The study of polyimide films and adhesion using a surface acoustic wave sensor

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The study of polyimide films and adhesion using a
surface acoustic wave sensor
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Introduction

• Application of the polyimide
• Features of the polyimide
• Techniques measured for film adhesion
  characterization
• mechanical method inaccuracy and destructive
• ultrasonic method accuracy and nondestructive
• Principle of acoustic wave probing interface
• Humidity effect on polyimide

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Experimental

• Substrate ST quartz
• Devices dual delay line
• Operating frequency 80 MHz

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• If SAW velocity increases, the film is said to
  stiffen the substrate
• If SAW velocity decreases, the film is said to
  load the substrate
• The relationship between adhesion and effective
  mass of the film
• Coating method spin-coating, d 1 ?m
• Film adhesion promoter(0.01?m) polyimide Cu
  mask(0.1?m, removed after the initial set of
  experiments)

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• Curing polyimide sample was heated to 400oC

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Results and discussion
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• Both uncured samples showed a relatively small
  and similar response to the humidity increase
• An stiffening response presents at lower RH
• A loading response presents at higher RH
• The cured samples exhibited a response similar in
  sharp but about 20 larger

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• The positive peak (stiffening effect) is still
  evident and is at about the same level as Fig.5
• The negative shift is about 50 larger and the
  stiffening is also seen during the drying cycle

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• The cured samples without Cu mask, exhibited
  similar and very large responses to water uptake,
  more than 20 times as great as when Cu mask was
  still on the polyimide film

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• The maximum response is sensitive to sample
  temperature, RH, and the times the sample has
  been humidity cycled

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• The largest relative velocity change to occur
  between 50 and 100RH in Fig.10
• The largest relative velocity change to occur
  between 0 and 50RH in Fig. 9
• In Fig. 9, the positive velocity difference
  change indicates that either a greater amount of
  loading as a function of humidity is taking place
  on the sample without promoter as compared to the
  sample with promoter, or that the sample with
  promoter is exhibiting a larger stiffening effect
  than the sample without

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• The stiffening (elastic constant increase) occurs
  when gases (lt 50) are absorbed in polymers 12
• The films with and without promoter load
  equally since being of identical size they would
  sorb equal amounts of water
• The sample with promoter is exhibiting a larger
  stiffening effect

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Conclusions

• The uncured sample has a very small response that
  might be indicative of a mass loading effect
  counterbalanced by changes in the viscoelastic
  properties of the film
• The large difference in the response between
  cured and uncured samples, this suggests the
  possible use of the SAW sensor as a cure
  indicator
• The main difference between promoter and
  unpromoter is attributed to a large stiffening
  effect for the sample with promoter at lower
  humidity level