Mechanical Testing of Aluminum Microbeams Using an AFM: End Semester Report

1
Mechanical Testing of Aluminum Microbeams Using
an AFM End Semester Report

• Adam Falk
• Advisor Dr. Gleixner
• Mate 198a
• November 29, 2001

2
Presentation Overview

• Review of concepts
• Project proposal
• Results of literature search
• Feasibility study overview
• Results of first lab work
• Conclusion

3
Atomic Force Microscope

• AFM is a surface analysis tool
• Accurately describes the topography of
  microscopic surfaces
• AFM can operate in two different modes contact
  and non-contact

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AFM Contact Mode

• Contact mode
• Atomically fine tip, attached to a cantilever, is
  dragged across the surface of the sample
• A laser beam is shot into the cantilever
• The reflection of the laser is read by a detector
• The detector maps out the surface of the sample

5
Schematic of AFM

• Description of AFM A.W.Marczewski Aug 24, 1995
  http//www.cheme.kyoto-u.ac.jp/2koza/awmarcz/AFM-s
  cheme.html

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Uses of Aluminum

• Aluminum is a common material used in thin film
  applications
• Interconnect wires in integrated circuits
• Metallic coatings for MEMS mirrors and optical
  switches
• Beneficial properties of aluminum are
• Good thermal conduction
• Good electrical conductor
• Corrosion resistance
• Good mechanical properties

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Mechanical Properties of Aluminum

• It is important to know the mechanical properties
  of aluminum because aluminum is under mechanical
  stress in ICs
• Mechanical properties are important to long term
  reliability of the devices
• There are not many techniques available to
  measure mechanical properties on such a small
  scale

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Project Proposal

• Manufacture aluminum microbeams on a silicon
  substrate
• Develop protocol for testing the beams with the
  AFM
• Conduct mechanical tests on the beams
• View the effects of annealing on the mechanical
  properties of the beams

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Goal of Experimental Results

• Develop a new application for the AFM
• Calculate Youngs modulus for Aluminum thin films
• Analyze the effect of grain size on Youngs
  modulus

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Microbeam Deflection

• Several articles talk about mechanical properties
  of thin films
• Most use micro-indenter techniques on microbeams
  of various compositions

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Results of Literature Search

• S.P. Baker, W.D. Nix microbeam deflection of
  Au-Ni using nano-indenter
• Constructed Au-Ni microbeams
• Measured microhardness, used mathematics to
  calculate Youngs modulus
• Problems with accuracy from influence of
  substrate on hardness values
• Other source of error difficulty of calculating
  the indentation modulus.

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Previous Work Using AFM

• B.T. Comella, M.R. Scanlon aluminum microbeam
  deflection using AFM
• Constructed microbeams
• AFM measures cantilever deflection. From
  mathematical calculation determined Youngs
  modulus
• Mathematical technique used tungsten spheres to
  calculate the spring constant of the cantilever
• Experienced problems in calculating the spring
  constant of the cantilever due to inaccuracies in
  calculating the geometry of the tungsten spheres
• Concluded that it is possible to receive accurate
  measurements using the AFM

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Microbeam Dimensions From Literature

• Beam thickness ranges between 1.0um and 2.5um
• Beam height ranges between 1.0um and 3.0um
• Beam width ranges between 14.0um and 20.0um
• Beam length ranges between 10.0um to 150.0um

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Feasibility Study Overview

• Determine method for producing aluminum beams
• Set experimental sample parameters
• Produce aluminum beams on silicon substrate
• Examine and measure beams for quality and
  accuracy
• Determine the best range of dimensions for future
  testing

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Feasibility Study Procedure

• Clean and scribe silicon wafer
• Grow SiO2 layer on the wafer and measure thickness

• Spin on photoresist and apply mask to wafer and
  remove unmasked photoresist

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Experimental Procedure Continued

• Etch off SiO2 layer that is not covered by photo
  resist

• Sputter on aluminum layer and measure the
  thickness

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Experimental Procedure Continued

• Pattern the aluminum, etch away the remainder,
  and measure the thickness

• Etch away the remainder of the SiO2

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Results to Date

• Cleaned and scribed five silicon wafers
• All five wafers are of similar type (p or n)
• Inspected wafers for preexisting defects
• Grown and measured SiO2 Layer

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Results to Date Continued

• Thickness was found to be an average of 1.286
  microns, with an average standard deviation of
  0.0036
• The variance in thickness between all of the
  samples was 0.02 microns
• There is a large degree of uniformity in the
  thickness of the oxide layer

20
Conclusion

• Background of AFM and aluminum thin films
• Project objectives
• Review of literature search
• Feasibility study
• Results of work done so far