STMicroelectronics MEMS Microphone -- Reverse Engineering Analysis

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This is a reverse engineering report of the STMicroelectronics MP34DT01 omnidirectional digital microphone. Details include a full description, tear down analysis and 3D model of the MEMS microphone with cross-sections and SEM images. The reports also includes a full review of the packaging strategy and a description of the sensor assembly process. Furthermore the report has 40 descriptive images, background on the application, performance specifications, interconnect strategies, materials used, EMC strategy description, an electrical schematic, chip attachment means, strengths and weaknesses of the design and links to the patent, data sheets and more.

The report is extremely useful for engineers and business leaders looking to better understand MEMS microphone design, packaging and assemblies processes. It is also beneficial within the MEMS microphone community as a competitive analysis tool.

Published in: Technology, Business

STMicroelectronics MEMS Microphone -- Reverse Engineering Analysis

  1. 1. STMicroelectronics MEMS Microphone Reverse Engineering Analysis 1 Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  2. 2. 2 Reverse Engineering of STMicroelectronics MEMS Microphone Introduction: This is a reverse engineering report of the STMicroelectronics MP34DT01 omnidirectional digital microphone. Details include a full description, tear down analysis and 3D model of the MEMS microphone with cross-sections and SEM images. The reports also includes a full review of the packaging strategy and a description of the sensor assembly process. Furthermore the report has 40 descriptive images, background on the application, performance specifications, interconnect strategies, materials used, EMC strategy description, an electrical schematic, chip attachment means, strengths and weaknesses of the design and links to the patent, data sheets and more. The report is extremely useful for engineers and business leaders looking to better understand MEMS microphone design, packaging and assemblies processes. It is also beneficial within the MEMS microphone community as a competitive analysis tool. Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  3. 3. Table of Contents 3 • • • • • • • • • Application Microphone Description and Key Manufacturer Specifications Description of Package Design and Function Description of Package Assembly Process MEMS Microphone Design Signal Conditioning Strategy Electrical Schematic Review of Product’s Strengths and Weaknesses Summary of Workmanship Quality Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  4. 4. 4 STMicroelectronics Omnidirectional Digital Microphone • Part Number: MP34DT01 STMM ro p ho ne ic US Pe nny Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  5. 5. Application 5 • • • Digital, omnidirectional microphones for mobile phones and tablets, portable computers and media players, VoIP, speech recognition, cameras and video equipment, gaming equipment and antitheft systems where limited height and size is critical and a top acoustical port is required STMicroelectronics provides a MEMS accelerometer and gyroscope for the Apple iPad but has yet to win a microphone socket with Apple STMicroelectronics provides MEMS microphones for Nokia mobile phones, HP and Asustek Computer laptops with 15 million units shipped in 2011 (Bouchaud 2012) Apple iPad Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  6. 6. 6 Microphone Description and Key Manufacturer Specifications • STMicroelectronics’ omnidirectional, digital MEMS microphone MP34DT01 – Capacitive sensing element and discrete IC for signal conditioning – Holed cap land grid array (HCLGA) plastic package (3 x 4 x 1 mm) top port design – EMI shielded – SMD and RoHS / ECOPACK compliant – Pulse Density Modulation (PDM) data output – Sensitivity: -29 to -23 dBFS (decibels with reference to full-scale digital output) – Acoustic Overload Point (AOP): 120 dBSPL (Sound Pressure Level) – Signal-to-noise Ratio: 63 dB (A-weighted at 1 KHz, 1 Pa) – Power Supply Rejection: -70 dBFS – Supply Voltage: 1.64 to 3.60 VDC – Operating Temperature: -40 to 85°C (storage: -40 to 125°C) – Current Draw: 0.6 mA (typical, short circuit 10 mA max) – Turn-on Time: 10 msec Adobe Acrobat Document – See technical data sheet for more product specifications Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  7. 7. Description of Package Design and Function 7 • • • Holed cap land grid array (HCLGA) package (3 x 4 x 1 mm) Adobe Acrobat Adobe Acrobat Top port design with plastic package specifically Bismaleimide Triazine (BT) Document Document Information regarding the design and process are provided in US Patent Applications US 2012/0153771A1 and 2011/0266640A1 (see pdf’s above) Bottom Isometric View Top Isometric View Acoustic Port BT Adhesive Seal Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com G5 X G5 Pin 1 Indicator Metallization For Mounting And Electrical Interconnect
  8. 8. Description of Package Design and Function 8 • Isometric View of Microphone Assembly without Bottom Plate Microphone Assembly Bottom Plate Solder Pads Solder Interconnects MEMS Microphone Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com EMI Shield ASIC
