Coating processes – Coating by vapor – gas – or smoke – Mixture of vapors or gases utilized
Reexamination Certificate
2000-10-18
2002-10-15
Jones, Deborah (Department: 1775)
Coating processes
Coating by vapor, gas, or smoke
Mixture of vapors or gases utilized
C427S058000, C427S255280
Reexamination Certificate
active
06465045
ABSTRACT:
BACKGROUND OF THE INVENTION
Polysilicon is one of the most widely used structural materials for microelectromechanical systems and devices. However, when deposited by low-pressure chemical vapor deposition (LPCVD) techniques, polysilicon films typically display high residual stresses and often stress gradients as well. These stresses, particularly when compressive, may cause released devices to bend and buckle, altering their original shapes and degrading their performances. While tensile stresses may promote planarity in doubly clamped designs, such stresses also increase stiffness and cause deformation of asymmetric features. Zero-stress polysilicon thin film structures would be optimal for many applications.
Prior artisans have attempted to produce thin silicon films with reduced stress levels. U.S. Pat. No. 5,753,134 entitled “Method for Producing a Layer With Reduced Mechanical Stresses” to Biebl, is directed to a method for producing a silicon layer having a reduced overall stress value, the layer being composed of two silicon sublayers. The first sublayer and the second sublayer are matched to one another such that the stresses in the two layers substantially compensate each other, and in effect, cancel each other out. However, Biebl requires that one or more auxiliary layers of silicon dioxide be provided between the sublayers. Those auxiliary layers require additional manufacturing or processing operations. Although satisfactory in some respects, a need still exists for an improved multi-layer polysilicon assembly and technique for forming, and particularly for an assembly that does not require the use of intermediate or auxiliary layers.
In addition, a disadvantage often associated with polysilicon films deposited by chemical vapor deposition techniques, pertains to the resulting relatively rough surface of the deposited film. Although approaches are known for producing films having relatively smooth finishes, typically, additional processing steps are necessary or critical process control schemes must be implemented. Accordingly, a need remains for a technique for producing films and multi-layer assemblies of such films having relatively smooth surfaces.
SUMMARY OF THE INVENTION
The present invention achieves the foregoing objectives and provides in a first aspect, a multi-layer thin film assembly comprising a first thin film including polysilicon, and a second thin film also comprising polysilicon. The first thin film has a devitrified microstructure and an internal tensile stress. The second thin film has a predominantly columnar microstructure and an internal compressive stress. The first and second thin films are disposed immediately adjacent to one another to form the multi-layer assembly, thereby avoiding the use of an intermediate layer between the first and second thin films. In addition, the resulting films have exceptionally smooth surfaces.
In yet another aspect of the present invention, a multi-layer thin film assembly is provided that includes at least three thin films, each having a thickness within a certain range, and each having a particular microstructure. The thin films are arranged such that they are immediately adjacent to at least one of the other thin films.
The present invention also provides a method of forming a multi-layer thin film assembly comprising a plurality of polysilicon thin films. The method enables the multi-layer assembly to be formed with a selectively determinable stress characteristic. The method comprises forming a first polysilicon thin film that has either (i) a devitrified microstructure and an internal tensile stress, or (ii) a predominantly columnar microstructure and an internal compressive stress. The method further comprises forming a second polysilicon thin film immediately adjacent to the first polysilicon thin film. The second thin film has either (i) a predominantly columnar microstructure and an internal compressive stress, or (ii) a devitrified microstructure and an internal tensile stress. The arrangement of the films is selected so that the microstructure of the second thin film is different from the microstructure of the first thin film, such that if the first thin film is devitrified, the second thin film is predominantly columnar and vice versa.
Further scope of the applicability of the present invention will become apparent from the detailed description given hereinafter. It should, however, be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art.
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Heuer Arthur H.
Kahn Harold
Yang Jie
Case Western Reserve University
Fay Sharpe Fagan Minnich & McKee LLP
Jones Deborah
Stein Stephen
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