Integrated micro-interferometer and method of making the same
Technical field of the invention
The present invention concerns an integrated micro-interferometer with wave guides reproducing a Mach-Zehnder configuration type on a substrate of LiNbO3 (Lithium Niobate), and a method for its realization by means of the ion implantation technique with high energy of medium-light ions. The invention is to be used as scanning interferometer for Fourier interferometry and spectral analysis, as sensor of gas in trace (DOAS techniques), as detector of electromagnetic field. State of the art
It is known the usage of optical interferometers of Mach-Zehnder type and one example of them is described in the patent application US-Al -2004/0004721. The disadvantage of these instruments is that some their parts are in motion and it doesn't permit their usage in all situations. To overcome these disadvantages compact wave guide interferometers like those described in applications EP-0940698-A and CA-2314997-A were proposed. Hence, they present different constructive difficulties and the impossibility to realize a series of them on the same substrate. Advantages of the invention
The main aim of the present invention is that to overcome the difficulties present in the state of the art. Moreover, applying the proposed method of making the present invention permits to obtain a device of small dimensions and weight (some gram for a single chip), with reduced consumptions (some mW), to have the possibility to be equipped with compact electronic parts (lay-outs smd) and with spectral resolution comparable to that of analogous conventional instruments.
Some characteristics and advantages of the invention will be clear from the following description regarding some methods of making, illustrated on the figures 1, 2, 3 and 4, which are given as a non- limitative example.
Brief description of the figures
Figure 1 represents a scheme of integrated micro-interferometer, object of the present invention.
Figure 2 represents a series of micro-interferometers, object of the present invention, integrated on a unique substrate.
Figure 3 represents a transversal section of an integrated channel guide, object of the present invention, when it is realized by means of ion implantation using a single-layer mask.
Figure 4 represents a transversal section of an integrated channel guide, object of the present invention, when it is realized by means of ion implantation using a multi-layer mask.
Detailed description of an embodiment of the invention
According to the figure 1, the waveguides (2) are obtained on a LiNbO3 (Lithium Niobate) (1) substrate. The waveguides are realized using the technique of ion implantation with high energy of medium-light ions by means of particle accelerator. The waveguides form reproduces a Mach-Zehnder type configuration, constituted by an input waveguide (3) which presents a first "Y" bifurcation (4), two parallel waveguides (5) which accord with a second overturned "Y" bifurcation (6) to output waveguide (7). On the sides of at least one of the two parallel waveguides (5) pilot electrodes (8) for a tension ramp application are placed.
Contrary to the classic solution in the configuration of Mach-Zehnder type which previews the phase shift of the optical paths by means of mobile glasses, in the case of integrated micro-interferometer the phase shift on the Lithium Niobate substrate (1) is realized making use of electro-optical properties of the material the waveguides (2) are made of.
The phase shift of the optical paths is realized applying to at least one of the arms of the interferometer an electrical field which modifies the refraction index of the waveguide. The obtained phase shift is proportional to applied tension: therefore, in order to perform a scanning, a tension ramp which amplitude depends on the dimensions of the pilot electrodes and spectral resolution to obtain is applied.
According to figure 2, the angle of the "Y" bifurcations (4)(6) of the Mach-Zehnder geometry can be even inferior to 2°, therefore, on the same substrate more than one device can be realized (for example, even more than 20 devices on a 3" commercial wafer).
According to figures 3 and 4, the ion implantation is performed using a mask realized by one or more layers, at least one of which is constituted by a metal (11) which density is > 2g/cm3. These layers make so that the ions which pass through the metal terminate their path on a precisely established depth (10) (more near to the substrate surface), different from the depth (13) achieved by the ions which make incidence in the points of the substrate where the mask does not cover the niobate surface. In such a way, choosing the opportune thicknesses according to the implanted ions and implantation energy, the lateral definition and at the same time the horizontal definition (parallel to the surface) of the channel guides are obtained, by means of the ion implantation process. The total thickness of such layers depends not only on the incident ions and their atomic weight, but also on the density of the elements which constitute them. Usually if gold, platinum or elements with elevated densities (e.g. >10 g/cm3) are used, then such thicknesses vary from some hundred of nanometers to two-three microns according to the used energy and, hence, to the implantation depth. For less dense materials such thicknesses are normally of the order of some micron.
