This document describes a project to develop a manufacturing process for thin film resistors with very high sheet resistance (TFRVHs) using tungsten-silicon-nitride (WSiN) for use in high-frequency integrated circuits. The goal was to produce TFRVHs with a sheet resistance of 2000 Ω/sq using reactive sputtering. Experimental results found that the developed process successfully produced TFRVHs within the target specifications for thickness, sheet resistance, and standard deviation, but uniformity was slightly outside the target range. The process is evaluated to be cost-effective and reliable for manufacturing the TFRVHs.

1. 0
EMS 188
Senior Design Project
June 3, 2015
Raymond Chen, Antonio Cruz, Martin Kwong, Jack Lam, Niteesh
Marathe, Camron Noorzad, Yongsheng Sun, Cheng Lun Wu,
Disheng Zheng;Ricardo Castro, Mike Powers
University of California, Davis, Department of Chemical Engineering and Materials
Science One Shields Ave, Davis, CA, 95616
REACTIVE SPUTTERING TO
INCREASE SHEET RESISTANCE OF
WSIN THIN FILM RESISTORS

2. 1
Table of Contents
EXECUTIVE SUMMARY 2
1. PROJECT INTRODUCTION
1.1. Objectives
1.2. Technical review
3
3
4
2. TFRVH MANUFACTURING PROCESS DESIGN
2.1. Design approach
2.2. Experimental procedures
2.3. Characterization
6
7
8
10
3. PROJECT OUTCOMES
3.1. Experiment results
3.2. TFRVH manufacturing process evaluation
3.3. Cost analysis and process assessment
3.4. Future work
13
13
14
20
22
4. CONCLUSION AND RECOMMENDATIONS 22
ACKNOWLEDGEMENTS 25
REFERENCES 26
APPENDIX A 27

3. 2
EXECUTIVE SUMMARY
Growth is good for business, and Keysight wants to grow by expanding into new
markets with two new testing and measurement platforms. The components of
these platforms include high-frequency monolithic microwave integrated circuits
(MMICs) which themselves incorporate TFRVHs (thin film resistors with very
high sheet resistance). These are precision components, so a robust, cost-effective
and reliable manufacturing process was needed.
Tungsten-silicon-nitride (WSiN) is a material system that is commonly used in
thin film resistors. Reactive sputtering is a physical vapor deposition method
frequently used to manufacture thin film resistors. Keysight has previous
experience with both the WSiN material system and reactive sputtering, which
proved very beneficial to this project.
The process developed over the course of the project successfully and reliably
fabricated WSiN TFRVHs with the target sheet resistance of 2000 Ω/sq.
Furthermore, the process repeated the results, both between batches and between
TFRVHs in the same batch. Specifically, the standard deviation of all successfully
manufactured TFRVHs were well below the 10% target; the thicknesses of the
TFRVHs fell within the prescribed range of 750 to 1500 Å. Uniformity values fell
just outside the target 10% value. Also worth noting are the variation in thickness
and a 3% margin of error between TFRVHs manufactured with identical
sputtering parameters. These unexpected results may come from the fact that the
WSi3N4 sputtering target was nearing the end of its usable life, and significant
wear had developed on the surface, causing unpredictable behavior during the
deposition.
With the exception of the uniformity, all targets were met. The standard deviation
values indicate that more than a 90% yield of TFRVHs manufactured with the
process as-is will have sheet resistance within 10 to 12% of the target 2000 Ω/sq.
Both the uniformity and the yield are expected to increase upon replacement of
the worn WSi3N4 target.
The project came under budget. The major factor driving the cost was labor.
Materials and equipment accounted for less than 1% of costs. Overall, the process
was evaluated to be cost-effective and reliable. It is the recommendation of this
project that Keysight begin adapting the process for large-scale manufacture of
WSiN TFRVHs.

4. 3
1. PROJECT INTRODUCTION
In the interest of diversifying the business and expanding into new markets,
Keysight Technologies wants to develop two new platforms of testing and
measurement equipment. A component of these new platforms is a high-
frequency monolithic microwave integrated circuit (MMIC). The MMIC requires a
TFRVH (thin film resistor very high). As the name suggests, these TFRVH exhibits
a large resistance—2000 Ω/sq. Tungsten-silicon-nitride (WSiN) is a material
system known to achieve high resistance values [1]. Non-reactive sputtering, the
previously-used technique used for making WSiN TFRVHs, produced WSiN thin
film resistors with sheet resistances on the order of 250 Ω/sq; thus, an order of
magnitude increase in sheet resistance was required.
1.1. Objectives
The goal of this project was to develop a robust, cost-effective and reliable process
for manufacturing WSiN TFRVHs. The fabrication process must consistently
produce TFRVHs with the desired characteristics. Table 1 below lists the
important characteristics and their desired values [1].
TABLE 1. Target values of important TFRVH parameters as
determined by the intended use in high-frequency MMICs.
PARAMETER SYMBOL TARGET VALUE
Sheet resistance 𝑅S 2000 Ω/sq
Standard deviation 𝜎 10%
Uniformity 𝑢 10%
Thickness 𝑑 750 Å < 𝑑 < 1500 Å
Sheet resistance is the most important property of these TFRs and achieving the
target value of 2000 Ω/sq was the main focus of the project. The standard
deviation and uniformity characterize the reliability of the process to produce
TFRVHs with the desired sheet resistance. The thickness requirement eliminates
substrate effects such as high residual stress and leakage current which can make
the electronic behavior of the resistor unpredictable.
This report will outline the project, beginning with identification of the problem,
including the problem scope, a technical review of relevant prior work and
detailed design requirements; to an overview of the design approach, including

