1. Field of the Invention
The invention relates to micro electromechanical systems, particularly to micro electromechanical switches and structures for testing the same. More specifically, the invention relates to test structures and test methods to acquire reliability and qualification data in order to characterize MEMS switch performance with statistical significance.
2. Description of Related Art
Micro Electromechanical Systems (MEMS) are being considered for possible switch structures in advanced high performance analog circuitry, in part, because of the improved switching characteristics over FET devices. For example, some MEMS-based RF switches are being developed with superior RF switching characteristics compared to other transistor-based switches, such as GaAs MESFETs, and the like.
While the development of these MEMS switches are in the early development stage, their performance must be empirically characterized; however, reliability and qualification methods for process enhancements and lifetime predictions are difficult to apply and require large sample sizes for accurate statistical determination.
In the qualification of MEMS relays, it is necessary to assess the overall performance of certain parameters including the degradation of performance over the life of the switch. These parameters will require quantitative measures with accompanying statistics in order to ascertain their longevity and reliability with statistical significance. Critical relays characteristics, such as activation and deactivation at certain activation/deactivation voltages, can be conveniently measured in a pass/fail fashion with the circuit design tolerance taken into account. These results are analyzed by plotting the cumulative fail in percentage versus lifetime under test in a lognormal scale. A statistical statement on the projected failure rate in normal operating lifetime can be obtained with an assigned level of confidence. In order meet higher and higher levels of reliability, statistical statements must be made with high precision and confidence. This means a larger amount of samples must be used in such test sequence.
Generally, the layout and fabrication of the MEMS devices makes the testing of large sample sizes impractical. For example, since each switch has at least four probe pads (two for the actuation and two for the contacts), an adequate sample size of switches would require either an extremely large number of I/O pads on the sample chip, or conversely, a large number of chips. These options quickly become expensive and impractical.
Bearing in mind the problems and deficiencies of the prior art, it is therefore an object of the present invention to provide an apparatus and method for testing MEMS relay devices using characteristic parameters to limit sample sizes and the number of I/O pads.
It is another object of the present invention to provide an apparatus and method for testing MEMS relay devices that accommodates the testing of a large number of devices and provides accurate measurements for certain device parameters.
A further object of the invention is to provide an apparatus for testing multiple MEMS switches on a semiconductor circuit chip without requiring a large number of probe pads.
Still other objects and advantages of the invention will in part be obvious and will in part be apparent from the specification.
The above and other objects, which will be apparent to those skilled in the art, are achieved in the present invention, which is directed to an apparatus for measuring contact and gap characteristics of MEMS switches comprising: a plurality of the MEMS switches in an array pattern configured in a serpentine circuit, having air gaps between combs of upper and lower actuation electrodes; a first pair of probe pads electrically connecting to the upper actuation electrodes; a second pair of probe pads electrically connecting to the lower actuation electrodes; and a third pair of probe pads electrically connecting to the MEMS switches in the serpentine circuit; such that all of the actuation electrodes are electrically configured in parallel, while the MEMS switches are electrically configured in series. Switch contacts of the MEMS switches may be arranged such that the MEMS switches close in series and the actuation electrodes in parallel so that an actuation leakage current which is a sum total of each individual actuation leakage current of each of the MEMS switches is measurable, or a contact resistance which is a sum total of each individual contact resistance of each of the MEMS switches is measurable.
In a second aspect, the present invention is directed to an apparatus for measuring characteristics of MEMS switches arranged in a cascaded electrical configuration comprising: a plurality of the MEMS switches, each of the MEMS switches having a signal line, a beam, and at least one actuation line; and via connections electrically connecting the signal line of a MEMS switch to the beam of an adjacent MEMS switch such that the plurality of MEMS switches are electrically linked to form a cascading chain when the actuation lines are biased; whereby, upon biasing the actuation lines of a first MEMS switch, biasing of each the adjacent MEMS switches is induced in a time delayed, linear fashion, until all of the plurality of MEMS switches are activated. The switch contacts of the MEMS switches are arranged to close in a cascading pattern so that a switch delay time which is a sum total of each individual switch delay time of the MEMS switch is measurable. The apparatus further comprises a frequency counter, an invertor, and an edge counter to form a ring oscillator of the MEMS switches arranged in the cascaded configuration, wherein the frequency counter yields a measurement of switch delay equal to a reciprocal of a product of frequency and number of the MEMS switches, satisfying an expression 1/f*N, and the edge counter counts rising and falling edges of a transmitted signal through the MEMS switches, the transmitted signal electrically circling back the cascading chain to reopen the MEMS switches. Moreover, the apparatus may further include having each of the MEMS switch signal lines connected to an adjacent MEMS switch actuator line, each of the beams being resistively connected to a voltage potential, and having a ring initiation pulse inputted to a first actuator line of a first MEMS switch in the cascading configuration.
