The Road to Integration Introduction Optoelectronic devices are by jennyyingdi


									                                The Road to Integration

1. Introduction

Optoelectronic devices are nowadays mainly produced by means of laser welding.
With this technology all parts have to be aligned on some sort of optical bench and
are then fixed by welding with a laser beam. This leads to rather bulky components
as all component holders are made of steel parts for weld ability and mechanical
stability. All components have to be aligned and fixed in the sub micrometer range
which leads to rather high material, personnel and machine costs as well as reduced
yield. The whole assembly is then packaged in a hermetic housing with high
frequency feed-troughs.

The whole assembly setup has an analogy to the “old days” of electronic components
when radio tubes were standard devices. Like in the picture shown above, in the
photonics world many devices are still manufactured in analogy to a radio tube with
lots of mechanical parts and alignment steps. The manufacturing technology in the
photonics world is some decades behind the electronics world. But the photonics
manufacturing will go the same way and in future photonic packaging we will and
have to get rid off these “watch-and-clock-maker” devices which means we have to
move towards photonic integration.

2. Integration Schemes

If one wants to move towards the integration of photonic devices there are several
options to achieve this:

   § Silicon Optical Bench (SOB) or Hybrid Integration (HIT);
   § Monolithic;
   § Heterogeneous.

            The Road to Integration / AIFOTEC GmbH / Dr. G. Vollrath / Page 1 of 4
Here Hybrid Integration means the integration of one or more photonic components,
e.g. made from the quaternary semiconductor Indium-Gallium-Arsenide-Phosphide
(InGaAsP), on a Silicon bench. The bench is made from Silicon because it’s an
standard material in the electronics world (standard processes for e.g. contact pads
and resistors are available), has a good thermal conductivity and it’s easy to integrate
so-called V-grooves for fiber alignment and attachment. The Silicon motherboard
itself can be structured with standard electronic semiconductor equipment and
standard processes used for electronics and MEMS (Micro Engineered Mechanical
Systems). This hybrid integration approach can be used for photonic components
integration only (Photonic Integrated Circuits, PIC) or for the integration of electronics
and photonics (Opto Electronic Integrated Circuits, OEIC), as electronics dies can be
placed on the motherboard, too. The components can consist of different material
systems, different vendors can be used and different technologies. Therefore this
approach is a “best out of breed” technology for integration.

In contrast to this technology, for Monolithic Integration all the photonic and electronic
components have to be manufactured in InGaAsP. This material system is well-
known for photonics, but in it’s early stage for electronics and difficult to process as
one has to control a quaternary semiconductor instead of an elementary
semiconductor like Silicon. This approach also lacks of flexibility as all building blocks
like electronics, active photonics (laser diodes, photo diodes) and passive photonics
(multiplexer, demultiplexer etc.) have to be designed on one piece of InGaAsP and
the technological requirements for these components are very different. We therefore
believe this approach is maybe only a long term solution.

To overcome the tradeoffs when realizing all components in InGaAsP a
Heterogeneous Integration approach is possible. Here the photonic InGaAsP
components can be directly grown on top of a Silicon substrate. Because of physical
principles problems like lattice mismatch of the semiconductors face scientists to
some big challenges with this technology and therefore it’s still in his infancy.

AIFOTEC has focused on the most promising approach in terms of flexibility, time to
market and costs, the hybrid integration approach, called HIT, which stands as an
abbreviation for Hybrid Integration Technology.

The tabular below shows a short summary of these integration approaches. For
comparison also the standard, discrete approach is shown.

                    Scalability     Flexibility    Fiber          On-Chip           Mature
                    Automation                    Coupling    Interconnection     Technology
  Discrete              –               +            –               –                +
  HIT                   +               +            +               +                o
  Monolithic            +               –            +               +                –
  Heterogeneous         +               o            +               o                –

             The Road to Integration / AIFOTEC GmbH / Dr. G. Vollrath / Page 2 of 4
3. Hybrid Integration Technology (HIT)

AIFOTEC has developed a Silicon Platform Technology which consists of the
following innovative technology platforms which gives us a toolset to solve the
tradeoff between smaller devices with better performance and lower costs.

