Q-Factor Enhancement for MEMS Devices: the Role of the Getter Film

G. Longoni, A. Conte, M. Amiotti, A. Renzo and M. Moraja

SAES Getters S.p.A. Viale Italia 77

20020 Lainate (Italy) Phone:+39.02.93.178.473 , Fax:+39.02.93.178.460

Email: giorgio_longoni@saes-group.com W. Reinert

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Abstract

The need to reach the highest possible values of the Q-factor is one of the most important issues of resonant MEMS in order to make high-performance sensors. The Q-factor is strongly influenced by the internal environment of the MEMS packaging, by total pressure, gas composition and eventually by the presence of a getter film that is able to chemically absorb active gasses under vacuum or in inert gas. Getter technical solution for wafer to wafer hermetically bonded MEMS systems is PaGeWafer, a silicon or glass wafer with patterned getter film, few microns thick. MEMS hermetically bonded devices such as gyroscopes, accelerometers, pressure and flow sensors, IR sensors, RF-MEMS and optical mirrors requires getter thin film solutions at wafer level to work properly. In this paper, first the theoretical evaluation of Q-factor of a MEMS resonant structure in presence of a getter film is investigated and compared to the results of a Residual Gas Analysis of the same MEMS resonant structure and with the conventional measurement of Q-factor. Using getter thin film technology, total pressures from 1 mbar down to 10-4 mbar with corresponding extremely high Q-factor have been achieved in MEMS resonant structures. We were therefore able to confirm that getter film can provide high Q-values, stability of sensor signal (phase stability), performances stability during the lifetime, removal of dangerous gases like H2 and H2O, reduction of mass-loading effect accounting for drift in resonant frequency in hermetically sealed MEMS resonant structures.

Key words: Getter, vacuum packaging, eutectic bonding, MEMS, Q-factor

1. Introduction Packaging microsensor is one of the most

important and challenging, technology areas [1]. Vacuum wafer bonding technology provides a number of very effective techniques to produce low-cost, hermetic sealed packages for micro-machined sensors and actuators. In particular, hermetic packaging on wafer level is a key technology of many micro-electro-mechanical systems (MEMS). MEMS need to be protected from outside environment stresses and also the package must provide an interior environment compatible with the device operation, increasing its performances, reliability and lifetime. In addition some MEMS need a specific gas or pressure environment within the

package to operate as specified and Optical MEMS should be protected against moisture related failures, see Tab. 1.

Sensor/Device type Vacuum level accelerometer, switch 300 – 700 mbar

absolute pressure sensor 1-10 mbar resonator (angular rate) 10-1 – 10-4 mbar

RF MEMS 10-1 – 10-4 mbar bolometer < 10-4 mbar

Optical MEMS moisture free DMD-DLP moisture free

Table 1: Internal environment requirements for several kinds of MEMS.

A number of technological topics must be explored to produce hermetic sealed, micro machined devices on wafer level with controlled cavity pressures ranging from 10-4 mbar to 1000 mbar. Wafer-level processes are particularly interesting for the MEMS packaging, since they can reduce the fabrication costs, open up possibilities for batch processing and assure protection during wafer dicing operation. The improved robustness of capped devices allows MEMS devices to be handled in existing standard semiconductor backend processes.

In particular, MEMS vacuum packaging at wafer level is really a challenging technology. A vacuum environment and a controlled and stable atmosphere within MEMS package allows stable resonant frequency, stable Q-values and phase stability of the signal for MEMS resonating structures. For example, the quality factor Q of RF-MEMS is strongly reduced by the operation in a non-vacuum environment due to the gas damping. This is also the case for accelerometers, where the capacitance readout between fingers with a narrow gap is used to detect changes in the acceleration. Other examples for MEMS devices requiring vacuum in the internal cavity are absolute pressure sensors, inertial sensors (accelerometers, angular rate sensors/ gyrometer) and optical devices (optical switches, bolometers, IR imaging sensors, DMD). To improve the life expectancy and reliability of certain devices, controlled atmosphere encapsulation is needed, as a changing atmosphere might change the working of the device. Other topics are related to moisture: stiction will occur if there is humidity in the packaged structure, giving raise to current leakage, corrosion and mass-loading effects.

