Sputtered AlN Thin Films on Si and Electrodes for MEMS Resonators:
Relationship Between Surface Quality Microstructure and Film Properties
S. Mishin, D. R. Marx and B. Sylvia,
Advanced Modular Sputtering, Inc,
93 S. La Patera Lane, Goleta, CA 93117
V. Lughi, K. L. Turner, and D. R. Clarke
Materials Department
University of California Santa Barbara
Santa Barbara, CA 93106
Abstract – Aluminum nitride thin films grown byreactive AC magnetron sputtering are characterizedusing several metrology techniques to examine thecorrelation between surface quality, microstructure
and piezoelectric properties. Atomic force
microscopy, X-ray diffraction and electron
microscopy, and are employed to characterize themicrostructure. . A range of substrate coatings isexplored to understand the impact of topography on
film crystallinity and piezoelectric performance. A
first order approximation model providing thepiezoelectric characteristics as a function of the c-axis
misorientation in the mosaic-structured wurtzite AlN
films is presented. While the model predicts a only
small e33,eff and kt change for a misorientationdistribution of FWHM of less than 5 degrees, itservices as a indication of the impact of AlN
crystallinity on film piezoelectric properties.
I. INTRODUCTION
As cell phone technology goes to higher and higher
frequencies and the MEMS market seeks to put
greater functionality into their systems, the need forhigh quality piezoelectric aluminum nitride, AlN, isgrowing. With different applications and device
designs, a wide range of substrate materials are being
considered. These range from Si, or GaAs wafers,wafers coated with poly silicon, SiO2 (thermal orPECVD) Si3N4, and sputtered Mo, Al alloys, Pt,W/Ti and W over these coatings. One measure of thequality of AlN is its crystallinity as measured by thefull wave-half maximum (FWHM) values from an
XRD rocking curve experiment.

0-7803-7922-5/03/$17.00 (c) 2003 IEEEAs indicated by Ruby [1], for good piezoelectric
properties, AlN crystals must grow in columns thatare perpendicular to the plane of the electrodes and inorder to achieve this, the surface should be a smooth,“mirror-like” surface otherwise the many facets of arough surface initiate crystal growth in a variety ofdirections.
A study has been undertaken in an attempt to evaluate
how the underlying surface morphology affects theAlN crystallinity. Using polished prime Si wafers as
the benchmark, comparison of structure, are madeand correlations to piezoelectric performance will beoffered.
II. EXPERIMENTAL
Prime (100) Si wafers were used as the standard
substrate. Oxide coatings were either thermallygrown or deposited using PECVD methods. Si3N4
coatings were also deposited by PECVD. With theexception of Mo, metal electrodes were sputter
deposited using DC magnetron sputtering. Mo wasdeposited using an AC magnetron process.
All AlN films were reactively deposited using an
Advanced Modular Sputtering, Inc, AMS 2003
cluster tool utilizing a dual-ring target configurationand employing AC rectification between the rings so
as to assure plasma stability and to eliminate issues
associated with the “disappearing anode effect”.

2003 IEEE ULTRASONICS SYMPOSIUM-2028

III. RESULTS
As a baseline, deposition of AlN directly on prime
silicon wafers with high quality low roughness
substrate (less than 0.5nm) surface is used. Figure 1shows the impact of film thickness on the XRD
rocking curves. In the range typically used for
BAW/FBAR and MEMS applications, values lessthan 2.0° is achieved.
Rocking Curve vs. AlN Thickness
on Polished Bare Si Wafers

considerably worse rocking curve. While the AFM
micrographs of the prime Si and the thermal oxide arefeatureless, the roughness of the as-deposited PECVD
oxide and Si3N4 is clearly observed.
Table 1. Dielectric coatings on Si wafers
PrimeSiDegrees

Si3N4As receivedSurface Roughness Ra (nm)
0.207
0.178
4.5

7.8

After CMP

1.3

0.5

2.50

2.05 µm AlN

2.00

0.8 µm 0.8 µm
Thermal PECVDOxideOxide

XRD FWHM (degrees)
1.27
1.23

6.2

1.50

1.00
0.00

0.50

1.00

1.50

2.00

2.50

3.00

AlN Thickness (µm)

Figure 1. XRD Rocking curve FWHM versus AlN film thickness

The surface roughness, or topography, tends toincrease as wafers are coated without any polishing.
Figure 2 illustrates the increase in the FWHM values
with the addition of a Mo electrode underlayer.

