Extrusion of polymer waveguides onto surfaces

A waveguide structure is directly extruded onto a surface from a nozzle placed a predetermined distance above the surface and which is moved relative to the surface, preferably by means of a translation table. The predetermined distance is preferably maintained constant and the speed of relative motion regulated to achieve a uniform degree of molecular orientation within the extruded material, thus maintaining a sufficiently uniform refractive index along the axis of the waveguide. Partitions within the nozzle allow the formation of a layered waveguide or the simultaneous formation of concentric cladding or protective layers. The waveguides are advantageously formed as a curtain which is later patterned, by direct writing on the surface or between chips mounted on an electronic module.

DESCRIPTION 
Background of the Invention 
1. Field of the Invention 
The present invention generally relates to the production of optical 
waveguides and, more particularly, to the formation of optical waveguides 
as a portion of another device, such as an electronic module. 
2. Description of the Prior Art 
Optical communications have become increasingly popular in recent years due 
to their large bandwidth and freedom from most forms of electromagnetic 
interference. Telephone and local or wide area digital communications 
networks are exemplary of such applications. Optical communication links 
have also been used in individual devices such as alarm systems where 
disturbance of the optical link is detected. Further, in some high 
performance electronic equipment, the high bandwidth and freedom from 
interference make optical communication very desirable for communication 
or distribution of clock and other high frequency digital signals. 
The structure of electronic circuit modules such as multi-layer modules of 
materials such as polymers or ceramics is also a suitable application for 
optical communication. These multilayer modules are capable of providing 
complex interconnection of a plurality of separate chips, each of which 
can be formed in accordance with mutually incompatible technologies and 
which may be operated over differing voltage ranges such as for bipolar 
and CMOS devices. The number of chips which may be accommodated is 
essentially arbitrary and it is not uncommon for a plurality of different 
clocks to be present on the same module. These clocks must usually be 
synchronized and may require synchronizing or master clock signals to be 
delivered at different voltages. If electrical signals are used, voltage 
conversion must often be provided which may, in turn, cause a significant 
delay of a synchronizing signal pulse. Therefore, optical communication of 
such signals is particularly desirable. 
Past attempts to provide a waveguide on a surface or within a layer of an 
electronic module have been less than fully successful since such a layer 
is typically applied by a so-called spin process. As an underlying surface 
of the device is rapidly rotated, a waveguide material is applied to the 
surface at the axis of rotation. The centrifugal forces due to the 
spinning causes a layer to be formed with high uniformity of thickness. 
The spin process is well-known and the process of choice for application 
of many diverse materials to different surfaces and in numerous 
applications. 
However, there are two principle drawbacks to the spin process for forming 
a waveguide. First, the spin process does not generally form a uniformly 
thick layer when the topology of the surface is other than planar. 
Differences in thickness or curvature of the surface of the layer may 
cause light loss or the pick-up of ambient light. Second, and more 
importantly, the spin process may radially stress the layer non-uniformly 
with distance from the axis of rotation. This stress, which is largely 
dependent on the molecular weight of the polymer, affects the alignment of 
molecules in the waveguide and causes the waveguide layer to be 
anisotropic. That is, a radial gradient of the refractive index over the 
distance from the spin axis may result. It has also been reported that a 
variation of the refractive index with angle about the spin axis will also 
be produced. Further, the optical waveguide generally must be patterned, 
and such patterning requires additional processing steps. Without 
patterning, the intensity of the communicated light (which may be outside 
the visual spectrum) diminishes significantly with distance from the 
source even when light losses can be kept low. On the other hand, the 
radial change of refractive index causes a change in refractive index at 
an angle to the axis of non-radial waveguides, causing increased light 
loss. Further, the surfaces formed by patterning are usually sufficiently 
irregular to scatter light and result in increased light loss. 
Additionally, spin coating cannot readily be accomplished after chips are 
in place on the module. 
It is known to form optical fiber waveguides by extrusion and to form 
cladding layers with differing refractive indices thereon by coextrusion 
as taught is U.S. Pat. Nos. 4,806,289 and 4,871,487, both issued to 
Laursen et al. However, the extrusion processes disclosed therein include 
steps for drawing the fiber in tension to establish a final 
cross-sectional size and uniformity of stress applied to achieve desired 
molecular orientation to obtain a uniform index of refraction along the 
optical fiber. This drawing process is therefore not compatible with the 
formation of an optical waveguide directly on a surface or to the solution 
of radial molecular orientation and resulting radial gradients of index of 
refraction due to application of optical waveguides to surfaces by spin 
coating. 