The most experienced and technically accepted way to keep constant the ambient of a hermetically sealed device is the getter material [2-5], which is able to chemically absorb active gasses under vacuum or in inert gas atmosphere for the lifetime of the MEMS devices. The present work focuses on the compatibility between Vacuum Wafer Level Packaging (VWLP) carried out through eutectic bonding techniques and getter technology. For the first time, we show that we obtained resonating angular rate MEMS sensor with controlled Q-factor, up to extremely high values (500000) and very good vacuum levels (10-4 mbar) by AuSi eutectic bonding with integrated thin film getter. 2. AuSi eutectic bonding for VWLP

Several bonding technologies can be used within the WLP techniques. However not all of these technologies can be compatible with vacuum environments or allow reaching stable and low pressure regimes inside the packaging. Getter material is thus required to reach the desired vacuum level or controlled atmosphere. Amongst the several hermetic bonding techniques, listed in the table 2, eutectic bonding is one of the most promising for the Micro-Systems Technology (MST) [6,7].

The eutectic bonding is a good candidate to high-vacuum level, as this technology uses only highly pure materials like silicon and well controlled metallic thin film layers. Due to the liquid melt formed, this technology is also compatible with non-perfect bonding areas with some small steps coming from previous processing operations.

Silicon/ glass

anodic bonding

High temperature silicon fusion

bonding

Low temperature silicon direct

bonding

Eutectic bonding

Glass frit bonding

bonding temperature [°C] 400 900 200 - 400

AuSi: 400 AuSn: 300 AlSi: 600

430

bond strength high high medium high low

outgassing during bond O2 H2, H2O H2O Noble gases CO, CxHy

bondframe width [µm] > 20 >30 % surface

coverage >30 % surface

coverage > 60 250 - 500

tolerance to topography [µm] almost 0 0 0 1,5 µm 1 µm (Poly-Si

line) vacuum suitability

without getter medium poor poor good medium

vacuum suitability with getter high high2 high2 high high

leak rate low very low unknown very low low

Table 2: Characteristics of hermetic wafer bonding technologies.

There are different approaches for coating of the materials forming the eutectic layer: - one material on one surface: this is the typical

situation of Au-Si, Au-Ge or Al-Si eutectic bonds, where a metal surface is brought in contact with a semiconductor surface;

- eutectic composition is on both surfaces: this is a typical situation for Au-Sn eutectic bonds, where on both surfaces are both metals in the form of multi-layer coating (alternating the two metals while keeping the ratio for forming the eutectic) or films sputtered from a target containing the two metals in the right ratio for forming the eutectic alloy.

The AuSi eutectic bonding is a technology using eutectic formation at 363 °C between a silicon wafer and gold deposited on another silicon substrate. The bond temperature used is in the range of 380-400°C. This is compatible with Al device metallizations. The technology is also compatible with extended outgassing cycles and the activation requirements for the deposited getter layer present in the device cavity. Compared to glass frit bonding, AuSi eutectic bonding has a very reduced outgassing during the wafer bonding cycle, and requires only very small bond frame widths, typically in the range of 60 – 100 µm [6,7]. This increases the throughput which is a major parameter for low-cost production.

3. Thin film getter technology for VWLP The use of Non Evaporable Getter (NEG)

material (Zr based alloy) is required to ensure suitable vacuum (total pressure down till 10-4 mbar) and long-term stability in MEMS devices. NEG can chemically sorb all active gases, including H2O, CO, CO2, O2, N2 and H2. The main constraints imposed by the device design and process are the compatibility of the getter with the fabrication process, the thickness of the getter film and an activation temperature compatible with the bonding process. Besides this, the geometrical layout of the MEMS design requires a patterned deposition of the getter material on the cap wafer. The getter solution for MEMS VWLP is a Si or glass wafer (PaGeWafer®), where thin getter films, few microns thick, are patterned to fit the particular structure of the MEMS devices [2-5]. The getter film can be selectively placed into the cavities without affecting the lateral regions of the wafer where the hermetic sealing is to be performed. The typical pattern lateral dimensions are in the range of the millimeter, while the getter film can be placed in the cavities with any depths, ranging from few microns up to hundreds of microns.

