Miniaturization as a key factor to the development and ...

Invited Paper

Miniaturization as a key factor to the development and application of advanced metrology systems Cosme Furlong 1,2,3, Ivo Dobrev 1, Ellery Harrington 1, Peter Hefti 1, and Morteza Khaleghi 1 1

Center for Holographic Studies and Laser micro-mechaTronics – CHSLT NanoEngineering Science Technology – NEST Mechanical Engineering Department Worcester Polytechnic Institute, Worcester MA 2 Eaton-Peabody Lab, Massachusetts Eye and Ear Infirmary, Boston MA 3 Harvard Medical School, Cambridge MA, USA ABSTRACT

Recent technological advances of miniaturization engineering are enabling the realization of components and systems with unprecedented capabilities. Such capabilities, which are significantly beneficial to scientific and engineering applications, are impacting the development and the application of optical metrology systems for investigations under complex boundary, loading, and operating conditions. In this paper, and overview of metrology systems that we are developing is presented. Systems are being developed and applied to high-speed and high-resolution measurements of shape and deformations under actual operating conditions for such applications as sustainability, health, medical diagnosis, security, and urban infrastructure. Systems take advantage of recent developments in light sources and modulators, detectors, microelectromechanical (MEMS) sensors and actuators, kinematic positioners, rapid prototyping fabrication technologies, as well as software engineering. Keywords: Optical Metrology, Miniaturization Engineering, Shape and Deformation Measurements.

1. INTRODUCTION The US National Academy of Engineering has identified 14 challenges awaiting engineering solutions in the near future [1]. Among these challenges is the need for the development of innovative tools to enable engineering and scientific discoveries in such areas as sustainability, health, medical diagnosis, security, urban infrastructure. To be effective, many of such tools have to be qualitative and quantitative as well as capable of measuring both the shape and the deformations of components or systems as a function of series of specific stimuli and environmental conditions. Shape is important because it is related to the function to be performed and deformations are related to the accuracy of operation, performance, and integrity. Development of tools to perform shape and deformations can be considered a challenge because of the required spatial and temporal resolutions needed in new applications. In addition, such new applications may require measurements under actual operating conditions or when components are subjected to multiphysics environmental and loading conditions. Optical metrology methodologies can meet both, spatial and temporal resolutions, theoretically. However, to be more effective and to be used under the operating conditions of a component of interest, recent technological advances should be incorporated into all of the aspects defining an optical metrology system. Namely, a metrology system is comprised of the following subsystems • • • •

Light-source, light-source modulator, and detector, Sample loading mechanisms as well as kinematic positioners for sample and/or measuring probes, Computer and software for control and data acquisition; and, because of the amount of recorded and processed data, Software for visualization, interpretation, and mining of large data sets.

Miniaturization, defined as the incorporation of millimeter, micrometer, and nanometer scale technologies, can have a significant impact into the metrology capabilities of most of these subsystems as scaling laws of physics enable the definition of components that can, among other characteristics, (a) operate at higher speeds than larger systems; (b) undergo minimal thermomechanical distortions; (c) have fewer problems in vibration because of inherent high natural Speckle 2012: V International Conference on Speckle Metrology, edited by Ángel F. Doval, Cristina Trillo, J. Carlos López-Vázquez, Proc. of SPIE Vol. 8413, 84130T © 2012 SPIE · CCC code: 0277-786X/12/$18 · doi: 10.1117/12.981668 Proc. of SPIE Vol. 8413 84130T-1 Downloaded From: http://proceedings.spiedigitallibrary.org/ on 07/11/2014 Terms of Use: http://spiedl.org/terms

frequencies; and (d) define complex systems within small footprints [2,3]. To illustrate the application of miniaturization, this paper shows examples of metrology systems to tackle some challenging metrology applications in engineering and medical fields.