5. 4
experimental procedures; to a comprehensive evaluation of the process and the
entire project, including a discussion of experimental results, an assessment of the
financial impact of the project, and a discussion of the project’s future. Finally, a
conclusion will summarize the project and its outcomes, making specific
recommendations for Keysight Technologies regarding the two new testing and
measurement platforms they hope to develop.
1.2. Technical review
A review of the literature on WSiN and reactive sputtering, vis-à-vis attempts to
increase the sheet resistance of thin film resistors, was performed.
1.2.1. W-Si-N as a material system for thin film resistors
WSiN is used as a material for thin film resistor because it can form an excellent
barrier layer even though extremely thin [2]. During sputtering, an RF bias is
applied to the substrate to increase the nitrogen content in the thin film, and
thereby increase the resistivity.
As-deposited WSiN thin film are typically amorphous. However, annealing at 800
℃ causes crystallization [3]. WSiN thin film is very effective at blocking atomic
diffusion. The nitrogen atoms occupying the interstitial sites of the Tungsten and
silicon amorphous network increase the resistivity. WSiN thin films must be kept
away from heat getting close to 800 ℃ since it would cause a significant drop in
resistivity due to crystallization. The coefficient of thermal expansion of WSiN is
6.37 × 10−6
℃−1
. The coefficient of thermal expansion of Si is 3.45 × 10−6
℃−1
.
This difference can result in significant thermal stresses if the Si substrate is heated
during deposition.
During film growth N preferentially bonds to Si rather than to W due to the higher
affinity. Affinity is the tendency of molecules to associate with each other. Once Si
binds N atoms, enough that a saturation is reached, N starts to either bind with W,
or is weakly trapped inside the matrix of the film. Temperatures over 750 ℃ will
break the bonds between N and W and cause losses in resistivity [4].

6. 5
1.2.2. RF magnetron sputtering for manufacturing thin film resistors
Reactive sputtering is a common method for fabricating thin film resistors. This
project made use of Keysight’s CVC 611 RF (radio frequency) Magnetron reactive
sputtering system. The CVC 611 has been used by Keysight for more than twenty
years. The information in this section was taken from Chapter 4: Sputtered Films
of Thin Film Technology by R. W. Berry, et al [5].
Broadly speaking, reactive sputtering is a kind of physical vapor deposition
process. Physical vapor deposition is distinguished from chemical vapor
deposition by the absence of a chemical reaction at the surface of the substrate.
Instead, a target is bombarded by energetic ions. The collisions knock or sputter
atoms from the target. The sputtered atoms travel through the sputtering chamber
and adsorb to the substrate. This deposition process continues until the
experiment is finished.
Argon gas was used because it is inert and massive enough to not react with other
atoms in the chamber and to give a high sputter yield. A high sputter yield is
necessary for the manufacturing process to be timely and efficient.
Specifically, Ar ions formed a plasma that served to sputter target atoms. The
power supplied to the system is oscillated at radio frequencies which sustains the
plasma even at the low pressures required for the vacuum in the chamber. The
high vacuum improves the uniformity of the deposited film by lowering the
sputtering rate and by increasing the mean free path of sputtered atoms so they
have more kinetic energy during adsorption. Furthermore, RF sputtering
resputters any insulating layers that might form during the process. A magnetron
is used during the process to sustain the plasma: electrons are trapped by the
magnetic field and so there are more electron-Ar collisions. Ultimately, the
inclusion of the magnetron leads to higher deposition rates as well as lower
substrate temperature due to less electron bombardment.
The substrate can be set at neutral, negative or positive electrical potential relative
to the target. If the substrate is neutral—that is, electrically isolated from the rest of
the chamber—the substrate will take on the potential of the plasma and neither
electrons nor the Ar ions will be pulled toward the film any significant amount:
bombardment becomes minimized. If the substrate is positively or negatively
biased, electrons or Ar ions, respectively, will be preferentially pulled toward the
substrate. Each of these cases would present changes in the microstructure of the
film.