The structure of the thin getter film is highly porous in order to improve sorption performances,

but at the same time there are no loose particles thanks to a proprietary manufacturing method. The getter thin film is composed of a patented Zr special alloy with a proper composition to optimize the sorption performances. The getter film has the absorption capacity to remove all the gas contamination coming from the outgassing of internal MEMS/OMEMS surfaces as well as possible small package leaks and permeations. The getter film is supplied in a stable, passivated form to protect the gettering surface and to ensure that it performs as specified. Therefore the getter film can be safely handled in clean room air.

Once the getter film is in a vacuum or noble gas environment, it needs to be activated. Activation is achieved by applying thermal energy to the getter to diffuse the passivation layer into the bulk, rendering the surface of the grains chemically active and ready to pump contaminants out of the MEMS package. The typical MEMS VWLP technologies provide a suitable combination of temperatures and process time to activate the thin getter film so that it can start absorbing gasses after the MEMS vacuum bonding at wafer level.

Figure 1: The schematic cross section of integration of eutectic bonding and PaGeWafer® for MEMS VWLP applications.

4. Q-factor dependence on the internal package atmosphere

The measure of the Q-factor is the most common method to evaluate the performances of resonant MEMS. The need to reach the highest possible values of the Q-factor is one of the most important issues of the MEMS, in order to make high-performance sensors. The damping of a vibrating system is mainly determined by the intrinsic dissipations in the mechanical spring (cantilever beams), by internal material losses (defects) and by the dissipations due to the interaction with the surrounding gas [8,9]. The nature of the damping depends on the gas pressure: at low pressures (under about 10 mbar) the damping is both of a molecular and viscous nature, whereas at high gas pressures, it

is viscous. The viscous damping is caused by the lateral displacement of the air surrounding the vibrating surface of the resonator, and it corresponds to the squeeze-film damping, while the origin of molecular (acoustic) losses is the perpendicular displacement of the air surrounding the vibrating surface.

However the relationship between Q-factor and gas pressure is not simple: the dependence of Q-value by both total pressure P and viscosity η can be summarized in the following formula [10,11]:

⎟⎠⎞

⎜⎝⎛ += 159.11

PQ β

ηα (1)

where α and β are two parameters taking into account the resonance frequency, geometrical factors and physical constants. Several experimental and theoretical studies confirm the Q-value of a silicon structure is order of magnitude higher in vacuum than in the presence of a damping gas [8]. However for pressures higher than about some tens of mbar, Q slightly depends on pressure P and is strongly influenced by gas viscosity. Instead for low pressures, Q is strongly influenced by both total pressure and gas viscosity. The gas viscosity is strongly dependent on gas species and temperature. Therefore the Q-value is influenced by the cavity atmosphere composition and by the device operational temperature. For example, assuming a constant pressure P, H2, instead of N2, guarantees a higher Q-factor, whilst, operating at a temperature of about 150 °C, the Q-value is decreased by about 30-45%.

5. Experimental results

5.1 Samples realized with AuSi eutectic bonding and getter technology

Several devices have been produced using the eutectic AuSi technology coupled to the thin film getter technology on the cap wafer. The Q-factor obtained using the getter film integrated within the cap wafer reached extremely high values, up to 500000. For comparison, the Q-factor of devices without getter is much lower, around 2000.

However, for the typical application of the yaw rate sensor, and in general for micro-resonating sensors, a lower vacuum level is required. The typical operating pressure of some of these devices is around 10-1, 10-2 mbar, corresponding to a Q-factor comprised between 5000 and 10000. The internal atmosphere needs in any case to be controlled and therefore the getter actions should theoretically be exploited in an environment made by noble gases. To realize a defined gas damping for resonating sensors, a controlled gas-filling procedure has to be established. Only inert gases or gases that do not

consume the getter or alter the getter sorption performance may be backfilled in the device cavities. The backfill operation is typically one step in the wafer bond cycle [7].

a)

b)

c) Figure 2: a) Q-factor map of a 6” wafer realized with AuSi eutectic bonding and thin film getter on the cap wafer. B) Q-factor statistical evaluation: the mean value is 6600 with an uncertainty of 10%. C) More than 70% of the dies have a total pressure around 1.7*10-1 mbar. Remark: the Q-factor distribution is widened by two different sensor designs with individual resonance frequency.

