The present invention relates generally to layered material compositions and related structures, in particular to photonic lattices, and more particularly to a method for fabricating photonic lattices having photonic bandgaps in the optical spectrum.
Layered material compositions are materials which exhibit spatial variation of physical properties, composition, or other tangible characteristics, where that spatial variation produces useful bulk properties of the layered material compositions, and the spatial variation can be subdivided a stack of structured layers (the stack can consist of a single layer), which are assembled atop one another with appropriate alignment between the various structured layers. An individual structured layer can exhibit one-, two-, or three-dimensional variation of physical properties, so long as the surfaces of the layers are substantially flat.
Such layered material compositions exhibit a wide range of fascinating, unique, and useful xe2x80x9cbulkxe2x80x9d physical properties which result from the collective interaction of the spatially varying properties of their constituent materials. One of the most interesting classes of layered material compositions is the photonic lattice, which is a layered material composition which has a spatially varying index of refraction. Photonic lattices will be used as an example throughout this disclosure, and are described in some detail below. It is sufficient at this point to describe two primary optical phenomena which can be exhibited by photonic lattices. A photonic bandgap can appear, being a region of photon energy in which photons cannot propagate. Also, many types of photonic lattices will exhibit rapidly varying xe2x80x9cbulkxe2x80x9d indices of refraction in certain wavelength regimes. Both of these phenomena are the basis for many useful optical devices.
Many other classes of interesting and useful layered material compositions exist. For example, if a layered material composition has an appropriate spatial variation in, e.g., sound velocity or mass density, it will exhibits a phononic bandgap, i.e., a solid which does not allow propagating sound waves with phonon energies inside the phononic bandgap. It is also possible to produce structures exhibiting unusual and useful electronic properties, such as are associated with superlattices and other layered structures, but where the spatial variation in electronic properties is two- or three dimensional in character. It is also possible to build up two- and three-dimensional active and passive circuitry using the present invention. A further example involves the ability to control mechanical properties, including strength, by introducing a spatial variation in material characteristics on a small size scale. All such compositions, where certain bulk material properties depend intrinsically on the presence of the spatially varying physical properties within and between structured layers, are layered material compositions.
Throughout this disclosure Applicants will focus on the application of the present invention to the fabrication of a particular class of layered material compositions, namely photonic lattices. The term xe2x80x9cphotonic latticexe2x80x9d is used to describe any structure or material having bulk optical properties associated with a layered spatial variation of refractive index. This includes periodic, quasiperiodic, and aperiodic structures.
The best known property exhibited by some photonic lattices is a photonic bandgap. A material shows a photonic bandgap if there exists a region in energy-momentum space wherein propagating photon modes do not exist. Various structures can exhibit a partial photonic bandgap (a bandgap along some directions), a complete photonic bandgap (a bandgap along all directions, but which do not necessarily overlap in energy), a photonic stopgap (a range of photon energy in which photon propagation is not allowed along any direction), or no photonic bandgap at all. Photonic lattices which do not exhibit a bandgap can still have anisotropic and strongly varying bulk dispersion associated with the spatially varying refractive index. Such bulk optical effects can appear in strictly dielectric layered material compositions, in compositions comprising discrete regions of dielectric and metallic materials, and in various intermediate cases. Any structure exhibiting spatial variation of the local optical properties herein called a photonic lattice. If said structure can be split into a stack of structured layers, it is then also a layered material composition.
Photonic lattices are under investigation for applications in which their unusual interactions with electromagnetic radiation are useful. In their simplest form, such photonic lattices are based on a one-, two-, or three-dimensional periodic refractive index. (Recall that such periodicity is not required.) In such structures the propagation of electromagnetic waves is governed by multiple interference effects leading to wavelength-energy dispersion relationships similar to those describing the motion of electrons in solids. Traditional electron-wave concepts such as reciprocal space, Brillouin zones, dispersion relations, Bloch wave functions, and semiconductor bandgap have electromagnetic counterparts in photonic lattices. Defect states (which allow propagation of very narrow bandwidths in particular directions) can be introduced into the photonic bandgap by adding or subtracting a small amount of material from the ideal structure.