According to figure 4, in the case where the layer of the material (11), suitable for the realization of the lateral walls of the guides, doesn't have a good level of chemical adhesion with the Lithium Niobate substrate (1), a layer of the material (12) of few tens of nanometers having direct contact with the Lithium Niobate substrate (1) is used. The main characteristics of this layer is that to realize a good adhesive between substrate and subsequent mask layers. If noble metals (Au, Pt) are used as subsequent layers, examples of such material are represented by titanium, Ni-Cr alloys ecc. The usage of such layer is functional to the adhesion properties of the subsequent layers and, hence, it is not necessarily requested. In fact, if the first of the layers has good adhesion capabilities on the niobate surface the accessory adhesive deposition will not be necessary. An example on this regard can be
given by materials like niobium and tantalum, able to be deposited directly on the Lithium Niobate surface to which they adhere without difficulties.
The channel guides (and, therefore, a certain number of integrated devices) are realized by means of a unique implantation process in the Lithium Niobate substrate (the mask defines all the guide walls) according to the requested geometry and used mask.
Afterwards, such mask may be a) or maintained to realise the pilot electrodes (8) directly, b) or eliminated and followed by further process of deposition and masking in case a different configuration of pilot electrodes is requested (8).
For example, the embodiment of integrated micro-interferometer previews the following steps: a) As substrates a 3" LiNbO3 wafer (1) with an "X-cut" orientation such that the guides are directed along the Y axis and the electric field is directed along the Z axis, or a wafer Of LiNbO3 (1) with an "Y-cut" orientation such that the guides are directed along the X axis and the electric field is directed along the Z axis are used. b) The mask deposition which in the case of Ti - Au mask has one Titanium layer with thickness comprised between IOOA and 5000A and one golden layer with thickness comprised between 5000A and 30000A is performed. c) The design of Ti - Au mask is defined by means of photolithographic techniques according to negative mask (the part of the material which will act as waveguide is not covered) d) The high energy ion implantation of medium-light ions of one of the following species: B, C, N, F, Al, Si, P and Cl is performed; e) The ions are implanted with energy values comprised between 500keV and 10MeV with fluences comprised between l*1012 ioni/cm2 and l*1017 ioni/cm2, and with flow comprised between l*107 ioni/cm2*s and l*1014 ioni/cm2*s.
f) The annealing in a not reducing atmosphere to temperatures comprised between 150 0C and 7000C is performed. g) Cutting and lapping of the surfaces are performed. h) It is proceeded with the elimination of the mask near the "Y" bifurcations (4) and (6). i) Optical input and output fibers are inserted.
This production method permits the maintaining of a good electro-optical coefficient, near to the nominal value of the virgin material. The ion implantation process followed by opportune annealing permits to maintain a linear electro-optical coefficient of r33 = 28.0 pm/V, while the nominal value of the virgin material is r33 =32.0 pm/V. The conventional techniques of channel waveguides realization by means of ion exchange (titanium, hydrogen, ecc.) reduce such coefficient to values inferior to the half of the initial value of the material. With conventional techniques only with very long annealing processes (efficient, by the way, only in the case of the protonation) such coefficient value will be higher.
Besides, using such method the geometry which approximates very well that of rectangular symmetrical channel guide is obtained; such geometry is obtainable only with ion implantation processes, and it can be realized with no diffusion process, which, given the characteristics of Lithium Niobate is always asymmetric (the diffusion depths depend on the crystallographic direction along which the drug diffuses). To this it is necessary to add the possibility to produce guides with reduced dimensions of the section (es. 3μm x 3μm), monomodal also to the wave lengths near to the inferior limit in wave length of the Lithium Niobate transmittance (λ=360 nm).
The guides produced with implantation don't present preferential polarizations, they are, namely, able to guide both TE and TM modes, what makes them particularly suitable for the launch in fibre of polychromatic light, avoiding the losses due to polarization states of injected light and making unuseful the usage of polarization maintaining fibres, with evident advantages during the fibre insertion processes.
The invention, certainly, is not limited, to the representation of the figures, but can receive perfections and modifications from men skilled in the art, without going out of the patent frame. The present invention permits numerous advantages and, particularly, allows to overcome the difficulties that could not be superated using the systems that are actually in commerce