7. 6
Another feature of the CVC 611 is its ability to rotate the substrates during the
deposition. Because the cloud of sputtered atoms is not isotropic—target atoms are
preferentially sputtered along the close-packed directions—rotation of the
substrates improves uniformity.
Deposition rate 𝑄 is proportional to the sputtering yield 𝛾, which is itself
dependent on the energy and mass of the sputtering species, as well as other
factors. The proportionality constants are the ion current 𝐼 and a constant 𝐶 that is
specific to the sputtering system:
𝑄 = 𝐼𝐶𝛾 (1)
For this project, 𝑄 was approximated as average rate 𝑄 ≡ 𝑄AVG, and generally
assumed to be constant at a given N2/Ar.
2. TFRVH MANUFACTURING PROCESS DESIGN
As described in §1.2 above, the reactive sputtering process incorporates a number
of different phenomena and physical effects, some of which are at odds with each
other. The parameters of a given deposition, then, must be carefully chosen so that
a reasonable deposition time will result in a film with appropriate thickness,
microstructure, composition, and electrical properties that serve its intended
application. This project made convenient use of Keysight’s practiced familiarity
with the CVC 611 system to fix several parameters, which have been tabulated
below:
TABLE 2. Fixed CVC 611 sputtering parameters,
determined heuristically by Keysight’s long use of
the system.
PARAMETER FIXED VALUE
RF power 750 W
Substrate bias −60 V
Total system pressure 10 mTorr
Total flow rate 40 sccm
Keysight has determined through many years of sputtering that these values
produce usable films with reasonable uniformity, low residual stress and low Ar
contamination [1]. With the parameters in Table 2 fixed, the deposition variables

8. 7
become the amount of the reactive gas N2 in the sputtering atmosphere and
deposition time, which actually controls thickness.
2.1. Design approach
To determine the appropriate N2/Ar ratio in the sputtering atmosphere, previous
work to increase the sheet resistance of thin films was investigated. S. M. Kang, et
al, showed that sheet resistance of TaN thin film resistors increased dramatically
as the N2/Ar+N2 gas flow ratio was raised above approximately 10%.
FIGURE 1. Sheet resistance increases with nitrogen partial
pressure in the sputtering atmosphere. Above approximately
10% N2, sheet resistance increases significantly. Source: [6]
N2 is the reactive gas that is supplied to the sputtering system; Ar is the inert gas
that forms the plasma and bombards the target. The increase in sheet resistance
with increased reactive gas concentration in the sputtering atmosphere is a
phenomenon that has been observed in other systems as well.
Thus, the principle mechanism for increasing the sheet resistance of WSiN thin
film resistors was determined to be increased N2 content in the sputtering
atmosphere. The sputtering system accepts N2 and Ar flow rates. Given the total
flow rate of 40 sccm, a desired N2/Ar ratio 𝑁 gives N2 and Ar flow rates
𝐹N2
= 40
𝑁
𝑁 + 1
(2a)

9. 8
and
𝐹Ar = 40 − 𝐹N2 (2b)
respectively. The secondary mechanism of controlling the sheet resistance was
thickness, according to Equation (3) below:
𝑅S = 𝜌
100
𝑑
(3)
where 𝜌 is resistivity in Ω ∙ cm and 𝑑 is thickness in Å to give a sheet resistance 𝑅S
in Ω/sq. As a process, sputtering offers control of film thickness via deposition
time. Keysight’s experience with the CVC 611 sputtering system established an
initial deposition time of 20 minutes to give a film thickness on the order of 103
Å.
It is worth noting that although Equation (3) indicates that an order-of-magnitude
decrease in thickness would give 𝑅S~2000 Ω/sq, such a thickness would result in
undesirable substrate effects, such as high residual stress, buckling, delamination,
and leakage current, as well as unpredictable resistivity due to the high degree of
strain in the atomic bonds. Therefore, simply reducing film thickness until the
target sheet resistance is reached is not enough; the resistivity must be changed
using reactive sputtering.
Given that, in general, using less time and material to manufacture something is
more cost-effective, this project’s goal was to select an N2/Ar ratio such that a
sheet resistance below the target value was achieved; then, deposition time and
film thickness were to be reduced according to Equation (3) to increase sheet
resistance to the target value.
2.2. Experimental procedures
There were two major phases of the experiment. The first phase determined the
appropriate N2/Ar ratio. The second phase varied deposition time to achieve the
desired sheet resistance. Each deposition involved a patterned wafer for thickness
measurements, and an unpatterned wafer which allowed both film stress and
sheet resistance measurements. Wafers were patterned using photolithography to

10. 9
create an array of rectangular WSiN films. The photoresist was removed using an
acetone wash; the wafers were soaked for about 10 minutes. Additional
characterization—scanning electron microscopy (SEM), energy dispersive X-ray
spectrometry (EDXS), and electron backscatter diffraction (EBSD)—was also
performed on select samples to determine microstructure and composition. Figure
2 below is a flowchart that provides an overview of the experiment.
FIGURE 2. Flowchart outlining the experiment design, including fabrication and characterization steps. Note that the
second stress measurement requires a user input from the thickness measurement..
2.2.1. Phase I: Varying N2/Ar
Depositions were carried out with a deposition time of 20 minutes [1] and input
parameters as outlined in Table 1. N2/Ar ratios of 0.1, 0.15, 0.17 and 0.2 were used.
Corresponding flow rates are tabulated below.
TABLE 3
TRIAL # RATIO 𝐹N2
(sccm) 𝐹Ar (sccm)
1,2,3 0.1 3.6 36.4
4 0.2 6.7 33.3
5 0.15 5.2 34.8
6 0.17 5.8 34.2
2.2.2. Phase II: Varying deposition time
For each deposition in Phase I, average deposition rate 𝑄AVG was calculated from
thickness and total deposition time 𝑇.
Stress
measurement 1
Fabrication
Stress
measurement 2
Thickness
measurement
Sheet resistance
measurement
SEM, EDXS,
EBSD, XRR
Tencor P12
profilometer
Tencor P2
profilometer
CVC 611 RF
Magnetron
Sputtering
system
Tencor P2
profilometer
4D Model 280C
4-point probe
FEI SCIOS Dual-
beam FIB, SEM
Si wafer
WSiN
film
Patterned
WSiN film

