This document summarizes an experiment conducted to develop a sputtering process to fabricate tungsten silicon nitride (WSiN) thin film resistors with a target sheet resistance of 2000 Ω/sq for use in new electronic testing equipment. The researchers varied the nitrogen to argon ratio in the sputtering atmosphere and the deposition time to control the film thickness and increase the sheet resistance from an initial 250 Ω/sq to the target value. Stress, thickness, sheet resistance, and uniformity values were measured for different depositions. The process was able to successfully produce WSiN thin film resistors with a sheet resistance of 2000 Ω/sq and a standard deviation below 10%, meeting the requirements.

1. Raymond Chen1, Antonio Cruz1, Martin Kwong1, Jack Lam1, Niteesh Marathe1, Camron Noorzad1, Yongsheng Sun1, Cheng Lun Wu1, Disheng Zheng1
Ricardo Castro1, Michael Powers1,2
1UC Davis Department of Chemical Engineering and Materials Science; 2Keysight Technologies
• Among others, Kang, et al showed
that 𝑅S increased with
concentration of N2 in sputtering
atmosphere [1].
• This experiment sought to develop
a process to increase the sheet
resistance ofW-Si-NTFRs to a
target value of 2000 Ω/sq.
Beginning with a deposition time of 20 minutes, N2/Ar was varied from 10% to
20% until the measured sheet resistance 𝑅S was close to the target value.
Deposition time was then adjusted to control thickness:Assuming resistivity 𝜌
and average deposition rate are constant at a given N2/Ar, 𝑅S varies with
thickness 𝑡 according to 𝑅S ∝ 𝑡−1
. Stress measurements were taken for each
wafer. Patterned wafers were made using photoresist to allow thickness
measurements for each deposition.
• Manufacturer of testing
and measurement
equipment
• Interested in expanding
into new markets with
two new platforms—
combined leverage
sales of $13M
• New platforms require
thin film resistors
(TFRs) with sheet
resistance (𝑅S) of 2000
Ω/sq
• Current fabrication
techniques yielded only
250 Ω/sq
26.5 GHz FieldFox Handheld Combination
Analyzer
1000
2000
3000
850 950 1050 1150
RS(Ω/sq)
Thickness (Å)
Results: Sheet Resistance
Results: Film Stress
Background and Motivation
Experiment Details
0%
25%
50%
75%
100%
0
1000
2000
Wafer 8 Wafer 9 Wafer 10
Ω/sq.
Sheet Resistance
Uniformity
Standard Deviation
The figure above shows a comparison of important film properties
across different depositions and wafers.Wafers 8, 9 and 10 used
the same deposition parameters.Wafers 8 and 9 were a batch-to-
batch comparison, while wafers 9 and 10 were a single-batch wafer-
to-wafer comparison.
Does the process work?
Results: Microstructure and Composition
EBSD showed no
crystallinity, suggesting
the film is amorphous.
Acetone
Wash
FEI SCIOS
Dual Beam
FIB, SEM
Stress
Measurement 2
Sheet Resistance
Measurement
Thickness
Measurement
Tencor P2
Long Scan
Profiler
CVC 601 Reactive
Sputtering System,
WSi3N4 target
4D Model
280C Four
Point Probe
SEM
EDXS
EBSD
Stress
Measurement 1
Fabrication
Tencor P2
Long Scan
Profiler
Si wafer
WSiN
film
WSiN
pattern
1000
2000
3000
0.1 0.12 0.14 0.16 0.18 0.2 0.22
RS(Ω/sq)
N2/Ar Ratio
Conclusions
By varying both the N2/Ar ratio of the sputtering
atmosphere and the thickness of the film, the
sheet resistance of aW-Si-N thin film resistor was
successfully increased from 250 Ω/sq to 2000 Ω/sq
with a standard deviation of less than 10%. Film
thickness was greater than 750 Å. For the CVC 601
RF magnetron sputtering system, the fixed
deposition parameters were
• RF Power: 750 W
• Substrate bias: −60 V
• Gas flow rate: 40 sccm
• Total system pressure: 10 mTorr
The values of the variable deposition parameters
as determined in this experiment were
• N2 flow rate: 5.2 sccm
• Ar flow rate: 34.8 sccm
• Deposition time: 1027 sec
Stress values were acceptable as no delamination
was observed in the samples. Uniformity values
for wafers 8, 9 and 10 were close to, but slightly
above, the target value of 10%. As discussed, a
possible source for these results is theWSi3N4
target nearing the end of its lifetime. A new target
should improve uniformity.
