Raymond Chen1, Antonio Cruz1, Martin Kwong1, Jack Lam1, Niteesh Marathe1, Camron Noorzad1, Yongsheng Sun1, Cheng Lun Wu1, Disheng Zheng1

Ricardo Castro1, Michael Powers1,21UC Davis Department of Chemical Engineering and Materials Science; 2Keysight Technologies

Among others, Kang, et al showed that S increased with concentration of N2 in sputtering atmosphere [1].

This experiment sought to develop a process to increase the sheet resistance of W-Si-N TFRs to a target value of 2000 /sq.

Beginning with a deposition time of 20 minutes, N2/Ar was varied from 10% to 20% until the measured sheet resistance S was close to the target value. Deposition time was then adjusted to control thickness: Assuming resistivity and average deposition rate are constant at a given N2/Ar, S varies with thickness according to S

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 platformscombined leverage sales of $13M

New platforms require thin film resistors (TFRs) with sheet resistance (S) of 2000 /sq

Current fabrication techniques yielded only 250 /sq

26.5 GHz FieldFox Handheld Combination Analyzer

1000

2000

3000

850 950 1050 1150

RS

(/s

q)

Thickness ()

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

/s

q. Sheet Resistance

UniformityStandard 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

WSiNfilm

WSiNpattern

1000

2000

3000

0.1 0.12 0.14 0.16 0.18 0.2 0.22

RS

(/s

q)

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 a W-Si-N thin film resistor was successfully increased from 250 /sq to 2000 /sqwith a standard deviation of less than 10%. Film thickness was greater than 750 . For the CVC 601 RF magnetron sputtering system, the fixed deposition parameters were

RF Power: 750W 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 the WSi3N4target nearing the end of its lifetime. A new target should improve uniformity.

Further investigation could determine the films 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 GaAssubstrates. 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 S = 100/, where resistivity was assumed constant, and 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

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0.1 0.12 0.14 0.16 0.18 0.2 0.22

-(M

Pa)

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 /sq

Standarddeviation

10%

Uniformity 10%

Thickness > 750

Vary N2/Ar, deposition time

Substrate

Film

The W- 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. The WSi3N4 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 Keysight Technologies, 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 of TaNthin 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 of W, WSi, WN and WSiN 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 sputtered W-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 of Thin Films in HFTC, Santa Rosa, CA: Keysight Technologies, 2015. (slides)

Side-by-side comparison of a used 4-inch Ti target and an unused 8-inch W 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 amorphous WSiN network: atoms are arranged randomly and have

only short-range order; N occupies interstitial-like sites.

Sources of ErrorThe 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-deposited WSiN 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 in WSiN films. Source: [3]

WSiNfilm