(a)(b)

(c)

(d)

(a)

(b)

Rocking Curve vs. Mo thickness (AlN - 2µm)
2.00
1.90
1.80

Degrees

1.70
1.60
1.50
1.40
1.30
1.20
1.10
1.00
0500

1000

1500

2000

2500

3000

3500

Mo Thickness (A)Figure 2. Impact of Mo underlayer on AlN rocking curve.

With Si wafers, coatings of thermal oxide, PECVD
oxide and Si3N4 are compared, Figure 3. Substratesurface roughness was measured using AFM. If done
properly, thermal oxide growth does not significantly
change the surface roughness as compared to a prime
Si wafer. The PECVD coating is considerablyrougher than the thermal oxide and results in aFigure 3. AFMs of substrates (a) prime Si, (b) thermal oxide,(c) PECVD oxide, (d) PECVD oxide after CMP, (e) Si3N4, (f)Si3N4 after CMP. Note Figures e and f are at lower magnification.

2003 IEEE ULTRASONICS SYMPOSIUM-2029

Metal electrode films are deposited on variouscoatings. In this evaluation, Mo (200 to 250nm),
Al/0.5Cu (180nm)/Ti (20nm), AlSi (200nm) and TiW(200nm) films were considered. Tables 2 and 3 show
the surface roughness and the FWHM results afterAlN deposition, respectively. With the Mo electrode,
it is necessary to provide a barrier between the metal
and the substrate to avoid interaction. Without abarrier, the result of the interaction is seen in the large
10° FWHM values.

surface roughness. While potentially not the best
electrode material due to its acoustic softness, room
temperature deposited (cold) AlCu/Ti electrodes on Si
exhibited a fairly smooth surface on to which highly
crystalline AlN could be grown. Warm deposition ofAlCu/Ti produced films with large grains structures.
AlCu/Ti on PECVD this resulted in poor crystallinity
FWHM versus Surface Roughness
25

Table 2. Surface roughness of several electrode materials
0.8 µm 0.8 µm
PECVD PECVD
0.8 µm Oxide Oxide Si3N4 Si3N4w/o with
Prime Thermal
w/owithSiOxideCMPCMP CMP CMP
0.2

0.2

Roughness (nm)
2.8
2.8

2.3
2.1
1.5

2.7
1.2

4.3
4.1
6.7

Cold AlCu/Ti

0.7

0.6

2.8

15

AlN onW/Ti on
SiAlN on
Mo on
Thermal

AlN onW/Ti onPECVD

AlN onMo on
PECVD1057.8
W/Ti10AlSi
Warm AlCu/Ti

FWHM (degrees)

200.5001234567Surface Roughness (nm)Figure 4. Summary of surface roughness versus XRD FWHM

1.3

Tables 3. FWHM of AlN on electrode materials
0.8 µm 0.8 µm
PECVD PECVD
0.8 µm Oxide Oxide Si3N4 Si3N4w/o with
Prime Thermal
w/owithOxide
SiCMP
CMP CMP CMP
AlN on Mo
AlN on Mo on
barrier

10.0
1.6

FWHM (degrees)
6.4
2.5

1.5

AlN on W/Ti10

14.0

22.0

AlN on AlSi
AlN on Warm
AlCu/Ti
AlN on Cold
AlCu/Ti

7.6

6.4

2.0

1.3

5.8

2.3

1.7

1.3

2.9

1.9

1.5

A summary of all data collected is shown in Figure 4.
While the data scatter is large the general trend is that
FWHM decreases as a function of substrate surface
roughness. It is observed the high surface roughness,of W/Ti, Figure 5, results in very poor crystallinity
independent of the substrate material.
Excellent crystallinity is observed for the AlN
deposited on Mo electrodes on Si, thermal oxide andSi3N4 when a barrier layer is employed. Withoutpolishing the AlN crystallinity is poor due to the highFigure 5. AFM of W/Ti on Si (left) and PECVD oxide (right)

The appearance of resulting AlN films are illustrated
in Figure 6. A fine uniform structure is observed for
the AlN on Mo and on the cold deposited AlCu/Ti on
Si. A less uniform structure is observed when thecold AlCu/Ti is deposited on PECVD oxde. A muchless uniform AlN structure is seen for the AlN on
W/Ti. The underlying grains structure is clearly seenfor the AlN on the warm AlCu/Ti on Si and a highly
irregular, non-uniform appearance is observed for thewarm AlCu/Ti on PECVD oxide.
Both PECVD oxide and Si3N4 are important to thefabrication of FBAR and MEMS structures. To
employ these materials, polishing is needed toproduce a smooth surface that, in turn, will provide
improved AlN rocking curve results. With Si3N4,

2003 IEEE ULTRASONICS SYMPOSIUM-2030

FWHM values decreased from 2.5° down to an
acceptable 1.5°. FWHM values dropper to less than
2.3° for polished PECVD for both cold and warm
AlCu /Ti.