2. EXAMPLES OF METROLOGY SYSTEMS In this Section, an overview of metrology systems that we are developing is presented. Specifically, (1) a 3D shape and deformations measurement system based on Micro-mirror Device (DMD) technologies [4,5], which is currently being applied to challenging metrology applications in art-conservation, civil engineering, and nondestructive testing; (2) a digital holographic otoscope, which is currently being applied and developed to measure both the shape and the 3D deformations of human tympanic membranes [6-14]; and (3) an interferometric readout system for a new class of MEMS-based infrared imaging detectors [15,16]. In these systems, miniaturization and the integration of multiple sensors and controls within suitable user-friendly hardware and software are highlighted. 2.1. 3D shape measurements with high-speed DMD-based fringe projection Noninvasive techniques for surface measurements have become paramount for quality analysis in industrial applications, art-conservation and restoration, as well as precision aid in medical procedures. Continued development of optical measurement systems enhances the versatility, applicability, and repeatability required by industry. Additionally, integration of 3D measurement techniques with computer aided design (CAD) software and computer aided manufacturing (CAM) equipment provides opportunities for reverse engineering [4,5]. The critical advantage of the fringe projection optical technique is its ability to provide full field-of-view (FOV) information. Although this technique is promising, limitations in speed and difficulties in achieving sinusoidal projection patterns have restricted many systems and their potential applications. For fringe projection, sinusoidal patterns can be critical because they minimize errors in shape reconstruction algorithms. 3D image reconstruction is achievable through several image unwrapping techniques. In our developments, we apply optimized Temporal Phase Unwrapping (TPU) algorithms that utilize varying fringe frequencies to recover shape information in the time domain [4,17]. Such algorithms were developed based on robustness and error analyses showing optimal projection patterns for TPU. The resulting system has unprecedented versatility to accommodate a variety of applications with the resolution and speed requirements. Hardware systems are integrated into user-friendly software [4,14]. 2.1.1. System setup The Fringe Projection system consists of two major components, a spatial light modulator (SLM) and a digital chargedcouple device (CCD) camera, shown in Fig. 1. The SLM contains a digital light processing (DLP®) unit from Texas Instruments called DLP Discovery™ [18]. The system uses a Digital Micro-mirror Device (DMD) with a 1080 × 1920 chip resolution. Each of the independent 10.8 × 10.8 μm2 micro-mirrors is controlled by a duty cycle representing a percentage of time each mirror is in the “on/off” state; thus, the SLM has intensity modulation control and the capability to produce sinusoidal fringe patterns. The second component of the system is a CCD camera with 640 × 480 chip resolution, 7.4 x 7.4 μm2 pixel size, a maximum frame rate of 208 frames per second at this resolution, and a bit-depth of 14-bit. Depending on the application and required field-of-view (FOV), the CCD camera can be interchangeable. Gray scale sinusoidal projection is achieved in our system by controlling each of the mirrors, or pixels, in the DMD, shown in Fig. 2, by setting the duty cycle for each of the mirrors appropriately for the desired gray scale. In addition, the camera’s exposure time is set to a level corresponding to the maximum time a mirror can be in the on-state to represent a completely light fringe. Over this exposure, the camera will integrate, or average, the light intensity of other pixels and produce an equivalent to a gray scale level [4,5,14]. Current developments enable the projector to change bit-depth rapidly from 5 to 14 bits at an equivalent effective range of 32 gray levels to 16384 gray levels. A calibration method determines the duty cycle to produce the most appropriate gray scale depending on fringe density. Higher bit-depths result in more accurate sinusoidal representations, but slower the acquisition speeds to a few frames per second (fps). Lower bit-depth projections can maintain speeds to process and display information on the order of 200 fps. Our software and controls have been designed to perform TPU at high speeds in order to display and record unwrapped phase and, therefore, enabling the capability to perform deformation measurements as well.

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AOI θ

CCD

SLM

PC (a)

(b)

Fig. 1. Fringe projection setup that we are developing: (a) a schematic of the system shows the SLM and the CCD camera separated by triangulation angle, θ, both interfaced into a laptop computer; and (b) realization of our system with an art sculpture under examination [4].

Number of mirrors: 1080 x 1920

20 μm (a)

(b)

Fig. 2. SLM developed by Texas Instruments and used in our shape measurement system [18]: (a) DMD chip; and (b) enlarged view of micro mirrors enabling sinusoidal fringe projection.

