Graduation Year

2008

Document Type

Dissertation

Degree

Ph.D.

Degree Granting Department

Mechanical Engineering

Major Professor

Alex Volinsky, Ph.D.

Keywords

Residual stress, Sputtering, X-ray diffraction, Nanoindentation, Wafer curvature

Abstract

Thin films are used for a variety of applications, which can include electronic devices, optical coatings and decorative parts. They are used for their physical, electrical, magnetic, optical and mechanical properties, and many times these properties are required simultaneously. Obtaining these desired properties starts with the deposition process and they are verified by a number of analysis techniques after deposition. A DC magnetron sputter system was used here to deposit tungsten films, with film thickness and residual stress uniformity being of primary interest. The film thickness was measured to vary by up to 45 % from the center to outer edge of a 4" wafer. Ar pressure was found to influence the thin film residual stress with lower Ar pressures leading to compressive residual stress (-1.5 GPa) and higher Ar pressures leading to tensile residual stress (1 GPa).

Residual stress measurements of the tungsten films were made using a wafer curvature technique and X-ray diffraction. The results of the two techniques were compared and found to be within 20 %. Nanoindentation was used to analyze the mechanical properties of several types of thin films that are commonly used in microelectronic devices. Thin film reduced modulus, hardness, interfacial toughness and fracture toughness were some of the mechanical properties measured. Difficulties with performing shallow indents (less than 100 nm) were addressed, with proper calibration procedures for the indentation equipment and tip area function detailed. Pile-up during the indentation of soft films will lead to errors in the indentation contact depth and area, leading to an overestimation of the films' reduced modulus and hardness.

A method was developed to account for pile-up in determining the indentation contact depth and calculating a new contact area for improving the analysis of reduced modulus and hardness. Residual stresses in thin films are normally undesired because in extreme cases they may result in thru-film cracking or interfacial film delamination. With the use of lithography techniques to pattern wafers with areas of an adhesion reducing layer, thin film delamination was controlled. The patterned delamination microchannels may be used as an alternative method of creating microchannels for fluid transport in MEMS devices. Delamination morphology was influenced by the amount of residual stress in the film and the critical buckling stress, which was primarily controlled by the width of the adhesion reducing layers.

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