Further investigation could determine the film’s
composition. RBS or XPS are viable options.
However, in light of the success of this
experiment, determination of the film
composition was not considered necessary.
Ultimately, these thin film resistors will be deposited on GaAs
substrates.The potential for adverse effects due to the
switching of the substrates was investigated.
Sheet resistance clearly increases with N2/Ar ratio. Given that thickness
could be reduced to increase RS, and using less material is better,
deposition time was decreased for the 0.15 N2/Ar case.
At a given N2/Ar ratio, sheet resistance increased as thickness
decreased, according to 𝑅S = 100𝜌/𝑡, where resistivity 𝜌 was assumed
constant, and 𝑡 is measured in angstroms. All RS values were within 10%
of the target.
Compositional EDXS analysis
was confounded by the
thickness of the film. At 10 keV,
electrons penetrated the sample
well beyond the film, reaching
into the Si substrate.0
500
1000
1500
2000
0.1 0.12 0.14 0.16 0.18 0.2 0.22
-σ(MPa)
N2/Ar
Fixed Sputtering Parameters
RF power 750 W
Substrate bias −60 V
Total system
pressure
10 mTorr
Total flow rate 40 sccm
Target Parameters
RS 2000 Ω/sq
Standard
deviation
±10%
Uniformity ±10%
Thickness > 750 Å
Vary N2/Ar,
deposition time
Substrate
Film
TheW- and N-rich areas in the above (a) top-down
and (b) edge-on views clearly indicate the location of
the film.
(a)
(b)
Si W N
Residual film stress decreased with increasing N2/Ar. Stress
values were higher than those measured by Lahav, et al [2]. An
effect of severely stressed films would be delamination from the
substrate; no delamination was observed.
The major goal of the study was to develop a reliable process for
manufacturing WSiN thin film resistors with the desired sheet
resistance. Once appropriate deposition parameters had been
determined, reproducibility of the results was examined.
The figure at left shows a comparison between
worn and new sputtering targets. On the left
side is an example of a worn sputtering target.
The wear pattern affects the direction of
sputtered atoms.TheWSi3N4 target used in
these depositions was nearing the end of its life
and likely had a similar wear pattern, resulting
in some of the observed variations in sheet
resistance and uniformity.
The authors would like to thank Nicholas Kiriaze at
KeysightTechnologies, for his help operating the
equipment; Rijuta Ravichandran at the University of
California, Davis for her expertise running the SEM;
Vache Harotoonian at the University of California, Davis
for his help during the characterization process; Steven
Zhang for enlightening discussions; and Michael Powers
and Ricardo Castro for their guidance.
Acknowledgements
References
1. S. M. Kang, et al, Control of electrical resistivity ofTaN
thin films by reactive sputtering for embedded passive
resistors, Thin Solid Films, vol. 516, no. 11, pp. 3568-3571,
April 2008
2. A. Lahav, et al, Measurement of thermal expansion
coefficients ofW,WSi, WN andWSiN thin film
metallizations, Journal of Applied Physics, vol. 67, no. 2,
pp. 734-738, January 1990
3. A.Vomiero, et al, Composition and resistivity changes of
reactively sputteredW-Si-N thin films under vacuum
annealing, Applied Physics Letters, vol. 88, no. 3, 031917-1-
031917-3, January 2006
4. M. Powers, Sputter Deposition ofThin Films in HFTC, Santa
Rosa, CA: KeysightTechnologies, 2015. (slides)
Side-by-side comparison of a used 4-inchTi target and an
unused 8-inchW target. Source: [4]
Sheet resistance increases with N2 content in
sputtering atmosphere. Source: [1]
Substrate
SEM micrograph,
edge-on view
SEM micrograph,
edge-on view
Si
N
WSchematic of amorphousWSiN network:
atoms are arranged randomly and have
only short-range order; N occupies
interstitial-like sites.
Sources of Error
The CVC 601 is a decades-old system. Slight variations in the results from depositions with
identical parameters may be linked to the age of the sputtering system.
Testing vs. Production
As-depositedWSiN films normally exhibit higher compressive stresses with GaAs substrates
than with Si substrates. Stress can be reduced via annealing at 400 °C for 20 minutes [2].The
figure above compares resistivity changes with annealing temperature for WSiN films with
various compositions: as a general rule, this heat treatment would not decrease the
resistivity [3].
Substrate
SEM micrograph,
edge-on view
Film
Film
Substrate Film
Variation of resistivity with annealing
time inWSiN films. Source: [3]
WSiN
film