   §   Silicon Optical Bench (SOB);
   §   Spot-Size Converter Laser (SSCL);
   §   Laser Soldering Technology (LST);
   §   V-Groove Gluing Technology (VGGT);
   §   Glob Top Technology (GTT).

As the name implies, the SOB serves as an optical alignment bench and carrier for
the devices. These benches can be produced in large quantities using standard
electronic and MEMS manufacturing tools.

By using the SSCL technology we can enhance the alignment tolerances of the chip-
fiber-coupling. Normally, the chip has to aligned to the fiber in the sub-micrometer
range. By using a SSCL these alignment tolerances are lying in the one-micrometer
range. This is a prerequisite for using a SOB for the integration of optical devices like
lasers and V-grooves for the fiber. These tolerances enables the alignment, like in
the electronics industry, only by means of image recognition instead of active
alignment processes with a turned-on laser. Also the coupling efficiency is enhanced,
which enables a direct “butt-coupling” of the fiber and the chip. Therefore no optical
elements like lenses or lensed fibers are necessary. The alignment process is
reduced from several steps down to only one step with this technology.

LST enables soldering of the optical component on the SOB with a small laser spot.
The soldering itself gives a very reliable interconnection also for flip-chip setups with
small contact areas. The soldering itself is performed from the downside of the SOB.
Because of the small spot size of the laser with this technology arrays of laser dies
can be assembled on a common motherboard.

VGGT is used to fix the optical fibers on the SOB. By using the right glue and
process parameters in combination with an automated process VGGT leads to very
reliable and reproducible devices.

GTT eliminates the costly and large hermetic housing normally used for fiberoptic
components. The whole device is covered with a glob top – a process widely used
and accepted in the electronics industry. Because the whole optical path is enclosed
with no free space regions, no particles, dust etc. can disturb the light and
consequently a hermetic housing is redundant.

4. The FGL - a first Application of HIT

AIFOTEC’s first product out of this HIT toolset is the so-called Fiber Grating Laser
(FGL) which consists of a semiconductor gain element integrated with a spot-size
converter, giving a Spot Size Converter Laser (SSCL). With this setup the light is
directly launched into the fiber without coupling elements like lenses, lensed fibers
etc. The wavelength is selected by the Bragg Grating in the fiber. Also integrated is a
Monitor Diode (MD) and a 25 Ohms Radio Frequency (RF) matching circuit for

             The Road to Integration / AIFOTEC GmbH / Dr. G. Vollrath / Page 3 of 4
frequencies up to 10 Gbit/s. This setup enables high transmission distances and high
modulation speeds at low costs in combination with a wavelength grid suitable for
Dense Wavelength Division Multiplexing (DWDM).

5. Outlook

As already said, the FGL is only the first product out of the HIT toolbox. In the next
step towards a HIT Multi-Channel DWDM device is under development. The next
step is targeting the integration of multiplexer functionality by using a planar
multiplexer like a low-cost coupler or an AWG (Arrayed Waveguide Grating) for
enhanced performance. In addition to the integration of photonic functionality we will
also integrate enhanced electronics functionality like laser drivers etc.

Beneath the FGL application, AIFOTEC opened it’s technology platform to other
applications and offers development and production services based on this
technology to other telecom and datacom applications and also new markets besides
tele- and datacom applications.

6. Glossary

AWG           Arrayed Waveguide Grating
CWDM          Coarse Wavelength Division Multiplexing
DWDM          Dense Wavelength Division Multiplexing
MD            Monitor Diode
GbE           Gigabit Ethernet
FGL           Fiber Grating Laser
GTT           Glob Top Technology
HIT           Hybrid Integration Technology
LST           Laser Soldering Technology
RF            Radio Frequency
SOB           Silicon Optical Bench
SSCL          Spot-Size Converter Laser
VGGT          V-Groove Gluing Technology

              The Road to Integration / AIFOTEC GmbH / Dr. G. Vollrath / Page 4 of 4

To top