A study on the performance and reliability of magnetostatic actuated RF MEMS switches

Microelectronics Reliability 49 (2009) 59–65

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A study on the performance and reliability of magnetostatic actuated RF MEMS switches Ta-Hsuan Lin a, Stephen Paul a, Susan Lu a,*, Huitian Lu b a

Department of Systems Science and Industrial Engineering, Tomas J. Watson School of Engineering and Applied Science, State University of New York at Binghamton, P.O. Box 6000, Vestal Pkwy East, Binghamton, NY 13902, United States b Department of Engineering Technology and Management, South Dakota State University, SD 57007, United States

a r t i c l e

i n f o

Article history: Received 3 November 2006 Received in revised form 27 June 2008 Available online 5 December 2008

a b s t r a c t The magnetostatic radio frequency micro-electro-mechanical system switch is a special latching type of switch that possesses substantially high performance due to low loss, high linearity, and broad bandwidth. This new technology targets applications where high electrical performance and reliability are required in a harsh environment. A study on switch performance and reliability under different environmental conditions is crucial to its applications. In order to assess the reliability under desired environmental and operational conditions, comprehensive humidity and temperature reliability tests were conducted. The humidity test was conducted under high relative humidity and temperature cycling conditions. The thermal aging test was conducted at different temperature levels, which was used to study the fitness of lifetime distribution and validate the suitability of Arrhenius model that can be used for the lifetime prediction at normal operation temperature. Ó 2008 Elsevier Ltd. All rights reserved.

1. Introduction The development of radio frequency micro-electro-mechanical system (RF MEMS) prospers vigorously among the MEMS devices. This is mainly driven by wireless communication and automotive applications. However, RF MEMS are still in their infancies of the market. In terms of RF MEMS devices, they have been reported in three types: the switch, the variable capacitor, and the antenna. Extensive studies on RF MEMS switches in manufacturing have been made, which are micromachined with mechanical movement to toggle RF signal between an open and short electric circuit [1,2]. RF MEMS switches offer substantially higher performance owing to their promising lower loss, higher linearity, and broader bandwidth comparing with field effect transistor diode switches. These advantages of MEMS indeed give the solution to the continuous increase of system performance requirements and their operation frequencies. RF MEMS switches have also demonstrated useful performance because of their low insertion loss and high isolation [3]. Today, the most common actuation mechanism of the switches is electrostatic which has been proved for lower power consumption. Nevertheless, there are still other developing types of actuation methods, such as magnetostatic, piezoelectric, and thermal actuation. Each of them has its advantages and drawbacks. RF switches have high isolation at open state, low insertion loss at closed state, and virtually zero power consumption. As MEMS is * Corresponding author. Tel.: +1 607 777 4908; fax: +1 607 777 4094. E-mail address: [email protected] (T.-H. Lin). 0026-2714/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved. doi:10.1016/j.microrel.2008.07.072

a new technology and the targeted applications require very high electrical performance and reliability, it is crucial to provide experimental study on the performance and reliability of the MEMS switches. For a contact type switch, literature has shown that the lifetime was not limited by mechanical considerations but by the degradation of ohmic contacts [4]. Also MEMS switches need a hermetic or near-hermetic package to prevent any residual moisture or organic contamination of the moving parts incorporated inside. A comprehensive study on both component reliability and electrical performance at various input levels and under various noise conditions, such as high temperature and moisture, needs to be performed. The influence of harsh environments on RF switch failure mechanisms, the electrical performance of the switch at various input frequency, control voltage, and power levels are important issues to be addressed. The research effort in this paper is to evaluate the reliability and performance of magnetic actuated RF MEMS switch that operates under a magnetic actuation principle. The studies focused are the thermal and moisture effects for a newly designed magnetic RF MEMS switch. The military temperature and humidity test standard, MIL-STD-202G, was used to assess the resistance and deleterious effects of the micro components. This study not only tested the performance of RF MEMS switch but also examined the qualification of test conditions for miniature MEMS devices. The following sections of the paper include a summary of magnetic type RF MEMS switches, basic failure mechanisms of MEMS packages, test vehicles and conducted experiments, evaluation of the performance of test samples, and qualification of the acceleration model.

