Specific Process Knowledge/Thin film deposition/Deposition of Germanium/Thermal Ge evaporation Thermal Evaporator
This page is written by Evgeniy Shkondin @DTU Nanolab if nothing else is stated.
All images and photos on this page belongs to DTU Nanolab.
The fabrication and characterization described below were conducted in 2023 by Evgeniy Shkondin, DTU Nanolab.
Evaporation of Ge in Thermal Evaporator
This page describes the resistive thermal evaporation method of Ge in Thermal Evaporator (NANO 36 THERMAL EVAPORATOR SYSTEM).
Photo of the Ge evaporated from EVS8B005W boat.
Photo of the 150 mm wafer (Si/300nm_SiO2) in the dedicated holder.
Boats selection
Germanium has a melting point of 937°C, which is significantly lower than the temperature required to reach 10-2 Torr where the reasonable deposition rate is expected. Consequently, germanium evaporates rather than sublimes under these conditions. For optimal germanium evaporation, we recommend a base pressure of 10-6 Torr or lower. By reaching an evaporation temperature of 1,400°C, an average deposition rate ranging from 1 to 5 angstroms per second can be achieved.
Germanium is not known to alloy with refractory metals. According to KJ Lesker, Germanium can be thermally evaporated using a tantalum or tungsten dimple-style boat, such as EVS8B005W or EVS8B005TA models. However, in this study, we did not had a great success with the EVS8B005W. The boat start to burn over once reaching the thicknesses of 50-60 nm, at 1Å/s, and it require too high power to operate (see the graphs below). Only a few times we could reach 100 nm setpoint. We strongly advice to use EVS20A015W instead. This is a simple solution that has proven to be reliable over time.
Two different boats have been tested:
Here is a photo of the EVS8B005W boat. Please be advised that using this boat is not recommended due to its high power requirement to achieve significant rates, and its limited durability, lasting only for a few tenths of a nanometer (maximum 50-70nm). Furthermore, these boats are intended for one-time use only.
Here is a photo of the EVS20A015W boat. We highly recommend using this boat. It requires much less power to operate, maintains a stable deposition rate, and enables achieving thicknesses above 100 nm. Additionally, these boats are reusable, providing added convenience and cost-effectiveness.
Rise-soak settings for two different boats. The Rise 2 power is sat to match 1 Å/s. We highly recommend using EVS20A015W boat. It requires much less power to operate, maintains a stable deposition rate, and enables achieving thicknesses above 100 nm. Additionally, these boats are reusable, providing added convenience and cost-effectiveness.
Uniformity across 150 mm wafer
Results have been obtained for <100> 150 mm Si wafers with 300nm thermal SiO2 oxide, based on ellipsometry study.
| Boat type | Deposition rate (nm/s) | Tooling Factor | Average thickness (nm) | Minimum thickness (nm) | Maximum thickness (nm) | Standard deviation | Uniformity (%) |
|---|---|---|---|---|---|---|---|
| EVS8B005W | 1 | 135% | 98.25 | 89.55 | 103.26 | 3.9151 | 6.9773 |
| EVS20A015W | 1 | 135% | 102.01 | 95.21 | 105.55 | 3.1227 | 5.0663 |
Measured and fitted Psi and Delta functions of Ge thin film deposited on Si wafer with 300 nm thermal SiO2 using the resistive thermal evaporation method.
Thickness uniformity of Ge evaporated on Si wafer with 300 nm SiO2 using resistive thermal evaporation from the boat EVS8B005W. Thickness setpoint sat to 100 nm.
Thickness uniformity of Ge evaporated on Si wafer with 300 nm SiO2 using resistive thermal evaporation from the boat EVS20A015W. Thickness setpoint sat to 100 nm.
Optical functions
Results have been obtained for <100> 150 mm Si wafers with 300 nm thermal SiO2, based on ellipsometry study.
Refractive index of 100 nm Ge evaporated on silicon with 300 nm thermal SiO2 using two different boats.
Absorption coefficient of 100 nm Ge evaporated on silicon with 300 nm thermal SiO2 using two different boats.