In a third aspect, the present invention is directed to an apparatus for measuring characteristics of MEMS switches using a resistor ladder comprising: a plurality of the MEMS switches; a plurality of resistors electrically configured such that each resistor has a corresponding MEMS switch, the resistor electrically connected in series with the MEMS switch, each resistor-MEMS switch pair electrically configured in parallel to one another; an actuation probe pad pair for applying an activation voltage; and a signal probe pad pair for collectively measuring output resistance of the resistor-MEMS switch pairs; such that when all of the MEMS switches are activated together, each of the MEMS switches close, one-by-one, incrementally decreasing measured resistance. Each of the plurality of resistors may have a different resistance values from one another, or an equivalent resistance value.
In a fourth aspect, the present invention is directed to an apparatus for measuring characteristic parameters of switches, comprising: a first set of switches comprised of a first technology; a second set of switches comprised of a second technology, the second technology different from the first technology; an actuation circuit in electromagnetic communication with the first set of switches; and a pair of actuation probe pads terminating the actuation circuit; wherein the first set of switches are configured in a closed-state and aligned in a series circuit when voltage is applied across the pair of actuation pads and the second set of switches are electrically held in an open-state, enabling a sum total contact resistance to be measured for the first set of switches or an open-state failure detected from at least one switch of the first set of switches. The second set of switches may be in a closed-state, electrically configuring the first set of switches in parallel, enabling a closed-state failure from at least one switch of the first set of switches when the first set of switches are activated to remain open. The first set of switches activates simultaneously when voltage is applied to the actuation pads. The first technology may include MEMS structure, while the second technology may include solid-state structure.
In a fifth aspect, the present invention is directed to an apparatus for increasing a MEMS switch sample size for quality assurance testing, comprising: a plurality of MEMS switches; an actuation circuit in electromagnetic communication with the plurality of MEMS switches, such that when the actuation circuit is activated at predetermined voltage levels, the MEMS switches are opened or closed; a shift register having a readout port and a plurality of data input registers, each of the data input registers corresponding to a MEMS switch of the plurality of MEMS switches, such that each of the data input registers is electrically in series with each of the MEMS switches, completing a series circuit when the MEMS switches are in a closed-state; and an electrical clock-pulse input to the shift register; wherein an open or close state of each of the plurality of MEMS switches is determined via a readout line of clock pulses from the shift register. An open/close status of each of the MEMS switches is determined from the shift register readout. The predetermined voltage comprises a step function of increasing voltage levels such that the shift register readout determines a pull-in voltage for each of the MEMS switches. Alternatively, the predetermined voltage comprises a step function of decreasing voltage levels such that the shift register readout determines a drop-out voltage for each of the MEMS switches.
The features of the invention believed to be novel and the elements characteristic of the invention are set forth with particularity in the appended claims. The figures are for illustration purposes only and are not drawn to scale. The invention itself, however, both as to organization and method of operation, may best be understood by reference to the detailed description which follows taken in conjunction with the accompanying drawings in which:
In describing the preferred embodiment of the present invention, reference will be made herein to
The objective is to test certain characteristic parameters on these relays in order to ascertain functionality during the entire design lifetime of the switches. These parameters include: pull-in and drop-out voltage; leakage current drawn by the actuation; resistance of the contacts; and the number of actuations (OPEN/CLOSE) before sticking. For example, unlike traditional BEOL structures where metal lines are imbedded in rigid insulating dielectrics, MEMS switches usually involve free-standing structures, such as cantilevers, fixed-fixed beams, or suspended bridge structures, that move in response to electrostatic forces from an applied voltage to the actuation components. At the application of the actuation voltage, the switch electrode contacts on the cantilever/beam make contact with the lower contact pads for electrical transmission, while the actuation electrodes remain separated by a narrow gap, as shown in
The total leakage current and total contact resistance of the entire population may be simultaneously measured, making the magnitude of the parameter measurements easier to obtain with more accuracy, which ultimately improves the value of the qualification process. Similarly, the change in total leakage current and contact resistance may be measured over the useful life of the switch population in order to ensure that these parameters stay within the design tolerance over the entire life of operation.