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2. MEMS switches and their properties There are two basic RF MEMS categories by contact methods: metal–to–metal contacts and metal–insulator–metal contacts. Metal to metal contacts are generally found on cantilever type switches and are called ohmic or direct contacts. It means that a signal propagated when two metal contacts touch each other and current passes through the interface [5,6]. Metal–insulator–metal contacts are generally found in membrane MEMS switches and are also called capacitive or indirect contacts [13]. Here the capacitance build-up on the insulator layer by an activation voltage is used as means of triggering the switch [6–8]. Based on the force required for mechanical movement and switch actuation principle, RF MEMS switches can be also classified as electrostatic, magnetostatic, piezoelectric, and thermal actuation. The electrostatic actuation is based on the Coulomb’s law of attraction between two parallel metal plates of opposite polarity [9,10]. Magnetostatic actuation is based on Lorentz force law between two parallel wires [11]. Switches thermally actuated are based on the behavioral changes of materials as they undergo thermal stresses [12]. The changes in length are resulted from thermal expansion. The research in this paper is performed on the cantilever type magnetic actuation switch. Fig. 1 presents the schematic drawings of this magnetostatic switches. With permanent magnets or ferromagnetic materials, magnetic actuator is capable of performing latching without any energy from outside. The permanent magnetic field holds the state of beam (up or down) so that no power is necessary during the latching state. The magnetic energy density allows actuators to provide high actuation and large force. With the device miniaturization, the magnetic force decreases. However, the magnetic actuators generate larger force than electrostatic actuator when the actuation gap is greater than 1 lm [14]. The magnetic circuit is composed of a permanent magnet creating the magnetomotive force. The circuit core is a type of soft magnetic material, perm-alloy NiFe, with high permeability to conduct the magnetic flux, and an air gap for a cantilever beam to be actuated. The switch uses a perm-alloy cantilever in a static permanent magnetic field. The switching between two stable states (on/off) is accomplished by momentarily energizing a planar coil that is located under the cantilever. The magnetic field generated by coil opposes to the magnetization of cantilever and causes the relay

to switch. Once switched, the device is latched by the permanent magnetic field and remains in this state until the coil is reenergized [15]. The mainly concerned performance characteristics of switches vary with the applications. They are often used as metrics to determine the performance of the devices. RF MEMS possesses functions on interacting electrical and mechanical functions, and it is critical to those parameters defined properly in parametric terms crossing the electrical–mechanical boundary [1]. Several function parameters are specific to RF MEMS switches. ‘Insertion loss’ of an RF switch is a measure of its efficiency for signal transmission. It is defined as high frequency signals traveling through a transmission line or a switch module attenuated by series resistance, dielectric absorption, and by reflections from impedance mismatches. This loss is collectively termed as insertion loss and is referred to the RF loss dissipated in the device, typically characterized by transmission coefficient, R21, in decibels (dB), between the input and output of the switch in its pass-through state (on-state of a switch). The main contributing factors include resistive loss due to the finite resistance of the signal lines and contact at low to medium frequencies, and loss due to the skin depth effect at high frequencies. For none of RF signal transmission between the input and output terminals, ‘Isolation’ is defined, typically characterized by S21 of the switch in blocking state, under the no-transmission or switch off-state. The main contributing factors to the ‘Isolation’ include capacitive coupling and surface leakage. Isolation is the measure of the ability to keep a signal on an unused channel from appearing on an active, terminated channel. Due to the capacitance between ports, there is always a certain level of unwanted signal transmission between isolated ports of interest. Isolation can also be defined as the amount of attenuation that a signal attains before it is detected at a termination port and is measured in decibel (dB). The independence of device impedance from the input RF signal power refers to ‘Linearity’, typically characterized by the third-order intersecting point, or IP3, in a two-tone RF inter-modulation measurement [16]. ‘Series resistance’ is directly considered for the resistance value of RF MEMS switch to represent the signal level for lower frequencies. It could be explained as resistance offered by the switch during signal transmission and would result in loss of signal level.

