An Outline of a Complementary Inspection System for Micro-Electro- Mechanical System (MEMS) Devices Based on Radiography and Plenoptic Camera Speaker: Shu-Mei Tan a Alvin Chong a , Guojin Feng a , Jamil Kanfoud a , Tat-Hean Gan a,1 a Brunel Innovation Centre (Brunel University London), Granta Park, Great Abington, Cambridge, CB21 6AL, United Kingdom 1 Corresponding Author. Prof. Tat-Hean Gan (email: Tat-Hean.Gan@brunel.ac.uk) 11.09.2018 - 13.09.2018
Content Introduction (MEMS and Project Overview) Samples Identified To Be Studied CITCOM System Overview Plenoptic Camera Subsystem Nano-focused X-ray Subsystem Inspection Procedure and Automated Defect Recognition (ADR) Conclusions and future work
Introduction to MEMS Micro-Electro-Mechanical Systems (MEMS) integrates miniaturized mechanical and electro-mechanical components. The components are typically integrated on a single chip using advanced micro fabrication techniques. Typical MEMS devices usage for sensing: accelerometer, Electrical Mechanical gyroscopes, flow sensor, microphone etc … Applications: Aerospace, automobile, medical, etc … Example of MEMS – Analog Devices three-axis accelerometer ADXL330
Types of MEMS applications Mechanical Chemical & Optical Medical /Thermal Biological • Strain gauges • Optical switch • Micro-fluidic • Gas sensors system • Accelerometers • Lens • Glucose (micropump) collimators & sensors • Pressure sensors focusers • Organ-on-Chip • Microphones devices • Tunable • Gyroscopes optical filter • Smart • Flow sensors catheter • Temperature sensors Network tunable filter Inertial measuring unit Smart contact lens Thermal infrared Gas (Co) sensor (X-IMU) – detect glacoma temperature sensor
Benefits of MEMS sensors Smaller in size Lighter in weight Can have lower power consumption Can have higher sensitivity to input variations Can have high integration capability with electric circuits Cost-effective (mass production) Advanced wafer fabrication process and innovative material such as Deep Reactive Ion- Etching (DRIE) and Cavity Silicon on Insulation (CSOI) has enabled high performance MEMS device. However, such components may be impaired in several ways during fabrication and assembly stages resulting in damages or/and structural failures!
Sample to be studied on this project Sample 1: Capacitive Micro-machined Ultrasound Transducer (CMUT) Scanning Intravascular Ultrasound (IVUS): Smart catheters used to aid measuring of diameter of vessels for angioplastry surgery. Capacitive Micro-Machined Ultrasound Transducers (CMUTs) gained increased popularity due to advantages of smaller size and permitting higher ultrasonic frequencies. IVUS scanning catheter and example of images produced CMUT is made by the deposition of three metal layers separated by dielectric layer Optical micrograph image of CMUT Top metal electrode can move freely after the sacrificial centre metal layer has been etched. When voltage is applied between the electrodes, they electrostatically attract each other thereby generating a pressure wave. Repeating this very quickly generating ultrasonic wave. Schematic cross section of CMUT
Cont. Sample to be studied on this project Sample 1: Capacitive Micro-machined Ultrasound Transducer (CMUT) The Flex-to-Rigid (F2R) technology such as flex-foil approach is employed to allow electronics to fold around or into the catheter tips as shown. (i) F2R structures are fabricated (wafer scale) (ii) After fabrication the partly flexible structures are removed from the silicon wafer and assembled around the catheter tip. (iii) Final assembled CMUTs Common defects such as scratches, foreign particles, lithography and etching problems.
