(C) 2000 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional
purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in
other works must be obtained from the IEEE
828
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 7, JULY 2000
Fiber Ribbon Alignment Structures Based on
Rhombus-Shaped Channels in Silicon
Martin Hoffmann, Member, IEEE, Sabine Dickhut, and Edgar Voges
Abstract—A fiber ribbon alignment structure based on bulk
silicon micromachining technique with precise rhombus-shaped
channels in 100 -silicon is demonstrated. The main advantages
are totally buried and thereby protected fibers and a less structured wafer surface suitable for the integration of components
on top. The presented structure allows a simplified assembly of
fiber ribbon arrays due to integrated funnels for fiber insertion.
The accuracy of the channels within an array was found to be
0; 2 m. Furthermore, this technique allows combining fiber
ribbons with a pitch of 250 m with photodiodes for system
monitoring in a single fiber pigtail.
f
g
6
Index Terms—Integrated optics, monitoring, optical fabrication,
optical fibers, photodetectors.
Fig. 1. Alignment structures in silicon for optical fibers. (a) V-groove. (b)
Rhombus-shaped channel. (c) U-groove (widths w are given for a core depth
of d f
62:5 m).
=
I. INTRODUCTION
RRAYS of components or multichannel devices such as
arrayed waveguide gratings have to be coupled to a large
number of fibers. Although passive technologies for the fiberwaveguide coupling have been investigated, most of the devices
are still actively aligned using multifiber arrays. Anisotropically etched V-grooves (e.g., [1]) in silicon or precision-machined glass blocks (e.g. [2]) are widely used for this purpose.
These technologies guarantee low-loss fiber coupling of optical
devices. A drawback of V-grooves is that the vulnerable bare
fibers are placed on top of the wafer. Also alternative technologies [3] were developed to fix the fibers mechanically within the
V-grooves.
Here, a rhombus-shaped channel is proposed for this purpose, which is fabricated in a bulk micromachining process.
The fibers are protected in tube-like cavities below the wafer
surface. f111g-equivalent planes within standard f100g-silicon
form the sidewalls of the tubes. A comparison of a V-groove,
a rhombus-shaped channel, and a U-shaped groove is shown in
Fig. 1. The trench width w at the wafer surface is given for a
standard fiber (diameter 125 m) that is totally buried below the
wafer surface. Arrows indicate the contact lines between fiber
and silicon.
The main advantages of the rhombus-shaped channels are the
encapsulation of the bare fiber, the flat wafer surface suitable for
the hybrid integration of other components, and, nevertheless,
an easier mounting of fiber ribbons. The assembly is simplified
by horn-like structures that appear during wet etching at the end
of the tubes.
A
Manuscript received February 7, 2000.
The authors are with the Universität Dortmund, Lehrstuhl für Hochfrequenztechnik, D-44227 Dortmund, Germany (email: [email protected]).
Publisher Item Identifier S 1041-1135(00)05613-5.
Fig. 2. Prestructuring for rhombus-shaped channels. The dashed lines indicate
the final f111g-etch stop planes.
II. RHOMBUS-SHAPED FIBER CHANNELS IN (100) SILICON
Rhombus-shaped channels occur whenever a mask opening
on f100g silicon that typically results in a V-groove is at first
structured in a vertically anisotropic step (Fig. 2) and then
etched in anisotropic etches such as KOH. Rhombic channels
were first demonstrated in silicon using laser melting [4].
Alternatively, deep silicon etching as well as wafer dicing
can be used for the prestructuring step. The etching stops at
f111g-planes, which are arranged under an angle of 654.7
against the surface forming a rhombus. The convex edge at the
wafer surface is not etched because a f111g-plane meets the
highly stable mask.
The vertical-anisotropic prestructuring step causes a deep
crystal damage that allows the wet etch to attack deeper
areas of the wafer. A rectangular damage such as a trench in
f100g-silicon aligned along the (110)-flat is surrounded by
f111g-planes in a rhombus-shaped manner (Fig. 2). The size
of the rectangular trench directly defines the shape and size of
the rhombus. A small but very deep trench for example results
in a nearly closed rhombic channel, while a broad but shallow
trench ends up in a V-groove.
Considering a rectangular trench, the final size of the
rhombus is defined by two parameters as shown in Fig. 2: the
width w of the opening at the wafer surface and the depth d of
the trench.
