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  1. 1. COVER STORY Detecting failure modes endless diversity in form and function of the devices. Each type of device Figure 2: Wyko NT profiler image has different complexity in the manufacturing process, a different geometry of a MeMS fabricated microphone. (Courtesy of of Texas Knowles Acoustics) (from tens of microns to tens of millimeters in width), and vastly different performance requirements or designed responses to external stimuli. Thus, while curvature, surface roughness, and switching time may be critical for in today’s MEMS micromirror arrays, co-planarity of the capacitor parts and linewidths may be the most crucial parameters for an accelerometer. Initial or long-term failure of a device may also be driven by an array of factors, such as geometric errors in production, contamina- tion, improper removal by Noushin Dowlatshahi and Bob Chanapan, Veeco instruments inc. M icroelectricalmechanical systems (MEMS) are taking a whole new role in our day-to- day life, and are much more widely used than ever before, due to a wide range of benefits including their low mass, fast mechanical response, low power consumption, and potential for lowering end costs. Meanwhile, test and quality control of such devices has become more critical to facilitate the insertion of this technology into critical applications. Therefore, it is not surpris- of sacrificial ing that precision metrology has a huge role in this steady layers, stiction, environmental advancement of MEMS technology. attack, fatigue, electrostatic Unlike traditional semiconductor devices, today’s MEMS devices clamping, fusing, require characterization in both their static state and under actuation. delamination, or Parameters of interest include shape, dimensions, surface roughness, side- electrical damage. wall angles, film thickness, residual stress, feature volumes, response times, During research thermal properties, resonance frequencies, stiction, environmental compat- and development, there ibility, and more. The greatest difficulty in MEMS metrology is the nearly is also a need to map the behavior of devices and materials placed under external stresses (e.g., tem- perature, pressure, or corro- Figure 1: Wyko NT sive agents) in order to fully dynamic measurement understand the time-course reveals a hidden defect in a micro-mirror. evolution of these processes. (Courtesy of of Texas Tech University) Early detection of failure modes not only enables improvement at 18 November/December 2008 19 November/December 2008 www.smalltimes.com | SMALL TiMeS www.smalltimes. com SMALL TiMeS |
  2. 2. Early detection of failure modes not only enables improvement at the next fabrication cycle and increased yields, but it also identifies the next fabrication cycle and increased yields, but it also These arrays typically consist of simple cantilever beams, has pros and cons, and thus the velocity of the moving part. Displacement can also identifies whether the problem is isolated or systematic, torsion beams, tethered (piston-style) beams, circular mem- selecting the proper solu- be measured through integration of this signal. These sys- whether the directly impacting field failure quality control. branes, and oval membranes. One technique of fabricating tion depends on a number tems are very sensitive to motion, with sub-nanometer reso- the micromirrors is to use low-temperature adhesive wafer of factors, from device type lution in the direction perpendicular to the test beam. More problem is isolated bonding to deposit a thin layer of monocrystalline silicon to stage of manufacturing. sophisticated systems enable the measurement point to be RF MeMS MEMS applications in the domain of radio-frequency (RF) device to a CMOS wafer. The digital micromirror device SEMs generally offer the scanned across the surface, to build up a complete picture of or systematic, circuits serve as a perfect example. For certain capaci- (DMD) developed by Texas Instruments, which incorporates highest lateral resolution the MEMS out-of-plane motion, though phase information tive MEMS switches, it is crucial to examine the stiction more than 500,000 individually addressable micromirrors, has available for static MEMS between each of the monitoring points is lost due to the time directly impacting between the metal layer (top electrode) and the dielectric made great strides in both performance and reliability of such imaging. These systems lag between measurements. The lasers on these systems can layer covering the bottom electrode. The charge build-up devices. Digital light processing technology based on DMD operate by focusing a beam also be coupled to other optical systems directly or via fiber field failure in the dielectric material can result in what would typi- has been used in such diverse products as projection displays