Etching a substrate: processes – Gas phase etching of substrate – Etching inorganic substrate
Reexamination Certificate
1999-10-26
2001-09-18
Stinson, Frankie L. (Department: 1746)
Etching a substrate: processes
Gas phase etching of substrate
Etching inorganic substrate
C216S073000, C252S079100
Reexamination Certificate
active
06290864
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
This invention lies in the technology of the manufacture of micro-structures, which include such devices as microelectromechanical structures and semiconductor devices. In particular, this invention addresses gas-phase etching procedures, focusing on those involving the etching of silicon by noble gas fluorides, halogen fluorides, or both, all in the gas phase.
2. Description of the Prior Art
The use of selective etchants to remove sacrificial layers or regions in a multilayer structure without removal of an adjacent layer or region is a necessary and common step in the manufacture of semiconductor devices and microelectromechanical structures (MEMS). MEMS have found applications in inertial measurement, pressure sensing, thermal measurement, micro-fluidics, optics, and radio-frequency communications, and the range of possibilities for these structures continues to grow. One example of such a structure is a reflective spatial light modulator, which is a device consisting of a planar array of electrostatically deflectable mirrors, each microscopic in size. The device is used as a microdisplay system for high-resolution, large-screen projection. The sacrificial layer in such a device is the layer over which the mirror material is deposited. Once the mirror structure is formed, the sacrificial layer is removed to leave gaps below the mirrors and a microhinge along one edge of each mirror to join the mirror to the remainder of the structure. The gap and the microhinge provide the mirror with the freedom of movement needed for its deflection. Devices of this type are described in U.S. Pat. No. 5,835,256 (issued Nov. 10, 1998, to Andrew Huibers, assignor to Reflectivity, Inc.). The contents of this patent are incorporated herein by reference.
The success of a manufacturing procedure for structures involving sacrificial layers depends on the selectivity of the etching process. The thicknesses and lateral dimensions of the layers, and in the case of the deflectable mirror structures the width of the gap and the integrity of the microhinges, are all critical to achieving uniform microstructure properties and a high yield of defect-free product. A factor in meeting this criticality is the quality of the etch. Performance, uniformity and yield can all be improved with increases in the etch selectivity of the sacrificial layer relative to the adjacent functional layers. Selectivity is important in both isotropic and anisotropic etching procedures. Isotropic etching is of particular interest, however, in structures where the sacrificial layer is an intervening layer between functional layers or between a functional layer and a substrate, since the bulk of the sacrificial layer in these structures is accessible to the etchant only through vias in the functional layer and etchant must proceed laterally outward from the vias. The structures described in U.S. Pat. No. 5,835,256 above require isotropic etchant for this reason. The “vias” in these structures are the narrow gaps between the facing edges of adjacent mirror elements or between a mirror edge and an adjacent feature.
Among the etchants that are used for the removal of sacrificial layers or regions in both isotropic and anisotropic etching procedures are noble gas fluorides and halogen fluorides. These materials, used in the gas phase, selectively etch silicon relative to other materials such as silicon-containing compounds, metallic elements, and compounds of metallic elements. The selectivity is not infinite, however, and can vary widely with the equipment, process, materials and reaction conditions. Xenon difluoride, for example, has demonstrated selectivities as high as 400:1 to 500:1, but any improvement in this ratio would significantly benefit the cost and reliability of the products manufactured.
SUMMARY OF THE INVENTION
It has now been discovered that the selectivity of the etching of silicon relative to other materials in a microstructure by use of an etchant gas which is either a noble gas fluoride, halogen fluoride, or a combination of the two can be increased significantly by using as the etching medium a gas mixture containing the etchant gas(es) and one or more of certain additional but non-etchant gaseous components. It has further been discovered that while the inclusion of non-etchant gaseous additives causes prolongation of the etch time, those additives whose molar-averaged formula weight is below that of nitrogen gas prolong the etch time to a much lesser extent than do those whose molar-averaged formula weight is equal to or greater than that of molecular nitrogen, while still achieving the same improvement in selectivity. The improvement in selectivity is achievable independently of the partial pressure of the etchant gas in the gas mixture. Likewise, the limitation on the increase in etch time when the averaged formula weight of the additive gas is less than that of molecular nitrogen is achievable independently of the partial pressure of the etchant gas in the gas mixture. Both the increase in selectivity and the limitation on the etch time are sufficiently great that significant improvements in uniformity, yield, and product reliability are achieved.
These discoveries offer numerous advantages, for example in overetching, i.e., etching purposely done to a degree beyond that which is strictly required for removal of the sacrificial silicon. Since the high selectivity allows one to overetch without introducing nonuniformity across the mirror array, this invention permits the use of overetching as a convenient and effective means of assuring complete removal of the sacrificial silicon while still preserving the integrity of the mirror elements. The invention thus eases the requirement for precise endpoint detection, i. e., precise detection of the point at which the last of the sacrificial silicon is removed. Another advantage stems from the dilution effect of the additive gas. Dilution improves the circulation of the gaseous mixture through the system by adding to the mass that flows through the recirculation system or agitator when such pieces of equipment are present. Also, the presence of the additive gas helps reduce high local concentrations of the etchant at the sample surface. Each of these factors improves microstructure uniformity and yield.
This invention is of particular interest in etching processes that are not performed in a plasma environment, i.e., etching processes performed without the use of externally imposed energy such as ultraviolet light or high frequency electromagnetic energy to excite the gases into a plasma state. The invention is also of particular interest in isotropic etching processes, notably those in which the silicon and the non-silicon portions (as defined below) of the microstructure are overlapping layers, coextensive or otherwise, or nonoverlapping layers, sharing a common boundary or separated but still simultaneously exposed to the etchant gas. The invention is particularly useful in structures in which the silicon is a layer positioned underneath a layer of the non-silicon material such that removal of the silicon by etching requires lateral access through vias in the non-silicon layer. The invention is also of particular interest in the manufacture of reflective spatial light modulators of the type described in U.S. Pat. No. 5,835,256, in which the mirror elements are formed of silicon nitride or silicon dioxide and the underlying sacrificial layer serving as the support to be removed by etching is polysilicon.
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MacDonald Douglas B.
Patel Satyadev R.
Schaadt Gregory P.
Muir Gregory R.
Olsen Allan
Reflectivity, Inc.
Stinson Frankie L.
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