11. 10
𝑄AVG =
𝑑
𝑇
(4)
Once the appropriate ratio had been determined—achieving a sheet resistance
value close to, but below the target value—that deposition was repeated, but with
a new deposition time. If 𝑄AVG is assumed to be constant at a given N2/Ar ratio,
Equation (4) shows that 𝑇 is directly proportional to 𝑑. Equation (3) shows that 𝑅S
is inversely proportional to 𝑑. Considering these relationships, it can be shown
that
𝑇 = 𝑇0
𝑅S0
𝑅S
(5)
for 𝑇0 and 𝑅S0 at a given N2/Ar ratio. Deposition time 𝑇 was then calculated for
the target 𝑅S = 2000 Ω/sq. Trial 7 was carried out.
However, 𝑄AVG is not exactly constant, and measurements showed that sheet
resistance was above the target value. A new 𝑄AVG was calculated and the
deposition was revised to be slightly larger. Sheet resistance measurements for
Trial 8 hit the target. Trials 9 and 10 were attempts to reproduce the results of Trial
8.
2.3. Characterization
Characterization was carried primarily at Keysight. Sheet resistance, standard
deviation, uniformity, film stress and thickness measurements were all taken on
Keysight’s equipment. A scanning electron microscope equipped with energy-
dispersive X-ray spectrometry and electron back-scatter diffraction instruments,
and an X-ray diffractometer, both located at University of California, Davis were
used for compositional and microstructural analysis.

12. 11
2.3.1. Tencor P2 profilometer
Each stress measurement is the result of fitted data [7]. The Tencor P2 Long-scan
Profiler was equipped with stress measurement software. Specifically, a recipe in
the software measured the stress of thin films deposited on 3-inch-diameter Si
wafers. Pre- and post-deposition measurements were taken, and the film thickness
was input by the user. The software then fitted the differential data to the equation
𝑠 =
1
6𝑟
𝐸
1 − 𝜈
𝑑S
2
𝑑F
(6)
to give a value for the residual film stress, 𝑠. In Equation (6), 𝐸 and 𝜈 are Young’s
modulus and Poisson’s ratio of the substrate, respectively; 𝑑S and 𝑑F are the
thicknesses of the substrate and deposited film, respectively; and 𝑟 is the radius of
curvature of the substrate, given by
𝑟 =
𝐿2
8𝐵
, 𝐿 ≫ 𝐵 (7)
where 𝐿 is the scan length and 𝐵 is the maximum distance between the trace and its
chord.
2.3.2. 4-point probe
Sheet resistance was measured with a 25-point recipe on a 4D Model 280C 4-point
probe. The probe measured voltage 𝑉 and current 𝐼 at 25 points around the surface
of the film, and sheet resistance was calculated with the relationship
𝑅S = 4.53
𝑉
𝐼
(8)
The probe then calculated a mean sheet resistance, along with standard deviation
and uniformity values. Standard deviation 𝜎 describes how the individual data
points are distributed relative to the mean. For this probe, the data points are sheet
resistance values, and there are 25 of them:
𝜎 = √
1
25
∑(𝑅S𝑖 − 𝑅̅S)
25
𝑖=1
(9)

13. 12
Uniformity 𝑢 describes the range of values relative to the sum of the maximum
and minimum values:
𝑢 =
max −min
max + min
(10)
2.3.3. Tencor P12 profilometer
Thicknesses were measured on the patterned wafer with a Tencor P12
profilometer. The profilometer drags a stylus over the wafer surface, and
measures the change in height between the substrate and the film. On a given
wafer, measurements were taken in the center and at the left and right and top
and bottom extrema. The mean value of these measurements was taken as the film
thickness.
2.3.4. SEM, EDXS, EBSD, XRR
Micrographs were taken with a FEI SCIOS Dual-beam FIB SEM. Scanning electron
microscopy uses an electron beam to raster across the surface of a sample and
generate an image. The SEM was equipped with both EDXS and EBSD
instruments which were used to characterize composition and microstructure,
respectively. EDXS counts the X-rays generated by excited electrons returning to
their ground state. Each type of atom exhibits a unique energy difference between
the excited and ground-level states. The generated X-rays, therefore, have energies
corresponding to types of atoms, and the intensity of each type of X-ray gives a
stoichiometric ratio of constituent atoms in the sample. EBSD relies on electrons
bouncing off the surface of the sample.
X-ray reflectivity data were taken with a PANalytical X-pert Pro diffractometer. X-
ray reflection impinges the sample surface with X-rays and collects both the
diffuse and specularly reflected waves. The intensity of the reflected X-rays
changes as a function of the angle of incidence, material density, surface
roughness and other sample properties. The critical angle of reflection gives
density information; the period and amplitude of oscillations give thickness and
roughness information, respectively.