y x

metal contact electrode coil

signal path permalloy cantilever signal path

(a) Top view

z Ho

x

Ho signal path

coil

m coil

substrate

substrate

permanent magnet

permanent magnet

(b) Side view switch-on

(c) Side view switch-off

Fig. 1. Schematic drawings of electromagnetic switch [15]. Planar coils are on the substrate underneath the cantilever. H0 is the magnetic field generated from permanent magnet. The changes of magnetization of the beam (m) result in a torque on the beam forcing it either down or up. The signal is actuated when the beam is in down position.

T.-H. Lin et al. / Microelectronics Reliability 49 (2009) 59–65

A high frequency switching device should be matched at both input and output sides for both on- and off-state of the switch in the operation. The match is to minimize the impact on the performance of the system. If improperly matched, it results in unwanted reflections within the circuit, which may cause the damage to other system. The ideal condition is to minimize the reflections within acceptance limits. 3. Magnetostatic actuation The magnetostatic actuation MEMS are relatively new to those traditional MEMS devices. Magnetostatic MEMS are formed with the electromagnetic interactions between magnetic materials and magnetic field source, such as permanent magnets. Niarchos [17] has given the arguments that the electromagnetic forces are dominant when compared the actuation field energy with electrostatic forces by simplified calculations. In considering the complexity of device structure, the electrostatic devices are simpler. They require higher voltage for actuation, which demands more controllable devices and dielectrics for conductor insulation [18]. The magnetostatic RF MEMS switch is a latching switch designed for switching RF signals from dc to tens of GHz in 50 X systems. It uses a bistable perm-alloy cantilever, an embedded planar coil, a permanent magnet, and the required electrical contacts to form the micro relay without long term bias voltage or current once actuated [17]. The operation principle of the device is presented in Fig. 2. The perm-alloy cantilever is supported by two sided torsion flexures. When it is exposed to an external magnetic field (H0), the cantilever self-aligns with the magnetic field and a torque exerted on the cantilever. Depending on the orientation of the beam with respect to the external field, the direction of the torque could be clockwise or counterclockwise. By applying the current through the planar coil, the cantilever moves to either ‘‘up” or ‘‘down” position that turns the switch in ‘‘off” (open) or ‘‘on” (close) state respectively. The main function of permanent magnet is to make a latching relay by holding the cantilever in a stable state after switching. To switch the cantilever from one state to the other, a momentarily magnetic field is required to overcome the external magnetic field (H0). The temporary magnetic field is generated by current pulse loop around the embedded planar coil. The value of momentarily magnetic field needs to be larger than the n-component of  the external magnetic field, H0n < H0sin/, typically with / 6 5 . For micro relay design with this minute u value, the switch current and power can be relatively low. Since the direction of the coil influences the generation of positive or negative n-field component, the position of the cantilever is determined by the direction of the current pulse [15].

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4. Reliability issues and concerns Reliability of an electrical component is the probability that the component will be operational properly for a given period of time. A critical part of understanding the reliability of a component is based on understanding the possible ways in which the component may fail. Basically, the reliability study on MEMS devices is characterized on their operational conditions at design and development stage. This focuses on the micromachined parts and electronics with concerns of material reliability, designed structural reliability, and fabrication process reliability [20]. For the reliability of RF MEMS, it requires designed structure to perform between 1 and 10 billion cycles or even higher. However, the well-studied failure mode of stiction usually limits the lifetime. It is caused by contact degradation that may result from mechanical contact wear or thermal/electrical effects resulting in contact material property changes [21]. The cause of stiction can also be due to capillary condensation that is mainly induced by humidity [22]. To avoid the capillary stiction, packaging the switch in a moisture free environment is very critical. This paper is to study the thermal and humidity effects on the RF MEMS switches. The thermal effect is a challenge for higher frequency, more functions and smaller volume microsystems, such as RF and photonic circuitry, which generate intrinsic thermal mass from moving parts over the operating life [23]. When the cantilever is in ‘‘on” position, the current flows and elevates the heat on contact spot. The heat generated in local contact spot could increase temperature at tens or hundreds degrees higher than the surrounding material [24]. Holm [25] implements Eq. (1) to evaluate the contact spot temperature Tc as a function of contact voltage Vc

rffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi Vc Tc ¼ þ T 20 4L

ð1Þ

where L = 2.45  108 W X/K2 is the Lorentz constant and T0 is the ambient temperature in K. When the contact temperature is constant applied, the contact is under annealing condition that results in insulation film delamination, melting or boiling. This will reduce the contact hardness, called contact softening, and increase the contact area under the load. Nevertheless, the contact point adhering is proportional to the degree of deformation and the size of contact area [26], the heat will cause micro melting of contact film and short the contact point permanently [27]. The major concern for environmental effect toward the failure of MEMS devices is moisture penetration. Pure moisture will not cause corrosion in the package, but will cause the capillary force for cantilever stiction. On the other hand, the moisture could carry contaminants such as positive (Na+, K+) and negative (Cl, F, NO 3) ions to cause the device performance degradation [28,29]. Thus, if not completely hermetic RF MEMS package is in operation, the thermal energy will increase with the penetration of moisture condition that will significantly reduce the time to component failures.

5. Experimental procedures and assessments

Fig. 2. Schematic drawing of magnetic operation device [15].

The test vehicle is a magnetic RF relay MEMS switch, as aforementioned. The basic characteristics of the switch are summarized in Table 1. The package design is a 5  5 mm hermetic package that consists of two alumina parts. The bottom substrate contains CuW vertical feed through to the package and a lid with internal cavity. The MEMS die is attached to the top of the substrate with wire bonds interconnecting to internal pads. The sealing utilizes AuSn alloy to prevent organic contaminations. The permanent magnet is attached to the top exterior surface. Schematic pad layout of the test component is as shown in Fig. 3. Input signal is supplied to the Common-in; for experimental purposes this signal is mixed

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Table 1 Characteristics of Maglatch RF MEMS, adopted from [19]. Technology

Magnetostatic, latching relay

Contact arrangement Series resistance (X) Bandwidth (GHz) Set/reset voltage (V) Switching pulse width (ls) Switching current (mA) Energy consumption/switching cycle (ls) SPDT die size (mm) Component package size (mm) Package technology Switching speed (ls) Insertion loss at 6 GHz (dB) Isolation at 6 GHz (dB) Contact life hot/cold

SPDT 0.5 DC to 6 ±5 100 100 50 (100 mA  100 ls  5 V) 1.7  2.0 55 Ceramic SMA, hermetic 200 0.5 45 1 M (at 10 V, 10 mA)/10 M

Fig. 4. Temperature–humidity test profile specifications.

with a DC bias voltage of 2 V. A 0–5 V constant current power supply is used for supplying DC bias voltage. The signal generator, Agilent 8645A, is used for generating high frequency input signal. The control voltage, a square wave signal of 10 volt peak to peak (Vpp) in 400 Hz, is supplied between Coil 1 and Coil 2. This signal can be generated by an oscilloscope, but it cannot retain a constant voltage if connected to more than two components at a time. Since the designed experiments need simultaneous testing of more than two components, a power supply circuit that can generate a square wave signal of 400 Hz and 10 Vpp, was designed. The output signals through R-out and L-out are continuously monitored using an Agilent 54641A dual channel oscilloscope.

