AlN Film Stress and Uniformity for BAW filters on
200mm wafers
Sergey Mishin
Yury Oshmyansky
Advanced Modular Systems, IncGoleta, CA/USA
smishin@amssb.com
yoshmyansky@amssb.com

Abstract— Most standard processes only require controlling
average stress on the sputter deposited films.
Recently, MEMS devices have moved into high volume
applications [1], [2]. Devices such as FBAR filters, MEMS
microphones, and cantilever structures have much more sensitivity
to stress variation across wafer and across the thickness of thedeposited film [3], [4], [5], [6].
Sputter deposited aluminum nitride (AlN) films havepolycrystalline and strongly oriented grain structure, starting withextremely compressive film at the initial few thousand angstroms
thickness and becoming more tensile as grains get larger. Also mostdeposition systems have high variation of film stress across wafer
due the magnetic field strength and electrical discharge at eachracetrack not being selected for the best stress control across wafer.
Because most systems have been designed with film thickness
uniformity and average stress on a wafer as primary goals, stressacross wafer and across film thickness is sometimes highly nonuniform.
In this paper we will show how it is possible to control
stress for all desired characteristics: average stress, stress uniformity
across wafer and across entire film by using appropriate magnetic
field design and in-situ ion beam trimming as part of the deposition
process.
Average stress control cross wafer is achieved by precise
adjustment of magnetic field of inner and outer target ringmagnetrons, see Figure 1.
Stress variation across wafer is achieved by varying theratio of magnetic field strength between two magnetrons as shown in
the Figure 2.
Stress variation across deposited films is achieved byusing multistep deposition/ion mill trimming processes, see Figure
3.
As result, film stress variation across wafer and acrossfilm is maintained at less than +/-100MPa range with thickness
uniformity on 200mm wafers maintained at below 0.2% one
standard deviation.

Figure 1. Average stress in AlN film vs. Magnetron magnetic
field for different materials; AlN (green) and Mo (red).

Figure 2. Stress as a function of Distance from the center ofthe wafer as a function of Magnetic field ratio between twomagnetrons.

Figure 3. Stress as a function of AlN film thickness

It is a reactive deposition process in deep poison modeusing pure Al targets. High purity research grade 99.9999%
argon and nitrogen process gasses we used for alldepositions.
Figure 4. AMSystems cluster tool with three modules(AlN deposition, Mo deposition and pre-clean/trimming)I.

INTRODUCTION

Many MEMS devices that use piezoelectric AlN
films require very tight stress control. There are few
reasons for such tight stress control.
1.

2.

On 200mm wafers, many processing tools (suchas steppers) require <120um maximum wafer
bow. This corresponds to <250 MPa stressrequirement for a 1um AlN film.
2. Many devices such as cantilever beam
structures, for example, require stress control
that is about +/-100MPa wafer-to-wafer as wellas across wafer and throughout the films
thickness.

All mentioned above issues have a great impact on device
yield, and as result, device cost.
Additional, for some devices such as FBAR, forexample, for which frequency depends on thickness ofdeposited layers, it is critical to have AlN (aluminumnitride) thickness control of +/-0.2% wafer-to-wafer andacross wafer.

Substrate rotation is used to reduce variation of nonuniformity of deposited film properties across the substrate.
Integrated Laser interferometer is used to control averagedeposition thickness of the AlN films during process.
The trimming module uses DC focused ion source withargon process gas to improve thickness/uniformity ofdeposited films. Film thickness trimming/tuning is processing
based on ion beam scanning across a wafer with power
variation based on film thickness map. Use of the trimming
process opens up a much wider process window for stress andother film properties control, because it allows avoiding ofspending too much effort on controlling thickness uniformity
during deposition.
Figure 5. Focus Ion Beam Profile

Considering the importance of AlN film sterss andfilm thickness uniformity, it is necessary to have
independent film stress and thickness control.
II.

EQUIPMEMTIn this investigation we used Advanced Modular
Systems cluster tool with three modules: AlN depositionchambers, electrode deposition chamber and focus ion
beam trimming module (shown in Figure 4).
AlN deposition process uses a high volume production
dual magnetron system with positive plasma column andwith AC power applied between targets (magnetrons).
Frequency of AC power is 40 kHz and power may varyfrom 3 to 10 kW.

