Specific Process Knowledge/Thin film deposition/Lesker

From LabAdviser

Film quality optimization

By Bjarke Thomas Dalslet @Nanotech.dtu.dk

The Lesker CMS 18 sputter system can produce films in a wide range of qualities. The quality of a film depends strongly on the substrate (lattice matching), but also on the energy the sputtered material can utilize for annealing.

Strain estimations was done on 30 nm NiFe thin films using low angle x-ray diffraction, for various substrates. It was found that the strain of the film influenced the resistance (R) and anisotropic magneto resistance (AMR) of the films (this relationship is also documented in literature); A Ta interface layer reduced R and increased AMR on both Si and SiO substrates while reducing strain.

This study was then done on 30 nm NiFe thin films deposited on 3 nm Ta on top of a SiO substrate, using R and AMR as an indication of strain. As seen in the tables, applying a substrate bias increases AMR and conductance (1/R). An equivalent effect is seen when heating the substrate during deposition. This heating can also be done after deposition without loosing the effect.

Name Substrate bias (W) AMR 1/R (S) Crystal strain
0029 NiFe3_stack_RF20 20 0.02724278 1.308044474
0018_NiFe1_stack_RF 10 0.025850358 0.898311175
0030 NiFe3_stack 0 0.020103598 0.71772052 0.8


Name Temperature (C) AMR 1/R (S) Crystal strain
0030 NiFe3_stack 25 0.020103598 0.71772052 0.8
BDT-NiFe1-blank30 200 0.019319002 1.095770327
BTD-NiFe-Blank22 250 0.021768497 1.047668937
BTD-NiFe-Blank14 300 0.02983617 1.724137931
BTD-NiFe-Blank13 350 0.033944331 1.887504719
BTD-NiFe-Blank15 400 0.031176801 1.655903295 0.2
BTD-NiFe-Blank16 450 0.030843457


Surface roughness optimization

By Bjarke Thomas Dalslet @Nanotech.dtu.dk

The Lesker CMS 18 sputter system provides thin films of varying surface roughness. This rougness was verified to be dependent on the sputtered material, sputter mode (DC or RF) and the substrate bias strength. Other probable factors include sputter power and pressure. Below is a table for three cases.

The "From SiO target (RF sputter)" study was done on clean Si substrates. The sputter power was 157W and the pressure 3.5 mTorr using RF sputtering of a SiO target. The film thicknesses were around 42 nm.

The "From Si target (DC sputter)" study was done on clean Si substrates. The sputter pressure 3 mTorr using DC reactive sputtering of a Si target. Oxygen was added to the argon sputter gas. Above 10% O the gun seems to oxidize (at this sputter power). The figure shows the difference in AFM images between no RF bias (wafer 12) and RF bias (Wafer 21)

The "Ta" study was done on clean Si substrates. The sputter pressure was 3 mTorr using DC sputtering of a Ta target. Some O was added to wafer 25 and 26 to make TaO. In order to get fully oxidized films, up to 30-45% O should be added. Consult the thesis of Carsten Christensen for details on TaO.

Other studies on metals (NiFe/MnIr) show only limited effect of the substrate bias on the roughness.


From SiO2 target (RF sputter)

Wafer nr RF bias (W) Reactive O2 (%) Power(W) Rq (RMS) (nm) Thickness
3 0 0 157 0.902
6 5 0 157 0.499 44
7 10 0 157 0.142 42
8 15 0 157 0.422 40


From Si target (DC sputter)

Wafer nr RF bias (W) Reactive O2 (%) Power(W) Rq (RMS) (nm) Thickness
12 0 5 135 1.44 123 nm (ellipsometry)
13 0 9 130 1.32 98 nm (ellipsometry)
14 0 13 100 1.37 71.5 nm (ellipsometry)
15 10 9 90 0.984 56.25 nm (ellipsometry)
21 20 9 90 0.112
22 15 9 90 0.509


Ta

Wafer nr RF bias (W) Reactive O2 (%) Power(W) Rq (RMS) (nm) Thickness
blank1 0 0 180 0.209
16 10 0 180 0.36 56
24 20 0 180 0.357
25 20 9 180 0.202 110
26 20 5 180 0.194 95
27 15 0 180 0.413
28 25 0 180 0.164
31 30 0 180 0.3