14. 13
3. PROJECT OUTCOMES
The outcomes of the project are discussed below. Experiment results are briefly
described; a comprehensive discussion of the results and an evaluation of the
process follow. This project per se was also evaluated.
3.1. Experiment results
Wafers 8, 9 and 10 were WSiN TFRVH’s with the desired target values. The
specific data are tabulated below.
TABLE 4. Target parameter results of three successful
depositions. All deposition parameters were identical. Wafers 8
and 9 represent a batch-to-batch comparison; wafers 9 and 10
represent a wafer-to-wafer comparison, within the same batch.
WAFER 𝑅S (Ω/sq) 𝜎 (%) 𝑢 (%) 𝑑 (Å)
8 1980 5.64 10.1 915
9 2060 5.86 10.1 974
10 2060 6.06 11.1 974
Each wafer exhibited a mean sheet resistance within a 3% margin of error of
2000 Ω/sq. The standard deviation over the surface of each wafer was well within
the 10% target. The thickness was between 750 and 1500 Å. Uniformity was
slightly outside the 10% range; possible reasons for this are discussed in §3.2.1
below. The deposition parameters that gave these results, including the fixed
parameters, are listed below.
TABLE 5. Deposition parameters
giving the desired TFRVH properties.
RF power 750 W
Substrate bias −60 V
Total system pressure 10 mTorr
N2 flow rate 5.2 sccm
Ar flow rate 34.8 sccm
Deposition time 1027 s

15. 14
3.2. TFRVH manufacturing process evaluation
The fabrication process developed over the course this project was evaluated with
respect to the objectives described in §1.1: in particular, sheet resistance, standard
deviation and uniformity, and film thickness. Residual film stress results are also
discussed below. Furthermore, the efficiency and cost-effectiveness of the process
and the reproducibility of the results were evaluated.
3.2.1. Sheet resistance, standard deviation, uniformity
Achieving the target 2000-Ω/sq sheet resistance was the primary goal of this
project. The sheet resistance values shown in Table 4 above indicate an effective
fabrication process, within an acceptable tolerance of ±3%. All standard deviation
values were within the 10% target, and all thickness values were well within the
range. Uniformity consistently fell outside the prescribed 10% limit.
The 3% tolerance was a process requirement. In the MMICs, designers set a 10%
margin of error; achieving a significantly smaller tolerance allows for greater
variations as the process is adapted for large-scale manufacturing.
Still, among Trials 8, 9 and 10 there was greater variation than expected for
identical input parameters. A possible reason for the variation is the age of the
sputtering system. Keysight has used the CVC 611 for decades; in fact, the CVC
611 is being retired, and operations will be shifted to a new system. This, however,
would not be expected to result in significant changes: systemic errors—e.g., those
caused by the age of the sputtering system—are regularly and carefully checked
for. Keysight maintains meticulous records on the output of their fabrication
equipment.
A more likely cause of the variation is the age of the sputtering target. The WSi3N4
target used for all the depositions was nearing the end of its usable lifetime, and
likely exhibited a wear pattern similar to the one in Figure 3 below.

16. 15
FIGURE 3. Comparison of a used 4-inch Ti target on left with an unused 8-inch
W target on the right. Note the pattern of severe wear on the Ti target,
characteristic of magnetron sputtering. Source: [1]
The 4-inch Ti target on the left is used. That wear pattern develops because the
magnetic field employed by the sputtering system to trap electrons also causes
preferential sputtering (the plasma is charged) in a characteristic circular pattern,
as is visible in the figure. As the topography of the target surface changes, the
interactions between the Ar+ and the target change, and sputtered atoms no
longer leave the surface, on average, uniformly, and the distribution of adatoms
over the surface of the growing film changes. Thus, there can be significant
changes in film composition and the corresponding properties over the surface of
the film.
Explanations for the poor uniformity, and larger-than-expected variation in sheet
resistance across identical depositions can appeal to the age of the sputtering
target; and, most importantly, such variation can be expected to disappear with
the installation of a new target.
3.2.2. Film thickness
Film thickness had the prescribed minimum value in order to prevent substrate
effects. If the film had been too thin, residual stress would have increased
dramatically, potentially causing buckling, delamination and unpredictable
electrical behavior, including the possibility of leakage current through the
substrate.