We have built up a MEMS yaw rate sensor with MEMS VWLP AuSi eutectic bonding and getter

technology. Fig. 2 shows the results of such tests. A yield of about 90% has been obtained onto 6” wafers (green dies are dies which Q-value is within one σ of the mean value). Q-factor appears now stable around about 6600 (Fig. 2b) with a statistical variation of about 10% (within one σ). Very few dies having a low Q-factor, lower than 5000, have been obtained. More than 700 dies revealed a total internal pressure narrowed around 1.7*10-1 mbar (Fig. 2c).

5.2 Q-factor measurements The choose of a yaw rate sensor as micro-

electromechanical resonator device has been taken because of its high sensitivity and therefore high dependency of the Q-factor on the total internal pressure. Fig. 3 shows the results of a calibration procedure performed on an opened device in a vacuum chamber, where a carrier gas (N2, Ar or Ne) is introduced and stabilized at precise pressure values. The cavity pressure can be tuned to any value between 10-4 mbar and 1000, even overpressure is possible depending on the wafer bonder infrastructure. With the conventional Q-factor electrical characterization, quality factors in the range of 1000 – 100000 can be measured.

0,01 0,1 1 10 100 1000

1000

10000

1000001E-4 1E-3 0,01 0,1 1 10

Pressure [mbar]

Q-fa

ctor

Pressure [Pa]

N2

Ne Ar

Figure 3: Dependency of the Q-factor versus cavity pressure for a typical surface micromachined renonator. However, as explained in section 4, the measured quality factor does not give the atmospheric gas composition inside the package, merely it is a measure of the total pressure in the device cavity. In order to correlate the Q-values with the internal cavity atmosphere and the getter sorption performances, a set of residual gas analyses have been carried out [4,12] on bonded samples with different Q-values.

5.3 Outgassing experiments Before showing the results coming from

residual gas analyses, it’s worthwhile to underline

some outgassing tests performed on materials and coatings present inside the final device.

Besides from any limitations of the device hermeticity, the cavity pressure is limited by any outgassing of noble gasses which can not be absorbed by the getter. The outgassing behavior of different device parts have been analyzed by thermal desorption spectrometry (TDS) during a simulation of a standard wafer bonding temperature profile. The TDS was focused on noble gases because all other gasses can be absorbed by the getter. The composition of the released gas was dominated by Argon which is typically used for sputtering and ion milling during the device preparation. Figure 4 summarizes the Ar content of the samples: both ion milling and sputtering processes are responsible for a significant Ar outgassing. On the contrary, layer deposition by evaporation is completely Ar free. It appears that using ion milling and sputtering, both incorporation within the metal layers and implantation at the semiconductor-metal interfaces are taking place.

For further analysis similar samples have been analyzed by second ion mass spectroscopy (SIMS) and electron probe micro analysis (EPMA). With both methods, a concentration of 1500 – 1700 ppm Ar have been found within the top Au layer of the sputtered plating base. The Ar content of all other layers is comparable small and can be neglected. The influence of ion milling was checked on a plain silicon sample. A high Ar concentration of 1000 – 1400 ppm was found within the first 0.6 µm, followed by a decrease over several hundred nm. Both results are in very good agreement with the outgassing experiments.

The whole process is now Ar free and no outgassing of any noble gasses is observed.

1E-7

1E-6

1E-5

1E-4

1E-3

0,01

sputteredplatingbase

evaporatedplatingbase

ion milledreference

Ar a

mou

nt (c

m3 ⋅m

bar/c

m2 )

sensorwafer

(Al-Pads) Figure 4: Outgassing of Ar from TDS experiments during a simulation of thermal wafer bond cycle for different samples

5.4 Residual Gas Analyses Only an accurate residual gas analysis

(RGA) carried out on the hermetic package of a resonant MEMS structure allows distinguishing the gas species present inside the cavity. Therefore RGA on MEMS samples is suggested to know the exact composition of the internal atmosphere, and thus the real viscosity η and the total pressure P. We developed a long experience in the field of RGA of hermetic packages. Our experimental apparatus can measure pressures down to 10-4 mbar in a cavity with volume lower than one mm3 [4,12].