  9. 9. Wirebond Pads from ASIC to Package 9 Gold Wirebonds Wirebonds start at ASIC And Continue to Lands (Next to Solder on Package Sidewall) Wirebond Broke When Opening Wirebond Pad Lands (Next to Solder) ASIC Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com Microphone
  10. 10. Gold Wirebonds from MEMS Microphone to ASIC 10 Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  11. 11. Wirebond Ball Bond on ASIC 11 ASIC Circuitry. BT Fragments From Opening Microphone Asm. 20 micron OD Gold Wire Used Gold Ball. Wirebond Pad Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  12. 12. Stitch Bond Over Gold Ball on MEMS Microphone 12 Stitch Bond. MEMS Microphone Back Plate BT Fragments From Opening Microphone Asm. Gold Ball. Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  13. 13. Cross Section of Microphone Assembly 13 Copper Via Lead-free Solder Interconnects Electrical Isolation Groves Back Chamber Adhesive Seal Bottom Plate Side Walls ASIC First Metallization Layer Second Metallization Layer EMI Shield Front Chamber MEMS Microphone Acoustic Port (0.4 mm diameter) Top Plate Soft Die Attach Thin Metal Film Seed Layer Note: Second metallization layer is also solder and wirebond pads Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  14. 14. Description of Microphone Assembly 14 • • • • • • BT is used for the external packaging as it is easily routed to desired size and geometry using high volume, low cost processes. The first and second metallization layers create a grounded EMI shield, minimizing the effect of electromagnetic disturbances, that surrounds the majority of the package. Only the area between the side wall and bottom plate (area of adhesive seal) does not have the EMI shield present (vertically). The acoustic port leads to the front chamber of the microphone. Front chamber geometry optimization provides improved sensitivity to the acoustic sensor. The back chamber is above the microphone in the cross section image and fills the reminder of the package providing negligible resistance to the air flowing through the holes in the back electrode / plate of the MEMS microphone. Both the ASIC and MEMS microphone are attached using a soft adhesive. An adhesive seal located between the bottom plate and package side walls surrounds the periphery of the device and seals it from the environment. The seal may flow into the electrical isolation groves. Lead-free solder is used to make electrical interconnect between the package and the bottom plate. Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  15. 15. Description of Package Assembly Process 15 • • The first metallization layer is applied to the top plate and core plate that will eventually become the package side walls An adhesive layer is applied to one side of the core plate over the first metallization layer First Metallization Layers Top Plate Assembly Core Plate Assembly Adhesive Layer Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  16. 16. Description of Package Assembly Process 16 • The top and core plates are bonded • A blind hole is cut forming side walls up to the first metallization layer on the top plate Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  17. 17. Description of Package Assembly Process 17 • Seed and second metallization layers added Seed Layer • Second Metallization Layer Acoustic port fabricated Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  18. 18. Description of Package Assembly Process 18 • • Isolation groves fabricated using a diamond-saw cutting tool Package cleaned Isolation Groves Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  19. 19. Description of Package Assembly Process 19 • • MEMS Microphone and ASIC attached to the top plate with soft adhesive Chips wirebonded to each other and the package (not shown) Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  20. 20. Description of Package Assembly Process 20 • Adhesive dispensed Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  21. 21. Description of Package Assembly Process 21 • • Bottom plate is assembled to the package with lead-free solder to electrically couple the package to the bottom plate and form an environmental seal Completed devices are diced (all preceding process steps are in panel form) Dicing Line Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com Dicing Line
  22. 22. MEMS Microphone Description and Function 22 • • • • STMicroelectronics collaborates with and sources their microphones from Omron Corporation The MEMS microphone is a capacitive based acoustic sensor with a polysilicon membrane that flexes with applied acoustic pressure from sound. The diaphragm is anchored to the handle wafer with silicon dioxide posts at the four corners and freely suspended else where around its circumference. A second fixed electrode (also polysilicon) completes the capacitor and is housed in a thicker silicon nitride back plate. Both the rigid electrode and the back plate are proliferated with holes to allow air flow to minimize the acoustic impedance as the distance between the plates oscillates. The holes also provide the etchant access to remove the sacrificial layers during assembly. Acoustic hole OD is ~18 micons. A barometric vent is created between the diaphragm, handle wafer and silicon nitride back plate around the diaphragms edges. This equalizes changes of ambient pressure on each side of the diaphragm. Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  23. 