17. 16
(a) (b) (c)
FIGURE 4. SEM micrographs showing sections of the flat (a) and middle (b), (c) of Wafer 9. The text gives
thickness measurements. On the upper parts of each micrograph, the various shapes are organic surface
contamination due to handling of the samples.
Film thickness was also constrained by the dimensions of the MMICs and HBTs
into which it would be incorporated; and, economically speaking, production
costs can be systemically reduced by using less material per TFRVH. Figure 4
above shows SEM micrographs of wafer 9, which give secondary thickness
measurements. The darkly-contrasted shapes on the upper part of each
micrograph represent organic surface contamination from handling the samples.
Such contamination is not expected during normal production of WSiN TFRVHs.
Film thickness depended on deposition rate and time. Deposition rate was
assumed to be constant, but as discussed in §1.2 above and shown in Equation (1),
deposition rate depends on sputtering yield, ion current, and a system-specific
constant. The ion current and system constant would not be expected to vary for
these depositions, so the sputtering yield must have changed to account for the
deviation in measured thickness from Equation (3). Figure 5 below illustrates the
deviations at a N2/Ar ratio of 0.15.
Substrate
Film
Organic contamination

18. 17
FIGURE 5. Plot of sheet resistance vs. film thickness for identical
depositions. Both films should have exhibited the same resistance and
thickness, but they did not.
Wafers 8 and 9 had the same deposition time, and were expected to have the same
thickness. The sputtering yield, then, must have changed; again, the age of the
target may be to blame. Interestingly, wafers 9 and 10—part of the same
deposition session—had exactly the same thickness, suggesting that the sputtering
yield changes between depositions, and does not vary significantly during a single
deposition.
3.2.3. Film stress
An interesting result of the experiments was the inverse relationship between film
stress and N2/Ar ratio. The figure below illustrates the observed dependence.
FIGURE 6. A plot of film stress vs. N2/Ar ratio shows a
decreasing trend with increased N content in the sputtering
atmosphere.
1500
2000
2500
3000
850 900 950 1000 1050 1100
SheetResistance(Ω/sq)
Thickness (Å)
RS vs. d
N2/Ar=0.15
Wafer 8
Wafer 9
0
500
1000
1500
2000
0.1 0.15 0.2 0.25
CompressiveStress(MPa)
N2/Ar
Film Stress vs. N2/Ar

19. 18
Kim, et al argued [8] that although Ar+ bombardment can unintentionally heat the
growing film, the contribution of thermally-induced stress to the total residual
film stress is negligible. Increasing negative substrate bias can increase the film
stress, which, Kim suggests, is due to increased Ar contamination in the film. It
may be the case for this project, then, that as the N2/Ar ratio increased, less Ar in
the sputtering atmosphere led to less Ar contamination in the film and an overall
reduction in stress. Similarly, less Ar in the sputtering atmosphere leads to less
bombardment. The massive Ar+ ions tend to densify the growing film during the
sputtering process, and, as more atoms are forced to occupy a smaller volume,
compressive stresses arise. Thus, less Ar bombardment can lead to less
compressive stress.
The microstructure may also have affected the film stress. A study [9] of stress in
crystalline WN films found that there is a threshold across which an increase in N
content led to a decrease in film stress. The authors suggested that the interstitial
N stretched the lattice to such an extent that it collapsed into an amorphous
network, allowing it to accommodate the N with less residual stress. A similar
process may be at work in the WSiN thin films: amorphous networks have some
short range order, which may affect the ability of a film to accommodate
interstitial atoms.
It is important to note that, ultimately, these WSiN TFRVHs will be deposited on
GaAs and InP substrates—not Si. Lahav, et al, found that residual film stress
increased, by a factor of approximately 2, in WSiN films when grown on GaAs
instead of Si. One possible explanation for this effect is the difference in the
thermal expansion coefficients: 6.40 × 10−6
℃−1
for GaAs, compared to
3.45 × 10−6
℃−1
for Si [10]. However, Kim, et al, argue that residual stress caused
by thermal expansion is negligible if the substrate is not intentionally heated
during the deposition [8]. Lahav also showed that residual stress could be reduced
significantly by annealing the sample at 400 °C for 20 minutes [3], offering a
possible solution to increased residual film stress.
3.2.4. SEM, EDXS, EBSD, XRR
SEM provided visual corroboration of the thickness measurements. Extensive
surface contamination was observed, but this was determined to be organic in
nature, and caused by handling the samples throughout the project. (See Figure 4
in §3.2.2.) EDXS analysis for wafer 9 was carried out, but due to a minimum
electron penetration depth of 1 μm for a beam energy of 10 keV, the interaction

20. 19
volume penetrated well beyond the 0.09 μm-thick film into the Si substrate and
results were skewed. See Figure A4 in Appendix A.
Because a specific composition was not an objective of this project, and timeliness
was a constraint, no further composition analyses were conducted.
EBSD analysis showed no crystallinity, suggesting the films were amorphous. This
result agreed with other studies of as-deposited WSiN thin films. X-ray reflectivity
data indicated a high degree of surface roughness. See Appendix A.
3.2.5. Reproducibility
It is common practice when designing a manufacturing process to examine the
reproducibility once input parameters have been established. Wafers 8, 9 and 10
are the result deposition trials with the same input parameters. Wafer 8 was a
different session and, a comparison of its properties with those of wafers 9 and 10
represents batch-to-batch reproducibility. Wafers 9 and 10 were part of the same
deposition session and a comparison of their properties represents wafer-to-wafer
reproducibility. The figure below illustrates these comparisions.
FIGURE 7. Comparison of important parameters for wafers 8, 9
and 10. Results were consistent, within an acceptable tolerance.
The process appeared to consistently reproduce the desired results, both between
depositions and between wafers in the same deposition; especially in light of the
0%
25%
50%
75%
100%
0
1000
2000
Wafer 8 Wafer 9 Wafer 10
Ω/sq.
Reproducibility
Sheet Resistance
Uniformity
Standard
Deviation

