Specific Process Knowledge/Thin film deposition/Lesker

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 Ni$_{81}$Fe$_{19}$ 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$_{2}$ substrates while reducing strain.

This study was then done on 30 nm Ni$_{81}$Fe$_{19}$ thin films deposited on 3 nm Ta on top of a SiO$_{2}$ 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 roughness 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$_{2}$ 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$_{2}$ 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$_{2}$ the gun seems to oxidize (at this sputter power). Figure 1 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$_{2}$ was added to wafer 25 and 26 to make Ta$_{2}$O$_{5}$. In order to get fully oxidized films, up to 30-45% O$_{2}$ should be added. Consult the thesis of Carsten Christensen for details on Ta$_{2}$O$_{5}$.

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

Figure 1:Left: no RF bias (wafer 12) gives high roughness. Right: RF bias (Wafer 21) gives low roughness

Oxide insulation analysis

The wafers in this analysis consisted of a Si substrate with no native oxide. A layer of SiO$_{2}$ was reactively sputtered (9% O2 90 W 3.5 mTorr). After that, using a shadow mask, 200nm thick gold rectangles was electro deposited on top of the oxide. Gold was also electro deposited on the back side. Then the impedance as a function of frequency was recorded.

The figure shows the measurements for different oxide thicknesses. Most of the measurements show perfect capacitors, although for illustration measurements with a few pinholes and with many pinholes is also shown for the 20 nm sample.

The success rate for the different thicknesses can be seen in the table, together with the number of samples measured and the number of perfect capacitors.

It is possible to make perfect capacitors with oxide thicknesses down to and including 5 nm and possibly even thinner, although the failure rate increases. Bear in mind, though that each structure measured here has an area of 8 mm² - for a 1 mm² structure the failure rate would be much lower, assuming the short circuits are not located on the sides of the structures.