## 3.6 Summary

### 3.6.1 Summary to: 3. Thin Films

| Semiconductor technology is almost synonymous with thin film technology. |  |
|  | A thin film is always adhering to a substrate and (at least originally) continuous. |
|  | Thin films may still be found in the product or may have been "sacrificed" during the making of the product. |
|  | An IC is a study of thin films in and on the Si substrate. |
|  | The same is true for pretty much every semiconductor product. |
| Thin always means "thin" relative to some intrinsic (internal) length scale. Examples are: |  |
Dimensions dx, y, z Grain size dgrain Lattice constants a0 l radiation (light, IR, UV) Absorption depths Mean free path lengths. Diffusion length | SCR width dSCR Debye length dDebye Critical thickness dcrit for electrical break down Critical thickness dtu for tunneling
|  | Structural length scales. |
|  | Wavelength and Interaction length scales. |
|  | Transport parameter length scales. |
|  | Electrical scales. |
| There are many thin film applications outside of semiconductor technology: |  |
|  | Optical, electrical, chemical, mechanical, magnetic technologies use thin films. |  |

| Thin films have other spatial properties besides their thickness. |  |
|  | Interface roughness and surface roughness R defined by their "root mean square": |
R | = | 1 N |  | N S i=1 | |zi|
--- | --- | --- | --- | --- | ---
| 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 film 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. |

| Deposition of a thin layer must start with a "clean" substrate surface on which the first atomic / molecular layer of the film must nucleate. |  |
| There are many possible interactions between the substrate and "first" incoming atoms. |
|  | As the interaction energy goes up we move from "some" absorption to physisorption (secondary bonds are formed) to chemisorption (full bonding) |
|  | The sticking coefficient is a measure of the likelihood to find an incoming atom in the thin film forming. |
|  | Immobilization by some bonding is more likely at defects (=more partners). The initial stage of nucleation is thus very defect sensitive. |
| Simple surface steps qualify as efficient "defects" for nucleation. |  |
|  | Small deviations from perfect orientation provide large step densities. Nucleation therefore can be very sensitive to the precise {hkl} of the surface |
|  | Intersections of (screw) dislocation lines with the surface also provide steps. |
|  | This may cause grain boundaries and other defects in the growing layer. |
|  | Scanning probe microscopy gives the experimental background |
| There is always a nucleation barrier that has to be overcome for the first B-clusters" to form on A |  |
|  | the three involved interface energies, all expressed in the "wetting angle", plus possibly some strain are the decisive inputs for the resulting growth mode. Frank - van der Merve: Smooth layer-by-layer growth Vollmer - Weber: Island growth Stranski - Krastonov: Layer plus island growth |

| Epitaxial layers are crucial for semiconductor technology. |  |
|  | Misfit of lattice constants will produce strained layers upon epitaxial growth; strain relief happens by the formation of misfit dislocations. |
|  | Misfit dislocations must be avoided at all costs! |
|  | Below a usually rather small critical thickness dcrit of the the thin layer no misfit dislocations will occur. |
|  | Rule of thumb: 0.5 % misfit Þ dcrit »10 nm |  |
| The internal structure of thin films can be anything known from bulk materials plus some (important!) specialities. |  |
a-Si: Micro electronics a-Si:H: Solar cells, LCD displays µc-Si:H: Solar cells
| Properties of thin films can be quite different from that of the bulk material. |  |
Much better in thin films Electrical break-down field strength of dielectrics. Critical current densities in conductors.
|  | The reason can be differences in length scales. |
|  | Semiconductor technology relies to some extent on superior thin film properties. |
Exercise 3.6-1
All Questions to 3

© H. Föll (Semiconductor Technology - Script)