SUMMARY OF THE INVENTION 
It is therefore an object of the present invention to provide an 
alternative to the formation of waveguides on surfaces by spin processes. 
It is another object of the invention to provide a generally linear 
waveguide on a surface in a manner and of a structure which can 
accommodate severe surface topology without substantial light loss or 
signal interference. 
It is a further object of the invention to provide any of a plurality of 
waveguide structures on a surface by a substantially common process 
involving only minor variations corresponding to particular waveguide 
structures. 
It is yet another object of the invention to provide a waveguide on a 
surface of an electronic module which is compatible with other processes 
which may be required in the fabrication of such a module. 
It is a yet further object of the invention to provide optical 
communication in an electronic circuit module in a manner consistent with 
the electronic and structural design thereof and requiring no displacement 
of electrical structure. 
In order to accomplish these and other objects, the invention, in essence, 
provides for the direct extrusion of an optical waveguide onto a desired 
surface. In the course of such extrusion, the radial forces will 
inherently be substantially constant and the axial forces can be 
sufficiently regulated to avoid significant changes in refractive index 
along the waveguide. Further, an extruded waveguide is inherently linear 
and will deliver light of substantially undiminished intensity from a 
transmitter to a receiver over the short distances involved in an 
electronic circuit module. The extruded waveguide can be applied over 
conductors or a passivation layer applied thereover and between pins of an 
electronic module and thus, since such space is not otherwise usable, such 
optical communication effectively requires no "footprint" on the module. 
Further, the extrusion process is applicable to a large number of 
materials which can be chosen to avoid conflicts with other processes 
involved in the fabrication of electronic modules. 
In accordance which one aspect of the invention, a method of forming a 
waveguide on a surface is provided including the steps of positioning an 
extrusion nozzle a predetermined distance above a surface, forcing a 
viscous material through the extrusion nozzle, relatively moving the 
nozzle and the surface and maintaining a predetermined level of tension in 
said viscous material between said nozzle and said surface. 
In accordance with another aspect of the invention, a waveguide formed on a 
surface is provided by a process including the steps of positioning an 
extrusion nozzle a predetermined distance above the surface, forcing a 
viscous material through the extrusion nozzle, relatively moving the 
nozzle and the surface, and maintaining a predetermined level of tension 
in the viscous material between said nozzle and said surface.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION 
Referring now to the drawings, and more particularly to FIG. 1, there is 
schematically shown an exemplary apparatus 100 by which the invention can 
be practiced. The basic arrangement of this apparatus 100 is a mechanical 
arrangement by which relative motion can be achieved between an extrusion 
nozzle 130 and the surface of a substrate or layer 150. It is deemed 
preferable to achieve this relative motion by means of a translation table 
which carries the substrate or layer and is movable, for example, in 
orthogonal directions at high precision and controllable speed under 
control of a programmed computer in a manner well-understood in the art. 
This arrangement is preferred in order to minimize vibration and 
independent movement of the extrusion nozzle 130 in directions parallel to 
the surface which could introduce irregularities in molecular orientation 
of the extruded material. 
The actual extrusion of the waveguide material 120 which is preferably a 
polymer such as polyimide is controlled by a material pump and reservoir 
110 which also preferably includes structure such as heaters or mixing 
apparatus for maintaining the condition (e.g. viscosity, solvent content, 
etc.) of the waveguide material. This extrusion apparatus may also include 
nozzle height sensor structure 140, 145 in order to regulate tension in 
the extruded material as it traverses topological features, if any, of the 
surface upon which the waveguide is deposited. The nozzle height 
transducer 145 could be of a mechanical, optical or other type and is not 
critical to the practice of the invention as long as it can resolve 
whatever topological features may exist on the surface 150. The 
maintenance of an approximately constant predetermined separation of the 
nozzle and the surface is important to the practice of the invention since 
the unsupported length of extruded viscous material is placed in viscous 
tension by gravity and the relative movement of the nozzle and surface. 
This viscous tension largely determines the finished transverse dimension 
of the waveguide and also provides regulation of the degree of molecular 
orientation in order to achieve a uniform refractive index along the axis 
of the waveguide. Therefore it is important to the practice of the 
invention to maintain a predetermined level of viscous tension in the 
unsupported length of extruded material by regulating nozzle height and 
writing speed. As a practical matter, this is accomplished principally 
through the gap or separation between the extrusion nozzle 130 and the 
surface 150. It should be noted, however, that no molecular orientation by 
tension is required in order to successfully practice the invention since 
the orientation of refraction gradients engendered by spin coating are 
avoided by direct writing. Preferably, however, the degree of molecular 
orientation should be as constant as possible. 