1041–1135/00$10.00 © 2000 IEEE
(C) 2000 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional
purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in
other works must be obtained from the IEEE
HOFFMANN et al.: ALIGNMENT STRUCTURES BASED ON CHANNELS IN SILICON
The minimal depth has to be
2r 0 w 1 tan() = p3 1 2r 0 p2 1 w
cos()
for = 54; 74 in f100g-silicon (1)
considering a fiber diameter 2r . A standard single-mode fiber
is completely buried below the wafer surface for d = 2r =
125 m and w = 2r tan(=2) = 64:5 m. A comparable
d=
V-groove would be as wide as 241 m, a U-groove requires
125 m (see Fig. 1). It should be noted that this calculation is
valid if the trench width is equal to the mask opening. For milled
trenches smaller than the mask opening, the depth has to be increased properly.
The mask opening w precisely defines the two upper
f g-planes of the rhombus. They provide the necessary
precision for the alignment of single-mode fibers. If w
varies by
p w , the fiber core position is vertically shifted by
1 w . For this reason, the achievable accuracy of a
y
fiber pigtail can directly be examined by measuring the widths
w of the slit after final KOH etching.
The position of the two f
g-planes at the bottom of the
rhombic channel is strongly affected by the shape and size of the
prestructured trench. These planes are not suitable for a precise
alignment of the fiber.
829
TABLE I
MEASURED ACCURACY OF THE WIDTH w WITHIN ARRAYS
RHOMBUS-SHAPED CHANNELS
OF
13
111
1
1 = 2 1
111
Fig. 3. Photograph of a single-mode fiber glued with UV-curing resin into a
rhombic channel.
fibers is highly precise due to the use of a lithographic mask
with a resolution of 50 nm. As shown before, the width w is a
direct measure for the absolute error of the f
g-plane positions. It is determined from a serial measurement of the trench
widths after KOH-etching using a Leitz MPV-CD2 linewidth
measuring system. The results of six arrays with 13 channels
each are given in Table I. wmin and wmax represent the smallest
and largest measured width w , respectively. The acceptable radial tolerance for an additional insertion loss of 0.2 dB per coupling is 60.7 m for single-mode fibers. All tested arrays are
far below this limit.
After fiber assembly, the quality of the mounting process is
proven by cutting the glued pigtail. No visible residual film
between the fiber and the upper V-groove is found and the
cavity is completely filled with hardened resin (Fig. 3). Note
that the rhombus is oversized and that the fiber is precisely
aligned against the upper f
g-planes.
The accuracy of an eight-channel fiber pigtail using
rhombus-shaped silicon channels is examined using an integrated-optical silica-on-silicon 1 2 8 splitter [5]. It is assumed
that the core positions of the 1 2 8 splitter are perfect, i.e., the
pitch is 250 m and all waveguides are arranged in a plane. The
1 2 8 splitter and the ribbon pigtail are aligned against each
other for an optimum coupling of the channels 1 and 8. After
that, each channel is separately tuned to maximum coupling
efficiency. The necessary change of the pigtail position is
read out. x is related to the accuracy of the pitch and y
represents the deviation of the core relative to the wafer surface.
The results are given in Table II. They include misalignments
caused by the rhombic channels itself, the fiber gluing process
and inaccuracies of the fibers (diameter and core position). The
small necessary adjustments show the high quality of these new
pigtails and their suitability for single-mode fiber coupling.
Extremely thin layers of glue between fiber and silicon cause
the slightly higher deviations in y -direction.
111
III. FABRICATION
Rhombus-shaped pigtails are fabricated on 100-mm
f100g-silicon wafers coated with silicon nitride. The trench
(110)
mask is carefully aligned along the
-flat and reactive ion
etching opens the silicon nitride mask. This step determines
the overall accuracy of the rhombic channels. After that, the
necessary trench is achieved by milling with a wafer-dicing
saw. The cut is centered within the mask opening. The typical
width of the cut is 50%–70% of the nitride mask opening to
avoid any damage of the nitride mask. Its depth is necessarily
somewhat larger than the value calculated from 1. For an easier
assembly, additional trenches are introduced perpendicular to
the channels. These cuts cause the forming of funnels at the
end of each chip. After milling, the rhombus is etched in 20%
KOH solution at 60 C in about 2 h. It must be stressed that the
sharp edges along the channels are not damaged.
The fibers are inserted from one end face using the funnels
at the perpendicular trenches. Single fibers as well as fiber ribbons can be threaded easily without further tools. Once introduced into the channels, the fibers are hold in the nearly closed
tube. After adding glue, the fibers are slightly pressed into the
upper V-groove. The excess glue can escape through the open
groove. A variety of glues can be used to fix the fibers in the
rhombus, especially epoxy resins and UV-curing glues. The
UV-transparent quartz fiber and the highly reflective silicon surfaces allow UV-curing even through the 65-m gap.