of electrons on a small area options, such as stroboscopic bright-field microscopes, so one cally be called a “failure mode.” The actuation voltage is with film-like projected images and photographic-quality and detecting the electrons can get the in-plane-motion through a separate series of mea- quality control. directly affecting this charge build-up. Researchers need printers. Reliability testing of the DMD has demonstrated that are scattered from the surements with that equip- to understand how an increase or decrease in the actua- greater than 100,000 operating hours and more than 1 tril- surface. Measurement time ment. However, all systems are Manufacturers tion voltage can impact the lifetime of typical MEMS. This lion mirror cycles. is typically between 5–10s sensitive to the roughness of actuation voltage is typically related to the device geometry, It is crucial for micromirror array manufacturers to exam- after the sample has been the surface, requiring enough have relied on a mechanical and material properties, and residual stresses ine the uniformity in how the micromirrors tilt in an array, loaded into the system. light to be scattered back into in the devices. When voltage is applied to a typical capaci- the time it takes them to tilt, and how repeatable this behav- Lateral resolutions down to the nanometer level are possible, the detector for adequate sig- range of methods tive RF MEMS switch, the electrostatic charges cause a dis- ior is. This is in addition to the traditional parameters of mir- and depth-of-focus of such systems can be quite large. SEMs nal-to-noise. Therefore, very tributed electrostatic force, leading to deformation of the ror curvature/deflection angle roughness and surface topog- provide an invaluable tool for evaluating sidewall angles and smooth or rough surfaces can to identify micro-structure. Later, due to the storage of elastic energy, raphy. Moreover, as the devices are packaged in hermetically roughness of very steep parts, and are often the only systems degrade accuracy. the structure tends to return to its original state. Accurate sealed packages, the encapsulation process can further affect capable of these measurements in such applications as micro- Digital holography (DH) structural failure metrology can play a significant role in characterizing such a the operational performance. Environmental factors includ- fluidics and capacitive accelerometers. The primary limitations has been employed for many switch by defining and evaluating the membrane’s deforma- ing temperature and humidity also impact on such devices, of the technique are that samples must be placed in vacuum, years to measure vibrating modes in MEMS. tion and analyzing thermal impact, as well as the material creating the need of a fourth-dimensional capable, non-con- and the penetration depth of electrons is very small, limiting devices. In this technique, and mechanical properties of the metal. tact metrology solution. Inspecting the device in a meaning- effective analysis on packaged parts. Also, parts must often a digital camera is used to ful manner can monitor and minimize fabrication errors and be cross-sectioned in order to achieve the proper orientation record a hologram produced positively impact the micromirrors components within an for best measurement, which can be cost prohibitive. Lastly, by interfering a high-quality reference beam with a beam Micromirrors Another widely used type of MEMS device that exhibits array. edges may produce artifacts if they become charged, leading reflected from the sample under test. As the test object is later particular metrology needs is the micromirror array, which to inaccurate data. deformed, the modified object beam is compared with the is finding use in adaptive optics for both space and consumer Laser Doppler vibrometers are frequently used for rapid original digitally recorded hologram, and the deformation can inspection strategies electronics, as well as in projectors, televisions, and digital Manufacturers have relied on a range of methods to iden- characterization of out-of-plane MEMS motion. These sys- be quantified using standard phase-shifting or other interfero- cameras. These mirror devices must be 100% reliable and tify structural failure modes in MEMS: optical microscopy, tems employ a beam of modulated laser light that is reflected metric techniques. These systems can therefore achieve nano- accurate. Because of variations in device geometry that result stylus profilometrey, infrared (IR) thermal imaging, focused from the moving test piece. The returned light is Doppler- meter-level measurement of out-of-plane motion of devices. from the micro-fabrication process, the performance from one ion beams (FIB), scanning electron microscopy (SEM), digital shifted from the original modulated beam, and the two beams Using high-speed cameras, deformations of up to several thou- device to the next can vary, making it difficult to reliably holography (DH), atomic force microscopy (AFM), and trans- are then compared. A spectrum analyzer or other electronics sand hertz can be measured. The systems can also be com- ensure accuracy and repeatability of the actuator positions. mission electron microscopy (TEM). Each of these techniques can be used to determine the magnitude of the shift, and bined with stroboscopic methods to measure motions up to 20 November/December 2008 21 November/December 2008 www.smalltimes.com | SMALL TiMeS www.smalltimes.com SMALL TiMeS |