Perhaps the most significant property which can be exhibited by a photonic lattice is the photonic bandgap, a range of photon energies for which no propagating photon modes exist. This effect is analogous to the semiconductor bandgap in solids, which defines a range of energies in which propagating electrons cannot exist. Not all photonic lattices exhibit such a bandgap. Prediction of the properties of a photonic lattice can be carried out using techniques known in the art which are again analogous to those used to calculate electronic band structures in solids. Qualitatively, however, a wide photonic bandgap is encouraged by a number of factors, including:
1. Large ratio between largest and smallest refractive index in the photonic lattice.
2. The existence of continuous sublattices of low and high refractive index throughout the photonic lattice.
3. The volume fraction of the high refractive index sublattice should be less than that of the low refractive index sublattice.
The above list has been simplified by using language which implies the photonic lattice comprises discrete regions having distinct refractive indices. Such discreteness is not required, and any effect which will be discussed in this specification can be found in a photonic lattice having continuously varying refractive index. Also note that whereas the structures which are easiest to analyze are also infinite in physical extent, real photonic lattices have limited spatial dimensions, and as such are technically distinct from theoretical photonic lattices of infinite extent. We shall consider structures with limited physical extent which can be embedded in a photonic lattice of infinite extent also to be a photonic lattice.
The physics which governs photonic lattices and the formation of photonic bandgaps scales with changes in wavelength in a manner which allows (at least in principle) photonic lattices which exhibit bandgaps to exist on any size scale. Indeed, the first demonstration materials were designed for microwave frequencies, and were assembled from bulk epoxy and Styrofoam pieces. Later, silicon micromachining was used to fabricate photonic lattices active in the millimeter wavelength range. Until the present invention was developed, however, only crude demonstrations of two- and three-dimensional photonic lattices had been made which produced a bandgap in what we are calling the optical regime, which comprises optical wavelengths from roughly 20xcexc down to perhaps 0.1xcexc. (The long wavelength end represents the ultimate capability of conventional micromachining approaches toward fabrication, and the short wavelength end is defined by the lack of materials having sufficiently large electronic energy gaps for transparency.)
In this optical regime, fabrication of photonic lattices prior to the instant invention has been limited to three general types. First, the conventional xe2x80x9cdielectric mirrorxe2x80x9d, which is a stack of uniform thin films with differing refractive indices. Second, the formation of one- and two-dimensional photonic lattices through definition of features (usually cylinders or cylindrical holes) in a semiconductor substrate using photolithography for long optical wavelengths (e-beam or x-ray lithography has been used for shorter wavelengths), followed by etching to remove the high refractive index material in unwanted regions. In the final type of procedure, a very thin layer (1-3 periods) of three-dimensional photonic lattice has been formed in semiconductors by placing an etch masking layer on the surface, lithographically defining a 2-d periodic array of small holes in that layer, then applying an anisotropic etch along several (usually three) lattice axes to produce a three-dimensional structure commonly called Yablonovite. Properly designed, this structure exhibits a narrow stop gap.
The procedures described above allowed fabrication of photonic lattices having sufficient quality to confirm the basic theoretical ideas underlying their design and operation, but had a number of practical limitations. Overlying the problem of fabricating large-scale high performance photonic lattices is the requirement that the variation in refractive index accurately follow the model structurexe2x80x94variations (especially cumulative variations) relative to the model structure can lead to poor and variable response. Consider the anisotropic etch fabrication of Yablonovite outlined above. Production of high-quality Yablonovite depends on being able to anisotropically etch away holes of constant cross-sectional shape and size precisely along the desired axes.
In practice, however, attempts to make this structure have yielded structures with mediocre performance. It is clear that the cross-sectional area of the etched holes cannot be constant unless the etch is infinitely anisotropic, which is not the case. The nature of the etched holes will change substantially as soon as holes etched along different directions overlap. In fact, any misalignment, taper, or scatter of the holes leads to a progressive deterioration in the quality of the photonic lattice. A point is quickly reached where additional layers cannot participate in the collective definition of the desired optical properties, and only serve to scatter the light. Such problems are encountered even in microwave structures fabricated by conventional machining of epoxies.