2.1.2. Representative applications 2.1.2.1. Road measurements at driving speeds Potholes, cracks and uneven pavement are costly to the average driver. These conditions can occur as a result of weather, wear, car accidents, or construction and are a danger to both drivers and pedestrians. Therefore, it is important to be able to record and evaluate these conditions so that a base for improvement can be identified. VOTERS (Versatile Onboard Traffic Embedded Roaming Sensors) is a project designed to provide an accurate, detailed road assessment and maintenance system [19]. The program is part of the National Institute of Standards and Technology’s (NIST) Technology Innovation Program and its overall goals involve sensing and detection systems that can be adhered to a vehicle in order to map out large areas. There are three main sensing subsystems to be developed for VOTERS in order to gather maximum information of road quality: acoustic systems to measure particle interaction with tires during normal driving; ground penetrating radar to detect subsurface delamination and corrosion; and optical profilometry to measure surface shape and detect anomalies. With the development of the 3D shape measurement system, our group was chosen for the detection and quantification of cracks at driving speeds. For this application, our shape measuring system has been incorporated as part of the developments for the VOTERS testing vehicle, as shown in Fig. 3.

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(a)

(bb)

Fig. 3. Frringe projectionn system being used as part of the VOTERS program p [19]: (aa) prototype proojector on testin ng vehicle; and (b) typicaal road surface measurement.

n 2.1.2.2. Scullpture digitizaation for art conservation Sculptures annd other three dimensional art forms porttray importantt parts of histoory dating bacck thousands of years whilee it is astoundiing that somee have survivved to date. The informattion and educcational valuees that they can c provide too students and art scientists are endless. Over the reecent years, cultural piecess have been tthreatened by many factorss including, buut not limited to, t population growth, urbann developmen nt, natural disaasters, man-m made environm mental hazardss. Therefore, prreservation of these pricelesss pieces relatees directly to the t scientific field f via 3D ddigitization.

(b)

100 mm m

(a)

(c)

(d)

F 4. Digitizattion of an Oran Fig. nt sculpture at thhe Worcester A Art Museum, Wo orcester Maassachusetts [200]: (a) sculpturee of interest; (b)) meshed data ccloud; (c) and (d d) surface rendered measu urements at diffferent magnificcations.

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Our group has been collaborating with the Worcester Art Museum, Worcester, Massachusetts in an effort to apply laser holography methods, including fringe projection methods to the investigation of important art pieces. With respect to fringe projection, we have applied our developments to the digitization of late 4th – 3rd century BC Orant sculptures. The objects are named Funerary Statue of a Young Maiden made of Terracotta with kaolin slip. Although the artist is unknown, it is evident that the pieces come from Canosa, South Italy [20]. Figure 4 shows representative 3D digitization results of one Oran sculpture as obtained with our fringe projection system. Among several applications, results will be used to further assess and understand the origins and the geometries embedded in the sculpture. 2.2. Digital holographic otoscope system for 3D displacements and shape measurements of tympanic membranes The rapid growth of the precision medical equipment industry has led to the development and use of even more accurate, sophisticated, and small-scale medical tools. Specifically, diagnosis and treatment of human ear disorders are challenging aspects and significant research and development activities have led to the study and the examination of ear mechanics more than ever before. Conventional ear examination methods asses the state of a patient’s hearing mainly based on qualitative visual estimation of the healthiness of the tympanic membrane (TM), which plays an important role in the transmission of sound into the inner-ear. More advanced procedures assess the response of the TM to controlled acoustic stimuli, and while giving valuable quantitative feedback of the healthiness of the patients hearing, these procedures are limited to providing information either of overall average responses or of the responses at only few points on the TM. Therefore, leaving many unknowns relative to the complex deformation patterns that unfold across the entire surface of the membrane when subjected to acoustic stimulation. In order to overcome such limitations, we are developing an advanced computercontrolled digital optoelectronic holographic system (DOEHS) that is capable of measuring both the shape and the 3D displacements of the entire surface of the TM [11-14]. Our DOEHS is capable of providing near real-time, full-field-of-view, quantitative measurements of the sound-induced nanometer scale motions of the TM. Two versions of the DOEHS have been deployed in the clinic for research on postmortem samples and one of these systems is being tested and optimized for measurements in-vivo. It is our intent to eventually bring DOEHS capabilities to an otology clinic for examinations of patients [12]. 2.2.1. DOEHS setup The system consists of physically independent modules allowing easy assembly/disassembly and mobility for transportation between examination rooms. In general, the main modules of a DOEHS include: (1) image-processing computer; (2) low-power laser-illumination and delivery; (3) otoscope head assembly comprised of a digital holographic interferometer together with a sound presentation device for the delivery of controlled acoustic stimulation and for the monitoring of presented sound pressure levels; and (4) mechatronic otoscope positioner for bringing and holding the otoscope head assembly near a patient’s ear.

