Specific Process Knowledge/Thin film deposition/Deposition of Carbon

We can deposit Carbon (C) by DC sputtering in Sputter-System (Lesker). A 2-inch target from gun 1 (DC) is used in the process. Since the material is known to possess cross-contamination issues it was decided to cover all other guns with the protection foil.

The fabrication and characterization described below were conducted in 2022 by Patama Pholprasit and Evgeniy Shkondin, DTU Nanolab. The prepared samples were investigated by the X-ray Reflectivity (XRR), Spectroscopi Ellipsometry (SE), Photo Electron Spectroscopy (XPS), Scanning Electron Microscopy (SEM), and Graizing Incident Diffraction (GiXRD) methods. The focus of the study was the deposition conditions.

Recipe:

• Power: 200 W
• Pressure: 3 mTorr
• Power rump: 0.3 W/s
• Deposition mode: DC (Src1)
• Deposition time: 10, 20, 30 min and 6000s for GiXRD measurement)
• Substrate: 4-inch Si and a variety of others (glass, silicon nitride) to study adhesion and conductivity.

Deposition Rate

  Deposition rate for C at 200W power (DC) and 3 mTorr presssure is 0.01 nm/s

• Carbon deposition rate.
• 
  Carbon thin films deposited on Si wafer and measured by XRR method. 10, 20 and 30 min of deposition time using 200W, 3 mTorr, Src1 DC.

X-ray Reflectivity

X-ray analysis (XRR) of samples (deposited at 10min, 20 min, and 30 min) have been performed to investigate the thicknesses, roughness, and density profiles.

The scans has been obtained using Rigaku XRD SmartLab equipment. The voltage and current settings for the Cu X-ray tube were standard 40kV and 30mA. The incident optics contained a IPS (incident parallel slit) adaptor with 5 ° Soller slit. Other slits: IS=0.03mm RS1=0.03mm and RS2=0.075mm. Step size: 0.01 and measurement time - 5s for each point. Fitting procesure was performed using commercial GlobalFit software assuming the model based on Si or SiO₂ substrates with native oxide/interlayer followed by the deposited C film with thin oxides and moisture surfaces. The results are summarized in a tables below.

Fitting resilts for Carbon films

• Carbon thickness measurements. X-ray reflectivity analysis.
• 
  10 min deposition from Src1.
• 
  20 min deposition from Src1.
• 
  30 min deposition from Src1.

XRR results for Carbon layers

Top layer (moisture) Main layer Si native oxide Si substrate Deposition time Moisture thickness (nm) Moisture density (g/cm3) Moisture roughness (nm) Carbon thickness (nm) Carbon density (g/cm3) Carbon roughness (nm) SiO2 thickness (nm) SiO2 density (g/cm3) SiO2 roughness (nm) Si thickness (nm) Si density (g/cm3) Si roughness (nm) 10 min 1.55 0.428 1.33 6.33 2.00 0.50 0.32 2.12 0.00 $\infty$ 2.33 0.00 20 min 1.16 0.43 0.88 12.34 2.00 0.67 0.17 2.19 0.00 $\infty$ 2.33 0.00 30 min 1.20 0.40 0.85 18.30 2.00 0.81 0.13 1.99 0.00 $\infty$ 2.33 0.00

Fitting parameters
Sample	Moisture		Carbon		Si native oxide		Si substrate		Fitting parameters
	Delta	Beta	Delta	Beta	Delta	Beta	Delta	Beta	Intensity	Background	Fitting area ($2\Theta$)	R	$\mathrm {X} ^{2}$
10 min	1.3932E-6	3.2352E-8	6.4587E-6	9.7010E-9	6.9134E-6	1.6054E-7	7.5860E-6	1.7616E-7	1.14309	7.02370E-8	0.2666 - 5.000	0.02092	0.01081
20 min	1.4031E-6	3.2581E-8	6.4279E-6	9.6547E-9	7.1423E-6	1.6585E-7	7.5860E-6	1.7616E-7	0.93079	1.96097E-8	0.2666 - 4.000	0.01987	0.00899
30 min	1.2905E-6	2.9968E-8	6.4412E-6	9.6747E-9	6.4832E-6	1.5055E-7	7.5860E-6	1.7616E-7	0.88610	3.11353E-8	0.3143 - 4.000	0.02722	0.01877

Graizing Incident Diffraction (GiXRD)

Graizing Incident Diffraction (GiXRD) of 60 nm C sample (deposited at 6000s, 200W, 3 mTprr, DC, Src1) has been performed to investigate the phase profile. The analysis revealed the amorphous film.

The scan has been obtained using Rigaku XRD SmartLab equipment. The voltage and current settings for the Cu X-ray tube were standard 40kV and 30mA. The incident optics contained a IPS (incident parallel slit) adaptor with 5 ° Soller slit. Other conditions: IS=1 mm, ω=2 ° PSA 0.5 ° RS1=RS2=open, Lenght-limiting slit=15mm, Speed: 0.5 ° /min. Angle range: 20-90 2θ.

• GiXRD analysis.
• 
  Amorphous film. No diffraction peaks.

X-ray Photoelectron Spectroscopy

Oxygen is almost non existing in the film (around or less than 1 at.%). The signal eventually rises during the depth profiling. It comes from the Carbon-Silicon interface (native oxide). No other contaminants have been found.

• Carbon XPS analysis.
• 
  Atomic concentartion as a fnction of depth. Etching parameters: 1000eV, low current, 10s@50 levels.
• 
  Survey spectrum after 10s Ar⁺ etch (1000eV, low current).

• Carbon XPS measurements. High-Resolution profiles.
• 
  C 1s.
• 
  O 1s.
• 
  Si 2p.

• Carbon XPS measurements. High-Resolution profiles in 2D
• 
  2D image of XPS depth profiles.

Scanning Electron Microscopy

• Carbon deposition. SEM inspection.
• 
  C deposited on high aspect ratio silicon structure (trenches) with 200W power and 3 mTorr, 20min. Cross-sectional tilted view.

Spectroscopic Ellipsometry

Results have been obtained for Si wafers with native oxide, based on ellipsometry study. The Bspline model has been used for analysis.

• Optical functions based on Spectroscopic Ellipsometry method.
• 
  Refractive index of carbon deposited at 30 min, 200W, 3 mTorr, DC, Src1.
• 
  Absorption coefficient of carbon deposited at 30 min, 200W, 3 mTorr, DC, Src1.

Conductivity

Sheet resistance has been performed using Four point probe – Jandel. The deposited carbon film (6nm at 600s, 200W, 3 mTorr) on top of a microscope glass slide showed 178-223 MOhm/sq range.

Deposition Recordings

• Deposition recordings (30min) Src1 DC.
• 
  Ar flow recording. Thin layer (1800s deposition time).
• 
  Pressure recording. Thin layer (1800s deposition time).
• 
  Power recording. Thin layer (1800s deposition time).
• 
  Voltage recording. Thin layer (1800s deposition time).