(a)(b)(c)(d)

(e)

(f)

Figure 6. Clockwise from upper left, (a) AlN surface structure
deposited on Mo on prime Si, (b) AlN on W/Ti on prime Si, (c)AlN on warm AlCu/Ti on Si, (d) AlN on warm AlCu/Ti on
PECVD oxide, (e) AlN on cold AlCu/Ti on Si, (f) AlN on cold
AlCu/Ti on PECVD oxide.

IV. DISCUSSION
When an optimized smooth surface structure isprovided highly, crystalline AlN can be grown. The
high quality film exhibits a fracture surface showing a
dense columnar structure as seen in Figure 7.

At higher magnification, Figure 8, TEM examination
reveals contrasts between a high and low quality film.
The former shows the development of the dense
columnthroughout
thestructure
while
the poor quality
film tends todevelop the welldefined columnar
struc-ture on aregion of lesswell-definedmaterial..
Figure 7.
Fracturesurface of a highquality AlN/electrodefilm stack showing
highlycolumnarstructure grown on a highly planar Mo electrode surface

Figure 8. TEM micrographs of high (left) and low (right) qualityAlN films.

In a thin film, the effective value of the piezoelectric
constants depends on the microstructure of thematerial. For FBAR geometries, the pertinent
constant is e33,eff, where the “3” direction is intended
to be the direction perpendicular to the free surface ofthe film. In order to estimate e33,eff, we schematize athin layer of AlN as shown in Figure 9.

2003 IEEE ULTRASONICS SYMPOSIUM-2031

This model resembles the typical highly textured
microstructure of sputtered AlN, shown in Figure 8.

Figure 9. Schematic of a cross section of a typical thin film of
sputtered AlN. The arrows represent the direction of the c-axis ofeach grain. Direction “3” is taken to be perpendicular to the freesurface of the film.

The grains are needle-shaped with the long axis –coincident with the wurtzite [0001] axispreferentially oriented approximately along thegrowth direction. Considering the piezoelectric
contribution of each individual grain along the “3”direction and taking the summation weighted by aGaussian distribution of the grains’ misorientation,
we find the result shown in Figure 10 where theeffective piezoelectric effect is represented as theratio between the calculated value and single crystal
value a function of the distribution’s FWHM. Thevalues for the piezoelectric tensor were taken fromNye [2].

It is observed that the effective piezoelectric effectdoes not change significantly even for grainorientation distributions with FWHM values up to
several degrees. Data from Lobl, et al [4], supports
contention.
While the model presented here is necessarily
simplistic in the sense that all interactions among
grains have been neglected, nevertheless, it helps toestimate, to a first approximation, the influence ofmicrostructure on the piezoelectric properties of thethin film
V. CONCLUSIONS
Obtaining high quality AlN films on different
electrode materials is a function of the surface quality
and to a much lesser extent on the nature of thematerial. Polishing using CMP techniques provides
an excellent avenue to develop a mirror like surface
eliminating nucleation sites that may allow growth indirections not perpendicular to the electrode surface.
The UCSB authors acknowledge the support of theUS Navy-MINT grant #N66001-01-1-8965
VI. REFERENCES[1] R. Ruby, “Cavity spanning bottom electrode of a
substrate-mounted bulk acoustic waveresonator.” US Patent No.6,384,697, May 7,
2002.
[2] J. F. Nye, Physical Properties of Crystals:
Their Representation by Tensors andMatrices,: Oxford University Press, Oxford
1985.
[3] A.Gualtieri and J.G. Ballato, “PiezoelectricMaterials for Acoustic Wave Applications.”
IEEE Transactions on Ultrasonics,
Ferroelectrics and Frequency Control, vol.
41, pp 53-59, January 1994.

Figure 10. Results of the calculation outlined in the text. It isshown that for a distribution FWHM of approximately 6°,
the effective piezoelectric coefficient drops by less than
0.5% of the original value.

[4] .H.P.Lobl, M.Klee, C. Metzmacher, W.Brand, R.
Milsom, P.Lok, F.van Straten "Piezoelectricmaterials for BAW resonator and filters",
2001 IEEE International Ultrasonics
Symposium, Atlanta, GA, Oct. 7-10, 2001,
paper 3E-2, pp. 807-811.

2003 IEEE ULTRASONICS SYMPOSIUM-2032