(a)

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(b) Fig. 5. Modular DOEHS under development for clinical use [12]: (a) overall schematic representation; and (b) examination of a patient at a clinical setting. Optical head (OH), mechatronic otoscope positioner (MOP), and head-restraining positioner (HRP) are shown near a patient.

The DOEHS can operate in lens and lensless digital holographic modes, can use different digital cameras depending on the required field-of-view and type of samples under investigation, and can operate in single or multiple lasers modes [12,13]. Figure 5 shows a schematic representation of a typical DOEHS configuration and its utilization in the clinic. One of the important challenges in the development of the DOEHS is the design and realization of the otoscope head assembly (OH), which has been synthesized and optimized to have: (a) sufficient field-of-view and working distances, which are constrained by the anatomy of a variety of ear canal dimensions; (b) sufficient thermo-mechanical and inertial stabilities to minimize undesirable effects on the interferometer and on the characteristic of the mechatronic otoscope positioner (MOP); (c) sufficiently small footprint to make it applicable for clinical use; and (d) capable to accommodate optical devices as well as actuators and sensors for the presentation and monitoring of sound. Figure 6 shows schematic of an optical head assembly arranged for measurements by lensless digital holography. (Alignment)

CCD camera (Holographic measurements)

FOV

10 mm

Fig. 6. Opto-mechanical arrangement of the optical head of DOEHS for TM measurements by lensless digital holography. Point source of illumination is realized by single-mode fibers. Sound presentation system (SPS) is embedded as well as a lipstick camera that is used for positioning and optical adjustments.

2.2.2. Representative measurements TM’s of different species, including humans have and are being investigated with DOEHS at the Mass Eye & Ear Infirmary (MEEI), Boston. Measurements are typically performed in time-averaged mode for identification of acoustically induced deformation and stroboscopic double-exposure mode for quantification of shape and static/dynamic deformations. Figure 7 shows representative measurements of chinchilla TM deformations obtained in time-averaged mode and Figs 8 and 9 show stroboscopic double-exposure measurements of the same sample. TM is approximately 6 mm in diameter and the field-of-view (FOV) shown is on the order of 3 mm.

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Fig. 7. R Representative time-averaged t i interferograms of chinchilla TM (FOV is ~ 3 mm) at differeent acoustical stimulation frequencies. We W have classiffied deformatioons as simple, co omplex, and orrdered since we have observed that deforrmation patternss undergo transsition from one to another as thhe frequency off excitation is increassed. Understannding this type of o behavior is work w in progresss. The manubriium of the maalleus is highligghted by dashedd lines. S – P arre the superior and a posterior axxes.

Fiig. 8. Represenntative strobosccopic double-exxposure measureements of chincchilla TM at thee sound excitatiion frequuency of 6,979 Hz at the soundd level of 91 dB B SPL: (a) modu ulation map; (bb) wrapped phasse map mod(2π); and defoormation map coorresponding too 389 nm peak-tto-peak. The manubrium m of thhe malleus is hiighlighted by daashed lines. S – P are the superior an nd posterior axees.

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Fig. 9. Meaasured displacem ment maps of chinchilla TM saample at the coonditions shownn in Fig. 8 for phaase positions 0--270° (a – d) off its vibration cyycle.

dout system for fo a MEMS infrared i imag ging detectorr 2.3. Interferrometric read A type of MEMS-based M u uncooled infrrared imagingg detector con nsists of an array a of bi-m metallic microsstructures thaat deform propoortional to inffrared radiatioon absorbed from f a scene [15,16,21,22]. In these miicrostructures, one materiaal needs to be an a efficient inffrared absorbeer and the diff fference betweeen coefficiennts of thermal expansion between the twoo materials neeeds to be as large as posssible to maxximize the deeflection of the t microstruuctures with respect r to thee temperature of a scene. An electronnic signal caan be measu ured through a change inn capacitance between thee microstructurres and the subbstrate. Howeever, such elecctronic readou ut is complicaated and costlyy to fabricate. Additionallyy, electronic siggnals can introoduce additionnal noise from m heat related effects e [15,16]]. Optical readoout systems prrovide a transsduction alternnative by defiining one of the t two metalllic layers to be b an efficiennt reflector of visible v light. In I such readouut system, lighht aimed at th he array is refllected off of tthe microstrucctures with thee intensity of the t reflected light l detected by a camera correspondin ng to the defoormation of a microstructurre and thus too spatial tempeerature distribuutions. In thiis case, measuuring noise caan be controllled by adjustm ments to the optical o readouut system and thhe design of thhe detector caan be simplifieed. An image of an individuual microstruccture element of an array of interest is shoown in Fig. 100 together withh a partial view w of the entiree array [15]. Ideally, everyy pixel on thee array shouldd undergo sim milar amounts of deformatiions at the sam me detected temperature t inn order to prodduce an accuraate optical reaadout. Howeever, because of potential geometrical g deeviations intro oduced duringg fabrication, the t intensity of the reflectted light withh respect to temperature t c changes can vvary across the t array andd, therefore, lim miting the meaasuring capabilities of the opptical readout system.