21. 20
systemic variability, as discussed in §3.2.1, the results are acceptable. Moreover,
the results would be expected to improve if the sputtering target were replaced.
3.3. Cost analysis and process assessment
A cost analysis of the project was performed, incorporating and revising the
analysis performed for the project proposal; an economic assessment of the
process was also performed.
3.3.1. Project cost analysis
The cost of the project was originally projected to be about $200K and driven
primarily by labor costs: nine engineers and one technician at $15K and $12K FTE,
respectively. The revised labor costs, over the nine-week project, totaled $56K, for
nine engineers who each spent 68 hours on the project, and one technician who
spent 18 hours on the project. Material costs were low. For the WSi3N4 target and
20 Si wafers, material costs were $800. Equipment time on the SEM and the XRR
cost a total of $600.

22. 21
FIGURE 8. Visual comparison of the relative costs for the different aspects of the project. The major driving
force, by a wide margin, was labor.
Thus, the experiment and process development costs came to $57K. Including the
time spent on research, which doubles the cost for the nine engineers, the total
project costs came to around $120K, well under the original projected budget. If
the estimated combined leverage sales of $13M remains valid, the return on
investment would increase to 0.96 and the payback period would decrease to just
over six months. This return on investment and payback period make use of a
rule-of-thumb method whereby production costs are estimated to be half of the
expected revenue. However, a 0.96 ROI and a half-year payback period are
unrealistic; more data on production is needed to improve the cost estimate and
give more a reasonable ROI and payback period.
3.3.2. TFRVH manufacturing process assessment
Assuming a normal distribution, the standard deviation of the TFRVHs
manufactured with the as-designed process would result in 68.2% of them having
a sheet resistance within 5 to 6% of 2000 Ω/sq, and 95.4% of them having sheet
resistance within 10 to 12%. Thus the yield of TFRVHs that fall within a 10%
margin of error is very high. Considering the expected improvements in
$400.00
$600.00
$56,000.00
$56,000.00
$1,350.00
PROJECT COSTS
Materials: WSi3N4 targets, Si
wafers
Equipment: SEM, XRD
Process Development:
Engineers, $15K/month FTE
Research: Engineers,
$15K/month FTE
Process Development:
Technicians, $12K/month FTE

23. 22
uniformity and standard deviation if the sputtering target is replaced, the yield of
usable TFRVHs should increase. Moreover, with the thickness closer to the
minimum value, the process uses a small amount of material, and takes a short
amount of time; particularly in light of the capacity of the sputtering system—
several wafers can be sputtered at once—the manufacturing time is extremely
short.
3.3.3. Health and environmental impacts
No significant or unusual health or environmental risks were identified during the
project. Once production shifts to GaAs and InP substrates, there may be some
toxicity issues as As is hazardous to humans. However, in the GaAs substrate, As
is unlikely to be liberated and increase the extant risk of exposure.
3.4. Future work
Investigations into how the results change when the depositions use GaAs and InP
substrates, rather than Si, should be carried out. Film stress would be expected to
change, and therefore, sheet resistance may change. An adjustment of the
deposition parameters might be required; or, perhaps a heat treatment may be
added to the process to reduce the stress in as-deposited films. A heat treatment
could, however, change the resistivity as well, so deposition parameters might be
changed regardless.
A more effective compositional analysis would be useful for determining more
precisely the relationship between N content and film properties such as sheet
resistance, film stress. Auger spectroscopy or rutherford backscattering would be
effective [4]. Rutherford backscattering would also reveal the film’s density.
Knowing the film’s density would aid in determining the thermal coefficient of
resistivity, which is important when characterizing the TFRVHs behavior under
actual operating conditions.
4. CONCLUSION AND RECOMMENDATIONS
This report concludes with a summary of the project and its outcomes, and a
recommendation regarding the stated problem.