Cont. Sample to be studied on this project Sample 2: Electrically conductive adhesives Excess glue shorting bond pads Standard high temperature soldering or wire bonding for electronic assembling and packaging are becoming incompatible with advanced MEMS devices. Example: organ-on-chip devices where the diagnostic medical chips may contain printed proteins. Insufficient glue Silver filled epoxies are used as an alternative. Could cause quality issues. Glue on top of cap (also misplaced)
Needs for CITCOM System Failure modes can developed during the microfabrication process and assembly of these MEMS devices into packages and Printed Circuit Boards (PCBs). Although there are planar inspection tools utilised for such MEMS devices quality assessment, most of the inspection is based on electrical parameters. This potentially poses a problem from the manufacturing point of view where critical defect go un-noticed. especially for transport and medical applications, as the reliability is paramount. Presence of subjective error dues to the use of manual or semi-autonomous inspection tool. Typically in a semi-autonomous inspection, operator have to manually orientate the MEMS devices and define the test points which could be randomly selected. The likelihood of missing critical defects can be high in such testing approach and inevitably adds cost to the production. The proposed system aims to tackle the reliability issues for MEMS devices (potential test cases identified above) by offering an automated 3D structural inspection system applied to inspect high value component production at both in-line and near-line processes which is reliable, accurate and cost-effective.
System Overview: Plenoptic Camera Subsystem Plenoptic camera (light field camera) to capture 2D image with depth information. This is achieved by using micro-lens array (MLA) in front of the image sensor. MLA re-group sensors (e.g. CCD, CMOS) into small groups. Capture information on the direction, colour and luminosity of individual rays of light. Computationally reconstruct the light field (3D). Integration of the plenoptic camera, all optical and opto- mechanical components, x-y translational stages, z-stage (for focus adjustment) and control software.
Cont. System Overview: Plenoptic Camera Subsystem Example of output – view in 3D point cloud of a component with foreign particle. Aim towards in-line inspection (relatively fast). Integration of the plenoptic camera, all optical and opto-mechanical components, x-y translational stages, z-stage (for focus adjustment) and control software. Automated Defect Recognition (ADR) based on classical image processing approach.
Cont. System Overview: Nano-focused X-ray Subsystem Aim towards near-line inspection. Preliminary results show a pixel based photon counting CMOS detector is promising due to image sharpness, absorption efficiency and Signal-Noise Ratio (SNR). Currently in-progress to select the optimum X-ray source and detector. Automated Defect Recognition (ADR) based on classical image processing approach.
Cont. System Overview: Inspection Procedure and Automated Defect Recognition (ADR) One area One MEMS Stitched scan comonent image Overlapped scan Silicon wafer Silicon wafer Silicon wafer Step and scan Stitched image Segmentation As an illustration, single wafer may contain hundreds of MEMS/micro device. Step and scan (with an overlap between adjacent scan) will be required to image the complete wafer. Image stitching is then performed to obtain a combined image. Subsequently, segmentation will be implemented for further post-process / analyze for feature extraction.
Cont. System Overview: Inspection Procedure and Automated Defect Recognition (ADR) Preliminary results: plenoptic camera image Plenoptic camera (R12) parameters Total focused image and its Region of Interest (ROI) 3D point cloud view showing anomaly Various noise reduction, features enhancement and potential machine learning algorithms will be studied. Estimated depth map and its ROI
Cont. System Overview: Inspection Procedure and Automated Defect Recognition (ADR) X-ray system Xradia 520 Versa machine X-ray excitation voltage 30 kV X-ray excitation current 67 µA Exposure time 160 s Pixel size 0.1 x 0.1 µm Inspected on CMUT sample. Some dark areas observed for certain CMUT highlighted in the red circles. These areas are enclosed so it’s unlikely to be foreign particles Suspected to be incomplete etching of the sacrificial aluminum layer. Such internal defect cannot be observed through optical inspection technique. Example of X-ray projection image result
Conclusions and Future Work Outline of the proposed CITCOM inspection system based on plenoptic camera and X-ray is described. Preliminary tests conducted on CMUT have shown the applicability of both plenoptic camera and X-ray as complementary techniques for the inspection of 3D MEMS devices. Further work on optimising the hardware for better resolution, efficiency, image processing and machine learning for ADR. Final trial planned to perform at End- Users’ facilities.
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