We prepared a series of samples similar to the ones showed in Fig. 2, but we differentiated the samples in function of presence or not of the getter layer and with and without noble gas backfilling during bonding.

Q Factor (measured) 3000 Q Factor

(measured) 5000

Q Factor (calculated) 3220 Q Factor

(calculated) 4180

Gas Pressure (mbar) Composition Gas Pressure

(mbar) Composition

H2 7.92E-01 81.7% H2

CH4 8.92E-02 9.2% CH4 5.66E-02 13.8%H2O 9.70E-04 0.1% H2O 3.90E-04 0.1%CO 5.53E-02 5.7% CON2 1.55E-02 1.6% N2

C2H6 5.82E-03 0.6% C2H6 2.46E-03 0.6%C3H8 7.76E-03 0.8% C3H8 3.77E-03 0.9%CO2 1.94E-03 0.2% CO2

Noble gases (He,Ar,etc...) 7.76E-04 0.1% Noble gases

(He,Ar,etc...) 3.47E-01 84.6%

Total 9.70E-01 100.0% Total 4.10E-01 100.0%

Mean Viscosity (10-5 Pa s)

0.984Mean Viscosity

(10-5 Pa s)2.056

Sample 1 Sample 2

Table 3: Results of RGA, measured and calculated Q-values and mean viscosities of sample 1 and sample 2.

RGA have been performed on two different samples (internal volume equal to 0.7 mm3), summarized as follows: • Sample 1: sample without getter and without

backfilling (Q=3000 and P=1 mbar) • Sample 2: sample with getter, without

backfilling, with sputtered Au bonding frame and plating base ion milled (Q=5000 and P=4.1*10-1 mbar).

The sample without getter showed a not high vacuum level, in the range of 1 mbar (viscous damping), mainly due to the outgassing of the internal surfaces (H2, CH4 and CO) during the heating of both the cap and sensor wafers for the bonding process. Many of the gases inside the cavity are active gases (like H2, CO, N2 and CO2) that can be eliminated by a gettering surface. On the other hand, the RGA on the second sample is showing that the getter is able to remove all the aforementioned gases, but now the atmosphere composition is dominated by the noble gases released by the cap wafer metallization processes carried out by physical methods (P=4.1*10-

1 mbar), as explained in section 5.3. In order to better understand effect of getter

film and of different backfilling and vacuum processes, we prepared three more samples such as follows: • Sample 3: sample with getter and with

backfilling (Q=10000 and P=1.8*10-1 mbar) • Sample 4: sample with getter and with

backfilling (Q=27000 and P=7*10-2 mbar) • Sample 5: sample with getter and without

backfilling (Q=500000 and P=4*10-4 mbar).

Q Factor (measured) 10000 Q Factor

(measured) 27000 Q Factor (measured) 500000

Q Factor (calculated) 10580 Q Factor

(calculated) 30920 Q Factor (calculated) 1.40E+07

Gas Pressure (mbar) Composition Gas Pressure

(mbar) Composition Gas Pressure (mbar) Composition

H2 H2 H2

CH4 1.84E-02 10.2% CH4 3.12E-03 4.5% CH4 3.92E-05 9.8%H2O 3.60E-04 0.2% H2O 6.94E-05 0.1% H2O 2.28E-05 5.7%CO CO CON2 N2 N2

C2H6 9.00E-04 0.5% C2H6 6.94E-05 0.1% C2H6 1.52E-05 3.8%C3H8 C3H8 C3H8 3.36E-05 8.4%CO2 CO2 CO2

Noble gases (He,Ar,etc...) 1.60E-01 89.1% Noble gases

(He,Ar,etc...) 6.61E-02 95.3% Noble gases (He,Ar,etc...) 2.89E-04 72.3%

Total 1.80E-01 100.0% Total 6.94E-02 100.0% Total 4.00E-04 100.0%

Mean Viscosity (10-5 Pa s)

2.107Mean Viscosity

(10-5 Pa s)2.177

Mean Viscosity (10-5 Pa s)

1.911

Sample 5Sample 3 Sample 4

Table 4: Results of RGA, measured and calculated Q-values and mean viscosities of samples 3, 4 and 5.

