Micromachined Silicon Periodic Structures For Millimeter- And Submillimeter-Wave Applications

MILLIMETER- AND SUBMILLIMETER -WAVE APPLICATIONSL.J. Cheng, G.T. Crotty, J. Farhoomand, K.M. Koliwad,and H.M. PickettJet Propulsion LaboratoryCalifornia Institute of TechnologyPasadena, California 91109ABSTRACTThe feasibility of fabricating periodic structures using silicon micromachining formillimeter- and submillimeter -wave applications is investigated. Diffraction gratings anddichroic filters are fabricated and tested.1. INTRODUCTIONSome chemical processes etch silicon crystals anisotropically depending on latticeorientation and impurity contentl. The utilization of this feature, combined withphotolithography, has created a unique technique of silicon micromachining withdimensional control down to the micron region. In the past few years, a wide range ofthree dimensional silicon microstructures, unattainable by any other means, have beenfabricated using this technique. These include valves, springs, mirrors, cantilevers,nozzles, channels, and diaphragms. This technique has been used to create new types ofsilicon sensing devices for innovative applications, particularly together withappropriate interface circuitry on a common silicon chip. These devices can be used tomeasure temperature, pressure, acceleration, tactile image, visible and infrared image.2This technique does have some restrictions, which are dictated by anisotropic etchingproperties of silicon crystals.Silicon micromachining uses almost the same technology as does the fabrication ofsilicon microelectronics and integrated circuits, including oxidation, photolithography,and pattern etching. This can be easily performed in an ordinary silicon integratedcircuit fabrication laboratory. The process starts with the same batch - fabricationtechniques and, therefore, it is simple and economical. Modern photolithography is veryaccurate and precise in the submicron scale. It has been experimentally shown that theanisotropic etching rate in silicon along the crystalline directions is about 600times slower than that along the directionsl. The basic idea of siliconmicromachining is to utilize this anisotropic etching property for fabricating the desiredthree dimensional structures. The large anisotropy in etching rate also makes possible tofabricate extremely smooth surfaces.In this paper, we report results of an exploratory investigation of fabricatingperiodic structures using silicon micromachining for millimeter- and submillimeter -waveapplications. Gold- plated triangular groove gratings and diamond -shape hole arraydichroic filters were fabricated. Both can be used to separate frequencies. As thewavelength becomes shorter, the manufacturing of these components using traditionalmachining becomes more difficult and time- consuming, if possible at all. Siliconmicromachining can be a valuable alternative tool.2. TRIANGULAR GROOVE DIFFRACTION GRATINGSConventional diamond -ruled grating techniques suffer from the limitations imposed bythe mechanical manufacturing processes. Such limitations are even more pronounced whensmaller dimensions are required over large areas. In addition, the manufacturing costincreases drastically for higher precision gratings. In recent years, holographicgratings with much higher precision and lower cost than the ruled gratings have beenavailable commercially3 -5. However, holographic gratings are not suitable forsubmillimeter wave applications, because they can not easily fabricated with the largerperiodicities required. As an alternative, silicon micromachining can produce gratingswith a high degree of precision at very low cost.Gold- plated triangular groove gratings were fabricated using commercially availablefloating -zone grown, (100)- oriented, boron -doped silicon wafers having an electrical