# Exercise 3.2-1

## Quick Questions to

### 3.2 Thin Films - Mechanical Properties

| Here are some quick questions:
|  | How would you define the roughness of the two thin films shown? Give an equation if possible and differentiate between the two cases. |  |
|  | Give examples for a thin layer of material B on substrate A for which you would expect good or bad adhesion, respectively: Give reasons for you expectation. |  |
|  | The "surface" energy of glass is around g(Glas) 300 mJ/m2, for a metal we might have g(Metal) » 2100 mJ/m2. You deposit a noble metal. On which substrate would you expect better adhesion? |  |
|  | Give an example of how one could measure the adhesion strength of a thin film. |  |
|  | A thin layer on some substrate is either under strong tensile or strong compressive stress. Discuss and scetch what might happen if The interface energy is small, i.e. the adherence is weak. The interface energy is large. In the second case something must "give" Discuss possible mechanisms of strain relaxation |  |
|  | Consider a thin SiO2 film on a thick Si (10 × 10) mm2 substrate that was made at 1.100 0K. The thermal expansion coefficients are aSi = 3 · 10–6 K–1 aquartz = 0.6 · 10–6 K–1 What will be the stress and strain in the film at 300 0K? How does the elastic energy stored in the film scale with its thickness dz? |  |
|  | Besides a mismatch in the coefficient of thermal expansion, other mechansim can produce stress in thin films, too. Do you know any or can you think of any? |  |
|  | The red thin layer (thickness dB) on the blue circular Si wafer substrate (thickness dA >> dB) is under compressive stress s; the wafer thus is warped with a radius of curvature = R. What would R be proportional to? Hint: It is a two-dimensional problem. |  |
|  | Imagine, that in the picture on the right you deposited the red film also on the backside. What would the radius of curvature be now? The stress in the two layers? Now imagine that yopu keep processing this system until, for exampe, you get a transitor. Thjis involves structuring the top layer, i.e. etching part of the layer off. When would you take the layer on one side off, if you don't need it for whatever you try to produce? |

With frame

3.2.4 Summary to: 3.2 Mechanical Properties

© H. Föll (Semiconductor Technology - Script)