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

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

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 /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 . 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 /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.

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 resistance2000 /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 /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 S 2000 /sq

Standard deviation 10%

Uniformity 10%

Thickness 750 < < 1500

Sheet resistance is the most important property of these TFRs and achieving the

target value of 2000 /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

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 projects 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--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

6.37 106 1. The coefficient of thermal expansion of Si is 3.45 106 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].

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 Keysights 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 neutralthat is, electrically isolated from the rest of

the chamberthe 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.

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 isotropictarget atoms are

preferentially sputtered along the close-packed directionsrotation of the

substrates improves uniformity.

Deposition rate is proportional to the sputtering yield , 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 and a constant that is

specific to the sputtering system:

= (1)

For this project, was approximated as average rate AVG, 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 Keysights 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 Keysights 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

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 gives N2 and Ar flow rates

N2 = 40

+ 1 (2a)

8

and

Ar = 40 N2 (2b)

respectively. The secondary mechanism of controlling the sheet resistance was

thickness, according to Equation (3) below:

S = 100

(3)

where is resistivity in cm and is thickness in to give a sheet resistance S

in /sq. As a process, sputtering offers control of film thickness via deposition

time. Keysights 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 .

It is worth noting that although Equation (3) indicates that an order-of-magnitude

decrease in thickness would give S~2000 /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 projects 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

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

characterizationscanning 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 N2 (sccm) Ar (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 AVG was calculated from

thickness and total deposition time .

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

10

AVG =

(4)

Once the appropriate ratio had been determinedachieving a sheet resistance

value close to, but below the target valuethat deposition was repeated, but with

a new deposition time. If AVG is assumed to be constant at a given N2/Ar ratio,

Equation (4) shows that is directly proportional to . Equation (3) shows that S

is inversely proportional to . Considering these relationships, it can be shown

that

= 0S0S

(5)

for 0 and S0 at a given N2/Ar ratio. Deposition time was then calculated for

the target S = 2000 /sq. Trial 7 was carried out.

However, AVG is not exactly constant, and measurements showed that sheet

resistance was above the target value. A new AVG 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

Keysights 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.

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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

=

1

6

1

S2

F (6)

to give a value for the residual film stress, . In Equation (6), and are Youngs

modulus and Poissons ratio of the substrate, respectively; S and F are the

thicknesses of the substrate and deposited film, respectively; and is the radius of

curvature of the substrate, given by

=

2

8, (7)

where is the scan length and 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 and current at 25 points around the surface

of the film, and sheet resistance was calculated with the relationship

S = 4.53

(8)

The probe then calculated a mean sheet resistance, along with standard deviation

and uniformity values. Standard deviation 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:

= 1

25(S S)

25

=1

(9)

12

Uniformity describes the range of values relative to the sum of the maximum

and minimum values:

=

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.

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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 TFRVHs 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 S (/sq) (%) (%) ()

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 /sq. The standard deviation over the surface of each wafer was well within

the 10% target. The thickness was between 750 and 1500 . 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

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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-/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 errorse.g., those

caused by the age of the sputtering systemare 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.

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.

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

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 10part of the same

deposition sessionhad 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

Sh

eet

Res

ista

nce

(

/sq

)

Thickness ()

RS vs. dN2/Ar=0.15

Wafer 8

Wafer 9

0

500

1000

1500

2000

0.1 0.15 0.2 0.25

Co

mp

ress

ive

Str

ess

(MP

a)

N2/Ar

Film Stress vs. N2/Ar

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 substratesnot 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: 6.40 106 1 for GaAs, compared to

3.45 106 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 m for a beam energy of 10 keV, the interaction

19

volume penetrated well beyond the 0.09 m-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

/

sq.

Reproducibility

Sheet Resistance

Uniformity

StandardDeviation

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.

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 /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

P R O J E CT C O S T S

Materials: WSi3N4 targets, Siwafers

Equipment: SEM, XRD

Process Development:Engineers, $15K/month FTE

Research: Engineers,$15K/month FTE

Process Development:Technicians, $12K/month FTE

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 oncethe 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 films density.

Knowing the films 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.

23

The project sought to increase the sheet resistance of WSiN TFRVHs by an order of

magnitude: from 250 /sq to 2000 /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 S (/sq) (%) (%) ()

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

/

sq.

Reproducibility

Sheet Resistance

Uniformity

StandardDeviation

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.

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.

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].

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 systemless 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.

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.

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.