Study On Key Technologies For Silicon-based 8×8MEMS Optical Switch Array

It is becoming the next development trend of the communication technology to construct multiwavelength all optical network (AON) so as to meet the further demands of capacity and velocity for the communication network. Optical switch plays an important role in AON, which can implement optical cross connect (OXC), optical add drop multiwavelength (OADM), besides switching protection and monitoring network. Among various schemes of optical switch, MEMS optical switch has many advantages, such as low insertion loss and low crosstalk, furthermore, MEMS optical switch matches with the trends of transparency and expansibility of the optical network in the future, because of its independence of format,wavelength,protocol,modulating method,polarization, and transmission direction of signals. So it is demonstrated that MEMS optical switch will become the main device in AON.2D(two-dimension) electrostatic actuating 8×8MEMS optical switch array is studied in this paper, which switch the signals by applying actuating voltages to the actuators associated micromirrors, as a result, the incoming optical signal is reflected into the selected output port. Optical switch array device is composed of upper electrodes chip, low electrodes chip and fiber collimators. Micromirrors and slanted low electrode array are fabricated on (110) silicon and tilting 4.5°(111) silicon by wet chemical etching, respectively. The whole device has compact size, simple fabrication processes, and low cost. Wet etching is the key technology of fabricating micromirrors of optical switch array. Characteristics of (110) silicon etched in KOH solution are studied in this paper. Results show that (110) plane etched in 40wt% KOH is more smooth than other concentrations, and the etching velocity increases with the temperature of KOH solution. Etched vertical {111} plane has low roughness and low etching velocity. It is high aspect ratio of (110) plane to {111} planes on (110) silicon in KOH solution. According to the effects of the temperature and the concentration of KOH solution on (110) plane and vertical {111} planes, the optimal condition of fabricating the micromirror array, namely, 40wt%KOH at the constant temperature of 70℃, is obtained,Self-aligned coupling fiber grooves are also studied on (110) silicon. Self-aligned coupling fiber grooves are difficult to design accurately, because of forming nonstandard V-groove or U-groove along some orientations. So fiber collimators must be used to couple in optical switch array, which are placed outside of the upper electrodes chip.During wet chemical etching on (110) silicon, emerging of the undercutting on convex corners would be one of the obstacles to the implementation of complicated structures. Convex corners undercutting and results of rhombus compensation patterns are carried out in 40% aqueous KOH solution and in KOH saturated with isopropanol (IPA) (KOH+IPA) solution. Undercuts take place on convex corners in both solutions. Moreover, the front etch planes governing undercut vary with solutions. Rhombus compensations are used to correct the undercut. Perfect acute corner without residue is obtained, and there are only some residue structures on both sides of obtuse convex corners in KOH +IPA solution, which have better results than those in pure aqueous KOH solution.According to the principle and the dimension limit of 8×8 MEMS optical switch array, the structure of optical switch unite is designed. Its parameters are given by analyzing the relation of the parameters to actuating voltage for the compact size of optical switch array. Actuating voltages of optical switch are analyzed based on the plane low electrode and on the slanted low electrode respectively. Slanted low electrode can greatly reduce the actuating voltage, so it is applied by 8×8 MEMS optical switch array.When beam from the simple mode fiber propagates in free-space, its radius increases rapidly with propagating distance. A suitable kind of lens must be provided in order to constrain the beam expansion, and consequently to maintain insertion losses within acceptable limits. Grin lenses are used as fiber collimators of optical switch array, because they are easily adapted to fibers and readily available. Coupling losses of fiber collimator to fiber collimator are analyzed, and losses are calculated caused by the dimension,roughness and verticality of micromirror. It is important that collimators to collimators and collimators to micromirrors are coupled precisely, for the loss is the most sensitive to the angle of collimators. Reflectivity and polarization of Al and Au are determined by the theory of film optics. Al is the better choice in the optical switch array, taking into account the level of surface micromaching technology in our county.Processes of fabricating the optical switch array are introduced in details.±0.5°warp of crystal orientation can't meet the precision of fabricating micromirrors on (110) silicon, therefore, fan-shape crystal direction alignment pattern is designed and fabricated. During fabricating the torsion beam actuating structure, the etched hillocks on (110) plane are obstacles to achieve smooth torsion beam. It can be settled by polishing in the reasonable ratio mixtures of HF,HNO3 and CH3COOH. As a result, it is proved a better way to improve the fabrication processes of torsion beam actuating structure. Due to the compact size of upper electrodes chip and anisotropic characteristics of (111) silicon etched in KOH solution, photomask of slant low electrode is designed, and slanted low electrode array is fabricated on tilting 4.5°(111) silicon that matches with the upper electrode array, moreover, metal-electrode down-leads are also fabricated on tilting 4.5°(111) silicon.Upper electrodes chip with micromirrors, slanted low electrodes chip with down-leads, and fiber collimators are assembled. Tests of the fabricated optical switch are carried out. It is presented that actuating voltage is 67.2V, which can be reduced to 5V by applying an increase-voltage circuit module. Insertion losses of different ports are greatly different, according to the diversity of collimators, the transmission distance, and the coupling error. The best insertion loss is 5dB. Switching times are obtained by comparing the actuating signals and the photo—electric signals, which are less than 6ms. Crosstalk of all channels are less than -50dB, and switching life is more than 10 million cycles.Every unit of fabricated 8×8 optical switch array has good consistency. All the micromirrors are parallel, and have low roughness, good verticality. Actuating voltage is lower due to the slant low electrode. The whole optical switch array device has simple structure, good integration and low cost. According to the experimental results, it shows that optical switch array can make the meet of the optical communication network.Some novelties of this thesis are as follows:1. Micromirrors are fabricated by anisotropic wet etching in KOH aqueous solution on (110) silicon, which simplifies the processes of fabricating optical switch array. All the micromirrors are parallel, and have high quality, good verticality.2. It is investigated that rhombus compensation for convex corners have better results in KOH saturated with isopropanol (IPA) (KOH+IPA) solution than those in 40wt% aqueous KOH solution and on (110) silicon. Large residues exists on the convex corners in KOH solution, however, perfect acute corner without residues is obtained, and there are only some residues on both sides of obtuse convex corners in KOH +IPA solution.3. It is put forward the mixtures of 6.5mlHF+54mlHNO3+25mlCH3COOH to improve the processes of fabricating torsion beam actuating structure. During fabricating the torsion beam actuating structure, the etched hillocks on (110) plane are obstacles to achieve smooth torsion beam. It can be settled by polishing in HF-HNO3 mixtures. As a result, it is proved a better way to improve the fabrication processes of torsion beam actuating structure.4. Slanted low electrode array is designed and fabricated on tilting 4.5°(111) silicon. Due to anisotropic characteristics of tilting 4.5°(111) silicon etched in KOH solution, photomask of slant low electrode is designed, the method and experimental results are provided. According to the designed photomask, slanted low electrode array is fabricated, which can guarantee the low electrodes aligning with one-to-one corresponding upper electrodes well.