### 3.2.4 Summary to: 3.2 Mechanical Properties

| Thin films have other spatial properties besides their thickness, i.e. roughness |  |
|  | Interface roughness and surface roughness R defined by their "root mean square". |
R | = | æ ç è | 1 N |  | N S i=1 | zi2 | ö ÷ ø | ½
| Useable thin films adhere to their substrate. |  |
|  | A direct measure of adhesion is the interfacial energy gAB between film A and substrate B. |
|  | The phase diagram provides some guideline. Complete miscibility = good adhesion, (eutectic)) decomposition =(?) low adhesion. Calculations of g are difficult. |
|  | Full adhesion can only be obtained for films grown on a substrate. Adhesion energies can be measured. |
| Generally, there will be stress s and strain e in a thin film and its substrate. |  |
Stress and strain in thin films can be large and problematic!
|  | A major source of strain is the difference of the thermal expansion coefficients a |
eTF | = | D T · D a
--- | --- | ---
sTF | = | Y · D T · D a
| Stress in thin films may relax by many mechanisms; and this might be good or bad: Cracking or buckling Plastic deformation Viscous flow Diffusion Bending of the whole system (Warpage) |  |
|  | Warpage can be a serious problem in semiconductor technology. |
Exercise 3.2-1
All Questions to 3.2

© H. Föll (Semiconductor Technology - Script)