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.

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. 5 nm to 10 nm thick 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.

	E-beam evaporation (Alcatel)	E-beam evaporation (Wordentec)	Sputter (Lesker)	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	E-beam deposition of Au	Sputter deposition of Au	Sputter deposition of Au
Pre-clean	RF Ar clean	RF Ar clean	RF Ar clean
Layer thickness	10 Å to 5000Å*	10 Å to 5000Å*	10 Å to	10Å to about 3000Å*
Deposition rate	2 Å/s to 10 Å/s	1 Å/s to 10 Å/s		From 5 Å/s up to 10Å/s	Not measured	Not measured
Batch size	Up to 1x4" wafers smaller pieces	24x2" wafers or 6x4" wafers or 6x6" wafers	Pieces or 1x4" wafer or 1x6" wafer	1x 2" wafer or 1x 4" wafers or Several smaller pieces	Several smaller samples 1x4" wafer	Several smaller samples 1x4" wafer
Allowed materials	Silicon oxide Silicon (oxy)nitride Photoresist PMMA Mylar SU-8 Metals	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	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 200 nm permission is required An adhesion layer (of Cr or Ti) is recommended under Au.	For thicknesses above 200 nm permission is required An adhesion layer (of Cr or Ti) is recommended under Au.			Used for gold sputter coating of samples before SEM inspection	Used for gold sputter coating of samples before SEM inspection

* For thicknesses above 200 nm permission from ThinFilm group (thinfilm@danchip.dtu.dk) is required.

Studies of Au deposition processes in the Wordentec

Roughness of Au layers - Roughness of Au layers deposited with different equipment and settings

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.

Thin Au layer deposition using 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 (click to email) and Radu Malureanu (click to email)