40 μm μ

(a)

(b)

Figg.10. Bi-metalllic structure of a type of MEM MS-based uncoo oled infrared im maging array: (a)) photomechaniical pixeel with thermal isolator; and (bb) partial view of o a 540 × 460 MEMS M pixels aarray.

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To overcome the effects produced by fabrication, holographic interferometry is implemented as the optical readout method. Specifically, in double-exposure holographic mode, a phase map is measured at a reference state of deformations and subsequent phase maps are subtracted from the reference phase map in order to measure optical phase differences introduced by only the deformations produced by time and space varying IR thermal loading. In our implementations, the IR absorbing side of a MEMS opto-mechanical detector is mounted at the focal point of a long wave infrared imaging lens. The reflective side of the array is focused on the object path of a Linnik interferometer with 4× magnification objectives. The MEMS detector, housed in a vacuum-sealed package, is viewed through an optical window that introduces an additional optical path length difference greater than the coherence length of the LED source of illumination used. Therefore, a compensation window of the same material and thickness as the optical window is placed in the reference path to correct for the optical path length difference. The measurements are performed in real-time and in full-field-of-view. Figures 11 and 12 show components of the implemented interferometric readout system, including a blackbody setup used for calibration/characterization and Fig. 13 shows representative thermograms that highlight thermal measuring capabilities.

Fig. 11. Interferometric-based thermal imaging system showing the configuration of the individual components.

Fig. 12. Thermal imaging system with blackbody calibration/characterization setup. Setup consists of a collimator, target wheel, and differential blackbody. Target wheel observed by the imaging system is controllable to 0.001 K.

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(a)

(c)

(b)

(d)

Fig. 13. Real-time recorded thermograms obtained by developed interferometric-based thermal imaging system (a)-(b) and by a commercially available thermal imaging camera (c)-(d) . Temperature difference between the skin and sleeve in (a) and (c) is ~20 K whereas the temperature difference between the skin and eyes in (b) and (d) is ~0.6 K. Spatial resolution in (a) and (b) is 40 × 40 MEMS pixels and in (c) and (d) is 640 × 480 bolometer pixels [16].

3. CONCLUSIONS AND RECOMMENTATIONS As progress in miniaturization engineering continues, more robust and capable metrology systems will be developed, which will be heavily based on the availability of new light sources and modulators, detectors, MEMS sensors and actuators, kinematic positioners, rapid prototyping fabrication technologies, as well as computer and software engineering. Though the development and application of optical metrology systems rely on the availability of such new technologies, multi-disciplinary engineering design, manufacturing, and materials science approaches that exploit the somehow conventionally different physical laws of miniaturization should be applied in order to realize innovative solutions to solve complex metrology problems. The overview presented in this paper showed progress in the development of metrology systems that are being applied to challenging problems related to shape and deformation measurements under actual operating conditions. The realization, the successful application, and the future improvements of these optical metrology systems will continue to be based on miniaturization engineering.

4. AKNOWLEDGEMENTS Portion of this work has been funded by the US National Institute on Deafness and Other Communication Disorders (NIDCD), the Massachusetts Eye and Ear Infirmary (MEEI), and the Mittal Fund. The authors also gratefully acknowledge the support of our sponsors and particularly of John J. Rosowski, Frank Pantuso, John Tyson, and Philip Klausmeyer as well as of the NanoEngineering, Science, and Technology (NEST) program at the Worcester Polytechnic Institute, Mechanical Engineering Department.

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