In addition, both the pull-in and drop-out voltage of each individual switch can be accurately measured, yielding a distribution of these and other important parameters for the entire population. A change in this distribution may be measured as a function of age or number of switch actions. Thus, it is important for the switches to be tested beyond the operational life by employing accelerated stress condition. Both OPEN and CLOSED conditions may be detected, but importantly, these conditions do not disable the test structure, so measurements may be continued until each and every switch in the test structure no longer functions, yielding a distribution of switch lifetime.
Method and Structure for Testing Contact and Gap Characteristics
By utilizing the test structure of
Cascaded Switch Chain
MEMS structures continue to be considered by persons of skill in the art as possible switch structures in advanced high performance analog circuitry, due mainly to their improved switching characteristics over FET devices. Typically, thick copper metal is used as the switching beam or cantilever, as shown schematically in
The performance and reliability of the MEMS switch structure depend critically on the choice of material and size. For example, the pull-down voltage and switching speed depend mainly on the mechanical properties of the cantilever material, as well as the dimensions of the beam. In the preferred embodiments, the device structures are allowed the use of shorter beams. This poses less of a stiction problem while adding more reliability, and exhibits reduced switching speed frequency, which can be used to provide proper time delay for switching in certain circuits.
Reliability measures associated with MEMS structures, such as fatigue, contact integrity, and stiction, are unique among conventional BEOL structures. A cascaded switch chain test structure is proposed to evaluate process yield, performance, and reliability. This test chain structure has been shown to greatly increase the sample size for testing and parameter/device characterization, including allowing easier, more accurate measurement of switching speed. The preferred cascaded switch chain embodiment offers flexible switch design and precise switching speed measurements. The cascade switch chain is also used as a test structure for evaluating yield performance and reliability of a MEMS switch. With the addition of inverters, edge counters, and frequency counters, the cascade switch chain structure may be modified to serve as a ring oscillator for automated lifetime measurement and precise switch speed characterization.
In addition, it is possible for the preferred cascade switch embodiment to function as a switch with specified switch delay characteristics for certain circuit applications. By increasing the total number of switches in the chain, the switch time can be properly delayed to match the time characteristics required for a given operation.
Cascade Switch Ring Oscillator
Pull-in and drop-out voltage levels are measured and verified by observing the presence or disappearance of the frequency signal when the actuation voltage is ramped up or down, respectively. The measured voltages represent the worst-case performance of the switch population, yielding the highest pull-in voltage and lowest drop-out voltage, because all switches must be functioning for the ring oscillator to operate.
Resistor Ladder Test Structures
Additionally, resistance measurement at the Rout terminals is capable of indicating a stuck-switch condition. If the resistance is too low when no activation voltage is applied, this indicates that at least one switch is in a closed position. The measured value of the total resistance will empirically show how many switches are in a closed position. Similarly, if maximum activation voltage is applied and resistance Rout is too high, the resistance will indicate how may switches are in an open position. Importantly, testing may continue even after some switches have been brought to failure. Furthermore, testing may continue until all switches have failed, when no resistance change at Rout is measured when the activation voltage is changed from zero to its maximum value. In this manner, the resistor ladder test structure is capable of yielding a distribution of switch lifetimes.
The preferred resistor ladder test structure is useful at both early and late points in the product qualification process. In the early stages, when the manufacturing process is not yet mature, it is useful to perform physical failure analysis on failed parts. This requires a failed switch to be precisely identified when the failure is detected. The preferred resistor ladder test structure technique accomplishes this by requiring each of the resistors in the ladder structure to have different values. The number of switches in the ladder also facilitates measuring and identifying specific switch failures. Preferably, ten to twenty switches per ladder are suitable for identifying specific switches upon failure, although the test structure may accommodate many more switches. When the manufacturing process has matured to a level where failure analysis of individual switches is no longer required, it becomes important for the test engineer to know how many switches have failed, and to be able to assign a statistically significant statement to the switch success rate. In this instance, many more switches may be built into the ladder structure, preferably 100 to 200 switches. An identical resistor is assigned for each switch. The number of closed switches is quantitatively defined by R/Rout, where R is the resistance of one of the identical ladder resistors. This method allows the measurement of more accurate distributions of switch pull-in and drop-out voltages, and lifecycle assessment, due to the availability of the large number of switches in the structure.