IV. RESULTS
A large number of rhombic channel arrays as well as complete
fiber ribbon pigtails using these arrays have been fabricated and
tested. It was found that the edge between f
g-plane and etch
mask at the wafer surface is very stable. The pitch between the
111
111
1
1
(C) 2000 IEEE. Personal use of this material is permitted. However, permission to reprint/republish this material for advertising or promotional
purposes or for creating new collective works for resale or redistribution to servers or lists, or to reuse any copyrighted component of this work in
other works must be obtained from the IEEE
830
IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 12, NO. 7, JULY 2000
TABLE II
MEASURED HORIZONTAL
x AND VERTICAL y ACCURACY OF THE
FIBER CORE POSITION WITHIN AN 8-CHANNEL PIGTAIL
(1 )
(1 )
V. APPLICATIONS
Rhombus-shaped channels can replace V-grooves in fiber
pigtails. Rhombic pigtails protect the fibers without requiring
an additional plate on top of the grooves. Therefore, the
thickness of the pigtail is given by the thickness of a single
silicon wafer (typically 525 m). The rhombi especially reduce
the effort for the fabrication of multifiber pigtails because the
fibers cannot change their position within the array.
The same chip can also be used for multifiber splices. In this
case, two fiber ribbons or fibers are introduced from both sides
and then glued. Alternatively, the fibers can be hold in place by
low pressure, e.g., during measurements. This allows the use of
rhombic channel arrays for temporary splices without gluing.
It is a fast and cost-effective alternative to conventional fiber
splicing, especially for temporary use. Furthermore, the position
of the fibers is precisely referenced to the surface of the wafer.
This makes the channels suitable as ferrules for fibers in microoptical systems and connectors.
The remaining surface of about 75% (for 250-m fiber pitch)
of the wafer after channel fabrication allows to integrate further components on the chip. One application is a dense monitoring pigtail for multichannel communication systems. It combines rhombic channels with short V-grooves as shown in Fig. 4.
Monitoring channels with an integrated mirror/photodiode array
are placed between the fibers for the through-traffic. The width
of the V-grooves is chosen with respect to the field of a gaussian
beam between waveguide endface and mirror. The remaining
silicon cantilevers have a thickness of approximately 35 m.
This dense monitoring pigtail does not require any modification
of the integrated optical chip to which it is coupled. In addition,
fibers between integrated optical chip and monitor diodes are
avoided. First silicon devices were fabricated. Photodiode arrays will be placed at the end of the V-grooves where the light
is reflected into the photodiodes.
(a)
(b)
Fig. 4. Dense monitor pigtail with integrated array of photodiodes for
coupling to i/o devices. (a) Principle. (b) SEM-photograph with fibers for
the through-channels and mirrors for the monitor channels (shown without
photodiodes).
VI. SUMMARY
A new type of precise alignment structures for optical fibers
is presented. The convex corner between upper f111g-planes
and wafer surface is highly stable during etching. The expected
average deviation of the vertical fiber position due to tolerances
of the rhombi is as low as 0.15 m. Rhombic channels simplify
the fabrication of multifiber ribbon pigtails and are also suitable
for temporary splices. As an advanced application, a ribbon pigtail with integrated photodiodes is demonstrated.
REFERENCES
[1] S. A. Bailey, C. A. Jones, M. W. Nield, K. Cooper, I. P. Hall, and A. C.
Thurlow, “Recent advances in fiber-to-fiber and fiber-to-waveguide interconnect technology,” Int. J. Optoelectron., vol. 9, no. 2, pp. 171–177,
1994.
[2] Y. Hibino, F. Hanawa, H. Nakagome, M. Ishii, and N. Takato, “High reliability optical splitters composed of silica-based planar lightwave circuits,” J. Lightwave Technol., vol. 13, pp. 1728–1735, Aug. 1995.
[3] C. Strandman and Y. Bäcklund, “Passive and fixed alignment of devices using flexible silicon elements formed by selective etching,” J.
Micromechan. Microeng., vol. 8, pp. 39–44, 1998.
[4] M. Alavi, S. Büttgenbach, A. Schumacher, and H.-J. Wagner, “Fabrication of microchannels by laser machining and ainisotropic etching of
silicon,” Sensors Actuators, vol. A32, pp. 299–302, 1992.
[5] M. Hoffmann, P. Kopka, and E. Voges, “Low-loss fiber-matched lowtemperature PECVD waveguides with small-core dimensions for optical communication systems,” IEEE Photon. Technol. Lett., vol. 8, pp.
1238–1240, Sept. 1997.