Chips 160 are shown as being attached to the surface 150 in FIG. 1 as is 
possible in some embodiments of the invention. In any event, the space 
between chips 160 is not otherwise used and the extrusion of waveguides at 
locations other than chip locations, as is more particularly shown in FIG. 
2, therefore, requires no additional space of the substrate or layer 
surface. This feature of the present invention also allows optical 
waveguides to be retrofit onto existing modular circuit components and 
other devices. The chips 160 are generally attached on the surface 150 in 
a matrix pattern as shown in FIG. 2. Thus, the spaces between the chips 
forms an orthogonal array of potential paths for the optical waveguides 
180 in order to communicate with clock chips 165 (indicated by a filled 
rectangle) which are typically centrally located within associated regions 
of the surface 150. Further, if desired, the optical waveguide can be 
brought out to the perimeter of the module as shown at 190 to allow 
communication to or from other modules either directly or through optical 
couplers and additional optical links, the design of which may be in 
accordance with that disclosed in "Module Interconnection by Optical 
Fibers", IBM Technical Disclosure Bulletin Vol. 28, No. 1, June 1985, pp. 
237-238. However, the design of the external optical link, if used, is not 
critical to the practice of the invention. 
Referring now to FIG. 3, one embodiment of the invention will now be 
discussed. In this embodiment, a nozzle having a generally linear 
footprint is used to extrude a pattern of waveguide of any arbitrary width 
up to and including the entire width of the surface 150. In this latter 
case, the linear footprint of the nozzle would extend along the entire 
length of dashed line 210 to extrude a "curtain" 200 of waveguide. In such 
a case, the waveguide is preferably patterned. The linear nozzle causes 
some stressing of the waveguide material in a direction parallel to the 
linear nozzle opening. However, since the patterning of the curtain 
waveguide would preferably follow orthogonal paths and if the nozzle is 
oriented along a direction of one of those orthogonal paths, the angle of 
any gradient in the refractive index of the waveguide to the axis of the 
waveguide is minimized and light loss can be kept within usable limits. 
Incidentally, the formation of waveguides as a "curtain" which is 
thereafter patterned is the only circumstance which is necessarily 
incompatible with the presence of chips 160 on the surface 150. Although 
the desired routing of a waveguide may traverse a chip location in some 
designs and which would necessitate removal of the chip at such a 
location, if the waveguides are confined to locations between chips, it is 
immaterial whether the chips are present or not. It is also possible that 
the size or design of the extrusion nozzle might require chips to be 
removed (or attached after the formation of the waveguide) but the 
extrusion nozzles which have been used to date are generally of 
cylindrical exterior shape and of a diameter which only slightly exceeds 
the finished diameter of the waveguide and thus can successfully extrude 
waveguides with the chips in place. 
Alternatively, the width W can be limited to any desired degree, in which 
case patterning can be avoided. Also, change of index of refraction across 
the width of the waveguide is reduced as the width of the waveguide is 
reduced. Provision for turning the nozzle to coincide with the orthogonal 
direction of relative motion between the nozzle and the surface thus 
results in an extruded waveguide of good uniformity and low light loss. 
Distribution structures to split the input light may also be readily 
formed in this way. In practice, it has been found that if contact can be 
made between different extruded segments prior to the evaporation of 
solvent from the segments, the segments will coalesce to form a single 
waveguide. Alternatively, the drying of solvent from one segment will 
allow one waveguide to be overlaid on or to cross another. 
Regardless of the waveguide width, one or more partitions, such as 240 may 
be advantageously formed within the nozzle so that a plurality of layers 
220, 230 with different refractive indices (e.g. for cladding above and/or 
below the waveguide) or protective properties (e.g. resistance to organic 
liquids and greases) can be simultaneously formed. By formation of such a 
compound nozzle so that one layer is slightly wider than an underlying 
layer, one or more such cladding or protective layers may be made to 
enclose the edges of the waveguide, as well. 
In accordance with another embodiment of the invention, as shown in FIG. 4, 
the extrusion nozzle 130' may be of circular (or rectangular, as a 
variation of the nozzle of FIG. 3) shape and partitions 240' may be 
provided concentrically therein, as shown in cutaway cross-section. If 
materials of different refractive indices are extruded through such a 
nozzle, a concentric cladding for the entire periphery of the section of 
the waveguide is formed simultaneously with the waveguide. Such a 
structure, having a lower refractive index material cladding surrounding a 
higher refractive index waveguide, is necessary if the waveguide is to 
operate in a total internal reflection mode and particularly if the 
surface on which the waveguide is to be deposited has a higher refractive 
index than that of the chosen waveguide material. Alternatively or 
additionally, a protective material can be extruded to surround the 
waveguide of waveguide and cladding simply by providing additional 
concentric partitions. Either cladding or protective material, if opaque, 
can protect the waveguide from susceptibility to ambient light. 