RGA have been carried out in a similar way as previous two samples (same internal MEMS volume of 0.7 mm3). The desired Q-value for the sensor application lies in the range between 5000 and 10000 (see section 5.1) and it can be successfully achieved by a backfilling procedure using an inert or noble gas. The Q-factor of sample 3 is the final goal for the application of interest and the atmosphere composition is mainly or totally due to noble gases (P=1.8*10-1 mbar). Traces of hydrocarbons can be found inside the device, mainly coming from some cleaning steps. The getter film succeeded in maintaining stable the cavity atmosphere removing all the active gases. The sample 4 is showing that a fine tuning of the backfilling procedure allows reaching good vacuum level (P=7*10-2 mbar) and higher Q-value (27000) with a lower contaminants concentrations. Finally, if no one backfilling procedure is applied to the bonding process, a total pressure down to 10-4 mbar can be achieved corresponding to extremely high Q-factors.

This result is fundamental because for the first time it was directly demonstrated that a pressure in the range of 10-4 mbar can be achieved by a VWLP, using the combination of eutectic bonding and getter technologies. This result is surely for interest for all kind of applications and devices that requires very good vacuum level to properly work.

6. Discussion Fig. 2 reports the Q-values measured on

open devices with different filling gases. The conditions under which the measurements have been performed are substantially different from the operating ones (bonded device). For an open device, the effects coming from the presence of the cap wafer and the getter film cannot be included in the experimental results. In this case, the resonating springs can move in a free volume and the viscous and molecular damping are modified. In other words, the squeeze-film damping, due to gas compressibility, and the gas lateral displacement around the mechanical cantilever beams are attenuated. Two side effects must therefore be considered. The first one is that the Q-factor is no more strongly dependent on the gas species that is filling the environment around the resonating structures. Fig. 3 is showing that Q has not a significant dependence on the gas species used for the calibrations. The second one is related to the mechanical effects: some differences can be introduced on the Q-factor evaluation, coming from the bonding process. Both the thermal load and mechanical coupling between cap and sensor wafers, can introduce variations in the internal mechanical stresses of silicon structure and thus also variation in the intrinsic dissipation upon which Q is dependent

when pressure is lower than a critical value. It’s worthwhile to carry out a possible correlation of Q of a bonded device with its internal pressure and gas composition.

In tables 3 and 4, the measured Q-factors (on bonded samples) are compared to the calculated ones. The Q-values are calculated on the basis of the Eq. (1) modified for pressure lower than few mbar. In such a case, the Eq. (1) can be simplified as:

159.1PQ

ηε

≈ (2)

where ε is a constant gathering all the physical and geometrical parameters, included the constants α and β of Eq. (1). In this way, we can really compare the Q-value measured on bonded sample with the theoretical predictions.

From Tab. 4, it is clear how the measured and calculated Q-factors are divergent for low pressures. This mismatch between eq. (2) and experimental values is ascribed to the fact that Q is not only dependent on the gas environment. It is well known that the general expression for Q is the following [8,9]:

imgas QQQQ1111

++= (3)

The damping of a vibrating system is mainly determined by the intrinsic dissipations in the mechanical spring Qm (cantilever beams), by internal material losses Qi (defects) and by the dissipations due to the interaction with the surrounding gas Qgas. When Q is pressure dependent, it is inversely proportional to the pressure, as confirmed also by Fig. 3. On the other hand, when the pressure is low, the effects of the intrinsic dissipations become predominant: the Q-factor cannot growth anymore and reach an asymptotic value (Fig. 5) [13].

Figure 5: An example of the relationship between pressure and resonant quality factor of microcantilevers.

