Specific Process Knowledge/Thin film deposition/Deposition of Gold

4th Level - Comparison

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Gold can be deposited by e-beam evaporation or sputtering. In the chart below you can compare the different deposition equipment. We also describe the temperature rise on the wafer during gold deposition, the adhesion layers that can be used for gold deposition, the roughness of gold deposited in the Wordentec, and issues with particulates on the gold films in the Temescal and the Wordentec.

Au deposition

Below you can compare the different equipment that allows Au deposition.

	E-beam evaporation (E-beam evaporator Temescal)	E-beam evaporation (Wordentec)	Sputter (Lesker)	Sputter (Cluster-based sputter system)	E-beam evaporation (Physimeca)	Sputter coater (Sputter coater 03)	Sputter coater (Sputter coater 04)
General description	E-beam deposition of Au	E-beam deposition of Au	Sputter deposition of Au	Sputter deposition of Au	E-beam deposition of Au	Sputter deposition of Au	Sputter deposition of Au
Pre-clean	Ar ion source	RF Ar clean	RF Ar clean	RF Ar clean
Layer thickness	10 Å to 1 µm *	10 Å to 5000 Å	10 Å to 5000 Å **	10 Å to 5000 Å **	10 Å to about 3000 Å **
Deposition rate	0.5-10 Å/s	1-10 Å/s	Depends on process parameters, 1-10 Å/s	Depends on process parameters	5-10 Å/s	Not measured	Not measured
Batch size	Up to 4x6" wafers or Up to 3x8" wafers (ask for special holder) many smaller pieces	24x2" wafers or 6x4" wafers or 6x6" wafers	Pieces or 1x4" wafer or 1x6" wafer	Up to 10x6" or 4" wafers Many small pieces	1x 2" wafer or 1x 4" wafers or Several smaller pieces	Several smaller samples 1x4" wafer	Several smaller samples 1x4" wafer
Allowed materials	Silicon Silicon oxide Silicon (oxy)nitride Photoresist PMMA Mylar Metals See also the cross-contamination sheet	Silicon oxide Silicon (oxy)nitride Photoresist PMMA Mylar SU-8 Metals	Silicon Silicon oxide Silicon nitride Silicon (oxy)nitride Photoresist PMMA Mylar SU-8 Metals Carbon	Almost any as long as it does not outgas - see cross-contamination sheets in Labmanager	III-V materials Silicon wafers Quartz wafers Pyrex wafers	All samples allowed in the SEM Supra 1	All samples allowed in the SEM Supra 2 or 3
Comment	For thicknesses above 600 nm permission is required Takes approx. 20 min to pump down.	Takes approx. 1 hour to pump down.	Takes approx. 10 minutes to load and transfer sample	Takes approx. 12 minutes to load and transfer samples	For thicknesses above 200 nm permission is required	Used for gold sputter coating of samples before SEM inspection Very fast.	Used for gold sputter coating of samples before SEM inspection Very fast.

* For thicknesses above 600 nm write to metal@nanolab.dtu.dk to ensure that there will be enough material in the machine.

** For thicknesses above 200 nm write to metal@nanolab.dtu.dk to ensure that there will be enough material in the machine.

Studies of Au deposition processes in the Wordentec

Roughness of Au layers - Roughness of Au layers deposited in the Wordentec

Wafer temperature

The wafer temperature during e-beam deposition of 200 nm Au on six wafers has been measured using thermal labels on the backside of the wafers. The following results were obtained:

Wafer	Temperature [C]
1	48
2	60
3	65
4	71
5	71
6	77

The temperatures are accurate within approximately +/- 3C and probably underestimating the actual wafer temperature slightly. It is observed that the wafer temperature increases with each wafer, thus if wafer temperature is of concern it is advised to reduce the number of wafers per run.

Adhesion of Au on Si

The adhesion of gold on Silicon or Silicon with native oxide is not very good. The Si substrate is often deposited an adhesion layer before the gold deposition. A few nm of Chromium or Titanium works well and they react with the Oxygen of Silicon oxide and present a metallic bond with gold. You can also use polymer or organosilane adhesion layers as exemplified by the work in the next section.

For Cr and Ti adhesion layers, a 5 nm to 10 nm thick layer of Cr or Ti is commonly used and it is important to deposit Cr or Ti and then immediately Au. If the vacuum chamber is opened in between, the surface of Cr or Ti will get oxidized and that will give a poor adhesion. If a gold layer needs to be deposited directly on Silicon, then native oxide has to be removed by deep in diluted HF and immediately load the evaporation chamber. And after the deposition, the wafer has to be heated op to get some Au-Si diffusion which improves the adhesion.

• Read more about Ti and Cr adhesion layers.

Below you can read about using an organosilane adhesion layer.

Thin Au layer using APTMS adhesion layer and sputter system Lesker

For depositing very thin, down to 6nm continuous Au layers on Si/SiO₂ substrates. Works also with ALD deposited Al₂O₃ and TiO₂ as substrate.

Adhesion promoter: aminopropyltrimethoxysilane (APTMS). MSDS here.
Adhesion promoter deposition: 3h immersion in 95%IPA, 2.5% H₂O, 2.5% APTMS.

NOTE: the APTMS layer is degrading quickly in atmosphere, so deposit it as close to the Au deposition as possible.

Lesker deposition parameters:

Gun #	Power [W]	Ramp rate [W/s]	Pressure [mtorr]	Atmosphere	Deposition rate [nm/s]
2	300	5	3	Ar	1

NOTE: As a general rule, the lower the pressure and higher the power (i.e. higher the deposition rate), the better.

Film characteristics (5-10 wafers for each thickness):

Thickness [nm]	Roughness min [nm]	Roughness max [nm]
6	0.25	0.4
10	0.3	0.5
24	0.3	0.5

NOTE: After depositing 10 layers of 10nm each, one on top of each other, the roughness increased to 0.8nm RMS

Work done by Johneph Sukham (@ DTU Fotonik) and Radu Malureanu (@ DTU Fotonik and DTU Nanolab) in 2016-2017.

Particulates in e-beam evaporated films

Au droplets on an Au film (image by Evgeniy Shkondin, Au identified by EDX).

We have found that the amount of particulates on the e-beam evaporated films depends on the deposition parameters. Specifically for gold it is hard to avoid tiny gold droplets on the films, but they can be minimized with careful attention to the sweep parameters, cleanliness of the target, etc.

The droplets appear to be inconsequential for many users, for instance if the Au layer is simply used as an electrical contact. However, for some users it is very important, for instance when the exact resistivity of the Au layer is critical.

You can read more about this issue here.