It should also be noted that distribution structures such as splitters and 
couplers can also be formed in the same manner as the layered structure of 
FIG. 3. Alternatively, of course, separate waveguides could be extruded 
from a single transmitter such as at 190 of FIG. 2 to each of a plurality 
of receivers such as clock chips 165. In this regard, it should be noted 
that the nozzles and waveguides can be made very small and a substantial 
number of such waveguides can be placed in the gap between chips on a 
module at typical spacings thereof. Specifically, diameters of the 
extrusion nozzles are preferably in the range of 15-50 mils and the 
waveguide may be reduced further by providing an increased writing speed 
(e.g. movement of translation table 170) relative to the speed of the 
extruded material through the nozzle as well as by the spacing, h, 
preferably in the range of 3-5 mils, of the nozzle tip from the surface 
150, as shown in FIG. 4, as is also the case with the thickness of the 
waveguide formed in accordance with FIG. 3. In this regard, the final 
transverse dimensions of the waveguide will also be affected by extrusion 
pressure and material viscosity. 
The above techniques of extruding waveguides onto surfaces are largely 
independent of the material of which the surface is composed. Relatively 
severe topologies may be accommodated by providing for sensing of surface 
height. Even if the surface is relatively planar, it may be desirable to 
provide for such surface height sensing in the event one waveguide is made 
to overlie or cross another, as discussed above. The techniques according 
to the invention are also applicable to a wide variety of waveguide, 
cladding and protective materials such as polyacrylates, polycarbonates, 
polystyrenes, polyimides and other polymers. Virtually the only constraint 
on the material used is that for the "curtain" form of waveguide of a 
width which requires patterning and subsequent attachment of chips to the 
module, the cured material must be able to withstand the temperatures 
involved in soldering or otherwise attaching the chips. 
Photosensitive polymers such as photosensitive polyimides also provide the 
advantage of selective curing since such materials may be caused to form 
cross-linkages and become cured by exposure to light. This provides the 
advantage of being able to cure the waveguide prior to other heat 
treatment steps or rapid curing treatments such as microwave curing which 
may be necessary in regard to the remainder of the module. Such 
differential curing may also be advantageously used to control wall angle 
and profile of the waveguide structure especially in such rapid curing 
processes which might otherwise cause distortion of the waveguide. 
As shown in FIGS. 5 and 6, the cross-sectional shape and wall angle of the 
waveguide as well as the area of attachment to the surface can be 
controlled to a substantial degree before curing by treatment of the 
surface with surfactants or by other processes such as reactive ion 
etching or plasma treatment. Depending on the materials of the surface and 
the outer layer of the waveguide, the surface will be wetted to a greater 
or lesser degree by the extruded material. This wetting action will 
determine the wall angle 300 where the waveguide is attached to the 
surface. The use of surfactants, etching or plasma treatment generally 
causes an increase in this wetting action and will cause the final 
cross-section of the waveguide to resemble FIG. 6 more than that of FIG. 
5. The edges of the waveguide produced in accordance with the embodiment 
of the invention illustrated in FIG. 3 can also be adjusted in the same 
manner. In either case, the adjustment may affect both the optical 
properties and the structure and adherence of the waveguide to the surface 
or these effects may be separated by the use of cladding 310 around the 
waveguide 320. It should also be noted that the waveguide cross-section 
may be altered independently of the wall angle at the base thereof by heat 
treatment (e.g. above the glass transition temperature of the waveguide 
material) or adjustment of the viscosity of the extruded material or both, 
allowing the extruded material to sag somewhat under its own weight. Thus, 
a substantial degree of control of both wall angle and cross-sectional 
shape of the waveguide is provided by the present invention. 
In view of the foregoing, it is seen that the extrusion of waveguides 
directly onto a surface in accordance with the present invention provides 
a simple and inexpensive alternative to the formation of waveguides 
through spin processes with improved performance and which can accommodate 
severe surface topology. Any of a number of waveguide shapes, profiles and 
cross-sections and claddings and protective coverings can be formed by 
variations of the invention involving only choice of materials and 
substitution of nozzle structures. No changes in design, materials or 
structure of electronic modules is required by the practice of the 
invention and waveguides may even be retrofit onto existing modules in 
accordance with the present invention. 
While the invention has been described in terms of a single preferred 
embodiment, those skilled in the art will recognize that the invention can 
be practiced with modification within the spirit and scope of the appended 
claims.