Optical waveguides – Integrated optical circuit
Reexamination Certificate
2002-04-23
2004-12-14
St. Cyr, Daniel (Department: 2876)
Optical waveguides
Integrated optical circuit
C385S052000, C385S105000
Reexamination Certificate
active
06832013
ABSTRACT:
This invention relates to an assembly structure and a method for assembling active and passive photonic and/or optoelectronic devices on a silicon board. The invention relates in particular to an assembly structure and a method for aligning the photonic devices during the assembling procedure. The resulting components will be used especially for broadband telecommunication components.
The assembly of photonic components is a very difficult and costly process. State of the art technology positions an optical fibre relative to a photonic component by an active alignment procedure. This means that during the alignment procedure the components are manipulated by highly accurate stages (±0.1 &mgr;m). The incoming or outgoing light signal is monitored during micromanipulation until the optimum of transmitted light has been determined. Then the fibre is mechanically fixed employing different techniques. The last assembly step is to put the fibre-terminated component into a housing, which then can be hermetically sealed in order to protect the semiconductor component. The housing provides electrical as well as optical feed-throughs. The fabrication of feed-throughs for the optical fibre is a process, which is difficult to manufacture, too.
The assembly is of photonic components is, due to the very demanding manufacturing steps time consuming and makes up typically 80% of the device cost. Much could be won if one used planar light guiding circuits (PLCs) for broadband telecommunication systems not only as passive components. PLCs have a very attractive potential to increase functionality by directly mounting semiconductor components onto the PLC board. But most importantly this concept, which in the literature is referred to as hybridisation could also facilitate the assembly of components and the subsequent hermetic sealing components to a high degree.
A typical application of a hybridly integrated photonic component is shown in
FIG. 1
where a pump laser device
2
is attached onto a PLC board
10
, which also hosts a waveguide
4
leading the light from the laser to other parts of an planar lightwave circuit (PLC). The alignment and assembly of the semiconductor component is achieved by a simple “clip-on” procedure, which does not require active alignment and therefore will greatly decrease assembly costs.
An inherent advantage of the hybrid integration concept is that it allows the light from the pump laser to be coupled directly into the waveguide whereas with conventional technologies each component has to be connected to a fibre separately, put into a housing and be connected using optical fibre connectors. The state-of-the art concept leads to bulky and difficult-to-handle components whereas the hybridisation leads to highly integrated and compact modules.
In the efforts to fully exploit the potential of PLCs one question is key; how does one obtain a long-term mechanical stability between the photonic component and the waveguide structure on the PLC board, and how can the active alignment procedure be avoided? This question has created a technical challenge for the fiberoptics components industry. The necessary coupling tolerance is in the submicron region and the fixation needs a mechanical stability of less than +/−0.1 &mgr;m under all possible operation and storage conditions.
In the prior art there are several attempts to achieve fastening of photonic components such as lasers to a PLC board by passive alignment.
Methods using highly accurate Flip-Chip Bonding machines have been developed. These make use of optical detection of fiducials (alignment marks) on the substrate and photonic component. Registration and positioning are however time consuming and equipment is very expensive.
Self-aligned bonding which make use of micromachined V-grooves for fibre fixation and the surface tension forces of the molten solder material, has been proposed and taken up by research facilities around the world. The method has been proven to reach the required tolerances. Nevertheless, the needed accuracy, which is within 1 &mgr;m, requires extremely well controlled process tolerances on soldering and V-groove micromachining, which will be costly to develop.
One of the alignment concepts that make use of etched alignment structures and surface tension forces of the molten solder material, is presented in U.S. Pat. No. 5,656,507. Here the silicon substrate is prepared with a waveguide, two alignment stops, a V-shaped groove and a trench with an L-shaped metal pad in the bottom. The bottom of the laser holds a ridge and an L-shaped metal pad with a solder bump so as to fit the V-shaped groove and the L-shaped metal pad on the silicon substrate. The principle is that when the laser is placed on the silicon substrate with the ridge inserted in the groove and an edge abutted to the alignment stops, the two L-shaped metal pads are slightly displaced though connected by the solder bump. When the solder is melted, it will draw the laser into alignment with the waveguide through surface tension forces.
Many companies have shifted their focus to alignment concepts, which require highly accurate pick and place machines in conjunction with alignment fiducials on the assembled parts (see H. L. Althaus et. al., “
Microsystems and Waferprocesses for Volumeproduction of Highly Reliable Fibre Optic Components for Telecom and Datacom Applications”,
47
th
ECTC Conf., San Jose, Calif., 1997, pp. 7-15). These concepts are very application specific and require large investments that only pay off for large production volumes. Parallel to that, an increasing number of publications emerged proposing a concept which makes use of dry- or wet-etched alignment structures, e.g. D. A. Ackerman (U.S. Pat. No. 5,023,881), J. Gates et al. (“
Hybrid Integrated Silicon Optical Bench Planar Lightguide Circuits”,
48
th
ECTC Conf., Seattle, Wash., 1998, pp. 551-559) and S. A Merrit (“
A Rapid Flip
-
Chip Bonding Method for Semiconductor Laser Diode Arrays”,
48
th
ECTC Conf., Seattle, Wash., 1998, pp. 775-779).
U.S. Pat. No. 5,023,881 covers the use of pedestals which initially forms a gap in-between the laser and the substrate. The vertical alignment is achieved by placing the laser on top of two pedestals, and the precision relies on the thickness of numerous individual layers. The specific alignment step consists of a cold welding for lacking the laser during the subsequent soldering. The horizontal alignment is not addressed in this patent and needs to be realised presumably by micromanipulation (“in a predetermined way”).
Another alignment concept, which makes use of etched alignment structures, is presented in U.S. Pat. No. 5,721,797. The patent discloses a method for aligning a laser relative to a fibre or a waveguide. Here, only the method relating to the waveguide is of interest. Two trenches are etched into the silicon substrate to later host waveguide structure (1
st
trench) and laser-mounting site (2
nd
trench). Then the second trench is filled with solder material, the first trench with cladding material for the optical waveguide. The procedure ensures that the waveguide core is at the same height as the light emitting core of the laser waveguide (vertical alignment). At the same time as the waveguide core material three alignment stops are formed to which the loser component will abut during assembly in order to achieve the horizontal alignment. The fabrication step is finalised by putting the top cladding onto the core waveguide material.
When the laser is then mounted on top of the solder (deposited into the 2
nd
trench) the component is slithered towards the three alignment structures to ensure horizontal alignment. It is important to note that it is the sides of the laser, which abut the alignment stops, and the thickness of the solder deposition, which defines the horizontal and vertical alignment respectively. Applying heat to the assembly, thereby melting the solder the laser is mechanically fixed onto the silicon substrate.
Often, hybrid integration apparatuses are not comp
Kuhmann Jochen Friedrich
Poulsen Mogens Rysholt
Cyr Daniel St.
Fish & Richardson P.C.
Hymite A/S
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