Serial/Parallel Structures
Switches 102, when closed, are electrically connected in series if switches 104 are simultaneously open.
A preferred method of operation of the serial/parallel structure is as follows: 1) exercise switches 102 for a defined number of actions; 2) measure activation leakage current, contact resistance, pull-in voltage, and drop-out voltage; 3) use switches 104 to check for any failures of switches 102 (open or closed state failures); and 4) repeat steps 1-3 above until failures are detected in switches 102.
Shift Register Structure
In general, the present invention involves different methods and structures for increasing the sample size of MEMS switches with a limited number of I/O pads. Another embodiment which may be employed towards this end is a shift register.
Referring to
For pull-in voltage, if the actuation voltage is slowly increased in small, discrete steeps, and a readout of the shift register is performed after each step, the activation voltage of each individual switch may be measured. The number of CLOSED switches may be counted by counting the number of 1's in the shift register chain. By plotting the number of closed switches against the applied actuation voltage, a histogram may be formed of the actuation voltages of the entire population. Moreover, since each cell of the shift register corresponds to a MEMS switch, physical failure analysis may be performed on any switches that fail to operate, or whose actuation voltage is no longer within specification.
For drop-out voltage, a similar activity is performed using the shift register structure; however, the applied actuation voltage is stepped down rather than increased, and the number of OPENED switches is counted.
For activation leakage current, since all of the activation contacts of all the switches are in parallel, the current drawn by the activation pads of the test structure is the sum total of the activation leakage of all active devices. The average leakage current may be calculated from the total leakage current measured divided by the number of CLOSED switches. Furthermore, a graph of the total leakage current against the number of CLOSED switches can be matched against a linear plot since this the linearity indicates uniformity of the actuation structures.
Lifetime measurements are derived from the number of actuations to physical failure of the MEMS device. The shift register structure may be used to indicate switches that do not close when the actuation voltage is applied or switches that remain closed when the actuation current is removed. The number and mode of failure may be plotted as a function of the number of actuation voltage pulses applied. This yields a histogram of the lifetime of the population of switches.
The present invention provides multiple test structures for performing reliability and qualification tests on MEMS switch devices. A test structure for contact and gap characteristic measurements having a serpentine layout simulates rows of upper and lower actuation electrodes. MEMS switches are electrically connected in series. A cascaded switch chain test is used to monitor process defects with large sample sizes. The entire chain closes only when all the individual switches are closed. The cascaded switch chain test will determine the characteristic switching speed of switches that have different dimensions and/or materials. With the addition of inverters, edge counters, and frequency counters, the cascade switch chain structure may be modified to serve as a ring oscillator. The ring oscillator is used to measure switch speed and switch lifetime. A resistor ladder test structure is configured having each resistor in series with a switch to be tested, and having each switch-resistor pair electrically connected in parallel. Pull-in voltage and drop-out voltages may be plotted for an entire population of switches. Serial/parallel test structures are proposed with MEMS switches working in tandem with switches of established technology. MEMS switches can be tested in series and in parallel. A shift register is used to monitor the open and close state of the MEMS switches. Pull-in voltage, drop-out voltage, activation leakage current, and switch lifetime measurements are performed using the shift register.
While the present invention has been particularly described, in conjunction with a specific preferred embodiment, it is evident that many alternatives, modifications and variations will be apparent to those skilled in the art in light of the foregoing description. It is therefore contemplated that the appended claims will embrace any such alternatives, modifications and variations as falling within the true scope and spirit of the present invention.
Number | Name | Date | Kind |
---|---|---|---|
6016092 | Qiu et al. | Jan 2000 | A |
6440767 | Loo et al. | Aug 2002 | B1 |
6624720 | Allison et al. | Sep 2003 | B1 |
6888420 | Schaffner et al. | May 2005 | B2 |
Number | Date | Country | |
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20070090902 A1 | Apr 2007 | US |