tion of organic materials, leaching out constituents of materials, and detrimental changes in electrical properties. A temperaturehumidity chamber by Cincinnati Sub-Zero (CSZ) is used to meet the temperature and humidity cycling specified in Fig. 4. The test profile follows military standard MIL-STD-202G (Method 106G) [30]. This test differs from the steady-state humidity test and derives its added effectiveness in its employment of temperature cycling, which provides alternate periods of condensation and drying essential to the development of corrosion process and, in addition, produces a ‘breathing’ action of moisture into partially sealed containers. Increased effectiveness is also obtained by use of a higher temperature, which intensifies the effect of humidity. The deterioration can be detected by measuring the electrical characteristics such as contact resistance and insertion loss. Components are not electrically loaded during the test. There were 14 sample components used in moisture resistance test. All the components were checked functionally well before introducing into the chamber. Components were back bonded to a 3 in by 3 in FR-4 platform. Seven components were covered with an epoxy resin coating to prevent moisture from entering the package. FR-4 was chosen since it does not contain formaldehyde or phenol in its composition and since it is not resiniferous as required by military standards. Also provisions were made to prevent water dripping from the ceiling onto the test specimen. Chamber door was opened for a short period of time for making measurements only during the 16th h through the 24th h of an individual cycle. This restricts opening of chamber only when temperature was 25 °C, and the relative humidity tolerance was maintained. De-mineralized water at room temperature was used to obtain specified humidity. Specimens were placed at the center of the chamber and were well spaced so that they do not contact each other, and each specimen receives essentially the same degree of humidity. As shown in Fig. 4, each cycle is 24 h and specimen shall be subjected to 10 continuous cycles. A preconditioned 24 h cycle was exercised as per the specification due to an unintentional test interruption.

5.1. Moisture resistance test

5.2. Thermal resistance test

Moisture resistance test is designed to evaluate the resistance of component parts and constituent materials to the deteriorative effects of high humidity in an accelerated time span [30]. RF MEMS switch application includes telecommunication and spatial applications. It is highly likely that these components have to work at high moisture levels and most degradation results directly or indirectly from absorption of moisture vapor and films by vulnerable insulating materials, and from surface wetting of metals and insulation. These phenomena produce many types of deterioration, including corrosion of metals, physical distortion and decomposi-

The CSZ temperature chamber was also used to meet the temperature requirements and test conditions. Wires were soldered to each of the six pads (Common-in, R-out, L-out, Coil 1, Coil 2, and Ground), and components were back bonded to the FR-4 coupons using adhesives. Specimens were equally distributed inside the chamber so that there is no obstruction to the airflow around each coupon. The first set of nine components was tested at 50 °C and the second set of nine components was tested at 105 °C. Both sets of components were tested until all the components were failed. This step was taken to avoid the problem of right

Fig. 3. Schematic drawing of switch layout (bottom view) [30].

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T.-H. Lin et al. / Microelectronics Reliability 49 (2009) 59–65 Table 2 Moisture test data (ohm) for unprotected components.

Isolation vs. Input Frequency 70 60

Isolation (dB)

50 40 30 20 RF in - Right out

10

RF in - Left out

0 0

2

4

6 8 Frequency (MHz)

10

12

14

Cycles

Component A1

A2

A3

A4

A5

A6

A7

1 2 3 4 5 6 7 8 9 10 Result

0.478 0.475 0.475 0.475 0.475 0.475 Fail

0.921 0.920 Fail

0.683 0.679 0.680 0.680 0.680 Fail

0.442 0.442 Fail

0.471 0.470 0.470 0.468 0.468 0.470 0.468 0.468 Fail

0.626 0.626 0.626 Fail

0.728 0.734 0.731 0.731 0.731 Fail

Fail

Fail

Fail

Fail

Fail

Fail

Fail

Fig. 5. Isolation versus input frequency.

complete sealed test vehicles survived through all 10 humidity and temperature cycles. All the failed units were electrically tested to study the failure mechanism. Components were not switching from one contact point to another even with a higher actuation voltage (15 Vpp); this kind of failure can be attributed as stiction failures. Careful examination of the data shows that there is no degradation of contact resistance after each stress cycles. Hence the effect of moisture can be concluded as follows.

