Specific Process Knowledge/Thin film deposition/Deposition of Vanadium Silicide/V3Si RF-DC Co-Sputtering in Cluster Lesker PC3

This page is written by Evgeniy Shkondin @ DTU Nanolab if nothing else is stated.
All images and data on this page belong to DTU Nanolab.
The exploratory deposition and characterization described below were performed in 2025.

Exploratory V-Si RF-DC co-sputtering in Cluster Lesker PC3

This page documents an early, non-systematic attempt to deposit vanadium silicide by simultaneous RF sputtering of a 4-inch Si target and DC sputtering of a 3-inch V target in the Cluster Lesker PC3 chamber. Two otherwise nominally identical depositions were compared: one without RF substrate bias and one with 30 W RF substrate bias.

The experiment gave a striking result. In the measured 30–80° GiXRD range, the unbiased sample showed only reflections assigned to cubic A15 V₃Si, whereas the sample deposited with 30 W RF substrate bias showed only reflections assigned to hexagonal C40 VSi₂. The two samples also had very different surface morphologies and sheet resistances.

This PC3 experiment is separate from the later, systematic V₃Si process-development study performed in PC1.

Substrate and deposition conditions

• Substrate: 150 mm Si(100) wafer
• Surface preparation: native SiO₂ removed by HF immediately before deposition
• Si source: 4-inch Si target, RF sputtering
• V source: 3-inch V target, DC sputtering
• Working pressure: 3 mTorr
• Deposition temperature: 600 °C
• Deposition time: 1200 s
• Substrate-bias comparison: no RF bias versus 30 W RF bias
• Recorded DC substrate-bias voltage at 30 W RF bias: 69 V

PC3 V-Si co-sputtering conditions

Condition	Si target	Si power	Displayed Si voltage	V target	V power	Displayed V voltage	Pressure	RF substrate power	Substrate DC bias	Temperature	Time
No substrate bias	4-inch Si	120 W RF	100 V	3-inch V	100 W DC	322 V	3 mTorr	0 W	Not applied	600 °C	1200 s
30 W RF substrate bias	4-inch Si	120 W RF	110 V	3-inch V	100 W DC	309 V	3 mTorr	30 W	69 V	600 °C	1200 s

Although the applied target powers were unchanged, the displayed Si and V discharge voltages changed when the substrate RF bias was enabled. This shows that the plasma impedance/coupling was not identical in the two runs. Consequently, the elemental rates measured in separate calibration depositions should be treated as nominal flux estimates rather than exact in-situ co-deposition rates.

Power calibration and estimated V:Si atomic-arrival ratio

The individual elemental deposition rates measured at 3 mTorr were:

Individual elemental-rate calibration

Element	Target size	Mode	Power	Pressure	Pure-element thickness rate	Density used	Molar mass used
Si	4 inch	RF	120 W	3 mTorr	0.024 nm/s	2.329 g/cm3	28.0855 g/mol
V	3 inch	DC	100 W	3 mTorr	0.0700 nm/s	6.11 g/cm3	50.9415 g/mol

The thickness-rate ratio was therefore:

$\frac{r_V}{r_{Si}}=\frac{0.0700}{0.024}=2.92$

This number cannot be compared directly with the 3:1 atomic stoichiometry of V₃Si. For an elemental rate calibration, the atomic arrival rate is proportional to:

${\mathrm{\Phi }}_i\propto \frac{{\rho }_i{r}_i}{M_i}$

where ${\rho }_i$ is the elemental density, $r_i$ is the pure-element thickness rate, and $M_i$ is the molar mass. The nominal target-derived atomic-arrival ratio is consequently:

$\frac{{\mathrm{\Phi }}_V}{{\mathrm{\Phi }}_{Si}}=\frac{{\rho }_V{r}_V/M_V}{{\rho }_{Si}{r}_{Si}/M_{Si}}=4.22$

Thus, the flux supplied by the two targets was nominally about 80.8 at.% V and 19.2 at.% Si, corresponding to V_4.22Si rather than V₃Si. The pure-element thickness-rate ratio required for an atomic V:Si ratio of 3 is approximately:

${\left(\frac{r_V}{r_{Si}}\right)}_{V_{3}Si}=3\frac{M_V/{\rho }_V}{M_{Si}/{\rho }_{Si}}=2.07$

The calibrated target flux was therefore substantially more V-rich than the ideal V₃Si value. The formation of V₃Si in the unbiased sample cannot be explained by the target flux alone; Si supplied by the oxide-stripped Si wafer must be considered.

Simple target/substrate mass balance

Using the calibrated rates and 1200 s deposition time, and assuming for the moment that all deposited V was retained:

• formation of V₃Si requires approximately 11.7 nm of substrate-equivalent Si in addition to the Si supplied by the target;
• the corresponding full-retention V₃Si layer thickness is approximately 105 nm using a bulk V₃Si density of 5.77 g/cm³;
• the measured no-bias XRR thickness was 94.046 nm, about 89% of this simple mass-balance value.

The agreement is reasonable for separately calibrated rates, compound formation, a simplified XRR model, and a process performed at 600 °C. It supports substrate participation but is not an independent composition measurement.

If all calibrated V instead formed VSi₂, the limiting mass balance would require approximately 214 nm of additional substrate-equivalent Si and would produce roughly 244 nm of VSi₂ using a density of 4.42 g/cm³. These are not measured values for the biased sample. They illustrate that formation of VSi₂ throughout the deposited material would require extensive Si uptake from the wafer, a change in retained V flux, or both.