23. MEMS Microphone Description and Function 23 • • • • The front chamber’s double angled geometry is fabricated using a TMAH wet etch process. This enables a larger chamber to be formed than a straight angled design seen in wet etched pressure sensors. The wirebond attachment on the MEMS microphone is a gold stitch on a gold ball. This was likely done to create a more robust joint. Depending on the metallization layers making up the wirebond pad, STMicroelectronics may have had some trouble with the robustness of using a stitch bond alone. Gold ball bonds have better adherence to substrates than stitches and the gold ball provides a thick, robust substrate for the gold stitch forming a stronger bond. Mechanical stops protrude from the silicon nitride back plate towards the diaphragm to prevent the diaphragm from making contact to the fixed electrode. This serves two purposes; it prevents 1) the diaphragm from sticking to (not able to return to normal position) the fixed electrode when excess voltage is applied due to electrostatic attractive forces and 2) a similar sticking phenomena due to surface tension when moisture enters the cavity and the electrodes come in contact. A gold guard ring appears to be used to minimize the effects of the non uniform electric field at the capacitors edges thus improving its performance. Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  24. 24. MEMS Microphone Images and Description 24 Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  25. 25. MEMS Microphone Images and Description 25 Anchor Point of Diaphragm MEMS Microphone Minor Damage From Cleaning Fixed Back Plate (see through) Guard Ring Wirebonds Polysilicon Electrode In Back Plate End of Polysilicon Electrode in Back Plate Diaphragm Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  26. 26. MEMS Microphone Images and Description 26 Back Plate Front Chamber Cross Section of Microphone EMI Shield Soft Die Attach BT Substrate Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  27. 27. MEMS Microphone Images and Description 27 Acoustic Holes In Back Plate Mechanical Stop to Prevent Electrode Contact Minor Debris from Opening Assembly Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  28. 28. MEMS Microphone Images and Description 28 Close Up of Microphone Corner Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  29. 29. MEMS Microphone Images and Description 29 Flexible Electrode Fixed Electrode and Back Plate Electrode Ends Here Electrode Interconnect To Wirebond Pad Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  30. 30. MEMS Microphone Images and Description 30 • Solid Model of MEMS Microphone Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  31. 31. MEMS Microphone Images and Description 31 • Cross Section Showing Diaphragm, Anchors and Vent A Front Chamber Section A - A A Acoustic Holes Fixed Electrode Barometric Vent (Green) Flexible Electrode SiO2 Anchor Silicon Nitride Back Plate Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com SiO2
  32. 32. MEMS Microphone Images and Description 32 • Enlarged Image of Cross Section Flexible Electrode SiO2 Anchor Silicon Nitride Back Plate Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  33. 33. 33 Confirmation that Silicon Nitride Was Used for Back Plate Note: Similar analysis was done for several materials in the design but is not shown. Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  34. 34. Signal Conditioning Strategy 34 • • • • Discrete ASIC provides signal conditioning Pulse Density Modulation (PDM) data output No glob top present Bare die used Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  35. 35. Electrical Schematic 35 Fixed Electrode Interconnect 1 4 5 4 1 3 2 5 3 2 Vias 5 5 5 Flexible Electrode Interconnect 1. 2. 3. 4. 5. Power Supply Left / Right Channel Select Synchronization Input Clock Left / Right PDM Data Output Ground Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  36. 36. Review of Product’s Strengths and Weaknesses 36 • • Strengths – Packaging fabrication approach extremely flexible to accommodate changing designs with minimal tooling cost while incorporating a robust EMI shield – Uses high volume, low cost processes for fabrication – Small envelope with room to go smaller (but not the smallest on the market) – Eliminates the need to attach wirebonds below the ASIC’s top surface where room is limited Weaknesses – EMI shield is not present around solder joints but there is an alternate configuration in the patent that addresses this issue (should it be a problem) – Wirebond pads and solder joints are in close proximity on same land and care must be taken to ensure wirebonds are not compromised – Significantly more process steps than traditional metal can approach and hence has the potential for lower yields Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com
  37. 37. Summary of Workmanship Quality 37 • • • Approximately 10 microphone assemblies were inspected at varying levels Workmanship quality was high for all of the device areas inspected MEMS microphone and ASIC soft attachment to the substrate is one area that showed less consistency than other processes but it is not expected to result in performance issues Copyright 2013 MEMS Journal, Inc. | Consulting Services Group | consulting@memsjournal.com

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