Insertion loss vs. Input Frequency 0.07

Insertion loss (dB)

0.06 0.05 0.04

 Moisture does not affect contact resistance.  Moisture particles penetrate the component package and cause stiction failure, when component is exposed to high humid environment.  The unsealed components are very sensitive to the humilities.

0.03 0.02 RF in - Right out

0.01

RF in - Left out

0 0

2

4

6 8 Frequency (MHz)

10

12

14

6.2. Thermal test results

Fig. 6. Insertion loss versus input frequency.

censoring and to use maximum sample size. Another set of eight components was tested at 85 °C until failure to verify the Arrhenius model. Contact resistance of two contact points (L-out and R-out) and isolation loss were component responses taken. Contact resistance was measured using a high precision HP multimeter. When modeled the switch as a simple two-port series resistive network, insertion loss can be calculated using Eq. (2). The transmission coefficient, R21, between two ports in the circuit is expressed in decibels as the insertion loss [31]

Insertion Loss ðILÞ ¼ 20 logðR21 Þ

ð2Þ

The measured isolation and insertion loss with regard to different input RF frequency are plotted in Figs. 5 and 6, respectively. The measurements were taken for both right and left signal transmission outputs. The test vehicles tested were prior to subjecting moisture and temperature conditions. 6. Results and discussion

Thermal test is performed to study the component performance and degradation over its lifetime. All the components were tested until a failure happens. End of life is defined as when the insertion loss becomes greater than or equal to average value, 0.46 dB, or when the contact resistance increment becomes more than one ohm over the time zero readings. This kind of failure is called a soft failure since the component is still operational [32]. Table 3 shows the time to failure for two sets of component testing conditions, one at 50 °C and another at 105 °C. With the collected data, the fittings of theoretical distributions were performed in three steps: (1) identifying candidate distributions; (2) estimating parameters; and (3) performing a goodness-of-fit test. 6.2.1. Data distribution identification Descriptive statistics computed from the sample data may be useful in either identifying a candidate distribution or eliminating some distributions [33]. Fig. 7 shows the goodness of various distributions fitting the failure data at different temperatures. The Anderson–Darling values in the figure are used to test the claim that the sample data from a population comes from a specific distribution. With a smaller value, the claim has lower potential to be rejected, which suggests the data fits the specific distribution better. By comparing the Anderson–Darling value of each data set of

6.1. Moisture resistance test results The contact resistance of components after each cycle of 24 h for the seven unprotected components was recorded and failures occurred when the contact resistances become infinite in reading. Table 2 shows the resistance readings of the components. The estimation of the mean time to failure is calculated as 5.43 cycles and the MTTF with 90% confidence interval is [7.06, 3.79]. The seven

Table 3 Failure time (in h) data at 50 °C and 105 °C. Component

1

2

3

4

5

6

7

8

9

50 °C 105 °C

1650 700

200 1200

200 200

700 250

650 1200

2000 450

250 2050

1900 150

1200 550

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T.-H. Lin et al. / Microelectronics Reliability 49 (2009) 59–65

Fig. 7. Evaluation of goodness of fit for various distributions.

samples in different distributions, it indicates Weibull distribution is a better one to represent the life test data collected in the thermal reliability test. Figs. 8 and 9 give the reliability plot and hazard rate plot, respectively, with shape parameter 1.22 and scale parameter 1051.42 for temperature 50 °C, and shape parameter 1.27 and scale parameter 810.85 for 105 °C. The shape parameters of test results are both greater than and close to one. This indicates the reliability is deteriorating but not dramatically. 6.2.2. Acceleration model validation and reliability prediction In order to interpolate the reliability performance of the tested RF MEMS switches in a normal operation condition, Arrhenius model was selected which is represented in the following equation

k ¼ AeB=T

ð3Þ

where k is the reaction or process rate, A and B are constants and T is temperature measured in Kelvin. Arrhenius model is a widely used acceleration model for macro electronic components under elevated thermal tests. However, the

