etch progresses, the electric field remains concentrated
                                                                                  Examples of deep macropores formed by the anodiza-
t Tel: +44 (0)1703-593737.      Fax:   +44    (0)1703-592773.    E-mail:      tion process are shown in figures 1 and 2. For the sake
mdcl @soton.ac.uk                                                             of clarity, figure 1 shows a relatively large scale structure,
                                                                               Figure   3. Free-standing   pillars.

      Figure   1. Macropores   7 pm pitch 57 pm deep.

                                                                                  Figure   4. Quantum wires.

                                                                 band structures. Structures based upon a cubic or triangular
                                                                 lattice of deep air rods in a background dielectric material
                                                                 can exhibit a photonic band gap [6] (PBG). The size
                                                                 and position of the band gap is dependent upon the
however arrays of pores can be grown to depths of over           wave polarization state, direction of wave propagation,
150 pm while maintaining a uniform diameter of the order         dimensions of the photonic crystal, and the dielectric
of 200 nm (figure 2).                                            contrast. The frequency extent of the band gap is of the
    After anodization the pores can be enlarged by repeated      order of the lattice spacing. Semiconductor materials are
oxidation and oxide stripping to form microstructures            ideal for the fabrication of PBGs because of their large
suitable for photonic band structure applications.               dielectric constant. It has also been shown that two-
                                                                 dimensional photonic lattices can have a three-dimensional
4. Quantum wires                                                 band gap. That is to say, the band gap remains open
                                                                 even when there is a large out-of-plane wave component
If the macropore structures are repeatedly oxidized, the         [71.
pores will eventually break through to their neighbours,              Photonic band structures with band gaps at optical
leaving free-standing silicon pillars at the comer sites         frequencies would have several interesting applications. An
between pores (figure 3). The diameter of the pillars can be     important property of PBGs is the ability to enhance or
reduced by further oxidation to form arrays of free-standing     inhibit spontaneous emission within the band gap energy
quantum wires (figure 4). Diameters as small as 150 nm           range. This has important implications for direct band gap
have so far been achieved.                                       optoelectronic devices such as semiconductor lasers and

5. Photonic band structures
                                                                 6. Experimental method
For certain periodic dielectric structures, the propagation of
electromagnetic radiation can become forbidden in certain        Due to the extremely small lattice dimensions and the
lattice directions. These structures are known as photonic       comparatively huge depth of air hole needed to create
                                          PBG transmittance

                                                                   angles measured fiom normal
                                                     3000             4000
                                                 Wavenumber (cm-l)
                                    Figure 5. Spectra scaled to blank silicon sample.

                                       Triangular    lattice of air rods in Si
                                                  (TE  modes f=45%)

                    i   6000                                                    .       . . . .I-I??h


                                     Figure 6. Band diagram for sample presented.

an optical band gap, it has proved extremely difficult to      of 40 pm with an estimated air filling fraction of 45%
fabricate photonic band structures in the near- or mid-        over a wide spectral range, and for various angles of
infrared (NIR/MIR) regions of the spectrum. We have            wave propagation. We used a specially modified Perkin-
applied the photoassisted anodic etching process to the        Elmer Fourier transform infrared (FTIR) spectrometer for
fabrication of a two-dimensional mid-infrared photonic         the measurements.
crystal based on a 0.81 pm triangular lattice of air               The PBG sample was mounted on a rotating stage and
rods in silicon.   We have tested the ‘through plane’          light from the ITIR spectrometer was focused onto the
transmission characteristics of a device grown to a depth      surface at the axis of rotation using a mirror and a reflecting
microscope objective. Part of the transmitted light was then      due to spectral noise, there is extremely good agreement
collected by an A& fibre which couples the light back into        between the detected and predicted band edges.
the spectrometer.     The fibre had an unusually large core
diameter of 150 pm. The light emerging from the other             7. Conclusion
fibre end was then reflected from a parabolic mirror and
focused onto a cadmium mercury telluride (CMT) detector           We have demonstrated            the fabrication    of extremely
within the FTIR spectrometer.                                     high aspect ratio silicon microstructures          by a process
     The ‘through plane’ transmission characteristics were        of photoassisted      anodization   used in conjunction      with
measured over a wide spectral range for various external          conventional     e-beam lithography.     We have shown these
angles of incidence with respect to the plane of the              structures to be suitable for applications           as quantum
structure. Spectral measurements were made with reference         wires and photonic band structures,           presenting  results
to a blank silicon spectra taken at each angle of                 which strongly imply that a photonic band gap has been
measurement.       This removes the effect of absorption          successfully fabricated in the mid-infrared region of the
bands in the fibre link and silicon substrate.         For this   spectrum. Spectral measurements of a structure based on a
particular sample, the PBG was found to be closed at              0.81 pm pitch triangular lattice of air rods with an air filling
normal incidence so we used this spectra as a reference           fraction of 45% show the presence of a band gap between
for measuring the level of attenuation (figure 5). For this       2 pm and 6.7 pm for wave propagation close to normal
sample we predict a band gap for TE modes only. It should         incidence to the plane of the structure. It has been shown
also be noted that the large external angular probing range       that the band gap for a two-dimensional           PBG structure
of 60” corresponds to an internal angular range of 14” due        can remain open as the out-of-plane wave component is
to refraction at the interface and accounting for the reduced     increased [7]. The angular range for which it remains open
refractive index of the device. Absorption bands within the       is dependent upon the geometry of the structure and air
fibre link appear as noise spikes on the spectra, the strongest   filling fraction. We find good agreement with the predicted
of which appears at 2500 cm-‘.                                    band gap.
     We clearly see that the PBG structure causes a strong
attenuation in the range 1500 to approximately 4800 cm-’
(2 pm to 6.7 pm) and that the level of attenuation increases
almost linearly as the angle of incidence moves away from         We thank Professor      H N Rutt (ECS Southampton
the normal to the plane of the lattice.                           University)  for the use of the FTIR spectrometer,        and
     Comparing the spectra with the theoretically predicted       also Dr D Hewak and T Schweizer (ORC Southampton
‘in-plane’ band diagram (figure 6) (calculated following          University)  for the loan of the spectrometer jig and
the plane wave method of Plihal er al [8] using 255               A$.$ fibre. Thanks also to Professor H Kemhadjian, P
plane waves), we find good agreement between the two.             Marchese and the staff at the silicon fabrication facility in
We notice however that the detected band gap extends              Southampton.
to a much lower wavenumber            than that predicted by
the band diagram.          Our optical arrangement     actually
launches uncollimated light into the device exciting all the      References
wave propagation modes within the plane of the lattice
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the lattice. We therefore compare the detected band edges         [41 Lehmann V 1993 J. Electrochem. Sot. 140 2836
with the average wavenumber of the lower two bands. The           PI Lau H W, Parker G J, Greef R and Helling M 1995 Appf.
                                                                        Phys. Lett. 67
‘average’ band gap extends from 1752 cm-’ to 4760 cm-‘.
                                                                  WI Yablonovitch E 1993 J. Opt. Sot. Am. B 10 283
The detected band gap extends from 1500-4800 cm-‘.                [71 Maradudin A A and McGum A R 1994 J. Mod. Opt. 41 275
Bearing in mind the ambiguity of the lower band edge              PI Plihal M and Maradudin A A 1991 Phys. Rev. B 44 8565

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