  3. 3. It is now possible to perform non-destructive, rapid, extremely precise MEMS characterization through packaging and under operation several megahertz, to the device under test and the other part to a high-quality similar to the strobed reference surface. The light recombines to form a pattern of on a single systems mentioned interference “fringes.” The reference surface is then trans- previously. Like the lated relative to the test surface, and multiple frames of data measuring tool. other optical meth- are obtained during the translation. The resulting series of ods, digital holo- fringe patterns are analyzed to calculate the surface profiler of graphic systems can the device. Optical profilers provide rapid, 3-D, non-contact measure parts through a transparent cover glass. How- sample surface characterization. Consequently, this method ever, as the source employed is a laser with long coher- has proven quite useful for MEMS production processes. How- ence length, they can suffer speckle problems from diffuse ever, this method has traditionally also been inadequate for MEMS surfaces and may also have problems with transpar- determining a device’s dynamic ability to perform, such as ent films on a surface. actuation, deformation, rotation, etc. Atomic force microscopes (AFM) employ nanoscale tips CalTech overcame this primary limitation of optical pro- attached to the end of a cantilever that is brought very close filing by combining it with stroboscopic illumination for to the test surface. Measurement of the tip deflection allows dynamic measurement of MEMS devices in motion. Strobo- full 3-D characterization of the surface, and van der Waals scopic illumination effectively “freezes” the motion of MEMS forces between the tip and sample can be detected. AFMs structures, allowing nanometer-resolution measurement of utilize a variety of measurement modes, including “contact their surface shape. By varying the amplitude, phase, and mode” (where the tip lightly touches the sample) and “tap- frequency of the drive signal, multiple measurements can be ping mode” (where high-frequency tip oscillations are used taken to describe the device’s full range of actuation/defor- to minimize contact with the sample while maintaining the mation. Moving MEMS structures can then be analyzed for high signal-to-noise ratio). AFMs have the highest lateral flatness variation, tilt, lateral translation, linearity of motion, and vertical resolutions available of any three-dimensional stiction, and other key device parameters. metrology instrumentation, with sub-nanometer features Today’s top-of-the-line optical profilers incorporate this visible in all three dimensions. However, the field of view technology along with other advances to provide a best mea- for a scan is typically only about 120µm2, and vertical limits surement solution for MEMS researchers and manufacturers. are about 10µm in height. In addition, AFM measurements Veeco’s ninth-generation Wyko NT profilers incorporate LED are relatively slow. Due to the scan speed and contact nature illumination for higher light levels and In-Motion capability for of the scan, these systems are incapable of measuring MEMS measuring devices under actuation from 0–2.4mHz with ang- while under actuation. strom-level resolution. Objective modules that can visualize Optical profiling (white light interferometry) has long parts through packaging are also commercially available. served as a standard technique for measuring surface topog- Thus, it is now possible to perform non-destructive, rapid, raphy of MEMS and optical MEMS devices. In a white-light extremely precise MEMS characterization through packaging optical microscope, the illumination source is traditionally a and under operation on a single measuring tool. tungsten-halogen bulb, coupled into an optical system with several interferometric microscope objectives. Light from the NoushiN Dowlatshahi is marketing product engineer at Veeco Instruments Inc. Bob Chanapan is marketing product manager at Veeco Instruments Inc. source is split in the objective, with part of the beam traveling 22 November/December 2008 www.smalltimes.com | SMALL TiMeS

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