Unlike back-sputter etch module in the traditional cluster
tools, AMS trimming module can be used to both, cleaning
the surface before deposition, as well as improving thickness

uniformity after the deposition. It is very cost effective
solution, because it doesn’t cost any more than RF sputter
etch chamber, but does a double duty as a pre-clean chamberand trimming chamber.
III. SYSTEM DESIGN FOR STRESS CONTROL IN
ALUMINUM NITRIDE DEPOSITION PROCESS

Wafer rotation is also used to improve both thickness
uniformity and stress variation across wafer. Figure 7
below shows a typical configurations (magnetic andelectrical) used for the 200mm wafers.
Figure 6. AlN film cross wafer stress for 200mm

When using a standard rotary magnetron deposition
system for the piezolectri AlN on 200mm wafer, stressvariation across wafer is typically greater than 350MPa.
This is partially due to the inherent geometric limitationsof the single target system, as well as the location of theracetracks on the target and the distance between
racetracks and the anode (or chamber). As a result, it
produces high density plasma with positive column in thecenter of the target and low density plasma (due toelimination of positive plasma column) at the edge of thetarget. Since more material on the center of the wafer isdeposited from the center of the target, and more materialon the edge of the wafer is deposited from the target edge,
it produces highly non-uniform center-to-edge AlN film
properties (such as stress and coupling coefficient). It isvery hard to optimize rotary magnetron system to give
both, good thickness uniformity and good stress variation
across wafer.
It is much easier to obtain good stress control andthickness uniformity of deposited film in a system with
two independent magnetrons (targets), that allow fairly
easy geometric manipulation of the target surface, as wellas independent magnetic field control on each target
surface. Effect of the magnetic filed to AlN film stress isshown in Figure 1.
AlN film, deposited from the inner target, effects onfilm properties, mostly on the wafer center, and AlN film,
deposited on the wafer edge, mostly come from the outertarget. Since, it is applied AC power between two targets
and shields or chamber do not participated in plasma
discharge, it creates symmetrical discharge and the positive
plasma columns are equels for both discharges, whenmaterial sputtered from inner target (inner target acts ascathode, and outer target acts as anode) and, when material
sputtered from outer target (outer target acts as cathode,and inner target acts as anode). As result of the samelength of plasma discharge and plasma density, AlN film
properties (stress for example) on the center fo the wafer
and on the wafer edge are the same. As result, it can bereached cross wafer stress uniformity less than +/- 75 MPafor 200mm wafers, see figure 6.
Additional DC power supply, in parallel with AC power
supply, allows to shift reference point for AC power andprecisely control center-to-edge thickness uniformity.

Figure 7. Typical configuration used for the 200mm wafers.

Trimming must be done without breaking vacuum.
If freshly trimmed surface is exposed to the atmosphere, it
can instantly oxidize and cause AlN film to grow with poor
crystal orientation.
Films, that are deposited and trimmed multiple times
show accellent stress uniformity throughout the filmthickness.
Standard depositions without trimming
typically have very compressive initial films andprogressively more tensile films as the films get thicker As
illustrated in Figure 3.

V.
IV.

SUMMARY

Manufacturable solution to producing highly
uniform AlN films for FBAR for 200mm wafer size wasdemonstrated using a dual magnetron deposition systemwith integrated pre-clean/trimming module.

TRIMMING FOR STRESS GRADIENT CONTROL

Trimming process [5], [6] can be used to smooth thesurface of the AlN as well as to get thickness uniformity
improved. Removing at least 200A of AlN reduces surface
roughness to <0.6nm, see figure 8 below.

Average Stress control wafer-to-wafer, stressuniformity across wafer and across the entire film thickness
was achieved without compromising thickness uniformity
or crystal orientation of the deposited AlN film.

Figure 8. Roughness reduction after Ion Mill Trimming
about 200Angstroms of AlN film

REFERENCES
[1]
[2]
[3]

When the followingg layer of AlN is deposited on asmoothed surface it tends to be more compressive than if
deposited on the un-trimmed surface.

[4]

Figure 9. Split Run for Average Stress of AlN film
with/without Trimming Process

[6]

[5]

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