When a photonic lattice is also a layered material composition, it is natural to try to simplify the fabrication process by growing individual structured layers, rather than trying to fabricate the entire structure at once. However, in practice the individual structured layers are severely impacted by variations in topography (e.g., lack of planarity) which appears as the result of nearly any growth technique or combination of techniques. These variations in topography disrupt the basic structure defined for each structured layer, and also alter the optimum alignment between structured layers from that predicted for a stack of the correct structured layers. These influences result in serious and cumulative structural errors when trying to fabricate photonic lattices using conventional multi-layer microelectronic fabrication techniques. These effects are essentially unavoidable when using conventional microelectronic fabrication techniques. An improved fabrication technique is sorely needed.
One can outline the requirements for a fabrication technique capable of making high performance photonic lattices (or other layered material compositions) of arbitrary dimensions. The difficulties involved with forming holes with large aspect ratios precisely directed along crystal axes are fundamental, and save for certain special purposes, such techniques are to be avoided. This suggests that a layer-by-layer process, in which the desired structure is grown and defined one structured layer at a time, should be developed. For such a fabrication process to be successful in making high-performance photonic lattices, however, it must allow precise definition and formation of features within each layer, it must allow precise control of layer thickness, it must insure layer planarity, and must allow the features within each layer to be precisely aligned relative to those on other layers in the desired structure. It is a given that any real fabrication technique will introduce errors. An additional requirement is then that such errors be small, and that their effect should preferably be random rather than cumulative in nature.
In the present invention, the thickness and planarity of each structured layer is controlled by a post-fabrication chemical-mechanical polishing step. In addition, the relative positions of subsequent structured layers are controlled by aligning the mask works either to a previous layer or to alignment marks on the substrate. (Global reference marks can be replicated as later structured layers are added so that alignment and position of structured layers can be traced to the came set of original reference marks.) As the remaining non-random structural errors do not accumulate fast enough to interfere with the proper function of the ultimate product, the present invention can be used to form layered material compositions of any dimensionality.
An advantage of the present invention is that layer-by-layer fabrication of layered material compositions with precise thickness, planarity, and alignment control is enabled thereby.
Another advantage of the present invention is that photonic lattices can be formed thereby having photon bandgaps within the range of several tens of microns down to perhaps 0.1 microns, thereby covering the regions of the optical spectrum commonly called the far-IR, the near-IR, the visible, and the ultraviolet.
An additional advantage of the present invention is that it allows fabrication of layered material compositions requiring structural features smaller than can be directly defined by the lithographic stepper mechanism.
A further advantage of the present invention is that it can be adapted to the fabrication of layered material compositions comprising a wide range of materials. Most metals, semiconductors, and insulators can be included in a layered material composition using the present invention.
Yet another advantage is that the present invention can be used to create a layered material composition over a large area. The ultimate area is that of the substrate used in the fabrication processxe2x80x94Applicants have demonstrated substantially uniform growth and optical properties in a photonic lattice grown over a 6 inch Si wafer.
These and other advantages of the present invention will become evident to one skilled in the art.
The present invention relates to a method for fabricating layered material compositions, said method comprising forming one or more structured layers exhibiting spatially varying physical properties; planarizing each structured layer by chemical-mechanical polishing after formation of that layer; and insuring proper relative alignment and positioning amongst the various structured layers.
Many structured layers can be formed by steps comprising depositing a layer of a first material, patterning the layer of first material to form an array of shaped openings therein, and depositing a second material to partially or completely cover the first material and to overfill all or a majority of the shaped openings. A fillet procedure and/or specialized etching techniques can often be adapted to fabricate structural features having feature size smaller than the resolution of the lithographic steppers.
The combined deposition can then be planarized with respect to the substrate surface by chemical-mechanical polishing to remove the combined deposit typically down to the level of the underlying first material. One of the first and second materials is used to form the spaced elements, and the other of the materials forms a spacer material separating or surrounding the elements. The spacer material can optionally be removed by a subsequent selective etching process step (e.g., removing an SiO2 spacer material using an HF/water etchant).
Additional advantages and novel features of the invention will become apparent to those skilled in the art upon examination of the following detailed description thereof when considered in conjunction with the accompanying drawings. The advantages of the invention can be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims and drawings.