24. 23
The project sought to increase the sheet resistance of WSiN TFRVHs by an order of
magnitude: from 250 Ω/sq to 2000 Ω/sq. Additionally, the sheet resistance values
must have standard deviation and uniformity values less than 10% over the
surface of each wafer, in order to ensure an acceptable level of reproducibility and
consistency. Thickness values were within the prescribed range. Figure 7 and
Table 4 are reproduced here for reference.
FIGURE 9. Reproduction of Figure 7, giving a visual indication
of the reproducibility of results.
TABLE 6. Reproduction of Table 4, showing specific values of
important TFRVH properties.
WAFER 𝑅S (Ω/sq) 𝜎 (%) 𝑢 (%) 𝑑 (Å)
8 1980 5.64 10.1 915
9 2060 5.86 10.1 974
10 2060 6.06 11.1 974
Based on these results, the process was evaluated and determined that the project
successfully developed a simple and consistent fabrication process as required: the
process is robust, cost-effective and reliable. Furthermore, a cost analysis revealed
that the project was completed under budget. The deposition parameters from
Table 5 are reproduced below.
0%
25%
50%
75%
100%
0
1000
2000
Wafer 8 Wafer 9 Wafer 10
Ω/sq.
Reproducibility
Sheet Resistance
Uniformity
Standard
Deviation

25. 24
TABLE 7. Reproduction of Table 5 showing
the process parameters determined by this
project to produce the desired results.
RF power 750 W
Substrate bias −60 V
Total system pressure 10 mTorr
N2 flow rate 5.2 sccm
Ar flow rate 34.8 sccm
Deposition time 1027 s
The poor uniformity and unexpected variation in the results can be ascribed to the
WSi3N4 sputtering target nearing the end of its usable life. Therefore, it is the
recommendation of this project to replace the sputtering target and use the
deposition parameters outlined in the table above.
With an efficient and effective method for producing WSiN TFRVHs, Keysight can
proceed with their integration into the development of new platforms and,
ultimately, new and lucrative markets.

26. 25
ACKNOWLEDGEMENTS
The authors wish to thank Nicholas Kiriaze at Keysight Technologies for his help
running the depositions and characterizations; Rijuta Ravichandran at University
of California, Davis, for her help performing all the SEM characterizations; Vache
Harotoonian, Steven Zhang, and Erkin Seker, for extensive consultation and
advice; and, finally, Drs. Ricardo Castro and Mike Powers, for their invaluable
help and guidance, as resources and mentors, throughout the entirety of the
project.

27. 26
References
[1] M. Powers, Sputter Deposition of Thin Films in HFTC, Santa Rosa, CA:
Keysight Technologies, 2015.
[2] A. Hirata, K. Machida, S. Maeyama, Y. Watanabe and H. Kyuragi, "Diffusion
Barrier Mechanism of Extremely Thin Tungsten Silicon Nitride Film Formed
by ECR Plasma Nitridation," Japanese Journal of Applied Physics, vol. 37, no. 3,
pp. 1251-1255, 1998.
[3] A. Lahav, K. A. Grim and I. A. Blech, "Measurement of thermal expansion
coefficients of W, Si, WN, and WSiN thin film metallizations," Journal of
Applied Physics, vol. 67, no. 2, pp. 734-738, 1990.
[4] A. Vomiero, et al, "Composition and resistivity changes of reactively
sputtered W-Si-N thin films under vacuum annealing," Applied Physics
Letters, vol. 88, no. 3, pp. 031917-1-031917-3, 2006.
[5] R. W. Berry, P. M. Hall and M. T. Harris, in Thin Film Technology, New York,
NY, Wan Nostrand Reinhold Company, 1968.
[6] S. M. Kang, et al, "Control of electrical resistivity of TaN thin films by reactive
sputtering for embedded passive resistors," Thin Solid Films, vol. 516, no. 11,
pp. 3568-3571, 2008.
[7] G. Franceschinis, "Surface Profilometry as a tool to Measure Thin Film Stress,
A Practical Approach," vol. 1, no. 1, pp. 1-5, 1999.
[8] J. H. Kim and K. W. Chung, "Microstructure and properties of silicon nitride
thin films deposited by reactive bias magnetron sputtering," Journal of Applied
Physics, vol. 83, no. 11, pp. 5831-5839, 1998.
[9] Y. G. Shen, et al, "Composition, residual stress, and structural properties of
thin tungsten nitride films deposited by reactive magnetron sputtering,"
Journal of Applied Physics, vol. 88, no. 3, pp. 1380-1388, 2000.
[10] "Semiconductors on NSM," [Online]. Available:
http://www.ioffe.ru/SVA/NSM/Semicond/. [Accessed 28 May 2015].

28. 27
APPENDIX A
Supplemental Information
EDXS was carried out on the SEM. The data below are representative of wafer 9.
FIGURE A1. EDXS results for wafer 9. Data is skewed
The high Si peak indicated that the interaction volume included the substrate.
There did not appear to be any Ar contamination; however, Ar contamination is
typically low for the WSiN material system—less than 1%--and may the EDXS
may not be sensitive enough to detect it.
XRR was carried out to examine microstructure and density. The data are
presented in the figures below. The prominent peak in Figure A2 correlated with
density of the film, but further characterization is needed to quantify it. Figure A3
shows a spike that correlates with Si, which was expected.

29. 28
FIGURE A2. Reflectivity curve for wafer 9. The prominent peak correlates with the density value.
However, without further characterization.
FIGURE A3. XRD data showing a Si peak.

30. 29
(a) (b) (c)
FIGURE A4. EDXS analysis showing relative distribution of Si (a), W (b) and N (c) atoms in a small rectangle
of WSiN thin film resistor from the patterned wafer.