Intrinsic damping

Molecular damping

Viscous damping

Fig. 6 shows, on the same plot, the experimentally measured Q-values (black square dots) and the calculated Q-value (red circle dots) versus the experimentally measured total pressures from RGA. On the same plot is also reported the theoretical trend of Q against P based on Eq. (2), assuming a mean gas viscosity equal to 2.00*10-5 Pa s, (red straight line) and the power law fit of the experimental data (black dashed line). From Fig. 6a), it is clearly visible how for pressures higher than 5*10-2 mbar, the agreement between Eq. (2) and experimental data is substantially good. It is also very important to underline how the calculated Q-factors are in good agreement with the experimental ones. In this case, in fact, the Q-factors are measured on bonded devices, where the gas influence (squeeze-film and lateral displacement) is significative, and therefore Eq. (1) can be applied.

0,01 0,1 1

10000

20000

30000

40000

50000

Experimental data fit

Theoretical curve

Q-fa

ctor

Pressure (mbar)

a)

0,0001 0,001 0,01 0,1 1

50000

100000

150000

200000

250000

300000

350000

400000

450000

500000

550000

600000

Experimental data fit

Theoretical curve

Q-fa

ctor

Pressure (mbar)

b) Figure 6: measured Q-values (■) and the calculated Q-value (●) versus the experimentally measured total pressures from RGA. The theoretical trend of Q against P based on Eq. (2) is the red straight line and the power law fit of the experimental data is black dashed line. This is a confirmation that the Q-factor measurement on open devices could not take into account some not

negligible environmental and mechanical effects (Fig. 3). Fig. 6b) is an enlarged view of the previous one. In this case, the discrepancy between the Q-factor trend predicted by Eq. (2) and the experimental data is absolutely not negligible. The Eq. (2) does not take into account the internal dissipation mechanisms related to elasticity and defects. These effects start becoming not negligible below 5*10-2 mbar and under 10-2 mbar they are practically dominant. In the region comprised between 10-3 mbar and 10-2 mbar the Q-factor growth is decreased, till under 10-3 mbar where the Q-factor is basically going to saturation.

It is important to consider this effect for each kind of devices and recognize the critical pressure under which the intrinsic dissipations are becoming not negligible, in order to correlate the Q-factor measurements to the right vacuum level inside the MEMS packages.

7. Conclusion We have shown that the combination of

AuSi eutectic bonding with getter technology at wafer level for vacuum packaging of MEMS can provide MEMS resonating structures working at 10-4 mbar environment with very high Q-factors (500000).

The Q-factor (and the internal total pressure) can be tuned and stabilized to the desired value for each application by backfilling procedure with noble gases coupled with integration of a getter film.

These experimental data show that the integration of getter film at wafer level for vacuum bonding of MEMS assures a high vacuum in small volumes for the entire lifetime of the MEMS devices. References [1]. M. Madou, “Fundamentals of

Microfabrication”, CRC Press, Boca Raton (2002).

[2]. M. Moraja, M. Amiotti and R.C. Kullberg, "New getter configuration at wafer level for assuring long term stability of MEMs" - Proceeding of SPIE Photonic West 2004, Vol.4980 (2003).

[3]. M. Moraja et al., "Impact of cleaning procedures on getter films", Proceeding of SPIE Vol.5343 (2004) pag. 87-93.

[4]. M. Moraja, M. Amiotti and G. Longoni - "Patterned getter film wafers for wafer level packaging of MEMS" MST 2003, Munich, October 2003.

[5]. G. Longoni et al., “Patterned Gas-Absorbing films for assuring Long-Term Reliability and Improving Performances of MEMS and

OMEMS”, in IEEE/LEOS International Conference on Optical MEMS and Their Applications, August 2005, Oulu, Finland.

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[11]. T. Corman, P. Enoksson, G. Stemme – “Gas damping of electrostatically excited resonators” – Sensors and Actuators A 61 (1997) 249-255

[12]. S. Tominetti, A. Renzo, “Determination of impurities in Organic Light Emitting Diode displays using a dedicated mass spectrometric technique”, SID’01 Digest (2001) 662-665

[13]. T. Itoh, H. Okada, H. Takagi, R. Maeda, T. Suga – “Room temperature vacuum sealing using surface activated bonding method” - Proceedings of TRANSDUCERS’03, (2003) 1828-1831.