Characterization methods

X-ray reflectivity (XRR)

XRR measurements were performed using a Rigaku SmartLab X-ray diffractometer. The Cu X-ray tube was operated at 40 kV and 30 mA. The incident optics included an IPS (incident parallel-slit) adapter with a 5° Soller slit. The slit configuration was IS = 0.05 mm, RS1 = 0.05 mm and RS2 = 0.075 mm. The programmed scan used a step size of 0.01° and a counting time of 2 s per point. The MATLAB analysis reads the actual exported angular coordinates and does not assume a fixed data interval.

Grazing-incidence X-ray diffraction (GiXRD)

GiXRD measurements were performed using the Rigaku SmartLab instrument with a Cu sealed X-ray tube operated at 40 kV and 30 mA. The scans covered 30–80° in 2θ, using a 0.5 mm incident slit, a step size of 0.01° and a scan speed of 2°/min. The incident angle was fixed at $\omega =2^{\circ }$ relative to the sample surface.

The shallow fixed incidence increases the film contribution relative to a conventional symmetric scan. Phase assignment is based on the observed reflection series. Reported lattice parameters are approximate because the scans were not subjected to a full instrumental-zero/refraction refinement and the sealed-tube spectrum can contain unresolved Cu Kα contributions.

Scanning electron microscopy (SEM)

Plan-view SEM images were recorded at 5.00 kV with the InLens detector. Images were acquired at 200,000× and 300,000× magnification. The original scale bars and acquisition metadata are retained in the TIFF images.

Sheet resistance

Room-temperature sheet resistance was measured for both samples. Resistivity is calculated only for the unbiased sample because only that sample has a defensible XRR thickness.

Results overview

Measured and fitted results

Condition	Crystalline phase detected by GiXRD	Lattice parameters from GiXRD	XRR thickness	XRR density	XRR roughness	Sheet resistance	Calculated resistivity
No substrate bias	V3Si, cubic A15	$a=4.731\pm 0.003$ Å1	94.046 nm	4.741 g/cm3	7.873 nm	29.34 Ω/sq	276 µΩ·cm
30 W RF substrate bias (69 V DC bias)	VSi2, hexagonal C40	$a=4.574$ Å, $c=6.376$ Å1	Not obtained; surface too rough for a defensible fit	Not obtained	Not obtained	6.64 Ω/sq	Not calculated because thickness is unknown

¹ Values are derived from pseudo-Voigt fits to several assigned GiXRD reflections using Cu Kα₁ = 1.540593 Å. They should be regarded as approximate GiXRD lattice parameters, not high-resolution lattice refinements.

Grazing-incidence X-ray diffraction results

• 
  GiXRD comparison of the two PC3 depositions. The temperature shown in the figure is corrected to 600 °C, even though the original data filenames contain 500C.

Unbiased film: A15 V₃Si

The unbiased sample contains the characteristic A15 V₃Si reflection series. The main fitted peaks are:

Reflection	Fitted 2θ	Approximate apparent coherence length2
V3Si (200)	38.03°	13.0 nm
V3Si (210)	42.70°	10.8 nm
V3Si (211)	46.99°	14.4 nm
V3Si (320)	71.95°	16.8 nm
V3Si (321)	75.00°	12.5 nm

The mean cubic lattice parameter obtained from the five reflections is $a=4.731\pm 0.003$ Å. This is slightly larger than the commonly reported room-temperature bulk value of approximately 4.72 Å, but the difference is too small to interpret confidently without a zero-shift/reference correction and a dedicated high-resolution scan.

30 W RF-biased film: C40 VSi₂

The biased sample contains the reflection series expected for C40 VSi₂. The fitted peaks used for the lattice calculation are:

Reflection	Fitted 2θ	Comment
VSi2 (110)	39.43°	Isolated peak
VSi2 (111)	41.96°	Fitted together with (003)
VSi2 (003)	42.50°	Fitted together with (111)
VSi2 (112)	48.98°	Several C40 reflections are close in this region
VSi2 (031)	73.12°	High-angle constraint
VSi2 (032)	78.32°	High-angle constraint

A simultaneous least-squares solution of the hexagonal d-spacing relation gives $a=4.574$ Å and $c=6.376$ Å, in good agreement with reported C40 VSi₂ lattice parameters.

In both samples, “single phase” means that no other crystalline phase was resolved above the detection/background limit within the measured 30–80° range. The scans do not exclude amorphous material, a thin interfacial phase, or a minor crystalline fraction hidden by peak overlap.

² The coherence lengths are Scherrer estimates based on uncorrected fitted FWHM values. Instrumental broadening, microstrain, texture, peak overlap, and the Cu Kα doublet were not separated. These numbers are therefore approximate coherent-domain sizes and must not be equated directly with SEM surface-feature size.

X-ray reflectivity results

• 
  XRR comparison and the exported fit for the unbiased sample. The strong damping/absence of useful fringes from the biased sample prevents a reliable thickness fit.

The unbiased sample could be represented by a simplified single vanadium-silicide layer on Si:

• thickness: 94.046 nm;
• fitted density: 4.741 g/cm³;
• fitted layer roughness: 7.873 nm;
• thickness-derived growth rate: 0.0784 nm/s.

The fitted density is about 82% of the 5.77 g/cm³ bulk value used for V₃Si. This can reflect porosity/voids, oxidation, model-density/roughness correlation, lateral non-uniformity, or a more complicated interface than the one-layer model. XRR density alone is not a chemical-composition measurement.

The 30 W RF-biased film was too rough for a defensible XRR model. No thickness, density, roughness or resistivity should be quoted from this scan. A cross-sectional measurement is needed.

Further characterization and data access

SEM images, electrical properties, sheet resistances, and the complete integrated interpretation are available on the corresponding LabIntra page or upon request. Please contact Evgeniy Shkondin, DTU Nanolab, for further details and results.