Fig. 9. Hazard rate plot for failure data at 50 °C and 105 °C.

suitability of the model has not yet been evaluated for MEMS devices according to the best knowledge of the authors. In order to validate the suitability of Arrhenius model for reliability prediction under different temperatures, the thermal reliability test was conducted at three different temperatures: 50, 85, and 105 °C. First the failure rates and the characteristic lifes are evaluated from the test results at stress conditions, 50 and 105 °C, and the acceleration factor can be determined from the following equation, which is the intuitive extension of Arrhenius model

AF ¼

Fig. 8. Survival plot for failure data at 50 °C and 105 °C.

AeB=T 2 AeB=T 1

   1 1 ¼ exp B  T1 T2

ð4Þ

where AF = h1/h2, with hi representing a scale parameter (characteristic life) at the stress level corresponding to Ti, A and B are constants and Ti is temperature measured in Kelvin. AF was calculated to be 1.3 and B equal to 577 by using Eq. (4) with the results from temperature tests, 50 °C (T1) and 105 °C (T2). Another set of accelerated test (with temperature 85 °C) was per-

T.-H. Lin et al. / Microelectronics Reliability 49 (2009) 59–65 Table 4 Failure time (in h) data at 85 °C.

References

Component

1

2

3

4

5

6

7

8

85 °C

1550

350

350

1600

600

400

1600

150

formed with eight test vehicles to evaluate the suitability of the Arrhenius model. Components were kept in the chamber until each of them failed. The failure time of components was recorded in Table 4. The distribution parameters estimated from the experimental test results at temperature 85 °C were with shape parameter b = 1.3 and scale parameter h = 894.7; while by Arrhenius model, AF was calculated equal to 1.09 and the predicted scale parameter h for the same temperature condition was 882.9. There was 1.32% deficiency of the characteristic life for the predicted results compared to the experimental results. The shape parameters estimated from the three different temperatures are very close, indicating the same failure mechanism for the failures occurred at different temperatures, which is also the prerequisite for applying Arrhenius model. From the above test and analysis, it is initially indicated that Arrhenius model is still valid as the acceleration model for MEMS devices at elevated thermal test. The predicted MTTF can be calculated by

  1 MTTF ¼ hC 1 þ b

65

ð5Þ

The shape parameter for 85 °C test condition was taken from the average shape parameter of 50 °C and 105 °C conditions. The result of MTTF for the tested RF MEMS switches at operation temperature at 85 °C was estimated for 822 h. 7. Conclusions Due to the difference of the application area and condition, the environmental situations give a variety of needs to reliability qualifications. These concerns are significant in MEMS device commercialization. The research adopts a novel magnetostatic RF MEMS switch device (in its prototype development stage) for reliability study. The focus is to understand the suitability of existing accelerated life test standards, acceleration model, and the fitness of statistical distribution. In this paper, two types of evaluation tests performed, moisture resistance test with thermal cycling condition and isothermal aging test. The results of moisture test delineate that moisture does not affect the contact resistance; however, the unprotected components are very sensitive to the humilities. It reveals that moisture particles penetrate the component package and cause stiction failure, when component is exposed to high humidity and temperature cycling profile. Under the temperature aging, the calculated MTTF for each condition is: 985 h, 822 h, and 751 h for the temperatures, 50, 85, and 105 °C, respectively. With the statistical distribution fitting process, the results of experiments on thermal test have been identified to be Weibull distribution. Another set of the experiment has been conducted to compare and validate the predicted results using Arrhenius model. It shows that the performance of the model fits the result of actual experimental data. In the future, reliability tests with more samples need to be conducted and failure mechanism studied.

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